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Sample records for aerogel thermal conductivity

  1. Study on Thermal Conductivities of Aromatic Polyimide Aerogels.

    PubMed

    Feng, Junzong; Wang, Xin; Jiang, Yonggang; Du, Dongxuan; Feng, Jian

    2016-05-25

    Polyimide aerogels for low density thermal insulation materials were produced by 4,4'-diaminodiphenyl ether and 3,3',4,4'-biphenyltetracarboxylic dianhydride, cross-linked with 1,3,5-triaminophenoxybenzene. The densities of obtained polyimide aerogels are between 0.081 and 0.141 g cm(-3), and the specific surface areas are between 288 and 322 m(2) g(-1). The thermal conductivities were measured by a Hot Disk thermal constant analyzer. The value of the measured thermal conductivity under carbon dioxide atmosphere is lower than that under nitrogen atmosphere. Under pressure of 5 Pa at -130 °C, the thermal conductivity is the lowest, which is 8.42 mW (m K)(-1). The polyimide aerogels have lower conductivity [30.80 mW (m K)(-1)], compared to the value for other organic foams (polyurethane foam, phenolic foam, and polystyrene foam) with similar apparent densities under ambient pressure at 25 °C. The results indicate that polyimide aerogel is an ideal insulation material for aerospace and other applications. PMID:27149155

  2. Study on Unit Cell Models and the Effective Thermal Conductivities of Silica Aerogel.

    PubMed

    Liu, He; Li, Zeng-Yao; Zhao, Xin-Peng; Tao, Wen-Quan

    2015-04-01

    In this paper, two modified unit cell models, truncated octahedron and cubic array of intersecting square rods with 45-degree rotation, are developed in consideration of the tortuous path of heat conduction in solid skeleton of silica aerogel. The heat conduction is analyzed for each model and the expressions of effective thermal conductivity of the modified unit cell models are derived. Considering the random microstructure of silica aerogel, the probability model is presented. We also discuss the effect of the thermal conductivity of aerogel backbone. The effective thermal conductivities calculated by the proposed probability model are in good agreement with available experimental data when the density of the aerogel is 110 kg/m3. PMID:26353566

  3. Mesoporous polyurethane aerogels for thermal superinsulation: Textural properties and thermal conductivity

    NASA Astrophysics Data System (ADS)

    Diascorn, N.; Sallee, H.; Calas, S.; Rigacci, A.; Achard, P.

    2015-07-01

    Organic aerogels based on polyurethane were elaborated via sol-gel synthesis and dried with supercritical carbon dioxide (CO2). The influence of the catalyst concentration was investigated, first in order to decrease the reaction kinetics, then to study its impact on the obtained materials properties. It was shown that this parameter also influences the global shrinkage and the bulk density of the resulting materials. Its effect on the dry materials was studied in terms of morphological, textural and thermal properties in order to determine the main correlations thanks to scanning electron microscopy (SEM), nitrogen adsorption, non-intrusive mercury porosimetry and thermal conductivity measurements. Results allowed us to demonstrate a correlation between the bulk density, the texture and the thermal conductivity of this family of polyurethane aerogels and to determine an optimal density range for thermal performance associated with a fine internal mesoporous texture.

  4. Thermal conductivity of a ZnO nanowire/silica aerogel nanocomposite

    NASA Astrophysics Data System (ADS)

    Xie, Jing; Frachioni, Anthony; Williams, D. S.; White, B. E.

    2013-05-01

    The thermal conductivity of 100 nm zinc oxide nanowires embedded in silica aerogel was measured using the 3ω method over a temperature range of 150 K to 300 K. Compared to 100 nm ZnO nanowires alone, the thermal conductivity of the nanocomposite was reduced by over an order of magnitude throughout this temperature range. We attribute this reduction to the scattering of ballistic phonons at the nanowire surface and the subsequent emission of and transport of energy by the scattered phonon into the silica aerogel, as predicted by the diffuse mismatch model.

  5. Hybrid Multifoil Aerogel Thermal Insulation

    NASA Technical Reports Server (NTRS)

    Sakamoto, Jeffrey; Paik, Jong-Ah; Jones, Steven; Nesmith, Bill

    2008-01-01

    This innovation blends the merits of multifoil insulation (MFI) with aerogel-based insulation to develop a highly versatile, ultra-low thermally conductive material called hybrid multifoil aerogel thermal insulation (HyMATI). The density of the opacified aerogel is 240 mg/cm3 and has thermal conductivity in the 20 mW/mK range in high vacuum and 25 mW/mK in 1 atmosphere of gas (such as argon) up to 800 C. It is stable up to 1,000 C. This is equal to commercially available high-temperature thermal insulation. The thermal conductivity of the aerogel is 36 percent lower compared to several commercially available insulations when tested in 1 atmosphere of argon gas up to 800 C.

  6. Improved Aerogel Vacuum Thermal Insulation

    NASA Technical Reports Server (NTRS)

    Ruemmele, Warren P.; Bue, Grant C.

    2009-01-01

    An improved design concept for aerogel vacuum thermal-insulation panels calls for multiple layers of aerogel sandwiched between layers of aluminized Mylar (or equivalent) poly(ethylene terephthalate), as depicted in the figure. This concept is applicable to both the rigid (brick) form and the flexible (blanket) form of aerogel vacuum thermal-insulation panels. Heretofore, the fabrication of a typical aerogel vacuum insulating panel has involved encapsulation of a single layer of aerogel in poly(ethylene terephthalate) and pumping of gases out of the aerogel-filled volume. A multilayer panel according to the improved design concept is fabricated in basically the same way: Multiple alternating layers of aerogel and aluminized poly(ethylene terephthalate) are assembled, then encapsulated in an outer layer of poly(ethylene terephthalate), and then the volume containing the multilayer structure is evacuated as in the single-layer case. The multilayer concept makes it possible to reduce effective thermal conductivity of a panel below that of a comparable single-layer panel, without adding weight or incurring other performance penalties. Implementation of the multilayer concept is simple and relatively inexpensive, involving only a few additional fabrication steps to assemble the multiple layers prior to evacuation. For a panel of the blanket type, the multilayer concept, affords the additional advantage of reduced stiffness.

  7. Thermal conductivity measurements of aerogel-impregnated shuttle tile at cryogenic temperatures

    NASA Astrophysics Data System (ADS)

    Helvensteijn, B. P. M.; Maddocks, J. R.; Salerno, L. J.; Roach, P. R.; Kittel, P.; White, S. M.

    2002-05-01

    New materials are being designed to allow the implementation of innovative concepts in future space transportation vehicles. In present day spacecraft, launch vehicles consist of an overall structure in which the cryogenic propellant tanks are mounted in a framework covered by an aeroshell. In NASA s next generation space transportation vehicles the innovative concept of conformal propellant tanks will be utilized. In addition to containing the propellants the cryogen tanks will serve as part of the main vehicle structure. This concept enables significant reduction in launch mass. Crucial to this approach will be the development of a material that will serve not only as cryogenic insulation on the launchpad and early in flight, but also as a thermal protection system (TPS), protecting the vehicle on re-entry. Prior to selection for this challenging application various material properties are to be assessed. The present paper discusses experimental results on the thermal conductivity of a preliminary candidate material (an aerogel-impregnated shuttle tile) in the presence of helium gas. Data have been taken at room temperature and near 80 K at pressures ranging from 1 atm down to 20 m Torr. The capability to extend the tests down to lower temperatures (4 K) was not utilized considering the noted adsorptive qualities of the open cell structure of the tile.

  8. Aerogels for Thermal Insulation of Thermoelectric Devices

    NASA Technical Reports Server (NTRS)

    Sakamoto, Jeffrey; Fleurial, Jean-Pierre; Snyder, Jeffrey; Jones, Steven; Caillat, Thierry

    2006-01-01

    Silica aerogels have been shown to be attractive for use as thermal-insulation materials for thermoelectric devices. It is desirable to thermally insulate the legs of thermoelectric devices to suppress lateral heat leaks that degrade thermal efficiency. Aerogels offer not only high thermal- insulation effectiveness, but also a combination of other properties that are especially advantageous in thermoelectric- device applications. Aerogels are synthesized by means of sol-gel chemistry, which is ideal for casting insulation into place. As the scale of the devices to be insulated decreases, the castability from liquid solutions becomes increasingly advantageous: By virtue of castability, aerogel insulation can be made to encapsulate devices having any size from macroscopic down to nanoscopic and possibly having complex, three-dimensional shapes. Castable aerogels can permeate voids having characteristic dimensions as small as nanometers. Hence, practically all the void space surrounding the legs of thermoelectric devices could be filled with aerogel insulation, making the insulation highly effective. Because aerogels have the lowest densities of any known solid materials, they would add very little mass to the encapsulated devices. The thermal-conductivity values of aerogels are among the lowest reported for any material, even after taking account of the contributions of convection and radiation (in addition to true thermal conduction) to overall effective thermal conductivities. Even in ambient air, the contribution of convection to effective overall thermal conductivity of an aerogel is extremely low because of the highly tortuous nature of the flow paths through the porous aerogel structure. For applications that involve operating temperatures high enough to give rise to significant amounts of infrared radiation, opacifiers could be added to aerogels to reduce the radiative contributions to overall effective thermal conductivities. One example of an opacifier is

  9. Multiscale Modeling of Heat Conduction in Carbon Nanotube Aerogels

    NASA Astrophysics Data System (ADS)

    Gong, Feng; Papavassiliou, Dimitrios; Duong, Hai

    Carbon nanotube (CNT) aerogels have attracted a lot of interest due to their ultrahigh strength/weight and surface area/weight ratios. They are promising advanced materials used in energy storage systems, hydrogen storage media and weight-conscious devices such as satellites, because of their ultralight and highly porous quality. CNT aerogels can have excellent electrical conductivity and mechanical strength. However, the thermal conductivity of CNT aerogels are as low as 0.01-0.1 W/mK, which is five orders of magnitude lower than that of CNT (2000-5000 W/mK). To investigate the mechanisms for the low thermal conductivity of CNT aerogels, multiscale models are built in this study. Molecular dynamic (MD) simulations are first carried out to investigate the heat transfer between CNT and different gases (e.g. nitrogen and hydrogen), and the thermal conductance at CNT-CNT interface. The interfacial thermal resistances of CNT-gas and CNT-CNT are estimated from the MD simulations. Mesoscopic modeling of CNT aerogels are then built using an off-lattice Monte Carlo (MC) simulations to replicate the realistic CNT aerogels. The interfacial thermal resistances estimated from MD simulations are used as inputs in the MC models to predict the thermal conductivity of CNT aerogels. The volume fractions and the complex morphologies of CNTs are also quantified to study their effects on the thermal conductivity of CNT aerogels. The quantitative findings may help researchers to obtain the CNT aerogels with expected thermal conductivity.

  10. Aerogel Composites for Aerospace Thermal Protection

    NASA Technical Reports Server (NTRS)

    White, Susan

    2003-01-01

    Aerogel composites formed by infiltrating organic and/or inorganic aerogels into fiber matrix materials enable us to exploit the low thermal conductivity and low density of aerogels while maintaining the strength, structure and other useful properties of a porous fiber matrix. New materials for extreme heating ranges are needed to insulate future spacecraft against the extreme heat of planetary atmospheric entry, but the insulation mass must be minimized in order to maximize the payload. A reusable system passively insulates to survive heating unchanged for relatively low heating. Ablators, which sacrifice mass to control heating, are used to protect vehicles against more extreme heating for a single use thermal protection system (TPS). Aerogel composites were fabricated and tested for spacecraft thermal protection. The high-temperaturey high heat flux tests described in this paper were performed in NASA Ames arc-jet facilities to simulate spacecraft atmospheric entry, and include heating conditions predicted for the forebody and backshell of the Mars Science Lander (MSL) entry probe. The aerogel composites tested showed excellent thermal performance in the arc-jet tests, functioning both as reusuable insulation under lower heat fluxes, and as ablative aerogels under the extreme heating predicted for the MSL forebody.

  11. Aerogel Beads as Cryogenic Thermal Insulation System

    NASA Technical Reports Server (NTRS)

    Fesmire, J. E.; Augustynowicz, S. D.; Rouanet, S.; Thompson, Karen (Technical Monitor)

    2001-01-01

    An investigation of the use of aerogel beads as thermal insulation for cryogenic applications was conducted at the Cryogenics Test Laboratory of NASA Kennedy Space Center. Steady-state liquid nitrogen boiloff methods were used to characterize the thermal performance of aerogel beads in comparison with conventional insulation products such as perlite powder and multilayer insulation (MLI). Aerogel beads produced by Cabot Corporation have a bulk density below 100 kilograms per cubic meter (kg/cubic m) and a mean particle diameter of 1 millimeter (mm). The apparent thermal conductivity values of the bulk material have been determined under steady-state conditions at boundary temperatures of approximately 293 and 77 kelvin (K) and at various cold vacuum pressures (CVP). Vacuum levels ranged from 10(exp -5) torr to 760 torr. All test articles were made in a cylindrical configuration with a typical insulation thickness of 25 mm. Temperature profiles through the thickness of the test specimens were also measured. The results showed the performance of the aerogel beads was significantly better than the conventional materials in both soft-vacuum (1 to 10 torr) and no-vacuum (760 torr) ranges. Opacified aerogel beads performed better than perlite powder under high-vacuum conditions. Further studies for material optimization and system application are in progress.

  12. Aerogel beads as cryogenic thermal insulation system

    NASA Astrophysics Data System (ADS)

    Fesmire, J. E.; Augustynowicz, S. D.; Rouanet, S.

    2002-05-01

    An investigation of the use of aerogel beads as thermal insulation for cryogenic applications was conducted at the Cryogenics Test Laboratory of NASA Kennedy Space Center. Steady-state liquid nitrogen boiloff methods were used to characterize the thermal performance of aerogel beads in comparison with conventional insulation products such as perlite powder and multilayer insulation (MLI). Aerogel beads produced by Cabot Corporation have a bulk density below 100 kilograms per cubic meter (kg/m3) and a mean particle diameter of 1 millimeter (mm). The apparent thermal conductivity values of the bulk material have been determined under steady-state conditions at boundary temperatures of approximately 293 and 77 kelvin (K) and at various cold vacuum pressures (CVP). Vacuum levels ranged from 10-5 torr to 760 torr. All test articles were made in a cylindrical configuration with a typical insulation thickness of 25 mm. Temperature profiles through the thickness of the test specimens were also measured. The results showed the performance of the aerogel beads was significantly better than the conventional materials in both soft-vacuum (1 to 10 torr) and no-vacuum (760 torr) ranges. Opacified aerogel beads performed better than perlite powder under high-vacuum conditions. Further studies for material optimization and system application are in progress.

  13. Uncooled thin film pyroelectric IR detector with aerogel thermal isolation

    DOEpatents

    Ruffner, Judith A.; Bullington, Jeff A.; Clem, Paul G.; Warren, William L.; Brinker, C. Jeffrey; Tuttle, Bruce A.; Schwartz, Robert W.

    1999-01-01

    A monolithic infrared detector structure which allows integration of pyroelectric thin films atop low thermal conductivity aerogel thin films. The structure comprises, from bottom to top, a substrate, an aerogel insulating layer, a lower electrode, a pyroelectric layer, and an upper electrode layer capped by a blacking layer. The aerogel can offer thermal conductivity less than that of air, while providing a much stronger monolithic alternative to cantilevered or suspended air-gap structures for pyroelectric thin film pixel arrays. Pb(Zr.sub.0.4 Ti.sub.0.6)O.sub.3 thin films deposited on these structures displayed viable pyroelectric properties, while processed at 550.degree. C.

  14. Thermal conductivity studies of a polyurea cross-linked silica aerogel-RTV 655 compound for cryogenic propellant tank applications in space

    NASA Astrophysics Data System (ADS)

    Sabri, F.; Marchetta, J.; Smith, K. M.

    2013-10-01

    Silica-based aerogel is an ideal thermal insulator with a makeup of up to 99% air associated with the highly porous nature of this material. Polyurea cross-linked silica aerogel (PCSA) has superior mechanical properties compared to the native aerogels yet retains the highly porous open pore network and functions as an ideal thermal insulator with added load-bearing capability necessary for some applications. Room temperature vulcanizing rubber-RTV 655—is a space qualified elastomeric thermal insulator and encapsulant with high radiation and temperature tolerance as well as chemical resistance. Storage and transport of cryogenic propellant liquids is an integral part of the success of future space exploratory missions and is an area under constant development. Limitations and shortcomings of current cryogenic tank materials and insulation techniques such as non-uniform insulation layers, self-pressurization, weight and durability issues of the materials used, has motivated the quest for alternative materials. Both RTV 655 and PCSA are promising space qualified materials with unique and tunable microscopic and macroscopic properties making them attractive candidates for this study. In this work, the effect of PCSA geometry and volume concentration on the thermal behavior of RTV 655—PCSA compound material has been investigated at room temperature and at a cryogenic temperature. Macroscopic and microscopic PCSA material was encapsulated at increasing concentrations in an RTV 655 elastomeric matrix. The effect of pulverization on the nanopores of PCSA as a method for creating large quantities of homogeneous PCSA microparticles has also been investigated and is reported. The PCSA volume concentrations ranged between 22% and 75% for both geometries. Thermal conductivity measurements were performed based on the steady state transient plane source method.

  15. Thermal properties of organic and modified inorganic aerogels

    SciTech Connect

    Pekala, R.W.; Hrubesh, L.W.

    1992-08-01

    Aerogels are open-cell foams that have already been shown to be among the best thermal insulating solid materials known. Improvements in the thermal insulating properties of aerogels are possible by synthesizing new organic varieties, by using additives within existing aerogel matrix, and by optimizing their nanostructures. We discuss these approaches and give some examples of aerogels which demonstrate the improvements.

  16. Low-Density, Aerogel-Filled Thermal-Insulation Tiles

    NASA Technical Reports Server (NTRS)

    Santos, Maryann; Heng, Vann; Barney, Andrea; Oka, Kris; Droege, Michael

    2005-01-01

    Aerogel fillings have been investigated in a continuing effort to develop low-density thermal-insulation tiles that, relative to prior such tiles, have greater dimensional stability (especially less shrinkage), equal or lower thermal conductivity, and greater strength and durability. In preparation for laboratory tests of dimensional and thermal stability, prototypes of aerogel-filled versions of recently developed low-density tiles have been fabricated by impregnating such tiles to various depths with aerogel formations ranging in density from 1.5 to 5.6 lb/ft3 (about 53 to 200 kg/cu m). Results available at the time of reporting the information for this article showed that the thermal-insulation properties of the partially or fully aerogel- impregnated tiles were equivalent or superior to those of the corresponding non-impregnated tiles and that the partially impregnated tiles exhibited minimal (<1.5 percent) shrinkage after multiple exposures at a temperature of 2,300 F (1,260 C). Latest developments have shown that tiles containing aerogels at the higher end of the density range are stable after multiple exposures at the said temperature.

  17. High Temperature Aerogels for Thermal Protection Systems

    NASA Technical Reports Server (NTRS)

    Hurwitz, Frances I.; Mbah, Godfrey C.

    2008-01-01

    High temperature aerogels in the Al2O3-SiO2 system are being investigated as possible constituents for lightweight integrated thermal protection system (TPS) designs for use in supersonic and hypersonic applications. Gels are synthesized from ethoxysilanes and AlCl3.6H2O, using an epoxide catalyst. The influence of Al:Si ratio, solvent, water to metal and water to alcohol ratios on aerogel composition, morphology, surface area, and pore size distribution were examined, and phase transformation on heat treatment characterized. Aerogels have been fabricated which maintain porous, fractal structures after brief exposures to 1000 C. Incorporation of nanofibers, infiltration of aerogels into SiC foams, use of polymers for crosslinking the aerogels, or combinations of these, offer potential for toughening and integration of TPS with composite structure. Woven fabric composites having Al2O3-SiO2 aerogels as a matrix also have been fabricated. Continuing work is focused on reduction in shrinkage and optimization of thermal and physical properties.

  18. Thermal Performance Testing of Order Dependancy of Aerogels Multilayered Insulation

    NASA Technical Reports Server (NTRS)

    Johnson, Wesley L.; Fesmire, James E.; Demko, J. A.

    2009-01-01

    Robust multilayer insulation systems have long been a goal of many research projects. Such insulation systems must provide some degree of structural support and also mechanical integrity during loss of vacuum scenarios while continuing to provide insulative value to the vessel. Aerogel composite blankets can be the best insulation materials in ambient pressure environments; in high vacuum, the thermal performance of aerogel improves by about one order of magnitude. Standard multilayer insulation (MU) is typically 50% worse at ambient pressure and at soft vacuum, but as much as two or three orders of magnitude better at high vacuum. Different combinations of aerogel and multilayer insulation systems have been tested at Cryogenics Test Laboratory of NASA Kennedy Space Center. Analysis performed at Oak Ridge National Laboratory showed an importance to the relative location of the MU and aerogel blankets. Apparent thermal conductivity testing under cryogenic-vacuum conditions was performed to verify the analytical conclusion. Tests results are shown to be in agreement with the analysis which indicated that the best performance is obtained with aerogel layers located in the middle of the blanket insulation system.

  19. Sprayable Aerogel Bead Compositions With High Shear Flow Resistance and High Thermal Insulation Value

    NASA Technical Reports Server (NTRS)

    Ou, Danny; Trifu, Roxana; Caggiano, Gregory

    2013-01-01

    A sprayable aerogel insulation has been developed that has good mechanical integrity and lower thermal conductivity than incumbent polyurethane spray-on foam insulation, at similar or lower areal densities, to prevent insulation cracking and debonding in an effort to eliminate the generation of inflight debris. This new, lightweight aerogel under bead form can be used as insulation in various thermal management systems that require low mass and volume, such as cryogenic storage tanks, pipelines, space platforms, and launch vehicles.

  20. Thermal Performance Of Space Suit Elements With Aerogel Insulation For Moon And Mars Exploration

    NASA Technical Reports Server (NTRS)

    Tang, Henry H.; Orndoff, Evelyne S.; Trevino, Luis A.

    2006-01-01

    Flexible fiber-reinforced aerogel composites were studied for use as insulation materials of a future space suit for Moon and Mars exploration. High flexibility and good thermal insulation properties of fiber-reinforced silica aerogel composites at both high and low vacuum conditions make it a promising insulation candidate for the space suit application. This paper first presents the results of a durability (mechanical cycling) study of these aerogels composites in the context of retaining their thermal performance. The study shows that some of these Aerogels materials retained most of their insulation performance after up to 250,000 cycles of mechanical flex cycling. This paper also examines the problem of integrating these flexible aerogel composites into the current space suit elements. Thermal conductivity evaluations are proposed for different types of aerogels space suit elements to identify the lay-up concept that may have the best overall thermal performance for both Moon and Mars environments. Potential solutions in mitigating the silica dusting issue related to the application of these aerogels materials for the space suit elements are also discussed.

  1. Thin aerogel films for optical, thermal, acoustic, and electronic applications

    SciTech Connect

    Hrubesh, L.W.; Poco, J.F.

    1994-09-01

    Aerogels are a special class of continuously porous solid materials which are characterized by nanometer size particles and pores. Typically, aerogels are made using sol-gel chemistry to form a solvent filled, high porosity gel that is dried by removing the solvent without collapsing the tenuous solid phase. As bulk materials, aerogels are known to have many exceptional, and even some unique physical properties. Aerogels provide the highest thermal insulation and lowest dielectric constant of any other material known. However, some important applications require the aerogels in the form of thin films or sheets. For example, electronic applications require micrometer thin aerogel films bonded to a substrate, and others require thicker films, either on a substrate or as free standing sheets. Special methods are required to make aerogel thin films or sheets. In this paper, the authors discuss the special conditions needed to fabricate thin aerogel films and they describe methods to make films and thin sheets. They also give some specific applications for which aerogel films are being developed.

  2. Polyimide-Foam/Aerogel Composites for Thermal Insulation

    NASA Technical Reports Server (NTRS)

    Williams, Martha; Fesmire, James; Sass, Jared; Smith, Trent; Weoser. Erol

    2009-01-01

    Composites of specific types of polymer foams and aerogel particles or blankets have been proposed to obtain thermal insulation performance superior to those of the neat polyimide foams. These composites have potential to also provide enhanced properties for vibration dampening or acoustic attenuation. The specific type of polymer foam is denoted "TEEK-H", signifying a series, denoted H, within a family of polyimide foams that were developed at NASA s Langley Research Center and are collectively denoted TEEK (an acronym of the inventors names). The specific types of aerogels include Nanogel aerogel particles from Cabot Corporation in Billerica, MA. and of Spaceloft aerogel blanket from Aspen Aerogels in Northborough, MA. The composites are inherently flame-retardant and exceptionally thermally stable. There are numerous potential uses for these composites, at temperatures from cryogenic to high temperatures, in diverse applications that include aerospace vehicles, aircraft, ocean vessels, buildings, and industrial process equipment. Some low-temperature applications, for example, include cryogenic storage and transfer or the transport of foods, medicines, and chemicals. Because of thermal cycling, aging, and weathering most polymer foams do not perform well at cryogenic temperatures and will undergo further cracking over time. The TEEK polyimides are among the few exceptions to this pattern, and the proposed composites are intended to have all the desirable properties of TEEK-H foams, plus improved thermal performance along with enhanced vibration or acoustic-attenuation performance. A composite panel as proposed would be fabricated by adding an appropriate amount of TEEK friable balloons into a mold to form a bottom layer. A piece of flexible aerogel blanket material, cut to the desired size and shape, would then be placed on the bottom TEEK layer and sandwiched between another top layer of polyimide friable balloons so that the aerogel blanket would become

  3. Comparative Investigation on Thermal Insulation of Polyurethane Composites Filled with Silica Aerogel and Hollow Silica Microsphere.

    PubMed

    Liu, Chunyuan; Kim, Jin Seuk; Kwon, Younghwan

    2016-02-01

    This paper presents a comparative study on thermal conductivity of PU composites containing open-cell nano-porous silica aerogel and closed-cell hollow silica microsphere, respectively. The thermal conductivity of PU composites is measured at 30 degrees C with transient hot bridge method. The insertion of polymer in pores of silica aerogel creates mixed interfaces, increasing the thermal conductivity of resulting composites. The measured thermal conductivity of PU composites filled with hollow silica microspheres is estimated using theoretical models, and is in good agreement with Felske model. It appears that the thermal conductivity of composites decreases with increasing the volume fraction (phi) when hollow silica microsphere (eta = 0.916) is used. PMID:27433652

  4. Compression molding of aerogel microspheres

    DOEpatents

    Pekala, R.W.; Hrubesh, L.W.

    1998-03-24

    An aerogel composite material produced by compression molding of aerogel microspheres (powders) mixed together with a small percentage of polymer binder to form monolithic shapes in a cost-effective manner is disclosed. The aerogel composites are formed by mixing aerogel microspheres with a polymer binder, placing the mixture in a mold and heating under pressure, which results in a composite with a density of 50--800 kg/m{sup 3} (0.05--0.80 g/cc). The thermal conductivity of the thus formed aerogel composite is below that of air, but higher than the thermal conductivity of monolithic aerogels. The resulting aerogel composites are attractive for applications such as thermal insulation since fabrication thereof does not require large and expensive processing equipment. In addition to thermal insulation, the aerogel composites may be utilized for filtration, ICF target, double layer capacitors, and capacitive deionization. 4 figs.

  5. Compression molding of aerogel microspheres

    DOEpatents

    Pekala, Richard W.; Hrubesh, Lawrence W.

    1998-03-24

    An aerogel composite material produced by compression molding of aerogel microspheres (powders) mixed together with a small percentage of polymer binder to form monolithic shapes in a cost-effective manner. The aerogel composites are formed by mixing aerogel microspheres with a polymer binder, placing the mixture in a mold and heating under pressure, which results in a composite with a density of 50-800 kg/m.sup.3 (0.05-0.80 g/cc). The thermal conductivity of the thus formed aerogel composite is below that of air, but higher than the thermal conductivity of monolithic aerogels. The resulting aerogel composites are attractive for applications such as thermal insulation since fabrication thereof does not require large and expensive processing equipment. In addition to thermal insulation, the aerogel composites may be utilized for filtration, ICF target, double layer capacitors, and capacitive deionization.

  6. Fabrication of hydrophobic, electrically conductive and flame-resistant carbon aerogels by pyrolysis of regenerated cellulose aerogels.

    PubMed

    Wan, Caichao; Lu, Yun; Jiao, Yue; Jin, Chunde; Sun, Qingfeng; Li, Jian

    2015-03-15

    In this paper, we reported miscellaneous carbon aerogels prepared by pyrolysis of regenerated cellulose aerogels that were fabricated by dissolution in a mild NaOH/PEG solution, freeze-thaw treatment, regeneration, and freeze drying. The as-prepared carbon aerogels were subsequently characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), nitrogen adsorption measurements, X-ray diffraction (XRD), Raman spectroscopy, and water contact angle (WCA) tests. The results showed that the carbon aerogels with pore diameters of 1-60 nm maintained interconnected three-dimensional (3D) network after the pyrolysis, and showed type-IV adsorption isotherm. The pyrolysis process leaded to the decomposition of oxygen-containing functional groups, the destruction of cellulose crystalline structure, and the formation of highly disordered amorphous graphite. Moreover, the carbon aerogels also had strong hydrophobicity, electrical conductivity and flame retardance, which held great potential in the fields of waterproof, electronic devices and fireproofing. PMID:25542115

  7. Multi-scale cellulose based new bio-aerogel composites with thermal super-insulating and tunable mechanical properties.

    PubMed

    Seantier, Bastien; Bendahou, Dounia; Bendahou, Abdelkader; Grohens, Yves; Kaddami, Hamid

    2016-03-15

    Bio-composite aerogels based on bleached cellulose fibers (BCF) and cellulose nanoparticles having various morphological and physico-chemical characteristics are prepared by a freeze-drying technique and characterized. The various composite aerogels obtained were compared to a BCF aerogel used as the reference. Severe changes in the material morphology were observed by SEM and AFM due to a variation of the cellulose nanoparticle properties such as the aspect ratio, the crystalline index and the surface charge density. BCF fibers form a 3D network and they are surrounded by the cellulose nanoparticle thin films inducing a significant reduction of the size of the pores in comparison with a neat BCF based aerogel. BET analyses confirm the appearance of a new organization structure with pores of nanometric sizes. As a consequence, a decrease of the thermal conductivities is observed from 28mWm(-1)K(-1) (BCF aerogel) to 23mWm(-1)K(-1) (bio-composite aerogel), which is below the air conductivity (25mWm(-1)K(-1)). This improvement of the insulation properties for composite materials is more pronounced for aerogels based on cellulose nanoparticles having a low crystalline index and high surface charge (NFC-2h). The significant improvement of their insulation properties allows the bio-composite aerogels to enter the super-insulating materials family. The characteristics of cellulose nanoparticles also influence the mechanical properties of the bio-composite aerogels. A significant improvement of the mechanical properties under compression is obtained by self-organization, yielding a multi-scale architecture of the cellulose nanoparticles in the bio-composite aerogels. In this case, the mechanical property is more dependent on the morphology of the composite aerogel rather than the intrinsic characteristics of the cellulose nanoparticles. PMID:26794770

  8. Silicon Oxycarbide Aerogels for High-Temperature Thermal Insulation

    NASA Technical Reports Server (NTRS)

    Evans, Owen; Rhine, Wendell; Coutinho, Decio

    2010-01-01

    This work has shown that the use of SOC-A35 leads to aerogel materials containing a significant concentration of carbidic species and limited amorphous free carbon. Substitution of the divalent oxide species in silica with tetravalent carbidic carbon has directly led to materials that exhibit increased network viscosity, reduced sintering, and limited densification. The SiOC aerogels produced in this work have the highest carbide content of any dense or porous SiOC glass reported in the literature at that time, and exhibit tremendous long-term thermal stability.

  9. Aerogel nanocomposite materials

    SciTech Connect

    Hunt, A.J.; Ayers, M.; Cao, W.

    1995-05-01

    Aerogels are porous, low density, nanostructured solids with many unusual properties including very low thermal conductivity, good transparency, high surface area, catalytic activity, and low sound velocity. This research is directed toward developing new nanocomposite aerogel materials for improved thermal insulation and several other applications. A major focus of the research has been to further increase the thermal resistance of silica aerogel by introducing infrared opacification agents into the aerogel to produce a superinsulating composite material. Opacified superinsulating aerogel permit a number of industrial applications for aerogel-based insulation. The primary benefits from this recently developed superinsulating composite aerogel insulation are: to extend the range of applications to higher temperatures, to provide a more compact insulation for space sensitive-applications, and to lower costs of aerogel by as much as 30%. Superinsulating aerogels can replace existing CFC-containing polyurethane in low temperature applications to reduce heat losses in piping, improve the thermal efficiency of refrigeration systems, and reduce energy losses in a variety of industrial applications. Enhanced aerogel insulation can also replace steam and process pipe insulation in higher temperature applications to substantially reduce energy losses and provide much more compact insulation.

  10. Polyimide Aerogel Thin Films

    NASA Technical Reports Server (NTRS)

    Meador, Mary Ann; Guo, Haiquan

    2012-01-01

    Polyimide aerogels have been crosslinked through multifunctional amines. This invention builds on "Polyimide Aerogels With Three-Dimensional Cross-Linked Structure," and may be considered as a continuation of that invention, which results in a polyimide aerogel with a flexible, formable form. Gels formed from polyamic acid solutions, end-capped with anhydrides, and cross-linked with the multifunctional amines, are chemically imidized and dried using supercritical CO2 extraction to give aerogels having density around 0.1 to 0.3 g/cubic cm. The aerogels are 80 to 95% porous, and have high surface areas (200 to 600 sq m/g) and low thermal conductivity (as low as 14 mW/m-K at room temperature). Notably, the cross-linked polyimide aerogels have higher modulus than polymer-reinforced silica aerogels of similar density, and can be fabricated as both monoliths and thin films.

  11. Strong, conductive, lightweight, neat graphene aerogel fibers with aligned pores.

    PubMed

    Xu, Zhen; Zhang, Yuan; Li, Peigang; Gao, Chao

    2012-08-28

    Liquid crystals of anisotropic colloids are of great significance in the preparation of their ordered macroscopic materials, for example, in the cases of carbon nanotubes and graphene. Here, we report a facile and scalable spinning process to prepare neat "core-shell" structured graphene aerogel fibers and three-dimensional cylinders with aligned pores from the flowing liquid crystalline graphene oxide (GO) gels. The uniform alignment of graphene sheets, inheriting the lamellar orders from GO liquid crystals, offers the porous fibers high specific tensile strength (188 kN m kg(-1)) and the porous cylinders high compression modulus (3.3 MPa). The porous graphene fibers have high specific surface area up to 884 m(2) g(-1) due to their interconnected pores and exhibit fine electrical conductivity (2.6 × 10(3) to 4.9 × 10(3) S m(-1)) in the wide temperature range of 5-300 K. The decreasing conductivity with decreasing temperature illustrates a typical semiconducting behavior, and the 3D interconnected network of 2D graphene sheets determines a dual 2D and 3D hopping conduction mechanism. The strong mechanical strength, high porosity, and fine electrical conductivity enable this novel material of ordered graphene aerogels to be greatly useful in versatile catalysts, supercapacitors, flexible batteries and cells, lightweight conductive fibers, and functional textiles. PMID:22799441

  12. Cellulose-silica aerogels.

    PubMed

    Demilecamps, Arnaud; Beauger, Christian; Hildenbrand, Claudia; Rigacci, Arnaud; Budtova, Tatiana

    2015-05-20

    Aerogels based on interpenetrated cellulose-silica networks were prepared and characterised. Wet coagulated cellulose was impregnated with silica phase, polyethoxydisiloxane, using two methods: (i) molecular diffusion and (ii) forced flow induced by pressure difference. The latter allowed an enormous decrease in the impregnation times, by almost three orders of magnitude, for a sample with the same geometry. In both cases, nanostructured silica gel was in situ formed inside cellulose matrix. Nitrogen adsorption analysis revealed an almost threefold increase in pores specific surface area, from cellulose aerogel alone to organic-inorganic composite. Morphology, thermal conductivity and mechanical properties under uniaxial compression were investigated. Thermal conductivity of composite aerogels was lower than that of cellulose aerogel due to the formation of superinsulating mesoporous silica inside cellulose pores. Furthermore, composite aerogels were stiffer than each of reference aerogels. PMID:25817671

  13. Ambient Dried Aerogels

    NASA Technical Reports Server (NTRS)

    Jones, Steven M.; Paik, Jong-Ah

    2013-01-01

    A method has been developed for creating aerogel using normal pressure and ambient temperatures. All spacecraft, satellites, and landers require the use of thermal insulation due to the extreme environments encountered in space and on extraterrestrial bodies. Ambient dried aerogels introduce the possibility of using aerogel as thermal insulation in a wide variety of instances where supercritically dried aerogels cannot be used. More specifically, thermoelectric devices can use ambient dried aerogel, where the advantages are in situ production using the cast-in ability of an aerogel. Previously, aerogels required supercritical conditions (high temperature and high pressure) to be dried. Ambient dried aerogels can be dried at room temperature and pressure. This allows many materials, such as plastics and certain metal alloys that cannot survive supercritical conditions, to be directly immersed in liquid aerogel precursor and then encapsulated in the final, dried aerogel. Additionally, the metalized Mylar films that could not survive the previous methods of making aerogels can survive the ambient drying technique, thus making multilayer insulation (MLI) materials possible. This results in lighter insulation material as well. Because this innovation does not require high-temperature or high-pressure drying, ambient dried aerogels are much less expensive to produce. The equipment needed to conduct supercritical drying costs many tens of thousands of dollars, and has associated running expenses for power, pressurized gasses, and maintenance. The ambient drying process also expands the size of the pieces of aerogel that can be made because a high-temperature, high-pressure system typically has internal dimensions of up to 30 cm in diameter and 60 cm in height. In the case of this innovation, the only limitation on the size of the aerogels produced would be in the ability of the solvent in the wet gel to escape from the gel network.

  14. New methods of characterizing the thermal properties of silica aerogel/xerogel

    NASA Astrophysics Data System (ADS)

    Stangl, R.; Platzer, Werner J.; Wittwer, Volker

    1994-09-01

    Integral as well as spectral methods to characterize the thermal properties of Silica Aerogel and Xerogel samples are presented. A new method to determine the effective thermal conductivity with the help of an infrared camera was developed, which allows for measuring small samples. Keeping the lower surface of the samples at a constant temperature, the IR camera monitors the upper surface temperature. With the help of a reference sample of known thermal conductivity, the total heat flux through the samples can be calculated. This is done under different pressure conditions in order to observe the Knudsen effect. As there is a complex coupling between the different modes of heat transport, the spectrum of the emitted radiation is of interest. Emission spectroscopy on Silica Aerogel has not been previously investigated in detail. An emission chamber was constructed, which permits measuring of the emitted spectra due to an applied temperature gradient using a Perkin Elmer Infrared Grating spectrometer. Different boundary emissivities and varying pressure conditions can be applied. For a correct interpretation of the measured spectra, a numerical treatment of the underlying heat transport mechanism is necessary. Measurements as well as computer simulations are presented. Computer simulations show that a variation of the total thermal conductivity in the range between 1 mW/mK and 30 mW/mK leads to a characteristic damping of the emitted radiation between 500 cm-1 and 2000 cm-1. This characteristic damping of the emitted radiation can be observed by measuring the emitted spectra of Silica Aerogel/Xerogel at varying pressure conditions. Therefore, in principle, it should be possible to extract the thermal conductivity through the gas pores and through the silica skeleton. Boundary effects, however, prevent this data from being extracted. Hopefully these problems will be solved in the near future.

  15. Competition between thermal fluctuations and disorder in the crystallization of 4He in aerogel.

    PubMed

    Nomura, Ryuji; Osawa, Aiko; Mimori, Tomohiro; Ueno, Ken-ichi; Kato, Haruko; Okuda, Yuichi

    2008-10-24

    The dynamical transition in the crystallization of 4He in aerogel has been investigated by direct visualization and dynamical phase diagrams have been determined. The crystal-superfluid interface in aerogel advances via creep at high temperatures and avalanches at low temperatures. The transition temperature is higher at a higher interface velocity and lower in higher porosity aerogels. The transition is due to competition between thermal fluctuations and disorder for the crystallization process. PMID:18999765

  16. Biomass-Based Mechanically Strong and Electrically Conductive Polymer Aerogels and Their Application for Supercapacitors.

    PubMed

    Zhao, Hai-Bo; Yuan, Lei; Fu, Zhi-Bing; Wang, Chao-Yang; Yang, Xi; Zhu, Jia-Yi; Qu, Jing; Chen, Hong-Bing; Schiraldi, David A

    2016-04-20

    A novel biomass-based mechanically strong and electrically conductive polymer aerogel was fabricated from aniline and biodegradable pectin. The strong hydrogen bonding interactions between polyaniline (PANI) and pectin resulted in a defined structure and enhanced properties of the aerogel. All the resultant aerogels exhibited self-surppoted 3D nanoporous network structures with high surface areas (207-331m(2)/g) and hierarchical pores. The results from electrical conductivity measurements and compressive tests revealed that these aerogels also had favorable conductivities (0.002-0.1 S/m) and good compressive modulus (1.2-1.4 MPa). The aerogel further used as electrode for supercapacitors showed enhanced capacitive performance (184 F/g at 0.5 A/g). Over 74% of the initial capacitance was maintained after repeating 1000 cycles of the cylic voltammetry test, while the capacitance retention of PANI was only 57%. The improved electrochemical performance may be attributed to the combinative properties of good electrical conductivity, BET surface areas, and stable nanoporous structure of the aerogel. Thus, this aerogel shows great potential as electrode materials for supercapacitors. PMID:27045343

  17. Polyolefin-Based Aerogels

    NASA Technical Reports Server (NTRS)

    Lee, Je Kyun; Gould, George

    2012-01-01

    radiation shielding materials due to their lower content of hydrogen element. The present invention relates to maleinized polybutadiene (or polybutadiene adducted with maleic anhydride)- based aerogel monoliths and composites, and the methods for preparation. Hereafter, they are collectively referred to as polybutadiene aerogels. Specifically, the polybutadiene aerogels of the present invention are prepared by mixing a maleinized polybutadiene resin, a hardener containing a maleic anhydride reactive group, and a catalyst in a suitable solvent, and maintaining the mixture in a quiescent state for a sufficient period of time to form a polymeric gel. After aging at elevated temperatures for a period of time to provide uniformly stronger wet gels, the micro porous maleinized polybutadiene- based aerogel is then obtained by removing interstitial solvent by supercritical drying. The mesoporous maleinized polybutadiene-based aerogels contain an open-pore structure, which provides inherently hydrophobic, flexible, nearly unbreakable, less dusty aerogels with excellent thermal and physical properties. The materials can be used as thermal and acoustic insulation, radiation shielding, and vibration-damping materials. The organic PB-based rubber aerogels are very flexible, no-dust, and hydrophobic organics that demonstrated the following ranges of typical properties: densities of 0.08 to 0.255 grams per cubic centimeters, shrinkage factor (raerogel/rtarget) = 1.2 to 2.84, and thermal conductivity values of 20.0 to 35.0 mW/m-K.

  18. Morphology effects on electrical and thermal properties of binderless graphene aerogels

    NASA Astrophysics Data System (ADS)

    Fan, Zeng; Tng, Daniel Zhi Yong; Nguyen, Son Truong; Feng, Jingduo; Lin, Chunfu; Xiao, Pengfei; Lu, Li; Duong, Hai M.

    2013-03-01

    Three-dimensional self-assembled graphene aerogels (GAs) are successfully developed by a simple chemical reduction with L-ascorbic acid at low temperature. And for the first time, the relationship among morphologies, electrical and thermal properties of GAs is studied comprehensively by controlling reaction conditions. The electrical conductivities of the GAs are also improved by four times after an annealing at 400 °C for 5 h under Ar environment. The results are very useful to optimize the best morphology, electrical and thermal properties of GAs for the best performance of GA-based electrodes of energy storage devices such as supercapacitors and batteries.

  19. Advanced Aerogel Technology

    NASA Technical Reports Server (NTRS)

    Jones, Steven

    2013-01-01

    The JPL Aerogel Laboratory has made aerogels for NASA flight missions, e.g., Stardust, 2003 Mars Exploration Rovers and the 2011 Mars Science Laboratory, as well as NASA research projects for the past 14 years. During that time it has produced aerogels of a range of shapes, sizes, densities and compositions. Research is ongoing in the development of aerogels for future sample capture and return missions and for thermal insulation for both spacecraft and scientific instruments. For the past several years, the JPL Aerogel Laboratory has been developing, producing and testing a new composite material for use as the high temperature thermal insulation in the Advanced Sterling Radioisotope Generator (ASRG) being developed by Lockheed Martin and NASA. The composite is made up of a glass fiber felt, silica aerogel, Titania powder, and silica powder. The oxide powders are included to reduce irradiative heat transport at elevated temperatures. These materials have thermal conductivity values that are the same as the best commercially produced high temperature insulation materials, and yet are 40% lighter. By greatly reducing the amount of oxide powder in the composite, the density, and therefore for the value of the thermal conductivity, would be reduced. The JPL Aerogel Laboratory has experimented with using glass fiber felt, expanded glass fiber felt and loose fibers to add structural integrity to silica aerogels. However, this work has been directed toward high temperature applications. By conducting a brief investigation of the optimal combination of fiber reinforcement and aerogel density, a durable, extremely efficient thermal insulation material for ambient temperature applications would be produced. If a transparent thermal insulation is desired, then aerogel is an excellent candidate material. At typical ambient temperatures, silica aerogel prevents the transport of heat via convection and conduction due to its highly porous nature. To prevent irradiative thermal

  20. Uncooled thin film pyroelectric IR detector with aerogel thermal isolation

    SciTech Connect

    Ruffner, J.A.; Clem, P.G.; Tuttle, B.A.

    1998-01-01

    Uncooled pyroelectric IR imaging systems, such as night vision goggles, offer important strategic advantages in battlefield scenarios and reconnaissance surveys. Until now, the current technology for fabricating these devices has been limited by low throughput and high cost which ultimately limit the availability of these sensor devices. We have developed and fabricated an alternative design for pyroelectric IR imaging sensors that utilizes a multilayered thin film deposition scheme to create a monolithic thin film imaging element on an active silicon substrate for the first time. This approach combines a thin film pyroelectric imaging element with a thermally insulating SiO{sub 2} aerogel thin film to produce a new type of uncooled IR sensor that offers significantly higher thermal, spatial, and temporal resolutions at a substantially lower cost per unit. This report describes the deposition, characterization and optimization of the aerogel thermal isolation layer and an appropriate pyroelectric imaging element. It also describes the overall integration of these components along with the appropriate planarization, etch stop, adhesion, electrode, and blacking agent thin film layers into a monolithic structure. 19 refs., 8 figs., 6 tabs.

  1. System and method for suppressing sublimation using opacified aerogel

    NASA Technical Reports Server (NTRS)

    Sakamoto, Jeff S. (Inventor); Snyder, G. Jeffrey (Inventor); Calliat, Thierry (Inventor); Fleurial, Jean-Pierre (Inventor); Jones, Steven M. (Inventor); Palk, Jong-Ah (Inventor)

    2008-01-01

    The present invention relates to a castable, aerogel-based, ultra-low thermal conductivity opacified insulation to suppress sublimation. More specifically, the present invention relates to an aerogel opacified with various opacifying or reflecting constituents to suppress sublimation and provide thermal insulation in thermoelectric modules. The opacifying constituent can be graded within the aerogel for increased sublimation suppression, and the density of the aerogel can similarly be graded to achieve optimal thermal insulation and sublimation suppression.

  2. Incorporation of graphene nanosheets into cellulose aerogels: enhanced mechanical, thermal, and oil adsorption properties

    NASA Astrophysics Data System (ADS)

    Wan, Caichao; Li, Jian

    2016-02-01

    In this paper, novel graphene/cellulose (GC) aerogels were prepared based on a green NaOH/PEG solution. Scanning electron microscope observation indicates that the three-dimensional network skeleton structure of cellulose aerogels is tightly covered by the compact sheet structure. X-ray diffraction and Raman spectroscopy analyses demonstrate that the graphene nanosheets have been successfully synthesized and embedded in the cellulose aerogels. The incorporation of graphene nanosheets gives rise to the significant improvement in the specific surface area and pore volume, thermal stability, mechanical strength, and oil adsorption efficiency of GC aerogels. Therefore, the green hybrid GC aerogels have more advantages over the pure cellulose aerogels in treating oil-containing wastewater or oil spills under the harsh environment.

  3. Strong, Thermally Superinsulating Biopolymer-Silica Aerogel Hybrids by Cogelation of Silicic Acid with Pectin.

    PubMed

    Zhao, Shanyu; Malfait, Wim J; Demilecamps, Arnaud; Zhang, Yucheng; Brunner, Samuel; Huber, Lukas; Tingaut, Philippe; Rigacci, Arnaud; Budtova, Tatiana; Koebel, Matthias M

    2015-11-23

    Silica aerogels are excellent thermal insulators, but their brittle nature has prevented widespread application. To overcome these mechanical limitations, silica-biopolymer hybrids are a promising alternative. A one-pot process to monolithic, superinsulating pectin-silica hybrid aerogels is presented. Their structural and physical properties can be tuned by adjusting the gelation pH and pectin concentration. Hybrid aerogels made at pH 1.5 exhibit minimal dust release and vastly improved mechanical properties while remaining excellent thermal insulators. The change in the mechanical properties is directly linked to the observed "neck-free" nanoscale network structure with thicker struts. Such a design is superior to "neck-limited", classical inorganic aerogels. This new class of materials opens up new perspectives for novel silica-biopolymer nanocomposite aerogels. PMID:26447457

  4. Study on the thermal resistance in secondary particles chain of silica aerogel by molecular dynamics simulation

    SciTech Connect

    Liu, M.; Qiu, L. E-mail: jzzhengxinghua@163.com; Zheng, X. H. E-mail: jzzhengxinghua@163.com; Zhu, J.; Tang, D. W.

    2014-09-07

    In this article, molecular dynamics simulation was performed to study the heat transport in secondary particles chain of silica aerogel. The two adjacent particles as the basic heat transport unit were modelled to characterize the heat transfer through the calculation of thermal resistance and vibrational density of states (VDOS). The total thermal resistance of two contact particles was predicted by non-equilibrium molecular dynamics simulations (NEMD). The defects were formed by deleting atoms in the system randomly first and performing heating and quenching process afterwards to achieve the DLCA (diffusive limited cluster-cluster aggregation) process. This kind of treatment showed a very reasonable prediction of thermal conductivity for the silica aerogels compared with the experimental values. The heat transport was great suppressed as the contact length increased or defect concentration increased. The constrain effect of heat transport was much significant when contact length fraction was in the small range (<0.5) or the defect concentration is in the high range (>0.5). Also, as the contact length increased, the role of joint thermal resistance played in the constraint of heat transport was increasing. However, the defect concentration did not affect the share of joint thermal resistance as the contact length did. VDOS of the system was calculated by numerical method to characterize the heat transport from atomic vibration view. The smaller contact length and greater defect concentration primarily affected the longitudinal acoustic modes, which ultimately influenced the heat transport between the adjacent particles.

  5. Mechanically Strong, Polymer Cross-linked Aerogels (X-Aerogels)

    NASA Technical Reports Server (NTRS)

    Leventis, Nicholas

    2006-01-01

    Aerogels comprise a class of low-density, high porous solid objects consisting of dimensionally quasi-stable self-supported three-dimensional assemblies of nanoparticles. Aerogels are pursued because of properties above and beyond those of the individual nanoparticles, including low thermal conductivity, low dielectric constant and high acoustic impedance. Possible applications include thermal and vibration insulation, dielectrics for fast electronics, and hosting of functional guests for a wide variety of optical, chemical and electronic applications. Aerogels, however, are extremely fragile materials, hence they have found only limited application in some very specialized environments, for example as Cerenkov radiation detectors in certain types of nuclear reactors, aboard spacecraft as collectors of hypervelocity particles (refer to NASA's Stardust program) and as thermal insulators on planetary vehicles on Mars (refer to Sojourner Rover in 1997 and Spirit and Opportunity in 2004). Along these lines, the X-Aerogel is a new NASA-developed strong lightweight material that has resolved the fragility problem of traditional (native) aerogels. X-Aerogels are made by applying a conformal polymer coating on the surfaces of the skeletal nanoparticles of native aerogels (see Scanning Electron Micrographs). Since the relative amounts of the polymeric crosslinker and the backbone are comparable, X-Aerogels can be viewed either as aerogels modified by the templated accumulation of polymer on the skeletal nanoparticles, or as nanoporous polymers made by remplated casting of polymer on a nanostructured framework. The most striking feature of X-Aerogels is that for a nominal 3-fold increase in density (still a ultralighweight material), the mechanical strength can be up to 300 times higher than the strength of the underlying native aerogel. Thus, X-Aerogels combine a multiple of the specific compressive strength of steel, with the the thermal conductivity of styrofoam. X-Aerogels

  6. Polyurea-Based Aerogel Monoliths and Composites

    NASA Technical Reports Server (NTRS)

    Lee, Je Kyun

    2012-01-01

    aerogel insulation material was developed that will provide superior thermal insulation and inherent radiation protection for government and commercial applications. The rubbery polyureabased aerogel exhibits little dustiness, good flexibility and toughness, and durability typical of the parent polyurea polymer, yet with the low density and superior insulation properties associated with aerogels. The thermal conductivity values of polyurea-based aerogels at lower temperature under vacuum pressures are very low and better than that of silica aerogels. Flexible, rubbery polyurea-based aerogels are able to overcome the weak and brittle nature of conventional inorganic and organic aerogels, including polyisocyanurate aerogels, which are generally prepared with the one similar component to polyurethane rubber aerogels. Additionally, with higher content of hydrogen in their structures, the polyurea rubber-based aerogels will also provide inherently better radiation protection than those of inorganic and carbon aerogels. The aerogel materials also demonstrate good hydrophobicity due to their hydrocarbon molecular structure. There are several strategies to overcoming the drawbacks associated with the weakness and brittleness of silica aerogels. Development of the flexible fiber-reinforced silica aerogel composite blanket has proven to be one promising approach, providing a conveniently fielded form factor that is relatively robust in industrial environments compared to silica aerogel monoliths. However, the flexible, silica aerogel composites still have a brittle, dusty character that may be undesirable, or even intolerable, in certain application environments. Although the cross - linked organic aerogels, such as resorcinol- formaldehyde (RF), polyisocyanurate, and cellulose aerogels, show very high impact strength, they are also very brittle with little elongation (i.e., less rubbery). Also, silica and carbon aerogels are less efficient radiation shielding materials due

  7. Improvements of reinforced silica aerogel nanocomposites thermal properties for architecture applications.

    PubMed

    Saboktakin, Amin; Saboktakin, Mohammad Reza

    2015-01-01

    An 1,4-cis polybutadiene rubber/carboxymethyl starch (CMS)-based silica aerogel nanocomposites as a insulation material was developed that will provide superior thermal insulation properties, flexibility, toughness, durability of the parent polymer, yet with the low density and superior insulation properties associated with the aerogels. In this study, reinforced 1,4-cis polybutadiene-CMS-silica aerogel nanocomposites were prepared from a silica aerogel with a surface area 710 m(2) g(-1), a pore size of 25.3 nm and a pore volume of 4.7 cm(3) g(-1). The tensile properties and dynamic mechanical properties of 1,4-cis polybutadiene/CMS nanocomposites were systematically enhanced at low silica loading. Similar improvements in tensile modulus and strength have been observed for 1,4-cis polybutadiene/CMS mesoporous silica aerogel nanocomposites. PMID:25172161

  8. Mechanically robust, electrically conductive ultralow-density carbon nanotube-based aerogels

    DOEpatents

    Worsley, Marcus A.; Baumann, Theodore F.; Satcher, Jr, Joe H.

    2016-07-05

    A method of making a mechanically robust, electrically conductive ultralow-density carbon nanotube-based aerogel, including the steps of dispersing nanotubes in an aqueous media or other media to form a suspension, adding reactants and catalyst to the suspension to create a reaction mixture, curing the reaction mixture to form a wet gel, drying the wet gel to produce a dry gel, and pyrolyzing the dry gel to produce the mechanically robust, electrically conductive ultralow-density carbon nanotube-based aerogel. The aerogel is mechanically robust, electrically conductive, and ultralow-density, and is made of a porous carbon material having 5 to 95% by weight carbon nanotubes and 5 to 95% carbon binder.

  9. Mechanically robust, electrically conductive ultralow-density carbon nanotube-based aerogels

    DOEpatents

    Worsley, Marcus A; Baumann, Theodore F; Satcher, Jr., Joe H

    2014-04-01

    A method of making a mechanically robust, electrically conductive ultralow-density carbon nanotube-based aerogel, including the steps of dispersing nanotubes in an aqueous media or other media to form a suspension, adding reactants and catalyst to the suspension to create a reaction mixture, curing the reaction mixture to form a wet gel, drying the wet gel to produce a dry gel, and pyrolyzing the dry gel to produce the mechanically robust, electrically conductive ultralow-density carbon nanotube-based aerogel. The aerogel is mechanically robust, electrically conductive, and ultralow-density, and is made of a porous carbon material having 5 to 95% by weight carbon nanotubes and 5 to 95% carbon binder.

  10. Conducting a thermal conductivity survey

    NASA Technical Reports Server (NTRS)

    Allen, P. B.

    1985-01-01

    A physically transparent approximate theory of phonon decay rates is presented starting from a pair potential model of the interatomic forces in an insulator or semiconductor. The theory applies in the classical regime and relates the 3-phonon decay rate to the third derivative of the pair potential. Phonon dispersion relations do not need to be calculated, as sum rules relate all the needed quantities directly to the pair potential. The Brillouin zone averaged phonon lifetime turns out to involve a dimensionless measure of the anharmonicity multiplied by an effective density of states for 3-phonon decay. Results are given for rare gas and alkali halide crystals. For rare gases, the results are in good agreement with more elaborate perturbation calculations. Comparison to experimental data on phonon linewidths and thermal conductivity are made.

  11. "Flexible aerogel as a superior thermal insulation for high temperature superconductor cable applications"

    SciTech Connect

    White, Shannon O.; Demko, Jonathan A; Tomich, A.

    2010-01-01

    High temperature superconducting (HTS) cables are an advanced technology that can both strengthen and improve the national electrical distribution infrastructure. HTS cables require sufficient cooling to overcome inherent low temperature heat loading. Heat loads are minimized by the use of cryogenic envelopes or cryostats. Cryostats require improvement in efficiency, reliability, and cost reduction to meet the demanding needs of HTS conductors (1G and 2G wires). Aspen Aerogels has developed a compression resistant aerogel thermal insulation package to replace compression sensitive multi-layer insulation (MLI), the incumbent thermal insulation, in flexible cryostats for HTS cables. Oak Ridge National Laboratory tested a prototype aerogel package in a lab-scale pipe apparatus to measure the rate of heat invasion. The lab-scale pipe test results of the aerogel solution will be presented and directly compared to MLI. A compatibility assessment of the aerogel material with HTS system components will also be presented. The aerogel thermal insulation solution presented will meet the demanding needs of HTS cables.

  12. Cryogenic Thermal Performance Testing of Bulk-Fill and Aerogel Insulation Materials

    NASA Technical Reports Server (NTRS)

    Scholtens, B. E.; Fesmire, J. E.; Sass, J. P.; Augustynowicz, S. D.; Heckle, K. W.

    2007-01-01

    The research testing and demonstration of new bulk-fill materials for cryogenic thermal insulation systems was performed by the Cryogenics Test Laboratory at NASA Kennedy Space Center. Thermal conductivity testing under actual-use cryogenic conditions is a key to understanding the total system performance encompassing engineering, economics, and materials factors. A number of bulk fill insulation materials, including aerogel beads, glass bubbles, and perlite powder, were tested using a new cylindrical cryostat. Boundary temperatures for the liquid nitrogen boil-off method were 293 K and 78 K. Tests were performed as a function of cold vacuum pressure from high vacuum to no vacuum conditions. Results are compared with other complementary test methods in the range of 300 K to 20 K. Various testing techniques are shown to be required to obtain a complete understanding of the operating performance of a material and to provide data for answers to design engineering questions.

  13. Cryogenic Thermal Performance Testing of Bulk-Fill and Aerogel Insulation Materials

    NASA Astrophysics Data System (ADS)

    Scholtens, B. E.; Fesmire, J. E.; Sass, J. P.; Augustynowicz, S. D.; Heckle, K. W.

    2008-03-01

    Thermal conductivity testing under actual-use conditions is a key to understanding how cryogenic thermal insulation systems perform in regard to engineering, economics, and materials factors. The Cryogenics Test Laboratory at NASA's Kennedy Space Center tested a number of bulk-fill insulation materials, including aerogel beads, glass bubbles, and perlite powder, using a new cylindrical cryostat. Boundary temperatures for the liquid nitrogen boiloff method were 78 K and 293 K. Tests were performed as a function of cold vacuum pressure under conditions ranging from high vacuum to no vacuum. Results were compared with those from complementary test methods in the range of 20 K to 300 K. Various testing techniques are required to completely understand the operating performance of a material and to provide data for answers to design engineering questions.

  14. Low thermal conductivity oxides

    SciTech Connect

    Pan, Wei; Phillpot, Simon R.; Wan, Chunlei; Chernatynskiy, Aleksandr; Qu, Zhixue

    2012-10-09

    Oxides hold great promise as new and improved materials for thermal-barrier coating applications. The rich variety of structures and compositions of the materials in this class, and the ease with which they can be doped, allow the exploration of various mechanisms for lowering thermal conductivity. In this article, we review recent progress in identifying specific oxides with low thermal conductivity from both theoretical and experimental perspectives. We explore the mechanisms of lowering thermal conductivity, such as introducing structural/chemical disorder, increasing material density, increasing the number of atoms in the primitive cell, and exploiting the structural anisotropy. We conclude that further systematic exploration of oxide crystal structures and chemistries are likely to result in even further improved thermal-barrier coatings.

  15. Manufacturable conducting rubber ambers and stretchable conductors from copper nanowire aerogel monoliths.

    PubMed

    Tang, Yue; Gong, Shu; Chen, Yi; Yap, Lim Wei; Cheng, Wenlong

    2014-06-24

    We report on a low-cost, simple yet efficient strategy to fabricate ultralightweight aerogel monoliths and conducting rubber ambers from copper nanowires (CuNWs). A trace amount of poly(vinyl alcohol) (PVA) substantially improved the mechanical robustness and elasticity of the CuNW aerogel while maintaining a high electrical conductivity. The resistivity was highly responsive to strains manifesting two distinct domains, and both followed a power law function consistent with pressure-controlled percolation theory. However, the values of the exponents were much less than the predicted value for 3D systems, which may be due to highly porous structures. Remarkably, the CuNW-PVA aerogels could be further embedded into PDMS resin, forming conducting rubber ambers. The ambers could be further manufactured simply by cutting into any arbitrary 1D, 2D, and 3D shapes, which were all intrinsically conductive without the need of external prewiring, a condition required in the previous aerogel-based conductors. The outstanding electrical conductivity in conjunction with high mechanical compliance enabled prototypes of the elastic piezoresistivity switches and stretchable conductors. PMID:24873318

  16. Mechanically robust, electrically conductive and stimuli-responsive binary network hydrogels enabled by superelastic graphene aerogels.

    PubMed

    Qiu, Ling; Liu, Diyan; Wang, Yufei; Cheng, Chi; Zhou, Kun; Ding, Jie; Truong, Van-Tan; Li, Dan

    2014-05-28

    The architecture of the nanofiller phase in polymer nanocomposites matters! Polymer hydrogels that can combine stimuli-responsiveness with excellent electrically conductivity and mechanical strength can be fabricated by incorporation of the polymer into an ultralight and superelastic graphene aerogel to form a binary network. PMID:24634392

  17. Epoxy Crosslinked Silica Aerogels (X-Aerogels)

    NASA Technical Reports Server (NTRS)

    fabrizio, Eve; Ilhan, Faysal; Meador, Mary Ann; Johnston, Chris; Leventis, Nicholas

    2004-01-01

    NASA is interested in the development of strong lightweight materials for the dual role of thermal insulator and structural component for space vehicles; freeing more weight for useful payloads. Aerogels are very-low density materials (0.010 to 0.5 g/cc) that, due to high porosity (meso- and microporosity), can be, depending on the chemical nature of the network, ideal thermal insulators (thermal conductivity approx. 15 mW/mK). However, aerogels are extremely fragile. For practical application of aerogels, one must increase strength without compromising the physical properties attributed to low density. This has been achieved by templated growth of an epoxy polymer layer that crosslinks the "pearl necklace" network of nanoparticles: the framework of a typical silica aerogel. The requirement for conformal accumulation of the epoxy crosslinker is reaction both with the surface of silica and with itself. After cross-linking, the strength of a typical aerogel monolith increases by a factor of 200, in the expense of only a 2-fold increase in density. Strength is increased further by coupling residual unreacted epoxides with diamine.

  18. Mechanical Properties of Aerogels

    NASA Technical Reports Server (NTRS)

    Parmenter, Kelly E.; Milstein, Frederick

    1995-01-01

    Aerogels are extremely low density solids that are characterized by a high porosity and pore sizes on the order of nanometers. Their low thermal conductivity and sometimes transparent appearance make them desirable for applications such as insulation in cryogenic vessels and between double paned glass in solar architecture. An understanding of the mechanical properties of aerogels is necessary before aerogels can be used in load bearing applications. In the present study, the mechanical behavior of various types of fiber-reinforced silica aerogels was investigated with hardness, compression, tension and shear tests. Particular attention was paid to the effects of processing parameters, testing conditions, storage environment, and age on the aerogels' mechanical response. The results indicate that the addition of fibers to the aerogel matrix generally resulted in softer, weaker materials with smaller elastic moduli. Furthermore, the testing environment significantly affected compression results. Tests in ethanol show an appreciable amount of scatter, and are not consistent with results for tests in air. In fact, the compression specimens appeared to crack and begin to dissolve upon exposure to the ethanol solution. This is consistent with the inherent hydrophobic nature of these aerogels. In addition, the aging process affected the aerogels' mechanical behavior by increasing their compressive strength and elastic moduli while decreasing their strain at fracture. However, desiccation of the specimens did not appreciably affect the mechanical properties, even though it reduced the aerogel density by removing trapped moisture. Finally, tension and shear test results indicate that the shear strength of the aerogels exceeds the tensile strength. This is consistent with the response of brittle materials. Future work should concentrate on mechanical testing at cryogenic temperatures, and should involve more extensive tensile tests. Moreover, before the mechanical response

  19. Mechanically stable thermally crosslinked poly(acrylic acid)/reduced graphene oxide aerogels.

    PubMed

    Ha, Heonjoo; Shanmuganathan, Kadhiravan; Ellison, Christopher J

    2015-03-25

    Graphene oxide (GO) aerogels, high porosity (>99%) low density (∼3-10 mg cm(-3)) porous materials with GO pore walls, are particularly attractive due to their lightweight, high surface area, and potential use in environmental remediation, superhydrophobic and superoleophilic materials, energy storage, etc. However, pure GO aerogels are generally weak and delicate which complicates their handling and potentially limits their commercial implementation. The focus of this work was to synthesize highly elastic, mechanically stable aerogels that are robust and easy to handle without substantially sacrificing their high porosity or low density. To overcome this challenge, a small amount of readily available and thermally cross-linkable poly(acrylic acid) (PAA) was intermixed with GO to enhance the mechanical integrity of the aerogel without disrupting other desirable characteristic properties. This method is a simple straightforward procedure that does not include multistep or complicated chemical reactions, and it produces aerogels with mass densities of about 4-6 mg cm(-3) and >99.6% porosity that can reversibly support up to 10,000 times their weight with full recovery of their original volume. Finally, pressure sensing capabilities were demonstrated and their oil absorption capacities were measured to be around 120 g oil per g aerogel(-1) which highlights their potential use in practical applications. PMID:25714662

  20. Graphene aerogels

    DOEpatents

    Pauzauskie, Peter J; Worsley, Marcus A; Baumann, Theodore F; Satcher, Jr., Joe H; Biener, Juergen

    2015-03-31

    Graphene aerogels with high conductivity and surface areas including a method for making a graphene aerogel, including the following steps: (1) preparing a reaction mixture comprising a graphene oxide suspension and at least one catalyst; (2) curing the reaction mixture to produce a wet gel; (3) drying the wet gel to produce a dry gel; and (4) pyrolyzing the dry gel to produce a graphene aerogel. Applications include electrical energy storage including batteries and supercapacitors.

  1. Highly Thermal Conductive Nanocomposites

    NASA Technical Reports Server (NTRS)

    Sun, Ya-Ping (Inventor); Connell, John W. (Inventor); Veca, Lucia Monica (Inventor)

    2015-01-01

    Disclosed are methods for forming carbon-based fillers as may be utilized in forming highly thermal conductive nanocomposite materials. Formation methods include treatment of an expanded graphite with an alcohol/water mixture followed by further exfoliation of the graphite to form extremely thin carbon nanosheets that are on the order of between about 2 and about 10 nanometers in thickness. Disclosed carbon nanosheets can be functionalized and/or can be incorporated in nanocomposites with extremely high thermal conductivities. Disclosed methods and materials can prove highly valuable in many technological applications including, for instance, in formation of heat management materials for protective clothing and as may be useful in space exploration or in others that require efficient yet light-weight and flexible thermal management solutions.

  2. Protective Skins for Aerogel Monoliths

    NASA Technical Reports Server (NTRS)

    Leventis, Nicholas; Johnston, James C.; Kuczmarski, Maria A.; Meador, Ann B.

    2007-01-01

    A method of imparting relatively hard protective outer skins to aerogel monoliths has been developed. Even more than aerogel beads, aerogel monoliths are attractive as thermal-insulation materials, but the commercial utilization of aerogel monoliths in thermal-insulation panels has been inhibited by their fragility and the consequent difficulty of handling them. Therefore, there is a need to afford sufficient protection to aerogel monoliths to facilitate handling, without compromising the attractive bulk properties (low density, high porosity, low thermal conductivity, high surface area, and low permittivity) of aerogel materials. The present method was devised to satisfy this need. The essence of the present method is to coat an aerogel monolith with an outer polymeric skin, by painting or spraying. Apparently, the reason spraying and painting were not attempted until now is that it is well known in the aerogel industry that aerogels collapse in contact with liquids. In the present method, one prevents such collapse through the proper choice of coating liquid and process conditions: In particular, one uses a viscous polymer precursor liquid and (a) carefully controls the amount of liquid applied and/or (b) causes the liquid to become cured to the desired hard polymeric layer rapidly enough that there is not sufficient time for the liquid to percolate into the aerogel bulk. The method has been demonstrated by use of isocyanates, which, upon exposure to atmospheric moisture, become cured to polyurethane/polyurea-type coats. The method has also been demonstrated by use of commercial epoxy resins. The method could also be implemented by use of a variety of other resins, including polyimide precursors (for forming high-temperature-resistant protective skins) or perfluorinated monomers (for forming coats that impart hydrophobicity and some increase in strength).

  3. Aerogel Development

    NASA Technical Reports Server (NTRS)

    Sahai, Rashmi K.

    2005-01-01

    Aerogel is one of the most promising materials of the future. It's unique properties, including high porosity, transparency, very high thermal tolerance, and environmental friendliness give it the potential of replacing many different products used in society today. However, the market for aerogel is still very limited because of the cost of producing the material and its fragility. The principle objective of my project has been to find new ways to apply aerogel in order to increase its practicality and appeal to different aspects of society. More specifically, I have focused on finding different chemicals that will coat aerogel and increase its durability. Because aerogel is so fragile and will crumble under the pressure of most coatings this has been no easy task. However, by experimenting with many different coatings and combinations of aerogel properties, I have made several significant discoveries. Aerogel (ideally, high density and hydrophobic) can be coated with several acrylic polymers, including artist's gel and nail polish. These materials provide a protective layering around the aerogel and keep it from breaking as easily. Because fragility is one of the main reasons applications of aerogel are limited, these discoveries will hopefully aid in finding future applications for this extraordinary material.

  4. Thermal conductivity of metals

    NASA Technical Reports Server (NTRS)

    Kazem, Sayyed M.

    1990-01-01

    The objective is to familiarize students with steady and unsteady heat transfer by conduction and with the effect of thermal conductivity upon temperature distribution through a homogeneous substance. The elementary heat conduction experiment presented is designed for associate degree technology students in a simple manner to enhance their intuition and to clarify many confusing concepts such as temperature, thermal energy, thermal conductivity, heat, transient and steady flows. The equipment set is safe, small, portable (10 kg) and relatively cheap (about $1200): the electric hot plate 2 kg (4.4 lb) for $175: the 24 channel selector and Thermocouple Digital Readout (Trendicator) 4.5 kg (10 lb) for about $1000; the three metal specimens (each of 2.5 cm diameter and 11 cm length), base plate and the bucket all about 3 kg (7 lb) for about $25. The experiment may take from 60 to 70 minutes. Although the hot plate surface temperature could be set from 90 to 370 C (maximum of 750 watts) it is a good practice to work with temperatures of 180 to 200 C (about 400 watts). They may experiment in squads of 2, 3 or even 4, or the instructor may demonstrate it for the whole class.

  5. Thermal radiation and conduction in microscale structures. Final report

    SciTech Connect

    Tien, C.L.

    1998-09-02

    The general objective of the current research program is to achieve a better understanding of the fundamental mechanisms of thermal radiation and heat conduction in microscale structures commonly encountered in engineering applications. Specifically, the program includes both experimental and analytical investigations of radiative heat transfer in microstructures, conductive heat transfer in micro devices, and short-pulse laser material interactions. Future work is planned to apply the knowledge of microscale heat transfer gained in this project to developing thermal insulating aerogel materials, thermal design schemes for quantum well lasers, and short-pulse laser micro-fabrication techniques. A listing of publications by Chang-Lin Tien is included.

  6. Composite Silica Aerogels Opacified with Titania

    NASA Technical Reports Server (NTRS)

    Paik, Jon-Ah; Sakamoto, Jeffrey; Jones, Steven; Fleurial, Jean-Pierre; DiStefano, Salvador; Nesmith, Bill

    2009-01-01

    A further improvement has been made to reduce the high-temperature thermal conductivities of the aerogel-matrix composite materials described in Improved Silica Aerogel Composite Materials (NPO-44287), NASA Tech Briefs, Vol. 32, No. 9 (September 2008), page 50. Because the contribution of infrared radiation to heat transfer increases sharply with temperature, the effective high-temperature thermal conductivity of a thermal-insulation material can be reduced by opacifying the material to reduce the radiative contribution. Therefore, the essence of the present improvement is to add an opacifying constituent material (specifically, TiO2 powder) to the aerogel-matrix composites.

  7. Thermally conductive polymers

    NASA Technical Reports Server (NTRS)

    Byrd, N. R.; Jenkins, R. K.; Lister, J. L. (Inventor)

    1971-01-01

    A thermally conductive polymer is provided having physical and chemical properties suited to use as a medium for potting electrical components. The polymer is prepared from hydroquinone, phenol, and formaldehyde, by conventional procedures employed for the preparation of phenol-formaldehyde resins. While the proportions of the monomers can be varied, a preferred polymer is formed from the monomers in a 1:1:2.4 molar or ratio of hydroquinone:phenol:formaldehyde.

  8. Uncooled thin film infrared imaging device with aerogel thermal isolation: Deposition and planarization techniques

    SciTech Connect

    Ruffner, J.A.; Clem, P.G.; Tuttle, B.A.; Brinker, C.J.; Sriram, C.S.; Bullington, J.A.

    1998-04-01

    The authors have successfully integrated a thermally insulating silica aerogel thin film into a new uncooled monolithic thin film infrared (IR) imaging device. Compared to other technologies (bulk ceramic and microbridge), use of an aerogel layer provides superior thermal isolation of the pyroelectric imaging element from the relatively massive heat sinking integrated circuit. This results in significantly higher thermal and temporal resolutions. They have calculated noise equivalent temperature differences of 0.04--0.10 C from a variety of Pb{sub x}Zr{sub y}Ti{sub 1{minus}y}O{sub 3} (PZT) and Pb{sub x}La{sub 1{minus}x}Zr{sub y}Ti{sub 1{minus}y}O{sub 3} (PLZT) pyroelectric imaging elements in monolithic structures. In addition, use of aerogels results in an easier, less expensive fabrication process and a more robust device. Fabrication of these monolithic devices entails sol-gel deposition of the aerogel, sputter deposition of the electrodes, and solution chemistry deposition of the pyroelectric imaging elements. Uniform pyroelectric response is achieved across the device by use of appropriate planarization techniques. These deposition and planarization techniques are described. Characterization of the individual layers and monolithic structure using scanning electron microscopy, atomic force microscopy and Byer-Roundy techniques also is discussed.

  9. Thermal Contact Conductance

    NASA Technical Reports Server (NTRS)

    Salerno, Louis J.; Kittel, Peter

    1997-01-01

    The performance of cryogenic instruments is often a function of their operating temperature. Thus, designers of cryogenic instruments often are required to predict the operating temperature of each instrument they design. This requires accurate thermal models of cryogenic components which include the properties of the materials and assembly techniques used. When components are bolted or otherwise pressed together, a knowledge of the thermal performance of such joints are also needed. In some cases, the temperature drop across these joints represents a significant fraction of the total temperature difference between the instrument and its cooler. While extensive databases exist on the thermal properties of bulk materials, similar databases for pressed contacts do not. This has often lead to instrument designs that avoid pressed contacts or to the over-design of such joints at unnecessary expense. Although many people have made measurements of contact conductances at cryogenic temperatures, this data is often very narrow in scope and even more often it has not been published in an easily retrievable fashion, if published at all. This paper presents a summary of the limited pressed contact data available in the literature.

  10. Electrical and Thermal Conductivity

    NASA Astrophysics Data System (ADS)

    Ventura, Guglielmo; Perfetti, Mauro

    After a Sect. 1.1 devoted to electrical conductivity and a section that deals with magnetic and dielectric losses ( 1.2 ), this chapter explores the theory of thermal conduction in solids. The examined categories of solids are: metals Sect. 1.3.2 , Dielectrics Sects. 1.3.3 and 1.3.4 and Nanocomposites Sect. 1.3.5 . In Sect. 1.3.6 the problem of thermal and electrical contact between materials is considered because contact resistance occurring at conductor joints in magnets or other high power applications can lead to undesirable electrical losses. At low temperature, thermal contact is also critical in the mounting of temperature sensors, where bad contacts can lead to erroneous results, in particular when superconductivity phenomena are involved.

  11. Self-Sensing, Ultralight, and Conductive 3D Graphene/Iron Oxide Aerogel Elastomer Deformable in a Magnetic Field.

    PubMed

    Xu, Xiang; Li, Hui; Zhang, Qiangqiang; Hu, Han; Zhao, Zongbin; Li, Jihao; Li, Jingye; Qiao, Yu; Gogotsi, Yury

    2015-04-28

    Three-dimensional (3D) graphene aerogels (GA) show promise for applications in supercapacitors, electrode materials, gas sensors, and oil absorption due to their high porosity, mechanical strength, and electrical conductivity. However, the control, actuation, and response properties of graphene aerogels have not been well studied. In this paper, we synthesized 3D graphene aerogels decorated with Fe3O4 nanoparticles (Fe3O4/GA) by self-assembly of graphene with simultaneous decoration by Fe3O4 nanoparticles using a modified hydrothermal reduction process. The aerogels exhibit up to 52% reversible magnetic field-induced strain and strain-dependent electrical resistance that can be used to monitor the degree of compression/stretching of the material. The density of Fe3O4/GA is only about 5.8 mg cm(-3), making it an ultralight magnetic elastomer with potential applications in self-sensing soft actuators, microsensors, microswitches, and environmental remediation. PMID:25792130

  12. Composite Aerogel Multifoil Protective Shielding

    NASA Technical Reports Server (NTRS)

    Jones, Steven M.

    2013-01-01

    New technologies are needed to survive the temperatures, radiation, and hypervelocity particles that exploration spacecraft encounter. Multilayer insulations (MLIs) have been used on many spacecraft as thermal insulation. Other materials and composites have been used as micrometeorite shielding or radiation shielding. However, no material composite has been developed and employed as a combined thermal insulation, micrometeorite, and radiation shielding. By replacing the scrims that have been used to separate the foil layers in MLIs with various aerogels, and by using a variety of different metal foils, the overall protective performance of MLIs can be greatly expanded to act as thermal insulation, radiation shielding, and hypervelocity particle shielding. Aerogels are highly porous, low-density solids that are produced by the gelation of metal alkoxides and supercritical drying. Aerogels have been flown in NASA missions as a hypervelocity particle capture medium (Stardust) and as thermal insulation (2003 MER). Composite aerogel multifoil protective shielding would be used to provide thermal insulation, while also shielding spacecraft or components from radiation and hypervelocity particle impacts. Multiple layers of foil separated by aerogel would act as a thermal barrier by preventing the transport of heat energy through the composite. The silica aerogel would act as a convective and conductive thermal barrier, while the titania powder and metal foils would absorb and reflect the radiative heat. It would also capture small hypervelocity particles, such as micrometeorites, since it would be a stuffed, multi-shock Whipple shield. The metal foil layers would slow and break up the impacting particles, while the aerogel layers would convert the kinetic energy of the particles to thermal and mechanical energy and stop the particles.

  13. Aerogel Blanket Insulation Materials for Cryogenic Applications

    NASA Technical Reports Server (NTRS)

    Coffman, B. E.; Fesmire, J. E.; White, S.; Gould, G.; Augustynowicz, S.

    2009-01-01

    Aerogel blanket materials for use in thermal insulation systems are now commercially available and implemented by industry. Prototype aerogel blanket materials were presented at the Cryogenic Engineering Conference in 1997 and by 2004 had progressed to full commercial production by Aspen Aerogels. Today, this new technology material is providing superior energy efficiencies and enabling new design approaches for more cost effective cryogenic systems. Aerogel processing technology and methods are continuing to improve, offering a tailor-able array of product formulations for many different thermal and environmental requirements. Many different varieties and combinations of aerogel blankets have been characterized using insulation test cryostats at the Cryogenics Test Laboratory of NASA Kennedy Space Center. Detailed thermal conductivity data for a select group of materials are presented for engineering use. Heat transfer evaluations for the entire vacuum pressure range, including ambient conditions, are given. Examples of current cryogenic applications of aerogel blanket insulation are also given. KEYWORDS: Cryogenic tanks, thermal insulation, composite materials, aerogel, thermal conductivity, liquid nitrogen boil-off

  14. Thermal Conductivity of Coated Paper

    SciTech Connect

    Kerr, Lei L; Pan, Yun-Long; Dinwiddie, Ralph Barton; Wang, Hsin; Peterson, Robert C.

    2009-01-01

    In this paper, we introduce a method for measuring the thermal conductivity of paper using a hot disk system. To the best of our knowledge, few publications are found discussing the thermal conductivity of a coated paper although it is important to various forms of today s digital printing where heat is used for imaging as well as for toner fusing. This motivates us to investigate the thermal conductivity of paper coating. Our investigation demonstrates that thermal conductivity is affected by the coat weight and the changes in the thermal conductivity affect ink gloss and density. As the coat weight increases, the thermal conductivity increases. Both the ink gloss and density decrease as the thermal conductivity increases. The ink gloss appears to be more sensitive to the changes in the thermal conductivity.

  15. Thermal Conductivity of Coated Paper

    NASA Astrophysics Data System (ADS)

    Kerr, Lei L.; Pan, Yun-Long; Dinwiddie, Ralph B.; Wang, Hsin; Peterson, Robert C.

    2009-04-01

    In this article, a method for measuring the thermal conductivity of paper using a hot disk system is introduced. To the best of our knowledge, few publications are found discussing the thermal conductivity of a coated paper, although it is important to various forms of today’s digital printing where heat is used for imaging, as well as for toner fusing. This motivated an investigation of the thermal conductivity of paper coating. This study demonstrates that the thermal conductivity is affected by the coating mass and the changes in the thermal conductivity affect toner gloss and density. As the coating mass increases, the thermal conductivity increases. Both the toner gloss and density decrease as the thermal conductivity increases. The toner gloss appears to be more sensitive to the changes in the thermal conductivity.

  16. Nonflammable, Hydrophobic Aerogel Composites for Insulation

    NASA Technical Reports Server (NTRS)

    Redouane, Begag

    2005-01-01

    Aerogel composites that are both nonflammable and hydrophobic have been developed for use as lightweight thermal- insulation materials for cryogenic systems. Aerogels are well known in the industry for their effectiveness as thermal insulators under cryogenic conditions, but the treatments used heretofore to render them hydrophobic also make them flammable. Nonflammability would make it safer to use aerogel insulation, especially in oxygen-rich environments and on cryogenic systems that contain liquid oxygen. A composite of this type is a silica aerogel reinforced with fibers. In comparison with unreinforced aerogels, the aerogel composite is about ten times as stiff and strong, better able to withstand handling, and more amenable to machining to required shapes. The composite can be made hydrophobic and nonflammable by appropriate design of a sol-gel process used to synthesize the aerogel component. In addition to very low thermal conductivity needed for insulation, aerogel composites of this type have been found to exhibit high resistance to moisture and nonflammability in oxygen-rich atmospheres: Samples floating on water for months gained no weight and showed no signs of deterioration. Samples were found to be nonflammable, even in pure oxygen at atmospheric pressure [14.7 psia (0.10 MPa)

  17. Foam/Aerogel Composite Materials for Thermal and Acoustic Insulation and Cryogen Storage

    NASA Technical Reports Server (NTRS)

    Williams, Martha K. (Inventor); Smith, Trent M. (Inventor); Fesmire, James E. (Inventor); Weiser, Erik S. (Inventor); Sass, Jared P. (Inventor)

    2011-01-01

    The invention involves composite materials containing a polymer foam and an aerogel. The composite materials have improved thermal insulation ability, good acoustic insulation, and excellent physical mechanical properties. The composite materials can be used, for instance, for heat and acoustic insulation on aircraft, spacecraft, and maritime ships in place of currently used foam panels and other foam products. The materials of the invention can also be used in building construction with their combination of light weight, strength, elasticity, ability to be formed into desired shapes, and superior thermal and acoustic insulation power. The materials have also been found to have utility for storage of cryogens. A cryogenic liquid or gas, such as N.sub.2 or H.sub.2, adsorbs to the surfaces in aerogel particles. Thus, another embodiment of the invention provides a storage vessel for a cryogen.

  18. Foam/aerogel composite materials for thermal and acoustic insulation and cryogen storage

    NASA Technical Reports Server (NTRS)

    Williams, Martha K. (Inventor); Smith, Trent M. (Inventor); Fesmire, James E. (Inventor); Weiser, Erik S. (Inventor); Sass, Jared P. (Inventor)

    2010-01-01

    The invention involves composite materials containing a polymer foam and an aerogel. The composite materials have improved thermal insulation ability, good acoustic insulation, and excellent physical mechanical properties. The composite materials can be used, for instance, for heat and acoustic insulation on aircraft, spacecraft, and maritime ships in place of currently used foam panels and other foam products. The materials of the invention can also be used in building construction with their combination of light weight, strength, elasticity, ability to be formed into desired shapes, and superior thermal and acoustic insulation power. The materials have also been found to have utility for storage of cryogens. A cryogenic liquid or gas, such as N.sub.2 or H.sub.2, adsorbs to the surfaces in aerogel particles. Thus, another embodiment of the invention provides a storage vessel for a cryogen.

  19. Shape memory thermal conduction switch

    NASA Technical Reports Server (NTRS)

    Vaidyanathan, Rajan (Inventor); Krishnan, Vinu (Inventor); Notardonato, William U. (Inventor)

    2010-01-01

    A thermal conduction switch includes a thermally-conductive first member having a first thermal contacting structure for securing the first member as a stationary member to a thermally regulated body or a body requiring thermal regulation. A movable thermally-conductive second member has a second thermal contacting surface. A thermally conductive coupler is interposed between the first member and the second member for thermally coupling the first member to the second member. At least one control spring is coupled between the first member and the second member. The control spring includes a NiTiFe comprising shape memory (SM) material that provides a phase change temperature <273 K, a transformation range <40 K, and a hysteresis of <10 K. A bias spring is between the first member and the second member. At the phase change the switch provides a distance change (displacement) between first and second member by at least 1 mm, such as 2 to 4 mm.

  20. Tailoring mechanical properties of aerogels for aerospace applications.

    PubMed

    Randall, Jason P; Meador, Mary Ann B; Jana, Sadhan C

    2011-03-01

    Silica aerogels are highly porous solid materials consisting of three-dimensional networks of silica particles and are typically obtained by removing the liquid in silica gels under supercritical conditions. Several unique attributes such as extremely low thermal conductivity and low density make silica aerogels excellent candidates in the quest for thermal insulation materials used in space missions. However, native silica aerogels are fragile at relatively low stresses. More durable aerogels with higher strength and stiffness are obtained by proper selection of silane precursors and by reinforcement with polymers. This paper first presents a brief review of the literature on methods of silica aerogel reinforcement and then discusses our recent activities in improving not only the strength but also the elastic response of polymer-reinforced silica aerogels. Several alkyl-linked bis-silanes were used in promoting flexibility of the silica networks in conjunction with polymer reinforcement by epoxy. PMID:21361281

  1. Thermal conductivity of zirconia thermal barrier coatings

    NASA Technical Reports Server (NTRS)

    Dinwiddie, R. B.; Beecher, S. C.; Nagaraj, B. A.; Moore, C. S.

    1995-01-01

    Thermal barrier coatings (TBC's) applied to the hot gas components of turbine engines lead to enhanced fuel efficiency and component reliability. Understanding the mechanisms which control the thermal transport behavior of the TBC's is of primary importance. Physical vapor description (PVD) and plasma spraying (PS) are the two most commonly used coating techniques. These techniques produce coatings with unique microstructures which control their performance and stability. The PS coatings were applied with either standard power or hollow sphere particles. The hollow sphere particles yielded a lower density and lower thermal conductivity coating. The thermal conductivity of both fully and partially stabilized zirconia, before and after thermal aging, will be compared. The thermal conductivity of the coatings permanently increase upon being exposed to high temperatures. These increases are attributed to microstructural changes within the coatings. Sintering of the as fabricated plasma sprayed lamellar structure is observed by scanning electron microscopy of coatings isothermally heat treated at temperatures greater than 1100 C. During this sintering process the planar porosity between lamella is converted to a series of small spherical pores. The change in pore morphology is the primary reason for the observed increase in thermal conductivity. This increase in thermal conductivity can be modeled using a relationship which depends on both the temperature and time of exposure. Although the PVD coatings are less susceptible to thermal aging effects, preliminary results suggest that they have a higher thermal conductivity than PS coatings, both before and after thermal aging. The increases in thermal conductivity due to thermal aging for partially stabilized plasma sprayed zirconia have been found to be less than for fully stabilized plasma sprayed zirconia coatings. The high temperature thermal diffusivity data indicates that if these coatings reach a temperature above

  2. Thermal conductivity of zirconia thermal barrier coatings

    NASA Technical Reports Server (NTRS)

    Dinwiddie, R. B.; Beecher, S. C.; Nagaraj, B. A.; Moore, C. S.

    1995-01-01

    Thermal barrier coatings (TBC's) applied to the hot gas components of turbine engines lead to enhanced fuel efficiency and component reliability. Understanding the mechanisms which control the thermal transport behavior of the TBC's is of primary importance. Physical vapor deposition (PVD) and plasma spraying (PS) are the two most commonly used coating techniques. These techniques produce coatings with unique microstructures which control their performance and stability. The PS coatings were applied with either standard powder or hollow sphere particles. The hollow sphere particles yielded a lower density and lower thermal conductivity coating. The thermal conductivity of both fully and partially stabilized zirconia, before and after thermal aging, will be compared. The thermal conductivity of the coatings permanently increases upon exposed to high temperatures. These increases are attributed to microstructural changes within the coatings. Sintering of the as-fabricated plasma sprayed lamellar structure is observed by scanning electron microscopy of coatings isothermally heat treated at temperatures greater than 1100 C. During this sintering process the planar porosity between lamella is converted to a series of small spherical pores. The change in pore morphology is the primary reason for the observed increase in thermal conductivity. This increase in thermal conductivity can be modeled using a relationship which depends on both the temperature and time of exposure. Although the PVD coatings are less susceptible to thermal aging effects, preliminary results suggest that they have a higher thermal conductivity than PS coatings, both before and after thermal aging. The increases in thermal conductivity due to thermal aging for partially stabilized plasma sprayed zirconia have been found to be less than for fully stabilized plasma sprayed zirconia coatings. The high temperature thermal diffusivity data indicate that if these coatings reach a temperature above 1100 C

  3. Fibrous-Ceramic/Aerogel Composite Insulating Tiles

    NASA Technical Reports Server (NTRS)

    White, Susan M.; Rasky, Daniel J.

    2004-01-01

    Fibrous-ceramic/aerogel composite tiles have been invented to afford combinations of thermal-insulation and mechanical properties superior to those attainable by making tiles of fibrous ceramics alone or aerogels alone. These lightweight tiles can be tailored to a variety of applications that range from insulating cryogenic tanks to protecting spacecraft against re-entry heating. The advantages and disadvantages of fibrous ceramics and aerogels can be summarized as follows: Tiles made of ceramic fibers are known for mechanical strength, toughness, and machinability. Fibrous ceramic tiles are highly effective as thermal insulators in a vacuum. However, undesirably, the porosity of these materials makes them permeable by gases, so that in the presence of air or other gases, convection and gas-phase conduction contribute to the effective thermal conductivity of the tiles. Other disadvantages of the porosity and permeability of fibrous ceramic tiles arise because gases (e.g., water vapor or cryogenic gases) can condense in pores. This condensation contributes to weight, and in the case of cryogenic systems, the heat of condensation undesirably adds to the heat flowing to the objects that one seeks to keep cold. Moreover, there is a risk of explosion associated with vaporization of previously condensed gas upon reheating. Aerogels offer low permeability, low density, and low thermal conductivity, but are mechanically fragile. The basic idea of the present invention is to exploit the best features of fibrous ceramic tiles and aerogels. In a composite tile according to the invention, the fibrous ceramic serves as a matrix that mechanically supports the aerogel, while the aerogel serves as a low-conductivity, low-permeability filling that closes what would otherwise be the open pores of the fibrous ceramic. Because the aerogel eliminates or at least suppresses permeation by gas, gas-phase conduction, and convection, the thermal conductivity of such a composite even at

  4. Thermal conductivity of zirconia thermal barrier coatings

    SciTech Connect

    Dinwiddie, R.B.; Beecher, S.C.; Nagaraj, B.A.; Moore, C.S.

    1995-10-01

    Thermal barrier coatings (TBC`s) applied to the hot gas components of turbine engines lead to enhanced fuel efficiency and component reliability. Understanding the mechanisms which control the thermal transport behavior of the TBC`s is of primary importance. Physical vapor deposition (PVD) and plasma spraying (PS) are the two most commonly used coating techniques. These techniques produce coatings with unique microstructures which control their performance and stability. The PS coatings were applied with either standard powder or hollow sphere particles. The hollow sphere particles yielded a lower density and lower thermal conductivity coating. The thermal conductivity of both fully and partially stabilized zirconia, before and after thermal aging, will be compared. The thermal conductivity of the coatings permanently increases upon exposed to high temperatures. These increases are attributed to microstructural changes within the coatings. Sintering of the as-fabricated plasma sprayed lamellar structure is observed by scanning electron microscopy of coatings isothermally heat treated at temperatures greater than 1100 C. During this sintering process the planar porosity between lamella is converted to a series of small spherical pores. The change in pore morphology is the primary reason for the observed increase in thermal conductivity. This increase in thermal conductivity can be modeled using a relationship which depends on both the temperature and time of exposure. Although the PVD coatings are less susceptible to thermal aging effects, preliminary results suggest that they have a higher thermal conductivity than PS coatings, both before and after thermal aging. The increases in thermal conductivity due to thermal aging for partially stabilized plasma sprayed zirconia have been found to be less than for fully stabilized plasma sprayed zirconia coatings.

  5. Thermal conductivity of boron carbides

    NASA Technical Reports Server (NTRS)

    Wood, C.; Emin, D.; Gray, P. E.

    1985-01-01

    Knowledge of the thermal conductivity of boron carbide is necessary to evaluate its potential for high-temperature thermoelectric energy conversion applications. Measurements have been conducted of the thermal diffusivity of hot-pressed boron carbide BxC samples as a function of composition (x in the range from 4 to 9), temperature (300-1700 K), and temperature cycling. These data, in concert with density and specific-heat data, yield the thermal conductivities of these materials. The results are discussed in terms of a structural model that has been previously advanced to explain the electronic transport data. Some novel mechanisms for thermal conduction are briefly discussed.

  6. Thermal Conductivity of Powder Insulations Below 180 K

    NASA Astrophysics Data System (ADS)

    Barrios, M. N.; Choi, Y. S.; Van Sciver, S. W.

    2008-03-01

    We have measured the thermal conductivity of aerogel beads and glass microspheres at average temperatures ranging from 30 K to 180 K. The measuring device consists of two closed, concentric cylinders suspended inside of a vacuum insulated cryostat. The insulation being tested occupies the annular space between the cylinders. A single stage Gifford-McMahon cryocooler, thermally anchored to the outer cylinder, cools the apparatus to a desired temperature range. A heater mounted on the inner cylinder generates uniform heat flux through the insulating material between the two cylinders. During each measurement, a temperature difference of roughly 10 K across the insulation is maintained. Fourier's law of heat conduction is used to relate the temperature difference between the two cylinders and the applied heating power to a bulk effective thermal conductivity of the powder insulation. Data were collected for aerogel beads between 30 K and 80 K and for glass bubbles between 30 K and 180 K. Results are compared to data from the literature.

  7. Thermal conductivity Measurements of Kaolite

    SciTech Connect

    Wang, H

    2003-02-21

    Testing was performed to determine the thermal conductivity of Kaolite 1600, which primarily consists of Portland cement and vermiculite. The material was made by Thermal Ceramics for refractory applications. Its combination of light weight, low density, low cost, and noncombustibility made it an attractive alternative to the materials currently used in ES-2 container for radioactive materials. Mechanical properties and energy absorption tests of the Kaolite have been conducted at the Y-12 complex. Heat transfer is also an important factor for the application of the material. The Kaolite samples are porous and trap moisture after extended storage. Thermal conductivity changes as a function of moisture content below 100 C. Thermal conductivity of the Kaolite at high temperatures (up to 700 C) are not available in the literature. There are no standard thermal conductivity values for Kaolite because each sample is somewhat different. Therefore, it is necessary to measure thermal conductivity of each type of Kaolite. Thermal conductivity measurements will help the modeling and calculation of temperatures of the ES-2 containers. This report focuses on the thermal conductivity testing effort at ORNL.

  8. Waterproofing Nanostructured Aerogel-Ceramic Fiber Composites

    NASA Technical Reports Server (NTRS)

    White, Susan; Hsu, Ming Ta; Arnold, Jim (Technical Monitor)

    2001-01-01

    Aerogels are nanoporous materials which can be used to enhance the transport properties of ceramic fiber materials, to exploit their unique properties such as high porosity, large surface area, low density and low thermal conductivity. Numerous applications have been investigated. major obstacle to commercialization is that the structure of aerogels collapses due to the adsorption of water. simple and relatively cheap process has been developed to waterproof silica, alumina and alumina-silica and carbon aerogels and composites incorporating them. Previous waterproofing methods are short lived or expensive and time consuming.

  9. Mechanically Strong Lightweight Materials for Aerospace Applications (x-aerogels)

    NASA Technical Reports Server (NTRS)

    Leventis, Nicholas

    2005-01-01

    The X-Aerogel is a new NASA-developed strong lightweight material made by reacting the mesoporous surfaces of 3-D networks of inorganic nanoparticles with polymeric crosslinkers. Since the relative amount of the crosslinker and the backbone are comparable, X-Aerogels can be viewed either as aerogels modified by templated accumulation of polymer on the skeletal nanoparticles, or as nanoporous polymers made by templated casting of polymeric precursors on a nanostructured framework. The most striking feature of X-Aerogels is that for a nominal 3-fold increase in density (still a ultralightweight material), the mechanical strength can be up to 300 times higher than the strength of the underlying native aerogel. Thus, X-Aerogels combine a multiple of the specific compressive strength of steel, with the thermal conductivity of styrofoam. XAerogels have been demonstrated with several polymers such as polyurethanes/polyureas, epoxies and polyolefins, while crosslinking of approximately 35 different oxide aerogels yields a wide variety of dimensionally stable, porous lightweight materials with interesting structural, magnetic and optical properties. X-Aerogels are evaluated for cryogenic rocket fuel storage tanks and for Advanced EVA suits, where they will play the dual role of the thermal insulator/structural material. Along the same lines, major impact is also expected by the use of X-Aerogels in structural components/thermal protection for small satellites, spacecrafts, planetary vehicles and habitats.

  10. Thermal conductivity of nanoparticle suspensions

    NASA Astrophysics Data System (ADS)

    Cahill, David; Putnam, Shawn

    2006-03-01

    We present our experimental study on the thermal conductivity of nanofluids loaded with small volume fractions of C60-C70 fullerenes and alkanethiolate-protected Au nanoparticles. We use an optical beam deflection technique that measures the thermal diffusivity of fluid mixtures and suspensions of nanoparticles with a precision of better than 1%. Our approach is tested using the thermal conductivity of ethanol-water mixtures; in nearly pure ethanol, the increase in thermal conductivity with water concentration is a factor of two larger than predicted by effective medium theory. The solutions of the C60-C70 fullerenes and the alkanethiolate-protected Au nanoparticles were measured to maximum volume fractions of 0.6% and 0.35 vol%, respectively. We do not observe anomalous enhancements of the thermal conductivity that have been reported in previous studies of nanofluids; the largest increase in thermal conductivity we have observed is 1.3 ±0.8% for 4 nm diameter Au particles suspended in ethanol. However, within the context of effective medium theory, these findings are expected: effective medium theory predicts that the largest possible increase in the thermal conductivity of a fluid loaded by a volume fraction φ1 of spherical particles will be 3φλ0, where λ0 is the thermal conductivity of the carrier fluid.

  11. Evaluating Dimethyldiethoxysilane for use in Polyurethane Crosslinked Silica Aerogels

    NASA Technical Reports Server (NTRS)

    Randall, Jason P.; Meador, Mary Ann B.; Jana, Sadhan C.

    2008-01-01

    Silica aerogels are highly porous materials which exhibit exceptionally low density and thermal conductivity. Their "pearl necklace" nanostructure, however, is inherently weak; most silica aerogels are brittle and fragile. The strength of aerogels can be improved by employing an additional crosslinking step using isocyanates. In this work, dimethyldiethoxysilane (DMDES) is evaluated for use in the silane backbone of polyurethane crosslinked aerogels. Approximately half of the resulting aerogels exhibited a core/shell morphology of hard crosslinked aerogel surrounding a softer, uncrosslinked center. Solid state NMR and scanning electron microscopy results indicate the DMDES incorporated itself as a conformal coating around the outside of the secondary silica particles, in much the same manner as isocyanate crosslinking. Response surface curves were generated from compression data, indicating levels of reinforcement comparable to that in previous literature, despite the core/shell morphology.

  12. Low Conductivity Thermal Barrier Coatings

    NASA Technical Reports Server (NTRS)

    Zhu, Dong-Ming

    2005-01-01

    Thermal barrier coatings will be more aggressively designed to protect gas turbine engine hot-section components in order to meet future engine higher fuel efficiency and lower emission goals. In this presentation, thermal barrier coating development considerations and requirements will be discussed. An experimental approach is established to monitor in real time the thermal conductivity of the coating systems subjected to high-heat-flux, steady-state and cyclic temperature gradients. Advanced low conductivity thermal barrier coatings have also been developed using a multi-component defect clustering approach, and shown to have improved thermal stability. The durability and erosion resistance of low conductivity thermal barrier coatings have been improved utilizing advanced coating architecture design, composition optimization, in conjunction with more sophisticated modeling and design tools.

  13. Thermal conductivity of nanoparticle suspensions

    NASA Astrophysics Data System (ADS)

    Putnam, Shawn A.; Cahill, David G.; Braun, Paul V.; Ge, Zhenbin; Shimmin, Robert G.

    2006-04-01

    We describe an optical beam deflection technique for measurements of the thermal diffusivity of fluid mixtures and suspensions of nanoparticles with a precision of better than 1%. Our approach is tested using the thermal conductivity of ethanol-water mixtures; in nearly pure ethanol, the increase in thermal conductivity with water concentration is a factor of 2 larger than predicted by effective medium theory. Solutions of C60-C70 fullerenes in toluene and suspensions of alkanethiolate-protected Au nanoparticles were measured to maximum volume fractions of 0.6% and 0.35 vol %, respectively. We do not observe anomalous enhancements of the thermal conductivity that have been reported in previous studies of nanofluids; the largest increase in thermal conductivity we have observed is 1.3%+/-0.8% for 4 nm diam Au particles suspended in ethanol.

  14. Radiative thermal conduction fronts

    NASA Technical Reports Server (NTRS)

    Borkowski, Kazimierz J.; Balbus, Steven A.; Fristrom, Carl C.

    1990-01-01

    The discovery of the O VI interstellar absorption lines in our Galaxy by the Copernicus observatory was a turning point in our understanding of the Interstellar Medium (ISM). It implied the presence of widespread hot (approx. 10 to the 6th power K) gas in disk galaxies. The detection of highly ionized species in quasi-stellar objects' absorption spectra may be the first indirect observation of this hot phase in external disk galaxies. Previous efforts to understand extensive O VI absorption line data from our Galaxy were not very successful in locating the regions where this absorption originates. The location at interfaces between evaporating ISM clouds and hot gas was favored, but recent studies of steady-state conduction fronts in spherical clouds by Ballet, Arnaud, and Rothenflug (1986) and Bohringer and Hartquist (1987) rejected evaporative fronts as the absorption sites. Researchers report here on time-dependent nonequilibrium calculations of planar conductive fronts whose properties match well with observations, and suggest reasons for the difference between the researchers' results and the above. They included magnetic fields in additional models, not reported here, and the conclusions are not affected by their presence.

  15. Radiative thermal conduction fronts

    NASA Astrophysics Data System (ADS)

    Borkowski, Kazimierz J.; Balbus, Steven A.; Fristrom, Carl C.

    1990-07-01

    The discovery of the O VI interstellar absorption lines in our Galaxy by the Copernicus observatory was a turning point in our understanding of the Interstellar Medium (ISM). It implied the presence of widespread hot (approx. 10 to the 6th power K) gas in disk galaxies. The detection of highly ionized species in quasi-stellar objects' absorption spectra may be the first indirect observation of this hot phase in external disk galaxies. Previous efforts to understand extensive O VI absorption line data from our Galaxy were not very successful in locating the regions where this absorption originates. The location at interfaces between evaporating ISM clouds and hot gas was favored, but recent studies of steady-state conduction fronts in spherical clouds by Ballet, Arnaud, and Rothenflug (1986) and Bohringer and Hartquist (1987) rejected evaporative fronts as the absorption sites. Researchers report here on time-dependent nonequilibrium calculations of planar conductive fronts whose properties match well with observations, and suggest reasons for the difference between the researchers' results and the above. They included magnetic fields in additional models, not reported here, and the conclusions are not affected by their presence.

  16. Thermal Conductances Of Metal Contacts

    NASA Technical Reports Server (NTRS)

    Salerno, L. J.; Kittel, P.; Scherkenbach, F. E.; Spivak, A. L.

    1988-01-01

    Report presents results of measurements of thermal conductances of aluminum and stainless-steel contacts at temperatures from 1.6 to 6.0 K. Measurement apparatus includes gearmotor assembly connected to rocker arm by music wire to load sample pair with forces up to 670 N. Heater placed above upper sample. Germanium resistance thermometers in upper and lower samples measured temperature difference across interface over range of heater powers from 0.1 to 10.0 mW. The thermal conductance calculated from temperature difference. Measurements provide data for prediction of thermal conductances of bolted joints in cryogenic infrared instruments.

  17. Aerogel-Based Multilayer Insulation with Micrometeoroid Protection

    NASA Technical Reports Server (NTRS)

    Begag, Redouane; White, Shannon

    2013-01-01

    aerogel panel. The impacted X-aerogel (the back specimen from the successful test) was further tested in comparison to another similar sample (not impacted) at Kennedy Space Center for thermal conductivity evaluation at cryogenic conditions. The specimens were tested under high vacuum and cryogenic temperatures, using Cryostat 500. The results show that the specimen did not lose a significant amount of thermal performance due to the impact test, especially at high vacuum.

  18. Low Thermal Conductivity Thermal Barrier Coatings Developed

    NASA Technical Reports Server (NTRS)

    Zhu, Dongming

    2003-01-01

    Thermal barrier coatings (TBCs) are used extensively in modern gas turbine engines to thermally insulate air-cooled metallic components from the hot gases in the engine. These coatings typically consist of a zirconia-yttria ceramic that has been applied by either plasma spraying or physical vapor deposition. Future engines will rely even more heavily on TBCs and will require materials that have even higher temperature capability with improved insulation (i.e., lower thermal conductivity even after many hours at high temperature). This report discusses new TBCs that have been developed with these future requirements in mind. The Ultra-Efficient Engine Technology Program at the NASA Glenn Research Center is funding this effort, which has been conducted primarily at Glenn with contractor support (GE and Howmet) for physical vapor deposition. As stated, the new TBC not only had to be more insulating but the insulation had to persist even after many hours of exposure-that is, the new TBC had to have both lower conductivity and improved sintering resistance. A new type of test rig was developed for this task. This new test approach used a laser to deliver a known high heat flux in an essentially uniform pattern to the surface of the coating, thereby establishing a realistic thermal gradient across its thickness. This gradient was determined from surface and backside pyrometry; and since the heat flux and coating thickness are known, this permitted continuous monitoring of thermal conductivity. Thus, this laser rig allowed very efficient screening of candidate low-conductivity, sinter-resistant TBCs. The coating-design approach selected for these new low-conductivity TBCs was to identify oxide dopants that had the potential to promote the formation of relatively large and stable groupings of defects known as defect clusters. This approach was used because it was felt that such clusters would reduce conductivity while enhancing stability. The approach proved to be

  19. Thermal Conductivity in Supercooled Water

    NASA Astrophysics Data System (ADS)

    Biddle, John; Holten, Vincent; Sengers, Jan; Anisimov, Mikhail

    2013-03-01

    The heat capacity of supercooled water, measured down to -37 C, shows an anomalous increase as temperature decreases. The thermal diffusivity, the ratio of thermal conductivity and the heat capacity per unit volume, shows a decrease. These anomalies may be associated with a hypothetical liquid-liquid critical point in metastable water below the line of homogeneous nucleation. The data suggest that the thermal conductivity does not show a significant critical enhancement, in contrast to what is observed near the vapor-liquid critical point. We have used mode-coupling theory to investigate the possible effect of critical fluctuations on the thermal conductivity of supercooled water, and shown that indeed this effect would be too small to be measurable at experimentally accessible temperatures. We discuss physical reasons for the striking difference between the vapor-liquid and liquid-liquid critical enhancements of thermal conductivity in water. We also discuss the discrepancy between the thermal conductivity calculated from experimental data and that obtained by computer simulations of the TIP5P water-like model. American Chemical Society Petroleum Research Fund Grant No. 52666-ND6

  20. Lightweight, Thermally Conductive Composite Material

    NASA Technical Reports Server (NTRS)

    Sharp, G. Richard; Loftin, Timothy A.

    1990-01-01

    Aluminum reinforced with carbon fibers superior to copper in some respects. Lightweight composite material has high thermal conductivity. Consists of aluminum matrix containing graphite fibers, all oriented in same direction. Available as sheets, tubes, and bars. Thermal conductivity of composite along fibers rises above that of pure copper over substantial range of temperatures. Graphite/aluminum composite useful in variety of heat-transfer applications in which reduction of weight critical. Used to conduct heat in high-density, high-speed integrated-circuit packages for computers and in base plates for electronic equipment. Also used to carry heat away from leading edges of wings in high-speed airplanes.

  1. Thermal conductivity of carbonate rocks

    USGS Publications Warehouse

    Thomas, J., Jr.; Frost, R.R.; Harvey, R.D.

    1973-01-01

    The thermal conductivities of several well-defined carbonate rocks were determined near 40??C. Values range from 1.2 W m-1 C-1 for a highly porous chalk to 5.1 W m-1 C-1 for a dolomite. The thermal conductivity of magnesite (5.0) is at the high end of the range, and that for Iceland Spar Calcite (3.2) is near the middle. The values for limestones decrease linearly with increasing porosity. Dolomites of comparable porosity have greater thermal conductivities than limestones. Water-sorbed samples have expected greater thermal conductivities than air-saturated (dry) samples of the same rock. An anomalously large increase in the thermal conductivity of a water-sorbed clayey dolomite over that of the same sample when dry is attributed to the clay fraction, which swells during water inhibition, causing more solid-to-solid contacts within the dolomite framework. Measurements were made with a Colora Thermoconductometer. Chemical and mineralogical analyses were made and tabulated. Porosity of the rocks was determined by mercury porosimetry and also from density measurements. The Iceland Spar Calcite and magnesite were included for reference. ?? 1973.

  2. Polyimide Cellulose Nanocrystal Composite Aerogels

    NASA Technical Reports Server (NTRS)

    Nguyen, Baochau N.; Meador, Mary Ann; Rowan, Stuart; Cudjoe, Elvis; Sandberg, Anna

    2014-01-01

    Polyimide (PI) aerogels are highly porous solids having low density, high porosity and low thermal conductivity with good mechanical properties. They are ideal for various applications including use in antenna and insulation such as inflatable decelerators used in entry, decent and landing operations. Recently, attention has been focused on stimuli responsive materials such as cellulose nano crystals (CNCs). CNCs are environmentally friendly, bio-renewable, commonly found in plants and the dermis of sea tunicates, and potentially low cost. This study is to examine the effects of CNC on the polyimide aerogels. The CNC used in this project are extracted from mantle of a sea creature called tunicates. A series of polyimide cellulose nanocrystal composite aerogels has been fabricated having 0-13 wt of CNC. Results will be discussed.

  3. Monte Carlo Study on Carbon-Gradient-Doped Silica Aerogel Insulation.

    PubMed

    Zhao, Y; Tang, G H

    2015-04-01

    Silica aerogel is almost transparent for wavelengths below 8 µm where significant energy is transferred by thermal radiation. The radiative heat transfer can be restricted at high temperature if doped with carbon powder in silica aerogel. However, different particle sizes of carbon powder doping have different spectral extinction coefficients and the doped carbon powder will increase the solid conduction of silica aerogel. This paper presents a theoretical method for determining the optimal carbon doping in silica aerogel to minimize the energy transfer. Firstly we determine the optimal particle size by combining the spectral extinction coefficient with blackbody radiation and then evaluate the optimal doping amount between heat conduction and radiation. Secondly we develop the Monte Carlo numerical method to study radiative properties of carbon-gradient-doped silica aerogel to decrease the radiative heat transfer further. The results indicate that the carbon powder is able to block infrared radiation and thus improve the thermal insulating performance of silica aerogel effectively. PMID:26353574

  4. Thermal conductivity of cane fiberboard

    SciTech Connect

    Leader, D.R.

    1995-05-01

    The thermal conductivity of cane fiberboard was measured in two planes; parallel to the surface and perpendicular to the surface of the manufactured sheet. The information was necessary to better understand the thermal response of a loaded shipping container. The tests demonstrated that the thermal conductivity of cane fiberboard in the plane parallel to the surface of the sheet was nearly twice as great as the conductivity of the same material in a plane perpendicular to the sheet. There was no significant difference in the conductivity in different directions within the plane parallel to the surface, and the presence of glue between layers of fiberboard did not significantly change the conductivity of the assembly. The tests revealed that the thermal conductivity measured in a direction perpendicular to the plane of the surface of a stack of cane fiberboard sheets not bonded together, decreases with an increase in the mean temperature. This was determined to be the result of air gaps between the sheets of fiberboard, and not related to the properties of the material itself

  5. Thermal conductance of superlattice junctions

    SciTech Connect

    Lu, Simon; McGaughey, Alan J. H.

    2015-05-15

    We use molecular dynamics simulations and the lattice-based scattering boundary method to compute the thermal conductance of finite-length Lennard-Jones superlattice junctions confined by bulk crystalline leads. The superlattice junction thermal conductance depends on the properties of the leads. For junctions with a superlattice period of four atomic monolayers at temperatures between 5 and 20 K, those with mass-mismatched leads have a greater thermal conductance than those with mass-matched leads. We attribute this lead effect to interference between and the ballistic transport of emergent junction vibrational modes. The lead effect diminishes when the temperature is increased, when the superlattice period is increased, and when interfacial disorder is introduced, but is reversed in the harmonic limit.

  6. Synthesis, Processing, and Characterization of Inorganic-Organic Hybrid Cross-Linked Silica, Organic Polyimide, and Inorganic Aluminosilicate Aerogels

    NASA Technical Reports Server (NTRS)

    Nguyen, Baochau N.; Guo, Haiquan N.; McCorkle, Linda S.

    2014-01-01

    As aerospace applications become ever more demanding, novel insulation materials with lower thermal conductivity, lighter weight and higher use temperature are required to fit the aerospace application needs. Having nanopores and high porosity, aerogels are superior thermal insulators, among other things. The use of silica aerogels in general is quite restricted due to their inherent fragility, hygroscopic nature, and poor mechanical properties, especially in extereme aerospace environments. Our research goal is to develop aerogels with better mechanical and environmental stability for a variety of aeronautic and space applications including space suit insulation for planetary surface missions, insulation for inflatable structures for habitats, inflatable aerodynamic decelerators for entry, descent and landing (EDL) operations, and cryotank insulation for advance space propulsion systems. Different type of aerogels including organic-inorganic polymer reinforced (hybrid) silica-based aerogels, polyimide aerogels and inorganic aluminosilicate aerogels have been developed and examined.

  7. Aerogel Projects Ongoing in MSFC's Engineering Directorate

    NASA Technical Reports Server (NTRS)

    Shular, David A.; Smithers, Gweneth A.; Plawsky, Joel L.; Whitaker, Ann F. (Technical Monitor)

    2000-01-01

    When we speak of an aerogel material, we are referring more to process and structure am to a specific substance. Aerogel, considered the lightest solid material, has been made from silica for seventy years. Resorcinol-formaldehyde, organic aerogels have been developed more recently. However, aerogel can be made from almost any type of substance, even lead. Because an aerogel is mostly air (about 99 %), the solid substance used will affect the weight very little. The term "aerogel" connotes the sol-gel process used to manufacture the material. The aerogel begins as a liquid "sol," becomes a solid "alcogel," and is then dried to become an "aerogel." The final product has a unique structure, useful for exploitation. It is an "open pore" system with nano-sized particles and pores, has very high surface area, and is highly interconnected. Besides low weight, aerogels have ultimate (lowest) values in other properties: thermal conductivity, refractive index, sound speed, and dielectric constant. Aerogels were first prepared in 1931 by Steven Kistler, who used a supercritical drying step to replace the liquid in a gel with air, preserving the structure (1). Kistler's procedure involved a water-to-alcohol exchange step; in the 1970's, this step was eliminated when a French investigator introduced the use of tetramethylorthosilicate. Still, alcohol drying involved dangerously high temperatures and pressures. In the 1980's, the Microstructured Materials Group at Berkeley Laboratory found that the alcohol in the gel could be replaced with liquid carbon dioxide before supercritical drying, which greatly improved safety (2). 'Me most recent major contribution has been that of Deshpande, Smith and Brinker in New Mexico, who are working to eliminate the supercritical drying step (3). When aerogels were first being developed, they were evaporatively dried. However, the wet gel, when dried, underwent severe shrinkage and cracking; this product was termed "xerogel." When the

  8. High thermal conductivity of diamond

    NASA Technical Reports Server (NTRS)

    Stephan, Patrick M.

    1993-01-01

    The objectives of this educational exercise were to demonstrate the high rate of heat flow from a synthetic diamond coupon and to compare it to a commonly used thermal conductor, such as copper. The principles of heat transfer by conduction and convection may also be demonstrated. A list of equipment and supplies and the procedure for the experiment are presented.

  9. Thermal Conductivity of Rubble Piles

    NASA Astrophysics Data System (ADS)

    Luan, Jing; Goldreich, Peter

    2015-11-01

    Rubble piles are a common feature of solar system bodies. They are composed of monolithic elements of ice or rock bound by gravity. Voids occupy a significant fraction of the volume of a rubble pile. They can exist up to pressure P≈ {ε }Yμ , where {ε }Y is the monolithic material's yield strain and μ its rigidity. At low P, contacts between neighboring elements are confined to a small fraction of their surface areas. As a result, the effective thermal conductivity of a rubble pile, {k}{con}≈ k{(P/({ε }Yμ ))}1/2, can be orders of magnitude smaller than the thermal conductivity of its monolithic elements, k. In a fluid-free environment, only radiation can transfer energy across voids. It contributes an additional component, {k}{rad}=16{\\ell }σ {T}3/3, to the total effective conductivity, {k}{eff}={k}{con}+{k}{rad}. Here ℓ, the inverse of the opacity per unit volume, is of the order of the size of the elements, and voids. An important distinction between {k}{con} and {k}{rad} is that the former is independent of the size of the elements, whereas the latter is proportional to it. Our expression for {k}{eff} provides a good fit to the depth dependence of thermal conductivity in the top 140 cm of the lunar regolith. It also offers a good starting point for detailed modeling of thermal inertias for asteroids and satellites. Measurement of the response of surface temperature to variable insolation is a valuable diagnostic of a regolith. There is an opportunity for careful experiments under controlled laboratory conditions to test models of thermal conductivity such as the one we outline.

  10. Aerogel-supported filament

    DOEpatents

    Wuest, Craig R.; Tillotson, Thomas M.; Johnson, III, Coleman V.

    1995-01-01

    The present invention is a thin filament embedded in a low density aerogel for use in radiation detection instruments and incandescent lamps. The aerogel provides a supportive matrix that is thermally and electrically nonconductive, mechanically strong, highly porous, gas-permeable, and transparent to ionizing radiation over short distances. A low density, open-cell aerogel is cast around a fine filament or wire, which allows the wire to be positioned with little or no tension and keeps the wire in place in the event of breakage. The aerogel support reduces the stresses on the wire caused by vibrational, gravitational, electrical, and mechanical forces.

  11. Aerogel-supported filament

    DOEpatents

    Wuest, C.R.; Tillotson, T.M.; Johnson, C.V. III

    1995-05-16

    The present invention is a thin filament embedded in a low density aerogel for use in radiation detection instruments and incandescent lamps. The aerogel provides a supportive matrix that is thermally and electrically nonconductive, mechanically strong, highly porous, gas-permeable, and transparent to ionizing radiation over short distances. A low density, open-cell aerogel is cast around a fine filament or wire, which allows the wire to be positioned with little or no tension and keeps the wire in place in the event of breakage. The aerogel support reduces the stresses on the wire caused by vibrational, gravitational, electrical, and mechanical forces. 6 Figs.

  12. Technical applications of aerogels

    SciTech Connect

    Hrubesh, L.W.

    1997-08-18

    Aerogel materials posses such a wide variety of exceptional properties that a striking number of applications have developed for them. Many of the commercial applications of aerogels such as catalysts, thermal insulation, windows, and particle detectors are still under development and new application as have been publicized since the ISA4 Conference in 1994: e.g.; supercapacitors, insulation for heat storage in automobiles, electrodes for capacitive deionization, etc. More applications are evolving as the scientific and engineering community becomes familiar with the unusual and exceptional physical properties of aerogels, there are also scientific and technical application, as well. This paper discusses a variety of applications under development at Lawrence Livermore National Laboratory for which several types of aerogels are formed in custom sizes and shapes. Particular discussions will focus on the uses of aerogels for physics experiments which rely on the exceptional, sometimes unique, properties of aerogels.

  13. Highly transparent, flexible, and thermally stable superhydrophobic ORMOSIL aerogel thin films.

    PubMed

    Budunoglu, Hulya; Yildirim, Adem; Guler, Mustafa O; Bayindir, Mehmet

    2011-02-01

    We report preparation of highly transparent, flexible, and thermally stable superhydrophobic organically modified silica (ORMOSIL) aerogel thin films from colloidal dispersions at ambient conditions. The prepared dispersions are suitable for large area processing with ease of coating and being directly applicable without requiring any pre- or post-treatment on a variety of surfaces including glass, wood, and plastics. ORMOSIL films exhibit and retain superhydrophobic behavior up to 500 °C and even on bent flexible substrates. The surface of the films can be converted from superhydrophobic (contact angle of 179.9°) to superhydrophilic (contact angle of <5°) by calcination at high temperatures. The wettability of the coatings can be changed by tuning the calcination temperature and duration. The prepared films also exhibit low refractive index and high porosity making them suitable as multifunctional coatings for many application fields including solar cells, flexible electronics, and lab on papers. PMID:21226471

  14. High-efficiency electrochemical thermal energy harvester using carbon nanotube aerogel sheet electrodes

    PubMed Central

    Im, Hyeongwook; Kim, Taewoo; Song, Hyelynn; Choi, Jongho; Park, Jae Sung; Ovalle-Robles, Raquel; Yang, Hee Doo; Kihm, Kenneth D.; Baughman, Ray H.; Lee, Hong H.; Kang, Tae June; Kim, Yong Hyup

    2016-01-01

    Conversion of low-grade waste heat into electricity is an important energy harvesting strategy. However, abundant heat from these low-grade thermal streams cannot be harvested readily because of the absence of efficient, inexpensive devices that can convert the waste heat into electricity. Here we fabricate carbon nanotube aerogel-based thermo-electrochemical cells, which are potentially low-cost and relatively high-efficiency materials for this application. When normalized to the cell cross-sectional area, a maximum power output of 6.6 W m−2 is obtained for a 51 °C inter-electrode temperature difference, with a Carnot-relative efficiency of 3.95%. The importance of electrode purity, engineered porosity and catalytic surfaces in enhancing the thermocell performance is demonstrated. PMID:26837457

  15. High-efficiency electrochemical thermal energy harvester using carbon nanotube aerogel sheet electrodes.

    PubMed

    Im, Hyeongwook; Kim, Taewoo; Song, Hyelynn; Choi, Jongho; Park, Jae Sung; Ovalle-Robles, Raquel; Yang, Hee Doo; Kihm, Kenneth D; Baughman, Ray H; Lee, Hong H; Kang, Tae June; Kim, Yong Hyup

    2016-01-01

    Conversion of low-grade waste heat into electricity is an important energy harvesting strategy. However, abundant heat from these low-grade thermal streams cannot be harvested readily because of the absence of efficient, inexpensive devices that can convert the waste heat into electricity. Here we fabricate carbon nanotube aerogel-based thermo-electrochemical cells, which are potentially low-cost and relatively high-efficiency materials for this application. When normalized to the cell cross-sectional area, a maximum power output of 6.6 W m(-2) is obtained for a 51 °C inter-electrode temperature difference, with a Carnot-relative efficiency of 3.95%. The importance of electrode purity, engineered porosity and catalytic surfaces in enhancing the thermocell performance is demonstrated. PMID:26837457

  16. High-efficiency electrochemical thermal energy harvester using carbon nanotube aerogel sheet electrodes

    NASA Astrophysics Data System (ADS)

    Im, Hyeongwook; Kim, Taewoo; Song, Hyelynn; Choi, Jongho; Park, Jae Sung; Ovalle-Robles, Raquel; Yang, Hee Doo; Kihm, Kenneth D.; Baughman, Ray H.; Lee, Hong H.; Kang, Tae June; Kim, Yong Hyup

    2016-02-01

    Conversion of low-grade waste heat into electricity is an important energy harvesting strategy. However, abundant heat from these low-grade thermal streams cannot be harvested readily because of the absence of efficient, inexpensive devices that can convert the waste heat into electricity. Here we fabricate carbon nanotube aerogel-based thermo-electrochemical cells, which are potentially low-cost and relatively high-efficiency materials for this application. When normalized to the cell cross-sectional area, a maximum power output of 6.6 W m-2 is obtained for a 51 °C inter-electrode temperature difference, with a Carnot-relative efficiency of 3.95%. The importance of electrode purity, engineered porosity and catalytic surfaces in enhancing the thermocell performance is demonstrated.

  17. Cellulose as an adhesion agent for the synthesis of lignin aerogel with strong mechanical performance, Sound-absorption and thermal Insulation.

    PubMed

    Wang, Chao; Xiong, Ye; Fan, Bitao; Yao, Qiufang; Wang, Hanwei; Jin, Chunde; Sun, Qingfeng

    2016-01-01

    The lignin aerogels that are both high porosity and compressibility would have promising implications for bioengineering field to sound-adsorption and damping materials; however, creating this aerogel had a challenge to adhesive lignin. Here we reported cellulose as green adhesion agent to synthesize the aerogels with strong mechanical performance. Our approach-straightforwardly dissolved in ionic liquids and simply regenerated in the deionized water-causes assembly of micro-and nanoscale and even molecule level of cellulose and lignin. The resulting lignin aerogels exhibit Young's modulus up to 25.1 MPa, high-efficiency sound-adsorption and excellent thermal insulativity. The successful synthesis of this aerogels developed a path for lignin to an advanced utilization. PMID:27562532

  18. High-Thermal-Conductivity Fabrics

    NASA Technical Reports Server (NTRS)

    Chibante, L. P. Felipe

    2012-01-01

    Heat management with common textiles such as nylon and spandex is hindered by the poor thermal conductivity from the skin surface to cooling surfaces. This innovation showed marked improvement in thermal conductivity of the individual fibers and tubing, as well as components assembled from them. The problem is centered on improving the heat removal of the liquid-cooled ventilation garments (LCVGs) used by astronauts. The current design uses an extensive network of water-cooling tubes that introduces bulkiness and discomfort, and increases fatigue. Range of motion and ease of movement are affected as well. The current technology is the same as developed during the Apollo program of the 1960s. Tubing material is hand-threaded through a spandex/nylon mesh layer, in a series of loops throughout the torso and limbs such that there is close, form-fitting contact with the user. Usually, there is a nylon liner layer to improve comfort. Circulating water is chilled by an external heat exchanger (sublimator). The purpose of this innovation is to produce new LCVG components with improved thermal conductivity. This was addressed using nanocomposite engineering incorporating high-thermalconductivity nanoscale fillers in the fabric and tubing components. Specifically, carbon nanotubes were added using normal processing methods such as thermoplastic melt mixing (compounding twin screw extruder) and downstream processing (fiber spinning, tubing extrusion). Fibers were produced as yarns and woven into fabric cloths. The application of isotropic nanofillers can be modeled using a modified Nielsen Model for conductive fillers in a matrix based on Einstein s viscosity model. This is a drop-in technology with no additional equipment needed. The loading is limited by the ability to maintain adequate dispersion. Undispersed materials will plug filtering screens in processing equipment. Generally, the viscosity increases were acceptable, and allowed the filled polymers to still be

  19. Carbon aerogel composites prepared by ambient drying and using oxidized polyacrylonitrile fibers as reinforcements.

    PubMed

    Feng, Junzong; Zhang, Changrui; Feng, Jian; Jiang, Yonggang; Zhao, Nan

    2011-12-01

    Carbon fiber-reinforced carbon aerogel composites (C/CAs) for thermal insulators were prepared by copyrolysis of resorcinol-formaldehyde (RF) aerogels reinforced by oxidized polyacrylonitrile (PAN) fiber felts. The RF aerogel composites were obtained by impregnating PAN fiber felts with RF sols, then aging, ethanol exchanging, and drying at ambient pressure. Upon carbonization, the PAN fibers shrink with the RF aerogels, thus reducing the difference of shrinkage rates between the fiber reinforcements and the aerogel matrices, and resulting in C/CAs without any obvious cracks. The three point bend strength of the C/CAs is 7.1 ± 1.7 MPa, and the thermal conductivity is 0.328 W m(-1) K(-1) at 300 °C in air. These composites can be used as high-temperature thermal insulators (in inert atmospheres or vacuum) or supports for phase change materials in thermal protection system. PMID:22047011

  20. Silica/Polymer and Silica/Polymer/Fiber Composite Aerogels

    NASA Technical Reports Server (NTRS)

    Ou, Danny; Stepanian, Christopher J.; Hu, Xiangjun

    2010-01-01

    Aerogels that consist, variously, of neat silica/polymer alloys and silica/polymer alloy matrices reinforced with fibers have been developed as materials for flexible thermal-insulation blankets. In comparison with prior aerogel blankets, these aerogel blankets are more durable and less dusty. These blankets are also better able to resist and recover from compression . an important advantage in that maintenance of thickness is essential to maintenance of high thermal-insulation performance. These blankets are especially suitable as core materials for vacuum- insulated panels and vacuum-insulated boxes of advanced, nearly seamless design. (Inasmuch as heat leakage at seams is much greater than heat leakage elsewhere through such structures, advanced designs for high insulation performance should provide for minimization of the sizes and numbers of seams.) A silica/polymer aerogel of the present type could be characterized, somewhat more precisely, as consisting of multiply bonded, linear polymer reinforcements within a silica aerogel matrix. Thus far, several different polymethacrylates (PMAs) have been incorporated into aerogel networks to increase resistance to crushing and to improve other mechanical properties while minimally affecting thermal conductivity and density. The polymethacrylate phases are strongly linked into the silica aerogel networks in these materials. Unlike in other organic/inorganic blended aerogels, the inorganic and organic phases are chemically bonded to each other, by both covalent and hydrogen bonds. In the process for making a silica/polymer alloy aerogel, the covalent bonds are introduced by prepolymerization of the methacrylate monomer with trimethoxysilylpropylmethacrylate, which serves as a phase cross-linker in that it contains both organic and inorganic monomer functional groups and hence acts as a connector between the organic and inorganic phases. Hydrogen bonds are formed between the silanol groups of the inorganic phase and the

  1. Organic and composite aerogels through ring opening metathesis polymerization (ROMP)

    NASA Astrophysics Data System (ADS)

    Mohite, Dhairyashil P.

    Aerogels are open-cell nanoporous materials, unique in terms of low density, low thermal conductivity, low dielectric constants and high acoustic attenuation. Those exceptional properties stem from their complex hierarchical solid framework (agglomerates of porous, fractal secondary nanoparticles), but they also come at a cost: low mechanical strength. This issue has been resolved by crosslinking silica aerogels with organic polymers. The crosslinking polymer has been assumed to form a conformal coating on the surface of the skeletal framework by covalent bridging elementary building blocks. However, "assuming" is not enough: for correlating nanostructure with bulk material properties, it is important to know the exact location of the polymer on the aerogel backbone. For that investigation, we synthesized a new norbornene derivative of triethoxysilane (Si-NAD) that can be attached to skeletal silica nanoparticles. Those norbornene-modified silica aerogels were crosslinked with polynorbornene by ring opening metathesis polymerization (ROMP). The detailed correlation between nanostructure and mechanical strength was probed with a wide array of characterization methods ranging from molecular to bulk through nano. Subsequently, it was reasoned that since the polymer dominates the exceptional mechanical properties of polymer crosslinked aerogels, purely organic aerogels with the same nanostructure and interparticle connectivity should behave similarly. That was explored and confirmed by: (a) synthesis of a difunctional nadimide monomer (bis-NAD), and preparation of robust polyimide aerogels by ROMP of its norbornene end-caps; and, (b) synthesis of dimensionally stable ROMP-derived polydicyclopentadiene aerogels by grafting the nanostructure with polymethylmethacrylate (PMMA) via free radical chemistry.

  2. Thermal management of thermoacoustic sound projectors using a free-standing carbon nanotube aerogel sheet as a heat source

    NASA Astrophysics Data System (ADS)

    Aliev, Ali E.; Mayo, Nathanael K.; Baughman, Ray H.; Avirovik, Dragan; Priya, Shashank; Zarnetske, Michael R.; Blottman, John B.

    2014-10-01

    Carbon nanotube (CNT) aerogel sheets produce smooth-spectra sound over a wide frequency range (1-105 Hz) by means of thermoacoustic (TA) sound generation. Protective encapsulation of CNT sheets in inert gases between rigid vibrating plates provides resonant features for the TA sound projector and attractive performance at needed low frequencies. Energy conversion efficiencies in air of 2% and 10% underwater, which can be enhanced by further increasing the modulation temperature. Using a developed method for accurate temperature measurements for the thin aerogel CNT sheets, heat dissipation processes, failure mechanisms, and associated power densities are investigated for encapsulated multilayered CNT TA heaters and related to the thermal diffusivity distance when sheet layers are separated. Resulting thermal management methods for high applied power are discussed and deployed to construct efficient and tunable underwater sound projector for operation at relatively low frequencies, 10 Hz-10 kHz. The optimal design of these TA projectors for high-power SONAR arrays is discussed.

  3. Flame Retardant Effect of Aerogel and Nanosilica on Engineered Polymers

    NASA Technical Reports Server (NTRS)

    Williams, Martha K.; Smith, Trent M.; Roberson, Luke B.; Yang, Feng; Nelson, Gordon L.

    2010-01-01

    Aerogels are typically manufactured vIa high temperature and pressure-critical-point drying of a colloidal metal oxide gel filled with solvents. Aerogel materials derived from silica materials represent a structural morphology (amorphous, open-celled nanofoams) rather than a particular chemical constituency. Aerogel is not like conventional foams in that it is a porous material with extreme microporosity and composed of individual features only a few nanometers in length with a highly porous dendriticlike structure. This unique substance has unusual properties such as low thermal conductivity, refractive index and sound suppression; in addition to its exceptional ability to capture fast moving dust. The highly porous nature of the aerogel's structure provides large amounts of surface area per unit weight. For instance, a silica aerogel material with a density of 100 kilograms per cubic meters can have surface areas of around 800 to 1500 square meters per gram depending on the precursors and process utilized to produce it. To take advantage of the unique properties of silica aerogels, especially the ultra light weight and low thermal conductivity, their composites with various engineering polymers were prepared and their flammability was investigated by Cone Calorimetry. The flammability of various polystyrene/silica aerogel nanocomposites were measured. The combination of these nanocomposites with a NASA patented flame retardant SINK were also studied. The results were compared with the base polymer to show the differences between composites with different forms of silica.

  4. THERMAL CONDUCTIVITY ANALYSIS OF GASES

    DOEpatents

    Clark, W.J.

    1949-06-01

    This patent describes apparatus for the quantitative analysis of a gaseous mixture at subatmospheric pressure by measurement of its thermal conductivity. A heated wire forms one leg of a bridge circuit, while the gas under test is passed about the wire at a constant rate. The bridge unbalance will be a measure of the change in composition of the gas, if compensation is made for the effect due to gas pressure change. The apparatus provides a voltage varying with fluctuations of pressure in series with the indicating device placed across the bridge, to counterbalance the voltage change caused by fluctuations in the pressure of the gaseous mixture.

  5. Effects of thermal efficiency in DCMD and the preparation of membranes with low thermal conductivity

    NASA Astrophysics Data System (ADS)

    Li, Zhehao; Peng, Yuelian; Dong, Yajun; Fan, Hongwei; Chen, Ping; Qiu, Lin; Jiang, Qi

    2014-10-01

    The effects of the membrane characteristics and operational conditions on the vapor flux and thermal efficiency in a direct contact membrane distillation (DCMD) process were studied with a mathematical simulation. The membrane temperature, driving force of vapor transfer, membrane distillation coefficient, etc. were used to analyze the effects. The operating conditions that increased the vapor flux improved the thermal efficiency. The membrane characteristics of four microporous membranes and their performances in DCMD were compared. A polysulfone (PSf) membrane prepared via vapor-induced phase separation exhibited the lowest thermal conductivity. The PSf and polyvinylidene difluoride (PVDF) membranes were modified using SiO2 aerogel blending and coating to reduce the thermal conductivity of the membrane. The coating process was more effective than the blending process toward this end. The changes in the structure of the modified membrane were observed with a scanning electron microscope. Si was found on the modified membrane surface with an energy spectrometer. The PVDF composite and support membranes were tested during the DCMD process; the composite membrane had a higher vapor flux and a better thermal efficiency than the support. A new method based on a 3ω technique was used to measure the thermal conductivity of the membranes.

  6. Polymer/graphene hybrid aerogel with high compressibility, conductivity, and "sticky" superhydrophobicity.

    PubMed

    Hu, Han; Zhao, Zongbin; Wan, Wubo; Gogotsi, Yury; Qiu, Jieshan

    2014-03-12

    The idea of extending functions of graphene aerogels and achieving specific applications has aroused wide attention recently. A solution to this challenge is the formation of a hybrid structure where the graphene aerogels are decorated with other functional nanostructures. An infiltration-evaporation-curing strategy has been proposed by the formation of hybrid structure containing poly(dimethylsiloxane) (PDMS) and compressible graphene aerogel (CGA), where the cellular walls of the CGA are coated uniformly with an integrated polymer layer. The resulting composite shows enhanced compressive strength and a stable Young's modulus that are superior to those of pure CGAs. This unique structure combines the advantages of both components, giving rise to an excellent electromechanical performance, where the bulk resistance repeatedly shows a synchronous and linear response to variation of the volume during compression at a wide range of compressed rates. Furthermore, the foamlike structure delivers a water droplet with "sticky" superhydrophobicity and a size as large as 32 μL that remains tightly pinned to the composite, even when it is turned upside-down. This is the first demonstration of superhydrophobicity with strong adhesion on a foamlike structure. These outstanding properties qualify the PDMS/CGA composites developed here as promising candidates for a wide range of applications such as in sensors, actuators, and materials used for biochemical separation and tissue engineering. PMID:24524309

  7. Aerogels for electronics

    SciTech Connect

    Hrubesh, L.W.

    1994-10-01

    In addition to their other exceptional properties, aerogels also exhibit unusual dielectric and electronic properties due to their nano-sized structures and high porosities. For example, aerogels have the lowest dielectric constants measured for a solid material (having values approaching 1.0); they have exceptionally high dielectric resistivities and strengths (i.e., ability to insulate very high voltages); they exhibit low dielectric loss at microwave frequencies; and some aerogels are electrically conductive and photoconductive. These properties are being exploited to provide the next generation of materials for energy storage, low power consumption, and ultra-fast electronics. We are working toward adapting these unusual materials for microelectronic applications, particularly, making thin aerogel films for dielectric substrates and for energy storage devices such as supercapacitors. Measurements are presented in this paper for the dielectric and electronic properties of aerogels, including the dielectric constant, loss factor, dielectric and electrical conductivity, volume resistivity, and dielectric strength. We also describe methods to form and characterize thin aerogel films which are being developed for numerous electronic applications. Finally, some of the electronic applications proposed for aerogels are presented. Commercialization of aerogels for electronics must await further feasibility, prototype development, and cost studies, but they are one of the key materials and are sure to have a major impact on future electronics.

  8. Aerogel Insulation Systems for Space Launch Applications

    NASA Technical Reports Server (NTRS)

    Fesmire, James E.

    2005-01-01

    New developments in materials science in the areas of solution gelation processes and nanotechnology have led to the recent commercial production of aerogels. Concurrent with these advancements has been the development of new approaches to cryogenic thermal insulation systems. For example, thermal and physical characterizations of aerogel beads under cryogenic-vacuum conditions have been performed at the Cryogenics Test Laboratory of the NASA Kennedy Space Center. Aerogel-based insulation system demonstrations have also been conducted to improve performance for space launch applications. Subscale cryopumping experiments show the thermal insulating ability of these fully breathable nanoporous materials. For a properly executed thermal insulation system, these breathable aerogel systems are shown to not cryopump beyond the initial cooldown and thermal stabilization phase. New applications are being developed to augment the thermal protection systems of space launch vehicles, including the Space Shuttle External Tank. These applications include a cold-boundary temperature of 90 K with an ambient air environment in which both weather and flight aerodynamics are important considerations. Another application is a nitrogen-purged environment with a cold-boundary temperature of 20 K where both initial cooldown and launch ascent profiles must be considered. Experimental results and considerations for these flight system applications are discussed.

  9. Flexible, Transparent and Conductive Carbon Nanotube Aerogels /PEDOT:PSS Electrodes created by Top-bottom Fabrication

    NASA Astrophysics Data System (ADS)

    Martinez, Patricia M.; Cerdan Pasaran, Andrea; Zakhidov, Anvar; University of Guanajuato, Mexico Collaboration

    The sheets of Carbon Nanotubes (CNT) have proven to be a good substitute for ITO. To improve their conductivity and increase optical transparency we have created composites which incorporate silver nanowires or other evaporated metals. Coating CNT/metals with PEDOT:PSS is important for creating hole transport/electron barrier layer functionality, but it is not easy to achieve using PEDOT:PSS solutions due to the hydrophobicity of CNT. We report a new top-to-bottom approach for the fabrication of highly flexible, transparent and conductive carbon nanotube-based electrodes using PDMS as a substrate. A uniform and smooth layer of approximately 50 nm of PEDOT:PSS was spin coated on top of a PDMS stamp followed by the deposition of vapor densified freestanding Multiwall Carbon Nanotube (MWNT) aerogels. An incorporation of silver nanowires, silver or Aluminum thin layer can be sprayed or evaporated on top of the freestanding MWNT aerogels in order to lower the sheet resistance even further. The PDMS substrate is drop cast on top of the configuration then the PDMS stamp is lifted-up. The PEDOT:PSS layer is selectively deposited on top of the MWNT only. The composite electrodes can be laminated on photovoltaic devices and on LEDs.

  10. Radiative magnetized thermal conduction fronts

    NASA Technical Reports Server (NTRS)

    Borkowski, Kazimierz J.; Balbus, Steven A.; Fristrom, Carl C.

    1990-01-01

    The evolution of plane-parallel magnetized thermal conduction fronts in the interstellar medium (ISM) was studied. Separating the coronal ISM phase and interstellar clouds, these fronts have been thought to be the site of the intermediate-temperature regions whose presence was inferred from O VI absorption-line studies. The front evolution was followed numerically, starting from the initial discontinuous temperature distribution between the hot and cold medium, and ending in the final cooling stage of the hot medium. It was found that, for the typical ISM pressure of 4000 K/cu cm and the hot medium temperature of 10 to the 6th K, the transition from evaporation to condensation in a nonmagnetized front occurs when the front thickness is 15 pc. This thickness is a factor of 5 smaller than previously estimated. The O VI column densities in both evaporative and condensation stages agree with observations if the initial hot medium temperature Th exceeds 750,000 K. Condensing conduction fronts give better agreement with observed O VI line profiles because of lower gas temperatures.

  11. Clay Nanocomposite/Aerogel Sandwich Structures for Cryotanks

    NASA Technical Reports Server (NTRS)

    Miller, Sandi; Leventis, Nicholas; Johnston, J. Chris; Meador, Michael

    2006-01-01

    GRC research has led to the development of epoxy-clay nanocomposites with 60-70% lower gas permeability than the base epoxy resin. Filament wound carbon fiber reinforced tanks made with this nanocomposite had a five-fold lower helium leak rate than the corresponding tanks made without clay. More recent work has produced new composites with more than a 100-fold reduction in helium permeability. Use of these advanced, high barrier composites would eliminate the need for a liner in composite cryotanks, thereby simplifying construction and reducing propellant leakage. Aerogels are attractive materials for use as cryotank insulation because of their low density and low thermal conductivity. However, aerogels are fragile and have poor environmental stability, which have limited their use to certain applications in specialized environments (e.g., in certain types of nuclear reactors as Cerenkov radiation detectors, and as thermal insulators aboard space rovers on Mars). New GRC developed polymer crosslinked aerogels (X-Aerogels) retain the low density of conventional aerogels, but they demonstrate a 300-fold increase in their mechanical strength. Currently, our strongest materials combine a density of approx. 0.45 g/cc, a thermal conductivity of approx. 0.04 W/mK and a compressive strength of 185 MPa. Use of these novel aerogels as insulation materials/structural components in combination with the low permeability of epoxy-clay nanocomposites could significantly reduce cryotank weight and improve durability.

  12. How to Measure Thermal Conductivity

    NASA Astrophysics Data System (ADS)

    Ventura, Guglielmo; Perfetti, Mauro

    The methods to measure the thermal conductivity at low temperature are described: the steady-state techniques, (Sect. 2.2 ); the 3ω technique (Sect. 2.3 ); and the thermal diffusivity measurement (Sect. 2.4 ). Each of these techniques has its own advantages as well as its inherent limitations, with some techniques more appropriate to specific sample geometry, such as the 3ω technique for thin films which is discussed in detail in Sect. 2.4.2 . The radial flux method is reported in Sect. 2.2.4 , the laser flash diffusivity method in Sect. 2.4.1 and the "pulsed power or Maldonado technique" in Sect. 2.3.2 .

  13. Thermal conductivity of sputtered amorphous Ge films

    SciTech Connect

    Zhan, Tianzhuo; Xu, Yibin; Goto, Masahiro; Tanaka, Yoshihisa; Kato, Ryozo; Sasaki, Michiko; Kagawa, Yutaka

    2014-02-15

    We measured the thermal conductivity of amorphous Ge films prepared by magnetron sputtering. The thermal conductivity was significantly higher than the value predicted by the minimum thermal conductivity model and increased with deposition temperature. We found that variations in sound velocity and Ge film density were not the main factors in the high thermal conductivity. Fast Fourier transform patterns of transmission electron micrographs revealed that short-range order in the Ge films was responsible for their high thermal conductivity. The results provide experimental evidences to understand the underlying nature of the variation of phonon mean free path in amorphous solids.

  14. High surface area silicon carbide-coated carbon aerogel

    DOEpatents

    Worsley, Marcus A; Kuntz, Joshua D; Baumann, Theodore F; Satcher, Jr, Joe H

    2014-01-14

    A metal oxide-carbon composite includes a carbon aerogel with an oxide overcoat. The metal oxide-carbon composite is made by providing a carbon aerogel, immersing the carbon aerogel in a metal oxide sol under a vacuum, raising the carbon aerogel with the metal oxide sol to atmospheric pressure, curing the carbon aerogel with the metal oxide sol at room temperature, and drying the carbon aerogel with the metal oxide sol to produce the metal oxide-carbon composite. The step of providing a carbon aerogel can provide an activated carbon aerogel or provide a carbon aerogel with carbon nanotubes that make the carbon aerogel mechanically robust. Carbon aerogels can be coated with sol-gel silica and the silica can be converted to silicone carbide, improved the thermal stability of the carbon aerogel.

  15. Thermal conductance of multilayered metallic sheets

    NASA Astrophysics Data System (ADS)

    Fletcher, L. S.; Blanchard, D. G.; Kinnear, K. P.

    1991-06-01

    An experimental investigation was conducted to determine the thermal conductivity, the overall thermal conductance, and the thermal contact conductance between layers of stacked aluminum alloy 3004, 5042, and 5182 sheet. Tests were conducted for aluminum sample thicknesses of 0.0305 to 0.3074 cm (0.012 to 0.121 in.), mean junction temperatures of 79.5 and 165.5 C (175 and 330 F), and contact pressures of 0.689 to 10.34 MPa (100 to 1500 psi). The overall thermal conductance increased with increasing contact pressure and increasing temperature. It decreased as the number of aluminum layers was increased. The experimental data were used to derive thermal contact conductance between layers of stacked aluminum sheet. From these derived values, a correlation for the thermal contact conductance was developed. The resulting expressions are presented as a function of dimensionless parameters for the layer material, apparent contact pressure, and mean junction temperature.

  16. Thermal Conductances of Pressed Copper Contacts

    NASA Technical Reports Server (NTRS)

    Salerno, L.; Kittel, P.; Spivak, A.

    1986-01-01

    Report describes investigation of thermal conductivities of smooth copper contacts pressed together at liquid-helium temperatures. Investigation prompted by need for accurate thermal models for infrared detectors and other cryogenic instruments.

  17. Mechanically Strong, Lightweight Porous Materials Developed (X-Aerogels)

    NASA Technical Reports Server (NTRS)

    Leventis, Nicholas

    2005-01-01

    Aerogels are attractive materials for a variety of NASA missions because they are ultralightweight, have low thermal conductivity and low-dielectric constants, and can be readily doped with other materials. Potential NASA applications for these materials include lightweight insulation for spacecraft, habitats, and extravehicular activity (EVA) suits; catalyst supports for fuel cell and in situ resource utilization; and sensors for air- and water-quality monitoring for vehicles, habitats, and EVA suits. Conventional aerogels are extremely fragile and require processing via supercritical fluid extraction, which adds cost to the production of an aerogel and limits the sizes and geometries of samples that can be produced from these materials. These issues have severely hampered the application of aerogels in NASA missions.

  18. Facilitated fabrication of high strength silica aerogels using cellulose nanofibrils as scaffold.

    PubMed

    Fu, Jingjing; Wang, Siqun; He, Chunxia; Lu, Zexiang; Huang, Jingda; Chen, Zhilin

    2016-08-20

    Monolithic cellulose nanofibrils (CNF)-silica composite aerogels were successfully prepared by immersing CNF aerogels into a silica solution in a two-step sol-gel process (initial hydrolysis of tetraethyl orthosilicate (TEOS) followed by condensation of silica particles). Aerogels were characterized by SEM, BET surface area test, bulk density and silica content analysis, FTIR spectroscopy, and compression test. The form of SiO2 existing in the composite aerogel was the spherical individual particles coated on CNF fibrils. The pH value of condensation solution was found to have great influence on the properties of the composite aerogels. By varying the pH value of condensation atmosphere from 8 to 12, the bulk densities of composite aerogels were able to be linearly increased from 0.059gcm(-3) to 0.29gcm(-3),and the silica content in the matrix sharply jumped from 3wt% to 79wt%. The porosities of the aerogels remained very high, between 85 and 96%, and the surface area of the composite aerogel reached up to 700.1m(2)g(-1). The compression properties of the composite aerogel improved greatly compared with those of the silica aerogel, about 8-30 times higher. Moreover, the compressive strength of the composite aerogel prepared in this work greatly exceeded the conventional insulation materials found in the recent commercial market, and without substantial increases in thermal conductivity. Hence, the findings of this research offer a promising application for composite aerogels and give a theoretical basis for developing new advanced materials. PMID:27178912

  19. Effective Thermal Conductivity of Corrugated Insulating Materials

    NASA Astrophysics Data System (ADS)

    Yamada, Etsuro; Kato, Masayasu; Tomikawa, Takayuki; Takahashi, Kaneko

    The effective thermal conductivity of corrugated insulating materials which are made by polypropylene or polycarbonate have been measured by employing steady state comparison method for several specimen having various thickness and specific weight. The thermal conductivity of them evaluated are also by using the thermal resistance models, and are compared with above measured values and raw materials' conductivity. The main results obtained in this paper are as follows: (1) In regard to the specimen in this paper, the effective thermal conductivity increases with increasing temperature, but the increasing rate of them is small. (2) There are considerable differences between the measured values and the predicted ones that are estimated by using the thermal resistance model in which heat flow by conduction only. This differences increase with increasing specimens' thickness. This difference become extinct by considering the coexistence heat flow of conduction and radiation in the air phase of specimen. (3) The thermal resistance of specimen increases linearly with increasing specimens' thickness.

  20. Thermal conductivity analysis of lanthanum doped manganites

    SciTech Connect

    Mansuri, Irfan; Shaikh, M. W.; Khan, E.; Varshney, Dinesh

    2014-04-24

    The temperature-dependent thermal conductivity of the doped manganites La{sub 0.7}Ca{sub 0.3}MnO{sub 3} is theoretically analyzed within the framework of Kubo formulae. The Hamiltonian consists of phonon, electron and magnon thermal conductivity contribution term. In this process we took defects, carrier, grain boundary, scattering process term and then calculate phonon, electron and magnon thermal conductivity.

  1. Thermal conductivity of nanostructured boron nitride materials.

    PubMed

    Tang, Chengchun; Bando, Yoshio; Liu, Changhong; Fan, Shoushan; Zhang, Jun; Ding, Xiaoxia; Golberg, Dmitri

    2006-06-01

    We have measured the thermal conductivity of bulky pellets made of various boron nitride (BN)-based nanomaterials, including spherical nanoparticles, perfectly structured, bamboo-like nanotubes, and collapsed nanotubes. The thermal conductivity strongly depends on the morphology of the BN nanomaterials, especially on the surface structure. Spherical BN particles have the lowest thermal conductivity while the collapsed BN nanotubes possess the best thermoconductive properties. A model was proposed to explain the experimental observations based on the heat percolation passage considerations. PMID:16722739

  2. Anisotropic Thermal Conductivity of Exfoliated Black Phosphorus.

    PubMed

    Jang, Hyejin; Wood, Joshua D; Ryder, Christopher R; Hersam, Mark C; Cahill, David G

    2015-12-22

    The anisotropic thermal conductivity of passivated black phosphorus (BP), a reactive two-dimensional material with strong in-plane anisotropy, is ascertained. The room-temperature thermal conductivity for three crystalline axes of exfoliated BP is measured by time-domain thermo-reflectance. The thermal conductivity along the zigzag direction is ≈2.5 times higher than that of the armchair direction. PMID:26516073

  3. Heat insulation performance, mechanics and hydrophobic modification of cellulose-SiO2 composite aerogels.

    PubMed

    Shi, Jianjun; Lu, Lingbin; Guo, Wantao; Zhang, Jingying; Cao, Yang

    2013-10-15

    Cellulose-SiO2 composite hydrogel was prepared by combining the NaOH/thiourea/H2O solvent system and the immersion method with controlling the hydrolysis-fasculation rate of tetraethyl orthosilicate (TEOS). The hydrophobic composite aerogels were obtained through the freeze-drying technology and the cold plasma modification technology. Composite SiO2 could obviously reduce the thermal conductivity of cellulose aerogel. The thermal conductivity could be as low as 0.026 W/(mK). The thermal insulation mechanism of the aerogel material was discussed. Composite SiO2 reduced hydrophilicity of cellulose aerogel, but environmental humidity had a significant influence on heat insulation performance. After hydrophobic modification using CCl4 as plasma was conducted, the surface of composite aerogel was changed from hydrophilic to hydrophobic and water contact angle was as high as 132°. The modified composite aerogel still kept good heat insulation performance. This work provided a foundation for the possibility of applying cellulose-SiO2 composite aerogel in the insulating material field. PMID:23987346

  4. Silica aerogel-polymer nanocomposites and new nanoparticle syntheses

    NASA Astrophysics Data System (ADS)

    Boday, Dylan Joseph

    Aerogels are extremely high surface area, low density materials with applications including thermal and acoustic insulators, radiation detectors and cometary dust particle traps. However, their low density and aggregate structure makes them extremely fragile and practically impossible to machine or handle without breaking. This has led to the development of aerogel composites with enhanced mechanical properties through the addition of polymers or surface modifiers. To date, attempts to strengthen aerogels have come with significant increases in density and processing time. Here I will describe our search for a solution to these problems with our invention using methyl cyanoacrylate chemical vapor deposition (CVD) to strengthen silica, aminated silica and bridged polysilsesquioxane aerogels. This approach led to a strength improvement of the composites within hours and the strongest composite prepared had a 100x strength improvement over the precursor aerogel. We also developed the first approach to control the molecular weight of the polymers that reinforce silica aerogels using surface-initiated atom transfer radical polymerization (SI-ATRP). Although PMMA reinforcement of silica aerogels improved the mechanical properties, further strength improvements were achieved by cross-linking the grafted PMMA. Additionally, we developed the first silica aerogels reinforced with polyaniline nanofibers that were strong and electrically conductive. Reinforcing silica aerogels with polyaniline allowed them to be used as a sensor for the detection of protonating and deprotonating gaseous species. Finally we developed a new approach for the synthesis of silica and bridged polysilsesquioxane spheres using a surfactant free synthesis. This approach allowed for the first in-situ incorporation of base sensitive functionalities during the sol-gel polymerization.

  5. Low lattice thermal conductivity of stanene

    PubMed Central

    Peng, Bo; Zhang, Hao; Shao, Hezhu; Xu, Yuchen; Zhang, Xiangchao; Zhu, Heyuan

    2016-01-01

    A fundamental understanding of phonon transport in stanene is crucial to predict the thermal performance in potential stanene-based devices. By combining first-principle calculation and phonon Boltzmann transport equation, we obtain the lattice thermal conductivity of stanene. A much lower thermal conductivity (11.6 W/mK) is observed in stanene, which indicates higher thermoelectric efficiency over other 2D materials. The contributions of acoustic and optical phonons to the lattice thermal conductivity are evaluated. Detailed analysis of phase space for three-phonon processes shows that phonon scattering channels LA + LA/TA/ZA ↔ TA/ZA are restricted, leading to the dominant contributions of high-group-velocity LA phonons to the thermal conductivity. The size dependence of thermal conductivity is investigated as well for the purpose of the design of thermoelectric nanostructures. PMID:26838731

  6. Low lattice thermal conductivity of stanene.

    PubMed

    Peng, Bo; Zhang, Hao; Shao, Hezhu; Xu, Yuchen; Zhang, Xiangchao; Zhu, Heyuan

    2016-01-01

    A fundamental understanding of phonon transport in stanene is crucial to predict the thermal performance in potential stanene-based devices. By combining first-principle calculation and phonon Boltzmann transport equation, we obtain the lattice thermal conductivity of stanene. A much lower thermal conductivity (11.6 W/mK) is observed in stanene, which indicates higher thermoelectric efficiency over other 2D materials. The contributions of acoustic and optical phonons to the lattice thermal conductivity are evaluated. Detailed analysis of phase space for three-phonon processes shows that phonon scattering channels LA + LA/TA/ZA ↔ TA/ZA are restricted, leading to the dominant contributions of high-group-velocity LA phonons to the thermal conductivity. The size dependence of thermal conductivity is investigated as well for the purpose of the design of thermoelectric nanostructures. PMID:26838731

  7. Aerogel/polymer composite materials

    NASA Technical Reports Server (NTRS)

    Williams, Martha K. (Inventor); Smith, Trent M. (Inventor); Fesmire, James E. (Inventor); Roberson, Luke B. (Inventor); Clayton, LaNetra M. (Inventor)

    2010-01-01

    The invention provides new composite materials containing aerogels blended with thermoplastic polymer materials at a weight ratio of aerogel to thermoplastic polymer of less than 20:100. The composite materials have improved thermal insulation ability. The composite materials also have better flexibility and less brittleness at low temperatures than the parent thermoplastic polymer materials.

  8. An effective thermal conductivity measurement system

    NASA Astrophysics Data System (ADS)

    Madrid, F.; Jordà, X.; Vellvehi, M.; Guraya, C.; Coleto, J.; Rebollo, J.

    2004-11-01

    In the technical literature, there is a lack of reliable thermal parameters and, often, it is necessary to do in situ measurements for every particular material. An effective thermal conductivity measurement system has been designed and implemented to provide reliable and accurate values for that thermal parameter. The thermal conductivity of a given material is deduced from thermal resistance differential measurements of two samples. All parts of the implemented system as well as practical and theoretical solutions are described, including a power controller circuit exclusively conceived for this application. Experimental considerations to reduce the measurement error are exposed, as well as some results obtained for three different materials.

  9. Thermal conductivity behavior of boron carbides

    NASA Technical Reports Server (NTRS)

    Wood, C.; Zoltan, A.; Emin, D.; Gray, P. E.

    1983-01-01

    Knowledge of the thermal conductivity of boron carbides is necessary to evaluate its potential for high temperature thermoelectric energy conversion applications. The thermal diffusivity of hot pressed boron carbide B/sub 1-x/C/sub x/ samples as a function of composition, temperature and temperature cycling was measured. These data in concert with density and specific heat data yield the thermal conductivities of these materials. The results in terms of a structural model to explain the electrical transport data and novel mechanisms for thermal conduction are discussed.

  10. Thermal conductivity of a zirconia thermal barrier coating

    NASA Astrophysics Data System (ADS)

    Slifka, A. J.; Filla, B. J.; Phelps, J. M.; Bancke, G.; Berndt, C. C.

    1998-03-01

    The conductivity of a thermal-barrier coating composed of atmospheric plasma sprayed 8 mass percent yttria partially stabilized zirconia has been measured. This coating was sprayed on a substrate of 410 stainless steel. An absolute, steady-state measurement method was used to measure thermal conductivity from 400 to 800 K. The thermal conductivity of the coating is 0.62 W/(m·K). This measurement has shown to be temperature independent.

  11. Electrically conductive fibers thermally isolate temperature sensor

    NASA Technical Reports Server (NTRS)

    De Waard, R.; Norton, B.

    1966-01-01

    Mounting assembly provides thermal isolation and an electrical path for an unbacked thermal sensor. The sensor is suspended in the center of a plastic mounting ring from four plastic fibers, two of which are coated with an electrically conductive material and connected to electrically conductive coatings on the ring.

  12. Binary Crystallized Supramolecular Aerogels Derived from Host-Guest Inclusion Complexes.

    PubMed

    Wang, Jin; Zhang, Xuetong

    2015-11-24

    Aerogels with low density and high porosity show outstanding properties such as large surface area and low thermal and acoustic conductivity. However, great challenges remain to convert hydrophilic polymer based hydrogels to corresponding aerogels. Here, we report a structurally new type of aerogels, supramolecular aerogels (SMAs), derived from supramolecular hydrogels formed by self-assembling of poly(ethylene glycol) and α-/γ-cyclodextrin. The SMAs posses a characteristic binary crystallized nanosheet structure due to their supramolecular cross-linking nature, and their specific surface areas and nanosheet structures are tunable. Furthermore, we demonstrated application of the aerogels as solid-solid phase change materials with tunable latent heat, reversible melting-crystallization cycle while keeping the microstructure of the SMAs unchanged. PMID:26513140

  13. Tailoring of Boehmite-Derived Aluminosilicate Aerogel Structure and Properties: Influence of Ti Addition

    NASA Technical Reports Server (NTRS)

    Hurwitz, Frances I.; Guo, Haiquan; Sheets, Erik J.; Miller, Derek R.; Newlin, Katy N.

    2010-01-01

    Aluminosilicate aerogels offer potential for extremely low thermal conductivities at temperatures greater than 900 C, beyond where silica aerogels reach their upper temperature limits. Aerogels have been synthesized at various Al:Si ratios, including mullite compositions, using Boehmite (AlOOH) as the Al source, and tetraethoxy orthosilicate as the Si precursor. The Boehmite-derived aerogels are found to form by a self-assembly process of AlOOH crystallites, with Si-O groups on the surface of an alumina skeleton. Morphology, surface area and pore size varies with the crystallite size of the starting Boehmite powder, as well as with synthesis parameters. Ternary systems, including Al-Si-Ti aerogels incorporating a soluble Ti precursor, are possible with careful control of pH. The addition of Ti influences sol viscosity, gelation time pore structure and pore size distribution, as well as phase formation on heat treatment.

  14. Electrostatic control of microstructure thermal conductivity

    NASA Astrophysics Data System (ADS)

    Supino, Ryan N.; Talghader, Joseph J.

    2001-03-01

    A technology for controlling the thermal conductivity of etch-released microstructures is proposed and demonstrated by placing test structures in and out of contact with their underlying substrate. By adjusting the duty cycle of a periodic actuation, the thermal conductivity can be adjusted linearly across a wide range. Experimental work with microfilaments in air has shown a continuous tuning range from approximately 1.7×10-4 W/K to 3.3×10-4 W/K. These numbers are limited by thermal conduction through air and thermal contact conductance, respectively. The fundamental tuning range is orders of magnitude wider, limited by radiation heat transfer and the thermal contact conductance of coated structures.

  15. Anisotropic thermal conductivity in uranium dioxide.

    PubMed

    Gofryk, K; Du, S; Stanek, C R; Lashley, J C; Liu, X-Y; Schulze, R K; Smith, J L; Safarik, D J; Byler, D D; McClellan, K J; Uberuaga, B P; Scott, B L; Andersson, D A

    2014-01-01

    The thermal conductivity of uranium dioxide has been studied for over half a century, as uranium dioxide is the fuel used in a majority of operating nuclear reactors and thermal conductivity controls the conversion of heat produced by fission events to electricity. Because uranium dioxide is a cubic compound and thermal conductivity is a second-rank tensor, it has always been assumed to be isotropic. We report thermal conductivity measurements on oriented uranium dioxide single crystals that show anisotropy from 4 K to above 300 K. Our results indicate that phonon-spin scattering is important for understanding the general thermal conductivity behaviour, and also explains the anisotropy by coupling to the applied temperature gradient and breaking cubic symmetry. PMID:25080878

  16. Anisotropic thermal conductivity in uranium dioxide

    NASA Astrophysics Data System (ADS)

    Gofryk, K.; Du, S.; Stanek, C. R.; Lashley, J. C.; Liu, X.-Y.; Schulze, R. K.; Smith, J. L.; Safarik, D. J.; Byler, D. D.; McClellan, K. J.; Uberuaga, B. P.; Scott, B. L.; Andersson, D. A.

    2014-08-01

    The thermal conductivity of uranium dioxide has been studied for over half a century, as uranium dioxide is the fuel used in a majority of operating nuclear reactors and thermal conductivity controls the conversion of heat produced by fission events to electricity. Because uranium dioxide is a cubic compound and thermal conductivity is a second-rank tensor, it has always been assumed to be isotropic. We report thermal conductivity measurements on oriented uranium dioxide single crystals that show anisotropy from 4 K to above 300 K. Our results indicate that phonon-spin scattering is important for understanding the general thermal conductivity behaviour, and also explains the anisotropy by coupling to the applied temperature gradient and breaking cubic symmetry.

  17. Effective thermal conductivity of damaged composites

    NASA Astrophysics Data System (ADS)

    Graham, Samuel, Jr.

    Ceramic matrix composites (CMCs) are susceptible to matrix cracking, fiber-matrix debonding, and oxidation processes as result of their application environments. These damage mechanisms act to degrade thermal conductivity and is a concern for many CMC applications. Prediction of this degradation relies on an accurate understanding of damage and thermal conductivity, and the development of analytical or numerical models. An experimental investigation into the degradation of thermal conductivity of CMCs was performed. In addition, an assessment was made of current micromechanics models for predicting thermal conductivity degradation. Experiments were performed on unidirectional reinforced Nicalon-LAS II glass-ceramic composites. Thermal conductivity was determined through flash diffusivity experiments. This procedure was also modified to treat orthotropic composite materials. Samples were subjected to mechanical loading-, oxidation-, and thermal shock-induced damage. The results showed that mechanical loading-induced damage resulted in no change in thermal conductivity transverse to the fiber axis and up to a 3.5% change parallel to the fibers. Mechanical loading followed by oxidation resulted in thermal conductivity degradation up to 26% and 10% transverse and parallel to the fibers, respectively. These data show the importance of the fiber-matrix interface in controlling both the longitudinal and transverse thermal conductivities of damaged composites. Predictions of thermal conductivity degradation parallel to the fiber direction were made with a shear-lag type micromechanics model. Results were in excellent agreement with experimental data. Thermal diffusivity data from isothermal oxidation and thermal shock experiments showed that this procedure is an effective nondestructive monitoring method. An assessment of transverse thermal conductivity rnicromechanics models was made through comparison with numerical solutions for random fiber inclusions with random

  18. The Electronic Thermal Conductivity of Graphene.

    PubMed

    Kim, Tae Yun; Park, Cheol-Hwan; Marzari, Nicola

    2016-04-13

    Graphene, as a semimetal with the largest known thermal conductivity, is an ideal system to study the interplay between electronic and lattice contributions to thermal transport. While the total electrical and thermal conductivity have been extensively investigated, a detailed first-principles study of its electronic thermal conductivity is still missing. Here, we first characterize the electron-phonon intrinsic contribution to the electronic thermal resistivity of graphene as a function of doping using electronic and phonon dispersions and electron-phonon couplings calculated from first-principles at the level of density-functional theory and many-body perturbation theory (GW). Then, we include extrinsic electron-impurity scattering using low-temperature experimental estimates. Under these conditions, we find that the in-plane electronic thermal conductivity κe of doped graphene is ∼300 W/mK at room temperature, independently of doping. This result is much larger than expected and comparable to the total thermal conductivity of typical metals, contributing ∼10% to the total thermal conductivity of bulk graphene. Notably, in samples whose physical or domain sizes are of the order of few micrometers or smaller, the relative contribution coming from the electronic thermal conductivity is more important than in the bulk limit, because lattice thermal conductivity is much more sensitive to sample or grain size at these scales. Last, when electron-impurity scattering effects are included we find that the electronic thermal conductivity is reduced by 30 to 70%. We also find that the Wiedemann-Franz law is broadly satisfied at low and high temperatures but with the largest deviations of 20-50% around room temperature. PMID:26907524

  19. A noncontact thermal microprobe for local thermal conductivity measurement.

    PubMed

    Zhang, Yanliang; Castillo, Eduardo E; Mehta, Rutvik J; Ramanath, Ganpati; Borca-Tasciuc, Theodorian

    2011-02-01

    We demonstrate a noncontact thermal microprobe technique for measuring the thermal conductivity κ with ∼3 μm lateral spatial resolution by exploiting quasiballistic air conduction across a 10-100 nm air gap between a joule-heated microprobe and the sample. The thermal conductivity is extracted from the measured effective thermal resistance of the microprobe and the tip-sample thermal contact conductance and radius in the quasiballistic regime determined by calibration on reference samples using a heat transfer model. Our κ values are within 5%-10% of that measured by standard steady-state methods and theoretical predictions for nanostructured bulk and thin film assemblies of pnictogen chalcogenides. Noncontact thermal microprobing demonstrated here mitigates the strong dependence of tip-sample heat transfer on sample surface chemistry and topography inherent in contact methods, and allows the thermal characterization of a wide range of nanomaterials. PMID:21361625

  20. A noncontact thermal microprobe for local thermal conductivity measurement

    NASA Astrophysics Data System (ADS)

    Zhang, Yanliang; Castillo, Eduardo E.; Mehta, Rutvik J.; Ramanath, Ganpati; Borca-Tasciuc, Theodorian

    2011-02-01

    We demonstrate a noncontact thermal microprobe technique for measuring the thermal conductivity κ with ˜3 μm lateral spatial resolution by exploiting quasiballistic air conduction across a 10-100 nm air gap between a joule-heated microprobe and the sample. The thermal conductivity is extracted from the measured effective thermal resistance of the microprobe and the tip-sample thermal contact conductance and radius in the quasiballistic regime determined by calibration on reference samples using a heat transfer model. Our κ values are within 5%-10% of that measured by standard steady-state methods and theoretical predictions for nanostructured bulk and thin film assemblies of pnictogen chalcogenides. Noncontact thermal microprobing demonstrated here mitigates the strong dependence of tip-sample heat transfer on sample surface chemistry and topography inherent in contact methods, and allows the thermal characterization of a wide range of nanomaterials.

  1. Electrically conductive and thermally conductive materials for electronic packaging

    NASA Astrophysics Data System (ADS)

    Liu, Zongrong

    The aim of this dissertation is to develop electrically or thermally conductive materials that are needed for electronic packaging and microelectronic cooling. These materials are in the form of coatings and are made from pastes. The research work encompasses paste formulation, studying the process of converting a paste to a conductive material, relating the processing conditions to the structure and performance, and evaluating performance attributes that are relevant to the application of these conductive materials. The research has resulted in new information that is valuable to the microelectronic industry. Work on electrically conductive materials emphasizes the development of electrical interconnection materials in the form of air-firable glass-free silver-based electrically conductive thick films, which use the Ti-Al alloy as the binder and are in contrast to conventional films that use glass as the binder. The air-firability, as enabled by minor additions of tin and zinc to the paste, is in contrast to previous glass-free films that are not firable. The recommended firing condition is 930°C in air. The organic vehicle in the paste comprises ethyl cellulose, which undergoes thermal decomposition during burnout of the paste. The ethyl cellulose is dissolved in ether, which facilitates the burnout. Excessive ethyl cellulose hinders the burnout. A higher heating rate results in more residue after burnout. The presence of silver particles facilitates drying and burnout. Firing in air gives lower resistivity than firing in oxygen. Firing in argon gives poor films. Compared to conventional films that use glass as the binder, these films, when appropriately fired, exhibit lower electrical resistivity (2.5 x 10-6 O.cm) and higher scratch resistance. Work on thermally conductive materials addresses thermal interface materials, which are materials placed at the interface between a heat sink and a heat source for the purpose of improving the thermal contact. Heat

  2. In situ synthesis of robust conductive cellulose/polypyrrole composite aerogels and their potential application in nerve regeneration.

    PubMed

    Shi, Zhuqun; Gao, Huichang; Feng, Jiao; Ding, Beibei; Cao, Xiaodong; Kuga, Shigenori; Wang, Yingjun; Zhang, Lina; Cai, Jie

    2014-05-19

    Nanostructured conductive polymers can offer analogous environments for extracellular matrix and induce cellular responses by electric stimulation, however, such materials often lack mechanical strength and tend to collapse under small stresses. We prepared electrically conductive nanoporous materials by coating nanoporous cellulose gels (NCG) with polypyrrole (PPy) nanoparticles, which were synthesized in situ from pyrrole monomers supplied as vapor. The resulting NCG/PPy composite hydrogels were converted to aerogels by drying with supercritical CO2, giving a density of 0.41-0.53 g cm(-3), nitrogen adsorption surface areas of 264-303 m(2) g(-1), and high mechanical strength. The NCG/PPy composite hydrogels exhibited an electrical conductivity of up to 0.08 S cm(-1). In vitro studies showed that the incorporation of PPy into an NCG enhances the adhesion and proliferation of PC12 cells. Electrical stimulation demonstrated that PC12 cells attached and extended longer neurites when cultured on NCG/PPy composite gels with DBSA dopant. These materials are promising candidates for applications in nerve regeneration, carbon capture, catalyst supports, and many others. PMID:24711342

  3. Thermal resistance and compressive strain of underwater aerogel syntactic foam hybrid insulation at atmospheric and elevated hydrostatic pressure

    NASA Astrophysics Data System (ADS)

    Bardy, Erik; Mollendorf, Joseph; Pendergast, David

    2006-05-01

    The purpose of this study was to present a new underwater thermal insulation designed for flexibility and high thermal resistance. The insulation was a hybrid composite of two constituents: syntactic foam and an insulating aerogel blanket. Methods for treating and combining the constituents into a hybrid insulation of several designs are presented. A final configuration was selected based on high thermal resistance and was tested for thermal resistance and compressive strain to a pressure of 1.2 MPa (107 msw, meters of sea water) for five continuous pressure cycles. The thermal resistance and compressive strain results were compared to foam neoprene and underwater pipeline insulation. It was found that the hybrid insulation has a thermal resistance significantly higher than both foam neoprene and underwater pipeline insulation at atmospheric and elevated hydrostatic pressures (1.2 MPa). The total thermal resistance of the hybrid insulation decreased 32% at 1.2 MPa and returned to its initial value upon decompression. It was concluded that the hybrid insulation, with modifications, could be used for wetsuit construction, shallow underwater pipeline insulation, or any underwater application where high thermal resistance, flexibility, and resistance to compression are desired.

  4. Lattice thermal conductivity crossovers in semiconductor nanowires.

    PubMed

    Mingo, N; Broido, D A

    2004-12-10

    For binary compound semiconductor nanowires, we find a striking relationship between the nanowire's thermal conductivity kappa(nwire), the bulk material's thermal conductivity kappa(bulk), and the mass ratio of the material's constituent atoms, r, as kappa(bulk)/kappa(nwire) (alpha) (1+1/r)(-3/2). A significant consequence is the presence of crossovers in which a material with higher bulk thermal conductivity than the rest is no longer the best nanowire thermal conductor. We show that this behavior stems from a change in the dominant phonon scattering mechanism with decreasing nanowire size. The results have important implications for nanoscale heat dissipation, thermoelectricity, and thermal conductivity of nanocomposites. PMID:15697834

  5. Conductivity-limiting bipolar thermal conductivity in semiconductors

    PubMed Central

    Wang, Shanyu; Yang, Jiong; Toll, Trevor; Yang, Jihui; Zhang, Wenqing; Tang, Xinfeng

    2015-01-01

    Intriguing experimental results raised the question about the fundamental mechanisms governing the electron-hole coupling induced bipolar thermal conduction in semiconductors. Our combined theoretical analysis and experimental measurements show that in semiconductors bipolar thermal transport is in general a “conductivity-limiting” phenomenon, and it is thus controlled by the carrier mobility ratio and by the minority carrier partial electrical conductivity for the intrinsic and extrinsic cases, respectively. Our numerical method quantifies the role of electronic band structure and carrier scattering mechanisms. We have successfully demonstrated bipolar thermal conductivity reduction in doped semiconductors via electronic band structure modulation and/or preferential minority carrier scatterings. We expect this study to be beneficial to the current interests in optimizing thermoelectric properties of narrow gap semiconductors. PMID:25970560

  6. Conductivity-limiting bipolar thermal conductivity in semiconductors.

    PubMed

    Wang, Shanyu; Yang, Jiong; Toll, Trevor; Yang, Jihui; Zhang, Wenqing; Tang, Xinfeng

    2015-01-01

    Intriguing experimental results raised the question about the fundamental mechanisms governing the electron-hole coupling induced bipolar thermal conduction in semiconductors. Our combined theoretical analysis and experimental measurements show that in semiconductors bipolar thermal transport is in general a "conductivity-limiting" phenomenon, and it is thus controlled by the carrier mobility ratio and by the minority carrier partial electrical conductivity for the intrinsic and extrinsic cases, respectively. Our numerical method quantifies the role of electronic band structure and carrier scattering mechanisms. We have successfully demonstrated bipolar thermal conductivity reduction in doped semiconductors via electronic band structure modulation and/or preferential minority carrier scatterings. We expect this study to be beneficial to the current interests in optimizing thermoelectric properties of narrow gap semiconductors. PMID:25970560

  7. Thermal conductivity of garnet laser crystals

    NASA Astrophysics Data System (ADS)

    Wang, B. S.; Jiang, H. H.; Zhang, Q. L.; Yin, S. T.

    2008-03-01

    The thermal conductivities of nine different synthetic garnet laser crystals at various temperatures, range from 273 to 393 K have been investigated by instantaneous measurement method. The results show that the thermal conductivity of each crystal decreases exponentially with the temperature increasing. It is notable that, different host crystals, such as YAG, GGG, and GSGG have different thermal conductivity, which is attributed to the crucial influence of crystal structure and composition on the absolute value of their thermal conductivity. Moreover, with respect to the same host crystals, the impurity scattering also results in the change of their thermal conductivities. This is because that a higher concentration of doped ions leads to a more phonon scattering modes, which results in a shorter mean free path of the phonons and a lower thermal conductivity. In addition, different host crystals have various dependences of thermal conductivity on dopant concentration. This works provides reliable and useful information for designing high power, high quality, and high stability laser devices.

  8. Thermal conductivity of synthetic garnet laser crystals

    NASA Astrophysics Data System (ADS)

    Wang, B. S.; Jiang, H. H.; Zhang, Q. L.; Yin, S. T.

    2007-07-01

    The thermal conductivities of nine different synthetic garnet laser crystals at various temperatures, range from 273 to 393K have been investigated by instantaneous measurement method. The results show that the thermal conductivity of each crystal decreases exponentially with the temperature increasing. It is notable that, different host crystals, such as YAG, GGG, and GSGG have different thermal conductivity, which is attributed to the crucial influence of crystal structure and composition on the absolute value of their thermal conductivity. Moreover, with respect to the same host crystals, the impurity scattering also results in the change of their thermal conductivities. This is because that a higher concentration of doped ions leads to a more phonon scattering modes, which results in a shorter mean free path of the phonons and a lower thermal conductivity. In addition, different host crystals have various dependences of thermal conductivity on dopant concentration. This works provides reliable and useful information for designing high power, high quality, and high stability laser devices.

  9. Thermal conductivity of uterine tissue in vitro.

    PubMed

    Olsrud, J; Friberg, B; Ahlgren, M; Persson, B R

    1998-08-01

    Thermotherapy of the uterus has emerged as an alternative to hysterectomy in the treatment of menorrhagia, from whence it follows that the thermal properties of uterine tissue have become of importance. This study presents measurements of the thermal conductivity and the water content of uterine tissue in vitro. A steady-state thermal conductivity apparatus, based on the comparison of test samples with a material with known thermal conductivity, is described. Measurements were conducted on tissue samples from eleven patients, directly after hysterectomy. Samples with and without endometrium, as well as coagulated samples, were examined. The thermal conductivity of myometrial tissue was found to be 0.536 +/- 0.012 W m(-1) K(-1) (mean +/- 1 SD) and the corresponding water content was 81.2 +/- 1.5% (mean +/- 1 SD). Measurements on samples with both endometrium and myometrium showed similar thermal conductivity (0.542 +/- 0.008 W m(-1) K(-1), mean +/- 1 SD) and water content (81.6 +/- 0.7%, mean +/- 1 SD). It was also indicated that coagulation causes dehydration, resulting in a lower thermal conductivity. PMID:9725614

  10. Thermal conductivity measurements of Summit polycrystalline silicon.

    SciTech Connect

    Clemens, Rebecca; Kuppers, Jaron D.; Phinney, Leslie Mary

    2006-11-01

    A capability for measuring the thermal conductivity of microelectromechanical systems (MEMS) materials using a steady state resistance technique was developed and used to measure the thermal conductivities of SUMMiT{trademark} V layers. Thermal conductivities were measured over two temperature ranges: 100K to 350K and 293K to 575K in order to generate two data sets. The steady state resistance technique uses surface micromachined bridge structures fabricated using the standard SUMMiT fabrication process. Electrical resistance and resistivity data are reported for poly1-poly2 laminate, poly2, poly3, and poly4 polysilicon structural layers in the SUMMiT process from 83K to 575K. Thermal conductivity measurements for these polysilicon layers demonstrate for the first time that the thermal conductivity is a function of the particular SUMMiT layer. Also, the poly2 layer has a different variation in thermal conductivity as the temperature is decreased than the poly1-poly2 laminate, poly3, and poly4 layers. As the temperature increases above room temperature, the difference in thermal conductivity between the layers decreases.

  11. Thermal Conductivity and Sintering Behavior of Advanced Thermal Barrier Coatings

    NASA Technical Reports Server (NTRS)

    Zhu, Dongming; Miller, Robert A.

    2002-01-01

    Advanced thermal barrier coatings, having significantly reduced long-term thermal conductivities, are being developed using an approach that emphasizes real-time monitoring of thermal conductivity under conditions that are engine-like in terms of temperatures and heat fluxes. This is in contrast to the traditional approach where coatings are initially optimized in terms of furnace and burner rig durability with subsequent measurement in the as-processed or furnace-sintered condition. The present work establishes a laser high-heat-flux test as the basis for evaluating advanced plasma-sprayed and physical vapor-deposited thermal barrier coatings under the NASA Ultra Efficient Engine Technology (UEET) Program. The candidate coating materials for this program are novel thermal barrier coatings that are found to have significantly reduced thermal conductivities due to an oxide-defect-cluster design. Critical issues for designing advanced low conductivity coatings with improved coating durability are also discussed.

  12. An Innovative High Thermal Conductivity Fuel Design

    SciTech Connect

    Jamil A. Khan

    2009-11-21

    Thermal conductivity of the fuel in today's Light Water Reactors, Uranium dioxide, can be improved by incorporating a uniformly distributed heat conducting network of a higher conductivity material, Silicon Carbide. The higher thermal conductivity of SiC along with its other prominent reactor-grade properties makes it a potential material to address some of the related issues when used in UO2 [97% TD]. This ongoing research, in collaboration with the University of Florida, aims to investigate the feasibility and develop a formal methodology of producing the resultant composite oxide fuel. Calculations of effective thermal conductivity of the new fuel as a function of %SiC for certain percentages and as a function of temperature are presented as a preliminary approach. The effective thermal conductivities are obtained at different temperatures from 600K to 1600K. The corresponding polynomial equations for the temperature-dependent thermal conductivities are given based on the simulation results. Heat transfer mechanism in this fuel is explained using a finite volume approach and validated against existing empirical models. FLUENT 6.1.22 was used for thermal conductivity calculations and to estimate reduction in centerline temperatures achievable within such a fuel rod. Later, computer codes COMBINE-PC and VENTURE-PC were deployed to estimate the fuel enrichment required, to maintain the same burnup levels, corresponding to a volume percent addition of SiC.

  13. Adjustable thermal conductivity in carbon nanotube nanofluids

    NASA Astrophysics Data System (ADS)

    Xie, Huaqing; Chen, Lifei

    2009-05-01

    Homogeneous and stable nanofluids have been produced by suspending well dispersible multi-walled carbon nanotubes (CNTs) into ethylene glycol base fluid. CNT nanofluids have enhanced thermal conductivity and the enhancement ratios increase with the nanotube loading and the temperature. Thermal conductivity enhancement was adjusted by ball milling and cutting the treated CNTs suspended in the nanofluids to relatively straight CNTs with an appropriate length distribution. Our findings indicate that the straightness ratio, aspect ratio, and aggregation have collective influence on the thermal conductivity of CNT nanofluids.

  14. Electrical and thermal conductivities in dense plasmas

    SciTech Connect

    Faussurier, G. Blancard, C.; Combis, P.; Videau, L.

    2014-09-15

    Expressions for the electrical and thermal conductivities in dense plasmas are derived combining the Chester-Thellung-Kubo-Greenwood approach and the Kramers approximation. The infrared divergence is removed assuming a Drude-like behaviour. An analytical expression is obtained for the Lorenz number that interpolates between the cold solid-state and the hot plasma phases. An expression for the electrical resistivity is proposed using the Ziman-Evans formula, from which the thermal conductivity can be deduced using the analytical expression for the Lorenz number. The present method can be used to estimate electrical and thermal conductivities of mixtures. Comparisons with experiment and quantum molecular dynamics simulations are done.

  15. Increased thermal conductivity monolithic zeolite structures

    DOEpatents

    Klett, James; Klett, Lynn; Kaufman, Jonathan

    2008-11-25

    A monolith comprises a zeolite, a thermally conductive carbon, and a binder. The zeolite is included in the form of beads, pellets, powders and mixtures thereof. The thermally conductive carbon can be carbon nano-fibers, diamond or graphite which provide thermal conductivities in excess of about 100 W/mK to more than 1,000 W/mK. A method of preparing a zeolite monolith includes the steps of mixing a zeolite dispersion in an aqueous colloidal silica binder with a dispersion of carbon nano-fibers in water followed by dehydration and curing of the binder is given.

  16. Thermal conductivity of bulk nanostructured lead telluride

    SciTech Connect

    Hori, Takuma; Chen, Gang; Shiomi, Junichiro

    2014-01-13

    Thermal conductivity of lead telluride with embedded nanoinclusions was studied using Monte Carlo simulations with intrinsic phonon transport properties obtained from first-principles-based lattice dynamics. The nanoinclusion/matrix interfaces were set to completely reflect phonons to model the maximum interface-phonon-scattering scenario. The simulations with the geometrical cross section and volume fraction of the nanoinclusions matched to those of the experiment show that the experiment has already reached the theoretical limit of thermal conductivity. The frequency-dependent analysis further identifies that the thermal conductivity reduction is dominantly attributed to scattering of low frequency phonons and demonstrates mutual adaptability of nanostructuring and local disordering.

  17. Thermal Conductivities of Crystalline Organic Semiconductors

    NASA Astrophysics Data System (ADS)

    Brill, Joseph

    2014-03-01

    As applications for organic semiconductors grow, it is becoming increasingly important to know their thermal conductivities, k. For example, for sub-micron electronic devices, values of k>k0 ~ 5 mW/cm/K are needed, while values kthermal conductivities below k0, many molecular organic crystals also have values of k below this value. We have started measurements of both the in-plane and interplane thermal diffusivities of layered crystalline organic semiconductors using frequency[2] and position dependent[3] ac-calorimetry; the thermal conductivities are then determined from the specific heats measured with differential scanning calorimetry. For rubrene, which has kthermal conductivity is several times smaller than the in-plane value, although its temperature dependence indicates that the phonon mean-free path is at least a few layers.[4] On the other hand, the in-plane thermal conductivity of TIPS-pentacene,[5] is several times greater than k0, similar to that of the quasi-one dimensional organic metal TTF-TCNQ.[6] Remarkably, its interlayer thermal conductivity is several times larger than its in-plane value,[7] perhaps due to interactions between the large (triisopropylsilylethynyl) side groups on the pentacene backbone. Research done with Hao Zhang and Yulong Yao and supported by NSF grants DMR-0800367, EPS-0814194, and DMR-1262261.

  18. Physicochemical properties of bamboo leaf aerogels synthesized via different modes of gelation

    NASA Astrophysics Data System (ADS)

    Kow, Kien-Woh; Yusoff, Rozita; Aziz, A. R. Abdul; Abdullah, E. C.

    2014-05-01

    Aerogels with ultralow silica concentration (3.5 %) was synthesized using bamboo leaf. The synthesis of aerogel was carried out in different pH to study the effect of gelation mechanisms on the properties of aerogel. Aerogel synthesized at acidic pH has generally exhibits more attractive properties, i.e. low shrinkage (24.3%), large specific surface area (547.2 m2 g-1), large pore volume (2.72 cm3 g-1) and low thermal conductivity (0.024 W m-1 K-1). As comparison, the aerogel synthesized at basic condition has different properties, in which it has small pore volume (0.287 cm3 g-1) and pore size (11.44 nm), large primary particles (6.69 nm), small specific surface area (247.7 m2 g-1), low degree of fractality (6.69), strong absorption of water and relatively high thermal conductivity (0.0415 W m-1 K-1). Properties of aerogels synthesized were also compared with aerogels synthesized using conventional TEOS precursor. The difference in the gelation mechanisms was discussed in detailed.

  19. Polyimide aerogels cross-linked through amine functionalized polyoligomeric silsesquioxane.

    PubMed

    Guo, Haiquan; Meador, Mary Ann B; McCorkle, Linda; Quade, Derek J; Guo, Jiao; Hamilton, Bart; Cakmak, Miko; Sprowl, Guilherme

    2011-02-01

    We report the first synthesis of polyimide aerogels cross-linked through a polyhedral oligomeric silsesquioxane, octa(aminophenyl)silsesquioxane (OAPS). Gels formed from polyamic acid solutions of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), bisaniline-p-xylidene (BAX) and OAPS were chemically imidized and dried using supercritical CO(2) extraction to give aerogels having density around 0.1 g/cm(3). The aerogels are greater than 90 % porous, have high surface areas (230 to 280 m(2)/g) and low thermal conductivity (14 mW/m-K at room temperature). Notably, the polyimide aerogels cross-linked with OAPS have higher modulus than polymer reinforced silica aerogels of similar density and can be fabricated as both monoliths and thin films. Thin films of the aerogel are flexible and foldable making them an ideal insulation for space suits, and inflatable structures for habitats or decelerators for planetary re-entry, as well as more down to earth applications. PMID:21294517

  20. Aerogel waveplates.

    PubMed

    Bhupathi, Pradeep; Hwang, Jungseek; Martin, Rodica M; Blankstein, Jackson; Jaworski, Lukas; Mulders, Norbert; Tanner, David B; Lee, Yoonseok

    2009-06-22

    Optical transmission measurements were made on 98% porosity silica aerogel samples under various degrees of uniaxial strain. Uniaxially compressed aerogels exhibit large birefringence, proportional to the amount of compression, up to the 15% strain studied. The birefringence is mostly reversible and reproducible through multiple compression-decompression cycles. Our study demonstrates that uniaxially strained high porosity aerogels can be used as tunable waveplates in a broad spectral range. PMID:19550455

  1. A Robust Approach to Lattice Thermal Conductivity

    NASA Astrophysics Data System (ADS)

    Nielson, Weston; Fei Zhou Team; Yi Xia Team; Vidvuds Ozolins Team

    2015-03-01

    Thermal conductivity is a key parameter in designing high performance thermoelectric materials. A multitude of computational methods have been developed to calculate lattice thermal conductivity. Molecular dynamics (MD) based techniques, including equilibrium and non-equilibrium methods, in addition to non MD-based solutions, such as the Boltzmann Transport Equation (BTE), are all capable of calculating thermal conductivity, but each comes with different sets of limitations and difficulties. After extensive use of these different methods, we have developed a robust set of tools for obtaining high-quality lattice thermal conductivity values of crystalline solids. The crux of our method involves a novel compressive sensing (CS) based approach for efficiently calculating high quality force constants for crystalline materials. The result is a technique for building lattice dynamical models that can treat compounds with large, complex unit cells and strong anharmonicity, including those with harmonically unstable phonon modes.

  2. Thermal Conductivity in Nanocrystalline Ceria Thin Films

    SciTech Connect

    Marat Khafizov; In-Wook Park; Aleksandr Chernatynskiy; Lingfeng He; Jianliang Lin; John J. Moore; David Swank; Thomas Lillo; Simon R. Phillpot; Anter El-Azab; David H. Hurley

    2014-02-01

    The thermal conductivity of nanocrystalline ceria films grown by unbalanced magnetron sputtering is determined as a function of temperature using laser-based modulated thermoreflectance. The films exhibit significantly reduced conductivity compared with stoichiometric bulk CeO2. A variety of microstructure imaging techniques including X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron analysis, and electron energy loss spectroscopy indicate that the thermal conductivity is influenced by grain boundaries, dislocations, and oxygen vacancies. The temperature dependence of the thermal conductivity is analyzed using an analytical solution of the Boltzmann transport equation. The conclusion of this study is that oxygen vacancies pose a smaller impediment to thermal transport when they segregate along grain boundaries.

  3. Anomalous thermal conductivity of monolayer boron nitride

    NASA Astrophysics Data System (ADS)

    Tabarraei, Alireza; Wang, Xiaonan

    2016-05-01

    In this paper, we use nonequilibrium molecular dynamics modeling to investigate the thermal properties of monolayer hexagonal boron nitride nanoribbons under uniaxial strain along their longitudinal axis. Our simulations predict that hexagonal boron nitride shows an anomalous thermal response to the applied uniaxial strain. Contrary to three dimensional materials, under uniaxial stretching, the thermal conductivity of boron nitride nanoribbons first increases rather than decreasing until it reaches its peak value and then starts decreasing. Under compressive strain, the thermal conductivity of monolayer boron nitride ribbons monolithically reduces rather than increasing. We use phonon spectrum and dispersion curves to investigate the mechanism responsible for the unexpected behavior. Our molecular dynamics modeling and density functional theory results show that application of longitudinal tensile strain leads to the reduction of the group velocities of longitudinal and transverse acoustic modes. Such a phonon softening mechanism acts to reduce the thermal conductivity of the nanoribbons. On the other hand, a significant increase in the group velocity (stiffening) of the flexural acoustic modes is observed, which counteracts the phonon softening effects of the longitudinal and transverse modes. The total thermal conductivity of the ribbons is a result of competition between these two mechanisms. At low tensile strain, the stiffening mechanism overcomes the softening mechanism which leads to an increase in the thermal conductivity. At higher tensile strain, the softening mechanism supersedes the stiffening and the thermal conductivity slightly reduces. Our simulations show that the decrease in the thermal conductivity under compressive strain is attributed to the formation of buckling defects which reduces the phonon mean free path.

  4. Engineering thermal conductivity in polymer blends

    NASA Astrophysics Data System (ADS)

    Rashidi, Vahid; Coyle, Eleanor; Kieffer, John; Pipe, Kevin

    Weak inter-chain bonding in polymers is believed to be a bottleneck for both thermal conductivity and mechanical strength. Most polymers have low thermal conductivity (~0.1 W/mK), hindering their performance in applications for which thermal management is critical (e.g., electronics packaging). In this work, we use computational methods to study how hydrogen bonding between polymer chains as well as water content can be used to engineer thermal transport in bulk polymers. We examine how changes in the number of hydrogen bonds, chain elongation, density, and vibrational density of states correlate with changes in thermal conductivity for polymer blends composed of different relative constituent fractions. We also consider the effects of bond strength, tacticity, and polymer chain mass. For certain blend fractions, we observe large increases in thermal conductivity, and we analyze these increases in terms of modifications to chain chemistry (e.g., inter-chain bonding) and chain morphology (e.g., chain alignment and radius of gyration). We observe that increasing the number of hydrogen bonds in the system results in better packing as well as better chain alignment and elongation that contribute to enhanced thermal conductivity. The Air Force Office of Scientific Research, Grant No. FA9550-14-1-0010.

  5. Thermal conductivity of Apollo 16 lunar fines

    NASA Technical Reports Server (NTRS)

    Cremers, C. J.; Hsia, H. S.

    1974-01-01

    The vacuum thermal conductivity of the Apollo 16 fines is presented as a function of temperature for the approximate range of diurnal temperatures on the moon. The density used is 1500 kg/cu m, which is approximately the density of the core-tube samples returned from the landing site. The thermal diffusivity was calculated from the conductivity, density, and specific heat and is also presented.

  6. Analysis and Testing of Multilayer and Aerogel Insulation Configurations

    NASA Astrophysics Data System (ADS)

    Johnson, W. L.; Demko, J. A.; Fesmire, J. E.

    2010-04-01

    Multilayer insulation systems that have robust operational characteristics have long been a goal of many research projects. Such thermal insulation systems may need to offer some degree of structural support and/or mechanical integrity during loss of vacuum scenarios while continuing to provide insulative value to the vessel. Aerogel-based composite blankets can be the best insulation materials in ambient pressure environments; in high vacuum, the thermal performance of aerogel improves by about one order of magnitude. Standard multilayer insulation (MLI) is typically 50% worse at ambient pressure and at soft vacuum, but as much as two or three orders of magnitude better at high vacuum. Different combinations of aerogel blanket and multilayer insulation materials have been tested at the Cryogenics Test Laboratory of NASA Kennedy Space Center. Analysis performed at Oak Ridge National Laboratory showed an importance to the relative location of the MLI and aerogel blankets. Apparent thermal conductivity testing under cryogenic-vacuum conditions was performed to verify the analytical conclusion. Tests results are shown to be in agreement with the analysis which indicated that the best performance is obtained with aerogel layers located in the middle of the blanket insulation system.

  7. Analysis and testing of multilayer and aerogel insulation configurations

    SciTech Connect

    Johnson, W L; Demko, Jonathan A; Fesmire, J. E.

    2010-01-01

    Multilayer insulation systems that have robust operational characteristics have long been a goal of many research projects. Such thermal insulation systems may need to offer some degree of structural support and/or mechanical integrity during loss of vacuum scenarios while continuing to provide insulative value to the vessel. Aerogel-based composite blankets can be the best insulation materials in ambient pressure environments; in high vacuum, the thermal performance of aerogel improves by about one order of magnitude. Standard multilayer insulation (MLI) is typically 50% worse at ambient pressure and at soft vacuum, but as much as two or three orders of magnitude better at high vacuum. Different combinations of aerogel blanket and multilayer insulation materials have been tested at the Cryogenics Test Laboratory of NASA Kennedy Space Center. Analysis performed at Oak Ridge National Laboratory showed an importance to the relative location of the MLI and aerogel blankets. Apparent thermal conductivity testing under cryogenicvacuum conditions was performed to verify the analytical conclusion. Tests results are shown to be in agreement with the analysis which indicated that the best performance is obtained with aerogel layers located in the middle of the blanket insulation system.

  8. Thermal Conductivity of Alumina and Silica Nanofluids

    NASA Astrophysics Data System (ADS)

    Castellanos, Julian G. Bernal

    This thesis studies the effects of the base fluid, particle type/size, and volumetric concentration on the thermal conductivity of Alumina and Silica nanofluids. The effects of base fluid were observed by preparing samples using ethylene glycol (EG), water, and mixtures of EG/water as the base fluid and Al2O3 (10 nm) nanoparticles. The particles type/size and volumetric concentration effects were tested by preparing samples of nanofluids using Al2O3 (10nm), Al2O3 (150nm), SiO2 (15 nm), and SiO2 (80 nm) nanoparticles and ionized water as base fluid at different volumetric concentrations. All samples were mixed using a sonicator for 30 minutes and a water circulator to maintain the sample at room temperature. The thermal conductivity was measured using a Thermtest Transient Plane Source TPS 500S. The effects of gravity, Brownian motion and thermophoresis were also studied. EG produced the highest thermal conductivity enhancement out of all base fluids tested. Smaller particle size produced a higher enhancement of thermal conductivity, while the volumetric concentration did not have a significant effect in the thermal conductivity enhancement. Finally, gravity, Brownian diffusion and thermophoresis effects played a role in the total enhancement of the thermal conductivity. The nanoparticles were observed to settle rapidly after sonication suggesting gravity effects may play a significant role.

  9. Polyethylene nanofibres with very high thermal conductivities.

    PubMed

    Shen, Sheng; Henry, Asegun; Tong, Jonathan; Zheng, Ruiting; Chen, Gang

    2010-04-01

    Bulk polymers are generally regarded as thermal insulators, and typically have thermal conductivities on the order of 0.1 W m(-1) K(-1). However, recent work suggests that individual chains of polyethylene--the simplest and most widely used polymer--can have extremely high thermal conductivity. Practical applications of these polymers may also require that the individual chains form fibres or films. Here, we report the fabrication of high-quality ultra-drawn polyethylene nanofibres with diameters of 50-500 nm and lengths up to tens of millimetres. The thermal conductivity of the nanofibres was found to be as high as approximately 104 W m(-1) K(-1), which is larger than the conductivities of about half of the pure metals. The high thermal conductivity is attributed to the restructuring of the polymer chains by stretching, which improves the fibre quality toward an 'ideal' single crystalline fibre. Such thermally conductive polymers are potentially useful as heat spreaders and could supplement conventional metallic heat-transfer materials, which are used in applications such as solar hot-water collectors, heat exchangers and electronic packaging. PMID:20208547

  10. Thermal Conductivity of Carbon Nanotube Composite Films

    NASA Technical Reports Server (NTRS)

    Ngo, Quoc; Cruden, Brett A.; Cassell, Alan M.; Walker, Megan D.; Koehne, Jessica E.; Meyyappan, M.; Li, Jun; Yang, Cary Y.

    2004-01-01

    State-of-the-art ICs for microprocessors routinely dissipate power densities on the order of 50 W/sq cm. This large power is due to the localized heating of ICs operating at high frequencies, and must be managed for future high-frequency microelectronic applications. Our approach involves finding new and efficient thermally conductive materials. Exploiting carbon nanotube (CNT) films and composites for their superior axial thermal conductance properties has the potential for such an application requiring efficient heat transfer. In this work, we present thermal contact resistance measurement results for CNT and CNT-Cu composite films. It is shown that Cu-filled CNT arrays enhance thermal conductance when compared to as-grown CNT arrays. Furthermore, the CNT-Cu composite material provides a mechanically robust alternative to current IC packaging technology.

  11. Lower-Conductivity Thermal-Barrier Coatings

    NASA Technical Reports Server (NTRS)

    Miller, Robert A.; Zhu, Dongming

    2003-01-01

    Thermal-barrier coatings (TBCs) that have both initial and post-exposure thermal conductivities lower than those of yttria-stabilized zirconia TBCs have been developed. TBCs are thin ceramic layers, generally applied by plasma spraying or physical vapor deposition, that are used to insulate air-cooled metallic components from hot gases in gas turbine and other heat engines. Heretofore, yttria-stabilized zirconia (nominally comprising 95.4 atomic percent ZrO2 + 4.6 atomic percent Y2O3) has been the TBC material of choice. The lower-thermal-conductivity TBCs are modified versions of yttria-stabilized zirconia, the modifications consisting primarily in the addition of other oxides that impart microstructural and defect properties that favor lower thermal conductivity.

  12. Thermal conductivity behavior of superatom molecular crystals

    NASA Astrophysics Data System (ADS)

    Ong, Wee-Liat; O'Brien, Evan; Dougherty, Patrick; Epstein, Jillian; Higgs, C. Fred; McGaughey, Alan; Roy, Xavier; Malen, Jonathan

    The room temperature thermal conductivity of several superatom molecular crystals (SMCs) are measured and found to be below 0.3 W/mK. The trend of room temperature thermal conductivity of the different crystals agree well with their sound speeds obtained independently using nano-indentation. These crystals, however, can exhibit non-crystalline thermal conductivity behavior depending on their constituent elements. A superatom is a cluster of atoms that acts as a stable entity [e.g., fullerenes (C60)]. By careful mixing and assembling these nano-sized superatoms, the resulting superatom-assembled materials hold promises for improving various technological devices. Organic-inorganic superatoms can assemble into unary SMCs or co-crystallized with C60 superatoms into binary SMCs. Thermal transport is of considerable interest with possible new physics in these hierarchically atomic precise crystals in the low temperature regime. The thermal conductivity of the SMCs are measured using the frequency domain thermoreflectance setup. Unary SMCs exhibit an almost invariant thermal conductivity down to a temperature of 150 K. Binary SMCs, however, can either show a crystalline-like increase or an amorphous-like decrease with decreasing temperature.

  13. Thermal contact conductance of spherical, rough metals

    SciTech Connect

    Lambert, M.A.; Fletcher, L.S.

    1996-12-31

    Junction thermal conductance is an important consideration in such applications as thermally induced stresses in supersonic and hypersonic flight vehicles, nuclear reactor cooling, electronics packaging, spacecraft thermal control, gas turbine and internal combustion engine cooling, and cryogenic liquid storage. A fundamental problem in analyzing and predicting junction thermal conductance is thermal contact conductance of non-flat, rough metals. Workable models have been previously derived for the limiting idealized cases of flat, rough and spherical, smooth surfaces. However, until now, no tractable models have been advanced for non-flat, rough surfaces (so called engineering surfaces) which are much more commonly dealt with in practice. The present investigation details the synthesis of previously derived models for macroscopically non-uniform thermal contact conductance and contact of non-flat, rough spheres, into a thermo-mechanical model, its refinement, and its presentation in an easily applied format. The present model is compared to representative experimental conductance results form the literature for stainless steel 304 with widely varying flatness deviation and roughness. Also included in the comparisons is a previous, often cited model for flat, rough metals to which the present model reduces in the limit of flat surfaces. The present model agrees well with the experimental results over the wide ranges of flatness deviation and roughness for which data was available. Conversely, the previous, well known model becomes increasingly overpredictive with increasing flatness deviation.

  14. Thermal Conductivity Of Natural Type IIa Diamond

    NASA Technical Reports Server (NTRS)

    Vandersande, Jan; Vining, Cronin; Zoltan, Andrew

    1992-01-01

    Report describes application of flash diffusivity method to measure thermal conductivity of 8.04 x 8.84 x 2.35-mm specimen of natural, white, type-IIa diamond at temperatures between 500 and 1,250 K. Provides baseline for comparison to isotopically pure (12C) diamond. Results used as reference against which diamond films produced by chemical-vapor deposition at low pressures can be compared. High thermal conductivity of diamond exploited for wide variety of applications, and present results also used to estimate heat-conduction performances of diamond films in high-temperature applications.

  15. Thermal conductivity of uterine tissue in vitro

    NASA Astrophysics Data System (ADS)

    Olsrud, Johan; Friberg, Britt; Ahlgren, Mats; Persson, Bertil R. R.

    1998-08-01

    Thermotherapy of the uterus has emerged as an alternative to hysterectomy in the treatment of menorrhagia, from whence it follows that the thermal properties of uterine tissue have become of importance. This study presents measurements of the thermal conductivity and the water content of uterine tissue in vitro. A steady-state thermal conductivity apparatus, based on the comparison of test samples with a material with known thermal conductivity, is described. Measurements were conducted on tissue samples from eleven patients, directly after hysterectomy. Samples with and without endometrium, as well as coagulated samples, were examined. The thermal conductivity of myometrial tissue was found to be and the corresponding water content was % . Measurements on samples with both endometrium and myometrium showed similar thermal conductivity (, ) and water content (%, ). It was also indicated that coagulation causes dehydration, resulting in a lower thermal conductivity.

  16. The Lattice and Thermal Radiation Conductivity of Thermal Barrier Coatings

    NASA Technical Reports Server (NTRS)

    Zhu, Dongming; Spuckler, Charles M.

    2008-01-01

    The lattice and radiation conductivity of thermal barrier coatings was evaluated using a laser heat flux approach. A diffusion model has been established to correlate the apparent thermal conductivity of the coating to the lattice and radiation conductivity. The radiation conductivity component can be expressed as a function of temperature and the scattering and absorption properties of the coating material. High temperature scattering and absorption of the coating systems can also be derived based on the testing results using the modeling approach. The model prediction is found to have good agreement with experimental observations.

  17. Comparison of thermal diffusivity and thermal conductivity methods

    SciTech Connect

    Sheffield, G.S.; Schorr, J.R. )

    1991-01-01

    This paper reports on applications involving ceramics which require that this diverse group of materials act as either thermal insulators or thermal conductors. Values of thermal conductivity can range over more than 4 orders of magnitude from 0.1 W/(m {center dot} K) (0.7 BTU {center dot} in./(h {center dot} ft{sup 2} {center dot} {degrees}F)) for fiberboard insulation to 1300 W/(M {center dot} K) (9013 BTU {center dot} in./(h {center dot} {degrees}F)) for boron nitride. The magnitude of temperature gradients in materials is governed by thermal diffusivity and thermal conductivity. In ceramic materials, gradients can create significant thermal stresses, cause heat flow (causing furnaces to heat but also causing usually undesirable heat losses) and be a controlling factor in reaction rates. In general, no single method is dominant because of the wide range of temperatures involved, the large variations in diffusivity and conductivity encountered, the differences in sampling requirements (homogeneity), and the duration of measurement time. Five thermal diffusivity and conductivity methods, all of which have been successfully applied to ceramic materials are reviewed. The methods covered are dynamic radial heat flow, laser flash, hot wire, calorimeter, and guarded hot plate. The dynamic radial heat flow and laser flash methods are diffusivity methods, whereas the remaining three are representative of conductivity methods.

  18. Thermal conductivity of tubrostratic carbon nanofiber networks

    DOE PAGESBeta

    Bauer, Matthew L.; Saltonstall, Chris B.; Leseman, Zayd C.; Beechem, Thomas E.; Hopkins, Patrick E.; Norris, Pamela M.

    2016-01-01

    Composite material systems composed of a matrix of nano materials can achieve combinations of mechanical and thermophysical properties outside the range of traditional systems. While many reports have studied the intrinsic thermal properties of individual carbon fibers, to be useful in applications in which thermal stability is critical, an understanding of heat transport in composite materials is required. In this work, air/ carbon nano fiber networks are studied to elucidate the system parameters influencing thermal transport. Sample thermal properties are measured with varying initial carbon fiber fill fraction, environment pressure, loading pressure, and heat treatment temperature through a bidirectional modificationmore » of the 3ω technique. The nanostructures of the individual fibers are characterized with small angle x-ray scattering and Raman spectroscopy providing insight to individual fiber thermal conductivity. Measured thermal conductivity varied from 0.010 W/(m K) to 0.070 W/(m K). An understanding of the intrinsic properties of the individual fibers and the interactions of the two phase composite is used to reconcile low measured thermal conductivities with predictive modeling. This methodology can be more generally applied to a wide range of fiber composite materials and their applications.« less

  19. Thermal conductivity of tubrostratic carbon nanofiber networks

    SciTech Connect

    Bauer, Matthew L.; Saltonstall, Chris B.; Leseman, Zayd C.; Beechem, Thomas E.; Hopkins, Patrick E.; Norris, Pamela M.

    2016-01-01

    Composite material systems composed of a matrix of nano materials can achieve combinations of mechanical and thermophysical properties outside the range of traditional systems. While many reports have studied the intrinsic thermal properties of individual carbon fibers, to be useful in applications in which thermal stability is critical, an understanding of heat transport in composite materials is required. In this work, air/ carbon nano fiber networks are studied to elucidate the system parameters influencing thermal transport. Sample thermal properties are measured with varying initial carbon fiber fill fraction, environment pressure, loading pressure, and heat treatment temperature through a bidirectional modification of the 3ω technique. The nanostructures of the individual fibers are characterized with small angle x-ray scattering and Raman spectroscopy providing insight to individual fiber thermal conductivity. Measured thermal conductivity varied from 0.010 W/(m K) to 0.070 W/(m K). An understanding of the intrinsic properties of the individual fibers and the interactions of the two phase composite is used to reconcile low measured thermal conductivities with predictive modeling. This methodology can be more generally applied to a wide range of fiber composite materials and their applications.

  20. Thermal conductivity of hydrate-bearing sediments

    USGS Publications Warehouse

    Cortes, D.D.; Martin, A.I.; Yun, T.S.; Francisca, F.M.; Santamarina, J.C.; Ruppel, C.

    2009-01-01

    A thorough understanding of the thermal conductivity of hydrate-bearing sediments is necessary for evaluating phase transformation processes that would accompany energy production from gas hydrate deposits and for estimating regional heat flow based on the observed depth to the base of the gas hydrate stability zone. The coexistence of multiple phases (gas hydrate, liquid and gas pore fill, and solid sediment grains) and their complex spatial arrangement hinder the a priori prediction of the thermal conductivity of hydrate-bearing sediments. Previous studies have been unable to capture the full parameter space covered by variations in grain size, specific surface, degree of saturation, nature of pore filling material, and effective stress for hydrate-bearing samples. Here we report on systematic measurements of the thermal conductivity of air dry, water- and tetrohydrofuran (THF)-saturated, and THF hydrate-saturated sand and clay samples at vertical effective stress of 0.05 to 1 MPa (corresponding to depths as great as 100 m below seafloor). Results reveal that the bulk thermal conductivity of the samples in every case reflects a complex interplay among particle size, effective stress, porosity, and fluid-versus-hydrate filled pore spaces. The thermal conductivity of THF hydrate-bearing soils increases upon hydrate formation although the thermal conductivities of THF solution and THF hydrate are almost the same. Several mechanisms can contribute to this effect including cryogenic suction during hydrate crystal growth and the ensuing porosity reduction in the surrounding sediment, increased mean effective stress due to hydrate formation under zero lateral strain conditions, and decreased interface thermal impedance as grain-liquid interfaces are transformed into grain-hydrate interfaces. Copyright 2009 by the American Geophysical Union.

  1. Measurement of the thermal contact conductance and thermal conductivity of anodized aluminum coatings

    SciTech Connect

    Peterson, G.P.; Fletcher, L.S. )

    1990-08-01

    An experimental investigation was conducted to determine the thermal contact conductance and effective thermal conductivity of anodized coatings. One chemically polished Aluminum 6061-T6 test specimen and seven specimens with anodized coatings varying in thickness from 60.9 {mu}m to 163.8 {mu}m were tested while in contact with a single unanodized aluminum surface. Measurements of the overall joint conductance, composed of the thermal contact conductance between the anodized coating and the bare aluminum surface and the bulk conductance of the coating material, indicated that the overall joint conductance decreased with increasing thickness of the anodized coating and increased with increasing interfacial load. Using the experimental data, a dimensionless expression was developed that related the overall joint conductance to the coating thickness, the surface roughness, the interfacial pressure, and the properties of the aluminum substrate. By subtracting the thermal contact conductance from the measured overall joint conductance, estimations of the effective thermal conductivity of the anodized coating as a function of pressure were obtained for each of the seven anodized specimens. At an extrapolated pressure of zero, the effective thermal conductivity was found to be approximately 0.02 W/m-K. In addition to this extrapolated value, a single expression for predicting the effective thermal conductivity as a function of both the interface pressure and the anodized coating thickness was developed and shown to be within {plus minus}5 percent of the experimental data over a pressure range of 0 to 14 MPa.

  2. Material Properties for Fiber-Reinforced Silica Aerogels

    NASA Technical Reports Server (NTRS)

    White, Susan; Rouanet, Stephane; Moses, John; Arnold, James O. (Technical Monitor)

    1994-01-01

    Ceramic fiber-reinforced silica aerogels are novel materials for high performance insulation, including thermal protection materials. Experimental data are presented for the thermal and mechanical properties, showing the trends exhibited over a range of fiber loadings and silica aerogel densities. Test results are compared to that of unreinforced bulk aerogels.

  3. Role of interfaces on thermal conductivity

    NASA Astrophysics Data System (ADS)

    Xue, Liping

    Using classical molecular dynamics (MD) simulations we study the role of interfaces on the thermal transport properties, and their role in overall heat flow in nanoscale materials. For the simple monoatomic liquid-solid interface, our simulations reveal that the key factor controlling interfacial thermal resistance is the strength of the bonding between liquid and solid atoms. The functional dependence of the thermal resistance on the strength of the liquid-solid interactions exhibits two distinct regimes. Surprisingly, ordering of the liquid at the liquid-solid interface shows no effect on the thermal transport either normal to the surface or parallel to the surface. The results suggest that the experimentally observed large enhancement of thermal conductivity in suspension of solid nanosized particles (nanofluids) can not be explained by altered thermal transport properties of the layered liquid. For the heat flow between a carbon nanotube and octane liquid interface, our simulation demonstrates the key role played by the soft vibrational modes in the mechanism of the heat flow. The results also imply that the thermal conductivity of carbon nanotube polymer composites and organic suspensions is limited by the interfacial thermal resistance and are consistent with recent experiments. We find that chemical functionalization of carbon nanotube reduces significantly the tube-matrix thermal boundary resistance, but at the same time decreases intrinsic tube conductivity. Interestingly, at high degrees of chemical functionalization, intrinsic tube conductivity becomes independent of the bond density, indicating important role of long-wavelength phonons in carbon nanotubes. Finally, we explore a different type of interfacial system: a bulk macromolecular liquid. In this case, the heat flow is controlled by the inter- and intra-chain thermal transport properties. Our simulations demonstrate that in order to significantly enhance the thermal conductivity of the liquid

  4. Anisotropic thermal conductivity of semiconducting graphene monoxide

    NASA Astrophysics Data System (ADS)

    Pu, H. H.; Rhim, S. H.; Hirschmugl, C. J.; Gajdardziska-Josifovska, M.; Weinert, M.; Chen, J. H.

    2013-06-01

    The intrinsic thermal conductivity of monolayer graphene monoxide is determined via first-principles calculations. The phonon transport in graphene monoxide is anisotropic, with the lattice thermal conductivity along the armchair direction (…C-2O-C…) about five times higher than that along the zigzag (…C-C…) direction. The predicted thermal conductivity (>3000 Wm-1K-1 at 300 K) of graphene monoxide is 80% of that of graphene along the armchair direction for large sample lateral sizes (>5 μm). In addition, heat is predominantly carried by longitudinal acoustic phonons along the armchair direction, while the contribution from the transverse acoustic phonon mode is prevalent along the zigzag direction.

  5. Gas storage carbon with enhanced thermal conductivity

    DOEpatents

    Burchell, Timothy D.; Rogers, Michael Ray; Judkins, Roddie R.

    2000-01-01

    A carbon fiber carbon matrix hybrid adsorbent monolith with enhanced thermal conductivity for storing and releasing gas through adsorption and desorption is disclosed. The heat of adsorption of the gas species being adsorbed is sufficiently large to cause hybrid monolith heating during adsorption and hybrid monolith cooling during desorption which significantly reduces the storage capacity of the hybrid monolith, or efficiency and economics of a gas separation process. The extent of this phenomenon depends, to a large extent, on the thermal conductivity of the adsorbent hybrid monolith. This invention is a hybrid version of a carbon fiber monolith, which offers significant enhancements to thermal conductivity and potential for improved gas separation and storage systems.

  6. Recent advances in thermal conductivity of nanofluids.

    PubMed

    Witharana, Sanjeeva; Weliwita, Jinendrika Anushi; Chen, Haisheng; Wang, Liang

    2013-11-01

    This paper presents the most recent review of research articles and patents on thermal conductivity on nanofluids. Larger portion of literature accounts for experimental investigations, which is a sign of the preference for hands-on work by experimentalists. Metallic, non-metallic as well as ceramic nanoparticles of different sizes and shapes were suspended in common heat transfer liquids and their thermal conductivities were measured. In contrast to previous belief, it has now been proven that when the nanoparticle concentration is kept low the degree of enhancement falls reasonably within the boundaries predicted by the effective medium theories. There are strong evidences to suggest that the main mechanisms driving the thermal conductivity behavior are nanoparticle aggregation and the particle Brownian motion in suspension. PMID:24330042

  7. Gas storage carbon with enhanced thermal conductivity

    SciTech Connect

    Burchell, T.D.; Rogers, M.R.; Judkins, R.R.

    2000-07-18

    A carbon fiber carbon matrix hybrid adsorbent monolith with enhanced thermal conductivity for storing and releasing gas through adsorption and desorption is disclosed. The heat of adsorption of the gas species being adsorbed is sufficiently large to cause hybrid monolith heating during adsorption and hybrid monolith cooling during desorption which significantly reduces the storage capacity of the hybrid monolith, or efficiency and economics of a gas separation process. The extent of this phenomenon depends, to a large extent, on the thermal conductivity of the adsorbent hybrid monolith. This invention is a hybrid version of a carbon fiber monolith, which offers significant enhancements to thermal conductivity and potential for improved gas separation and storage systems.

  8. Enhancing thermal conductivity of fluids with nanoparticles

    SciTech Connect

    Choi, S.U.S.; Eastman, J.A.

    1995-10-01

    Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids that are required in many industrial applications. In this paper we propose that an innovative new class of heat transfer fluids can be engineered by suspending metallic nanoparticles in conventional heat transfer fluids. The resulting {open_quotes}nanofluids{close_quotes} are expected to exhibit high thermal conductivities compared to those of currently used heat transfer fluids, and they represent the best hope for enhancement of heat transfer. The results of a theoretical study of the thermal conductivity of nanofluids with copper nanophase materials are presented, the potential benefits of the fluids are estimated, and it is shown that one of the benefits of nanofluids will be dramatic reductions in heat exchanger pumping power.

  9. Thermal contact conductance of pressurized surfaces

    NASA Technical Reports Server (NTRS)

    Voss, Fred E.

    1988-01-01

    Thermal vacuum testing has demonstrated the feasibility of the concept of pressurizing contact surfaces for the transfer of waste heat on the Space Station. Data show a thin inflatable bladder design to provide a greater contact conductance than an expandable bellows approach, with substantially less interface volume to transfer the same amount of heat. Extended vacuum testing of the Radiator-to-Thermal Bus assembly indicated a continual increase of conductance as the interface was maintained at 150 psi for a 100-hour period.

  10. An Innovative High Thermal Conductivity Fuel Design

    SciTech Connect

    PI: James S. Tulenko; Co-PI: Ronald H. Baney,

    2007-10-14

    Uranium dioxide (UO2) is the most common fuel material in commercial nuclear power reactors. UO2 has the advantages of a high melting point, good high-temperature stability, good chemical compatibility with cladding and coolant, and resistance to radiation. The main disadvantage of UO2 is its low thermal conductivity. During a reactor’s operation, because the thermal conductivity of UO2 is very low, for example, about 2.8 W/m-K at 1000 oC [1], there is a large temperature gradient in the UO2 fuel pellet, causing a very high centerline temperature, and introducing thermal stresses, which lead to extensive fuel pellet cracking. These cracks will add to the release of fission product gases after high burnup. The high fuel operating temperature also increases the rate of fission gas release and the fuel pellet swelling caused by fission gases bubbles. The amount of fission gas release and fuel swelling limits the life time of UO2 fuel in reactor. In addition, the high centerline temperature and large temperature gradient in the fuel pellet, leading to a large amount of stored heat, increase the Zircaloy cladding temperature in a lost of coolant accident (LOCA). The rate of Zircaloy-water reaction becomes significant at the temperature above 1200 oC [2]. The ZrO2 layer generated on the surface of the Zircaloy cladding will affect the heat conduction, and will cause a Zircaloy cladding rupture. The objective of this research is to increase the thermal conductivity of UO2, while not affecting the neutronic property of UO2 significantly. The concept to accomplish this goal is to incorporate another material with high thermal conductivity into the UO2 pellet. Silicon carbide (SiC) is a good candidate, because the thermal conductivity of single crystal SiC is 60 times higher than that of UO2 at room temperature and 30 times higher at 800 oC [3]. Silicon carbide also has the properties of low thermal neutron absorption cross section, high melting point, good chemical

  11. Thermal conductivity and diffusivity of soil

    SciTech Connect

    Sorour, M.M. ); Saleh, M.M.; Mahmoud, R.A. )

    1990-03-01

    The thermal conductivity and diffusivity of soil has been experimentally measured using the line heat source transient method. Representative samples of different textures were collected and analyzed from different localities and depths. The effect of temperatures, in the range of {minus}10{degrees}C {yields} 35{degrees}C and moisture content up to 40% on the conductivity and diffusivity were investigated. The results of this investigation indicate some interesting results and confirm some previously published data.

  12. Optothermal nonlinearity of silica aerogel

    NASA Astrophysics Data System (ADS)

    Braidotti, Maria Chiara; Gentilini, Silvia; Fleming, Adam; Samuels, Michiel C.; Di Falco, Andrea; Conti, Claudio

    2016-07-01

    We report on the characterization of silica aerogel thermal optical nonlinearity, obtained by z-scan technique. The results show that typical silica aerogels have nonlinear optical coefficient similar to that of glass (≃10-12 m2/W), with negligible optical nonlinear absorption. The nonlinear coefficient can be increased to values in the range of 10-10 m2/W by embedding an absorbing dye in the aerogel. This value is one order of magnitude higher than that observed in the pure dye and in typical highly nonlinear materials like liquid crystals.

  13. Local measurement of thermal conductivity and diffusivity

    SciTech Connect

    Hurley, David H.; Schley, Robert S.; Khafizov, Marat; Wendt, Brycen L.

    2015-12-01

    Simultaneous measurement of local thermal diffusivity and conductivity is demonstrated on a range of ceramic samples. This was accomplished by measuring the temperature field spatial profile of samples excited by an amplitude modulated continuous wave laser beam. A thin gold film is applied to the samples to ensure strong optical absorption and to establish a second boundary condition that introduces an expression containing the substrate thermal conductivity. The diffusivity and conductivity are obtained by comparing the measured phase profile of the temperature field to a continuum based model. A sensitivity analysis is used to identify the optimal film thickness for extracting the both substrate conductivity and diffusivity. Proof of principle studies were conducted on a range of samples having thermal properties that are representative of current and advanced accident tolerant nuclear fuels. It is shown that by including the Kapitza resistance as an additional fitting parameter, the measured conductivity and diffusivity of all the samples considered agree closely with literature values. Lastly, a distinguishing feature of this technique is that it does not require a priori knowledge of the optical spot size which greatly increases measurement reliability and reproducibility.

  14. Thermal Conductance of Nanoscale VOx Epitaxial Layers

    NASA Astrophysics Data System (ADS)

    Oh, Dong-Wook; Petrov, Ivan; Cahill, David

    2010-03-01

    We use time-domain thermoreflectance to measure the thermal conductance of VOx layers in epitaxial Pt/VOx/Pt structures. In particular, the metal-insulator-transition of VO2 at 70^oC allows us to systematically explore channels for heat transport between metals and correlated-electron systems. Pt/VOx/Pt layers are deposited on a sapphire substrates by reactive DC sputtering with O2 partial pressure varied from 0% to 13%. The thermal conductance has a strong dependence on thickness, 3-50 nm, and oxygen content, pure V to V2O5. The thermal conductance of ˜10 nm thick layers of V in series with the two Pt/V interfaces is 1 GW/m^2-K, comparable to what is expected based on the diffuse-mismatch model for electron transport at interfaces. The conductance of ˜10 nm thick layers of VO2 at room temperatures is remarkably high, 0.5 GW/m^2-K, for the series conductance of two metal-dielectric interfaces. At the metal-insulator-transition, the conductance of VO2 layers increases by only 10%, indicating that electrons in Pt and electrons in metallic VO2 are not strongly coupled.

  15. Local measurement of thermal conductivity and diffusivity.

    PubMed

    Hurley, David H; Schley, Robert S; Khafizov, Marat; Wendt, Brycen L

    2015-12-01

    Simultaneous measurement of local thermal diffusivity and conductivity is demonstrated on a range of ceramic samples. This was accomplished by measuring the temperature field spatial profile of samples excited by an amplitude modulated continuous wave laser beam. A thin gold film is applied to the samples to ensure strong optical absorption and to establish a second boundary condition that introduces an expression containing the substrate thermal conductivity. The diffusivity and conductivity are obtained by comparing the measured phase profile of the temperature field to a continuum based model. A sensitivity analysis is used to identify the optimal film thickness for extracting the both substrate conductivity and diffusivity. Proof of principle studies were conducted on a range of samples having thermal properties that are representatives of current and advanced accident tolerant nuclear fuels. It is shown that by including the Kapitza resistance as an additional fitting parameter, the measured conductivity and diffusivity of all the samples considered agreed closely with the literature values. A distinguishing feature of this technique is that it does not require a priori knowledge of the optical spot size which greatly increases measurement reliability and reproducibility. PMID:26724041

  16. Local measurement of thermal conductivity and diffusivity

    NASA Astrophysics Data System (ADS)

    Hurley, David H.; Schley, Robert S.; Khafizov, Marat; Wendt, Brycen L.

    2015-12-01

    Simultaneous measurement of local thermal diffusivity and conductivity is demonstrated on a range of ceramic samples. This was accomplished by measuring the temperature field spatial profile of samples excited by an amplitude modulated continuous wave laser beam. A thin gold film is applied to the samples to ensure strong optical absorption and to establish a second boundary condition that introduces an expression containing the substrate thermal conductivity. The diffusivity and conductivity are obtained by comparing the measured phase profile of the temperature field to a continuum based model. A sensitivity analysis is used to identify the optimal film thickness for extracting the both substrate conductivity and diffusivity. Proof of principle studies were conducted on a range of samples having thermal properties that are representatives of current and advanced accident tolerant nuclear fuels. It is shown that by including the Kapitza resistance as an additional fitting parameter, the measured conductivity and diffusivity of all the samples considered agreed closely with the literature values. A distinguishing feature of this technique is that it does not require a priori knowledge of the optical spot size which greatly increases measurement reliability and reproducibility.

  17. Local measurement of thermal conductivity and diffusivity

    SciTech Connect

    Hurley, David H.; Schley, Robert S.; Khafizov, Marat; Wendt, Brycen L.

    2015-12-15

    Simultaneous measurement of local thermal diffusivity and conductivity is demonstrated on a range of ceramic samples. This was accomplished by measuring the temperature field spatial profile of samples excited by an amplitude modulated continuous wave laser beam. A thin gold film is applied to the samples to ensure strong optical absorption and to establish a second boundary condition that introduces an expression containing the substrate thermal conductivity. The diffusivity and conductivity are obtained by comparing the measured phase profile of the temperature field to a continuum based model. A sensitivity analysis is used to identify the optimal film thickness for extracting the both substrate conductivity and diffusivity. Proof of principle studies were conducted on a range of samples having thermal properties that are representatives of current and advanced accident tolerant nuclear fuels. It is shown that by including the Kapitza resistance as an additional fitting parameter, the measured conductivity and diffusivity of all the samples considered agreed closely with the literature values. A distinguishing feature of this technique is that it does not require a priori knowledge of the optical spot size which greatly increases measurement reliability and reproducibility.

  18. Local measurement of thermal conductivity and diffusivity

    DOE PAGESBeta

    Hurley, David H.; Schley, Robert S.; Khafizov, Marat; Wendt, Brycen L.

    2015-12-01

    Simultaneous measurement of local thermal diffusivity and conductivity is demonstrated on a range of ceramic samples. This was accomplished by measuring the temperature field spatial profile of samples excited by an amplitude modulated continuous wave laser beam. A thin gold film is applied to the samples to ensure strong optical absorption and to establish a second boundary condition that introduces an expression containing the substrate thermal conductivity. The diffusivity and conductivity are obtained by comparing the measured phase profile of the temperature field to a continuum based model. A sensitivity analysis is used to identify the optimal film thickness formore » extracting the both substrate conductivity and diffusivity. Proof of principle studies were conducted on a range of samples having thermal properties that are representative of current and advanced accident tolerant nuclear fuels. It is shown that by including the Kapitza resistance as an additional fitting parameter, the measured conductivity and diffusivity of all the samples considered agree closely with literature values. Lastly, a distinguishing feature of this technique is that it does not require a priori knowledge of the optical spot size which greatly increases measurement reliability and reproducibility.« less

  19. Infrared Detector System with Controlled Thermal Conductance

    NASA Technical Reports Server (NTRS)

    Cunningham, Thomas J. (Inventor)

    2000-01-01

    A thermal infrared detector system includes a heat sink, a support member, a connection support member connecting the support member to the heat sink and including a heater unit is reviewed. An infrared detector element is mounted on the support member and a temperature signal representative of the infrared energy contacting the support member can then be derived by comparing the temperature of the support member and the heat sink. The temperature signal from a support member and a temperature signal from the connection support member can then be used to drive a heater unit mounted on the connection support member to thereby control the thermal conductance of the support member. Thus, the thermal conductance can be controlled so that it can be actively increased or decreased as desired.

  20. Thermal Conductivity of Al-Salt Composites

    NASA Astrophysics Data System (ADS)

    Li, Peng; Zhang, Mei; Wang, Lijun; Seetharaman, Seshadri

    2015-11-01

    With a view to examine the possibility of estimating the content of entrapped metallic aluminium in the salt cake from aluminium remelting, the thermal diffusivity of reference composites of KCl-NaCl-Al was measured as a function of aluminium metal content at room temperature. The thermal conductivity of the reference composites was found to increase with the metallic Al content. The lumped parameter model approach was carried out to discuss the influence of different geometry arrangements of each phase, viz. air, salts and metallic aluminium on the thermal conductivity. Application of the present results to industrial samples indicates that factors such as the interfacial condition of metallic Al particles have to be considered in order to estimate the amount of entrapped Al in the salt cake.

  1. Thermal and electrical contact conductance studies

    NASA Technical Reports Server (NTRS)

    Vansciver, S. W.; Nilles, M.

    1985-01-01

    Prediction of electrical and thermal contact resistance for pressed, nominally flat contacts is complicated by the large number of variables which influence contact formation. This is reflected in experimental results as a wide variation in contact resistances, spanning up to six orders of magnitude. A series of experiments were performed to observe the effects of oxidation and surface roughness on contact resistance. Electrical contact resistance and thermal contact conductance from 4 to 290 K on OFHC Cu contacts are reported. Electrical contact resistance was measured with a 4-wire DC technique. Thermal contact conductance was determined by steady-state longitudinal heat flow. Corrections for the bulk contribution ot the overall measured resistance were made, with the remaining resistance due solely to the presence of the contact.

  2. Thermal Conductivity of Polyimide/Nanofiller Blends

    NASA Technical Reports Server (NTRS)

    Ghose, S.; Watson, K. A.; Delozier, D. M.; Working, D. c.; Connell, J. W.; Smith, J. G.; Sun, Y. P.; Lin, Y.

    2006-01-01

    In efforts to improve the thermal conductivity of Ultem(TM) 1000, it was compounded with three carbon based nano-fillers. Multiwalled carbon nanotubes (MWCNT), vapor grown carbon nanofibers (CNF) and expanded graphite (EG) were investigated. Ribbons were extruded to form samples in which the nano-fillers were aligned. Samples were also fabricated by compression molding in which the nano-fillers were randomly oriented. The thermal properties were evaluated by DSC and TGA, and the mechanical properties of the aligned samples were determined by tensile testing. The degree of dispersion and alignment of the nanoparticles were investigated with high-resolution scanning electron microscopy. The thermal conductivity of the samples was measured in both the direction of alignment as well as perpendicular to that direction using the Nanoflash technique. The results of this study will be presented.

  3. Thermal conductivity of BN-C nanostructures

    NASA Astrophysics Data System (ADS)

    Kınacı, Alper; Haskins, Justin B.; Sevik, Cem; Çaǧın, Tahir

    2012-09-01

    Chemical and structural diversity present in hexagonal boron nitride (h-BN) and graphene hybrid nanostructures provide avenues for tuning various properties for their technological applications. In this paper we investigate the variation of thermal conductivity (κ) of hybrid graphene/h-BN nanostructures: stripe superlattices and BN (graphene) dots embedded in graphene (BN) are investigated using equilibrium molecular dynamics. To simulate these systems, we have parametrized a Tersoff type interaction potential to reproduce the ab initio energetics of the B-C and N-C bonds for studying the various interfaces that emerge in these hybrid nanostructures. We demonstrate that both the details of the interface, including energetic stability and shape, as well as the spacing of the interfaces in the material, exert strong control on the thermal conductivity of these systems. For stripe superlattices, we find that zigzag configured interfaces produce a higher κ in the direction parallel to the interface than the armchair configuration, while the perpendicular conductivity is less prone to the details of the interface and is limited by the κ of h-BN. Additionally, the embedded dot structures, having mixed zigzag and armchair interfaces, affect the thermal transport properties more strongly than superlattices. The largest reduction in thermal conductivity is observed at 50% dot concentration, but the dot radius appears to have little effect on the magnitude of reduction around this concentration.

  4. Reducing Thermal Conduction In Acoustic Levitators

    NASA Technical Reports Server (NTRS)

    Lierke, Ernst G.; Leung, Emily W.; Bhat, Balakrishna T.

    1991-01-01

    Acoustic transducers containing piezoelectric driving elements made more resistant to heat by reduction of effective thermal-conductance cross sections of metal vibration-transmitting rods in them, according to proposal. Used to levitate small objects acoustically for noncontact processing in furnaces. Reductions in cross sections increase amplitudes of transmitted vibrations and reduce loss of heat from furnaces.

  5. Indium Foil Serves As Thermally Conductive Gasket

    NASA Technical Reports Server (NTRS)

    Eastman, G. Yale; Dussinger, Peter M.

    1993-01-01

    Indium foil found useful as gasket to increase thermal conductance between bodies clamped together. Deforms to fill imperfections on mating surfaces. Used where maximum temperature in joint less than melting temperature of indium. Because of low melting temperature of indium, most useful in cryogenic applications.

  6. Tunable Interfacial Thermal Conductance by Molecular Dynamics

    NASA Astrophysics Data System (ADS)

    Shen, Meng

    We study the mechanism of tunable heat transfer through interfaces between solids using a combination of non-equilibrium molecular dynamics simulation (NEMD), vibrational mode analysis and wave packet simulation. We investigate how heat transfer through interfaces is affected by factors including pressure, interfacial modulus, contact area and interfacial layer thickness, with an overreaching goal of developing fundamental knowledge that will allow one to tailor thermal properties of interfacial materials. The role of pressure and interfacial stiffness is unraveled by our studies on an epitaxial interface between two Lennard-Jones (LJ) crystals. The interfacial stiffness is varied by two different methods: (i) indirectly by applying pressure which due to anharmonic nature of bonding, increases interfacial stiffness, and (ii) directly by changing the interfacial bonding strength by varying the depth of the potential well of the LJ potential. When the interfacial bonding strength is low, quantitatively similar behavior to pressure tuning is observed when the interfacial thermal conductance is increased by directly varying the potential-well depth parameter of the LJ potential. By contrast, when the interfacial bonding strength is high, thermal conductance is almost pressure independent, and even slightly decreases with increasing pressure. This decrease can be explained by the change in overlap between the vibrational densities of states of the two crystalline materials. The role of contact area is studied by modeling structures comprised of Van der Waals junctions between single-walled nanotubes (SWCNT). Interfacial thermal conductance between SWCNTs is obtained from NEMD simulation as a function of crossing angle. In this case the junction conductance per unit area is essentially a constant. By contrast, interfacial thermal conductance between multiwalled carbon nanotubes (MWCNTs) is shown to increase with diameter of the nanotubes by recent experimental studies [1

  7. Tunable Interfacial Thermal Conductance by Molecular Dynamics

    NASA Astrophysics Data System (ADS)

    Shen, Meng

    We study the mechanism of tunable heat transfer through interfaces between solids using a combination of non-equilibrium molecular dynamics simulation (NEMD), vibrational mode analysis and wave packet simulation. We investigate how heat transfer through interfaces is affected by factors including pressure, interfacial modulus, contact area and interfacial layer thickness, with an overreaching goal of developing fundamental knowledge that will allow one to tailor thermal properties of interfacial materials. The role of pressure and interfacial stiffness is unraveled by our studies on an epitaxial interface between two Lennard-Jones (LJ) crystals. The interfacial stiffness is varied by two different methods: (i) indirectly by applying pressure which due to anharmonic nature of bonding, increases interfacial stiffness, and (ii) directly by changing the interfacial bonding strength by varying the depth of the potential well of the LJ potential. When the interfacial bonding strength is low, quantitatively similar behavior to pressure tuning is observed when the interfacial thermal conductance is increased by directly varying the potential-well depth parameter of the LJ potential. By contrast, when the interfacial bonding strength is high, thermal conductance is almost pressure independent, and even slightly decreases with increasing pressure. This decrease can be explained by the change in overlap between the vibrational densities of states of the two crystalline materials. The role of contact area is studied by modeling structures comprised of Van der Waals junctions between single-walled nanotubes (SWCNT). Interfacial thermal conductance between SWCNTs is obtained from NEMD simulation as a function of crossing angle. In this case the junction conductance per unit area is essentially a constant. By contrast, interfacial thermal conductance between multiwalled carbon nanotubes (MWCNTs) is shown to increase with diameter of the nanotubes by recent experimental studies [1

  8. THERMAL CONDUCTIVITY OF SIC AND C FIBERS

    SciTech Connect

    Youngblood, Gerald E.; Senor, David J.; Kowbel, W.; Webb, J.; Kohyama, Akira

    2000-09-01

    Several rod-shaped specimens with uniaxially packed fibers (Hi-Nicalon, Hi-Nicalon Type S, Tyranno SA and Amoco K1100 types) and a pre-ceramic polymer matrix have been fabricated. By using appropriate analytic models, the bare fiber thermal conductivity (Kf) and the interface thermal conductance (h) will be determined as a function of temperature up to 1000?C before and after irradiation for samples cut from these rods. Initial results are: (1) for unirradiated Hi-Nicalon SiC fiber, Kf varied from 4.3 up to 5.9 W/mK for the 27-1000?C range, (2) for unirradiated K1100 graphite fiber, Kf varied from 576 down to 242 W/mK for the 27-1000?C range, and (3) h = 43 W/cm2K at 27?C as a typical fiber/matrix interface conductance.

  9. Aerogel Composites: Strong and Waterproof

    NASA Technical Reports Server (NTRS)

    White, Susan; Hsu, Ming-ta; Arnold, James O. (Technical Monitor)

    1999-01-01

    Aerogels are exotic materials having superior thermal and physical properties with great potential for both space and industrial uses. Although aerogels are excellent low-density insulators with unique acoustic and optical properties, their commercialization potential is currently limited by moisture absorption, fragility, and cost. This paper describes useful, easily scaled-up solutions to the first two of these three problems. The waterproofing and water-repellent method described here is a cheaper and simpler improvement over previous permanent methods.

  10. Flexible Polyimide Aerogel Cross-linked by Poly(maleic Anhydride-alt-alkylene)

    NASA Technical Reports Server (NTRS)

    Guo, Haiquan; Meador, Mary Ann B.; Wilkewitz, Brittany Marie

    2014-01-01

    Aerogels are potential materials for aerospace applications due to their lower thermal conductivity, lighter weight, and low dielectric constant. However, silica aerogels are restricted due to their inherent fragility, hygroscopic nature, and poor mechanical properties, especially in extreme aerospace environments. In order to fit the needs of aerospace applications, developing new thermal insulation materials that are flexible, and moisture resistant is needed. To this end, we fabricated a series of polyimide aerogels crosslinked with different poly(maleic anhydride-alt-alkylene)s as seen in Scheme 1. The polyimide oligomers were made with 3,3,4,4-biphenyltetracarboxylic dianhydride (BPDA), and different diamines or diamine combinations. The resulting aerogels have low density (0.06 gcm3 to 0.16 gcm3) and high surface area (240-440 m2g). The effect of the different backbone structures on density, shrinkage, porosity, surface area, mechanical properties, moisture resistance and thermal properties will be discussed. These novel polyalkylene-imide aerogels may be potential candidates for applications such as space suit insulation for planetary surface missions, insulation for inflatable structures for habitats, inflatable aerodynamic decelerators for entry, descent and landing (EDL) operations, and cryotank insulation for advance space propulsion systems. Scheme 1. Network of polyimide aerogels crosslinked with deifferent poly(maleic anhydride).

  11. Thermal Conductivity Measurements on consolidated Soil Analogs

    NASA Astrophysics Data System (ADS)

    Seiferlin, K.; Heimberg, M.; Thomas, N.

    2007-08-01

    Heat transport in porous media such as soils and regolith is significantly reduced compared to the properties of compact samples of the same material. The bottle neck for solid state heat transport is the contact area between adjacent grains. For "dry" and unconsolidated materials the contact areas and thus the thermal conductivity are extremely small. Sintering and cementation are two processes that can increase the cross section of interstitial bonds signifcantly. On Mars, cementation can be caused by condensation of water or carbon dioxide ice from the vapor phase, or from salts and minerals that fall out from aqueous solutions. We produced several artificially cemented samples, using small glass beads of uniform size as soil analog. The cementation is achieved by initially molten wax that is mixed with the glass beads while liqiud. The wax freezes preferably at the contact points between grains, thus minimizing surface energy, and consolidates the samples. The thermal conductivity of these samples is then measured in vacuum. We present the results of these measurements and compare them with theoretical models. The observed range of thermal conductivity values can explain some, but not all of the variations in thermal intertia that can be seen in TES remote sensing data.

  12. The Apparent Thermal Conductivity of Pozzolana Concrete

    NASA Astrophysics Data System (ADS)

    Bessenouci, M. Z.; Triki, N. E. Bibi; Khelladi, S.; Draoui, B.; Abene, A.

    The recent development of some lightweight construction materials, such as light concrete, can play an important role as an insulator, while maintaining sufficient levels of mechanical performance. The quality of insulation to provide depends on the climate, the exposure of the walls and also the materials used in the construction. The choice of a material to be used as an insulator, obviously, depends on its availability and its cost. This is a study of natural pozzolanas as basic components in building materials. It is intended to highlight their thermal advantage. It is economically advantageous to use pozzolana in substitution for a portion of the clinker as hydraulically active additions, as well as in compositions of lightweight concretes in the form of pozzolanic aggregate mixtures, which provide mechanical strengths that comply with current standards. A theoretical study is conducted on the apparent thermal conductivity of building materials, namely concrete containing pozzolana. Thermal modeling, apparent to that commonly used for porous materials, has been applied to pozzolana concrete. Experimental results on measurements of the apparent thermal conductivity of pozzolana concrete are reported in this study, using an approach that considers that concrete is composed of two solid ingredients, a binding matrix (hydrated cement paste) and all aggregates. A second comparative theoretical approach is used for the case where concrete consists of a solid phase and a fluid phase (air).

  13. Universal thermal and electrical conductivity from holography

    NASA Astrophysics Data System (ADS)

    Jain, Sachin

    2010-11-01

    It is known from earlier work of Iqbal, Liu [1] that the boundary transport coefficients such as electrical conductivity (at vanishing chemical potential), shear viscosity etc. at low frequency and finite temperature can be expressed in terms of geometrical quantities evaluated at the horizon. In the case of electrical conductivity, at zero chemical potential gauge field fluctuation and metric fluctuation decouples, resulting in a trivial flow from horizon to boundary. In the presence of chemical potential, the story becomes complicated due to the fact that gauge field and metric fluctuation can no longer be decoupled. This results in a nontrivial flow from horizon to boundary. Though horizon conductivity can be expressed in terms of geometrical quantities evaluated at the horizon, there exist no such neat result for electrical conductivity at the boundary. In this paper we propose an expression for boundary conductivity expressed in terms of geometrical quantities evaluated at the horizon and thermodynamic quantities. We also consider the theory at finite cutoff recently constructed in [2], at radius r c outside the horizon and give an expression for cutoff dependent electrical conductivity ( σ( r c )), which interpolates smoothly between horizon conductivity σ H ( r c → r h ) and boundary conductivity σ B ( r c → ∞). Using the results about the conductivity we gain much insight into the universality of thermal conductivity to viscosity ratio proposed in [3].

  14. Inhomogeneous thermal conductivity enhances thermoelectric cooling

    NASA Astrophysics Data System (ADS)

    Lu, Tingyu; Zhou, Jun; Li, Nianbei; Yang, Ronggui; Li, Baowen

    2014-12-01

    We theoretically investigate the enhancement of thermoelectric cooling performance in thermoelectric refrigerators made of materials with inhomogeneous thermal conductivity, beyond the usual practice of enhancing thermoelectric figure of merit (ZT) of materials. The dissipation of the Joule heat in such thermoelectric refrigerators is asymmetric which can give rise to better thermoelectric cooling performance. Although the thermoelectric figure of merit and the coefficient-of-performance are slightly enhanced, both the maximum cooling power and the maximum cooling temperature difference can be enhanced significantly. This finding can be used to increase the heat absorption at the cold end. We further find that the asymmetric dissipation of Joule heat leads to thermal rectification.

  15. Thermal conductivity of lower-mantle minerals

    NASA Astrophysics Data System (ADS)

    Goncharov, Alexander F.; Beck, Pierre; Struzhkin, Viktor V.; Haugen, Benjamin D.; Jacobsen, Steven D.

    2009-05-01

    Geodynamic models of heat transport and the thermal evolution of Earth's interior require knowledge of thermal conductivity for high-pressure phases at relevant temperatures and pressures. Here we present new data on radiative and lattice heat transfer in mantle materials determined from optical spectroscopy and time-resolved optical radiometry. The pressure dependence of optical absorption in ferropericlase (Mg,Fe)O, and silicate perovskite (Mg,Fe)SiO 3, has been determined in the IR through UV regions up to 133 GPa. Whereas (Mg,Fe)O exhibits a strong pressure dependence of absorption and spectral changes associated with the high-spin (HS) to low-spin (LS) transition of Fe 2+ [Goncharov, A.F., Struzhkin, V.V., Jacobsen, S.D. 2006. Reduced radiative conductivity of low-spin (Mg,Fe)O in the lower mantle. Science 312, 1205-1208], the pressure dependence of optical absorption in (Mg,Fe)SiO 3 is relatively weak. We observe a moderate increase in absorption with pressure for (Mg,Fe)SiO 3 in the visible and infrared spectral range due to a red-shift of absorption in ultraviolet, however the crystal-field transitions of Fe 2+ become weaker with pressure and disappear above 50 GPa as a result of the HS-LS transition in (Mg,Fe)SiO 3. Intervalence charge-transfer transitions in silicate perovskite shift to higher energies with pressure. The temperature dependence of the optical absorption of (Mg,Fe)O measured up to 65 GPa and 800 K is moderate below 30 GPa and weak above 30 GPa. Thus, the temperature correction of the radiative conductivity is insignificant. The estimated total pressure-dependent radiative conductivity (in approximation of a large grain size) is lower than expected from the pressure extrapolation of the ambient and low-pressure data [Hofmeister, A.M., 1999. Mantle values of thermal conductivity and the geotherm from phonon lifetimes. Science 283, 1699-1706; Hofmeister, A.M., 2005. Dependence of diffusive radiative transfer on grain-size, temperature, and Fe

  16. Improved Silica Aerogel Composite Materials

    NASA Technical Reports Server (NTRS)

    Paik, Jong-Ah; Sakamoto, Jeffrey; Jones, Steven

    2008-01-01

    A family of aerogel-matrix composite materials having thermal-stability and mechanical- integrity properties better than those of neat aerogels has been developed. Aerogels are known to be excellent thermal- and acoustic-insulation materials because of their molecular-scale porosity, but heretofore, the use of aerogels has been inhibited by two factors: (1) Their brittleness makes processing and handling difficult. (2) They shrink during production and shrink more when heated to high temperatures during use. The shrinkage and the consequent cracking make it difficult to use them to encapsulate objects in thermal-insulation materials. The underlying concept of aerogel-matrix composites is not new; the novelty of the present family of materials lies in formulations and processes that result in superior properties, which include (1) much less shrinkage during a supercritical-drying process employed in producing a typical aerogel, (2) much less shrinkage during exposure to high temperatures, and (3) as a result of the reduction in shrinkage, much less or even no cracking.

  17. Thermal effects in microfluidics with thermal conductivity spatially modulated

    NASA Astrophysics Data System (ADS)

    Vargas Toro, Agustín.

    2014-05-01

    A heat transfer model on a microfluidic is resolved analytically. The model describes a fluid at rest between two parallel plates where each plate is maintained at a differentially specified temperature and the thermal conductivity of the microfluidic is spatially modulated. The heat transfer model in such micro-hydrostatic configuration is analytically resolved using the technique of the Laplace transform applying the Bromwich Integral and the Residue theorem. The temperature outline in the microfluidic is presented as an infinite series of Bessel functions. It is shown that the result for the thermal conductivity spatially modulated has as a particular case the solution when the thermal conductivity is spatially constant. All computations were performed using the computer algebra software Maple. It is claimed that the analytical obtained results are important for the design of nanoscale devices with applications in biotechnology. Furthermore, it is suggested some future research lines such as the study of the heat transfer model in a microfluidic resting between coaxial cylinders with radially modulated thermal conductivity in order to achieve future developments in this area.

  18. Graphene oxide/cellulose aerogels nanocomposite: Preparation, pyrolysis, and application for electromagnetic interference shielding.

    PubMed

    Wan, Caichao; Li, Jian

    2016-10-01

    Hybrid aerogels consisting of graphene oxide (GO) and cellulose were prepared via a solution mixing-regeneration-freeze drying process. The presence of GO affected the micromorphology of the hybrid aerogels, and a self-assembly behavior of cellulose was observed after the incorporation of GO. Moreover, there is no remarkable modification in the crystallinity index and thermal stability after the insertion of GO. After the reduction of GO in the hybrid aerogels by l-ascorbic acid and the subsequent pyrolysis of the aerogels, the resultant displays some interesting characteristics, including good electromagnetic interference (EMI) shielding capacity (SEtotal=58.4dB), high electrical conductivity (19.1Sm(-1)), hydrophobicity, and fire resistance, which provide an opportunity for some advanced applications such as EMI protection, electrochemical devices, water-proofing agents, and fire retardants. Moreover, this work possibly helps to facilitate the development of both cellulose and GO-based materials and expand their application scope. PMID:27312627

  19. Thermal Boundary Conductance: A Materials Science Perspective

    NASA Astrophysics Data System (ADS)

    Monachon, Christian; Weber, Ludger; Dames, Chris

    2016-07-01

    The thermal boundary conductance (TBC) of materials pairs in atomically intimate contact is reviewed as a practical guide for materials scientists. First, analytical and computational models of TBC are reviewed. Five measurement methods are then compared in terms of their sensitivity to TBC: the 3ω method, frequency- and time-domain thermoreflectance, the cut-bar method, and a composite effective thermal conductivity method. The heart of the review surveys 30 years of TBC measurements around room temperature, highlighting the materials science factors experimentally proven to influence TBC. These factors include the bulk dispersion relations, acoustic contrast, and interfacial chemistry and bonding. The measured TBCs are compared across a wide range of materials systems by using the maximum transmission limit, which with an attenuated transmission coefficient proves to be a good guideline for most clean, strongly bonded interfaces. Finally, opportunities for future research are discussed.

  20. Senftleben-Beenakker thermal conductivity effect apparatus.

    PubMed

    Vevai, J E; Elliot, D G; Honeywell, W I

    1978-08-01

    An apparatus for measuring the influence of magnetic fields on the thermal conductivity of gases, the Senftleben-Beenakker effect, is described. Conventional ''cold finger type'' cooling with appropriate cryogenic fluids is utilized with a helium exchange gas chamber to establish the general temperature level of the cell holder. Automatic temperature control of the trim heater maintains the desired temperature to +/-10(-3) degrees C in the 77-300 K range. A concentric cylinder cell adaptable for use with either conventional electromagnet or superconducting solenoid is described. The use of thin Mylar polyester film end seals minimizes cell ''end effect'' corrections and provides a resolution of 5x10(-6) in gas thermal conductivity. PMID:18699265

  1. The contact area dependent interfacial thermal conductance

    SciTech Connect

    Liu, Chenhan; Wei, Zhiyong; Bi, Kedong; Yang, Juekuan; Chen, Yunfei; Wang, Jian

    2015-12-15

    The effects of the contact area on the interfacial thermal conductance σ are investigated using the atomic Green’s function method. Different from the prediction of the heat diffusion transport model, we obtain an interesting result that the interfacial thermal conductance per unit area Λ is positively dependent on the contact area as the area varies from a few atoms to several square nanometers. Through calculating the phonon transmission function, it is uncovered that the phonon transmission per unit area increases with the increased contact area. This is attributed to that each atom has more neighboring atoms in the counterpart of the interface with the increased contact area, which provides more channels for phonon transport.

  2. Ultralow Thermal Conductivity in Full Heusler Semiconductors.

    PubMed

    He, Jiangang; Amsler, Maximilian; Xia, Yi; Naghavi, S Shahab; Hegde, Vinay I; Hao, Shiqiang; Goedecker, Stefan; Ozoliņš, Vidvuds; Wolverton, Chris

    2016-07-22

    Semiconducting half and, to a lesser extent, full Heusler compounds are promising thermoelectric materials due to their compelling electronic properties with large power factors. However, intrinsically high thermal conductivity resulting in a limited thermoelectric efficiency has so far impeded their widespread use in practical applications. Here, we report the computational discovery of a class of hitherto unknown stable semiconducting full Heusler compounds with ten valence electrons (X_{2}YZ, X=Ca, Sr, and Ba; Y=Au and Hg; Z=Sn, Pb, As, Sb, and Bi) through high-throughput ab initio screening. These new compounds exhibit ultralow lattice thermal conductivity κ_{L} close to the theoretical minimum due to strong anharmonic rattling of the heavy noble metals, while preserving high power factors, thus resulting in excellent phonon-glass electron-crystal materials. PMID:27494488

  3. Ultralow Thermal Conductivity in Full Heusler Semiconductors

    NASA Astrophysics Data System (ADS)

    He, Jiangang; Amsler, Maximilian; Xia, Yi; Naghavi, S. Shahab; Hegde, Vinay I.; Hao, Shiqiang; Goedecker, Stefan; OzoliĆš, Vidvuds; Wolverton, Chris

    2016-07-01

    Semiconducting half and, to a lesser extent, full Heusler compounds are promising thermoelectric materials due to their compelling electronic properties with large power factors. However, intrinsically high thermal conductivity resulting in a limited thermoelectric efficiency has so far impeded their widespread use in practical applications. Here, we report the computational discovery of a class of hitherto unknown stable semiconducting full Heusler compounds with ten valence electrons (X2Y Z , X =Ca , Sr, and Ba; Y =Au and Hg; Z =Sn , Pb, As, Sb, and Bi) through high-throughput ab initio screening. These new compounds exhibit ultralow lattice thermal conductivity κL close to the theoretical minimum due to strong anharmonic rattling of the heavy noble metals, while preserving high power factors, thus resulting in excellent phonon-glass electron-crystal materials.

  4. A low temperature thermal conductivity database

    NASA Astrophysics Data System (ADS)

    Woodcraft, Adam L.; Gray, Adam

    2009-12-01

    Low temperature detectors necessarily require low temperature instruments. Constructing good low temperature instruments requires information on the properties of materials used in their construction, in particular the thermal conductivity. Unfortunately, this is poorly known for many materials. Collections of data in text books tend to be incomplete and in the worst cases are misleading. For most materials, what information is known is scattered through the literature. Searching out this data is time consuming, and in any case often results in conflicting information. We have started a programme to locate, consolidate and critically analyse thermal conductivity measurements from the literature, particularly for the challenging temperature range below 1 K. This has already produced useful results. We present some preliminary results here.

  5. Multiscale Modeling of UHTC: Thermal Conductivity

    NASA Technical Reports Server (NTRS)

    Lawson, John W.; Murry, Daw; Squire, Thomas; Bauschlicher, Charles W.

    2012-01-01

    We are developing a multiscale framework in computational modeling for the ultra high temperature ceramics (UHTC) ZrB2 and HfB2. These materials are characterized by high melting point, good strength, and reasonable oxidation resistance. They are candidate materials for a number of applications in extreme environments including sharp leading edges of hypersonic aircraft. In particular, we used a combination of ab initio methods, atomistic simulations and continuum computations to obtain insights into fundamental properties of these materials. Ab initio methods were used to compute basic structural, mechanical and thermal properties. From these results, a database was constructed to fit a Tersoff style interatomic potential suitable for atomistic simulations. These potentials were used to evaluate the lattice thermal conductivity of single crystals and the thermal resistance of simple grain boundaries. Finite element method (FEM) computations using atomistic results as inputs were performed with meshes constructed on SEM images thereby modeling the realistic microstructure. These continuum computations showed the reduction in thermal conductivity due to the grain boundary network.

  6. Thermally activated conductivity in gapped bilayer graphene

    NASA Astrophysics Data System (ADS)

    Trushin, Maxim

    2012-05-01

    This is a theoretical study of electron transport in gated bilayer graphene —a novel semiconducting material with a tunable band gap. It is shown that the which-layer pseudospin coherence enhances the subgap conductivity and facilitates the thermally activated transport. The mechanism proposed can also lead to the non-monotonic conductivity vs. temperature dependence at a band gap size of the order of 10 meV. The effect can be observed in gapped bilayer graphene sandwiched in boron nitride where the electron-hole puddles and flexural phonons are strongly suppressed.

  7. Design principles of interfacial thermal conductance

    NASA Astrophysics Data System (ADS)

    Polanco, Carlos; Rastgarkafshgarkolaei, Rouzbeh; Zhang, Jingjie; Le, Nam; Norris, Pamela; Ghosh, Avik

    We explore fundamental principles to design the thermal conductance across solid interfaces by changing the composition and disorder of an intermediate matching layer. In absence of phonon-phonon interactions, the layer addition involves two competing effects that influence the conductance. The layer can act as an impedance matching 'bridge' to increase the mode-averaged phonon transmission. However, it also reduces the relevant modes that conserve their momenta transverse to the interface, so that the net result depends on features such as the overlap of conserving modes and the dispersivity of the transverse subbands. Moving into the interacting anharmonic regime, we find that the added layer aids conductance when the decreased resistances at the contact-layer boundaries compensate for the layer resistance. In fact, we show that the maximum conductance corresponds to an exact matching of the two separate contact-layer resistances. For instance, if we vary just the atomic mass across layers, then maximum conductance happens when the intervening layer mass is the geometric mean of the contact masses. We conjecture that the best interfacial layer is one that is compositionally graded into many geometric means - in other words, an exponential variation in thermal impedance.

  8. X-Aerogels for Structural Components and High Temperature Applications

    NASA Technical Reports Server (NTRS)

    2005-01-01

    Future NASA missions and space explorations rely on the use of materials that are strong ultra lightweight and able to withstand extreme temperatures. Aerogels are low density (0.01-0.5 g/cu cm) high porosity materials that contain a glass like structure formed through standard sol-gel chemistry. As a result of these structural properties, aerogels are excellent thermal insulators and are able to withstand temperatures in excess of l,000 C. The open structure of aerogels, however, renders these materials extremely fragile (fracturing at stress forces less than 0.5 N/sq cm). The goal of NASA Glenn Research Center is to increase the strength of these materials by templating polymers and metals onto the surface of an aerogel network facilitating the use of this material for practical applications such as structural components of space vehicles used in exploration. The work this past year focused on two areas; (1) the research and development of new templated aerogels materials and (2) process development for future manufacturing of structural components. Research and development occurred on the production and characterization of new templating materials onto the standard silica aerogel. Materials examined included polymers such as polyimides, fluorinated isocyanates and epoxies, and, metals such as silver, gold and platinum. The final properties indicated that the density of the material formed using an isocyanate is around 0.50 g/cc with a strength greater than that of steel and has low thermal conductivity. The process used to construct these materials is extremely time consuming and labor intensive. One aspect of the project involved investigating the feasibility of shortening the process time by preparing the aerogels in the templating solvent. Traditionally the polymerization used THF as the solvent and after several washes to remove any residual monomers and water, the solvent around the aerogels was changed to acetonitrile for the templating step. This process

  9. THERMAL CONDUCTIVITY OF THE POTENTIAL REPOSITORY HORIZON

    SciTech Connect

    J.E. BEAN

    2004-09-27

    The primary purpose of this report is to assess the spatial variability and uncertainty of bulk thermal conductivity in the host horizon for the repository at Yucca Mountain. More specifically, the lithostratigraphic units studied are located within the Topopah Spring Tuff (Tpt) and consist of the upper lithophysal zone (Tptpul), the middle nonlithophysal zone (Tptpmn), the lower lithophysal zone (Tptpll), and the lower nonlithophysal zone (Tptpln). Design plans indicate that approximately 81 percent of the repository will be excavated in the Tptpll, approximately 12 percent in the Tptpmn, and the remainder in the Tptul and Tptpln (BSC 2004 [DIRS 168370]). This report provides three-dimensional geostatistical estimates of the bulk thermal conductivity for the four stratigraphic layers of the repository horizon. The three-dimensional geostatistical estimates of matrix and lithophysal porosity, dry bulk density, and matrix thermal conductivity are also provided. This report provides input to various models and calculations that simulate heat transport through the rock mass. These models include the ''Drift Degradation Analysis, Multiscale Thermohydrologic Model, Ventilation Model and Analysis Report, Igneous Intrusion Impacts on Waste Packages and Waste Forms, Drift-Scale Coupled Processes (DST and TH Seepage) Models'', and ''Drift Scale THM Model''. These models directly or indirectly provide input to the total system performance assessment (TSPA). The main distinguishing characteristic among the lithophysal and nonlithophysal units is the percentage of large-scale (centimeters-meters) voids within the rock. The Tptpul and Tptpll, as their names suggest, have a higher percentage of lithophysae than the Tptpmn and the Tptpln. Understanding the influence of the lithophysae is of great importance to understanding bulk thermal conductivity.

  10. Mechanisms for thermal conduction in hydrogen hydrate

    NASA Astrophysics Data System (ADS)

    English, Niall J.; Gorman, Paul D.; MacElroy, J. M. D.

    2012-01-01

    Extensive equilibrium molecular dynamics simulations have been performed to investigate thermal conduction mechanisms via the Green-Kubo approach for (type II) hydrogen hydrate, at 0.05 kbar and between 30 and 250 K, for both lightly filled H2 hydrates (1s4l) and for more densely filled H2 systems (2s4l), in which four H2 molecules are present in the large cavities, with respective single- and double-occupation of the small cages. The TIP4P water model was used in conjunction with a fully atomistic hydrogen potential along with long-range Ewald electrostatics. It was found that substantially less damping in guest-host energy transfer is present in hydrogen hydrate as is observed in common type I clathrates (e.g., methane hydrate), but more akin in to previous results for type II and H methane hydrate polymorphs. This gives rise to larger thermal conductivities relative to common type I hydrates, and also larger than type II and H methane hydrate polymorphs, and a more crystal-like temperature dependence of the thermal conductivity.

  11. Computer Simulation of Fracture in Aerogels

    NASA Technical Reports Server (NTRS)

    Good, Brian S.

    2006-01-01

    Aerogels are of interest to the aerospace community primarily for their thermal properties, notably their low thermal conductivities. While the gels are typically fragile, recent advances in the application of conformal polymer layers to these gels has made them potentially useful as lightweight structural materials as well. In this work, we investigate the strength and fracture behavior of silica aerogels using a molecular statics-based computer simulation technique. The gels' structure is simulated via a Diffusion Limited Cluster Aggregation (DLCA) algorithm, which produces fractal structures representing experimentally observed aggregates of so-called secondary particles, themselves composed of amorphous silica primary particles an order of magnitude smaller. We have performed multi-length-scale simulations of fracture in silica aerogels, in which the interaction b e e n two secondary particles is assumed to be described by a Morse pair potential parameterized such that the potential range is much smaller than the secondary particle size. These Morse parameters are obtained by atomistic simulation of models of the experimentally-observed amorphous silica "bridges," with the fracture behavior of these bridges modeled via molecular statics using a Morse/Coulomb potential for silica. We consider the energetics of the fracture, and compare qualitative features of low-and high-density gel fracture.

  12. Evaluation of New Thermally Conductive Geopolymer in Thermal Energy Storage

    NASA Astrophysics Data System (ADS)

    Černý, Matěj; Uhlík, Jan; Nosek, Jaroslav; Lachman, Vladimír; Hladký, Radim; Franěk, Jan; Brož, Milan

    This paper describes an evaluation of a newly developed thermally conductive geopolymer (TCG), consisting of a mixture of sodium silicate and carbon micro-particles. The TCG is intended to be used as a component of high temperature energy storage (HTTES) to improve its thermal diffusivity. Energy storage is crucial for both ecological and economical sustainability. HTTES plays a vital role in solar energy technologies and in waste heat recovery. The most advanced HTTES technologies are based on phase change materials or molten salts, but suffer with economic and technological limitations. Rock or concrete HTTES are cheaper, but they have low thermal conductivity without incorporation of TCG. It was observed that TCG is stable up to 400 °C. The thermal conductivity was measured in range of 20-23 W m-1 K-1. The effect of TCG was tested by heating a granite block with an artificial fissure. One half of the fissure was filled with TCG and the other with ballotini. 28 thermometers, 5 dilatometers and strain sensors were installed on the block. The heat transport experiment was evaluated with COMSOL Multiphysics software.

  13. Analytical estimation of skeleton thermal conductivity of a geopolymer foam from thermal conductivity measurements

    NASA Astrophysics Data System (ADS)

    Henon, J.; Alzina, A.; Absi, J.; Smith, D. S.; Rossignol, S.

    2015-07-01

    The geopolymers are alumino-silicate binders. The addition of a high pores volume fraction, gives them a thermal insulation character desired in the building industry. In this work, potassium geopolymer foams were prepared at room temperature (< 70 ∘C) by a process of in situ gas release. The porosity distribution shows a multiscale character. However, the thermal conductivity measurements gave values from 0.35 to 0.12 Wm-1.K-1 for a pore volume fraction values between 65 and 85%. In the aim to predict the thermal properties of these foams and focus on the relationship "thermal-conductivity/microstructure", knowledge of the thermal conductivity of their solid skeleton (λ s ) is paramount. However, there is rare work on the determination of this value depending on the initial composition. By the formulation used, the foaming agent contributes to the final network, and it is not possible to obtain a dense material designate to make a direct measurement of λ s . The objective of this work is to use inverse analytical methods to identify the value of λ s . Measurements of thermal conductivity by the fluxmetre technique were performed. The obtained value of the solid skeleton thermal conductivity by the inverse numerical technique is situated in a framework between 0.95 and 1.35 Wm-1.K-1 and is in agreement with one issue from the literature.

  14. Aerogel: Tile Composites Toughen a Brittle Superinsulation

    NASA Technical Reports Server (NTRS)

    White, Susan; Rasky, Daniel; Arnold, James O. (Technical Monitor)

    1998-01-01

    Pure aerogels, though familiar in the laboratory for decades as exotic lightweight insulators with unusual physical properties, have had limited industrial applications due to their low strength and high brittleness. Composites formed of aerogels and the ceramic fiber matrices like those used as space shuttle tiles bypass the fragility of pure aerogels and can enhance the performance of space shuttle tiles in their harsh operating environment. Using a layer of aerogel embedded in a tile may open up a wide range of applications where thermal insulation, gas convection control and mechanical strength matter.

  15. Thermal radiation properties and thermal conductivity of lunar material.

    PubMed

    Birkebak, R C; Cremers, C J; Dawson, J P

    1970-01-30

    The thermal radiation properties were measured for lunar fines and chips from three different lunar rocks. Measurements for the fines were made at atmospheric pressure and at a pressure of 10(-5) torr or lower. The directional reflectance was obtained over a wavelength range of 0.5 to 2.0 microns for angles of incidence up to 60 degrees. The bidirectional reflectance-the distribution of reflected light-was measured for white light angles of illumination up to 60 degrees. The thermal conductivity was measured over a temperature range 200 to 400 degrees K under vacuum conditions. PMID:17781563

  16. Thermal Expansion and Thermal Conductivity of Rare Earth Silicates

    NASA Technical Reports Server (NTRS)

    Zhu, Dongming; Lee, Kang N.; Bansal, Narottam P.

    2006-01-01

    Rare earth silicates are considered promising candidate materials for environmental barrier coatings applications at elevated temperature for ceramic matrix composites. High temperature thermophysical properties are of great importance for coating system design and development. In this study, the thermal expansion and thermal conductivity of hot-pressed rare earth silicate materials were characterized at temperatures up to 1400 C. The effects of specimen porosity, composition and microstructure on the properties were also investigated. The materials processing and testing issues affecting the measurements will also be discussed.

  17. Method for producing metal oxide aerogels

    DOEpatents

    Tillotson, T.M.; Poco, J.F.; Hrubesh, L.W.; Thomas, I.M.

    1995-04-25

    A two-step hydrolysis-condensation method was developed to form metal oxide aerogels of any density, including densities of less than 0.003g/cm{sup 3} and greater than 0.27g/cm{sup 3}. High purity metal alkoxide is reacted with water, alcohol solvent, and an additive to form a partially condensed metal intermediate. All solvent and reaction-generated alcohol is removed, and the intermediate is diluted with a nonalcoholic solvent. The intermediate can be stored for future use to make aerogels of any density. The aerogels are formed by reacting the intermediate with water, nonalcoholic solvent, and a catalyst, and extracting the nonalcoholic solvent directly. The resulting monolithic aerogels are hydrophobic and stable under atmospheric conditions, and exhibit good optical transparency, high clarity, and homogeneity. The aerogels have high thermal insulation capacity, high porosity, mechanical strength and stability, and require shorter gelation times than aerogels formed by conventional methods. 8 figs.

  18. Method for producing metal oxide aerogels

    DOEpatents

    Tillotson, Thomas M.; Poco, John F.; Hrubesh, Lawrence W.; Thomas, Ian M.

    1995-01-01

    A two-step hydrolysis-condensation method was developed to form metal oxide aerogels of any density, including densities of less than 0.003g/cm.sup.3 and greater than 0.27g/cm.sup.3. High purity metal alkoxide is reacted with water, alcohol solvent, and an additive to form a partially condensed metal intermediate. All solvent and reaction-generated alcohol is removed, and the intermediate is diluted with a nonalcoholic solvent. The intermediate can be stored for future use to make aerogels of any density. The aerogels are formed by reacting the intermediate with water, nonalcoholic solvent, and a catalyst, and extracting the nonalcoholic solvent directly. The resulting monolithic aerogels are hydrophobic and stable under atmospheric conditions, and exhibit good optical transparency, high clarity, and homogeneity. The aerogels have high thermal insulation capacity, high porosity, mechanical strength and stability, and require shorter gelation times than aerogels formed by conventional methods.

  19. Thermal contact conductance of adhered microcantilevers

    NASA Astrophysics Data System (ADS)

    Huxtable, Scott T.; Cahill, David G.; Phinney, Leslie M.

    2004-02-01

    The thermal contact conductance G for polycrystalline silicon cantilever beams that are adhered to an underlying substrate is examined using two different optical techniques. Using time-domain thermoreflectance, we measure G=9±2 MW m-2 K-1 at 25 °C and G=4±1 MW m-2 K-1 at 150 °C. The room temperature value is confirmed using a modified Ångström method, which establishes a lower limit of G>5 MW m-2 K-1. This contact conductance is a factor of 10-105 greater than values reported for metal-metal and ceramic-ceramic interfaces. The large interfacial conductance is consistent with the presence of a thin layer of water trapped between the cantilever and the substrate. The thermal conductivity Λ of the phosphorus doped polysilicon cantilever is nearly isotropic with Λcrossplane=65 W m-1 K-1, and Λin plane=70 W m-1 K-1 at room temperature.

  20. Thermal conductivities of thin, sputtered optical films

    SciTech Connect

    Henager, C.H. Jr.; Pawlewicz, W.T.

    1991-05-01

    The normal component of the thin film thermal conductivity has been measured for the first time for several advanced sputtered optical materials. Included are data for single layers of boron nitride (BN), aluminum nitride (AIN), silicon aluminum nitride (Si-Al-N), silicon aluminum oxynitride (Si-Al-O-N), silicon carbide (SiC), and for dielectric-enhanced metal reflectors of the form Al(SiO{sub 2}/Si{sub 3}N{sub 4}){sup n} and Al(Al{sub 2}O{sub 3}/AIN){sup n}. Sputtered films of more conventional materials like SiO{sub 2}, Al{sub 2}O{sub 3}, Ta{sub 2}O{sub 5}, Ti, and Si have also been measured. The data show that thin film thermal conductivities are typically 10 to 100 times lower than conductivities for the same materials in bulk form. Structural disorder in the amorphous or very fine-grained films appears to account for most of the conductivity difference. Conclusive evidence for a film/substrate interface contribution is presented.

  1. Effective thermal conductivity of a thin composite material

    SciTech Connect

    Phelan, P.E.; Niemann, R.C.

    1996-12-31

    The thermal conductivity of a randomly oriented composite material is modeled using a probabilistic approach in order to determine if a size effect exists for the thermal conductivity at small composite thickness. The numerical scheme employs a random number generator to position the filler elements, which have a relatively high thermal conductivity, within a matrix having a relatively low thermal conductivity. Results indicate that, below some threshold thickness, the composite thermal conductivity increases with decreasing thickness, while above the threshold the thermal conductivity is independent of thickness. The threshold thickness increases for increasing filler fraction and increasing k{sub f}/k{sub m}, the ratio between filler and matrix thermal conductivities.

  2. Treating Fibrous Insulation to Reduce Thermal Conductivity

    NASA Technical Reports Server (NTRS)

    Zinn, Alfred; Tarkanian, Ryan

    2009-01-01

    A chemical treatment reduces the convective and radiative contributions to the effective thermal conductivity of porous fibrous thermal-insulation tile. The net effect of the treatment is to coat the surfaces of fibers with a mixture of transition-metal oxides (TMOs) without filling the pores. The TMO coats reduce the cross-sectional areas available for convection while absorbing and scattering thermal radiation in the pores, thereby rendering the tile largely opaque to thermal radiation. The treatment involves a sol-gel process: A solution containing a mixture of transition-metal-oxide-precursor salts plus a gelling agent (e.g., tetraethylorthosilicate) is partially cured, then, before it visibly gels, is used to impregnate the tile. The solution in the tile is gelled, then dried, and then the tile is fired to convert the precursor salts to the desired mixed TMO phases. The amounts of the various TMOs ultimately incorporated into the tile can be tailored via the concentrations of salts in the solution, and the impregnation depth can be tailored via the viscosity of the solution and/or the volume of the solution relative to that of the tile. The amounts of the TMOs determine the absorption and scattering spectra.

  3. Measuring Thermal Conductivity at LH2 Temperatures

    NASA Technical Reports Server (NTRS)

    Selvidge, Shawn; Watwood, Michael C.

    2004-01-01

    For many years, the National Institute of Standards and Technology (NIST) produced reference materials for materials testing. One such reference material was intended for use with a guarded hot plate apparatus designed to meet the requirements of ASTM C177-97, "Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus." This apparatus can be used to test materials in various gaseous environments from atmospheric pressure to a vacuum. It allows the thermal transmission properties of insulating materials to be measured from just above ambient temperature down to temperatures below liquid hydrogen. However, NIST did not generate data below 77 K temperature for the reference material in question. This paper describes a test method used at NASA's Marshall Space Flight Center (MSFC) to optimize thermal conductivity measurements during the development of thermal protection systems. The test method extends the usability range of this reference material by generating data at temperatures lower than 77 K. Information provided by this test is discussed, as are the capabilities of the MSFC Hydrogen Test Facility, where advanced methods for materials testing are routinely developed and optimized in support of aerospace applications.

  4. Sorption Properties of Aerogel in Liquid Nitrogen

    NASA Technical Reports Server (NTRS)

    Johnson, Wesley L.

    2006-01-01

    Aerogel products are now available as insulation materials of the future. The Cryogenics Test Laboratory at the NASA Kennedy Space Center is developing aerogel-based thermal insulation systems for space launch applications. Aerogel beads (Cabot Nanogel ) and aerogel blankets (Aspen Aerogels Spaceloft ) have outstanding ambient pressure thermal performance that makes them useful for applications where sealing is not possible. Aerogel beads are open-celled silicone dioxide and have tiny pores that run throughout the body of the bead. It has also recently been discovered that aerogel beads can be used as a filtering device for aqueous compounds at room temperature. With their hydrophobic covering, the beads absorb any non-polar substance and they can be chemically altered to absorb hot gases. The combination of the absorption and cryogenic insulating properties of aerogel beads have never been studied together. For future cryogenic insulation applications, it is crucial to know how the beads react while immersed in cryogenic liquids, most notably liquid nitrogen. Aerogel beads in loose-fill situation and aerogel blankets with composite fiber structure have been tested for absorption properties. Depending on the type of aerogel used and the preparation, preliminary results show the material can absorb up to seven times its own weight of liquid nitrogen, corresponding to a volumetric ratio of 0.70 (unit volume nitrogen per unit volume aerogel). These tests allow for an estimate on how much insulation is needed in certain situations. The theory behind the different processes of sorption is necessary for a better understanding of the preparation of the beads before they are used in an insulation system.

  5. 3D Printing of Graphene Aerogels.

    PubMed

    Zhang, Qiangqiang; Zhang, Feng; Medarametla, Sai Pradeep; Li, Hui; Zhou, Chi; Lin, Dong

    2016-04-01

    3D printing of a graphene aerogel with true 3D overhang structures is highlighted. The aerogel is fabricated by combining drop-on-demand 3D printing and freeze casting. The water-based GO ink is ejected and freeze-cast into designed 3D structures. The lightweight (<10 mg cm(-3) ) 3D printed graphene aerogel presents superelastic and high electrical conduction. PMID:26861680

  6. Thermal conductivity of hybrid short fiber composites

    SciTech Connect

    Dunn, M.L.; Taya, M.; Hatta, H.; Takei, T.; Nakajima, Y. Inst. of Space and Astronautical Science, Sagamihara Three-D Composites Research, Tsukuba Tohoku Univ., Sendai )

    1993-01-01

    A combined analytical/experimental study has been undertaken to investigate the effective thermal conductivity of hybrid composite materials. The analysis utilizes the equivalent inclusion approach for steady state heat conduction (Hatta and Taya, 1986) through which the interaction between the various reinforcing phases at finite concentrations is approximated by the Mori-Tanaka (1973) mean field approach. The multiple reinforcing phases of the composite are modeled as ellipsoidal in shape and thus can simulate a wide range of microstructural geometries ranging from thin platelet to continuous fiber reinforcement. The case when one phase of the composite is penny-shaped microcracks is studied in detail. Multiphase composites consisting of a Kerimid matrix and Al2O3 short fibers and Si3N4 whiskers were fabricated and, after a careful study of their microstructure, their thermal conductivities were measured. Analytical predictions are shown to be in good agreement with experimental results obtained for the Al2O3/Si3N4/Kerimid short fiber composites. 26 refs.

  7. Estimating stomatal conductance with thermal imagery.

    PubMed

    Leinonen, I; Grant, O M; Tagliavia, C P P; Chaves, M M; Jones, H G

    2006-08-01

    Most thermal methods for the study of drought responses in plant leaves are based on the calculation of 'stress indices'. This paper proposes and compares three main extensions of these for the direct estimation of absolute values of stomatal conductance to water vapour (gs) using infrared thermography (IRT). All methods use the measured leaf temperature and two environmental variables (air temperature and boundary layer resistance) as input. Additional variables required, depending on the method, are the temperatures of wet and dry reference surfaces, net radiation and relative humidity. The methods were compared using measured gs data from a vineyard in Southern Portugal. The errors in thermal estimates of conductance were of the same order as the measurement errors using a porometer. Observed variability was also compared with theoretical estimates of errors in estimated gs determined on the basis of the errors in the input variables (leaf temperature, boundary layer resistance, net radiation) and the partial derivatives of the energy balance equations used for the gs calculations. The full energy balance approach requires accurate estimates of net radiation absorbed, which may not be readily available in field conditions, so alternatives using reference surfaces are shown to have advantages. A new approach using a dry reference leaf is particularly robust and recommended for those studies where the specific advantages of thermal imagery, including its non-contact nature and its ability to sample large numbers of leaves, are most apparent. Although the results suggest that estimates of the absolute magnitude of gs are somewhat subjective, depending on the skill of the experimenter at selecting evenly exposed leaves, relative treatment differences in conductance are sensitively detected by different experimenters. PMID:16898014

  8. Thermal conductivity of partially ionized gas mixtures

    NASA Astrophysics Data System (ADS)

    Armaly, B. F.; Sutton, K.

    1981-06-01

    A method is proposed for predicting the translational component of the thermal conductivity of partially ionized gas mixtures. It is approximate but simple in form and offers a significant improvement over commonly utilized approximations. It does not require large computer run times nor storage, thus it is suitable for use with complex flow fields and heat transfer calculations. Results for gas mixtures which are representative of the atmosphere of Jupiter, Earth, and Venus are presented and they compare favorably with results from detailed kinetic theory analyses.

  9. Thermal contact conductance of nominaly flat surfaces

    NASA Astrophysics Data System (ADS)

    Yüncü, H.

    2006-11-01

    The variation of thermal conductance of contact has been investigated as a function of apparent contact pressure experimentally. Experimental data has been obtained for steel, brass, copper and aluminum test pieces having different surface roughness over a wide range of contact pressures. Experimental results are compared with the theoretical predictions of an existing theory. Comparison revealed good agreement of trend with the experimental data, however, numerical values vary widely. Theory can predict the experimental results within an over-all error of less than 35%.

  10. Measuring Contact Thermal Conductances at Low Temperatures

    NASA Technical Reports Server (NTRS)

    Salerno, Louis J.; Kittel, Peter; Brooks, Walter; Spivak, Alan L.; Marks, William G., Jr.

    1987-01-01

    Instrument measures thermal conductance of pressed contacts in liquid helium. Makes measurements automatically as function of force on pairs of brass samples having various surface finishes. Developed as part of effort to determine heat-transfer characteristics of bolted joints on cryogenically cooled focal planes in infrared equipment. Cylindrical chamber hangs from cover plate in bath of liquid helium. Inside chamber rocker arm applies controlled force to samples. Upper sample made slightly wider than lower one so two samples remain in complete contact even under slight lateral misalignment.

  11. Electronic thermal conductivity of suspended graphene

    SciTech Connect

    Begum, K. Rizwana Sankeshwar, N. S.

    2014-04-24

    Electronic thermal conductivity, κ{sub e}, of suspended graphene is studied for 20K 100K, becoming dominant for T > 250K. Good agreement with recent experimental data is obtained.

  12. On the theory of nonlinear thermal conductivity

    NASA Astrophysics Data System (ADS)

    Gladkov, S. O.; Bogdanova, S. B.

    2016-02-01

    A nonlinear differential equation of thermal conductivity is derived phenomenologically from the general principles of construction of functional Q invariant to the inversion operation I(r →-r), and the temperature evolution dynamics is analyzed in the nonstationary case. The proposed method makes it possible to reveal some general regularities in the physical behavior of such systems for describing irreversible phenomena in self-organization processes. It is noted that an analogous situation may take place, for example, in strongly inhomogeneous structures with stochastic internal heat fluxes.

  13. Effective thermal conductivity in thermoelectric materials

    SciTech Connect

    Baranowski, LL; Snyder, GJ; Toberer, ES

    2013-05-28

    Thermoelectric generators (TEGs) are solid state heat engines that generate electricity from a temperature gradient. Optimizing these devices for maximum power production can be difficult due to the many heat transport mechanisms occurring simultaneously within the TEG. In this paper, we develop a model for heat transport in thermoelectric materials in which an "effective thermal conductivity" (kappa(eff)) encompasses both the one dimensional steady-state Fourier conduction and the heat generation/consumption due to secondary thermoelectric effects. This model is especially powerful in that the value of kappa(eff) does not depend upon the operating conditions of the TEG but rather on the transport properties of the TE materials themselves. We analyze a variety of thermoelectric materials and generator designs using this concept and demonstrate that kappa(eff) predicts the heat fluxes within these devices to 5% of the exact value. (C) 2013 AIP Publishing LLC.

  14. Thermal conductance of metallic interface in vacuum

    SciTech Connect

    Mortazavi, P.; Shu, D.

    1985-10-01

    In most heat transfer applications, the deposited heat is transfered by any of the following classical methods: conduction, convection, radiation, or any combinations of these three. Depending on how critical the nature is of the designed equipment, the response time must be short enough in order to safeguard the proper performance of the devices. For instance, currently at the National Synchrotron Light Source (NSLS), various hardware equipment are being designed to intercept or to stop intense radiation beams induced by insertion devices such as wiggler and undulators. Due to the nature of some of these designs, the deposited high flux thermal load must be transferred across unbonded contact surfaces. Since any miscalculation would result in the disintegration of exposed material and therefore cause substantial problems, a true actual conductance measurement of the material in question is highly desirable. In the following three sections, background summary, the method of measurement, and the obtained results are discussed.

  15. Thermal conductance of metallic interface in vacuum

    SciTech Connect

    Mortazavi, P.; Shu, D.

    1985-01-01

    In most heat transfer applications, the deposited heat is transferred by any of the following classical methods: conduction, convection, radiation, or any combinations of these three. Depending on how critical the nature is of the designed equipment, the response time must be short enough in order to safeguard the proper performance of the devices. For instance, currently at the National Synchrotron Light Source (NSLS), various hardware equipment are being designed to intercept or to stop intense radiation beams induced by insertion devices such as wiggler and undulators. Due to the nature of some of these designs, the deposited high flux thermal load must be transferred across unbonded contact surfaces. Since any miscalculation would result in the disintegration of exposed material and therefore cause substantial problems, a true actual conductance measurement of the material in question is highly desirable. In the following three sections, background summary, the method of measurement, and the obtained results are discussed.

  16. Effective thermal conductivity in thermoelectric materials

    NASA Astrophysics Data System (ADS)

    Baranowski, Lauryn L.; Jeffrey Snyder, G.; Toberer, Eric S.

    2013-05-01

    Thermoelectric generators (TEGs) are solid state heat engines that generate electricity from a temperature gradient. Optimizing these devices for maximum power production can be difficult due to the many heat transport mechanisms occurring simultaneously within the TEG. In this paper, we develop a model for heat transport in thermoelectric materials in which an "effective thermal conductivity" (κeff) encompasses both the one dimensional steady-state Fourier conduction and the heat generation/consumption due to secondary thermoelectric effects. This model is especially powerful in that the value of κeff does not depend upon the operating conditions of the TEG but rather on the transport properties of the TE materials themselves. We analyze a variety of thermoelectric materials and generator designs using this concept and demonstrate that κeff predicts the heat fluxes within these devices to 5% of the exact value.

  17. Polyolefin-based aerogels

    NASA Technical Reports Server (NTRS)

    Lee, Je Kyun (Inventor); Gould, Gerogle L. (Inventor)

    2010-01-01

    The present invention relates to cross-linked polyolefin aerogels in simple and fiber-reinforced composite form. Of particular interest are polybutadiene aerogels. Especially aerogels derived from polybutadienes functionalized with anhydrides, amines, hydroxyls, thiols, epoxies, isocyanates or combinations thereof.

  18. Thermally unstable perturbations in stratified conducting atmospheres

    NASA Astrophysics Data System (ADS)

    Reale, Fabio; Serio, Salvatore; Peres, Giovanni

    1994-10-01

    We investigate the thermal stability of isobaric perturbations in a stratified isothermal background atmosphere with solar abundances, as resulting from the competition of optically thin plasma radiative cooling and of heating conducted from the surrounding atmosphere. We have analyzed the threshold line between stable and unstable perturbations, in the plane of the two important control parameters: the initial size of the perturbation and the temperature of the unperturbed medium; this line changes with the pressure of the unperturbed atmosphere. We have extended the results of linear perturbation analysis by means of numerical calculations of the evolution of spherical isobaric perturbations, using a two-dimensional hydrodynamic code including Spitzer heat conduction. We explore a wide range of the parameters appropriate to the solar and stellar upper atmospheres: the background uniform temperature is between 105 K and 107 K, the initial pressure betweeen 0.1 and 10 dyn/sq cm, and the perturbation size between 105 and 1010 cm. The numerical results are in substantial agreement with the linear analysis. We discuss possible implications of our results also in terms of observable effects, especially concerning plasma downflows, and propose thermal instability as a possible candidate to explain the observed redshifts in solar and stellar transition region lines.

  19. How We 3D-Print Aerogel

    SciTech Connect

    2015-04-23

    A new type of graphene aerogel will make for better energy storage, sensors, nanoelectronics, catalysis and separations. Lawrence Livermore National Laboratory researchers have made graphene aerogel microlattices with an engineered architecture via a 3D printing technique known as direct ink writing. The research appears in the April 22 edition of the journal, Nature Communications. The 3D printed graphene aerogels have high surface area, excellent electrical conductivity, are lightweight, have mechanical stiffness and exhibit supercompressibility (up to 90 percent compressive strain). In addition, the 3D printed graphene aerogel microlattices show an order of magnitude improvement over bulk graphene materials and much better mass transport.

  20. Multifunctional Lattices with Low Thermal Expansion and Low Thermal Conductivity

    NASA Astrophysics Data System (ADS)

    Xu, Hang; Liu, Lu; Pasini, Damiano

    Systems in space are vulnerable to large temperature changes when travelling into and out of the Earth's shadow. Variations in temperature can lead to undesired geometric changes in susceptible applications requiring very fine precision. In addition, temperature-sensitive electronic equipment hosted in a satellite needs adequate thermal-control to guarantee a moderate ambient temperature. To address these specifications, materials with low coefficient of thermal expansion (CTE) and low coefficient of thermal conductivity (CTC) over a wide range of temperatures are often sought, especially for bearing components in satellites. Besides low CTE and low CTC, these materials should also provide desirable stiffness, strength and extraordinarily low mass. This work presents ultralightweight bi-material lattices with tunable CTE and CTC, besides high stiffness and strength. We show that the compensation of the thermal expansion and joint rotation at the lattice joints can be used as an effective strategy to tailor thermomechanical performance. Proof-of-concept lattices are fabricated from Al and Ti alloy sheets via a simple snap-fit technique and vacuum brazing, and their CTE and CTC are assessed via a combination of experiments and theory. Corresponding Author.

  1. Solid phase microextraction device using aerogel

    DOEpatents

    Miller, Fred S.; Andresen, Brian D.

    2005-06-14

    A sample collection substrate of aerogel and/or xerogel materials bound to a support structure is used as a solid phase microextraction (SPME) device. The xerogels and aerogels may be organic or inorganic and doped with metals or other compounds to target specific chemical analytes. The support structure is typically formed of a glass fiber or a metal wire (stainless steel or kovar). The devices are made by applying gel solution to the support structures and drying the solution to form aerogel or xerogel. Aerogel particles may be attached to the wet layer before drying to increase sample collection surface area. These devices are robust, stable in fields of high radiation, and highly effective at collecting gas and liquid samples while maintaining superior mechanical and thermal stability during routine use. Aerogel SPME devices are advantageous for use in GC/MS analyses due to their lack of interfering background and tolerance of GC thermal cycling.

  2. Effective thermal conductivity of a thin, randomly oriented composite material

    SciTech Connect

    Phelan, P.E.; Niemann, R.C.

    1997-10-01

    The thermal conductivity of a randomly oriented composite material is modeled using a probabilistic approach in order to determine if a size effect exists for the thermal conductivity at small composite thicknesses. The numerical scheme employs a random number generator to position the filler elements, which have a relatively high thermal conductivity, within a matrix having a relative low thermal conductivity. The results indicate that, below some threshold thickness, the composite thermal conductivity is independent of thickness. The threshold thickness increases for increasing filler fraction and increasing k{sub f}/k{sub m}, the ratio between the filler and matrix thermal conductivities.

  3. Aerogel sorbents

    DOEpatents

    Begag, Redouane; Rhine, Wendell E; Dong, Wenting

    2016-04-05

    The current invention describes methods and compositions of various sorbents based on aerogels of various silanes and their use as sorbent for carbon dioxide. Methods further provide for optimizing the compositions to increase the stability of the sorbents for prolonged use as carbon dioxide capture matrices.

  4. Composite ceria-coated aerogels and methods of making the same

    DOEpatents

    Eyring, Edward M; Ernst, Richard D; Turpin, Gregory C; Dunn, Brian C

    2013-05-07

    Ceria-coated aerogels can include an aerogel support material having a stabilized ceria coating thereon. The ceria coating can be formed by solution or vapor deposition of alcogels or aerogels. Additional catalytic metal species can also be incorporated into the coating to form multi-metallic compounds having improved catalytic activity. Further, the ceria coated aerogels retain high surface areas at elevated temperatures. Thus, improvements in catalytic activity and thermal stability can be achieved using these ceria-coated composite aerogels.

  5. Organic aerogel microspheres

    DOEpatents

    Mayer, Steven T.; Kong, Fung-Ming; Pekala, Richard W.; Kaschmitter, James L.

    1999-01-01

    Organic aerogel microspheres which can be used in capacitors, batteries, thermal insulation, adsorption/filtration media, and chromatographic packings, having diameters ranging from about 1 micron to about 3 mm. The microspheres can be pyrolyzed to form carbon aerogel microspheres. This method involves stirring the aqueous organic phase in mineral oil at elevated temperature until the dispersed organic phase polymerizes and forms nonsticky gel spheres. The size of the microspheres depends on the collision rate of the liquid droplets and the reaction rate of the monomers from which the aqueous solution is formed. The collision rate is governed by the volume ratio of the aqueous solution to the mineral oil and the shear rate, while the reaction rate is governed by the chemical formulation and the curing temperature.

  6. Organic aerogel microspheres

    DOEpatents

    Mayer, S.T.; Kong, F.M.; Pekala, R.W.; Kaschmitter, J.L.

    1999-06-01

    Organic aerogel microspheres are disclosed which can be used in capacitors, batteries, thermal insulation, adsorption/filtration media, and chromatographic packings, having diameters ranging from about 1 micron to about 3 mm. The microspheres can be pyrolyzed to form carbon aerogel microspheres. This method involves stirring the aqueous organic phase in mineral oil at elevated temperature until the dispersed organic phase polymerizes and forms nonstick gel spheres. The size of the microspheres depends on the collision rate of the liquid droplets and the reaction rate of the monomers from which the aqueous solution is formed. The collision rate is governed by the volume ratio of the aqueous solution to the mineral oil and the shear rate, while the reaction rate is governed by the chemical formulation and the curing temperature.

  7. Thermal contact conductance of actuated interfaces

    NASA Astrophysics Data System (ADS)

    Song, Woo-Bin; Sutton, Michael S.; Talghader, Joseph J.

    2002-08-01

    The thermal contact conductance (TCC) of microactuated mechanical interfaces has been characterized using an electronic technique, where micromachined test structures were heated with a current and the TCC was inferred from the change in resistance. For every device tested, the TCC was higher in vacuum than in air. This is in stark contrast to the behavior of bulk interfaces, and several experiments suggest that it may be the result of a decreased solid-solid contact area in air caused by the pressure of the interstitial gas. The average effective TCC of a polysilicon/nitride interface brought together by electrostatic actuation varies about values of 6.0 x104 W/(K m2) in air and 9.5 x104 W/(K m2) under vacuum for applied pressures of 1 MPa. These values are significantly higher than commonly reported for nonmetallic materials and probably reflect the very smooth surfaces of deposited thin films.

  8. Electronic thermal conduction in suspended graphene

    NASA Astrophysics Data System (ADS)

    Rizwana Begum, K.; Sankeshwar, N. S.

    2015-09-01

    The electronic thermal conductivity (ETC), κe, of suspended graphene (SG) is studied for 15

  9. Thermally conductive porous element-based recuperators

    NASA Technical Reports Server (NTRS)

    Du, Jian Hua (Inventor); Chow, Louis C (Inventor); Lin, Yeong-Ren (Inventor); Wu, Wei (Inventor); Kapat, Jayanta (Inventor); Notardonato, William U. (Inventor)

    2012-01-01

    A heat exchanger includes at least one hot fluid flow channel comprising a first plurality of open cell porous elements having first gaps there between for flowing a hot fluid in a flow direction and at least one cold fluid flow channel comprising a second plurality of open cell porous elements having second gaps therebetween for flowing a cold fluid in a countercurrent flow direction relative to the flow direction. The thermal conductivity of the porous elements is at least 10 W/mK. A separation member is interposed between the hot and cold flow channels for isolating flow paths associated these flow channels. The first and second plurality of porous elements at least partially overlap one another to form a plurality of heat transfer pairs which transfer heat from respective ones of the first porous elements to respective ones of the second porous elements through the separation member.

  10. Efficient Reformulation of HOTFGM: Heat Conduction with Variable Thermal Conductivity

    NASA Technical Reports Server (NTRS)

    Zhong, Yi; Pindera, Marek-Jerzy; Arnold, Steven M. (Technical Monitor)

    2002-01-01

    Functionally graded materials (FGMs) have become one of the major research topics in the mechanics of materials community during the past fifteen years. FGMs are heterogeneous materials, characterized by spatially variable microstructure, and thus spatially variable macroscopic properties, introduced to enhance material or structural performance. The spatially variable material properties make FGMs challenging to analyze. The review of the various techniques employed to analyze the thermodynamical response of FGMs reveals two distinct and fundamentally different computational strategies, called uncoupled macromechanical and coupled micromechanical approaches by some investigators. The uncoupled macromechanical approaches ignore the effect of microstructural gradation by employing specific spatial variations of material properties, which are either assumed or obtained by local homogenization, thereby resulting in erroneous results under certain circumstances. In contrast, the coupled approaches explicitly account for the micro-macrostructural interaction, albeit at a significantly higher computational cost. The higher-order theory for functionally graded materials (HOTFGM) developed by Aboudi et al. is representative of the coupled approach. However, despite its demonstrated utility in applications where micro-macrostructural coupling effects are important, the theory's full potential is yet to be realized because the original formulation of HOTFGM is computationally intensive. This, in turn, limits the size of problems that can be solved due to the large number of equations required to mimic realistic material microstructures. Therefore, a basis for an efficient reformulation of HOTFGM, referred to as user-friendly formulation, is developed herein, and subsequently employed in the construction of the efficient reformulation using the local/global conductivity matrix approach. In order to extend HOTFGM's range of applicability, spatially variable thermal

  11. New organic aerogels based upon a phenolic-furfural reaction

    SciTech Connect

    Hrubesh, L.W.

    1994-09-01

    The aqueous polycondensation of (1) resorcinol with formaldehyde and (2) melamine with formaldehyde are two proven synthetic routes for the formation of organic aerogels. Recently, we have discovered a new type of organic aerogel based upon a phenolic-furfural (PF) reaction. This sol-gel polymerization has a major advantage over past approaches since it can be conducted in alcohol (e.g., 1-propanol), thereby eliminating the need for a solvent exchange step prior to supercritical drying from carbon dioxide. The resultant aerogels are dark brown in color and can be converted to a carbonized version upon pyrolysis in an inert atmosphere. BET surface areas of 350--600 m{sup 2}/g have been measured, and transmission electron microscopy reveals an interconnected structure of irregularly-shaped particles or platelets with {approximately}10 nm dimensions. Thermal conductivities as low as 0.015 W/m-K have been recorded for PF aerogels under ambient conditions. This paper describes the chemistry-structure-property relationships of these new materials in detail.

  12. Method for Measuring Thermal Conductivity of Small Samples Having Very Low Thermal Conductivity

    NASA Technical Reports Server (NTRS)

    Miller, Robert A.; Kuczmarski, Maria a.

    2009-01-01

    This paper describes the development of a hot plate method capable of using air as a standard reference material for the steady-state measurement of the thermal conductivity of very small test samples having thermal conductivity on the order of air. As with other approaches, care is taken to ensure that the heat flow through the test sample is essentially one-dimensional. However, unlike other approaches, no attempt is made to use heated guards to block the flow of heat from the hot plate to the surroundings. It is argued that since large correction factors must be applied to account for guard imperfections when sample dimensions are small, it may be preferable to simply measure and correct for the heat that flows from the heater disc to directions other than into the sample. Experimental measurements taken in a prototype apparatus, combined with extensive computational modeling of the heat transfer in the apparatus, show that sufficiently accurate measurements can be obtained to allow determination of the thermal conductivity of low thermal conductivity materials. Suggestions are made for further improvements in the method based on results from regression analyses of the generated data.

  13. Origin of reduction in phonon thermal conductivity of microporous solids

    NASA Astrophysics Data System (ADS)

    Hopkins, Patrick E.; Rakich, Peter T.; Olsson, Roy H.; El-kady, Ihab F.; Phinney, Leslie M.

    2009-10-01

    Porous structures have strong tunable size effects due to increased surface area. Size effects on phonon thermal conductivity have been observed in porous materials with periodic voids on the order of microns. This letter explores the origin of this size effect on phonon thermal conductivity observed in periodic microporous membranes. Pore-edge boundary scattering of low frequency phonons explains the temperature trends in the thermal conductivity; further reduction in thermal conductivity is explained by the porosity.

  14. Multiscale Computer Simulation of Failure in Aerogels

    NASA Technical Reports Server (NTRS)

    Good, Brian S.

    2008-01-01

    Aerogels have been of interest to the aerospace community primarily for their thermal properties, notably their low thermal conductivities. While such gels are typically fragile, recent advances in the application of conformal polymer layers to these gels has made them potentially useful as lightweight structural materials as well. We have previously performed computer simulations of aerogel thermal conductivity and tensile and compressive failure, with results that are in qualitative, and sometimes quantitative, agreement with experiment. However, recent experiments in our laboratory suggest that gels having similar densities may exhibit substantially different properties. In this work, we extend our original diffusion limited cluster aggregation (DLCA) model for gel structure to incorporate additional variation in DLCA simulation parameters, with the aim of producing DLCA clusters of similar densities that nevertheless have different fractal dimension and secondary particle coordination. We perform particle statics simulations of gel strain on these clusters, and consider the effects of differing DLCA simulation conditions, and the resultant differences in fractal dimension and coordination, on gel strain properties.

  15. Strain-controlled thermal conductivity in ferroic twinned films

    PubMed Central

    Li, Suzhi; Ding, Xiangdong; Ren, Jie; Moya, Xavier; Li, Ju; Sun, Jun; Salje, Ekhard K. H.

    2014-01-01

    Large reversible changes of thermal conductivity are induced by mechanical stress, and the corresponding device is a key element for phononics applications. We show that the thermal conductivity κ of ferroic twinned thin films can be reversibly controlled by strain. Nonequilibrium molecular dynamics simulations reveal that thermal conductivity decreases linearly with the number of twin boundaries perpendicular to the direction of heat flow. Our demonstration of large and reversible changes in thermal conductivity driven by strain may inspire the design of controllable thermal switches for thermal logic gates and all-solid-state cooling devices. PMID:25224749

  16. Particle Tracks in Aerogel

    NASA Technical Reports Server (NTRS)

    2005-01-01

    In an experiment using a special air gun, particles are shot into aerogel at high velocities. Closeup of particles that have been captured in aerogel are shown here. The particles leave a carrot-shaped trail in the aerogel. Aerogel was used on the Stardust spacecraft to capture comet particles from Comet Wild 2.

  17. Surface modified aerogel monoliths

    NASA Technical Reports Server (NTRS)

    Leventis, Nicholas (Inventor); Johnston, James C. (Inventor); Kuczmarski, Maria A. (Inventor); Meador, Mary Ann B. (Inventor)

    2013-01-01

    This invention comprises reinforced aerogel monoliths such as silica aerogels having a polymer coating on its outer geometric surface boundary, and to the method of preparing said aerogel monoliths. The polymer coatings on the aerogel monoliths are derived from polymer precursors selected from the group consisting of isocyanates as a precursor, precursors of epoxies, and precursors of polyimides. The coated aerogel monoliths can be modified further by encapsulating the aerogel with the polymer precursor reinforced with fibers such as carbon or glass fibers to obtain mechanically reinforced composite encapsulated aerogel monoliths.

  18. Deterioration in effective thermal conductivity of aqueous magnetic nanofluids

    NASA Astrophysics Data System (ADS)

    Altan, Cem L.; Gurten, Berna; Sommerdijk, Nico A. J. M.; Bucak, Seyda

    2014-12-01

    Common heat transfer fluids have low thermal conductivities, which decrease their efficiency in many applications. On the other hand, solids have much higher thermal conductivity values. Previously, it was shown that the addition of different nanoparticles to various base fluids increases the thermal conductivity of the carrier fluid remarkably. However, there are limited studies that focus on the thermal conductivity of magnetic fluids. In this study, thermal conductivity of magnetic nanofluids composed of magnetite nanoparticles synthesized via co-precipitation and thermal decomposition methods is investigated. Results showed that the addition of magnetite nanoparticles decreased the thermal conductivity of water and ethylene glycol. This decrease was found to increase with increasing particle concentration and to be independent of the synthesis method, the type of surfactant, and the interfacial thermal resistance.

  19. Aerogel Derived Nanostructured Thermoelectric Materials

    SciTech Connect

    Wendell E Rhine, PI; Dong, Wenting; Greg Caggiano, PM

    2010-10-08

    America’s dependence on foreign sources for fuel represents a economic and security threat for the country. These non renewable resources are depleting, and the effects of pollutants from fuels such as oil are reaching a problematic that affects the global community. Solar concentration power (SCP) production systems offer the opportunity to harness one of the United States’ most under utilized natural resources; sunlight. While commercialization of this technology is increasing, in order to become a significant source of electricity production in the United States the costs of deploying and operating SCP plants must be further reduced. Parabolic Trough SCP technologies are close to meeting energy production cost levels that would raise interest in the technology and help accelerate its adoption as a method to produce a significant portion of the Country’s electric power needs. During this program, Aspen Aerogels will develop a transparent aerogel insulation that can replace the costly vacuum insulation systems that are currently used in parabolic trough designs. During the Phase I program, Aspen Aerogels will optimize the optical and thermal properties of aerogel to meet the needs of this application. These properties will be tested, and the results will be used to model the performance of a parabolic trough HCE system which uses this novel material in place of vacuum. During the Phase II program, Aspen Aerogels will scale up this technology. Together with industry partners, Aspen Aerogels will build and test a prototype Heat Collection Element that is insulated with the novel transparent aerogel material. This new device will find use in parabolic trough SCP applications.

  20. Ultralight multiwalled carbon nanotube aerogel.

    PubMed

    Zou, Jianhua; Liu, Jianhua; Karakoti, Ajay Singh; Kumar, Amit; Joung, Daeha; Li, Qiang; Khondaker, Saiful I; Seal, Sudipta; Zhai, Lei

    2010-12-28

    Ultralight multiwalled carbon nanotube (MWCNT) aerogel is fabricated from a wet gel of well-dispersed pristine MWCNTs. On the basis of a theoretical prediction that increasing interaction potential between CNTs lowers their critical concentration to form an infinite percolation network, poly(3-(trimethoxysilyl) propyl methacrylate) (PTMSPMA) is used to disperse and functionalize MWCNTs where the subsequent hydrolysis and condensation of PTMSPMA introduces strong and permanent chemical bonding between MWCNTs. The interaction is both experimentally and theoretically proven to facilitate the formation of a MWCNT percolation network, which leads to the gelation of MWCNT dispersion at ultralow MWCNT concentration. After removing the liquid component from the MWCNT wet gel, the lightest ever free-standing MWCNT aerogel monolith with a density of 4 mg/cm(3) is obtained. The MWCNT aerogel has an ordered macroporous honeycomb structure with straight and parallel voids in 50-150 μm separated by less than 100 nm thick walls. The entangled MWCNTs generate mesoporous structures on the honeycomb walls, creating aerogels with a surface area of 580 m(2)/g which is much higher than that of pristine MWCNTs (241 m(2)/g). Despite the ultralow density, the MWCNT aerogels have an excellent compression recoverable property as demonstrated by the compression test. The aerogels have an electrical conductivity of 3.2 × 10(-2) S·cm(-1) that can be further increased to 0.67 S·cm(-1) by a high-current pulse method without degrading their structures. The excellent compression recoverable property, hierarchically porous structure with large surface area, and high conductivity grant the MWCNT aerogels exceptional pressure and chemical vapor sensing capabilities. PMID:21090673

  1. Size effects in thermal conduction by phonons

    NASA Astrophysics Data System (ADS)

    Allen, Philip B.

    2014-08-01

    Heat transport in nanoscale systems is both hard to measure microscopically, and hard to interpret. Ballistic and diffusive heat flow coexist, adding confusion. This paper looks at a very simple case: a nanoscale crystal repeated periodically. This is a popular model for simulation of bulk heat transport using classical molecular dynamics (MD), and is related to transient thermal grating experiments. Nanoscale effects are seen in perhaps their simplest form. The model is solved by an extension of standard quasiparticle gas theory of bulk solids. Both structure and heat flow are constrained by periodic boundary conditions. Diffusive transport is fully included, while ballistic transport by phonons of a long mean free path is diminished in a specific way. Heat current J (x) and temperature gradient ∇T (x') have a nonlocal relationship, via κ (x-x'), over a distance |x-x'| determined by phonon mean free paths. In MD modeling of bulk conductivity, finite computer resources limit system size. Long mean free paths, comparable to the scale of heating and cooling, cause undesired finite-size effects that have to be removed by extrapolation. The present model allows this extrapolation to be quantified. Calculations based on the Peierls-Boltzmann equation, using a generalized Debye model, show that extrapolation involves fractional powers of 1/L. It is also argued that heating and cooling should be distributed sinusoidally [ė∝cos(2πx/L)] to improve convergence of numerics.

  2. Synthesis and Properties of Cross-Linked Polyamide Aerogels

    NASA Technical Reports Server (NTRS)

    Williams, Jarrod C.; Meador, Mary Ann; McCorkle, Linda

    2015-01-01

    We report the first synthesis of cross-linked polyamide aerogels through step growth polymerization using a combination of diamines, diacid chloride and triacid chloride. Polyamide oligomers endcapped with amines are prepared as stable solutions in N-methylpyrrolidinone from several different diamine precursors and 1,3-benzenedicarbonyl dichloride. Addition of 1,3,5-benzenetricarbonyl trichloride yields gels which form in under five minutes according to the scheme shown. Solvent exchange of the gels into ethanol, followed by drying using supercritical CO2 extraction gives colorless aerogels with densities around 0.1 to 0.2 gcm3. Thicker monolithes of the polyamide aerogels are stiff and strong, while thin films of certain formulations are highly flexible, durable, and even translucent. These materials may have use as insulation for deployable space structures, rovers, habitats or extravehicular activity suits as well as in many terrestrial applications. Strucure property relationships of the aerogels, including surface area, mechanical properties, and thermal conductivity will be discussed.

  3. Preparation of Biopolymer Aerogels Using Green Solvents.

    PubMed

    Subrahmanyam, Raman; Gurikov, Pavel; Meissner, Imke; Smirnova, Irina

    2016-01-01

    Although the first reports on aerogels made by Kistler(1) in the 1930s dealt with aerogels from both inorganic oxides (silica and others) and biopolymers (gelatin, agar, cellulose), only recently have biomasses been recognized as an abundant source of chemically diverse macromolecules for functional aerogel materials. Biopolymer aerogels (pectin, alginate, chitosan, cellulose, etc.) exhibit both specific inheritable functions of starting biopolymers and distinctive features of aerogels (80-99% porosity and specific surface up to 800 m(2)/g). This synergy of properties makes biopolymer aerogels promising candidates for a wide gamut of applications such as thermal insulation, tissue engineering and regenerative medicine, drug delivery systems, functional foods, catalysts, adsorbents and sensors. This work demonstrates the use of pressurized carbon dioxide (5 MPa) for the ionic cross linking of amidated pectin into hydrogels. Initially a biopolymer/salt dispersion is prepared in water. Under pressurized CO2 conditions, the pH of the biopolymer solution is lowered to 3 which releases the crosslinking cations from the salt to bind with the biopolymer yielding hydrogels. Solvent exchange to ethanol and further supercritical CO2 drying (10 - 12 MPa) yield aerogels. Obtained aerogels are ultra-porous with low density (as low as 0.02 g/cm(3)), high specific surface area (350 - 500 m(2)/g) and pore volume (3 - 7 cm(3)/g for pore sizes less than 150 nm). PMID:27403649

  4. Hypervelocity capture of particles in aerogel: Dependence on aerogel properties

    NASA Astrophysics Data System (ADS)

    Burchell, M. J.; Fairey, S. A. J.; Foster, N. J.; Cole, M. J.

    2009-01-01

    Capture of high-speed (hypervelocity) particles in aerogel at ambient temperatures of 175-763 K is reported. This extends previous work which has mostly focussed on conducting experiments at ambient laboratory temperatures, even though aerogels are intended for use in cosmic dust capture cells in space environments which may experience a range of temperatures (e.g., the NASA Stardust mission which collected dust at 1.81 AU and putative Mars atmospheric sampling missions). No significant change in track length (normalised to impactor size) was found over the range 175-600 K, although at 763 K a significant reduction (30%) was found. By contrast, entrance hole diameter remained constant only up to 400 K, above this sudden changes of up to 50% were observed. Experiments were also carried out at normal laboratory temperature using a wide range of aerogel densities and particle sizes. It was found that track length normalised to particle size varies inversely with aerogel density. This is a power law dependence and not linear as previously reported, with longer tracks at lower densities. Glass projectiles (up to 100 μm size) were found to undergo a variety of degrees of damage during capture. In addition to the well known acquisition of a coating (partial or complete) of molten aerogel the mechanical damage includes pitting and meridian fractures. Larger (500 μm diameter) stainless steel spheres also showed damage during capture. In this case melting and ablation occurs, suggesting surficial temperatures during impact in excess of 1400 °C. The response of the aerogel itself to passage of particles through it is reported. The presence of fan-like fractures around the tracks is attributed to cone cracking similar to that in glasses of normal density, with the difference that here it is a repetitive process as the particles pass through the aerogel.

  5. High Thermal Conductivity Aligned Polyethylene-Graphene Nanocomposites

    NASA Astrophysics Data System (ADS)

    Garg, Jivtesh; Saeidijavash, Mortaza

    We investigate enhancement of thermal conductivity in polyethylene-graphene nanocomposites. The effect of alignment of both the polymer chains and the dispersed graphene flakes on thermal conductivity enhancement will be reported. In this work nanocomposites are prepared through microextrusion of polyethylene pellets and graphene nanopowder. Alignment is achieved through mechanical stretching of the nanocomposites. Thermal conductivity is measured using both Angstrom method and Laser flash. Variables involved in the study are the draw ratio and the weight percentage of graphene nanopowder. Results will shed light on the role of alignment of graphene flakes on enhancing thermal transport and provide new avenues to achieve ultra-high thermal conductivity in polymeric materials.

  6. Experiments on thermal contact conductance between metals below 100 K

    NASA Astrophysics Data System (ADS)

    Choi, Yeon Suk; Kim, Myung Su

    2014-01-01

    The thermal contact conductance is one of the important components in heat transfer mechanism. The accurate estimation of thermal contact conductance is necessary for the development of a conduction-cooled superconducting magnet system because the metallic materials are thermally connected to the refrigerator or heat sink to cool the superconducting magnet without cryogen. The contact resistance occurs at the interface between metals and the amount of conductance can be affected by various factors, such as surface roughness, contact area, and contact pressure. In the superconducting magnet system, there are several metal components in contact with each other for cooling magnet as well as supplying current to the magnet. Therefore, a temperature gradient exists between superconducting magnet and cryocooler. In this study, we have developed the thermal contact conductance measurement system and used the steady state method to measure the thermal conductance between metals. The effects of temperature, contact pressure and interfacial materials on the thermal contact conductance are also discussed.

  7. Flexible Fabrics with High Thermal Conductivity for Advanced Spacesuits

    NASA Technical Reports Server (NTRS)

    Trevino, Luis A.; Bue, Grant; Orndoff, Evelyne; Kesterson, Matt; Connel, John W.; Smith, Joseph G., Jr.; Southward, Robin E.; Working, Dennis; Watson, Kent A.; Delozier, Donovan M.

    2006-01-01

    This paper describes the effort and accomplishments for developing flexible fabrics with high thermal conductivity (FFHTC) for spacesuits to improve thermal performance, lower weight and reduce complexity. Commercial and additional space exploration applications that require substantial performance enhancements in removal and transport of heat away from equipment as well as from the human body can benefit from this technology. Improvements in thermal conductivity were achieved through the use of modified polymers containing thermally conductive additives. The objective of the FFHTC effort is to significantly improve the thermal conductivity of the liquid cooled ventilation garment by improving the thermal conductivity of the subcomponents (i.e., fabric and plastic tubes). This paper presents the initial system modeling studies, including a detailed liquid cooling garment model incorporated into the Wissler human thermal regulatory model, to quantify the necessary improvements in thermal conductivity and garment geometries needed to affect system performance. In addition, preliminary results of thermal conductivity improvements of the polymer components of the liquid cooled ventilation garment are presented. By improving thermal garment performance, major technology drivers will be addressed for lightweight, high thermal conductivity, flexible materials for spacesuits that are strategic technical challenges of the Exploration

  8. Thermal contact conductance across filled polyimide films at cryogenic temperatures

    NASA Astrophysics Data System (ADS)

    Zhao, L.; Phelan, P. E.

    1999-12-01

    Thermal contact resistance arises in the region of contact where two solid specimens are pressed together. The thermal resistance can be controlled by inserting an interstitial material at the interface, such as Kapton MT, a polyimide film containing alumina particles, which has a relatively low thermal resistance, but yet a high voltage standoff capability. The thermal resistance consists of two components: thermal contact resistance at the copper/Kapton MT interfaces, and the thermal conduction resistance across the Kapton MT film. The measured thermal resistance at low temperatures indicates that increasing the contact pressure reduces the thermal resistance, to a limit determined by the film conduction resistance. The effects of the contact pressure, the average interface temperature and the thickness of the interstitial layer are evaluated. A novel dimensionless correlation is derived from the experimental results that describes the thermal contact conductance of joints which include a soft interstitial material, at cryogenic temperatures.

  9. Phonon thermal conductivity of monolayer MoS2

    NASA Astrophysics Data System (ADS)

    Wang, Xiaonan; Tabarraei, Alireza

    2016-05-01

    We use nonequilibrium molecular dynamics modeling using Stillinger-Weber interatomic potential to investigate the thermal properties of monolayer molybdenum disulfide (MoS2) nanoribbons. We study the impact of factors such as length, edge chirality, monovacancies, and uniaxial stretching on the thermal conductivity of MoS2 nanoribbons. Our results show that longer ribbons have a higher thermal conductivity, and the thermal conductivity of infinitely long zigzag and armchair MoS2 nanoribbons is, respectively, 54 W/mK and 33 W/mK. This is significantly lower than the thermal conductivity of some other graphene-like two-dimensional materials such as graphene and boron nitride. While the presence of molybdenum or sulfur vacancies reduces the thermal conductivity of ribbons, molybdenum vacancies have a more deteriorating effect on thermal conductivities. We also have studied the impact of uniaxial stretching on the thermal conductivity of MoS2 nanoribbons. The results show that in contrast to three dimensional materials, thermal conductivity of MoS2 is fairly insensitive to stretching. We have used the phonon dispersion curves and group velocities to investigate the mechanism of this unexpected behavior. Our results show that tensile strain does not alter the phonon dispersion curves and hence the thermal conductivity does not change.

  10. Thermal Conductivity of Functional Citrus Tree Wood 1

    PubMed Central

    Turrell, F. M.; Austin, S. W.; McNee, Dan; Park, W. J.

    1967-01-01

    Thermal conductivity coefficients have been determined for longitudinal and transverse flow in 4 varieties of fresh Citrus wood using steady state-methods. Equations were developed from which thermal conductivity could be rapidly estimated from moisture content or electrical conductivity. The heat balance of large and small tree trunks on a freezing night has been calculated on the basis of the coefficients. PMID:16656610

  11. Effective thermal conductivity determination for low-density insulating materials

    NASA Technical Reports Server (NTRS)

    Williams, S. D.; Curry, D. M.

    1978-01-01

    That nonlinear least squares can be used to determine effective thermal conductivity was demonstrated, and a method for assessing the relative error associated with these predicted values was provided. The differences between dynamic and static determination of effective thermal conductivity of low-density materials that transfer heat by a combination of conduction, convection, and radiation were discussed.

  12. Final Report: Thermal Conductance of Solid-Liquid Interfaces

    SciTech Connect

    Cahil, David, G.; Braun, Paul, V.

    2006-05-31

    Research supported by this grant has significantly advanced fundamental understanding of the thermal conductance of solid-liquid interfaces, and the thermal conductivity of nanofluids and nanoscale composite materials. • The thermal conductance of interfaces between carbon nanotubes and a surrounding matrix of organic molecules is exceptionally small and this small value of the interface conductance limits the enhancement in thermal conductivity that can be achieved by loading a fluid or a polymer with nanotubes. • The thermal conductance of interfaces between metal nanoparticles coated with hydrophilic surfactants and water is relatively high and surprisingly independent of the details of the chemical structure of the surfactant. • We extended our experimental methods to enable studies of planar interfaces between surfactant-coated metals and water where the chemical functionalization can be varied between strongly hydrophobic and strongly hydrophilic. The thermal conductance of hydrophobic interfaces establishes an upper-limit of 0.25 nm on the thickness of the vapor-layer that is often proposed to exist at hydrophobic interfaces. • Our high-precision measurements of fluid suspensions show that the thermal conductivity of fluids is not significantly enhanced by loading with a small volume fraction of spherical nanoparticles. These experimental results directly contradict some of the anomalous results in the recent literature and also rule-out proposed mechanisms for the enhanced thermal conductivity of nanofluids that are based on modification of the fluid thermal conductivity by the coupling of fluid motion and the Brownian motion of the nanoparticles.

  13. Thermal conductivity degradation of graphites irradiated at low temperature

    SciTech Connect

    Snead, L.L.; Burchell, T.D.

    1995-04-01

    The objective of this work is to study the thermal conductivity degradation of new, high thermal conductivity graphites and to compare these results to more standard graphites irradiated at low temperatures. Several graphites and graphite composites (C/C`s) have been irradiated near 150{degree}C and at fluences up to a displacement level of 0.24 dpa. The materials ranged in unirradiated room temperature thermal conductivity of these materials varied from 114 W/m-K for H-451 isotropic graphite, to 670 W/m-K for unidirectional FMI-1D C/C composite. At the irradiation temperature a saturation reduction in thermal conductivity was seen to occur at displacement levels of approximately 0.1 dpa. All materials were seen to degrade to approximately 10 to 14 % of their original thermal conductivity after irradiation. The effect of post irradiation annealing on the thermal conductivity was also studied.

  14. Thermal conductivity of graphene nanoribbons in noble gaseous environments

    SciTech Connect

    Zhong, Wei-Rong Xu, Zhi-Cheng; Zheng, Dong-Qin; Ai, Bao-Quan

    2014-02-24

    We investigate the thermal conductivity of suspended graphene nanoribbons in noble gaseous environments using molecular dynamics simulations. It is reported that the thermal conductivity of perfect graphene nanoribbons decreases with the gaseous pressure. The decreasing is more obvious for the noble gas with large atomic number. However, the gaseous pressure cannot change the thermal conductivity of defective graphene nanoribbons apparently. The phonon spectra of graphene nanoribbons are also provided to give corresponding supports.

  15. Thermophysical Properties of Polymer Materials with High Thermal Conductivity

    NASA Astrophysics Data System (ADS)

    Lebedev, S. M.; Gefle, O. S.; Dneprovskii, S. N.; Amitov, E. T.

    2015-06-01

    Results of studies on the main thermophysical properties of new thermally conductive polymer materials are presented. It is shown that modification of polymer dielectrics by micron-sized fillers allows thermally conductive materials with thermal conductivity not less than 2 W/(m K) to be produced, which makes it possible to use such materials as cooling elements of various electrical engineering and semiconductor equipment and devices.

  16. Thermal conductivity of Rene 41 honeycomb panels. [space transportation vehicles

    NASA Technical Reports Server (NTRS)

    Deriugin, V.

    1980-01-01

    Effective thermal conductivities of Rene 41 panels suitable for advanced space transportation vehicle structures were determined analytically and experimentally for temperature ranges between 20.4K (423 F) and 1186K (1675 F). The cryogenic data were obtained using a cryostat whereas the high temperature data were measured using a heat flow meter and a comparative thermal conductivity instrument respectively. Comparisons were made between analysis and experimental data. Analytical methods appear to provide reasonable definition of the honeycomb panel effective thermal conductivities.

  17. Thermal Conductivity of Alumina-Toughened Zirconia Composites

    NASA Technical Reports Server (NTRS)

    Bansal, Narottam P.; Zhu, Dong-Ming

    2003-01-01

    10-mol% yttria-stabilized zirconia (10YSZ)-alumina composites containing 0 to 30 mol% alumina were fabricated by hot pressing at 1500 C in vacuum. Thermal conductivity of the composites, determined at various temperatures using a steady-state laser heat flux technique, increased with increase in alumina content. Composites containing 0, 5, and 10-mol% alumina did not show any change in thermal conductivity with temperature. However, those containing 20 and 30-mol% alumina showed a decrease in thermal conductivity with increase in temperature. The measured values of thermal conductivity were in good agreement with those calculated from simple rule of mixtures.

  18. Modelling of thermal conductance during microthermal machining with scanning thermal microscope using an inverse methodology

    NASA Astrophysics Data System (ADS)

    Yang, Yu-Ching; Chang, Win-Jin; Fang, Te-Hua; Fang, Shih-Chung

    2008-01-01

    In this study, a general methodology for determining the thermal conductance between the probe tip and the workpiece during microthermal machining using Scanning Thermal Microscopy (SThM) has been proposed. The processing system was considered as an inverse heat conduction problem with an unknown thermal conductance. Temperature dependence for the material properties and thermal conductance in the analysis of heat conduction is taken into account. The conjugate gradient method is used to solve the inverse problem. Furthermore, this methodology can also be applied to estimate the thermal contact conductance in other transient heat conduction problems, like metal casting process, injection molding process, and electronic circuit systems.

  19. Thermal conductivity of penta-graphene from molecular dynamics study

    NASA Astrophysics Data System (ADS)

    Xu, Wen; Zhang, Gang; Li, Baowen

    2015-10-01

    Using classical equilibrium molecular dynamics simulations and applying the original Tersoff interatomic potential, we study the thermal transport property of the latest two dimensional carbon allotrope, penta-graphene. It is predicted that its room-temperature thermal conductivity is about 167 W/mK, which is much lower than that of graphene. With normal mode decomposition, the accumulated thermal conductivity with respect to phonon frequency and mean free path is analyzed. It is found that the acoustic phonons make a contribution of about 90% to the thermal conductivity, and phonons with mean free paths larger than 100 nm make a contribution over 50%. We demonstrate that the remarkably lower thermal conductivity of penta-graphene compared with graphene results from the lower phonon group velocities and fewer collective phonon excitations. Our study highlights the importance of structure-property relationship and provides better understanding of thermal transport property and valuable insight into thermal management of penta-graphene.

  20. Thermal conductivity of penta-graphene from molecular dynamics study.

    PubMed

    Xu, Wen; Zhang, Gang; Li, Baowen

    2015-10-21

    Using classical equilibrium molecular dynamics simulations and applying the original Tersoff interatomic potential, we study the thermal transport property of the latest two dimensional carbon allotrope, penta-graphene. It is predicted that its room-temperature thermal conductivity is about 167 W/mK, which is much lower than that of graphene. With normal mode decomposition, the accumulated thermal conductivity with respect to phonon frequency and mean free path is analyzed. It is found that the acoustic phonons make a contribution of about 90% to the thermal conductivity, and phonons with mean free paths larger than 100 nm make a contribution over 50%. We demonstrate that the remarkably lower thermal conductivity of penta-graphene compared with graphene results from the lower phonon group velocities and fewer collective phonon excitations. Our study highlights the importance of structure-property relationship and provides better understanding of thermal transport property and valuable insight into thermal management of penta-graphene. PMID:26493918

  1. High thermal conductivity of hexagonal boron nitride laminates

    NASA Astrophysics Data System (ADS)

    Zheng, Jin-Cheng; Zhang, Liang; Kretinin, A. V.; Morozov, S. V.; Wang, Yi Bo; Wang, Tun; Li, Xiaojun; Ren, Fei; Zhang, Jingyu; Lu, Ching-Yu; Chen, Jia-Cing; Lu, Miao; Wang, Hui-Qiong; Geim, A. K.; Novoselov, K. S.

    2016-03-01

    Two-dimensional materials are characterised by a number of unique physical properties which can potentially make them useful to a wide diversity of applications. In particular, the large thermal conductivity of graphene and hexagonal boron nitride (hBN) has already been acknowledged and these materials have been suggested as novel core materials for thermal management in electronics. However, it was not clear if mass produced flakes of hBN would allow one to achieve an industrially-relevant value of thermal conductivity. Here we demonstrate that laminates of hBN exhibit thermal conductivity of up to 20 W/m·K, which is significantly larger than that currently used in thermal management. We also show that the thermal conductivity of laminates increases with the increasing volumetric mass density, which creates a way of fine tuning its thermal properties.

  2. Electrochemically tunable thermal conductivity of lithium cobalt oxide

    NASA Astrophysics Data System (ADS)

    Cho, Jiung; Losego, Mark D.; Zhang, Hui Gang; Kim, Honggyu; Zuo, Jianmin; Petrov, Ivan; Cahill, David G.; Braun, Paul V.

    2014-06-01

    Using time-domain thermoreflectance, the thermal conductivity and elastic properties of a sputter deposited LiCoO2 film, a common lithium-ion cathode material, are measured as a function of the degree of lithiation. Here we report that via in situ measurements during cycling, the thermal conductivity of a LiCoO2 cathode reversibly decreases from ~5.4 to 3.7 W m-1 K-1, and its elastic modulus decreases from 325 to 225 GPa, as it is delithiated from Li1.0CoO2 to Li0.6CoO2. The dependence of the thermal conductivity on lithiation appears correlated with the lithiation-dependent phase behaviour. The oxidation-state-dependent thermal conductivity of electrolytically active transition metal oxides provides opportunities for dynamic control of thermal conductivity and is important to understand for thermal management in electrochemical energy storage devices.

  3. Electrochemically tunable thermal conductivity of lithium cobalt oxide.

    PubMed

    Cho, Jiung; Losego, Mark D; Zhang, Hui Gang; Kim, Honggyu; Zuo, Jianmin; Petrov, Ivan; Cahill, David G; Braun, Paul V

    2014-01-01

    Using time-domain thermoreflectance, the thermal conductivity and elastic properties of a sputter deposited LiCoO2 film, a common lithium-ion cathode material, are measured as a function of the degree of lithiation. Here we report that via in situ measurements during cycling, the thermal conductivity of a LiCoO2 cathode reversibly decreases from ~5.4 to 3.7 W m(-1) K(-1), and its elastic modulus decreases from 325 to 225 GPa, as it is delithiated from Li1.0CoO2 to Li0.6CoO2. The dependence of the thermal conductivity on lithiation appears correlated with the lithiation-dependent phase behaviour. The oxidation-state-dependent thermal conductivity of electrolytically active transition metal oxides provides opportunities for dynamic control of thermal conductivity and is important to understand for thermal management in electrochemical energy storage devices. PMID:24892640

  4. Anisotropic thermal conductivity of thin polycrystalline oxide samples

    SciTech Connect

    Tiwari, A.; Boussois, K.; Nait-Ali, B.; Smith, D. S.; Blanchart, P.

    2013-11-15

    This paper reports about the development of a modified laser-flash technique and relation to measure the in-plane thermal diffusivity of thin polycrystalline oxide samples. Thermal conductivity is then calculated with the product of diffusivity, specific heat and density. Design and operating features for evaluating in-plane thermal conductivities are described. The technique is advantageous as thin samples are not glued together to measure in-plane thermal conductivities like earlier methods reported in literature. The approach was employed to study anisotropic thermal conductivity in alumina sheet, textured kaolin ceramics and montmorillonite. Since it is rare to find in-plane thermal conductivity values for such anisotropic thin samples in literature, this technique offers a useful variant to existing techniques.

  5. Thermal conductance of pressed contacts at liquid helium temperatures

    NASA Technical Reports Server (NTRS)

    Salerno, L. J.; Kittel, P.; Spivak, A. L.

    1983-01-01

    It is pointed out that the optimum design of cryogenic instruments requires accurate thermal models. The present models are limited by a lack of knowledge of the low temperature thermal conductance of the bolted joints which are typically used in the instrument-to-system interface. In connection with studies of pressed contacts, it has been found that the thermal conductance does not obey the Wiedemann-Franz law. The present investigation is concerned with the characterization of the thermal conductance of pressed contacts at liquid helium-4 temperatures, taking into account the dependence of thermal contact conductance on applied force and temperature. It is shown that for the 0.4 micron OFHC copper pressed contact pair, the thermal conductance varies roughly as the second power of the temperature, and increases with increasing applied force.

  6. Thermal conductance of space suit insulations, thermal micrometeroid garments, and other insulations

    NASA Technical Reports Server (NTRS)

    Richardson, D. L.; Stevens, J. M.

    1976-01-01

    The thermal protection capabilities of development and operational thermal micrometeroid garments and other insulations were evaluated. The relationship among sample thermal conductance, surface temperature, and compressive loads was empirically defined.

  7. Optical shock waves in silica aerogel.

    PubMed

    Gentilini, S; Ghajeri, F; Ghofraniha, N; Di Falco, A; Conti, C

    2014-01-27

    Silica aerogels are materials well suited for high power nonlinear optical applications. In such regime, the non-trivial thermal properties may give rise to the generation of optical shock waves, which are also affected by the structural disorder due to the porous solid-state gel. Here we report on an experimental investigation in terms of beam waist and input power, and identify various regimes of the generation of wave-breaking phenomena in silica aerogels. PMID:24515173

  8. Preliminary experimental evaluation of thermal conductivity of ceramic pebble beds

    NASA Astrophysics Data System (ADS)

    Aquaro, D.; Lo Frano, R.

    2014-04-01

    This paper illustrates the preliminary experimental tests for determining the effective thermal conductivity of ceramic pebble beds versus temperature and compression strains. Ceramic pebble beds are promising candidates to be used in breeding blankets for nuclear fusion reactor as breeder and neutron multiplier. The tests were performed with an experimental rig, built at the DICI-University of Pisa, which permits to determine the thermal conductivity of pebble beds in steady state conditions, at several temperatures and compression forces. The values of thermal conductivity of pebble beds are obtained as function of a known conductivity of an alumina disc. The assessment of the method has been performed determining the effective thermal conductivity of alumina pebbles beds of different diameters. Void fraction and compression strains are the parameters that mainly influence the variability of the thermal conductivity of the beds.

  9. Effective electrical and thermal conductivity of multifilament twisted superconductors

    NASA Astrophysics Data System (ADS)

    Chechetkin, V. R.

    2013-06-01

    The effective electrical and thermal conductivity of composite wire with twisted superconducting filaments embedded into normal metal matrix is calculated using the extension of Bruggeman method. The resistive conductivity of superconducting filaments is described in terms of symmetric tensor, whereas the conductivity of a matrix is assumed to be isotropic and homogeneous. The dependence of the resistive electrical conductivity of superconducting filaments on temperature, magnetic field, and current density is implied to be parametric. The resulting effective conductivity tensor proved to be non-diagonal and symmetric. The non-diagonal transverse-longitudinal components of effective electrical conductivity tensor are responsible for the redistribution of current between filaments. In the limits of high and low electrical conductivity of filaments the transverse effective conductivity tends to that of obtained previously by Carr. The effective thermal conductivity of composite wires is non-diagonal and radius-dependent even for the isotropic and homogeneous thermal conductivities of matrix and filaments.

  10. Experimental investigation of thermal conductance of finned tube contacts

    NASA Astrophysics Data System (ADS)

    Sheffield, J. W.; Wood, R. A.; Sauer, H. J., Jr.

    1989-01-01

    An experimental study was conducted to investigate the contact conductance of plate finned tubes. The basic theory of thermal contact conductance supports the use of measurable parameters including interference, fin spacing, fin thickness, tube hardness, and tube diameter as prediction parameters. Thirty-one coils were tested in a vacuum chamber. A correlation was developed that predicts the thermal contact conductance. The heat transfer results of the experimental study are presented.

  11. Ultralight nanofibre-assembled cellular aerogels with superelasticity and multifunctionality

    NASA Astrophysics Data System (ADS)

    Si, Yang; Yu, Jianyong; Tang, Xiaomin; Ge, Jianlong; Ding, Bin

    2014-12-01

    Three-dimensional nanofibrous aerogels (NFAs) that are both highly compressible and resilient would have broad technological implications for areas ranging from electrical devices and bioengineering to damping materials; however, creating such NFAs has proven extremely challenging. Here we report a novel strategy to create fibrous, isotropically bonded elastic reconstructed (FIBER) NFAs with a hierarchical cellular structure and superelasticity by combining electrospun nanofibres and the fibrous freeze-shaping technique. Our approach causes the intrinsically lamellar deposited electrospun nanofibres to assemble into elastic bulk aerogels with tunable densities and desirable shapes on a large scale. The resulting FIBER NFAs exhibit densities of >0.12 mg cm-3, rapid recovery from deformation, efficient energy absorption and multifunctionality in terms of the combination of thermal insulation, sound absorption, emulsion separation and elasticity-responsive electric conduction. The successful synthesis of such fascinating materials may provide new insights into the design and development of multifunctional NFAs for various applications.

  12. Ultralight nanofibre-assembled cellular aerogels with superelasticity and multifunctionality.

    PubMed

    Si, Yang; Yu, Jianyong; Tang, Xiaomin; Ge, Jianlong; Ding, Bin

    2014-01-01

    Three-dimensional nanofibrous aerogels (NFAs) that are both highly compressible and resilient would have broad technological implications for areas ranging from electrical devices and bioengineering to damping materials; however, creating such NFAs has proven extremely challenging. Here we report a novel strategy to create fibrous, isotropically bonded elastic reconstructed (FIBER) NFAs with a hierarchical cellular structure and superelasticity by combining electrospun nanofibres and the fibrous freeze-shaping technique. Our approach causes the intrinsically lamellar deposited electrospun nanofibres to assemble into elastic bulk aerogels with tunable densities and desirable shapes on a large scale. The resulting FIBER NFAs exhibit densities of >0.12 mg cm(-3), rapid recovery from deformation, efficient energy absorption and multifunctionality in terms of the combination of thermal insulation, sound absorption, emulsion separation and elasticity-responsive electric conduction. The successful synthesis of such fascinating materials may provide new insights into the design and development of multifunctional NFAs for various applications. PMID:25512095

  13. Measurement of the anisotropic thermal conductivity of the porcine cornea.

    PubMed

    Barton, Michael D; Trembly, B Stuart

    2013-10-01

    Accurate thermal models for the cornea of the eye support the development of thermal techniques for reshaping the cornea and other scientific purposes. Heat transfer in the cornea must be quantified accurately so that a thermal treatment does not destroy the endothelial layer, which cannot regenerate, and yet is responsible for maintaining corneal transparency. We developed a custom apparatus to measure the thermal conductivity of ex vivo porcine corneas perpendicular to the surface and applied a commercial apparatus to measure thermal conductivity parallel to the surface. We found that corneal thermal conductivity is 14% anisotropic at the normal state of corneal hydration. Small numbers of ex vivo feline and human corneas had a thermal conductivity perpendicular to the surface that was indistinguishable from the porcine corneas. Aqueous humor from ex vivo porcine, feline, and human eyes had a thermal conductivity nearly equal to that of water. Including the anisotropy of corneal thermal conductivity will improve the predictive power of thermal models of the eye. PMID:23933570

  14. Aerogel in Hand

    NASA Technical Reports Server (NTRS)

    2005-01-01

    Though ghostly in appearance like an hologram, aerogel is very solid. It feels like hard styrofoam to the touch. Aerogel was used on the Stardust spacecraft to capture comet particles from Comet Wild 2.

  15. Thermal conduction effects in human skin.

    PubMed

    Stoll, A M; Chianta, M A; Piergallini, J R

    1979-08-01

    To determine the maximum permissible temperature any material may attain without causing pain or burn on contact with bare skin, over 2000 observations were made of pain threshold during contact with materials at elevated temperatures. Six materials were used representing the full range of thermal properties from good conductors to good insulators. Time to pain threshold was converted to time to threshold blister on the basis of the relationship between pain and burn established earlier for radiant and for convective heating. Calculated times to blister were used to predict the material temperatures causative of "touch-burn". Experimentally produced threshold blisters at the predicted temperature-times verified the predictions. Graphs and equations were generated for determining safe temperatures for any material in contact with bare skin for 1-5 s solely from a knowledge of its thermal properties. Conversely, the thermal inertia (k rho c) of the optimal material for a specific use and skin contact can be predicted from a knowledge of the maximum material temperature and length of contact time anticipated. PMID:496745

  16. Superior thermal conductivity in suspended bilayer hexagonal boron nitride

    NASA Astrophysics Data System (ADS)

    Wang, Chengru; Guo, Jie; Dong, Lan; Aiyiti, Adili; Xu, Xiangfan; Li, Baowen

    2016-05-01

    We reported the basal-plane thermal conductivity in exfoliated bilayer hexagonal boron nitride h-BN that was measured using suspended prepatterned microstructures. The h-BN sample suitable for thermal measurements was fabricated by dry-transfer method, whose sample quality, due to less polymer residues on surfaces, is believed to be superior to that of PMMA-mediated samples. The measured room temperature thermal conductivity is around 484 Wm‑1K‑1(+141 Wm‑1K‑1/ ‑24 Wm‑1K‑1) which exceeds that in bulk h-BN, providing experimental observation of the thickness-dependent thermal conductivity in suspended few-layer h-BN.

  17. Thermal conductivity and other properties of cementitious grouts

    SciTech Connect

    Allan, M.

    1998-08-01

    The thermal conductivity and other properties cementitious grouts have been investigated in order to determine suitability of these materials for grouting vertical boreholes used with geothermal heat pumps. The roles of mix variables such as water/cement ratio, sand/cement ratio and superplasticizer dosage were measured. In addition to thermal conductivity, the cementitious grouts were also tested for bleeding, permeability, bond to HDPE pipe, shrinkage, coefficient of thermal expansion, exotherm, durability and environmental impact. This paper summarizes the results for selected grout mixes. Relatively high thermal conductivities were obtained and this leads to reduction in predicted bore length and installation costs. Improvements in shrinkage resistance and bonding were achieved.

  18. THERMAL CONDUCTIVITY AND OTHER PROPERTIES OF CEMENTITIOUS GROUTS

    SciTech Connect

    ALLAN,M.

    1998-05-01

    The thermal conductivity and other properties cementitious grouts have been investigated in order to determine suitability of these materials for grouting vertical boreholes used with geothermal heat pumps. The roles of mix variables such as water/cement ratio, sand/cement ratio and superplasticizer dosage were measured. In addition to thermal conductivity, the cementitious grouts were also tested for bleeding, permeability, bond to HDPE pipe, shrinkage, coefficient of thermal expansion, exotherm, durability and environmental impact. This paper summarizes the results for selected grout mixes. Relatively high thermal conductivities were obtained and this leads to reduction in predicted bore length and installation costs. Improvements in shrinkage resistance and bonding were achieved.

  19. Thermal conductivity of halide solid solutions: measurement and prediction.

    PubMed

    Gheribi, Aïmen E; Poncsák, Sándor; St-Pierre, Rémi; Kiss, László I; Chartrand, Patrice

    2014-09-14

    The composition dependence of the lattice thermal conductivity in NaCl-KCl solid solutions has been measured as a function of composition and temperature. Samples with systematically varied compositions were prepared and the laser flash technique was used to determine the thermal diffusivity from 373 K to 823 K. A theoretical model, based on the Debye approximation of phonon density of state (which contains no adjustable parameters) was used to predict the thermal conductivity of both stoichiometric compounds and fully disordered solid solutions. The predictions obtained with the model agree very well with our measurement. A general method for predicting the thermal conductivity of different halide systems is discussed. PMID:25217938

  20. High conductance thermal interface concept for space applications

    NASA Technical Reports Server (NTRS)

    Poulin, Elizabeth C.; Horan, D. C.

    1991-01-01

    An interface concept has been developed which produces high conductance at a thermal/mechanical joint without resorting to high clamping forces or potentially contaminating fillers such as thermal grease. This paper discusses the characteristics of several variations of the high conductance interface concept and compares them to those of existing interface concepts proposed for several Space Station applications. The application of the high conductance concept to thermal joints such as internal coldplate interfaces and external equipment module to heat acquisition plate interfaces would reduce the weight and complexity and increase the efficiency of the Space Station Thermal Management System.

  1. Coherent thermal conductance of 1-D photonic crystals

    NASA Astrophysics Data System (ADS)

    Tschikin, Maria; Ben-Abdallah, Philippe; Biehs, Svend-Age

    2012-10-01

    We present an exact calculation of coherent thermal conductance in 1-D multilayer photonic crystals using the S-matrix method. In particular, we study the thermal conductance in a bilayer structure of Si/vacuum or Al2O3/vacuum slabs by means of the exact radiative heat flux expression. Based on the results obtained for the Al2O3/vacuum structure we show by comparison with previous works that the material losses and (localized) surface modes supported by the inner layers play a fundamental role and cannot be omitted in the definition of thermal conductance. Our results could have significant implications in the conception of efficient thermal barriers.

  2. Depressing thermal conductivity of fullerene by caging rare gas

    NASA Astrophysics Data System (ADS)

    Li, Jian; Zheng, Dong-Qin; Zhong, Wei-Rong

    2016-01-01

    We have investigated the thermal conductivity of C60 and its derivatives caged with rare gas by using the nonequilibrium molecular dynamics method. It is reported that embedding C60 with different rare gas atoms has a significant impact on its thermal conductivity. We analyze the phenomenon through the phonon spectra of rare gas atom and the C-C bonds length of C60. When the number of atoms inside the C60 increases, the phonon spectra band width of rare gas expands and the length of C-C bonds becomes longer, which contributes to the depression of the thermal conductivity of C60. The method is applied to control the thermal conductivity of C60 chains, which maybe a kind of potential materials in thermal circuits. Our results also provide a controllable method for the thermal management in nanoscale materials.

  3. Di-Isocyanate Crosslinked Aerogels with 1, 6-Bis (Trimethoxysilyl) Hexane Incorporated in Silica Backbone

    NASA Technical Reports Server (NTRS)

    Vivod, Stephanie L.; Meador, Mary Ann B.; Nguyen, Baochau N.; Quade, Derek; Randall, Jason; Perry, Renee

    2008-01-01

    Silica aerogels are desirable materials for many applications that take advantage of their light weight and low thermal conductivity. Addition of a conformal polymer coating which bonds with the amine decorated surface of the silica network improves the strength of the aerogels by as much as 200 times. Even with vast improvement in strength they still tend to undergo brittle failure due to the rigid silica backbone. We hope to increase the flexibility and elastic recovery of the silica based aerogel by altering the silica back-bone by incorporation of more flexible hexane links. To this end, we investigated the use of 1,6-bis(trimethoxysilyl)hexane (BTMSH), a polysilsesquioxane precursor3, as an additional co-reactant to prepare silica gels which were subsequently cross-linked with di-isocyanate. Previously, this approach of adding flexibility by BTMSH incorporation was demonstrated with styrene cross-linked aerogels. In our study, we varied silane concentration, mol % of silicon from BTMSH and di-isocyanate concentration by weight percent to attempt to optimize both the flexibility and the strength of the aerogels.

  4. Low-Thermal-Conduction Links For Silicon Sensors

    NASA Technical Reports Server (NTRS)

    Mott, D. Brent

    1991-01-01

    Simple method of texturing surface of silicon reduces thermal conductivities of links in silicon x-ray calorimeters and infrared bolometers. Gives links high density of phonon scattering sites reducing conduction of heat. Links made shorter and more robust. Used in making x-ray calorimeters and infrared bolometers. Applicable to any microelectronic device in which high degree of thermal isolation needed.

  5. Prediction of effective thermal conductivity of cellular ceramics

    SciTech Connect

    Fu, X.; Viskanta, R.; Gore, J.P.

    1998-02-01

    Two unit cell based models have been developed to predict the effective thermal conductivity of porous materials using the thermal-circuit method. The first unit cell is a cubic-shaped box. The second unit cell is formed by a solid cube which is voided centrally by a sphere. The model predictions of the effective thermal conductivity of cellular ceramics are compared with experimental data.

  6. Thermal conductivity of vertically aligned boron nitride nanotubes

    NASA Astrophysics Data System (ADS)

    Essedik Belkerk, Boubakeur; Achour, Amine; Zhang, Dongyan; Sahli, Salah; Djouadi, M.-Abdou; Khin Yap, Yoke

    2016-07-01

    For the first time, we report the thermal conductivity of vertically aligned boron nitride nanotube (BNNT) films produced by catalytic chemical vapor deposition. High-quality BNNTs were synthesized at 1200 °C on fused silica substrates precoated with Pt thin-film thermometers. The thermal conductivity of the BNNTs was measured at room temperature by using a pulsed photothermal technique. The apparent thermal conductivity of the BNNT coatings increased from 55 to 170 W m‑1 K‑1 when the thickness increased from 10 to 28 µm, while the thermal conductivity attained a value as high as 2400 W m‑1 K‑1. These results suggested that BNNTs, which are highly thermally conductive, but electrically insulating, are promising materials with unique properties.

  7. Thermal Conductivity of Alumina-reinforced Zirconia Composites

    NASA Technical Reports Server (NTRS)

    Bansal, Narottam P.

    2005-01-01

    10-mol% yttria-stabilized zirconia (10SZ) - alumina composites containing 0-30 mol% alumina were fabricated by hot pressing at 1500 C in vacuum. Thermal conductivity was determined at various temperatures using a steady-state laser heat flux technique. Thermal conductivity of the composites increased with increase in alumina content. Composites containing 0, 5, and 10-mol% alumina did not show any change in thermal conductivity with temperature. However, those containing 20 and 30-mol% alumina showed a decrease in thermal conductivity with increase in temperature. The measured values of thermal conductivity were in good agreement with those calculated from the Maxwell-Eucken model where one phase is uniformly dispersed within a second major continuous phase.

  8. Thermal Conductivity of Polyimide/Carbon Nanofiller Blends

    NASA Technical Reports Server (NTRS)

    Delozier, D. M.; Watson, K. A.; Ghose, S.; Working, D. C.; Connell, J. W.; Smith, J. G.; Sun, Y. P.; Lin, Y.

    2006-01-01

    Ultem(TM) was mixed with three different carbon-based nanofillers in efforts to increase the thermal conductivity of the polymer. After initial mixing, the nanocomposites were extruded or processed via the Laboratory Mixing Molder (LMM) process. High resolution scanning electron microscopy (HRSEM) revealed significant alignment of the nanofillers in the extruded samples. Thermal conductivity measurements were made both in the direction and perpendicular to the direction of alignment of nanofillers as well as for unaligned samples. It was found that the largest improvement in thermal conductivity was achieved in the case of aligned samples when the measurement was performed in the direction of alignment. Unaligned samples also showed a significant improvement in thermal conductivity and may be useful in applications when it is not possible to align the nanofiller. However the improvements in thermal conductivity did not approach those expected based on a rule of mixtures. This is likely due to poor phonon transfer through the matrix.

  9. Carbon aerogel electrodes for direct energy conversion

    DOEpatents

    Mayer, S.T.; Kaschmitter, J.L.; Pekala, R.W.

    1997-02-11

    A direct energy conversion device, such as a fuel cell, using carbon aerogel electrodes is described, wherein the carbon aerogel is loaded with a noble catalyst, such as platinum or rhodium and soaked with phosphoric acid, for example. A separator is located between the electrodes, which are placed in a cylinder having plate current collectors positioned adjacent the electrodes and connected to a power supply, and a pair of gas manifolds, containing hydrogen and oxygen positioned adjacent the current collectors. Due to the high surface area and excellent electrical conductivity of carbon aerogels, the problems relative to high polarization resistance of carbon composite electrodes conventionally used in fuel cells are overcome. 1 fig.

  10. Carbon aerogel electrodes for direct energy conversion

    DOEpatents

    Mayer, Steven T.; Kaschmitter, James L.; Pekala, Richard W.

    1997-01-01

    A direct energy conversion device, such as a fuel cell, using carbon aerogel electrodes, wherein the carbon aerogel is loaded with a noble catalyst, such as platinum or rhodium and soaked with phosphoric acid, for example. A separator is located between the electrodes, which are placed in a cylinder having plate current collectors positioned adjacent the electrodes and connected to a power supply, and a pair of gas manifolds, containing hydrogen and oxygen positioned adjacent the current collectors. Due to the high surface area and excellent electrical conductivity of carbon aerogels, the problems relative to high polarization resistance of carbon composite electrodes conventionally used in fuel cells are overcome.

  11. Therma1 Conductivity and Durability of Advanced Thermal Barrier Coatings

    NASA Technical Reports Server (NTRS)

    Zhu, Dong-Ming; Miller, Robert A.

    2003-01-01

    Thermal barrier coatings (TBCs) will play a crucial role in advanced gas turbine engines because of their ability to further increase engine operating temperature and reduce cooling, thus helping to achieve engine emission and efficiency goals. Future TBCs must be designed with increased phase stability, lower thermal conductivity, and improved sintering and thermal stress resistance in order to effectively protect engine hot-section components. Advanced low conductivity TBCs are being developed at NASA by incorporating multi-component oxide dopants into zirconia-yttria or hafnia-yttria to promote the formation of thermodynamically stable defect clusters within the coating structures. This presentation will primarily focus on thermal conductivity and durability of the novel defect cluster thermal barrier coatings for turbine airfoil and combustor applications, determined by a unique CO2 laser heat-flux approach. The laser heat-flux testing approach emphasizes the real-time monitoring and assessment of the coating thermal conductivity under simulated engine temperature and thermal gradient conditions. The conductivity increase due to coating sintering (and/or phase change) and the conductivity decrease due to coating delamination have been determined under steady-state, cyclic, uniform or non-uniform heat-flux conditions. The coating radiation flux resistance has been evaluated by varying coating thermal gradients, and also by using a laser-heated radiative-flux source. Advanced multi-component TBC systems have been shown to have significantly reduced thermal conductivity and improved high temperature stability due to the nano-sized, low mobility defect clusters associated with the paired rare earth dopant additions. The effect of oxide defect cluster dopants on coating thermal conductivity, thermal stability and furnace cyclic durability will also be discussed. The current low conductivity TBC systems have demonstrated long-term cyclic durability at very high

  12. A theoretical model for lunar surface material thermal conductivity.

    NASA Technical Reports Server (NTRS)

    Khader, M. S.; Vachon, R. I.

    1973-01-01

    This paper presents a theoretical thermal conductivity model for the uppermost layer of lunar surface material under the lunar vacuum environment. The model assumes that the lunar soil can be simulated by spherical particles in contact with each other and that the effective thermal conductivity is a function of depth, temperature, porosity, particle dimension, and mechanical-thermal properties of the solid particles. Two modes of heat transport are considered, conduction and radiation - with emphasis on the contact resistance between particles. The model gives effective conductivity values that compare favorably with the experimental data from lunar surface samples obtained on Apollo 11 and 12 missions.

  13. Measurement of thermal conductivity in proton irradiated silicon

    SciTech Connect

    Marat Khafizov; Clarissa Yablinsky; Todd Allen; David Hurley

    2014-04-01

    We investigate the influence of proton irradiation on thermal conductivity in single crystal silicon. We apply laser based modulated thermoreflectance technique to extract the change in conductivity of the thin layer damaged by proton irradiation. Unlike time domain thermoreflectance techniques that require application of a metal film, we perform our measurement on uncoated samples. This provides greater sensitivity to the change in conductivity of the thin damaged layer. Using sample temperature as a parameter provides a means to deduce the primary defect structures that limit thermal transport. We find that under high temperature irradiation the degradation of thermal conductivity is caused primarily by extended defects.

  14. Thermally Conductive Tape Based on Carbon Nanotube Arrays

    NASA Technical Reports Server (NTRS)

    Kashani, Ali

    2011-01-01

    To increase contact conductance between two mating surfaces, a conductive tape has been developed by growing dense arrays of carbon nanotubes (CNTs, graphite layers folded into cylinders) on both sides of a thermally conductive metallic foil. When the two mating surfaces are brought into contact with the conductive tape in between, the CNT arrays will adhere to the mating surface. The van der Waals force between the contacting tubes and the mating surface provides adhesion between the two mating surfaces. Even though the thermal contact conductance of a single tube-to-tube contact is small, the tremendous amount of CNTs on the surface leads to a very large overall contact conductance. Interface contact thermal resistance rises from the microroughness and the macroscopic non-planar quality of mating surfaces. When two surfaces come into contact with each other, the actual contact area may be much less than the total area of the surfaces. The real area of contact depends on the load, the surface roughness, and the elastic and inelastic properties of the surface. This issue is even more important at cryogenic temperatures, where materials become hard and brittle and vacuum is used, which prevents any gas conduction through the interstitial region. A typical approach to increase thermal contact conductance is to use thermally conducting epoxies or greases, which are not always compatible with vacuum conditions. In addition, the thermal conductivities of these compounds are often relatively low. The CNTs used in this approach can be metallic or semiconducting, depending on the folding angle and diameter. The electrical resistivity of multiwalled carbon nanotubes (MWCNTs) has been reported. MWCNTs can pass a current density and remain stable at high temperatures in air. The thermal conductivity of a MWCNT at room temperature is measured to be approximately 3,000 W/m-K, which is much larger than that of diamond. At room temperature, the thermal conductance of a 0.3 sq cm

  15. Self-assembled and pyrolyzed carbon aerogels: an overview of their preparation mechanisms, properties and applications

    NASA Astrophysics Data System (ADS)

    Allahbakhsh, Ahmad; Bahramian, Ahmad Reza

    2015-08-01

    An overview of the synthesis conditions and mechanisms for the fabrication of different types of carbon aerogels, as well as the structural and functional properties of these materials, is presented here. In this overview, carbon aerogels are classified into three major categories: (i) conventional pyrolyzed organic-based carbon aerogels, which are products of the pyrolysis process of organic aerogels; (ii) self-assembled carbon aerogels, which are products of a reduction process; and (iii) nanocomposite carbon aerogels. Synthesis mechanisms for the sol-gel process of organic aerogels are reviewed using different mechanisms suggested in the literature. Moreover, the overall fabrication process of self-assembled carbon aerogels (graphene and carbon nanotube aerogels) is covered and the suggested mechanism for the gelation process of self-assembled carbon aerogels during the reduction process is investigated using reported mechanisms. The structural performance and functional properties (electrochemical and thermal properties) of different types of carbon aerogels are covered in detail. Moreover, different structural features of carbon aerogels and the influence of synthesis conditions on these structural characteristics are assessed and compared. Based on the literature results covered in this review paper, carbon aerogels are perfect candidates for the fabrication of ultra-low density supercapacitors, as well as thermal insulating materials.

  16. Self-assembled and pyrolyzed carbon aerogels: an overview of their preparation mechanisms, properties and applications.

    PubMed

    Allahbakhsh, Ahmad; Bahramian, Ahmad Reza

    2015-09-14

    An overview of the synthesis conditions and mechanisms for the fabrication of different types of carbon aerogels, as well as the structural and functional properties of these materials, is presented here. In this overview, carbon aerogels are classified into three major categories: (i) conventional pyrolyzed organic-based carbon aerogels, which are products of the pyrolysis process of organic aerogels; (ii) self-assembled carbon aerogels, which are products of a reduction process; and (iii) nanocomposite carbon aerogels. Synthesis mechanisms for the sol-gel process of organic aerogels are reviewed using different mechanisms suggested in the literature. Moreover, the overall fabrication process of self-assembled carbon aerogels (graphene and carbon nanotube aerogels) is covered and the suggested mechanism for the gelation process of self-assembled carbon aerogels during the reduction process is investigated using reported mechanisms. The structural performance and functional properties (electrochemical and thermal properties) of different types of carbon aerogels are covered in detail. Moreover, different structural features of carbon aerogels and the influence of synthesis conditions on these structural characteristics are assessed and compared. Based on the literature results covered in this review paper, carbon aerogels are perfect candidates for the fabrication of ultra-low density supercapacitors, as well as thermal insulating materials. PMID:26245296

  17. Flexible nanofiber-reinforced aerogel (xerogel) synthesis, manufacture, and characterization.

    PubMed

    Li, Lichun; Yalcin, Baris; Nguyen, Baochau N; Meador, Mary Ann B; Cakmak, Miko

    2009-11-01

    Silica aerogels are sol-gel-derived materials consisting of interconnected nanoparticle building blocks that form an open and highly porous three-dimensional silica network. Flexible aerogel films could have wide applications in various thermal insulation systems. However, aerogel thin films produced with a pure sol-gel process have inherent disadvantages, such as high fragility and moisture sensitivity, that hinder wider applications of these materials. We have developed synthesis and manufacturing methods to incorporate electrospun polyurethane nanofibers into the cast sol film prior to gelation of the silica-based gel in order to reinforce the structure and overcome disadvantages such as high fragility and poor mechanical strength. In this method, a two-stage sol-gel process was employed: (1) acid-catalyzed tetraethyl orthosilicate hydrolysis and (2) base-catalyzed gelation. By precisely controlling the sol gelation kinetics with the amount of base present in the formulation, nanofibers were electrospun into the sol before the onset of the gelation process and uniformly embedded in the silica network. Nanofiber reinforcement did not alter the thermal conductivity and rendered the final composite film bendable and flexible. PMID:20356119

  18. Effect of Aggregation on Thermal Conduction in Colloidal Nanofluids

    SciTech Connect

    R Prasher; W Evans; J Fish; P Meakin; P Phelan; Pawel Keblinski

    2006-08-10

    Using effective medium theory we demonstrate that the thermal conductivity of nanofluids can be significantly enhanced by the aggregation of nanoparticles into clusters. The enhancement is based purely on conduction and does not require a novel mechanism. Predictions of the effective medium theory are in excellent agreement with detailed numerical calculations on model nanofluids involving fractal clusters and show the importance of cluster morphology on thermal conductivity enhancements.

  19. Acoustic transducer apparatus with reduced thermal conduction

    NASA Technical Reports Server (NTRS)

    Lierke, Ernst G. (Inventor); Leung, Emily W. (Inventor); Bhat, Balakrishna T. (Inventor)

    1990-01-01

    A horn is described for transmitting sound from a transducer to a heated chamber containing an object which is levitated by acoustic energy while it is heated to a molten state, which minimizes heat transfer to thereby minimize heating of the transducer, minimize temperature variation in the chamber, and minimize loss of heat from the chamber. The forward portion of the horn, which is the portion closest to the chamber, has holes that reduce its cross-sectional area to minimize the conduction of heat along the length of the horn, with the entire front portion of the horn being rigid and having an even front face to efficiently transfer high frequency acoustic energy to fluid in the chamber. In one arrangement, the horn has numerous rows of holes extending perpendicular to the length of horn, with alternate rows extending perpendicular to one another to form a sinuous path for the conduction of heat along the length of the horn.

  20. Flexible aerogel composite for mechanical stability and process of fabrication

    SciTech Connect

    Coronado, P.R.; Poco, J.F.

    1999-10-26

    A flexible aerogel and process of fabrication are disclosed. An aerogel solution is mixed with fibers in a mold and allowed to gel. The gel is then processed by supercritical extraction, or by air drying, to produce a flexible aerogel formed to the shape of the mold. The flexible aerogel has excellent thermal and acoustic properties, and can be utilized in numerous applications, such as for energy absorption, insulation (temperature and acoustic), to meet the contours of aircraft shapes, and where space is limited since an inch of aerogel is a 4--5 times better insulator than an inch of fiberglass. The flexible aerogel may be of an inorganic (silica) type or an organic (carbon) type, but containing fibers, such as glass or carbon fibers.

  1. Flexible aerogel composite for mechanical stability and process of fabrication

    DOEpatents

    Coronado, Paul R.; Poco, John F.

    2000-01-01

    A flexible aerogel and process of fabrication. An aerogel solution is mixed with fibers in a mold and allowed to gel. The gel is then processed by supercritical extraction, or by air drying, to produce a flexible aerogel formed to the shape of the mold. The flexible aerogel has excellent thermal and acoustic properties, and can be utilized in numerous applications, such as for energy absorption, insulation (temperature and acoustic), to meet the contours of aircraft shapes, and where space is limited since an inch of aerogel is a 4-5 times better insulator than an inch of fiberglass. The flexible aerogel may be of an inorganic (silica) type or an organic (carbon) type, but containing fibers, such as glass or carbon fibers.

  2. Flexible aerogel composite for mechanical stability and process of fabrication

    DOEpatents

    Coronado, Paul R.; Poco, John F.

    1999-01-01

    A flexible aerogel and process of fabrication. An aerogel solution is mixed with fibers in a mold and allowed to gel. The gel is then processed by supercritical extraction, or by air drying, to produce a flexible aerogel formed to the shape of the mold. The flexible aerogel has excellent thermal and acoustic properties, and can be utilized in numerous applications, such as for energy absorption, insulation (temperature and acoustic), to meet the contours of aircraft shapes, and where space is limited since an inch of aerogel is a 4-5 times better insulator than an inch of fiberglass. The flexible aerogel may be of an inorganic (silica) type or an organic (carbon) type, but containing fibers, such as glass or carbon fibers.

  3. Highly compressible 3D periodic graphene aerogel microlattices.

    PubMed

    Zhu, Cheng; Han, T Yong-Jin; Duoss, Eric B; Golobic, Alexandra M; Kuntz, Joshua D; Spadaccini, Christopher M; Worsley, Marcus A

    2015-01-01

    Graphene is a two-dimensional material that offers a unique combination of low density, exceptional mechanical properties, large surface area and excellent electrical conductivity. Recent progress has produced bulk 3D assemblies of graphene, such as graphene aerogels, but they possess purely stochastic porous networks, which limit their performance compared with the potential of an engineered architecture. Here we report the fabrication of periodic graphene aerogel microlattices, possessing an engineered architecture via a 3D printing technique known as direct ink writing. The 3D printed graphene aerogels are lightweight, highly conductive and exhibit supercompressibility (up to 90% compressive strain). Moreover, the Young's moduli of the 3D printed graphene aerogels show an order of magnitude improvement over bulk graphene materials with comparable geometric density and possess large surface areas. Adapting the 3D printing technique to graphene aerogels realizes the possibility of fabricating a myriad of complex aerogel architectures for a broad range of applications. PMID:25902277

  4. Highly compressible 3D periodic graphene aerogel microlattices

    PubMed Central

    Zhu, Cheng; Han, T. Yong-Jin; Duoss, Eric B.; Golobic, Alexandra M.; Kuntz, Joshua D.; Spadaccini, Christopher M.; Worsley, Marcus A.

    2015-01-01

    Graphene is a two-dimensional material that offers a unique combination of low density, exceptional mechanical properties, large surface area and excellent electrical conductivity. Recent progress has produced bulk 3D assemblies of graphene, such as graphene aerogels, but they possess purely stochastic porous networks, which limit their performance compared with the potential of an engineered architecture. Here we report the fabrication of periodic graphene aerogel microlattices, possessing an engineered architecture via a 3D printing technique known as direct ink writing. The 3D printed graphene aerogels are lightweight, highly conductive and exhibit supercompressibility (up to 90% compressive strain). Moreover, the Young's moduli of the 3D printed graphene aerogels show an order of magnitude improvement over bulk graphene materials with comparable geometric density and possess large surface areas. Adapting the 3D printing technique to graphene aerogels realizes the possibility of fabricating a myriad of complex aerogel architectures for a broad range of applications. PMID:25902277

  5. Highly compressible 3D periodic graphene aerogel microlattices

    SciTech Connect

    Zhu, Cheng; Han, T. Yong-Jin; Duoss, Eric B.; Golobic, Alexandra M.; Kuntz, Joshua D.; Spadaccini, Christopher M.; Worsley, Marcus A.

    2015-04-22

    Graphene is a two-dimensional material that offers a unique combination of low density, exceptional mechanical properties, large surface area and excellent electrical conductivity. Recent progress has produced bulk 3D assemblies of graphene, such as graphene aerogels, but they possess purely stochastic porous networks, which limit their performance compared with the potential of an engineered architecture. Here we report the fabrication of periodic graphene aerogel microlattices, possessing an engineered architecture via a 3D printing technique known as direct ink writing. The 3D printed graphene aerogels are lightweight, highly conductive and exhibit supercompressibility (up to 90% compressive strain). Moreover, the Young’s moduli of the 3D printed graphene aerogels show an order of magnitude improvement over bulk graphene materials with comparable geometric density and possess large surface areas. Ultimately, adapting the 3D printing technique to graphene aerogels realizes the possibility of fabricating a myriad of complex aerogel architectures for a broad range of applications.

  6. Steady heat conduction-based thermal conductivity measurement of single walled carbon nanotubes thin film using a micropipette thermal sensor

    NASA Astrophysics Data System (ADS)

    Shrestha, R.; Lee, K. M.; Chang, W. S.; Kim, D. S.; Rhee, G. H.; Choi, T. Y.

    2013-03-01

    In this paper, we describe the thermal conductivity measurement of single-walled carbon nanotubes thin film using a laser point source-based steady state heat conduction method. A high precision micropipette thermal sensor fabricated with a sensing tip size varying from 2 μm to 5 μm and capable of measuring thermal fluctuation with resolution of ±0.01 K was used to measure the temperature gradient across the suspended carbon nanotubes (CNT) film with a thickness of 100 nm. We used a steady heat conduction model to correlate the temperature gradient to the thermal conductivity of the film. We measured the average thermal conductivity of CNT film as 74.3 ± 7.9 W m-1 K-1 at room temperature.

  7. Steady heat conduction-based thermal conductivity measurement of single walled carbon nanotubes thin film using a micropipette thermal sensor.

    PubMed

    Shrestha, R; Lee, K M; Chang, W S; Kim, D S; Rhee, G H; Choi, T Y

    2013-03-01

    In this paper, we describe the thermal conductivity measurement of single-walled carbon nanotubes thin film using a laser point source-based steady state heat conduction method. A high precision micropipette thermal sensor fabricated with a sensing tip size varying from 2 μm to 5 μm and capable of measuring thermal fluctuation with resolution of ±0.01 K was used to measure the temperature gradient across the suspended carbon nanotubes (CNT) film with a thickness of 100 nm. We used a steady heat conduction model to correlate the temperature gradient to the thermal conductivity of the film. We measured the average thermal conductivity of CNT film as 74.3 ± 7.9 W m(-1) K(-1) at room temperature. PMID:23556837

  8. Highly anisotropic thermal conductivity of arsenene: An ab initio study

    NASA Astrophysics Data System (ADS)

    Zeraati, Majid; Vaez Allaei, S. Mehdi; Abdolhosseini Sarsari, I.; Pourfath, Mahdi; Donadio, Davide

    2016-02-01

    Elemental two-dimensional (2D) materials exhibit intriguing heat transport and phononic properties. Here we have investigated the lattice thermal conductivity of newly proposed arsenene, the 2D honeycomb structure of arsenic, using ab initio calculations. Solving the Boltzmann transport equation for phonons, we predict a highly anisotropic thermal conductivity of 30.4 and 7.8 W/mK along the zigzag and armchair directions, respectively, at room temperature. Our calculations reveal that phonons with mean free paths between 20 nm and 1 μ m provide the main contribution to the large thermal conductivity in the zigzag direction; mean free paths of phonons contributing to heat transport in the armchair directions range between 20 and 100 nm. The obtained anisotropic thermal conductivity and feasibility of synthesis, in addition to high electron mobility reported elsewhere, make arsenene a promising material for nanoelectronic applications and thermal management.

  9. The best nanoparticle size distribution for minimum thermal conductivity

    PubMed Central

    Zhang, Hang; Minnich, Austin J.

    2015-01-01

    Which sizes of nanoparticles embedded in a crystalline solid yield the lowest thermal conductivity? Nanoparticles have long been demonstrated to reduce the thermal conductivity of crystals by scattering phonons, but most previous works assumed the nanoparticles to have a single size. Here, we use optimization methods to show that the best nanoparticle size distribution to scatter the broad thermal phonon spectrum is not a similarly broad distribution but rather several discrete peaks at well-chosen nanoparticle radii. For SiGe, the best size distribution yields a thermal conductivity below that of amorphous silicon. Further, we demonstrate that a simplified distribution yields nearly the same low thermal conductivity and can be readily fabricated. Our work provides important insights into how to manipulate the full spectrum of phonons and will guide the design of more efficient thermoelectric materials. PMID:25757414

  10. Resorcinol-formaldehyde and carbon aerogel microspheres

    SciTech Connect

    Alviso, C.T.; Pekela, R.W.; Gross, J.; Lu, X.; Caps, R.; Fricke, J

    1996-04-01

    Aerogels are a unique class of materials possessing an open-cell structure with ultrafine cells/pores (<100nm), high surface area (400--1100 m{sup 2}/g), and a solid matrix composed of interconnected particles, fibers, or platelets with characteristic dimensions of 10nm. Although monolithic aerogels are ideal candidates for many applications (e.g. transparent window insulation), current processing methods have limited their introduction into the commercial marketplace. Our research focuses on the formation of resorcinol-formaldehyde (RF) aerogel microspheres which offer an attractive alternative to monolith production. An inverse emulsion polymerization is used to produce these spherical gel particles which undergo solvent exchange followed by supercritical drying with carbon dioxide. This process yields aerogel microspheres (10--80{mu} diameter) which can be used as loosely packed powders, compression molded into near-net shapes using a polymer binder, or used as additives in conventional foaming operations to produce new aerogel composites with superior thermal properties. The emulsification procedure, thermal characterization, mechanical properties, and potential applications of RF aerogel microspheres will be discussed.

  11. Conductive ink containing thermally exfoliated graphite oxide and method a conductive circuit using the same

    NASA Technical Reports Server (NTRS)

    Prud'Homme, Robert K. (Inventor); Aksay, Ilhan A. (Inventor)

    2011-01-01

    A conductive ink containing a conductive polymer, wherein the conductive polymer contains at least one polymer and a modified graphite oxide material, which is a thermally exfoliated graphite oxide with a surface area of from about 300 sq m/g to 2600 sq m/g, and it use in a method for making a conductive circuit.

  12. Computer Modeling of the Thermal Conductivity of Cometary Ice

    NASA Technical Reports Server (NTRS)

    Bunch, Theodore E.; Wilson, Michael A.; Pohorille, Andrew

    1998-01-01

    The main objective of this research was to estimate the thermal conductivity of cometry ices from computer simulations of model amorphous ices. This was divided into four specific tasks: (1) Generating samples of amorphous water ices at different microporosities; (2) Comparing the resulting molecular structures of the ices with experimental results, for those densities where data was available; (3) Calculating the thermal conductivities of liquid water and bulk amorphous ices and comparing these results with experimentally determined thermal conductivities; and (4) Investigating how the thermal conductivity of amorphous ice depends upon the microscopic porosity of the samples. The thermal conductivity was found to be only weakly dependent on the microstructure of the amorphous ice. In general, the amorphous ices were found to have thermal conductivities of the same order of magnitude as liquid water. This is in contradiction to recent experimental estimates of the thermal conductivity of amorphous ice, and it is suggested that the extremely low value obtained experimentally is due to larger-scale defects in the ice, such as cracks, but it is not an intrinsic property of the bulk amorphous ice.

  13. Fractal prediction model of thermal contact conductance of rough surfaces

    NASA Astrophysics Data System (ADS)

    Ji, Cuicui; Zhu, Hua; Jiang, Wei

    2013-01-01

    The thermal contact conductance problem is an important issue in studying the heat transfer of engineering surfaces, which has been widely studied since last few decades, and for predicting which many theoretical models have been established. However, the models which have been existed are lack of objectivity due to that they are mostly studied based on the statistical methodology characterization for rough surfaces and simple partition for the deformation formats of contact asperity. In this paper, a fractal prediction model is developed for the thermal contact conductance between two rough surfaces based on the rough surface being described by three-dimensional Weierstrass and Mandelbrot fractal function and assuming that there are three kinds of asperity deformation modes: elastic, elastoplastic and fully plastic. Influences of contact load and contact area as well as fractal parameters and material properties on the thermal contact conductance are investigated by using the presented model. The investigation results show that the thermal contact conductance increases with the increasing of the contact load and contact area. The larger the fractal dimension, or the smaller the fractal roughness, the larger the thermal contact conductance is. The thermal contact conductance increases with decreasing the ratio of Young's elastic modulus to the microhardness. The results obtained indicate that the proposed model can effectively predict the thermal contact conductance at the interface, which provide certain reference to the further study on the issue of heat transfer between contact surfaces.

  14. Nano-casted Metal Oxide Aerogels as Dual Purpose Structural Components for Space Exploration

    NASA Technical Reports Server (NTRS)

    Vassilaras, Plousia E.

    2004-01-01

    NASA missions and space exploration rely on strong, ultra lightweight materials. Such materials are needed for building up past and present space vehicles such as the Sojourner Rover (1997) or the two MERs (2003), but also for a number of components and/or systems including thermal insulators, Solar Sails, Rigid Aeroshells, and Ballutes. The purpose of my internship here at Glenn Research Center is to make dual purpose materials; materials that in addition to being lightweight have electronic, photophysical and magnetic properties and, therefore, act as electronic components and sensors as well as structural components. One type of ultra lightweight material of great interest is aerogels, which have densities ranging from 0.003 g/cc to 0.8 g/cc . However, aerogels are extremely fragile and, as a result, have limited practical applications. Recently, Glenn Research Center has developed a process of nano-casting polymers onto the inorganic network of silica-based aerogels increasing the strength 300 fold while only increasing the density 3 fold. By combining the process of nano-casting polymers with inorganic oxide networks other than silica, we are actively pursuing lightweight dual purpose materials. To date, thirty different inorganic oxide aerogels have been prepared using either standard sol-gel chemistry or a non-alkoxide method involving metal chloride precursors and an epoxide; epichlorohydrin, propylene oxide or trimethylene oxide, as proton scavengers. More importantly, preliminary investigations show that the residual surface hydroxyl groups on each of these inorganic oxide aerogels can be successfully crosslinked with urethane. In addition to characterizing physical and mechanical properties such as density, strength and flexibility, each of these metal oxide aerogels are being characterized for thermal and electronic conductivity and magnetic and optical properties.

  15. Structure-Property Relationships in Porous 3-D Nanostructures as a Function of Preparation Conditions: Isocyanate Cross-Linked Silica Aerogels

    NASA Technical Reports Server (NTRS)

    Meador, Mary Ann B.; Capadona, Lynn A.; McCorkle, Linda; Papadopoulos, Demetrios S.; Leventis, Nicholas

    2007-01-01

    Sol-gel derived silica aerogels are attractive candidates for many unique thermal, optical, catalytic, and chemical applications because of their low density and high mesoporosity. However, their inherent fragility has restricted use of aerogel monoliths to applications where they are not subject to any load. We have previously reported cross-linking the mesoporous silica structure of aerogels with di-isocyanates, styrenes or epoxies reacting with amine decorated silica surfaces. These approaches have been shown to significantly increase the strength of aerogels with only a small effect on density or porosity. Though density is a prime predictor of properties such as strength and thermal conductivity for aerogels, it is becoming clear from previous studies that varying the silica backbone and size of the polymer cross-link independently can give rise to combinations of properties which cannot be predicted from density alone. Herein, we examine the effects of four processing parameters for producing this type of polymer cross-linked aerogel on properties of the resulting monoliths. We focus on the results of 13C CP-MAS NMR which gives insight to the size and structure of polymer cross-link present in the monoliths, and relates the size of the cross-links to microstructure, mechanical properties and other characteristics of the materials obtained.

  16. Structure-Property Relationships in Porous 3-D Nanostructures as a Function of Preparation Conditions: Isocyanate Cross-Linked Silica Aerogels

    NASA Technical Reports Server (NTRS)

    Meador, Mary Ann B.; Capadona, Lynn A.; McCorkle, Linda; Padadopoulos, Demetrios S.; Leventis, Nicholas

    2007-01-01

    Sol-gel derived silica aerogels are attractive candidates for many unique thermal, optical, catalytic, and chemical applications because of their low density and high mesoporosity. However, their inherent fragility has restricted use of aerogel monoliths to applications where they are not subject to any load. We have previously reported cross-linking the mesoporous silica structure of aerogels with di-isocyanates, styrenes or epoxies reacting with amine decorated silica surfaces. These approaches have been shown to significantly increase the strength of aerogels with only a small effect on density or porosity. Though density is a prime predictor of properties such as strength and thermal conductivity for aerogels, it is becoming clear from previous studies that varying the silica backbone and size of the polymer cross-link independently can give rise to combinations of properties which cannot be predicted from density alone. Herein, we examine the effects of four processing parameters for producing this type of polymer cross-linked aerogel on properties of the resulting monoliths. We focus on the results of C-13 CP-MAS NMR which gives insight to the size and structure of polymer cross-link present in the monoliths, and relates the size of the cross-links to microstructure, mechanical properties and other characteristics of the materials obtained.

  17. Aerogel dust collection for in situ mass spectrometry analysis

    NASA Astrophysics Data System (ADS)

    Jones, S. M.; Anderson, M. S.; Davies, A. G.; Kirby, J. P.; Burchell, M. J.; Cole, M. J.

    2015-02-01

    The current technique for conducting in situ mass spectroscopic analysis of dust around extraterrestrial bodies is to have the dust impact a solid plate and analyze the atoms and molecular fragments resulting from the high speed impact. Due to the fact that the kinetic energy from the impact is converted primarily to thermal energy, much of the organic compounds present in the dust may be significantly altered or destroyed. To avoid this problem, aerogel could be used to capture the dust grains, largely intact, maintaining the integrity of the organic compounds in the interior of the dust grains. To demonstrate that organic molecules, present as minor components of silica particles, would survive hypervelocity capture in aerogel and can then be analyzed with mass spectrometry, several light gas gun impact tests and analyses were conducted. Fine particles containing polycyclic aromatic hydrocarbons (PAHs) were captured in aerogel at 5.5 km s-1. The flow of metastable helium from a Direct Analysis Real Time (DART) source was used to desorb and ionize the organics, which were then analyzed with a mass spectrometer. The PAHs were detected and identified by the DART-MS, demonstrating that this method could be used on future flight instruments.

  18. Thermal Conductivity of Quantum Wires with Surface Roughness

    NASA Astrophysics Data System (ADS)

    Hershfield, Selman; Muttalib, Khandker

    Quantum wires have been shown to have greatly reduced thermal conductivity compared to bulk systems because of the increased role of surface scattering. The lattice thermal conductance and conductivity is calculated in the harmonic approximation for a long quantum wire placed between two heat baths using the Landauer formula for phonons and a recursive Green function technique to compute the transmission probabilities. The width of the wires is varied in the transverse direction so as to have a root mean square value σ and correlation length L. As observed experimentally, we find that the thermal conductance is decreased with increasing σ and increased as L increases. The full scaling of the thermal conductance as a function of σ, L, the width and the length of the sample is discussed. The simulations are also compared to approximate techniques such as modeling the surfaces as having diffusive scattering.

  19. Extremes of heat conduction-Pushing the boundaries of the thermal conductivity of materials

    SciTech Connect

    Cahill, DG

    2012-09-12

    Thermal conductivity is a familiar property of materials: silver conducts heat well, and plastic does not. In recent years, an interdisciplinary group of materials scientists, engineers, physicists, and chemists have succeeded in pushing back long-established limits in the thermal conductivity of materials. Carbon nanotubes and graphene are at the high end of the thermal conductivity spectrum due to their high sound velocities and relative lack of processes that scatter phonons. Unfortunately, the superlative thermal properties of carbon nanotubes have not found immediate application in composites or interface materials because of difficulties in making good thermal contact with the nanotubes. At the low end of the thermal conductivity spectrum, solids that combine order and disorder in the random stacking of two-dimensional crystalline sheets, so-called "disordered layered crystals," show a thermal conductivity that is only a factor of 2 larger than air. The cause of this low thermal conductivity may be explained by the large anisotropy in elastic constants that suppresses the density of phonon modes that propagate along the soft direction. Low-dimensional quantum magnets demonstrate that electrons and phonons are not the only significant carriers of heat. Near room temperature, the spin thermal conductivity of spin-ladders is comparable to the electronic thermal conductivities of metals. Our measurements of nanoscale thermal transport properties employ a variety of ultrafast optical pump-probe metrology tools that we have developed over the past several years. We are currently working to extend these techniques to high pressures (60 GPa), high magnetic fields (5 T), and high temperatures (1000 K).

  20. Low temperature thermal hall conductivity of a nodal chiral superconductor

    NASA Astrophysics Data System (ADS)

    Yip, Sungkit

    2016-08-01

    Motivated by Sr2RuO4, we consider a chiral superconductor where the gap is strongly suppressed along certain momentum directions. We evaluate the thermal Hall conductivity in the gapless regime, i.e., at low temperature compared with the impurity band width γ, taking the simplest model of isotropic impurity scattering. We find that, under favorable circumstances, this thermal Hall conductivity can be quite significant and is smaller than the diagonal component (the universal thermal conductivity) only by a factor of 1/{ln}(2{{{Δ }}}M/γ ), where {{{Δ }}}M is the maximum gap.

  1. A thermal conductivity cell for small powdered samples.

    NASA Technical Reports Server (NTRS)

    Cremers, C. J.

    1971-01-01

    A thermal conductivity cell is described for making measurements of the thermal conductivity and thermal diffusivity of small samples of powdered dielectric materials. The principle used is that of the line heat source. A novel way is described for applying this method so that much smaller samples than normal may be tested. This size requirement is necessary for investigations involving limited samples as does the Lunar Science Program. The method is checked by measuring the conductivity of standard samples and comparing the results with those found in the literature.

  2. Mechanisms and models of effective thermal conductivities of nanofluids.

    SciTech Connect

    Yu, W.; France, D. M.; Singh, D.; Timofeeva, E. V.; Smith, D. S.; Routbort, J. L.; Univ. of Illinois

    2010-08-01

    The physical mechanisms and mathematical models of the effective thermal conductivities of nanofluids have long been of interest to the nanofluid research community because the effective thermal conductivities of nanofluids cannot generally be fully explained and predicted by classical effective medium theories. This review article summarizes considerable progress made on this topic. Specifically, the physical mechanisms and mathematical models of the effective thermal conductivities of nanofluids are reviewed, the potential contributions of those physical mechanisms are evaluated, and the comparisons of the theoretical predictions and experimental data are presented along with opportunities for future research.

  3. Thermal conductivity of mechanically joined semiconducting/metal nanomembrane superlattices.

    PubMed

    Grimm, Daniel; Wilson, Richard B; Teshome, Bezuayehu; Gorantla, Sandeep; Rümmeli, Mark H; Bublat, Thomas; Zallo, Eugenio; Li, Guodong; Cahill, David G; Schmidt, Oliver G

    2014-05-14

    The decrease of thermal conductivity is crucial for the development of efficient thermal energy converters. Systems composed of a periodic set of very thin layers show among the smallest thermal conductivities reported to-date. Here, we fabricate in an unconventional but straightforward way hybrid superlattices consisting of a large number of nanomembranes mechanically stacked on top of each other. The superlattices can consist of an arbitrary composition of n- or p-type doped single-crystalline semiconductors and a polycrystalline metal layer. These hybrid multilayered systems are fabricated by taking advantage of the self-rolling technique. First, differentially strained nanomembranes are rolled into three-dimensional microtubes with multiple windings. By applying vertical pressure, the tubes are then compressed and converted into a planar hybrid superlattice. The thermal measurements show a substantial reduction of the cross-sectional heat transport through the nanomembrane superlattice compared to a single nanomembrane layer. Time-domain thermoreflectance measurements yield thermal conductivity values below 2 W m(-1) K(-1). Compared to bulk values, this represents a reduction of 2 orders of magnitude by the incorporation of the mechanically joined interfaces. The scanning thermal atomic force microscopy measurements support the observation of reduced thermal transport on top of the superlattices. In addition, small defects with a spatial resolution of ∼100 nm can be resolved in the thermal maps. The low thermal conductivity reveals the potential of this approach to fabricate miniaturized on-chip solutions for energy harvesters in, e.g., microautonomous systems. PMID:24738656

  4. Microstructure modifications of silicates induced by the collection in aerogel: Experimental approach and comparison with Stardust results

    NASA Astrophysics Data System (ADS)

    Stodolna, Julien; Jacob, Damien; Leroux, Hugues; Burchell, Mark J.

    2012-04-01

    Particles from comet 81P/Wild 2 were captured with silica aerogel during the flyby Stardust mission. A significant part of the collection was damaged during the impact at hypervelocity in the aerogel. In this study, we conducted impact experiments into aerogel of olivine and pyroxene powder using a light-gas gun in similar conditions as that of the comet Wild 2 particles collection. The shot samples were investigated using transmission electron microscopy to characterize their microstructure. Both olivine and pyroxene samples show evidence of thermal alteration due to friction with the aerogel. All the grains have rounded edges after collection, whereas their shape was angular in the initial shot powder set. This is probably associated with mass loss of particles. The rims of the grains are clearly melted and mixed with aerogel. The core of olivine grains is fairly well preserved, but some grains contain dislocations in glide configuration. We interpret these dislocations as generated by the thermal stresses that have emerged due to the high temperature gradients between the core and the rim of the grains. Most of the pyroxene grains have been fully melted. Their high silica concentration reflects a strong impregnation with melted aerogel. The preferential melting of pyroxene compared with olivine is due to a difference in melting temperatures of 300°. This melting point difference probably induces a bias in the measurements of the ratio olivine/pyroxene in the Wild 2 comet. The proportion of pyroxene was probably higher on Wild 2 than expected from the samples collected into aerogel.

  5. In-Pile Thermal Conductivity Measurement Method for Nuclear Fuels

    SciTech Connect

    Joy L. Rempe; Brandon Fox; Heng Ban; Joshua E. Daw; Darrell L. Knudson; Keith G. Condie

    2009-08-01

    Thermophysical properties of advanced nuclear fuels and materials during irradiation must be known prior to their use in existing, advanced, or next generation reactors. Thermal conductivity is one of the most important properties for predicting fuel and material performance. A joint Utah State University (USU) / Idaho National Laboratory (INL) project, which is being conducted with assistance from the Institute for Energy Technology at the Norway Halden Reactor Project, is investigating in-pile fuel thermal conductivity measurement methods. This paper focuses on one of these methods – a multiple thermocouple method. This two-thermocouple method uses a surrogate fuel rod with Joule heating to simulate volumetric heat generation to gain insights about in-pile detection of thermal conductivity. Preliminary results indicated that this method can measure thermal conductivity over a specific temperature range. This paper reports the thermal conductivity values obtained by this technique and compares these values with thermal property data obtained from standard thermal property measurement techniques available at INL’s High Test Temperature Laboratory. Experimental results and material properties data are also compared to finite element analysis results.

  6. Remarkable reduction of thermal conductivity in phosphorene phononic crystal

    NASA Astrophysics Data System (ADS)

    Xu, Wen; Zhang, Gang

    2016-05-01

    Phosphorene has received much attention due to its interesting physical and chemical properties, and its potential applications such as thermoelectricity. In thermoelectric applications, low thermal conductivity is essential for achieving a high figure of merit. In this work, we propose to reduce the thermal conductivity of phosphorene by adopting the phononic crystal structure, phosphorene nanomesh. With equilibrium molecular dynamics simulations, we find that the thermal conductivity is remarkably reduced in the phononic crystal. Our analysis shows that the reduction is due to the depressed phonon group velocities induced by Brillouin zone folding, and the reduced phonon lifetimes in the phononic crystal. Interestingly, it is found that the anisotropy ratio of thermal conductivity could be tuned by the ‘non-square’ pores in the phononic crystal, as the phonon group velocities in the direction with larger projection of pores is more severely suppressed, leading to greater reduction of thermal conductivity in this direction. Our work provides deep insight into thermal transport in phononic crystals and proposes a new strategy to reduce the thermal conductivity of monolayer phosphorene.

  7. Remarkable reduction of thermal conductivity in phosphorene phononic crystal.

    PubMed

    Xu, Wen; Zhang, Gang

    2016-05-01

    Phosphorene has received much attention due to its interesting physical and chemical properties, and its potential applications such as thermoelectricity. In thermoelectric applications, low thermal conductivity is essential for achieving a high figure of merit. In this work, we propose to reduce the thermal conductivity of phosphorene by adopting the phononic crystal structure, phosphorene nanomesh. With equilibrium molecular dynamics simulations, we find that the thermal conductivity is remarkably reduced in the phononic crystal. Our analysis shows that the reduction is due to the depressed phonon group velocities induced by Brillouin zone folding, and the reduced phonon lifetimes in the phononic crystal. Interestingly, it is found that the anisotropy ratio of thermal conductivity could be tuned by the 'non-square' pores in the phononic crystal, as the phonon group velocities in the direction with larger projection of pores is more severely suppressed, leading to greater reduction of thermal conductivity in this direction. Our work provides deep insight into thermal transport in phononic crystals and proposes a new strategy to reduce the thermal conductivity of monolayer phosphorene. PMID:27033566

  8. Predicting lattice thermal conductivity with help from ab initio methods

    NASA Astrophysics Data System (ADS)

    Broido, David

    2015-03-01

    The lattice thermal conductivity is a fundamental transport parameter that determines the utility a material for specific thermal management applications. Materials with low thermal conductivity find applicability in thermoelectric cooling and energy harvesting. High thermal conductivity materials are urgently needed to help address the ever-growing heat dissipation problem in microelectronic devices. Predictive computational approaches can provide critical guidance in the search and development of new materials for such applications. Ab initio methods for calculating lattice thermal conductivity have demonstrated predictive capability, but while they are becoming increasingly efficient, they are still computationally expensive particularly for complex crystals with large unit cells . In this talk, I will review our work on first principles phonon transport for which the intrinsic lattice thermal conductivity is limited only by phonon-phonon scattering arising from anharmonicity. I will examine use of the phase space for anharmonic phonon scattering and the Grüneisen parameters as measures of the thermal conductivities for a range of materials and compare these to the widely used guidelines stemming from the theory of Liebfried and Schölmann. This research was supported primarily by the NSF under Grant CBET-1402949, and by the S3TEC, an Energy Frontier Research Center funded by the US DOE, office of Basic Energy Sciences under Award No. DE-SC0001299.

  9. Processing dependent thermal conductivity of nanoporous silica xerogel films

    NASA Astrophysics Data System (ADS)

    Jain, Anurag; Rogojevic, Svetlana; Ponoth, Shom; Gill, William N.; Plawsky, Joel L.; Simonyi, Eva; Chen, Shyng-Tsong; Ho, P. S.

    2002-03-01

    Sintered xerogel films (porous SiO2) show a much higher thermal conductivity than other low dielectric constant (low-K) materials available for the same value of K. The thermal conductivity of xerogels which we have processed using different methods is compared with that of other low-K materials such as silica hybrid (silsesquioxanes) and polymeric low-K materials. The methods used were: (1) single solvent (ethanol) method, (2) binary solvent (mixture of ethanol and ethylene glycol) method, (3) sintering. For the xerogel films, we show that process history is as important as the chemistry of the solid matrix or the porosity in determining the thermal conductivity. The thermal conductivity, measured by the 3-ω method or the photothermal deflection method, is affected by phonon scattering, which in turn is effected by the size and distribution of pores and particles and the presence of imperfections such as interfaces, substituted chemical species, impurities, microcracks, and microporosity. The thermal conductivity extrapolated to zero porosity for porous sintered xerogel films approaches that of thermally grown SiO2 indicating the least phonon scattering of all processing methods. For these films, the elastic modulus is proportional to thermal conductivity squared, in agreement with theories developed for materials with few defects and a connected matrix.

  10. Fabrication and Characterization of a Conduction Cooled Thermal Neutron Filter

    SciTech Connect

    Heather Wampler; Adam Gerth; Heng Ban; Donna Post Guillen; Douglas Porter; Cynthia Papesch

    2010-06-01

    Installation of a conduction cooled thermal (low-energy) neutron filter in an existing domestic test reactor would provide the U.S. the capability to test new reactor fuels and materials for advanced fast (high-energy) reactor concepts. A composite consisting of Al3Hf-Al has been proposed for the neutron filter due to both the neutron filtering properties of hafnium and the conducting capabilities of aluminum. Knowledge of the thermal conductivity of the Al3Hf-Al composite is essential for the design of the filtering system. The present objectives are to identify a suitable fabrication technique and to measure the thermophysical properties of the Al3Hf intermetallic, which has not been done previous to this study. A centrifugal casting method was used to prepare samples of Al3Hf. X-ray diffraction and Rietveld analysis were conducted to determine the structural make-up of each of the samples. Thermophysical properties were measured as follows: specific heat by a differential scanning calorimeter (DSC), thermal diffusivity by a laser flash thermal diffusivity measuring system, thermal expansion by a dilatometer, and thermal conductivity was calculated based on the previous measurements. All measurements were acquired over a temperature range of 90°C - 375°C with some measurements outside these bounds. The average thermal conductivity of the intermetallic Al3Hf (~7 at.% Hf) was found to be ~ 41 W/m-K for the given temperature range. This information fills a knowledge gap in the thermophysical properties of the intermetallic Al3Hf with the specified percentage of hafnium. A model designed to predict composite properties was used to calculate a thermal conductivity of ~177 W/m-K for an Al3Hf-Al composite with 23 vol% Al3Hf. This calculation was based upon the average thermal conductivity of Al3Hf over the specified temperature range.

  11. Submicron Mapping of Thermal Conductivity of Thermoelectric Thin Films

    NASA Astrophysics Data System (ADS)

    Lo, Hsinyi; Ram, Rajeev J.

    2012-06-01

    A tool for evaluating thin-film thermal conductivity to submicron spatial resolution has been developed. The micro-instrumentation utilizes the thermoreflectance (TR) technique to characterize thermal conductivity and material uniformity. The instrument consists of a heating element for creating temperature gradients and an Invar bar with in situ temperature monitoring for heat flux measurements. The thin-film sample is sandwiched between the heater and Invar bar while a microscope is used to direct light onto a cross-section of the sample and reflected light is collected with a camera. By using this technique, we can achieve submicron spatial resolution for thermal conductivity and eliminate contributions from thermal contact resistance, thereby also eliminating the need for sample preparation other than cleaving. The method offers temperature resolution of 10 mK, spatial resolution of 200 nm, and thermal conductivity measurement with 0.01 ± 0.001 W/mK resolution. The thermal conductivity of a 0.6% ErAs:InGaAlAs thermoelectric (TE) element, prepared by molecular beam epitaxy (MBE) growth, obtained with the new instrument is 2.3 W/mK, while the average thermal conductivity obtained with the 3-omega method is 2.5 W/mK. Energy-dispersive x-ray (EDX) spectroscopy is also used to prove that the elemental composition has uniformity consistent with the material variation observed by the TR technique. Moreover, a temperature profile across a 0.6% ErAs:InGaAlAs TE element on InP substrate is imaged. Two different slopes, corresponding to different thermal conductivities, have been observed, showing that the thermal conductivity of the TE element is lower than that of the InP substrate as expected.

  12. Supercapacitors Based on Three-Dimensional Hierarchical Graphene Aerogels with Periodic Macropores.

    PubMed

    Zhu, Cheng; Liu, Tianyu; Qian, Fang; Han, T Yong-Jin; Duoss, Eric B; Kuntz, Joshua D; Spadaccini, Christopher M; Worsley, Marcus A; Li, Yat

    2016-06-01

    Graphene is an atomically thin, two-dimensional (2D) carbon material that offers a unique combination of low density, exceptional mechanical properties, thermal stability, large surface area, and excellent electrical conductivity. Recent progress has resulted in macro-assemblies of graphene, such as bulk graphene aerogels for a variety of applications. However, these three-dimensional (3D) graphenes exhibit physicochemical property attenuation compared to their 2D building blocks because of one-fold composition and tortuous, stochastic porous networks. These limitations can be offset by developing a graphene composite material with an engineered porous architecture. Here, we report the fabrication of 3D periodic graphene composite aerogel microlattices for supercapacitor applications, via a 3D printing technique known as direct-ink writing. The key factor in developing these novel aerogels is creating an extrudable graphene oxide-based composite ink and modifying the 3D printing method to accommodate aerogel processing. The 3D-printed graphene composite aerogel (3D-GCA) electrodes are lightweight, highly conductive, and exhibit excellent electrochemical properties. In particular, the supercapacitors using these 3D-GCA electrodes with thicknesses on the order of millimeters display exceptional capacitive retention (ca. 90% from 0.5 to 10 A·g(-1)) and power densities (>4 kW·kg(-1)) that equal or exceed those of reported devices made with electrodes 10-100 times thinner. This work provides an example of how 3D-printed materials, such as graphene aerogels, can significantly expand the design space for fabricating high-performance and fully integrable energy storage devices optimized for a broad range of applications. PMID:26789202

  13. Ultralow thermal conductivity in highly anion-defective aluminates.

    PubMed

    Wan, Chunlei; Qu, Zhixue; He, Yong; Luan, Dong; Pan, Wei

    2008-08-22

    Ultralow thermal conductivity (1.1 W/m.K, 1000 degrees C) in anion-deficient Ba2RAlO5 (R=Dy, Er, Yb) compounds was reported. The low thermal conductivity was then analyzed by kinetic theory. The highly defective structure of Ba2RAlO5 results in weak atomic bond strength and low sound speeds, and phonon scattering by large concentration of oxygen vacancies reduces the phonon mean free path to the order of interatomic distance. Ba2DyAlO5 exhibits the shortest phonon mean free path and lowest thermal conductivity among the three compositions investigated, which can be attributed to additional phonon scattering by DyO6 octahedron tilting as a result of a low tolerance factor. The Ba2RAlO5 (R=Dy, Er, Yb) compounds have shown great potential in high-temperature thermal insulation applications, particularly as a thermal barrier coating material. PMID:18764638

  14. Engineering thermal conductance using a two-dimensional phononic crystal

    PubMed Central

    Zen, Nobuyuki; Puurtinen, Tuomas A.; Isotalo, Tero J.; Chaudhuri, Saumyadip; Maasilta, Ilari J.

    2014-01-01

    Controlling thermal transport has become relevant in recent years. Traditionally, this control has been achieved by tuning the scattering of phonons by including various types of scattering centres in the material (nanoparticles, impurities, etc). Here we take another approach and demonstrate that one can also use coherent band structure effects to control phonon thermal conductance, with the help of periodically nanostructured phononic crystals. We perform the experiments at low temperatures below 1 K, which not only leads to negligible bulk phonon scattering, but also increases the wavelength of the dominant thermal phonons by more than two orders of magnitude compared to room temperature. Thus, phononic crystals with lattice constants ≥1 μm are shown to strongly reduce the thermal conduction. The observed effect is in quantitative agreement with the theoretical calculation presented, which accurately determined the ballistic thermal conductance in a phononic crystal device. PMID:24647049

  15. Indirect measurement of thermal conductivity in silicon nanowires

    SciTech Connect

    Pennelli, Giovanni Nannini, Andrea; Macucci, Massimo

    2014-02-28

    We report indirect measurements of thermal conductivity in silicon nanostructures. We have exploited a measurement technique based on the Joule self-heating of silicon nanowires. A standard model for the electron mobility has been used to determine the temperature through the accurate measurement of the nanowire resistance. We have applied this technique to devices fabricated with a top-down process that yields nanowires together with large silicon areas used both as electrical and as thermal contacts. As there is crystalline continuity between the nanowires and the large contact areas, our thermal conductivity measurements are not affected by any temperature drop due to the contact thermal resistance. Our results confirm the observed reduction of thermal conductivity in nanostructures and are comparable with those previously reported in the literature, achieved with more complex measurement techniques.

  16. Manipulating Steady Heat Conduction by Sensu-shaped Thermal Metamaterials

    PubMed Central

    Han, Tiancheng; Bai, Xue; Liu, Dan; Gao, Dongliang; Li, Baowen; Thong, John T. L.; Qiu, Cheng-Wei

    2015-01-01

    The ability to design the control of heat flow has innumerable benefits in the design of electronic systems such as thermoelectric energy harvesters, solid-state lighting, and thermal imagers, where the thermal design plays a key role in performance and device reliability. In this work, we employ one identical sensu-unit with facile natural composition to experimentally realize a new class of thermal metamaterials for controlling thermal conduction (e.g., thermal concentrator, focusing/resolving, uniform heating), only resorting to positioning and locating the same unit element of sensu-shape structure. The thermal metamaterial unit and the proper arrangement of multiple identical units are capable of transferring, redistributing and managing thermal energy in a versatile fashion. It is also shown that our sensu-shape unit elements can be used in manipulating dc currents without any change in the layout for the thermal counterpart. These could markedly enhance the capabilities in thermal sensing, thermal imaging, thermal-energy storage, thermal packaging, thermal therapy, and more domains beyond. PMID:25974383

  17. Fiber/Matrix Interfacial Thermal Conductance Effect on the Thermal Conductivity of SiC/SiC Composites

    SciTech Connect

    Nguyen, Ba Nghiep; Henager, Charles H.

    2013-04-20

    SiC/SiC composites used in fusion reactor applications are subjected to high heat fluxes and require knowledge and tailoring of their in-service thermal conductivity. Accurately predicting the thermal conductivity of SiC/SiC composites as a function of temperature will guide the design of these materials for their intended use, which will eventually include the effects of 14-MeV neutron irradiations. This paper applies an Eshelby-Mori-Tanaka approach (EMTA) to compute the thermal conductivity of unirradiated SiC/SiC composites. The homogenization procedure includes three steps. In the first step EMTA computes the homogenized thermal conductivity of the unidirectional (UD) SiC fiber embraced by its coating layer. The second step computes the thermal conductivity of the UD composite formed by the equivalent SiC fibers embedded in a SiC matrix, and finally the thermal conductivity of the as-formed SiC/SiC composite is obtained by averaging the solution for the UD composite over all possible fiber orientations using the second-order fiber orientation tensor. The EMTA predictions for the transverse thermal conductivity of several types of SiC/SiC composites with different fiber types and interfaces are compared to the predicted and experimental results by Youngblood et al.

  18. Thermal conductance modulator based on folded graphene nanoribbons

    SciTech Connect

    Ouyang, Tao; Chen, Yuanping; Xie, Yuee; Stocks, George Malcolm; Zhong, Jianxin

    2011-01-01

    Based on folded graphene nanoribbons, we report a thermal conductance modulator which performs analogous operations as the rheostat in electronic circuits. This fundamental device can controllably and reversibly modulate the thermal conductance by varying the geometric structures and its tuning range can be up to 40% of the conductance of unfolded nanoribbons ( 1 nm wide and 7 15 nm long). Under this modulation, the conductance shows a linearly dependence on the folded angle, while undergoes a transition with the variation of the inter-layer distance. This primary thermal device may have great potential applications for phononic circuits and nanoscale thermal management. VC2011 American Institute of Physics. [doi:10.1063/1.3665184

  19. Comparative Study of the Thermal Conductivity of Solid Biomass Fuels

    PubMed Central

    2016-01-01

    The thermal conductivity of solid biomass fuels is useful information in the investigation of biomass combustion behavior and the development of modeling especially in the context of large scale power generation. There are little published data on the thermal conductivity of certain types of biomass such as wheat straw, miscanthus, and torrefied woods. Much published data on wood is in the context of bulk materials. A method for determining the thermal conductivities of small particles of biomass fuels has been developed using a custom built test apparatus. Fourteen different samples of various solid biomass fuel were processed to form a homogenized pellet for analysis. The thermal conductivities of the pelletized materials were determined and compared against each other and to existing data. PMID:27041819

  20. Thermal Conductivity of Gas Mixtures in Chemical Equilibrium

    NASA Technical Reports Server (NTRS)

    Brokaw, Richard S.

    1960-01-01

    The expression for the thermal conductivity of gas mixtures in chemical equilibrium is presented in a simpler and less restrictive form. This new form is shown to be equivalent to the previous equations.

  1. Lattice thermal conductance of quantum wires with disorder

    NASA Astrophysics Data System (ADS)

    Vyhmeister, Erik; Hershfield, Selman

    We model the lattice thermal conductance in long quantum wires connected to two large heat baths at different temperatures in the harmonic approximation. The thermal conductance is computed with the Landauer formula for phonons, where it is related to the sum over all transmission probabilities for phonons through the wire. The net transmission probability is computed using a recursive Green function technique, which allows one to study long wires efficiently. We consider several different kinds of disorder to reduce the lattice thermal conductivity: periodic rectangular holes of varying sizes and shapes, periodic triangular holes, and narrow bands, averaged over randomness to account for variance in manufacturing. Depending on the model, the thermal conductance was reduced by 80 percent or more from the perfectly ordered wire case. Funded by NSF grant DMR-1461019.

  2. Experimental Determination of Thermal Conductivity of Low-Density Ice

    NASA Technical Reports Server (NTRS)

    Coles, Willard D.

    1954-01-01

    The thermal conductivity of low-density ice has been computed from data obtained in an experimental investigation of the heat transfer and mass transfer by sublimation for an iced surface on a flat plate in a high-velocity tangential air stream. The results are compared with data from several sources on the thermal conductivity of packed snow and solid glaze ice. The results show good agreement with the equations for the thermal conductivity of packed snow as a function of snow density. The agreement of the curves for packed snow near the solid ice regime with the values of thermal conductivity, of ice indicates that the curves are applicable over the entire-ice-density range.

  3. Anisotropic lattice thermal conductivity in chiral tellurium from first principles

    NASA Astrophysics Data System (ADS)

    Peng, Hua; Kioussis, Nicholas; Stewart, Derek A.

    2015-12-01

    Using ab initio based calculations, we have calculated the intrinsic lattice thermal conductivity of chiral tellurium. We show that the interplay between the strong covalent intrachain and weak van der Waals interchain interactions gives rise to the phonon band gap between the lower and higher optical phonon branches. The underlying mechanism of the large anisotropy of the thermal conductivity is the anisotropy of the phonon group velocities and of the anharmonic interatomic force constants (IFCs), where large interchain anharmonic IFCs are associated with the lone electron pairs. We predict that tellurium has a large three-phonon scattering phase space that results in low thermal conductivity. The thermal conductivity anisotropy decreases under applied hydrostatic pressure.

  4. Thermal conductance of pressed brass contacts at liquid helium temperatures

    NASA Technical Reports Server (NTRS)

    Salerno, L. J.; Kittel, P.; Brooks, W. F.; Spivak, A. L.; Marks, W. G., Jr.

    1986-01-01

    An apparatus has been designed and fabricated which will measure the thermal conductance of pressed contacts at liquid helium temperatures as a function of applied force, with surface finish as a parameter. The apparatus is automated and was used to measure thermal conductance at temperatures from 1.5 to 6.5 K at applied forces up to 700 N for brass sample pairs having surface finishes from 0.1 to 1.6 micron rms. The experimental data were found to fit a simple power law where the thermal conductance is given by k = alpha T exp n, where k is the thermal conductance, T is the absolute temperature, and alpha and n are empirically determined constants.

  5. Enhanced thermal conductivity through the development of nanofluids

    SciTech Connect

    Eastman, J.A.; Choi, U.S.; Li, S.; Thompson, L.J.; Lee, S.

    1996-11-01

    Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids required in many industrial applications. To overcome this limitation, a new class of heat transfer fluids is being developed by suspending nanocrystalline particles in liquids such as water or oil. The resulting nanofluids possess extremely high thermal conductivities compared to the liquids without dispersed nanocrystalline particles. For example, 5 volume % of nanocrystalline copper oxide particles suspended in water results in an improvement in thermal conductivity of almost 60% compared to water without nanoparticles. Excellent suspension properties are also observed, with no significant settling of nanocrystalline oxide particles occurring in stationary fluids over time periods longer than several days. Direct evaporation of Cu nanoparticles into pump oil results in similar improvements in thermal conductivity compared to oxide-in-water systems, but importantly, requires far smaller concentrations of dispersed nanocrystalline powder.

  6. Calculation of the lattice thermal conductivity in granular crystals

    SciTech Connect

    Kazan, M.; Volz, S.

    2014-02-21

    This paper provides a general model for the lattice thermal conductivity in granular crystals. The key development presented in this model is that the contribution of surface phonons to the thermal conductivity and the interplay between phonon anharmonic scattering and phonon scattering by boundaries are considered explicitly. Exact Boltzmann equation including spatial dependence of phonon distribution function is solved to yield expressions for the rates at which phonons scatter by the grain boundaries in the presence of intrinsic phonon scattering mechanisms. The intrinsic phonon scattering rates are calculated from Fermi's golden rule, and the vibration parameters of the model are derived as functions of temperature and crystallographic directions by using a lattice dynamics approach. The accuracy of the model is demonstrated with reference to experimental measurements regarding the effects of surface orientation and isotope composition on the thermal conductivity in single crystals, and the effect of grains size and shape on the thermal conductivity tensor in granular crystals.

  7. Anisotropic lattice thermal conductivity in chiral tellurium from first principles

    SciTech Connect

    Peng, Hua; Kioussis, Nicholas; Stewart, Derek A.

    2015-12-21

    Using ab initio based calculations, we have calculated the intrinsic lattice thermal conductivity of chiral tellurium. We show that the interplay between the strong covalent intrachain and weak van der Waals interchain interactions gives rise to the phonon band gap between the lower and higher optical phonon branches. The underlying mechanism of the large anisotropy of the thermal conductivity is the anisotropy of the phonon group velocities and of the anharmonic interatomic force constants (IFCs), where large interchain anharmonic IFCs are associated with the lone electron pairs. We predict that tellurium has a large three-phonon scattering phase space that results in low thermal conductivity. The thermal conductivity anisotropy decreases under applied hydrostatic pressure.

  8. Toward nanofluids of ultra-high thermal conductivity

    PubMed Central

    2011-01-01

    The assessment of proposed origins for thermal conductivity enhancement in nanofluids signifies the importance of particle morphology and coupled transport in determining nanofluid heat conduction and thermal conductivity. The success of developing nanofluids of superior conductivity depends thus very much on our understanding and manipulation of the morphology and the coupled transport. Nanofluids with conductivity of upper Hashin-Shtrikman (H-S) bound can be obtained by manipulating particles into an interconnected configuration that disperses the base fluid and thus significantly enhancing the particle-fluid interfacial energy transport. Nanofluids with conductivity higher than the upper H-S bound could also be developed by manipulating the coupled transport among various transport processes, and thus the nature of heat conduction in nanofluids. While the direct contributions of ordered liquid layer and particle Brownian motion to the nanofluid conductivity are negligible, their indirect effects can be significant via their influence on the particle morphology and/or the coupled transport. PMID:21711677

  9. Thermal conductance of pressed contacts at liquid helium temperatures

    SciTech Connect

    Salerno, L.J.; Kittel, P.; Spivak, A.L.

    1983-05-01

    The thermal contact conductance of a 0.4 micrometer surface finish OFHC copper sample pair has been investigated from 1.6 to 3.8 K for a range of applied contact forces up to 670 N. Experimental data have been fitted to the relation for the integral alpha T to the nth power dt by assuming that the thermal contact conductance is a simple power function of the sample temperature. It has been found that the conductance is proportional to T squared and that conductance increases with an increase in applied contact force. These results confirm earlier work.

  10. Thermal conductance of pressed contacts at liquid helium temperatures

    NASA Technical Reports Server (NTRS)

    Salerno, L. J.; Kittel, P.; Spivak, A. L.

    1983-01-01

    The thermal contact conductance of a 0.4 micrometer surface finish OFHC copper sample pair has been investigated from 1.6 to 3.8 K for a range of applied contact forces up to 670 N. Experimental data have been fitted to the relation Q = the integral alpha T to the nth power dt by assuming that the thermal contact conductance is a simple power function of the sample temperature. It has been found that the conductance is proportional to T squared and that conductance increases with an increase in applied contact force. These results confirm earlier work.

  11. Analysis of effective thermal conductivity of fibrous materials

    NASA Technical Reports Server (NTRS)

    Futschik, Michael W.; Witte, Larry C.

    1993-01-01

    The objective of this research is to gain a better understanding of the various mechanisms of heat transfer through fibrous materials and to gain insight into how fill-gas pressure influences the effective thermal conductivity. By way of first principles and some empiricism, two mathematical models are constructed to correlate experimental data. The data are obtained from a test series measuring the effective thermal conductivity of Nomex using a two-sided guarded hot-plate heater apparatus. Tests are conducted for certain mean temperatures and fill-gases over a range of pressures varying from vacuum to atmospheric conditions. The models are then evaluated to determine their effectiveness in representing the effective thermal conductivity of a fibrous material. The models presented herein predict the effective thermal conductivity of Nomex extremely well. Since the influence of gas conduction is determined to be the most influential component in predicting the effective thermal conductivity of a fibrous material, an improved representation of gas conduction is developed. Finally, some recommendations for extension to other random-oriented fiber materials are made concerning the usefulness of each model depending on their advantages and disadvantages.

  12. Si/Ge superlattice nanowires with ultralow thermal conductivity.

    PubMed

    Hu, Ming; Poulikakos, Dimos

    2012-11-14

    The engineering of nanostructured materials with very low thermal conductivity is a necessary step toward the realization of efficient thermoelectric devices. We report here the main results of an investigation with nonequilibrium molecular dynamics simulations on thermal transport in Si/Ge superlattice nanowires aiming at taking advantage of the inherent one dimensionality and the combined presence of surface and interfacial phonon scattering to yield ultralow values for their thermal conductivity. Our calculations revealed that the thermal conductivity of a Si/Ge superlattice nanowire varies nonmonotonically with both the Si/Ge lattice periodic length and the nanowire cross-sectional width. The optimal periodic length corresponds to an order of magnitude (92%) decrease in thermal conductivity at room temperature, compared to pristine single-crystalline Si nanowires. We also identified two competing mechanisms governing the thermal transport in superlattice nanowires, responsible for this nonmonotonic behavior: interface modulation in the longitudinal direction significantly depressing the phonon group velocities and hindering heat conduction, and coherent phonons occurring at extremely short periodic lengths counteracting the interface effect and facilitating thermal transport. Our results show trends for superlattice nanowire design for efficient thermoelectrics. PMID:23106449

  13. Identification of temperature-dependent thermal conductivity and experimental verification

    NASA Astrophysics Data System (ADS)

    Pan, Weizhen; Yi, Fajun; Zhu, Yanwei; Meng, Songhe

    2016-07-01

    A modified Levenberg–Marquardt method (LMM) for the identification of temperature-dependent thermal conductivity is proposed; the experiment and structure of the specimen for identification are also designed. The temperature-dependent thermal conductivities of copper C10200 and brass C28000 are identified to verify the effectiveness of the proposed identification method. The comparison between identified results and the measured data of laser flash diffusivity apparatus indicates the fine consistency and potential usage of the proposed method.

  14. Solid spot thermal conductance of Zircaloy-2/uranium dioxide interfaces

    SciTech Connect

    Madhusudana, C.V.; Fletcher, L.S.

    1983-03-01

    Simple power-law correlations have been deduced to predict the dependence of nondimensional thermal contact conductance on the nondimensional contact pressure for Zircaloy-2/uranium dioxide surfaces in contact. These correlations are based on the experimental results of several workers. These results indicate that further experimental work is needed to cover a broader range of pressures and temperatures and to ascertain the effect of mean junction temperature on the thermal contact conductance.

  15. Automation of a guarded hot plate thermal conductivity instrument

    SciTech Connect

    Holland, L.L.

    1980-06-01

    The Thermo-Physics Corporation's GP-1800 guarded hot plate thermal conductivity instrument has been automated using a Digital Equipment Corporation PDP 11/35 minicomputer with an Industrial Control Subsystem Remote. Automation included constructing a hardware link between the instrument and the minicomputer system and designing, writing, and documenting software to perform equipment control, data acquisition, data reduction, and report generation. The software was designed and written so that non-programmers can run the thermal conductivity experiment.

  16. Modeling the thermal conductivity of fiber-reinforced ceramic composites

    SciTech Connect

    Beecher, S.C.; Dinwiddie, R.B.

    1993-06-01

    A review of models for the prediction of the thermal conductivity of uni-directional fiber-reinforced composites will be presented. The ability of these models to give an accurate prediction of the composite thermal conductivity depends on the amount of information known about the constituent phase properties under the assumption that these properties do not change as a result of processing. Also presented are models that take into account the effects of fiber coatings.

  17. Thermal conductivity anisotropy of metasedimentary and igneous rocks

    NASA Astrophysics Data System (ADS)

    Davis, Michael G.; Chapman, David S.; van Wagoner, Thomas M.; Armstrong, Phillip A.

    2007-05-01

    Thermal conductivity anisotropy was determined for three sets of metasedimentary and igneous rocks from central Utah, USA. Most conductivity measurements were made in transient mode with a half-space, line source instrument oriented in two orthogonal directions on a flat face cut perpendicular to bedding. One orientation of the probe yields thermal conductivity parallel to bedding (kpar) directly, the other orientation of the probe measures a product of conductivities parallel and perpendicular to bedding from which the perpendicular conductivity (kperp) is calculated. Some direct measurements of kpar and kperp were made on oriented cylindrical discs using a conventional divided bar device in steady state mode. Anisotropy is defined as kpar/kperp. Precambrian argillites from Big Cottonwood Canyon have anisotropy values from 0.8 to 2.1 with corresponding conductivity perpendicular to bedding of 2.0 to 6.2 W m-1 K-1. Anisotropy values for Price Canyon sedimentary samples are less than 1.2 with a mean of 1.04 although thermal conductivity perpendicular to bedding for the samples varied from 1.3 to 5.0 W m-1 K-1. The granitic rocks were found to be essentially isotropic with thermal conductivity perpendicular to bedding having a range of 2.2 to 3.2 W m-1 K-1 and a mean of 2.68 W m-1 K-1. The results confirm the observation by Deming [1994] that anisotropy is negligible for rocks having kperp greater than 4.0 W m-1 K-1 and generally increases for low conductivity metamorphic and clay-rich rocks. There is little evidence, however, for his suggestion that thermal conductivity anisotropy of all rocks increases systematically to about 2.5 for low thermal conductivity rocks.

  18. Maneuvering thermal conductivity of magnetic nanofluids by tunable magnetic fields

    NASA Astrophysics Data System (ADS)

    Patel, Jaykumar; Parekh, Kinnari; Upadhyay, R. V.

    2015-06-01

    We report an experimental investigation of magnetic field dependent thermal conductivity of a transformer oil base magnetic fluid as a function of volume fractions. In the absence of magnetic field, thermal conductivity increases linearly with an increase in volume fraction, and magnitude of thermal conductivity thus obtained is lower than that predicted by Maxwell's theory. This reveals the presence of clusters/oligomers in the system. On application of magnetic field, it exhibits a non-monotonous increase in thermal conductivity. The results are interpreted using the concept of a two-step homogenization method (which is based on differential effective medium theory). The results show a transformation of particle cluster configuration from long chain like prolate shape to the aggregated drop-like structure with increasing concentration as well as a magnetic field. The aggregated drop-like structure for concentrated system is supported by optical microscopic images. This shape change of clusters reduces thermal conductivity enhancement. Moreover, this structure formation is observed as a dynamic phenomenon, and at 226 mT field, the length of the structure extends with time, becomes maximum, and then reduces. This change results in the increase or decrease of thermal conductivity.

  19. Metallic coatings for enhancement of thermal contact conductance

    SciTech Connect

    Lambert, M.A.; Fletcher, L.S. )

    1994-04-01

    The reliability of standard electronic modules may be improved by decreasing overall module temperature. This may be accomplished by enhancing the thermal contact conductance at the interface between the module frame guide rib and the card rail to which the module is clamped. Some metallic coatings, when applied to the card rail, would deform under load, increasing the contact area and associated conductance. This investigation evaluates the enhancements in thermal conductance afforded by vapor deposited silver and gold coatings. Experimental thermal conductance measurements were made for anodized aluminum 6101-T6 and electroless nickel-plated copper C11000-H03 card materials to the aluminum A356-T61 rail material. Conductance values for the electroless nickel-plated copper junction ranged from 600 to 2800 W/m(exp 2)K and those for the anodized aluminum junction ranged from 25 to 91 W/m(exp 2)K for contact pressures of 0.172-0.862 MPa and mean junction temperatures of 20-100 C. Experimental thermal conductance values of vapor deposited silver- and gold-coated aluminum A356-T61 rail surfaces indicate thermal enhancements of 1.25-2.19 for the electroless nickel-plated copper junctions and 1.79-3.41 for the anodized aluminum junctions. The silver and gold coatings provide significant thermal enhancement; however, these coating-substrate combinations are susceptible to galvanic corrosion under some conditions. 25 refs.

  20. Metallic coatings for enhancement of thermal contact conductance

    NASA Astrophysics Data System (ADS)

    Lambert, M. A.; Fletcher, L. S.

    1994-04-01

    The reliability of standard electronic modules may be improved by decreasing overall module temperature. This may be accomplished by enhancing the thermal contact conductance at the interface between the module frame guide rib and the card rail to which the module is clamped. Some metallic coatings, when applied to the card rail, would deform under load, increasing the contact area and associated conductance. This investigation evaluates the enhancements in thermal conductance afforded by vapor deposited silver and gold coatings. Experimental thermal conductance measurements were made for anodized aluminum 6101-T6 and electroless nickel-plated copper C11000-H03 card materials to the aluminum A356-T61 rail material. Conductance values for the electroless nickel-plated copper junction ranged from 600 to 2800 W/m(exp 2)K and those for the anodized aluminum junction ranged from 25 to 91 W/m(exp 2)K for contact pressures of 0.172-0.862 MPa and mean junction temperatures of 20-100 C. Experimental thermal conductance values of vapor deposited silver- and gold-coated aluminum A356-T61 rail surfaces indicate thermal enhancements of 1.25-2.19 for the electroless nickel-plated copper junctions and 1.79-3.41 for the anodized aluminum junctions. The silver and gold coatings provide significant thermal enhancement; however, these coating-substrate combinations are susceptible to galvanic corrosion under some conditions.

  1. Low-temperature thermal conductivity of terbium-gallium garnet

    SciTech Connect

    Inyushkin, A. V. Taldenkov, A. N.

    2010-11-15

    Thermal conductivity of paramagnetic Tb{sub 3}Ga{sub 5}O{sub 12} (TbGG) terbium-gallium garnet single crystals is investigated at temperatures from 0.4 to 300 K in magnetic fields up to 3.25 T. A minimum is observed in the temperature dependence {kappa}(T) of thermal conductivity at T{sub min} = 0.52 K. This and other singularities on the {kappa}(T) dependence are associated with scattering of phonons from terbium ions. The thermal conductivity at T = 5.1 K strongly depends on the magnetic field direction relative to the crystallographic axes of the crystal. Experimental data are considered using the Debye theory of thermal conductivity taking into account resonance scattering of phonons from Tb{sup 3+} ions. Analysis of the temperature and field dependences of the thermal conductivity indicates the existence of a strong spin-phonon interaction in TbGG. The low-temperature behavior of the thermal conductivity (field and angular dependences) is mainly determined by resonance scattering of phonons at the first quasi-doublet of the electron spectrum of Tb{sup 3+} ion.

  2. Thermal Conductivity Changes in Titanium-Graphene Composite upon Annealing

    NASA Astrophysics Data System (ADS)

    Jagannadham, Kasichainula

    2016-02-01

    Ti-graphene composite films were prepared on polished Ti substrates by deposition of graphene platelets from suspension followed by deposition of Ti by magnetron sputtering. The films were annealed at different temperatures up to 1073 K (800 °C) and different time periods in argon atmosphere. The annealed films were characterized by X-ray diffraction for phase identification, scanning electron microscopy for microstructure, energy-dispersive spectrometry for chemical analysis, atomic force microscopy for surface roughness, and transient thermoreflectance for thermal conductivity and interface thermal conductance. The results showed that the interface between the composite film and Ti substrate remained continuous with the absence of voids. Oxygen concentration in the composite films has increased for higher temperature and time of annealing. TiO2 and TiC phases are formed only in the film annealed at 1073 K (800 °C). The thermal conductivity of the composite film decreased with increasing oxygen concentration. The effective thermal conductance of the film annealed at 1073 K (800 °C) was significantly lower. The interface thermal conductance between the composite film and the Ti substrate is also reduced for higher oxygen concentration. Formation of microscopic TiO2 phase bound by interface boundaries and oxygen incorporation is considered responsible for the lower thermal conductance of the Ti-graphene composite annealed at 1073 K (800 °C).

  3. Advanced Low Conductivity Thermal Barrier Coatings: Performance and Future Directions

    NASA Technical Reports Server (NTRS)

    Zhu, Dongming; Miller, Robert A.

    2008-01-01

    Thermal barrier coatings will be more aggressively designed to protect gas turbine engine hot-section components in order to meet future engine higher fuel efficiency and lower emission goals. In this presentation, thermal barrier coating development considerations and performance will be emphasized. Advanced thermal barrier coatings have been developed using a multi-component defect clustering approach, and shown to have improved thermal stability and lower conductivity. The coating systems have been demonstrated for high temperature combustor applications. For thermal barrier coatings designed for turbine airfoil applications, further improved erosion and impact resistance are crucial for engine performance and durability. Erosion resistant thermal barrier coatings are being developed, with a current emphasis on the toughness improvements using a combined rare earth- and transition metal-oxide doping approach. The performance of the toughened thermal barrier coatings has been evaluated in burner rig and laser heat-flux rig simulated engine erosion and thermal gradient environments. The results have shown that the coating composition optimizations can effectively improve the erosion and impact resistance of the coating systems, while maintaining low thermal conductivity and cyclic durability. The erosion, impact and high heat-flux damage mechanisms of the thermal barrier coatings will also be described.

  4. Reduction of Thermal Conductivity in Wafer-Bonded Silicon

    SciTech Connect

    ZL Liau; LR Danielson; PM Fourspring; L Hu; G Chen; GW Turner

    2006-11-27

    Blocks of silicon up to 3-mm thick have been formed by directly bonding stacks of thin wafer chips. These stacks showed significant reductions in the thermal conductivity in the bonding direction. In each sample, the wafer chips were obtained by polishing a commercial wafer to as thin as 36 {micro}m, followed by dicing. Stacks whose starting wafers were patterned with shallow dots showed greater reductions in thermal conductivity. Diluted-HF treatment of wafer chips prior to bonding led to the largest reduction of the effective thermal conductivity, by approximately a factor of 50. Theoretical modeling based on restricted conduction through the contacting dots and some conduction across the planar nanometer air gaps yielded fair agreement for samples fabricated without the HF treatment.

  5. Thermal and Electrical Conductivity Measurements of CDA 510 Phosphor Bronze

    NASA Technical Reports Server (NTRS)

    Tuttle, James E.; Canavan, Edgar; DiPirro, Michael

    2009-01-01

    Many cryogenic systems use electrical cables containing phosphor bronze wire. While phosphor bronze's electrical and thermal conductivity values have been published, there is significant variation among different phosphor bronze formulations. The James Webb Space Telescope (JWST) will use several phosphor bronze wire harnesses containing a specific formulation (CDA 510, annealed temper). The heat conducted into the JWST instrument stage is dominated by these harnesses, and approximately half of the harness conductance is due to the phosphor bronze wires. Since the JWST radiators are expected to just keep the instruments at their operating temperature with limited cooling margin, it is important to know the thermal conductivity of the actual alloy being used. We describe an experiment which measured the electrical and thermal conductivity of this material between 4 and 295 Kelvin.

  6. Thermal conductivity enhancement of nanofluids containing graphene nanosheets

    NASA Astrophysics Data System (ADS)

    Sen Gupta, Soujit; Manoj Siva, V.; Krishnan, Sreenath; Sreeprasad, T. S.; Singh, Pawan K.; Pradeep, T.; Das, Sarit K.

    2011-10-01

    This paper envisages a mechanism of heat conduction behind the thermal conductivity enhancement observed in graphene nanofluids. Graphene nanofluids have been prepared, characterized, and their thermal conductivity was measured using the transient hot wire method. The enhancements in thermal conductivity are substantial even at lower concentrations and are not predicted by the classical Maxwell model. The enhancement also shows strong temperature dependence which is unlike its carbon predecessors, carbon nanotube (CNT) and graphene oxide nanofluids. It is also seen that the magnitude of enhancement is in-between CNT and metallic/metal oxide nanofluids. This could be an indication that the mechanism of heat conduction is a combination of percolation in CNT and Brownian motion and micro convection effects in metallic/metal oxide nanofluids, leading to a strong proposition of a hybrid model.

  7. Pore morphology study of silica aerogels

    SciTech Connect

    Hua, D.W.; Anderson, J.; Haereid, S.; Smith, D.M.

    1994-12-31

    Silica aerogels have numerous properties which suggest applications such as ultra high efficiency thermal insulation. These properties relate directly to the aerogel`s pore size distribution. The micro and meso pore size ranges can be investigated by normal small angle x-ray scattering and possibly, nitrogen adsorption. However, the measurement of larger pores (> 250 {angstrom}) is more difficult. Due to their limited mechanical strength, mercury porosimetry and nitrogen condensation can disrupt the gel structure and electron microscopy provides only limited large scale structure information. The use of small angle light scattering techniques seems to have promise, the only hurdle is that aerogels exhibit significant multiple scattering. This can be avoided if one observes the gels in the wet stage since the structure of the aerogel should be very similar to the wet gel (as the result of supercritical drying). Thus, if one can match the refractive index, the morphology can be probed. The combination of certain alcoholic solvents fit this index matching criteria. Preliminary results for the gel network (micron range) and primary particle structure (manometer) are reported by using small angle light scattering and ultra-small angle x-ray scattering. The effects on structure over the length scale range of <1 nm to >5 {mu}m under different conditions (precursors, pH, etc.) are presented. The change in structure of an aerogel during isostatic compaction to 228 MPa (to simulate drying from wetting solvents) are also discussed.

  8. Quasi-ballistic Electronic Thermal Conduction in Metal Inverse Opals.

    PubMed

    Barako, Michael T; Sood, Aditya; Zhang, Chi; Wang, Junjie; Kodama, Takashi; Asheghi, Mehdi; Zheng, Xiaolin; Braun, Paul V; Goodson, Kenneth E

    2016-04-13

    Porous metals are used in interfacial transport applications that leverage the combination of electrical and/or thermal conductivity and the large available surface area. As nanomaterials push toward smaller pore sizes to increase the total surface area and reduce diffusion length scales, electron conduction within the metal scaffold becomes suppressed due to increased surface scattering. Here we observe the transition from diffusive to quasi-ballistic thermal conduction using metal inverse opals (IOs), which are metal films that contain a periodic arrangement of interconnected spherical pores. As the material dimensions are reduced from ∼230 nm to ∼23 nm, the thermal conductivity of copper IOs is reduced by more than 57% due to the increase in surface scattering. In contrast, nickel IOs exhibit diffusive-like conduction and have a constant thermal conductivity over this size regime. The quasi-ballistic nature of electron transport at these length scales is modeled considering the inverse opal geometry, surface scattering, and grain boundaries. Understanding the characteristics of electron conduction at the nanoscale is essential to minimizing the total resistance of porous metals for interfacial transport applications, such as the total electrical resistance of battery electrodes and the total thermal resistance of microscale heat exchangers. PMID:26986050

  9. Method of simultaneous measurement of radiative and lattice thermal conductivity.

    NASA Technical Reports Server (NTRS)

    Schatz, J. F.; Simmons, G.

    1972-01-01

    A new technique of high-temperature thermal-conductivity measurement is described. A CO2 gas laser is used to generate a low-frequency temperature wave at one face of a small disk-shaped sample, and an infrared detector views the opposite face to detect the phase of the emerging radiation. A mathematical expression is derived which enables phase data at several frequencies to be used for the simultaneous determination of thermal diffusivity and mean extinction coefficient. Lattice and radiative thermal conductivities are then calculated. Test results for sintered aluminum oxide at temperatures from 530 to 1924 K are within the range of error of previously existing data.

  10. Thermal conductivity measurements of particulate materials under Martian conditions

    NASA Technical Reports Server (NTRS)

    Presley, M. A.; Christensen, P. R.

    1993-01-01

    The mean particle diameter of surficial units on Mars has been approximated by applying thermal inertia determinations from the Mariner 9 Infrared Radiometer and the Viking Infrared Thermal Mapper data together with thermal conductivity measurement. Several studies have used this approximation to characterize surficial units and infer their nature and possible origin. Such interpretations are possible because previous measurements of the thermal conductivity of particulate materials have shown that particle size significantly affects thermal conductivity under martian atmospheric pressures. The transfer of thermal energy due to collisions of gas molecules is the predominant mechanism of thermal conductivity in porous systems for gas pressures above about 0.01 torr. At martian atmospheric pressures the mean free path of the gas molecules becomes greater than the effective distance over which conduction takes place between the particles. Gas particles are then more likely to collide with the solid particles than they are with each other. The average heat transfer distance between particles, which is related to particle size, shape and packing, thus determines how fast heat will flow through a particulate material.The derived one-to-one correspondence of thermal inertia to mean particle diameter implies a certain homogeneity in the materials analyzed. Yet the samples used were often characterized by fairly wide ranges of particle sizes with little information about the possible distribution of sizes within those ranges. Interpretation of thermal inertia data is further limited by the lack of data on other effects on the interparticle spacing relative to particle size, such as particle shape, bimodal or polymodal mixtures of grain sizes and formation of salt cements between grains. To address these limitations and to provide a more comprehensive set of thermal conductivities vs. particle size a linear heat source apparatus, similar to that of Cremers, was assembled to

  11. Revisiting the block method for evaluating thermal conductivities of clay and granite

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Determination of thermal conductivities of porous media using the contact method is revisited and revalidated with consideration of thermal contact resistance. Problems that limit the accuracy of determination of thermal conductivities of porous media are discussed. Thermal conductivities of granite...

  12. THERMAL CONDUCTIVITY OF NON-REPOSITORY LITHOSTRATIGRAPHIC LAYERS

    SciTech Connect

    R. JONES

    2004-10-22

    This model report addresses activities described in ''Technical Work Plan for: Near-Field Environment and Transport Thermal Properties and Analysis Reports Integration'' (BSC 2004 [DIRS 171708]). The model develops values for thermal conductivity, and its uncertainty, for the nonrepository layers of Yucca Mountain; in addition, the model provides estimates for matrix porosity and dry bulk density for the nonrepository layers. The studied lithostratigraphic units, as identified in the ''Geologic Framework Model'' (GFM 2000) (BSC 2004 [DIRS 170029]), are the Timber Mountain Group, the Tiva Canyon Tuff, the Yucca Mountain Tuff, the Pah Canyon Tuff, the Topopah Spring Tuff (excluding the repository layers), the Calico Hills Formation, the Prow Pass Tuff, the Bullfrog Tuff, and the Tram Tuff. The deepest model units of the GFM (Tund and Paleozoic) are excluded from this study because no data suitable for model input are available. The parameter estimates developed in this report are used as input to various models and calculations that simulate heat transport through the rock mass. Specifically, analysis model reports that use product output from this report are: (1) Drift-scale coupled processes (DST and TH seepage) models; (2) Drift degradation analysis; (3) Multiscale thermohydrologic model; and (4) Ventilation model and analysis report. In keeping with the methodology of the thermal conductivity model for the repository layers in ''Thermal Conductivity of the Potential Repository Horizon'' (BSC 2004 [DIRS 169854]), the Hsu et al. (1995 [DIRS 158073]) three-dimensional (3-D) cubic model (referred to herein as ''the Hsu model'') was used to represent the matrix thermal conductivity as a function of the four parameters (matrix porosity, thermal conductivity of the saturating fluid, thermal conductivity of the solid, and geometric connectivity of the solid). The Hsu model requires input data from each test specimen to meet three specific conditions: (1) Known value for

  13. Voltage tunability of thermal conductivity in ferroelectric materials

    DOEpatents

    Ihlefeld, Jon; Hopkins, Patrick Edward

    2016-02-09

    A method to control thermal energy transport uses mobile coherent interfaces in nanoscale ferroelectric films to scatter phonons. The thermal conductivity can be actively tuned, simply by applying an electrical potential across the ferroelectric material and thereby altering the density of these coherent boundaries to directly impact thermal transport at room temperature and above. The invention eliminates the necessity of using moving components or poor efficiency methods to control heat transfer, enabling a means of thermal energy control at the micro- and nano-scales.

  14. Atomistic Modeling of Thermal Conductivity of Epoxy Nanotube Composites

    NASA Astrophysics Data System (ADS)

    Fasanella, Nicholas A.; Sundararaghavan, Veera

    2016-05-01

    The Green-Kubo method was used to investigate the thermal conductivity as a function of temperature for epoxy/single wall carbon nanotube (SWNT) nanocomposites. An epoxy network of DGEBA-DDS was built using the `dendrimer' growth approach, and conductivity was computed by taking into account long-range Coulombic forces via a k-space approach. Thermal conductivity was calculated in the direction perpendicular to, and along the SWNT axis for functionalized and pristine SWNT/epoxy nanocomposites. Inefficient phonon transport at the ends of nanotubes is an important factor in the thermal conductivity of the nanocomposites, and for this reason discontinuous nanotubes were modeled in addition to long nanotubes. The thermal conductivity of the long, pristine SWNT/epoxy system is equivalent to that of an isolated SWNT along its axis, but there was a 27% reduction perpendicular to the nanotube axis. The functionalized, long SWNT/epoxy system had a very large increase in thermal conductivity along the nanotube axis (~700%), as well as the directions perpendicular to the nanotube (64%). The discontinuous nanotubes displayed an increased thermal conductivity along the SWNT axis compared to neat epoxy (103-115% for the pristine SWNT/epoxy, and 91-103% for functionalized SWNT/epoxy system). The functionalized system also showed a 42% improvement perpendicular to the nanotube, while the pristine SWNT/epoxy system had no improvement over epoxy. The thermal conductivity tensor is averaged over all possible orientations to see the effects of randomly orientated nanotubes, and allow for experimental comparison. Excellent agreement is seen for the discontinuous, pristine SWNT/epoxy nanocomposite. These simulations demonstrate there exists a threshold of the SWNT length where the best improvement for a composite system with randomly oriented nanotubes would transition from pristine SWNTs to functionalized SWNTs.

  15. Thermal and Electrical Conductivities of Porous Si Membranes

    NASA Astrophysics Data System (ADS)

    Hagino, Harutoshi; Tanaka, Saburo; Tanimura, Naoki; Miyazaki, Koji

    2015-11-01

    The microstructure of materials affects thermal and electrical transport as well as the physical properties. The effects of the microstructure on both thermal and electrical transport in silicon membranes with periodic microporous structures produced from silicon-on-insulator wafers using microfabrication processes were studied. The in-plane thermal and electrical conductivities of the Si membranes were measured simultaneously by using a self-heating method. The measured thermal conductivity was compared with the result from the periodically laser-heating method. The thermal and electrical conductivities were much lower in the porous membranes than in the non-porous membrane. The measured thermal conductivity was much lower than expected based on values determined using classical models. A significant phonon size effect was observed even in microsized structures, and the mean free path for phonons was very long. It was concluded that phonon transport is quasi-ballistic and electron transport is diffuse in microporous Si structures. It was suggested that the microstructure had a different effect on thermal and electrical transport.

  16. Method for preparing a solid phase microextraction device using aerogel

    DOEpatents

    Miller, Fred S.; Andresen, Brian D.

    2006-10-24

    A sample collection substrate of aerogel and/or xerogel materials bound to a support structure is used as a solid phase microextraction (SPME) device. The xerogels and aerogels may be organic or inorganic and doped with metals or other compounds to target specific chemical analytes. The support structure is typically formed of a glass fiber or a metal wire (stainless steel or kovar). The devices are made by applying gel solution to the support structures and drying the solution to form aerogel or xerogel. Aerogel particles may be attached to the wet layer before drying to increase sample collection surface area. These devices are robust, stable in fields of high radiation, and highly effective at collecting gas and liquid samples while maintaining superior mechanical and thermal stability during routine use. Aerogel SPME devices are advantageous for use in GC/MS analyses due to their lack of interfering background and tolerance of GC thermal cycling.

  17. Multiscale Modeling of Thermal Conductivity of Polymer/Carbon Nanocomposites

    NASA Technical Reports Server (NTRS)

    Clancy, Thomas C.; Frankland, Sarah-Jane V.; Hinkley, Jeffrey A.; Gates, Thomas S.

    2010-01-01

    Molecular dynamics simulation was used to estimate the interfacial thermal (Kapitza) resistance between nanoparticles and amorphous and crystalline polymer matrices. Bulk thermal conductivities of the nanocomposites were then estimated using an established effective medium approach. To study functionalization, oligomeric ethylene-vinyl alcohol copolymers were chemically bonded to a single wall carbon nanotube. The results, in a poly(ethylene-vinyl acetate) matrix, are similar to those obtained previously for grafted linear hydrocarbon chains. To study the effect of noncovalent functionalization, two types of polyethylene matrices. -- aligned (extended-chain crystalline) vs. amorphous (random coils) were modeled. Both matrices produced the same interfacial thermal resistance values. Finally, functionalization of edges and faces of plate-like graphite nanoparticles was found to be only modestly effective in reducing the interfacial thermal resistance and improving the composite thermal conductivity

  18. Thermal Conductivity Based on Modified Laser Flash Measurement

    NASA Technical Reports Server (NTRS)

    Lin, Bochuan; Ban, Heng; Li, Chao; Scripa, Rosalia N.; Su, Ching-Hua; Lehoczky, Sandor L.

    2005-01-01

    The laser flash method is a standard method for thermal diffusivity measurement. It employs single-pulse heating of one side of a thin specimen and measures the temperature response of the other side. The thermal diffusivity of the specimen can be obtained based on a one-dimensional transient heat transfer analysis. This paper reports the development of a theory that includes a transparent reference layer with known thermal property attached to the back of sample. With the inclusion of heat conduction from the sample to the reference layer in the theoretical analysis, the thermal conductivity and thermal diffusivity of sample can be extracted from the temperature response data. Furthermore, a procedure is established to select two points from the data to calculate these properties. The uncertainty analysis indicates that this method can be used with acceptable levels of uncertainty.

  19. Accurate calculation of conductive conductances in complex geometries for spacecrafts thermal models

    NASA Astrophysics Data System (ADS)

    Garmendia, Iñaki; Anglada, Eva; Vallejo, Haritz; Seco, Miguel

    2016-02-01

    The thermal subsystem of spacecrafts and payloads is always designed with the help of Thermal Mathematical Models. In the case of the Thermal Lumped Parameter (TLP) method, the non-linear system of equations that is created is solved to calculate the temperature distribution and the heat power that goes between nodes. The accuracy of the results depends largely on the appropriate calculation of the conductive and radiative conductances. Several established methods for the determination of conductive conductances exist but they present some limitations for complex geometries. Two new methods are proposed in this paper to calculate accurately these conductive conductances: The Extended Far Field method and the Mid-Section method. Both are based on a finite element calculation but while the Extended Far Field method uses the calculation of node mean temperatures, the Mid-Section method is based on assuming specific temperature values. They are compared with traditionally used methods showing the advantages of these two new methods.

  20. Effective thermal conductivity of composites with fibre-matrix debonding

    NASA Technical Reports Server (NTRS)

    Fadale, T. D.; Taya, M.

    1991-01-01

    Debonding of the fiber-matrix interface is a major cause for the degradation of the mechanical properties and the loss of thermal conductivity of fiber-reinforced composites. This paper discusses two analytical approaches for modeling the thermal conduction problem of composites. One is based on the concept of modeling the thermal barrier by an equivalent heat transfer coefficient at the fiber-matrix interface, as described by Hasselman and Johnson (1987) and Benveniste and Miloh (1986). The other approach, suggested by Hatta and Taya (1986), is by treating a composite with debonded interface as a coated-fiber composite. The major advantage of the latter aproach is that the thickness of the fiber coating can be realistically modeled depending upon the extent of degradation of the composite with the thermal conductivity of the coating as that of air.

  1. Low Frequency Thermal Conductivity in Micro Phononic Crystals

    NASA Astrophysics Data System (ADS)

    Anjos, Virgilio; Arantes, Alison

    2015-03-01

    We study theoretically the cumulative thermal conductivity of a micro phononic crystal at low temperature regime. The phononic crystal considered presents carbon microtubes inclusions arranged periodically in a two-dimensional square lattice embebed in soft elastic matrix. Moderate and high impedance mismatch are considered concerning the material composition. The low frequency phonon spectra (up to tens of GHz) are obtained solving the generalized wave equation for inhomogeneous media within the Plane Wave Expansion method. We consider low temperatures in order to increase the participation of GHz thermal phonons. We observed suppression in the cumulative thermal conductivity at the band gap region and thus a reduction of thermal conductivity of the phononic crystal when compared with the bulk matrix. The authors would like to thank the Brazilian agencies, National Council of Technological and Scientific Development (CNPq), Foundation for Research Support of Minas Gerais (FAPEMIG) and CAPES for their support.

  2. Thermal Conductivity behavior of MWCNT based PMMA and PC composites

    NASA Astrophysics Data System (ADS)

    Dubey, Girija; Jindal, Prashant; Bhandari, Rajiv; Dhiman, Neha; Bajaj, Chetan; Jindal, Vijay

    Poly methyl methacrylate (PMMA) and Polycarbonate (PC) are low cost polymer materials which can be easily transformed into desired shapes for various applications. However they have poor mechanical, thermal and electrical properties which are required to be enhanced to widen their scope of applications specifically where along with high strength, rapid heat transfer is essential. Multi Walled Carbon nanotubes (MWCNTs) are excellent new materials having extraordinary mechanical and transport properties. We will report results of fabricating composites of varying compositions of MWCNTs with PMMA and PC and their thermal conductivity behaviour using simple transient heat flow methods. The samples in disk shapes of around 2 cm diameters and 0.2 cm thickness with MWCNT compositions varying up to 10 wt% were fabricated. We found that both PMMA and PC measured high thermal conductivity with increase in the composition of CNTs. The thermal conductivity of 10wt% MWCNT/PMMA composite increased by nearly two times in comparison to pure PMMA.

  3. Superior thermal conductivity in suspended bilayer hexagonal boron nitride

    PubMed Central

    Wang, Chengru; Guo, Jie; Dong, Lan; Aiyiti, Adili; Xu, Xiangfan; Li, Baowen

    2016-01-01

    We reported the basal-plane thermal conductivity in exfoliated bilayer hexagonal boron nitride h-BN that was measured using suspended prepatterned microstructures. The h-BN sample suitable for thermal measurements was fabricated by dry-transfer method, whose sample quality, due to less polymer residues on surfaces, is believed to be superior to that of PMMA-mediated samples. The measured room temperature thermal conductivity is around 484 Wm−1K−1(+141 Wm−1K−1/ −24 Wm−1K−1) which exceeds that in bulk h-BN, providing experimental observation of the thickness-dependent thermal conductivity in suspended few-layer h-BN. PMID:27142571

  4. Robustly Engineering Thermal Conductivity of Bilayer Graphene by Interlayer Bonding

    NASA Astrophysics Data System (ADS)

    Zhang, Xiaoliang; Gao, Yufei; Chen, Yuli; Hu, Ming

    2016-02-01

    Graphene and its bilayer structure are the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. Their realistic applications in emerging nanoelectronics usually call for thermal transport manipulation in a controllable and precise manner. In this paper we systematically studied the effect of interlayer covalent bonding, in particular different interlay bonding arrangement, on the thermal conductivity of bilayer graphene using equilibrium molecular dynamics simulations. It is revealed that, the thermal conductivity of randomly bonded bilayer graphene decreases monotonically with the increase of interlayer bonding density, however, for the regularly bonded bilayer graphene structure the thermal conductivity possesses unexpectedly non-monotonic dependence on the interlayer bonding density. The results suggest that the thermal conductivity of bilayer graphene depends not only on the interlayer bonding density, but also on the detailed topological configuration of the interlayer bonding. The underlying mechanism for this abnormal phenomenon is identified by means of phonon spectral energy density, participation ratio and mode weight factor analysis. The large tunability of thermal conductivity of bilayer graphene through rational interlayer bonding arrangement paves the way to achieve other desired properties for potential nanoelectronics applications involving graphene layers.

  5. Discussion on the thermal conductivity enhancement of nanofluids

    PubMed Central

    2011-01-01

    Increasing interests have been paid to nanofluids because of the intriguing heat transfer enhancement performances presented by this kind of promising heat transfer media. We produced a series of nanofluids and measured their thermal conductivities. In this article, we discussed the measurements and the enhancements of the thermal conductivity of a variety of nanofluids. The base fluids used included those that are most employed heat transfer fluids, such as deionized water (DW), ethylene glycol (EG), glycerol, silicone oil, and the binary mixture of DW and EG. Various nanoparticles (NPs) involving Al2O3 NPs with different sizes, SiC NPs with different shapes, MgO NPs, ZnO NPs, SiO2 NPs, Fe3O4 NPs, TiO2 NPs, diamond NPs, and carbon nanotubes with different pretreatments were used as additives. Our findings demonstrated that the thermal conductivity enhancements of nanofluids could be influenced by multi-faceted factors including the volume fraction of the dispersed NPs, the tested temperature, the thermal conductivity of the base fluid, the size of the dispersed NPs, the pretreatment process, and the additives of the fluids. The thermal transport mechanisms in nanofluids were further discussed, and the promising approaches for optimizing the thermal conductivity of nanofluids have been proposed. PMID:21711638

  6. Thermal conductivity modeling of core-shell and tubular nanowires.

    PubMed

    Yang, Ronggui; Chen, Gang; Dresselhaus, Mildred S

    2005-06-01

    The heteroepitaxial growth of crystalline core-shell nanostructures of a variety of materials has become possible in recent years, allowing the realization of various novel nanoscale electronic and optoelectronic devices. The increased surface or interface area will decrease the thermal conductivity of such nanostructures and impose challenges for the thermal management of such devices. In the meantime, the decreased thermal conductivity might benefit the thermoelectric conversion efficiency. In this paper, we present modeling results on the lattice thermal conductivity of core-shell and tubular nanowires along the wire axis direction using the phonon Boltzmann equation. We report the dependence of the thermal conductivity on the surface conditions and the core-shell geometry for silicon core-germanium shell and tubular silicon nanowires at room temperature. The results show that the effective thermal conductivity changes not only with the composition of the constituents but also with the radius of the nanowires and nanopores due to the nature of the ballistic phonon transport. The results in this work have implications for the design and operation of a variety of nanoelectronic devices, optoelectronic devices, and thermoelectric materials and devices. PMID:15943452

  7. Monodisperse magnetite nanofluids: Synthesis, aggregation, and thermal conductivity

    NASA Astrophysics Data System (ADS)

    Jiang, Wei; Wang, Liqiu

    2010-12-01

    Magnetic nanofluids possess some unique properties that can significantly affect their thermal conductivity. We synthesize monodispersed magnetite (Fe3O4) nanofluids in toluene with the particle size from 4 to 12 nm and obtain aqueous nanofluids by a simple "one-step" phase transfer. Even without the effect of external field, the magnetic-interaction-induced self-assembled aggregation can still be significant in magnetite nanofluids. Investigation of the microstructures of self-assembled aggregation is carried out by the dynamic light scattering, which unveils the variation of aggregated configurations with particle concentration and time. Based on the calculation from the existing models, the aggregates decrease the thermal conductivity of both themselves and the entire system, mainly due to the less solid contents and weaker mobility compared with the single particles as well as the increase in interfacial thermal resistance. As the manifestation of the aggregation-structure variation, the measured thermal conductivity is of a wavelike shape as a function of particle concentration. The particle coating layers are also of importance in cluster formation so that nanofluid thermal conductivity can be manipulated for some nanofluids by changing the stabilizer used and thus controlling the particle aggregated structures. Due to the effects of temperature, viscosity and coating layers, the thermal conductivity for aqueous system varies in a different way as that for the toluene system.

  8. Beating the amorphous limit in thermal conductivity by superlattices design

    PubMed Central

    Mizuno, Hideyuki; Mossa, Stefano; Barrat, Jean-Louis

    2015-01-01

    The value measured in the amorphous structure with the same chemical composition is often considered as a lower bound for the thermal conductivity of any material: the heat carriers are strongly scattered by disorder, and their lifetimes reach the minimum time scale of thermal vibrations. An appropriate design at the nano-scale, however, may allow one to reduce the thermal conductivity even below the amorphous limit. In the present contribution, using molecular-dynamics simulation and the Green-Kubo formulation, we study systematically the thermal conductivity of layered phononic materials (superlattices), by tuning different parameters that can characterize such structures. We have discovered that the key to reach a lower-than-amorphous thermal conductivity is to block almost completely the propagation of the heat carriers, the superlattice phonons. We demonstrate that a large mass difference in the two intercalated layers, or weakened interactions across the interface between layers result in materials with very low thermal conductivity, below the values of the corresponding amorphous counterparts. PMID:26374147

  9. Robustly Engineering Thermal Conductivity of Bilayer Graphene by Interlayer Bonding

    PubMed Central

    Zhang, Xiaoliang; Gao, Yufei; Chen, Yuli; Hu, Ming

    2016-01-01

    Graphene and its bilayer structure are the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. Their realistic applications in emerging nanoelectronics usually call for thermal transport manipulation in a controllable and precise manner. In this paper we systematically studied the effect of interlayer covalent bonding, in particular different interlay bonding arrangement, on the thermal conductivity of bilayer graphene using equilibrium molecular dynamics simulations. It is revealed that, the thermal conductivity of randomly bonded bilayer graphene decreases monotonically with the increase of interlayer bonding density, however, for the regularly bonded bilayer graphene structure the thermal conductivity possesses unexpectedly non-monotonic dependence on the interlayer bonding density. The results suggest that the thermal conductivity of bilayer graphene depends not only on the interlayer bonding density, but also on the detailed topological configuration of the interlayer bonding. The underlying mechanism for this abnormal phenomenon is identified by means of phonon spectral energy density, participation ratio and mode weight factor analysis. The large tunability of thermal conductivity of bilayer graphene through rational interlayer bonding arrangement paves the way to achieve other desired properties for potential nanoelectronics applications involving graphene layers. PMID:26911859

  10. Aerogel/Particle Composites for Thermoelectric Devices

    NASA Technical Reports Server (NTRS)

    Paik, Jong-Ah; Sakamoto, Jeffrey; Jones, Steven

    2006-01-01

    Optimizing solution chemistry and the addition of titania and fumed silica powder reduces shrinkage. These materials would serve to increase thermal efficiency by providing thermal insulation to suppress lateral heat leaks. They would also serve to prolong operational lifetime by suppressing sublimation of certain constituents of thermoelectric materials (e.g., sublimation of Sb from CoSb3) at typical high operating temperatures. [The use of pure silica aerogels as cast-in-place thermal-insulation and sublimation-suppression materials was described in "Aerogels for Thermal Insulation of Thermoelectric Devices" (NPO-40630), NASA Tech Briefs, Vol. 30, No. 7 (July 2006), page 50.] A silica aerogel is synthesized in a solgel process that includes preparation of a silica sol, gelation of the sol, and drying of the gel in a solvent at a supercritical temperature and pressure. The utility of pure silica aerogel is diminished by a tendency to shrink (and, therefore, also to crack) during the gelation and supercritical-drying stages. Moreover, to increase suppression of sublimation, it is advantageous to make an aerogel having greater density, but shrinkage and cracking tend to increase with density. A composite material of the type under investigation consists mostly of titania oxide powder particles and a small addition of fumed silica powder, which are mixed into the sol along with other ingredients prior to the gelation stage of processing. The silica aerogel and fumed silica act as a binder, gluing the titania particles together. It is believed that the addition of fumed silica stiffens the aerogel network and reduces shrinkage during the supercritical-drying stage. Minimization of shrinkage enables establishment of intimate contact between thermoelectric legs and the composite material, thereby maximizing the effectiveness of the material for thermal insulation and suppression of sublimation. To some extent, the properties of the composite can be tailored via the

  11. Rock thermal conductivity as key parameter for geothermal numerical models

    NASA Astrophysics Data System (ADS)

    Di Sipio, Eloisa; Chiesa, Sergio; Destro, Elisa; Galgaro, Antonio; Giaretta, Aurelio; Gola, Gianluca; Manzella, Adele

    2013-04-01

    The geothermal energy applications are undergoing a rapid development. However, there are still several challenges in the successful exploitation of geothermal energy resources. In particular, a special effort is required to characterize the thermal properties of the ground along with the implementation of efficient thermal energy transfer technologies. This paper focuses on understanding the quantitative contribution that geosciences can receive from the characterization of rock thermal conductivity. The thermal conductivity of materials is one of the main input parameters in geothermal modeling since it directly controls the steady state temperature field. An evaluation of this thermal property is required in several fields, such as Thermo-Hydro-Mechanical multiphysics analysis of frozen soils, designing ground source heat pumps plant, modeling the deep geothermal reservoirs structure, assessing the geothermal potential of subsoil. Aim of this study is to provide original rock thermal conductivity values useful for the evaluation of both low and high enthalpy resources at regional or local scale. To overcome the existing lack of thermal conductivity data of sedimentary, igneous and metamorphic rocks, a series of laboratory measurements has been performed on several samples, collected in outcrop, representative of the main lithologies of the regions included in the VIGOR Project (southern Italy). Thermal properties tests were carried out both in dry and wet conditions, using a C-Therm TCi device, operating following the Modified Transient Plane Source method.Measurements were made at standard laboratory conditions on samples both water saturated and dehydrated with a fan-forced drying oven at 70 ° C for 24 hr, for preserving the mineral assemblage and preventing the change of effective porosity. Subsequently, the samples have been stored in an air-conditioned room while bulk density, solid volume and porosity were detected. The measured thermal conductivity

  12. Scanning thermal microscopy with heat conductive nanowire probes.

    PubMed

    Timofeeva, Maria; Bolshakov, Alexey; Tovee, Peter D; Zeze, Dagou A; Dubrovskii, Vladimir G; Kolosov, Oleg V

    2016-03-01

    Scanning thermal microscopy (SThM), which enables measurement of thermal transport and temperature distribution in devices and materials with nanoscale resolution is rapidly becoming a key approach in resolving heat dissipation problems in modern processors and assisting development of new thermoelectric materials. In SThM, the self-heating thermal sensor contacts the sample allowing studying of the temperature distribution and heat transport in nanoscaled materials and devices. The main factors that limit the resolution and sensitivities of SThM measurements are the low efficiency of thermal coupling and the lateral dimensions of the probed area of the surface studied. The thermal conductivity of the sample plays a key role in the sensitivity of SThM measurements. During the SThM measurements of the areas with higher thermal conductivity the heat flux via SThM probe is increased compared to the areas with lower thermal conductivity. For optimal SThM measurements of interfaces between low and high thermal conductivity materials, well defined nanoscale probes with high thermal conductivity at the probe apex are required to achieve a higher quality of the probe-sample thermal contact while preserving the lateral resolution of the system. In this paper, we consider a SThM approach that can help address these complex problems by using high thermal conductivity nanowires (NW) attached to a tip apex. We propose analytical models of such NW-SThM probes and analyse the influence of the contact resistance between the SThM probe and the sample studied. The latter becomes particularly important when both tip and sample surface have high thermal conductivities. These models were complemented by finite element analysis simulations and experimental tests using prototype probe where a multiwall carbon nanotube (MWCNT) is exploited as an excellent example of a high thermal conductivity NW. These results elucidate critical relationships between the performance of the SThM probe on

  13. Thermal conductivity of Si{endash}Ge superlattices

    SciTech Connect

    Lee, S.; Cahill, D.G.; Venkatasubramanian, R.

    1997-06-01

    The thermal conductivity of Si{endash}Ge superlattices with superlattice periods 30{lt}L{lt}300 {Angstrom}, and a Si{sub 0.85}Ge{sub 0.15} thin film alloy is measured using the 3{omega} method. The alloy film shows a conductivity comparable to bulk SiGe alloys while the superlattice samples have a thermal conductivity that is smaller than the alloy. For 30{lt}L{lt}70 {Angstrom}, the thermal conductivity decreases with decreasing L; these data provide a lower limit to the interface thermal conductance G of epitaxial Si{endash}Ge interfaces: G{gt}2{times}10{sup 9}Wm{sup {minus}2}K{sup {minus}1} at 200 K. Superlattices with relatively longer periods, L{gt}130 {Angstrom}, have smaller thermal conductivities than the short-period samples. This unexpected result is attributed to a strong disruption of the lattice vibrations by extended defects produced during lattice-mismatched growth. {copyright} {ital 1997 American Institute of Physics.}

  14. Process for fabricating composite material having high thermal conductivity

    DOEpatents

    Colella, Nicholas J.; Davidson, Howard L.; Kerns, John A.; Makowiecki, Daniel M.

    2001-01-01

    A process for fabricating a composite material such as that having high thermal conductivity and having specific application as a heat sink or heat spreader for high density integrated circuits. The composite material produced by this process has a thermal conductivity between that of diamond and copper, and basically consists of coated diamond particles dispersed in a high conductivity metal, such as copper. The composite material can be fabricated in small or relatively large sizes using inexpensive materials. The process basically consists, for example, of sputter coating diamond powder with several elements, including a carbide forming element and a brazeable material, compacting them into a porous body, and infiltrating the porous body with a suitable braze material, such as copper-silver alloy, thereby producing a dense diamond-copper composite material with a thermal conductivity comparable to synthetic diamond films at a fraction of the cost.

  15. Lattice dynamics and lattice thermal conductivity of thorium dicarbide

    NASA Astrophysics Data System (ADS)

    Liao, Zongmeng; Huai, Ping; Qiu, Wujie; Ke, Xuezhi; Zhang, Wenqing; Zhu, Zhiyuan

    2014-11-01

    The elastic and thermodynamic properties of ThC2 with a monoclinic symmetry have been studied by means of density functional theory and direct force-constant method. The calculated properties including the thermal expansion, the heat capacity and the elastic constants are in a good agreement with experiment. Our results show that the vibrational property of the C2 dimer in ThC2 is similar to that of a free standing C2 dimer. This indicates that the C2 dimer in ThC2 is not strongly bonded to Th atoms. The lattice thermal conductivity for ThC2 was calculated by means of the Debye-Callaway model. As a comparison, the conductivity of ThC was also calculated. Our results show that the ThC and ThC2 contributions of the lattice thermal conductivity to the total conductivity are 29% and 17%, respectively.

  16. Electric and thermal conductivities of quenched neutron star crusts

    NASA Technical Reports Server (NTRS)

    Ogata, Shuji; Ichimaru, Setsuo

    1990-01-01

    The electric and thermal conductivities in the outer crustal matter of a neutron star quenched into a solid state by cooling are estimated using a Monte Carlo simulation of freezing transition for dense plasmas. The conductivities are calculated by the precise evaluation of the scattering integrals, using the procedure of Ichimaru et al. (1983) and Iyetomi and Ichimaru (1983). The results predict the conductivities lower, by a factor of about 3, than those with the single-phonon approximation.

  17. Method of casting aerogels

    DOEpatents

    Poco, J.F.

    1993-09-07

    The invention describes a method for making monolithic castings of transparent silica aerogel with densities in the range from 0.001 g/cm[sup 3] to 0.6 g/cm[sup 3]. Various shapes of aerogels are cast in flexible polymer molds which facilitate removal and eliminate irregular surfaces. Mold dimensions are preselected to account for shrinkage of aerogel which occurs during the drying step of supercritical extraction of solvent. 2 figures.

  18. Synthesis of Yttria-Stabilized Zirconia Aerogels by a Non-Alkoxide Sol-Gel Route

    SciTech Connect

    Chervin, C N; Clapsaddle, B J; Chiu, H W; Gash, A E; Satcher, Jr., J H; Kauzlarich, S M

    2005-02-11

    Homogeneous, nanocrystalline powders of yttria-stabilized zirconia were prepared using a nonalkoxide sol-gel method. Monolithic gels, free of precipitation, were prepared by addition of propylene oxide to aqueous solutions of Zr{sup 4+} and Y{sup 3+} chlorides at room temperature. The gels were dried with supercritical CO{sub 2}(l), resulting in amorphous aerogels that crystallized into cubic stabilized ZrO{sub 2} following calcination at 500 C. The aerogels and resulting crystalline products were characterized using in-situ temperature profile X-ray diffraction, thermal analysis, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and nitrogen adsorption/desorption analysis. TEM and N{sub 2} adsorption/desorption analysis of an aerogel indicated a porous network structure with a high surface area (409 m{sup 2}/g). The crystallized yttria-stabilized zirconia maintained high surface area (159 m{sup 2}/g) upon formation of homogeneous, nanoparticles ({approx}10 nm). Ionic conductivity at 1000 C of sintered YSZ (1500 C, 3 hours) prepared by this method, was 0.13 {+-} 0.02 {Omega}{sup -1} cm{sup -1}. Activation energies for the conduction processes from 1000-550 C and 550-400 C, were 0.95 {+-} 0.09 and 1.12 {+-} 0.05 eV, respectively. This is the first reported synthesis and characterization of yttria-stabilized zirconia via an aerogel precursor.

  19. Lattice thermal conductivity of dense silicate glass at high pressures

    NASA Astrophysics Data System (ADS)

    Chang, Y. Y.; Hsieh, W. P.

    2015-12-01

    The layered structure of the Earth's interior is generally believed to develop through the magma ocean differentiation in the early Earth. Previous seismic studies revealed the existence of ultra low velocity zones above the core mantle boundary (CMB) which was inferred to be associated with the remnant of a deep magma ocean. The heat flux through the core mantle boundary therefore would strongly depend on the thermal conductivity, both lattice (klat) and radiative (krad) of dense silicate melts and major constituent minerals of the lower mantle. Recent experimental results on the radiative thermal conductivity of dense silicate glasses and lower-mantle minerals suggest that krad of dense silicate glasses could be remarkably lower than krad of the surrounding solid mantle phases. In this case, the dense silicate melts will act as a trap for heat from the Earth's outer core. However, this conclusion remains uncertain because of the lack of direct measurements on lattice thermal conductivities of silicate glasses/melts under lower mantle pressures up to date. Here we report experimental results on lattice thermal conductivities of dense silicate glass with basaltic composition under pressures relevant to the Earth's lower mantle in a diamond-anvil cell using time-domain thermoreflectance method. The study will assist the comprehension of thermal transport properties of silicate melts in the Earth's deep interior and is crucial for understanding the dynamic and thermal evolution of the Earth's internal structure.

  20. Thermal conductivity of III-V semiconductor superlattices

    SciTech Connect

    Mei, S. Knezevic, I.

    2015-11-07

    This paper presents a semiclassical model for the anisotropic thermal transport in III-V semiconductor superlattices (SLs). An effective interface rms roughness is the only adjustable parameter. Thermal transport inside a layer is described by the Boltzmann transport equation in the relaxation time approximation and is affected by the relevant scattering mechanisms (three-phonon, mass-difference, and dopant and electron scattering of phonons), as well as by diffuse scattering from the interfaces captured via an effective interface scattering rate. The in-plane thermal conductivity is obtained from the layer conductivities connected in parallel. The cross-plane thermal conductivity is calculated from the layer thermal conductivities in series with one another and with thermal boundary resistances (TBRs) associated with each interface; the TBRs dominate cross-plane transport. The TBR of each interface is calculated from the transmission coefficient obtained by interpolating between the acoustic mismatch model (AMM) and the diffuse mismatch model (DMM), where the weight of the AMM transmission coefficient is the same wavelength-dependent specularity parameter related to the effective interface rms roughness that is commonly used to describe diffuse interface scattering. The model is applied to multiple III-arsenide superlattices, and the results are in very good agreement with experimental findings. The method is both simple and accurate, easy to implement, and applicable to complicated SL systems, such as the active regions of quantum cascade lasers. It is also valid for other SL material systems with high-quality interfaces and predominantly incoherent phonon transport.

  1. Thermal Conduction in Vertically Aligned Copper Nanowire Arrays and Composites.

    PubMed

    Barako, Michael T; Roy-Panzer, Shilpi; English, Timothy S; Kodama, Takashi; Asheghi, Mehdi; Kenny, Thomas W; Goodson, Kenneth E

    2015-09-01

    The ability to efficiently and reliably transfer heat between sources and sinks is often a bottleneck in the thermal management of modern energy conversion technologies ranging from microelectronics to thermoelectric power generation. These interfaces contribute parasitic thermal resistances that reduce device performance and are subjected to thermomechanical stresses that degrade device lifetime. Dense arrays of vertically aligned metal nanowires (NWs) offer the unique combination of thermal conductance from the constituent metal and mechanical compliance from the high aspect ratio geometry to increase interfacial heat transfer and device reliability. In the present work, we synthesize copper NW arrays directly onto substrates via templated electrodeposition and extend this technique through the use of a sacrificial overplating layer to achieve improved uniformity. Furthermore, we infiltrate the array with an organic phase change material and demonstrate the preservation of thermal properties. We use the 3ω method to measure the axial thermal conductivity of freestanding copper NW arrays to be as high as 70 W m(-1) K(-1), which is more than an order of magnitude larger than most commercial interface materials and enhanced-conductivity nanocomposites reported in the literature. These arrays are highly anisotropic, and the lateral thermal conductivity is found to be only 1-2 W m(-1) K(-1). We use these measured properties to elucidate the governing array-scale transport mechanisms, which include the effects of morphology and energy carrier scattering from size effects and grain boundaries. PMID:26284489

  2. Highly thermally conductive papers with percolative layered boron nitride nanosheets.

    PubMed

    Zhu, Hongli; Li, Yuanyuan; Fang, Zhiqiang; Xu, Jiajun; Cao, Fangyu; Wan, Jiayu; Preston, Colin; Yang, Bao; Hu, Liangbing

    2014-04-22

    In this work, we report a dielectric nanocomposite paper with layered boron nitride (BN) nanosheets wired by one-dimensional (1D) nanofibrillated cellulose (NFC) that has superior thermal and mechanical properties. These nanocomposite papers are fabricated from a filtration of BN and NFC suspensions, in which NFC is used as a stabilizer to stabilize BN nanosheets. In these nanocomposite papers, two-dimensional (2D) nanosheets form a thermally conductive network, while 1D NFC provides mechanical strength. A high thermal conductivity has been achieved along the BN paper surface (up to 145.7 W/m K for 50 wt % of BN), which is an order of magnitude higher than that in randomly distributed BN nanosheet composites and is even comparable to the thermal conductivity of aluminum alloys. Such a high thermal conductivity is mainly attributed to the structural alignment within the BN nanosheet papers; the effects of the interfacial thermal contact resistance are minimized by the fact that the heat transfer is in the direction parallel to the interface between BN nanosheets and that a large contact area occurs between BN nanosheets. PMID:24601534

  3. Thermal conductivity of III-V semiconductor superlattices

    NASA Astrophysics Data System (ADS)

    Mei, S.; Knezevic, I.

    2015-11-01

    This paper presents a semiclassical model for the anisotropic thermal transport in III-V semiconductor superlattices (SLs). An effective interface rms roughness is the only adjustable parameter. Thermal transport inside a layer is described by the Boltzmann transport equation in the relaxation time approximation and is affected by the relevant scattering mechanisms (three-phonon, mass-difference, and dopant and electron scattering of phonons), as well as by diffuse scattering from the interfaces captured via an effective interface scattering rate. The in-plane thermal conductivity is obtained from the layer conductivities connected in parallel. The cross-plane thermal conductivity is calculated from the layer thermal conductivities in series with one another and with thermal boundary resistances (TBRs) associated with each interface; the TBRs dominate cross-plane transport. The TBR of each interface is calculated from the transmission coefficient obtained by interpolating between the acoustic mismatch model (AMM) and the diffuse mismatch model (DMM), where the weight of the AMM transmission coefficient is the same wavelength-dependent specularity parameter related to the effective interface rms roughness that is commonly used to describe diffuse interface scattering. The model is applied to multiple III-arsenide superlattices, and the results are in very good agreement with experimental findings. The method is both simple and accurate, easy to implement, and applicable to complicated SL systems, such as the active regions of quantum cascade lasers. It is also valid for other SL material systems with high-quality interfaces and predominantly incoherent phonon transport.

  4. Development of Advanced Low Conductivity Thermal Barrier Coatings

    NASA Technical Reports Server (NTRS)

    Zhu, Dong-Ming; Miller, Robert A.

    2004-01-01

    Advanced multi-component, low conductivity oxide thermal barrier coatings have been developed using an approach that emphasizes real-time monitoring of thermal conductivity under conditions that are engine-like in terms of temperatures and heat fluxes. This is in contrast to the traditional approach where coatings are initially optimized in terms of furnace and burner rig durability with subsequent measurement in the as-processed or furnace-sintered condition. The present work establishes a laser high-heat-flux test as the basis for evaluating advanced plasma-sprayed and electron beam-physical vapor deposited (EB-PVD) thermal barrier coatings under the NASA Ultra-Efficient Engine Technology (UEET) Program. The candidate coating materials for this program are novel thermal barrier coatings that are found to have significantly reduced thermal conductivities and improved thermal stability due to an oxide-defect-cluster design. Critical issues for designing advanced low conductivity coatings with improved coating durability are also discussed.

  5. Effects of lithium insertion on thermal conductivity of silicon nanowires

    SciTech Connect

    Xu, Wen; Zhang, Gang; Li, Baowen

    2015-04-27

    Recently, silicon nanowires (SiNWs) have been applied as high-performance Li battery anodes, since they can overcome the pulverization and mechanical fracture during lithiation. Although thermal stability is one of the most important parameters that determine safety of Li batteries, thermal conductivity of SiNWs with Li insertion remains unclear. In this letter, using molecular dynamics simulations, we study room temperature thermal conductivity of SiNWs with Li insertion. It is found that compared with the pristine SiNW, there is as much as 60% reduction in thermal conductivity with 10% concentration of inserted Li atoms, while under the same impurity concentration the reduction in thermal conductivity of the mass-disordered SiNW is only 30%. With lattice dynamics calculations and normal mode decomposition, it is revealed that the phonon lifetimes in SiNWs decrease greatly due to strong scattering of phonons by vibrational modes of Li atoms, especially for those high frequency phonons. The observed strong phonon scattering phenomenon in Li-inserted SiNWs is similar to the phonon rattling effect. Our study serves as an exploration of thermal properties of SiNWs as Li battery anodes or weakly coupled with impurity atoms.

  6. Effective Thermal Conductivity of High Porosity Open Cell Nickel Foam

    NASA Technical Reports Server (NTRS)

    Sullins, Alan D.; Daryabeigi, Kamran

    2001-01-01

    The effective thermal conductivity of high-porosity open cell nickel foam samples was measured over a wide range of temperatures and pressures using a standard steady-state technique. The samples, measuring 23.8 mm, 18.7 mm, and 13.6 mm in thickness, were constructed with layers of 1.7 mm thick foam with a porosity of 0.968. Tests were conducted with the specimens subjected to temperature differences of 100 to 1000 K across the thickness and at environmental pressures of 10(exp -4) to 750 mm Hg. All test were conducted in a gaseous nitrogen environment. A one-dimensional finite volume numerical model was developed to model combined radiation/conduction heat transfer in the foam. The radiation heat transfer was modeled using the two-flux approximation. Solid and gas conduction were modeled using standard techniques for high porosity media. A parameter estimation technique was used in conjunction with the measured and predicted thermal conductivities at pressures of 10(exp -4) and 750 mm Hg to determine the extinction coefficient, albedo of scattering, and weighting factors for modeling the conduction thermal conductivity. The measured and predicted conductivities over the intermediate pressure values differed by 13%.

  7. Thermal Conductivity of Nonazeotropic Gaseous Mixtures of Fluorocarbon Refrigerants

    NASA Astrophysics Data System (ADS)

    Tanaka, Yoshiyuki; Ueno, Hiroshi; Kubota, Hironobu; Makita, Tadashi

    The thermal conductivity of four binary gaseous mixtures of R22 (CHCIF2) with R13(CClF3), R23(CHF3), R12(CCl2F2) and R114(CClF2·CClF2) has been measured at temperatures 298.15 and 323.15K under pressures from atmospheric to saturated pressures by a coaxial cylinder cell. The precision of the thermal conductivity obtained is within 2%. The thermal conductivity of mixtures increases with increasing temperature and pressure at a constant composition. The thermal conductivity in each mixture changes almost linearly with the concentration of R22 at a constant temperature and pressure, although the thermal conductivity at each composition is slightly larger than the calculated values by a simple molefraction average method. The experimental results were correlated with composition and pressure by empirical equations and compared with several kinds of prediction methods. The Brokaw's equation is found to reproduce the experimental data most successfully with a mean deviation of 0.7%.

  8. Characterization of the thermal conductivity for ceramic pebble beds

    NASA Astrophysics Data System (ADS)

    Lo Frano, R.; Aquaro, D.; Scaletti, L.; Olivi, N.

    2015-11-01

    The evaluation of the thermal conductivity of breeder materials is one of the main goals to find the best candidate material for the fusion reactor technology. The aim of this paper is to evaluate experimentally the thermal conductivity of a ceramic material by applying the hot wire method at different temperatures, ranging from 50 to about 800°C. The updated experimental facility, available at the Department of Civil and Industrial Engineering (DICI) of the University of Pisa, used to determine the thermal conductivity of a ceramic material (alumina), will be described along with the measurement acquisition system. Moreover it will be also provided an overview of the current state of art of the ceramic pebble bed breeder thermos-mechanics R&D (e.g. Lithium Orthosilicate (Li4SiO4) and Lithium Metatitanate (Li2TiO3)) focusing on the up-to-date analysis. The methodological approach adopted is articulated in two phase: the first one aimed at the experimental evaluation of thermal conductivity of a ceramic material by means of hot wire method, to be subsequently used in the second phase that is based on the test rig method, through which is measured the thermal conductivity of pebble bed material. In this framework, the experimental procedure and the measured results obtained varying the temperature, are presented and discussed.

  9. Thermal conductivity of solid thiophene in an incommensurate orientational state

    NASA Astrophysics Data System (ADS)

    Korolyuk, O. A.; Krivchikov, A. I.; Vdovichenko, G. A.; Romantsova, O. O.; Horbatenko, Yu. V.

    2016-01-01

    The thermal conductivity of solid thiophene at equilibrium vapor pressure between 2 K < T < 170 K, has been measured in a sequence of incommensurate metastable orientationally disordered phases II, II1, II2, and II2g with different degrees of orientational ordering of the molecules. It is found that in phase states II, II1 and II2 with dynamic orientational disorder of the molecules, the thermal conductivity does not depend on the temperature. It is shown that the temperature dependence of the thermal conductivity κ(T) of orientational glass Vg and II2g (incommensurate) does not have any of the anomalies that are typical for amorphous materials and glasses. The temperature dependence κ(T) of the incommensurate state of orientational glass II2g is bell-shaped, which is typical for the thermal conductivity of crystals with long-range orientational order. In the II2g state, as temperature drops from Tg to almost 10 K, the thermal conductivity increases according to κ(T) = A/T + B, where the first term describes the input of the propagating phonons, wherein the average length of their mean free path is greater than half of the phonon wavelength. The B term is associated with the input of localized short-wave, or "diffuse" vibrational modes. At low temperatures T ≤ 7 K, κ(T) ∝ T3 is observed with increasing temperatures, which corresponds to the boundary scattering of phonons.

  10. Thermal Conductivity Measurement of Liquid-Quenched Higher Manganese Silicides

    NASA Astrophysics Data System (ADS)

    Nishino, Shunsuke; Miyata, Masanobu; Ohdaira, Keisuke; Koyano, Mikio; Takeuchi, Tsunehiro

    2016-03-01

    Higher manganese silicides (HMSs, MnSi γ , γ ˜ 1.75) show promise for use as low-cost and environmentally friendly thermoelectric materials. To reduce their thermal conductivity, we partially substituted the Mn site with heavy elements using liquid quenching. Fabricated samples possess a curly ribbon-shape with about a 10- μm thickness and 1-mm width, with high surface roughness. In this study, we determined the thermal conductivity of the curly-ribbon-shaped samples using two independent methods: the 3 ω method with two heat flow models, and the steady-state method using a physical property measurement system (PPMS; Quantum Design). We succeeded in estimating the thermal conductivity at the temperature range of 100-200 K using the PPMS. The estimated thermal conductivity of non-doped HMSs shows a constant value without temperature dependence of 2.2 ± 0.8 W K-1m-1 at 100-200 K. The difference of thermal conductivities of W-doped and non-doped HMSs was not recognized within the measurement error.

  11. Anisotropic thermal conduction with magnetic fields in galaxy clusters

    NASA Astrophysics Data System (ADS)

    Arth, Alexander; Dolag, Klaus; Beck, Alexander; Petkova, Margarita; Lesch, Harald

    2015-08-01

    Magnetic fields play an important role for the propagation and diffusion of charged particles, which are responsible for thermal conduction. In this poster, we present an implementation of thermal conduction including the anisotropic effects of magnetic fields for smoothed particle hydrodynamics (SPH). The anisotropic thermal conduction is mainly proceeding parallel to magnetic fields and suppressed perpendicular to the fields. We derive the SPH formalism for the anisotropic heat transport and solve the corresponding equation with an implicit conjugate gradient scheme. We discuss several issues of unphysical heat transport in the cases of extreme ansiotropies or unmagnetized regions and present possible numerical workarounds. We implement our algorithm into the cosmological simulation code GADGET and study its behaviour in several test cases. In general, we reproduce the analytical solutions of our idealised test problems, and obtain good results in cosmological simulations of galaxy cluster formations. Within galaxy clusters, the anisotropic conduction produces a net heat transport similar to an isotropic Spitzer conduction model with low efficiency. In contrast to isotropic conduction our new formalism allows small-scale structure in the temperature distribution to remain stable, because of their decoupling caused by magnetic field lines. Compared to observations, strong isotropic conduction leads to an oversmoothed temperature distribution within clusters, while the results obtained with anisotropic thermal conduction reproduce the observed temperature fluctuations well. A proper treatment of heat transport is crucial especially in the outskirts of clusters and also in high density regions. It's connection to the local dynamical state of the cluster also might contribute to the observed bimodal distribution of cool core and non cool core clusters. Our new scheme significantly advances the modelling of thermal conduction in numerical simulations and overall gives

  12. Analytical model for thermal boundary conductance and equilibrium thermal accommodation coefficient at solid/gas interfaces.

    PubMed

    Giri, Ashutosh; Hopkins, Patrick E

    2016-02-28

    We develop an analytical model for the thermal boundary conductance between a solid and a gas. By considering the thermal fluxes in the solid and the gas, we describe the transmission of energy across the solid/gas interface with diffuse mismatch theory. From the predicted thermal boundary conductances across solid/gas interfaces, the equilibrium thermal accommodation coefficient is determined and compared to predictions from molecular dynamics simulations on the model solid-gas systems. We show that our model is applicable for modeling the thermal accommodation of gases on solid surfaces at non-cryogenic temperatures and relatively strong solid-gas interactions (εsf ≳ kBT). PMID:26931716

  13. Method to produce alumina aerogels having porosities greater than 80 percent

    SciTech Connect

    Poco, John F.; Hrubesh, Lawrence W.

    2003-09-16

    A two-step method for producing monolithic alumina aerogels having porosities of greater than 80 percent. Very strong, very low density alumina aerogel monoliths are prepared using the two-step sol-gel process. The method of preparing pure alumina aerogel modifies the prior known sol method by combining the use of substoichiometric water for hydrolysis, the use of acetic acid to control hydrolysis/condensation, and high temperature supercritical drying, all of which contribute to the formation of a polycrystalline aerogel microstructure. This structure provides exceptional mechanical properties of the alumina aerogel, as well as enhanced thermal resistance and high temperature stability.

  14. Thermal distribution in high power optical devices with power-law thermal conductivity

    NASA Astrophysics Data System (ADS)

    Zhou, Chuanle; Grayson, M.

    2012-01-01

    We introduce a power-law approximation to model non-linear ranges of the thermal conductivity, and under this approximation derive a simple analytical expression for calculating the temperature profile in high power quantum cascade lasers and light emitting diodes. The thermal conductivity of a type II InAs/GaSb superlattice (T2SL) is used as an example, having negative or positive power-law exponents depending on the thermal range of interest. The result is an increase or decrease in the temperature, respectively, relative to the uniform thermal conductivity assumption.

  15. Thermal conductivity and thermal expansion of graphite fiber-reinforced copper matrix composites

    NASA Technical Reports Server (NTRS)

    Ellis, David L.; Mcdanels, David L.

    1993-01-01

    The high specific conductivity of graphite fiber/copper matrix (Gr/Cu) composites offers great potential for high heat flux structures operating at elevated temperatures. To determine the feasibility of applying Gr/Cu composites to high heat flux structures, composite plates were fabricated using unidirectional and cross-plied pitch-based P100 graphite fibers in a pure copper matrix. Thermal conductivity of the composites was measured from room temperature to 1073 K, and thermal expansion was measured from room temperature to 1050 K. The longitudinal thermal conductivity, parallel to the fiber direction, was comparable to pure copper. The transverse thermal conductivity, normal to the fiber direction, was less than that of pure copper and decreased with increasing fiber content. The longitudinal thermal expansion decreased with increasing fiber content. The transverse thermal expansion was greater than pure copper and nearly independent of fiber content.

  16. Thermal conductivity and thermal expansion of graphite fiber/copper matrix composites

    NASA Technical Reports Server (NTRS)

    Ellis, David L.; Mcdanels, David L.

    1991-01-01

    The high specific conductivity of graphite fiber/copper matrix (Gr/Cu) composites offers great potential for high heat flux structures operating at elevated temperatures. To determine the feasibility of applying Gr/Cu composites to high heat flux structures, composite plates were fabricated using unidirectional and cross-plied pitch-based P100 graphite fibers in a pure copper matrix. Thermal conductivity of the composites was measured from room temperature to 1073 K, and thermal expansion was measured from room temperature to 1050 K. The longitudinal thermal conductivity, parallel to the fiber direction, was comparable to pure copper. The transverse thermal conductivity, normal to the fiber direction, was less than that of pure copper and decreased with increasing fiber content. The longitudinal thermal expansion decreased with increasing fiber content. The transverse thermal expansion was greater than pure copper and nearly independent of fiber content.

  17. Numerical Investigation of the Thermal Conductivity of Graphite Nanofibers

    NASA Astrophysics Data System (ADS)

    Hakak Khadem, Masoud

    The thermal conductivity of graphite nano-fibers (GNFs) with different styles is predicted computationally. GNFs are formed as basal planes of graphene stacked based on the catalytic configuration. The large GNF thermal conductivity relative to a base phase change material (PCM) may lead to improved PCM performance when embedded with GNFs. Three different types of GNFs are modeled: platelet, ribbon, and herringbone. Molecular dynamics (MD) simulations are used in this study as a means to predict the thermal conductivity tensor based on atomic behavior. The in-house MD code, Molecular Dynamics in Arbitrary Geometries (MDAG), was updated with the features required to create the predictions. To model both interlayer van-der Waals and intralayer covalent bonding of carbon atoms in GNFs, a combination of the optimized Tersoff potential function for atoms within the layers and a pairwise Lennard-Jones (LJ) potential function to model the interactions between the layers was used. Tests of energy conservation in the NVE ensemble have been performed to validate the employed potential model. Nose-Hoover, Andersen, and Berendsen thermostats were also incorporated into MDAG to enable MD simulations in NVT ensembles, where the volume, number of atoms, and temperature of the system are conserved. Equilibrium MD with Green-Kubo (GK) relations was then employed to extract the thermal conductivity tensor for symmetric GNFs (platelet and ribbon). The thermal conductivity of solid argon at different temperatures was calculated and compared to other studies to validate the GK implementation. Different heat current formulations, as a result of using the three-body Tersoff potential, were considered and the discrepancy in the calculated thermal conductivity values of graphene using each formula was resolved by employing a novel comparative technique that identifies the most accurate formulation. The effect of stacking configuration on the thermal conductivity of platelet and ribbon GNFs

  18. A model for including thermal conduction in molecular dynamics simulations

    NASA Technical Reports Server (NTRS)

    Wu, Yue; Friauf, Robert J.

    1989-01-01

    A technique is introduced for including thermal conduction in molecular dynamics simulations for solids. A model is developed to allow energy flow between the computational cell and the bulk of the solid when periodic boundary conditions cannot be used. Thermal conduction is achieved by scaling the velocities of atoms in a transitional boundary layer. The scaling factor is obtained from the thermal diffusivity, and the results show good agreement with the solution for a continuous medium at long times. The effects of different temperature and size of the system, and of variations in strength parameter, atomic mass, and thermal diffusivity were investigated. In all cases, no significant change in simulation results has been found.

  19. Anisotropic Effective Thermal Conductivity of Particle Beds Under Uniaxial Compression

    NASA Astrophysics Data System (ADS)

    Mo, Jingwen; Garrett, Daniel; Ban, Heng

    2015-11-01

    The effective thermal conductivity (ETC) of particle beds has been shown to be anisotropic under uniaxial compression. The objective of this study is to validate the assumption that vertical and horizontal effective thermal conductivities of particle beds under uniaxial (vertical) compression can be correlated to compressive stresses in each direction. The horizontal compressive pressure, which is perpendicular to the applied compressive pressure, can be calculated with the use of the at-rest pressure coefficient and subsequently used in a modified macro-contact thermal resistance model to predict the horizontal ETC. The vertical ETC is obtained using the same model by substituting the vertical compressive pressure into the macro-contact thermal resistance. The modified model prediction is shown to match well with experimental data of both vertical and horizontal ETCs of particle beds under uniaxial compression in air and vacuum.

  20. Thermal conductivity and electrical resistivity of porous material

    NASA Technical Reports Server (NTRS)

    Koh, J. C. Y.; Fortini, A.

    1971-01-01

    Thermal conductivity and electrical resistivity of porous materials, including 304L stainless steel Rigimesh, 304L stainless steel sintered spherical powders, and OFHC sintered spherical powders at different porosities and temperatures are reported and correlated. It was found that the thermal conductivity and electrical resistivity can be related to the solid material properties and the porosity of the porous matrix regardless of the matrix structure. It was also found that the Wiedermann-Franz-Lorenz relationship is valid for the porous materials under consideration. For high conductivity materials, the Lorenz constant and the lattice component of conductivity depend on the material and are independent of the porosity. For low conductivity, the lattice component depends on the porosity as well.

  1. Thermal conductivity of graphene in corbino membrane geometry.

    PubMed

    Faugeras, Clement; Faugeras, Blaise; Orlita, Milan; Potemski, M; Nair, Rahul R; Geim, A K

    2010-04-27

    Local laser excitation and temperature readout from the intensity ratio of Stokes to anti-Stokes Raman scattering signals are employed to study the thermal properties of a large graphene membrane. The concluded value of the heat conductivity coefficient kappa approximately 600 W/(m.K) is smaller than previously reported but still validates the conclusion that graphene is a very good thermal conductor. PMID:20218666

  2. Relationship between the thermal conductivity and shear viscosity of nanofluids

    NASA Astrophysics Data System (ADS)

    Ding, Yulong; Chen, Haisheng; Musina, Zenfira; Jin, Yi; Zhang, Tianfu; Witharana, Sanjeeva; Yang, Wei

    2010-05-01

    Nanofluids are dilute liquid suspensions of nanoparticles. Nanoparticles and liquid media, in such fluids, mix and interact at the nanoscale. Interactions between nanoparticles in nanofluids can lead to structuring of the particles. This paper discusses how the nanoparticle structuring affects the thermal conductivity and viscosity of nanofluids and how the two transport properties are related through the nanoparticle structuring. It is shown that the experimentally measured thermal conductivity enhancement and the viscosity increase due to the presence of nanoparticles can be interpreted by the aggregation of nanoparticles. It is also shown that modification of the conventional form of the effective medium theory by taking into account nanoparticle structuring information from the rheological analyses gives good agreement with experimentally measured thermal conductivity.

  3. Increased Thermal Conductivity in Metal-Organic Heat Carrier Nanofluids

    NASA Astrophysics Data System (ADS)

    Nandasiri, Manjula I.; Liu, Jian; McGrail, B. Peter; Jenks, Jeromy; Schaef, Herbert T.; Shutthanandan, Vaithiyalingam; Nie, Zimin; Martin, Paul F.; Nune, Satish K.

    2016-06-01

    Metal-organic heat carriers (MOHCs) are recently developed nanofluids containing metal-organic framework (MOF) nanoparticles dispersed in various base fluids including refrigerants (R245Fa) and methanol. Here, we report the synthesis and characterization of MOHCs containing nanoMIL-101(Cr) and graphene oxide (GO) in an effort to improve the thermo-physical properties of various base fluids. MOHC/GO nanocomposites showed enhanced surface area, porosity, and nitrogen adsorption compared with the intrinsic nanoMIL-101(Cr) and the properties depended on the amount of GO added. MIL-101(Cr)/GO in methanol exhibited a significant increase in the thermal conductivity (by approximately 50%) relative to that of the intrinsic nanoMIL-101(Cr) in methanol. The thermal conductivity of the base fluid (methanol) was increased by about 20%. The increase in the thermal conductivity of nanoMIL-101(Cr) MOHCs due to GO functionalization is explained using a classical Maxwell model.

  4. Thermal conductivity at a disordered quantum critical point

    NASA Astrophysics Data System (ADS)

    Hartnoll, Sean A.; Ramirez, David M.; Santos, Jorge E.

    2016-04-01

    Strongly disordered and strongly interacting quantum critical points are difficult to access with conventional field theoretic methods. They are, however, both experimentally important and theoretically interesting. In particular, they are expected to realize universal incoherent transport. Such disordered quantum critical theories have recently been constructed holographically by deforming a CFT by marginally relevant disorder. In this paper we find additional disordered fixed points via relevant disordered deformations of a holographic CFT. Using recently developed methods in holographic transport, we characterize the thermal conductivity in both sets of theories in 1+1 dimensions. The thermal conductivity is found to tend to a constant at low temperatures in one class of fixed points, and to scale as T 0.3 in the other. Furthermore, in all cases the thermal conductivity exhibits discrete scale invariance, with logarithmic in temperature oscillations superimposed on the low temperature scaling behavior. At no point do we use the replica trick.

  5. Experimental investigation of thermal conductivity of magnetic nanofluids

    NASA Astrophysics Data System (ADS)

    Parekh, Kinnari; Lee, H. S.

    2012-06-01

    Two different magnetic nanofluids comprising of magnetite and Mn-Zn ferrite particles were synthesized in light hydrocarbon oil using continuous chemical process. Powder XRD and TEM image show single phase spinel structure with size of 10 nm and 6.7 nm, respectively for magnetite and Mn-Zn ferrite. Thermal conductivity of nanofluids has been studied as a function of volume fraction under transverse magnetic field. Magnetite nanofluid shows 17% enhancements in thermal conductivity for 4.7% volume fraction while Mn-Zn ferrite shows 45% enhancement at 10% volume fraction. In presence of transverse magnetic field the magnetite nanofluids shows further enhancement from 17% to 30% while no change in thermal conductivity has been observed for Mn-Zn ferrite. These results are explained considering the dipolar coupling co-efficient which for magnetite particles favors chain structures.

  6. Effect of variable thermal conductivity on isotherms in Bridgman growth

    NASA Technical Reports Server (NTRS)

    Naumann, R. J.; Lehoczky, S. L.

    1983-01-01

    A change in thermal conductivity associated with melting or solidification can have a profound influence on the isotherms near the solidification interface if the material is being directionally solidified in an ampoule whose walls carry a substantial portion of the heat. This analysis was prompted by a recent discovery that the thermal conductivity of Hg(1-x)CD(x)Te increased dramatically as the material is heated above the solidus curve. An illustrative example is shown in which the sample is approximated as an infinite cylinder with constant but diffferent thermal properties in the solid and melt. The boundary conditions are fixed on the surface by a conductive ampoule in a two-zone Bridgman furnace with an adiabatic region separating the two zones. The effect of the adiabatic zone in this case is to intensify the curvature of the interface rather than to lessen it.

  7. Lattice thermal conductivity of nanograined half-Heusler solid solutions

    SciTech Connect

    Geng, Huiyuan Meng, Xianfu; Zhang, Hao; Zhang, Jian

    2014-05-19

    We report a phenomenological model of atomic weight, lattice constant, temperature, and grain size to calculate the high-temperature lattice thermal conductivity of nanograined solid solutions. The theoretical treatment developed here is reasonably consistent with the experimental results of n-type MNiSn and p-type MCoSb alloys, where M is the combination of Hf, Zr, and Ti. For disordered half-Heusler alloys with moderated grain sizes, we predict that the reduction in lattice thermal conductivity due to grain boundary scattering is independent of the scattering parameter, which characterizes the phonon scattering cross section of point defects. In addition, the lattice thermal conductivity falls off with temperature as T{sup –1∕2} around the Debye temperature.

  8. Increased Thermal Conductivity in Metal-Organic Heat Carrier Nanofluids.

    PubMed

    Nandasiri, Manjula I; Liu, Jian; McGrail, B Peter; Jenks, Jeromy; Schaef, Herbert T; Shutthanandan, Vaithiyalingam; Nie, Zimin; Martin, Paul F; Nune, Satish K

    2016-01-01

    Metal-organic heat carriers (MOHCs) are recently developed nanofluids containing metal-organic framework (MOF) nanoparticles dispersed in various base fluids including refrigerants (R245Fa) and methanol. Here, we report the synthesis and characterization of MOHCs containing nanoMIL-101(Cr) and graphene oxide (GO) in an effort to improve the thermo-physical properties of various base fluids. MOHC/GO nanocomposites showed enhanced surface area, porosity, and nitrogen adsorption compared with the intrinsic nanoMIL-101(Cr) and the properties depended on the amount of GO added. MIL-101(Cr)/GO in methanol exhibited a significant increase in the thermal conductivity (by approximately 50%) relative to that of the intrinsic nanoMIL-101(Cr) in methanol. The thermal conductivity of the base fluid (methanol) was increased by about 20%. The increase in the thermal conductivity of nanoMIL-101(Cr) MOHCs due to GO functionalization is explained using a classical Maxwell model. PMID:27302196

  9. Thermally conductive cementitious grout for geothermal heat pump systems

    DOEpatents

    Allan, Marita

    2001-01-01

    A thermally conductive cement-sand grout for use with a geothermal heat pump system. The cement sand grout contains cement, silica sand, a superplasticizer, water and optionally bentonite. The present invention also includes a method of filling boreholes used for geothermal heat pump systems with the thermally conductive cement-sand grout. The cement-sand grout has improved thermal conductivity over neat cement and bentonite grouts, which allows shallower bore holes to be used to provide an equivalent heat transfer capacity. In addition, the cement-sand grouts of the present invention also provide improved bond strengths and decreased permeabilities. The cement-sand grouts can also contain blast furnace slag, fly ash, a thermoplastic air entraining agent, latex, a shrinkage reducing admixture, calcium oxide and combinations thereof.

  10. Increased Thermal Conductivity in Metal-Organic Heat Carrier Nanofluids

    PubMed Central

    Nandasiri, Manjula I.; Liu, Jian; McGrail, B. Peter; Jenks, Jeromy; Schaef, Herbert T.; Shutthanandan, Vaithiyalingam; Nie, Zimin; Martin, Paul F.; Nune, Satish K.

    2016-01-01

    Metal-organic heat carriers (MOHCs) are recently developed nanofluids containing metal-organic framework (MOF) nanoparticles dispersed in various base fluids including refrigerants (R245Fa) and methanol. Here, we report the synthesis and characterization of MOHCs containing nanoMIL-101(Cr) and graphene oxide (GO) in an effort to improve the thermo-physical properties of various base fluids. MOHC/GO nanocomposites showed enhanced surface area, porosity, and nitrogen adsorption compared with the intrinsic nanoMIL-101(Cr) and the properties depended on the amount of GO added. MIL-101(Cr)/GO in methanol exhibited a significant increase in the thermal conductivity (by approximately 50%) relative to that of the intrinsic nanoMIL-101(Cr) in methanol. The thermal conductivity of the base fluid (methanol) was increased by about 20%. The increase in the thermal conductivity of nanoMIL-101(Cr) MOHCs due to GO functionalization is explained using a classical Maxwell model. PMID:27302196

  11. Dynamical thermal conductivity of the spin Lieb lattice

    NASA Astrophysics Data System (ADS)

    Yarmohammadi, Mohsen

    2016-05-01

    In the ferromagnetic insulator with the Dzyaloshinskii-Moriya interaction (DMI), we have theoretically investigated the dynamical thermal conductivity (DTC). In other words, we have investigated the frequency dependence of thermal conductivity, κ, of the Lieb lattice, a face-centered square lattice, subjected to a time dependence temperature gradient. Using linear response theory and Green's function approach, DTC has been obtained in the context of Heisenberg Hamiltonian. At low frequencies, DTC is found to be monotonically increasing with DMI strength (DMIS), temperature and next-nearest-neighbor (NNN) coupling. Also we have found that DTC includes a peak for different values of temperature, DMIS and NNN coupling. Furthermore we study the temperature dependence of thermal conductivity of Lieb lattice for different values of DMIS, NNN coupling and external magnetic filed. We witness a decrease in DTC with temperature due to the quantum effects in the system.

  12. Upper bound to the thermal conductivity of carbon nanotube pellets

    NASA Astrophysics Data System (ADS)

    Chalopin, Yann; Volz, Sebastian; Mingo, Natalio

    2009-04-01

    Using atomistic Green's function calculations, we find that the phonon thermal conductivity of pellets composed of ˜μm long carbon nanotubes has an upper bound of a few W/m K. This is in striking contrast with the extremely high thermal conductivity of individual nanotubes (˜3000 W/m K). We show that, at room temperature, this upper bound does not depend on the nanotube diameter. Conversely, for low temperatures, an inverse proportionality with nanotube diameter is predicted. We present concrete results as a function of nanotube length and chirality, pellet density, and temperature. These results imply that carbon nanotube pellets belong to the category of thermal insulators, contrasting with the good conducting properties of parallel nanotube arrays, or individual nanotubes.

  13. Thermal Conductivity of AlN-Ethanol Nanofluids

    NASA Astrophysics Data System (ADS)

    Hu, Peng; Shan, Wan-Liang; Yu, Fei; Chen, Ze-Shao

    2008-12-01

    Aluminum nitride (AlN) particles of 20 nm diameter were dispersed into ethanol by a two-step process, first magnetic striation and then ultrasonic agitation. Castor oil was added as a dispersant to improve the stability of the AlN suspension. The thermal conductivities of AlN-ethanol nanofluids were measured by a hot-disk method from 0.5 vol% to 4.0 vol% at temperatures of 273.15 K and 297.15 K. Results show about 20% increase in the thermal conductivity of ethanol with the addition of 4.0 vol% at 273.15 K, and a strong temperature dependence of the thermal conductivity.

  14. Reduction of Thermal Conductivity by Nanoscale 3D Phononic Crystal

    PubMed Central

    Yang, Lina; Yang, Nuo; Li, Baowen

    2013-01-01

    We studied how the period length and the mass ratio affect the thermal conductivity of isotopic nanoscale three-dimensional (3D) phononic crystal of Si. Simulation results by equilibrium molecular dynamics show isotopic nanoscale 3D phononic crystals can significantly reduce the thermal conductivity of bulk Si at high temperature (1000 K), which leads to a larger ZT than unity. The thermal conductivity decreases as the period length and mass ratio increases. The phonon dispersion curves show an obvious decrease of group velocities in 3D phononic crystals. The phonon's localization and band gap is also clearly observed in spectra of normalized inverse participation ratio in nanoscale 3D phononic crystal. PMID:23378898

  15. Reduction of thermal conductivity by nanoscale 3D phononic crystal.

    PubMed

    Yang, Lina; Yang, Nuo; Li, Baowen

    2013-01-01

    We studied how the period length and the mass ratio affect the thermal conductivity of isotopic nanoscale three-dimensional (3D) phononic crystal of Si. Simulation results by equilibrium molecular dynamics show isotopic nanoscale 3D phononic crystals can significantly reduce the thermal conductivity of bulk Si at high temperature (1000 K), which leads to a larger ZT than unity. The thermal conductivity decreases as the period length and mass ratio increases. The phonon dispersion curves show an obvious decrease of group velocities in 3D phononic crystals. The phonon's localization and band gap is also clearly observed in spectra of normalized inverse participation ratio in nanoscale 3D phononic crystal. PMID:23378898

  16. Spectral mapping of thermal conductivity through nanoscale ballistic transport

    NASA Astrophysics Data System (ADS)

    Hu, Yongjie; Zeng, Lingping; Minnich, Austin J.; Dresselhaus, Mildred S.; Chen, Gang

    2015-08-01

    Controlling thermal properties is central to many applications, such as thermoelectric energy conversion and the thermal management of integrated circuits. Progress has been made over the past decade by structuring materials at different length scales, but a clear relationship between structure size and thermal properties remains to be established. The main challenge comes from the unknown intrinsic spectral distribution of energy among heat carriers. Here, we experimentally measure this spectral distribution by probing quasi-ballistic transport near nanostructured heaters down to 30 nm using ultrafast optical spectroscopy. Our approach allows us to quantify up to 95% of the total spectral contribution to thermal conductivity from all phonon modes. The measurement agrees well with multiscale and first-principles-based simulations. We further demonstrate the direct construction of mean free path distributions. Our results provide a new fundamental understanding of thermal transport and will enable materials design in a rational way to achieve high performance.

  17. Spectral mapping of thermal conductivity through nanoscale ballistic transport.

    PubMed

    Hu, Yongjie; Zeng, Lingping; Minnich, Austin J; Dresselhaus, Mildred S; Chen, Gang

    2015-08-01

    Controlling thermal properties is central to many applications, such as thermoelectric energy conversion and the thermal management of integrated circuits. Progress has been made over the past decade by structuring materials at different length scales, but a clear relationship between structure size and thermal properties remains to be established. The main challenge comes from the unknown intrinsic spectral distribution of energy among heat carriers. Here, we experimentally measure this spectral distribution by probing quasi-ballistic transport near nanostructured heaters down to 30 nm using ultrafast optical spectroscopy. Our approach allows us to quantify up to 95% of the total spectral contribution to thermal conductivity from all phonon modes. The measurement agrees well with multiscale and first-principles-based simulations. We further demonstrate the direct construction of mean free path distributions. Our results provide a new fundamental understanding of thermal transport and will enable materials design in a rational way to achieve high performance. PMID:26030656

  18. Thermoelectric Polymers and their Elastic Aerogels.

    PubMed

    Khan, Zia Ullah; Edberg, Jesper; Hamedi, Mahiar Max; Gabrielsson, Roger; Granberg, Hjalmar; Wågberg, Lars; Engquist, Isak; Berggren, Magnus; Crispin, Xavier

    2016-06-01

    Electronically conducting polymers constitute an emerging class of materials for novel electronics, such as printed electronics and flexible electronics. Their properties have been further diversified to introduce elasticity, which has opened new possibility for "stretchable" electronics. Recent discoveries demonstrate that conducting polymers have thermoelectric properties with a low thermal conductivity, as well as tunable Seebeck coefficients - which is achieved by modulating their electrical conductivity via simple redox reactions. Using these thermoelectric properties, all-organic flexible thermoelectric devices, such as temperature sensors, heat flux sensors, and thermoelectric generators, are being developed. In this article we discuss the combination of the two emerging fields: stretchable electronics and polymer thermoelectrics. The combination of elastic and thermoelectric properties seems to be unique for conducting polymers, and difficult to achieve with inorganic thermoelectric materials. We introduce the basic concepts, and state of the art knowledge, about the thermoelectric properties of conducting polymers, and illustrate the use of elastic thermoelectric conducting polymer aerogels that could be employed as temperature and pressure sensors in an electronic-skin. PMID:26836440

  19. Estimating interfacial thermal conductivity in metamaterials through heat flux mapping

    SciTech Connect

    Canbazoglu, Fatih M.; Vemuri, Krishna P.; Bandaru, Prabhakar R.

    2015-04-06

    The variability of the thickness as well as the thermal conductivity of interfaces in composites may significantly influence thermal transport characteristics and the notion of a metamaterial as an effective medium. The consequent modulations of the heat flux passage are analytically and experimentally examined through a non-contact methodology using radiative imaging, on a model anisotropic thermal metamaterial. It was indicated that a lower Al layer/silver interfacial epoxy ratio of ∼25 compared to that of a Al layer/alumina interfacial epoxy (of ∼39) contributes to a smaller deviation of the heat flux bending angle.

  20. Thermal conductivity of model zeolites: molecular dynamics simulation study

    NASA Astrophysics Data System (ADS)

    Murashov, Vladimir V.

    1999-02-01

    The thermal conductivity of model zeolites was investigated using non-equilibrium molecular dynamics calculations. This type of calculation was found to overestimate the thermal conductivity of low-density silica polymorphs. A better reproduction of the experimental results was found for zeolites, and this was related to the lower phonon mean free path. The thermal conductivity of framework silicates was shown to be determined primarily by the vibrations of the continuous oxygen sublattice. Thus, the most drastic suppression of the heat transfer was related to alterations of the O-O distances; for example, a sixfold reduction in thermal conductivity compared to that of siliceous LTA zeolite was found for LTA-A1PO4. Framework cations were shown to affect the heat transfer by changing the vibrational modes of the structural building units of the framework and non-framework counter-cations, by disturbing the oxygen sublattice locally and acting as Rayleigh and resonant scatterers. A model assuming the heat transfer to be due only to non-dispersive acoustic phonons failed to reproduce the dependence of the thermal conductivity on the mass of the cations and the unit-cell dimension, thus suggesting a more sophisticated mechanism of heat transfer to be operative in framework materials. The effect of non-framework non-ionic species on the thermal conductivity was shown to be determined by their effect on the characteristics of the oxygen framework vibrations. Thus, repulsive interactions between the oxygen sublattice and Xe8 clusters, reducing the anisotropy and anharmonicity of the oxygen vibrations, give rise to enhanced heat transfer in LTA-SiO2 at ambient conditions.