Flame exposure time on Langmuir probe degradation, ion density, and thermionic emission for flame temperature.

PubMed

Doyle, S J; Salvador, P R; Xu, K G

2017-11-01

The paper examines the effect of exposure time of Langmuir probes in an atmospheric premixed methane-air flame. The effects of probe size and material composition on current measurements were investigated, with molybdenum and tungsten probe tips ranging in diameter from 0.0508 to 0.1651 mm. Repeated prolonged exposures to the flame, with five runs of 60 s, resulted in gradual probe degradations (-6% to -62% area loss) which affected the measurements. Due to long flame exposures, two ion saturation currents were observed, resulting in significantly different ion densities ranging from 1.16 × 10 16 to 2.71 × 10 19 m -3 . The difference between the saturation currents is caused by thermionic emissions from the probe tip. As thermionic emission is temperature dependent, the flame temperature could thus be estimated from the change in current. The flame temperatures calculated from the difference in saturation currents (1734-1887 K) were compared to those from a conventional thermocouple (1580-1908 K). Temperature measurements obtained from tungsten probes placed in rich flames yielded the highest percent error (9.66%-18.70%) due to smaller emission current densities at lower temperatures. The molybdenum probe yielded an accurate temperature value with only 1.29% error. Molybdenum also demonstrated very low probe degradation in comparison to the tungsten probe tips (area reductions of 6% vs. 58%, respectively). The results also show that very little exposure time (<5 s) is needed to obtain a valid ion density measurement and that prolonged flame exposures can yield the flame temperature but also risks damage to the Langmuir probe tip.

First-Principles Studies of Structure-Property Relationships: Enabling Design of Functional Materials

NASA Astrophysics Data System (ADS)

Zhou, Qunfei

First-principles calculations based on quantum mechanics have been proved to be powerful for accurately regenerating experimental results, uncovering underlying myths of experimental phenomena, and accelerating the design of innovative materials. This work has been motivated by the demand to design next-generation thermionic emitting cathodes and techniques to allow for synthesis of photo-responsive polymers on complex surfaces with controlled thickness and patterns. For Os-coated tungsten thermionic dispenser cathodes, we used first-principles methods to explore the bulk and surface properties of W-Os alloys in order to explain the previously observed experimental phenomena that thermionic emission varies significantly with W-Os alloy composition. Meanwhile, we have developed a new quantum mechanical approach to quantitatively predict the thermionic emission current density from materials perspective without any semi-empirical approximations or complicated analytical models, which leads to better understanding of thermionic emission mechanism. The methods from this work could be used to accelerate the design of next-generation thermionic cathodes. For photoresponsive materials, we designed a novel type of azobenzene-containing monomer for light-mediated ring-opening metathesis polymerization (ROMP) toward the fabrication of patterned, photo-responsive polymers by controlling ring strain energy (RSE) of the monomer that drives ROMP. This allows for unprecedented remote, noninvasive, instantaneous spatial and temporal control of photo-responsive polymer deposition on complex surfaces.This work on the above two different materials systems showed the power of quantum mechanical calculations on predicting, understanding and discovering the structures and properties of both known and unknown materials in a fast, efficient and reliable way.

Effects of nanoscale vacuum gap on photon-enhanced thermionic emission devices

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Yuan; Liao, Tianjun; Zhang, Yanchao

2016-01-28

A new model of the photon-enhanced thermionic emission (PETE) device with a nanoscale vacuum gap is established by introducing the quantum tunneling effect and the image force correction. Analytic expressions for both the thermionic emission and tunneling currents are derived. The electron concentration and the temperature of the cathode are determined by the particle conservation and energy balance equations. The effects of the operating voltage on the maximum potential barrier, cathode temperature, electron concentration and equilibrium electron concentration of the conduction band, and efficiency of the PETE device are discussed in detail for different given values of the vacuum gapmore » length. The influence of the band gap of the cathode and flux concentration on the efficiency is further analyzed. The maximum efficiency of the PETE and the corresponding optimum values of the band gap and the operating voltage are determined. The results obtained here show that the efficiency of the PETE device can be significantly improved by employing a nanoscale vacuum gap.« less

Thermionic Properties of Carbon Based Nanomaterials Produced by Microhollow Cathode PECVD

NASA Technical Reports Server (NTRS)

Haase, John R.; Wolinksy, Jason J.; Bailey, Paul S.; George, Jeffrey A.; Go, David B.

2015-01-01

Thermionic emission is the process in which materials at sufficiently high temperature spontaneously emit electrons. This process occurs when electrons in a material gain sufficient thermal energy from heating to overcome the material's potential barrier, referred to as the work function. For most bulk materials very high temperatures (greater than 1500 K) are needed to produce appreciable emission. Carbon-based nanomaterials have shown significant promise as emission materials because of their low work functions, nanoscale geometry, and negative electron affinity. One method of producing these materials is through the process known as microhollow cathode PECVD. In a microhollow cathode plasma, high energy electrons oscillate at very high energies through the Pendel effect. These high energy electrons create numerous radical species and the technique has been shown to be an effective method of growing carbon based nanomaterials. In this work, we explore the thermionic emission properties of carbon based nanomaterials produced by microhollow cathode PECVD under a variety of synthesis conditions. Initial studies demonstrate measureable current at low temperatures (approximately 800 K) and work functions (approximately 3.3 eV) for these materials.

Temperature dependence of photon-enhanced thermionic emission from GaAs surface with nonequilibrium Cs overlayers

NASA Astrophysics Data System (ADS)

Zhuravlev, A. G.; Alperovich, V. L.

2017-02-01

The temperature influence on the Cs/GaAs surface electronic properties, which determine the photon-enhanced thermionic emission (PETE), is studied. It was found that heating to moderate temperatures of about 100 °С leads to substantial changes in the magnitude and shape of Cs coverage dependences of photoemission current and surface band bending, along with the changes of relaxation kinetics after Cs deposition. A spectral proof of the PETE process is obtained under thermal cycling of the Cs/GaAs surface with 0.45 monolayer (ML) of Cs.

Fabrication, Densification and Thermionic Emission Property of Lanthanum Hexaboride

NASA Astrophysics Data System (ADS)

Yu, Yiping; Wang, Song; Li, Wei; Chen, Hongmei; Chen, Zhaohui

2018-03-01

An effective way to improve sintering densification of LaB6 was proposed and confirmed experimentally. Firstly, LaB6 nanopowders with a cube-like shape of 94.7 nm were fabricated by molten salt synthesis route at 800 °C for 1 h. Then, LaB6 bulk material of 98% density was prepared by hot pressing sintering of as-synthesized LaB6 nanopowders under 1800 °C/50 MPa/30 min. The acquired LaB6 bulk material had a work function of 2.87 eV and exhibited an excellent thermionic emission property. The saturation emission current density at 1500 and 1600 °C reached 37.4 and 44.3 A/cm2, respectively.

Evidence for cluster shape effects on the kinetic energy spectrum in thermionic emission.

PubMed

Calvo, F; Lépine, F; Baguenard, B; Pagliarulo, F; Concina, B; Bordas, C; Parneix, P

2007-11-28

Experimental kinetic energy release distributions obtained for the thermionic emission from C(n) (-) clusters, 10< or =n< or =20, exhibit significant non-Boltzmann variations. Using phase space theory, these different features are analyzed and interpreted as the consequence of contrasting shapes in the daughter clusters; linear and nonlinear isomers have clearly distinct signatures. These results provide a novel indirect structural probe for atomic clusters associated with their thermionic emission spectra.

Analysis of thermionic bare tether operation regimes in passive mode

NASA Astrophysics Data System (ADS)

Sanmartín, J. R.; Chen, Xin; Sánchez-Arriaga, G.

2017-01-01

A thermionic bare tether (TBT) is a long conductor coated with a low work-function material. In drag mode, a tether segment extending from anodic end A to a zero-bias point B, with the standard Orbital-motion-limited current collection, is followed by a complex cathodic segment. In general, as bias becomes more negative in moving from B to cathodic end C, one first finds space-charge-limited (SCL) emission covering up to some intermediate point B*, then full Richardson-Dushman (RD) emission reaching from B* to end C. An approximate analytical study, which combines the current and voltage profile equations with results from asymptotic studies of the Vlasov-Poisson system for emissive probes, is carried out to determine the parameter domain covering two limit regimes, which are effectively controlled by just two dimensionless parameters involving ambient plasma and TBT material properties. In one such limit regime, no point B* is reached and thus no full RD emission develops. In an opposite regime, SCL segment BB* is too short to contribute significantly to the current balance.

High efficiency thermionic converter studies

NASA Technical Reports Server (NTRS)

Huffman, F. N.; Sommer, A. H.; Balestra, C. L.; Briere, D. P.; Oettinger, P. E.

1976-01-01

The objective is to improve thermionic converter performance by means of reduced interelectrode losses, greater emitter capabilities, and lower collector work functions until the converter performance level is suitable for out-of-core space reactors and radioisotope generators. Electrode screening experiments have identified several promising collector materials. Back emission work function measurements of a ZnO collector in a thermionic diode have given values less than 1.3 eV. Diode tests were conducted over the range of temperatures of interest for space power applications. Enhanced mode converter experiments have included triodes operated in both the surface ionization and plasmatron modes. Pulsed triodes were studied as a function of pulse length, pulse potential, inert gas fill pressure, cesium pressure, spacing, emitter temperature and collector temperature. Current amplifications (i.e., mean output current/mean grid current) of several hundred were observed up to output current densities of one amp/sq cm. These data correspond to an equivalent arc drop less than 0.1 eV.

Thermodynamics of photon-enhanced thermionic emission solar cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Reck, Kasper, E-mail: kasper.reck@nanotech.dtu.dk; Hansen, Ole, E-mail: ole.hansen@nanotech.dtu.dk; CINF Center for Individual Nanoparticle Functionality, Technical University of Denmark, Kgs. Lyngby 2800

2014-01-13

Photon-enhanced thermionic emission (PETE) cells in which direct photon energy as well as thermal energy can be harvested have recently been suggested as a new candidate for high efficiency solar cells. Here, we present an analytic thermodynamical model for evaluation of the efficiency of PETE solar cells including an analysis of the entropy production due to thermionic emission of general validity. The model is applied to find the maximum efficiency of a PETE cell for given cathode and anode work functions and temperatures.

Photo-assisted electron emission from illuminated monolayer graphene

NASA Astrophysics Data System (ADS)

Upadhyay Kahaly, M.; Misra, Shikha; Mishra, S. K.

2017-05-01

We establish a formalism to address co-existing and complementing thermionic and photoelectric emission from a monolayer graphene sheet illuminated via monochromatic laser radiation and operating at a finite temperature. Taking into account the two dimensional Fermi-Dirac statistics as is applicable for a graphene sheet, the electron energy redistribution due to thermal agitation via laser irradiation, and Fowler's approach of the electron emission, along with Born's approximation to evaluate the tunneling probability, the expressions for the photoelectric and thermionic emission flux have been derived. The cumulative emission flux is observed to be sensitive to the parametric tuning of the laser and material specifications. Based on the parametric analysis, the photoemission flux is noticed to dominate over its coexisting counterpart thermionic emission flux for smaller values of the material work function, surface temperature, and laser wavelength; the analytical estimates are in reasonably good agreement with the recent experimental observations [Massicotte et al., Nat. Commun. 7, 12174 (2016)]. The results evince the efficient utilization of a graphene layer as a photo-thermionic emitter.

Current transmission and nonlinear effects in un-gated thermionic cathode RF guns

DOE Office of Scientific and Technical Information (OSTI.GOV)

Edelen, J. P.; Harris, J. R.

Un-gated thermionic cathode RF guns are well known as a robust source of electrons for many accelerator applications. These sources are in principle scalable to high currents without degradation of the transverse emittance due to control grids but they are also known for being limited by back-bombardment. While back-bombardment presents a significant limitation, there is still a lack of general understanding on how emission over the whole RF period will affect the nature of the beams produced from these guns. In order to improve our understanding of how these guns can be used in general we develop analytical models thatmore » predict the transmission efficiency as a function of the design parameters, study how bunch compression and emission enhancement caused by Schottky barrier lowering affect the output current profile in the gun, and study the onset of space-charge limited effects and the resultant virtual cathode formation leading to a modulation in the output current distribution.« less

Photoemission experiments of a large area scandate dispenser cathode

NASA Astrophysics Data System (ADS)

Zhang, Huang; Liu, Xing-guang; Chen, Yi; Chen, De-biao; Jiang, Xiao-guo; Yang, An-min; Xia, Lian-sheng; Zhang, Kai-zhi; Shi, Jin-shui; Zhang, Lin-wen

2010-09-01

A 100-mm-diameter scandate dispenser cathode was tested as a photocathode with a 10 ns Nd:YAG laser (266 nm) on an injector test stand for linear induction accelerators. This thermionic dispenser cathode worked at temperatures ranging from room temperature to 930 °C (below or near the thermionic emission threshold) while the vacuum was better than 4×10 -7 Torr. The laser pulse was synchronized with a 120 ns diode voltage pulse stably and they were in single pulse mode. Emission currents were measured by a Faraday cup. The maximum peak current collected at the anode was about 100 A. The maximum quantum efficiency measured at low laser power was 2.4×10 -4. Poisoning effect due to residual gas was obvious and uninterrupted heating was needed to keep cathode's emission capability. The cathode was exposed to air one time between experiments and recovered after being reconditioned. Photoemission uniformity of the cathode was also explored by changing the laser spot's position.

Role of deuterium desorption kinetics on the thermionic emission properties of polycrystalline diamond films with respect to kinetic isotope effects

DOE Office of Scientific and Technical Information (OSTI.GOV)

Paxton, W. F., E-mail: william.f.paxton@vanderbilt.edu; Howell, M.; Kang, W. P.

2014-06-21

The desorption kinetics of deuterium from polycrystalline chemical vapor deposited diamond films were characterized by monitoring the isothermal thermionic emission current behavior. The reaction was observed to follow a first-order trend as evidenced by the decay rate of the thermionic emission current over time which is in agreement with previously reported studies. However, an Arrhenius plot of the reaction rates at each tested temperature did not exhibit the typical linear behavior which appears to contradict past observations of the hydrogen (or deuterium) desorption reaction from diamond. This observed deviation from linearity, specifically at lower temperatures, has been attributed to non-classicalmore » processes. Though no known previous studies reported similar deviations, a reanalysis of the data obtained in the present study was performed to account for tunneling which appeared to add merit to this hypothesis. Additional investigations were performed by reevaluating previously reported data involving the desorption of hydrogen (as opposed to deuterium) from diamond which further indicated this reaction to be dominated by tunneling at the temperatures tested in this study (<775 °C). An activation energy of 3.19 eV and a pre-exponential constant of 2.3 × 10{sup 12} s{sup −1} were determined for the desorption reaction of deuterium from diamond which is in agreement with previously reported studies.« less

Photon-enhanced thermionic emission for solar concentrator systems.

PubMed

Schwede, Jared W; Bargatin, Igor; Riley, Daniel C; Hardin, Brian E; Rosenthal, Samuel J; Sun, Yun; Schmitt, Felix; Pianetta, Piero; Howe, Roger T; Shen, Zhi-Xun; Melosh, Nicholas A

2010-09-01

Solar-energy conversion usually takes one of two forms: the 'quantum' approach, which uses the large per-photon energy of solar radiation to excite electrons, as in photovoltaic cells, or the 'thermal' approach, which uses concentrated sunlight as a thermal-energy source to indirectly produce electricity using a heat engine. Here we present a new concept for solar electricity generation, photon-enhanced thermionic emission, which combines quantum and thermal mechanisms into a single physical process. The device is based on thermionic emission of photoexcited electrons from a semiconductor cathode at high temperature. Temperature-dependent photoemission-yield measurements from GaN show strong evidence for photon-enhanced thermionic emission, and calculated efficiencies for idealized devices can exceed the theoretical limits of single-junction photovoltaic cells. The proposed solar converter would operate at temperatures exceeding 200 degrees C, enabling its waste heat to be used to power a secondary thermal engine, boosting theoretical combined conversion efficiencies above 50%.

Electromechanical Characterization of Single GaN Nanobelt Probed with Conductive Atomic Force Microscope

NASA Astrophysics Data System (ADS)

Yan, X. Y.; Peng, J. F.; Yan, S. A.; Zheng, X. J.

2018-04-01

The electromechanical characterization of the field effect transistor based on a single GaN nanobelt was performed under different loading forces by using a conductive atomic force microscope (C-AFM), and the effective Schottky barrier height (SBH) and ideality factor are simulated by the thermionic emission model. From 2-D current image, the high value of the current always appears on the nanobelt edge with the increase of the loading force less than 15 nN. The localized (I-V) characteristic reveals a typical rectifying property, and the current significantly increases with the loading force at the range of 10-190 nN. The ideality factor is simulated as 9.8 within the scope of GaN nano-Schottky diode unity (6.5-18), therefore the thermionic emission current is dominant in the electrical transport of the GaN-tip Schottky junction. The SBH is changed through the piezoelectric effect induced by the loading force, and it is attributed to the enhanced current. Furthermore, a single GaN nanobelt has a high mechanical-induced current ratio that could be made use of in a nanoelectromechanical switch.

Electromechanical Characterization of Single GaN Nanobelt Probed with Conductive Atomic Force Microscope

NASA Astrophysics Data System (ADS)

Yan, X. Y.; Peng, J. F.; Yan, S. A.; Zheng, X. J.

2018-07-01

The electromechanical characterization of the field effect transistor based on a single GaN nanobelt was performed under different loading forces by using a conductive atomic force microscope (C-AFM), and the effective Schottky barrier height (SBH) and ideality factor are simulated by the thermionic emission model. From 2-D current image, the high value of the current always appears on the nanobelt edge with the increase of the loading force less than 15 nN. The localized ( I- V) characteristic reveals a typical rectifying property, and the current significantly increases with the loading force at the range of 10-190 nN. The ideality factor is simulated as 9.8 within the scope of GaN nano-Schottky diode unity (6.5-18), therefore the thermionic emission current is dominant in the electrical transport of the GaN-tip Schottky junction. The SBH is changed through the piezoelectric effect induced by the loading force, and it is attributed to the enhanced current. Furthermore, a single GaN nanobelt has a high mechanical-induced current ratio that could be made use of in a nanoelectromechanical switch.

Characterization and Modeling Analysis for Metal-Semiconductor-Metal GaAs Diodes with Pd/SiO2 Mixture Electrode

PubMed Central

Tan, Shih-Wei; Lai, Shih-Wen

2012-01-01

Characterization and modeling of metal-semiconductor-metal (MSM) GaAs diodes using to evaporate SiO2 and Pd simultaneously as a mixture electrode (called M-MSM diodes) compared with similar to evaporate Pd as the electrode (called Pd-MSM diodes) were reported. The barrier height (φ b) and the Richardson constant (A*) were carried out for the thermionic-emission process to describe well the current transport for Pd-MSM diodes in the consideration of the carrier over the metal-semiconductor barrier. In addition, in the consideration of the carrier over both the metal-semiconductor barrier and the insulator-semiconductor barrier simultaneously, thus the thermionic-emission process can be used to describe well the current transport for M-MSM diodes. Furthermore, in the higher applied voltage, the carrier recombination will be taken into discussion. Besides, a composite-current (CC) model is developed to evidence the concepts. Our calculated results are in good agreement with the experimental ones. PMID:23226352

Two-diode behavior in metal-ferroelectric-semiconductor structures with bismuth titanate interfacial layer

NASA Astrophysics Data System (ADS)

Durmuş, Perihan; Altindal, Şemsettin

2017-10-01

In this study, electrical parameters of the Al/Bi4Ti3O12/p-Si metal-ferroelectric-semiconductor (MFS) structure and their temperature dependence were investigated using current-voltage (I-V) data measured between 120 K and 300 K. Semi-logarithmic I-V plots of the structure revealed that fabricated structure presents two-diode behavior that leads to two sets of ideality factor, reverse saturation current and zero-bias barrier height (BH) values. Obtained results of these parameters suggest that current conduction mechanism (CCM) deviates strongly from thermionic emission theory particularly at low temperatures. High values of interface states and nkT/q-kT/q plot supported the idea of deviation from thermionic emission. In addition, ln(I)-ln(V) plots suggested that CCM varies from one bias region to another and depends on temperature as well. Series resistance values were calculated using Ohm’s law and Cheungs’ functions, and they decreased drastically with increasing temperature.

Investigation of miniaturized radioisotope thermionic power generation for general use

NASA Astrophysics Data System (ADS)

Duzik, Adam J.; Choi, Sang H.

2016-04-01

Radioisotope thermoelectric generators (RTGs) running off the radioisotope Pu238 are the current standard in deep space probe power supplies. While reliable, these generators are very inefficient, operating at only ~7% efficiency. As an alternative, more efficient radioisotope thermionic emission generators (RTIGs) are being explored. Like RTGs, current RTIGs concepts use exotic materials for the emitter, limiting applicability to space and other niche applications. The high demand for long-lasting mobile power sources would be satisfied if RTIGs could be produced inexpensively. This work focuses on exposing several common materials, such as Al, stainless steel, W, Si, and Cu, to elevated temperatures under vacuum to determine the efficiency of each material as inexpensive replacements for thermoelectric materials.

Thermionic field emission in gold nitride Schottky nanodiodes

NASA Astrophysics Data System (ADS)

Spyropoulos-Antonakakis, N.; Sarantopoulou, E.; Kollia, Z.; Samardžija, Z.; Kobe, S.; Cefalas, A. C.

2012-11-01

We report on the thermionic field emission and charge transport properties of gold nitride nanodomains grown by pulsed laser deposition with a molecular fluorine laser at 157 nm. The nanodomains are sandwiched between the metallic tip of a conductive atomic force microscope and a thin gold layer forming thus a metal-semiconductor-metal junction. Although the limited existing data in the literature indicate that gold nitride was synthesized previously with low efficiency, poor stability, and metallic character; in this work, it is shown that gold nitride nanodomains exhibit semiconducting behavior and the metal-semiconductor-metal contact can be modeled with the back-to-back Schottky barrier model. From the experimental I-V curves, the main charge carrier transport process is found to be thermionic field emission via electron tunneling. The rectifying, near symmetric and asymmetric current response of nanocontacts is related to the effective contact area of the gold nitride nanodomains with the metals. A lower limit for the majority charge carriers concentration at the boundaries of nanodomains is also established using the full depletion approximation, as nanodomains with thickness as low as 6 nm were found to be conductive. Current rectification and charge memory effects are also observed in "quite small" conductive nanodomains (6-10 nm) due to stored charges. Indeed, charges near the surface are identified as inversion domains in the phase shift mapping performed with electrostatic force microscopy and are attributed to charge trapping at the boundaries of the nanodomains.

Barium-Dispenser Thermionic Cathode

NASA Technical Reports Server (NTRS)

Wintucky, Edwin G.; Green, M.; Feinleib, M.

1989-01-01

Improved reservoir cathode serves as intense source of electrons required for high-frequency and often high-output-power, linear-beam tubes, for which long operating lifetime important consideration. High emission-current densities obtained through use of emitting surface of relatively-low effective work function and narrow work-function distribution, consisting of coat of W/Os deposited by sputtering. Lower operating temperatures and enhanced electron emission consequently possible.

Temperature dependent electrical transport behavior of InN/GaN heterostructure based Schottky diodes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Roul, Basanta; Kumar, Mahesh; Central Research Laboratory, Bharat Electronics, Bangalore 560013

InN/GaN heterostructure based Schottky diodes were fabricated by plasma-assisted molecular beam epitaxy. The temperature dependent electrical transport properties were carried out for InN/GaN heterostructure. The barrier height and the ideality factor of the Schottky diodes were found to be temperature dependent. The temperature dependence of the barrier height indicates that the Schottky barrier height is inhomogeneous in nature at the heterostructure interface. The higher value of the ideality factor and its temperature dependence suggest that the current transport is primarily dominated by thermionic field emission (TFE) other than thermionic emission (TE). The room temperature barrier height obtained by using TEmore » and TFE models were 1.08 and 1.43 eV, respectively.« less

Study of a contracted glow in low-frequency plasma-jet discharges operating with argon

DOE Office of Scientific and Technical Information (OSTI.GOV)

Minotti, F.; Giuliani, L.; Xaubet, M.

2015-11-15

In this work, we present an experimental and theoretical study of a low frequency, atmospheric plasma-jet discharge in argon. The discharge has the characteristics of a contracted glow with a current channel of submillimeter diameter and a relatively high voltage cathode layer. In order to interpret the measurements, we consider the separate modeling of each region of the discharge: main channel and cathode layer, which must then be properly matched together. The main current channel was modeled, extending a previous work, as similar to an arc in which joule heating is balanced by lateral heat conduction, without thermal equilibrium betweenmore » electrons and heavy species. The cathode layer model, on the other hand, includes the emission of secondary electrons by ion impact and by additional mechanisms, of which we considered emission due to collision of atoms excited at metastable levels, and field-enhanced thermionic emission (Schottky effect). The comparison of model and experiment indicates that the discharge can be effectively sustained in its contracted form by the secondary electrons emitted by collision of excited argon atoms, whereas thermionic emission is by far insufficient to provide the necessary electrons.« less

Investigation of Miniaturized Radioisotope Thermionic Power Generation for General Use

NASA Technical Reports Server (NTRS)

Duzik, Adam J.; Choi, Sang H.

2016-01-01

Radioisotope thermoelectric generators (RTGs) running off the radioisotope Pu238 are the current standard in deep space probe power supplies. While reliable, these generators are very inefficient, operating at only approx.7% efficiency. As an alternative, more efficient radioisotope thermionic emission generators (RTIGs) are being explored. Like RTGs, current RTIGs concepts use exotic materials for the emitter, limiting applicability to space and other niche applications. The high demand for long-lasting mobile power sources would be satisfied if RTIGs could be produced inexpensively. This work focuses on exposing several common materials, such as Al, stainless steel, W, Si, and Cu, to elevated temperatures under vacuum to determine the efficiency of each material as inexpensive replacements for thermoelectric materials.

Barrier inhomogeneities and electronic transport of Pt contacts to relatively highly doped n-type 4H-SiC

DOE Office of Scientific and Technical Information (OSTI.GOV)

Huang, Lingqin, E-mail: lqhuang@jsnu.edu.cn, E-mail: dwang121@dlut.edu.cn; Wang, Dejun, E-mail: lqhuang@jsnu.edu.cn, E-mail: dwang121@dlut.edu.cn

The barrier characteristics of Pt contacts to relatively highly doped (∼1 × 10{sup 18 }cm{sup −3}) 4H-SiC were investigated using current-voltage (I-V) and capacitance-voltage (C-V) measurements in the temperature range of 160–573 K. The barrier height and ideally factor estimated from the I-V characteristics based on the thermionic emission model are abnormally temperature-dependent, which can be explained by assuming the presence of a double Gaussian distribution (GD) of inhomogeneous barrier heights. However, in the low temperature region (160–323 K), the obtained mean barrier height according to GD is lower than the actual mean value from C-V measurement. The values of barrier height determined from themore » thermionic field emission model are well consistent with those from the C-V measurements, which suggest that the current transport process could be modified by electron tunneling at low temperatures.« less

Particle-In-Cell Simulations of a Thermionic Converter

NASA Astrophysics Data System (ADS)

Clark, S. E.

2017-12-01

Simulations of thermionic converters are presented where cesium is used as a work function reducing agent in a nano-fabricated triode configuration. The cathode and anode are spaced on the order of 100 μm, and the grid structure has features on the micron scale near the anode. The hot side is operated near 1600 K, the cold side near 600 K, and the converter has the potential to convert heat to DC electrical current upwards of 20% efficiency. Affordable and robust thermionic converters have the potential to displace century old mechanical engines and turbines as a primary means of electrical power generation in the near future. High efficiency converters that operate at a small scale could be used to generate power locally and alleviate the need for large scale power transmission systems. Electron and negative cesium ion back emission from the anode are considered, as well as device longevity and fabrication feasibility.

Particle-In-Cell Simulations of a Thermionic Converter

NASA Astrophysics Data System (ADS)

Clark, Stephen

2017-10-01

Simulations of thermionic converters are presented where cesium is used as a work function reducing agent in a nano-fabricated triode configuration. The cathode and anode are spaced on the order of 100 μm, and the grid structure has features on the micron scale near the anode. The hot side is operated near 1600 K, the cold side near 600 K, and the converter has the potential to convert heat to DC electrical current upwards of 20% efficiency. Affordable and robust thermionic converters have the potential to displace century old mechanical engines and turbines as a primary means of electrical power generation in the near future. High efficiency converters that operate at a small scale could be used to generate power locally and alleviate the need for large scale power transmission systems. Electron and negative cesium ion back emission from the anode are considered, as well as device longevity and fabrication feasibility.

Real-time first-principles simulations of thermionic emission from N-doped diamond surfaces

NASA Astrophysics Data System (ADS)

Shinozaki, Tomoki; Hagiwara, Satoshi; Morioka, Naoya; Kimura, Yuji; Watanabe, Kazuyuki

2018-06-01

We investigate thermionic emission from N-doped C(100) surfaces terminated with H or Li atoms using finite-temperature real-time density functional theory simulations. The current–temperature characteristics are found to follow the Richardson–Dushman (RD) equation, which was derived from a semiclassical theory. However, the Richardson constants are two orders of magnitude smaller than the ideal values from the RD theory. This considerable reduction is attributed primarily to the extremely low transmission probability of electrons from the surfaces toward the vacuum. The present method enables straightforward evaluation of the ideal efficiency of a thermionic energy converter.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tierno, S. P., E-mail: sp.tierno@upm.es; Donoso, J. M.; Domenech-Garret, J. L.

The interaction between an electron emissive wall, electrically biased in a plasma, is revisited through a simple fluid model. We search for realistic conditions of the existence of a non-monotonic plasma potential profile with a virtual cathode as it is observed in several experiments. We mainly focus our attention on thermionic emission related to the operation of emissive probes for plasma diagnostics, although most conclusions also apply to other electron emission processes. An extended Bohm criterion is derived involving the ratio between the two different electron densities at the potential minimum and at the background plasma. The model allows amore » phase-diagram analysis, which confirms the existence of the non-monotonic potential profiles with a virtual cathode. This analysis shows that the formation of the potential well critically depends on the emitted electron current and on the velocity at the sheath edge of cold ions flowing from the bulk plasma. As a consequence, a threshold value of the governing parameter is required, in accordance to the physical nature of the electron emission process. The latter is a threshold wall temperature in the case of thermionic electrons. Experimental evidence supports our numerical calculations of this threshold temperature. Besides this, the potential well becomes deeper with increasing electron emission, retaining a fraction of the released current which limits the extent of the bulk plasma perturbation. This noninvasive property would explain the reliable measurements of plasma potential by using the floating potential method of emissive probes operating in the so-called strong emission regime.« less

Work function determination of promising electrode materials for thermionic energy converters

NASA Technical Reports Server (NTRS)

Jacobson, D.; Storms, E.; Skaggs, B.; Kouts, T.; Jaskie, J.; Manda, M.

1976-01-01

The work function determinations of candidate materials for low temperature (1400 K) thermionics through vacuum emission tests are discussed. Two systems, a vacuum emission test vehicle and a thermionic emission microscope are used for emission measurements. Some nickel and cobalt based super alloys were preliminarily examined. High temperature physical properties and corrosion behavior of some super alloy candidates are presented. The corrosion behavior of sodium is of particular interest since topping cycles might use sodium heat transfer loops. A Marchuk tube was designed for plasma discharge studies with the carbide and possibly some super alloy samples. A series of metal carbides and other alloys were fabricated and tested in a special high temperature mass spectrometer. This information coupled with work function determinations was evaluated in an attempt to learn how electron bonding occurs in transition alloys.

Device for providing high-intensity ion or electron beam

DOEpatents

McClanahan, Edwin D.; Moss, Ronald W.

1977-01-01

A thin film of a low-thermionic-work-function material is maintained on the cathode of a device for producing a high-current, low-pressure gas discharge by means of sputter deposition from an auxiliary electrode. The auxiliary electrode includes a surface with a low-work-function material, such as thorium, uranium, plutonium or one of the rare earth elements, facing the cathode but at a disposition and electrical potential so as to extract ions from the gas discharge and sputter the low-work-function material onto the cathode. By continuously replenishing the cathode film, high thermionic emissions and ion plasmas can be realized and maintained over extended operating periods.

Solar energy conversion with photon-enhanced thermionic emission

NASA Astrophysics Data System (ADS)

Kribus, Abraham; Segev, Gideon

2016-07-01

Photon-enhanced thermionic emission (PETE) converts sunlight to electricity with the combined photonic and thermal excitation of charge carriers in a semiconductor, leading to electron emission over a vacuum gap. Theoretical analyses predict conversion efficiency that can match, or even exceed, the efficiency of traditional solar thermal and photovoltaic converters. Several materials have been examined as candidates for radiation absorbers and electron emitters, with no conclusion yet on the best set of materials to achieve high efficiency. Analyses have shown the complexity of the energy conversion and transport processes, and the significance of several loss mechanisms, requiring careful control of material properties and optimization of the device structure. Here we survey current research on PETE modeling, materials, and device configurations, outline the advances made, and stress the open issues and future research needed. Based on the substantial progress already made in this young topic, and the potential of high conversion efficiency based on theoretical performance limits, continued research in this direction is very promising and may yield a competitive technology for solar electricity generation.

CdS-metal contact at higher current densities.

NASA Technical Reports Server (NTRS)

Stirn, R. J.; Boeer, K. W.; Dussel, G. A.

1973-01-01

An investigation is conducted concerning the mechanisms by which a steady flow of current proceeds through the contact when an external voltage is applied. The main characteristics of current mechanisms are examined, giving attention to photoemission from the cathode, thermionic emission, minority-carrier extraction, and the tunneling of electrons. A high-field domain analysis is conducted together with experimental studies. Particular attention is given to the range in which tunneling predominates.

Effect of thermionic cathode heating current self-magnetic field on gaseous plasma generator characteristics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lopatin, I. V., E-mail: lopatin@opee.hcei.tsc.ru; Akhmadeev, Yu. H.; Koval, N. N.

2015-10-15

The performance capabilities of the PINK, a plasma generator with a thermionic cathode mounted in the cavity of a hollow cathode, depending for its operation on a non-self-sustained low-pressure gas discharge have been investigated. It has been shown that when a single-filament tungsten cathode 2 mm in diameter is used and the peak filament current is equal to or higher than 100 A, the self-magnetic field of the filament current significantly affects the discharge current and voltage waveforms. This effect is due to changes in the time and space distributions of the emission current density from the hot cathode. Whenmore » the electron mean free path is close to the characteristic dimensions of the thermionic cathode, the synthesized plasma density distribution is nonuniform and the cathode is etched nonuniformly. The cathode lifetime in this case is 8–12 h. Using a cathode consisting of several parallel-connected tungsten filaments ∼0.8 mm in diameter moderates the effect of the self-magnetic field of the filament current and nearly doubles the cathode lifetime. The use of this type of cathode together with a discharge igniting electrode reduces the minimum operating pressure in the plasma generator to about one third of that required for the generator operation with a single-filament cathode (to 0.04 Pa)« less

Life Model of Hollow Cathodes Using a Barium Calcium Aluminate Impregnated Tungsten Emitter

NASA Technical Reports Server (NTRS)

Kovaleski, S. D.; Burke, Tom (Technical Monitor)

2001-01-01

Hollow cathodes with barium calcium aluminate impregnated tungsten emitters for thermionic emission are widely used in electric propulsion. These high current, low power cathodes are employed in ion thrusters, Hall thrusters, and on the International Space Station in plasma contactors. The requirements on hollow cathode life are growing more stringent with the increasing use of electric propulsion technology. The life limiting mechanism that determines the entitlement lifetime of a barium impregnated thermionic emission cathode is the evolution and transport of barium away from the emitter surface. A model is being developed to study the process of barium transport and loss from the emitter insert in hollow cathodes. The model accounts for the production of barium through analysis of the relevant impregnate chemistry. Transport of barium through the approximately static gas is also being treated. Finally, the effect of temperature gradients within the cathode are considered.

High efficiency and non-Richardson thermionics in three dimensional Dirac materials

NASA Astrophysics Data System (ADS)

Huang, Sunchao; Sanderson, Matthew; Zhang, Yan; Zhang, Chao

2017-10-01

Three dimensional (3D) topological materials have a linear energy dispersion and exhibit many electronic properties superior to conventional materials such as fast response times, high mobility, and chiral transport. In this work, we demonstrate that 3D Dirac materials also have advantages over conventional semiconductors and graphene in thermionic applications. The low emission current suffered in graphene due to the vanishing density of states is enhanced by an increased group velocity in 3D Dirac materials. Furthermore, the thermal energy carried by electrons in 3D Dirac materials is twice of that in conventional materials with a parabolic electron energy dispersion. As a result, 3D Dirac materials have the best thermal efficiency or coefficient of performance when compared to conventional semiconductors and graphene. The generalized Richardson-Dushman law in 3D Dirac materials is derived. The law exhibits the interplay of the reduced density of states and enhanced emission velocity.

Experiments on transient melting of tungsten by ELMs in ASDEX Upgrade

NASA Astrophysics Data System (ADS)

Krieger, K.; Balden, M.; Coenen, J. W.; Laggner, F.; Matthews, G. F.; Nille, D.; Rohde, V.; Sieglin, B.; Giannone, L.; Göths, B.; Herrmann, A.; de Marne, P.; Pitts, R. A.; Potzel, S.; Vondracek, P.; ASDEX-Upgrade Team; EUROfusion MST1 Team

2018-02-01

Repetitive melting of tungsten by power transients originating from edge localized modes (ELMs) has been studied in ASDEX Upgrade. Tungsten samples were exposed to H-mode discharges at the outer divertor target plate using the divertor manipulator II (DIM-II) system (Herrmann et al 2015 Fusion Eng. Des. 98-9 1496-9). Designed as near replicas of the geometries used also in separate experiments on the JET tokamak (Coenen et al 2015 J. Nucl. Mater. 463 78-84 Coenen et al 2015 Nucl. Fusion 55 023010; Matthews et al 2016 Phys. Scr. T167 7), the samples featured a misaligned leading edge and a sloped ridge respectively. Both structures protrude above the default target plate surface thus receiving an increased fraction of the parallel power flux. Transient melting by ELMs was induced by moving the outer strike point to the sample location. The temporal evolution of the measured current flow from the samples to vessel potential confirmed transient melting. Current magnitude and dependency from surface temperature provided strong evidence for thermionic electron emission as main origin of the replacement current driving the melt motion. The different melt patterns observed after exposures at the two sample geometries support the thermionic electron emission model used in the MEMOS melt motion code, which assumes a strong decrease of the thermionic net current at shallow magnetic field to surface angles (Pitts et al 2017 Nucl. Mater. Energy 12 60-74). Post exposure ex situ analysis of the retrieved samples show recrystallization of tungsten at the exposed surface areas to a depth of up to several mm. The melt layer transport to less exposed surface areas leads to ratcheting pile up of re-solidified debris with zonal growth extending from the already enlarged grains at the surface.

EFFECT OF THERMIONIC EMISSION AT ROOM TEMPERATURES IN PHOTOSENSITIVE GEIGERMULLER TUBES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Grotowski, K.; Hrynkiewicz, A.Z.; Niewodniczanski, H.

1953-01-01

The temperature-dependence of the background of Geiger-Mueller counting tubes was compared for nonsensitized and sensitized (treated by electric discharge) tubes. A strong increase of background with increasing temperature was observed for phatosensitive counters, while no change was observed in nonsensetized counters. It is shown that the increase is due to thermionic emission of the brass cathode. (T.R.H.)

Solid-State Thermionic Nuclear Power for Megawatt Propulsion, Planetary Surface and Commercial Power Project

NASA Technical Reports Server (NTRS)

George, Jeffrey

2014-01-01

Thermionic (TI) power conversion is a promising technology first investigated for power conversion in the 1960's, and of renewed interest due to modern advances in nanotechnology, MEMS, materials and manufacturing. Benefits include high conversion efficiency (20%), static operation with no moving parts and potential for high reliability, greatly reduced plant complexity, and the potential for reduced development costs. Thermionic emission, credited to Edison in 1880, forms the basis of vacuum tubes and much of 20th century electronics. Heat can be converted into electricity when electrons emitted from a hot surface are collected across a small gap. For example, two "small" (6 kWe) Thermionic Space Reactors were flown by the USSR in 1987-88 for ocean radar reconnaissance. Higher powered Nuclear-Thermionic power systems driving Electric Propulsion (Q-thruster, VASIMR, etc.) may offer the breakthrough necessary for human Mars missions of < 1 yr round trip. Power generation on Earth could benefit from simpler, moe economical nuclear plants, and "topping" of more fuel and emission efficient fossil-fuel plants.

Low work function materials for microminiature energy conversion and recovery applications

DOEpatents

Zavadil, Kevin R.; Ruffner, Judith A.; King, Donald B.

2003-05-13

Low work function materials are disclosed together with methods for their manufacture and integration with electrodes used in thermionic conversion applications (specifically microminiature thermionic conversion applications). The materials include a mixed oxide system and metal in a compositionally modulated structure comprised of localized discontinuous structures of material that are deposited using techniques suited to IC manufacture, such as rf sputtering or CVD. The structures, which can include layers are then heated to coalescence yielding a thin film that is both durable and capable of electron emission under thermionic conversion conditions used for microminiature thermionic converters. Using the principles of the invention, thin film electrodes (emitters and collectors) required for microconverter technology are manufactured using a single process deposition so as to allow for full fabrication integration consistent with batch processing, and tailoring of emission/collection properties. In the preferred embodiment, the individual layers include mixed BaSrCaO, scandium oxide and tungsten.

Amplified Thermionic Cooling Using Arrays of Nanowires

NASA Technical Reports Server (NTRS)

Yang, Eui-Hyeok; Choi, Daniel; Shcheglov, Kirill; Hishinuma, Yoshikazu

2007-01-01

A class of proposed thermionic cooling devices would incorporate precise arrays of metal nanowires as electron emitters. The proposed devices could be highly miniaturized, enabling removal of heat from locations, very close to electronic devices, that have previously been inaccessible for heat-removal purposes. The resulting enhancement of removal of heat would enable operation of the devices at higher power levels and higher clock speeds. Moreover, the mass, complexity, and bulk of electronic circuitry incorporating these highly miniaturized cooling devices could be considerably reduced, relative to otherwise equivalent circuitry cooled by conventional electromechanical, thermoelectric, and fluidic means. In thermionic cooling, one exploits the fact that because only the highest-energy electrons are thermionically emitted, collecting those electrons to prevent their return to the emitting electrode results in the net removal of heat from that electrode. Collection is effected by applying an appropriate positive bias potential to another electrode placed near the emitting electrode. The concept underlying the proposal is that the thermionic-emission current and, hence, the cooling effect attainable by use of an array of nanowires could be significantly greater than that attainable by use of a single emitting electrode or other electron- emitting surface. The wires in an array according to the proposal would protrude perpendicularly from a planar surface and their heights would be made uniform to within a sub-nanometer level of precision

Theory of thermionic emission from a two-dimensional conductor and its application to a graphene-semiconductor Schottky junction

NASA Astrophysics Data System (ADS)

Trushin, Maxim

2018-04-01

The standard theory of thermionic emission developed for three-dimensional semiconductors does not apply to two-dimensional materials even for making qualitative predictions because of the vanishing out-of-plane quasiparticle velocity. This study reveals the fundamental origin of the out-of-plane charge carrier motion in a two-dimensional conductor due to the finite quasiparticle lifetime and huge uncertainty of the out-of-plane momentum. The theory is applied to a Schottky junction between graphene and a bulk semiconductor to derive a thermionic constant, which, in contrast to the conventional Richardson constant, is determined by the Schottky barrier height and Fermi level in graphene.

Work function determination of promising electrode materials for thermionic converters

NASA Technical Reports Server (NTRS)

Jacobson, D.

1977-01-01

Work performed on this contract was primarily for the evaluation of selected electrode materials for thermionic energy converters. The original objective was to characterize selected nickel based superalloys up to temperatures of 1400 K. It was found that an early selection, Inconel 800 produced a high vapor pressure which interfered with the vacuum emission measurements. The program then shifted to two other areas. The first area was to obtain emission from the superalloys in a cesiated atmosphere. The cesium plasma helps to suppress the vaporization interference. The second area involved characterization of the Lanthanum-Boron series as thermionic emitters. These final two areas resulted in three journal publications which are attached to this report.

A Novel Ni/WOX/W Resistive Random Access Memory with Excellent Retention and Low Switching Current

NASA Astrophysics Data System (ADS)

Chien, Wei-Chih; Chen, Yi-Chou; Lee, Feng-Ming; Lin, Yu-Yu; Lai, Erh-Kun; Yao, Yeong-Der; Gong, Jeng; Horng, Sheng-Fu; Yeh, Chiao-Wen; Tsai, Shih-Chang; Lee, Ching-Hsiung; Huang, Yu-Kai; Chen, Chun-Fu; Kao, Hsiao-Feng; Shih, Yen-Hao; Hsieh, Kuang-Yeu; Lu, Chih-Yuan

2011-04-01

The behavior of WOX resistive random access memory (ReRAM) is a strong function of the top electrode material, which controls the conduction mechanism and the forming process. When using a top electrode with low work function, the current conduction is limited by space charges. On the other hand, the mechanism becomes thermionic emission for devices with a high work function top electrode. These (thermionic) devices are also found to have higher initial resistance, reduced forming current, and larger resistance window. Based on these insights and considering the compatibility to complementary metal-oxide-semiconductor (CMOS) process, we proposed to use Ni as the top electrode for high performance WOX ReRAM devices. The new Ni/WOX/W device can be switched at a low current density less than 8×105 A/cm2, with RESET/SET resistance ratio greater than 100, and extremely good data retention of more than 300 years at 85 °C.

Investigation of significantly high barrier height in Cu/GaN Schottky diode

DOE Office of Scientific and Technical Information (OSTI.GOV)

Garg, Manjari, E-mail: meghagarg142@gmail.com; Kumar, Ashutosh; Singh, R.

2016-01-15

Current-voltage (I-V) measurements combined with analytical calculations have been used to explain mechanisms for forward-bias current flow in Copper (Cu) Schottky diodes fabricated on Gallium Nitride (GaN) epitaxial films. An ideality factor of 1.7 was found at room temperature (RT), which indicated deviation from thermionic emission (TE) mechanism for current flow in the Schottky diode. Instead the current transport was better explained using the thermionic field-emission (TFE) mechanism. A high barrier height of 1.19 eV was obtained at room temperature. X-ray photoelectron spectroscopy (XPS) was used to investigate the plausible reason for observing Schottky barrier height (SBH) that is significantlymore » higher than as predicted by the Schottky-Mott model for Cu/GaN diodes. XPS measurements revealed the presence of an ultrathin cuprous oxide (Cu{sub 2}O) layer at the interface between Cu and GaN. With Cu{sub 2}O acting as a degenerate p-type semiconductor with high work function of 5.36 eV, a high barrier height of 1.19 eV is obtained for the Cu/Cu{sub 2}O/GaN Schottky diode. Moreover, the ideality factor and barrier height were found to be temperature dependent, implying spatial inhomogeneity of barrier height at the metal semiconductor interface.« less

Excited-state thermionic emission in III-antimonides: Low emittance ultrafast photocathodes

NASA Astrophysics Data System (ADS)

Berger, Joel A.; Rickman, B. L.; Li, T.; Nicholls, A. W.; Andreas Schroeder, W.

2012-11-01

The normalized rms transverse emittance of an electron source is shown to be proportional to √m* , where m* is the effective mass of the state from which the electron is emitted, by direct observation of the transverse momentum distribution for excited-state thermionic emission from two III-V semiconductor photocathodes, GaSb and InSb, together with a control experiment employing two-photon emission from gold. Simulations of the experiment using an extended analytical Gaussian model of electron pulse propagation are in close agreement with the data.

Thermionic Emission of Single-Wall Carbon Nanotubes Measured

NASA Technical Reports Server (NTRS)

Landis, Geoffrey A.; Krainsky, Isay L.; Bailey, Sheila G.; Elich, Jeffrey M.; Landi, Brian J.; Gennett, Thomas; Raffaelle, Ryne P.

2004-01-01

Researchers at the NASA Glenn Research Center, in collaboration with the Rochester Institute of Technology, have investigated the thermionic properties of high-purity, single-wall carbon nanotubes (SWNTs) for use as electron-emitting electrodes. Carbon nanotubes are a recently discovered material made from carbon atoms bonded into nanometer-scale hollow tubes. Such nanotubes have remarkable properties. An extremely high aspect ratio, as well as unique mechanical and electronic properties, make single-wall nanotubes ideal for use in a vast array of applications. Carbon nanotubes typically have diameters on the order of 1 to 2 nm. As a result, the ends have a small radius of curvature. It is these characteristics, therefore, that indicate they might be excellent potential candidates for both thermionic and field emission.

Photon enhanced thermionic emission

DOEpatents

Schwede, Jared; Melosh, Nicholas; Shen, Zhixun

2014-10-07

Photon Enhanced Thermionic Emission (PETE) is exploited to provide improved efficiency for radiant energy conversion. A hot (greater than 200.degree. C.) semiconductor cathode is illuminated such that it emits electrons. Because the cathode is hot, significantly more electrons are emitted than would be emitted from a room temperature (or colder) cathode under the same illumination conditions. As a result of this increased electron emission, the energy conversion efficiency can be significantly increased relative to a conventional photovoltaic device. In PETE, the cathode electrons can be (and typically are) thermalized with respect to the cathode. As a result, PETE does not rely on emission of non-thermalized electrons, and is significantly easier to implement than hot-carrier emission approaches.

Evaluation of single crystal LaB6 cathodes for use in a high frequency backward wave oscillator tube

NASA Technical Reports Server (NTRS)

Swanson, L. W.; Davis, P. R.; Schwind, G. A.

1984-01-01

The results of thermionic emission and evaporation studies of single crystal LaB6 cathodes are given. A comparison between the (100), (210) and (310) crystal planes shows the (310) and (210) planes to possess a work function approx 0.2 eV lower than (100). This translates into a significant increase in current density, J, at a specified temperature. Comparison with a state-of-the-art impregnated dispenser cathode shows that LaB6 (310) is a superior cathode in nearly all respects except operating temperature at j 10 A/sq cm. The 1600 K thermionic and room temperature retarding potential work functions for LaB6 (310) are 2.42 and 2.50 respectively.

Power-Efficient, High-Current-Density, Long-Life Thermionic Cathode Developed for Microwave Amplifier Applications

NASA Technical Reports Server (NTRS)

Wintucky, Edwin G.

2002-01-01

A power-efficient, miniature, easily manufactured, reservoir-type barium-dispenser thermionic cathode has been developed that offers the significant advantages of simultaneous high electron-emission current density (>2 A/sq cm) and very long life (>100,000 hr of continuous operation) when compared with the commonly used impregnated-type barium-dispenser cathodes. Important applications of this cathode are a wide variety of microwave and millimeter-wave vacuum electronic devices, where high output power and reliability (long life) are essential. We also expect it to enable the practical development of higher purveyance electron guns for lower voltage and more reliable device operation. The low cathode heater power and reduced size and mass are expected to be particularly beneficial in traveling-wave-tube amplifiers (TWTA's) for space communications, where future NASA mission requirements include smaller onboard spacecraft systems, higher data transmission rates (high frequency and output power) and greater electrical efficiency.

Thermionic Power Cell To Harness Heat Energies for Geothermal Applications

NASA Technical Reports Server (NTRS)

Manohara, Harish; Mojarradi, Mohammad; Greer, Harold F.

2011-01-01

A unit thermionic power cell (TPC) concept has been developed that converts natural heat found in high-temperature environments (460 to 700 C) into electrical power for in situ instruments and electronics. Thermionic emission of electrons occurs when an emitter filament is heated to gwhite hot h temperatures (>1,000 C) allowing electrons to overcome the potential barrier and emit into the vacuum. These electrons are then collected by an anode, and transported to the external circuit for energy storage.

Gate-bias and temperature dependence of charge transport in dinaphtho[2,3-b:2‧,3‧-d]thiophene thin-film transistors with MoO3/Au electrodes

NASA Astrophysics Data System (ADS)

Shaari, Safizan; Naka, Shigeki; Okada, Hiroyuki

2018-04-01

We investigated the gate-bias and temperature dependence of the voltage-current (V-I) characteristics of dinaphtho[2,3-b:2‧,3‧-d]thiophene with MoO3/Au electrodes. The insertion of the MoO3 layer significantly improved the device performance. The temperature dependent V-I characteristics were evaluated and could be well fitted by the Schottky thermionic emission model with barrier height under forward- and reverse-biased regimes in the ranges of 33-57 and 49-73 meV, respectively. However, at a gate voltage of 0 V, at which a small activation energy was obtained, we needed to consider another conduction mechanism at the grain boundary. From the obtained results, we concluded that two possible conduction mechanisms governed the charge injection at the metal electrode-organic semiconductor interface: the Schottky thermionic emission model and the conduction model in the organic thin-film layer and grain boundary.

Gate oxide thickness dependence of the leakage current mechanism in Ru/Ta2O5/SiON/Si structures

NASA Astrophysics Data System (ADS)

Ťapajna, M.; Paskaleva, A.; Atanassova, E.; Dobročka, E.; Hušeková, K.; Fröhlich, K.

2010-07-01

Leakage conduction mechanisms in Ru/Ta2O5/SiON/Si structures with rf-sputtered Ta2O5 with thicknesses ranging from 13.5 to 1.8 nm were systematically studied. Notable reaction at the Ru/Ta2O5 interface was revealed by capacitance-voltage measurements. Temperature-dependent current-voltage characteristics suggest the bulk-limited conduction mechanism in all metal-oxide-semiconductor structures. Under gate injection, Poole-Frenkel emission was identified as a dominant mechanism for 13.5 nm thick Ta2O5. With an oxide thickness decreasing down to 3.5 nm, the conduction mechanism transforms to thermionic trap-assisted tunnelling through the triangular barrier. Under substrate injection, the dominant mechanism gradually changes with decreasing thickness from thermionic trap-assisted tunnelling to trap-assisted tunnelling through the triangular barrier; Poole-Frenkel emission was not observed at all. A 0.7 eV deep defect level distributed over Ta2O5 is assumed to be responsible for bulk-limited conduction mechanisms and is attributed to H-related defects or oxygen vacancies in Ta2O5.

Electron Thermionic Emission from Graphene and a Thermionic Energy Converter

NASA Astrophysics Data System (ADS)

Liang, Shi-Jun; Ang, L. K.

2015-01-01

In this paper, we propose a model to investigate the electron thermionic emission from single-layer graphene (ignoring the effects of the substrate) and to explore its application as the emitter of a thermionic energy converter (TIC). An analytical formula is derived, which is a function of the temperature, work function, and Fermi energy level. The formula is significantly different from the traditional Richardson-Dushman (RD) law for which it is independent of mass to account for the supply function of the electrons in the graphene behaving like massless fermion quasiparticles. By comparing with a recent experiment [K. Jiang et al., Nano Res. 7, 553 (2014)] measuring electron thermionic emission from suspended single-layer graphene, our model predicts that the intrinsic work function of single-layer graphene is about 4.514 eV with a Fermi energy level of 0.083 eV. For a given work function, a scaling of T3 is predicted, which is different from the traditional RD scaling of T2. If the work function of the graphene is lowered to 2.5-3 eV and the Fermi energy level is increased to 0.8-0.9 eV, it is possible to design a graphene-cathode-based TIC operating at around 900 K or lower, as compared with the metal-based cathode TIC (operating at about 1500 K). With a graphene-based cathode (work function=4.514 eV ) at 900 K and a metallic-based anode (work function=2.5 eV ) like LaB6 at 425 K, the efficiency of our proposed TIC is about 45%.

Characterization of sputter deposited thin film scandate cathodes for miniaturized thermionic converter applications

NASA Astrophysics Data System (ADS)

Zavadil, Kevin R.; Ruffner, Judith H.; King, Donald B.

1999-01-01

We have successfully developed a method for fabricating scandate-based thermionic emitters in thin film form. The primary goal of our effort is to develop thin film emitters that exhibit low work function, high intrinsic electron emissivity, minimum thermal activation properties and that can be readily incorporated into a microgap converter. Our approach has been to incorporate BaSrO into a Sc2O3 matrix using rf sputtering to produce thin films. Diode testing has shown the resulting films to be electron emissive at temperatures as low as 900 K with current densities of 0.1 mA.cm-2 at 1100 K and saturation voltages. We calculate an approximate maximum work function of 1.8 eV and an apparent emission constant (Richardson's constant, A*) of 36 mA.cm-2.K-2. Film compositional and structural analysis shows that a significant surface and subsurface alkaline earth hydroxide phase can form and probably explains the limited utilization and stability of Ba and its surface complexes. The flexibility inherent in sputter deposition suggests alternate strategies for eliminating undesirable phases and optimizing thin film emitter properties.

Experimental investigation of high temperature high voltage thermionic diode for the space power nuclear reactor

NASA Astrophysics Data System (ADS)

Onufriyev, Valery. V.

2001-02-01

It is well known that the rise of arc from the dense glow discharge is connected with the thermion and secondary processes on the cathode surface (Granovsky, 1971; Leob, 1953; Engel, 1935). First model of breakdown of the cathode layer is connected with the increase of the cathode temperature in consequence of the ion bombardment that leads to the grows its thermo-emissive current. Other model shows the main role of the secondary effects on the cathode surface-the increase of the secondary ion emission coefficient-γi with the grows of glow discharge voltage. But the author of this investigation work of breakdown in Cs vapor (a transmission the glow discharge into self-maintaining arc discharge) discovered the next peculiarity: the value of breakdown voltage is constant when the values of vapor temperature (its pressure pcs) and cathode temperature Tk is constant too (Ub=constant with Tk=constant and pcs=constant) and it is not a statistical value (Onufryev, Grishin, 1996) (that was observed in gas glow discharges other authors (Granovsky, 1971; Leob, 1953; Engel, 1935)). The investigations of thermion high voltage high temperature diode (its breakdown characteristics in closed state and voltage-current characteristics in disclosed state) showed that the value of the breakdown voltage is depended on the vapor pressure in inter-electrode gap (IEG)-pcs and cathode temperature-Tk and is independent on IEG length-Δieg. On this base it was settled that the main role in transition of glow discharge to self-maintaining arc discharge plays an ion cathode layer but more exactly-the region of excited atoms-``Aston glow.'' .

Precision control of high temperature furnaces using an auxiliary power supply and charged practice current flow

DOEpatents

Pollock, George G.

1997-01-01

Two power supplies are combined to control a furnace. A main power supply heats the furnace in the traditional manner, while the power from the auxiliary supply is introduced as a current flow through charged particles existing due to ionized gas or thermionic emission. The main power supply provides the bulk heating power and the auxiliary supply provides a precise and fast power source such that the precision of the total power delivered to the furnace is improved.

Thermionic energy converter investigations

NASA Technical Reports Server (NTRS)

Goodale, D. B.; Lee, C.; Lieb, D.; Oettinger, P. E.

1979-01-01

This paper presents evaluation of a variety of thermionic converter configurations to obtain improved efficiency. A variable-spacing diode using an iridium emitter gave emission properties comparable to platinum, but the power output from a sintered LaB6 collector diode was not consistent with its work function. Reflectivities above 0.5 were measured at thermal energies on oxygenated-cesiated surfaces using a field emission retarding potential gun. Performance of converters with structured electrodes and the characteristics of a pulsed triode were studied as a function of emitter, collector, cesium reservoir, interelectrode spacing, xenon pressure, and pulsing parameters.

Attractive potential around a thermionically emitting microparticle.

PubMed

Delzanno, G L; Lapenta, G; Rosenberg, M

2004-01-23

We present a simulation study of the charging of a dust grain immersed in a plasma, considering the effect of thermionic electron emission from the grain. It is shown that the orbit motion limited theory is no longer reliable when electron emission becomes large: screening can no longer be treated within the Debye-Huckel approach and an attractive potential well can form, leading to the possibility of attractive forces on other grains with the same polarity. We suggest to perform laboratory experiments where emitting dust grains could be used to create nonconventional dust crystals or macromolecules.

Schottky barrier parameters and low frequency noise characteristics of graphene-germanium Schottky barrier diode

NASA Astrophysics Data System (ADS)

Khurelbaatar, Zagarzusem; Kil, Yeon-Ho; Shim, Kyu-Hwan; Cho, Hyunjin; Kim, Myung-Jong; Lee, Sung-Nam; Jeong, Jae-chan; Hong, Hyobong; Choi, Chel-Jong

2016-03-01

We investigated the electrical properties of chemical vapor deposition-grown monolayer graphene/n-type germanium (Ge) Schottky barrier diodes (SBD) using current-voltage (I-V) characteristics and low frequency noise measurements. The Schottky barrier parameters of graphene/n-type Ge SBDs, such as Schottky barrier height (VB), ideality factor (n), and series resistance (Rs), were extracted using the forward I-V and Cheung's methods. The VB and n extracted from the forward ln(I)-V plot were found to be 0.63 eV and 1.78, respectively. In contrast, from Cheung method, the VB and n were calculated to be 0.53 eV and 1.76, respectively. Such a discrepancy between the values of VB calculated from the forward I-V and Cheung's methods indicated a deviation from the ideal thermionic emission of graphene/n-type Ge SBD associated with the voltage drop across graphene. The low frequency noise measurements performed at the frequencies in the range of 10 Hz-1 kHz showed that the graphene/n-type Ge SBD had 1/f γ frequency dependence, with γ ranging from 1.09 to 1.12, regardless of applied forward biases. Similar to forward-biased SBDs operating in the thermionic emission mode, the current noise power spectral density of graphene/n-type Ge SBD was linearly proportional to the forward current.

Temperature dependence of current-voltage characteristics in highly doped Ag/p-GaN/In Schottky diodes

NASA Astrophysics Data System (ADS)

Ćınar, K.; Yıldırım, N.; Coşkun, C.; Turut, A.

2009-10-01

To obtain detailed information about the conduction process of the Ag/p-GaN Schottky diodes (SDs) fabricated by us, we measured the I-V characteristics over the temperature range of 80-360 K by the steps of 20 K. The slope of the linear portion of the forward bias I-V plot and nkT =E0 of the device remained almost unchanged as independent of temperature with an average of 25.71±0.90 V-1 and 41.44±1.38 meV, respectively. Therefore, it can be said that the experimental I-V data quite well obey the field emission model rather than the thermionic emission or thermionic field emission model. The study is a very good experimental example for the FE model. Furthermore, the reverse bias saturation current ranges from 8.34×10-8 A at 80 K to 2.10×10-7 A at 360 K, indicating that the charge transport mechanism in the Ag/p-GaN SD is tunneling due to the weak temperature dependence of the saturation current. The possible origin of high experimental characteristic tunneling energy of E00=39 meV, which is ten times larger than possible theoretical value of 3.89 meV, is attributed to the accumulation of a large amount of defect states near the GaN surface or to the deep level defect band induced by high doping or to any mechanism which enhances the electric field and the state density at the semiconductor surface.

Ab initio investigation of the surface properties of dispenser B-type and scandate thermionic emission cathodes

NASA Astrophysics Data System (ADS)

Vlahos, Vasilios; Lee, Yueh-Lin; Booske, John H.; Morgan, Dane; Turek, Ladislav; Kirshner, Mark; Kowalczyk, Richard; Wilsen, Craig

2009-05-01

Scandate cathodes (BaxScyOz on W) are important thermionic electron emission materials whose emission mechanism remains unclear. Ab initio modeling is used to investigate the surface properties of both scandate and traditional B-type (Ba-O on W) cathodes. We demonstrate that the Ba-O dipole surface structure believed to be present in active B-type cathodes is not thermodynamically stable, suggesting that a nonequilibrium steady state dominates the active cathode's surface structure. We identify a stable, low work function BaxScyOz surface structure, which may be responsible for some scandate cathode properties and demonstrate that multicomponent surface coatings can lower cathode work functions.

The advanced thermionic converter with microwave power as an auxiliary ionization source

NASA Technical Reports Server (NTRS)

Manikopoulos, C. N.; Hatziprocopiou, M.; Chiu, H. S.; Shaw, D. T.

1978-01-01

In the search for auxiliary sources of ionization for the advanced thermionic converter plasma, as required for terrestial applications, the use of externally applied microwave power is considered. The present work is part of the advanced model thermionic converter development research currently performed at the laboratory for Power and Environmental Studies at SUNY Buffalo. Microwave power in the frequency range 1-3 GHz is used to externally pump a thermionic converter and the results are compared to the theoretical model proposed by Lam (1976) in describing the thermionic converter plasma. The electron temperature of the plasma is found to be raised considerably by effective microwave heating which results in the disappearance of the double sheath ordinarily erected in front of the emitter. The experimental data agree satisfactorily with theory in the low current region.

Hollow-Cathode Source Generates Plasma

NASA Technical Reports Server (NTRS)

Deininger, W. D.; Aston, G.; Pless, L. C.

1989-01-01

Device generates argon, krypton, or xenon plasma via thermionic emission and electrical discharge within hollow cathode and ejects plasma into surrounding vacuum. Goes from cold start up to full operation in less than 5 s after initial application of power. Exposed to moist air between operations without significant degradation of starting and running characteristics. Plasma generated by electrical discharge in cathode barrel sustained and aided by thermionic emission from emitter tube. Emitter tube does not depend on rare-earth oxides, making it vulnerable to contamination by exposure to atmosphere. Device modified for use as source of plasma in laboratory experiments or industrial processes.

Thermionic energy conversion technology - Present and future

NASA Technical Reports Server (NTRS)

Shimada, K.; Morris, J. F.

1977-01-01

Aerospace and terrestrial applications of thermionic direct energy conversion and advances in direct energy conversion (DEC) technology are surveyed. Electrode materials, the cesium plasma drop (the difference between the barrier index and the collector work function), DEC voltage/current characteristics, conversion efficiency, and operating temperatures are discussed. Attention is centered on nuclear reactor system thermionic DEC devices, for in-core or out-of-core operation. Thermionic fuel elements, the radiation shield, power conditions, and a waste heat rejection system are considered among the thermionic DEC system components. Terrestrial applications include topping power systems in fossil fuel and solar power generation.

Precision control of high temperature furnaces using an auxiliary power supply and charged particle current flow

DOEpatents

Pollock, G.G.

1997-01-28

Two power supplies are combined to control a furnace. A main power supply heats the furnace in the traditional manner, while the power from the auxiliary supply is introduced as a current flow through charged particles existing due to ionized gas or thermionic emission. The main power supply provides the bulk heating power and the auxiliary supply provides a precise and fast power source such that the precision of the total power delivered to the furnace is improved. 5 figs.

Thermionic cooling devices based on resonant-tunneling AlGaAs/GaAs heterostructure

NASA Astrophysics Data System (ADS)

Bescond, M.; Logoteta, D.; Michelini, F.; Cavassilas, N.; Yan, T.; Yangui, A.; Lannoo, M.; Hirakawa, K.

2018-02-01

We study by means of full quantum simulations the operating principle and performance of a semiconductor heterostructure refrigerator combining resonant tunneling filtering and thermionic emission. Our model takes into account the coupling between the electric and thermal currents by self-consistently solving the transport equations within the non-equilibrium Green’s function framework and the heat equation. We show that the device can achieve relatively high cooling power values, while in the considered implementation, the maximum lattice temperature drop is severely limited by the thermal conductivity of the constituting materials. In such an out-of-equilibrium structure, we then emphasize the significant deviation of the phonon temperature from its electronic counterpart which can vary over several hundred Kelvin. The interplay between those two temperatures and the impact on the electrochemical potential is also discussed. Finally, viable options toward an optimization of the device are proposed.

Thermionic cooling devices based on resonant-tunneling AlGaAs/GaAs heterostructure.

PubMed

Bescond, M; Logoteta, D; Michelini, F; Cavassilas, N; Yan, T; Yangui, A; Lannoo, M; Hirakawa, K

2018-02-14

We study by means of full quantum simulations the operating principle and performance of a semiconductor heterostructure refrigerator combining resonant tunneling filtering and thermionic emission. Our model takes into account the coupling between the electric and thermal currents by self-consistently solving the transport equations within the non-equilibrium Green's function framework and the heat equation. We show that the device can achieve relatively high cooling power values, while in the considered implementation, the maximum lattice temperature drop is severely limited by the thermal conductivity of the constituting materials. In such an out-of-equilibrium structure, we then emphasize the significant deviation of the phonon temperature from its electronic counterpart which can vary over several hundred Kelvin. The interplay between those two temperatures and the impact on the electrochemical potential is also discussed. Finally, viable options toward an optimization of the device are proposed.

Electrical characterization of the temperature dependence in CdTe/CdS heterojunctions deposited in-situ by pulsed laser deposition

NASA Astrophysics Data System (ADS)

Avila-Avendano, Jesus; Quevedo-Lopez, Manuel; Young, Chadwin

2018-02-01

The I-V and C-V characteristics of CdTe/CdS heterojunctions deposited in-situ by Pulsed Laser Deposition (PLD) were evaluated. In-situ deposition enables the study of the CdTe/CdS interface by avoiding potential impurities at the surface and interface as a consequence of exposure to air. The I-V and C-V characteristics of the resulting junctions were obtained at different temperatures, ranging from room temperature to 150 °C, where the saturation current (from 10-8 to 10-4 A/cm2), ideality factor (between 1 and 2), series resistance (from 102 to 105 Ω), built-in potential (0.66-0.7 V), rectification factor (˜106), and carrier concentration (˜1016 cm-3) were obtained. The current-voltage temperature dependence study indicates that thermionic emission is the main transport mechanism at the CdTe/CdS interface. This study also demonstrated that the built-in potential (Vbi) calculated using a thermionic emission model is more accurate than that calculated using C-V extrapolation since C-V plots showed a Vbi shift as a function of frequency. Although CdTe/CdS is widely used for photovoltaic applications, the parameters evaluated in this work indicate that CdTe/CdS heterojunctions could be used as rectifying diodes and junction field effect transistors (JFETs). JFETs require a low PN diode saturation current, as demonstrated for the CdTe/CdS junction studied here.

Pulsed thermionic converter study

NASA Technical Reports Server (NTRS)

1976-01-01

A nuclear electric propulsion concept using a thermionic reactor inductively coupled to a magnetoplasmadynamic accelerator (MPD arc jet) is described, and the results of preliminary analyses are presented. In this system, the MPD thruster operates intermittently at higher voltages and power levels than the thermionic generating unit. A typical thrust pulse from the MPD arc jet is characterized by power levels of 1 to 4 MWe, a duration of 1 msec, and a duty cycle of approximately 20%. The thermionic generating unit operates continuously but with a lower power level of approximately 0.4 MWe. Energy storage between thrust pulses is provided by building up a large current in an inductor using the output of the thermionic converter array. Periodically, the charging current is interrupted, and the energy stored in the magnetic field of the inductor is utilized for a short duration thrust pulse. The results of the preliminary analysis show that a coupling effectiveness of approximately 85 to 90% is feasible for a nominal 400 KWe system with an inductive unit suitable for a flight vehicle.

500(deg)C electronics for harsh environments

NASA Technical Reports Server (NTRS)

Sadwick, Laurence P.; Hwu, R. Jennifer; Chern, J. H. Howard; Lin, Ching-Hsu; Castillo, Linda Del; Johnson, Travis

2005-01-01

Solid state vacuum devices (SSVDs) are a relatively new class of electronic devices. Innosys is a leading producer of high frequency SSVDs for a number of applications, including RF communications. SSVDs combine features inherent to both solid state and vacuum transistors. Electron transport can be by solid state or vacuum or both. The focus of this talk is on thermionic SSVDs, in which the primary vacuum transport is by thermionically liberated electron emission.

Design Considerations for Heavily-Doped Cryogenic Schottky Diode Varactor Multipliers

NASA Technical Reports Server (NTRS)

Schlecht, E.; Maiwald, F.; Chattopadhyay, G.; Martin, S.; Mehdi, I.

2001-01-01

Diode modeling for Schottky varactor frequency multipliers above 500 GHz is presented with special emphasis placed on simple models and fitted equations for rapid circuit design. Temperature- and doping-dependent mobility, resistivity, and avalanche current multiplication and breakdown are presented. Next is a discussion of static junction current, including the effects of tunneling as well as thermionic emission. These results have been compared to detailed measurements made down to 80 K on diodes fabricated at JPL, followed by a discussion of the effect on multiplier efficiency. Finally, a simple model of current saturation in the undepleted active layer suitable for inclusion in harmonic balance simulators is derived.

Direct Determination of Field Emission across the Heterojunctions in a ZnO/Graphene Thin-Film Barristor.

PubMed

Mills, Edmund M; Min, Bok Ki; Kim, Seong K; Kim, Seong Jun; Kang, Min-A; Song, Wooseok; Myung, Sung; Lim, Jongsun; An, Ki-Seok; Jung, Jongwan; Kim, Sangtae

2015-08-26

Graphene barristors are a novel type of electronic switching device with excellent performance, which surpass the low on-off ratios that limit the operation of conventional graphene transistors. In barristors, a gate bias is used to vary graphene's Fermi level, which in turn controls the height and resistance of a Schottky barrier at a graphene/semiconductor heterojunction. Here we demonstrate that the switching characteristic of a thin-film ZnO/graphene device with simple geometry results from tunneling current across the Schottky barriers formed at the ZnO/graphene heterojunctions. Direct characterization of the current-voltage-temperature relationship of the heterojunctions by ac-impedance spectroscopy reveals that this relationship is controlled predominantly by field emission, unlike most graphene barristors in which thermionic emission is observed. This governing mechanism makes the device unique among graphene barristors, while also having the advantages of simple fabrication and outstanding performance.

Why do aged fluorescent tubes flicker?

NASA Astrophysics Data System (ADS)

Plihon, Nicolas; Ferrand, Jérémy; Guyomar, Tristan; Museur, Flavien; Taberlet, Nicolas

2017-11-01

Our everyday experience of aged and defective fluorescent tubes or bulbs informs us that they may flicker and emit a clicking sound while struggling to light up. In this article, the physical mechanisms controlling the initial illumination of a functioning fluorescent tube are investigated using a simple and affordable experimental setup. Thermionic emission from the electrodes of the tube controls the startup of fluorescent tubes. The origin of the faulty startup of aged fluorescent tubes is discussed and flickering regimes using functional tubes are artificially produced using a dedicated setup that decreases electron emission by the thermionic effect in a controlled manner. The physical parameters controlling the occurrence of flickering light are discussed, and their temporal statistics are reported.

Single-size thermometric measurements on a size distribution of neutral fullerenes.

PubMed

Cauchy, C; Bakker, J M; Huismans, Y; Rouzée, A; Redlich, B; van der Meer, A F G; Bordas, C; Vrakking, M J J; Lépine, F

2013-05-10

We present measurements of the velocity distribution of electrons emitted from mass-selected neutral fullerenes, performed at the intracavity free electron laser FELICE. We make use of mass-specific vibrational resonances in the infrared domain to selectively heat up one out of a distribution of several fullerene species. Efficient energy redistribution leads to decay via thermionic emission. Time-resolved electron kinetic energy distributions measured give information on the decay rate of the selected fullerene. This method is generally applicable to all neutral species that exhibit thermionic emission and provides a unique tool to study the stability of mass-selected neutral clusters and molecules that are only available as part of a size distribution.

Electronic transition imaging of carbon based materials: The photothreshold of melanin and thermionic field emission from diamond

NASA Astrophysics Data System (ADS)

Garguilo, Jacob

This study explores electronic transitions in carbon based materials through the use of a custom built, non rastering electron emission microscope. The specifics and history of electron emission are described as well as the equipment used in this study. The materials examined fall into two groups, melanosome films isolated from the human body and polycrystalline diamond tip arrays. A novel technique for determining the photothreshold of a heterogeneous material on a microscopic or smaller scale is developed and applied to melanosome films isolated from the hair, eyes, and brain of human donors. The conversion of the measured photothreshold on the vacuum scale to an electrochemical oxidation potential is discussed and the obtained data is considered based on this conversion. Pheomelanosomes isolated from human hair are shown to have significantly lower photoionization energy than eumelanosomes, indicating their likelihood as sources of oxidative stress. The ionization energies of the hair melanosomes are checked with complimentary procedures. Ocular melanosomes from the retinal pigment epithelium are measured as a function of patient age and melanosome shape. Lipofuscin, also found in the eye, is examined with the same microscopy technique and shown to have a significantly lower ionization threshold than RPE melanosomes. Neuromelanin from the substantia nigra is also examined and shown to have an ionization threshold close to that of eumelanin. A neuromelanin formation model is proposed based on these results. Polycrystalline diamond tip arrays are examined for their use as thermionic energy converter emitters. Thermionic energy conversion is accomplished through the combination of a hot electron emitter in conjunction with a somewhat cooler electron collector. The generated electron current can be used to do work in an external load. It is shown that the tipped structures of these samples result in enhanced emission over the surrounding flat areas, which may prove valuable in limiting the negative space charge effect in vacuum energy converting devices. Additionally, the effects of exceeding a threshold temperature for the films are shown, establishing a maximum operating regime for any device which incorporates hydrogen terminated diamond.

Thermionic gas switch

DOEpatents

Hatch, G.L.; Brummond, W.A.; Barrus, D.M.

1984-04-05

The present invention is directed to an improved temperature responsive thermionic gas switch utilizing a hollow cathode and a folded emitter surface area. The folded emitter surface area of the thermionic switch substantially increases the on/off ratio by changing the conduction surface area involved in the two modes thereof. The improved switch of this invention provides an on/off ratio of 450:1 compared to the 10:1 ratio of the prior known thermionic switch, while providing for adjusting the on current. In the improved switch of this invention the conduction area is made small in the off mode, while in the on mode the conduction area is made large. This is achieved by utilizing a folded hollow cathode configuration and utilizing a folded emitter surface area, and by making the dimensions of the folds small enough so that a space charge will develop in the convolutions of the folds and suppress unignited current, thus limiting the current carrying surface in the off mode.

Diamond Thin-Film Thermionic Generator

NASA Astrophysics Data System (ADS)

Clewell, J. M.; Ordonez, C. A.; Perez, J. M.

1997-03-01

Since the eighteen-hundreds scientists have sought to develop the highest thermal efficiency in heat engines such as thermionic generators. Modern research in the emerging diamond film industry has indicated the work functions of diamond thin-films can be much less than one electron volt, compelling fresh investigation into their capacity as thermionic generators and inviting new methodology for determining that efficiency. Our objective is to predict the efficiency of a low-work-function, degenerate semiconductor (diamond film) thermionic generator operated as a heat engine between two constant-temperature thermal reservoirs. Our presentation will focus on a theoretical model which predicts the efficiency of the system by employing a Monte Carlo computational technique from which we report results for the thermal efficiency and the thermionic current densities of diamond thin-films.

Electrical characteristics of multilayer MoS2 FET's with MoS2/graphene heterojunction contacts.

PubMed

Kwak, Joon Young; Hwang, Jeonghyun; Calderon, Brian; Alsalman, Hussain; Munoz, Nini; Schutter, Brian; Spencer, Michael G

2014-08-13

The electrical properties of multilayer MoS2/graphene heterojunction transistors are investigated. Temperature-dependent I-V measurements indicate the concentration of unintentional donors in exfoliated MoS2 to be 3.57 × 10(11) cm(-2), while the ionized donor concentration is determined as 3.61 × 10(10) cm(-2). The temperature-dependent measurements also reveal two dominant donor levels, one at 0.27 eV below the conduction band and another located at 0.05 eV below the conduction band. The I-V characteristics are asymmetric with drain bias voltage and dependent on the junction used for the source or drain contact. I-V characteristics of the device are consistent with a long channel one-dimensional field-effect transistor model with Schottky contact. Utilizing devices, which have both graphene/MoS2 and Ti/MoS2 contacts, the Schottky barrier heights of both interfaces are measured. The charge transport mechanism in both junctions was determined to be either thermionic-field emission or field emission depending on bias voltage and temperature. On the basis of a thermionic field emission model, the barrier height at the graphene/MoS2 interface was determined to be 0.23 eV, while the barrier height at the Ti/MoS2 interface was 0.40 eV. The value of Ti/MoS2 barrier is higher than previously reported values, which did not include the effects of thermionic field emission.

Thermionic cogeneration burner design

NASA Astrophysics Data System (ADS)

Miskolczy, G.; Goodale, D.; Moffat, A. L.; Morgan, D. T.

Since thermionic converters receive heat at very high temperatures (approximately 1800 K) and reject heat at moderately high temperatures (approximately 800 K), they are useful for cogeneration applications involving high temperature processes. The electric power from thermionic converters is produced as a high amperage, low-voltage direct current. An ideal cogeneration application would be to utilize the reject heat at the collector temperature and the electricity without power conditioning. A cogeneration application in the edible oil industry fulfills both of these requirements since both direct heat and hydrogen gas are required in the hydrogenation of the oils. In this application, the low-voltage direct current would be used in a hydrogen electrolyzer.

Thermionic refrigeration at CNT-CNT junctions

NASA Astrophysics Data System (ADS)

Li, C.; Pipe, K. P.

2016-10-01

Monte Carlo (MC) simulation is used to study carrier energy relaxation following thermionic emission at the junction of two van der Waals bonded single-walled carbon nanotubes (SWCNTs). An energy-dependent transmission probability gives rise to energy filtering at the junction, which is predicted to increase the average electron transport energy by as much as 0.115 eV, leading to an effective Seebeck coefficient of 386 μV/K. MC results predict a long energy relaxation length (˜8 μm) for hot electrons crossing the junction into the barrier SWCNT. For SWCNTs of optimal length, an analytical transport model is used to show that thermionic cooling can outweigh parasitic heat conduction due to high SWCNT thermal conductivity, leading to a significant cooling capacity (2.4 × 106 W/cm2).

The TEF modeling and analysis approach to advance thermionic space power technology

NASA Astrophysics Data System (ADS)

Marshall, Albert C.

1997-01-01

Thermionics space power systems have been proposed as advanced power sources for future space missions that require electrical power levels significantly above the capabilities of current space power systems. The Defense Special Weapons Agency's (DSWA) Thermionic Evaluation Facility (TEF) is carrying out both experimental and analytical research to advance thermionic space power technology to meet this expected need. A Modeling and Analysis (M&A) project has been created at the TEF to develop analysis tools, evaluate concepts, and guide research. M&A activities are closely linked to the TEF experimental program, providing experiment support and using experimental data to validate models. A planning exercise has been completed for the M&A project, and a strategy for implementation was developed. All M&A activities will build on a framework provided by a system performance model for a baseline Thermionic Fuel Element (TFE) concept. The system model is composed of sub-models for each of the system components and sub-systems. Additional thermionic component options and model improvements will continue to be incorporated in the basic system model during the course of the program. All tasks are organized into four focus areas: 1) system models, 2) thermionic research, 3) alternative concepts, and 4) documentation and integration. The M&A project will provide a solid framework for future thermionic system development.

Enhanced nonlinear current-voltage behavior in Au nanoparticle dispersed CaCu 3 Ti 4 O 12 composite films

NASA Astrophysics Data System (ADS)

Chen, Cong; Wang, Can; Ning, Tingyin; Lu, Heng; Zhou, Yueliang; Ming, Hai; Wang, Pei; Zhang, Dongxiang; Yang, Guozhen

2011-10-01

An enhanced nonlinear current-voltage behavior has been observed in Au nanoparticle dispersed CaCu 3Ti 4O 12 composite films. The double Schottky barrier model is used to explain the enhanced nonlinearity in I-V curves. According to the energy-band model and fitting result, the nonlinearity in Au: CCTO film is mainly governed by thermionic emission in the reverse-biased Schottky barrier. This result not only supports the mechanism of double Schottky barrier in CCTO, but also indicates that the nonlinearity of current-voltage behavior could be improved in nanometal composite films, which has great significance for the resistance switching devices.

Study and modeling of the transport mechanism in a semi insulating GaAs Schottky diode

NASA Astrophysics Data System (ADS)

Resfa, A.; Smahi, Bourzig Y.; Menezla, Brahimi. R.

2012-09-01

The current through a metal-semiconductor junction is mainly due to the majority carriers. Three distinctly different mechanisms exist in a Schottky diode: diffusion of carriers from the semiconductor into the metal, thermionic emission-diffusion (TED) of carriers across the Schottky barrier and quantum-mechanical tunneling through the barrier. The insulating layer converts the MS device in an MIS device and has a strong influence on its current-voltage (I-V) and the parameters of a Schottky barrier from 3.7 to 15 eV. There are several possible reasons for the error that causes a deviation of the ideal behavior of Schottky diodes with and without an interfacial insulator layer. These include the particular distribution of interface states, the series resistance, bias voltage and temperature. The GaAs and its large concentration values of trap centers will participate in an increase of the process of thermionic electrons and holes, which will in turn the IV characteristic of the diode, and an overflow maximum value [NT = 3 × 1020] is obtained. The I-V characteristics of Schottky diodes are in the hypothesis of a parabolic summit.

Developmental status of thermionic materials.

NASA Technical Reports Server (NTRS)

Yang, L.; Chin, J.

1972-01-01

Description of the reference materials selected for the major components of the unit cell of a thermionic pile element (TFE), the out-of-pile and in-pile test results, and current efforts for improving the life and performance of thermionic fuel elements. The component materials are required to withstand the fuel burnup and fast neutron fluence dictated by the thermionic reactor system. Tungsten was selected as the cladding material because of its compatibility with both the carbide and the oxide fuel materials. Niobium was selected as the collector material because its thermal expansion coefficient matches closely with that of the thin aluminum oxide layer used to electrically insulate the collector from the TFE sheath. An unfueled converter has performed stably over 41,000 hr. Accelerated irradiation tests have attained burnups equivalent to that for 40,000 hr of the thermionic reactor under consideration.

Optimize out-of-core thermionic energy conversion for nuclear electric propulsion

NASA Technical Reports Server (NTRS)

Morris, J. F.

1978-01-01

Thermionic energy conversion (TEC) potentialities for nuclear electric propulsion (NEP) are examined. Considering current designs, their limitations, and risks raises critical questions about the use of TEC for NEP. Apparently a reactor cooled by hotter-than-1675 K heat pipes has good potentialities. TEC with higher temperatures and greater power densities than the currently proposed 1650 K, 5-to-6 W/sq cm version offers substantial gains. Other approaches to high-temperature electric isolation appear also promising. A high-power-density, high-temperature TEC for NEP appears, therefore, attainable. It is recommended to optimize out-of-core thermionic energy conversion for nuclear electric propulsion. Although current TEC designs for NEP seem unnecessary compared with Brayton versions, large gains are apparently possible with increased temperatures and greater power densities.

A NEW THERMIONIC RF ELECTRON GUN FOR SYNCHROTRON LIGHT SOURCES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kutsaev, Sergey; Agustsson, R.; Hartzell, J

A thermionic RF gun is a compact and efficient source of electrons used in many practical applications. RadiaBeam Systems and the Advanced Photon Source at Argonne National Laboratory collaborate in developing of a reliable and robust thermionic RF gun for synchrotron light sources which would offer substantial improvements over existing thermionic RF guns and allow stable operation with up to 1A of beam peak current at a 100 Hz pulse repetition rate and a 1.5 μs RF pulse length. In this paper, we discuss the electromagnetic and engineering design of the cavity and report the progress towards high power testsmore » of the cathode assembly of the new gun.« less

Harvesting Electricity From Wasted Heat

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schwede, Jared

Scientists as SLAC National Laboratory explain the concept, Photon Enhanced Thermionic Emission (PETE), and how this process can capture more energy from photovoltaic panels by harnessing heat energy from sunlight.

Harvesting Electricity From Wasted Heat

ScienceCinema

Schwede, Jared

2018-01-16

Scientists as SLAC National Laboratory explain the concept, Photon Enhanced Thermionic Emission (PETE), and how this process can capture more energy from photovoltaic panels by harnessing heat energy from sunlight.

Laser and Electron Beam Processing of Semiconductors: CW Beam- Recrystallized Polysilicon as a Device-Worthy Material

DTIC Science & Technology

1980-12-31

boundary is given by the thermionic emission current, kT 1 /2 qVB qVa Jth = qn (m) exp (- [exp (Kn)- 1 ] ( 1 ) where Va is the applied voltage, q is the...small applied voltage, qVa << kT, Eq. ( 1 ) reduces to Jth = LVa (2) where = q n exp - -T- q.na (3) which gives the effective grain boundary resistance...POLYSILICON AS A DEVICE-WORTHY MATERIAL BY STANFORD UNIVERSITY STANFORD, CALIFORNIA 94305 FOR THE PERIOD JANUARY 1 , 1978 THROUGH DECEMBER 31, 1980 Dr

System Design for a Nuclear Electric Spacecraft Utilizing Out-of-core Thermionic Conversion

NASA Technical Reports Server (NTRS)

Estabrook, W. C.; Phillips, W. M.; Hsieh, T.

1976-01-01

Basic guidelines are presented for a nuclear space power system which utilizes heat pipes to transport thermal power from a fast nuclear reactor to an out of core thermionic converter array. Design parameters are discussed for the nuclear reactor, heat pipes, thermionic converters, shields (neutron and gamma), waste heat rejection systems, and the electrical bus bar-cable system required to transport the high current/low voltage power to the processing equipment. Dimensions are compatible with shuttle payload bay constraints.

Leakage current transport mechanism under reverse bias in Au/Ni/GaN Schottky barrier diode

NASA Astrophysics Data System (ADS)

Peta, Koteswara Rao; Kim, Moon Deock

2018-01-01

The leakage current transport mechanism under reverse bias of Au/Ni/GaN Schottky diode is studied using temperature dependent current-voltage (I-V-T) and capacitance-voltage (C-V) characteristics. I-V measurement in this study is in the range of 140 K-420 K in steps of 10 K. A reduction in voltage dependent barrier height and a strong internal electric field in depletion region under reverse bias suggested electric field enhanced thermionic emission in carrier transport via defect states in Au/Ni/GaN SBD. A detailed analysis of reverse leakage current revealed two different predominant transport mechanisms namely variable-range hopping (VRH) and Poole-Frenkel (PF) emission conduction at low (<260 K) and high (>260 K) temperatures respectively. The estimated thermal activation energies (0.20-0.39 eV) from Arrhenius plot indicates a trap assisted tunneling of thermally activated electrons from a deep trap state into a continuum of states associated with each conductive threading dislocation.

Thermionic combustor application to combined gas and steam turbine power plants

NASA Astrophysics Data System (ADS)

Miskolczy, G.; Wang, C. C.; Lieb, D. P.; Margulies, A. E.; Fusegni, L. J.; Lovell, B. J.

A design for the insertion of thermionic converters into the wall of a conventional combustor to produce electricity in a topping cycle is described, and a study for applications in gas and steam generators of 70 and 30 MW is evaluated for engineering and economic feasibility. Waste heat from the thermionic elements is used to preheat the combustor air; the heat absorbed by the elements plus further quenching of the exhaust gases with ammonia is projected to reduce NO(x) emissions to acceptable levels. Schematics, flow diagrams, and components of a computer model for cost projections are provided. It was found that temperatures around the emitters must be maintained above 1,600 K, with maximum efficiency and allowable temperature at 1,800 K, while collectors generate maximally at 950 K, with a corresponding work function of 1.5 eV. Cost sensitive studies indicate an installed price of $475/kW for the topping cycle, with improvements in thermionic converter characteristics bringing the cost to $375/kW at a busbar figure of 500 mills/kWh.

Tunneling contact IGZO TFTs with reduced saturation voltages

NASA Astrophysics Data System (ADS)

Wang, Longyan; Sun, Yin; Zhang, Xintong; Zhang, Lining; Zhang, Shengdong; Chan, Mansun

2017-04-01

We report a tunneling contact indium-gallium-zinc oxide (IGZO) thin film transistor (TFT) with a graphene interlayer technique in this paper. A Schottky junction is realized between a metal and IGZO with a graphene interlayer, leading to a quantum tunneling of the TFT transport in saturation regions. This tunneling contact enables a significant reduction in the saturation drain voltage Vdsat compared to that of the thermionic emission TFTs, which is usually equal to the gate voltage minus their threshold voltages. Measured temperature independences of the subthreshold swing confirm a transition from the thermionic emission to quantum tunneling transports depending on the gate bias voltages in the proposed device. The tunneling contact TFTs with the graphene interlayer have implications to reduce the power consumptions of certain applications such as the active matrix OLED display.

Bohm velocity in the presence of a hot cathode

DOE Office of Scientific and Technical Information (OSTI.GOV)

Palacio Mizrahi, J. H.; Krasik, Ya. E.

2013-08-15

The spatial distribution of the plasma and beam electrons in a region whose extension from a hot cathode is larger than the Debye length, but smaller than the electron mean free path, is analyzed. In addition, the influence of electrons thermionically emitted from a hot cathode and the ratio of electron-to-ion mass on the Bohm velocity and on the ion and electron densities at the plasma-sheath boundary in a gas discharge are studied. It is shown that thermionic emission has the effect of increasing the Bohm velocity, and this effect is more pronounced for lighter ions. In addition, it ismore » shown that the Bohm velocity cannot be increased to more than 24% above its value when there is no electron emission.« less

Development of multi-pixel x-ray source using oxide-coated cathodes.

PubMed

Kandlakunta, Praneeth; Pham, Richard; Khan, Rao; Zhang, Tiezhi

2017-07-07

Multiple pixel x-ray sources facilitate new designs of imaging modalities that may result in faster imaging speed, improved image quality, and more compact geometry. We are developing a high-brightness multiple-pixel thermionic emission x-ray (MPTEX) source based on oxide-coated cathodes. Oxide cathodes have high emission efficiency and, thereby, produce high emission current density at low temperature when compared to traditional tungsten filaments. Indirectly heated micro-rectangular oxide cathodes were developed using carbonates, which were converted to semiconductor oxides of barium, strontium, and calcium after activation. Each cathode produces a focal spot on an elongated fixed anode. The x-ray beam ON and OFF control is performed by source-switching electronics, which supplies bias voltage to the cathode emitters. In this paper, we report the initial performance of the oxide-coated cathodes and the MPTEX source.

Gate-dependent asymmetric transport characteristics in pentacene barristors with graphene electrodes.

PubMed

Hwang, Wang-Taek; Min, Misook; Jeong, Hyunhak; Kim, Dongku; Jang, Jingon; Yoo, Daekyung; Jang, Yeonsik; Kim, Jun-Woo; Yoon, Jiyoung; Chung, Seungjun; Yi, Gyu-Chul; Lee, Hyoyoung; Wang, Gunuk; Lee, Takhee

2016-11-25

We investigated the electrical characteristics and the charge transport mechanism of pentacene vertical hetero-structures with graphene electrodes. The devices are composed of vertical stacks of silicon, silicon dioxide, graphene, pentacene, and gold. These vertical heterojunctions exhibited distinct transport characteristics depending on the applied bias direction, which originates from different electrode contacts (graphene and gold contacts) to the pentacene layer. These asymmetric contacts cause a current rectification and current modulation induced by the gate field-dependent bias direction. We observed a change in the charge injection barrier during variable-temperature current-voltage characterization, and we also observed that two distinct charge transport channels (thermionic emission and Poole-Frenkel effect) worked in the junctions, which was dependent on the bias magnitude.

A Robust High Current Density Electron Gun

NASA Astrophysics Data System (ADS)

Mako, F.; Peter, W.; Shiloh, J.; Len, L. K.

1996-11-01

Proof-of-principle experiments are proposed to validate a new concept for a robust, high-current density Pierce electron gun (RPG) for use in klystrons and high brightness electron sources for accelerators. This rugged, long-life electron gun avoids the difficulties associated with plasma cathodes, thermionic emitters, and field emission cathodes. The RPG concept employs the emission of secondary electrons in a transmission mode as opposed to the conventional mode of reflection, i.e., electrons exit from the back face of a thin negative electron affinity (NEA) material, and in the same direction as the incident beam. Current amplification through one stage of a NEA material could be over 50 times. The amplification is accomplished in one or more stages consisting of one primary emitter and one or more secondary emitters. The primary emitter is a low current density robust emitter (e.g., thoriated tungsten). The secondary emitters are thin NEA electrodes which emit secondary electrons in the same direction as the incident beam. Specific application is targeted for a klystron gun to be used by SLAC with a cold cathode at 30-40 amps/cm^2 output from the secondary emission stage, a ~2 μs pulse length, and ~200 pulses/second.

Observation of Repetition-Rate Dependent Emission From an Un-Gated Thermionic Cathode Rf Gun

DOE Office of Scientific and Technical Information (OSTI.GOV)

Edelen, J. P.; Sun, Y.; Harris, J.R.

Recent work at Fermilab in collaboration with the Advanced Photon Source and members of other national labs, designed an experiment to study the relationship between the RF repetition rate and the average current per RF pulse. While existing models anticipate a direct relationship between these two parameters we observed an inverse relationship. We believe this is a result of damage to the barium coating on the cathode surface caused by a change in back-bombardment power that is unaccounted for in the existing theories. These observations shed new light on the challenges and fundamental limitations associated with scaling an ungated thermionicmore » cathode RF gun to high average current.« less

Emission efficiency optimization of RE 2O 3 doped molybdenum thermionic cathode by application of pattern recognition method

NASA Astrophysics Data System (ADS)

Wang, Jinshu; Liu, Wei; Liu, Yanqin; Zhou, Meiling

2005-09-01

As an alternative for thoriated tungsten thermionic cathodes, molybdenum doped with either a single rare earth oxide such as La 2O 3, Y 2O 3 and Sc 2O 3 or a mixture thereof has been produced by powder metallurgy. It is shown that carbonization can greatly improve the emission properties (i.e. emission capability and stability) of RE 2O 3 doped molybdenum due to the formation of a (metallic) rare earth atomic layer on the surface of the cathode by the reduction reaction of molybdenum carbide and rare earth oxide. Among all the carbonized samples, La 2O 3 and Y 2O 3 co-doped molybdenum cathode showed the best performance in emission. In addition, computer pattern recognition technique has been used to optimize the composition of the material and of the cathode preparation technique. We derive the equation of the emission efficiency as a function of cathode composition and carbonization degree. Based on the projecting coordinates obtained from the equation, the optimum projection region was identified, which can serve as guide for the composition and carbonization degree design.

X-ray Diffraction Studies of the Structure and Thermochemistry of Alkaline-Earth Oxide-Coated Thermionic Cathodes

NASA Technical Reports Server (NTRS)

Karikari, E. K.; Bassey, E.; Wintucky, Edwin G.

1998-01-01

NASA LeRC has a broad, active cathode technology development program in which both experimental and theoretical studies are being employed to further development of thermionic cathodes for use as electron sources in vacuum devices for communications and other space applications. One important type of thermionic cathode under development is the alkaline-earth oxide-coated (BaO, SrO, CaO) cathode. Significant improvements in the emission characteristics of this cathode have been obtained through modification of the chemical composition and morphology of the oxide coating, with the best result thus far coming from the addition of In2O3 and Sc2O3. Whereas the In2O3 produces a finer, more uniform particle structure, the exact chemical state and role of the Sc2O3 in the emission enhancement is unknown. The purpose of this cooperative agreement is to combine the studies of the surface chemistry and electron emission at NASA LeRC of chemically modified oxide coatings with a study of the thermochemistry and crystal structure using X-ray diffraction equipment and expertise at Clark Atlanta University (CAU). The study at CAU is intended to provide the description and understanding of the structure and thermochemistry needed for further improvement and optimization of the modified coatings. A description of the experimental procedure, preliminary X-ray diffraction test results, together with the design of an ultrahigh vacuum chamber necessary for high temperature thermochemistry studies will be presented.

X-ray generation using carbon nanotubes

NASA Astrophysics Data System (ADS)

Parmee, Richard J.; Collins, Clare M.; Milne, William I.; Cole, Matthew T.

2015-01-01

Since the discovery of X-rays over a century ago the techniques applied to the engineering of X-ray sources have remained relatively unchanged. From the inception of thermionic electron sources, which, due to simplicity of fabrication, remain central to almost all X-ray applications, there have been few fundamental technological advances. However, with the emergence of ever more demanding medical and inspection techniques, including computed tomography and tomosynthesis, security inspection, high throughput manufacturing and radiotherapy, has resulted in a considerable level of interest in the development of new fabrication methods. The use of conventional thermionic sources is limited by their slow temporal response and large physical size. In response, field electron emission has emerged as a promising alternative means of deriving a highly controllable electron beam of a well-defined distribution. When coupled to the burgeoning field of nanomaterials, and in particular, carbon nanotubes, such systems present a unique technological opportunity. This review provides a summary of the current state-of-the-art in carbon nanotube-based field emission X-ray sources. We detail the various fabrication techniques and functional advantages associated with their use, including the ability to produce ever smaller electron beam assembles, shaped cathodes, enhanced temporal stability and emergent fast-switching pulsed sources. We conclude with an overview of some of the commercial progress made towards the realisation of an innovative and disruptive technology.

Anomalous Temperature Dependence in Metal-Black Phosphorus Contact.

PubMed

Li, Xuefei; Grassi, Roberto; Li, Sichao; Li, Tiaoyang; Xiong, Xiong; Low, Tony; Wu, Yanqing

2018-01-10

Metal-semiconductor contact has been the performance limiting problem for electronic devices and also dictates the scaling potential for future generation devices based on novel channel materials. Two-dimensional semiconductors beyond graphene, particularly few layer black phosphorus, have attracted much attention due to their exceptional electronic properties such as anisotropy and high mobility. However, due to its ultrathin body nature, few layer black phosphorus-metal contact behaves differently than conventional Schottky barrier (SB) junctions, and the mechanisms of its carrier transport across such a barrier remain elusive. In this work, we examine the transport characteristic of metal-black phosphorus contact under varying temperature. We elucidated the origin of apparent negative SB heights extracted from classical thermionic emission model and also the phenomenon of metal-insulator transition observed in the current-temperature transistor characteristic. In essence, we found that the SB height can be modulated by the back-gate voltage, which beyond a certain critical point becomes so low that the injected carrier can no longer be described by the conventional thermionic emission theory. The transition from transport dominated by a Maxwell-Boltzmann distribution for the high energy tail states, to that of a Fermi distribution by low energy Fermi sea electrons, is the physical origin of the observed metal-insulator transition. We identified two distinctive tunneling limited transport regimes in the contact: vertical and longitudinal tunneling.

Performance of a thermionic converter module utilizing emitter and collector heat pipes

NASA Technical Reports Server (NTRS)

Kroeger, E. W.; Morris, J. F.; Miskolczy, G.; Lieb, D. P.; Goodale, D. B.

1978-01-01

A thermionic converter module simulating a configuration for an out-of-core thermionic nuclear reactor was designed, fabricated, and tested. The module consists of three cylindrical thermionic converters. The tungsten emitter of the converter is heated by a tungsten, lithium heat pipe. The emitter heat pipes are immersed in a furnace, insulated by MULTI-FOIL thermal insulation, and heated by tungsten radiation filaments. The performance of each thermionic converter was characterized before assembly into the module. Dynamic voltage, current curves were taken using a 60 Hz sweep and computerized data acquisition over a range of emitter, collector, and cesium-reservoir temperatures. An output power of 215 W was observed at an emitter temperature of 1750 K and a collector temperature of 855 K for a two diode module. With a three diode module, an output power of 270 W was observed at an average emitter temperature of 1800 K and a Collector temperature of 875 K.

Ballistic and resonant negative photocurrents in semiconducting carbon nanotubes

NASA Astrophysics Data System (ADS)

Karnetzky, Christoph; Sponfeldner, Lukas; Engl, Max; Holleitner, Alexander W.

2017-04-01

Ultrafast photocurrent experiments are performed on semiconducting, single-walled carbon nanotubes under a resonant optical excitation of their subbands. The photogenerated excitons are dissociated at large electric fields and the resulting transport of the charge carriers turns out to be ballistic. Thermionic emission processes to the contacts dominate the photocurrent amplitude. The charge current without laser excitation is well described by a Fowler-Nordheim tunneling. The time-averaged photocurrent changes polarity as soon as sufficient charge carriers are injected from the contacts, which can be explained by an effective population inversion in the optically pumped subbands.

Ballistic and resonant negative photocurrents in single carbon nanotubes

NASA Astrophysics Data System (ADS)

Karnetzky, Christoph; Sponfeldner, Lukas; Engl, Max; Holleitner, Alexander W.

We present ultrafast photocurrent experiments on semiconducting, single-walled carbon nanotubes under a resonant optical excitation of their subbands. We demonstrate that a ballistic transport of the photogenerated charge carriers can be achieved. Moreover, thermionic emission processes to the contacts dominate the photocurrent. In contrast, the charge current without laser excitation is well described by a Fowler-Nordheim tunneling. The time-averaged photocurrent changes polarity as soon as sufficient charge carriers are injected from the contacts, which can be explained by an effective population inversion in the optically pumped subbands. We acknowledge the ERC via the project NanoREAL.

Thermionic gas switch

DOEpatents

Hatch, George L.; Brummond, William A.; Barrus, Donald M.

1986-01-01

A temperature responsive thermionic gas switch having folded electron emitting surfaces. An ionizable gas is located between the emitter and an interior surface of a collector, coaxial with the emitter. In response to the temperature exceeding a predetermined level, sufficient electrons are derived from the emitter to cause the gas in the gap between the emitter and collector to become ionized, whereby a very large increase in current in the gap occurs. Due to the folded emitter surface area of the switch, increasing the "on/off" current ratio and adjusting the "on" current capacity is accomplished.

ANALYTICAL MODELING OF ELECTRON BACK-BOMBARDMENT INDUCED CURRENT INCREASE IN UN-GATED THERMIONIC CATHODE RF GUNS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Edelen, J. P.; Sun, Y.; Harris, J. R.

In this paper we derive analytical expressions for the output current of an un-gated thermionic cathode RF gun in the presence of back-bombardment heating. We provide a brief overview of back-bombardment theory and discuss comparisons between the analytical back-bombardment predictions and simulation models. We then derive an expression for the output current as a function of the RF repetition rate and discuss relationships between back-bombardment, fieldenhancement, and output current. We discuss in detail the relevant approximations and then provide predictions about how the output current should vary as a function of repetition rate for some given system configurations.

Compact modeling of SiC Schottky barrier diode and its extension to junction barrier Schottky diode

NASA Astrophysics Data System (ADS)

Navarro, Dondee; Herrera, Fernando; Zenitani, Hiroshi; Miura-Mattausch, Mitiko; Yorino, Naoto; Jürgen Mattausch, Hans; Takusagawa, Mamoru; Kobayashi, Jun; Hara, Masafumi

2018-04-01

A compact model applicable for both Schottky barrier diode (SBD) and junction barrier Schottky diode (JBS) structures is developed. The SBD model considers the current due to thermionic emission in the metal/semiconductor junction together with the resistance of the lightly doped drift layer. Extension of the SBD model to JBS is accomplished by modeling the distributed resistance induced by the p+ implant developed for minimizing the leakage current at reverse bias. Only the geometrical features of the p+ implant are necessary to model the distributed resistance. Reproduction of 4H-SiC SBD and JBS current-voltage characteristics with the developed compact model are validated against two-dimensional (2D) device-simulation results as well as measurements at different temperatures.

The Demon in a Vacuum Tube

NASA Astrophysics Data System (ADS)

D'Abramo, Germano

2013-05-01

In the present paper, several issues concerning the second law of thermodynamics, Maxwell's demon and Landauer's principle are dealt with. I argue that if the demon and the system on which it operates without dissipation of external energy are made of atoms and molecules (gas, liquid or solid) in thermal equilibrium (whose behaviour is described by a canonical distribution), then the unavoidable reason why the demon cannot successfully operate resides in the ubiquity of thermal fluctuations and friction. Landauer's principle appears to be unnecessary. I also suggest that if the behaviour of the demon and the system on which it acts is not always describable by a canonical distribution, as would happen for instance with the ballistic motion of electrons at early stages of thermionic emission, then a successful working demon cannot be ruled out a priori. A critical review of two recent experiments on thermionic emission Maxwell's demons is also given.

Recent progress on RE2O3-Mo/W emission materials.

PubMed

Wang, Jinshu; Zhang, Xizhu; Liu, Wei; Cui, Yuntao; Wang, Yiman; Zhou, Meiling

2012-08-01

RE2O3-Mo/W cathodes were prepared by powder metallurgy method. La2O3-Y2O3-Mo cermet cathodes prepared by traditional sintering method and spark plasma sintering (SPS) exhibit different secondary emission properties. The La2O3-Y2O3-Mo cermet cathode prepared by SPS method has smaller grain size and exhibits better secondary emission performance. Monte carlo calculation results indicate that the secondary electron emission way of the cathode correlates with the grain size. Decreasing the grain size can decrease the positive charging effect of RE2O3 and thus is favorable for the escaping of secondary electrons to vacuum. The Scandia doped tungsten matrix dispenser cathode with a sub-micrometer microstructure of matrix with uniformly distributed nanometer-particles of Scandia has good thermionic emission property. Over 100 A/cm2 full space charge limited current density can be obtained at 950Cb. The cathode surface is covered by a Ba-Sc-O active surface layer with nano-particles distributing mainly on growth steps of W grains, leads to the conspicuous emission property of the cathode.

Power Management and Distribution System Developed for Thermionic Power Converters

NASA Technical Reports Server (NTRS)

Baez, Anastacio N.

1998-01-01

A spacecraft solar, bimodal system combines propulsion and power generation into a single integrated system. An Integrated Solar Upper Stage (ISUS) provides orbital transfer capabilities, power generation for payloads, and onboard propulsion to the spacecraft. A key benefit of a bimodal system is a greater payload-to-spacecraft mass ratio resulting in lower launch vehicle requirements. Scaling down to smaller launch vehicles increases space access by reducing overall mission cost. NASA has joined efforts with the Air Force Phillips Laboratory to develop enabling technologies for such a system. The NASA/Air Force bimodal concept uses solar concentrators to focus energy into an integrated power plant. This power plant consists of a graphite core that stores thermal energy within a cavity. An array of thermionic converters encircles the graphite cavity and provides electrical energy conversion functions. During the power generation phase of the bimodal system, the thermionic converters are exposed to the heated cavity and convert the thermal energy to electricity. Near-term efforts of the ISUS bimodal program are focused on a ground demonstration of key technologies in order to proceed to a full space flight test. Thermionic power generation is one key technology of the bimodal concept. Thermionic power converters impose unique operating requirements upon a power management and distribution (PMAD) system design. Single thermionic converters supply large currents at very low voltages. Operating voltages can vary over a range of up to 3 to 1 as a function of operating temperature. Most spacecraft loads require regulated 28-volts direct-current (Vdc) power. A combination of series-connected converters and powerprocessing boosters is required to deliver power to the spacecraft's payloads at this level.

Negative space charge effects in photon-enhanced thermionic emission solar converters

DOE Office of Scientific and Technical Information (OSTI.GOV)

Segev, G.; Weisman, D.; Rosenwaks, Y.

2015-07-06

In thermionic energy converters, electrons in the gap between electrodes form a negative space charge and inhibit the emission of additional electrons, causing a significant reduction in conversion efficiency. However, in Photon Enhanced Thermionic Emission (PETE) solar energy converters, electrons that are reflected by the electric field in the gap return to the cathode with energy above the conduction band minimum. These electrons first occupy the conduction band from which they can be reemitted. This form of electron recycling makes PETE converters less susceptible to negative space charge loss. While the negative space charge effect was studied extensively in thermionicmore » converters, modeling its effect in PETE converters does not account for important issues such as this form of electron recycling, nor the cathode thermal energy balance. Here, we investigate the space charge effect in PETE solar converters accounting for electron recycling, with full coupling of the cathode and gap models, and addressing conservation of both electric and thermal energy. The analysis shows that the negative space charge loss is lower than previously reported, allowing somewhat larger gaps compared to previous predictions. For a converter with a specific gap, there is an optimal solar flux concentration. The optimal solar flux concentration, the cathode temperature, and the efficiency all increase with smaller gaps. For example, for a gap of 3 μm the maximum efficiency is 38% and the optimal flux concentration is 628, while for a gap of 5 μm the maximum efficiency is 31% and optimal flux concentration is 163.« less

OBSERVATION OF REPETITION-RATE DEPENDANT EMISSION FROM AN UN-GATED THERMIONIC CATHODE RF GUN

DOE Office of Scientific and Technical Information (OSTI.GOV)

Edelen, J. P.; Sun, Y.; Harris, J. R.

Recent work at Fermilab in collaboration with the Advanced Photon Source and members of other national labs, designed an experiment to study the relationship between the RF repetition rate and the average current per RF pulse. While existing models anticipate a direct relationship between these two parameters we observed an inverse relationship. We believe this is a result of damage to the barium coating on the cathode surface caused by a change in back-bombardment power that is unaccounted for in the existing theories. These observations shed new light on the challenges and fundamental limitations associated with scaling an ungated thermionicmore » cathode RF gun to high average current machines.« less

Conceptual Design for CLIC Gun Pulser

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tang, Tao

The Compact Linear Collider (CLIC) is a proposed future electron-positron collider, designed to perform collisions at energies from 0.5 to 5 TeV, with a nominal design optimized for 3 TeV (Dannheim, 2012). The Drive Beam Accelerator consists of a thermionic DC gun, bunching section and an accelerating section. The thermionic gun needs deliver a long (~143us) pulse of current into the buncher. A pulser is needed to drive grid of the gun to generate a stable current output. This report explores the requirements of the gun pulser and potential solutions to regulate grid current.

Graphene for thermoelectronic solar energy conversion

NASA Astrophysics Data System (ADS)

De, Dilip K.; Olukunle, Olawole C.

2017-08-01

Graphene is a high temperature material which can stand temperature as high as 4600 K in vacuum. Even though its work function is high (4.6 eV) the thermionic emission current density at such temperature is very high. Graphene is a wonderful material whose work function can be engineered as desired. Kwon et al41 reported a chemical approach to reduce work function of graphene using K2CO3, Li2CO3, Rb2CO3, Cs2CO3. The work functions are reported to be 3.7 eV, 3.8 eV, 3.5 eV and 3.4 eV. Even though they did not report the high temperature tolerance of such alkali metal carbonate doped graphene, their works open a great promise for use of pure graphene and doped graphene as emitter (cathode) and collector (anode) in a solar thermionic energy converter. This paper discusses the dynamics of solar energy conversion to electrical energy using thermionic energy converter with graphene as emitter and collector. We have considered parabolic mirror concentrator to focus solar energy onto the emitter to achieve temperature around 4300 K. Our theoretical calculations and the modelling show that efficiency as high as 55% can easily be achieved if space-charge problem can be reduced and the collector can be cooled to certain proper temperature. We have discussed methods of controlling the associated space-charge problems. Richardson-Dushman equation modified by the authors have been used in this modelling. Such solar energy conversion would reduce the dependence on silicon solar panel and has great potential for future applications.

Space charge effects on the current-voltage characteristics of gated field emitter arrays

NASA Astrophysics Data System (ADS)

Jensen, K. L.; Kodis, M. A.; Murphy, R. A.; Zaidman, E. G.

1997-07-01

Microfabricated field emitter arrays (FEAs) can provide the very high electron current densities required for rf amplifier applications, typically on the order of 100 A/cm2. Determining the dependence of emission current on gate voltage is important for the prediction of emitter performance for device applications. Field emitters use high applied fields to extract current, and therefore, unlike thermionic emitters, the current densities can exceed 103A/cm2 when averaged over an array. At such high current densities, space charge effects (i.e., the influence of charge between cathode and collector on emission) affect the emission process or initiate conditions which can lead to failure mechanisms for field emitters. A simple model of a field emitter will be used to calculate the one-dimensional space charge effects on the emission characteristics by examining two components: charge between the gate and anode, which leads to Child's law, and charge within the FEA unit cell, which gives rise to a field suppression effect which can exist for a single field emitter. The predictions of the analytical model are compared with recent experimental measurements designed to assess space charge effects and predict the onset of gate current. It is shown that negative convexity on a Fowler-Nordheim plot of Ianode(Vgate) data can be explained in terms of field depression at the emitter tip in addition to reflection of electrons by a virtual cathode created when the anode field is insufficient to extract all of the current; in particular, the effects present within the unit cell constitute a newly described effect.

Vertical electron transport in van der Waals heterostructures with graphene layers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ryzhii, V., E-mail: v-ryzhii@riec.tohoku.ac.jp; Center for Photonics and Infrared Engineering, Bauman Moscow State Technical University and Institute of Ultra High Frequency Semiconductor Electronics of RAS, Moscow 111005; Otsuji, T.

We propose and analyze an analytical model for the self-consistent description of the vertical electron transport in van der Waals graphene-layer (GL) heterostructures with the GLs separated by the barriers layers. The top and bottom GLs serve as the structure emitter and collector. The vertical electron transport in such structures is associated with the propagation of the electrons thermionically emitted from GLs above the inter-GL barriers. The model under consideration describes the processes of the electron thermionic emission from and the electron capture to GLs. It accounts for the nonuniformity of the self-consistent electric field governed by the Poisson equationmore » which accounts for the variation of the electron population in GLs. The model takes also under consideration the cooling of electrons in the emitter layer due to the Peltier effect. We find the spatial distributions of the electric field and potential with the high-electric-field domain near the emitter GL in the GL heterostructures with different numbers of GLs. Using the obtained spatial distributions of the electric field, we calculate the current-voltage characteristics. We demonstrate that the Peltier cooling of the two-dimensional electron gas in the emitter GL can strongly affect the current-voltage characteristics resulting in their saturation. The obtained results can be important for the optimization of the hot-electron bolometric terahertz detectors and different devices based on GL heterostructures.« less

Experimental investigation of electron guns for THz microwave vacuum amplifiers

NASA Astrophysics Data System (ADS)

Burtsev, A. A.; Grigor'ev, Yu. A.; Navrotsky, I. A.; Rogovin, V. I.; Sakhadzhi, G. V.; Shumikhin, K. V.

2016-05-01

Single-sheet and multiple beam electron emitters based on thermionic minicathodes for terahertz traveling-wave tubes have been studied. Data are presented for impregnated blade thermionic cathode with dimensions 0.1 × 0.7 mm and a maximum current density of 114 A/cm2 in a pulsed mode. A variant of the five-beam electron gun with 0.25-mm-diameter cylindrical minicathodes in cells of a control grid is proposed that provides a current density of 85.5 A/cm2 at a grid potential of 900-1000 V.

Bulk heterojunction polymer memory devices with reduced graphene oxide as electrodes.

PubMed

Liu, Juqing; Yin, Zongyou; Cao, Xiehong; Zhao, Fei; Lin, Anping; Xie, Linghai; Fan, Quli; Boey, Freddy; Zhang, Hua; Huang, Wei

2010-07-27

A unique device structure with a configuration of reduced graphene oxide (rGO) /P3HT:PCBM/Al has been designed for the polymer nonvolatile memory device. The current-voltage (I-V) characteristics of the fabricated device showed the electrical bistability with a write-once-read-many-times (WORM) memory effect. The memory device exhibits a high ON/OFF ratio (10(4)-10(5)) and low switching threshold voltage (0.5-1.2 V), which are dependent on the sheet resistance of rGO electrode. Our experimental results confirm that the carrier transport mechanisms in the OFF and ON states are dominated by the thermionic emission current and ohmic current, respectively. The polarization of PCBM domains and the localized internal electrical field formed among the adjacent domains are proposed to explain the electrical transition of the memory device.

Electrical current at micro-/macro-scale of undoped and nitrogen-doped MWPECVD diamond films

NASA Astrophysics Data System (ADS)

Cicala, G.; Velardi, L.; Senesi, G. S.; Picca, R. A.; Cioffi, N.

2017-12-01

Chemical, structural, morphological and micro-/macro-electrical properties of undoped and nitrogen-(N-)doped diamond films are determined by X-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopies, field emission scanning electron microscopy, atomic force microscopy, scanning capacitance microscopy (SCM) and two points technique for I-V characteristics, respectively. The characterization results are very useful to examine and understand the relationship among these properties. The effect of the nitrogen incorporation in diamond films is investigated through the evolution of the chemical, structural, morphological and topographical features and of the electrical behavior. The distribution of the electrical current is first assessed at millimeter scale on the surface of diamond films and then at micrometer scale on small regions in order to establish the sites where the carriers preferentially move. Specifically, the SCM images indicate a non-uniform distribution of carriers on the morphological structures mainly located along the grain boundaries. A good agreement is found by comparing the electrical currents at the micro- and macro-scale. This work aims to highlight phenomena such as photo- and thermionic emission from N-doped diamond useful for microelectronic engineering.

Optimize out-of-core thermionic energy conversion for nuclear electric propulsion

NASA Technical Reports Server (NTRS)

Morris, J. F.

1977-01-01

Current designs for out of core thermionic energy conversion (TEC) to power nuclear electric propulsion (NEP) were evaluated. Approaches to improve out of core TEC are emphasized and probabilities for success are indicated. TEC gains are available with higher emitter temperatures and greater power densities. Good potentialities for accommodating external high temperature, high power density TEC with heat pipe cooled reactors exist.

High heat-flux self-rotating plasma-facing component: Concept and loading test in TEXTOR

NASA Astrophysics Data System (ADS)

Terra, A.; Sergienko, G.; Hubeny, M.; Huber, A.; Mertens, Ph.; Philipps, V.; The Textor Team

2015-08-01

This contribution reports on the concept of a circular self-rotating and temperature self-stabilising plasma-facing component (PFC), and test of a related prototype in TEXTOR tokamak. This PFC uses the Lorentz force induced by plasma current and magnet field (J × B) to create a torque applied on metallic discs which produce a rotational movement. Additional thermionic current, present at high operation temperatures, brings additional temperature stabilisation ability. This self-rotating disk limiter was exposed to plasma in the TEXTOR tokamak under different radial positions to vary the heat flux. This disk structure shows the interesting ability to stabilise its maximum temperature through the fact that the self-induced rotation is modulated by the thermal emission current. It was observed that the rotation speed increased following both the current collected by the limiter, and the temperature of the tungsten disks.

Photophysics of C60 Colloids

DTIC Science & Technology

2012-11-28

strong linear absorption giving rise to thermionic emission, resulting in avalanche ionization and thus, nanoplasmas that absorb and scatter the...point of water is reached before the sublimation point of carbon black. If nanoplasmas are the source of the nonlinear absorption seen in CBS-1, then

High power densities from high-temperature materials interactions. [thermionic energy conversion and metallic fluid heat pipes

NASA Technical Reports Server (NTRS)

Morris, J. F.

1981-01-01

Thermionic energy converters and metallic-fluid heat pipes are well suited to serve together synergistically. The two operating cycles appear as simple and isolated as their material problems seem forebodingly deceptive and complicated. Simplified equations verify material properties and interactions as primary influences on the operational effectiveness of both. Each experiences flow limitations in thermal emission and vaporization because of temperature restrictions redounding from thermophysicochemical stability considerations. Topics discussed include: (1) successful limitation of alkali-metal corrosion; (2) protection against external hot corrosive gases; (3) coping with external and internal vaporization; (4) controlling interfacial reactions and diffusion; and (5) meeting other thermophysical challenges; expansion matches and creep.

Electron beam dynamics in an ultrafast transmission electron microscope with Wehnelt electrode.

PubMed

Bücker, K; Picher, M; Crégut, O; LaGrange, T; Reed, B W; Park, S T; Masiel, D J; Banhart, F

2016-12-01

High temporal resolution transmission electron microscopy techniques have shown significant progress in recent years. Using photoelectron pulses induced by ultrashort laser pulses on the cathode, these methods can probe ultrafast materials processes and have revealed numerous dynamic phenomena at the nanoscale. Most recently, the technique has been implemented in standard thermionic electron microscopes that provide a flexible platform for studying material's dynamics over a wide range of spatial and temporal scales. In this study, the electron pulses in such an ultrafast transmission electron microscope are characterized in detail. The microscope is based on a thermionic gun with a Wehnelt electrode and is operated in a stroboscopic photoelectron mode. It is shown that the Wehnelt bias has a decisive influence on the temporal and energy spread of the picosecond electron pulses. Depending on the shape of the cathode and the cathode-Wehnelt distance, different emission patterns with different pulse parameters are obtained. The energy spread of the pulses is determined by space charge and Boersch effects, given by the number of electrons in a pulse. However, filtering effects due to the chromatic aberrations of the Wehnelt electrode allow the extraction of pulses with narrow energy spreads. The temporal spread is governed by electron trajectories of different length and in different electrostatic potentials. High temporal resolution is obtained by excluding shank emission from the cathode and aberration-induced halos in the emission pattern. By varying the cathode-Wehnelt gap, the Wehnelt bias, and the number of photoelectrons in a pulse, tradeoffs between energy and temporal resolution as well as beam intensity can be made as needed for experiments. Based on the characterization of the electron pulses, the optimal conditions for the operation of ultrafast TEMs with thermionic gun assembly are elaborated. Copyright © 2016 Elsevier B.V. All rights reserved.

Metal-insulator-semiconductor heterostructures for plasmonic hot-carrier optoelectronics.

PubMed

García de Arquer, F Pelayo; Konstantatos, Gerasimos

2015-06-01

Plasmonic hot-electron devices are attractive candidates for light-energy harvesting and photodetection applications. For solid state devices, the most compact and straightforward architecture is the metal-semiconductor Schottky junction. However convenient, this structure introduces limitations such as the elevated dark current associated to thermionic emission, or constraints for device design due to the finite choice of materials. In this work we theoretically consider the metal-insulator-semiconductor heterojunction as a candidate for plasmonic hot-carrier photodetection and solar cells. The presence of the insulating layer can significantly reduce the dark current, resulting in increased device performance with predicted solar power conversion efficiencies up to 9%. For photodetection, the sensitivity can be extended well into the infrared by a judicious choice of the insulating layer, with up to 300-fold expected enhancement in detectivity.

Electrical characterization of n/p-type nickel silicide/silicon junctions by Sb segregation.

PubMed

Jun, Myungsim; Park, Youngsam; Hyun, Younghoon; Choi, Sung-Jin; Zyung, Taehyung; Jang, Moongyu

2011-08-01

In this paper, n/p-type nickel-silicided Schottky diodes were fabricated by incorporating antimony atoms near the nickel silicide/Si junction interface and the electrical characteristics were studied through measurements and simulations. The effective Schottky barrier height (SBH) for electron, extracted from the thermionic emission model, drastically decreased from 0.68 to less than 0.1 eV while that for hole slightly increased from 0.43 to 0.53 eV. In order to identify the current conduction mechanisms, the experimental current-temperature-voltage characteristics for the n-type diode were fitted based on various models for transport of charge carrier in Schottky diodes. As the result, the large change in effective SBH for electron is ascribed to trap-assisted tunneling rather than barrier height inhomogeneity.

Carbon Nanotube Field Emitters Synthesized on Metal Alloy Substrate by PECVD for Customized Compact Field Emission Devices to Be Used in X-Ray Source Applications.

PubMed

Park, Sangjun; Gupta, Amar Prasad; Yeo, Seung Jun; Jung, Jaeik; Paik, Sang Hyun; Mativenga, Mallory; Kim, Seung Hoon; Shin, Ji Hoon; Ahn, Jeung Sun; Ryu, Jehwang

2018-05-29

In this study, a simple, efficient, and economical process is reported for the direct synthesis of carbon nanotube (CNT) field emitters on metal alloy. Given that CNT field emitters can be customized with ease for compact and cold field emission devices, they are promising replacements for thermionic emitters in widely accessible X-ray source electron guns. High performance CNT emitter samples were prepared in optimized plasma conditions through the plasma-enhanced chemical vapor deposition (PECVD) process and subsequently characterized by using a scanning electron microscope, tunneling electron microscope, and Raman spectroscopy. For the cathode current, field emission (FE) characteristics with respective turn on (1 μA/cm²) and threshold (1 mA/cm²) field of 2.84 and 4.05 V/μm were obtained. For a field of 5.24 V/μm, maximum current density of 7 mA/cm² was achieved and a field enhancement factor β of 2838 was calculated. In addition, the CNT emitters sustained a current density of 6.7 mA/cm² for 420 min under a field of 5.2 V/μm, confirming good operational stability. Finally, an X-ray generated image of an integrated circuit was taken using the compact field emission device developed herein.

On thermionic emission and the use of vacuum tubes in the advanced physics laboratory

NASA Astrophysics Data System (ADS)

Angiolillo, Paul J.

2009-12-01

Two methods are outlined for measuring the charge-to-mass ratio e /me of the electron using thermionic emission as exploited in vacuum tube technology. One method employs the notion of the space charge in the vacuum tube diode as described by the Child-Langmuir equation; the other method uses the electron trajectories in vacuum tube pentodes with cylindrical electrodes under conditions of orthogonally related electric and magnetic fields (the Hull magnetron method). The vacuum diode method gave e /me=1.782±0.166×10+11 C/kg (averaged over the vacuum diodes studied), and the Hull magnetron method gave e /me=1.779±0.208×10+11 C/kg (averaged over both pentodes and the anode voltages studied). These methods afford opportunities for students to determine the e /me ratio without using the Bainbridge tube method and to become familiar with phenomena not normally covered in a typical experimental methods curriculum.

Development of a high current 60 keV neutral lithium beam injector for beam emission spectroscopy measurements on fusion experiments.

PubMed

Anda, G; Dunai, D; Lampert, M; Krizsanóczi, T; Németh, J; Bató, S; Nam, Y U; Hu, G H; Zoletnik, S

2018-01-01

A 60 keV neutral lithium beam system was designed and built up for beam emission spectroscopy measurement of edge plasma on the KSTAR and EAST tokamaks. The electron density profile and its fluctuation can be measured using the accelerated lithium beam-based emission spectroscopy system. A thermionic ion source was developed with a SiC heater to emit around 4-5 mA ion current from a 14 mm diameter surface. The ion optic is following the 2 step design used on other devices with small modifications to reach about 2-3 cm beam diameter in the plasma at about 4 m from the ion source. A newly developed recirculating sodium vapour neutralizer neutralizes the accelerated ion beam at around 260-280 °C even during long (<20 s) discharges. A set of new beam diagnostic and manipulation techniques are applied to allow optimization, aiming, cleaning, and beam modulation. The maximum 60 keV beam energy with 4 mA ion current was successfully reached at KSTAR and at EAST. Combined with an efficient observation system, the Li-beam diagnostic enables the measurement of the density profile and fluctuations on the plasma turbulence time scale.

Development of a high current 60 keV neutral lithium beam injector for beam emission spectroscopy measurements on fusion experiments

NASA Astrophysics Data System (ADS)

Anda, G.; Dunai, D.; Lampert, M.; Krizsanóczi, T.; Németh, J.; Bató, S.; Nam, Y. U.; Hu, G. H.; Zoletnik, S.

2018-01-01

A 60 keV neutral lithium beam system was designed and built up for beam emission spectroscopy measurement of edge plasma on the KSTAR and EAST tokamaks. The electron density profile and its fluctuation can be measured using the accelerated lithium beam-based emission spectroscopy system. A thermionic ion source was developed with a SiC heater to emit around 4-5 mA ion current from a 14 mm diameter surface. The ion optic is following the 2 step design used on other devices with small modifications to reach about 2-3 cm beam diameter in the plasma at about 4 m from the ion source. A newly developed recirculating sodium vapour neutralizer neutralizes the accelerated ion beam at around 260-280 °C even during long (<20 s) discharges. A set of new beam diagnostic and manipulation techniques are applied to allow optimization, aiming, cleaning, and beam modulation. The maximum 60 keV beam energy with 4 mA ion current was successfully reached at KSTAR and at EAST. Combined with an efficient observation system, the Li-beam diagnostic enables the measurement of the density profile and fluctuations on the plasma turbulence time scale.

Gridded thermionic gun and integral superconducting ballistic bunch compression cavity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schultheiss, Thomas

Electron-Ion colliders such as the Medium energy Electron Ion Collider (MEIC) being developed by JLAB require high current electrons with low energy spread for electron cooling of the collider ring. Accelerator techniques for improving bunch charge, average current, emittance, and energy spread are required for Energy Recovery Linacs (ERLs) and Circulator Rings (CR) for next generation colliders for nuclear physics experiments. Example candidates include thermionic-cathode electron guns with RF accelerating structures. Thermionic cathodes are known to produce high currents and have excellent lifetime. The success of the IR and THz Free-Electron Laser (FEL) designed and installed by Advanced Energy Systemsmore » at the Fritz Haber Institute (FHI) of the Max Planck Society in Berlin [1,2] demonstrates that gridded thermionic cathodes and rf systems be considered for next generation collider technology. In Phase 1 Advanced Energy Systems (AES) developed and analyzed a design concept using a superconducting cavity pair and gridded thermionic cathode. Analysis included Beam Dynamics and thermal analysis to show that a design of this type is feasible. The latest design goals for the MEIC electron cooler were for electron bunches of 420 pC at a frequency of 952.6 MHz with a magnetic field on the cathode of 2kG. This field magnetizes the beam imparting angular momentum that provides for helical motion of the electrons in the cooling solenoid. The helical motion increases the interaction time and improves the cooling efficiency. A coil positioned around the cathode providing 2kG field was developed. Beam dynamics simulations were run to develop the particle dynamics near the cathode and grid. Lloyd Young added capability to Tstep to include space charge effects between two plates and include image charge effects from the grid. He also added new pepper-pot geometry capability to account for honeycomb grids. These additions were used to develop the beam dynamics for this gun. The general design is a modified ballistic compression cavity pair with two independently powered cells [3]. The first is a cathode cell that includes the thermionic cathode and grid to provide for beam bunching. The second is a full cell with independent phasing and field levels designed to minimize energy spread. The primary goal for Phase II is to manufacture a superconducting gun with a thermionic cathode and imbedded coil. The system developed here is applicable to many high current electron accelerators. The analysis and design constraints imposed by the magnetized cathode make the cathode system developed here more complicated and limited than one without the magnetized beam constraints. High power ERLs would benefit by a gun with the capabilities shown here, 400 mA or more of current. ERLs hold great promise for electron cooling experiments, advanced light sources and Free Electron Lasers. This high current electron injector is a technological advance that will place the requirements for an ERL capable of providing quality bunches needed for cooling within the MEIC circulator ring within reach. This injector would have application to future ERLs around the world.« less

A description of the new 3D electron gun and collector modeling tool: MICHELLE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Petillo, J.; Mondelli, A.; Krueger, W.

1999-07-01

A new 3D finite element gun and collector modeling code is under development at SAIC in collaboration with industrial partners and national laboratories. This development program has been designed specifically to address the shortcomings of current simulation and modeling tools. In particular, although there are 3D gun codes that exist today, their ability to address fine scale features is somewhat limited in 3D due to disparate length scales of certain classes of devices. Additionally, features like advanced emission rules, including thermionic Child's law and comprehensive secondary emission models also need attention. The program specifically targets problems classes including gridded-guns, sheet-beammore » guns, multi-beam devices, and anisotropic collectors. The presentation will provide an overview of the program objectives, the approach to be taken by the development team, and a status of the project.« less

Simulation of real I-V characteristics of metal/GaN/AlGaN heterostructure based on the 12-EXT model of trap-assisted tunnelling

NASA Astrophysics Data System (ADS)

Racko, Juraj; Benko, Peter; Mikolášek, Miroslav; Granzner, Ralf; Kittler, Mario; Schwierz, Frank; Harmatha, Ladislav; Breza, Juraj

2017-02-01

The contribution employs electrical simulation to assess the effect of the distribution of aluminium in the metal/GaN/AlGaN heterostructure on the leakage current. The heterostructure is characterized by a high density of traps causing an increase of the leakage current consisting of the thermionic emission component and of a non-negligible contribution of trap-assisted tunnelling. The leakage current is highly sensitive to the bending of the potential barrier Ec in the subsurface region of the GaN/AlGaN structure. The band bending is strongly affected by the sheet bound charge at the first GaN/AlGaN/GaN interface due to spontaneous and piezoelectric polarization. The overall charge depends on the concentration of Al, the distribution of Al at the first heterointerface having a strong effect on the formation of the potential barrier.

NUCLEAR REACTOR AND THERMIONIC FUEL ELEMENT THEREFOR

DOEpatents

Rasor, N.S.; Hirsch, R.L.

1963-12-01

The patent relates to the direct conversion of fission heat to electricity by use of thermionic plasma diodes having fissionable material cathodes, said diodes arranged to form a critical mass in a nuclear reactor. The patent describes a fuel element comprising a plurality of diodes each having a fissionable material cathode, an anode around said cathode, and an ionizable gas therebetween. Provision is made for flowing the gas and current serially through the diodes. (AEC)

Carbon-containing cathodes for enhanced electron emission

DOEpatents

Cao, Renyu; Pan, Lawrence; Vergara, German; Fox, Ciaran

2000-01-01

A cathode has electropositive atoms directly bonded to a carbon-containing substrate. Preferably, the substrate comprises diamond or diamond-like (sp.sup.3) carbon, and the electropositive atoms are Cs. The cathode displays superior efficiency and durability. In one embodiment, the cathode has a negative electron affinity (NEA). The cathode can be used for field emission, thermionic emission, or photoemission. Upon exposure to air or oxygen, the cathode performance can be restored by annealing or other methods. Applications include detectors, electron multipliers, sensors, imaging systems, and displays, particularly flat panel displays.

In situ current-voltage characterization of swift heavy ion irradiated Au/n-GaAs Schottky diode at low temperature

NASA Astrophysics Data System (ADS)

Singh, R.; Arora, S. K.; Singh, J. P.; Kanjilal, D.

A Au/n-GaAs(100) Schottky diode was irradiated at 80 K by a 180 MeV Ag-107(14+) ion beam. In situ current-voltage (I--V) characterization of the diode was performed at various irradiation fluences ranging from 1x10(10) to 1x10(13) ions cm(-2) . The semiconductor was heavily doped (carrier concentration=1x10(18) cm(-3)), hence thermionic field emission was assumed to be the dominant current transport mechanism in the diode. Systematic variations in various parameters of the Schottky diode like characteristic energy E-0 , ideality factor n , reverse saturation current I-S , flatband barrier height Phi(bf) and reverse leakage current I-R have been observed with respect to the irradiation fluence. The nuclear and electronic energy losses of the swift heavy ion affect the interface state density at the metal-semiconductor interface resulting in observed variations in Schottky diode parameters.

Analysis of Schottky Barrier Parameters and Current Transport Properties of V/p-Type GaN Schottky Junction at Low Temperatures

NASA Astrophysics Data System (ADS)

Asha, B.; Harsha, Cirandur Sri; Padma, R.; Rajagopal Reddy, V.

2018-05-01

The electrical characteristics of a V/p-GaN Schottky junction have been investigated by current-voltage (I-V) and capacitance-voltage (C-V) characteristics under the assumption of the thermionic emission (TE) theory in the temperature range of 120-280 K with steps of 40 K. The zero-bias barrier height (ΦB0), ideality factor (n), flat-band barrier height (ΦBF) and series resistance (R S) values were evaluated and were found to be strongly temperature dependent. The results revealed that the ΦB0 values increase, whereas n, ΦFB and R S values decrease, with increasing temperature. Using the conventional Richardson plot, the mean barrier height (0.39 eV) and Richardson constant (8.10 × 10-10 Acm-2 K-2) were attained. The barrier height inhomogeneities were demonstrated by assuming a Gaussian distribution function. The interface state density (N SS) values were found to decrease with increasing temperature. The reverse leakage current mechanism of the V/p-GaN Schottky junction was found to be governed by Poole-Frenkel emission at all temperatures.

Work function and surface stability of tungsten-based thermionic electron emission cathodes

NASA Astrophysics Data System (ADS)

Jacobs, Ryan; Morgan, Dane; Booske, John

2017-11-01

Materials that exhibit a low work function and therefore easily emit electrons into vacuum form the basis of electronic devices used in applications ranging from satellite communications to thermionic energy conversion. W-Ba-O is the canonical materials system that functions as the thermionic electron emitter commercially used in a range of high-power electron devices. However, the work functions, surface stability, and kinetic characteristics of a polycrystalline W emitter surface are still not well understood or characterized. In this study, we examined the work function and surface stability of the eight lowest index surfaces of the W-Ba-O system using density functional theory methods. We found that under the typical thermionic cathode operating conditions of high temperature and low oxygen partial pressure, the most stable surface adsorbates are Ba-O species with compositions in the range of Ba0.125O-Ba0.25O per surface W atom, with O passivating all dangling W bonds and Ba creating work function-lowering surface dipoles. Wulff construction analysis reveals that the presence of O and Ba significantly alters the surface energetics and changes the proportions of surface facets present under equilibrium conditions. Analysis of previously published data on W sintering kinetics suggests that fine W particles in the size range of 100-500 nm may be at or near equilibrium during cathode synthesis and thus may exhibit surface orientation fractions well described by the calculated Wulff construction.

Strong and reversible modulation of carbon nanotube-silicon heterojunction solar cells by an interfacial oxide layer.

PubMed

Jia, Yi; Cao, Anyuan; Kang, Feiyu; Li, Peixu; Gui, Xuchun; Zhang, Luhui; Shi, Enzheng; Wei, Jinquan; Wang, Kunlin; Zhu, Hongwei; Wu, Dehai

2012-06-21

Deposition of nanostructures such as carbon nanotubes on Si wafers to make heterojunction structures is a promising route toward high efficiency solar cells with reduced cost. Here, we show a significant enhancement in the cell characteristics and power conversion efficiency by growing a silicon oxide layer at the interface between the nanotube film and Si substrate. The cell efficiency increases steadily from 0.5% without interfacial oxide to 8.8% with an optimal oxide thickness of about 1 nm. This systematic study reveals that formation of an oxide layer switches charge transport from thermionic emission to a mixture of thermionic emission and tunneling and improves overall diode properties, which are critical factors for tailoring the cell behavior. By controlled formation and removal of interfacial oxide, we demonstrate oscillation of the cell parameters between two extreme states, where the cell efficiency can be reversibly altered by a factor of 500. Our results suggest that the oxide layer plays an important role in Si-based photovoltaics, and it might be utilized to tune the cell performance in various nanostructure-Si heterojunction structures.

Tunneling Electroresistance Effect with Diode Characteristic for Cross-Point Memory.

PubMed

Lee, Hong-Sub; Park, Hyung-Ho

2016-06-22

Cross-point memory architecture (CPMA) by using memristors has attracted considerable attention because of its high-density integration. However, a common and significant drawback of the CPMA is related to crosstalk issues between cells by sneak currents. This study demonstrated the sneak current free resistive switching characteristic of a ferroelectric tunnel diode (FTD) memristor for a CPMA by utilizing a novel concept of a ferroelectric quadrangle and triangle barrier switch. A FTD of Au/BaTiO3 (5 nm)/Nb-doped SrTiO3 (100) was used to obtain a desirable memristive effect for the CPMA. The FTD could reversibly change the shape of the ferroelectric potential from a quadrangle to a triangle. The effect included high nonlinearity and diode characteristics. It was derived from utilizing different sequences of carrier transport mechanisms such as the direct tunneling current, Fowler-Nordheim tunneling, and thermionic emission. The FTD memristor demonstrated the feasibility of sneak current-free high-density CPMA.

On the AlGaInP-bulk and AlGaInP/GaAs-superlattice confinement effects for heterostructure-emitter bipolar transistors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tsai, Jung-Hui, E-mail: jhtsai@nknucc.nknu.edu.tw

2015-02-09

The confinement effect and electrical characteristics of heterostructure-emitter bipolar transistors with an AlGaInP bulk-confinement layer and an AlGaInP/GaAs superlattice-confinement layer are first demonstrated and compared by experimentally results. In the two devices, the relatively large valence band discontinuity at AlGaInP/GaAs heterojunction provides excellent confinement effect for holes to enhance current gain. As to the AlGaInP/GaAs superlattice-confinement device, part of thermionic-emission electrons will be trapped in the GaAs quantum wells of the superlattice. This will result in lower collector current and current gain as compared with the bulk-confinement device. Nevertheless, the superlattice-confinement device exhibits a larger current-gain cutoff frequency, which canmore » be attributed that the tunneling behavior is included in the carrier transportation and transporting time across the emitter region could be substantially reduced.« less

Satellite Charge Control with Lithium Ion Source and Electron Emission

DTIC Science & Technology

1990-12-01

for the spacecraft charge control. C. THERMIONIC ELECTRON EMISSION Electrons may be emitted by surfaces at high temperature in a process, called...data in the high voltage region and 1300 to 1600 °K temperature range may be fitted to the following equation, for a 50 % lithium sample: log01 =logos...in Figure 15, is similar to a high - temperature quartz structure, yet differs from it in that half of the silicon atoms are repiaced by aluminum atoms

Application of CaCu3Ti4O12 based quadruple perovskites as a promising candidate for optoelectronic devices

NASA Astrophysics Data System (ADS)

Pal, Kamalesh; Jana, Rajkumar; Dey, Arka; Ray, Partha P.; Seikh, Md Motin; Gayen, Arup

2018-05-01

We report the synthesis of nanosized (40-50 nm) CaCu3-xMnxTi4-xMnxO12 (x = 0, 0.5 and 1) quadruple perovskite (QP) semiconductor via a modified combustion method for use as Schottky barrier diode (SBD) at the Al/QP junction. The fabricated SBD is analysed on the basis of thermionic emission theory to observe its quality and some important diode parameters. For insight analysis of charge transport mechanism through metal-semiconductor junction, theory of space charge limited currents is applied and discussed in the light of parameters like carrier concentration, mobility-lifetime product and diffusion length. The Mn-doped exhibit better device performance compared to parent material.

Effect of Cl2 plasma treatment and annealing on vanadium based metal contacts to Si-doped Al0.75Ga0.25N

NASA Astrophysics Data System (ADS)

Lapeyrade, Mickael; Alamé, Sabine; Glaab, Johannes; Mogilatenko, Anna; Unger, Ralph-Stephan; Kuhn, Christian; Wernicke, Tim; Vogt, Patrick; Knauer, Arne; Zeimer, Ute; Einfeldt, Sven; Weyers, Markus; Kneissl, Michael

2017-09-01

In order to understand the electrical properties of V/Al/Ni/Au metal contacts to Si-doped Al0.75Ga0.25N layers, X-ray photoelectron spectroscopy analysis was performed on differently treated AlGaN:Si surfaces before metal deposition, and transmission electron microscopy was used to study the semiconductor-metal interface after contact annealing at 900 °C. Cl2 plasma etching of AlGaN increases the aluminum/nitrogen ratio at the surface, and Al oxide or oxynitride is always formed by any surface treatment applied after etching. After contact annealing, a complex interface structure including amorphous AlOx and different metal phases such as Al-Au-Ni, V-Al, and V2N were found. The electrical properties of the contacts were determined by thermionic emission and/or thermionic field emission in the low voltage regime. Nearly ohmic contacts on AlGaN surfaces exposed to a Cl2 plasma were only obtained by annealing the sample at a temperature of 815 °C under N2/NH3 prior to metallization. By this treatment, the oxygen contamination on the surface could be minimized, resulting in a larger semiconductor area to be in direct contact with metal phases such as Al-rich Al-Au-Ni or V-Al and leading to a contact resistivity of 2.5 × 10-2 Ω cm2. This treatment can be used to significantly reduce the operating voltage of current deep ultraviolet light emitting diodes which will increase their wall plug efficiency and lower the thermal stress during their operation.

Detection of alpha particles using DNA/Al Schottky junctions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Al-Ta'ii, Hassan Maktuff Jaber, E-mail: hassankirkukly@gmail.com, E-mail: vengadeshp@um.edu.my; Department of Physics, Faculty of Science, University of Al-Muthana, Al-Muthana 66001; Periasamy, Vengadesh, E-mail: hassankirkukly@gmail.com, E-mail: vengadeshp@um.edu.my

2015-09-21

Deoxyribonucleic acid or DNA can be utilized in an organic-metallic rectifying structure to detect radiation, especially alpha particles. This has become much more important in recent years due to crucial environmental detection needs in both peace and war. In this work, we fabricated an aluminum (Al)/DNA/Al structure and generated current–voltage characteristics upon exposure to alpha radiation. Two models were utilized to investigate these current profiles; the standard conventional thermionic emission model and Cheung and Cheung's method. Using these models, the barrier height, Richardson constant, ideality factor and series resistance of the metal-DNA-metal structure were analyzed in real time. The barriermore » height, Φ value calculated using the conventional method for non-radiated structure was 0.7149 eV, increasing to 0.7367 eV after 4 min of radiation. Barrier height values were observed to increase after 20, 30 and 40 min of radiation, except for 6, 8, and 10 min, which registered a decrease of about 0.67 eV. This was in comparison using Cheung and Cheung's method, which registered 0.6983 eV and 0.7528 eV for the non-radiated and 2 min of radiation, respectively. The barrier height values, meanwhile, were observed to decrease after 4 (0.61 eV) to 40 min (0.6945 eV). The study shows that conventional thermionic emission model could be practically utilized for estimating the diode parameters including the effect of series resistance. These changes in the electronic properties of the Al/DNA/Al junctions could therefore be utilized in the manufacture of sensitive alpha particle sensors.« less

Nano-scale investigations of electric-dipole-layer enhanced field and thermionic emission from high current density cathodes

NASA Astrophysics Data System (ADS)

Vlahos, Vasilios

Cesium iodide coated graphitic fibers and scandate cathodes are two important electron emission technologies. The coated fibers are utilized as field emitters for high power microwave sources. The scandate cathodes are promising thermionic cathode materials for pulsed power vacuum electron devices. This work attempts to understand the fundamental physical and chemical relationships between the atomic structure of the emitting cathode surfaces and the superior emission characteristics of these cathodes. Ab initio computational modeling in conjunction with experimental investigations was performed on coated fiber cathodes to understand the origin of their very low turn on electric field, which can be reduced by as much as ten-fold compared to uncoated fibers. Copious amounts of cesium and oxygen were found co-localized on the fiber, but no iodine was detected on the surface. Additional ab initio studies confirmed that cesium oxide dimers could lower the work function significantly. Surface cesium oxide dipoles are therefore proposed as the source of the observed reduction in the turn on electric field. It is also proposed that emission may be further enhanced by secondary electrons from cesium oxide during operation. Thermal conditioning of the coated cathode may be a mechanism by which surface cesium iodide is converted into cesium oxide, promoting the depletion of iodine by formation of volatile gas. Ab initio modeling was also utilized to investigate the stability and work functions of scandate structures. The work demonstrated that monolayer barium-scandium-oxygen surface structures on tungsten can dramatically lower the work function of the underlying tungsten substrate from 4.6 eV down to 1.16 eV, by the formation of multiple surface dipoles. On the basis of this work, we conclude that high temperature kinetics force conventional dispenser cathodes (barium-oxygen monolayers on tungsten) to operate in a non-equilibrium compositional steady state with higher than optimal work functions of ˜2 eV. We hypothesize that scandium enables the barium-oxygen surface monolayer kinetics to access a more thermodynamically stable phase with reported work functions as low as ˜1.3 eV.

Planar edge Schottky barrier-tunneling transistors using epitaxial graphene/SiC junctions.

PubMed

Kunc, Jan; Hu, Yike; Palmer, James; Guo, Zelei; Hankinson, John; Gamal, Salah H; Berger, Claire; de Heer, Walt A

2014-09-10

A purely planar graphene/SiC field effect transistor is presented here. The horizontal current flow over one-dimensional tunneling barrier between planar graphene contact and coplanar two-dimensional SiC channel exhibits superior on/off ratio compared to conventional transistors employing vertical electron transport. Multilayer epitaxial graphene (MEG) grown on SiC(0001̅) was adopted as the transistor source and drain. The channel is formed by the accumulation layer at the interface of semi-insulating SiC and a surface silicate that forms after high vacuum high temperature annealing. Electronic bands between the graphene edge and SiC accumulation layer form a thin Schottky barrier, which is dominated by tunneling at low temperatures. A thermionic emission prevails over tunneling at high temperatures. We show that neglecting tunneling effectively causes the temperature dependence of the Schottky barrier height. The channel can support current densities up to 35 A/m.

Intrinsic inhomogeneous barrier height at the n-TiO2/p-Si hole-blocking junction

NASA Astrophysics Data System (ADS)

Kumar, Mohit; Singh, Ranveer; Som, Tapobrata

2018-01-01

Using Kelvin probe force microscopy (KPFM) and temperature-dependent current-voltage characteristics, we study the charge transport across an n-TiO2/p-Si heterojunction. In particular, the KPFM result shows a variation in the work function at the TiO2 surface. On the other hand, temperature-dependent current-voltage characteristics depict a non-ideal hole-blocking behaviour of the same. In addition, the measured barrier height is found to decrease with temperature and does not follow the thermionic emission theory, strongly suggesting an inhomogeneous nature of the barrier. The observed barrier inhomogeneity is attributed to the nanoscale height modulation, arising due to the growth dynamics of TiO2 and corroborates well with the KPFM map. The presented results will open a new avenue to understand the charge transport in TiO2-based nanoscale devices.

Thermion: Verification of a thermionic heat pipe in microgravity

NASA Technical Reports Server (NTRS)

1991-01-01

The design and development is examined of a small excore heat pipe thermionic space nuclear reactor power system (SEHPTR). The need was identified for an in-space flight demonstration of a solar powered, thermionic heat pipe element. A demonstration would examine its performance and verify its operation in microgravity. The design of a microsatellite based technology demonstration experiment is proposed to measure the effects of microgravity on the performance of an integrated thermionic heat pipe device in low earth orbit. The specific objectives are to verify the operation of the liquid metal heat pipe and the cesium reservior in the space environment. Two design configurations are described; THERMION-I and THERMION-II. THERMION-I is designed for a long lifetime study of the operations of the thermionic heat pipe element in low earth orbit. Heat input to the element is furnished by a large mirror which collects solar energy and focuses it into a cavity containing the heat pipe device. THERMION-II is a much simpler device which is used for short term operation. This experiment remains attached to the Delta II second stage and uses energy from 500 lb of alkaline batteries to supply heat energy to the heat pipe device.

Research of thermionic converter collector properties in model experiments with surface control

NASA Astrophysics Data System (ADS)

Agafonov, Valerii R.; Vizgalov, Anatolii V.; Iarygin, Valerii I.

Consideration was given to a possible scheme of phenomena on electrodes leading to changes in emission properties (EP) of a thermionic converter (TEC) collector. It was based on technology and materials typical of the TOPAZ-type reactor-converter (TRC). The element composition (EC), near-surface layer (NSL) structure, and work function (WF) of a collector made from niobium-based polycrystal alloy were studied within this scheme experimentally. The influence of any media except for the interelectrode gap (IEG) medium was excluded when investigating the effect of thermovacuum treatment (TVT) as well as the influence of carbon monoxide, hydrogen, and methane on the NSL characteristics. Experimental data and analytical estimates of the impact of fission products of the nuclear fuel on collector EP are presented. The calculation of possible TRC electrical power decrease was also carried out.

Delayed Ionization in Transition Metal Carbon Clusters

NASA Astrophysics Data System (ADS)

Kooi, S. E.; Castleman, A. W., Jr.

1997-03-01

Mass spectrometric studies of several single and binary transition metal carbon cluster systems, produced in a laser vaporization source, reveal several species that undergo delayed ionization. Pulsed extraction and blocking electric fields, in a time-of-flight mass spectrometer, allow the study of delayed ionization over a time window after excitation with a pulsed laser. In systems where metallocarbohedrenes (Met-Cars) are produced, the Met-Cars are the dominate delayed species. Delayed ionization of binary metal Met-Cars Ti_xM_yC_12 (M=Zr,Nb,Y; x+y=8) is dependent on the ratio of the two metals. Delayed behavior is investigated over a range of photoionization wavelengths and fluences. In order to determine the degree to which the delayed ionization is thermionic in character, the experimental data have been compared to Klots's model for thermionic emission from small particles.

Experimental analysis of the Schottky barrier height of metal contacts in black phosphorus field-effect transistors

NASA Astrophysics Data System (ADS)

Chang, Hsun-Ming; Fan, Kai-Lin; Charnas, Adam; Ye, Peide D.; Lin, Yu-Ming; Wu, Chih-I.; Wu, Chao-Hsin

2018-04-01

Compared to graphene and MoS2, studies on metal contacts to black phosphorus (BP) transistors are still immature. In this work, we present the experimental analysis of titanium contacts on BP based upon the theory of thermionic emssion. The Schottky barrier height (SBH) is extracted by thermionic emission methods to analyze the properties of Ti-BP contact. To examine the results, the band gap of BP is extracted followed by theoretical band alignment by Schottky-Mott rule. However, an underestimated SBH is found due to the hysteresis in electrical results. Hence, a modified SBH extraction for contact resistance that avoids the effects of hysteresis is proposed and demonstrated, showing a more accurate SBH that agrees well with theoretical value and results of transmission electron microscopy and energy-dispersive x-ray spectroscopy.

Influence of etching current density on microstructural, optical and electrical properties of porous silicon (PS):n-Si heterostructure

NASA Astrophysics Data System (ADS)

Das, M.; Nath, P.; Sarkar, D.

2016-02-01

In this article effect of etching current density (J) on the microstructural, optical and electrical properties of photoelectrochemically prepared heterostructure is reported. Prepared samples are characterized by FESEM, XRD, UV-Visible, Raman and photoluminescence (PL) spectra and current-voltage (I-V) characteristics. FESEM shows presence of mixture of randomly distributed meso- and micro-pores. Porous layer thickness determined by cross section view of SEM is proportional to J. XRD shows crystalline nature but gradually extent of crystallinity decreases with increasing J. Raman spectra show large red-shift and asymmetric broadening with respect to crystalline silicon (c-Si). UV-visible reflectance and PL show blue shift in peaks with increasing J. The I-V characteristics are analyzed by the conventional thermionic emission (TE) model and Cheung's model to estimate the barrier height (φb), ideality factor (n) and series resistance (Rs) for comparison between the two models. The latter model is found to fit better.

Thermionic nuclear reactor with internal heat distribution and multiple duct cooling

DOEpatents

Fisher, C.R.; Perry, L.W. Jr.

1975-11-01

A Thermionic Nuclear Reactor is described having multiple ribbon-like coolant ducts passing through the core, intertwined among the thermionic fuel elements to provide independent cooling paths. Heat pipes are disposed in the core between and adjacent to the thermionic fuel elements and the ribbon ducting, for the purpose of more uniformly distributing the heat of fission among the thermionic fuel elements and the ducts.

Tuning of Schottky barrier height of Al/n-Si by electron beam irradiation

NASA Astrophysics Data System (ADS)

Vali, Indudhar Panduranga; Shetty, Pramoda Kumara; Mahesha, M. G.; Petwal, V. C.; Dwivedi, Jishnu; Choudhary, R. J.

2017-06-01

The effect of electron beam irradiation (EBI) on Al/n-Si Schottky diode has been studied by I-V characterization at room temperature. The behavior of the metal-semiconductor (MS) interface is analyzed by means of variations in the MS contact parameters such as, Schottky barrier height (ΦB), ideality factor (n) and series resistance (Rs). These parameters were found to depend on the EBI dose having a fixed incident beam of energy 7.5 MeV. At different doses (500, 1000, 1500 kGy) of EBI, the Schottky contacts were prepared and extracted their contact parameters by applying thermionic emission and Cheung models. Remarkably, the tuning of ΦB was observed as a function of EBI dose. The improved n with increased ΦB is seen for all the EBI doses. As a consequence of which the thermionic emission is more favored. However, the competing transport mechanisms such as space charge limited emission, tunneling and tunneling through the trap states were ascribed due to n > 1. The analysis of XPS spectra have shown the presence of native oxide and increased radiation induced defect states. The thickness variation in the MS interface contributing to Schottky contact behavior is discussed. This study explains a new technique to tune Schottky contact parameters by metal deposition on the electron beam irradiated n-Si wafers.

Detailed modeling of electron emission for transpiration cooling of hypersonic vehicles

NASA Astrophysics Data System (ADS)

Hanquist, Kyle M.; Hara, Kentaro; Boyd, Iain D.

2017-02-01

Electron transpiration cooling (ETC) is a recently proposed approach to manage the high heating loads experienced at the sharp leading edges of hypersonic vehicles. Computational fluid dynamics (CFD) can be used to investigate the feasibility of ETC in a hypersonic environment. A modeling approach is presented for ETC, which includes developing the boundary conditions for electron emission from the surface, accounting for the space-charge limit effects of the near-wall plasma sheath. The space-charge limit models are assessed using 1D direct-kinetic plasma sheath simulations, taking into account the thermionically emitted electrons from the surface. The simulations agree well with the space-charge limit theory proposed by Takamura et al. for emitted electrons with a finite temperature, especially at low values of wall bias, which validates the use of the theoretical model for the hypersonic CFD code. The CFD code with the analytical sheath models is then used for a test case typical of a leading edge radius in a hypersonic flight environment. The CFD results show that ETC can lower the surface temperature of sharp leading edges of hypersonic vehicles, especially at higher velocities, due to the increase in ionized species enabling higher electron heat extraction from the surface. The CFD results also show that space-charge limit effects can limit the ETC reduction of surface temperatures, in comparison to thermionic emission assuming no effects of the electric field within the sheath.

Photon-enhanced thermionic emission from p-GaAs with nonequilibrium Cs overlayers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhuravlev, A. G.; Romanov, A. S.; Alperovich, V. L., E-mail: alper@isp.nsc.ru

2014-12-22

Photon-enhanced thermionic emission (PETE), which is promising for increasing the efficiency of solar energy conversion, is studied during cesium deposition on the As- and Ga-rich p-GaAs(001) surfaces and subsequent relaxation in the nonequilibrium Cs overlayer by means of photoemission quantum yield spectroscopy adapted for systems with time-variable parameters. Along with direct photoemission of “hot” electrons excited by light above the vacuum level, the spectra contain PETE contribution of “thermalized” electrons, which are excited below the vacuum level and emit in vacuum due to thermalization up in energy by phonon absorption. Comparing the measured and calculated spectra, the effective electron affinitymore » and escape probabilities of hot and thermalized electrons are obtained as functions of submonolayer Cs coverage. The minima in the affinity and pronounced peaks in the escape probabilities are observed for Cs deposition on both the As- and Ga-rich surfaces. Possible reasons for the low mean values of the electron escape probabilities and for the observed enhancement of the probabilities at certain Cs coverages are discussed, along with the implications for the PETE device realization.« less

Performance of a carbon nanotube field emission electron gun

NASA Astrophysics Data System (ADS)

Getty, Stephanie A.; King, Todd T.; Bis, Rachael A.; Jones, Hollis H.; Herrero, Federico; Lynch, Bernard A.; Roman, Patrick; Mahaffy, Paul

2007-04-01

A cold cathode field emission electron gun (e-gun) based on a patterned carbon nanotube (CNT) film has been fabricated for use in a miniaturized reflectron time-of-flight mass spectrometer (RTOF MS), with future applications in other charged particle spectrometers, and performance of the CNT e-gun has been evaluated. A thermionic electron gun has also been fabricated and evaluated in parallel and its performance is used as a benchmark in the evaluation of our CNT e-gun. Implications for future improvements and integration into the RTOF MS are discussed.

Ultra-high vacuum photoelectron linear accelerator

DOEpatents

Yu, David U.L.; Luo, Yan

2013-07-16

An rf linear accelerator for producing an electron beam. The outer wall of the rf cavity of said linear accelerator being perforated to allow gas inside said rf cavity to flow to a pressure chamber surrounding said rf cavity and having means of ultra high vacuum pumping of the cathode of said rf linear accelerator. Said rf linear accelerator is used to accelerate polarized or unpolarized electrons produced by a photocathode, or to accelerate thermally heated electrons produced by a thermionic cathode, or to accelerate rf heated field emission electrons produced by a field emission cathode.

Effect of doping on all TMC vertical heterointerfaces

NASA Astrophysics Data System (ADS)

Nair, Salil; Joy, Jolly; Patel, K. D.; Pataniya, Pratik; Solanki, G. K.; Pathak, V. M.; Sumesh, C. K.

2018-05-01

The present work reports the growth and basic characterizations of GeSePbx (x=0, 0.02, 0.04) layered mono chalcogenide single crystal substrates for preparation of heterojunction devices. These crystals are grown by Direct Vapour Transport (DVT) Technique [1,2]. Heterojunction interfaces on these substrates are prepared using thermal evaporation of nanocrystalline SnSe thin films having 5kÅ thickness. The electrical characterizations reveal the rectifying behavior of the devices based on which its ideality factor, barrier height, saturation current, series resistance etc. have been determined using thermionic emission model [3,4]. The device parameters have been determined and analyzed by three different methods viz. LnI-V, Cheung's method and Norde method [5]. The variation in the device parameters in light of doping is reported in the present work.

Elemental boron-doped p(+)-SiGe layers grown by molecular beam epitaxy for infrared detector applications

NASA Technical Reports Server (NTRS)

Lin, T. L.; George, T.; Jones, E. W.; Ksendzov, A.; Huberman, M. L.

1992-01-01

SiGe/Si heterojunction internal photoemission (HIP) detectors have been fabricated utilizing molecular beam epitaxy of p(+)-SiGe layers on p(-)-Si substrates. Elemental boron from a high-temperature effusion cell was used as the dopant source during MBE growth, and high doping concentrations have been achieved. Strong infrared absorption, mainly by free-carrier absorption, was observed for the degenerately doped SiGe layers. The use of elemental boron as the dopant source allows a low MBE growth temperature, resulting in improved crystalline quality and smooth surface morphology of the Si(0.7)Ge(0.3) layers. Nearly ideal thermionic emission dark current characteristics have been obtained. Photoresponse of the HIP detectors in the long-wavelength infrared regime has been demonstrated.

Mechanism of carrier injection in (Ni/Au)/p-AlxGa1-xN:Mg(0<=x<0.1) Ohmic contacts

NASA Astrophysics Data System (ADS)

Nikishin, S.; Chary, I.; Borisov, B.; Kuryatkov, V.; Kudryavtsev, Yu.; Asomoza, R.; Karpov, S. Yu.; Holtz, M.

2009-10-01

We report the mechanism of current injection in (Ni/Au)/p-AlxGa1-xN:Mg(0≤x<0.1) Ohmic contacts based on the temperature dependence of hole concentrations (p) and specific contact resistance (ρc). The injection mechanism is found to be thermionic emission in all cases. A model is developed to describe the temperature dependences of p and ρc for Mg concentrations from 1019 to 1020 cm-3. The model takes into account splitting in the valence band structure, hole activation energy, and Schottky barrier height. For GaN (AlGaN) these are found to be 132-140 (135-150) meV and 66-88 (84-93) meV, respectively.

Features of current-voltage characteristic of nonequilibrium trench MOS barrier Schottky diode

NASA Astrophysics Data System (ADS)

Mamedov, R. K.; Aslanova, A. R.

2018-06-01

The trench MOS barrier Schottky diodes (TMBS diode) under the influence of the voltage drop of the additional electric field (AEF) appearing in the near-contact region of the semiconductor are in a nonequilibrium state and their closed external circuit flows currents in the absence of an external voltage. When an external voltage is applied to the TMBS diode, the current transmission is described by the thermionic emission theory with a specific feature. Both forward and reverse I-V characteristics of the TMBS diode consist of two parts. In the initial first part of the forward I-V characteristic there are no forward currents, but reverse saturation currents flow, in its subsequent second part the currents increase exponentially with the voltage. In the initial first part of the reverse I-V characteristic, the currents increase in an abrupt way and in the subsequent second part the saturation currents flow under the action of the image force. The mathematical expressions for forward and reverse I-V characteristic of the TMBS diode and also narrow or nanostructure Schottky diode are proposed, which are in good agreement with the results of experimental and calculated I-V characteristics.

Characterization of Pb-Doped GaN Thin Films Grown by Thermionic Vacuum Arc

NASA Astrophysics Data System (ADS)

Özen, Soner; Pat, Suat; Korkmaz, Şadan

2018-03-01

Undoped and lead (Pb)-doped gallium nitride (GaN) thin films have been deposited by a thermionic vacuum arc (TVA) method. Glass and polyethylene terephthalate were selected as optically transparent substrates. The structural, optical, morphological, and electrical properties of the deposited thin films were investigated. These physical properties were interpreted by comparison with related analysis methods. The crystalline structure of the deposited GaN thin films was hexagonal wurtzite. The optical bandgap energy of the GaN and Pb-doped GaN thin films was found to be 3.45 eV and 3.47 eV, respectively. The surface properties of the deposited thin films were imaged using atomic force microscopy and field-emission scanning electron microscopy, revealing a nanostructured, homogeneous, and granular surface structure. These results confirm that the TVA method is an alternative layer deposition system for Pb-doped GaN thin films.

Uranium nitride behavior at thermionic temperatures

NASA Technical Reports Server (NTRS)

Phillips, W. M.

1973-01-01

The feasibility of using uranium nitride for in-core thermionic applications was evaluated in electrically heated thermal gradient tests and in flat plate thermionic converters. These tests indicated that grain boundary penetration of uranium nitride into both tungsten and rhenium will occur under thermal gradient conditions. In the case of the tungsten thermionic converter, this led to grain boundary rupture of the emitter and almost total loss of electrical output from the converter. It appears that uranium nitride is unsuitable for thermionic applications at the 2000 K temperatures used in these tests.

Thermionic emission and tunneling at carbon nanotube-organic semiconductor interface.

PubMed

Sarker, Biddut K; Khondaker, Saiful I

2012-06-26

We study the charge carrier injection mechanism across the carbon nanotube (CNT)-organic semiconductor interface using a densely aligned carbon nanotube array as electrode and pentacene as organic semiconductor. The current density-voltage (J-V) characteristics measured at different temperatures show a transition from a thermal emission mechanism at high temperature (above 200 K) to a tunneling mechanism at low temperature (below 200 K). A barrier height of ∼0.16 eV is calculated from the thermal emission regime, which is much lower compared to the metal/pentacene devices. At low temperatures, the J-V curves exhibit a direct tunneling mechanism at low bias, corresponding to a trapezoidal barrier, while at high bias the mechanism is well described by Fowler-Nordheim tunneling, which corresponds to a triangular barrier. A transition from direct tunneling to Fowler-Nordheim tunneling further signifies a small injection barrier at the CNT/pentacene interface. Our results presented here are the first direct experimental evidence of low charge carrier injection barrier between CNT electrodes and an organic semiconductor and are a significant step forward in realizing the overall goal of using CNT electrodes in organic electronics.

Cogeneration Technology Alternatives Study (CTAS). Volume 6: Computer data. Part 1: Coal-fired nocogeneration process boiler, section A

NASA Technical Reports Server (NTRS)

Knightly, W. F.

1980-01-01

Various advanced energy conversion systems (ECS) are compared with each other and with current technology systems for their savings in fuel energy, costs, and emissions in individual plants and on a national level. About fifty industrial processes from the largest energy consuming sectors were used as a basis for matching a similar number of energy conversion systems that are considered as candidates which can be made available by the 1985 to 2000 time period. The sectors considered included food, textiles, lumber, paper, chemicals, petroleum, glass, and primary metals. The energy conversion systems included steam and gas turbines, diesels, thermionics, stirling, closed cycle and steam injected gas turbines, and fuel cells. Fuels considered were coal, both coal and petroleum based residual and distillate liquid fuels, and low Btu gas obtained through the on-site gasification of coal. Computer generated reports of the fuel consumption and savings, capital costs, economics and emissions of the cogeneration energy conversion systems (ECS's) heat and power matched to the individual industrial processes are presented for coal fired process boilers. National fuel and emissions savings are also reported for each ECS assuming it alone is implemented.

Identification of the limiting factors for high-temperature GaAs, GaInP, and AlGaInP solar cells from device and carrier lifetime analysis

NASA Astrophysics Data System (ADS)

Perl, E. E.; Kuciauskas, D.; Simon, J.; Friedman, D. J.; Steiner, M. A.

2017-12-01

We analyze the temperature-dependent dark saturation current density and open-circuit voltage (VOC) for GaAs, GaInP, and AlGaInP solar cells from 25 to 400 °C. As expected, the intrinsic carrier concentration, ni, dominates the temperature dependence of the dark currents. However, at 400 °C, we measure VOC that is ˜50 mV higher for the GaAs solar cell and ˜60-110 mV lower for the GaInP and AlGaInP solar cells compared to what would be expected from commonly used solar cell models that consider only the ni2 temperature dependence. To better understand these deviations, we measure the carrier lifetimes of p-type GaAs, GaInP, and AlGaInP double heterostructures (DHs) from 25 to 400 °C using time-resolved photoluminescence. Temperature-dependent minority carrier lifetimes are analyzed to determine the relative contributions of the radiative recombination, interface recombination, Shockley-Read-Hall recombination, and thermionic emission processes. We find that radiative recombination dominates for the GaAs DHs with the effective lifetime approximately doubling as the temperature is increased from 25 °C to 400 °C. In contrast, we find that thermionic emission dominates for the GaInP and AlGaInP DHs at elevated temperatures, leading to a 3-4× reduction in the effective lifetime and ˜40× increase in the surface recombination velocity as the temperature is increased from 25 °C to 400 °C. These observations suggest that optimization of the minority carrier confinement layers for the GaInP and AlGaInP solar cells could help to improve VOC and solar cell efficiency at elevated temperatures. We demonstrate VOC improvement at 200-400 °C in GaInP solar cells fabricated with modified AlGaInP window and back surface field layers.

Identification of the limiting factors for high-temperature GaAs, GaInP, and AlGaInP solar cells from device and carrier lifetime analysis

DOE PAGES

Perl, E. E.; Kuciauskas, D.; Simon, J.; ...

2017-12-21

We analyze the temperature-dependent dark saturation current density and open-circuit voltage (VOC) for GaAs, GaInP, and AlGaInP solar cells from 25 to 400 degrees C. As expected, the intrinsic carrier concentration, ni, dominates the temperature dependence of the dark currents. However, at 400 degrees C, we measure VOC that is ~50 mV higher for the GaAs solar cell and ~60-110 mV lower for the GaInP and AlGaInP solar cells compared to what would be expected from commonly used solar cell models that consider only the ni2 temperature dependence. To better understand these deviations, we measure the carrier lifetimes of p-typemore » GaAs, GaInP, and AlGaInP double heterostructures (DHs) from 25 to 400 degrees C using time-resolved photoluminescence. Temperature-dependent minority carrier lifetimes are analyzed to determine the relative contributions of the radiative recombination, interface recombination, Shockley-Read-Hall recombination, and thermionic emission processes. We find that radiative recombination dominates for the GaAs DHs with the effective lifetime approximately doubling as the temperature is increased from 25 degrees C to 400 degrees C. In contrast, we find that thermionic emission dominates for the GaInP and AlGaInP DHs at elevated temperatures, leading to a 3-4x reduction in the effective lifetime and ~40x increase in the surface recombination velocity as the temperature is increased from 25 degrees C to 400 degrees C. These observations suggest that optimization of the minority carrier confinement layers for the GaInP and AlGaInP solar cells could help to improve VOC and solar cell efficiency at elevated temperatures. We demonstrate VOC improvement at 200-400 degrees C in GaInP solar cells fabricated with modified AlGaInP window and back surface field layers.« less

Identification of the limiting factors for high-temperature GaAs, GaInP, and AlGaInP solar cells from device and carrier lifetime analysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Perl, E. E.; Kuciauskas, D.; Simon, J.

We analyze the temperature-dependent dark saturation current density and open-circuit voltage (VOC) for GaAs, GaInP, and AlGaInP solar cells from 25 to 400 degrees C. As expected, the intrinsic carrier concentration, ni, dominates the temperature dependence of the dark currents. However, at 400 degrees C, we measure VOC that is ~50 mV higher for the GaAs solar cell and ~60-110 mV lower for the GaInP and AlGaInP solar cells compared to what would be expected from commonly used solar cell models that consider only the ni2 temperature dependence. To better understand these deviations, we measure the carrier lifetimes of p-typemore » GaAs, GaInP, and AlGaInP double heterostructures (DHs) from 25 to 400 degrees C using time-resolved photoluminescence. Temperature-dependent minority carrier lifetimes are analyzed to determine the relative contributions of the radiative recombination, interface recombination, Shockley-Read-Hall recombination, and thermionic emission processes. We find that radiative recombination dominates for the GaAs DHs with the effective lifetime approximately doubling as the temperature is increased from 25 degrees C to 400 degrees C. In contrast, we find that thermionic emission dominates for the GaInP and AlGaInP DHs at elevated temperatures, leading to a 3-4x reduction in the effective lifetime and ~40x increase in the surface recombination velocity as the temperature is increased from 25 degrees C to 400 degrees C. These observations suggest that optimization of the minority carrier confinement layers for the GaInP and AlGaInP solar cells could help to improve VOC and solar cell efficiency at elevated temperatures. We demonstrate VOC improvement at 200-400 degrees C in GaInP solar cells fabricated with modified AlGaInP window and back surface field layers.« less

Thermionic photovoltaic energy converter

NASA Technical Reports Server (NTRS)

Chubb, D. L. (Inventor)

1985-01-01

A thermionic photovoltaic energy conversion device comprises a thermionic diode mounted within a hollow tubular photovoltaic converter. The thermionic diode maintains a cesium discharge for producing excited atoms that emit line radiation in the wavelength region of 850 nm to 890 nm. The photovoltaic converter is a silicon or gallium arsenide photovoltaic cell having bandgap energies in this same wavelength region for optimum cell efficiency.

Goals of thermionic program for space power

NASA Technical Reports Server (NTRS)

English, R. E.

1981-01-01

The thermionic and Brayton reactor concepts were compared for application to space power. For a turbine inlet temperature of 15000 K the Brayton powerplant weighted 5 to 40% less than the thermionic concept. The out of core concept separates the thermionic converters from their reactor. Technical risks are diminished by: (1) moving the insolator out of the reactor; (2) allowing a higher thermal flux for the thermionic converters than is required of the reactor fuel; and (3) eliminating fuel swelling's threat against lifetime of the thermionic converters. Overall performance can be improved by including power processing in system optimization for design and technology on more efficient, higher temperature power processors. The thermionic reactors will be larger than those for competitive systems with higher conversion efficiency and lower reactor operating temperatures. It is concluded that although the effect of reactor size on shield weight will be modest for unmanned spacecraft, the penalty in shield weight will be large for manned or man-tended spacecraft.

Apparatus for Study of Ion-Thruster Propellant Ionization

DTIC Science & Technology

2006-12-01

extracted . Releasing these electrons can be done in several ways. Thermionic photo emission and field emissions are valid options. All the...50.9 80.7 106 159 208 503]; %% pressure in millitorr P2=P2/1000; %% pressure in torr Vb2 =[296 276 248 258 258 274 324]; %% voltage in volts pd2=d2...262 276 348]; %% voltage in volts pd3=d3*P3; figure(1) plot(pd1,Vb1,’bo-’,’linewidth’,2) hold on plot(pd2, Vb2 ,’rs:’,’linewidth’,2) hold on 42

Study and modeling of the transport mechanism in a Schottky diode on the basis of a GaAs semiinsulator

NASA Astrophysics Data System (ADS)

Resfa, A.; Smahi, Bourzig Y.; Menezla, Brahimi R.

2011-12-01

The current through a metal—semiconductor junction is mainly due to the majority carriers. Three distinctly different mechanisms exist in a Schottky diode: diffusion of the semiconductor carriers in metal, thermionic emission-diffusion (TED) of carriers through a Schottky gate, and a mechanical quantum that pierces a tunnel through the gate. The system was solved by using a coupled Poisson—Boltzmann algorithm. Schottky BH is defined as the difference in energy between the Fermi level and the metal band carrier majority of the metal—semiconductor junction to the semiconductor contacts. The insulating layer converts the MS device in an MIS device and has a strong influence on its current—voltage (I—V) and the parameters of a Schottky barrier from 3.7 to 15 eV. There are several possible reasons for the error that causes a deviation of the ideal behaviour of Schottky diodes with and without an interfacial insulator layer. These include the particular distribution of interface states, the series resistance, bias voltage and temperature. The GaAs and its large concentration values of trap centers will participate in an increase in the process of thermionic electrons and holes, which will in turn act on the I—V characteristic of the diode, and an overflow maximum value [NT = 3 × 1020] is obtained. The I—V characteristics of Schottky diodes are in the hypothesis of a parabolic summit.

Theoretical Current-Voltage Curve in Low-Pressure Cesium Diode for Electron-Rich Emission

NASA Technical Reports Server (NTRS)

Coldstein, C. M.

1964-01-01

Although considerable interest has been shown in the space-charge analysis of low-pressure (collisionless case) thermionic diodes, there is a conspicuous lack in the presentation of results in a way that allows direct comparison with experiment. The current-voltage curve of this report was, therefore, computed for a typical case within the realm of experimental interest. The model employed in this computation is shown in Fig. 1 and is defined by the limiting potential distributions [curves (a) and (b)]. Curve (a) represents the potential V as a monotonic function of position with a slope of zero at the anode; curve (b) is similarly monotonic with a slope of zero at the cathode. It is assumed that by a continuous variation of the anode voltage, the potential distributions vary continuously from one limiting form to the other. Although solutions for infinitely spaced electrodes show that spatically oscillatory potential distributions may exist, they have been neglected in this computation.

Theoretical studies of thermionic conversion of solar energy with graphene as emitter and collector

NASA Astrophysics Data System (ADS)

Olawole, Olukunle C.; De, Dilip Kumar

2018-01-01

Thermionic energy conversion (TEC) using nanomaterials is an emerging field of research. It is known that graphene can withstand temperatures as high as 4600 K in vacuum, and it has been shown that its work function can be engineered from a high value (for monolayer/bilayer) of 4.6 eV to as low as 0.7 eV. Such attractive electronic properties (e.g., good electrical conductivity and high dielectric constant) make engineered graphene a good candidate as an emitter and collector in a thermionic energy converter for harnessing solar energy efficiently. We have used a modified Richardson-Dushman equation and have adopted a model where the collector temperature could be controlled through heat extraction in a calculated amount and a magnet can be attached on the back surface of the collector for future control of the space-charge effect. Our work shows that the efficiency of solar energy conversion also depends on power density falling on the emitter surface, and that a power conversion efficiency of graphene-based solar TEC as high as 55% can be easily achieved (in the absence of the space-charge effect) through proper choice of work functions, collector temperature, and emissivity of emitter surfaces. Such solar energy conversion would reduce our dependence on silicon solar panels and offers great potential for future renewable energy utilization.

Electrical Characterization of Graphite/InP Schottky Diodes by I-V-T and C-V Methods

NASA Astrophysics Data System (ADS)

Tiagulskyi, Stanislav; Yatskiv, Roman; Grym, Jan

2018-02-01

A rectifying junction was prepared by casting a drop of colloidal graphite on the surface of an InP substrate. The electrophysical properties of graphite/InP junctions were investigated in a wide temperature range. Temperature-dependent I-V characteristics of the graphite/InP junctions are explained by the thermionic emission mechanism. The Schottky barrier height (SBH) and the ideality factor were found to be 0.9 eV and 1.47, respectively. The large value of the SBH and its weak temperature dependence are explained by lateral homogeneity of the junction, which is related to the structure of the graphite layer. The moderate disagreement between the current-voltage and capacitance-voltage measurements is attributed to the formation of interfacial native oxide film on the InP surface.

Observation of quantum oscillation of work function in ultrathin-metal/semiconductor junctions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Takhar, Kuldeep; Meer, Mudassar; Khachariya, Dolar

2015-09-15

Quantization in energy level due to confinement is generally observed for semiconductors. This property is used for various quantum devices, and it helps to improve the characteristics of conventional devices. Here, the authors have demonstrated the quantum size effects in ultrathin metal (Ni) layers sandwiched between two large band-gap materials. The metal work function is found to oscillate as a function of its thickness. The thermionic emission current bears the signature of the oscillating work function, which has a linear relationship with barrier heights. This methodology allows direct observation of quantum oscillations in metals at room temperature using a Schottkymore » diode and electrical measurements using source-measure-units. The observed phenomena can provide additional mechanism to tune the barrier height of metal/semiconductor junctions, which are used for various electronic devices.« less

Conduction Mechanisms in Multiferroic Multilayer BaTiO3/NiFe2O4/BaTiO3 Memristors

NASA Astrophysics Data System (ADS)

Samardzic, N.; Bajac, B.; Srdic, V. V.; Stojanovic, G. M.

2017-10-01

Memristive devices and materials are extensively studied as they offer diverse properties and applications in digital, analog and bio-inspired circuits. In this paper, we present an important class of memristors, multiferroic memristors, which are composed of multiferroic multilayer BaTiO3/NiFe2O4/BaTiO3 thin films, fabricated by a spin-coating deposition technique on platinized Si wafers. This cost-effective device shows symmetric and reproducible current-voltage characteristics for the actuating voltage amplitude of ±10 V. The origin of the conduction mechanism was investigated by measuring the electrical response in different voltage and temperature conditions. The results indicate the existence of two mechanisms: thermionic emission and Fowler-Nordheim tunnelling, which alternate with actuating voltage amplitude and operating temperature.

Doping enhanced barrier lowering in graphene-silicon junctions

NASA Astrophysics Data System (ADS)

Zhang, Xintong; Zhang, Lining; Chan, Mansun

2016-06-01

Rectifying properties of graphene-semiconductor junctions depend on the Schottky barrier height. We report an enhanced barrier lowering in graphene-Si junction and its essential doping dependence in this paper. The electric field due to ionized charge in n-type Si induces the same type doping in graphene and contributes another Schottky barrier lowering factor on top of the image-force-induced lowering (IFIL). We confirm this graphene-doping-induced lowering (GDIL) based on well reproductions of the measured reverse current of our fabricated graphene-Si junctions by the thermionic emission theory. Excellent matching between the theoretical predictions and the junction data of the doping-concentration dependent barrier lowering serves as another evidence of the GDIL. While both GDIL and IFIL are enhanced with the Si doping, GDIL exceeds IFIL with a threshold doping depending on the as-prepared graphene itself.

The NASA thermionic-conversion (TEC-ART) program

NASA Technical Reports Server (NTRS)

Morris, J. F.

1977-01-01

The current emphasis is on out-of-core thermionic conversion (TEC). The additional degrees of freedom offer new potentialities, but high-temperature material effects determine the level and lifetime of TEC performance: New electrodes not only raise power outputs but also maintain them regardless of emitter-vapor deposition on collectors. In addition, effective electrodes serve compatibly with hot-shell alloys. Space TEC withstands external and internal high-temperature vaporization problems, and terrestrial TEC tolerates hot corrosive atmospheres outside and near-vacuum inside. Finally, reduction of losses between converter electrodes is essential even though rather demanding geometries appear to be required for some modes of enhanced operation.

A program-management plan with critical-path definition for Combustion Augmentation with Thermionic Energy Conversion (CATEC)

NASA Technical Reports Server (NTRS)

Morris, J. F.; Merrill, O. S.; Reddy, H. K.

1981-01-01

Thermionic energy conversion (TEC) is discussed. In recent TEC-topping analyses, overall plant efficiency (OPE) and cost of electricity (COE) improved slightly with current capabilities and substantially with fully matured technologies. Enhanced credibility derives from proven hot-corrosion protection for TEC by silicon-carbide clads in fossil fuel combustion products. Combustion augmentation with TEC (CATEC) affords minimal cost and plant perturbation, but with smaller OPE and COE improvements than more conventional topping applications. Risk minimization as well as comparative simplicity and convenience, favor CATEC for early market penetration. A program-management plan is proposed. Inputs, characteristics, outputs and capabilities are discussed.

A program-management plan with critical-path definition for Combustion Augmentation with Thermionic Energy Conversion (CATEC)

NASA Astrophysics Data System (ADS)

Morris, J. F.; Merrill, O. S.; Reddy, H. K.

Thermionic energy conversion (TEC) is discussed. In recent TEC-topping analyses, overall plant efficiency (OPE) and cost of electricity (COE) improved slightly with current capabilities and substantially with fully matured technologies. Enhanced credibility derives from proven hot-corrosion protection for TEC by silicon-carbide clads in fossil fuel combustion products. Combustion augmentation with TEC (CATEC) affords minimal cost and plant perturbation, but with smaller OPE and COE improvements than more conventional topping applications. Risk minimization as well as comparative simplicity and convenience, favor CATEC for early market penetration. A program-management plan is proposed. Inputs, characteristics, outputs and capabilities are discussed.

Solid-State Thermionic Power Generators: An Analytical Analysis in the Nonlinear Regime

NASA Astrophysics Data System (ADS)

Zebarjadi, M.

2017-07-01

Solid-state thermionic power generators are an alternative to thermoelectric modules. In this paper, we develop an analytical model to investigate the performance of these generators in the nonlinear regime. We identify dimensionless parameters determining their performance and provide measures to estimate an acceptable range of thermal and electrical resistances of thermionic generators. We find the relation between the optimum load resistance and the internal resistance and suggest guidelines for the design of thermionic power generators. Finally, we show that in the nonlinear regime, thermionic power generators can have efficiency values higher than the state-of-the-art thermoelectric modules.

Storage Reliability Missile Materiel Program. Storage Reliability Summary Report. Volume 2. Electromechanical Devices

DTIC Science & Technology

1978-02-01

ii•t, difforonc wit h ono data not’ hoV14. a nq I til i. It n 1.1 y Hho i~r fitltrevi rato, TIhil # da tai ou t W~AP i’eiwivod and th I’ll oeitnniciw...consideration. Wolfram, molybdenum, and carbon are able to sustain an arc without reaching their melting point (sufficient thermionic emission occurs at

Microscopic study on the carrier distribution in optoelectronic device structures: experiment and modeling

NASA Astrophysics Data System (ADS)

Huang, Wenchao; Xia, Hui; Wang, Shaowei; Deng, Honghai; Wei, Peng; Li, Lu; Liu, Fengqi; Li, Zhifeng; Li, Tianxin

2011-12-01

Scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM) both are capable of mapping the 2-demensional carrier distribution in semiconductor device structures, which is essential in determining their electrical and optoelectronic performances. In this work, cross-sectional SCM1,2 is used to study the InGaAs/InP P-i-N junctions prepared by area-selective p-type diffusion. The diffusion lengths in the depth as well as the lateral directions are obtained for junctions under different window sizes in mask, which imply that narrow windows may result in shallow p-n junctions. The analysis is beneficial to design and fabricate focal plane array of near infrared photodetectors with high duty-cycle and quantum efficiency. On the other hand, SSRM provides unparalleled spatial resolution (<10 nm) in electrical characterization3 that is demanded for studying low-dimensional structures. However, to derive the carrier density from the measured local conductance in individual quantum structures, reliable model for SSRM is necessary but still not well established. Based on the carrier concentration related transport mechanisms, i.e. thermionic emission and thermionic field emission4,5, we developed a numerical model for the tip-sample Schottky contact4. The calculation is confronted with SSRM study on the dose-calibrated quantum wells (QWs).

Thermionic/AMTEC cascade converter concept for high-efficiency space power

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hagan, T.H. van; Smith, J.N. Jr.; Schuller, M.

1996-12-31

This paper presents trade studies that address the use of the thermionic/AMTEC cell--a cascaded, high-efficiency, static power conversion concept that appears well-suited to space power applications. Both the thermionic and AMTEC power conversion approaches have been shown to be promising candidates for space power. Thermionics offers system compactness via modest efficiency at high heat rejection temperatures, and AMTEC offers high efficiency at modest heat rejection temperature. From a thermal viewpoint the two are ideally suited for cascaded power conversion: thermionic heat rejection and AMTEC heat source temperatures are essentially the same. In addition to realizing conversion efficiencies potentially as highmore » as 35--40%, such a cascade offers the following perceived benefits: survivability; simplicity; technology readiness; and technology growth. Mechanical approaches and thermal/electric matching criteria for integrating thermionics and AMTEC into a single conversion device are described. Focusing primarily on solar thermal space power applications, parametric trends are presented to show the performance and cost potential that should be achievable with present-day technology in cascaded thermionic/AMTEC systems.« less

Excitation of Ion Acoustic Waves in Plasmas with Electron Emission from Walls

NASA Astrophysics Data System (ADS)

Khrabrov, A. V.; Wang, H.; Kaganovich, I. D.; Raitses, Y.; Sydorenko, D.

2015-11-01

Various plasma propulsion devices exhibit strong electron emission from the walls either as a result of secondary processes or due to thermionic emission. To understand details of electron kinetics in plasmas with strong emission, we have performed kinetic simulations of such plasmas using EDIPIC code. We show that excitation of ion acoustic waves is ubiquitous phenomena in many different plasma configurations with strong electron emission from walls. Ion acoustic waves were observed to be generated near sheath if the secondary electron emission from the walls is strong. Ion acoustic waves were also observed to be generated in the plasma bulk due to presence of an intense electron beam propagating from the cathode. This intense electron beam can excite strong plasma waves, which in turn drive the ion acoustic waves. Research supported by the U.S. Air Force Office of Scientific Research.

High current density sheet-like electron beam generator

NASA Astrophysics Data System (ADS)

Chow-Miller, Cora; Korevaar, Eric; Schuster, John

Sheet electron beams are very desirable for coupling to the evanescent waves in small millimeter wave slow-wave circuits to achieve higher powers. In particular, they are critical for operation of the free-electron-laser-like Orotron. The program was a systematic effort to establish a solid technology base for such a sheet-like electron emitter system that will facilitate the detailed studies of beam propagation stability. Specifically, the effort involved the design and test of a novel electron gun using Lanthanum hexaboride (LaB6) as the thermionic cathode material. Three sets of experiments were performed to measure beam propagation as a function of collector current, beam voltage, and heating power. The design demonstrated its reliability by delivering 386.5 hours of operation throughout the weeks of experimentation. In addition, the cathode survived two venting and pump down cycles without being poisoned or losing its emission characteristics. A current density of 10.7 A/sq cm. was measured while operating at 50 W of ohmic heating power. Preliminary results indicate that the nearby presence of a metal plate can stabilize the beam.

Temperature Dependence Of Current-Voltage Characteristics Of Au/p-GaAsN Schottky Barrier Diodes, With Small N Content

NASA Astrophysics Data System (ADS)

Rangel-Kuoppa, Victor-Tapio; Reentilä, Outi; Sopanen, Markku; Lipsanen, Harri

2011-12-01

The temperature dependent current-voltage (IVT) measurements on Au Schottky barrier diodes made on intrinsically p-type GaAs1-xNx were carried out. Three samples with small N content (x = 0.5%, 0.7% and 1%) were studied. The temperature range was 10-320 K. All contacts were found to be of Schottky type. The ideality factor and the apparent barrier height calculated by using thermionic emission (TE) theory show a strong temperature dependence. The current voltage (IV) curves are fitted based on the TE theory, yielding a zero-bias carrier height (ΦB0) and a ideality factor (n) that decrease and increase with decreasing temperature, respectively. The linear fitting of ΦB0 vs n and its subsequent evaluation for n = 1 give a zero-bias ΦB0 in the order of 0.35-0.4 eV. From the reverse-bias IV study, it is found that the experimental carrier density (NA) values increase with increasing temperature and are in agreement with the intrinsic carrier concentration for GaAs.

Investigation of the short argon arc with hot anode. I. Numerical simulations of non-equilibrium effects in the near-electrode regions

NASA Astrophysics Data System (ADS)

Khrabry, A.; Kaganovich, I. D.; Nemchinsky, V.; Khodak, A.

2018-01-01

The atmospheric pressure arcs have recently found application in the production of nanoparticles. The distinguishing features of such arcs are small length and hot ablating anode characterized by intensive electron emission and radiation from its surface. We performed a one-dimensional modeling of argon arc, which shows that near-electrode effects of thermal and ionization non-equilibrium play an important role in the operation of a short arc, because the non-equilibrium regions are up to several millimeters long and are comparable to the arc length. The near-anode region is typically longer than the near-cathode region and its length depends more strongly on the current density. The model was extensively verified and validated against previous simulation results and experimental data. The Volt-Ampere characteristic (VAC) of the near-anode region depends on the anode cooling mechanism. The anode voltage is negative. In the case of strong anode cooling (water-cooled anode) when the anode is cold, temperature and plasma density gradients increase with current density, resulting in a decrease of the anode voltage (the absolute value increases). Falling VAC of the near-anode region suggests the arc constriction near the anode. Without anode cooling, the anode temperature increases significantly with the current density, leading to a drastic increase in the thermionic emission current from the anode. Correspondingly, the anode voltage increases to suppress the emission, and the opposite trend in the VAC is observed. The results of simulations were found to be independent of sheath model used: collisional (fluid) or collisionless model gave the same plasma profiles for both near-anode and near-cathode regions.

Investigation of the short argon arc with hot anode. I. Numerical simulations of non-equilibrium effects in the near-electrode regions

DOE PAGES

Khrabry, A.; Kaganovich, I. D.; Nemchinsky, V.; ...

2018-01-22

The atmospheric pressure arcs have recently found application in the production of nanoparticles. The distinguishing features of such arcs are small length and hot ablating anode characterized by intensive electron emission and radiation from its surface. We performed a one-dimensional modeling of argon arc, which shows that near-electrode effects of thermal and ionization non-equilibrium play an important role in the operation of a short arc, because the non-equilibrium regions are up to several millimeters long and are comparable to the arc length. The near-anode region is typically longer than the near-cathode region and its length depends more strongly on themore » current density. The model was extensively verified and validated against previous simulation results and experimental data. The Volt-Ampere characteristic (VAC) of the near-anode region depends on the anode cooling mechanism. The anode voltage is negative. In the case of strong anode cooling (water-cooled anode) when the anode is cold, temperature and plasma density gradients increase with current density, resulting in a decrease of the anode voltage (the absolute value increases). Falling VAC of the near-anode region suggests the arc constriction near the anode. Without anode cooling, the anode temperature increases significantly with the current density, leading to a drastic increase in the thermionic emission current from the anode. Correspondingly, the anode voltage increases to suppress the emission, and the opposite trend in the VAC is observed. Here, the results of simulations were found to be independent of sheath model used: collisional (fluid) or collisionless model gave the same plasma profiles for both near-anode and near-cathode regions.« less

Investigation of the short argon arc with hot anode. I. Numerical simulations of non-equilibrium effects in the near-electrode regions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Khrabry, A.; Kaganovich, I. D.; Nemchinsky, V.

The atmospheric pressure arcs have recently found application in the production of nanoparticles. The distinguishing features of such arcs are small length and hot ablating anode characterized by intensive electron emission and radiation from its surface. We performed a one-dimensional modeling of argon arc, which shows that near-electrode effects of thermal and ionization non-equilibrium play an important role in the operation of a short arc, because the non-equilibrium regions are up to several millimeters long and are comparable to the arc length. The near-anode region is typically longer than the near-cathode region and its length depends more strongly on themore » current density. The model was extensively verified and validated against previous simulation results and experimental data. The Volt-Ampere characteristic (VAC) of the near-anode region depends on the anode cooling mechanism. The anode voltage is negative. In the case of strong anode cooling (water-cooled anode) when the anode is cold, temperature and plasma density gradients increase with current density, resulting in a decrease of the anode voltage (the absolute value increases). Falling VAC of the near-anode region suggests the arc constriction near the anode. Without anode cooling, the anode temperature increases significantly with the current density, leading to a drastic increase in the thermionic emission current from the anode. Correspondingly, the anode voltage increases to suppress the emission, and the opposite trend in the VAC is observed. Here, the results of simulations were found to be independent of sheath model used: collisional (fluid) or collisionless model gave the same plasma profiles for both near-anode and near-cathode regions.« less

Numerical model of the plasma formation at electron beam welding

NASA Astrophysics Data System (ADS)

Trushnikov, D. N.; Mladenov, G. M.

2015-01-01

The model of plasma formation in the keyhole in liquid metal as well as above the electron beam welding zone is described. The model is based on solution of two equations for the density of electrons and the mean electron energy. The mass transfer of heavy plasma particles (neutral atoms, excited atoms, and ions) is taken into account in the analysis by the diffusion equation for a multicomponent mixture. The electrostatic field is calculated using the Poisson equation. Thermionic electron emission is calculated for the keyhole wall. The ionization intensity of the vapors due to beam electrons and high-energy secondary and backscattered electrons is calibrated using the plasma parameters when there is no polarized collector electrode above the welding zone. The calculated data are in good agreement with experimental data. Results for the plasma parameters for excitation of a non-independent discharge are given. It is shown that there is a need to take into account the effect of a strong electric field near the keyhole walls on electron emission (the Schottky effect) in the calculation of the current for a non-independent discharge (hot cathode gas discharge). The calculated electron drift velocities are much bigger than the velocity at which current instabilities arise. This confirms the hypothesis for ion-acoustic instabilities, observed experimentally in previous research.

Progress Toward a Gigawatt-Class Annular Beam Klystron with a Thermionic Electron Gun

NASA Astrophysics Data System (ADS)

Fazio, M.; Carlsten, B.; Farnham, J.; Habiger, K.; Haynes, W.; Myers, J.; Nelson, E.; Smith, J.; Arfin, B.; Haase, A.

2002-08-01

In an effort to reach the gigawatt power level in the microsecond pulse length regime Los Alamos, in collaboration with SLAC, is developing an annular beam klystron (ABK) with a thermionic electron gun. We hope to address the causes of pulse shortening in very high peak power tubes by building a "hard-vacuum" tube in the 10-10 Torr range with a thermionic electron gun producing a constant impedance electron-beam. The ABK has been designed to operate at 5 Hz pulse repetition frequency to allow for RF conditioning. The electron gun has a magnetron injection gun configuration and uses a dispenser cathode running at 1100 degC to produce a 4 kA electron beam at 800 kV. The cathode is designed to run in the temperature-limited mode to help maintain beam stability in the gun. The beam-stick consisting of the electron gun, an input cavity, an idler cavity, and drift tube, and the collector has been designed collaboratively, fabricated at SLAC, then shipped to Los Alamos for testing. On the test stand at Los Alamos a low voltage emission test was performed, but unfortunately as we prepared for high voltage testing a problem with the cathode heater was encountered that prevented the cathode from reaching a high enough temperature for electron emission. A post-mortem examination will be done shortly to determine the exact cause of the heater failure. The RF design has been proceeding and is almost complete. The output cavity presents a challenging design problem in trying to efficiently extract energy from the low impedance beam while maintaining a gap voltage low enough to avoid breakdown and a Q high enough to maintain mode purity. In the next iteration, the ABK will have a new cathode assembly installed along with the remainder of the RF circuit. This paper will discuss the electron gun and the design of the RF circuit along with a report on the status of the work.

Mathematical Modeling Of A Nuclear/Thermionic Power Source

NASA Technical Reports Server (NTRS)

Vandersande, Jan W.; Ewell, Richard C.

1992-01-01

Report discusses mathematical modeling to predict performance and lifetime of spacecraft power source that is integrated combination of nuclear-fission reactor and thermionic converters. Details of nuclear reaction, thermal conditions in core, and thermionic performance combined with model of swelling of fuel.

FUSION WELDING METHOD AND APPARATUS

DOEpatents

Wyman, W.L.; Steinkamp, W.I.

1961-01-17

An apparatus for the fusion welding of metal pieces at a joint is described. The apparatus comprises a highvacuum chamber enclosing the metal pieces and a thermionic filament emitter. Sufficient power is applied to the emitter so that when the electron emission therefrom is focused on the joint it has sufficient energy to melt the metal pieces, ionize the metallic vapor abcve the molten metal, and establish an arc discharge between the joint and the emitter.

Computer controlled performance mapping of thermionic converters: effect of collector, guard-ring potential imbalances on the observed collector current-density, voltage characteristics and limited range performance map of an etched-rhenium, niobium planar converter

NASA Technical Reports Server (NTRS)

Manista, E. J.

1972-01-01

The effect of collector, guard-ring potential imbalance on the observed collector-current-density J, collector-to-emitter voltage V characteristic was evaluated in a planar, fixed-space, guard-ringed thermionic converter. The J,V characteristic was swept in a period of 15 msec by a variable load. A computerized data acquisition system recorded test parameters. The results indicate minimal distortion of the J,V curve in the power output quadrant for the nominal guard-ring circuit configuration. Considerable distortion, along with a lowering of the ignited-mode striking voltage, was observed for the configuration with the emitter shorted to the guard ring. A limited-range performance map of an etched-rhenium, niobium, planar converter was obtained by using an improved computer program for the data acquisition system.

Effect on Non-Uniform Heat Generation on Thermionic Reactions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schock, Alfred

The penalty resulting from non-uniform heat generation in a thermionic reactor is examined. Operation at sub-optimum cesium pressure is shown to reduce this penalty, but at the risk of a condition analogous to burnout. For high pressure diodes, a simple empirical correlation between current, voltage and heat flux is developed and used to analyze the performance penalty associated with two different heat flux profiles, for series-and parallel-connected converters. The results demonstrate that series-connected converters require much finer power flattening than parallel converters. For example, a ±10% variation in heat generation across a series array can result in a 25 tomore » 50% power penalty.« less

Advanced thermionic energy conversion

NASA Technical Reports Server (NTRS)

1979-01-01

Developments towards space and terrestrial applications of thermionic energy conversion are presented. Significant accomplishments for the three month period include: (1) devised a blade-type distributed lead design with many advantages compared to the stud-type distributed lead; (2) completed design of Marchuk tube test apparatus; (3) concluded, based on current understanding, that residual hydrogen should not contribute to a negative space charge barrier at the collector; (4) modified THX design program to include series-coupled designs as well as inductively-coupled designs; (5) initiated work on the heat transfer technology, THX test module, output power transfer system, heat transfer system, and conceptual plant design tasks; and (6) reached 2200 hours of operation in JPL-5 cylindrical converter envelope test.

Excitation of Ion Acoustic Waves in Confined Plasmas with Untrapped Electrons

NASA Astrophysics Data System (ADS)

Schamis, Hanna; Dow, Ansel; Carlsson, Johan; Kaganovich, Igor; Khrabrov, Alexander

2015-11-01

Various plasma propulsion devices exhibit strong electron emission from the walls either as a result of secondary processes or due to thermionic emission. To understand the electron kinetics in plasmas with strong emission, we have performed simulations using a reduced model with the LSP particle-in-cell code. This model aims to show the instability generated by the electron emission, in the form of ion acoustic waves near the sheath. It also aims to show the instability produced by untrapped electrons that propagate across the plasma, similarly to a beam, and can drive ion acoustic waves in the plasma bulk. This work was made possible by funding from the Department of Energy for the Summer Undergraduate Laboratory Internship (SULI) program. This work is supported by the US DOE Contract No.DE-AC02-09CH11466.

High power cascade diode lasers emitting near 2 μm

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hosoda, Takashi; Feng, Tao; Shterengas, Leon, E-mail: leon.shterengas@stonybrook.edu

2016-03-28

High-power two-stage cascade GaSb-based type-I quantum well diode lasers emitting near 2 μm were designed and fabricated. Coated devices with cavity length of 3 mm generated about 2 W of continuous wave power from 100-μm-wide aperture at the current of 6 A. The power conversion efficiency peaked at 20%. Carrier recycling between quantum well gain stages was realized using band-to-band tunneling in GaSb/AlSb/InAs heterostructure complemented with optimized electron and hole injector regions. Design optimization eliminated parasitic optical absorption and thermionic emission, and included modification of the InAs quantum wells of electron and composition and doping profile of hole injectors. Utilization of the cascade pumpingmore » scheme yielded 2 μm lasers with improved output power and efficiency compared to existing state-of-the-art diodes.« less

Plasma-anode electron gun

NASA Astrophysics Data System (ADS)

Santoru, Joseph; Schumacher, Robert W.; Gregoire, Daniel J.

1994-11-01

The plasma-anode electron gun (PAG) is an electron source in which the thermionic cathode is replaced with a cold, secondary-electron-emitting electrode. Electron emission is stimulated by bombarding the cathode with high-energy ions. Ions are injected into the high-voltage gap through a gridded structure from a plasma source (gas pressure less than or equal to 50 mTorr) that is embedded in the anode electrode. The gridded structure serves as both a cathode for the plasma discharge and as an anode for the PAG. The beam current is modulated at near ground potential by modulating the plasma source, eliminating the need for a high-voltage modulator system. During laboratory tests, the PAG has demonstrated square-wave, 17-microsecond-long beam pulses at 100 kV and 10 A, and it has operated stably at 70 kV and 2.5 A for 210 microsecond pulse lengths without gap closure.

thin film capacitors

NASA Astrophysics Data System (ADS)

Bodeux, Romain; Gervais, Monique; Wolfman, Jérôme; Gervais, François

2014-09-01

CaCu3Ti4O12 (CCTO) thin films were grown by pulsed laser deposition on Pt and La0.9Sr1.1NiO4 (LSNO) bottom electrodes. The electrical characteristics of the CCTO/Pt and CCTO/LSNO Schottky junctions have been analyzed by impedance spectroscopy, capacitance-voltage (C-V) and current-voltage (I-V) measurements as a function of frequency (40 Hz-1 MHz) and temperature (300-475 K). Similar results were obtained for the two Schottky diodes. The conduction mechanism through the Schottky junctions was described using a thermionic emission model and the electrical parameters were determined. The strong deviation from the ideal I-V characteristics and the increase in capacitance at low frequency for -0.5 V bias are in agreement with the presence of traps near the interfaces. Results point toward the important effect of defects generated at the interface by deposition of CCTO.

Organic/Inorganic Hybrid p-n Junction with PEDOT Nanoparticles for Light-Emitting Diode

NASA Astrophysics Data System (ADS)

Kim, M. S.; Jin, S. M.; Cho, M. Y.; Choi, H. Y.; Kim, G. S.; Jeon, S. M.; Yim, K. G.; Kim, H. G.; Shim, K. B.; Kang, B. K.; Kim, Y.; Lee, D. Y.; Kim, J. S.; Kim, J. S.; Leem, J. Y.

2011-12-01

A heavily Si-doped GaN/polymer hybrid structure with p-type poly(3,4-ethylene-dioxythiophene):beta-1,3-glucan (PEDOT nanoparticle) interface layer has been fabricated. The Si-doped GaN thin film with carrier concentration of 1×1019 cm-3 was grown by metal-organic chemical vapor deposition (MOCVD). The PEDOT nanoparticle with various sizes ranging from 60 to 120 nm was synthesized via a miniemulsion polymerization process. The electrical conductivity of the PEDOT nanoparticle is less than 1.2 S/cm. The current-voltage (I-V) characteristic of the hybrid structure shows diode-like behavior. The I-V characteristic was examined in the framework of the thermionic emission model. The ideality factor and barrier height of the hybrid structure were obtained as 5.6 and 0.41 eV, respectively. The value of ideality factor is decreased by inserting the PEDOT nanoparticle interface layer.

Barrier inhomogeneities at vertically stacked graphene-based heterostructures.

PubMed

Lin, Yen-Fu; Li, Wenwu; Li, Song-Lin; Xu, Yong; Aparecido-Ferreira, Alex; Komatsu, Katsuyoshi; Sun, Huabin; Nakaharai, Shu; Tsukagoshi, Kazuhito

2014-01-21

The integration of graphene and other atomically flat, two-dimensional materials has attracted much interest and been materialized very recently. An in-depth understanding of transport mechanisms in such heterostructures is essential. In this study, vertically stacked graphene-based heterostructure transistors were manufactured to elucidate the mechanism of electron injection at the interface. The temperature dependence of the electrical characteristics was investigated from 300 to 90 K. In a careful analysis of current-voltage characteristics, an unusual decrease in the effective Schottky barrier height and increase in the ideality factor were observed with decreasing temperature. A model of thermionic emission with a Gaussian distribution of barriers was able to precisely interpret the conduction mechanism. Furthermore, mapping of the effective Schottky barrier height is unmasked as a function of temperature and gate voltage. The results offer significant insight for the development of future layer-integration technology based on graphene-based heterostructures.

High efficiency thermionic converter studies

NASA Technical Reports Server (NTRS)

Huffman, F. N.; Sommer, A. H.; Balestra, C. L.; Briere, T. R.; Lieb, D.; Oettinger, P. E.; Goodale, D. B.

1977-01-01

Research in thermionic energy conversion technology is reported. The objectives were to produce converters suitable for use in out of core space reactors, radioisotope generators, and solar satellites. The development of emitter electrodes that operate at low cesium pressure, stable low work function collector electrodes, and more efficient means of space charge neutralization were investigated to improve thermionic converter performance. Potential improvements in collector properties were noted with evaporated thin film barium oxide coatings. Experiments with cesium carbonate suggest this substance may provide optimum combinations of cesium and oxygen for thermionic conversion.

Translations on USSR Science and Technology, Physical Sciences and Technology, Number 28.

DTIC Science & Technology

1978-02-07

some part of the planet. However, this may not be a cause-and-effect situation, but a simple coin- cidence. For example, at Tashkent in 1966, some...to observe such thermionic emission separately from photoemission in the most simple experiment; superposition of both mechanisms leads to a...is realized: 83 Parameter y on the left side of the inequality has a simple meaning [15, 50]: it is equal to the ratio of light frequency to

Preparation and surface characteristics of Re3W matrix scandate cathode: An experimental and theoretical study

NASA Astrophysics Data System (ADS)

Lai, Chen; Wang, Jinshu; Zhou, Fan; Liu, Wei; Hu, Peng; Wang, Changhao; Wang, Ruzhi; Miao, Naihua

2018-05-01

The Scandia doped thermionic cathodes have received great attention owing to their high electron emission density in past two decades. Here, Scandia doped Re3W matrix scandate (RS) cathodes are fabricated by using Sc2O3 doped Re3W powders that prepared by spray drying method. The micromorphology, surface composition and chemical states of RS cathode are investigated with various modern technologies. It reveals that the reduction temperature of RS powders is dramatically increased by Sc2O3. On the surface of RS cathode, a certain amount of Sc2O3 nanoparticles and barium salt submicron particles are observed. According to the in situ Auger electron spectroscopy analysis, the concentration ratio of Ba:Sc:O is determined to be 2.9:1.1:2.7. The X-ray photoelectron spectroscopy data indicates that low oxidation state of Sc is clearly observed in scandate cathodes. The high atomic ratio of Ba on RS cathode surface is suggested due to the high adsorption of Re3W to Ba. Moreover, RS cathode shows better adsorption to Sc by comparison with conventional tungsten matrix scandate cathode. For RS cathode, the main depletion of Sc is suggested to -OSc desorbing from RS cathode surface. RS cathode is expected to be an impressive thermionic cathode with good emission properties and ion anti-bombarding insensitivity.

Electrical transport and capacitance characteristics of metal-insulator-metal structures using hexagonal and cubic boron nitride films as dielectrics

NASA Astrophysics Data System (ADS)

Teii, Kungen; Kawamoto, Shinsuke; Fukui, Shingo; Matsumoto, Seiichiro

2018-04-01

Metal-insulator-metal capacitor structures using thick hexagonal and cubic boron nitride (hBN and cBN) films as dielectrics are produced by plasma jet-enhanced chemical vapor deposition, and their electrical transport and capacitance characteristics are studied in a temperature range of 298 to 473 K. The resistivity of the cBN film is of the order of 107 Ω cm at 298 K, which is lower than that of the hBN film by two orders of magnitude, while it becomes the same order as the hBN film above ˜423 K. The dominant current transport mechanism at high fields (≥1 × 104 V cm-1) is described by the Frenkel-Poole emission and thermionic emission models for the hBN and cBN films, respectively. The capacitance of the hBN film remains stable for a change in alternating-current frequency and temperature, while that of the cBN film has variations of at most 18%. The dissipation factor as a measure of energy loss is satisfactorily low (≤5%) for both films. The origin of leakage current and capacitance variation is attributed to a high defect density in the film and a transition interlayer between the substrate and the film, respectively. This suggests that cBN films with higher crystallinity, stoichiometry, and phase purity are potentially applicable for dielectrics like hBN films.

Advanced thermionic converter developments with microwave external pumping

NASA Technical Reports Server (NTRS)

Chiu, H. S.; Shaw, D. T.; Manikopulos, C. N.; Lee, C. H.

1977-01-01

This work reports ion generation in a cesium thermionic converter as part of advanced-model thermionic converter development research. A microwave with frequency in the range between 1-2 GHz is used to externally pump a thermionic converter as part of our effort in the verification of Lam's theory. It is found that the motive peak as predicted in the theory disappears whenever microwave power is used to excite the cesium plasma of the converter. The electron temperature is effectively heated by the microwave and the experimental data agrees with theory in the low-power output region.

Analysis of electrical properties of heterojunction based on ZnIn2Se4

NASA Astrophysics Data System (ADS)

Attia, A. A.; Ali, H. A. M.; Salem, G. F.; Ismail, M. I.; Al-Harbi, F. F.

2017-04-01

Heterojunction of n-ZnIn2Se4/p-Si was fabricated using thermal evaporation of ZnIn2Se4 thin films of thickness 473 nm onto p-Si substrate at room temperature. The characteristics of current-voltage (I-V) for n-ZnIn2Se4/p-Si heterojunction were investigated at different temperatures ranged from 308 K to 363 K. The junction parameters namely are; rectification ratio (RR), series resistance (Rs), shunt resistance (Rsh) and diode ideality factor (n) were calculated from the analysis of I-V curves. The forward current showed two conduction mechanisms operating, which were the thermionic emission and the single trap space charge limited current in low (0 ≤ V ≤ 0.5 V) and high (V ≥ 0.7 V) ranges of voltage, respectively. The reverse current was due to the generation through Si rather than the ZnIn2Se4 film. The built-in voltage and the width of the depletion region were determined from the capacitance-voltage (C-V) measurements. The photovoltaic characteristics of the junction were also studied through the (I-V) measurements under illumination of 40 mW/cm2. The cell parameters; the short-circuit current, the open-circuit voltage and the fill factor were estimated at room temperature.

Forward Current Transport Mechanisms of Ni/Au—InAlN/AlN/GaN Schottky Diodes

NASA Astrophysics Data System (ADS)

Wang, Xiao-Feng; Shao, Zhen-Guang; Chen, Dun-Jun; Lu, Hai; Zhang, Rong; Zheng, You-Dou

2014-05-01

We fabricate two Ni/Au-In0.17Al0.83N/AlN/GaN Schottky diodes on substrates of sapphire and Si, respectively, and investigate their forward-bias current transport mechanisms by temperature-dependent current-voltage measurements. In the temperature range of 300-485 K, the Schottky barrier heights (SBHs) calculated by using the conventional thermionic-emission (TE) model are strongly positively dependent on temperature, which is in contrast to the negative-temperature-dependent characteristic of traditional semiconductor Schottky diodes. By fitting the forward-bias I-V characteristics using different current transport models, we find that the tunneling current model can describe generally the I-V behaviors in the entire measured range of temperature. Under the high forward bias, the traditional TE mechanism also gives a good fit to the measured I-V data, and the actual barrier heights calculated according to the fitting TE curve are 1.434 and 1.413 eV at 300K for InAlN/AlN/GaN Schottky diodes on Si and the sapphire substrate, respectively, and the barrier height shows a slightly negative temperature coefficient. In addition, a formula is given to estimate SBHs of Ni/Au—InAlN/AlN/GaN Schottky diodes taking the Fermi-level pinning effect into account.

Mechanical Modulation of Tunneling Current in Transition Metal Dichalcogenides Heterostructures: A First Principles Study

NASA Astrophysics Data System (ADS)

Kuroda, Marcelo

Recent experiments in MoS2 heterostructures reported that out-of-plane tunneling piezoresistivity (TPR) - mechanical modulation of the tunneling current - achieves sensitivities of one decade per Ådisplacement. Owing to their nanometer scale, a quantitative theoretical framework providing the TPR structure-property relationship is necessary to further improve sensitivities. To this end, first principles calculations within density functional theory are used to characterize the phenomenon in MoX2 (with X = S, Se). The TPR is quantified in relation to electrode composition and film thickness showing remarkable agreement with experiments. The origin of the TPR is attributed to the heterostructure compliance rather than band alignment changes with strain, and differs from mechanisms in other nanometer-thick bulk films. Large work function metals (Pt, Au) are singled out as best candidates for enhanced TPR gauges due to weak bonding and negligible thermionic emission; compliant bilayers show larger stress-sensitivity than monolayers. By accounting for the atomistic details and material composition of 2D material-based heterostructures, this work has the potential to advance sensor and nano-electro-mechanical system technologies.

Numerical model of the plasma formation at electron beam welding

DOE Office of Scientific and Technical Information (OSTI.GOV)

Trushnikov, D. N., E-mail: trdimitr@yandex.ru; The Department for Welding Production and Technology of Constructional Materials, Perm National Research Polytechnic University, Perm 614990; Mladenov, G. M., E-mail: gmmladenov@abv.bg

2015-01-07

The model of plasma formation in the keyhole in liquid metal as well as above the electron beam welding zone is described. The model is based on solution of two equations for the density of electrons and the mean electron energy. The mass transfer of heavy plasma particles (neutral atoms, excited atoms, and ions) is taken into account in the analysis by the diffusion equation for a multicomponent mixture. The electrostatic field is calculated using the Poisson equation. Thermionic electron emission is calculated for the keyhole wall. The ionization intensity of the vapors due to beam electrons and high-energy secondarymore » and backscattered electrons is calibrated using the plasma parameters when there is no polarized collector electrode above the welding zone. The calculated data are in good agreement with experimental data. Results for the plasma parameters for excitation of a non-independent discharge are given. It is shown that there is a need to take into account the effect of a strong electric field near the keyhole walls on electron emission (the Schottky effect) in the calculation of the current for a non-independent discharge (hot cathode gas discharge). The calculated electron drift velocities are much bigger than the velocity at which current instabilities arise. This confirms the hypothesis for ion-acoustic instabilities, observed experimentally in previous research.« less

Transmission Electron Microscope Measures Lattice Parameters

NASA Technical Reports Server (NTRS)

Pike, William T.

1996-01-01

Convergent-beam microdiffraction (CBM) in thermionic-emission transmission electron microscope (TEM) is technique for measuring lattice parameters of nanometer-sized specimens of crystalline materials. Lattice parameters determined by use of CBM accurate to within few parts in thousand. Technique developed especially for use in quantifying lattice parameters, and thus strains, in epitaxial mismatched-crystal-lattice multilayer structures in multiple-quantum-well and other advanced semiconductor electronic devices. Ability to determine strains in indivdual layers contributes to understanding of novel electronic behaviors of devices.

Dexter - A one-dimensional code for calculating thermionic performance of long converters.

NASA Technical Reports Server (NTRS)

Sawyer, C. D.

1971-01-01

This paper describes a versatile code for computing the coupled thermionic electric-thermal performance of long thermionic converters in which the temperature and voltage variations cannot be neglected. The code is capable of accounting for a variety of external electrical connection schemes, coolant flow paths and converter failures by partial shorting. Example problem solutions are given.

HEAT Sensor: Harsh Environment Adaptable Thermionic Sensor

DOE Office of Scientific and Technical Information (OSTI.GOV)

Limb, Scott J.

2016-05-31

This document is the final report for the “HARSH ENVIRONMENT ADAPTABLE THERMIONIC SENSOR” project under NETL’s Crosscutting contract DE-FE0013062. This report addresses sensors that can be made with thermionic thin films along with the required high temperature hermetic packaging process. These sensors can be placed in harsh high temperature environments and potentially be wireless and self-powered.

The influence of voltage applied between the electrodes on optical and morphological properties of the InGaN thin films grown by thermionic vacuum arc.

PubMed

Özen, Soner; Şenay, Volkan; Pat, Suat; Korkmaz, Şadan

2016-01-01

The aim of this research is to investigate the optical and morphological properties of the InGaN thin films deposited onto amorphous glass substrates in two separate experiments with two different voltages applied between the electrodes, i.e. 500 and 600 V by means of the thermionic vacuum arc technique. This technique is original for thin film deposition and it enables thin film production in a very short period of time. The optical and morphological properties of the films were investigated by using field emission scanning electron microscope, atomic force microscope, spectroscopic ellipsometer, reflectometer, spectrophotometer, and optical tensiometer. Optical properties were also supported by empirical relations. The deposition rates were calculated as 3 and 3.3 nm/sec for 500 and 600 V, respectively. The increase in the voltage also increased the refractive index, grain size, root mean square roughness and surface free energy. According to the results of the wetting experiments, InGaN samples were low-wettable, also known as hydrophobic. © Wiley Periodicals, Inc.

Diminiode thermionic conversion with 111-iridium electrodes

NASA Technical Reports Server (NTRS)

Koeger, E. W.; Bair, V. L.; Morris, J. F.

1976-01-01

Preliminary data indicating thermionic-conversion potentialities for a 111-iridium emitter and collector spaced 0.2 mm apart are presented. These results comprise output densities of current and of power as functions of voltage for three sets of emitter, collector, and reservoir temperatures: 1553, 944, 561 K; 1605, 898, 533 K; and 1656, 1028, 586 K. For the 1605 K evaluation, estimates produced work-function values of 2.22 eV for the emitter and 1.63 eV for the collector with a 2.0-eV barrier index (collector work function plus interelectrode voltage drop) corresponding to the maximum output of 5.5 W/sq cm at 0.24 volt. The current, voltage curve for the 1656 K 111-iridium diminiode yields a 6.2 W/sq cm maximum at 0.25 volt and is comparable with the 1700 K envelope for a diode with an etched-rhenium emitter and a 0.025-mm electrode gap made by TECO and evaluated by NASA.

High-temperature, high-power-density thermionic energy conversion for space

NASA Technical Reports Server (NTRS)

Morris, J. F.

1977-01-01

Theoretic converter outputs and efficiencies indicate the need to consider thermionic energy conversion (TEC) with greater power densities and higher temperatures within reasonable limits for space missions. Converter-output power density, voltage, and efficiency as functions of current density were determined for 1400-to-2000 K emitters with 725-to-1000 K collectors. The results encourage utilization of TEC with hotter-than-1650 K emitters and greater-than-6W sq cm outputs to attain better efficiencies, greater voltages, and higher waste-heat-rejection temperatures for multihundred-kilowatt space-power applications. For example, 1800 K, 30 A sq cm TEC operation for NEP compared with the 1650 K, 5 A/sq cm case should allow much lower radiation weights, substantially fewer and/or smaller emitter heat pipes, significantly reduced reactor and shield-related weights, many fewer converters and associated current-collecting bus bars, less power conditioning, and lower transmission losses. Integration of these effects should yield considerably reduced NEP specific weights.

DEXTER: A one-dimensional code for calculating thermionic performance of long converters

NASA Technical Reports Server (NTRS)

Sawyer, C. D.

1971-01-01

A versatile code is described for computing the coupled thermionic electric-thermal performance of long thermionic converters in which the temperature and voltage variations cannot be neglected. The code is capable of accounting for a variety of external electrical connection schemes, coolant flow paths and converter failures by partial shorting. Example problem solutions are included along with a user's manual.

Fabrication and surface characterization of composite refractory compounds suitable for thermionic converters

NASA Technical Reports Server (NTRS)

Davis, P. R.; Swanson, L. W.

1979-01-01

The techniques of fabricating and characterizing the surface properties of electrode materials were investigated. The basic surface properties of these materials were studied with respect to their utilization as thermionic energy converter electrodes. Emphasis was placed on those factors (e.g, cesium disorption kinetic and mechanisms of low work function production) which are of primary concern to thermionic converter performance.

Thermionic converter

DOEpatents

Fitzpatrick, G.O.

1987-05-19

A thermionic converter is set forth which includes an envelope having an electron collector structure attached adjacent to a wall. An electron emitter structure is positioned adjacent the collector structure and spaced apart from opposite wall. The emitter and collector structures are in a common chamber. The emitter structure is heated substantially only by thermal radiation. Very small interelectrode gaps can be maintained utilizing the thermionic converter whereby increased efficiency results. 10 figs.

Electric energy production by particle thermionic-thermoelectric power generators

NASA Technical Reports Server (NTRS)

Oettinger, P. E.

1980-01-01

Thermionic-thermoelectric power generators, composed of a thin layer of porous, low work function material separating a heated emitter electrode and a cooler collector electrode, have extremely large Seebeck coefficients of over 2 mV/K and can provide significant output power. Preliminary experiments with 20-micron thick (Ba Sr Ca)O coatings, limited by evaporative loss to temperatures below 1400 K, have yielded short circuit current densities of 500 mA/sq cm and power densities of 60 mW/ sq cm. Substantially more output is expected with cesium-coated refractory oxide particle coatings operating at higher temperatures. Practical generators will have thermal-to-electrical efficiencies of 10 to 20%. Further increases can be gained by cascading these high-temperature devices with lower temperature conventional thermoelectric generators.

Lunar in-core thermionic nuclear reactor power system conceptual design

NASA Technical Reports Server (NTRS)

Mason, Lee S.; Schmitz, Paul C.; Gallup, Donald R.

1991-01-01

This paper presents a conceptual design of a lunar in-core thermionic reactor power system. The concept consists of a thermionic reactor located in a lunar excavation with surface mounted waste heat radiators. The system was integrated with a proposed lunar base concept representative of recent NASA Space Exploration Initiative studies. The reference mission is a permanently-inhabited lunar base requiring a 550 kWe, 7 year life central power station. Performance parameters and assumptions were based on the Thermionic Fuel Element (TFE) Verification Program. Five design cases were analyzed ranging from conservative to advanced. The cases were selected to provide sensitivity effects on the achievement of TFE program goals.

Thermionic switched self-actuating reactor shutdown system

DOEpatents

Barrus, Donald M.; Shires, Charles D.; Brummond, William A.

1989-01-01

A self-actuating reactor shutdown system incorporating a thermionic switched electromagnetic latch arrangement which is responsive to reactor neutron flux changes and to reactor coolant temperature changes. The system is self-actuating in that the sensing thermionic device acts directly to release (scram) the control rod (absorber) without reference or signal from the main reactor plant protective and control systems. To be responsive to both temperature and neutron flux effects, two detectors are used, one responsive to reactor coolant temperatures, and the other responsive to reactor neutron flux increase. The detectors are incorporated into a thermionic diode connected electrically with an electromagnetic mechanism which under normal reactor operating conditions holds the the control rod in its ready position (exterior of the reactor core). Upon reaching either a specified temperature or neutron flux, the thermionic diode functions to short-circuit the electromagnetic mechanism causing same to lose its holding power and release the control rod, which drops into the reactor core region under gravitational force.

Arrays of Nano Tunnel Junctions as Infrared Image Sensors

NASA Technical Reports Server (NTRS)

Son, Kyung-Ah; Moon, Jeong S.; Prokopuk, Nicholas

2006-01-01

Infrared image sensors based on high density rectangular planar arrays of nano tunnel junctions have been proposed. These sensors would differ fundamentally from prior infrared sensors based, variously, on bolometry or conventional semiconductor photodetection. Infrared image sensors based on conventional semiconductor photodetection must typically be cooled to cryogenic temperatures to reduce noise to acceptably low levels. Some bolometer-type infrared sensors can be operated at room temperature, but they exhibit low detectivities and long response times, which limit their utility. The proposed infrared image sensors could be operated at room temperature without incurring excessive noise, and would exhibit high detectivities and short response times. Other advantages would include low power demand, high resolution, and tailorability of spectral response. Neither bolometers nor conventional semiconductor photodetectors, the basic detector units as proposed would partly resemble rectennas. Nanometer-scale tunnel junctions would be created by crossing of nanowires with quantum-mechanical-barrier layers in the form of thin layers of electrically insulating material between them (see figure). A microscopic dipole antenna sized and shaped to respond maximally in the infrared wavelength range that one seeks to detect would be formed integrally with the nanowires at each junction. An incident signal in that wavelength range would become coupled into the antenna and, through the antenna, to the junction. At the junction, the flow of electrons between the crossing wires would be dominated by quantum-mechanical tunneling rather than thermionic emission. Relative to thermionic emission, quantum mechanical tunneling is a fast process.

Fuel elements of thermionic converters

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hunter, R.L.; Gontar, A.S.; Nelidov, M.V.

1997-01-01

Work on thermionic nuclear power systems has been performed in Russia within the framework of the TOPAZ reactor program since the early 1960s. In the TOPAZ in-core thermionic convertor reactor design, the fuel element`s cladding is also the thermionic convertor`s emitter. Deformation of the emitter can lead to short-circuiting and is the primary cause of premature TRC failure. Such deformation can be the result of fuel swelling, thermocycling, or increased unilateral pressure on the emitter due to the release of gaseous fission products. Much of the work on TRCs has concentrated on preventing or mitigating emitter deformation by improving themore » following materials and structures: nuclear fuel; emitter materials; electrical insulators; moderator and reflector materials; and gas-exhaust device. In addition, considerable effort has been directed toward the development of experimental techniques that accurately mimic operational conditions and toward the creation of analytical and numerical models that allow operational conditions and behavior to be predicted without the expense and time demands of in-pile tests. New and modified materials and structures for the cores of thermionic NPSs and new fabrication processes for the materials have ensured the possibility of creating thermionic NPSs for a wide range of powers, from tens to several hundreds of kilowatts, with life spans of 5 to 10 years.« less

Veritable electronic characteristics in ZnO nanowire circuits uncovered by the four-terminal method at a low temperature

NASA Astrophysics Data System (ADS)

Li, Xin; Zhang, Qi

2017-04-01

Understanding the natural electrical properties in semiconductor channels and the carrier transport across the metal-semiconductor contact is essential to improve the performance of nanowire devices. This work presents the true electronic characteristics of ZnO nanowire devices measured by a four-electrode method at a low-temperature environment. The temperature rise leads to the decrease in near-band-gap emission, which is attributed to two non-radiative recombination processes. For ZnO circuits, thermionic emission carrier transport mechanism plays a dominant role at Ti-Au/ZnO interface and the transport mechanism in ZnO nanowires is governed by two competitive thermal activation conduction processes: optical or acoustic phonons assisting hopping.

An overview of thermionic power conversion technology

DOE Office of Scientific and Technical Information (OSTI.GOV)

White, Morgan C.

1996-12-01

Thermionic energy conversion is one of the many concepts which make up the direct power conversion technologies. Specifically, thermionics is the process of changing heat directly into electricity via a material`s ability to emit electrons when heated. This thesis presents a broad overview of the engineering and physics necessary to make thermionic energy conversion (TEC) a practical reality. It begins with an introduction to the technology and the history of its development. This is followed by a discussion of the physics and engineering necessary to develop practical power systems. Special emphasis is placed on the critical issues which are stillmore » being researched. Finally, there is a discussion of the missions which this technology may fulfill.« less

Ion and advanced electric thruster research

NASA Technical Reports Server (NTRS)

Wilbur, P. J.

1980-01-01

A phenomenological model of the orificed, hollow cathode based on the field enhanced, thermionic mechanism of electron emission is presented. High frequency oscillations associated with the orificed, hollow cathode are shown to be a consequence of current flow through the cathode orifice. A procedure for Langmuir probing of the hollow cathode discharge and analyzing the resulting probe characteristics is discussed. The results of sputter yield measurements made for molybdenum, tantalum, type 304 stainless steel and copper surfaces being bombarded by low energy argon or mercury ions are also given. The effects of nitrogen and alternated copper layers on the sputter yields of molybdenum, tantalum and 304 stainless steel are also discussed. A dynamic model of electrothermal rocket and ramjet thrusters is developed. The gross performance of these devices is compared to that of an electromagnetic gun for the case of a high acceleration, Earth launch mission. The theoretical performance of electrothermal rockets and ramjets is shown to be comparable to that of the electromagnetic gun.

Electron transport characteristics of silicon nanowires by metal-assisted chemical etching

NASA Astrophysics Data System (ADS)

Qi, Yangyang; Wang, Zhen; Zhang, Mingliang; Wang, Xiaodong; Ji, An; Yang, Fuhua

2014-03-01

The electron transport characteristics of silicon nanowires (SiNWs) fabricated by metal-assisted chemical etching with different doping concentrations were studied. By increasing the doping concentration of the starting Si wafer, the resulting SiNWs were prone to have a rough surface, which had important effects on the contact and the electron transport. A metal-semiconductor-metal model and a thermionic field emission theory were used to analyse the current-voltage (I-V) characteristics. Asymmetric, rectifying and symmetric I-V curves were obtained. The diversity of the I-V curves originated from the different barrier heights at the two sides of the SiNWs. For heavily doped SiNWs, the critical voltage was one order of magnitude larger than that of the lightly doped, and the resistance obtained by differentiating the I-V curves at large bias was also higher. These were attributed to the lower electron tunnelling possibility and higher contact barrier, due to the rough surface and the reduced doping concentration during the etching process.

100 Years of the Physics of Diodes

NASA Astrophysics Data System (ADS)

Luginsland, John

2013-10-01

The Child-Langmuir Law (CL), discovered 100 years ago, gives the maximum current that can be transported across a planar diode in the steady state. As a quintessential example of the impact of space-charge shielding near a charged surface, it is central to the studies of high current diodes, such as high power microwave sources, vacuum microelectronics, electron and ion sources, and high current drivers used in high-energy density physics experiments. CL remains a touchstone of fundamental sheath physics, including contemporary studies of nano-scale quantum diodes and plasmonic devices. Its solid state analog is the Mott-Gurney law, governing the maximum charge injection in solids, such as organic materials and other dielectrics, which is important to energy devices, such as solar cells and light-emitting diodes. This paper reviews the important advances in the physics of diodes since the discovery of CL, including virtual cathode formation and extension of CL to multiple dimensions, to the quantum regime, and to ultrafast processes. We will review the influence of magnetic fields, multiple species in bipolar flow, electromagnetic and time dependent effects in both short pulse and high frequency THz limits, and single electron regimes. Transitions from various emission mechanisms (thermionic, field, and photo-emission) to the space charge limited state (CL) will be addressed, especially highlighting important simulation and experimental developments in selected contemporary areas of study. This talk will stress the fundamental physical links between the physics of beams to limiting currents in other areas, such as low temperature plasmas, laser plasmas, and space propulsion. Also emphasized is the role of non-equilibrium phenomena associated with materials and plasmas in close contact. Work supported by the Air Force Office of Scientific Research.

Helicon plasma generator-assisted surface conversion ion source for the production of H- ion beams at the Los Alamos Neutron Science Centera)

NASA Astrophysics Data System (ADS)

Tarvainen, O.; Rouleau, G.; Keller, R.; Geros, E.; Stelzer, J.; Ferris, J.

2008-02-01

The converter-type negative ion source currently employed at the Los Alamos Neutron Science Center (LANSCE) is based on cesium enhanced surface production of H- ion beams in a filament-driven discharge. In this kind of an ion source the extracted H- beam current is limited by the achievable plasma density which depends primarily on the electron emission current from the filaments. The emission current can be increased by increasing the filament temperature but, unfortunately, this leads not only to shorter filament lifetime but also to an increase in metal evaporation from the filament, which deposits on the H- converter surface and degrades its performance. Therefore, we have started an ion source development project focused on replacing these thermionic cathodes (filaments) of the converter source by a helicon plasma generator capable of producing high-density hydrogen plasmas with low electron energy. In our studies which have so far shown that the plasma density of the surface conversion source can be increased significantly by exciting a helicon wave in the plasma, and we expect to improve the performance of the surface converter H- ion source in terms of beam brightness and time between services. The design of this new source and preliminary results are presented, along with a discussion of physical processes relevant for H- ion beam production with this novel design. Ultimately, we perceive this approach as an interim step towards our long-term goal, combining a helicon plasma generator with an SNS-type main discharge chamber, which will allow us to individually optimize the plasma properties of the plasma cathode (helicon) and H- production (main discharge) in order to further improve the brightness of extracted H- ion beams.

Helicon plasma generator-assisted surface conversion ion source for the production of H(-) ion beams at the Los Alamos Neutron Science Center.

PubMed

Tarvainen, O; Rouleau, G; Keller, R; Geros, E; Stelzer, J; Ferris, J

2008-02-01

The converter-type negative ion source currently employed at the Los Alamos Neutron Science Center (LANSCE) is based on cesium enhanced surface production of H(-) ion beams in a filament-driven discharge. In this kind of an ion source the extracted H(-) beam current is limited by the achievable plasma density which depends primarily on the electron emission current from the filaments. The emission current can be increased by increasing the filament temperature but, unfortunately, this leads not only to shorter filament lifetime but also to an increase in metal evaporation from the filament, which deposits on the H(-) converter surface and degrades its performance. Therefore, we have started an ion source development project focused on replacing these thermionic cathodes (filaments) of the converter source by a helicon plasma generator capable of producing high-density hydrogen plasmas with low electron energy. In our studies which have so far shown that the plasma density of the surface conversion source can be increased significantly by exciting a helicon wave in the plasma, and we expect to improve the performance of the surface converter H(-) ion source in terms of beam brightness and time between services. The design of this new source and preliminary results are presented, along with a discussion of physical processes relevant for H(-) ion beam production with this novel design. Ultimately, we perceive this approach as an interim step towards our long-term goal, combining a helicon plasma generator with an SNS-type main discharge chamber, which will allow us to individually optimize the plasma properties of the plasma cathode (helicon) and H(-) production (main discharge) in order to further improve the brightness of extracted H(-) ion beams.

Convectively cooled electrical grid structure

DOEpatents

Paterson, J.A.; Koehler, G.W.

1980-11-10

Undesirable distortions of electrical grid conductors from thermal cycling are minimized and related problems such as unwanted thermionic emission and structural failure from overheating are avoided by providing for a flow of fluid coolant within each conductor. The conductors are secured at each end to separate flexible support elements which accommodate to individual longitudinal expansion and contraction of each conductor while resisting lateral displacements, the coolant flow preferably being directed into and out of each conductor through passages in the flexible support elements. The grid may have a modular or divided construction which facilitates manufacture and repairs.

Tunneling-assisted transport of carriers through heterojunctions.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wampler, William R.; Myers, Samuel M.; Modine, Normand A.

The formulation of carrier transport through heterojunctions by tunneling and thermionic emission is derived from first principles. The treatment of tunneling is discussed at three levels of approximation: numerical solution of the one-band envelope equation for an arbitrarily specified potential profile; the WKB approximation for an arbitrary potential; and, an analytic formulation assuming constant internal field. The effects of spatially varying carrier chemical potentials over tunneling distances are included. Illustrative computational results are presented. The described approach is used in exploratory physics models of irradiated heterojunction bipolar transistors within Sandia's QASPR program.

Thermally actuated thermionic switch

DOEpatents

Barrus, Donald M.; Shires, Charles D.

1988-01-01

A thermally actuated thermionic switch which responds to an increase of temperature by changing from a high impedance to a low impedance at a predictable temperature set point. The switch has a bistable operation mode switching only on temperature increases. The thermionic material may be a metal which is liquid at the desired operation temperature and held in matrix in a graphite block reservoir, and which changes state (ionizes, for example) so as to be electrically conductive at a desired temperature.

Thermionic converter

DOEpatents

Fitzpatrick, Gary O.

1987-05-19

A thermionic converter (10) is set forth which includes an envelope (12) having an electron collector structure (22) attached adjacent to a wall (16). An electron emitter structure (24) is positioned adjacent the collector structure (22) and spaced apart from opposite wall (14). The emitter (24) and collector (22) structures are in a common chamber (20). The emitter structure (24) is heated substantially only by thermal radiation. Very small interelectrode gaps (28) can be maintained utilizing the thermionic converter (10) whereby increased efficiency results.

Thermally actuated thermionic switch

DOEpatents

Barrus, D.M.; Shires, C.D.

1982-09-30

A thermally actuated thermionic switch which responds to an increase of temperature by changing from a high impedance to a low impedance at a predictable temperature set point. The switch has a bistable operation mode switching only on temperature increases. The thermionic material may be a metal which is liquid at the desired operation temperature and held in matrix in a graphite block reservoir, and which changes state (ionizes, for example) so as to be electrically conductive at a desired temperature.

High-Performance Solid-State Thermionic Energy Conversion Based on 2D van der Waals Heterostructures: A First-Principles Study.

PubMed

Wang, Xiaoming; Zebarjadi, Mona; Esfarjani, Keivan

2018-06-18

Two-dimensional (2D) van der Waals heterostructures (vdWHs) have shown multiple functionalities with great potential in electronics and photovoltaics. Here, we show their potential for solid-state thermionic energy conversion and demonstrate a designing strategy towards high-performance devices. We propose two promising thermionic devices, namely, the p-type Pt-G-WSe 2 -G-Pt and n-type Sc-WSe 2 -MoSe 2 -WSe 2 -Sc. We characterize the thermionic energy conversion performance of the latter using first-principles GW calculations combined with real space Green's function (GF) formalism. The optimal barrier height and high thermal resistance lead to an excellent performance. The proposed device is found to have a room temperature equivalent figure of merit of 1.2 which increases to 3 above 600 K. A high performance with cooling efficiency over 30% of the Carnot efficiency above 450 K is achieved. Our designing and characterization method can be used to pursue other potential thermionic devices based on vdWHs.

A hybrid model describing ion induced kinetic electron emission

NASA Astrophysics Data System (ADS)

Hanke, S.; Duvenbeck, A.; Heuser, C.; Weidtmann, B.; Wucher, A.

2015-06-01

We present a model to describe the kinetic internal and external electron emission from an ion bombarded metal target. The model is based upon a molecular dynamics treatment of the nuclear degree of freedom, the electronic system is assumed as a quasi-free electron gas characterized by its Fermi energy, electron temperature and a characteristic attenuation length. In a series of previous works we have employed this model, which includes the local kinetic excitation as well as the rapid spread of the generated excitation energy, in order to calculate internal and external electron emission yields within the framework of a Richardson-Dushman-like thermionic emission model. However, this kind of treatment turned out to fail in the realistic prediction of experimentally measured internal electron yields mainly due to the restriction of the treatment of electronic transport to a diffusive manner. Here, we propose a slightly modified approach additionally incorporating the contribution of hot electrons which are generated in the bulk material and undergo ballistic transport towards the emitting interface.

Thermionic reactors for space nuclear power

NASA Technical Reports Server (NTRS)

Homeyer, W. G.; Merrill, M. H.; Holland, J. W.; Fisher, C. R.; Allen, D. T.

1985-01-01

Thermionic reactor designs for a variety of space power applications spanning the range from 5 kWe to 3 MWe are described. In all of these reactors, nuclear heat is converted directly to electrical energy in thermionic fuel elements (TFEs). A circulating reactor coolant carries heat from the core of TFEs directly to a heat rejection radiator system. The recent design of a thermionic reactor to meet the SP-100 requirements is emphasized. Design studies of reactors at other power levels show that the same TFE can be used over a broad range in power, and that design modifications can extend the range to many megawatts. The design of the SP-100 TFE is similar to that of TFEs operated successfully in test reactors, but with design improvements to extend the operating lifetime to seven years.

Back-bombardment compensation in microwave thermionic electron guns

NASA Astrophysics Data System (ADS)

Kowalczyk, Jeremy M. D.; Madey, John M. J.

2014-12-01

The development of capable, reliable, and cost-effective compact electron beam sources remains a long-standing objective of the efforts to develop the accelerator systems needed for on-site research and industrial applications ranging from electron beam welding to high performance x-ray and gamma ray light sources for element-resolved microanalysis and national security. The need in these applications for simplicity, reliability, and low cost has emphasized solutions compatible with the use of the long established and commercially available pulsed microwave rf sources and L-, S- or X-band linear accelerators. Thermionic microwave electron guns have proven to be one successful approach to the development of the electron sources for these systems providing high macropulse average current beams with picosecond pulse lengths and good emittance out to macropulse lengths of 4-5 microseconds. But longer macropulse lengths are now needed for use in inverse-Compton x-ray sources and other emerging applications. We describe in this paper our approach to extending the usable macropulse current and pulse length of these guns through the use of thermal diffusion to compensate for the increase in cathode surface temperature due to back-bombardment.

Electrical properties of Si-Si interfaces obtained by room temperature covalent wafer bonding

NASA Astrophysics Data System (ADS)

Jung, A.; Zhang, Y.; Arroyo Rojas Dasilva, Y.; Isa, F.; von Känel, H.

2018-02-01

We study covalent bonds between p-doped Si wafers (resistivity ˜10 Ω cm) fabricated on a recently developed 200 mm high-vacuum system. Oxide- and void free interfaces were obtained by argon (Ar) or neon (Ne) sputtering prior to wafer bonding at room temperature. The influence of the sputter induced amorphous Si layer at the bonding interface on the electrical behavior is accessed with temperature-dependent current-voltage measurements. In as-bonded structures, charge transport is impeded by a potential barrier of 0.7 V at the interface with thermionic emission being the dominant charge transport mechanism. Current-voltage characteristics are found to be asymmetric which can tentatively be attributed to electric dipole formation at the interface as a result of the time delay between the surface preparation of the two bonding partners. Electron beam induced current measurements confirm the corresponding asymmetric double Schottky barrier like band-alignment. Moreover, we demonstrate that defect annihilation at a low temperature of 400 °C increases the electrical conductivity by up to three orders of magnitude despite the lack of recrystallization of the amorphous layer. This effect is found to be more pronounced for Ne sputtered surfaces which is attributed to the lighter atomic mass compared to Ar, inducing weaker lattice distortions during the sputtering.

Enhancement of memory margins in the polymer composite of [6,6]-phenyl-C61-butyric acid methyl ester and polystyrene.

PubMed

Sun, Yanmei; Lu, Junguo; Ai, Chunpeng; Wen, Dianzhong; Bai, Xuduo

2016-11-09

Memory devices based on composites of polystyrene (PS) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) were investigated with bistable resistive switching behavior. Current-voltage (I-V) curves for indium-tin-oxide (ITO)/PS + PCBM/Al devices with 33 wt% PCBM showed non-volatile, rewritable, flash memory properties with a maximum ON/OFF current ratio of 1 × 10 4 , which was 100 times larger than the ON/OFF ratio of the device with 5 wt% PCBM. For ITO/PS + PCBM/Al devices with 33 wt% PCBM, the write-read-erase-read test cycles demonstrated the bistable devices with ON and OFF states at the same voltage. The programmable ON and OFF states endured up to 10 4 read pulses and possessed a retention time of over 10 5 s, indicative of the memory stability of the device. In the OFF state, the I-V curve at lower voltages up to 0.45 V was attributed to the thermionic emission mechanism, and the I-V characteristics in the applied voltage above 0.5 V dominantly followed the space-charge-limited-current behaviors. In the ON state, the curve in the applied voltage range was related to an Ohmic mechanism.

Magnetic field induced suppression of the forward bias current in Bi2Se3/Si Schottky barrier diodes

NASA Astrophysics Data System (ADS)

Jin, Haoming; Hebard, Arthur

Schottky diodes formed by van der Waals bonding between freshly cleaved flakes of the topological insulator Bi2Se3 and doped silicon substrates show electrical characteristics in good agreement with thermionic emission theory. The motivation is to use magnetic fields to modulate the conductance of the topologically protected conducting surface state. This surface state in close proximity to the semiconductor surface may play an important role in determining the nature of the Schottky barrier. Current-voltage (I-V) and capacitance-voltage (C-V) characteristics were obtained for temperatures in the range 50-300 K and magnetic fields, both perpendicular and parallel to the interface, as high as 7 T. The I-V curve shows more than 6 decades linearity on semi-logarithmic plots, allowing extraction of parameters such as ideality (η), zero-voltage Schottky barrier height (SBH), and series resistance (Rs). In forward bias we observe a field-induced decrease in current which becomes increasingly more pronounced at higher voltages and lower temperature, and is found to be correlated with changes in Rs rather than other barrier parameters. A comparison of changes in Rs in both field direction will be made with magnetoresistance in Bi2Se3 transport measurement. The work is supported by NSF through DMR 1305783.

Alternative model of space-charge-limited thermionic current flow through a plasma

NASA Astrophysics Data System (ADS)

Campanell, M. D.

2018-04-01

It is widely assumed that thermionic current flow through a plasma is limited by a "space-charge-limited" (SCL) cathode sheath that consumes the hot cathode's negative bias and accelerates upstream ions into the cathode. Here, we formulate a fundamentally different current-limited mode. In the "inverse" mode, the potentials of both electrodes are above the plasma potential, so that the plasma ions are confined. The bias is consumed by the anode sheath. There is no potential gradient in the neutral plasma region from resistivity or presheath. The inverse cathode sheath pulls some thermoelectrons back to the cathode, thereby limiting the circuit current. Thermoelectrons entering the zero-field plasma region that undergo collisions may also be sent back to the cathode, further attenuating the circuit current. In planar geometry, the plasma density is shown to vary linearly across the electrode gap. A continuum kinetic planar plasma diode simulation model is set up to compare the properties of current modes with classical, conventional SCL, and inverse cathode sheaths. SCL modes can exist only if charge-exchange collisions are turned off in the potential well of the virtual cathode to prevent ion trapping. With the collisions, the current-limited equilibrium must be inverse. Inverse operating modes should therefore be present or possible in many plasma devices that rely on hot cathodes. Evidence from past experiments is discussed. The inverse mode may offer opportunities to minimize sputtering and power consumption that were not previously explored due to the common assumption of SCL sheaths.

Investigation of efficiency enhancement in InGaN MQW LED with compositionally step graded GaN/InAlN/GaN multi-layer barrier

NASA Astrophysics Data System (ADS)

Prajoon, P.; Anuja Menokey, M.; Charles Pravin, J.; Ajayan, J.; Rajesh, S.; Nirmal, D.

2018-04-01

The advantage of InGaN multiple Quantum well (MQW) Light emitting diode (LED) on a SiC substrate with compositionally step graded GaN/InAlN/GaN multi-layer barrier (MLB) is studied. The Internal quantum efficiency, Optical power, current-voltage characteristics, spontaneous emission rate and carrier distribution profile in the active region are investigated using Sentaurus TCAD simulation. An analytical model is also developed to describe the QW carrier injection efficiency, by including carrier leakage mechanisms like carrier overflow, thermionic emission and tunnelling. The enhanced electron confinement, reduced carrier asymmetry, and suppressed carrier overflow in the active region of the MLB MQW LED leads to render a superior performance than the conventional GaN barrier MQW LED. The simulation result also elucidates the efficiency droop behaviour in the MLB MQW LED, it suggests that the efficiency droop effect is remarkably improved when the GaN barrier is replaced with GaN/InAlN/GaN MLB barrier. The analysis shows a dominating behaviour of carrier escape mechanism due to tunnelling. Moreover, the lower lattice mismatching of SiC substrate with GaN epitaxial layer is attributed with good crystal quality and reduced polarization effect, ultimately enhances the optical performance of the LEDs.

Poole-Frenkel effect in sputter-deposited CuAlO2+x nanocrystals

NASA Astrophysics Data System (ADS)

Narayan Banerjee, Arghya; Joo, Sang Woo

2013-04-01

Field-assisted thermionic emission within a sputter-deposited, nanocrystalline thin film of CuAlO2.06 is observed for the first time, and explained in terms of the Poole-Frenkel model. The presence of adsorbed oxygen ions as trap-states at the grain boundary regions of the nanostructured thin film is considered to manifest this phenomenon. Under an applied field, the barrier of the trap potential is lowered and thermal emission of charge carriers takes place at different sample temperatures to induce nonlinearity in the current (I)-voltage (V) characteristics of the nanomaterial. Fitting of the Poole-Frenkel model with the I-V data shows that the nonlinearity is effective above 50 V under the operating conditions. Calculations of the energy of the trap level, acceptor level and Fermi level reveal the existence of deep level trap-states and a shallow acceptor level with acceptor concentration considerably higher than the trap-states. Hall measurements confirm the p-type semiconductivity of the film, with a hole concentration around 1018 cm-3. Thermopower measurements give a room-temperature Seebeck coefficient around 130 μV K-1. This temperature-dependent conductivity enhancement within CuAlO2 nanomaterial may find interesting applications in transparent electronics and high-voltage applications for power supply networks.

Poole-Frenkel effect in sputter-deposited CuAlO(2+x) nanocrystals.

PubMed

Banerjee, Arghya Narayan; Joo, Sang Woo

2013-04-26

Field-assisted thermionic emission within a sputter-deposited, nanocrystalline thin film of CuAlO2.06 is observed for the first time, and explained in terms of the Poole-Frenkel model. The presence of adsorbed oxygen ions as trap-states at the grain boundary regions of the nanostructured thin film is considered to manifest this phenomenon. Under an applied field, the barrier of the trap potential is lowered and thermal emission of charge carriers takes place at different sample temperatures to induce nonlinearity in the current (I)-voltage (V) characteristics of the nanomaterial. Fitting of the Poole-Frenkel model with the I-V data shows that the nonlinearity is effective above 50 V under the operating conditions. Calculations of the energy of the trap level, acceptor level and Fermi level reveal the existence of deep level trap-states and a shallow acceptor level with acceptor concentration considerably higher than the trap-states. Hall measurements confirm the p-type semiconductivity of the film, with a hole concentration around 10(18) cm(-3). Thermopower measurements give a room-temperature Seebeck coefficient around 130 μV K(-1). This temperature-dependent conductivity enhancement within CuAlO2 nanomaterial may find interesting applications in transparent electronics and high-voltage applications for power supply networks.

Optical Properties and Junction Characteristics of 6-(5-Bromothiohen-2-yl)-2,3-Dihydro-1-Methyl-3-Oxo-2-Phenyl-1 H-Pyrazolo[4,3-b]Pyridine-5-Carbonitrile Films

NASA Astrophysics Data System (ADS)

Zedan, I. T.; El-Taweel, F. M. A.; Abu El-Enein, R. A. N.; Nawar, H. H.; El-Menyawy, E. M.

2016-11-01

In this study, 6-(5-bromothiohen-2-yl)-2,3-dihydro-1-methyl-3-oxo-2-phenyl-1 H-pyrazolo[4,3-b]pyridine-5-carbonitrile (BDPC) powder was synthesized. BDPC powder showed a polycrystalline structure, whereas the thermally evaporated films had an amorphous structure. The optical parameters such as absorption coefficient and refractive index were calculated in the spectral range 200-500 nm. Spectral distribution analysis of the absorption coefficient revealed that the films had an indirect band transitions with energy gaps of 2.57 eV and 3.5 eV. According to the single oscillator model, the oscillation energy, dispersion energy, and dielectric constant were estimated. The room-temperature current-voltage characteristics of the fabricated Au/BDPC/p-Si/Al heterojunction showed diode-like behavior. The ideality factor, the barrier height and series resistance were determined based on thermionic emission theory and Norde's function. At reverse bias, the current was interpreted in terms of the Schottky and pool-Frenkle effects in low and high voltages, respectively. The built-in voltage, carrier concentration and barrier height were obtained using capacitance-voltage characteristics.

Towards high frequency heterojunction transistors: Electrical characterization of N-doped amorphous silicon-graphene diodes

NASA Astrophysics Data System (ADS)

Strobel, C.; Chavarin, C. A.; Kitzmann, J.; Lupina, G.; Wenger, Ch.; Albert, M.; Bartha, J. W.

2017-06-01

N-type doped amorphous hydrogenated silicon (a-Si:H) is deposited on top of graphene (Gr) by means of very high frequency (VHF) and radio frequency plasma-enhanced chemical vapor deposition (PECVD). In order to preserve the structural integrity of the monolayer graphene, a plasma excitation frequency of 140 MHz was successfully applied during the a-Si:H VHF-deposition. Raman spectroscopy results indicate the absence of a defect peak in the graphene spectrum after the VHF-PECVD of (n)-a-Si:H. The diode junction between (n)-a-Si:H and graphene was characterized using temperature dependent current-voltage (IV) and capacitance-voltage measurements, respectively. We demonstrate that the current at the (n)-a-Si:H-graphene interface is dominated by thermionic emission and recombination in the space charge region. The Schottky barrier height (qΦB), derived by temperature dependent IV-characteristics, is about 0.49 eV. The junction properties strongly depend on the applied deposition method of (n)-a-Si:H with a clear advantage of the VHF(140 MHz)-technology. We have demonstrated that (n)-a-Si:H-graphene junctions are a promising technology approach for high frequency heterojunction transistors.

Graphene-based vdW heterostructure Induced High-efficiency Thermoelectric Devices

NASA Astrophysics Data System (ADS)

Liang, Shijun; Ang, Lay Kee

Thermoelectric material (TE) can convert the heat into electricity to provide green energy source and its performance is characterized by a figure of merit (ZT) parameter. Traditional TE materials only give ZT equal to around 1 at room temperature. But, it is believed that materials with ZT >3 will find wide applications at this low temperature range. Prior studies have implied that the interrelation between electric conductivity and lattice thermal conductivity renders the goal of engineering ZT of bulk materials to reach ZT >3. In this work, we propose a high-efficiency van del Waals (vdW) heterostructure-based thermionic device with graphene electrodes, which is able to harvest wasted heat (around 400K) based on the newly established thermionic emission law of graphene electrodes instead of Seebeck effect, to boost the efficiency of power generation over 10% around room temperature. The efficiency can be above 20% if the Schottky barrier height and cross-plane lattice thermal conductivity of transition metal dichacogenides (TMD) materials can be fine-engineered. As a refrigerator at 260 K, the efficiency is 50% to 80% of Carnot efficiency. Finally, we identify two TMD materials as the ideal candidates of graphene/TMD/graphene devices based on the state-of-art technology.

An out-of-core thermionic-converter system for nuclear space power

NASA Technical Reports Server (NTRS)

Breitwieser, R.

1972-01-01

Design of the nuclear thermionic space power system, 40 50 70 Kw(e) power range, are given. The design configuration (1) meets the constraints of readily available launch vehicles; (2) allows for off-design operation including startup, shutdown, and possible emergency conditions; (3) provides tolerance of failure by extensive use of modular, redundant elements; (4) incorporates and uses heat pipes in a fashion that reduces the need for extensive in-pile testing of system components; and (5) uses thermionic converters, nuclear fuel elements, and heat transfer devices in a geometrical form adapted from existing incore thermionic system designs. Designs and in some cases performance data for elements and groups of the elements of the system are included. Benefits of the highly modular system approach to reliability, safety, economy of development, and flexibility are discussed.

Experimental detailed power distribution in a fast spectrum thermionic reactor fuel element at the core/BeO reflector interface region

NASA Technical Reports Server (NTRS)

Klann, P. G.; Lantz, E.

1973-01-01

A zero-power critical assembly was designed, constructed, and operated for the prupose of conducting a series of benchmark experiments dealing with the physics characteristics of a UN-fueled, Li-7-cooled, Mo-reflected, drum-controlled compact fast reactor for use with a space-power conversion system. The critical assembly was modified to simulate a fast spectrum advanced thermionics reactor by: (1) using BeO as a reflector in place of some of the existing molybdenum, (2) substituting Nb-1Zr tubing for some of the existing Ta tubing, and (3) inserting four full-scale mockups of thermionic type fuel elements near the core and BeO reflector boundary. These mockups were surrounded with a buffer zone having the equivalent thermionic core composition. In addition to measuring the critical mass of this thermionic configuration, a detailed power distribution in one of the thermionic element stages in the mixed spectrum region was measured. A power peak to average ratio of two was observed for this fuel stage at the midplane of the core and adjacent to the reflector. Also, the power on the outer surface adjacent to the BeO was slightly more than a factor of two larger than the power on the inside surface of a 5.08 cm (2.0 in.) high annular fuel segment with a 2.52 cm (0.993 in. ) o.d. and a 1.86 cm (0.731 in.) i.d.

Probing the Limits: Collected Works on the Second Law of Thermodynamics and Special Relativity

NASA Astrophysics Data System (ADS)

D'Abramo, Germano

2017-01-01

Synopsis: This book brings together the chief results of the research work carried out by the author on the second law of thermodynamics and the theory of special relativity since 2008. The first six chapters are devoted to the research on the epistemological status of the second law of thermodynamics and the connection between thermionic/photoelectric phenomena and the second law: evidence is provided that thermionic emission could, in principle, violate the second law. More precisely, the photoelectric emission induced by the high-frequency tail of black-body radiation at room temperature (heat) can be harnessed to charge a capacitor and provide readily usable energy from a single heat reservoir. Chapter 7 contains some reflections on special relativity. It is the most speculative part of the book and it has not been published elsewhere. Two thought experiments on time dilation in the framework of special relativity are presented. The main contention in this part of the book is that if both postulates of special relativity are assumed to hold concurrently, then the prediction of asymmetric ageing made by Einstein in his 1905 relativity paper appears to be in fact incompatible with them and the fact that time dilation (which is intimately related to "asymmetric ageing") seems to have been experimentally confirmed provides, paradoxically, a refutation rather than a confirmation of the theory of special relativity, at least as interpreted today. A critical assessment of Purcell's basic explanation of magnetic forces, which basically relies on special relativity, is also given at the end of the book.

An assessment of the hardness of miniature vacuum tubes to high-voltage transients

DOE Office of Scientific and Technical Information (OSTI.GOV)

Orvis, W.J.

1990-03-01

Miniature vacuum tubes are vacuum switching and control devices fabricated on a silicon wafer, using the same technology as is used to make integrated circuits. They operate in much the same manner as conventional vacuum tubes, but with two important differences: they are micron sized devices, and they employ field emission instead of thermionic emission as the electron source. As these devices have a vacuum as their active region, they will be extremely hard to nuclear radiation and relatively insensitive to temperature effects, they are also expected to be extremely fast devices. We have estimated here that their hardness tomore » high-voltage transients will be at least as good as existing semiconductor devices and possibly better. 5 figs.« less

Nanomaterial-based x-ray sources

NASA Astrophysics Data System (ADS)

Cole, Matthew T.; Parmee, R. J.; Milne, William I.

2016-02-01

Following the recent global excitement and investment in the emerging, and rapidly growing, classes of one and two-dimensional nanomaterials, we here present a perspective on one of the viable applications of such materials: field electron emission based x-ray sources. These devices, which have a notable history in medicine, security, industry and research, to date have almost exclusively incorporated thermionic electron sources. Since the middle of the last century, field emission based cathodes were demonstrated, but it is only recently that they have become practicable. We outline some of the technological achievements of the past two decades, and describe a number of the seminal contributions. We explore the foremost market hurdles hindering their roll-out and broader industrial adoption and summarise the recent progress in miniaturised, pulsed and multi-source devices.

Structural defects and recombination behavior of excited carriers in Cu(In,Ga)Se2 solar cells

NASA Astrophysics Data System (ADS)

Yang, J.; Du, H. W.; Li, Y.; Gao, M.; Wan, Y. Z.; Xu, F.; Ma, Z. Q.

2016-08-01

The carriers' behavior in neutral region (NTR) and space charged region (SCR) of Cu(In,Ga)Se2 thin film based solar cells has been investigated by temperature dependent photoluminescence (PL-T), electroluminescence (EL-T) and current-voltage (IV-T) from 10 to 300 K. PL-T spectra show that three kinds of defects, namely VSe, InCu and (InCu+VCu), are localized within the band gap of NTR and SCR of CIGS layer, corresponding to the energy levels of EC-0.08, EC-0.20 and EC-0.25 eV, respectively. The InCu and (InCu+VCu) deep level defects are non-radiative recombination centers at room temperature. The IV-T and EL-T analysis reveals that the injection modes of electrons from ZnO conduction band into Cu(In,Ga)Se2 layer are tunneling, thermally-excited tunneling and thermionic emission under 10-40, 60-160, and 180-300 K, respectively. At 10-160 K, the electrons tunnel into (InCu+VCu) and Vse defect levels in band gap of SCR and the drifting is involved in the emission bands at 0.96 and 1.07 eV, which is the direct evidence for a tunneling assisted recombination. At 180-300 K, the electrons are directly injected into the Cu(In,Ga)Se2 conduction band, and the emission of 1.13 eV are ascribed to the transitions from the conduction band to the valence band.

Life Testing and Diagnostics of a Planar Out-of-Core Thermionic Converter

NASA Astrophysics Data System (ADS)

Thayer, Kevin L.; Ramalingam, Mysore L.; Young, Timothy J.; Lamp, Thomas R.

1994-07-01

This paper details the design and performance of an automated computer data acquisition system for a planar, out-of-core thermionic converter with CVD rhenium electrodes. The output characteristics of this converter have been mapped for emitter temperatures ranging from approximately 1700K to 2000K, and life testing of the converter is presently being performed at the design point of operation. An automated data acquisition system has been constructed to facilitate the collection of current density versus output voltage (J-V) and temperature data from the converter throughout the life test. This system minimizes the amount of human interaction necessary during the lifetest to measure and archive the data and present it in a usable form. The task was accomplished using a Macintosh Ilcx computer, two multiple-purpose interface boards, a digital oscilloscope, a sweep generator, and National Instrument's LabVIEW application software package.

Junction formation and current transport mechanisms in hybrid n-Si/PEDOT:PSS solar cells

PubMed Central

Jäckle, Sara; Mattiza, Matthias; Liebhaber, Martin; Brönstrup, Gerald; Rommel, Mathias; Lips, Klaus; Christiansen, Silke

2015-01-01

We investigated hybrid inorganic-organic solar cells combining monocrystalline n-type silicon (n-Si) and a highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS). The build-in potential, photo- and dark saturation current at this hybrid interface are monitored for varying n-Si doping concentrations. We corroborate that a high build-in potential forms at the hybrid junction leading to strong inversion of the n-Si surface. By extracting work function and valence band edge of the polymer from ultraviolet photoelectron spectroscopy, a band diagram of the hybrid n-Si/PEDOT:PSS heterojunction is presented. The current-voltage characteristics were analyzed using Schottky and abrupt pn-junction models. The magnitude as well as the dependence of dark saturation current on n-Si doping concentration proves that the transport is governed by diffusion of minority charge carriers in the n-Si and not by thermionic emission of majorities over a Schottky barrier. This leads to a comprehensive explanation of the high observed open-circuit voltages of up to 634 mV connected to high conversion efficiency of almost 14%, even for simple planar device structures without antireflection coating or optimized contacts. The presented work clearly shows that PEDOT:PSS forms a hybrid heterojunction with n-Si behaving similar to a conventional pn-junction and not, like commonly assumed, a Schottky junction. PMID:26278010

Analysis of high reverse currents of 4H-SiC Schottky-barrier diodes

NASA Astrophysics Data System (ADS)

Okino, Hiroyuki; Kameshiro, Norifumi; Konishi, Kumiko; Shima, Akio; Yamada, Ren-ichi

2017-12-01

Nickel (Ni), titanium (Ti), and molybdenum (Mo) 4H-silicon carbide Schottky-barrier diodes (SiC SBDs) were fabricated and used to investigate the relation between forward and reverse currents. Temperature dependence of reverse current follows a theory that includes tunneling in regard to thermionic emission, namely, temperature dependence is weak at low temperature but strong at high temperatures. On the other hand, the reverse currents of the Ni and Mo SBDs are higher than their respective currents calculated from their Schottky barrier heights (SBHs), whereas the reverse current of the Ti SBD agrees well with that calculated from its SBH. The cause of the high reverse currents was investigated from the viewpoints of low barrier patch, Gaussian distribution of barrier height (GD), thin surface barrier, and electron effective mass. The high reverse current of the Ni and Mo SBDs can be explained not in terms of a low-barrier patch, GD, or thin surface barrier but in terms of small effective masses. Investigation of crystal structures at the Schottky interface revealed a large lattice mismatch between the metals (Ni, Ti, or Mo) and SiC for the Ni and Mo SBDs. The small effective mass is possibly attributed to the large lattice mismatch, which might generate transition layers at the Schottky interface. It is concluded from these results that the lattice constant as well as the work function is an important factor in selecting the metal species as the Schottky metal for wide band-gap SBDs, for which tunneling current dominates reverse current.

Thermionic converter temperature controller

DOEpatents

Shaner, Benjamin J [McMurray, PA; Wolf, Joseph H [Pittsburgh, PA; Johnson, Robert G. R. [Trafford, PA

2001-04-24

A method and apparatus for controlling the temperature of a thermionic reactor over a wide range of operating power, including a thermionic reactor having a plurality of integral cesium reservoirs, a honeycomb material disposed about the reactor which has a plurality of separated cavities, a solid sheath disposed about the honeycomb material and having an opening therein communicating with the honeycomb material and cavities thereof, and a shell disposed about the sheath for creating a coolant annulus therewith so that the coolant in the annulus may fill the cavities and permit nucleate boiling during the operation of the reactor.

Space power reactor in-core thermionic multicell evolutionary (S-prime) design

NASA Astrophysics Data System (ADS)

Determan, William R.; Van Hagan, Tom H.

1993-01-01

A 5- to 40-kWe moderated in-core thermionic space nuclear power system (TI-SNPS) concept was developed to address the TI-SNPS program requirements. The 40-kWe baseline design uses multicell Thermionic Fuel Elements (TFEs) in a zirconium hydride moderated reactor to achieve a specific mass of 18.2 We/kg and a net end-of-mission (EOM) efficiency of 8.2%. The reactor is cooled with a single NaK-78 pumped loop, which rejects the heat through a 24 m2 heat pipe space radiator.

Advanced thermionic converter development

NASA Technical Reports Server (NTRS)

Huffman, F. N.; Lieb, D.; Briere, T. R.; Sommer, A. H.; Rufeh, F.

1976-01-01

Recent progress at Thermo Electron in developing advanced thermionic converters is summarized with particular attention paid to the development of electrodes, diodes, and triodes. It is found that one class of materials (ZnO, BaO and SrO) provides interesting cesiated work functions (1.3-1.4 eV) without additional oxygen. The second class of materials studied (rare earth oxides and hexaborides) gives cesiated/oxygenated work functions of less than 1.2 eV. Five techniques of oxygen addition to thermionic converters are discussed. Vapor deposited tungsten oxide collector diodes and the reflux converter are considered.

Comparison of Tungsten and Molybdenum Based Emitters for Advanced Thermionic Space Nuclear Reactors

NASA Astrophysics Data System (ADS)

Lee, Hsing H.; Dickinson, Jeffrey W.; Klein, Andrew C.; Lamp, Thomas R.

1994-07-01

Variations to the Advanced Thermionic Initiative thermionic fuel element are analyzed. Analysis included neutronic modeling with MCNP for criticality determination and thermal power distribution, and thermionic performance modeling with TFEHX. Changes to the original ATI configuration include the addition of W-HfC wire to the emitter for high temperature creep resistance improvement and substitution of molybdenum for the tungsten base material. Results from MCNP showed that all the tungsten used in the coating and base material must be 100% W-184 to obtain criticality. The presence of molybdenum in the emitter base affects the neutronic performance of the TFE by increasing the emitter neutron absorption cross section. Due to the reduced thermal conductivity for the molybdenum based emitter, a higher temperature is obtained resulting in a greater electrical power production. The thermal conductivity and resistivity of the composite emitter region were derived for the W-Mo composite and used in TFEHX.

Extreme sub-threshold swing in tunnelling relays

NASA Astrophysics Data System (ADS)

AbdelGhany, M.; Szkopek, T.

2014-01-01

We propose and analyze the theory of the tunnelling relay, a nanoscale active device in which tunnelling current is modulated by electromechanical actuation of a suspended membrane above a fixed electrode. The tunnelling current is modulated exponentially with vacuum gap length, permitting an extreme sub-threshold swing of ˜10 mV/decade breaking the thermionic limit. The predicted performance suggests that a significant reduction in dynamic energy consumption over conventional field effect transistors is physically achievable.

Convectively cooled electrical grid structure

DOEpatents

Paterson, James A.; Koehler, Gary W.

1982-01-01

Undesirable distortions of electrical grid conductors (12) from thermal cycling are minimized and related problems such as unwanted thermionic emission and structural failure from overheating are avoided by providing for a flow of fluid coolant within each conductor (12). The conductors (12) are secured at each end to separate flexible support elements (16) which accommodate to individual longitudinal expansion and contraction of each conductor (12) while resisting lateral displacements, the coolant flow preferably being directed into and out of each conductor through passages (48) in the flexible support elements (16). The grid (11) may have a modular or divided construction which facilitates manufacture and repairs.

Alternative model of space-charge-limited thermionic current flow through a plasma

DOE PAGES

Campanell, M. D.

2018-04-19

It is widely assumed that thermionic current flow through a plasma is limited by a “space-charge-limited” (SCL) cathode sheath that consumes the hot cathode's negative bias and accelerates upstream ions into the cathode. In this paper, we formulate a fundamentally different current-limited mode. In the “inverse” mode, the potentials of both electrodes are above the plasma potential, so that the plasma ions are confined. The bias is consumed by the anode sheath. There is no potential gradient in the neutral plasma region from resistivity or presheath. The inverse cathode sheath pulls some thermoelectrons back to the cathode, thereby limiting themore » circuit current. Thermoelectrons entering the zero-field plasma region that undergo collisions may also be sent back to the cathode, further attenuating the circuit current. In planar geometry, the plasma density is shown to vary linearly across the electrode gap. A continuum kinetic planar plasma diode simulation model is set up to compare the properties of current modes with classical, conventional SCL, and inverse cathode sheaths. SCL modes can exist only if charge-exchange collisions are turned off in the potential well of the virtual cathode to prevent ion trapping. With the collisions, the current-limited equilibrium must be inverse. Inverse operating modes should therefore be present or possible in many plasma devices that rely on hot cathodes. Evidence from past experiments is discussed. Finally, the inverse mode may offer opportunities to minimize sputtering and power consumption that were not previously explored due to the common assumption of SCL sheaths.« less

Alternative model of space-charge-limited thermionic current flow through a plasma

DOE Office of Scientific and Technical Information (OSTI.GOV)

Campanell, M. D.

It is widely assumed that thermionic current flow through a plasma is limited by a “space-charge-limited” (SCL) cathode sheath that consumes the hot cathode's negative bias and accelerates upstream ions into the cathode. In this paper, we formulate a fundamentally different current-limited mode. In the “inverse” mode, the potentials of both electrodes are above the plasma potential, so that the plasma ions are confined. The bias is consumed by the anode sheath. There is no potential gradient in the neutral plasma region from resistivity or presheath. The inverse cathode sheath pulls some thermoelectrons back to the cathode, thereby limiting themore » circuit current. Thermoelectrons entering the zero-field plasma region that undergo collisions may also be sent back to the cathode, further attenuating the circuit current. In planar geometry, the plasma density is shown to vary linearly across the electrode gap. A continuum kinetic planar plasma diode simulation model is set up to compare the properties of current modes with classical, conventional SCL, and inverse cathode sheaths. SCL modes can exist only if charge-exchange collisions are turned off in the potential well of the virtual cathode to prevent ion trapping. With the collisions, the current-limited equilibrium must be inverse. Inverse operating modes should therefore be present or possible in many plasma devices that rely on hot cathodes. Evidence from past experiments is discussed. Finally, the inverse mode may offer opportunities to minimize sputtering and power consumption that were not previously explored due to the common assumption of SCL sheaths.« less

Microminiature thermionic converters

DOEpatents

King, Donald B.; Sadwick, Laurence P.; Wernsman, Bernard R.

2001-09-25

Microminiature thermionic converts (MTCs) having high energy-conversion efficiencies and variable operating temperatures. Methods of manufacturing those converters using semiconductor integrated circuit fabrication and micromachine manufacturing techniques are also disclosed. The MTCs of the invention incorporate cathode to anode spacing of about 1 micron or less and use cathode and anode materials having work functions ranging from about 1 eV to about 3 eV. Existing prior art thermionic converter technology has energy conversion efficiencies ranging from 5-15%. The MTCs of the present invention have maximum efficiencies of just under 30%, and thousands of the devices can be fabricated at modest costs.

Thermionic Energy Conversion (TEC) topping thermoelectrics

NASA Technical Reports Server (NTRS)

Morris, J. F.

1981-01-01

Performance expectations for thermionic and thermoelectric energy conversion systems are reviewed. It is noted that internal radiation effects diminish thermoelectric figures of merit significantly at 1000 K and substantially at 2000 K; the effective thermal conductivity contribution of intrathermoelectric radiative dissipation increases with the third power of temperature. It is argued that a consideration of thermoelectric power generation with high temperature heat sources should include utilization of thermionic energy conversion (TEC) topping thermoelectrics. However TEC alone or TEC topping more efficient conversion systems like steam or gas turbines, combined cycles, or Stirling engines would be more desirable generally.

Test development for the thermionic system evaluation test (TSET) project

NASA Astrophysics Data System (ADS)

Morris, D. Brent; Standley, Vaughn H.; Schuller, Michael J.

1992-01-01

The arrival of a Soviet TOPAZ-II space nuclear reactor affords the US space nuclear power (SNP) community the opportunity to study an assembled thermionic conversion power system. The TOPAZ-II will be studied via the Thermionic System Evaluation Test (TSET) Project. This paper is devoted to the discussion of TSET test development as related to the objectives contained in the TSET Project Plan (Standley et al. 1991). The objectives contained in the Project Plan are the foundation for scheduled TSET tests on TOPAZ-II and are derived from the needs of the Air Force Thermionic SNP program. Our ability to meet the objectives is bounded by unique constraints, such as procurement requirements, operational limitations, and necessary interaction between US and Soviet Scientists and engineers. The fulfillment of the test objectives involves a thorough methodology of test scheduling and data managment. The overall goals for the TSET program are gaining technical understanding of a thermionic SNP system and demonstrating the capabilities and limitations of such a system while assisting in the training of US scientist and engineers in preparation for US SNP system testing. Tests presently scheduled as part of TSET include setup, demonstration, and verification tests; normal and off-normal operating test, and system and component performance tests.

The electronic structures and work functions of (100) surface of typical binary and doped REB6 single crystals

NASA Astrophysics Data System (ADS)

Liu, Hongliang; Zhang, Xin; Xiao, Yixin; Zhang, Jiuxing

2018-03-01

The density function theory been used to calculate the electronic structures of binary and doped rare earth hexaborides (REB6), which exhibits the large density of states (DOS) near Fermi level. The d orbital elections of RE element contribute the electronic states of election emission near the Fermi level, which imply that the REB6 (RE = La, Ce, Gd) with wide distribution of high density d orbital electrons could provide a lower work function and excellent emission properties. Doping RE elements into binary REB6 can adjust DOS and the position of the Fermi energy level. The calculated work functions of considered REB6 (100) surface show that the REB6 (RE = La, Ce, Gd) have lower work function and doping RE elements with active d orbital electrons can significantly reduce work function of binary REB6. The thermionic emission test results are basically accordant with the calculated value, proving the first principles calculation could provide a good theoretical guidance for the study of electron emission properties of REB6.

Nonthermal model for ultrafast laser-induced plasma generation around a plasmonic nanorod

NASA Astrophysics Data System (ADS)

Labouret, Timothée; Palpant, Bruno

2016-12-01

The excitation of plasmonic gold nanoparticles by ultrashort laser pulses can trigger interesting electron-based effects in biological media such as production of reactive oxygen species or cell membrane optoporation. In order to better understand the optical and thermal processes at play, we modeled the interaction of a subpicosecond, near-infrared laser pulse with a gold nanorod in water. A nonthermal model is used and compared to a simple two-temperature thermal approach. For both models, the computation of the transient optical response reveals strong plasmon damping. Electron emission from the metal into the water is also calculated in a specific way for each model. The dynamics of the resulting local plasma in water is assessed by a rate equation model. While both approaches provide similar results for the transient optical properties, the simple thermal one is unable to properly describe electron emission and plasma generation. The latter is shown to mostly originate from electron-electron thermionic emission and photoemission from the metal. Taking into account the transient optical response is mandatory to properly calculate both electron emission and local plasma dynamics in water.

Survey of Current and Next Generation Space Power Technologies

DTIC Science & Technology

2006-06-26

different thermodynamic cycles, such as the Brayton, Rankine, and Stirling cycles, alkali metal thermal electric converters ( AMTEC ) and thermionic...efficiencies @ 1700K. The primary issue with this system is the integration of the converter technology into the nuclear reactor core. AMTEC (static...Alkali metal thermal to electric converters ( AMTECs ) are thermally powered electrochemical concentration cells that convert heat energy directly to DC

Electronic structure of BaO/W cathode surfaces

NASA Technical Reports Server (NTRS)

Muller, Wolfgang

1989-01-01

The local electronic structure of the emissive layer of barium dispenser thermionic cathodes is investigated theoretically using the relativistic scattered-wave approach. The interaction of Ba and O with W, Os, and W-Os alloy surfaces is studied with atomic clusters modeling different absorption environments representative of B- and M-type cathodes. Ba is found to be strongly oxidized, while O and the metal substrate are in a reduced chemical state. The presence of O enhances the surface dipole and Ba binding energy relative to Ba on W. Model results for W-Os alloy substrates show only relatively small changes in Ba and O for identical geometries, but very large charge redistributions inside the substrate, which are attributed to the electronegativity difference between Os and W. If Os is present in the surface layer, the charge transfer from Ba to the substrate and the Ba binding energy increase relative to W. Explanations are offered for the improved electron emission from alloy surfaces and the different emission enhancement for different alloy substrates.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fukao, Shinji; Nakanishi, Yoshikazu; Mizoguchi, Tadahiro

X-rays are radiated due to the bremsstrahlung caused by the collision of electrons with a metal target placed opposite the negative electric surface of a crystal by changing the temperature of a LiNbO{sub 3} single crystal uniaxially polarized in the c-axis direction. It is suggested that both electric field intensity and electron density determine the intensity of X-ray radiation. Electrons are supplied by the ionization of residual gas in space, field emission from a case inside which a crystal is located, considered to be due to the high electric-field intensity formed by the surface charges on the crystal, and anmore » external electron source, such as a thermionic source. In a high vacuum, it was found that the electrons supplied by electric-field emission mainly contribute to the radiation of X-rays. It was found that the integrated intensity of X-rays can be maximized by supplying electrons both external and by electric-field emission. Furthermore, the integrated intensity of the X-rays is stable for many repeated temperature changes.« less

Preliminary plan for testing a thermionic reactor in the Plum Brook Space Power Facility

NASA Technical Reports Server (NTRS)

Haley, F. A.

1972-01-01

A preliminary plan is presented for testing a thermionic reactor in the Plum Brook Space Power Facility (SPF). A technical approach, cost estimate, manpower estimate, and schedule are presented to cover a 2 year full power reactor test.

Thermionic system evaluated test (TSET) facility description

NASA Astrophysics Data System (ADS)

Fairchild, Jerry F.; Koonmen, James P.; Thome, Frank V.

1992-01-01

A consortium of US agencies are involved in the Thermionic System Evaluation Test (TSET) which is being supported by the Strategic Defense Initiative Organization (SDIO). The project is a ground test of an unfueled Soviet TOPAZ-II in-core thermionic space reactor powered by electrical heat. It is part of the United States' national thermionic space nuclear power program. It will be tested in Albuquerque, New Mexico at the New Mexico Engineering Research Institute complex by the Phillips Laboratoty, Sandia National Laboratories, Los Alamos National Laboratory, and the University of New Mexico. One of TSET's many objectives is to demonstrate that the US can operate and test a complete space nuclear power system, in the electrical heater configuration, at a low cost. Great efforts have been made to help reduce facility costs during the first phase of this project. These costs include structural, mechanical, and electrical modifications to the existing facility as well as the installation of additional emergency systems to mitigate the effects of utility power losses and alkali metal fires.

Semiconductor nanostructures for plasma energetic systems

NASA Astrophysics Data System (ADS)

Mustafaev, Alexander; Smerdov, Rostislav; Klimenkov, Boris

2017-10-01

In this talk we discuss the research results of the three types of ultrasmall electrodes namely the nanoelectrode arrays based on composite nanostructured porous silicon (PS) layers, porous GaP and nanocrystals of ZnO. These semiconductor materials are of great interest to nano- and optoelectronic applications by virtue of their high specific surface area and extensive capability for surface functionalization. The use of semiconductor (GaN) cathodes in photon-enhanced thermionic emission systems has also proved to be effective although only a few (less than 1%) of the incident photons exceed the 3.3 eV GaN band gap. This significant drawback provided us with a solid foundation for our research in the field of nanostructured PS, and composite materials based on it exhibiting nearly optimal parameters in terms of the band gap (1.1 eV). The band gap modification for PS nanostructured layers is possible in the range of less than 1 eV and 3 eV due to the existence of quantum confinement effect and the remarkable possibilities of PS surface alteration thus providing us with a suitable material for both cathode and anode fabrication. The obtained results are applicable for solar concentration and thermionic energy conversion systems. Dr. Sci., Ph.D, Principal Scientist, Professor.

Optimal thermionic energy conversion with established electrodes for high-temperature topping and process heating. [coal combustion product environments

NASA Technical Reports Server (NTRS)

Morris, J. F.

1980-01-01

Applied research-and-technology (ART) work reveals that optimal thermionic energy conversion (TEC) with approximately 1000 K to approximately 1100 K collectors is possible using well established tungsten electrodes. Such TEC with 1800 K emitters could approach 26.6% efficiency at 27.4 W/sq cm with approximately 1000 K collectors and 21.7% at 22.6 W/sq cm with approximately 1100 K collectors. These performances require 1.5 and 1.7 eV collector work functions (not the 1 eV ultimate) with nearly negligible interelectrode losses. Such collectors correspond to tungsten electrode systems in approximately 0.9 to approximately 6 torr cesium pressures with 1600 K to 1900 K emitters. Because higher heat-rejection temperatures for TEC allow greater collector work functions, interelectrode loss reduction becomes an increasingly important target for applications aimed at elevated temperatures. Studies of intragap modifications and new electrodes that will allow better electron emission and collection with lower cesium pressures are among the TEC-ART approaches to reduced interelectrode losses. These solutions will provide very effective TEC to serve directly in coal-combustion products for high-temperature topping and process heating. In turn this will help to use coal and to use it well.

Investigation of the structural, surface, optical and electrical properties of the Indium doped CuxO thin films deposited by a thermionic vacuum arc

NASA Astrophysics Data System (ADS)

Musaoğlu, Caner; Pat, Suat; Özen, Soner; Korkmaz, Şadan; Mohammadigharehbagh, Reza

2018-03-01

In this study, investigation of some physical properties of In-doped CuxO thin films onto amorphous glass substrates were done. The thin films were depsoied by thermionic vacuum arc technique (TVA). TVA technique gives a thin film with lower precursor impurity according to the other chemical and physical depsoition methods. The microstructural properties of the produced thin films was determined by x-ray diffraction device (XRD). The thickness values were measured as to be 30 nm and 60 nm, respectively. The miller indices of the thin films’ crystalline planes were determined as to be Cu (111), CuO (\\bar{1} 12), CuInO2 (107) and Cu2O (200), Cu (111), CuO (\\bar{1} 12), CuO (\\bar{2} 02), CuInO2 (015) for sample C1 and C2, respectively. The produced In-doped CuO thin films are in polycrystalline structure. The surface properties of produced In doped CuO thin films were determined by using an atomic force microscope (AFM) and field emission scanning electron microscope (FESEM) tools. The optical properties of the In doped CuO thin films were determined by UV–vis spectrophotometer, interferometer, and photoluminescence devices. p-type semiconductor thin film was obtained by TVA depsoition.

Direct observation of the carrier transport process in InGaN quantum wells with a pn-junction

NASA Astrophysics Data System (ADS)

Wu, Haiyan; Ma, Ziguang; Jiang, Yang; Wang, Lu; Yang, Haojun; Li, Yangfeng; Zuo, Peng; Jia, Haiqiang; Wang, Wenxin; Zhou, Junming; Liu, Wuming; Chen, Hong

2016-11-01

A new mechanism of light-to-electricity conversion that uses InGaN/GaN QWs with a p-n junction is reported. According to the well established light-to-electricity conversion theory, quantum wells (QWs) cannot be used in solar cells and photodetectors because the photogenerated carriers in QWs usually relax to ground energy levels, owing to quantum confinement, and cannot form a photocurrent. We observe directly that more than 95% of the photoexcited carriers escape from InGaN/GaN QWs to generate a photocurrent, indicating that the thermionic emission and tunneling processes proposed previously cannot explain carriers escaping from QWs. We show that photoexcited carriers can escape directly from the QWs when the device is under working conditions. Our finding challenges the current theory and demonstrates a new prospect for developing highly efficient solar cells and photodetectors. Project supported by the National Natural Science Foundation of China (Grant Nos. 11574362, 61210014, and 11374340) and the Innovative Clean-energy Research and Application Program of Beijing Municipal Science and Technology Commission, China (Grant No. Z151100003515001).

Helium temperature measurements in a hot filament magnetic mirror plasma using high resolution Doppler spectroscopy

NASA Astrophysics Data System (ADS)

Knott, S.; McCarthy, P. J.; Ruth, A. A.

2016-09-01

Langmuir probe and spectroscopic diagnostics are used to routinely measure electron temperature and density over a wide operating range in a reconfigured Double Plasma device at University College Cork, Ireland. The helium plasma, generated through thermionic emission from a negatively biased tungsten filament, is confined by an axisymmetric magnetic mirror configuration using two stacks of NdFeB permanent magnets, each of length 20 cm and diameter 3 cm placed just outside the 15 mm water cooling jacket enclosing a cylindrical vacuum vessel of internal diameter 25 cm. Plasma light is analysed using a Fourier Transform-type Bruker spectrometer with a highest achievable resolution of 0.08 cm-1 . In the present work, the conventional assumption of room temperature ions in the analysis of Langmuir probe data from low temperature plasmas is examined critically using Doppler spectroscopy of the 468.6 nm He II line. Results for ion temperatures obtained from spectroscopic data for a variety of engineering parameters (discharge voltage, gas pressure and plasma current) will be presented.

Temperature dependent electrical characterization of organic Schottky diode based on thick MgPc films

NASA Astrophysics Data System (ADS)

Singh, J.; Sharma, R. K.; Sule, U. S.; Goutam, U. K.; Gupta, Jagannath; Gadkari, S. C.

2017-07-01

Magnesium phthalocyanine (MgPc) based Schottky diode on indium tin oxide (ITO) substrate was fabricated by thermal evaporation method. The dark current voltage characteristics of the prepared ITO-MgPc-Al heterojunction Schottky diode were measured at different temperatures. The diode showed the non-ideal rectification behavior under forward and reverse bias conditions with a rectification ratio (RR) of 56 at  ±1 V at room temperature. Under forward bias, thermionic emission and space charge limited conduction (SCLC) were found to be the dominant conduction mechanisms at low (below 0.6 V) and high voltages (above 0.6 V) respectively. Under reverse bias conditions, Poole-Frenkel (field assisted thermal detrapping of carriers) was the dominant conduction mechanism. Three different approaches namely, I-V plots, Norde and Cheung methods were used to determine the diode parameters including ideality factor (n), barrier height (Φb), series resistance (R s) and were compared. SCLC mechanism showed that the trap concentration is 5.52  ×  1022 m-3 and it lies at 0.46 eV above the valence band edge.

Universal diffusion-limited injection and the hook effect in organic thin-film transistors

NASA Astrophysics Data System (ADS)

Liu, Chuan; Huseynova, Gunel; Xu, Yong; Long, Dang Xuan; Park, Won-Tae; Liu, Xuying; Minari, Takeo; Noh, Yong-Young

2016-07-01

The general form of interfacial contact resistance was derived for organic thin-film transistors (OTFTs) covering various injection mechanisms. Devices with a broad range of materials for contacts, semiconductors, and dielectrics were investigated and the charge injections in staggered OTFTs was found to universally follow the proposed form in the diffusion-limited case, which is signified by the mobility-dependent injection at the metal-semiconductor interfaces. Hence, real ohmic contact can hardly ever be achieved in OTFTs with low carrier concentrations and mobility, and the injection mechanisms include thermionic emission, diffusion, and surface recombination. The non-ohmic injection in OTFTs is manifested by the generally observed hook shape of the output conductance as a function of the drain field. The combined theoretical and experimental results show that interfacial contact resistance generally decreases with carrier mobility, and the injection current is probably determined by the surface recombination rate, which can be promoted by bulk-doping, contact modifications with charge injection layers and dopant layers, and dielectric engineering with high-k dielectric materials.

Universal diffusion-limited injection and the hook effect in organic thin-film transistors.

PubMed

Liu, Chuan; Huseynova, Gunel; Xu, Yong; Long, Dang Xuan; Park, Won-Tae; Liu, Xuying; Minari, Takeo; Noh, Yong-Young

2016-07-21

The general form of interfacial contact resistance was derived for organic thin-film transistors (OTFTs) covering various injection mechanisms. Devices with a broad range of materials for contacts, semiconductors, and dielectrics were investigated and the charge injections in staggered OTFTs was found to universally follow the proposed form in the diffusion-limited case, which is signified by the mobility-dependent injection at the metal-semiconductor interfaces. Hence, real ohmic contact can hardly ever be achieved in OTFTs with low carrier concentrations and mobility, and the injection mechanisms include thermionic emission, diffusion, and surface recombination. The non-ohmic injection in OTFTs is manifested by the generally observed hook shape of the output conductance as a function of the drain field. The combined theoretical and experimental results show that interfacial contact resistance generally decreases with carrier mobility, and the injection current is probably determined by the surface recombination rate, which can be promoted by bulk-doping, contact modifications with charge injection layers and dopant layers, and dielectric engineering with high-k dielectric materials.

Characterization of WB/SiC Schottky Barrier Diodes Using I-V-T Method

NASA Astrophysics Data System (ADS)

Aldridge, James; Oder, Tom

2009-04-01

The importance of silicon carbide (SiC) semiconductor for high temperature and high power microelectronic device applications has long been established. We have fabricated SiC Schottky barrier diodes using tungsten boride (WB) as the Schottky contact. The diodes were characterized using the current-voltage-temperature method. The sample was mounted on a heated stage and the temperature varied from about 25 ^oC to 300 ^oC at intervals of 25 ^oC. From the Richardson's plot, we obtained an energy barrier height of 0.96 eV and a Richardson's constant of 71.2 AK-1cm-2. Using the modified Richardson's plot, we obtained a barrier height of 1.01 eV. From the variation of the ideality factor and the temperature, we determined a characteristic energy of 0.02 eV to 0.04 eV across the range of the measurement temperature. This implies that thermionic emission is dominant in the low measurement temperature range. Our results confirm the excellent thermal stability of WB/SiC Schottky barrier diodes.

Cogeneration Technology Alternatives Study (CTAS). Volume 5: Cogeneration systems results

NASA Technical Reports Server (NTRS)

Gerlaugh, H. E.; Hall, E. W.; Brown, D. H.; Priestley, R. R.; Knightly, W. F.

1980-01-01

The use of various advanced energy conversion systems is examined and compared with each other and with current technology systems for savings in fuel energy, costs, and emissions in individual plants and on a national level. About fifty industrial processes from the largest energy consuming sectors were used as a basis for matching a similar number of energy conversion systems that are considered as candidate which can be made available by the 1985 to 2000 time period. The sectors considered included food, textiles, lumber, paper, chemicals, petroleum, glass, and primary metals. The energy conversion systems included steam and gas turbines, diesels, thermionics, stirling, closed cycle and steam injected gas turbines, and fuel cells. Fuels considered were coal, both coal and petroleum based residual and distillate liquid fuels, and low Btu gas obtained through the on site gasification of coal. The methodology and results of matching the cogeneration energy conversion systems to approximately 50 industrial processes are described. Results include fuel energy saved, levelized annual energy cost saved, return on investment, and operational factors relative to the noncogeneration base cases.

Universal diffusion-limited injection and the hook effect in organic thin-film transistors

PubMed Central

Liu, Chuan; Huseynova, Gunel; Xu, Yong; Long, Dang Xuan; Park, Won-Tae; Liu, Xuying; Minari, Takeo; Noh, Yong-Young

2016-01-01

The general form of interfacial contact resistance was derived for organic thin-film transistors (OTFTs) covering various injection mechanisms. Devices with a broad range of materials for contacts, semiconductors, and dielectrics were investigated and the charge injections in staggered OTFTs was found to universally follow the proposed form in the diffusion-limited case, which is signified by the mobility-dependent injection at the metal-semiconductor interfaces. Hence, real ohmic contact can hardly ever be achieved in OTFTs with low carrier concentrations and mobility, and the injection mechanisms include thermionic emission, diffusion, and surface recombination. The non-ohmic injection in OTFTs is manifested by the generally observed hook shape of the output conductance as a function of the drain field. The combined theoretical and experimental results show that interfacial contact resistance generally decreases with carrier mobility, and the injection current is probably determined by the surface recombination rate, which can be promoted by bulk-doping, contact modifications with charge injection layers and dopant layers, and dielectric engineering with high-k dielectric materials. PMID:27440253

Diagnostics for a 1.2 kA, 1 MeV, electron induction injector

NASA Astrophysics Data System (ADS)

Houck, T. L.; Anderson, D. E.; Eylon, S.; Henestroza, E.; Lidia, S. M.; Vanecek, D. L.; Westenskow, G. A.; Yu, S. S.

1998-12-01

We are constructing a 1.2 kA, 1 MeV, electron induction injector as part of the RTA program, a collaborative effort between LLNL and LBNL to develop relativistic klystrons for Two-Beam Accelerator applications. The RTA injector will also be used in the development of a high-gradient, low-emittance, electron source and beam diagnostics for the second axis of the Dual Axis Radiographic Hydrodynamic Test (DARHT) Facility. The electron source will be a 3.5″-diameter, thermionic, flat-surface, m-type cathode with a maximum shroud field stress of approximately 165 kV/cm. Additional design parameters for the injector include a pulse length of over 150 ns flat top (1% energy variation), and a normalized edge emittance of less than 200 π-mm-mr. Precise measurement of the beam parameters is required so that performance of the RTA injector can be confidently scaled to the 4 kA, 3 MeV, and 2-microsecond pulse parameters of the DARHT injector. Planned diagnostics include an isolated cathode with resistive divider for direct measurement of current emission, resistive wall and magnetic probe current monitors for measuring beam current and centroid position, capacitive probes for measuring A-K gap voltage, an energy spectrometer, and a pepperpot emittance diagnostic. Details of the injector, beam line, and diagnostics are presented.

Theory and simulation of backbombardment in single-cell thermionic-cathode electron guns

DOE Office of Scientific and Technical Information (OSTI.GOV)

Edelen, J.  P.; Biedron, S.  G.; Harris, J.  R.

This paper presents a comparison between simulation results and a first principles analytical model of electron back-bombardment developed at Colorado State University for single-cell, thermionic-cathode rf guns. While most previous work on back-bombardment has been specific to particular accelerator systems, this work is generalized to a wide variety of guns within the applicable parameter space. The merits and limits of the analytic model will be discussed. This paper identifies the three fundamental parameters that drive the back-bombardment process, and demonstrates relative accuracy in calculating the predicted back-bombardment power of a single-cell thermionic gun.

Theory and simulation of backbombardment in single-cell thermionic-cathode electron guns

DOE PAGES

Edelen, J.  P.; Biedron, S.  G.; Harris, J.  R.; ...

2015-04-01

This paper presents a comparison between simulation results and a first principles analytical model of electron back-bombardment developed at Colorado State University for single-cell, thermionic-cathode rf guns. While most previous work on back-bombardment has been specific to particular accelerator systems, this work is generalized to a wide variety of guns within the applicable parameter space. The merits and limits of the analytic model will be discussed. This paper identifies the three fundamental parameters that drive the back-bombardment process, and demonstrates relative accuracy in calculating the predicted back-bombardment power of a single-cell thermionic gun.

Design, construction and measurements of an alpha magnet as a solution for compact bunch compressor for the electron beam from Thermionic RF Gun

NASA Astrophysics Data System (ADS)

Rajabi, A.; Jazini, J.; Fathi, M.; Sharifian, M.; Shokri, B.

2018-03-01

The beam produced by a thermionic RF gun has wide energy spread that makes it unsuitable for direct usage in photon sources. Here in the present work, we optimize the extracted beam from a thermionic RF gun by a compact economical bunch compressor. A compact magnetic bunch compressor (Alpha magnet) is designed and constructed. A comparison between simulation results and experimental measurements shows acceptable conformity. The beam dynamics simulation results show a reduction of the energy spread as well as a compression of length less than 1 ps with 2.3 mm-mrad emittance.

Venting of fission products and shielding in thermionic nuclear reactor systems

NASA Technical Reports Server (NTRS)

Salmi, E. W.

1972-01-01

Most thermionic reactors are designed to allow the fission gases to escape out of the emitter. A scheme to allow the fission gases to escape is proposed. Because of the low activity of the fission products, this method should pose no radiation hazards.

Stable synthesis of few-layered boron nitride nanotubes by anodic arc discharge.

PubMed

Yeh, Yao-Wen; Raitses, Yevgeny; Koel, Bruce E; Yao, Nan

2017-06-08

Boron nitride nanotubes (BNNTs) were successfully synthesized by a dc arc discharge using a boron-rich anode as synthesis feedstock in a nitrogen gas environment at near atmospheric pressure. The synthesis was achieved independent of the cathode material suggesting that under such conditions the arc operates in so-called anodic mode with the anode material being consumed by evaporation due to the arc heating. To sustain the arc current by thermionic electron emission, the cathode has to be at sufficiently high temperature, which for a typical arc current density of ~100 A/cm 2 , is above the boron melting point (2350 K). With both electrodes made from the same boron-rich alloy, we found that the arc operation unstable due to frequent sticking between two molten electrodes and formation of molten droplets. Stable and reliable arc operation and arc synthesis were achieved with the boron-rich anode and the cathode made from a refractory metal which has a melting temperature above the melting point of boron. Ex-situ characterization of synthesized BNNTs with electron microscopy and Raman spectroscopy revealed that independent of the cathode material, the tubes are primarily single and double walled. The results also show evidence of root-growth of BNNTs produced in the arc discharge.

Stable synthesis of few-layered boron nitride nanotubes by anodic arc discharge

DOE PAGES

Yeh, Yao-Wen; Raitses, Yevgeny; Koel, Bruce E.; ...

2017-06-08

Boron nitride nanotubes (BNNTs) were successfully synthesized by a dc arc discharge using a boron-rich anode as synthesis feedstock in a nitrogen gas environment at near atmospheric pressure. The synthesis was achieved independent of the cathode material suggesting that under such conditions the arc operates in so-called anodic mode with the anode material being consumed by evaporation due to the arc heating. In order to sustain the arc current by thermionic electron emission, the cathode has to be at sufficiently high temperature, which for a typical arc current density of similar to 100 A/cm 2, is above the boron meltingmore » point (2350 K). With both electrodes made from the same boron-rich alloy, we found that the arc operation unstable due to frequent sticking between two molten electrodes and formation of molten droplets. We achieved a stable and reliable arc operation and arc synthesis with the boronrich anode and the cathode made from a refractory metal which has a melting temperature above the melting point of boron. Ex-situ characterization of synthesized BNNTs with electron microscopy and Raman spectroscopy revealed that independent of the cathode material, the tubes are primarily single and double walled. Our results also show evidence of root-growth of BNNTs produced in the arc discharge.« less

Stable synthesis of few-layered boron nitride nanotubes by anodic arc discharge

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yeh, Yao-Wen; Raitses, Yevgeny; Koel, Bruce E.

Boron nitride nanotubes (BNNTs) were successfully synthesized by a dc arc discharge using a boron-rich anode as synthesis feedstock in a nitrogen gas environment at near atmospheric pressure. The synthesis was achieved independent of the cathode material suggesting that under such conditions the arc operates in so-called anodic mode with the anode material being consumed by evaporation due to the arc heating. In order to sustain the arc current by thermionic electron emission, the cathode has to be at sufficiently high temperature, which for a typical arc current density of similar to 100 A/cm 2, is above the boron meltingmore » point (2350 K). With both electrodes made from the same boron-rich alloy, we found that the arc operation unstable due to frequent sticking between two molten electrodes and formation of molten droplets. We achieved a stable and reliable arc operation and arc synthesis with the boronrich anode and the cathode made from a refractory metal which has a melting temperature above the melting point of boron. Ex-situ characterization of synthesized BNNTs with electron microscopy and Raman spectroscopy revealed that independent of the cathode material, the tubes are primarily single and double walled. Our results also show evidence of root-growth of BNNTs produced in the arc discharge.« less

Design and validation of the ball-pen probe for measurements in a low-temperature magnetized plasma

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bousselin, G.; Cavalier, J.; Pautex, J. F.

Ball-pen probes have been used in fusion devices for direct measurements of the plasma potential. Their application in low-temperature magnetized plasma devices is still subject to studies. In this context, a ball-pen probe has been recently implemented on the linear plasma device Mirabelle. Produced by a thermionic discharge, the plasma is characterized by a low electron temperature and a low density. Plasma confinement is provided by an axial magnetic field that goes up to 100 mT. The principle of the ball-pen probe is to adjust the saturation current ratio to 1 by reducing the electron current contribution. In that case,more » the floating potential of the probe is close to the plasma potential. A thorough study of the ball-pen probe operation is performed for different designs of the probe over a large set of plasma conditions. Comparisons between ball-pen, Langmuir, and emissive probes are conducted in the same plasma conditions. The ball-pen probe is successfully measuring the plasma potential in these specific plasma conditions only if an adapted electronics and an adapted probe size to the plasma characteristic lengths ({lambda}{sub D}, {rho}{sub ce}) are used.« less

The impact of nanocontact on nanowire based nanoelectronics.

PubMed

Lin, Yen-Fu; Jian, Wen-Bin

2008-10-01

Nanowire-based nanoelectronic devices will be innovative electronic building blocks from bottom up. The reduced nanocontact area of nanowire devices magnifies the contribution of contact electrical properties. Although a lot of two-contact-based ZnO nanoelectronics have been demonstrated, the electrical properties bringing either from the nanocontacts or from the nanowires have not been considered yet. High quality ZnO nanowires with a small deviation and an average diameter of 38 nm were synthesized to fabricate more than thirty nanowire devices. According to temperature behaviors of current-voltage curves and resistances, the devices could be grouped into three types. Type I devices expose thermally activated transport in ZnO nanowires and they could be considered as two Ohmic nanocontacts of the Ti electrode contacting directly on the nanowire. For those nanowire devices having a high resistance at room temperatures, they can be fitted accurately with the thermionic-emission theory and classified into type II and III devices according to their rectifying and symmetrical current-voltage behaviors. The type II device has only one deteriorated nanocontact and the other one Ohmic contact on single ZnO nanowire. An insulating oxide layer with thickness less than 20 nm should be introduced to describe electron hopping in the nanocontacts, so as to signalize one- and high-dimensional hopping conduction in type II and III devices.

Performance of GAASP/GAAS Superlattice Photocathodes in High Energy Experiments using Polarized Electrons

DOE Office of Scientific and Technical Information (OSTI.GOV)

Brachmann, A.; Clendenin, J.E.; Maruyama, T.

2006-02-27

The GaAsP/GaAs strained superlattice photocathode structure has proven to be a significant advance for polarized electron sources operating with high peak currents per microbunch and relatively low duty factor. This is the characteristic type of operation for SLAC and is also planned for the ILC. This superlattice structure was studied at SLAC [1], and an optimum variation was chosen for the final stage of E-158, a high-energy parity violating experiment at SLAC. Following E-158, the polarized source was maintained on standby with the cathode being re-cesiated about once a week while a thermionic gun, which is installed in parallel withmore » the polarized gun, supplied the linac electron beams. However, in the summer of 2005, while the thermionic gun was disabled, the polarized electron source was again used to provide electron beams for the linac. The performance of the photocathode 24 months after its only activation is described and factors making this possible are discussed.« less

Improved understanding of the hot cathode current modes and mode transitions [Mechanism of the hot cathode current mode transitions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Campanell, Michael D.; Umansky, M. V.

Hot cathodes are crucial components in a variety of plasma sources and applications, but they induce mode transitions and oscillations that are not fully understood. It is often assumed that negatively biased hot cathodes have a space-charge limited (SCL) sheath whenever the current is limited. Here, we show on theoretical grounds that a SCL sheath cannot persist. First, charge-exchange ions born within the virtual cathode (VC) region get trapped and build up. After the ion density reaches the electron density at a point in the VC, a new neutral region is formed and begins growing in space. In planar geometry,more » this 'new plasma' containing cold trapped ions and cold thermoelectrons grows towards the anode and fills the gap, leaving behind an inverse cathode sheath. This explains how transitions from temperature-limited mode to anode glow mode occur in thermionic discharge experiments with magnetic fields. If the hot cathode is a small filament in an unmagnetized plasma, the trapped ion region is predicted to grow radially in both directions, get expelled if it reaches the cathode, and reform periodically. Filament-induced current oscillations consistent with this prediction have been reported in experiments. Here, we set up planar geometry simulations of thermionic discharges and demonstrate several mode transition phenomena for the first time. Lastly, our continuum kinetic code lacks the noise of particle simulations, enabling a closer study of the temporal dynamics.« less

Improved understanding of the hot cathode current modes and mode transitions [Mechanism of the hot cathode current mode transitions

DOE PAGES

Campanell, Michael D.; Umansky, M. V.

2017-11-22

Hot cathodes are crucial components in a variety of plasma sources and applications, but they induce mode transitions and oscillations that are not fully understood. It is often assumed that negatively biased hot cathodes have a space-charge limited (SCL) sheath whenever the current is limited. Here, we show on theoretical grounds that a SCL sheath cannot persist. First, charge-exchange ions born within the virtual cathode (VC) region get trapped and build up. After the ion density reaches the electron density at a point in the VC, a new neutral region is formed and begins growing in space. In planar geometry,more » this 'new plasma' containing cold trapped ions and cold thermoelectrons grows towards the anode and fills the gap, leaving behind an inverse cathode sheath. This explains how transitions from temperature-limited mode to anode glow mode occur in thermionic discharge experiments with magnetic fields. If the hot cathode is a small filament in an unmagnetized plasma, the trapped ion region is predicted to grow radially in both directions, get expelled if it reaches the cathode, and reform periodically. Filament-induced current oscillations consistent with this prediction have been reported in experiments. Here, we set up planar geometry simulations of thermionic discharges and demonstrate several mode transition phenomena for the first time. Lastly, our continuum kinetic code lacks the noise of particle simulations, enabling a closer study of the temporal dynamics.« less

InGaP Heterojunction Barrier Solar Cells

NASA Technical Reports Server (NTRS)

Welser, Roger E.

2010-01-01

A new solar-cell structure utilizes a single, ultra-wide well of either gallium arsenide (GaAs) or indium-gallium-phosphide (InGaP) in the depletion region of a wide bandgap matrix, instead of the usual multiple quantum well layers. These InGaP barrier layers are effective at reducing diode dark current, and photogenerated carrier escape is maximized by the proper design of the electric field and barrier profile. With the new material, open-circuit voltage enhancements of 40 and 100 mV (versus PIN control systems) are possible without any degradation in short-circuit current. Basic tenets of quantum-well and quantum- dot solar cells are utilized, but instead of using multiple thin layers, a single wide well works better. InGaP is used as a barrier material, which increases open current, while simultaneously lowering dark current, reducing both hole diffusion from the base, and space charge recombination within the depletion region. Both the built-in field and the barrier profile are tailored to enhance thermionic emissions, which maximizes the photocurrent at forward bias, with a demonstrated voltage increase. An InGaP heterojunction barrier solar cell consists of a single, ultra-wide GaAs, aluminum-gallium-arsenide (AlGaAs), or lower-energy-gap InGaP absorber well placed within the depletion region of an otherwise wide bandgap PIN diode. Photogenerated electron collection is unencumbered in this structure. InGaAs wells can be added to the thick GaAs absorber layer to capture lower-energy photons.

100 years of the physics of diodes

NASA Astrophysics Data System (ADS)

Zhang, Peng; Valfells, Ágúst; Ang, L. K.; Luginsland, J. W.; Lau, Y. Y.

2017-03-01

The Child-Langmuir Law (CL), discovered a century ago, gives the maximum current that can be transported across a planar diode in the steady state. As a quintessential example of the impact of space charge shielding near a charged surface, it is central to the studies of high current diodes, such as high power microwave sources, vacuum microelectronics, electron and ion sources, and high current drivers used in high energy density physics experiments. CL remains a touchstone of fundamental sheath physics, including contemporary studies of nanoscale quantum diodes and nano gap based plasmonic devices. Its solid state analog is the Mott-Gurney law, governing the maximum charge injection in solids, such as organic materials and other dielectrics, which is important to energy devices, such as solar cells and light emitting diodes. This paper reviews the important advances in the physics of diodes since the discovery of CL, including virtual cathode formation and extension of CL to multiple dimensions, to the quantum regime, and to ultrafast processes. We review the influence of magnetic fields, multiple species in bipolar flow, electromagnetic and time dependent effects in both short pulse and high frequency THz limits, and single electron regimes. Transitions from various emission mechanisms (thermionic-, field-, and photoemission) to the space charge limited state (CL) will be addressed, especially highlighting the important simulation and experimental developments in selected contemporary areas of study. We stress the fundamental physical links between the physics of beams to limiting currents in other areas, such as low temperature plasmas, laser plasmas, and space propulsion.

Improved understanding of the hot cathode current modes and mode transitions

NASA Astrophysics Data System (ADS)

Campanell, M. D.; Umansky, M. V.

2017-12-01

Hot cathodes are crucial components in a variety of plasma sources and applications, but they induce mode transitions and oscillations that are not fully understood. It is often assumed that negatively biased hot cathodes have a space-charge limited (SCL) sheath whenever the current is limited. Here, we show on theoretical grounds that a SCL sheath cannot persist. First, charge-exchange ions born within the virtual cathode (VC) region get trapped and build up. After the ion density reaches the electron density at a point in the VC, a new neutral region is formed and begins growing in space. In planar geometry, this ‘new plasma’ containing cold trapped ions and cold thermoelectrons grows towards the anode and fills the gap, leaving behind an inverse cathode sheath. This explains how transitions from temperature-limited mode to anode glow mode occur in thermionic discharge experiments with magnetic fields. If the hot cathode is a small filament in an unmagnetized plasma, the trapped ion region is predicted to grow radially in both directions, get expelled if it reaches the cathode, and reform periodically. Filament-induced current oscillations consistent with this prediction have been reported in experiments. Here, we set up planar geometry simulations of thermionic discharges and demonstrate several mode transition phenomena for the first time. Our continuum kinetic code lacks the noise of particle simulations, enabling a closer study of the temporal dynamics.

Electrical properties of CZTS pellets made from microwave-processed powder

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ghediya, Prashant R., E-mail: prashantghediya@yahoo.co.in; Chaudhuri, Tapas K.

2015-06-24

Electrical properties of the kesterite copper zinc tin sulphide (CZTS) pellets in the temperature range from 300 K to 500 K are reported. The pellets are p-type with thermoelectric power (TEP) of + 175 µV/K. Electrical conductivity (σ) increases with the temperatures and is found to be due to thermionic emission (TE) over grain boundary (GB) barriers with activation energy of 170 meV. CZTS pellets are made from micropowders synthesized by microwave irradiation of precursor solution. Formation of kesterite CZTS is confirmed by X-ray diffraction (XRD) and Raman spectroscopy. Scanning Electron Microscope (SEM) shows that powder is micron sized spherical particles.

Terrella for Advanced Undergraduate Physics Laboratory

NASA Astrophysics Data System (ADS)

Reardon, Jim; Endrizzi, Douglass; Forest, Cary; Oliva, Steven

2017-10-01

A terrella has been in use in the Advanced Laboratory for undergraduates in the Physics Department at the University of Wisconsin-Madison since spring 2016. Our terrella is a permanent magnet on a pedestal which may be biased in various ways. In the vacuum region B <= 200 gauss; for typical operation p10-4 Torr. Plasma may be created by thermionic emission from a filament or by an S-band magnetron. Students are guided through diagnosis of the terrella plasma using spectroscopy and swept Langmuir probes. A suite of supporting experiments has been developed to introduce basic plasma phenomena, such as the Child-Langmuir law. University of Wisconsin-Madison.

Ionization studies in laser-excited alkaline-earth vapors.

PubMed

Hermann, J P; Wynne, J J

1980-06-01

We report on the time behavior of ionization signals produced by laser excitation of Ca and Ba atomic vapor to high-Rydberg states. A space-charge-limited thermionic diode detector shows a long-lived (>I-msec) ionization signal. However, optical detection of atomic ions (Ca+, Ba+) shows that these species live for much shorter times (<100 microsec). These results, in conjunction with published results on mass-spectrometric studies of high-density atomic beams, suggest that our ionization signal is primarily due to molecular species (Ca2+, Ba2+). We also observed optically pumped amplified spontaneous emission and stimulated electronic Raman scattering in Ca+ and Ba+.

Graphite based Schottky diodes formed semiconducting substrates

NASA Astrophysics Data System (ADS)

Schumann, Todd; Tongay, Sefaattin; Hebard, Arthur

2010-03-01

We demonstrate the formation of semimetal graphite/semiconductor Schottky barriers where the semiconductor is either silicon (Si), gallium arsenide (GaAs) or 4H-silicon carbide (4H-SiC). The fabrication can be as easy as allowing a dab of graphite paint to air dry on any one of the investigated semiconductors. Near room temperature, the forward-bias diode characteristics are well described by thermionic emission, and the extracted barrier heights, which are confirmed by capacitance voltage measurements, roughly follow the Schottky-Mott relation. Since the outermost layer of the graphite electrode is a single graphene sheet, we expect that graphene/semiconductor barriers will manifest similar behavior.

Kinetic energy spectra in thermionic emission from small tungsten cluster anions: evidence for nonclassical electron capture.

PubMed

Concina, Bruno; Baguenard, Bruno; Calvo, Florent; Bordas, Christian

2010-03-14

The delayed electron emission from small mass-selected anionic tungsten clusters W(n)(-) has been studied for sizes in the range 9 < or = n < or = 21. Kinetic energy spectra have been measured for delays of about 100 ns after laser excitation by a velocity-map imaging spectrometer. They are analyzed in the framework of microreversible statistical theories. The low-energy behavior shows some significant deviations with respect to the classical Langevin capture model, which we interpret as possibly due to the influence of quantum dynamical effects such as tunneling through the centrifugal barrier, rather than shape effects. The cluster temperature has been extracted from both the experimental kinetic energy spectrum and the absolute decay rate. Discrepancies between the two approaches suggest that the sticking probability can be as low as a few percent for the smallest clusters.

Studies of thermionic materials for space power applications

NASA Technical Reports Server (NTRS)

1971-01-01

Service life tests of LC-8 and LC-9 carbide-fueled thermionic converters are discussed. Post operational tests of the converters to show emitter diametric change, microstructures of cladding and fuel, and analysis of fuel composition are described. The fabrication and performance of high temperature thermocouples used in the test procedures are included.

Chemical regeneration of emitter surface increases thermionic diode life

NASA Technical Reports Server (NTRS)

Breiteieser, R.

1966-01-01

Chemical regeneration of sublimated emitter electrode increases the operating efficiency and life of thermionic diodes. A gas which forms chemical compounds with the sublimated emitter material is introduced into the space between the emitter and the collector. The compounds migrate to the emitter where they decompose and redeposit the emitter material.

Energy conversion research and development with diminiodes

NASA Technical Reports Server (NTRS)

Morris, J. F.

1974-01-01

Diminiodes are variable-gap cesium diodes with plane miniature guarded electrodes. These converters allow thermionic evaluations of tiny pieces of rare solids. In addition to smallness, diminiode advantages comprise simplicity, precision, fabrication ease, parts interchangeability, cleanliness, full instrumentation, direct calibration, ruggedness, and economy. Diminiodes with computerized thermionic performance mapping make electrode screening programs practical.

The diminiode: A research and development tool for nuclear thermionics

NASA Technical Reports Server (NTRS)

Morris, J. F.

1972-01-01

Diminiodes are fixed-or variable-gap cesium diodes with plane miniature emitters and guarded collectors. In addition to smallness, their relative advantages are simplicity, precision, ease of fabrication, interchangeability of parts, cleanliness, full instrumentation, ruggedness, and economy. With diminiodes and computers used in thermionic performance mapping, a thorough electrode screening program becomes practical.

ENVIRONMENTAL TECHNOLOGY VERIFICATION REPORT, EXPLOSIVES DETECTION TECHNOLOGY, SRI INSTRUMENTS, MODEL 8610C, GAS CHROMATOGRAPH/THERMIONIC IONIZATION DETECTION

EPA Science Inventory

The SRI Model 86 1 OC gas chromatograph (GC) is a transportable instrument that can provide on-site analysis of soils for explosives. Coupling this transportable gas chromatograph with a thermionic ionization detector (TID) allows for the determination of explosives in soil matri...

Thermionic Energy Conversion Based on Graphene van der Waals Heterostructures

PubMed Central

Liang, Shi-Jun; Liu, Bo; Hu, Wei; Zhou, Kun; Ang, L. K.

2017-01-01

Seeking for thermoelectric (TE) materials with high figure of merit (or ZT), which can directly converts low-grade wasted heat (400 to 500 K) into electricity, has been a big challenge. Inspired by the concept of multilayer thermionic devices, we propose and design a solid-state thermionic devices (as a power generator or a refrigerator) in using van der Waals (vdW) heterostructure sandwiched between two graphene electrodes, to achieve high energy conversion efficiency in the temperature range of 400 to 500 K. The vdW heterostructure is composed of suitable multiple layers of transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, WS2 and WSe2. From our calculations, WSe2 and MoSe2 are identified as two ideal TMDs (using the reported experimental material’s properties), which can harvest waste heat at 400 K with efficiencies about 7% to 8%. To our best knowledge, this design is the first in combining the advantages of graphene electrodes and TMDs to function as a thermionic-based device. PMID:28387363

High-Current-Density Thermionic Cathodes and the Generation of High-Voltage Electron Beams

DTIC Science & Technology

1989-04-30

Cathode Temperature =1700 OC Figure 37: Peak gun voltage = 90 kV -57- 60- 0 EGUN 327 ~40 0S 20’ Vacuum 5 .2 x 10 Tor 0 o 0 15202 30 Time (jis...by modeling the filament as a thin disk. The shape of the H - V -, 2 actual filament is sketched in Fig. 2. The EGUN code 1 131 is used to calculate

A Systematic Cathode Study-Activation of a Thermionic Cathode, and Measuring Cesium Evaporation from a Dispenser Photocathode

DTIC Science & Technology

2010-06-01

QCM Quartz Crystal Deposition Monitor SEM Scanning Electron Microscope SRF Superconducting Radio Frequency T Torr Ti Titanium UHV Ultra...High Vacuum ( ᝺-7 Torr) UM University of Maryland QCM Quartz Crystal Deposition Monitor V Volt VAC Voltage-Alternating Current xvii...event. The two originally had problems with their tungsten filaments crystallizing and breaking. Being experimentalists, they added thorium in an

Close-Spaced High Temperature Knudsen Flow.

DTIC Science & Technology

1986-07-15

work~was a study of discharge processes in Knudsen mode (collisionless), thermionic energy converters. Areas of research involve’mechanisms for reducing ...power densities. The mechanisms/we have chosen to study are: reduction of space-charge through a very close interelectrode gap (less than 10 microns...In order to operate at practical current densities, the effect of electron space charge must be reduced . This can be done through very close

Steep-slope hysteresis-free negative capacitance MoS2 transistors

NASA Astrophysics Data System (ADS)

Si, Mengwei; Su, Chun-Jung; Jiang, Chunsheng; Conrad, Nathan J.; Zhou, Hong; Maize, Kerry D.; Qiu, Gang; Wu, Chien-Ting; Shakouri, Ali; Alam, Muhammad A.; Ye, Peide D.

2018-01-01

The so-called Boltzmann tyranny defines the fundamental thermionic limit of the subthreshold slope of a metal-oxide-semiconductor field-effect transistor (MOSFET) at 60 mV dec-1 at room temperature and therefore precludes lowering of the supply voltage and overall power consumption1,2. Adding a ferroelectric negative capacitor to the gate stack of a MOSFET may offer a promising solution to bypassing this fundamental barrier3. Meanwhile, two-dimensional semiconductors such as atomically thin transition-metal dichalcogenides, due to their low dielectric constant and ease of integration into a junctionless transistor topology, offer enhanced electrostatic control of the channel4-12. Here, we combine these two advantages and demonstrate a molybdenum disulfide (MoS2) two-dimensional steep-slope transistor with a ferroelectric hafnium zirconium oxide layer in the gate dielectric stack. This device exhibits excellent performance in both on and off states, with a maximum drain current of 510 μA μm-1 and a sub-thermionic subthreshold slope, and is essentially hysteresis-free. Negative differential resistance was observed at room temperature in the MoS2 negative-capacitance FETs as the result of negative capacitance due to the negative drain-induced barrier lowering. A high on-current-induced self-heating effect was also observed and studied.

Numerical analysis of direct-current microdischarge for space propulsion applications using the particle-in-cell/Monte Carlo collision (PIC/MCC) method

NASA Astrophysics Data System (ADS)

Kong, Linghan; Wang, Weizong; Murphy, Anthony B.; Xia, Guangqing

2017-04-01

Microdischarges are an important type of plasma discharge that possess several unique characteristics, such as the presence of a stable glow discharge, high plasma density and intense excimer radiation, leading to several potential applications. The intense and controllable gas heating within the extremely small dimensions of microdischarges has been exploited in micro-thruster technologies by incorporating a micro-nozzle to generate the thrust. This kind of micro-thruster has a significantly improved specific impulse performance compared to conventional cold gas thrusters, and can meet the requirements arising from the emerging development and application of micro-spacecraft. In this paper, we performed a self-consistent 2D particle-in-cell simulation, with a Monte Carlo collision model, of a microdischarge operating in a prototype micro-plasma thruster with a hollow cylinder geometry and a divergent micro-nozzle. The model takes into account the thermionic electron emission including the Schottky effect, the secondary electron emission due to cathode bombardment by the plasma ions, several different collision processes, and a non-uniform argon background gas density in the cathode-anode gap. Results in the high-pressure (several hundreds of Torr), high-current (mA) operating regime showing the behavior of the plasma density, potential distribution, and energy flux towards the hollow cathode and anode are presented and discussed. In addition, the results of simulations showing the effect of different argon gas pressures, cathode material work function and discharge voltage on the operation of the microdischarge thruster are presented. Our calculated properties are compared with experimental data under similar conditions and qualitative and quantitative agreements are reached.

Thermionic fuel element for the S-prime reactor

NASA Astrophysics Data System (ADS)

Van Hagan, Thomas H.; Drees, Elizabeth A.

1993-01-01

Technical aspects of the thermionic fuel element (TFE) design proposed for the S-PRIME space nuclear power system are discussed. Topics covered include the rational for selecting a multicell TFE approach, a technical description of the S-PRIME TFE and its estimated performance, and the technology readiness of the design, which emphasizes techology maturity and low risk.

Distributed electrical leads for thermionic converter

DOEpatents

Fitzpatrick, Gary O.; Britt, Edward J.

1979-01-01

In a thermionic converter, means are provided for coupling an electrical lead to at least one of the electrodes thereof. The means include a bus bar and a plurality of distributed leads coupled to the bus bar each of which penetrates through one electrode and are then coupled to the other electrode of the converter in spaced apart relation.

Thermionic reactor power conditioner design for nuclear electric propulsion.

NASA Technical Reports Server (NTRS)

Jacobsen, A. S.; Tasca, D. M.

1971-01-01

Consideration of the effects of various thermionic reactor parameters and requirements upon spacecraft power conditioning design. A basic spacecraft is defined using nuclear electric propulsion, requiring approximately 120 kWe. The interrelationships of reactor operating characteristics and power conditioning requirements are discussed and evaluated, and the effects on power conditioner design and performance are presented.

Nuclear thermionic power plants in the 50-300 kWe range.

NASA Technical Reports Server (NTRS)

Van Hoomissen, J. E.; Sawyer, C. D.; Prickett, W. Z.

1972-01-01

This paper reviews the results of recent studies performed by General Electric on in-core thermionic reactor power plants in the 50-300 kWe range. In particular, a 100 kWe manned Space Base mission and a 240 kWe unmanned electric propulsion mission are singled out as representative design points for this concept.

Thermionic energy converters

DOEpatents

Monroe, Jr., James E.

1977-08-09

A thermionic device for converting nuclear energy into electrical energy comprising a tubular anode spaced from and surrounding a cylindrical cathode, the cathode having an outer emitting surface of ruthenium, and nuclear fuel on the inner cylindrical surface. The nuclear fuel is a ceramic composition of fissionable material in a metal matrix. An axial void is provided to collect and contain fission product gases.

Design of an external-fueled thermionic diode for in-pile testing.

NASA Technical Reports Server (NTRS)

Ernst, D. M.; Peelgren, M. L.

1971-01-01

Description of an external-fueled thermionic diode suitable for in-pile testing in a research reactor. The active electrode area is 94 sq cm. The 10-in. long, 1.5-in.-OD emitter body is tungsten 2% thoria. The fuel is contained in six 0.4-in.-diam holes equally spaced about the 0.5-in. central emitter hole. The collector is niobium-1% zirconium. The expected diode performance is 6 W/sq cm at 2000 K. In addition to following the constraints imposed by the in-pile testing and the electrically heated performance mapping prior to insertion in-pile, the diode will have end configurations prototypical of those anticipated for a flow-through, NaK-cooled, external-fuel thermionic reactor.

Split-core heat-pipe reactors for out-of-pile thermionic power systems.

NASA Technical Reports Server (NTRS)

Niederauer, G.; Lantz, E.; Breitweiser, R.

1971-01-01

Description of the concept of splitting a heat-pipe reactor for out-of-core thermionics into two identical halves and using the resulting center gap for reactivity control. Short Li-W reactor heat pipes penetrate the axial reflectors and form a heat exchanger with long heat pipes which wind through the shield to the thermionic diodes. With one reactor half anchored to the shield, the other is attached to a long arm with a pivot behind the shield and swings through a small arc for reactivity control. A safety shim prevents large reactivity inputs, and a fueled control arm drive shaft acts as a power stabilizer. Reactors fueled with U-235C and with U-233C have been studied.-

Low work function silicon collector for thermionic converters

NASA Technical Reports Server (NTRS)

Chang, K. H.; Shimada, K.

1976-01-01

To improve the efficiency of present thermionic converters, single crystal silicon was investigated as a low work function collector material. The experiments were conducted in a test vehicle which resembled an actual thermionic converter. Work function as low as 1.0eV was obtained with an n-type silicon. The stabilities of the activated surfaces at elevated temperatures were tested by raising the collector temperature up to 829 K. By increasing the Cs arrival rate, it was possible to restore the originally activated low work function of the surface at elevated surface temperatures. These results, plotted in the form of Rasor-Warner curve, show a behavior similar to that of metal electrode except that the minimum work function was much lower with silicon than with metals.

Radiation of X-Rays Using Uniaxially Polarized LiNbO3 Single Crystal

NASA Astrophysics Data System (ADS)

Fukao, Shinji; Nakanishi, Yoshikazu; Mizoguchi, Tadahiro; Ito, Yoshiaki; Nakamura, Toru; Yoshikado, Shinzo

2009-03-01

X-rays are radiated due to the bremsstrahlung caused by the collision of electrons with a metal target placed opposite the negative electric surface of a crystal by changing the temperature of a LiNbO3 single crystal uniaxially polarized in the c-axis direction. It is suggested that both electric field intensity and electron density determine the intensity of X-ray radiation. Electrons are supplied by the ionization of residual gas in space, field emission from a case inside which a crystal is located, considered to be due to the high electric-field intensity formed by the surface charges on the crystal, and an external electron source, such as a thermionic source. In a high vacuum, it was found that the electrons supplied by electric-field emission mainly contribute to the radiation of X-rays. It was found that the integrated intensity of X-rays can be maximized by supplying electrons both external and by electric-field emission. Furthermore, the integrated intensity of the X-rays is stable for many repeated temperature changes.

Characterization of the inhomogeneous barrier distribution in a Pt/(100)β-Ga2O3 Schottky diode via its temperature-dependent electrical properties

NASA Astrophysics Data System (ADS)

Jian, Guangzhong; He, Qiming; Mu, Wenxiang; Fu, Bo; Dong, Hang; Qin, Yuan; Zhang, Ying; Xue, Huiwen; Long, Shibing; Jia, Zhitai; Lv, Hangbing; Liu, Qi; Tao, Xutang; Liu, Ming

2018-01-01

β-Ga2O3 is an ultra-wide bandgap semiconductor with applications in power electronic devices. Revealing the transport characteristics of β-Ga2O3 devices at various temperatures is important for improving device performance and reliability. In this study, we fabricated a Pt/β-Ga2O3 Schottky barrier diode with good performance characteristics, such as a low ON-resistance, high forward current, and a large rectification ratio. Its temperature-dependent current-voltage and capacitance-voltage characteristics were measured at various temperatures. The characteristic diode parameters were derived using thermionic emission theory. The ideality factor n was found to decrease from 2.57 to 1.16 while the zero-bias barrier height Φb0 increased from 0.47 V to 1.00 V when the temperature was increased from 125 K to 350 K. This was explained by the Gaussian distribution of barrier height inhomogeneity. The mean barrier height Φ ¯ b0 = 1.27 V and zero-bias standard deviation σ0 = 0.13 V were obtained. A modified Richardson plot gave a Richardson constant A* of 36.02 A.cm-2.K-2, which is close to the theoretical value of 41.11 A.cm-2.K-2. The differences between the barrier heights determined using the capacitance-voltage and current-voltage curves were also in line with the Gaussian distribution of barrier height inhomogeneity.

Energy broadening in electron beams: A comparison of existing theories and Monte Carlo simulation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jansen, G.H.; Groves, T.R.; Stickel, W.

1985-01-01

Different theories on the Boersch effect are applied to a simple beam geometry with one crossover in drift space.The results are compared with each other, with Monte Carlo simulations, and with the experiment. The most complete and accurate theory is given by van Leeuwen and Jansen. This theory predicts energy spreads within 10% of the Monte Carlo results for operating conditions usually given in systems with thermionic emission sources. No comprehensive theory, however, of energy broadening in electron guns has yet been presented. Nevertheless, the theory of van Leeuwen and Jansen was found to predict the experimental values by trendmore » and within a factor of 2.« less

Structural defects and recombination behavior of excited carriers in Cu(In,Ga)Se{sub 2} solar cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, J.; Du, H. W.; Li, Y.

2016-08-15

The carriers’ behavior in neutral region (NTR) and space charged region (SCR) of Cu(In,Ga)Se{sub 2} thin film based solar cells has been investigated by temperature dependent photoluminescence (PL-T), electroluminescence (EL-T) and current-voltage (IV-T) from 10 to 300 K. PL-T spectra show that three kinds of defects, namely V{sub Se}, In{sub Cu} and (In{sub Cu}+V{sub Cu}), are localized within the band gap of NTR and SCR of CIGS layer, corresponding to the energy levels of E{sub C}-0.08, E{sub C}-0.20 and E{sub C}-0.25 eV, respectively. The In{sub Cu} and (In{sub Cu}+V{sub Cu}) deep level defects are non-radiative recombination centers at room temperature.more » The IV-T and EL-T analysis reveals that the injection modes of electrons from ZnO conduction band into Cu(In,Ga)Se{sub 2} layer are tunneling, thermally-excited tunneling and thermionic emission under 10-40, 60-160, and 180-300 K, respectively. At 10-160 K, the electrons tunnel into (In{sub Cu}+V{sub Cu}) and V{sub se} defect levels in band gap of SCR and the drifting is involved in the emission bands at 0.96 and 1.07 eV, which is the direct evidence for a tunneling assisted recombination. At 180-300 K, the electrons are directly injected into the Cu(In,Ga)Se{sub 2} conduction band, and the emission of 1.13 eV are ascribed to the transitions from the conduction band to the valence band.« less

A search for space energy alternatives

NASA Technical Reports Server (NTRS)

Gilbreath, W. P.; Billman, K. W.

1978-01-01

This paper takes a look at a number of schemes for converting radiant energy in space to useful energy for man. These schemes are possible alternatives to the currently most studied solar power satellite concept. Possible primary collection and conversion devices discussed include the space particle flux devices, solar windmills, photovoltaic devices, photochemical cells, photoemissive converters, heat engines, dielectric energy conversion, electrostatic generators, plasma solar collectors, and thermionic schemes. Transmission devices reviewed include lasers and masers.

Direct Electricity from Heat: A Solution to Assist Aircraft Power Demands

NASA Technical Reports Server (NTRS)

Goldsby, Jon C.

2010-01-01

A thermionic device produces an electrical current with the application of a thermal gradient whereby the temperature at one electrode provides enough thermal energy to eject electrons. The system is totally predicated on the thermal gradient and the work function of the electrode collector relative to the emitter electrode. Combined with a standard thermoelectric device high efficiencies may result, capable of providing electrical energy from the waste heat of gas turbine engines.

High Current Density Cathodes for Future Vacuum Electronics Applications

DTIC Science & Technology

2008-05-30

Tube - device for generating high levels of RF power DARPA Defense Advanced Research Agency PBG Photonic band gap W- Band 75-111 GHz dB Decibels GHz...Extended interaction klystron 1. Introduction All RF vacuum electron sources require a high quality electron beam for efficient operation. Research on...with long life. Pres- ently, only thermionic dispenser cathodes are practical for high power RF sources. Typical thermi- onic cathodes consists of a

Thermionic converter

DOEpatents

Rasor, Ned S.; Britt, Edward J.

1976-01-01

A gas-filled thermionic converter is provided with a collector and an emitter having a main emitter region and an auxiliary emitter region in electrical contact with the main emitter region. The main emitter region is so positioned with respect to the collector that a main gap is formed therebetween and the auxiliary emitter region is so positioned with respect to the collector that an auxiliary gap is formed therebetween partially separated from the main gap with access allowed between the gaps to allow ionizable gas in each gap to migrate therebetween. With heat applied to the emitter the work function of the auxiliary emitter region is sufficiently greater than the work function of the collector so that an ignited discharge occurs in the auxiliary gap and the work function of the main emitter region is so related to the work function of the collector that an unignited discharge occurs in the main gap sustained by the ions generated in the auxiliary gap. A current flows through a load coupled across the emitter and collector due to the unignited discharge in the main gap.

Electrical properties and interface state energy distributions of Cr/n-Si Schottky barrier diode

NASA Astrophysics Data System (ADS)

Karataş, Şükrü; Yildirim, Nezir; Türüt, Abdülmecit

2013-12-01

In this study, the electrical characteristics of the Cr/n-type Si (MS) Schottky barrier diode have been investigated by the current-voltage (I-V) and capacitance-voltage (C-V) measurements at 300 K temperature. Using the thermionic emission theory, the values of ideality factor and the barrier height have been obtained to be 1.22, 0.71 and 1.01, 0.83 eV, from the results of the I-V and C-V measurements, respectively. The barrier height (Φb) and the series resistance (RS) obtained from Norde’s function have been compared with those obtained from Cheung functions, and a good agreement between the results of both methods was seen. The interface state density (NSS) calculated without the RS is obtained to be increasing exponentially with bias from 2.40 × 1012 cm-2 eV-1 in (EC-0.623) eV to 1.94 × 1014 cm-2 eV-1 in (EC-0.495) eV, also, the NSS obtained taking into account the RS has increased exponentially with bias from 2.07 × 1012 cm-2 eV-1 to 1.47 × 1014 cm-2 eV-1 in the same interval.

Transport Physics Mechanisms in Thin-Film Oxides.

NASA Astrophysics Data System (ADS)

Tierney, Brian D.; Hjalmarson, Harold P.; Jacobs-Gedrim, Robin B.; James, Conrad D.; Marinella, Matthew M.

A physics-based model of electron transport mechanisms in metal-insulating oxide-metal (M-I-M) systems is presented focusing on transport through the metal-oxide interfaces and in the bulk of the oxide. Interface tunneling, such as electron tunneling between the metal and the conduction band, or to oxide defect states, is accounted for via a WKB model. The effects of thermionic emission are also included. In the bulk of the oxide, defect-site hopping is dominant. Corresponding continuum calculations are performed for Ta2O5 M-I-M systems utilizing two different metal electrodes, e.g., platinum and tantalum. Such an asymmetrical M-I-M structure, applicable to resistive memory applications or oxide-based capacitors, reveals that the current can be either bulk or interface limited depending on the bias polarity and concentration of oxygen vacancy defects. Also, the dominance of some transport mechanisms over others is shown to be due to a complex interdependence between the vacancy concentration and bias polarity. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Asymmetric band offsets in silicon heterojunction solar cells: Impact on device performance

DOE Office of Scientific and Technical Information (OSTI.GOV)

Seif, Johannes Peter; Menda, Deneb; Descoeudres, Antoine

Here, amorphous/crystalline silicon interfaces feature considerably larger valence than conduction band offsets. In this article, we analyze the impact of such band offset asymmetry on the performance of silicon heterojunction solar cells. To this end, we use silicon suboxides as passivation layers -- inserted between substrate and (front or rear) contacts -- since such layers enable intentionally exacerbated band-offset asymmetry. Investigating all topologically possible passivation layer permutations and focussing on light and dark current-voltage characteristics, we confirm that to avoid fill factor losses, wider-bandgap silicon oxide films (of at least several nanometer thin) should be avoided in hole-collecting contacts. Asmore » a consequence, device implementation of such films as window layers -- without degraded carrier collection -- demands electron collection at the front and hole collection at the rear. Furthermore, at elevated operating temperatures, once possible carrier transport barriers are overcome by thermionic (field) emission, the device performance is mainly dictated by the passivation of its surfaces. In this context, compared to the standard amorphous silicon layers, the wide-bandgap oxide layers applied here passivate remarkably better at these temperatures, which may represent an additional benefit under practical operation conditions.« less

Asymmetric band offsets in silicon heterojunction solar cells: Impact on device performance

DOE PAGES

Seif, Johannes Peter; Menda, Deneb; Descoeudres, Antoine; ...

2016-08-01

Here, amorphous/crystalline silicon interfaces feature considerably larger valence than conduction band offsets. In this article, we analyze the impact of such band offset asymmetry on the performance of silicon heterojunction solar cells. To this end, we use silicon suboxides as passivation layers -- inserted between substrate and (front or rear) contacts -- since such layers enable intentionally exacerbated band-offset asymmetry. Investigating all topologically possible passivation layer permutations and focussing on light and dark current-voltage characteristics, we confirm that to avoid fill factor losses, wider-bandgap silicon oxide films (of at least several nanometer thin) should be avoided in hole-collecting contacts. Asmore » a consequence, device implementation of such films as window layers -- without degraded carrier collection -- demands electron collection at the front and hole collection at the rear. Furthermore, at elevated operating temperatures, once possible carrier transport barriers are overcome by thermionic (field) emission, the device performance is mainly dictated by the passivation of its surfaces. In this context, compared to the standard amorphous silicon layers, the wide-bandgap oxide layers applied here passivate remarkably better at these temperatures, which may represent an additional benefit under practical operation conditions.« less

Asymmetric band offsets in silicon heterojunction solar cells: Impact on device performance

DOE Office of Scientific and Technical Information (OSTI.GOV)

Seif, Johannes Peter, E-mail: johannes.seif@alumni.epfl.ch; Ballif, Christophe; De Wolf, Stefaan

Amorphous/crystalline silicon interfaces feature considerably larger valence than conduction band offsets. In this article, we analyze the impact of such band offset asymmetry on the performance of silicon heterojunction solar cells. To this end, we use silicon suboxides as passivation layers—inserted between substrate and (front or rear) contacts—since such layers enable intentionally exacerbated band-offset asymmetry. Investigating all topologically possible passivation layer permutations and focussing on light and dark current-voltage characteristics, we confirm that to avoid fill factor losses, wider-bandgap silicon oxide films (of at least several nanometer thin) should be avoided in hole-collecting contacts. As a consequence, device implementation ofmore » such films as window layers—without degraded carrier collection—demands electron collection at the front and hole collection at the rear. Furthermore, at elevated operating temperatures, once possible carrier transport barriers are overcome by thermionic (field) emission, the device performance is mainly dictated by the passivation of its surfaces. In this context, compared to the standard amorphous silicon layers, the wide-bandgap oxide layers applied here passivate remarkably better at these temperatures, which may represent an additional benefit under practical operation conditions.« less

Cogeneration Technology Alternatives Study (CTAS). Volume 3: Industrial processes

NASA Technical Reports Server (NTRS)

Palmer, W. B.; Gerlaugh, H. E.; Priestley, R. R.

1980-01-01

Cogenerating electric power and process heat in single energy conversion systems rather than separately in utility plants and in process boilers is examined in terms of cost savings. The use of various advanced energy conversion systems are examined and compared with each other and with current technology systems for their savings in fuel energy, costs, and emissions in individual plants and on a national level. About fifty industrial processes from the target energy consuming sectors were used as a basis for matching a similar number of energy conversion systems that are considered as candidate which can be made available by the 1985 to 2000 time period. The sectors considered included food, textiles, lumber, paper, chemicals, petroleum, glass, and primary metals. The energy conversion systems included steam and gas turbines, diesels, thermionics, stirling, closed cycle and steam injected gas turbines, and fuel cells. Fuels considered were coal, both coal and petroleum based residual and distillate liquid fuels, and low Btu gas obtained through the on site gasification of coal. An attempt was made to use consistent assumptions and a consistent set of ground rules specified by NASA for determining performance and cost. Data and narrative descriptions of the industrial processes are given.

Charge Carrier Transport Mechanism Based on Stable Low Voltage Organic Bistable Memory Device.

PubMed

Ramana, V V; Moodley, M K; Kumar, A B V Kiran; Kannan, V

2015-05-01

A solution processed two terminal organic bistable memory device was fabricated utilizing films of polymethyl methacrylate PMMA/ZnO/PMMA on top of ITO coated glass. Electrical characterization of the device structure showed that the two terminal device exhibited favorable switching characteristics with an ON/OFF ratio greater than 1 x 10(4) when the voltage was swept between - 2 V and +3 V. The device maintained its state after removal of the bias voltage. The device did not show degradation after a 1-h retention test at 120 degrees C. The memory functionality was consistent even after fifty cycles of operation. The charge transport switching mechanism is discussed on the basis of carrier transport mechanism and our analysis of the data shows that the charge carrier trans- port mechanism of the device during the writing process can be explained by thermionic emission (TE) and space-charge-limited-current (SCLC) mechanism models while erasing process could be explained by the FN tunneling mechanism. This demonstration provides a class of memory devices with the potential for low-cost, low-power consumption applications, such as a digital memory cell.

Extension of the general thermal field equation for nanosized emitters

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kyritsakis, A., E-mail: akyritsos1@gmail.com; Xanthakis, J. P.

2016-01-28

During the previous decade, Jensen et al. developed a general analytical model that successfully describes electron emission from metals both in the field and thermionic regimes, as well as in the transition region. In that development, the standard image corrected triangular potential barrier was used. This barrier model is valid only for planar surfaces and therefore cannot be used in general for modern nanometric emitters. In a recent publication, the authors showed that the standard Fowler-Nordheim theory can be generalized for highly curved emitters if a quadratic term is included to the potential model. In this paper, we extend thismore » generalization for high temperatures and include both the thermal and intermediate regimes. This is achieved by applying the general method developed by Jensen to the quadratic barrier model of our previous publication. We obtain results that are in good agreement with fully numerical calculations for radii R > 4 nm, while our calculated current density differs by a factor up to 27 from the one predicted by the Jensen's standard General-Thermal-Field (GTF) equation. Our extended GTF equation has application to modern sharp electron sources, beam simulation models, and vacuum breakdown theory.« less

Extreme Carrier Depletion and Superlinear Photoconductivity in Ultrathin Parallel-Aligned ZnO Nanowire Array Photodetectors Fabricated by Infiltration Synthesis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nam, Chang-Yong; Stein, Aaron

Ultrathin semiconductor nanowires enable high-performance chemical sensors and photodetectors, but their synthesis and device integration by standard complementary metal-oxide-semiconductor (CMOS)-compatible processes remain persistent challenges. This work demonstrates fully CMOS-compatible synthesis and integration of parallel-aligned polycrystalline ZnO nanowire arrays into ultraviolet photodetectors via infiltration synthesis, material hybridization technique derived from atomic layer deposition. The nanowire photodetector features unique, high device performances originating from extreme charge carrier depletion, achieving photoconductive on–off ratios of >6 decades, blindness to visible light, and ultralow dark currents as low as 1 fA, the lowest reported for nanostructure-based photoconductive photodetectors. Surprisingly, the low dark current is invariantmore » with increasing number of nanowires and the photodetector shows unusual superlinear photoconductivity, observed for the first time in nanowires, leading to increasing detector responsivity and other parameters for higher incident light powers. Temperature-dependent carrier concentration and mobility reveal the photoelectrochemical-thermionic emission process at grain boundaries, responsible for the observed unique photodetector performances and superlinear photoconductivity. Here, the results elucidate fundamental processes responsible for photogain in polycrystalline nanostructures, providing useful guidelines for developing nanostructure-based detectors and sensors. Lastly, the developed fully CMOS-compatible nanowire synthesis and device fabrication methods also have potentials for scalable integration of nanowire sensor devices and circuitries.« less

Extreme Carrier Depletion and Superlinear Photoconductivity in Ultrathin Parallel-Aligned ZnO Nanowire Array Photodetectors Fabricated by Infiltration Synthesis

DOE PAGES

Nam, Chang-Yong; Stein, Aaron

2017-11-15

Ultrathin semiconductor nanowires enable high-performance chemical sensors and photodetectors, but their synthesis and device integration by standard complementary metal-oxide-semiconductor (CMOS)-compatible processes remain persistent challenges. This work demonstrates fully CMOS-compatible synthesis and integration of parallel-aligned polycrystalline ZnO nanowire arrays into ultraviolet photodetectors via infiltration synthesis, material hybridization technique derived from atomic layer deposition. The nanowire photodetector features unique, high device performances originating from extreme charge carrier depletion, achieving photoconductive on–off ratios of >6 decades, blindness to visible light, and ultralow dark currents as low as 1 fA, the lowest reported for nanostructure-based photoconductive photodetectors. Surprisingly, the low dark current is invariantmore » with increasing number of nanowires and the photodetector shows unusual superlinear photoconductivity, observed for the first time in nanowires, leading to increasing detector responsivity and other parameters for higher incident light powers. Temperature-dependent carrier concentration and mobility reveal the photoelectrochemical-thermionic emission process at grain boundaries, responsible for the observed unique photodetector performances and superlinear photoconductivity. Here, the results elucidate fundamental processes responsible for photogain in polycrystalline nanostructures, providing useful guidelines for developing nanostructure-based detectors and sensors. Lastly, the developed fully CMOS-compatible nanowire synthesis and device fabrication methods also have potentials for scalable integration of nanowire sensor devices and circuitries.« less

Effect of Light Intensity and Temperature on the Current Voltage Characteristics of Al/ SY/ p- Si Organic-Inorganic Heterojunction

NASA Astrophysics Data System (ADS)

Imer, Arife Gencer; Ocak, Yusuf Selim

2016-10-01

An organic-inorganic contact was fabricated by forming a thin film of sunset yellow dye ( SY) on a p- Si wafer. The device showed a good rectification property, and the sunset yellow thin film modified the barrier height (Φb) of Al/ p- Si contact by influencing the space charge region. The heterojunction had a strong response to the different illumination intensities and showed that it can be suitable for photodiode applications. The I- V measurements of the device were also applied in the temperature range of 100-500 K. It was seen that characteristic parameters of the device were strongly dependent upon temperature. While the value of Φb increased, the ideality factor ( n) decreased with the increase in temperature. This variation was attributed to spatial inhomogeneity at the interface. The Norde function was used to determine the temperature-dependent series resistance and Φb values, and there was a good agreement with that of ln I- V data. The values of the Richardson constant ( A*) and mean Φb were determined as 29.47 Acm-2 K-2 by means of a modified activation energy plot, matching with a theoretical one, and 1.032 eV, respectively. Therefore, it was stated that the current voltage characteristic with the temperature can be explained by thermionic emission theory with Gaussian distribution of the Φb at the interface.

In Situ Chemical Modification of Schottky Barrier in Solution-Processed Zinc Tin Oxide Diode.

PubMed

Son, Youngbae; Li, Jiabo; Peterson, Rebecca L

2016-09-14

Here we present a novel in situ chemical modification process to form vertical Schottky diodes using palladium (Pd) rectifying bottom contacts, amorphous zinc tin oxide (Zn-Sn-O) semiconductor made via acetate-based solution process, and molybdenum top ohmic contacts. Using X-ray photoelectron spectroscopy depth profiling, we show that oxygen plasma treatment of Pd creates a PdOx interface layer, which is then reduced back to metallic Pd by in situ reactions during Zn-Sn-O film annealing. The plasma treatment ensures an oxygen-rich environment in the semiconductor near the Schottky barrier, reducing the level of oxygen-deficiency-related defects and improving the rectifying contact. Using this process, we achieve diodes with high forward current density exceeding 10(3)A cm(-2) at 1 V, rectification ratios of >10(2), and ideality factors of around 1.9. The measured diode current-voltage characteristics are compared to numerical simulations of thermionic field emission with sub-bandgap states in the semiconductor, which we attribute to spatial variations in metal stoichiometry of amorphous Zn-Sn-O. To the best of our knowledge, this is the first demonstration of vertical Schottky diodes using solution-processed amorphous metal oxide semiconductor. Furthermore, the in situ chemical modification method developed here can be adapted to tune interface properties in many other oxide devices.

Developing the OEIC solutions using two section light-emitting transistor

NASA Astrophysics Data System (ADS)

Liang, Shan-Fong; Hsu, Yuan-Fu; Cheng, Gong-Sheng; Wu, Chao-Hsin

2016-02-01

An integrated on-chip optical device composed of a multiple quantum-well light-emitter and photodetector in the lightemitting transistor (LET) platform is fabricated. The two devices are 400 μm in length and electrically isolated by dry etching with 4.9 μm gap. The two facets are formed by cleaving for optical output. In this report, we discuss the characteristics of the two-section device and demonstrate the optical detection by the heterojunction phototransistor (HPT) under different operation points (IB and VCE) and injected optical powers. The collector current of the HPT is 74.88 mA without illumination and 83.87 mA under illumination of 7.46μW at VCE = 3 V and IB = 12 mA, which exhibits 12% increment. The responsivity of the InGaP/GaAs HPT can reach to 711.74 A/W. At the electrical modulation bandwidth of phototransistor fT is enhanced from 1.4 GHz to 1.51 GHz under illumination. This is attributed to the Franz-Keldysh photon-assisted absorption at base-collector junction of light-emitting transistor, which produces additional holes and electrons to enhance the current gain. Through the analysis of small-signal equivalent circuit models, we can show the transit time by de-embedding the circuit parasitic effect. Extracting those parameters can clearly know the thermionic emission lifetime in the quantum well.

Temperature dependent current-voltage characteristics of Au/n-Si Schottky barrier diodes and the effect of transition metal oxides as an interface layer

NASA Astrophysics Data System (ADS)

Mahato, Somnath; Puigdollers, Joaquim

2018-02-01

Temperature dependent current-voltage (I‒V) characteristics of Au/n-type silicon (n-Si) Schottky barrier diodes have been investigated. Three transition metal oxides (TMO) are used as an interface layer between gold and silicon. The basic Schottky diode parameters such as ideality factor (n), barrier height (ϕb 0) and series resistance (Rs) are calculated and successfully explained by the thermionic emission (TE) theory. It has been found that ideality factor decreased and barrier height increased with increased of temperature. The conventional Richardson plot of ln(I0/T2) vs. 1000/T is determined the activation energy (Ea) and Richardson constant (A*). Whereas value of 'A*' is much smaller than the known theoretical value of n-type Si. The temperature dependent I-V characteristics obtained the mean value of barrier height (ϕb 0 bar) and standard deviation (σs) from the linear plot of ϕap vs. 1000/T. From the modified Richardson plot of ln(I0/T2) ˗ (qσ)2/2(kT)2 vs. 1000/T gives Richardson constant and homogeneous barrier height of Schottky diodes. Main observation in this present work is the barrier height and ideality factor shows a considerable change but the series resistance value exhibits negligible change due to TMO as an interface layer.

Near room temperature operation of a highly strained short wavelength (2.1 µm) AlAs/In0.84Ga0.16As/AlAs/InAlAs QWIP

NASA Astrophysics Data System (ADS)

Mohamed, A. H.; Missous, M.; Lai, K. T.; Haywood, S. K.

2006-06-01

A strain-compensated AlAs/InxGa1-xAs/AlAs/InyAl1-yAs (x ap 0.8, y ap 0.5) quantum well infrared photodetector (QWIP) structure was grown by molecular beam epitaxy (MBE). Conditions of exact stoichiometric growth were applied at a temperature of ~420 °C to produce structures capable of detecting IR radiation in the 2-5 µm mid-infrared spectrum. Double crystal x-ray diffraction (DCXRD) and room temperature photoluminescence (PL) experiments confirmed the excellent structural characteristics of the grown material system. A strong room temperature intersubband absorption peak was observed at a wavelength of 2.16 µm. Current-voltage (I-V) measurements as a function of temperature were carried out to electrically characterize the fabricated QWIP devices yielding devices working under background limited infrared photodetection (BLIP) conditions at 270 K. From the I-V curves, an activation energy of 270 meV at zero bias was extracted. This is in good agreement with a current transport mechanism which is dominated by thermionic emission. Photocurrent measurements were carried out and we demonstrate devices that are capable of working at a temperature as high as 270 K at a wavelength of 2.1 µm. The experimental results are in excellent agreement with the modelled values.

DOE/JPL advanced thermionic technology program

NASA Technical Reports Server (NTRS)

1979-01-01

Progress made in different tasks of the advanced thermionic technology program is described. The tasks include surface and plasma investigations (surface characterization, spectroscopic plasma experiments, and converter theory); low temperature converter development (tungsten emitter, tungsten oxide collector and tungsten emitter, nickel collector); component hardware development (hot shell development); flame-fired silicon carbide converters; high temperature and advanced converter studies; postoperational diagnostics; and correlation of design interfaces.

Thermionic Energy

NASA Technical Reports Server (NTRS)

1985-01-01

Thermionic energy conversion is the production of energy from a nuclear source. It is a technology advanced by SNSO, a joint research and development organization formed by NASA and the AEC. SNSO contracted with Thermo Electron Corporation to develop high temperature applications, i.e., metals with high melting points. Thermo Electron Corporation's expertise resulted in contracts for products made from exotic metals such as bone implants, artificial hips, and heart pacemakers.

Diminiode thermionic energy conversion with lanthanum-hexaboride electrodes

NASA Technical Reports Server (NTRS)

Kroeger, E. W.; Bair, V. L.; Morris, J. F.

1978-01-01

Thermionic conversion data obtained from a variable gap cesium diminiode with a hot pressed, sintered lanthanum hexaboride emitter and an arc melted lanthanum hexaboride collector are presented. Performance curves cover a range of temperatures: emitter 1500 to 1700 K, collector 750 to 1000 K, and cesium reservoir 370 to 510 K. Calculated values of emitter and collector work functions and barrier index are also given.

High temperature fuel/emitter system for advanced thermionic fuel elements

NASA Astrophysics Data System (ADS)

Moeller, Helen H.; Bremser, Albert H.; Gontar, Alexander; Fiviesky, Evgeny

1997-01-01

Specialists in space applications are currently focusing on bimodal power systems designed to provide both electric power and thermal propulsion (Kennedy, 1994 and Houts, 1995). Our work showed that thermionics is a viable technology for nuclear bimodal power systems. We demonstrated that materials for a thermionic fuel-emitter combination capable of performing at operating temperatures of 2473 K are not only possible but available. The objective of this work, funded by the US Department of Energy, Office of Space and Defense Power Systems, was to evaluate the compatibility of fuel material consisting of an uranium carbide/tantalum carbide solid solution with an emitter material consisting of a monocrystalline tungsten-niobium alloy. The uranium loading of the fuel material was 70 mole% uranium carbide. The program was successfully accomplished by a B&W/SIA LUTCH team. Its workscope was integrated with tasks being performed at both Babcock & Wilcox, Lynchburg Research Center, Lynchburg, Virginia, and SIA LUTCH, Podolsk, Russia. Samples were fabricated by LUTCH and seven thermal tests were performed in a hydrogen atmosphere. The first preliminary test was performed at 2273 K by LUTCH, and the remaining six tests were performed At B&W. Three tests were performed at 2273 K, two at 2373 K, and the final test at 2473 K. The results showed that the fuel and emitter materials were compatible in the presence of hydrogen. No evidence of liquid formation, dissolution of the uranium carbide from the uranium carbide/tantalum carbide solid solution, or diffusion of the uranium into the monocrystalline tungsten alloy was observed. Among the highlights of the program was the successful export of the fuel samples from Russia and their import into the US by commercial transport. This paper will discuss the technical aspects of this work.

Preparation of W–Sc{sub 2}O{sub 3} targets and scandate cathodes with film prepared by pulsed laser deposition

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Xizhu; Wang, Jinshu, E-mail: wangjsh@bjut.edu.cn; Liu, Wei

2013-12-15

Graphical abstract: - Highlights: • W–Sc{sub 2}O{sub 3} film containing 5% Sc{sub 2}O{sub 3} and 95% W were prepared by pulsed laser deposition. • W–Sc{sub 2}O{sub 3} film on scandate cathode surface improves emission property. • The film improves Sc distribution uniformity and is favorable for forming Ba–Sc–O layer. - Abstract: Sub-micrometer Sc{sub 2}O{sub 3}–W powder with a narrow particle size distribution has been obtained by a sol–gel method combined with two-step hydrogen reduction process. Based on the obtained powder, the W–Sc{sub 2}O{sub 3} targets have been sintered via spark plasma sintering (SPS) at 1300 °C. The W–Sc{sub 2}O{sub 3}more » targets have the average grain size of about 1 μm. Both the sintering temperature and holding time are much lower than those of the targets prepared with micrometer sized powders. The obtained W–Sc{sub 2}O{sub 3} targets have a high comparative density of 96.4% and rockwell hardness of 86.4 HRC. Using the target, the scandate cathode deposited with a film containing 5% Sc{sub 2}O{sub 3} and 95% W has been obtained by pulsed laser deposition (PLD) method. This cathode has good emission property, i.e., the highest thermionic emission current density reaches 43.09 A/cm{sup 2} of J{sub div} at 900 °C{sub b} after being activated for 8 h, which is much higher than that of scandate cathode without film. Scandium (Sc) supplied by the film on the surface during the activation forms a Ba–Sc–O active layer, which helps to the emission.« less

Reducing Energy Degradation Due to Back-bombardment Effect with Modulated RF Input in S-band Thermionic RF Gun

NASA Astrophysics Data System (ADS)

Kii, Toshiteru; Nakai, Yoko; Fukui, Toshio; Zen, Heishun; Kusukame, Kohichi; Okawachi, Norihito; Nakano, Masatsugu; Masuda, Kai; Ohgaki, Hideaki; Yoshikawa, Kiyoshi; Yamazaki, Tetsuo

2007-01-01

Energy degradation due to back-bombardment effect is quite serious to produce high-brightness electron beam with long macro-pulse with thermionic rf gun. To avoid the back-bombardment problem, a laser photo cathode is used at many FEL facilities, but usually it costs high and not easy to operate. Thus we have studied long pulse operation of the rf gun with thermionic cathode, which is inexpensive and easy to operate compared to the photocathode rf gun. In this work, to reduce the energy degradation, we controlled input rf power amplitude by controlling pulse forming network of the power modulator for klystron. We have successfully increased the pulse duration up to 4 μs by increasing the rf power from 7.8 MW to 8.5 MW during the macro pulse.

Study of the collector/heat pipe cooled externally configured thermionic diode

NASA Technical Reports Server (NTRS)

1973-01-01

A collector/heat pipe cooled, externally configured (heated) thermionic diode module was designed for use in a laboratory test to demonstrate the applicability of this concept as the fuel element/converter module of an in-core thermionic electric power source. During the course of the program, this module evolved from a simple experimental mock-up into an advanced unit which was more reactor prototypical. Detailed analysis of all diode components led to their engineering design, fabrication, and assembly, with the exception of the collector/heat pipe. While several designs of high power annular wicked heat pipes were fabricated and tested, each exhibited unexpected performance difficulties. It was concluded that the basic cause of these problems was the formation of crud which interfered with the liquid flow in the annular passage of the evaporator region.

Annual Summary Report on Thermionic Cathode Project.

DTIC Science & Technology

1986-01-09

Voltage Operation The electron gun cathode is driven negative by a high voltageRadiation pulse modulator in the circuit of Figure 3-1. Typical current...tungsten filament. The bombardment heating system is stabilized by a feed- back control circuit . The power required to heat tne cathode is 315 W bom...project. The primary purpose of the first phase was to develop the bombardment heating circuit used to heat the LaB 6 cathode, and to test the beam

Heat Recuperator Engineering for an ARL Liquid-Fueled Thermophotovoltaic Power Source Demonstrator

DTIC Science & Technology

2014-09-01

using logistics and multiple other fuels. Some potential technologies include thermoelectric , thermophotovoltaic (TPV), and thermionic. For these... thermoelectric , thermophotovoltaic (TPV), and thermionic. For these technologies, thermal efficiency is critical to achieve high energy density and thermal-to... thermoelectric and TPV. The exhaust gas will be above this temperature, but more than 50% of the thermal power of the combustor can be lost to the exhaust

User interface and operational issues with thermionic space power systems

NASA Technical Reports Server (NTRS)

Dahlberg, R. C.; Fisher, C. R.

1987-01-01

Thermionic space power systems have unique features which facilitate predeployment operations, provide operational flexibility and simplify the interface with the user. These were studied in some detail during the SP-100 program from 1983 to 1985. Three examples are reviewed in this paper: (1) system readiness verification in the prelaunch phase; (2) startup, shutdown, and dormancy in the operations phase; (3) part-load operation in the operations phase.

Design, fabrication, and testing of an external fuel (UO2), full-length thermionic converter

NASA Technical Reports Server (NTRS)

Schock, A.; Raab, B.

1971-01-01

The development of a full-length external-fuel thermionic converter for in-pile testing is described. The development program includes out-of-pile performance testing of the fully fueled-converter, using RF-induction heating, before its installation in the in-pile test capsule. The external-fuel converter is cylindrical in shape, and consists of an inner, centrally cooled collector, and an outer emitter surrounded by nuclear fuel. The term full-length denotes that the converter is long enough to extend over the full height of the reactor core. Thus, the converter is not a scaled-down test device, but a full-scale fuel element of the thermionic reactor. The external-fuel converter concept permits a number of different design options, particularly with respect to the fuel composition and shape, and the collector cooling arrangement. The converter described was developed for the Jet Propulsion Laboratory, and is based on their concept for a thermionic reactor with uninsulated collector cooling as previously described. The converter is double-ended, with through-flow cooling, and with ceramic seals and emitter and collector power take-offs at both ends. The design uses a revolver-shaped tungsten emitter body, with the central emitter hole surrounded by six peripheral fuel holes loaded with cylindrical UO2 pellets.

Cogeneration Technology Alternatives Study (CTAS). Volume 6: Computer data. Part 2: Residual-fired nocogeneration process boiler

NASA Technical Reports Server (NTRS)

Knightly, W. F.

1980-01-01

About fifty industrial processes from the largest energy consuming sectors were used as a basis for matching a similar number of energy conversion systems that are considered as candidate which can be made available by the 1985 to 2000 time period. The sectors considered included food, textiles, lumber, paper, chemicals, petroleum, glass, and primary metals. The energy conversion systems included steam and gas turbines, diesels, thermionics, stirling, closed cycle and steam injected gas turbines, and fuel cells. Fuels considered were coal, both coal and petroleum based residual and distillate liquid fuels, and low Btu gas obtained through the on site gasification of coal. Computer generated reports of the fuel consumption and savings, capital costs, economics and emissions of the cogeneration energy conversion systems (ECS's) heat and power matched to the individual industrial processes are presented. National fuel and emissions savings are also reported for each ECS assuming it alone is implemented. Two nocogeneration base cases are included: coal fired and residual fired process boilers.

Cogeneration Technology Alternatives Study (CTAS). Volume 6: Computer data. Part 1: Coal-fired nocogeneration process boiler, section A

NASA Technical Reports Server (NTRS)

Knightly, W. F.

1980-01-01

About fifty industrial processes from the largest energy consuming sectors were used as a basis for matching a similar number of energy conversion systems that are considered as candidate which can be made available by the 1985 to 2000 time period. The sectors considered included food, textiles, lumber, paper, chemicals, petroleum, glass, and primary metals. The energy conversion systems included steam and gas turbines, diesels, thermionics, stirling, closed cycle and steam injected gas turbines, and fuel cells. Fuels considered were coal, both coal and petroleum based residual and distillate liquid fuels, and low Btu gas obtained through the on site gasification of coal. Computer generated reports of the fuels consumption and savings, capital costs, economics and emissions of the cogeneration energy conversion systems (ECS's) heat and power matched to the individual industrial processes are presented. National fuel and emissions savings are also reported for each ECS assuming it alone is implemented. Two nocogeneration base cases are included: coal fired and residual fired process boilers.

Cogeneration Technology Alternatives Study (CTAS). Volume 6: Computer data. Part 1: Coal-fired nocogeneration process boiler, section B

NASA Technical Reports Server (NTRS)

Knightly, W. F.

1980-01-01

About fifty industrial processes from the largest energy consuming sectors were used as a basis for matching a similar number of energy conversion systems that are considered as candidate which can be made available by the 1985 to 2000 time period. The sectors considered included food, textiles, lumber, paper, chemicals, petroleum, glass, and primary metals. The energy conversion systems included steam and gas turbines, diesels, thermionics, stirling, closed cycle and steam injected gas turbines, and fuel cells. Fuels considered were coal, both coal and petroleum based residual and distillate liquid fuels, and low Btu gas obtained through the on site gasification of coal. Computer generated reports of the fuel consumption and savings, capital costs, economics and emissions of the cogeneration energy conversion systems (ECS's) heat and power matched to the individual industrial processes are presented. National fuel and emissions savings are also reported for each ECS assuming it alone is implemented. Two nocogeneration base cases are included: coal fired and residual fired process boilers.

Visible sub-band gap photoelectron emission from nitrogen doped and undoped polycrystalline diamond films

NASA Astrophysics Data System (ADS)

Elfimchev, S.; Chandran, M.; Akhvlediani, R.; Hoffman, A.

2017-07-01

In this study the origin of visible sub-band gap photoelectron emission (PEE) from polycrystalline diamond films is investigated. The PEE yields as a function of temperature were studied in the wavelengths range of 360-520 nm. Based on the comparison of electron emission yields from diamond films deposited on silicon and molybdenum substrates, with different thicknesses and nitrogen doping levels, we suggested that photoelectrons are generated from nitrogen related centers in diamond. Our results show that diamond film thickness and substrate material have no significant influence on the PEE yield. We found that nanocrystalline diamond films have low electron emission yields, compared to microcrystalline diamond, due to the presence of high amount of defects in the former, which trap excited electrons before escaping into the vacuum. However, the low PEE yield of nanocrystalline diamond films was found to increase with temperature. The phenomenon was explained by the trap assisted photon enhanced thermionic emission (ta-PETE) model. According to the ta-PETE model, photoelectrons are trapped by shallow traps, followed by thermal excitation at elevated temperatures and escape into the vacuum. Activation energies of trap levels were estimated for undoped nanocrystalline, undoped microcrystalline and N-doped diamond films using the Richardson-Dushman equation, which gives 0.13, 0.39 and 0.04 eV, respectively. Such low activation energy of trap levels makes the ta-PETE process very effective at elevated temperatures.

Enhanced Semiconductor Nanocrystal Conductance via Solution Grown Contacts

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sheldon, Matthew T.; Trudeau, Paul-Emile; Mokari, Taleb

We report a 100,000-fold increase in the conductance of individual CdSe nanorods when they are electrically contacted via direct solution phase growth of Au tips on the nanorod ends. Ensemble UV-Vis and X-Ray photoelectron spectroscopy indicate this enhancement does not result from alloying of the nanorod. Rather, low temperature tunneling and high temperature (250-400 K) thermionic emission across the junction at the Au contact reveal a 75percent lower interface barrier to conduction compared to a control sample. We correlate this barrier lowering with the electronic structure at the Au-CdSe interface. Our results emphasize the importance of nanocrystal surface structure formore » robust device performance and the advantage of this contact method.« less

Apparatus for in situ cleaning of carbon contaminated surfaces

DOEpatents

Klebanoff, Leonard E.; Grunow, Philip; Graham, Jr., Samuel

2004-08-10

Activated gaseous species generated adjacent a carbon contaminated surface affords in-situ cleaning. A device for removing carbon contamination from a surface of the substrate includes (a) a housing defining a vacuum chamber in which the substrate is located; (b) a source of gaseous species; and (c) a source of electrons that are emitted to activate the gaseous species into activated gaseous species. The source of electrons preferably includes (i) a filament made of a material that generates thermionic electron emissions; (ii) a source of energy that is connected to the filament; and (iii) an electrode to which the emitted electrons are attracted. The device is particularly suited for photolithography systems with optic surfaces, e.g., mirrors, that are otherwise inaccessible unless the system is dismantled.

Thermionic modules

DOEpatents

King, Donald B.; Sadwick, Laurence P.; Wernsman, Bernard R.

2002-06-18

Modules of assembled microminiature thermionic converters (MTCs) having high energy-conversion efficiencies and variable operating temperatures manufactured using MEMS manufacturing techniques including chemical vapor deposition. The MTCs incorporate cathode to anode spacing of about 1 micron or less and use cathode and anode materials having work functions ranging from about 1 eV to about 3 eV. The MTCs also exhibit maximum efficiencies of just under 30%, and thousands of the devices and modules can be fabricated at modest costs.

Thermionic converter output as a function of collector temperature

NASA Technical Reports Server (NTRS)

Stark, G.; Saunders, M.; Lieb, D.

1980-01-01

Surprisingly few data are available on the variation of thermionic converter output with collector temperature. In this study the output power density has been measured as a function of collector temperature (at a fixed emitter temperature of 1650 K) for six converters with different electrode combinations. Collector temperatures ranged from 750 to 1100 K. For collector temperatures below 900 K, converters built with sublimed molybdenum oxide collectors gave the best performance.

New Concepts for Compact Space Reactor Power Systems for Space Based Radar Applications: A Feasibility Study

DTIC Science & Technology

1989-12-01

SPENT FUEL REPROCESSING COULD ALSO BE EMPLOYED IRRADIATION EXPERIENCE - EXTREMELY LIMITED - JOINT US/UK PROGRAM (ONGOING) - TUI/KFK PROGRAM (CANCELED...only the use of off-the-shelf technologies. For example, conventional fuel technology (uranium dioxide), conventional thermionic conversion...advanced fuel (Americium oxide, A1TI2O3) and advanced thermionic conversion. Concept C involves use of an advanced fuel (Americium oxide, Arri203

Space station systems analysis study. Part 3: Documentation. Volume 7: SCB alternate EPS evaluation, task 10

NASA Technical Reports Server (NTRS)

1977-01-01

Power levels up to 100 kWe average were baselined for the electrical power system of the space construction base, a long-duration manned facility capable of supporting manufacturing and large scale construction projects in space. Alternatives to the solar array battery systems discussed include: (1) solar concentrator/brayton; (2) solar concentrator/thermionic; (3) isotope/brayton; (4) nuclear/brayton; (5) nuclear thermoelectric; and (6) nuclear thermionic.

Irradiation test of tungsten clad uranium carbide-zirconium carbide ((U,Zr)C) specimens for thermionic reactor application at conditions conductive to long-term performance

NASA Technical Reports Server (NTRS)

Creagh, J. W. R.; Smith, J. R.

1973-01-01

Uranium carbide fueled, thermionic emitter configurations were encapsulated and irradiated. One capsule contained a specimen clad with fluoride derived chemically vapor deposited (CVD) tungsten. The other capsule used a duplex clad specimen consisting of chloride derived on floride derived CVD tungsten. Both fuel pins were 16 millimeters in diameter and contained a 45.7-millimeter length of fuel.

Processing of thermionic power on an electrically propelled spacecraft

NASA Technical Reports Server (NTRS)

Macie, T. W.

1973-01-01

A study to define the power processing equipment required between a thermionic reactor and an array of mercury-ion thrusters for a nuclear electric propulsion system is reported. Observations and recommendations that resulted from this study were: (1) the preferred thermionic-fuel-element source voltages are 23 V or higher; (2) transistor characteristics exert a strong effect on power processor mass; (3) the power processor mass could be considerably reduced should the magnetic materials that exhibit low losses at high frequencies, that have a high Curie point, and that can operate at 15 to 20 kG become avaliable; (4) electrical component packaging on the radiator could reduce the area that is sensitive to meteoroid penetration, thereby reducing the meteoroid shielding mass requirement; (5) an experimental model of the power processor design should be built and tested to verify the efficiencies, masses, and all the automatic operational aspects of the design.

A cusp electron gun for millimeter wave gyrodevices

NASA Astrophysics Data System (ADS)

Donaldson, C. R.; He, W.; Cross, A. W.; Li, F.; Phelps, A. D. R.; Zhang, L.; Ronald, K.; Robertson, C. W.; Whyte, C. G.; Young, A. R.

2010-04-01

The experimental results of a thermionic cusp electron gun, to drive millimeter and submillimeter wave harmonic gyrodevices, are reported in this paper. Using a "smooth" magnetic field reversal formed by two coils this gun generated an annular-shaped, axis-encircling electron beam with 1.5 A current, and an adjustable velocity ratio α of up to 1.56 at a beam voltage of 40 kV. The beam cross-sectional shape and transported beam current were measured by a witness plate technique and Faraday cup, respectively. These measured results were found to be in excellent agreement with the simulated results using the three-dimensional code MAGIC.

Temperature dependent current transport of Pd/ZnO nanowire Schottky diodes

NASA Astrophysics Data System (ADS)

Gayen, R. N.; Bhattacharyya, S. R.; Jana, P.

2014-09-01

Zinc oxide (ZnO) nanowire based Schottky barrier diodes are fabricated by depositing Pd metal contact on top of vertically well-aligned ZnO nanowire arrays. A vertical array of ZnO nanowires on indium tin oxide (ITO) coated glass substrates is synthesized by hybrid wet chemical route. Scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) measurement confirm the formation of stoichiometric well-aligned hexagonal (h-ZnO) nanowire arrays with wurtzite structure. Temperature dependent current-voltage (I-V) measurements on palladium-ZnO (Pd/ZnO) nanowire Schottky junctions in the temperature range 303-383 K exhibit excellent rectifying character. From these nonlinear I-V plots, different electrical parameters of diode-like reverse saturation current, barrier height and ideality factor are determined as a function of temperature assuming pure thermionic emission model. The ideality factor is found to decrease while the barrier height increases with the increase in temperature. The series resistance values calculated from Cheung’s functions also show temperature dependency. Such behavior can be attributed to the presence of defects that traps carriers, and barrier height inhomogeneity at the interface of the barrier junction. After barrier height inhomogeneity correction, considering a Gaussian distributed barrier height fluctuation across the Pd/ZnO interface, the estimated values of mean barrier height and modified Richardson constant are more closely matched to the theoretically predicted value for Pd/ZnO Schottky barrier diodes. The variation of density of interface states as a function of interface state energy is also calculated.

An X-band high-impedance relativistic klystron amplifier with an annular explosive cathode

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, Danni; Zhang, Jun, E-mail: zhangjun@nudt.edu.cn; Zhong, Huihuang

2015-11-15

The feasibility of employing an annular beam instead of a solid one in the X-band high-impedance relativistic klystron amplifier (RKA) is investigated in theory and simulation. Small-signal theory analysis indicates that the optimum bunching distance, fundamental current modulation depth, beam-coupling coefficient, and beam-loaded quality factor of annular beams are all larger than the corresponding parameters of solid beams at the same beam voltage and current. An annular beam RKA and a solid beam RKA with almost the same geometric parameters are compared in particle-in-cell simulation. Output microwave power of 100 MW, gain of 50 dB, and power conversion efficiency of 42% aremore » obtained in an annular beam RKA. The annular beam needs a 15% lower uniform guiding magnetic field than the solid beam. Our investigations demonstrate that we are able to use a simple annular explosive cathode immersed in a lower uniform magnetic field instead of a solid thermionic cathode in a complicated partially shielding magnetic field for designing high-impedance RKA, which avoids high temperature requirement, complicated electron-optical system, large area convergence, high current density, and emission uniformity for the solid beam. An equivalent method for the annular beam and the solid beam on bunching features is proposed and agrees with the simulation. The annular beam has the primary advantages over the solid beam that it can employ the immersing uniform magnetic field avoiding the complicated shielding magnetic field system and needs a lower optimum guiding field due to the smaller space charge effect.« less

Current-voltage characteristics of the semiconductor nanowires under the metal-semiconductor-metal structure

NASA Astrophysics Data System (ADS)

Wen, Jing; Zhang, Xitian; Gao, Hong; Wang, Mingjiao

2013-12-01

We present a method to calculate the I-V characteristics of semiconductor nanowires under the metal-semiconductor-metal (MSM) structure. The carrier concentration as an important parameter is introduced into the expression of the current. The subband structure of the nanowire has been considered for associating it with the position of the Fermi level and circumventing the uncertainties of the contact areas in the contacts. The tunneling and thermionic emission currents in the two Schottky barriers at the two metal-semiconductor contacts are discussed. We find that the two barriers have different influences on the I-V characteristics of the MSM structure, one of which under the forward bias plays the role of threshold voltage if its barrier height is large and the applied voltage is small, and the other under the reverse bias controls the shapes of I-V curves. Our calculations show that the shapes of the I-V curves for the MSM structure are mainly determined by the barrier heights of the contacts and the carrier concentration. The nearly identical I-V characteristics can be obtained by using different values of the barrier heights and carrier concentration, which means that the contact type conversion can be ascribed not only to the changes of the barrier heights but also that of the carrier concentration. We also discuss the mechanisms of the ohmic-Schottky conversions and clarify the ambiguity in the literature. The possibility about the variation of the carrier concentration under the applied fields has been confirmed by experimental results.

Calculated power distribution of a thermionic, beryllium oxide reflected, fast-spectrum reactor

NASA Technical Reports Server (NTRS)

Mayo, W.; Lantz, E.

1973-01-01

A procedure is developed and used to calculate the detailed power distribution in the fuel elements next to a beryllium oxide reflector of a fast-spectrum, thermionic reactor. The results of the calculations show that, although the average power density in these outer fuel elements is not far from the core average, the power density at the very edge of the fuel closest to the beryllium oxide is about 1.8 times the core avearge.

Microfabricated thermionic detector

DOEpatents

Lewis, Patrick R; Manginell, Ronald P; Wheeler, David R; Trudell, Daniel E

2012-10-30

A microfabricated TID comprises a microhotplate and a thermionic source disposed on the microhotplate. The microfabricated TID can provide high sensitivity and selectivity to nitrogen- and phosphorous-containing compounds and other compounds containing electronegative function groups. The microfabricated TID can be microfabricated with semiconductor-based materials. The microfabricated TID can be combined with a microfabricated separation column and used in microanalytical system for the rapid on-site detection of pesticides, chemical warfare agents, explosives, pharmaceuticals, and other organic compounds that contain nitrogen or phosphorus.

Schottky barrier diode based on β-Ga2O3 (100) single crystal substrate and its temperature-dependent electrical characteristics

NASA Astrophysics Data System (ADS)

He, Qiming; Mu, Wenxiang; Dong, Hang; Long, Shibing; Jia, Zhitai; Lv, Hangbing; Liu, Qi; Tang, Minghua; Tao, Xutang; Liu, Ming

2017-02-01

The Pt/β-Ga2O3 Schottky barrier diode and its temperature-dependent current-voltage characteristics were investigated for power device application. The edge-defined film-fed growth (EFG) technique was utilized to grow the (100)-oriented β-Ga2O3 single crystal substrate that shows good crystal quality characterized by X-ray diffraction and high resolution transmission electron microscope. Ohmic and Schottky electrodes were fabricated by depositing Ti and Pt metals on the two surfaces, respectively. Through the current-voltage (I-V) measurement under different temperature and the thermionic emission modeling, the fabricated Pt/β-Ga2O3 Schottky diode was found to show good performances at room temperature, including rectification ratio of 1010, ideality factor (n) of 1.1, Schottky barrier height (ΦB) of 1.39 eV, threshold voltage (Vbi) of 1.07 V, ON-resistance (RON) of 12.5 mΩ.cm2, forward current density at 2 V (J@2V) of 56 A/cm2, and saturation current density (J0) of 2 × 10-16 A/cm2. The effective donor concentration Nd - Na was calculated to be about 2.3 × 1014 cm3. Good temperature dependent performance was also found in the device. The Schottky barrier height was estimated to be about 1.3 eV-1.39 eV at temperatures ranging from room temperature to 150 °C. With increasing temperature, parameters such as RON and J@2V become better, proving that the diode can work well at high temperature. The EFG grown β-Ga2O3 single crystal is a promising material to be used in the power devices.

Thermionic fast spectrum reactor-converter on the basis of multi-cell TFE

NASA Astrophysics Data System (ADS)

Ponomarev-Stepnoi, N. N.; Kompaniets, G. V.; Poliakov, D. N.; Stepennov, B. S.; Andreev, P. V.; Zhabotinsky, E. E.; Nikolaev, Yu. V.; Lapochkin, N. V.

2001-02-01

Today Russian experts have technological experience in development of in-core thermionic converters for reactors of space nuclear power plants. Such a converter contains nuclear fuel inside and really represents a fuel element of a reactor. Two types of reactors can be considered on the basis of these thermionic fuel elements: with thermal or intermediate neutron spectrum, and with fast neutron spectrum. The first type is characterized by the presence of moderator in core that ensures most economical usage of nuclear fuel. The estimation shows that moderated system is the most effective in the power range of about 5 ... 100 kWe. The power systems of higher level are characterized by larger dimensions due to the presence of moderator. The second type of reactor is considered for higher power levels. This power range is about hundreds kWe. Dimensions of the fast reactor and core configuration are determined by the necessity to ensure the required net output power, on the one hand, and the necessity to ensure critical state on the other hand. In the case of using in-core thermionic fuel elements of the specified design, minimal reactor output power is determined by reactor criticality condition, and maximum reactor power output is determined by specifications and launcher capabilities. In the present paper the effective multiplication factor of a fast spectrum reactor on the basis of a multi-cell TFE developed by ``Lutch'' is considered a function of the total number of TFEs in the reactor. The MCU Monte-Carlo code, developed in Russia (Alekseev, et al., 1991), was used for computations. TFE computational models are placed in the nodes of a uniform triangular lattice and surrounded with pressure vessel and a side reflector. Ordinary fuel pins without thermionic converters were used instead of some TFEs to optimize criticality parameters, dimensions and output power of the reactor. General weight parameters of the reactor are presented in the paper. .

Low-frequency noise properties in Pt-indium gallium zinc oxide Schottky diodes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Jiawei; Zhang, Linqing; Ma, Xiaochen

2015-08-31

The low-frequency noise properties of Pt-indium gallium zinc oxide (IGZO) Schottky diodes at different forward biases are investigated. The IGZO layer and Pt contact were deposited by RF sputtering at room temperature. The diode showed an ideality factor of 1.2 and a barrier height of 0.94 eV. The current noise spectral density exhibited 1/f behavior at low frequencies. The analysis of the current dependency of the noise spectral density revealed that for the as-deposited diode, the noise followed Luo's mobility and diffusivity fluctuation model in the thermionic-emission-limited region and Hooge's empirical theory in the series-resistance-limited region. A low Hooge's constant ofmore » 1.4 × 10{sup −9} was found in the space-charge region. In the series-resistance-limited region, the Hooge's constant was 2.2 × 10{sup −5}. After annealing, the diode showed degradation in the electrical performance. The interface-trap-induced noise dominated the noise spectrum. By using the random walk model, the interface-trap density was obtained to be 3.6 × 10{sup 15 }eV{sup −1 }cm{sup −2}. This work provides a quantitative approach to analyze the properties of Pt-IGZO interfacial layers. These low noise properties are a prerequisite to the use of IGZO Schottky diodes in switch elements in memory devices, photosensors, and mixer diodes.« less

Electronic Characterization of Au/DNA/ITO Metal-Semiconductor-Metal Diode and Its Application as a Radiation Sensor.

PubMed

Al-Ta'ii, Hassan Maktuff Jaber; Periasamy, Vengadesh; Amin, Yusoff Mohd

2016-01-01

Deoxyribonucleic acid or DNA molecules expressed as double-stranded (DSS) negatively charged polymer plays a significant role in electronic states of metal/silicon semiconductor structures. Electrical parameters of an Au/DNA/ITO device prepared using self-assembly method was studied by using current-voltage (I-V) characteristic measurements under alpha bombardment at room temperature. The results were analyzed using conventional thermionic emission model, Cheung and Cheung's method and Norde's technique to estimate the barrier height, ideality factor, series resistance and Richardson constant of the Au/DNA/ITO structure. Besides demonstrating a strongly rectifying (diode) characteristic, it was also observed that orderly fluctuations occur in various electrical parameters of the Schottky structure. Increasing alpha radiation effectively influences the series resistance, while the barrier height, ideality factor and interface state density parameters respond linearly. Barrier height determined from I-V measurements were calculated at 0.7284 eV for non-radiated, increasing to about 0.7883 eV in 0.036 Gy showing an increase for all doses. We also demonstrate the hypersensitivity phenomena effect by studying the relationship between the series resistance for the three methods, the ideality factor and low-dose radiation. Based on the results, sensitive alpha particle detectors can be realized using Au/DNA/ITO Schottky junction sensor.

InP MOS capacitor and E-mode n-channel FET with ALD Al2O3-based high- k dielectric

NASA Astrophysics Data System (ADS)

Yen, Chih-Feng; Yeh, Min-Yen; Chong, Kwok-Keung; Hsu, Chun-Fa; Lee, Ming-Kwei

2016-07-01

The electrical characteristics of atomic-layer-deposited Al2O3/TiO2/Al2O3 on (NH4)2S-treated InP MOS capacitor and related MOSFET were studied. The electrical characteristics were improved from the reduction of native oxides and sulfur passivation on InP by (NH4)2S treatment. The high bandgap Al2O3 on TiO2 can reduce the thermionic emission, and the Al2O3 under TiO2 improves the interface-state density by self-cleaning. The high dielectric constant TiO2 is used to lower the equivalent oxide thickness. The leakage currents can reach 2.3 × 10-8 and 2.2 × 10-7 A/cm2 at ±2 MV/cm, respectively. The lowest interface-state density is 4.6 × 1011 cm-2 eV-1 with a low-frequency dispersion of 15 %. The fabricated enhancement-mode n-channel sulfur-treated InP MOSFET exhibits good electrical characteristics with a maximum transconductance of 146 mS/mm and effective mobility of 1760 cm2/V s. The subthreshold swing and threshold voltage are 117 mV/decade and 0.44 V, respectively.

Experimental and numerical investigation of contact-area-limited doping for top-contact pentacene thin-film transistors with Schottky contact.

PubMed

Noda, Kei; Wada, Yasuo; Toyabe, Toru

2015-10-28

Effects of contact-area-limited doping for pentacene thin-film transistors with a bottom-gate, top-contact configuration were investigated. The increase in the drain current and the effective field-effect mobility was achieved by preparing hole-doped layers underneath the gold contact electrodes by coevaporation of pentacene and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), confirmed by using a thin-film organic transistor advanced simulator (TOTAS) incorporating Schottky contact with a thermionic field emission (TFE) model. Although the simulated electrical characteristics fit the experimental results well only in the linear regime of the transistor operation, the barrier height for hole injection and the gate-voltage-dependent hole mobility in the pentacene transistors were evaluated with the aid of the device simulation. This experimental data analysis with the simulation indicates that the highly-doped semiconducting layers prepared in the contact regions can enhance the charge carrier injection into the active semiconductor layer and concurrent trap filling in the transistor channel, caused by the mitigation of a Schottky energy barrier. This study suggests that both the contact-area-limited doping and the device simulation dealing with Schottky contact are indispensable in designing and developing high-performance organic thin-film transistors.

UV/ozone assisted local graphene (p)/ZnO(n) heterojunctions as a nanodiode rectifier

NASA Astrophysics Data System (ADS)

Sahatiya, Parikshit; Badhulika, Sushmee

2016-07-01

Here we report the fabrication of a novel graphene/ZnO nanodiode by UV/ozone assisted oxidation of graphene and demonstrate its application as a half-wave rectifier to generate DC voltage. The method involves the use of electrospinning for one-step in situ synthesis and alignment of single Gr/ZnO nanocomposite across metal electrodes. On subsequent UV illumination, graphene oxidizes, which induces p type doping and ZnO being an n type semiconductor, thus resulting in the formation of a nanodiode. The as-fabricated device shows strong non-linear current-voltage characteristic similar to that of conventional semiconductor p-n junction diodes. Excellent rectifying behavior with a rectification ratio of ~103 was observed and the nanodiodes were found to exhibit long-term repeatability in their performance. Ideality factor and barrier height, as calculated by the thermionic emission model, were found to be 1.6 and 0.504 eV respectively. Due to the fact that diodes are the basic building blocks in the electronics and semiconductor industry, the successful fabrication of these nanodiodes based on UV assisted p type doping of graphene indicates that this approach can be used for developing highly scalable and efficient components for nanoelectronics, such as rectifiers and logic gates that find applications in numerous fields.

JSUS solar thermal thruster and its integration with thermionic power converter

NASA Astrophysics Data System (ADS)

Shimizu, Morio; Eguchi, Kunihisa; Itoh, Katsuya; Sato, Hitoshi; Fujii, Tadayuki; Okamoto, Ken-Ichi; Igarashi, Tadashi

1998-01-01

This paper describes solar heating test results of a single crystal Mo thruster of solar thermal propulsion (STP) with super high-temperature brazing of Mo/Ru for hydrogen-gas sealing, using the paraboloidal concentrator of 1.6 m diameter newly installed in NAL in the Japan Solar Upper Stage (JSUS) research program. The designed thruster has a target Isp about 800 sec for 2,250 K or higher temperatures of hydrogen propellant. Additionally, tungsten CVD-coating was applied to a outer surface of the thruster in order to prevent vaporization of the wall material and Mo/Ru under the condition of high temperature over 2,500K and high vacuum. Also addressed in our paper is solar thermionic power module design for the integration with the STP receiver. The thermionic converter (TIC) module is of a planar type in a Knudsen-mode operation and provides a high conversion efficiency of 23% at the TIC emitter temperature of nearly 1,850 K for a heat input flux of 24 W/cm2.

Inductive storage for quasi-steady MPD thrusters

NASA Technical Reports Server (NTRS)

Clark, K. E.

1978-01-01

Experiments in which a quasi-steady MPD thruster is driven by a large inductor demonstrate the feasibility of using inductive energy storage to couple an intermittent high power plasma thruster to a lower power steady state supply, such as a thermionic converter. Switching between inductor charging and MPD thrusting phases of the current cycle occurs smoothly, with the voltage spike generated during switching sufficient to initiate the arc discharge in the thruster without an auxiliary starting circuit. Further, the current waveforms delivered by the inductor are of a shape suitable for the quasi-steady thrusting process, and they agree with analytical estimates, indicating that the interaction between the thruster impedance and the inductive source is dynamically stable.

Development of a High Average Current Thermionic Injector for Free-Electron Lasers

DTIC Science & Technology

2013-02-11

high   average   power   FEL   should   produce   high ...The  cathode  heater   is   powered  by  a  60  Hz  AC   feed  that  floats  on  the   high  voltage  pulse... high -­‐voltage   power  supply  for  the  IOT  gun  is  a  70  kV  Rockwell  hard  tube   modulator   with  

A pilot level decision analysis of thermionic reactor development strategy for nuclear electric propulsion

NASA Technical Reports Server (NTRS)

Menke, M. M.; Judd, B. R.

1973-01-01

The development policy for thermionic reactors to provide electric propulsion and power for space exploration was analyzed to develop a logical procedure for selecting development alternatives that reflect the technical feasibility, JPL/NASA project objectives, and the economic environment of the project. The partial evolution of a decision model from the underlying philosophy of decision analysis to a deterministic pilot phase is presented, and the general manner in which this decision model can be employed to examine propulsion development alternatives is illustrated.

Chemical vapor deposition techniques and related methods for manufacturing microminiature thermionic converters

DOEpatents

King, Donald B.; Sadwick, Laurence P.; Wernsman, Bernard R.

2002-06-25

Methods of manufacturing microminiature thermionic converters (MTCs) having high energy-conversion efficiencies and variable operating temperatures using MEMS manufacturing techniques including chemical vapor deposition. The MTCs made using the methods of the invention incorporate cathode to anode spacing of about 1 micron or less and use cathode and anode materials having work functions ranging from about 1 eV to about 3 eV. The MTCs also exhibit maximum efficiencies of just under 30%, and thousands of the devices can be fabricated at modest costs.

Impact of interlayer processing conditions on the performance of GaN light-emitting diode with specific NiOx/graphene electrode.

PubMed

Chandramohan, S; Kang, Ji Hye; Ryu, Beo Deul; Yang, Jong Han; Kim, Seongjun; Kim, Hynsoo; Park, Jong Bae; Kim, Taek Yong; Cho, Byung Jin; Suh, Eun-Kyung; Hong, Chang-Hee

2013-02-01

This paper reports on the evaluation of the impact of introducing interlayers and postmetallization annealing on the graphene/p-GaN ohmic contact formation and performance of associated devices. Current-voltage characteristics of the graphene/p-GaN contacts with ultrathin Au, Ni, and NiO(x) interlayers were studied using transmission line model with circular contact geometry. Direct graphene/p-GaN interface was identified to be highly rectifying and postmetallization annealing improved the contact characteristics as a result of improved adhesion between the graphene and the p-GaN. Ohmic contact formation was realized when interlayer is introduced between the graphene and p-GaN followed by postmetallization annealing. Temperature-dependent I-V measurements revealed that the current transport was modified from thermionic field emission for the direct graphene/p-GaN contact to tunneling for the graphene/metal/p-GaN contacts. The tunneling mechanism results from the interfacial reactions that occur between the metal and p-GaN during the postmetallization annealing. InGaN/GaN light-emitting diodes with NiO(x)/graphene current spreading electrode offered a forward voltage of 3.16 V comparable to that of its Ni/Au counterpart, but ended up with relatively low light output power. X-ray photoelectron spectroscopy provided evidence for the occurrence of phase transformation in the graphene-encased NiO(x) during the postmetallization annealing. The observed low light output is therefore correlated to the phase change induced transmittance loss in the NiO(x)/graphene electrode. These findings provide new insights into the behavior of different interlayers under processing conditions that will be useful for the future development of opto-electronic devices with graphene-based electrodes.

Separation of paralytic shellfish poisoning toxins on Chromarods-SIII by thin-layer chromatography with the Iatroscan (mark 5) and flame thermionic detection.

PubMed

Indrasena, W M; Ackman, R G; Gill, T A

1999-09-10

Thin-layer chromatography (TLC) on Chromarods-SIII with the Iatroscan (Mark-5) and a flame thermionic detector (FTID) was used to develop a rapid method for the detection of paralytic shellfish poisoning (PSP) toxins. The effect of variation in hydrogen (H2) flow, air flow, scan time and detector current on the FTID peak response for both phosphatidylcholine (PC) and PSP were studied in order to define optimum detection conditions. A combination of hydrogen and air flow-rates of 50 ml/min and 1.5-2.0 l/min respectively, along with a scan time of 40 s/rod and detector current of 3.0 A (ampere) or above were found to yield the best results for the detection of PSP compounds. Increasing the detector current level to as high as 3.3 A gave about 130 times more FTID response than did flame ionization detection (FID), for PSP components. Quantities of standards as small as 1 ng neosaxitoxin (NEO), 5 ng saxitoxin (STX), 5 ng B1-toxins (B1), 2 ng gonyautoxin (GTX) 2/3, 6 ng GTX 1/4 and 6 ng C-toxins (C1/C2) could be detected with the FTID. The method detection limits for toxic shellfish tissues using the FTID were 0.4, 2.1, 0.8 and 2.5 micrograms per g tissue for GTX 2/3, STX, NEO and C toxins, respectively. The FTID response increased with increasing detector current and with increasing the scan time. Increasing hydrogen and air flow-rates resulted in decreasing sensitivity within defined limits. Numerous solvent systems were tested, and, solvent consisting of chloroform: methanol-water-acetic acid (30:50:8:2) could separate C toxins from GTX, which eluted ahead of NEO and STX. Accordingly, TLC/FTID with the Iatroscan (Mark-5) seems to be a promising, relatively inexpensive and rapid method of screening plant and animal tissues for PSP toxins.

Low-temperature performance of semiconducting asymmetric nanochannel diodes

NASA Astrophysics Data System (ADS)

Akbas, Y.; Savich, G. R.; Jukna, A.; Plecenik, T.; Ďurina, P.; Plecenik, A.; Wicks, G. W.; Sobolewski, Roman

2017-10-01

We present our studies on fabrication and electrical and optical characterization of semiconducting asymmetric nanochannel diodes (ANCDs), focusing mainly on the temperature dependence of their current-voltage (I-V) characteristics in the range from room temperature to 77 K. These measurements enable us to elucidate the electron transport mechanism in a nanochannel. Our test devices were fabricated in a GaAs/AlGaAs heterostructure with a two-dimensional electron gas layer and were patterned using electron-beam lithography. The 250-nm-wide, 70-nm-deep trenches that define the nanochannel were ion-beam etched using the photoresist as a mask, so the resulting nanostructure consisted of approximately ten ANCDs connected in parallel with 2-µm-long, 230-nm-wide nanochannels. The ANCD I-V curves collected in the dark exhibited nonlinear, diode-type behavior at all tested temperatures. Their forward-biased regions were fitted to the classical diode equation with a thermionic barrier, with the ideality factor n and the saturation current as fitting parameters. We have obtained very good fits, but with n as large as ˜50, suggesting that there must be a substantial voltage drop likely at the contact pads. The thermionic energy barrier was determined to be 56 meV at high temperatures. We have also observed that under optical illumination our ANCDs at low temperatures exhibited, at low illumination powers, a very strong photoresponse enhancement that exceeded that at room temperature. At 78 K, the responsivity was of the order of 104 A/W at the nW-level light excitation.

Hybrid thermionic-photovoltaic converter

DOE Office of Scientific and Technical Information (OSTI.GOV)

Datas, A.

2016-04-04

A conceptual device for the direct conversion of heat into electricity is presented. This concept hybridizes thermionic (TI) and thermophotovoltaic (TPV) energy conversion in a single thermionic-photovoltaic (TIPV) solid-state device. This device transforms into electricity both the electron and photon fluxes emitted by an incandescent surface. This letter presents an idealized analysis of this device in order to determine its theoretical potential. According to this analysis, the key advantage of this converter, with respect to either TPV or TI, is the higher power density in an extended temperature range. For low temperatures, TIPV performs like TPV due to the negligiblemore » electron flux. On the contrary, for high temperatures, TIPV performs like TI due to the great enhancement of the electron flux, which overshadows the photon flux contribution. At the intermediate temperatures, ∼1650 K in the case of this particular study, I show that the power density potential of TIPV converter is twice as great as that of TPV and TI. The greatest impact concerns applications in which the temperature varies in a relatively wide range, for which averaged power density enhancement above 500% is attainable.« less

Electrical characteristics of n-GaN Schottky contacts on cleaved surfaces of free-standing substrates: Metal work function dependence of Schottky barrier height

NASA Astrophysics Data System (ADS)

Imadate, Hiroyoshi; Mishima, Tomoyoshi; Shiojima, Kenji

2018-04-01

We report the electrical characteristics of Schottky contacts with nine different metals (Ag, Ti, Cr, W, Mo, Au, Pd, Ni, and Pt) formed on clean m-plane surfaces by cleaving freestanding GaN substrates, compared with these of contacts on Ga-polar c-plane n-GaN surfaces grown on GaN substrates. The n-values from the forward current–voltage (I–V) characteristics are as good as 1.02–1.18 and 1.02–1.09 for the m- and c-plane samples, respectively. We found that the reverse I–V curves of both samples can be explained by the thermionic field emission theory, and that the Schottky barrier height of the cleaved m-plane contacts shows a metal work function dependence.

Influence of the Interaction Between Graphite and Polar Surfaces of ZnO on the Formation of Schottky Contact

NASA Astrophysics Data System (ADS)

Yatskiv, R.; Grym, J.

2018-03-01

We show that the interaction between graphite and polar surfaces of ZnO affects electrical properties of graphite/ZnO Schottky junctions. A strong interaction of the Zn-face with the graphite contact causes interface imperfections and results in the formation of laterally inhomogeneous Schottky contacts. On the contrary, high quality Schottky junctions form on the O-face, where the interaction is significantly weaker. Charge transport through the O-face ZnO/graphite junctions is well described by the thermionic emission model in both forward and reverse directions. We further demonstrate that the parameters of the graphite/ZnO Schottky diodes can be significantly improved when a thin layer of ZnO2 forms at the interface between graphite and ZnO after hydrogen peroxide surface treatment.

Electrical characterization of ZnO/NiO p-n junction prepared by the sol-gel method

NASA Astrophysics Data System (ADS)

Merih Akyuzlu, A.; Dagdelen, Fethi; Gultek, Ahmet; Hendi, A. A.; Yakuphanoglu, Fahrettin

2017-04-01

ZnO and NiO films were synthesized on fluourine-doped tin oxide (FTO) glass substrate by the sol-gel method. The surface morphology of the films was investigated by atomic force microscopy. The optical band gaps of the ZnO and NiO films were found to be 3.198 and 3.827eV, respectively. A ZnO/NiO p-n junction diode was prepared and electrical charge transport mechanism of the diode was analyzed using thermionic emission and Norde functions. The ideality factor, barrier height and series resistance of the diode were determined to be 6.46, 1.036eV and 39.1 M {Ω} , respectively. The obtained results indicate that ZnO/NiO p-n junction can be used as transparent diode for optic communications.

Design and optimization of the heat rejection system for a liquid cooled thermionic space nuclear reactor power system

DOE Office of Scientific and Technical Information (OSTI.GOV)

Moriarty, M.P.

1993-01-15

The heat transport subsystem for a liquid metal cooled thermionic space nuclear power system was modelled using algorithms developed in support of previous nuclear power system study programs, which date back to the SNAP-10A flight system. The model was used to define the optimum dimensions of the various components in the heat transport subsystem subjected to the constraints of minimizing mass and achieving a launchable package that did not require radiator deployment. The resulting design provides for the safe and reliable cooling of the nuclear reactor in a proven lightweight design.

Design and optimization of the heat rejection system for a liquid cooled thermionic space nuclear reactor power system

NASA Astrophysics Data System (ADS)

Moriarty, Michael P.

1993-01-01

The heat transport subsystem for a liquid metal cooled thermionic space nuclear power system was modelled using algorithms developed in support of previous nuclear power system study programs, which date back to the SNAP-10A flight system. The model was used to define the optimum dimensions of the various components in the heat transport subsystem subjected to the constraints of minimizing mass and achieving a launchable package that did not require radiator deployment. The resulting design provides for the safe and reliable cooling of the nuclear reactor in a proven lightweight design.

In-pile and out-of-pile testing of a molybdenum-uranium dioxide cermet fueled themionic diode

NASA Technical Reports Server (NTRS)

Diianni, D. C.

1972-01-01

The behavior of Mo-UO2 cermet fuel in a diode for thermionic reactor application was studied. The diode had a Mo-0.5 Ti emitter and niobium collector. Output power ranged from 1.4 to 2.8 W/cm squared at emitter and collector temperatures of 1500 deg and 540 C. Thermionic performance was stable within the limits of the instrumentation sensitivity. Through 1000 hours of in-pile operation the emitter was dimensionally stable. However, some fission gases (15 percent) leaked through an inner clad imperfection that occurred during fuel fabrication.

Carbide fuel pin and capsule design for irradiations at thermionic temperatures

NASA Technical Reports Server (NTRS)

Siegel, B. L.; Slaby, J. G.; Mattson, W. F.; Dilanni, D. C.

1973-01-01

The design of a capsule assembly to evaluate tungsten-emitter - carbide-fuel combinations for thermionic fuel elements is presented. An inpile fuel pin evaluation program concerned with clad temperture, neutron spectrum, carbide fuel composition, fuel geometry,fuel density, and clad thickness is discussed. The capsule design was a compromise involving considerations between heat transfer, instrumentation, materials compatibility, and test location. Heat-transfer calculations were instrumental in determining the method of support of the fuel pin to minimize axial temperature variations. The capsule design was easily fabricable and utilized existing state-of-the-art experience from previous programs.

Spatial inhomogeneous barrier heights at graphene/semiconductor Schottky junctions

NASA Astrophysics Data System (ADS)

Tomer, Dushyant

Graphene, a semimetal with linear energy dispersion, forms Schottky junction when interfaced with a semiconductor. This dissertation presents temperature dependent current-voltage and scanning tunneling microscopy/spectroscopy (STM/S) measurements performed on graphene Schottky junctions formed with both three and two dimensional semiconductors. To fabricate Schottky junctions, we transfer chemical vapor deposited monolayer graphene onto Si- and C-face SiC, Si, GaAs and MoS2 semiconducting substrates using polymer assisted chemical method. We observe three main type of intrinsic spatial inhomogeneities, graphene ripples, ridges and semiconductor steps in STM imaging that can exist at graphene/semiconductor junctions. Tunneling spectroscopy measurements reveal fluctuations in graphene Dirac point position, which is directly related to the Schottky barrier height. We find a direct correlation of Dirac point variation with the topographic undulations of graphene ripples at the graphene/SiC junction. However, no such correlation is established at graphene/Si and Graphene/GaAs junctions and Dirac point variations are attributed to surface states and trapped charges at the interface. In addition to graphene ripples and ridges, we also observe atomic scale moire patterns at graphene/MoS2 junction due to van der Waals interaction at the interface. Periodic topographic modulations due to moire pattern do not lead to local variation in graphene Dirac point, indicating that moire pattern does not contribute to fluctuations in electronic properties of the heterojunction. We perform temperature dependent current-voltage measurements to investigate the impact of topographic inhomogeneities on electrical properties of the Schottky junctions. We observe temperature dependence in junction parameters, such as Schottky barrier height and ideality factor, for all types of Schottky junctions in forward bias measurements. Standard thermionic emission theory which assumes a perfect smooth interface fails to explain such behavior, hence, we apply a modified emission theory with Gaussian distribution of Schottky barrier heights. The modified theory, applicable to inhomogeneous interfaces, explains the temperature dependent behavior of our Schottky junctions and gives a temperature independent mean barrier height. We attribute the inhomogeneous barrier height to the presence of graphene ripples and ridges in case of SiC and MoS2 while surface states and trapped charges at the interface is dominating in Si and GaAs. Additionally, we observe bias dependent current and barrier height in reverse bias regime also for all Schottky junctions. To explain such behavior, we consider two types of reverse bias conduction mechanisms; Poole-Frenkel and Schottky emission. We find that Poole-Frenkel emission explains the characteristics of graphene/SiC junctions very well. However, both the mechanism fails to interpret the behavior of graphene/Si and graphene/GaAs Schottky junctions. These findings provide insight into the fundamental physics at the interface of graphene/semiconductor junctions.

Electrical transport characteristics of single-layer organic devices from theory and experiment

NASA Astrophysics Data System (ADS)

Martin, S. J.; Walker, Alison B.; Campbell, A. J.; Bradley, D. D. C.

2005-09-01

An electrical model based on drift diffusion is described. We have explored systematically how the shape of the current density-voltage (J-V) curves is determined by the input parameters, information that isessential when deducing values of these parameters by fitting to experimental data for an ITO/PPV/Al organic light-emitting device (OLED), where ITO is shorthand for indium tin oxide and PPV is poly(phenylene vinylene). Our conclusion is that it is often possible to obtain a unique fit even with several parameters to fit. Our results allowing for a tunneling current show remarkable resemblance to experimental data before and after the contacts are conditioned. We have demonstrated our model on single-layer devices with ITO/PFO/Au and ITO/PEDOT/PFO/Au at room temperature and ITO/TPD/Al over temperatures from 130 to 290 K. PFO is shorthand for poly(9,9'-dialkyl-fluorene-2,7-dyl) and TPD is shorthand for N,N'-diphenyl-N,N'-bis(3-methylphenyl)1-1'-biphenyl-4,4'-diamine. Good fits to experimental data have been obtained, but in the case of the TPD device, only if a larger value for the relative permittivity ɛs than would be expected is used. We infer that a layer of dipoles at the ITO/TPD interface could be responsible for the observed J-V characteristics by locally causing changes in ɛs. The strong temperature dependence of the hole barrier height from fitting J-V characteristics to the experimental data may indicate that the temperature dependence of the thermionic emission model is incorrect.

Photovoltaic and thermal energy conversion for solar powered satellites

NASA Technical Reports Server (NTRS)

Von Tiesenhausen, G. F.

1976-01-01

A summary is provided concerning the most important aspects of present investigations related to a use of solar power satellites (SPS) as a future source of terrestrial energy. General SPS characteristics are briefly considered, early work is reviewed, and a description of current investigations is presented. System options presently under study include a photovoltaic array, a thermionic system, and a closed Brayton cycle. Attention is given to system reference options, basic building blocks, questions of system analysis and engineering, photovoltaic conversion, and the utility interface. It is concluded that an SPS may be cost effective compared to terrestrial systems by 1995.

Characterization of deposited CdS thin films by Spray Pyrolysis method and used in Cd/CdS/p-Si/Al structure

NASA Astrophysics Data System (ADS)

Özakın, Oǧuzhan; Aktaş, Şeydanur; Güzeldir, Betül; Saǧlam, Mustafa

2017-04-01

In our study, as p-type crystalline Si substrate was used. Omic contact was performed by evaporating Al metal on the matt surface of crystal. On the other surface of it CdS thin film were enlarged with the technique of Spray Pyrolysis. Structural characteristics of the grown thin film was examined SEM and EDAX image. When examining SEM image of CdS thin film were totally covered the p-Si crystal surface of it was nearly homogeneous and The EDAX spectra showed that the expected different ratios metal percent exist in the alloys, approximately. On the CdS films whose surface features were investigated, at 10-7 torr pressure was obtained Cd/CdS/p-Si/Al sandwich structure by evaporating Cd. Firstly, the I-V (current-voltage) characteristics on 80K between 320K at room temperature of this structure was measured. I-V characteristics of the examined at parameters diodes calculation, Thermionic Emission, were used. The characteristic parameters such as barrier height and ideality factor of this structure have been calculated from the forward bias I-V characteristics. Consequently, it was seen that CdS thin film grown on p-Si semiconductor will be used confidently in Cd/p-Si metal-semiconductor contacts thanks to Spray Pyrolysis method.

Research on the electrical characteristics of the Pt/CdS Schottky diode

NASA Astrophysics Data System (ADS)

Ding, Jia-xin; Zhang, Xiang-feng; Yao, Guansheng

2013-08-01

With the development of technology, the demand for semiconductor ultraviolet detector is increasing day by day. Compared with the traditional infrared detector in missile guidance, ultraviolet/infrared dual-color detection can significantly improve the anti-interference ability of the missile. According to the need of missile guidance and other areas of the application of ultraviolet detector, the paper introduces a manufacture of the CdS Schottky barrier ultraviolet detector. By using the radio frequency magnetron sputtering technology, a Pt thin film layer is sputtered on CdS basement to form a Schottky contact firstly. Then the indium ohmic contact electrode is fabricated by thermal evaporation method, and eventually a Pt/CdS/In Schottky diode is formed. The I-V characteristic of the device was tested at room temperature, its zero bias current and open circuit voltage is -0.578nA and 130mV, respectively. Test results show that the the Schottky contact has been formed between Pt and CdS. The device has good rectifying characteristics. According to the thermionic emission theory, the I-V curve fitting analysis of the device was studied under the condition of small voltage. The ideality factor and Schottky barrier height is 1.89 and 0.61eV, respectively. The normalized spectral responsivity at zero bias has been tested. The device has peak responsivity at 500nm, and it cutoff at 510nm.

Thermionic performance of a cesium diminiode with relatively impure 110-tungsten electrodes

NASA Technical Reports Server (NTRS)

Smith, A. L.; Manista, E. J.; Morris, J. F.

1974-01-01

Thermionic performance data from a miniature plane cesium diode (diminiode) with 110-tungsten electrodes are presented. The diminiode has a guard-ringed collector and a spacing of 0.23 mm. The data were obtained by using a computerized acquisition system. The diode was tested at increments between 1700 and 1900 K for the emitter, 694 and 1101 K for the collector, and 519 and 650 K for the reservoir. A maximum power density of 4.5 W/sq cm was obtained at an emitter temperature of 1900 K. This relatively low output probably results from high carbon and sodium impurities in the electrode materials.

Thermionic reactor ion propulsion system /TRIPS/ - Its multi-mission capability.

NASA Technical Reports Server (NTRS)

Peelgren, M. L.

1972-01-01

The unmanned planetary exploration to be conducted in the last two decades of this century includes many higher energy missions which tax all presently available propulsion systems beyond their limit. One candidate with the versatility and performance to meet these mission objectives is nuclear electric propulsion (NEP). Additionally, the NEP System is feasible in orbit raising operations with the Shuttle or Shuttle/Tug combination. A representative planetary mission is described (Uranus-Neptune flyby with probe), and geocentric performance and tradeoffs are discussed. The NEP System is described in more detail with particular emphasis on the power subsystem consisting of the thermionic reactor, heat rejection subsystem, and neutron shield.

Nuclear radiation problems, unmanned thermionic reactor ion propulsion spacecraft

NASA Technical Reports Server (NTRS)

Mondt, J. F.; Sawyer, C. D.; Nakashima, A.

1972-01-01

A nuclear thermionic reactor as the electric power source for an electric propulsion spacecraft introduces a nuclear radiation environment that affects the spacecraft configuration, the use and location of electrical insulators and the science experiments. The spacecraft is conceptually configured to minimize the nuclear shield weight by: (1) a large length to diameter spacecraft; (2) eliminating piping penetrations through the shield; and (3) using the mercury propellant as gamma shield. Since the alumina material is damaged by the high nuclear radiation environment in the reactor it is desirable to locate the alumina insulator outside the reflector or develop a more radiation resistant insulator.

High-efficiency, low-temperature cesium diodes with lanthanum-hexaboride electrodes

NASA Technical Reports Server (NTRS)

Morris, J. F.

1974-01-01

Lanthanum hexaboride electrodes in 1700 K cesium diodes may triple power outputs compared with those demonstrated for nuclear thermionic space applications. Still greater relative gains seem possible for emitters below 1700 K. Further improvements in cesium diode performance should result from the lower collector temperatures allowed for earth and low power space duties. Decreased temperatures will lessen thermal transport losses that attend thermionic conversion mechanisms. Such advantages will add to those from collector Carnot and electrode effects. If plasma ignition difficulties impede diode temperature reductions, recycling small fractions of the output power could provide ionization. So high efficiency, low temperature cesium diodes with lanthanum hexaboride electrodes appear feasible.

Fundamental studies on a heat driven lamp

NASA Technical Reports Server (NTRS)

Lawless, J. L.

1985-01-01

A detailed theoretical study of a heat-driven lamp has been performed. This lamp uses a plasma produced in a thermionic diode. The light is produced by the resonance transition of cesium. An important result of this study is that up to 30% of the input heat is predicted to be converted to light in this device. This is a major improvement over ordinary thermionic energy converters in which only approx. 1% is converted to resonance radiation. Efficiencies and optimum inter-electrode spacings have been found as a function of cathode temperature and the radiative escape factor. The theory developed explains the operating limits of the device.

High power densities from high-temperature material interactions. [in thermionic energy conversion and metallic fluid heat pipes

NASA Technical Reports Server (NTRS)

Morris, J. F.

1981-01-01

Thermionic energy conversion (TEC) and metallic-fluid heat pipes (MFHPs), offering unique advantages in terrestrial and space energy processing by virtue of operating on working-fluid vaporization/condensation cycles that accept great thermal power densities at high temperatures, share complex materials problems. Simplified equations are presented that verify and solve such problems, suggesting the possibility of cost-effective applications in the near term for TEC and MFHP devices. Among the problems discussed are: the limitation of alkali-metal corrosion, protection against hot external gases, external and internal vaporization, interfacial reactions and diffusion, expansion coefficient matching, and creep deformation.

Analyzing indirect secondary electron contrast of unstained bacteriophage T4 based on SEM images and Monte Carlo simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ogura, Toshihiko, E-mail: t-ogura@aist.go.jp

2009-03-06

The indirect secondary electron contrast (ISEC) condition of the scanning electron microscopy (SEM) produces high contrast detection with minimal damage of unstained biological samples mounted under a thin carbon film. The high contrast image is created by a secondary electron signal produced under the carbon film by a low acceleration voltage. Here, we show that ISEC condition is clearly able to detect unstained bacteriophage T4 under a thin carbon film (10-15 nm) by using high-resolution field emission (FE) SEM. The results show that FE-SEM provides higher resolution than thermionic emission SEM. Furthermore, we investigated the scattered electron area within themore » carbon film under ISEC conditions using Monte Carlo simulation. The simulations indicated that the image resolution difference is related to the scattering width in the carbon film and the electron beam spot size. Using ISEC conditions on unstained virus samples would produce low electronic damage, because the electron beam does not directly irradiate the sample. In addition to the routine analysis, this method can be utilized for structural analysis of various biological samples like viruses, bacteria, and protein complexes.« less

Electron dynamics and prompt ablation of aluminum surface excited by intense femtosecond laser pulse

NASA Astrophysics Data System (ADS)

Ionin, A. A.; Kudryashov, S. I.; Makarov, S. V.; Seleznev, L. V.; Sinitsyn, D. V.

2014-12-01

Thin aluminum film homogeneously heated by intense IR femtosecond laser pulses exhibits on the excitation timescale consequent fluence-dependent rise and drop of the IR-pump self-reflectivity, followed by its final saturation at higher fluences F > 0.3 J/cm2. This prompt optical dynamics correlates with the initial monotonic increase in the accompanying laser-induced electron emission, which is succeeded by its non-linear (three-photon) increase for F > 0.3 J/cm2. The underlying electronic dynamics is related to the initial saturation of IR resonant interband transitions in this material, followed by its strong instantaneous electronic heating via intraband transitions during the pump pulse resulting in thermionic emission. Above the threshold fluence of 0.3 J/cm2, the surface electronic heating is balanced during the pump pulse by simultaneous cooling via intense plasma removal (prompt ablation). The relationship between the deposited volume energy density in the film and its prompt electronic temperature derived from the self-reflection measurements using a Drude model, demonstrates a kind of electron "liquid-vapor" phase transition, driven by strong cubic optical non-linearity of the photo-excited aluminum.

A novel electron accelerator for MRI-Linac radiotherapy.

PubMed

Whelan, Brendan; Gierman, Stephen; Holloway, Lois; Schmerge, John; Keall, Paul; Fahrig, Rebecca

2016-03-01

MRI guided radiotherapy is a rapidly growing field; however, current electron accelerators are not designed to operate in the magnetic fringe fields of MRI scanners. As such, current MRI-Linac systems require magnetic shielding, which can degrade MR image quality and limit system flexibility. The purpose of this work was to develop and test a novel medical electron accelerator concept which is inherently robust to operation within magnetic fields for in-line MRI-Linac systems. Computational simulations were utilized to model the accelerator, including the thermionic emission process, the electromagnetic fields within the accelerating structure, and resulting particle trajectories through these fields. The spatial and energy characteristics of the electron beam were quantified at the accelerator target and compared to published data for conventional accelerators. The model was then coupled to the fields from a simulated 1 T superconducting magnet and solved for cathode to isocenter distances between 1.0 and 2.4 m; the impact on the electron beam was quantified. For the zero field solution, the average current at the target was 146.3 mA, with a median energy of 5.8 MeV (interquartile spread of 0.1 MeV), and a spot size diameter of 1.5 mm full-width-tenth-maximum. Such an electron beam is suitable for therapy, comparing favorably to published data for conventional systems. The simulated accelerator showed increased robustness to operation in in-line magnetic fields, with a maximum current loss of 3% compared to 85% for a conventional system in the same magnetic fields. Computational simulations suggest that replacing conventional DC electron sources with a RF based source could be used to develop medical electron accelerators which are robust to operation in in-line magnetic fields. This would enable the development of MRI-Linac systems with no magnetic shielding around the Linac and reduce the requirements for optimization of magnetic fringe field, simplify design of the high-field magnet, and increase system flexibility.

A novel electron accelerator for MRI-Linac radiotherapy

PubMed Central

Whelan, Brendan; Gierman, Stephen; Holloway, Lois; Schmerge, John; Keall, Paul; Fahrig, Rebecca

2016-01-01

Purpose: MRI guided radiotherapy is a rapidly growing field; however, current electron accelerators are not designed to operate in the magnetic fringe fields of MRI scanners. As such, current MRI-Linac systems require magnetic shielding, which can degrade MR image quality and limit system flexibility. The purpose of this work was to develop and test a novel medical electron accelerator concept which is inherently robust to operation within magnetic fields for in-line MRI-Linac systems. Methods: Computational simulations were utilized to model the accelerator, including the thermionic emission process, the electromagnetic fields within the accelerating structure, and resulting particle trajectories through these fields. The spatial and energy characteristics of the electron beam were quantified at the accelerator target and compared to published data for conventional accelerators. The model was then coupled to the fields from a simulated 1 T superconducting magnet and solved for cathode to isocenter distances between 1.0 and 2.4 m; the impact on the electron beam was quantified. Results: For the zero field solution, the average current at the target was 146.3 mA, with a median energy of 5.8 MeV (interquartile spread of 0.1 MeV), and a spot size diameter of 1.5 mm full-width-tenth-maximum. Such an electron beam is suitable for therapy, comparing favorably to published data for conventional systems. The simulated accelerator showed increased robustness to operation in in-line magnetic fields, with a maximum current loss of 3% compared to 85% for a conventional system in the same magnetic fields. Conclusions: Computational simulations suggest that replacing conventional DC electron sources with a RF based source could be used to develop medical electron accelerators which are robust to operation in in-line magnetic fields. This would enable the development of MRI-Linac systems with no magnetic shielding around the Linac and reduce the requirements for optimization of magnetic fringe field, simplify design of the high-field magnet, and increase system flexibility. PMID:26936713

Silicon carbide multilayer protective coating on carbon obtained by thermionic vacuum arc method

NASA Astrophysics Data System (ADS)

Ciupină, Victor; Lungu, Cristian Petrica; Vladoiu, Rodica; Prodan, Gabriel; Porosnicu, Corneliu; Belc, Marius; Stanescu, Iuliana M.; Vasile, Eugeniu; Rughinis, Razvan

2014-01-01

Thermionic vacuum arc (TVA) method is currently developing, in particular, to work easily with heavy fusible material for the advantage presented by control of directing energy for the elements forming a plasma. The category of heavy fusible material can recall C and W (high-melting point materials), and are difficult to obtain or to control by other means. Carbon is now used in many areas of special mechanical, thermal, and electrical properties. We refer in particular to high-temperature applications where unwanted effects may occur due to oxidation. Changed properties may lead to improper functioning of the item or device. For example, increasing the coefficient of friction may induce additional heat on moving items. One solution is to protect the item in question by coating with proper materials. Silicon carbide (SiC) was chosen mainly due to compatibility with coated carbon substrate. Recently, SiC has been used as conductive transparent window for optical devices, particularly in thin film solar cells. Using the TVA method, SiC coatings were obtained as thin films (multilayer structures), finishing with a thermal treatment up to 1000°C. Structural properties and oxidation behavior of the multilayer films were investigated, and the measurements showed that the third layer acts as a stopping layer for oxygen. Also, the friction coefficient of the protected films is lower relative to unprotected carbon films.

Correlation between microstructure and thermionic electron emission from Os-Ru thin films on dispenser cathodes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Swartzentruber, Phillip D.; John Balk, Thomas, E-mail: john.balk@uky.edu; Effgen, Michael P.

2014-07-01

Osmium-ruthenium films with different microstructures were deposited onto dispenser cathodes and subjected to 1000 h of close-spaced diode testing. Tailored microstructures were achieved by applying substrate biasing during deposition, and these were evaluated with scanning electron microscopy, x-ray diffraction, and energy dispersive x-ray spectroscopy before and after close-spaced diode testing. Knee temperatures determined from the close-spaced diode test data were used to evaluate cathode performance. Cathodes with a large (10-11) Os-Ru film texture possessed comparatively low knee temperatures. Furthermore, a low knee temperature correlated with a low effective work function as calculated from the close-spaced diode data. It is proposedmore » that the formation of strong (10-11) texture is responsible for the superior performance of the cathode with a multilayered Os-Ru coating.« less

Ferroelectric-Domain-Patterning-Controlled Schottky Junction State in Monolayer MoS 2

DOE PAGES

Xiao, Zhiyong; Song, Jingfeng; Ferry, David K.; ...

2017-06-08

Here, we exploit scanning probe controlled domain patterning in a ferroelectric top-layer to induce nonvolatile modulation of the conduction characteristic of monolayer MoS 2 between a transistor and a junction state. In the presence of a domain wall, MoS 2 exhibits rectified I-V that is well described by the thermionic emission model. The induced Schottky barrier height Φ eff Β varies from 0.38 eV to 0.57 eV and is tunabe by a SiO 2 global back-gate, while the tuning range of Φ eff Β the barrier height depends sensitively on the conduction band tail trapping states. Our work points tomore » a new route to achieve programmable functionalities in van der Waals materials and sheds light on the critical performance limiting factors in these hybrid systems.« less

Cogeneration Technology Alternatives Study (CTAS). Volume 1: Summary

NASA Technical Reports Server (NTRS)

Barna, G. J.; Burns, R. K.; Sagerman, G. D.

1980-01-01

Various advanced energy conversion systems that can use coal or coal-derived fuels for industrial cogeneration applications were compared to provide information needed by DOE to establish research and development funding priorities for advanced-technology systems that could significantly advance the use of coal or coal-derived fuels in industrial cogeneration. Steam turbines, diesel engines, open-cycle gas turbines, combined cycles, closed-cycle gas turbines, Stirling engines, phosphoric acid fuel cells, molten carbonate fuel cells, and thermionics were studied with technology advancements appropriate for the 1985-2000 time period. The various advanced systems were compared and evaluated for wide diversity of representative industrial plants on the basis of fuel energy savings, annual energy cost savings, emissions savings, and rate of return on investment as compared with purchasing electricity from a utility and providing process heat with an on-site boiler. Also included in the comparisons and evaluations are results extrapolated to the national level.

Cogeneration Technology Alternatives Study (CTAS). Volume 2: Comparison and evaluation of results

NASA Technical Reports Server (NTRS)

1984-01-01

CTAS compared and evaluated various advanced energy conversion systems that can use coal or coal-derived fuels for industrial cogeneration applications. The principal aim of the study was to provide information needed by DOE to establish research and development (R&D) funding priorities for advanced-technology systems that could significantly advance the use of coal or coal-derived fuels in industrial cogeneration. Steam turbines, diesel engines, open-cycle gas turbines, combined cycles, closed-cycle gas turbines, Stirling engines, phosphoric acid fuel cells, molten carbonate fuel cells, and thermionics were studied with technology advancements appropriate for the 1985-2000 time period. The various advanced systems were compared and evaluated for a wide diversity of representative industrial plants on the basis of fuel energy savings, annual energy cost savings, emissions savings, and rate of return on investment (ROI) as compared with purchasing electricity from a utility and providing process heat with an on-site boiler.

Ohmic contact formation between metal and AlGaN/GaN heterostructure via graphene insertion

NASA Astrophysics Data System (ADS)

Sung Park, Pil; Reddy, Kongara M.; Nath, Digbijoy N.; Yang, Zhichao; Padture, Nitin P.; Rajan, Siddharth

2013-04-01

A simple method for the creation of Ohmic contact to 2D electron gas in AlGaN/GaN high electron-mobility transistors using Cr/graphene layer is demonstrated. A weak temperature dependence of this Ohmic contact observed in the range 77 to 300 K precludes thermionic emission or trap-assisted hopping as possible carrier-transport mechanisms. It is suggested that the Cr/graphene combination acts akin to a doped n-type semiconductor in contact with AlGaN/GaN heterostructure, and promotes carrier transport along percolating Al-lean paths through the AlGaN layer. This use of graphene offers a simple method for making Ohmic contacts to AlGaN/GaN heterostructures, circumventing complex additional processing steps involving high temperatures. These results could have important implications for the fabrication and manufacturing of AlGaN/GaN-based microelectronic and optoelectronic devices/sensors of the future.

Ferroelectric-Domain-Patterning-Controlled Schottky Junction State in Monolayer MoS 2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xiao, Zhiyong; Song, Jingfeng; Ferry, David K.

Here, we exploit scanning probe controlled domain patterning in a ferroelectric top-layer to induce nonvolatile modulation of the conduction characteristic of monolayer MoS 2 between a transistor and a junction state. In the presence of a domain wall, MoS 2 exhibits rectified I-V that is well described by the thermionic emission model. The induced Schottky barrier height Φ eff Β varies from 0.38 eV to 0.57 eV and is tunabe by a SiO 2 global back-gate, while the tuning range of Φ eff Β the barrier height depends sensitively on the conduction band tail trapping states. Our work points tomore » a new route to achieve programmable functionalities in van der Waals materials and sheds light on the critical performance limiting factors in these hybrid systems.« less

Monte Carlo modelling of Schottky diode for rectenna simulation

NASA Astrophysics Data System (ADS)

Bernuchon, E.; Aniel, F.; Zerounian, N.; Grimault-Jacquin, A. S.

2017-09-01

Before designing a detector circuit, the electrical parameters extraction of the Schottky diode is a critical step. This article is based on a Monte-Carlo (MC) solver of the Boltzmann Transport Equation (BTE) including different transport mechanisms at the metal-semiconductor contact such as image force effect or tunneling. The weight of tunneling and thermionic current is quantified according to different degrees of tunneling modelling. The I-V characteristic highlights the dependence of the ideality factor and the current saturation with bias. Harmonic Balance (HB) simulation on a rectifier circuit within Advanced Design System (ADS) software shows that considering non-linear ideality factor and saturation current for the electrical model of the Schottky diode does not seem essential. Indeed, bias independent values extracted in forward regime on I-V curve are sufficient. However, the non-linear series resistance extracted from a small signal analysis (SSA) strongly influences the conversion efficiency at low input powers.

Nuclear safety considerations in the conceptual design of a fast reactor for space electric power and propulsion

NASA Technical Reports Server (NTRS)

Hsieh, T.-M.; Koenig, D. R.

1977-01-01

Some nuclear safety aspects of a 3.2 mWt heat pipe cooled fast reactor with out-of-core thermionic converters are discussed. Safety related characteristics of the design including a thin layer of B4C surrounding the core, the use of heat pipes and BeO reflector assembly, the elimination of fuel element bowing, etc., are highlighted. Potential supercriticality hazards and countermeasures are considered. Impacts of some safety guidelines of space transportation system are also briefly discussed, since the currently developing space shuttle would be used as the primary launch vehicle for the nuclear electric propulsion spacecraft.

Beam dynamics studies of a 30 MeV RF linac for neutron production

NASA Astrophysics Data System (ADS)

Nayak, B.; Krishnagopal, S.; Acharya, S.

2018-02-01

Design of a 30 MeV, 10 Amp RF linac as neutron source has been carried out by means of ASTRA simulation code. Here we discuss details of design simulations for three different cases i.e Thermionic , DC and RF photocathode guns and compare them as injectors to a 30 MeV RF linac for n-ToF production. A detailed study on choice of input parameters of the beam from point of view of transmission efficiency and beam quality at the output have been described. We found that thermionic gun isn't suitable for this application. Both DC and RF photocathode gun can be used. RF photocathode gun would be of better performance.

Trace-transform invariants of tracks of high-velocity jets from the surface of tungsten droplets in the plasma flow

NASA Astrophysics Data System (ADS)

Gulyaev, P.; Jordan, V.; Gulyaev, I.; Dolmatov, A.

2017-05-01

The paper presents the analysis of the recorded tracks of high-velocity emission in the air-argon plasma flow during breaking up of tungsten microdroplets. This new physical effect of optical emission involves two stages. The first one includes thermionic emission of electrons from the surface of the melted tungsten droplet of 100-200 μm size and formation of the charged sphere of 3-5 mm diameter. After it reaches the breakdown electric potential, it collapses and produces a spherical shock wave and luminous radiation. The second stage includes previously unknown physical phenomenon of narrowly directed energy jet with velocity exceeding 4000 m/s from the surface of the tungsten droplet. The luminous spherical collapse and high-velocity jets were recorded using CMOS photo-array operating in a global shutter charge storage mode. Special features of the CMOS array scanning algorithm affect formation of distinctive signs of the recorded tracks, which stay invariant to trace transform (TT) with specific functional. The series of concentric circles were adopted as primitive object models (patterns) used in TT at the spherical collapse stage and linear segment of fixed thickness - at the high-velocity emission stage. The two invariants of the physical object, motion velocity and optical brightness distribution in the motion front, were adopted as desired identification features of tracks. The analytical expressions of the relation of 2D TT parameters and physical object motion invariants were obtained. The equations for spherical collapse stage correspond to Radon-Nikodym transform.

The electron trap parameter extraction-based investigation of the relationship between charge trapping and activation energy in IGZO TFTs under positive bias temperature stress

NASA Astrophysics Data System (ADS)

Rhee, Jihyun; Choi, Sungju; Kang, Hara; Kim, Jae-Young; Ko, Daehyun; Ahn, Geumho; Jung, Haesun; Choi, Sung-Jin; Myong Kim, Dong; Kim, Dae Hwan

2018-02-01

Experimental extraction of the electron trap parameters which are associated with charge trapping into gate insulators under the positive bias temperature stress (PBTS) is proposed and demonstrated for the first time in amorphous indium-gallium-zinc-oxide thin-film transistors. This was done by combining the PBTS/recovery time-evolution of the experimentally decomposed threshold voltage shift (ΔVT) and the technology computer-aided design (TCAD)-based charge trapping simulation. The extracted parameters were the trap density (NOT) = 2.6 × 1018 cm-3, the trap energy level (ΔET) = 0.6 eV, and the capture cross section (σ0) = 3 × 10-19 cm2. Furthermore, based on the established TCAD framework, the relationship between the electron trap parameters and the activation energy (Ea) is comprehensively investigated. It is found that Ea increases with an increase in σ0, whereas Ea is independent of NOT. In addition, as ΔET increases, Ea decreases in the electron trapping-dominant regime (low ΔET) and increases again in the Poole-Frenkel (PF) emission/hopping-dominant regime (high ΔET). Moreover, our results suggest that the cross-over ΔET point originates from the complicated temperature-dependent competition between the capture rate and the emission rate. The PBTS bias dependence of the relationship between Ea and ΔET suggests that the electric field dependence of the PF emission-based electron hopping is stronger than that of the thermionic field emission-based electron trapping.

Performance of an electron gun for a high-brightness X-ray generator.

PubMed

Sugimura, Takashi; Ohsawa, Satoshi; Ikeda, Mitsuo

2008-05-01

A prototype thermionic electron gun for a high-brightness X-ray generator has been developed. Its extraction voltage and design current are 60 kV and 100 mA (DC), respectively. The X-ray generator aims towards a maximum brilliance of 60 kW mm(-2). The beam sizes at the rotating anticathode must therefore be within 1.0 mm x 0.1 mm and a small beam emittance is required. The fabricated electron gun optimizes an aperture grid and a Whenelt electrode. The performance of the prototype electron gun measured using pulsed-beam tests is as follows: maximum beam current, 85.7 mA; beam focus size at the rotating anticathode, 0.79 mm x 0.13 mm. In DC beam tests, FWHM beam sizes were measured to be 0.65 mm x 0.08 mm at the rotating anticathode with a beam current of 45 mA. The beam current recently reached approximately 60 mA with some thermal problems.

A power propulsion system based on a second-generation thermionic NPS of the ``Topaz'' type

NASA Astrophysics Data System (ADS)

Gryaznov, Georgi M.; Zhabotinski, Eugene E.; Andreev, Pavel V.; Zaritski, Gennadie a.; Koroteev, Anatoly S.; Martishin, Viktor M.; Akimov, Vladimir N.; Ponomarev-Stepnoi, Nikolai N.; Usov, Veniamin A.; Britt, Edward J.

1992-01-01

The paper considers the concept of power propulsion systems-universal space platforms (USPs) on the basis of second-generation thermionic nuclear power system (NPSs) and stationary plasma electric thrusters (SPETs). The composition and the principles of layout of such a system, based on a thermionic NPS with a continuous power of up to 30 kWe allowing power augmentation by a factor of 2-2.5 as long as during a year, as well as SPETs with a specific impulse of at least 20 km/s and a propulsion efficiency of 0.6-0.7 are discussed. The layouts and the basic parameters are presented for a power propulsion system ensuring cargo transportation from an initial radiation-safe 800 km high orbit into a geostationary one using the ``Zenit'' and ``Proton'' launch systems for injection into an initial orbit. It is shown that the mass of mission-oriented equipment in the geostationary orbit in the cases under consideration ranges from 2500 to 5500 kg on condition that the flight time is not longer than a year. The power propulsion system can be applied to autonomous power supply of various spacecraft including remote power delivery. It can be also used for deep space exploration.

Super-giant magnetoresistance at room-temperature in copper nanowires due to magnetic field modulation of potential barrier heights at nanowire-contact interfaces

NASA Astrophysics Data System (ADS)

Hossain, Md I.; Maksud, M.; Palapati, N. K. R.; Subramanian, A.; Atulasimha, J.; Bandyopadhyay, S.

2016-07-01

We have observed a super-giant (∼10 000 000%) negative magnetoresistance at 39 mT field in Cu nanowires contacted with Au contact pads. In these nanowires, potential barriers form at the two Cu/Au interfaces because of Cu oxidation that results in an ultrathin copper oxide layer forming between Cu and Au. Current flows when electrons tunnel through, and/or thermionically emit over, these barriers. A magnetic field applied transverse to the direction of current flow along the wire deflects electrons toward one edge of the wire because of the Lorentz force, causing electron accumulation at that edge and depletion at the other. This lowers the potential barrier at the accumulated edge and raises it at the depleted edge, causing a super-giant magnetoresistance at room temperature.

Super-giant magnetoresistance at room-temperature in copper nanowires due to magnetic field modulation of potential barrier heights at nanowire-contact interfaces.

PubMed

Hossain, Md I; Maksud, M; Palapati, N K R; Subramanian, A; Atulasimha, J; Bandyopadhyay, S

2016-07-29

We have observed a super-giant (∼10 000 000%) negative magnetoresistance at 39 mT field in Cu nanowires contacted with Au contact pads. In these nanowires, potential barriers form at the two Cu/Au interfaces because of Cu oxidation that results in an ultrathin copper oxide layer forming between Cu and Au. Current flows when electrons tunnel through, and/or thermionically emit over, these barriers. A magnetic field applied transverse to the direction of current flow along the wire deflects electrons toward one edge of the wire because of the Lorentz force, causing electron accumulation at that edge and depletion at the other. This lowers the potential barrier at the accumulated edge and raises it at the depleted edge, causing a super-giant magnetoresistance at room temperature.

Computer acquired performance data from an etched-rhenium, molybdenum planar diode

NASA Technical Reports Server (NTRS)

Manista, E. J.

1972-01-01

Performance data from an etched-rhenium, molybdenum thermionic converter are presented. The planar converter has a guard-ringed collector and a fixed spacing of 0.254 mm (10 mils). The data were acquired by using a computer and are available on microfiche as individual or composite parametric current, voltage curves. The parameters are the temperatures of the emitter T sub E, collector T sub C and cesium reservoir T sub R. The composite plots have constant T sub E, and varying T sub C or T sub R, or both. The envelope and composite plots having constant I sub E are presented. The diode was tested at increments between 1500 and 2000 K for the emitter, 750 and 1100 K for the collector, and 540 and 640 K for the reservoir. In all, 774 individual current, voltage curves were obtained.

Back-gated graphene anode for more efficient thermionic energy converters

DOE PAGES

Yuan, Hongyuan; Riley, Daniel C.; Shen, Zhi-Xun; ...

2016-12-15

Thermionic energy converters (TECs) are a direct heat-to-electricity conversion technology with great potential for high efficiency and scalability. However, space charge barrier in the inter-electrode gap and high anode work function are major obstacles toward realizing high efficiency. Here, we demonstrate for the first time a prototype TEC using a back-gated graphene anode, a barium dispenser cathode, and a controllable inter-electrode gap as small as 17 µm, which simultaneously addresses these two obstacles. This leads to an electronic conversion efficiency of 9.8% at cathode temperature of 1000 °C, the highest reported by far. We first demonstrate that electrostatic gating ofmore » graphene by a 20 nm HfO 2 dielectric layer changes the graphene anode work function by 0.63 eV, as observed from the current-voltage characteristics of the TEC. Next, we show that the efficiency increases by a factor of 30.6 by reducing the gap from 1 mm down to 17 µm, after a mono-layer of Ba is deposited on graphene by the dispenser cathode. Lastlu, we show that electrostatic gating of graphene further reduces the graphene work function from 1.85 to 1.69 eV, leading to an additional 67% enhancement in TEC efficiency. Note that the overall efficiency using the back-gated graphene anode is 6.7 times higher compared with that of a TEC with a tungsten anode and the same inter-electrode gap.« less

Development of a novel thermionic RF electron gun applied on a compact THz-FEL facility

NASA Astrophysics Data System (ADS)

Hu, T. N.; Pei, Y. J.; Qin, B.; Liu, K. F.; Feng, G. Y.

2018-04-01

The current requirements from civil and commercial applications lead to the development of compact free-electron laser (FEL)-based terahertz (THz) radiation sources. A picosecond electron gun plays an important role in an FEL-THz facility and attracts significant attention, as machine performance is very sensitive to initial conditions. A novel thermionic gun with an external cathode (EC) and two independently tunable cavities (ITCs) has been found to be a promising alternative to conventional electron sources due to its remarkable characteristics, and correspondingly an FEL injector can achieve a balance between a compact layout and high brightness benefitting from the velocity bunching properties and RF focusing effects in the EC-ITC gun. Nevertheless, the EC-ITC gun has not been extensively examined as part of the FEL injector in the past years. In this regard, to fill this gap, a development focusing on the experimental setup of an FEL injector based on an EC-ITC gun is described in detail. Before assembly, dynamic beam simulations were performed to investigate the optimal mounting position for the Linac associated with the focusing coils, and a suitable radio-frequency (RF) system was established based on a power coupling design and allocation. The testing bench proved to be fully functional through basic experiments using typical diagnostic approaches for estimating primary parameters. Associated with dynamic beam calculations, a performance evaluation for an EC-ITC gun was established while providing indirect testing results for an FEL injector.

Back-gated graphene anode for more efficient thermionic energy converters

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yuan, Hongyuan; Riley, Daniel C.; Shen, Zhi-Xun

Thermionic energy converters (TECs) are a direct heat-to-electricity conversion technology with great potential for high efficiency and scalability. However, space charge barrier in the inter-electrode gap and high anode work function are major obstacles toward realizing high efficiency. Here, we demonstrate for the first time a prototype TEC using a back-gated graphene anode, a barium dispenser cathode, and a controllable inter-electrode gap as small as 17 µm, which simultaneously addresses these two obstacles. This leads to an electronic conversion efficiency of 9.8% at cathode temperature of 1000 °C, the highest reported by far. We first demonstrate that electrostatic gating ofmore » graphene by a 20 nm HfO 2 dielectric layer changes the graphene anode work function by 0.63 eV, as observed from the current-voltage characteristics of the TEC. Next, we show that the efficiency increases by a factor of 30.6 by reducing the gap from 1 mm down to 17 µm, after a mono-layer of Ba is deposited on graphene by the dispenser cathode. Lastlu, we show that electrostatic gating of graphene further reduces the graphene work function from 1.85 to 1.69 eV, leading to an additional 67% enhancement in TEC efficiency. Note that the overall efficiency using the back-gated graphene anode is 6.7 times higher compared with that of a TEC with a tungsten anode and the same inter-electrode gap.« less

Design and testing a high fuel volume fraction, externally finned, thermionic emitter.

NASA Technical Reports Server (NTRS)

Peelgren, M. L.; Ernst, D. M.

1971-01-01

A prototypical, high fuel volume fraction, thermionic emitter body was designed and tested. The emitter body is all tungsten, with a 1.40-cm ID, a 3.23-cm OD, and eight full-length axial fins. The emitter thickness is 0.15 cm while the fins and outer clad are 0.075 cm thick. Different methods of fabrication were used in making the test samples. Stress analysis was performed with a three-dimensional elastic code. Thermal testing of the samples, duplicating calculated radial temperature gradients, heatup and cooldown rates, and emitter body temperatures in operation, was performed with no structural failures noted (six heatup and cooldown cycles per sample). Further emitter analysis and testing is planned.

Comparative assessment of out-of-core nuclear thermionic power systems

NASA Technical Reports Server (NTRS)

Estabrook, W. C.; Koenig, D. R.; Prickett, W. Z.

1975-01-01

The hardware selections available for fabrication of a nuclear electric propulsion stage for planetary exploration were explored. The investigation was centered around a heat-pipe-cooled, fast-spectrum nuclear reactor for an out-of-core power conversion system with sufficient detail for comparison with the in-core system studies completed previously. A survey of competing power conversion systems still indicated that the modular reliability of thermionic converters makes them the desirable choice to provide the 240-kWe end-of-life power for at least 20,000 full power hours. The electrical energy will be used to operate a number of mercury ion bombardment thrusters with a specific impulse in the range of about 4,000-5,000 seconds.

IECEC '83; Proceedings of the Eighteenth Intersociety Energy Conversion Engineering Conference, Orlando, FL, August 21-26, 1983. Volume 1 - Thermal energy systems

NASA Astrophysics Data System (ADS)

Among the topics discussed are the nuclear fuel cycle, advanced nuclear reactor designs, developments in central status power reactors, space nuclear reactors, magnetohydrodynamic devices, thermionic devices, thermoelectric devices, geothermal systems, solar thermal energy conversion systems, ocean thermal energy conversion (OTEC) developments, and advanced energy conversion concepts. Among the specific questions covered under these topic headings are a design concept for an advanced light water breeder reactor, energy conversion in MW-sized space power systems, directionally solidified cermet electrodes for thermionic energy converters, boron-based high temperature thermoelectric materials, geothermal energy commercialization, solar Stirling cycle power conversion, and OTEC production of methanol. For individual items see A84-30027 to A84-30055

Fabrication and surface characterization of composite refractory compounds suitable for thermionic converters

NASA Technical Reports Server (NTRS)

Davis, P. R.; Swanson, L. W.

1980-01-01

Thermal faceting was observed for the high index planes of LaB6. The (100), (110), and (111) planes were found to be the most thermodynamically stable faces in vacuum in a study of electrode materials for thermionic emitters. The properties of adsorbed carbon, cesium, and cesium-oxygen layers were investigated on LaB6 single crystal surfaces as well as on Zr/0/W(100) and W(100). Cesium was found to increase electron reflection near the collision threshold on LaB6(100) and W(100) and to decrease the reflection on Zr/0/W(100). This difference may be explained by the unusually high threshold reflection coefficient of Zr/0/W without adsorbed cesium.

Nuclear thermionic converter. [tungsten-thorium oxide rods

NASA Technical Reports Server (NTRS)

Phillips, W. M.; Mondt, J. F. (Inventor)

1977-01-01

Efficient nuclear reactor thermionic converter units are described which can be constructed at low cost and assembled in a reactor which requires a minimum of fuel. Each converter unit utilizes an emitter rod with a fluted exterior, several fuel passages located in the bulges that are formed in the rod between the flutes, and a collector receiving passage formed through the center of the rod. An array of rods is closely packed in an interfitting arrangement, with the bulges of the rods received in the recesses formed between the bulges of other rods, thereby closely packing the nuclear fuel. The rods are constructed of a mixture of tungsten and thorium oxide to provide high power output, high efficiency, high strength, and good machinability.

Graphene base heterojunction transistor: An explorative study on device potential, optimization, and base parasitics

NASA Astrophysics Data System (ADS)

Di Lecce, Valerio; Grassi, Roberto; Gnudi, Antonio; Gnani, Elena; Reggiani, Susanna; Baccarani, Giorgio

2015-12-01

The Graphene-Base Heterojunction Transistor (GBHT) is a novel device concept with a high potential for analog high-frequency RF operation, in which the current is due to both thermionic emission and tunneling. In this paper we study through numerical simulations the influence of previously uninvestigated aspects of Si- and Ge-based GBHTs-namely, crystallographic orientation and doping density values-on the device performance; a comparison with an aggressively scaled HBT structure is then reported. The simulations are carried out with an in-house developed code based on a 1-D quantum transport model within the effective mass approximation and the assumptions of ballistic transport with non-parabolic corrections and ideal semiconductor-graphene interface. We show that crystallographic orientation has a negligible effect on the GBHT performance. The doping density values in the GBHT emitter and collector regions can be tailored to maximize the device performance: the Si device shows better overall performance than the Ge one, yielding a peak cut-off frequency fT higher than 4 THz together with an intrinsic voltage gain above 10, or even higher fT at the cost of a lower gain. The Si-based GBHT can potentially outperform the SiGe HBT by a 2.8 higher fT . For a Si-based GBHT with a circular active region of diameter 50-100 nm, a theoretical balanced value for fT and fmax above 2 THz can be achieved, provided the base parasitics are carefully minimized.

Schottky junction interfacial properties at high temperature: A case of AgNWs embedded metal oxide/p-Si

NASA Astrophysics Data System (ADS)

Mahala, Pramila; Patel, Malkeshkumar; Gupta, Navneet; Kim, Joondong; Lee, Byung Ha

2018-05-01

Studying the performance limiting parameters of the Schottky device is an urgent issue, which are addressed herein by thermally stable silver nanowire (AgNW) embedded metal oxide/p-Si Schottky device. Temperature and bias dependent junction interfacial properties of AgNW-ITO/Si Schottky photoelectric device are reported. The current-voltage-temperature (I-V-T), capacitance-voltage-temperature (C-V-T) and impedance analysis have been carried out in the high-temperature region. The ideality factor and barrier height of Schottky junction are assessed using I-V-T characteristics and thermionic emission, to reveal the decrease of ideality factor and increase of barrier height by the increasing of temperature. The extracted values of laterally homogeneous Schottky (ϕb) and ideality factor (n) are approximately 0.73 eV and 1.58, respectively. Series resistance (Rs) assessed using Cheung's method and found that it decreases with the increase of temperature. A linear response of Rs of AgNW-ITO/Si Schottky junction is observed with respect to change in forward bias, i.e. dRS/dV from 0 to 0.7 V is in the range of 36.12-36.43 Ω with a rate of 1.44 Ω/V. Impedance spectroscopy is used to study the effect of bias voltage and temperature on intrinsic Schottky properties which are responsible for photoconversion efficiency. These systematic analyses are useful for the AgNWs-embedding Si solar cells or photoelectrochemical cells.

Effect of temperature and ridge-width on the lasing characteristics of InAs/InP quantum-dash lasers: A thermal analysis view

NASA Astrophysics Data System (ADS)

Alkhazraji, E.; Khan, M. T. A.; Ragheb, A. M.; Fathallah, H.; Qureshi, K. K.; Alshebeili, S.; Khan, M. Z. M.

2018-01-01

We investigate the thermal characteristics of multi-stack chirped barrier thickness InAs/InGaAlAs/InP quantum-dash-in-a-well lasers of different ridge widths 2, 3, 4 and 15 μm. The effect of varying this geometrical parameter on the extracted thermal resistance and characteristic temperature, and their stability with temperature are examined. The results show an inverse relation of ridge-width with junction temperature with 2 μm device exhibiting the largest junction temperature buildup owing to an associated high thermal resistance of ∼45 °C/W. Under the light of this thermal analysis, lasing behavior of different ridge-width quantum-dash (Qdash) lasers with injection currents and operating temperatures, is investigated. Thermionic carrier escape and phonon-assisted tunneling are found to be the dominant carrier transport mechanisms resulting in wide thermal spread of carriers across the available transition states of the chirped active region. An emission coverage of ∼75 nm and 3 dB bandwidth of ∼55 nm is exhibited by the 2 μm device, thus possibly exploiting the inhomogeneous optical transitions to the fullest. Furthermore, successful external modulation of a single Qdash Fabry-Perot laser mode via injection locking is demonstrated with eye diagrams at bit rates of 2-12 Gbit/s incorporating various modulation schemes. These devices are being considered as potential light sources for future high-speed wavelength-division multiplexed optical communication systems.

High-Performance Schottky Diode Gas Sensor Based on the Heterojunction of Three-Dimensional Nanohybrids of Reduced Graphene Oxide-Vertical ZnO Nanorods on an AlGaN/GaN Layer.

PubMed

Minh Triet, Nguyen; Thai Duy, Le; Hwang, Byeong-Ung; Hanif, Adeela; Siddiqui, Saqib; Park, Kyung-Ho; Cho, Chu-Young; Lee, Nae-Eung

2017-09-13

A Schottky diode based on a heterojunction of three-dimensional (3D) nanohybrid materials, formed by hybridizing reduced graphene oxide (RGO) with epitaxial vertical zinc oxide nanorods (ZnO NRs) and Al 0.27 GaN 0.73 (∼25 nm)/GaN is presented as a new class of high-performance chemical sensors. The RGO nanosheet layer coated on the ZnO NRs enables the formation of a direct Schottky contact with the AlGaN layer. The sensing results of the Schottky diode with respect to NO 2 , SO 2 , and HCHO gases exhibit high sensitivity (0.88-1.88 ppm -1 ), fast response (∼2 min), and good reproducibility down to 120 ppb concentration levels at room temperature. The sensing mechanism of the Schottky diode can be explained by the effective modulation of the reverse saturation current due to the change in thermionic emission carrier transport caused by ultrasensitive changes in the Schottky barrier of a van der Waals heterostructure between RGO and AlGaN layers upon interaction with gas molecules. Advances in the design of a Schottky diode gas sensor based on the heterojunction of high-mobility two-dimensional electron gas channel and highly responsive 3D-engineered sensing nanomaterials have potential not only for the enhancement of sensitivity and selectivity but also for improving operation capability at room temperature.

Temperature dependence of current-and capacitance-voltage characteristics of an Au/4H-SiC Schottky diode

NASA Astrophysics Data System (ADS)

Gülnahar, Murat

2014-12-01

In this study, the current-voltage (I-V) and capacitance-voltage (C-V) measurements of an Au/4H-SiC Schottky diode are characterized as a function of the temperature in 50-300 K temperature range. The experimental parameters such as ideality factor and apparent barrier height presents to be strongly temperature dependent, that is, the ideality factor increases and the apparent barrier height decreases with decreasing temperature, whereas the barrier height values increase with the temperature for C-V data. Likewise, the Richardson plot deviates at low temperatures. These anomaly behaviors observed for Au/4H-SiC are attributed to Schottky barrier inhomogeneities. The barrier anomaly which relates to interface of Au/4H-SiC is also confirmed by the C-V measurements versus the frequency measured in 300 K and it is interpreted by both Tung's lateral inhomogeneity model and multi-Gaussian distribution approach. The values of the weighting coefficients, standard deviations and mean barrier height are calculated for each distribution region of Au/4H-SiC using the multi-Gaussian distribution approach. In addition, the total effective area of the patches NAe is obtained at separate temperatures and as a result, it is expressed that the low barrier regions influence meaningfully to the current transport at the junction. The homogeneous barrier height value is calculated from the correlation between the ideality factor and barrier height and it is noted that the values of standard deviation from ideality factor versus q/3kT curve are in close agreement with the values obtained from the barrier height versus q/2kT variation. As a result, it can be concluded that the temperature dependent electrical characteristics of Au/4H-SiC can be successfully commented on the basis of the thermionic emission theory with both models.

Current transport in Pd2Si/n-Si(100) Schottky barrier diodes at low temperatures

NASA Astrophysics Data System (ADS)

Chand, Subhash; Kumar, Jitendra

1996-08-01

The forward current-voltage ( I V) characteristics of Pd2Si/n-Si(100) Schottky barrier diodes are shown to follow the Thermionic Emission-Diffusion (TED) mechanism in the temperature range of 52-295 K. The evaluation of the experimental I V data reveals a decrease of the zero-bias barrier height (ϕ b0) and an increase of the ideality factor (η) with decreasing temperature. Further, the changes in ϕ b0 and η become quite significant below 148 K. It is demonstrated that the findings cannot be explained on the basis of tunneling, generation-recombination and/or image force lowering. Also, the concepts of flat band barrier height and “ T 0-effect” fail to account for the temperature dependence of the barrier parameters. The 1n( I s / T 2) vs 1/ T plot exhibits nonlinearity below 185 K with the linear portion corresponding to an activat ion energy of 0.64 eV, a value smaller than the zero-bias barrier height energy (0.735 eV) of Pd2Si/n-Si Schottky diodes. Similarly, the value of the effective Richardson constant A** turns out to be 1.17 × 104 A m-2 K-2 against the theoretical value of 1.12 × 106 A m-2 K-2. Finally, it is demonstrated that the observed trends result due to barrier height inhomogeneities prevailing at the interface which, in turn, cause extra current such that the I V characteristics continue to remain consistent with the TED process even at low temperatures. The inhomogeneities are believed to have a Gaussian distribution with a mean barrier height of 0.80 V and a standard deviation of 0.05 V at zero-bias. Also, the effect of bias is shown to homogenize barrier heights at a slightly higher mean value.

Effect of cathode cooling efficiency and oxygen plasma gas pressure on the hafnium cathode wall temperature

NASA Astrophysics Data System (ADS)

Ashtekar, Koustubh; Diehl, Gregory; Hamer, John

2012-10-01

The hafnium cathode is widely used in DC plasma arc cutting (PAC) under an oxygen gas environment to cut iron and iron alloys. The hafnium erosion is always a concern which is controlled by the surface temperature. In this study, the effect of cathode cooling efficiency and oxygen gas pressure on the hafnium surface temperature are quantified. The two layer cathode sheath model is applied on the refractive hafnium surface while oxygen species (O2, O, O+, O++, e-) are considered within the thermal dis-equilibrium regime. The system of non-linear equations comprising of current density balance, heat flux balance at both the cathode surface and the sheath-ionization layer is coupled with the plasma gas composition solver. Using cooling heat flux, gas pressure and current density as inputs; the cathode wall temperature, electron temperature, and sheath voltage drop are calculated. Additionally, contribution of emitted electron current (Je) and ions current (Ji) to the total current flux are estimated. Higher gas pressure usually reduces Ji and increases Je that reduces the surface temperature by thermionic cooling.

On the work function and the charging of small ( r ≤ 5 nm) nanoparticles in plasmas

NASA Astrophysics Data System (ADS)

Kalered, E.; Brenning, N.; Pilch, I.; Caillault, L.; Minéa, T.; Ojamäe, L.

2017-01-01

The growth of nanoparticles (NPs) in plasmas is an attractive technique where improved theoretical understanding is needed for quantitative modeling. The variation of the work function W with size for small NPs, rN P≤ 5 nm, is a key quantity for modeling of three NP charging processes that become increasingly important at a smaller size: electron field emission, thermionic electron emission, and electron impact detachment. Here we report the theoretical values of the work function in this size range. Density functional theory is used to calculate the work functions for a set of NP charge numbers, sizes, and shapes, using copper for a case study. An analytical approximation is shown to give quite accurate work functions provided that rN P > 0.4 nm, i.e., consisting of about >20 atoms, and provided also that the NPs have relaxed close to spherical shape. For smaller sizes, W deviates from the approximation, and also depends on the charge number. Some consequences of these results for nanoparticle charging are outlined. In particular, a decrease in W for NP radius below about 1 nm has fundamental consequences for their charge in a plasma environment, and thereby on the important processes of NP nucleation, early growth, and agglomeration.

The HelCat Helicon-Cathode Device at UNM

NASA Astrophysics Data System (ADS)

Cyrin, Bricette; Watts, Christopher; Gilmore, Mark; Hayes, Tiffany; Kelly, Ralph; Leach, Christopher; Lynn, Alan; Sanchez, Andrew; Xie, Shuangwei; Yan, Lincan; Zhang, Yue

2009-11-01

The HelCat helicon-cathode device is a dual-source linear plasma device for investigating a wide variety of basic plasma phenomena. HelCat is 4 m long, 50 cm diameter, with axial magnetic field < 2.2 kG. An RF helicon source is at one end of the device, and a thermionic BaO-Ni cathode is at the other end. Current research topics include the relationship of turbulence to sheared plasma flows, deterministic chaos, Alfv'en wave propagation and damping, and merging plasma interaction. We present an overview of the ongoing research, and focus on recent results of merging helicon and cathode plasma. We will present some really cool movies.

Beam Measurement of 11.424 GHz X-Band Linac for Compton Scattering X-ray Source

NASA Astrophysics Data System (ADS)

Natsui, Takuya; Mori, Azusa; Masuda, Hirotoshi; Uesaka, Mitsuru; Sakamoto, Fumito

2010-11-01

An inverse Compton scattering X-ray source for medical applications, consisting of an X-band (11.424 GHz) linac and Q-switched Nd:YAG laser, is currently being developed at the University of Tokyo. This system uses an X-band 3.5-cell thermionic cathode RF gun for electron beam generation. We can obtain a multi-bunch electron beam with this gun. The beam is accelerated to 30 MeV by a traveling-wave accelerating tube. So far, we have verified stable beam generation (around 2.3 MeV) by using the newly designed RF gun and we have succeeded in beam transportation to a beam dump.

Space nuclear power systems; Proceedings of the 8th Symposium, Albuquerque, NM, Jan. 6-10, 1991. Pts. 1-3

NASA Technical Reports Server (NTRS)

El-Genk, Mohamed S. (Editor); Hoover, Mark D. (Editor)

1991-01-01

The present conference discusses NASA mission planning for space nuclear power, lunar mission design based on nuclear thermal rockets, inertial-electrostatic confinement fusion for space power, nuclear risk analysis of the Ulysses mission, the role of the interface in refractory metal alloy composites, an advanced thermionic reactor systems design code, and space high power nuclear-pumped lasers. Also discussed are exploration mission enhancements with power-beaming, power requirement estimates for a nuclear-powered manned Mars rover, SP-100 reactor design, safety, and testing, materials compatibility issues for fabric composite radiators, application of the enabler to nuclear electric propulsion, orbit-transfer with TOPAZ-type power sources, the thermoelectric properties of alloys, ruthenium silicide as a promising thermoelectric material, and innovative space-saving device for high-temperature piping systems. The second volume of this conference discusses engine concepts for nuclear electric propulsion, nuclear technologies for human exploration of the solar system, dynamic energy conversion, direct nuclear propulsion, thermionic conversion technology, reactor and power system control, thermal management, thermionic research, effects of radiation on electronics, heat-pipe technology, radioisotope power systems, and nuclear fuels for power reactors. The third volume discusses space power electronics, space nuclear fuels for propulsion reactors, power systems concepts, space power electronics systems, the use of artificial intelligence in space, flight qualifications and testing, microgravity two-phase flow, reactor manufacturing and processing, and space and environmental effects.

THERMIONIC AC GENERATION

DTIC Science & Technology

is shown that the maximum ac efficiency is equal to approximately 70% of the corresponding dc value. An illustrative example, including a proposed design for a rather unconventional transformer, is appended. (Author)

Nitride based quantum well light-emitting devices having improved current injection efficiency

DOEpatents

Tansu, Nelson; Zhao, Hongping; Liu, Guangyu; Arif, Ronald

2014-12-09

A III-nitride based device provides improved current injection efficiency by reducing thermionic carrier escape at high current density. The device includes a quantum well active layer and a pair of multi-layer barrier layers arranged symmetrically about the active layer. Each multi-layer barrier layer includes an inner layer abutting the active layer; and an outer layer abutting the inner layer. The inner barrier layer has a bandgap greater than that of the outer barrier layer. Both the inner and the outer barrier layer have bandgaps greater than that of the active layer. InGaN may be employed in the active layer, AlInN, AlInGaN or AlGaN may be employed in the inner barrier layer, and GaN may be employed in the outer barrier layer. Preferably, the inner layer is thin relative to the other layers. In one embodiment the inner barrier and active layers are 15 .ANG. and 24 .ANG. thick, respectively.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Matsubara, Y.; Tahara, H.; Nogawa, S.

A new type of electron source for ion sources, which serves as a cathode has been developed. In this cathode, a high-density microwave plasma is produced under the electron-cyclotron-resonance (ECR) condition, and a high electron current of several amperes can be extracted from it. The structure of this microwave plasma (MP) cathode is very simple and compact. A rod antenna connected to a coaxial line for introducing the microwave power (2.45 GHz) and a rare-earth metal permanent magnet for producing the ECR condition are major components. Since there is no filament in this MP cathode, it has a longer lifetimemore » than the equivalent thermionic filament electron emitter. It offers a great advantage to the operation with reactive as well as inert gases. This MP cathode has been adapted in Kaufman-type ion source and have successfully obtained an argon ion-beam current of 110 mA and an oxygen ion-beam current of 43 mA in 25 mm diameter.« less

Effects of P3HT concentration on the electrical properties of the Au/PEDOT:PSS/P3HT/n-GaN hybrid junction structure

NASA Astrophysics Data System (ADS)

Noh, Ji-yeon; Lee, Ha Young; Lim, Kyung-won; Ahn, Hyung Soo; Yi, Sam Nyung; Jeon, Hunsoo; Shin, Min Jeong; Yu, Young Moon; Ha, Dong Han

2017-09-01

An inorganic-organic hybrid junction has been fabricated by spin coating the p-type poly(3- hexylthiophene-2,5-diyl)(P3HT) and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) on an n-type GaN layer. The GaN layer was formed on Al2O3 by metal organic chemical vapor deposition(MOCVD) method. To investigate the effects of P3HT concentration on the electrical properties, we changed P3HT solution concentration and speed of spin coater. The currentvoltage (I-V ) characteristic of Au/PEDOT:PSS/P3HT/n-GaN shows rectifying behavior. The I-V characteristic was examined in the frame work of the thermionic emission model. The most proper rectifying behavior was obtained for 0.6 wt% and thickness below 65 nm of P3HT used diode. We expect that such hybrid structures, suitably developed, might be enable the fabrication of highquality electronic and optoelectronic devices.

Method for in-situ cleaning of carbon contaminated surfaces

DOEpatents

Klebanoff, Leonard E.; Grunow, Philip; Graham, Jr., Samuel

2006-12-12

Activated gaseous species generated adjacent a carbon contaminated surface affords in-situ cleaning. A device for removing carbon contamination from a surface of the substrate includes (a) a housing defining a vacuum chamber in which the substrate is located; (b) a source of gaseous species; and (c) a source of electrons that are emitted to activate the gaseous species into activated gaseous species. The source of electrons preferably includes (i) a filament made of a material that generates thermionic electron emissions; (ii) a source of energy that is connected to the filament; and (iii) an electrode to which the emitted electrons are attracted. The device is particularly suited for photolithography systems with optic surfaces, e.g., mirrors, that are otherwise inaccessible unless the system is dismantled. A method of removing carbon contaminants from a substrate surface that is housed within a vacuum chamber is also disclosed. The method employs activated gaseous species that react with the carbon contaminants to form carbon containing gaseous byproducts.

Effects of bias and temperature on the intersubband absorption in very long wavelength GaAs/AlGaAs quantum well infrared photodetectors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, X. H.; Zhou, X. H., E-mail: xhzhou@mail.sitp.ac.cn; Li, N.

2014-03-28

The temperature- and bias-dependent photocurrent spectra of very long wavelength GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) are studied using spectroscopic measurements and corresponding theoretical calculations. It is found that the peak response wavelength will shift as the bias and temperature change. Aided by band structure calculations, we propose a model of the double excited states and explain the experimental observations very well. In addition, the working mechanisms of the quasi-bound state confined in the quantum well, including the processes of tunneling and thermionic emission, are also investigated in detail. We confirm that the first excited state, which belongs to themore » quasi-bound state, can be converted into a quasi-continuum state induced by bias and temperature. These obtained results provide a full understanding of the bound-to-quasi-bound state and the bound-to-quasi-continuum state transition, and thus allow for a better optimization of QWIPs performance.« less

InGaAs-based planar barrier diode as microwave rectifier

NASA Astrophysics Data System (ADS)

Farhani Zakaria, Nor; Rizal Kasjoo, Shahrir; Zailan, Zarimawaty; Mohamad Isa, Muammar; Arshad, Mohd Khairuddin Md; Taking, Sanna

2018-06-01

In this report, we proposed and simulated a new planar nonlinear rectifying device fabricated using InGaAs substrate and referred to as a planar barrier diode (PBD). Using an asymmetrical inverse-arrowhead-shaped structure between the electrodes, a nonuniform depletion region is developed, which creates a triangular energy barrier in the conducting channel. This barrier is voltage dependent and can be controlled by the applied voltage across the PBD, thus resulting in nonlinear diode-like current–voltage characteristics; thus it can be used as a rectifying device. The PBD’s working principle is explained using thermionic emission theory. Furthermore, by varying the PBD’s geometric design, the asymmetry of the current–voltage characteristics can be optimized to realize superior rectification performance. By employing the optimized structural parameters, the obtained cut-off frequency of the device was approximately 270 GHz with a curvature coefficient peak of 14 V‑1 at a low DC bias voltage of 50 mV.

A Rapid Method for Deposition of Sn-Doped GaN Thin Films on Glass and Polyethylene Terephthalate Substrates

NASA Astrophysics Data System (ADS)

Pat, Suat; Özen, Soner; Korkmaz, Şadan

2018-01-01

We report the influence of Sn doping on microstructure, surface, and optical properties of GaN thin films deposited on glass and polyethylene terephthalate (PET) substrate. Sn-doped GaN thin films have been deposited by thermionic vacuum arc (TVA) at low temperature. TVA is a rapid deposition technology for thin film growth. Surface and optical properties of the thin films were presented. Grain size, height distribution, roughness values were determined. Grain sizes were calculated as 20 nm and 13 nm for glass and PET substrates, respectively. Nano crystalline forms were shown by field emission scanning electron microscopy. Optical band gap values were determined by optical methods and photoluminescence measurement. The optical band gap values of Sn doped GaN on glass and PET were determined to be approximately ˜3.40 eV and ˜3.47 eV, respectively. As a result, TVA is a rapid and low temperature deposition technology for the Sn doped GaN deposited on glass and PET substrate.

Composition and work function relationship in Os–Ru–W ternary alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Swartzentruber, Phillip D.; Detisch, Michael J.; Balk, T. John, E-mail: john.balk@uky.edu

2015-03-15

Os–Ru thin films with varying concentrations of W were sputter deposited in order to investigate their structure–property relationships. The films were analyzed with x-ray diffraction to investigate their crystal structures, and a Kelvin probe to investigate their work functions. An Os–Ru–W film with ∼30 at. % W yielded a work function maximum of approximately 5.38 eV. These results align well with other studies that found work function minima from thermionic emission data on M-type cathodes with varying amounts of W in the coatings. Furthermore, the results are consistent with other work explaining energy-level alignment and charge transfer of molecules on metalmore » oxides. This may shed light on the mechanism behind the “anomalous effect” first reported by Zalm et al., whereby a high work function coating results in a low work function for emitting cathode surfaces. An important implication of this work is the potential for the Kelvin probe to evaluate the effectiveness of dispenser cathode coatings.« less

Advanced thermionic energy conversion

NASA Technical Reports Server (NTRS)

Britt, E. J.; Fitzpatrick, G. D.; Hansen, L. K.; Rasor, N. S.

1974-01-01

Basic analytical and experimental exploration was conducted on several types of advanced thermionic energy converters, and preliminary analysis was performed on systems utilizing advanced converter performance. The Pt--Nb cylindrical diode which exhibited a suppressed arc drop, as described in the preceding report, was reassembled and the existence of the postulated hydrid mode of operation was tentatively confirmed. Initial data obtained on ignited and unignited triode operation in the demountable cesium vapor system essentially confirmed the design principles developed in earlier work, with a few exceptions. Three specific advanced converter concepts were selected as candidates for concentrated basic study and for practical evaluation in fixed-configuration converters. Test vehicles and test stands for these converters and a unique controlled-atmosphere station for converter assembly and processing were designed, and procurement was initiated.

Construction and performance of the magnetic bunch compressor for the THz facility at Chiang Mai University

NASA Astrophysics Data System (ADS)

Saisut, J.; Kusoljariyakul, K.; Rimjaem, S.; Kangrang, N.; Wichaisirimongkol, P.; Thamboon, P.; Rhodes, M. W.; Thongbai, C.

2011-05-01

The Plasma and Beam Physics Research Facility at Chiang Mai University has established a THz facility to focus on the study of ultra-short electron pulses. Short electron bunches can be generated from a system that consists of a radio-frequency (RF) gun with a thermionic cathode, an alpha magnet as a magnetic bunch compressor, and a linear accelerator as a post-acceleration section. The alpha magnet is a conventional and simple instrument for low-energy electron bunch compression. With the alpha magnet constructed in-house, several hundred femtosecond electron bunches for THz radiation production can be generated from the thermionic RF gun. The construction and performance of the alpha magnet, as well as some experimental results, are presented in this paper.

The plasmatron: Advanced mode thermionic energy conversion

NASA Technical Reports Server (NTRS)

Hansen, L. K.; Hatch, G. L.; Rasor, N. S.

1976-01-01

A theory of the plasmatron was developed. Also, a wide range of measurements were obtained with two versatile, research devices. To gain insight into plasmatron performance, the experimental results are compared with calculations based on the theoretical model of plasmatron operation. Results are presented which show that the plasma arc drop of the conventional arc (ignited) mode converter can be suppressed by use of an auxiliary ion source. The improved performance, however, is presently limited to low current densities because of voltage losses due to plasma resistance. This resistance loss could be suppressed by an increase in the plasma electron temperature or a decrease in spacing. Plasmatron performance characteristics for both argon and cesium are reported. The argon plasmatron has superior performance. Results are also presented for magnetic cutoff effects and for current distributing effects. These are shown to be important factors for the design of practical devices.

The electrical, elemental, optical, and surface properties of Si-doped ZnO thin films prepared by thermionic vacuum arc

NASA Astrophysics Data System (ADS)

Mohammadigharehbagh, Reza; Özen, Soner; Yudar, Hafizittin Hakan; Pat, Suat; Korkmaz, Şadan

2017-09-01

The purpose of this work is to study the properties of Si-doped ZnO (SZO) thin films, which were prepared using the non-reactive thermionic vacuum arc technique. The analysis of the elemental, optical, and surface properties of ZnO:Si thin films was carried out using energy dispersive x-ray spectroscopy, UV-VIS spectrophotometry, atomic force microscopy, and scanning electron microscopy, respectively. The current-voltage measurement was employed in order to study the electrical properties of the films. The effect of Si doping on the physical properties of ZnO films was investigated. The film thicknesses were measured as 55 and 35 nm for glass and PET substrates, respectively. It was clearly observed from the x-ray diffraction results that the Si and ZnO peaks were present in the coated SZO films for all samples. The morphological studies showed that the deposited surfaces are homogenous, dense, and have a uniform surface, with the existence of some cracks only on the glass substrate. The elemental composition has confirmed the existence of Zn, Si, and O elements within the prepared films. Using a UV-VIS spectrophotometer, the optical parameters such as transmittance, absorbance, refractive index, and reflectance were calculated. It should be noted that the transparency and refractive indices obtained from the measurements decrease with increasing Si concentration. The obtained optical bandgap values using transmittance spectra were determined to be 3.74 and 3.84 eV for the glass and PET substrates, respectively. An increase in the bandgap results demonstrates that the Si doping concentration is comparable to the pure ZnO thin films. The current versus voltage curves revealed the ohmic nature of the films. Subsequently, the development and fabrication of excellent transparent conducting electrodes enabled the appropriate use of Si-doped ZnO thin films.

Performance of an electron gun for a high-brightness X-ray generator

PubMed Central

Sugimura, Takashi; Ohsawa, Satoshi; Ikeda, Mitsuo

2008-01-01

A prototype thermionic electron gun for a high-brightness X-ray generator has been developed. Its extraction voltage and design current are 60 kV and 100 mA (DC), respectively. The X-ray generator aims towards a maximum brilliance of 60 kW mm−2. The beam sizes at the rotating anticathode must therefore be within 1.0 mm × 0.1 mm and a small beam emittance is required. The fabricated electron gun optimizes an aperture grid and a Whenelt electrode. The performance of the prototype electron gun measured using pulsed-beam tests is as follows: maximum beam current, 85.7 mA; beam focus size at the rotating anticathode, 0.79 mm × 0.13 mm. In DC beam tests, FWHM beam sizes were measured to be 0.65 mm × 0.08 mm at the rotating anticathode with a beam current of 45 mA. The beam current recently reached ∼60 mA with some thermal problems. PMID:18421153

EXPERIMENTAL MEASUREMENT AND INTERPRETATION OF VOLT-AMPERE CURVES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gingrich, J.E.; Warner, C.; Weeks, C.C.

1962-07-01

Cylindrical and parallel-plane cesium vapor thermionic converters were used for obtaining volt-ampere curves for systematic variations of emitter, collector, and cesium reservoir temperatures, with electrode spacings ranging from a few to many mean free paths, and with space charge conditions varying from electron-rich to ion-rich. The resulting curves exhibit much variety. The saturation currents agree well with the data of Houston and Aamodt for the space charge neutralized, few-mean-free-path cases. Apparent'' saturation currents for space charge limited cases were observed and were always less than the currents predicted by Houston and Aamodt. Several discontinuities in slope were observed in themore » reverse current portion of the curves and these have tentatively been identified with volume ionization of atoms in both the ground and excited states. Similar processes may be important for obtaining the ignited mode. The methods used to measure static and dynamic volt-ampere curves are described. The use of a controlled-current load has yielded a negative resistance'' region in the curves which show the ignited mode. The curves obtained with poor current control do not show this phenomenon. Extinction is considered from the standpoint of Kaufmann' s criterion for stability. (auth)« less

The anodic emitter effect and its inversion demonstrated by temperature measurements at doped and undoped tungsten electrodes

NASA Astrophysics Data System (ADS)

Hoebing, T.; Bergner, A.; Hermanns, P.; Mentel, J.; Awakowicz, P.

2016-04-01

The admixture of a small amount of emitter oxides, e.g. \\text{Th}{{\\text{O}}2} , \\text{L}{{\\text{a}}2}{{\\text{O}}3} or \\text{C}{{\\text{e}}2}{{\\text{O}}3} to tungsten generates the so-called emitter effect. It reduces the work function of tungsten cathodes, that are applied in high intensity discharge (HID) lamps. After leaving the electrode bulk and moving to the surface, a monolayer of Th, La, or Ce atoms is formed on the surface, which reduces the effective work function ϕ. Depending on the coverage of the electrode, the effective reduction in ϕ is subjected to the thermal desorption of the monolayer from the hot electrode surface. The thermal desorption of emitter atoms from the cathode is compensated not only by the supply from the interior of the electrode and by surface diffusion of the emitter material to its tip, but also to a large extent by a repatriation of the emitter ions from the plasma by the strong electric field in front of the cathode. Yet, an emitter ion current from the arc discharge to the anode may only be present, if the anode is cold enough to refrain from thermionic emission. Therefore, the ability of emitter oxides to reduce the temperature of tungsten anodes is only given for a moderate temperature so that the thermal desorption is low and an additional ion current is present in front of the anode. A higher electrode temperature leads to their evaporation and to an inversion of the emitter effect, which increases the temperature of the respective anodes in comparison with pure tungsten anodes. Within this article, the emitter effect of doped tungsten anodes and the transition to its inversion is investigated for thoriated, lanthanated, and ceriated tungsten electrodes by measurements of the electrode temperature in dependence on the discharge current. It is shown for a lanthanated and a ceriated anode that the emitter effect is sustained by an ion current at anode temperatures at which the thermal evaporation of emitter material is completed.

Modeling of Electron Transpiration Cooling for Leading Edges of Hypersonic Vehicles

NASA Astrophysics Data System (ADS)

Hanquist, Kyle Matthew

The development of aeronautics has been largely driven by the passion to fly faster. From the flight of the Wright Flyer that flew 48 km/hr to the recent advances in hypersonic flight, most notably NASA's X-43A that flew at over 3 km/s, the velocity of flight has steadily increased. However, as these hypersonic speeds are reached and increased, contradicting aerothermodynamic design requirements present themselves. For example, a hypersonic cruise vehicle requires sharp leading edges to decrease the drag in order to maximize the range. However, the aerodynamic performance gains obtained by having a sharp leading edge come at the cost of very high, localized heating rates. There is currently no ideal way to manage these heating loads for sustained hypersonic flight, especially as flight velocities continue to increase. An approach that has been recently proposed involves using thermo-electric materials on these sharp leading edges to manage the heating loads. When exposed to high convective heating rates, these materials emit a current of electrons that leads to a cooling effect of the surface of the vehicle called electron transpiration cooling (ETC). This dissertation focuses on developing a modeling approach to investigate this phenomenon. The research includes developing and implementing an approach for ETC into a computational fluid dynamics code for simulation of hypersonic flow that accounts for electron emission from the surface. Models for space-charge-limited emission are also developed and implemented in order to accurately determine the level of emission from the surface. This work involves developing analytic models and assessing them using a direct-kinetic plasma sheath solver. Electric field effects are also implemented in the modeling approach, which accounts for forced diffusion and Joule heating. Finally, the modeling approach is coupled to a material response code in order to model the heat transfer into the material surface. Using this modeling approach, ETC is investigated as a viable technology for a wide range of hypersonic operating conditions. This includes altitudes between 30 and 60 km, freestream velocities between 4 and 8 km/s, and leading edge radii between 1 mm and 10 cm. The results presented in this study show that ETC can reduce the leading edge temperature significantly for certain conditions, most notably from 3120 to 1660 K for Mach 26 flight for a sharp leading edge (1 cm). However, at lower velocities, the cooling effect can be diminished by space-charge limits in the plasma sheath. ETC is shown to be most effective at cooling hotter surfaces (e.g. high freestream velocities and sharp leading edges) and the level of ionization in the flowfield can help the emission overcome space-charge limits. The modeling approach is assessed using experiments from the 1960s where thermionic emission was investigated as a mode of power generation for reentry vehicles. The computational results produce a wide range of emitted current due to the uncertainty in the freestream conditions and material properties, but they still agree well with the experiments. Overall, this work indicates that ETC is a viable method of managing the immense heat loads on sharp leading edges during hypersonic flight for certain conditions and motivates future work in the area both computationally and experimentally.

Novel Low-Cost, Low-Power Miniature Thermionic Cathode Developed for Microwave/Millimeter Wave Tube and Cathode Ray Tube Applications

NASA Technical Reports Server (NTRS)

Wintucky, Edwin G.

1999-01-01

A low cost, small size and mass, low heater power, durable high-performance barium dispenser thermionic cathode has been developed that offers significant advancements in the design, manufacture, and performance of the electron sources used in vacuum electronic devices--such as microwave (and millimeter wave) traveling-wave tubes (TWT's)--and in display devices such as high-brightness, high-resolution cathode ray tubes (CRT's). The lower cathode heater power and the reduced size and mass of the new cathode are expected to be especially beneficial in TWT's for deep space communications, where future missions are requiring smaller spacecraft, higher data transfer rates (higher frequencies and radiofrequency output power), and greater electrical efficiency. Also expected to benefit are TWT's for commercial and government communication satellites, for both low and geosynchronous Earth orbit, with additional benefits offered by lower cost and potentially higher cathode current loading. A particularly important TWT application is in the microwave power module (MPM), which is a hybrid microwave (or millimeter wave) amplifier consisting of a low-noise solid state driver, a vacuum power booster (small TWT), and an electronic power conditioner integrated into a single compact package. The attributes of compactness and potentially high electrical efficiency make the MPM very attractive for many commercial and government (civilian and defense) applications in communication and radar systems. The MPM is already finding application in defense electronic systems and is under development by NASA for deep space communications. However, for the MPM to become competitive and commercially successful, a major reduction in cost must be achieved.