These are representative sample records from Science.gov related to your search topic.
For comprehensive and current results, perform a real-time search at Science.gov.
1

Ice crystallization in ultrafine water-salt aerosols: nucleation, ice-solution equilibrium, and internal structure.  

PubMed

Atmospheric aerosols have a strong influence on Earth's climate. Elucidating the physical state and internal structure of atmospheric aqueous aerosols is essential to predict their gas and water uptake, and the locus and rate of atmospherically important heterogeneous reactions. Ultrafine aerosols with sizes between 3 and 15 nm have been detected in large numbers in the troposphere and tropopause. Nanoscopic aerosols arising from bubble bursting of natural and artificial seawater have been identified in laboratory and field experiments. The internal structure and phase state of these aerosols, however, cannot yet be determined in experiments. Here we use molecular simulations to investigate the phase behavior and internal structure of liquid, vitrified, and crystallized water-salt ultrafine aerosols with radii from 2.5 to 9.5 nm and with up to 10% moles of ions. We find that both ice crystallization and vitrification of the nanodroplets lead to demixing of pure water from the solutions. Vitrification of aqueous nanodroplets yields nanodomains of pure low-density amorphous ice in coexistence with vitrified solute rich aqueous glass. The melting temperature of ice in the aerosols decreases monotonically with an increase of solute fraction and decrease of radius. The simulations reveal that nucleation of ice occurs homogeneously at the subsurface of the water-salt nanoparticles. Subsequent ice growth yields phase-segregated, internally mixed, aerosols with two phases in equilibrium: a concentrated water-salt amorphous mixture and a spherical cap-like ice nanophase. The surface of the crystallized aerosols is heterogeneous, with ice and solution exposed to the vapor. Free energy calculations indicate that as the concentration of salt in the particles, the advance of the crystallization, or the size of the particles increase, the stability of the spherical cap structure increases with respect to the alternative structure in which a core of ice is fully surrounded by solution. We predict that micrometer-sized particles and nanoparticles have the same equilibrium internal structure. The variation of liquid-vapor surface tension with solute concentration is a key factor in determining whether a solution-embedded ice core or vapor-exposed ice cap is the equilibrium structure of the aerosols. In agreement with experiments, we predict that the structure of mixed-phase HNO3-water particles, representative of polar stratospheric clouds, consists of an ice core surrounded by freeze-concentrated solution. The results of this work are important to determine the phase state and internal structure of sea spray ultrafine aerosols and other mixed-phase particles under atmospherically relevant conditions. PMID:24820354

Hudait, Arpa; Molinero, Valeria

2014-06-01

2

Triangular ice crystals  

NASA Astrophysics Data System (ADS)

We are all familiar with the hexagonal form of snow crystals and it is well established that this shape is derived from the arrangement of water molecules in the crystal lattice. However, crystals with a triangular form are often found in the Earth's atmosphere and the reason for this non-hexagonal shape has remained elusive. Recent laboratory work has shed light on why ice crystals should take on this triangular or three-fold scalene habit. Studies of the crystal structure of ice have shown that ice which initially crystallises can be made of up of hexagonal layers which are interlaced with cubic layers to produce a 'stacking disordered ice'. The degree of stacking disorder can vary from crystals which are dominantly hexagonal with a few cubic stacking faults, through to ice where the cubic and hexagonal sequences are fully randomised. The introduction of stacking disorder to ice crystals reduces the symmetry of the crystal from 6-fold (hexagonal) to 3-fold (triangular); this offers an explanation for the long standing problem of why some atmospheric ice crystals have a triangular habit. We discuss the implications of triangular crystals for halos, radiative properties, and also discuss the implications for our understanding of the nucleation and early stages of ice crystal growth for ice crystals in the atmosphere.

Murray, Benjamin; Salzmann, Christoph; Heymsfield, Andrew; Neely, Ryan

2014-05-01

3

Development of Measurement System for Three-Dimensional Structure of Ice Crystals in Raw Beef Samples  

NASA Astrophysics Data System (ADS)

Micro-Slicer Image Processing System (MSIPS) has been developed for measuring the three-dimensional(3-D) structure and distribution of ice crystals formed in biological materials. The system has functions to reconstruct the 3-D image based on the image data of exposed cross sections obtained by multi-slicing of a frozen sample with the minimum thickness of 1?m and to display the internal structure as well as an arbitrary cross section of the sample choosing observation angles. The effects of freezing conditions on the morphology and distribl1tion of ice crystals were demonstrated quantitatively from the observations of raw beef stained by fluorescent indicator. The 3-D image of the sample demonstrated that the growth of ice columns was restricted by the intrinsic structure of muscle fibers. The proposed method provided a new tool to investigate the effects of freezing conditions on the size, morphology and distribution of ice crystals.

Do, Gab-Soo; Sagara, Yasuyuki; Tabata, Mizuho; Kudoh, Ken-Ichi; Higuchi, Toshiro

4

Crystal structure of an insect antifreeze protein and its implications for ice binding.  

PubMed

Antifreeze proteins (AFPs) help some organisms resist freezing by binding to ice crystals and inhibiting their growth. The molecular basis for how these proteins recognize and bind ice is not well understood. The longhorn beetle Rhagium inquisitor can supercool to below -25 °C, in part by synthesizing the most potent antifreeze protein studied thus far (RiAFP). We report the crystal structure of the 13-kDa RiAFP, determined at 1.21 ? resolution using direct methods. The structure, which contains 1,914 nonhydrogen protein atoms in the asymmetric unit, is the largest determined ab initio without heavy atoms. It reveals a compressed ?-solenoid fold in which the top and bottom sheets are held together by a silk-like interdigitation of short side chains. RiAFP is perhaps the most regular structure yet observed. It is a second independently evolved AFP type in beetles. The two beetle AFPs have in common an extremely flat ice-binding surface comprising regular outward-projecting parallel arrays of threonine residues. The more active, wider RiAFP has four (rather than two) of these arrays between which the crystal structure shows the presence of ice-like waters. Molecular dynamics simulations independently reproduce the locations of these ordered crystallographic waters and predict additional waters that together provide an extensive view of the AFP interaction with ice. By matching several planes of hexagonal ice, these waters may help freeze the AFP to the ice surface, thus providing the molecular basis of ice binding. PMID:23486477

Hakim, Aaron; Nguyen, Jennifer B; Basu, Koli; Zhu, Darren F; Thakral, Durga; Davies, Peter L; Isaacs, Farren J; Modis, Yorgo; Meng, Wuyi

2013-04-26

5

Crystal Structure of an Insect Antifreeze Protein and Its Implications for Ice Binding*  

PubMed Central

Antifreeze proteins (AFPs) help some organisms resist freezing by binding to ice crystals and inhibiting their growth. The molecular basis for how these proteins recognize and bind ice is not well understood. The longhorn beetle Rhagium inquisitor can supercool to below ?25 °C, in part by synthesizing the most potent antifreeze protein studied thus far (RiAFP). We report the crystal structure of the 13-kDa RiAFP, determined at 1.21 ? resolution using direct methods. The structure, which contains 1,914 nonhydrogen protein atoms in the asymmetric unit, is the largest determined ab initio without heavy atoms. It reveals a compressed ?-solenoid fold in which the top and bottom sheets are held together by a silk-like interdigitation of short side chains. RiAFP is perhaps the most regular structure yet observed. It is a second independently evolved AFP type in beetles. The two beetle AFPs have in common an extremely flat ice-binding surface comprising regular outward-projecting parallel arrays of threonine residues. The more active, wider RiAFP has four (rather than two) of these arrays between which the crystal structure shows the presence of ice-like waters. Molecular dynamics simulations independently reproduce the locations of these ordered crystallographic waters and predict additional waters that together provide an extensive view of the AFP interaction with ice. By matching several planes of hexagonal ice, these waters may help freeze the AFP to the ice surface, thus providing the molecular basis of ice binding. PMID:23486477

Hakim, Aaron; Nguyen, Jennifer B.; Basu, Koli; Zhu, Darren F.; Thakral, Durga; Davies, Peter L.; Isaacs, Farren J.; Modis, Yorgo; Meng, Wuyi

2013-01-01

6

Ice Crystal Terminal Velocities  

Microsoft Academic Search

Terminal velocities of different ice crystal forms were calculated using the most recent ice crystal drag coefficients, aspect ratios and densities. The equations derived were primarily for use in calculating precipitation rates by sampling particles with an aircraft in cirrus clouds, and determining particle size in cirrus clouds by Doppler radar. However, the equations are sufficiently general for determining particle

Andrew Heymsfield

1972-01-01

7

Viewing Ice Crystals Using Polarized Light.  

ERIC Educational Resources Information Center

Describes a method for identifying and examining single ice crystals by photographing a thin sheet of ice placed between two inexpensive polarizing filters. Suggests various natural and prepared sources for ice that promote students' insight into crystal structures, and yield colorful optical displays. Includes directions, precautions, and sample…

Kinsman, E. M.

1992-01-01

8

Ice crystal terminal velocities.  

NASA Technical Reports Server (NTRS)

Terminal velocities of different ice crystal forms were calculated, using the most recent ice crystal drag coefficients, aspect ratios, and densities. The equations derived were primarily for use in calculating precipitation rates by sampling particles with an aircraft in cirrus clouds, and determining particle size in cirrus clouds by Doppler radar. However, the equations are sufficiently general for determining particle terminal velocity at any altitude, and almost any crystal type. Two sets of equations were derived. The 'general' equations provide a good estimate of terminal velocities at any altitude. The 'specific' equations are a set of equations for ice crystal terminal velocities at 1000 mb. The calculations are in good agreement with terminal velocity measurements. The results from the present study were also compared to prior calculations by others and seem to give more reasonable results, particularly at higher altitudes.

Heymsfield, A.

1972-01-01

9

Bacterial Ice Crystal Controlling Proteins  

PubMed Central

Across the world, many ice active bacteria utilize ice crystal controlling proteins for aid in freezing tolerance at subzero temperatures. Ice crystal controlling proteins include both antifreeze and ice nucleation proteins. Antifreeze proteins minimize freezing damage by inhibiting growth of large ice crystals, while ice nucleation proteins induce formation of embryonic ice crystals. Although both protein classes have differing functions, these proteins use the same ice binding mechanisms. Rather than direct binding, it is probable that these protein classes create an ice surface prior to ice crystal surface adsorption. Function is differentiated by molecular size of the protein. This paper reviews the similar and different aspects of bacterial antifreeze and ice nucleation proteins, the role of these proteins in freezing tolerance, prevalence of these proteins in psychrophiles, and current mechanisms of protein-ice interactions. PMID:24579057

Lorv, Janet S. H.; Rose, David R.; Glick, Bernard R.

2014-01-01

10

Ice crystal ingestion by turbofans  

NASA Astrophysics Data System (ADS)

This Thesis will present the problem of inflight icing in general and inflight icing caused by the ingestion of high altitude ice crystals produced by high energy mesoscale convective complexes in particular, and propose a new device to prevent it based on dielectric barrier discharge plasma. Inflight icing is known to be the cause of 583 air accidents and more than 800 deaths in more than a decade. The new ice crystal ingestion problem has caused more than 100 flights to lose engine power since the 1990's, and the NTSB identified it as one of the causes of the Air France flight 447 accident in 1-Jun2008. The mechanics of inflight icing not caused by ice crystals are well established. Aircraft surfaces exposed to supercooled liquid water droplets will accrete ice in direct proportion of the droplet catch and the freezing heat transfer process. The multiphase flow droplet catch is predicted by the simple sum of forces on each spherical droplet and a droplet trajectory calculation based on Lagrangian or Eulerian analysis. The most widely used freezing heat transfer model for inflight icing caused by supercooled droplets was established by Messinger. Several computer programs implement these analytical models to predict inflight icing, with LEWICE being based on Lagrangian analysis and FENSAP being based on Eulerian analysis as the best representatives among them. This Thesis presents the multiphase fluid mechanics particular to ice crystals, and explains how it differs from the established droplet multiphase flow, and the obstacles in implementing the former in computational analysis. A new modification of the Messinger thermal model is proposed to account for ice accretion produced by ice crystal impingement. Because there exist no computational and experimental ways to fully replicate ice crystal inflight icing, and because existing ice protections systems consume vast amounts of energy, a new ice protection device based on dielectric barrier discharge plasma is proposed and built in this Thesis, called DBDAIS, with a complete description of the anti-ice cycle. Contrary to existing ice protection systems, which either heat the aircraft surfaces, or mechanically remove the accreted ice, the DBDAIS employs non-thermal plasma discharges to prevent ice accretion. A new apparatus that mimics inflight icing based on combining the liquid sprays of liquid nitrogen and water was designed and fabricated, named LNITA. The apparatus produces ice similar to glaze ice and rime ice, the two characteristic types of ice from inflight icing, at the cost of 1% of similar tests in icing wind tunnels. Nineteen experiments of the DBDAIS were performed in the LNITA. The results from the experiments point to 32 kV and 4 kHz being adequate to prevent ice accretion, with a power consumption of 1 W/cm2. This compares favorably to existing ice protection systems, which typically run at 10 W/cm2, and to the power consumption of a typical electric stove burner at maximum power, which is 5 W/cm2. To complete this Thesis, a design and development project is proposed to implement the DBDAIS in Unmanned Aircraft Systems (UAS), with the selection of standard FAA inflight icing conditions, the run of 240 LEWICE simulations, and an analysis of the run results. The computational results lead to the design of a wing boot covering the airfoil from 20% of the lower pressure surface to 4% of the upper suction surface as the optimal protection for a UAS.

Rios Pabon, Manuel A.

11

Dependence on accelerating voltage of crystal structural changes in water ice thin film under electron beam irradiation  

NASA Astrophysics Data System (ADS)

The dependence on accelerating voltage of crystal structural changes in water ice thin film under electron beam irradiation was investigated by cryogenic transmission electron microscopy (cryo-TEM) at 95 K and accelerating voltages of 25, 50, 75, 100, or 125 kV. Ice thin film was prepared by depositing residual moisture in the TEM column onto both sides of a carbon membrane at 95 K. The major phase of the deposited film at 95 K was identified as ice Ic by transmission electron diffractometry. We found that the mass loss rate of the ice thin film decreased sharply as the accelerating voltage was increased. From this result, we conclude the mass loss mechanism was the ionization of water by inelastic scattering of incident electrons. Moreover, the phase transition from ice Ic to ice Ih was observed at accelerating voltages of 75 kV or greater. At 50 kV or lower, however, the phase transition was hardly observed by TEM. Because the phase transition can also be attributed to inelastic scattering of incident electrons, the results suggest that whether mass loss or a phase transition occurs depends primarily on the accelerating voltage.

Kobayashi, Keita; Yasuda, Hidehiro

2013-02-01

12

Effects of Ice-Crystal Structure on Halo Formation: Cirrus Cloud Experimental and Ray-Tracing Modeling Studies  

NASA Technical Reports Server (NTRS)

During the 1986 Project FIRE (First International Satellite Cloud Climatology Project Regional Experiment) field campaign, four 22 deg halo-producing cirrus clouds were studied jointly from a ground-based polarization lidar and an instrumented aircraft. The lidar data show the vertical cloud structure and the relative position of the aircraft, which collected a total of 84 slides by impaction, preserving the ice crystals for later microscopic examination. Although many particles were too fragile to survive impaction intact, a large fraction of the identifiable crystals were columns and radial bullet rosettes, with both displaying internal cavitations and radial plate-column combinations. Particles that were solid or displayed only a slight amount of internal structure were relatively rare, which shows that the usual model postulated by halo theorists, i.e., the randomly oriented, solid hexagonal crystal, is inappropriate for typical cirrus clouds. With the aid of new ray-tracing simulations for hexagonal hollow-ended column and bullet-rosette models, we evaluate the effects of more realistic ice-crystal structures on halo formation and lidar depolarization and consider why the common halo is not more common in cirrus clouds.

Sassen, Kenneth; Knight, Nancy C.; Takano, Yoshihide; Heymsfield, Andrew J.

1994-01-01

13

Ice crystallization during the manufacture of ice cream  

Microsoft Academic Search

Control of ice crystallization during the manufacture of ice cream is important for the development of proper texture, product quality and storage stability. Improving our somewhat limited understanding of the mechanisms that control ice-crystal formation, as well as of the effects of formulation and process factors, may lead to improvements in processing techniques.

Richard W. Hartel

1996-01-01

14

Evolution of crystal fabric: Ice-Age ice versus Holocene ice  

Microsoft Academic Search

Ice-Age ice has smaller crystals and higher concentrations of impurities than Holocene ice; these properties cause it to develop a more strongly-aligned crystal-orientation fabric. In many regions of the Antarctic and Greenland ice sheets, the Ice-Age ice is now at depth and its flow properties may dominate the ice flow patterns, particularly where sliding is minimal. We use a fabric

J. H. Kennedy; E. C. Pettit

2009-01-01

15

Relationships between ice cream mix viscoelasticity and ice crystal growth in ice cream  

Microsoft Academic Search

The relationship between ice cream mix viscoelasticity and ice crystal growth in ice cream as a function of stabilizer addition was studied by a simulation of freeze-concentration using a series of ice cream mixes containing reduced quantities of water. Ice cream mixes were formulated with guar gum concentrations ranging from 0 to 0.25% and a series of concentrated mixes from

S. Bolliger; H. Wildmoser; H. D. Goff; B. W. Tharp

2000-01-01

16

Prismatic Structure in Ice  

Microsoft Academic Search

``In one of the March numbers of NATURE I see a letter over your signature on the prismatic structure of ice, and as our climate gives us favourable opportunities of observing this and other curious facts respecting ice, I am induced to address a few words to you on the subject.

John Langters

1870-01-01

17

Mixing of the Immiscible: Hydrocarbons in Water-Ice near the Ice Crystallization Temperature.  

PubMed

Structural changes in hydrocarbon-doped water-ice during amorphous to crystalline phase conversion are investigated using polycyclic aromatic hydrocarbons (PAHs) as probes. We show that aggregation of impurity molecules occurs due to the amorphous-crystalline transition in ice, especially when they are hydrophobic molecules such as PAHs. Using ultraviolet-visible (UV-vis), Fourier-transform Infrared (FTIR), and laser-induced-fluorescence (LIF) spectroscopic techniques, we show that, although ice infrared absorption features change from a broad structureless band corresponding to amorphous ice to a sharp structured crystalline ice bands, simultaneously, sharper isolated PAH UV absorption features measured in the amorphous ice host turn broad upon ice crystallization. A simultaneous decrease in the monomer fluorescence and increase in the excimer emission band is observed, a clear indication for the formation of PAH molecular aggregates when amorphous ice is converted to crystalline ice at higher temperatures. Similar to the irreversible amorphous-crystalline phase transitions, the UV, fluorescence, and excimer emissions indicate that PAHs undergo irreversible aggregation. Our studies suggest that organic impurities exist as aggregates rather than monomers trapped in crystalline water-ice when cycled through temperatures that convert amorphous ice to crystalline ice, rendering a better insight into phenomena such as the formation of cometary crust. This aggregate formation also may significantly change the secondary reaction pathways and rates in impurity-doped ices in the lab, on Earth, in the solar system, and in the interstellar medium. PMID:25302532

Lignell, Antti; Gudipati, Murthy S

2014-10-27

18

Ice interaction with offshore structures  

SciTech Connect

Oil platforms and other offshore structures being built in the arctic regions must be able to withstand icebergs, ice islands, and pack ice. This reference explain the effect ice has on offshore structures and demonstrates design and construction methods that allow such structures to survive in harsh, ice-ridden environments. It analyzes the characteristics of sea ice as well as dynamic ice forces on structures. Techniques for ice modeling and field testing facilitate the design and construction of sturdy, offshore constructions. Computer programs included.

Cammaert, A.B.; Muggeridge, D.B.

1988-01-01

19

Evolution of crystal fabric: Ice-Age ice versus Holocene ice  

NASA Astrophysics Data System (ADS)

Ice-Age ice has smaller crystals and higher concentrations of impurities than Holocene ice; these properties cause it to develop a more strongly-aligned crystal-orientation fabric. In many regions of the Antarctic and Greenland ice sheets, the Ice-Age ice is now at depth and its flow properties may dominate the ice flow patterns, particularly where sliding is minimal. We use a fabric evolution model, based on that developed by Thorsteinsson (2002), to explore the evolution of Ice-Age ice fabric along particle paths for ice within Taylor Glacier, a cold-based outlet glacier of the East Antarctic Ice Sheet. The bulk of the ice within Taylor Glacier consists of Ice-Age and older ice because the Holocene ice has ablated away (there is no Holocene ice remaining within 25km of the terminus, Aciego, 2007). We initialize the evolving fabric based on fabric measurements from Taylor Dome where available (DiPrinzio, 2003) and other ice core records. We compare model results with thin-section data from shallow cores taken near the terminus. As expected, crystal alignment strengthens along the ice particle path. Due to lateral shearing along valley walls and the ice cliffs (terminal ice cliffs are cold in winter and present a resistance to flow), a tilted single maximum is common near the terminus. The highly-aligned fabric of Ice-Age ice is significantly softer than Holocene ice in simple shear parallel to the bed, this softness not only results in faster flow rates for glaciers and ice sheets such as Taylor, but creates a climate-flow-fabric feedback loop through concentrating ice-sheet flow within the Ice-Age ice. Thorsteinsson, T. (2002), Fabric development with nearest-neighbor interaction and dynamic recrystallization, J. Geophys. Res., 107(B1), 2014, doi:10.1029/2001JB000244. S.M. Aciego, K.M. Cuffey, J.L. Kavanaugh, D.L. Morse, J.P. Severinghaus, Pleistocene ice and paleo-strain rates at Taylor Glacier, Antarctica, Quaternary Research, Volume 68, Issue 3, November 2007, Pages 303-313, ISSN 0033-5894, DOI: 10.1016/j.yqres.2007.07.013. DiPrinzio, Eos Trans. AGU, 84(46), Fall Meet. Suppl., Abstract C11C-0834, 2003

Kennedy, J. H.; Pettit, E. C.

2009-12-01

20

Crystallization of CO2 ice and the absence of amorphous CO2 ice in space  

PubMed Central

Carbon dioxide (CO2) is one of the most relevant and abundant species in astrophysical and atmospheric media. In particular, CO2 ice is present in several solar system bodies, as well as in interstellar and circumstellar ice mantles. The amount of CO2 in ice mantles and the presence of pure CO2 ice are significant indicators of the temperature history of dust in protostars. It is therefore important to know if CO2 is mixed with other molecules in the ice matrix or segregated and whether it is present in an amorphous or crystalline form. We apply a multidisciplinary approach involving IR spectroscopy in the laboratory, theoretical modeling of solid structures, and comparison with astronomical observations. We generate an unprecedented highly amorphous CO2 ice and study its crystallization both by thermal annealing and by slow accumulation of monolayers from the gas phase under an ultrahigh vacuum. Structural changes are followed by IR spectroscopy. We also devise theoretical models to reproduce different CO2 ice structures. We detect a preferential in-plane orientation of some vibrational modes of crystalline CO2. We identify the IR features of amorphous CO2 ice, and, in particular, we provide a theoretical explanation for a band at 2,328 cm?1 that dominates the spectrum of the amorphous phase and disappears when the crystallization is complete. Our results allow us to rule out the presence of pure and amorphous CO2 ice in space based on the observations available so far, supporting our current view of the evolution of CO2 ice. PMID:23858474

Escribano, Rafael M.; Muñoz Caro, Guillermo M.; Cruz-Diaz, Gustavo A.; Rodríguez-Lazcano, Yamilet; Maté, Belén

2013-01-01

21

Ice crystals in high clouds and contrails  

NASA Astrophysics Data System (ADS)

In honor of Dr. Helmut K. Weickmann's lifetime achievements, some of his early discoveries on ice crystals in high clouds are resurrected and discussed. It is demonstrated that the shapes and sizes of cloud meteors convey information related to cloud type, cloud duration, cloud amount, cloud radiative forcing, and the environment in which the cloud forms. For example, persistence of a contrail behind an aircraft depends on ice crystal concentration and size distribution, which are governed by the ambient temperature, humidity, and the concentration of ice nuclei. It is suggested that increased air traffic, especially high-flying jets over the past three decades, may have modified global cirrus cloud amount, which in turn may affect surface temperature and global climate.

Parungo, F.

22

Ice cream structural elements that affect melting rate and hardness.  

PubMed

Statistical models were developed to reveal which structural elements of ice cream affect melting rate and hardness. Ice creams were frozen in a batch freezer with three types of sweetener, three levels of the emulsifier polysorbate 80, and two different draw temperatures to produce ice creams with a range of microstructures. Ice cream mixes were analyzed for viscosity, and finished ice creams were analyzed for air cell and ice crystal size, overrun, and fat destabilization. The ice phase volume of each ice cream were calculated based on the freezing point of the mix. Melting rate and hardness of each hardened ice cream was measured and correlated with the structural attributes by using analysis of variance and multiple linear regression. Fat destabilization, ice crystal size, and the consistency coefficient of the mix were found to affect the melting rate of ice cream, whereas hardness was influenced by ice phase volume, ice crystal size, overrun, fat destabilization, and the rheological properties of the mix. PMID:14765804

Muse, M R; Hartel, R W

2004-01-01

23

Carbon dioxide enhances fragility of ice crystals  

NASA Astrophysics Data System (ADS)

Ice caps and glaciers cover 7% of the Earth, greater than the land area of Europe and North America combined, and play an important role in global climate. The small-scale failure mechanisms of ice fracture, however, remain largely elusive. In particular, little understanding exists about how the presence and concentration of carbon dioxide molecules, a significant component in the atmosphere, affects the propensity of ice to fracture. Here we use atomic simulations with the first-principles based ReaxFF force field capable of describing the details of chemical reactions at the tip of a crack, applied to investigate the effects of the presence of carbon dioxide molecules on ice fracture. Our result shows that increasing concentrations of carbon dioxide molecules significantly decrease the fracture toughness of the ice crystal, making it more fragile. Using enhanced molecular sampling with metadynamics we reconstruct the free energy landscape in varied chemical microenvironments and find that carbon dioxide molecules affect the bonds between water molecules at the crack tip and decrease their strength by altering the dissociation energy of hydrogen bonds. In the context of glacier dynamics our findings may provide a novel viewpoint that could aid in understanding the breakdown and melting of glaciers, suggesting that the chemical composition of the atmosphere can be critical to mediate the large-scale motion of large volumes of ice.

Qin, Zhao; Buehler, Markus J.

2012-11-01

24

Disturbed basal ice seen in radio echo images coincide with zones of big interlocking ice crystals.  

NASA Astrophysics Data System (ADS)

Improvement of the depth sounding radio echo sounding (RES) over Antarctica and Greenland Ice Sheet has made it possible to map the near basal layers that have not been 'seen' earlier due to the very high demand of attenuation needed to reach through more than 3000m of ice. The RES internal reflectors show that the near basal ice at many locations has disturbed layering. At the locations where ice cores reach the bedrock both in Greenland and Antarctica studies of the ice crystal size and orientation show that the near basal ice has big and interlocking ice crystals which suggests the ice is not actively deforming. These observations challenge the often used constitutive equations like Glens flow law in ice sheet modelling. A discussion of the impact of the RES findings on ice sheet modeling and the quest to find the oldest ice in Antarctic based on the anisotropy of the basal ice will follow.

Dahl-Jensen, Dorthe; Gogineni, Sivaprasad; Panton, Christian

2014-05-01

25

An immune response to ice crystals in North Atlantic fishes.  

PubMed

In mammals, the presence of crystals composed of small organic molecules, including urate and related compounds, has been shown to trigger an inflammatory response and the subsequent production of specific immunoglobulins (Ig's). Many fishes that are exposed to ice crystals in cold temperate and polar oceans may harbour ice crystals internally. Here, we report evidence for a specific immune response to ice crystals in cold-ocean marine fishes. Using ice nucleation activity as an assay, anti-ice Ig's were detected in the sera of the cold-ocean marine fish species, ocean pout (Macrozoarces americanus) and Atlantic herring (Clupea harengus harengus), but not in the sera of species that are not exposed to ice. Purified Ig's isolated from ocean pout serum using two different protocols showed ice nucleation activity, thus demonstrating the presence of ice binding specificity among these Ig's. PMID:8944760

Verdier, J M; Ewart, K V; Griffith, M; Hew, C L

1996-11-01

26

Factors Affecting the Changes of Ice Crystal Form in Ice Cream  

NASA Astrophysics Data System (ADS)

In this study, the shape of ice crystals in ice cream was quantitatively evaluated by introducing fractal analysis. A small droplet of commercial ice cream mix was quickly cooled to about -30°C on the cold stage of microscope. Subsequently, it was heated to -5°C or -10°C and then held for various holding time. Based on the captured images at each holding time, the cross-sectional area and the length of circumference for each ice crystal were measured to calculate fractal dimension using image analysis software. The results showed that the ice crystals were categorized into two groups, e.g. simple-shape and complicated-shape, according to their fractal dimensions. The fractal dimension of ice crystals became lower with increasing holding time and holding temperature. It was also indicated that the growing rate of complicated-shape ice crystals was relatively higher because of aggregation.

Wang, Xin; Watanabe, Manabu; Suzuki, Toru

27

Probing the structure of cometary ice  

Microsoft Academic Search

Computer simulations of bulk and vapor deposited amorphous ices are presented. The structure of the bulk low density amorphous ice is in good agreement with experiments on pressure disordered amorphous ice. Both the low density bulk ice and the vapor deposited ices exhibit strong ordering. Vapor deposition of hot (300 K) water molecules onto a cold (77 K) substrate yields

Michael A. Wilson; Rew Pohorille; Peter Jenniskens; David F. Blake

1995-01-01

28

New metastable form of ice and its role in the homogeneous crystallization of water  

NASA Astrophysics Data System (ADS)

The homogeneous crystallization of water at low temperature is believed to occur through the direct nucleation of cubic (Ic) and hexagonal (Ih) ices. Here, we provide evidence from molecular simulations that the nucleation of ice proceeds through the formation of a new metastable phase, which we name Ice 0. We find that Ice 0 is structurally similar to the supercooled liquid, and that on growth it gradually converts into a stacking of Ice Ic and Ih. We suggest that this mechanism provides a thermodynamic explanation for the location and pressure dependence of the homogeneous nucleation temperature, and that Ice 0 controls the homogeneous nucleation of low-pressure ices, acting as a precursor to crystallization in accordance with Ostwald’s step rule of phases. Our findings show that metastable crystalline phases of water may play roles that have been largely overlooked.

Russo, John; Romano, Flavio; Tanaka, Hajime

2014-07-01

29

Light Scattering by Ice Crystals and Mineral Dust Aerosols in the Atmosphere  

E-print Network

regular faceted particles (such as hexagon columns, plates, etc.) and imperfect ice crystals. Modeling of the scattering by regular ice crystals is straightforward, as their morphologies can be easily de?ned. For imperfect ice crystals, the morphology...

Bi, Lei

2012-07-16

30

Ice crystal number concentration versus temperature for climate studies  

Microsoft Academic Search

ABSTRACT Ice crystal number concentration (Ni) is an important parameter, having a strong influence on the calculation of cloud ,optical and ,microphysical ,parameters. Cloud and ,precipitation parameterizations within climate and weather forecasting models, affecting the heat and moisture budget of the atmosphere, cannot be determined accurately if Ni is not estimated correctly. Previous studies of ice ,crystal number ,concentration versus

I. Gultepe; G. A. Isaac; S. G. Cober

2001-01-01

31

Ice mechanics and risks to offshore structures  

SciTech Connect

This volume brings together the results of all salient research development in ice engineering, from smaller scale to full size tests on ice strength and ice mechanics which is essential criteria for the design of safe, cost effective structures. Much of the data has been released from confidential industry files and thus allows, for the first time, a full appraisal of the subject. Contents include - Types and Distribution of Ice, Mechanical Properties, Measurements of Ice-Structure Interaction, and Analysis of Ice Failure and Design Ice Loads. This work is completed with a full literary review and subject index.

Sanderson, T.J.O.

1988-01-01

32

Ice-Crystal Fallstreaks from Supercooled Liquid Water Parent Clouds  

NASA Technical Reports Server (NTRS)

On 31 December 2001, ice-crystal fallstreaks (e.g., cirrus uncinus, or colloquially "Mare's Tails") from supercooled liquid water parent clouds were observed by ground-based lidars pointed vertically from the Atmospheric Radiation Measurement Southern Great Plains (SGP) facility near Lamont, Oklahoma. The incidence of liquid phase cloud with apparent ice-phase precipitation is investigated. Scenarios for mixed-phase particle nucleation, and fallstreak formation and sustenance are discussed. The observations are unique in the context of the historical reverence given to the commonly observed c h s uncinus fallstreak (wholly ice) versus this seemingly contradictory coincidence of liquid water begetting ice-crystal streaks.

Campbell, James R.; O'C. Starr, David; Welton, Ellsworth J.; Spinhirne, James D.; Ferrare, Richard A.

2003-01-01

33

Scattering Properties of Oriented Hexagonal Ice Crystals  

E-print Network

. In this study, the dipole approximation (DDA) method is employed to the scattering of light on oriented hexagonal ice columns and plates with various tilting angles. It is found that the oriented hexagonal ice particles tend to have strong backscattering...

Zhang, Feng

2010-01-14

34

Forces Generated by High Velocity Impact of Ice on a Rigid Structure  

NASA Technical Reports Server (NTRS)

Tests were conducted to measure the impact forces generated by cylindrical ice projectiles striking a relatively rigid target. Two types of ice projectiles were used, solid clear ice and lower density fabricated ice. Three forms of solid clear ice were tested: single crystal, poly-crystal, and "rejected" poly-crystal (poly-crystal ice in which defects were detected during inspection.) The solid ice had a density of approximately 56 lb/cu ft (0.9 gm/cu cm). A second set of test specimens, termed "low density ice" was manufactured by molding shaved ice into a cylindrical die to produce ice with a density of approximately 40 lb/cu ft (0.65 gm/cu cm). Both the static mechanical characteristics and the crystalline structure of the ice were found to have little effect on the observed transient response. The impact forces generated by low density ice projectiles, which had very low mechanical strength, were comparable to those of full density solid ice. This supports the hypothesis that at a velocity significantly greater than that required to produce fracture in the ice, the mechanical properties become relatively insignificant, and the impact forces are governed by the shape and mass of the projectile.

Pereira, J. Michael; Padula, Santo A., II; Revilock, Duane M.; Melis, Matthew E.

2006-01-01

35

Steady-state and stability analysis of a population balance based nonlinear ice cream crystallization model  

E-print Network

Steady-state and stability analysis of a population balance based nonlinear ice cream the key phenomenons of the crystallization process. In ice cream crystallization, it is well known that the quality of the product, that is the hardness and the texture of the ice cream, depends on the ice crystal

Boyer, Edmond

36

Preferred Ice Crystal Orientation Fabric Measurements within the Greenland Ice Sheet Using Multi-Polarization Radar Data  

NASA Astrophysics Data System (ADS)

Discharge of ice from the Greenland Ice Sheet to the ocean has increased significantly over the last 25 years due to the acceleration of important outlet glaciers. It was reported that the Greenland Ice Sheet contributed about 2.5 m out of about 6 m of sea-level rise during the Eemian interglacial period. The temperatures during Eemian were reported to be about 8o×4o C higher than the mean of the past millennium. Laboratory measurements have shown that glacial ice, characterized by preferred crystal orientation fabric (COF), is three times more deformable than ice with randomly oriented crystalline structures. Layers characterized by preferred ice COF can influence the flow behavior of a glacier or ice sheet. However, COF measurements are typically obtained from ice cores, and thus are very spatially limited and mostly constrained to areas with little ice flow. A more efficient technique to map the extent of ice fabric over larger regions of ice sheets is needed to better understand the effects on large scale ice flow processes. Radar measurements are capable of discriminating between reflections caused by changes in density, electrical permittivity and COF by exploiting the anisotropic and birefringent properties of ice crystals. For this investigation two radar datasets were collected during the survey of the Greenland Eemian Ice Drilling Site (77.45°N 51.06°W) in August 2008, using a ground-based and chirped-pulse Multi-Channel Radar Depth Sounder (MCRDS) developed by the Center for Remote Sensing of Ice Sheets (CReSIS). The radar used two transmit and eight receive antennas at the center frequency of 150 MHz with a bandwidth of 30 MHz. The first data set consisted of polarimatric measurements acquired in a circular pattern (radius: 35 m) with two co-polarized antenna orientations (one transmitter and four receivers oriented with 90° offsets in the directions of the incident H-Field and E-Field, respectively). Analysis of the circular data shows a periodic power variation with four distinct extinction patterns occurring at 90 degree intervals starting at approximately 700 m depth. Furthermore a 20 degree phase change is observed between the E- and H-field data. Both observations suggest that approximately 72% of the 2542m ice column exhibits birefringent anisotropy caused by preferred ice crystal orientation. The second dataset was acquired in a grid pattern consisting of twenty 10-Km 2D lines (NW to SE) spaced at 0.5-Km and three 10-Km lines (NE to SW) spaced at 2.5-Km. Both transmit and eight receive antenna were oriented parallel to the vehicle track, resulting in E-Field co-polarized data. We will determine the dominant COF relative to the ice divide for a 100 square Km region around the NEEM camp using the results from both datasets. The results of this investigation will be compared to the NEEM ice core observations to determine the accuracy of the analysis. In this investigation we will provide a brief overview of the system and experiments and present the results of data analysis.

Velez-Gonzalez, J. A.; JiLu, L.; Leuschen, C.; Gogineni, P.; Van der Veen, C. J.; Tsoflias, G. P.; Drews, R.; Harish, A. R.

2013-12-01

37

Ice-templated structures for biomedical tissue repair: From physics to final scaffolds  

NASA Astrophysics Data System (ADS)

Ice-templating techniques, including freeze-drying and freeze casting, are extremely versatile and can be used with a variety of materials systems. The process relies on the freezing of a water based solution. During freezing, ice nucleates within the solution and concentrates the solute in the regions between the growing crystals. Once the ice is removed via sublimation, the solute remains in a porous structure, which is a negative of the ice. As the final structure of the ice relies on the freezing of the solution, the variables which influence ice nucleation and growth alter the structure of ice-templated scaffolds. Nucleation, the initial step of freezing, can be altered by the type and concentration of solutes within the solution, as well as the set cooling rate before freezing. After nucleation, crystal growth and annealing processes, such as Ostwald ripening, determine the features of the final scaffold. Both crystal growth and annealing are sensitive to many factors including the set freezing temperature and solutes. The porous structures created using ice-templating allow scaffolds to be used for many diverse applications, from microfluidics to biomedical tissue engineering. Within the field of tissue engineering, scaffold structure can influence cellular behavior, and is thus critical for determining the biological stimulus supplied by the scaffold. The research focusing on controlling the ice-templated structure serves as a model for how other ice-templating systems might be tailored, to expand the applications of ice-templated structures to their full potential.

Pawelec, K. M.; Husmann, A.; Best, S. M.; Cameron, R. E.

2014-06-01

38

Mixed Crystals of Ice and Ammonium Fluoride  

Microsoft Academic Search

THE ``inability of forming solid solutions with any compound'' was attributed to water two years ago by Mironov and Bergman1, who refuted previous reports by Giguere and Maass2 and by Kubaschewski and Weber3 of the solubility of hydrogen peroxide in ice. Yet Kathleen Lonsdale4 directed attention to the fact that ice and ammonium fluoride are not only isomorphous but also

R. Brill; S. Zaromb

1954-01-01

39

Superheating of ice crystals in antifreeze protein solutions.  

PubMed

It has been argued that for antifreeze proteins (AFPs) to stop ice crystal growth, they must irreversibly bind to the ice surface. Surface-adsorbed AFPs should also prevent ice from melting, but to date this has been demonstrated only in a qualitative manner. Here we present the first quantitative measurements of superheating of ice in AFP solutions. Superheated ice crystals were stable for hours above their equilibrium melting point, and the maximum superheating obtained was 0.44 degrees C. When melting commenced in this superheated regime, rapid melting of the crystals from a point on the surface was observed. This increase in melting temperature was more appreciable for hyperactive AFPs compared to the AFPs with moderate antifreeze activity. For each of the AFP solutions that exhibited superheating, the enhancement of the melting temperature was far smaller than the depression of the freezing temperature. The present findings clearly show that AFPs adsorb to ice surfaces as part of their mechanism of action, and this absorption leads to protection of ice against melting as well as freezing. PMID:20215465

Celik, Yeliz; Graham, Laurie A; Mok, Yee-Foong; Bar, Maya; Davies, Peter L; Braslavsky, Ido

2010-03-23

40

Growth of ice discs from the vapor and the mechanism of habit change of ice crystals  

NASA Technical Reports Server (NTRS)

Ice crystals nucleated on a liquid nitrogen cooled glass fiber grow first as thin disks which subsequently transform to plates and columns as they thicken and extend to regions of higher supersaturation. Crystals are often found to be dislocation-free, which suggests that growth results from surface nucleation, the habit depending on preferential nucleation in an adsorbed multilayer on basal or prism face.

Keller, V. W.; Mcknight, C. V.; Hallett, J.

1980-01-01

41

On the correlation between ice water content and ice crystal size and its application to radiative transfer and general circulation models  

E-print Network

observations from aircraft, and attests to the fact that small and large ice crystals are related to coldOn the correlation between ice water content and ice crystal size and its application to radiative analysis involving ice water content (IWC) and mean effective ice crystal size (De) intended

Liou, K. N.

42

The effect of ice crystal shape on aircraft contrails  

NASA Astrophysics Data System (ADS)

Aircraft contrails are a common phenomenon observed in the sky. They are formed mainly of water, from the ambient atmosphere and as a by-product of the combustion process, in the form of ice crystals. They have been identified as a potential contributor to global warming. Some contrails can be long-lived and create man-made cloud cover, thus possibly altering the radiative balance of the earth. There has been a great deal of research on various aspects of contrail development, but to date, little has been done on the influence of ice crystal shapes on the contrail evolution. In-situ studies have reported that young contrails are mainly quasi-spherical crystals while older contrails can have a much more diverse spectrum of possible shapes. The most common shapes found in contrails are quasi-spherical, hexagonal columns, hexagonal plates, and bullet rosettes. Numerical simulations of contrails to date typically have assumed "spherical" as the default ice shape. This work simulated contrail development with a large eddy simulation (LES) model that implemented both spherical and non-spherical shapes to examine the effects. The included shape effect parameters, such as capacitance coefficient, ventilation factor, Kelvin effect, fall velocity and ice crystal surface area, help to establish the shape difference in the results. This study also investigated initial sensitivities to an additional ice parameter, the ice deposition coefficient. The literature shows conflicting values for this coefficient over a wide range. In the course of this investigation a comparison of various ice metrics was made for simulations with different assumed crystal shapes (spheres, hexagonal columns, hexagonal plates, bullet rosettes and combination of shapes). The simulations were performed at early and late contrail time, with a range of ice crystal sizes, and with/without coupled radiation. In young and older contrails and without coupled radiation, the difference from the shape effect in ice crystal number, N(t), is not significant compared with the level of uncertainty. In young contrails, the difference between spherical and non-spherical shapes in N(t) is less than 7% for relatively large ice particles and 23% for relatively small ice particles. The ice mass, M(t), is not significantly affected by the crystal shapes, with less than 8% difference. However, the ice surface area, S(t), is the ice metric more sensitive to crystal shape, with a maximum difference of 68%. It increases at late time, though it is mainly governed by geometrical rather than dynamical effects. The small sensitivity to shape effects in the ice contrail metrics when radiation is not included suggests that the spherical shape will provide a reasonable representation for all shapes found in the in-situ studies. The radiation is included at late time, when the lasting effects of contrails are more critical. The inclusion of coupled radiation increases the level of dispersion in the results and hence increases slightly the differences due to shape effects. The small difference is also observed in the infrared heating rates of contrails.

Meza Castillo, Omar E.

43

Variability in millimeter wave scattering properties of dendritic ice crystals  

NASA Astrophysics Data System (ADS)

A detailed electromagnetic scattering model for ice crystals is necessary for calculating radar reflectivity from cloud resolving model output in any radar simulator. The radar reflectivity depends on the backscattering cross sections and size distributions of particles in the radar resolution volume. The backscattering cross section depends on the size, mass and distribution of mass within the crystal. Most of the available electromagnetic scattering data for ice hydrometeors rely on simple ice crystal types and a single mass-dimensional relationship for a given type. However, a literature survey reveals that the mass-dimensional relationships for dendrites cover a relatively broad region in the mass-dimensional plane. This variability of mass and mass distribution of dendritic ice crystals cause significant variability in their backscattering cross sections, more than 10 dB for all sizes (0.5-5 mm maximum dimension) and exceeding 20 dB for the larger ones at X-, Ka-, and W-band frequencies. Realistic particle size distributions are used to calculate radar reflectivity and ice water content (IWC) for three mass-dimensional relationships. The uncertainty in the IWC for a given reflectivity spans an order of magnitude in value at all three frequencies because of variations in the unknown mass-dimensional relationship and particle size distribution. The sensitivity to the particle size distribution is reduced through the use of dual frequency reflectivity ratios, e.g., Ka- and W-band frequencies, together with the reflectivity at one of the frequencies for estimating IWC.

Botta, Giovanni; Aydin, Kültegin; Verlinde, Johannes

2013-12-01

44

Modeling variability in dendritic ice crystal backscattering cross sections at millimeter wavelengths using a modified Rayleigh-Gans theory  

NASA Astrophysics Data System (ADS)

Using the Generalized Multi-particle Mie-method (GMM), Botta et al. (in this issue) [7] created a database of backscattering cross sections for 412 different ice crystal dendrites at X-, Ka- and W-band wavelengths for different incident angles. The Rayleigh-Gans theory, which accounts for interference effects but ignores interactions between different parts of an ice crystal, explains much, but not all, of the variability in the database of backscattering cross sections. Differences between it and the GMM range from -3.5 dB to +2.5 dB and are highly dependent on the incident angle. To explain the residual variability a physically intuitive iterative method was developed to estimate the internal electric field within an ice crystal that accounts for interactions between the neighboring regions within it. After modifying the Rayleigh-Gans theory using this estimated internal electric field, the difference between the estimated backscattering cross sections and those from the GMM method decreased to within 0.5 dB for most of the ice crystals. The largest percentage differences occur when the form factor from the Rayleigh-Gans theory is close to zero. Both interference effects and neighbor interactions are sensitive to the morphology of ice crystals. Improvements in ice-microphysical models are necessary to predict or diagnose internal structures within ice crystals to aid in more accurate interpretation of radar returns. Observations of the morphology of ice crystals are, in turn, necessary to guide the development of such ice-microphysical models and to better understand the statistical properties of ice crystal morphologies in different environmental conditions.

Lu, Yinghui; Clothiaux, Eugene E.; Aydin, Kültegin; Botta, Giovanni; Verlinde, Johannes

2013-12-01

45

Dynamic pressure-induced dendritic and shock crystal growth of ice VI  

PubMed Central

Crystal growth mechanisms are crucial to understanding the complexity of crystal morphologies in nature and advanced technological materials, such as the faceting and dendrites found in snowflakes and the microstructure and associated strength properties of structural and icy planetary materials. In this article, we present observations of pressure-induced ice VI crystal growth, which have been predicted theoretically, but had never been observed experimentally to our knowledge. Under modulated pressure conditions in a dynamic-diamond anvil cell, rough single ice VI crystal initially grows into well defined octahedral crystal facets. However, as the compression rate increases, the crystal surface dramatically changes from rough to facet, and from convex to concave because of a surface instability, and thereby the growth rate suddenly increases by an order of magnitude. Depending on the compression rate, this discontinuous jump in crystal growth rate or “shock crystal growth” eventually produces 2D carpet-type fractal morphology, and moreover dendrites form under sinusoidal compression, whose crystal morphologies are remarkably similar to those predicted in theoretical simulations under a temperature gradient field. The observed strong dependence of the growth mechanism on compression rate, therefore, suggests a different approach to developing a comprehensive understanding of crystal growth dynamics. PMID:17296943

Lee, Geun Woo; Evans, William J.; Yoo, Choong-Shik

2007-01-01

46

Snow crystal imaging using scanning electron microscopy: III. Glacier ice, snow and biota  

USGS Publications Warehouse

Low-temperature scanning electron microscopy (SEM) was used to observe metamorphosed snow, glacial firn, and glacial ice obtained from South Cascade Glacier in Washington State, USA. Biotic samples consisting of algae (Chlamydomonas nivalis) and ice worms (a species of oligochaetes) were also collected and imaged. In the field, the snow and biological samples were mounted on copper plates, cooled in liquid nitrogen, and stored in dry shipping containers which maintain a temperature of -196??C. The firn and glacier ice samples were obtained by extracting horizontal ice cores, 8 mm in diameter, at different levels from larger standard glaciological (vertical) ice cores 7.5 cm in diameter. These samples were cooled in liquid nitrogen and placed in cryotubes, were stored in the same dry shipping container, and sent to the SEM facility. In the laboratory, the samples were sputter coated with platinum and imaged by a low-temperature SEM. To image the firn and glacier ice samples, the cores were fractured in liquid nitrogen, attached to a specimen holder, and then imaged. While light microscope images of snow and ice are difficult to interpret because of internal reflection and refraction, the SEM images provide a clear and unique view of the surface of the samples because they are generated from electrons emitted or reflected only from the surface of the sample. In addition, the SEM has a great depth of field with a wide range of magnifying capabilities. The resulting images clearly show the individual grains of the seasonal snowpack and the bonding between the snow grains. Images of firn show individual ice crystals, the bonding between the crystals, and connected air spaces. Images of glacier ice show a crystal structure on a scale of 1-2 mm which is considerably smaller than the expected crystal size. Microscopic air bubbles, less than 15 ??m in diameter, clearly marked the boundaries between these crystal-like features. The life forms associated with the glacier were easily imaged and studied. The low-temperature SEM sample collecting and handling methods proved to be operable in the field; the SEM analysis is applicable to glaciological studies and reveals details unattainable by conventional light microscopic methods.Low temperature scanning electron microscopy (SEM) was used to observe metamorphosed snow, glacial firn, and glacial ice obtained from South Cascade Glacier in Washington State, USA. Biotic samples consisting of algae and ice worms were also collected and imaged. The SEM images provide a clear and unique view of the surface of the samples because they are generated from electrons emitted or reflected only from the surface of the sample. The SEM has a great depth of field with a wide range of magnifying capabilities.

Rango, A.; Wergin, W.P.; Erbe, E.F.; Josberger, E.G.

2000-01-01

47

The scavenging of high altitude aerosol by small ice crystals  

NASA Astrophysics Data System (ADS)

There have been several global models developed for the theoretical investigation of the removal of high altitude aerosol from the atmosphere, following concern about the injection of particulate material by nuclear explosions and volcanic events. These models lack a knowledge of the scavenging efficiencies of the small ice crystals associated with cirus clouds and storm ice anvils. These are the only hydrometers that could remove the injected particles. In the past there have been a number of practical studies into the scavenging efficiencies of large ice crystaks and snowflakes. A comparison of the extrapolated results of these findings and the theoretical models of Martin et al. (1980, Pure appl. Phys.188, 1109-1129, J. atmos. Sci.37, 1628-1638) for the small crystal situation has been made. It was found that in general the extrapolated results gave efficiencies that were significantly higher than the predicted value. This difference was found to be enhanced as the crystal diameter decreased. Experiments used small ice plates grown at ˜-18.5°C in a cloud chamber, which were then permitted to fall through a dense aerosol cloud, to provide the first direct measurements of the scavenging efficiencies of this small crystals under cloud conditions. Initial results are presented for mono-disperse NaCl aerosol particles of size 4-6 ?m.

Andrew Bell, D.; Saunders, Clive P. R.

48

Computation of the scattering properties of nonspherical ice crystals  

E-print Network

This thesis is made up of three parts on the computation of scattering properties of nonspherical particles in the atmosphere. In the first part, a new crystal type-droxtal-is introduced to make a better representation of the shape of small ice...

Zhang, Zhibo

2004-11-15

49

Laboratory Investigation of Direct Measurement of Ice Water Content, Ice Surface Area, and Effective Radius of Ice Crystals Using a Laser-Diffraction Instrument  

NASA Technical Reports Server (NTRS)

The aircraft microphysics probe, PVM-100A, was tested in the Colorado State University dynamic cloud chamber to establish its ability to measure ice water content (IWC), PSA, and Re in ice clouds. Its response was compared to other means of measuring those ice-cloud parameters that included using FSSP-100 and 230-X 1-D optical probes for ice-crystal concentrations, a film-loop microscope for ice-crystal habits and dimensions, and an in-situ microscope for determining ice-crystal orientation. Intercomparisons were made in ice clouds containing ice crystals ranging in size from about 10 microns to 150 microns diameter, and ice crystals with plate, columnar, dendritic, and spherical shapes. It was not possible to determine conclusively that the PVM accurately measures IWC, PSA, and Re of ice crystals, because heat from the PVM evaporated in part the crystals in its vicinity in the chamber thus affecting its measurements. Similarities in the operating principle of the FSSP and PVM, and a comparison between Re measured by both instruments, suggest, however, that the PVM can make those measurements. The resolution limit of the PVM for IWC measurements was found to be on the order of 0.001 g/cubic m. Algorithms for correcting IWC measured by FSSP and PVM were developed.

Gerber, H.; DeMott, P. J.; Rogers, D. C.

1995-01-01

50

Dimensions and aspect ratios of natural ice crystals  

NASA Astrophysics Data System (ADS)

During the 2006 Tropical Warm Pool International Cloud Experiment (TWP-ICE) in the Tropics, the 2008 Indirect and Semi-Direct Aerosol Campaign (ISDAC) in the Arctic, and the 2010 Small PARTicles In CirrUS (SPARTICUS) campaign in mid-latitudes, high-resolution images of ice crystals were recorded by a Cloud Particle Imager at temperatures (T) between -87 and 0 °C. The projected maximum dimension (D'), length (L'), and width (W') of pristine columns, plates, and component bullets of bullet rosettes were measured using newly developed software, the Ice Crystal Ruler. The number of bullets in each bullet rosette was also measured. Column crystals were further distinguished as either horizontally oriented columns or columns with other orientations to eliminate any orientation effect on the measured dimensions. Dimensions and aspect ratios (AR, dimension of major axis divided by dimension of minor axis) of crystals were determined as functions of temperature, geophysical location, and type of cirrus. Dimensions of crystals generally increased as temperature increased. Columns and bullets had larger dimensions (i.e., W') of the minor axis (i.e., a axis) for a given dimension (i.e., D' or L') of the major axis (i.e., c axis), and thus smaller AR, as T increased, whereas this trend did not occur for plate crystals. The average number of branches in bullet rosettes was 5.50±1.35 during three campaigns and 6.32±1.34 (5.46±1.34; 4.95±1.01) during TWP-ICE (SPARTICUS; ISDAC). The AR of bullets increased with the number of branches in bullet rosettes. Most dimensions of crystals and ARs of columnar crystals measured during SPARTICUS were larger than those measured during TWP-ICE and ISDAC at -67 < T < -35 °C and at -40 < T < -15 °C, respectively. The relative occurrence of varying pristine habits depended strongly on cirrus type (i.e., anvil or non-anvil clouds), with plates especially occurring more frequently in anvils. The L-W relationships of columns derived using current data exhibited a strong dependence on temperature; similar relationship determined in previous studies were within the range of the current data.

Um, J.; McFarquhar, G. M.; Hong, Y. P.; Lee, S.-S.; Jung, C. H.; Lawson, R. P.; Mo, Q.

2014-12-01

51

Steady-state and stability analysis of a population balance based nonlinear ice cream crystallization model  

E-print Network

Steady-state and stability analysis of a population balance based nonlinear ice cream that adequately describes the key phenomena of the crystallization process. In ice cream crystallization, it is well known that the quality of the product, that is the hardness and the texture of the ice cream

Boyer, Edmond

52

Charge Transfer Process During Collision of Riming Graupel Pellet with Small Ice Crystals within a Thundercloud  

NASA Technical Reports Server (NTRS)

A charge transfer process during the collision of a riming graupel pellet and an ice-crystal at low temperature is proposed. During riming, the surface structure of graupel deviates from perfect crystalline structure. A concept of quasi-solid layer (QSL) formation on the surface is introduced. This QSL contains defects formed during riming. In absence of impurities, positively charged X-defect abundance is considered in the outer layer. These defects are assumed to be the charge carriers during the charge transfer process. Some part of the QSL is stripped off by the colliding ice crystals, which thereby gain some positive charge, leaving the graupel pellet negatively charged. With the proposed model, fC to pC of charge transfer is observed per collision. A transition temperature between -10 C to -15 C is also noted beyond which the QSL concept does not hold. This transition temperature is dependent on the bulk liquid water content of the cloud.

Datta, Saswati; De, Utpal K.; Goswami, K.; Jones, Linwood

1999-01-01

53

The Structural Properties of Vapor Deposited Water Ice and Astrophysical Implications  

NASA Technical Reports Server (NTRS)

Films of vapor deposited water ice at low temperature (T<30 K) show a number of interesting structural changes during a gradual warmup. We would like to talk about the structure of the low temperature high density amorphous form of water ice, the process of crystallization, and some recent work on the morphological changes of water ice films at high temperature. The studies of the high density amorphous form are from in-situ electron microscopy as well as numerical simulations of molecular dynamics and have lead to new insights into the physical distinction between this high density amorphous form and the low density amorphous form. For the process of crystallization, we propose a model that describes the crystallization of water ice from the amorphous phase to cubic ice in terms of the nucleation of small domains in the ice. This model agrees well with the behavior of water ice in our electron microscopy studies and finds that pure water above the glass transition is a strong liquid. In more recent work, we have concentrated on temperatures above the crystallization temperature and we find interesting morphological changes related to the decrease in viscosity of the amorphous component in the cubic crystalline regime. Given enough time, we would like to put these results in an astrophysical context and discuss some observed features of the frost on interstellar grains and the bulk ice in comets.

Jenniskens, P.; Blake, D. F.; Chang, Sherwood (Technical Monitor)

1996-01-01

54

Are longitudinal ice-surface structures on the Antarctic Ice Sheet indicators of long-term ice-flow configuration?  

NASA Astrophysics Data System (ADS)

Continent-wide mapping of longitudinal ice-surface structures on the Antarctic Ice Sheet reveals that they originate in the interior of the ice sheet and are arranged in arborescent networks fed by multiple tributaries. Longitudinal ice-surface structures can be traced continuously down-ice for distances of up to 1200 km. They are co-located with fast-flowing glaciers and ice streams that are dominated by basal sliding rates above tens of m yr-1 and are strongly guided by subglacial topography. Longitudinal ice-surface structures dominate regions of converging flow, where ice flow is subject to non-coaxial strain and simple shear. Associating these structures with the AIS' surface velocity field reveals (i) ice residence times of ~ 2500 to 18 500 years, and (ii) undeformed flow-line sets for all major flow units analysed except the Kamb Ice Stream and the Institute and Möller Ice Stream areas. Although it is unclear how long it takes for these features to form and decay, we infer that the major ice-flow and ice-velocity configuration of the ice sheet may have remained largely unchanged for several thousand years, and possibly even since the end of the last glacial cycle. This conclusion has implications for our understanding of the long-term landscape evolution of Antarctica, including large-scale patterns of glacial erosion and deposition.

Glasser, N. F.; Jennings, S. J. A.; Hambrey, M. J.; Hubbard, B.

2014-07-01

55

Optical Properties of Small Ice Crystals with Black Carbon Inclusions  

NASA Astrophysics Data System (ADS)

The optical properties of ice crystals play a fundamental role in modeling atmospheric radiation and hydrological cycle, which are critical in monitoring climate change. While Black Carbon (BC) is recognized as the dominant absorber with positive radiative forcing (warming) (Ramanathan & Carmichael, 2008), in-situ observations (Cappa, et al, 2012) indicate that the characterization of the mixing state of BC with ice crystals and other non-BC particles in global climate models (Ghan & Schwartz, 2007) needs further investigation. The limitation in the available mixing models is due to the drastically different absorbing properties of BC compared to other aerosols. We explore the scattering properties of ice crystals (in shapes commonly found in cirrus clouds and contrails - Yang, et al. 2012) with the inclusion of BC particles. The Discrete Dipole Approximation (DDA) (Yurkin & Hoekstra, 2011) is utilized to directly calculate the optical properties of the crystals with multiple BC inclusions, modeled as a distribution of spheres. The results are then compared with the most popular models of internal and external mixing (Liou, et al. 2011). The DDA calculations are carried out over a broad range of BC particle sizes and volume fractions within the crystal at the 532 nm wavelength and for ice crystals smaller than 50 ?m. The computationally intensive database generated in this study is critical for understanding the effect of different types of BC inclusions on the atmosphere radiative forcing. Examples will be discussed to illustrate the modification of BC optical properties by encapsulation in ice crystals and how the parameterization of the BC mixing state in global climate models can be improved. Acknowledgements Support by Sandia National Laboratories' LDRD (Laboratory Directed Research and Development) is gratefully acknowledged. Sandia National Laboratories is a multi-program 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. Cappa, C.D., Onasch, T.B., Massoli, et al. (2012). Radiative absorption enhancements due to the mixing state of atmospheric black carbon. Science, 337(6098), 1078-1081. Ghan, S.J., & Schwartz, S.E. (2007). Aerosol properties and processes: A path from field and laboratory measurements to global climate models. Bulletin of the American Meteorological Society, 88(7), 1059-1083. Liou, K.N., Takano, Y., & Yang, P. (2011). Light absorption and scattering by aggregates: Application to black carbon and snow grains. Journal of Quantitative Spectroscopy and Radiative Transfer, 112(10), 1581-1594. Ramanathan, V., & Carmichael, G. (2008). Global and regional climate changes due to black carbon. Nature Geoscience, 1(4), 221-227. Yang, P., Bi, L., Baum, B.A., et al. (2013). Spectrally Consistent Scattering, Absorption, and Polarization Properties of Atmospheric Ice Crystals at Wavelengths from 0.2 to 100 ? m. Journal of the Atmospheric Sciences, 70(1), 330-347. Yurkin, M.A., & Hoekstra, A.G. (2011). The discrete-dipole-approximation code ADDA: capabilities and known limitations. Journal of Quantitative Spectroscopy and Radiative Transfer, 112(13), 2234-2247.

Yang, X.; Geier, M.; Arienti, M.

2013-12-01

56

Optical detection and characterization of ice crystals in LACIS  

NASA Astrophysics Data System (ADS)

Tropospheric ice and mixed phase clouds are an integral part of the earth system and their microphysical and radiative properties are strongly coupled e.g. through the complexities of the ice nucleation process. Therefore the investigation of influences of different aerosol particles which act as ice nuclei (IN) on the freezing behaviour of cloud droplets is important and still poses unresolved questions. The Leipzig Aerosol and Cloud Interaction Simulator (LACIS) is used to investigate the IN activity of different natural and artificial aerosol particles (mineral dust, soot etc.) in heterogeneous freezing processes (immersion or deposition freezing). A critical part of LACIS is the particle detection system allowing for size-resolved counting of activated seed particles and discrimination between ice crystals and water droplets. Recently, two instruments have been developed to provide these measurements at the LACIS facility. The Thermally-stabilized Optical Particle Spectrometer (TOPS) is measuring the particle size based on the intensity of light scattered by individual particles into a near-forward (15° to 45°) direction. Two symmetrical forward scattering channels allow for optical determination of the sensing volume, thus reducing the coincidence counting error and the edge zone effect. The backscatter channel (162° to 176°) equipped with a rotatable cross polarizer allows for establishing the change in linear polarization state of the scattered light. The backscatter elevation angle is limited so that the linear depolarization of light scattered by spherical particles of arbitrary size is zero. Any detectable signal in the depolarization channel can be therefore attributed to non-spherical particles (ice crystals). With consideration of the signal in the backscatter channel the separate counting of water drops and ice particle is possible. The Leipzig Ice Scattering Apparatus (LISA) is a modified version of the Small Ice Detector (SID3), developed at the Science and Technology Research Institute at the University of Hertfordshire, UK. The SID instruments have been developed primarily as wing-mounted systems for airborne studies of cloud ice particles. SID3 records the forward scattered light pattern with high angular resolution using an intensified CCD (780 by 582 pixels) at a rate of 20 images per second. In addition to the SID3 capabilities, LISA is able to measure the circular depolarization ratio in the range of scattering angles from 166° to 172°. Whereas particle size, shape and orientation are characterized by the angular distribution of forward-scattered light, the measured value of the circular depolarization can be used to validate the existing theoretical models of light scattering by irregular particles (RTDF, GSVM, T-Matrix, DDA). The first measurements done at the LACIS facility have demonstrated a promising sensitivity of LISA's depolarization channel to the shape of ice crystals. Results showed an increase of the mean circular depolarization ratio from 1.5 (characteristic for the liquid water droplets above 3 µm) to 2.5 for the "just frozen" almost-spherical droplets in the same size range. The presentation will describe details of instruments set up and present some exemplary results from experiments carried out at LACIS and AIDA (KIT) facilities.

Kiselev, Alexei; Clauß, Tina; Niedermeier, Dennis; Hartmann, Susan; Wex, Heike; Stratmann, Frank

2010-05-01

57

Ice-Crystallization Kinetics during Fuel-Cell Cold-Start  

NASA Astrophysics Data System (ADS)

Proton-exchange-membrane fuel cells (PEMFCs) show promise in automotive applications because of their high efficiency, high power density, and potentially low emissions. To be successful in automobiles, PEMFCs must permit rapid startup with minimal energy from subfreezing temperatures, known as cold-start. In a PEMFC, reduction of oxygen to water occurs in the cathode catalyst layer (CL). Under subfreezing conditions, water generated during startup solidifies and hinders access of gaseous oxygen to the catalytic sites in the cathode CL, severely inhibiting cell performance and potentially causing cell failure. Achieving cold-start is difficult in practice, due to potential flooding, sluggish reaction kinetics, durability loss, and rapid ice crystallization. Currently, however, few studies focus on the fundamentals of ice crystallization during cold-start. Elucidation of the mechanisms and kinetics of ice formation within PEMFC porous media is, therefore, critical to successful cell startup and high performance at low temperatures. First, an experimental method is presented for obtaining isothermal ice-crystallization kinetics in water-saturated gas-diffusion layers (GDLs). Ice formation is initially studied in the GDL because this layer retains a significant amount of product water during cold-start. Isothermal ice-crystallization and ice-nucleation rates are obtained in commercial Toray GDLs as functions of subcooling using differential scanning calorimetry (DSC). A nonlinear ice-crystallization rate expression is developed using Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory, in which the heat-transfer-limited growth rate is determined from the moving-boundary Stefan problem. Predicted ice-crystallization rates are in excellent agreement with experiment. A validated rate expression is thus available for predicting ice-crystallization kinetics in GDLs. Ice-crystallization kinetics is also considered under experimental settings similar to real PEMFC operating conditions where ice invariably forms non-isothermally. Non-isothermal ice-crystallization rates and ice-crystallization temperatures are obtained in water-saturated GDLs as a function of cooling rate. Our previously developed ice-crystallization rate expression is extended to non-isothermal crystallization to predict ice-crystallization kinetics at various cooling rates. For non-isothermal ice formation, we find that cooling rate has a negligible effect on the crystallization rate when crystallization times are much faster than the time to decrease the sample temperature by the subcooling. Therefore, a pseudo-isothermal method is proposed for non-isothermal crystallization kinetics using isothermal crystallization kinetics evaluated at the non-isothermal crystallization temperature. Catalyst layers also retain a significant amount of product water during cold-start. Accordingly, ice nucleation and growth in PEMFC CLs are investigated using isothermal DSC and compared to isothermal galvanostatic membrane-electrode assembly (MEA) cold-starts. Measured ice-crystallization and ice-nucleation rates follow expected trends from classical nucleation theory. Following our previous approach, a quantitative nonlinear ice-crystallization rate expression is developed from the JMAK framework. To validate ice-crystallization kinetics within PEMFCs, we further measure and predict MEA cell-failure time during isothermal galvanostatic cold-start. Using a simplified PEMFC isothermal cold-start continuum model, MEA cell-failure times predicted using the newly obtained rate expression are compared to that predicted using a traditional thermodynamics-based approach. From this comparison, conditions are identified under which including ice-crystallization kinetics is critical and to elucidate the impact of freezing kinetics on low-temperature PEMFC operation. During cold-start, the time for recovering cell performance strongly depends on the rate of melting residual ice by reactive heat generation. Non-isothermal ice melting in water-saturated GDLs is investigated using DSC with various he

Dursch, Thomas James, Jr.

58

Surface and bulk diffusion of HDO on ultrathin single-crystal ice multilayers on Ru(001)  

Microsoft Academic Search

The kinetics of HDO surface and bulk diffusion on ultrathin (25-192 BL; 90-700 Å) single-crystal H216O ice multilayers were studied using a combination of laser-induced thermal desorption (LITD) probing and isothermal desorption depth-profiling. The single-crystal hexagonal ice multilayers were grown epitaxially on a single-crystal Ru(001) metal substrate with the basal (001) facet of ice parallel to the Ru(001) surface. HDO

Frank E. Livingston; Galen C. Whipple; Steven M. George

1998-01-01

59

Surface and bulk diffusion of HDO on ultrathin single-crystal ice multilayers on Ru(001)  

Microsoft Academic Search

The kinetics of HDO surface and bulk diffusion on ultrathin (25–192 BL; 90–700 Å) single-crystal H216O ice multilayers were studied using a combination of laser-induced thermal desorption (LITD) probing and isothermal desorption depth-profiling. The single-crystal hexagonal ice multilayers were grown epitaxially on a single-crystal Ru(001) metal substrate with the basal (001) facet of ice parallel to the Ru(001) surface. HDO

Frank E. Livingston; Galen C. Whipple; Steven M. George

1998-01-01

60

Ice Cream Structural Elements that Affect Melting Rate and Hardness  

Microsoft Academic Search

Statistical models were developed to reveal which structural elements of ice cream affect melting rate and hardness. Ice creams were frozen in a batch freezer with three types of sweetener, three levels of the emulsifier polysorbate 80, and two different draw temperatures to produce ice creams with a range of microstructures. Ice cream mixes were analyzed for viscosity, and finished

M. R. Muse; R. W. Hartel

2004-01-01

61

Crystallization of Cubic Ice in Liquid Water and Aqueous Solutions  

Microsoft Academic Search

It is often assumed that the only natural phase of ice that forms on Earth is that of hexagonal ice (ice Ih). However, the rare observation of haloes around the sun at 28o and cloud particles of cubic habit indicates that the metastable crystalline form of ice, cubic ice (ice Ic), may form in Earth's atmosphere. The conditions used to

B. J. Murray; D. A. Knopf; A. K. Bertram

2004-01-01

62

Inorganic Crystal Structure Database (ICSD)  

National Institute of Standards and Technology Data Gateway

SRD 84 FIZ/NIST Inorganic Crystal Structure Database (ICSD) (PC database for purchase)   The Inorganic Crystal Structure Database (ICSD) is produced cooperatively by the Fachinformationszentrum Karlsruhe(FIZ) and the National Institute of Standards and Technology (NIST). The ICSD is a comprehensive collection of crystal structure data of inorganic compounds containing more than 140,000 entries and covering the literature from 1915 to the present.

63

Structure and energetics of extended defects in ice Ih  

NASA Astrophysics Data System (ADS)

We consider the molecular structure and energetics of extended defects in proton-disordered hexagonal ice Ih. Using plane-wave density functional theory (DFT) calculations, we compute the energetics of stacking faults and determine the structure of the 30? and 90? partial dislocations on the basal plane. Consistent with experimental data, the formation energies of all fully reconstructed stacking faults are found to be very low. This is consistent with the idea that basal-plane glide dislocations in ice Ih are dissociated into partial dislocations separated by an area of stacking fault. For both types of partial dislocation we find a strong tendency toward core reconstruction through pairwise hydrogen-bond reformation. In the case of the 30? dislocation, the pairwise hydrogen-bond formation leads to a period-doubling core structure equivalent to that seen in zinc-blende semiconductor crystals. For the 90? partial we consider two possible core reconstructions, one in which the periodicity of the structure along the core remains unaltered and another in which it is doubled. The latter is preferred, although the energy difference between both is rather small, so that a coexistence of both reconstructions appears plausible. Our results imply that a mobility theory for dislocations on the basal plane in ice Ih should be based on the idea of reconstructed partial dislocations.

Silva Junior, Domingos L.; de Koning, Maurice

2012-01-01

64

Radar Sounding of Layering in Polar Ice Sheets: Possibilities and Limitations Considering the Dielectric Properties of Ice Crystals  

NASA Astrophysics Data System (ADS)

Radio frequency radar sounding is now an essential part of methodology to observe the ice sheets on polar continents. We can detect physical conditions within the several thousand meters thick ice sheets using proper platforms such as ground-based vehicles or airplanes. We can also extend information of ice-core data from a drilling point to wider regions of polar ice sheets. Ice sounding technique has its basis in the high-frequency dielectric properties of the target object, that is, ice and snow. In the past decade, new progress has appeared in understanding of the high-frequency electrical properties of water ice (Ih) through laboratory measurements. This progress consequently brought about significant progress in interpretation of the radar data from Antarctica. In particular, using multiple frequencies, we can distinguish reflections due to changes in dielectric permittivity and due to changes in electrical conductivity. Through this discrimination, we can deduce that what factor (density, impurity or crystal orientation fabrics) cause electromagnetic scattering. We introduce the physical principles of the ice-radar-data interpretation using actual data from a traverse line Dome F region to marginal area in Antarctica, and present knowledge of the dielectric properties of ice and its limitations. We mainly put stress on the knowledge of water ice and data from Antarctic ice sheet, considering application and extension of the knowledge to the Mars polar region.

Fujita, Shuji

2000-08-01

65

Crystal structure of propaquizafop  

PubMed Central

The title compound, C22H22ClN3O5 {systematic name: 2-(propan-2-yl­idene­amino­oxy)ethyl (R)-2-[4-(6-chloro­quin­oxalin-2-yl­oxy)phen­oxy]propionate}, is a herbicide. The asymmetric unit comprises two independent mol­ecules in which the dihedral angles between the phenyl ring and the quinoxaline ring plane are 75.93?(7) and 82.77?(8)°. The crystal structure features C—H?O, C—H?N, and C—H?Cl hydrogen bonds, as well as weak ?–? inter­actions [ring-centroid separation = 3.782?(2) and 3.5952?(19)?Å], resulting in a three-dimensional architecture. PMID:25553037

Jeon, Youngeun; Kim, Jineun; Lee, Sangjin; Kim, Tae Ho

2014-01-01

66

Long-term climatic changes indicated by crystal growth in polar ice  

Microsoft Academic Search

A new and independent way to obtain data on paleotemperatures from polar ice samples by using crystal size change profiles is proposed. From central Antarctica Dome C and Vostok ice cores data, it is suggested that crystal growth rate is mainly driven by a built-in 'memory' of the surface temperature conditions at the time of deposition. A semiempirical model of

J. R. Petit; P. Duval; C. Lorius

1987-01-01

67

Ice Crystal Concentration in Cumulus Clouds: Influence of the Drop Spectrum  

Microsoft Academic Search

Secondary ice crystals are thrown off when supercooled cloud drops are captured and freeze on a moving target in a cloud at -5 degrees C. The rate of production of these ice crystals is proportional to the rate of accretion of drops of diameter >= 24 micrometers.

S. C. Mossop; J. Hallett

1974-01-01

68

Microfluidic experiments reveal that antifreeze proteins bound to ice crystals suffice to prevent their growth.  

PubMed

Antifreeze proteins (AFPs) are a subset of ice-binding proteins that control ice crystal growth. They have potential for the cryopreservation of cells, tissues, and organs, as well as for production and storage of food and protection of crops from frost. However, the detailed mechanism of action of AFPs is still unclear. Specifically, there is controversy regarding reversibility of binding of AFPs to crystal surfaces. The experimentally observed dependence of activity of AFPs on their concentration in solution appears to indicate that the binding is reversible. Here, by a series of experiments in temperature-controlled microfluidic devices, where the medium surrounding ice crystals can be exchanged, we show that the binding of hyperactive Tenebrio molitor AFP to ice crystals is practically irreversible and that surface-bound AFPs are sufficient to inhibit ice crystal growth even in solutions depleted of AFPs. These findings rule out theories of AFP activity relying on the presence of unbound protein molecules. PMID:23300286

Celik, Yeliz; Drori, Ran; Pertaya-Braun, Natalya; Altan, Aysun; Barton, Tyler; Bar-Dolev, Maya; Groisman, Alex; Davies, Peter L; Braslavsky, Ido

2013-01-22

69

Modeling of Commercial Turbofan Engine With Ice Crystal Ingestion: Follow-On  

NASA Technical Reports Server (NTRS)

The occurrence of ice accretion within commercial high bypass aircraft turbine engines has been reported under certain atmospheric conditions. Engine anomalies have taken place at high altitudes that have been attributed to ice crystal ingestion, partially melting, and ice accretion on the compression system components. The result was degraded engine performance, and one or more of the following: loss of thrust control (roll back), compressor surge or stall, and flameout of the combustor. As ice crystals are ingested into the fan and low pressure compression system, the increase in air temperature causes a portion of the ice crystals to melt. It is hypothesized that this allows the ice-water mixture to cover the metal surfaces of the compressor stationary components which leads to ice accretion through evaporative cooling. Ice accretion causes a blockage which subsequently results in the deterioration in performance of the compressor and engine. The focus of this research is to apply an engine icing computational tool to simulate the flow through a turbofan engine and assess the risk of ice accretion. The tool is comprised of an engine system thermodynamic cycle code, a compressor flow analysis code, and an ice particle melt code that has the capability of determining the rate of sublimation, melting, and evaporation through the compressor flow path, without modeling the actual ice accretion. A commercial turbofan engine which has previously experienced icing events during operation in a high altitude ice crystal environment has been tested in the Propulsion Systems Laboratory (PSL) altitude test facility at NASA Glenn Research Center. The PSL has the capability to produce a continuous ice cloud which is ingested by the engine during operation over a range of altitude conditions. The PSL test results confirmed that there was ice accretion in the engine due to ice crystal ingestion, at the same simulated altitude operating conditions as experienced previously in flight. The computational tool was utilized to help guide a portion of the PSL testing, and was used to predict ice accretion could also occur at significantly lower altitudes. The predictions were qualitatively verified by subsequent testing of the engine in the PSL. In a previous study, analysis of select PSL test data points helped to calibrate the engine icing computational tool to assess the risk of ice accretion. This current study is a continuation of that data analysis effort. The study focused on tracking the variations in wet bulb temperature and ice particle melt ratio through the engine core flow path. The results from this study have identified trends, while also identifying gaps in understanding as to how the local wet bulb temperature and melt ratio affects the risk of ice accretion and subsequent engine behavior.

Jorgenson, Philip C. E.; Veres, Joseph P.; Coennen, Ryan

2014-01-01

70

Demonstration of novel polarization lidar technique for identifying horizontally oriented ice crystals  

NASA Astrophysics Data System (ADS)

Ice crystals are known to horizontally orient in the atmosphere when drag forces overcome the randomizing effects of Brownian motion. Such ice crystals have been shown to have an impact on radiative transfer, reflecting a greater portion of incident sunlight than their randomly oriented counter parts. However, regular identification of oriented ice crystals in the atmosphere has proven challenging. Existing lidar techniques rely on detection of strong specular backscatter from oriented platelets. These measurements are not common to most lidar systems, and are in fact, frequently avoided because such strong specular signals generally overwhelm lidar detector systems designed for typical cloud and aerosol studies. When lidars are tilted to avoid these specular returns, the low polarization ratio observed in some clouds consisting of oriented ice crystals will cause researchers to incorrectly conclude they are composed of liquid water, thereby skewing cloud phase statistics and providing an incorrect estimate of the cloud's impact on radiative transfer. To address these problems, we apply a novel lidar configuration, which provides a unique polarization capability that detects oriented ice crystals. By tilting the lidar off zenith and performing three polarization measurements, diattenuation, a polarization attribute only exhibited by oriented ice crystals, can be measured. This allows us to disambiguate clouds consisting of oriented ice crystals and water. We present here some of the first measurements of diattenuation for detection of oriented ice crystals as performed by the CAPABL lidar system in Summit Camp, Greenland. This polarization technique avoids detecting the strong specular reflections commonly used to identify oriented ice crystals, allowing return signals from oriented crystals to remain in the same dynamic range as other clouds and aerosols. This feature makes it possible for CAPABL to perform accurate, high performance measurements of all clouds and aerosols, even when oriented crystals are present.

Hayman, M. M.; Thayer, J. P.; Neely, R. R.; O'Neill, M.; Stillwell, R.

2011-12-01

71

Ice breaking in GPCR structural biology  

PubMed Central

G-protein-coupled receptors (GPCRs) are one of the most challenging targets in structural biology. To successfully solve a high-resolution GPCR structure, several experimental obstacles must be overcome, including expression, extraction, purification, and crystallization. As a result, there are only a handful of unique structures reported from this protein superfamily, which consists of over 800 members. In the past few years, however, there has been an increase in the amount of solved GPCR structures, and a few high-impact structures have been determined: the peptide receptor CXCR4, the agonist bound receptors, and the GPCR-G protein complex. The dramatic progress in GPCR structural studies is not due to the development of any single technique, but a combination of new techniques, new tools and new concepts. Here, we summarize the progress made for GPCR expression, purification, and crystallization, and we highlight the technical advances that will facilitate the future determination of GPCR structures. PMID:22286917

Zhao, Qiang; Wu, Bei-li

2012-01-01

72

Enhanced high-temperature ice nucleation ability of crystallized aerosol particles after preactivation at low temperature  

NASA Astrophysics Data System (ADS)

cloud chamber experiments with crystallized aqueous ammonium sulfate, oxalic acid, and succinic acid solution droplets, we have studied a preactivation mechanism that markedly enhances the particles' heterogeneous ice nucleation ability. First cloud expansion experiments were performed at a high temperature (267-244 K) where the crystallized particles did not promote any heterogeneous ice nucleation. Ice nucleation at this temperature, however, could be triggered by temporarily cooling the crystallized particles to a lower temperature. This is because upon crystallization, residuals of the aqueous solution are trapped within the crystals. These captured liquids can freeze when cooled below their respective homogeneous or heterogeneous freezing temperature, leading to the formation of ice pockets in the crystalline particles. When warmed again to the higher temperature, ice formation by the preactivated particles occurred via depositional and deliquescence-induced ice growth, with ice active fractions ranging from 1 to 4% and from 4 to 20%, respectively. Preactivation disappeared above the eutectic temperature, which for the organic acids are close to the melting point of ice. This mechanism could therefore contribute to the very small fraction of atmospheric aerosol particles that are still ice active well above 263 K.

Wagner, Robert; Möhler, Ottmar; Saathoff, Harald; Schnaiter, Martin

2014-07-01

73

Self-oscillatory ice crystal growth in antifreeze protein (AFP) and glycoprotein (AFGP) solutions  

NASA Astrophysics Data System (ADS)

AFPs and AFGPs allow many organisms including fish, plants and insects to survive sub-freezing environments. They occur in a wide range of compositions and structure, but to some extent they all accomplish the same functions: they suppress the freezing temperature, inhibit recrystallization, and modify ice crystal growth. A complete description of the AFGP/AFP surface mechanism as well as other ice surface phenomenon has eluded scientists primarily due to a lack of direct surface studies. We study ice crystal growth in AFGP and AFP solutions with phase contrast microscopy during free solution growth under various conditions including microgravity. Free-solution growth experiments show an anisotropic self-oscillatory growth mode of the steps and interface near the freezing temperature and enhancement of the growth rates in the c-axis. These results contradict the previous ?tight-binding? mechanism thought to be responsible for antifreeze function. To study the effects of temperature driven convective flows on the interface kinetics, microgravity experiments were performed in a jet airplane during a parabolic flight path. Step propagation on the basal plane slows down considerably when entering the microgravity condition and reaches a critical condition just below 0.2g.

Zepeda, Salvador; Nakaya, Hiroyuki; Uda, Yukihiro; Yokoyama, Etsuro; Furukawa, Yoshinori

2006-03-01

74

Demonstration of Crystal Structure.  

ERIC Educational Resources Information Center

Describes an experiment where equal parts of copper and aluminum are heated then cooled to show extremely large crystals. Suggestions are given for changing the orientation of crystals by varying cooling rates. Students are more receptive to concepts of microstructure after seeing this experiment. (DH)

Neville, Joseph P.

1985-01-01

75

Effects of ice crystals on the FSSP measurements in mixed phase clouds  

NASA Astrophysics Data System (ADS)

In this paper, we show that in mixed phase clouds FSSP-100 measurements may be contaminated by ice crystals, inducing wrong interpretation of particle size and subsequent bulk parameters. This contamination is generally revealed by a bimodal feature of the particle size distribution; in other words, in mixed phase clouds bimodal features could be an indication of the presence of ice particles. The combined measurements of the FSSP-100 and the Polar Nephelometer give a coherent description of the effect of the ice crystals on the FSSP-100 response. The FSSP-100 particle size distributions are characterized by a bimodal shape with a second mode peaked between 25 and 35 ?m related to ice crystals. This feature is observed with the FSSP-100 at airspeed up to 200 m s-1 and with the FSSP-300 series. In order to assess the size calibration for clouds of ice crystals the response of the FSSP-100 probe has been numerically simulated using a light scattering model of randomly oriented hexagonal ice particles and assuming both smooth and rough crystal surfaces. The results suggest that the second mode measured between 25 ?m and 35 ?m, does not necessarily represent true size responses but likely corresponds to bigger aspherical ice particles. According to simulation results, the sizing understatement would be neglected in the rough case but would be major with the smooth case. Qualitatively, the Polar Nephelometer phase function suggests that the rough case is the more suitable to describe real crystals. Quantitatively, however, it is difficult to conclude. Previous cloud in situ measurements suggest that the FSSP-100 secondary mode, peaked in the range 25-35 ?m, is likely to be due to the shattering of large ice crystals on the probe tips. This finding is supported by the rather good relationship between the concentration of particles larger than 20 ?m (hypothesized to be ice shattered-fragments measured by the FSSP) and the concentration of (natural) ice particles larger than 100 ?m (CPI data). The shattering efficiency is defined as the ratio of the measured ice shattered-fragments to the number of natural ice particles (with d>100 ?m) impacting the probe leading edge. In the present study the shattering efficiency is evaluated to ~7%. It is found that about 400 ice fragments may result from the shattering of one equivalent irregular shaped ice crystal with a mean volume diameter of 310 ?m. Obviously, these values could be strongly dependent on the inlet design, the airspeed and the robustness of ice crystals via the impact kinetic energy to surface energy ratio providing the particle breakup.

Febvre, G.; Gayet, J.-F.; Shcherbakov, V.; Gourbeyre, C.; Jourdan, O.

2012-03-01

76

Ikaite crystal distribution in Arctic winter sea ice and implications for CO2 system dynamics  

NASA Astrophysics Data System (ADS)

The precipitation of ikaite (CaCO3·6H2O) in polar sea ice is critical to the efficiency of the sea ice-driven carbon pump and potentially important to the global carbon cycle, yet the spatial and temporal occurrence of ikaite within the ice is poorly known. We report unique observations of ikaite in unmelted ice and vertical profiles of ikaite abundance and concentration in sea ice for the crucial season of winter. Ice was examined from two locations: a 1 m thick land-fast ice site and a 0.3 m thick polynya site, both in the Young Sound area (74° N, 20° W) of NE Greenland. Ikaite crystals, ranging in size from a few µm to 700 µm were observed to concentrate in the interstices between the ice platelets in both granular and columnar sea ice. In vertical sea-ice profiles from both locations, ikaite concentration determined from image analysis, decreased with depth from surfaceice values of 700-900 µmol kg-1 ice (~ 25 × 106 crystals kg-1) to bottom-layer values of 100-200 µmol kg-1 ice (1-7 × 106 kg-1), all of which are much higher (4-10 times) than those reported in the few previous studies. Direct measurements of total alkalinity (TA) in surface layers fell within the same range as ikaite concentration whereas TA concentrations in bottom layers were twice as high. This depth-related discrepancy suggests interior ice processes where ikaite crystals form in surface sea ice layers and partly dissolved in bottom layers. From these findings and model calculations we relate sea ice formation and melt to observed pCO2 conditions in polar surface waters, and hence, the air-sea CO2 flux.

Rysgaard, S.; Søgaard, D. H.; Cooper, M.; Pu?ko, M.; Lennert, K.; Papakyriakou, T. N.; Wang, F.; Geilfus, N. X.; Glud, R. N.; Ehn, J.; McGinnnis, D. F.; Attard, K.; Sievers, J.; Deming, J. W.; Barber, D.

2012-12-01

77

Soap Froths and Crystal Structures  

Microsoft Academic Search

We propose a physical mechanism to explain the crystal symmetries found in macromolecular and supramolecular micellar materials. We argue that the packing entropy of the hard micellar cores is frustrated by the entropic interaction of their brushlike coronas. The latter interaction is treated as a surface effect between neighboring Voronoi cells. The observed crystal structures correspond to the Kelvin and

Primoz Ziherl; Randall D. Kamien

2000-01-01

78

Structure of Water Ice in the Solar System  

NASA Technical Reports Server (NTRS)

Nearly all of the properties of solar system ices (chemical reaction rates, volatile retention and release, vaporization behavior, thermal conductivity, infrared spectral characteristics and the like) are a direct consequence of ice structure. However, the characterization of astrophysical ices and their laboratory analogs has typically utilized indirect measurements which yield phenomenological interpretations. When water ice is vapor-deposited at 14 K and warmed until it volatilizes in moderate vacuum, the ice undergoes a series of amorphous to amorphous and amorphous to crystalline structural transitions which we have characterized by diffraction methods. These structural transitions correlate with and underlie many phenomena observed in laboratory infrared and gas release experiments. The elucidation of the dynamic structural changes which occur in vapor-deposited water ice as a function of time, temperature and radiation history allows for the more complete interpretation of remote observations of astrophysical ices and their laboratory analogs.

Blake, David; Jenniskens, Peter; Chang, Sherwood (Technical Monitor)

1996-01-01

79

On the importance of small ice crystals in tropical anvil cirrus  

Microsoft Academic Search

In situ measurements of ice crystal concentrations and sizes made with aircraft instrumentation over the past two decades have often indicated the presence of numerous relatively small (< 50 mum diameter) crystals in cirrus clouds. Further, these measurements frequently indicate that small crystals account for a large fraction of the extinction in cirrus clouds. The fact that the instruments used

E. J. Jensen; P. Lawson; B. Baker; B. Pilson; Q. Mo; A. J. Heymsfield; A. Bansemer; T. P. Bui; M. McGill; D. Hlavka; G. Heymsfield; S. Platnick; G. T. Arnold; S. Tanelli

2009-01-01

80

Some effects of ice crystals on the FSSP measurements in mixed phase clouds  

NASA Astrophysics Data System (ADS)

In this paper, we show that in mixed phase clouds, the presence of ice crystals may induce wrong FSSP 100 measurements interpretation especially in terms of particle size and subsequent bulk parameters. The presence of ice crystals is generally revealed by a bimodal feature of the particle size distribution (PSD). The combined measurements of the FSSP-100 and the Polar Nephelometer give a coherent description of the effect of the ice crystals on the FSSP-100 response. The FSSP-100 particle size distributions are characterized by a bimodal shape with a second mode peaked between 25 and 35 ?m related to ice crystals. This feature is observed with the FSSP-100 at airspeed up to 200 m s-1 and with the FSSP-300 series. In order to assess the size calibration for clouds of ice crystals the response of the FSSP-100 probe has been numerically simulated using a light scattering model of randomly oriented hexagonal ice particles and assuming both smooth and rough crystal surfaces. The results suggest that the second mode, measured between 25 ?m and 35 ?m, does not necessarily represent true size responses but corresponds to bigger aspherical ice particles. According to simulation results, the sizing understatement would be neglected in the rough case but would be significant with the smooth case. Qualitatively, the Polar Nephelometer phase function suggests that the rough case is the more suitable to describe real crystals. Quantitatively, however, it is difficult to conclude. A review is made to explore different hypotheses explaining the occurrence of the second mode. However, previous cloud in situ measurements suggest that the FSSP-100 secondary mode, peaked in the range 25-35 ?m, is likely to be due to the shattering of large ice crystals on the probe inlet. This finding is supported by the rather good relationship between the concentration of particles larger than 20 ?m (hypothesized to be ice shattered-fragments measured by the FSSP) and the concentration of (natural) ice particles (CPI data). In mixed cloud, a simple estimation of the number of ice crystals impacting the FSSP inlet shows that the ice crystal shattering effect is the main factor in observed ice production.

Febvre, G.; Gayet, J.-F.; Shcherbakov, V.; Gourbeyre, C.; Jourdan, O.

2012-10-01

81

Soap froths and crystal structures  

PubMed

We propose a physical mechanism to explain the crystal symmetries found in macromolecular and supramolecular micellar materials. We argue that the packing entropy of the hard micellar cores is frustrated by the entropic interaction of their brushlike coronas. The latter interaction is treated as a surface effect between neighboring Voronoi cells. The observed crystal structures correspond to the Kelvin and Weaire-Phelan minimal foams. We show that these structures are stable for reasonable areal entropy densities. PMID:11030938

Ziherl; Kamien

2000-10-16

82

Stability relationship for water droplet crystallization with the NASA Lewis icing spray  

NASA Technical Reports Server (NTRS)

In order to produce small droplets for icing cloud simulation, high pressure air atomizing nozzles are used. For certain icing testing applications, median drop sizes as small as 5 mm are needed, which require air atomizing pressures greater than 3000 kPa. Isentropic expansion of the ambient temperature atomizing air to atmospheric pressure can result in air stream temperatures of -160 C which results in ice crystals forming in the cloud. To avoid such low temperatures, it is necessary to heat the air and water to high initial temperatures. An icing spray research program was conducted to map the temperatures below which ice crystals form. A soot slide technique was used to determine the presence of crystals in the spray.

Marek, C. John; Bartlett, C. Scott

1987-01-01

83

The structure of internal stresses in the uncompacted ice cover  

SciTech Connect

Interactions between engineering structures and sea ice cover are associated with an inhomogeneous space/time field of internal stresses. Field measurements (e.g., Coon, 1989; Tucker, 1992) have revealed considerable local stresses depending on the regional stress field and ice structure. These stresses appear in different time and space scales and depend on rheologic properties of the ice. To estimate properly the stressed state a knowledge of a connection between internal stress components in various regions of the ice cover is necessary. To develop reliable algorithms for estimates of ice action on engineering structures new experimental data are required to take into account both microscale (comparable with local ice inhomogeneities) and small-scale (kilometers) inhomogeneities of the ice cover. Studies of compacted ice (concentration N is nearly 1) are mostly important. This paper deals with the small-scale spatial distribution of internal stresses in the interaction zone between the ice covers of various concentrations and icebergs. The experimental conditions model a situation of the interaction between a wide structure and the ice cover. Field data on a drifting ice were collected during the Russian-US experiment in Antarctica WEDDELL-I in 1992.

Sukhorukov, K.K. [Arctic and Antarctic Research Inst., St. Petersburg (Russian Federation)

1995-12-31

84

Arabidopsis thaliana ICE2 gene: Phylogeny, structural evolution and functional diversification from ICE1.  

PubMed

The ability to tolerate environmental stresses is crucial for all living organisms, and gene duplication is one of the sources for evolutionary novelties. Arabidopsis thaliana INDUCER OF CBF EXPRESSION1 and 2 (ICE1 and ICE2) encode MYC-type bHLH (basic helix-loop-helix) transcription factors. They confer cold stress tolerance by induction of the CBF/DREB1 regulon and regulate stomata formation. Although ICE2 is closely related to ICE1, its origin and role in cold response remains uncertain. Here, we used a bioinformatics/phylogenetic approach to uncover the ICE2 evolutionary history, structural evolution and functional divergence from the putative ancestral gene. Sequence diversification from ICE1 included the gain of cis-acting elements in ICE2 promoter sequence that may provide meristem-specific and defense-related gene expression. By analyzing transgenic Arabidopsis lines with ICE2 over-expression we showed that it contributes to stomata formation, flowering time regulation and cold response. Constitutive ICE2 expression led to induced meristem freezing tolerance, resulting from activation of CBF1 and CBF3 genes and ABA biosynthesis by NCED3 induction. We presume that ICE2 gene has originated from a duplication event about 17.9MYA followed by sub- and neofunctionalization of the ancestral ICE1 gene. Moreover, we predict its role in pathogen resistance and flowering time regulation. PMID:25443829

Kurbidaeva, Amina; Ezhova, Tatiana; Novokreshchenova, Maria

2014-12-01

85

II. Properties of Water 1. Ice and Liquid water structure  

E-print Network

II. Properties of Water 1. Ice and Liquid water structure 2. Cohesion / Surface Tension 3. High and Properties Water's molecular structure and capacity to donate and accept hydrogen bonds give it unusual.6: Five Critical Properties of Water 1. Ice and liquid water structure Temperature 0°C Temperature > 0°C

Frey, Terry

86

Modal Analysis of the Ice-Structure Interaction Problem  

E-print Network

Modal Analysis of the Ice-Structure Interaction Problem LT Michael Anthony Venturella A thesis-structure interaction, modal analysis, Poincaré mapping, recurrence plot) Copyright 2008, Michael A. Venturella #12;Modal Analysis of the Ice-Structure Interaction Problem Michael A. Venturella Abstract In the present

Patil, Mayuresh

87

A theoretical spongy spray icing model with surficial structure  

NASA Astrophysics Data System (ADS)

In an attempt to improve the predictive capability of atmospheric icing models, we have developed a theoretical model of spongy ice formation, including the surficial morphology under a falling supercooled liquid film. A steady-state model of freshwater spongy spray icing for a stationary vertical cylinder is presented. A falling film submodel accounts for the flow of excess liquid on the icing surface. Traditional heat and mass balance equations at the outer surface of the falling film are formulated, along with heat and mass balances for the falling film and for the dendritic freezing zone. The rate of advance of the icing interface is calculated by analogy with the rate of advance of freely-growing ice crystals in bulk supercooled liquid. This allows, for the first time, the prediction of ice accretion flux and accretion sponginess, for a specific icing configuration and environmental conditions. An analysis of the model's sensitivity to spray temperature reveals that spray supercooling enhances both the rate of accretion and its sponginess. A comparison of the model's performance with experiments shows rather good agreement, and suggests that further research into the nature of the icing surface and its effect on the accreted ice is warranted.

Blackmore, R. Z.; Lozowski, E. P.

88

Average ice crystal size and bulk short-wave single-scattering properties of cirrus clouds  

Microsoft Academic Search

The bulk single-scattering properties of cirrus clouds required for driving the radiation scheme in large-scale climate models are computed with respect to various size distributions and ice crystal shapes. It is shown that the average ice crystal size, defined as the ratio of total volume to the total projected area, can well-characterize the effect of various size distributions in determining

Klaus Wyser; Ping Yang

1998-01-01

89

A Theoretical Determination of the Capture Efficiency of Small Columnar Ice Crystals by Large Cloud Drops  

Microsoft Academic Search

A theoretical model has been formulated to study by numerical techniques the efficiency E with which columnar ice crystals grown at temperatures between 3 and 8°C are captured in a cloud by relatively large, supercooled cloud drops. The ice crystals studied had lengths L of 15 L 240 m and diameters D of 1.5 D 240 m. and L\\/D values

Jeffrey K. Lew; Hans R. Pruppacher

1983-01-01

90

Nanoscale structure of the magnetic induction at monopole defects in artificial spin-ice lattices.  

SciTech Connect

Artificially frustrated spin-ice systems are of considerable interest since they simulate the spin frustration and concomitant rich behavior exhibited by atoms on a crystal lattice in naturally occurring spin-ice systems such as pyrochlores. As a result of the magnetic frustration, these systems can exhibit 'magnetic monopole' type defects, which are an example of an exotic emergent quasiparticle. The local magnetization structure of such monopole defects determines their stability and thus is critical to understanding their behavior. In this paper, we report on the direct observation at room temperature of the nanoscale magnetic structure of individual magnetic monopoles in an artificially frustrated two-dimensional square spin-ice lattice, using high-resolution aberration-corrected Lorentz transmission electron microscopy. By combining the high-resolution microscopy with micromagnetic simulation, we demonstrate how nucleation of defect strings, reminiscent of Dirac strings, connecting monopole defects controls the demagnetization process in these spin-ice lattices.

Phatak, C.; Petford-Long, A. K.; Tanase, M.; Heinonen, O.; De Graef, M. (Center for Nanoscale Materials); ( MSD); (Carnegie Mellon Univ.); (Northwestern Univ.); (NIST)

2011-05-18

91

Trapping of trace gases in growing ice crystals  

Microsoft Academic Search

An analytical model describing the combined effect of mass accomodation and net adsorption of trace gases on the surfaces of growing ice particles (trapping) is developed. An approximate solution for the release of trapped trace gases from evaporating ice particles is also given. The model fully accounts for the fact that atmospheric ice particles frequently experience substantial subsaturations and supersaturations.

B. Kärcher; M. M. Basko

2004-01-01

92

Structural Basis for Antifreeze Activity of Ice-binding Protein from Arctic Yeast*  

PubMed Central

Arctic yeast Leucosporidium sp. produces a glycosylated ice-binding protein (LeIBP) with a molecular mass of ?25 kDa, which can lower the freezing point below the melting point once it binds to ice. LeIBP is a member of a large class of ice-binding proteins, the structures of which are unknown. Here, we report the crystal structures of non-glycosylated LeIBP and glycosylated LeIBP at 1.57- and 2.43-? resolution, respectively. Structural analysis of the LeIBPs revealed a dimeric right-handed ?-helix fold, which is composed of three parts: a large coiled structural domain, a long helix region (residues 96–115 form a long ?-helix that packs along one face of the ?-helix), and a C-terminal hydrophobic loop region (243PFVPAPEVV251). Unexpectedly, the C-terminal hydrophobic loop region has an extended conformation pointing away from the body of the coiled structural domain and forms intertwined dimer interactions. In addition, structural analysis of glycosylated LeIBP with sugar moieties attached to Asn185 provides a basis for interpreting previous biochemical analyses as well as the increased stability and secretion of glycosylated LeIBP. We also determined that the aligned Thr/Ser/Ala residues are critical for ice binding within the B face of LeIBP using site-directed mutagenesis. Although LeIBP has a common ?-helical fold similar to that of canonical hyperactive antifreeze proteins, the ice-binding site is more complex and does not have a simple ice-binding motif. In conclusion, we could identify the ice-binding site of LeIBP and discuss differences in the ice-binding modes compared with other known antifreeze proteins and ice-binding proteins. PMID:22303017

Lee, Jun Hyuck; Park, Ae Kyung; Do, Hackwon; Park, Kyoung Sun; Moh, Sang Hyun; Chi, Young Min; Kim, Hak Jun

2012-01-01

93

Tropical tropopause ice clouds: A new approach to answer the mystery of low crystal numbers  

NASA Astrophysics Data System (ADS)

Water vapour is the most important natural green house gas. However, in the stratosphere an increase in water vapour would possibly result in a net cooling of the earth-atmosphere system. The major entrance pathway of trace substances into the stratosphere is the tropical tropopause layer (TTL). The TTL water vapor budget, and thus the exchange between troposphere and stratosphere, depends crucially on the occurrence and properties of ice clouds in this cold region (T < 200 K). New observations indicate that very low ice crystal numbers frequently occur in the TTL. This phenomenon is not yet understood and is not compatible with the idea that homogeneous freezing of solution droplets is the major pathway of ice formation. These low ice number concentrations are consistent with observed persistent high ice supersaturations inside cold TTL cirrus clouds, which in turn control the exchange of water vapor with the stratosphere. Here, we reproduce in-situ measurements of frequencies of occurrence of ice crystal concentrations by extensive model simulations, driven by the special dynamical conditions in the TTL, namely the superposition of slow large-scale updrafts with high-frequency short waves. The simulations show that about 80% of the observed incidences of low ice crystal concentrations can be explained by 'classical' homogeneous ice nucleation in the very slow updrafts (< 1cm/s), about 19% stem from heterogeneous freezing, while the remaining of about 1% originates from homogeneous freezing in slightly faster updrafts (> 1cm/s). The mechanism limiting the ice crystal production from homogeneous freezing in an environment full of gravity waves is that freezing events are stalled -due to the shortness of the gravity waves- before a higher number concentration of ice crystals can be formed.

Spichtinger, Peter; Krämer, Martina

2013-04-01

94

Ice-binding proteins: a remarkable diversity of structures for stopping and starting ice growth.  

PubMed

Antifreeze proteins (AFPs) were discovered in marine fishes that need protection from freezing. These ice-binding proteins (IBPs) are widespread across biological kingdoms, and their functions include freeze tolerance and ice adhesion. Consistent with recent independent evolution, AFPs have remarkably diverse folds that rely heavily on hydrogen- and disulfide-bonding. AFP ice-binding sites are typically flat, extensive, relatively hydrophobic, and are thought to organize water into an ice-like arrangement that merges and freezes with the quasi-liquid layer next to the ice lattice. In this article, the roles, properties, and structure-function interactions of IBPs are reviewed, and their relationship to ice nucleation proteins, which promote freezing at high subzero temperatures, is explored. PMID:25440715

Davies, Peter L

2014-11-01

95

Spray-ice islands evaluated for Arctic-drilling structures  

SciTech Connect

Comparisons of spray-ice drilling structures for the Beaufort Sea with drilling structures presently in use there show that over a wide range of water depths drilling from spray-ice islands offers significant benefits over the existing alternatives. Both technical and economic comparisons were made. In a water depth of 50 ft, a gravel island for drilling an exploration hole would likely cost between $40 and $60 million. A spray-ice island in the same location would cost less than $10 million. ''Spray-ice'' as used in this article refers to ice made by pumping sea water high up into the air for rapid heat transfer and freezing. The resulting ice has a granular structure and is quite porous.

Juvkam-Wold, H.C.

1986-04-21

96

A lattice model to simulate ice-structure interaction  

E-print Network

at drawing some conclusions about the forces exerted on large-scale structures such as oil rigs such a model seems able to capture the main trends involved in ice sheets cracking. This is an essential step of the forces exerted by ice sheets on oil rigs is of great interest for designers of offshore structures. Over

Boyer, Edmond

97

Crystal structure refinement with SHELXL  

PubMed Central

The improvements in the crystal structure refinement program SHELXL have been closely coupled with the development and increasing importance of the CIF (Crystallographic Information Framework) format for validating and archiving crystal structures. An important simplification is that now only one file in CIF format (for convenience, referred to simply as ‘a CIF’) containing embedded reflection data and SHELXL instructions is needed for a complete structure archive; the program SHREDCIF can be used to extract the .hkl and .ins files required for further refinement with SHELXL. Recent developments in SHELXL facilitate refinement against neutron diffraction data, the treatment of H atoms, the determination of absolute structure, the input of partial structure factors and the refinement of twinned and disordered structures. SHELXL is available free to academics for the Windows, Linux and Mac OS X operating systems, and is particularly suitable for multiple-core processors. PMID:25567568

Sheldrick, George M.

2015-01-01

98

Crystal structure refinement with SHELXL.  

PubMed

The improvements in the crystal structure refinement program SHELXL have been closely coupled with the development and increasing importance of the CIF (Crystallographic Information Framework) format for validating and archiving crystal structures. An important simplification is that now only one file in CIF format (for convenience, referred to simply as `a CIF') containing embedded reflection data and SHELXL instructions is needed for a complete structure archive; the program SHREDCIF can be used to extract the .hkl and .ins files required for further refinement with SHELXL. Recent developments in SHELXL facilitate refinement against neutron diffraction data, the treatment of H atoms, the determination of absolute structure, the input of partial structure factors and the refinement of twinned and disordered structures. SHELXL is available free to academics for the Windows, Linux and Mac OS X operating systems, and is particularly suitable for multiple-core processors. PMID:25567568

Sheldrick, George M

2015-01-01

99

Crystal structure of fenpropathrin  

PubMed Central

In the title compound [systematic name: cyano­(3-phen­oxy­phen­yl)methyl 2,2,3,3-tetra­methyl­cyclo­propane­carboxyl­ate], C22H23NO3, which is the pyrethroid insecticide fenpropathrin, the dihedral angle between the cyclo­propane ring plane and the carboxyl­ate group plane is 88.25?(11)°. The dihedral angle between the benzene and phenyl rings in the phen­oxy­benzyl group is 82.99?(4)°. In the crystal, C—H?N hydrogen bonds and weak C—H?? inter­actions link adjacent mol­ecules, forming loop chains along the b-axis direction. PMID:25553036

Kang, Gihaeng; Jeon, Youngeun; Lee, Sangjin; Kim, Tae Ho

2014-01-01

100

The Ice Selective Inlet: a novel technique for exclusive extraction of pristine ice crystals in mixed-phase clouds  

NASA Astrophysics Data System (ADS)

Climate predictions are affected by high uncertainties partially due to an insufficient knowledge of aerosol-cloud interactions. One of the poorly understood processes is formation of mixed-phase clouds (MPCs) via heterogeneous ice nucleation. Field measurements of the atmospheric ice phase in MPCs are challenging due to the presence of supercooled liquid droplets. The Ice Selective Inlet (ISI), presented in this paper, is a novel inlet designed to selectively sample pristine ice crystals in mixed-phase clouds and extract the ice residual particles contained within the crystals for physical and chemical characterisation. Using a modular setup composed of a cyclone impactor, droplet evaporation unit and pumped counterflow virtual impactor (PCVI), the ISI segregates particles based on their inertia and phase, exclusively extracting small ice particles between 5 and 20 ?m in diameter. The setup also includes optical particle spectrometers for analysis of the number size distribution and shape of the sampled hydrometeors. The novelty of the ISI is a droplet evaporation unit, which separates liquid droplets and ice crystals in the airborne state, thus avoiding physical impaction of the hydrometeors and limiting potential artifacts. The design and validation of the droplet evaporation unit is based on modelling studies of droplet evaporation rates and computational fluid dynamics simulations of gas and particle flows through the unit. Prior to deployment in the field, an inter-comparison of the WELAS optical particle size spectrometers and a characterisation of the transmission efficiency of the PCVI was conducted in the laboratory. The ISI was subsequently deployed during the Cloud and Aerosol Characterisation Experiment (CLACE) 2013 - an extensive international field campaign encompassing comprehensive measurements of cloud microphysics, as well as bulk aerosol, ice residual and ice nuclei properties. The campaign provided an important opportunity for a proof of concept of the inlet design. In this work we present the setup of the ISI, including the modelling and laboratory characterisation of its components, as well as a case study demonstrating the ISI performance in the field during CLACE 2013.

Kupiszewski, P.; Weingartner, E.; Vochezer, P.; Bigi, A.; Rosati, B.; Gysel, M.; Schnaiter, M.; Baltensperger, U.

2014-12-01

101

Technical Note: Formation of airborne ice crystals in a wall independent reactor (WIR) under atmospheric conditions  

NASA Astrophysics Data System (ADS)

Both, gas and particle scavenging contribute to the transport of organic compounds by ice crystals in the troposphere. To simulate these processes an experimental setup was developed to form airborne ice crystals under atmospheric conditions. Experiments were performed in a wall independent reactor (WIR) installed in a walk-in cold chamber maintained constantly at -20°C. Aerosol particles were added to the carrier gas of ambient air by an aerosol generator to allow heterogeneous ice formation. Temperature variations and hydrodynamic conditions of the WIR were investigated to determine the conditions for ice crystal formation and crystal growth by vapour deposition. In detail, the dependence of temperature variations from flow rate and temperature of the physical wall as well as temperature variations with an increasing reactor depth were studied. The conditions to provide a stable aerosol concentration in the carrier gas flow were also studied. The temperature distribution inside the reactor was strongly dependent on flow rate and physical wall temperature. At an inlet temperature of -20°C, a flow rate of 30 L•min-1 and a physical wall temperature of +5°C turned out to provide ideal conditions for ice formation. At these conditions a sharp and stable laminar down draft "jet stream" of cold air in the centre of the reactor was produced. Temperatures measured at the chamber outlet were kept well below the freezing point in the whole reactor depth of 1.0 m. Thus, melting did not affect ice formation and crystal growth. The maximum residence time for airborne ice crystals was calculated to at 40 s. Ice crystal growth rates increased also with increasing reactor depth. The maximum ice crystal growth rate was calculated at 2.82 mg• s-1. Further, the removal efficiency of the cleaning device for aerosol particles was 99.8% after 10 min. A reliable particle supply was attained after a preliminary lead time of 15 min. Thus, the minimum lead time was determined at 25 min. Several test runs revealed that the WIR is suitable to perform experiments with airborne ice crystals.

Fries, E.; Haunold, W.; Starokozhev, E.; Palitzsch, K.; Sitals, R.; Jaeschke, W.; Püttmann, W.

2008-07-01

102

The role of sea ice in structuring Antarctic ecosystems  

Microsoft Academic Search

This paper focusses on the links between growth, persistence and decay of sea ice and the structure of Antarctic marine ecosystems on different spatial and temporal scales. Sea-ice growth may divide an oceanic ecosystem into two dissimilar compartments: (1) the water column, with primary production controlled by the reduction of irradiative fluxes due to the snow-laden sea-ice cover and thermo-haline

Hajo Eicken

1992-01-01

103

Arctic ice shelves and ice islands: Origin, growth and disintegration, physical characteristics, structural-stratigraphic variability, and dynamics  

SciTech Connect

Ice shelves are thick, floating ice masses most often associated with Antarctica where they are seaward extensions of the grounded Antarctic ice sheet and sources of many icebergs. However, there are also ice shelves in the Arctic, primarily located along the north coast of Ellesmere Island in the Canadian High Arctic. The only ice shelves in North America and the most extensive in the north polar region, the Ellesmere ice shelves originate from glaciers and from sea ice and are the source of ice islands, the tabular icebergs of the Arctic Ocean. The present state of knowledge and understanding of these ice features is summarized in this paper. It includes historical background to the discovery and early study of ice shelves and ice islands, including the use of ice islands as floating laboratories for polar geophysical research. Growth mechanisms and age, the former extent and the twentieth century disintegration of the Ellesmere ice shelves, and the processes and mechanisms of ice island calving are summarized. Surface features, thickness, thermal regime, and the size, shape, and numbers of ice islands are discussed. The structural-stratigraphic variability of ice islands and ice shelves and the complex nature of their growth and development are described. Large-scale and small-scale dynamics of ice islands are described, and the results of modeling their drift and recurrence intervals are presented. The conclusion identifies some unanswered questions and future research opportunities and needs. 97 refs., 18 figs.

Jeffries, M.O. (Univ. of Alaska, Fairbanks (United States))

1992-08-01

104

Crystal structure of difenoconazole.  

PubMed

In the title compound difenoconazole [systematic name: 1-({2-[2-chloro-4-(4-chloro-phen-oxy)phen-yl]-4-methyl-1,3-dioxolan-2-yl}meth-yl)-1H-1,2,4-triazole], C19H17Cl2N3O3, the dihedral angle between the planes of the 4-chloro-phenyl and 2-chloro-phenyl rings is 79.34?(9)°, while the dihedral angle between the planes of the triazole ring and the dioxolanyl group is 59.45?(19)°. In the crystal, pairs of C-H?N hydrogen bonds link adjacent mol-ecules, forming dimers with R 2 (2)(6) loops. In addition, the dimers are linked by C-H?O hydrogen bonds, resulting in a three-dimensional architecture. Disorder was modeled for one C atom of the dioxolanyl group over two sets of sites with an occupancy ratio of 0.566?(17):0.434?(17). PMID:25484812

Cho, Seonghwa; Kang, Gihaeng; Lee, Sangjin; Kim, Tae Ho

2014-11-01

105

Ice rule correlations in stuffed spin ice  

NASA Astrophysics Data System (ADS)

Stuffed spin ice is a chemical variation of a spin ice material like Ho2Ti2O7 in which extra magnetic ions are inserted into the crystal structure. Previous studies have shown that the degree of stuffing has very little effect on the residual entropy in the system, which takes a value very close to the spin ice entropy. We argue, however, that the observation of this entropy does not imply long range coherence of the ice rules, that determine the local spin configurations. We have characterized deviations from the ice rules by means of a polarized neutron diffraction study of a single crystal of Ho2+?Ti2-?O7-?/2 with ? = 0.3. Our results demonstrate that the ice rules in stuffed spin ice are strictly valid only over a relatively short range, and that at longer range stuffed spin ice exhibits some characteristics of a ‘cluster glass’, with a tendency to more conventional ferromagnetic correlations.

Aldus, R. J.; Fennell, T.; Deen, P. P.; Ressouche, E.; Lau, G. C.; Cava, R. J.; Bramwell, S. T.

2013-01-01

106

An experimental study of the ice column habit transitions. [crystal growth in atmosphere  

NASA Technical Reports Server (NTRS)

The influence of supersaturation on column growth of ice crystals forming from atmospheric water vapor was investigated. A high density of crystals was generated on a glass fiber cooled by liquid N2 in a thermal diffusion chamber. Attention was focused on a neighbor-free hollow prism during a stepwise decrease in supersaturation while the crystal temperature was maintained constant. Another experiment involved epitaxial growth of ice crystals on CuS, where nonthickening crystals could only be grown below -7 C. A critical supersaturation was found to be necessary for growth of the basal plane. Beyound the critical value, surface kinetics do not control the growth rate, which is then dominated by the penetration of water molecules through the diffusion field surrounding the crystal.

Cho, N.; Hallett, J.

1982-01-01

107

The Crystal Structure of Talc  

Microsoft Academic Search

The crystal structure of a sample of talc from Harford County, Maryland, has been deter- mined by least squares refinement from X-ray diffraction photographs. A triclinic cell with a = 5.293, b = 9.179, c = 9.496A, a = 90'57 ~ = 98-91 ~ y = 90.03, space group cT is adopted. The layers of the structure have almost monoclinic

J. H. Rayner; G. BROWN

1973-01-01

108

Effects of impurities and their redistribution during recrystallization of ice crystals  

NASA Astrophysics Data System (ADS)

In order to examine the effects of solutes on recrystallization and subsequent grain growth in ice, both doped and undoped ice single crystals were extruded through a 120° equal-channel angular extrusion jig, in order to impart a large shear strain (˜1.15). Upon subsequent annealing at -3°C, the original single crystals recrystallized, in most cases to a new single crystal with a different orientation. Increasing the solute concentration (for H2SO4 to ˜200-300 ppb, and for NaCl, KCl and MgSO4 to >5 ppm) was found to significantly retard the growth and possibly, for H2SO4-doped ice, the nucleation of new grains in the strained ice single crystals. This is indicative of how soluble impurities can retard grain growth in ice cores. It was also found that the migrating grain boundaries surrounding the newly formed grains contained large concentrations of impurities, often observed as filaments. These could have formed by the grain boundaries sweeping up impurities from the lattice into the boundary or by their diffusion to the boundary - mechanisms whereby impurities could be concentrated into the grain boundaries in ice cores - although the latter mechanism seems unlikely since it would require very high diffusion rates.

Iliescu, D.; Baker, I.

109

Crystal structure of guggulsterone Z  

SciTech Connect

The crystal structure of the title compound (4,17(20)-trans-pregnadiene-3,16-dione, C{sub 21}H{sub 28}O{sub 2}) has been determined by direct methods using single-crystal X-ray diffraction data. The compound crystallizes into the orthorhombic space group P2{sub 1}2{sub 1}2{sub 1} with the unit cell parameters a = 7.908(2) A, b = 13.611(3) A, c = 16.309(4) A, and Z = 4. The structure has been refined to R = 0.058 for 3667 observed reflections. The bond distances and angles are in good agreement with guggulsterone E and other related steroid molecules. Ring A exists in the distorted sofa conformation, while rings B and C adopt the distorted chair conformation. Five-membered ring D is intermediate between the half-chair and envelope conformations. The A/B ring junction is quasi-trans, while ring systems B/C and C/D are trans fused about the C(8)-C(9) and C(13)-C(14) bonds, respectively. The steroid nucleus has a small twist, as shown by the C(19)-C(10)...C(13)-C(18) pseudo-torsion angle of 7.2{sup o}. The crystal structure is stabilized by intra-and intermolecular C-H...O hydrogen bonds.

Gupta, V. K., E-mail: vivek_gupta2k2@hotmail.com; Bandhoria, P. [University of Jammu, Post Graduate Department of Physics (India); Gupta, B. D.; Gupta, K. K. [Regional Research Laboratory (India)

2006-03-15

110

Ice structure monitoring with an optical fiber sensing system  

Microsoft Academic Search

Ice has been used as an effective and economical material for constructions of roads and platforms in cold regions. However, the practical applications of this brittle material are limited by the fact that ice structures can suddenly crack due to low tensile strength, be crushed due to excessive compression, melt and become soften as temperature elevates. In this paper, an

Zhi Zhou; Minghua Huang; Jianping He; Genda Chen; Jinping Ou

2010-01-01

111

The capacitance of pristine ice crystals and aggregate snowflakes  

E-print Network

A new method of accurately calculating the capacitance of realistic ice particles is described: such values are key to accurate estimates of sublimation rates in numerical weather models. We have directly simulated the trajectories of diffusing water molecules, using random `walkers'. By counting how many of these trajectories intersect the surface of the ice particle (which may be any shape) and how many escape outside a spherical boundary far from the particle, we have estimated the capacitance of a number of model ice particle habits including hexagonal columns and plates, `scalene' columns and plates, bullets, bullet-rosettes, dendrites, and realistic aggregate snowflakes. For ice particles with sharp edges and corners this method is an efficient and straightforward way of solving Laplace's equation for the capacitance. Provided that a large enough number of random walkers are used to sample the particle geometry ($\\sim10^4$) we expect the calculated capacitances to be accurate to within $\\sim1%$. The cap...

Westbrook, C D; Illingworth, A J

2006-01-01

112

Phase transition of the hydrogen-bonded crystals and ice  

Microsoft Academic Search

The order–disorder phase transition for KH2PO4 (KDP) and ice VIII is studied in terms of the dipole–hydrogen coupling model. The effective interactions between hydrogens are obtained by averaging out the dipole’s degrees of freedom. It is found that the bonding energy of ice VIII obtained in the present theory compares with the hydrogen bond energy determined by Whalley. The isotope

Katsuhiko Fujii

1995-01-01

113

The capacitance of pristine ice crystals and aggregate snowflakes  

Microsoft Academic Search

A new method of accurately calculating the capacitance of realistic ice\\u000aparticles is described: such values are key to accurate estimates of deposition\\u000aand evaporation rates in NWP models. The trajectories of diffusing water\\u000amolecules are directly sampled, using random `walkers'. By counting how many of\\u000athese trajectories intersect the surface of the ice particle (which may be any\\u000ashape)

Christopher David Westbrook; Robin J. Hogan; Anthony J. Illingworth

2006-01-01

114

STUDY OF PHOTONIC CRYSTAL STRUCTURES YUGUANG ZHAO  

E-print Network

STUDY OF PHOTONIC CRYSTAL STRUCTURES BY THz-TDS By YUGUANG ZHAO Bachelor of Science Northwest CRYSTAL STRUCTURES BY THz-TDS Thesis Approved: Dr. Daniel Grischkowsky . Thesis Adviser Dr. Yumin Zhang photonic crystal structures ....................................................... 25 3.2 MEMS technology

Oklahoma State University

115

American Mineralogist Crystal Structure Database  

NSDL National Science Digital Library

This database provides access to information on every crystal structure published in the American Mineralogist, the Canadian Mineralogist, European Journal of Mineralogy, and Physics and Chemistry of Minerals, as well as selected datasets from other journals. The data are searchable by mineral name, author, chemistry, cell parameters and symmetry, diffraction pattern, and a general search. There are also lists of minerals represented in the database and authors of publications cited.

R. T. Downs

116

Structural characterization of ice polymorphs from self-avoiding walks  

NASA Astrophysics Data System (ADS)

Topological properties of crystalline ice structures are studied by means of self-avoiding walks on their H-bond networks. The number of self-avoiding walks, Cn, for eight ice polymorphs has been obtained by direct enumeration up to walk length n=27. This has allowed us to determine the ‘connective constant' or effective coordination number ? of these structures as the limit of the ratio Cn/Cn-1 for large n. This structure-dependent parameter ? is related with other topological characteristics of ice polymorphs, such as the mean and minimum ring size, or the topological density of network sites. A correlation between the connective constant and the configurational entropy of hydrogen-disordered ice structures is discussed.

Herrero, Carlos P.

2014-08-01

117

Topological characterization of crystalline ice structures from coordination sequences.  

PubMed

Topological properties of crystalline ice structures are studied by considering ring statistics, coordination sequences, and topological density of different ice phases. The coordination sequences (number of sites at topological distance k from a reference site) have been obtained by direct enumeration until at least 40 coordination spheres for different ice polymorphs. This allows us to study the asymptotic behavior of the mean number of sites in the k-th shell, Mk, for high values of k: Mk~ak(2), a being a structure-dependent parameter. Small departures from a strict parabolic dependence have been studied by considering first and second differences of the series {Mk} for each structure. The parameter a ranges from 2.00 for ice VI to 4.27 for ice XII, and is used to define a topological density for these solid phases of water. Correlations between such topological density and the actual volume of ice phases are discussed. Ices Ih and Ic are found to depart from the general trend in this correlation due to the large void space in their structures. PMID:23986009

Herrero, Carlos P; Ramírez, Rafael

2013-10-21

118

American Mineralogist Crystal Structure Database  

NSDL National Science Digital Library

The American Mineralogist Crystal Structure Database website, maintained by the Mineralogical Society of America and the Mineralogical Association of Canada and sponsored by the National Science Foundation, "includes every structure published in the American Mineralogist, The Canadian Mineralogist, and the European Journal of Mineralogy." The authors are also currently adding data from Physics and Chemistry of Minerals. Users can search the data by minerals, authors, chemistry, cell parameter and symmetry, or by a simple general search. This no frills website allows users to easily find and download data.

119

Influence of snow and ice crystal formation and accumulation on mercury deposition to the Arctic.  

PubMed

Mercury is deposited to the Polar Regions during springtime atmospheric mercury depletion events (AMDEs) but the relationship between snow and ice crystal formation and mercury deposition is not well understood. The objective of this investigation was to determine if mercury concentrations were related to the type and formation of snow and ice crystals. On the basis of almost three hundred analyses of samples collected in the Alaskan Arctic, we suggestthat kinetic crystals growing from the vapor phase, including surface hoar, frost flowers, and diamond dust, yield mercury concentrations that are typically 2-10 times higher than that reported for snow deposited during AMDEs (approximately 80 ng/L). Our results show that the crystal type and formation affect the mercury concentration in any given snow sample far more than the AMDE activity prior to snow collection. We present a conceptual model of how snow grain processes including deposition, condensation, reemission, sublimation, and turbulent diffusive uptake influence mercury concentrations in snow and ice. These processes are time dependent and operate collectively to affect the retention and fate of mercury in the cryosphere. The model highlights the importance of the formation and postdeposition crystallographic history of snow or ice crystals in determining the fate and concentration of mercury in the cryosphere. PMID:18441801

Douglas, Thomas A; Sturm, Matthew; Simpson, William R; Blum, Joel D; Alvarez-Aviles, Laura; Keeler, Gerald J; Perovich, Donald K; Biswas, Abir; Johnson, Kelsey

2008-03-01

120

Influence of air velocity on the habit of ice crystal growth from the vapor  

NASA Technical Reports Server (NTRS)

The effect of air velocity on the growth behavior of ice crystals growing from water vapor was investigated at temperatures between 0 and -35 C and at supersaturation levels ranging from 2 to 40 percent, using a laboratory chamber in which it was possible to make these variations. It was found that crystal growth was most sensitive to changes in the air velocity at temperatures near -4 C and -15 C where, near water saturation, the introduction of only a 5 cm/s air velocity induced skeletal transitions (columns to needles near -4 C and plates to dendrites near -15 C). The experiments provide conditions which simulate growth of ice crystals in the atmosphere, where crystal growth takes place at or somewhat below water saturation.

Keller, V. W.; Hallett, J.

1982-01-01

121

Crystal Structures Classifier for an Evolutionary Algorithm Structure Predictor  

E-print Network

Crystal Structures Classifier for an Evolutionary Algorithm Structure Predictor Mario Valle Data of Crystallography, Department of Materials ETH Z¨urich ABSTRACT USPEX is a crystal structure predictor based on an evolutionary algorithm. Every USPEX run produces hundreds or thousands of crystal structures, some of which may

Oganov, Artem R.

122

Best face forward: crystal-face competition at the ice-water interface.  

PubMed

The ice-water interface plays an important role in determining the outcome of both biological and environmental processes. Under ambient pressure, the most stable form of ice is hexagonal ice (Ih). Experimentally probing the surface free energy between each of the major faces of Ih ice and the liquid is both experimentally and theoretically challenging. The basis for the challenge is the near-equality of the surface free energy for the major faces along with the tendency of water to supercool. As a result, morphology from crystallization initiated below 0 °C is kinetically controlled. The reported work circumvents supercooling consequences by providing a polycrystalline seed, followed by isothermal, equilibrium growth. Natural selection among seeded faces results in a single crystal. A record of the growth front is preserved in the frozen boule. Crystal orientation at the front is revealed by examining the boule cross section with two techniques: (1) viewing between crossed polarizers to locate the optical axis and (2) etching to distinguish the primary-prism face from the secondary-prism face. Results suggest that the most stable ice-water interface at 0 °C is the secondary-prism face, followed by the primary-prism face. The basal face that imparts the characteristic hexagonal shape to snowflakes is a distant third. The results contrast with those from freezing the vapor where the basal and primary-prism faces have comparable free energy followed by the secondary-prism face. PMID:24784996

Shultz, Mary Jane; Bisson, Patrick J; Brumberg, Alexandra

2014-07-17

123

Flow in Polycrystalline Ice  

NSDL National Science Digital Library

This is a virtual journal article about polycrystalline ice. It focuses on plastic deformation, specific flow characteristics and crystallographic preferred orientations associated with polycrystalline ice within glaciers. Part I covers Polycrystalline aggregates deformed in pure-shear; Dynamic recrystallisation; Grain shape and preferred orientation change; Fabric; Evolution of glacial ice during deformation. Part II covers: Time lapse photography; Glaciers; Dislocations; Bernal-Fowler rule; Generation of defect structures; Crystal structure; Ice; Basal glide; Strain rate for glide on basal systems; Critical resolved shear stress; Non-basal glide; Diffusional flow; Plastic deformation; Primary creep; Secondary creep; Tertiary creep; Deformation maps; Grain growth; Grain size reduction; Anisotropic flow law for ice.

Chris Wilson

124

Crystal structure of lignin peroxidase.  

PubMed Central

The crystal structure of lignin peroxidase (LiP) from the basidiomycete Phanerochaete chrysosporium has been determined to 2.6 A resolution by usine multiple isomorphous replacement methods and simulated annealing refinement. Of the 343 residues, residues 3-335 have been accounted for in the electron density map, including four disulfide bonds. The overall three-dimensional structure is very similar to the only other peroxidase in this group for which a high-resolution crystal structure is available, cytochrome c peroxidase, despite the fact that the sequence identity is only approximately 20%, LiP has four disulfide bonds, while cytochrome c peroxidase has none, and LiP is larger (343 vs. 294 residues). The basic helical fold and connectivity defined by 11 helical segments with the heme sandwiched between the distal and proximal helices found in cytochrome c peroxidase is maintained in LiP. Both enzymes have a histidine as a proximal heme ligand, which is hydrogen bonded to a buried aspartic acid side chain. The distal or peroxide binding pocket also is similar, including the distal arginine and histidine. The most striking difference is that, whereas cytochrome c peroxidase has tryptophans contacting the distal and proximal heme surfaces, LiP has phenylalanines. This in part explains why, in the reaction with peroxides, cytochrome c peroxidase forms an amino acid-centered free radical, whereas LiP forms a porphyrin pi cation radical. Images PMID:11607355

Edwards, S L; Raag, R; Wariishi, H; Gold, M H; Poulos, T L

1993-01-01

125

NASA Glenn Propulsion Systems Lab: 2012 Inaugural Ice Crystal Cloud Calibration Procedure and Results  

NASA Technical Reports Server (NTRS)

The inaugural calibration of the ice crystal and supercooled liquid water clouds generated in NASA Glenn's engine altitude test facility, the Propulsion Systems Lab (PSL) is reported herein. This calibration was in support of the inaugural engine ice crystal validation test. During the Fall of 2012 calibration effort, cloud uniformity was documented via an icing grid, laser sheet and cloud tomography. Water content was measured via multi-wire and robust probes, and particle sizes were measured with a Cloud Droplet Probe and Cloud Imaging Probe. The environmental conditions ranged from 5,000 to 35,000 ft, Mach 0.15 to 0.55, temperature from +50 to -35 F and relative humidities from less than 1 percent to 75 percent in the plenum.

VanZante, Judith F.; Rosine, Bryan M.

2014-01-01

126

Radiative influences on ice crystal and droplet growth within mixed-phase stratus clouds  

E-print Network

Radiative influences on ice crystal and droplet growth within mixed-phase stratus clouds Z. J. Lebo and droplet growth within mixed-phase stratus clouds, J. Geophys. Res., 113, D09203, doi:10.1029/2007JD009262-tropospheric levels in the sub-Arctic and Arctic (Arctic stratus and stratocumulus, [e.g., Curry et al., 1996

Johnson, Nat

127

An Investigation of Light Scattering by Irregular Ice Crystals via PSTD  

E-print Network

generated from different power functions.(a) P (k) = 1/k2 (b) P (k) = 1/k4 (c) Write noise: P (k) = 1 (d)Gaussian type power function: P (k) = e?(k/?k) 2 . . . . . . . . . . . . . . . . . 19 3.2 Hexagonal ice plate... Hexagonal Mono-crystal model iv TABLE OF CONTENTS Page ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii NOMENCLATURE...

Zhang, Jianing

2014-07-28

128

American Mineralogist Crystal Structure Database  

NSDL National Science Digital Library

This database includes the crystal structure for every mineral published in the American Mineralogist, The Canadian Mineralogist, European Journal of Mineralogy and Physics, and Chemistry of Minerals. Data is now being imported from Acta Crystallographica as well. The database is maintained under the care of the Mineralogical Society of America and the Mineralogical Association of Canada. The data can be displayed or downloaded and are searchable by mineral, author, mineral chemistry, unit cell parameters and symmetry, diffraction pattern, or a general search. Links are provided to additional information and to crystallographic software.

129

Kinetics of conversion of air bubbles to air hydrate crystals in antarctic ice.  

PubMed

The depth dependence of bubble concentration at pressures above the transition to the air hydrate phase and the optical scattering length due to bubbles in deep ice at the South Pole are modeled with diffusion-growth data from the laboratory, taking into account the dependence of age and temperature on depth in the ice. The model fits the available data on bubbles in cores from Vostok and Byrd and on scattering length in deep ice at the South Pole. It explains why bubbles and air hydrate crystals coexist in deep ice over a range of depths as great as 800 meters and predicts that at depths below approximately 1400 meters the AMANDA neutrino observatory at the South Pole will operate unimpaired by light scattering from bubbles. PMID:17775808

Price, P B

1995-03-24

130

Structure order, local potentials, and physical anomalies of water ice  

E-print Network

Hydrogen-bond forms a pair of asymmetric, coupled, H-bridged oscillators with ultra-short-range interactions and memory. hydrogen bond cooperative relaxation and the associated binding electron entrapment and nonbonding electron polarization discriminate water and ice from other usual materials in the physical anomalies. As a strongly correlated fluctuating system, water prefers the statistically mean of tetrahedrally-coordinated structure with a supersolid skin that is elastic, polarized, ice like, hydrophobic, with 3/4 density.

Chang Q Sun

2014-07-11

131

Ice surfaces: macroscopic effects of microscopic structure  

E-print Network

, Michael Faraday began a 20 year inves- tigation into the properties of snow and ice. Faraday's publications based on this research (see, for example, Faraday 1860) clearly demonstrate the notion that melt solid, the general temperature remaining the same. Although Faraday and Tyndall's (Tyndall 1858

Wettlaufer, John S.

132

Photonic Band Structure of fcc Colloidal Crystals  

Microsoft Academic Search

Polystyrene colloidal crystals form three dimensional periodic dielectric structures which can be used for photonic band structure measurements in the visible regime. From transmission measurements the photonic band structure of an fcc crystal has been obtained along the directions between the L point and the W point. Kossel line patterns were used for locating the symmetry points of the lattice

I. Inanç Tarhan; George H. Watson

1996-01-01

133

Simulations of Photonic Crystal and Dielectric Structures  

SciTech Connect

Dielectric materials and photonic crystal structures have electromagnetic properties that could potentially offer great benefits for accelerators. Computer simulation plays a critical role in designing, understanding, and optimizing these structures, especially the non-intuitive photonic crystal structures for which there is no relevant zeroth-order analytic model.

Werner, G. R. [Center for Integrated Plasma Studies, University of Colorado, Boulder, CO 80309 (United States)

2010-11-04

134

Seismicity within a propagating ice shelf rift: the relationship between icequake locations and ice shelf structure  

USGS Publications Warehouse

Iceberg calving is a dominant mass loss mechanism for Antarctic ice shelves, second only to basal melting. An important known process involved in calving is the initiation and propagation of through-penetrating fractures called rifts; however, the mechanisms controlling rift propagation remain poorly understood. To investigate the mechanics of ice-shelf rifting, we analyzed seismicity associated with a propagating rift tip on the Amery Ice Shelf, using data collected during the Austral summers of 2004-2007. We investigated seismicity associated with fracture propagation using a suite of passive seismological techniques including icequake locations, back projection, and moment tensor inversion. We confirm previous results that show that seismicity is characterized by periods of relative quiescence punctuated by swarms of intense seismicity of one to three hours. However, even during periods of quiescence, we find significant seismic deformation around the rift tip. Moment tensors, calculated for a subset of the largest icequakes (MW?>?-2.0) located near the rift tip, show steeply dipping fault planes, horizontal or shallowly plunging stress orientations, and often have a significant volumetric component. They also reveal that much of the observed seismicity is limited to the upper 50?m of the ice shelf. This suggests a complex system of deformation that involves the propagating rift, the region behind the rift tip, and a system of rift-transverse crevasses. Small-scale variations in the mechanical structure of the ice shelf, especially rift-transverse crevasses and accreted marine ice, play an important role in modulating the rate and location of seismicity associated with propagating ice shelf rifts.

Heeszel, David S.; Fricker, Helen A.; Bassis, Jeremy N.; O'Neel, Shad; Walter, Fabian

2014-01-01

135

The physical-optics approximation and its application to light backscattering by hexagonal ice crystals  

NASA Astrophysics Data System (ADS)

The physical-optics approximation in the problem of light scattering by large particles is so defined that it includes the classical physical optics concerning the problem of light penetration through a large aperture in an opaque screen. In the second part of the paper, the problem of light backscattering by quasi-horizontally oriented atmospheric ice crystals is considered where conformity between the physical-optics and geometric-optics approximations is discussed. The differential scattering cross section as well as the polarization elements of the Mueller matrix for quasi-horizontally oriented hexagonal ice plates has been calculated in the physical-optics approximation for the case of vertically pointing lidars.

Borovoi, A.; Konoshonkin, A.; Kustova, N.

2014-10-01

136

Formation of Large (Approximately 100 micrometers) Ice Crystals Near the Tropical Tropopause  

NASA Technical Reports Server (NTRS)

Recent high-altitude aircraft measurements with in situ imaging instruments indicated the presence of relatively large (approx.100 microns length), thin (aspect ratios of approx.6:1 or larger) hexagonal plate ice crystals near the tropical tropopause in very low concentrations (<0.01/L). These crystals were not produced by deep convection or aggregation. We use simple growth-sedimentation calculations as well as detailed cloud simulations to evaluate the conditions required to grow the large crystals. Uncertainties in crystal aspect ratio leave a range of possibilities, which could be constrained by knowledge of the water vapor concentration in the air where the crystal growth occurred. Unfortunately, water vapor measurements made in the cloud formation region near the tropopause with different instruments ranged from <2 ppmv to approx.3.5 ppmv. The higher water vapor concentrations correspond to very large ice supersaturations (relative humidities with respect to ice of about 200%). If the aspect ratios of the hexagonal plate crystals are as small as the image analysis suggests (6:1, see companion paper (Lawson et al., 2008)) then growth of the large crystals before they sediment out of the supersaturated layer would only be possible if the water vapor concentration were on the high end of the range indicated by the different measurements (>3 ppmv). On the other hand, if the crystal aspect ratios are quite a bit larger (approx.10:1), then H2O concentrations toward the low end of the measurement range (approx.2-2.5 ppmv) would suffice to grow the large crystals. Gravity-wave driven temperature and vertical wind perturbations only slightly modify the H2O concentrations needed to grow the crystals. We find that it would not be possible to grow the large crystals with water concentrations less than 2 ppmv, even with assumptions of a very high aspect ratio of 15 and steady upward motion of 2 cm/s to loft the crystals in the tropopause region. These calculations would seem to imply that the measurements indicating water vapor concentrations less than 2ppmv are implausible, but we cannot rule out the possibility that higher humidity prevailed upstream of the aircraft measurements and the air was dehydrated by the cloud formation. Simulations of the cloud formation with a detailed model indicate that homogeneous freezing should generate ice concentrations larger than the observed concentrations (20/L), and even concentrations as low as 20/L should have depleted the vapor in excess of saturation and prevented growth of large crystals. It seems likely that the large crystals resulted from ice nucleation on effective heterogeneous nuclei at low ice supersaturations. Improvements in our understanding of detailed cloud microphysical processes require resolution of the water vapor measurement discrepancies in these very cold, dry regions of the atmosphere.

Jensen, E. J.; Pfister, L.; Bui, T. V.; Lawson, P.; Baker, B.; Mo, Q.; Baumgardner, D.; Weinstock, E. M.; Smith, J. B.; Moyer, E. J.; Hanisco, T. F.; Sayres, D. S.; SaintClair, J. M.; Alexander, M.; Toon, O. B.; Smith, J. A.

2008-01-01

137

Arctic ice island and sea ice movements and mechanical properties: Fourteenth quarterly report, 1 January 1987-31 March 1987. [Sea spray ice bonds to offshore structures  

SciTech Connect

The research program on ice islands has four elements: (1) through the use of satellite imagery, historical records, and aerial photography, to establish a time history of all of the Arctic ice shelves, and thus an historically verified source for ice islands; (2) to establish postioning buoys on the known existing ice islands to track their trajectories daily and to telemeter daily barometer pressure and temperature, via System Argos; (3) to calculate geostrophic winds from global pressure maps and barometric pressure data from the buoys, and relate the observed ice island trajectories to the winds and the internal pack ice forces; (4) to construct a model for ice island motion which will enable a determination of the probability of interaction between ice islands and offshore structures, and which will be verified by comparsion with the experimentally observed trajectory data. Research activities covered in the fourteenth quarter include; ice islands (buoy operation and ice island motion); and mechanical properties of sea spray ice bonds to structures. 2 figs., 6 tabs.

Sackinger, W.M.; Jeffries, M.O.

1987-01-01

138

A modified scheme that parameterizes depositional growth of ice crystal: A modeling study of pre-summer torrential rainfall case over Southern China  

NASA Astrophysics Data System (ADS)

Depositional growth of cloud ice is estimated and its parameterization schemes are compared through the two-dimensional cloud-resolving modeling analysis of pre-summer heavy rainfall over southern China. Hsie et al. (1980) and Krueger et al. (1995) developed parameterization schemes to calculate depositional growth of cloud ice by estimating the growth timescale under the assumption that the ice crystal concentration is independent of crystal size. A new scheme is proposed by Zeng et al. (2008) under the assumption that the ice crystal concentration is proportional to the mass of ice crystal. Hsie's and Krueger's schemes produce small amount of cloud ice similar to what Zeng's scheme with low ice crystal concentration does. When ice crystal concentration is increased to a high value in Zeng's scheme, the simulation generates anomalous depositional growth of cloud ice and thus anomalous area expansion of stratiform rainfall. Zeng's scheme is modified by changing radius of base ice crystal from 0 to 40 ?m in the calculation of depositional growth of cloud ice. The modified scheme with high ice crystal concentration greatly reduces growth of cloud ice and thus fractional coverage of stratiform rainfall.

Shen, Xinyong; Huang, Wei; Qing, Tao; Huang, Wenyan; Li, Xiaofan

2014-03-01

139

Stacking disorder in ice I.  

PubMed

Traditionally, ice I was considered to exist in two well-defined crystalline forms at ambient pressure: stable hexagonal ice (ice Ih) and metastable cubic ice (ice Ic). However, it is becoming increasingly evident that what has been called cubic ice in the past does not have a structure consistent with the cubic crystal system. Instead, it is a stacking-disordered material containing cubic sequences interlaced with hexagonal sequences, which is termed stacking-disordered ice (ice Isd). In this article, we summarise previous work on ice with stacking disorder including ice that was called cubic ice in the past. We also present new experimental data which shows that ice which crystallises after heterogeneous nucleation in water droplets containing solid inclusions also contains stacking disorder even at freezing temperatures of around -15 °C. This supports the results from molecular simulations, that the structure of ice that crystallises initially from supercooled water is always stacking-disordered and that this metastable ice can transform to the stable hexagonal phase subject to the kinetics of recrystallization. We also show that stacking disorder in ice which forms from water droplets is quantitatively distinct from ice made via other routes. The emerging picture of ice I is that of a very complex material which frequently contains stacking disorder and this stacking disorder can vary in complexity depending on the route of formation and thermal history. PMID:25380218

Malkin, Tamsin L; Murray, Benjamin J; Salzmann, Christoph G; Molinero, Valeria; Pickering, Steven J; Whale, Thomas F

2015-01-01

140

Size and location of ice crystals in pork frozen by high-pressure-assisted freezing as compared to classical methods  

Microsoft Academic Search

In high-pressure-assisted freezing, samples are cooled under pressure (200 MPa) to ? 20 °C without ice formation then pressure is released (0.1 MPa) and the high super-cooling reached (approx. 20 °C), promotes uniform and rapid ice nucleation. The size and location of ice crystals in large meat pieces (Longissimus dorsi pork muscle) as a result of high-pressure-assisted freezing were compared

M. N. Martino; L. Otero; P. D. Sanz; N. E. Zaritzky

1998-01-01

141

Crystal structure analysis of intermetallic compounds  

NASA Technical Reports Server (NTRS)

Study concerns crystal structures and lattice parameters for a number of new intermetallic compounds. Crystal structure data have been collected on equiatomic compounds, formed between an element of the Sc, Ti, V, or Cr group and an element of the Co or Ni group. The data, obtained by conventional methods, are presented in an easily usable tabular form.

Conner, R. A., Jr.; Downey, J. W.; Dwight, A. E.

1968-01-01

142

A model predicting the evolution of ice particle size spectra and radiative properties of cirrus clouds. Part 2: Dependence of absorption and extinction on ice crystal morphology  

NASA Technical Reports Server (NTRS)

This study builds upon the microphysical modeling described in Part 1 by deriving formulations for the extinction and absorption coefficients in terms of the size distribution parameters predicted from the micro-physical model. The optical depth and single scatter albedo of a cirrus cloud can then be determined, which, along with the asymmetry parameter, are the input parameters needed by cloud radiation models. Through the use of anomalous diffraction theory, analytical expressions were developed describing the absorption and extinction coefficients and the single scatter albedo as functions of size distribution parameters, ice crystal shapes (or habits), wavelength, and refractive index. The extinction coefficient was formulated in terms of the projected area of the size distribution, while the absorption coefficient was formulated in terms of both the projected area and mass of the size distribution. These properties were formulated as explicit functions of ice crystal geometry and were not based on an 'effective radius.' Based on simulations of the second cirrus case study described in Part 1, absorption coefficients predicted in the near infrared for hexagonal columns and rosettes were up to 47% and 71% lower, respectively, than absorption coefficients predicted by using equivalent area spheres. This resulted in single scatter albedos in the near-infrared that were considerably greater than those predicted by the equivalent area sphere method. Reflectances in this region should therefore be underestimated using the equivalent area sphere approach. Cloud optical depth was found to depend on ice crystal habit. When the simulated cirrus cloud contained only bullet rosettes, the optical depth was 142% greater than when the cloud contained only hexagonal columns. This increase produced a doubling in cloud albedo. In the near-infrared (IR), the single scatter albedo also exhibited a significant dependence on ice crystal habit. More research is needed on the geometrical properties of ice crystals before the influence of ice crystal shape on cirrus radiative properties can be adequately understood. This study provides a way of coupling the radiative properties of absorption, extinction, and single scatter albedo to the microphysical properties of cirrus clouds. The dependence of extinction and absorption on ice crystal shape was not just due to geometrical differences between crystal types, but was also due to the effect these differences had on the evolution of ice particle size spectra. The ice particle growth model in Part 1 and the radiative properties treated here are based on analytical formulations, and thus represent a computationally efficient means of modeling the microphysical and radiative properties of cirrus clouds.

Mitchell, David L.; Arnott, W. Patrick

1994-01-01

143

Solvation structure of ice-binding antifreeze proteins  

NASA Astrophysics Data System (ADS)

Antifreeze proteins (AFPs) can be found in organisms which survive at subzero temperatures. They were first discovered in polar fishes since the 1950's [1] and have been isolated meanwhile also from insects, plants, and bacteria. While AFPs shift the freezing point of water below the bulk melting point and hence can prevent recrystallization; the effect is non-colligative and there is a pronounced hysteresis between freezing and melting. For many AFPs it is generally accepted that they function through an irreversible binding to the ice-water interface which leads to a piecewise convex growth front with a lower nonequilibrium freezing point due to the Kelvin effect. Recent molecular dynamics simulations of the AFP from Choristoneura fumiferana reveal that the solvation structures of water at ice-binding and non-ice-binding faces of the protein are crucial for understanding how the AFP binds to the ice surface and how it is protected from being overgrown [2]. We use density functional theory of classical fluids in order to assess the microscopic solvent structure in the vicinity of protein faces with different surface properties. With our method, binding energies of different protein faces to the water-ice-interface can be computed efficiently in a simplified model. [1] Y. Yeh and R.E. Feeney, Chem. Rev. 96, 601 (1996). [2] D.R. Nutt and J.C. Smith, J. Am. Chem. Soc. 130, 13066 (2008).

Hansen-Goos, Hendrik; Wettlaufer, John

2009-03-01

144

Mimicry of ice structure by surface hydroxyls and water of a beta-helix antifreeze protein.  

PubMed

Insect antifreeze proteins (AFP) are much more effective than fish AFPs at depressing solution freezing points by ice-growth inhibition. AFP from the beetle Tenebrio molitor is a small protein (8.4 kDa) composed of tandem 12-residue repeats (TCTxSxxCxxAx). Here we report its 1.4-A resolution crystal structure, showing that this repetitive sequence translates into an exceptionally regular beta-helix. Not only are the 12-amino-acid loops almost identical in the backbone, but also the conserved side chains are positioned in essentially identical orientations, making this AFP perhaps the most regular protein structure yet observed. The protein has almost no hydrophobic core but is stabilized by numerous disulphide and hydrogen bonds. On the conserved side of the protein, threonine-cysteine-threonine motifs are arrayed to form a flat beta-sheet, the putative ice-binding surface. The threonine side chains have exactly the same rotameric conformation and the spacing between OH groups is a near-perfect match to the ice lattice. Together with tightly bound co-planar external water, three ranks of oxygen atoms form a two-dimensional array, mimicking an ice section. PMID:10917536

Liou, Y C; Tocilj, A; Davies, P L; Jia, Z

2000-07-20

145

Chemical Characterization of Individual Particles and Residuals of Cloud Droplets and Ice Crystals Collected On Board Research Aircraft in the ISDAC 2008 Study  

SciTech Connect

Although it has been shown that size of atmospheric particles has a direct correlation with their ability to act as cloud droplet and ice nuclei, the influence of composition of freshly emitted and aged particles in nucleation processes is poorly understood. In this work we combine data from field measurements of ice nucleation with chemical imaging of the sampled particles to link aerosol composition with ice nucleation ability. Field measurements and sampling were conducted during the Indirect and Semidirect Aerosols Campaign (ISDAC) over Barrow, Alaska, in the springtime of 2008. In-situ ice nucleation measurements were conducted using a Continuous Flow Diffusion Chamber (CFDC). Measured number concentrations of ice nuclei (IN) varied from frequent values of 0.01 per liter to more than 10 per liter. Residuals of airborne droplets and ice crystals were collected through a counterflow virtual impactor (CVI). The compositions of individual atmospheric particles and the residuals were studied using Computer Controlled Scanning Electron Microscopy with Energy Dispersive X-ray analysis (CCSEM/EDX) and Scanning Transmission X-ray Microscopy coupled with Near Edge X-ray Absorption Fine Structure spectroscopy (STXM/NEXAFS). Chemical analysis of cloud particle residuals collected during an episode of high ice nucleation suggests that both size and composition may influence aerosol's ability to act as IN. The STXM/NEXAFS chemical composition maps of individual residuals have characteristic structures of either inorganic or black carbon cores coated by organic materials. In a separate flight, particle samples from a biomass burning plume were collected. Although it has previously been suggested that episodes of biomass burning contribute to increased numbers of highly effective ice nuclei, in this episode we observed that only a small fraction were effective ice nuclei. Most of the particles from the biomass plume episode were smaller in size and were composed of homogeneous organic material without identifiable cores.

Hiranuma, Naruki; Brooks, Sarah D.; Moffet, Ryan C.; Glen, Andrew; Laskin, Alexander; Gilles, Marry K.; Liu, Peter; MacDonald, A. M.; Strapp, J. Walter; McFarquhar, Greg

2013-06-24

146

Sea Ice  

NSDL National Science Digital Library

In this resource, students will discover that there are notable differences between sea ice and fresh-water ice, such as density. In on segment, students learn that the first sign of freezing on the sea is an oily appearance of the water caused by the formation of needle-like crystals. The site explains the relationship between growth and the rate at which heat flows from the water and that the ice pack can alter its shape and dimension due to the movement of winds, currents, thermal expansion, and contraction of the ice. Types of ice described here include new ice, nilas, young ice, first-year ice, and old ice while the forms of ice covered include pancake ice, brash ice, ice cake, floe, and fast ice. The site also explains the meteorological and oceanographic factors that control the amount and movement of ice.

147

The mystery of low ice crystal numbers in the TTL and implications for the UTLS water vapor budget  

NASA Astrophysics Data System (ADS)

Water vapour is the most important natural green house gas. However, in the stratosphere an increase in water vapour would possibly result in a net cooling of the earth-atmosphere system. The major entrance pathway of trace substances into the stratosphere is the tropical tropopause layer (TTL). The TTL water vapor budget, and thus the exchange between troposphere and stratosphere, depends crucially on the occurrence and properties of ice clouds in this cold region (T < 200 K). New observations indicate that very low ice crystal numbers frequently occur in the TTL. This phenomenon is not yet understood and is not compatible with the idea that homogeneous freezing of solution droplets is the major pathway of ice formation. These low ice number concentrations are consistent with observed persistent high ice supersaturations inside cold TTL cirrus clouds, which in turn control the exchange of water vapor with the stratosphere. Here, we reproduce in-situ measurements of frequencies of occurrence of ice crystal concentrations by extensive model simulations, driven by the special dynamical conditions in the TTL, namely the superposition of slow large-scale updrafts with high-frequency short waves. The simulations show that about 80% of the observed incidences of low ice crystal concentrations can be explained by 'classical' homogeneous ice nucleation in the very slow updrafts (< 1cm/s), about 19% stem from heterogeneous freezing, while the remaining of about 1% originates from homogeneous freezing in slightly faster updrafts (> 1cm/s). The mechanism limiting the ice crystal production from homogeneous freezing in an environment full of gravity waves is that freezing events are stalled -due to the shortness of the gravity waves- before a higher number concentration of ice crystals can be formed. Furthermore, the very few ice crystals cannot efficiently reduce the gas phase water vapor inside of the cirrus. As a result, high supersaturations can last for many hours thus hindering the downward transport of water by sedimenting ice crystals. Based on our new insights in both the low ice crystal numbers and subsequent persistent high supersatuartions, we propose to reasses the water transport to the stratosphere in the TTL.

Kraemer, M.; Spichtinger, P.

2012-12-01

148

The Structure of Ice Nanoclusters and Thin-films of Water Ice: Implications for Icy Grains in Cold Molecular Clouds  

NASA Technical Reports Server (NTRS)

The cubic to hexagonal phase transformation in water ice (I(sub c) yields I(sub h)) is used to measure the extent to which surface structure and impurities control bulk properties. In pure crystalline (I(sub c)) water ice nanoclusters and in thin-films of impure water ice, I(sub c) yields I(sub h) occurs at lower temperatures than in thin-films of pure water ice. The disordered surface of the 20 nm diameter nanoclusters promotes transformations or reactions which would otherwise be kinetically hindered. Likewise, impurities such as methanol introduce defects into the ice network, thereby allowing sluggish structural transitions to proceed. Such surface-related phenomena play an important role in promoting chemical reactions on interstellar ice grains within cold molecular clouds, where the first organic compounds are formed.

Delzeit, Lance; Blake, David; Uffindell, Christine; DeVincenzi, Donald L. (Technical Monitor)

2000-01-01

149

Ice  

NSDL National Science Digital Library

When a chunk of ice "twice the size of Manhattan" broke away from the northernmost part of the Antarctic Peninsula in February, ice was at the forefront of scientific news. Now, with the spectacular discovery of bacteria in Antarctic ice and with new evidence of slush beneath the frozen surface of Jupiter's moon Europa, water in its frozen form is once again in the news. The discovery of living organisms in the Antarctic ecosystem, described in the June 26, 1998 issue of Science, is significant because it presents a model for "how life may have arisen and persisted on other worlds." Scientists speculate that if organisms can thrive in the hard ice of Antarctica, they may possibly have done so on Europa and Mars. Galileo's closest approach to Europa occurred on July 21, 1998, offering new images of ice in space. The nine sites listed offer insights and details of the recent findings and discoveries related to ice.

Harris, Kathryn L.

150

Terahertz Time Domain Spectroscopy of Simple Astrophysically Relevant Ices: the Structure of the Ice  

NASA Astrophysics Data System (ADS)

International astronomical facilities, in particular the Herschel Space Telescope, SOFIA and ALMA, are currently characterizing the interstellar medium (ISM) by collecting a huge amount of new THz spectral data that must be compared to THz laboratory spectra to be interpreted. The latter, however, are largely lacking, and this severely restricts the scientific impact of the astronomical observations. We have recently constructed a new THz time-domain spectroscopy system to investigate the spectra of interstellar relevant ice analogs in the range between 0.3 - 7 THz. The system is coupled to a FT-IR spectrometer to monitor the ices in the mid-IR (4000 - 500 cm^{-1}). The THz region of the electromagnetic spectrum is dominated by large amplitude motions, such as phonon modes and intermolecular vibrations, along with high-frequency torsional motions of individual species. This talk will focus on the laboratory investigation of the composition and structure of the bulk phases of interstellar ice analogs (i.e., H_{2}O, CO_{2}, CO, CH_{3}OH, NH_{3}, CH_{4}). Different temperatures, mixing ratios, and matrix isolation experiments will be shown. The ultimate goal of this research project is to provide the scientific community with an extensive THz ice-database, which will allow quantitative studies of the ISM, and guide future astronomical observations of species in the solid phase.

Ioppolo, Sergio; Allodi, Marco A.; McGuire, Brett A.; Kelley, Matthew J.; Blake, Geoffrey A.

2013-06-01

151

Ice friction: The effects of surface roughness, structure, and hydrophobicity  

SciTech Connect

The effect of surface roughness, structure, and hydrophobicity on ice friction is studied systematically over a wide range of temperature and sliding speeds using several metallic interfaces. Hydrophobicity in combination with controlled roughness at the nanoscale is achieved by femtosecond laser irradiation to mimic the lotus effect on the slider's surface. The controlled roughness significantly increases the coefficient of friction at low sliding speeds and temperatures well below the ice melting point. However, at temperatures close to the melting point and relatively higher speeds, roughness and hydrophobicity significantly decrease ice friction. This decrease in friction is mainly due to the suppression of capillary bridges in spite of the presence of surface asperities that facilitate their formation. Finally, grooves oriented in the sliding direction also significantly decrease friction in the low velocity range compared to scratches and grooves randomly distributed over a surface.

Kietzig, Anne-Marie; Hatzikiriakos, Savvas G.; Englezos, Peter [Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia V6T 1Z3 (Canada)

2009-07-15

152

Crystal structure from one-electron theory  

Microsoft Academic Search

We have studied the crystal structures of all the 3d, 4d, and 5d transition metals at zero pressure and temperature by means of the linear muffin-tin orbital method and Andersen's force theorem. We find that, although the structural energy differences seem to be overestimated by the theory, the predicted crystal structures are in accord with experiment in all cases except

Hans L. Skriver

1985-01-01

153

Librarians, crystal structures and drug design.  

PubMed

There are now 355,000 published crystal structures of organic and metal-organic compounds, all of which have been acquired, validated, chemically annotated and organised for searching in the Cambridge Structural Database (CSD). The CSD is used in rational drug design and is beginning to answer important questions relevant to the formulation of pharmaceutical active ingredients. The value and credibility of this research are ultimately dependent on the accuracy and completeness of the underlying crystal-structure data. PMID:16228017

Allen, Frank H; Taylor, Robin

2005-11-01

154

Evaluation of Morphological Change and Aggregation Process of Ice Crystals in Frozen Food by Using Fractal Analysis  

NASA Astrophysics Data System (ADS)

Size and shape of ice crystals in frozen food materials are very important because they affect not only quality of foods but also the viability of industrial processing such as freeze-drying of concentration. In this study, 30%wt sucrose solution is used as test samples. For examining the effect of stabilizerspectine and xantan gum is added to the sucrose solution. They are frozen on the cold stage of microscope to be observed their growing ice crystals under the circumstance of -10°C. Their size and shape are measured and quantitatively evaluated by applying fractal analysis. lce crystal of complicated shape has large fractal dimension, and vice versa. It successflly categorized the ice crystals into two groups; one is a group of large size and complicated shape, and the other is a group of small size and plain shape. The critical crystal size between the two groups is found to become larger with increasing holding time. It suggests a phenomenological model for metamorphoses process of ice crystals. Further, it is indicated that xantan gum is able to suppress the smoothing of ice crystals.

Koshiro, Yoko; Watanabe, Manabu; Takai, Rikuo; Hagiwara, Tomoaki; Suzuki, Toru

155

Communication: Anti-icing characteristics of superhydrophobic surfaces investigated by quartz crystal microresonators  

NASA Astrophysics Data System (ADS)

We investigated the anti-icing characteristics of superhydrophobic surfaces with various morphologies by using quartz crystal microresonators. Anodic aluminum oxide (AAO) or ZnO nanorods were synthesized directly on gold-coated quartz crystal substrates and their surfaces were rendered hydrophobic via chemical modifications with octyltrichlorosilane (OTS), octadecyltrichlorosilane (ODS), or octadecanethiol (ODT). Four different hydrophobic nanostructures were prepared on the quartz crystals: ODT-modified hydrophobic plain gold (C18-Au), an OTS-modified AAO nanostructure (C8-AAO), an ODS-modified AAO nanostructure (C18-AAO), and ODT-modified ZnO nanorods (C18-ZnO). The water contact angles on the C18-Au, C8-AAO, C18-AAO, and C18-ZnO surfaces were measured to be 91.4°, 147.2°, 156.3°, and 157.8°, respectively. A sessile water droplet was placed on each quartz crystal and its freezing temperature was determined by monitoring the drastic changes in the resonance frequency and Q-factor upon freezing. The freezing temperature of a water droplet was found to decrease with decreases in the water contact radius due to the decreases in the number of active sites available for ice nucleation.

Lee, Moonchan; Yim, Changyong; Jeon, Sangmin

2015-01-01

156

Sea ice salinity and structure: A winter time series of salinity and its distribution  

NASA Astrophysics Data System (ADS)

We present a winter time series of Antarctic sea ice salinity from eastern McMurdo Sound, an area close to an ice shelf where a subice platelet layer forms below the sea ice late in winter. This dramatically changes the sea ice structure as the sea ice grows into the subice platelet layer. Every 2 weeks during the 5 months of sea ice formation, salinity profiles were measured, along with detailed measurements of ice structure and growth rates. Once the influence of growth rate on sea ice bulk salinity is removed, the data from 69 cores and the results of a basic parameterization demonstrate that bulk salinity for platelet ice is higher than that for columnar sea ice. We also present measurements of the salinity profile close to the ice-water interface and use these to investigate the expected regime of fluid flow within the permeable portions of the sea ice, with particular reference to mushy layer and percolation theory. Finally, we provide a new distribution of sea ice salinity from 740 measurements, which can be interpreted as the sum of two spatial fields that we attribute to sea ice samples with and without brine channels and which should be reproduced by any realistic sea ice models. This distribution suggests that two measurements of quantities linearly linked to sea ice salinity must differ by 29% if they are to be considered different with 90% confidence.

Gough, A. J.; Mahoney, A. R.; Langhorne, P. J.; Williams, M. J. M.; Haskell, T. G.

2012-03-01

157

Ab initio molecular crystal structures, spectra, and phase diagrams.  

PubMed

Conspectus Molecular crystals are chemists' solids in the sense that their structures and properties can be understood in terms of those of the constituent molecules merely perturbed by a crystalline environment. They form a large and important class of solids including ices of atmospheric species, drugs, explosives, and even some organic optoelectronic materials and supramolecular assemblies. Recently, surprisingly simple yet extremely efficient, versatile, easily implemented, and systematically accurate electronic structure methods for molecular crystals have been developed. The methods, collectively referred to as the embedded-fragment scheme, divide a crystal into monomers and overlapping dimers and apply modern molecular electronic structure methods and software to these fragments of the crystal that are embedded in a self-consistently determined crystalline electrostatic field. They enable facile applications of accurate but otherwise prohibitively expensive ab initio molecular orbital theories such as Møller-Plesset perturbation and coupled-cluster theories to a broad range of properties of solids such as internal energies, enthalpies, structures, equation of state, phonon dispersion curves and density of states, infrared and Raman spectra (including band intensities and sometimes anharmonic effects), inelastic neutron scattering spectra, heat capacities, Gibbs energies, and phase diagrams, while accounting for many-body electrostatic (namely, induction or polarization) effects as well as two-body exchange and dispersion interactions from first principles. They can fundamentally alter the role of computing in the studies of molecular crystals in the same way ab initio molecular orbital theories have transformed research practices in gas-phase physical chemistry and synthetic chemistry in the last half century. In this Account, after a brief summary of formalisms and algorithms, we discuss applications of these methods performed in our group as compelling illustrations of their unprecedented power in addressing some of the outstanding problems of solid-state chemistry, high-pressure chemistry, or geochemistry. They are the structure and spectra of ice Ih, in particular, the origin of two peaks in the hydrogen-bond-stretching region of its inelastic neutron scattering spectra, a solid-solid phase transition from CO2-I to elusive, metastable CO2-III, pressure tuning of Fermi resonance in solid CO2, and the structure and spectra of solid formic acid, all at the level of second-order Møller-Plesset perturbation theory or higher. PMID:24754304

Hirata, So; Gilliard, Kandis; He, Xiao; Li, Jinjin; Sode, Olaseni

2014-09-16

158

crystal: growth, crystal structure perfection, piezoelectric, and acoustic properties  

NASA Astrophysics Data System (ADS)

A five-component crystal of lanthanum-gallium silicate group La3Ga5.3Ta0.5Al0.2O14 (LGTA) was grown by the Czochralski method. The LGTA crystal possesses unique thermal properties and substitution of Al for Ga in the unit cell leads to a substantial increase of electrical resistance at high temperatures. The unit cell parameters of LGTA were determined by powder diffraction. X-ray topography was used to study the crystal structure perfection: the growth banding normal to the growth axis were visualized. The independent piezoelectric constants d 11 and d 14 were measured by X-ray diffraction in the Bragg and Laue geometries. Excitation and propagation of surface acoustic waves were studied by the double-crystal X-ray diffraction at the BESSY II synchrotron radiation source. The analysis of the diffraction spectra of acoustically modulated crystals permitted the determination of the velocity of acoustic wave propagation and the power flow angles in different acoustic cuts of the LGTA crystal.

Roshchupkin, Dmitry; Ortega, Luc; Plotitcyna, Olga; Irzhak, Dmitry; Emelin, Evgenii; Fahrtdinov, Rashid; Alenkov, Vladimir; Buzanov, Oleg

2014-09-01

159

Ice island creation, drift, recurrences, mechanical properties, and interactions with arctic offshore oil production structures  

SciTech Connect

Research and engineering studies on first-year sea ice for over two decades has resulted in the design, construction, and operation of jacket platforms, of artificial islands, and of massive gravity structures which routinely withstand moving sea ice of thickness up to 2 meters. However, the less-common interactions between such structures and moving multiyear ice ([ge]3 meters thick), and also moving ice islands (10 to 60 meters thick) remain as the unknown and potentially most serious hazard for Arctic offshore structures. In this study, research was addressed across the complete span of remaining questions regarding such features. Ice island components, thickness distributions, scenarios and models for the interactions of massive ice features with offshore structures, all were considered. Ice island morphology and calving studies were directed at the cluster of 19 ice islands produced in a calving from the Ward Hunt Ice Shelf on Ellesmere Island in 1983, and also at a calving from the Milne Ice Shelf in 1988. The statistics of ice island dynamics, on both a short-term small-scale basis and also on a long-term basis, were studied. Typical wind velocities of 5 to 7.5 meters per second led to ice island speeds of about 0.014 of the wind speed, at an angle of 20[degrees] to the right of the wind direction. Ice island samples were tested for their stress/strain characteristics. Compressive strength values ranged from 1.64 MPa at a strain rate of 2 [times] 10[sup [minus]7] s[sup [minus]1] to 6.75 MPa at a strain rate of 1 [times] 10[sup [minus]3] s[sup [minus]1]. Scenarios for ice island/structure interactions were developed, and protective countermeasures such as spray ice and ice rubble barriers were suggested. Additional computer modeling of structure/ice interactions for massive ice features is recommended.

Sackinger, W.M.; Jeffries, M.O.; Li, Fucheng; Lu, Mingchi.

1991-03-01

160

Photonic-crystal fibre: Mapping the structure  

NASA Astrophysics Data System (ADS)

The demonstration of real-time and non-destructive Doppler-assisted tomography of the internal structure of photonic-crystal fibres could aid the fabrication of high-quality fibres with enhanced performance.

Markos, Christos

2015-01-01

161

Transformation of the snow crystal to a particle of ice  

Microsoft Academic Search

To study the physical properties of snow under different meteorological conditions a mathematical model and numerical computer\\u000a program were created and applied for some numerical modelling estimates. The non-linear mathematical model consists of partial\\u000a differential equations and can be subdivided into a thermal part with phase changes in porous media, diffusion, structural\\u000a transformation and mechanical parts. The model was applied

Elena Guseva-Lozinski

1999-01-01

162

Predicting crystal structure by merging data mining with quantum mechanics  

E-print Network

ARTICLES Predicting crystal structure by merging data mining with quantum mechanics CHRISTOPHER C crystal structures will form in an alloy system. Crystal structure can only be predicted effectively the stable crystal structure of materials. C rystal structure occupies a central and often critical role

Ceder, Gerbrand

163

The American Mineralogist Crystal Structure Database is a compilation of every crystal structure potentially of  

E-print Network

and symmetry, and a list of elements in the structure. This element list contains the atomic coordinates, siteABSTRACT The American Mineralogist Crystal Structure Database is a compilation of every crystal structure potentially of mineralogic or geologic interest. The database, seen as an outreach service

Downs, Robert T.

164

Using MODIS data to detect the presence of ice crystals in and above super-cooled liquid water clouds over the Arctic  

NASA Astrophysics Data System (ADS)

Determining cloud properties from satellite data over the Arctic is difficult due to low solar elevation angles and the large extent of snow and ice cover. Although it is well established that ice clouds can be discriminated from those having a liquid phase, the mixed-phase clouds represent a problem since they appear nearly identical to liquid phase clouds in satellite data. This is due to the liquid-dominant top found in many mixed-phase stratus clouds. The Terra MODIS particle size ratio between 1.6 and 3.7 ?m for super-cooled liquid water (SLW) clouds is useful for determining ice crystal presence. Since the 1.6 ?m channel detects radiation from deeper inside the clouds, particle size retrievals using this channel are expected to be larger than 3.7 ?m retrievals if ice crystals are embedded inside the cloud. A technique utilizing the MODIS 6.7, 7.3, 8.5, 11 and 12 ?m channels is also used to determine ice crystal presence in SLW clouds. After determining the phase occurring at the cloud top, the internal cloud phase is parameterized in terms of the thermal and water vapor structure above the cloud. This method is useful since it can be applied to nighttime and twilight scenes in addition to daytime scenes. Although the focus of this study is on single layer cloud systems, a multilayer cloud algorithm, which discriminates thin, high ice crystal clouds from low level SLW clouds, is also run. Microwave radiometer, cloud radar and rawinsonde data collected at the Atmospheric Radiation Measurement DOE site in Barrow, Alaska and surface observations from the National Weather Service collected at the Barrow airport are used to evaluate MODIS cloud phase, and to determine whether MODIS can detect differences in the amount of ice embedded in SLW clouds. Accurately assigning cloud phase over large areas of the remote Arctic will benefit the aviation community for aircraft icing detection. Also, since mixed-phase clouds usually precipitate, the likely occurrence of snowfall can be determined remotely. The assignment of cloud phase is also critical in retrieving a cloud’s optical depth, height, particle size, and water path.

Spangenberg, D.; Minnis, P.; Palikonda, R.; Chang, F.; Shupe, M.

2010-12-01

165

Crystal structure of trismorpholino phosphiniminocyclotrithiazene  

Microsoft Academic Search

The title compound (OC4H8N)3P=N–S3N3 crystallizes in a monoclinic crystal system with unit cell parameters a = 8.9996(3), b = 17.2895(7), and c = 12.3648(9) Å, ß = 90.63(5)°, Z = 4, and space group P21\\/n. Strikingly the exocylic S1–N4 bond length is 1.545(3) ÅR and is accompanied by the largest angle at P–N4–S1 as 131.2(2)°. The tricoordinated sulfur atom of

J. Srinivas; G. Sreenivasa Murthy; U. Swarnalatha; M. N. Sudheendra Rao

2001-01-01

166

Structures of cyano-biphenyl liquid crystals  

NASA Technical Reports Server (NTRS)

The structures of p-alkyl- p'-cyano- bicyclohexanes, C(n)H(2n+1) (C6H10)(C6H10) CN (n-CCH), and p-alkyl- p'-cyano- biphenyls, C(n)H(2n+1) (C6H4)(C6H4) CN (n-CBP), were studied. It is convenient to use an x ray image intensification device to search for symmetric x ray diffraction patterns. Despite the similarities in molecular structures of these compounds, very different crystal structures were found. For the smectic phase of 2CCH, the structure is close to rhombohedral with threefold symmetry. In contrast, the structure is close to hexagonal close-packed with two molecules per unit cell for 4CCH. Since intermolecular forces may be quite weak for these liquid crystals systems, it appears that crystal structures change considerably when the alkyl chain length is slightly altered. Different structures were also found in the crystalline phase of n-CBP for n = 6 to 9. For n = 7 to 9, the structures are close to monclinic. The structures are reminiscent of the smectic-A liquid crystal structures with the linear molecules slightly tilted away from the c-axis. In contrast, the structure is quite different for n = 6 with the molecules nearly perpendicular to the c-axis.

Chu, Yuan-Chao; Tsang, Tung; Rahimzadeh, E.; Yin, L.

1989-01-01

167

Variation of ice crystal size, shape, and asymmetry parameter in tops of tropical deep convective clouds  

NASA Astrophysics Data System (ADS)

The variation of ice crystal properties in the tops of deep convective clouds off the north coast of Australia is analyzed. Cloud optical thickness, ice effective radius, aspect ratio of ice crystal components, crystal distortion parameter and asymmetry parameter are simultaneously retrieved from combined measurements of the Moderate Resolution Imaging Spectroradiometer (MODIS) and Polarization and Directionality of the Earth's Reflectances (POLDER) satellite instruments. The data are divided into periods with alternating weak and strong convection. Mostly plate-like particle components with aspect ratios closer to unity and lower asymmetry parameters characterize strongly convective periods, while weakly convective periods generally show lower aspect ratios, relatively more column-like shapes and somewhat greater asymmetry parameters. Results for strongly convective periods show that, with increasing cloud top temperature, the distortion parameter generally decreases, while the asymmetry parameter and effective radius increase. For one of the strongly convective periods, the rate at which effective radii increase with cloud top temperature is more than double that of the other periods, while the temperature dependence of the other microphysical quantities for this period is substantially weaker. Atmospheric state analysis indicates that these differences are concurrent with differences in middle-to-upper tropospheric zonal wind shear. The observed variation of microphysical properties may have significant effects on the shortwave radiative fluxes and cloud absorption associated with deep convection. Additionally, MODIS collection 5 effective radii are estimated to be biased small with an artificially narrow range. Collection 6 products are expected to have less severe biases that depend on cloud top temperature and atmospheric conditions.

van Diedenhoven, Bastiaan; Fridlind, Ann M.; Cairns, Brian; Ackerman, Andrew S.

2014-10-01

168

Crystal engineering: From design of crystal structures to fabrication of composite crystals  

NASA Astrophysics Data System (ADS)

This thesis reports how to design and control co-crystal structures from a kinetic point of view, and demonstrates the control of crystal morphology through understanding the kinetics and crystal structures. In chapter one, the in-situ atomic force microscope (AFM) was utilized to investigate how side chain on a glycine 2,5-diketopiperazine (GLYDKP) backbone can affect the assembly of GLYDKP, and showed that methyl groups cause larger energy barrier for crystallization. Because the introduction of functional group on the side chain could inevitably slow down the assembly process, a different approach should be considered. Chapter two shows that formic acid at low concentration can accelerate the assembly process without incorporating into the crystal structure. Because formic acid only crystallizes with GLYDKP in concentrated solution, these results prove that co-crystallization is a better method for incorporating functionalized molecules into a solid than direct modification of molecule itself. Chapter three focuses on the rational design of GLYDKP cocrystals by utilizing the observation found in chapter two. Structure of GLYDKP and formic acid crystal was analyzed to search possible guest molecules for cocrystal studies. This method successfully identified eleven molecules that crystallize with GLYDKP, and proved that crystal structure can be controlled through weak interactions such as C-H•••O=C and C-H•••Cl interactions. Chapter four and chapter five explore the possibility of using self-assembled process to control morphology of crystals and surface epitaxy. Metal(II) bis(imidazolium 2,b-pyridinedicarboxylate) complexes were chosen and two morphologies associated with different metal ions were found: rhombohedral (Type I) and rectangular (Type II) crystals. In this study, an additive was found to change the morphology of crystal from type I to type II, and then methods of producing various shapes of composite crystals were also established. These self-assembled procedures of making composite crystals at micron scale are very promising, because the fabrication will only relies on solvent, additives, or combination of them without using sophisticated crystallizers.

Luo, Tzy-Jiun Mark

169

Liquid crystal light valve structures  

NASA Technical Reports Server (NTRS)

An improved photosensor film and liquid crystal light valves embodying said film is provided. The photosensor film and liquid crystal light valve is characterized by a significant lower image retention time while maintaining acceptable photosensitivity. The photosensor film is produced by sputter depositing CdS onto an ITO substrate in an atmosphere of argon/H2S gas while maintaining the substrate at a temperature in the range of about 130 C to about 200 C and while introducing nitrogen gas into the system to the extent of not more than about 1% of plasma mixture. Following sputter deposition of the CdS, the film is annealed in an inert gas at temperatures ranging from about 300 C to about 425 C.

Koda, N. J. (inventor)

1985-01-01

170

High-temperature crystal structure of pyroxmangite  

Microsoft Academic Search

AssrRAcr High-temperature crystal-structure analysis of MnSiO, pyroxmangite indicates that ex- pansion of the structure takes place by expansion of the M cation

171

A discussion of mechanisms proposed to explain habit changes of vapor-grown ice crystals  

NASA Astrophysics Data System (ADS)

In the present work, surface kinetics processes that can contribute to the growth behavior of ice crystals from the vapor phase are revised and proposed interpretations of crystal habit changes are discussed. Following the main initial papers on this subject by Hallet, Mason et al. and Kobayashi, relationships are considered between linear growth rate and step velocity. More recent results obtained by Sei and Gonda (SG2) for molecular steps naturally formed on basal and prism surfaces are shown to confirm Hallet's interpretation of previous curves obtained for the velocity of giant steps that were artificially formed on basal surfaces only. The different behavior of the condensation coefficient ?( T) characterizing growth in pure water vapor, observed by Lamb and Scott for surfaces intersecting a substrate and by Sei and Gonda for free surfaces, is discussed by considering that ? is the product of the adsorption and accommodation coefficients ? and ?, respectively. It is noted that, as in previous works, ?=1 was assumed, the variations of ? discussed to interpret crystal habit changes were made to coincide with variations of ?. However, Sei and Gonda's results show that in the temperature range where crystal habit changes are observed, values of ?( T)?1 are found. As these depend on surface orientation, they should play an important role in the phenomenon. The dependence of crystal habits on two-dimensional nucleation is also discussed on the basis of measurements carried out by Nelson and Knight of the critical supersaturation ?cr on the basal and prism surfaces. A possible relationship between the curves for ?cr( T) and those of ?( T) derived from Sei and Gonda's results is suggested. The mechanisms determining the large anisotropy exhibited by crystals grown in atmospheric conditions are discussed by taking into account that the growth rate curves R( T) on the basal and prism surfaces show a correlation between maximum and minimum values, which are not observed for crystals grown in pure vapor.

Levi, Laura; Nasello, Olga B.

172

Crystal Structure of A-amylose: a Revisit from Synchrotron Microdiffraction Analysis of Single Crystals  

E-print Network

1 Crystal Structure of A-amylose: a Revisit from Synchrotron Microdiffraction Analysis of Single;2 Abstract The three-dimensional structure of A-amylose crystals, as a model of the crystal domains of A-sized single crystals. The resulting datasets allowed a determination of the structure with conventional X

Paris-Sud XI, Université de

173

Scattering of partially coherent electromagnetic beams by water droplets and ice crystals  

NASA Astrophysics Data System (ADS)

The conventional Lorenz-Mie theory is generalized for a case when the light source is partially spatially coherent. The influence of the degree of coherence of the incident field on the generalized Mueller matrix and the spectral degree of coherence of the scattered light is analytically studied by using the vector field instead of the scalar field to extend previous results on the angular intensity distribution. The results are compared with the Mueller matrix obtained from the Discrete Dipole Approximation (DDA) method, which is an average over an ensemble of stochastic incident beams. Special attention is paid to the Mueller matrix elements in the backward direction, and the results show some Mueller matrix elements, such as P22, depend monotonically on the coherence length of the incident beam. Therefore, detecting back scattering Mueller matrix elements may be a promising method to measure the degree of coherence. The new formalism is applied to cases of large spherical droplets in water clouds and hexagonal ice crystals in cirrus clouds. The corona and glory phenomena due to spheres and halos associated with hexagonal ice crystals are found to disappear if the incident light tends to be highly incoherent.

Liu, Jianping; Bi, Lei; Yang, Ping; Kattawar, George W.

2014-02-01

174

Radiative properties of visible and subvisible Cirrus: Scattering on hexagonal ice crystals  

NASA Technical Reports Server (NTRS)

One of the main objectives of the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) is to provide a better understanding of the physics of upper level clouds. The focus is on just one specific aspect of cirrus physics, namely on characterizing the radiative properties of single, nonspherical ice particles. The basis for further more extensive studies of the radiative transfer through upper level clouds is provided. Radiation provides a potential mechanism for strong feedback between the divergence of in-cloud radiative flux and the cloud microphysics and ultimately on the dynamics of the cloud. Some aspects of ice cloud microphysics that are relevant to the radiation calculations are described. Next, the Discrete Dipole Approximation (DDA) is introduced and some new results of scattering by irregular crystals are presented. The Anomalous Diffraction Theory (ADT) was adopted to investigate the scattering properties of even larger crystals. In this way the scattering properties of nonspherical particles were determined over a range of particle sizes.

Flatau, Piotr J.; Stephens, Graeme L.; Draine, Bruce T.

1990-01-01

175

Crystal structures of sialyltransferase from Photobacterium damselae.  

PubMed

Sialyltransferase structures fall into either GT-A or GT-B glycosyltransferase fold. Some sialyltransferases from the Photobacterium genus have been shown to contain an additional N-terminal immunoglobulin (Ig)-like domain. Photobacterium damselae ?2-6-sialyltransferase has been used efficiently in enzymatic and chemoenzymatic synthesis of ?2-6-linked sialosides. Here we report three crystal structures of this enzyme. Two structures with and without a donor substrate analog CMP-3F(a)Neu5Ac contain an immunoglobulin (Ig)-like domain and adopt the GT-B sialyltransferase fold. The binary structure reveals a non-productive pre-Michaelis complex, which are caused by crystal lattice contacts that prevent the large conformational changes. The third structure lacks the Ig-domain. Comparison of the three structures reveals small inherent flexibility between the two Rossmann-like domains of the GT-B fold. PMID:25451227

Huynh, Nhung; Li, Yanhong; Yu, Hai; Huang, Shengshu; Lau, Kam; Chen, Xi; Fisher, Andrew J

2014-12-20

176

Flies expand the repertoire of protein structures that bind ice.  

PubMed

An antifreeze protein (AFP) with no known homologs has been identified in Lake Ontario midges (Chironomidae). The midge AFP is expressed as a family of isoforms at low levels in adults, which emerge from fresh water in spring before the threat of freezing temperatures has passed. The 9.1-kDa major isoform derived from a preproprotein precursor is glycosylated and has a 10-residue tandem repeating sequence xxCxGxYCxG, with regularly spaced cysteines, glycines, and tyrosines comprising one-half its 79 residues. Modeling and molecular dynamics predict a tightly wound left-handed solenoid fold in which the cysteines form a disulfide core to brace each of the eight 10-residue coils. The solenoid is reinforced by intrachain hydrogen bonds, side-chain salt bridges, and a row of seven stacked tyrosines on the hydrophobic side that forms the putative ice-binding site. A disulfide core is also a feature of the similar-sized beetle AFP that is a ?-helix with seven 12-residue coils and a comparable circular dichroism spectrum. The midge and beetle AFPs are not homologous and their ice-binding sites are radically different, with the latter comprising two parallel arrays of outward-pointing threonines. However, their structural similarities is an amazing example of convergent evolution in different orders of insects to cope with change to a colder climate and provide confirmation about the physical features needed for a protein to bind ice. PMID:25561557

Basu, Koli; Graham, Laurie A; Campbell, Robert L; Davies, Peter L

2015-01-20

177

Data mining chemistry and crystal structure  

NASA Astrophysics Data System (ADS)

The availability of large amounts of data generated by high-throughput computing and experimentation has generated interest in the application of machine learning techniques to materials science. Machine learning of materials behavior requires the use of feature vectors that capture compositional or structural information influence a target property. We present methods for assessing the similarity of compositions, substructures, and crystal structures. Similarity measures are important for the classification and clustering of data points, allowing for the organization of data and the prediction of materials properties. The similarity functions between ions, compositions, substructures and crystal structure are based upon a data-mined probability with which two ions will substitute for each other within the same structure prototype. The composition similarity is validated via the prediction of crystal structure prototypes for oxides from the Inorganic Crystal Structure Database. It performs particularly well on the quaternary oxides, predicting the correct prototype within 5 guesses 90% of the time. The sustructural similarity is validated via the prediction of Li insertion sites in the oxides; it finds all of the Li sites with less than 8 incorrect guesses 90% of the time.

Yang, Lusann W.

178

Crystal structure and chirality of natural floridoside.  

PubMed

The crystal structure and absolute configuration of natural floridoside (2-O-alpha-D-galactopyranosylglycerol) were determined by single-crystal X-ray diffraction analysis. The space group is orthorhombic P2(1)2(1)2(1) with Z=4, a=4.885(1), b=9.734(1), c=23.886(2) A at 296 +/- 2 K. The structure was solved by a direct method and refined to R=0.0351 from 1914 reflections of Cu Kalpha radiation. PMID:14572727

Simon-Colin, Christelle; Michaud, François; Léger, Jean-Michel; Deslandes, Eric

2003-10-31

179

The crystal structure and crystal chemistry of fernandinite and corvusite  

USGS Publications Warehouse

Using type material of fernandinite from Minasragra, Peru, and corvusite from the Jack Claim, La Sal Mountains, Utah, the properties and crystal chemistry of these minerals have been determined by Rietveld analysis of the powder X-ray-diffraction patterns. The crystal structure of both species is isotypic with the V2O5 -type layer first found for ??-Ag0.68V2O5; it consists of chains of VO6 octahedra linked by opposite corners (parallel to b) condensed by edge-sharing to form the layer. The vanadium has average valence 4.8, and the resulting layer-charge is balanced by varying amounts of Ca, Na, and K in the interlayer region accompanied by labile water. This study has confirmed the validity of fernandinite as a unique mineral species. It is closely related to corvusite, from which it is distinguished on the basis of the dominant interlayer cation: Ca for fernandinite, Na for curvusite. -Authors

Evans, H.T., Jr.; Post, J.E.; Ross, D.R.; Nelen, J.A.

1994-01-01

180

Automated protein crystal structure determination using elves  

PubMed Central

Efficient determination of protein crystal structures requires automated x-ray data analysis. Here, we describe the expert system elves and its use to determine automatically the structure of a 12-kDa protein. Multiwavelength anomalous diffraction analysis of a selenomethionyl derivative was used to image the Asn-16-Ala variant of the GCN4 leucine zipper. In contrast to the parallel, dimeric coiled coil formed by the WT sequence, the mutant unexpectedly formed an antiparallel trimer. This structural switch reveals how avoidance of core cavities at a single site can select the native fold of a protein. All structure calculations, including indexing, data processing, locating heavy atoms, phasing by multiwavelength anomalous diffraction, model building, and refinement, were completed without human intervention. The results demonstrate the feasibility of automated methods for determining high-resolution, x-ray crystal structures of proteins. PMID:14752198

Holton, James; Alber, Tom

2004-01-01

181

Solar radiative transfer in cirrus clouds. I - Single-scattering and optical properties of hexagonal ice crystals. II - Theory and computations of multiple scattering in an anisotropic medium  

NASA Technical Reports Server (NTRS)

The light scattering and absorption programs of Cai and Liou (1982) and Takano and Jaweera (1985) are extended to include hexagonal ice crystals randomly and horizontally oriented in space. The scattering and polarization results for the ice crystals are calculated. The results are compared with measurement data. The single-scattering properties for horizontally oriented columns and plates are presented and used to explain halos and arcs observed in the atmopshere. In the second section, the theory and computations for multiple scattering in cirrus clouds containing oriented ice crystals are presented. The radiative transfer in clouds composed of horizontally oriented ice crystals is formulated. Also, reflected and transmitted intensities, planetary albedo, and polarization in multiple scattering by ice crystals are discussed.

Takano, Yoshihide; Liou, Kuo-Nan

1989-01-01

182

A design protocol for tailoring ice-templated scaffold structure.  

PubMed

In this paper, we show, for the first time, the key link between scaffold architecture and latent heat evolution during the production of porous biomedical collagen structures using freeze-drying. Collagen scaffolds are used widely in the biomedical industry for the repair and reconstruction of skeletal tissues and organs. Freeze-drying of collagen slurries is a standard industrial process, and, until now, the literature has sought to characterize the influence of set processing parameters including the freezing protocol and weight percentage of collagen. However, we are able to demonstrate, by monitoring the local thermal events within the slurry during solidification, that nucleation, growth and annealing processes can be controlled, and therefore we are able to control the resulting scaffold architecture. Based on our correlation of thermal profile measurements with scaffold architecture, we hypothesize that there is a link between the fundamental freezing of ice and the structure of scaffolds, which suggests that this concept is applicable not only for collagen but also for ceramics and pharmaceuticals. We present a design protocol of strategies for tailoring the ice-templated scaffold structure. PMID:24402916

Pawelec, K M; Husmann, A; Best, S M; Cameron, R E

2014-03-01

183

Response of salt structures to ice-sheet loading: implications for ice-marginal and subglacial processes  

NASA Astrophysics Data System (ADS)

During the past decades the effect of glacioisostatic adjustment has received much attention. However, the response of salt structures to ice-sheet loading and unloading is poorly understood. Our study aims to test conceptual models of the interaction between ice-sheet loading and salt structures by finite-element modelling. The results are discussed with regard to their implications for ice-marginal and subglacial processes. Our models consist of 2D plane-strain cross-sections, which represent simplified geological cross-sections from the Central European Basin System. The model layers represent (i) sedimentary rocks of elastoplastic rheology, (ii) a viscoelastic diapir and layer of salt and (iii) an elastoplastic basement. On top of the model, a temporarily variable pressure simulates the advance and retreat of an ice sheet. The durations of the individual loading phases were defined to resemble the durations of the Pleistocene ice advances in northern central Europe. The geometry and rheology of the model layers and the magnitude, spatial distribution and timing of ice-sheet loading were systematically varied to detect the controlling factors. All simulations indicate that salt structures respond to ice-sheet loading. An ice advance towards the diapir causes salt flow from the source layer below the ice sheet towards the diapir, resulting in an uplift of up to +4 m. The diapir continues to rise as long as the load is applied to the source layer but not to the crest of the diapir. When the diapir is transgressed by the ice sheet the diapir is pushed down (up to -36 m) as long as load is applied to the crest of the diapir. During and after ice unloading large parts of the displacement are compensated by a reversal of the salt flow. Plastic deformation of the overburden is restricted to the area immediately above the salt diapir. The displacements after unloading range between -3.1 and +2.7 m. Larger displacements are observed in models with deep-rooted diapirs, thicker ice sheets, longer duration of the loading phase, thicker salt source layers and lower viscosity of the salt. The rise or fall of diapirs triggered or amplified by ice-sheet loading are likely to affect glacigenic deformation, erosion and deposition above the diapir and within the rim synclines. Ice-load induced uplift in front of an ice sheet will provide favourable conditions for the formation of push moraines, for example by creating a topographic obstacle and inclining potential detachments. Subglacial subsidence of salt structures will enhance erosion by providing a preferential drainage pathway and fracturing of the overburden of the salt structure and thereby contribute to the incision of tunnel valleys. However, the resulting displacements are probably too low to have a marked effect on the advance or retreat pattern of the ice sheets.

Lang, Jörg; Hampel, Andrea; Brandes, Christian; Winsemann, Jutta

2014-10-01

184

Development of self-actuated in-flight de-icing technology utilizing smart structure concepts  

NASA Astrophysics Data System (ADS)

Ice accretions on aircraft components have severe and sometimes fatal effects. Aircraft wings are one of the many components that are prone to severe ice accretions. The de-icing/anti-icing technologies currently being used are bulky, cover the airfoil surface and consume high energies. Addressing these drawbacks, the current study proposes a novel de-icing technique utilizing lightweight piezoelectric actuators to break the weak adhesive shear bond of ice-substrate interface. When structures are excited at their natural frequencies, high shear stresses are generated in certain modes. These high shear stresses can break the weak adhesive shear bond of ice-substrate interface with minimal energy inputs. The proposed de-icing technique is applied on two structures, (1) a laminated composite cantilever rectangular plate, and (2) a prototyped aluminum leading edge. Theoretical investigations are fist performed to determine the frequencies and modes in which high amount of shear stresses and debonding of the ice layers occurred. After determining optimal actuator locations, experimental set-up is designed and structures are built. Experimentation of the proposed technique is carried out inside a freezer by forming two types of ice layers on the surfaces and exciting the structures to the determined frequencies. Testing is carried out at five different temperatures ranging from 5°F to 25°F. De-icing is observed for both types of ice layers in both the structures. While the average de-icing times increased with decreased temperatures, longer de-icing times are noted for the aluminum leading edge. In addition, energy requirements of the piezoelectric actuators to actuate an adaptive composite wing structure are evaluated and a composite material is designed to improve deicing of the leading edge.

Venna, Suresh Venkata

185

Requirements for structure determination of aperiodic crystals  

SciTech Connect

Using computer simulation, we compared the Patterson functions of one-dimensional (1D) randomly packed and quasiperiodic Fibonacci lattices with or without disorder, and a 2D Penrose lattice and random packing of pentagons (icosahedral glass model). Based on these comparisons, we derived some empirical guidelines for distinguishing ideal quasicrystals from aperiodic crystals with disorder using diffraction data. In contrast to periodic crystals, it is essential to include the background to obtain correct Patterson functions of the average structure since the background contains unresolved peaks. In particular, a Bragg peak scattering measurement {ital cannot}, in general, determine the structure of aperiodic crystals. Instead, a diffuse scattering measurement is required, which determines the absolute value of the diffraction background, in addition to the Bragg peaks. We further estimate that, dependent upon the disorder present, it is necessary to include up to 75% of the total diffracted intensity in any analysis.

Li, X.; Stern, E.A.; Ma, Y. (Department of Physics, FM-15, University of Washington, Seattle, Washington 98195 (US))

1991-01-15

186

Nonlinear dynamic response of a simple ice-structure interaction model  

SciTech Connect

The problem addressed in the continuous indentation of a ship or offshore structure into an ice sheet. The impacting ship or offshore structure is represented by a mass-spring-dashpot system having a constant velocity relative to the ice sheet. The dynamic response of this simple analogue model of ice-structure interaction is studied in considerable detail. The complicated, highly nonlinear dynamic response is due to intermittent ice breakage and intermittent contact of the structure with the ice. Periodic motions are found and the periodicity for a particular system is dependent upon initial conditions. For a representative system, a Poincare map is presented showing the fixed points. A description of some of the effects of random variations in system parameters is also presented. Some implications of these findings regarding structural design for ice interaction are discussed.

Karr, D.G.; Troesch, A.W.; Wingate, W.C. (Univ. of Michigan, Ann Arbor, MI (United States). Dept. of Naval Architecture and Marine Engineering)

1993-11-01

187

Shear induced structures in crystallizing cocoa butter  

NASA Astrophysics Data System (ADS)

Cocoa butter is the main structural component of chocolate and many cosmetics. It crystallizes in several polymorphs, called phases I to VI. We used Synchrotron X-ray diffraction to study the effect of shear on its crystallization. A previously unreported phase (phase X) was found and a crystallization path through phase IV under shear was observed. Samples were crystallized under shear from the melt in temperature controlled Couette cells, at final crystallization temperatures of 17.5^oC, 20^oC and 22.5^oC in Beamline X10A of NSLS. The formation of phase X was observed at low shear rates (90 s-1) and low crystallization temperature (17.5^oC), but was absent at high shear (720 s-1) and high temperature (20^oC). The d-spacing and melting point suggest that this new phase is a mixture rich on two of the three major components of cocoa butter. We also found that, contrary to previous reports, the transition from phase II to phase V can happen through the intermediate phase IV, at high shear rates and temperature.

Mazzanti, Gianfranco; Guthrie, Sarah E.; Sirota, Eric B.; Marangoni, Alejandro G.; Idziak, Stefan H. J.

2004-03-01

188

Crystal Structure of Tetraphenyltin(1~) (a Redetermination)  

Microsoft Academic Search

The crystal structure of tetraphenyltin(1v) has been redetermined at 295(1) K, full-matrix least-squares refinement yielding a residual of 0.031 for 557 independent 'observed' reflections. The tin-phenyl carbon distance is 2.143(5) A, with the angles subtended at the tin atom being 108.9(2) and 110.5(2)\\

Lutz M. Engelhardt; Wing-Por Leung; Colin L. Raston; Allan H. White; W. A. Nedlands

189

Hammerhead Ribozyme Crystal Structures and Catalysis  

E-print Network

are comparatively large and complex catalytic RNAs, the identification of the hammerhead ribozyme offered hopeCHAPTER 4 Hammerhead Ribozyme Crystal Structures and Catalysis WILLIAM SCOTT Center in fact those of hammerhead ribozymes, but they seemed to create more questions than compelling

Scott, William

190

Crystallization and preliminary X-ray crystallographic studies of the ice-binding protein from the Arctic [correction of Aantarctic] yeast Leucosporidium sp. AY30.  

PubMed

Freezing is dangerous to cellular organisms because it causes an increase in the concentration of ions and other solutes in the plasma, denatures biomolecules and ruptures cell membranes. Some cold-adapted organisms can survive at subzero temperatures by producing proteins that bind to and inhibit the growth of ice crystals. To better understand the structure and function of these proteins, the ice-binding protein from Leucosporidium sp. AY30 (LeIBP) was overexpressed, purified and crystallized. The native crystal belonged to space group P4(3)2(1)2, with unit-cell parameters a=b=98.05, c=106.13?Å. Since LeIBP lacks any cysteine or methionine residues, two leucine residues (Leu69 and Leu155) were substituted by methionine residues in order to obtain selenomethionine-substituted LeIBP for use in multiple-wavelength anomalous diffraction (MAD) phasing. The selenomethionine-substituted mutant crystallized in the same space group as the native protein. PMID:21795798

Park, Ae Kyung; Park, Kyoung Sun; Kim, Hak Jun; Park, Hyun; Ahn, In Young; Chi, Young Min; Moon, Jin Ho

2011-07-01

191

Importance of aggregation and small ice crystals in cirrus clouds, based on observations and an ice particle growth model  

NASA Technical Reports Server (NTRS)

The 1 November 1986 FIRE I case study was used to test an ice particle growth model which predicts bimodal size spectra in cirrus clouds. The model was developed from an analytically based model which predicts the height evolution of monomodal ice particle size spectra from the measured ice water content (IWC). Size spectra from the monomodal model are represented by a gamma distribution, N(D) = N(sub o)D(exp nu)exp(-lambda D), where D = ice particle maximum dimension. The slope parameter, lambda, and the parameter N(sub o) are predicted from the IWC through the growth processes of vapor diffusion and aggregation. The model formulation is analytical, computationally efficient, and well suited for incorporation into larger models. The monomodal model has been validated against two other cirrus cloud case studies. From the monomodal size spectra, the size distributions which determine concentrations of ice particles less than about 150 mu m are predicted.

Mitchell, David L.; Chai, Steven K.; Dong, Yayi; Arnott, W. Patrick; Hallett, John

1993-01-01

192

Crystal structure of a plectonemic RNA supercoil  

SciTech Connect

Genome packaging is an essential housekeeping process in virtually all organisms for proper storage and maintenance of genetic information. Although the extent and mechanisms of packaging vary, the process involves the formation of nucleic-acid superstructures. Crystal structures of DNA coiled coils indicate that their geometries can vary according to sequence and/or the presence of stabilizers such as proteins or small molecules. However, such superstructures have not been revealed for RNA. Here we report the crystal structure of an RNA supercoil, which displays one level higher molecular organization than previously reported structures of DNA coiled coils. In the presence of an RNA-binding protein, two interlocking RNA coiled coils of double-stranded RNA, a 'coil of coiled coils', form a plectonemic supercoil. Molecular dynamics simulations suggest that protein-RNA interaction is required for the stability of the supercoiled RNA. This study provides structural insight into higher order packaging mechanisms of nucleic acids.

Stagno, Jason R.; Ma, Buyong; Li, Jess; Altieri, Amanda S.; Byrd, R. Andrew; Ji, Xinhua (NCI); (Maryland)

2012-12-14

193

Crystal-structure calculations with distorted ions  

NASA Astrophysics Data System (ADS)

We present the polarization-included electron-gas (PEG) model for crystal structures, which is similar to the modified-electron-gas (MEG) model for crystal structures, but in which the anions can distort from spherical symmetry. This nonspherical distortion is important when the anions occupy low-symmetry positions. For SiO2 quartz, SiO2 cristobalite, BeF2 quartz, and the zeolite sodalite, which have open crystal structures, the structures and energies calculated with the PEG model are in much better agreement with experiment than those calculated with the MEG model. The improved structural results are due mainly to smaller and more accurate cation-anion-cation bond angles. For SiO2 stishovite, TiO2 rutile, and Mg2SiO4 spinel, which have more closely packed crystal structures, the structures are modeled well with both the PEG and MEG models, but the energies are more accurately calculated with the PEG model. The improved results for the energies are due to the stronger bonds formed when charge density moves into the bonding regions. Electron-distribution plots are in good agreement with those from accurate band-structure calculations for the cristobalite and stishovite phases of silica. The electron-distribution plots show that the nonspherical distortions increase from BeF2 to TiO2 to SiO2, demonstrating that the extent of covalent bonding increases from BeF2 to TiO2 to SiO2, in agreement with electronegativity differences. We find that covalent effects are not as important in MgSiO3 perovskite as they are in the silica polymorphs quartz, cristobalite and stishovite, and Mg2SiO4 spinel.

Lacks, Daniel J.; Gordon, Roy G.

1993-08-01

194

Crystal Structure of Human Enterovirus 71  

SciTech Connect

Enterovirus 71 is a picornavirus associated with fatal neurological illness in infants and young children. Here, we report the crystal structure of enterovirus 71 and show that, unlike in other enteroviruses, the 'pocket factor,' a small molecule that stabilizes the virus, is partly exposed on the floor of the 'canyon.' Thus, the structure of antiviral compounds may require a hydrophilic head group designed to interact with residues at the entrance of the pocket.

Plevka, Pavel; Perera, Rushika; Cardosa, Jane; Kuhn, Richard J.; Rossmann, Michael G. (Purdue); (Sentinext)

2013-04-08

195

Crystal structure of human enterovirus 71.  

PubMed

Enterovirus 71 is a picornavirus associated with fatal neurological illness in infants and young children. Here, we report the crystal structure of enterovirus 71 and show that, unlike in other enteroviruses, the "pocket factor," a small molecule that stabilizes the virus, is partly exposed on the floor of the "canyon." Thus, the structure of antiviral compounds may require a hydrophilic head group designed to interact with residues at the entrance of the pocket. PMID:22383808

Plevka, Pavel; Perera, Rushika; Cardosa, Jane; Kuhn, Richard J; Rossmann, Michael G

2012-06-01

196

Crystal structures of dibromochloromethane and bromodichloromethane  

Microsoft Academic Search

The crystal structures of CHB2Cl and CHBrCl2 have been investigated using neutron powder profile techniques. These structures were found to be similar to that of CHBr3 in its lowest temperature triclinic phase. Whereas CHBr3 has three phases, both CHBr2Cl and CHBrCl2 have only one. The two CHBr3 phases which are absent in these compounds require the molecules to have threefold

B. H. Torrie; O. S. Binbrek; I. P. Swainson; B. M. Powell

1999-01-01

197

Crystal structures of dibromochloromethane and bromodichloromethane  

Microsoft Academic Search

The crystal structures of CHB2C1 and CHBrC12 have been investigated using neutron powder profile techniques. These structures were found to be similar to that of CHBr3 in its lowest temperature triclinic phase. Whereas CHBr3 has three phases, both CHBr2C1 and CHBrC12 have only one. The two CHBr3 phases which are absent in these compounds require the molecules to have threefold

B. H. TORRIE; O. S. BINBREK; I. P. SWAINSON; B. M. POWELL

1999-01-01

198

Crystal Structure of Cesium-V  

NASA Astrophysics Data System (ADS)

The crystal structure of the high-pressure phase cesium-V was investigated using monochromatic synchrotron x-ray diffraction. Full profile refinements of powder diffraction data resulted in a solution with space group Cmca and 16 atoms in the orthorhombic unit cell. The Cs-V structure can be viewed as a distorted fcc structure. Atoms occupy two different Wyckoff positions with 10-fold and 11-fold coordination, respectively. This new structure type is considered a possible candidate for high-pressure phases of other elemental metals.

Schwarz, U.; Takemura, K.; Hanfland, M.; Syassen, K.

1998-09-01

199

Photonic Crystal Laser-Driven Accelerator Structures  

SciTech Connect

Laser-driven acceleration holds great promise for significantly improving accelerating gradient. However, scaling the conventional process of structure-based acceleration in vacuum down to optical wavelengths requires a substantially different kind of structure. We require an optical waveguide that (1) is constructed out of dielectric materials, (2) has transverse size on the order of a wavelength, and (3) supports a mode with speed-of-light phase velocity in vacuum. Photonic crystals---structures whose electromagnetic properties are spatially periodic---can meet these requirements. We discuss simulated photonic crystal accelerator structures and describe their properties. We begin with a class of two-dimensional structures which serves to illustrate the design considerations and trade-offs involved. We then present a three-dimensional structure, and describe its performance in terms of accelerating gradient and efficiency. We discuss particle beam dynamics in this structure, demonstrating a method for keeping a beam confined to the waveguide. We also discuss material and fabrication considerations. Since accelerating gradient is limited by optical damage to the structure, the damage threshold of the dielectric is a critical parameter. We experimentally measure the damage threshold of silicon for picosecond pulses in the infrared, and determine that our structure is capable of sustaining an accelerating gradient of 300 MV/m at 1550 nm. Finally, we discuss possibilities for manufacturing these structures using common microfabrication techniques.

Cowan, Benjamin M.

2007-08-22

200

Finite-difference time domain method for light scattering by small ice crystals in three-dimensional space  

Microsoft Academic Search

The finite-difference time domain (FDTD) method for the solution of light scattering by nonspherical particles has been developed for small ice crystals of hexagonal shapes including solid and hollow columns, plates, and bullet rosettes commonly occurring in cirrus clouds. To account for absorption, we have introduced the effec- tive permittivity and conductivity to circumvent the required complex calculations in the

Ping Yang; K. N. Liou

1996-01-01

201

Integrable structure of modified melting crystal model  

E-print Network

Our previous work on a hidden integrable structure of the melting crystal model (the U(1) Nekrasov function) is extended to a modified crystal model. As in the previous case, "shift symmetries" of a quantum torus algebra plays a central role. With the aid of these algebraic relations, the partition function of the modified model is shown to be a tau function of the 2D Toda hierarchy. We conjecture that this tau function belongs to a class of solutions (the so called Toeplitz reduction) related to the Ablowitz-Ladik hierarchy.

Kanehisa Takasaki

2012-08-22

202

The crystal structure of nickel arsenide  

Microsoft Academic Search

The crystal structure has been redetermined using X-ray diffraction data collected from a triply twinned crystal, a=10.8568(8), b=6.2682(5), c=5.0340(7)AA, Cmc21 grown using I2 as the vapour transport agent. The weak orthorhombic superlattice originally observed by convergent-beam electron diffraction, corresponding to the freezing in of a q=1\\/3(1120) Sigma 6 sub-cell phonon mode, is confirmed, and magnitudes of the atomic displacements from

J. G. Thompson; A. D. Rae; R. L. Withers; T. R. Welberry; A. C. Willis

1988-01-01

203

T-1020 NaI crystal test for DM-Ice  

SciTech Connect

This is a memorandum of understanding between the Fermi National Accelerator Laboratory (Fermilab) and the experiments of the NaI Crystal Test for DM-Ice from the University of Wisconsin who have committed to participate in detector tests to be carried out during the 2011-2012 Fermilab Neutrino program. The memorandum is intended primarily for the purpose of recording expectations for budget estimates and work allocations for Fermilab, the funding agencies and the participating institutions. It reflects an arrangement that currently is satisfactory to the parties; however, it is recognized and anticipated that changing circumstances of the evolving research program will necessitate revisions. The parties agree to modify this memorandum to reflect such required adjustments. Actual contractual obligations will be set forth in separate documents. The DM-Ice collaboration is designing a sodium-iodide (NaI) based detector for a direct dark matter search. The detectors should have low readout noise and background levels to carry out a sensitive search. A 17-kg version of the experiment is running at the South Pole, 2500 m deep in the Antarctic ice, and a large scale experiment is currently being designed. One of the keys to the success of the experiment is to have a good understanding of the background levels intrinsic in the NaI detectors. To measure the background level, the detectors have to be shielded against cosmic rays. The lead shielding used for DAMIC in the Minos Underground Areas is a well-suited location for this test since it offers enough overburden to shield against cosmic rays, lead shielding, and experimental infrastructure. The goal of the test is to assess the background levels in the detector and to assess the characteristics of phosphorescence induced by muons and 100 keV-3 MeV gamma rays.

Maruyama, Reina; Heeger, Karsten; Pierpoint, Zachary; Pettus, Walter; Broerman, Benjamin; Hilgenberg, Chris; Webber, David; /Wisconsin U., Madison

2011-11-03

204

Ice island creation, drift, recurrences, mechanical properties, and interactions with arctic offshore oil production structures  

Microsoft Academic Search

Research and engineering studies on first-year sea ice for over two decades has resulted in the design, construction, and operation of jacket platforms, of artificial islands, and of massive gravity structures which routinely withstand moving sea ice of thickness up to 2 meters. However, the less-common interactions between such structures and moving multiyear ice ([ge]3 meters thick), and also moving

W. M. Sackinger; M. O. Jeffries; Fucheng Li; Mingchi. Lu

1991-01-01

205

Millimeter wave scattering from ice crystals and their aggregates: Comparing cloud model simulations with X- and Ka-band radar measurements  

NASA Astrophysics Data System (ADS)

Arctic clouds are often mixed-phase, such that the radiative properties of the clouds are a strong function of the relative amounts of cloud liquid and ice. Modeling studies have shown that the poorly understood ice phase processes are the regulators of the liquid water fraction. However, evaluating the fidelity of the model ice parameterizations has proven to be a difficult task. This study evaluates results of different ice microphysics representations in a cloud resolving model (CRM) using cloud radar measurements. An algorithm is presented to generate realistic ice crystals and their aggregates from which radar backscattering cross sections may be calculated using a generalized solution for a cluster of spheres. The aggregate is composed of a collection of ice crystals, each of which is constructed from a cluster of tiny ice spheres. Each aggregate satisfies the constraints set by the component crystal type and the mass-dimensional relationship used in the cloud resolving model, but is free to adjust its aspect ratio. This model for calculating radar backscattering is compared to two spherical and two spheroidal (bulk model) representations for ice hydrometeors. It was found that a refined model for representing the ice hydrometeors, both pristine crystals and their aggregates, is required in order to obtain good comparisons between the CRM calculations and the radar measurements. The addition of the radar-CRM comparisons to CRM-in situ measurements comparisons allowed conclusions about the appropriateness of different CRM ice microphysics parameterizations.

Botta, Giovanni; Aydin, Kultegin; Verlinde, Johannes; Avramov, Alexander E.; Ackerman, Andrew S.; Fridlind, Ann M.; McFarquhar, Greg M.; Wolde, Mengistu

2011-01-01

206

Convective Troposphere-Stratosphere Transport in the Tropics and Hydration by ice Crystals Geysers  

NASA Astrophysics Data System (ADS)

Twenty-five years ago the suggestion was made by Danielsen of direct fast convective penetration of tropospheric air in the stratosphere over land convective systems. Although the existence of the mechanism is accepted, it was thought to be rare and thus its contribution to Troposphere-Stratosphere Transport (TST) of chemical species and water vapour at global scale unimportant at global scale. In contrast to this assumption, observations of temperature, water vapour, ice particles, long-lived tropospheric species during HIBISCUS, TROCCINOX and SCOUT-O3 over Brazil, Australia and Africa and more recently CALIPSO aerosols observations suggest that it is a general feature of tropical land convective regions in the summer. Particularly relevant to stratospheric water vapour is the observation of geyser like ice crystals in the TTL over overshooting events which may result in the moistening of the stratosphere. Although such events successfully captured by small scale Cloud-Resolving Models may have a significant impact on stratospheric ozone chemistry and climate, they are currently totally ignored by NWPs, CTMs and CCMs. Several recent balloon and aircraft observations of overshoots and CRM simulations will be shown illustrating the mechanism, as well as observations from a variety of satellites suggesting a significant impact at global scale.

Pommereau, J.

2008-12-01

207

Crystal structure of low-symmetry rondorfite  

SciTech Connect

The crystal structure of an aluminum-rich variety of the mineral rondorfite with the composition Ca{sub 16}[Mg{sub 2}(Si{sub 7}Al)(O{sub 31}OH)]Cl{sub 4} from the skarns of the Verkhne-Chegemskoe plateau (the Kabardino-Balkarian Republic, the Northern Caucasus Region, Russia) was solved in the triclinic space group with the unit-cell parameters a = 15.100(2) Angstrom-Sign , b = 15.110(2) Angstrom-Sign , c = 15.092(2) Angstrom-Sign , {alpha} = 90.06(1) Degree-Sign , {beta} = 90.01(1) Degree-Sign , {gamma} = 89.93(1) Degree-Sign , Z = 4, sp. gr. P1. The structural model consisting of 248 independent atoms was determined by the phase-correction method and refined to R = 3.8% with anisotropic displacement parameters based on all 7156 independent reflections with 7156 F > 3{sigma}(F). The crystal structure is based on pentamers consisting of four Si tetrahedra linked by the central Mg tetrahedron. The structure can formally be refined in the cubic space group (a = 15.105 Angstrom-Sign , sp. gr. Fd 3 bar , seven independent positions) with anisotropic displacement parameters to R = 2.74% based on 579 reflections with F > 3{sigma}(F) without accounting for more than 1000 observed reflections, which are inconsistent with the cubic symmetry of the crystal structure.

Rastsvetaeva, R. K., E-mail: rast@ns.crys.ras.ru [Russian Academy of Sciences, Shubnikov Institute of Crystallography (Russian Federation); Zadov, A. E. [NPO Neokhim (Russian Federation); Chukanov, N. V. [Russian Academy of Sciences, Institute of Problems of Chemical Physics (Russian Federation)

2008-03-15

208

Crystal structure of low-symmetry rondorfite  

SciTech Connect

The crystal structure of an aluminum-rich variety of the mineral rondorfite with the composition Ca{sub 16}[Mg{sub 2}(Si{sub 7}Al)(O{sub 31}OH)]Cl{sub 4} from the skarns of the Verkhne-Chegemskoe plateau (the Kabardino-Balkarian Republic, the Northern Caucasus Region, Russia) was solved in the triclinic space group with the unit-cell parameters a = 15.100(2) A, b = 15.110(2) A, c = 15.092(2) A, {alpha} = 90.06(1) deg., {beta} = 90.01(1) deg., {gamma} = 89.93(1) deg., Z = 4, sp. gr. P1. The structural model consisting of 248 independent atoms was determined by the phase-correction method and refined to R = 3.8% with anisotropic displacement parameters based on all 7156 independent reflections with 7156 F > 3{sigma}(F). The crystal structure is based on pentamers consisting of four Si tetrahedra linked by the central Mg tetrahedron. The structure can formally be refined in the cubic space group (a = 15.105 A, sp. gr. Fd-bar 3, seven independent positions) with anisotropic displacement parameters to R = 2.74% based on 579 reflections with F > 3{sigma}(F) without accounting for more than 1000 observed reflections, which are inconsistent with the cubic symmetry of the crystal structure.

Rastsvetaeva, R. K. [Russian Academy of Sciences, Shubnikov Institute of Crystallography (Russian Federation)], E-mail: rast@ns.crys.ras.ru; Zadov, A. E. [NPO Neokhim (Russian Federation); Chukanov, N. V. [Russian Academy of Sciences, Institute of Problems of Chemical Physics (Russian Federation)

2008-03-15

209

Persistent hydrogen bonding in polymorphic crystal structures.  

PubMed

The significance of hydrogen bonding and its variability in polymorphic crystal structures is explored using new automated structural analysis methods. The concept of a chemically equivalent hydrogen bond is defined, which may be identified in pairs of structures, revealing those types of bonds that may persist, or not, in moving from one polymorphic form to another. Their frequency and nature are investigated in 882 polymorphic structures from the Cambridge Structural Database. A new method to compare conformations of equivalent molecules is introduced and applied to derive distinct subsets of conformational and packing polymorphs. The roles of chemical functionality and hydrogen-bond geometry in persistent interactions are systematically explored. Detailed structural comparisons reveal a large majority of persistent hydrogen bonds that are energetically crucial to structural stability. PMID:19155561

Galek, Peter T A; Fábián, László; Allen, Frank H

2009-02-01

210

Bathymetry and geological structures beneath the Ross Ice Shelf at the mouth of Whillans Ice Stream, West Antarctica, modeled from ground-based gravity measurements  

NASA Astrophysics Data System (ADS)

Grounding zones of ice sheets and contiguous ice shelves are important in understanding ice sheet dynamics, as key processes that influence the grounded ice and its discharge into the ocean occur in these regions. Ice-ocean interactions are controlled by the relatively poorly known bathymetry and the configuration of the cavity beneath ice shelves. In addition, knowledge of submarine geological structures and their distributions contributes to understanding the dynamic history of the glaciers and ice streams feeding the ice shelves. However, detailed geophysical surveys of these areas remain scarce due largely to the logistic difficulties of obtaining observational data about the subglacial environment beneath an ice shelf. Here we present a 3D model of the bathymetry and geological structures beneath the Ross Ice Shelf in an embayment at the mouth of Whillans Ice Stream. We use gravity data collected at 82 locations with a portable gravimeter, in conjunction with high-resolution active-source seismic and ice-penetrating radar data to constrain thicknesses of the water column, sediments, ice and firn where the data sets overlap. The active-source seismic survey revealed a shallow water column and soft sediments approximately 15 km seaward of the grounding zone. We explore the extent of such water and sedimentary columns, and the density of the sediment, in a ~500 km-2 embayment that is roughly triangular in shape. Finally, we discuss the uncertainties and trade-offs of the various methods.

Muto, A.; Christianson, K.; Horgan, H.; Anandakrishnan, S.; Alley, R. B.

2012-12-01

211

5.841 Crystal Structure Refinement, Fall 2006  

E-print Network

This course in crystal structure refinement examines the practical aspects of crystal structure determination from data collection strategies to data reduction and basic and advanced refinement problems of organic and ...

Mueller, Peter

212

5.067 Crystal Structure Refinement, Fall 2007  

E-print Network

This course in crystal structure refinement examines the practical aspects of crystal structure determination from data collection strategies to data reduction and basic and advanced refinement problems of organic and ...

Mueller, Peter

213

CALYPSO: A method for crystal structure prediction  

NASA Astrophysics Data System (ADS)

We have developed a software package CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) to predict the energetically stable/metastable crystal structures of materials at given chemical compositions and external conditions (e.g., pressure). The CALYPSO method is based on several major techniques (e.g. particle-swarm optimization algorithm, symmetry constraints on structural generation, bond characterization matrix on elimination of similar structures, partial random structures per generation on enhancing structural diversity, and penalty function, etc.) for global structural minimization from scratch. All of these techniques have been demonstrated to be critical to the prediction of global stable structure. We have implemented these techniques into the CALYPSO code. Testing of the code on many known and unknown systems shows high efficiency and the highly successful rate of this CALYPSO method [Y. Wang, J. Lv, L. Zhu, Y. Ma, Phys. Rev. B 82 (2010) 094116] [29]. In this paper, we focus on descriptions of the implementation of CALYPSO code and why it works.

Wang, Yanchao; Lv, Jian; Zhu, Li; Ma, Yanming

2012-10-01

214

Crystal structure of riboflavin synthase  

SciTech Connect

Riboflavin synthase catalyzes the dismutation of two molecules of 6,7-dimethyl-8-(1'-D-ribityl)-lumazine to yield riboflavin and 4-ribitylamino-5-amino-2,6-dihydroxypyrimidine. The homotrimer of 23 kDa subunits has no cofactor requirements for catalysis. The enzyme is nonexistent in humans and is an attractive target for antimicrobial agents of organisms whose pathogenicity depends on their ability to biosynthesize riboflavin. The first three-dimensional structure of the enzyme was determined at 2.0 {angstrom} resolution using the multiwavelength anomalous diffraction (MAD) method on the Escherichia coli protein containing selenomethionine residues. The homotrimer consists of an asymmetric assembly of monomers, each of which comprises two similar {beta} barrels and a C-terminal {alpha} helix. The similar {beta} barrels within the monomer confirm a prediction of pseudo two-fold symmetry that is inferred from the sequence similarity between the two halves of the protein. The {beta} barrels closely resemble folds found in phthalate dioxygenase reductase and other flavoproteins. The three active sites of the trimer are proposed to lie between pairs of monomers in which residues conserved among species reside, including two Asp-His-Ser triads and dyads of Cys-Ser and His-Thr. The proposed active sites are located where FMN (an analog of riboflavin) is modeled from an overlay of the {beta} barrels of phthalate dioxygenase reductase and riboflavin synthase. In the trimer, one active site is formed, and the other two active sites are wide open and exposed to solvent. The nature of the trimer configuration suggests that only one active site can be formed and be catalytically competent at a time.

Liao, D.-I.; Wawrzak, Z.; Calabrese, J.C.; Viitanen, P.V.; Jordan, D.B. (DuPont); (NWU)

2010-03-05

215

Crystal Structure of a Cyclotetraicosaphenylene by Peter Mllera  

E-print Network

Crystal Structure of a Cyclotetraicosaphenylene by Peter Müllera ), Isabel Uso�na ), Volker Henselb-rigid macrocycle is the cyclotetraicosaphenylene 1. Determining the X-ray crystal structure was a challenge which]. Although of great interest, a three-dimensional structure determination ± preferably a single-crystal X

Müller, Peter

216

Structural Evolution of Colloidal Crystals with Increasing Ionic Strength  

E-print Network

Structural Evolution of Colloidal Crystals with Increasing Ionic Strength Michael A. Bevan. In Final Form: June 5, 2004 We have directly observed the structural evolution of colloidal crystals-driven transport that also melted initial single domain crystals to yield polycrystalline and gel structures

Braun, Paul

217

Isothermal ice crystallization kinetics in the gas-diffusion layer of a proton-exchange-membrane fuel cell.  

PubMed

Nucleation and growth of ice in the fibrous gas-diffusion layer (GDL) of a proton-exchange membrane fuel cell (PEMFC) are investigated using isothermal differential scanning calorimetry (DSC). Isothermal crystallization rates and pseudo-steady-state nucleation rates are obtained as a function of subcooling from heat-flow and induction-time measurements. Kinetics of ice nucleation and growth are studied at two polytetrafluoroethylene (PTFE) loadings (0 and 10 wt %) in a commercial GDL for temperatures between 240 and 273 K. A nonlinear ice-crystallization rate expression is developed using Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory, in which the heat-transfer-limited growth rate is determined from the moving-boundary Stefan problem. Induction times follow a Poisson distribution and increase upon addition of PTFE, indicating that nucleation occurs more slowly on a hydrophobic fiber than on a hydrophilic fiber. The determined nucleation rates and induction times follow expected trends from classical nucleation theory. A validated rate expression is now available for predicting ice-crystallization kinetics in GDLs. PMID:22133053

Dursch, T J; Ciontea, M A; Radke, C J; Weber, A Z

2012-01-17

218

Fourier Analysis and Structure Determination--Part III: X-ray Crystal Structure Analysis.  

ERIC Educational Resources Information Center

Discussed is single crystal X-ray crystal structure analysis. A common link between the NMR imaging and the traditional X-ray crystal structure analysis is reported. Claims that comparisons aid in the understanding of both techniques. (MVL)

Chesick, John P.

1989-01-01

219

Crystal structure of the bacterial conjugation  

E-print Network

. Here we present the 2.0 Ã? resolution X-ray crystal structure of FinO, lacking its flexible N to interact with traJ mRNA to occlude its ribo- some binding site, blocking traJ translation and thereby of the inhibition sys- tem, protects FinP against degradation. It binds to FinP and steri- cally blocks access

Glover, Mark

220

Observations on the crystal structures of lueshite  

NASA Astrophysics Data System (ADS)

Laboratory powder XRD patterns of the perovskite-group mineral lueshite from the type locality (Lueshe, Kivu, DRC) and pure NaNbO3 demonstrate that lueshite does not adopt the same space group ( Pbma; #57) as the synthetic compound. The crystal structures of lueshite (2 samples) from Lueshe, Mont Saint-Hilaire (Quebec, Canada) and Sallanlatvi (Kola, Russia) have been determined by single-crystal CCD X-ray diffraction. These room temperature X-ray data for all single-crystal samples can be satisfactorily refined in the orthorhombic space group Pbnm (#62). Cell dimensions, atomic coordinates of the atoms, bond lengths and octahedron tilt angles are given for four crystals. Conventional neutron diffraction patterns for Lueshe lueshite recorded over the temperature range 11-1,000 K confirm that lueshite does not adopt space group Pbma within these temperatures. Neutron diffraction indicates no phase changes on cooling from room temperature to 11 K. None of these neutron diffraction data give satisfactorily refinements but suggest that this is the space group Pbnm. Time-of-flight neutron diffraction patterns for Lueshe lueshite recorded from room temperature to 700 °C demonstrate phase transitions above 550 °C from Cmcm through P4 /mbm to above 650 °C. Cell dimensions and atomic coordinates of the atoms are given for the three high-temperature phases. The room temperature to 400 °C structures cannot be satisfactorily resolved, and it is suggested that the lueshite at room temperature consists of domains of pinned metastable phases with orthorhombic and/or monoclinic structures. However, the sequence of high-temperature phase transitions observed is similar to those determined for synthetic NaTaO3, suggesting that the equilibrated room temperature structure of lueshite is orthorhombic Pbnm.

Mitchell, Roger H.; Burns, Peter C.; Knight, Kevin S.; Howard, Christopher J.; Chakhmouradian, Anton R.

2014-06-01

221

Crystal Structure of a Phosphorylated Smad2  

Microsoft Academic Search

Ligand-induced phosphorylation of the receptor-regulated Smads (R-Smads) is essential in the receptor Ser\\/Thr kinase-mediated TGF-? signaling. The crystal structure of a phosphorylated Smad2, at 1.8 Å resolution, reveals the formation of a homotrimer mediated by the C-terminal phosphoserine (pSer) residues. The pSer binding surface on the MH2 domain, frequently targeted for inactivation in cancers, is highly conserved among the Co-

Jia-Wei Wu; Min Hu; Jijie Chai; Joan Seoane; Morgan Huse; Carey Li; Daniel J. Rigotti; Saw Kyin; Tom W. Muir; Robert Fairman; Joan Massagué; Yigong Shi

2001-01-01

222

Photosensitive liquid crystals with nanoparticulate internal structure  

NASA Astrophysics Data System (ADS)

Stabilization of thermodynamic relaxation of photoinduced cis isomers of azobenzene liquid-crystal molecules is observed in nanoparticulate networks. The phenomenon permits bistability of the phase state (anisotropic and isotropic) of the material and reversible all-optical switching between those states, resulting in strong changes in the light-scattering properties of the material. Recording of complex optical structures with high spatial resolution with the aid of laser beams of different wavelengths is demonstrated.

Tabiryan, N.; Grozhik, V.; Serak, S.

2002-11-01

223

Predicting polymeric crystal structures by evolutionary algorithms.  

PubMed

The recently developed evolutionary algorithm USPEX proved to be a tool that enables accurate and reliable prediction of structures. Here we extend this method to predict the crystal structure of polymers by constrained evolutionary search, where each monomeric unit is treated as a building block with fixed connectivity. This greatly reduces the search space and allows the initial structure generation with different sequences and packings of these blocks. The new constrained evolutionary algorithm is successfully tested and validated on a diverse range of experimentally known polymers, namely, polyethylene, polyacetylene, poly(glycolic acid), poly(vinyl chloride), poly(oxymethylene), poly(phenylene oxide), and poly (p-phenylene sulfide). By fixing the orientation of polymeric chains, this method can be further extended to predict the structures of complex linear polymers, such as all polymorphs of poly(vinylidene fluoride), nylon-6 and cellulose. The excellent agreement between predicted crystal structures and experimentally known structures assures a major role of this approach in the efficient design of the future polymeric materials. PMID:25338876

Zhu, Qiang; Sharma, Vinit; Oganov, Artem R; Ramprasad, Ramamurthy

2014-10-21

224

Predicting polymeric crystal structures by evolutionary algorithms  

NASA Astrophysics Data System (ADS)

The recently developed evolutionary algorithm USPEX proved to be a tool that enables accurate and reliable prediction of structures. Here we extend this method to predict the crystal structure of polymers by constrained evolutionary search, where each monomeric unit is treated as a building block with fixed connectivity. This greatly reduces the search space and allows the initial structure generation with different sequences and packings of these blocks. The new constrained evolutionary algorithm is successfully tested and validated on a diverse range of experimentally known polymers, namely, polyethylene, polyacetylene, poly(glycolic acid), poly(vinyl chloride), poly(oxymethylene), poly(phenylene oxide), and poly (p-phenylene sulfide). By fixing the orientation of polymeric chains, this method can be further extended to predict the structures of complex linear polymers, such as all polymorphs of poly(vinylidene fluoride), nylon-6 and cellulose. The excellent agreement between predicted crystal structures and experimentally known structures assures a major role of this approach in the efficient design of the future polymeric materials.

Zhu, Qiang; Sharma, Vinit; Oganov, Artem R.; Ramprasad, Ramamurthy

2014-10-01

225

Winter ice processes and pool habitat associated with two types of constructed instream structures  

USGS Publications Warehouse

There is little information on the winter features of salmonid habitats associated with constructed instream structures to provide guidance when planning habitat improvement projects. We assessed winter habitat features for trout of the genera Oncorhynchus and Salvelinus in pools associated with two types of instream structures constructed on a low-gradient reach of a mountain stream in western Wyoming with a mean wetted width of 6.4 m. Pool habitat was affected by temporal variability in ice formations from fall into winter. As surface ice and snow accumulated with the progression of winter, variation in ice formations was less frequent and winter habitat conditions became more stable. However, groundwater inflow that maintained water temperatures at 0.2-0.6??C in a portion of the study reach appeared to contribute to incomplete surface ice cover and variation in ice formations in pools through most of the winter. Hanging dams and anchor ice dams were the primary ice features that affected winter habitat in pools associated with constructed instream structures. Trout were observed in these pools in the fall but tended to abandon pools with variation in ice formations as winter progressed. The potential impacts of groundwater inflow and winter ice processes on trout habitat in pools associated with instream structures should be considered when planning habitat improvement projects. ?? Copyright by the American Fisheries Society 2005.

Barrineau, C.E.; Hubert, W.A.; Dey, P.D.; Annear, T.C.

2005-01-01

226

Ice-templated structures for biomedical tissue repair: From physics to final scaffolds  

E-print Network

temperature and solutes. The porous structures created using ice-templating allow scaffolds to be used for many diverse applications, from microfluidics to biomedical tissue engineering. Within the field of tissue engineering, scaffold structure can influence...

Pawelec, K. M.; Husmann, A; Best, Serena Michelle; Cameron, Ruth Elizabeth

2014-04-11

227

Crystal structure and DFT calculations of andrographiside  

NASA Astrophysics Data System (ADS)

Crystal and molecular structure of a labdane diterpenoid glucoside, andrographiside ( 1) is determined from 2D-NMR and X-ray diffraction data. The 2D-NMR study indicates that the carbohydrate moiety is in ?-linkage and the sugar moiety is linked to C-19 of the aglycon. These observations are further confirmed from the X-ray diffraction studies. Both the six-membered rings are in chair conformation whereas the glucose ring adopts a twist-boat conformation. The molecular geometries and electronic structure of ( 1) were calculated at the DFT level using the hybrid exchange-correlation functional, BLYP, PW91 and PBE.

Seth, Saikat Kumar; Banerjee, Sukdeb; Kar, Tanusree

2010-02-01

228

Formation of ridges on Europa above crystallizing water bodies inside the ice shell  

NASA Astrophysics Data System (ADS)

Jupiter’s second Galilean satellite, Europa, is a Moon-sized body with an icy shell and global ocean approximately 100 km thick surrounding a rocky interior. Its surface displays extensive tectonic activity in a geologically recent past. Europa’s most ubiquitous surface features, double ridges, have a central trough flanked by two raised edifices. Double ridges can extend hundreds of kilometers and appear genetically related to cracks formed in the Europan ice shell. The origin of the raised flanks has been the center of much debate and many models have been proposed. There are also ridges without a central trough, single ridges. These ridges are far less common than their double ridge counterparts. However, there are locations where along-strike changes in ridge type appear to occur. We explore an elastic model in which the ridges form in response to crystallization of a liquid water intrusion. In our model, liquid water fills tension cracks that open in the Europan crust in response to tidal stress or perhaps overpressure of a subsurface ocean. The crack would be long and essentially continuous, similar to dikes on Earth, explaining the remarkable continuity and lack of segmentation of Europan ridges. The freezing of the water would cause a volume expansion, compressing and buckling the adjacent crust. We find that the geometry of the intruding water body controls the shape of the resulting ridges, with single ridges forming above sill-like intrusions and double ridges above dike-like intrusions. In order to match the ridge heights observed for double ridges we would need approximately 1.5 km2 of water intruded at a shallow depth in the ice shell, potentially over the course of multiple events. Deeper intrusions result in a broader, lower amplitude ridge than shallow intrusions.

Johnston, Stephanie A.; Montési, Laurent G. J.

2014-07-01

229

Examining Crystal Fabric Develoment in Ice: Cryo EBSD, Deformation Experiments and the Link to En-glacial Reflectivity  

NASA Astrophysics Data System (ADS)

Over the past few years, cryogenic electron back-scatter diffraction (Cryo-EBSD) has been increasingly used to examine micro-structures in both natural and experimentally deformed ice samples on a micron-scale. Experiments that investigate grain size-sensitive behavior require working on fine-grained ice. These samples present a number of experimental challenges. Issues that present particular difficulties include stable mounting of ice samples, transport of mounted samples and producing a planar, frost-free and damage-free surface. Recent work at the new Otago Ice Deformation Lab has led to the development of a number of experimental methods that help overcome these challenges and enable routine EBSD analysis of fine-grained ice. A brief outline of these experimental methods and some EBSD results from variably deformed ice samples will be presented. Shear deformation experiments on polycrystalline ice are being conducted in order to further explore the relationships between deformation, processes of recrystallization, and the development of anisotropic fabrics that lead to en-glacial seismic reflections. Self-contained, refrigerated deformation units fitted with digital controllers and hardware have been designed for unconfined deformation experiments on polycrystalline synthetic ice. Methods of analysis, from time-lapse photography to random-point tracking, are being employed for monitoring strain in real time. Various methods have been explored for monitoring the development of anisotropic fabrics in ice during progressive deformation. Ultra-sonic transducers can be used to monitor wave velocity changes in various orientations in materials under strain. These experiments have been designed with the objective of quantifying the relationship between deformation fabrics and en-glacial seismic reflectivity observed in thick grounded ice sheets.

Vaughan, Matthew; Prior, David; Seidemann, Meike; Gorman, Andrew; Lilly, Kat; Langhorne, Pat; Easingwood, Richard; Golding, Narayana; Durham, Bill

2014-05-01

230

Crystal Structure of Cryptosporidium parvum Pyruvate Kinase  

PubMed Central

Pyruvate kinase plays a critical role in cellular metabolism of glucose by serving as a major regulator of glycolysis. This tetrameric enzyme is allosterically regulated by different effector molecules, mainly phosphosugars. In response to binding of effector molecules and substrates, significant structural changes have been identified in various pyruvate kinase structures. Pyruvate kinase of Cryptosporidium parvum is exceptional among known enzymes of protozoan origin in that it exhibits no allosteric property in the presence of commonly known effector molecules. The crystal structure of pyruvate kinase from C. parvum has been solved by molecular replacement techniques and refined to 2.5 Å resolution. In the active site a glycerol molecule is located near the ?-phosphate site of ATP, and the protein structure displays a partially closed active site. However, unlike other structures where the active site is closed, the ?6' helix in C. parvum pyruvate kinase unwinds and assumes an extended conformation. In the crystal structure a sulfate ion is found at a site that is occupied by a phosphate of the effector molecule in many pyruvate kinase structures. A new feature of the C. parvum pyruvate kinase structure is the presence of a disulfide bond cross-linking the two monomers in the asymmetric unit. The disulfide bond is formed between cysteine residue 26 in the short N-helix of one monomer with cysteine residue 312 in a long helix (residues 303–320) of the second monomer at the interface of these monomers. Both cysteine residues are unique to C. parvum, and the disulfide bond remained intact in a reduced environment. However, the significance of this bond, if any, remains unknown at this time. PMID:23056503

Cook, William J.; Senkovich, Olga; Aleem, Khadijah; Chattopadhyay, Debasish

2012-01-01

231

PROTEIN STRUCTURE REPORT Crystal structure of the Yersinia type III  

E-print Network

Yersinia pestis utilizes a contact-dependent (type III) secretion system (T3SS) to transport virulence of oligomerization is discussed. Keywords: Yersinia pestis; plague; type III secretion; YscE; crystal structure Yersinia pestis, the causative agent of plague, utilizes a type III secretion system (T3SS) to inject

232

Crystal structures and freezing of dipolar fluids  

E-print Network

We investigate the crystal structure of classical systems of spherical particles with an embedded point dipole at T=0. The ferroelectric ground state energy is calculated using generalizations of the Ewald summation technique. Due to the reduced symmetry compared to the nonpolar case the crystals are never strictly cubic. For the Stockmayer (i.e., Lennard-Jones plus dipolar) interaction three phases are found upon increasing the dipole moment: hexagonal, body-centered orthorhombic, and body-centered tetragonal. An even richer phase diagram arises for dipolar soft spheres with a purely repulsive inverse power law potential $\\sim r^{-n}$. A crossover between qualitatively different sequences of phases occurs near the exponent $n=12$. The results are applicable to electro- and magnetorheological fluids. In addition to the exact ground state analysis we study freezing of the Stockmayer fluid by density-functional theory.

B. Groh; S. Dietrich

2000-10-21

233

Crystal structures and freezing of dipolar fluids.  

PubMed

We investigate the crystal structure of classical systems of spherical particles with an embedded point dipole at T=0. The ferroelectric ground state energy is calculated using generalizations of the Ewald summation technique. Due to the reduced symmetry compared to the nonpolar case the crystals are never strictly cubic. For the Stockmayer (i.e., Lennard-Jones plus dipolar) interaction three phases are found upon increasing the dipole moment: hexagonal, body-centered orthorhombic, and body-centered tetragonal. An even richer phase diagram arises for dipolar soft spheres with a purely repulsive inverse power law potential approximately r(-n). A crossover between qualitatively different sequences of phases occurs near the exponent n=12. The results are applicable to electro- and magnetorheological fluids. In addition to the exact ground state analysis we study freezing of the Stockmayer fluid by density-functional theory. PMID:11308482

Groh, B; Dietrich, S

2001-02-01

234

Crystal structure of MboIIA methyltransferase.  

SciTech Connect

DNA methyltransferases (MTases) are sequence-specific enzymes which transfer a methyl group from S-adenosyl-L-methionine (AdoMet) to the amino group of either cytosine or adenine within a recognized DNA sequence. Methylation of a base in a specific DNA sequence protects DNA from nucleolytic cleavage by restriction enzymes recognizing the same DNA sequence. We have determined at 1.74 {angstrom} resolution the crystal structure of a {beta}-class DNA MTase MboIIA (M {center_dot} MboIIA) from the bacterium Moraxella bovis, the smallest DNA MTase determined to date. M {center_dot} MboIIA methylates the 3' adenine of the pentanucleotide sequence 5'-GAAGA-3'. The protein crystallizes with two molecules in the asymmetric unit which we propose to resemble the dimer when M {center_dot} MboIIA is not bound to DNA. The overall structure of the enzyme closely resembles that of M {center_dot} RsrI. However, the cofactor-binding pocket in M {center_dot} MboIIA forms a closed structure which is in contrast to the open-form structures of other known MTases.

Osipiuk, J.; Walsh, M. A.; Joachimiak, A.; Biosciences Division; Univ. of Gdansk; Medical Research Council France

2003-09-15

235

Spatial and temporal variations in the age structure of Arctic sea ice  

USGS Publications Warehouse

Spatial and temporal variations in the age structure of Arctic sea ice are investigated using a new reverse chronology algorithm that tracks ice-covered pixels to their location and date of origin based on ice motion and concentration data. The Beaufort Gyre tends to harbor the oldest (>10 years old) sea ice in the western Arctic while direct ice advection pathways toward the Transpolar Drift Stream maintain relatively young (10 years old (10+ year age class) were observed during 1989-2003. Since the mid-1990s, losses to the 10+ year age class lacked compensation by recruitment due to a prior depletion of all mature (6-10 year) age classes. Survival of the 1994 and 1996-1998 sea ice generations reestablished most mature age classes, and thereby the potential to increase extent of the 10+ year age class during the mid-2000s.

Belchansky, G.I.; Douglas, D.C.; Platonov, N.G.

2005-01-01

236

Crystal structure of a human GABAA receptor.  

PubMed

Type-A ?-aminobutyric acid receptors (GABAARs) are the principal mediators of rapid inhibitory synaptic transmission in the human brain. A decline in GABAAR signalling triggers hyperactive neurological disorders such as insomnia, anxiety and epilepsy. Here we present the first three-dimensional structure of a GABAAR, the human ?3 homopentamer, at 3?Å resolution. This structure reveals architectural elements unique to eukaryotic Cys-loop receptors, explains the mechanistic consequences of multiple human disease mutations and shows an unexpected structural role for a conserved N-linked glycan. The receptor was crystallized bound to a previously unknown agonist, benzamidine, opening a new avenue for the rational design of GABAAR modulators. The channel region forms a closed gate at the base of the pore, representative of a desensitized state. These results offer new insights into the signalling mechanisms of pentameric ligand-gated ion channels and enhance current understanding of GABAergic neurotransmission. PMID:24909990

Miller, Paul S; Aricescu, A Radu

2014-08-21

237

The crystal structure of calcite III Joseph R. Smyth  

E-print Network

The crystal structure of calcite III Joseph R. Smyth Department of Geological Sciences, University of Technology, Pasadena, CA. Abstract. The crystal structure of calcite III has been deduced from existing high of the calcite I structure. The structure is monoclinic with space group C2 and a Z of 6. There are two Ca

Smyth, Joseph R.

238

Tightly integrated single- and multi-crystal data collection strategy calculation and parallelized data processing in JBluIce beamline control system  

PubMed Central

The calculation of single- and multi-crystal data collection strategies and a data processing pipeline have been tightly integrated into the macromolecular crystallographic data acquisition and beamline control software JBluIce. Both tasks employ wrapper scripts around existing crystallographic software. JBluIce executes scripts through a distributed resource management system to make efficient use of all available computing resources through parallel processing. The JBluIce single-crystal data collection strategy feature uses a choice of strategy programs to help users rank sample crystals and collect data. The strategy results can be conveniently exported to a data collection run. The JBluIce multi-crystal strategy feature calculates a collection strategy to optimize coverage of reciprocal space in cases where incomplete data are available from previous samples. The JBluIce data processing runs simultaneously with data collection using a choice of data reduction wrappers for integration and scaling of newly collected data, with an option for merging with pre-existing data. Data are processed separately if collected from multiple sites on a crystal or from multiple crystals, then scaled and merged. Results from all strategy and processing calculations are displayed in relevant tabs of JBluIce. PMID:25484844

Pothineni, Sudhir Babu; Venugopalan, Nagarajan; Ogata, Craig M.; Hilgart, Mark C.; Stepanov, Sergey; Sanishvili, Ruslan; Becker, Michael; Winter, Graeme; Sauter, Nicholas K.; Smith, Janet L.; Fischetti, Robert F.

2014-01-01

239

Tightly integrated single- and multi-crystal data collection strategy calculation and parallelized data processing in JBluIce beamline control system.  

PubMed

The calculation of single- and multi-crystal data collection strategies and a data processing pipeline have been tightly integrated into the macromolecular crystallographic data acquisition and beamline control software JBluIce. Both tasks employ wrapper scripts around existing crystallographic software. JBluIce executes scripts through a distributed resource management system to make efficient use of all available computing resources through parallel processing. The JBluIce single-crystal data collection strategy feature uses a choice of strategy programs to help users rank sample crystals and collect data. The strategy results can be conveniently exported to a data collection run. The JBluIce multi-crystal strategy feature calculates a collection strategy to optimize coverage of reciprocal space in cases where incomplete data are available from previous samples. The JBluIce data processing runs simultaneously with data collection using a choice of data reduction wrappers for integration and scaling of newly collected data, with an option for merging with pre-existing data. Data are processed separately if collected from multiple sites on a crystal or from multiple crystals, then scaled and merged. Results from all strategy and processing calculations are displayed in relevant tabs of JBluIce. PMID:25484844

Pothineni, Sudhir Babu; Venugopalan, Nagarajan; Ogata, Craig M; Hilgart, Mark C; Stepanov, Sergey; Sanishvili, Ruslan; Becker, Michael; Winter, Graeme; Sauter, Nicholas K; Smith, Janet L; Fischetti, Robert F

2014-12-01

240

Ice island creation, drift, recurrences, mechanical properties, and interactions with arctic offshore oil production structures. Final report  

SciTech Connect

Research and engineering studies on first-year sea ice for over two decades has resulted in the design, construction, and operation of jacket platforms, of artificial islands, and of massive gravity structures which routinely withstand moving sea ice of thickness up to 2 meters. However, the less-common interactions between such structures and moving multiyear ice ({ge}3 meters thick), and also moving ice islands (10 to 60 meters thick) remain as the unknown and potentially most serious hazard for Arctic offshore structures. In this study, research was addressed across the complete span of remaining questions regarding such features. Ice island components, thickness distributions, scenarios and models for the interactions of massive ice features with offshore structures, all were considered. Ice island morphology and calving studies were directed at the cluster of 19 ice islands produced in a calving from the Ward Hunt Ice Shelf on Ellesmere Island in 1983, and also at a calving from the Milne Ice Shelf in 1988. The statistics of ice island dynamics, on both a short-term small-scale basis and also on a long-term basis, were studied. Typical wind velocities of 5 to 7.5 meters per second led to ice island speeds of about 0.014 of the wind speed, at an angle of 20{degrees} to the right of the wind direction. Ice island samples were tested for their stress/strain characteristics. Compressive strength values ranged from 1.64 MPa at a strain rate of 2 {times} 10{sup {minus}7} s{sup {minus}1} to 6.75 MPa at a strain rate of 1 {times} 10{sup {minus}3} s{sup {minus}1}. Scenarios for ice island/structure interactions were developed, and protective countermeasures such as spray ice and ice rubble barriers were suggested. Additional computer modeling of structure/ice interactions for massive ice features is recommended.

Sackinger, W.M.; Jeffries, M.O.; Li, Fucheng; Lu, Mingchi

1991-03-01

241

Ice Cover as a Factor Driving Microbial Community Structure in the Laurentian Great Lakes  

NASA Astrophysics Data System (ADS)

Lakes serve as rapid responding sentinels of human influence on the natural environment rendering them powerful tools to advance our understanding of a changing climate on microbial community structure and function. Whereas we possess a baseline knowledge of microbial diversity in the Great Lakes, we know little about how these communities respond to the manifestations of climate change. Through collaboration with U.S.- and Canadian Coast Guards, winter surveys have been conducted on Lake Erie since 2007. The surveys have captured extremes in ice extent ranging from expansive ice cover through 2011 to nearly ice-free waters in winter 2012, a condition driven by a warm positive Arctic Oscillation. We showed that dramatic changes in annual ice cover were accompanied by equally dramatic shifts in phytoplankton community structure. Expansive ice cover documented for Lake Erie in winters 2010 and 2011 supported ice-associated phytoplankton blooms dominated by physiologically robust, filamentous centric diatoms. Transcriptomic analysis of the winter bloom offers insights into the success of this psychrophilic community. By comparison, ice free conditions promoted the growth of small-sized cells supported by analysis of size-fractionated chlorophyll a and flow cytometry. The phytoplankton community in winter 2013 was dominated by microplankton-sized filamentous diatoms, coincident with expansive ice cover and thus returning to the size structure of the 2010 and 2011 communities. Reduced size is recognized as a universal ecological response to global warming in aquatic systems although it usually marks a response to climate warming over multiple years, not a single season as reported here. Fig. 1. Winter surveys conducted on Lake Erie over two years demonstrated tight coupling between microplankton Chl a biomass and total Chl a during winter 2010-11 (purple, green), a year of expansive ice cover. A warm positive Arctic Oscillation resulted in negligible ice cover on Lake Erie in 2011-12. Coincident with the ice-free conditions, a strong departure from a microplankton-dominated system was documented (red, yellow).

McKay, R. M.; Beall, B.; Oyserman, B.; Smith, D.; Bullerjahn, G.; Morris, P.; Twiss, M. R.

2013-12-01

242

Ice crystallization and freeze tolerance in embryonic stages of the tardigrade Milnesium tardigradum.  

PubMed

In tardigrades, tolerance to low temperature is well known and allows them to cope with subzero temperatures in their environment. Although the ability to tolerate freezing body water has been demonstrated in some tardigrades, freeze tolerance of embryonic stages has been little studied, although this has ecological significance. In this study, we evaluated the subzero temperature survival of five different developmental stages of the eutardigrade species Milnesium tardigradum after freezing to -30 degrees C. Embryos were exposed to five different cooling rates between room temperature and -30 degrees C at 1 degrees C/h, 3 degrees C/h, 5 degrees C/h, 7 degrees C/h, and 9 degrees C/h followed by a warming period at 10 degrees C/h. The results showed that the developmental stage and the cooling rate have a significant effect on the hatching rate. Less developed embryonic stages were more sensitive to freezing at higher freezing rates than more developed stages. Differential Scanning Calorimetry (DSC) was used to determine the temperature of crystallization (Tc) in single embryos of the different developmental stages and revealed no differences between the stages. Based on the calorimetric data, we also conclude that the ice nucleation is homogeneous in embryonic stages in tardigrades, as also recently shown for fully developed tardigrades, and not triggered by nucleating agents. PMID:20116441

Hengherr, S; Reuner, A; Brümmer, F; Schill, R O

2010-05-01

243

The effect of ice crystal surface roughness on the retrieval of ice cloud microphysical and optical properties  

E-print Network

Extinction and absorption eficiencies of hexagonal columns from the composite method based on the FDTD, Mie and IGOM (?=6.7 µm)…………… 8 2 Comparison of bidirectional reflectances from DISORT with true coeficients, ?-fit truncation and ?-M truncation... by cloud optical thicknes, efective particle size, cloud-top temperature and ice water path in general circulation models. Satelite-based cloud retrieval algorithms often require a function of cloud particle types in the forward radiative transfer models...

Xie, Yu

2007-09-17

244

Prediction of binary hard-sphere crystal structures  

NASA Astrophysics Data System (ADS)

We present a method based on a combination of a genetic algorithm and Monte Carlo simulations to predict close-packed crystal structures in hard-core systems. We employ this method to predict the binary crystal structures in a mixture of large and small hard spheres with various stoichiometries and diameter ratios between 0.4 and 0.84. In addition to known binary hard-sphere crystal structures similar to NaCl and AlB2 , we predict additional crystal structures with the symmetry of CrB, ?CuTi , ?IrV , HgBr2 , AuTe2 , Ag2Se , and various structures for which an atomic analog was not found. In order to determine the crystal structures at infinite pressures, we calculate the maximum packing density as a function of size ratio for the crystal structures predicted by our GA using a simulated annealing approach.

Filion, Laura; Dijkstra, Marjolein

2009-04-01

245

Prediction of binary hard-sphere crystal structures.  

PubMed

We present a method based on a combination of a genetic algorithm and Monte Carlo simulations to predict close-packed crystal structures in hard-core systems. We employ this method to predict the binary crystal structures in a mixture of large and small hard spheres with various stoichiometries and diameter ratios between 0.4 and 0.84. In addition to known binary hard-sphere crystal structures similar to NaCl and AlB2, we predict additional crystal structures with the symmetry of CrB, gammaCuTi, alphaIrV, HgBr2, AuTe2, Ag2Se, and various structures for which an atomic analog was not found. In order to determine the crystal structures at infinite pressures, we calculate the maximum packing density as a function of size ratio for the crystal structures predicted by our GA using a simulated annealing approach. PMID:19518387

Filion, Laura; Dijkstra, Marjolein

2009-04-01

246

Structural Analysis of the Redesigned Ice/Frost Ramp Bracket  

NASA Technical Reports Server (NTRS)

This paper describes the interim structural analysis of a redesigned Ice/Frost Ramp bracket for the Space Shuttle External Tank (ET). The proposed redesigned bracket consists of mounts for attachment to the ET wall, supports for the electronic/instrument cables and propellant repressurization lines that run along the ET, an upper plate, a lower plate, and complex bolted connections. The eight nominal bolted connections are considered critical in the summarized structural analysis. Each bolted connection contains a bolt, a nut, four washers, and a non-metallic spacer and block that are designed for thermal insulation. A three-dimensional (3D) finite element model of the bracket is developed using solid 10-node tetrahedral elements. The loading provided by the ET Project is used in the analysis. Because of the complexities associated with accurately modeling the bolted connections in the bracket, the analysis is performed using a global/local analysis procedure. The finite element analysis of the bracket identifies one of the eight bolted connections as having high stress concentrations. A local area of the bracket surrounding this bolted connection is extracted from the global model and used as a local model. Within the local model, the various components of the bolted connection are refined, and contact is introduced along the appropriate interfaces determined by the analysts. The deformations from the global model are applied as boundary conditions to the local model. The results from the global/local analysis show that while the stresses in the bolts are well within yield, the spacers fail due to compression. The primary objective of the interim structural analysis is to show concept viability for static thermal testing. The proposed design concept would undergo continued design optimization to address the identified analytical assumptions and concept shortcomings, assuming successful thermal testing.

Phillips, D. R.; Dawicke, D. S.; Gentz, S. J.; Roberts, P. W.; Raju, I. S.

2007-01-01

247

Development of a structural concept to resist impacts from multiyear ice floes, ridges, and icebergs  

SciTech Connect

Large multi-year ice features and icebergs may have masses ranging up to 20 million tons or more and may move in the open water at speeds up to 1 knot, thus developing tremendous kinetic energy. A stepped structure concept has been developed to resist these impacts and to transfer the resultant forces and moments into the foundation thus developing a relatively high concentrated reaction force against the ice tending to spall and split it, thus causing a multi-modal failure of the ice, as well as using up kinetic energy at a relatively controlled rate. Calculated ice forces and ice failure modes will be presented. The stepped structure concept is primarily applicable to production platforms in water depths of 50 to 200 meters, which are subject to impact of large ice features such as multi-year ridges, floes, and icebergs The resulting global ice loads are reduced by 50 percent or more as compared to those developed by a vertical or a steep sided structure. Overturning moments are reduced, thus reducing maxima soil bearing values. The structure itself is efficient in its use of structural materials, and is practicable for construction in prestressed reinforced concrete or steel/ concrete hybrid construction. It has acceptable draft during tow and can carry a fully outfitted deck. It is stable during all stages of installation.

Gerwick, B.C.; Potter, R.E.; Rojansky, M.

1984-05-01

248

Crystal structure of yeast Sco1  

SciTech Connect

The Sco family of proteins are involved in the assembly of the dinuclear CuA site in cytochrome c oxidase (COX), the terminal enzyme in aerobic respiration. These proteins, which are found in both eukaryotes and prokaryotes, are characterized by a conserved CXXXC sequence motif that binds copper ions and that has also been proposed to perform a thiol:disulfide oxidoreductase function. The crystal structures of Saccharomyces cerevisiae apo Sco1 (apo-ySco1) and Sco1 in the presence of copper ions (Cu-ySco1) were determined to 1.8- and 2.3-{angstrom} resolutions, respectively. Yeast Sco1 exhibits a thioredoxin-like fold, similar to that observed for human Sco1 and a homolog from Bacillus subtilis. The Cu-ySco1 structure, obtained by soaking apo-ySco1 crystals in copper ions, reveals an unexpected copper-binding site involving Cys181 and Cys216, cysteine residues present in ySco1 but not in other homologs. The conserved CXXXC cysteines, Cys148 and Cys152, can undergo redox chemistry in the crystal. An essential histidine residue, His239, is located on a highly flexible loop, denoted the Sco loop, and can adopt positions proximal to both pairs of cysteines. Interactions between ySco1 and its partner proteins yeast Cox17 and yeast COX2 are likely to occur via complementary electrostatic surfaces. This high-resolution model of a eukaryotic Sco protein provides new insight into Sco copper binding and function.

Abajian, Carnie; Rosenzweig, Amy C. (NWU)

2010-03-05

249

Crystal Distortion of Dy2Ti2O7 at the Spin Ice Transition Temperature  

SciTech Connect

Dy2Ti2O7 with the pyrochlore structure shows a spin ice transition at about 1 K. The Dy3+ ion in this compound has an effective spin Seff=1/2. The Dy ions reside on the vertices of corner-linked tetrahedra. Due to a strong single-ion anisotropy, the ground state of Dy3+ is well expressed by an Ising doublet with local <111> quantization axes. We measured the x-ray diffraction for a Dy2Ti2O7 powder sample between 0.15 K. and 20 K. The lattice spacing increases drastically between 1.5 K and about 1 K, with decreasing temperature. The lattice spacing of the (222) diffraction is considerably larger than the other directions.

Suzuki, H.; Hata, F.; Xue, Y.; Kaneko, H.; Hosomichi, A.; Abe, S. [Department of Physics, Kanazawa University, Kakuma-machi, Kanazawa 920-1192 (Japan); Higashinaka, R.; Nakatsuji, S.; Maeno, Y. [Department of Physics, Kyoto University, Kyoto 606-8502 (Japan)

2006-09-07

250

Radiation effects in water ice: A near-edge x-ray absorption fine structure study  

SciTech Connect

The changes in the structure and composition of vapor-deposited ice films irradiated at 20 K with soft x-ray photons (3-900 eV) and their subsequent evolution with temperatures between 20 and 150 K have been investigated by near-edge x-ray absorption fine structure spectroscopy (NEXAFS) at the oxygen K edge. We observe the hydroxyl OH, the atomic oxygen O, and the hydroperoxyl HO{sub 2} radicals, as well as the oxygen O{sub 2} and hydrogen peroxide H{sub 2}O{sub 2} molecules in irradiated porous amorphous solid water (p-ASW) and crystalline (I{sub cryst}) ice films. The evolution of their concentrations with the temperature indicates that HO{sub 2}, O{sub 2}, and H{sub 2}O{sub 2} result from a simple step reaction fuelled by OH, where O{sub 2} is a product of HO{sub 2} and HO{sub 2} a product of H{sub 2}O{sub 2}. The local order of ice is also modified, whatever the initial structure is. The crystalline ice I{sub cryst} becomes amorphous. The high-density amorphous phase (I{sub a}h) of ice is observed after irradiation of the p-ASW film, whose initial structure is the normal low-density form of the amorphous ice (I{sub a}l). The phase I{sub a}h is thus peculiar to irradiated ice and does not exist in the as-deposited ice films. A new 'very high density' amorphous phase--we call I{sub a}vh--is obtained after warming at 50 K the irradiated p-ASW ice. This phase is stable up to 90 K and partially transforms into crystalline ice at 150 K.

Laffon, C.; Lacombe, S.; Bournel, F.; Parent, Ph. [Laboratoire de Chimie-Physique, Matiere et Rayonnement, UMR 7614, Universite Pierre et Marie Curie et CNRS, 11 Rue Pierre et Marie Curie, 75231 Paris, Cedex 05 (France); Laboratoire des Collisions Atomiques et Moleculaires, UMR 8625, Universite Paris Sud 11, 91405 Orsay Cedex (France); Laboratoire de Chimie-Physique, Matiere et Rayonnement, UMR 7614, Universite Pierre et Marie Curie et CNRS, 11 Rue Pierre et Marie Curie, 75231 Paris, Cedex 05 (France)

2006-11-28

251

The Crystal Structure of Human Argonaute2  

SciTech Connect

Argonaute proteins form the functional core of the RNA-induced silencing complexes that mediate RNA silencing in eukaryotes. The 2.3 angstrom resolution crystal structure of human Argonaute2 (Ago2) reveals a bilobed molecule with a central cleft for binding guide and target RNAs. Nucleotides 2 to 6 of a heterogeneous mixture of guide RNAs are positioned in an A-form conformation for base pairing with target messenger RNAs. Between nucleotides 6 and 7, there is a kink that may function in microRNA target recognition or release of sliced RNA products. Tandem tryptophan-binding pockets in the PIWI domain define a likely interaction surface for recruitment of glycine-tryptophan-182 (GW182) or other tryptophan-rich cofactors. These results will enable structure-based approaches for harnessing the untapped therapeutic potential of RNA silencing in humans.

Schirle, Nicole T.; MacRae, Ian J. (Scripps)

2012-07-18

252

Crystal structure of glucokinase regulatory protein.  

PubMed

Glucokinase (GK) plays a major role in the regulation of blood glucose homeostasis in both the liver and the pancreas. In the liver, GK is controlled by the GK regulatory protein (GKRP). GKRP in turn is activated by fructose 6-phosphate (F6P) and inactivated by fructose 1-phosphate (F1P). Disrupting the GK-GKRP complex increases the activity of GK in the cytosol and is considered an attractive concept for the regulation of blood glucose. We have determined the crystal structure of GKRP in its inactive F1P-bound form. The binding site for F1P is located deeply buried at a domain interface, and H-D exchange experiments confirmed that F1P and F6P compete for this site. The structure of the inactive GKRP-F1P complex provides a starting point for understanding the mechanism of fructose phosphate-dependent GK regulation at an atomic level. PMID:23621087

Pautsch, Alexander; Stadler, Nadja; Löhle, Adelheid; Rist, Wolfgang; Berg, Adina; Glocker, Lucia; Nar, Herbert; Reinert, Dirk; Lenter, Martin; Heckel, Armin; Schnapp, Gisela; Kauschke, Stefan G

2013-05-21

253

Crystal structures of dibromochloromethane and bromodichloromethane  

NASA Astrophysics Data System (ADS)

The crystal structures of CHB2Cl and CHBrCl2 have been investigated using neutron powder profile techniques. These structures were found to be similar to that of CHBr3 in its lowest temperature triclinic phase. Whereas CHBr3 has three phases, both CHBr2Cl and CHBrCl2 have only one. The two CHBr3 phases which are absent in these compounds require the molecules to have threefold axes. The degree of disorder of the halogen atoms has been examined and the results indicate that the Br and Cl atoms have definite preferences for some sites over others, so that the molecules on average do not have a threefold axis.

Torrie, B. H.; Binbrek, O. S.; Swainson, I. P.; Powell, B. M.

254

Crystal-like low frequency phonons in the low-density amorphous and high-density amorphous ices.  

PubMed

The structure and vibrational properties of high- and low-density amorphous (HDA and LDA, respectively) ices have been determined using reverse Monte Carlo, molecular dynamics, and lattice dynamics simulations. This combined approach leads to a more accurate and detailed structural description of HDA and LDA ices when compared to experiment than was previously possible. The water molecules in these ices form well connected hydrogen-bond networks that exhibit modes of vibration that extend throughout the solid and can involve up to 70% of all molecules. However, the networks display significant differences in their dynamical behavior. In HDA, the extended low-frequency vibrational modes occur in dense parallel two dimensional layers of water that are approximately 10 nm thick. In contrast, the extended modes in LDA resemble a holey structure that encapsulates many small pockets of nonparticipating water molecules. PMID:19044969

Belosludov, R V; Subbotin, O S; Mizuseki, H; Rodger, P M; Kawazoe, Y; Belosludov, V R

2008-09-21

255

Influence of freezing conditions on ice crystallisation in ice cream  

Microsoft Academic Search

Successful optimisation of the ice cream freezing process to deliver a product with small ice crystals, and therefore a smooth texture, requires an understanding of the mechanisms of ice crystallisation. The purpose of this work was to relate the processing variables available to the ice cream manufacturer to measured ice crystal size distributions, with a view to elucidating the dominant

A. B. Russell; P. E. Cheney; S. D. Wantling

1999-01-01

256

Sensitivity of Cirrus Bidirectional Reflectance at MODIS Bands to Vertical Inhomogeneity of Ice Crystal Habits and Size Distribution  

NASA Technical Reports Server (NTRS)

A common assumption in satellite imager-based cirrus retrieval algorithms is that the radiative properties of a cirrus cloud may be represented by those associated with a specific ice crystal shape (or habit) and a single particle size distribution. However, observations of cirrus clouds have shown that the shapes and sizes of ice crystals may vary substantially with height within the clouds. In this study we investigate the sensitivity of the top-of-atmosphere bidirectional reflectances at two MODIS bands centered at 0.65 micron and 2.11 micron to the cirrus models assumed to be either a single homogeneous layer or three distinct but contiguous, layers. First, we define the single- and three-layer cirrus cloud models with respect to ice crystal habit and size distribution on the basis of in situ replicator data acquired during the First ISCCP Regional Experiment (FIRE-II), held in Kansas during the fall of 1991. Subsequently, fundamental light scattering and radiative transfer theory is employed to determine the single scattering and the bulk radiative properties of the cirrus cloud. Regarding the radiative transfer computations, we present a discrete form of the adding/doubling principle by introducing a direct transmission function, which is computationally straightforward and efficient an improvement over previous methods. For the 0.65 micron band, at which absorption by ice is negligible, there is little difference between the bidirectional reflectances calculated for the one- and three-layer cirrus models, suggesting that the vertical inhomogeneity effect is relatively unimportant. At the 2.11 micron band, the bidirectional reflectances computed for both optically thin (tau = 1) and thick (tau = 10) cirrus clouds show significant differences between the results for the one- and three-layer models. The reflectances computed for the three-layer cirrus model are substantially larger than those computed for the single-layer cirrus. Finally, we find that cloud reflectance is very sensitive to the optical properties of the small crystals that predominate in the top layer of the three-layer cirrus model. It is critical to define the most realistic geometric shape for the small "quasi-spherical" ice crystals in the top layer for obtaining reliable single-scattering parameters and bulk radiative properties of cirrus.

Yang, P.; Gao, B.-C.; Baum, B. A.; Wiscombe, W.; Hu, Y.; Nasiri, S. L.; Soulen, P. F.; Heymsfield, A. J.; McFarquhar, G. M.; Miloshevich, L. M.

2000-01-01

257

Crystal structure of an amphiphilic foldamer reveals a 48-mer assembly comprising a hollow truncated octahedron  

PubMed Central

Foldamers provide an attractive medium to test the mechanisms by which biological macromolecules fold into complex three-dimensional structures, and ultimately to design novel protein-like architectures with properties unprecedented in nature. Here, we describe a large cage-like structure formed from an amphiphilic arylamide foldamer crystallized from aqueous solution. Forty eight copies of the foldamer assemble into a 5 nm cage-like structure, an omnitruncated octahedron filled with well-ordered ice-like water molecules. The assembly is stabilised by a mix of arylamide stacking interaction, hydrogen bonding and hydrophobic forces. The omnitruncated octahedra tessellate to form a cubic crystal. These findings may provide an important step towards the design of nanostructured particles resembling spherical viruses. PMID:24705140

Pavone, Vincenzo; Zhang, Shao-Qing; Merlino, Antonello; Lombardi, Angela; Wu, Yibing; DeGrado, William F.

2014-01-01

258

Crystal structure of an amphiphilic foldamer reveals a 48-mer assembly comprising a hollow truncated octahedron.  

PubMed

Foldamers provide an attractive medium to test the mechanisms by which biological macromolecules fold into complex three-dimensional structures, and ultimately to design novel protein-like architectures with properties unprecedented in nature. Here, we describe a large cage-like structure formed from an amphiphilic arylamide foldamer crystallized from aqueous solution. Forty-eight copies of the foldamer assemble into a 5-nm cage-like structure, an omnitruncated octahedron filled with well-ordered ice-like water molecules. The assembly is stabilized by a mix of arylamide stacking interaction, hydrogen bonding and hydrophobic forces. The omnitruncated octahedra tessellate to form a cubic crystal. These findings may provide an important step towards the design of nanostructured particles resembling spherical viruses. PMID:24705140

Pavone, Vincenzo; Zhang, Shao-Qing; Merlino, Antonello; Lombardi, Angela; Wu, Yibing; DeGrado, William F

2014-01-01

259

Crystal and Electronic Structure of Copper Sulfides  

NASA Astrophysics Data System (ADS)

Because of the complexity of the crystal structure of Cu2-xS, no electronic band structure studies have been performed in the past. These materials have S atoms on a (hcp) or at high-temperature (fcc) close packed lattice but the Cu atoms occupy various low-symmetry Wyckoff sites of which only the statistical distribution is known from X-ray diffraction experiments. Here, we constructed supercell models for the cubic and hexagonal phases with the Cu positions determined by a weighted random number generator. The electronic structure of both these models and the monoclinic structure are studied using the full-potential linearized muffin-tin orbital method in the local density approximation (LDA). Both LDA and GW quasiparticle calculations give a zero band gap for the latter. The supercell models gave small band gaps of order 0.1--0.2 eV. Adding a Cu-s shift as suggested by the antifluorite structure GW calculation and an analysis in terms of atomic orbitals, increases the gap to about 0.5 eV.

Lukashev, Pavel; Lambrecht, Walter R. L.

2007-03-01

260

Crystal Structure of Human Spermine Synthase  

PubMed Central

The crystal structures of two ternary complexes of human spermine synthase (EC 2.5.1.22), one with 5?-methylthioadenosine and spermidine and the other with 5?-methylthioadenosine and spermine, have been solved. They show that the enzyme is a dimer of two identical subunits. Each monomer has three domains: a C-terminal domain, which contains the active site and is similar in structure to spermidine synthase; a central domain made up of four ?-strands; and an N-terminal domain with remarkable structural similarity to S-adenosylmethionine decarboxylase, the enzyme that forms the aminopropyl donor substrate. Dimerization occurs mainly through interactions between the N-terminal domains. Deletion of the N-terminal domain led to a complete loss of spermine synthase activity, suggesting that dimerization may be required for activity. The structures provide an outline of the active site and a plausible model for catalysis. The active site is similar to those of spermidine synthases but has a larger substrate-binding pocket able to accommodate longer substrates. Two residues (Asp201 and Asp276) that are conserved in aminopropyltransferases appear to play a key part in the catalytic mechanism, and this role was supported by the results of site-directed mutagenesis. The spermine synthase·5?-methylthioadenosine structure provides a plausible explanation for the potent inhibition of the reaction by this product and the stronger inhibition of spermine synthase compared with spermidine synthase. An analysis to trace possible evolutionary origins of spermine synthase is also described. PMID:18367445

Wu, Hong; Min, Jinrong; Zeng, Hong; McCloskey, Diane E.; Ikeguchi, Yoshihiko; Loppnau, Peter; Michael, Anthony J.; Pegg, Anthony E.; Plotnikov, Alexander N.

2008-01-01

261

Midlatitude Cirrus Clouds Derived from Hurricane Nora: A Case Study with Implications for Ice Crystal Nucleation and Shape  

NASA Technical Reports Server (NTRS)

Hurricane Nora traveled up the Bala Peninsula coast in the unusually warm El Nino waters of September 1997, until rapidly decaying as it approached Southern California on 24 September. The anvil cirrus blowoff from the final surge of tropical convection became embedded in subtropical flow that advected the cirrus across the western US, where it was studied from the Facility for Atmospheric Remote Sensing (FARS) in Salt Lake City, Utah. A day later, the cirrus shield remnants were redirected southward by midlatitude circulations into the Southern Great Plains, providing a case study opportunity for the research aircraft and ground-based remote sensors assembled at the Clouds and Radiation Testbed (CART) site in northern Oklahoma. Using these comprehensive resources and new remote sensing cloud retrieval algorithms, the microphysical and radiative cloud properties of this unusual cirrus event are uniquely characterized. Importantly, at both the FARS and CART sites the cirrus generated spectacular optical displays, which acted as a tracer for the hurricane cirrus, despite the limited lifetimes of individual ice crystals. Lidar polarization data indicate widespread regions of uniform ice plate orientations, and in situ particle masticator data show a preponderance of pristine, solid hexagonal plates and columns. It is suggested that these unusual aspects are the result of the mode of cirrus particle nucleation, presumably involving the lofting of sea-salt nuclei in thunderstorm updrafts into the upper troposphere. This created a reservoir of haze particles that continued to produce halide-saltcontaminated ice crystals during the extended period of cirrus cloud maintenance. The reference that marine microliters are embedded in the replicas of ice crystals collected over the CART site points to the longevity of marine effects. Various nucleation scenarios proposed for cirrus clouds based on this and other studies, and the implications for understanding cirrus radiative properties or a global scale, are discussed.

Sassen, Kenneth; Arnott, W. Patrick; OCStarr, David; Mace, Gerald G.; Wang, Zhien; Poellot, Michael R.

2002-01-01

262

Some Lower Valence Vanadium Fluorides: Their Crystal Distortions, Domain Structures, Modulated Structures, Ferrimagnetism, and Composition Dependence.  

ERIC Educational Resources Information Center

Describes some contemporary concepts unique to the structure of advanced solids, i.e., their crystal distortions, domain structures, modulated structures, ferrimagnetism, and composition dependence. (Author/CS)

Hong, Y. S.; And Others

1980-01-01

263

A study of fat and air structures in ice cream  

Microsoft Academic Search

Three ice cream mixes of conventional composition with varying emulsifier content (no emulsifier; 0.15% mono- and di-glycerides; 0.15% mono- and di-glycerides plus 0.06% polysorbate 80) were frozen using three different freezing regimes (continuous freezer at low and high back pressure and batch freezer) in order to prepare a series of ice cream samples with varying levels of fat destabilization and

H. D. Goff; E. Verespej; A. K. Smith

1999-01-01

264

Broadband super-collimation in a hybrid photonic crystal structure  

E-print Network

Broadband super-collimation in a hybrid photonic crystal structure Rafif E. Hamam, Mihai Ibanescu, USA rafif@mit.edu Abstract: We propose a two dimensional (2D) photonic crystal (PhC) structure of holes). We theoretically and numerically investigate the collimation mechanism in our 2D structure

Soljaèiæ, Marin

265

Cloud structure and crystal growth in nimbostratus clouds. Mengistu Wolde*  

E-print Network

1 Cloud structure and crystal growth in nimbostratus clouds. Mengistu Wolde* , Gabor Vali-mail: mengistu.wolde@nrc.ca. #12;2 Abstract Cloud structure and crystal growth in two nimbostratus were examined using in situ and airborne radar observations. In both cases, structure throughout the cloud depth

Vali, Gabor

266

Data Mining Analysis of HIV1 Protease Crystal Structures  

Microsoft Academic Search

A data mining study has been done using HIV-1 protease crystal structures complexed with FDA approved HIV-1 protease inhibitor drugs. Chemical descriptors have been computed for the binding pockets of each crystal structure, yielding approximately 600 constitutional, topological, geometric, elecrotostatic, and quantum mechanical descriptors for each structure. Several supervised (hybrid binary particle swarm optimization- artificial neural network and random forest)

Gene M. Ko; A. Srinivas Reddy; Sunil Kumar; Rajni Garg

267

Cyanuryl-PNA monomer: synthesis and crystal structure.  

PubMed

[structure: see text] The chemical synthesis and crystal structure of the peptide nucleic acid (PNA) monomer 11 having cyanuric acid as the nucleobase is reported. The crystal structure of 11 shows molecular tapes arising from continuous intermolecular dimeric hydrogen bonding, with successive tapes held by single hydrogen bonds in the backbone. PMID:10964375

Sanjayan, G J; Pedireddi, V R; Ganesh, K N

2000-09-01

268

Crystal structure of the ?-secretase component nicastrin  

PubMed Central

?-Secretase is an intramembrane protease responsible for the generation of amyloid-? (A?) peptides. Aberrant accumulation of A? leads to the formation of amyloid plaques in the brain of patients with Alzheimer's disease. Nicastrin is the putative substrate-recruiting component of the ?-secretase complex. No atomic-resolution structure had been identified on ?-secretase or any of its four components, hindering mechanistic understanding of ?-secretase function. Here we report the crystal structure of nicastrin from Dictyostelium purpureum at 1.95-Å resolution. The extracellular domain of nicastrin contains a large lobe and a small lobe. The large lobe of nicastrin, thought to be responsible for substrate recognition, associates with the small lobe through a hydrophobic pivot at the center. The putative substrate-binding pocket is shielded from the small lobe by a lid, which blocks substrate entry. These structural features suggest a working model of nicastrin function. Analysis of nicastrin structure provides insights into the assembly and architecture of the ?-secretase complex. PMID:25197054

Xie, Tian; Yan, Chuangye; Zhou, Rui; Zhao, Yanyu; Sun, Linfeng; Yang, Guanghui; Lu, Peilong; Ma, Dan; Shi, Yigong

2014-01-01

269

STRUCTURE NOTE Crystal Structure of a Truncated Version of the Phage  

E-print Network

STRUCTURE NOTE Crystal Structure of a Truncated Version of the Phage Protein gpD Changsoo Chang,1 utilized in this technique are of considerable interest. The high-resolution crystal structure of gp crystallized in a larger cell with two trimers in the asymmetric unit, the structure of this truncated version

270

Low modulus polymer packaged optical fiber sensor for macrocrack monitoring in ice structures of cold regions  

NASA Astrophysics Data System (ADS)

Ice structures provide load-bearing capability for energy exploitation and transportation in cold regions. Meanwhile, staff and facilities take a risk due to large amounts of distributed macrocracks in ice roads, ice bridges, and ice platforms. It is critical to monitor macrocracks for detecting and understanding the fracture process under such a harsh environment. Aiming to obtain real-time, long-term, and quantitative crack opening information for ice structures, this paper presents a feasibility study on monitoring macrocracks with a low modulus polymer packaged optical fiber sensor. Brillouin optical time-domain analysis-based sensing technology is utilized for the distributed strain measurement. According to in situ monitoring requirements, a type of silicone rubber material with appropriate mechanical properties is selected to fabricate the sensor. On this basis, a strain transfer analysis on the packaged and embedded sensor is carried out to derive the relation between the optical measurement and the increment of the crack width. The prototypes have been evaluated by demonstration tests on a tensile device and an ice road model. The experimental results show the sensor can survive in a cold environment and under the large strain resulting from the macrocrack opening. These measured data agree well with the linear calibration. The macrocracks opening in large-scale ice structures can be characterized based on the optical sensor.

Ren, Peng; Zhou, Zhi

2014-09-01

271

Crystal Structures of Respiratory Pathogen Neuraminidases  

SciTech Connect

Currently there is pressing need to develop novel therapeutic agents for the treatment of infections by the human respiratory pathogens Pseudomonas aeruginosa and Streptococcus pneumoniae. The neuraminidases of these pathogens are important for host colonization in animal models of infection and are attractive targets for drug discovery. To aid in the development of inhibitors against these neuraminidases, we have determined the crystal structures of the P. aeruginosa enzyme NanPs and S. pneumoniae enzyme NanA at 1.6 and 1.7 {angstrom} resolution, respectively. In situ proteolysis with trypsin was essential for the crystallization of our recombinant NanA. The active site regions of the two enzymes are strikingly different. NanA contains a deep pocket that is similar to that in canonical neuraminidases, while the NanPs active site is much more open. The comparative studies suggest that NanPs may not be a classical neuraminidase, and may have distinct natural substrates and physiological functions. This work represents an important step in the development of drugs to prevent respiratory tract colonization by these two pathogens.

Hsiao, Y.; Parker, D; Ratner, A; Prince, A; Tong, L

2009-01-01

272

Revisiting the blind tests in crystal structure prediction: accurate energy ranking of molecular crystals.  

PubMed

In the 2007 blind test of crystal structure prediction hosted by the Cambridge Crystallographic Data Centre (CCDC), a hybrid DFT/MM method correctly ranked each of the four experimental structures as having the lowest lattice energy of all the crystal structures predicted for each molecule. The work presented here further validates this hybrid method by optimizing the crystal structures (experimental and submitted) of the first three CCDC blind tests held in 1999, 2001, and 2004. Except for the crystal structures of compound IX, all structures were reminimized and ranked according to their lattice energies. The hybrid method computes the lattice energy of a crystal structure as the sum of the DFT total energy and a van der Waals (dispersion) energy correction. Considering all four blind tests, the crystal structure with the lowest lattice energy corresponds to the experimentally observed structure for 12 out of 14 molecules. Moreover, good geometrical agreement is observed between the structures determined by the hybrid method and those measured experimentally. In comparison with the correct submissions made by the blind test participants, all hybrid optimized crystal structures (apart from compound II) have the smallest calculated root mean squared deviations from the experimentally observed structures. It is predicted that a new polymorph of compound V exists under pressure. PMID:19950907

Asmadi, Aldi; Neumann, Marcus A; Kendrick, John; Girard, Pascale; Perrin, Marc-Antoine; Leusen, Frank J J

2009-12-24

273

Ice Cream  

NSDL National Science Digital Library

In this chemistry activity, learners use the lowered freezing point of water to chill another mixture (ice cream) to the solid state. Learners will record the temperature of the ice before and after mixing it with the ice cream ingredients and discover that adding a solute to a solvent lowers the freezing point of that solvent (also known as a colligative property). This activity can also be used to introduce learners to crystallization.

The Science House

2014-01-28

274

Crystal structure of betulinic acid methanol monosolvate  

PubMed Central

The title compound [systematic name: 3?-hy­droxy­lup-20(29)-en-28-oic acid methanol monosolvate], C30H48O3·CH3OH, is a solvent pseudopolymorph of a naturally occurring plant-derived lupane-type penta­cyclic triterpenoid, which was isolated from the traditional Chinese medicinal plant Syzygium jambos (L.) Alston. The dihedral angle between the planes of the carb­oxy­lic acid group and the olefinic group is 12.17?(18)°. The A/B, B/C, C/D and D/E ring junctions are all trans-fused. In the crystal, O—H?O hydrogen bonds involving the hy­droxy and carb­oxy­lic acid groups and the methanol solvent mol­ecule give rise to a two-dimensional network structure lying parallel to (001). PMID:25553022

Tang, Wei; Chen, Neng-Hua; Li, Guo-Qiang; Wang, Guo-Cai; Li, Yao-Lan

2014-01-01

275

Crystal structure of betulinic acid methanol monosolvate.  

PubMed

The title compound [systematic name: 3?-hy-droxy-lup-20(29)-en-28-oic acid methanol monosolvate], C30H48O3·CH3OH, is a solvent pseudopolymorph of a naturally occurring plant-derived lupane-type penta-cyclic triterpenoid, which was isolated from the traditional Chinese medicinal plant Syzygium jambos (L.) Alston. The dihedral angle between the planes of the carb-oxy-lic acid group and the olefinic group is 12.17?(18)°. The A/B, B/C, C/D and D/E ring junctions are all trans-fused. In the crystal, O-H?O hydrogen bonds involving the hy-droxy and carb-oxy-lic acid groups and the methanol solvent mol-ecule give rise to a two-dimensional network structure lying parallel to (001). PMID:25553022

Tang, Wei; Chen, Neng-Hua; Li, Guo-Qiang; Wang, Guo-Cai; Li, Yao-Lan

2014-12-01

276

Spatial and temporal variations in the age structure of Arctic sea ice  

USGS Publications Warehouse

Spatial and temporal variations in the age structure of Arctic sea ice are investigated using a new reversechronology algorithm that tracks ice-covered pixels to their location and date of origin based on ice motion and concentration data. The Beaufort Gyre tends to harbor the oldest (>10 years old) sea ice in the western Arctic while direct ice advection pathways toward the Transpolar Drift Stream maintain relatively young (???5 years) ice in the eastern Arctic. Persistent net losses (-4.2% yr-1) in extent of ice >10 years old (10+ year age class) were observed during 1989-2003. Since the mid-1990s, losses to the 10+ year age class lacked compensation by recruitment due to a prior depletion of all mature (6-10 year) age classes. Survival of the 1994 and 1996-1998 sea ice generations reestablished most mature age classes, and thereby the potential to increase extent of the 10+ year age class during the mid-2000s. Copyright 2005 by the American Geophysical Union.

Belchansky, G.I.; Douglas, D.C.; Platonov, N.G.

2005-01-01

277

Southern Ocean frontal structure and sea-ice formation rates revealed by elephant seals.  

PubMed

Polar regions are particularly sensitive to climate change, with the potential for significant feedbacks between ocean circulation, sea ice, and the ocean carbon cycle. However, the difficulty in obtaining in situ data means that our ability to detect and interpret change is very limited, especially in the Southern Ocean, where the ocean beneath the sea ice remains almost entirely unobserved and the rate of sea-ice formation is poorly known. Here, we show that southern elephant seals (Mirounga leonina) equipped with oceanographic sensors can measure ocean structure and water mass changes in regions and seasons rarely observed with traditional oceanographic platforms. In particular, seals provided a 30-fold increase in hydrographic profiles from the sea-ice zone, allowing the major fronts to be mapped south of 60 degrees S and sea-ice formation rates to be inferred from changes in upper ocean salinity. Sea-ice production rates peaked in early winter (April-May) during the rapid northward expansion of the pack ice and declined by a factor of 2 to 3 between May and August, in agreement with a three-dimensional coupled ocean-sea-ice model. By measuring the high-latitude ocean during winter, elephant seals fill a "blind spot" in our sampling coverage, enabling the establishment of a truly global ocean-observing system. PMID:18695241

Charrassin, J-B; Hindell, M; Rintoul, S R; Roquet, F; Sokolov, S; Biuw, M; Costa, D; Boehme, L; Lovell, P; Coleman, R; Timmermann, R; Meijers, A; Meredith, M; Park, Y-H; Bailleul, F; Goebel, M; Tremblay, Y; Bost, C-A; McMahon, C R; Field, I C; Fedak, M A; Guinet, C

2008-08-19

278

Southern Ocean frontal structure and sea-ice formation rates revealed by elephant seals  

PubMed Central

Polar regions are particularly sensitive to climate change, with the potential for significant feedbacks between ocean circulation, sea ice, and the ocean carbon cycle. However, the difficulty in obtaining in situ data means that our ability to detect and interpret change is very limited, especially in the Southern Ocean, where the ocean beneath the sea ice remains almost entirely unobserved and the rate of sea-ice formation is poorly known. Here, we show that southern elephant seals (Mirounga leonina) equipped with oceanographic sensors can measure ocean structure and water mass changes in regions and seasons rarely observed with traditional oceanographic platforms. In particular, seals provided a 30-fold increase in hydrographic profiles from the sea-ice zone, allowing the major fronts to be mapped south of 60°S and sea-ice formation rates to be inferred from changes in upper ocean salinity. Sea-ice production rates peaked in early winter (April–May) during the rapid northward expansion of the pack ice and declined by a factor of 2 to 3 between May and August, in agreement with a three-dimensional coupled ocean–sea-ice model. By measuring the high-latitude ocean during winter, elephant seals fill a “blind spot” in our sampling coverage, enabling the establishment of a truly global ocean-observing system. PMID:18695241

Charrassin, J.-B.; Hindell, M.; Rintoul, S. R.; Roquet, F.; Sokolov, S.; Biuw, M.; Costa, D.; Boehme, L.; Lovell, P.; Coleman, R.; Timmermann, R.; Meijers, A.; Meredith, M.; Park, Y.-H.; Bailleul, F.; Goebel, M.; Tremblay, Y.; Bost, C.-A.; McMahon, C. R.; Field, I. C.; Fedak, M. A.; Guinet, C.

2008-01-01

279

The Crystal Structure of Triuranyl Diphosphate Tetrahydrate  

NASA Astrophysics Data System (ADS)

The hydrated neutral uranyl phosphate, (UO2)3(PO4)2(H2O)4, was synthesized by hydrothermal methods. Intensity data were collected using MoK? radiation and a CCD-based area detector. The crystal structure was solved by direct methods and refined by full-matrix least-squares techniques to agreement indices wR2=0.116 for all data, and R1=0.040, calculated for the 2764 unique observed reflections (?Fo??4?F). The compound is orthorhombic, space group Pnma, Z=4, a=7.063(1) Å, b=17.022(3) Å, c=13.172(3) Å, V=1583.5(5) Å3. The structure consists of sheets of phosphate tetrahedra and uranyl pentagonal bipyramids, with composition [(UO2)(PO4)]- and the uranophane sheet anion topology. The sheets are connected by a uranyl pentagonal bipyramid in the interlayer that shares corners with a phosphate tetrahedron on each of two adjacent sheets, resulting in an open framework with isolated H2O groups in the larger cavities of the structure.

Locock, Andrew J.; Burns, Peter C.

2002-01-01

280

Lactose Crystallization in Ice Cream. IV. Factors Responsible for Reduced Incidence of Sandiness  

Microsoft Academic Search

SUMMARY An explanation was sought for the virtual disappearance of sandiness from commercial ice cream. Only five of 36 commercial samples became sandy when stored seven months at 12 F. Nuclei formation was accelerated by drawing ice cream from freezers at low temperatures and hardening it rapidly. Partial sub- stitution of corn syrup solids for sucrose neither delayed the development

T. A. Nickerson

1962-01-01

281

The hierarchical structure of glacial climatic oscillations: Interactions between ice-sheet dynamics and climate  

SciTech Connect

Abrupt climatic oscillations around the North Atlantic have been identified recently in Greenland ice cores as well as in North Atlantic marine sediment cores. The good correlation between the {open_quote}Dansgaard Oeschger events{close_quote} in the ice and the {open_quote}Heinrich events{close_quote} in the ocean suggests climate, in the North Atlantic region, underwent several massive reorganizations in the last glacial period. A characteristic feature seems to be their hierarchical structure. Every 7 to 10-thousand years, when the temperature is close to its minimum, the ice-sheet undergoes a massive iceberg discharge. This Heinrich event is followed by an abrupt warming. then by other oscillations, each lasting between one and two thousand years. These secondary oscillations do not have a clear signature in marine sediments but constitute most of the{open_quote} Dansgaard-Oeschger events{close_quote} found in the ice. A simplified model coupling an ice-sheet and an ocean basin, to illustrate how the interactions between these two components can lead to such a hierarchical structure. The ice-sheet model exhibits internal oscillations composed of growing phases and basal ice melting phases that induce massive iceberg discharges. These fresh water inputs in the ocean stop for a moment the thermohaline circulation, enhancing the temperature contrast between low- and high-latitudes. Just after this event, the thermohaline circulation restarts and an abrupt warming of high-latitude regions is observed. For some parameter values, these warmer temperatures have some influence on the ice-sheet, inducing secondary oscillations similar to those found in paleoclimatic records. Although the mechanism presented here may be too grossly simplified. it nevertheless underlines the potential importance of the coupling between ice-sheet dynamics and oceanic thermohaline circulation on the structure of the climatic records during the last glacial period. 33 refs., 14 figs., 1 tab.

Paillard, D. [Centre d`Etudes de Saclay, Gif sur Yvette (France)] [Centre d`Etudes de Saclay, Gif sur Yvette (France)

1995-04-01

282

Research on TGS single crystal growth with modulated structure  

NASA Astrophysics Data System (ADS)

The experimental results of TGS crystal growth with modulated structure (periodic laminar of ferroelectric domains) is briefly discussed in this paper. The method used here is to add some DL-alanine additive in the solution of TGS and to apply the electric field during crystal growth, the direction of which is periodically varied. By using this method, TGS single crystals containing a nearly regular and flat periodic laminar structure of ferroelectric domains have been fabricated.

Wang, Wenshan; Qi, Ming

1986-12-01

283

Sea Ice, an Antarctic Habitat  

NSDL National Science Digital Library

A 'click-and-learn' sub site hosted by the Alfred Wegener Institute Foundation for Polar and Marine Research (AWI), this is a succinct, educational tour of sea-ice and its associated ecological communities. Short synopses introduce the dynamics of sea-ice formation, the microstructure of sea-ice (including crystal structure, brine channels, and ice algae), the effects of ice melt on resident organisms, the logistics of sea-ice research, and _land fast-ice_ and platelet ice habitats. Introductions also exist for the following organisms: krill; whales (i.e., Orcas, southern bottlenosesd dolphins, minke whales); sea birds (i.e., skuas and snow petrals), penguins (i.e., emperor, adelie, and chinstraps), and seals (i.e., weddell, crabeater, leopard, and ross.) Enlargeable thumbnail images accompany the habitat and inhabitant descriptions. Further investigations (at an accelerated level) are prompted with the inclusion of bibliographic references and scientific research presentations (in PDF format) on fast-ice and platelet ice, as well as links to the main site for the AWI.

284

Crystallization and Characterization of Galdieria sulphuraria RUBISCO in Two Crystal Forms: Structural Phase Transition Observed in P21 Crystal Form  

PubMed Central

We have isolated ribulose-1,5-bisphosphate-carboxylase/oxygenase (RUBISCO) from the red algae Galdieria Sulphuraria. The protein crystallized in two different crystal forms, the I422 crystal form being obtained from high salt and the P21 crystal form being obtained from lower concentration of salt and PEG. We report here the crystallization, preliminary stages of structure determination and the detection of the structural phase transition in the P21 crystal form of G. sulphuraria RUBISCO. This red algae enzyme belongs to the hexadecameric class (L8S8) with an approximate molecular weight 0.6MDa. The phase transition in G. sulphuraria RUBISCO leads from two hexadecamers to a single hexadecamer per asymmetric unit. The preservation of diffraction power in a phase transition for such a large macromolecule is rare.

Baranowski, Michael; Stec, Boguslaw

2007-01-01

285

Porous ice phases with VI and distorted VII structures constrained in nanoporous silica.  

PubMed

High-pressure compression of water contained in nanoporous silica allowed fabrication of novel porous ice phases as a function of pressure. The starting liquid nanoporous H2O transformed to ice VI and VII at 1.7 and 2.5 GPa, respectively, which are 0.6 and 0.4 GPa higher than commonly accepted pressures for bulk H2O. The continuous increase of pressure drives the formation of a tetragonally distorted VII structure with the space group I4mm, rather than a cubic Pn3m phase in bulk ice. The enhanced incompressibility of the tetragonal ice is related to the unique nanoporous configuration, and the distortion ratio c/a gradually increases with increasing pressure. The structural changes and enhanced thermodynamic stability may be interpreted by the two-dimensional distribution of silanol groups on the porous silica surfaces and the associated anisotropic interactions with H2O at the interfaces. PMID:25338300

Zhu, Jinlong; Quan, Zewei; Lin, Yu-Shen; Jiang, Ying-Bing; Wang, Zhongwu; Zhang, Jianzhong; Jin, Changqing; Zhao, Yusheng; Liu, Zhenxian; Brinker, C Jeffrey; Xu, Hongwu

2014-11-12

286

Undergraduates Improve upon Published Crystal Structure in Class Assignment  

ERIC Educational Resources Information Center

Recently, 57 undergraduate students at the University of Michigan were assigned the task of solving a crystal structure, given only the electron density map of a 1.3 Å crystal structure from the electron density server, and the position of the N-terminal amino acid. To test their knowledge of amino acid chemistry, the students were not given the…

Horowitz, Scott; Koldewey, Philipp; Bardwell, James C.

2014-01-01

287

Use of Pom Pons to Illustrate Cubic Crystal Structures.  

ERIC Educational Resources Information Center

Describes a method that uses olefin pom pons to illustrate cubic crystal structure. Facilitates hands-on examination of different packing arrangements such as hexagonal close-packed and cubic close-packed structures. (JRH)

Cady, Susan G.

1997-01-01

288

Crystal structure of a Trypanosoma brucei metacaspase  

PubMed Central

Metacaspases are distantly related caspase-family cysteine peptidases implicated in programmed cell death in plants and lower eukaryotes. They differ significantly from caspases because they are calcium-activated, arginine-specific peptidases that do not require processing or dimerization for activity. To elucidate the basis of these differences and to determine the impact they might have on the control of cell death pathways in lower eukaryotes, the previously undescribed crystal structure of a metacaspase, an inactive mutant of metacaspase 2 (MCA2) from Trypanosoma brucei, has been determined to a resolution of 1.4 Å. The structure comprises a core caspase fold, but with an unusual eight-stranded ?-sheet that stabilizes the protein as a monomer. Essential aspartic acid residues, in the predicted S1 binding pocket, delineate the arginine-specific substrate specificity. In addition, MCA2 possesses an unusual N terminus, which encircles the protein and traverses the catalytic dyad, with Y31 acting as a gatekeeper residue. The calcium-binding site is defined by samarium coordinated by four aspartic acid residues, whereas calcium binding itself induces an allosteric conformational change that could stabilize the active site in a fashion analogous to subunit processing in caspases. Collectively, these data give insights into the mechanistic basis of substrate specificity and mode of activation of MCA2 and provide a detailed framework for understanding the role of metacaspases in cell death pathways of lower eukaryotes. PMID:22529389

McLuskey, Karen; Rudolf, Jana; Proto, William R.; Isaacs, Neil W.; Coombs, Graham H.; Moss, Catherine X.; Mottram, Jeremy C.

2012-01-01

289

Discovery of Crystallized Water Ice in a Silhouette Disk in the M43 Region  

E-print Network

We present the 1.9--4.2um spectra of the five bright (Lsilhouette disks with moderate to high inclination angle of 39--80deg in the M42 and M43 regions. The water ice absorption is seen toward d121-1925 and d216-0939, while the spectra of d182-316, d183-405, and d218-354 show no water ice feature around 3.1um within the detection limits. By comparing the water ice features toward nearby stars, we find that the water ice absorption toward d121-1925 and d216-0939 most likely originates from the foreground material and the surrounding disk, respectively. The angle of the disk inclination is found to be mainly responsible for the difference of the optical depth of the water ice among the five young stars. Our results suggest that there is a critical inclination angle between 65deg and 75deg for the circumstellar disk where the water ice absorption becomes strong. The average density at the disk surface of d216-0939 was found to be 6.38x10^(-18) g cm^(-3). The water ice absorp...

Terada, Hiroshi

2012-01-01

290

Part A: Cirrus ice crystal nucleation and growth. Part B: Automated analysis of aircraft ice particle data  

NASA Technical Reports Server (NTRS)

Specific measurement of cirrus crystals by aircraft and temperature modified CN are used to specify measurements necessary to provide a basis for a conceptual model of cirrus particle formation. Key to this is the ability to measure the complete spectrum of particles at cirrus levels. The most difficult regions for such measurement is from a few to 100 microns, and uses a replicator. The details of the system to automate replicator data analysis are given, together with an example case study of the system provided from a cirrus cloud in FIRE 2, with particles detectable by replicator and FSSP, but not 2DC.

Arnott, William P.; Hallett, John; Hudson, James G.

1995-01-01

291

The effect of structural porosity on the ablation of sea ice ridges  

NASA Astrophysics Data System (ADS)

Observations reveal that the decrease in ice thickness through melting in summer is much more rapid for ridges than for surrounding level ice. A physical model that represents internal melting within ridge keels has been developed to explain this observed draft-dependent ablation for first-year pack ice in the Beaufort Sea. The porous structure of a ridge keel permits percolation of a substantial fraction of the oncoming oceanic flow, up to 20% for a feature with 30% porosity and 9-m draft. The percolating flow delivers oceanic heat to a large surface area deep within the keel and increases melt rates relative to surrounding level ice by a factor of 5 when seawater temperatures are 0.18 degrees above freezing. Melt rates are sensitive to the internal geometry of ridges through keel porosity and block dimensions, characteristics that vary widely between ridge features. However, the average rate of melting as a function of draft, calculated for a realistic population of keels with average cross-sectional shape and differing draft, has the same draft-dependence as the observations. This concurrence suggests that the process of internal melting may be dominant in the ablation of ridged ice. In addition, internal melting during the summer may well hasten structural consolidation of surviving ridge keels through freezing during the following winter. It appears that the evolution of the thickest ice within the Arctic ice pack is dependent on the small-scale structural characteristics of the ridged ice and its interaction with the upper layer of the ocean.

Amundrud, Trisha L.; Melling, Humfrey; Ingram, R. Grant; Allen, Susan E.

2006-06-01

292

Synthesis, crystal structure and superconductivity of LaNiPO  

Microsoft Academic Search

Single crystals of LaNiPO were synthesized by reacting La, P and NiO at 1173K in a tin flux under argon atmosphere. A phase analysis and crystal structure determination was carried out using X-ray powder and single crystal methods. The quaternary phosphide oxide crystallizes in the tetragonal ZrCuSiAs structure (P4\\/nmm, a=404.53(1)pm, c=810.54(3)pm, Z=2), which is characterized by layers of edge-sharing [La4\\/4O]?2

Marcus Tegel; Daniel Bichler; Dirk Johrendt

2008-01-01

293

Dielectric constant adjustments in computations of the scattering properties of solid ice crystals using the Generalized Multi-particle Mie method  

NASA Astrophysics Data System (ADS)

Ice crystal scattering properties at microwave radar wavelengths can be modeled with the Generalized Multi-particle Mie (GMM) method by decomposing an ice crystal into a cluster of tiny spheres composed of solid ice. In this decomposition the mass distribution of the tiny spheres in the cluster is no longer equivalent to that in the original ice crystal because of gaps between the tiny spheres. To compensate for the gaps in the cluster representation of an ice crystal in the GMM computation of crystal scattering properties, the Maxwell Garnett approximation is used to estimate what the dielectric function of the tiny spheres (i.e., the inclusions) in the cluster must be to make the cluster of tiny spheres with associated air gaps (i.e., the background matrix) dielectrically equivalent to the original solid ice crystal. Overall, compared with the T-matrix method for spheroids outside resonance regions this approach agrees to within mostly 0.3 dB (and often better) in the horizontal backscattering cross section ?hh and the ratio of horizontal and vertical backscattering cross sections ?hh/?vv, and 6% for the amplitude scattering matrix elements Re{S22-S11} and Im{S22} in the forward direction. For crystal sizes and wavelengths near resonances, where the scattering parameters are highly sensitive to the crystal shape, the differences are generally within 1.2 dB for ?hh and ?hh/?vv, 20% for Re{S22-S11} and 6% for Im{S22}. The Discrete Dipole Approximation (DDA) results for the same spheroids are generally closer than those of GMM to the T-matrix results. For hexagonal plates the differences between GMM and the DDA at a W-band wavelength (3.19 mm) are mostly within 0.6 dB for ?hh, 1 dB for ?hh/?vv, 11% for Re{S22-S11} and 12% for Im{S22}. For columns the differences are within 0.3 dB for ?hh and ?hh/?vv, 8% for Re{S22-S11} and 4% for Im{S22}. This method shows higher accuracy than an alternative method that artificially increases the thickness of ice plates to provide the same mass as the original ice crystal.

Lu, Yinghui; Aydin, Kültegin; Clothiaux, Eugene E.; Verlinde, Johannes

2014-03-01

294

Crystal structure of 2-pentyl­oxybenzamide  

PubMed Central

In the title mol­ecule, C12H17NO2, the amide NH2 group is oriented toward the pent­yloxy substituent and an intra­molecular N—H?O hydrogen bond is formed with the pent­yloxy O atom. The benzene ring forms dihedral angles of 2.93?(2) and 5.60?(2)° with the amide group and the pent­yloxy group mean planes, respectively. In the crystal, mol­ecules are linked by pairs of N—H?O hydrogen bonds, forming inversion dimers with their mol­ecular planes parallel, but at an offset of 0.45?(1)?Å to each other. These dimers are ordered into two types of symmetry-related columns extended along the a axis, with the mean plane of one set of dimers in a column approximately parallel to (121) and the other in a column approximately parallel to (1-21). The two planes form a dihedral angle of 85.31?(2)°, and are linked via C—H?O hydrogen bonds and C—H?? inter­actions, forming a three-dimensional framework structure. PMID:25484660

Bugenhagen, Bernhard; Al Jasem, Yosef; Thiemann, Thies

2014-01-01

295

Crystal structure, spectral, thermal and dielectric studies of a new zinc benzoate single crystal  

NASA Astrophysics Data System (ADS)

Single crystals of zinc benzoate with a novel structure were grown in gel media. Sodium metasilicate of gel density 1.04 g/cc at pH 6 was employed to yield transparent single crystals. The crystal structure of the compound was ascertained by single crystal X-ray diffractometry. It was noted that the crystal belongs to monoclinic system with space group P21/c with unit cell parameters a = 10.669(1) Å, b = 12.995(5) Å, c = 19.119(3) Å, and ? = 94.926(3)°. The crystal was seen to possess a linear polymeric structure along b-axis; with no presence of coordinated or lattice water. CHN analysis established the stoichiometric composition of the crystal. The existence of functional groups present in the single crystal system was confirmed by FT-IR studies. The thermal characteristic of the sample was analysed by TGA-DTA techniques, and the sample was found to be thermally stable up to 280 °C. The kinetic and thermodynamic parameters were also determined. UV-Vis spectroscopy corroborated the transparency of the crystal and revealed the optical band gap to be 4 eV. Dielectric studies showed decrease in the dielectric constant of the sample with increase in frequency.

Bijini, B. R.; Prasanna, S.; Deepa, M.; Nair, C. M. K.; Rajendra Babu, K.

2012-11-01

296

Ice water content retrievals using an estimation theory approach: examples from the NASA CRYSTAL-FACE experiment  

NASA Astrophysics Data System (ADS)

This study introduces a new, robust and reliable method to estimate ice cloud microphysical properties from cloud radar reflectivities and visible optical depth. The retrieval is formulated in an estimation theory framework which permits the introduction of optimal combinations of different measurements and a complete characterization of retrieval errors. The sensitivity of the retrieval to the assumed error statistics is assessed performing experiments with variablea priori, optical depth and forward model uncertainties. Quantitative estimates of the uncertainties show that the average ice water content is retrieved with errors varying between 20--30%. The relative error on ice water path is of the same order of magnitude. The retrieval is applied to synthetic and real observations. Retrieved products are checked against other retrieval methods andin situ observations when available. The results compare well with results from other methods. The retrieval appears to be robust and can be applied successfully to a variety of cirrus clouds without suffering from the problems often encountered when using empirically--based methods. As part of ongoing research the method is being evaluated using data from the NASA CRYSTAL--FACE experiment and the Atmospheric Radiation Measurement - Unmanned Aerospace Vehicle (ARM-UAV) Fall 2002 experiment.

Benedetti, A.; Stephens, G.; Haynes, J.

2003-04-01

297

Photonic crystals for monitoring fatigue phenomena in steel structures  

NASA Astrophysics Data System (ADS)

This paper introduces the concept and development of a strain sensing system for structural application based on the properties of photonic crystals. Photonic crystals are artificially created periodic structures, usually produced in the thinfilm form, where optical properties are tailored by a periodicity in the refractive index. The idea of using the crystal as a sensor is based on the observation that a distortion in the crystal structure produces a change in the reflected bandwidth. When a photonic crystal is designed to operate in the visible part of the spectrum, a permanent distortion of the film results in a change in its apparent color. This property makes photonic crystals suitable for permanent monitoring of structural elements, as any critical changes in the strain field can be promptly and easily detected by visual inspection. A simple and low-cost example of photonic crystals consists of opals synthesized by vertical deposition. In this contribution we introduce a target application for the fatigue monitoring of wind turbines, and then provide the reader with some basic information concerning modeling of the crystal architecture and fabrication of these structures. Next we discuss their application to strain measurement, specifying how reflection and transmission properties of the opals have to be designed to satisfy the expected strain response of the sensor. Finally, we present the preliminary results of a laboratory validation carried out on thin films applied to a rubber support.

Zonta, Daniele; Chiappini, Andrea; Chiasera, Alessandro; Ferrari, Maurizio; Pozzi, Matteo; Battisti, Lorenzo; Benedetti, Matteo

2009-03-01

298

Structural and Thermoelectric Properties of Tungsten Diselenide Crystals  

NASA Astrophysics Data System (ADS)

Crystals of Tungsten diselenide (WSe2) have been grown by direct vapour transport (DVT) technique using micro processor controlled dual zone horizontal furnace. The chemical composition and structure of grown crystals were confirmed using energy dispersive analysis of X-ray (EDAX) and X-ray diffraction (XRD). In the present investigation thermoelectric power measurements (TEP) have been carried out on the grown crystals. Different electrical transport parameters of semiconductors have been determined and discussed in the paper.

Patel, K. K.; Patel, K. D.; Patel, Mayur; Patel, C. A.; Pathak, V. M.; Srivastava, R.

2011-12-01

299

Crystal and molecular structure of organophosphorus insecticides. 12. Dowco 214  

Microsoft Academic Search

The crystal and molecular structure of Dowco 214 (O,O-dimethyl O-(3,5,6-trichloro-2-pyridyl) phosphorothioate) has been determined by single crystal X-ray techniques. It crystallizes in space group P1 with ..cap alpha.. = 11.598 (2), b = 13.619 (3), c = 8.281 (1) A, ..cap alpha.. = 94.65 (1), ..beta.. = 94.87 (2), and ..gamma.. = 79.97 (2)° with four molecules per unit cell

Debra E. Beckman; Robert A. Jacobson

1979-01-01

300

laser ultrasonic characterization of ice cores  

NASA Astrophysics Data System (ADS)

We present preliminary measurements on ice cores of elastic wave forms at ultrasonic frequencies. The aim of the project is to map out internal properties of the ice to improve our understanding of the processes responsible for the ice structure. Annual layering is one of the targets, but the alignment of ice crystals is another.We use a system based on laser sources and receivers for a number of reasons. First, the lasers allow us to probe the ice in a non-destructive and non-contacting matter through optical windows into our cold room. Second, the lasers/core system is controlled via computerized stages, which allow us to have unprecedented data density, repeatable data acquisition, and high fidelity in each waveform. We calibrated layering properties with man-made ice structures, and we will present ongoing tests on Antarctic cores from various depths and locations.

van Wijk, K.; Otheim, L. T.; Marshall, H.; Kurbatov, A.; Spaulding, N. E.

2013-12-01

301

Photonic crystal structures with tunable structure color as colorimetric sensors.  

PubMed

Colorimetric sensing, which transduces environmental changes into visible color changes, provides a simple yet powerful detection mechanism that is well-suited to the development of low-cost and low-power sensors. A new approach in colorimetric sensing exploits the structural color of photonic crystals (PCs) to create environmentally-influenced color-changeable materials. PCs are composed of periodic dielectrics or metallo-dielectric nanostructures that affect the propagation of electromagnetic waves (EM) by defining the allowed and forbidden photonic bands. Simultaneously, an amazing variety of naturally occurring biological systems exhibit iridescent color due to the presence of PC structures throughout multi-dimensional space. In particular, some kinds of the structural colors in living organisms can be reversibly changed in reaction to external stimuli. Based on the lessons learned from natural photonic structures, some specific examples of PCs-based colorimetric sensors are presented in detail to demonstrate their unprecedented potential in practical applications, such as the detections of temperature, pH, ionic species, solvents, vapor, humidity, pressure and biomolecules. The combination of the nanofabrication technique, useful design methodologies inspired by biological systems and colorimetric sensing will lead to substantial developments in low-cost, miniaturized and widely deployable optical sensors. PMID:23539027

Wang, Hui; Zhang, Ke-Qin

2013-01-01

302

Photonic Crystal Structures with Tunable Structure Color as Colorimetric Sensors  

PubMed Central

Colorimetric sensing, which transduces environmental changes into visible color changes, provides a simple yet powerful detection mechanism that is well-suited to the development of low-cost and low-power sensors. A new approach in colorimetric sensing exploits the structural color of photonic crystals (PCs) to create environmentally-influenced color-changeable materials. PCs are composed of periodic dielectrics or metallo-dielectric nanostructures that affect the propagation of electromagnetic waves (EM) by defining the allowed and forbidden photonic bands. Simultaneously, an amazing variety of naturally occurring biological systems exhibit iridescent color due to the presence of PC structures throughout multi-dimensional space. In particular, some kinds of the structural colors in living organisms can be reversibly changed in reaction to external stimuli. Based on the lessons learned from natural photonic structures, some specific examples of PCs-based colorimetric sensors are presented in detail to demonstrate their unprecedented potential in practical applications, such as the detections of temperature, pH, ionic species, solvents, vapor, humidity, pressure and biomolecules. The combination of the nanofabrication technique, useful design methodologies inspired by biological systems and colorimetric sensing will lead to substantial developments in low-cost, miniaturized and widely deployable optical sensors. PMID:23539027

Wang, Hui; Zhang, Ke-Qin

2013-01-01

303

Novel photonic crystal cavities and related structures.  

SciTech Connect

The key accomplishment of this project is to achieve a much more in-depth understanding of the thermal emission physics of metallic photonic crystal through theoretical modeling and experimental measurements. An improved transfer matrix technique was developed to enable incorporation of complex dielectric function. Together with microscopic theory describing emitter radiative and non-radiative relaxation dynamics, a non-equilibrium thermal emission model is developed. Finally, experimental methodology was developed to measure absolute emissivity of photonic crystal at high temperatures with accuracy of +/-2%. Accurate emissivity measurements allow us to validate the procedure to treat the effect of the photonic crystal substrate.

Luk, Ting Shan

2007-11-01

304

Ice island creation, drift, recurrences, mechanical properties, and interactions with arctic offshore oil production structures. Final report  

Microsoft Academic Search

Research and engineering studies on first-year sea ice for over two decades has resulted in the design, construction, and operation of jacket platforms, of artificial islands, and of massive gravity structures which routinely withstand moving sea ice of thickness up to 2 meters. However, the less-common interactions between such structures and moving multiyear ice (â¥3 meters thick), and also moving

W. M. Sackinger; M. O. Jeffries; Fucheng Li; Mingchi Lu

1991-01-01

305

Hertzian Fracture in Single Crystals with the Diamond Structure  

Microsoft Academic Search

Extension of an earlier theory of Hertzian fracture in brittle isotropic materials is here made to include the case of brittle single crystals, with particular reference to crystals having the diamond structure. A detailed description is first given of the inhomogeneous stress field in a flat, elastic specimen loaded normally with a hard sphere. The geometry of cracks growing in

B. R. Lawn

1968-01-01

306

Crystal Structure, Physical Properties, and Electrochemistry of Copper Substituted LiFePO4 Single Crystals  

E-print Network

indicate that the dark brown color originates from interionic d-d transitions. A part of the lithium ions phosphate, with the olivine structure, has been synthesized hydrothermally as dark-brown single crystals

Ceder, Gerbrand

307

The crystal structure of SnP3 and a note on the crystal structure of GeP3  

Microsoft Academic Search

SnP3 crystallizes in the trigonal space group R3m with six formula units in a unit cell of dimensions a = 7.378Å and c = 10.512Å. The detailed atomic arrangement has been determined from three-dimensional single crystal X ray data. The structure is characterized as a layer structure related to the As-type structure (A7). The identical layers consist of puckered P6

Jan Gullman; Olle Olofsson

1972-01-01

308

Meth (Crank, Ice) Facts  

MedlinePLUS

Listen to this page Meth (Crank, Ice) Facts Methamphetamine—meth for short—is a white, bitter powder. ... names for meth are: Crank Ice Crystal Glass Chalk En español "Heart disease runs in some families. ...

309

Band structures of bilayer radial phononic crystal plate with crystal gliding  

NASA Astrophysics Data System (ADS)

Lamb wave propagation in bilayer radial phononic crystal plate with crystal gliding is investigated. Axial symmetric model in cylindrical coordinate is applied to the bilayer radial phononic crystal plate for band structure calculation and transmission spectra. Gliding in radial direction and direction vertical to plate thickness is analyzed to modulate band gaps. Physical mechanism of gliding effects on radial phononic crystal plate is also studied with displacement fields of super cells. Numerical results show that crystal gliding both in radial direction and direction vertical to plate thickness can significantly tune omnidirectional band gaps. New lower band gaps occur and attenuation areas in transmission spectra are in good agreement with gaps of band structure calculation. Band structure evolution together with eigenmodes indicate that gliding effect converts lamb wave modes resulting in separations or interactions of adjacent bands to open new gaps or close the original ones. In addition, band gaps' sensitivity to crystal gliding is also investigated. Higher gaps are more sensitive to crystal gliding in thickness direction, and lowest gap extends in the map. Crystal gliding in radial direction can open new lowest order gap and open or close another two higher gaps, while the fourth gap is insensitive to it. The omnidirectional band gaps properties have potential application in acoustic device with isotropic gap characters.

Ma, Ting; Chen, Tianning; Wang, Xiaopeng; Li, Yinggang; Wang, Peng

2014-09-01

310

An unconventional bilayer ice structure on a NaCl(001) film  

NASA Astrophysics Data System (ADS)

Water-solid interactions are of broad importance both in nature and technology. The hexagonal bilayer model based on the Bernal-Fowler-Pauling ice rules has been widely adopted to describe water structuring at interfaces. Using a cryogenic scanning tunnelling microscope, here we report a new type of two-dimensional ice-like bilayer structure built from cyclic water tetramers on an insulating NaCl(001) film, which is completely beyond this conventional bilayer picture. A novel bridging mechanism allows the interconnection of water tetramers to form chains, flakes and eventually a two-dimensional extended ice bilayer containing a regular array of Bjerrum D-type defects. Ab initio density functional theory calculations substantiate this bridging growth mode and reveal a striking proton-disordered ice structure. The formation of the periodic Bjerrum defects with unusually high density may have a crucial role as H donor sites in directing multilayer ice growth and in catalysing heterogeneous chemical reactions on water-coated salt surfaces.

Chen, Ji; Guo, Jing; Meng, Xiangzhi; Peng, Jinbo; Sheng, Jiming; Xu, Limei; Jiang, Ying; Li, Xin-Zheng; Wang, En-Ge

2014-05-01

311

An unconventional bilayer ice structure on a NaCl(001) film.  

PubMed

Water-solid interactions are of broad importance both in nature and technology. The hexagonal bilayer model based on the Bernal-Fowler-Pauling ice rules has been widely adopted to describe water structuring at interfaces. Using a cryogenic scanning tunnelling microscope, here we report a new type of two-dimensional ice-like bilayer structure built from cyclic water tetramers on an insulating NaCl(001) film, which is completely beyond this conventional bilayer picture. A novel bridging mechanism allows the interconnection of water tetramers to form chains, flakes and eventually a two-dimensional extended ice bilayer containing a regular array of Bjerrum D-type defects. Ab initio density functional theory calculations substantiate this bridging growth mode and reveal a striking proton-disordered ice structure. The formation of the periodic Bjerrum defects with unusually high density may have a crucial role as H donor sites in directing multilayer ice growth and in catalysing heterogeneous chemical reactions on water-coated salt surfaces. PMID:24874452

Chen, Ji; Guo, Jing; Meng, Xiangzhi; Peng, Jinbo; Sheng, Jiming; Xu, Limei; Jiang, Ying; Li, Xin-Zheng; Wang, En-Ge

2014-01-01

312

Size, separation, structural order, and mass density of molecules packing in water and ice  

PubMed Central

The structural symmetry and molecular separation in water and ice remain uncertain. We present herewith a solution to unifying the density, the structure order and symmetry, the size (H-O length dH), and the separation (dOO = dL + dH or the O:H length dL) of molecules packing in water and ice in terms of statistic mean. This solution reconciles: i) the dL and the dH symmetrization of the O:H-O bond in compressed ice, ii) the dOO relaxation of cooling water and ice and, iii) the dOO expansion of a dimer and between molecules at water surface. With any one of the dOO, the density ?(g·cm?3), the dL, and the dH, as a known input, one can resolve the rest quantities using this solution that is probing conditions or methods independent. We clarified that: i) liquid water prefers statistically the mono-phase of tetrahedrally-coordinated structure with fluctuation, ii) the low-density phase (supersolid phase as it is strongly polarized with even lower density) exists only in regions consisting molecules with fewer than four neighbors and, iii) repulsion between electron pairs on adjacent oxygen atoms dictates the cooperative relaxation of the segmented O:H-O bond, which is responsible for the performance of water and ice. PMID:24141643

Huang, Yongli; Zhang, Xi; Ma, Zengsheng; Li, Wen; Zhou, Yichun; Zhou, Ji; Zheng, Weitao; Sun, Chang Q.

2013-01-01

313

Predicting the melting temperature of ice-Ih with only electronic structure information as input  

NASA Astrophysics Data System (ADS)

The melting temperature of ice-Ih was calculated with only electronic structure information as input by creating a problem-specific force field. The force field, Water model by AFM for Ice and Liquid (WAIL), was developed with the adaptive force matching (AFM) method by fitting to post-Hartree-Fock quality forces obtained in quantum mechanics/molecular mechanics calculations. WAIL predicts the ice-Ih melting temperature to be 270 K. The model also predicts the densities of ice and water, the temperature of maximum density of water, the heat of vaporizations, and the radial distribution functions for both ice and water in good agreement with experimental measurements. The non-dissociative WAIL model is very similar to a flexible version of the popular TIP4P potential and has comparable computational cost. By customizing to problem-specific configurations with the AFM approach, the resulting model is remarkably more accurate than any variants of TIP4P for simulating ice-Ih and water in the temperature range from 253 K and 293 K under ambient pressure.

Pinnick, Eric R.; Erramilli, Shyamsunder; Wang, Feng

2012-07-01

314

Prediction and Observation of Crystal Structures of Oppositely Charged Colloids  

NASA Astrophysics Data System (ADS)

We studied crystal structures in mixtures of large and small oppositely charged spherical colloids with size ratio 0.31 using Monte Carlo simulations and confocal microscopy. We developed an interactive method based on simulated annealing to predict new binary crystal structures with stoichiometries from 1 to 8. Employing these structures in Madelung energy calculations using a screened Coulomb potential, we constructed a ground-state phase diagram, which shows a remarkably rich variety of crystals. Our phase diagram displays colloidal analogs of simple-salt structures and of the doped fullerene C60 structures, but also novel structures that do not have an atomic or molecular analog. We found three of the predicted structures experimentally, which provides confidence that our method yields reliable results.

Hynninen, A.-P.; Christova, C. G.; van Roij, R.; van Blaaderen, A.; Dijkstra, M.

2006-04-01

315

A machine learning approach to crystal structure prediction  

E-print Network

This thesis develops a machine learning framework for predicting crystal structure and applies it to binary metallic alloys. As computational materials science turns a promising eye towards design, routine encounters with ...

Fischer, Christopher Carl

2007-01-01

316

Structural, optical and electrical characteristics of a new NLO crystal  

NASA Astrophysics Data System (ADS)

A new nonlinear optical (NLO) organic crystal 1-[4-({(E)-[4-(methylsulfanyl)phenyl]methylidene}amino)phenyl]ethanone (MMP) has been grown by slow evaporation technique at ambient temperature. The crystal structure of MMP was determined by single crystal X-ray diffraction. MMP crystallizes in non-centrosymmetric monoclinic system with space group P21. The FT-IR spectrum recorded for new crystal confirmed the presence of various functional groups in the material. MMP was found to be thermally stable up to 300 °C. The grown crystal was optically transparent in the wavelength range of 400-1100 nm. The second harmonic generation (SHG) efficiency of the crystal was measured by the classical powder technique using Nd:YAG laser and was found to be 4.13 times more efficient than reference material, urea. Third order nonlinear parameters were measured by employing the Z-scan technique. The laser damage threshold for MMP crystal was determined to be 4.26 GW/cm2. The Brewster angle technique was employed to measure the refractive index of the crystal and the values for green and red wavelengths were found to be 1.35 and 1.33, respectively. The dielectric and electrical measurements were carried out to study the different polarization mechanisms and conductivity of the crystal.

D'silva, E. D.; Krishna Podagatlapalli, G.; Venugopal Rao, S.; Dharmaprakash, S. M.

2012-09-01

317

Investigating the « ice mélange » in an ice-shelf coastal rift along the Princess Ragnhild Coast (Antarctica)  

NASA Astrophysics Data System (ADS)

This paper presents the first results of a glaciological investigation conducted in the vicinity of the new Belgian Antarctic research station "Princess Elisabeth" during the 2008-2009 Antarctic field season. The study is part of the BELISSIMA project which aim is to investigate the dynamics of transition zones at the grounding line and the interaction of the ice sheet and the ice-shelf with the ocean, with respect to the stability of the ice sheet. The studied site is a conspicuous rift zone developed in a short floating ice shelf, a few kilometres downstream from the grounding line associated with the presence of a coastal ice dome. The rift, very close to the location of the old Belgian Station "Base Roi Baudouin", is about 10 km long and between 0.5 and 4 km wide. A natural ramp on the eastern apex of the rift allowed access to the rift base, from where a series of five, 10-38 m-long cores were recovered. Visual observation of these cores indicates that they consist of heterogeneous ice types, which is typical of what is often referred to as the "ice mélange". Wind-blown snow, firn and ice dominate outside the rift and within the rift's apex ramp. However, within the base of the rift proper, where episodic tensional stresses dominate, the ice is correspondingly more heavily crevassed and shows clear surface albedo contrasts, suggesting material heterogeneity. Ice cores from these areas show an abrupt transition within a few metres of the surface from surface-derived firn and ice to a sharply contrasting ice type that is translucent, greenish, and bubble-free -interpreted as marine ice. Such ice results from the consolidation of frazil ice crystals which are known to be forming in Ice Shelf Water through ice-ocean interactions in other regions of Antarctica (e.g. Filchner-Ronne Ice Shelf, Amery ice Shelf, Nansen Ice Shelf). One of our drill sites was located in a surface outcrop of marine ice, yielding 13 m of solid translucent ice, overlying ~0.5 m of fragile and loosely consolidated ice before the sub-shelf interface was reached. Borehole images from below this interface reveal an additional thickness of at least 5 m of loose platelet ice crystals located below the shelf, suggesting an active thermohaline convection in the region. The paper presents textural, structural, bulk salinity, bulk density and stable isotopes (DeltaD, Delta18O) results from the five ice cores and discusses origin and transformation of the various ice types forming the "ice mélange" and their potential impact on the welding efficiency of the rift.

Depoorter, Mathieu; Samyn, Denis; Hubbard, Bryn; Pattyn, Frank; Matsuoka, Kenny; Dierckx, Marie; Tison, Jean-Louis

2010-05-01

318

Determination of channeling perspectives for complex crystal structures  

SciTech Connect

Specification of the atomic arrangement for axes and planes of high symmetry is essential for crystal alignment using Rutherford backscattering and for studies of the lattice location of impurities in single crystals. By rotation of an inscribed orthogonal coordinate system, a visual image for a given perspective of a crystal structure can be specified. Knowledge of the atomic arrangement permits qualitative channeling perspectives to be visualized and calculation of continuum potentials for channeling. Channeling angular-yield profiles can then be analytically modeled and, subsequently, shadowing by host atoms of positions within the unit cell predicted. Software to calculate transformed atom positions for a channeling perspective in a single crystal are described and illustrated for the spinel crystal structure.

Allen, W.R.

1993-03-01

319

Teaching Mineralogy with Crystal Structure Databases and Visualization Software  

NSDL National Science Digital Library

This resource collection provides information on how to incorporate crystal structure databases and 3-D visualization software into undergraduate mineralogy courses. Features include background information for teachers on how to teach with data, models, and visualizations; commentary on the benefits of physical versus virtual models; and a set of links to online crystal structure databases. There are also links to visualization software, tutorials, and example exercises and activities.

Kent Ratajeski

2002-01-01

320

More accurate determination of the quantity of ice crystallized at low cooling rates in the glycerol and 1,2-propanediol aqueous solutions: comparison with equilibrium.  

PubMed

It is generally assumed that when cells are cooled at rates close to those corresponding to the maximum of survival, once supercooling has ceased, above the eutectic melting temperature the extracellular ice is in equilibrium with the residual solution. This did not seem evident to us due to the difficulty of ice crystallization in cryoprotective solutions. The maximum quantities of ice crystallized in glycerol and 1,2-propanediol solutions have been calculated from the area of the solidification and fusion peaks obtained with a Perkin-Elmer DSC-2 differential scanning calorimeter. The accuracy has been improved by several corrections: better defined baseline, thermal variation of the heat of fusion of the ice, heat of solution of the water from its melting with the residual solution. More ice crystallizes in the glycerol than in the 1,2-propanediol solutions, of which the amorphous residue contains about 40 to 55% 1,2-propanediol. The equilibrium values are unknown in the presence of 1,2-propanediol. With glycerol, in our experiments, the maximum is first lower than the equilibrium but approaches it as the concentration increases. It is not completely determined by the colligative properties of the solutes. PMID:6713947

Boutron, P

1984-04-01

321

Fractal structure of the equilibrium crystal shape S. E. Burkov  

E-print Network

317 Fractal structure of the equilibrium crystal shape S. E. Burkov Landau Institute décroît avec la distance plus rapidement que r-4. Nous montrons que la surface du cristal a une structure continue aux arêtes. Le bord d'une face a également une structure fractale. Elle consiste en un nombre

Paris-Sud XI, Université de

322

The crystal structure of faustite and its copper analogue turquoise  

Microsoft Academic Search

The crystal structure of faustite, ZnAI6(P04MOHhAH20, was determined using single-crystal data (Mo-KIX X-radiation, CCD area detector, 1624 unique reflections, RI = 4.91 %, wR2 = 9.23%), and compared with results of a reinvestigation of the structure of its copper analogue turquoise, CuAI6(P04MOH)gAH20 (2737 unique reflections, RI = 2.81%, wR2 = 6.90%). Both are isostructural and crystallize in space group PI,

U. Kolitsch; G. Giester

2000-01-01

323

Quantitative crystal structure descriptors from multiplicative congruential generators.  

PubMed

Special types of number-theoretic relations, termed multiplicative congruential generators (MCGs), exhibit an intrinsic sublattice structure. This has considerable implications within the crystallographic realm, namely for the coordinate description of crystal structures for which MCGs allow for a concise way of encoding the numerical structural information. Thus, a conceptual framework is established, with some focus on layered superstructures, which proposes the use of MCGs as a tool for the quantitative description of crystal structures. The multiplicative congruential method eventually affords an algorithmic generation of three-dimensional crystal structures with a near-uniform distribution of atoms, whereas a linearization procedure facilitates their combinatorial enumeration and classification. The outlook for homometric structures and dual-space crystallography is given. Some generalizations and extensions are formulated in addition, revealing the connections of MCGs with geometric algebra, discrete dynamical systems (iterative maps), as well as certain quasicrystal approximants. PMID:22338652

Hornfeck, Wolfgang

2012-03-01

324

High density amorphous ice at room temperature  

PubMed Central

The phase diagram of water is both unusual and complex, exhibiting a wide range of polymorphs including proton-ordered or disordered forms. In addition, a variety of stable and metastable forms are observed. The richness of H2O phases attests the versatility of hydrogen-bonded network structures that include kinetically stable amorphous ices. Information of the amorphous solids, however, is rarely available especially for the stability field and transformation dynamics—but all reported to exist below the crystallization temperature of approximately 150–170 K below 4–5 GPa. Here, we present the evidence of high density amorphous (HDA) ice formed well above the crystallization temperature at 1 GPa—well inside the so-called “no-man’s land.” It is formed from metastable ice VII in the stability field of ice VI under rapid compression using dynamic-diamond anvil cell (d-DAC) and results from structural similarities between HDA and ice VII. The formation follows an interfacial growth mechanism unlike the melting process. Nevertheless, the occurrence of HDA along the extrapolated melt line of ice VII resembles the ice Ih-to-HDA transition, indicating that structural instabilities of parent ice VII and Ih drive the pressure-induced amorphization. PMID:21518902

Chen, Jing-Yin; Yoo, Choong-Shik

2011-01-01

325

Cloud Resolving Simulations of Mixed-Phase Arctic Stratus Observed during BASE: Sensitivity to Concentration of Ice Crystals and Large-Scale Heat and Moisture Advection  

Microsoft Academic Search

The authors' previous idealized, two-dimensional cloud resolving model (CRM) simulations of Arctic stratus revealed a surprising sensitivity to the concentrations of ice crystals. In this paper, simulations of an actual case study observed during the Beaufort and Arctic Seas Experiment are performed and the results are compared to the observed data. It is again found in the CRM simulations that

Hongli Jiang; William R. Cotton; James O. Pinto; Judy A. Curry; Michael J. Weissbluth

2000-01-01

326

Differential propagation constants on slant paths through snow and ice crystals as measured by 16.5 GHz polarization-diversity radar  

Microsoft Academic Search

Radar measurements of differential propagation constants at a wavelength of 1.82 cm on slant paths through heavy snow and ice crystals are described. The data from three snow storms show that the differential phase shift per kilometer through heavy snow may be considerably larger on slant paths with elevation angles of 10 to 30 deg than it is on terrestrial

A. Hendry; Y. M. M. Antar; G. C. McCormick

1981-01-01

327

Impact of surface nanostructure on ice nucleation  

NASA Astrophysics Data System (ADS)

Nucleation of water on solid surface can be promoted noticeably when the lattice parameter of a surface matches well with the ice structure. However, the characteristic length of the surface lattice reported is generally less than 0.5 nm and is hardly tunable. In this paper, we show that a surface with nanoscale roughness can also remarkably promote ice nucleation if the characteristic length of the surface structure matches well with the ice crystal. A series of surfaces composed of periodic grooves with same depth but different widths are constructed in molecular dynamics simulations. Water cylinders are placed on the constructed surfaces and frozen at constant undercooling. The nucleation rates of the water cylinders are calculated in the simulation using the mean first-passage time method and then used to measure the nucleation promotion ability of the surfaces. Results suggest that the nucleation behavior of the supercooled water is significantly sensitive to the width of the groove. When the width of the groove matches well with the specific lengths of the ice crystal structure, the nucleation can be promoted remarkably. If the width does not match with the ice crystal, this kind of promotion disappears and the nucleation rate is even smaller than that on the smooth surface. Simulations also indicate that even when water molecules are adsorbed onto the surface structure in high-humidity environment, the solid surface can provide promising anti-icing ability as long as the characteristic length of the surface structure is carefully designed to avoid geometric match.

Zhang, Xiang-Xiong; Chen, Min; Fu, Ming

2014-09-01

328

Vegetation structure in gullies developed by the melting of ice wedges along Kolyma River, northern Siberia  

Microsoft Academic Search

Vegetation structure was surveyed in gullies developed by the melting of ice wedges along the Kolyma River, northern Siberia, using 72–50 × 50 cm plots. The mean total plant cover was approximately 50% on gley soils, which were only distributed in the gullies. Based on twinspan cluster analysis, four vegetation types were recognized: (i) Agrostis purpurascens grassland with Ceratodon purpureus

Shiro Tsuyuzaki; Takeshi Ishizaki; Toshiyuki Sato

1999-01-01

329

The Arctic Cone Exploration Structure: A Mobile Offshore Drilling Unit for Heavy Ice Cover  

Microsoft Academic Search

This paper reports on the results of a rig development program which began in 1981 to design a mobile drilling unit which could operate beyond the 50 ft depth contour in the most exposed ice conditions. The Arctic Cone Exploration Structure (ACES) project has produced the design of what is likely to be the prototype for heavy-duty, bottom-founded mobile rigs

R. C. Byrd; R. Coleman; R. Weiss; L. Boaz; E. Sauve; R. M. White

1984-01-01

330

Strong anti-ice ability of nanohairs over micro-ratchet structures.  

PubMed

A strong anti-ice property of nanohairs over micro-ratchet surfaces is observed. A long freezing delay of more than 185 min is achieved for a droplet on the nanohairs over ratchet structure with a period of ?290 ?m under -10 °C, which is attributed to the effective cooperation of the nano- and microstructures. PMID:24122128

Guo, Peng; Wen, Mengxi; Wang, Lei; Zheng, Yongmei

2014-04-21

331

Characterization of ice cream structure by direct optical microscopy. Influence of freezing parameters  

Microsoft Academic Search

The main objective of this study was to develop and to set up a new optical direct microscopy method, based on the reflected light flux differences, with episcopic axial lighting to characterize the different phases structure of commercial overrun ice creams. Firstly, the results obtained have been validated by two others methods, a destructive method by dispersion and observation by

Alexandre Caillet; Claudia Cogné; Julien Andrieu; Pierre Laurent; Alain Rivoire

2003-01-01

332

On the development of ice-templated silicon carbide scaffolds for nature-inspired structural materials  

E-print Network

On the development of ice-templated silicon carbide scaffolds for nature-inspired structural investigated the architecture of silicon carbide (SiC) scaffolds produced by this technique over a range: Ceramics; Silicon carbide; Bioinspired materials; Freeze casting; Scaffolds 1. Introduction Freeze casting

Ritchie, Robert

333

Effects of an intense ice storm on the structure of a northern hardwood forest1  

Microsoft Academic Search

A major ice storm in January 1998 provided an opportunity to study the effects of a rare, intense distur- bance on the structure of the northern hardwood forest canopy. Canopy damage was assessed using visual damage classes within watersheds of different ages at the Hubbard Brook Experimental Forest (HBEF) and changes in leaf area index in two of these watersheds.

Anne G. Rhoads; Steven P. Hamburg; Timothy J. Fahey; Thomas G. Siccama; Elizabeth N. Hane; John Battles; Charles Cogbill; Jesse Randall; Geoff Wilson

334

Submicrometer Single Crystal Diffractometry for Highly Accurate Structure Determination  

NASA Astrophysics Data System (ADS)

Submicrometer single crystal diffractometry for highly accurate structure determination was developed using the extremely stable and highly brilliant synchrotron radiation from SPring-8. This was achieved using a microbeam focusing system and the submicrometer precision low-eccentric goniometer system. We demonstrated the structure analyses with 2×2×2 ?m3 cytidine, 600×600×300 nm3 BaTiO3, and 1×1×1 ?m3 silicon. The observed structure factors of the silicon crystal were in agreement with the structure factors determined by the Pendellösung method and do not require absorption and extinction corrections.

Yasuda, Nobuhiro; Fukuyama, Yoshimitsu; Toriumi, Koshiro; Kimura, Shigeru; Takata, Masaki

2010-06-01

335

Submicrometer Single Crystal Diffractometry for Highly Accurate Structure Determination  

SciTech Connect

Submicrometer single crystal diffractometry for highly accurate structure determination was developed using the extremely stable and highly brilliant synchrotron radiation from SPring-8. This was achieved using a microbeam focusing system and the submicrometer precision low-eccentric goniometer system. We demonstrated the structure analyses with 2x2x2 {mu}m{sup 3} cytidine, 600x600x300 nm{sup 3} BaTiO{sub 3}, and 1x1x1 {mu}m{sup 3} silicon. The observed structure factors of the silicon crystal were in agreement with the structure factors determined by the Pendelloesung method and do not require absorption and extinction corrections.

Yasuda, Nobuhiro; Fukuyama, Yoshimitsu; Kimura, Shigeru [Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo, Sayo, Hyogo 679-5198 (Japan); Japan Science and Technology Agency, CREST, 5, Sanbancho, Chiyoda, Tokyo 102-0075 (Japan); Toriumi, Koshiro [Japan Science and Technology Agency, CREST, 5, Sanbancho, Chiyoda, Tokyo 102-0075 (Japan); University of Hyogo, 3-2-1 Kouto, Kamigori, Ako, Hyogo 678-1297 (Japan); Takata, Masaki [Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo, Sayo, Hyogo 679-5198 (Japan); Japan Science and Technology Agency, CREST, 5, Sanbancho, Chiyoda, Tokyo 102-0075 (Japan); RIKEN SPring-8 Center, Harima Institute, 1-1-1 Kouto, Sayo, Sayo, Hyogo 679-5148 (Japan)

2010-06-23

336

Crystal structure prediction for cyclotrimethylene trinitramine (RDX) from first principles.  

PubMed

Crystal structure prediction and molecular dynamics methods were applied to the cyclotrimethylene trinitramine (RDX) crystal to explore the stability rankings of various polymorphs using a recently developed nonempirical potential energy function that describes the RDX dimer interactions. The energies of 500 high-density structures resulting from molecular packing were minimized and the 14 lowest-energy structures were subjected to isothermal-isostress molecular dynamics (NsT-MD) simulations. For both crystal structure prediction methods and molecular dynamics simulations, the lowest-energy polymorph corresponded to the experimental structure; furthermore, the lattice energy of this polymorph was lower than that of the other polymorphs by at least 1.1 kcal mol(-1). Crystal parameters and densities of the low-energy crystal produced by the NsT-MD simulations matched those of the experimental crystal to within 1% of density and cell edge lengths and 0.01 degrees of the cell angle. The arrangement of the molecules within the time-averaged unit cell were in equally outstanding agreement with experiment, with the largest deviation of the location of the molecular mass centers being less than 0.07 A and the largest deviation in molecular orientation being less than 2.8 degrees . NsT-MD simulations were also used to calculate crystallographic parameters as functions of temperature and pressure and the results were in a reasonable agreement with experiment. PMID:19551222

Podeszwa, Rafal; Rice, Betsy M; Szalewicz, Krzysztof

2009-07-14

337

STRUCTURE NOTE Crystal Structure of Gene Locus At3g16990 from  

E-print Network

STRUCTURE NOTE Crystal Structure of Gene Locus At3g16990 from Arabidopsis thaliana Paul G. Blommel thaliana was given a suitable score for study, with the only major demerits being a large cysteine residue, we report the crystal structure of the protein from Arabidopsis thaliana gene locus At3g16990

Rayment, Ivan

338

Changes in the firn structure of the Greenland Ice Sheet caused by recent warming  

NASA Astrophysics Data System (ADS)

Atmospheric warming over the Greenland Ice Sheet during the last two decades has increased the amount of surface meltwater production, resulting in the migration of melt and percolation regimes to higher altitudes and an increase in the amount of solid ice from refrozen meltwater found in the firn above the equilibrium line. Here we present observations of near-surface (0-20 m) firn conditions in western Greenland obtained from campaigns between 1998 and 2014. We find a sharp increase in firn ice content in the form of thick widespread layers in the percolation zone, which decreases the capacity of the firn to store meltwater. The estimated total annual ice content retained in the firn in areas with positive surface mass balance west of the ice divide in Greenland reached a maximum of 74 ± 25 Gt in 2012, compared to the 1958-1999 average of 13 ± 2 Gt, while the percolation zone area more than doubled between 2003 and 2012. Increased melt and column densification resulted in surface lowering averaging -0.80 ± 0.39 m yr-1 between 1800 and 2800 m in the accumulation zone of western Greenland. Since 2007, annual melt and refreezing rates in the percolation zone at elevations below 2100 m surpass the annual snowfall from the previous year, implying that mass gain in the region is now in the form of refrozen meltwater. If current melt trends over high elevation regions continue, subsequent changes in firn structure will have implications for the hydrology of the ice sheet and related abrupt seasonal densification could become increasingly significant for altimetry-derived ice sheet mass balance estimates.

de la Peña, S.; Howat, I. M.; Nienow, P. W.; van den Broeke, M. R.; Mosley-Thompson, E.; Price, S. F.; Mair, D.; Noël, B.; Sole, A. J.

2015-01-01

339

Predicting inclusion behaviour and framework structures in organic crystals.  

PubMed

We have used well-established computational methods to generate and explore the crystal structure landscapes of four organic molecules of well-known inclusion behaviour. Using these methods, we are able to generate both close-packed crystal structures and high-energy open frameworks containing voids of molecular dimensions. Some of these high-energy open frameworks correspond to real structures observed experimentally when the appropriate guest molecules are present during crystallisation. We propose a combination of crystal structure prediction methodologies with structure rankings based on relative lattice energy and solvent-accessible volume as a way of selecting likely inclusion frameworks completely ab initio. This methodology can be used as part of a rational strategy in the design of inclusion compounds, and also for the anticipation of inclusion behaviour in organic molecules. PMID:19876969

Cruz-Cabeza, Aurora J; Day, Graeme M; Jones, William

2009-12-01

340

Isothermal Ice-Crystallization Kinetics in the Gas-Diffusion Layer of a Proton-Exchange-Membrane Fuel Cell  

SciTech Connect

Nucleation and growth of ice in the fibrous gas-diffusion layer (GDL) of a proton-exchange membrane fuel cell (PEMFC) are investigated using isothermal differential scanning calorimetry (DSC). Isothermal crystallization rates and pseudo-steady-state nucleation rates are obtained as a function of subcooling from heat-flow and induction-time measurements. Kinetics of ice nucleation and growth are studied at two polytetrafluoroethylene (PTFE) loadings (0 and 10 wt %) in a commercial GDL for temperatures between 240 and 273 K. A nonlinear icecrystallization rate expression is developed using Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory, in which the heat-transfer-limited growth rate is determined from the moving-boundary Stefan problem. Induction times follow a Poisson distribution and increase upon addition of PTFE, indicating that nucleation occurs more slowly on a hydrophobic fiber than on a hydrophilic fiber. The determined nucleation rates and induction times follow expected trends from classical nucleation theory. A validated rate expression is now available for predicting icecrystallization kinetics in GDLs.

Dursch, Thomas J.; Ciontea, Monica A.; Radke, Clayton J.; Weber, Adam Z.

2011-11-11

341

Magnetic vortex crystal formation in the antidot complement of square artificial spin ice  

SciTech Connect

We have studied ferromagnetic nickel thin films patterned with square lattices of elongated antidots that are negative analogues of square artificial spin ice. Micromagnetic simulations and direct current magnetic moment measurements reveal in-plane anisotropy of the magnetic hysteresis loops, and the formation of a dense array of magnetic vortices with random polarization and chirality. These multiply-connected antidot arrays could be superior to lattices of disconnected nanodisks for investigations of vortex switching by applied electric current.

Araujo, C. I. L. de, E-mail: dearaujo@ufv.br; Silva, R. C.; Ribeiro, I. R. B.; Nascimento, F. S.; Felix, J. F.; Ferreira, S. O.; Moura-Melo, W. A.; Pereira, A. R. [Departamento de Física, Universidade Federal de Viçosa, Viçosa 36570-900, Minas Gerais (Brazil); Mól, L. A. S. [Departamento de Física, ICEx, Universidade Federal de Minas Gerais, Belo Horizonte 31270-901, Minas Gerais (Brazil)

2014-03-03

342

Electron-stimulated desorption of D+ from D2O ice: Surface structure and electronic excitations  

NASA Astrophysics Data System (ADS)

We present a study of the electron-stimulated desorption of deuterium cations (D+) from thin (1-40 ML) D2O ice films vapor deposited on a Pt(111) substrate. Measurements of the total yield and velocity distributions as a function of temperature from 90 to 200 K show that the D+ yield changes with film thickness, surface temperature, and ice phase. We observe two energy thresholds for cation emission, near 25 and 40 eV, which are weakly dependent upon the ice temperature and phase. The cation time-of-flight (TOF) distribution is at least bimodal, indicating multiple desorption channels. A decomposition of the TOF distributions into ``fast'' and ``slow'' channels shows structure as a function of excitation energy, film thickness, and temperature. The D+ yield generally increases with temperature, rising near 120 K on amorphous ice, and near 135 K on crystalline ice. The amorphous-crystalline phase transition at ~160 K causes a drop in total desorption yield. The temperature dependence of D- desorption via the 2B1 dissociative electron attachment resonance is very similar to the slow D+ yield, and likely involves similar restructuring and lifetime effects. The data collectively suggest that a thermally activated reduction of surface hydrogen bonding increases the lifetime of the excited states responsible for ion desorption, and that these lifetime effects are strongest for excited states involving a1 bands

Sieger, M. T.; Simpson, W. C.; Orlando, T. M.

1997-08-01

343

Structural characteristics and second order nonlinear optical properties of borate crystals  

E-print Network

Structural characteristics and second order nonlinear optical properties of borate crystals D. Xue optical (NLO) responses of some typical borate crystals with various crystal structures have been the reported inorganic crystal structures there are in total only 15% of noncentrosymmetric structure

Osnabrück, Universität

344

Terrestrial Sea Ice Morphology: Considerations for Europa  

NASA Astrophysics Data System (ADS)

The Galileo mission has returned the first high-resolution (21 m/pixel) images of the surface of Europa. These images reveal structures with morphologies reminiscent of those seen on terrestrial sea ice. Although it is premature to make one-to-one analogies between sea ice and Europa's surface, a review of the types of surface features commonly formed on Earth and of various sea-ice processes can provide insight into the complex geology of Europa. For example, deformation of terrestrial sea ice results from winds, tides, and currents and from thermally induced stresses; the resulting features include fractures ranging in width from millimeters to kilometers, pressure ridges, shear ridges, and rafted ice. Potential agents of deformation on Europa are more likely to be limited to tidal flexing and possibly convection, but could produce similar features and perhaps account for the ridges and fractures seen in many areas. Subtle differences in albedo and color in terrestrial sea ice result from differences in ice thickness and grain size, attributed to factors such as the rate of ice-crystal growth, water turbulence, age of the ice, and deformation. Similar factors could account for differences observed in the bright icy plains of Europa. Moreover, salts in both the solid form and as brine vary in concentration and composition as a function of space and time on Earth, leading to differences in density and the strength of ice sheets. Salts are also suspected in the europan ice and could lead to similar differences, enhancing the creation of topographic relief from density contrasts and the formation of fractures from brittle failure of the ice. Differences in the environments between Europa and terrestrial sea ice in terms of parameters such as temperature, gravity, time, and ice compositions suggest caution in drawing direct analogies. Future work by the planetary and sea-ice communities must include understanding the terrestrial processes sufficiently for extrapolation to Europa.

Greeley, Ronald; Sullivan, Robert; Coon, Max D.; Geissler, Paul E.; Tufts, B. Randall; Head, James W.; Pappalardo, Robert T.; Moore, Jeffrey M.

1998-09-01

345

Recrystallization of ice during bulk storage of ice cream  

Microsoft Academic Search

Ice recrystallization was studied in 1.9 L containers of ice cream stored so that surface temperature of ice cream was controlled with fluctuations of ±1.0 °C. Core and surface samples were taken at regular intervals and analyzed for ice crystal size by cold-stage microscopy and image analysis. Mean ice crystal size plotted vs. time0.33 resulted in a straight line, with

Daniel P. Donhowe; Richard W. Hartel

1996-01-01

346

The crystal structure of some rhenium and technetium dichalcogenides  

Microsoft Academic Search

The crystal structures of ReSe., ReS., ReSSe and TeS2 are determined using single crystal X-ray diffraction. The compounds are triclinic with space groupPl. ReSe., ReS, and ReSSe have a distorted CdCl2-type structure: TeS2 has a distorted Cd(OH)-type structure. In the case of ReS, there are two sandwiches in the unit cell, related by symmetry centers. The other compounds have one

H.-J. Lamfers; A. Meetsma; G. A. Wiegers; J. L. de Boer

1996-01-01

347

Crystal structure solution from experimentally determined atomic pair distribution functions  

Microsoft Academic Search

The paper describes an extension of the Liga algorithm for structure solution from atomic pair distribution function (PDF), to handle periodic crystal structures with multiple elements in the unit cell. The procedure is performed in 2 separate steps - at first the Liga algorithm is used to find unit cell sites consistent with pair distances extracted from the experimental PDF.

Pavol Juhas; Luke Granlund; Saurabh R. Gujarathi; Phillip M. Duxbury; Simon J. L. Billinge

2010-01-01

348

Nitric Oxide Myoglobin: Crystal Structure and Analysis of Ligand Geometry  

E-print Network

Nitric Oxide Myoglobin: Crystal Structure and Analysis of Ligand Geometry Eric Allen Brucker,1 John School of Medicine, Cleveland, Ohio ABSTRACT The structure of the ferrous nitric oxide form of native oxide ligand is bent with respect to the heme plane: the Fe-N-O angle is 112°. This angle is smaller

Phillips, George N. Jr.

349

Formation, structure, and crystallization of Al-rich metallic glasses  

Microsoft Academic Search

The formation, structure, and the crystallization of Al85YxNi15?x are studied using x-ray diffraction and differential scanning calorimetry. The results show two distinct glasses depending on composition. Y-rich glasses (x?8) are homogeneous with a well-defined glass transition. The x-ray diffraction patterns have a single main peak. These glasses crystallize through a nucleation and growth process. Y-poor glasses (x<8) do not show

R. Sabet-Sharghi; Z. Altounian; W. B. Muir

1994-01-01

350

Impact of ice crystal habit on the parameterization of cloud microphysical properties when using 94ghz polarimetric scanning cloud radar during STORMVEX  

NASA Astrophysics Data System (ADS)

Through the analysis of scanning polarimetric W-band cloud radar data collected during STORMVEX, an algorithm has been developed to both identify and parameterize various ice crystal habits present within mixed-phase clouds. Armed with a unique dataset, the development of the algorithm took advantage of a slant 45° linear depolarization ratio (SLDR) measurement that was made as a function of the radar elevation angle when in range height indicator (RHI) scanning mode. This measurement technique proved to be invaluable in that it limited the influence of the particle's maximum dimension on the measured depolarization, which instead became more a function of the ice particle's shape. Validated through in situ measurements; pristine dendrites, lightly rimed dendrites, rimed stellar crystals, aggregates of dendrites, columns, and graupel particles were identified and matched with specific SLDR signatures. With a known ice particle habit and SLDR signature, the ice particle habit identification segment of the newly developed algorithm was then applied to the entire dataset consisting of 38,190 individual scans, in order to identify ice particle habits at a combined 849,745 range-heights and scanning angles. Through this analysis and the use of a chi-square test statistic, the predominant ice particle habit could be determined. Of primary interest in this study were the parameterizations of the ice particle mass and radar backscatter cross section. Through the modeling of the chosen ice particle habit as an oblate spheroid, these parameterizations were carried out in part by relying on previously published empirical studies as well as T-matrix scattering calculations of oblate spheroids composed of an ice/air mixture. Due to the computational expense of T-matrix calculations, however, a new T-matrix scaling factor was derived from the Clausius-Mossotti relation, which relates the refractive index of a material to its polarizability. With this scaling factor, new T-matrix results could be found, still functions of ice particle mass and shape. Using this new parameterization scheme, a radar-based cloud microphysical property retrieval algorithm was then executed for two cases and compared to generic parameterizations. Results show that the potential difference in the retrieved microphysical properties for the generic versus the ice particle habit-based parameterization could be as high as a factor of two.

Hammonds, Kevin Don

351

The Cloud Particle Spectrometer with Polarization Detection (CPSPD): A next generation open-path cloud probe for distinguishing liquid cloud droplets from ice crystals  

NASA Astrophysics Data System (ADS)

The differentiation of small water droplets and ice crystals by in situ measurements, in the size range < 50 ?m, remains a challenge and the lack of such measurements is an obstacle to progress in understanding ice formation in clouds. A new microphysical instrument, the Cloud Particle Spectrometer with Polarization Detection (CPSPD), has been developed that measures light intensity scattered (in forward and backward directions) by individual cloud particles that pass through a focused laser beam and derives their size and thermodynamic phase (liquid or ice) in the optical diameter range from 2 to 50 ?m. The optical equivalent diameter is derived from the light scattered in the forward direction. The change in polarization state of the incident light, caused by interaction with the cloud particle, is determined from the polarized components of the backscattered light. The CPSPD, along with several other cloud microphysical probes, has been flown on the University of North Dakota Citation aircraft in mixed phase clouds. It has also been deployed and operated at the Zugspitze research station studying mountain clouds. The preliminary results show that liquid cloud droplets can be distinguished from ice crystals and that the ice fraction can be estimated; an important parameter for better understanding of cloud processes, particularly that of glaciation.

Baumgardner, Darrel; Newton, Roy; Krämer, Martina; Meyer, Jessica; Beyer, Alexander; Wendisch, Manfred; Vochezer, Paul

2014-06-01

352

Hydrohalite in cold sea ice: Laboratory observations of single crystals, surface accumulations, and migration rates under a temperature gradient, with application to “Snowball Earth”  

NASA Astrophysics Data System (ADS)

When NaCl precipitates out of a saturated solution, it forms anhydrous crystals of halite at temperatures above +0.11°C, but at temperatures below this threshold it instead precipitates as the dihydrate "hydrohalite," NaCl · 2H2O. When sea ice is cooled, hydrohalite begins to precipitate within brine inclusions at about -23°C. In this work, hydrohalite crystals are examined in laboratory experiments: their formation, their shape, and their response to warming and desiccation. Sublimation of a sea ice surface at low temperature leaves a lag deposit of hydrohalite, which has the character of a fine powder. The precipitation of hydrohalite in brine inclusions raises the albedo of sea ice, and the subsequent formation of a surface accumulation further raises the albedo. Although these processes have limited climatic importance on the modern Earth, they would have been important in determining the surface types present in regions of net sublimation on the tropical ocean in the cold phase of a Snowball Earth event. However, brine inclusions in sea ice migrate downward to warmer ice, so whether salt can accumulate on the surface depends on the relative rates of sublimation and migration. The migration rates are measured in a laboratory experiment at temperatures from -2°C to -32°C; the migration appears to be too slow to prevent formation of a salt crust on Snowball Earth.

Light, Bonnie; Brandt, Richard E.; Warren, Stephen G.

2009-07-01

353

Polymorphism of Scyllo-Inositol: Joining Crystal Structure Prediction with Experiment to Elucidate the Structures of Two  

E-print Network

Polymorphism of Scyllo-Inositol: Joining Crystal Structure Prediction with Experiment to Elucidate, 2006; ReVised Manuscript ReceiVed July 19, 2006 ABSTRACT: We report on the crystal structures of two in parallel with the crystallization experiments. When a single crystal was finally grown, its structure

de Gispert, Adrià

354

The Crystal Structures of the Tryparedoxin-Tryparedoxin Peroxidase Couple Unveil the Structural Determinants of  

E-print Network

The Crystal Structures of the Tryparedoxin-Tryparedoxin Peroxidase Couple Unveil the Structural-swapped dimer. In LmTXNPx, crystallized in reducing condition, both the locally unfolded (LU) and fully folded cysteine which facilitates Cys52 to form an inter-chain disulfide bond with the resolving cysteine (Cys173

Paris-Sud XI, Université de

355

Ytterbium- and neodymium-doped vanadate laser hose crystals having the apatite crystal structure  

DOEpatents

Yb.sup.3+ and Nd.sup.3+ doped Sr.sub.5 (VO.sub.4).sub.3 F crystals serve as useful infrared laser media that exhibit low thresholds of oscillation and high slope efficiencies, and can be grown with high optical quality. These laser media possess unusually high absorption and emission cross sections, which provide the crystals with the ability to generate greater gain for a given amount of pump power. Many related crystals such as Sr.sub.5 (VO.sub.4).sub.3 F crystals doped with other rare earths, transition metals, or actinides, as well as the many structural analogs of Sr.sub.5 (VO.sub.4).sub.3 F, where the Sr.sup.2+ and F.sup.- ions are replaced by related chemical species, have similar properties.

Payne, Stephen A. (Castro Valley, CA); Kway, Wayne L. (Fremont, CA); DeLoach, Laura D. (Manteca, CA); Krupke, William F. (Pleasanton, CA); Chai, Bruce H. T. (Oviedo, FL)

1994-01-01

356

Ytterbium- and neodymium-doped vanadate laser hose crystals having the apatite crystal structure  

DOEpatents

Yb[sup 3+] and Nd[sup 3+] doped Sr[sub 5](VO[sub 4])[sub 3]F crystals serve as useful infrared laser media that exhibit low thresholds of oscillation and high slope efficiencies, and can be grown with high optical quality. These laser media possess unusually high absorption and emission cross sections, which provide the crystals with the ability to generate greater gain for a given amount of pump power. Many related crystals such as Sr[sub 5](VO[sub 4])[sub 3]F crystals doped with other rare earths, transition metals, or actinides, as well as the many structural analogs of Sr[sub 5](VO[sub 4])[sub 3]F, where the Sr[sup 2+] and F[sup [minus

Payne, S.A.; Kway, W.L.; DeLoach, L.D.; Krupke, W.F.; Chai, B.H.T.

1994-08-23

357

Structural colored gels for tunable soft photonic crystals.  

PubMed

A periodically ordered interconnecting porous structure can be embodied in chemical gels by using closest-packed colloidal crystals as templates. The interconnecting porosity not only provides a quick response but also endows the porous gels with structural color arising from coherent Bragg optical diffraction. The structural colors revealed by porous gels can be regulated by several techniques, and thus, it is feasible to obtain desirable, smart, soft materials. A well-known thermosensitive monomer, N-isopropylacrylamide (NIPA), and other minor monomers were used to fabricate various structural colored gels. The selection of minor monomers depended on the targeted properties. This review focuses on the synthesis of templates, structural colored porous gels, and the applications of structural colored gel as smart soft materials for tunable photonic crystals. PMID:19306332

Harun-Ur-Rashid, Mohammad; Seki, Takahiro; Takeoka, Yukikazu

2009-01-01

358

Quench crystallization of linear polyethylene: Crystallization kinetics, morphology and structure investigation  

NASA Astrophysics Data System (ADS)

The behavior of semi-crystalline polymers at large supercoolings has always been of great interest in the industry and academia alike. It is well known that the resulting crystal morphology and microstructure are strongly dependent on supercooling or quench depth, and ultimately control a variety of physical, mechanical, chemical and optical properties. One of the main goals of the current research was to extend the crystallization range of linear polyethylene beyond currently accessible supercoolings. The current study reports the development of a simple yet effective "depolarized reflection light microscopy" technique that allows quench-crystallization at unprecedented onset crystallization temperatures well below 90°C, but more importantly allows concurrent spherulitic and bulk growth rates to be measured for kinetic analyses. Application of this technique to linear polyethylene, which is perhaps the fastest crystallizing polymer, generated new insights into the morphology, microstructure, crystallization kinetics, and reorganization-thickening processes prior to the melting of the crystals formed under deep quench conditions. The predominant morphology was found to vary substantially with the crystallization temperature from fully developed and impinged spherulites in isothermal conditions to a very high density of small isolated, un-impinged spherulites in deep quench. At the lowest temperatures the micrographs revealed sizeable areas of amorphous material, consistent with the low degree of crystallinity (˜35%) measured by X-ray diffraction. Predominant surface nucleation gave rise to hemispheres or "half-spherulites" on both surfaces in contact with dissimilar substrates. The quenched samples exhibited crystallite orientation effects; in particular the growth of the b axis direction of the unit cell was approximately perpendicular to the film surface. A quantitative analysis of the Herman's orientation factor revealed that the degree of orientation was fairly moderate and therefore had no major influence on the growth rate of the crystals. Crystallization kinetics measurements of bulk and spherulitic growth rates demonstrated for the first time, the (elusive) characteristic maximum and the crossover to the diffusion-controlled side of the growth rate-temperature dependence. The growth rates at temperatures below the maximum conform to a single linear fit characteristic of regime III which suggests conformity with the mechanism of the regime theory of secondary nucleation. However the stability and nature of the fold surface of the quenched crystals that are capable of further crystallization was substantially altered. Combined DSC, transmitted light intensity method, and temperature-resolved X-ray scattering methods were used to investigate the structural reorganization processes that take place prior to melting. Finally the mechanism of the recrystallization-lamellar thickening of quenched crystals was proposed.

Patki, Rahul P.

359

Compact Couplers for Photonic Crystal Laser-Driven Accelerator Structures  

SciTech Connect

Photonic crystal waveguides are promising candidates for laser-driven accelerator structures because of their ability to confine a speed-of-light mode in an all-dielectric structure. Because of the difference between the group velocity of the waveguide mode and the particle bunch velocity, fields must be coupled into the accelerating waveguide at frequent intervals. Therefore efficient, compact couplers are critical to overall accelerator efficiency. We present designs and simulations of high-efficiency coupling to the accelerating mode in a three-dimensional photonic crystal waveguide from a waveguide adjoining it at 90{sup o}. We discuss details of the computation and the resulting transmission. We include some background on the accelerator structure and photonic crystal-based optical acceleration in general.

Cowan, Benjamin; /Tech-X, Boulder; Lin, M.C.; /Tech-X, Boulder; Schwartz, Brian; /Tech-X, Boulder; Byer, Robert; /Stanford U., Phys. Dept.; McGuinness, Christopher; /Stanford U., Phys. Dept.; Colby, Eric; /SLAC; England, Robert; /SLAC; Noble, Robert; /SLAC; Spencer, James; /SLAC

2012-07-02

360

Crystal structure tuning in GaAs nanowires using HCl  

NASA Astrophysics Data System (ADS)

The use of HCl during growth of nanowires presents new possibilities for controlling the growth dynamics and resulting nanowire properties. In this paper, we investigate the effects of in situ HCl on the growth of Au-seeded GaAs nanowires in a growth regime where both wurtzite and zinc blende crystal structures are possible to achieve. We find that HCl changes the crystal structure of the nanowires from pure wurtzite to defect-free zinc blende. By comparing the growth of wurtzite-zinc blende heterostructures with and without the addition of HCl, it is deduced that HCl mainly interacts with Ga species prior incorporation, reducing the amount of Ga available to contribute to the growth. We conclude that the change in crystal structure is related to the reduction of Ga adatoms, and demonstrate the realization of wurtzite-zinc blende heterostructures with atomically sharp interfaces achieved only by adding HCl.

Jacobsson, Daniel; Lehmann, Sebastian; Dick, Kimberly A.

2014-06-01

361

Dynamic structure of superionic protons in hydrogen fluoride crystal  

NASA Astrophysics Data System (ADS)

Hydrogen fluoride crystal forms zig-zag chains of hydrogen fluoride molecules forming covalent bond between them. Goldman et al.(J. Chem. Phys.125,044501(2006).) have found the superionic state of the protons in the hydrogen fluoride crystal at 900 K and beyond the pressures at 33 GPa. The present study elucidates the dynamic structure of the protons in the superionic state of the crystal at the extreme conditions with the first principles molecular dynamics method. The strong covalent bond between the proton and the fluorine in the conductor has shown a different dynamic structure from that in the ?-CuI; The protons in the conductor are bonded with the nearest fluorine and the other protons are located at incommensurate sites of the bcc fluorine lattice. This is a different dynamic structure from the formation of the incommensurate dynamic copper dimers in the ?-CuI.(Tsumuraya et al. J. Phys. Soc. Jpn. 81,055603(2012).)

Ohde, Yoshiyuki; Tsumuraya, Kazuo

2013-03-01

362

Modeling liquid crystal bilayer structures with minimal surfaces J. D. Enlowa)  

E-print Network

Modeling liquid crystal bilayer structures with minimal surfaces J. D. Enlowa) and R. L. Enlow crystal phase used in familiar liquid crystal displays, but the term also refers to structures with far that surfactant liquid crystals can have structures whose shapes are based on minimal surfaces. II. MINIMAL

Gruner, Sol M.

363

J. Mol. Biol. (1978) 123, 607-630 Crystal Structure of Yeast Phenylalanine Transfer RNA  

E-print Network

J. Mol. Biol. (1978) 123, 607-630 Crystal Structure of Yeast Phenylalanine Transfer RNA I phenylalanine transfer RNA in an orthorhombic crystal form. The crystal structure of the transfer KNA has been to the crystal structure of yeast phenylalanine tRNA are described in this paper. A complete list of atomic co

Church, George M.

364

Crystal Structure of Streptococcus mutans Pyrophosphatase  

Microsoft Academic Search

Background:Streptococcus mutans pyrophosphatase (Sm-PPase) is a member of a relatively uncommon but widely dispersed sequence family (family II) of inorganic pyrophosphatases. A structure will answer two main questions: is it structurally similar to the family I PPases, and is the mechanism similar?Results: The first family II PPase structure, that of homodimeric Sm-PPase complexed with metal and sulfate ions, has been

Michael C Merckel; Igor P Fabrichniy; Anu Salminen; Nisse Kalkkinen; Alexander A Baykov; Reijo Lahti; Adrian Goldman

2001-01-01

365

The crystal structure of GXGD membrane protease FlaK  

SciTech Connect

The GXGD proteases are polytopic membrane proteins with catalytic activities against membrane-spanning substrates that require a pair of aspartyl residues. Representative members of the family include preflagellin peptidase, type 4 prepilin peptidase, presenilin and signal peptide peptidase. Many GXGD proteases are important in medicine. For example, type 4 prepilin peptidase may contribute to bacterial pathogenesis, and mutations in presenilin are associated with Alzheimer's disease. As yet, there is no atomic-resolution structure in this protease family. Here we report the crystal structure of FlaK, a preflagellin peptidase from Methanococcus maripaludis, solved at 3.6 {angstrom} resolution. The structure contains six transmembrane helices. The GXGD motif and a short transmembrane helix, helix 4, are positioned at the centre, surrounded by other transmembrane helices. The crystal structure indicates that the protease must undergo conformational changes to bring the GXGD motif and a second essential aspartyl residue from transmembrane helix 1 into close proximity for catalysis. A comparison of the crystal structure with models of presenilin derived from biochemical analysis reveals three common transmembrane segments that are similarly arranged around the active site. This observation reinforces the idea that the prokaryotic and human proteases are evolutionarily related. The crystal structure presented here provides a framework for understanding the mechanism of the GXGD proteases, and may facilitate the rational design of inhibitors that target specific members of the family.

Hu, Jian; Xue, Yi; Lee, Sangwon; Ha, Ya (Yale-MED)

2011-09-20

366

The Crystal Structure of GXGD Membrane Protease FlaK  

SciTech Connect

The GXGD proteases are polytopic membrane proteins with catalytic activities against membrane-spanning substrates that require a pair of aspartyl residues. Representative members of the family include preflagellin peptidase, type 4 prepilin peptidase, presenilin and signal peptide peptidase. Many GXGD proteases are important in medicine. For example, type 4 prepilin peptidase may contribute to bacterial pathogenesis, and mutations in presenilin are associated with Alzheimer's disease. As yet, there is no atomic-resolution structure in this protease family. Here we report the crystal structure of FlaK, a preflagellin peptidase from Methanococcus maripaludis, solved at 3.6 {angstrom} resolution. The structure contains six transmembrane helices. The GXGD motif and a short transmembrane helix, helix 4, are positioned at the centre, surrounded by other transmembrane helices. The crystal structure indicates that the protease must undergo conformational changes to bring the GXGD motif and a second essential aspartyl residue from transmembrane helix 1 into close proximity for catalysis. A comparison of the crystal structure with models of presenilin derived from biochemical analysis reveals three common transmembrane segments that are similarly arranged around the active site. This observation reinforces the idea that the prokaryotic and human proteases are evolutionarily related. The crystal structure presented here provides a framework for understanding the mechanism of the GXGD proteases, and may facilitate the rational design of inhibitors that target specific members of the family.

J Hu; Y Xue; S Lee; Y Ha

2011-12-31

367

Why biomolecules prefer only a few crystal structures  

NASA Astrophysics Data System (ADS)

We have shown that, in determining the biomolecule-crystal symmetry, the occupation of low-site-symmetry Wyckoff positions is crucial, which contrasts with the overwhelming majority of nonmolecular, inorganic crystals where atoms mainly reside in high-symmetry Wyckoff positions. We consider the general relation between the symmetry of an isolated molecule and the possible symmetries of biomolecular crystals it can generate. We reveal that the improper symmetry operations (inversion and mirror symmetries) are prohibited in the chirally pure biomolecular crystals. Next, we show that the low (C1) symmetry of large biological molecules substantially decreases the space in a crystal where the molecules can reside. The space “forbidden” for molecule centers is found to be in the R vicinity of the higher-symmetry Wyckoff positions on symmetry lines, where R is the molecule characteristic size. The remaining free space and hence the probability for the structure to exist are shown to be drastically increased when replacing any rotation axis by a screw one. Based on the proposed model, we have explained the peculiar distribution of biomolecular crystals over the space groups, which can be obtained from biomolecule-crystal databases.

Kitaev, Yu. E.; Panfilov, A. G.; Smirnov, V. P.; Tronc, P.

2003-01-01

368

Microscopic characterization of defect structure in RDX crystals.  

PubMed

Three batches of the commercial energetic material RDX, as received from various production locations and differing in sensitivity towards shock initiation, have been characterized with different microscopic techniques in order to visualize the defect content in these crystals. The RDX crystals are embedded in an epoxy matrix and cross-sectioned. By a treatment of grinding and polishing of the crystals, the internal defect structure of a multitude of energetic crystals can be visualized using optical microscopy, scanning electron microscopy and confocal scanning laser microscopy. Earlier optical micrographs of the same crystals immersed in a refractive index matched liquid could visualize internal defects, only not in the required detail. The combination of different microscopic techniques allows for a better characterization of the internal defects, down to inclusions of approximately 0.5 ?m in size. The defect structure can be correlated to the sensitivity towards a high-amplitude shock wave of the RDX crystals embedded in a polymer bonded explosive. The obtained experimental results comprise details on the size, type and quantity of the defects. These details should provide modellers with relevant and realistic information for modelling defects in energetic materials and their effect on the initiation and propagation of shock waves in PBX formulations. PMID:24117989

Bouma, R H B; Duvalois, W; Van der Heijden, A E D M

2013-12-01

369

INVESTIGATION OF THE CRYSTAL STRUCTURES OF NUCLEOSIDES CONTAINING SULFUR  

Microsoft Academic Search

The crystal structures of three nucleosides containing sulfur have been determined by X-ray analysis: 4'-thiouridine, 5-fluoro-4'-thio-(alpha)-2'-deoxyuridine and 2,2'-anhydro-3'-O-acetyl-2'-thio-1-(beta)-D-arabinofuranosylcytosine hydrochloride.\\u000aThe study of the crystal structures of 4'-thiouridine and 5-fluoro-4'-thio-(alpha)-2'-deoxyuridine showed that the substitution of the ring oxygen by sulfur alters substantially the stereochemical features of furanoside nucleosides. The g('-) orientation of O(5') is favored more and the g('+) less. The

Jacqueline Vitali

1986-01-01

370

Theoretical and Experimental Study of Photonic Crystal Based Structures for Optical Communication Applications  

E-print Network

Theoretical and Experimental Study of Photonic Crystal Based Structures for Optical Communication 01854 ABSTRACT Photonic crystal based structures have been considered for optical communication applications. A class of novel symmetric structures consisting of cavities and waveguides have been proposed

Chen, Ray

371

Strong anti-ice ability of nanohairs over micro-ratchet structures  

NASA Astrophysics Data System (ADS)

A strong anti-ice property of nanohairs over micro-ratchet surfaces is observed. A long freezing delay of more than 185 min is achieved for a droplet on the nanohairs over ratchet structure with a period of ~290 ?m under -10 °C, which is attributed to the effective cooperation of the nano- and microstructures.A strong anti-ice property of nanohairs over micro-ratchet surfaces is observed. A long freezing delay of more than 185 min is achieved for a droplet on the nanohairs over ratchet structure with a period of ~290 ?m under -10 °C, which is attributed to the effective cooperation of the nano- and microstructures. Electronic supplementary information (ESI) available. See DOI: 10.1039/c3nr04061e

Guo, Peng; Wen, Mengxi; Wang, Lei; Zheng, Yongmei

2014-03-01

372

Formation of the structure of gold nanoclusters during crystallization  

SciTech Connect

The structure formation in gold nanoparticles 1.6-5.0 nm in diameter is studied by molecular dynamics simulation using a tight-binding potential. The simulation shows that the initial fcc phase in small Au clusters transforms into other structural modifications as temperature changes. As the cluster size increases, the transition temperature shifts toward the melting temperature of the cluster. The effect of various crystallization conditions on the formation of the internal structure of gold nanoclusters is studied in terms of microcanonical and canonical ensembles. The stability boundaries of various crystalline isomers are analyzed. The obtained dependences are compared with the corresponding data obtained for copper and nickel nanoparticles. The structure formation during crystallization is found to be characterized by a clear effect of the particle size on the stability of a certain isomer modification. Nickel and copper clusters are shown to exhibit common features in the formation of their structural properties, whereas gold clusters demonstrate much more complex behavior.

Gafner, Yu. Ya., E-mail: ygafner@khsu.ru; Goloven'ko, Zh. V.; Gafner, S. L. [Khakassian State University (Russian Federation)] [Khakassian State University (Russian Federation)

2013-02-15

373

Crystal structure of the human spastin AAA domain  

PubMed Central

Hereditary spastic paraplegia (HSP) is a motor neuron disease caused by a progressive degeneration of the motor axons of the corticospinal tract. Point mutations or exon deletions in the microtubule-severing ATPase, spastin, are responsible for approximately 40% of cases of autosomal dominant HSP. Here, we report the 3.3 Å X-ray crystal structure of a hydrolysis- deficient mutant (E442Q) of the human spastin protein AAA domain. This structure is analyzed in the context of the existing Drosophila melanogaster spastin AAA domain structure and crystal structures of other closely related proteins in order to build a more unifying framework for understanding the structural features of this group of microtubule-severing ATPases. PMID:22446388

Taylor, Jennifer L.; White, Susan Roehl; Lauring, Brett; Kull, F. Jon

2012-01-01

374

Crystal structure of the human spastin AAA domain.  

PubMed

Hereditary spastic paraplegia (HSP) is a motor neuron disease caused by a progressive degeneration of the motor axons of the corticospinal tract. Point mutations or exon deletions in the microtubule-severing ATPase, spastin, are responsible for approximately 40% of cases of autosomal dominant HSP. Here, we report the 3.3 Å X-ray crystal structure of a hydrolysis-deficient mutant (E442Q) of the human spastin protein AAA domain. This structure is analyzed in the context of the existing Drosophila melanogaster spastin AAA domain structure and crystal structures of other closely related proteins in order to build a more unifying framework for understanding the structural features of this group of microtubule-severing ATPases. PMID:22446388

Taylor, Jennifer L; White, Susan Roehl; Lauring, Brett; Kull, F Jon

2012-08-01

375

Bathymetry and geological structures beneath the Ross Ice Shelf at the mouth of Whillans Ice Stream, West Antarctica, modeled from ground-based gravity measurements  

NASA Astrophysics Data System (ADS)

gravity data reveal important geologic controls on the location and behavior of the grounding zone of Whillans Ice Stream (WIS), West Antarctica. Grounding zones of ice sheets and contiguous ice shelves are important for understanding ice sheet dynamics, as key processes that influence the grounded ice and its discharge into the ocean occur in these regions. Here, we model the bathymetry and shallow geological structures beneath the Ross Ice Shelf in an embayment of the WIS grounding zone using gravity data collected on the ground, in conjunction with seismic and radar data. We find that the region of shallow ocean water (<~50 m) is extensive; oceanographic models suggest that grounding zones exhibiting thin ocean cavities with gently sloping ice-ocean interfaces are likely to be tidally well mixed, leading to slower basal melting than would occur in a thicker, stratified water column. Beneath the ocean water column, we model a fault and a sedimentary basin in a half-graben, filled with two layers of sediments. The total thickness of the sediment layer is 900 to 1250 m in the half-graben, and 600 to 800 m on the upthrown block, and the basement depth is no more than 2000 m. We observe that the upper, softer sediment is thinnest near the modern grounding line and may possibly pinch out near our grid, and that the modeled fault is roughly parallel to part the grounding. We therefore hypothesize that the WIS grounding line stabilized in its current location in part due to the subglacial geology.

Muto, Atsuhiro; Christianson, Knut; Horgan, Huw J.; Anandakrishnan, Sridhar; Alley, Richard B.

2013-08-01

376

The crystal structure of URu3B2  

NASA Astrophysics Data System (ADS)

The crystal structure of URu 3B 2 has been determined by single crystal X-ray analysis. URu 3B 2 crystallizes in the trigonal space group P3¯ (C 131) with hexagonal lattice a = 1.09531(14), c = 0.59353 (8) nm, Z = 8. Intensity measurements were obtained from a fourcircle diffractometer. The structure was solved by Patterson methods and refined by full matrix least squares calculation. The final R-value, R = ? | ?F|/? F0, is 0.052 for an asymètric set of 962 independent reflections ( l- F0l > 2 ? ( F0)). The crystal structure is a twofold superstructure (distortion-derivative) of the CeCo3B2-type cell ( a = 2 a', c = 2 c' and thus closely related to the CaCu5 type structure. The coordination numbers of U are 2 U + 12 Ru + (6 B) and those of Ru atoms 4 U + 6 Ru + 4 B. The isolated boron atoms have tetrakaidekahedral metal coordination 6 Ru + 3 U; no boron-boron contacts occur. The structural chemistry of ( Th, U, RE) Ru3B2 phases is discussed.

Rogl, Peter

1980-09-01

377

CRYSTAL STRUCTURE ANALYSIS OF A PUTATIVE OXIDOREDUCTASE FROM KLEBSIELLA PNEUMONIAE  

SciTech Connect

Klebsiella pneumoniae, a gram-negative enteric bacterium, is found in nosocomial infections which are acquired during hospital stays for about 10% of hospital patients in the United States. The crystal structure of a putative oxidoreductase from K. pneumoniae has been determined. The structural information of this K. pneumoniae protein was used to understand its function. Crystals of the putative oxidoreductase enzyme were obtained by the sitting drop vapor diffusion method using Polyethylene glycol (PEG) 3350, Bis-Tris buffer, pH 5.5 as precipitant. These crystals were used to collect X-ray data at beam line X12C of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). The crystal structure was determined using the SHELX program and refi ned with CNS 1.1. This protein, which is involved in the catalysis of an oxidation-reduction (redox) reaction, has an alpha/beta structure. It utilizes nicotinamide adenine dinucleotide phosphate (NADP) or nicotine adenine dinucleotide (NAD) to perform its function. This structure could be used to determine the active and co-factor binding sites of the protein, information that could help pharmaceutical companies in drug design and in determining the protein’s relationship to disease treatment such as that for pneumonia and other related pathologies.

Baig, M.; Brown, A.; Eswaramoorthy, S.; Swaminathan, S.

2009-01-01

378

Detections of Trans-Neptunian Ice in Protoplanetary Disks  

NASA Astrophysics Data System (ADS)

We present Herschel Space Observatory PACS spectra of T Tauri stars, in which we detect amorphous and crystalline water ice features. Using irradiated accretion disk models, we determine the disk structure and ice abundance in each of the systems. Combining a model-independent comparison of the ice feature strength and disk size with a detailed analysis of the model ice location, we estimate that the ice emitting region is at disk radii >30 AU, consistent with a proto-Kuiper belt. Vertically, the ice emits most below the photodesorption zone, consistent with Herschel observations of cold water vapor. The presence of crystallized water ice at a disk location (1) colder than its crystallization temperature and (2) where it should have been re-amorphized in ~1 Myr suggests that localized generation is occurring; the most likely cause appears to be micrometeorite impact or planetesimal collisions. Based on simple tests with UV models and different ice distributions, we suggest that the SED shape from 20 to 50 ?m may probe the location of the water ice snowline in the disk upper layers. This project represents one of the first extra-solar probes of the spatial structure of the cometary ice reservoir thought to deliver water to terrestrial planets.

McClure, M. K.; Espaillat, C.; Calvet, N.; Bergin, E.; D'Alessio, P.; Watson, D. M.; Manoj, P.; Sargent, B.; Cleeves, L. I.

2015-02-01

379

Ice forming experiment  

NASA Technical Reports Server (NTRS)

A low gravity experiment to assess the effect of the presence of supercooled cloud droplets on the diffusional growth rate of ice crystals is described. The theoretical work and the feasibility studies are summarized. The nucleation of ice crystals in supercooled clouds is also discussed.

Vali, G.

1982-01-01

380

Ice Spikes  

NSDL National Science Digital Library

This webpage from SnowCrystals.com provides instructions for growing ice spikes in ordinary freezer. The page explains the physics of growing these spikes and concludes useful information on water purity and growing temperature. Related photos and drawings are provided.

Libbrecht, Kenneth

2012-02-28

381

Crystal structures of carbonates up to Mbar pressures determined by single crystal synchrotron radiation diffraction  

NASA Astrophysics Data System (ADS)

The recent improvements at synchrotron beamlines, currently allow single crystal diffraction experiments at extreme pressures and temperatures [1,2] on very small single crystal domains. We successfully applied such technique to determine the crystal structure adopted by carbonates at mantle pressures. The knowledge of carbon-bearing phases is in fact fundamental for any quantitative modelling of global carbon cycle. The major technical difficulty arises after first order transitions or decomposition reactions, since original crystal (apx. 10x10x5 ?m3) is transformed in much smaller crystalline domains often with random orientation. The use of 3D reciprocal space visualization software and the improved resolution of new generation flat panel detectors, however, allow both identification and integration of each single crystal domain, with suitable accuracy for ab-initio structure solution, performed with direct and charge-flipping methods and successive structure refinements. The results obtained on carbonates, indicate two major crystal-chemistry trends established at high pressures. The CO32- units, planar and parallel in ambient pressure calcite and dolomite structures, becomes non parallel in calcite- and dolomite-II and III phases, allowing more flexibility in the structures with possibility to accommodate strain arising from different cation sizes (Ca and Mg in particular). Dolomite-III is therefore also observed to be thermodynamically stable at lower mantle pressures and temperatures, differently from dolomite, which undergoes decomposition into pure end-members in upper mantle. At higher pressure, towards Mbar (lowermost mantle and D'' region) in agreement with theoretical calculations [3,4] and other experimental results [5], carbon coordination transform into 4-fold CO4 units, with different polymerisation in the structure depending on carbonate composition. The second important crystal chemistry feature detected is related to Fe2+ in Fe-bearing magnesite, which spontaneously oxidises at HP/HT, forming Fe3+ carbonates, Fe3+ oxides and reduced carbon (diamonds). Single crystal diffraction approach allowed full structure determination of these phases, yielding to the discovery of few unpredicted structures, such as Mg2Fe2C4O13 and Fe13O19, which can be well reproduced in different experiments. Mg2Fe2C4O13 carbonate present truncated chain C4O13 groups, and Fe13O19 oxide, whose stoichiometry is intermediate between magnetite and hematite, is a one-layer structure, with features encountered in superconducting materials. The results fully support the ideas of unexpected complexities in the mineralogy of the lowermost mantle, and single crystal technique, once properly optimized in ad-hoc synchrotron beamlines, is fundamental for extracting accurate structural information, otherwise rarely accessible with other experimental techniques. References: [1] Merlini M., Hanfland M. (2013). Single crystal diffraction at Mbar conditions by synchrotron radiation. High Pressure Research, in press. [2] Dubrovinsky et al., (2010). High Pressure Research, 30, 620-633. [3] Arapan et al. (1997). Phys. Rev. Lett., 98, 268501. [4] Oganov et al. (2008) EPSL, 273, 38-47. [5] Boulard et al. (2011) PNAS, 108, 5184-5187.

Merlini, M.

2013-12-01

382

On the effects of anisotropic rheology on ice flow, internal structure, and the age-depth relationship at ice divides  

Microsoft Academic Search

We use numerical modeling with a full-system Stokes solver to elucidate the effects of nonlinear rheology and strain-induced anisotropy on ice flow at ice divides. We find that anisotropic rheology profoundly affects the shape of both isochrone layering and surface topography. Anisotropic effects cause the formation of a downward curving fold, i.e., a syncline, in isochrones in the lower central

Carlos Martín; G. Hilmar Gudmundsson; Hamish D. Pritchard; Olivier Gagliardini

2009-01-01

383

Crystal growth, structure analysis and characterisation of 2 - (1, 3 - dioxoisoindolin - 2 - yl) acetic acid single crystal  

SciTech Connect

Single crystal of dielectric material 2 - (1, 3 - dioxoisoindolin - 2 - yl) acetic acid has been grown by slow evaporation solution growth method. The grown crystal was harvested in 25 days. The crystal structure was analyzed by Single crystal X - ray diffraction. UV-vis-NIR analysis was performed to examine the optical property of the grown crystal. The thermal property of the grown crystal was studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The dielectric measurements were carried out and the dielectric constant was calculated and plotted at all frequencies.

Sankari, R. Siva, E-mail: sivasankari.sh@act.edu.in [Department of Physics, Agni College of Technology, Thalambur, Chennai-603103 (India); Perumal, Rajesh Narayana [Department of Physics, SSN College of Engineering, Kalavakkam, Chennai-603110 (India)

2014-04-24

384

Magnetic properties and crystal structure of ?-Ta  

NASA Astrophysics Data System (ADS)

Polycrystalline samples of ?-Ta with Frank-Kasper ?-phase structure prepared by electrolysis are studied. The atomic parameters are determined by the Rietveld method. The magnetic susceptibility is measured in the temperature range 4.2 < T < 273 K and the dependence of the magnetization on the magnetic field strength is measured at 4.2 and 77 K. The analysis of interatomic distances and the results of magnetic measurements indicate that clusters with a localized excess charge exist in ?-Ta. The data obtained in this study suggest structural changes in ?-Ta at T < 77 K.

Shamra?, V. F.; Warhulska, J. K.; Arakcheeva, A. V.; Grinevich, V. V.

2004-11-01

385

[Band electronic structures and crystal packing forces  

SciTech Connect

We investigated the electronic and structural properties of low-dimensional materials and explored the structure-property correlations governing their physical properties. Progress was made on how to interpret the scanning tunneling microscopy and atomic force microscopy images of layered materials and on how to account for charge density wave instabilities in 2-D metals. Materials studied included transition metal chalcogenides, transition metal halides, organic conducting salts, Mo bronzes, A[sub 2]PdH[sub 2], fullerenes, squarate tetrahydrate polymers Fe, Cu(C[sub 4]O[sub 4])4[center dot]H[sub 2]O, BEDT salts, etc.

Not Available

1993-01-01

386

The structure and dynamics of carbon dioxide and water containing ices investigated via THz and mid-IR spectroscopy.  

PubMed

Icy dust grains play a key role in the chemistry of the interstellar medium. The cumulative outcome of recent observations, laboratory studies, and astrochemical models indicates that solid-phase reaction mechanisms may dominate the formation of complex organic molecules such as amino acids and sugars in space. Consequently, the composition and structure of the icy grain mantle may significantly influence solid-phase reaction pathways. In this work, we present a new experimental setup capable of studying astrochemical ice analogs in both the TeraHertz (THz), or far-Infrared (far-IR), region (0.3-7.5 THz; 10-250 cm(-1)) and the mid-IR (400-4000 cm(-1)). The instruments are capable of performing a variety of spectroscopic studies that can provide especially relevant laboratory data to support astronomical observations from telescopes such as Herschel, SOFIA, and ALMA. Experimental spectra of astrochemical ice analogs of water and carbon dioxide in pure, mixed, and layered ices were collected at different temperatures under high vacuum conditions with the goal of investigating the structure of the ice. We tentatively observe a new feature in both amorphous solid water and crystalline water at 33 cm(-1) (1 THz). In addition, our studies of mixed and layered ices show how it is possible to identify the location of carbon dioxide as it segregates within the ice by observing its effect on the THz spectrum of water ice. The THz spectra of mixed and layered ices are further analyzed by fitting their spectral features to those of pure amorphous solid water and crystalline water ice to quantify the effects of temperature changes on structure. From the results of this work, it appears that THz spectroscopy is potentially well suited to study thermal transformations within the ice. PMID:24394213

Allodi, Marco A; Ioppolo, Sergio; Kelley, Matthew J; McGuire, Brett A; Blake, Geoffrey A

2014-02-28

387

Ice-volume changes (1936-1990) and structure of Aldegondabreen, Spitsbergen  

NASA Astrophysics Data System (ADS)

Aldegondabreen is a small valley glacier, ending on land, located in the Grønfjorden area of Spitsbergen, Svalbard. Airborne radio-echo sounding in 1974/75, using a 440 MHz radar, revealed a polythermal two-layered structure, which has been confirmed by detailed ground-based radio-echo sounding done in 1999 using a 15 MHz monopulse radar. The 1999 radar data reveal an upper cold layer extending down to 90 m depth in the southern part of the glacier, where the thickest ice (216 m) was also found. A repeated pattern of diffractions from the southern part of the glacier, at depths of 50-80 m and dipping down-glacier, has been interpreted as an englacial channel which originates in the temperate ice. From joint analysis of the 1936 topographic map, a digital elevation model constructed from 1990 aerial photographs and the subglacial topography determined from radar data, a severe loss of mass during the period 1936-90 has been estimated: a glacier tongue retreat of 930 m, a decrease in area from 8.9 to 7.6 km2, in average ice thickness from 101 to 73 m and in ice volume from 0.950 to 0.558 km3, which are equivalent to an average annual balance of -0.7 m w.e. This is comparable with the only available data of net mass balance for Aldegondabreen (-1.1 and -1.35 m w.e. for the balance years 1976/77 and 2002/03) and consistent with the 0.27°C increase in mean summer air temperature in this zone during 1936-90, as well as the warming in Spitsbergen following the end of the Little Ice Age (LIA), and the general glacier recession trend observed in this region.

Navarro, F. J.; Glazovsky, A. F.; Macheret, Y. U. Y. A.; Vasilenko, E. V.; Corcuera, M. I.; Cuadrado, M. L.

388

Timing of sea ice retreat can alter phytoplankton community structure in the western Arctic Ocean  

NASA Astrophysics Data System (ADS)

This study assesses the response of phytoplankton assemblages to recent climate change, especially with regard to the shrinking of sea ice in the northern Chukchi Sea of the western Arctic Ocean. Distribution patterns of phytoplankton groups in the late summers of 2008-2010 were analyzed based on HPLC pigment signatures and, the following four major algal groups were inferred via multiple regression and cluster analyses: prasinophytes, diatoms, haptophytes and dinoflagellates. A remarkable interannual difference in the distribution pattern of the groups was found in the northern basin area. Haptophytes dominated and dispersed widely in warm surface waters in 2008, whereas prasinophytes dominated in cold water in 2009 and 2010. A difference in the onset date of sea ice retreat was evident among years - the sea ice retreat in 2008 was 1-2 months earlier than in 2009 and 2010. The spatial distribution of early sea ice retreat matched the areas in which a shift in algal community composition was observed. Steel-Dwass's multiple comparison tests were used to assess the physical, chemical and biological parameters of the four clusters. We found a statistically significant difference in temperature between the haptophyte-dominated cluster and the other clusters, suggesting that the change in the phytoplankton communities was related to the earlier sea ice retreat in 2008 and the corollary increase in sea surface temperatures. Longer periods of open water during the summer, which are expected in the future, may affect food webs and biogeochemical cycles in the western Arctic due to shifts in phytoplankton community structure.

name prefix surname suffix, given; Fujiwara, A.; Hirawake, T.; Suzuki, K.; Imai, I.; Saitoh, S.-I.

2013-09-01

389

RECENT CRYSTAL STRUCTURE DETERMINATIONS BY NEUTRON DIFFRACTION AT OAK RIDGE  

E-print Network

469. RECENT CRYSTAL STRUCTURE DETERMINATIONS BY NEUTRON DIFFRACTION AT OAK RIDGE By GEORGE M. BROWN and HENRI A. LEVY, Chemistry Division Oak Ridge National Laboratory (1), Oak Ridge, Tennessee, U. S. A ont été relevées grace au diffractomètre à neutrons d'Oak Ridge position- nant automatiquement les

Paris-Sud XI, Université de

390

Crystal structures and morphologies of fractionated milk fat in nanoemulsions.  

PubMed

The triacylglycerol (TAG) crystal structures and morphologies of fractionated milk lipids in nanoemulsions were investigated at 4°C. Droplet size (0.17 versus 1.20 ?m), lipid composition (stearin versus olein) and cooling rate (1 versus 10°C min(-1)) had an influence on the structural properties. Five crystal polymorphs (?, ?'1, ?'2, ?1, and ?2) were formed with either triple and/or double chain length structures in the solid phases of the emulsified systems. X-ray scattering peak intensities were reduced with the nanoemulsion particles. The internal structure of TAG exhibited stacking of individual lamellar layers (3.8-4.2 nm). Various anisometric shapes of fat nanoparticles were formed due to a highly sharp curvature of the nano-size droplets. The shape of olein nanoparticles was more polyhedral compared to the stearin. TAG crystals arranged in a planar-layered organisation at the slower cooling rate. These differences imply that the nanometric confinement of oil droplets modifies the fat crystal habit. PMID:25308656

Truong, Tuyen; Morgan, Garry P; Bansal, Nidhi; Palmer, Martin; Bhandari, Bhesh

2015-03-15

391

The diammoniate of diborane: crystal structure and hydrogen release.  

PubMed

[(NH(3))(2)BH(2)](+)[BH(4)](-) is formed from the room temperature decomposition of NH(4)(+)BH(4)(-), via a NH(3)BH(3) intermediate. Its crystal structure has been determined and contains disordered BH(4)(-) ions in 2 distinct sites. Hydrogen release is similar to that from NH(3)BH(3) but with faster kinetics. PMID:20941402

Bowden, Mark; Heldebrant, David J; Karkamkar, Abhi; Proffen, Thomas; Schenter, Gregory K; Autrey, Tom

2010-12-01

392

~ Animation of Crystal Structure Variations with Pressure, Temperature and Composition  

E-print Network

~ Animation of Crystal Structure Variations with Pressure, Temperature and Composition Robert T as a function of temperature, pressure and composition. Examples of these animations are found on the cover another is an effective way to make the computer animations. This paper presents an outline

Downs, Robert T.

393

Diffusion-Driven Crystal Structure Transformation: Synthesis of Heusler Alloy  

E-print Network

Diffusion-Driven Crystal Structure Transformation: Synthesis of Heusler Alloy Fe3Si Nanowires Materials Research Team, KBSI, Daejeon 305-333, Korea ABSTRACT We report fabrication of Heusler alloy Fe3Si transformation, magnetic materials, Heusler alloy M etal silicide nanowires (NWs) can have diverse metal

Kim, Bongsoo

394

Materials research at Stanford University. [composite materials, crystal structure, acoustics  

NASA Technical Reports Server (NTRS)

Research activity related to the science of materials is described. The following areas are included: elastic and thermal properties of composite materials, acoustic waves and devices, amorphous materials, crystal structure, synthesis of metal-metal bonds, interactions of solids with solutions, electrochemistry, fatigue damage, superconductivity and molecular physics and phase transition kinetics.

1975-01-01

395

Crystal Structures of Thermostable Xylose Isomerases from Thermus caldophilus and  

E-print Network

of XI makes it a useful enzyme for converting glucose to fructose for the industrial production of high-fructose corn syrup. However, XIs in general have higher KM and lower kcat for D-glucose than for D of Science and Technology, Daejon 305-333, Korea The crystal structures of highly thermostable xylose

Suh, Se Won

396

Crystal shape-dependent magnetic susceptibility and Curie law crossover in the spin ices Dy2Ti2O7 and Ho2Ti2O7  

NASA Astrophysics Data System (ADS)

We present an experimental determination of the isothermal magnetic susceptibility of the spin ice materials Dy2Ti2O7 and Ho2Ti2O7 in the temperature range 1.8-300 K. The use of spherical crystals has allowed accurate correction for demagnetizing fields and allowed the true bulk isothermal susceptibility ?T(T) to be estimated. This has been compared against a theoretical expression based on a Husimi tree approximation to the spin ice model. Agreement between experiment and theory is excellent at T > 10 K, but systematic deviations occur below that temperature. Our results largely resolve an apparent disagreement between neutron scattering and bulk measurements that has been previously noted. They also show that the use of non-spherical crystals in magnetization studies of spin ice may introduce very significant systematic errors, although we note some interesting—and possibly new—systematics concerning the demagnetizing factor in cuboidal samples. Finally, our results show how experimental susceptibility measurements on spin ices may be used to extract the characteristic energy scale of the system and the corresponding chemical potential for emergent magnetic monopoles.

Bovo, L.; Jaubert, L. D. C.; Holdsworth, P. C. W.; Bramwell, S. T.

2013-09-01

397

Crystal shape-dependent magnetic susceptibility and Curie law crossover in the spin ices Dy2Ti2O7 and Ho2Ti2O7.  

PubMed

We present an experimental determination of the isothermal magnetic susceptibility of the spin ice materials Dy2Ti2O7 and Ho2Ti2O7 in the temperature range 1.8-300 K. The use of spherical crystals has allowed accurate correction for demagnetizing fields and allowed the true bulk isothermal susceptibility ?T(T) to be estimated. This has been compared against a theoretical expression based on a Husimi tree approximation to the spin ice model. Agreement between experiment and theory is excellent at T > 10 K, but systematic deviations occur below that temperature. Our results largely resolve an apparent disagreement between neutron scattering and bulk measurements that has been previously noted. They also show that the use of non-spherical crystals in magnetization studies of spin ice may introduce very significant systematic errors, although we note some interesting--and possibly new--systematics concerning the demagnetizing factor in cuboidal samples. Finally, our results show how experimental susceptibility measurements on spin ices may be used to extract the characteristic energy scale of the system and the corresponding chemical potential for emergent magnetic monopoles. PMID:23988470

Bovo, L; Jaubert, L D C; Holdsworth, P C W; Bramwell, S T

2013-09-25

398

Domain Structures in Nematic Liquid Crystals on a Polycarbonate Surface  

PubMed Central

Alignment of nematic liquid crystals on polycarbonate films obtained with the use of solvents with different solvations is studied. Domain structures occurring during the growth on the polymer surface against the background of the initial thread-like or schlieren texture are demonstrated. It is established by optical methods that the domains are stable formations visualizing the polymer surface structures. In nematic droplets, the temperature-induced transition from the domain structure with two extinction bands to the structure with four bands is observed. This transition is shown to be caused by reorientation of the nematic director in the liquid crystal volume from the planar alignment to the homeotropic state with the pronounced radial configuration of nematic molecules on the surface. The observed textures are compared with different combinations of the volume LC orientations and the radial distribution of the director field and the disclination lines at the polycarbonate surface. PMID:23965955

Parshin, Alexander M.; Gunyakov, Vladimir A.; Zyryanov, Victor Y.; Shabanov, Vasily F.

2013-01-01

399

High pressure ices  

PubMed Central

H2O will be more resistant to metallization than previously thought. From computational evolutionary structure searches, we find a sequence of new stable and meta-stable structures for the ground state of ice in the 1–5 TPa (10 to 50 Mbar) regime, in the static approximation. The previously proposed Pbcm structure is superseded by a Pmc21 phase at p = 930 GPa, followed by a predicted transition to a P21 crystal structure at p = 1.3 TPa. This phase, featuring higher coordination at O and H, is stable over a wide pressure range, reaching 4.8 TPa. We analyze carefully the geometrical changes in the calculated structures, especially the buckling at the H in O-H-O motifs. All structures are insulating—chemistry burns a deep and (with pressure increase) lasting hole in the density of states near the highest occupied electronic levels of what might be component metallic lattices. Metallization of ice in our calculations occurs only near 4.8 TPa, where the metallic C2/m phase becomes most stable. In this regime, zero-point energies much larger than typical enthalpy differences suggest possible melting of the H sublattice, or even the entire crystal. PMID:22207625

Hermann, Andreas; Ashcroft, N. W.; Hoffmann, Roald

2012-01-01

400

Hydrohalite in cold sea ice: Laboratory observations of single crystals, surface accumulations, and migration rates under a temperature  

E-print Network

the fractional volumes of ice, gas, brine, and precipitated salt. [3] Temperatures within sea ice exhibit a wide and desiccation. Sublimation of a sea ice surface at low temperature leaves a lag deposit of hydrohalite, which depend strongly on temperature, because of freezing-equilibrium relationships which dictate

Warren, Stephen

401

Differential propagation constants on slant paths through snow and ice crystals as measured by 16.5 GHz polarization-diversity radar  

Microsoft Academic Search

Radar determinations of differential propagation constants at a wavelength of 1.82 cm on slant paths through heavy snow and\\u000a ice crystals are reported. Also reported are measurements of differential propagation effects in the melting layer. The slant\\u000a path results indicate an increase of differential phase shift with height, to a value at 2000 m which may exceed 1.3 deg\\/\\u000a km,

Archibald Hendry; Glendon C. McCormick; Yahia M. M. Antar

1981-01-01

402

Crystal structure of cyclin-dependent kinase 2  

Microsoft Academic Search

Cyclin-dependent kinase 2 (CDK2) is a member of a highly conserved family of protein klnases that regulate the eukaryotic cell cycle. The crystal structures of the human CDK2 apoenzyme and its Mg2+ATP complex have been determined to 2.4Å resolution. The structure is bi-lobate, like that of the cyclic AMP-dependent protein kinase, but contains a unique helix-loop segment that interferes with

Hendrik L. de Bondt; Jody Rosenblatt; Jarmila Jancarik; Heather D. Jones; David O. Morgant; Sung-Hou Kim

1993-01-01

403

Crystal structure and chemical bonding in tin(II) acetate  

Microsoft Academic Search

Tin(II) acetate was prepared and its crystal structure was solved from X-ray powder diffraction data. Tin(II) acetate adopts a polymeric structure consisting of infinite Sn(CH3COO)2 chains running along the c-axis which are packed into groups of four. The acetate groups bridge the Sn atoms along the chains. The Sn atoms are asymmetrically surrounded by four oxygen atoms with two short

Varvara S. Stafeeva; Alexander S. Mitiaev; Artem M. Abakumov; Alexander A. Tsirlin; Artem M. Makarevich; Evgeny V. Antipov

2007-01-01

404

Crystal Structure of the Phosphorus Oxynitride P 4ON 6  

Microsoft Academic Search

The ab initio crystal structure determination of the phosphorus oxynitride P4ON6 has been performed by the X-ray powder diffraction technique (space group Pnnm; a = 6.8424(1)Å, b = 6.0714(2)Å, c = 6.8176(1)Å, Z = 2; RBragg = 4.8%, Gof = 1.35) A structure refinement of neutron data incorporating anisotropic temperature coefficients for oxygen and nitrogen reduces the RBragg-factor down to

J. Ronis; B. Bondars; A. Vitola; T. Millers; J. Schneider; F. Frey

1995-01-01

405

Crystal Structure of Chaperonin-60 from Paracoccus denitrificans  

Microsoft Academic Search

The crystal structure of chaperonin-60 from Paracoccus denitrificans (P.cpn60) has been determined at 3.2Å resolution by the molecular replacement method. Two heptameric rings of identical subunits of P.cpn60 in adjacent asymmetric units are stacked in a back-to-back manner and form a cylinder, as found in GroEL, cpn60 from Escherichia coli. With respect to the unliganded GroEL structure, each subunit of

Takaaki A. Fukami; Masafumi Yohda; Hideki Taguchi; Masasuke Yoshida; Kunio Miki

2001-01-01

406

TUNABLE PHOTONIC STRUCTURES BASED ON SILICON AND LIQUID CRYSTALS  

Microsoft Academic Search

This paper is focused on the design, fabrication and characterization of the conventional and tunable photonic devices based on grooved silicon, serving as one-dimensional (1D) photonic crystal. The advantages of these photonic structures are as follows: the large refractive index contrast, in-plane moulding of the light flow, the possibility to fabricate a composite photonic structures by filling the grooves with

Tatiana S. Perova; Vladimir A. Tolmacheva; Ekaterina V. Astrova

407

Intermetallic crystal structures as foams. Beyond frank-kasper.  

PubMed

In many intermetallic structures, the atoms and bonds divide space into tilings by tetrahedra. The well-known Frank-Kasper phases are examples. The dual tilings divide space into a tiling by polyhedra that is topologically a foam. The number of faces of the dual polyhedron corresponds to the atom coordination number in the direct structure, and face sharing by adjacent polyhedra corresponds to bonds in the direct structure. A number of commonly occurring intermetallic crystal structures are shown as their duals. A major advantage of this alternative mode of depiction is that coordination of all of the atoms can be seen simultaneously. PMID:25247234

Bonneau, Charlotte; O'Keeffe, Michael

2015-02-01

408

Solution Structures, Dynamics, and Ice Growth Inhibitory Activity of Peptide Fragments Derived from an Antarctic Yeast Protein  

PubMed Central

Exotic functions of antifreeze proteins (AFP) and antifreeze glycopeptides (AFGP) have recently been attracted with much interest to develop them as commercial products. AFPs and AFGPs inhibit ice crystal growth by lowering the water freezing point without changing the water melting point. Our group isolated the Antarctic yeast Glaciozyma antarctica that expresses antifreeze protein to assist it in its survival mechanism at sub-zero temperatures. The protein is unique and novel, indicated by its low sequence homology compared to those of other AFPs. We explore the structure-function relationship of G. antarctica AFP using various approaches ranging from protein structure prediction, peptide design and antifreeze activity assays, nuclear magnetic resonance (NMR) studies and molecular dynamics simulation. The predicted secondary structure of G. antarctica AFP shows several ?-helices, assumed to be responsible for its antifreeze activity. We designed several peptide fragments derived from the amino acid sequences of ?-helical regions of the parent AFP and they also showed substantial antifreeze activities, below that of the original AFP. The relationship between peptide structure and activity was explored by NMR spectroscopy and molecular dynamics simulation. NMR results show that the antifreeze activity of the peptides correlates with their helicity and geometrical straightforwardness. Furthermore, molecular dynamics simulation also suggests that the activity of the designed peptides can be explained in terms of the structural rigidity/flexibility, i.e., the most active peptide demonstrates higher structural stability, lower flexibility than that of the other peptides with lower activities, and of lower rigidity. This report represents the first detailed report of downsizing a yeast AFP into its peptide fragments with measurable antifreeze activities. PMID:23209600

Asmawi, Azren A.; Rahman, Mohd Basyaruddin A.; Murad, Abdul Munir A.; Mahadi, Nor M.; Basri, Mahiran; Rahman, Raja Noor Zaliha A.; Salleh, Abu B.; Chatterjee, Subhrangsu; Tejo, Bimo A.; Bhunia, Anirban

2012-01-01

409

Tailor-made force fields for crystal-structure prediction.  

PubMed

A general procedure is presented to derive a complete set of force-field parameters for flexible molecules in the crystalline state on a case-by-case basis. The force-field parameters are fitted to the electrostatic potential as well as to accurate energies and forces generated by means of a hybrid method that combines solid-state density functional theory (DFT) calculations with an empirical van der Waals correction. All DFT calculations are carried out with the VASP program. The mathematical structure of the force field, the generation of reference data, the choice of the figure of merit, the optimization algorithm, and the parameter-refinement strategy are discussed in detail. The approach is applied to cyclohexane-1,4-dione, a small flexible ring. The tailor-made force field obtained for cyclohexane-1,4-dione is used to search for low-energy crystal packings in all 230 space groups with one molecule per asymmetric unit, and the most stable crystal structures are reoptimized in a second step with the hybrid method. The experimental crystal structure is found as the most stable predicted crystal structure both with the tailor-made force field and the hybrid method. The same methodology has also been applied successfully to the four compounds of the fourth CCDC blind test on crystal-structure prediction. For the five aforementioned compounds, the root-mean-square deviations between lattice energies calculated with the tailor-made force fields and the hybrid method range from 0.024 to 0.053 kcal/mol per atom around an average value of 0.034 kcal/mol per atom. PMID:18642947

Neumann, Marcus A

2008-08-14

410

Crystal Structure and Calculated Electronic Band Structure of ZrTe 3  

Microsoft Academic Search

Because of inconsistencies in literature data, the crystal structure of ZrTe3was redetermined from single-crystal data and the electronic band structure was calculated using density functional theory in the local density approximation (LDA) and the linear muffin tin orbital method (LMTO). ZrTe3crystallizes in the monoclinic space groupP21\\/mwitha=589.8(1) pm,b=392.69(3) pm,c=1010.3(1) pm, and?=97.81(1)° (Z=2) in the ZrSe3structure type (?data collection,Rw=1.88%). In the layer

Klaus Stöwe; Frank R Wagner

1998-01-01

411

Structural analysis of three-dimenstionl photonic crystals in nature  

NASA Astrophysics Data System (ADS)

We studied the structural origin of the color and photonic band structure in exoskeletons of Eupholus weevils and dorsal wings of lycaenids butterflies. The internal structures of the insects were systematically investigated using focused ion beam (FIB) milling, and the optical response of the insects was observed by optical microscopy and a microspectrophotometer. A series of sequential SEM images were obtained during the FIB milling process and 3D structures were reconstructed by image processing. The correlation of the structures and the optical responses were studied by theoretical modeling. Diamond-based 3D photonic crystal lattice existed in Eupholus weevils, while gyroid structure was in lycaenids butterflies. The calculated photonic band structures matched the measured optical response. Aluminum oxide and titanium oxide were deposited on the weevils and the butterflies in order to study the effect of refractive index contrast to the photonic band structure and the optical response.

Yoon, Beom-Jin; Park, Jung Ok; Srinivasarao, Mohan

2012-02-01

412

Crystal structures of dibromodichloromethane and bromotrichloromethane  

NASA Astrophysics Data System (ADS)

The neutron powder profiles for CBr2Cl2 and CBrCl3 have been recorded at temperatures ranging from about 260 K to 5 K. The profiles at the highest temperatures are consistent with fcc structures with a = 8.597(2)A and a = 8.526(2)A respectively. CBrCl3 has a second plastic phase observed at 245 K. The remaining profiles, below 250 K for CBr2Cl2, or below 225 K for CBrCl3, can be analysed in terms of the C2/c space group with Z = 32. This is the same as for the ordered phases of CBr4 and CCl4. Orientational disorder of the molecules leads to the structural similarity of all members of the family. Tests for partial ordering were not successful.

Lee-Dadswell, S. E.

413

Crystal structure of human chorionic gonadotropin  

Microsoft Academic Search

The three-dimensional structure of human chorionic gonadotropin shows that each of its two different subunits has a similar topology, with three disulphide bonds forming a cystine knot. This same folding motif is found in some protein growth factors. The heterodimer is stabilized by a segment of the beta-subunit which wraps around the alpha-subunit and is covalently linked like a seat

A. J. Lapthorn; D. C. Harris; A. Littlejohn; J. W. Lustbader; R. E. Canfield; K. J. Machin; F. J. Morgan; N. W. Isaacs

1994-01-01

414

The EMBO Journal Vol.16 No.13 pp.37873796, 1997 Crystal structure of a deubiquitinating enzyme  

E-print Network

processing (OlveraKeywords: crystal structure/cysteine protease/substrate and Wool, 1993; Haas et al., 1996The EMBO Journal Vol.16 No.13 pp.3787­3796, 1997 Crystal structure of a deubiquitinating enzyme-terminus of ubiquitin. We have modification of chromatin structure (Bradbury, 1992),determined the crystal structure

Hill, Chris

415

Smectic Layer Structures in Complex Geometries—Modelling Complex Layer Structures in Smectic Liquid Crystals  

Microsoft Academic Search

In order to understand the operation of liquid crystal devices based on tilted smectic materials it is highly important to also understand the formation of the smectic liquid crystal layer structures. However, in many situations this is not easy to do. It is well established that in devices filled with materials that have an upper lying SmA phase a “chevron”

Steve J. Elston; Lesley A. Parry-Jones

2005-01-01

416

Crystal structure of tin(IV) chloride octa­hydrate  

PubMed Central

The title compound, [SnCl4(H2O)2]·6H2O, was crystallized according to the solid–liquid phase diagram at lower temperatures. It is built-up of SnCl4(H2O)2 octa­hedral units (point group symmetry 2) and lattice water mol­ecules. An intricate three-dimensional network of O—H?O and O—H?Cl hydrogen bonds between the complex molecules and the lattice water molecules is formed in the crystal structure. PMID:25552971

Hennings, Erik; Schmidt, Horst; Voigt, Wolfgang

2014-01-01

417

Crystal structure of tin(IV) chloride octa-hydrate.  

PubMed

The title compound, [SnCl4(H2O)2]·6H2O, was crystallized according to the solid-liquid phase diagram at lower temperatures. It is built-up of SnCl4(H2O)2 octa-hedral units (point group symmetry 2) and lattice water mol-ecules. An intricate three-dimensional network of O-H?O and O-H?Cl hydrogen bonds between the complex molecules and the lattice water molecules is formed in the crystal structure. PMID:25552971

Hennings, Erik; Schmidt, Horst; Voigt, Wolfgang

2014-12-01

418

Impact of Ice Ages on the genetic structure of trees and shrubs.  

PubMed Central

Data on the genetic structure of tree and shrub populations on the continental scale have accumulated dramatically over the past decade. However, our ability to make inferences on the impact of the last ice age still depends crucially on the availability of informative palaeoecological data. This is well illustrated by the results from a recent project, during which new pollen fossil maps were established and the variation in chloroplast DNA was studied in 22 European species of trees and shrubs. Species exhibit very different levels of genetic variation between and within populations, and obviously went through very different histories after Ice Ages. However, when palaeoecological data are non-informative, inferences on past history are difficult to draw from entirely genetic data. On the other hand, as illustrated by a study in ponderosa pine, when we can infer the species' history with some certainty, coalescent simulations can be used and new hypotheses can be tested. PMID:15101576

Lascoux, Martin; Palmé, Anna E; Cheddadi, Rachid; Latta, Robert G

2004-01-01

419

Projection structure of frog rhodopsin in two crystal forms.  

PubMed Central

Rhodopsin is the G protein-coupled receptor that upon light activation triggers the visual transduction cascade. Rod cell outer segment disc membranes were isolated from dark-adapted frog retinas and were extracted with Tween detergents to obtain two-dimensional rhodopsin crystals for electron crystallography. When Tween 80 was used, tubular structures with a p2 lattice (a = 32 A, b = 83 A, gamma = 91 degrees) were formed. The use of a Tween 80/Tween 20 mixture favored the formation of larger p22(1)2(1) lattices (a = 40 A, b = 146 A, gamma = 90 degrees). Micrographs from frozen hydrated frog rhodopsin crystals were processed, and projection structures to 7-A resolution for the p22(1)2(1) form and to 6-A resolution for the p2 form were calculated. The maps of frog rhodopsin in both crystal forms are very similar to the 9-A map obtained previously for bovine rhodopsin and show that the arrangement of the helices is the same. In a tentative topographic model, helices 4, 6, and 7 are nearly perpendicular to the plane of the membrane. In the higher-resolution projection maps of frog rhodopsin, helix 5 looks more tilted than it appeared previously. The quality of the two frog rhodopsin crystals suggests that they would be suitable to obtain a three-dimensional structure in which all helices would be resolved. Images Fig. 1 Fig. 2 Fig. 6 PMID:8524807

Schertler, G F; Hargrave, P A

1995-01-01

420

Use of Pom Pons To Illustrate Cubic Crystal Structures  

NASA Astrophysics Data System (ADS)

In general chemistry classes, students are introduced to the ways in which atoms are arranged in cubic crystal structures. Transposing the textbook illustrations into three dimensional structures is difficult for some students. This transitions is easier if a three dimensional model is available for examination. Several 3D models are cited. A quick to assemble, inexpensive, colorful, and durable alternative to these models and styrofoam balls is the use of olefin pom pons. Different sized pom pons can be used to demonstrate how the atomic radius will vary when comparing the different types of cubic crystal unit cells. Being made of a coarse material, pom pons can be stacked to illustrate different packing arrangements such as hexagonal close-packed and cubic close-packed structures. Pom pons make great atoms.

Cady, Susan G.

1997-07-01