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1

Structure of ice crystallized from supercooled water  

PubMed Central

The freezing of water to ice is fundamentally important to fields as diverse as cloud formation to cryopreservation. At ambient conditions, ice is considered to exist in two crystalline forms: stable hexagonal ice and metastable cubic ice. Using X-ray diffraction data and Monte Carlo simulations, we show that ice that crystallizes homogeneously from supercooled water is neither of these phases. The resulting ice is disordered in one dimension and therefore possesses neither cubic nor hexagonal symmetry and is instead composed of randomly stacked layers of cubic and hexagonal sequences. We refer to this ice as stacking-disordered ice I. Stacking disorder and stacking faults have been reported earlier for metastable ice I, but only for ice crystallizing in mesopores and in samples recrystallized from high-pressure ice phases rather than in water droplets. Review of the literature reveals that almost all ice that has been identified as cubic ice in previous diffraction studies and generated in a variety of ways was most likely stacking-disordered ice I with varying degrees of stacking disorder. These findings highlight the need to reevaluate the physical and thermodynamic properties of this metastable ice as a function of the nature and extent of stacking disorder using well-characterized samples.

Malkin, Tamsin L.; Murray, Benjamin J.; Brukhno, Andrey V.; Anwar, Jamshed; Salzmann, Christoph G.

2012-01-01

2

Structure of ice crystallized from supercooled water.  

PubMed

The freezing of water to ice is fundamentally important to fields as diverse as cloud formation to cryopreservation. At ambient conditions, ice is considered to exist in two crystalline forms: stable hexagonal ice and metastable cubic ice. Using X-ray diffraction data and Monte Carlo simulations, we show that ice that crystallizes homogeneously from supercooled water is neither of these phases. The resulting ice is disordered in one dimension and therefore possesses neither cubic nor hexagonal symmetry and is instead composed of randomly stacked layers of cubic and hexagonal sequences. We refer to this ice as stacking-disordered ice I. Stacking disorder and stacking faults have been reported earlier for metastable ice I, but only for ice crystallizing in mesopores and in samples recrystallized from high-pressure ice phases rather than in water droplets. Review of the literature reveals that almost all ice that has been identified as cubic ice in previous diffraction studies and generated in a variety of ways was most likely stacking-disordered ice I with varying degrees of stacking disorder. These findings highlight the need to reevaluate the physical and thermodynamic properties of this metastable ice as a function of the nature and extent of stacking disorder using well-characterized samples. PMID:22232652

Malkin, Tamsin L; Murray, Benjamin J; Brukhno, Andrey V; Anwar, Jamshed; Salzmann, Christoph G

2012-01-09

3

The structure of ice crystallized from supercooled water  

NASA Astrophysics Data System (ADS)

The freezing of water to ice is fundamentally important to fields as diverse as cloud formation to cryopreservation. Traditionally ice was thought to exist in two well-crystalline forms: stable hexagonal ice and metastable cubic ice. It has recently been shown, using X-ray diffraction data, that ice which crystallizes homogeneously and heterogeneously from supercooled water is neither of these phases. The resulting ice is disordered in one dimension and therefore possesses neither cubic nor hexagonal symmetry and is instead composed of randomly stacked layers of cubic and hexagonal sequences. We refer to this ice as stacking-disordered ice I (ice Isd). This result is consistent with a number of computational studies of the crystallization of water. Review of the literature reveals that almost all ice that has been identified as cubic ice in previous diffraction studies and generated in a variety of ways was most likely stacking-disordered ice I with varying degrees of stacking disorder, which raises the question of whether cubic ice exists. New data will be presented which shows significant stacking disorder (or stacking faults on the order of 1 in every 100 layers of ice Ih) in droplets which froze heterogeneously as warm as 257 K. The identification of stacking-disordered ice from heterogeneous ice nucleation supports the hypothesis that the structure of ice that initially crystallises from supercooled water is stacking-disordered ice I, independent of nucleation mechanism, but this ice can relax to the stable hexagonal phase subject to the kinetics of recrystallization. The formation and persistence of stacking disordered ice in the Earth's atmosphere will also be discussed.

Murray, Benjamin

2013-03-01

4

Crystal Ice  

Microsoft Academic Search

IN reference to the ``crystal ice'' proposed by Dr. Calantarients, of Scarborough, for skating upon with ordinary skates, it may not be generally known that more than thirty years ago a skating pond was constructed in Liverpool, consisting, I believe, entirely of crystallised Glauber's salt. I have a perfect recollection of this miniature lake with its grotto-like surroundings, of its

R. H

1880-01-01

5

Structural transformation in supercooled water controls the crystallization rate of ice  

Microsoft Academic Search

One of water's unsolved puzzles is the question of what determines the lowest temperature to which it can be cooled before freezing to ice. The supercooled liquid has been probed experimentally to near the homogeneous nucleation temperature TH{\\\\approx}232 K, yet the mechanism of ice crystallization - including the size and structure of critical nuclei - has not yet been resolved.

Emily B. Moore; Valeria Molinero

2011-01-01

6

Snow Ice Crystals  

NSDL National Science Digital Library

This article from Physics Today by Yoshinori Furukawa and John S. Wettlaufer and John S. Wettlaufer describes how ice crystals form on the earth. The resource includes graphics depicting how different shapes of ice crystals are formed.

Furukawa, Y.; Wettlaufer, John S., 1963-

2010-03-12

7

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

2013-03-12

8

Sublimation of Ice Crystals  

Microsoft Academic Search

Recent experiments on the sublimation of single crystals of ice in an atmosphere of air indicate that the sublimation rate is diffusion limited and initially solid prismatic crystals evolve into time-independent shapes similar to confocal ellipses rotated about their major or minor axis (prolate or oblate spheroids). Step formation at crystal edges and vapor diffusion easily explain these observations.

Jon Nelson

1998-01-01

9

Crystal structure, stable isotopes (delta O-18), and development of sea ice in the Ross, Amundsen, and Bellingshausen seas, Antarctica  

NASA Astrophysics Data System (ADS)

The crystal structure and oxygen isotopic composition of ice cores obtained from floes at the end of summer in the eastern Ross Sea, the Amundsen Sea, and the western Bellingshausen Sea were investigated to determine the ice growth processes and conditions that contribute to sea ice development in the eastern Pacific sector of the southern ocean. The isotope data indicate that a moderate amount of snow contributes to the development of the sea ice. However, even the combined use of isotopes and crystal structure analysis does not unambiguously explain the means by which all of the snow is entrained in the ice. Nevertheless, it seems clear that much of the snow is contained in granular snow-ice that results from seawater flooding of floes and the base of the snow cover. The snow cover in the Ross-Amundsen region was as much as 2 m deep and supported by 7- to 8-m-thick floes primarily composed of frazil ice. In the Bellingshausen region the snow cover and the floes were thinner than in the Ross-Amundsen region. The Bellingshausen cores were composed primarily of multiple layers of frazil and congelation ice. In addition, in both regions there were numerous tipped or inclined blocks of congelation ice and layers of rafted nilas in the cores. The data indicate that the sea ice develops by multiple mechanisms in a turbulent environment.

Jeffries, Martin O.; Shaw, Raymond A.; Morris, Kim; Veazey, Alice L.; Krouse, H. Roy

1994-01-01

10

Modeling, Simulation and Comparison Study of Cirrus Clouds' Ice Crystals  

Microsoft Academic Search

Various methods and techniques to estimate ice crystals radar response have been developed to study the structure of cirrus clouds. Most methods assume a spherical shape for the ice crystals. This assumption leads to mistakes on the parameter estimation related to the particles' size. In this work, we modeled the shape of ice particles found in cirrus cloud as measured

Jorge M. Villa; Sandra L. Cruz-Pol; Jos Colom-Ustriz; Stephen M. Sekelsky

11

Modeling, simulation, and comparison study of cirrus cloud ice crystals  

Microsoft Academic Search

Various methods and techniques to estimate ice crystals radar response have been developed to study the structure of cirrus clouds. Most methods assume a spherical shape for the ice crystals. This assumption leads to mistakes on the parameter estimation related to the particles' size. In this work, we modeled the shape of ice particles found in cirrus cloud as measured

Jorge M. Villa; Sandra L. Cruz-Pol; Jose G. Colom-Ustariz; Stephen M. Sekelsky

2003-01-01

12

Radar Anisotropy of sea ice due to preferred Azimuthal orientation of the horizontal c axes of ice crystals  

Microsoft Academic Search

Results of impulse radar, ice crystal c axis, and subice current measurements on the fast ice near Narwhal Island, Alaska, are presented. The crystal structure of the ice was found to have a horizontal crystal c axis with a preferred azimuthal orientation. This orientation was found to align with the direction of the current at the ice-water interface. Impluse radar

Austin Kovacs; Rexford M. Morey

1978-01-01

13

Effects of ice-crystal structure on halo formation: cirrus cloud experimental and ray-tracing modeling studies.  

PubMed

During the 1986 Project FIRE (First International Satellite Cloud Climatology Project Regional Experiment) field campaign, four 22 halo-producing cirrus clouds were studied jointly from a groundbased 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. PMID:20935827

Sassen, K; Knight, N C; Takano, Y; Heymsfield, A J

1994-07-20

14

Fundamental Ice Crystal Accretion Physics Studies.  

National Technical Information Service (NTIS)

Due to numerous engine power-loss events associated with high-altitude convective weather, ice accretion within an engine due to ice-crystal ingestion is being investigated. The National Aeronautics and Space Administration (NASA) and the National Researc...

A. P. Broeren D. Fuleki D. Knezevici J. Tsao M. Vargas P. M. Struk T. Currie W. Wright

2011-01-01

15

Fundamental Ice Crystal Accretion Physics Studies.  

National Technical Information Service (NTIS)

Due to numerous engine power-loss events associated with high-altitude convective weather, ice accretion within an engine due to ice crystal ingestion is being investigated. The National Aeronautics and Space Administration (NASA) and the National Researc...

A. P. Broeren D. Fuleki D. Knezevici J. C. Tsao M. Vargas P. M. Struk T. Currie W. B. Wright

2012-01-01

16

Impurities in Spin Ice Crystals  

NASA Astrophysics Data System (ADS)

Spin ice crystals (and pyrochlore oxides in general) have raised a lot of interest of late thanks to their exotic properties, including emergent gauge symmetries, possible spin liquid behavior, and magnetic monopole excitations. Theoretical and experimental efforts in the study of these materials have benefited from the relative ease of growth of large clean single crystals. Even in such clean systems, however, impurities can play a crucial role in determining the properties at very low temperatures (see e.g., C. Henley, http://arxiv.org/abs/1210.8137). Here we investigate this issue both experimentally and theoretically. We study how controlled non-magnetic Y-dilution in Dy2Ti2O7 gradually alters the effective monopole description and the thermodynamic properties of the system at low temperature (extending earlier work by other authors to regimes that have not been investigated so far). We also study how oxygen deficiency affects spin ice samples, and we discuss how the oxygen stoichiometry can be quantified and controlled experimentally.

Sala, Gabriele; Castelnovo, Claudio; Goff, Jon; Gutmann, Matthias; Dharmalingam, Prabhakaran

2013-03-01

17

Overview of NASA Engine Ice-Crystal Icing Research.  

National Technical Information Service (NTIS)

Ice accretions that have formed inside gas turbine engines as a result of flight in clouds of high concentrations of ice crystals in the atmosphere have recently been identified as an aviation safety hazard. NASA s Aviation Safety Program (AvSP) has made ...

H. E. Addy J. P. Veres

2011-01-01

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

Artic ice and drilling structures  

SciTech Connect

The sea ice in the southern Beaufort Sea is examined and subdivided into three zones: the fast ice zone, the seasonal pack-ice zone, an the polar pack ice zone. Each zone requires its own type of system. Existing floating drilling systems include ice-strengthened drill ships, conical drilling systems, and floating ice platforms in deep-water land-fast ice. The development of hydrocarbon resources in the Arctic presents great challenges to engineers, since the structures are required to operate safely under various conditions. Significant progress has yet to be made in understanding the behavior of ice.

Sodhl, D.S.

1985-04-01

20

Nucleation and Growth of Ice Crystals.  

National Technical Information Service (NTIS)

The basic information regarding nucleation in supercooled water was reviewed. Measurements were performed with water drops and water in bulk. Ice growth was measured as a function of crystal orientation, temperature, flow velocity of solution, salt concen...

J. Farrar W. S. Hamilton

1965-01-01

21

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

22

Scattering of light by polyhedral ice crystals.  

PubMed

The single-scattering phase functions of polyhedral-shaped ice particles are calculated by means of geometric optics and the diffraction theory. Particle orientation is assumed to be random in space. Particle shapes are taken both from ice-crystal classifications and from in situ measurements. The effects of particle concavity on the scattering signature are discussed in detail. A common feature is the pronounced forward-scattering peak, as well as different halo peaks that are due to a minimum deviation at corresponding ice prisms. An unusual halo phenomena, which results from a minimum deviation in a double-prism configuration, is found and verified. The comparison of different particle types shows that backscattering is a sensitive indicator for the identification of types of ice-crystal. Aggregate particles, like bullet rosettes, basically show the scattering characteristics of their individual components. PMID:20820442

Macke, A

1993-05-20

23

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

PubMed

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; Muoz Caro, Guillermo M; Cruz-Diaz, Gustavo A; Rodrguez-Lazcano, Yamilet; Mat, Beln

2013-07-15

24

Modeling, simulation, and comparison study of cirrus cloud ice crystals  

NASA Astrophysics Data System (ADS)

Various methods and techniques to estimate ice crystals radar response have been developed to study the structure of cirrus clouds. Most methods assume a spherical shape for the ice crystals. This assumption leads to mistakes on the parameter estimation related to the particles' size. In this work, we modeled the shape of ice particles found in cirrus cloud as measured by airborne instruments, specifically ice bullets. These can be found depending on the temperature and cloud altitude, isolated or in groups of two or more bullets, called bullet rosettes. The model of the bullets was developed using the parameters obtained by airborne measurements from the National Center for Atmospheric Research (NCAR) Video Ice Particle Sampler (VIPS). This is an airborne instrument that takes samples of the cirrus cloud particles sizes. With these sample parameters we created a bullet function in DDSCAT with the actual shape of the bullets. This software allows us to create irregular models of particles using the Discrete Dipole Approximation method. With this model we can analyze the backscattering produced by the bullet and rosette model or reflectivity and compute the total volume backscattering coefficient from the cirrus clouds. Various models of ice crystal habits are compared.

Villa, Jorge M.; Cruz-Pol, Sandra L.; Colom-Ustariz, Jose G.; Sekelsky, Stephen M.

2003-04-01

25

Visual Simulation of Ice Crystal Growth  

Microsoft Academic Search

The beautiful, branching structure of ice is one of the most striking visual phenomena of the winter landscape. Yet there is little study about modeling this effect in computer graphics. In this paper, we present a novel approach for visual simulation of ice growth. We use a numerical simulation technique from computational physics, the \\

Theodore Kim; Ming C. Lin

2003-01-01

26

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

27

Impact of small ice crystal assumptions on ice sedimentation rates in cirrus clouds and GCM simulations  

Microsoft Academic Search

In the prediction of climate change, the greatest uncertainty lies in the representation of clouds. Ice clouds are particularly challenging, and to date there is no accepted method for measuring the smaller ice crystals (D < 60 ?m). This study examines the sensitivity of a global climate model to different assumptions regarding the number concentrations of small ice crystals when

David L. Mitchell; Philip Rasch; Dorothea Ivanova; Greg McFarquhar; Timo Nousiainen

2008-01-01

28

Properties of Columnar Ice Crystals Precipitating front Layer Clouds  

Microsoft Academic Search

Single unrimed columnar ice crystals (>200 m in length) from shallow layer clouds were collected in silicone oil, photographed under a microscope, and melted to determine their mass. These ice crystals were representative of those growing in a water sub-saturated environment in the temperature range 4 to 10C. The axial lengths of the crystals were related by the expression D

K. O. L. F. Jayaweera; T. Ohtake

1974-01-01

29

Interaction of turbulence, ice crystals, and water vapour in cirrus clouds  

NASA Astrophysics Data System (ADS)

The EMERALD-1 airborne campaign to investigate cirrus clouds was conducted at Adelaide Australia during September 2001. This involved the Egrett aircraft flying within cirrus clouds for in situ measurements of ice crystal properties, water vapour, wind, temperature and pressure. A second aircraft, a King Air, flew directly below the Egrett with an upward viewing lidar that mapped the structure of the clouds. Results are presented here that show the interaction between turbulence, humidity, ice crystal nucleation, growth, and sublimation. The maximum intensity of turbulence was measured at the top and bottom edges of the cloud, where ice crystals were either forming or sublimating. At the cloud top the largest ice crystal concentrations were associated with the greatest turbulence intensity. The width of the statistical distribution of relative humidity over ice was inversely proportional to the crystal concentration. The RHi distributions were approximately Gaussian in form and did not exhibit skewing toward supersaturation.

Cook, C. R.; Whiteway, J.; Busen, R.; Connolly, P.; Choularton, T.; Gallagher, M.; Bower, K.; Flynn, M.; Hacker, J.

2006-12-01

30

Epitaxial Growth of Ice Crystals on the Muscovite Cleavage Plane and Their Relation to Partial Dislocations.  

National Technical Information Service (NTIS)

The arrangement of oxygen atoms in hexagonal rings in crystals of ice and muscovite gives two possible alternatives of epitaxial connection between these two structures. Which one of these two alternatives is favored depends entirely on the location of po...

J. L. Caslavsky K. Vedam

1970-01-01

31

Microscopic pattern of ice crystal growth in the presence of thermal hysteresis proteins  

SciTech Connect

This study examines the effect of thermal hysteresis proteins (THPs) from the winter flounder (Psuedopleuronectes americanus) on the ice-water interface morphology during freezing of aqueous solutions. Experiments were performed using a directional solidification stage, and the development of the two-phase interface was observed through a microscope and recorded by a video system. Unusual ice crystal morphologies were observed, including faceted ice crystal growth along the (1100) crystal plane; spicular or needlelike growth in the (1010) direction; and growth parallel to the c-axis, (0001), consisting of incorporated liquid inclusions bounded by hexagonal prism faces. The observed crystallographic structures can be explained as an effect of the interaction between the THPs and the primary prism faces of ice crystals. This results in an increase in the Gibbs free energy of these planes, followed by ice growth into the supercooled liquid adjacent to these faces.

Coger, R.; Rubinsky, B. (Univ. of California, Berkeley, CA (United States). Dept. of Mechanical Engineering); Fletcher, G. (Memorial Univ. of Newfoundland, St. John's, Newfoundland (Canada))

1994-08-01

32

The Structure of Ice  

Microsoft Academic Search

A consistent set of unit cell parameters at various temperatures is not yet available for ordinary ice, but the mean of the most precise measurements leads to a density of 0\\\\cdot 9164 g\\/cm3 at 0 degrees C (atmospheric pressure) with a cubical expansion coefficient of 11 10-5, increasing to 0\\\\cdot 9414 and 21 10-5 at liquid air temperatures.

Dame Kathleen Lonsdale

1958-01-01

33

Aircraft Observations of Ice Crystal Evolution in an Altostratus Cloud  

Microsoft Academic Search

Observations from a Lagrangian spiral descent within altostratus cloud associated with a cold front were used to study the evolution of ice particle spectra by following populations of ice crystals as they fell through the cloud. The flight track was corrected for wind effects and was divided into distinct regions for spatial comparison of ice particle spectra. Analysis of size

Paul R. Field

1999-01-01

34

Design of Breakup Ice Control Structures  

Microsoft Academic Search

ABSTRACT The primary purpose of a breakup ice control structure (ICS) is to retain a breakup ice run upstream of a traditional ice jam problem area and thereby miti- gate ice-jam flooding. By controlling ice-jam location, breakup ICSs also can prevent ice-related scour associated with dam removals or contaminated sedi- ment remediation projects. This report briefly describes basic ICS types,

Andrew M. Tuthill; James H. Lever

35

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

36

Ice Accumulation on Ocean Structures.  

National Technical Information Service (NTIS)

A literature search was made for information on the accretion of ice on ocean structures and on methods for control. The bulk of the reports were in Russian, with some additional Japanese, British, American, Canadian, and Icelandic sources. Analysis of ic...

L. D. Minsk

1977-01-01

37

Effect of biopolymers on structure and ice recrystallization in dynamically frozen ice cream model systems.  

PubMed

Ice crystal growth and microstructure of sugarsolutions prepared with stabilizers (carboxymethyl cellulose [CMC], xanthan gum, locust bean gum [LBG], and gelatin) with or without milk solids-nonfat (MSNF) after freezing in a scraped surface heat exchanger and temperature cycling (5 cycles from -6 degrees C to -20 degrees C) were studied. Ice crystal growth was calculated from brightfield microscopic images acquired from samples before and after cycling. Freeze-substitution and low-temperature embedding (LR-Gold resin) were sample preparation techniques utilized for structure analyses by light microscopy and transmission electron microscopy. Differential staining for carbohydrates and proteins allowed the identification of stabilizer gel-like structures in LBG, gelatin, and gelatin/MSNF solutions. In the absence of milk proteins, xanthan and LBG were the most effective at retarding recrystallization, while in their presence, only xanthan had an effect. Cryo-gelation of the LBG was observed but is not the only mechanism of stabilizer action. Thermodynamic incompatibility between biopolymers was observed to promote localized high concentrations of milk proteins located at the ice crystal interface, probably exerting a water-holding action that significantly enhanced the stabilizer effect. Qualitatively, solution heterogeneity (phase separation) was directly proportional to ice crystal growth inhibition. It is suggested that water-holding by stabilizer and proteins, and in some cases steric hindrance induced by a stabilizer gel-like network, caused a reduction in the kinetics of the ice recrystallization phenomena and promoted mechanisms of melt-regrow instead of melt-diffuse-grow recrystallization, thus resulting in the preservation of the ice crystal size and in a small span of the ice crystal size distribution. PMID:12487439

Regand, A; Goff, H D

2002-11-01

38

Artic ice and drilling structures  

Microsoft Academic Search

The sea ice in the southern Beaufort Sea is examined and subdivided into three zones: the fast ice zone, the seasonal pack-ice zone, an the polar pack ice zone. Each zone requires its own type of system. Existing floating drilling systems include ice-strengthened drill ships, conical drilling systems, and floating ice platforms in deep-water land-fast ice. The development of hydrocarbon

Sodhl

1985-01-01

39

Acoustic detection of ice crystals in Antarctic waters  

NASA Astrophysics Data System (ADS)

During the voyage of the RSV Aurora Australis to the region of Prydz Bay, Antarctica in January-March 1991, ice crystals were encountered at depths from the surface to 125-m in the western area of the bay. On two occasions, crystals were retrieved by netting, and echo sounder records have been used to infer additional regions of occurrence. Acoustic target strength estimates made on the ice crystal assemblies encountered show significant spatial variation, which may relate to crystal size and/or aggregation. Data from a suite of conductivity-temperature-depth casts have been used to map regions of the study area where in situ water temperatures fell below the computed freezing point. Such regions correlate well with those selected on the basis of echogram type and imply that ice crystals occurred at depth over large areas of the bay during the cruise period. The ice crystal distribution described is consistent with that expected from a plume of supercooled water emerging from under the Amery Ice Shelf and forming part of the general circulation of the bay. The magnitude of the supercooled water plume is greater than those reported previously in the Prydz Bay region. If misinterpreted as biota on echo sounder records, ice crystals could significantly bias biomass estimates based on echo integration in this and potentially other areas.

Penrose, John D.; Conde, M.; Pauly, T. J.

1994-06-01

40

Secondary Ice Nucleation and Crystallizer-Performance Optimization.  

National Technical Information Service (NTIS)

A batch crystallizer was used to investigate the secondary nucleation of ice in NaCl and KCl solutions. The kinetics of nucleation were determined over a range of salt concentrations, agitation power inputs, and solution subcoolings. The nucleation kineti...

E. Woltz

1975-01-01

41

The characteristics of mid-latitude and low-latitude ice cloud crystals  

NASA Astrophysics Data System (ADS)

An accurate understanding of the dimensional characteristics of atmospheric ice crystals is important for weather and climate models. Ice crystal fall speed which partially governs cloud lifetime is dependent on crystal mass and projected area. Ice cloud radiative properties are dependent on crystal shape as well as cloud optical thickness which can vary widely depending on local conditions and cloud formation mechanisms. These are some of the reasons that cirrus clouds are considered to be one of the most uncertain elements in the earth's climate system. This thesis addresses some of these uncertainties. Mid-latitude cirrus clouds are frequently composed of bullet rosette shaped ice crystals. Bullet rosettes can grow with hollow ends which affects their radiative properties. In chapter 2, the frequency of occurrence of bullet rosettes with hollows ends is investigated. The radiative properties of hollow crystals are investigated in Chapter 3. For a thin cloud (optical depth of unity) with hollow crystals would lead to a difference of 5 W/m2 in short wave radiation at the surface compared to solid crystals. The properties of low latitude sub-visible cirrus cloud particles have been poorly investigated due to the difficulty of reaching them with instrumented aircraft. The properties of sub-visible cirrus clouds are investigated through the analysis of a large dataset of aircraft observations in chapter 4. Parameterizations for particle area, mass, size distributions and mass weighted fall speeds are developed. Mass weighted fall speeds were found to be lower than predicted by previous parameterizations due to the lack of large particles compared to previous studies. Most atmospheric ice cloud particles are irregular in shape. The final two research topics addressed in this thesis investigate the dimensional characteristics of irregularly shaped ice particles. In chapter 5, the total surface area of irregular ice crystals is investigated. Knowledge of particle surface area is important for atmospheric chemistry applications. In Chapter 6, ice crystal projected area and mass are investigated using fractal geometry techniques. Ice crystal aggregation was simulated to investigate the structure of ice crystal aggregates. The fractal analysis allows the determination of mass dimensional relationships from particle projected area measurements.

Schmitt, Carl George

42

An experimental study of crystallization and crystal growth of methane hydrates from melting ice  

Microsoft Academic Search

An experiment with well defined gas-water interfacial surface area was developed to study the crystallization and crystal growth of methane hydrates. Measurable formation rates were observed only when melting ice was involved. No hydrates nucleated from liquid water or from non-melting ice. It is concluded that melting ice, which like hydrate water is hydrogen-bonded, provides a template for hydrate nucleation

M. J. Hwang; D. A. Wright; A. Kapur; G. D. Holder

1990-01-01

43

The Backscattering Linear Depolarization Ratio of Ice Clouds Composed of Small Ice Crystals  

NASA Astrophysics Data System (ADS)

The importance of small ice crystals (< 50 m) for cirrus cloud radiative properties is a matter of controversial debate, mainly because some measurements seemed to clearly overestimate the number concentrations of small ice particles due to particle shattering on the instrument inlets. On the other hand, there is no doubt that small micrometer-sized ice crystals dominate the particle size distributions of contrails and cirrus clouds emerging from contrails. Polarisation LIDAR is frequently used to investigate the microphysics of contrails and contrail cirrus remotely. These investigations reveal unusually high maximum linear depolarization ratios of 0.5 - 0.7. The knowledge of the link between ice crystal depolarization and their size and shape is a prerequisite for the interpretation of these LIDAR data. Since young contrails consist of relatively small ice crystals with sizes typically less than 10 m, the scattering matrix of these non-spherical particles can be calculated by the T-matrix method. In order to investigate the relation between the linear backscattering depolarization ratio and the microphysical properties of small ice particles that closely resemble those found in contrails and young cirrus, we started to run dedicated ice crystal nucleation and growth experiments at the large cloud simulation chamber AIDA of Forschungszentrum Karlsruhe. Such studies became feasible after the installation of the new in situ laser scattering and depolarization set up SIMONE at the chamber in 2006. The light scattering measurements are analyzed in the context of the microphysical properties of the ice clouds measured by optical cloud particle spectrometers, single particle imaging, and in situ infrared extinction spectroscopy. We compare our experimental results with theoretical results generated by the T-matrix method for finite cylinders. The results give new insight into the scattering depolarisation properties of small ice crystals grown under simulated contrail and cirrus formation conditions.

Schnaiter, M.; Abdelmonem, A.; Benz, S.; Leisner, T.; Mhler, O.; Wagner, R.

2009-04-01

44

Acoustic detection of ice crystals in Antarctic waters  

Microsoft Academic Search

During the voyage of the RSV Aurora Australis to the region of Prydz Bay, Antarctica in January-March 1991, ice crystals were encountered at depths from the surface to 125-m in the western area of the bay. On two occasions, crystals were retrieved by netting, and echo sounder records have been used to infer additional regions of occurrence. Acoustic target strength

John D. Penrose; M. Conde; T. J. Pauly

1994-01-01

45

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

2010-03-09

46

Population structure of ice-breeding seals.  

PubMed

The development of population genetic structure in ice-breeding seal species is likely to be shaped by a combination of breeding habitat and life-history characteristics. Species that return to breed on predictable fast-ice locations are more likely to exhibit natal fidelity than pack-ice-breeding species, which in turn facilitates the development of genetic differentiation between subpopulations. Other aspects of life history such as geographically distinct vocalizations, female gregariousness, and the potential for polygynous breeding may also facilitate population structure. Based on these factors, we predicted that fast-ice-breeding seal species (the Weddell and ringed seal) would show elevated genetic differentiation compared to pack-ice-breeding species (the leopard, Ross, crabeater and bearded seals). We tested this prediction using microsatellite analysis to examine population structure of these six ice-breeding species. Our results did not support this prediction. While none of the Antarctic pack-ice species showed statistically significant population structure, the bearded seal of the Arctic pack ice showed strong differentiation between subpopulations. Again in contrast, the fast-ice-breeding Weddell seal of the Antarctic showed clear evidence for genetic differentiation while the ringed seal, breeding in similar habitat in the Arctic, did not. These results suggest that the development of population structure in ice-breeding phocid seals is a more complex outcome of the interplay of phylogenetic and ecological factors than can be predicted on the basis of breeding substrate and life-history characteristics. PMID:18494764

Davis, Corey S; Stirling, Ian; Strobeck, Curtis; Coltman, David W

2008-05-20

47

Ice Crystals Produced by Expansion: Experiments and Application to Aircraft-produced Ice  

Microsoft Academic Search

The production of ice crystals as a result of the expansion and cooling of moist air was investigated by laboratory experiment. In particular, the warmest expanded air temperature that produces crystals was sought as a function of the initial temperature. The results fit the standard theory of homogeneous nucleation of water droplets, as long as the droplets remain at a

T. C. Foster; J. Hallett

1993-01-01

48

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

NASA Astrophysics Data System (ADS)

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.

1980-07-01

49

The characteristics of mid-latitude and low-latitude ice cloud crystals  

Microsoft Academic Search

An accurate understanding of the dimensional characteristics of atmospheric ice crystals is important for weather and climate models. Ice crystal fall speed which partially governs cloud lifetime is dependent on crystal mass and projected area. Ice cloud radiative properties are dependent on crystal shape as well as cloud optical thickness which can vary widely depending on local conditions and cloud

Carl George Schmitt

2009-01-01

50

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.

51

Mechanism of Ice Crystal Growth Habit and Shape Instability Development Below Water Saturation.  

National Technical Information Service (NTIS)

Ice phase process in supercooled clouds play dominant roles in development of updrafts and downdrafts as well as various forms of precipitation. In a supercooled cloud, an ice phase process begins normally with vapor diffusional growth of ice crystals. Co...

G. D. Swoboda

1981-01-01

52

Physical and structural properties of the Greenland Ice Sheet Project 2 ice core: A review  

NASA Astrophysics Data System (ADS)

Substantial data sets have been collected on the relaxation characteristics, density, grain size, c axis fabrics, and ultrasonic velocities of the Greenland Ice Sheet Project 2 (GISP2) core to its contact with bedrock at 3053.4 m. Changes in all these properties paralleled closely those found in cores from Byrd Station, Antarctica, and Dye 3, Greenland. Density increased progressively with depth to a maximum of 0.921 Mg/m3 at about 1400 m, at which depth the ice became bubble free. Below about 2000 m, in situ densities began to decrease in response to increasing ice sheet temperatures. Since drilling, much of the ice core has undergone significant volume expansion (relaxation) due to microcracking and the exsolving of enclathratized gases, especially in the brittle ice zone between 650 and 1400 m. Grain size increased linearly to about 1000 m, thereafter remaining fairly constant until the Younger Dryas event at 1678 m where a twofold to threefold decrease in grain size occurred. These grain size changes were accompanied by a progressive clustering of crystal c axes toward the vertical, including a small increase in c axis concentration across the Younger Dryas/Holocene boundary. Increased dust levels in the Wisconsin ice have contributed to the maintenance of a fine-grained texture which, with its strong vertical c axis fabric, persisted to nearly 3000 m. However, beginning at about 2800 m, layers of coarse-grained ice intermixed with the much finer-grained matrix ice are observed. Below 3000 m the ice became very coarse grained. This change, attributed to annealing recrystallization at elevated temperatures in the ice sheet, was accompanied by a dispersed or ring-like redistribution of the c axes about the vertical. Ultrasonic measurements of vertical and horizontal P wave velocities made at 10-m intervals along the entire length of the GISP2 core fully confirmed the results of the crystallo-optical observations. A return to fine-grained ice coincided with the first appearance of brown, silty ice 13 m above bedrock. Bedrock material consisted of 48 cm of till, including boulders and cobbles, overlying gray biotite granite comprising the true bedrock. There is evidence that disturbed structure in the GISP2 cores begins little more than 70% of the way through the ice sheet. This disturbance increases with depth until it becomes large enough to cast suspicion on features lasting centuries or more in the bottom 10% of the ice sheet.

Gow, A. J.; Meese, D. A.; Alley, R. B.; Fitzpatrick, J. J.; Anandakrishnan, S.; Woods, G. A.; Elder, B. C.

1997-11-01

53

Ice crystal properties retrieval using radar spectral polarimetric measurements within ice\\/mixed-phase clouds  

Microsoft Academic Search

In the field of atmospheric research, ground-based radar systems are often employed to study ice\\/mixed-phase cloud properties based on retrieval techniques. These techniques convert the radar signal backscattered by each bulk of ice crystals being probed within the same radar resolution volume to clouds microphysical characteristics. However, the size of a radar resolution volume is often too large compared to

Y. Dufournet

2010-01-01

54

Mixing antifreeze protein types changes ice crystal morphology without affecting antifreeze activity  

Microsoft Academic Search

All three fish antifreeze protein types (I, II and III) inhibit the growth of ice to form hexagonal bipyramidal ice crystals of characteristic morphology. Mixtures of these different antifreezes produced ice crystals of hybrid shapes and dimensions, consistent with the different antifreeze types binding to the same ice surfaces. The activity of the mixtures was independent of the proportions of

Chao Heman; Carl I. DeLuca; Peter L. Davies

1995-01-01

55

Dependence of the single-scattering properties of small ice crystals on idealized shape models  

NASA Astrophysics Data System (ADS)

Small ice crystals (with maximum dimension <50 ?m) appear quasi-circular when imaged by probes on aircraft flying through cloud. Therefore, idealized models constructed to calculate their single-scattering properties have included quasi-spherical models such as Chebyshev particles, Gaussian random spheres, and droxtals. Recently, an ice analogue grown from sodium fluorosilicate solution on a glass substrate, with several columns emanating from a common center of mass, was shown to be quasi-circular when imaged by state-of-the-art cloud probes. In this study, a new idealized model, called the budding Bucky ball (3B) that resembles the shape of the small ice analogue is developed. The corresponding single-scattering properties (scattering phase function P11 and asymmetry parameter g) are computed by a ray-tracing code. Compared with previosly used models, 3B scatters less light in the forward and more light in the lateral and backward directions. The Chebyshev particles and Gaussian random spheres show smooth and featureless P11, whereas droxtals and 3Bs, which have a faceted structure, show several peaks in P11 associated with angles of minimum deviation. Overall, the difference in the forward (lateral; backward) scattering between models are up to 22% (994%; 132%), 20% (510%; 101%), and 16% (146%; 156%) for small ice crystals with repective area ratios of 0.85, 0.77, and 0.69. The g for different models varies by up to 25%, 23%, and 19% for particles with area ratios of 0.85, 0.77, and 0.69, respectively. Becuase the single-scattering properties of small ice crystals depend both on the choice of the idealized model and the area ratios used to characterize the small ice crystals, higher resolution observations of small ice crystals or direct observations of their single-scattering properties are required.

Um, J.; McFarquhar, G. M.

2010-11-01

56

Dependence of the single-scattering properties of small ice crystals on idealized shape models  

NASA Astrophysics Data System (ADS)

The projections of small ice crystals (with maximum dimension <50 ?m) appear quasi-circular when imaged by probes on aircraft flying through cloud. Therefore, idealized models constructed to calculate their single-scattering properties have included quasi-spherical models such as Chebyshev particles, Gaussian random spheres, and droxtals. Recently, an ice analogue grown from sodium fluorosilicate solution on a glass substrate, with several columns emanating from a common center of mass, was shown to be quasi-circular when imaged by state-of-the-art cloud probes. In this study, a new idealized model, called the budding Bucky ball (3B) that resembles the shape of the small ice analogue is developed. The corresponding single-scattering properties (scattering phase function P11 and asymmetry parameter g) are computed by a ray-tracing code. Compared with previously used models, 3B scatters less light in the forward and more light in the lateral and backward directions. The Chebyshev particles and Gaussian random spheres show smooth and featureless P11, whereas droxtals and 3Bs, which have a faceted structure, show several peaks in P11 associated with angles of minimum deviation. Overall, the difference in the forward (lateral; backward) scattering between models are up to 22% (994%; 132%), 20% (510%; 101%), and 16% (146%; 156%) for small ice crystals with respective area ratios of 0.85, 0.77, and 0.69. The g for different models varies by up to 25%, 23%, and 19% for particles with area ratios of 0.85, 0.77, and 0.69, respectively. Because the single-scattering properties of small ice crystals depend both on the choice of the idealized model and the area ratios used to characterize the small ice crystals, higher resolution observations of small ice crystals or direct observations of their single-scattering properties are required.

Um, J.; McFarquhar, G. M.

2011-04-01

57

Structure and dynamics of amorphous water ice.  

PubMed

Further insight into the structure and dynamics of amorphous water ice, at low temperatures, was obtained by trapping in it Ar, Ne, H2, and D2. Ballistic water-vapor deposition results in the growth of smooth, approximately 1 x 0.2 micrometer2, ice needles. The amorphous ice seems to exist in at least two separate forms, at T < 85 K and at 85 < T < 136.8 K, and transform irreversibly from one form to the other through a series of temperature-dependent metastable states. The channels formed by the water hexagons in the ice are wide enough to allow the free penetration of H2 and D2 into the ice matrix even in the relatively compact cubic ice, resulting in H2-(D2-) to-ice ratios (by number) as high as 0.63. The larger Ar atoms can penetrate only into the wider channels of amorphous ice, and Ne is an intermediate case. Dynamic percolation behavior explains the emergence of Ar and Ne (but not H2 and D2) for the ice, upon warming, in small and big gas jets. The big jets, each containing approximately 5 x 10(10) atoms, break and propel the ice needles. Dynamic percolation also explains the collapse of the ice matrix under bombardment by Ar , at a pressure exceeding 2.6 dyn cm-2, and the burial of huge amounts of gas inside the collapsed matrix, up to an Ar-to-ice of 3.3 (by number). The experimental results could be relevant to comets, icy satellites, and icy grain mantles in dense interstellar clouds. PMID:9942788

Laufer, D; Kochavi, E; Bar-Nun, A

1987-12-15

58

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.

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

2007-01-01

59

Atmospheric Ice Crystals over the Antarctic Plateau in Winter.  

NASA Astrophysics Data System (ADS)

Falling ice crystals were collected daily on a gridded glass slide at South Pole Station, Antarctica, during the Antarctic winter of 1992 and were photographed through a microscope. Nine types of ice crystals are identified, which fall into three main categories: `diamond dust,' blowing snow, and snow grains. The dimensions of about 20 000 crystals were measured on scanned images of the photomicrographs. The predominant crystal types are hexagonal columns and plates (diamond dust) and rounded particles of blowing snow. Diamond-dust crystals have a large range of lengths (2-1000 ?m) and aspect ratios (0.1-100). Diamond-dust crystals can usually be classified as either columns or plates; nearly equidimensional crystals are rare. `Long prism' crystals with aspect ratios greater than 5 were collected often, and very long prisms (`Shimizu' crystals), 1000 ?m long but only 10 ?m thick, were collected occasionally. The extreme Shimizu crystals were predominant on only one winter day, but the meteorological conditions on that day were not unusual. Some precipitation was observed on every day; even when the dominant crystal type was blowing snow, there were always, in addition, some snow grains or diamond dust. Blowing-snow particles dominate by number and contribute nearly one-half of the total surface area. Bullet clusters and blowing snow each contribute about one-third of the total volume of atmospheric ice. Size distributions of the equivalent spherical radius are obtained for each of the nine crystal types, as well as for the three main categories of crystals, using the volume-to-area ratio to specify the equivalent spheres. In addition, the effective radius for each day when crystals were sampled is computed. Many of the distributions are approximately lognormal. The effective radius (area-weighted mean radius) of the entire size distribution of diamond dust is 12 ?m in winter, somewhat smaller than in summer (15 ?m). The small size of wintertime blowing snow allows it to reach heights of tens of meters in winter, as compared with only a few meters in summer. The average effective radius was 11 ?m for blowing snow and 24 ?m for snow grains. The most probable effective radius for any given day in winter is about 11 ?m.

Walden, Von P.; Warren, Stephen G.; Tuttle, Elizabeth

2003-10-01

60

Ice-binding structure and mechanism of an antifreeze protein from winter flounder.  

PubMed

Antifreeze proteins provide fish with protection against the freezing effect of polar environments by binding to ice surfaces and inhibiting growth of ice crystals. We present the X-ray crystal structure at 1.5 A resolution of a lone alpha-helical antifreeze protein from winter flounder, which provides a detailed look at its ice-binding features. These consist of four repeated ice-binding motifs, the side chains of which are inherently rigid or restrained by pair-wise side-chain interactions to form a flat binding surface. Elaborate amino- and carboxy-terminal cap structures are also present, which explain the protein's rich alpha-helical content in solution. We propose an ice-binding model that accounts for the binding specificity of the antifreeze protein along the <0112> axes of the (2021) ice planes. PMID:7760940

Sicheri, F; Yang, D S

1995-06-01

61

Computer simulation of bilayer ice: structures and thermodynamics  

NASA Astrophysics Data System (ADS)

A series of molecular dynamics simulations is performed in order to examine in more detail the results of a previous simulation which shows that a thin film of water, when confined to a hydrophobic slit nanopore, freezes into a bilayer ice crystal composed of two layers of hexagonal rings. Three simulations are carried out and each starts with a different initial configuration but has the same number of molecules and the area density. Using a previously introduced solid-like cluster definition, we monitor the dynamic process of crystallization. We find that only in one case the confined water completely freezes into perfect bilayer of ice whereas in other two cases, an imperfect crystalline structure consisting of hexagons of slightly different shapes is observed and this imperfection apparently hinders the growth of perfect bilayer of crystal. After adjusting the area density to match spatial arrangements of molecules, the latter two systems are able to crystallize completely. As a result, we obtain three forms of bilayer crystal differing in the area density and hexagonal rings alignment. Further analyses of these bilayer crystals provide more insightful explanation on the influence of the boundary condition and the simulation-cell size on the diversity of possible crystallographic structures.

Slovk, Jan; Tanaka, Hideki; Koga, Kenichiro; Zeng, Xiao C.

2003-03-01

62

Components of ice nucleation structures of bacteria.  

PubMed Central

Nonprotein components attached to the known protein product of the inaZ gene of Pseudomonas syringae have been identified and shown to be necessary for the most efficient ice nucleation of supercooled H2O. Previous studies have shown that cultures of Ina+ bacteria have cells with three major classes of ice-nucleating structures with readily differentiated activities. Further, some cells in the culture have nucleating activities intermediate between those of the different classes and presumably have structures that are biosynthetic intermediates between those of the different classes. Since these structures cannot be readily isolated and analyzed, their components have been identified by the use of specific enzymes or chemical probes, by direct incorporation of labeled precursors, and by stimulation of the formation of specific classes of freezing structures by selective additions to the growth medium. From these preliminary studies it appears that the most active ice nucleation structure (class A) contains the ice nucleation protein linked to phosphatidylinositol and mannose, probably as a complex mannan, and possibly glucosamine. These nonprotein components are characteristic of those used to anchor external proteins to cell membranes of eucaryotic cells and suggest that a similar but not identical anchoring mechanism is required for efficient ice nucleation structure. The class B structure has been found to contain protein presumably linked to the mannan and glucosamine moieties but definitely not to the phosphatidylinositol. The class C structure, which has the poorest ice nucleation activity, appears to be the ice nucleation protein linked to a few mannose residues and to be partially imbedded in the outer cell membrane. Images FIG. 1 FIG. 2 FIG. 5 FIG. 9 FIG. 15 FIG. 16

Turner, M A; Arellano, F; Kozloff, L M

1991-01-01

63

Artificial antifreeze polypeptides: ?-helical peptides with KAAK motifs have antifreeze and ice crystal morphology modifying properties  

Microsoft Academic Search

Antifreeze polypeptides from fish are generally thought to inhibit ice crystal growth by specific adsorption onto ice surfaces and preventing addition of water molecules to the ice lattice. Recent studies have suggested that this adsorption results from hydrogen bonding through the side chains of polar amino acids as well as hydrophobic interactions between the non-polar domains on the ice-binding side

Wei Zhang; Richard A. Laursen

1999-01-01

64

Ice island structure and drilling method  

Microsoft Academic Search

An off-shore ice island structure for location over a submerged drill site in waters which normally freeze in winter. The structure includes a buoyant protective caisson which freezes in position over the drill site upon onset of winter. A barge floats on water kept unfrozen within the caisson, and is connected to the caisson so it can be swivelled generally

1984-01-01

65

Nanoscale Ice: Spectroscopic Ellipsometry of Epitaxially-Grown Crystals  

NASA Astrophysics Data System (ADS)

A new laboratory technique has been developed to examine the surface characteristics and kinetics of ice crystals at the nanoscale. Uncertainties remain regarding the fundamental physics of nucleation and depositional growth in atmospheric ice crystals. These molecular-scale uncertainties propagate upward into modeling outcomes at all scales of atmospheric interest: particle models, cloud models, mesoscale models, and climate models. Molecular-scale growth mechanisms and kinetics have been mainly inferred from bulk and particle-scale experiments as well as crystal-growth theory. The precarious nature of the ice surface resisted the first generation of direct nanoscale probing technologies, but new in-situ techniques including ESEM, AFM, and ellipsometry promise to divulge a wealth of new knowledge. Spectroscopic ellipsometry measures changes in the polarization state of light as it reflects off the surface of a thin film. This non-destructive technique is capable of measuring layer thicknesses as small as a single monolayer (~1 ) and up to thicknesses of ~10 ?m. Other physical parameters including index of refraction and surface roughness are also accessible. At the TCNJ Cloud Physics Laboratory, a Horiba Scientific Auto-SE ellipsometer (440 - 1000 nm spectral range) has been adapted for in-situ measurements of ice crystals. The ice crystals are grown epitaxially on various horizontal substrates in a custom-built static diffusion chamber. The diffusion chamber is housed within a vacuum chamber and an optical path is provided from the ellipsometer light source to sample stage and back to the ellipsometer analyzer at 75 from normal. The diffusion chamber is cooled in two stages, with initial cooling accomplished with a fluid-chilled block and final chilling controlled by two independent thermoelectric cells. A wide range of temperatures, pressures, and saturation ratios are accessible: from 0C to -30C, 50mb to atmospheric pressure, and from subsaturated to greater that 200% RHi. Temperature and moisture profiles are continuously determined by platinum resistance thermometers. Optimization of cooling efficiencies are under way and should permit extension of temperature range to -60C. Ongoing efforts are targeted at kinetic measurements of thickness changes in order to identify growth thresholds as a function of ambient conditions and nucleation mechanism.

Cumiskey, A.; Grippaldi, J.; Magee, N. B.

2011-12-01

66

An uncoupled multiphase approach towards modeling ice crystals in jet engines  

NASA Astrophysics Data System (ADS)

A recent series of high altitude turbofan engine malfunctions, characterized by flameout and sudden power losses have been reported in recent years. The source of these incidents has been hypothesized to be due to the presence of ice crystals at high altitudes. Ice crystals have been shown to have ballistic trajectories and consequently enter the core engine flow, without getting centrifuged out towards the engine bypass as droplets do. The crystals may melt as they move downstream to higher temperatures in successive stages, or hit a heated surface. The wetted surface may then act as an interface for further crystal impingement, which locally reduces the temperature and could lead to an ice accretion on the components. Ice can accrete to dangerously high levels, causing compressor surge due to blockage of the primary flowpath, vibrational instabilities due to load imbalances of ice on rotating components, mechanical damage of components downstream due to large shed ice fragments, or performance losses if ice enters the combustor, causing a decreased burner efficiency and an eventual flame-out. In order to provide a numerical tool to analyze such situations, FENSAP-ICE has been extended to model mixed-phase flows that combine air, water and ice crystals, and the related ice accretion. DROP3D has been generalized to calculate particle impingement, concentration, and field velocities in an uncoupled approach that neglects any phase change by assuming both ice crystals and supercooled droplets are in thermodynamic equilibrium. ICE3D then accounts for the contribution of ice crystals that stick and melt on an existing water-film and promote ice accretion. The extended ice crystal impingement and ice accretion model has been validated against test data from Cox and Co. and National Research Council icing tests conducted on a NACA0012 airfoil and unheated non-rotating cylinder respectively. The tests show a consistent agreement with respect to experimental profiles in terms of capturing the overall shape, although some of the ice profiles were conservative since they over-predicted the amount of ice accreted. The experimental observations suggest that ice crystals cause splashing of an existing film, and erosion effects when they impact an iced surface, and cause an overall loss in the amount of ice, as well as a general streamlining of the ice profile. This has not been taken into account in the present numerical model. The overall predictions in comparison with other numerical models, however, have improved and are a promising step towards simulating ice-shedding characteristics in a turbomachine.

Nilamdeen, Mohamed Shezad

67

Ice island structure and drilling method  

SciTech Connect

An off-shore ice island structure for location over a submerged drill site in waters which normally freeze in winter. The structure includes a buoyant protective caisson which freezes in position over the drill site upon onset of winter. A barge floats on water kept unfrozen within the caisson, and is connected to the caisson so it can be swivelled generally about a vertical axis to adjust the circumferential location of the drilling axis of drilling apparatus carried on the barge. The drilling apparatus is movable relative to the barge to enable further adjustment of the drilling axis location. The arrangement enables the drilling axis to be maintained in substantial vertical alignment with the drill site despite movement of the caisson caused by the surrounding shelf ice. The caisson is part of an ice island structure whose mass is built up by successive flooding and freezing steps to ground it on the sea bed. The capability for fixing the location of the drilling axis despite shelf ice movement permits drilling operations to commence long prior to grounding of the ice island. Various arrangements are disclosed for moving the barge from within the caisson for reuse at another drill site.

Bishop, G.H.

1984-06-26

68

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 m) 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; Clau, Tina; Niedermeier, Dennis; Hartmann, Susan; Wex, Heike; Stratmann, Frank

2010-05-01

69

Mitigation of ice crystallization fouling in stationary and circulating liquidsolid fluidized bed heat exchangers  

Microsoft Academic Search

Liquidsolid fluidized bed heat exchangers are attractive ice crystallizers since they are able to mitigate ice crystallization fouling and exhibit high heat transfer coefficients. Experiments show that the fouling removal ability of stationary fluidized beds increases with decreasing bed voidage (9580%) and increasing particle size (24mm). The removal of ice crystallization fouling appears to be more effective in circulating fluidized

P. Pronk; C. A. Infante Ferreira; G. J. Witkamp

2010-01-01

70

Numerical Simulation of Three-Dimensional Unsteady Flow past Ice Crystals  

Microsoft Academic Search

The unsteady flow fields around falling columnar ice crystals, hexagonal ice plates, and broad-branch crystals are simulated by numerically solving the time-dependent Navier-Stokes equations appropriate for these geometries in the primitive equation form. A predictor-corrector method and a quadratic interpolation for convective kinematics (QUICK) scheme are applied on nonuniform grids to determine the velocity fields. The ice crystals are held

Pao K. Wang; Wusheng Ji

1997-01-01

71

Modeling the effects of frazil ice crystals on the dynamics and thermodynamics of Ice Shelf Water plumes  

NASA Astrophysics Data System (ADS)

The seawater that comes into contact with the base of a floating ice shelf is modified as a result of the phase changes that occur. Melting is prevalent in the deepest parts of the subice cavity, and this drives a buoyant flow of Ice Shelf Water (ISW) along the sloping ice shelf base. The ascent of the ISW toward the surface of the ocean causes supercooling, because the freezing point rises with the falling pressure, and this induces a change from melting to freezing. Assuming that seed crystals exist, the ISW now fulfills the condition for the rapid growth of disc-shaped frazil ice crystals, which may subsequently settle (upward) out of suspension under the action of gravity. A simple numerical model of these processes has been developed, based on the theory of inclined plumes. The ISW is treated as a turbulent, particle-laden gravity current ascending a reactive boundary and containing a suspended crystal load which evolves in response to the supercooling of the water and the inverted sedimentation of the crystals. The frazil ice has two important effects on the behavior of the ISW plume. Because the generation of crystals through-out the plume provides such a large area over which phase changes can occur, the conversion of supercooling into ice happens much more readily than is possible through turbulent transfer of heat and mass at the ice shelf base. A suspended crystal load reduces the bulk density of the ISW, so that a growing suspension causes the plume to accelerate, while deposition of crystals onto the ice shelf effects a deceleration. There are positive feedbacks in that an acceleration of the plume induces more rapid crystal growth, while a deceleration allows suspended crystals to settle out more easily.

Jenkins, Adrian; Bombosch, Andreas

1995-04-01

72

Cryo-Microscopic Analysis of the Effects of Extra Cellular Proteins on Polycrystalline Ice Structure  

NASA Astrophysics Data System (ADS)

Recent work has demonstrated that microorganisms can occupy the liquid filled inter-crystalline veins in ice and maintain their metabolic activity under these conditions. While these discoveries have increased the extent of the biosphere to include the large continental ice sheets of Antarctica and Greenland as biomes, the habitat of the microorganisms within the inter-crystalline liquid veins is poorly understood. Certain cold tolerant organisms produce extra cellular proteins (i.e., ice-binding proteins) that have the ability to bind to the prism face of an ice crystal and inhibit recrystallization of ice. This phenotype affects the physical ice structure and the liquid vein network, potentially providing ice-inhabiting species a protective mechanism with which to control their habitat. One such microorganism is Chryseobacterium sp. V3519-10, a bacterium isolated from a depth of 3519 m in the Vostok Ice Core. Our investigation is examining the impact of extra cellular proteins from this ice-adapted bacterium on the formation of ice crystals and characterizing the inter-crystalline liquid filled vein network using cryo-microscopy.

Brox, T.; Skidmore, M. L.; Christner, B. C.; Achberger, A.

2010-12-01

73

Crystal structure of magnesioneptunite  

NASA Astrophysics Data System (ADS)

The crystal structure of the new mineral magnesioneptunite (K0.8Na0.1-0.1)Na2Li(Ti0.39Mg0.34Fe0.27)2(Ti0.59Mg0.22Fe0.19)2[Si4O11]2(O,OH) from the xenolith of Verkhnechegemskaya caldera (Lakarga Mountain, North Caucasus) has been investigated by X-ray diffraction (XCalibur-S diffractometer, R = 0.0244): a = 16.3271(7) , b = 12.4788(4) , c = 9.9666(4) ? = 115.651(5), V = 1830.5(1) 3, sp. gr. C2/ c, Z = 4, and ?calcd = 3.152 g/cm3. The disordered distribution of Ti, Mg, and Fe atoms in the octahedra forming the basis of the cationic framework is established. It is shown that the isomorphic occupation of octahedral positions by cations of three types corresponds to the centrosymmetric crystal structure and is likely caused by the high-temperature crystallization of the mineral.

Karimova, O. V.; Yakubovich, O. V.; Zadov, A. E.; Ivanova, A. G.; Urusov, V. S.

2012-07-01

74

Study of Ice Crystal Formation in Tubes Under Steady Flow Conditions as Relating to Freeze Desalination.  

National Technical Information Service (NTIS)

A test loop was assembled to investigate several designs for indirect freeze crystallization of brine solutions. Various parameters were monitored to determine the conditions that allow successful production of ice crystals in the flowing brine. At certai...

W. S. Schoerner T. R. Carbery T. J. Nowak

1981-01-01

75

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

76

Growth Rates and Habits of Ice Crystals between -20 and -70C  

Microsoft Academic Search

A laboratory study of ice crystal growth characteristics at temperatures between -20 and -70C has been performed at ice supersaturations and pressures comparable with those in the atmosphere using a horizontal static diffusion chamber. Maximum dimension, projected area, and volume growth rates, in addition to habit frequency, have been measured for individual habit types as functions of temperature, ice supersaturation,

Matthew Bailey; John Hallett

2004-01-01

77

Fluidized bed heat exchangers to prevent fouling in ice slurry systems and industrial crystallizers  

Microsoft Academic Search

Ozon layer depletion and global warming by synthetic refrigerants forces refrigeration industries to switch over to natural but hazardous refrigerants like ammonia and hydrocarbons. A promising technology to safely use the latter refrigerants is the application of indirect refrigeration systems with ice slurry as heat transfer fluid. Ice slurry, a suspension of aqueous solution and small ice crystals, has a

P. Pronk

2006-01-01

78

Vaterite crystals contain two interspersed crystal structures.  

PubMed

Calcite, aragonite, and vaterite are the three anhydrous polymorphs of calcium carbonate, in order of decreasing thermodynamic stability. Although vaterite is not commonly found in geological settings, it is an important precursor in several carbonate-forming systems and can be found in biological settings. Because of difficulties in obtaining large, pure, single crystals, the crystal structure of vaterite has been elusive for almost a century. Using aberration-corrected high-resolution transmission electron microscopy, we found that vaterite is actually composed of at least two different crystallographic structures that coexist within a pseudo-single crystal. The major structure exhibits hexagonal symmetry; the minor structure, existing as nanodomains within the major matrix, is still unknown. PMID:23620047

Kabalah-Amitai, Lee; Mayzel, Boaz; Kauffmann, Yaron; Fitch, Andrew N; Bloch, Leonid; Gilbert, Pupa U P A; Pokroy, Boaz

2013-04-26

79

Tropical tropopause ice clouds: a dynamic approach to the mystery of low crystal numbers  

NASA Astrophysics Data System (ADS)

The occurrence of high, persistent ice supersaturation inside and outside cold cirrus in the tropical tropopause layer (TTL) remains an enigma that is intensely debated as the "ice supersaturation puzzle". However, it was recently confirmed that observed supersaturations are consistent with very low ice crystal concentrations, which is incompatible with the idea that homogeneous freezing is the major method of ice formation in the TTL. Thus, the tropical tropopause "ice supersaturation puzzle" has become an "ice nucleation puzzle". To explain the low ice crystal concentrations, a number of mainly heterogeneous freezing methods have been proposed. Here, we reproduce in situ measurements of frequencies of occurrence of ice crystal concentrations by extensive model simulations, driven by the special dynamic conditions in the TTL, namely the superposition of slow large-scale updraughts with high-frequency short waves. From the simulations, it follows that the full range of observed ice crystal concentrations can be explained when the model results are composed from scenarios with consecutive heterogeneous and homogeneous ice formation and scenarios with pure homogeneous ice formation occurring in very slow (< 1 cm s-1) and faster (> 1 cm s-1) large-scale updraughts, respectively. This statistical analysis shows that about 80% of TTL cirrus can be explained by "classical" homogeneous ice nucleation, while the remaining 20% stem from heterogeneous and homogeneous freezing occurring within the same environment. The mechanism limiting ice crystal production via homogeneous freezing in an environment full of gravity waves is the shortness of the gravity waves, which stalls freezing events before a higher ice crystal concentration can be formed.

Spichtinger, P.; Krmer, M.

2013-10-01

80

Stable growth mechanisms of ice disk crystals in heavy water.  

PubMed

Ice crystal growth experiments in heavy water were carried out under microgravity to investigate the morphological transition from a disk crystal to a dendrite. Surprisingly, however, no transition was observed, namely, the disk crystal or dendrite maintained its shape throughout the experiments, unlike the results obtained on the ground. Therefore, we introduce a growth model to understand disk growth. The Gibbs-Thomson effect is taken into account as a stabilization mechanism. The model is numerically solved by varying both an interfacial tension of the prism plane and supercooling so that the final sizes of the crystals can become almost the same to determine the interfacial tension. The results are compared with the typical experimental ones and thus the interfacial tension is estimated to be 20 mJ/m(2). Next, the model is solved under two supercooling conditions by using the estimated interfacial tension to understand stable growth. Comparisons between the numerical and experimental results show that our model explains well the microgravity experiments. It is also found that the experimental setup has the capability of controlling temperature on the order of 1/100 K. PMID:22181428

Adachi, Satoshi; Yoshizaki, Izumi; Ishikawa, Takehiko; Yokoyama, Etsuro; Furukawa, Yoshinori; Shimaoka, Taro

2011-11-22

81

An uncoupled multiphase approach towards modeling ice crystals in jet engines  

Microsoft Academic Search

A recent series of high altitude turbofan engine malfunctions, characterized by flameout and sudden power losses have been reported in recent years. The source of these incidents has been hypothesized to be due to the presence of ice crystals at high altitudes. Ice crystals have been shown to have ballistic trajectories and consequently enter the core engine flow, without getting

Mohamed Shezad Nilamdeen

2010-01-01

82

75 FR 8116 - Notice Pursuant to the National Cooperative Research and Production Act of 1993-Ice Crystal...  

Federal Register 2010, 2011, 2012, 2013

...National Cooperative Research and Production Act of 1993--Ice Crystal Consortium Notice is hereby given that, on December 31, 2009...15 U.S.C. 4301 et seq. (``the Act''), the Ice Crystal Consortium (``ICC'') has filed written...

2010-02-23

83

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

Microsoft Academic Search

We performed correlation analysis involving ice water content (IWC) and mean effective ice crystal size (De) intended for application to climate models. For this purpose, ice crystal size distributions obtained from in situ measurements conducted from numerous field campaigns in the tropics, midlatitude, and Arctic regions were used and we show that IWC and De are well-correlated in this regional

K. N. Liou; Y. Gu; Q. Yue; G. McFarguhar

2008-01-01

84

Ice/hydrohalite crystallization structures in sub-eutectic freezing experiments in the system NaCl-H20 and possible implications for the properties of frozen brines in Europa: A preliminary report  

NASA Astrophysics Data System (ADS)

Sulfates are likely to be the most abundant solutes in the subsurface Europan liquid ocean. NaCl may also be a significant component of such liquids based on the compositions of stony meteorites like those thought to be among the source materials for the silicates in Europa's interior. The system NaCl-H20 exhibits a eutectic at -20.8C and 23.3 weight percent NaCl between ice Ih and hydrohalite (NaCl.2H20). This low eutectic temperature compared to Mg and Na sulfate hydrate/ice eutectics indicates that hydrohalite should be among the last salts to crystallize in brine upwellings along rifts and other places where resurfacing by melt extrusion occurs on Europa. We conducted a suite of freezing experiments on NaCl brines with 20.3, 23.3, and 26.6 (saturated) weight percent NaCl by holding these liquids at a few degrees below the eutectic temperature. These runs produced ice-rich, eutectic and hydrohalite-rich aggregates of both phases, respectively, as confirmed by cryogenic x-ray diffraction and x-ray fluorescence spectroscopy. Based on direct observations of crystals forming at the tops and bottoms of the sample chambers and on refractive index measurements of subsequently melted sample material, marked fractional crystallization and segregation by density of ice, hydrohalite, and residual liquids occurred in the 20.3 and 26.6% samples and less so in for the eutectic composition. Crystallization of very fine grained eutectic intergrowths was recognized in cryogenic SEM images of all these samples and they were especially prominent in samples frozen from saturated brine. These samples were very difficult to cleave compared to pure polycrystalline ice, and hence are likely to have high fracture toughness. Direct measurements of this property and also the effects of partial melting on ductile flow rates are planned on such samples. Refracturing of such regions of fine eutectoid ice/hydrohalite intergrowths is likely to be inhibited in refrozen rifts compared to more ice-rich regions on Europa.

Rieck, K.; Kirby, S. H.; Stern, L. A.

2005-12-01

85

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

2013-01-08

86

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

PubMed Central

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.

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

2013-01-01

87

Light scattering by absorbing hexagonal ice crystals in cirrus clouds.  

PubMed

An improved ray-optics theory for single scattering and polarization of hexagonal columns and plates randomly oriented in space has been developed by considering absorption and by using the Chebyshev solution for diffraction integrals. The vector-tracing method and statistics technique of random sampling are employed. The equivalent forms of Snell's law and Fresnel formulas for absorbing ice crystals are derived, and two equivalent optical constants, m' and m?, are obtained. Comparison is made of the computed results of our model and the Takano and Liou model for asymmetry factors, single-scattering albedos, and scattering phase matrix elements. Some characteristics of our model are discussed, and these analyses demonstrate that our ray-optics model is practical and much improved. PMID:21060422

Zhang, J; Xu, L

1995-09-01

88

Cold Regions Science and Engineering Monograph 3, Section Blb: Ice Pressure on Engineering Structures.  

National Technical Information Service (NTIS)

The monograph summarizes existing knowledge on forces exerted by an expanding ice sheet, impact forces of ice on structures, and vertical forces exerted by ice on hydraulic structures. Sections are also devoted to icebreakers and ice models. (Author)

B. Michel

1970-01-01

89

Crystal Structure Lab  

NSDL National Science Digital Library

This activity is concerned with crystals, the basic building units that make up rocks and minerals. Students construct a model of the silicon-oxygen tetrahedron and discover that the smallest whole unit that could form a unique crystal is called a unit cell. They learn that a unit cell would have all the properties of a large crystal such as a diamond, but would be only molecular size (submicroscopic). If a crystal starts to form from a slowly cooling magma or from a drying up pool of salty sea water, unit cells add themselves one on top of another in order to develop the large crystals we can see and handle.

Fetcho, Ray

90

Morphological investigations of disaccharide molecules for growth inhibition of ice crystals  

NASA Astrophysics Data System (ADS)

Freezing of solutions including disaccharides (trehalose, sucrose, and maltose) has been investigated by microscopic observations of freeze-fractured replicas using FE-TEM. Three typical features were observed: the smooth surface considered as the ice crystal, fine particles as the precipitated disaccharide molecules, and remaining part as the glass state of the solution. The expanded observations of fine particle and its distribution investigations suggested that it was larger than 10 nm in size and averaged approximately 20 30 nm in diameter. The smallest particle was estimated to include several hundred disaccharide molecules. Based on systematic observations of trehalose solutions regarding concentrations and freezing rates, we concluded that ice crystal growth was inhibited by trehalose molecules. Since the ice crystal size reduced exponentially with increase in trehalose concentration, we could control ice crystal size formed in the frozen material. The growth inhibition of ice crystals with trehalose resulted both from a reduction in the free water in the solution due to a significant hydration effect and from an enhancement of nucleation of the ice crystals. It was confirmed that trehalose was more effective than the other disaccharide solutions examined for inhibiting the growth of ice crystals.

Uchida, Tsutomu; Nagayama, Masafumi; Shibayama, Tamaki; Gohara, Kazutoshi

2007-02-01

91

Using polarimetric remote sensing measurements to estimate ice particle size, optical depth and ice water path during CRYSTAL-FACE  

NASA Astrophysics Data System (ADS)

In situ observations made during the CRYSTAL-FACE field experiment have indicated that ice crystals have smaller sizes and are more reflective than is commonly assumed in most current climate models. The size of the particles appears to be principally determined by temperature with the smallest particles being found at the coldest temperatures. Previous analyses of polarimetric measurements in non-absorbing bands have suggested that either bubble inclusions (inhomogeneous hexagonal mono-crystals) or distortions of the hexagonal crystal shape (distorted chain aggregates) are responsible for the observed general absence of haloes, smooth angular variation of reflectance and brightness of ice clouds. In this paper we use multi-angle measurements made by the Research Scanning Polarimeter (RSP) to examine the polarized and unpolarized reflectance of cirrus clouds in bands where ice is non-absorbing (670 and 865 nm) and absorbing (1590, 1880 and 2250 nm). During CRSYTAL-FACE the RSP scan was biased so that the view angle range was from 0 to 75 degrees to the rear of the Proteus aircraft and from 0 to 45 degrees to the front and was oriented to scan along the groundtrack of the aircraft. This allowed observations of a single target over a wide scattering angle range particularly when consecutive flight legs could be combined which allows for basic discrimination of crystal habit using the non-absorbing bands, similar to previous studies (at least in the gross sense of being able to separate columns from plates from distorted crystals from spheroidal shapes). However, compared with non-absorbing bands, the reflectance in absorbing bands is different depending on whether ice particles are geometrically distorted or contain air bubble inclusions because the path length of light inside an ice crystal is quite short which limits scattering off the bubble inclusions. Consequently the retrieved particle size is also sensitive to whether ice crystals are modeled as being distorted or containing air bubbles. We examine how the RSP size retrievals, with an appropriate vertical weighting determined by Green's function calculations, compare with in situ measurements and examine the angular and spectral polarized and unpolarized residuals from the retrievals. This allows us to identify the most appropriate crystal habit for use in the remote sensing of cirrus clouds formed by convection over land, such as those observed during CRYSTAL-FACE, and provide best estimates for the particle size, optical depth and ice water path determined using solar reflectance measurements.

Geogdzhayev, I.; Cairns, B.; Mishchenko, M. I.; Travis, L. D.

2006-12-01

92

Growth of epitaxial ice crystals on covellite (CuS) under reduced air pressure  

Microsoft Academic Search

The advance velocity of nonthickening crystals and basal steps were measured as a function of temperature, supersaturation, and air pressure, using an optical interference technique. Ice crystal growth at -7 C in a thermal diffusion chamber was also investigated as a function of supersaturation. The radial growth rates of nonthickening crystals and the advance velocity of a 0.08 micrometer step

N. K. Cho

1982-01-01

93

Importance of small ice crystals to cirrus properties: Observations from the Tropical Warm Pool International Cloud Experiment (TWP-ICE)  

NASA Astrophysics Data System (ADS)

During the Department of Energy Atmospheric Radiation Measurement Program (DOE ARM) sponsored Tropical Warm Pool International Cloud Experiment (TWP-ICE), ice crystals with maximum dimensions (D) < 50 ?m were measured in aged cirrus and fresh anvils by a Cloud and Aerosol Spectrometer (CAS) and a Cloud Droplet Probe (CDP). The CAS/CDP ratio of the number concentrations of droplets with 3 < D < 50 ?m, N3-50, averaged 0.98 +/- 0.69 in liquid clouds. However, N3-50, measured by the CAS averaged 91 +/- 127 times larger than N3-50 from the CDP in ice clouds. The CAS/CDP N3-50 ratio had a correlation coefficient of 0.387 with the concentration of particles with D > 100 ?m measured by the Cloud Imaging Probe, suggesting that ice crystals may have been shattering or bouncing on the CAS inlet or protruding airflow shroud enhancing N>3-50,CAS. During the Costa Rica Aura Validation Experiment N3-50,CAS measured by a CAS without an airflow shroud were an order of magnitude less than those observed during TWP-ICE. This, and estimates of the maximum shattering based on the inlet and shroud sizes, suggest that the airflow shroud used during TWP-ICE was responsible for much of the shattering or bouncing.

McFarquhar, Greg M.; Um, Junshik; Freer, Matt; Baumgardner, Darrel; Kok, Gregory L.; Mace, Gerald

2007-07-01

94

Growthmelt asymmetry in ice crystals under the influence of spruce budworm antifreeze protein  

Microsoft Academic Search

Here we describe studies of the crystallization behavior of ice in an aqueous solution of spruce budworm antifreeze protein (sbwAFP) at atmospheric pressure. SbwAFP is an ice binding protein with high thermal hysteresis activity, which helps protect Choristoneura fumiferana (spruce budworm) larvae from freezing as they overwinter in the spruce and fir forests of the north eastern United States and

Natalya Pertaya; Yeliz Celik; Carlos L DiPrinzio; J S Wettlaufer; Peter L Davies; Ido Braslavsky

2007-01-01

95

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

Microsoft Academic Search

This study uses a box model and a Lagrangian microphysical parcel model to investigate the influences of radiative heating and cooling on the vapor diffusional growth of liquid drops and ice crystals within mixed-phase clouds. Without radiative effects, the combined influences of drop and ice vapor diffusion lead to slight supersaturations with respect to liquid despite the rapid growth of

Z. J. Lebo; N. C. Johnson; J. Y. Harrington

2008-01-01

96

The Conversion of Aircraft Ice Crystal Measurements into Terms of Liquid Water Using Simulated Data.  

National Technical Information Service (NTIS)

This investigation presents simulated environments of ice crystals to computer analysis as a means of imitating particle measurements made by electro-optical devices such as the Particle Measuring System's 1-D instrument. A mathematical model is developed...

R. O. Berthel

1981-01-01

97

Nanoscale structure of the magnetic induction at monopole defects in artificial square 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); (Northwestern Univ.); (Carnegie Mellon Univ.)

2011-05-01

98

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

99

Determination of the Combined Ventilation Factor and Capacitance for Ice Crystal Aggregates from Airborne Observations in a Tropical Anvil Cloud  

Microsoft Academic Search

The ventilation factor and capacitance used in numerical models to represent ice crystal aggregates directly affects the growth rate of the ice crystal aggregates, and consequently the sink of atmospheric water vapor. Currently, numerical models that prognose ice water content (IWC) and water vapor mixing ratio represent the capacitance and ventilation factor of precipitation-sized particles with simplified geometries, such as

Paul R. Field; J. Heymsfield; Aaron Bansemer; Cynthia H. Twohy

2008-01-01

100

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 96115 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.

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

2012-01-01

101

Geometric-optics integral-equation method for light scattering by nonspherical ice crystals  

Microsoft Academic Search

A new geometric-optics model has been developed for the calculation of the single-scattering and polar- ization properties for arbitrarily oriented hexagonal ice crystals. The model uses the ray-tracing tech- nique to solve the near field on the ice crystal surface, which is then transformed to the far field on the basis of the electromagnetic equivalence theorem. From comparisons with the

Ping Yang; K. N. Liou

1996-01-01

102

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

103

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; Krmer, Martina

2013-04-01

104

Dynamic interactions between floating ice and offshore structures  

SciTech Connect

The interactions between an offshore structure and floating ice are investigated. Attention is directed to the case where ice fails by crushing at the face of the structure. A formulation suitable for investigating the dynamic response of platform systems with surrounding floating ice sheets or ice features is developed and applied. Material properties of freshwater- and sea-ice relevant to the problem are first reviewed and summarized. Korzhavin's crushing formula, which is the basis of the proposed analytical model, is then studied in detail. The review discusses plasticity theory, yield criteria for ice, strain rate, the contact factor and the shape factor. Three types of problems are studied in order to demonstrate the effectiveness of the formulation and to gain some insights into the physical phenomena: 1) the impact of large ice floes against fixed gravity-type offshore drilling platforms; 2) the response of slender stuctures subjected to continuous crushing by a uniform ice sheet; and 3) the earthquake response of an offshore structure in the presence of floating ice. Under the assumed conditions, the ice is found to have a favorable effect.

Croteau, P.

1983-01-01

105

Spray-ice islands evaluated for Arctic-drilling structures  

Microsoft Academic Search

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

Juvkam-Wold

1986-01-01

106

On the effects of ice crystal porosity on the radiative characteristics of cirrus clouds  

NASA Astrophysics Data System (ADS)

This paper attempts to illustrate how the porous nature of ice crystals might influence the relationships between the radiative properties of ice clouds and the ice water path. Solutions of the electromagnetic wave equation were obtained for a simplified, hollow, infinitely long circular cylinder. On the basis of these solutions it was implied that the effects of ice crystal porosity on the particle-scattering parameters can be expected to be small and that the major influence of porosity on radiation-ice water path (IWP) relationships will be primarily on the definition of IWP. It is suggested that this effect probably accounts for the existing discrepancy noted in this paper between the theoretical and observed ?-IWP relationships.

Stephens, Graeme L.

1987-04-01

107

Critical Supersaturation for Ice Crystal Growth: Laboratory Measurements and Atmospheric Modeling Implications  

NASA Astrophysics Data System (ADS)

An improved understanding of ice crystal growth, particularly at low temperatures, is much in demand for the advancement of numerical modeling of atmospheric processes. Cirrus models must contend with the complexity of ice crystals growing in cold temperatures, low pressures, low supersaturations, and with multiple nucleation mechanisms. Recent observations have allowed increasingly realistic parameterizations of cirrus ice crystal microphysics, but these observations need to be supplemented by a fundamental understanding of growth processes affecting low-temperature crystals. Several experimental studies have demonstrated that certain ice crystals require a minimum "critical" supersaturation before exhibiting detectable growth. These crystals are presumed to be essentially defect-free, preventing vicinal hillock growth at the site of crystal dislocations. In the case of crystal growth by spiral dislocation, advancement of faces begins as soon as supersaturation is present. The finding of conditional critical supersaturations have analogies in other materials (metals, semiconductors, potassium dihydrogen phosphate) and are thermodynamically predicted given a two-dimensional nucleation growth mechanism. Previous measurements have determined the critical supersaturation for ice as a function of temperature and crystallographic face from 0 to --15 C with extrapolation to --30 C. For both basal and prism faces, critical supersaturation is seen to increase with decreasing temperature, suggesting that low-temperature, low-supersaturation processes are most likely to be affected by this critical contingency. We present laboratory results to verify and extend prior critical supersaturation measurements using a novel approach for supersaturation generation, control, and measurement. The crystals are grown on the tip of a fine glass fiber ( 10 microns in diameter) under varying conditions of temperature, pressure, and saturation. Supersaturation is generated when a pre-saturated airflow passes over a coil of ice warmed by electrical resistance upstream from the growing crystal. Supersaturation is determined by a system of differential thermocouples calibrated to sulfuric acid drop size measurements. Measurements follow those made in earlier studies, but also extend to temperatures of --45 C, mimicking conditions found in some high altitude clouds.

Magee, N.; Moyle, A.; Lamb, D.

2003-12-01

108

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

109

Experimental investigation of the interactions of hyperactive antifreeze proteins with ice crystals  

NASA Astrophysics Data System (ADS)

Antifreeze proteins (AFPs) evolved in cold-adapted organisms and serve to protect them against freezing cold conditions by arresting ice crystal growth and inhibiting ice recrystallization. The freezing point depression by AFPs is defined as thermal hysteresis (TH) and AFPs are classified as hyperactive (hypAFPs) and moderate according to their TH activities. The mechanism of action of AFPs is not well understood. In particular, it is not clear what determines the concentration dependence of TH and whether the binding of AFP to ice is irreversible. Additionally, it is not known why some types of AFP are hyperactive compared to others and it was suggested that hyperactivity might be related to basal plane affinity of hypAFP to ice. The present study utilizes the techniques of microfluidic devices and fluorescence microscopy to study the interaction of AFPs with ice crystals. With novel temperature controlled microfluidic devices, we showed the accumulation and affinity of hypAFPs on the basal plane of ice. This supports the view that hypAFPs adhere to the basal plane. Additionally, for the first time in literature, small ice crystals of 30-50 mum sizes covered with adsorbed GFP tagged hypAFPs were stabilized in supercooled non-AFP solutions for hours with no observed ice growth in temperature controlled microfluidic devices. Repeated TH experiments of ice crystals incubated in AFP solutions before and after the exchange of liquids in microfluidic devices gave the same TH activity. This finding clarifies our understanding of concentration dependence of TH. Furthermore, we found that hypAFPs protect ice against melting as well as freezing, resulting in superheated ice. Ice crystals were superheated up to 0.5C above their equilibrium melting temperatures and remained stable in this superheated state for hours. Measurements of fast melting velocities added additional evidence to the observed superheating of ice in AFP solutions. The experimental results of the current study provide strong evidence that AFPs bind to ice surfaces via irreversible binding. We have demonstrated that the use of microfluidics in combination with fluorescence microscopy is a valuable technique to study the binding mechanisms of AFPs and the concentration dependence of AFP activity.

Celik, Yeliz

110

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

111

The capacitance of pristine ice crystals and aggregate snowflakes  

Microsoft Academic Search

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)

C. D. Westbrook; R. J. Hogan; A. J. Illingworth

2006-01-01

112

Aircraft Measurements of Icing in Supercooled and Water Droplet\\/Ice Crystal Clouds  

Microsoft Academic Search

Icing measurements were carried out in Spain during the Precipitation Enhancement Project experiment in 1979, with an instrumented DC-7 aircraft.The energy balance at the riming surface of a cylinder allows the prediction of the ice growth regime (wet or dry) for the present measurements with 85% success.The mixed conditions encountered show that the ice phase plays an important role in

M. Bain; J. F. Gayet

1982-01-01

113

Measurements of the aerosol and ice crystal populations in tropical stratospheric cumulonimbus anvils  

NASA Astrophysics Data System (ADS)

A pair of particle size spectrometers was flown aboard the NASA U-2 operating from the Canal Zone to make measurements of the aerosol and ice crystal budgets in cirrus produced by thunderstorms in the tropics. Measurements indicate that fairly large crystals up to 1 mm in size are injected into the stratosphere during cumulonimbus activity. Ice water contents range from a few thousandths to a few hundredths of a gram per cubic meter. Because the ambient temperature is typically around -80C the mass of the larger crystals largely returns to lower altitudes before evaporating. Aerosol size distributions indicate a curious narrow growth mode between 0.15 and 0.2 m in cirrus anvils which is absent outside these clouds. The presence of this narrow mode is attributed to near water saturation produced at cloud top. Nucleation of new ice crystals as well as aerosol is hypothesized.

Knollenberg, R. G.; Dascher, A. J.; Huffman, D.

114

Crystallization, melting, and structure of water nanoparticles at atmospherically relevant temperatures.  

PubMed

Water nanoparticles play an important role in atmospheric processes, yet their equilibrium and nonequilibrium liquid-ice phase transitions and the structures they form on freezing are not yet fully elucidated. Here we use molecular dynamics simulations with the mW water model to investigate the nonequilibrium freezing and equilibrium melting of water nanoparticles with radii R between 1 and 4.7 nm and the structure of the ice formed by crystallization at temperatures between 150 and 200 K. The ice crystallized in the particles is a hybrid form of ice I with stacked layers of the cubic and hexagonal ice polymorphs in a ratio approximately 2:1. The ratio of cubic ice to hexagonal ice is insensitive to the radius of the water particle and is comparable to that found in simulations of bulk water around the same temperature. Heating frozen particles that contain multiple crystallites leads to Ostwald ripening and annealing of the ice structures, accompanied by an increase in the amount of ice at the expense of the liquid water, before the particles finally melt from the hybrid ice I to liquid, without a transition to hexagonal ice. The melting temperatures T(m) of the nanoparticles are not affected by the ratio of cubic to hexagonal layers in the crystal. T(m) of the ice particles decreases from 255 to 170 K with the particle size and is well described by the Gibbs-Thomson equation, T(m)(R) = T(m)(bulk) - K(GT)/(R - d), with constant K(GT) = 82 5 Knm and a premelted liquid of width d = 0.26 0.05 nm, about one monolayer. The freezing temperatures also decrease with the particles' radii. These results are important for understanding the composition, freezing, and melting properties of ice and liquid water particles under atmospheric conditions. PMID:22452637

Johnston, Jessica C; Molinero, Valeria

2012-04-05

115

Structural and dynamic changes of Wilkins Ice Shelf, Antarctic Peninsula  

NASA Astrophysics Data System (ADS)

Wilkins Ice Shelf (WIS) has shown considerable ice front retreat since 1990. This retreat includes various break-up events, such as recently in 2008 (Feb: 425 km, May: 160 km, Jul: 1220 km) and in 2009 (790 km). The break-up events took place under contrasting surface conditions, which indicates potentially different mechanisms for break-up. WIS shows quite specific peculiarities like a high amount of ice rises, highly variable ice thicknesses across the ice shelf, tributary glaciers draining into inlets as well as only limited nourishing by direct inflow from tributary glaciers. The present study aims to better understand the dynamics and mechanisms leading to disintegration and break-up of WIS. We hence investigate satellite data to reveal changes of glaciological structures like fractures and shear margins, the position of the grounding line, changes of frontal positions and ice surface velocities. Very few in situ measurements are available at WIS, which emphasizes the use of satellite data. Especially Synthetic Aperture Radar (SAR) data show high potential for glaciological purposes. We use SAR data (ALOS PALSAR, TerraSAR-X, TanDEM-X, ERS-1/2) in order to calculate surface velocities of the ice shelf and its tributaries at different times using SAR offset tracking procedures. The combined use of TanDEM-X InSAR surface elevations and IceSAT, CryoSat and NASA Ice Bridge ATM data enables the estimation of ice thickness assuming a constant ice density. First results show surface velocities before and after the break-up events in 2008 and 2009 as well as changing flow velocities of tributary glaciers. The combination of InSAR surface elevation and altimeter data allows for a comprehensive estimation of ice thickness across WIS. Both data sets can be used for subsequent ice dynamic modeling and fracture mechanics.

Rankl, Melanie; Helm, Veit; Braun, Matthias

2013-04-01

116

Abrasion of Concrete by Ice in Arctic Sea Structures.  

National Technical Information Service (NTIS)

In arctic sea regions a concrete sea structure is subjected to heavy mechanical loads near the water level due to the moving ice sheets. The determination of the abrasion depth is based on laboratory tests for measuring ice pressures against protruding ag...

S. Huovinen

1990-01-01

117

The Growth of Atmospheric Ice Crystals: A Summary of Findings in Vertical Supercooled Cloud Tunnel Studies  

Microsoft Academic Search

Measurements of ice crystal growth under free fall in a generation of vertical supercooled cloud tunnels and some static cloud chambers as well as the related theoretical works are summarized.Growth parameters, that is, mass (m), dimensions, apparent density, and fall velocity (w), show extrema at about 5, 10, and 15C where crystals are predominantly column-needle, isometric, and plate-stellar-dendrite, respectively. Crystal

Norihiko Fukuta; Tsuneya Takahashi

1999-01-01

118

A probabilistic basis for selecting design ice pressures and ice loads for Arctic offshore structures  

SciTech Connect

This paper presents a probabilistic procedure for determining the maximum loads and pressures on a fixed offshore structure due to multi-year ice floes during winter loading events. The approach depends on a computerized simulation procedure which is described together with mechanistic models for estimating the floe contact width, peak indentation pressure, pressure vs. displacement, and interaction between the multi-year floe and the surrounding first-year ice. Several load scenarios involving different assumptions are investigated and the predicted loads compared on the basis of their cumulative probability distributions. The paper also presents a procedure for selecting the design ice pressure as a function of the loaded area size.

Vivatrat, V.; Slomski, S.

1983-05-01

119

Teaching With Crystal Structures  

NSDL National Science Digital Library

Classifying a particle requires an understanding of the type of bonding that exists within and among the particles, which requires an understanding of atomic structure and electron configurations, which requires an understanding of the elements of periodi

Smithenry, Dennis W.

2009-09-01

120

Progress in Computing the Scattering and Absorption Properties of Nonspherical Ice Crystals: A Review  

NASA Astrophysics Data System (ADS)

Cirrus clouds consist of ice crystals that are exclusively nonspherical with their common habits including hexagonal plate, solid and hollow hexagonal columns, bullet rosette, aggregate, and more irregular shapes with various degrees of surface roughness. An outstanding scientific problem in remote sensing and climate research involving cirrus clouds has been to correctly compute or measure the single-scattering properties of these nonspherical ice crystals. Significant advances and substantial progress have been made by the radiative transfer and optics communities in the last three decades to understand and determine the fundamental scattering and absorption properties of ice crystals. In this presentation, we will review these advances and progress in the studies of the single-scattering properties (i.e., extinction coefficient, single- scattering albedo, and phase matrix) of nonspherical ice crystals in the Earth's atmosphere from the theoretical and computational perspectives, but will focus on the geometric optics method (GOM) in terms of the ray-tracing technique, including its relevant improvements, and the finite-difference time domain (FDTD) method. The GOM and FDTD are applicable to the computation of the scattering and absorption properties of large and small particles compared with the incident wavelength, respectively. Furthermore, we will also briefly review application of the single-scattering properties of ice crystals to the remote sensing of cirrus cloud optical depth and compositions.

Yang, P.; Liou, K.

2006-12-01

121

Pattern information extraction from crystal structures  

Microsoft Academic Search

Abstract Determining the crystal structure parameters of a material is an important issue in crystallography and material science. Knowing,the crystal structure parameters helps in understanding the physical behavior of material. It can be difficult to obtain crystal parameters for complex structures, particularly those materials that show local symmetry,as well as global symmetry. This work provides a tool that extracts crystal

Erhan Okuyan; Ugur Gdkbay; Oguz Glseren

2007-01-01

122

Surface melting of ice Ih single crystals revealed by glancing angle x-ray scattering  

Microsoft Academic Search

We present glancing angle x-ray scattering experiments at [00.1], [10.0], and [11.0] surfaces of ice Ih single crystals. The temperature dependence of the evanescent Bragg scattering upon heating reveals a quasiliquid surface layer well below the melting point of each investigated ice surface. At [10.0] and [11.0] surfaces, thermal faceting is observed, which is briefly discussed. The ``oxygen-forbidden'' (00.4) Bragg

A. Lied; H. Dosch; J. H. Bilgram

1994-01-01

123

Dependence of the single-scattering properties of small ice crystals on idealized shape models  

Microsoft Academic Search

Small ice crystals (with maximum dimension <50 mum) appear quasi-circular when imaged by probes on aircraft flying through cloud. Therefore, idealized models constructed to calculate their single-scattering properties have included quasi-spherical models such as Chebyshev particles, Gaussian random spheres, and droxtals. Recently, an ice analogue grown from sodium fluorosilicate solution on a glass substrate, with several columns emanating from a

J. Um; G. M. McFarquhar

2010-01-01

124

Dependence of the single-scattering properties of small ice crystals on idealized shape models  

Microsoft Academic Search

The projections of small ice crystals (with maximum dimension <50 mum) appear quasi-circular when imaged by probes on aircraft flying through cloud. Therefore, idealized models constructed to calculate their single-scattering properties have included quasi-spherical models such as Chebyshev particles, Gaussian random spheres, and droxtals. Recently, an ice analogue grown from sodium fluorosilicate solution on a glass substrate, with several columns

J. Um; G. M. McFarquhar

2011-01-01

125

Cirrus Clouds Millimeter-Wave Reflectivity Comparison with In-Situ Ice Crystal Airborne Data  

Microsoft Academic Search

In an effort to evaluate scattering models for particle size distributions of ice crystals within cirrus clouds, simultaneous data was collected during the Department of Energy (DoE) Atmospheric Radiation Measurement (ARM) Cloud Intensive operational period (Cloud IOP) at the Cloud and Radiation Testbed (CART) site in Lamont, Oklahoma, from the NCAR Video Ice Particle Sampler (VIPS) and UMass 33GHz\\/95GHz Cloud

Jos Morales; Jorge Trabal; Sandra L. Cruz-Pol; Stephen M. Sekelsky

126

Structural mechanisms of DIAP1 auto-inhibition and DIAP1-mediated inhibition of drICE.  

PubMed

The Drosophila inhibitor of apoptosis protein DIAP1 exists in an auto-inhibited conformation, unable to suppress the effector caspase drICE. Auto-inhibition is disabled by caspase-mediated cleavage of DIAP1 after Asp20. The cleaved DIAP1 binds to mature drICE, inhibits its protease activity, and, presumably, also targets drICE for ubiquitylation. DIAP1-mediated suppression of drICE is effectively antagonized by the pro-apoptotic proteins Reaper, Hid, and Grim (RHG). Despite rigorous effort, the molecular mechanisms behind these observations are enigmatic. Here we report a 2.4 crystal structure of uncleaved DIAP1-BIR1, which reveals how the amino-terminal sequences recognize a conserved surface groove in BIR1 to achieve auto-inhibition, and a 3.5 crystal structure of active drICE bound to cleaved DIAP1-BIR1, which provides a structural explanation to DIAP1-mediated inhibition of drICE. These structures and associated biochemical analyses, together with published reports, define the molecular determinants that govern the interplay among DIAP1, drICE and the RHG proteins. PMID:21811237

Li, Xiaochun; Wang, Jiawei; Shi, Yigong

2011-08-02

127

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; Ramrez, Rafael

2013-08-28

128

Crystal Structure of Beryllium Borohydride.  

National Technical Information Service (NTIS)

The crystal structure of Be(BH4)2 consists of a helical polymer of BH4Be and BH4 units. Within the BH4Be units there are two hydrogen bridges between B and Be, and in the helical polymer each B...Be contact has two hydrogen bridges. The hydrogen arrangeme...

D. S. Marynick W. N. Lipscomb

1971-01-01

129

Piezoelectricity, ferroelectricity, and crystal structure  

Microsoft Academic Search

By visualizing polar crystals as a network of permanent dipole moments, the piezo- and ferroelectric properties of dielectrics may be derived from the standpoint of molecular symmetry. This approach is used to clarify the relation between the sphalerite and wurtzite structures, the ferroelectric feedback effect in barium titanate, aspects of domain formation, and the interrelationship between ferro and piezoelectricity.

A. von Hippel

1952-01-01

130

Crystal structure of sorbitol dehydrogenase  

Microsoft Academic Search

Sorbitol dehydrogenase (SDH) is a distant relative to the alcohol dehydrogenases (ADHs) with sequence identities around 20%. SDH is a tetramer with one zinc ion per subunit. We have crystallized rat SDH and determined the structure by molecular replacement using a tetrameric bacterial ADH as search object. The conformation of the bound coenzyme is extended and similar to NADH bound

Kenth Johansson; Mustafa El-Ahmad; Christina Kaiser; Hans Jrnvall; Hans Eklund; Jan-Olov Hg; S. Ramaswamy

2001-01-01

131

Refinement of Hematite Crystal Structure.  

National Technical Information Service (NTIS)

The crystal structure of hematite, alpha-Fe2O3, has been refined as part of a continuing study of the behavior of phases in the Fe-O system during reduction and oxidation. Three-dimensional diffraction intensities were collected on two spherical single cr...

R. L. Blake T. Zoltai R. E. Hessevick L. W. Finger

1970-01-01

132

An Investigation of Ice Forces on Vertical Structures.  

National Technical Information Service (NTIS)

Ice forces are the principal concern in designing offshore structures such as drilling platforms, transloading points, or bridge peers in arctic regions. Although some results have been obtained by field measurements in recent years, there are still signi...

K. Hirayama J. Schwarz H. C. Wu

1974-01-01

133

Crystal structure of lignin peroxidase  

Microsoft Academic Search

The crystal structure of lignin peroxidase (LiP) from the basidiomycete Phanerochaete chrysosporium has been determined to 2.6 [Angstrom] resolution by using 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

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

1993-01-01

134

Mechanical interactions between ice crystals and red blood cells during directional solidification.  

PubMed

Experiments in which red blood cells were frozen on a directional solidification stage under a microscope show that there is a mechanical interaction between ice crystals and cells in which cells are pushed and deformed by the ice crystals. The mechanical interaction occurs during freezing of cells in physiological saline and is significantly inhibited by the addition of 20% v/v glycerol to the solution. The addition of osmotically insignificant quantities of antifreeze proteins from the winter flounder or ocean pout to the physiological saline with 20% v/v glycerol generates strong mechanical interactions between the ice and the cells. The cells were destroyed during freezing in physiological saline, survived freezing in physiological saline with glycerol, and were completely destroyed by the addition of antifreeze proteins to the solution with glycerol. The difference in cell survival through freezing and thawing appears to be related, in part, to the habit of ice crystal growing in the suspension of red blood cells and the nature of mechanical interaction between the ice crystal and the cells. This suggests that mechanical damage may be a factor during cryopreservation of cells. PMID:7988158

Ishiguro, H; Rubinsky, B

1994-10-01

135

Crystal Structure of Bi  

SciTech Connect

The room temperature structures of the two-layer Aurivillius phases Bi{sub 2.5}Me{sub 0.5}Nb{sub 2}O{sub 9} (Me=Na, K) have been refined with the Rietveld method from powder neutron diffraction data ({lambda}=1.470 {angstrom}). They consist of (Bi{sub 2}O{sub 2}){sup 2+} layers interleaved with perovskite (Bi{sub 0.5}Me{sub 0.5}Nb{sub 2}O{sub 7}){sup 2-} (Me=Na, K) slabs. The structures were refined in the orthorhombic space group A2{sub 1}am, Z=4, and the unit cell parameters of the two oxides are a= 5.4937(3), b=5.4571(4), c=24.9169(14) {angstrom} and a=5.5005(8), b=5.4958(8), c=25.2524(16) {angstrom}, respectively. The orthorhombic distortion increases with decreasing Me+ cation size in the perovskite layer (Bi/Me){sup 2+} site and the lone pair electrons from the Bi{sup 3+} cation are influencing the site distortion. This is in agreement with other two-layer Aurivillius phases and originates from bonding requirements depending on size and electronic environment.

Borg, Stefan; Svensson, Goeran

2001-02-15

136

Crystal alignments in the fast ice of Arctic Alaska  

SciTech Connect

Field observations at 60 sites located in the fast or near-fast ice along a 1200-km stretch of the north coast of Alaska between the Bering Strait and Barter Island have shown that the great majority of the ice samples (95%) exhibit striking c axis alignments within the horizontal plane. In all cases the degree of preferred orientation increased with depth in the ice. Representative standard deviations around a mean direction in the horizontal plane are commonly less than +- 10/sup 0/ for samples collected near the bottom of the ice. At a given site the mean c axis direction X-bar/sub 0/ may vary as much as 20/sup 0/ with vertical location in the ice sheet. The c axis allignments in the nearshore region generally parallel the coast, with strong alignments occurring in the lagoon systems between the barrier islands and the coast and seaward of the barrier islands. In passes between islands and in entrances such as the opening to Kotzebue Sound the alignment is parallel to the channel. Only limited observations are available farther seaward over the inner (10- to 50-m isobaths) and outer (50-m isobath to shelf break) shelf regions. These indicate Ne-SW and E-W alignments, respectively, in the Beaufort Sea north of Prudhoe Bay.

Weeks, W.F.; Gow, A.J.

1980-02-20

137

From the single-scattering properties of ice crystals to climate prediction: A way forward  

NASA Astrophysics Data System (ADS)

Cirrus is composed of non-spherical ice crystals, and against the blue background of the sky, they appear as tenuous wispy clouds, usually located at altitudes greater than about 6 km. Their spatial and temporal distribution about the Earth's atmosphere is significant. With such distributions, their contributions to the Earth's natural greenhouse effect and hydrological cycle are important. Therefore, it is important that climate models are able to predict the radiative effect of cirrus, as well as their contribution to the total amount of ice mass that occurs in the Earth's atmosphere. However, cirrus is composed of ice crystals that can take on a variety of geometrical shapes, from pristine habits such as hexagonal ice columns, hexagonal ice plates and bullet-rosettes, to highly randomized habits, which may have roughened surfaces and/or air cavities. These habits also aggregate together, to form chains of aggregates and compact aggregates. The sizes of these habits may also vary, from about less than 10 ?m, to several cm, with the smaller ice crystals usually existing toward cloud-top and the larger ice crystals existing toward the cloud-bottom. Due to this variability of geometrical complexity, size, and ice mass, predicting the magnitude of the cirrus greenhouse effect has proven problematic. To try to constrain these radiative and hydrological uncertainties, since about 2006 there is now available the A-train constellation of satellites, which attempt to quantify the radiative and hydrological contributions of cirrus to the Earth's atmosphere. The A-train obtains nearly simultaneous measurements of cirrus from across the electromagnetic spectrum. Such simultaneous measurements pose challenges for theoretical scattering models of cirrus, as these models must conserve ice mass and be physically consistent across the electromagnetic spectrum. In this review paper, the microphysical properties of cirrus are summarized. The current idealized habit mixture models that have been proposed to represent the observed variability in ice crystal shape, size and mass are discussed. The theoretical light scattering methods that are currently applied to the idealized habit mixture models to solve for their scattering and absorption properties are discussed. The physical inconsistency of the current approach to parameterize the bulk scattering and absorption properties of cirrus in climate models is highlighted. An alternative parameterization, which couples cloud physics more directly with radiation, is proposed. Such a coupling is required, if climate models are to be physically consistent and radiatively interactive.

Baran, Anthony J.

2012-08-01

138

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

Microsoft Academic Search

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 (8.4kDa) composed of tandem 12-residue repeats (TCTxSxxCxxAx). Here we report its 1.4- resolution crystal structure, showing that this repetitive sequence translates into an exceptionally regular beta-helix. Not only are the 12-amino-acid

Yih-Cherng Liou; Ante Tocilj; Peter L. Davies; Zongchao Jia

2000-01-01

139

Structural and spectroscopic characterization of mixed planetary ices.  

PubMed

Mixed ices play a central role in characterizing the origin, evolution, stability and chemistry of planetary ice surfaces. Examples include the polar areas of Mars, the crust of the Jupiter moon Europa, or atmospheres of planets and their satellites, particularly in the outer solar system. Atomistic simulations using accurate representations of the interaction potentials have recently shown to be suitable to quantitatively describe both, the mid- and the far-infrared spectrum of mixed H2O/CO amorphous ices. In this work, molecular dynamics simulations are used to investigate structural and spectroscopic properties of mixed and crystalline ices containing H2O, CO and CO2. Particular findings include: (a) the sensitivity of the water bending mode to the local environment of the water molecules which, together with structural insights from MD simulations, provides a detailed picture for the relationship between spectroscopy and structure; and (b) the sensitivity of the low-frequency spectrum to the structure of the mixed CO2/H2O ice. Specifically, for mixed H2O/CO2 ices with low water contents isolated water molecules are found which give rise to a band shifted by only 12 cm(-1) from the gas-phase value whereas for increasing water concentration (for a 1 : 1 mixture) the band progressively shifts to higher frequency because water clusters can form. More generally it is found that changes in the ice structure due to the presence of CO2 are larger compared to changes induced by the presence of CO and that this difference is reflected in the shape of the water bending vibration. Thus, the water bending vibration appears to be a suitable diagnostic for structural and chemical aspects of mixed ices. PMID:21302549

Plattner, Nuria; Lee, Myung Won; Meuwly, Markus

2010-01-01

140

Crystal Structure of Neotame Anhydrate Polymorph G  

Microsoft Academic Search

Purpose. To determine the crystal structure of the neotame anhydrate polymorph G and to evaluate X-ray powder diffractometry (XRPD) with molecular modeling as an alternative method for determining the crystal structure of this conformationally flexible dipeptide.

Zedong Dong; Victor G. Young; Agam Sheth; Eric J. Munson; Steve A. Schroeder; Indra Prakash; David J. W. Grant

2002-01-01

141

Ice-Internal and Sedimenetary Structures in the Ekstrmisen Grounding Line Region Detected With Multi-offset Seismics.  

NASA Astrophysics Data System (ADS)

Ekstrmisen is a small catchment area in coastal Dronning Maud Land, Antarctica, terminating in the Ekstrm ice shelf, which is bounded by a narrow embayment formed by two ice ridges. A seismic survey has been performed along a flow line on Ekstrmisen over about 22 km, crossing the grounding line between ice sheet and shelf approximately midway of the profile. The measurements were performed with explosives in shallow firn holes as seismic sources and a 60 channel 1.5 km snow streamer for data recording. The data has been resorted to form a virtual 120 channel 3 km streamer, consisting of 150 shots. The maximum shot-receiver offset is thus about three times larger then the ice thickness, yielding wide angle information for intra-ice and bedrock reflections. Standard seismic data processing yields 862 common depth points in total, with an increment of 25~m. This provides a 20-fold coverage of each common depth point. In addition to yielding the distribution of seismic velocity within the firn, ice and sediment, the data clearly images ice and sedimentary layers. Within the bottom part of the ice, a number of continuous internal layers are visible upstream of the grounding line. Currently, our favorite explanation is abrupt changes in the crystal orientation fabric caused by a combination of laterally compressional flow and vertical shear, as observed with radio-echo sounding at other places in Antarctica. Upstream of and at the grounding line, structures are visible within the bedrock, which we interpret as sedimentary deposits related to glacial activity. Downstream of the grounding line the seismic data suggests the existence of a pinning point under the ice shelf, formed by sedimentary deposits. The data provide the base for interpretations of the ice-dynamic and sedimentary processes occurring in the basal ice layer and at the ice-bedrock boundary, of relevance for further understanding details of the ice sheet-to-shelf transition area.

Hofstede, C. M.; Burnie, I.; Uenzelmann-Neben, G.; Nixdorf, U.; Eisen, O.

2008-12-01

142

Simplification for Fraunhofer diffracting pattern of various randomly oriented ice crystals in cirrus.  

PubMed

This paper deals with Fraunhofer diffraction by an ensemble of independent randomly oriented ice crystals of assorted shapes, like those of cirrus clouds. There is no restriction on the shape of each crystal. It is shown that light flux density in the Fourier plane is azimuth-invariant and varies as 1/sin(4)?, ? being the angle of diffraction. The analytical formula proposed is exact. The key point of this study is conservation of electromagnetic energy. PMID:23201960

Pujol, Olivier; Brogniez, Grard; Labonnote, Laurent

2012-09-01

143

Elemental composition and morphology of ice-crystal residual particles in cirrus clouds and contrails  

Microsoft Academic Search

Aircraft sampling of residual particles from evaporated ice crystals was performed using a Counterflow Virtual Impactor. Samples of crystals taken in both contrails and cirrus clouds were compared with interstitial aerosols found in natural cirrus. The samples were analyzed with a scanning electron microscope which was equipped with a windowless energy-dispersive X-ray detector (SEM\\/EDX). In the contrail and cirrus cases

A Petzold; J Strm; S Ohlsson; F. P Schrder

1998-01-01

144

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.

Downs, R. T.; Hall-Wallace, M.

145

Structural Transitions in Amorphous Water Ice and Astrophysical Implications  

Microsoft Academic Search

Selected area electron diffraction is used to monitor structural changes of vapor-deposited water ice in vacuum during warm-up from 15 to 188 K. A progression of three amorphous forms of water ice is found with well-defined transitions. The formation of a high-density amorphous form (I_ah) at 15 K is confirmed, and the transition to the more familiar low-density form (I_aI)

Peter Jenniskens; David F. Blake

1994-01-01

146

A renewed argument for crystal size control of ice sheet strain rates  

Microsoft Academic Search

At present, it is generally believed that crystal size has no direct influence on strain rate in the ice sheets and that the fraction of strain rate enhancement there which is not ascribable to c axis fabric is due to impurity content. Here we challenge this view because it is not consistent with recent results from analyses of deformation at

K. M. Cuffey; T. Thorsteinsson; E. D. Waddington

2000-01-01

147

Time-varying ice crystal orientation in thunderstorms observed with multiparameter radar  

Microsoft Academic Search

Repeated changes associated with lightning have been observed with multiparameter radar in the echoes from the tops of Florida thunderstorms. These lightning-related radar signatures are interpreted as changes in the orientation of ice crystals being preferentially aligned parallel to the in-cloud electric field. The changes occur at intervals on the order of 10 s and are easily observed in the

I. Jeff Caylor; V. Chandrasekar

1996-01-01

148

The Deposition of Ice Crystals on Cooling Surfaces in Low Temperature Plant  

Microsoft Academic Search

A study is made of the conditions governing the deposition of ice crystals from a moist air stream on to the inside surface of a cooled tube. The promoting effect of impurities such as oil vapour in the air stream is demonstrated. Series of experiments were made to establish the effect of air velocity, and of the temperature difference between

G. G. Haselden

1950-01-01

149

Platinum-Palladium Crystal Structures  

NASA Astrophysics Data System (ADS)

Being able to predict Platinum-Palladium ordering is important in discovering new alloys that have commercial and industrial applications. Using direct quantum mechanical calculations coupled with a lattice-based Hamiltonian called a cluster expansion, we can predict which crystal structures are thermodynamically stable for. In addition, a Monte Carlo simulation can be used in this model to determine the order-disorder transition temperatures. Knowing which structures are thermodynamically stable and their respective transition temperatures may help develop useful platinum palladium alloys.

Pratt, Weston

2009-10-01

150

The crystal structure of hopeite  

Microsoft Academic Search

The crystal structure of hopeite, Zns(POa)2.4Hro, has been solved by the Heavy Atom method from l42l graphite-monochromatized MoKa data and refined by full matrix least- squares to R = 0.026 (R. = 0.036). The structure is orthorhombic, pnma, a = 10.597(3), b : l8.3lE(8), c: 5.031(l) A, and Z: 4.Thezn atoms occur in two crystallographically distinct sites, one six-coordinated and

RooEnrcr J. Hrr; J. B. JoNes

151

Bedrock structure and the interpretation of palaeo ice stream footprints: examples from the Pleistocene British Ice Sheet  

NASA Astrophysics Data System (ADS)

To model past and future behaviour of ice sheets, a good understanding of both modern and ancient ice streams is required. The study of present-day ice streams provides detailed data of short-term dynamic changes, whilst the study of Pleistocene palaeo-ice streams can provide crucial constraints on the longer-term evolution of ice sheets. To date, palaeo-ice streams, such as the classical Dubawnt Lake palaeo-ice stream of the former Laurentide Ice Sheet, have been recognised largely on the basis of extremely elongate drumlins and megascale glacial lineations; all soft-sediment features. Whilst it appears that topographically unconstrained ice streams (eg. within the West Antarctic Ice Sheet) are generally underlain by deformable till, topographically constrained ice streams such as Jakobshavn Isbrae do not require deformable sediment and may occur on a bedrock-dominated bed. Analysis of DEM data and geomorphology and structural geology fieldwork in Northern Scotland and Northern England has shown the occurrence of highly streamlined bedforms in bedrock of the former base of topographically controlled palaeo-ice streams, which drained parts of the British Ice Sheet. The bedforms are predominantly bedrock megagrooves with asymmetric cross-profiles. In the Ullapool tributary of the Minch palaeo ice stream, bedrock megagrooves form the dominant evidence for ice streaming. The megagrooves are typically 5-15 m deep, 10-30 m wide and 500 - 3000 m long. Spacing of megagrooves is typically 100 - 200 m. In both study areas, the bedrock is strongly anisotropic, either consisting of thin-bedded strata or strongly foliated metasedimentary rocks, with the strata or foliation having a gentle dip. Megagrooves are best developed where the strike of the anisotropy is sub-parallel (within 10 - 20) with palaeo ice flow. The bedrock in both areas has a well-developed, relatively densely spaced (< 1m), conjugate joint system. We suggest that asymmetric megagrooves are formed by "lateral plucking", facilitated by the combination of strong bedding/foliation and the joint pattern. Glacial erosion was laterally more effective than vertically; so that stepped faces subparallel to palaeo ice flow are enhanced rather that destroyed. We propose that: a) Lateral plucking is an effective mechanism to produce streamlined bedrock bedforms by fast ice flow, providing the bedrock and bedrock structure are suitable; b) some topographically controlled palaeo-ice stream beds are dominated by bedrock rather than soft-sediment; c) the recognition of palaeo-ice streams may be dependent on the type of bedrock and the orientation of bedrock structure with respect to palaeo ice flow; d) palaeo-ice stream footprints may have been underestimated in formerly glaciated areas.

Krabbendam, M.; Bradwell, T.

2009-04-01

152

A global classification of snow crystals, ice crystals, and solid precipitation based on observations from middle latitudes to polar regions  

NASA Astrophysics Data System (ADS)

This paper presents an extensive revision of Magono and Lee's (1966) classification of natural snow crystals, which has been widely used in snow and ice studies to describe snow crystal shapes. The new classification catalogs snow crystals and other solid precipitation particles into 121 categories, in contrast to Magono and Lee's 80 categories. Of these, 28 categories were created to classify new types of snow crystals that have been discovered in polar regions since 1968, seven were created after reconsidering the original categories, and six categories were created to classify solid precipitation particles such as frozen cloud particles and small raindrops. Because our observational area extended from middle latitudes (Japan) to polar regions, we refer to our new classification scheme as 'global-scale classification' or 'global classification'. The global classification consists of three levels general, intermediate, and elementary which are composed of 8, 39, and 121 categories, respectively. This paper describes the characteristics of each type of snow crystal, ice crystal, and solid precipitation particle.

Kikuchi, Katsuhiro; Kameda, Takao; Higuchi, Keiji; Yamashita, Akira

2013-10-01

153

Shear induced structures in crystallizing cocoa butter  

Microsoft Academic Search

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

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

2004-01-01

154

Ice crystal size estimation using multiple-wavelength radar  

Microsoft Academic Search

Measurements and simulations of multiple-wavelength radar scattering have demonstrated the feasibility of using multiple-wavelength systems to estimate effective hydrometeor size in ice-phase clouds, and in various forms of precipitation. Radar reflectivity differences occur when the higher frequency experiences non-Rayleigh scattering. For cloud particle sizing, the higher frequency must exhibit non-Rayleigh scattering from small hydrometeors. This occurs at mm-wavelengths where radar

Stephen M. Sekelsky; Robert E. McIntosh; Warner L. Ecklund; Kenneth S. Gage

1998-01-01

155

Lattice Boltzmann Simulation of Water Isotope Fractionation During Growth of Ice Crystals in Clouds  

NASA Astrophysics Data System (ADS)

The isotopic composition of precipitation, especially that of snow, plays a special role in the global hydrological cycle and in reconstruction of past climates using polar ice cores. The fractionation of the major water isotope species (HHO, HDO, HHO-18) during ice crystal formation is critical to understanding the global distribution of isotopes in precipitation. Ice crystal growth in clouds is traditionally treated with a spherically- symmetric steady state diffusion model, with semi-empirical modifications added to account for ventilation and for complex crystal morphology. Although it is known that crystal growth rate, which depends largely on the degree of vapor over-saturation, determines crystal morphology, there are no existing quantitative models that directly relate morphology to the vapor saturation factor. Since kinetic (vapor phase diffusion-controlled) isotopic fractionation also depends on growth rate, there should be a direct relationship between vapor saturation, crystal morphology, and crystal isotopic composition. We use a 2D Lattice-Boltzmann model to simulate diffusion-controlled ice crystal growth from vapor- oversaturated air. In the model, crystals grow solely according to the diffusive fluxes just above the crystal surfaces, and hence crystal morphology arises from the initial and boundary conditions in the model and does not need to be specified a priori. The input parameters needed are the isotope-dependent vapor deposition rate constant (k) and the water vapor diffusivity in air (D). The values of both k and D can be computed from kinetic theory, and there are also experimentally determined values of D. The deduced values of k are uncertain to the extent that the sticking coefficient (or accommodation coefficient) for ice is uncertain. The ratio D/k is a length that determines the minimum scale of dendritic growth features and allows us to scale the numerical calculations to atmospheric conditions using a dimensionless Damkohler number: Da = kh/D, where h is the width of the 2D calculation domain. Varying the nondimensional Da in the model is equivalent to varying the scale (h) in the model. Our calculations confirm that the crystal/vapor isotopic fractionation approaches the equilibrium value, and the crystals are compact (circular in 2D) as the saturation factor approaches unity (S= 1.0). At higher oversaturation (e.g. S = 1.2), dendritic crystals of millimeter size develop on timescales appropriate to cloud processes, the isotopic fractionations are dominated by kinetic effects, and similar to those predicted by the spherical diffusion model. Dendritic crystals are constrained to be relatively large, with dimension much greater than D/k. The most difficult aspect of the modeling is to account for the large density difference between air and ice, which requires us to use a fictitious higher density for the vapor-oversaturated air and scale the crystal growth time accordingly. A different approach, using a larger scale simulation to derive boundary conditions for a nested smaller scale calculation is in progress. The results to date clarify the controls on dendritic crystal growth, the relationships between saturation state, growth rate, crystal morphology and isotopic fractionation, and provide limits on the value of the accommodation coefficient.

Lu, G.; Depaolo, D.; Kang, Q.; Zhang, D.

2006-12-01

156

Ice-internal and sedimentary structures in the Ekstrmisen grounding line region detected with multi-offset seismics  

NASA Astrophysics Data System (ADS)

Ekstrmisen is a small catchment area in coastal Dronning Maud Land, Antarctica, terminating in the Ekstrm ice shelf, which is bounded by a narrow embayment formed by two ice ridges. A seismic survey has been performed along a flow line on Ekstrmisen over about 22 km, crossing the grounding line between ice sheet and shelf approximately midway of the profile. The measurements were performed with explosive in shallow firn holes as seismic sources and a 60 channel 1.5 km snow streamer for data recording. The data has been resorted to form a virtual 120 channel 3 km streamer, consisting of 150 shots. The maximum shot-receiver offset is thus about three times larger then the ice thickness, yielding wide angle information for intra-ice and bedrock reflections. Standard seismic data processing yields 862 common depth points in total, with an increment of 25 m. This provides a 20-fold coverage of each common depth point. In addition to yielding the distribution of seismic velocity within the firn, ice and sediment, the data clearly images ice and sedimentary layers. Within the bottom part of the ice, a number of continuous internal layers are visible upstream of the grounding line. Currently, our favorite explanation is abrupt changes in the crystal orientation fabric caused by a combination of laterally compressional flow and vertical shear, as observed with radio-echo sounding at other places in Antarctica. Upstream of and at the grounding line, structures are visible within the bedrock, which we interpret as sedimentary deposits related to glacial activity. The data provide the base for interpretations of the ice-dynamic and sedimentary processes occurring in the basal ice layer and at the ice-bedrock boundary, of relevance for further understanding details of the ice sheet-to-shelf transition area.

Hofstede, C.; Uenzelmann-Neben, G.; Nixdorf, U.; Eisen, O.; Kulessa, B.

2009-04-01

157

Ice-breaking off-shore drilling and production structure  

Microsoft Academic Search

An ice-breaking drilling and production structure is described for use with an off-shore vertically disposed well shaft and the like having one end extending downwardly toward a sea floor and having an opposite end extending upwardly to a point above sea level. The structure consists of: (a) a platform member having outer edges and a generally downwardly facing lower surface

Loire

1986-01-01

158

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

159

Crystal Structure of Human ?-Galactosidase  

PubMed Central

GM1 gangliosidosis and Morquio B are autosomal recessive lysosomal storage diseases associated with a neurodegenerative disorder or dwarfism and skeletal abnormalities, respectively. These diseases are caused by deficiencies in the lysosomal enzyme ?-d-galactosidase (?-Gal), which lead to accumulations of the ?-Gal substrates, GM1 ganglioside, and keratan sulfate. ?-Gal is an exoglycosidase that catalyzes the hydrolysis of terminal ?-linked galactose residues. This study shows the crystal structures of human ?-Gal in complex with its catalytic product galactose or with its inhibitor 1-deoxygalactonojirimycin. Human ?-Gal is composed of a catalytic TIM barrel domain followed by ?-domain 1 and ?-domain 2. To gain structural insight into the molecular defects of ?-Gal in the above diseases, the disease-causing mutations were mapped onto the three-dimensional structure. Finally, the possible causes of the diseases are discussed.

Ohto, Umeharu; Usui, Kimihito; Ochi, Toshinari; Yuki, Kenjiro; Satow, Yoshinori; Shimizu, Toshiyuki

2012-01-01

160

Elementary steps at the surface of ice crystals visualized by advanced optical microscopy  

PubMed Central

Due to the abundance of ice on earth, the phase transition of ice plays crucially important roles in various phenomena in nature. Hence, the molecular-level understanding of ice crystal surfaces holds the key to unlocking the secrets of a number of fields. In this study we demonstrate, by laser confocal microscopy combined with differential interference contrast microscopy, that elementary steps (the growing ends of ubiquitous molecular layers with the minimum height) of ice crystals and their dynamic behavior can be visualized directly at air-ice interfaces. We observed the appearance and lateral growth of two-dimensional islands on ice crystal surfaces. When the steps of neighboring two-dimensional islands coalesced, the contrast of the steps always disappeared completely. We were able to discount the occurrence of steps too small to detect directly because we never observed the associated phenomena that would indicate their presence. In addition, classical two-dimensional nucleation theory does not support the appearance of multilayered two-dimensional islands. Hence, we concluded that two-dimensional islands with elementary height (0.37 and 0.39nm on basal and prism faces, respectively) were visualized by our optical microscopy. On basal and prism faces, we also observed the spiral growth steps generated by screw dislocations. The distance between adjacent spiral steps on a prism face was about 1/20 of that on a basal face. Hence, the step ledge energy of a prism face was 1/20 of that on a basal face, in accord with the known lower-temperature roughening transition of the prism face.

Sazaki, Gen; Zepeda, Salvador; Nakatsubo, Shunichi; Yokoyama, Etsuro; Furukawa, Yoshinori

2010-01-01

161

Crystal structure of lignin peroxidase  

SciTech Connect

The crystal structure of lignin peroxidase (LiP) from the basidiomycete Phanerochaete chrysosporium has been determined to 2.6 [Angstrom] resolution by using 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 [approx]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. 42 refs., 4 figs., 2 tabs.

Edwards, S.L. (Center for Advanced Research in Biotechnology, Rockville, MD (United States) National Institutes of Health, Bethesda, MD (United States)); Raag, R. (Center for Advanced Research in Biotechnology, Rockville, MD (United States)); Wariishi, Hiroyuki; Gold, M.H. (Oregon Graduate Institute of Science and Technology, Beaverton (United States)); Poulos, T.L. (Center for Advanced Reseaarch in Biotechnology, Rockville, MD (United States) Univ. of California, Irvine (United States))

1993-01-15

162

Multi-fractal structure in Arctic sea ice satellite data  

NASA Astrophysics Data System (ADS)

Since 21th century, Arctic sea ice has shown significant decrease especially during summer season. These decline has been considered by climate scientists as a critical sign of the effect of global warming. Especially, IPCC global climate models confirmed the recent decline of Arctic sea ice and predict the more decline in the near future. Even with rather significant amount of inter-model variability in climate models, satellite data during around 30 years support the results of climate models. However, the decline of Arctic sea ice is confirmed based on monthly averaged data and linear regression after erasing seasonal cycle. Even with strong conjecture of the real decline of Arctic sea ice, we cannot exclude one part of natural oscillations affecting Arctic sea ice physics due to strong non-stationarity. It is also possible not to make any conclusive statements using the monthly averaged data due to a lack of data or multi-fractal structure of data excluded in the data. Here, we will use MF-DFA (Multi-Fractal Detrended Fluctuation Analysis) to determine the multi-scale structure of Arctic sea ice area and albedo using AVHRR Polar Pathfinder Twice-Daily 5km EASE-grid composites, which is expected to decide whether the decline shown in the linear regression of the monthly averaged data can be considered as the real decline caused by global warming or not. Also, the multi-scale structure information drawn from this analysis is expected to give us the guideline for selecting significant physics for low-order Arctic sea ice

Moon, W.; Wettlaufer, J. S.

2011-12-01

163

Photonic band structure of bcc colloidal crystals  

Microsoft Academic Search

Self-ordering colloidal systems are being recognized as possible candidates for optical photonic crystals. Such structures exhibit three-dimensional periodicity in refractive index, and possess lattice dimensions comparable to optical wavelengths. Recently, fcc colloidal crystals of water-suspended polystyrene microspheres were investigated in the context of photonic band structure. At lower polystyrene volume fractions, these colloidal suspensions crystallize in the bcc crystal lattice.

Ranjit D. Pradhan; John A. Bloodgood; George H. Watson

1997-01-01

164

Crystal structure of metarhodopsin II.  

PubMed

G-protein-coupled receptors (GPCRs) are seven transmembrane helix (TM) proteins that transduce signals into living cells by binding extracellular ligands and coupling to intracellular heterotrimeric G proteins (G???). The photoreceptor rhodopsin couples to transducin and bears its ligand 11-cis-retinal covalently bound via a protonated Schiff base to the opsin apoprotein. Absorption of a photon causes retinal cis/trans isomerization and generates the agonist all-trans-retinal in situ. After early photoproducts, the active G-protein-binding intermediate metarhodopsin II (Meta?II) is formed, in which the retinal Schiff base is still intact but deprotonated. Dissociation of the proton from the Schiff base breaks a major constraint in the protein and enables further activating steps, including an outward tilt of TM6 and formation of a large cytoplasmic crevice for uptake of the interacting C terminus of the G? subunit. Owing to Schiff base hydrolysis, Meta?II is short-lived and notoriously difficult to crystallize. We therefore soaked opsin crystals with all-trans-retinal to form Meta?II, presuming that the crystal's high concentration of opsin in an active conformation (Ops*) may facilitate all-trans-retinal uptake and Schiff base formation. Here we present the 3.0? and 2.85? crystal structures, respectively, of Meta?II alone or in complex with an 11-amino-acid C-terminal fragment derived from G? (G?CT2). G?CT2 binds in a large crevice at the cytoplasmic side, akin to the binding of a similar G?-derived peptide to Ops* (ref. 7). In the Meta?II structures, the electron density from the retinal ligand seamlessly continues into the Lys?296 side chain, reflecting proper formation of the Schiff base linkage. The retinal is in a relaxed conformation and almost undistorted compared with pure crystalline all-trans-retinal. By comparison with early photoproducts we propose how retinal translocation and rotation induce the gross conformational changes characteristic for Meta?II. The structures can now serve as models for the large GPCR family. PMID:21389988

Choe, Hui-Woog; Kim, Yong Ju; Park, Jung Hee; Morizumi, Takefumi; Pai, Emil F; Krauss, Norbert; Hofmann, Klaus Peter; Scheerer, Patrick; Ernst, Oliver P

2011-03-09

165

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.

166

Characterizing Single-Scattering Properties of Non-Spherical Ice-Crystal Ensembles  

NASA Astrophysics Data System (ADS)

Retrievals of atmospheric particulate constituents using remote sensing means rely on the characterization of the single-scattering properties of these particulate matters. We are able to characterize with fairly high accuracies the single-scattering properties involving spherical particles, due to the symmetry of their shape. For example, it has been demonstrated that liquid water content, effective radius, and effective variance adequately characterize the single-scattering of an ensemble of spherical cloud water droplets in the visible and near-infrared wavelengths (Hansen and Travis 1974). Such is not the case, however, for non-spherical particles such as ice crystals. In order to characterize the scattering of these non-spherical particles we need to first find realistic representations of their shapes. The Snowfake (Gravner and Griffeath 2009) model is a numerical growth model for ice crystals based on diffusion and vapor deposition. It is capable of growing realistic ice crystals with the exquisite fine features observed in nature. Before the advent of Snowfake, single-scattering calculations are performed on shapes described by simplified mathematical formulae, which bear only a rough resemblance to the real ones. With crystals grown by the Snowfake model we can be more confident with the representativeness of the single-scattering properties obtained. Single-scattering properties of these particles are calculated using the open-source DDSCAT software developed by Draine and Flatau (2008), based on discrete dipole approximation (DDA). In this presentation we report early results obtained from the effort of characterizing the single-scattering of ensembles of pristine ice crystals using geometric parameters derived from the particle size distribution (PSD) of the ensembles at the wavelengths of current spaceborne radars.

Kuo, K.; Han, Q.; Smith, E. A.

2009-12-01

167

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.

168

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

169

On the observation of unusual high concentration of small chain-like aggregate ice crystals and large ice water contents near the top of a deep convective cloud during the CIRCLE-2 experiment  

NASA Astrophysics Data System (ADS)

During the CIRCLE-2 experiment carried out over Western Europe in May 2007, combined in situ and remote sensing observations allowed to describe microphysical and optical properties near-top of an overshooting convective cloud (11 080 m/-58 C). The airborne measurements were performed with the DLR Falcon aircraft specially equipped with a unique set of instruments for the extensive in situ cloud measurements of microphysical and optical properties (Polar Nephelometer, FSSP-300, Cloud Particle Imager and PMS 2-D-C) and nadir looking remote sensing observations (DLR WALES Lidar). Quasi-simultaneous space observations from MSG/SEVIRI, CALIPSO/CALIOP-WFC-IIR and CloudSat/CPR combined with airborne RASTA radar reflectivity from the French Falcon aircraft flying above the DLR Falcon depict very well convective cells which overshoot by up to 600 m the tropopause level. Unusual high values of the concentration of small ice particles, extinction, ice water content (up to 70 cm-3, 30 km-1 and 0.5 g m-3, respectively) are experienced. The mean effective diameter and the maximum particle size are 43 ?m and about 300 ?m, respectively. This very dense cloud causes a strong attenuation of the WALES and CALIOP lidar returns. The SEVIRI retrieved parameters confirm the occurrence of small ice crystals at the top of the convective cell. Smooth and featureless phase functions with asymmetry factors of 0.776 indicate fairly uniform optical properties. Due to small ice crystals the power-law relationship between ice water content (IWC) and radar reflectivity appears to be very different from those usually found in cirrus and anvil clouds. For a given equivalent reflectivity factor, IWCs are significantly larger for the overshooting cell than for the cirrus. Assuming the same prevalent microphysical properties over the depth of the overshooting cell, RASTA reflectivity profiles scaled into ice water content show that retrieved IWC up to 1 g m-3 may be observed near the cloud top. Extrapolating the relationship for stronger convective clouds with similar ice particles, IWC up to 5 g m-3 could be experienced with reflectivity factors no larger than about 20 dBZ. This means that for similar situations, indication of rather weak radar echo does not necessarily warn the occurrence of high ice water content carried by small ice crystals. All along the cloud penetration the shape of the ice crystals is dominated by chain-like aggregates of frozen droplets. Our results confirm previous observations that the chains of ice crystals are found in a continental deep convective systems which are known generally to generate intense electric fields causing efficient ice particle aggregation processes. Vigorous updrafts could lift supercooled droplets which are frozen extremely rapidly by homogeneous nucleation near the -37 C level, producing therefore high concentrations of very small ice particles at upper altitudes. They are sufficient to deplete the water vapour and suppress further nucleation as confirmed by humidity measurements. These observations address scientific issues related to the microphysical properties and structure of deep convective clouds and confirm that particles smaller than 50 ?m may control the radiative properties in convective-related clouds. These unusual observations may also provide some possible insights regarding engineering issues related to the failure of jet engines commonly used on commercial aircraft during flights through areas of high ice water content. However, large uncertainties of the measured and derived parameters limit our observations.

Gayet, J.-F.; Mioche, G.; Bugliaro, L.; Protat, A.; Minikin, A.; Wirth, M.; Drnbrack, A.; Shcherbakov, V.; Mayer, B.; Garnier, A.; Gourbeyre, C.

2012-01-01

170

On the observation of unusual high concentration of small chain-like aggregate ice crystals and large ice water contents near the top of a deep convective cloud during the CIRCLE-2 experiment  

NASA Astrophysics Data System (ADS)

During the CIRCLE-2 experiment carried out over Western Europe in May 2007, combined in situ and remote sensing observations allowed to describe microphysical and optical properties near-top of an overshooting convective cloud (11 080 m/-58 C). The airborne measurements were performed with the DLR Falcon aircraft specially equipped with a unique set of instruments for the extensive in situ cloud measurements of microphysical and optical properties (Polar Nephelometer, FSSP-300, Cloud Particle Imager and PMS 2D-C) and nadir looking remote sensing observations (DLR WALES Lidar). Quasi-simultaneous space observations from MSG/SEVIRI, CALIPSO/CALIOP-WFC-IIR and CloudSat/CPR combined with airborne RASTA radar reflectivity from the French Falcon aircraft flying above the DLR Falcon depict very well convective cells which overshoot by up to 600 m the tropopause level. Unusual high values of the concentration of small ice particles, extinction, ice water content (up to 70 cm-3, 30 km-1 and 0.5 g m-3, respectively) are experienced. This very dense cloud causes a strong attenuation of the WALES and CALIOP lidar returns. The mean effective diameter is of 43 ?m and the maximum particle size is about 300 ?m. The SEVIRI retrieved parameters confirm the occurrence of small ice crystals at the top of the convective cell. Smooth and featureless phase functions with asymmetry factors of 0.776 indicate fairly uniform optical properties. Due to small ice crystals the power-law relationship between ice water content (IWC) and radar reflectivity appears to be very different from those usually found in cirrus and anvil clouds. For a given equivalent reflectivity factor, IWCs are significantly larger for the overshooting cell than for the cirrus. Assuming the same prevalent microphysical properties over the depth of the overshooting cell, RASTA reflectivity profiles scaled into ice water content show that retrieved IWC up to 1 g m-3 may be observed near the cloud top. Extrapolating the relationship for stronger convective clouds with similar ice particles, IWC up to 5 g m-3 could be experienced with reflectivity factors no larger than about 20 dBZ. This means that for similar situations, indication of rather weak radar echo does not necessarily warn the occurrence of high ice water content carried by small ice crystals. All along the cloud penetration the shape of the ice crystals is dominated by chain-like aggregates of frozen droplets. Our results confirm previous observations that the chains of ice crystals are found in a continental deep convective systems which are known generally to generate intense electric fields causing efficient ice particle aggregation processes. Vigorous updrafts could lift supercooled droplets which are frozen extremely rapidly by homogeneous nucleation near the -37 C level, producing therefore high concentrations of very small ice particles at upper altitudes. They are sufficient to deplete the water vapour and suppress further nucleation as confirmed by humidity measurements. These observations address scientific issues related to the microphysical properties and structure of deep convective clouds and confirm that particles smaller than 50 ?m may control the radiative properties in convective-related clouds. These unusual observations may also provide some possible insights regarding engineering issues related to the failure of jet engines commonly used on commercial aircraft during flights through areas of high ice water content.

Gayet, J.-F.; Mioche, G.; Bugliaro, L.; Protat, A.; Minikin, A.; Wirth, M.; Drnbrack, A.; Shcherbakov, V.; Mayer, B.; Garnier, A.; Gourbeyre, C.

2011-08-01

171

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

172

Possible Evidence for Crystallization of Astrophysical Ice Analogs by Heavy and Energetic Cosmic Rays  

NASA Astrophysics Data System (ADS)

We present an experimental study about the alteration of the 3300 cm-1 band (?_1 vibration mode) in the infrared spectra of water-rich ices due to the bombardments with heavy, highly-charged, and energetic ions (15.7 MeV 16O5+; 46 MeV 58Ni13+). The experiments simulate the physical chemistry as well possible morphological changes induced by heavy-ion cosmic rays at water-rich astrophysical ices. The measurements were performed inside a high vacuum chamber at the heavy-ion accelerator GANIL (Grand Accelerateur National d'Ions Lourds) in Caen, France. The experiments employed pure amorphous water ice and mixed H_2O:CO_2 amorphous ices at 13 K. In-situ analysis was performed by a Fourier transform infrared spectrometer (FTIR) at different ion fluences. After the ion bombardment the center of this water band is shifted to lower frequencies (longer wavelength).We suggest this behavior may be attributed to the destruction of small water clusters (n=2,3), as well the production of larger clusters (n>5), both as a result of energy delivered by the fast ions (and its secondary electrons) along the neighborhood of the ion tracks inside the ices. The vibration of individual small water clusters are representative for the left wing of the ?_1 band in water ice while larger individual clusters are important for the right wing (lower energy). An experiment employing H_2O:CO_2 ice at 80 K, showing a small crystallization degree, does not show changes in the water ?_1 profile during the ion bombardment. The results suggest that a small degree of crystallization may be achieved in the amorphous astrophysical ices after the extensive bombardment with heavy and energetic ions. This may give us some clues about the crystalline water features observed at some cold regions in the interstellar medium such as the ices around young stellar objects, and also at some frozen surfaces of outer solar system bodies. Both regions are highly exposed to galactic cosmic rays.

Pilling, S.; Seperuelo Duarte, E.; da Silveira, E. F.; Rothard, H.; Domaracka, A.; Boduch, P.

2011-05-01

173

FAST TRACK COMMUNICATION: Growth melt asymmetry in ice crystals under the influence of spruce budworm antifreeze protein  

Microsoft Academic Search

Here we describe studies of the crystallization behavior of ice in an aqueous solution of spruce budworm antifreeze protein (sbwAFP) at atmospheric pressure. SbwAFP is an ice binding protein with high thermal hysteresis activity, which helps protect Choristoneura fumiferana (spruce budworm) larvae from freezing as they overwinter in the spruce and fir forests of the north eastern United States and

Natalya Pertaya; Yeliz Celik; Carlos L. Di Prinzio; J. S. Wettlaufer; Peter L. Davies; Ido Braslavsky

2007-01-01

174

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. Inan Tarhan; George H. Watson

1996-01-01

175

Study of the Structure and Electrical Properties of the Liquid-Like Layer on Ice/Air, Ice/Solid and Ice/Liquid Interfaces  

NASA Astrophysics Data System (ADS)

We have studied the thickness and electrical properties of the liquid-like layer (LLL) on the interfaces between single crystals of ice on the one side and air, hexane and solids on the other side. NanoScope III and Auto Probe SA scanning force microscopes were used in the SFM experiments. Force-versus-distance curves (FC) were used to determine the LLL thickness and the adhesion force between ice and microscope tips made of silicon and silicon nitride. These measurements were carried out in the temperature range from -1 dgr C to -30 dgrC on the basal and prismatic planes of pure ice and on ice doped with KOH and NaCl. Similar experiments were also directed on ice/hexane interface. We found that the LLL thickness depends strongly on temperature, doping level and an interface material type. The LLL thickness found in SFM experiments was then compared with the thickness of an ice subsurface layer characterized with high-density space charge. The thickness and density of this electric layer were calculated from data on the layer capacitance, C, and conductance, G. To measure the C and G we developed field effect transistors made of ice. The paper discusses the important role that this electrical layer plays in forming the LLL on the ice surface. Work supported by ARO

Petrenko, V. F.; Khusnatdinov, N. N.; Petrenko, V. V.; Nickolayev, O. V.

1996-03-01

176

Particle habit in tropical ice clouds during CRYSTAL-FACE: Comparison of two remote sensing techniques with in situ observations  

Microsoft Academic Search

Ice crystal shapes in tropical ice clouds are estimated with two different remote sensing methods and compared with measurements from an in situ cloud aerosol spectrometer (CAS) during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE) campaign conducted in Florida during July 2002. The remote sensing techniques use dual-satellite reflectances and lidar linear depolarization

H. Chepfer; V. Noel; P. Minnis; D. Baumgardner; L. Nguyen; G. Raga; M. J. McGill; P. Yang

2005-01-01

177

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

178

THE MECHANICAL PROPERTIES OF SINGLE CRYSTALS OF ICE AT LOW TEMPERATURES  

Microsoft Academic Search

The mechanical properties of single crystals of ice have been investigated at various temperatures down to 90 C. Two methods have been used: creep tests in tension and constant strain-rate tests in compression. Results show that the activation energy for creep varies with temperature from 0.41 -_t 0.03 eV between 50 C and 90 C to 0.68

S. J. JONES; J. W. GLEN

179

Geometric-optics-integral-equation method for light scattering by nonspherical ice crystals.  

PubMed

A new geometric-optics model has been developed for the calculation of the single-scattering and polarization properties for arbitrarily oriented hexagonal ice crystals. The model uses the ray-tracing technique to solve the near field on the ice crystal surface, which is then transformed to the far field on the basis of the electromagnetic equivalence theorem. From comparisons with the results computed by the finite-difference time domain method, we show that the novel geometric-optics method can be applied to the computation of the extinction cross section and single-scattering albedo for ice crystals with size parameters along the minimum dimension as small as ~6. Overall agreement has also been obtained for the phase function when size parameters along the minimum dimension are larger than ~20. We demonstrate that the present model converges to the conventional ray-tracing method for large size parameters and produces single-scattering results close to those computed by the finite-difference time domain method for size parameters along the minimum dimension smaller than ~20. The present geometric-optics method can therefore bridge the gap between the conventional ray-tracing and the exact numerical methods that are applicable to large and small size parameters, respectively. PMID:21127681

Yang, P; Liou, K N

1996-11-20

180

Geometric-optics{endash}integral-equation method for light scattering by nonspherical ice crystals  

SciTech Connect

A new geometric-optics model has been developed for the calculation of the single-scattering and polarization properties for arbitrarily oriented hexagonal ice crystals. The model uses the ray-tracing technique to solve the near field on the ice crystal surface, which is then transformed to the far field on the basis of the electromagnetic equivalence theorem. From comparisons with the results computed by the finite-difference time domain method, we show that the novel geometric-optics method can be applied to the computation of the extinction cross section and single-scattering albedo for ice crystals with size parameters along the minimum dimension as small as {approximately}6. Overall agreement has also been obtained for the phase function when size parameters along the minimum dimension are larger than {approximately}20. We demonstrate that the present model converges to the conventional ray-tracing method for large size parameters and produces single-scattering results close to those computed by the finite-difference time domain method for size parameters along the minimum dimension smaller than {approximately}20. The present geometric-optics method can therefore bridge the gap between the conventional ray-tracing and the exact numerical methods that are applicable to large and small size parameters, respectively. {copyright} {ital 1996 Optical Society of America.}

Yang, P.; Liou, K.N. [Center for Atmospheric and Remote Sounding Studies, University of Utah, 809 William C. Browning Building, Salt Lake City, Utah 84112 (United States)

1996-11-01

181

Ice Construction--Bottom Freezing Techniques for Constructing Shore and Near-Shore Ice Structures.  

National Technical Information Service (NTIS)

New methods and equipment for thickening and strengthening natural sea ice are needed to advance polar operational capabilities. Of the basic categories of techniques for inducing ice growth (ice injection, cold fluid injection, and recirculating fluid), ...

T. L. Culbertson

1971-01-01

182

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

183

Laboratory Growth of Ice Crystals Under Simulated Polar Stratospheric Cloud and High Altitude Cirrus Conditions at Temperatures Below -70 C  

NASA Astrophysics Data System (ADS)

A static diffusion chamber has been used to grow ice crystals at temperatures below -70 C under controlled conditions of temperature, pressure, and ice supersaturation. Type 1 polar stratospheric cloud (PSC) particle growth was performed with frozen nitric acid solution drops in the presence of nitric acid and water vapor at temperatures between -75 C and -85 C. Type 2 PSC particle growth was performed with predominantly pure water at temperatures below -85 C. Ice crystals were also grown from pure water vapor over the same range of temperatures for comparison, nucleating on frozen sulfuric acid solution drops and on mineral dust particles. Linear, projected area, and volume growth rates are presented.

Bailey, M.; Hallett, J.; Peterson, H.; Petersen, D.

2006-12-01

184

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

185

Crystal structures of proline-derived enamines  

PubMed Central

The isolation and structural characterization of both aldehyde- and ketone-derived proline enaminones are reported and discussed. Crystal structures of 10 proline enamines provide information on stereochemical aspects, i.e., double bond configuration and syn- vs. anti-positioning of the carboxylate relative to the enamine double bond. Furthermore, the obtained crystal structures are compared with the density functional theory-calculated structures of the ground and transition state and the postulated SeebachEschenmoser transition state.

Bock, Dominique Anna; Lehmann, Christian W.; List, Benjamin

2010-01-01

186

Photonic Crystal Laser-Driven Accelerator Structures  

SciTech Connect

We discuss simulated photonic crystal structure designs for laser-driven particle acceleration, focusing on three-dimensional planar structures based on the so-called ''woodpile'' lattice. We demonstrate guiding of a speed-of-light accelerating mode by a defect in the photonic crystal lattice and discuss the properties of this mode. We also discuss particle beam dynamics in the structure, presenting a novel method for focusing the beam. In addition we describe some potential coupling methods for the structure.

Cowan, B.; /SLAC

2005-09-19

187

Structure-Function Studies of Native and Recombinant Fish Antifreeze Proteins.  

National Technical Information Service (NTIS)

This project investigates the structures of several fish antifreeze proteins, and how they interact with ice crystals and inhibit ice growth. Formation of hexagonal pit formation on ice crystal basal plane in the presence of fish antifreeze proteins was e...

C. Cheng-DeVries A. L. DeVries

2000-01-01

188

A global view of horizontally oriented crystals in ice clouds from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)  

Microsoft Academic Search

We analyze optical signatures in 18 months of CALIOP layer-integrated backscatter and depolarization ratio to investigate the geographical and seasonal distribution of oriented crystals in ice clouds on a global scale. Oriented crystals are found to be rare: they appear in ?6% of all ice cloud layers, and inside these layers the proportion of oriented crystals is estimated below 5%,

Vincent Noel; Helene Chepfer

2010-01-01

189

Ice-binding site of snow mold fungus antifreeze protein deviates from structural regularity and high conservation.  

PubMed

Antifreeze proteins (AFPs) are found in organisms ranging from fish to bacteria, where they serve different functions to facilitate survival of their host. AFPs that protect freeze-intolerant fish and insects from internal ice growth bind to ice using a regular array of well-conserved residues/motifs. Less is known about the role of AFPs in freeze-tolerant species, which might be to beneficially alter the structure of ice in or around the host. Here we report the 0.95- high-resolution crystal structure of a 223-residue secreted AFP from the snow mold fungus Typhula ishikariensis. Its main structural element is an irregular ?-helix with six loops of 18 or more residues that lies alongside an ?-helix. ?-Helices have independently evolved as AFPs on several occasions and seem ideally structured to bind to several planes of ice, including the basal plane. A novelty of the ?-helical fold is the nonsequential arrangement of loops that places the N- and C termini inside the solenoid of ?-helical coils. The ice-binding site (IBS), which could not be predicted from sequence or structure, was located by site-directed mutagenesis to the flattest surface of the protein. It is remarkable for its lack of regularity and its poor conservation in homologs from psychrophilic diatoms and bacteria and other fungi. PMID:22645341

Kondo, Hidemasa; Hanada, Yuichi; Sugimoto, Hiroshi; Hoshino, Tamotsu; Garnham, Christopher P; Davies, Peter L; Tsuda, Sakae

2012-05-29

190

Crystal Structure of Chabazite K  

SciTech Connect

The crystal structure of the chabazite K with the formula (K{sub 1.33}Na{sub 1.02}Ca{sub 0.84})[Al{sub 4}Si{sub 8}O{sub 24}] {center_dot} 12.17H{sub 2}O from late hydrothermalites in the Khibiny alkaline massif (Kola Peninsula) is established by X-ray diffraction analysis (CAD4 four-circle diffractometer, {lambda}MoK{sub {alpha}} radiation, graphite monochromator, T = 193 K, 2{theta}{sub max} = 70 deg., R{sub 1} = 0.047 for 4745 reflections) on the basis of experimental data (6265 reflections) obtained from a twin (twinning parameter 0.535(1)): a = 13.831(3) A, c = 15.023(5) A, sp. gr. R3-barm, Z = 3, {rho}{sub calcd} = 2.016 g/cm{sup 3} . It is shown that cations occupy five independent positions in large cavities of the tetrahedral Al,Si,O anionic framework in potassium-rich chabazite. A comparative crystallochemical analysis of chabazites of different composition and origin is performed.

Yakubovich, O.V.; Gavrilenko, P.G.; Pekov, I.V. [Moscow State University, Vorob'evy gory, Moscow, 119992 (Russian Federation); Massa, W. [Philipps University, Marburg (Germany)

2005-07-15

191

Crystallization of amorphous ice as the cause of comet P/Halley's outburst at 14 AU.  

PubMed

The post-perihelion eruption of comet P/Halley, detected in Feb. 1991 and believed to have started 3 months earlier, can be explained by crystallization of amorphous ice taking place in the interior of the porous nucleus, at depths a few tens of meters, accompanied by the release of trapped gases. Numerical calculations show that for a bulk density of 0.5 g cm-3 and a pore size of 1 millimicron crystallization occurs on the outbound leg of comet P/Halley's orbit, at heliocentric distances between 5 AU and 17 AU. The trapped gas is released and flows to the surface through the porous medium. It may also open wider channels, as the internal pressures obtained surpass the tensile strength of cometary ice. The outflowing gas carries with it grains of ice and dust, and thus can explain the large amounts of dust observed in the coma at 14.3 AU and beyond. The typical decline time of the process is found to be on the order of months, in agreement with observations. The rate of outgassing is two or three orders of magnitude higher than in quiescence. In an asymmetric, non-uniform nucleus--in contrast to the one-dimensional spherical model--the process should occur intermittently, such as was observed for comet P/Halley beyond 5 AU. PMID:11538062

Prialnik, D; Bar-Nun, A

1992-01-01

192

Crystallization and preliminary X-ray crystallographic analysis of an ice-binding protein (FfIBP) from Flavobacterium frigoris PS1.  

PubMed

Ice growth in a cold environment is fatal for polar organisms, not only because of the physical destruction of inner cell organelles but also because of the resulting chemical damage owing to processes such as osmotic shock. The properties of ice-binding proteins (IBPs), which include antifreeze proteins (AFPs), have been characterized and IBPs exhibit the ability to inhibit ice growth by binding to specific ice planes and lowering the freezing point. An ice-binding protein (FfIBP) from the Gram-negative bacterium Flavobacterium frigoris PS1, which was isolated from the Antarctic, has recently been overexpressed. Interestingly, the thermal hysteresis activity of FfIBP was approximately 2.5?K at 50?M, which is ten times higher than that of the moderately active IBP from Arctic yeast (LeIBP). Although FfIBP closely resembles LeIBP in its amino-acid sequence, the antifreeze activity of FfIBP appears to be much greater than that of LeIBP. In an effort to understand the reason for this difference, an attempt was made to solve the crystal structure of FfIBP. Here, the crystallization and X-ray diffraction data of FfIBP are reported. FfIBP was crystallized using the hanging-drop vapour-diffusion method with 0.1?M sodium acetate pH 4.4 and 3?M sodium chloride as precipitant. A complete diffraction data set was collected to a resolution of 2.9?. The crystal belonged to space group P4(1)22, with unit-cell parameters a = b = 69.4, c = 178.2?. The asymmetric unit contained one monomer. PMID:22750870

Do, Hackwon; Lee, Jun Hyuck; Lee, Sung Gu; Kim, Hak Jun

2012-06-28

193

Microphysical and Optical Properties of Atmospheric Ice Crystals at South Pole Station  

NASA Astrophysics Data System (ADS)

In early February 2001 (during the austral summer), over 900 000 digital images of ice crystals were recorded at the South Pole using two ground-based cloud particle imagers (CPIs). Of these, 721 572 crystals >50 ?m were classified into crystal habits. When sorted by number, 30% of the crystals were rosette shaped (mixed-habit rosettes, platelike polycrystals, and rosette shapes with side planes), 45% were diamond dust (columns, thick plates, and plates), and 25% were irregular. When sorted by area, rosette shapes comprised 50%, diamond dust 30% and irregular 20%. By mass, the percentages were 57% rosette shapes, 23% diamond dust, and 20% irregular. Particle size distributions as a function of maximum dimension and equivalent radius are compared with previous studies. Particles are generally found to be slightly larger than previous austral wintertime studies. In 2002, a polar nephelometer (PN) that measures scattering phase function was incorporated with one of the CPIs. Correlated measurements between the two instruments showed that 22 and 46 peaks in the phase function were present when diamond dust was recorded by the CPI, but not when rosette shapes were present. Visual observations confirmed the presence of 22 and 46 atmospheric halos in some, but not all, of the diamond dust events. No visual halos were observed when rosette shapes were precipitating. Average PN phase functions are presented for diamond dust and rosette shapes. The diamond dust and rosette-shaped ice crystals appear to be very similar in shape to those observed by CPIs in cirrus clouds. Cloud conditions at the South Pole that were associated with various crystal types are discussed, as are some effects of blowing snow.

Lawson, R. Paul; Baker, Brad A.; Zmarzly, Patrick; O'Connor, Darren; Mo, Qixu; Gayet, Jean-Francois; Shcherbakov, Valery

2006-11-01

194

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 clouds optical depth, height, particle size, and water path.

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

2010-12-01

195

Using a quartz crystal microbalance to probe formation of Xe hydrate in thin ice films  

NASA Astrophysics Data System (ADS)

We have demonstrated the ability of the quartz crystal microbalance to successfully measure the formation and dissociation of thin films of Xe hydrate formed from ice films in the presence of Xe gas or from a cooled mixture of water vapor and Xe gas. By monitoring the uptake of mass, we have measured the formation of Xe hydrates on ice films in the temperature range 180

Chan, J.; Forrest, J. A.; Torrie, B. H.

2004-09-01

196

Structural characterization of thin film photonic crystals  

SciTech Connect

We quantitatively analyze the structure of thin film inverse-opal photonic crystals composed of ordered arrays of air pores in a background of titania. Ordering of the sphere template and introduction of the titania background were performed simultaneously in the thin film photonic crystals. Nondestructive optical measurements of backfilling with high refractive index liquids, angle-resolved reflectivity, and optical spectroscopy were combined with band-structure calculations. The analysis reveals a thin film photonic crystal structure with a very high filling fraction (92{endash}94%) of air and a substantial compression along the c axis ({similar_to}22{endash}25%).

Subramania, G.; Biswas, R.; Constant, K.; Sigalas, M. M.; Ho, K. M.

2001-06-15

197

Accuracy of the Anomalous Diffraction approximation to light scattering by column-like ice crystals  

NASA Astrophysics Data System (ADS)

The wavelength and the particle size effects on the accuracy of ADA (Anomalous Diffraction Approximation) method are discussed by comparing the scattering results between ADA method and Mie theory for spherical particles, which indicates that the ADA accuracy depends mainly on the particle size parameter, and is not sensitive to the condition of | m - 1 | ? 1. The results comparison between ADA method and ray tracing method for hexagonal column-like ice crystal particles shows that within the visible wavelength range, results from both methods agree well, and within the infrared wavelength range, there is large discrepancy for co-albedo result due to different assumptions of the ray path inside the particle.

Liu, C.; Jonas, P. R.; Saunders, C. P. R.

198

Refinement of the Crystal Structure of Nacrite  

Microsoft Academic Search

The crystal structure of nacrite from Pike's Peak district, Colorado, has been refined by least squares and electron density difference maps utilizing ten levels of data. Complete refinement was inhibited by thick domains involving a\\/3 interlayer shifts in the \\

Alice M. Blount; I. M. THREADGOLD; S. W. BAILEY

1969-01-01

199

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

Microsoft Academic Search

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

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

2006-01-01

200

High crystalline quality of large single crystals of subglacial ice above Lake Vostok (Antarctica) revealed by hard X-ray diffraction  

Microsoft Academic Search

X-ray diffraction experiments carried out on large single crystals from accreted ice above Lake Vostok, a subglacial lake lying beneath the East Antarctic ice sheet, revealed a surprisingly very low lattice distortion. This crystalline quality does not seem to be affected by impurities. Chloride and sodium appear to be homogeneously distributed in the ice lattice in spite of their relatively

Maurine Montagnat; Paul Duval; P. Bastie; B. Hamelin; M. de Angelis; J. R. Petit; V. Ya Lipenkov

2003-01-01

201

Sensitivity of the Global Distribution of Cirrus Ice Crystal Concentration to Heterogeneous Freezing (Invited)  

NASA Astrophysics Data System (ADS)

This study presents the sensitivity of global ice crystal number concentration, Nc, to the parameterization of heterogeneous ice nuclei (IN). Simulations are carried out with the NASA Global Modeling Initiative (GMI) chemical and transport model coupled to an analytical ice microphysics parameterization. Heterogeneous freezing is described using nucleation spectra derived from theoretical and empirical considerations, considering dust, black carbon, ammonium sulfate, and glassy aerosol as IN precursors. When competition between homogeneous and heterogeneous freezing is considered, global mean Nc vary by up to a factor of twenty depending on the heterogeneous freezing spectrum used. IN effects on Nc strongly depend on dust and black carbon concentrations, and are strongest under conditions of weak updraft and high temperature. Regardless of the heterogeneous spectrum used, dust is an important contributor of IN over large regions of the northern hemisphere. Black carbon however exhibits appreciable effects on when the freezing fraction is greater than 1%. Compared to in situ observations, Nc is overpredicted at temperatures below 205 K, even if a fraction of liquid aerosol is allowed to act as glassy IN. Assuming that cirrus formation is forced by weak updraft addressed this overprediction but promoted heterogeneous freezing effects to the point where homogeneous freezing is inhibited even for IN concentrations as low as 1 L-1. Chemistry and dynamics must be considered to explain cirrus characteristics at low temperature. Only cloud formation scenarios where competition between homogeneous and heterogeneous freezing is the dominant feature would result in maximum supersaturation levels consistent with observations.

Nenes, A.; Barahona, D.; Rodriguez, J. M.

2010-12-01

202

Turbulence beneath sea ice and leads: A coupled sea ice/large-eddy simulation study  

NASA Astrophysics Data System (ADS)

The importance of leads, sea ice motion, and frazil ice on the wintertime ocean boundary layer was examined by using a large-eddy simulation turbulence model coupled to a thermodynamic slab ice model. Coupling was achieved through exchange coefficients that accounted for the differing diffusion rates of heat and salinity. Frazil ice concentrations were modeled by using an ice crystal parameterization with constant crystal size and shape. Stationary ice without leads produced cellular structures similar to atmospheric convection without winds. Ice motion caused this pattern to break down into a series of streaks aligned with the flow. Eddy fluxes were strongly affected by ice motion with relatively larger entrainment fluxes at the mixed layer base under moving ice, whereas stationary ice produced larger fluxes near the top of the boundary layer. Opening of leads caused significant changes in the turbulent structure of the boundary layer. Leads in stationary ice produced concentrated plumes of higher-salinity water beneath the lead. Ice motion caused the lead convection to follow preexisting convective rolls, enhancing the roll circulation salinity and vertical velocity under the lead. Comparison of model time series data with observations from the Arctic Leads Experiment showed general agreement for both pack ice and lead conditions. Simulated heat flux carried by frazil ice had a prominent role in the upper boundary layer, suggesting that frazil ice is important in the heat budget of ice-covered oceans.

Skyllingstad, Eric D.; Denbo, Donald W.

2001-02-01

203

Ice-crystal absorption: a comparison between theory and implications for remote sensing.  

PubMed

The problem of the disagreement between cirrus crystal sizes determined remotely and by in situ measurements is shown to be due to inappropriate application of Mie theory. We retrieved the absorption optical depth at 8.3 and 11.1 mum from 11 tropical anvil cirrus clouds, using data from the High Resolution Infrared Radiation Sounder (HIRS). We related the absorption optical depth ratio between the two wavelengths to crystal size (the size was defined in terms of the crystal median mass dimension) by assuming Mie theory applied to ice spheres and anomalous diffraction theory (ADT) applied to hexagonal columns, hexagonal plates, bullet rosettes, and aggregates (polycrystals). The application of Mie theory to retrievals yielded crystal sizes approximately one third those obtained with ADT. The retrievals of crystal size by use of HIRS data are compared with measurements of habit and crystal size obtained from in situ measurements of tropical anvil cirrus particles. The results of the comparison show that ADT provides the more realistic retrieval. Moreover, we demonstrate that at infrared wavelengths retrieval of crystal size depends on assumed habit. The reason why Mie theory predicts smaller sizes than ADT is shown to result from particle geometry and enhanced absorption owing to the capture of photons from above the edge of the particle (tunneling). The contribution of particle geometry to absorption is three times greater than from tunneling, but this process enhances absorption by a further 35%. The complex angular momentum and T-matrix methods are used to show that the contribution to absorption by tunneling is diminished as the asphericity of spheroidal particles is increased. At an aspect ratio of 6 the contribution to the absorption that is due to tunneling is substantially reduced for oblate particles, whereas for prolate particles the tunneling contribution is reduced by 50% relative to the sphere. PMID:18273143

Baran, A J; Foot, J S; Mitchell, D L

1998-04-20

204

Determination of the ice load on elements of marine hydraulic structures  

SciTech Connect

The purpose of the present work is to investigate the limit state of sea ice interacting with vertical elements of hydraulic structures and to develop a reliable and simple method of determining the maximum ice load on marine structures. The interaction of the ice floe with an obstacle is modeled by the scheme of pressing of rigid punch into an elastoplastic plate. Both the condition of free slipping and condition of adhesion (adfreezing) are examined at the ice-punch interface. The coordinate system and tests for measuring the characteristics of the strength of an ice plate under a uniaxial load are shown.

Gol'din, A.L.; Gladkov, M.G.

1987-01-01

205

Role of small ice crystals in radiative properties of cirrus: A case study, FIRE II, November 22, 1991  

NASA Astrophysics Data System (ADS)

Aircraft observations of cirrus cloud were made near Coffeyville, Kansas, during November 1991 as part of the FIRE II project. Cloud ice particle spectra measurements were made using both a PMS 2DC probe and an ice particle replicator. Particles larger than 200 ?m were column rosettes. The replicator shows the presence of large numbers of ice crystals smaller than 66 ?m (two PMS size bins) that are not recorded by the PMS 2DC probe. Calculations based on the replicator data of the geometrical blocked area and absorption cross section of the cloud per unit volume show that small particles can contribute significantly to and sometimes dominate both the solar extinction and the infrared emission. Intercomparison is made of the ice particle size, area, and mass distributions determined by these different instruments. Power law relationships for area occluded by a crystal as a function of crystal maximum dimension were computed from the PMS 2DC data. The wavelength-dependent infrared absorption cross section per volume was computed using a simple model based on anomalous diffraction and area and mass dimensional relationships for the ice crystals.

Arnott, W. Patrick; Dong, Ya Yi; Hallett, John; Poellot, Michael R.

1994-01-01

206

Sierra Nevada Winter Storms: a Study Using Microwave Radiometry, Ice Crystal and Isotopic Analysis Technique  

NASA Astrophysics Data System (ADS)

An observational study has been made of ice-phase winter storm clouds over the Sierra Nevada mountains. In Part I, two microwave radiometers, one designed with a spinning reflector to shed precipitation particles while the other radiometer's reflector was fixed, are compared. The absence/presence of contaminated periods in the data was attributed to difference in design. These apparent contaminated periods led to lower correlation coefficients between the radiometers. Comparison of radiometer and rawinsonde resulted in a correlation coefficient of 0.97 for the spinning reflector as opposed to 0.8 for the fixed reflector radiometer. In Part II, stable water isotopes were used to study mesoscale and microscale storm modifications by the Sierra Nevada. Initially, a low level warm front lay across the region and its elevation lowered with time from 2.5 km to 1.7 km. This decrease of frontal surface height was accompanied by a steady increase in the delta ^{18}O values. In the pre-cold frontal period, the delta^{18 }O values at the upwind site signified warmer origin ice crystals than the downwind site. This is explained by orographic effects and the production of supercooled liquid water at low elevations on the upslope side. The delta^{18}O value peaked around -13perthous which translates to an "equivalent temperature" of -10.7^circC for ice phase water capture at the upwind site. At the downwind site, this was some 5 to 6 centigrade degrees colder. During surface cold front passage, the differences in delta^{18}O at the two sites are small probably because, during frontal passage, the orography plays a less significant role in the precipitation production process. In Part III, observations of precipitation rates, ice crystals, wind and supercooled liquid water (SLW) upwind and downwind of the Sierra Nevada are presented. Observations show that the stage of development of the storms was important in the liquid and vapor development. High SLW, and increased riming were located before the frontal passage. Duration of SLW as observed by the radiometers, was always shorter over the downwind station. Heavy riming was associated with precipitation decrease while high precipitation rates were correlated with high number fraction of aggregate crystals. Aggregation was found to be an important process for precipitation development over the downwind station.

Demoz, Belay Berhane

207

The Structure of Surface H2O Layers of Ice-covered Planets with High-pressure Ice  

NASA Astrophysics Data System (ADS)

Many extrasolar (bound) terrestrial planets and free-floating (unbound) planets have been discovered. While the existence of bound and unbound terrestrial planets with liquid water is an important question, of particular importance is the question of these planets' habitability. Even for a globally ice-covered planet, geothermal heat from the planetary interior may melt the interior ice, creating an internal ocean covered by an ice shell. In this paper, we discuss the conditions that terrestrial planets must satisfy for such an internal ocean to exist on the timescale of planetary evolution. The question is addressed in terms of planetary mass, distance from a central star, water abundance, and abundance of radiogenic heat sources. In addition, we investigate the structure of the surface H2O layers of ice-covered planets by considering the effects of ice under high pressure (high-pressure ice). As a fiducial case, a 1 M ? planet at 1 AU from its central star and with 0.6-25 times the H2O mass of the Earth could have an internal ocean. We find that high-pressure ice layers may appear between the internal ocean and the rock portion on a planet with an H2O mass over 25 times that of the Earth. The planetary mass and abundance of surface water strongly restrict the conditions under which an extrasolar terrestrial planet may have an internal ocean with no high-pressure ice under the ocean. Such high-pressure ice layers underlying the internal ocean are likely to affect the habitability of the planet.

Ueta, S.; Sasaki, T.

2013-10-01

208

Pholcodine monohydrate: Crystal structure and polymorphism  

NASA Astrophysics Data System (ADS)

The first crystal structure elucidation of pholcodine monohydrate, an important antitussive active pharmaceutical ingredient is reported herein. The studied compound crystallizes in the orthorhombic system in the space group P212121. Each H2O molecule is shared by two pholcodine molecules via three strong hydrogen bonds. The detailed crystallization screening from several different organic solvents afforded single crystals with various quality, all exhibiting prism-to-needlelike micro morphology. The investigation of the obtained single crystals by means of several physico-chemical, solid-state instrumental techniques (FT-IR, DSC, TG/DTG and XRPD) proved that pholcodine monohydrate exists in a single crystalline modification, identical to the commercial form of the compound.

Petruevski, Gjorgji; Zba?nik, Marija; Kajdanoska, Marina; Ugarkovic, Sonja; Trim?eski, Vase; Kaitner, Branko; Jovanovski, Gligor; Makreski, Petre

2013-07-01

209

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

210

The structure and dynamics of amorphous and crystalline phases of ice  

SciTech Connect

The structures of the high and low-density amorphous phases of ice are studied using several techniques. The diffraction patterns of high and low density amorphous ice are analyzed using reverse Monte Carlo methods and compared with molecular dynamics simulations of these phases. The spectra of crystalline and amorphous phases of ice obtained by Raman and incoherent inelastic neutron scattering are analyzed to yield structural features for comparison with the results of molecular dynamics and Reverse Monte Carlo analysis. The structural details obtained indicate that there are significant differences between the structure of liquid water and the amorphous phases of ice.

Klug, D. D.; Tse, J. S.; Tulk, C. A.; Svensson, E. C.; Swainson, I.; Loong, C.-K.

2000-07-14

211

Natural photonic crystals: formation, structure, function  

NASA Astrophysics Data System (ADS)

The structure and properties of natural photonic crystals are discussed using the colored scales of the beetle Lamprocyphus augustus as an example. While the exact mechanism behind the formation of these biopolymeric photonic structures has yet to be fully explored, similarities of these structures to intracellular cubic membrane architectures are introduced. Some crucial parameters behind the formation of cubic membranes are discussed. Using these insights, intracellular cubic membrane structures are transformed into an extracellular environment.

Bartl, Michael H.; Dahlby, Michael R.; Barrows, Frank P.; Richens, Zachary J.; Terooatea, Tommy; Jorgensen, Matthew R.

2012-02-01

212

Crystal structure of plant photosystem I  

Microsoft Academic Search

Oxygenic photosynthesis is the principal producer of both oxygen and organic matter on Earth. The conversion of sunlight into chemical energy is driven by two multisubunit membrane protein complexes named photosystem I and II. We determined the crystal structure of the complete photosystem I (PSI) from a higher plant (Pisum sativum var. alaska) to 4.4 resolution. Its intricate structure shows

Adam Ben-Shem; Felix Frolow; Nathan Nelson

2003-01-01

213

Structure of crystals of hard colloidal spheres  

SciTech Connect

We report light-scattering measurements of powder diffraction patterns of crystals of essentially hard colloidal spheres. These are consistent with structures formed by stacking close-packed planes of particles in a sequence of permitted lateral positions, {ital A},{ital B},{ital C}, which shows a high degree of randomness. Crystals grown slowly, while still containing many stacking faults, show a tendency towards face-centered-cubic packing: possible explanations for this observation are discussed.

Pusey, P.N.; van Megen, W.; Bartlett, P.; Ackerson, B.J.; Rarity, J.G.; Underwood, S.M. (Royal Signals and Radar Establishment, Malvern, WR14 3PS, United Kingsom (GB) Department of Applied Physics, Royal Melbourne Institute of Technology, Melbourne, Victoria, Australia School of Chemistry, Bristol University, Bristol, BS8 1TS, United Kingdom Department of Physics, Oklahoma State University, Stillwater, Oklahoma 74078)

1989-12-18

214

Structure of crystals of hard colloidal spheres  

Microsoft Academic Search

We report light-scattering measurements of powder diffraction patterns of crystals of essentially hard colloidal spheres. These are consistent with structures formed by stacking close-packed planes of particles in a sequence of permitted lateral positions, {ital A},{ital B},{ital C}, which shows a high degree of randomness. Crystals grown slowly, while still containing many stacking faults, show a tendency towards face-centered-cubic packing:

P. N. Pusey; W. van Megen; P. Bartlett; B. J. Ackerson; J. G. Rarity; S. M. Underwood

1989-01-01

215

Structure of crystals of hard colloidal spheres  

Microsoft Academic Search

We report light-scattering measurements of powder diffraction patterns of crystals of essentially hard colloidal spheres. These are consistent with structures formed by stacking close-packed planes of particles in a sequence of permitted lateral positions, A,B,C, which shows a high degree of randomness. Crystals grown slowly, while still containing many stacking faults, show a tendency towards face-centered-cubic packing: possible explanations for

P. N. Pusey; W. van Megen; P. Bartlett; B. J. Ackerson; J. G. Rarity; S. M. Underwood

1989-01-01

216

The Crystal and Molecular Structure of Fluorene  

Microsoft Academic Search

Fluorene, C13H10, crystallizes in the orthorhombic system, space group Pnam with four molecules per unit cell. The molecule possesses a plane of symmetry which is parallel to the (001) plane of the crystal. The structure has been determined by trial-and-error methods, followed (i) by a least-squares refinement and (ii) by two-dimensional Fourier syntheses. The results from the two methods of

D. M. Burns; J. Iball

1955-01-01

217

Crystal growth investigations of ice?water interfaces from molecular dynamics simulations: Profile functions and average properties.  

PubMed

Attempts to simulate crystal growth of ice from liquid water and to provide a consistent microscopic description of this process have been challenging tasks. In this paper we have adapted our previously developed molecular dynamics simulation methodology to enable the investigation of steady-state directional crystal growth?melting of ice. Specifically, we examine ice?water systems of the (001), (110), and (111) faces of ice Ic and the (0001), (1010), and (1120) faces of ice Ih, where the TIP4P, TIP4P-Ew, and SPC?E water models have been utilized. The influence of different growth?melting conditions (temperature gradients and growth velocities) is investigated. Profile functions of properties of interest across the interface are obtained from nonequilibrium steady-state simulations and provide consistent descriptions of ice?water interfaces. The widths of the various crystallographic faces are found to increase in the apparent order Ic111, Ih0001 < Ih1010 < Ih1120 < Ic001 < Ic110. The observed growth rates were in agreement with experimental values and the possible dependence on the various faces is explored. The melting temperatures obtained with the present methodology for the different models are in good agreement with estimates from other work. PMID:21219023

Razul, M S Gulam; Kusalik, P G

2011-01-01

218

A laboratory study on the uptake of HCl, HNO 3, and SO 2 gas by ice crystals and the effect of these gases on the evaporation rate of the crystals  

Microsoft Academic Search

The results of our new and earlier laboratory studies on the uptake of gases by ice crystals are summarized in terms of (1) the equilibrium phase diagram for a system gas\\/H2O, (2) the effect of these gases on the evaporation rate of ice crystals, and (3) in terms of the uptake of the gases by water drops. It is shown

K Diehl; S. K Mitra; H. R Pruppacher

1998-01-01

219

Crystal structure of recombinant bovine neurocalcin  

Microsoft Academic Search

The crystal structure of calcium-bound unmyristoylated bovine neurocalcin from Escherichia coli has been determined at 2.4 resolution. The three-dimensional structure reveals a highly compact structure consisting of: (i) two pairs of calcium-binding EF-hands (EF1-EF2 and EF3-EF4); (ii) a calcium ion bound at EF2, EF3 and EF4 sites; and (iii) an EF1-hand that is disabled from calcium-binding due to a

Senadhi Vijay-Kumar; Vinod D. Kumar

1999-01-01

220

Retrieval of cirrus optical thickness and assessment of ice crystal shape from ground-based imaging spectrometry  

NASA Astrophysics Data System (ADS)

A ground-based hyperspectral imaging spectrometer (AisaEAGLE, manufactured by Specim Ltd., Finland) is applied to measure downward spectral radiance fields with high spatial (1024 spatial pixels within 36.7 field of view), spectral (488 spectral pixels, 400-970 nm, 1.25 nm full width at half maximum), and temporal (4-30 Hz) resolution. The calibration, measurement and data evaluation procedures are introduced. A new method is presented to retrieve the cirrus optical thickness (?ci) using the spectral radiance data collected by AisaEAGLE. The data were collected during the Cloud Aerosol Radiation and tuRbulence of trade wInd cumuli over BArbados (CARRIBA) project in 2011. The spatial inhomogeneity of the investigated cirrus is characterised by the standard deviation of the retrieved ?ci as well as the width of its frequency distribution. By comparing measured and simulated downward solar spectral radiance as a function of scattering angle, some evidence of the prevailing cirrus ice crystal shape can be obtained and subsequently used to substantiate the retrieval of ?ci. The sensitivity of the retrieval method with respect to surface albedo, effective radius (reff), cloud height and ice crystal shape is quantified. An enhanced sensitivity of the retrieved ?ci is found with respect to the surface albedo (up to 30%) and ice crystal shape (up to 90%). The sensitivity with regard to the effective ice crystal radius (≤ 5%) and the cloud height (≤ 0.5%) is rather small and can be neglected.

Schfer, M.; Bierwirth, E.; Ehrlich, A.; Heyner, F.; Wendisch, M.

2013-08-01

221

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

National Technical Information Service (NTIS)

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. Howev...

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

2000-01-01

222

Inhibition of bacterial ice nucleators by fish antifreeze glycoproteins.  

PubMed

Certain bacteria promote the formation of ice in super-cooled water by means of ice nucleators which contain a unique protein associated with the cell membrane. Ice nucleators in general are believed to act by mimicking the structure of an ice crystal surface, thus imposing an ice-like arrangement on the water molecules in contact with the nucleating surface and lowering the energy necessary for the initiation of ice formation. Quantitative investigation of the bacterial ice-nucleating process has recently been made possible by the discovery of certain bacteria that shed stable membrane vesicles with ice nucleating activity. The opposite effect, inhibition of ice formation, has been described for a group of glycoproteins found in different fish and insect species. This group of substances, termed antifreeze glycoproteins (AFGPs), promotes the supercooling of water with no appreciable effect on the equilibrium freezing point or melting temperature. Substantial evidence now indicates that AFGPs act by binding to a growing ice crystal and slowing crystal growth. As the ice-nucleating protein surface is believed to have a structure similar to an embryonic ice crystal, AFGPs might be predicted to interact directly with a bacterial ice-nucleating site. We report here that AFGPs from the antarctic fish Dissostichus mawsoni inhibit the ice-nucleating activity of membrane vesicles from the bacterium Erwinia herbicola. The inhibition effect shows saturation at high concentration of AFGP and conforms to a simple binding reaction between the AFGP and the nucleation centre. PMID:3386720

Parody-Morreale, A; Murphy, K P; Di Cera, E; Fall, R; DeVries, A L; Gill, S J

1988-06-23

223

Factors controlling the bifurcation structure of sea ice retreat  

NASA Astrophysics Data System (ADS)

The contrast in surface albedo between sea ice and open ocean suggests the possibility of an unstable climate state flanked by two separate stable climate states. Previous studies using idealized single-column models and comprehensive climate models have considered the possibility of abrupt thresholds during sea ice retreat associated with such multiple states, and they have produced a wide range of results. When the climate is warmed such that the summer minimum Arctic sea ice cover reaches zero, some models smoothly transition to seasonally ice-free conditions, others discontinuously transition to seasonally ice-free conditions, and others discontinuously transition to annually ice-free conditions. Among the models that simulate a continuous transition to seasonally ice-free conditions, further warming causes some to smoothly lose the remaining wintertime-only sea ice cover and others to discontinuously lose it. Here, we use a toy model representing the essential physics of thermodynamic sea ice in a single column to investigate the factors controlling which of these scenarios occurs. All of the scenarios are shown to be possible in the toy model when the parameters are varied, and physical mechanisms giving rise to each scenario are investigated. We find that parameter shifts that make ice thicker or open ocean warmer under a given climate forcing make models less prone to stable seasonally ice-free conditions and more prone to bistability and hence bifurcations. The results are used to interpret differences in simulated sea ice stability in comprehensive climate models.

Eisenman, Ian

2012-01-01

224

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

225

Density and surface temperature of graupel and the charge separation during ice crystal interactions  

NASA Astrophysics Data System (ADS)

Significant amounts of charge are separated when vapor-grown ice crystals interact with riming graupel a mechanism widely believed to be responsible for the generation of electric charge in thunderstorms. This study shows that the density of graupel, by itself, is not an effective parameter in determining the sign of the separated charge. The well-known charge sign reversal temperature is found to be strongly influenced by the surface temperature of the rime at low cloud water content (CWC). At a cloud temperature of -10C and a CWC of 0.4 g m-3 the charging current to the graupel reversed from positive to negative as the surface temperature of the graupel exceeded -6C. The maximum CWC at which the sign could thus be reversed increased as the cloud temperature decreased, ranging from less than 0.7 g m-3 at -10C to less than 0.9 g m-3 at -20C. At higher cloud water contents the rime surface temperature had no effect on the charging sign. The observations are shown to be broadly explicable in terms of the hypothesis where, during an interaction between two ice particles, the particle that is growing faster from the vapor acquires the positive charge.

Jayaratne, E. R.

1998-06-01

226

Ice flow field over Lake Vostok, East Antarctica inferred by structure tracking  

NASA Astrophysics Data System (ADS)

Here, we present a flow field for the East Antarctic ice sheet over Lake Vostok based on tracking structures across the lake inherited from the western, upstream shoreline. The structures, distinctive peaks and troughs identified in the ice sheet internal layers, are imaged in ice-penetrating radar data collected in a systematic grid over the lake. These structures are resolved in three distinct internal layers at depths between 900 and 3750 m. Ridges in the internal layers are associated with prominent topographic highs along the western shoreline, and troughs are correlated with shoreline topographic lows. Lake Vostok is bisected by a topographic ice divide; south of this topographic divide the ice flow is dominantly NNW to SSE and north of the divide the ice flow is W to E. The flow over the lake is also deflected by the small (0.02%) southward lake surface slope. Accretion ice, lake water frozen to the bottom of the ice sheet, is preferentially imaged along flowlines emanating from topographic ridges. The coincidence of the accretion ice reflector with our flowlines both provides independent support for our new flow field, and suggests focused accretion along the western shoreline. Global positioning system (GPS) data provide additional independent support for our new flow field in the southern lake. The new flow field will be crucial for subsequent analyses of mass exchange between the ice sheet and the lake, identifying areas of melting above the lake and estimating residence times.

Tikku, Anahita A.; Bell, Robin E.; Studinger, Michael; Clarke, Garry K. C.

2004-11-01

227

Crystal structure of Arabidopsis thaliana cytokinin dehydrogenase  

SciTech Connect

Since first discovered in Zea mays, cytokinin dehydrogenase (CKX) genes have been identified in many plants including rice and Arabidopsis thaliana, which possesses CKX homologues (AtCKX1-AtCKX7). So far, the three-dimensional structure of only Z. mays CKX (ZmCKX1) has been determined. The crystal structures of ZmCKX1 have been solved in the native state and in complex with reaction products and a slowly reacting substrate. The structures revealed four glycosylated asparagine residues and a histidine residue covalently linked to FAD. Combined with the structural information, recent biochemical analyses of ZmCKX1 concluded that the final products of the reaction, adenine and a side chain aldehyde, are formed by nonenzymatic hydrolytic cleavage of cytokinin imine products resulting directly from CKX catalysis. Here, we report the crystal structure of AtCKX7 (gene locus At5g21482.1, UniProt code Q9FUJ1).

Bae, Euiyoung; Bingman, Craig A.; Bitto, Eduard; Aceti, David J.; Phillips, Jr., George N. (UW)

2008-08-13

228

A temperature-dependent, structural-optical model of first-year sea ice  

Microsoft Academic Search

A model has been developed that relates the structural properties of first-year sea ice to its inherent optical properties, quantities needed by detailed radiative transfer models. The structural-optical model makes it possible to calculate absorption coefficients, scattering coefficients, and phase functions for the ice from information about its physical properties. The model takes into account scattering by brine inclusions in

B. Light; G. A. Maykut; T. C. Grenfell

2004-01-01

229

Requirements for structure determination of aperiodic crystals  

NASA Astrophysics Data System (ADS)

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 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, Xiao-Ou; Stern, Edward A.; Ma, Yanjun

1991-01-01

230

Crystal structure of methane oxidation enzyme determined  

SciTech Connect

A team of chemists has determined to 2.2-[angstrom] resolution the crystal structure of the hydroxylase protein of methane monooxygenase, the enzyme system responsible for the biological oxidation of methane. The hydroxylase, at a molecular weight of 251,000 daltons, if by far the largest component of methane monooxygenase. Although the crystal structure of the hydroxylase did not reveal any startling surprises about the enzyme-many features of the hydroxylase had been inferred previously from modeling and spectroscopic studies -- obtaining it is a significant achievement. For one thing, the crystal structure unambiguously confirms aspects of the enzyme structure that been at least somewhat speculative. The three-dimensional structure of the enzyme, the chemist say, also provides important insight into biological methane oxidation, including how methane, a relatively inert gas, might diffuse to and bind near the active site of the enzyme. The structure points to particular amino acid residues that are likely to participate in catalysis, and clarifies the structure of the dinuclear iron core of the enzyme.

Baum, R.

1994-01-10

231

The peculiarities of water crystallization and ice melting processes in the roots of one-year plants (Plantago major L.).  

PubMed

Results are presented of a water phase transition study in plantain (Plantago major L.) roots, which were used as a model system to research the peculiarities of water crystallization and ice melting processes in complex heterogeneous biological systems. It was confirmed that water in such systems is crystallized in two clearly distinguished temperature ranges: -10 to -25 degree capital ES, Cyrillic and -25 to -45 degree capital ES, Cyrillic. These water fractions are conditionally attributed to extracellular (-10 to -25 degree capital ES, Cyrillic) and intracellular (-25 to -45 degree capital ES, Cyrillic) solutions. A possible explanation is given for such significant supercooling of the intracellular solution. The values of osmotic pressures of extra- and intracellular solutions were determined according to ice melting curves. It is noted that the intracellular solution, which crystallized at lower temperatures, had a lower osmotic pressure. PMID:18754062

Bakradze, N; Kiziria, E; Sokhadze, V; Gogichaishvili, S

232

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

233

Crystal and molecular structure of paclitaxel (taxol).  

PubMed Central

Paclitaxel (formerly called taxol), an important anticancer drug, inhibits cell replication by binding to and stabilizing microtubule polymers. As drug-receptor interactions are governed by the three-dimensional stereochemistries of both participants, we have determined the crystal structure of paclitaxel to identify its conformational preferences that may be related to biological activity. The monoclinic crystals contain two independent paclitaxel molecules in the asymmetric unit plus several water and dioxane solvent molecules. Taxane ring conformation is very similar in both paclitaxel molecules and is similar to the taxane ring conformation found in the crystal structure of the paclitaxel analogue docetaxel (formerly called taxotere). The two paclitaxel molecules have carbon-13 side-chain conformations that differ from each other and from that of the corresponding side chain in the docetaxel crystal structure. The carbon-13 side-chain conformation of one paclitaxel molecule is similar to what was proposed from NMR studies done in polar solvents, while that of the other paclitaxel molecule is different and hitherto unobserved. The paclitaxel molecules interact with each other and with solvent atoms through an extensive network of hydrogen bonds. Analysis of the hydrogen-bonding network together with structure-activity studies may suggest which atoms of paclitaxel are important for binding to microtubule receptors.

Mastropaolo, D; Camerman, A; Luo, Y; Brayer, G D; Camerman, N

1995-01-01

234

Crystal Structure of a Plectonemic RNA Supercoil  

PubMed Central

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

2012-01-01

235

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

236

Structure analysis on synthetic emerald crystals  

NASA Astrophysics Data System (ADS)

Single crystals of emerald synthesized by means of the flux method were adopted for crystallographic analyses. Emerald crystals with a wide range of Cr3+-doping content up to 3.16 wt% Cr2O3 were examined by X-ray single crystal diffraction refinement method. The crystal structures of the emerald crystals were refined to R 1 (all data) of 0.019-0.024 and w R 2 (all data) of 0.061-0.073. When Cr3+ substitutes for Al3+, the main adjustment takes place in the Al-octahedron and Be-tetrahedron. The effect of substitution of Cr3+ for Al3+ in the beryl structure results in progressively lengthening of the Al-O distance, while the length of the other bonds remains nearly unchanged. The substitution of Cr3+ for Al3+ may have caused the expansion of a axis, while keeping the c axis unchanged in the emerald lattice. As a consequence, the Al-O-Si and Al-O-Be bonding angles are found to decrease, while the angle of Si-O-Be increases as the Al-O distance increases during the Cr replacement.

Lee, Pei-Lun; Lee, Jiann-Shing; Huang, Eugene; Liao, Ju-Hsiou

2013-05-01

237

Crystal Structure of Dipotassium Carbonate Trihydrate  

Microsoft Academic Search

The crystal structure of 2K2CO3?3H2O was determined by a sign-correlation procedure using three-dimensional data and refined by anisotropic least squares. The space group is C2?c with unit cell dimensions a = 11.887,b = 13.827,c = 7.112 and ? = 120.56. The structure consists of columns of hydrogen-bonded carbonate ions and water molecules within and between which lie the cations.

F. D. Hunter; G. A. Jeffrey

1967-01-01

238

Crystal structure of the 4-chromanone derivative.  

PubMed

The title compound was extracted from a natural product and its structure was characterized by an X-ray diffraction method. It crystallizes in the tetragonal space group P41 with cell parameters a = 15.832(10)A, c = 11.622(10)A, Z = 4; the final residual factor is R1 = 0.0769. The structure has both intra and intermolecular hydrogen bonds. PMID:15055979

Doreswamy, Beeranahally H; Mahendra, Madegowda; Devarajegowda, Hirihally C; Devaru, Venkatesh B; Anandalwar, Sridhar M; Prasad, Javaregowda S

2004-02-01

239

Complex crystal structure of cesium-III.  

PubMed

The structure of Cs-III, stable between 4.2 and 4.3 GPa at room temperature, has been determined from single-crystal x-ray diffraction data. Rather than the simple fcc structure previously reported [Hall et al., Science 146, 1297 (1964)], the data yield a complex new type of elemental structure which is orthorhombic (space group C222(1)) with 84 atoms in the unit cell. No evidence could be found for the fcc form reported previously, even in a further experiment, conducted under conditions close to those used by Hall et al., which also yielded the 84-atom structure. PMID:11736587

McMahon, M I; Nelmes, R J; Rekhi, S

2001-11-30

240

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

241

Evolution of crystal structures in metallic elements  

NASA Astrophysics Data System (ADS)

Crystal structures of metals are often treated as dense packing of atomic spheres. Face-centered cubic and hexagonal close-packed structures are favored in many metals. Long-period-stacking structures such as 9R are sometimes formed. However, nonclose packed structures such as body centered cubic and ? are formed depending upon chemistry and process conditions. Even in metallic elements, it is a priori unknown how such close/nonclose packed structures are formed and what are their interrelationships. In the present study we show a simple algorithm for automated searching of the phase-transition pathway based upon first-principles calculations, which is applied to systematically pursue the evolution of crystal structures. Following the present algorithm, dynamical stability and interrelationships of different structures generated from a simple cubic structure are revealed for seven metallic elements. Effects of pressure are examined as well. The powerfulness of the automated method to investigate the nature of the phase transition and to predict as-yet-unknown metastable structures is demonstrated.

Togo, Atsushi; Tanaka, Isao

2013-05-01

242

The crystal structures of ? and ? * nitrogen  

NASA Astrophysics Data System (ADS)

The crystal structures of the high-pressure ? and ? * phases of nitrogen have been investigated using single-crystal x-ray diffraction. The structure of the ? phase is very similar to isostructural ?-O2 and comprises spherically disordered molecules, with a preference for avoiding pointing along the cubic <100> directions, and disklike molecules with a uniform distribution of orientations. The structure of the ? * phase is tetragonal and the space group is identified unambiguously as P42/ncm with unit cell parameters of a=8.603(5) A? and c=5.685(5) A? at 14.5 GPa. The orientations of the partially disordered molecules have been experimentally determined for the first time and are similar to those predicted on the basis of molecular dynamics simulations.

Stinton, G. W.; Loa, I.; Lundegaard, L. F.; McMahon, M. I.

2009-09-01

243

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

244

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

245

Crystal Structure of Toxoplasma gondii Porphobilinogen Synthase  

PubMed Central

Porphobilinogen synthase (PBGS) is essential for heme biosynthesis, but the enzyme of the protozoan parasite Toxoplasma gondii (TgPBGS) differs from that of its human host in several important respects, including subcellular localization, metal ion dependence, and quaternary structural dynamics. We have solved the crystal structure of TgPBGS, which contains an octamer in the crystallographic asymmetric unit. Crystallized in the presence of substrate, each active site contains one molecule of the product porphobilinogen. Unlike prior structures containing a substrate-derived heterocycle directly bound to an active site zinc ion, the product-bound TgPBGS active site contains neither zinc nor magnesium, placing in question the common notion that all PBGS enzymes require an active site metal ion. Unlike human PBGS, the TgPBGS octamer contains magnesium ions at the intersections between pro-octamer dimers, which are presumed to function in allosteric regulation. TgPBGS includes N- and C-terminal regions that differ considerably from previously solved crystal structures. In particular, the C-terminal extension found in all apicomplexan PBGS enzymes forms an intersubunit ?-sheet, stabilizing a pro-octamer dimer and preventing formation of hexamers that can form in human PBGS. The TgPBGS structure suggests strategies for the development of parasite-selective PBGS inhibitors.

Jaffe, Eileen K.; Shanmugam, Dhanasekaran; Gardberg, Anna; Dieterich, Shellie; Sankaran, Banumathi; Stewart, Lance J.; Myler, Peter J.; Roos, David S.

2011-01-01

246

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

247

Analysis and design of an ice wall framing system for an arctic drilling structure  

SciTech Connect

The exterior shell of a concrete base structure for an arctic oil drilling platform must be designed to resist extremely high local ice pressures. Stringent draft criteria for deployment of these structures in shallow waters require that the exterior shells, commonly called ice walls, have minimal weight in conjunction with maximum strength. These conflicting requirements are satisfactorily balanced by the selection of an arch shape on the interior face of the wall. This geometry induces arching action within the ice wall plate to resist ice loads in compression, thereby minimizing principal tension within the concrete. The development of principal tension in a concrete structure limits its ability to resist external forces. This paper describes the design of a prestressed concrete ice wall which satisfies these load, weight, and material constraints.

Schlechten, J.R.; Bivens, H.R.; Dolan, D.K.; Fernandes, R.L.

1984-05-01

248

Winter phytoplankton community structure in three shallow temperate lakes during ice cover  

Microsoft Academic Search

The general model of seasonal phytoplankton succession in temperate lakes suggests that winter phytoplankton growth is minimal under ice-cover. However, some studies have found diverse phytoplankton communities during winter. The primary objectives of this study were to determine the species composition and the changes in the winter phytoplankton community structure under the ice. For 2 consecutive winters, phytoplankton samples were

Karen A. Phillips; Marvin W. Fawley

2002-01-01

249

Structural map of flow variability and propagation behavior in the Ross Ice Shelf  

NASA Astrophysics Data System (ADS)

Fracture geometries in the Ross Ice Shelf, observable using visible band satellite imagery from the MODIS Mosaic of Antarctica (MOA) and the Landsat Image Mosaic of Antarctica (LIMA) provide a unique opportunity to study fracture propagation behavior and discharge variability in the ice streams and outlet glaciers feeding the shelf. Propagation is driven by changes in fracture length, near-field stress conditions, and the material properties of the ice. Changes in ice stream discharge and the development of "sticky spots," in both ice streams and within the shelf, lead to redirection of flow, changes in lateral gradients of ice velocity, and the propagation of fractures in response to changes in near-field stresses. The propagation behaviors most commonly observed in the ice shelf are the growth in the transverse direction of a fracture that formed within a shear zone and mechanical interactions between adjacent fracture tips. We use fracture mechanics theory and remote-sensed imagery to categorize fracture patterns and longitudinal zones of fractured ice in the Ross Ice Shelf. Near current sites of formation, simple fracture geometries and principal stresses are used to illustrate physical processes related to the formation and propagation of fractures. To compute flow lines and principal stresses, we derive a velocity map of the Ross Ice Shelf by merging two velocity datasets using a combination of statistical methods. A structural map of fracture geometries, relict shear margins, and structural boundaries is constructed. Using the ice shelf features, present-day flow lines, and principal stresses, we investigate the manner in which principal stresses affect fracture formation and propagation behavior and the variability of ice stream discharge into the shelf.

LeDoux, C. M.; Hulbe, C. L.

2011-12-01

250

Changes in the Velocity Structure of the Greenland Ice Sheet  

Microsoft Academic Search

Using satellite radar interferometry observations of Greenland, we detected widespread glacier acceleration below 66 north between 1996 and 2000, which rapidly expanded to 70 north in 2005. Accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year. As more glaciers accelerate

Eric Rignot; Pannir Kanagaratnam

2006-01-01

251

Photonic crystal and photonic wire device structures  

NASA Astrophysics Data System (ADS)

Photonic devices that exploit photonic crystal (PhC) principles in a planar environment continue to provide a fertile field of research. 2D PhC based channel waveguides can provide both strong confinement and controlled dispersion behaviour. In conjunction with, for instance, various electro-optic, thermo-optic and other effects, a range of device functionality is accessible in very compact PhC channel-guide devices that offer the potential for high-density integration. Low enough propagation losses are now being obtained with photonic crystal channel-guide structures that their use in real applications has become plausible. Photonic wires (PhWs) can also provide strong confinement and low propagation losses. Bragg-gratings imposed on photonic wires can provide dispersion and frequency selection in device structures that are intrinsically simpler than 2D PhC channel guides--and can compete with them under realistic conditions.

De La Rue, Richard; Sorel, Marc; Johnson, Nigel; Rahman, Faiz; Ironside, Charles; Cronin, Lee; Watson, Ian; Martin, Robert; Jin, Chongjun; Pottier, Pierre; Chong, Harold; Gnan, Marco; Jugessur, Aju; Camargo, Edilson; Erwin, Grant; Md Zain, Ahmad; Ntakis, Iraklis; Hobbs, Lois; Zhang, Hua; Armenise, Mario; Ciminelli, Caterina; Coquillat, Dominique

2005-09-01

252

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

253

Fission fragment damage to crystal structures  

Microsoft Academic Search

AlO and ZrSiO show little change in properties when ; exposed to 10¹⁸ to 10¹⁹ nvt of thermal neutrons. When, however, a ; source of fission fragments is provided in the form of a UO dispersant, ; the grain boundaries and the peaks of the x-ray-diffraction profile disappear on ; irradiation. The crystal structure of UO is also destroyed when

B. Berman; M. L. Bleiberg; W. Yeniscavich

1960-01-01

254

Crystal structure of a Rad51 filament  

Microsoft Academic Search

Rad51, the major eukaryotic homologous recombinase, is important for the repair of DNA damage and the maintenance of genomic diversity and stability. The active form of this DNA-dependent ATPase is a helical filament within which the search for homology and strand exchange occurs. Here we present the crystal structure of a Saccharomyces cerevisiae Rad51 filament formed by a gain-of-function mutant.

Adam B Conway; Thomas W Lynch; Ying Zhang; Gary S Fortin; Cindy W Fung; Lorraine S Symington; Phoebe A Rice

2004-01-01

255

Crystal Structure of Kaolinite: Dimethylsulfoxide Intercalate  

Microsoft Academic Search

The crystal structure of the kaolinite : dimethylsulfoxide (DMSO) intercalate (P 1, a = 5.187(2), b = 8.964(3), c = 11.838(4) A, a = 91.53(1) ~ = 108.59(2) ~ T = 89.92(1)*) has been determined using spectroscopic and X-ray and neutron powder diffraction data. Both the X-ray and neutron powder diffraction patterns were refined. Solid-state ~3C, 298i, and 27A1 nuclear

J. G. Thompson; C. CUFF

1985-01-01

256

The crystal structure of potassium dimolybdate hydrate  

Microsoft Academic Search

K2Mo2O7 . H2O crystallizes in the triclinic system with unit-cell dimensions a = 7.640(1), b = 8.909(1), c = 7.654(1), , alpha = 109.42(1), beta = 95.75(1), gamma = 119.19(2), and space group P1 with Z = 2. The structure was solved by Patterson and Fourier methods. Of the 2361 unique reflections measured by counter techniques, 2229 with I >=

B. M. Gatehouse; A. J. Jozsa

1987-01-01

257

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

258

High-frequency microwave anti-/de-icing system for carbon-reinforced airfoil structures  

NASA Astrophysics Data System (ADS)

An aircraft may be subjected to icing for a variety of meteorological reasons during the flight. Ice formation on the plane and in particular on the aerodynamically carrying structures adversely affects the flight behaviour. Conventional de-icing methods for aluminum wings are characterised by a high energy consumption during the flight and slow ice melting due to thermal diffusion of the heat in the wing material. In addition to advanced turbines, novel materials and composites have to be used in order to reduce the weight and, hence, the fuel consumption. These composite materials have a far worse thermal conductivity than metals and undergo delamination when hot air systems, resistance or ohmic heating mats are used. In the paper, the unique advantages of a novel High Frequency Microwave Anti-/De-icing System for large future aircraft with carbon reinforced leading edge structures are presented.

Feher, Lambert; Thumm, Manfred

2001-08-01

259

Imaging ice-like structures formed on HOPG at room temperature.  

PubMed

In this work, ice was viewed at the nanoscale by scanning an atomic force microscopy tip over a highly oriented pyrolytic graphite (HOPG) surface in air. At low scan velocities, the tip exhibited stick-slip motion with a period of 0.13 nm corresponding to the scanner step; at higher velocities, the HOPG lattice and the periodicity of the ice were visible. A hexagonal structure with a 0.45 0.04 nm periodicity was observed in which the distance between the second neighbors of the HOPG coincided with the distance of the first neighbors for the ice-like arrangement. Small water clusters were also nucleated with an ice-Ic structure (0.34 0.03 nm), and thus, the ice layers consisted of extensive sets composed of arrangements of hexamers and tetramers. PMID:20932040

Teschke, Omar

2010-10-08

260

Crystal-field states of Pr3+ in the candidate quantum spin ice Pr2Sn2O7  

NASA Astrophysics Data System (ADS)

Neutron time-of-flight spectroscopy has been employed to study the crystal-field splitting of Pr3+ in the pyrochlore stannate Pr2Sn2O7. The crystal field has been parameterized from a profile fit to the observed neutron spectrum. The single-ion ground state is a well-isolated non-Kramers doublet of ?3+ symmetry with a large Ising-like anisotropy, ?zz/???60 at 10 K, but with a significant admixture of terms |MJ?J>, which can give rise to quantum zero-point fluctuations. This magnetic state satisfies the requirements for quantum spin-ice behavior.

Princep, A. J.; Prabhakaran, D.; Boothroyd, A. T.; Adroja, D. T.

2013-09-01

261

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

262

Crystal Structure, Chemical Binding, and Lattice Properties  

NASA Astrophysics Data System (ADS)

This chapter starts with an overview of the ZnO crystal structure and its conjunction to the chemical binding. ZnO commonly occurs in the wurtzite structure. This fact is closely related to its tetrahedral bond symmetry and its prominent bond polarity. The main part of the first section deals with the ZnO wurtzite crystal lattice, its symmetry properties, and its geometrical parameters. Besides wurtzite ZnO, the other polytypes, zinc-blende and rocksalt ZnO are also briefly discussed. Subsequently, lattice constant variations and crystal lattice deformations are treated. This discussion starts with static lattice constant variations, induced by temperature or by pressure, as well as strain-induced static lattice deformation, which reduces the crystal symmetry. The impact of this symmetry reduction on the electrical polarization is the piezo effect, which is very much pronounced in ZnO and is exploited in many applications. See also Chap. 13. Dynamic lattice deformations manifest themselves as phonons and, in case of doping, as phonon-plasmon mixed states. The section devoted to phonons starts with a consideration of the vibration eigenmodes and their dispersion curves. Special attention is paid to the investigation of phonons by optical spectroscopy. The methods applied for this purpose are infrared spectroscopy and, more often, Raman spectroscopy. The latter method is very common for the structural quality assessment of ZnO bulk crystals and layers; it is also frequently used for the study of the incorporation of dopant and alloying atoms in the ZnO crystal lattice. Thus, it plays an important role with regard to possible optoelectronics and spintronics applications of ZnO. The final section of this chapter focuses on phonon-plasmon mixed states. These eigenstates occur in doped ZnO due to the strong coupling between collective free-carrier oscillations and lattice vibrations, which occurs due to the high bond polarity. Owing to the direct correlation of the plasmon-phonon modes to the electronic doping, they are an inherent property of ZnO samples, when applied in (opto-) electronics and spintronics. See also Chap. 12.

Geurts, J.

263

Application of ground-based hyperspectral imaging to retrieve ice crystal shape and fields of cirrus optical thickness  

NASA Astrophysics Data System (ADS)

A ground-based hyperspectral imaging spectrometer (AisaEAGLE) is applied to measure downward spectral radiance fields with high spatial (1024 spatial pixels within 36.7 field of view), spectral (488 spectral pixels, 400-970 nm, 1.25 nm full width at half maximum) and temporal (4-30 Hz) resolution. The calibration, measurement, and data evaluation procedures are introduced. A method is presented to retrieve the cirrus optical thickness ?ci using ground-based spectral radiance data collected by AisaEAGLE. On the basis of four measurement cases during the second campaign of the Cloud Aerosol Radiation and tuRbulence of trade wInd cumuli over BArbados (CARRIBA) project in 2011 the spatial inhomogeneity of the investigated cirrus is characterized by the standard deviation of the retrieved ?ci, as well as the width of the frequency distribution of the retrieved ?ci. By comparing measured and simulated downward solar radiance as a function of scattering angle, a first estimation of the detected cirrus ice crystal shape is given and used in the retrieval of the ?ci. The sensitivity of the retrieval method with respect to surface albedo, effective radius reff, cloud height, and ice crystal shape was characterized. Significant sensitivities of the retrieval method were found for the assumed surface albedo (up to 30%) and ice crystal shape (up to 90%). The sensitivity with regard to the effective radius (≤ 5%) and the cloud height (≤ 0.5%) is rather small and can be neglected.

Schfer, M.; Bierwirth, E.; Ehrlich, A.; Heyner, F.; Wendisch, M.

2013-02-01

264

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

NASA Astrophysics Data System (ADS)

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 HO2 radicals, as well as the oxygen O2 and hydrogen peroxide H2O2 molecules in irradiated porous amorphous solid water (p-ASW) and crystalline (Icryst) ice films. The evolution of their concentrations with the temperature indicates that HO2, O2, and H2O2 result from a simple step reaction fuelled by OH, where O2 is a product of HO2 and HO2 a product of H2O2. The local order of ice is also modified, whatever the initial structure is. The crystalline ice Icryst becomes amorphous. The high-density amorphous phase (Iah) of ice is observed after irradiation of the p-ASW film, whose initial structure is the normal low-density form of the amorphous ice (Ial). The phase Iah 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 Iavh-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.

2006-11-01

265

Geostatistical Characterization of Snow-Depth Structures on Sea Ice Near Point Barrow, AlaskaA Contribution to the AMSR-Ice03 Field Validation Campaign  

Microsoft Academic Search

The objective of this paper is to characterize spatial properties of snow-depth structures and their role as indicators of sea-ice properties and sea-ice-morphogenetic processes, and to provide quantitative measures of sea-ice properties that may be utilized in analyses of passive-microwave data. Snow-depth data collected near Point Barrow, Alaska, as part of the AMSRIce03 Field Validation Campaign for Advanced Microwave Scanning

Ute C. Herzfeld; James A. Maslanik; Matthew Sturm

2006-01-01

266

Physical and structural properties of the Greenland Ice Sheet Project 2 ice core: A review  

Microsoft Academic Search

Substantial data sets have been collected on the relaxation characteristics, density, grain size, c axis fabrics, and ultrasonic velocities of the Greenland Ice Sheet Project 2 (GISP2) core to its contact with bedrock at 3053.4 m. Changes in all these properties paralleled closely those found in cores from Byrd Station, Antarctica, and Dye 3, Greenland. Density increased progressively with depth

A. J. Gow; D. A. Meese; R. B. Alley; J. J. Fitzpatrick; S. Anandakrishnan; G. A. Woods; B. C. Elder

1997-01-01

267

Physical and structural properties of the Greenland Ice Sheet Project 2 ice core: A review  

Microsoft Academic Search

Substantial data sets have been collected on the relaxation characteristics, density, grain size, c axis fabrics, and ultrasonic velocities of the Greenland Ice Sheet Project 2 (GISP2) core to its contact with bedrock at 3053.4 m. Changes in all these properties paralleled closely those found in cores from Byrd Station, Antarctica, and Die 3, Greenland. Density increased progressively with depth

A. J. Gow; D. A. Meese; R. B. Alley; J. J. Fitzpatrick; S. Anandakrishnan; G. A. Woods; B. C. Elder

1997-01-01

268

The effects of small ice crystals on the infrared radiative properties of cirrus clouds. Semiannual status report, 1 October 1989-31 March 1990  

SciTech Connect

To be successful in the development of satellite retrieval methodologies for the determination of cirrus cloud properties, fundamental scattering and absorption data on nonspherical ice crystals that are found in cirrus clouds must be available. Recent aircraft observations (Platt et al.) reveal that there is a large amount of small ice particles, on the order of 10 micron, in cirrus clouds. Thus it is important to explore the potential differences in the scattering and absorption properties of ice crystals with respect to their sizes and shapes. In this study the effects of nonspherical small ice crystals on the infrared radiative properties of cirrus clouds are investigated using light scattering properties of spheroidal particles. In Section 2, using the anomalous diffraction theory for spheres and results from the exact spheroid scattering program, efficient parameterization equations are developed for calculations of the scattering and absorption properties for small ice crystals. Parameterization formulas are also developed for large ice crystals using results computed from the geometric ray-tracing technique and the Fraunhofer diffraction theory for spheroids and hexagonal crystals. This is presented in Section 3. Finally, applications to the satellite remote sensing are described in Section 4.

Takano, Y.; Liou, K.N.; Asano, S.; Heymsfield, A.; Minnis, P.

1990-04-01

269

Mars Water-Ice Clouds and Precipitation  

Microsoft Academic Search

The light detection and ranging instrument on the Phoenix mission observed water-ice clouds in the atmosphere of Mars that were similar to cirrus clouds on Earth. Fall streaks in the cloud structure traced the precipitation of ice crystals toward the ground. Measurements of atmospheric dust indicated that the planetary boundary layer (PBL) on Mars was well mixed, up to heights

J. A. Whiteway; L. Komguem; C. Dickinson; C. Cook; M. Illnicki; J. Seabrook; V. Popovici; T. J. Duck; R. Davy; P. A. Taylor; J. Pathak; D. Fisher; A. I. Carswell; M. Daly; V. Hipkin; A. P. Zent; M. H. Hecht; S. E. Wood; L. K. Tamppari; N. Renno; J. E. Moores; M. T. Lemmon; F. Daerden; P. H. Smith

2009-01-01

270

Crystal structures of the benzene and ethanol solvates of neotame  

Microsoft Academic Search

The benzene and ethanol solvates of neotame crystallized from solutions of neotame anhydrate in benzene and ethanol, respectively. The crystal structures of the two solvates were determined by single-crystal X-ray diffraction using synchrotron radiation. The benzene solvate crystallizes in the monoclinic space group, P21, Z = 2, with one neotame molecule and one benzene molecule per asymmetric unit. The cell

Zedong Dong; Victor G. Young; Eric J. Munson; Steve A. Schroeder; Indra Prakash; David J. W. Grant

2003-01-01

271

Terrestrial Ice Sheets: Studies of Climate History, Internal Structure, Surface, and Bedrock  

NASA Astrophysics Data System (ADS)

Recently drilled deep ice cores from Central Greenland (GRIP and GISP2) provide the most detailed results available on climatic variation in the northern hemisphere during the last 100,000 years, a period that includes the Holocene (0-11.5 ka) and most of the Wisconsin glacial period. Summer-winter variation in various physical and chemical properties of polar ice allows dating of ice cores by annual layer counting. Several such methods are currently being employed on an ice core drilled by the new North Greenland Ice Core Project (NGRIP), which is aimed at extending the Greenland ice palaeoclimatic record through the last interglacial, the Eemian. Two examples will be presented: (1) visual and photographic studies of seasonal variation in stratigraphic layering, crystal size, air bubble and clathrate concentration, and (2) studies of electric stratigraphy, using the method of dielectric profiling (DEP). This method records the AC conductivity of ice cores, which is negatively correlated with the concentration of airborne dust in the ice but positively correlated with volcanic and marine aerosols. Comprehensive surface traverse programs, which include shallow coring and ice velocity measurements, have recently been carried out by the Alfred Wegener Institute in previously little-investigated regions of Greenland and Antarctica. Serving partly as reconnaissance prior to deep drilling projects, such studies also help to reduce considerable uncertainties in the mass balance of the two large polar ice sheets and thus in their estimated response to climate change. Main results of a recent traverse in North Greenland include the following: (1) A new map of the accumulation distribution on the ice sheet indicates a large low-accumulation region in Northeast-Greenland; (2) North Greenland records show significantly greater climatic variability during the last 500 yr than corresponding records from the southern part of the ice sheet; and (3) data on variation in accumulation rates do not indicate a definite trend in the region during this century. The Alfred Wegener Institute has in recent years employed both airborne and ground-penetrating ice radar systems to map the bedrock around deep drilling sites in Central and North Greenland, as well as in a planned Antarctic site in Dronning Maud Land. The radar also records shallow and deep internal echoes, caused by rapid variation in density and ice acidity in layers of certain ages, allowing isochrones to be traced over wide reaches of the ice sheet. Disturbances in regular stratigraphic layering, due to ice flow over an irregular bed, were observed in the lowest 200-300 m of the GRIP and GISP2 ice cores. Since the aim of the new NGRIP coring program is to obtain an ice core reaching further back in time than the Central Greenland cores, this site was chosen in a region where the bedrock is relatively flat. Echo-sounding surveys between GRIP and NGREP show that the isochrones lie 100-200 in higher above the bed at NGRIP, indicating that the Eemian layer is unlikely to have been disturbed by ice flow at this location. Due to the flow pattern of ice sheets, layers forming a vertical sequence in the interior regions of an ice sheet can, under favorable conditions, be traced on horizontal profiles at the margins. Some meaningful correlations have already been established between Greenland deep ice core climatic records and corresponding records from ice margins. In these regions, a clear contrast is observed between ice of Holocene origin and significantly darker-looking ice dating from the Wisconsin glacial period, which displays summertime ablation rates 2-4x higher than the Holocene ice. This difference is due to higher concentrations of dust and other impurities in the Wisconsin ice, by 1-2 orders of magnitude, leading to reduced albedo. Furthermore, smaller crystal sizes in the Wisconsin ice lead to a more homogeneous distribution of impurities on the surface, which probably contributes to lowering the albedo. Comprehensive studies of ice crystal size and c-axis orientations on the

Thorsteinsson, Th.; Kipfstuhl, J.; Nixdorf, U.; Oerter, H.; Miller, H.; Fritsche, D.; Jung-Rothenhaeusler, F.; Mayer, C.; Schwager, M.; Wilhelms, F.; Steinhage, D.; Goektas, F.

1998-01-01

272

Crystal structure of Cryptosporidium parvum pyruvate kinase.  

PubMed

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 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

2012-10-09

273

Energy benchmarks for water clusters and ice structures from an embedded many-body expansion  

NASA Astrophysics Data System (ADS)

We show how an embedded many-body expansion (EMBE) can be used to calculate accurate ab initio energies of water clusters and ice structures using wavefunction-based methods. We use the EMBE described recently by Bygrave et al. [J. Chem. Phys. 137, 164102 (2012)], in which the terms in the expansion are obtained from calculations on monomers, dimers, etc., acted on by an approximate representation of the embedding field due to all other molecules in the system, this field being a sum of Coulomb and exchange-repulsion fields. Our strategy is to separate the total energy of the system into Hartree-Fock and correlation parts, using the EMBE only for the correlation energy, with the Hartree-Fock energy calculated using standard molecular quantum chemistry for clusters and plane-wave methods for crystals. Our tests on a range of different water clusters up to the 16-mer show that for the second-order Mller-Plesset (MP2) method the EMBE truncated at 2-body level reproduces to better than 0.1 mEh/monomer the correlation energy from standard methods. The use of EMBE for computing coupled-cluster energies of clusters is also discussed. For the ice structures Ih, II, and VIII, we find that MP2 energies near the complete basis-set limit reproduce very well the experimental values of the absolute and relative binding energies, but that the use of coupled-cluster methods for many-body correlation (non-additive dispersion) is essential for a full description. Possible future applications of the EMBE approach are suggested.

Gillan, M. J.; Alf, D.; Bygrave, P. J.; Taylor, C. R.; Manby, F. R.

2013-09-01

274

Crystal and molecular structure of Destruxin B.  

PubMed

The cyclic hexadepsipeptide mycotoxin Destruxin B, produced by Metarrhizium anisopliae, crystallizes in the orthorhombic space group P212121, with a = 11.010(2)A, b = 14.679(5)A, c = 21.273(7)A and Z = 4. The structure was solved by direct methods and refined by least-squares technique to a final unweighted R value of 0.051, for 3361 reflections with I greater than 3 sigma (I). The backbone of the peptide is asymmetric and is made of 5 trans peptide and ester units and 1 cis peptide unit. The backbone conformation of this cyclic depsipeptide is very similar to that of Roseotoxin B, an analogous mycotoxin produced by Trichothecium roseum. The conformation in the crystalline state also correlates well with the solution conformation, as reported from proton n.m.r. studies. The crystal packing is directed by van der Waals contacts. PMID:3366552

Steiner, J R; Barnes, C L

1988-02-01

275

Chemical bond analysis of the correlation between crystal structure and nonlinear optical properties of complex crystals  

Microsoft Academic Search

Second order nonlinear optical (NLO) properties of single crystals with complex structures are studied, from the chemical bond viewpoint. Contributions of each type of constituent chemical bond to the total linearity and nonlinearity are calculated from the actual crystal structure, using the chemical bond theory of complex crystals and the modified bond charge model. We have quantitatively proposed certain relationships

Dongfeng Xue; Siyuan Zhang

1999-01-01

276

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

277

Ab initio simulation of the ice II structure  

Microsoft Academic Search

We have carried out ab initio simulations on the high-pressure polymorph of solid water, ice II, a phase for which there is a surprising lack of experimental data. We report our calculated third-order Birch-Murnaghan equation of state for ice II: the zero pressure and temperature density, rho0=1240.27+\\/-0.62 kg m-3, bulk modulus, K0=16.18+\\/-0.12 GPa, with the first pressure derivative of the

A. D. Fortes; I. G. Wood; J. P. Brodholt; L. Vocadlo

2003-01-01

278

Changes in the velocity structure of the Greenland Ice Sheet.  

PubMed

Using satellite radar interferometry observations of Greenland, we detected widespread glacier acceleration below 66 degrees north between 1996 and 2000, which rapidly expanded to 70 degrees north in 2005. Accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year. As more glaciers accelerate farther north, the contribution of Greenland to sea-level rise will continue to increase. PMID:16484490

Rignot, Eric; Kanagaratnam, Pannir

2006-02-17

279

Snow Crystals  

NSDL National Science Digital Library

This site describes snow crystals and snowflakes. Although a common meteorological phenomenon, snow crystal growth is a fascinating and poorly understood process, in which remarkably complex and beautifully symmetric structures appear, quite literally, out of thin air. The many facets of snow crystals are described here, along with the attempts to understand their formation. Site highlights include research on creating designer snow crystals in the laboratory, the history of early snow crystal observations, snow crystal photography, properties of frozen precipitation, and a snow crystal primer for a short course in snow crystal physics - what snow crystals are, how they form, and why they form the way they do. Information is offered on snow crystal classification, preservation, and unusual crystal forms. An extensive image gallery of lab-created crystal forms is available, with enlargeable thumbnail images. There are even instructions for users on how to create crystals. This could be made into a classroom activity, as the science of the growth is explained. Snowflake Physics discusses diffusion, dendrite growth, ice surface physics, electric growth, and ice properties. A vast list of related links is also provided.

Libbrecht, Kenneth

280

New constraints on the structure and dynamics of the East Antarctic Ice Sheet from the joint IPY/Ice Bridge ICECAP aerogeophysical project  

NASA Astrophysics Data System (ADS)

Ice within marine basins of East Antarctica, and their outlets, represent the ultimate limit on sea level change. The region of East Antarctica between the Ross Sea and Wilkes Land hosts a number of major basin, but has been poorly understood. Long range aerogeophysics from US, Australian and French stations, with significant British and IceBridge support, has, under the banner of the ICECAP project, greatly improved our knowledge of ice thickness, surface elevation, and crustal structure of the Wilkes and Aurora Subglacial Basins, as well as the Totten Glacier, Cook Ice Shelf, and Byrd Glacier. We will discuss the evolution of the Wilkes and Aurora Subglacial Basins, new constraints on the geometry of the major outlet glaciers, as well as our results from surface elevation change measurements over dynamic regions of the ice sheet. We will discuss the implications of our data for the presence of mid Pleistocene ice in central East Antarctica. Future directions for ICECAP will be discussed.

Blankenship, D. D.; Young, D. A.; Siegert, M. J.; van Ommen, T. D.; Roberts, J. L.; Wright, A.; Warner, R. C.; Holt, J. W.; Young, N. W.; Le Meur, E.; Legresy, B.; Cavitte, M.; Icecap Team

2010-12-01

281

Crystal structure of plant photosystem I  

NASA Astrophysics Data System (ADS)

Oxygenic photosynthesis is the principal producer of both oxygen and organic matter on Earth. The conversion of sunlight into chemical energy is driven by two multisubunit membrane protein complexes named photosystem I and II. We determined the crystal structure of the complete photosystem I (PSI) from a higher plant (Pisum sativum var. alaska) to 4.4 resolution. Its intricate structure shows 12 core subunits, 4 different light-harvesting membrane proteins (LHCI) assembled in a half-moon shape on one side of the core, 45 transmembrane helices, 167 chlorophylls, 3 Fe-S clusters and 2 phylloquinones. About 20 chlorophylls are positioned in strategic locations in the cleft between LHCI and the core. This structure provides a framework for exploration not only of energy and electron transfer but also of the evolutionary forces that shaped the photosynthetic apparatus of terrestrial plants after the divergence of chloroplasts from marine cyanobacteria one billion years ago.

Ben-Shem, Adam; Frolow, Felix; Nelson, Nathan

2003-12-01

282

Molecular and crystal structure of polycyclic nitramines  

Microsoft Academic Search

An X-ray structural investigation has been carried out for four polycyclic nitramines with an isowurtzitane structure. These\\u000a compounds are high-density, high-energy materials: 4,10-dinitro-2,6,8,12-tetraoxa-4,10-diaza-tetracyclo(5.5.0.03,11.05,9)dodecane (4), 4,8,10,12-tetranitro-2,6-dioxa-4,8,10,12-tetrazatetracyclo (5.5.0.03,11.05,9)dodecane (5), and 4,6,10,12-tetranitro-2,8-dioxa-4,6,10,12-tetrazatetracyclo(5.5.0.03,11.05,9)-dodecane (6). Nitramine 5 crystallizes as triclinic (form ?, d\\u000a calc = 1.966 g\\/cm3) and trigonal (form ?, d\\u000a calc = 2.014 g\\/cm3) modifications. All amine nitrogen atoms have a nonplanar structure;

Yu. V. Gatilov; T. V. Rybalova; O. A. Efimov; A. A. Lobanova; G. V. Sakovich; S. V. Sysolyatin

2005-01-01

283

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.

House, The S.

2013-05-15

284

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

2008-08-11

285

Combined seismic and radar investigation to define ice properties and structure of a cold alpine site  

NASA Astrophysics Data System (ADS)

The cold alpine saddle Colle Gnifetti, Monte Rosa, Swiss-Italian Alps resembles very much polar and subpolar ice masses in terms of glaciological conditions. It has been the site for several ice-core drilling campaigns over more than 20 years to determine paleoclimatological and glaciological conditions. To investigate the feasibility of geophysical methods for improved characterization of ice masses surrounding borehole and ice-core sites, a combined active reflection seismic and ground-penetrating radar pilot study has been carried out in summer 2008. Aims are the characterization of density, internal layering, seismic and radar wave speed and attenuation, identification of anisotropic features (like crystal orientation or bubble content and shape). Here we present the overall setup and first results. Seismic and GPR profiles were centered on an existing borehole location covering the full ice thickness of 62 m. Active seismics was carried out with 24-channel 3-m spacing recording, using a Seismic Impulse Source System (SISSY) along two profiles parallel and perpendicular to the ice-flow direction. The same profiles were complemented with GPR measurements utilizing 250, 500 MHz frequencies. Additionally, circular profiles with 250, 500 and 800 MHz were carried out circumferencing the borehole to detect anisotropic features.

Eisen, O.; Bohleber, P.; Drews, R.; Heilig, A.; Hofstede, C.

2009-04-01

286

Nucleation and growth of ice crystals inside cultured hepatocytes during freezing in the presence of dimethyl sulfoxide.  

PubMed Central

A three-part, coupled model of cell dehydration, nucleation, and crystal growth was used to study intracellular ice formation (IIF) in cultured hepatocytes frozen in the presence of dimethyl sulfoxide (DMSO). Heterogeneous nucleation temperatures were predicted as a function of DMSO concentration and were in good agreement with experimental data. Simulated freezing protocols correctly predicted and explained experimentally observed effects of cooling rate, warming rate, and storage temperature on hepatocyte function. For cells cooled to -40 degrees C, no IIF occurred for cooling rates less than 10 degrees C/min. IIF did occur at faster cooling rates, and the predicted volume of intracellular ice increased with increasing cooling rate. Cells cooled at 5 degrees C/min to -80 degrees C were shown to undergo nucleation at -46.8 degrees C, with the consequence that storage temperatures above this value resulted in high viability independent of warming rate, whereas colder storage temperatures resulted in cell injury for slow warming rates. Cell damage correlated positively with predicted intracellular ice volume, and an upper limit for the critical ice content was estimated to be 3.7% of the isotonic water content. The power of the model was limited by difficulties in estimating the cytosol viscosity and membrane permeability as functions of DMSO concentration at low temperatures. Images FIGURE 1

Karlsson, J O; Cravalho, E G; Borel Rinkes, I H; Tompkins, R G; Yarmush, M L; Toner, M

1993-01-01

287

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

288

Physical, structural, and isotopic characteristics and growth processes of fast sea ice in Ltzow-Holm Bay, Antarctica  

NASA Astrophysics Data System (ADS)

A sea-ice/ocean study was conducted off Queen Maud Land and Enderby Land, Antarctica, from 1990 to 1991 by the Japanese Antarctic Research Expedition. Observations of multiyear land fast sea ice were made in Ltzow-Holm Bay over a period of 2 years to determine the snow and ice characteristics and ice growth processes. The snow depth in the bay reached large values of 1.0 to 1.5 m during the winter season at offshore locations. From the analysis of ice thickness measurements, it is confirmed that the fast ice with deep snow cover grew little in winter but substantially thickened during the summer months. On the basis of ice core structure, salinity, and stable isotopic composition, we conclude that the summer growth was caused by upward growth at the top of the ice to which snow ice and superimposed ice formation contribute. These processes were the primary contributors to sea-ice growth and characteristics only where the snow accumulation was large. In areas of low snow accumulation, there was no surface growth. Superimposed ice formation on sea ice in Antarctica has not been reported previously. Evidence for snow cover melting, which is a prerequisite for superimposed ice formation, was also found.

Kawamura, T.; Ohshima, K. I.; Takizawa, T.; Ushio, S.

1997-02-01

289

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  

Microsoft Academic Search

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

W. M. Sackinger; M. O. Jeffries

1987-01-01

290

The crystal structure of vyuntspakhite: A redetermination  

NASA Astrophysics Data System (ADS)

The crystal structure of the mineral vyuntspakhite (Y, TR)6{Al2(OH)3[H1.48Si1.88O7][SiO4][SiO3(OH)]}2( a = 5.7551(11) , b = 14.752(3) , c = 15.906(4) , ? = 96.046(4), sp. gr. P21/ n, Z = 2), which had been established earlier in the pseudo-unit cell, is redetermined by X-ray diffraction ( R = 0.040, T = 100 K). The redetermination of the structure shows that pronounced pseudotranslation along the axis c' = c/3 is associated with the fact that Y( TR) atoms are related by a 1/3 translation along the [001] direction. Most of the hydrogen atoms are located. The crystal-chemical function of hydrogen bonds is analyzed. In the unit cell of vyuntspakhite, the cationic layers consisting of edge-sharing (Y, TR) eight-vertex polyhedra alternate along the b axis with mixed anionic layers composed of isolated Si tetrahedra (orthotetrahedra), Si2O7 double-tetrahedra (diortho) groups, Al five-vertex polyhedra, and Al2O8 double-tetrahedra groups linked by shared vertices and through hydrogen bonding.

Yakubovich, O. V.; Steele, I. M.

2009-09-01

291

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

292

Structure and dynamics of orientational defects in ice I  

NASA Astrophysics Data System (ADS)

Orientational defects in hexagonal ice were investigated using molecular dynamics simulations. Energy relaxation during L- and D-defect migration was shown to be associated with improved alignment of water molecules along the local electric fields. Two new forms of defects, an ``L+D complex,'' and a ``5+7 defect,'' were characterized. These forms appear in ice trajectories close to the melting point, and in the course of L- and D-pair recombination process. Defect pair recombination was shown to be a complex process, involving collective H-bond changes in groups of molecules.

Grishina, N.; Buch, V.

2004-03-01

293

Structure and dynamics of orientational defects in ice I.  

PubMed

Orientational defects in hexagonal ice were investigated using molecular dynamics simulations. Energy relaxation during L- and D-defect migration was shown to be associated with improved alignment of water molecules along the local electric fields. Two new forms of defects, an "L+D complex," and a "5+7 defect," were characterized. These forms appear in ice trajectories close to the melting point, and in the course of L- and D-pair recombination process. Defect pair recombination was shown to be a complex process, involving collective H-bond changes in groups of molecules. PMID:15267393

Grishina, N; Buch, V

2004-03-15

294

Velocity structure, flow instability and mass flux on a large Arctic ice cap from satellite radar interferometry  

Microsoft Academic Search

Satellite radar interferometry allows calculation of the ice-surface velocity distribution throughout the largest ice cap in the Eurasian Arctic: Austfonna in eastern Svalbard. Data on ice-cap velocity structure show strong spatial variations observed in unprecedented detail. Ice-cap drainage basins have clearly defined fast-flowing units with marked shear margins at the surface, associated with troughs in the subglacial bedrock identified from

J. A Dowdeswell; B Unwin; A.-M Nuttall; D. J Wingham

1999-01-01

295

Cryogenic structure and ice content of lacustrine sediments in the Yukon River Basin, Alaska  

NASA Astrophysics Data System (ADS)

Lacustrine sediments often present a significant part of the upper permafrost of plains and lowlands of Alaska. Lacustrine sediments in their contemporary state vary from ice-poor to extremely ice-rich. The ice content depends on initial conditions of freezing and further history of permafrost development, which can include partial degradation and recovery of permafrost. In Alaska, the primary mechanism of freezing and cryogenic structure formation of lacustrine sediments is para-syngenetic (term by E.M. Katasonov, 1978), typical of sediments accumulated in lakes surrounded by permafrost. Though the freezing of such sediments occurs before or immediately after the termination of sedimentation (similar to syngenetic permafrost), the freezing conditions and cryogenic structure of para-syngenetic permafrost are similar to epigenetic permafrost. The ice lenses in para-syngenetic sediments are often inclined, and the ice content is relatively small in the central parts of refrozen taliks. This type of ice distribution is generally governed by the migration of water to multidirectional fronts of freezing. We studied the cryogenic structure of lacustrine sediments across different regions of the Yukon River Basin, Alaska, including Koyukuk Flats (Hozatka Lake area), Innoko Lowlands, and Tanana-Kuskokwim Lowlands (Lake Minchumina area). Study sites are located in the discontinuous permafrost zone, where permafrost was encountered mainly within uplifted peat plateaus. Field work included study of natural exposures and drilling. The upper part of studied sections is formed by frozen organic soils up to 2-3 -m -thick underlain by lacustrine silt, which is mostly ice-rich. Volume of visible ice in silt reaches at places 40% and more. A combination of layered and reticulate cryostructures is the most typical and common cryostructure assemblage. The thickness of ice lenses generally varies from 1 to 5 cm and occasionally reaches 10 cm. Aggradation of ice during the freezing of lacustrine silt caused a sufficient heave of the ground surface. Remnants of peat plateaus are surrounded by unfrozen bogs and fens, a result of thawing and settling of ice-rich lacustrine silt. Thermokarst scars initially form at places where ice-rich silt is not protected by a thick layer of organic soil. Further development of thermokarst bogs is related mostly to lateral enlargement of thaw bulbs and collapsing of the margins of peat plateaus. Lacustrine silt within taliks is covered by woody peat accumulated under forests during the permafrost plateau stage and then by aquatic sphagnum peat accumulated after collapse.

Kanevskiy, M. Z.; Jorgenson, M. T.; Shur, Y.; O'Donnell, J.; Harden, J. W.; Fortier, D.

2009-12-01

296

Hydraulics and Sediment Transport of a Proposed Ice Control Structure  

Microsoft Academic Search

The Leesville Dam in East Haddam, Connecticut was lowered by 10 feet in the late 1950s. As a result, ice jams that had previously formed behind the dam, began to occur further downstream, causing damage to private property. The US Army Corps of Engineers have design a series of piers to be constructed upstream of the lowered dam to help

Alaina L. Briggs

2003-01-01

297

Polarimetric radar observation of ice crystals and aggregates: Backscattering modeling of signatures from C to Ka band  

NASA Astrophysics Data System (ADS)

Hydrometeor classification using polarimetric Doppler weather radar is based on the characteristic polarimetric signature for each hydrometeor class. This signature can be obtained by either experimental campaigns or by proper electromagnetic modelling. Both approaches have advantages and drawbacks: the experimental approach is not easy to conduct as it requires co-located measurements of a weather radar with in-situ sampler (usually installed aboard an aircraft); moreover, it is generally strictly related to the measurement configuration (e.g., frequency, range) it is performed. Of course, experimental campaigns are needed for definitive validation, but the modelling approach exhibits a high flexibility in terms of system and meteorological parameters very well suited for retrieval algorithm design. On the other hand, a model approach is heavily dependent on the model capability to represent hydrometeor volumes in a realistic way. Within the electromagnetic scattering modelling of hydrometeor radar response, a well known technique to simulate the radar backscattering from an ensemble of particle is based on the T-matrix algorithm (Kim, 2006). The T-matrix model is based on the equivalence principle and can ensure numerical convergence for a small set of canonical shapes such as ellipsoids. These shapes are useful to represent raindrops and vertically-oriented small crystals, but are largely unrealistic when dealing with ice aggregates and crystals. In this work we use a different approach to the scattering modelling that fits well for classes like ice crystals and aggregates of different shapes and sizes: the discrete dipole approximation (DDA). The DDA model lets us simulate almost any kind of particle under the hypothesis it can be approximated as a three dimensional array of dipoles that generate the scattering field (the wavelength should be large compared to the distance between dipoles). The DDA code used is DDSCAT, developed by Draine and Flatau (2004), which computes the scattering by a single randomly oriented particle. With this approach a variety of hydrometeor shapes have been simulated: cylindrical ice crystals, aggregates of ice cylinders, snow crystals, mixed-phase particles, etc. From DDA it has been possible to obtain the polarimetric signature for ground-based radars at C and X band for these hydrometeor classes after solving some heavy computational issues. An equivalent spheroid model has been also developed for the ice hydrometeor classes in order to use a T-matrix code, faster than DDA, to simulate ice crystals-equivalent spheroids (Weinman and Kim, 2007). Numerical results will be discussed analyzing the sensitivity of the DDA model to the particle shape, wavelength, size distribution and orientation. The accuracy of T-matrix approximation of the ensemble particle polarimetric signature will be also evaluated within the context of hydrometeor classification schemes based on either fuzzy-logic or Bayesian techniques.

Botta, G.; Montopoli, M.; Marzano, F. S.

2009-04-01

298

Crystal structure of Junin virus nucleoprotein.  

PubMed

Junin virus (JUNV) has been identified as the aetiological agent of Argentine haemorrhagic fever (AHF), which is a serious public health problem with approximately 5 million people at risk. It is treated as a potential bioterrorism agent because of its rapid transmission by aerosols. JUNV is a negative-sense ssRNA virus that belongs to the genus Arenavirus within the family Arenaviridae, and its genomic RNA contains two segments encoding four proteins. Among these, the nucleoprotein (NP) has essential roles in viral RNA synthesis and immune suppression, but the molecular mechanisms of its actions are only partially understood. Here, we determined a 2.2 crystal structure of the C-terminal domain of JUNV NP. This structure showed high similarity to the Lassa fever virus (LASV) NP C-terminal domain. However, both the structure and function of JUNV NP showed differences compared with LASV NP. This study extends our structural insight into the negative-sense ssRNA virus NPs. PMID:23884367

Zhang, Yinjie; Li, Le; Liu, Xiang; Dong, Shishang; Wang, Wenming; Huo, Tong; Guo, Yu; Rao, Zihe; Yang, Cheng

2013-07-24

299

Structures of Complex Crystals of Alkylammonium Salts with Aromatic Molecules  

Microsoft Academic Search

The complex crystal structures of dodecyltrimethylammonium chloride (DTAC) with catechol and hydroquinone were analysed by an X-ray diffraction method. Both complexes have isomorphous layered structures. The guest molecules locate between the interdigitated host molecules. Crystal structures are stabilized by mainly hydrogen bonds including water molecules. A cross-section balance between hydrophilic and hydrophobic parts is important for an energetically stable packing.

Keiichi Noguchi; Kenji Okuyama; Kulthida Vongbupnimit

1996-01-01

300

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

301

Crystal structure of level zero extremal weight modules  

Microsoft Academic Search

We consider the crystal structure of the level zero extremal weight modules $V(\\\\lambda)$ using the crystal base of the quantum affine algebra constructed by Beck, Chari and Pressley. This approach yields an explicit form for the U^- extremal weight vectors in each connected component of the crystal of $V(\\\\lambda)$, which are given as Schur functions in the imaginary root vectors.

Jonathan Beck

2002-01-01

302

Tunable photonic structures from liquid crystal elastomers  

NASA Astrophysics Data System (ADS)

We investigated one-dimensional and two-dimensional optical diffraction structures fabricated in thin films of a sidechain light-sensitive liquid crystal elastomer (SC-LS-LCEs) by optical holographic lithography methods. The emphasis was on analysis of modifications of the periodicity of the recorded patterns induced by application of an external strain and by temperature modifications. The results show that due to rubber elasticity of the LCE films, relative modifications of the periodicity by 10% can easily be reached. In most cases tuning is reversible and linear with respect to the strain. Temperature induced tuning is most efficient in the region of phase transition from the nematic to the paranematic phase and provides relative periodicity modifications up to 30%.

Gregorc, Marko; Li, Hui; Domenici, Valentina; Drevenek-Olenik, Irena

2012-11-01

303

Crystal structures of saposins A and C.  

PubMed

Saposins A and C are sphingolipid activator proteins required for the lysosomal breakdown of galactosylceramide and glucosylceramide, respectively. The saposins interact with lipids, leading to an enhanced accessibility of the lipid headgroups to their cognate hydrolases. We have determined the crystal structures of human saposins A and C to 2.0 Angstroms and 2.4 Angstroms, respectively, and both reveal the compact, monomeric saposin fold. We confirmed that these two proteins were monomeric in solution at pH 7.0 by analytical centrifugation. However, at pH 4.8, in the presence of the detergent C(8)E(5), saposin A assembled into dimers, while saposin C formed trimers. Saposin B was dimeric under all conditions tested. The self-association of the saposins is likely to be relevant to how these small proteins interact with lipids, membranes, and hydrolase enzymes. PMID:16823039

Ahn, Victoria E; Leyko, Paul; Alattia, Jean-Ren; Chen, Lu; Priv, Gilbert G

2006-07-05

304

Crystal structure of a snake venom cardiotoxin  

SciTech Connect

Cardiotoxin V/sup II/4 from Naja mossambica crystallizes in space group P6/sub 1/ (a = b = 73.9 A; c = 59.0 A) with two molecules of toxin (molecular mass = 6715 Da) in the asymmetric unit. The structure was solved by using a combination of multiple isomorphous replacement and density modification methods. Model building and least-squares refinement led to an agreement factor of 27% for a data set to 3-A resolution prior to any inclusion of solvent molecules. The topology of the molecule is similar to that found in short and long snake neurotoxins, which block the nicotinic acetylcholine receptor. Major differences occur in the conformation of the central loop, resulting in a change in the concavity of the molecule. Hydrophobic residues are clustered in two distinct areas. The existence of stable dimeric entities in the crystalline state, with the formation of a six-stranded antiparallel ..beta.. sheet, may be functionally relevant.

Rees, B.; Samama, J.P.; Thierry, J.C.; Gilibert, M.; Fischer, J.; Schweitz, H.; Lazdunski, M.; Moras, D.

1987-05-01

305

Constrained evolutionary algorithm for structure prediction of molecular crystals: methodology and applications.  

PubMed

Evolutionary crystal structure prediction proved to be a powerful approach for studying a wide range of materials. Here we present a specifically designed algorithm for the prediction of the structure of complex crystals consisting of well defined molecular units. The main feature of this new approach is that each unit is treated as a whole body, which drastically reduces the search space and improves the efficiency, but necessitates the introduction of new variation operators described here. To increase the diversity of the population of structures, the initial population and part (~20%) of the new generations are produced using space-group symmetry combined with random cell parameters, and random positions and orientations of molecular units. We illustrate the efficiency and reliability of this approach by a number of tests (ice, ammonia, carbon dioxide, methane, benzene, glycine and butane-1,4-diammonium dibromide). This approach easily predicts the crystal structure of methane A containing 21 methane molecules (105 atoms) per unit cell. We demonstrate that this new approach also has a high potential for the study of complex inorganic crystals as shown on examples of a complex hydrogen storage material Mg(BH(4))(2) and elemental boron. PMID:22610672

Zhu, Qiang; Oganov, Artem R; Glass, Colin W; Stokes, Harold T

2012-05-17

306

EFFECT OF LASER LIGHT ON MATTER. LASER PLASMAS: Effects of CO2 laser radiation on large orthophosphoric acid and water drops and on spherical ice crystals  

NASA Astrophysics Data System (ADS)

An experimental investigation is reported of the conditions present during evaporation of suspended orthophosphoric acid and water drops, and of spherical ice crystals with a radius of the order of 1 mm when the laser radiation power density was 20-104 W cm-2 at the wavelength of 10.6 ?m. The lower limit of explosive evaporation was determined for H3PO4 drops and ice crystals. Only one evaporation mechanism of H3PO4 drops was observed (this mechanism was explosive), but there were two mechanisms in the case of water drops (convective with vapour ejection and explosive) and spherical ice crystals (melting followed by evaporation of a water drop and explosive evaporation). Repeated explosions of H2O drops were observed for a power density w = 104 W cm-2 when the beam diameter was 10 mm.

Rudash, V. K.

1994-02-01

307

Crystal Structure of Human Kynurenine Aminotransferase ll*  

SciTech Connect

Human kynurenine aminotransferase II (hKAT-II) efficiently catalyzes the transamination of knunrenine to kynurenic acid (KYNA). KYNA is the only known endogenous antagonist of N-methyl-d-aspartate (NMDA) receptors and is also an antagonist of 7-nicotinic acetylcholine receptors. Abnormal concentrations of brain KYNA have been implicated in the pathogenesis and development of several neurological and psychiatric diseases in humans. Consequently, enzymes involved in the production of brain KYNA have been considered potential regulatory targets. In this article, we report a 2.16 Angstroms crystal structure of hKAT-II and a 1.95 Angstroms structure of its complex with kynurenine. The protein architecture of hKAT-II reveals that it belongs to the fold-type I pyridoxal 5-phosphate (PLP)-dependent enzymes. In comparison with all subclasses of fold-type I-PLP-dependent enzymes, we propose that hKAT-II represents a novel subclass in the fold-type I enzymes because of the unique folding of its first 65 N-terminal residues. This study provides a molecular basis for future effort in maintaining physiological concentrations of KYNA through molecular and biochemical regulation of hKAT-II.

Han,Q.; Robinson, H.; Li, J.

2008-01-01

308

Effects of cirrus spatial heterogeneity and ice particle shape on remote sensing of cirrus optical thickness and effective crystal radius - A case study  

NASA Astrophysics Data System (ADS)

The relative importance of three-dimensional (3D) effects and ice crystal shape of spatially heterogeneous cirrus on the remote-sensing of optical thickness and effective crystal radius is evaluated. In current ice cloud retrievals, the single-scattering properties of ice crystals have to be assumed a-priori. Likewise, the effects of spatial cloud heterogeneity are ignored in current techniques. Both simplifications introduce errors in the retrievals. The study is based on 3D and independent pixel approximation (IPA) radiative transfer calculations. As model input a cloud case that was generated from data collected during the NASA Tropical Composition, Cloud, and Climate Coupling (TC4) experiment is used. First, spectral upwelling radiance fields from the input cloud as they would be sensed by airborne or spaceborne radiometers were determined with 3D radiative transfer simulations. Then the cirrus optical thickness and ice particle effective radius that would be obtained in standard satellite techniques under the IPA assumption were retrieved. The ratios between retrieved and original fields are used as a metric for cloud heterogeneity effects on retrievals. Second, in the retrieval single-scattering properties (crystal shapes) different from those in the radiance calculations were used. In order to isolate ice crystal habit effects, the net horizontal photon transport was disabled here. Thus, the ratios between retrieved and original values of optical thickness and effective radius serve as metric for ice crystal habit effects. When comparing the two metrics, it is found that locally both can be of the same magnitude (up to 50% over- and underestimation), with different dependencies on cirrus optical thickness, effective radius, and optical thickness variability. On domain average, shape effects bias the retrievals more strongly than 3D effects.

Eichler, Heike; Schmidt, Konrad Sebastian; Buras, Robert; Wendisch, Manfred; Mayer, Bernhard; Emde, Claudia; Pilewskie, Peter; King, Michael; Platnick, Steven

2010-05-01

309

Anomalous variations of crystal habits and solution properties in the context of the crystallization medium structure  

NASA Astrophysics Data System (ADS)

The effect of the real structure of solutions on crystallization is one of the basic issues of crystallogenesis, which is also important for resolving problems of genetic mineralogy. The study of the NaNO3-H2O and KNO3-H2O model systems yielded new data on anomalous characteristics of crystal-forming systems, including morphological and kinetic properties of crystals, crystal-solution equilibrium, and physical properties of solutions (light scattering, thermal properties, IR parameters, pH), providing information on the structure of solutions. The internally consistent data confirm the previously suggested variations in structural heterogeneity of solutions related to minor (2-4%) variations in their composition, which result in numerous disturbances of monotonicity (thermal-concentration oscillations) in the liquidus curves of salts. It is shown that these variations can be caused by variable size and composition of crystal hydrate clusters. The experimental data indicate that the effect of the real solution structure on crystal morphology and crystal-solution equilibrium is enhanced in multicomponent systems, including natural crystal-forming systems. Anomalous faceting and habit, zoning, a sectorial structure of crystals, and nonuniform entrapment of admixtures cannot be ruled out in these systems.

Kiryanova, E. V.; Ugolkov, V. L.; Pyankova, L. A.; Filatov, S. K.

2009-12-01

310

Predicting crystal structure by merging data mining with quantum mechanics  

Microsoft Academic Search

Modern methods of quantum mechanics have proved to be effective tools to understand and even predict materials properties. An essential element of the materials design process, relevant to both new materials and the optimization of existing ones, is knowing which crystal structures will form in an alloy system. Crystal structure can only be predicted effectively with quantum mechanics if an

Christopher C. Fischer; Kevin J. Tibbetts; Dane Morgan; Gerbrand Ceder

2006-01-01

311

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 ?(gcm?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.

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

2013-01-01

312

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

PubMed

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 ?(gcm(-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

2013-10-21

313

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

NASA Astrophysics Data System (ADS)

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.

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

2013-10-01

314

Investigating the ice mlange 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 mlange". 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 mlange" 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

315

Preparation and Crystal Structure of Paeonol  

Microsoft Academic Search

Paeonol was prepared by the extraction method from Moutan Cortex. Its crystal struc- ture was determined by single-crystal X-ray diffraction. The compound crystallizes in the monoclinic sys- tem, space group P21\\/c with a = 6.724(4), b = 8.792(6), c = 14.689(10) , ? = 100.138(11), V = 854.8(10) 3, Mr = 166.17, Z = 4, F(000) = 352, Dc =

XU Xing-Youa

316

Refined solution structure of type III antifreeze protein: hydrophobic groups may be involved in the energetics of the proteinice interaction  

Microsoft Academic Search

Background Antifreeze proteins are found in certain fish inhabiting polar sea water. These proteins depress the freezing points of blood and body fluids below that of the surrounding sea water by binding to and inhibiting the growth of seed ice crystals. The proteins are believed to bind irreversibly to growing ice crystals in such a way as to change the

Frank D Snnichsen; Carl I DeLuca; Peter L Davies; Brian D Sykes

1996-01-01

317

Microstructure and Crystal Structure in TAGS Compositions  

SciTech Connect

GeTe, a small bandgap semiconductor that has native p-type defects due to Ge vacancies, is an important constituent in the thermoelectric material known as TAGS. TAGS is an acronym for alloys of GeTe with AgSbTe{sub 2}, and compositions are normally designated as TAGS-x, where x is the fraction of GeTe. TAGS-85 is the most important with regard to applications, and there is also commercial interest in TAGS-80. The crystal structure of GeTe{sub 1+{delta}} has a composition-dependent phase transformation at a temperature ranging from 430 C ({delta} = 0) to {approx}400 C ({delta} = 0.02). The high-temperature form is cubic. The low-temperature form is rhombohedral for {delta} < 0.01, as is the case for good thermoelectric performance. Addition of AgSbTe{sub 2} shifts the phase transformation to lower temperatures, and one of the goals of this work is a systematic study of the dependence of transformation temperature on the parameter x. We present results on phase transformations and associated instabilities in TAGS compositions in the range of 70 at.% to 85 at.% GeTe.

Thompson, A. J. [Marlow Industries, Inc; Sharp, J [Marlow Industries, Inc; Rawn, Claudia J [ORNL

2009-01-01

318

Highly transparent ceramics with disordered crystal structure  

NASA Astrophysics Data System (ADS)

A highly transparent ceramic has been synthesized from Nd3+:Y2O3 to which 6 mol. % ZrO2 and 25 mol. % Sc2O3 or Lu2O3 were added for disordering the crystal structure. Nanopowders with an average particle size of 10-15 nm served as an initial material. They were compacted by the method of uniaxial static pressing combined with ultrasonic action on nanoparticles. The compacting pressure was 200 MPa; the power of the ultrasonic generator was 1.5 kW. It has been shown that the replacement of Y by isovalent Sc and Lu ions and by heterovalent Zr ions reduces the content of pores and the sizes of crystallites. The transparency of the Nd3+:Y2O3 ceramic with these additives reaches a maximum of 82.2%, and the 40% intensity level spectral band corresponding to the 4F3/2 ? 4I11/2 transition widens from 11.4 to 40 nm.

Osipov, V. V.; Khasanov, O. L.; Solomonov, V. I.; Shitov, V. A.; Orlov, A. N.; Platonov, V. V.; Spirina, A. V.; Luk'yashin, K. E.; Dvilis, E. S.

2010-08-01

319

Preparation of iridescent colloidal crystal coatings with variable structural colors.  

PubMed

Iridescent colloidal crystal coatings with variable structural colors were fabricated by incorporating carbon black nanoparticles (CB-NPs) into the voids of polystyrene (PS) colloidal crystals. The structural color of the colloid crystal coatings was not only greatly enhanced after the composition but also varied with observation angles. By changing the diameter of monodisperse PS colloids in the composites, colloidal crystal coatings with three primary colors for additive or subtractive combination were obtained. After incorporation of the PS/CB-NPs hybrid coatings into polydimethylsiloxane (PDMS) matrix, manmade opal jewelry with variable iridescent colors was made facilely. PMID:23938656

Cong, Hailin; Yu, Bing; Wang, Shaopeng; Qi, Limin; Wang, Jilei; Ma, Yurong

2013-07-29

320

Supercooled Droplets and Ice Crystals in Mixed-Phase Clouds: Numerical Simulations Considering Isotropic Turbulence of the Ambient Flow Field  

NASA Astrophysics Data System (ADS)

In midlatitudes the formation and evolution of precipitation is the result of a chain of processes taking place in mixed-phase clouds. Due to the coexistence of supercooled water drops and ice particles in such clouds mutual interactions by collisions, i.e. riming and aggregation, take place leading to ice hydrometeors of a large precipitation size. In the past these collision mechanisms have been investigated - besides laboratory measurements - by numerical simulations of the collision process where trajectories of the participating hydrometeors have been calculated as occurring in an environment at rest (Pruppacher and Klett, Kluwer Academic Publishers, Dordrecht, 1997). However, as it is well-known the flow field in clouds is almost always turbulent (Siebert et al., Atmos. Res. 97 (2010) 426-437) except in undiluted updrafts of single strong convective clouds. And it has been argued that turbulence may enhance precipitation formation. As a consequence turbulence effects on the collisional interaction of cloud and other heavy particles came into focus during the last decade and gave rise to the description in terms of radial distribution function, mean radial relative velocity and the collection efficiency all derived from numerical simulations. Up to now mostly the turbulence influence on cloud droplet/cloud droplet collisions has been investigated (Ayala et al., New J. Phys. 10 (2008) 075015), (Bec et al., J. Fluid Mech. 646 (2010) 527-536). Much less is known about the influence of turbulence on particles in mixed phase clouds. This is mainly due to the various and complex shapes of the ice particles depending on the temperature, the supersaturation, and their life time. Hence, our knowledge about the behavior of ice crystals in turbulence is based on wind tunnel experiments. In the early stage ice crystals often have the shape of hexagonal plates or needles. In theoretical and numerical studies these are commonly approximated by ellipsoids. However, except in (Pinsky and Khain, Atmos. Res. 47-48 (1998) 69-86) only laminar flows have been considered so far. Therefore we have developed a numerical experiment with a novel setup (Kunnen et al., under review in Atmos. Res. (2013)). Therein synthetic turbulence is generated at the inflow and is then advected by a mean flow through the domain. The full Navier-Stokes equations are solved using a DNS method on an Eulerian Cartesian grid. The evolving decaying turbulence shares similarities with the grid-generated turbulence of wind tunnels. In this flow several million particle spheres as well as ellipsoids are advanced in a Lagrangian manner in order to represent the supercooled droplets and ice crystals out of a small region of a mixed-phase cloud. Statistics will be gathered about the orientation, the sedimentation velocities, the clustering, and the relative velocities of these particles. From this basis collision kernels can be calculated. These are input parameters for cloud models estimating the evolution of precipitation.

Siewert, Christoph; Kunnen, Rudie; Meinke, Matthias; Schrder, Wolfgang; Beheng, Klaus

2013-04-01

321

Synthesis, crystal structure, and transistor performance of tetracene derivatives.  

PubMed

The substitution of chloro or bromo groups in tetracene gives rise to the change of crystal structure, having a substantial effect on carrier transport. Halogenated tetracene derivatives were synthesized and grown into single crystals. Monosubstituted 5-bromo- and 5-chlorotetracenes have the herringbone-type structure, while 5,11-dichlorotetracene has the slipped pi stacking structure. Mobility of 5,11-dichlorotetracene was measured to be as high as 1.6 cm2/V.s in single-crystal transistors. The pi stacking structure, which enhances pi orbital overlap and facilitates carrier transport, may thus be responsible for this high mobility. PMID:15563126

Moon, Hyunsik; Zeis, Roswitha; Borkent, Evert-Jan; Besnard, Celine; Lovinger, Andrew J; Siegrist, Theo; Kloc, Christian; Bao, Zhenan

2004-12-01

322

Refinement of the crystal structure of rutherfordine.  

SciTech Connect

Rutherfordine, UO{sub 2}CO{sub 3} is orthorhombic, a 4.840(1), b 9.273(2), c 4.298(1) Angstroms, V192.90(7) Angstroms{sup 3}, space group lmm2,Z=2. The structure was refined to an R index of 2.2% on the basis of 306 unique data [|F{sub o}|/{sigma}(|F{sub o}|)>5] measured with MoK{alpha} X-radiation on a single-crystal diffractometer. The structure consists of neutral sheets of edge- and corner-sharing (UO{sub 8}) hexagonal bipyramids and (CO{sub 3}) triangles, as originally proposed by Christ et al. (1955); our refinement, however shows that (CO{sub 3}) groups in alternate layers have the same orientation, not opposite orientations as originally reported. The refined value of the U-O (uranyl) distance is strongly affected by the details of the absorption correction, ranging from 1.71 to 1.80 Angstroms as a function of the plate-glancing angle used in an empirical psi-scan absorption correction and as a function of the type of weighting scheme used in the refinement. The Gaussian-quadrature method of integration also shows similar problems, but they are less extreme. The preferred value for the U-O (uranyl) distance in rutherfordine is {approx}1.745 Angstroms; as rutherfordine contains no H atoms, the O(uranyl) atom is [1]-coordinated, and should have the shortest U-O(uranyl) distance stereochemically possible. The current work suggests that U-O(uranyl) values less than 1.745 Angstrom reported in other studies are adversely affected by less-than-optimum absorption corrections.

Finch, R. J.; Cooper, M. A.; Hawthorne, F. C.; Ewing, R. C.; Chemical Engineering; Univ. of Manitoba; Univ. of Michigan

1999-01-01

323

Requirements for structure determination of aperiodic crystals  

Microsoft Academic Search

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,

Xiao-Ou Li; Edward A. Stern; Yanjun Ma

1991-01-01

324

De-icing: recovery of diffraction intensities in the presence of ice rings  

SciTech Connect

Macromolecular structures are routinely determined at cryotemperatures using samples flash-cooled in the presence of cryoprotectants. However, sometimes the best diffraction is obtained under conditions where ice formation is not completely ablated, with the result that characteristic ice rings are superimposed on the macromolecular diffraction. In data processing, the reflections that are most affected by the ice rings are usually excluded. Here, an alternative approach of subtracting the ice diffraction is tested. High completeness can be retained with little adverse effect upon the quality of the integrated data. This offers an alternate strategy when high levels of cryoprotectant lead to loss of crystal quality.

Chapman, Michael S.; Somasundaram, Thayumanasamy (Oregon State U.); (FSU)

2010-11-03

325

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 dynamicsbut all reported to exist below the crystallization temperature of approximately 150170K below 45GPa. Here, we present the evidence of high density amorphous (HDA) ice formed well above the crystallization temperature at 1GPawell inside the so-called no-mans 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.

Chen, Jing-Yin; Yoo, Choong-Shik

2011-01-01

326

On the Crystal Structure of Ln  

SciTech Connect

The crystal structures of La{sub 2}O{sub 2}CO{sub 3} II and Nd{sub 2}O{sub 2}CO{sub 3} II have been shown by means of high-resolution powder neutron (PND) and synchrotron X-ray diffraction (SXRD) combined with selected area electron diffraction (SAED) studies to be far more complex than earlier anticipated, owing to ordering of carbonate groups between (Ln{sub 2}O{sub 2}{sup +2}){sub n} layers. In contrast to earlier descriptions, the carbonate groups appear to be rather regular. Relative to an average model, the SAED patterns show additional scattering in the form of closely distributed, but essentially discrete, spots along < 1/3, 1/3, 1 >. Most of the observed scattering, H, can be described as H=G{+-}m q{sub 1}+n q{sub 2}, where G is the Bragg reflections of the underlying average P6{sub 3}/mmc lattice, q1=[1/3, 1/3, {+-}1/2]*, q2=[1/3, 1/3, {+-}2/3]*, and m and n are integers. The additional scattering reflects ordering of the carbonate groups into trigonal layers between the (Ln{sub 2}O{sub 2}{sup +2}){sub n} layers, but it remains open whether q{sub 1} and q{sub 2} represent two separate structures with different stacking sequences of such layers or whether they correspond to an even more complex stacking sequence. In any case, some disorder and rotational domain twinning are present. Two structure models, one for each modulation wave vector, were constructed. Rietveld-type refinements of PND data of La{sub 2}O{sub 2}CO{sub 3} II were performed, approximating the complex, and at least partly disordered, stacking sequence as a two-phase mixture of the two modulated phases. Satisfactory convergence was achieved with R{sub p}=6.4%, R{sub wp}=8.3%, and {chi}{sup 2}=3.32. The isothermal expansivities, {alpha}{sub p}, for La{sub 2}O{sub 2}CO{sub 3} II and Nd{sub 2}O{sub 2}CO{sub 3} II between 298 and 893 K were determined as 2.92x10{sup {minus}5} and 2.70x10{sup {minus}5} K{sup {minus}1}, respectively.

Olafsen, Anja; Larsson, Ann-Kristin; Fjellvaag, Helmer; Hauback, Bjoern C.

2001-04-01

327

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.

Wang, Hui; Zhang, Ke-Qin

2013-01-01

328

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

2013-03-28

329

Ordering of crystal structure by ionizing radiation  

NASA Astrophysics Data System (ADS)

We have studied the action of ionizing radiation on defect-containing semiconductor crystals, metals, and alloys. Using modern methods for investigation of solids, Rutherford back scattering of channeled charged particles, x-ray diffraction, electron microscopy, and also calorimetric methods, we have established: a) irradiation (by x-ray beams, gamma rays, and electrons) of metals and alloys with an equivalent radiation dose less than 105 J/kg and of semiconductor crystals with a dose less than 103 J/kg does not lead to additional accumulation of defects but conversely leads to elimination of defects and transition of the crystal to a more equilibrium state; b) ionization processes play a determining role in rearrangment of defects in crystals exhibiting both semiconductor and metallic conductivity. We show that rearrangment of the crystal occurs as a result of stored energy in the crystal which is liberated due to chain reactions of annihilation of defects, initiated by ionization. Transition of the crystal to the equilibrium state is accompanied by improvement of its physical properties.

Chernov, I. P.; Momontov, A. P.; Cherdantsev, P. A.; Chakhlov, B. V.

1994-12-01

330

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

331

Two Types of Anchoring Structure in Smectic Liquid Crystal Molecules  

Microsoft Academic Search

The anchoring structures of smectic liquid crystals, n-alkylcyanobiphenyl (mCB: m{=}8, 10, 12), on molybdenum disulfide (MoS2) are directly observed by scanning tunneling microscopy (STM) in order to analyze the alignment mechanism of liquid crystals. For 8CB, the anchoring structure is of the periodic monolayer type, while 10CB and 12CB take a bilayer structure in which there is no interdigitation of

Yasushi Iwakabe; Masahiko Hara; Katumi Kondo; Kenji Tochigi; Akio Mukoh; Anthony F. Garito; Hiroyuki Sasabe; Akira Yamada

1990-01-01

332

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

333

Internal Structure in Supercooled Water Aerosols and Their Role in the Formation of Ice Clouds  

Microsoft Academic Search

The chemistry and physics of processes occurring in supercooled cloud droplets is a sensitive function of the structure of the liquid. Such properties as uptake and diffusion coefficients, viscosity and ice nucleation behaviour all change rapidly with the amount of supercooling in samples of pure water. Direct measurements of these properties can only be made in micron-sized droplets. This presentation

A. F. Khalizov; M. E. Earle; A. Y. Zasetsky; J. J. Sloan

2004-01-01

334

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

335

Structural characterisation of hyperquenched glassy water and vapour-deposited amorphous ice  

Microsoft Academic Search

Neutron diffraction experiments have been made on hyperquenched glassy water (D2O). After subtraction of a small crystalline component (Ic) it is found that the structure is based on a continuous random network (CRB) of hydrogen-bonded molecules similar to that describing the low-density form of amorphous ice.

A. Hallbrucker; E. Mayer; L. P. O'Mard; J. C. Dore; P. Chieux

1991-01-01

336

In vivo protein crystallization opens new routes in structural biology  

PubMed Central

Protein crystallization in cells has been observed several times in nature. However, owing to their small size these crystals have not yet been used for X-ray crystallographic analysis. We prepared nano-sized in vivogrown crystals of Trypanosoma brucei enzymes and applied the emerging method of free-electron laser-based serial femtosecond crystallography to record interpretable diffraction data. This combined approach will open new opportunities in structural systems biology.

Koopmann, Rudolf; Cupelli, Karolina; Redecke, Lars; Nass, Karol; DePonte, Daniel P; White, Thomas A; Stellato, Francesco; Rehders, Dirk; Liang, Mengning; Andreasson, Jakob; Aquila, Andrew; Bajt, Sasa; Barthelmess, Miriam; Barty, Anton; Bogan, Michael J; Bostedt, Christoph; Boutet, Sebastien; Bozek, John D; Caleman, Carl; Coppola, Nicola; Davidsson, Jan; Doak, R Bruce; Ekeberg, Tomas; Epp, Sascha W; Erk, Benjamin; Fleckenstein, Holger; Foucar, Lutz; Graafsma, Heinz; Gumprecht, Lars; Hajdu, Janos; Hampton, Christina Y; Hartmann, Andreas; Hartmann, Robert; Hauser, Gunter; Hirsemann, Helmut; Holl, Peter; Hunter, Mark S; Kassemeyer, Stephan; Kirian, Richard A; Lomb, Lukas; Maia, Filipe R N C; Kimmel, Nils; Martin, Andrew V; Messerschmidt, Marc; Reich, Christian; Rolles, Daniel; Rudek, Benedikt; Rudenko, Artem; Schlichting, Ilme; Schulz, Joachim; Seibert, M Marvin; Shoeman, Robert L; Sierra, Raymond G; Soltau, Heike; Stern, Stephan; Struder, Lothar; Timneanu, Nicusor; Ullrich, Joachim; Wang, Xiaoyu; Weidenspointner, Georg; Weierstall, Uwe; Williams, Garth J; Wunderer, Cornelia B; Fromme, Petra; Spence, John C H; Stehle, Thilo; Chapman, Henry N; Betzel, Christian; Duszenko, Michael

2012-01-01

337

Optical Response of Liquid Crystals in Colloid-Templated Structures  

Microsoft Academic Search

We report optical switching studies on nematic liquid crystals in self-assembled colloids. The liquid crystal may be imbibed into interstitial cavities of the assembled colloid, or can be incorporated into a network of positionally-ordered cavities obtained from a polymer replica of the colloid crystal. Our periodic host structures have lattice constants in the range of 0.5 to 1.5 microns. We

P. Mach; K. H. Lin; A. G. Yodh; D. A. Weitz; P. Wiltzius

2001-01-01

338

Stabilities of filled ice II structure of hydrogen and helium hydrates at low temperatures and high pressures  

NASA Astrophysics Data System (ADS)

Hydrogen hydrate is expected to be a hydrogen storage material, because it can contain relatively high hydrogen and its synthetic condition is mild comparable to industrial production. Three phases of hydrogen hydrate have been known so for. One is a clathrate hydrate sII [1], and others are filled ice II structure and filled ice Ic structure [2]. The ratio of water to hydrogen molecules for these phases are1:3, 1:6, 1:1, respectively. The clathrate sII containing only hydrogen molecules is stable only in a lower temperature region. At room temperature, above about 0.8 GPa filled ice II and above 2.5 GPa filled ice Ic are formed. The latter one survives at least up to 90 GPa [3]. However, investigations in low temperature and high pressure region have been limited. In this study, low temperature and high pressure experiments were performed by using diamond anvil cells and a helium-refrigeration cryostat in a region of 0.2 to 4.5 GPa and 130 to 300 K. X-ray diffractometry (XRD) showed a series of phase change from sII to filled ice Ic via filled ice II. For example, at 220K, sII transformed to filled ice II at approximately 0.7 GPa and further transformed to filled ice Ic structure at about 2.0 GPa. The present results experimentally confirmed the previously predicted phase boundaries. For filled ice II structure, Raman spectroscopy revealed that pressure dependency of vibration mode of guest hydrogen molecules and OH stretching mode of host water molecules changed at approximately 2.5 GPa. The XRD also showed change in axial ratio at the same pressure. These result suggested that state of filled ice II structure changed at about 2.5 GPa. Helium hydrate is known to form filled ice II structure [4], but high pressure study has not been yet fully performed. Similar experiments were carried out in a region of 0.2 to 5.0 GPa and 200 to 300 K. The results showed that the filled ice II structure did not transformed to filled ice Ic structure, but decomposed into helium and ice VI or VIII without transition to filled ice Ic structure as expected. [1] W. L. Mao et al., Science, 2002, 297, 2247-2249. [2] W. L. Vos et al, Phys. Rev. Lett. 1993, 71, 3150-3153. [3] H. Hirai et al., Amer. Mineralogist, 2006, 91, 826-830. [4] D. Londono et al., J. Chem. Phys., 1992, 97, 547.-552.

Hirai, H.; Umeda, A.; Fujii, T.; Machida, S.; Shinozaki, A.; Kawamura, T.; Yamamoto, Y.; Yagi, T.

2011-12-01

339

Crystal structure of interleukin 8: Symbiosis of NMR and crystallography  

Microsoft Academic Search

The crystal structure of a host defense system chemotactic factor, interleukin 8, has been solved by molecular replacement using as a model the solution structure derived from nuclear magnetic resonance experiments. The structure was refined with 2 x-ray data to an R factor of 0.817. A comparison indicates some potential differences between the structure in solution and in the

E. T. Baldwin; I. T. Weber; R. St. Charles; Jiancheng Xuan; Kouji Matsushima; A. Wlodawer; E. Appella; G. M. Clore; A. M. Gronenborn; Masaki Yamada; B. F. P. Edwards

1991-01-01

340

Ice Flora (Bottom Type): A Mechanism of Primary Production in Polar Seas and the Growth of Diatoms in Sea Ice.  

National Technical Information Service (NTIS)

A field survey off Barrow in the summer of 1964 revealed that sea ice in the Arctic develops a layered structure through the growth of diatoms. The diatoms increase in brine solutions which occur in the microfissures between fine crystals of sea ice and f...

H. Meguro K. Ito H. Fukushima

1967-01-01

341

Thermal Expansion and Structure of Crystallizing Glass.  

National Technical Information Service (NTIS)

The author carried out a complex study of the glasses with a composition close to spodumene during the isothermal crystallization process. Original samples differed in the type of catalyzer only. The results cover the changes in compression deformation, m...

E. S. Sorkin

1972-01-01

342

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

343

Order-disorder transition in self-clathrate structures of ice  

NASA Astrophysics Data System (ADS)

Phase boundaries among various forms of ice are difficult to determine experimentally because of the large hysteresis involved. For several reasons they are also challenging to theoretical studies. The order-disorder (OD) facet of some of these transitions is one of these challenges. We present a first principles study of a phase transition between the self-clathrate structures ice VII and ice VIII, a typical OD phenomenon. Since this transition has been reasonably well constrained experimentally, it is a good reference point for first principles studies. Hence, this calculation offers benchmarks for the current ability of first principles theory to address this class of solids. Research supported by NSF/EAR 013533, 0230319, and NSF/ITR 0428774 (VLab).

Umemoto, K.; Wentzcovitch, R. M.; Gironcoli, S. D.; Baroni, S.

2006-12-01

344

Crystal structure of cholesteryl butanoate at 123 K  

Microsoft Academic Search

Cholesteryl butanoate has a complex crystal struc- ture that differs from those of the three main structure types for cholesteryl esters. It contains four molecules (C3LH5202) un- related by crystal symmetry. The molecules are packed in almost planar sheets and have molecular long axes nearly parallel. However, the molecules have different orientations about their long axes and furthermore, in a

Gye Won Han; B. M. Craven; David A. Langs

345

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

346

Cavity-enhanced structural colour in extrudeable photonic crystals  

Microsoft Academic Search

Photonic crystals remain of significant interest because of the opportunity to modify a host of optical properties by structuring the material at the sub-wavelength scale, including enhanced light emission and absorption, superprism performance, and negative refraction. However until now photonic crystals have remained expensive to fabricate and control, and inconceivable to adopt in industrial processes because their assembly requires expensive

Jeremy Baumberg; David Snoswell; Andreas Kontogeorgos; Peter Spahn; Otto Pursiainen

2009-01-01

347

Crystal structures of drugs: advances in determination, prediction and engineering  

Microsoft Academic Search

Most marketed pharmaceuticals consist of molecular crystals. The arrangement of the molecules in a crystal determines its physical properties and, in certain cases, its chemical properties, and so greatly influences the processing and formulation of solid pharmaceuticals, as well as key drug properties such as dissolution rate and stability. A thorough understanding of the relationships between physical structures and the

Sharmistha Datta; David J. W. Grant

2004-01-01

348

Crystal structure and phototransistor behavior of N-substituted heptacence.  

PubMed

6,8,15,17-Tetraaza-1.18,4.5,9.10,13.14-tetrabenzoheptacene (TTH, 1) has been prepared and characterized by single-crystal X-ray structure analysis. A phototransistor device based on TTH single crystal demonstrated that TTH showed a good performance in signal amplification under the photoconductive effect as well as photocontrolled switches. PMID:22475002

Wu, Yuechao; Yin, Zongyou; Xiao, Jinchong; Liu, Yi; Wei, Fengxia; Tan, Ke Jie; Kloc, Christian; Huang, Ling; Yan, Qingyu; Hu, Fangzhong; Zhang, Hua; Zhang, Qichun

2012-04-09

349

Structural, spectral and mechanical studies of bimetallic crystal: cadmium manganese thiocyanate single crystals  

NASA Astrophysics Data System (ADS)

A nonlinear optical bimetallic thiocyanate complex crystal, cadmium manganese thiocyanate (CMTC) has been successfully synthesized. The growth of single crystals of cadmium manganese thiocyanate has been accomplished from aqueous solution using slow evaporation method. The presence of manganese and cadmium in the synthesized material was confirmed through energy dispersive spectrum (EDS) analysis. Structural analysis was carried out using powder X-ray diffractometer (PXRD) and crystalline perfection of the grown crystals was ascertained by high-resolution X-ray diffraction (HRXRD) analysis. Fourier transform infrared (FTIR) spectrum was taken to confirm the functional groups. The transmittance spectrum of the crystal in the UV-visible region has been recorded and the cutoff wavelength has been determined. The dielectric measurements for the crystals were performed for various frequencies and temperatures. The mechanical properties were evaluated by Vickers microhardness testing, which reveals hardness and stiffness constant of the crystals.

Manikandan, M.; Vijaya Prasath, G.; Bhagavannarayan, G.; Vijayan, N.; Mahalingam, T.; Ravi, G.

2012-09-01

350

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

351

Structural and mechanical studies of cadmium manganese thiocyanate crystal  

NASA Astrophysics Data System (ADS)

Single crystals of cadmium manganese thiocyanate (CMTC) have been synthesized successfully and grown by slow evaporation method. The structural perfection of the grown crystals has been analyzed by High resolution X-ray diffraction (HRXRD), which shows the crystalline perfection of the grown crystal is quite good. Optical behavior was assessed by UV-Vis analysis and found that no absorption in the UV visible region and it may be useful for second harmonic applications. The mechanical hardness of the grown crystals was studied and Vicker's microhardness, Stiffness constant was calculated.

Manikandan, M. R.; Vijayaprasath, G.; babu, G. Anandha; Bhagavannarayan, G.; Vijayan, N.; Ravi, G.

2012-06-01

352

Discrete breathers in crystals with the NaCl structure  

NASA Astrophysics Data System (ADS)

In the recent decade, the spatially localized large-amplitude vibrational modes in defect-free crystals, referred to as discrete breathers (DBs), are intensively investigated in the materials science. This review reports the main results on gap DBs in crystals with the NaCl structure. The experimental proof of their existence in a NaI crystal is described. A number of molecular dynamics simulation results are presented, including new so far unpublished data. The properties of crystals potentially affected by the DBs are discussed.

Baimova, J. A.; Dmitriev, S. V.; Kistanov, A. A.; Potekaev, A. I.

2013-07-01

353

Macroturbulent coherent structures and helical cell motions in an ice-covered meander bend  

NASA Astrophysics Data System (ADS)

In cold climate environments, rivers have an ice cover that can persist for a significant period of time. The addition of an ice cover on an open channel creates a flow in which two distinctive boundary layers develop and contribute to turbulence production. It is still unclear how turbulent coherent flow structures rising from both boundaries interact and modify the overall structure of the flow. Moreover, the presence of two boundary layers radically changes the three-dimensionality of the flow field at the reach scale. Flume studies have shown the presence of two counter-rotating helical cells at ice-covered meander bends, but field evidences to support these observation is still lacking. We measured the field of flow velocities with a Pulse-Coherent Acoustic Doppler Profiler (PC-ADP) in a sand-bedded meander reach in the Neigette River (Eastern Qubec, Canada). The surveys were conducted during two successive winter periods presenting contrasting ice conditions. Velocity profiles collected in ice-covered conditions are compared to data collected during open channel conditions. Analysis of the velocity data shows weaker overall coherency of the flow structures and the disappearance of classical macroturbulent structures often characterized by high and low speed wedges in open channels. Integral time scale, integral length scales and cross-correlation analyses revealed a lack of vertical continuity of the turbulent velocity fluctuations. Vertical exchanges of momentum and overall mixing are greatly weakened and macroturbulent coherent flow structures remain constrained within their respective boundary layers. At the meander bend scale, the field of flow shows clear evidence of two stacked counter rotating helical cell pattern occurring at the bend entrance. The pattern rapidly evolves downstream, reducing to one helical cell rotating in an opposite direction than the one observed in open channel flows. During the second winter survey, coherent flow patterns rapidly form in the lee of a massive obstruction due to frazil ice that accumulated in the bend. Frazil ice is shown to have a non-persistent impact on secondary flow patterns and on the dynamics of helical cells that seem to be resilient features of the flow pattern in river bends.

Buffin-Belanger, T. K.; Demers, S.; Roy, A. G.

2009-12-01

354

Remote Sounding of Cirrus Cloud Optical Depths and Ice Crystal Sizes from AVHRR Data: Verification Using FIRE II IFO Measurements.  

NASA Astrophysics Data System (ADS)

Using the data obtained from the Advanced Very High Resolution Radiometer (AVHRR) 3.7-m and 10.9-m channels, a retrieval scheme has been developed to simultaneously infer cirrus cloud optical depth and mean effective ice crystal size based on the theory of radiative transfer and parameterizations. A numerical scheme is further developed to remove the solar component in the 3.7-m radiance for applications to daytime satellite data. This scheme is based on the correlation between the 3.7-m (solar) and 0.63-m reflectances. Validation of the algorithm has been performed by using various datasets that were collected during the FIRE-II IFO (Nov-Dec 1991) at Coffeyville, Kansas. We have focused on the 26 November and 5 December cases. The retrieval analysis over a 0.51.0 area is performed around Coffeyville for each case based on AVHRR-HRPT data. For validation the authors analyze the photomicrograph data collected by the balloonborne replicator, determine the microphysical and optical properties of the sampled cirrus clouds, and derive their position at the satellite overpass based on sounding data. It is demonstrated that the retrieved cirrus cloud temperature, mean effective ice crystal size, and optical depth closely match the observed values. Further, the retrieved cirrus cloud properties are applied to the computation of surface radiative fluxes using a radiative transfer program that involves a consistent representation of cirrus cloud fields. The computed values are compared with the data measured from ground-based radiometers, and it is shown that the computed downward surface IR and solar fluxes are within 5 and 10 W m2 of the measured values, respectively, near the time of satellite overpass.

Ou, S. C.; Liou, K. N.; Takano, Y.; Rao, N. X.; Fu, Q.; Heymsfield, A. J.; Miloshevich, L. M.; Baum, B.; Kinne, S. A.

1995-12-01

355

The crystal structure and crystallization temperature of equilibrium and metastable alloys and compounds  

NASA Astrophysics Data System (ADS)

The crystallization temperature of equilibrium V and Nb alloys and compounds is correlated with their crystal structure and phase diagrams. The production of superconducting materials under high pressure, in a thin-film state, or with superfast cooling requires increased superconductivity parameters. The temperature of the transition from a compound to a superconductor under superfast cooling and tempering depends on the chemical composition of the alloys and their crystal structure; the effects of superfast hardening on phase composition and transition temperature are analyzed for V and Nb binary and ternary alloys.

Frolova, T. M.; Efimov, Iu. V.; Savitskii, E. M.

1980-10-01

356

Solvent effects on morphology and crystal structure of solution-grown organic crystals  

NASA Astrophysics Data System (ADS)

Based on the measured solubility data of acenaphthene, fluoranthene and pyrene in six halogen derivative solvents, the solutes activity coefficients ((gamma) s) have been determined from Scatchard-Hildebrand solution theory and used as a measure of solute-solvent interactions for determination of observed solvent effect on growth morphology and intrinsic structure of solution grown crystals of these hydrocarbons. The growth morphologies of these crystals have been interpreted on the basis of PBC theory assumptions. Correlation between activity coefficients of the investigated solutes and morphology as well as structure of obtained crystals, are found.

Marciniak, Bernard; Rozycka-Sokolowska, Ewa; Pawliuk, W.

2001-04-01

357

Molecular structures and crystal packings of 2-styrylquinoxaline derivatives  

NASA Astrophysics Data System (ADS)

The crystal and molecular structures of 2-styrylquinoxaline derivatives with different substituents in the styryl fragment are determined. The degree of planarity of the molecules studied varies in a very wide range, from 1.7 to 33.5. In the ethylene fragment, the double bond is essentially localized. The bicycle-pedal disordering of the ethylene fragment is found in the crystals of the methoxy and oxyacetyl derivatives of 2-styrylquinoxaline. None of the packings contains packing motifs favorable for the photocycloaddition (PCA) reaction with single crystal retention. The crystal packings of these compounds and that of 2-(4-methylstyryl)quinoxaline are characterized by a stacking motif of the head-to-head type, which eliminates the possibility of PCA taking place with single crystal retention but is suitable for this reaction in polycrystalline films. The crystal packing of 2-(3,4-dimethoxystyryl)quinoxaline does not contain elements with stacking interactions.

Kuz'mina, L. G.; Sitin, A. G.; Gulakova, E. N.; Fedorova, O. A.; Lermontova, E. Kh.; Churakov, A. V.

2012-01-01

358

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

359

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

360

Structural Reorganization of Liquid Crystals Revealled by Fast Scanning Calorimeter  

NASA Astrophysics Data System (ADS)

Liquid crystal glass of 4-Cyano-4'-octylbiphenyl is obtained by rapid cooling with rates over 2000 Kelvin per second (K/s) on the chip calorimeter. The glass can crystallize easily upon heated above its glass transition temperature. Depending on the prior cooling rate and annealing history thereafter, melting-structural reorganization-remelting behavior similar to that of semicrystalline polymer can be observed during subsequent heating. The complex melting behavior is attributed to the transformation of metastable crystal forms formed during annealing or heating induced cold crystallization. Increasing the heating rate (>15000 K/s) can suppress the transformation and, additionally, enables us to capture the multiple N-I transition. This implies the coexistence of two different types of nematic states. To avoid above complex structural reorganization, one can anneal the sample at 260K for 2 seconds to get the stable crystal form.

Zhou, Dongshan; Jiang, Jing; Wang, Xiaoliang; Xue, Gi

2011-03-01

361

Detergent structure in tetragonal crystals of OmpF porin  

Microsoft Academic Search

Background: The high-resolution structures of five porins have been solved by X-ray crystallography including the trigonal crystal form of the trimeric OmpF porin from Escherichia coli. In an accompanying article, the structure of the tetragonal form of OmpF porin is presented. In contrast to the trigonal crystal form, the protein surfaces normally in contact with lipids in the membrane are

E Pebay-Peyroula; RM Garavito; JP Rosenbusch; M Zulauf; PA Timmins

1995-01-01

362

Crystal structure of a phospholipase D family member  

Microsoft Academic Search

The first crystal structure of a phospholipase D (PLD) family member has been determined at 2.0 resolution. The PLD superfamily is defined by a common sequence motif, HxK(x)4D(x)6GSxN, and includes enzymes involved in signal transduction, lipid biosynthesis, endonucleases and open reading frames in pathogenic viruses and bacteria. The crystal structure suggests that residues from two sequence motifs form a

Jeanne A. Stuckey; Jack E. Dixon

1999-01-01

363

The spin ice Ho(2)Ti(2)O(7) versus the spin liquid Tb(2)Ti(2)O(7): field-induced magnetic structures.  

PubMed

We studied the field-induced magnetic structures of Ho(2)Ti(2)O(7) spin ice by means of single-crystal neutron diffraction with a magnetic field applied along a [110] direction. These structures are compared to those of the spin liquid Tb(2)Ti(2)O(7) previously measured in similar experimental conditions. For both compounds, magnetic structures of two types with k = 0 and k = (0, 0, 1) propagation vectors coexist at low temperature (1.6K) and high applied field (7T). The k = 0 structures are described by the basis functions of the same irreducible representation for both Tb(2)Ti(2)O(7) and Ho(2)Ti(2)O(7). On the other hand, the k = (0, 0, 1) structures of Tb(2)Ti(2)O(7) and Ho(2)Ti(2)O(7) correspond to different irreducible representations, leading to different magnetic structures. PMID:21471634

Sazonov, A P; Gukasov, A; Mirebeau, I

2011-04-06

364

Measurements of HNO3 and NOy on Cirrus Ice Particles and in Solution Aerosols During CRYSTAL-FACE  

NASA Astrophysics Data System (ADS)

NOx (NO + NO2) is central to tropospheric O3 production. The larger family of gases with which NOx interconverts, NOy, or total reactive nitrogen, includes HNO3. One potential mechanism for the removal and/or vertical transport of NOy is via uptake on ice particles and aerosols. NOx is unlikely to be subject to such uptake, and HNO3 is likely the most significant NOy species subject to uptake. Thus the uptake of HNO3 has the potential to affect indirectly the O3 budget of the atmospehre [Lawrence and Crutzen, Tellus, 1998; Meier and Hendricks, JGR, 2002]. To assess the degree of uptake, condensed-phase HNO3 and NOy were measured during CRYSTAL-FACE by instruments in adjacent pallets on the WB57. The instruments employed nearly identical inlets for the enhanced (anisokinetic) sampling of condensed-phase species. Each instrument employed a second inlet for the measurement of the gas-phase plus small-particle fraction. Differences in the small-particle sampling by these latter inlets has proven beneficial for the inference of condensed-phase HNO3 on particles with diameters of order 1 micron. This talk will emphasize the southern survey flight of 9 July 2002, for which we infer the presence of condensed-phase HNO3 in ternary solutions of H2O, H2SO4, and HNO3. Ice was found in greater and lesser amounts in the presence of the aerosols, but ice appears to have played little role in the uptake of HNO3 here, as the aerosols compete very effectively for the HNO3 in this particular case, due to the relatively large particle volume and the low temperatures. In addition, the condensed-phase NOy measurements are compared to the condensed-phase HNO3 measurements described in another presentation at this meeting (Popp et al.) Overall the condensed-phase HNO3 and NOy amounts are similar to one another and give no indication that other NOy species are taken up by cirrus ice particles.

Weinheimer, A. J.; Knapp, D. J.; Montzka, D. D.; Ridley, B. A.; Popp, P. J.; Gao, R. S.; Marcy, T. P.; Fahey, D. W.; Baumgardner, D.; Anderson, B. E.; Wilson, J. C.; Lee, S. H.; Reeves, J. M.; Lafleur, B. G.; Hilbert, H.; Schmit, O.; Herman, R. L.; Weinstock, E. M.; Smith, J. B.; Sayres, D. W.; Vellovic, J.; Anderson, J. G.; Bui, T. P.; Bowen, S. W.; Pfister, L.; Dean-Day, J.; Chang, C.

2003-12-01

365

Rhombohedral crystal structure of compounds containing boron-rich icosahedra  

Microsoft Academic Search

The crystal structures of several icosahedral boron containing compounds have been refined using Mo K? intensity data. Though these compounds, ?-boron, boron carbide, boron phosphide and boron arsenide, differ chemically, all have the same basic rhombohedral structure. The structures consist of icosahedral units bonded together with direct B-B bonds as well as other linkage units. Similarities in electron distributions are

B. Morosin; A. W. Mullendore; D. Emin; G. A. Slack

1986-01-01

366

Rhombohedral crystal structure of compounds containing boron-rich icosahedra  

Microsoft Academic Search

The crystal structures of several icosahedral boron containing compounds have been refined using Mo Kalpha intensity data. Though these compounds, alpha-boron, boron carbide, boron phosphide and boron arsenide, differ chemically, all have the same basic rhombohedral structure. The structures consist of icosahedral units bonded together with direct B-B bonds as well as other linkage units. Similarities in electron distributions are

B. Morosin; A. W. Mullendore; D. Emin; G. A. Slack

1986-01-01

367

Structure and vertical modes in finite 2D plasma crystals  

Microsoft Academic Search

The formation of finite 2D plasma crystals was numerically simulated to allow the investigation of structural symmetries for systems with various Debye lengths. For the case of increasing Debye length, a transition was discovered between a fully hexagonal structure and a structure having concentric rings along the outer edge and a hexagonal lattice in the interior. Additionally, the vertical as

K. Qiao; T. W. Hyde

2007-01-01

368

Structural Patterns in Polymer-Dispersed Liquid Crystal Films  

Microsoft Academic Search

Different methods for the creation of structural patterns in Polymer Dispersed Liquid Crystals (PDLC) are discussed and compared, with a special attention to the recent techniques of induced director pre-orientation and inhomogeneities creation in PDLC films. Examples of the periodic structural patterns obtained by means of different methods adopted during the PDLC preparation are reported. These PDLC with intrinsic structure

S. J. K?osowicz; O. Francescangeli; S. Di Bella

1999-01-01

369

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 222 ?m3 cytidine, 600600300 nm3 BaTiO3, and 111 ?m3 silicon. The observed structure factors of the silicon crystal were in agreement with the structure factors determined by the Pendellsung method and do not require absorption and extinction corrections.

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

2010-06-01

370

Crystal structure transfer in core/shell nanowires.  

PubMed

Structure engineering is an emerging tool to control opto-electronic properties of semiconductors. Recently, control of crystal structure and the formation of a twinning superlattice have been shown for III-V nanowires. This level of control has not been obtained for Si nanowires, the most relevant material for the semiconductor industry. Here, we present an approach, in which a designed twinning superlattice with the zinc blende crystal structure or the wurtzite crystal structure is transferred from a gallium phosphide core wire to an epitaxially grown silicon shell. These materials have a difference in lattice constants of only 0.4%, which allows for structure transfer without introducing extra defects. The twinning superlattices, periodicity, and shell thickness can be tuned with great precision. Arrays of free-standing Si nanotubes are obtained by a selective wet-chemical etch of the core wire. PMID:21417242

Algra, Rienk E; Hocevar, Mora; Verheijen, Marcel A; Zardo, Ilaria; Immink, George G W; van Enckevort, Willem J P; Abstreiter, Gerhard; Kouwenhoven, Leo P; Vlieg, Elias; Bakkers, Erik P A M

2011-03-21

371

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

372

Quasi-crystals-random structures or twins?  

Microsoft Academic Search

This paper discusses different ways of tiling a plane without defects by using two decorating elements. Four classes of structure with different tiling rules and different relative concentrations of the two elements are compared: Penrose tiling, microcrystalline twins, a random structure and a precipitated structure. The four structures have similar diffraction patterns exhibiting tenfold symmetry, but they differ in many

J. Wolny; L. Pytlik; B. Lebech

1988-01-01

373

Ordering of crystal structure by ionizing radiation  

Microsoft Academic Search

We have studied the action of ionizing radiation on defect-containing semiconductor crystals, metals, and alloys. Using modern methods for investigation of solids, Rutherford back scattering of channeled charged particles, x-ray diffraction, electron microscopy, and also calorimetric methods, we have established: a) irradiation (by x-ray beams, gamma rays, and electrons) of metals and alloys with an equivalent radiation dose less than

I. P. Chernov; A. P. Momontov; P. A. Cherdantsev; B. V. Chakhlov

1994-01-01

374

Crystal Structures of Two Putative Phosphoheptose  

SciTech Connect

Structural genomic centers use both NMR spectroscopic and X-ray crystallographic methods to determine three-dimensional structures of proteins on a genomic scale in a high-throughput mode and to deposit in the PDB. The main goal of structural genomics is to determine a large number of protein structures to complement the ever-expanding database of genome sequences. Another role of structural genomics is to delineate the correspondence between sequence and structure space; a number of protein structures from otherwise unrelated (i.e., 8-10% sequence identity) families often prove to have remarkably similar folds. This finding, in turn, allows better understanding of the structure-function relationships in those proteins for which either structures are not available or cannot be experimentally determined.

Seetharaman,J.; Rajashankar, K.; Solarzano, V.; Kniewel, R.; Lima, C.; Bonanno, J.; Burley, S.; Swaminathan, S.

2006-01-01

375

Thermal Qpalescence in Crystals and the Colour of Ice in Glaciers  

Microsoft Academic Search

IN a previous communication to NATURE (vol. 109, page 42) it was pointed out that the thermal agitation of the atoms in crystals causes optical heterogeneity which should give rise to a noticeable scattering when a beam of light is sent through the substance, and that this effect may actually be observed with suitable arrangements in clear quartz or rock-salt.

C. V. Raman

1923-01-01

376

Ice-Crystal Absorption: A Comparison Between Theory and Implications for Remote Sensing  

Microsoft Academic Search

The problem of the disagreement between cirrus crystal sizes determined remotely and by in situ measurements is shown to be due to inappropriate application of Mie theory. We retrieved the absorption optical depth at 8.3 and 11.1 m from 11 tropical anvil cirrus clouds, using data from the High Resolution Infrared Radiation Sounder (HIRS). We related the absorption optical depth

Anthony J. Baran; John S. Foot; David L. Mitchell

1998-01-01

377

Frazil ice formation in an ice shelf water plume  

NASA Astrophysics Data System (ADS)

We present a model for the growth of frazil ice crystals and their accumulation as marine ice at the base of Antarctic ice shelves. The model describes the flow of buoyant water upward along the ice shelf base and includes the differential growth of a range of crystal sizes. Frazil ice formation starts when the rising plume becomes supercooled. Initially, the majority of crystals have a radius of 0.3 mm and concentrations are below 0.1 g/L. Depending on the ice shelf slope, which controls the plume speed, frazil crystals increase in size and number. Typically, crystals up to 1.0 mm in radius are kept in suspension, and concentrations reach a maximum of 0.4 g/L. The frazil ice in suspension decreases the plume density and thus increases the plume speed. Larger crystals precipitate upward onto the ice shelf base first, with smaller crystals following as the plume slows down. In this way, marine ice is formed at rates of up to 4 m/yr in some places, consistent with areas of observed basal accumulation on Filchner-Ronne Ice Shelf. The plume continues below the ice shelf as long as it is buoyant. If the plume reaches the ice front, its rapid rise produces high supercooling and the ice crystals attain a radius of several millimeters before reaching the surface. Similar ice crystals have been trawled at depth north of Antarctic ice shelves, but otherwise no observations exist to verify these first predictions of ice crystal sizes and volumes.

Smedsrud, Lars H.; Jenkins, Adrian

2004-03-01

378

Modeling Hail Ice Impacts and Predicting Impact Damage Initiation in Composite Structures  

Microsoft Academic Search

Hail ice impacts are a realistic threat to exposed composite structures such as aircraft fuselage and wing skins, leading-edgeandcontrolsurfaces,enginenacelles,andfanblades.Toshedsomelightontothislittle-understood(and unavoidable) threat, experimental, numerical, and analytical investigations have been conducted. Experiments in whichcarbon\\/epoxy composite panelswere impacted byicespheres athigh velocity (30-200 m\\/s)were conducted to measure 1)the impact energy at whichdamage initiates, 2) the multiplefailure modes exhibited by thin composite panels over a

Hyonny Kimand; Keith T. Kedward

2000-01-01

379

Crystal structure of a theta-class glutathione transferase.  

PubMed Central

Glutathione S-transferases (GSTs) are a family of enzymes involved in the cellular detoxification of xenotoxins. Cytosolic GSTs have been grouped into four evolutionary classes for which there are representative crystal structures of three of them. Here we report the first crystal structure of a theta-class GST. So far, all available GST crystal structures suggest that a strictly conserved tyrosine near the N-terminus plays a critical role in the reaction mechanism and such a role has been convincingly demonstrated by site-directed mutagenesis. Surprisingly, the equivalent residue in the theta-class structure is not in the active site, but its role appears to have been replaced by either a nearby serine or by another tyrosine residue located in the C-terminal domain of the enzyme. Images

Wilce, M C; Board, P G; Feil, S C; Parker, M W

1995-01-01

380

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

381

Optical and structural properties of chalcone NLO single crystals  

NASA Astrophysics Data System (ADS)

Organic compound (E)-1-(4-methoxyphenyl)-3-(2,3,5-trichlorophenyl)prop-2-en-1-one [MPTCPP] with molecular formula C16H11Cl3O2 was synthesized using Claisen-Schmidt condensation reaction method. 1H NMR spectra was recorded to identify the various functional groups present in the compound and confirm the chemical structure. The single crystals were grown using slow evaporation solution growth technique. The UV-Visible spectrum study reveals that the crystal is transparent in the entire visible region and the absorption is observed at 364 nm. The Kurtz powder second harmonic generation (SHG) test shows that the MPTCPP is NLO active and its SHG efficiency is three times that of urea. Single crystal XRD study shows that the compound crystallizes in the monoclinic system with a space group Cc. The corresponding lattice parameters of the crystal are a = 28.215(5) , b = 3.9740(4) , c = 16.178(3) and V = 1503.0(4) 3. The micro hardness test was carried out and the work hardening coefficient value (n) of the crystal was found to be 1.48. This indicates that the crystal is hard and is suitable for device application. The thermal study reveals that the thermal stability of the crystal is good.

Rajesh Kumar, P. C.; Ravindrachary, V.; Janardhana, K.; Manjunath, H. R.; Karegouda, Prakash; Crasta, Vincent; Sridhar, M. A.

2011-11-01

382

Mars water-ice clouds and precipitation.  

PubMed

The light detection and ranging instrument on the Phoenix mission observed water-ice clouds in the atmosphere of Mars that were similar to cirrus clouds on Earth. Fall streaks in the cloud structure traced the precipitation of ice crystals toward the ground. Measurements of atmospheric dust indicated that the planetary boundary layer (PBL) on Mars was well mixed, up to heights of around 4 kilometers, by the summer daytime turbulence and convection. The water-ice clouds were detected at the top of the PBL and near the ground each night in late summer after the air temperature started decreasing. The interpretation is that water vapor mixed upward by daytime turbulence and convection forms ice crystal clouds at night that precipitate back toward the surface. PMID:19574386

Whiteway, J A; Komguem, L; Dickinson, C; Cook, C; Illnicki, M; Seabrook, J; Popovici, V; Duck, T J; Davy, R; Taylor, P A; Pathak, J; Fisher, D; Carswell, A I; Daly, M; Hipkin, V; Zent, A P; Hecht, M H; Wood, S E; Tamppari, L K; Renno, N; Moores, J E; Lemmon, M T; Daerden, F; Smith, P H

2009-07-01

383

Hexagonal structures for two-dimensional photonic crystals  

Microsoft Academic Search

Periodic dielectric structures have been recently proposed to inhibit spontaneous emission in semiconductors. From this suggestion, the new concepts of photonic band gap and photonic crystal have been developed. Zero-threshold lasers, waveguides, antenna substrates, filters and polarizers are promising applications. We propose a new class of two-dimensional periodic dielectric structures with hexagonal symmetry. We study the gap opening according to

D. Cassagne; C. Jouanin; D. Bertho

1996-01-01

384

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

385

Photonic crystals of diamond spheres with the opal structure  

NASA Astrophysics Data System (ADS)

Opal samples consisting of diamond spheres have been synthesized by chemical deposition in microwave plasma from a CH4/H2 mixture using templates of silicon inverse opal. The optical investigations have confirmed that the periodic structures prepared are structurally perfect three-dimensional photonic crystals.

Sovyk, D. N.; Ralchenko, V. G.; Kurdyukov, D. A.; Grudinkin, S. A.; Golubev, V. G.; Khomich, A. A.; Konov, V. I.

2013-05-01

386

On automation of the procedure for crystal structure model refinement  

NASA Astrophysics Data System (ADS)

The methods of automation of the procedure for crystal structure model refinement from experimental diffraction data, implemented in the ASTRA program package, are described. Such tools as statistical tests, parameter scanning, and data scanning reduce the time necessary for structural investigation. At strong correlations between parameters, especially when the data set is limited, parameter scanning has an advantage over the full-matrix refinement.

Dudka, A. P.

2008-03-01

387

Liquid crystal based electrically switchable Bragg structure for THz waves.  

PubMed

We demonstrate the first electronically switchable Bragg structure for THz frequencies. The structure works as stop-band filter and as mirror. It exhibits the 60 GHz broad stop-band around 300 GHz which can be removed by reorienting liquid crystal molecules in an external electric field. Our first proof-of-principle experiments agree very well with transfer matrix calculations. PMID:19399116

Wilk, Rafal; Vieweg, Nico; Kopschinski, Olaf; Koch, Martin

2009-04-27

388

Crystal Structure of Purine Nucleoside Phosphorylase from Thermus thermophilus  

Microsoft Academic Search

The purine nucleoside phosphorylase from Thermus thermophilus crystallized in space group P43212 with the unit cell dimensions a=131.9, and c=169.9, and one biologically active hexamer in the asymmetric unit. The structure was solved by the molecular replacement method and refined at a 1.9 resolution to an Rfree value of 20.8%. The crystals of the binary complex with sulfate ion and

Tahir H Tahirov; Eiji Inagaki; Noriyasu Ohshima; Tomoe Kitao; Chizu Kuroishi; Yoko Ukita; Koji Takio; Masanori Kobayashi; Seiki Kuramitsu; Shigeyuki Yokoyama; Masashi Miyano

2004-01-01

389

Crystal Structure of L-Histidinium 2-Nitrobenzoate  

PubMed Central

A new nonlinear optical organic compound, namely, L-histidinium 2-nitrobenzoate (abbreviated as LH2NB (I); ([C6H10N3O2]+ [C7H4NO4]?)), was synthesized. The molecular structure of LH2NB (I) was elucidated using single crystal X-ray diffraction technique. The second harmonic generation (SHG) efficiency of this compound is about two times that of the standard potassium dihydrogen phosphate crystals.

Natarajan, Subramanian; Moovendaran, Kalimuthu; Kalyana Sundar, Jeyaperumal; Ravikumar, Krishnan

2012-01-01

390

Crystal structure of a calcium sulfate-urea complex  

Microsoft Academic Search

The crystal structure of CaSO4 4(CH4N2) has been determined from 2695 x-ray intensities measured on a diffractometer, and refined anisotropically to anR-factor of 0.059. The pseudotetragonal crystals contain linear chains of CaSO4 which strongly resemble those in other CaSO4 phases. A common dodecahedral coordination of calcium ions in all these phases is demonstrated, and its implications in the conversion

J. P. R. De Villiers; J. C. A. Boeyens

1975-01-01

391

Boron-Oxygen Polyanion in the Crystal Structure of Tunellite.  

PubMed

The crystal structure of tunellite, SrO.3B(2)O(3).4H(2)O, with infinite sheets of composition n[B(6)O(9)(OH)(2)](2-), has cations and water molecules in the spaces within the sheets. Adjacent sheets are held together by hydrogen bonding through the water molecules. The boron-oxygen polyanions provide the first example in hydrated borate crystals of one oxygen linked to three borons. PMID:17751796

Clark, J R

1963-09-20

392

Crystal and molecular structure of glutaconic acid  

Microsoft Academic Search

Crystals of Glutaconic acid, C5O4H6, are triclinic, space group P\\u000a$${\\\\bar 1}$$\\u000a, with the cell dimensions (294 K), a = 4.843(1) , b = 10.188(1) , c = 12.609(1) , a = 83.46(1), = 80.02(1), ? = 78.71(1), V = 598.8(3) 3 with Dm = 1.47, Dx = 1.443 gcm -3. There are two independent molecules in the

Leonard Thomas; Thamarapu Srikrishnan

2003-01-01

393

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

394

Fine Structure of Electron Diffraction Beams from a Gold Crystal and from a Silver Film on a Gold Crystal  

Microsoft Academic Search

A study of the diffraction of low-speed electrons from a gold crystal has been made under the same conditions as those previously used for a silver crystal to determine the extent to which the previously observed fine structure characteristics depend on the nature of the atoms composing the crystal. For normal incidence there are many differences in the positions, structure,

H. E. Farnsworth

1933-01-01

395

Ensemble refinement of protein crystal structures  

PubMed Central

Summary X-ray crystallography typically uses a single set of coordinates and B-factors to describe macromolecular conformations. Refinement of multiple copies of the entire structure has been previously used in specific cases as an alternative means of representing structural flexibility. Here, we systematically validate this method using simulated diffraction data, and find ensemble refinement produces better representations of the distributions of atomic positions in the simulated structures than single conformer refinements. Comparison of principal components calculated from the refined ensembles and simulations shows that concerted motions are captured locally, but correlations dissipate over long distances. Ensemble refinement is also used on 50 experimental structures of varying resolution, and leads to decreases in R-free, implying that improvements in the representation of flexibility observed for the simulated structures may apply to real structures. These gains are essentially independent of resolution or data-to-parameter ratio, suggesting even structures at moderate resolution can benefit from ensemble refinement.

Levin, Elena J.; Kondrashov, Dmitry A.; Wesenberg, Gary E.; Phillips, George N.

2007-01-01

396

Synthesis and Structure Characterization of Forsterite Single Crystal  

NASA Astrophysics Data System (ADS)

Forsterite (Mg2SiO4), the low-pressure polymorph of magnesium orthosilicate, is of great importance in the upper mantle due to its aboundance. Up to now, only powder samples of forsterite can be synthesized due to the difficulty of its crystal growth. Therefore, the exact crystal structure of forsterite is still an open question. The crystal structure of forsterite was firstly studied in 1926 by Brown and Bragg. Numerous experimental investigations have been performed in order to get the structure of the olivine group minerals at ambient conditions and a variety of temperature and pressures by using the advent of the computer, the single crystal diffractometer and the diamond cell. However, there are still considerable uncertaintes regarding the accuracy of its unit-cell parameter values. In this study, we synthesized for the first time high quality single crystals of forsterite using the Quickpress piston-cylinder apparatus. The single crystal of forsterite was synthesized by direct reaction of stoichiometric amounts of MgO and amorphous SiO2 (Alfa Aesar, 99.999%) in the presence of ~10-11 wt% distilled water at 2.0GPa, 1723 K for 12h. A colorless single crystal of Mg2SiO4 with size dimensions 0.160.110.04 mm was selected for single crystal X-ray diffraction analysis. The intensity data were collected with a Rigaku R-AXIS RAPID IP diffractometer by oscillation scans using graphite- monochromated Mo-K?0?6?0?6?7677?0?6?0?6?7699 radiation (?=0.71073 ). Cell refinement and data reduction were accomplished with RAPID AUTO program. The crystal structure was solved by direct methods with the SHELXL crystallographic software package. Single crystal X-ray diffraction analysis shows a crystal structure of orthorhombic space group Pnma (No. 62) with a = 10.2073(11) , b = 5.9863(5) , c = 4.7611(4) and Z = 4. Our new data provides new constraints for theoretical investigations of the physical and chemistry properties and behaviors of forsterite under various pressures.

Wang, C.; Jin, S.; Wang, X.; Liu, X.; Fleet, M. E.; Jin, Z.

2006-12-01

397

ANS Fluorescence Detects Widespread Perturbations of Protein Tertiary Structure in Ice  

PubMed Central

Freeze-induced perturbations of the protein native fold are poorly understood owing to the difficulty of monitoring their structure in ice. Here, we report that binding of the fluorescence probe 1-anilino-8-naphthalene sulfonate (ANS) to proteins in ice can provide a general monitor of ice-induced alterations of their tertiary structure. Experiments conducted with copper-free azurin from Pseudomonas aeruginosa and mutants I7S, F110S, and C3A/C26A correlate the magnitude of the ice-induced perturbation, as inferred from the extent of ANS binding, to the plasticity of the globular fold, increasing with less stable globular folds as well as when the flexibility of the macromolecule is enhanced. The distortion of the native structure inferred from ANS binding was found to draw a parallel with the extent of irreversible denaturation by freeze-thawing, suggesting that these altered conformations play a direct role on freeze damage. ANS binding experiments, extended to a set of proteins including serum albumin, ?-amylase, ?-galactosidase, alcohol dehydrogenase from horse liver, alcohol dehydrogenase from yeast, lactic dehydrogenase, and aldolase, confirmed that a stressed condition of the native fold in the frozen state appears to be general to most proteins and pointed out that oligomers tend to be more labile than monomers presumably because the globular fold can be further destabilized by subunit dissociation. The results of this study suggest that the ANS binding method may find practical utility in testing the effectiveness of various additives employed in protein formulations as well as to devise safer freeze-drying protocols of pharmaceutical proteins.

Gabellieri, Edi; Strambini, Giovanni B.

2006-01-01

398

Sea Level: Ice Volume Changes  

NSDL National Science Digital Library

In this exercise, students observe simulations of melting sea ice and a melting continental ice sheet in order to investigate the relationship between the melting of the ice and the water level in the tank. The water tanks simulate the world oceans. In the first example, the ice is floating in water. This would be an example of icebergs or Arctic ice floating on the ocean. In the second example the ice lies on a wood structure. The structure simulates a continent. The block of ice on top of the structure simulates ice grounded on top of a continent. This would be an example of ice sheets in Greenland and Antarctica.

399

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 1.8300 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 interestingand possibly newsystematics 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

400

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

401

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

402

Glacial-isostatic Adjustment and the Viscosity Structure Underlying the Vatnajkull Ice Cap, Iceland  

NASA Astrophysics Data System (ADS)

We examine the dependence of glacial-isostatic adjustment (GIA) due to changes in the Vatnajkull Ice Cap, Iceland, on the underlying viscosity structure. Iceland offers a unique case study for GIA research, with a thinner elastic lithosphere underlain by a low-viscosity zone or asthenosphere, as opposed to regions such as Fennoscandia or North America described by a thicker lithosphere, while not necessarily featuring an asthenosphere. A laterally homogeneous spherical earth model is used consisting of an elastic lithosphere, a viscoelastic asthenosphere, a viscoelastic upper and lower mantle and a fluid core. We examine the response of the earth model to three ice models with circular plans and cross-section profiles based on the assumption of perfectly plastic material, but with different load histories. These are: (1) A history where the ice cap grows from a AD 900 minimum to a maximum at 1890, followed by a uniform decrease until 1991, continuing to the present day at an average rate based on recent mass-balance measurements, (2) a history that is the same as the first, except for constant ice volumes prior to 1890, and (3) a history that is again the same as the first model, except that the post-1991 changes correspond to the measured mass-balance values. We first compare the response to each ice model using typical earth-model parameters for Iceland presented in the literature. We then undertake a parameter-space search, where we assess the importance of lithosphere thickness, asthenosphere viscosity and basal asthenosphere depth, to predicted vertical-displacement rates, and compare them to rates determined from GPS measurements obtained from campaigns conducted between 1991 and 1999. The earth-viscosity structure that provides the optimum predictions with respect to the GPS-derived vertical-displacement rates consists of an elastic lithosphere with a thickness of between 20 and 30 km, an asthenosphere viscosity between 1 and 2 1018 Pa s, and a basal asthenosphere depth between 250 km and possibly greater than 400 km. We find that the very low asthenosphere viscosity values of ca. 1017 Pa s sometimes suggested in the literature are not necessary to account for the rapid vertical-displacement rates observed, which are the result of the contemporary decrease in the mass of the ice cap not considered previously.

Fleming, Kevin; Martinec, Zden?k; Wolf, Detlef

2007-04-01

403

Gastric lipase: crystal structure and activity  

Microsoft Academic Search

Fat digestion in humans requires not only the classical pancreatic lipase but also gastric lipase, which is stable and active despite the highly acidic stomach environment. We have solved the structure of recombinant human gastric lipase at 3.0 resolution, the first structure to be described within the mammalian acid lipase family. This globular enzyme (379 residues) consists of a

Stphane Canaan; Alain Roussel; Robert Verger; Christian Cambillau

1999-01-01

404

Ice Tank Experiments Highlight Changes in Sea Ice Types  

NASA Astrophysics Data System (ADS)

With the current and likely continuing reduction of summer sea ice extent in the Arctic Ocean, the predominant mechanism of sea ice formation in the Arctic is likely to change in the future. Although substantial new ice formation occurred under preexisting ice in the past, the fraction of sea ice formation in open water likely will increase significantly. In open water, sea ice formation starts with the development of small ice crystals, called frazil ice, which are suspended in the water column [World Meteorological Organization, 1985]. Under quiescent conditions, these crystals accumulate at the surface to form an unbroken ice sheet known in its early stage as nilas. Under turbulent conditions, caused by wind and waves, frazil ice continues to grow and forms into a thick, soupy mixture called grease ice. Eventually the frazil ice will coalesce into small, rounded pieces known as pancake ice, which finally consolidate into an ice sheet with the return of calm conditions. This frazil/pancake/ice sheet cycle is currently frequently observed in the Antarctic [Lange et al., 1989]. The cycle normally occurs in regions that have a significant stretch of open water, because this allows for the formation of larger waves and hence increased turbulence. Given the increase of such open water in the Arctic Ocean caused by retreating summer sea ice, the frazil/pancake/ice sheet cycle may also become the dominant ice formation process during freezeup in the Arctic.

Wilkinson, Jeremy P.; DeCarolis, Giacomo; Ehlert, Iris; Notz, Dirk; Evers, Karl-Ulrich; Jochmann, Peter; Gerland, Sebastian; Nicolaus, Marcel; Hughes, Nick; Kern, Stefan; de la Rosa, Sara; Smedsrud, Lars; Sakai, Shigeki; Shen, Hayley; Wadhams, Peter

2009-03-01

405

Ice recrystallization inhibition in ice cream by propylene glycol monostearate.  

PubMed

The effectiveness of propylene glycol monostearate (PGMS) to inhibit ice recrystallization was evaluated in ice cream and frozen sucrose solutions. PGMS (0.3%) dramatically reduced ice crystal sizes in ice cream and in sucrose solutions frozen in a scraped-surface freezer before and after heat shock, but had no effect in quiescently frozen solutions. PGMS showed limited emulsifier properties by promoting smaller fat globule size distributions and enhanced partial coalescence in the mix and ice cream, respectively, but at a much lower level compared to conventional ice cream emulsifier. Low temperature scanning electron microscopy revealed highly irregular crystal morphology in both ice cream and sucrose solutions frozen in a scraped-surface freezer. There was strong evidence to suggest that PGMS directly interacts with ice crystals and interferes with normal surface propagation. Shear during freezing may be required for its distribution around the ice and sufficient surface coverage. PMID:19021802

Aleong, J M; Frochot, S; Goff, H D

2008-11-01

406

The crystal structure and superconducting properties of monatomic bromine  

Microsoft Academic Search

The crystal structure and superconducting properties of monatomic bromine under high pressure have been studied by first-principles calculations. We have found the following phase transition sequence with increasing pressure: from body-centered orthorhombic (bco, phase II) to body-centered tetragonal structure (bct, phase III) at 126 GPa, then to face-centered cubic structure (fcc, phase IV) at 157 GPa, which is stable at

Defang Duan; Xing Meng; Fubo Tian; Changbo Chen; Liancheng Wang; Yanming Ma; Tian Cui; Bingbing Liu; Zhi He; Guangtian Zou

2010-01-01

407

Structure of Widmanstatten crystals of ferrite and cementite  

Microsoft Academic Search

Structural analysis of Widmanstatten crystals in ?8, 10, and 5? carbon steel shows that Widmanstatten ferrite is first deposited\\u000a at the austenite grain boundaries in cast 50? steel. The intervals are filled with polyhedral ferrite. The plates of Widmanstatten\\u000a ferrite and cementite are laminar in structure. The layer thickness is ?10 3 nm or less. Analysis of the fine structure

I. A. Bataev; A. A. Bataev; V. G. Burov; Ya. S. Lizunkova; E. E. Zakharevich

2008-01-01

408

A uniform stress, multi-grain model for migration recrystallization in polar ice  

NASA Astrophysics Data System (ADS)

A multi-grain model for a migration recrystallization process in polar ice is presented. The model is based on the Sachs-Reuss approximation of the stress homogeneity in a polycrystalline aggregate. An individual crystal of ice is treated as a transversely isotropic and incompressible medium which deforms by viscous creep. The highly anisotropic viscous behaviour of the ice crystal is described by a constitutive law expressing microscopic strain-rate in terms of the deviatoric stress and three fluidity parameters that define different viscous resistances of the crystal in different glide directions. It is assumed that the recrystallization occurs in those crystals in the aggregate which are most slowly deforming, and new crystals are nucleated at orientations which favour the crystal deformation by basal glide. The model predictions are illustrated by results of numerical simulations of simple flows, showing the evolution of the microscopic structure of ice and the variation of macroscopic viscosities with increasing deformations.

Staroszczyk, Ryszard

2011-10-01

409

Petrographic and Salinity Characteristics of Brackish Water Ice in the Bay of Bothnia.  

National Technical Information Service (NTIS)

Field observations made during the March 1988 BEPERS (Bothnian Experiment in Preparation for ERS-1) remote sensing experiment included measurements of the snow and ice thickness, temperature, salinity and crystal structure profiles of the different types ...

A. J. Gow W. F. Weeks P. Kosloff S. Carsey

1992-01-01

410

Defectiveness of the crystal structure of electroerosion powders  

SciTech Connect

The fine structure and defectiveness of metal powder crystal lattices produced by electroerosion dispersion were examined. Dispersion was performed on granulated aluminum, Armco iron, carbon steels, and tungsten. The fine structure was examined by x-ray diffraction. Harmonic analysis was performed using a computer and a program which calculates not only the expansion coefficients of the functions into a Fourier series but also the microdistortions and the dimensions of the mosaic blocks. Electroerosion powders were found to have higher density of crystal lattice defects which can increase their chemical and catalytic activity, improve the metallic electroerosion powder passivation, and increase their corrosion resistance.

Fominskii, L.P.; Myuller, A.S.; Levchuk, M.V.

1988-03-01

411

Crystal structure of copper bis(tenuazonate) monohydrate  

Microsoft Academic Search

The crystal structure of copper bis(tenuazonate) monohydrate has been determined by X-ray diffraction from 968 counter-measured intensities. The thin flat crystals are monoclinic,P21, witha = 13.77(1),b = 15.51(1),c = 11.14(1) , = 109.5(2) , and two formula units, C20H26CuN2O6H2O, per asymmetric unit. The structure was refined by full-matrix least squares toR = 0.056. Because of the dimerization one copper

A. Dippenaar; C. W. Holzapfel; J. C. A. Boeyens

1977-01-01

412

Crystal chemistry and structure refinement of five hydrated calcium borates  

USGS Publications Warehouse

The crystal structures of the five known members of the series Ca2B6O11??xH2O (x = 1, 5, 5, 7, 9, and 13) have been refined by full-matrix least-squares techniques, yielding bond distances and angles with standard error