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NASA NTRS2002

Phase-Shifting Liquid Crystal Interferometers for Microgravity Fluid Physics

The initial focus of this project was to eliminate both of these problems in the Liquid Crystal Point-Diffraction Interferometer (LCPDI). Progress toward that goal will be described, along with the demonstration of a phase shifting Liquid Crystal Shearing Interferometer (LCSI) that was developed as part of this work. The latest LCPDI, other than a lens to focus the light from a test section onto a diffracting microsphere within the interferometer and a collimated laser for illumination, the pink region contained within the glass plates on the rod-mounted platform is the complete interferometer. The total width is approximately 1.5 inches with 0.25 inches on each side for bonding the electrical leads. It is 1 inch high and there are only four diffracting microspheres within the interferometer. As a result, it is very easy to align, achieving the first goal. The liquid crystal electro-optical response time is a function of layer thickness, with thinner devices switching faster due to a reduction in long-range viscoelastic forces between the LC molecules. The LCPDI has a liquid crystal layer thickness of 10 microns, which is controlled by plastic or glass microspheres embedded in epoxy 'pads' at the corners of the device. The diffracting spheres are composed of polystyrene/divinyl benzene polymer with an initial diameter of 15 microns. The spheres deform slightly when the interferometer is assembled to conform to the spacing produced by the microsphere-filled epoxy spacer pads. While the speed of this interferometer has not yet been tested, previous LCPDIs fabricated at the Laboratory for Laser Energetics switched at a rate of approximately 3.3 Hz, a factor of 10 slower than desired. We anticipate better performance when the speed of these interferometers is tested since they are approximately three times thinner. Phase shifting in these devices is a function of the AC voltage level applied to the liquid crystal. As the voltage increases, the dye in the liquid crystal tends to become more transparent, thus introducing a rather large amount of error into the phase-shifting measurement. While that error can be greatly reduced by normalization, we prefer eliminating the source of the error. To that end, we have pursued development of a 'blend' of custom dyes that will not exhibit these properties. That goal has not yet been fully achieved. Guardalben, et al, presented a similar set of interferograms in a paper partially funded by this grant. Shearing interferometers are a second class of common path interferometers. Typically they consist of a thick glass plate optimized for equal reflection from the front and back surface. While not part of the original thrust of the project, through the course of laboratory work, we demonstrated a prototype of a shearing interferometer capable of phase shifting using a commercial liquid crystal retardation plate. A schematic of this liquid crystal shearing interferometer (LCSI) and a sample set of interferograms are in the reference. This work was also supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460, the University of Rochester, and the New York State Energy Research and Development Authority. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article. Additional information is included in the original extended abstract.

Griffin, DeVon W.↗
NASA NTRS2003

Kinetics of Nucleation and Crystal Growth in Glass Forming Melts in Microgravity

This flight definition project has the specific objective of investigating the kinetics of nucleation and crystal growth in high temperature inorganic oxide, glass forming melts in microgravity. It is related to one1 of our previous NASA projects that was concerned with glass formation for high temperature containerless melts in microgravity. The previous work culminated in two experiments which were conducted aboard the space shuttle in 1983 and 1985 and which consisted of melting (at 1500 C) and cooling levitated 6 to 8 mm diameter spherical samples in a Single Axis Acoustic Levitator (SAAL) furnace. Compared to other types of materials, there have been relatively few experiments, 6 to 8, conducted on inorganic glasses in space. These experiments have been concerned with mass transport (alkali diffusion), containerless melting, critical cooling rate for glass formation, chemical homogeneity, fiber pulling, and crystallization of glass forming melts. One of the most important and consistent findings in all of these experiments has been that the glasses prepared in microgravity are more resistant to crystallization (better glass former) and more chemically homogeneous than equivalent glasses made on earth (1g). The chemical composition of the melt appears relatively unimportant since the same general results have been reported for oxide, fluoride and chalcogenide melts. These results for space-processed glasses have important implications, since glasses with a higher resistance to crystallization or higher chemical homogeneity than those attainable on earth can significantly advance applications in areas such as fiber optics communications, high power laser glasses, and other photonic devices where glasses are the key functional materials. The classical theories for nucleation and crystal growth for a glass or melt do not contain any parameter that is directly dependent upon the g-value, so it is not readily apparent why glasses prepared in microgravity should be more resistant to crystallization than equivalent glasses prepared on earth. Similarly, the gravity-driven convection in a fluid melt is believed to be the primary force field that is responsible for melt homogenization on earth. Thus, it is not obvious why a glass prepared in space, where gravity-driven convection is ideally absent, would be more chemically homogeneous than a glass identically prepared on earth. The primary objective of the present research is to obtain experimental data for the nucleation rate and crystal growth rate for a well characterized silicate melt (lithium disilicate) processed entirely in space (low gravity) and compare these rates with the nucleation and crystal growth rates for a similar glass prepared identically on earth (1g).

Day, Delbert E.↗
NASA NTRS2003

Towards the Structure Determination of a Modulated Protein Crystal: The Semicrystalline State of Profilin:Actin

One of the remaining challenges to structural biology is the solution of modulated structures. While small molecule crystallographers have championed this type of structure, to date, no modulated macromolecular structures have been determined. Modulation of the molecular structures within the crystal can produce satellite reflections or a superlattice of reflections in reciprocal space. We have developed the data collection methods and strategies that are needed to collect and analyze these data. If the macromolecule's crystal lattice is composed of physiologically relevant packing contacts, structural changes induced under physiological conditions can cause distortion relevant to the function and biophysical processes of the molecule making up the crystal. By careful measurement of the distortion, and the corresponding three-dimensional structure of the distorted molecule, we will visualize the motion and mechanism of the biological macromolecule(s). We have measured the modulated diffraction pattern produced by the semicrystalline state of profilin:actin crystals using highly parallel and highly monochromatic synchrotron radiation coupled with fine phi slicing (0.001-0.010 degrees) for structure determination. These crystals present these crystals present a unique opportunity to address an important question in structural biology. The modulation is believed to be due to the formation of actin helical filaments from the actin beta ribbon upon the pH-induced dissociation of profilin. To date, the filamentous state of actin has resisted crystallization and no detailed structures are available. The semicrystalline state profilin:actin crystals provides a unique opportunity to understand the many conformational states of actin. This knowledge is essential for understanding the dynamics underlying shape changes and motility of eukaryotic cells. Many essential processes, such as cytokinesis, phagocytosis, and cellular migration depend upon the capacity of the actin microfilament system to be restructured in a controlled manner via polymerization, depolymerization, severing, cross-linking, and anchorage. The structure the semicrystalline state of profilin:actin will challenge and validate current models of muscle contraction and cell motility. The methodology and theory under development will be easily extendable to other systems.

Borgstahl, G.↗
NASA NTRS2003

A Theory of the von Weimarn Rules Governing the Average Size of Crystals Precipitated from a Supersaturated Solution

More than 75 years ago, von Weimarn summarized his observations of the dependence of the average crystal size on the initial relative concentration supersaturation prevailing in a solution from which crystals were growing. Since then, his empirically derived rules have become part of the lore of crystal growth. The first of these rules asserts that the average crystal size measured at the end of a crystallization increases as the initial value of the relative supersaturation decreases. The second rule states that for a given crystallization time, the average crystal size passes through a maximum as a function of the initial relative supersaturation. Using a theory of nucleation and growth due to Buyevich and Mansurov, we calculate the average crystal size as a function of the initial relative supersaturation. We confirm the von Weimarn rules for the case where the nucleation rate is proportional to the third power or higher of the relative supersaturation.

Barlow, Douglas A.↗
NASA NTRS2004

Anisotropic Piezocomposite Actuator Incorporating Machined PMN-PT Single Crystal Fibers

The design, fabrication, and testing of a flexible, planar, anisotropic piezoelectric composite actuator utilizing machined PMN-32%PT single crystal fibers is presented. The device consists of a layer of rectangular single crystal piezoelectric fibers in an epoxy matrix, packaged between interdigitated electrode polyimide films. Quasistatic free-strain measurements of the single crystal device are compared with measurements from geometrically identical specimens incorporating polycrystalline PZT-5A and PZT-5H piezoceramic fibers. Free-strain actuation of the single crystal actuator at low bipolar electric fields (+/- 250 V/mm) is approximately 400% greater than that of the baseline PZT-5A piezoceramic device, and 200% greater than that of the PZT-5H device. Free-strain actuation under high unipolar electric fields (0-4kV/mm) is approximately 200% of the PZT-5A baseline device, and 150% of the PZT-5H alternate piezoceramic device. Performance increases at low field are qualitatively consistent with predicted increases based on scaling the low-field d(sub 33) piezoelectric constants of the respective piezoelectric materials. High-field increases are much less than scaled d(sub 33) estimates, but appear consistent with high-field freestrain measurements reported for similar bulk single-crystal and piezoceramic compositions. Measurements of single crystal actuator capacitance and coupling coefficient are also provided. These properties were poorly predicted using scaled bulk material dielectric and coupling coefficient data. Rules-of-mixtures calculations of the effective elastic properties of the single crystal device and estimated actuation work energy densities are also presented. Results indicate longitudinal stiffnesses significantly lower (50% less) than either piezoceramic device. This suggests that single-crystal piezocomposite actuators will be best suited to low induced-stress, high strain and deflection applications.

Wilkie, W. Keats↗
NASA NTRS2003

New Programs Utilizing Light Scattering and Flow Imaging Techniques for Macromolecular Crystal Growth and Fluid Dynamics Studies

Dr. Phil Segre, a physicist by training, is a recent addition to the Biotech group, SD46, having joined NASA in August of 2000. Over the past two years he has been developing a laboratory for the study of macromolecular and protein crystal growth. The main apparatus for this work is a Dynamic Light Scattering apparatus, DLS, which is capable of making highly precise measurements of size distributions of both protein solutions and protein crystals. With Drs. Chernov and Thomas (USRA), he has begun a collaboration studying the affects of protein impurities on protein crystal growth and subsequent crystal quality. One of the hypotheses behind the differences between Earth and space grown protein crystals is that the absorption of harmful impurities is reduced in space due to the absence of convective flows. Using DLS measurements we are examining crystal growth with varying amounts of impurities and testing whether there is a strong physical basis behind this hypothesis. With Dr. Joe Ng of UAH he has been collaborating on a project to examine the folding/unfolding dynamics of large RNA complexes. A detailed understanding of this process is necessary for the handling of RNA in biotech applications, and the DLS instrument gives details and results beyond that of other instruments. With Prof. Jim McClymer of the University of Maine (summer faculty visitor to NASA in 2001, 2002), we have been studying the crystallization process in model colloidal suspensions whose behavior in some cases can mimic that of much smaller protein solutions. An understanding of the self-assembly of colloids is the first step in the process of engineering novel materials for photonic and light switching applications. Finally, he has begun an investigation into the physics of particle sedimentation. In addition to the DLS instrument he also has an instrument (called PIV) that can measure flow fields of fluids. The applications are to the dynamics of protein crystal motions both on earth and in low-gravity.

Source record↗
NASA NTRS2004

Kinetic Roughening and Energetics of Tetragonal Lysozyme Crystal Growth: A Preliminary Atomic Force Microscopy Investigation

We examined particulars of crystal growth from measurements obtained at both microscopic and molecular levels. The crystal growth measurements performed at the microscopic level are well characterized by a model that balances the flux of macromolecules towards the crystal surface with the flux of the crystal surface. Numerical evaluation of model with measurements of crystal growth, in time, provided accurate estimates for the average growth velocities. Growth velocities thus obtained were also interpreted using well-established phenomenological theories. Moreover, we find that microscopic measurements of growth velocity measurements obtained as a function of temperature best characterizes changes in crystal growth modes, when present. We also examined the possibility of detecting a change in crystal growth modes at the molecular level using atomic force microscopy, AFM. From preliminary AFM measurements performed at various supersaturations, we find that magnitude of surface height fluctuations, h(x), increases with supersaturation. Further examination of surface height fluctuations using methods established for fluctuation spectroscopy also enabled the discovery of the existence of a characteristic length, c, which may possibly determine the mode of crystal growth. Although the results are preliminary, we establish the non- critical divergence of 5 and the root-mean-square (rms) magnitude of height-height fluctuations as the kinetic roughening transition temperatures are approached. Moreover, we also examine approximate models for interpreting the non-critical behavior of both 6 and rms magnitude of height-height fluctuations, as the solution supersaturation is increased towards the kinetic roughening supersaturation.

Gorti, Sridhar↗
NASA NTRS2003

Effects of Convective Transport of Solute and Impurities on Defect-Causing Kinetics Instabilities in Protein Crystallization

Insight into the crystallization processes of biological macromolecules into crystals or aggregates can provide valuable guidelines in many fundamental and applied fields. Such insight will prompt new means to regulate protein phase transitions in-vivo, e.g., polymerization of hemoglobin S in the red cells, crystallization of crystallins in the eye lens, etc. Understanding of protein crystal nucleation will help achieve narrow crystallite size distributions, needed for sustained release of pharmaceutical protein preparations such as insulin or interferon. Traditionally, protein crystallization studies have been related to the pursuit of crystal perfection needed to improve the structure details provided by x-ray, electron or neutron diffraction methods. Crystallization trials for the purposes of structural biology carried out in space have posed an intriguing question related to the inconsistency of the effects of the microgravity growth on the quality of the crystals.

Vekilov, Peter G.↗
NASA NTRS2005

Liquid Between Macromolecules in Protein Crystals: Static Versus Dynamics

Protein crystals are so fragile that they often can not be handled by tweezers. Indeed, measurements of the Young modulus, E, of lysozyme crystals resulted in E approx. equals 0.1 - 1 GPa, the lower figures, 0.1 - 0.5 GPa, being obtained from triple point bending of as-grown and not cross-linked crystals sitting in solution. The bending strength was found to be approx.10(exp -2) E. On the other hand, ultrasound speed and Mandelstam-Raman-Brilloin light scattering experiments led to much higher figures, E approx. equals 2.7 GPa. The lower figures for E were found from static or low frequency crystal deformations measurements, while the higher moduli are based on high frequency lattice vibrations, 10(exp 7) - 10(exp 10) 1/s. The physical reason for the about an order of magnitude discrepancy is in different behavior of water filling space between protein molecules. At slow lattice deformation, the not-bound intermolecular water has enough time to flow from the compressed to expanded regions of the deformed crystal. At high deformation frequencies in the ultra- and hypersound waves, the water is confined in the intermolecular space and, on that scale, behaves like a solid, thus contributing to the elastic crystal moduli. In this case, the reciprocal crystal modulus is expected to be an average of the water protein and water compressibilities (reciprocal compressibilities): the bulk modulus for lysozyme is 26 GPa, for water it is 7 GPa. Anisotropy of the crystal moduli comes from intermolecular contacts within the lattice while the high frequency hardness comes from the bulk of protein molecules and water bulk moduli. These conclusions are based on the analysis of liquid flow in porous medium to be presented.

Chernov, A. A.↗
NASA NTRS2002

Dynamically controlled crystal growth system

Crystal growth can be initiated and controlled by dynamically controlled vapor diffusion or temperature change. In one aspect, the present invention uses a precisely controlled vapor diffusion approach to monitor and control protein crystal growth. The system utilizes a humidity sensor and various interfaces under computer control to effect virtually any evaporation rate from a number of different growth solutions simultaneously by means of an evaporative gas flow. A static laser light scattering sensor can be used to detect aggregation events and trigger a change in the evaporation rate for a growth solution. A control/follower configuration can be used to actively monitor one chamber and accurately control replicate chambers relative to the control chamber. In a second aspect, the invention exploits the varying solubility of proteins versus temperature to control the growth of protein crystals. This system contains miniature thermoelectric devices under microcomputer control that change temperature as needed to grow crystals of a given protein. Complex temperature ramps are possible using this approach. A static laser light scattering probe also can be used in this system as a non-invasive probe for detection of aggregation events. The automated dynamic control system provides systematic and predictable responses with regard to crystal size. These systems can be used for microgravity crystallization projects, for example in a space shuttle, and for crystallization work under terrestial conditions. The present invention is particularly useful for macromolecular crystallization, e.g. for proteins, polypeptides, nucleic acids, viruses and virus particles.

Bray, Terry L.↗
NASA NTRS2006

Anisotropic Laminar Piezocomposite Actuator Incorporating Machined PMN-PT Single Crystal Fibers

The design, fabrication, and testing of a flexible, laminar, anisotropic piezoelectric composite actuator utilizing machined PMN-32%PT single crystal fibers is presented. The device consists of a layer of rectangular single crystal piezoelectric fibers in an epoxy matrix, packaged between interdigitated electrode polyimide films. Quasistatic free-strain measurements of the single crystal device are compared with measurements from geometrically identical specimens incorporating polycrystalline PZT-5A and PZT-5H piezoceramic fibers. Free-strain actuation of the single crystal actuator at low bipolar electric fields (+/- 250 V/mm) is approximately 400% greater than that of the baseline PZT-5A piezoceramic device, and 200% greater than that of the PZT-5H device. Free-strain actuation under high unipolar electric fields (0-4kV/mm) is approximately 200% of the PZT-5A baseline device, and 150% of the PZT-5H alternate piezoceramic device. Performance increases at low field are qualitatively consistent with predicted increases based on scaling the low-field d33 piezoelectric constants of the respective piezoelectric materials. High-field increases are much less than scaled d33 estimates, but appear consistent with high-field freestrain measurements reported for similar bulk single-crystal and piezoceramic compositions. Measurements of single crystal actuator capacitance and coupling coefficient are also provided. These properties were poorly predicted using scaled bulk material dielectric and coupling coefficient data. Rules-of-mixtures calculations of the effective elastic properties of the single crystal device and estimated actuation work energy densities are also presented. Results indicate longitudinal stiffnesses significantly lower (50% less) than either piezoceramic device. This suggests that single-crystal piezocomposite actuators will be best suited to low induced-stress, high strain and deflection applications.

Wilkie, W. Keats↗
NASA NTRS2015

Shape Evolution of Detached Bridgman Crystals Grown in Microgravity

Detached (or dewetted) Bridgman crystal growth defines that process in which a gap exists between a growing crystal and the crucible wall. In microgravity, the parameters that influence the existence of a stable gap are the growth angle of the solidifying crystal, the contact angle between the melt and the crucible wall, and the pressure difference across the meniscus. During actual crystal growth, the initial crystal radius will not have the precise value required for stable detached growth. Beginning with a crystal diameter that differs from stable conditions, numerical calculations are used to analyze the transient crystal growth process. Depending on the initial conditions and growth parameters, the crystal shape will either evolve towards attachment at the crucible wall, towards a stable gap width, or inwards towards eventual collapse of the meniscus. Dynamic growth stability is observed only when the sum of the growth and contact angles exceeds 180 degrees.

Volz, M. P.↗
NASA NTRS2015

Shape Evolution of Detached Bridgman Crystals Grown in Microgravity

A theory describing the shape evolution of detached Bridgman crystals in microgravity has been developed. A starting crystal of initial radius r0 will evolve to one of the following states: Stable detached gap; Attachment to the crucible wall; Meniscus collapse. Only crystals where alpha plus omega is great than 180 degrees will achieve stable detached growth in microgravity. Results of the crystal shape evolution theory are consistent with predictions of the dynamic stability of crystallization (Tatarchenko, Shaped Crystal Growth, Kluwer, 1993). Tests of transient crystal evolution are planned for ICESAGE, a series of Ge and GeSi crystal growth experiments planned to be conducted on the International Space Station (ISS).

Volz, M. P.↗
NASA NTRS2015

Flight Experiments of Physical Vapor Transport of ZnSe: Growth of Crystals in Various Convective Conditions

A low gravity material experiment will be performed in the Material Science Research Rack (MSRR) on International Space Station (ISS). The flight experiment will conduct crystal growths of ZnSe and related ternary compounds, such as ZnSeS and ZnSeTe, by physical vapor transport (PVT). The main objective of the project is to determine the relative contributions of gravity-driven fluid flows to the compositional distribution, incorporation of impurities and defects, and deviation from stoichiometry observed in the grown crystals as results of buoyancy-driven convection and growth interface fluctuations caused by irregular fluid-flows on Earth. The investigation consists of extensive ground-based experimental and theoretical research efforts and concurrent flight experimentation. The objectives of the ground-based studies are (1) obtain the experimental data and conduct the analyses required to define the optimum growth parameters for the flight experiments, (2) perfect various characterization techniques to establish the standard procedure for material characterization, (3) quantitatively establish the characteristics of the crystals grown on Earth as a basis for subsequent comparative evaluations of the crystals grown in a low-gravity environment and (4) develop theoretical and analytical methods required for such evaluations. ZnSe and related ternary compounds have been grown by vapor transport technique with real time in-situ non-invasive monitoring techniques. The grown crystals have been characterized extensively by various techniques to correlate the grown crystal properties with the growth conditions. This talk will focus on the ground-based studies on the PVT crystal growth of ZnSe and related ternary compounds, especially the effects of different growth orientations related to gravity direction on the grown crystals.

Su, Ching-Hua↗
NASA NTRS2015

Crystal Growth of Germanium-Silicon Alloys on the ISS

A series of Ge(1-x)Si(x) crystal growth experiments are planned to be conducted in the Low Gradient Furnace (LGF) onboard the International Space Station. The experiments are part of the investigation "Influence of Containment on the Growth of Silicon-Germanium" (ICESAGE). The primary objective of the research is to determine the influence of containment on the processing-induced defects and impurity incorporation in germanium-silicon alloy crystals. A comparison will be made between crystals grown by the normal and "detached" Bridgman methods and the ground-based float zone technique. Crystals grown without being in contact with a container have superior quality to otherwise similar crystals grown in direct contact with a container, especially with respect to impurity incorporation, formation of dislocations, and residual stress in crystals. "Detached" or "dewetted" Bridgman growth is similar to regular Bridgman growth in that most of the melt is in contact with the crucible wall, but the crystal is separated from the wall by a small gap, typically of the order of 10-100 microns. Long duration reduced gravity is essential to test the proposed theory of detached growth. Detached growth requires the establishment of a meniscus between the crystal and the ampoule wall. This meniscus can exist over a much larger range of processing parameters in microgravity and the meniscus is more stable under microgravity conditions. The plans for the flight experiments will be described.

Volz, M. P.↗
NASA NTRS2021

Absolute throughput calibration of multiple spherical crystals for the Orion High-REsolution X-ray spectrometer (OHREX)

We present absolute throughput analysis of several crystals for the Orion High-REsolution X-ray (OHREX) imaging crystal spectrometer using ray tracing and experimental measurements. The OHREX spectrometer is a high-resolution x-ray spectrometer designed to measure spectral line shapes at the Orion laser facility. The spectrometer is fielded with up to two spherical crystals simultaneously covering two independent spectral ranges. Each crystal has a nominal radius of curvature of R = 67.2 cm and is fielded at a nominal Bragg angle of 51.3°. To cover different bands of interest, several different crystals are available, including Ge (111), KAP, and several cuts of quartz, whose resolving power λ/Δλ exceeds 10 000. The calibrated response of the available crystals has previously been reported from measurements at the EBIT-I electron beam ion trap at Lawrence Livermore National Laboratory. Here, we model the absolute throughput of each crystal using ray tracing and verify the results using experimental data for the quartz (10¯11) crystal.

M. J. MacDonald↗
NASA NTRS2014

Structural Characterization of Lateral-grown 6H-SiC am-plane Seed Crystals by Hot Wall CVD Epitaxy

The performance of commercially available silicon carbide (SiC) power devices is limited due to inherently high density of screw dislocations (SD), which are necessary for maintaining polytype during boule growth and commercially viable growth rates. The NASA Glenn Research Center (GRC) has recently proposed a new bulk growth process based on axial fiber growth (parallel to the c-axis) followed by lateral expansion (perpendicular to the c-axis) for producing multi-faceted m-plane SiC boules that can potentially produce wafers with as few as one SD per wafer. In order to implement this novel growth technique, the lateral homoepitaxial growth expansion of a SiC fiber without introducing a significant number of additional defects is critical. Lateral expansion is being investigated by hot wall chemical vapor deposition (HWCVD) growth of 6H-SiC am-plane seed crystals (0.8mm x 0.5mm x 15mm) designed to replicate axially grown SiC single crystal fibers. The post-growth crystals exhibit hexagonal morphology with approximately 1500 m (1.5 mm) of total lateral expansion. Preliminary analysis by synchrotron white beam x-ray topography (SWBXT) confirms that the growth was homoepitaxial, matching the polytype of the respective underlying region of the seed crystal. Axial and transverse sections from the as grown crystal samples were characterized in detail by a combination of SWBXT, transmission electron microscopy (TEM) and Raman spectroscopy to map defect types and distribution. X-ray diffraction analysis indicates the seed crystal contained stacking disorders and this appears to have been reproduced in the lateral growth sections. Analysis of the relative intensity for folded transverse acoustic (FTA) and optical (FTO) modes on the Raman spectra indicate the existence of stacking faults. Further, the density of stacking faults is higher in the seed than in the grown crystal. Bundles of dislocations are observed propagating from the seed in m-axis lateral directions. Contrast extinction analysis of these dislocation lines reveals they are edge type basal plane dislocations that track the growth direction. Polytype phase transition and stacking faults were observed by high-resolution TEM (HRTEM), in agreement with SWBXT and Raman scattering.

Single Crystal↗
NASA NTRS1996

Crystallization of amorphous water ice in the solar system

Electron diffraction studies of vapor-deposited water ice have characterized the dynamical structural changes during crystallization that affect volatile retention in cometary materials. Crystallization is found to occur by nucleation of small domains, while leaving a significant part of the amorphous material in a slightly more relaxed amorphous state that coexists metastably with cubic crystalline ice. The onset of the amorphous relaxation is prior to crystallization and coincides with the glass transition. Above the glass transition temperature, the crystallization kinetics are consistent with the amorphous solid becoming a "strong" viscous liquid. The amorphous component can effectively retain volatiles during crystallization if the volatile concentration is approximately 10% or less. For higher initial impurity concentrations, a significant amount of impurities is released during crystallization, probably because the impurities are trapped on the surfaces of micropores. A model for crystallization over long timescales is described that can be applied to a wide range of impure water ices under typical astrophysical conditions if the fragility factor D, which describes the viscosity behavior, can be estimated.

NASA Center ARC↗