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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Large Colloids in Cholesteric Liquid Crystals

We describe a coarse-grained Landau-de Gennes model of liquid crystals (LCs) including hydrodynamics based on the Beris–Edwards equations. The model is employed to study the impact of large colloids on the long range LC defect structure in the cholesteric LC blue phases. ‘Large’ here means that the particle size is comparable to the cholesteric pitch, the length scale on which the LC order undergoes a helical twist. We investigate the case of a single particle, with either normal or degenerate planar anchoring, placed initially in an equilibrium blue phase LC. It is found that in some cases, well defined steady disclination structure emerges at the particle surface, while in other cases no clear steady state is reached in the simulations, and disclination reorganisation appears to proliferate through the bulk LC. These systems are of potential interest in the context of using LCs to template self-assembly of colloid structure, e.g., for opto-electronic devices. Computationally, we demonstrate a parallel approach using mixed message-passing and threaded model on graphical processing units allows effective and efficient progress for this problem.

Physics↗
DOE OSTI2015

Defect Engineering in SrI 2 :Eu 2+ Single Crystal Scintillators

Eu 2+ -activated strontium iodide is an excellent single crystal scintillator used for gamma-ray detection and significant effort is currently focused on the development of large-scale crystal growth techniques. A new approach of molten-salt pumping or so-called melt aging was recently applied to optimize the crystal quality and scintillation performance. Nevertheless, a detailed understanding of the underlying mechanism of this technique is still lacking. The main purpose of this paper is to conduct an in-depth study of the interplay between microstructure, trap centers and scintillation efficiency after melt aging treatment. Three SrI 2 :2 mol% Eu2+ single crystals with 16 mm diameter were grown using the Bridgman method under identical growth conditions with the exception of the melt aging time (e.g. 0, 24 and 72 hours). Using energy-dispersive X-ray spectroscopy, it is found that the matrix composition of the finished crystal after melt aging treatment approaches the stoichiometric composition. The mechanism responsible for the formation of secondary phase inclusions in melt-aged SrI 2 :Eu 2+ is discussed. Simultaneous improvement in light yield, energy resolution, scintillation decay-time and afterglow is achieved in melt-aged SrI 2 :Eu 2+ . The correlation between performance improvement and defect structure is addressed. The results of this paper lead to a better understanding of the effects of defect engineering in control and optimization of metal halide scintillators using the melt aging technique.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1994

Ice nucleation in the upper troposphere: Sensitivity to aerosol number density, temperature, and cooling rate

We have investigated the processes that control ice crystal nucleation in the upper troposphere using a numerical model. Nucleation of ice resulting from cooling was simulated for a range of aerosol number densities, initial temperatures, and cooling rates. In contrast to observations of stratus clouds, we find that the number of ice crystals that nucleate in cirrus is relatively insensitive to the number of aerosols present. The ice crystal size distribution at the end of the nucleation process is unaffected by the assumed initial aerosol number density. Essentially, nucleation continues until enough ice crystals are present such that their deposition growth rapidly depletes the vapor and shuts off any further nucleation. However, the number of ice crystals nucleated increases rapidly with decreasing initial temperature and increasing cooling rate. This temperature dependence alone could explain the large ice crystal number density observed in very cold tropical cirrus.

Jensen, E. J.↗
NASA NTRS1999

Identification of Gravity-Related Effects on Crystal Growth From Melts With an Immiscibility Gap

This work involves an experimental-numerical approach to study the effects of natural and Marangoni convections on solidification of single crystals from a silicate melt with a liquid-liquid immiscibility gap. Industrial use of crystals grown from silicate melts is becoming increasingly important in electronic, optical, and high temperature structural applications. Even the simplest silicate systems like Al203-SiO2 have had, and will continue to have, a significant role in the development of traditional and advanced ceramics. A unique feature of crystals grown from the silicate systems is their outstanding linear electro-optic properties. They also exhibit exceptionally high optical rotativity. As a result, these crystals are attractive materials for dielectric, optical, and microwave applications. Experimental work in our laboratory has indicated that directional solidification of a single crystal mullite appears to be preceded by liquid-liquid phase separation in the melt. Disruption of the immiscible state results in crystallization of a two phase structure. There is also evidence that mixing in the melt caused by density-driven convection can significantly affect the stability of the immiscible liquid layers and result in poly-crystalline growth. On earth, the immiscible state has only been observed for small diameter crystals grown in float zone systems where natural convection is almost negligible. Therefore, it is anticipated that growth of large single crystals from silicate melts would benefit from microgravity conditions because of the reduction of the natural convective mixing. The main objective of this research is to determine the effects of transport processes on the phase separation in the melt during growth of a single crystal while addressing the following issues: (1) When do the immiscible layers form and are they real?; (2) What are the main physical characteristics of the immiscible liquids?; and (3) How mixing by natural or Marangoni convection affects the stability of the phase separated melt.

Kassemi, M.↗
DOE OSTI2019

Observation of a Large Magnetic Anisotropy and a Field-Induced Magnetic State in SrCo(VO 4 )(OH): A Structure with a Quasi One-Dimensional Magnetic Chain

A new member of the descloizite family, a cobalt vanadate, SrCo(VO 4 )(OH), has been synthesized as large single crystals using high-temperature and high-pressure hydrothermal methods. SrCo(VO 4 )(OH) crystallizes in the orthorhombic crystal system in space group P 2 1 2 1 2 1 with the following unit cell parameters: a = 6.0157(2) Å, b = 7.645(2) Å, c = 9.291(3) Å, V = 427.29(2) Å 3 , and Z = 4. It contains one-dimensional Co–O–Co chains of edge-sharing CoO 6 octahedra along the a -axis connected to each other via VO 4 tetrahedra along the b -axis forming a three-dimensional structure. The magnetic susceptibility of SrCo(VO 4 )(OH) indicates an antiferromagnetic transition at 10 K as well as unusually large spin orbit coupling. Single-crystal magnetic measurements in all three main crystallographic directions displayed a significant anisotropy in both temperature- and field-dependent data. Single-crystal neutron diffraction at 4 K was used to characterize the magnetically ordered state. The Co 2+ magnetic spins are arranged in a staggered configuration along the chain direction, with a canting angle that follows the tipping of the CoO 6 octahedra. The net magnetization along the chain direction, resulting in ferromagnetic coupling of the a -axis spin components in each chain, is compensated by an antiferromagnetic interaction between nearest neighbor chains. A metamagnetic transition appears in the isothermal magnetization data at 2 K along the chain direction, which seems to correspond to a co-alignment of the spin directions of the nearest neighbor chain. We propose a phenomenological spin Hamiltonian that describes the canted spin configuration of the ground state and the metamagnetic transition in SrCo(VO 4 )(OH).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

A Low-Temperature Structural Transition in Canfieldite, Ag 8 SnS 6 , Single Crystals

Canfieldite, Ag 8 SnS 6 , is a semiconducting mineral notable for its high ionic conductivity, photosensitivity, and low thermal conductivity. In this paper, we report the solution growth of large single crystals of Ag 8 SnS 6 of mass up to 1 g from a ternary Ag–Sn–S melt. On cooling from high temperature, Ag 8 SnS 6 undergoes a known cubic ($F\bar{4}3m$) to orthorhombic ($Pna2_1$) phase transition at ≈460 K. By studying the magnetization and thermal expansion between 5–300 K, we discover a second structural transition at ≈120 K. Single crystal X-ray diffraction reveals the low-temperature phase adopts a different orthorhombic structure with space group $Pmn2_1$ ($\textit{a}$ = 7.662 9(5) Å, $\textit{b}$ = 7.539 6(5) Å, $\textit{c}$ = 10.630 0(5) Å, Z = 2 at 90 K) that is isostructural to the room-temperature forms of the related Se-based compounds Ag 8 SnSe 6 and Ag 8 GeSe 6 . The 120 K transition is first-order and has a large thermal hysteresis. On the basis of the magnetization and thermal expansion data, the room-temperature polymorph can be kinetically arrested into a metastable state by rapidly cooling to temperatures below 40 K. We last compare the room- and low-temperature forms of Ag8SnS6 with its argyrodite analogues, Ag 8 TQ 6 ($\textit{T}$ = Si, Ge, Sn; $\textit{Q}$ = S, Se), and identify a trend relating the preferred structures to the unit cell volume, suggesting smaller phase volume favors the $Pna2_1$ arrangement. We support this picture by showing that the transition to the $Pmn2_1$ phase is avoided in Ge alloyed Ag 8 Sn 1–x Ge x S 6 samples as well as in pure Ag 8 GeS 6 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Topological quantum properties of chiral crystals

Chiral crystals are materials with a lattice structure that has a well-defined handedness due to the lack of inversion, mirror or other roto-inversion symmetries. Although it has been shown that the presence of crystalline symmetries can protect topological band crossings, the topological electronic properties of chiral crystals remain largely uncharacterized. In this paper we show that Kramers–Weyl fermions are a universal topological electronic property of all non-magnetic chiral crystals with spin–orbit coupling and are guaranteed by structural chirality, lattice translation and time-reversal symmetry. Unlike conventional Weyl fermions, they appear at time-reversal-invariant momenta. We identify representative chiral materials in 33 of the 65 chiral space groups in which Kramers–Weyl fermions are relevant to the low-energy physics. We determine that all point-like nodal degeneracies in non-magnetic chiral crystals with relevant spin–orbit coupling carry non-trivial Chern numbers. Kramers–Weyl materials can exhibit a monopole-like electron spin texture and topologically non-trivial bulk Fermi surfaces over an unusually large energy window.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Annual Report: Property Improvement in CZT via Modeling and Processing Innovations

The objective of this project is to develop growth models of CZT crystals from the melt using vertical gradient freeze (VGF) or vertical Bridgman growth as a typical process. Further, the project will perform critical experiments including single crystal growth to validate the growth models and to provide detailed data for modeling and simulation. Ideally, the project will develop growth models that will provide, for the first time, choices for optimal CZT single crystal growth from the melt based on model input. The overarching goal that guides this research proposal is to produce large, single crystals of CZT with good yield and reproducible properties. In our view this depends on 1) understanding crystal growth processes, including annealing and cool-down processing, and 2) understanding the role of defects on detector response since it is not possible, yet, to produce defect-free materials. Models of defect structure and formation are addressed. Validated models and experiments on reducing defects in melt-grown crystals are used to guide our understanding of growth processes and in-furnace annealing plus cool-down.

36 MATERIALS SCIENCE↗
DOE OSTI2012

EMPIRICAL MODEL FOR FORMULATION OF CRYSTAL-TOLERANT HLW GLASSES

Historically, high-level waste (HLW) glasses have been formulated with a low liquidus temperature (TL), or temperature at which the equilibrium fraction of spinel crystals in the melt is below 1 vol % (T0.01), nominally below 1050°C. These constraints cannot prevent the accumulation of large spinel crystals in considerably cooler regions (~ 850°C) of the glass discharge riser during melter idling and significantly limit the waste loading, which is reflected in a high volume of waste glass, and would result in high capital, production, and disposal costs. A developed empirical model predicts crystal accumulation in the riser of the melter as a function of concentration of spinel-forming components in glass, and thereby provides guidance in formulating crystal-tolerant glasses that would allow high waste loadings by keeping the spinel crystals small and therefore suspended in the glass.

Empirical model↗
DOE OSTI2025

Understanding the Effect of Sample Geometry on Temperature Distribution during Optical Floating Zone Crystal Growth in Vacuum Environment through Heat Transfer Modeling

Optical floating zone furnaces (OFZ) have had a transformative impact on fundamental science due to their ability to rapidly produce large single crystals of a wide variety of complex materials. However, a quantitative understanding of the OFZ growth environment is generally lacking due to the difficulty of measuring the local sample temperatures during OFZ growth, as well as to the general lack of information about the temperature-dependent physical parameters needed to model heat transfer. To overcome these challenges, we apply a physics-based heat transfer model, parametrized by measurements from synchrotron experiments and a machine-learning (ML) algorithm, to simulate the temperature distributions of samples heated in an OFZ furnace in a vacuum environment. This model is used to quantitatively understand how the sample maximum temperature and temperature gradient (key parameters that influence the success of crystal growth) are affected by the rod size, rod shape, and heat-zone position on the rod. The results of this study can be applied to make informed decisions on how crystal growth parameters can be tuned to modify temperature profiles and to optimize crystal growth outcomes even when data on internal sample temperature profiles (e.g., those obtained through in situ synchrotron experiments) are not accessible.

36 MATERIALS SCIENCE↗
DOE OSTI2024

High‐Pressure Synthesis and Recovery of Single Crystals of the Metastable Manganese Carbide, MnC x

Abstract Transition metal carbides find widespread use throughout industry due to their high strength and resilience under extreme conditions. However, they remain largely limited to compounds formed from the early d‐block elements, since the mid‐to‐late transition metals do not form thermodynamically stable carbides. We report here the high‐pressure bulk synthesis of large single crystals of a novel metastable manganese carbide compound, MnC x (P6 3 /mmc), which adopts the anti‐NiAs‐type structure with significant substoichiometry at the carbon sites. We demonstrate how synthesis pressure modulates the carbon loading, with ~40 % occupancy being achieved at 9.9 GPa.

Chemistry↗
DOE OSTI2016

Influence of triethanolamine on the hydration product of portlandite in cement paste and the mechanism

The influences of triethanolamine (TEA) on the portlandite in hardened cement pastes (HCPs) were systematically investigated. Results show that the addition of TEA in cement pastes leads to a visible reduction of Ca(OH){sub 2} (CH) content and considerably alters the morphology of CH crystals from large and parallel-stacked lamellar shape to smaller and distorted actinomorphic one. For the first time, the CH micro-crystals and even non-crystalline CH in HCPs were observed in the presence of TEA. Due to integration of CH micro-crystals in C–S–H phase, remarkable higher Ca/Si ratio of C–S–H phase was found. The formation of TEA-Ca{sup 2+} complex via the interaction between Ca{sup 2+} and the oxygen atoms in TEA molecule was evidenced by the results of NMR and UV. It is believed that TEA can be introduced into the crystallization process of portlandite and thus significantly alters the morphology of CH crystals and even the content of the crystalline CH phase.

36 MATERIALS SCIENCE↗
NASA NTRS2002

Microencapsulated Bioactive Agents and Method of Making

Microcapsules prepared by encapsulating an aqueous solution of a protein, drug or other bioactive substance inside a semi-permeable membrane by are disclosed. The microcapsules are formed by interfacial coacervation under conditions where the shear forces are limited to 0- 100 dynes per sq cm at the interface. By placing the microcapsules in a high osmotic watering solution, the protein solution is gradually made saturated and then supersaturated, and the controlled nucleation and crystallization of the protein is achieved. The crystal-filled microcapsules prepared by this method can be conveniently harvested and stored while keeping the encapsulated crystals in essentially pristine condition due to the rugged, protective membrane. Because the membrane components themselves are x-ray transparent, large crystal-containing microcapsules can be individually selected. mounted in x-ray capillary tubes and subjected to high energy x-ray diffraction studies to determine the 3-D structure of the protein molecules. Certain embodiments of the microcapsules of the invention have composite polymeric outer membranes which are somewhat elastic, water insoluble, permeable only to water, salts, and low molecular weight molecules and are structurally stable in fluid shear forces typically encountered in the human vascular system.

Dennis R Morrison↗
NASA NTRS2004

X-Ray Crystallography Reagent

Microcapsules prepared by encapsulating an aqueous solution of a protein, drug or other bioactive substance inside a semi-permeable membrane by are disclosed. The microcapsules are formed by interfacial coacervation under conditions where the shear forces are limited to 0-100 dynes per square centimeter at the interface. By placing the microcapsules in a high osmotic dewatering solution. the protein solution is gradually made saturated and then supersaturated. and the controlled nucleation and crystallization of the protein is achieved. The crystal-filled microcapsules prepared by this method can be conveniently harvested and stored while keeping the encapsulated crystals in essentially pristine condition due to the rugged. protective membrane. Because the membrane components themselves are x-ray transparent, large crystal-containing microcapsules can be individually selected, mounted in x-ray capillary tubes and subjected to high energy x-ray diffraction studies to determine the 3-D smucture of the protein molecules. Certain embodiments of the microcapsules of the invention have composite polymeric outer membranes which are somewhat elastic, water insoluble, permeable only to water, salts, and low molecular weight molecules and are structurally stable in fluid shear forces typically encountered in the human vascular system.

Dennis R Morrison↗
NASA NTRS2004

Method for determining the three-dimensional structure of a protein

Microcapsules prepared by encapsulating an aqueous solution of a protein, drug or other bioactive substance inside a semi-permeable membrane by are disclosed. The microcapsules are formed by interfacial coacervation under conditions where the shear forces are limited to 0-100 dynes/cm.sup.2 at the interface. By placing the microcapsules in a high osmotic dewatering solution, the protein solution is gradually made saturated and then supersaturated, and the controlled nucleation and crystallization of the protein is achieved. The crystal-filled microcapsules prepared by this method can be conveniently harvested and stored while keeping the encapsulated crystals in essentially pristine condition due to the rugged, protective membrane. Because the membrane components themselves are x-ray transparent, large crystal-containing microcapsules can be individually selected, mounted in x-ray capillary tubes and subjected to high energy x-ray diffraction studies to determine the 3-D structure of the protein molecules. Certain embodiments of the microcapsules of the invention have composite polymeric outer membranes which are somewhat elastic, water insoluble, permeable only to water, salts, and low molecular weight molecules and are structurally stable in fluid shear forces typically encountered in the human vascular system.

Morrison, Dennis R.↗
DOE OSTI2012

EMPIRICAL MODEL FOR FORMULATION OF CRYSTAL-TOLERANT HLW GLASSES

Historically, high-level waste (HLW) glasses have been formulated with a low liquideus temperature (T{sub L}), or temperature at which the equilibrium fraction of spinel crystals in the melt is below 1 vol % (T{sub 0.01}), nominally below 1050 C. These constraints cannot prevent the accumulation of large spinel crystals in considerably cooler regions ({approx} 850 C) of the glass discharge riser during melter idling and significantly limit the waste loading, which is reflected in a high volume of waste glass, and would result in high capital, production, and disposal costs. A developed empirical model predicts crystal accumulation in the riser of the melter as a function of concentration of spinel-forming components in glass, and thereby provides guidance in formulating crystal-tolerant glasses that would allow high waste loadings by keeping the spinel crystals small and therefore suspended in the glass.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2018

Evolutionary selection growth of two-dimensional materials on polycrystalline substrates

There is a demand for the manufacture of two-dimensional (2D) materials with high-quality single crystals of large size. Usually, epitaxial growth is considered the method of choice in preparing single-crystalline thin films, but it requires single-crystal substrates for deposition. Here in this paper we present a different approach and report the synthesis of single-crystal-like monolayer graphene films on polycrystalline substrates. The technological realization of the proposed method resembles the Czochralski process and is based on the evolutionary selection approach, which is now realized in 2D geometry. The method relies on ‘self-selection’ of the fastest-growing domain orientation, which eventually overwhelms the slower-growing domains and yields a single-crystal continuous 2D film. Here we have used it to synthesize foot-long graphene films at rates up to 2.5 cm h -1 that possess the quality of a single crystal. We anticipate that the proposed approach could be readily adopted for the synthesis of other 2D materials and heterostructures.

36 MATERIALS SCIENCE↗
NASA NTRS1992

Protein crystal growth (5-IML-1)

Proteins (enzymes, hormones, immunoglobulins) account for 50 pct. or more of the dry weight of most living systems. A detailed understanding of the structural makeup of a protein is essential to any systematic research pertaining to it. Most macromolecules are extremely difficult to crystallize, and many otherwise exciting projects have terminated at the crystal growth stage. In principle, there are several aspects of microgravity that might be exploited to enhance protein crystal growth. The major factor is the elimination of density driven convective flow. Other factors that can be controlled in the absence of gravity is the sedimentation of growing crystals in a gravitational field, and the potential advantage of doing containerless crystal growth. As a result of these theories and facts, one can readily understand why the microgravity environment of an Earth orbiting vehicle seems to offer unique opportunities for the protein crystallographer. This perception has led to the establishment of the Protein Crystal Growth in a Microgravity Environment (PCG/ME) project. The results of experiments already performed during STS missions have in many cases resulted in large protein crystals which are structurally correct. Thus, the near term objective of the PCG/ME project is to continue to improve the techniques, procedures, and hardware systems used to grow protein crystals in Earth orbit.

Bugg, Charles E.↗