Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “liquid crystals”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,297 records · Page 72
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Shock Initiation Microscopy with High Time and Space Resolution

In this paper, we describe studies of shock initiation and shock-to-detonation transitions in energetic materials using a tabletop shock compression microscope with nanosecond time resolution and micrometer spatial resolution. Planar input shocks with durations of 4–20 ns are produced using 0–4.5 km/s laser-launched flyer plates. Emphasis is on measurements of temperature, velocities, pressure, and microstructure using photon Doppler velocimetry (PDV), optical pyrometry and high-speed videography. Techniques are discussed for fabricating disposable shock target arrays of tiny plastic-bonded explosives (PBX), liquid and powder explosives, and single-crystal explosives for high-throughput studies. Optical temperature measurements of shocked triaminotrinitrobenzene (TATB) are discussed. Since TATB is yellow, we developed methods to correct for the blue absorption to obtain more accurate temperatures. Hot spots in shocked polymer-encased HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) crystals are observed in real-time, showing a hot spot produced in a collapsing void that ignites a deflagration. Despite the small dimensions of our explosive charges (typically 1 mm diameter and 250 μm length), we produced reproducible detonation states in solid and liquid explosives using short-duration shocks near the von Neumann spike (VNS) pressure. We show the VNS pressure is associated with a transition to high-efficiency gas production from the explosives. In studies of NM, prior to detonation, we see reaction originating at hot spots which coalesce to form a superdetonation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1985

Containerless Processing of Undercooled Melts

The physical mechanisms controlling liquid undercooling were studied for application to solidification processing methods. The undercooling potential of containerless processing was assessed on droplet samples of high melting temperature metals drop tube and drop tower facilities and in a laboratory-scale apparatus. New insight into nucleation and crystal growth will be obtained in undercooled liquids of high melting temperature iron and nickel-base systems. The processing parameters include melt superheat, droplet size and particle statistics and droplet surface coating. The solidification behavior is determined by thermal analysis and by structural and metallographic characterization.

Perepezko, J. H.↗
NASA NTRS1978

Fractional crystallization of iron meteorites, an experimental study

Measurements of the distribution coefficients of various elements (Ni, Co, P, Pt, Ir, Ge, Cr, and Au) between solid and liquid phases were made for FeNi alloys. A high thermal gradient crystal growing furnace was employed in order to promote plane front solidification, rapid mixing in the liquid and a minimum of diffusion in the solid. Eight measurements of the nickel distribution coefficient gave an average value of 0.91 plus or minus 0.03 and this value was independent of the Ni concentration in the range 5.5 to 8.75 wt. % Ni. Equilibrium distribution coefficients were also calculated by correcting for the diffusion boundary layer present in the liquid. These values are within 20% of the measured distribution coefficient values. Except for Co and Ni, all the measured distribution coefficient values are closer to 1.0 than those calculated by Scott (1972) for the IIIAB irons. Using the experimental values and the calculated equilibrium values for ternary additions to FeNi melts, it appears that fractional crystallization processes will not explain the observed chemical variations in the major iron meteorite groups. Alternative explanations are suggested which also recognize the fact that cooling rates vary widely in several chemical groups.

Goldstein, J. I.↗
NASA NTRS2013

Anhydrous Liquid Line of Descent of Yamato 980459 and Evolution of Martian Parental Magmas

Martian basalts represented by the shergottite meteorites reflect derivation from highly depleted mantle sources (high Nd, strong LREE depletions, low fO2)[1-3], with evidence of mixing with a much more enriched and oxidized reservoir, most likely a late-stage product of crystallization of an initial martian magma ocean [3-6]. The martian basaltic meteorites Yamato 980459 (Y98) and QUE 94201 (QUE) have bulk compositions that appear to represent bonafide liquids, rather than products of protracted crystallization. These two meteorites also represent the most primitive and evolved examples of the depleted basaltic shergottite suite. Magmatic liquids serve as effective probes of their source regions, and thus studying the potential relationship between magmas represented by Y98 and QUE can yield important information on the formation and evolution of martian basalts. Although the ages of these meteorites preclude that they are petrogenetically related to each other, they represent the best existing candidates for genuine liquids (other meteorites are suggested to represent liquid compositions, including LAR 06319 [7] and NWA 5789 [8], but only Y98 and QUE have been verified experimentally). They span much of the bulk-compositional range of martian basaltic meteorites, and represent end-member liquid compositions likely to arise from partial melting of the martian mantle. Recent efforts to model Y98-like parent liquid evolution by fractional crystallization using MELTS [6] produced a derivative liquid composition that closely matches QUE bulk composition, although it required a some-what unusual crystallization sequence. Experimental endeavours to verify this result at 1 bar have, however, been inconclusive [9].

Rapp, J. F.↗
NASA NTRS1976

Melting behavior and phase relations of lunar samples

An attempt was made to show that feldspar would float during melting. Large anorthite crystals were placed beneath a silicate glass representative of liquid in which plagioclase accumulation is thought to have occurred. In less than 3 hours at 1,300 C, the crystals rose to the top in a Pt crucible 3 cm deep equilibrated in air and in a Mo crucible 1.5 cm deep equilibrated in an H2/CO2 gas stream of log PO2 = -10.9 (below Fe/FeO). These results suggest that lunar crustal formation by feldspar flotation is possible without special recourse to differential sinking of plagioclase versus mafic minerals or selective elutriation of plagioclase.

Hays, J. F.↗
NASA NTRS1977

Convection and dendrite crystallization

The convection and thermal conditions in aqueous and metallic liquid systems under conditions of the Dendrite Remelting Rocket Experiment were assessed to help establish the relevance of the rocket experiment to the metals casting phenomena. The results of the study indicate that aqueous or metallic convection velocities in the cell are of insignificant magnitudes at the 0.0001 to 0.00001 g levels of the experiment. The crystallization phenomena observed in the rocket experiment, therefore, may be indicative of how metals will solidify in low-g. The influence of possibly differing thermal fields, however, remains to be assessed. The rocket experiment may also be relevant to how metals solidify on the ground at temperature differences and in cell configurations such that the flow velocities are not high enough to break or bend delicate dendrite arms. Again, however, the influence of the thermal fields must be assessed.

Grodzka, P. G.↗
NASA NTRS1979

LSA Large Area Silicon Sheet Task. Continuous Liquid Feed Czochralski Growth

The design and development of equipment and processes to demonstrate continuous growth of crystals by the Czochralski method suitable for producing single silicon crystals for use in solar cells is presented. The growth of at least 150 kg of mono silicon crystal, 150 mm in diameter is continuous from one growth container. A furnace with continuous liquid replenishment of the growth crucible, accomplished by a meltdown system with a continuous solid silicon feed mechanism and a liquid transfer system, with associated automatic feedback controls is discussed. Due to the silicon monoxide build up in the furnace and its retarding effect on crystal growth the furnace conversion for operation in the low pressure range is described. Development of systems for continuous solid recharging of the meltdown chamber for various forms of poly silicon is described.

Fiegl, G.↗
DOE OSTI2025

Crystal Nucleation Kinetics and Mechanism: Influence of Interaction Potential

Modulating liquid-to-solid transitions and the resulting crystalline structure for tailored properties is much desired. Colloidal systems are exemplary to this end, but the fundamental knowledge gaps in relating the influence of intermolecular interactions to crystallization behavior continue to hinder progress. In this study, we address this knowledge gap by studying nucleation and growth in systems with modified Lennard-Jones potential. Specifically, we study the commonly used 12-6 potential and a softer 7-6 potential. The thermodynamic state point for the study is chosen such that both systems are investigated at the same level of supercooling and pressure. Under these conditions, we find that the nucleation rate for both systems is comparable. Interestingly, the nucleation pathways and resulting crystal structures are different. In the 12-6 system, nucleation and growth occur predominantly through the FCC structure. Softening the potential alters the critical nucleus composition and introduces two distinct nucleation pathways. One pathway predominantly leads to the nucleus with a body-centered cubic (BCC) structure, while the other favors the face-centered cubic (FCC) arrangement. Our study illustrates that polymorph selection can be achieved through modifications to intermolecular interactions without impacting nucleation kinetics. The results have significant implications in designing approaches for polymorph selection and modulating self-assembly mechanisms.

36 MATERIALS SCIENCE↗
NASA NTRS1996

Forced Oscillations of Supported Drops

Oscillations of supported liquid drops are the subject of wide scientific interest, with applications in areas as diverse as liquid-liquid extraction, synthesis of ceramic powders, growing of pure crystals in low gravity, and measurement of dynamic surface tension. In this research, axisymmetric forced oscillations of arbitrary amplitude of viscous liquid drops of fixed volume which are pendant from or sessile on a rod with a fixed or moving contact line and surrounded by an inviscid ambient gas are induced by moving the rod in the vertical direction sinusiodally in time. In this paper, a preliminary report is made on the computational analysis of the oscillations of supported drops that have 'clean' interfaces and whose contact lines remain fixed throughout their motions. The relative importance of forcing to damping can be increased by either increasing the amplitude of rod motion A or Reynolds number Re. It is shown that as the ratio of forcing to damping rises, for drops starting from an initial rest state a sharp increase in deformation can occur when they are forced to oscillate in the vicinity of their resonance frequencies, indicating the incipience of hysteresis. However, it is also shown that the existence of a second stable limit cycle and the occurrence of hysteresis can be observed if the drop is subjected to a so-called frequency sweep, where the forcing frequency is first increased and then decreased over a suitable range. Because the change in drop deformation response is abrupt in the vicinity of the forcing frequencies where hysteresis occurs, it should be possible to exploit the phenomenon to accurately measure the viscosity and surface tension of the drop liquid.

Wilkes, Edward D.↗
NASA NTRS1986

Optical and transient capacitance study of EL2 in the absence and presence of other midgap levels

A high-resolution optical study was carried out on GaAs crystals grown by horizontal Bridgman and liquid-encapsulated-Czochralski methods. An excellent correlation was found between the intensity of the 1.039-eV no-phonon line and the characteristic absorption of EL2, the major deep donor level in GaAs. A correlation was also found between the characteristic optical absorption of EL2 and its concentration as determined by junction capacitance measurements. The presence of EL0, another midgap level contained in heavily oxygen-doped crystals at concentration always less than those of EL2, had no effect on the optical spectra, but altered the capacitance measurements. Accordingly, an accurate calibration for the determination of EL2 by optical absorption was obtained from capacitance measurements on crystals containing only EL2; in this way the uncertainties introduced by other midgap levels were eliminated.

Skowronski, M.↗
NASA NTRS1985

Crystal Growth Research in Space

The objectives of this work are to develop growth techniques and theory leading to improved bulk growth of semiconductor single crystals. Ground based experiments will be complemented by experiments carried out in the low-g environment provided by the space shuttle. Analytical studies and laboratory investigations are being conducted to better define the causes of crystalline defects and inhomogeneities. The compound semiconductor lead-tin-telluride is being used as the modeling material. Theoretical techniques are being developed to predict the thermal and solutal fields which are present during bulk growth from a melt. Techniques for measuring the thermophysical properties of semiconductors at high temperatures have been developed. During the past year electrochemical etching techniques have been developed for delineation of inhomogeneous regions in crystals. Thermal diffusivity measurements have been completed for the solid and liquid phases of PbTe and PbSnTe. Preliminary results have been obtained on the effects on crystal morphology of gravity, interface shape and interaction between the melt and the container.

Crouch, R. K.↗
NASA NTRS1994

Two-Zone Bridgman Furnace With Sharp Thermal Gradient

Two-zone vertical directional-solidification furnace designed and built to grow crystals from stoichiometric and nonstoichiometric melts and from solutions. Includes conventional wire heater in lower zone, tubular silicon carbide heating element in upper zone, and thermal baffle between zones. Temperature gradients up to 125 degrees centigrade per centimeter achieved in the crystal-growth region. Sharper gradient enables both faster growth and better separation between solid and liquid. Furnace used in laboratory or industrial setting for growth of crystals from congruently melting materials as well as for growth of compounds formed by peritectic reactions.

Borshchevsky, Alex↗
NASA NTRS1977

Dendritic web silicon for solar cell application

The dendritic web process for growing long thin ribbon crystals of silicon and other semiconductors is described. Growth is initiated from a thin wirelike dendrite seed which is brought into contact with the melt surface. Initially, the seed grows laterally to form a button at the melt surface; when the seed is withdrawn, needlelike dendrites propagate from each end of the button into the melt, and the web portion of the crystal is formed by the solidification of the liquid film supported by the button and the bounding dendrites. Apparatus used for dendritic web growth, material characteristics, and the two distinctly different mechanisms involved in the growth of a single crystal are examined. The performance of solar cells fabricated from dendritic web material is indistinguishable from the performance of cells fabricated from Czochralski grown material.

Seidensticker, R. G.↗
NASA NTRS1993

Growth of semiconductor compound single crystal InSb by floating zone method (M-3)

Floating zone methods have potential applications in growing single high-quality semi-conductor crystals. In this method, melts can be sustained without containers and, therefore, are free from contamination from the containers. The main objective of this project is to use the Image Furnace to study a large diameter, (20 mm) single crystal of InSb under microgravity conditions. The behavior of the liquid column is recorded on the VTR tapes and is compared with what is expected theoretically. The single crystal grown in space is characterized by comparing it with single crystals grown on the ground with respect to crystallographic and electronic properties. The goal of this project is to confirm the effects of the microgravity on the single crystals.

Nakatani, I.↗
NASA NTRS2002

Seeing the Heat: Preliminary Studies of Cryocrystallography Using Infrared Imaging

As preparation for an extensive study imaging the cryocooling process of macromolecular crystals we have demonstrated the ability to thermally image solid objects and liquids at temperatures far below 273 K. In the case of a large lysozyme crystal qualitative measurements show the cooling process to take about 0.6s with the cooling taking place in a wave from the face of the crystal nearest to the origin of the cryostream, to the point furthest away from the origin. Annealing of this lysozyme crystal, cooled under good cryoprotectant conditions, showed cold striations formed perpendicular to the cooling stream. These striations became more pronounced after successive annealing. Cryocooling of a non-cryoprotected crystal of glucose isomerase displayed an S-shaped cold front wave traveling across the sample. These preliminary results are qualitative but show the power of infrared imaging as a new tool for fundamental and practical cryocrystallography studies.

Snell, E. H.↗
DOE OSTI2021

Phase Diagram of Active Brownian Spheres: Crystallization and the Metastability of Motility-Induced Phase Separation

Motility-induced phase separation (MIPS), the phenomenon in which purely repulsive active particles undergo a liquid-gas phase separation, is among the simplest and most widely studied examples of a nonequilibrium phase transition. Here, we show that states of MIPS coexistence are in fact only metastable for three-dimensional active Brownian particles over a very broad range of conditions, decaying at long times through an ordering transition we call active crystallization. At an activity just above the MIPS critical point, the liquid-gas binodal is superseded by the crystal-fluid coexistence curve, with solid, liquid, and gas all coexisting at the triple point where the two curves intersect. Nucleating an active crystal from a disordered fluid, however, requires a rare fluctuation that exhibits the nearly close-packed density of the solid phase. The corresponding barrier to crystallization is surmountable on a feasible timescale only at high activity, and only at fluid densities near maximal packing. The glassiness expected for such dense liquids at equilibrium is strongly mitigated by active forces, so that the lifetime of liquid-gas coexistence declines steadily with increasing activity, manifesting in simulations as a facile spontaneous crystallization at incredibly high activity.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Spinon mediation of witness spin dynamics in herbertsmithite

The kagome lattice of spin-1/2 copper atoms in herbertsmithite is conjectured to sustain a quantum spin liquid state with spinon quasiparticles. Ideally, the kagome crystal planes are each separated by a plane of spinless zinc atoms. However, in real crystals, some spin-1/2 copper atoms substitute randomly onto these inter-kagome zinc sites. Here we reconceptualize such ‘impurity’ atoms as quantum witness spins whose dynamics is designed to probe the spin liquid state. We then introduce spin noise spectroscopy to measure the frequency and temperature dependence of witness spin dynamics, demonstrating that their phenomenology is consistent with extensive interactions between witness spins mediated by propagation of spinons through a quantum spin liquid. Ultimately, a sharp transition occurs at around 260 mK, below which the properties of both spin noise and magnetic susceptibility suggest that the witness spins form a spin glass phase. Among the theoretical models considered, we demonstrate that our observations are only consistent with spinon-mediated interactions between witness spins by either a Z 2 or U(1) quantum spin liquid, with the former model more closely matching the data. Our work demonstrates that quantum mechanical witness spins may now conceivably be used as a widely applicable probe of quantum spin liquid physics.

Takahashi, Hiroto [University of Oxford (United Ki↗
NASA NTRS2000

All-Optical Micro Motors Based on Moving Gratings in Photosensitive Media

An all-optical micro motor with a rotor driven by a traveling wave of surface deformation of a stator being in contact with the rotor is being studied. Instead of an ultrasonic wave produced by an electrically driven piezoelectric actuator as in ultrasonic motors, the wave is a result of a photo induced surface deformation of a photosensitive material produced by a traveling holographic grating. Two phase modulated coherent optical beams generate the grating. Several types of photosensitive materials are studied such as photorefractive crystals, photosensitive piezoelectric ceramics, and side-chain liquid crystalline polyesters. In order to be considered as a possible candidate for micro motors, the material should exhibit surface deformation produced by moving grating of the order of 10 micron. Deformations produced by static holographic gratings are studied in photorefractive crystals of LiNbO3 using high vertical resolution surface profilometer Dektak 3 and surface interferometer WYKO. An experimental set-up with moving grating has been developed. The set-up uses a two-beam interferometry configuration with one beam being reflected by a thin mirror mounted on a loud speaker. A ramp voltage signal generator drives the speaker. Changing voltage, polarity, and frequency of the signal can easily generate vibrating gratings or moving gratings in both directions. A vibrating grating has been applied to a photorefractive crystal of BSO controlled by an external electric field of the order of 104 V/cm. We have additionally studied effects of moving grating interaction with light absorbing fluids such as solutions of 2,9,16,23-Tetrakis(phenylthio)-29H, 31 Hphthalocyanine in chlorobenzene in capillary tubes. The purpose of using a liquid is to show that the moving gratings can force a liquid to shift. The interaction of a single low power focused laser beam at 633 nm with such fluid produced an intensive circular motion, which also might be applied to all-optical micro motors or actuators.

Curley, M.↗