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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 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Roughening Surfaces Of Solar Cells

Proposed treatment involving bombardment by ions gives silicon solar cells rough surfaces and increases amount of light absorbed by cells. First step of treatment, wafer of single-crystal silicon held at temperature of 70 degree C or higher while bombarded with argon or hydrogen ions at kinetic energies between 100 and 2,000 eV. This high dose produces damaged layer on surface, consisting of outer polycrystalline sublayer and underlying sublayer containing extended defects. Bombarded wafer then etched chemically. Photovoltaic conversion efficiency increases with number of reflections at surface of silicon cell because large part of light absorbed at each reflection. Deeply textured surface provides opportunities for multiple reflections.

Singh, Ranbir↗
NASA NTRS1979

Comparison of dynamic crystallization techniques on Apollo 15 quartz normative basalts

The paper compares dynamic crystallization techniques used on Apollo 15 quartz normative basalts which show differences related to their method. The variation of pyroxene core major element chemistry with cooling rate is confirmed, and it is shown that the Fe loss from liquid is shifted toward the En-Di joint; it is suggested that the nucleation temperature and breakdown of equilibrium partitioning as a function of cooling rate dictate the core composition. It is concluded that the different crystallization behavior that results from the Fe-capsule and Fe-loop container methods is related to the nucleation kinetics.

Lofgren, G. E.↗
NASA NTRS1981

Synthesis and characteristics of polyarylene ether sulfones

A method utilizing potassium carbonate/dimethyl acetamide, as base and solvent respectively, was used for the synthesis of several homopolymers and copolymers derived from various bisphenols. It is demonstrated that this method deviates from simple second order kinetics; this deviation being due to the heterogeneous nature of the reaction. Also, it is shown that a liquid induced crystallization process can improve the solvent resistance of these polymers. Finally, a Monte Carlo simulation of the triad distribution of monomers in nonequilibrium copolycondensation is discussed.

Viswanathan, R.↗
NASA NTRS1992

Continuing adventures in lysozyme crystal growth

Results obtained on the crystal nucleation and growth of lysozyme are presented. Special attention is given to the anion-protein binding, the nucleation kinetics, the mechanisms of aggregation, and the conditions that promote or inhibit lysozyme (110)-face growth rate. The emerging theory that is currently being used for data interpretation and for designing new experimental approaches is outlined.

Pusey, Marc L.↗
NASA NTRS1995

Laser-Induced Photochromic Centers in Ce(3+):La2Be2O5

Intense 355-nm excitation of the lowest 4f to 5d transition of Ce(3+):La2Be2O5 leads to a brownish coloration of the crystal due to the formation of stable photochromic centers. These centers are characterized by a broadband absorption spectrum that spans the 220-840 nm spectral range. Since they can be readily bleached by optical excitation into their absorption band, the centers are photochromic. They are produced by a two-step photoionization of Ce(3+) ions followed by attachment of the detached electrons at trapping sites in the crystal. The dependence of the number of centers on laser intensity and length of exposure have been measured, and the kinetics of growth have been studied.

Cheung, Y. M.↗
NASA NTRS1974

Crystallization behavior of anorthite

The growth rate of anorthite crystals from the melt is studied as a function of temperature with undercooling in the ranges 52-152 and 402-652 degrees C. The triclinic form is invariably observed as the crystallization product, growth is preferentially in the c direction, and the interface morphology is faceted. Significant growth rate anisotropy is indicated. The maximum growth rate of anorthite from the melt is higher than for anorthite-rich lunar compositions. Recent computer studies are combined with experimental data to estimate the heat of fusion of anorthite as 28000-45000 cal/mol; the corresponding range for entropy of fusion is (7.8-12)R (where R is the gas constant). The observations and kinetic data support Jackson's predictions concerning materials with large entropies of fusion and his suggestion that entropy of fusion is an important parameter for characterizing the crystal-liquid interface and the nature of the crystallization process.

Klein, L.↗
NASA NTRS1994

Kinetics of phase transformations in glass forming systems

A nucleation rate like curve for a glass can be determined from the functional dependence of the maximum height of its DTA crystallization peak, (delta T)(sub p), on the nucleation temperature, T(sub n). This nucleation rate curve provides information for the temperature range where nucleation for the glass can occur and the temperature where the nucleation rate is a maximum. However, this curve does not provide information for the nucleation rate, I, for the glass at different temperatures. A method for estimating I at different temperatures from (delta T)(sub p) was developed using a Li2O.2SiO2 (LS2) glass. Also, the dielectric constant (epsilon) and the loss factor (tan delta) of a glass-ceramic depend, in part, upon the amount of crystallinity which, in turn, depends upon the nucleation density in the starting glass. It is therefore expected that epsilon and tan delta should have a relationship with nucleation density and hence on the nucleation rate.

Ray, Chandra S.↗
NASA NTRS1987

Crystallization behavior of a melt-spun Fe-Ni based steel

Whether Fe-Ni-based alloys solidify with a bcc or fcc structure has been observed by many investigators to be a stronger function of kinetics and undercooling than strictly free-energy minimization. Such behavior has been observed in an Fe(52.8)Ni(28.7)Al(3.4)Ti(6.1)B(9.0) alloy. The alloy was cast as ribbons about 45 microns thick using a dual free-jet variation of chillbock melt spinning against a Cu wheel. Optical, X-ray, and electron analyses of the as-cast and annealed ribbons were performed. A microstructure of at least four layers containing combinations of ecc, bcc, and amorphous phases in differing proportions was observed in the as-cast ribbon. The midthickness layer had the most unusual features, containing fcc grains about 75 nm in size encompassing spherulitic regions as large as 15 microns comprised of fcc grains about 25 nm in size. The crystallization sequence responsible for the as-cast microstructure is discussed in terms of the competition between the formation of bcc and fcc phases as influenced by undercooling, recalescence, and variations in cooling rate experienced by the as-cast ribbon.

Michal, G. M.↗
NASA NTRS1976

Formation of iron sulphide in solar nebula

Noting that the iron sulfide in the Orgueil carbonaceous meteorite is an Fe-deficient monosulfide (pyrrhotite), it is suggested that such mineral chemistry is inconsistent with equilibrium condensation of the solar nebula and that the course of condensation may have been modified by kinetic effects. The effect of Ni on the reaction between Fe and S to produce FeS is examined, and possible reasons are considered for the fact that the cited meteorite differs in both crystal structure and Ni content from the predictions of equilibrium condensation. It is proposed that sulfide formation in the solar nebula may have been inhibited by sluggish diffusion, so that sulfur began to react with previously condensed troilite to form pyrrhotite. On this basis, observations of the Orgueil sulfides are shown to suggest that the course of solar-system condensation was modified by kinetic effects below about 700 K and that equilibrium may not have been achieved.

Kerridge, J. F.↗
NASA NTRS2004

Challenges in Physics of Biocrystals and Biocrystallization

Selected problems in biocrystal mechanics(1), nucleation(2) and growth(3) will be addressed. (1) Elastic moduli of protein crystals found from triple bonding, rod vibrations, ultrasound speed and Brillion light scattering vary in a wide range of 0.3 - 3 GPa. This diversity comes from behaviour of intracrystalline liquid. Namely, liquid flow through the narrow intermolecular channels between compressed and expanded regions within an inhomogeneously deformed crystal was considered. The deformation comes from bending or ultrasound wave propagation. (2) Wide scattering in the number of crystals nucleated in the batch technique may come from liquid flow turbulence associated with pipetting; the Reynolds number these injections reaches 10(exp 3). (3) Poor reproducibility in crystal size and, probably, perfection is supposed to come from a variety of growth conditions in the same batch. Growth in a kinetic growth mode in laminar flow, e.g., in a microfluidic device, would allow to better understand growth kinetics and defect formation.

Chernov, Alexander A.↗
NASA NTRS1986

Nonradiative relaxation in tunable solid state laser crystals

The characteristics of nonradiative transitions between the 4T2 and 2E excited states of trivalent-chromium-ion-activated ruby (containing 0.04 percent Cr2O3 by weight) and alexandrite (containing 0.4 at. percent chromium ion) laser crystals were studied using the technique described by Gayen et al. (1985). In this technique, a 527-nm pulse excites the 4T2 band of the Cr(3+), and the subsequent population kinetics among excited states is monitored by an IR picosecond probe pulse as a function of pump-probe delay. In ruby, a resolution-limited sharp rise in the excited state population was followed by a long-lifetime decay, leading to an upper limit of 7 ps for the 4T2-state nonradiative lifetime. In alexandrite, a longer rise time was followed by a multicomponent decay. A theoretical model is proposed for explaining the induced absorption and the transition dynamics observed in these crystals.

Gayen, S. K.↗
NASA NTRS1975

Materials research at Stanford University

Research activity related to the science of materials is described. The following areas are included: elastic and thermal properties of composite materials, acoustic waves and devices, amorphous materials, crystal structure, synthesis of metal-metal bonds, interactions of solids with solutions, electrochemistry, fatigue damage, superconductivity and molecular physics and phase transition kinetics.

Source record↗
NASA NTRS1984

A morphological study of silicon carbide prepared by chemical vapor deposition

Surface morphologies of SiC deposits obtained via chemical vapor deposition (CVD) were studied as functions of substrate surface temperature and the concentration of the silicon and carbon source material, methyltrichlorosilane. Substrates of graphite and alpha-SiC crystals were used. Explanations of the observed morphologies on graphite substrates and their marked changes with temperature are given in terms of chemical kinetics and mass transport arguments. The results of thermodynamic calculations were used to help explain the observed morphologies of the deposits on alpha-SiC substrates.

Tsui, P.↗
NASA NTRS1989

Superconducting Bi1.5Pb0.5Sr2Ca2Cu3O(x) ceramics by rapid melt quenching and glass crystallization

A glass of nominal Bi(1.5)Pb(0.5)Sr2Ca2Cu3O(x) composition, prepared by rapid quenching of the melt, showed a glass transition temperature of 383 C, crystallization temperature of 446 C, melting temperature of 855 C, and bulk density of 5.69 g/cu cm in air. The activation energy for crystallization of the glass was estimated to be 292kJ/mol from non-isothermal DSC. On heating in oxygen, the glass showed a slow and continuous weight gain starting at approximately 530 C which reached a plateau at approximately 820 C. The weight gained during heating was retained on cooling to ambient conditions indicating an irreversible oxidation step. The influence of annealing conditions on the formation of various phases in the glass has been investigated. The Bi(2)Sr(2)Ca(0)Cu(1)O(6) phase crystallized out first followed by formation of other phases at higher temperatures. The high-T(sub c) phase, isostructural with Bi(2)Sr(2)Ca(2)Cu(3)O(10) was not detected below 840 C, but its fraction increased with the annealing time at 840 C. A sample annealed at 840 C for 243h in air and furnace cooled showed the highest T(sub c)(R=0) of 107.2K and a narrow transition width, delta T(sub c)(10 to 90 percent), of approximately 2 K. The high T(sub c) phase does not seem to crystallize out directly from the glass but is rather produced at high temperature by reaction between the phases formed at lower temperatures. The kinetics of 110K phase formation was sluggish. It appears that the presence of lead helps in the formation and/or stabilization of the 110 K phase.

Bansal, Narottam P.↗
NASA NTRSDate not supplied

Reactivity of Calcium-Bearing Minerals Under Simulated Venus Conditions

•SO2 is reactive with several common elements e.g. calcium [1-4] •Knowledge on chemical reactions has implications for the past and current state of Venus - Venus may have had liquid water on its surface [5], thus hydrous silicates may have formed at that time [6]. - The Venus Emissivity Mapper (on VERITAS and EnVision) will be used to determine bulk composition of the surface by observing transition metal content [8-9] - Calcium diffuses through basalt to react with CO2 and SO2, changing the bulk composition and decreasing the emissivity from orbit [8] - Experiments investigating kinetics will be informative for future emissivity data •Goal: - Constrain reaction rate between several calcium minerals and SO2 - Determine the effect, if any, crystal lattices may have on these rates

S.T. Port↗
NASA NTRS2003

The Impact of Aerosols on Cloud and Precipitation Processes: Cloud-Resolving Model Simulations

Cloud microphysics are inevitably affected by the smoke particle (CCN, cloud condensation nuclei) size distributions below the clouds. Therefore, size distributions parameterized as spectral bin microphysics are needed to explicitly study the effects of atmospheric aerosol concentration on cloud development, rainfall production, and rainfall rates for convective clouds. Recently, two detailed spectral-bin microphysical schemes were implemented into the Goddard Cumulus Ensemble (GCE) model. The formulation for the explicit spectral-bin microphysical processes is based on solving stochastic kinetic equations for the size distribution functions of water droplets (i.e., cloud droplets and raindrops), and several types of ice particles [i.e.,pristine ice crystals (columnar and plate-like), snow (dendrites and aggregates), graupel and frozen drops/hail]. Each type is described by a special size distribution function containing many categories (i.e. 33 bins). Atmospheric aerosols are also described using number density size-distribution functions.A spectral-bin microphysical model is very expensive from a from a computational point of view and has only been implemented into the 2D version of the GCE at the present time. The model is tested by studying the evolution of deep tropical clouds in the west Pacific warm pool region using identical thermodynamic conditions but with different concentrations of CCN: a low "clean" concentration and a high "dirty" concentration. Besides the initial differences in aerosol concentration, preliminary results indicate that the low CCN concentration case produces rainfall at the surface sooner than the high CCN case but has less cloud water mass aloft. Because the spectral-bin model explicitly calculates and allows for the examination of both the mass and number concentration of species in each size categor, a detailed analysis of the instantaneous size spectrum can be obtained for the two cases. It is shown that since the low CCN case produces fewer droplets, larger sized develop due to the greater condensational and collectional growth, leading to a broader size spectrum in comparison to the high CCN case.

Tao, Wei-Kuo↗