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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS2005

Crystallization Kinetics of a Solid Oxide Fuel Cell Seal Glass by Differential Thermal Analysis

Crystallization kinetics of a barium calcium aluminosilicate glass (BCAS), a sealant material for planar solid oxide fuel cells, have been investigated by differential thermal analysis (DTA). From variation of DTA peak maximum temperature with heating rate, the activation energy for glass crystallization was calculated to be 259 kJ/mol. Development of crystalline phases on thermal treatments of the glass at various temperatures has been followed by powder x-ray diffraction. Microstructure and chemical composition of the crystalline phases were investigated by scanning electron microscopy and energy dispersive spectroscopic (EDS) analysis. BaSiO3 and hexacelsian (BaAl2Si2O8) were the primary crystalline phases whereas monoclinic celsian (BaAl2Si2O8) and (Ba(x), Ca(y))SiO4 were also detected as minor phases. Needle-shaped BaSiO3 crystals are formed first, followed by the formation of other phases at longer times of heat treatments. The glass does not fully crystallize even after long term heat treatments at 750 to 900 C.

Bansal, Narottam P.↗
NASA NTRS1992

Crystallization of lithium borate glasses

The glass-forming ability and crystallization behavior of lithium borate compositions, in the diborate-to-metaborate-range, were studied. In particular, the nature and sequence of formation of crystalline phases and the tendency toward devitrification were investigated as functions of temperature, thermal history and batch composition. It was found that the sequence of crystalline phase formation was sensitive to all of the three latter factors, and it was observed that under certain conditions metastable defect structures of the metaborate can appear.

Goktas, A. A.↗
NASA NTRS2024

Microgravity Effects on Nonequilibrium Melt Processing of Neodymium Titanate: Thermophysical Properties, Atomic Structure, Glass Formation and Crystallization

The relationships between materials processing and structure can vary between terrestrial and reduced gravity environments. As one case study, we compare the nonequilibrium melt processing of a rare-earth titanate, nominally 83TiO 2 -17Nd 2 O 3 , and the structure of its glassy and crystalline products. Density and thermal expansion for the liquid, supercooled liquid, and glass are measured over 300–1850 °C using the Electrostatic Levitation Furnace (ELF) in microgravity, and two replicate density measurements were reproducible to within 0.4%. Cooling rates in ELF are 40–110 °C s −1 lower than those in a terrestrial aerodynamic levitator due to the absence of forced convection. X-ray/neutron total scattering and Raman spectroscopy indicate that glasses processed on Earth and in microgravity exhibit similar atomic structures, with only subtle differences that are consistent with compositional variations of ~2 mol. % Nd 2 O 3 . The glass atomic network contains a mixture of corner- and edge-sharing Ti-O polyhedra, and the fraction of edge-sharing arrangements decreases with increasing Nd 2 O 3 content. X-ray tomography and electron microscopy of crystalline products reveal substantial differences in microstructure, grain size, and crystalline phases, which arise from differences in the melt processes.

thermophysical properties↗
NASA NTRS1969

Impurity deposits on alumina crystals.

Silicate glass impurity droplet formations on alumina single crystals polished surfaces analyzed by electron microprobe as function of high temperature

Scott, W. D.↗
NASA NTRS1979

Crystal nucleation in glass-forming alloy and pure metal melts under containerless and vibrationless conditions

Crystal nucleation behavior in metallic alloys known to form glasses in melt quenching was characterized and from this characterization the possibility that massive amounts of certain alloys could be slow cooled to the glass state was assessed. Crystal nucleation behavior of pure liquid metals was examined experimentally, under containerless conditions, and theoretically.

Turnbull, D.↗
NASA NTRS1978

Petrographic and petrological studies of lunar rocks

Thin sections and polished electron probe mounts of Apollo 15 glasscoated breccias 15255, 15286, 15466, and 15505 were examined optically and analyzed by sem/microprobe. Sections from breccias 15465 and 15466 were examined in detail, and chemical and mineralogical analyses of several larger lithic clasts, green glass, and partly crystallized green glass spheres are presented. Area analyses of 33 clasts from the above breccias were also done using the SEM/EDS system. Mineralogical and bulk chemical analyses of clasts from the Apollo 15 glass-coated breccias reveal a diverse set of potential rock types, including plutonic and extrusive igneous rocks and impact melts. Examination of the chemistry of the clasts suggests that many of these clasts, like those found in 61175, are impact melts. Their variability suggests formation by several small local impacts rather than by a large basin-forming event.

Winzer, S. R.↗
NASA NTRS1984

Crystal nucleation and glass formation in metallic alloy melts

Homogeneous nucleation, containerless solidification, and bulk formation of metallic glasses are discussed. Homogeneous nucleation is not a limiting factor for metallic glass formation at slow cooling rates if the reduced glass transition temperature is high enough. Such glasses can be made in bulk if heterogeneous nucleants are removed. Containerless processing eleminates potential sources of nucleants, but as drop tube experiments on the Pd-Si alloys show, the free surface may still be a very effective heterogeneous nucleant. Combination of etching and heating in vacuum or fluxing can be effective for cleaning fairly large ingots of nucleants. Reduced gravity processing has a potentially useful role in the fluxing technique, for example to keep large metallic ingots surrounded by a low density, low fluidity flux if this proved difficult under ground conditions. For systems where heterogeneous nucleants in the bulk of the ingot need gravity to segregate to the flux-metal interface, reduced gravity processing may not be appropriate for bulk glass formation.

Spaepen, F.↗
NASA NTRS2006

XRD, TEM, IR, Raman and NMR Spectroscopy of In Situ Crystallization of Lithium Disilicate Glass

The structure of a Li2O-2SiO2 (LS2) glass was investigated as a function of pressure and temperature up to 6 GPa and 750 C respectively, using XRD, TEM, IR, Raman and NMR spectroscopy. Glass densified at 6 GPa has an average Si-O-Si bond angle approx.7deg lower than that found in glass processed at 4.5 GPa. At 4.5 GPa, lithium disilicate crystallizes from the glass, while at 6 GPa a new high pressure form of lithium metasilicate crystallizes. The new phase, while having lithium metasilicate crystal symmetry, contains at least 4 different Si sites. NMR results for 6 GPa sample indicate the presence of Q4 species with (Q(sup 4))Si-O-Si(Q(sup 4)) bond angles of approx.157deg. This is the first reported occurrence of Q(sup 4) species with such large bond angles in alumina free alkali silicate glass. No five- or six- coordinated Si are found.

Fuss, T.↗
NASA NTRS1994

Crystallization Kinetics of Barium and Strontium Aluminosilicate Glasses of Feldspar Composition

Crystallization kinetics of BaO.Al2O3.2SiO2 (BAS) and SrO.Al2O3.2SiO2 (SAS) glasses in bulk and powder forms have been studied by non-isothermal differential scanning calorimetry (DSC). The crystal growth activation energies were evaluated to be 473 and 451 kJ/mol for bulk samples and 560 and 534 kJ/mol for powder specimens in BAS and SAS glasses, respectively. Development of crystalline phases on thermal treatments of glasses at various temperatures has been followed by powder x-ray diffraction. Powder samples crystallized at lower temperatures than the bulk and the crystallization temperature was lower for SAS glass than BAS. Crystallization in both glasses appeared to be surface nucleated. The high temperature phase hexacelsian, MAl2Si2O8 (M = Ba or Sr), crystallized first by nucleating preferentially on the glass surface. Also, monoclinic celsian does not nucleate directly in the glass, but is formed at higher temperatures from the transformation of the metastable hexagonal phase. In SAS the transformation to monoclinic celsian occurred rapidly after 1 h at 1100 C. In contrast, in BAS this transformation is sluggish and difficult and did not go to completion even after 10 h heat treatment at 1400 C. The crystal growth morphologies in the glasses have been observed by optical microscopy. Some of the physical properties of the two glasses are also reported.

Hyatt, Mark J.↗
NASA NTRS1993

Gas flow-field induced director alignment in polymer dispersed liquid crystal microdroplets deposited on a glass substrate

Polymer dispersed liquid crystal thin films have been deposited on glass substrates by the processes of polymerization and solvent evaporation induced phase separation. The electron and the optical polarization microscopies of the films reveal that PDLC microdroplets formed during the process of phase separation near the top surface of the film remain exposed and respond to shear stress due to air or gas flow on the surface. Optical response of the film to an air flow-induced shear stress input on the free surface has been measured. Director orientation in the droplets changes with the applied shear stress leading to time varying transmitted light intensity. Director dynamics of the droplet for an applied step shear stress has been discussed from free energy considerations. Results on the measurement of light transmission as a function of the gas flow parameter unambiguously demonstrate the potential of these systems for use as boundary layer and gas flow sensors.

Parmar, D. S.↗
NASA NTRS1993

Numerical Model for Isothermal and Non-Isothermal Crystallization of Liquids and Glasses

A new numerical model of isothermal and non-isothermal first order phase transformations, such as the crystallization of liquids and glasses, is presented. This model computes directly the volume fraction transformed, taking into account time-dependent nucleation rates and cluster-size-dependent growth velocities. The model is applied to the crystallization of lithium disilicate glass, using the appropriate kinetic and thermodynamic parameters. The model is used (1) to determine the validity of common methods for computing the volume fraction transformed as a function of time in isothermal experiments when a time-dependent nucleation rate is expected, (2) to simulate non-isothermal differential scanning calorimetry (DSC) studies of glass devitrification as a function of scan rate, and (3) to compute the effect of preannealing on the DSC peak parameters. A novel behavior of the nucleation rate with scan rate is predicted, arising because the relaxation of the cluster distribution cannot be described by a single relaxation time. Comparisons of the calculations with experimental data on this glass demonstrate the validity of the model.

Kelton, K. F.↗
NASA NTRS1978

The formation kinetics of lunar glasses

The kinetic treatment of crystallization and glass formation, involving the construction of time-temperature-transformation curves (TTT) corresponding to a given degree of crystallinity, is extended to permit the description of crystallization of a body initially cooled to a glassy state. The key assumption is that if at any time and temperature a crystallite is smaller than the critical size corresponding to that temperature, it will melt completely and can be ignored in any further calculations of the crystal distribution. This approach is used to predict the temperature of maximum crystallization rate for the matrix composition of lunar breccia 67975; results are shown to be in excellent agreement with experiment. Theoretical results obtained for anorthite indicate a barrier to nucleation in the range of 75 kT when the ratio of the undercooling to the liquidus temperature is 0.2. Measured nucleation barriers for the 67975 matrix composition are in the range of 42 to 45 kT.

Uhlmann, D. R.↗
NASA NTRS1986

Crystallization of baria-titania-silica glasses

The critical cooling rate for glass formation, Rc, and the crystallization kinetics of the compositions (1/2)(100-x)BaO-(1/2)(100-x)TiO2-(x)SiO2 with x = 20, 25, 30, 33.3, and 40 mol pct were studied using a thermal image furnace. Crystallization was studied under nonisothermal conditions, and the data were analyzed using the Johnson-Mehl-Avrami equation. The Rc and activation energy for crystallization both decrease with increasing silica content. Fresnoite, Ba2TiSi2O8, crystallized from all of the glasses when they were reheated. The infrared absorption spectra of the glasses and crystals show that they both contain (Si2O7) and square pyramidal (TiO5) groups.

Ray, Chandra S.↗
NASA NTRS2004

The Effects of Gravity on the Crystallization Behavior of Heavy Metal Fluoride Glasses

Heavy metal fluoride glasses are used in such applications as fiber lasers and laser amplifiers. ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) is one of the more commonly used heavy metal fluoride glasses. ZBLAN is an infrared transmitter and has a theoretical attenuation coefficient of 0.002 db/km. However, due to impurities and small crystallites this attenuation coefficient has not been achieved to date. ZBLAN is a fragile glass which can lead to rapid crystallization, if the glass is not cooled rapidly to below the glass transition temperature or if the glass is reheated near the crystallization temperature for any period of time. Studies carried on at Marshall Space Flight Center and the University of Alabama in Huntsville since 1993 have shown that heating ZBLAN glass at the crystallization temperature in reduced gravity results in a suppression of crystallization when compared to ZBLAN processed in unit gravity. These studies utilized NASA's KC-135 aircraft and the Conquest sounding rocket. In the first series of experiments, short lengths of ZBLAN fiber were heated to the crystallization temperature in reduced gravity on board the KC- 135 and the Conquest sounding rocket and compared with fibers heated in unit gravity. The fibers processed in reduced gravity showed no evidence of crystallization when studied with x-ray diffraction and scanning electron microscopy. However, the fibers processed in unit gravity were completely crystallized. Subsequent experiments included heating small pieces of ZBLAN glass at the crystallization temperature while viewing with a video camera to follow the crystallization phenomenon. In this experiment crystallization was observed in reduced gravity, however, it was suppressed when compared to heating in unit gravity. In the most recent experiment on board the KC-135, rapid thermal analysis of ZBLAN was performed. A mechanism to explain the observations has been proposed. This mechanism is based on shear thinning whereby, the glass undergoes a reduction in viscosity in the 10(exp 5) - 10(exp 7) poise range, allowing more rapid diffusion and thus crystallization. It is proposed that this mechanism is suppressed in reduced gravity. An experiment is presently being conducted to test this theory. With increased knowledge of ZBLAN behavior in reduced gravity, three low earth orbit tiber drawing facilities have been designed. One would be suitable for use on the International Space Station, another while aboard the Space Shuttle and the third system is a fully automated facility which would operate independently of the ISS or Shuttle in a free float mode. The primary benefits of free floating a facility in LEO includes a higher quality of microgravity and reduced safety concerns since it is not in a manned environment.

Tucker, Dennis S.↗
NASA NTRS2004

Disordered Zinc in Zn4Sb3 with Phonon-Glass and Electron-Crystal Thermoelectric Properties

By converting waste heat into electricity, thermoelectric generators could be an important part of the solution to today's energy challenges. The compound Zn4Sb3 is one of the most efficient thermoelectric materials known. Its high efficiency results from an extraordinarily low thermal conductivity in conjunction with the electronic structure of a heavily doped semiconductor. Previous structural studies have been unable to explain this unusual combination of properties. Here, we show through a comprehensive structural analysis using single-crystal X-ray and powder-synchrotron-radiation diffraction methods, that both the electronic and thermal properties of Zn4Sb3 can be understood in terms of unique structural features that have been previously overlooked. The identification of Sb3- ions and Sb-2(4-) dimers reveals that Zn4Sb3 is a valence semiconductor with the ideal stoichiometry Zn13Sb10. In addition, the structure contains significant disorder, with zinc atoms distributed over multiple positions. The discovery of glass-like interstitial sites uncovers a highly effective mechanism for reducing thermal conductivity. Thus Zn4Sb3 is in many ways an ideal 'phonon glass, electron crystal' thermoelectric material.

phases↗
NASA NTRS1985

Glass Research

Research efforts span three general areas of glass science: glass refining, gel-derived glasses, and nucleation and crystallization of glasses. Gas bubbles which are present in a glass product are defects which may render the glass totally useless for the end application. For example, optical glasses, laser host glasses, and a variety of other specialty glasses must be prepared virtually defect free to be employable. Since a major mechanism of bubble removal, buoyant rise, is virtually inoperative in microgravity, glass fining will be especially difficult in space. On the other hand, the suppression of buoyant rise and the ability to perform containerless melting experiments in space allows the opportunity to carry out several unique bubble experiments in space. Gas bubble dissolution studies may be performed at elevated temperatures for large bubbles with negligible bubble motion. Also, bubble nucleation studies may be performed without the disturbing feature of heterogeneous bubble nucleation at the platinum walls. Ground based research efforts are being performed in support of these potential flight experiments.

Weinberg, M. C.↗