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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 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Elevated Temperature Deformation of Cr3Si Alloyed with Mo

Four-point bend, constant load compressive creep and constant engineering strain rate tests were conducted on arc-melted and powder-metallurgy (PM) processed Cr40Mo30Si30 specimens in the temperature range 1400 and 1700 K. This is a two phase alloy consisting of (CrMo)3Si and (Cr,Mo)5Si3 phases. The PM specimens, which were substantially weaker than the arc-melted materials, exhibited a stress exponent n, of about 2 and an apparent activation energy for creep, Q(sub 3), of 485 kJ/mol. The mechanism in these specimens appeared to be controlled by creep of a glassy phase. In the case of arc-melted specimens for which nis approximately 3 and Q(sub 3) is approximately 430 kJ/mol, the rate-controlling creep mechanism appeared to be that dominant in the (Cr,Mo)5Si3 phase. In this case, it is suggested that the Nabarro creep mechanism, where dislocation climb is controlled by Bardeen-Herring vacancy sources, is the dominant creep mechanism. Finally, an analysis of the present and literature data on Cr3Si alloyed with Mo appeared to suggest that the creep rate decreases sharply with an increase in the Mo/Si ratio.

Raj, S. V.↗
NASA NTRS1999

Petrology and Geochemistry of a Mg- and Al-Rich Orthopyroxenite Xenolith in the EETA79001 Shergottite: Implications for Mars Crustal Evolution

EETA79001 is a Mars meteorite (SNC) consisting of multiple rock types, including two basalt types, olivine and pyroxene xenocrysts, and ultramafic xenoliths. This study is focused on the petrology and geochemistry of one orthopyroxenite xenolith in PTS 68, designated X-1. It consists of chemically homogeneous orthopyroxene cores with exceptionally high Mg/Fe (mg#=85) and Al. Cores are permeated by minute high-Si+Al glassy inclusions, some with augite microlites. Magnesian core areas are mantled by more Fe-rich orthopyroxene rims grading to pigeonite away from cores. The xenolith is transected by cross-cutting shear planes, some of pre-incorporation origin. Major and minor element composition and variation suggest that core areas are primarily igneous, crystallized from a high temperature mafic melt. However, nearly constant mg# across cores suggest metamorphic equilibration. Si+Al inclusions may result from, among other processes, exsolution of feldspathic material during subsolidus cooling, or may be solid materials (alkali feldspar) poikilitically enclosed by growing igneous orthopyroxene crystals. Late reaction with more fractionated melts produced Fe-rich mantles, the whole assemblage later cut by tectonic micro-shear planes. Raw electron microprobe data produced during this study are available on request from the author.

Berkley, John L.↗
NASA NTRS1999

Determination of the Wetting Angle of Germanium and Germanium-Silicon Melts on Different Substrate Materials

During Bridgman growth of semiconductors detachment of the crystal and the melt meniscus has occasionally been observed, mainly under microgravity (microg) conditions. An important factor for detached growth is the wetting angle of the melt with the crucible material. High contact angles are more likely to result in detachment of the growing crystal from the ampoule wall. In order to achieve detached growth of germanium (Ge) and germanium-silicon (GeSi) crystals under 1g and microg conditions, sessile drop measurements were performed to determine the most suitable ampoule material as well as temperature dependence of the surface tension for GeSi. Sapphire, fused quartz, glassy carbon, graphite, SiC, pyrolytic Boron Nitride (pBN), AIN, and diamond were used as substrates. Furthermore, different cleaning procedures and surface treatments (etching, sandblasting, etc.) of the same substrate material and their effect on the wetting behavior were studied during these experiments. pBN and AIN substrates exhibited the highest contact angles with values around 170 deg.

Kaiser, Natalie↗
NASA NTRS1995

Freezing Behavior of Stratospheric Sulfate Aerosols Inferred from Trajectory Studies

Temperature histories based on 10-day back growth trajectories for six ER-2 flights during AASE I (1989) and AAOE (1987) are presented. These trajectories along with the properties of the observed PSC (polar stratospheric cloud) particles are used here to infer the physical state of the pre-existing sulfuric acid aerosols. Of all the ER-2 flights described here, only the PSCs observed on the flights of January 24 and 25, 1989 are consistent with the thermodynamics of liquid ternary solutions of H2SO4/HNO3/H2O Ib PSCs). For these two days, back trajectories indicate that the air mass was exposed to SAT (sulfuric acid tetrahydrate) melting temperatures about 24 hours prior to being sampled by the ER-2. For the remaining ER-2 flights (January, 16, 19, and 20 for the AASE I campaign and August 17 for the AAOE campaign), the observed PSCs were probably composed of amorphous solid solutions of HNO3 and H2O (Type Ic PSCs). Formation of such Type Ic PSCs requires the presence of solid H2SO4 aerosols since liquid aerosols yield ternary solutions. The 10-day back trajectories of these flights indicate that the air mass was not exposed to SAT melting temperatures during the past week and had experienced cooling/warming cycles prior to being sampled by the ER-2. These temperature histories, recent laboratory measurements and the properties of glassy solids suggest that stratospheric H2SO4 aerosols may undergo a phase transition to SAT upon warming at approximately 198 K after going through a cooling cycle to about 194 K or lower.

Tabazadeh, A.↗
NASA NTRS2000

New Alloys for Electroformed Replicated X-Ray Optics

The process of electroforming x-ray mirror shells off a superpolished mandrel has been widely used. The recently launched XMM mission is a good example of this, containing 174 such mirror shells of diameters ranging from 0.3-0.7 m and thicknesses of 0.47-1.07 mm. To continue to utilize this technique for the next generation of x-ray observatories, where ever-larger collecting areas will be required within the constraints of tight weight budgets, demands that new alloys be developed that can withstand the large stresses imposed on very thin shells by the replication and handling processes. Towards this end, we began a development program in late 1997 to produce a high-strength alloy suitable for electroforming very thin high-resolution x-ray optics. The requirements for this task are quite severe; not only must the electroformed deposit be very strong, it must also have extremely low residual stresses to prevent serious figure distortions in large thin-walled shells. Further, the electroforming must be performed at near room temperature, as large temperature changes will modify the figure of the mandrel, in an environment that is not corrosive for the mandrel. The figure of merit for the strength of the electroformed deposit is its Precision Elastic Limit (PEL). This is a measure of permanent strain, at the few parts per million level, under applied stress. Pure nickel is very ductile and will permanently deform, at the parts-per-million level under loads of a few x 10(exp 7) Pa. These stresses are easily exceeded when thin-walled shells (150 micron thick) are replicated. Our goal was to develop an alloy an order of magnitude stronger than this. We will present the results of our development program, showing the evolution of our plating baths through to our present 'glassy' nickel alloy that satisfies the goals above. For each we will show the electroforming characteristics of the bath and the PEL measurements for the resulting alloys. We estimate the ultimate limit on shell thickness and mass for x-ray mirrors produced in these baths.

Engelhaupt, D.↗
NASA NTRS1999

Space Weathering in the Fine Size Fractions of Lunar Soils: Soil Maturity Effects

The effects of space weathering on the optical properties of lunar materials have been well documented. These effects include a reddened continuum slope, lowered albedo, and attenuated absorption features in reflectance spectra of lunar soils as compared to finely comminuted rocks from the same Apollo sites. However, the regolith processes that cause these effects are not well known, nor is the petrographic setting of the products of these processes fully understood. A Lunar Soil Characterization Consortium has been formed with the purpose of systematically integrating chemical and mineralogical data with the optical properties of lunar soils. Understanding space-weathering effects is critical in order to fully integrate the lunar sample collection with remotely-sensed data from recent robotic missions (e.g., Lunar Prospector, Clementine, and Galileo) We have shown that depositional processes (condensation of impact-derived vapors, sputter deposits, accreted impact material, e.g., splash glass, spherules, etc.) are a major factor in the modification of the optical surfaces of lunar regolith materials. In mature soils, it is the size and distribution of the nanophase metal in the soil grains that has the major effect on optical properties. In this report, we compare and contrast the space-weathering effects in an immature and a mature soil with similar elemental compositions. For this study, we analyzed <10 micron sieve fractions of two Apollo 17 soils, 79221 (mature, Is/FeO = 81) and 71061 (immature, Is/FeO = 14). Details of the sieving procedures and allocation scheme are given else where. The results of other detailed chemical, mineralogical, and spectroscopic analyses of these soil samples are reported elsewhere. A representative sample of each soil was embedded in low-viscosity epoxy, and thin sections (about 70nm thick) were obtained through ultra microtomy. The thin sections used for these analyses typically contained cross sections of up to 500 individual grains. The thin sections were studied using a JEOL 2010 transmission electron microscope (TEM) equipped with a thin window energy-dispersive X-ray (EDX) spectrometer. An individual thin section was selected from each soil, and for each grain in the section we determined (1) the elemental composition by EDX; (2) whether the grain was crystalline or glassy using electron diffraction and darkfield imaging; (3) the presence or absence of rims and accreted material; and (4) the distribution of nanophase Fe where present. Most of the categories are self-evident; however, we divide the agglutinate derived material into agglutinitic glass (glass with approximately the same composition as the bulk soil that contains nanophase Fe with or without vesicles) and agglutinate fragments, which are composed of crystalline grains and agglutinitic glass. Lithic fragments are defined as polymineralic grains with no glass. Pyroxene grains have been divided into high- and low-Ca groups. As expected, there are a number of differences in the petrography of the <10-microns fractions of 79221 and 71061 given the great difference in their respective maturities, but we focus here on two major distinctions: agglutinate content and the number of grains with micropatina. Slightly over 50% of the particles in 79221 consist of agglutinitic glass and agglutinate fragments, while the remainder are predominantly crystalline mineral grains. The agglutinic glass particles contain abundant nanophase Fe and vesicles. Angular particles are rare, with most showing smooth, rounded exteriors, Of the mineral grains analyzed thus far, over 90% of the grains have amorphous rims that contain nanophase Fe (these rims are believed to have formed by vapor deposition and irradiation effects). The nanophase Fe in these rims probably accounts for a significant fraction of the increase in Is/FeO measured in these size fractions. In addition to the rims, the majority of particles also show abundant accreted material in the form of glass splashes and spherules that also contain nanophase Fe. In stark contrast, the surfaces of the mineral grains in the 71061 sample are relatively prisitine, as only about 14% of the mineral grains in the sample exhibited amorphous rims. Furthermore, the mineral particles are more angular and show greater surface roughness than in the mature sample. Accreted material on particle surfaces is rare. Agglutinitic material is a major component of the 71061 sample; however, nanophase Fe and vesicles are not as well developed as in the 79221 sample. It is now recognized that nanophase Fe is probably the main agent in modifying the optical properties of lunar soil grains. The most important result of this study is the observation that in the fine size fractions of mature soils, nearly every grain has nanophase Fe within 100 run of the particle surface. (Additional Information contained in original)

Keller, L. P.↗
NASA NTRS1999

Petrology of the Indian Eucrite Piplia Kalan

Piplia Kalan is an equilibrated eucrite consisting of 60-80 vol.% lithic clasts in a subordinate brecciated matrix. Ophitic/subophitic lithic clasts fall into two groups: finer-grained lithology A and coarser-grained lithology B. Very fine-grained clasts (lithology C) also occur and originally were hypocrystalline in texture. The variety of materials represented in Piplia Kalan suggests cooling histories ranging from quenching or fast crystallization to slower crystallization. Despite textural differences, clasts and matrix have similar mineral and bulk compositions. Thus. Piplia Kalan is probably best classified as a genomict breccia that could represent fragments of a single lava flow or shallow intrusive body, including fine-grained or glassy outer margin and more slowly cooled coarser-grained interior. Piplia Kalan displays evidence of an early shock event, including brecciated matrix and areas of lithic clasts that contain fine-grained, equigranular pyroxene between deformed feldspar laths. The meteorite also displays evidence of at least one episode of thermal metamorphism: hypocrystalline materials are recrystallized to hornfelsic textures and the matrix has a nonporous texture similar to those of eucrites that were affected by post-brecciation heating. Veins of brown glass transect both lithic clasts and brecciated matrix and indicate a second, post-metamorphism shock event.

Buchanan, Paul C.↗
NASA NTRS2000

Experimental Study of Lunar and SNC Magmas

The research described in this progress report involved the study of petrological, geochemical and volcanic processes that occur on the Moon and the SNC parent body, generally accepted to be Mars. The link between these studies is that they focus on two terrestrial-type parent bodies somewhat smaller than earth, and the fact that they focus on the role of volatiles in magmatic processes and on processes of magma evolution on these planets. The work on the lunar volcanic glasses has resulted in some exciting new discoveries over the years of this grant. During the tenure of the present grant, we discovered a variety of metal blebs in the A17 orange glass. Some of these Fe-Ni metal blebs occur in the glass; others were found in olivine phenocrysts which we find make up about 2 vol % of the orange glass magma. The importance of these metal spheres is that they fix the oxidation state of the parent magma during the eruption, and also indicate changes during the eruption. They also yield important information about the composition of the gas phase present, the gas which drove the lunar fire-fountaining. In an Undergraduate senior thesis project, Nora Klein discovered a melt inclusion that remained in a glassy state in one of the olivine phenocrysts. Analyses of this inclusion gave additional information on the CO2, CO and S contents of the orange glass magma prior to its reaching the lunar surface. The composition of lunar volcanic gases has long been one of the puzzles of lunar magmatic processes. One of the more exciting findings in our research over the past year has been the study of magmatic processes linking the SNC meteorite source magma composition with the andesitic composition rocks found at the Pathfinder site. In this project, graduate student Michelle Minitti showed that there was a clear petrologic link between these two magma types via fractional removal of crystals from the SNC parent melt, but the process only worked if there was at least 1 wt % dissolved water in the melt.

Rutherford, Malcolm J.↗
NASA NTRS2000

In-plane and Interlaminar Shear Strength of a Unidirectional Hi-nicalon Fiber-reinforced Celsian Matrix Composite

In-plane and interlaminar shear strength of a unidirectional SiC fiber-reinforced (BaSr)Al2Si2O8 celsian composite were measured by the double-notch shear test method between room temperature and 1200 C. The interlaminar shear strength was lower than the in-plane shear strength at all temperatures. Stress analysis, using finite element modeling, indicated that shear stress concentration was not responsible for the observed difference in strength. Instead, the difference in layer architecture and thus, the favorable alignment of fiber-rich layers with the shear plane in the interlaminar specimens appears to be the reason for the low strength of this composite. A rapid decrease in strength was observed with temperature due to softening of the glassy phase in the material.

Uenal, O.↗
NASA NTRS2000

The Effect of Sliding Speed on Film Thickness and Pressure Supporting Ability of a Point Contact Under Zero Entrainment Velocity Conditions

A unique tribometer is used to study film forming and pressure supporting abilities of point contacts at zero entrainment velocity (ZEV). Film thickness is determined using a capacitance technique, verified through comparisons of experimental results and theoretical elastohydrodynamic lubrication (EHL) predictions for rolling contacts. Experiments are conducted using through hardened AISI 52 100 steel balls, Polyalphaolefin (PAO) 182 and Pentaerythritol Tetraheptanoate (PT) lubricants, and sliding speeds between 2.0 to 12.0 m/s. PAO 182 and PT are found to support pressures up to 1. 1 GPa and 0.67 GPa respectively. Protective lubricant films ranging in thickness between 90 to 2 10 nm for PAO 182 and 220 to 340 nm for PT are formed. Lubricants experience shear stresses between 14 to 22 MPa for PAO 182 and 7 to 16 MPa for PT at shear rates of 10(exp 7)/sec. The lubricant's pressure supporting ability most likely results from the combination of immobile films and its transition to a glassy solid at high pressures.

Thompson, Peter M.↗
NASA NTRS2001

Wetting Angle and Surface Tension of Germanium Melts on Different Substrate Materials

The sessile drop technique has been used to measure the wetting angle and the surface tension of molten germanium (Ge) on various substrate materials. Sapphire, fused silica, glassy carbon, graphite, SiC, carbon-based aerogel, pyrolytic boron nitride (pBN), AlN, Si3N4, and CVD diamond were used as substrate materials. In addition, the effects of different cleaning procedures and surface treatments on the wetting behavior were investigated. The highest wetting angles with values around 170 deg. were found for pBN substrates under active vacuum or with a slight overpressure of 5N Argon or forming gas (2% Hydrogen in 5N Argon). The measurement of the surface tension and its temperature dependence for Ge under a forming gas atmosphere resulted in gamma(T) = 591 - 0.077 (T-T(sub m).

Kaiser, N.↗
NASA NTRS2001

Contact Angles and Surface Tension of Germanium-Silicon Melts

Precise knowledge of material parameters is more and more important for improving crystal growth processes. Two important parameters are the contact (wetting) angle and the surface tension, determining meniscus shapes and surface-tension driven flows in a variety of methods (Czochralski, EFG, floating-zone, detached Bridgman growth). The sessile drop technique allows the measurement of both parameters simultaneously and has been used to measure the contact angles and the surface tension of Ge(1-x)Si(x) (0 less than or equal to x less than or equal to 1.3) alloys on various substrate materials. Fused quartz, Sapphire, glassy carbon, graphite, SiC, carbon-based aerogel, pyrolytic boron nitride (pBN), AIN, Si3N4, and polycrystalline CVD diamond were used as substrate materials. In addition, the effect of different cleaning procedures and surface treatments on the wetting behavior were investigated. Measurements were performed both under dynamic vacuum and gas atmospheres (argon or forming gas), with temperatures up to 1100 C. In some experiments, the sample was processed for longer times, up to a week, to investigate any changes of the contact angle and/or surface tension due to slow reactions with the substrate. For pure Ge, stable contact angles were found for carbon-based substrates and for pBN, for Ge(1-x)Si(x) only for pBN. The highest wetting angles were found for pBN substrates with angles around 170deg. For the surface tension of Ge, the most reliable values resulted in gamma(T) = (591- 0.077 (T-T(sub m)) 10(exp -3)N/m. The temperature dependence of the surface tension showed similar values for Ge(1-x)Si(x), around -0.08 x 10(exp -3)N/m K, and a compositional dependence of 2.2 x 10(exp -3)N/m at%Si.

Croell, A.↗
NASA NTRS2001

Ultrahigh Temperature Ceramics for Thermal Protection of Next Generation Space Vehicles

Materials with improved properties are needed for thermal protection of next generation space vehicles. Sharp leading edges on these vehicles will have to withstand exposure to high temperatures (> 2200 C or 4000 F) and severe thermal cycling in both neutral and oxidizing environments. These extreme conditions will require materials that possess superior oxidation resistance, low creep, and excellent thermal shock properties. This presentation will first discuss the system requirements for thermal protection of advanced space vehicles and then show how they are driving development of new materials systems. The presentation will focus on ultrahigh temperature ceramics (UHTCs) that are promising candidates for such applications. ZrB2 and HfB2 and composites of those ceramics with SiC are two particular families of UHTCs that are currently under development for sharp leading edges. These ceramics are appealing because their melting temperatures are 3245 C (5873 F) for ZrB2 and 3380 C (6116 F) for HfB2 and because they may form protective, oxidation resistant coatings in use. The mechanical properties of the UHTCs are strongly dependent on phase purity and the processing route used to make them, both of which factors are being actively investigated. For example, oxide impurities could form glassy grain boundary phases that soften at high temperatures and make the ceramic susceptible to creep deformation. Results from scanning and transmission electron microscopic studies of the UHTCs have shown how their processing can be improved to give better properties. This presentation will discuss the UHTC characterization results in some detail, focusing particularly on the structure and composition of the ceramic grain boundaries. The presentation will conclude with some remarks on how the properties of these promising UHTCs can be further improved and how they might be made more economically.

Loehman, R. E.↗
NASA NTRS1995

Determination of Quantum Chemistry Based Force Fields for Molecular Dynamics Simulations of Aromatic Polymers

Ab initio quantum chemistry calculations for model molecules can be used to parameterize force fields for molecular dynamics simulations of polymers. Emphasis in our research group is on using quantum chemistry-based force fields for molecular dynamics simulations of organic polymers in the melt and glassy states, but the methodology is applicable to simulations of small molecules, multicomponent systems and solutions. Special attention is paid to deriving reliable descriptions of the non-bonded and electrostatic interactions. Several procedures have been developed for deriving and calibrating these parameters. Our force fields for aromatic polyimide simulations will be described. In this application, the intermolecular interactions are the critical factor in determining many properties of the polymer (including its color).

Jaffe, Richard↗
NASA NTRS2002

Wetting Angles and Surface Tension of Ge(1-x)Si(x) Melts on Different Substrate Materials

The wetting angles and the surface tension of Ge(1-x)Si(x) melts (0.02 less than x less than 0.13) have been measured on various substrate materials using the sessile drop technique. Fused quartz, sapphire, SiC, glassy carbon, pBN, AIN, and Si3N4 have been used as substrates. The highest and most stable wetting angles were found for pBN substrates with 164 +/- 8 deg., either under forming gas with an additional carbon getter in the system or under active vacuum. The surface tension measurements resulted in a value of +2.2 x 10(exp -3) N/m.at%Si for the concentration dependence delta(gamma)/(delta)C. For the composition range measured, the temperature dependence (delt)gamma/(delta)T showed values similar to those of pure Ge, on average -0.07 x 10(exp -3) N/mK.

Croell, A.↗
NASA NTRS1998

The Generation of Lighting in the Solar Nebula

The process that melted and formed the chondrules, mm-sized glassy beads within meteorites, has not been conclusively identified. Origin by lightning in the solar nebula is consistent with many features of chondrules, but no viable model of lightning has yet been advanced. We present a model demonstrating how lightning could be generated in the solar nebula which differs from previous models in three important aspects. First, we identify a new', powerful charging mechanism that is based on the differences in contact potentials between particles of different composition, a form of triboelectric charging. In the presence of fine silicate grains and fine iron metal grains, large silicate particles (the chondrules) can acquire charges of +10(exp 5) e. Second, we assume that the chondrule precursor particles are selectively concentrated in clumps 1 - 100 km in size by the turbulent concentration mechanism described by Cuzzi et al. (1996). The concentration of these highly charged particles into clumps, in a background of negatively charged metal grains, is what generates the strong electric fields. Third, we make refinements in the estimates of the breakdown electric field and the ionization rate. We calculate that electric fields large enough to trigger breakdown easily could have existed over regions large enough (approx. 100km) to generate very large discharges of electrical energy (approx. 10(exp 16)erg). The discharges would have been sufficiently energetic and frequent to have formed the chondrules. We place constraints on the generation of lightning and conclude that it could not be generated if the abundance of Al-26 in chondrules was as high as the level in the CAls. This conclusion is consistent with isotopic analyses of chondrules. This possibly implies that Al-26 was non-uniformly distributed in the solar nebula or that the chondrules formed several Myr after the CAIs.

Cuzzi, Jeffrey↗
NASA NTRS2002

A Thermodynamic Theory Of Solid Viscoelasticity. Part 1: Linear Viscoelasticity.

The present series of three consecutive papers develops a general theory for linear and finite solid viscoelasticity. Because the most important object for nonlinear studies are rubber-like materials, the general approach is specified in a form convenient for solving problems important for many industries that involve rubber-like materials. General linear and nonlinear theories for non-isothermal deformations of viscoelastic solids are developed based on the quasi-linear approach of non-equilibrium thermodynamics. In this, the first paper of the series, we analyze non-isothermal linear viscoelasticity, which is applicable in a range of small strains not only to all synthetic polymers and bio-polymers but also to some non-polymeric materials. Although the linear case seems to be well developed, there still are some reasons to implement a thermodynamic derivation of constitutive equations for solid-like, non-isothermal, linear viscoelasticity. The most important is the thermodynamic modeling of thermo-rheological complexity , i.e. different temperature dependences of relaxation parameters in various parts of relaxation spectrum. A special structure of interaction matrices is established for different physical mechanisms contributed to the normal relaxation modes. This structure seems to be in accord with observations, and creates a simple mathematical framework for both continuum and molecular theories of the thermo-rheological complex relaxation phenomena. Finally, a unified approach is briefly discussed that, in principle, allows combining both the long time (discrete) and short time (continuous) descriptions of relaxation behaviors for polymers in the rubbery and glassy regions.

Freed, Alan D.↗