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Results for “single-crystal X-ray study”

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 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

The crystal chemistry of armalcolites from Apollo 17

Single-crystal X-ray studies of armacolites from the 2 to 5 mm fraction of Apollo 17 soils 75082 and 78502 are evaluated. The two types of armacolites studied show slight composition differences and identical structures. Precession photographs of each crystal show the two forms, ortho and para, to have the same space group and nearly the same cell dimensions. The analyses conducted support the proposition that the difference in optical properties is caused by differing MgO and Cr2O3 contents.

Smyth, J. R.↗
NASA NTRS1976

Mineralogy of the Ibitira eucrite and comparison with other eucrites and lunar samples

The mineralogy of the Ibitira meteorite is studied using single-crystal X-ray and energy-dispersive electron-probe techniques. It is shown that this strongly metamorphosed unbrecciated vesicular eucrite has a primary variolitic and secondary hornfelsic texture dominated by equant grains of pale-brown pyroxene (60%) set in an irregular matrix of plagioclase (30%). Minor phases are detected which indicate igneous crystallization, a prolonged metamorphic history, and vapor deposition. The following sequence of events is suggested as the simplest explanation for the mineralogical features observed: (1) crystallization of the meteorite in a lava flow; (2) prolonged annealing, reduction of primary spinel, and vapor deposition; (3) strong shock sufficient to modify the texture of the plagioclase grains; and (4) sufficient annealing to allow crystallization of the plagioclase as a subparallel mosaic and partial conversion to the low structural state. Evidence for this sequence is examined in detail, particularly in relation to the general features of eucrites, howardites, and mesosiderites

Steele, I. M.↗
NASA NTRS1975

Relative cooling rates of mare basalts at the Apollo 12 and 15 sites as estimated from pyroxene exsolution data

X-ray single-crystal diffraction studies, supplemented by electron microprobe analyses of core pigeonites and rim augites from rocks 12031, 15085, 15475, and 15495, have been used to suggest cooling rates for these and other Apollo 12 and 15 rocks studied by similar methods. The extent of subsolidus phase separation of pyroxenes is used as a measure of the cooling rate. The results were interpreted in terms of model cooling histories of a lava flow whose thickness was estimated from the absolute cooling rates obtained by cooling rate experiments and by temperature-time variation through an extrusive flow computed by employing simplified theory of Jaeger. All available data on the exsolution and cation distributions of pyroxenes from Apollo 12 and 15 samples at present are consistent with the hypothesis that these rocks were derived from the top and interior of lava flows with thicknesses of 4-10 m. A process for exsolution of augite from a host pigeonite on (100) has been related to its low-calcium content.

Takeda, H.↗
NASA NTRS1972

Petrology of basaltic and monomineralic soil fragments from the Sea of Fertility.

Basaltic and monomineralic fragments from the 150-425 micron size fractions of the Luna 16 core obtained from the Sea of Fertility were studied by optical petrographic, electron microprobe, and single-crystal X-ray diffraction techniques. Three textural varieties were identified in the basalts (intersertal, subophitic, and recrystallized). These textures, the assemblages, and individual mineral compositions (especially pyroxenes) indicate that these basalts crystallized under conditions very similar to those from Apollo 11. The pyroxenes have slightly lower Ti/Al (atomic) ratios than the Apollo 11 pyroxenes. We interpret the presence of octahedral Al as being due to the low TiO2/Al2O3 (weight %) ratio in the bulk rock. All observations made on the fine-grained basalt fragments are consistent with a rapid, near-surface, one-stage crystallization history.

Bence, A. E.↗
NASA NTRS1982

Surface chemistry and wear behavior of single-crystal silicon carbide sliding against iron at temperatures to 1500 C in vacuum

X-ray photoelectron and Auger electron spectroscopy analyses and morphological studies of wear and metal transfer were conducted with a single-crystal silicon carbide 0001 surface in contact with iron at various temperatures to 1500 C in a vacuum of 10 to the minus 8th power pascal. The results indicate that below 800 C, carbide-carbon and silicon are primarily seen on the silicon carbide surface. Above 800 C the graphite increases rapidly with increase in temperature. The outermost surficial layer, which consists mostly of graphite and little silicon at temperatures above 1200 C is about 2 nm thick. A thicker layer, which consists of a mixture of graphite, carbide, and silicon is approximately 100 nm thick. The closer the surface sliding temperature is to 800 C, the more the metal transfer produced. Above 800 C, there was a transfer of rough, discontinuous, and thin iron debris instead of smooth, continuous and thin iron film which was observed to transfer below 800 C. Two kinds of fracture pits were observed on the silicon carbide surface: (1) a pit with a spherical asperity; and (2) multiangular shaped pits.

Miyoshi, K.↗
NASA NTRS1996

TEM Study of Precipitation in a NiAl-3Ti-0.5Hf Single-Crystal Alloy

Samples for transmission electron microscopy (TEM) were prepared from 3 mm diameter cylinders electro-discharge machined from the heat-treated ingots. Slices sectioned from the cylinders were mechanically ground and electrochemically thinned in a twinjet Tenupol-3 polisher. Microstructural and energy-dispersive X-ray spectroscopy (EDXS) studies were conducted in a Philips 400T TEM equipped with a double tilt goniometer and a KEVEX Si/Li X-ray detector.

Garg, A.↗
NASA NTRS1994

Structure and stability of hydrous minerals at high pressure

The presence of even small amounts of hydrogen in the Earth's deep interior may have profound effects on mantle melting, rheology, and electrical conductivity. The recent discovery of a large class of high-pressure H-bearing silicates further underscores the potentially important role for hydrous minerals in the Earth's mantle. Hydrogen may also be a significant component of the Earth's core, as has been recently documented by studies of iron hydride at high pressure. In this study, we explore the role of H in crystal structures at high pressure through detailed Raman spectroscopic and x ray diffraction studies of hydrous minerals compressed in diamond anvil cells. Brucite, Mg(OH)2, has a simple structure and serves as an analogue for the more complex hydrous silicates. Over the past five years, this material has been studied at high pressure using shock-compression, powder x ray diffraction, infrared spectroscopy, Raman spectroscopy, and neutron diffraction. In addition, we have recently carried out single-crystal synchrotron x-ray diffraction on Mg(OH)2 and Raman spectroscopy on Mg(OD)2 at elevated pressure. From all these studies, an interesting picture of the crystal chemical behavior of this material at high pressure is beginning to emerge. Some of the primary conclusions are as follows: First, hydrogen bonding is enhanced by the application of pressure. Second, layered minerals which are elastically anisotropic at low pressure may not be so at high pressure. Furthermore, the brucite data place constraints on the effect of hydrogen on seismic velocities and density at very high pressure. Third, the stability of hydrous minerals may be enhanced at high P by subtle structural rearrangements that are difficult to detect using traditional probes and require detailed spectroscopic analyses. Finally, brucite appears to be unique in that it undergoes pressure-induced disordering that is confined solely to the H-containing layers of the structure.

Duffy, T. 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 NTRS1986

Crack Growth in Single-Crystal Silicon

Report describes experiments on crack growth in single-crystal silicon at room temperature in air. Crack growth in (111) cleavage plane of wafers, 50 by 100 by 0.76 mm in dimension, cut from Czochralski singlecrystal silicon studied by double-torsion load-relaxation method and by acoustic-emission measurements. Scanning electron microscopy and X-ray topography also employed. Results aid in design and fabrication of silicon photovoltaic and microelectronic devices.

Chen, C. P.↗
NASA NTRS1988

High-pressure phase diagram and equation of state of solid helium from single-crystal X-ray diffraction to 23.3 GPa

Single-crystal X-ray diffraction measurements have been performed on solid He-4 from 15.6 to 23.3 GPa at 300 K with synchrotron radiation. The diffraction patterns demonstrate that the structure of the solid is hexagonal close packed over this pressure-temperature range, contrary to both the interpretation of high-pressure optical studies and to theoretical predictions. The solid is more compressible than is indicated by equations of state calculated with recently determined helium pair potentials. The results suggest that a significant revision of current views of the phase diagram and energetics of dense solid helium is in order.

Mao, H. K.↗
NASA NTRS1981

Silicide surface phases on gold

The crystalline silicide layers formed on (111) and (100) surfaces of Au films on various Si single-crystal substrates are studied by LEED and AES in conjunction with sputter-depth profiling as a function of annealing temperature. On the (111) surface, three basic silicide structures are obtained corresponding to layers of various thicknesses as obtained by different preparation conditions. The (100) surface shows only two different structures. None of the structures is compatible with the various bulk silicide structures deduced from X-ray diffraction. Using LEED as a criterion for the presence or absence of silicide on the surface, smaller layer thicknesses are obtained than reported previously on the basis of AES studies.

Green, A. K.↗
NASA NTRS1978

Measurements of the spectrum and energy dependence of X-ray transition radiation

The results of experiments designed to test the theory of X-ray transition radiation and to verify the predicted dependence of the characteristic features of the radiation on the radiator dimensions are presented. The X-ray frequency spectrum produced by 5- to 9-GeV electrons over the range 4 to 30 keV was measured with a calibrated single-crystal Bragg spectrometer, and at frequencies up to 100 keV with an NaI scintillator. The interference pattern in the spectrum and the hardening of the radiation with increasing foil thickness are clearly observed. The energy dependence of the total transition-radiation intensity was studied using a radiator with large dimensions designed to yield energy-dependent signals at very high particle energies, up to E/mc-squared approximately equal to 100,000. The results are in good agreement with the theoretical predictions.

Cherry, M. L.↗
NASA NTRS2002

Failure Morphologies of Cyclically Oxidized ZrO2-Based Thermal Barrier Coatings

Advanced and baseline thermal barrier coatings (TBCs) were thermal cycle tested in air at 1163 C until delamination or spallation of the ceramic top coat. The top coat of the advanced TBC s consisted of ZrO2 with various amounts of Y2O3, Yb2O3, Gd2O3, or Nd2O3 dopants. The composition of the top coat of the baseline TBC was ZrO2-8wt.%Y2O3. All top coats were deposited by air plasma spraying. A NiCrAlY or NiCoCrAlY bond coat was deposited by low pressure plasma spraying onto a single-crystal, Ni-base superalloy. The TBC lifetime for the baseline coatings was approximately 190 cycles (45 minutes at 1163 C per cycle) while the lifetime for the advanced coatings was as high as 425 cycles. The fracture surfaces and sample cross sections were examined after TBC failure by SEM and optical microscopy, and the top coats were further examined by X-ray diffraction. These post-test studies revealed that the fracture path largely followed splat boundaries with some trans-splat fracture. However, there were no obvious distinguishing features which explained the difference in TBC lifetimes between some of the advanced and baseline coatings.

Nesbitt, James A.↗
NASA NTRS1988

Experimental study of planetary gases with applications to planetary interior models

High-pressure experimental data on planetary materials are critical in developing planetary models and in addressing otherwise insoluble problems of the internal structure of the major planets. Progress in the last five years has been particularly marked. Maximum static pressure of 550 GPa was achieved. For the first time, X-ray diffraction of solidified gases (Ne, Xe) and ices (H2O) were obtained at pressures above one megabar, single-crystal diffraction of ultralight elements (H2, He) were detected up to 25 GPa, pressures over 200 GPa at 77 K were reached in solid hydrogen, including the discovery of a phase transformation in the molecular solid. Advances in instrumentation and new measurements performed during 1983 to 1988 are summarized.

Bell, Peter M.↗
NASA NTRS1973

Petrology of fine-grained rock fragments and petrologic implications of single crystal from the Luna 20 soil.

Only fine-grained rocks are present in the Luna 20 samples, and coarser-grained rocks are represented by fragments of single crystals. A petrologic study has been made of 47 fine-grained crystalline rocks, microbreccias, and glassy aggregates. In addition, a total of 33 single crystals of pyroxene, plagioclase, olivine, and spinel, in the size range from 125 to 500 microns, have been examined using electron microprobe and single-crystal X-ray diffraction techniques. The most abundant fine-grained crystalline rocks in the samples examined are recrystallized anorthositic norite and anorthositic troctolite. Gabbroic rocks, anorthosite, and KREEP basalt are present but not common. Most of the single crystals of pyroxene and plagioclase could have been derived from coarser-grained noritic, troctolitic, and anorthositic rocks. However, three of the 14 pyroxene crystals, and two of the five olivine crystals have Fe/(Fe + Mg) contents greater than 0.45 and are believed to have been derived from mare basalts or related rocks.

Cameron, K. L.↗
NASA NTRS1982

XPS, AES and friction studies of single-crystal silicon carbide

The surface chemistry and friction behavior of a single crystal silicon carbide surface parallel to the 0001 plane in sliding contact with iron at various temperatures to 1500 C in a vacuum of 3 x 10 nPa are investigated using X-ray photoelectron and Auger electron spectroscopy. Results show that graphite and carbide-type carbon are seen primarily on the silicon carbide surface in addition to silicon at temperatures to 800 C by both types of spectroscopy. The coefficients of friction for iron sliding against a silicon carbide surface parallel to the 0001 plane surface are found to be high at temperatures up to 800 C, with the silicon and carbide-type carbon at maximum intensity in the X-ray photoelectron spectroscopy at 800 C. The concentration of the graphite increases rapidly on the surface as the temperature is increased above 800 C, while the concentrations of the carbide-type carbon and silicon decrease rapidly and this presence of graphite is accompanied by a significant decrease in friction. Preheating the surfaces to 1500 C also gives dramatically lower coefficients of friction when reheating in the sliding temperature range of from room temperature to 1200 C, with this reduction in friction due to the graphite layer on the silicon carbide surface.

Miyoshi, K.↗
NASA NTRS1988

Crystal structure and density of helium to 232 kbar

The properties of helium and hydrogen at high pressure are topics of great interest to the understanding of planetary interiors. These materials constitute 95 percent of the entire solar system. A technique was presented for the measurement of X-ray diffraction from single-crystals of low-Z condenses gases in a diamond-anvil cell at high pressure. The first such single-crystal X-ray diffraction measurements on solid hydrogen to 26.5 GPa were presented. The application of this technique to the problem of the crystal structure, equation of state, and phase diagram of solid helium is reported. Crucial for X-ray diffraction studies of these materials is the use of a synchrotron radiation source which provides high brillance, narrow collimation of the incident and diffracted X-ray beams to reduce the background noise, and energy-dispersive diffraction techniques with polychromatic (white) radiation, which provides high detection efficiency.

Mao, H. K.↗