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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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NASA NTRS2016

Experimental Insights into the Origin of Defect-Structured Hibonites Found in Meteorites

Hibonite (CaAl12O19) is a primary, highly refractory phase occurring in many Ca-Al-rich inclusions (CAIs). Previous microstructural studies of hibonite in CAIs and their Wark-Lovering (WL) rims showed the presence of numerous stacking defects in hibonites. These defects are interpreted as the modification of the stacking sequences of spinel and Ca-containing blocks within the ideal hexagonal hibonite structure due to the presence of wider spinel blocks [3], as shown by experimental studies of reaction-sintered compounds in the CaO-Al2O3 system. We performed a series of experiments in the CaO-Al2O3-MgO system in order to provide additional in-sights into the formation processes and conditions of defect-structured hibonites found in meteorites.

Han. J.↗
NASA NTRS1996

Defect Structure of Beta NiAl Using the BFS Method for Alloys

The semiempirical BFS method for alloys is generalized by replacing experimental input with first-principles results thus allowing for the study of complex systems. In order to examine trends and behavior of a system in the vicinity of a given point of the phase diagram a search procedure based on a sampling of selected configurations is employed. This new approach is applied to the study of the beta phase of the Ni-Al system, which exists over a range of composition from 45-60 at.% Ni. This methodology results in a straightforward and economical way of reproducing and understanding the basic features of this system. At the stoichiometric composition, NiAl should exist in a perfectly ordered B2 structure. Ni-rich alloys are characterized by antisite point defects (with Ni atoms in the Al sites) with a decrease in lattice parameters. On the Al-rich side of stoichiometry there is a steep decrease in lattice parameter and density with increasing Al content. The presence of vacancies in Ni sites would explain such behavior. Recent X-ray diffraction experiments suggest a richer structure: the evidence, while strongly favoring the presence of vacancies in Ni sites, also suggests the possibility of some vacancies in Al sites in a 3:1 ratio. Moreover, local ordering of vacant sites may be preferred over a random distribution of individual point defects.

Bozzolo, Guillermo↗
NASA NTRS1981

Lattice defects in semiconducting Hg/1-x/Cd/x/Te alloys. III - Defect structure of undoped Hg0.6Cd0.4Te

Hall effect measurements were carried out on undoped Hg0.6Cd0.4Te crystals quenched to room temperature subsequent to equilibration at temperatures varying from 450 to 720 C under various partial pressures of Hg. The variation of the hole concentration as a function of the partial pressure of Hg indicates that the native acceptor defects are doubly ionized. Native donor defects are found to be negligible in concentration and the p-type to n-type conversion is shown to be due to residual donors and not due to native donor defects. Thermodynamic constant for the incorporation of the doubly ionized native acceptor defect has been established.

Vydyanath, H. R.↗
NASA NTRS1971

The observation of structural defects in neutron-irradiated lithium-doped silicon solar cells

Electron microscopy has been used to observe the distribution and morphology of lattice defects introduced into lithium-doped silicon solar cells by neutron irradiation. Upon etching the surface of the solar cells after irradiation, crater-like defects are observed that are thought to be associated with the space charge region around vacancy clusters. Thermal annealing experiments showed that the crater defects were stable in the temperature range 300 to 1200 K in all of the lithium-doped samples. Some annealing of the crater defects was observed to occur in the undoped cells which were irradiated at the lowest doses.

Sargent, G. A.↗
NASA NTRS1984

Wuestite (Fe/1-x/O) - A review of its defect structure and physical properties

Such complexities of the Wustite structure as nonstoichiometry, ferric iron variable site distribution, long and short range ordering, and exsolution, yield complex physical properties. Magnesiowustite, a phase which has been suggested to occur in the earth's lower mantle, is also expected to exhibit many of these complexities. Geophysical models including the properties of (Mg, Fe)O should accordingly take into account the uncertainties associated with the synthesis and measurement of iron-rich oxides. Given the variability of the Fe(1-x)O structure, it is important that future researchers define the structural state and extent of exsolution of their samples.

Hazen, R. M.↗
NASA NTRS1985

Structural defects in crystalline silicon

The basic photovoltaic properties of a given crystalline silicon specimen seem to be governed by density and nature of two to three dimensional lattice defects. These are mainly generated by primary growth conditions as grain boundaries of more or less intrinsic character or second phase precipitates from supersaturated solutions of carbon or oxygen. Considerably high values of both solubility and diffusivity in connection with their abundance in common refractory material systems account for the predominance of the two particular elements. Unsaturated dislocations of different types very often can be seen as a consequence of the existence of more dimensional defects as described initially. The final performance of a solar cell is dependent of the concentration and distribution of recombination active centers in the different regions of this device. Typical representatives are fast diffusing transition metals in form of either single atoms or simple complexes. Their avoidance, annihilation, or removal is of great concern in different fields of electronic materials development.

Sirtl, E.↗
NASA NTRS1998

Gaseous Etching for Characterization of Structural Defects in Silicon Carbide Single Crystals

Single crystal SiC substrates were subjected to high-temperature H2/C3H8 gaseous etches. The etches resulted in a variety of surface features on 4H-SiC substrates that included elongated hillocks from 10 to more than 100 microns in length by a few microns in width. In some 4H- and 6H-SiC substrates, the etches resulted in a continuous coverage of macrosteps. We conclude that the morphology observed after the etching process is influenced by the local Si-C bilayer stacking sequence on the surface of off-(0001)oriented substrates. A model is presented for the formation of the hillocks, based on localized transformations of the 4H substrates during the high temperature etch process.

Powell, J. A.↗