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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Advances in silicon carbide Chemical Vapor Deposition (CVD) for semiconductor device fabrication

Improved SiC chemical vapor deposition films of both 3C and 6H polytypes were grown on vicinal (0001) 6H-SiC wafers cut from single-crystal boules. These films were produced from silane and propane in hydrogen at one atmosphere at a temperature of 1725 K. Among the more important factors which affected the structure and morphology of the grown films were the tilt angle of the substrate, the polarity of the growth surface, and the pregrowth surface treatment of the substrate. With proper pregrowth surface treatment, 6H films were grown on 6H substrates with tilt angles as small as 0.1 degrees. In addition, 3C could be induced to grow within selected regions on a 6H substrate. The polarity of the substrate was a large factor in the incorporation of dopants during epitaxial growth. A new growth model is discussed which explains the control of SiC polytype in epitaxial growth on vicinal (0001) SiC substrates.

Powell, J. Anthony↗
NASA NTRS1991

Fabrication and performance analysis of 4-sq cm indium tin oxide/InP photovoltaic solar cells

Large-area photovoltaic solar cells based on direct current magnetron sputter deposition of indium tin oxide (ITO) into single-crystal p-InP substrates demonstrated both the radiation hardness and high performance necessary for extraterrestrial applications. A small-scale production project was initiated in which approximately 50 ITO/InP cells are being produced. The procedures used in this small-scale production of 4-sq cm ITO/InP cells are presented and discussed. The discussion includes analyses of performance range of all available production cells, and device performance data of the best cells thus far produced. Additionally, processing experience gained from the production of these cells is discussed, indicating other issues that may be encountered when large-scale productions are begun.

Gessert, T. A.↗
NASA NTRS1990

1300 K compressive properties of a reaction milled NiAl-AlN composites

When B2 crystal-structure nickel aluminide is subjected to high-intensity mechanical ball milling in a liquid nitrogen bath, or 'cryomilling', an NiAl composite is obtained which contains about 10 vol pct AlN particles. This composition arises from the incorporation of N during cryomilling; during subsequent thermomechanical processing, the N reacts with Al. While compressive testing of extruded or isostatically pressed specimens at 1300 K indicated that strength at relatively fast strain rates is slightly dependent on consolidation method, slower strain rates indicate no clear dependency on densification technique: four different consolidation methods were found to yield similar creep strengths. The creep properties of NiAl-AlN are similar to those of the single-crystal Ni-base superalloy NASAIR 100.

Whittenberger, J. Daniel↗
NASA NTRS1991

A model for the high-temperature transport properties of heavily doped n-type silicon-germanium alloys

A model is presented for the high-temperature transport properties of large-grain-size, heavily doped n-type silicon-germanium alloys. Electron and phonon transport coefficients are calculated using standard Boltzmann equation expressions in the relaxation time approximation. Good agreement with experiment is found by considering acoustic phonon and ionized impurity scattering for electrons, and phonon-phonon, point defect, and electron-phonon scattering for phonons. The parameters describing electron transport in heavily doped and lightly doped materials are significantly different and suggest that most carriers in heavily doped materials are in a band formed largely from impurity states. The maximum dimensionless thermoelectric figure of merit for single-crystal, n-type Si(0.8)Ge(0.2) at 1300 K is estimated at ZT about 1.13 with an optimum carrier concentration of n about 2.9 x 10 to the 20th/cu cm.

Vining, Cronin B.↗
NASA NTRS1991

Reaction of Ti and Ti-Al alloys with alumina

The reaction of single-crystal Al2O3 with pure Ti and Ti-Al alloys with different Al concentrations was examined in the temperature range of 1173 to 1573 K. Significant reaction occurred between Al2O3 and the Ti-Al alloys with Al concentrations lower than that corresponding to the gamma-TiAl phase. The reaction mechanism was determined to be simultaneous diffusion of Al and atomic oxygen from Al2O3 into Ti and the Ti-Al alloys.

Misra, Ajay K.↗
NASA NTRS1991

Temperature dependence of hardness in yttria-stabilized zirconia single crystals

The temperature dependence of hardness and microcracking in single-crystal 9.5-mol pct-Y2O3-fully-stabilized cubic-ZrO2 was studied as a function of orientation. Crack lengths increased with increased temperature up to 500 C; above 800 C, no cracks were found, indicating an indentation brittle-to-ductile transition of about 800 C. The temperature dependence of hardness was reduced around 500 C. Etching studies to delineate the plastic zone around and below indents identified the operative slip systems. The role of dislocations and their interactions within the plastic zone on the hardness and indentation fracture behavior of cubic-ZrO2 are discussed.

Morscher, Gregory N.↗
NASA NTRS1991

Stability of Y-Ba-Cu-oxide phases in salt flux

The results of an investigation into the possibility of growing YBa2Cu3O7 (or Y123) single crystals from a NaCl-KCl flux (salt flux) are presented. They are compared with results for Y123 crystals grown from a Ba-Cu-oxide flux system. The chosen salt flux composition was 1:1 (NaCl:KCl). The preparation method of the solutes is presented, with particular attention given to the compounds surrounding the Y123 phase, such as Y2BaCuO6 (Y211). Optical microscopy was introduced to study the morphology of the sample; X-ray diffraction was used for phase identification; and a chemical analysis was effected. The Y123, Y211, and BaCuO2 phases were shown to be dissociated in the NaCl-KCl flux, and could not be recrystallized. A small amount of the flux could be added to create large-grain Y123 crystals, but single-crystal growth was not possible in a top-seeded NaCl-KCl solution. This is attributed to significant differences in solubility of the crystal elements, and phase separation caused by CuO crystal formation.

Tao, Y. K.↗
NASA NTRS1990

Experimental approaches for improving SiGe thermoelectric efficiency at JPL

Efforts to improve the thermoelectric efficiency of silicon-germanium alloys are discussed. These efforts are essentially focused on substantially enhancing carrier concentration levels and thus increasing the power factor values of n-type materials. With the help of thermodynamic phase diagram calculations, a combination of various crystal growth and dopant saturation techniques is used for this purpose. Bulk polycrystal growth of double-doped materials is achieved by zone leveling while a new method (the traveling heater method) for high-quality single-crystal growth of SiGe is being developed. Thin films of homogeneous single-crystalline multidoped Si80Ge20 have also been successfully grown from low-temperature melts to investigate combined dopant solid solubility variations. Very high carrier concentration levels were obtained by (P+As)-combined diffusion in zone-leveled and hot-pressed materials. The good overall understanding of doping mechanisms which has been achieved should enable optimization of the thermoelectric properties over this extended carrier concentration range. Reductions in thermal conductivity are being sought by introduction of very small phonon scattering centers (inert material) in the initial powders-precursors of Si-Ge alloys. This approach should benefit both n- and p-type materials.

Borshchevsky, Alex↗
NASA NTRS1990

Peeled film GaAs solar cell development

Thin-film, single-crystal gallium arsenide (GaAs) solar cells could exhibit a specific power approaching 700 W/kg including coverglass. A simple process has been described whereby epitaxial GaAs layers are peeled from a reusable substrate. This process takes advantage of the extreme selectivity of the etching rate of aluminum arsenide (AlAs) over GaAs in dilute hydrofluoric acid. The feasibility of using the peeled film technique to fabricate high-efficiency, low-mass GaAs solar cells is presently demonstrated. A peeled film GaAs solar cell was successfully produced. The device, although fractured and missing the aluminum gallium arsenide window and antireflective coating, had a Voc of 874 mV and a fill factor of 68 percent under AM0 illumination.

Wilt, D. M.↗
NASA NTRS1990

Buried homojunction solar cells formed in p-InP during sputter deposition and hydrogen plasma processing

Although it is apparent that direct current (DC) magnetron sputter deposition of indium tin oxide (ITO) leads to the formation of a buried homojunction in single-crystal p-type InP, the actual mechanism of type conversion of the InP surface is not clear. Since sputter deposition of In2O3 has also led to high-quality cells, it appears that Sn is not the cause of type conversion. To investigate the junction formation process further, experiments involving the use of a pure hydrogen plasma have been performed, resulting in type conversion of Fe-doped InP surfaces and in solar cells with efficiencies over 16 percent (global). The results confirm that sputter deposition is not necessary to form this type of buried homojunction solar cell, and they suggest a fabrication process that may be used with other relevant photovoltaic materials.

Gessert, T. A.↗
NASA NTRS1990

23.5 percent thin-film space concentrator cells

Thin-film AlGaAs-GaAs double-heterostructure concentrator cells were fabricated which exhibit total-area conversion efficiencies as high as 23.5 percent AM0 at 100 suns, 25 C. This is one of the best space concentrators measured to date at NASA and is designed for a thin-film cell without a prismatic coverglass. This solar cell structure consists of a GaAs/AlGaAs film less than 5 micron thick mounted to a glass cover/superstrate, with coplanar back-side contacts. The coverglass is not prismatic. The CLEFT process, a method for mechanically separating epitaxial layers from their substrate, is used to process these cells into thin films. The advantages of single-crystal GaAs are thereby retained, while reducing weight and cutting cost by allowing for substrate reuse. Thin-film cells also have better thermal management capabilities and can be stacked for use in tandem structures. Cell fabrication and performance are described, and directions for further improvements are identified.

Dingle, B. D.↗
NASA NTRS1991

Selective and low temperature synthesis of polycrystalline diamond

Polycrystalline diamond thin films have been deposited on single-crystal silicon substrates at low temperatures (not above 600 C) using a mixture of hydrogen and methane gases by high-pressure microwave plasma-assisted chemical vapor deposition. Low-temperature deposition has been achieved by cooling the substrate holder with nitrogen gas. For deposition at reduced substrate temperature, it has been found that nucleation of diamond will not occur unless the methane/hydrogen ratio is increased significantly from its value at higher substrate temperature. Selective deposition of polycrystalline diamond thin films has been achieved at 600 C. Decrease in the diamond particle size and growth rate and an increase in surface smoothness have been observed with decreasing substrate temperature during the growth of thin films. As-deposited films are identified by Raman spectroscopy, and the morphology is analyzed by scanning electron microscopy.

Ramesham, R.↗
NASA NTRS1991

Controlled growth of 3C-SiC and 6H-SiC films on low-tilt-angle vicinal (0001) 6H-SiC wafers

It has been found that, with proper pregrowth surface treatment, 6H-SiC single-crystal films can be grown by chemical vapor deposition (CVD) at 1450 C on vicinal (0001) 6H-SiC with tilt angles as small as 0.1 deg. Previously, tilt angles of greater than 1.5 deg were required to achieve 6H on 6H at this growth temperature. In addition, 3C-SiC could be induced to grow within selected regions on the 6H substrate. the 3C regions contained few (or zero) double-positioning boundaries and a low density of stacking faults. A new growth model is proposed to explain the control of SiC polytype in this epitaxial film growth process.

Powell, J. A.↗
NASA NTRS1991

Application of Oxidation to the Structural Characterization of Sic Epitaxial Films

Both 3C-SiC and 6H-SiC single-crystal films can be grown on vicinal (0001) 6H-SiC wafers. It is found that oxidation can be a powerful diagnostic process for (1) 'color mapping' the 3C and 6H regions of these films, (2) decorating stacking faults in the films, (3) enhancing the decoration of double positioning boundaries, and (4) decorating polishing damage. Contrary to previously published oxidation results, proper oxidation conditions can yield interference colors that provide a definitive map of the polytype distribution for both the Si face and C face of SiC films.

Powell, J. A.↗
NASA NTRS1991

Fracture of single crystals of the nickel-base superalloy PWA 1480E in hydrogen at 22 C

The present study investigates the hydrogen-induced fracture behavior of notched single crystals of the PWA 1480E nickel-based superalloy. Notched single crystals with seven different crystal orientations were tensile tested at 22 C in a hydrogen atmosphere at 34 MPa. Hydrogen-enhanced cleavage along 100-line planes in the notched region was found to be due to cracking along the gamma/gamma-prime interfaces. Cleavage on 111-line-type planes was observed only outside the notch region. The occurrence of 100- and 111-line-types of cleavage was independent of the single-crystal orientation. Outside the notch region, the fracture surfaces of specimens tested in hydrogen appeared very similar to those tested in helium. Differences in the fracture surfaces were located primarily near the notch region.

Chen, P. S.↗
NASA NTRS1991

Controllable surface-plasmon resonance in engineered nanometer epitaxial silicide particles embedded in silicon

Epitaxial CoSi2 particles in a single-crystal silicon matrix are grown by molecular-beam epitaxy using a technique that allows nanometer control over particle size in three dimensions. These composite layers exhibit resonant absorption predicted by effective-medium theory. Selection of the height and diameter of disklike particles through a choice of growth conditions allows tailoring of the depolarization factor and hence of the surface-plasmon resonance energy. Resonant absorption from 0.49 to 1.04 eV (2.5 to 1.2 micron) is demonstrated and shown to agree well with values predicted by the Garnett (1904, 1906) theory using the bulk dielectric constants for CoSi2 and Si.

Fathauer, R. W.↗
NASA NTRS1990

Dissociation of O(2-)2 defects into paramagnetic O(-) in wide band-gap insulators - A magnetic susceptibility study of magnesium oxide

The magnetic susceptibility of single-crystal MgO has been measured in the temperature range 300-1000 K, using a Faraday balance. The high-purity crystal (less than 100 ppm transition metals), grown from the melt in a H2O-containing atmosphere, was found to be paramagnetic due to the presence of defects on the O(2-) sublattice. The defects derive from OH(-) introduced into the MgO matrix by the dissolution of traces of H2O during crystal growth. The OH(-) converts into O(2-)2 and H2. Each O(2-)2 represents two coupled, spin-paired O(-) states. The observed strongly temperature-dependent paramagnetism can be described by three contributions that overlay the intrinsic diamagnetism of MgO and arise from the low level of transition-metal impurities, O(-) generated by 0(2-)2 dissociation, and O(-) states trapped by quenching from high temperatures from previous experiments.

Batllo, F.↗
NASA NTRS1991

Growth of Sr(0.61)Ba(0.39)Nb2O6 fibers - New results regarding orientation

The paper describes stable growth of Sr(0.61)Ba(0.39)Nb2O6 (SBN) single-crystal optical fibers (grown by the laser-heated pedestal growth method) along the 100-line and 110-line crystallographic axes. The orientation of SBN fibers was investigated using transmission holograms recorded by focusing two separate, but mutually coherent, optical wavefronts into one end of the fiber. Results showed that the crystal quality of 100-line and 110-line SBN fibers grown at a given pull velocity strongly depended on the fiber diameter; generally, the quality improves with decreasing diameter.

Wilde, Jeffrey P.↗