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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Low-cost polycrystalline process for solar cells

Economical batch process includes resolidification step to improve conversion efficiency. Experimental models made by process vary in conversion efficiency from 1.4 to more than 6 percent.

Chu, T. L.↗
NASA NTRS1977

The vibrational spectrum and lattice dynamics of polycrystalline ammonium hydrosulfide

The infrared spectra of alkali metal and ammonium hydrosulfides have been recorded from wavenumbers of 200 to 4000 at liquid nitrogen temperatures, and the Raman spectra of these substances have been recorded from wavenumbers of 0 to 4000 over the temperature range 83-390 K. No evidence of a second solid phase was obtained. Internal and external fundamentals were assigned, in detail, based on selection rules, isotopic frequency shifts, and analogy with structurally similar salts. Barriers to anion and cation reorientation of 3.8 and 1.9 kcal/mole, respectively, have been calculated from librational assignments. The implications of the infrared spectrum of ammonium hydrosulfide for the possible spectroscopic detection of this substance in the atmosphere of the planet Jupiter are discussed.

Bragin, J.↗
NASA NTRS1969

Method for the preparation of inorganic single crystal and polycrystalline electronic materials

Large area, semiconductor crystals selected from group 3-5 compounds and alloys are provided for semiconductor device fabrication by the use of a selective etching operation which completely removes the substrate on which the desired crystal was deposited. The substrate, selected from the same group as the single crystal, has a higher solution rate than the epitaxial single crystal which is essentially unaffected by the etching solution. The preparation of gallium phosphide single crystals using a gallium arsenide substrate and a concentrated nitric acid etching solution is described.

Groves, W. O.↗
NASA NTRS1979

Polycrystalline silicon on tungsten substrates

Thin films of electron-beam-vaporized silicon were deposited on fine-grained tungsten substrates under a pressure of about 1 x 10 to the -10th torr. Mass spectra from a quadrupole residual-gas analyzer were used to determine the partial pressure of 13 residual gases during each processing step. During separate silicon depositions, the atomically clean substrates were maintained at various temperatures between 400 and 780 C, and deposition rates were between 20 and 630 A min. Surface contamination and interdiffusion were monitored by in situ Auger electron spectrometry before and after cleaning, deposition, and annealing. Auger depth profiling, X-ray analysis, and SEM in the topographic and channeling modes were utilized to characterize the samples with respect to silicon-metal interface, interdiffusion, silicide formation, and grain size of silicon. The onset of silicide formation was found to occur at approximately 625 C. Above this temperature tungsten silicides were formed at a rate faster than the silicon deposition. Fine-grain silicon films were obtained at lower temperatures.

Bevolo, A. J.↗
NASA NTRS1979

X-ray measurements of stresses and defects in EFG and large grained polycrystalline silicon ribbons

The Bond method was employed to measure the lattice parameter in an area of 0.4 mm in diameter of EFG Si-ribbons to an accuracy of + or - 0.00008 A. A Bond goniometer was built which included a goniostat with a special specimen holder to mount ribbons 1 m in length and 75 mm in width which could be rotated about two orthogonal axes, and a Leitz microscope for precision alignment of a particular area into the center of the goniostat and the small primary X-ray beam. The (321) planes were found to be parallel to the surface of the ribbons with an angular spread of about 15 deg. The poles of the (111) planes clustered about an angle of 25 deg away from the surface normal, again with a spread of 10 deg. The lattice parameter of a small piece of ribbon material was found to be a sub o = 5.43075 A. A maximum stress of 115 MPa was observed in a fractured ribbon which corresponded to the fracture stress of single crystals of Si.

Wagner, C. N. J.↗
NASA NTRS1982

The Orthogonal In-Situ Machining of Single and Polycrystalline Aluminum and Copper, Volume 1

Metal cutting is a unique deformation process characterized by large strains, exceptionally high strain rates and few constraints to the deformation. These factors, along with the difficulty of directly measuring the shear angle, make chip formation difficult to model and understand. One technique for skirting the difficulty of post mortem chip measurement is to perform a cutting experiment dynamically in a scanning electron microscope. The performance of the in-situ experiment with full instrumentation allows for component force measurement, orientation measurement (on a round single crystal disk) and a timing device, all superimposed below the deformation on the TV monitor and recorded for future viewing. This allows the sher angle to be directly measured for the screen along with the other needed information.

Cohen, P. H.↗
NASA NTRS1982

An EBIC study of HEM polycrystalline silicon

Low-cost silicon for solar cells grown by the heat exchanger method (HEM) was studied in the electron beam induced current (EBIC) mode of a scanning electron microscope (SEM). Comparisons were made between the defects observed optically and the recombination centers visible in EBIC. Much of the HEM material was single crystalline, but structural defects were found from areas near the corners of the grown material. Most of these defects consisted of linear twin boundaries and grain boundaries. The electrical activity of these boundaries was dependent on symmetry of the boundaries. Symmetric twin boundaries did not exhibit recombination activity while unsymmetric twin boundaries were electrically active.

Koch, T.↗
NASA NTRS1983

Measuring Diffusion and Recombination in Polycrystalline Silicon

Light-beam-induced currents yield information about solar cell material. Apparatus measures short-circuit current generated when spot of concentrated light is scanned across grains and grain boundaries in material under test. Technique used to evaluate SOC samples for diffusion and recombination effects of cell processing and chemical and structural defects.

Zook, J. D.↗
NASA NTRS1982

The strength and ductility of polycrystalline NiAl in tension

Experiments at temperatures from 20 C to 400 C at two strain rates (.0001/s and .000005/s) establish that: (1) at room temperatures, binary and microalloyed ( 1000 ppm La, Y, Mo, Ti) NiAl shows negligible ductility, independent of grain size over the range 5 to 140 micrometers; (2) at 295 C the tensile elongation of binary 51 Ni/49 Al increases from 1% to about 5% upon decreasing the grain size to below approximately 10 micrometers; (3) similarly, at 400 C the ductility increases from about 2% to 15% upon decreasing the grain size to below 15 micrometers; (4) the ductility of fine grained (7 micrometer) binary aggregates deformed at 295 C increases from approximately 5% to 12% upon decreasing the strain rate from .0001/s to .000005/s; (5) partial recrystallization (10% to 20%) of warm extruded binary and microalloyed material imparts 1% to 2% ductility at room temperature where fully recrystallized material is brittle; (6) the yield strength obeys a Hall-Petch relationship; and (7) when ductility is not observed, fracture coincides with yielding. The mechanisms underlying the flow and fracture of NiAl are discussed in terms of the nucleation and growth of microcracks. The concept of a critical grain size is considered in the light of the results.

Schulson, E. M.↗
NASA NTRS1983

Micromechanically Based Constitutive Relations for Polycrystalline Solids

A basic method to estimate the overall mechanical response of solids which contain periodically distributed defects is presented. The method estimates the shape and growth pattern of voids periodically distributed over the grain boundaries in a viscous matrix. The relaxed moduli are obtained for a polycrytalline solid that undergoes relaxation by grain boundary sliding which accounts for the interaction effects. The overall inelastic nonlinear response at elevated temperatures in terms of a model which considers nonlinear power law creep within the grains, and linear viscous flow in the grain boundaries is discussed.

Nemat-Nasser, S.↗
NASA NTRS1985

The effect of tantalum on the structure/properties of two polycrystalline nickel-base superalloys: B-1900 + Hf MAR-M247

The microstructure, phase compositions, and phase fractions were studied in conventionally cast B-1900 + Hf and both conventionally cast and directionally solidified MAR-M247 as a function of tantalum concentration. The hot tensile and creep rupture properties of the solutionized and aged MAR-M247-type alloys were also determined as a function of tantalum level. The effects of tantalum on the microstructure and phase compositions of B-1900 + Hf and MAR-M247 (conventionally cast and directionally solidified) were found to be very similar. The addition of tantalum to the as cast and heat treated alloys was shown to cause the partial replacement of the Hf in the MC carbides by Ta, although the degree of replacement was decreased by the solutionizing and aging heat treatment. The gamma prime and minor phase fractions (primarily MC type carbides) both increased approximately linearly with tantalum concentration. The gamma prime phase compositions were relatively insensitive to tantalum variations with the exception of the tantalum and/or hafnium levels. Bulk tantalum additions increased the tantalum, chromium, and cobalt levels of the gamma phase in both alloy series. The increase in the concentrations of the latter two elements in the gamma phase was a result of the decrease in the gamma phase fraction with increasing bulk tantalum concentration and constant gamma/gamma prime partitioning ratio. Tantalum additions increased the yield stress and ultimate tensile strength of the directionally solidified MAR-M247 type alloys and had no significant effect on ductility.

Janowski, G. M.↗
NASA NTRS1984

Advances in large area polycrystalline solar cells

Polycrystaline silicon is presently routinely processed into low cost 10 cm by 10 cm solar cells. However assuming minimal handling difficulties, only minor equipment modifications and no increase in processing complexity, the fabrication of even larger geometries would be economically advantageous. This investigation addressed the feasibility of developing 10 cm by 15 cm solar cells from research through pilot line production stages. The major thrust was to minimize costing by using existing production equipment and proven techniques wherever possible. Accordingly methods were developed to section larger substrates from existing cast ingots, a simple solar cell was designed and low cost processes implemented. After numerous preliminary experiments, pilot line production of 1500 cells was completed. Of these approximately 62 percent exhibited efficiencies greater than 7.0 percent, 53 percent were greater than 8.0 percent and 15 percent were greater than 9.0 percent. Based on this study, the regular production of 10 cm by 15 cm solar cells was determined to be both feasible and cost effective using existing processing methods.

Vendura, G. J., Jr.↗