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At least 433 records · Page 24

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NASA NTRS1974

Influence of degree of ambient medium rarefaction on deformation of copper and aluminum

The influence of high vacuum 1 x 10 to the minus 9th power torr on the plastic deformation of copper and aluminum was studied. When stretching polycrystalline aluminum and copper specimens in vacuum, it was found that the deformation at failure increases and the ultimate strength decreases. Deformation in vacuum is accompanied by the evolution of hydrogen. For copper, gas evolution is observed only at failure, while for aluminum hydrogen evolution takes place in the initial stage and the gas evolution peaks correspond to small steps on the deformation curve.

Lyubarskiy, I. M.↗

NASA NTRS1974

Auger electron spectroscopy and depth profile study of oxidation of modified 440C steel

Auger electron spectroscopy (AES) and sputtering were used to study selective oxidation of modified 440C steel. The sample was polycrystalline. Oxidation was performed on initially clean surfaces for pressures ranging from 1 x 10 to the minus 7th power to 1 x 10 to the minus 5th power torr and temperatures ranging from room temperature to 800 C. AES traces were taken during oxidation. In situ sputtering depth profiles are also obtained. A transition temperature is observed in the range 600 to 700 C for which the composition of the outer surface oxide changed from iron oxide to chromium oxide. Heating in vacuum about 5 x 10 to the minus 10 power torr to 700 C causes conversion of the iron oxide surface to chromium oxide.

Ferrante, J.↗

NASA NTRS1973

Electromotive force measurements on cells involving beta-alumina solid electrolyte

Open-circuit emf measurements have been made to demonstrate that a two-phase, polycrystalline mixture of beta-alumina and alpha-alumina could be used as a solid electrolyte in galvanic cells with reversible electrodes fixing oxygen or aluminum chemical potentials. These measurements indicate that such a two-phase solid electrolyte may be used to monitor oxygen chemical potentials as low as that corresponding to Al and Al2O3 coexistence (potentials of about 10 to the minus 47th power atm at 1000 K). The activity of Na2O in beta-alumina in coexistence with alpha-alumina was also determined by emf measurements.

Choudhury, N. S.↗

NASA NTRS1973

Solute effects on deformation and fracture of beta brass

It is shown that the ductility of several ternary beta brass alloys in air and in several liquid metals can be related to the operative slip and grain boundary relaxation processes. Nickel and manganese were chosen as alloying elements because they are expected to respectively enhance and suppress cross slip in beta brass. Single-phase binary and ternary beta brass alloys were used in both polycrystalline and single crystal form.

Shea, M. M.↗

NASA NTRS1974

Some interesting aspects of physisorption stay-time measurements obtained using molecular-beam techniques

Stay-time distributions have been obtained for Xe physisorbing on polycrystalline nickel as a function of the target temperature using a pulsed molecular-beam technique. Some interesting effects due to ion bombardment of the surface using He, Ar, and Xe ions are presented. Measured detector signal shapes are found to deviate from those predicted for first-order desorption with velocities corresponding to Maxwellian effusion at the surface temperature. Evidence is found for interaction between beam pulse adsorption and steady-state adsorption of beam species background atoms.

Wilmoth, R. G.↗

NASA NTRS1974

A mass spectrometric system for analyzing thermal desorption spectra of ion-implanted argon and cesium in tungsten

A mass spectrometric system for determining the characteristics of materials used in instrumental development and aerospace applications was developed. The desorption spectra of cesium that was ion-implanted into polycrystalline tungsten and the effects on the spectra of bombardment of the tungsten by low energy (70 eV) electrons were investigated. Work function changes were measured by the retarding potential diode method. Flash desorption characteristics were observed and gas-reaction mechanisms of the surface of heated metal filaments were studied. Desorption spectra were measured by linearly increasing the sample temperature at a selected rate, the temperature cycling being generated from a ramp-driven dc power supply, with the mass spectrometer tuned to a mass number of interest. Results of the study indicate an anomolous desorption mechanism following an electron bombardment of the sample surface. The enhanced spectra are a function of the post-bombardment time and energy and are suggestive of an increased concentration of cesium atoms, up to 10 or more angstroms below the surface.

Wood, G. M., Jr.↗

NASA NTRS1975

Reaction of sulfur dioxide with modified 440C, studied by Auger electron spectroscopy and depth profiling

Auger electron spectroscopy and sputtering were used to study the interaction of SO2 with modified 440C, which is a nominally 77-wt%-Fe, 14-wt%-Cr, and 4-wt%-Mo bearing steel with C, S, Si, Ni, V, P, and Mn making up the balance. The sample was polycrystalline. Three temperatures were used: room temperature, 500 C, and 600 C. The reaction time was varied from 30 minutes to 2 hours. A surface cleaned of oxides was the starting point for each reaction. For reactions at 500 C, the major constituents Cr, O, Fe, and S were present in the surface film. At 600 C, the principal constituents of the film were Cr, O, and S with no Fe present. Therefore, a transition in film composition occurred between 500 and 600 C. Oxides were the primary constituents of the films at both temperatures. Room-temperature reactions indicated that SO2 adsorbed dissociatively, with approximately equal quantities of S and O on the surface. For the same reaction time (1 hr) and pressure, a strong temperature dependence of film thickness was observed. The film formed at 600 C was approximately seven times thicker than that formed at 500 C.

Ferrante, J.↗

NASA NTRS1975

Friction and wear of tin and tin alloys from minus 100 C to 150 C

Sliding friction experiments were conducted with an iron (110) single-crystal pin sliding on single and polycrystalline tin and tin alloys. Specimens were examined at various ambient temperatures from -100 to 150 C. Applied loads varied from 1 to 50 grams, and sliding velocity was constant at 0.7 mm/min. Results indicate that the crystal transformation of tin influences the friction coefficient. Friction was higher for the diamond structure (gray tin) than it was for the body-centered tetragonal structure (white tin). Bismuth arrested the crystal transformation, which resulted in constant friction over the temperature range -100 to 150 C. Both copper and aluminum enhanced the kinetics of transformation, with aluminum producing a nearly twofold change in friction with the crystal transformation.

Buckley, D. H.↗

NASA NTRS1975

EPR study of electron bombarded alkali- and alkaline-earth halide crystal surfaces

An EPR study of electron bombarded LiF, NaCl, KCl, CaF2 and BaF2 polycrystalline surfaces has shown that small metal particles are formed on the surfaces of the crystals. Identification was made from CESR signals. The symmetric line-shape of the signals, even at 77 K, indicated that the particles were less than 0.5 micron in diameter. Signals due to F centers were observed in LiF but not in the other halides. Implications to metal deposition are considered.

Fryburg, G. C.↗

NASA NTRS1974

Knudsen measurements of the sublimation and the heat of formation of GeSe

Knudsen effusion studies of the sublimation of polycrystalline GeSe have been performed employing mass spectrometry. The results demonstrate that GeSe vaporizes congruently under present experimental conditions according to the reaction: GeSe(s) yields GeSe(g). The mean values for the third-law heat and second-law entropy of reaction based on direct mass-loss data are 42.0 + or - 1.5 kcal/mole and 42.3 + or - 1.6 eu respectively. From these data the standard heat of formation was calculated to be -10.1 + or - 2.0 kcal/mole, and the standard absolute entropy was determined to be 16.9 + or - 2.0 eu.

Wiedemeier, H.↗

NASA NTRS1975

Auger spectroscopy of HCl interaction with Al at low pressures

The purpose of the present study was to determine some adsorption characteristics of HCl with partially clean polycrystalline Al by Auger electron spectroscopy. The adsorption was found to exhibit a Freundlich-type behavior pattern in which the number of molecules adsorbed varied with HCl pressure linearly on a log-log plot. Assuming that the adsorption behaves according to the Clausius-Clapeyron equation, the isosteric heat of adsorption increases with coverage at low pressures, appearing to approach a constant value of 4-5 kcal/mole.

Lassiter, W. S.↗

NASA NTRS1975

The preparation and properties of aluminum nitride films

Aluminum nitride films have been deposited on silicon substrates at 800-1200 C by the pyrolysis of an aluminum trichloride-ammonia complex, AlCl3.3NH3, in a gas flow system. The deposit was transparent, tightly adherent to the substrate, and was confirmed to be aluminum nitride by X-ray and electron diffraction techniques. The deposited aluminum nitride films were found to be polycrystalline with the crystallite size increasing with increasing temperature of deposition. Other properties of aluminum nitride films relevant to device applications, including density, refractive index, dissolution rate, dielectric constant, and masking ability, have been determined. These properties indicate that aluminum nitride films have potential as a dielectric in electronic devices.

Chu, T. L.↗

NASA NTRS1975

A 15% efficient antireflection-coated metal-oxide-semiconductor solar cell

A new effect is being developed which significantly improves the conversion efficiency of antireflection-coated metal-oxide-semiconductor (AMOS) solar cells. The effect, a marked increase in the open-circuit voltage, is produced by the addition of an oxide layer to the semiconductor. Cells using gold on n-type gallium arsenide have been made with efficiencies up to 15% in terrestrial sunlight. All processing steps are amenable to the use of low-cost polycrystalline films of GaAs in place of the single crystals now used.

Stirn, R. J.↗

NASA NTRS1975

Electromagnetic levitation facility incorporating electron beam

An electromagnetic levitation apparatus incorporating an electron beam for auxiliary heating and melting has been developed for experiments on containerless vacuum purification and undercooled solidification of high melting materials. Stable levitation of 10-g specimens of molten tungsten has been achieved and a variety of containerless solidification experiments is being performed, including pure polycrystalline castings and single tungsten crystals grown from the undercooled levitated melts.

Wouch, G.↗

NASA NTRS1976

Comparative thermal fatigue resistance

Nineteen cast nickel-base alloys, five cast cobalt-base alloys, and two wrought nickel-base alloys are included in study. Five nickel-base alloys have directionally-solidified polycrystalline grain structure. Three diffusion coatings and vapor-deposited overlay coating are also included in investigation.

Bizon, P. T.↗

NASA NTRS1975

Feasibility and process scale-up low cost alumina fibers for advanced Re-usable Surface Insulation (RSI)

The objective of this program was to establish feasibility of a process to produce low cost aluminum oxide fibers having sufficient strength, flexibility, and thermal stability for multiple re-use at temperatures to 1480 C in advanced RSI type heat shields for reentry vehicles. Using bench-scale processing apparatus, the Alcoa 'Saphiber' process was successfully modified to produce nominally 8 microns diameter polycrystalline alpha-alumina fiber. Thermal stability was demonstrated in vacuum reheating tests to 1371 C and in atmospheric reheating to 1483 C. Individual fiber properties of strength, modulus, and flexibility were not determined because of friability and short length of the fiber. Rigidized tile produced from fiber of nominally 8, 20 and 40 micron diameter had thermal conductivities significantly higher than those of RSI SiO2 or mullite at relatively low temperature but became comparable above about 1000 C. Tile densities were high due to short fiber length, especially in the coarser diameter fiber. No significant effect of fiber diameter on thermal properties could be determined form the data. Mechanical properties of tiles deteriorated as fiber diameter increased.

Pearson, A.↗

NASA NTRS1976

Magnetic field generated resistivity maximum in graphite

In zero magnetic field, B, the electrical resistivity, rho(O,T) of highly oriented pyrolytic (polycrystalline) graphite drops smoothly with decreasing T, becoming constant below 4 K. However, in a fixed applied magnetic field B, the resistivity rho(B,T) goes through a maximum as a function of T, with larger maximum for larger B. The temperature of the maximum increases with B, but saturates to a constant value near 25 K (exact T depends on sample) at high B. In single crystal graphite a maximum in rho(B,T) as a function of T is also present, but has the effects of Landau level quantization superimposed. Several possible explanations for the rho(B,T) maximum are proposed, but a complete explanation awaits detailed calculations involving the energy band structure of graphite, and the particular scattering mechanisms involved.

Wollam, J. A.↗

NASA NTRS1975

Dip coating process: Silicon sheet growth development for the large-area silicon sheet task of the low-cost silicon solar array project

The research program to investigate the technical and economic feasibility of producing solar-cell-quality sheet silicon by dip-coating one surface of carbonized ceramic substrates with a thin layer of large-grain polycrystalline silicon is reported. The initial effort concentrated on the design and construction of the experimental dip-coating facility. The design was completed and its experimental features are discussed. Current status of the program is reported, including progress toward solar cell junction diffusion and miscellaneous ceramic substrate procurement.

Heaps, J. D.↗