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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

The study of 'microsurfaces' using thermal desorption spectroscopy

The use of a newly combined ultrahigh vacuum technique for studying continuous and particulate evaporated thin films using thermal desorption spectroscopy (TDS), transmission electron microscopy (TEM), and transmission electron diffraction (TED) is discussed. It is shown that (1) CO thermal desorption energies of epitaxially deposited (111) Ni and (111) Pd surfaces agree perfectly with previously published data on bulk (111) single crystal, (2) contamination and surface structural differences can be detected using TDS as a surface probe and TEM as a complementary technique, and (3) CO desorption signals from deposited metal coverages of one-thousandth of a monolayer should be detectable. These results indicate that the chemisorption properties of supported 'microsurfaces' of metals can now be investigated with very high sensitivity. The combined use of TDS and TEM-TED experimental methods is a very powerful technique for fundamental studies in basic thin film physics and in catalysis.

Thomas, M. E.↗
NASA NTRS1979

Phase 2 of the array automated assembly task for the low cost solar array project

The process sequence for the fabrication of dendritic web silicon into solar panels was modified to include aluminum back surface field formation. Plasma etching was found to be a feasible technique for pre-diffusion cleaning of the web. Several contacting systems were studied. The total plated Pd-Ni system was not compatible with the process sequence; however, the evaporated TiPd-electroplated Cu system was shown stable under life testing. Ultrasonic bonding parameters were determined for various interconnect and contact metals but the yield of the process was not sufficiently high to use for module fabrication at this time. Over 400 solar cells were fabricated according to the modified sequence. No sub-process incompatibility was seen. These cells were used to fabricate four demonstration modules. A cost analysis of the modified process sequence resulted in a selling price of $0.75/peak watt.

Campbell, R. B.↗
NASA NTRS1980

Optimization of solar cells for air mass zero operation and study of solar cells at high temperatures, phase 4

The Pd contact to GaAs was studied using backscattering, Auger analysis, and sheet resistance measurements. Several metallurgical phases were present at low temperatures, but PdGa was the dominant phase in samples annealed at 500 C. Ti/Pd/Ag contacts appeared to have the lowest contact resistance. Etchback epitaxy (EBE) was compared to saturated melt epitaxy (SME) method of growing liquid phase epitaxial layers. The SME method resulted in a lower density of Ga microdroplets in the grown layer, although the best solar cells were made by the EBE method. Photoluminescence was developed as a tool for contactless analysis of GaAs cells. Efficiencies of over 8 percent were measured at 250 C.

Hovel, H. J.↗
NASA NTRS1979

New isotopic clues to solar system formation

The presence of two new extinct nuclides Al-26 and Pd-107 with half lives of approximately one million years in the early solar system implies that there were nucleosynthetic activities involving a great many elements almost at the instant of solar system formation. Rate gas and oxygen isotopic abundance variations ('anomalies') relative to the 'cosmic' composition were observed in a variety of planetary objects, which indicates that isotopic heterogeneities caused by the incomplete mixing of distinct nucleosynthesis components permeate the entire solar system. These new results have major implications for cosmochronology, nucleosynthesis theory, star formation, planetary heating, and the genetic relationship between different planetary bodies

Lee, T.↗
NASA NTRS1979

Breccias 73215 and 73255 - Siderophile and volatile trace elements

Fifteen siderophile and volatile trace elements (Os, Re, Ir, Pd, Ni, Au, Sb, Ge, Se, Ag, In, Zn, Cd, Bi, Tl) and U were determined by radiochemical neutron activation analysis in a spheroidal aphanitic clast and a clast of coarse-grained anorthositic gabbro from breccia 73215 and in three types of aphanite and two clasts of fine-grained anorthositic gabbro from breccia 73255. In common with most Apollo 17 fragment-laden melt rocks, the aphanites from 73215 and 73255 predominantly contain a Group 2 meteoritic component, which is apparently derived from the Serenitatis impact. All aphanitic lithologies contain the same meteoritic component, and are probably cogenetic. The clasts of fine-grained anorthositic gabbro contain substantial amounts (2% to 6% Cl equivalent based on Au) of a pre-Serenitatis Group 3 component. The clast of coarse-grained anorthositic gabbro is low in siderophile elements (0.4% Cl equivalent), and the meteoritic component (Group 5) is not well-defined. A strong correlation exists between Ir and Au in both the aphanites and the anorthositic gabbro clasts, which argues against the breccias 73215 and 73255 being open systems for Au

Morgan, J. W.↗
NASA NTRS1979

The asteroids - Accretion, differentiation, fragmentation, and irradiation

Various types of meteorites have experienced processes of condensation, accretion, metamorphism, differentiation, brecciation, irradiation and fragmentation. A typical view of meteorite formation has been that the processes following accretion take place in a few asteroidal-sized (approximately 100 km) objects. Discovery of decay products of now extinct Al-26 and Pd-107 in meteorites, discovery of isotopic heterogeneity among meteorite types, re-analysis of meteorite cooling rates, and continuing study of meteoritic compositions have led some meteoriticists to conclude that meteorites obtained their chemical, isotopic, and some textural characteristics in objects initially less than 10 km in diameter. Such a scenario, which is described in this paper, raises the possibility that some of these small planetesimals may have been 'condensation nuclei' for the formation of comets as well as the precursors of asteroids.

Wilkening, L. L.↗
NASA NTRS1980

Chemical fractionations in meteorites. XI - C2 chondrites

Measurements of the compositions of 20 trace elements in the representative C2 chondrites Boriskino, Cold Bokkeveld, Erakot, Essebi, Haripura, Santa Cruz and Al Rais are reported. The contents of Ag, Au, Bi, Cd, Cs, Ge, In, Ir, Ni, Os, Pd, Rb, Re, Sb, Se, Sn, Te, Tl, U, and Zn were determined by radiochemical neutron activation analysis. The siderophile abundances of the C2 chondrites are found to be less uniform than those of other carbonaceous chondrites, while the C2R chondrite Al Rais is systematically lower in 12 volatiles than the C2M chondrites. Enrichment of Bi and Tl found in Erakot and Haripura indicate the possible presence of the late condensate mysterite. Volatile abundances are shown to agree with matrix contents for meteorites that have suffered little aqueous alteration, however to be 20-30% lower for the more altered meteorites. Finally, the decline of element abundance with volatility is shown to be consistent with the sigmoid curve explained by the two-component model.

Wolf, R.↗
NASA NTRS1980

Meteoritic material at four Canadian impact craters

Eleven impact melt and six basement rock samples from four craters were analyzed by neutron activation for Au, Co, Cr, Fe, Ge, Ir, Ni, Os, Pd, Re and Se. Wanapitei Lake, Ontario: the impact melts show uniform enrichments corresponding to 1-2% C1-chondrite material. Interelement ratios (Co/Cr, Ni/Cr, Ni/Ir) suggest that the impacting body was a C1-, C2-, or LL-chondrite. Nicholson Lake, North West Territory: Ni, Cr and Co are distinctly more enriched than Ir and Au which tentatively suggests an olivine-rich achondrite (nakhlite or ureilite). Gow Lake, Saskatchewan and Mistastin, Labrador: small enrichments in Ir and Ni; both the low Ir/Ni ratios and low Cr content suggest iron meteorites, but the signals are too weak for conclusive identification. A tentative comparison of meteoritic signatures at 10 large, greater than or equal to 4 km craters and their presumed celestial counterparts (13 Apollo and Amor asteroids) shows more irons and achondrites among known projectile types, and a preponderance of S-type objects, having no known meteoritic equivalent, among asteroids. It is not yet clear that these differences are significant, in view of the tentative nature of the crater identifications and the limited statistics.

Wolf, R.↗
NASA NTRS1980

Development of low cost contacts to silicon solar cells

A copper based contact system using plated Pd-Cr-Cu was developed. Good cells were made but cells degraded under low temperature (300 C) heat treatments. The degradation was identified as copper migration into the cells junction region. A paper study was conducted to find a proper barrier to the copper migration problem. Nickel was identified as the best candidate barrier and this was verified in a heat treatment study using evaporated metal layers. An electroless nickel solution was substituted for the electroless chromium solution in the original process.

Tanner, D. P.↗
NASA NTRS1980

The origin of the solar system

A review is presented of the recent evidence that a supernova outburst may have played a decisive role in the origin of the solar system. This evidence has been largely based on the precision isotopic abundance measurements made in meteoritic material. These measurements are reviewed; emphasis is placed on the observed short lived isotopes Al-26 and Pd-107.

Schramm, D. N.↗
NASA NTRS1981

Performance of room temperature mercuric iodide /HgI2/ detectors in the ultralow-energy X-ray region

Experiments have been done to study the performance of mercuric iodide (HgI2) detectors in the ultralow-energy X-ray region. Energy resolution values of 245 eV (FWHM) for the Mg K-alpha X-ray line at 1.25 keV and 225 eV (FWHM) for the electronic noise linewidth have been obtained for an HgI2 detector with painted carbon contacts using a pulsed-light feedback preamplifier; the whole system was operated at room temperature. The resolution values in the ultralow-energy region are still limited by electronic noise of the system. In an attempt to minimize X-ray attenuation in the front contact, detectors were prepared with thin evaporated Pd contacts. These detectors show a pronounced low-energy tailing of the photopeak below a few keV, in contrast to the spectra obtained by detectors with carbon contact. An attempt has been made to explain the tailing effect starting with models wich have been proposed to describe similar effects in Ge detectors.

Dabrowski, A. J.↗
NASA NTRS1981

Calculation of the critical cooling rate for amorphous Pd/77.5/Si/16.5/Cu/6/

The viscosity of the molten alloy Pd(77.5)Si(16.5)Cu(6) has been determined from 744 to 1000 C using an oscillating cup viscometer. Using these data (together with other pertinent physical data available for this alloy) in existing crystallization theory indicates that it could be made amorphous at cooling rates as low as 5-10 K/sec. This analysis assumes strictly homogeneous nucleation. Containerless solidification will be one good testing ground for the results of this analysis.

Steinberg, J.↗
NASA NTRS1981

Composition of the earth's upper mantle. I - Siderophile trace elements in ultramafic nodules

The considered investigation is concerned with a reexamination of the question of the distribution of siderophile elements in the earth's upper mantle, taking into account a more unified data base which is now available. A comprehensive suite of ultramafic inclusions was collected as part of the Basaltic Volcanism Study Project and has been analyzed by instrument neutron activation analysis for major, minor, and some lithophile trace elements. In addition, 18 of these rocks and the important sheared garnet lherzolite PHN 1611 have been analyzed by means of radiochemical neutron activation analysis for 7 siderophile elements (Au, Ge, Ir, Ni, Os, Pd, and Re) and 9 volatile elements (Ag, Bi, Cd, In, Sb, Se, Te, Tl, and Zn). The siderophile element data reveal interesting inter-element correlations, which were not apparent from the compiled abundance tables of Ringwood and Kesson (1976) and Chou (1978).

Morgan, J. W.↗
NASA NTRS1981

Effect of particle size on the chemisorption and decomposition of carbon monoxide by palladium and nickel clusters

The chemisorption of gases on well-defined, supported metal particles is a model for basic processes in heterogeneous catalysis. In this study, the chemisorption and decomposition of carbon monoxide on palladium and nickel particles was examined as a function of particle size. Particulate films with average particle sizes ranging from 1 to 10 nm were grown by vapor deposition on UHV-cleaved mica. Successive CO adsorption-desorption cycles resulted in the accumulation of carbon on the particles, which suppressed CO adsorption. The rate of carbon accumulation was strongly dependent on particle size and was higher for Ni than for Pd over the same size range. Carbon was removed from both metals by oxygen treatments at elevated temperatures. However, a mixture of CO and O2 was effective for monitoring the removal of carbon from palladium.

Doering, D. L.↗
NASA NTRS1982

Silicon dendritic web material

The development of a low cost and reliable contact system for solar cells and the fabrication of several solar cell modules using ultrasonic bonding for the interconnection of cells and ethylene vinyl acetate as the potting material for module encapsulation are examined. The cells in the modules were made from dendritic web silicon. To reduce cost, the electroplated layer of silver was replaced with an electroplated layer of copper. The modules that were fabricated used the evaporated Ti, Pd, Ag and electroplated Cu (TiPdAg/Cu) system. Adherence of Ni to Si is improved if a nickel silicide can be formed by heat treatment. The effectiveness of Ni as a diffusion barrier to Cu and the ease with which nickel silicide is formed is discussed. The fabrication of three modules using dendritic web silicon and employing ultrasonic bonding for interconnecting calls and ethylene vinyl acetate as the potting material is examined.

Meier, D. L.↗
NASA NTRS1982

Multi-100 kW: Planar low cost solar array development

The development of a 100 kW planar low cost solar array is presented. Tasks are defined, objectives are stated, and test results are given. A study of alternate contact configuration concluded that gridded back contact cells sharply reduced solar absorptance, that copper contacts will require a developmental program before interface can be achieved, and that gridded back contact cells using Ti-Pd-Ag as the contact materials appear to be the most cost-effective design.

Source record↗
NASA NTRS1981

Thermal stability of GaAs heteroface solar cells with differing contact materials

GaAlAs/GaAs heteroface solar cells used in space offer advantages of higher operating temperatures and recovery from radiation damage using thermal annealing. Elevated temperature experiments were conducted to evaluate the electrical stability of cells with different contact materials. These experiments indicate that for operation of GaAs heteroface solar cells at elevated temperatures, front contact metals must be carefully chosen. The short circuit current varied by only about 3% for cells with Pd/Ag contacts that were heated to 240 C for a total of 500 hours. However, a total decrease in the open circuit voltage of about 20% was observed for these cells. After heating cells to 400 C, large changes in open circuit voltage were observed for cells with Pd/Ag, Pd/Au, Pd, Ag, and Ti contacts; however, preliminary results indicate more stable open circuit voltages for cells with Au, Cr, Zn, and Cr/Au contacts.

Friesen, W. J.↗