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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 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Modelling of photodegradation in solar cell modules of substrate and superstrate design made with ethylene-vinyl acetate as pottant material

A computer model which simulates, in principle, the chemical changes in the photooxidation of hydrocarbons using as input data a set of elementary reactions, corresponding kinetic rate data and appropriate initial conditions was developed. The Model was refined and exploited to examine more closely the photooxidation and photostabilization of a hydrocarbon polymer. The results lead to the following observations. (1) The time to failure, tau sub f (chosen as the level of 5% C-H bond oxidation which is within the range anticipated for marked change in mechanical properties) varies as the inverse square root of the light intensity. However, tau sub f is almost unaffected by both the photoinitiator type and concentration. (2) The time to failure decreases with the rate of abstraction of C-H by peroxy radicals but increases with the rate of bimolecular radical termination controlled by diffusion. (3) Of the various stabilization mechanisms considered, the trapping of peroxy radicals is distinctly the most effective, although the concommitant decomposition of hydroperoxide is also desirable.

Somersall, A. C.↗
NASA NTRS1984

Effect of substrate chemical pretreatment on the tribological properties of graphite films

Rubbed films of natural flake Madagascar graphite were applied to ASTM A-355(D) steel with chemical surface pretreatments of zinc phosphate, gas nitride, salt nitride, sulfo-nitride, and with mechanical pretreatment (sandblasting). SAE 1045 steel pins were slid against these films using a pin-on-disk tribometer. The results indicate that two different lubricating mechanisms can occur. In the chemical surface pretreatment, the graphite can mix together to form a surface layer of the two constituents and this plasticity flowing layer provides the lubrication. The longest endurance lives and the lowest pin wear rates were obtained with this mechanism. In the other, surface topography appeared to control the mechanism. A rough surface was necessary to serve as a reservoir to supply the graphite to the flat metallic plateaus where it was sheared in very thin films between the plateaus and the sliding pin surface. For this mechanism, chemical pretreatment seemed to do little more than serve as a means for roughening the surface. Mean friction was not significantly influenced by chemical pretreatment, but surface roughness effects were observed.

Fusaro, R. L.↗
NASA NTRS1984

Substrate orientation dependence of enhanced epitaxial regrowth of silicon

This work extends the study of dopant-enhanced epitaxial regrowth rate of amorphized Si from the 100 to the 110 and 111 line orientations of Si. Boron and phosphorus dopants are considered. The annealing temperatures are 500 and 550 C. Phosphorus enhances the growth rates in all three orientations by a constant factor of 8.1 + or - 0.9. Boron produces a higher enhancement factor of 12.2 + or - 1.2, except in the case of 100. Implications of the results on various growth models are considered. The crystalline quality of regrown 111 layers is improved in the doped samples.

Ho, K. T.↗
NASA NTRS1985

Fabrication of ceramic substrate-reinforced and free forms

Components fabricated of, or coated with, ceramics have lower parasitic cooling requirements. Techniques are discussed for fabricating thin-shell ceramic components and ceramic coatings for applications in rocket or jet engine environments. Thin ceramic shells with complex geometric forms involving convolutions and reentrant surfaces were fabricated by mandrel removal. Mandrel removal was combined with electroplating or plasma spraying and isostatic pressing to form a metal support for the ceramic. Rocket engine thrust chambers coated with 0.08 mm (3 mil) of ZrO2-8Y2O3 had no failures and a tenfold increase in engine life. Some measured mechanical properties of the plasma-sprayed ceramic are presented.

Quentmeyer, R. J.↗
NASA NTRS1983

Modelling of photodegradation in solar cell modules of substrate and superstrate design made with ethylene-vinyl acetate as pottant material

A computer model was developed which can generate realistic concentration versus time profiles of the chemical species formed during photooxidation of hydrocarbon polymers using as input data a set of elementary reactions with corresponding rate constants and initial conditions. The results of computer simulation have been shown to be consistent with the general experimental observations of the photooxidation of polyethylene exposed to sunlight at ambient temperatures. The useful lifetime (5% oxidation) of the unstabilized polyethylene is predicted to vary from a few months in hot weather (100 F) to almost two years in cool weather (45 F) with an apparent net activation energy of 10 kcal/mol. Modelling studies of alternate mechanisms for stabilization of clear, amorphous, linear polyethylene suggest that the optimum stabilizer would be a molecularly dispensed additive in very low concentration which can trap peroxy radicals and also decompose hydroperoxides.

Somersall, A. C.↗
NASA NTRS1984

Microbial ecology of extreme environments: Antarctic yeasts and growth in substrate-limited habitats

An extreme environment is by definition one with a depauperate biota. While the Ross Desert is by no means homogeneous, the most exposed and arid habitats, soils in the unglaciated high valleys, do indeed contain a very sparse biota of low diversity. So sparse that the natives could easily be outnumbered by airborne exogenous microbes. Native biota must be capable of overwintering as well as growing in the high valley summer. Tourists may undergo a few divisions before contributing their enzymes and, ultimately, elements to the soil - or may die before landing. The simplest way to demonstrate the indigenicity of a particular microbe is therefore to establish unique distribution; occurrence only in the habitat in question precludes foreign origin.

Vishniac, H. S.↗
NASA NTRS1985

Microbial ecology of extreme environments: Antarctic yeasts and growth in substrate-limited habitats

The high, dry valleys of the Ross Desert of Antarctic, characterized by extremely low temperatures, aridity and a depauperate biota, are used as an analog of the postulated extreme climates of other planetary bodies of the Solar System to test the hypothesis that if life could be supported by Ross, it might be possible where similar conditions prevail. The previously considered sterility of the Ross Desert soil ecosystem has yielded up an indigenous yeast, Cryptoccus vishniacci, which is able to resist the extremes of cold, wet and dry freezing, and long arid periods, while making minimal nutritional demands on the soil.

Vishniac, H. S.↗
NASA NTRS1984

Reaction of amorphous Ni-W and Ni-N-W films with substrate silicon

Wiley et al. (1982) have studied sputtered amorphous films of Nb-Ni, Mo-Ni, Si-W, and Si-Mo. Kung et al. (1984) have found that amorphous Ni-Mo films as diffusion barriers between multilayer metallizations on silicon demonstrate good electrical and thermal stability. In the present investigation, the Ni-W system was selected because it is similar to the Ni-Mo system. However, W has a higher silicide formation temperature than Mo. Attention is given to aspects of sample preparation, sample characterization, the interaction between amorphous Ni-W films and Si, the crystallization of amorphous Ni(36)W(64) films on SiO2, amorphous Ni-N-W films, silicide formation and phase separation, and the crystallization of amorphous Ni(36)W(64) and Ni(30)N(21)W(49) layers.

Zhu, M. F.↗
NASA NTRS1984

Substrate-induced orientational order in the isotropic phase of liquid crystals

Nematic order induced near a solid boundary in an otherwise isotropic liquid crystal is studied theoretically, at temperatures just above the bulk nematic-isotropic phase transition. Three distinct regimes are found, depending on the strength of orientational torques at the boundary: (1) strong orientational order, (2) strong orientational order followed by a first-order transition to a state of weak orientational order as temperature is raised, and (3) a state of weak orientational order.

Mauger, A.↗
NASA NTRS1984

The scattering of ultrasonic third sound from substrate surface defects

The interaction of third sound with isolated surface defects is examined. Phase-inverted reflections are observed to occur at surface defects even when the wavelength of the third sound is much greater than the dimensions of the defect. The origin of the scattering mechanism is believed to be due to the change in the helium film thickness in the vicinity of the topological variations introduced by the presence of the surface defects. Data are obtained for ultrasonic frequencies in the 20-200 kHz range.

Generazio, E. R.↗
NASA NTRS1985

Fabrication of ceramic substrate-reinforced and free forms by mandrel plasma spraying metal-ceramic composites

Components fabricated of, or coated with, ceramics have lower parasitic cooling requirements. Techniques are discussed for fabricating thin-shell ceramic components and ceramic coatings for applications in rocket or jet engine environments. Thin ceramic shells with complex geometric forms involving convolutions and reentrant surfaces were fabricated by mandrel removal. Mandrel removal was combined with electroplating or plasma spraying and isostatic pressing to form a metal support for the ceramic. Rocket engine thrust chambers coated with 0.08 mm (3 mil) of ZrO2-8Y2O3 had no failures and a tenfold increase in engine life. Some measured mechanical properties of the plasma-sprayed ceramic are presented.

Quentmeyer, R. J.↗
NASA NTRS1985

Endfire tapered slot antennas on dielectric substrates

Endfire-tapered slot antennas are suitable for many integrated circuit applications, imaging and phased arrays. An investigation of single elements of such antennas, including slots which are exponentially tapered (Vivaldi), linearly tapered, and constant width. For antennas of all types, a good general agreement is obtained for curves of beamwidth-versus-length, normalized to wavelength, when one compares the data with that for traveling-wave antennas published by Zucker (1961). An important condition for this agreement is that the effective dielectric thickness, defined in the text, is in a certain optimum range. This condition is qualitatively explained in terms of the theory for traveling-wave antennas.

Yngvesson, K. S.↗
NASA NTRS1987

Interactions of amorphous Ta(x)Cu(1-x) (x = 0.93 and 0.80) alloy films with Au overlayers and GaAs substrates

Amorphous Ta(93)Cu(7) and Ta(80)Cu(20) alloy films are prepared by cosputtering of pure Ta and pure Cu targets with a rotating sample holder table. To investigate the possible application of these materials as diffusion barriers for the Au-GaAs system, vacuum annealings are made in the temperature range from 200 to 800 C. Resistivity change, X-ray diffraction, and Auger electron spectroscopy measurements are performed to find the chemical and metallurgical stabilities of these materials in this system. The reaction temperature for Ta(x)Cu(1-x) in contact with GaAs lies between 500 and 700 C. For Au in contact with Ta(x)Cu(1-x) the reaction occurs at about 600 C. Amorphous Ta(93)Cu(7) shows different interdiffusion characteristics with surrounding elements than does Ta(80)Cu(20).

Oh, Jae E.↗
NASA NTRS1988

Acrolein Microspheres Are Bonded To Large-Area Substrates

Reactive cross-linked microspheres produced under influence of ionizing radiation in aqueous solutions of unsaturated aldehydes, such as acrolein, with sodium dodecyl sulfate. Diameters of spheres depend on concentrations of ingredients. If polystyrene, polymethylmethacrylate, or polypropylene object immersed in solution during irradiation, microspheres become attached to surface. Resulting modified surface has grainy coating with reactivity similar to free microspheres. Aldehyde-substituted-functional microspheres react under mild conditions with number of organic reagents and with most proteins. Microsphere-coated macrospheres or films used to immobilize high concentrations of proteins, enzymes, hormones, viruses, cells, and large number of organic compounds. Applications include separation techniques, clinical diagnostic tests, catalytic processes, and battery separators.

Rembaum, Alan↗
NASA NTRS1988

Organic molecules on meteoritic solid substrates

The production of C1-C3 monocarboxylic acids from carbonaceous meteorites was investigated by the radiolysis of air-free CO2 and alkane solutions, utilizing a Cs-137 gamma-ray source. Samples were first loaded on a vacuum system, irradiated, and then analyzed by the Hewlett Packard model 5890A gas chromatograph. Moreover, the samples were irradiated with and without added ferrous iron.

Sweeney, Michael A.↗
NASA NTRS1988

Effect of substrate preparation on the structure and chemisorption properties of Pd/MgO model catalyst

Pd/MgO model catalysts are prepared by vapor deposition of Pd particles on MgO thin films which are deposited, at different temperatures, on UHV cleaved mica. MgO films and Pd particles are characterized in situ by Auger electron spectroscopy and energy-loss spectroscopy and ex situ by transmission electron microscopy. Succesive temperature programmed desorption (TPD) of CO shows a different evolution of the morphology of the Pd particles deposited on MgO films prepared at low and high temperature. In addition, on MgO prepared at low temperature, the smallest particles show a continuous decay of CO adsorption during TPD due to contamination by support species.

Henry, C. R.↗
NASA NTRS1989

Composite Semiconductor Substrates

Epitaxial structure of three semiconductor materials - silicon, gallium arsenide, and cadmium telluride - makes possible integrated monolithic focal-plane arrays of photodectors. Silicon layer contains charge-coupled devices, gallium arsenide layer contains other fast electronic circuitry, and cadmium telluride layer serves as base for array of mercury cadmium telluride infrared sensors. Technique effectively combines two well-established techniques; metalorganic chemical-vapor deposition (MOCVD) and molecular-beam epitaxy (MBE). Multilayer structure includes HgCdTe light sensors with Si readout devices and GaAs signal-processing circuits. CdTe layer provides base for building up HgCdTe layer.

Nouhi, Akbar↗