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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

TiO2 film properties as a function of processing temperature, volume 3

Thin film TiO2 was produced at 150 C by chemical vapor deposition using hydrolysis of tetraisopropyl titanate. Films were amorphous as grown, but annealing in air caused crystallization, with anatase formed beginning at 350 C and rutile at 700 C. Density and index of refraction increased substantially with increasing anneal temperature, while etch susceptibility in HF and H2SO4 decreased. Comparison with literature data showed two groups of processes. One group yields films having properties that gradually approach those of rutile with increasing process temperature. The other group gives rutile directly at moderate temperatures. Deposition of amorphous film followed by etching and annealing is suggested as a means for pattern definition.

Fitzgibbons, E. T.↗
NASA NTRS1973

Friction, wear, and transfer of carbon and graphite to copper, chromium, and aluminum metal surfaces in vacuum

Sliding friction experiments were conducted with amorphous and fully graphitized carbons sliding on copper and on films of chromium and aluminum on copper. Auger emission spectroscopy analysis was used to monitor carbon transfer to the metal surfaces. Friction and wear were also measured. Metal surfaces were examined both in the clean state and with normal oxides present. Results indicate that different metals have an important effect on friction, wear, and transfer characteristics. With amorphous carbon, the least chemically active metal gave the highest wear and amount of carbon transfer. Both forms of carbon gave lower friction and wear and lower transfer rates when in contact with clean, as opposed to oxide-covered, chromium surfaces. With copper, the reverse was true; cleaning was detrimental.

Buckley, D. H.↗
NASA NTRS1973

The influence of defects and impurities on the nucleation and growth of oriented films by evaporation

The influence of substrate imperfections on the nucleation and growth of fcc metals on alkali halides is discussed. Films deposited on well characterized substrated under well defined vacuum evaporation conditions are investigated. The experimental results of this work are correlated with similar work by other investigators. Models which have been proposed by various authors to explain experimental results are critically examined and areas of difficulty are pointed out. The influence of defects on nucleation rate and the orientation of the film is emphasized. Specific examples of impurity effects, irradiation effects and the influence of amorphous layers are discussed in detail. Evidence is shown that the formation of multiply twinned particles is a result of coalescence and growth. The only consistent model for the orienting influence of impurities is shown to be a chemical reaction effect. It is demonstrated that an alkali metal impurity is very likely responsible for the orienting influence of both water vapor exposure and irradiation. A negative result is found for the reported possibility of an orienting influence being transmitted through an amorphous layer.

Green, A. K.↗
NASA NTRS1973

Fused Silica Surface Coating for a Flexible Silica Mat Insulation System

Fused silica insulation coatings have been developed for application to a flexible mat insulation system. Based on crystalline phase nucleation and growth kinetics, a 99+% SiO2 glass was selected as the base composition. A coating was developed that incorporated the high emissivity phase NiCr2O4 as a two phase coating with goals of high emittance and minimum change in thermal expansion. A second major coating classification has a plasma sprayed emittance coating over a sealed pure amorphous SiO2 layer. A third area of development centered on extremely thin amorphous SiO2 coatings deposited by chemical vapor deposition. The coating characterization studies presented are mechanical testing of thin specimens extracted from the coatings, cyclic arc exposures, and emittance measurements before and after arc exposures.

Rhodes, W. H.↗
NASA NTRS1974

Energy source for comet outbursts

Development of a mechanism explaining the internal source of energy of comet outbursts. A mechanism is proposed which automatically provides a source of particulate matter which creates a huge surface area which contains a substantial percentage of amorphous ice, so that the phase transition of the amorphous ice to a cubic structure provides a release of energy which may be responsible for the outbursts observed in many comets. In addition, the volume into which the transition can propagate is estimated for a spherical comet with a radius of 5 km.

Patashnick, H.↗
NASA NTRS1973

Optical investigations of noncrystalline semiconductors

Three areas of investigation into the properties of amorphous silicon and boron are reported: (1) optical properties of elemental amorphous semiconductors; (2) Mossbauer studies of disordered systems; and (3) theoretical aspects of disordered semiconductors.

Blum, N. A.↗
NASA NTRS1975

Direct observation of small cluster mobility and ripening

Direct evidence is reported for the simultaneous occurrence of Ostwald ripening and short-distance cluster mobility during annealing of discontinuous metal films on clean amorphous substrates. The annealing characteristics of very thin particulate deposits of silver on amorphized clean surfaces of single crystalline thin graphite substrates were studied by in-situ transmission electron microscopy (TEM) under controlled environmental conditions (residual gas pressure of 10 to the minus 9th power torr) in the temperature range from 25 to 450 C. Sputter cleaning of the substrate surface, metal deposition, and annealing were monitored by TEM observation. Pseudostereographic presentation of micrographs in different annealing stages, the observation of the annealing behavior at cast shadow edges, and measurements with an electronic image analyzing system were employed to aid the visual perception and the analysis of changes in deposit structure recorded during annealing. Slow Ostwald ripening was found to occur in the entire temperature range, but the overriding surface transport mechanism was short-distance cluster mobility.

Heinemann, K.↗
NASA NTRS1975

Ovonic type switching in tin selenide thin films

Amorphous tin selenide thin films which possess Ovonic type switching properties are fabricated using vacuum deposition techniques. The devices are fabricated in a planar configuration and consist of amorphous tin selenide deposited over silver contacts. Results obtained indicate that Ovonic type memory switching does occur in these films with the energy density required for switching from a high impedance to a low impedance state being dependent on the spacing between the electrodes of the device. There is also a strong implication that the switching is a function of the magnitude of the applied voltage pulse.

Baxter, C. R.↗
NASA NTRS1975

Ultraviolet optical constants of water and ammonia ices

A method reported by Tempelmeyer and Mills (1968) for the determination of real refractive indices of transparent films was used in the investigation. It was found that the fringe period for a transparent film can be used to approximate the period for a semitransparent film near its extremum. The experimental apparatus is discussed along with details regarding the employed technique. Graphs are presented concerning the refractive index and the absorption coefficients for amorphous water ice and amorphous ammonia ice.

Browell, E. V.↗
NASA NTRS1976

Direct observation of small cluster mobility and ripening

Direct evidence is reported for the simultaneous occurrence of Ostwald ripening and short-distance cluster mobility during annealing of discontinuous metal films on clean amorphous substrates. The annealing characteristics of very thin particulate deposits of silver on amorphized clean surfaces of single-crystalline thin graphite substrates have been studied by in situ transmission electron microscopy (TEM) under controlled environmental conditions in the temperature range from 25 to 450 C. It was possible to monitor all stages of the experiments by TEM observation of the same specimen area. Slow Ostwald ripening was found to occur over the entire temperature range, but the overriding surface transport mechanism was short-distance cluster mobility. This was concluded from in situ observations of individual particles during annealing and from measurements of cluster size distributions, cluster number densities, area coverages, and mean cluster diameters.

Heinemann, K.↗
NASA NTRS1977

Ionization induced damage in crystalline silicon

Close examination of the interaction of the energetic knock-on atoms with the local lattice environment reveals a damage mechanism which does satisfy the experimental data on proton irradiation of silicon. A proton-atom interaction with high energy transfer is considered where the proton path is delineated by a trail of ionization, and the silicon ion path is characterized by much heavier ionization terminating in a dense displacement cluster. At collision, many of the silicon electrons are stripped off, and the resulting energetic ion subsequently loses energy rapidly by Coulomb interaction with bound electrons. The rate of energy loss depends on the charge state and velocity of the knock-on ion. For ion energies in excess of 1 MeV, the intensity of ionization is sufficient to permit lattice atoms, stripped of their binding electrons, to reorient randomly before having an opportunity to recombine with electrons and re-establish the lattice. The path of a knock-on ion thus becomes a thin cylinder of amorphous material within the crystal. Amorphous silicon has a Fermi level closer to mid-band than does single crystal silicon, and a strong field therefore, results around this damaged region. The field produces a large depletion region, representing a very large capture cross section for minority carriers.

Meulenberg, A., Jr.↗
NASA NTRS1978

An experimental investigation of the condensation of silicate grains

Results are presented for a series of laboratory experiments designed to investigate the nucleation of small silicate grains from a vapor of astrophysically significant elements and compounds. In the experiments, magnesium silicate grains were condensed by simultaneously evaporating Mg and SiO solids into an atmosphere of argon or hydrogen at a pressure of a few torr. The results show that at low temperatures (up to a few hundred degrees C) the condensates are amorphous grains and have widely varying stoichiometries. The thermodynamically most stable compounds (Mg2SiO4, MgSiO3, SiO2) do not form readily, but all initial condensates can be converted to crystalline forsterite (Mg2SiO4) by heating to 1000 C in vacuum. At higher temperatures (above 700 K) it becomes more difficult to nucleate any silicates, and those that do form are amorphous, indicating that surface energies and kinetic effects are very important in determining under what conditions condensation will occur. The IR spectra of the experimentally produced magnesium silicates are found to have a strong resemblance to those observed in many astronomical clouds.

Day, K. L.↗
NASA NTRS1978

Synthesis of Silane and Silicon in a Non-equilibrium Plasma Jet

The original objective of this program was to determine the feasibility of high volume, low-cost production of high purity silane or solar cell grade silicon using a non equilibrium plasma jet. The emphasis was changed near the end of the program to determine the feasibility of preparing photovoltaic amorphous silicon films directly using this method. The non equilibrium plasma jet should be further evaluated as a technique for producing high efficiency photovoltaic amorphous silicon films.

Calcote, H. F.↗
NASA NTRS1979

Emission and extinction of ground and vapor-condensed silicates from 4 to 14 microns and the 10 micron silicate feature

Emission and absorption spectra from 4 to 14 microns of ground and laser-vaporized olivine and enstatite silicates are compared with the 10-micron emission feature of the Orion Trapezium. The agreement in band center and shape between the amorphous laser-vaporized olivine sample and the Trapezium feature suggests that amorphous silicate grains of approximately olivine composition may be a major constituent of interstellar dust. Differences between the emission and absorption spectral profiles (absorption plus scattering) show characteristics that could be used as a sensitive probe of the morphology of interstellar grain systems when high signal-to-noise ratio (30-100) observational spectra become available.

Stephens, J. R.↗
NASA NTRS1979

Development of RF sputtered chromium oxide coating for wear application

The radio frequency sputtering technique was used to deposite a hard refractory, chromium oxide coating on an Inconel X-750 foil 0.1 mm thick. Optimized sputtering parameters for a smooth and adherent coating were found to be as follows: target-to-substrate spacing, 41.3 mm; argon pressure, 5-10 mTorr; total power to the sputtering module, 400 W (voltage at the target, 1600 V), and a water-cooled substrate. The coating on the annealed foil was more adherent than that on the heat-treated foil. Substrate biasing during the sputter deposition of Cr2O3 adversely affected adherence by removing naturally occurring interfacial oxide layers. The deposited coatings were amorphous and oxygen deficient. Since amorphous materials are extremely hard, the structure was considered to be desirable.

Bhushan, B.↗
NASA NTRS1982

Friction and surface chemistry of some ferrous-base metallic glasses

The friction properties of some ferrous-base metallic glasses were measured both in argon and in vacuum to a temperature of 350 C. The alloy surfaces were also analyzed with X-ray photoelectron spectroscopy to identify the compounds and elements present on the surface. The results of the investigation indicate that even when the surfaces of the amorphous alloys, or metallic glasses, are atomically clean, bulk contaminants such as boric oxide and silicon dioxide diffuse to the surfaces. Friction measurements in both argon and vacuum indicate that the alloys exhibit higher coefficients of friction in the crystalline state than they do in the amorphous state.

Miyoshi, K.↗
NASA NTRS1983

Sliding induced crystallization of metallic glass

Sliding friction and wear experiments, electron microscopy, and diffraction studies were conducted with an Fe67Co18B14Si1 ferrous-base metallic glass in sliding contact with aluminum oxide at room temperature in air. The results indicate that the amorphous alloy can be crystallized during the sliding process. Crystallization of the wear surface causes high friction. Plastic flow occurred on the amorphous alloy with sliding, and the flow film of the alloy transferred to the aluminum oxide surface. Two distinct types of wear debris were observed as a result of sliding: an alloy wear debris, and powdery and whiskery oxide debris. Generation of oxide wear debris particles on an alloy can cause transitions in friction behavior.

Miyoshi, K.↗
NASA NTRS1982

Spectral evidence for the mineralogy of high-albedo soils and dust on Mars

Laboratory spectroscopic observations are presented which further constrain the mineralogy and origin of the high albedo Martian soils and dust, and suggest that nontronite is not a major component of Martian soils, although the presence of other iron-poor clays cannot be excluded on the basis of current observational data. Because the best of the known spectral analogs for the high albedo Martian material is a type of palagonite from Hawaii, it is thought that ferric iron is likely to occur in poorly defined Martian crystallographic sites producing X-ray amorphous weathering products of mafic volcanic glass. These materials form slowly, under semiarid conditions, at ambient temperatures. Since the amorphous Hawaiian soils exist metastably for thousands of years, their Martian analogs may be expected to survive even longer under the present cold and dry climatic conditions.

Singer, R. B.↗