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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Adhesion and transfer of polytetrafluorethylene to metals studied by Auger emission spectroscopy

The adhesion and transfer of polytetrafluoroethylene (PTFE) to metals in ultrahigh vacuum were studied. The transfer was effected both by compressive static contact and by sliding contact. The transfer observed after static contact was independent of the chemical constitution of the substrate. Electron-induced desorption of the fluorine in the transferred PTFE showed that the fluorine had no chemical interaction with the metal substrate. The coefficient of friction on metals was independent of the chemical constitution of the substrate. However, sliding PTFE on soft metals, such as aluminum, generated wear fragments that lodged in the PTFE and machined the substrate.

Pepper, S. V.↗
NASA NTRS1971

Generation of long time creep data on refractory alloys at elevated temperatures

Ultrahigh vacuum creep tests were conducted on tantalum, tungsten, and molybdenum alloys to develop creep data and to evaluate the influence of liquid lithium exposure on the creep behavior of the tantalum alloy T-111. Test conditions were generally selected to provide 1% creep in 1000 to 10,000 hours, with test temperatures ranging from 1600 to 2912 F (1144 to 1873 K). Tests on T-111 specimens exposed to vacuum and to liquid lithium for various times and temperatures showed large reductions in creep strength for exposures in the 1800 to 1900 F (1255 to 1310 K) range. Tests on the ASTAR 811c showed a significant influence of grain size on the creep strength of this material, with larger grain size specimens being significantly stronger above the equicohesive temperature of 2000 F (1366 K).

Sheffler, K. D.↗
NASA NTRS1971

Generation of long time creep data on refractory alloys at elevated temperatures

Ultrahigh vacuum creep tests were performed on tungsten, molybdenum, and tantalum alloys to develop design creep data and to evaluate the influence of liquid lithium exposure on the creep resistance of a tantalum alloy. Test conditions were generally selected to provide 1% creep in 1000 to 10,000 hours, with the test temperatures ranging between 1600 and 2900 F (1144 K and 1866 K). One percent creep life data from a tantalum-base T-111 alloy (Ta-8%W-2%Hf) were analyzed using a station function method to provide an improved parametric representation of the T-111 data. In addition, the minimum creep rate data from an ASTAR 811C alloy (Ta-8%W-1%Re-0.7%Hf0.025%C) were analyzed to determine the stress and temperature dependence of creep rate. Results of this analysis indicated that the activation energy for creep decreased from about 150 Kcal/mole (5130 J/mole) above 2400 F (1589 K) to about 110 Kcal/mole (3760 J/mole) below 2000 F (1361 K). This temperature range corresponds to the range where the creep mechanism changes from grain boundary sliding to intragranular creep.

Sheffler, K. D.↗
NASA NTRS1971

Microbial survival in deep space environment.

Review of the knowledge available on the extent to which microorganisms (mainly microbial spores, vegetative cells, and fungi) are capable of surviving the environment of deep space, based on recent simulation experiments of deep space. A description of the experimental procedures used is followed by a discussion of deep space ecology, the behavior of microorganisms in ultrahigh vacuum, and factors influencing microbial survival. It is concluded that, so far, simulation experiments have proved far less lethal to microorganisms than to other forms of life. There are, however, wide gaps in the knowledge available, and no accurate predictions can as yet be made on the degree of lethality that might be incurred by a microbial population on a given mission. Therefore, sterilization of spacecraft surfaces is deemed necessary if induced panspermia (i.e., interplanetary life propagation) is to be avoided.

Silverman, G. J.↗
NASA NTRS1971

Photoemf in cadmium sulfide

Theoretical and experimental investigations on CdS single crystals and CuxS:CdS photovoltaic cells prepared from CdS single crystals by a chemical-dip procedure are described. The studies are aimed at clarifying cell mechanisms which affect key cell properties (efficiency, reliability, and lifetime) by examining the properties of intrinsic and extrinsic defects in the junction and surface regions and their effects on carrier transport through these regions. The experimental research described includes studies of thermal, infrared, and field quenching of acceptor-doped CdS crystals; investigation of optical and electrical properties of CuxS:CdS photovoltaic cells (current-voltage characteristics, spectral distribution of photocurrent and photovoltage) and the dependence of these properties on temperature and light intensity; measurement of changes, as a result of heat treatment in ultrahigh vacuum, in the spectral distribution of photoconductivity at room temperature and liquid nitrogen temperature, the luminescence spectrum at liquid nitrogen temperature, and the thermally stimulated current curves of CdS crystals; determination of the effect of irradiation with 150 keV (maximum) X-rays on the spectral distribution of photoconductivity and thermally-stimulated current of CdS crystals; and studies of the effect of growth conditions on the photoconductive properties of CdS crystals.

Boeer, K. W.↗
NASA NTRS1973

UHV outgassing measurements on various carbons.

Mass spectral analyses were made of gases desorbed from carbon samples at increasing temperatures up to 2200 K. Samples were initially outgassed above 2000 K, exposed to air for several hours at room temperature, and then tested in ultrahigh vacuum following a 24-hr bake at 300 C. Four forms of carbon tested were pyrolytic carbon, Grafoil, vitreous carbon, and pyrolyzed-phenolic fibers. The equivalent of several monolayers of gas (mostly H2 and CO) are evolved from all of the carbons except vitreous carbon. Less than a monolayer of gas is evolved from vitreous carbon during outgassing to 2000 K.

Beitel, G. A.↗
NASA NTRS1973

Evaporation rate and vapor pressure of selected polymeric lubricating oils.

A recently developed ultrahigh-vacuum quartz spring mass sorption microbalance has been utilized to measure the evaporation rates of several low-volatility polymeric lubricating oils at various temperatures. The evaporation rates are used to calculate the vapor pressures by the Langmuir equation. A method is presented to accurately estimate extended temperature range evaporation rate and vapor pressure data for polymeric oils, incorporating appropriate corrections for the increases in molecular weight and the change in volatility of the progressively evaporating polymer fractions. The logarithms of the calculated data appear to follow linear relationships within the test temperature ranges, when plotted versus 1000/T. These functions and the observed effusion characteristics of the fluids on progressive volatilization are useful in estimating evaporation rate and vapor pressure changes on evaporative depletion.

Gardos, M. N.↗
NASA NTRS1974

Electrical properties of lunar soil sample 15301,38

Electrical property measurements have been made on an Apollo 15 lunar soil sample in ultrahigh vacuum from room temperature to 827 C for the frequency spectrum from 100 Hz through 1 MHz. The dielectric constant, the total ac loss tangent, and the dc conductivity were measured. The dc conductivity showed no thermal hysteresis, but an irreversible (in vacuum) thermal effect was found in the dielectric loss tangent on heating above 700 C and during the subsequent cooling. This appears to be related to several effects associated with lunar glass above 700 C. The sample also showed characteristic low-frequency dispersion in the dielectric constant with increasing temperature, presumably due to Maxwell-Wagner intergranular effects. The dielectric properties may be fitted to a model involving a Cole-Cole frequency distribution that is relatively temperature-independent below 200 C and follows a Boltzmann temperature distribution with an activation energy of 2.5 eV above 200 C. The dc conductivity is fitted by an exponential temperature distribution and becomes the dominant loss above 700 C.

Olhoeft, G. R.↗
NASA NTRS1974

Magnetospheric particle composition experiment

Thin foils of aluminum and platinum were mounted on the exterior of the Skylab spacecraft where they were bombarded by particles that penetrate to the 400-km spacecraft altitude, including auroral particles precipitating from the earth magnetosphere. These foils have a high trapping efficiency for both charged and neutral particles above 1 keV and a decreasing but appreciable efficiency below that energy. Some sections of the foils were double layered to obtain an estimate of the energy of the entrapped particles. The foils were retrieved and returned for analysis in an ultrahigh-vacuum mass spectrometer. So far, the noble gas components of the entrapped particles have been analyzed in selected sections of two foils. Helium-4, helium-3, neon-20, and neon-22 have been detected.

Geiss, J.↗
NASA NTRS1975

Effect of adsorbed chlorine and oxygen on shear strength of iron and copper junctions

Static friction experiments were performed in ultrahigh vacuum at room temperature on copper, iron, and steel contacts selectively contaminated with oxygen and chlorine in submonolayer amounts. The concentration of the adsorbates was determined with Auger electron spectroscopy and was measured relative to the saturation concentration of oxygen on iron (concentration 1.0). The coefficient of static friction decreased with increasing adsorbate concentration. It was independent of the metal and the adsorbate. The results compared satisfactorily with an extension of the junction growth theory to heterogeneous interfaces. The reduction in interfacial shear strength was measured by the ratio sub a/sub m where sub a is the shear strength of the interface with an adsorbate concentration of 1.0, and sub m is the strength of the clean metal interface. This ratio was 0.835 + or - 0.012 for all the systems tested.

Wheeler, D. R.↗
NASA NTRS1974

The progeny of Skylab - The pioneering phase of our expansion into space

The opportunities provided by such near-earth space resources as the continuous view of the earth, sun, and stars, the presence of ultrahigh vacuum, and the absence of gravitational stress are briefly reviewed, along with the significance of manned observation and experimentation in near-earth space. The stimulation triggered in young minds by a week or summer in space, as the Apollo and Skylab experiences show, simply defies the imagination. The plea is made not to miss these opportunities.

Schmitt, H. H.↗
NASA NTRS1974

Photoelectron spectra of the decomposition of ethylene on /110/ tungsten

The experimental apparatus used in the investigation consisted of an ultrahigh-vacuum chamber, a triple-grid, a microwave-excited resonance lamp, and an electron energy analyzer. The chemical nature of the chemisorbed species was studied, taking into account the energy distribution of photoemitted electrons, work function determinations, and low-energy electron diffraction patterns.

Plummer, E. W.↗
NASA NTRS1975

Oxide nucleation on thin films of copper during in situ oxidation in an electron microscope

Single-crystal copper thin films were oxidized at an isothermal temperature of 425 C and at an oxygen partial pressure of 0.005 torr. Specimens were prepared by epitaxial vapor deposition onto polished faces of rocksalt and were mounted in a hot stage inside the ultrahigh-vacuum chamber of a high-resolution electron microscope. An induction period of roughly 30 min was established which was independent of the film thickness but depended strongly on the oxygen partial pressure and to exposure to oxygen prior to oxidation. Neither stacking faults nor dislocations were found to be associated with the Cu2O nucleation sites. The experimental data, including results from oxygen dissolution experiments and from repetitive oxidation-reduction-oxidation sequences, fit well into the framework of an oxidation process involving the formation of a surface charge layer, oxygen saturation of the metal with formation of a supersaturated zone near the surface, and nucleation followed by surface diffusion of oxygen and bulk diffusion of copper for lateral and vertical oxide growth, respectively.

Heinemann, K.↗
NASA NTRS1976

Exoelectron emission from a clean, annealed magnesium single crystal during oxygen adsorption

Exoelectron emission was observed from a clean, annealed Mg (0001) surface during oxygen and chlorine adsorption at pressures of 6.5x10 0.00001- N/sq m and lower. the studies were performed in an ultrahigh vacuum system. The crystals were cleaned by argon ion bombardment and annealed at 300 C. Auger electron spectroscopy was used to verify surface cleanliness, and low energy electron diffraction was used to verify that the surface was annealed. The emission was found to be oxygen arrival rate dependent. Two peaks were observed in the electron emission with exposure. Evidence is presented that the formation of the second peak corresponds to oxidation of the Mg surface. No emission was observed from clean aluminum during adsorption. Results verify that electron emission occurs from a strain free surface simply upon adsorption of oxygen. A qualitative explanation for the mechanisms of emission in terms of chemical effects is presented.

Ferrante, J.↗
NASA NTRS1976

Shear strength of metal-sapphire contacts

The shear strength of polycrystalline Ag, Cu, Ni, and Fe contacts on clean (0001) sapphire has been studied in ultrahigh vacuum. Both clean metal surfaces and surfaces exposed to O2, Cl2, and C2H4 were used. The results indicate that there are two sources of strength of Al2O3-metal contacts: an intrinsic one that depends on the particular clean metal in contact with Al2O3 and an additional one due to intermediate films. The shear strength of the clean metal contacts correlated directly with the free energy of oxide formation for the lowest metal oxide, in accord with the hypothesis that a chemical bond is formed between metal cations and oxygen anions in the sapphire surface. Contacts formed by metals exposed to chlorine exhibited uniformly low shear strength indicative of van der Waals bonding between chlorinated metal surfaces and sapphire. Contacts formed by metals exposed to oxygen exhibited enhanced shear strength, in accord with the hypothesis that an intermediate oxide layer increases interfacial strength.

Pepper, S. V.↗
NASA NTRS1976

Effect of adsorbed chlorine and oxygen on the shear strength of iron and copper junctions

Static-friction experiments were performed in ultrahigh vacuum at room temperature on copper, iron, and steel contacts selectively contaminated with oxygen and chlorine in submonolayer amounts. The concentration of the adsorbates was determined with Auger electron spectroscopy and was measured relative to the saturation concentration of oxygen on iron (concentration, 1.0). The coefficient of static friction decreased with increasing adsorbate concentration; however, it was independent of the type of metal and the adsorbate species. The results compared satisfactorily with an extension of the junction growth theory to heterogeneous interfaces. The reduction in interfacial shear strength was measured by the ratio of the shear strength of the interface with an adsorbate concentration of 1.0 and the strength of the clean metal interface. This ratio was about 0.835 for all the systems tested.

Wheeler, D. R.↗
NASA NTRS1976

Effect of adsorbed films on friction of Al2O3-metal systems

The kinetic friction of polycrystalline Al2O3 sliding on Cu, Ni, and Fe in ultrahigh vacuum was studied as a function of the surface chemistry of the metal. Clean metal surfaces were exposed to O2, Cl2, C2H4, and C2H3Cl, and the change in friction due to the adsorbed species was observed. Auger electron spectroscopy assessed the elemental composition of the metal surface. It was found that the systems exposed to Cl2 exhibited low friction, interpreted as the van der Waals force between the Al2O3 and metal chloride. The generation of metal oxide by oxygen exposures resulted in an increase in friction, interpreted as due to strong interfacial bonds established by reaction of metal oxide with Al2O3 to form the complex oxide (spinel). The only effect of C2H4 was to increase the friction of the Fe system, but C2H3Cl exposures decreases friction in both Ni and Fe systems, indicating the dominance of the chlorine over the ethylene complex on the surface

Pepper, S. V.↗
NASA NTRS1976

Symposium on Space Industrialization, Huntsville, Ala., May 26, 27, 1976, Proceedings

Space habitats are considered, with attention given the evolution of space station systems, space station habitability, space settlement planning methodology, and orbital assembly. Various aspects of the Space Transportation System are discussed, including Shuttle booster/propulsion growth concept, advanced earth orbital transportation systems technology, single-stage-to-orbit vehicles and aeromaneuvering orbit transfer vehicles. Materials processing in space is examined, with emphasis on biological materials, metallurgical materials, the uses of space ultrahigh vacuum, and extraterrestrial mining and industrial processing. Solar space power is investigated, with attention given the potential of satellite solar power stations, thermal engine power satellites and microwave power transmission to earth. Individual items are announced in this issue.

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