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At least 397 records · Page 22

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NASA NTRS1977

Sputtering in interstellar shocks - A model for heavy element depletion

High-velocity (at least 100 km/s) shock fronts are found to provide an environment where grains can be destroyed by thermal sputtering. Application is made to supernova remnants. Sputtering of refractory grains in shocks associated with high-velocity clouds leads to cosmic abundances of heavy elements and if followed by deceleration of the high-velocity gas and dilution with ambient depleted matter. A correlation of depletion with systematic velocity is predicted. It is proposed that there is a residual underlying depletion associated with refractory grain formation and a subsequent trapping of gas-phase metallic species by adsorption. Sputtering of adsorbed monolayers in intermediate-velocity shocks (not exceeding 50 km/s) can account for a range of depletions in diffuse interstellar matter.

Barlow, M. J.↗

NASA NTRS1977

Cluster calculation of carbon monoxide on a Cu surface using the self-consistent-field-X-alpha-scattered-wave method

Two geometries of CO chemisorbed on a Cu(100) surface have been studied using the self-consistent-field-X-alpha-scattered-wave method. Both geometries assume the CO molecule is bonded to the Cu surface with the C end down; one geometry is in the fourfold-symmetric hole site and one is directly on top of a Cu atom. The calculated valence density of states for the fourfold-symmetry case gives a satisfactory interpretation of the two main peaks below the copper d band in the UV photoemission spectra (UPS). A third weaker peak of the UPS data can be correlated with a CO molecule adsorbed directly on top of a copper atom.

Yu, H. L.↗

NASA NTRS1976

The excess fission xenon problem in lunar samples

This paper presents noble-gas (neon, krypton, and xenon) data obtained in stepwise heating of five lunar breccias: 14063, 14082, 15205, 15405, and 15445. These data are discussed in terms of spallation gas content, cosmic-ray exposure ages, and content of fission xenon. None of these samples is rich in solar-wind gases, and none contains more fission xenon than would be expected for in situ fission of U-238 and Pu-244. The essential features of the excess-fission-xenon phenomenon seen in several Apollo 14 samples are reviewed, as are models which might account for this phenomenon and criteria by which these models may be evaluated. The lack of excess fission xenon in 14063 and 14082 eliminates models relying on microscale xenon redistribution. A model is proposed in which the excess fission xenon originates in local material and percolates through and is adsorbed on regolith material.

Drozd, R. J.↗

NASA NTRS1977

Surface properties of a north ray crater soil /Apollo 16/

Surface properties of lunar fines sample 67481 have been investigated by measuring the adsorptions of nitrogen (at -196 C) and water (at 20 C). Characteristics of this sample are similar to those of samples from other locations on the lunar surface and include the more typical alteration reaction with adsorbed water. Although their maturities are markedly different, the surface properties of 67481 are very much like those of the more mature 63341 from the adjacent station 13. These results indicate that the surface properties of lunar soils attain an equilibrium state faster than other properties used to indicate maturity.

Holmes, H. F.↗

NASA NTRS1977

Low-work-function surfaces produced by cesium carbonate decomposition

Cesium carbonate (Cs2CO3) was heated to the decomposition temperature of approximately 600 C. The nonvolatile decomposition products were condensed on a nickel substrate while the carbon dioxide was removed by pumping. The deposited material is characterized by an effective work function of between 1.05 and 1.15 eV at 450 K and by photoemission in the visible and near-infrared region of the spectrum. It is suggested that the deposited material consists of Cs2O, possibly Cs2O2, and adsorbed cesium. Silver, evaporated from a heated silver bead, produced the typical photoemissive and thermionic properties of a silver-oxygen-cesium (S-1) photocathode. The material may be of interest for thermionic energy converters and for the formation of silver-oxygen-cesium photocathodes.

Briere, T. R.↗

NASA NTRS1977

The development and testing of a regenerable CO2 and humidity control system for Shuttle

A regenerable CO2 and humidity control system is presently being developed for potential use on Shuttle as an alternate to the baseline lithium hydroxide (LiOH) system. The system utilizes a sorbent material (designated 'HS-C') to adsorb CO2 and water vapor from the cabin atmosphere and desorb the CO2 and water vapor overboard when exposed to a space vacuum. Continuous operation is achieved by utilizing two beds which are alternately cycled between adsorption and desorption. This paper presents the significant hardware development and test accomplishments of the past year. A half-size breadboard system utilizing a flight configuration canister was successfully performance tested in simulated Shuttle missions. A vacuum desorption test provided considerable insight into the desorption phenomena and allowed a significant reduction of the Shuttle vacuum duct size. The fabrication and testing of a flight prototype canister and flight prototype vacuum valves have proven the feasibility of these full-size, flight-weight components.

Boehm, A. M.↗

NASA NTRS1977

Variation in the thermionic work function of semiconductor powders exposed to electromagnetic radiation

The study of the variation of thermoelectronic work function potential of TiO2 in the presence of isobutane shows that this gas is not adsorbed on this solid, in either the presence or the absence of ultraviolet radiation. These results, as well as those obtained in a previous work, lead to the mechanism of the photo-oxidation of isobutane at room temperature, in which excited atomic oxygen is the active species.

Bourasseau, S.↗

NASA NTRS1977

Effect of trichlorofluoromethane and molecular chlorine on ozone formation by simulated solar radiation

Mixtures of air with either Cl2 or CFCl3 were photolyzed in a reaction chamber by simulated solar radiation. Ozone formation was temporarily inhibited by Cl2 and permanently inhibited by CFCl3. A chemical mechanism including gas phase and wall reactions is proposed to explain these results. The CFCl3 is assumed to be adsorbed on the chamber walls and to poison the sites for Cl destruction.

Bittker, D. A.↗

NASA NTRS1977

Some developments in the feeding of coal to fluidized bed combustors

Research is being done in the development of fluid bed combustors for high sulphur coal, using limestone or dolomite in the bed for removal of the sulphur. Operating units to date have proven the inadequacies of available material handling techniques for introduction and control of the coal and adsorbent to the beds. Larger units now being contemplated will pose formidable problems in this area. Some of the techniques which were developed for the existing pilot units and novel ideas under consideration for future, large production units are illustrated and described.

Caldwell, L. G.↗

NASA NTRS1977

Flight prototype CO2 and humidity control system

A regenerable CO2 and humidity control system is presently being developed for potential use on the space shuttle as an alternative to the baseline lithium hydroxide system. The system utilizes a sorbent material (designated HS-C) to adsorb CO2 and water vapor from the cabin atmosphere and desorb the CO2 and water vapor overboard when exposed to a space vacuum. Continuous operation is achieved by utilizing two beds which are alternately cycled between adsorption and desorption. A shuttle vehicle integration study showed that the HS-C system offers substantial weight advantages compared to the baseline shuttle orbiter expendable lithium hydroxide CO2 removal system for extended missions beyond the nominal design of four men for seven days. This study defined a system packaging envelope in the area presently occupied by the LiOH cartridges.

Rudy, K. M.↗

NASA NTRS1978

Interaction of hydrogen chloride with alumina

The influence of temperature, pressure, and outgas conditions on the absorption of hydrogen chloride and water vapor on both alpha and gamma alumina was studied. Characterization of the adsorbents was performed using X-ray powder diffraction, scanning electron microscopy (SEM), low temperature nitrogen adsorption desorption measurements, BET nitrogen surface area measurements and electron spectroscopy for chemical analysis (ESCA). Water vapor adsorption isotherms at 30, 40, and 50 C were measured on alpha and gamma alumina after outgassing at 80, 200, and 400 C. Both outgas temperature and adsorption temperature influenced the adsorption of water vapor on the aluminas. The water vapor adsorption was completely reversible. Alpha alumina absorbed more water per unit area than gamma alumina. Differences in the adsorption capacity for water vapor of the two aluminas were explained on the basis of ideal surface models of alpha and gamma alumina. Isosteric heats of adsorption for water vapor on the aluminas were determined over a limited range of surface coverage.

Bailey, R. R.↗

NASA NTRS1978

Some effects of composition on friction and wear of graphite-fiber-reinforced polyimide liners in plain spherical bearings

Oscillating, plain spherical bearings with graphite-fiber-reinforced polyimide (GFRPI) liners were tested for friction and wear from 25 to 315 C. A condensation polymer was compared with an addition polymer, and a high-modulus fiber was compared with a lower cost, low-modulus fiber. All polymer-fiber combinations gave friction coefficients from 0.05 to 0.18 and low wear. Adding CdO and CdI2 reduced the wear of degassed bearings in dry air. These additives were not needed when the bearing liners contained adsorbed moisture. Although, at 25 C, MoS2 reduced the friction and wear of the base composite at unit loads above 70,000,000 N/m squared (10,000 psi), it had no beneficial effect at lighter loads.

Sliney, H. E.↗

NASA NTRS1978

The effect of load in a contact with boundary lubrication

The effect of the transition load on the wear in a contact with boundary lubrication was investigated. An experimental method was developed for this purpose, and parameters affecting the boundary lubrication under industrial operating conditions were identified. These parameters are the adsorbed boundary film, the contact microgeometry (surface roughness), macrogeometry, and hardness of materials used. It was found that the curve of the tops of the surface protrustion affect the transition load, and thus the boundary lubrication. The transition load also depends on the chemical nature of the contact and its geometrical and mechanical aspects.

Georges, J. M.↗

NASA NTRS1978

Effect of oxygen and nitrogen interactions on friction of single-crystal silicon carbide

Friction studies were conducted with single-crystal silicon carbide contacting silicon carbide and titanium after having been exposed to oxygen and nitrogen in various forms. After they had been sputter cleaned, the surfaces were (1) exposed to gaseous oxygen and nitrogen (adsorption), (2) ion bombarded with oxygen and nitrogen, or (3) reacted with oxygen (SiC only). Auger emission spectroscopy was used to determine the presence of oxygen and nitrogen. The results indicate that the surfaces of silicon carbide with reacted and ion-bombarded oxygen ions give higher coefficients of friction than do argon sputter-cleaned surfaces. The effects of oxygen on friction may be related to the relative chemical, thermodynamic properties of silicon, carbon, and titanium for oxygen. The adsorbed films of oxygen, nitrogen, and mixed gases of oxygen and nitrogen on sputter-cleaned, oxygen-ion bombarded, and oxygen-reacted surfaces generally reduce friction. Adsorption to silicon carbide is relatively weak.

Miyoshi, K.↗

NASA NTRS1978

Breadboard wash water renovation system

A total wash water renovation system concept was developed for removing objectionable materials from spacecraft wash water in order to make the water reusable. The breadboard model system described provides for pretreatment with ferric chloride to remove soap by chemical precipitation, carbon adsorption to remove trace dissolved organics, and ion exchange for removal of dissolved salts. The entire system was put into continuous operation and carefully monitored to assess overall efficiency and equipment maintenance problems that could be expected in actual use. In addition, the capacity of the carbon adsorbers and the ion-exchange resin was calculated and taken into consideration in the final evaluation of the system adequacy. The product water produced was well within the Tentative Wash Water Standards with regard to total organic carbon, conductivity, urea content, sodium chloride content, color, odor, and clarity.

Source record↗

NASA NTRS1977

The effects of cations and anions on hydrogen chemisorption at Pt

Experimental evidence based on linear sweep voltammetry is presented to substantiate the view that ionic adsorption substantially shifts electrode potentials in addition to the relative heights of the hydrogen adsorption peaks. HClO4 and HF are chosen as better reference electrolytes for anion studies. The voltammetry curves for 0.1M HF and 0.1M HClO4 as well as the effect of adding successively increasing amounts of H2SO4 to these electrolytes are discussed. The measurements are also extended to alkaline solutions. Mechanisms whereby the addition of various cations and anions to electrolytes such as HF and HClO4 can induce changes in the structure of the hydrogen adsorption region in the voltammetry curves are identified: (1) blocking of sites by anion adsorption and coupling of hydrogen adsorption and anion desorption, (2) modification in the hydrogen adsorption energies for sites adjacent to adsorbed anions, (3) changes in the potential distribution across the interface, and (4) surface restructuring.

Huang, J. C.↗

NASA NTRS1978

Chemical reactions in low-g

The Apollo-Soyuz flight experiment, 'Chemical Foams' demonstrated that foams and air/liquid dispersions are much more stable in low-gravity than on the ground. It thus should be possible to conduct unique chemical reactions in space foams. The low-g results and subsequent ground work on the formaldehyde clock reaction indicate that the reaction is strongly influenced by (1) dissociated and undissociated solution species being adsorbed at solid/liquid and gas/liquid surfaces and (2) chemical reaction rates apparently being affected by long-range forces determined by the liquid mass and the extent and nature of all surface interfaces.

Grodzka, P. G.↗

NASA NTRS1978

Mars - Regolith adsorption and the relative concentrations of atmospheric rare gases

Laboratory measurements of Kr and Xe adsorption on samples of ground montmorillonite, limonite and basalt at -77 C have been performed. Results suggest that (1) most degassed martian Xe could be adsorbed on a regolith with a large effective surface area, and (2) if this is the case the elemental composition of martian nonradiogenic rare gas (counting that in the regolith) may be virtually identical to that of ordinary chondrites.

Fanale, F. P.↗