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Davis, P. R.

Publications and source records attributed to Davis, P. R..

9 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

The effect of oxygen pressure on volatility and morphology of LaB6 single crystal cathodes

The effect of oxygen pressure on the volatility and morphology of single crystal LaB6 cathodes, heated to different temperatures, was investigated. At a temperature of 1600 K, an increase of oxygen pressure from 1 x 10 to the -8th torr to 1 x 10 to the -6th torr has led to a 100-fold enhancement in cathode volatility. The enhancement effect of oxygen pressure diminished with increasing temperature: at a cathode operating temperature of 1900 K, the volatility enhancement due to the same oxygen pressure was negligible. It was shown that the faceting frequently observed during evaporation of conically shaped emitters is due to a crystallograpic anisotropy of the oxidation rate of LaB6. No facet formation occurs during evaporation at oxygen pressures below -110 to the -8th torr.

Davis, P. R.↗
NASA NTRS1984

Evaluation of single crystal LaB6 cathodes for use in a high frequency backward wave oscillator tube

The results of thermionic emission and evaporation studies of single crystal LaB6 cathodes are given. A comparison between the (100), (210) and (310) crystal planes shows the (310) and (210) planes to possess a work function approx 0.2 eV lower than (100). This translates into a significant increase in current density, J, at a specified temperature. Comparison with a state-of-the-art impregnated dispenser cathode shows that LaB6 (310) is a superior cathode in nearly all respects except operating temperature at j 10 A/sq cm. The 1600 K thermionic and room temperature retarding potential work functions for LaB6 (310) are 2.42 and 2.50 respectively.

Swanson, L. W.↗
NASA NTRS1980

Fabrication and surface characterization of composite refractory compounds suitable for thermionic converters

Thermal faceting was observed for the high index planes of LaB6. The (100), (110), and (111) planes were found to be the most thermodynamically stable faces in vacuum in a study of electrode materials for thermionic emitters. The properties of adsorbed carbon, cesium, and cesium-oxygen layers were investigated on LaB6 single crystal surfaces as well as on Zr/0/W(100) and W(100). Cesium was found to increase electron reflection near the collision threshold on LaB6(100) and W(100) and to decrease the reflection on Zr/0/W(100). This difference may be explained by the unusually high threshold reflection coefficient of Zr/0/W without adsorbed cesium.

Davis, P. R.↗
NASA NTRS1980

The adsorption of cesium on lanthanum hexaboride surfaces

The adsorption/desorption characteristics of cesium on clean and oxygen-covered LaB6 (100) surfaces were studied using various surface analysis techniques. On the initially clean surface (phi = 2.77 eV), adsorption produces a minimum work function of 1.96 eV and a saturation work function of 2.07 eV. For the oxygen-saturated surface, the cesium adsorption curve shows no minimum, the lowest work function (1.35 eV) occurring at cesium saturation. The utility of LaB6 (100) as an emitter or collector in thermionic converter applications (particularly for space nuclear electric propulsion) is evaluated.

Davis, P. R.↗
NASA NTRS1979

Fabrication and surface characterization of composite refractory compounds suitable for thermionic converters

The techniques of fabricating and characterizing the surface properties of electrode materials were investigated. The basic surface properties of these materials were studied with respect to their utilization as thermionic energy converter electrodes. Emphasis was placed on those factors (e.g, cesium disorption kinetic and mechanisms of low work function production) which are of primary concern to thermionic converter performance.

Davis, P. R.↗
NASA NTRS1978

The co-adsorption of copper and oxygen on a tungsten 100 plane-type surface

The coadsorption of Cu on O2 and a W 100 plane-type surface is studied by Auger electron spectroscopy, thermal desorption, low energy electron diffraction and by work function change measurements. It is shown that the presence of Cu on the surface initially decreases the sticking coefficient of O2. For longer oxygen exposures and for higher adsorption temperatures, the coverage of preadsorbed oxygen reaches values larger than those on the clean surface for the same O2 exposure. Except at the highest values and temperatures of the coverage of preadsorbed oxygen, the sticking coefficient for copper is unity and is independent of the oxygen coverage in the range studied. Coadsorption at room temperatures does not produce any long range order while coadsorption at elevated temperature leads to ordered structures. The saturation coverage of the two dimensional coadsorbate at 800 K is given by a relation. The work function is a complicated function of the coverage of preadsorbed oxygen and the coverage of preadsorbed Cu and is determined predominantly by the temperature at which oxygen is adsorbed. At high temperatures the sequence of adsorption has no influence, in contrast to the room temperature behavior.

Bauer, E.↗
NASA NTRS1977

The adsorption and early stages of condensation of Ag and Au on W single-crystal surfaces

Silver and gold were evaporated up to several monolayers in thickness onto W single-crystal surfaces of the 110 and 100 plane types in the temperature range between 300 and 1300 K. The mode of growth and the structure of the layers were studied by Auger electron spectroscopy and low-energy electron diffraction. The results are compared with thermal desorption spectroscopy data and work-function change measurements. They show certain similarities but also some striking differences between the two systems Ag/W and Au/W as well as a strong crystallographic anisotropy of adsorption and condensation.

Bauer, E.↗