Surface studies have been made of multilayer and monolayer films of barium and barium oxide on a tungsten substrate. The purpose of the investigation was to synthesize the surface conditions that exist on an activated impregnated tungsten cathode and obtain a better understanding of the mechanism of operation of such cathodes. The techniques employed in these measurements were Auger spectroscopy and work-function measurements. The results of this study show that the surface of an impregnated cathode is identical to that observed for a synthesized monolayer or partial monolayer of barium on oxidized tungsten, by evaluating Auger spectra and work-function measurements. Data obtained from desorption studies of barium monolayers on a tungsten substrate in conjunction with Auger and work-function results have been interpreted to show that throughout most of its life an impregnated cathode has a partial monolayer, rather than a monolayer, of barium on its surface.

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This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 155.33.16.124 On: Sat, 29 Nov 2014 08:17:22 82 C. A. NEUGEBAUER AND M. B. WEBBeffect. If one prepares a series of increasingly thicker films, which consist of increasingly bigger particles, the "free space"between particles not filled with metal will decrease. This means that for any applied field across the film, the thicker films will actually have a much higher E. since there are fewer particles and therefore a much greater voltage drop across each gap. Equation (A3) would then predict that, at any one temperature, the conductance of the thick films should change more rapidly with applied voltage than the thinner ones, since E. changes more rapidly with voltage. This is indeed always observed, and is illustrated in Fig. 11, where the change of conductance with field is plotted logarithmically against the square root of the applied voltage for nickel films of different thickness at 77°K. Small tantalum carbide objects may be made by heating the objects made from tantalum in dilute hydrocarbon atmospheres at temperatures > 20000K. Carburization proceeds quite rapidly above 2300 0 K by diffusion of carbon initially deposited on the surface into the metal, principally along grain boundaries, to form a thin layer of Ta2C which is quickly converted to the TaC. The latter has a tensile strength of 21-25 kg/mm!, a work function <1>=3.17 ev, an emissivity of 0.40-0.46 between 2400 0 and 3500 o K, rising with increasing temperature, and a resistivity p = 102+0.0178 T (OK) ohm em between 1400° and 3500°K.

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