Work function measurements in experimental tubes containing cathodes of uranium and zirconium carbides

Abstract

Work function measurements are reported for carburized tantalum cathodes coated electrophoretically with uranium and zirconium carbides. Thermionic methods for the determination of work function have been used together with the Zisman modification of the Kelvin technique. The poor agreement found between the two sets of results is thought to indicate that thermionic emission takes place from only a fraction of the total cathode area. Work function measurements have also been made on the thin films that evaporate from these cathodes during their operation at high temperatures on to fized anodes. When the cathode is uranium carbide, our measurements can be interpreted in terms of an evaporated film of uranium metal and uranium monoxide on the anode surface. All of this work has been performed under ultra-high vacuum conditions. The mean work function values for tubes containing uranium carbide cathodes are as follows: the work function from the Kelvin measurements at 300°K is 3·81 ± 0·01 eV and that from the slope of a Richardson line plotted from retarding field data and applying to 0°K is 2·9 ± 0·1 eV. The temperature coefficient of the work function of uranium carbide is 5 × 10 −4 eV per deg. For zirconium carbide cathodes the mean values are: 3·62 ± 0·01, 2·5 ± 0·1 and 7 × 10 −4 eV per deg. The work function of the anode in a tube containing a uranium carbide cathode reaches a constant value in a few hours of 3·0 ± 0·1 eV. This, like the Kelvin results quoted above, is an arithmetic average over the area of any high and low work function patches and in this case applied to the unmeasured temperature of the anode when the cathode was operating at 1300°K.