Work Function of Uranium Carbide

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URANIUM carbide has assumed importance recently as a cathode material for use in a thermionic converter. In this context it combines the advantage of stability in an atmosphere of caesium (though recent American reports indicate that this may not be the case), which will probably be required for the neutralization of space charge, with the possibility of fission heating in a neutron flux. Uranium carbide has been claimed to have good emission properties and at least two attempts have been made to determine the thermionic constants1,2. Work functions of 4.57 and 2.94 eV. have been reported; the former for bulk uranium carbide1, the latter for the powder deposited on a tungsten filament2. In both cases high accelerating potentials were used and the zero field emission was determined by the extrapolation of the linear region of a Schottky plot which is found at the highest fields. The application of Richardson's equation then produced the work functions detailed here. Criticism of most of the standard techniques for the measurement of work function is not difficult to produce, especially when patchy cathodes are involved3,4. The aim of the present work, part of which is described in this communication, is to use a technique which is subject to fewer objections than can be raised against the previous measurements of the work function of uranium carbide. To this end, the Zisman modification of the Kelvin vibrating condenser technique has been applied to measure the contact potential difference between a clean tungsten reference surface and the uranium carbide cathode.