Improved Thermionic Emitter Using Uniaxially Oriented Tungsten

Abstract

Emission from most practical electron sources is nonuniform over the emitting area. Ordinary metallic surfaces are polycrystalline and their emission comes from many different small crystal facets whose work functions can vary over a wide range of values. As an example, the range of work functions of tungsten goes from a low of 4.3 eV for the (116) face to a high of 5.9 eV for the (110) face. Substantially improved uniformity of work function and current density would result if the entire emitting surface contained only a single‐crystal plane. The use of a single crystal as a means of obtaining such a surface is costly and wasteful since its unique bulk properties are superfluous and do not in any way contribute towards the improved performance. Even more important, using a single crystal inherently limits one to planar geometries. Many polycrystalline materials of arbitrary shape, among them the refractory metals, when prepared by chemical vapor deposition can be made to exhibit a high degree of uniaxial crystal orientation with their preferred axis centered about the direction of growth. Given proper treatment after deposition, the surface of such materials predominantly consists of a single‐crystal plane. In this paper, X‐ray, optical, and thermionic‐emission measurements are described which were made on vapor‐deposited tungsten prepared by the hydrogen reduction of tungsten hexafluoride. By comparing volt‐ampere characteristics of high‐vacuum diodes, one of which contains a conventional tungsten filament and the other a chemically vapor deposited one, of identical dimensions, it is found that there is an expected improvement in emission uniformity due to the uniaxial grain orientation in the latter. By a similar comparison of cesium‐neutralized energy‐converter diodes, it is shown that the improved surface uniformity results in significantly improved power quadrant characteristics.