Electron emission from oxide coated cathodes

Abstract

The work has been mainly concerned with the irregularities which are found in the electron images of emitting oxide cathodes. An electron emission microscope has been designed to produce pictures at magnifications of up to x 120. Electron pictures from oxide cathodes have been compared with the optical picture of the cathode surface and a correlation has been found to exist between the two types of picture from the same part of a cathode. This result is in contradiction to the findings of earlier investigators. In view of this new evidence of the correlation, the causes of irregularities in the emission picture have been reconsidered and a revised list of the possible causes has been given.In an attempt to produce a cathode which emits uniformly, a technique has been developed for coating the core of the cathode. The polished core is coated by evaporating the oxide on to it. The cathodes produced in this way had smooth surfaces and uniform emission pictures were obtained. The emission pictures did not remain uniform but altered in character during the life of the cathode. Experiments were performed to establish the reasons for the observed sequence of emission pictures.A theoretical explanation of the emission pictures hasbeen attempted using a semiconductor model for the oxide coating. An equation for the saturated emission current is derived for different forms of the semiconductor model and it is shown that all the models lead to an equation of the form [equation] where A, n and O are constants which depend on the model used. The emission from a metal coated with a semiconductor is considered and it is shown that with a model of the contact between the metal and the semiconductor which assumes space charge barriers, an explanation of the sequence of emission patterns is possible.