Thermionic Emission from Oxide Coated Cathodes

It is shown experimentally that with Ba/Sr oxide cathodes, the saturated emission depends on the core material in use, and there is some correlation between total coating resistance and saturated pulsed emission. The steady-state D. C. emission is lower than the pulsed emission, but the D. C. emission measured immediately on application of the anode voltage is similar to the pulsed emission. The difference is due to a decay effect with a time constant in the range 1/1,000-1/10 sec., which is similar in many respects to the decay effect studied in an earlier paper in conductivity measurements. It is shown that the application of semiconductor theory can give a consistent account of coating conductivity and emission, provided the concentration of barium in stoichiometric excess is about 3 × 10 17 atoms/cm 3 . This figure has some experimental support. It is shown that the presence of an interface layer of the type actually detected should cause decay phenomena with a time constant less than 1 μsec. and in most cases less than 1/10 μsec., so that the decay from pulsed to D. C. emission is not due to the capacity effects introduced by the presence of the layer. With thoria there is little difference thermionically when studied as a coating on tantalum or on tungsten, or as a ceramic tube, and the pulsed emission is about twice the D. C. emission. Experiment and theory indicate that thoria is an excess semiconductor containing a stoichiometric excess of about 10 18 atoms/cm 3 of thorium. There is very little sensitivity to oxygen, however, at temperatures above 1,850° K.