The outgassing properties of mica with particular reference to the life performance of thermionic valves

Abstract Muscovite (ruby) mica, used to locate the various electrodes in a thermionic valve, has been found to be the largest potential source of gas in a valve. Total gas quantities of about 5000 ml. Torr g −1 have been measured. Most of this gas is evolved at temperatures above 750°C when the mica decomposes and exfoliates. By the use of clamps the exfoliation can be avoided and the mica outgassed completely. Using such outgassed mica spacers in thermionic valves it is found that there is no improvement in performance when compared with valves using mica that has not been de-gassed—even though a small part of the mica is in contact with the cathode core operating at 730°C. It is deduced that the gas evolution from the standard processed mica spacers during normal running of the valve must be negligibly small. However, the presence of mica does limit the degree of processing that valves can be subjected to on the pump though the worst effects of outgassing can be avoided by careful mechanical design.