STUDY OF THE OXIDE CATHODE IN DEMOUNTABLE VACUUM SYSTEMS

摘要

A study has been made of the decay in emission of oxide cathodes as they are repeatedly exposed to air. The results indicate that the decay is largely due to flaking of the oxide coating and can be greatly reduced by keeping the cathode above 100°C during the air exposure. This pre?vents the formation of the hydrate and allows the oxide to be converted only to the simple hydroxide, thereby decreasing the distortion in the crys-tal. A slight poisoning effect is observed due to the buildup of evapora-tion products on the anode if the tube parts cannot be cleaned or baked after each air exposure. However, this can be somewhat reduced by starting the cathode from the hydroxide rather than from the carbonate. Even if none of these precautions are taken, the oxide cathodes would not decay to less than half their initial value after twelve air exposures, whereas in tungsten matrix cathodes operated under similar conditions, this occurred after the second exposure. The formation of the simple hydroxide and subsequent conversion back to the oxide does not appear to appreciably change the thermionic capabilities if the poisoning effects are minimized by baking, since the emission level, even after twelve exposures to air, is comparable to that of active oxide cathodes which have never been opened to the atmosphere.