The life of oxide cathodes in modern receiving valves

The paper attempts to integrate all information on oxide cathodes which may have a bearing on life span. Original research results are included in addition to published work from other sources. Two broad problems are distinguished as prerequisites to effective control of valve quality from the aspect of longevity. First, production processes must be more clearly understood in order that they may be closely related to valve life, and secondly, a short-term method for assessment of valve life must be developed. In considering a solution of the first problem, functional failure and cathode failure are recognized as the two main causes of valve faults. Only the latter is considered in detail, and this is subdivided into failures due to gas attack, to excessive interface feedback and to excessive evaporation of the activated cathode. Production processes (developed on a laboratory scale by the Post Office) which go far towards excluding gas attack are outlined, and valves have been produced which show no deterioration in total emission after 10 000 h of use. Interface feedback is identified with impurities in the core, and control of the impurity concentration may set a limit to the magnitude of the feedback. Cathode evaporation may be reduced to negligible amounts in gas-free valves by appropriate reduction of running temperature, which also achieves a considerable saving in heater power. The efficacy of these attempts to avoid valve failure is judged by the short-term methods for assessing valve life, which are advanced as solutions of the second broad problem. These take the form of short life-tests making use of total-emission measurements and a.c. measurements of interface resistance. The more usual measurements of space-charge-limited mutual conductance are used in correlation of the newer tests.