The Use of Radioactive Isotopes in a Study of Evaporation from Thermionic Cathodes

Abstract

Radioactive tracers have been used in a study of evaporation of the alkaline earths, barium, strontium, and calcium from oxide coatings on passive nickel bases. Several percent of the total cathode Ba, Sr, and Ca leave a triple oxide cathode during processing and aging. The rates of transfer of these elements from the cathode during life tests of 118 hours at true cathode temperatures from 1175 to 1279°K obey equations of the form, logW=A−(B/T), where W is the rate of transfer in g/cm²/sec, T is the absolute temperature of the cathode, A and B are constants. The transfer of Ba from a BaO cathode obeys a similar law. The mass of Ba transferred in 118 hours of life from a triple oxide cathode at 1123°K true as 2.9×10⁻⁶g/cm², and from a BaO cathode 1.4×10⁻⁵g/cm². The masses of Sr and Ca transferred from a triple oxide cathode at the same temperature were 2.1×10⁻⁷ and 1.2×10⁻⁸g/cm², respectively. Bombardment of the anode by an electron beam during life tests decreased the quantity of Sr observed on the anode. Anode current densities of 5 to 10 ma/cm² at electron energies of approximately 100 volts produced a fiftyfold reduction in the quantity of Sr on the anode. Experiments indicated that this decrease was not due to heating of the anode. The quantity of Ba deposited on the anode was not decreased by electron bombardment. The very considerable transfer of Ba from the cathode during processing was greatly reduced by breaking down the carbonates and high temperature flashing the cathodes in the presence of a few mm pressure of argon gas.