Study of the Enhanced Thermionic Emission of Molybdenum, Hafnium and Zirconium due to Dispenser Cathode Exposure

摘要

Hafnium, zirconium and molybdenum were studied in a tube environment to determine their levels of thermionic emission following exposure to a 4:1:1 dispenser cathode operated at a brightness temperature of 1050 degree. In order to obtain a curve of equilibrium emission density versus temperature, the temperature of the sample under study was varied above and below that of the source. Once equilibrium was reached at each temperature, the emission was recorded. Variations in surface temperature alter the Ba/BaO coverage, and hence the work function. The change in work function with coverage is much greater for low coverage and the measured emission therefore followed the typical S-shaped variation of metals placed in alkali and alkaline-earth vapor streams. From emission densities the effectiveness work functions of the surfaces studied were calculated. Before exposure to the source cathode, calculated effective work functions were in good agreement and published values. Thermal emission densities of hafnium and zirconium, after equilibrium exposure at the same temperature as the source, were found to be on the order of only a few microamperes.