Chemical stability of barium calcium aluminate dispenser cathode impregnants

Abstract

A thermodynamic model has been formulated to describe the effects of temperature and composition on the chemistry of barium and barium oxide (Ba/BaO) generation from dispenser cathode impregnants. Our approach is to make use of the BaO-CaO-Al 2O 3 phase diagram and BaO vapor pressures of impregnant phases to calculate the BaO vapor pressure of the impregnant. This BaO vapor pressure is used to calculate the Ba vapor pressure resulting from reactions between BaO vapor and the tungsten cathode. The Ba/BaO vapor pressures are used to calculate Ba/BaO generation and evaporation rates from a wide range of impregnant compositions. Ba/BaO evaporation rates from dispenser cathodes, reported in the literature over a range of Ba and calcium (Ca) concentrations, are consistent with the rates predicted from this study. Two trends are evident. First, increasing the Ba concentration in the impregnant generally forms high vapor pressure Ba-rich phases leading to high Ba/BaO evaporation rates. Second, increasing Ca concentration in the impregnant lowers the Ba/BaO vapor pressures by forming barium calcium aluminate solid solutions. Ca has little effect on Ba/BaO vapor pressure once its solubility limit is reached and a separate CaO phase is formed. This model illustrates the possibility of extending cathode life by selecting impregnants composed of fairly stable solid-solution phases for use in cathodes that must be operated at high temperature for high current density.