Evaporation of Barium from Cathodes Impregnated with Barium–Calcium–Aluminate

ABSTRACT The evaporation of barium from cathodes impregnated with barium-calcium-aluminate, 3BaO. Al2O3½CaO has been observed during the life of cathodes made with various thicknesses of two types of porous tungsten. The slow decay observed in thin cathodes of very porous tungsten appears to be duo mainly to the reaction producing barium being retarded by the reaction products accumulating at the interface between the tungsten and the impregnant. The more rapid decrease for thick cathodes of lower porosity is due to the escape of barium becoming the limiting factor. This results in the reaction going to completion at increasingly greater depths within the cathode during life. The transport of barium to the surface of the cathode appears to be mainly by migration along the crystal boundaries in the impregnant and reaction products.