Evaporation and diffusion rate measurements on cathodes of sintered nickel containing alkaline-earth oxides

The evaporation rate of barium from a sintered nickel cathode and the diffusion rate of barium through it have been measured using 140Ba and tracer techniques. Confirmation of the evaporation rates thus determined have been obtained by comparison with measurements made by a method due to Becker. The activation energy of evaporation is 2.2 to 2.5 eV. Two diffusion processes operate, one predominant in the range 875 to 1020° C and one below 875° C. The activation energies are 2.5 and 0.5 eV respectively. The mechanism of diffusion at the higher temperatures appears to be Knudsen flow following evaporation from the inner surfaces, and that for the lower temperatures probably by surface diffusion.