The conductivity of oxide cathodes. Part 16: Thermodynamic considerations of matrix stability

Under conditions of free evaporation in vacuo from a platinum substrate the sublimate from barium oxide is almost wholly oxide, whereas that from strontium oxide is a mixture containing a high proportion of metallic strontium. The present Part suggests that this difference in behaviour can be explained in terms of the energy difference between the heats of formation and sublimation of the two oxides. This hypothesis predicts that both calcium oxide and magnesium oxide should behave in a manner similar to strontium oxide, and experiments show that the prediction is correct. A further prediction suggests that barium oxide should itself behave like the other three alkaline-earth oxides if its evaporation is physically restricted, and this again turns out to be demonstrable. The hypothesis is finally used to provide a satisfactory explanation for the emission of excited strontium-metal vapour from a mixed-oxide matrix under heavy current loading.