Excess Ba Content of Practical Oxide Coated Cathodes and Thermionic Emission

Using methods given in paper I, emission and Ba content are given on (a) 125 coated cathodes, (b) 15 coated filaments; H2 evolution by 19 uncoated cathodes was also measured. For analysis, each was isolated from the remainder of its tube. Coatings varied in composition from pure SrO to pure BaO and in density from 1 to 5 mg per cm2. Ba was also measured in cathode coatings removed from their support metal. Essentially all cathodes contained excess Ba, but no cathode structure gave any correlation between Ba content and emission. These experiments contribute the following toward understanding practical oxide cathodes: If excess Ba (or Sr) is important, the significant amounts are probably less than one atom of Ba in 106 molecules of (BaSr)O. Because essentially all cathodes, including some with extremely low emission, contained at least this amount, we believe that performance is seldom limited by excess Ba. The concentration of Ba (or Sr) was not affected by using pure BaO or SrO coatings or Pt supports, although each of these steps led to very low emission; the highest emissions were obtained by operating at relatively low temperature during life, but this retained no more Ba in the coat. These major effects require interpretation.