On the Mechanism of Operation of the Barium Aluminate Impregnated Cathode

Emission and evaporation characteristics of a porous tungsten cathode impregnated with the composition 5BaO·2Al₂O₃ are presented and are interpreted in terms of the cathode mechanism. Barium necessary for activation is generated by the reaction, $$\frac{2}{3}\hspace{0.17em}{\mathrm{Ba}}_3{\mathrm{Al}}_2{\mathrm{O}}_6+\frac{1}{3}\hspace{0.17em}\mathrm{W}=\frac{1}{3}\hspace{0.17em}{\mathrm{BaWO}}_4+\frac{2}{3}\hspace{0.17em}{\mathrm{BaAl}}_2{\mathrm{O}}_4+\mathrm{Ba}$$ , and is transported through partially clogged pores, the length of which increases with time, predominantly via Knudsen flow. During transport, oxygen is acquired from the tungsten, leading to a substantial content of BaO in the evaporant. The BaAl₂O₄ component of the impregnant is inert. Emission is substantially lower than that of an L cathode, presumably because of release of a poisoning agent accompanying the activator.