Studies on the Mechanism of Operation of the L Cathode. II

An experimental study of the L cathode is presented, including the chemistry of carbonate decomposition and barium generation, the origin of BaO in the evaporant, the barium transport mechanism through the porous plug, and the factors determining cathode life. Carbonate decomposition is accompanied in low porosity cathodes by the deleterious reaction $${\mathrm{BaCO}}_3+\frac{1}{3}\mathrm{W}=\frac{1}{3}{\mathrm{Ba}}_3{\mathrm{WO}}_6+\mathrm{CO}$$ . Barium is generated in the cavity at an equilibrium pressure by the reaction $$2\mathrm{BaO}+\frac{1}{3}\mathrm{W}=\frac{1}{3}{\mathrm{Ba}}_3{\mathrm{WO}}_6+\mathrm{Ba}$$ and is transported continuously through the porous tungsten predominantly by Knudsen flow. During transport the barium acquires oxygen from the tungsten, producing in the evaporant a large BaO content which helps to activate the surface. Termination of life coincides with completion of the BaO–W reaction and onset of the subsequent reaction $$\frac{2}{3}{\mathrm{Ba}}_3{\mathrm{WO}}_6+\frac{1}{3}\mathrm{W}={\mathrm{BaWO}}_4+\mathrm{Ba}$$ , which is accompanied by release of a poisoning agent. Material decrease in evaporation rate and a corresponding increase in cathode life may be gained by addition of SrCO₃.