Thermionic Emission from Sintered Cathode of Thoria and Tungsten Mixture

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Abstract
To obtain suitable conductivity for direct heating, tungsten powder is added to sintered thoria cathodes. The thermionic properties of a cathode, sintered from a mixture of 67 percent thoria and 33 percent tungsten, were investigated. Emission constants were determined and the emission was found to be somewhat lower than that of cathodes sintered from pure thoria. Change of cathode activity with temperature was observed and studied in detail. The cathode showed high activity at temperatures which would bring about deactivation of ordinary thoriated tungsten and decreased its activity when the temperature was lowered to the range where thoriated tungsten would be activated. A theory is proposed for the mechanism of the change in activity, involving the production of free thorium due to reduction of thoria by tungsten, diffusion and evaporation of thorium from thoria. Evaporation of thorium as well as thoria was proved by the use of a tungsten filament probe which detected evaporation of materials from the cathode by the change of its own emission. The energy of evaporation of free thorium from thoria is estimated from the experiments to be 46600 cal./g‐atom. The energy of evaporation of thoria is found to be 184000 cal./g‐molecule. The behavior of the cathode is satisfactorily explained by the proposed theory.