Chemical Reactions in Barium Oxide on Tungsten Emitters

Abstract

When barium carbonate is heated in vacuum in contact with tungsten, the following reactions may occur in sequence as progressively higher temperatures are reached: (1) WO₃+BaCO₃⇄BaWO₄+CO₂. (2) 3BaCO₃+W⇄Ba₃WO₆+3CO. (3) BaCO₃⇄BaO+CO₂. (4) 6BaO+W⇄Ba₃WO₆+3Ba. At a temperature of approximately 600°C, reaction (2) begins to occur with appreciable speed. Reaction (3) proceeds at 800–900°C. Reactions (2) and (3) may be made to proceed simultaneously, the preponderance of one over the other depending upon state of subdivision and intimacy of admixture of the reactants. Under favorable conditions, reaction (2) may be driven to completion with almost total absence of reaction (3). Reaction (4) progresses with appreciable speed at temperatures over approximately 1000°C. Activated cathodes of BaO on W will have an interface of Ba₃WO₆ formed by reactions (2) or (4), or both, unless unusual precautions are taken to avoid this. The end result of heating the cathode is the conversion of the entire thickness of the oxide coating to the basic tungstate. The resulting coating of Ba₃WO₆ on W is unreactive and nonvolatile at temperatures up to 1300°C, and provides no significant thermionic emission.