Emission and evaporation characteristics of Ba_1.8 Sr_0.2 CaWO₆ emitters

Abstract

Emission and evaporation characteristics of Ba_1.8 Sr_0.2 CaWO₆ coated emitters are affected by the addition of aluminum oxide and yttrium oxide. An emitter consisting of a mixture of Ba_1.8 Sr_0.2 CaWO₆ and 11.5% (wt.) aluminum oxide has a higher emission current and a higher evaporation rate of active barium than does an emitter of the tungstate alone. X-ray diffraction analysis of the surface film of the emitter clearly indicates that this high evaporation rate is due to the presence of BaAl₂O₄ produced by the reaction of the tungstate with aluminum oxide. Yttrium oxide does not react with the tungstate. An emitter consisting of a mixture of Ba_1.8 Sr_0.2 CaWO₆ and 21.9% (wt.) yttrium oxide has a lower emission current and a lower barium evaporation rate than does an emitter of the tungstate alone, presumably because the rate of barium formation in the emitter decreases with the addition of yttrium oxide. When these mixtures are applied to the electrodes of 400-watt high-pressure mercury lamps, the light output maintenance of the lamps using the latter mixture is superior to those using the former mixture. This is related to the evaporation rates of the mixtures.