The effect of oxygen pressure on volatility and morphology of LaB6 single crystal cathodes

The effect of oxygen pressure on the volatility and morphology of single crystal LaB6 cathodes, heated to different temperatures, was investigated. At a temperature of 1600 K, an increase of oxygen pressure from 1 x 10 to the -8th torr to 1 x 10 to the -6th torr has led to a 100-fold enhancement in cathode volatility. The enhancement effect of oxygen pressure diminished with increasing temperature: at a cathode operating temperature of 1900 K, the volatility enhancement due to the same oxygen pressure was negligible. It was shown that the faceting frequently observed during evaporation of conically shaped emitters is due to a crystallograpic anisotropy of the oxidation rate of LaB6. No facet formation occurs during evaporation at oxygen pressures below -110 to the -8th torr.