The surface of thermionic emitters was investigated as a function of temperature by means of XPS, Auger electron spectrosopy and the Kelvin method. The hexaborides were sensitive to oxidation at room temperature, a thin layer of rare earth oxide and boron oxide having been detected at their surfaces. This study revealed the influence of surface carbon upon the Auger line intensities of lanthanum and boron. Surface cleaning by ionic bombardment led to preferential erosion of the rare earth with respect to boron. It was concluded that the emission mechanism in hexaborides was an intrinsic property. A previous hypothesis, that the low work function was due to the presence of a layer of rare earth metal on the LaB6 substrate, was not confirmed.

Thermionic Emission Properties of LaB6 and CeB6 in Connection with their Surface States, Examination by XPS, Auger Spectroscopy and the Kelvin Method. Berrada, A., Mercurio, J.P., Etourneau, J., Alexandre, F., Theeten, J.B., Duc, T.M.: Surface Science, 1978, 72[1], 177-88