Thermionic emission properties of LaB6 and CeB6 in connection with their surface states, examination by XPS, Auger spectroscopy and the Kelvin method

Abstract The surface of the thermionic emitters LaB 6 and CeB 6 was investigated as a function of temperature by means of XPS, Auger electron spectrosopy and the Kelvin method. The hexaborides are sensitive to oxidation at room temperature, a thin layer of rare earth oxide and boron oxide having been detected at their surfaces. This study has shown the influence of surface carbon on the Auger line intensities of lanthanum and boron in the case of LaB 6 . A surface cleaning by ionic bombardment leads to a preferential erosion of the rare earth with respect to boron. One may conclude that the emission mechanism in hexaborides is an intrinsic property of the material. The hypothesis previously proposed by other authors, that the low work function may be due to the presence of a layer of rare earth metal on the LaB 6 substrate was not confirmed.