Model scandate cathodes investigated by thermionic emission microscopy

Model scandate thermionic cathodes have been prepared by rf reactive sputtering of scandium oxide and barium oxide onto tungsten foil. By preparing separate, 200 nm thick layers of these oxides, sputtered through different sized masks, the function of barium oxide and scandium oxide can be identified directly in the thermionic electron emission image. Scandium oxide is sputtered through a 100 micron square mask, with barium oxide sputtered through a 25 micron square mask. The location and deposition order of the oxide layers can be varied and observed directly in the image. Emission current is measured by a Faraday cup located at the center of the detector used to form the thermionic emission image. Based on these images, we show that the scandium oxide-barium oxide thermionic cathode operates due to a two-step work function reduction mechanism.