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Model thermionic cathodes with different oxide coatings have been surveyed using a combination of photoelectron emission microscopy and thermionic electron emission microscopy. The model cathodes are imaged directly at operating temperatures and the electron yield is measured with a Faraday cup located in the image plane. Cathodes with combinations of Ba, Sc, BaO, Sc2O3, and pulsed-laser-deposited Sc2O3 on W substrates have been examined. BaO deposited on Sc2O3 is the most stable cathode combination. Measurement of the W–Sc2O3–BaO cathode shows that the electron yield enhancement produced by BaO and Sc2O3 is a two-step process which requires that the BaO be on top of the Sc2O3. BaO does not need to be in direct contact with the W substrate.

Crystalline compounds found at the surface of model Ba-Sc-O-W thermionic cathodes (“scandate”) are uniquely identified using Raman spectroscopy. Thin films of sputtered BaO and Sc2O3 on W have been observed in thermionic emission microscopy, field emission scanning electron microscopy, optical microscopy, and Raman Spectroscopy. While the best thermionic electron emission is observed from areas that at the end of the cathode life are completely devoid of thin film BaO, Sc2O3 or observable bulk oxide or tungstate material, the poor emission areas are characterized by BaWO4, Ba2WO5 and long chain linear tungstates (νas = 860 cm−1) that are related to Scx-WOy components. There is no evidence from Raman spectroscopy that tetrahedral Sc2(WO4)3 is present or forms on the surface of the model cathode, or for the presence of Ba3WO6.