Surface coating influence on scandate cathode performance

It was found that the electron emission from scandate-impregnated cathodes coated with W-scandia films depends on scandium concentration and film thickness. W-scandia cathodes with low Sc-concentration films (2.8 at.% Sc) produce high electron emission when film thickness is 670 nm. On the contrary, the cathodes with high Sc-concentration films (13, 22, or 33 at.% Sc) showed a tendency that the thinner film (10 nm) provided higher electron emission than a thicker film (30–220 nm). The electron emission dependence is related to surface scandia density. It was shown that almost all of the scandia in the high Sc-concentration films is segregated to the surface from the inside. A surface covered by too much scandia can not provide high electron emission, because the Ba–Sc–O monolayer on the scandia area might not produce a low work function. However, scandia segregation in a low Sc-concentration film might be less than that in a high Sc-concentration film, so a large tungsten area might remain on the cathode surface. If so, the Ba–Sc–O monolayer can be formed on the tungsten area and produce a low work function.