CAO SURFACE SEGREGATION AS AN EMISSION-LOSS MECHANISM

The formation of high work function patches on a cathode surface can lead to cathode failures in certain applications even though the cathode as a whole may show adequate overall emission properties. This type of failure was found for an oxide cathode used in a Vidicon tube wherein the beam current originated from a small portion of the cathode surface. Surface analytical techniques such as AES, used in conjuction with SLEEP, showed that the high work function areas could be correlated with high surface calcium concentration and low surface barium concentration. Auger sputter profiling also revealed that these calcium-rich regions (high work function patches) are actually thin surface layers with a thickness the order of 100 Å while the bulk cathode composition appears to be uniform. A possible explanation for these patches is the formation of a CaO slag on the surface caused by BaO/BaCO3 eutectic melting during the conversion process. © 1979.