Influence of oxide layers on the microwave surface resistance of niobium

We have investigated the effects of oxide layers on the microwave surface resistance of 8.6 Ghz superconducting niobium cavities. Cavities were UHV fired to remove oxides and then tested before and after exposure to oxygen. Microwave measurements were made on oxide layers grown and maintained room temperature and on layers which had been heated to 300C. The contribution to residual resistance from oxide layers kept at or below room temperature is less than 2 or 3 n-ohm. Heated layers showed a 20% decrease in B.C.S. resistance, and a 40 n-ohm increase in residual resistance. Heating unoxidized cavities to 300C has no effect, demonstrating that these cavities were actually oxide free.