Study of niobium nitrides for superconducting rf cavities

This paper deals with the first results of a study on Nb-N system having as the main goal the performance of niobium nitride rf cavities. The nitrides were obtained by diffusion of nitrogen in bulk niobium at high temperature. Metallurgical and resistive measurements have been done to determine the phases and the critical temperature of the samples. Furthermore, in order to study the rf behavior of the material, a 4.5-GHz niobium nitride cavity has been built and tested. The rf measurements showed, above 4 K, a surface resistance in good agreement with the computed values (1.15 μΩ at 4.2 K), i.e., about a factor of 10 better than the value obtained for niobium at the same temperature and frequency. At temperatures below 4 K, the residual resistance became prevalent, and its value was found to be 0.69 μΩ.