NbN Materials Development for Practical Superconducting Devices

摘要

Power switches such as a Superconducting Fault Current Limiter require large cross sectional area superconductors with both high critical current density J/sub c/ and normal state resistivity rho/sub n/. Large values of J/sub c/ and rho/sub n/ have been previously reported in small cross sectional area weak links of NbN. We report on reactively sputtered NbN films up to 5 mu m thick and 2.2 cm wide which have rho/sub n/ > 200 mu omega cm and a self-field J/sub c/ up to 10 exp 6 A/cm exp 2 . Severe degradation in J/sub c/ was observed with increasing film width and for millisecond current pulses. This degradation could be substantially reduced by stabilization with either low rho/sub n/ normal metal or the use of a sapphire substrate. The resistivity and critical current dependence both imply Josephson coupled grains and the results will be discussed within that model. (ERA citation 81:026017)