Fault current limiting properties of YBCO-films on sapphire substrates
We have studied the switching of YBCO thin film resistive fault current limiting devices. Films of 300 nm thickness were deposited on 2 inch and 4 inch sapphire substrates by thermal co-evaporation. Bridges 10 mm wide and 22 mm long (2 inch) or 42 mm long (4 inch) were structured by standard photolithography. Contacts were made by in-situ gold overlayers and soft solder. The gold film was removed from the switching area so that the YBCO film was not shunted. The films were tested by 30 /spl mu/s DC pulses and 50 Hz AC pulses for 50 ms. We find evidence that at the AC tests heat propagates over several cm under these conditions in sapphire so that hot spots can be avoided even without shunt layer with the prospect of higher switching power. The highest destruction free switching power-the RMS critical current times RMS voltage after switching-was 57 kVA. The highest switching power density achieved was 2.5 kVA/cm/sup 2/. To our knowledge, these are the highest switching power densities observed so far.