A fundamental behavior on mechanical contacts of YBCO bulks

A persistent current switch (PCS) is a key component for superconducting magnetic energy storage (SMES) system. In past research on a mechanical PCS, a metallic superconductor made of Nb and NbTi is used as a switching material. However, recently the research on SMES coil of a high temperature superconductor (HTS) is widely carried out, in which a PCS is also expected to work in liquid nitrogen temperature. In this paper, yttrium barium copper oxide (YBCO) bulk of HTS was proposed as a switch material, and the test on the fundamental characteristics of a mechanical contact between two YBCO bulks was carried out. As a result, a switching phenomenon between low and high resistance ranges was observed at a threshold current value in the V-I characteristics of the YBCO contact. Furthermore, the tests indicate that a certain amount of current needs to flow before the switching phenomenon. Because the discontinuous transition occurred after the surface-insulating layer formed by degradation of YBCO was destroyed due to current effect in the contact area.