Contact resistance characteristics of YBaCuO superconductors with deposited metal layer

A persistent current switch (PCS) is used for superconducting applications, such as superconducting magnetic energy storage (SMES) system. The authors have proposed a mechanical switch of Y-Ba-Cu-O (YBCO) bulk as a mechanical PCS. From the previous study, it turned out that there were improvements of reducing contact resistance and load in the YBCO switch. The sample surfaces were carefully polished and deposited with silver in order to reduce the contact resistance. As a result, the transport current exceeded 50A, and the contact resistance reduced to 0.64μΩ with applying constant load of 500N. The resistance was reduced by increasing the thickness of the deposited metal layers, but it saturated when the thickness reached a certain level. In this paper, the authors measured the contact resistance of Ag block with convex curvature and Ag, YBCO sample, and Agdeposited YBCO sample (dAg/YBCO) to clarify the main source of the resistance. The study was performed with the development of a PCS in mind. The contact resistance was the largest in the Ag/YBCO contact. It was lowered with increasing the thickness of deposited-Ag layer for the Ag/dAg/YBCO contact. However, the smallest resistance of Ag/dAg/YBCO was even larger than that of the Ag/Ag contact. This is probably due to the fact that Ag/dAg/YBCO contact has two interfaces. The present result suggests that the contact area is dominant in controlling the contact resistance. Simulation study of the contact area with a finite element method (FEM) also supported this idea.