High Current Capacity Joints of HTSC and Normal Metals

Abstract

Design considerations and experimental data of high current capacity contact joints between monolithic ceramic high temperature superconductor (HTSC) and normal metals are presented. Practical applications of HTSC in devices such as cryogenic current leads require the ability to fabricate low cost HTSC/normal metal joints which can carry currents in excess of hundreds of amperes and with minimal Joule heating loss. Contact designs to achieve this goal require considerations of current transfer in the contact interface, current diffusion in the normal metal, heat transfer characteristics, as well as the thermophysical properties of HTSC and normal metals in the temperature range covering contact fabrication and device operation. Low cost contact joints of BSCCO (2212) to copper terminal (with silver sheet interface to BSCCO) have been fabricated, tested and implemented in prototype vapor-cooled current leads employing high temperature superconductors. These contacts showed overall contact joint resistances in the µΩ range when carrying up to 180A of dc current in LN₂, and these resistances were found to decrease significantly with the decrease of temperature, i.e., the joints have a metallic characteristic. In addition, the contact joint resistances degrade only marginally after 60 cycles of fast thermal cycling between room temperature and LN₂ temperature. These data demonstrate the feasibility of the joint design for power device applications.