Performance degradation due to thermal cycling in soldered Faraday and SuperPower HTS tape stacks

Abstract

Soldering stacks of high temperature superconducting (HTS) tapes results in a current carrying matrix with high electrical, thermal, and mechanical stability. In some applications, these tape stacks may be exposed to elevated temperatures during thermal heat cycles. Previous research on thermal degradation of high temperature superconductors has been performed with isolated crystals or individual HTS tapes, showing that oxygen-out diffusion from the rare-earth barium copper oxide (REBCO) results in a reduction of the superconducting performance. In this work, the performance of Pb37Sn63 soldered HTS tape stacks with thermal cycling to 170∘ C is quantified through determination of the critical current Ic , the n-value, the linear resistance R in the pre-transition region of the tape stack current–voltage (I–V) traces, and the diffusion coefficient D associated with oxygen-out diffusion. The effect of tape manufacturer and nickel electroplating were considered; three soldered stacks each of unplated Faraday, nickel electroplated Faraday, unplated SuperPower, and nickel electroplated SuperPower tapes were fabricated. Across 20–30 one hour thermal cycles, there are no significant differences in degradation profiles observed between the unplated and nickel electroplated tape stacks for a given manufacturer. There is however a marked difference in degradation profiles observed between the SuperPower and Faraday tape stacks. For the unplated and plated SuperPower tape stacks, the degradation in Ic generally follows the model to describe oxygen-out diffusion from the superconducting REBCO layer. Significantly less degradation is observed in the unplated and plated Faraday tape stacks, and in some sections Ic even increases slightly across cycles. Further, cases of increasing Ic accompanied with decreasing n are observed, a decoupling between the two parameters. Across all samples, the linear resistance was highest in current-redistribution regions near the leads. Consistent with the model of oxygen-out diffusion and the formation of an increasingly wide oxygen deficient layer in the REBCO, the linear resistance increased with cycle number.