Thermal degradation analysis and optimization of controlled heat treatment for stacked high temperature superconducting tapes

High temperature superconducting (HTS) tapes have crucial applications for generating high magnetic fields with minimal power input. Given a single tape has a limited current-carrying capacity, stacked tapes are common, fabricated through methods like solder soldering or epoxy impregnation requiring heat treatment. In this work, we have investigated the efficient region for vacuum heat treatment applicable of commercial HTS tapes, analysed the thermal degradation principles and accordingly proposed a controlled heat treatment process for stacked HTS tapes to achieve more precise regulation of the critical current ( Ic). This heat treatment process was explored using a specialized multi-temperature zone vacuum system. Critical parameters in this process include heat treatment temperature, duration and pressure on the tape. A series of experiments were conducted at 77 K in a self-field to investigate how these parameters affect the superconductivity performance of stacked HTS tapes. Based on the experimental results, an optimal heat treatment process has been proposed. Under the current process, with the heat treatment temperature set at 200 °C, duration at 20 min, and pressure on the tape at 12 MPa, the samples exhibit favourable properties characterized by a smooth and neat appearance without defects such as pinholes or false soldering, and the superconductivity performance can be consistently maintained at more than 97%. The obtained measurements were compared with simulated results, demonstrating an error margin within 0.5%. Moreover, precise control of Ic is achieved, tailored to tape and stacking specifications, allowing manageable degradation as required. This heat treatment process for stacked HTS tapes holds significant importance, especially in the context of designing cable-in-conduit conductors fabricated with stacked HTS tapes. It serves as a valuable reference for further advancements in this field.