Fundamental Research on Magnetic Shielding of High-Temperature Superconducting Bulk Materials for Magnetic Refrigeration Applications

When considering cooling of high-temperature superconducting equipment, it is not always possible to install the refrigerator and the superconducting equipment close to each other. For this reason, we are researching technology to cool superconducting equipment efficiently by circulating a refrigerant. We aim to achieve high efficiency by combining this circulating cooling with magnetic refrigeration technology. Magnetic refrigeration requires a magnetic field change to be applied to the magnetic material. To achieve high magnetic refrigeration capacity it is essential to include a superconducting magnet in the system. Consequently, we are investigating the feasibility of a system that inserts and removes a magnetic shield to provide the required magnetic field variation. We have previously investigated the possibility of using superconducting materials as magnetic shielding materials, and reported the magnetic shielding effect of YBCO bulk material and YBCO wire laminates at 77 K. In this study, we confirmed the shielding effect at approximately 65 K under reduced pressure using liquid nitrogen for YBCO bulk materials with different outer diameters and thicknesses. From the results obtained from these experiments, we estimated the temperature dependence of the critical current density of the bulk material, and through analysis, estimated the magnetic shielding ability up to 20 K. It was suggested that a magnetic shield exceeding 5 T is possible at the 20 K level, and that a high refrigeration performance applicable to the reliquefaction of liquid hydrogen can be achieved.