Breakthrough in the reduction of oxygen-annealing time for REBCO melt-textured bulks under an oxygen atmosphere containing water vapor

Abstract

Since REBCO melt-textured bulks are produced in an extremely carrier underdoped state after melt-growth, oxygen-annealing is required for a few hundred hours to control the oxygen composition throughout the bulks due to their slow oxygen-diffusivity. In this study, we discovered that the oxygen-annealing time can be largely reduced with the introduction of water vapor to the oxygen-annealing process for Ag-added YBCO melt-textured bulks without any degradation of the superconducting properties. It was revealed that highly dense stacking faults were induced to the matrix during this wet oxygen-annealing, which are considered to act as fast diffusion paths of oxygen. The estimated chemical diffusion coefficients of oxygen at 400 °C for the bulk annealed under wet O2 was ∼4.3 × 10−11 m2s−1, which is almost twice as high as that annealed under standard dry O2 atmosphere. High Jc without a decrease in Tc was confirmed for the sample annealed under wet O2, when the wet O2 annealing was performed for a moderate time. In addition, a combined oxygen-annealing process, where wet O2 was supplied for the first 24 h followed by a dry O2 process for the remaining period, was found to be more effective to achieve both enhanced oxygen-diffusion and maintain high field trapping properties. These improvements are probably because moderate amounts of defects were introduced to the bulk as a function of the duration of the wet O2 annealing. The introduction of water vapor to the oxygen-annealing process will become a versatile technique to reduce the total processing time of REBCO melt-textured bulks.