Effect of pre-annealing on microstructure, mechanical properties and current-carrying properties of Bi-2212 wires

Abstract

Abstract In order to solve the problem of excessive porosity of Bi-2212 Cable in Conduit Conductor (CICC) after the overpressure heat treatment, which leads to a significant decrease of mechanical stability of the conductor, a pre-annealing of Bi-2212 wires with a total pressure of 50 bar has been carried out in this paper. After the pre-annealing, the density of oxide cores of Bi-2212 wires was close to the theoretical density, which avoided the problem of diameter reduction of the wires in Bi-2212 CICC during overpressure heat treatment. At the same time, the mechanical properties, microstructures and current carrying properties of pre-annealed wires have been also analyzed. It showed that after pre-annealing, the Bi-2212 wires had not only high yield strength σ0.2, which reached 167 Mpa, but also great tensile strainse, which reached 7.4 %. High yield strength of the wires could effectively avoid serious scratches on the surface of wires during the preparing process of Bi-2212 CICC. Large tensile strain of the wires could prevent the crack of the wires caused by large compressive deformation or twisting deformation of the wires during preparation of Bi-2212 CICC, so that the conductors with smaller pitch and low hysteresis loss could be prepared. In addition, the phase composition of oxide cores in Bi-2212 wires did not change obviously during the pre-annealing process, but the crystallinity of Bi-2212 powder of the cores increased significantly, which was beneficial to the simultaneous melting of Bi-2212 powder of the cores during the overpressure heat treatment and the improvement of the content of Bi-2212 phase of the final wires. As a result, the current carrying capacity of the wires was slightly improved, and its engineer critical current density (Je) reached 825 A/mm2 (4.2K, 12T), which could meet the requirements of future fusion systems. In summary, it is expected that Bi-2212 CICC with less surface defects of the wires, low hysteresis loss, high filling ratio and excellent current-carrying performance can be obtained by the pre-annealing method.