Thermal-magnetic-stress stability of trapped-field bulk superconductors: Dependence on growth anisotropy and seed number

Abstract

During magnetization, the peculiar phenomenon of thermal-magnetic-stress instability frequently takes place, manifested as a sudden puncture of magnetic flux, a sharp increase of temperature, and a rapid change of stress, for single-grain bulk superconductors, which are a brittle ceramic material prepared by the top-seeded melt growth process with growth sector boundaries (GSBs) and growth sector regions (GSRs). Based on the magnetic field and heat diffusion equations and elasticity theory, we build a thermal-magnetic-stress model to analyze the interaction mechanism between magnetic flux and Joule heating and accompanying elastic stress for cylindrical superconductors under three magnetization processes. This model can not only reproduce the theoretical magnetostriction for 2D homogeneous samples but also predict the thermal-magnetic-stress stability for 2D or even 3D inhomogeneous multi-seed samples. It is found that as the critical current density is higher in GSBs than in GSRs, the magnetic flux will penetrate preferentially into GSRs, resulting in a large trapped field. Once the amplitude or variation rate of the magnetic field exceeds a threshold, the local temperature and stress in GSRs will increase dramatically and the largest trapped field will shift from GSRs to GSBs due to the flux jump. For a 2D sample, the flux jump is prone to occur at the midline between two seeds, and a large field is trapped by GSBs passing through two seeds in the a–b plane. For a 3D sample, due to the inhomogeneity of critical current density around the a–b plane and along the c-axis, the maximum field is trapped at the bulk center, while the maximum stress appears at the seed site. The edges of GSRs on the upper and lower surfaces are most susceptible to cracking if the flux jump occurs.