Influence of two-stage annealing treatment on critical current of bronze-processed multifilamentary Nb 3Sn superconducting materials

Abstract

The influence of changes of volume fraction of Nb 3 Sn, grain size and upper critical magnetic field due to two-stage annealing treatment (low temperature annealing to form fine grains + high temperature annealing to raise upper critical magnetic field) on overall critical current and critical current density of the bronze-processed Nb 3Sn compound were studied at the magnetic field of 3–15 T. When annealing temperature was low (773–923 K) and the volume fraction of Nb 3 Sn was low in the first stage annealing, the second stage annealing could raise the overall critical current over the whole range of applied magnetic field of 3–15 T due to increase in upper critical magnetic field H c2 and due to increase in the volume fraction of Nb 3Sn accompanying with reduction in Sn concentration in the bronze matrix, which played a role to reduce residual strain in Nb 3Sn, leading to high H du,c2 although the loss in pinning force arose from the coarsening of the grains. When the annealing temperature was high (973 K) and the Nb 3Sn was formed until the Sn in the bronze was consumed in the first stage, the second stage annealing could not raise the critical current due to increase in grain size and due to no effective increase in H c2. The critical current density at low magnetic fields below several Testas was reduced by the second stage annealing due to increase in grain size but that at high fields was raised due to increase in high H c2. The reverse two-stage annealing treatment (high temperature annealing in the first stage + low temperature annealing in the second stage) reduced the H c2 slightly with increasing second stage annealing temperature and time. The critical current density at low magnetic fields was determined mainly by the grain size but that at high fields was determined by the combination of the upper critical field and grain size. The present results could be accounted for by the empirical equation based on the Suenaga's speculation, indicating that the global pinning force is proportional to the product of inverse grain size and (1 − h) 2h 1 2 taken from the Kramer's equation where h is the reduced magnetic field.