The critical current of high temperature superconducting inclined-substrate coated conductors under biaxial strain.

Abstract

A new biaxial strain probe which allows in situ x-strain variation and independent fixed y-strains has been commissioned and biaxial strain measurements of critical
current density (Jc) of an inclined-substrate high temperature superconducting coated conductor (CC) completed.

The first measurements of the effect of strain on interplanar currents with a c-directed component are presented, showing no strong difference in behaviour from ab-oriented currents. This indicates the effect of inter-plane coupling on Jc does not change dramatically with strain at 77 K.

Measurements of Jc were made using mechanical scribing to reduce the cross section of the CCs. It was shown the decrease in critical current is proportional to the sample
width from 12 mm down to 1 mm, enabling higher critical current densities to be measured.

Short samples (12mm long) were investigated at high currents to enable rotation of samples on the strain board, although current shunting prevented reliable measurements at larger strains.

Calculations of self-field in thin superconducting tapes were made using a Josephson junction model for local Jc(B), and self-consistent current and field spatial distributions derived, that explain the low-field behaviour of CCs.

A theoretical model for the strain behaviour of CCs is presented which uses single-crystal strain measurements and elastic constants from the literature to calculate 1/Jc(0)
dJc/dε for both current and strain along and perpendicular to the tape length on a copper-beryllium sample holder. Qualitative agreement is found between the model values of between +1.0 and -9.2 and between +1.9 and -4.0 for the two
geometries considered, compared to experimental measurements of -7.0(3) and -7.6(13) respectively.

The model results suggest crystallographic normal strains determine the strain behaviour of Jc in CCs in contrast to deviatoric strain that is known to control Nb3Sn. These results suggest different mechanisms operate in these two materials.