Effect of tensile strain on grain connectivity and flux pinning in Bi_2Sr_2Ca_2Cu_3O_x tapes

摘要

The grain-to-grain connectivity in Bi_2Sr_2Ca_2Cu_3O_x tapes is still poorly understood, even though they have been commercially available in long lengths for several years. This letter explains the effects of tensile strain on the grain-to-grain connectivity in Bi_2Sr_2Ca_2Cu_3O_x tapes. The different length scales at which damage to the grain structure occurs are studied with magneto-optical imaging, scanning-electron microscopy, and transport current. These data show that the initial degradation in critical current when strain exceeds the irreversible strain limit is caused by microcracks (～100-500 nm in width) that form mainly at high-angle grain boundaries. Filament-wide cracks (～5-10 μm in width) form at locations of lower grain density in the filaments at strains far exceeding the irreversible strain limit. However, in contrast to previous reports, a careful analysis of the pinning force as a function of tensile strain, taking into account current sharing with the normal matrix by using the offset criterion, shows that intragranular flux pinning is not affected by strain in any significant way.