Residual Stress/Strain Analysis of Bulk YBCO Superconductors Using EBSD

Flux pinning of bulk YBa$_2$Cu$_3$O$_{7-\delta }$ (Y-123) superconductors is commonly done using secondary phase particles, mainly Y$_2$BaCuO$_5$ (Y-211) particles, which are created during the growth of the samples or additionally added. However, the crystallographic parameters of Y-211 particles indicate a misfit to the ones of Y-123, there will be residual strain around such particles within the Y-123 matrix. Here, we perform a dedicated analysis of electron backscatter diffraction (EBSD) and transmission Kikuchi diffraction (TKD) data obtained on various Y-123 superconductor samples (Y-123 bulks prepared by melt-texture, melt-growth and the infiltration growth, YBCO foam struts and commercial YBCO coated conductors). This enables the strain distribution around such embedded secondary phase particles to be revealed down to the nanometer scale. The results are presented in the form of Kernel Average Misorientation (KAM) mappings. Around large Y-211 particles and around clusters, the strain can be so large that even subgrains in the Y-123 matrix are formed. The impact of the strain distribution on the flux pinning properties is discussed, as stress/strain can deliver an extra source of flux pinning. Therefore, it is essential to properly control the particle size as well as their distribution within the bulk sample to achieve the best superconducting performance.