Y₂BaCuO₅ addition and its effects on critical currents in large grains of YBa₂Cu₃O_7-δ: A quantitative microstructural study

The addition of Y₂BaCuO₅ (211) particles to large grain melt textured YBa₂Cu₃O_7-δ (Y123) has significantly improved the critical current density (J_c) of this material. Here, a systematic quantitative analysis on the effects of the 211 addition was performed on a microscopic scale with a systematic variation of the initial volume percent of 211. From the correlation between critical current measurements and quantitative microscopy of both (001) and (110) sections, a maximum value of J_c is observed, corresponding to a measured Y123 volume percent of 20% ± 3%. Accounting for the loss of liquid phase for the present processing, the corresponding optimum initial volume of 211 for the highest measured J_c is 40%. Further comparison between the weighted J_c and the true flux pinning force (F_p) also shows a maximum pinning force for an initial 211 addition of 40%. Although the weighted J_c starts to decrease with an initial 211 volume of above 40%, the pinning efficiency at higher magnetic fields (2-4 T) of the superconducting Y123 matrix was actually improved with an ever increasing 211 addition to at least 50%. Though an increasing addition of 211 is effective in producing efficient flux pinning sites in the Y123 matrix, percolation paths in the Y123 matrix become limited for supercurrent. Hence, a measured 211 volume corresponding to 80% 211 is proved to give the best possible critical current density. Furthermore, crack opening and crack spacing of the superficial cracks are found to decrease with an increasing 211 addition and with a decreasing 211 interparticle spacing. The penetration and surface length of each of these superficial cracks are hence reduced, which leads to a better electrical connectivity in the Y123 matrix.