Twin engineering for high critical current densities in bulk YBa2Cu3O7−δ

We have exploited the positive entropy of the twin boundary energy to refine twin structure in YBa2Cu3O7−x (Y123) which is the matrix phase in melt-textured grown (MTG) Y–Ba–Cu–O (YBCO) pellets. Refinement of twin microstructure in MTG pellets is achieved by annealing at higher temperatures. Decrease of twin spacing with increasing temperature is confirmed by electron microscopy. The critical current density (Jc), the pinning force and the magnetic field at maximum pinning-force all increase with increasing annealing temperature of oxygenation. This thermal method provides a viable way to improve these superconducting properties that are evidently extrinsic and strong functions of the microstructure. For the MTG YBCO samples with initial composition of 28.6 wt% Y2BaCuO5/71.4 wt% Y123/0.5 wt% PtO2, twin spacing reduces from 1900 to 540 Å as the oxygenation temperature increases from 450 to 680 °C. Concomitantly, the Jc (77 K, 1 T) is improved by 22,120–31,920 A/cm2 and the maximum flux-pinning force nearly triples from 1.2 × 108 to 3.4 × 108 N/m3.