New pinning strategies for second-generation wires

In the last several years, second-generation (2G) superconducting wires have been considered for applications in rotating machines operating in the 20-40 K temperature range in 1-3 T magnetic fields. Here, we outline several novel strategies for improving the low-temperature performance of second-generation wires by utilizing the in-plane strain of thick YBCO layers manufactured by the reel-to-reel metal-organic deposition (MOD) method. First, we show that he strain-induced pinning mechanism analysis, based on the Eshelby model of the elastically-strained composites, predicts that small YBCO grain size is a critical component of a strong pinning architecture. Second, we describe how the in-plane strain can be controlled by processing parameters. Systematic changes of the in-plane structure and YBCO grain size are mapped with respect to the YBCO stability line and the Cu₂O-CuO line on the Bormann-Hammond diagram. It is demonstrated that the optimum critical current density is the result of a trade-off between YBCO grain coupling and the strain-induced pinning.