Two-Phase Microstructure Generated by Reaction of Nano WO₃ Addition and its Effect on Flux Pinning in Bi 2212 Composites

In the present work, Tungsten trioxide (WO₃) nanoparticles are added (up to 5 wt%) to Bi₂Sr_2.14Ca_0.86Cu₂O_y (Bi 2212) compound using a sol-casting method that facilitates their uniform distribution in the Bi 2212 matrix. Sintering at 870 ^oC, close to the melting point, has led to the formation of nearly spherical particles of WSr₂CaO₆ (W-alloy) phase (20–80 nm) by local reaction of distributed nano WO₃ particles with Bi 2212 grains. These particles are well dispersed and located preferentially at the platelet-like grain boundaries in the superconducting Bi 2212 composites. This is of interest in analogy to the widely studied YBa₂Cu₃O_y (Y123) superconductor with two-phase microstructure with Y₂BaCuO₅ (Y211) precipitates, which is known to provide flux pinning and considerable enhancement of the critical current densities (J_c). The superconducting properties of the present Bi 2212 composites with varying amounts of second phase content are assessed by recording M-H loops at different temperatures 5 to 77 K. Critical current densities are found to be non-zero at 5 K in all the samples up to 9 T applied field, suggesting irreversibility fields to be above 9T. A considerable enhancement of J_c and flux pinning force are observed at temperatures 15–50 K, compared to pure 2212 phase at low concentrations (0.1 wt%) of nano WO₃ addition in the composites. A minor drop in the superconducting transition temperature T_c (onset) from 90 to 80 K observed in the sample with higher WO₃ addition (5 wt%) caused a lowering of flux pinning force at temperatures 50 K and above. Scaling laws of flux pinning show that normal surface pinning is the dominant mechanism in all the samples, which can be attributed to the structural defects at the 2212 platelet boundaries. Flux pinning is observed to be sustained in a broader field range and up to 50 K in all the samples with WO₃ addition. This provides evidence that the interfacial defects associated with the second phase (W-alloy) particles, created at the platelet boundaries, are effective in providing flux pinning.