Abstract

A compatibility between Bi₂Sr₂CaCu₂O_8 + δ (Bi-2212) and MgO was investigated, and the Bi-2212–Mg_1 − xCu_xO composite materials were obtained and characterised. It was established that Bi-2212 interacts with the MgO powder at 850 °C producing (Sr,Ca)_4 − yBi₂O_z and the Mg_1 − xCu_xO solid solution, while single crystal MgO is stable against Bi-2212 solid and melt up to 1100 °C. The Bi-2212 phase is found to coexist with Mg_1 − xCu_xO, where x ≥ 0.08, at 850 °C in air. Magnesium oxide content is estimated to be under 1 mol% in the solid Bi-2212 phase, as well as in the phases formed via the Bi-2212 peritectic decomposition, and in the Bi–Sr–Ca–Cu–O melt. The Bi-2212–Mg_1 − xCu_xO superconducting composites were prepared by melt-processing using different precursors. The effect of the precursor on the material microstructure and superconducting properties was investigated. The Mg_1 − xCu_xO submicron particles were found to be rather uniformly distributed in the Bi-2212 matrix, their size depending on the preparation conditions. Application of the nanosize MgO powder as a precursor indicated substantial coarsening of the magnesium oxide particles during the composite processing, the average particle size exceeding 0.2 µm. The composites exhibited higher critical current density as well as better field stability at enhanced temperatures in comparison with the Bi-2212 samples.