Microstructures and superconducting properties of MgB2 bulk samples processed by ultra-high pressure-assisted sintering

Nanocrystalline MgB2 bulk superconductors have been fabricated by ultra-high pressure-assisted sintering (∼ 5 GPa) over a range of temperatures (700 °C-1100 °C). Phase evolution and morphology, grain size and lattice defects were systematically investigated. The superconducting performance was measured using magnetization methods and linked to the corresponding microstructural features. A sample processed at 900 °C and 5 GPa achieved a Jc value of 4.5 × 107 A/m2 at 4.2 K, 6 T, 30 times and more than 100 times higher than those prepared at 800 and 1100 °C respectively, under similar pressures. Its superior superconducting properties arise from the combination of limited grain growth, retained crystal defects and complete densification achieved in a rapid process by the application of ultra-high pressure. This study reveals the importance of microstructure in controlling the superconducting behavior in sintered MgB2, especially the sample homogeneity that can affect the length scales over which the supercurrents flow.