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Nanocrystalline MgB 2 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 J c 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 MgB 2 , especially the sample homogeneity that can affect the length scales over which the supercurrents flow. [ABSTRACT FROM AUTHOR]

Abstract: The relaxation of the irreversible magnetization of MgB2 bulk samples obtained by electric-field assisted sintering was investigated using the SQUID magnetometry for a magnetic field H up to 50kOe applied in zero-field-cooling conditions. We observed a crossover plastic creep at high temperatures T-elastic creep at low T, described by H ∝ T −2 in the low T range, which appears to be caused by the macroscopic currents induced in the sample during magnetization measurements. By decreasing T below this line the determined creep exponent rapidly overcomes the widely accepted theoretical values for elastic (collective) pinning. This behaviour can easily be explained through the occurrence of micro flux jumps, leading to a finite magnetization relaxation rate in the low-T limit. [Copyright &y& Elsevier]