Effects of oxide particle addition on superconductivity in nanocrystalline MgB2 bulk samples

In order to improve the extrinsic superconducting properties of MgB2 a combination of nanocrystalline microstructure and particle doping was studied with regard to the enhancement of flux pinning. For comparison, MgO and SiO2 particles were separately introduced into the MgB2 matrix by the in situ preparation technique of mechanical alloying. The oxides do not affect the formation of MgB2 but influence characteristically the critical current density Jc and the irreversibility field Hirr in hot pressed bulk samples due to a change of microstructure and lattice constants. Optimal doping levels are 5 wt.% for MgO and 2 wt.% for SiO2 addition, which lead to improved Jc values of 1.3 × 106 and 1.2 × 106 A/cm2 in self-field at 7.5 K, respectively, but do not affect the critical temperature Tc. Microstructural investigations revealed a nanocrystalline MgB2 matrix with homogeneously distributed impurity particles.