Nano-sized Al2O3 doping effects on the critical current density of MgB2 superconductors

MgB2 polycrystalline bulk samples with addition of 5 and 10 wt.% nano-sized Al2O3 powders were prepared by solid state reaction using pre-reacted MgB2. All the samples were sintered at 850 °C for 40 min in Ar. All the samples were characterized by X-ray diffraction, scanning electron microscopy and magnetic measurements. Results show that the critical current density and irreversibility fields decrease significantly with increasing Al2O3 level. Jc values decreased significantly by more than one order of magnitude. The Tc drops slightly from 37.9 to 36.6 K, but acquires a very wide transition width of more than 20 K. Furthermore, the amount of MgO was found to increase with increasing Al2O3 level, probably indicating that Mg was replaced by Al, with the excess Mg forming extra MgO. The field dependence of Jc for all the samples is also presented.