Effect of starting composition and annealing temperature on irreversibility field and critical current density in Mg xB2

Bulk samples of Mg xB2 with starting composition of 0.5 ≤ x ≤ 1.3 were prepared using a solid-state reaction route. Their structure and critical current density (J c) were investigated. The experimental results show that the starting composition Mg xB2, as well as the sintering temperature, has a significant influence on J c and H irr. The lattice parameter a increases with x and c shows a maximum value around x = 0.9. All the samples possess a J c value about 106 A/cm2 in self-field for x > 0.5. However, at high magnetic field the Mg-deficient samples exhibit higher J c values. The highest irreversibility field of H irr = 5.2 T at 20 K was reached for x = 0.8 with sintering temperature of 800 deg. C, which is 0.8 T higher than that of the stoichiometric MgB2 sample. It is suggested that the formation of MgB4 nanoparticles is responsible for the increase of H irr and J c