Magnetic characterization of sintered MgB2samples: effect of substitution or ‘doping’ with Li, Al and Si

Abstract

Powdered and sintered MgB₂ samples, both pure and substituted or doped, have been prepared and characterized through magnetic measurements performed from T = 5 K up to a few degrees above the transition temperature of about 39 K. For all the samples, the irreversibility line (IL) appears much lower than the H_c2–T line and very far from that of both high-T_c (YBCO) and low-T_c (Nb₃Sn) materials indicating the need to increase the pinning in this material to make it attractive for technological applications. Moreover, we have verified through different procedures that the sintered samples behave as well-connected bodies, showing no trace of granularity; therefore, the critical current density values J_c may be obtained by applying the 'critical state model' in a straightforward way. The hysteresis loop measurements allowed estimation of J_c both for powders and sintered samples and confirmed the strong field decrease of J_c, implicit in IL behaviour. We attempt to introduce defects in the MgB₂ structure by different chemical treatments like substitution of lithium on the magnesium site and doping of the precursor boron powders with aluminium and silicon. J_c always increased in doped or substituted samples (up to a factor 3) and this fact is meaningful, in particular in the light of the small level of substitution or doping we performed. The best result in terms of J_c is achieved by silicon doping that, moreover, does not significantly decrease the transition temperature.