Magnetic characterization of sintered MgB2 samples:: effect of substitution or 'doping' with Li, Al and Si

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.