  Powdered and sintered MgB2 samples have been characterized through magnetic
measurements performed from T = 5 K up to few degrees above the transition
temperature of about 39 K. We found that the sintered samples behave as
well-connected bodies, showing no trace of granularity. In order to obtain the
critical current density value Jc the Critical State Model has been therefore
employed in a straightforward way. With the aim either to decrease the electron
mean free path or to increase its Jc we have attempted to introduce defects in
the MgB2 structure by different procedures: substitution of Lithium on the
Magnesium site and doping of the precursor Boron powders with Aluminum and
Silicon. The best result in terms of Jc has been achieved by Silicon doping
that, moreover, does not significantly affect the transition temperature.
