  An improvement of the transport critical current density, Jc, of MgB2 wires
was obtained after addition of 10 wt.% B4C powders, after reaction at 800 C: Jc
values of 1.10^4 A/cm2 at 4.2 K and 9T were obtained for wires of 1.11 mm
diameter in a Fe matrix. The starting mixture of Mg and B was doped with
sub-micrometric B4C, the ratio being Mg : B : B4C = 1: 2 : 0.08, corresponding
to 10 wt.% B4C. For T > 800 C, a decrease of Jc was found, due to the reaction
with the Fe sheath. In order to investigate the origin of the improvement of
the transport properties for heat treatments up to 800 C, X-ray diffraction
measurements were performed. A decrease of the lattice constant a from 0.30854
nm to 0.30797 nm was found, thus suggesting an effect of the substitution of
Carbon on the properties of the wires. A comparison with the literature data
shows that the addition of B4C powders leads to the second highest improvement
of Jc reported so far after SiC, thus constituting an alternative for future
applications.
