  The effect of nanoscale-SiC doping of MgB2 was investigated using transport
and magnetic measurements. It was found that there is a clear correlation
between the critical temperature Tc, the resistivity r, the residual
resistivity ratio, RRR = R(300K)/R(40K), the irreversibility field H* and the
alloying state in the samples. SiC-doping introduced many nano-scale
precipitates, provoking an increase of r(40K) from 1 mW-cm (RRR = 15) for the
clean limit sample to 300 mW-cm (RRR = 1.75) for the SiC-doped sample, leading
to significant enhancement of Hc2 and H* with only minor effect on Tc. EELS
analysis revealed a number of nano-scale impurity phases: Mg2Si, MgO, MgB4,
BOx, SixByOz, BC and unreacted SiC in the doped sample. TEM study showed an
extensive domain structure of 2-4nm domains induced by SiC doping. The Jc for
the 10% nano-SiC doped sample increased substantially at all fields and
temperatures compared to the undoped samples, due to the strong increase in Hc2
and H* produced by SiC doping.
