  In this study, Si and C were incorporated into polycrystalline MgB2 via in
situ reaction of Mg and B with either SiC or with separate Si and C (Si+C). The
electrical transport and magnetic properties of the two series of samples were
compared. The corrected resistivity at 40K, Rho A(40K), is higher for the SiC
reacted samples regardless of carbon (C) substitution level, indicating larger
intragrain scattering because of the simultaneous reaction between Mg and SiC
and carbon substitution during the formation of MgB2. In addition, because of
the cleaner reaction route for the SiC reacted samples, the calculated active
area that carries current, AF, is twice that of the (Si+C) samples. On the
other hand, the upper critical field, Hc2, was similar for both sets of samples
despite their different C substitution levels which proves the importance of
defect scattering in addition to C substitution level. Hence, the form of the
precursor reactants is critical for tuning the form of Hc2(T).
