Optimizing C Incorporation Into Magnesium Diboride

In this work, either SiC or separate Si and C (Si+C) powders of up to 10 weight percentage were in situ reacted with Mg and B (molar ratio of 1:2) and the superconducting properties compared. The latter shows a smaller
a
-axis lattice cell parameter as compared to the same level of SiC additions, indicating a higher level of C substitution. In those samples, a larger increase in the
c
-axis is also noticeable and a more severe degradation in the superconducting transition temperature is observed. However, reaction with SiC causes a greater broadening in the transition width. At both 5 K and 20 K, for samples reacted with SiC (up to 5 wt.%) a stronger improvement in the magnitude of critical current density,
J
_c
, is obtained. On the other hand, samples reacted with (Si+C) show a weaker
J
_c
dependence on field at 5 K because of higher C substitution. At 20 K, the
J
_c
decreases more rapidly with field for the same level of SiC additions. Hence, for high temperature (20 K) and low field ( <;5 T) applications, reaction with SiC is preferred for obtaining higher
J
_c
(
H
).