Synthesis of Mg(B$_{1-x}$C$_x$)$_2$ Powders

We have synthesized bulk Mg(B$_{1-x}$C$_x$)$_2$ from a mixture of elemental Mg, B, and the binary compound B$_4$C. Carbon incorporation was dramatically improved by a two step reaction process at an elevated temperature of 1200 $^o$C. This reaction process results in a solubility limit near x$sim$0.07. We found that impurities in the starting B cause an additive suppression of T$_c$. We combine these data with T$_c$ and H$_{c2}$(T=0) data from CVD wires as well as plasma spray synthesized powders and present a unifying H$_{c2}$ and T$_c$ versus x plot.