Synthesis and optimization of Mg(B1−xCx)2 wire segments

We have succeeded in increasing the upper critical field of polycrystalline MgB2 wire segments to near 36 T by the substitution of 5.2% carbon for boron. We present fabrication details and physical properties of Mg(B1−xCx)2 for x up to 0.052. Carbon doping was achieved using a two step reaction technique which involved an initial carbon doping of boron wire segments using chemical vapor deposition (CVD) followed by exposure of the carbon doped boron segments to magnesium vapor. The carbon content was determined by the relative contraction of the a-lattice parameter. The superconducting transition temperature is suppressed by approximately 1 K/%C while Hc2⊥c(T=0) rises at a rate of nearly 4 T/%C.