Understanding the route to high critical current density in mechanically alloyed Mg(B1−xCx)2

We have systematically doped pre-reacted MgB 2 with C by high-energy ball milling, and obtained peak J c (8 T, 4.2 K)>500 A mm −2 in a form compatible with powder-in-tube wire manufacture. Effects of carbon doping by this method were qualitatively similar to single crystals and CVD (chemical-vapour-deposited) filaments, but T c , H c2 , and resistivity measurements indicate more electron scattering than in single crystals and CVD filaments of similar composition. We attribute both the high J c ( H ) and the large degree of electron scattering to the high grain boundary density associated with our very fine ∼50 nm grain size. At high C concentrations (C/(B+C)>0.07) J c fell sharply. We identified the cause of the J c decline to be a sharp decline in connectivity. This work demonstrates how J c ( H ) of pre-reacted MgB 2 can be improved by simultaneously alloying with C and refining grains through cold work and further improvement of connectivity.