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

We have systematically doped pre-reacted MgB2 with C by high-energy ball milling, and obtained peak Jc (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 Tc, Hc2, and resistivity measurements indicate more electron scattering than in single crystals and CVD filaments of similar composition. We attribute both the high Jc(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) Jc fell sharply. We identified the cause of the Jc decline to be a sharp decline in connectivity. This work demonstrates how Jc(H) of pre-reacted MgB2 can be improved by simultaneously alloying with C and refining grains through cold work and further improvement of connectivity