Abstract. We have systematically doped pre-reacted MgB₂ 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 MgB₂ can be improved by simultaneously alloying with C and refining grains through cold work and further improvement of connectivity.

Print publication: Issue 7 (July 2007)
Received 20 March 2007, in final form 30 April 2007
Published 4 June 2007