Optimization of critical currents in MgB/sub 2/ wires and coils

We present an analysis of the effect of SiC doping and heat treatments, including hot isostatic pressing (HIPing), on the microstructure, magnetization and transport properties of MgB/sub 2/ wires and coils prepared by the powder in tube (PIT) method. The HIPed wires have higher critical current densities (J/sub c/) than the annealed wires, especially at high temperatures and magnetic fields, and higher irreversibility field. The improvements are due to the reduction of porosity and to a high density of structural defects, which are the likely source of vortex pinning. We demonstrate that the high-quality properties mentioned above can be retained in longer wires. At 25 K our champion 6-layer coil, made with 25 meters of wire, was able to generate a field of 1 T at self-field. We were able to achieve even higher J/sub c/'s in another coil by combining the addition of SiC and HIPing. We also observed a larger upper critical field due to the SiC doping, which drives the sample to the dirty limit.