High transport critical current density and large Hc2 and Hirr in nanoscale SiC doped MgB2 wires sintered at low temperature

We report a systematic study on the effect of sintering temperature on the phase formation, critical current density, upper critical field and irreversibility field of nanoscale SiC doped MgB2. Bulk and Fe sheathed wires doped with different nano-SiC particle sizes have been made and heat treated at temperatures ranging from 580 to 1000 degrees C. A systematic correlation between the sintering temperature, normal state resistivity, RRR, J(c), H-c2, and H-irr has been found in all samples of each batch. Samples sintered at a lower temperature have a very fine and well consolidated grain structure while samples sintered at a high temperature contain large grains with easily distinguishable grain boundaries. Low temperature sintering resulted in a higher concentration of impurity precipitates, larger resistivity, higher J(c) up to 15 T and lower T-c values. These samples show higher H-c2 and H-irr at T near T-c but lower H-c2 near T = 0 than samples sintered at high temperature. It is proposed that huge local strains produced by nano-precipitates and grain boundary structure are the dominant mechanism responsible for higher H-c2 at T near T-c. However, higher impurity scattering due to C substitution is responsible for higher H-c2 in the low temperature regime for samples sintered at a higher temperature. In addition to high H-c2, it is also proposed that the large number of nano-impurities serve as pinning centres and improve the flux pinning, resulting in higher J(c) values at high magnetic fields up to 15 T.