Nanoscale-SiC doping for enhancing Jc and Hc2 in superconducting MgB2

The effect of nanoscale-SiC doping of MgB2 was investigated in comparison with undoped, clean-limit, and Mg-vapor-exposed samples using transport and magnetic measurements. It was found that there are two distinguishable but related mechanisms that control the critical current-density-field J(c)(H) behavior: increase of upper critical field H-c2 and improvement of flux pinning. There is a clear correlation between the critical temperature T-c, the resistivity rho, the residual resistivity ratio RRR=R(300 K)/R(40 K), the irreversibility field H*, and the alloying state in the samples. The H-c2 is about the same within the measured field range for both the Mg-vapor-treated and the SiC-doped samples. However, the J(c)(H) for the latter is higher than the former in a high-field regime by an order of magnitude. Mg vapor treatment induced intrinsic scattering and contributed to an increase in H-c2. SiC doping, on the other hand, introduced many nanoscale precipitates and disorder at B and Mg sites, provoking an increase of rho(40 K) from 1 muOmega cm (RRR=15) for the clean-limit sample to 300 muOmega cm (RRR=1.75) for the SiC-doped sample, leading to significant enhancement of both H-c2 and H* with only a minor effect on T-c. Electron energy-loss spectroscope and transmission electron microscope analysis revealed impurity phases: Mg2Si, MgO, MgB4, BOx, SixByOz, and BC at a scale below 10 nm and an extensive domain structure of 2-4-nm domains in the doped sample, which serve as strong pinning centers. (C) 2004 American Institute of Physics.