Enhancement of Hc2 and Jc by carbon-based chemical doping
In the past 5 years, various kinds of doping of MgB2, including single elements (metal and non-metal), silicates, various carbon sources, and other compounds have been investigated and reported. Most nanoparticle doping leads to improvement of critical current density, Jc(H), and performance, but some types show a negative effect. In this paper, the effect of carbon doping on Jc and the upper critical field, Hc2, of MgB2 is reviewed. Carbon substitution effects make two distinguishable contributions to the enhancement of Jc field performance: increase of Hc2 and improvement of flux pinning, both because carbon substitutes for boron in the MgB2 lattice. Among all the carbon sources so far, nano-SiC has been confirmed to be the most effective dopant to enhance the Jc in magnetic fields and Hc2. An irreversibility field, Hirr, of 10 T has been achieved with nano-SiC doping at 20 K, exceeding Hirr of NbTi at 4.2 K. Besides that, Hc2 of carbon alloyed MgB2 film has reached the value of 71 T. The significant enhancement in Jc(H) and Hc2 via carbon substitution has provided great potential for practical applications of MgB2. The dual reaction model proposed by the authors group provides a comprehensive understanding of the mechanism of enhancement in Jc and Hc2 by chemical doping. Further improvement in self-field Jc performance while maintaining the already achieved in-field performance remains as a major challenge in the development of MgB2.