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, J(c)(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, H-c2, of MgB2 is reviewed. Carbon substitution effects make two distinguishable contributions to the enhancement of J(C) field performance: increase of H-c2 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 J(c), in magnetic fields and H-c2. An irreversibility field, Hi, of 10 T has been achieved with nano-SiC doping at 20 K, exceeding Hi, of NbTi at 4.2 K. Besides that, H-c2 of carbon alloyed MgB2 film has reached the value of 71 T. The significant enhancement in J(c)(H) and H-c2 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 J(c) and H-c2 by chemical doping. Further improvement in self-field J(c) performance while maintaining the already achieved in-field performance remains as a major challenge in the development of MgB2. (c) 2007 Elsevier B.V. All rights reserved.