Measurements of the superconducting upper critical field, H_{c2}, and critical current density, J_c, have been carried out for MgB_2 doped with Ti and/or C in order to explore the problems encountered if these dopants are used to enhance the superconducting performance. Carbon replaces boron in the MgB_2 lattice and apparently shortens the electronic mean free path thereby raising H_c2. Titanium forms precipitates of either TiB or TiB_2 that enhance the flux pinning and raise J_c. Most of these precipitates are intra-granular in the MgB_2 phase. If approximately 0.5% Ti and approximately 2% C are co-deposited with B to form doped boron fibers and these fibers are in turn reacted in Mg vapor to form MgB_2, the resulting superconductor has H_{c2}(T=0) ~ 25 T and J_c ~ 10,000 A/cm**2 at 5 K and 2.2 T.
@article{arxiv.cond-mat/0411531,
title = {Superconductivity in MgB_2 doped with Ti and C},
author = {R. H. T. Wilke and S. L. Bud'ko and P. C. Canfield and M. J. Kramer and Y. Q. Wu and D. K. Finnemore and R. J. Suplinskas and J. V. Marzik and S. T. Hannahs},
journal= {arXiv preprint arXiv:cond-mat/0411531},
year = {2009}
}