Prospects for improving the intrinsic and extrinsic properties of magnesium diboride superconducting strands

The magnetic and transport properties of MgB2 films present performance goals yet to be attained for powder-processed bulk samples and conductors. Carbon-doped films have exhibited upper critical fields, mu H-0(c2), as high as 60 T and a possible upper limit of more than twice this value has been predicted. Very high critical current densities, J(c), have also been measured for films, e.g. 25 MA cm(-2) in the self-field and 7 kA cm(-2) in 15 T. Such performance limits are still out of reach for even the best MgB2 magnet wire. In discussing the present status and prospects for improving the performance of powder-based wire we focus attention on (1) the intrinsic (intragrain) superconducting properties of MgB2-H-c2 and flux pinning, and (2) the factors that control the efficiency with which current is transported from grain to grain in the conductor, an extrinsic (intergrain) property. With regard to item (1), the role of dopants in H-c2 enhancement is discussed and examples presented. On the other hand their roles in increasing J(c), both via H-c2 enhancement as well as via direct fluxoid/pinning center interaction, are discussed and a comprehensive survey of H-c2 dopants and flux pinning additives is presented. Dopant selection, chemistry, methods of introduction (inclusion), and homogeneity of distribution (via the rounding of the superconducting electronic specific heat transition) are considered. Current transport through the powder-processed wire (an extrinsic property) is partially blocked by the inherent granularity of the material itself and the chemical or other properties of the intergrain surfaces. Overall porosity, including reduced density and intergranular blocking, is quantified in terms of the measured temperature dependence of the normal-state resistivity compared to that of a clean single crystal. Several experimental results are presented in terms of per cent effective cross-sectional area for current transport. These and other such results indicate that in many cases less than 15% of the conductor's cross-sectional area is able to carry transport current. It is pointed out that densificationin association with the elimination of grain-boundary blocking phases would yield fivefold-to-tenfold increases in J(c) in relevant regimes, enabling the performance of MgB2 in selected applications to compete with that of Nb3Sn.