Optimization of Nb₃Sn and MgB₂ wires

The critical current density in industrial Nb3Sn and MgB2 wires is currently optimized by introducing various kinds of additives, either Ta and/or Ti for Nb3Sn wires or SiC or C for MgB2 wires. In the following, several problems linked to the presence of additives in the two classes of compounds are discussed. A reinvestigation of the site occupancy of Ta and Ti additives in Nb3Sn wires shows that the Ta atoms occupy the 6c chain sites, while the Ti atoms are located on the cubic 2a sites. It follows that in perfectly ordered A15 compounds A1−βBβ, the relation ρo versus β exhibits a 'universal' behavior: the effect of the chemical nature of the constituents on ρo is negligible. The slopes of ρ0 versus the Ti, Ga and Ni contents in the A15 layer coincide and are much steeper than for the Ta additive, corresponding to the three times higher number of 6c sites with respect to 2a A15 lattice sites. The presence of two grain morphologies, e.g. equiaxial and columnar, is observed in Nb3Sn wires produced by the bronze route only. The nonlinearity of the Kramer plot in multifilamentary Nb3Sn bronze route wires is explained by the presence of these two different grain types, which have distinctly different Sn contents and sizes. For these wires, the total pinning force can be represented as the superposition of two contributions with different scaling fields. Simultaneous addition of different additives on 'in situ' Fe/MgB2 wires is presented as an attempt to combine different possible mechanisms influencing Jc. The substitution of boron by carbon is known to enhance the value of ρo and thus of the critical field. In addition, the pinning behavior is expected to be improved by grain boundary effects or nanosize precipitations, caused by the presence of appropriate additives during the MgB2 phase formation. Since the two mechanisms are independent, their effect on Jc is expected to be cumulative. In the present paper, the results on the additive combination B4C+LaB6 in monofilamentary Fe sheathed MgB2 wires are reported. The data are compared with the additives B4C+SiC and show that simultaneous additives could be promising in view of applications at 20 K