Optimization of Nb3Sn and MgB2 wires

Abstract

The critical current density in industrialNb3Sn and MgB2 wires is currently optimized by introducing various kinds of additives, either Ta and/or Ti forNb3Sn wires orSiC or C for MgB2 wires. In the following, several problems linked to the presence of additives in the twoclasses of compounds are discussed.A reinvestigation of the site occupancy of Ta and Ti additives inNb3Sn wires shows that the Ta atoms occupy the 6c chain sites, while the Ti atoms arelocated on the cubic 2a sites. It follows that in perfectly ordered A15 compoundsA1−βBβ, therelation ρo versus β exhibits a ‘universal’ behavior: the effect of the chemical nature of the constituents onρo is negligible.The slopes of ρ0 versus the Ti, Ga and Ni contents in the A15 layer coincide and are much steeper than forthe Ta additive, corresponding to the three times higher number of 6c sites with respect to2a A15 lattice sites.The presence of two grain morphologies, e.g. equiaxial and columnar, is observed inNb3Sn wires produced by the bronze route only. The nonlinearity of the Kramer plot in multifilamentaryNb3Sn bronze route wires is explained by the presence of these two different grain types, whichhave distinctly different Sn contents and sizes. For these wires, the total pinning force canbe 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 influencingJc. The substitution of boron by carbon is known to enhance the value ofρo and thus of the critical field. In addition, the pinning behavior is expected to be improved by grainboundary effects or nanosize precipitations, caused by the presence of appropriate additives duringthe MgB2 phase formation. Since the two mechanisms are independent, their effect onJc is expected to be cumulative. In the present paper, the results on the additive combinationB4C+LaB6 in monofilamentaryFe sheathed MgB2 wires are reported. The data are compared with the additivesB4C+SiC and show that simultaneous additives could be promising in view of applications at 20 K.