Optimization of Nb₃Sn and MgB₂wires

Abstract

The critical current density in industrial Nb 3 Sn and MgB 2 wires is currently optimized by introducing various kinds of additives, either Ta and/or Ti for Nb 3 Sn wires or SiC or C for MgB 2 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 Nb 3 Sn 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 A 1−β B β , the relation ρ o versus β exhibits a 'universal' behavior: the effect of the chemical nature of the constituents on ρ o is negligible. The slopes of ρ 0 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 Nb 3 Sn wires produced by the bronze route only. The nonlinearity of the Kramer plot in multifilamentary Nb 3 Sn 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/MgB 2 wires is presented as an attempt to combine different possible mechanisms influencing J c . The substitution of boron by carbon is known to enhance the value of ρ o 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 MgB 2 phase formation. Since the two mechanisms are independent, their effect on J c is expected to be cumulative. In the present paper, the results on the additive combination B 4 C + LaB 6 in monofilamentary Fe sheathed MgB 2 wires are reported. The data are compared with the additives B 4 C + SiC and show that simultaneous additives could be promising in view of applications at 20 K.

Temperature and time scaling of the peak-effect vortex configuration in FeTe0.7Se0.3

“…This procedure has been successfully used in the case of wires, polycrystalline samples, and crystals. [70][71][72][73] However, if pinning is ruled only by defects whose dimensions are smaller than the intervortex distance, it is expected J 1/2 c ∝ (1 − h). 67 In this case, H irr may be determined by doing a linear extrapolation down to zero of the J These results indicate that, for the single crystal sample B, the pinning mechanisms are ruled by defects whose dimensions are smaller that the intervortex distance in the investigated field range and as a consequence, the most appropriate procedure for determining H irr (T ) is from the J 1/2 c vs H curve.…”

Superconducting Materials and Conductors: Fabrication and Limiting Parameters

“…The difference is due to the fact that Ta replaces Nb and Ti replaces Sn in the Nb 3 Sn lattice (suggested in Ref. 105 and confirmed by Ref. 106).…”

An experimental and computational study of srain sensitivity in superconducting Nb3Sn

“…In experiments by Goldacker et al [134,135], it was observed that the martensitic transformation in stoichiometric Nb 3 Sn could be prevented through various additions, including tantalum, titanium, nickel, gallium, and hydrogen. The martensitic transformation was completely suppressed in samples with 0.6 at.% H, 2.8 at.% Ta, 1.3 at.% Ti, and 1 at.% Ni, with normal state resistivities of 37 µΩcm, 29 µΩcm, 33 µΩcm, and 30 µΩcm, respectively.…”