The Effect of Silicon Addition to The Interfilamentary Copper on J_C, Compound Formation and Interdiffusion

Abstract

One of the reasons why high critical current density is difficult to achieve in fine filament Nb-Ti superconducting wire is that a reaction occurs between the copper matrix and Nb-Ti filaments. A diffusion barrier around each filament was introduced in the processing of fine filamentary wire in order to achieve J_c values close to the intrinsic ones.¹ One study of diffusional reaction rates through the Nb barrier² has indicated that, for typical SSC composites, a barrier area of 4% and 9% is necessary for producing Oμm and 2.5μm diameter filaments respectively. Consequently, if diffusional interactions can be eliminated without adding a large volume of barrier material, it is possible to achieve higher J_c’s at lower cost. Another limitation on the J_c in fine filament Nb-Ti superconducting wire results from the mismatch in mechanical properties of Nb-Ti filaments and copper matrix at high wire strains. The hardness and ultimate tensile strength (UTS) of Nb-Ti filaments increase with increasing amount of the cold work and no UTS saturation has been seen³, whereas the UTS of copper saturates. An improper filament array also adversely affects J_c, but this can be resolved by changing the filament distribution geometry, i.e. by reducing the interfilamentary spacing.⁴ Improving mechanical strength of copper matrix is important for reducing the amount of fine filament sausaging. Recently, in work that was primarily directed towards the development of material for ac applications, it was reported that, when silicon is added to the copper matrix, the formation of intermetallic compounds can be greatly reduced^5,6. Cu-Si alloy also has mechanical properties more compatible with NbTi than copper. If the above results can be verified, the technique can probably be applied to the manufacture of high J_c SSC type conductors and large filamentary NbTi superconductor materials for general use. The work described here was to investigate these effects of silicon additions to the Cu matrix.