Niobium core composition

Fig. 3. Cross section of a copper-stabilized composite strand at 3.86-mm diameter during the wire-drawing sequence. The finned tantalum barrier surrounds a 4453-filament bronze and niobium core.

Hashimoto et al. [ ] have described a multifilamentary NbaSn superconductor produced by heat treatment of a composite consisting of niobium cores and a two-component matrix of pure copper and a high-tin bronze (Sn-20 wt.% Cu). The technical feasibility and economic advantages of this technique have been evaluated in the present study. This study included consideration of the likely cost per A m and the overall current densities achievable with this new conductor design. Particular attention was devoted to high current (10 kA at 12 T) conductors suitable for application in magnets intended for experimental controlled thermonuclear reaction devices, such as the Fusion Engineering Research Facility (FERF) [ ] at Lawrence Livermore Laboratory and the tokamak-type experimental reactors. 

There are, however, limitations on the degree to which the matrix/NbaSn volume ratio can be reduced in these high-tin bronze composites. For example, the niobium cores need to be spaced sufficiently for uniform reaction throughout the filament groups and niobium filaments must not be engulfed by Kirkendall voids or non-a-bronze phases [ ] during the homogenization heat treatment, which distributes the tin prior to reaction. A purpose of this study was to determine the quantity of matrix material necessary to fulfill these requirements. 

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