The influence of gallium on external-diffusion-processed Nb3Sn composites

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Abstract
The effect of gallium on the processing and properties of multifilamentary Nb3Sn‐based superconducting composites was studied. Diffusion in the copper (bronze) matrix and reaction layer formation were studied in pure copper (bronze) wires, in composites with large (15 μ) fibers, and in composites with ultrafine (0.1 μ) fibers closely spaced. Using the external‐diffusion method, Ga was introduced at various stages of the Sn diffusion process. The results show that Ga markedly increased the activity coefficient for Sn in the bronze matrix, to the point where ‘‘uphill’’ diffusion occurs. Thus, Ga can be used to control the Sn distribution. Whether or not Ga additions improve superconducting properties depends crucially on the manner and timing of the addition. Higher critical current densities, particularly at high magnetic fields, were demonstrated for external‐diffusion composites. Similar improvements should be feasible for all types of Nb3Sn‐based filametary materials using small Ga additions appropriately timed.