Multifilamentary Nb3Sn by an Improved External Diffusion Method
The external diffusion technique permits easy mechanical fabrication of Nb3Sn-based superconducting multifilamentary composites due to the lower work-hardening rate of Cu compared to Cu-Sn bronze. However, Kirkendall porosity can appear to a disastrous extent in external diffusion composites rather than the lesser degree apparent in internal diffusion products.1 Figure 1 shows a cross section of the extremely fragile material resulting from the application of the external diffusion technique to multi filamentary composites with no precautions taken to suppress Kirkendall porosity. Figure 2 shows the vastly improved cross section resulting from the incorporation of a solution preanneal prior to electroplating in the external diffusion method. This approach was developed in our laboratory2 upon recognition of the heterogeneous nucleation mechanism in the observed Kirkendall porosity. The heterogeneously nucleated voids precipitate from the temporary supersaturation of lattice vacancies caused by the differing diffusion mobilities of copper and tin in the copper matrix. The nucleation substrates are mainly particles of oxide, tarnish, or other foreign matter adherent to the surfaces of the original composite components or included in the copper itself.
KeywordsCritical Current Density Fibre Size External Diffusion Nb3Sn Layer Fragile Material
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