The Diffusion Process in Nb3Sn-Cu Superconducting Wire Made by the External Tin Method
The external Sn process for producing Nb3Sn-Cu superconducting wire generally employs a solid state diffusion process for the Sn diffusion step. The latter process is reviewed and shown to have limitations at larger wire sizes due to both long time requirements and a blister formation problem. New experimental data are presented on wire geometries and compared to previous results on sheet geometries. It is concluded that the blister formation problem results from nucleation of rows of voids at the ε/α interface during stage I of the process. The void and blister formation problem is found to be more severe with a wire versus a sheet geometry, which is consistent with the idea that phase transformation induced tensile stresses play a key role in nucleation of the void rows. Methods for minimizing the blister problem, and implications for the Sn core process are discussed.
KeywordsVoid Formation Sheet Sample Blister Formation Solid State Process Wire Sample
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