The relationship between superconducting properties and microstructures of the bronze-processed Nb
3Sn composite wires is reviewed in connection with the stress effect, the martensitic transformation, the pinning mechanism and the nature of the Nb
3Sn layer (i.e. grain size, and grain boundary segregation and structure etc.).
The Nb
3Sn layer in the composite wire is compressively prestrained at cryogenic temperatures because of the difference of the thermal contraction between the Nb
3Sn layer and the bronze matrix. This resultant compressive prestrain affects superconducting properties and the martensitic transformation of the bronze-processed Nb
3Sn compounds.
Martensitic transformation is inevitable for the pure Nb
3Sn compound at and near the stoichiometric composition, and this transformation brings about a derease in
Hc2 of Nb
3Sn. The addition of Ti and Ta to Nb
3Sn not only prevents the martensitic transformation but increases its electrical resistivity and
Hc2. Therefore, the high field performance of the multi-filamentary Nb
3Sn composites is improved by the addition of Ti and Ta.
The possibility of an electron scattering flux pinning mechanism has been recently proposed for Nb
3Sn compounds. Although
Jc of the bronze-processed Nb
3Sn wires depends much on the nature of Nb
3Sn layers, there are few studies about it. More detailed studies are desired for understanding and improving superconducting properties of the bronze-processed Nb
3Sn wires.
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