Nb₃Sn超電導線材のマイクロストラクチャーと特性

抄録

The relationship between superconducting properties and microstructures of the bronze-processed Nb₃Sn composite wires is reviewed in connection with the stress effect, the martensitic transformation, the pinning mechanism and the nature of the Nb₃Sn layer (i.e. grain size, and grain boundary segregation and structure etc.).
The Nb₃Sn layer in the composite wire is compressively prestrained at cryogenic temperatures because of the difference of the thermal contraction between the Nb₃Sn layer and the bronze matrix. This resultant compressive prestrain affects superconducting properties and the martensitic transformation of the bronze-processed Nb₃Sn compounds.
Martensitic transformation is inevitable for the pure Nb₃Sn compound at and near the stoichiometric composition, and this transformation brings about a derease in H_c2 of Nb₃Sn. The addition of Ti and Ta to Nb₃Sn not only prevents the martensitic transformation but increases its electrical resistivity and H_c2. Therefore, the high field performance of the multi-filamentary Nb₃Sn composites is improved by the addition of Ti and Ta.
The possibility of an electron scattering flux pinning mechanism has been recently proposed for Nb₃Sn compounds. Although J_c of the bronze-processed Nb₃Sn wires depends much on the nature of Nb₃Sn layers, there are few studies about it. More detailed studies are desired for understanding and improving superconducting properties of the bronze-processed Nb₃Sn wires.