Enhancement of jc at 10-12 T in Nb3Sn wires by artificial Ta inclusions distributed at a nanometre scale

Abstract

A new type of multifilamentary Nb3Sn wires prepared by the bronze route but with composite cores is found to exhibit considerably enhanced jc values with respect to conventional Ta alloyed wires in the intermediate field range, i.e. >30% between 10 and 12 T. The enhancement of jc is obtained by artificially introduced continuous ribbon-like Ta inclusions of <or=10 nm thickness, i.e. close to the coherence length. These inclusions are distributed at a nanometre scale over the filament cross section. The enhanced-pinning at the present wires ('artificial pinning') is a consequence of the new Nb3Sn/Ta interfaces with a total area comparable to that of the already existing grain boundary interface. The present method is an extension of the original powder-metallurgical composite core method recently developed in the authors laboratory. Considerable progress was achieved in view of industrial fabrication, the power-metallurgical Nb/Ta cores being replaced by continuous cores consisting of Ta rods surrounded by Nb tubes and bundled together. At the final stage, the Ta-Ta distance is estimated to be approximately=20 nm, i.e. close to the calculated flux line distance at 10 T. Measurements of jc up to 20 T as well as the effects of uniaxial tensile strain on jc are presented. Further enhancement of jc is expected by optimising the Ta content and the separations of the Ta-Ta ribbons.