Growth kinetics and characterization of superconducting properties of multifilament materials made by the ECN powder method

Analysis of the growth of the niobium-tin phases during heat treatment of multifilament material containing niobium tube and NbSn2powder has revealed that the occurrence of an Nb6Sn5intermediate phase has a great influence on the superconducting properties of the final A-15 phase. It is found that a very high Jccombined with high Tcand Bc2values can be obtained. On the other hand, the homogeneity of the structure in both radial and axial direction and the current carrying capacity of the A-15 layer is strongly dictated by the heat treatment temperature and the composition of the original powder core. From this study an improved filament composition is found which now guarantees excellent and well controllable superconducting properties. Further experimental work to find the dependence of the overall current density on the powder core size is underway. This should eventually give a tool for further improvement. Also the determination of the critical current degradation under application of mechanical strain and bending is proceeding. Provisional results are given in this paper.