The effect of heat treatment on the superconducting properties of a multifilamentary Nb3Sn composite

The superconducting properties of a 2046 filament IMI multifilamentary Nb3Sn superconductor have been investigated. The top of the superconducting transition increases with time and temperature of annealing until a fairly steady value near 18K is achieved. Removal of the bronze increases the transition temperature but still leaves it depressed in very thin Nb3Sn layers. Microprobe analysis shows that the thicker layers are stoichiometric and contain only ∼ 0.25 weight % copper. The critical current increases with time and temperature of annealing at 630°C and 690°C, but longer anneals at 730°C and 780°C produce decreases from the maximum values. For the longer anneals at 630-730°C the critical current density in the Nb3Sn is relatively constant at a given field, but Nb3Sn formed at 780°C has a lower current density. Flux pinning follows a similar dependence on grain size to that reported by West and Rawlings, and the shape of this graph is explained by Luhman and Suenaga´s suggestion that the stress produced by the differential contraction of the composite induces the martensitic transformation and consequently more pinning centres in the Nb3Sn.