The study on the critical temperature, composition, and microstresses in a Nb3Sn layer Original Research Article

Monofilamentary Nb3Sn wires of large diameter with niobium tube, which were obtained by the method of solid-phase diffusion, are well suited for the study of the distribution of the critical temperature Tc in Nb3Sn layers. Three regions with different gradients of Sn and Nb concentration and different Cu content can be distinguished in Nb3Sn layer. In the central part of the layer, the Sn content comprises 24.5 at.% and the gradient of Sn concentration is negligibly small. Measurements on specimens of 1.2 mm in diameter with a slit cut along the cylinder generatrix showed that the critical temperature of the Nb3Sn region adjacent to Cu(Sn) bronze is lower than the critical temperature of the central part of the layer. Fluctuations of Tc in the central part of the layer exceed the change of Tc related to the gradient of the Sn concentration, which is very small. These fluctuations spread both the R(T) curve and the high-temperature part of the temperature transition registered by the inductive method.