Effect of Sn Concentration in Bronze Matrix on the Pre-Reaction Formation of Nb3Sn Layers in Bronze-Processed Superconducting Strands of Different Design

The goal of this paper is to study and analyze the effect of Sn concentration (14% or 16%), the mode of Ti-doping and the shape of Nb filaments on the thickness of the “parasitic” layers of prereacted Nb 3 Sn grains (formed before the diffusion annealing) in the ITER-type strands and strands for high field applications processed by bronze technology. The use of short (of total duration not more than 1 h) intermediate heat treatments at 500 °С does not allow avoiding the prereaction formation of Nb 3 Sn in all the samples under study. Sn concentration in bronze decreases by 1.5–2.0% after heat treatments compared to the initial state in all samples. In the samples with Ti-doped bronze matrix and coupled Nb filaments, the average thickness of “parasitic” layers is 112 nm in the case of 14% Sn in bronze and 140 nm for a bronze matrix with 16% Sn. In the sample with 16% Sn in matrix and extended Nb filaments of intricate shape the average thickness of prereacted layers reaches 200 nm. It is concluded that to minimize the parasitic layers formation the strands with high tin bronze matrix should be subjected to short intermediate heat treatments at temperatures lower than 500 °C.