Interface Structure, Grain Morphology, and Kinetics of Growth of the Superconducting Intermetallic Compound Nb3Sn Doped with ZrO2and Copper

Abstract

AbstractOne method for the fabrication of the superconducting compound Nb3Sn involves interdiffusion of a surface coating of Sn alloyed with Cu on Nb containing Zr and O. In this study, the kinetics and microstructure associated with this reaction have been studied in detail. The results show that small Nb3Sn grains nucleate at the Nb3Sn/Nb interface, and that the Nb3Sn grains experience grain growth immediately after they are formed. ZrO2precipitates are observed in the Nb3Sn at the Nb3Sn/Nb interface and throughout the Nb3Sn. The ZrO2precipitates occur in the form of small partially-coherent spheres in the Nb3Sn. No ZrO2precipitates are observed by TEM in the unreacted Nb. The grain boundaries in the Nb3Sn region are coated with a Sn-Nb-Cu alloy which would have been liquid at the diffusion/reaction temperature. The thickness of the Nb3Sn reaction layer formed during the isothermal diffusion anneal is proportional to time to the first power, indicating “reaction”-controlled rather than “diffusion”-controlled kinetics. The absence of diffusion-controlled kinetics can be explained by the presence of the liquid coating on the Nb3Sn grains. Diffusion of Sn in this liquid layer is apparently fast enough to not be the limiting kinetic step.