Effect of oxygen and zirconium on the growth and superconducting properties of Nb3sn

Abstract

The intermetallic compound Nb3Sn is a type II superconductor of interest because of its high superconducting critical current density in high magnetic fields. One technique for forming Nb3Sn is to react a molten tin alloy with a solid niobium-zirconium-oxygen alloy. It was found that the properties of Nb3Sn are directly related to its microstructure, which is in turn directly related to the O: Zr atom ratio in the starting niobium foil. For a niobium alloy foil with an O:Zr atom ratio of 2, the resulting Nb3Sn layer is fine grained and grows linearly with reaction anneal time until the entire Nb-Zr-O alloy core is consumed. This leads to a linear increase in critical current with time and a relatively constant critical current density. For a niobium foil without oxygen, the resulting Nb3Sn grains are large and columnar and grow with a diffusion-limited layer growth rate. The resulting critical current density is low and decreases with reaction time. For a niobium alloy foil with an O:Zr ratio of >0 but <2, fine-grained Nb3Sn is formed initially and grows with a linear layer growth rate, followed by a second layer of large, columnar-grained Nb3Sn growing with a diffusion-limited rate. As a function of reaction anneal time and similar to the grain growth, the critical current initially increases linearly and then decreases.