EFFECT OF OXYGEN AND ZIRCONIUM ON THE GROWTH AND SUPERCONDUCTING PROPERTIES OF NB3SN

The intermetallic compound Nb3Sn is a type II superconductor of intere st because of its high superconducting critical current density in hig h magnetic fields. One technique for forming Nb3Sn is to react a molte n tin alloy with a solid niobium-zirconium-oxygen alloy. It was found that the properties of Nb3Sn are directly related to its microstructur e, which is in turn directly related to the O:Zr atom ratio in the sta rting niobium foil. For a niobium alloy foil with an O:Zr atom ratio o f 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 r elatively constant critical current density. For a niobium foil withou t oxygen, the resulting Nb3Sn grains are large and columnar and grow w ith a diffusion-limited layer growth rate. The resulting critical curr ent density is low and decreases with reaction time. For a niobium all oy 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 sec ond layer of large, columnar-grained Nb3Sn growing with a diffusion-li mited rate. As a function of reaction anneal time and similar to the g rain growth, the critical current initially increases linearly and the n decreases.