Microstructure, Diffusion and Growth Mechanism of Nb3Sn Superconductor by Bronze Technique

Nb-Ti is a widely used superconductor material. However, its use is limited to applications of magnetic field upto 8T (Sharma, 1987). At the present, Nb3Sn intermetallic compound with A15 structure is considered to be one of the most suitable superconductors for the applications where field requirements go beyond the limit of Nb-Ti superconductors. However, intermetallic compounds are in general brittle and cannot be drawn as wire. To circumvent this problem, different manufacturing technologies have been developed for Nb3Sn, such as bronze method, internal tin process, powder metallurgy route, Jelly roll process, ECN technique (Suenaga, 1981; Sharma, 1987) etc. In this chapter, we shall discuss mainly the growth and diffusion mechanism of Nb3Sn fabricated by the bronze technique. In this method, several Nb rods are inserted inside Cu(Sn) bronze alloy and drawn as a multifilamentary wire. The product phase Nb3Sn is grown during the subsequent annealing by solid state diffusion. The efficiency of a superconductor wire largely depends on the presence of microstructural defects, such as grain boundaries and Kirkendall pores, grain size distribution, the variation of chemical composition over the cross section (Suenaga, 1981; Suenaga & Jansen, 1983; Lee & Larbalestier, 2005; Lee & Larbalestier, 2008) and so on. The application of pure Nb3Sn compound has been found to be limited to magnetic fields of 12 T, since the increase in the field drastically reduces the critical current density, Jc. Further improvements have been achieved by alloying Nb3Sn with different elements, such as Ti, Ta, Zr, Mg etc. (Suenaga et al., 1986). Thus, the aim of this chapter is to discuss and analyse various factors which affect the growth of the product phase by diffusion controlled process. The evolution of microstructure is detemined by the thermodynamics and kinetics of the system. The combined thermodynamic-kinetic approach will be discussed, which provides a feasible tool to rationalize the formation of the observed reaction structures.