Preparation of Nb₃Al by high-energy ball milling and superconductivity

The A15 phase superconductor Nb₃Al has been considered as an alternative to Nb₃Sn for high field and large scale applications. However, to prepare a stoichiometric Nb₃Al with fine grain structures is very difficult. High-energy ball milling is a solid state powder processing technique and is a very useful for preparing Nb-Al alloys (Nb₃Al). The effects of ball milling time and annealing temperature on the formation of Nb₃Al superconducting phase have been studied. Pure Nb and Al powders with stoichiometric ratio of Nb₃Al were mixed and milled, and the charging and milling were performed in an inert atmosphere. Phase formation and structural evolution during high-energy ball milling have been examined by X-ray diffraction. Al disappeared and Nb peaks broadened after about one hour of milling. With increasing milling time, the peaks of Nb became considerably broader and intensities decreased, the Nb-Al solid solution phase was extensive when milled about 3 hours. In order to obtain Nb₃Al superconducting phase, a subsequent anneal was required. We have annealed the as-milled powders at 800-900°C for different times to prepared Nb₃Al superconducting alloy. The results indicated that Nb₃Al with small amount of impurity phase can be obtained on annealing the Nb-Al solid solution phase and the superconducting transition temperature was about 15K, but it is difficult to obtain a homogeneous Nb₃Al phase by annealing the amorphous powder.