Synthesis of nano intermetallic Nb_3Sn by mechanical alloying and annealing at low temperature

摘要

In this study, intermetallic Nb_3Sn of nanometer-sized grains was synthesized by powder metallurgy route. Elemental powders of Nb and Sn in the stoichiometric proportions were mechanically alloyed for 3 h in a high-energy mill under a protective atmosphere of argon. X-ray diffraction patterns of milled powders confirmed the formation of a Nb(Sn) solid solution evidenced by the presence of Nb peaks only, which are shifted to higher angles. Rietveld refinements used to analyze this XRD pattern indicated a better fit when a tetragonal structure with the space group I4/mmm is used instead the Nb cubic lattice with space group Im-3m. Size-strain analysis from line-broadening of peak profiles by using "double-Voigt" approaches showed that the broadening is due to both a small crystallite size (around 6nm) and microstrains. Subsequent heat treatment of the Nb(Sn) powder mixture was required for the formation of the Nb_3Sn ordered phase. X-ray diffraction patterns obtained after a thermal treatment at 700 °C for 1 h were fitted using a multiphase Rietveld refinement. Although the resulting powders are composed mainly by Nb_3Sn (up to 87 weight%), certain amount of other intermetallic phases like Nb_6Sn_5, NbSn_2 and Nb and Sn oxides were also determined. In agreement with the Rietveld refinement analysis, microprobe analysis also revealed that changes in chemical composition at different sites of powder particles are preserved even after annealing at 700 °C. Magnetic properties measured at 300 K on resulted Nb_3Sn powders evidenced the soft super paramagnetic materials nature through its magnetization hysteresis loops.