Facile Synthesis of Nb 3 Sn Via a Hydrogen Reduction Process
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NB3SN intermetallic compound has a wide range of applications in advanced technologies due to its superconductivity in large current and strong magnetic field, such as magnets for NMR spectrometers, fusion reactors, and particle accelerators.[1–3] The brittleness and high melting point (2403 K (2130 C) of Nb3Sn, as shown in Figure 1),[1] however, are major technical obstacles resulting in the complex manufacture techniques of powder-in-tube process, bronze process, and internal tin process.[4–6] All the processes demand precursor powders, such as Nb3Sn, Nb6Sn5, or NbSn2. Moreover, the critical current density of a superconductor is increased obviously with decreasing the grain size of Nb3Sn.[7] If nanopowders can be used, a better superconductor with the grain size of Nb3Sn less than 200 nm can be achieved.[8,9] Therefore, there is a strong demand but a big challenge to develop a facile method for the synthesis of Nb3Sn nanopowders with a high melting point. There are many approaches to prepare Nb3Sn powder, such as mechanical alloying, plasma process, metallothermic reduction of oxides, FFC Cambridge
JUN ZHU, Postdoctoral Fellow, is with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Xueyuan Road 30, Beijing 100083, China, and School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Xueyuan Road 30, Beijing 100083, China. SHUQIANG JIAO and HONGMIN ZHU, Professors, and LONG ZHANG, Master Student, are with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing. Contact e-mails: [email protected] [email protected] YANXIANG LI, Professor, is with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Xueyuan Road 30 Beijing 100083 P.R. China. Manuscript submitted October 24, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
process, and chemical vapor synthesis (CVS).[10–13] Among them, CVS is a promising approach for synthesizing nanosized products, because a large amount of dispersed particulates can deposit from gas phase in short time by homogeneous nucleation.[14–16] Hydrogen reduction of metal halides vapor is a typical process of CVS. Comparatively, the size, microstructure, and component of the finally formed powders can be controlled properly by the reduction parameters, such as precursor concentration, reaction temperature, and carrier gas flow rate.[17–19] There are few works, however, concerning the preparation of Nb3Sn nanopowders by hydrogen reduction. Kato et al.[20] have synthesized Nb3Sn powders with the particle sizes ranging from 0.05 to 0.2 lm by hydrogen-reducing NbBr5–SnCl4 vapor at 1673 K to 1773 K (1400 C to 1500 C). However, NbBr5 is uncommon and expensive, and the high vapor pressure of SnCl4 [boiling at 330 K (57 C)] results in the difficulty of controlling the precursor concentration, which is a key to determine the size and composition of the nanopowders. Therefore, alternative precursors should be developed. The
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