Transformation of mechanically alloyed Nb-Sn powder to Nb 3 Sn
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Communications Transformation of Mechanically Alloyed Nb-Sn Powder to Nb3Sn S.N. PATANKAR and F.H. (SAM) FROES Nb3Sn was processed via mechanical alloying (MA). The powder mixture comprised of stoichiometric proportions of elemental niobium and tin powder was mechanically alloyed for 3 hours and the mechanically alloyed powder mixture was heat treated. While MA resulted in Nb-Sn solid solution, the reaction leading to the formation of Nb3Sn occurs during the subsequent heat treatment of the powder mixture.
Superconductivity is generally regarded as a “macroscopic quantum phenomenon” wherein an element, intermetallic alloy, or compound conducts electricity without resistance. Type I superconductors are comprised of pure metals, whereas type II superconductors are comprised primarily of alloys or intermetallic compounds.[1–5] Nb3Sn with A-15 type structure has high critical current density, jc, at very high magnetic fields; it is a much sought after type II superconductor. Because of their brittle nature, all the routes involving manufacturing of A-15 type superconductors make use of ductile precursors that are heat treated to form the A-15 compound by solid-state solution. In the case of the Nb3Sn superconductor, the Nb3Sn builds up at the Sn(bronze)/Nb interface and gradually spreads inside the Nb. The rate of progression is slow and the reaction requires an exceedingly long time to reach completion. Use of the mechanically alloyed Nb-Sn powder mixture could potentially ensure complete and near instantaneous transformation of the Nb-Sn powder mixture to Nb3Sn, thereby eliminating the need for independent long duration heat treatments that are normally required during the processing of Nb3Sn superconductors. In our present study dealing with processing of Nb3Sn, mechanically alloyed Nb-Sn was found to transform into Nb3Sn during the heat treatment that followed. The objective of this article is to discuss the formation of Nb3Sn during the heat treatment of the MA Nb-Sn powder mixture. The materials used in the present study were 99.9 pct pure niobium powder and 99.9 pct pure tin, with sieve size of 325 mesh (44 m). For high-energy milling, a model 8000 Spex (Spex CertiPrep, Metuchen, NJ) mill was employed for MA. Milling was carried out “dry” in a carbide-lined steel cylinder, under an Ar atmosphere to minimize oxidation using 0.25-in.-diameter stainless steel balls. The chargeto-ball ratio was 1:10, the charge being comprised of 10 g of a stoichiometric powder mixture. Structural transformation was monitored using the PHILIPS* X-ray diffractometer *PHILIPS is a trademark of Philips Electronic Instruments Corp., Mahwah, NJ.
S.N. PATANKAR, Visiting Scientist, and F.H. (Sam) FROES, Director, are with the Institute for Materials and Advanced Processes, University of Idaho, Moscow, ID 83844-3026. Contact e-mail:[email protected] Manuscript submitted February 3, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
(XRD) with Cu K radiation. A Perkin-Elmer differential thermal analyzer (D
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