The role of oxygen and zirconium in the formation and growth of Nb 3 sn grains
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I.
INTRODUCTION
T H E intermetallic compound Nb3Sn is a type 1I metallic superconductor of interest because of its high values of superconducting critical current density in high magnetic fields. I~-sI In type II superconductors, the critical current density is controlled by microstructural heterogeneities that pin the supercurrent flux lines, with strong pinning leading to high transport currents. In Nb3Sn, it has been suggested that grain boundaries are the primary flux-pinning centers, and thus control of grain size is essential to the superconducting properties of this material, tS,6j One technique developed for forming NbaSn is to react liquid tin with solid niobium. The most successful approach developed thus far for controlling the grain size of Nb3Sn formed by this technique uses alloy additions of zirconium and oxygen, tT'Sm However, the mechanism by which zirconium and oxygen control the grain size of Nb3Sn has not been addressed. This article describes a study of the microstructural aspects of Nb3Sn made by this technique that was undertaken in an effort to understand this mechanism. All compositions given are in atomic percent. II.
EXPERIMENTAL PROCEDURE
A. Nb3Sn Processing Nb3Sn foil processing requires multiple steps. The general method has been described elsewhere, ts,s,~~ and a detailed procedure can be found in References 11 and 12. L E RUMANER, formerly with GE Corporate Research & Development, Schenectady, NY, is a Graduale Smdenl, Universily of Washington, Sea~tle, WA, 98195. M.G. BENZ, MetalIurgis~, and E.L. HALL, Microscopist, are with GE Corporate Research & Development, Schenectady, NY 12301. Manuscript submitted November 17, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS A
Nb-lZr foil and Nb-2Zr, 0.00254-cm thick by 2.54-cm wide, and pure niobium foil, 0.0051-cm thick by 1.91-cm wide, were used to produce Nb3Sn. The asrolled foil was found to contain surface oils associated with tbe rolli,g operation and was cleaned using a H2SO4-Hf-H20 solution. Specific amounts of oxygen were added to the niobium alloy foil surface by anodization. The process used an electrolyte solution of 7 g of sodium sulfate (NaSO4) per liter of distilled water at 25 ~ The thickness of the oxide film was controlled by the voltage of the anodization process. To prevent the tin alloy from running off the surface of the foil during the reaction anneal, a foil surfaceroughening operation was incorporated at this stage. The effective thickness of the foil was increased from approximately 25.4 to 35.6 /xm because of the surfaceroughening operation. Following the roughening, the foil was annealed for 60 seconds by passing it through a 1000 -+ 5 ~ furnace in an argon atmosphere to allow the oxygen to diffuse into the foil. During the anneal, it is energetically favorable for the surface oxide to dissociate according to the reaction: "Nb2Os" = 2Nb + 50 where Q represents dissolved oxygen in the niobium alloy foil. The oxygen-containing niobium alloy foil was then passed through a molten Sn-17Cu bath at approximately 1050
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