Metallurgy, Fabrication, and Superconducting Properties of Multifilamentary Nb sub 3 Al Composites

摘要

The control of metallurgical structure during fabrication that will improve the superconducting properties of multifilamentary, aluminium-stabilized, Nb sub 3 Al composites is described. Composites are fabricated by placing niobium rods in an aluminum matrix, and then drawing to wire. Nb sub 3 Al is formed at temperatures exceeding 1800 exp 0 C for about 5 s and ordered at 750 exp 0 C for 48 h. A critical current, J/sub c/(H), in excess of 10 exp 5 A/cm exp 2 (F/sub p/ approximately equal to 7 x 10 exp 8 dynes/cm exp 3 ) at 7 T and a T/sub c/ to 18.2 K are obtained. Attempts to improve J/sub c/(H) by controlling the grain size in the Nb sub 3 Al diffused layer are discussed. Precipitates, arising from the addition of carbon during Nb sub 3 Al layer growth, do not appear to be effective as grain-boundary or flux pinners. When 1 percent Zr is added to the Nb, the growth of the Nb sub 3 Al layer is accelerated, T/sub c/ is lowered and J/sub c/(H) is not significantly improved. J/sub c/(H) rapidly decreases with an increase in Nb sub 3 Al or (Nb-Zr) sub 3 Al layer thickness, d. J/sub c/(H) is independent of d in composites with d greater than or approximately equal to 1.5 mu m. In general, the Nb sub 3 Al grain size appears comparable to d for d less than or equal to 1 mu m. Significant improvement of J/sub c/(H) for Nb sub 3 Al superconducting composites reacted at temperatures above 1800 exp 0 C (to achieve T/sub c/ greater than 17 K) is achieved only by maintaining the layer thickness well below d approximately equal to 1.0 mu m. (ERA citation 02:026969)