High-rate sputtering of Nb-Al-Ge and Nb-Al superconductors

High quality Nb-Al-Ge and Nb-Al superconductor deposits, up to 1 mm thick, were made at a rate of 1 μm/min using high-rate sputtering techniques. High-rate sputtering eliminates most of the fabrication problems generally encountered with these materials. Reasonable quantities of high-field superconductors can be made at acceptable cost for practical applications by high-rate sputtering. Highly reflective metallic mirror surfaces on these deposits indicate a high quality, even in thicknesses up to 1 mm. The deposits were completely free of voids and they did not seem to be overly brittle either before or after heat treatment. Crystal structures for deposits made at 20°C were not clearly defined, but probably were body-centered-cubic (bcc). Heat treatment between 550°C and 850°C completely transformed the bcc deposits to extremely small (350 Å) grains of the A-15 crystal structure. The A-15 phase formed by heat treatment for 1 hr to 5 days at 750°C was metastable and supersaturated with Al and Ge. Deposition and heat conditions for decomposition of the metastable A-15 phase were identified. The highest critical temperatures (18°K) were observed only in those deposits containing undecomposed metastable A-15 phase. To assess practical use of the sputtered superconductors, 1- to 2-m lengths of ribbon and filamentary Nb-Al-Ge superconductor were produced on ribbon and wire substrates. It also was found that an excellent superconductor-stabilizer bond can be formed by high-rate sputter deposition of copper stabilizer onto heat-treated Nb-Al-Ge and Nb-Al superconductors.