High-rate sputter deposition and heat treatment of thick Nb-Al-Ge and Nb-Al superconductors

Abstract

Crystal structures, microstructures and critical temperatures were determined for Nb-Al-Ge and Nb-Al sputter-deposits in order to evaluate their dependence on sputter-deposition conditions, heat treatment procedures and composition. High-rate sputter deposition techniques were used to make the deposits at rates up to 1 Μm/min. Compositions studied were Nb3(Al0.6Ge0.4), Nb3(Al0.75Ge0.25), Nb3Al, Nb2.52(Al0.84)Ge0.16), Nb2.33Al, Nb3.07(Al0.75Ge0.25), and Nb4.15(Al0.71Ge0.29). The investigation indicated it is feasible to make practical A-15 phase superconductors by high-rate sputter deposition. Of all the deposits studied, Nb3(Al0.75Ge0.25),i.e., Nb2Al3Ge, deposited at 15°C and heat treated at 750°C for 1 to 5 days had the highest critical temperature (18.5 K), and it had a very high critical current density;e.g., 4.4 × 105 A/cm2 at 100 kOe and 4.2 K. Deposits having the highest critical temperatures consisted only of undecomposed metastable A-15 phase. The high current density was attributed to the presence of very small A-15 phase grains, which were observed to be about 350a in diameter by transmission electron microscopy. The crystal structures for deposits made at 15°C were not always clearly defined, but probably were all body-centered-cubic. Body-centered-cubic phases were transformed by heat treatment for short times at 550°C to 850°C to an A-15 phase that was supersaturated with Al and Ge. If heat treatment temperatures were too high or heat treatment times were too long, however, minor phases formed as the A-15 phase decomposed. Nb3(Al0.75Ge0.25) deposits made at elevated temperatures (485°C and 750°C) predominantly consisted of the A-15 phase, but the presence of minor phases even before heat treatment indicated Al and Ge were not completely retained in the A-15 phase solid solution during deposition. Deposits made with-20 V substrate-anode bias had nearly the same composition as the sputtering target, but deposits made with -50 V and -75 V bias had significantly lower Al and Ge contents than the target. The compactness of the A-15 crystal structure relative to the bcc structure was noted in a comparison of average atomic volumes for the two structures in Nb3Al. The average atomic volume in the A-15 phase formed by heat treatment was 1.36 pct less than the average atomic volume in the bcc phase that existed before heat treatment in sputter-deposited Nb3Al made at 15°C.