Growth of A15 Nb-Si by epitaxy and composition grading

Films of niobium-rich A15 Nb-Si were grown epitaxially on polycrystalline A15 Nb-Ir using electron beam evaporation. The Nb-Si films were made niobium rich in order to match lattice parameters with the Nb-Ir. Some samples were then brought toward the 3:1 stoichiometry by a composition-grading technique. Composition grading can enhance transition temperatures Tsub(c) by in some cases more than 6 K. The maximum resistive Tsub(c) onset is 13.2 K. The highest Tsub(c)s are achieved when the Nb-Ir lattice is tending to expand the Nb-Si overgrowth beyond its own natural lattice parameter. Transmission electron microscopy experiments show that the ability of composition grading to bring the A15 phase toward the 3:1 stoichiometry is limited by the growth of silicon-rich phases in the A15 grain boundaries. (Auth.)