| Niobium-silicon compounds with the A15 crystal structure have been of interest for the possibility that they would have superior superconducting properties. The maximum superconducting transition temperature is expected to occur for a well-ordered A15 phase containing 25 atomic percent silicon. The A15 phase does not appear in the niobium-silicon equilibrium phase diagrams, so the structure must be synthesized by nonequilibrium means.; Electron beam evaporation has been used to synthesize A15 niobium-silicon films. The addition of oxygen during evaporation stabilizes the A15 structure, and permits the formation of a homogeneous, well-ordered, single-phase film for compositions up to approximately 18.5 atomic percent silicon. The maximum transition temperature is 8.3 K.; In order to understand the role played by oxygen, metastable phase diagrams were prepared, and the initial growth of films was studied as a function of substrate temperature and oxygen pressure. It is suggested that the stabilization is due either to oxygen atoms retarding grain boundary diffusion, or to oxygen atoms entering the lattice in small amounts and creating a more stable ternary.; A technique combining polycrystalline epitaxy with composition grading also stabilizes the A15 phase. The resulting A15 phases have higher silicon than do the films prepared with oxygen, and the transition temperatures can be as high as 13.2 K. However, the films produced by the epitaxy-plus-grading method are not single-phase.; Structures and transition temperatures have been monitored at different stages of the epitaxial and graded film growth. The ability of this synthesis method to stabilize the A15 structure was found to be limited by the growth of silicon-rich phases in the A15 grain boundaries. |
