Investigations of the chemistry and bonding at niobiumsapphire interfaces

Spatially resolved electron energy-loss data have been recorded at the interface between niobium and sapphire (α-Al₂O₃), a model metal/ceramic couple. The spatial-difference technique is used to extract interface specific components of the energy-loss near-edge structure (ELNES), which are dependent on the chemistry and bonding across the interface. Multiple scattering calculations of aluminum, oxygen, and niobium clusters were performed to simulate the measured Al L_2,3 ELNES. Two samples fabricated by different techniques were examined. The first interface was made by diffusion bonding pure crystals. Its interface spectrum is identified with tetrahedral coordination of the Al ions at the interface. The calculations match the experimental edge structures, supporting the notion of aluminum to niobium metal bonding and concurring with a structural model in which the basal plane of sapphire at the interface is terminated by a full monolayer (i.e., 67% excess) of aluminum. The second sample was produced by molecular beam epitaxy. The spectrum of this interface is consistent with an atomistic structure in which the interfacial basal plane of sapphire is terminated by oxygen. An unoccupied band of states within the band gap of Al₂O₃ is observed, signifying chemical bonding between metal and ceramic.