Investigations of the chemistry and bonding at niobiumsapphire interfaces
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R. Brydson Department of Materials Science, University of Surrey, Guildford GU2 5XH, United Kingdom
H. Miillejans, J. Mayer, G. Gutekunst, W. Mader,b) D. Knauss, and M. Riihle Max-Planck-Institut fur Metallforschung, Institutfur Werkstoffwissenschaft, 70174 Stuttgart, Germany (Received 20 December 1993; accepted 21 June 1994)
Spatially resolved electron energy-loss data have been recorded at the interface between niobium and sapphire ( a - A l 2 O 3 ) , 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 L2>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 A12O3 is observed, signifying chemical bonding between metal and ceramic.
I. INTRODUCTION The heterogeneous interface between metals and ceramics is of particular interest because of its widespread occurrence in structural, electronic, and functional materials, as well as in catalyst systems.1"3 Selected as a model metal-ceramic system, niobium-on-sapphire is well suited for several reasons.4 The metal exhibits strong adhesion to the sapphire; in certain "special" orientations, there is small mismatch (less than 2%) between lattice planes in the two materials; the coefficients of thermal expansion are very similar and there is no evidence for the formation of reaction phases in pure materials; and atomically and chemically abrupt interfaces can be readily fabricated. A number of interfaces in this system have been studied by high resolution electron microscopy (HREM) in recent years, a major goal being to elucidate the atomistic structure and determine its
^Present address: Department of Materials Science, Whitaker Laboratory, Lehigh University, Bethlehem, Pennsylvania 18015. ^Present address: Anorganische Materialforschung, Institut fur Anorganische Chemie der Universitat Bonn, R6merstra/?e 164, 53117 Bonn, Germany. 2574
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J. Mater. Res., Vol. 9, No. 10, Oct 1994
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role on physical properties such as fracture strength.5 n Various methods can be employed to produce the atomically flat, clean interfaces, includi
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