Model Systems for Metal-Ceramic Interface Studies
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termination of atomistic structure, chemical composition, and bonding. Theoretical methods are then used to calculate bonding and atomistic structures of these interfaces.10"12 We summarize here studies of model insulating and semiconducting oxide substrates and interfaces with strongly and weakly interacting metals.
The Metal-Alumina System A common ceramic material and one that is often used as a model for weakly
interacting metal supports is alumina.13 The geometric structure of the a-Al2O3 surfaces have been well-characterized using low-energy electron diffraction and reflection high-energy electron diffraction.14"17 While most electronicstructure measurements can be interpreted in terms of the bulk properties,1819 the Al-O bond ionicity at the surface with respect to the bulk is lower.20 Ultraviolet photoelectron spectroscopy of single-crystal A^CyOOOl) shows the valence band to be dominated by the oxygen 2p levels with total bandwidth of 8.0 eV.21 Further, substantial increases in conductivity cannot be induced through doping. The interaction of metals with the alumina surface tends to be dominated by the overlayer's ability to reduce the surface cations. Thus many transition metals interact only weakly with the surface due to the extremely high oxygen affinity of Al. One example of a weak metal-ceramic interaction is the Cu(lll)/a-Al2O3(0001) interface.22 Figure 1 shows an HRTEM image of such an interface. Information about the bonding mechanism and electronic structure of this interface was derived by quantitative energy-loss nearedge structure (ELNES) studies indicat-
Figure 1. Atomic-resolution image of the copper/sapphire interface obtained on the Stuttgart JEM-ARM1250 operating at 1250 kV. The (022)Cu planes with a lattice spacing of 1.28 A are clearly resolved in this cross-sectional micrograph. The Cu/AI2O3 interface shows no strain field. The interface is atomically sharp. The orientation relationship is (111)cull(0001)s, [2M]Cu\\[21 10]s, (s: sapphire).2*
MRS BULLETIN/AUGUST 1997
Model Systems for Metal-Ceramic Interface Studies
Figure 2. Cross-sectional transmission electron micrograph of a Cu/CuAIO2/AI2O3 interface obtained with a JEM-2000FX operating at 200 kV. The orientation relationship between Al£)3 and the CuAIO2 reaction layer is determined to be (0001)sll(0001)cuAio2, [1120]S\\[1010]CUMO2. (Unpublished results by A. Liedtke.)
pressures of KT3 mbar at 1000°C. Thus the Cu has ample oxygen with which to form compounds during interface formation. Figure 2 shows a TEM image of such a CU/AI2O3 interface; the interlayer phase formed is CuAlO2. Therefore the Cu at the interface of the Cu/Al2O3 system under these conditions is part of a Cu-Al-O compound. A second model system, the Nb/ a-Al2O3 interfaces with different crystallographic orientations, has been investigated in detailed studies.728"38 Nb/Al2O3 serves as a model system for metals on £¥-Al2O3 surfaces since the thermal-expansion coefficients of both materials are nearly identical. The interfaces were produced by different methods
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