Ab Initio Study of Electronic and Geometric Structures of Metal/Ceramic Heterophase Boundaries
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S. KOSTLMEIER*, C. ELSASSER-, B. MEYER", M. W. FINNIS*** Max-Planck-Institut ffir Metallforschung, Seestrasse 92, D-70174 Stuttgart, Germany; Max-Planck-Institut ffir Metallforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany. Atomistic Simulation Group, School of Mathematics and Physics, Queen's University of Belfast, Belfast BT7 INN, Northern Ireland. ABSTRACT The adhesion geometries of coherent cube-on-cube interfaces between spinel (MgAI2 0 4) and the two metals Al and Ag were determined by density functional band structure calculations in the local density approximation (LDA). For AI/MgAI 2 0 4 , for which experimental data are available, the calculated optimum interface geometry is in excellent agreement with HRTEM measurements (distance dit: 1.90 A calc., 1.90+0.04 A exp.). The work of adhesion Wad is calculated for three different high-symmetry translation states between an AI-O terminated (001) surface of the spinel and the (001) surface of each of the metals. The binding energy curves display a distinct optimum for the adhesion of aluminum atoms on top of the 2 spinel oxygen ions at a Wad value of 2.4 J/m . For silver several adsorption sites are isoenergetic 2 at 1.1 J/m and the intersections of the Wad(dint) curves indicate a low-energy dissociation path. A further analysis of the electronic structure in the Al/MgAI2 0 4 system reveals the charge redistribution in the metal towards the oxygen ions as the main contribution to bonding. On the contrary, polarization of the metal film is the major effect observed on the adhesion of Ag to the spinel substrate.
INTRODUCTION The goal of the present investigations is a basic understanding of the factors that enhance or inhibit bonding at metal-ceramic interfaces at a microscopic scale. For this purpose ab-initio electronic structure theory was employed to study both the geometric and the electronic structures of two model adhesion systems, namely A1/MgAI 20 4 (001) and Ag/MgAI20 4 (001). These systems were carefully chosen to avoid a large lattice misfit between the two constituents of the interface (0.27% for AI/MgA120 4 , 1.25% for Ag/MgAl204). In this manner, a nearly unperturbed 1:1 relation between the metal atoms and the high-symmetry adhesion sites on a spinel AI-O-surface is obtained, which makes the strain play a minor role in the experiment and which also helps to keep the repeat unit cell for the ab-initio calculations at a manageable size. By high resolution transmission electron microscopy (HRTEM) it was observed that Al exhibits strong coherent bonding on top of atom columns of the spinel side with a contracted interface distance of 1.90 ± 0.04 A(ideal: ao/4 = 2.02 A; a, is the lattice constant of bulk spinel) [1]. From comparison with other interface systems containing a main group (Al) or an early transition metal (Ti, V, Nb) on a main group oxide (MgO [2,3], A120 3 [4-6]) it can be assumed that the metal atom bonds to the oxygen ions at the spinel surface. However, adhesion on spinel Al ions can not be ruled out on the basis of available
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