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HIGH-RESOLUTION MICROSCOPY OF CERAMICS C.B. CARTER, S. MCKERNAN, D.R. RASMUSSEN, Y.K. SIMPSON, S.R. SUMMERFELT, D.W. SUSNITZKY AND L.A. TIETZ Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853

ABSTRACT The use of high-resolution electron microscopy in the study of ceramic materials is illustrated with examples from current research. Particular emphasis is placed on the preparation of the sample and the difficulties in interpretation which may result from the techniques which are necessary for these hard, chemically inert materials.

INTRODUCTION Electron microscopy is an established tool for the study of structures of ceramic materials. However, the question can still be asked regarding the role of high-resolution techniques in this study. One aspect of this review paper will be to discuss where has it been and where is it going? Two problems are particularly pressing: the first is the problem of sample preparation, the importance of this factor became clear with the extensive effort devoted to the study by TEM of high-Tc superconducting oxides. The second problem is related to the uniqueness of image interpretation, and involves the use of image simulation/processing. Of course, these problems are not unique to ceramic materials but, because of the nature of the material (hardness and structural complexity), they assume even greater importance. Many of the properties of ceramic materials are governed by the chemistry and structure of interfaces, whether these be surfaces, grain boundaries or phase boundaries. The status of the understanding of interface structure has been greatly improved in recent years through the application of high-resolution electron microscopy (HREM) and this advance forms the basis of the present talk. Particularly important for the interpretation of high-resolution images is the application of image simulation and image processing; both of these aspects will therefore be discussed. Illustrations will be presented from current work on the surfaces and grain boundaries in alumina and spinel, and phase boundaries between materials with similar structures or between materials having very different structures. For the former, a striking case is the NiO/NiFe 2 O4 phase boundary, where the anion sublattice can be continuous across the interface. For the case where the structures are very different, the alumina/spinel interface is particularly relevant with many new orientation relationships having recently been identified. It is emphasized that, in these studies, HREM is only one of the imaging techniques which must be used in order to understand the structures fully.

EXPERIMENTAL The experimental observations discussed in this paper were made on a number of very different materials. Full accounts of the sample preparation methods used are given in the cited references. However, wherever the details of the preparation technique are particularly important factors in the discussion, these will be given explicitly. All of the samples were thinned by mechanically