High-resolution electron microscopy of grain boundary structures in yttria-stabilized cubic zirconia
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High-resolution electron microscopy of grain boundary structures in yttria-stabilized cubic zirconia K. L. Merkle, L. J. Thompson, G.-R. Bai, and J. A. Eastman Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, U. S. A. ABSTRACT The atomic-scale structure of grain boundaries (GBs) in yttria-stabilized cubic zirconia (YSZ) was investigated by high-resolution electron microscopy (HREM). Non-stoichiometric oxides have found a wide range of applications and therefore it is of importance to explore the role of GBs and their atomic-scale relaxation modes. [001] and [110] tilt GBs were examined by HREM in highly textured thin films of YSZ grown by metal-organic chemical vapor deposition (MOCVD). In addition, a special technique was developed to also allow the HREM study of twist and general GBs. GBs and triple junctions show quite dense arrangements of cation atomic columns. The GB core structures in YSZ can be contrasted to the more open structures in stoichiometric cubic oxides, such NiO, which are characterized by a relatively large GB excess volume. This appears to be due to several factors, including the necessary rearrangement of the oxygen sublattice near GBs in a CsCl2 type structure, the redeployment of oxygen vacancies near GBs, and the segregation of Y to the GB. Relative to stoichiometric oxides, such mechanisms provide additional degrees of freedom for atomic relaxations at GBs and the development of low-energy GBs. These additional relaxation modes, which result in GB cation arrangements more akin to metallic systems, are also reflected by Burgers vector dissociations observed in low-angle YSZ GBs. INTRODUCTION Zirconia based materials are of great importance in a broad range of technological applications. In many of these areas GBs play an important and often dominant role in determining the properties of the material and functionality of the technical systems. In the Y2O3-ZrO2 system the cubic phase can be stabilized to low temperature over a wide range of composition, from roughly 8 to 30 mol.% Y2O3. Figure 1 illustrates the cubic unit cell of the YSZ lattice. The Y3+and Zr4+ cations are arranged on an fcc lattice and consequently a high concentration of oxygen vacancies is present in the bulk to maintain charge neutrality. When two rigid blocks of YSZ material are put together to form a grain boundary, it is immediately clear, as illustrated in figure 2, that a considerable amount of restructuring of the oxygen sublattice is necessary to avoid energetically impossible configurations associated with having ions with like charges in close proximity to each other. To study the atomic-scale structure of GBs by HREM it is necessary that adjoining grains are closely aligned to low index zone axes. In the oxides, the scattering power of oxygen typically is too weak to give any distinctive features that can be related to the position of the oxygen sublattice, however cation columns can under favorable conditions give rise to motifs that can be directly related to their position. A detaile
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