Inversion domain boundaries in ZnO ceramics
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Inversion domain boundaries in ZnO ceramics M. A. McCoy Department of Materials, Imperial College of Science, Technology and Medicine, London, and Department of Engineering Materials, University of Sheffield, Sheffield, United Kingdom
R. W. Grimes Department of Materials, Imperial College of Science, Technology and Medicine, London, United Kingdom
W. E. Lee Department of Engineering Materials, University of Sheffield, Sheffield, United Kingdom (Received 22 May 1995; accepted 4 April 1996)
Inversion domain boundaries (IDB’s) in ZnO ceramics, associated with Sb2 O3 doping, have been characterized using a range of electron microscopy techniques. The ¯ IDB’s lie primarily on basal planes, but frequently are stepped along prismatic h1210j planes. The basal IDB can be characterized as (i) an inversion that causes an antisite exchange of cations and anions across the boundary, (ii) an effective displacement ≠ Æ of the sixfold screw axis in the wurtzite structure vectors by a translation of 1y3 1010 , and (iii) a displacement normal to the boundary. Significant Sb segregation is detected in the basal IDB segments in agreement with previous work, and in ceramics doped with Sb2 O3 and Bi2 O3 . These IDB’s contained both Sb and Bi, suggesting that while Bi does not participate in IDB nucleation, it resides in the boundary. Comparison of experimental and calculated HREM images suggests that the IDB is composed of a monolayer of Type I (111) zinc antimonate spinel, consisting of a single layer of octahedrally coordinated zinc and antimony cations. I. INTRODUCTION
Zinc oxide ceramics have been widely studied because of their use as varistors (nonohmic resistors) for electronic device protection. A typical varistor composition contains a number of different dopants, each added to optimize different aspects of varistor performance.1 The most important dopant is bismuth oxide, which acts to promote liquid-phase sintering as well as to produce an insulating grain boundary layer, the key to the varistor behavior; this layer exists either as a discrete intergranular phase,2 or as a segregation layer less than a nanometer thick which is incorporated into the ZnO grain-boundary structure.3 The bismuth oxide is thought to act as a grain boundary donor which sufficiently alters the local oxygen vacancy and zinc interstitial concentration to create a barrier to electronic transport through the grain boundary.4 Antimony oxide (Sb2 O3 ) is typically added to inhibit grain growth, thereby controlling the grain boundary area/volume ratio and subsequent varistor breakdown voltage.5 In addition to controlling grain growth, grain-boundary segregation of Sb2 O3 has been shown to produce varistor behavior in the binary ZnO–Sb2 O3 system.6 A typical feature in the microstructure of a commercial varistor is a twin-like structure which bisects essentially every ZnO grain (Fig. 1). These defects are associated with Sb2 O3 doping7 (Fig. 2), although similar faults have been observed in ZnO ceramics doped
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