Ceramic systems theoretical models

The degree of anisotropy of a property may be negligible, but this is not usually the case in indentation hardness measurements on ceramic crystals. Later we will consider the phenomenological aspect of hardness anisotropy to demonstrate that, whatever the ramifications of the theoretical models, the nature of anisotropy is consistent and reproducible for a wide range of ceramics. Then we shall consider the models based on a resolved shear stress analysis and discuss their implications in terms of the role of plastic deformation and indentification of active dislocation slip systems. 

This chapter discusses materials known as electroactive composites. These are mixtures of an electroactive (or in the present context, ferroelectric) ceramic phase and a polymer phase. Early sections will look at the individual phases, making use of piezoelectric parameters defined in Chapter 5. A summary of theoretical models will then be given, before preparation and characterization are discussed. Finally, the problems associated with producing an active device and examples of working systems will be considered. 

The electrical properties of heterogeneous media have been modeled for over 100 years. Meredith and Tobias [1962], Mitoff [1968], and McLachlan et al. [1990] have given clear accounts of their scope and vahdity. However, since these articles cover the case where the conductivity or permittivity are real, which normally means dc conductivity or permittivity of loss-free dielectrics, they are not directly apphca-ble to IS. AC properties have been discussed by Wimmer, Graham, and Tallan [1974] with special reference to ceramics. The dielectric literature has been reviewed by van Beek [1965], while Dukhin and Shilov [1974] have described models that include the effects of the interfacial double layer. Sihvola [1999] has produced a comprehensive survey of the properties of mixed phase systems with coverage of the historical and theoretical background. 

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