iii Introduction In all materials, the physical and even the chemical properties of a polycrystalline body are not exactly the same as those of a single crystal of the same materials. Many materials are anisotropic -their properties depend upon the orientation of the measuring system with respect to the crystallographic axes -and the polycrystalline properties are some form of an average over the crystal directions. Thus the dielectric constant, elastic constants, index of refraction, magnetic susceptibility, and many other "bulk" properties depend to some extent on the microstructure of the specimen . Beyond this, some properties depend on the motion of various entities through the material-transport of atoms and ions in diffusion, transport of phonons in thermal conduction and electrons and ions in electrical conduction, motion of dislocations and other defects in plastic deformation and of domain walls in ferromagnetic and ferroelectric switching, and even the propagation of cracks in fracture.In these transport processes, the grain boundaries between the crystals in a polycrystalline body behave differently from the bulk material, and their presence markedly affects the resulting properties.Diffusion is usually faster at grain boundaries, especially at low temperatures, so that diffusion is enhanced in polycrystalline bodies. Electrons and phonons are scattered by grain boundaries, so that electrical and thermal conduction tends to be lower in the polycrystals . The movements of dislocations across grain boundaries are impeded, so that plastic deformation is inhibited by their presence, and polycrystalline bodies tend to be stiffer and less ductile than the corresponding single crystals.Finally, the presence of grain boundaries not only modifies the behavior, but sometimes even introduces new elements.Thus in brittle fracture the grain boundaries provide sources of cracks, making polycrystals weaker in general than single crystals.The presence of strain and of impurities at grain boundaries raises the local free energy, so that chemical effects, such as etching rates, are enhanced.It is clear, then, that a knowledge of the microstructure of a polycrystalline body is essential in any attempt to study and control its properties. This is particularly important in the field of ceramics, where the overwhelmingly important form is the polycrystalline body. In order to review the problems involved in specifying and studying microstructure in ceramics and the factors involved in the interaction between microstructure and physical properties of ceramics, this Symposium on Microstructure of Ceramic Materials was held. The papers presented are published in this volume. Primary responsibility for their technical content must rest, of course, with the individual authors and their organizations.In the first two Chapters, Prof. Van Vlack reviews the geometry of microstructures and how they can be specified and Prof. Frechette describes the principal experimental techniques by which observations of microstructures are made....
