Ceramics and Composite Materials

Book Preface

In stabilized zirconia, which is one of the representative solid electrolytes, the grainboundary resistivity is known to be ~102-104 times higher than the grain-interior one. For the applications such as solid oxide fuel cells and electrochemical gas sensors operated at the intermediate (~600oC) and low temperature (~400oC) regime, the improvement in ionic conduction across the grain boundary becomes significant. For this, a precise estimation of the grain-boundary conduction via impedance spectroscopy is essential. In Chapter 1, New Approaches for Estimating and Improving the Grain-Boundary Conduction in Stabilized Zirconia, Jong-Heun Lee suggests the new methods for improving and estimating the grainboundary conduction in stabilized zirconia. In the first part, the various approaches to improve the grain-boundary conduction are discussed. The addition of Al2O3 is known to scavenge the siliceous grain-boundary phase. However, this might deteriorate the graininterior conduction when the sintering temperature becomes very high (>1600°C). Therefore, new routes for improving the grain-boundary conduction using two-stage sintering process are suggested. The formation of a Si-containing phase in a discrete configuration and the dewetting of the intergranular liquid phase were suggested to be the mechanisms for scavenging via pre- and post-sintering heat treatments, respectively. In the second part, a local impedance technique using a sub-millimeter-scale electrode array was suggested to estimate the spatially uneven distribution of the grain-boundary resistivity, which was named as ‘Millicontact Impedance Spectroscopy’. The fundamentals and validity of the technique were explained, and the analyses of the dynamic rearrangement of an intergranular liquid phase are given as an example.

In the second chapter, Effective Elastic Moduli of Alumina, Zirconia and Alumina- Zirconia Composite Ceramics, Willi Pabst and Eva Gregorová investigate from the theoretical point of view, with a side-glance on experimental results and applications. In the first section alumina, zirconia and alumina-zirconia composites are introduced as structural materials, relations of elastic moduli to other properties are recalled and targets of microstructural design are formulated. In the second section elastic properties are defined from the viewpoint of rational mechanics for anisotropic and isotropic materials in general. The difference between adiabatic and isothermal elastic moduli is explained and estimated for alumina and zirconia. In the third section effective elastic properties are defined and discussed from the viewpoint of micromechanics and composite theory. General formulae are given for the calculation of effective elastic moduli of polycrystalline materials from monocrystal data. Further, the Voigt-Reuss bounds for the effective elastic moduli of multiphase materials are given, as well the Hashin-Shtrikman bounds for the special case of two phase materials. For porous materials the dilute approximations are recalled as well as the predictions following from the self-consistent, Mori-Tanaka, differential, Gibson-Ashby and Coble-Kingery approach as well as the functional equation approach recently advocated by the authors. A comprehensive survey of model relations for the porosity dependence of elastic moduli is given, including exponential and power-law relations and a new relation recently proposed by the authors, which seems to be the simplest relation allowing for the occurrence of a percolation threshold (critical porosity). In the fourth section all these micromechanical bounds and relations are applied to the alumina-zirconia system. Using a theoretically sound and experimentally confirmed set of elastic moduli (and Poisson ratios) for dense (i.e. fully sintered) polycrystalline alumina and zirconia the Hashin Shtrikman bounds of dense alumina-zirconia composites are calculated and compared to experimentally measured values. Several predictions for porous alumina, zirconia and alumina-zirconia composites are compared to the data measured for ceramics with convex interconnected pores prepared by the starch consolidation casting technique. A master fit curve is given for porous ceramics with this type of matrix-inclusion microstructure and explicit numerical expressions are given throughout. The last section gives examples of the mathematical modeling of other effective properties and their dependence on composition and microstructure and an outlook is given to future research aims. In particular, the significance of interfaces is emphasized and ideas on the way from micromechanics to nanoscience − towards a general mixture theory − are outlined.

A strong interest in the use of ceramics for biomedical engineering applications developed in the late 1960’s. Used initially as alternatives to metallic materials in order to increase the biocompatibility of implants, bioceramics have become a diverse class of biomaterials presently including three basic types: relatively bioinert ceramics maintain their physical and mechanical properties in the host and form a fibrous tissue of variable thickness; surface reactive bioceramics which form a direct chemical bonds with the host; and bioresorbable ceramics that are dissolved with the time and the surrounding tissue replaces it. In Progress in Bioceramic Materials, P. N. De Aza gives a review of the composition, physicochemical properties and biological behaviour of the principal types of bioceramics, based on the literature and some of our own data. The materials include, in addition to bioceramics, bioglasses and bio-glass-ceramics. Special attention is given to structure as the main physical parameter determining nor only the properties of the materials, but also their reaction with the surrounding tissue.