Important Ceramic Phase Systems

In this chapter, the phase relations of those oxide components will be discussed that most commonly occur in ceramics, namely silica, alumina, calcia, magnesia, potassia, and iron oxide. The chapter is introduced by a cursory analysis of the anatomy and construction of phase diagrams based on Gibbs phase rule, progressing from one component to two components, and from three components to multicomponent systems. The thermodynamics of these phase assemblies will be described, some important phase diagrams discussed, and conclusions drawn. 

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Being the only stable intermediate phase in the Al203-Si02 system at atmospheric pressure, mullite is one of the most important ceramic materials. Mullite has been fabricated into transparent, translucent, and opaque bulk forms. These materials may have optical and electronic device applications. Mullite s temperature stability and... 

Figure 4.22 Schematic phase diagram for the MgO-Al203-Si02 system, which contains several important ceramic materials...

Since the most important glass-forming systems are based on silicate compositions, the key crystalline components of glass-ceramics are therefore silicates. Certain oxide minerals, however, are important, both in controlling nucleation as well as forming accessory phases in the final product. 

The relation between conductivity, chemical and phase compositions, and the structure of separate phases of solid electrolytes is very important In many systems, the maximum in conductivity closely coincides with the existing F-phase (fluorite-phase) limit TEM image of Figure 13.3 shows the formation of a dense single-phase polycrystalline microstructure for the sintered ceramics [1]. X-ray diffraction and analysis of electron diffractograms of this sintered ceramic confirmed the fluorite structure. 

The interactions between the polymer and ceramic phases of a composite are paramount if the material is to be used in any practical device. This is particularly important in 0-3 composites, where the polymer-filler interface within the system is considerable. Inadequate bonding between the components due to physical and chemical incompatibility, contaminants and defects on either surface can have profound effects on the ultimate integrity and performance of the composite. Careful selection of compatible components and the optimization of processing conditions are therefore of prime importance. 

Although the dioxides of zirconium and hafnium crystallize at room temperature in the monoclinic baddeleyite-type structure, this is closely related to fluorite, and they do form fluorite-type solid solutions with rare earth oxides which have important ceramic properties. Compared with Th, U", Ce etc., Zr " and Hf have considerably smaller ionic radii which are close to that of the smallest ion (Sc ), and this fact has an important influence on the phase relationships in these systems, leading to the appearance of intermediate phases not encountered in other MO2-R2O3 systems. They have been studied extensively, particularly in the first instance by Collongues et al. (1965 and references therein). Work published prior to 1964 has been reviewed by Mobius (1964). 

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