Silica-alumina phase diagram

The silica-alumina phase diagram ss denotes solid solution. (Adapted from F. J. Klug, S. Prochazka, and R. H. Doremus, Alumina-Silica Phase Diagram in the Mullite Region, /. Am. Ceram. Soc., 70[10],758 (1987). Reprinted by permission of the American Ceramic Society.)... 

The lime (C), silica (S), and alumina (A) phase diagram is shown in Fig. 17.1a as mol%, and where the area of Portland cement and high alumina cement are indicated. Identify on the phase diagram the location for the following minerals found in (a) Portland cement clinker, C3S, C2S, and C3A (b) found in high alumina clinkers, CA, and CA2. 

Take the silica-alumina system as an example. It is convenient to treat the components as the two pure oxides SiOj and AI2O3 (instead of the three elements Si, A1 and O). Then the phase diagram is particularly simple, as shown in Fig. 16.6. There is a compound, mullite, with the composition (Si02)2 (Al203)3, which is slightly more stable than the simple solid solution, so the alloys break up into mixtures of mullite and alumina, or mullite and silica. The phase diagram has two eutectics, but is otherwise straightforward. 

Silica and aluminum phosphate have much in common. They are isoelec-tronic and isostructural, the phase diagrams being nearly identical even down to the transition temperatures. Therefore, aluminum phosphate can replace silica as a support to form an active polymerization catalyst (79,80). However, their catalytic properties are quite different, because on the surface the two supports exhibit quite different chemistries. Hydroxyl groups on A1P04 are more varied (P—OH and A1—OH) and more acidic, and of course the P=0 species has no equivalent on silica. The presence of this third species seems to reduce the hydroxyl population, as can be seen in Fig. 21, so that Cr/AP04 is somewhat more active than Cr/silica at the low calcining temperatures, and it is considerably more active than Cr/alumina. 

The phase diagram for the Na20-Al203-Si02 system illustrates the thermochemical relationships between these three components [2-4], Knowledge of the equilibrium relations in this system is pertinent to the attack of alumina-silica ceramics by alkali vapors. 

Figure 4.19. Phase diagram of the silica/alumina system. 134...

Figure 2. The binary Al203-Si02 system, (a) Equilibrium phase diagram by S. Aramaki and A. Roy, J. Amer. Ceram. Soc., 45, 229-242 (1962) reproduced with permission of The American Ceramic Society, Westerville, OH. (b) Non-equiibrium phase sequences from heating sol-gel silica aluminas [44] reproduced with permission of the Institute of Materials, London.

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. 

Commercially, the silica-alumina system is an important one because the principal constituents of many ceramic refractories are these two materials. Figure 12.25 shows the Si02-Al203 phase diagram. The polymorphic form of silica that is stable at these... 

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