Alumina Phase diagrams

Fig. 1. CryoHte—alumina phase diagram from 0 to 18.5% alumina. L, Hquid S, cryoHte S2, comndum O, Hquid , Hquid and soHd , soHd (14).

G. K. Duncan and A. R. West, The stoichiometry of p"-alumina phase diagram studies in the system NazO-MgO-LizO-AlzOj, Solid State Ionics 28-30 (1988) 338-43. 

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.)... 

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. 

Supported Cu-Pd catalysts have the potential to provide new alternatives to conventional commercial methanol synthesis catalysts (based on the Cu-ZnO-alumina system). Cu-Pd catalysts are also of industrial interest in hydrogenolysis and CO oxidation (Bulatov 1995). To interpret the catalyst behaviour and selectivity, including CO hydrogenation, a fundamental understanding of the structure, surface structure and stability of the phases in this system is required. The Cu-Pd phase diagram indicates that at temperatures greater than 600 °C, Cu... 

From this point we can proliferate other additions in limited amounts which further depress the fusion temperature. Notable of these are lime, alumina, soda and potash. The limitation in amounts is shown by phase diagrams to the facts that, above a limit, fusion temperatures climb rapidly. For example, an addition of 20% lime to the sio2-70% FeO mixture will lower the fusion temperature to 1150 C but a further addition up to 40% will bring the fusion temperature to over 1400 C. 

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 shows that the most refractory mixtures contain 72 wt % to 100 wt % A1203 (high-alumina compositions). A liquid phase develops in these mixtures (if pure) at a temperature above 1840°C. Refractories in this composition range can be used to temperatures near 1840°C. 

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. 

Fig. 12. The high-alumina portion of the Na20-Al203 phase diagram [57], Reprinted by permission of the American Ceramic Society.

FIGURE 8.21 Phase diagram for the system Na02-Al203-H20 at four different temperatures. The precipitation of Gibbsite, Bayerite, Boehmite, and alumina from supersaturated aqueous solutions is dependent on the temperature. (From Wefers, K. and Misra, C., Report 19, Alcoa Laboratories, 1987, 44. With permission.)... 

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