Microstructure alumina solution

Concrete made with calcium aluminate cement at a properly low w/c ratio is highly resistant to sulphate solutions, sea water or dilute acid solutions with pH > 4, including natural waters in which CO2 is the only significant solute. Resistance may extend to pH 3 if the salt formed is of sufficiently low solubility. Midgley (M96) showed that, for fully converted material exposed to a sulphate ground water for 18 years, penetration with formation of a substituted ettringite was limited to a depth of 5 mm. These properties are consistent with Lea s (L6) view that the resistance is due to the formation of a protective coating of alumina gel, coupled with the absence of CH. No fundamental studies, e.g. on microstructural effects, appear to have been reported. 

Uhlhorn et al. [12] studied the effect of PVA with a molecular weight of 72000 Dalton on the microstructure of y-alumina membranes by gas absorption, water permeability and solute retention measurements and concluded that the structure of supported thin alumina films with and without PVA is similar. In a more recent study Ziiter et al. [15] found no significant effect of the molecular weight of the PVA (range 3000-155000 Da, fixed concentration) on the pore characteristics of the final (calcined) alumina or alumina-zirconia membranes. Pore diameters between 3.2 and 3.8 nm (with porosities of 52-56%) were found after calcination at 600°C for 3 h. 

Thin films of the 1-2-3 superconductor were prepared on zirconla and alumina substrates by adding ca. 10% solutions of the citric acld/ethylene glycol precursor to the surface of the substrate followed by drying and pyrolysis in oxygen to 950 C. XRD patterns indicate that the 1-2-3 orthorhombic phase is the only crystalline phase formed under these conditions. Detailed microstructural and physical property studies of these films are in progress. 

Platinum was deposited by impregnation into the framework of y-alumina membrane tubes with an asymmetric configuration, using ammoniac-hexachloroplatinic solutions at different pH values and dipping times. Metallic platinum was obtained after calcination and reduction. The microstructure of the membranes was studied by SEM and BET their gas permeabilities were measured as well. The heat delivered during the formation of PtO on membranes prepared in different conditions were measured in order to compare their activities. Cyclohexane dehydrogenation reaction was carried out on these membranes. Tlie effect of the preparation conditions on the catalytic activities is discussed. 

For some powder compositions, such as sodium (3-alumina and lead-based electroceramics, evaporation of volatile components (e.g., Na and Pb) can occur during sintering, thereby making it difficult to control the chemical composition and the microstructure of the sintered material. A common solution is to surround the powder compact with a coarse powder having the same composition as the compact, which leads to the establishment of an equilibrium partial pressure of the volatile component, thereby reducing the tendency for evaporation from the powder compact. As an example of this approach. Fig. 12.21 shows a schematic illustration of the system used for the sintering of sodiiun (3-alumina in a labora-... 

The results of development work on processes indicate that the two main methods of preventing the duplex microstructure from forming appear to be fast-firing, or increasing the amount of / "-alumina at low temperatures. Based on these results, Duncan et al. [21] and Zyl et al. [22] have described production processes starting from aluminum oxyhydroxides or aluminum hydroxides as precursors for the synthesis of the solid electrolyte "-alumina. Duncan et al. described an alumina precursor which substitutes in part or wholly for a-alumina in an established slurry solution spray-drying process. As a precursor, hydrothermal boehmite, Cera hydrate, has been used. A calcination step is important at the begirming of the process. Boehmite was used both in the as-received condition and after calcination. The effect of the calcination temperature on the fired properties of /S"-alumina can be seen in Table 21.5. 

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