Boehmite sintering

The starting material is a state-of-the-art flat y-alumina membrane prepared by dipcoating of a boehmite solution on a macroporous a-alumina support and subsequent firing at 600°C as described in [4], The a-alumina support is prepared from AKP-30 powder by making a colloidal suspension of this powder in diluted nitric acid and subsequent filtration. After filtration the wet cake is dried overnight and sintered for 1 hour at 1100°C. The resulting flat a-alumina supports have a mean pore diameter of 80 nm. A detailed description of the support synthesis is provided in chapter 4. 

Figure 1. The Vickers hardness (testing load 1 kg) of fine-grained sintered alumina ceramics prepared by a sol/gel approach starting with boehmite (7-seeding).

Figure 2. Microstructure with typical cluster of micropores in a submicrometer sintered alumina ceramic produced from a boehmite sol with corundum (a-Al203) seeds <0.2 pm.

Because the performance of spinel windows is also very sensitive to the quality of the precursor materials (both purity and morphology), there have been researches that can be found in the open literature. To produce better spinel precursor powders, various methods have been developed. For example, a new method has been used to synthesize spinel powders from boehmite, in which Mg " ion was metal exchanged onto the surface of boehmite particles [208]. In this method, particle size, size distribution, purity, and stoichiometry of the Mg-doped boehmite powders could be well controlled. Such powders exhibited good sinterability and resulted in spinel ceramics with desired optical properties. 

The sintering behavior ofa diphasic mullite powder starting from colloidal silica (surface area 200 m /gm) and two different Al sources as given in Table 6-12 were studied by Kara and Little (1996). The boehmite-sihca mixture (Powder A) formed mullite at 1250°C in the presence of 5-A1203 and an amorphous phase, and the aluminum sulfate silica mixture (Powder B) formed mullite at 1200°C in the presence of 5/-AI2O3 and an amorphous phase. The bulk density of the compact (Powder A) increased sharply at 1100°C, with the complete elimination of open porosity at 1250° C. 

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