Synthesis of a Mullite Precursor from Aluminum Nitrate and Tetraethoxysilane via Aqueous Homogeneous Precipitation: An 27Al and 29Si Liquid- and Solid-State NMR Spectroscopic Study |
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Authors: | Isabelle Jaymes,ré Douy,Dominique Massiot,Jean-Pierre Busnel |
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Affiliation: | CNRS, Centre de Recherches sur la Physique des Hautes Températures, 45071 Orleans Cedex 02, France;Universitédu Maine, Laboratoire de Physico-Chimie Macromoléculaire, URA 509, 72017 Le Mans Cedex, France |
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Abstract: | A simple aqueous process is described for the preparation of aluminosilicate colloids and chemically homogeneous mullite precursor gel. Starting from a solution of aluminum nitrate and silicic acid, aluminum is slowly hydrolyzed at 80–100°C by in situ generation of ammonia. A silica gel is rapidly made, probably by a catalytic effect of urea, the base generator. This gel is then slowly digested by partially hydrolyzed aluminum species which break the Si-O-Si bonds and link to the gel by Si-O-Al bonds. Progressively a clear colloidal sol is obtained and the colloidal particle size continues to decrease toward aluminosilicate species where the silicon atoms are in a single environment and may be linked to three hexacoordinated aluminum atoms and a hydroxyl group, by reference to natural imogolite. When the hydrolysis of aluminum is nearly complete, these particles are cross-linked and a final gel precursor of mullite is obtained. This gel is chemically very homogeneous and crystallizes to mullite at 980°C. The structural evolution, from the first gel to the ceramic, has been followed by 27Al and 29Si liquid- and solid-state MAS NMR spectroscopy. |
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