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Chemically bonded composite SOL-Gel ceramics : a study of alumina-phosphate reaction products Moorlag, Carolyn


Chemical bonding is an alternative bonding process to sintering and fusion and can usually be carried out at relatively low temperatures. This study focuses on the analysis of the aluminum phosphate reaction products formed by reaction of alumina and phosphates (phosphoric acid or monoaluminum phosphate) in low-temperature (100-600°C) systems to form chemically-bonded ceramics. The main objectives are the identification of products formed, determination of the influence of alumina sol phases, and characterization of some chemically-bonded coatings. Characterization studies are carried out by X-ray diffraction, scanning electron microscopy, electron dispersive spectroscopy, infrared spectroscopy, and magic angle spinning nuclear magnetic resonance spectroscopy. It is found that two product types, aluminum orthophosphates (AIPO4) and aluminum metaphosphates (A1(P03)3), are formed by separate reaction sequences. Aluminum metaphosphates (meta-A and meta-B) are formed from monoaluminum phosphate via aluminum triphosphate hydrate (ATH), and are favored by high phosphate-loading conditions (A1/P<1) and phosphate reaction with heat-treated (500°C) alumina sol. Aluminum orthophosphates form directly and are favored by low phosphate loading conditions (A1/P>1) and phosphate reaction with hydrated alumina sol phase. Analysis of morphology of mixed aluminum phosphate products generally shows 50-100 urn particles composed of alumina and AIPO4 phase with clusters of fine particles of phosphate rich phases (ATH, meta-B, meta-A) surrounding. When monoaluminum phosphate is used as a phosphate source rather than phosphoric acid, the reaction products are affected less by the Al/P ratio or the hydration of alumina sol. Heat-treated alumina sol phases, composed of either microcrystalline boehmite or y-alumina, are shown to affect the type and crystallinity of the aluminum phosphate reaction products. MAS NMR experiments are used to identify and quantify aluminum phosphate products formed. Microhardness of composite sol-gel ceramics impregnated with phosphoric acid is -2.5-3.0 GPa after heat treatment at 400-500°C. Decrease in microhardness with heat treatment at temperatures >400°C is correlated with crystallization of aluminum metaphosphate phases. Analysis of some ceramic coatings on metals analyzed for product content indicate that phosphate content in the coatings is low (Al/P>2). The information obtained from the study may be used to further develop energy-efficient alternatives to high-temperature firing of certain industrial ceramics.

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