| Summary: | 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. === Applied Science, Faculty of === Materials Engineering, Department of === Graduate
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