Optimum Synthetic Conditions of Ammonium Aluminum Carbonate Hydroxide (AACH) as Starting Material for α-Alumina Fine Powders

Abstract

Ammonium aluminum carbonate hydroxide (ARCH) as starting material to obtain α-alumina fine powders was synthesized between aqueous solutions of ammonium hydrogen carbonate (AHC) and ammonium alum (AA). The effects of pH of the AHC aqueous solution and reaction temperature on the formation of AACH were discussed. Three products, amorphous aluminum hydroxide, pseudoboehmite (=low crystalline boehmite) and AACH were formed depending on pH. AACH was mainly formed in alkaline aqueous solutions. The optimum synthetic condition of AACH was discussed in terms of the relation between pH and concentration of species calculated by the master variable technique of ionic equilibrium in AHC and AA aqueous solutions. NH4+, AlO(OH)2- and HCO3- species were neccessary to form AACH. Crystallite size of AACH decreased with increasing concentrations of ionic species, [NH4+]⋅[AlO(OH)2-]⋅[HCO3-] and decreasing reaction temperature. Homogeneous, spherical α-alumina particles with a particle size of 0.2μm were obtained by firing at 1050°C for 6h from the AACH with a crystallite size of 50Å synthesized at a pH of 10.0 at 5°C.