PHYSICO-CHEMICAL STUDY OF THE BEHAVIOR OF A MULLITE PRECURSOR SYNTHESIZED WITH CO-PRECIPITATION
Abstract
The co-precipitation of hydrated forms of aluminum oxide and silica with the addition of ammonia has been studied. Thermal curves of a dried co-precipitated product showed that at low temperature (up to ~300 °C) there was a set of clearly defined endothermic peaks which resulted from dehydration of adsorbed and hydrated water. Then, a gradual removal of water was observed up to ~ 600 °C, which corresponded to the transition aluminum hydroxide Al(OH)3 → monohydrate (boehmite) γ-AlOOH. After it, the sample weight remained constant, and exothermic peaks were because of the spinel formation (about 900 °C) and the mullite crystallization from the spinel phase (above 1200 °C). After calcination at 900–1000 °C, the Al–Si spinel phase of γ-Al2O3 type was dominated, though some slight signs of crystalline mullite were already appeared. The high dispersion of hydrated alumina and silica particles determined their considerable reactivity. It resulted in the mullite appearance at a rather low temperature. Sharp peaks which concerned to single orthorhombic mullite were registered since 1150–1200 °C. At the same time, the spinel reflexes practically disappeared. The most intensive phase changes were in the range of 1100–1200 °C. The peak positions and intensity of calcined products agreed well with the reference data for mullite. Its lattice parameters were determined. The average crystallite size was ranged from 6.3 nm at 1100 °C to 7.4 nm at 1200 °C. The effective activation energy was calculated by non-isothermal method (Avrami equation) as (740 ± 40) kJ/mol. This magnitude was in a well accordance with the activation energy values for the diffusion of the Si4+ ions in the mullite layer which were estimated to range from 730 to 780 kJ/mol (literature data). So, one might assume that the limiting stage was just the diffusion of the Si4+ ions.
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