Activation energy of mullitization from various starting materials

The activation energy (Ea) of mullitization from various starting materials was investigated using both our own data and data from the literature. The starting materials included mixtures of kaolinite and alumina, sol mixtures, diphasic gels, hybrid gels, monophasic gels, glasses and glass fibers. The first crystalline phase formed by heating these starting materials can be of two types, i.e. (1) γ-Al2O3, which is formed in mixtures of kaolinite and alumina, sol mixtures, diphasic gels and hybrid gels and (2) Al-rich mullite, formed in monophasic gels, glasses and glass fibers. Irrespective of the initial crystalline phase, the Ea values for mullitization range from 800 to 1400 kJ/mol and show a maximum at about 1000 °C. These Ea values are higher than for other silicates, for example, <800 kJ/mol found for the CaO–Al2O3–SiO2 system, and are thought to be due to the overlap of the Ea values for nucleation and nucleation-growth (crystallization). The Ea values for both these processes are high for mullitization, reflecting the high Ea value for diffusion of Si ions in the Al2O3–SiO2 system. The steps controlling mullitization in type (1) starting materials are thought to change at higher temperatures from being dominated by the transfer of alumina from γ-Al2O3 to amorphous silica, to diffusion-controlled processes. By contrast, reactions in type (2) starting materials proceed in three stages, nucleation, nucleation-growth and coalescence, as the heating temperature increases.