engleski

Crystallization kinetics of mullite from single-phase gel determined by isothermal differential scanning calorimetry

Transformation kinetics of single- phase gel with mullite composition was studied by isothermal differential scanning calorimetry (DSC) in temperature range from 937 to 959oC. Single exotherm was observed for annealing temperatures below 947oC, and two overlapped exothermic peaks were seen abowe this temperature. According to XRD analysis, mullite was the only phase scrystalized either under non-isothermal or isothermal heat treatment. Johnson-Mehl-Avrami (JMA) equation for nucleation and growth could not describe mullite crystallization adequately, even below 947oC. Using bimodal JMA-type model, that proposes mullite crystalization in two steps, the fitting was remarkably good in the whole temperature range. Obtained kinetics data do not allow one to characterize the gel, either as typical single phase one (nucleation-controlled process with two rate constants and small apparent activation energies), or as a hybrid gel (mullite formation via spinell and high apparent activation energies). The rate constants were an order of magnitude smaller than is proposed for single-phase gel. The apparent activation energies, however (Ea1=1053+-51 kJ/mol, and Ea2=1028+-22 kJ/mol), were in great discrepanci to those already cited for single phase gels, but they were in good agreement with data evaluated for diphasic and hybrid gels. Mullite a-axis length and effective fraction of mullite that is formed in the first and second step of the process provided an insight in the mechanism of mullite crystallization. It is assumed that not the nucleation and crystallization limitations, rather the phase separation is the controling process in mullite formation from single phase gel under applied experimental conditions.

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