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Influence of transition metal cations on reaction sequences in diphasic gels with 3:2 mullite composition

The effect of Co2+, Ni2+ and Zn2+ on phase formation, reaction sequence and microstructure evolution in diphasic gels with 3:2 mullite composition has been investigated. In order to determine the crystallization path and to evaluate crystallization kinetic parameters, differential scanning calorimetry (DSC) has been applied. X-ray diffraction (XRD) was used to determine the development of crystal phases with temperature increase. The course of thermal reactions is dominated by the intermediate formation of two faint crystallized phases having different compositions and activation energies. The former phase was attributed to transition metal-containing spinel structure and the latter to Al-Si spinel. With temperature increase metal-containing spinels transform progressively to MeAl2O4, while Al-Si spinel forms mullite above 1100°C. Mullite stoichiometry, microstructure and chemical composition of samples sintered at 1600°C were obtained by Rietveld structure refinement, scanning and transmission electron microscopy (SEM, TEM) and energy dispersive X-ray spectroscopy (EDS). It has been established that transition metal cations predominantly form metal-aluminate spinels. Small saturation levels of each metal cation in mullite were obtained.

Mullite; sol-gel; transition metals; XRD; SEM; TEM

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