engleski

Diphasic aluminosilicate gels with two stage mullitization in temperature range of 1200-1300 ^oC

The crystallization of mullite in amorphous diphasic gel aged for 6 months has been studied using non-isothermal differential scanning calorimetry (DSC) and powder X-ray diffraction with Rietveld structure refinement analysis. The diphasic premullite gels undergo structural changes by aging even when they are calcined at 700 ^oC. These changes imply segregation of the sample to Al_2O_3-rich and SiO_2-rich regions. From the Al_2O_3-rich region crystallizes poorly defined Al-Si spinel at 977 ^oC followed by two-step mullite crystallization in the temperature interval of 1200-1300 ^oC. Two overlapped exothermic peaks on DSC scan of aged gels were observed ; the first at 1233 ^oC and the second at 1261 ^oC. The former is attributed to mullite crystallization af Al-Si spinel, by which excess alumina occurs, which in the second step of mullitization reacts with amorphous SiO_2-rich phase. The activation energy for mullite crystallization in the first step was E_a=935+-14 kJ mol^(-1) and the Avrami Exponent n=2.5. The values E_a=1119+-25kJ mol^(-1) and n=1.2 were obtained for mullite formation in the second step. If amorphous SiO_2-rich phase is extracted from the sample, the value E_a=805+-26kJ mol^(-1) is obtained. Mullite crystallizing from Al-Si spinel (when SiO_2-rich phase has been extracted) differentiates compositionally from that formed by both reactions. Smaller unit cell parameters and higher amount of oxygen vacancies are incorporated into tetrahedral positions of mullite structure, as was determined by Rietveld structure refinement method.

Mullite; X-ray diffraction; Rietveld structure refinement; Phase development; Kinetics; Gels

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