Diphasic aluminosilicate gels with two stage mullitization in temperature range of 1200-1300 °C

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 °C. These changes imply segregation of the sample to Al2O3-rich and SiO2-rich regions. From the Al2O3-rich region crystallizes poorly defined AlSi spinel at 977 °C followed by two-step mullite crystallization in the temperature interval of 1200-1300 °C. Two overlapped exothermic peaks on DSC scan of aged gel were observed; the first at 1233 °C and the second at 1261 °C. The former is attributed to mullite crystallization by transformation of AlSi spinel, by which excess alumina occurs, which in the second step of mullitization reacts with amorphous SiO2-rich phase. The activation energy for mullite crystallization in the first step was Ea=935±14 kJ mol−1 and the Avrami exponent n=2.5. The values Ea=1119±25 kJ mol−1 and n=1.2 were obtained for mullite formation in the second step. If amorphous SiO2-rich phase is extracted from the sample, the value Ea=805±26 kJ mol−1 is obtained. Mullite crystallizing from AlSi spinel (when SiO2-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.