Formation Mechanisms of Sol-Gel Derived Cordierite

Gels with cordierite stoichiometry, Mg2Al4Si5O18, were prepared by sol-gel technique using three different starting aluminum sources: nano-sized γ-Al2O3, aluminium nitrate nonahydrate and Al-sec-butoxide, as aluminum sources, using tetraetoxysilane (TEOS) and magnesium nitrate hexahydrate in all three gels as sources for silicon and magnesium. The influence of Al-precursors on formation mechanism and sintering behavior of gels was studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and dilatometry. Gels calcined at 700°C were characterized by different specific surface area, pore sizes and different crystallization sequences. For gel with nano-sized γ-Al2O3 α-cordierite was formed at 1220°C through intermediate MgAl2O4 phase. If aluminum was introduced as Al-sec-butoxide μ-cordierite crystallized in two steps (760 and 1070°C) and the transformation μ-cordierite→α-cordierite occurred at 1250°C. When Al(NO3)3·9H2O was introduced as intermediate phase crystallized MgAl2O4 at ~870°C and μ- cordierite at ~950°C. Both phases reacted with amorphous silica forming α-cordierite at ~1210°C. Compacts prepared from gels with aluminum-nitrates and Al-sec-butoxide, respectively, sintered between 700 and 1000°C, whereas the gel derived from γ-Al2O3 sintered in two steps; between 700- 1000°C and 1010 and 1250°C.