The Study Of Monophasic Mullitization’s Reaction Mechanism

Because of excellent high-temperature mechanical properties, impact resistance,low thermal expansion coefficient, chemical stability, low dielectric constant andgood infrared-transparent coefficient, mullite has been widely used in structuralceramics and functional ceramics. In this paper, typeâ…  mullite precursors weresuccessfully synthesized by the sol-gel process, monophasic mullitization was studiedby thermodynamic theory and kinetics software, thus got the mullitization’s reactionrate equation for scientific research and actual production providing the theoreticalbasis.Typeâ…  m ullite precursors were prepared by sol-gel, taking tetraethoxysilance,aluminum nitrate nonahydrate and anhydrous ethanol as raw materials, and withSimultaneous thermal analysis, X-ray diffraction and Scanning electron microscopy itwas verified. As the increase of temperature, it gradually converts into stableorthorhombic mullite with the composition of3Al2O3·2SiO2. In orderto reveal monophasic mullitization’s reaction mechanism, the paper envisages Al-richmullite with the composition of2Al2O3·SiO2(Al2O3content is about67mol%). Thefirst time illustrates the rationality of it from the thermodynamic point and gives thethermodynamic data of Al-rich mullite. The results indicated that amorphous SiO2andAl2O3transformed into the tendency of Al-rich mullite bigger than orthorhombicmullite at the solid phase reaction point, and got the thermodynamic processes ofmonophasic mullitization:Sol-g elâ†'M onophasic a bou t985℃â†'A l-rich mullite+S i-rich phase t emp eretu re ri seâ†'Orthorhombic mulliteSample size and sample quality are studied to the influence of the thermalanalysis curves. With the help of kinetics software this paper studied the dynamics ofmonophasic. The results showed that monophasic mullitization’s the most probablekinetic model wasA1â†'B2â†'C, the first step was An, the kinetic model functionwasf (a)=n (1-a)[-ln(1-a)]n1/n, activation energy was982KJ/mol, kinetic index(n)was4.38; the second step was D3, the kinetic model functionwasf (a)=3/2(1-a)2/3[1-(1-a)1/3]1, activation energy was998KJ/mol. Usingkinetics software’s dynamic response prediction module compared the actualmeasurement curve and the model curve. It was found that monophasic solid phase reaction’s kinetic triplet was credible.