Characterization and mechanisms of the phase's formation evolution in sol-gel derived mullite/cordierite composite

Abstract

In this work, mullite/cordierite precursor powder was prepared through a technology of low-temperature synthesis by using the sol-gel process, tetraethyl orthosilicate (TEOS) as a source of silicon oxide SiO2, and aluminum nitrate nonahydrate Al (NO3)3.9H2O as a source of aluminum oxide (Al2O3) and magnesium nitrate hexahydrate Mg (NO3)2.6H2O as a source of magnesium oxide MgO was used as raw materials to synthesize mullite/cordierite precursor gel with a concentration (sample containing 50 wt% of cordierite and 50 wt% of mullite) and named as (MC50). The objective of this study is to find a suitable kinetic model to study the phases and the mechanisms of their formation in mixtures, with the prediction of the system's behavior under selected thermal conditions, including finding the kinetic and thermodynamic media that describe these interactions. To follow and characterize the crystalline phases and their transformation as a function of temperature utilizing differential thermal analysis (DTA), Dilatometry (DIL), and powder X-ray diffraction (XRD). The results show that the crystallization process occurred in the temperature interval between (900–1350) °C. In the temperature range of (900–1000) °C, spinels between Al–Si and Al–Mg with the chemical formulas (Al4Si3O12 and MgAl2O4) were formed. When the thermal treatment temperature increases from (1000–1100) °C, mullite is produced. As the temperature increases, the amount of Mg–Al spinel decreases to form amorphous silica, and μ-cordierite has appeared at 1250 °C. With an increase in temperature up to 1350 °C, α-cordierite appeared as a stable phase. The reason for this is the presence of the spinel (Al–Mg) phase that helped it form.To determine the reaction kinetics of these transformations at high temperatures, the mixture 50/50 mullite/cordierite was selected to study its kinetics. The activation energy values (Ea/Tm) (Tm is the maximum temperature of the transformation, i.e., the maximum peak temperature is not related to the crystallization fraction α) calculated by Ozawa, Boswell, and Kissinger methods are in good agreement with the evident activation energy (Eα/Tα) (Tα is the degree of the heat of transformation in terms of crystallization fraction α changes from 0<α < 1) calculated using the KAS and FWO methods.For the purpose of calculating the interaction model and finding the media that determine the interaction model based on the experimental data, Malék's methodology method was used. The best kinetic model is the Šesták - Berggren model to describe the reaction process to form spinel, mullite, and α-cordierite. From the SB model, the equations Kinetics and all kinetic parameters (n, m, ln(k0)) that describe the kinetics of the reactions and mechanisms of formation of spinel, mullite, and α-cordierite in the mixture are, respectively, (2.14, 0.023, 65.21), (1.62, 0.1232, 81.76), and (1.41, 0.2859, 91.13). While the values of Gibbs free energy ΔG#, enthalpy ΔH#, and entropy ΔS# were as follows: 407.254 kJ mol−1, 976.756 kJ mol−1 and 415.561 J mol−1K−1 for Mullite formation, and 471.64 kJ mol−1, 1255.16 kJ.mol-1 and 491.75 J mol−1K−1 for the formation of α-cordierite.Comparison of simulation curves with experimental data obtained at different temperatures gives good agreement with the thermal analysis data (Experimental), which indicates that the Model of Šestak − Berggren, is the best suitable kinetic model for studying and describing the reaction technique for MC50 prepared by the sol-gel method.