Impact of Temperature and MgO on Cordierite Ceramics Prepared Through Conventional Reaction Sintering of Nano-powders of Al₂O₃, SiO₂, and MgO

This study examined, for the first time, the impact of excess magnesia (0–8 wt%) on phase formation and transformation in non-stoichiometric cordierite ceramics prepared through conventional reaction sintering of nano-powders of Al₂O₃, SiO₂, and MgO. Diffraction and thermal analysis methods were used to characterize the formed phases and their subsequent transformations. Activation energy (E_a) values for the formation of enstatite and cordierite were determined through non-isothermal analysis using the Kissinger equation. The density, coefficient of thermal expansion (CTE), and hardness of sintered samples were measured using a densimeter, dilatometer, and hardness tester, respectively. Additionally, the fracture surface of sintered specimens was characterized using a field emission scanning electron microscope (FE-SEM) coupled with energy dispersive spectroscopy (EDS). It was found that the temperatures at which enstatite and cordierite form increase with heating rate and decrease with the increase in excess magnesia. The formation of enstatite in sample (MAS0M) requires an activation energy of 655 kJ mol⁻¹. This energy increased to 748 and 698 kJ mol⁻¹ for samples MAS2M and MAS4M, and then decreased to 644 and 645 kJ mol⁻¹ for samples MAS8M and MAS8M. The formation of α-cordierite in sample MAS0M requires an activation energy of 684 kJ mol⁻¹. This energy increases to 869, 904, 950, and 894 kJ mol⁻¹ for samples MAS2M, MAS4M, MAS6M, and MAS8M. The prepared materials demonstrated similar phase transformations, ultimately resulting in the formation of α-cordierite single phase from the alumina-silica-magnesia powder mixture of stoichiometric composition. Cordierite, sapphirine, and enstatite were formed in the mixtures with an excess of magnesia. The bulk density of samples sintered at 1350 °C for 2 h increased from 2.58 to 2.88 g cm⁻³ as the excess magnesia content increased from 0 to 8 wt%, and the CTE also increased from 1.16 × 10^–6 to 2.53 × 10^–6 K⁻¹. The sample with 4 wt% excess magnesia exhibited the highest hardness of 10 GPa.