Reaction heat utilization in aluminosilicate-based ceramics synthesis and sintering

Self-propagating high-temperature synthesis (SHS) is a widely known and extensively studied highly exothermicreaction- utilizing technique for making certain advanced composites and intermetallic compounds. However, only few studies have been published about the SHS of pure aluminosilicate ceramics. In the current work, possibilities for aluminosilicate ceramic synthesis and sintering requiring less energy based on the utilization of SHS in air was studied. Kaolinite powder and exothermically reactive metallic aluminium powder were used as raw materials. Thermodynamic calculations for the possible reactions and reaction paths were performed to show the theoretical possibilities for SHS utilization. The chemical reactions, thermal expansion behaviour and formed phase- and microstructures after SHS were compared to the conventional reaction sintering of mullite. Results conclude that highly exothermic reactions above 900 °C relating mainly to aluminium oxidation can ignite the SHS reaction in air atmosphere. After initialization, the reaction proceeded in a self-sustaining manner through entire test pieces, resulting in the formation of an Al₂O₃ - Si phase structure. Thermodynamic calculations showed the total energy balance for mullite formation from aluminium and kaolinite mixtures as highly exothermic in nature only if sufficient oxygen is available to complete the reactions. However, future research is needed to fully utilize SHS in aluminosilicate ceramics processing.