Wang Shengjin

Progress on the formation of ceramics and ceramic-based composites through geopolymer precursors

Geopolymers have the advantages of being low cost, environmentally friendly, fabricated at low temperature, and possessing sound heat resistant and tunable thermal properties. They have extensive potential applications in the aerospace, nuclear, and chemical industries. As geopolymers are prepared at room temperature, diverse kinds of reinforcement, such as ceramic fillers, carbon nanotubes, and short or continuous fibers, can be easily applied. Therefore, together with proper high-temperature heat treatment, the geopolymer technique provides a novel route for the preparation of high-performance ceramics and their composites. This would be coupled with the advantages of being environmentally friendly and low cost. Upon heating, the geopolymer underwent evaporation of water, dehydroxylation, particle rearrangement, viscous sintering, nucleation and crystallization, and a new kind of glass ceramic, leucite, with high-strength, high melting point, and adjustable coefficient of thermal expansion, was obtained. On increasing the cesium substitution in place of the potassium, the amount of stabilized leucite increased and with 20 at% cesium substitution, leucite was fully stabilized in the cubic phase and the thermal expansion coefficient decreased sharply. After proper heat treatment, the Cf/geopolymer composite could be directly converted into the carbon fiber-reinforced leucite ceramic matrix composites, and the mechanical properties were greatly enhanced. For the carbon-fiber-reinforced K-based geopolymer composite heat treated at 1100℃, flexural strength, work of fracture and Youngs modulus reach their highest values, increasing by 102.3%, 29.1%, and 84.7%, respectively, relative to their original state before heat treatment. The increased strength was attributed to the cracks blunding and release of residual strength based on the integrity of the carbon fiber. The impregnated composites also showed considerable isothermal oxidation resistance, and after being oxidized at 900℃ for 60 min, the composites still retained approximately 50% of the original value. Therefore, geopolymer technology together with high-temperature heat treatment provides a new method to fabricate composites with low cost and high specific strength, which might be attractive for applications at elevated temperature. We report the progress on the thermal evolution, crystallization kinetics, and evolution of the microstructure and properties of geopolymers and carbon-fiber- reinforced geopolymer composites. Future research directions are also presented.