Study On Molten Salts Assisted Preparation And Properties Of Potassium Based Polymer Ceramics And Its Composites

Because of its excellent three-dimensional oxide network structure,geopolymers have excellent mechanical and thermal properties,but the defects such as high brittleness and poor reliability have limited the application of geopolymers in industrial buildings and so on.The use of a variety of reinforcing phases to fabricate geopolymer composites is an effective way to solve the question.On the other hand,geopolymers transform into ceramic materials with a dense structure and more excellent mechanical properties after high temperature heated.Molten salt method is a synthetic method for fabricating ceramic powder that has attracted inclusive attention in recent years.The advantage lies its ability to reduce the synthesis temperature.Simultaneously,the prepared material has a specific morphology.Therefore,the paper mainly introduct the molten salt method to synthesize potassium-based geopolymer?KGP?ceramics and mullite fiber mats/KGP ceramic composites.In this paper,KGP was fabricated with metakaolin,KOH and nano-SiO₂,and mullite fiber mats/KGP composites were prepared with mullite fiber mats as reinforcement phase.The KGP and its composite materials were heated at a high temperature by the molten salt method to obtain KGP ceramic and mullite fiber mat-reinforced KGP ceramic composite material.Using X-ray diffractometry,Scanning electron microscopy,thermal expansion analysis,thermal conductivity analysis,and other testing methods,the microstructure,mechanical properties and thermal properties of KGP and its composites under high temperature heat treatment with KCl as molten salts were studied systematically.In order to characterize the promoting effect of the molten salt method on ceramization,KGP ceramics were also prepared using the traditional sintering method without molten salts.The study of ceramization shows that KCl molten salt acts as a liquid-phase accelerator and KGP can generate ceramics with a crystal structure at 800?.With the Si/Al=2,leucite is formed;Si/Al=1,kalsilite is generated.Moreover,with the temperature rising,the ceramization reaction intensifies until a fully-developed crystal is formed,and the mechanical properties of the product are improved significantly.Compared with the samples fabricated by the traditional sintering method,the product structure obtained by the molten salt method is denser,and it shows the largest shrinkage at the sintering densification stage,and the compressive strength and the thermal conductivity are also significantly improved.Compared with KGP materials that have not been heat-treated at high temperatures,the properties of the ceramized products have been qualitatively improved.Compared with kalsilite,the mechanicl properties and thermal conductivity of leucite are better,so the research group mainly studies the mechanical and thermal properties of leucite-based composite materials.In further analysis of the mechanism of molten salt promotion of KGP ceramization and the chemical mechanism of leucite formation.The mullite fiber mats/KGP composite material can be directly transformed into a mullite fiber mats/leucite ceramic composite material after heat treatment.With the help of liquid phase accelarator KCl,the reactants can generate ceramic matrix composites more effectivly.The toughening of fiber mat can significantly improve the bending performance of the composites.The flexural strength of the mullite fiber mats/KGP composites kept at 900?for 8 hours attain 142.3MPa.On the other hand,the thermal properties such as thermal expansion coefficient and thermal conductivity also change with the increase of holding time.And as the crystallization of ceramic products increased,the connection between the fiber mats and the substrate is more stable,and the thermal expansion coefficient of the composite material is continuously changing.the thermal conductivity also showed an upward trend,when the holding time reaches 8h,the thermal conductivity is 0.67w·m^-1·k^-1.