Interfacial Modification And Performance Study Of SiC/Geopolymer Composites

Geopolymer is a kind of three-dimensional network gel with amorphous and quasi-crystalline characteristics,which is composed of silicon oxygen tetrahedron and aluminum oxygen tetrahedron.However,the research and development of geopolymers are not yet mature,and the research on their mechanical properties is yet to be carried out.Meanwhile,their high temperature resistance and wave absorption properties are still lacking of experimental research and verification.Silicon carbide whisker（SiCw）is a wideband gap semiconductor ceramic fiber with high strength,good thermal stability,adjustable and controllable conductivity and dielectric properties.Therefore,the development of SiCw toughened polymer composites has important research value in the fields of high-temperature structure,indoor absorbing materials and wall protection.In view of the existing research analysis indicated that the investigations of the SiCw/geopolymer composites in main several problems:on the one hand,SiCw dispersion and surface coating of composite material preparation and interface control play a key role,and the introduction of interface coating can effectively improve the mechanical properties of polymer composites,high temperature resistant performance,has not yet been reported;on the other hand,there are a lot of research about the electromagnetic wave absorbing material absorption performance,but most of them is in view of the radar absorbing military defense security,but in civil,including indoor absorbing material and almost no protective wall integral formula,SiCw is one of the important components,preparation of absorbing material will SiCw application to polymer materials for its absorbing performance is improved in the study still stay.Based on the review of the research status of geopolymer and fiber toughened geopolymer composites,dense and uniform SiCw/geopolymer composites were designed,the dispersion behavior of SiCw in water medium was studied,and the stable dispersion conditions were determined.The homogeneous SiCw/geopolymer composite powders with superior mechanical properties were prepared by introducing the ball mill dispersion process.Heat treatment and sol-gel method were used to modify SiCw,and then the mechanical properties,high temperature resistance and wave absorption properties of SiCw/geopolymer composites were analyzed.The toughening mechanism of SiCw in geopolymer was studied,and the correlation between preparation process,interface and properties was revealed.The main research contents and conclusions are as follows:（1）The effects of SiCw content and dispersion process on the microstructure and room temperature mechanical properties of SiCw/slag-based geopolymer composites were studied,and the effects of SiCw and different dispersion processes on the early hydration reaction degree,hydration product structure and composition of slag-based polymer were also studied.The results show that the dispersion stability of SiCw in aqueous solution can be guaranteed by the proper amount of dispersant AMP-95 combined with the appropriate mechanical dispersion method.With the increase of SiCw content,the flexural strength of geosynthetics first increases and then decreases.Among them,the optimal whisker content to improve the bending performance of composite materials is 1%,which is 63%higher than that of samples without whisker content.Mixing SiCw,slag and dispersant into mixed powder under the process of ball grinding dispersion can ensure the optimal mechanical properties of geomolymer composites at room temperature and avoid secondary agglomeration of whiskers.Due to the high specific surface area of SiCw,SiCw acts as a nucleating point to accelerate the hydration reaction of the composites.Meanwhile,the better the dispersion effect of SiCw,the more the hydration reaction of the composites can be promoted.（2）The microstructure,surface chemical state and interfacial chemical bond properties of the coating were studied before and after the modification of SiCw.The results showed that after mild oxidation of SiCw surface by heat treatment（800℃-1h）,it was found that the Si-O bonds on the whisker surface increased,while the Si-C and C-C bonds on the whisker surface decreased,and O elements were evenly distributed on the whisker surface.BN coating was successfully prepared by sol-gel method with urea and boric acid as precursors at 850℃.Aluminum sol was obtained by aluminum nitrate hydrolysis method to coat SiCw,andγ-Al₂O₃andα-Al₂O₃ coatings were prepared on the whisker surface at 800℃and 950℃respectively.The interface between the coating and whisker is a chemical bond,and all of them are bonded by chemical bonds connected by bridging oxygen.（3）The influence law and mechanism of different modified SiCw on the high temperature resistance and wave absorption of SiCw/geopolymer composites at room temperature were studied.The results show that,at 900℃,only SiC/GP and SiC/BN/GP remain intact,and the flexural strength of the two groups is 84.8%and 92.7%respectively at room temperature.It can be seen that the unmodified SiCw/geopolymer composites and the modified SiCw/geopolymer composites with BN coating can withstand at least 600～900℃.The fundamental reason for the degradation of the physical properties of the geopolymer after 900℃treatment is that the hydration product of the geopolymer,the hydrated calcium silicate gel,is gradually decomposed and new crystal phase products such as tablet zeolite are generated.Whisker deflection,bridging and pulling out are the main toughening mechanisms of SiCw/geopolymer composites.SiCw after BN coating modification has the best toughening effect on geopolymer.The reason is that BN interface achieves load transfer from matrix to whisker,thus promoting whisker pulling out.SiCw/geopolymer composites have the capability of electromagnetic wave loss.Compared with pure geopolymer,the effective absorption bandwidth of SiCw/geopolymer composites to electromagnetic wave in X band increases by 0.26～1GHz.Different modification treatments of SiCw also have an impact on the absorption performance of SiCw/gepolymer composites.The absorption performance of SiC/BN/GP sample is the best.In the x-band,the lowest reflection coefficient of SiC/BN/GP is-55.4dB,and the effective absorption bandwidth is 4.11GHz.