Investigation Of Carbon-rich SiAlCN Precursor And Its Structural Evolution During The Ceramization Process

SiAlCN ceramics have been widely used in aerospace,temperature and pressure sensors,ceramic micro devices and many other fields due to their excellent high temperature resistance,oxidation resistance,corrosion resistance,high temperature semiconductor properties,dielectric properties and piezoresistive properties.Among their many preparation methods,the polymer-derived ceramics is one of the most widely used methods because of its high product purity,excellent performance,safety and efficiency,low cost,low preparation temperature,easy to mass production and many other excellent characteristics.Meanwhile,carbon-rich structure can improve the mechanical properties,high temperature properties,electrical properties and many other properties of ceramics.Therefore,it is of great significance to research the preparation process,structural changes and influencing factors of carbon rich SiAlCN by precursor conversion method.In this thesis,Polyaluminocarbosilane（LPACS）was synthesized by the reaction between polycarbosilane（PSCS）and aluminum acetylacetonate（Al（acac）₃）in the autoclave.The effects of reaction temperature,reaction time and feed ratio on the structure of LPACS were investigated by FT-IR and GPC.The reaction mechanism of LPACS was also studied by ²⁹SiNMR,which was the reaction of Si-H bond in PSCS and Al-O bond in Al（acac）₃.LPACS and vinylsilazane（VSN）were crosslinked and cured with castor catalyst as initiator.The effects of initiator type,concentration and raw material ratio on curing yield and structure of cured product were investigated by FT-IR.The reaction mechanism is also proposed,which is the hydrosilylation reaction between Si-H from LPACS and C=C from VSN.The best ratio of ingredient between them is 5 g:6 g,and the curing yield is 93.05%at the same time.The crosslinked product was smooth,compact,crack free,carbon rich and have fluorescence properties,which have great potential applications in fluorescent devices.TG-DTG,XRD,Raman,²⁷AlNMR,SEM,TEM and so on were used to study on the pyrolysis process of SiAlCN and the structural forms of each element.The reaction can be divided into three stages:（1）with the release of H₂,CH₄ and other gases,the inorganic reaction takes place before 800℃.（2）The order of structure enhanced from 800℃to 1200℃.（3）Crystallization after 1200℃.The pyrolysis product after 1800℃still has 55.1%ceramic yield,which is mainly composed of highly crystallineβ-SiC and crystalline carbon,and a small amount ofα-SiC,AlN and Al₂O₃.