Fabrication Of SiCN(O) Ceramic Precursors Via Novel Sol-gel Method And Their Thermal Behavios At High Temperatures

The silicon carbonitride based ceramics, which are fabricated via the novel sol-gelprocess, possess excellent mechanical properties, oxidation and corrosion resistance athigh temperature and other functional properties, and they are promising materials forthe application as high-temperature structural and functional materials. In order toknow further the relationship between their structure and properties of this kind ofsilicon based precursor derived ceramics, different functional groups-containingpolysilylcarbodiimides precursor ceramics were fabricated by the design at amolecular level. The thermolysis behavior and relevant structure transformation ofthis precursor derived ceramics were investigated.Silicon oxycarbonitride （SiCON） ceramic precursors were fabricated by thereaction of hexachlorodisiloxane and bis（trimethylsilyl）carbodiimide （BTSC）. Theoxygen was controllably introduced into Si-[NCN] network. The influence of pyridineas a catalyst for the cross-linking reaction was studied. The crystal structure ofCl₃Si-O-SiCl₃（pyridine）₂is presented. The result of quantum chemical calculations isin support of this adduct being a potential intermediate in the pyridine catalyzedsol-gel process. The degree of cross-linking is accelerated by the addition of pyridineand also depends on the aging time and temperature. The ceramic yield after pyrolysisat1000°C reaches up to50%. The ceramic reveals a porous SiCON matrixcomprised of Si₂N₂O phases. Further heat-treatment up to1500°C caused thetransformation of Si₂N₂O phase to crystalline Si₃N₄phase. The carbon in theSiCON ceramic matrix was consumed by Si₂N₂O with the increasing temperature.Thus, the final materials contain majority of Si₃N₄and minor Si₂N₂O crystallinephases, and no SiC phase was observed.In order to enhance the ceramic yield of the silicon carbonitride precursors, activevinyl-containing polysilylcarbodiimides were synthesized by the reaction ofvinylchlorosilane and BTSC. The high ceramic yield of70%was obtained after thepyrolysis of the SiCN precursor. The SiCN ceramic exhibits an amorphousnano-porous structure with pore size of tens of nanometer. The thermolysis behaviorof the SiCN ceramics between1000and1500°C indicates that the pore size decreasesas the temperature rises and the separation of crystalline Si₃N₄phase from the amorphous SiCN ceramic matrix is observed.In order to fabricate porous silicon carbonitride ceramics without any templates, anovel double functional groups-containing polysilylcarbodiimides were synthesizedby the reaction of a mixture of vinyltrichlorosilane and methyldichlorosilane withBTSC. The poly（R1R2silylcarbodiimide） precursor possesses three-andtwo-dimension structure units. The polymeric precursor was converted into anamorphous porous SiCN ceramic with85%porosity and about10micrometer poresby pyrolysis at1000°C in nitrogen. The porous structure keeps up to1500°C andpore size decreases slightly as the temperature rises. The separation of crystallineSi₃N₄from the amorphous SiCN ceramic matrix is observed at1500°C.In order to study the influence of the content of carbon on the structure andproperties of polymer derived ceramics, polysilylcarbodiimides were synthesized bydifferent chlorosilanes （ethyltri-, hexyl-and benzyl-trichlorosilane） in reaction withBTSC. The ceramic yields of the three samples after pyrolysis at1000°C differ fromeach other due to various functional groups in the polysilylcarbodiimides. The SiCNceramics exhibit an amorphous porous structure at this temperature. All three SiCNmaterials convert from amorphous station to crystalline structure above1400°C.However, the temperature of crystallization depends on the content of carbon in theceramic matrix. In the sample with higher content of carbon the separation of Si₃N₄from the amorphous SiCN matrix is observed at higher temperature, demonstratingthe higher thermal stability against crystallization.