Synthesis Of SiCNO Precursor And Preparation Of Sicno Ceramic Bulks

SiCNO precursor was synthesized with solid ammonium carbamate asammonia resourse via solov-thermal route. The SiCNO bulk ceramics were preparedby hot press sintering after the precurosr were pyrolyzed and billed. The synthesisand pyrolys mechanisms of SiCNO precursor as well as the thermal stability werestudied by XRD, elemental analysis, FT-IR, STA-IR-MS, XPS, SEM, TEM and EDS.In-Situ TiCxNycrystal reinforcing amorphous SiCNO ceramic bulks were alsodiscussed. The coefficient of thermal diffusion as well as the thermal expansion andthe electrical conductivity of SiCNO ceramic bulks were measured.One of benefits from solvo-thermal route synthesizing SiCNO precursor wasthat the reaction can be forced to occur according to chemical stoichiometric ratios.In addition, the experiments were simplified and the safty improved largely. Solidammonium carbamate and solvent were first put into autoclave, then methyl vinyldichlorosilane (MS) and vinyl triclorosilicane were dropped into the autoclaveaccording to the designed chemical mole ratios. After heating36h at70±10oC inclosed circumstance, SiCNO precursors were finally obtained.When mole ratios of MS to TS were above1.5, the liquid precursor yields werealmost the same. At the mole ratios below1.5, the liquid precursor yields would bedropped with a decrease in mole ratio. For the reason of three Si-Cl groups in TS,the ammonolysis of TS behaved body poly-condensation, however, the ammonolysisof MS behaved linear poly-condensation.Under stronger agitation, TS can not touchitself and the final precursors were liquid for the absence of body poly-condensationwhen mole ratios of TS: MS were above3:2. On the contrary, when the ratios werebelow3:2, SiCNO precursors began to precipitate in solid state from the solvent forthe massive TS touched itself, the body poly-condenstation led to an increase incross-linking and the precursor with big molecular weight was produced. With anincrease in TS content, it was showed there was a linear decreasing tendency for theyield of SiCNO liquid precursor.The content of TS had an important effect on the ceramic yield for SiCNOliquid precursor. When mole ratios of MS to TS were above1.5, the ceramic yieldsof SiCNO precursors were improved with an increase in TS content. However, whenthe mole ratios were below1.5, the chance of excessive TS fragment contactingitself was increased. Once they were connected with themselves rather than DS,solid precursor precipited. So the composition of liquid precursors was much as thesame. It was almost the same for the ceramic yields of liquid precursors.At temperatures over58oC, ammonium carbamate will decomose into ammonia and carbon dioxide. After ammonia reacted with chlorosilane quickly,carbon dioxide could react with polysilazane and polysilane ammonium carbamatebe produced. When it was heated, a dehydration occured and siloxane isocyanatewas obtained.There were many differences between traditional polysilazane and siloxaneisocyanate. Siloxane isocyanate was lower polymer. For low molecular, largenumbers of small molecurlars, isocyanate and methyl as well as part of vinyl groupswere evaporated during pyrolysis.During pyrolysis, the self polycondensation of isocyanate groups occurred witha further increase in temperature. The higher molecular made the thermal stability ofthe polymer improved. A transformation of organic—inorganic was finished at800oC and SiCNO ceramic could be obtained after being pyrolyzed at1100oC.Amorphous SiCN(M) ceramic bulks can be obtained by “warm press” sintering.However, only small size of parts was prepared by this process. According to lowviscosity of SiCN materials at high teperature, this work tried to use hot-presssintering and prepared amorphous SiCNO ceramic bulks with a diameter of50mm.The effect of sintering temperature on structure and properites was studied. Whenthe sintering temerpature was at1500oC, the mechanical properties of SiCNOceramic bulks were in accordance with the related literature except elastic modulus.Free carbon precipitating from the matrix and clusting were observed by SEM andresposible for the low elastic modulus. At sintering temperature above1500oC,Si2N2O crystal phase was observed by TEM.Titanium was filled into SiCNO and In-situ TiCxNyreiforcing SiCNO ceramicbulk was prepared. With the aid of active free carbon, titanium atom deprivednitrogen atom from the amorphous SiCNO matrix and TiCxNywas produced. Freecarbon atom substituted the deprived nitrogen atom. As titanium atom diffusedtowards amorphous matrix, the macro holes were produced, which had an effect onthe composite mechanical properties. When a bit of titanium was filled, themechanical properties were improved largely. With an increase in titanium content,the bending strength and fracture toughness were decreased, but the elastic moduluswas increased for the production of high elastic modulus phase TiCxNy.Amorphous SiCNO ceramic bulks expressed low coefficient of thermalexpansion, high thermal conductivity and high electrical conductivity (102S·cm-1).The successful preparation of amorphous SiCNO ceramic bulks with large scalemade the practical applicaiton possible for the other related materials.