Investigation On Synthesis And Application Of A Novel Polymetalcarbosilane

Silicon carbide ceramics(SiC ceramics),which have excellent performances on creep resistance,corrosion resistance,mechanical properties and oxidation resistance at high temperature,have been applied to many advanced fields such as chemical engineering,nuclear industry,defense industry,aviation and aerospace industry.Especially,SiC fibers and their composites,as two of the most important members of advanced ceramic materials,have been used in manned spacecraft cabin,high-heat units of aero engine,guided missile ablation parts and high radiation and temperature units in nuclear industry successfully.Organic precursor derived SiC technique is one of the most attractive methods for manufacturing SiC fibers and their composite materials because of its controllable ingredient,diverse operation,lower pyrolysis temperature and other advantages,and the highlight of this method is to synthesize outstanding polycarbosilanes(PCS)as the precursor for SiC ceramics.With the rapidly growing demand for SiC ceramics in industry,the synthetic routes of PCS have been studied widely and deeply by researchers all over the world,and an increasing number of new routes have been reported one after another,for example,the Two-Step Method developed by Japanese scientist S.Yajima and Ring-Opening polymerization invented by L.Interrante and his coworkers from USA.However,for the reason that stable silene molecules are still too difficult to produce for their thermal lability,PCS cannot be massively prepared via catalytic polymerization of 1-silenes,just like polymerization of olefin,and it is still a worldwide problem in production and application of silenes on large scale.Building on the comparison of major reported routes,a novel route of synthesizing PCS is presented in this study,and furthermore chemical mechanism and industry application of the synthetic reaction are intensively researched.Following are the research content and major conclusions:(1)Synthesis of PCS by one-pot reaction.In this study,PCS was synthesized via dichloromethylsilanes(SiRMeCl2,R=Me,Ph,Et,H)and metallocenes(MCp2Cl2,M= Ti,Zr,Hf)catalytic polymerization in a one-pot reaction,which removes the requirements for an additional Kumada rearrangement of polydimethylsilanes(PDMS).Meanwhile,introduction of extra transition metal element into the products allows conversion to metal carbide ceramics during pyrolysis together with SiC forming a homogeneously distributed composite ceramics on nanoscale,which would help to enhance the final ceramic product's performance.Quite high reaction conversion,mild reaction condition,simple operating steps and secure production environment make this new route outstanding,and the products are expected to be a cost-effective raw material for high-performance SiC fiber and its composites.(2)Investigation on physical properties,chemical structure and composition of PZCS.PZCS,a black brown brittle solid,has a stable softening point.Its density is 2.52 g/cm3 and is able to dissolve in toluene,xylene,THF and other organic solvents.Its solid form is quite stable while solving in some solvent makes it muchmore sensitive in the air.Chemical structure and elementary composition were characterized by XDS,XPS,FT-IR,29Si-NMR,UV,GPC and MALDI-TOF-MS.Results showed that,PZCS is consist of Si,Zr,C,H,and bits of O and Cl,and presents a linear molecular structure,in which Si-H and Si-CH2-Si units can be clearly found.PZCS's molecular weight is varying from 900 to 1500,with a quite small polymolecularity(Mw/Mn=1.31).Test results of TG,XRD and SEM indicate that when PZCS is pyrolyzed under 1000?,the ceramic yield was over 58%.The final ceramic product mainly contained two kinds of phase composition:SiC and ZrC,and thereinto,nano-sized ZrC was uniformly distributed in continuous SiC.(3)Proposed chemical mechanism of synthetic reaction.A catalytic rearrangement polymerization of methylsilylene using metallocene as catalyst yielding PCS took place during the synthesis procedure.On the basis of experimental data,test results analysis and DFT analog computation,a catalytic rearrangement polymerization mechanism,characterized by synergistic effect between diffusion-controlled and high reactivity,is proposed.Details as followed:i)formation of methylsilylene and active center in metallocenes via dechloridation by sodium initiated the reaction;ii)methylsilene(CH2=SiHR)and methylsilylene(RMeSi:)are tautomeric intermediates,and the methylsilene moiety was rapidly trapped in active center in metallocene to form Mt-C bond,waiting for other monomers inserted one by one;iii)surface deactivation of sodium by metallocenes and sodium chloride limited the diffusion of dichloromethylsilane,and as a result,competition from methylsilylene polymerization was strongly inhibited simply because of a very low concentration of this moiety on the surface of sodium.(4)Investigation on modification process of PZCS.i)Catalytic copolymerization between dichlorodimethylsilanes and dichloromethylsilanes was investigated.The results demonstrated that carbon content in the PCS could be adjusted by changing the copolymerization ratio of the two silanes.The free carbon content in final ceramic products reduced from 23%to 6%.ii)Dechloridation process of PZCS was studied.Lithium Aluminum Hydride(LiAlH4)and ammonia(NH3)were used respectively as dechlorinating agent to remove Cl element in PZCS.As results showed,LiAlH4 made Cl element in PZCS reduced by 35%?50%,regardless of its damage to the backbone of PZCS,resulting in considerably smaller molecular weight and ceramic yields;NH3 removed less Cl element from PZCS,but the introduction of N element allowed conversion to extra Si-C-N ceramic after pyrolysis,which improved the performance of final ceramic products.iii)Heat pretreatment process of PZCS was studied.Results showed that after 200?260? heat pretreatment,ceramic yield of PZCS increased from 58%to 67%and average molecular weight raised from 930 to 2300.The reason was that crosslinking reactions,related to Si-H bond,occurred during the pretreatment.However,once excessive crosslinking happened,PZCS could not be dissolved or melted,resulting in useless.(5)Preparation of SiC_f/SiC-ZrC composites by PIP technique,and investigation on their microstructure,mechanical properties,and oxidation resistance.SiC_f/SiC-ZrC composites was prepared by PIP technique,using PZCS dissolved in xylene by 55%concentration as the impregnant,and low density SiC/C-SiC from CVI technique as the preform.It was found that an interface layer consist of pyrolytic Carbon(PyC)and SiC ceramic that connected SiC fibers reinforcement and compact SiC-ZrC composite ceramic matrix,in which ZrC homogeneously distributed on the nanometer size regime.It was measured that bending strength and breaking tenacity of SiC_f/SiC-ZrC composites were 495.2±71.6 MPa and 16.9±2.05 MPa·m1/2 respectively.The fracture surface exhibited quite irregular morphology with obviously fibers and fiber bundles pull-out,indicating a typical ductile fracture.Analyzing the reason for that,the interface between fibers and matrix made a big difference.When there was a load on the composites,appropriate bonding strength of interface layer ensured little load well transmited from matrix to fibers,as well as big load well dispersed via breaking the bond and leading the crack extending along the fiber surface,resulted in absorbing the overmuch fracture energy.Rearch on oxidation behaviours showed that,the composites presented well oxidation resistance at high temperature(>1000?)for the formation of compact oxide film from the SiC and ZrC oxidation,preventing oxygen from diffusing into the matrix to do damage to material strength.While,the composites showed poor oxidation resistance at low temperature(<800?)just because the compact oxide film couldnot be formed at such temperature and matrix was heavily oxided by oxygen,making mechanical properties of composites drop down badly.