Synthesis And Characterization Of Organosilicon Polymers For SiC-Based Ceramic Fibers

SiC fiber is one of the most promising candidates as the reinforcing fiber of ceramic matrix composites(CMCs) for high temperature applications,because of its high tensile strength,thermal and oxidation resistance.In this dissertation,different precursors for fine diameter continuous SiC-based fibers were prepared and studied.In chapter 3,the control of structure formation of polycarbosilane(PCS) synthesized from polydimethylsilane(PDMS) was studied.It was found that the molecular structure of PCS was strongly dependent on the reaction temperature and heating rate.Higher reaction temperature and fast heating rate tended to give PCS with highly cross-linked skeleton,while lower reaction temperature and slow heating rate tended to result in PCS with linear backbone.A scissor-couple rearrangement model was proposed for the formation of the PCS skeleton,which was confirmed by²⁹Si NMR,GPC and UV analysis.On the base of this model,PCS with controllable molecular structure and good spinnability can be obtained.Moreover,when vacuum heating is used to remove the low molecular weight fraction in the as-synthesized PCS to increase its softening point,the linearity,hence spinnability of the as-synthesized PCS is reduced,because of the continuing reaction during this process.This problem can be avoided by solvent extraction.The ability of solvents to exact low molecular weight fraction of PCS is in the ascending order of methanol,N,N-dimethylformamide,2-methoxyethanol,ethanol,2-ethoxyethanol, 2-propanol,ethyl formate,methyl acetate and n-propanol;iso-propanol,acetone, n-butanol,1,4-dioxane,butanone,1,2-dimethoxyethane and ether.Through solvent extraction of the as-synthesized PCS,PCS with optimal molecular weight can be obtained.The good spinnability of Markâ…¡and Markâ…¢PCS was also explained by scissor-couple rearrangement model in chapter 3. In chapter 4,a new method to prepare polyaluminocarbosilane(PACS) directly from the one-pot reaction of PDMS with Al(acac)₃ in autoclave was developed.In this closed system,the sublimation of Al(acac)₃ is prevented and all of Al in Al(acac)₃ are converted into PACS.Therefore,the content of Al can be readily controlled quantitatively.On the base of the FT-IR,¹H NMR,¹³C NMR,²⁹Si NMR and ²⁷Al MAS NMR analysis,the reaction mechanism was proposed as follows:PDMS dissociated during pyrolysis to generate silicon free radicals,and then they reacted with Al(acac)₃ to give PACS containing(Si-O)_nAl groups(n=4,5,6).Meanwhile,these reactions resulted in the cleavage of O-C and/or O=C bonds in Al(acac)₃.Some of free radical fragments generated by this cleavage continued to react with the silicon free radicals and were incorporated into the structure units of PACS;the rest of them may convert into small oxygen-containing compounds,which were removed in the subsequent processing after the reactions.The low molecular weight fraction in the as-synthesized PACS can be also removed by solvent exaction.The ability of solvents to exact low molecular weight fraction of PACS was found in the ascending order of methanol,95%ethanol,2-methoxyethanol, N,N-dimethylformamide,ethanol,acetone,ethyl formate,2-propanol and methyl acetate.In chapter 5,polyyttrocarbosilane(PYCS),polyzirconocarbosilane(PZCS), polyferrocarbosilane(PFCS) and nitrogen-containing polycarbosilane(PNCS) were successfully prepared using the method similar to the synthesis of PACS.The reaction mechanism of PYCS and PZCS were supported to be similar to PACS.However,it was found that the reproducibility of preparation of PFCS was very poor.Meanwhile,the reaction mechanism of PNCS synthesized from melamine and PDMS was studied by FT-IR,¹H,¹³C and ²⁹Si NMR analysis and described in chapter 5.Preparation of SiC(Fe) fiber had been attempt by using PFCS as polymer precursor, producing ceramic fibers with high tensile strength in this chapter.However,attempt to produce Si-N-O fiber was given up,because it failed to produce Si-N-O ceramic by oxidation of PNCS in air.Si-N-O ceramic was not obtained because of the loss of nitrogen during the oxidation curing of PNCS in air.Si-B-N-C polymer precursor was prepared by the the reaction of triphenylborazine with PDMS.It was found that triphenylborazine can serve as the catalyst to prepare Markâ…¢PCS.No oxygen was incorporated into the resultant precursor.It can serve as the starting materials to prepare Si-B-N-C ceramics or fiber.