Synthesis And Ceramicization Of Polyzirconosilane/Polyzirconosilazane Precursors

Ultra-high temperature ceramic（UHTC）fibers and composites are urgenty required materials in the propulsion and thermal protection systems of advanced aerospace vehicles.Polymer-derived ceramics technology can realize the molding and structure regulation of ceramic materials by rationally designing the molecular structure,and thus represents one of the most effective ways to prepare UHTC fibers and composites.In this work,the polyzirconosilane（PZCS）precursor with Zr-C-Si main chain was synthesized through stepwise Grignard reaction,and its synthesis mechanism and ceramic transformation mechanism were studied.By further adjusting the molecular structure,a novel polyzirconosilazane（PZCSN）precursor with Zr-C-Si-N main chain was synthesized with improved rheological propertie for melt spinning.Furthermore,an efficient method was explored to prepare Zr CSi BN multicomponent UHTC fibers by blending PZCSN with polyborosilazane（PBSZ）combining with subsequent melt spinning and inorganic treatments.In the stepwise Grignard reaction,the active Cp₂Zr（Ⅱ）monomer was prepared using Cp₂Zr Cl₂and Mg as raw materials,which further reacted with（Cl CH₂）₂Si（CH₃）₂to obtain PZCS precursor.The PZCS is soluble in common solvent,such as THF,DMF and toluene,etc.,and its number and weight average molecular weight are 848 and 1002g/mol,respectively.The ceramic yield of PZCS was 45.28 wt%at 850℃.When pyrolyzed at 1600℃,Zr C/Si C/C multiphase ceramics were obtained with a"concrete like"structure of Zr C and a small amount of Si C nanocrystals dispersed in the carbon matrix.Such a"concrete like"structure is helpful to improve the mechanical property and high temperature stability of Zr C/Si C/C ceramics.In order to optimize the spinning performance of the precursor,PZCSN with good dissolution and melting preperties was synthesized by introducing flexible-N-segment structure into the Zr-C-Si main chain.Its number average molecular weight and weight average molecular weight are 1447 g/mol and 1653 g/mol respectively.and its softening point is in the range of 80.8～89.2℃.The PZCSN precursor presented shear thinning behavior in the temperature range of 120～180℃,preliminarily meeting the requirements of melting spinning for producing micro-sized green fibers.The ceramic yield of PZCSN precursor was 51.48 wt%at 850℃,and Zr C/Si C multiphase ceramics were pyrolyzed at 1600℃.In order to improve the comprehensive performance of the final ceramic fiber,PBSZ was chosen to mix with PZCSN as a blending polyzirconosilazane precursor（m PZCSN）to prepare Zr CSi BN multicomponent UHTC fibers.When the mass ratio of PBSZ and PZCSN ranged from 5:1 to 1:1,the ceramic yield of all the blending precursors increased to more than 65.00 wt%at 850℃.The green fibers are in a diameter range of 15～25μm,and after crosslinking in active atmosphere,the ceramic yield of the m PZCSN（PBSZ:PZCSN=1:1）crosslinked fibers increased to 84.34 wt%.The final m PZCSN ceramic fibers after 1600℃treatment are mainly composed of Zr B₂/Zr C crystalline phase with Zr content up to 80.0 wt%.However,the introduction of much oxygen elements in the process of melt spinning and post-treatment would cause complex reactions in the high-temperature treatment and thus bring out evident holes and cracks in the final ceramic fibers.In conclusion,this thesis puts forward a new method to synthesize UHTCs precursor containing refractory metal main chain.The melt spinning performance of the precursor is improved by effective regulation of molecular structure.Moreover,the explorations of polymer-derived UHTC fibers in this work lay important foundations for the design of new UHTCs precursors and fibers.