Research On Continuous Preparation Of SA Type SiC Fiber

Polymer-derived SiC fiber is one of the most important reinforcing materials for high performance ceramic matrix composites(CMC).Ultra-high-temperature performance requirement is keeping proposed for CMC reinforcements with the development of aerospace technology.Among the strategies for improving the high-temperature performance,the most accessible method is to prepare near-stoichiometric SiC fiber by introducing small amount of aluminum(SA type SiC fiber) through ultra-high-temperature sintering.In this work,the whole preparation processes for SA type SiC fiber were completely investigated.The synthesis mechanism of PACS was revealed and the composition and quality of PACS were under well control based on the understanding of the reaction and improved techniques.In addition,the main factors of sintering, including compositions of cured precursor fiber and continuously sintering process, were studied.As a result,a new sintering process was designed and studied.Finally, grain coarsening was avoided during the sintering process and the SA type SiC fiber can be prepared more stably.The reaction between PSCS and Al(AcAc)₃ was traced by FTIR,¹H NMR and UV-Vis and compared with the reaction without Al(AcAc)₃.The results suggest that the synthesis of PACS can be divided into 3 stages and the main reactions during the stages were clarified.At the first stage(RT～325℃),the main reaction is Kumada rearrangements of PSCS,where the Si-Si-Si chains transform into Si-C-Si,and the chains grow;At the second stage(325～420℃),the main reaction is the one between Al(AcAc)₃ and SioH bonds of PSCS;At the third stage(～420℃),the main reaction is condensation between chains,where the(?) and branching degree are dramatically increased due to the multifunctional structure of Al(AcAc)₃.Based on the studies above,Al content in PACS was well controlled by prolonging reaction time of the first stage.To overcome the unfavorable long reaction time,fillings were introduced into the pyrolysis system,which promotes the Kumada rearrangement reaction and the chains growth significantly.In addition,PACS obtained by this method is characterized by lower Si-Si-Si chains,higher(?) and narrower molecular weight distribution.The rheology study shows that,the melting PACS is a pseudo-plasticity liquid.Its apparent viscosity energy is at the range of 190～250 kJ/mol when its softening point is at the range of 190～220℃.The relationship between the spirmability and the viscosity of PACS was established.It is revealed that the higher Al content in PACS will increase the viscosity,leading a higher spinning temperature and poorer spinnability.The spinnability is improved by blending PCS with PACS.As a result,the average fiber diameter becomes finer from 18 to 12μm while the high molecular weight parts are kept in the blends.Air curing of PACS fiber is the crucial procedure in the processes for SA type SiC fiber.The oxygen introduced during air curing plays complicated roles.Firstly,fiber shape will be kept in the sintering process aider fiber being crosslinked by oxygen; secondly,near-stiochiometric compositions can be achieved since the excess carbon will connect with oxygen to release from the fiber as CO gas.Whereas,the study suggests that over-high oxygen content in cured PACS fiber brings some disadvantages:pore faults will come forth in the obtained fiber,even the fiber will be rich in silicon;grain coarsening during sintering process tends to happen due to the oxygen consumes Al. When the weight gains of PACS fiber during curing is about 8.5wt%,the obtained fiber is near-stoichiometric.In addition,it was found that the curing of PACS fiber is affected by its Al content.Its gel content is increasing before Al content increases to a threshold, and then its gel content is decreasing.Finally,aider an oxygen range of cured PACS fibers was determined by balance the positive and negative effects of oxygen,the curing degree has been precisely controlled by an air curing equipment with a simultaneous weighing system.By the studies on continuously sintering of cured PACS fibers for SA type SiC fiber,some results were concluded as following.Grain growth ofβ-SiC can be controlled during sintering when Al content of PACS fiber is more than 0.2wt%.But excessive aluminum does not help to control grain sizes.The EFTEM investigation found that aluminum tends to concentrate at grain boundary.It proved the mechanism of the Al in controlling the grain growth.It suggests that oxygen would connect with aluminum when oxygen content is high.As a result,aluminum can not enter toβ-SiC lattice,which cause its mechanism controlling grain growth not to work.In addition, properly higher content of aluminum favors to lower curing degree and oxygen introducing during curing,as the cross-linkage of aluminum in PACS.The one step sintering process demands higher oxygen in cured PACS fibers.And SiC_xO_y phase decomposition rate is over high.So,pore faults and grain coarsening tend to happen during one step sintering.During the two steps sintering process,the carbon layer on Si-C-O fiber tends to prohibit the gas release so that SA type SiC fiber preparation is unstable.Therefore,the problems of grain coarsening and pore faults can not be avoided by the existing continuously sintering processes.Base on the studies above,a new sintering method was designed and studies.It demands lower oxygen content for cured PACS fiber.In addition,it can control decomposition of SiC_xO_y phase and densifieation process.The sintering method can be interpreted as follows:a cured PACS fiber was continuously pyrolyzed at the range of 1500～1600℃,where SiC_xO_y phase decomposed slowly;The obtained fiber was continuously sintered at about 1800℃where the fiber can be densified.The SA type SiC fiber prepared this way can avoid grain coarsening with good reproducibility.The strength reservation of the obtained fiber is 77%after been heated at 1800℃for 1hr in Ar.