【作者】 赵大方；

【导师】 李效东；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2008， 博士

【摘要】 先驱体法制备的SiC纤维是高性能陶瓷基复合材料（CMC）的关键增强材料。随着航空航天事业的发展,要求用于CMC的增强纤维具有优异的高温性能。通过在先驱体中引入铝元素烧结制备的近化学计量比SA型SiC纤维具有耐超高温特性,将在高技术复合材料领域发挥重要作用。本文对SA型SiC纤维的连续制备工艺进行了全面分析,重点对先驱体PACS的合成、高温烧结中存在问题和各个工艺之间的相互关系进行了系统研究。揭示了PACS主要合成机理,在此基础上改进了合成工艺,提高PACS中铝引入的稳定性。在研究了先驱体纤维组成和连续烧结过程对纤维烧结影响的基础上,设计了新的连续烧结工艺,可避免纤维在烧结过程中发生晶粒粗化,提高了SA型SiC纤维连续化制备过程的稳定性。采用FTIR、¹H NMR和UV-Vis等手段对PSCS和Al（AcAc）₃的反应过程进行实时跟踪,发现合成过程三个阶段的主要反应:在RT～325℃的第Ⅰ阶段,主要发生PSCS裂解重排和扩链反应,Si-Si-Si链转变为Si-C-Si链,并生成Si-H键;在325～420℃的第Ⅱ阶段,主要是PSCS的Si-H键与Al（AcAc）₃之间的反应;在420℃以上的第Ⅲ阶段,主要发生分子间缩合反应,由于Al（AcAc）₃的多官能团交联作用,这一阶段分子量和支化度急剧增加。在以上研究基础上,解决了反应过程中铝含量波动的问题,并缩短了合成时间。研究表明,延长反应时间可减少铝的损失并提高（?）,而延长第Ⅰ阶段反应时间最为有利;据此调整合成升温制度并稳定精制工艺后,实现了PACS铝含量的可控性。针对合成时间过长不利于实际生产的问题,在裂解柱中填充填料来促进裂解重排和扩链反应,使整个合成时间缩短了近一半。并且,由此合成的PACS的Si-Si-Si链含量更低,（?）更大,分子量分布更窄。对PACS的流变性研究表明,PACS熔体是假塑性流体。当软化点在190～220℃时,PACS的粘流活化能为190～260 kJ/mol。PACS的粘度在100 Pa.s左右时可纺性好。随着铝含量增加,PACS粘度增大,导致纺丝温度过高,可纺性变差。采用PCS与PACS共混提高了可纺性,同时保持PACS的高分子量部分,纺出纤维的平均直径可从18μm左右下降到12μm左右。PACS纤维的预氧化是整个制备过程中的重要步骤。纤维中由此引入的氧具有多重作用,既使纤维交联确保下一步高温烧结时不并丝,又会在高温下带走纤维中富余碳使SA型SiC纤维达到近化学计量比。但研究表明,纤维中氧含量过高将造成过多孔缺陷,甚至使纤维富硅,同时也会使铝失效导致SA型SiC纤维发生晶粒粗化。一般当PACS纤维的预氧化增重率约8.5wt%时,能制备出近化学计量比的连续纤维。研究还发现铝含量变化对PACS纤维的预氧化过程具有较大的影响,体现在铝含量提高其预氧化程度增加,但铝含量超过临界值后交联度反而下降。最终,本文在综合氧的多重作用并确定最佳氧含量范围后,通过实时称重预氧化装置实现了预氧化程度的精确控制。在对PACS预氧化纤维高温连续烧结制备SA型SiC纤维的研究过程中发现以下规律。PACS纤维中含有少量的铝（＞0.2wt%）就能有效抑制β-SiC晶粒增长,但当铝含量进一步增加时,其抑制晶粒长大作用基本不变。通过EFTEM分析发现铝原子富集于晶界,证实了铝的抑晶机理;由此认为,连续烧结时,纤维中氧含量过高将与铝结合,使铝不能有效进入β-SiC晶格,导致铝的抑制晶粒长大的作用失效。此外,铝在PACS纤维中起到交联作用,故适当过量的铝有助于降低预氧化程度,减少预氧化过程中引入的氧;一步连续烧结对PACS预氧化纤维氧含量要求高,并且SiC_xO_y相分解过快是导致纤维存在较多孔缺陷和发生晶粒粗化的重要原因;二步烧结过程中SiC_xO_y纤维富碳层的存在导致气体扩散受阻是纤维制备不稳定的重要原因。根据以上研究,设计了新的烧结工艺。此工艺对PACS纤维氧含量要求适中,并可协调和控制SiC_xO_y相分解和致密化过程:第一步将PACS预氧化纤维在1500～1600℃下连续预烧,使SiC_xO_y相分解速度降低;第二步将纤维在1800℃左右连续烧结,使纤维尽可能致密化。此烧结新工艺具有较好的性能重复性,制备出的连续SA型SiC纤维在1800℃氩气中热处理1小时不出现晶粒粗化,强度保留率为77%。更多还原

【Abstract】 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.更多还原