【作者】 刘琳；

【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】 制备出陶瓷产率高,使用方便,元素组成合理,成本较低的先驱体一直是SiC先驱体技术发展的方向。聚甲基硅烷（Polymethylsilane,PMS）具有室温液态、分子元素组成近化学计量比等优点,在SiC先驱体方面有很好的应用潜力。但PMS分子量小,交联度低,加热过程中大量小分子逸出造成失重,且PMS以Si-Si为主链,在Si-Si的断裂重排为Si-C过程中也容易造成小分子逸出失重。PMS的结构缺陷决定了其不适合直接用作SiC先驱体。本文通过改性使PMS由不可用变为可用,探索了改性PMS在制备SiC基复合材料、SiC纤维和SiC纳米线方面的应用,取得了很好的效果。由于PMS合成条件苛刻,合成规模一般较小,不能满足制备SiC材料的要求。本研究首先通过合成工艺改进将PMS的Wurtz合成由10g/次放大到近200g/次,最佳反应条件为二氯甲基硅烷（Dichloromethylsilane,DCMS）:Na:甲苯=1mol:2mol:350ml,反应温度80℃,反应时间24hr。在此条件下合成的PMS数均分子量860,在空气中易氧化,陶瓷产率35%,热解产物富硅,Si/C（atom）=1.23。PMS与SbCl₃室温反应得到了液态先驱体锑改性聚硅烷（Antimony substitutedpolymethylsilane,A-PMS）。SbCl₃活性较低,避免了A-PMS的过度交联,使其在室温保持液态,具有较好的可加工性。SbCl₃与Si-H的反应消耗了PMS中的Si-H,使A-PMS在空气中具有较好的稳定性。Sb元素的引入使A-PMS具有较高的分子量和交联度,同时促进了Si-Si的重排,A-PMS在320℃热交联,产物形成以SiC₄和SiC₃H为主的高度交联结构,陶瓷产率高达91%。A-PMS热解得到β-SiC,Si/C原子比为1.12。A-PMS作为先驱体制备SiC基复合材料,具有不需要溶剂、低温固化增重等优点,将A-PMS用于制备C_f/SiC复合材料,根据A-PMS 320℃交联后陶瓷产率高的特点改进了PIP工艺,实现了低温增重,减少了高温烧结次数,经过4次高温烧结制备出密度1.76g/cm³的C_f/SiC复合材料,弯曲强度381MPa。以A-PMS为先驱体,通过改性的PIP制备C_f/SiC复合材料,将复合材料的制备周期由120hr左右缩短至60hr左右,提高了制备效率,降低了制备成本。用A-PMS为先驱体,国产KD-Ⅰ型SiC纤维为增韧相制备的SiC_f/SiC复合材料,密度2.05g/cm³,弯曲强度353MPa。PMS与聚碳硅烷（Polycarbosilane,PCS）两者结构、性质各有特点,将两者共混,有望得到物理状态、性质可调的先驱体,并利用PMS与PCS热解产物分别富硅和富碳的特点得到近化学计量比的SiC。将PMS与PCS室温共混,两者不发生反应,PMS/PCS陶瓷产率只有48-54%。将PMS/PCS加热,PMS与PCS发生Si-H热交联反应,使产物熔点升高,在二甲苯中的溶解度降低,陶瓷产率提高。PMS/PCS 320℃热交联产物陶瓷产率高达81-83%,表现出协同效应。PMS/PCS经1250℃热解,产物C/Si比值在0.81-1.29之间,与纯PMS或PCS相比更接近化学计量比。C/Si比值与PMS/PCS中PMS比例基本成线性关系。经1600℃热解,得到近化学计量比的SiC。将8wt%的PMS加入聚铝碳硅烷（Polyaluminocarbosilane,PACS）,PMS的加入降低了PACS的可纺性,纺丝速度由250-300r/min降低至60r/min。经1250℃烧结,得到的纤维直径15.3μmm,强度1.54GPa。1800℃热处理后保持较好的形貌。用PMS/PCS共混后在N₂中1250℃裂解,在不用催化剂的条件下生成了高纯度的超长SiC纳米线,长度3mm左右,直径250nm。更多还原

【Abstract】 Polymethylsilane （PMS）, is once looked upon as an attractive precursor because it is liquid and the C:Si ratio is 1:1. However, PMS suffers from the low ceramic yield and the poor processability. Moreover, the synthesis of PMS is in a very low scale of about 10g/time. In this paper, our work is to make PMS usable as a precursor. The study is in the following aspects: enlarging the scale of the synthesis, modifying PMS to overcome the disadvantages, and then preparing materials with PMS.First, the scale of the synthesis of PMS through Wurtz reaction was enlarged from 10g/time to near 200g/time when the reaction was carried on at 80℃for 24hr, and Dichloromethylsilane （MeSiHCl₂, DCMS） : Na: toluene = 1mol: 2 mol: 350 ml. The molecular weight and the ceramic yield of the resulted PMS is 860 and 35%, respectively. Pyrolyzed in N₂ at 1250℃, PMS derives ceramic with the Si/C =1.23.A novel SiC precursor, A-PMS, was synthesized through a reaction of polymethylsilane （PMS） with SbCl₃, where the Si-H in PMS reacts with Sb-Cl to form Si-Sb bond with HCl evaporated.It is evident that SbCl₃ plays a very important role in the promoting chain crosslinking, in the transforming of the Si-Si into Si-C bonds and in the stabilizing PMS from very high oxidation trend of the active Si-H bonds. A-PMS keeps liquid at room temperature that is suitable for the infiltration in the absence of any solvent. When heated, the crosslinking degree increases of A-PMS with the curing temperature. A-PMS can be cured into a fully crosslinked structure at 320℃that leads a very high ceramic yield up to 91%. After pyrolysis, the new precursor resulted in near stoichiometricβ-SiC with the Si/C ratio near 1.12.A-PMS was used as precursor to prepare C_f/SiC and SiC_f/SiC composites via polymer infiltration and pyrolysis （PIP） process. The PIP process was modified according to the characteristics of A-PMS. The resulted C_f/SiC samples reaches the density of 1.76g/cm³ and the flexural strength of 381MPa after only 4 pyrolysis cycles, which will be not achieved less than 10 cycles using PCS and the traditional PIP process. A-PMS can also be used preparing SiC_f/SiC composites. The resulted composite has the density of 2.05g/cm³ and the flexural strength of 353MPa.PMS and PCS were blended to adjust the C/Si ratio of the derived SiC. Analysis shows that when PMS and PCS are mixed in xylene, there is no reaction between the two precursors and the ceramic yield of the blended polymer is only 48～54%. But when the blended polymer is heated, Si-H groups in PMS and PCS react with each other to give high crosslinked structure. The ceramic yield of PMS/PCS cured at 320℃is 81～83%. The C/Si ratio of PMS/PCS derived ceramics （pyrolyzed at 1250℃） is linear to the content of PMS in PMS/PCS.SiC fibers were obtained from the blended polymers of PMS（8wt%） and PACS （Polyaluminocarbosilane）. The diameter and the tensile strength of the resulted fibers are 15.3μm and 1.54GPa, respectively.After the pyrolysis of PMS/PCS at 1250℃, ultra long SiC nanowires with high purity were produced. The length is about 3mm and the diameter is 250nm.更多还原