【作者】 蔡溪南；

【导师】 谢征芳；

【作者基本信息】 国防科学技术大学 ， 高分子化学与物理， 2010， 硕士

【摘要】 航空、航天、兵器、能源等高技术领域的快速发展,对轻质、耐高温材料提出了迫切需求。单纯的SiC陶瓷越来越难于满足耐超高温度的使用要求。人们在SiC陶瓷中引入其它元素,形成复相陶瓷,以提高陶瓷的耐高温性能。Si-B-C-N体系陶瓷中由于B元素的引入,其高温性能得到很大的提高。但由于B元素在高温氧化环境下易生成挥发性的B₂O₃,限制了其在氧化环境中的应用。Al是与B同主族的元素,其原子尺寸与Si原子相差不大,且AlN与SiC具有相似的晶格常数,能够形成“固溶体”,具有较好的耐高温和抗氧化性。本文选用甲基乙烯基二氯硅烷和含有Al-H的铝烷——二异丁基氢化铝为原料,合成Si-Al-C-N陶瓷先驱体。对先驱体的合成工艺、组成结构、反应机理、交联及陶瓷化过程进行了研究,并研究了Si-Al-C-N陶瓷的组成结构及耐高温和抗氧化性能。通过先驱体的分子设计与计算,确定了先驱体的合成路线,即先将氯硅烷通过氨气氨解得到聚硅氮烷,再将聚硅氮烷与铝烷利用Al-H与N-H、Al-H与C=C和Al-C与N-H等机理合成Si-Al-C-N先驱体。先驱体的合成工艺研究表明,Si-Al-C-N先驱体较佳的合成工艺为:原料中Si/Al=1,反应温度为室温,反应时间为24h。合成的先驱体的产率83.4%,为淡黄色粘稠状的液体,分子量为300⁸00,能够溶于四氢呋喃、苯、甲苯、二甲苯、氯仿等有机溶剂,易与水、醇等物质反应而交联。先驱体的组成结构研究表明,合成的先驱体中主要由Si、C、Al、N、H等元素组成,主要成键基团为:Si-C、Si-N、Al-N、Al-C。推测主要的合成反应机理为Al-H与N-H的脱氢耦合反应。先驱体的交联研究主要采用热交联和催化交联（H₂PtCl₆·H₂O和DCP）两种方式。合适的交联工艺为:采用DCP交联,DCP添加量为1.0wt%,交联温度为140℃、交联时间为12h。交联产物为淡黄色的、致密的、透明的固体,凝胶含量为88.3%。Si-Al-C-N陶瓷先驱体的裂解过程大体可以分为三步:残留溶剂的逸出及小分子的脱除阶段（200℃以下）、初步陶瓷化阶段（200⁶00℃）和深度陶瓷化及SiC等晶粒长大阶段（600℃以上）。Si-Al-C-N先驱体的裂解特性表明,当Si/Al=1时,陶瓷产率为49.1%。不同的升温速率对先驱体的裂解有很大影响,升温速率越快,陶瓷产率越低。XRD分析表明,Si-Al-C-N陶瓷先驱体裂解时,其主要无定形结构可以保持到1500℃,1600℃才有比较明显的结晶峰,AlN的结晶峰也在1600℃才比较明显。裂解动力学研究表明,先驱体裂解过程中第一阶段的表观活化能为100.6kJ/mol,属于由Ginstiling-Brounshtein方程主导的三维扩散过程;第二阶段的表观活化能为219.5kJ/mol,为属于由Avrami EqII控制的随机成核过程,第三阶段的表观活化能为389.3kJ/mol,该阶段为随机成核、部分粒子成为成核的中心的过程。Si-Al-C-N陶瓷耐高温性能研究表明,Si-Al-C-N陶瓷具有比较好的耐高温性,在惰性气氛保护下,1800℃处理之后的陶瓷产物的失重为8.3wt%,1500℃时β-SiC的晶粒尺寸只有2.31nm,升温到1600℃以上,有AlN和α-SiC结晶生成,AlN与SiC晶格尺寸比较接近,能够形成固溶体。Si-Al-C-N陶瓷抗氧化性能研究表明,Si-Al-C-N陶瓷的抗氧化性能也比较优异。在空气中高温氧化处理之后,Si-Al-C-N陶瓷的重量并无明显变化,XRD的结果显示,氧化后陶瓷中的晶粒种类及大小并无明显变化。1200℃以上氧化处理之后,通过SEM分析发现Si-Al-C-N陶瓷表面生成了一层以Al₂O₃为主要成分的氧化层,元素分析结果表明,陶瓷的元素组成基本不变,陶瓷的抗氧化性能好。更多还原

【Abstract】 With the development of high technique areas, such as aviation, aerospace, weapons, energy, etc., advanced materials with high temperature resistance and excellent oxidation resistance at high temperature are urgently required. The grains in the pure SiC ceramics grow rapidly with increasing the temperature. It is difficult to meet the application requirements under ultra-high temperature. To improve the high temperature resistance and high temperature oxidation resistance of SiC ceramics, hetero-elements are introduced into SiC ceramics to form multiphase ceramics.The high temperature resistance of Si-B-C-N ceramics improve drastically through introducing the elemental boron. In oxidative atmosphere, however, boron tends to produce B2O3, which is volatile species at high temperatures.Aluminium is the higher homologue element of boron. Furthermore, AlN and SiC can form the solid solutions for their similiar crystal lattices, whcich show excellent high temperature resistance and high temperature oxidation resistance.In this paper, the Si-Al-C-N preceramic polymers were synthesized by polymethylvinylsilazane （which is obtained by the ammonolysis of dichloromethylvinylsilane） and diisobutyl aluminium hydride. The synthesis conditions, composition, structure of the preceramic polymers, reaction mechanism, crosslinking conditions, and the composition, structure, high temperature resitance, and high temperature oxidation resitance of the derived ceramics were investigated.The molecular design and thermodynamics calculation showed that, the reactions between Al-H and N-H, Al-H and C=C, N-H and Al-C, to synthesize the Si-Al-C-N preceramic polymers were possible in theory. The study of synthesis conditions of preceramic polymers showed that, the optimal synthesis conditions were the followings, the molar ratio of between Si and Al about 1, room temperature, holding time 24h. The preceramic polymers were light yellow viscous liquid with yields about 83.4%. The as-synthesized polymer could be easily dissolved in organic solvents such as THF, benzene, toluene, xylene, and chloroform. The number average molecular weight of the polymer was 300⁸00.The structure of preceramic polymers was characterized by FT-IR, NMR, XPS, LC-MS, respectively. The results showed that, Si-N, Si-C, Al-N, Al-C groups were included in the polymer. The main reaction mechanism was dehydrogenation coupling between Al-H and N-H.The crosslinking of the preceramic polymers were researched by thermal crosslinking and catalyst aided crosslinking （H₂PtCl₆·H₂O and DCP）, respectively. The optimal crosslinking conditions were, the dosage of DCP about 1.0wt%, reaction temperature 140℃, holding time 12h. The crossliked products were light yellow transparent compacts with gel content about 88.3%.The pyrolysis process of the crossliked products was studied. The composition and structure of the derived ceramics were characterized by elemental analysis, FT-IR, TG, XRD, XPS, respectively. The results showed that, the pyrolysis process could be devided to three steps. The volatilization of the residue solvent occurred under 200℃. During 200℃to 600℃, the polymer was decomposed with the release of organic species. The pyrolysis of the polymers were almost completed at about 600℃with the ceramic yield about 49.1%. The pyrolysis continued over 600℃with the release of a little of gasous species. The derived ceramics were amorphous till 1500℃. The grain of SiC and AlN were observed while the temperature over 1600℃. Pyrolysis kinetic study indicated that, the stage I was dominated by Ginstiling-Brounshtein equation-controlled three dimensional diffusion with the apparent activation energy （E_a） about 100.6kJ/mol. The second stage was a random nuclearation controlled process which abided by Avrami EqII with the Ea about 219.5kJ/mol. The stage III was a random nuclearation process, one nuclear for one particle with the Ea about 389.3kJ/mol.The high temperature resistance study showed that, the derived ceramics showed good high temperature performance. The weight loss of the derived ceramic was 8.3wt% at 1800℃under argon atmosphere. The grain size of SiC at 1500℃was 2.31nm. The grain of AlN andα-SiC formed at 1600℃. The lattice constants of AlN were close to those of SiC. Therefore, solid solution could be easily formed.The derived Si-Al-C-N ceramics showed excellent high temperature oxidation resistance. After exposed to air at 1400℃, the weight change of ceramics was not obvious. The results of XRD showed that, the species and the grain size of Si-Al-C-N ceramics were maintained. Aluminium oxide layer on the surface of Si-Al-C-N ceramics were observed after exposed to air at 1400℃, while the elemental composition of the ceramics changed little.更多还原