【作者】 徐天恒；

【导师】 陈朝辉； 马青松；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2011， 博士

【摘要】 鉴于聚硅氧烷转化SiOC陶瓷的广阔应用前景,近年来对SiOC陶瓷的研究热度不断升温。SiOC陶瓷可以看作是一种SiO2无定形网络中二价氧被四价碳部分取代的产物。这种改变可以增加其热稳定性和机械性能,使其适合用作高温结构材料。由于SiOC陶瓷需要在高温环境中工作,因此有必要揭示其组成、结构和性能在不同环境中的演变规律,从而对SiOC陶瓷的应用提供技术支持。然后基于对SiOC陶瓷结构演变规律的认识,有的放矢地对SiOC陶瓷结构进行改性设计,从而最终满足更高的使用要求,拓宽应用范围。研究了硅树脂转化SiOC陶瓷在各种高温环境中结构与组成的演变行为。尤其是SiOC陶瓷微观结构随温度的变化规律。结果发现在常压惰性环境中,当温度升高到1000℃时,硅树脂基本完成无机化,生成富碳的SiOC陶瓷。其组成为Si:35.96wt%,O:26.87 wt %,C:34.85 wt %。在惰性环境中,在1000-1500℃温度区间, SiOC陶瓷中发生了Si-C键和Si-O键的重排反应,重排反应导致了SiOxC4-x（3≥x≥2）单元分解生成SiO4和SiC4单元。在键重排反应过程中SiOC陶瓷的体积和质量均无明显变化。当温度高于1450℃,SiOC陶瓷中的SiO2和自由碳之间的碳热还原反应激活,生成β-SiC和α-SiC混合结晶。碳热还原反应导致了明显的质量损失和体积收缩。当温度达到1700℃时,碳热还原反应基本反应完全。此时样品为SiC和自由碳所组成的一种疏松多孔材料。较低的环境压力将会降低SiOC陶瓷中的SiO2与自由碳之间的碳热还原反应激活温度,并且加速键重排反应的进行。在低压环境中（10Pa）,SiOC陶瓷中Si-C键和Si-O键的重排反应的完成温度降低至1400℃。SiOC陶瓷中的碳热还原反应激活温度降低至1250℃,碳热还原反应完成温度降低至1500℃。环境中氧元素的存在将会导致SiOC陶瓷中氧化反应的发生,并且影响键重排反应。当温度大于1000℃时,SiOC陶瓷的表面和开孔的孔壁上发生了SiOxC1-0.5x网络的氧化和自由碳的氧化。随着温度的升高,氧化反应加剧,并在样品表明形成一层致密的SiO2。在1000-1400℃温度区间,SiOC陶瓷中发生了Si-C键和Si-O键的重排反应,重排反应由于氧的存在,导致了SiO_xC_4-x（3≥x≥2）单元分解只生成SiO4单元。在研究SiOC陶瓷基体结构演变的基础上,表征了C_f/SiOC复合材料在高温环境中的性能与结构随温度的演变规律,揭示SiOC陶瓷基体结构与组成的演变行与C_f/SiOC复合材料力学性能的演变规律之间的关系。结果表明无论是在惰性常压环境还是在低压环境中,C_f/SiOC复合材料的力学性能受基体中的的分解反应所控制。当SiOC陶瓷基体中只发生键重排反应时, C_f/SiOC复合材料的力学性能基本保持不变。而一旦基体中的碳热还原反应激活后,基体将会变得疏松多孔,难以有效传递载荷。同时纤维也会与基体发生反应,造成纤维损伤。最终使得C_f/SiOC复合材料强度和弹性模量快速下降。因此要提高C_f/SiOC复合材料的高温稳定性,就必须提高基体中的碳热还原反应温度。以提高SiOC陶瓷的碳热还原反应温度为目的,在SiOC陶瓷中引入异质元素Al,形成SiAlOC陶瓷。并且对SiAlOC陶瓷在各种高温环境中结构与组成的演变行为进行表征。结果表明通过溶胶-凝胶法可以对硅树脂进行改性,得到SiAlOC先驱体。这种先驱体中主要包含七种Si单元和两种Al单元。随着温度的升高,Si-OH键之间的缩合以及Si-O键,Si-C键和Si-H键之间的重排反应均有可能发生,导致生成含H和C的挥发性气体（如CH4和H2）。当温度升高到1000℃时,SiAlOC先驱体完成无机化生成SiAlOC陶瓷。其组成为Si:31.26wt%,Al:15.53 wt %,O:46.89 wt %,C:6.32 wt %。在整个无机化过程中,先驱体中的Si单元只有Q4和T单元保留了下来,同时还发生了AlO6向AlO4单元的转化。在低压环境中1000℃到1300℃之间,Al-O键,Si-O键和Si-C键发生重排反应。Al-O自身发生重排反应,导致AlO4单元重排生成AlO6单元;Si-O键和Si-C键之间发生重排反应,导致SiO3C单元分解生成SiO4和SiC4单元;Si-O键和Al-O键之间发生重排反应,引起Q4（nAl）（1≤n≤4）四个基团的相对含量变化。当温度升高到1300℃,SiO3C单元分解完全。当温度升高到1400℃时,样品中生成大量的Q4（nAl） （1≤n≤4）单元,同时形成莫来石结晶。当温度超过1500℃时,样品中莫来石与自由碳之间发生碳热还原反应,生成Al2O3。此反应在1600℃时结束。SiAlOC陶瓷的碳热还原反应温度激活温度随环境压力的升高而升高。在常压惰性环境中,样品中莫来石与自由碳之间的碳热还原反应温度可以提高到1600℃以上。但是环境压力Al-O键,Si-O键和Si-C键之间的重排反应以及莫来石的结晶行为均无明显影响。环境中氧元素的存在将会导致SiAlOC陶瓷中氧化反应的发生,当温度超过1200℃时,在SiAlOC陶瓷样品的表面和开孔的孔壁上发生了氧化反应。随着温度的升高,氧化反应加剧,并在样品表面形成一层厚度随温度升高而增加的白色混有无定形SiO2的莫来石晶体。采用改性先驱体制备了C_f/SiAlOC复合材料,并表征了C_f/SiAlOC复合材料在高温环境中性能与结构随温度的演变规律。结果表明无论是惰性环境还是低压环境中,C_f/SiAlOC复合材料的力学性能受基体中的分解反应以及基体与纤维之间的反应所控制。SiAlOC基体中所发生Si-O,Si-C和Al-O之间的重排反应以及莫来石的结晶均对C_f/SiAlOC复合材料的力学性能没有明显影响。而一旦基体中莫来石与自由碳之间的碳热还原反应激活,或者基体与碳纤维之间的反应激活后,复合材料的力学性能便会快速降低。相比于C/SiOC体系,无论是在惰性环境还是低压环境中,C/SiAlOC体系都具有更好的高温稳定性,是一种适宜的高温结构材料体系。更多还原

【Abstract】 Due to the wide range of possible applications, an increased attention has been paid to silicon oxycarbide(SiOC) ceramics. Those ceramics can be considered as anionic modification of silica network in which two divalent O atoms are partially replaced by one tetravalent C atom. This modification is expected to improve the thermal and mechanical properties suitable for applications at high temperatures. Because some applications concern high temperature utilization, in order to offer technologies for applications of the SiOC ceramics, there is a strong need to understand their high temperature behavior completely in various environments. Subsequently, based on the structural evolutions of SiOC ceramics, the structure of SiOC ceramics have been modified with the aim of a further improvement of their high temperature stability.The structure and composition evolutions of SiOC ceramics in various environments have been investigated. It is found that carbon rich SiOC ceramics were obtained through pyrolysis in inert atmosphere at 1000℃of silicon resin. Its composition are Si:35.96wt%, O:26.87 wt % and C:34.85 wt %. In inert environment, SiOC ceramics undergo a redistribution of Si-O bonds and Si-C bonds leading to the formation of SiC4 and SiO4 units at temperature from 1000 to 1500℃. The weight and volume of SiOC ceramics are almost constant during redistribution reactions. At temperature above 1450℃, carbothermal reduction is active leading to the formation ofβ-SiC andα-SiC. In same temperature range, the weight loss and volume shrinkage are obvious. At 1700℃, carbothermal reduction is almost finished, and the samples are porous materials which consist of SiC and free carobn.A lower pressure will shift the start of carbothermal reduction to lower temperatures, and promotes the distribution reactions. In reduced pressure environment (10Pa), carbothermal reduction was active at 1250℃, and finished at 1500℃. Distribution reactions finished at 1400℃. The presence of oxygen in environment will induce the oxidation of SiOC ceramics, and affect the redistribution reactions. At temperature above 1000℃, Oxidations of SiOxC1-0.5x network and free carbon take place at the surface and the wall of open pores of SiOC samples. On increasing temperature, a dense coating of SiO2 formed at the surface of SiOC samples. At temperature above 1000℃, redistribution reactions are active. Because of the presence oxygen, only SiO4 unit can be observed during the decomposition of SiOxC4-x(3≥x≥2)units.C_f/SiOC composites were obtained through PIP process. Their high temperature behavior in inert and reduced pressure environment was investigated, and the relationship between the structure and composition evolution of SiOC ceramics matrix and mechanical property evolution of C_f/SiOC composites were revealed. It was found that the decomposition behavior of SiOC ceramics matrix play a crucial role on the mechanical property of C_f/SiOC composites. When the redistribution reactions are active in matrix, the mechanical property of C_f/SiOC composites are almost constant. Once the carbothermal reduction is active in matrix, the mechanical property of C_f/SiOC composites decrease rapidly due to the loose matrix and the reaction between fiber and matrix. Thus, to improve the high temperature stability of C_f/SiOC composites, shifting the start of carbothermal reduction to high temperature is necessary.Aluminum has been introduced into the structure SiOC ceramics with the aim of shift the start of carbothermal reduction to higher temperatures. The structure and composition evolutions of SiAlOC ceramics have been investigated. It was found an Al-doped SiAlOC polymer can be obtained through sol-gel process. There are at least seven different Si and two different Al units in SiAlOC polymer. On increasing temperature, condensation reactions of Si-OH group and the cleavage of Si-O, Si-C, Si-H bonds may occur, leading to the evolution of C and H containing volatile species. The conversion of the polymer into an inorganic network is achieved at 1000℃, and the composition of SiAlOC ceramics are Si:31.26wt%, Al: 15.53wt%, O:46.89wt % and C:6.32 wt %. In the mineralization process from polymer to glass, two different Si units remained at the inorganic network and the transition from AlO6 to AlO4 occurred.In reduced pressure environment, Al-O, Si-O, and Si-C bonds redistribution reactions are active from 1000 to 1300℃. Al-O bond redistribution will lead to the conversion from AlO4 to AlO6 units. Bonds redistribution between Si-O and Si-C bonds will lead to the consumption of SiO3C unit. Bonds redistribution between Si-O and Al-O bonds will lead to the contents change of Q4(nAl)(1≤n≤4)units. At 1300℃, the consumption of SiO3C unit finished. At 1400℃, the contents of Q4(nAl)(1≤n≤4)units increased rapidly, and the crystallization of mullite occur. Once the temperature is above 1500℃, the carbothermal reduction between mullite and free carbon is active leading to the formation of Al2O3, and the reaction finished at 1600℃.A higher pressure will shift the start of carbothermal reduction to higher temperatures. In flowing Ar atmosphere(101KPa), the carbothermal reduction between mullite and free carbon is not active at temperature below 1600℃. However, environment pressure has fewer effects on the Al-O, Si-O, and Si-C bonds redistribution reactions and the crystallization of mullite. The presence of oxygen in environment will induce the oxidation of SiAlOC ceramics. At temperature above 1200℃, Oxidation of free carbon takes place at the surface and the wall of open pores of SiAlOC samples. On increasing temperature, a white coating consists of mullite and amorphous SiO2 formed at the surface, and the thickness increased with increasing temperatures.C_f/SiAlOC composites were obtained through PIP process, and their high temperature behavior in inert and reduced pressure environment was investigated. It was found that the decomposition of matrix and the reaction between fiber and matrix play crucial roles on the mechanical property of C_f/SiAlOC composites. Al-O, Si-O, and Si-C bonds redistribution reactions are active in matrix or the crystallization of mullite occur, the mechanical property of C_f/SiAlOC composites are almost constant. Once the carbothermal reduction or the reaction between fiber and matrix are active in matrix, the mechanical property of C_f/SiAlOC composites decrease rapidly. Compared with the C/SiOC system, C/SiAlOC system shows improved thermal stability both in inert and reduced pressure environment. Thus C/SiAlOC system is a suitable high temperature structure material system.更多还原