| 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.
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