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Research of Si-Al-C-N Preceramic Polymers

Author: CaiXiNan
Tutor: XieZhengFang
School: National University of Defense Science and Technology
Course: Polymer Chemistry and Physics
Keywords: Si-Al-C-N precursor Methyl vinyl dichlorosilane Diisobutyl aluminum hydride Ceramics High temperature performance Antioxidant properties
CLC: TQ174.1
Type: Master's thesis
Year: 2010
Downloads: 92
Quote: 0
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Abstract


The rapid development of high-tech fields such as aviation, aerospace, weapons, energy, light weight, high temperature materials proposed urgent needs. The pure SiC ceramic increasingly difficult to meet the requirements of the use of ultra-high temperature resistance. SiC ceramics introduce other elements to form composite ceramics, to improve the performance of high temperature ceramic. Si-BCN ceramics of the system due to the introduction of the element B, the high temperature performance is greatly improved. But because of the element B in a high temperature oxidizing environments easily generated the volatile B 2 O 3 , limiting its application in an oxidizing environment. Al is and B of the same main group elements, atomic size and Si atoms or less, and AlN and SiC has a similar lattice constant, it is possible to form a \This selection of the methyl vinyl dichlorosilane aluminoxane and containing Al-H - di-iso-butyl aluminum hydride, as raw materials, Synthesis of Si-Al-CN preceramic. Of the the precursor synthesis process, the composition, reaction mechanism, cross-linking and ceramic process research and study the structure of the composition of the Si-Al-CN ceramics and high temperature and oxidation resistance. Through the precursor molecular design and calculation, to determine the precursor synthesis route, ammonia ammonolysis polysilazane first chlorosilane, and then polysilazane alane Al-H and NH Al -H and C = C and Al-C with NH mechanism Precursor Synthesis of Si-Al-CN. Precursor synthesis process research shows that the Si-Al-CN Precursor preferred synthetic process as follows: the raw material in the Si / Al = 1, the reaction temperature is room temperature, and the reaction time was 24 h. Synthetic Precursor Yield 83.4%, as a pale yellow viscous liquid, the molecular weight of 300 8 00, can be dissolved in tetrahydrofuran, benzene, toluene, xylene, chloroform and other organic solvents, easy reaction with water, alcohol, and other substances and crosslinked. Precursor composition studies show that synthetic Precursor composed mainly composed of Si, C, Al, N, H and other elements, the main bonding groups: Si-C, Si-N, Al-N, Al-C . Speculated that the main reaction mechanism for the Al-H and NH dehydrogenation coupling reaction. Precursor crosslinking studies using thermal cross-linking and catalytic crosslinking (H 2 the PtCl 6 · H 2 O and DCP) two kinds way. Suitable crosslinking process: using DCP crosslinked, DCP added amount of 1.0wt% crosslinking temperature of 140 ° C, crosslinking time of 12h. The crosslinked product as a pale yellow, dense, transparent, solid, a gel content of 88.3%. Si-Al-CN preceramic cracking process can be roughly divided into three steps: escape residual solvent and small molecule removal stage (200 ° C or less), preliminary ceramic phase (200 6 00 ℃) and depth of the ceramic and SiC grain growth stage (600 ° C or above). The cleavage of the Si-Al-CN Precursor characteristics show that, when the Si / Al = 1, the ceramic yield of 49.1%. Different heating rates of cleavage of the precursor, the faster the heating rate, the lower ceramic yield. XRD analysis showed that the Si-Al-CN preceramic cracking mainly amorphous structure can be maintained until 1500 ° C, 1600 ° C have obvious crystallization peak, AlN crystallization peak is also 1600 ℃ is rather obvious. Cracking kinetic studies show that the the PYROLYSIS the first stage of the process apparent activation energy 100.6kJ/mol, and belongs to the three-dimensional diffusion process dominated by the Ginstiling-Brounshtein equation; apparent activation energy of the second phase of 219.5kJ/mol , as belonging to the controlled by Avrami EqII, random nucleation process, the apparent activation energy of the third stage of 389.3kJ/mol stage random nucleation, some particles become the nucleation centers in the process. Si-Al-CN ceramic high temperature performance study shows that the Si-Al-CN ceramic having a relatively good heat resistance, and the ceramic product under inert atmosphere, 1800 ° C after the treatment the weight loss of 8.3 wt%, 1500 ° C β -SiC grain size of only 2.31nm, the temperature was raised to above 1600 ℃, AlN and α-SiC crystal formation, AlN and SiC lattice relatively close in size, capable of forming a solid solution. Si-Al-CN ceramic antioxidant properties studies have shown that the antioxidant properties of the Si-Al-CN ceramics is relatively excellent. After the high temperature oxidation treatment in air, the weight of the Si-Al-CN ceramic did not change significantly, the results of XRD showed no significant change in the type and size of the crystal grains in the ceramic after oxidation. 1200 ℃ above oxidation treatment after the Si-Al-CN ceramic surface generated the layer In Al 2 O 3 as a main component of the oxide layer, and the elemental analysis was found by SEM analysis The results showed that the elemental composition of the ceramic is substantially constant, the oxidation resistance of the ceramic.

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