Research Of Si-al-c-n Preceramic Polymers

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.