Preparation And Characterization Of Silicon Oxycarbide Aerogels/Rigid Porous Fibers Composite

Thermal protection materials and structures is the key technology to heat protect the security of hypersonic spacecraft. Ceramic fiber tile, which is rigid porous fibrous insulation material, forming a strong bond fibers at the overlap, with porosity rate of 80-95%, high temperature resistance, high strength, low density, low thermal conductivity, corrosion erosion and stability, etc. It is the necessary mandatory thermal protection materials for the space shuttle and X series large hypersonic vehicles.In the paper, Si-O-C airgel / rigid porous fiber composites is made by initially impregnating Si-O-C airgel into the interior space of the rigid porous fibrous skeleton, and then drying, temperature pyrolysis and other process steps. There are two categories of carbonyl-base and quartz-base in rigid porous fibrous skeleton, it is prepared by fiber dispersion, wet billet molding and high temperature sintering process and made of carbon fiber and quartz fiber. Different from the ceramic body formed through the conventional power sinter, Si-O-C ceramics are three-dimensional network structures made of silicon, carbon and oxygen atoms by silicon-containing polymer precursor condensation reaction, dehydrogenation crosslinking, inorganic and ceramic pyrogenation, Si-O-C ceramics are formed through substituting O in Si O2 partially by C. Both of two valence electrons of the oxygen atom can form chemical bonds, to strengthen the network structure, which can improve the mechanical properties and thermal stability of the material and effectively solve the problem of low mechanical properties caused by carbides oxidized easily and oxides high creep variable rate under the high temperature environment. This study intends to combine the two materials together aimed to producing ceramic based thermal conductivity insulating composite materials with better mechanical properties, higher temperature resistance and lower thermal conductivity.In this research, precursor, which includes dimethyl diethoxy silane(DMDES), methyl trimethoxysilane(MTMS) and tetraethylorthosilicate(TEOS), is made into Si-O-C airgel in anhydrous ethanol using precursor transformation. Precursor takes hydrolysis reaction in an acidic environment(PH=3), then polycondensation reaction in an alkaline environment(PH=8) using 1.5ml catalyst to effectively control the sol particle size of the Si-O-C airgel made by this technology, presents nanoparticle accumulation porous structures and mainly consist of amorphous Si OC amorphous structures. After pyrolysis under 1000℃, 1100℃and 1200℃, infrared, XRD, Raman, XPS, and EDS were tested and the results showed that as pyrolysis temperature raising, Si-O bond reducing while Si-C bond, the structure of β-Si C structure, D and G peak significantly increasing, the free carbon content gradually increased, the form Si-O of Si in the material decreases, while the C-C of C elements increases gradually, Si element increasing at first and then decreasing, as well as O element, and C reducing first and then rising. To sum up, through pyrolysis, a high temperature environment, due to the chemical reaction inside the material, Si O2 and Si C, tend to converse from Si-O to Si-C, and material occurs carbonized, so the C major element in the form of free carbon.In conclusion, the research described using vacuum impregnation process to produce aerogels Si-O-C / rigid porous to obtain insulation composite material. In addition, another type of partition hot composite materials is made by mechanical dispersion process to disperse the short-cut quartz fibers uniformly into Si-O-C airgel. Characterizations of the composite material after pyrolysis under temperature of 1000 ℃, 1100 ℃, and 1200 ℃ show that Si-O-C airgel can be filled into the fiber skeleton, in particulate accumulation state, and between the fiber framework does not chemically react, just superimposed physically. quartz composite material existed Si O2 crystal before temperature pyrolysis due to the presence of silica fiber, Because carbon-based composite material consisting of carbon fiber is more, the higher the content of free carbon. Compared with porous fiber backbone, the compressive strength of the composites increased by 20%-30%, the thermal conductivity was reduced by about 20 percent, dipped 1 # airgel density increased by 50%, dipped 8 # airgel density fairly porous fiber, TG showed less than 1000 ℃, the material is about 0.2% weight loss and better thermal stability.