| Aerogel is a kind of solid material composed of nano particles or nano porous framework structure formed by cross-linking polymer moleculars, which possesses special nano-scale framwork and pore distribution characterization, enduing that it has excellent thermal physical properties and wide applications in many fields. However,the instrisic fracture of aerogel seriously limits its further applications. Therefore, the study on improving the flexibility of aerogel become an important issue. In this thesis, the phase separation during hydrolysis condensation of methyltrimethoxysilane was resolved by using DMSO as solvent, and the mechanisms of DMSO to control phase separation were revealed. A flexible Si O2 aerogel with high strength was prepared. Based on above, the MFC was employed as an enhancement, a flexible Si O2/MFC aerogel composite with high strength and light quality was prepared, by which two questions including that fracture of Si O2 aerogel and low compression strength of cellulose aerogel were resolved. In additation, based on the material systems of Al2O3 of thermo stability, a flexible Al2O3/HEC aerogel composite with higher strength and anisotropic struxtures was prepared.Based on the principles where the variety of polymerization of silanols, depending on the precursor effecting in different concentrations of DMSO, affects the phase separation, a new technique of controlling phase separation during hydrolysis condensation of methyltrimethoxysilane was obtained. A flexible Si O2 aerogel was prepared by using supercritical drying method and the mechanisms of controlling phase separation by DMSO was revealed. The effecting mechanisms of DMSO solvent and ammonia catalyzer contents on gel time and microstructures were studied. The results show that the framework of flexible aerogel was formed by Si-O and Si-C, resulting in the hydrophobic characterization and flexibility of aerogel. The best ratio of MTMS, DMSO, H2 O, ammonia and oxalic acid of flexible Si O2 aerogel is 1:6:8:0.05:0.6. The phase composition is amorphous Si O2, and this structure can remarkablely improve the toughness of materials. In the range of 50%, it can achieve completely elastic recovery, and the mechanical compression performance is four times higher than that of traditional Si O2 aerogels. The thermal conductivity, ratio surface area and hydrophobic angle of this aerogel are 0.0302W/(m·K), 458.4m2/g and 152°, respectively. Because of the organic group crack, at 400℃ the Si O2 aerogel will take place quality decline, whereas after 500℃ the residual quality is more than 80% and tents to be steady. The effecting mechanisms of DMSO contents on ratio surface area, pore diameter and thermal conductivity of aerogels were studied. The results show that the ratio surface area and thermal conductivity increase with the increase of solvent contents.Based on the the principles where the polymer cross-linking can make aerogels flexible, some proportion of MFC were mixed into above Si O2 aerogel, and a flexible Si O2/MFC composite aerogel was prepared by using supercritical drying method. The effecting of cellulose on ratio surface area and microstructures and the effecting of content on phase composition and chemical structures were studied. The results show that, in composite aerogel, the Si-C bonds are still existent, and Si O2 exists in form of amorphous and MFC are natural celluloses. The inner network structures of this composite aerogel are Si O2 particles, where the MFC are uniformly distributed and form entangled networks. This aerogel structure is hydrophobic, and the hydrophobic angle decline with the increase of cellulose content. Comparing with flexible Si O2 aerogel, the additation of celluloses can effectively improve the mechanical and thermal properties, but has little influence on denity of aerogels.Based on the directional control principle of microstructures, the long chains of HEC was mixed into the material system of Al2O3 of thermostability, and a flexible anisotropic Al2O3/HEC composite aerogel was prepared by freezing process. Especially, a regular layer structure, where the inter-layers are linked, was obtained. The mechanical and thermal properties along different directions were studied, and the formation mechanisms of regular structure and effecting mechanisms of variety of cellulose content on microstructure and properties were revealed. The results show that with the increase of cellulose content, the disancet between layers changes to be small and the linking fashions change. There are no influences on phase components and chemical structures by changing cellulose content. The framework of hybrid aerogels is cellulose, on which there is a layer of boehmite Al2O3. This aerogel keeps good thermal stability below 400℃. The effecting mechanisms of cellulose content on density and compression properties are the same as that of Si O2/MFC hybrid aerogels, but the breakages are different. This Al2O3/HEC aerogel along the regular array direction presents good mechanical performances. |
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