| Silica aerogels are quite unique material because of their marvelous properties such as high porosity, high specific surface area, low bulk density, low thermal conductivity. The focuses of this thesis was first on the properties of silica aerogels synthesized with water glass precursor and modified by different contents of trimethylchlorosilane. The thermal conductivity of granular silica aerogels was measured by Guarded Hot Plate method, while thermal conductivity of monolith silica aerogels was tested by Transient Hot-wire method. The water glass based wet gels were then exchanged with t-butanol follow dried by freezing drying. The fractal geometric characteristics of aerogels and cryogels were analyzed by Frenkel-Halsey-Hill model. Then, chitosan-silica hybrid aerogels were prepared by using water glass and tetraethoxysilane as precursors. Furthermore, grafting aniline on the backbone of the chitosan, the co-polymer chitaline was synthesized and combined with silica gel to form a new aerogel composite. Easily reduced ions such as Au(Ⅲ), an Au(0)-chitaline-silica composite were finally prepared. The surface group, thermal stability, morphology and microstructure of silica aerogels and hybrid aerogels were investigated by FT-IR, DTA-TG, XRD, SEM, TEM, HRTEM and BET adsorption method. The redox property of the chitaline was tested by cyclic voltammetry. UV/VIS spectrometry as well as four-probe Van der Pauw conductivity method was used to test the chemical properties and electrical conductivity of the chitaline-silica aerogels, respectively.Water glass was used as the silica precursor, the synthesis procedures as well as structural properties and thermal conductivity of the ambient dried hydrophobic silica aerogels with Ethanol (EtOH)/Trimethylchlorosilane (TMCS)/n-Hexane as surface modification agent. The research was first on the S/W and pH of the sol to determine the optimum experimental parameters. Then, One-step solvent exchange and surface modification were simultaneously progressed by immersing the hydrogels in EtOH/TMCS/n-Hexane solution. When P(VTMCs/VHydrogels) ranges from 10% to 20%, the final product is hydrophilic xerogel, which is porous material with porosity of 78-88% and the specific density is over 0.24 g/cm3. When P(VTMCS/VHydrogels) is at the scope of 75-100%, the specific densities of the aerogels range from 0.10 to 0.14 g/cm3 and the porosities are in range of 93.5-95.8%. The silica aerogels modified with 75-100% TMCS (V/V) present good performance with the specific surface area in range of 745-770m2/g and the average pore sizes are about 20nm. Furthermore, using granular silica aerogels (P(VTMCS/VHydrogels)=100%) as the fillings, the thermal conductivities of granular aerogels are analyzed by Guarded Hot Plate Method. When temperature ranges from 303 to 333K, thermal conductivity coefficients of the samples are lower than 0.03 W/(m K).Silica cryogels were synthesized in the thesis via sol-gel process and freeze-drying was carried out with t-butanol. In the whole process, Na2SiO3 aqueous solutions with different SiO2 contents were used as the precursor (1:4,1:9 and 1:14). The obtained silica hydrogels were freezed-dried after exchanging the water included in the structure to t-butanol, and the finally dry samples maintaining their wet state structures were obtained. The microstructural morphology and the porous properties of the silica cryogels are observed just like aerogels, but several smaller pores distributes in gels. The particle size distribution was tested in the range of 0-75μm by laser particle size analyzer, when S/W ratio was 1:4, pH was 5.92, the particle size distribution was narrowest. The BET results show that the isotherms of the cryogels was type 4, the pore size distribution was in the range of 10-100nm. The thermal conductivities of the samples was about 0.2 W/(m K), measured by transient hot-wire method.Fractal geometric characteristics of porous silica materials suffered two different drying processes (one-step solvent exchange/surface modification & ambient pressure drying as well as freeze-drying) were investigated based on the N2 gas adsorption method. The overall surface fractal dimensions of the aerogel and cryogel specimens were determined from the analysis of the N2 gas adsorption isotherms and the Frenkel-Halsey-Hill equation was used to determine surface fractal dimension Df. It was found that Df values of cryogels were observed larger than that of the aerogels, which indicated that freeze drying method might induce a rough surface of the gels.Novel hybrid aerogel composite consisting of the bioderived polymer chitosan and water glass or tetraethoxysilane as precursor was synthesized as monolith by solvent exchange/surface modification and ambient drying. The hybrid aerogel from TEOS was shown type 4 isotherms, with a clear hysteresis loop to be seen on the isotherms, while water glass based hybrid aerogels have almost no hysteresis loop and can be approximated as Type 2 isotherms. The reason of the difference was the pore size distribution of TEOS based hybrid gels shown narrower than water glass based gels.A composite of a silica aerogel and a copolymer of chitosan and polyaniline (chitaline) with Au nanoparticles were prepared by an electrodeless and surfactant-free process. By grafting polyaniline on the chitosan backbones, the copolymer chitaline was synthesized, which is soluble in aqueous solution. It was combined with silica gel to form an aerogel composite. Upon protonic doping, the chitaline-silica aerogel conductivities rise from less than 10"8 S/cm to 10"4 S/cm. Chitaline possesses primary amines in the backbone structure that can bind metal ions. Easily reduced ions such as Au(Ⅲ) are spontaneously reduced to form surfactant-free nanostructured zero valent metal particles which are homogenously dispersed in the aerogels. The porosity and chemical inertness of the silica aerogel matrix may be expected to be favorable as an easily recoverable and reusable catalytic media.
|
PDF Full Text Request