【作者】 王慧；

【导师】 曾令可；

【作者基本信息】 华南理工大学 ， 材料学， 2010， 博士

【摘要】 气凝胶(aerogel)是指以纳米量级超微颗粒相互聚集构成纳米多孔网络结构,并在网络孔隙中充满气态分散介质的轻质纳米固态材料。由于气凝胶具有独特的性能,气凝胶被广泛应用于催化剂及催化剂载体、高效隔热材料、切伦科夫传感器、集成电路、电容器以及宇宙尘埃搜集器等。因此,气凝胶具有重要的理论研究价值及广阔的应用前景。本文以正硅酸乙酯(TEOS)为原料,乙醇和水为溶剂,利用溶胶—凝胶法,首次采用水／正丁醇／正己烷三重溶液共沸交换的表面改性的方法,采用常压干燥工艺制备了SiO2气凝胶及TiO2／SiO2气凝胶、陶瓷纤维／SiO2气凝胶复合材料；对它们的结构及性能进行了测试分析,利用氮气吸附脱附法、X射线小角散射法研究了其分形特性。通过前期实验以及国内外研究学者的理论分析,确定了正硅酸已酯、乙醇、水的摩尔比为1：4：6,采用二步法制备SiO2气凝胶,研究了酸碱加入的间隔时间、温度对凝胶时间的影响,重点分析了非超临界干燥条件下制备SiO2气凝胶所需要的老化方式、表面改性过程,用三甲基氯硅烷(TMCS)做表面改性剂,对醇凝胶进行表面改性制备SiO2气凝胶,TMCS/H2O的最佳摩尔比为0.4。确定了采用水／正丁醇／正己烷三重溶液共沸的常压干燥工艺,分别在60℃和80℃干燥。通过XRD、SEM、TEM、FTIR、TG-DSC、N2吸附脱附等测试方法对SiO2气凝胶的物相、疏水性能、热稳定性能、微观结果及孔结构进行了测试分析得出一下结论：试验制得了组分为非晶无序的、具有良好的疏水性能的SiO2气凝胶的,且经过750℃热处理后的SiO2气凝胶仍呈非晶态。通过接触角的测试以及红外光谱分析得到,为了保持SiO2气凝胶的疏水性能,其热处理的温度不能超过400℃。本试验制得的SiO2气凝胶微观上是由SiO2气凝胶的圆形颗粒堆积而成的,颗粒大小均匀,比表面积达到868.5 m2／g,且孔径分布窄。样品经750℃煅烧后颗粒尺寸有所减小,孔径增大,整体颗粒与孔结构趋于均匀,但是整体微观形貌变化不大,说明750℃下,SiO2气凝胶的网络结构仍比较稳定,仍具有良好的纳米孔结构。采用直接添加TiO2粉体以及先驱体添加钛酸丁酯两种实验方法制备TiO2／SiO2气凝胶复合材料。TiO2粉末的添加可以得到具有很高的比表面积的SiO2气凝胶,当添加量为15%时,高达1107.38 m2／g。而添加钛酸丁酯后,比表面积急剧下降,具有相当高的微孔体积,存在大量的微孔(2nm以下的孔)。对甲基橙溶液的光催化活性实验表明：TiO2粉体掺杂SiO2气凝胶,提高了TiO2的光催化效果。当TiO2粉体的添加量为15%时,降解率最大；比较纯TiO2与TiO2／SiO2气凝胶的活性可知,将TiO2粉体分散在介孔材料中其催化活性可以得到充分发挥。采用先驱体加莫来石陶瓷纤维以及SiO2溶胶浸渍莫来石纤维预制体两种方法制备最大直径为103mm的纤维／SiO2气凝胶复合材料,两种方法得到的复合材料中,纤维都很好的起到了骨架的作用。方法一得到的复合材料中,纤维的添加,并没有影响气凝胶本身的特性,其孔隙率以及SiO2的颗粒形状良好；从孔体积来看,当添加纤维的量为20%时,其孔体积与纯SiO2气凝胶相近,此时的吸附—脱附曲线表明：具有较高的吸附量和孔容。方法二得到的复合材料中纤维预制体中的孔洞被SiO2气凝胶填充,但纤维与气凝胶的结合比较紧密,具有一定的强度。SAXS对SiO2气凝胶的研究中表明,热处理后由于颗粒变大使得低散射区的散射强度降低,而在反应釜中密闭的状态下制备的SiO2,由于颗粒小且均匀,得到的低散射区的散射强度就高；本实验所制得的SiO2气凝胶体系的小角散射均不遵守Porod定理,表现为不同程度的正偏离,这表明散射体系中有第二相(界而层)存在；反应釜中密闭制备、常压下制备、经400℃热处理的三种样品在高散射强度区呈现出表面分形,表面分形维数Ds分别为：2.65、2.48、2.11；在低散射强度区呈现出质量分形,分形质量分形维数Dm分别2.23、2.36、2.43。利用氮气吸附脱附法对复合材料的分形研究表明,所有样品的直线拟合相关度很高,所得分形维数Ds值均在2.4～2.9之间,因而均具有表面分形特性。其中添加纤维的气凝胶分形维数与纯凝胶非常接近,变化很小,说明表面复杂程度相似。添加钛酸丁酯的气凝胶的分形维数很高,在2.6～2.9之间,说明其具有比较复杂的结构。两种方法对纯SiO2气凝胶的分析得到的结构基本一致,且跟前面的测试分析结果相吻合,说明利用这两种方法得到的数据真实可靠。更多还原

【Abstract】 Aerogels are high dispersive solid materials which consist of colloid particles or high polymer molecule and have continuous random network structure filling with gaseous dispersive medium. According to their low densities, low refractive indices, small pore sizes, and good transparencies, Much attention have been given to the aerogels for use in several applications:catalyst supprot, thermal insulators, Cherenkov detectors, monolithic, capacitors and cosmic dust capture etc.Silica aerogels, TiO2/SiO2 aerogels and Fiber/SiO2 were synthesized by two-step sol-gel polymerization of tetraethoxysilane (TEOS) in alcohol and using ambient pressure drying with ternary azeotropes as components of pore fluid. The physical properties such as density, appearance, hydrophobicity, surface area, pore size distribution and thermal stability were measured. According to the data of absorption-desorption experimental and Small Angle X-ray Scattering, the surface fractal of silica serogels and the composites were studied.The wet gels were synthesized via acid-base catalysis using tetraethyl orthosilicate as silica precursor and alcohol as solvent. The molar rate of TEOS, alcohol and water is 1:4:6. Alcohol was exchanged by n-butanol, and the gel surface was modified by a mixture mixed by n-butanol and trimethylchlorosilane (TMCS).Then the solvent was exchanged by several steps by saturated hydrocarbon, so as to obtain the pore fluids which contain azeotropic mixtures such as water, n-butanol and hexane. Ambient pressure drying was performed in two steps, at the boiling points of the ternary azeotropes (60℃)and the hydrocarbons(80℃), respectively.The structure and physical properties of these aerogels were investigated by several experimental methods, such as X-ray diffraction (XRD), scanning electron microscopy (SEM),thermo-gravimetric and differential scanning calorimetry (TG-DSC), Fourier transform infrared spectroscopy (FTIR), specific surface area analyzer,pore size analyzer and so on. The result shows that silica aerogels are coherent, amorphous form, hydrophobic and nanosized porous solids. And its high specific surface achieves 838.6 m2/g.Titania-silica aerogels with different Titania content were prepared. Two preparation methods differing mainly in approach to precursor hydrolysis were applied. The high specific surface area is obtained by adding TiO2 powder, and the maximal one is 1107.38 m2/g while the content of it is 15wt%. While adding tetraisopropyl orthotitanate as one of precursor, the specific surface area is sharp decline and the N2 adsorption and desorption experiment shows that there are a lot of micropores (<2nm). Methyl orange photocatalytic experimental result shows that:photodegradation rate of Methyl Orange is higher by TiO2 powder mixed in SiO2 aerogels than pure TiO2 powder, and when the content of TiO2 powder accounts for 15wt%, the best photocatalytic properties appears. Compare the photocatalytic experiment using TiO2 and TiO2/SiO2 as catalyst. It’s easy to know that when mixed in the SiO2 aerogels, the catalytic of TiO2 is mucher better.Fiber/SiO2 aerogels with maximal diameter 103mm were synthesized using two different methods:the first one is adding fibers in precursors while the second one is dipping fiber’s preformed units in the SiO2 sol. The characteristic of SiO2 aerogels haven’t changed because of the adding of the fibers. When the content of fiber was 20%, the pore volume was similar to pure SiO2. Nitrogen adsorption and desorption isotherms indicated that the composite material had a big adsorptive capacity and pore volume.The results of SAXS shows that at low q-domain, the scattering intensity of SiO2 aerogels was low, because of heat treatment, the particles of SiO2 became bigger. While the the scattering intensity of SiO2 aerogels prepared in airproof reactor was higher. It also shows that the SAXS profiles do not agree with Porod’s law and have positive slopes, suggesting that the dispersive interfacial layer exists between sol particle and dispersing agent. The three kinds of silica aerogels prepared in airproof reactor,ambient pressure, and heat treated at 400℃have different surface fractal dimension Ds at high scattering intensity region, the surface fractal dimension Ds were 2.65,2.48 and 2.11 respectively. And all of the three has have different mass fractal dimension Dm at low scattering intensity region, the mass fractal dimension Dm were 2.23,2.36 and 2.43 respectively. The results of N2 adsorption and desorption shows that the surface fractal dimension Ds of pure silica aerogels and composites were between 2.4-2.9. The Ds of fiber/SiO2 and pure silica aerogels were uniform, so their structures were very similar. The silica aerogels by adding tetrabutyl titanate have complicated surface, whose fractal dimension were between 2.6 and 2.9. The results from SAXS and N2 adsorption and desorption were so uniform, it indicated that the results were believable.更多还原