气凝胶（Aerogel）是一种内含大量空气、低密度、高孔隙率的轻质纳米孔结构的非晶固态材料。由于它在力学、声学、热学、光学等诸方面均显示独特的性质，因此在许多领域蕴藏着广泛的应用前景。本文以含硅量很高且价格便宜的稻壳灰作为硅源，替代传统生产方法中昂贵的有机硅化合物，制备二氧化硅气凝胶。提出了以稻壳灰为硅源，超临界干燥制备的硅气凝胶的疏水化改性方法和常压干燥制备硅气凝胶的方法，着重对其中的关键工艺步骤进行了研究。以稻壳灰为原料通过溶胶-凝胶法结合超临界干燥制备出的硅气凝胶是亲水性的。本文采用表面修饰的方法对亲水性硅气凝胶进行处理，成功地制备出了疏水性的硅气凝胶，其结构参数可以达到：表观密度为0.11 g/cm3，孔隙率为95.7%，BET比表面积为360 m2/g，孔容为2.9 cm3/g，孔基本分布在2~60 nm之间。采用工业上测量气凝胶疏水性的方法，即气凝胶在甲醇水溶液中开始下沉时的甲醇溶液浓度来衡量疏水性，这一浓度越高，表明疏水性越好。本论文方法制备出的气凝胶这一疏水性指标可以达到43%。本文研究了几种不同的有机修饰试剂及其他因素对形成疏水性的影响，获得了较好的制备疏水性二氧化硅气凝胶的工艺条件。超临界干燥制备出的硅气凝胶虽然结构参数很好，但是由于超临界干燥时压力很高，对设备的要求较高，实现以稻壳灰为原料常压干燥来制备硅气凝胶的方法可以很大程度的简化生产工艺路线和降低生产成本。通过溶胶-凝胶法，在溶胶中加入有机硅溶剂，可以在常压下干燥来制备硅气凝胶，其结构参数可以达到：表观密度为0.33 g/cm3，孔隙率为87.6%，BET比表面积为499.2 m2/g，孔容为3.3 cm3/g，孔基本分布在10~60 nm之间。本文对干燥前对溶胶－凝胶的处理方法进行了研究，考察多个因素的影响，得出了常压干燥制备硅气凝胶的工艺参数。

Silica aerogel is a kind of highly porous, low-density solid nano-material, which has potential applications in many fields such as thermal insulation, catalysis, acoustic delay liners and so on. Since its microstructure consists of nano-sized pores and linked primary particles, it has unique properties, e.g. very low thermal conductivity, low refractive index, and low sound velocity, and ultra-low dielectric constant.In this work, the hydrophobic silica aerogel was prepared by modifying the surface of the hydrophilic silica aerogel, which was prepared using sol-gel followed supercritical drying method with the rice hull ash as the silicon source. The process with drying gel at the ambient pressure for preparing silica aerogel from rice hull ash was also developed. The prepared hydrophobic silica aerogel is a lightweight mesoporous solid material with a bulk density as low as 0.11 g/cm3, porosity as high as 95.7%, the distributed pores from 2 to 60 nm, a BET surface area up to 360 m2/g and a pore volume up to 2.9 cm3/g. Standard industry practice to measure the hydrophobicity as a function of methanol concentration in water, above which the aerogel starts to settle. By this method, the hydrophobic degree of aerogel was up to 43%. The key step of preparing hydrophobic aerogel was to modify the pores’ surface. The effects of different agents for the modification were investigated. The effects of process parameters on the hydrophobic feature of the aerogel were determined. The silica aerogel obtained by ambient drying of a gel from rice hull ash is a mesoporous solid material with a bulk density as low as 0.33 g/cm3, porosity as high as 87.6%, the distributed pores from 10 to 60 nm, a BET surface area up to 499.2 m2/g and a pore volume up to 3.3 cm3/g. The key step of ambient drying method was decreasing the surface tension in the gel pores by dipping organic silicon monomer, tetramethylorthosilicate, into the sol. The optimal conditions for the aerogel preparation by atmospheric drying were proposed.