SiO2气凝胶的非超临界干燥法制备及其形成过程

通过对正硅酸乙酯的两步水解-缩聚反应速率的调控，使生成的醇凝胶具有比较完整的网络结构，配合乙醇溶剂替换和正硅酸乙酯乙醇溶液浸泡和陈化，改善和增强凝胶的结构和强度，在分级干燥下实现了SiO2气凝胶的非超临界干燥制备，并采用SEM、TEM、TG-DTA、XRD和吸附-脱附技术等手段对所得气凝胶样品进行表征.结果表明, 该气凝胶是由粒径约10 nm均匀球状纳米粒子构成的具有连续网络结构的低密度多孔材料，密度为200～400 kg•m－3，孔径分布在10～30 nm范围内，孔隙率约为91％，比表面高达625.65 m2•g－1.外观及微观构造与应用超临界干燥制得的气凝胶完全一致.调节反应体系中各组分的配比以及控制两步水解-缩聚过程中酸与碱的加入量可以获得不同密度的块状SiO2气凝胶.

Preparation of Silica Aerogels via Non-supercritical Drying and Their Formation Process

The preparation of monolithic silica aerogels via non-supercritical drying is realized by the following procedures:(1) formation of silica alcogels with perfect network structure by controlling two-step hydrolysis and condensation reaction rates of tetraethoxysilane； (2) improvement and reinforcement of network structure and strength of silica gels through exchanging the pore fluid with ethanol solvent thoroughly, immersing of gels in the ethanol solutions of tetraethyoxysilane and aging; (3) gradual drying under ethanol fluid whose surface tension is much lower than water under ambient pressure. The resulting aerogels were characterized by means of SEM， TEM, TG-DTA, XRD, and adsorption-desorption techniques. It is found that the aerogels, which are made up of uniform spherical nanoparticles with diameters of about 10 nm, are of coherent network structures. Furthermore, the aerogels are porous, and the porosity is as high as 91％. The diameter of the pore sizes is in the range of 10～30 nm, and the density of the aerogels is low（200～400 kg•m－3）. The specific surface area of the aerogels can be as high as 625.65 m2•g－1. The appearance， microstructure，diameters of primary particles and pore size distributions of the aerogels are in concordance with those prepared by conventional supercritical drying. It is demonstrated that the quality of the resulting aerogels is affected by the preparation conditions adopted in the preparation process. In fact, by adjusting the proportions of the reaction systems， and controlling the adding amounts of acid and base during the two-step hydrolysis and condensation process，silica aerogels with required density， definite strength and ideal morphology can be produced.