热负载条件对SiO2气凝胶组成及微观结构的影响

通过阶梯升温并结合傅里叶变换红外光谱(FT-IR)、X射线光电子能谱(XPS)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)等表征手段，研究了不同温度所得SiO2气凝胶的组成和微观结构，在程序升温条件下通过TG-DTG曲线和无模式函数法研究了SiO2气凝胶热重行为的变化.结果表明：SiO2气凝胶主要由7~9 nm球状颗粒构成类线形团簇，进而以团簇为骨架构成三维网络多孔结构；阶梯升温下，随着热处理温度的升高，气凝胶中Si—O—Si基团的摩尔分数逐渐增加，Si—OH和Si—OC2H5的摩尔分数逐渐降低，1 073 K时Si—OH基本消失，但存在6.59%的Si—OC2H5基团；气凝胶颗粒逐渐长大，部分骨架坍塌，温度达到1 273 K时，颗粒长大至约50 nm，团簇彻底消失，材料发生明显烧结.程序升温下，升温速率越高，气凝胶的热稳定性越好；材料的失重过程分为3个阶段：当反应转化率α < 30%时，主要发生硅羟基(Si—OH)间的缩合；当30% < α < 70%时，主要是硅羟基(Si—OH)与/或硅乙氧基(Si—OC2H5)之间的缩合；当α>70%时，主要是硅乙氧基(Si—OC2H5)之间的缩合.
Composition and microstructure of SiO2 aerogels treated at different temperatures were studied using the ladder-elevating temperature combined with FT-IR, XPS, SEM, and TEM. The change of thermogravimetric behavior of SiO2 aerogels under temperature-programmed conditions was investigated using TG-DTG curve and model-free kinetics. Results showed that SiO2 aerogel was mainly composed of spherical particles ranged between 7 and 9 nm. These particles formed the class alignment clusters, which continued to work as skeletons to from a three-dimensional network porous structure. Under ladder-elevating temperature condition, with the increase of treatment temperatures, content of Si—O—Si group increased while that of Si—OH and Si—OC2H5 groups both decreased. When the temperature reached 1 073 K, Si—OH group almost disappeared and content of Si—OC2H5 group decreased to 6.59%. Nanoparticles in SiO2 aerogel grew slowly, resulting in the collapse of skeletons. When the temperature rose to 1 273 K, the particle size grew about 50 nm, clusters disappeared completely, and SiO2 aerogels sintered obviously. Under temperature-programmed condition, the faster the heating rate, the better the thermal stability of SiO2 aerogel. The weight-loss process of SiO2 aerogel was mainly divided into three steps: when α < 30%, the weight-loss process was controlled via condensation reaction between Si—OH; when 30% < α < 70%, it was mainly controlled via the condensation reaction between Si—OH and/or Si—OC2H5; when α>70%, it was mainly controlled via condensation reaction of Si—OC2H5.