热负载条件对SiO_2气凝胶组成及微观结构的影响

Influence of heating loading conditions on the composition and microstructure of SiO_2 aerogel

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SiO_2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SiO_2气凝胶热重行为的变化.结果表明:SiO_2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—OC_2H_5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—OC_2H_5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—OC_2H_5)之间的缩合;当α> 70%时,主要是硅乙氧基(Si—OC_2H_5)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 theermogravimetric 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 strutting. 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 temperaturerose to 1 273 K, the particle size glow 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.