Origin of the springback effect in ambient pressure dried silica and other metal-oxide aerogels

回弹效应是一个有趣的现象，因为它代表了陶瓷材料惊人的巨大体积变化。这一现象使孔隙率类似于在常压下实现的超临界干燥(SCD)。在常压下干燥时，材料可能会收缩到其大小的一半。热处理和表面改性可以使干凝胶膨胀到几乎原始尺寸。凝胶表面的硅烷化是通过三甲基氯硅烷(TMCs)的表面修饰完成的，通过诱导回弹效应实现了这种收缩的可逆性，从而获得了与APD和SCD衍生材料类似的结果。必须彻底研究这一现象，才能预测众所周知的二氧化硅系统的行为，并将这一知识转移到其他系统。本提案的主要目标是(1)研究气凝胶回弹效应的发生和来源，(2)了解从纳米到微米尺度的回弹效应，(3)在常温下气凝胶的加工过程中控制回弹效应。我们假设气凝胶的收缩和回弹效应的发生与气凝胶骨架的强度有关，这可以与分维相关。后者可以通过气凝胶合成过程中的原位和非原位SAXS实验来确定。在前期工作中，我们证明了合成所提出的二氧化硅系统是可能的，在定制的测量单元中可视化了回弹效应，并成功地测试了我们的方法能够在气凝胶加工的几个步骤中检测到纳米结构特征。在该项目中，将进一步加强这一点，以便在加工路线上应用现场方法。我们将应用一种以X射线散射为重点的原位多方法方法来研究纳米结构特征，特别是在气凝胶干燥过程中，特别是在回弹效应期间结构层次的所有级别的变化。在此基础上，我们阐明了在环境条件下气凝胶整个形成过程中的纳米尺度结构和功能关系。该模型将建立二氧化硅、二氧化钛和氧化锆气凝胶的模型，这些气凝胶是通过相应的烷氧基化合物的水解而合成的。之后，该模型被验证为二氧化硅-二氧化钛和二氧化硅-氧化锆气凝胶。

The springback effect is an interesting phenomenon, because it represents a surprisingly large volume change for a ceramic material. This phenomenon enables porosities similar to those of supercritical drying (SCD), achieved at ambient pressure. While drying at ambient pressure (APD), the material might shrink up to half its size. Heat treatment and surface modification may expand the dried gel to almost the original size. Silylation of the surface of the gel, which is done by surface modification with i.e. trimethylchlorosilane (TMCS), grants reversibility of this shrinkage by inducing the springback effect, leading to comparable results of APD and SCD derived materials. The phenomenon has to be studied thoroughly for the well-known silica system to predict the behaviour and transfer this knowledge to other systems. The main goals of the present proposal are (i) to research the occurrence and origin of the springback effect in aerogels, (ii) to understand the springback effect from nano- to micro-scale, (iii) to control it also during processing of aerogels at ambient conditions. We hypothesize that the shrinkage of aerogels and the occurrence of the springback effect are linked to the strength of the aerogel skeleton which can be associated to fractal dimension. The latter can be determined by SAXS experiments, both ex-situ and in-situ during the aerogel synthesis. In preliminary work we showed that it is possible to synthesize the proposed silica system, visualized the springback effect in a custom-made measurement cell and tested successfully that our methods are able to detect nanostructural characteristics during several steps of processing of aerogels. In this project, this will be further enhanced to apply in-situ methodology along the processing route. We will apply an in-situ multi-method approach with a strong focus on X-ray scattering to investigate nanostructural features especially during the drying of aerogels and in particular alterations at all levels of structural hierarchy during the spring back effect. With this we elucidate nanoscale structure function relations during the entire formation route of aerogels at ambient conditions. The model will be established for silica, titania and zirconia aerogels, synthesized by hydrolysis of the corresponding alkoxydes. Afterwards the model be verified for silica-titania and silica-zirconia aerogels.