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令人惊讶的大体积变化。这种现象使孔隙率类似于超临界干燥（SCD），在环境压力下实现。当在环境压力（APD）下干燥时，材料可能会收缩到其尺寸的一半。热处理和表面改性可以使干凝胶膨胀到几乎原始尺寸。通过用三甲基氯硅烷（TMCS）进行表面改性来进行凝胶表面的硅烷化，通过诱导回弹效应来赠款这种收缩的可逆性，从而导致APD和SCD衍生材料的可比结果。对于众所周知的二氧化硅系统，必须彻底研究这种现象，以预测其行为并将这些知识转移到其他系统。本提案的主要目标是（i）研究气凝胶中回弹效应的发生和起源，（ii）了解从纳米到微米尺度的回弹效应，（iii）在环境条件下气凝胶加工过程中也对其进行控制。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，而骨架的强度与分形维数有关。后者可以通过SAXS实验来确定，在气凝胶合成过程中，既可以是非原位的也可以是原位的。在初步工作中，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纳米结构特征。在本项目中，将进一步加强这一点，以便沿着加工路线采用现场方法。我们将采用原位多方法方法，重点关注X射线散射，以研究纳米结构特征，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各个层次的变化。有了这一点，我们阐明了在环境条件下气凝胶的整个形成路线的纳米结构功能关系。该模型将建立二氧化硅，二氧化钛和氧化锆气凝胶，通过水解相应的烷氧基化合物合成。并以硅钛和硅锆气凝胶为例对模型进行了验证。

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.