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

• Applicants: Professor Dr. Peter Fratzl, since 1/2024; Professor Dr. Aleksander Gurlo
• Subject Area: Synthesis and Properties of Functional Materials
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 454019637

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.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Wolfgang Wagermaier, Ph.D., until 1/2024