Abstract
Silica aerogels are known to be materials with exceptional characteristics, such as ultra-low density, high surface area, high porosity, high adsorption, and low-thermal conductivity. In addition, these unique properties are mainly related to their specific processing. Depending on the aerogel synthesis procedure, the aerogels texture can be tailored with meso and/or macroporosity. Fractal geometry has been observed and used to describe silica aerogels at nanoscales in certain conditions. In this review paper, we describe the fractal structure of silica aerogels that can develop depending on the synthesis conditions. X-ray and neutron scattering measurements allow to show that silica aerogels can exhibit a fractal structure over one or even more than two orders of magnitude in length. The fractal dimension does not depend directly on the material density but can vary with the synthesis conditions. It ranges typically between 1.6 and 2.4. The effect of the introduction of silica particles or of further thermal treatment or compression of the silica aerogels on their microstructure and their fractal characteristics is also resumed.
Highlights
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.license.Aerogels have drawn increasing interest in different fields ranging from fundamental physics research to applications as specific materials
A shown in the literature [32,33,43,44], small angle X-ray or neutron scattering experiments can provide information on three different aspects of aerogels fractal geometry: the mean size of the fractal clusters (ξ), the mean size of the primary particles (a) which stick together to build the cluster, and the fractal dimension Df which expresses the clusters compactness
The fractal dimensions deduced from the results presented above are different for
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. It has been proposed that they can develop a fractal structure [4,5] These properties and features are essentially due to the very large pore volume of these materials, which can be tailored up to 99% by the sol-gel synthesis conditions [6] (i.e., the siloxane monomer content, pH), but which can be modified by sintering [7] or compression processing [8]. They are ideal materials to analyze the change in their physical properties as a function of their structure which can be studied experimentally over an exceptionally large range of porosity from 0% to 99%. We will show the effect of synthesis parameters, such as the organosilane concentration, pH of the solution, and the effect of silica particles addition, and the effect of porous collapse induced either by aerogels sintering or by aerogels compression
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
