Multiscale structure of super insulation nano-fumed silicas studied by SAXS, tomography and porosimetry

We focus on describing the multi-scale structure of a fumed silica characterized by remarkably low thermal conductivity (ca. 2–5 mW m −1 K −1 ) when used as a core material in vacuum insulating panels. While such powders are known to be highly polydisperse at different lengthscales (hardly quantifiable), we propose to adapt a recent methodology based on small-angle X-ray scattering experiments with the aim of providing simple criteria for characterizing the morphology of these nanostructured silicas. Combining this technique with transmission electron microscopy, electron-tomography and mercury intrusion porosimetry then allows assigning the origin of the super-insulation to the low dimensionality of the silica aggregates at lengthscales smaller than 500 nm. Remarkably, by using independently these three techniques, we always find the compacity of the aggregates (radius of ca. 40 nm) to be equal to 0.29 ± 0.01. This study proposes therefore a robust methodology, potentially of a great interest for industrial applications. © 2019 Acta Materialia Inc.