Some comments on the effect of skeletal features on the properties of aerogels

Abstract

Generally, for porous materials like aerogels, the density is considered to be the responsible factor dictating their mechanical properties. In this contribution, we will systematically analyse three parameters, viz., the density, the pore-size distribution, and the pore-wall thickness, for their effect on the bulk mechanical properties. It will be demonstrated that although density remains an important parameter governing the mechanical stiffness, the other two parameters do not have a negligible effect. Furthermore, most aerogels do not show a constant cross-section for their pore walls due to the pearl-necklace-like morphology. In the second part, the effect of this pearl-necklace-like morphology on the pore sizes and the mechanical properties will be elucidated. First, mathematical models for their pore-sizes will be described. Three scenarios will be discussed, where the pore sizes are calculated for cells in two dimensions, three dimensions (3-d) and in 3-d with overlapping particles representing necks. The dependency of the skeletal features on the resulting pore size will be shown. The model for pore walls with 3-d overlapping spheres will be subjected to axial stretching, bending and buckling. The effect of the particle sizes and the amount of overlap between the particles on the corresponding mechanical features will be illustrated. The results will also be compared with models that assume constant cross-section of pore-walls. It will be demonstrated that neglecting the corrugations arising from the pearl-necklace-like morphology in aerogel-like materials can result in serious misinterpretations of their mechanical properties.