Hierarchically structured porous silica xerogels: influence of mesoporosity on thermal conductivity and mechanical strength

Abstract The thermal conductivity and mechanical strength of porous materials are critically influenced by their internal architecture. Beyond the total porosity, the hierarchical arrangement of pore sizes governs whether heat transfer is dominated by continuum conduction or Knudsen effects, and whether the solid network can efficiently carry mechanical loads. Despite this recognized importance, the specific contribution of mesopores within a macroporous scaffold has remained unclear. In this study, we systematically investigate the role of a secondary mesopore system in hierarchically structured silica xerogels. By combining controlled sol-gel synthesis with polymer-induced phase separation and subsequent calcination, we generate bimodal monoliths with tailored mesopore volumes and structurally comparable macroporous frameworks. Selective reduction of mesopores at 950 °C yields macropore-dominated reference samples, enabling direct assessment of mesoporosity effects. Morphological analysis confirms that mesopore reduction leads to densification within the silica struts, accompanied by moderate macropore shrinkage but preservation of the overall network geometry. Thermal conductivity measurements reveal a pronounced decrease of up to 53% with the addition of mesopore volume, attributed to enhanced Knudsen scattering. In contrast, compressive strength correlates positively with bulk density and inversely with mesoporosity, indicating a trade-off between insulation performance and mechanical stability. A power-law relationship between strength and density is observed, modulated by pore hierarchy. These findings demonstrate that mesoporosity is a powerful yet nuanced design parameter in silica-based monoliths, enabling functional tuning of heat transport and structural integrity. The insights presented here provide a quantitative foundation for the targeted design of advanced thermal insulation materials with application-specific property profiles.