Understanding Thermal Insulation in Porous, Particulate Materials

Silica hollow nanosphere colloidal crystals feature a uniquely well‐defined structure across multiple length scales. This contribution elucidates the intricate interplay between structure and atmosphere on the effective thermal diffusivity as well as the effective thermal conductivity. Using silica hollow sphere assemblies, one can independently alter the particle geometry, the density, the packing symmetry, and the interparticle bonding strength to fabricate materials with an ultralow thermal conductivity. Whereas the thermal diffusivity decreases with increasing shell thickness, the thermal conductivity behaves inversely. However, the geometry of the colloidal particles is not the only decisive parameter for thermal insulation. By a combination of reduced packing symmetry and interparticle bonding strength, the thermal conductivity is lowered by additionally 70% down to only 8 mW m −1 K −1 in vacuum. The contribution of gaseous transport, even in these tiny pores (<200 nm), leads to minimum thermal conductivities of ≈35 and ≈45 mW m −1 K −1 for air and helium atmosphere, respectively. The influence of the individual contributions of the solid and (open‐ and closed‐pore) gaseous conductions is further clarified by using finite element modeling. Consequently, these particulate materials can be considered as a non‐flammable and dispersion‐processable alternative to commercial polymer foams.