Understanding Thermal Insulation in Porous, Particulate Materials

Abstract

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 dispersionprocessable alternative to commercial polymer foams.

Investigation of the Thermal Conductivity of Resin-Based Lightweight Composites Filled with Hollow Glass Microspheres

“…According to the second law of thermodynamics, heat transfer will occur as long as there is a temperature difference in the system [31]. The mechanism of heat insulation materials is blocking the heat transfer and slowing the heat exchange [32]. In this paper, HGM/EP LWTI composites are a three-phase composite material, including the resin matrix phase, the spherical shell of HGMs, and the gas phase inside the Figure 4 also illustrates that the thermal conductivity of HGM/EP LWTI composites decreases with the increasing HGM content of the composites, when the same HGMs are used.…”

A 0D Lead‐Free Hybrid Crystal with Ultralow Thermal Conductivity

“…The lower limit of thermal conductivity of amorphous solids is well explained by the Einstein limit where the mean free path is assumed to be the same as half of the wavelength of phonons . However, the major bottleneck of employing amorphous low thermal conductivity materials such as silica aerogels is their brittleness, which leads to short lifetimes and higher maintenance costs . As a result, there have been efforts to search for low thermal conductivity materials through nanostructure engineering, i.e., using thin films, superlattices, nanostructured samples, and disordered crystals .…”

Thermal transport in binary colloidal glasses: Composition dependence and percolation assessment

“…For a colloidal glass consisting of hollow silica spheres (0P), the thermal conductivity is extremely low with a value of only 13 mW m −1 K −1 . The heat transport is limited in these colloidal glasses due to several structural properties on the 022612-3 nano-and mesoscale [18,19]. These are (i) the hollow structure, which reduces the density to only 0.30 g cm −3 , (ii) the large amount of interfaces due to the particulate structure, which increases the thermal resistance by means of interparticle constriction, and (iii) the low packing parameter of the particles to reduce the density and interparticle contact points even further.…”