Correlation of microstructure and thermal conductivity in nanoporous solids: The case of polyurea aerogels synthesized from an aliphatic tri-isocyanate and water

• Correlation of structure and thermal transport properties in a micro-porous solid.
• Model system is based on polyurea aerogels.
• Microstructure changes from a network of fibers to a string-of-pearls-like structure.
• An exceptionally low exponent α = 1 was found for λsolid∝ρα.
• Particle connectivity has a significant impact on λsolid.

This study correlates microstructure with thermal transport properties in nanoporous solids. The model system is based on polyurea (PUA) aerogels. Those aerogels demonstrate a dramatic change in microstructure with density. Low density aerogels consist of entangled nano-fibers changing into interconnected nanoparticles as the density increases. The nanostructure was probed in terms of both particle size and network interconnectivity with scanning electron microscopy and small angle X-ray scattering. Thermal conductivity values between 0.027 and 0.066 W/mK were obtained with the hot-wire method for PUA samples with densities between 0.04 and 0.53 g/cm3. Both, pressure and temperature dependent experiments were performed for the deconvolution of total thermal conductivity into gaseous, radiative, and transport-through-the-solid-framework contributions. Subsequently, thermal conductivity along the solid framework was considered as a function of microstructure. That leads to a quantitative evaluation of the impact of primary particle characteristics and network interconnectivity on the solid thermal conductivity.