Thermal transport in solar-reflecting and infrared-transparent polyethylene aerogels

Polyethylene aerogels (PEAs) present a unique combination of optical (solar-reflecting but infrared-transparent) and thermal properties that are ideally suited for applications requiring infrared-transparent thermal insulation such as atmospheric radiative cooling and condensation-free radiant cooling. As the material's thermal resistance plays a large role in these systems’ performance, better understanding of thermal transport within the material is crucial to guide future material development. In this work, we characterize thermal transport in PEAs, elucidating contributions of different heat transfer mechanisms and identifying pathways to further improve their thermal insulation performance. We first present a theoretical model that separates thermal transport via solid conduction through the polyethylene backbone, gaseous conduction within the porous structure and radiative transfer through the semi-transparent material. We then fabricated PEA samples of densities ranging from 12.0 kg/m³ to 82.2 kg/m³ and experimentally characterized their thermal conductivity using a custom-built guarded-hot-plate thermal conductivity setup, with pressures ranging from vacuum to 1 atm, in three gas environments (nitrogen, argon and carbon dioxide), and with low and high emissivity boundaries. The experimental and modeling results indicate that conduction in the gas phase and radiative transfer (for the high emissivity boundary case) dominate heat transfer through the material. We find that reducing the pore size and gas pressure, using lower thermal conductivity gases, adding infrared opacifiers and maintaining low density could all provide significant reduction in thermal conductivity.