Low-Density and Flexible Silicone with Tunable Transparency for Thermal Insulation and Passive Cooling

Optimizing the performance of windows is central to reducing energy consumption. However, existing passive radiative cooling technologies primarily focus on reflecting solar radiation, often at the expense of visible light transmittance, which hinders their practical application. Herein, we report a low-density, flexible silicone with tunable transparency fabricated through a facile sol-gel transition and atmospheric pressure drying process. Utilizing polyethylene glycol-isocyanatopropyltriethoxysilane as a functional crosslinker, methyltrimethoxysilane and dimethyldimethoxysilane as copolymerized monomers, we achieved precise control over the material's microstructure and optical properties by tuning its content. The resulting transparent silicone samples exhibit outstanding overall performance: an average visible light transmittance as high as 92.97%, surpassing that of commercial ultra-clear glass, and an infrared emissivity of 94.33% within the atmospheric window. Additionally, the silicone demonstrates a low density (0.343-0.857 g/cm³), low thermal conductivity (0.073-0.214 W/(m·K)), and machinability with a bending toughness up to 4.89 × 10⁵ J/m³. Benefiting from the silicone's selective spectral transmittance and high infrared emissivity, a transparent silicone film-coated substrate achieved a temperature drop up to 11.6°C compared to an uncoated substrate. This work integrates high transparency and passive cooling into a single material, providing a strategy for the development of transparent energy-saving materials.