Switchable Smart Window Passive Radiative Cooling via Mechanical Stress

Recently, passive radiative cooling (PRC) technology has emerged as a promising energy-saving technology capable of regulating room temperature without external energy input. It is based on the principle of radiative cooling, where a hotter body (~300 K) dissipates heat by emitting thermal radiation into the cold outer space (~3 K) through atmospheric window. PRC has been developed in various ways, improving cooling performance or altering optical properties. It has also expanded its application to optically switchable smart windows. However, previous research has shown slow optical switching times or has been unsuitable for flexible or stretchable applications. Therefore, there is a need to develop optically switchable PRCs that can easily change their optical properties in response to slight external stimuli and adapt to deformable devices. Here, we report a mechanically switchable smart window passive radiative cooler (SW-PRC) of which transparency is reversibly converted between opaque and transparent states in response to external mechanical stress, while maintaining efficient heat dissipation in both optical states. To achieve these properties, we first treated the surface of silicon dioxide nanoparticles (SiO₂ NPs) with trimethylchlorosilane (TMSCl). As a result, the hydrophobicity of SiO₂ increased and the agglomeration of SiO₂ NPs was reduced in the polydimethylsiloxane (PDMS) matrix. The SW-PRC film fabricated from a SiO₂ NPs/PDMS solution shows high visible light transmission (≥ 90%) and a low haze factor (20-30%). It demonstrates high radiative cooling performance, and when stretched by more than 5%, the cooling power increases while it becomes opaque. Finally, by demonstrating the reversible stretching and restoration of switchable SW-PRC over 20 cycles, we propose a mechano-responsive switchable smart window that represents a new direction in energy-efficient exterior window design.


Acknowledgement

This research was supported by the Digital Research Innovation Institution Program Through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (RS-2023-00283597)