Crystal Hydrogel-Based Switchable Radiative Cooling Materials for Smart Windows

Abstract

Smart windows can effectively balance the space temperature of buildings without compromising the essential functions of windows. However, conventional thermochromic windows have limited sunlight regulation capabilities and face challenges with switching as desired. Herein, A class of novel smart windows based on crystal hydrogels is introduced that achieve free switching between transparent (for heating) and opaque (for radiative cooling) states through thermal and mechanical stimuli. The crystal hydrogels are made from cross-linked polyacrylamide (PAM) and sodium acetate (NaAc). By optimizing the sodium acetate concentration and sample thickness, The combination of excellent cooling ability is achieved at the opaque state and good low-temperature stability at the transparent state in the hydrogels. Using the optimized hydrogel to prepare a smart window equipped with a heater and a mechanical trigger tip, the rapid on-demand transition between transparent and opaque states is demonstrated. The results indicate that the smart window lowers temperatures by up to 9.4 °C compared to ordinary windows and maintains stable emissivity and reflectivity even after 100 cycles due to its robust solar modulation capabilities. This technology provides new energy-saving solutions for smart buildings but also explores future applications of smart materials, showcasing innovative advantages and technical strengths in smart windows.