Integrated BN@ZnO/cellulose acetate composite films with enhanced thermal conductivity and daytime passive radiative cooling

Although the utilization of radiant cooling technology achieves a cooling effect in sunlight, it is hardly able to alleviate the heat inside electronic devices operating in outdoor environments, and even leads to an increase in temperature due to its inherent low thermal conductivity. Here, we developed a novel high thermal conductivity radiant cooling film BN@ZnO/cellulose acetate, which was achieved by employing a composite filler strategy to construct thermal conduction pathways on optical scatterers. The BN@ZnO fillers, synthesized by in situ deposition, construct a micron/nano hierarchical structure conducive to Mie scattering, resulting in a solar spectral reflectance of 93.3 %. Meanwhile, the hierarchical structure that ZnO nanorods bridging the BN micron sheet significantly enhanced the in-plane (3.5 W/(m·K)) and through-plane (1.2 W/(m·K)) thermal conductivity of the composite film. In the hot midday, the BN@ZnO/cellulose acetate composite film achieves a sub-ambient cooling performance of 4.1 °C. More importantly, the high thermal conductivity of the film significantly facilitates the efficient utilization of the cold source. The BN@ZnO/cellulose acetate film achieves a practical cooling of 6.3 °C for the outdoor operating equipment with internal heat sources, while the low thermal conductivity traditional porous cellulose acetate film with advanced radiative cooling performance shows an effect of only 0.4 °C. This work provides a novel approach to the design of highly thermally conductive radiant cooling materials and offers significant implications for reducing energy consumption, improving operational reliability and extending service life of outdoor equipment. The BN@ZnO/CA with high thermal conductivity could deliver exceptional cooling performance for outdoor equipment with internal heat sources during the daytime.