Nanoporous silica microspheres–ploymethylpentene (TPX) hybrid films toward effective daytime radiative cooling

Abstract The metamaterial radiative cooling films based on nanoporous SiO2 microsphere-Poly-4-methyl -1-pentene (TPX) hybrid system were deposited on fluorine doped tin oxide (FTO) substrates via tape casting. Effects of the fabrication parameters, film thickness, SiO2/TPX volumetric ratio and microsphere size on the crystallinity, microstructure, optical transmittance and infrared emittance of the hybrid films were examined. Results show that the crystallinity of the TPX matrix is influenced by the deposition temperature, and the microstructure of the hybrid film is influenced by the film thickness, with the film of 50 μm thickness exhibiting porous microstructure on the surface. The optical transmittance in the visible-near infrared band decreases with the increase of the film thickness and the SiO2/TPX volumetric ratio, and with the decrease of the microsphere size. The absolute quantity of SiO2 microspheres in hybrid film determines the transmittance and the scattering effect. The emittance at 8–13 μm increases with the increase of the film thickness and the SiO2/TPX ratio and with the decrease of microsphere size. The film with 50 μm thickness, 5% SiO2/TPX ratio and 8 μm microspheres exhibits both high emittance (up to 0.91) and high selectivity in the atmospheric window. The cooling device integrating the optimal hybrid film and silver reflective layer demonstrates the lowest temperature in the diurnal test, about 20 °C lower than the black surface, 12 °C lower than the Ag-coated glass and 8 °C lower than the FTO sample. In the nocturnal test, the maximum 4.5 °C reduction relative to the ambient temperature is observed.