The famous "10 ℃ law" indicates that the reliability of electronic devices is closely related to the temperature, i.e., the reliability can be reduced by 50% for every 10 ℃ increase in temperature when the temperature of electronic devices exceeds 80 ℃. It is thus extremely important to discharge the waste heat of electronic devices in time to achieve an efficient device cooling. Heat transfer methods include heat conduction, heat convection and heat radiation. Conventional air-cooled and liquid-cooled achieve cooling via heat convection and heat conduction. In addition, the cooling technologies also include magnetic cooling (i.e., magneto-thermal effect), phase change cooling (i.e., phase change heat storage) and thermoelectric cooling (i.e., the Peltier effect). With the high integration and miniaturization of electronic devices, especially flexible electronic devices limited by size and deformation, conventional cooling technologies are no longer applicable. The newly flexible radiative cooling films with zero power consumption become potential candidates for cooling electronic devices and are expected to promote the development of the national dual-carbon cause, which can emit heat to outer space through thermal radiation and reject solar irradiance. However, it is extremely difficult to combine outstanding radiative cooling performance with excellent thermostability limited by materials and preparation process. In this paper, Al₂O₃-doped colorless polyimide hybrid films were prepared by a sol-gel method. The infrared radiation enhanced by Al₂O₃ phonon-enhanced resonance and the higher thermostability because of the interaction of Al₂O₃ nanoparticles with the molecular chains were analyzed.

TFMB (0.365 7 g) was mixed with DMAc (6 mL) under electromagnetic stirring at 0 ℃. Al(OH)₃ sol at different contents (i.e., 0%, 5%, 10%, 15%, and 20%, in mass fraction) was added to the solution above. An accurately weighed 6FDA (0.503 6 g) was sequentially added to a trident flask in batches for 1 h, and then stirred continuously for 30 min. Subsequently, the mixture was stirred at room temperature for 4 h. The obtained precursor poly (amido acid) (PAA) solution was rested for 12 h.

The PAA solution was coated onto a dry clean glass substrate by a spin-coating method and dried in a vacuum tube furnace at 80 ℃ for 1 h and then at 50 ℃ for 12 h to ensure that the solvent was removed completely. The sample was heated in a vacuum tube furnace in respective heating steps (i.e., at 110 ℃ for 30 min; at 140 ℃ for 30 min; at 170 ℃ for 30 min; at 200 ℃ for 30 min; at 220 ℃ for 30 min; and at 280 ℃ for 60 min).

The obtained Al₂O₃-doped colorless polyimide (CPI) films were ultrasonically cleaned with absolute alcohol and deionized water for 30 min. The films were blown dry and put into a vacuum chamber. A 300 nm thick silver (Ag) film was deposited on the backside of the Al₂O₃-doped CPI film via direct current sputtering at 6×10^-4 Pa. The argon flow rate, sputtering pressure and Ag target power were 50 cm³/min, 0.5 Pa and 100 W, respectively.

The mid-infrared emissivities (5-20 µm) of Al₂O₃-doped CPI/Ag hybrid films increase with increasing Al₂O₃ content. The mid-infrared emissivities are measured as ε(0%)=87.54%, ε(5%)=92.15%, ε(10%)=92.23%, ε(15%)=93.25%, and ε(20%)=94.51%, respectively. The solar reflectivities (0.4-2.5 µm) decrease with increasing Al₂O₃ content. The solar reflectivities (R) are measured as R(0%)=94.81%, R(5%)=91.12%, R(10%)=89.31%, R(15%)=88.36%, and R(20%)=83.87%, respectively. This is because the scattering effect of the hybrid films increases with increasing Al₂O₃ content. The visible reflectivities (0.40-0.75 µm) firstly increase and then decrease with increasing Al₂O₃ content. The 5% Al₂O₃-doped CPI/Ag hybrid film has the maximum reflectance in the visible band, which can effectively improve the net cooling power of the hybrid film. Meanwhile, the intense interaction of Al₂O₃ nanoparticles with the molecular chains can effectively enhance the thermostability of the hybrid films. Compared with 0%Al₂O₃-doped CPI film, the glass-transition temperature (T_g) of 5% Al₂O₃-doped CPI film is 328 ℃, which is increased by 22 ℃.

The Al₂O₃-doped CPI hybrid film was prepared by a sol-gel method. The infrared radiation increased due to the phonon-enhanced resonance, and the thermostability improved due to the interaction between Al₂O₃ nanoparticles and molecular chains. The 5% Al₂O₃-doped CPI/Ag hybrid film had a high solar (0.4-2.5 µm) reflectivity (i.e., 91.12%) and a mid-infrared (5-20 µm) emissivity (i.e., 92.15%). The maximum cooling capability of 7.7 ℃ and 11.1 ℃ that were lower than that of bare aluminum device was achieved under sunlight. The Al₂O₃-doped CPI hybrid film exhibited superior thermostability (i.e., T_g~328 ℃), and thermal decomposition temperature (i.e., T_5%~554 ℃). The results indicated an effective strategy to realize efficient radiative cooling for electronic devices and appendages in spacecraft.