Development of a selective absorbent SiAlON/Si-Al thin film system for radiative cooling

Abstract

Radiative Cooling (RC), which can reduce the surrounding temperature with no extra energy input, has attracted much attention in recent decades. The key principle of this approach involves the development of a system that selectively emits the light within the Atmosphere Window (AW) in a wavelength of 3~5 μm and 8~13 μm, while minimizing the absorption in the region from 400 nm to 2.5 μm. Traditionally, RC films have employed organic materials as the absorbent layer, characterized by intrinsic transparency in the short wavelengths and absorption in the InfraRed (IR) range due to the vibrational and rotational modes of abundant chemical groups presented within their molecular structures. However, they have inherent limitations, including large film thickness, unavailable spectrum selectivity and limited lifetime. Conversely, contemporary system design based on inorganic materials also faces challenges related to the complex film structures. These complexities mainly stem from either the fixed, narrow absorption band, or the limited Refractive Index (RI) of the materials. Presently, the foremost challenge in the development of inorganic film systems lies in simplifying their structural composition while preserving the cooling efficiency as much as possible. Hence, the primary objective of this research project is to demonstrate an ultrathin coating for RC by designing and preparing novel materials used in the absorbent and light confinement layers.
Thus, this research is separated into three parts. Firstly, SiAlON composite is fabricated through magnetron co-sputtering method (Si3N4 and AlN targets are used), enabling the formation of multiple absorption bonds through chemical intermixing. This composite exhibits broadband absorption characteristics ranging from 8 to 16 μm, which can be readily manipulated by tuning the film composition in response to the deposition parameters. Extensive analyses suggest that regulation upon the absorption feature is attributed to the precise control over chemical composition as well as the chemical bonding states. Secondly, the spacer is based on Al NanoWire (NW) arrays-embedded Si metamaterial film, serving to confine incident light and establish Fabry-Pérot (F-P) resonance. Molecular Dynamics (MD) simulations are conducted to reveal the forming mechanism of vertical aluminum nanowire arrays, which is ascribed to the mobility variance between the two phases (Si and Al). Additionally, ion bombardment due to the substrate bias supplies external energy, facilitating adatom diffusion and reevaporation during the deposition process, thereby increasing the upper limit of aluminum filling fraction required for the formation of the metamaterial structure. The Si-Al NWs system exhibits metallic behavior within the Visible (Vis) to near-IR region, characterized by high reflectivity (up to 70%). Besides, the composite demonstrates dielectric properties within the mid-IR range because of its deep-subwavelength NW structure, marked by a tunable high RI ranging from 6 to 8. Finally, a double-layer RC coating is developed by combining the two materials mentioned above onto a 120 nm thick aluminum mirror. The thickness of each layer is optimized through the application of a Particle Swarm Optimization (PSO) algorithm, taking the theoretical performance in 8~13 μm computed by the Transfer-Matrix Method (TMM) program executed by MATLAB as the criterion. The film system shows an average emissivity of 0.806 in the AW and achieves a maximum temperature reduction of approximately 20°C compared to the bare Cu plate under low-humidity conditions.

Date of Award	17 Mar 2025
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Hao Chen (Supervisor), Hongtao Cao (Supervisor) & Ming Li (Supervisor)