All-inorganic multilayer film with broadband radiation refrigeration performance

Abstract

The invention discloses a design and preparation method of an all-inorganic multilayer film for broadband radiative refrigeration, the multilayer film has a similar one-dimensional photonic crystal film layer structure, and belongs to the technical field of optical function materials and energy conservation. The thin film adopts a four-layer alternate SiO2 / Al2O3 film layer structure, the uppermost layer is a Si3N4 coupling layer, the bottom layer is an Ag reflecting layer protected by AlN, the emissivity of an atmospheric window is enhanced through impedance mismatch of a high-refractive-index layer and a low-refractive-index layer, and the mid-infrared radiance is improved through the Si3N4 phonon coupling effect and the refractive index gradient design. The low-refractive-index layer is thinned, and the mid-infrared emission spectrum is widened through coherent interference of the high-refractive-index layer. The solar spectral reflectivity of the film layer structure is 97.42%, the atmospheric window emissivity is 90.65%, the broadband infrared emissivity is 90.59%, and the thermal stability reaches up to 1450 DEG C. According to the multilayer film, cooling at the highest temperature of 10.9 DEG C in the horizontal direction and at the highest temperature of 3.2 DEG C in the vertical direction is achieved in the actual environment, and the net cooling power is 288W / m < 2 >. The material is suitable for building energy saving, electronic device heat dissipation and spacecraft heat management, overcomes the defect that a traditional inorganic material is poor in spectral selectivity, and provides an innovative radiation refrigeration cooling scheme.

Description

Technical Field

[0001] This invention relates to the field of sustainable energy, specifically to a high-performance inorganic multilayer film for broadband radiative cooling. In recent years, with the increasing demand for energy-saving and environmentally friendly technologies, radiative cooling technology has received widespread attention due to its passive heat dissipation and cooling characteristics. Background Technology

[0002] Climate change is causing extreme heat, glacial melting, and rising sea levels, leading to a significant increase in reliance on energy-intensive cooling systems. Traditional cooling systems not only consume approximately 10% of the world's electricity but also emit over 1 billion tons of carbon dioxide annually, creating a severe environmental burden. As a sustainable alternative, passive daytime radiative cooling (PDRC) technology reflects sunlight and radiates heat into space through atmospheric windows of 8–13 μm, achieving energy-free cooling. High-performance PDRC materials typically possess solar reflectivity exceeding 95% and thermal emissivity exceeding 90%, significantly reducing object temperature and demonstrating potential for energy conservation and emission reduction.

[0003] Early breakthroughs, such as the seven-layer HfO2 / SiO2 photonic crystal film design proposed by Raman et al., were landmark achievements, but their infrared emission bands were narrow and their cooling capabilities were limited.

[0004] While subsequent design improvements have enhanced performance, challenges remain, including high material costs, complex manufacturing processes, poor adaptability, and insufficient optical response. For example, the TiO2 / SiO2 design by Liu et al., with its high emissivity, still falls short in reflectivity and cooling capacity. All-inorganic radiation-cooled materials exhibit good temperature resistance but typically lack broadband response, limiting their application in space environments to achieve broad-spectrum radiation. Metamaterials, while possessing extremely high spectral selectivity, suffer from complex and expensive manufacturing processes (based on etching), making large-scale scaling and integration into micro-devices difficult. Organic materials (such as polymer μDE and nano-PE sawtooth gratings) can provide broad-spectrum radiation, but they are prone to rapid degradation under long-term photothermal exposure, severely impacting their durability. Therefore, research focus is gradually shifting towards low-cost and durable inorganic thin films. These materials combine high solar spectral reflectivity, high emissivity through atmospheric windows, broadband radiation performance, and high net cooling power, making them suitable for various applications, including space and vertical surfaces. Summary of the Invention

[0005] This invention aims to provide an inorganic multilayer thin film with a simple structure and excellent radiative cooling performance. This thin film possesses high solar reflectivity, high atmospheric emissivity, and broadband emissivity, exhibiting excellent cooling performance and high thermal stability, making it suitable for various applications, including vertical surfaces.

[0006] To achieve the above objectives, this invention deposits a designed four-layer inorganic thin film structure (Si3N4 / 1D / AlN / Ag) from bottom to top on a quartz substrate that has undergone ultrasonic cleaning and vacuum degassing pretreatment. The deposition process is carried out under strictly controlled conditions to ensure uniform film layer and optimized thickness. The influence of interface roughness is not considered in this study.

[0007] The specific functions and preparation methods of each layer are as follows:

[0008] The first layer is a reflective silver (Ag) metal film, deposited by DC magnetron sputtering using a pure silver target and argon inert gas. This layer has high reflectivity and low absorptivity within the solar spectrum, effectively reflecting incident sunlight and reducing solar thermal gain.

[0009] The second layer is a protective aluminum nitride (AlN) film, deposited via reactive RF magnetron sputtering using a pure aluminum target, argon as the sputtering working gas, and nitrogen as the reactive gas. This layer prevents the silver layer from oxidizing, enhances structural stability, and does not affect optical performance. In an oxygen-free deposition environment, this protective layer can be omitted.

[0010] The third layer is a one-dimensional photonic crystal-like film composed of four periodically alternating layers made of silicon dioxide (SiO2, low refractive index) and aluminum oxide (Al2O3, high refractive index). It is deposited via reactive RF magnetron sputtering using pure silicon and pure aluminum targets, with argon as the sputtering working gas and oxygen as the reactive gas. The two materials exhibit intrinsic absorption differences in the 9μm and 15-17μm wavelength bands, leading to a strong impedance mismatch and thus enhancing the emissivity in the atmospheric window band. By thinning the low-refractive-index layer and utilizing the coherent interference of the high-refractive-index layer, the emission spectral width in the mid-infrared band is extended.

[0011] The fourth layer is a silicon nitride (Si3N4) coupling film, deposited via reactive RF magnetron sputtering using a pure silicon target, argon as the sputtering working gas, and nitrogen as the reactive gas. This layer is directly exposed to air, primarily achieving radiative heat exchange through its surface. Its active optical phonons exhibit strong absorption at approximately 11 μm wavelength, coinciding with the atmospheric window band. This mode reduces impedance mismatch and back reflection at the film surface, thereby enhancing thermal radiation. Furthermore, its optical properties create a refractive index gradient on the surface, further improving impedance matching and increasing the emissivity of the atmospheric window.

[0012] The low-cost all-inorganic multilayer film of the present invention has the characteristics of low solar absorption, high atmospheric window and broadband emission, high net cooling power and thermal stability. The preparation process is simple and it is suitable for cooling and heat dissipation of spacecraft, electronic devices, buildings and vertical surface components. Attached Figure Description

[0013] The attached figure is a schematic cross-sectional view of a thermally stable all-inorganic multilayer thin film used for high-performance broadband radiative cooling on vertical surfaces. Detailed Implementation

[0014] This invention employs magnetron sputtering deposition technology to prepare high-performance all-inorganic multilayer films for radiation cooling. Before deposition, the quartz substrate is thoroughly pre-cleaned to remove surface contaminants. The distance between the target and the substrate is maintained at 40 mm, and the background pressure of the vacuum chamber during deposition is below 6 × 10⁻⁶. -4 The working pressure is 0.5 Pa. The preparation steps for each layer are as follows:

[0015] (1) Using a silver target with a purity of 99.99%, a 100 nm silver (Ag) thin film (film 2) was deposited on a quartz substrate (film 1) by DC magnetron sputtering under the conditions of target power of 100 W and argon flow rate of 40 sccm. The quartz substrate can be replaced with other materials.

[0016] (2) Using an aluminum target with a purity of 99.99%, a 20 nm aluminum nitride (AlN) film was deposited on a metal Ag film by reactive RF magnetron sputtering at 100 W, with an Ar:N2 flow ratio of 40:4 (film 3).

[0017] (3) Using aluminum and silicon targets with a purity of 99.99%, a one-dimensional photonic crystal structure film was deposited onto the AlN layer by reactive RF magnetron sputtering at 250W. This multilayer consists of alternating 620nm thick aluminum oxide (Al2O3) and 170nm thick silicon dioxide (SiO2) films (layers 4-1 to 4-4). During deposition, the argon to oxygen flow rate ratio for Al2O3 was 50:6, and the argon to oxygen flow rate ratio for SiO2 was 40:4 (film 4).

[0018] (4) Using a silicon target with a purity of 99.99%, a 260 nm silicon nitride (Si3N4) thin film was deposited on a photonic crystal layer by reactive RF magnetron sputtering at 200 W, with an Ar:N2 flow ratio of 50:10 (film 5).

[0019] The coating exhibits a reflectance of 97.42% in the solar spectrum (AM1.5), and emissivity of 90.65% and 90.59% in the atmospheric window (8–13 μm) and (8–20 μm) broadband range, respectively, achieving 288 W / m². 2 It boasts high net cooling power and a maximum temperature drop of 19.83℃, while maintaining excellent thermal stability even at temperatures as high as 1450℃. Under actual environmental conditions including wind, clouds, and humidity, the horizontal temperature reduction reached 10.9℃, and the vertical temperature reduction was 3.2℃, effectively verifying its cooling effect in different directions.

Claims

1. A high-performance broadband radiation-cooled inorganic multilayer thin film, consisting of a four-layer thin film structure: The first layer is a metallic reflective film, the second layer is a metallic protective barrier layer, the third layer is a photonic crystal layer composed of two materials with a large refractive index difference arranged periodically, and the fourth layer is an atmospheric window radiation enhancement layer. Its fabrication process includes the following steps: (1) The first layer is a silver (Ag) metal reflective film, which serves as a reflective substrate to effectively reflect incident solar radiation in order to minimize heat absorption. (2) The second layer is an AlN protective layer, which provides anti-oxidation protection for the reflective metal Ag film, ensuring stability, while not affecting its optical performance; (3) The third layer is a photonic crystal-like structure layer composed of SiO2 (low refractive index) and Al2O3 (high refractive index) materials with a large refractive index difference, which can generate strong radiation peaks within the atmospheric window range. At the same time, thinning the low refractive index layer can enhance the coherent interference effect of the high refractive index layer, thereby broadening the radiation spectrum in the mid-infrared band; (4) The fourth layer is a Si3N4 coupling layer, which enhances the emissivity of the atmospheric window through interaction with the atmosphere and refractive index gradient design.

2. A method for preparing high-performance broadband radiation-cooled inorganic multilayer films includes four steps: (1) Using a pure silver target, an Ag reflective film is deposited on a quartz substrate by direct current (DC) magnetron sputtering with argon as the sputtering working gas; the substrate can be replaced with other materials according to actual application requirements. (2) Using a pure aluminum target, radio frequency reactive magnetron sputtering is used on the metal film, with argon as the sputtering working gas and nitrogen as the reactive gas to deposit an AlN protective layer. (3) Using high-purity aluminum and silicon targets, with argon as the sputtering working gas and nitrogen as the reaction gas, a periodic four-layer photonic crystal structure multilayer film is deposited on the nitride protective layer by radio frequency reactive sputtering. (4) Using a pure silicon target, a Si3N4 coupling layer is deposited on a photonic crystal multilayer film by reactive radio frequency sputtering. The sputtering working gas is argon and the reactive gas is nitrogen.

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