A transparent radiative cooling device based on optical thin films and a method of manufacturing

Abstract

The application discloses a radiation cooling device based on an optical film, comprising a substrate, an optical film with high visible light transmittance and high infrared emissivity arranged on one side of the substrate, the optical film comprising an infrared reflection layer arranged on the substrate and an optical antireflection film arranged on the surface of the infrared reflection layer; the transmittance of the device in the visible light band is above 60%, and the emissivity in the atmospheric window band is above 50%. The radiation cooling device based on the optical film has simple structure and good stability, and is suitable for large-scale and low-cost production; the good thermal conductivity of silicon carbide and the high visible light transmittance and high infrared emissivity after regulation and control enable the silicon carbide to be applied to various devices, and the silicon carbide is more energy-saving and simple than active heat dissipation components; especially for virtual / augmented reality devices, if the silicon carbide is used as a lens, the silicon carbide can be well thermally conducted and radiated, and the unique high refractive index of the silicon carbide can further increase the field angle of view of the device and improve the optical performance.

Description

Technical Field

[0001] This invention relates to the field of radiation cooling technology, specifically to a transparent radiation cooling device based on thin film design and its fabrication method. Background Technology

[0002] For a system with a temperature above absolute zero, the process of electromagnetic waves generated by the thermal motion of internal particles radiating energy outward is called thermal radiation. According to Planck's law of blackbody radiation, when an object near room temperature (around 300K) undergoes thermal radiation, the electromagnetic waves radiated outward are concentrated around a wavelength of 10μm. Simultaneously, due to the absorption characteristics of gas molecule vibrations in the atmosphere, electromagnetic waves of 3-5μm and 8-13μm can travel long distances through the atmosphere. Based on these characteristics, by controlling the electromagnetic wavelength radiated by an object, not only can applications related to information transmission such as infrared detection and infrared stealth be realized, but it can also achieve radiative cooling by exchanging heat with the low-temperature environment of outer space. Compared to the large energy consumption of active cooling devices such as air conditioners and fans, objects using radiative cooling technology can achieve a cooling effect for 24 hours without consuming energy, making it a pollution-free, energy-saving, and green new cooling technology.

[0003] The sun, as the primary heat source, transfers heat to Earth in the form of visible light. To better achieve radiative cooling, it is necessary to increase the radiant power of electromagnetic waves in the atmospheric window band (especially 8-13 μm) and reduce the absorption of electromagnetic waves in the visible light band. According to Kirchhoff's laws, at a given temperature, the emissivity of an object is always equal to its absorptivity; simultaneously, according to electromagnetic wave theory, the sum of the reflection, transmission, and absorption rates of light by a medium is always equal to 1. Based on these characteristics, by controlling devices to specifically reflect, transmit, and absorb light of different wavelengths, the device can achieve the desired cooling effect. Currently, the application of radiative cooling mainly targets the surfaces of buildings or clothing that require cooling. This is achieved through micro- and nano-fabrication, growth, or coating of thin films to increase the reflectivity or transmittance of the object's surface in the visible light band, while simultaneously increasing the absorptivity in the atmospheric window band. This is currently the most important application of radiative cooling.

[0004] Radiation cooling also holds significant promise for device heat dissipation. Firstly, with the increasing development of virtual and augmented reality technology, higher demands are being placed on the heat dissipation performance of head-mounted devices while prioritizing portability and comfort. Compared to active cooling components with complex designs, passive cooling based on radiation cooling better meets the weight and size requirements of head-mounted devices. Secondly, in the solar energy field, battery lifespan is significantly affected by ambient temperature; waste heat can drastically reduce the lifespan of solar cells, and the energy consumption introduced by active cooling is detrimental to the efficient energy conversion of solar cells. In this case, passive cooling is crucial. Furthermore, for electronic devices such as mobile phones and computers, screen heat is a limiting factor affecting device performance. Using transparent materials with high thermal conductivity and regulating radiation cooling to increase emissivity can also achieve effective heat dissipation.

[0005] In summary, for the three types of devices mentioned above, if a material and design scheme can be found that enables the heat dissipation component to have high transmittance in the visible light range, high absorption in the atmospheric window band, and good thermal conductivity while achieving a large radiation area, then it can be widely used in device heat dissipation to improve the performance of the equipment. Summary of the Invention

[0006] The purpose of this invention is to find radiation-cooling materials suitable for the aforementioned application scenarios and to achieve the modulation of the emissivity of electromagnetic waves in different bands through optical design, thereby overcoming the shortcomings of existing technologies in these application scenarios. This invention provides a radiation-cooling device based on an optical thin film, which has high visible light transmittance, high infrared emissivity, and higher cooling efficiency.

[0007] The present invention also provides a method for fabricating a radiation cooling device based on an optical thin film. This method is simple and can be mass-produced industrially.

[0008] A radiation cooling device based on an optical thin film, comprising:

[0009] The substrate is made of a transparent material with high thermal conductivity;

[0010] An optical thin film with high visible light transmittance and high infrared emissivity is disposed on one side of a substrate. The optical thin film includes an infrared reflective layer disposed on the substrate and an optical antireflective film disposed on the surface of the infrared reflective layer. The device has a transmittance of more than 60% in the visible light band (460-660nm) and an emissivity of more than 50% in the atmospheric window band (8-13μm).

[0011] Preferably, the substrate is silicon carbide. More preferably, it is an undoped silicon carbide wafer. Even more preferably, the substrate thickness is 0.5-5 mm.

[0012] The substrate of this invention uses silicon carbide, a transparent material with high thermal conductivity; one side of the substrate has an optical thin film with high visible light transmittance and high infrared emissivity, which simultaneously modulates the visible light band and the atmospheric window band. On the one hand, it can ensure transparency under visible light and have a low absorption rate of sunlight. On the other hand, it can improve the heat dissipation effect by using high emissivity in the atmospheric window band, thereby further improving the performance of radiation cooling.

[0013] Silicon carbide, as a third-generation semiconductor, possesses superior physical properties such as high thermal conductivity, high electrical conductivity, high breakdown electric field, high power density, and high temperature resistance, giving it irreplaceable advantages in high-power electronic devices and extreme environment applications such as aerospace and nuclear energy. Simultaneously, silicon carbide exhibits good transmittance in the visible light band, meeting the requirements of specific applications. This invention utilizes silicon carbide's excellent thermal conductivity and high visible light transmittance, based on the principle of radiative cooling, through scheme design and simple fabrication, to apply it to electronic devices such as virtual and augmented reality, solar cells, and mobile phone and computer screens. This not only improves the heat dissipation capacity of the devices but also provides advantages that active cooling methods lack.

[0014] Specifically, for virtual and augmented reality (VR) devices, weight and size are often important indicators when evaluating the quality of head-mounted displays, considering the comfort of wearing them. Replacing the heavy and bulky active cooling components with simple and lightweight passive cooling will improve the comfort of the head-mounted display under normal cooling conditions, while also increasing its battery life. To improve the performance of passive cooling, key components need to have good thermal conductivity and maximize emissivity and heat dissipation area. If silicon carbide is used for the lenses in VR devices, its excellent thermal conductivity will allow heat generated during operation to be effectively conducted to the lenses. Furthermore, silicon carbide's good transmittance in the visible light spectrum, combined with a multi-layer optical film design, can further enhance visible light transmittance and increase emissivity at atmospheric windows, effectively transferring heat away. In addition, silicon carbide has a refractive index of approximately 2.6, which significantly contributes to improving the field of view and portability of head-mounted displays. Similarly, for solar cells and electronic screens, heat dissipation components not only need to have high thermal conductivity and high emissivity, but also high transmittance of visible light, in order to ensure that the device can meet heat dissipation requirements while maintaining normal device performance.

[0015] Preferably, the present invention designs a silicon carbide radiative cooling device based on an optical thin film, the structure of which includes:

[0016] The substrate is made of silicon carbide.

[0017] An optical thin film possesses high visible light transmittance and high infrared emissivity. Based on transmittance control in the visible light band, it increases emissivity in the atmospheric window band. The optical thin film consists of an infrared reflective layer and an optical antireflective film. The visible light band is between 460-660 nm; the atmospheric window band is between 8-13 μm; the high visible light transmittance can reach 0.96-0.99 on the dielectric surface between the silicon carbide substrate and air; the high infrared emissivity reaches 0.6-0.9 in the atmospheric window band.

[0018] Furthermore, the optical thin film structure comprises an infrared reflective layer and an optical antireflective film. The infrared reflective layer uses a material with high transmittance in the visible light range and a reflectance of over 50% in the atmospheric window band, such as indium tin oxide (ITO, thickness unlimited, for example, 5-50 nm), which can effectively improve infrared absorption capability between the substrate and the optical antireflective film. The optical antireflective film adopts a structure with alternating distribution of high and low refractive index materials and absorption in the atmospheric window band. The optical antireflective film adopts a film stack structure with alternating high and low refractive index materials. Preferably, the optical thin film includes an infrared reflective layer and an oxide thin film with alternating high and low refractive indices. In the optical antireflective film, the high refractive index material is selected from titanium dioxide (TiO2), hafnium dioxide (HfO2), etc.; the low refractive index material is selected from silicon dioxide (SiO2), etc. Preferably, the optical antireflective film can adopt a film system structure such as alternating silicon dioxide and titanium dioxide, which can not only improve visible light transmittance, but also increase the radiation of the device in the atmospheric window band due to the absorption of infrared by the oxide. The overall visible light transmittance is above 0.6, the emissivity to the atmospheric window band (8-13μm) is above 0.5, the number of layers is 5-51 (i.e., the total number of layers of the optical antireflective film is 4-50), and the total thickness is 0.5-5μm.

[0019] As a preferred embodiment, the high refractive index material is selected from titanium dioxide; the low refractive index material is selected from silicon dioxide; the total number of layers of the optical antireflective film is 6 to 20, and the thickness of each layer is 80 to 300 nm.

[0020] As a specific preferred embodiment, the structure of the optical antireflective film is TiO2\SiO2\TiO2\SiO2\TiO2\SiO2\TiO2\SiO2\TiO2\SiO2, with the thickness of each layer being: 150~160nm\175~185nm\100~110nm\180~185nm\125~130nm\205~210nm\125~135nm\205~210nm\132.0~135nm\100~110nm.

[0021] The transmittance of the radiation-cooled device using this structure can reach about 70% in the visible light range, and the absorption rate of this device is about 80% in the 8-13μm band.

[0022] This invention also provides a method for fabricating a radiation-cooling device based on an optical thin film as described in any of the above technical solutions, comprising:

[0023] (1) Using simulation software, determine the optimal optical thin film material and optical thin film composition that meet the target transmittance and emissivity;

[0024] During the simulation calculation, some parameters can be set manually, while the remaining parameters can be optimized using existing simulation software. As a preferred option, the thickness of the infrared reflective layer, the high and low refractive index materials, and the number of layers are set to constant values. The thickness of each layer is optimized using simulation software. The optimization goal is to ensure that the transmittance and emissivity meet the set requirements.

[0025] As a preferred option, the simulation software may include, but is not limited to, Macleod and Comsol.

[0026] (2) The infrared reflective layer and the optical anti-reflective film are processed sequentially on the substrate using a coating method.

[0027] Preferably, the infrared reflective layer is processed by magnetron sputtering, and the optical antireflective film is processed by electron beam evaporation deposition.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] Compared to the high requirements of micro-nano fabrication and the complex steps in the preparation of doped particle thin films, optical thin films are simple to prepare, have a wide range of material sources, excellent stability, low cost, and are suitable for large-scale production.

[0030] Silicon carbide's excellent thermal conductivity, along with its high visible light transmittance and high infrared emissivity after regulation, allows it to be used in various devices based on the principle of radiative cooling. This enables the heat generated during device operation to be transferred away, achieving heat dissipation at a lower temperature than the ambient temperature. Compared to active cooling, it is more energy-efficient and lightweight, improving space and energy utilization.

[0031] In particular, for virtual / augmented reality devices, using silicon carbide as lenses not only achieves good thermal conductivity and heat dissipation, but its high refractive index can further increase the field of view of the lenses, providing a larger optical display range. This allows head-mounted devices to meet performance and heat dissipation requirements while significantly reducing weight and size. Attached Figure Description

[0032] Figure 1: A schematic diagram of the structure of the radiation cooling device in this invention.

[0033] Figure 2A Simulation results of transmittance of SiC devices without film in the visible light range (460-660nm).

[0034] Figure 2B Simulation results of single-sided transmittance of devices with designed film structures added on SiC substrates in the visible light range (460-660nm).

[0035] Figure 2C Simulation results of the overall transmittance of devices with added film structures on SiC substrates in the visible light range (460-660nm).

[0036] Figure 3 Simulation results of the absorption rate of devices with additional designed film structures on SiC substrates in the atmospheric window band (8-13μm).

[0037] Figure 4 The transmittance of the device fabricated according to the design parameters in the 460-660nm wavelength band is experimentally obtained.

[0038] Figure 5 Experimental results of the absorption rate of the device fabricated according to the design parameters in the 8-13μm band.

[0039] Figure 6A The experimental results of the device manufactured according to the design parameters and captured by an infrared camera show that the substrate temperature was 45℃.

[0040] Figure 6B The experimental results of the device fabricated according to the design parameters and captured by an infrared camera show that the substrate temperature was 80℃. Detailed Implementation

[0041] The design concept of using optical thin films to prepare radiation cooling devices in this invention is as follows: under the condition of silicon carbide as substrate, according to the design principle of optical antireflection film, materials with alternating high and low refractive indices are used as thin films, and the film thickness and number of layers are controlled to achieve high transmittance of visible light and high absorption of infrared light at the same time.

[0042] The present invention will be described in detail below with reference to embodiments. These examples will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way. Those skilled in the art can make several adjustments and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the present invention can be purchased commercially.

[0043] like Figure 1The diagram shows a schematic of a radiation cooling device based on an optical thin film according to the present invention, including a substrate 30, an infrared reflective layer 20 disposed on the top surface of the substrate 30, and an optical antireflective film 10 disposed on the top surface of the infrared reflective layer 20.

[0044] The substrate 30 can be a silicon carbide wafer. The infrared reflective layer 20 can be an indium tin oxide thin film. The optical antireflective coating 10 can be a film stack structure with alternating silicon dioxide and titanium dioxide.

[0045] In actual production, simulation software can be used to optimize the thickness of each layer of silicon dioxide and titanium dioxide (during optimization, the thickness of the infrared reflective layer 20 is kept constant), and then the actual production can be carried out based on the obtained thickness parameters.

[0046] In this embodiment, based on the simulation results of Macleod and Comsol, the specific film structure is shown in the table below:

[0047] membrane thickness ITO 10nm TiO2 156.65nm SiO2 177.92nm TiO2 104.02nm SiO2 182.59nm TiO2 128.31nm SiO2 207.29nm TiO2 130.87nm SiO2 207.46nm TiO2 132.65nm SiO2 106.34nm

[0048] Simulations of SiC devices were performed using Macleod software. The transmittance of SiC devices without any added films in the visible light range (460-660 nm) is shown in [reference needed]. Figure 2A ,Depend on Figure 2A It can be seen that the transmittance of SiC devices without any added film is approximately 66%; while after adding a film structure with the parameters set above, the single-sided transmittance of SiC devices in the 460-660nm range is shown in the figure. Figure 2B It can achieve 98% single-sided transmittance; at the same time, the overall transmittance of this device in air is about 78% (see...). Figure 2C ).

[0049] Using Comsol software to simulate the SiC device, and performing film system simulation according to the design parameters, it was found that the absorption rate of this device can reach approximately 75% in the atmospheric window band of 8-13 μm. Please refer to [the relevant documentation] for details. Figure 3 .

[0050] After obtaining the above-mentioned film thickness, the actual device is fabricated:

[0051] Prepare an undoped silicon carbide wafer with a thickness of 500 μm. After cleaning, use magnetron sputtering to grow an indium tin oxide thin film on its surface as an infrared reflective layer with a thickness of about 10 nm. Then, use electron beam evaporation to grow an alternating film of silicon dioxide and titanium dioxide on the indium tin oxide film, with each layer having a thickness of 100-200 nm and a total thickness of about 1.5 μm.

[0052] The transmittance results of the device fabricated using the method of this invention in the 460-660nm wavelength band are shown below. Figure 4 The testing equipment used was Nicolet iS50, and the experimental results showed that the transmittance in the visible light range could reach about 70%.

[0053] To obtain the absorbance of this device in the atmospheric window band, Fourier transform infrared spectroscopy was used for testing, and the results are shown in [Figure number missing]. Figure 5 The device model is Bruker Vertex 70. The experimental results show that the absorption rate of this device is about 80% in the 8-13μm band.

[0054] To better understand the heat dissipation effect of the device, it was photographed using an infrared camera (Blackbird Precision SL, Jenoptik). The infrared camera results were obtained when the substrate temperature was 45°C. Figure 6A When the substrate temperature is 80℃, the infrared camera's capture results are shown below. Figure 6B In the diagram, A ( Figure 6A and 6B The top left (middle) shows silicon carbide without coating, B ( Figure 6A and 6B The lower left (middle) is an approximate black body, C( Figure 6A and 6B (Top right) is silicon carbide after coating according to the above-described film structure of the present invention, D( Figure 6A and 6B (Lower right) is quartz glass. (From) Figure 6A and Figure 6B As can be seen, for silicon carbide, the thermal radiation effect of the device is significantly improved before and after coating. At a substrate temperature of 45°C, it increases from 35.5°C to 40.2°C, which is close to the radiation capability of a blackbody. More significantly, at a substrate temperature of 80°C, the coated silicon carbide can achieve a thermal radiation effect of 70.8°C.

Claims

1. A transparent radiative cooling device based on an optical thin film, characterized in that, include: The substrate is made of silicon carbide. An optical thin film with high visible light transmittance and high infrared emissivity is disposed on one side of a substrate. The optical thin film includes an infrared reflective layer disposed on the substrate and an optical antireflective film disposed on the surface of the infrared reflective layer. The device has a transmittance of more than 60% in the visible light band and an emissivity of more than 50% in the atmospheric window band. The optical antireflective coating adopts an oxide film stack structure with alternating high and low refractive index materials; the total thickness of the optical thin film is 0.5-5μm, and the total number of layers of the optical antireflective coating is 4-50.

2. The transparent radiation cooling device based on optical thin films as described in claim 1, characterized in that, The substrate thickness is 0.5-5mm.

3. The transparent radiation cooling device based on optical thin films as described in claim 1, characterized in that, The infrared reflective layer material is indium tin oxide.

4. The transparent radiation cooling device based on optical thin films as described in claim 3, characterized in that, In the optical antireflective coating, the high refractive index material is selected from titanium dioxide and hafnium dioxide; the low refractive index material is selected from silicon dioxide.

5. The transparent radiation cooling device based on optical thin films as described in claim 3, characterized in that, In the optical antireflective coating, the high refractive index material is selected from titanium dioxide; the low refractive index material is selected from silicon dioxide.

6. The transparent radiation cooling device based on optical thin films as described in claim 5, characterized in that, The total number of layers in the optical antireflective coating is 6 to 20, and the thickness of each layer is 80 to 300 nm.

7. A method for fabricating a transparent radiation-cooling device based on an optical thin film as described in any one of claims 1 to 6, characterized in that, include: (1) Using simulation software, determine the optimal optical thin film material and composition that meet the target transmittance and emissivity; (2) The infrared reflective layer and the optical anti-reflective film are processed sequentially on the substrate using a coating method.

8. The preparation method according to claim 7, characterized in that, The infrared reflective layer is fabricated using magnetron sputtering, and the optical antireflective film is fabricated using electron beam evaporation.

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