Radiative cooling and photothermal heating switchable films, methods of making and using the same

Abstract

The application discloses a switchable film capable of radiative cooling and photothermal heating and a preparation method and application thereof. The switchable film comprises, from top to bottom, a scattering layer, a PDMS film layer and a photothermal heating layer. The scattering layer is a PE film doped with nano zinc oxide. The photothermal heating layer is a PE film doped with carbon black. The switchable film can realize switching between radiative cooling and photothermal heating, solves the phenomenon of supercooling of a traditional radiative cooling device in winter, separates radiative cooling and solar radiation scattering, minimizes the influence of heat convection and heat conduction on the radiative cooling device, and improves the actual radiative cooling effect of the device. The film device has the advantages of simple structure, simple preparation method, low cost, good chemical stability and the like, and has a bright application prospect in textile clothes, building materials and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the fields of radiation refrigeration, photothermal heating, multilayer film, etc., and particularly relates to a radiation refrigeration and photothermal heating switchable film, a preparation method and application thereof. BACKGROUND

[0002] Energy crisis and global warming are two big problems in the world today. With the continuous rise of the average temperature of the earth, the refrigeration energy consumption is also rising. According to statistics, the current refrigeration energy consumption accounts for about 20% of the total electricity consumption of the global building, and accounts for about 10% of the total electricity consumption of the global building. In daily life, the strong solar radiation in summer will greatly increase the temperature of the building, and increase the air conditioning refrigeration energy consumption. Exploring a refrigeration material that can be widely applied to the surface of the building is the current research direction that people focus on.

[0003] Using the radiation refrigeration phenomenon to cool the building is a good response method. In recent years, the radiation refrigeration phenomenon has attracted more and more attention. When the temperature of an object is greater than absolute zero, it will radiate heat in the form of thermal radiation. The average temperature of the earth's surface is 300K, which is a huge heat radiator, and the average temperature of outer space is 3K, which is a huge cold source. The earth's object surface can reduce the temperature of the object surface by radiating heat to outer space in the form of thermal radiation, which is the radiation refrigeration phenomenon. Thermal radiation will be affected by carbon dioxide, water vapor and other gases in the atmosphere when it penetrates the atmosphere. Studies have shown that when the wavelength of thermal radiation is 8-13um, thermal radiation can penetrate the atmosphere to outer space to achieve radiation refrigeration, so 8-13um is also called atmospheric window. The radiation refrigeration phenomenon is a spontaneous process, and its greatest advantage is that it does not need any external energy input, so it has attracted more and more attention from researchers.

[0004] The radiation refrigeration device widely used at present can achieve very good radiation refrigeration effect in summer through special design, effectively reducing the energy consumption of the refrigeration device. However, in cold winter, continuous radiation refrigeration will take away the indoor heat, reduce the indoor temperature, and increase the heating energy consumption, causing unnecessary energy waste. In addition, radiation refrigeration is very susceptible to thermal convection and heat conduction. Most of the existing radiation refrigeration devices directly contact the external environment, and thermal convection and heat conduction have a great influence on the refrigeration effect, which makes the actual radiation refrigeration effect of the existing radiation refrigeration device poor. SUMMARY

[0005] To address the aforementioned problems, this invention proposes a switchable thin film for radiative cooling and photothermal heating, possessing dual functions. In the hot summer, the radiative cooling surface faces the sky, utilizing radiative cooling to lower the film's surface temperature; in the cold winter, the photothermal heating surface faces the sky, leveraging the film's excellent absorption of solar radiation to achieve thermal insulation and reduce energy consumption of the heating device in winter. Furthermore, the design of the radiative cooling surface separates the solar scattering layer and the radiative cooling layer, ensuring that the radiative cooling performance of the thin film device is unaffected by heat convection and heat conduction during operation, maximizing the film's radiative cooling effect.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] The first objective of this invention is to provide a switchable thin film for radiative cooling and photothermal heating. This switchable thin film, from top to bottom, comprises a scattering layer, a PDMS thin film layer, and a photothermal heating layer; the scattering layer is a PE film doped with nano-zinc oxide; the PDMS thin film layer is the radiative cooling layer; and the photothermal heating layer is a PE film doped with carbon black. Wherein:

[0008] The scattering layer is composed of multiple scattering unit films stacked together, with a preferred thickness of 200µm-500µm. Each scattering unit film consists of a pure PE film and a PE film doped with nano-zinc oxide. The nano-zinc oxide consists of oleophilic nano-zinc oxide particles. The mass fraction of nano-zinc oxide in the PE film is 3%-5%, and the particle size is 300nm-500nm. PE is highly transparent in both solar radiation and atmospheric window wavelengths. Doping with nanoparticles can alter the solar radiation transmittance of the PE film. This layer, composed of a PE film doped with nano-zinc oxide, allows for high reflection of solar radiation while maintaining high transparency in the atmospheric window, thus avoiding any impact on radiative cooling. This structure ensures both solar radiation scattering and atmospheric window transmittance, while separating the functions of radiative cooling and solar scattering. The low thermal conductivity of PE ensures that the underlying radiative cooling layer is unaffected by heat convection and conduction during actual operation, maximizing the mitigation of the decline in the film's radiative cooling performance during actual operation.

[0009] The PDMS thin film layer serves as a radiative cooling layer, acting as an intermediate layer between the scattering layer and the photothermal heating layer. It is a pure PDMS thin film layer with a thickness of 80-120 μm; more specifically, a thickness of 100 μm is preferred, as this is currently the most common thickness for PDMS products, balancing cooling performance and cost. PDMS (polydimethylsiloxane) contains numerous Si-O bonds, resulting in strong infrared absorption at atmospheric windows. According to Kirchhoff's laws, PDMS exhibits approximately 0.9 infrared emissivity at atmospheric windows, demonstrating excellent radiative cooling potential and meeting the requirements for infrared emissivity at atmospheric windows in daytime radiative cooling materials. Besides its emissivity advantage, PDMS also boasts low cost, simple preparation process, and strong chemical stability. In this invention, PDMS is positioned as the intermediate layer of the entire film. The infrared radiation emitted by PDMS at the atmospheric window can penetrate the top high-scattering layer to reach the cold outer space, reducing the film's temperature and achieving energy savings in summer.

[0010] The thickness of the photothermal heating layer is 80-120 μm, more preferably 100 μm, as a 100 μm thickness balances heating effect and cost. Common radiative cooling devices have good cooling effects in summer, but cause overcooling in winter. In this invention, a photothermal heating layer is provided below the aforementioned high scattering layer and radiative cooling layer; this layer is a PE film doped with carbon black. PE film has good chemical stability, and carbon black can absorb a large amount of solar radiation and infrared radiation. In winter, this layer can absorb a large amount of heat to achieve a heat preservation effect. This design allows the invention to achieve radiative cooling in summer, reducing cooling power consumption, and to achieve photothermal heating in winter, raising the film temperature to achieve a heat preservation effect. Existing carbon black-doped PE film production technology is very mature, which also makes the film described in this invention easy to industrialize.

[0011] A second objective of this invention is to provide a method for preparing a switchable thin film for radiative cooling and photothermal heating as described in the first objective, comprising the following steps:

[0012] After melting PE polymer particles, nano zinc oxide particles are doped into them. The PE masterbatch doped with nano zinc oxide is obtained by extrusion granulation. Then, the PE masterbatch doped with nano zinc oxide and the pure PE masterbatch are added to two extruders respectively. After the two masterbatches are melted, a scattering unit film is formed by combining the PE film doped with nano zinc oxide and the pure PE film. The scattering unit film is stacked by a stack multiplier to form a multilayer structure of up to 128 layers to obtain the scattering layer.

[0013] After the PDMS pre-cured material and curing agent are mixed evenly, they are coated on the surface of the scattering layer and cured to form a PDMS thin film layer on the surface of the scattering layer.

[0014] A black PE film, which is a PE film doped with carbon black, is laminated onto the surface of the PDMS film layer to obtain the target product.

[0015] A third objective of this invention is to provide an application of a switchable film for radiative cooling and photothermal heating as described in the first objective, which has promising applications in building materials such as textiles, clothing, and corrugated steel sheets.

[0016] The beneficial effects of this invention are as follows:

[0017] The switchable thin film for radiative cooling and photothermal heating provided by this invention enables switching between the two modes, solving the problem of overcooling in traditional radiative cooling devices during winter. Furthermore, it separates radiative cooling from solar radiation scattering, minimizing the impact of heat convection and conduction on the radiative cooling device and improving its actual radiative cooling effect. The thin film device described in this invention possesses numerous advantages, including a simplified structure, simple fabrication method, low cost, and good chemical stability, and has the potential for large-scale application. It shows promising application prospects in textiles, clothing, and building materials. Attached Figure Description

[0018] Figure 1 A schematic diagram of the structure of the switchable radiative cooling and photothermal heating thin film provided by the present invention;

[0019] Figure 2 for Figure 1 Schematic diagram of the intermediate scattering layer structure;

[0020] Figure 3 for Figure 1 Schematic diagram of the principle of the solar thermal heating layer;

[0021] Figure 4 A schematic diagram of the process flow for preparing a scattering layer using micro-nano multilayer co-extrusion technology;

[0022] Figure 5 The theoretical reflectance of a PE film doped with 5% nano zinc oxide in the 300-2500 nm wavelength range;

[0023] Figure 6 The standard infrared absorption spectrum of PDMS;

[0024] Figure 7 The diagram shows the theoretical cooling power.

[0025] Figure 8 This is a schematic diagram of the switchable thin-film radiative cooling function provided by the present invention.

[0026] Figure 9A schematic diagram of the switchable thin-film photothermal heating function provided by the present invention;

[0027] Figure reference numerals: 1-scattering layer, 101-pure PE film, 102-PE film doped with nano zinc oxide, 2-PDMS film layer, 3-photothermal heating layer, 4-nano zinc oxide, 5-PE, 6-carbon black. Detailed Implementation

[0028] The present invention will be further described below with reference to embodiments, so that those skilled in the art can better understand and implement the present invention, but the embodiments are not intended to limit the present invention.

[0029] In addition, unless otherwise specified, the preparation processes in the following embodiments are all conventional methods in the prior art, and therefore will not be described in detail.

[0030] The types and suppliers of the materials used in the following examples are as follows:

[0031] The nano zinc oxide is lipophilic nano zinc oxide particles, model number Z112848, and the supplier is Aladdin.

[0032] The PDMS pre-cured material is model A189544, and the supplier is Aladdin; the curing agent is a supporting raw material for the PDMS pre-cured material and is provided by the manufacturer when purchasing the PDMS product.

[0033] The commercial black PE film is a high-viscosity black PE film with a thickness of 100cm x 0.07mm, supplied by Foshan Longmo New Material Technology Co., Ltd.

[0034] The reagents described above are only intended to illustrate the source and composition of the materials used in the experiments of this invention, so as to provide full disclosure, and do not imply that the invention cannot be achieved by using other similar materials or reagents provided by other suppliers.

[0035] Example 1

[0036] refer to Figure 1 This is a schematic diagram of the structure of the switchable radiative cooling and photothermal heating thin film provided by the present invention, which includes, from top to bottom, a scattering layer 1, a PDMS thin film layer 2, and a photothermal heating layer 3; wherein:

[0037] The structural principle diagram of scattering layer 1 is as follows: Figure 2As shown, when solar radiation reaches the scattering layer 1, the nano-zinc oxide 4 particles within it cause high scattering of the solar radiation. PE 5 can maintain high transparency through the atmospheric window, thus not affecting radiative cooling. This structure ensures both solar radiation scattering rate and atmospheric window transmittance, while separating the two functions of radiative cooling and solar scattering. The low thermal conductivity of PE ensures that the underlying radiative cooling layer is not affected by heat convection and heat conduction during actual operation, maximizing the mitigation of the decline in the radiative cooling performance of the film during actual operation.

[0038] The PDMS thin film layer 2 contains a large number of Si-O bonds, which can undergo strong infrared absorption in the atmospheric window. According to Kirchhoff's law, PDMS has an infrared emissivity of about 0.9 in the atmospheric window, which has excellent radiative cooling effect and meets the requirements of infrared emissivity in the atmospheric window of daytime radiative cooling materials.

[0039] The structural principle of the photothermal heating layer 3 is as follows: Figure 3 As shown, the PE film exhibits good chemical stability, and carbon black 6 can absorb a large amount of solar and infrared radiation. In winter, this layer can absorb a significant amount of heat to achieve a heat preservation effect. This design enables the invention to achieve radiative cooling in summer, reducing cooling power consumption, and to achieve photothermal heating in winter, raising the film temperature to achieve a heat preservation effect. Existing carbon black-doped PE film production technology is very mature, which also makes the film described in this invention easy to industrialize.

[0040] Example 2

[0041] The preparation method of the switchable thin film for radiative cooling and photothermal heating is as follows:

[0042] Preparation of the scattering layer: PE polymer particles and zinc oxide nanoparticles were selected. The zinc oxide nanoparticles were lipophilic nano-zinc oxide particles. The PE polymer particles were melted and then doped with zinc oxide nanoparticles at a mass fraction of 5%. The mixture was then granulated using an extruder for later use. Using nanoparticle-doped polymer particles and pure PE polymer particles as raw materials, the high-scattering film was prepared using micro-nano multilayer co-extrusion technology. The preparation process is as follows: Figure 4As shown, PE polymer particles doped with nanoparticles and pure PE polymer particles were respectively added to two extruders, melted, and extruded to form a PE film 102 doped with nano-zinc oxide and a pure PE film 101. The film was then multiplied by a stack multiplier to increase the number of layers, resulting in a final film with a total of 128 layers and a total thickness of 500 μm, comprising 64 layers of the PE film doped with nano-zinc oxide and 64 layers of the pure PE film. The film is white in color, exhibits spectral selectivity, and can effectively scatter solar radiation while transmitting infrared radiation through an atmospheric window. Traditional doping techniques simply add nanoparticles to polymers. This invention combines polymers with doped and undoped nanoparticles, offering the following advantages: (1) During the multilayer stacking process, the two polymer fluids flow through the stack multiplier, and the flow channels have a cutting effect on the nanoparticles, effectively alleviating the agglomeration of nanoparticles and enhancing their dispersion in the polymer; (2) When preparing polymer films of the same thickness, the stacking process can reduce the amount of nanoparticles by half. In this invention, nanoparticles are filled in certain polymer layers, allowing the original function to be achieved with half the amount of nanoparticles. Yes, that is, the stacking process can double the concentration of nanoparticles in certain layers. In this invention, nanoparticles are used to scatter solar radiation. The stacking process can double the concentration of nanoparticles in certain layers to achieve a better scattering effect. (3) Nanoparticles doping into polymers will affect the mechanical properties of polymers. In particular, when the doping amount is large, the mechanical properties of polymers will drop significantly. Using the stacking process of this invention, the undoped PE layer is equivalent to the adhesive layer of the PE layer doped with nanoparticles, which is used to enhance the overall mechanical properties. It is precisely because of this that the stacking technology can make the nanoparticle content in certain layers larger, thereby making the light scattering effect better.

[0043] Preparation of PDMS thin film layer 2: PDMS pre-cured material and curing agent are mixed at a mass ratio of 10:1. After mixing, air bubbles are removed in a vacuum chamber. Then, the mixture is coated onto the pre-prepared scattering layer using a wire bar coater. The coating thickness is 100 μm. After coating, the PDMS is completely cured by vacuum drying at 80 °C for 6 h, forming a PDMS thin film layer on the surface of the scattering layer.

[0044] Preparation of Photothermal Heating Layer 3: The photothermal heating layer is a PE film doped with carbon black. The preparation process of this film is mature. The photothermal heating layer described in this invention directly uses a common commercial black PE film with a thickness of 100 μm. The PDMS film has a certain degree of adhesion. The commercial black PE film can be laminated onto the PDMS surface to complete the bonding, thus obtaining the target product, namely, a film that can switch between radiative cooling and photothermal heating.

[0045] Performance testing

[0046] Using the micro-nano multilayer co-extrusion technology in Example 2 of this invention, a pure PE layer is used as the adhesive layer for the doped layer, which significantly enhances the mechanical properties and solar radiation scattering ability of the thin film. Figure 5 This is the theoretical reflectance diagram of the PE scattering film doped with 5% nano-zinc oxide prepared in Example 2 in the 300-2500nm wavelength range. Figure 5 It is known that the scattering film has a strong reflectivity in the 300-1200nm wavelength range where solar radiation is strong, especially in the 300-1000nm wavelength range, where its average reflectivity reaches more than 90%. This allows the scattering film to effectively scatter solar radiation and prevent solar radiation from heating objects.

[0047] Infrared standard spectra of compounds from the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences ( Figure 6 As can be seen from the data, PDMS has strong absorption of infrared radiation in the 8-13µm range. According to Kirchhoff's law, when an object is in thermal equilibrium, its infrared absorptivity is equal to its infrared emissivity. Therefore, it can be concluded that PDMS has a high emissivity in the atmospheric window band of 8-13µm, which indicates that PDMS has good radiative cooling potential.

[0048] Figure 7 This is a diagram showing the theoretical cooling power of the present invention during the day. As can be seen from the diagram, the maximum theoretical cooling power of the present invention during the day is 40W / m². 2 Theoretically, during the day, it can achieve a temperature reduction of 3.5℃ below the ambient temperature (assuming an overall heat transfer coefficient of 5W / (m²)). 2 ·K)) This indicates that the present invention has a good cooling effect and can effectively reduce cooling energy consumption in summer.

[0049] Carbon black has extremely high absorption of solar radiation, which ensures that the present invention has a good heat preservation effect in winter and can effectively reduce heating energy consumption in winter.

[0050] It should be noted that in other embodiments, the objective of this invention can be achieved when the experimental process meets the following conditions:

[0051] The preferred particle size for zinc oxide nanoparticles is 300nm-500nm, specifically 300nm, 400nm, or 500nm.

[0052] The doping amount of zinc oxide nanoparticles in the scattering layer is preferably 3%-5%, specifically 3%, 4% or 5%.

[0053] The thickness of the scattering layer is preferably 200um-500um, specifically 200um, 400um, or 500um.

[0054] Those skilled in the art can make appropriate selections of the above process parameters according to actual needs, and all of them can achieve the purpose of this invention.

[0055] The switchable thin-film device for radiative cooling and photothermal heating provided by this invention has excellent photothermal heating function in winter, and its photothermal heating function principle diagram is shown below. Figure 9 As shown. In winter, simply flip the thin-film device so that the black carbon black-doped PE film faces the sky. PE has a very high thermal resistance, which can effectively reduce heat loss. The carbon black nanoparticles can absorb a large amount of solar radiation and infrared radiation, raising the temperature of the thin-film device in winter. The coupling effect greatly enhances the heating performance of the thin-film device in winter, solving the overcooling problem of existing radiative cooling materials in winter, reducing heating energy consumption in winter, and providing good heating and heat preservation functions.

[0056] The switchable thin-film device for radiative cooling and photothermal heating provided by this invention adopts a structural design that separates solar radiation scattering from radiative cooling. A scattering layer covers the surface of the radiative cooling layer (PDMS thin film layer), which can reduce the impact of heat conduction and heat convection on the device's radiative cooling performance. Existing radiative cooling materials require a transparent cover plate to reduce the influence of heat conduction and heat convection on the measurement results when testing their radiative cooling performance. However, in practical applications, the radiative cooling layer is in contact with air, and heat conduction and heat convection significantly reduce the material's actual radiative cooling performance. This invention uses a PE film doped with zinc oxide nanoparticles to separate the radiative cooling layer from the air, avoiding contact between the atmospheric environment and the radiative cooling layer, minimizing the impact of the environment on the material's radiative cooling power. Compared with traditional radiative cooling materials, the thin-film device of this invention has better cooling performance. Its radiative cooling function principle diagram is shown below. Figure 8 As shown.

[0057] Application Example 1

[0058] The switchable film prepared in Example 2 above can be applied in the textile and clothing industry. The film is laminated on both sides of cotton fibers, with the high-scattering layer and the photothermal heating layer on each side facing outwards. The laminated cotton fibers are then used to make clothing. In summer, with the high-scattering layer facing outwards, solar radiation can be effectively scattered, and the body surface temperature can be lowered through radiative cooling, improving the comfort of summer clothing. In winter, with the photothermal heating side facing outwards, the heating layer absorbs a large amount of solar and atmospheric radiation, raising the temperature of the clothing and achieving a warming effect. Clothing using the film described in this invention can achieve cooling in summer and insulation in winter, possessing significant application value and market potential.

[0059] Application Example 2

[0060] The switchable film prepared in Example 2 above can be applied in the construction field. Corrugated steel roof houses have advantages such as convenient installation, low cost, energy saving, and environmental protection. They are widely used in temporary offices and dormitories at construction sites, as well as in large-scale field exploration and construction work for railways, transportation, water conservancy, oil, and natural gas industries; temporary commercial or other buildings in urban areas; leisure villas in tourist areas; holiday homes; earthquake relief and disaster relief facilities; and military applications. Traditional corrugated steel roof houses use steel tiles that do not provide thermal insulation, often resulting in excessively high indoor temperatures in summer and rapid heat loss in winter, severely reducing the living comfort of the houses. This invention effectively addresses the problems of existing corrugated steel roof houses. By applying the film described in this invention to the inner and outer surfaces of the corrugated steel roof, in summer, with the high-reflectivity layer facing the sky, the film undergoes a radiative cooling process, reducing the surface temperature of the corrugated steel roof and significantly reducing solar radiation entering the room, thus slowing the rise in room temperature. In winter, with the heating layer facing the sky, the heating layer can absorb a large amount of solar radiation, raising the indoor temperature. In summary, the application of this invention to the surface of color steel tiles can significantly improve the living conditions of traditional steel tile houses, and it has broad application prospects in the field of building materials.

[0061] Obviously, the described embodiments are some embodiments and application examples of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A thin film that can switch between radiative cooling and photothermal heating, characterized in that: From top to bottom, it includes a scattering layer, a PDMS thin film layer, and a photothermal heating layer; the scattering layer is a PE film doped with nano-zinc oxide; the photothermal heating layer is a PE film doped with carbon black; The scattering layer is composed of multiple scattering unit films stacked together; the scattering unit film is composed of a pure PE film and a PE film doped with nano-zinc oxide; the nano-zinc oxide is oleophilic nano-zinc oxide particles; The doping mass fraction of the zinc oxide nanoparticles in the PE film is 3%-5%; the particle size of the zinc oxide nanoparticles is 300 nm-500 nm. The thickness of the PDMS thin film layer is 80-120 μm.

2. The switchable thin film for radiative cooling and photothermal heating according to claim 1, characterized in that: The thickness of the scattering layer is 200-500 μm.

3. The switchable thin film for radiative cooling and photothermal heating according to claim 1, characterized in that: The thickness of the photothermal heating layer is 80-120 μm.

4. The method for preparing a switchable thin film for radiative cooling and photothermal heating according to any one of claims 1 to 3, characterized in that: Includes the following steps: After melting PE polymer particles, nano zinc oxide particles are doped into them. The PE masterbatch doped with nano zinc oxide is obtained by extrusion granulation. Then, the PE masterbatch doped with nano zinc oxide and the pure PE masterbatch are added to two extruders respectively. After the two masterbatches are melted, a scattering unit film is formed by combining the PE film doped with nano zinc oxide and the pure PE film. The scattering unit film is stacked by a stack multiplier to form a multilayer structure of up to 128 layers to obtain the scattering layer. After the PDMS pre-cured material and curing agent are mixed evenly, they are coated on the surface of the scattering layer and cured to form a PDMS thin film layer on the surface of the scattering layer. A black PE film, which is a PE film doped with carbon black, is laminated onto the surface of the PDMS film layer to obtain the target product.

5. A type of textile garment, characterized in that: The textile garment includes a cotton fiber layer, and two sides of the cotton fiber layer are respectively laminated with a switchable film for radiative cooling and photothermal heating as described in any one of claims 1 to 3; among the switchable films located on both sides of the cotton fiber layer, the scattering layer of one switchable film faces outward, and the photothermal heating layer of the other switchable film faces outward.

6. A type of color steel sheet, characterized in that: The two sides of the color steel tile are respectively attached with a switchable film for radiative cooling and photothermal heating as described in any one of claims 1 to 3. The scattering layer of one switchable film on the two sides of the color steel tile faces outward, and the photothermal heating layer of the other switchable film faces outward.

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