A multi-layer thermal management flexible material and preparation method thereof

Abstract

The present invention belongs to the field of thermal management and radiative cooling technology, and specifically relates to a multilayer thermal management flexible material and its preparation method that achieves all-season thermal management performance through radiative cooling and photon recovery. The present invention provides a multilayer thermal management flexible material, which at least includes: a radiative cooling fabric layer facing the sky, the radiative cooling fabric layer is prepared by coating boron nitride particles on the surface of glass fiber bundles on glass fiber cloth, the radiative cooling fabric layer has high reflectivity in the 0.3-2.5μm band of solar radiation, with an average reflectivity of up to 95.6%, and high emissivity in the 8-13μm infrared band, with an average emissivity of up to 96.3%, and has cooling performance during the summer daytime; and a flexible metal base layer facing the object to be thermally managed, the flexible metal base layer has high reflectivity in both the 0.3-2.5μm band of solar radiation and the mid-infrared band, and achieves a heat preservation function by recovering heat radiation on winter nights.

Description

Technical Field

[0001] The present invention belongs to the technical field of thermal management and radiative cooling, and specifically relates to a multilayer thermal management flexible material that achieves all-season thermal management performance through radiative cooling and photon recycling, and a preparation method thereof. Background Art

[0002] Thermal management is crucial for energy conservation and environmental protection. Currently, heating, ventilation, and air-conditioning systems account for approximately 50% of building energy consumption and approximately 20% of the world's total energy consumption. Extreme weather and high energy costs are placing an urgent demand on the development of low-cost, high-efficiency thermal management technologies. Passive thermal management, which can cool and insulate the indoor environment of buildings and vehicles without the use of external power sources, is a promising technology for future carbon-neutral energy systems. Passive temperature regulation is also crucial for extraterrestrial applications such as spacecraft thermal control and ecosystem modification in extraterrestrial habitats. The main challenge of passive thermal management is that traditional materials cannot fully function at opposite temperature extremes, such as cooling during the summer daytime and warmth during the winter nights. According to guidelines from the American Society of Heating, Ventilation, and Air-Conditioning Engineers (ASHRAE), minimizing temperature differences during extreme weather conditions will significantly reduce annual energy consumption for HVAC energy costs and mitigate the impact of peak demand on the power grid.

[0003] Radiative cooling is a passive cooling technology that blocks solar radiation (0.3-2.5 μm) and transmits infrared photons through the atmospheric window (8-13 μm) to the cold outer space at 3k, thereby achieving a cooling effect. Nighttime radiative cooling technology has been widely studied and applied to specific scenarios such as cooling buildings. This technology only requires the design of a surface with high emissivity at the atmospheric window. However, compared to the night when there is no solar radiation, the demand for daytime radiative cooling is more realistic. In order to achieve daytime radiative cooling that can be lower than the ambient temperature, the designed surface needs to meet both high reflectivity in the solar radiation band and high emissivity at the atmospheric window. In addition, in order to adapt to specific application scenarios, such as outdoors, the prepared materials also have different degrees of requirements for strength, flexibility, wear resistance, fire resistance and large-scale preparation.

[0004] In previous studies, radiative cooling materials with the potential for large-scale preparation are mainly based on polymer-based materials. Whether it is a film with a polymer as the matrix or a fabric material spun and woven by itself, the polymer itself is easily degraded, easy to wear, and not resistant to high temperatures, which has great limitations in practical applications. The structure with a metal substrate on which the film is deposited is complex and precise to prepare, which is not conducive to large-scale production. Therefore, there is an urgent need to study a new material with efficient refrigeration performance, which can be prepared on a large scale and has good weather resistance. In addition, radiative cooling cannot be turned off automatically, resulting in overcooling and additional energy consumption in cold weather. To achieve radiative cooling throughout the season, other mechanisms need to be combined to achieve thermal insulation performance on cold winter nights. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a multilayer thermal management flexible material and a preparation method thereof that achieves all-season thermal management performance through radiative cooling and photon recovery, so as to realize passive all-season thermal management while overcoming the problem that radiative cooling cannot be automatically shut down.

[0006] To achieve the above objectives, the solutions adopted by the present invention are as follows:

[0007] In a first aspect, the present invention provides a multi-layer thermal management flexible material comprising at least:

[0008] A sky-facing radiant cooling fabric layer, made by coating the surface of glass fiber strands of a glass fiber cloth with boron nitride particles. The radiant cooling fabric layer has high reflectivity in the 0.3-2.5 μm solar radiation band and high transmittance in the 8-13 μm infrared band, providing cooling performance during summer daytime; and

[0009] The flexible metal base layer faces the heat-managed object and has high reflectivity in both the 0.3-2.5 μm band and the mid-infrared band of solar radiation, achieving a heat preservation function by recycling heat radiation on winter nights.

[0010] Preferably, the glass fiber cloth is classified according to the stitching method into uniaxial fabric, biaxial fabric, triaxial fabric and quadriaxial fabric, classified according to the single-layer weaving method into plain weave, twill, satin, rib and mat weave, and classified according to the material into alkali-free glass fiber cloth, medium-alkali glass fiber cloth and high-alkali glass fiber cloth, wherein the high-purity glass fiber cloth basically does not contain alkali metal oxides and the silicon dioxide content is greater than 99.99%; the flexible metal base layer includes at least one of aluminum foil, silver foil and copper foil.

[0011] Preferably, the thickness of the radiant cooling fabric layer is 0.1 mm-3 mm, and the thickness of the flexible metal base layer is 0.001 mm-0.1 mm.

[0012] Preferably, the surface of the glass fiber bundle is further treated with a silane coupling agent before being coated with the boron nitride particles.

[0013] Preferably, the invention further comprises a resin adhesive layer located between the radiant cooling fabric layer and the flexible metal base layer.

[0014] In a second aspect, the present invention further provides a method for preparing the multilayer thermal management flexible material as described above, comprising the following steps:

[0015] Dispersing boron nitride particles in a solvent to prepare a uniformly dispersed suspension;

[0016] A layer of silane coupling agent is coated on the surface of the glass fiber bundle of the glass fiber cloth, and after the solvent evaporates, the boron nitride particles in the boron nitride suspension are evenly coated on the surface of the glass fiber bundle of the glass fiber cloth coated with the silane coupling agent;

[0017] After the solvent evaporates, the radiant cooling fabric layer is obtained by high-temperature treatment;

[0018] The flexible metal base layer is compounded onto the radiant cooling fabric layer.

[0019] Preferably, the particle size of the boron nitride particles is 1-10 μm, the concentration of the prepared suspension is 1-50 mg / ml, and the solvent is selected from anhydrous ethanol, ethylene glycol or other polar solvents; the silane coupling agent includes all coupling agents with a molecular formula of YR-Si(OR)3, wherein Y is an organic functional group and SiOR is a silaneoxy group, preferably selected from at least one of A151 (vinyltriethoxysilane), A171 (vinyltrimethoxysilane), A172 (vinyltri(β-methoxyethoxy)silane) and KH550 (γ-aminopropyltriethoxysilane).

[0020] Preferably, the coating method of a layer of silane coupling agent on the surface of the glass fiber bundle of the glass fiber cloth includes a drop coating method, a spraying method or a dipping method; the method of uniformly coating the boron nitride particles in the boron nitride suspension on the surface of the glass fiber bundle of the glass fiber cloth coated with the silane coupling agent includes a drop coating method, a spraying method or a dipping method.

[0021] Preferably, the high temperature treatment step uses a muffle furnace or a vacuum tube furnace to heat at 600° C.-800° C. for 3-5 hours.

[0022] Preferably, the flexible metal base layer is bonded to the radiant cooling fabric layer by a hot pressing process using a chemical bonding method of a resin adhesive, or is assembled with the radiant cooling fabric layer by a physical bonding method such as clamping, riveting, or splicing, or the flexible metal base layer is deposited on the surface of the radiant cooling fabric layer by a pulsed laser deposition method.

[0023] According to the present invention, a multilayer thermal management flexible material is provided, comprising a radiative cooling fabric layer facing the sky and a metal base layer facing the object being heat-managed. The radiative cooling fabric layer is prepared by coating the surface of glass fiber strands of a glass fiber cloth with boron nitride particles. In the fabric, core-shell fibers encapsulated by dense boron nitride particles form a completely opaque flexible film material with high reflectivity in the 0.3-2.5μm wavelength band of solar radiation. Both silicon dioxide and boron nitride are ceramic materials with high emissivity in the 8-13μm infrared wavelength band, meeting the spectral properties required for radiative cooling materials. The spectral properties of the fabric enable the multilayer thermal management flexible material to provide cooling performance during summer daytime. Furthermore, the fabric maintains stable spectral performance after extreme temperature testing, simulated aerospace environment testing, and corrosive environment testing. The metal base layer is made of flexible metal foils such as aluminum foil, silver foil, and copper foil. Through a photon recycling mechanism, it prevents excessive heat loss from the object being heat-managed during winter nights, providing thermal insulation. The photon recycling mechanism is a concept that has been widely used in devices and systems in recent years. It is used to describe the reflection and reabsorption of photons in various wavelength ranges. Flexible metal foil has high reflectivity in both the solar band and the mid-infrared band, which can further reduce the absorption of sunlight during the summer daytime. At the same time, the heat preservation function is achieved by recycling thermal radiation on winter nights, thereby effectively turning off the radiation cooling. There is no phase change or manual flipping, which plays a role in keeping warm on winter nights. On cold nights, the heat of the thermally managed object will be emitted into the surrounding environment in the form of thermal radiation, causing its temperature to gradually decrease. The flexible metal substrate has high reflectivity throughout the mid-infrared band, so that the photons in the mid-infrared band are reflected back to the thermally managed object, realizing its recycling. On a macro scale, it is manifested as the thermal radiation is recycled to achieve a thermal insulation effect.

[0024] Compared with the existing technology, the beneficial effects of the present invention are:

[0025] (1) The present invention provides a multi-layer thermal management flexible material, which consists of a radiant cooling fabric layer facing the sky and a metal base layer facing the object to be thermally managed. The radiant cooling fabric layer is prepared by coating boron nitride particles on the surface of the glass fiber bundle of the glass fiber cloth, and has high-efficiency radiant cooling performance, providing cooling performance during the summer daytime. The metal base layer is a flexible metal foil such as aluminum foil, which prevents the heat-managed object from excessively losing heat during the winter nights through a photon recovery mechanism, thereby providing thermal insulation performance. In the designed outdoor experiment, the double-sided flexible material can reduce the temperature of the electric vehicle cabin by 27.7°C on hot daytime and keep warm by 4.6°C on cold nights. The multi-layer thermal management flexible material provided by the present invention has all-season thermal management performance, can achieve cooling during the summer daytime and heat preservation during the winter nights, is low-cost, can be prepared in batches, has strong weather resistance, and has broad application prospects.

[0026] (2) The present invention provides a radiative cooling fabric composed of ceramic fibers, with an average reflectivity of up to 95.6% in the 0.3-2.5μm solar radiation band and an average emissivity of up to 96.3% in the 8-13μm infrared band, exhibiting the spectral properties required for efficient radiative cooling. The fabric maintains stable spectral properties after undergoing extreme temperature testing, simulated aerospace environment testing, and corrosive environment testing. It exhibits strong weather resistance and high stability, and has broad potential for outdoor applications, such as thermal management on the surfaces of buildings, automobiles, electric vehicles, and containers. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a schematic structural diagram of the multi-layer thermal management flexible material involved in the present invention; in the figure, 1 is the radiant cooling fabric layer facing the sky, and 2 is the flexible metal base layer facing the object to be thermally managed.

[0028] Figure 2 SEM image of a single glass fiber bundle of the radiative cooling fabric of the multi-layer thermal management flexible material according to the present invention.

[0029] Figure 3 : These are the reflection spectra of the radiative cooling fabrics of samples 1 and 2 in the embodiments of the present invention.

[0030] Figure 4 : This is the emission spectrum of the radiative cooling fabrics of samples 1 and 2 in the embodiments of the present invention.

[0031] Figure 5 It is the ratio of the solar band emissivity / atmospheric window emissivity of samples 3, 4, 5, 6, 7, and 8 of the radiative cooling fabrics in the embodiments of the present invention before and after the corresponding treatment.

[0032] Figure 6 : This is the reflection spectrum of the double-sided flexible material of Sample 9 in the embodiment of the present invention.

[0033] Figure 7 This is the emission spectrum of the double-sided flexible material of Sample 9 in the embodiment of the present invention.

[0034] Figure 8 The data in the figure shows the temperature of the electric vehicle cabin covered with the double-sided flexible material and the temperature of the electric vehicle cabin without any covering.

[0035] Figure 9 Temperature data from a 36-hour outdoor test in cold weather demonstrates its thermal insulation performance on winter nights. The data in the figure shows the temperature of an electric vehicle cabin covered with double-sided flexible material and the temperature of an electric vehicle cabin without any covering. DETAILED DESCRIPTION

[0036] The present invention provides a multilayer thermal management flexible material and a preparation method thereof that achieves all-season thermal management performance through radiative cooling and photon recycling, so as to realize passive all-season thermal management and overcome the problem that radiative cooling cannot be automatically shut down.

[0037] The present invention provides a multi-layer thermal management flexible material, such as Figure 1 As shown, including:

[0038] The radiation cooling fabric layer 1 facing the sky is prepared by coating the surface of the glass fiber bundles of the glass fiber cloth with boron nitride particles (such as Figure 2 As shown), the radiative cooling fabric layer has high reflectivity in the solar radiation band of 0.3-2.5 μm and high emissivity in the infrared band of 8-13 μm, and has cooling performance during the day in summer; and

[0039] The flexible metal base layer 2 faces the object to be heat-managed. The flexible metal base layer has high reflectivity in the 0.3-2.5 μm band of solar radiation and the mid-infrared band, and achieves a heat preservation function by recycling heat radiation on winter nights.

[0040] The technical solution of the present invention is further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited to these embodiments. Any changes or equivalent substitutions that do not deviate from the concept of the present invention are included in the scope of protection of the present invention.

[0041] Example 1:

[0042] The glass fiber cloth used as the substrate is a 2D plain weave cloth, the yarn is high-purity glass fiber, and the silica content is greater than 99.99%. The boron nitride particles are selected from h-BN with a particle size of 1 to 2 μm, and the prepared suspension concentration is 10 mg / ml. The solvent is anhydrous ethanol, and the mixture is ultrasonically mixed for 1 hour to achieve uniformity. The silane coupling agent is selected from γ-aminopropyltriethoxysilane (KH550) with a concentration of 5wt% and anhydrous ethanol as the solvent. The glass fiber cloth is cut into 20cm*20cm square pieces of cloth. First, a layer of silane coupling agent is sprayed on. After the solvent evaporates, the boron nitride suspension is sprayed multiple times to prepare a coating. After the solvent evaporates, the fabric is treated at 600°C in a muffle furnace for three hours to produce radiant cooling fabric sample 1 (hereinafter referred to as sample 1).

[0043] Example 2:

[0044] The glass fiber cloth used as the substrate is a 2.5D shallow cross-linked cloth, with high-purity glass fiber yarns and a silica content greater than 99.99%. The boron nitride particles are h-BN with a particle size of 1-2 μm, and the suspension is prepared at a concentration of 10 mg / ml in anhydrous ethanol as the solvent, and ultrasonic mixing is performed for 1 hour to achieve uniformity. The silane coupling agent is γ-aminopropyltriethoxysilane (KH550) at a concentration of 5 wt% in anhydrous ethanol as the solvent. The glass fiber cloth is cut into 20 cm x 20 cm square pieces. A layer of silane coupling agent is first sprayed on. After the solvent evaporates, the boron nitride suspension is sprayed multiple times to prepare a coating. After the solvent evaporates, the fabric is treated at 600°C in a muffle furnace for three hours to produce radiant cooling fabric sample 2 (hereinafter referred to as sample 2).

[0045] Example 3:

[0046] Samples 1 and 2 in the examples were cut into 4 cm*4 cm square pieces, and then the reflectivity and transmittance tests of the UV-Vis-NIR spectrum (0.3-2.5 μm) and the emissivity and transmittance tests of the Fourier transform infrared spectrum (2.5-16.7 μm) were performed. The emissivity of the corresponding bands was calculated (emissivity (%) = 1-reflectivity (%) - transmittance (%)). The corresponding spectrum results are shown in Figure 3 ,4.

[0047] Example 4:

[0048] Sample 2 from Example 2 was cut into 4 cm x 4 cm square pieces. To test its high-temperature resistance, it was burned for 20 seconds using a butane torch, with a flame temperature reaching 1400°C. The sample did not burn and remained intact. The treated fabric became radiant cooling fabric Sample 3 (hereinafter referred to as Sample 3).

[0049] Example 5:

[0050] Sample 2 from Example 2 was cut into 4 cm x 4 cm square pieces. To test its low-temperature resistance, Sample 2 was submerged in liquid nitrogen (-196°C) for 10 minutes for cryogenic treatment. The fabric retained its intact morphology and structure after treatment. This treated fabric is designated as radiant cooling fabric Sample 4 (hereinafter referred to as Sample 4).

[0051] Example 6:

[0052] Sample 2 in the embodiment was cut into 4cm*4cm square pieces and subjected to a hypergravity oscillation test to test its suitability for use in a space environment. In the vibration experiment, the overall acceleration level of the three-axis random vibration (Grms) was 10 times the acceleration of gravity (g), which is equivalent to the gravity acceleration level when a rocket takes off, and the frequency range was between 0 and 2000Hz. After the oscillation, no boron nitride coating on the fiber surface was found to have fallen off. The treated fabric is radiant cooling fabric sample 5 (hereinafter referred to as sample 5).

[0053] Example 7:

[0054] Sample 2 from Example 2 was cut into 4cm x 4cm square pieces and subjected to surface tension testing to verify its suitability for space environments. In the surface tension test, the metafabric was immersed in deionized water, left to stand for 24 hours, and then air-dried. The treated fabric became radiant cooling fabric sample 6 (hereinafter referred to as sample 6).

[0055] Example 8:

[0056] Sample 2 from Example 2 was cut into 4 cm x 4 cm square pieces. To test its resistance to strong acid, the sample was immersed in a strong acid solution (HCl, pH = 1) for 24 hours and then rinsed with deionized water. The treated fabric became radiant cooling fabric sample 7 (hereinafter referred to as sample 7).

[0057] Example 9:

[0058] Sample 2 from Example 2 was cut into 4 cm x 4 cm square pieces. To test its resistance to strong alkaline conditions, the sample was immersed in a strong alkaline solution (NaOH, pH = 13) for 24 hours and then rinsed with deionized water. The treated fabric became radiant cooling fabric Sample 8 (hereinafter referred to as Sample 8).

[0059] Example 10:

[0060] Samples 3, 4, 5, 6, 7, and 8 in the embodiment were tested for reflectivity and transmittance of the UV-Vis-NIR spectrum (0.3-2.5 μm) and emissivity and transmittance of the Fourier infrared spectrum (2.5-16.7 μm) before and after treatment, and the emissivity of the corresponding bands was calculated (emissivity (%) = 1-reflectivity (%)-transmittance (%)), as well as the average reflectivity of the solar band (0.3-2.5 μm) and the average emissivity of the atmospheric window band (8-13 μm). The spectral performance stability of the samples after extreme treatment was judged based on the ratio results. The results show that the average reflectivity of the solar band and the average emissivity of the atmospheric window are almost unchanged before and after treatment, which proves the stability of its spectral performance. The ratio spectrum results are shown in Figure 5 .

[0061] Example 11:

[0062] A radiant cooling fabric was prepared according to the sample preparation method of Sample 2 in Example 2, using aluminum foil as the metal substrate, and the two were assembled into a double-sided flexible material by hot pressing (hereinafter referred to as Sample 9).

[0063] Example 12:

[0064] Sample 9 in Example 11 was cut into 4 cm*4 cm square pieces, and the reflectivity and transmittance tests of the UV-Vis-NIR spectrum (0.3-2.5 μm) and the emissivity and transmittance tests of the Fourier transform infrared spectrum (2.5-16.7 μm) were performed on the front and back sides respectively. The emissivity of the corresponding bands was calculated (emissivity (%) = 1-reflectivity (%) - transmittance (%)). The corresponding spectrum results are shown in Figure 6 ,7.

[0065] Example 13:

[0066] Design an outdoor experiment to test the thermal management performance of the double-sided flexible material. To investigate the thermal management effect of the material on electric vehicles, two commercial electric vehicles (stainless steel structure, 3 doors and 2 seats, length 2488mm, width 1506mm, height 1607mm, battery capacity 28kWh) were parked in an open area outdoors. One was completely covered with sample 9, and the other was not covered with any material. A T-type thermocouple was used to detect the temperature inside the cabins of the two vehicles, and a data acquisition instrument was used to record the temperature. The tests were carried out in hot weather and cold weather, and the temperature results are shown in the table below. Figure 8 ,9.

[0067] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A multi-layer thermal management flexible material, characterized in that: At least: A sky-facing radiant cooling fabric layer, made by coating the surface of glass fiber strands of a glass fiber cloth with boron nitride particles. The radiant cooling fabric layer has high reflectivity in the 0.3-2.5 μm solar radiation band and high transmittance in the 8-13 μm infrared band, providing cooling performance during summer daytime; and A flexible metal substrate facing the object being heat-managed has high reflectivity in both the 0.3-2.5 μm solar radiation band and the mid-infrared band, achieving thermal insulation on winter nights by recycling thermal radiation. The preparation method of the multilayer thermal management flexible material is as follows: Dispersing boron nitride particles in a solvent to prepare a uniformly dispersed suspension; the particle size of the boron nitride particles is 1-10 μm, the concentration of the prepared suspension is 1-50 mg / ml, and the solvent is selected from anhydrous ethanol, ethylene glycol or other polar solvents; A layer of silane coupling agent is coated on the surface of the glass fiber bundle of the glass fiber cloth, and after the solvent evaporates, the boron nitride particles in the boron nitride suspension are evenly coated on the surface of the glass fiber bundle of the glass fiber cloth coated with the silane coupling agent; the silane coupling agent includes all coupling agents with a molecular formula of YR-Si(OR)3, wherein Y is an organic functional group and SiOR is a silaneoxy group; After the solvent evaporates, the radiant cooling fabric layer is obtained by high temperature treatment, that is, heating at 600-800 degrees Celsius for 3-5 hours; The flexible metal base layer is compounded onto the radiant cooling fabric layer via a resin adhesive layer.

2. The multi-layer thermal management flexible material according to claim 1, characterized in that The glass fiber cloth is classified into uniaxial fabric, biaxial fabric, triaxial fabric and quadriaxial fabric according to the stitching method, and is classified into plain weave, twill, satin weave, rib and mat weave according to the single-layer weaving method. It is classified into alkali-free glass fiber cloth, medium-alkali glass fiber cloth and high-alkali glass fiber cloth according to the material. The flexible metal base layer includes at least one of aluminum foil, silver foil and copper foil.

3. The multi-layer thermal management flexible material according to claim 1, characterized in that The thickness of the radiant cooling fabric layer is 0.1 mm to 3 mm, and the thickness of the flexible metal base layer is 0.001 mm to 0.1 mm.

4. The multi-layer thermal management flexible material according to claim 1, characterized in that The silane coupling agent is selected from at least one of A151 (vinyl triethoxysilane), A171 (vinyl trimethoxysilane), A172 (vinyl tris(β-methoxyethoxy) silane) and KH550 (γ-aminopropyl triethoxysilane).

5. The multi-layer thermal management flexible material according to claim 1, characterized in that The coating method of coating a layer of silane coupling agent on the surface of the glass fiber bundle of the glass fiber cloth includes a drop coating method, a spraying method or a dipping method; the method of making the boron nitride particles in the boron nitride suspension uniformly coated on the surface of the glass fiber bundle of the glass fiber cloth coated with the silane coupling agent includes a drop coating method, a spraying method or a dipping method.

6. The multi-layer thermal management flexible material according to claim 1, characterized in that The high temperature treatment step uses a heating device such as a muffle furnace or a vacuum tube furnace.

7. The multi-layer thermal management flexible material according to claim 1, characterized in that The flexible metal base layer is bonded to the radiant cooling fabric layer by a hot pressing process using a chemical bonding method of a resin adhesive, or assembled with the radiant cooling fabric layer by a physical bonding method such as clamping, riveting, or splicing, or the flexible metal base layer is deposited on the surface of the radiant cooling fabric layer by a pulsed laser deposition method.

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