Composite thermochromic film and preparation and application thereof

Abstract

The invention discloses a composite thermochromic film as well as preparation and application thereof. The composite thermochromic film is prepared from tungsten-doped vanadium dioxide, polyvinyl alcohol and paraffin. The film can be tightly adhered to an adhesion surface, and has a function of adjusting the transmissivity of visible light and near-infrared light.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sunlight transmittance materials, in particular to a preparation and application of a composite thermochromic film, and more particularly to a preparation and application of a composite thermochromic film capable of adjusting the transmittance of near-infrared light while adjusting the transmittance of visible light. BACKGROUND

[0002] Thermochromic materials are special materials that change color under thermal stimulation. The basic principle mainly involves phase transition and crystal structure change of the material. When the temperature of the material reaches a certain critical value, the crystal structure of the material changes, which leads to changes in its optical properties. This phenomenon is particularly evident in solid-state thermochromic materials, such as vanadium dioxide and certain polymer materials. That is, the substance exhibits reversible color change when the temperature changes.

[0003] Adhesion of such thermochromic films to glass becomes thermochromic glass, which is widely used in the fields of architecture, automobiles, electronic devices, etc. Because it does not need to be driven by additional energy, it has the characteristics of energy saving and environmental friendliness. By automatically adjusting the light transmittance, thermochromic glass can provide a more comfortable indoor environment for users and reduce the discomfort caused by direct sunlight. In some applications, thermochromic glass can provide privacy protection when needed, while in other cases it can remain transparent and provide good visibility. When the temperature rises, the light transmittance of the glass changes, affecting its color and transparency.

[0004] As we all know, sunlight plays an important role in human life. 99.9% of the energy in solar radiation is concentrated in the ultraviolet region (<0.4 μm), the visible light region (0.4-0.76 μm), and the infrared region (>0.76 μm). The visible light region accounts for about 50% of the total solar radiation energy, the infrared region accounts for about 43%, and the ultraviolet region accounts for only about 7%. Therefore, the visible light region and the infrared region account for the vast majority of solar radiation.

[0005] Currently, the research on thermochromic glass mostly focuses on controlling the transmittance of visible light or controlling the transmittance of near-infrared light, that is, there is no material that can regulate both visible light and near-infrared light, and the single regulation of visible light transmittance or near-infrared light transmittance cannot meet the cooling demand in all scenarios, for example: the design of single regulation of visible light transmittance but unable to regulate near-infrared light transmittance will significantly limit the cooling effect; and the design of single regulation of near-infrared light transmittance but unable to regulate visible light transmittance still has limitations in specific scenarios. If two kinds of functional materials are simply layered, they cannot withstand such high temperatures because polyvinyl alcohol film is not resistant to ultra-high temperature and will be directly scalded. On the other hand, the currently popular thermochromic glass on the market has a high manufacturing cost, with high core material costs: high-quality glass substrates, and conductive films, electrolyte layers, ion storage layers, and other multi-layer structure materials. The manufacturing process is complex: involving magnetron sputtering, vacuum coating, high-temperature treatment, thin film coating, and other complex processes, which require expensive equipment and high production environment (such as a dust-free workshop). SUMMARY

[0006] The present application aims to overcome the above-mentioned defects and designs a composite thermochromic film which can not only adjust the transmittance of visible light but also change the transmittance of near-infrared region, significantly reducing the heating ability of sunlight on the interior of the building. The specific design concept is: prevent visible light from penetrating by paraffin phase change, and prevent near-infrared light from penetrating by vanadium dioxide phase change. Such a thermochromic film can make the interior of a house model drop by more than 10℃ under 1 times solar radiation, and can maintain high-efficiency and stable performance after 100 cycles of testing.

[0007] To achieve the above-mentioned purpose, the present application provides the following specific technical solutions:

[0008] The present application provides a composite thermochromic film prepared from tungsten-doped vanadium dioxide, polyvinyl alcohol, and paraffin.

[0009] In addition, the present application also provides a preparation method of the above-mentioned composite thermochromic film, which is as follows:

[0010] After the tungsten-doped vanadium dioxide particles are uniformly dispersed in the solution, they are mixed with polyvinyl alcohol and added with paraffin solution treated by ultrasonic emulsification to fully disperse the components, and then the mixed system is left to evaporate, finally obtaining the composite thermochromic film.

[0011] The ultrasonic emulsification treated paraffin solution is a micro-droplet of the paraffin in the continuous phase emulsified into a micro-nano size by using an ultrasonic homogenizer. Through the process, the paraffin is changed from a macroscopic layered state into a microcosmic uniform dispersion system, so that the paraffin can stably exist in the water in the form of micro-droplets. Thus, at low temperature, the film will not become abnormally hard and inflexible due to solidification of the continuous phase paraffin, and the process is irreversible, that is, even if the film surface is heated, no change will occur. Therefore, the film must be treated by ultrasonic emulsification before use.

[0012] Further, the preparation method of the composite thermochromic film provided by the present application has the following characteristics and specific steps:

[0013] S1. After adding deionized water to the vanadium dioxide particles doped with tungsten elements to form a mixed solution, fully dispersed by stirring and ultrasonic treatment;

[0014] S2. Add polyvinyl alcohol powder to the dispersed S1 mixed solution and stir uniformly;

[0015] S3. After the solid paraffin is melted into liquid paraffin, the temperature is kept unchanged, and the polyvinyl alcohol is mixed;

[0016] S4. Homogenize and emulsify the paraffin polyvinyl alcohol mixed solution obtained in S3 and stand still;

[0017] S5. Add cooling water to the emulsion of S4 and stand still;

[0018] S6. Heat the solution obtained in S5 and stir to make the solution uniform and not stratified, then add the vanadium dioxide precursor solution doped with tungsten elements obtained in S2, and continue to heat and stir; preferably, the heat preservation temperature is maintained at about 50℃, which is the melting point of paraffin;

[0019] S7. Pour the solution obtained in S6 into a culture dish and stand still until the water in the culture dish evaporates, to obtain a composite thermochromic film.

[0020] Further, the preparation method of the composite thermochromic film provided by the present application has the following characteristics and specific steps:

[0021] The concentration of the vanadium dioxide solution is 0.1wt‰-6wt‰.

[0022] Further, the preparation method of the composite thermochromic film provided by the present application has the following characteristics and specific steps:

[0023] Further, the preparation method of the composite thermochromic film also has the characteristics that in S3, the phase transition temperature of the paraffin is 45-50 DEG C, the paraffin concentration in the mixed solution is 5-15wt%, the temperature of the mixed solution is maintained at 50-55 DEG C, the polyvinyl alcohol solution concentration used is 0.5-2wt%.

[0024] Further, the preparation method of the composite thermochromic film also has the characteristics that in S4, the power of the ultrasonic homogenizer is 300-400W, the ultrasonic is opened for 13-17s, the ultrasonic is closed for 3-7s, and the process is continued for 5-8min, and the solution after processing is placed for 10-30min.

[0025] Further, the preparation method of the composite thermochromic film also has the characteristics that in S5, the cooling water temperature is 0-10 DEG C, and the volume added is 1-2 times the volume of the S4 mixed solution.

[0026] Further, the preparation method of the composite thermochromic film also has the characteristics that in S6, the volume ratio of the vanadium dioxide precursor solution obtained in S2 to the solution obtained in S5 is 3-5:1.

[0027] Further, the preparation method of the composite thermochromic film also has the characteristics that in S7, the heating temperature of the solution is 50-55 DEG C.

[0028] In addition, the application also provides the application of the composite thermochromic film as a cooling material.

[0029] Effects and advantages of the present invention:

[0030] The composite thermochromic film is adhered to the glass of the house model, under the irradiation of sunlight, the film temperature gradually rises, when reaching the phase transition temperature of the vanadium dioxide doped with tungsten elements, the reversible metal-insulator phase transition of the vanadium dioxide metal particles occurs, at this time, the characteristics of weakening the near-infrared light transmittance are excited, and then the ability of the near-infrared light to heat the house interior is weakened.

[0031] Under the further heating of sunlight, the temperature of the thin film will be higher, and when the phase transition temperature of the paraffin is reached, the paraffin particles will change from solid particles to liquid droplets, at this time, the refractive index of paraffin and polyvinyl alcohol will change from matching state to mismatching state, thereby realizing the adjustment of the transmittance of the thin film to visible light. Specifically, the refractive index of polyvinyl alcohol is 1.49-1.52, the refractive index of solid paraffin is 1.490-1.510, and the refractive index of liquid paraffin is 1.4178-1.4600, that is, at low temperature, the refractive index of the two is matched, and the transmittance of visible light is high, and at high temperature, the refractive index of the two is not matched, and the transmittance of visible light is low. Therefore, the color of the composite thermochromic film will change significantly: from transparent color to pure white, specifically, at this time, the paraffin has been emulsified into small particles, here the color of the film depends on whether the refractive index of polyvinyl alcohol and paraffin is matched, at low temperature, the two are matched, that is, transparent, visible light can pass through, at high temperature, the two are not matched, visible light cannot pass through, the color of the film becomes opaque pure white, therefore, at this time, the visible light transmittance is reduced, and the visible light is blocked to heat the interior of the house. In summary, the ability of the house with the composite thermochromic glass to be heated by sunlight is greatly attenuated, and the temperature rise in the house is reduced.

[0032] In summary, the composite thermochromic film of the present application uses raw materials such as polyvinyl alcohol, paraffin and tungsten-doped vanadium dioxide particles. The above materials are simple to obtain and low in cost, the preparation process is simple and pollution-free, and no later maintenance is required, greatly reducing the cost and having good application prospect.

[0033] The composite thermochromic film of the present application uses paraffin and vanadium dioxide which are sensitive to temperature, and can respond to stimulation faster when heated by sunlight, so that it can weaken the heating of sunlight in time.

[0034] The composite thermochromic film of the present application has a reasonable structure, and after the final water evaporation, a very stable film is formed.

[0035] In the composite thermochromic film of the present application, the thermal conductivity of paraffin is 0.13-0.2 W / (m·k), which is much lower than the thermal conductivity of glass 1 W / (m·k), so it can also play a good heat insulation effect, effectively solving the problem of heat conduction from outdoor hot summer to indoor low temperature environment through windows, and the temperature in the house with the composite thermochromic glass is more stable. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 The preparation flow chart of the composite thermochromic film provided in the embodiment is shown in the figure.

[0037] Figure 2Images of the sample prepared in Example 1, the top image showing the state at low temperature and the bottom image showing the state at high temperature;

[0038] Figure 3 Temperature curves inside houses with ordinary glass panels under 1x solar radiation in Examples 1, 2, and 3. Detailed Implementation

[0039] This invention is capable of various modifications and embodiments, and therefore specific embodiments are illustrated and described in the accompanying drawings. However, this is not intended to limit the invention to specific implementations, but should be understood to include all modifications, equivalents, and even substitutions that fall within the spirit and scope of this invention.

[0040] like Figure 1 As shown, the composite thermochromic film provided in this embodiment is prepared by the following method:

[0041] S1. Preparation of vanadium dioxide precursor solution doped with tungsten: Place an appropriate amount of vanadium dioxide particles doped with tungsten into a beaker, add deionized water, and place the vanadium dioxide particle solution doped with tungsten into a magnetic stirrer and stir.

[0042] The proportion of tungsten dopant in vanadium dioxide is 2wt%, the concentration of vanadium dioxide solution ranges from 0.1wt‰ to 6wt‰, the stirring time with a magnetic stirrer is 12 to 18 hours, and the temperature is room temperature (25℃).

[0043] S2. The vanadium dioxide particle solution doped with tungsten obtained in step S1 is placed in an ultrasonic instrument for further processing to make the particles more evenly dispersed. Then, polyvinyl alcohol powder is added and stirred. By stirring the vanadium dioxide particle solution doped with tungsten for a long time, it can be evenly dispersed in the solution.

[0044] The ultrasonic treatment time is 8-10 min, the polyvinyl alcohol concentration is 7-12 wt%, and the stirring environment temperature is 85-100℃.

[0045] Regarding the concentration of polyvinyl alcohol, when the concentration is too high, it is not conducive to dissolution and it becomes viscous, making it difficult to remove air bubbles; when the concentration is too low, it is not suitable for film formation.

[0046] S3. Emulsified paraffin solution: Solid paraffin is heated and melted into liquid paraffin, and then mixed with a polyvinyl alcohol solution while maintaining a constant temperature.

[0047] The phase transition temperature of paraffin is 45~50℃, the paraffin concentration in the mixed solution is 5-15wt%, the temperature of the mixed solution is maintained at 50~55℃, and the concentration of polyvinyl alcohol solution used is 0.5-2wt%.

[0048] S4. Place the paraffin polyvinyl alcohol mixture obtained in step S3 into an ultrasonic homogenizer for emulsification. After the solution is completely ultrasonically dispersed, remove the beaker and then let the emulsified solution stand.

[0049] The ultrasonic homogenizer has a power of 300-400W, with ultrasonic on for 13-17s, ultrasonic off for 3-7s, and a duration of 5-8min. The treated solution is then left to stand for 10-30min.

[0050] The purpose of ultrasonic treatment of the paraffin solution is to emulsify the continuous phase of paraffin into micro- and nano-sized droplets, so that the paraffin changes from a macroscopic layered state to a microscopic, uniformly dispersed system that exists stably in water.

[0051] S5. Pour the cooling water into the emulsion treated in step S4, and then let it stand;

[0052] In step S5, the cooling water temperature is 0-10℃, and the added volume is 1-2 times the volume of the mixture in step S4.

[0053] S6. Heat the solution obtained in step S5 and stir it to make the solution uniform and no longer separate into layers. Add the vanadium dioxide precursor solution doped with tungsten obtained in step S2 and continue to heat and stir. Preferably, the heat preservation temperature is maintained at around 50°C, the melting point of paraffin.

[0054] In step S6, the heating temperature of the solution is 50~55℃;

[0055] The volume ratio of the vanadium dioxide precursor solution obtained in step S2 to the solution obtained in step S5 is 3-5:1.

[0056] S7. Pour the solution obtained in step S6 into a petri dish and let it stand until the water in the petri dish evaporates to obtain a composite thermochromic film.

[0057] In step S7, the evaporation temperature of the water in the mixed solution is room temperature 25°C, and the standing time is determined based on the evaporation results. During the above-mentioned standing evaporation and passive compaction process, the water in the mixed solution evaporates at room temperature to form a film on the adhesion surface. During this evaporation process, the film is tightly adhered to the adhesion surface due to atmospheric pressure.

[0058] Preferred embodiment 1.

[0059] This embodiment 1 provides a composite thermochromic film, which is prepared by the following method:

[0060] S1: Preparation of vanadium dioxide precursor solution doped with tungsten: Place an appropriate amount of vanadium dioxide particles doped with tungsten into a beaker, add deionized water, and place the vanadium dioxide particle solution doped with tungsten into a magnetic stirrer and stir at room temperature for 12 hours; specifically, the ratio of vanadium dioxide doped with tungsten is 2wt%, and the concentration of vanadium dioxide solution is 1wt‰.

[0061] S2: Place the vanadium dioxide particle solution doped with tungsten obtained in step S1 into an ultrasonic instrument and sonicate for 10 minutes to make the particles more evenly dispersed. Then add polyvinyl alcohol powder (to a concentration of 10 wt%) and stir at 95°C.

[0062] S3: Emulsified paraffin solution: Melt 10g of solid paraffin at 55℃ to become liquid paraffin, keep the temperature between 50-55℃, and add 100ml of 1wt% polyvinyl alcohol solution and mix.

[0063] S4: Place the 300W ultrasonic homogenizer into the paraffin polyvinyl alcohol mixed solution obtained in step S3 for emulsification. The emulsification process is to turn on the ultrasonicator for 15 seconds, turn off the ultrasonicator for 5 seconds, and continue for 5 minutes. After the solution is completely dispersed by ultrasonication, remove the beaker and let the emulsified solution stand for 10 minutes.

[0064] S5: Pour 150ml of 10℃ cooling water into the emulsion treated in step S4, and then let it stand.

[0065] S6: Heat the solution obtained in step S5 to about 50°C and stir it to prevent the white part in the solution from separating into layers. Add 4 times the volume of the vanadium dioxide precursor solution obtained in step S2 and keep it at 50°C while stirring.

[0066] S7: Pour the solution obtained in step S6 into a petri dish and let it stand. After the water in the petri dish evaporates at room temperature, a composite thermochromic film is obtained. The standing time is generally about 5 days, but it may vary depending on the size and container.

[0067] The finished product image is as follows: Figure 2 As shown, this composite thermochromic film can adhere tightly to the surface and can adjust the transmittance of visible and near-infrared light.

[0068] Preferred embodiment 2.

[0069] The composite thermochromic film provided in Example 2 is prepared by the following method:

[0070] S1: Preparation of vanadium dioxide precursor solution doped with tungsten: Place an appropriate amount of vanadium dioxide particles doped with tungsten into a beaker, add deionized water, and place the vanadium dioxide particle solution doped with tungsten into a magnetic stirrer and stir at room temperature for 12 hours; specifically, the proportion of tungsten doping in step S1 is 2wt%, and the concentration of vanadium dioxide solution is 3wt‰.

[0071] S2: Place the vanadium dioxide particle solution doped with tungsten obtained in step S1 into an ultrasonic instrument for 10 minutes to make the particles more evenly dispersed. Then add polyvinyl alcohol powder (to a concentration of 10 wt%) and stir at 95°C.

[0072] S3: Emulsified paraffin solution: Melt 10g of solid paraffin at 55℃ to become liquid paraffin, keep the temperature between 50-55℃, and add 100ml of 1wt% polyvinyl alcohol solution and mix.

[0073] S4: Place a 300W ultrasonic homogenizer into the paraffin-polyvinyl alcohol mixture obtained in step S3 for emulsification. The emulsification process involves ultrasonication on for 15 seconds, ultrasonication off for 5 seconds, and continuing for 5 minutes. After the solution is completely ultrasonically dispersed, remove the beaker. Let the emulsified solution stand for 10 minutes.

[0074] S5: Pour 150ml of 10℃ cooling water into the emulsion treated in step S4, and then let it stand.

[0075] S6: Heat the solution obtained in step S5 to about 55°C and stir it to prevent the white part in the solution from separating into layers. Add 4 times the volume of the vanadium dioxide precursor solution obtained in step S2 and keep it at 50°C while stirring.

[0076] S7: Pour the solution obtained in step S6 into a petri dish and let it stand. After the water in the petri dish evaporates at room temperature, a composite thermochromic film is obtained. The standing time is generally about 5 days, but it may vary depending on the size and container.

[0077] Preferred Example 3

[0078] A method for preparing a composite thermochromic thin film is basically the same as that in Example 1, except that...

[0079] The concentration of vanadium dioxide solution in S1 is 6 wt‰.

[0080] Comparative Example 1 (Vanadium dioxide composition without tungsten doping)

[0081] A method for preparing a composite thermochromic film is basically the same as that in Example 1, except that step S1 is omitted; step S2 is to prepare a 10 wt% polyvinyl alcohol solution; in step S6, the vanadium dioxide precursor solution obtained in step S2 is not required to be added, but the 10 wt% polyvinyl alcohol solution prepared in step S2 is added, and the volume ratio of the polyvinyl alcohol solution obtained in step S2 to the solution obtained in step S5 is 4:1.

[0082] Comparative Example 2 (without paraffin components)

[0083] A method for preparing a composite thermochromic film is basically the same as that in Example 2, except that steps S3-S6 are omitted. Step S7 involves pouring the solution obtained after step S2 into a petri dish and letting it stand at room temperature until the water in the petri dish evaporates, thereby obtaining a vanadium dioxide thermochromic film.

[0084] Product effect comparison experiment

[0085] 1. Product color

[0086] The films prepared in Examples 1, 2, and 3 have colors ranging from light to dark, and can be applied to corresponding scenarios. Example 1 is suitable for scenarios with high light requirements at low temperatures, such as windows in houses. Example 2, because its color is darker than that of Example 1, is suitable for rooms where privacy is important. Example 3 is suitable for outdoor scenarios with independent light sources to avoid direct sunlight.

[0087] 2. Temperature control performance test

[0088] The composite thermochromic films prepared in Example 2, Comparative Example 1, and Comparative Example 2 were tested on the windows of a house model. The temperature changes on the film surface and inside the house under 1x solar radiation are shown in the graph. Figure 3 As shown, the ambient temperature during the test was 23-25℃.

[0089] As can be seen from the temperature curves, when the indoor temperature gradually flattens out (the indoor temperature is relatively constant), due to the presence of paraffin in Comparative Example 1, the vanadium dioxide film in Comparative Example 2 undergoes a phase transition. The indoor temperature changes in these two sets of data are approximately 8°C lower than those in the ordinary glass group. Comparing Example 2 with Comparative Examples 1 and 2, the indoor temperature change is significantly lower than that of the ordinary glass control group, reaching 11°C. This demonstrates a clear cooling effect on the room temperature.

[0090] The function and effect of this embodiment:

[0091] This embodiment uses low-cost paraffin wax as a base, polyvinyl alcohol as a stabilizer and film-forming agent, and vanadium dioxide particles as a co-existing component to develop a novel composite thermochromic film based on the principle of thermochromism. Tungsten-doped vanadium dioxide metal particles, which regulate near-infrared light transmittance, are introduced. Below their phase transition temperature, they are in the monoclinic (M) phase, not weakening near-infrared light transmission; however, above their phase transition temperature, they are in the rutile (R) phase, suppressing near-infrared light transmission. Polyvinyl alcohol is a biodegradable polymer in natural environments, reducing environmental pollution. Paraffin wax is a phase change material; at low temperatures, it is a solid and opaque, but at high temperatures, it melts into a liquid, becoming colorless and transparent. Therefore, paraffin wax regulates visible light, but this optical property differs from the optical properties required in our daily lives, so some treatment is needed to reverse its optical properties. Solid paraffin and polyvinyl alcohol have relatively small differences in refractive index, while liquid paraffin and polyvinyl alcohol have relatively large differences in refractive index. However, if paraffin is mixed directly with polyvinyl alcohol without treatment, it cannot adjust the transmittance of visible and near-infrared light bands. Therefore, by emulsifying paraffin into small particles and mixing it with polyvinyl alcohol, the transmittance of visible and near-infrared light bands can be adjusted simultaneously.

[0092] Those skilled in the art should understand that variations can be implemented by combining existing technology with the above embodiments, which will not be elaborated here. Such variations do not affect the essence of the present invention, and will not be elaborated here either.

[0093] The preferred embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and the devices and structures not described in detail should be understood as being implemented in a manner common to the art. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention using the methods and techniques disclosed above, or modify them into equivalent embodiments with equivalent changes, without departing from the scope of the present invention. This does not affect the essential content of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the present invention are still within the scope of protection of the present invention.

Claims

1. A composite thermochromic film, characterized in that: It is made of tungsten doped with vanadium dioxide, polyvinyl alcohol, and paraffin.

2. The method for preparing a composite thermochromic thin film as described in claim 1, characterized in that: After tungsten-doped vanadium dioxide particles are evenly dispersed in a solution, they are mixed with polyvinyl alcohol and an ultrasonically emulsified paraffin solution is added to fully disperse the components. Then, the mixture is allowed to stand and evaporate to finally obtain a composite thermochromic film.

3. The method for preparing a composite thermochromic thin film as described in claim 1, characterized in that, It includes the following specific steps: S1. After adding deionized water to vanadium dioxide particles doped with tungsten to form a mixture, the mixture is then subjected to stirring and ultrasonic treatment to achieve full dispersion. S2. Add polyvinyl alcohol powder to the dispersed S1 mixture and stir until homogeneous; S3. After heating solid paraffin to melt it into liquid paraffin, mix it with polyvinyl alcohol while keeping the temperature constant; S4. Homogenize and emulsify the paraffin-polyvinyl alcohol mixture obtained in S3 and let it stand. S5. Add cooling water to the emulsion in S4 and let it stand; S6. Heat the solution obtained in S5 and stir until the solution is homogeneous and no longer separates into layers. Then add the vanadium dioxide precursor solution doped with tungsten obtained in S2 and continue to heat and stir. S7. Pour the solution obtained in S6 into a petri dish and let it stand until the water in the petri dish evaporates to obtain a composite thermochromic film.

4. The method for preparing a composite thermochromic thin film as described in claim 3, characterized in that: In S1, the proportion of tungsten doping is 2 wt%; The concentration range of the vanadium dioxide solution is 0.1wt‰ to 6wt‰.

5. The method for preparing a composite thermochromic thin film as described in claim 3, characterized in that: In S2, the ultrasonic treatment time is 8-10 min, the polyvinyl alcohol concentration is 7-12 wt%, and the stirring environment temperature is 85-100℃.

6. The method for preparing a composite thermochromic thin film as described in claim 3, characterized in that: In S3, the phase transition temperature of paraffin is 45~50℃, the paraffin concentration in the mixed solution is 5-15wt%, the temperature of the mixed solution is maintained at 50~55℃, and the concentration of the polyvinyl alcohol solution used is 0.5-2wt%.

7. The method for preparing a composite thermochromic thin film as described in claim 3, characterized in that: In S4, the power of the ultrasonic homogenizer is 300-400W, the ultrasonic on is 13-17s, the ultrasonic off is 3-7s, the duration is 5-8min, and the treated solution is left to stand for 10-30min.

8. The method for preparing a composite thermochromic thin film as described in claim 3, characterized in that: The cooling water temperature in S5 is 0-10℃, and the volume added is 1-2 times the volume of the S4 mixture.

9. The method for preparing a composite thermochromic thin film as described in claim 3, characterized in that: The heating temperature of the solution in S7 is 50~55℃.

10. The application of the composite thermochromic film as described in claim 1 or the composite thermochromic film prepared by the method described in any one of claims 2-9 as a cooling material.

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