Asymmetric spectrum composite film as well as preparation method and application thereof

Abstract

The invention discloses an asymmetric spectrum composite film as well as a preparation method and application thereof. The asymmetric spectrum composite film comprises an MXene substrate film and an infrared high-emissivity layer which is deposited on the substrate film and consists of hollow MXene spheres or a carbon-based material. Through a reasonable structural design, the two surfaces of the asymmetric spectrum composite film present different spectrum characteristics: the MXene substrate film surface has the characteristics of high absorption of sunlight wave bands and low emission of infrared wave bands; and the infrared high-emissivity layer has the characteristic of high infrared band emission. Based on the asymmetric spectrum composite film, efficient solar energy capture and targeted infrared energy transfer can be achieved, and the resistance in the water vapor escape process can be reduced, the mass transfer effect can be enhanced and the agricultural product drying efficiency can be remarkably improved by combining with the evenly-distributed microchannel structure.

Description

Technical Field

[0001] This invention relates to the field of materials, and more particularly to an asymmetric spectral composite film, its preparation method, and its applications. Background Technology

[0002] Post-harvest preservation and loss reduction of agricultural products are major issues in the field of food safety. However, fresh fruits and vegetables and other agricultural products generally have high moisture content, making them highly susceptible to severe losses due to mold, pests, and other contaminants, resulting in resource waste and economic losses. Drying, as an effective means of extending shelf life, inhibits the reproduction of microorganisms and pests by reducing water activity; however, traditional hot air drying consumes a huge amount of energy, accounting for about 15% of total industrial energy consumption, and has drawbacks such as high cost and high carbon emissions.

[0003] Direct solar drying offers a clean and low-consumption technological path for this purpose. This method utilizes direct sunlight to irradiate materials, using photon energy to excite the agricultural products' own photothermal conversion to achieve moisture evaporation. However, most agricultural products have poor spectral absorption characteristics, especially high reflectivity in the visible light band, resulting in low efficiency in converting solar energy into heat energy. This leads to a slow and limited drying process, fundamentally restricting its large-scale application.

[0004] To overcome this bottleneck, research has led to the development of solar-powered hot air drying devices. These devices absorb solar energy through independent collectors, heat the air, and then force convection to transfer the heat to the material, forming a multi-stage energy transfer chain of "solar energy - collector heat energy - air internal energy - material heat energy." However, in this indirect method, a large amount of heat energy is consumed in the air heating itself. The hot air also experiences heat loss during transport due to pipe cooling and incomplete heat exchange with the material, resulting in a low percentage of effective energy ultimately used for moisture evaporation. Furthermore, heating a large volume of air requires a long preheating time, the system starts slowly, and it exhibits poor responsiveness under intermittent sunlight conditions, limiting its adaptability. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide an asymmetric spectral composite film that exhibits different spectral characteristics on both sides and can perform efficient solar photothermal conversion; the second purpose is to provide a method for preparing the asymmetric spectral composite film and its applications.

[0006] Technical solution: The asymmetric spectral composite film of the present invention includes an MXene substrate film and an infrared high emissivity layer deposited on the substrate film, wherein the infrared high emissivity layer is composed of hollow MXene spheres or carbon-based materials.

[0007] Preferably, the thickness of the MXene substrate film is 0.2-10 μm, and the thickness of the infrared high emissivity layer is 10-100 μm.

[0008] Preferably, the MXene is a monolayer and / or few-layer Ti3C2; more preferably, the Ti3C2 is a few-layer structure with no more than 5 layers and a lateral sheet diameter of 0.5~10 μm; its surface termination group T x Including -O, -OH and / or -F; the Ti3C2 can be obtained by etching the Ti3AlC2MAX phase, and the etching method includes direct HF etching or in-situ etching of LiF with an acid system.

[0009] Preferably, the carbon-based material is graphene.

[0010] Preferably, the composite membrane is also uniformly provided with a plurality of through micropores, and the distance between any adjacent micropores is not less than 50 μm.

[0011] Preferably, the diameter of the micropores is 10~50 μm.

[0012] The method for preparing the asymmetric spectral composite film of the present invention includes the following steps: (a1) Mix and stir an MXene dispersion with a concentration of 1~1.5 mg / mL and a polymer microsphere dispersion with a concentration of 2~3 mg / mL to obtain an MXene microsphere dispersion; (a2) Filter the MXene dispersion with a concentration of 0.5~1 mg / mL to obtain the MXene base membrane, then add 1~1.5 times the volume of the MXene sphere dispersion obtained in step a1, filter and dry to obtain a bilayer membrane; (a3) The bilayer film obtained in step a2 is calcined under an inert atmosphere to obtain an asymmetric spectral composite film; or, (b1) Filter the MXene dispersion with a concentration of 0.5~1 mg / mL to obtain the MXene substrate membrane, then add 1~1.5 times the volume of the graphene dispersion with a concentration of 0.8~1.2 mg / mL, filter and dry to obtain the asymmetric spectral composite membrane.

[0013] Preferably, in the MXene dispersion, the polymer microsphere dispersion, and the graphene dispersion, the dispersion system is water.

[0014] Preferably, the diameter of the polymer microspheres in step a1 is 1~5 μm.

[0015] Preferably, the heating and calcination in step a3 specifically involves heating to 450-550°C at a heating rate of 4.5-5.5°C / min and holding the temperature for calcination for 0.5-2 hours.

[0016] Preferably, steps a3 and b1 further include: constructing uniformly distributed micropores on the composite membrane.

[0017] The application of the asymmetric spectral composite film described in this invention in solar photothermal conversion.

[0018] Preferably, the application is the drying of agricultural products using solar photothermal conversion.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: 1. Through reasonable structural design, the two sides of the asymmetric spectral composite membrane exhibit different spectral characteristics. The MXene substrate membrane has the characteristics of high absorption in the solar band and low emission in the infrared band, while the infrared high emissivity layer has the characteristics of high emission in the infrared band; 2. Based on the asymmetric spectral composite membrane, it is possible to achieve efficient solar energy capture and targeted infrared energy transfer. Combined with the uniformly distributed microporous structure, it can simultaneously reduce the resistance in the water vapor escape process, enhance the mass transfer effect, and significantly improve the drying efficiency of agricultural products. Attached Figure Description

[0020] Figure 1 This is a flowchart of the preparation process for asymmetric spectral composite films. Figure 2 Scanning electron microscopy image of the cross section of the asymmetric spectral composite film prepared in Example 1; Figure 3 Scanning electron microscopy image of the surface of the asymmetric spectral composite film prepared in Example 1; Figure 4 The spectral characterization diagram of the asymmetric spectral composite film prepared in Example 1; Figure 5 This is a schematic diagram of a photothermal dehydration device based on an asymmetric spectral composite membrane. Figure 6 Moisture content-time curve of the photothermal dehydration device based on the asymmetric spectral composite film prepared in Example 1 for drying double-petaled chamomile; Figure 7 The moisture content-time curve of the photothermal dehydration device based on the asymmetric spectral composite membrane prepared in Example 2 for the drying of Angelica sinensis; Figure 8 The spectral characterization diagram of the asymmetric spectral composite film prepared in Example 3; Figure 9 Moisture content-time curve of the photothermal dehydration device based on the asymmetric spectral composite film prepared in Example 3 for drying banana chips. Detailed Implementation

[0021] The technical solution of the present invention will be further described below.

[0022] Example 1: Preparation and Application of Asymmetric Spectroscopic Composite Films 1. Preparation of asymmetric spectral composite films The preparation process of asymmetric spectral composite films is as follows: Figure 1 As shown.

[0023] (1) Take 1.4 mL of Ti3C2 monolayer / few-layer dispersion with a concentration of 5 mg / L (purchased from Jilin Yiyi Technology Co., Ltd., catalog number YY201753LS), disperse it in 5 mL of deionized water to obtain dispersion A; take 2 mL of Ti3C2 monolayer dispersion with a concentration of 5 mg / L, disperse it in 10 mL of deionized water to obtain dispersion B; take 28 mg of polymethyl methacrylate (PMMA) microspheres with a diameter of 3 μm (purchased from Xi'an Qiyue Chuangke Biotechnology Co., Ltd., catalog number Q-0166053), disperse them in 10 mL of deionized water to obtain PMMA dispersion; (2) Place dispersion A and PMMA dispersion in a round-bottom flask and mix for 24 h to allow Ti3C2 monolayer and PMMA to self-assemble through electrostatic attraction to form MXene sphere dispersion; (3) Place dispersion B in a vacuum filtration flask, filter to obtain MXene base membrane, add MXene ball dispersion, continue filtration, and after filtration, dry at room temperature for 12 h, peel off the dried double membrane from the filter membrane. (4) The double-layer film obtained in step 3 is placed in a tube furnace and heated to 500°C at a heating rate of 5°C / min under an argon atmosphere, and kept at this temperature for 1 h to obtain an asymmetric spectral composite film. (5) The asymmetric spectral composite film was processed using laser direct writing technology to construct uniformly distributed through-hole micropores with a diameter of 25 μm.

[0024] 2. Characterization of asymmetric spectral composite films 2.1 Characterization of the morphology of asymmetric spectral composite films The microstructure of the asymmetric spectral composite film was characterized by scanning electron microscopy. Before testing, the sample was cut and fixed on the sample stage. The sample surface was then subjected to metal sputtering treatment. Subsequently, the surface and cross-sectional morphology of the sample were scanned and observed under accelerating voltages of 3–10 kV, and the film thickness, pore size, and pore spacing were measured and analyzed.

[0025] Scanning electron microscope images of asymmetric spectral composite films, as shown below Figures 2-3 As shown, the thickness of the MXene substrate film is about 2.5 μm, the thickness of the hollow MXene sphere structure layer is about 25 μm, the diameter of the interconnected micropore array distributed on the asymmetric spectral composite film is about 25 μm, and the pore spacing is about 100 μm.

[0026] 2.2 Characterization of Spectral Characteristics of Asymmetric Spectral Composite Films (1) The reflectance R and transmittance T of each surface were measured in the solar spectral band (0.28-2.5 μm) using the integrating sphere mode of the UV-VIS ultraviolet-visible spectrophotometer. (2) The reflectance R and transmittance T of each surface were measured in the infrared band (2.5-17 μm) using the integrating sphere mode of a Fourier transform infrared spectrometer (FTIR). (3) Calculate the absorptivity of each surface at each wavelength, using the following formula: absorptivity = 1 - TR.

[0027] The spectral characteristic curves plotted based on the absorbance are as follows: Figure 4 As shown, the MXene substrate film exhibits a selective spectrum, characterized by high absorption in the solar band (absorption rate ~0.87), low emission in the mid-infrared band (8-14 μm) (absorption rate ~0.25), and high infrared emissivity in the 2.5-17 μm infrared band (emissivity ~0.9), which meets the spectral requirements.

[0028] 3. Applications of asymmetric spectral composite films A photothermal dehydration device was constructed using an asymmetric spectral composite membrane as the evaporator for solar-driven infrared flower drying. The structure is as follows: Figure 5 As shown in the figure, the attached figures are labeled as follows: evaporator-1, air layer-2, agricultural product-3, container-4, and insulating foam-5.

[0029] The non-contact drying unit is configured as follows: evaporator diameter 40 mm, air layer 1 mm, inner wall diameter 34 mm, outer wall diameter 45 mm, container depth 7 mm, and insulating foam (low thermal conductivity porous foam with a thermal conductivity of 0.02 W·m). -1 ·K -1 (The thickness is 30 mm). Fresh double-flowered chamomile is placed in a container, and the evaporator is placed on top of the container, not in contact with the double-flowered chamomile.

[0030] The direct drying unit is configured as follows: no evaporator is installed, and all other conditions are the same as those of the non-contact drying unit, using direct solar energy for drying.

[0031] Both tests were conducted indoors under 1-day sun conditions, and the changes in moisture content over time during the drying process were recorded.

[0032] The curve of moisture content changing over time was fitted using exponential simulation, and the results are as follows: Figure 6 As shown, the drying constant of the non-contact drying group is 1.35, while that of the direct drying group is 0.3, a difference of 4.49 times. This proves that the rate of non-contact solar drying of double-petal chamomile based on this evaporator is much higher than that of direct solar drying of double-petal chamomile.

[0033] Example 2: Preparation and Application of Asymmetric Spectroscopic Composite Films 1. Preparation of asymmetric spectral composite films (1) Take 1.4 mL of Ti3C2 monolayer / few-layer dispersion with a concentration of 5 mg / L and disperse it in 5 mL of deionized water to obtain dispersion A; take 2 mL of Ti3C2 monolayer dispersion with a concentration of 5 mg / L and disperse it in 10 mL of deionized water to obtain dispersion B; take 28 mg of polymethyl methacrylate (PMMA) beads with a diameter of 3 μm and disperse them in 10 mL of deionized water to obtain PMMA dispersion; (2) Place dispersion A and PMMA dispersion in a round-bottom flask and mix for 24 h to allow Ti3C2 monolayer and PMMA to self-assemble through electrostatic attraction to form MXene sphere dispersion; (3) Place dispersion B in a vacuum filtration flask, filter to obtain MXene base membrane, add MXene ball dispersion, continue filtration, and after filtration, dry at room temperature for 12 h, peel off the dried double membrane from the filter membrane. (4) The double-layer film obtained in step 3 is placed in a tube furnace and heated to 500°C at a heating rate of 5°C / min under an argon atmosphere, and kept at this temperature for 1 h to obtain an asymmetric spectral composite film. (5) The asymmetric spectral composite film was processed using laser direct writing technology to construct uniformly distributed through-hole micropores with a diameter of 25 μm.

[0034] 2. Characterization of the morphology of asymmetric spectral composite films Scanning electron microscope images of the asymmetric spectral composite film were acquired. The results showed that the thickness of the MXene base film was about 2.5 μm, the thickness of the hollow MXene sphere structure layer was about 25 μm, the diameter of the interconnected micropore array distributed on the asymmetric spectral composite film was about 25 μm, and the pore spacing was about 100 μm.

[0035] 3. Applications of asymmetric spectral composite films A photothermal dehydration device was constructed using an asymmetric spectral composite membrane as the evaporator for solar-driven infrared drying of fresh Chinese medicinal herbs. The structure is the same as in Example 1.

[0036] The non-contact drying unit is configured as follows: evaporator diameter 40 mm, air layer 1 mm, inner wall diameter 34 mm, outer wall diameter 45 mm, container depth 7 mm, and insulating foam (low thermal conductivity porous foam with a thermal conductivity of 0.02 W·m). -1 ·K -1 Fresh 3mm thick slices of angelica are placed in a container, with the evaporator placed on top of the container, ensuring that the evaporator does not come into contact with the angelica.

[0037] The direct drying unit is configured as follows: no evaporator is installed, and all other conditions are the same as those of the non-contact drying unit, using direct solar energy for drying.

[0038] Both tests were conducted indoors under 1-day sun conditions, and the changes in moisture content over time during the drying process were recorded.

[0039] The curve of moisture content changing over time was fitted using exponential simulation, and the results are as follows: Figure 6 As shown, the drying constant of the non-contact drying group is 0.51, while that of the direct drying group is 0.23, a difference of 2.2 times. This proves that the rate of non-contact solar drying of Angelica sinensis based on this evaporator is much higher than that of direct solar drying of Angelica sinensis.

[0040] Example 3: Preparation and Application of Asymmetric Spectroscopic Composite Films 1. Preparation of asymmetric spectral composite films (1) Take 5 mL of 1 mg / L graphene dispersion (dispersion A, purchased from Jiangsu Xianfeng Nanomaterials Technology Co., Ltd., item number XF019); take 2 mL of 5 mg / L Ti3C2 monolayer / few-layer dispersion and disperse it in 10 mL of deionized water to obtain dispersion B; (2) Dispersion B was placed in a vacuum filtration flask and filtered to obtain an MXene substrate membrane. Then dispersion A was added and the filtration was continued. After filtration was completed, the membrane was dried at room temperature for 12 h. The dried asymmetric spectral composite membrane was peeled off from the filter membrane. (3) The asymmetric spectral composite film prepared in step 2 is processed using laser direct writing technology to construct uniformly distributed through-hole micropores with a diameter of 25 μm.

[0041] 2. Characterization of spectral characteristics of asymmetric spectral composite films (1) The reflectance R and transmittance T of each surface were measured in the solar spectral band (0.28-2.5 μm) using the integrating sphere mode of the UV-VIS ultraviolet-visible spectrophotometer. (2) The reflectance R and transmittance T of each surface were measured in the infrared band (2.5-17 μm) using the integrating sphere mode of a Fourier transform infrared spectrometer (FTIR). (3) Calculate the absorptivity of each surface at each wavelength, using the following formula: absorptivity = 1 - TR.

[0042] The spectral characteristic curves plotted based on the absorbance are as follows: Figure 8As shown, the MXene substrate film exhibits a selective spectrum, characterized by high absorption in the solar band (absorption rate ~0.87), low emission in the mid-infrared band (8-14 μm) (absorption rate ~0.25), and high infrared emissivity in the 2.5-17 μm infrared band (emissivity ~0.94), which meets the spectral requirements.

[0043] 3. Applications of asymmetric spectral composite films A photothermal dehydration device was constructed using an asymmetric spectral composite membrane as the evaporator for solar-driven infrared drying of fresh fruit. The structure is the same as in Example 1.

[0044] The non-contact drying unit is configured as follows: evaporator diameter 40 mm, air layer 1 mm, inner wall diameter 34 mm, outer wall diameter 45 mm, container depth 7 mm, and insulating foam (low thermal conductivity porous foam with a thermal conductivity of 0.02 W·W·m). -1 ·K -1 Fresh 3mm thick banana slices are placed in a container, with the evaporator placed on top of the container, ensuring that the evaporator does not contact the banana slices.

[0045] The direct drying unit is configured as follows: no evaporator is installed, and all other conditions are the same as those of the non-contact drying unit, using direct solar energy for drying.

[0046] Both tests were conducted indoors under 1-day sun conditions, and the changes in moisture content over time during the drying process were recorded.

[0047] The curve of moisture content changing over time was fitted using exponential simulation, and the results are as follows: Figure 9 As shown, the drying constant of the non-contact drying group is 0.50, while that of the direct drying group is 0.29, a difference of 1.7 times. This proves that the rate of non-contact solar drying of banana chips based on this evaporator is much higher than that of direct solar drying of banana chips.

Claims

1. An asymmetric spectral composite film, characterized in that, It includes an MXene substrate film and an infrared high emissivity layer deposited on the substrate film, the infrared high emissivity layer being composed of hollow MXene spheres or carbon-based materials.

2. The asymmetric spectral composite film according to claim 1, characterized in that, The MXene is an MXene with selective spectral properties.

3. The asymmetric spectral composite film according to claim 1, characterized in that, The composite membrane is also uniformly provided with multiple through-holes, and the distance between any two adjacent micropores is not less than 50 μm.

4. The asymmetric spectral composite film according to claim 3, characterized in that, The diameter of the micropores is 10~50μm.

5. A method for preparing the asymmetric spectral composite film according to claim 1, characterized in that, step include: (a1) Mix and stir an MXene dispersion with a concentration of 1~1.5 mg / mL and a polymer microsphere dispersion with a concentration of 2~3 mg / mL to obtain an MXene microsphere dispersion; (a2) Filter the MXene dispersion with a concentration of 0.5~1 mg / mL to obtain the MXene base membrane, then add 1~1.5 times the volume of the MXene sphere dispersion obtained in step a1, filter and dry to obtain a bilayer membrane; (a3) The bilayer film obtained in step a2 is calcined under an inert atmosphere to obtain an asymmetric spectral composite film; or, (b1) Filter the MXene dispersion with a concentration of 0.5~1 mg / mL to obtain the MXene substrate membrane, then add 1~1.5 times the volume of the graphene dispersion with a concentration of 0.8~1.2 mg / mL, filter and dry to obtain the asymmetric spectral composite membrane.

6. The preparation method according to claim 5, characterized in that, The diameter of the polymer microspheres described in step a1 is 1~5 μm.

7. The preparation method according to claim 5, characterized in that, The heating and calcination described in step a3 specifically involves heating to 450-550℃ at a heating rate of 4.5-5.5℃ / min and holding the temperature for calcination for 0.5-2 hours.

8. The preparation method according to claim 5, characterized in that, Steps a3 and b1 further include: constructing uniformly distributed micropores on the composite membrane.

9. The application of the asymmetric spectral composite film according to any one of claims 1 to 4 in solar photothermal conversion.

10. The application according to claim 9, characterized in that, The application involves using solar thermal conversion for drying agricultural products.

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