Photo-thermal conversion coating capable of being used for solar evaporator, preparation method of photo-thermal conversion coating and solar evaporator

Abstract

The invention belongs to the technical field of seawater desalination equipment, and particularly discloses a photothermal conversion coating capable of being used for a solar evaporator, a preparation method of the photothermal conversion coating and the solar evaporator. The specific preparation method of the photothermal conversion coating comprises the following steps: firstly, mixing the carbon nano tube, the black titanium dioxide, the dispersing agent, the defoaming agent and the volatilizable solvent to obtain mixed slurry; and then spraying the mixed slurry for multiple times by utilizing a hot air-assisted air spraying method to obtain the photothermal conversion coating for the solar evaporator. And the thickness is 30-80 [mu] m. The solar evaporator can be obtained by sequentially spraying waterborne polyurethane and a photothermal conversion coating on the substrate material with the water absorption rod. The prepared photothermal conversion coating has high hydrophilicity and high light absorption capacity, and the solar evaporator has high evaporation rate.

Description

Technical Field

[0001] The present invention relates to the technical field of seawater desalination equipment, and in particular to a photothermal conversion coating that can be used for a solar evaporator, a preparation method thereof, and a solar evaporator. Background Art

[0002] Water is the source of life, but at present, only 2.8% of the fresh water resources on the earth. In recent years, with the development of human society, the problem of fresh water shortage has become increasingly serious. In contrast, seawater resources account for as high as 97.2%. How to reasonably utilize seawater resources and convert seawater into available fresh water has become one of the important issues today. As a new type of clean energy, solar energy has the advantages of universality, pollution-free, and renewable, and shows great potential in the field of seawater desalination. Researchers have mainly studied seawater desalination from two aspects: photovoltaic and photothermal, and the rapid development has given birth to new technologies for seawater desalination photothermal materials. However, the realization of efficient seawater desalination by photothermal materials still faces technical and economic limitations. The complex preparation process, unstable materials, and high cost have become the main problems. Therefore, a simple and efficient photothermal conversion material is needed to assist seawater evaporation.

[0003] In response to the issue of seawater desalination, many new materials have emerged, especially new nanomaterials with high efficiency. Currently, the commonly used materials for seawater desalination are metal nanoparticles such as gold and copper, alumina nanoparticles, silica particles, graphene and its derivatives, and so on. Among them, black titanium dioxide has shown great application potential in the photothermal field in recent years due to its high light absorption ability and broad spectral absorption characteristics. Some research work has designed and prepared a low-temperature photothermal conversion material of black TiO2 / carbonized melamine sponge (CMF) by adjusting the energy band gap of titanium oxide nanoparticles, and encapsulated a supercapacitor with an activated carbon electrode in this conversion layer, and the specific capacitance of the capacitor has been significantly improved at low temperature. The excellent photothermal conversion performance of black titanium dioxide is also expected to be applied in the field of seawater desalination. Some research has wrapped a black titanium dioxide / graphene oxide nanocomposite film on expandable polyethylene foam, and the solar energy absorption is about 90% in the range of 200-1000 nm, and the solar thermal conversion efficiency of 69.1% can be achieved under one sun intensity. It is found that the combination of AgCu nanoparticles on the surface of the black titanium dioxide (BT) matrix can generate strong random plasma coupling, resulting in broadband absorption, and the absorbance is up to about 90% in the range of 190-2500 nm. Under the irradiation of one sun, the evaporation flux is 1.4 kg·m -2 *h -1 , and the efficiency is as high as 95.7%. Although the photothermal conversion performance of black titanium dioxide is excellent, such materials show strong hydrophobic properties, which greatly affects the rapid spreading of the water source at the bottom of the solar evaporator on the surface of the evaporator during the water evaporation process, thereby affecting the water evaporation efficiency.

[0004] Due to its high thermal conductivity, good light absorption performance, low cost and other advantages, carbon nanotubes have been widely concerned in the research of photothermal conversion. Using chemical vapor deposition method to grow a "Vantablack" coating with a carbon nanotube forest structure with multiple scattering characteristics on aluminum foil can absorb 99.96% of light, which is an excellent manifestation of the light absorption performance of carbon nanotubes. In addition, researchers constructed zinc oxide nanoneedle structures on the surface of carbon nanotube films based on electrochemical methods. The obtained films have excellent light absorption ability and energy transfer efficiency (76.71%), and can be applied to the surfaces of vehicles, ships, etc. to achieve photothermal anti-icing. At the same time, there are also studies on loading multi-walled carbon nanotubes on filter paper for seawater desalination. Using cotton thread as the water rising channel to assemble a multi-walled carbon nanotube disk evaporator, the evaporation rate of the obtained 0.7-MCED can reach 1.31 kg·m -2 *h -1 。Another researcher prepared a strong and super-black all-carbon nanotube hybrid membrane through vacuum filtration. The hybrid membrane consists of hydrophilic SWCNT bundles at the bottom and hydrophobic MWCNT with a nano-micro hierarchical structure at the top. Under one sun intensity, the thermal efficiency of the membrane with a small area (~4.5 cm 2 ) can reach 87.4%, and the heating efficiency of the large area (40 cm 2 ) is greater than 80%. It can be seen that carbon nanotubes not only have good light absorption performance, but also can be used as water transmission channels to achieve rapid spreading of water. However, it is usually difficult to prepare coatings with ultra-large sizes using the above processes, the preparation environment requirements are strict, the cost is high, and the light absorption performance and photothermal conversion efficiency are lower compared with the coatings of the present invention, and the solar energy cannot be utilized more efficiently.

[0005] Therefore, the present invention discloses a photothermal conversion coating for a solar evaporator, a preparation method thereof, and a solar evaporator, which ensure the high hydrophilicity of the photothermal conversion coating and improve the light absorption ability and evaporation rate. Summary of the Invention

[0006] In view of this, the present invention provides a photothermal conversion coating for a solar evaporator, a preparation method thereof, and a solar evaporator to solve the problems existing in the existing solutions, such as difficulty in preparing ultra-large size coatings, high cost, low photothermal conversion efficiency, and low solar energy utilization rate.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A preparation method of a photothermal conversion coating for a solar evaporator includes the following steps:

[0009] 1) Mix carbon nanotubes, black titanium dioxide, a dispersant, an antifoaming agent, and a volatile solvent to obtain a mixed slurry;

[0010] 2) Use hot air-assisted air spraying to spray the mixed slurry multiple times to obtain a photothermal conversion coating for a solar evaporator;

[0011] Among them, the thickness of the photothermal conversion coating for the solar evaporator is 30 - 80 μm.

[0012] Preferably, the mass ratio of the carbon nanotubes, black titanium dioxide, dispersant, antifoaming agent, and volatile solvent is 0.9 - 1.1: 1.5 - 3: 0.4 - 0.6: 0.4 - 0.6: 95 - 96.5.

[0013] Preferably, the length of the carbon nanotubes is 1.5 - 50 μm;

[0014] The particle size of the black titanium dioxide is 15 - 45 μm;

[0015] The dispersant includes polyoxyethylene polyoxypropylene polymer and / or ethoxylated polyether;

[0016] The antifoaming agent includes one or more of polydimethylsiloxane, propylene glycol methyl ether, and polyoxyethylene polyoxypropylene amine ether;

[0017] The volatile solvent includes ethanol and / or isopropanol.

[0018] Preferably, in step 2), the spraying temperature for the multiple sprays is independently 150 - 200 °C, the fixed spraying angle is independently 40 - 90°, and the thickness of each spray is 5 - 10 μm.

[0019] Preferably, for each spray in the multiple sprays, spraying is carried out after the volatile solvent in the mixed slurry after the previous spray has completely volatilized.

[0020] Another object of the present invention is to provide a photothermal conversion coating for a solar evaporator prepared by the above preparation method.

[0021] Another object of the present invention is to provide a preparation method for a solar evaporator, including the following steps:

[0022] S1. Assemble a water-absorbing rod and a substrate material to obtain a substrate material with a water-absorbing rod;

[0023] S2. Sequentially spray waterborne polyurethane and a photothermal conversion coating for a solar evaporator on the substrate material with a water-absorbing rod;

[0024] The photothermal conversion coating for the solar evaporator is the above-mentioned photothermal conversion coating for the solar evaporator.

[0025] Another object of the present invention is to provide a solar evaporator prepared by the above preparation method.

[0026] As can be seen from the above technical solutions, compared with the prior art, the present invention has the following beneficial effects:

[0027] The base material of the solar evaporator supports the evaporator on the water surface. The excellent heat insulation can avoid the loss of heat on the surface of the light absorption layer. The water under the base material is quickly transported to the evaporator surface through the water absorption rod. The photothermal conversion coating composed of carbon nanotubes and black titanium dioxide obtained by hot air-assisted air spraying has a loose and porous structure on the surface. Through the adhesion between CNTs, the rapid diffusion of water can be realized, guiding the incident of sunlight into the coating, and achieving almost total absorption of sunlight from multiple angles through multiple reflections. This characteristic, combined with the excellent photothermal conversion characteristics of black titanium dioxide (Black TiO2), realizes the rapid temperature rise of the surface light absorption layer, evaporates the water spreading on the coating surface, enhances the utilization rate of solar energy by the coating, and greatly improves the evaporation efficiency of water. Description of the Drawings

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained according to the provided drawings.

[0029] Figure 1 is the light reflectivity of photothermal conversion coatings with different thicknesses;

[0030] Figure 2 is the comparison diagram of solar radiation energy density and light absorption image of the photothermal conversion coating with a thickness of 43.30 μm;

[0031] Figure 3 is the contact angle test results of photothermal conversion coatings with different thicknesses;

[0032] Figure 4 is the coating adhesion test results of the photothermal conversion coating described in Embodiment 1 of the present invention;

[0033] Figure 5 is the surface microstructure of photothermal conversion coatings with different thicknesses, where, Figure 5Among them, (a1) is a 100 - fold magnification image of the coating with a thickness of 27.65μm, (a2) is a 1000 - fold magnification image of the coating with a thickness of 27.65μm, (a3) is a 20000 - fold magnification image of the coating with a thickness of 27.65μm, (b1) is a 100 - fold magnification image of the coating with a thickness of 43.3μm, (b2) is a 1000 - fold magnification image of the coating with a thickness of 43.3μm, and (b3) is a 20000 - fold magnification image of the coating with a thickness of 43.3μm;

[0034] Figure 6 is the cross - sectional microscopic morphology of the photothermal conversion coating prepared in Example 1; among them, Figure 6 in (a) is the SEM image of the cross - section of the coating with a thickness of 43.3μm, Figure 6 in (b) is the high - magnification SEM image of the cross - section of the coating with a thickness of 43.3μm, Figure 6 in (c) is the TEM image of the cross - section of the coating with a thickness of 43.3μm, Figure 6 in (d) is the high - magnification TEM image of the cross - section of the coating with a thickness of 43.3μm;

[0035] Figure 7 is the water absorption test diagram of the solar evaporator obtained in Example 1 of the present invention;

[0036] Figure 8 is the evaporation performance test of photothermal conversion coatings with different thicknesses;

[0037] Figure 9 is the comparative test result of the seawater desalination performance of evaporator samples with coatings of different surface materials. Detailed implementation manners

[0038] The present invention provides a preparation method of a photothermal conversion coating that can be used for a solar evaporator, including the following steps:

[0039] 1) Mix carbon nanotubes, black titanium dioxide, a dispersant, an antifoaming agent, and a volatile solvent to obtain a mixed slurry;

[0040] 2) Use the hot - air assisted air spraying method to spray the mixed slurry multiple times to obtain a photothermal conversion coating that can be used for a solar evaporator;

[0041] Among them, the thickness of the photothermal conversion coating that can be used for a solar evaporator is 30 - 80μm, and specifically can be 32.9μm, 36.3μm, 38.0μm, 40.2μm, 56.3μm, 76.8μm.

[0042] In the present invention, the surface microstructure of the photothermal conversion coating has an important influence on the light absorption rate of the photothermal conversion coating. The continuous increase in the thickness of the photothermal conversion coating causes the continuous change of the surface microstructure, thereby affecting the light absorption rate of the photothermal conversion coating.

[0043] In the present invention, the mass ratio of the carbon nanotubes, black titanium dioxide, dispersant, defoamer and volatile solvent is 0.9 to 1.1: 1.5 to 3: 0.4 to 0.6: 0.4 to 0.6: 95 to 96.5, preferably 1: 1.8 to 2.5: 0.5: 0.5: 95.5 to 96.2, and more preferably 1: 2: 0.5: 0.5: 96.

[0044] In the present invention, the length of the carbon nanotubes is 1.5 to 50 μm, specifically it can be 5 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm.

[0045] In the present invention, the particle size of the black titanium dioxide is 15 to 45 μm, specifically it can be 20 μm, 25 μm, 30 μm, 35 μm, 40 μm.

[0046] In the present invention, the dispersant includes polyoxyethylene polyoxypropylene polymer and / or ethoxylated polyether.

[0047] In the present invention, the defoamer includes one or more of polydimethylsiloxane, propylene glycol methyl ether and polyoxyethylene polyoxypropylene amine ether.

[0048] In the present invention, the volatile solvent includes ethanol and / or isopropanol.

[0049] In the present invention, the mixing in step 1) is preferably carried out under ultrasonic conditions to ensure the full dispersion of the carbon nanotubes and black titanium dioxide.

[0050] In the present invention, the spraying temperature for each of the multiple sprayings in step 2) is independently 150 to 200 °C, specifically it can be 155 °C, 160 °C, 165 °C, 170 °C, 175 °C, 180 °C, 185 °C, 190 °C, 195 °C; the fixed spraying angle is independently 40 to 90°, specifically it can be 45°, 50°, 55°, 60°, 65°, 70°, 75°, 80°, 85°; the thickness of each spraying is 5 to 10 μm, specifically it can be 6 μm, 7 μm, 8 μm, 9 μm.

[0051] In the present invention, for each of the multiple sprayings, spraying is carried out after ensuring that the volatile solvent in the mixed slurry after the previous spraying has completely volatilized.

[0052] In the present invention, multiple sprayings are beneficial for the carbon nanotubes to settle and adhere on the resin surface to build a porous structure; one-time long-time spraying is not conducive to the further volatilization of the anhydrous ethanol remaining on the surface of the carbon nanotubes in the coating, resulting in the influence of the tension of ethanol, making it impossible for the carbon nanotubes to uniformly build a porous structure on the resin surface.

[0053] In the present invention, in the hot air-assisted air spraying method, the use of hot air assistance enables the rapid evaporation of the volatile solvents in the mixed slurry, thereby forming a uniform porous structure. At the same time, the exposed carbon nanotubes effectively reduce the refractive index difference between the surface of the photothermal conversion coating and the air.

[0054] The present invention also provides a photothermal conversion coating prepared by the above preparation method and can be used for a solar evaporator.

[0055] The present invention also provides a preparation method of a solar evaporator, comprising the following steps:

[0056] S1. Assemble a water-absorbing rod and a substrate material to obtain a substrate material with a water-absorbing rod;

[0057] S2. Sequentially spray waterborne polyurethane and a photothermal conversion coating that can be used for a solar evaporator on the substrate material with a water-absorbing rod.

[0058] In the present invention, the purpose of spraying the waterborne polyurethane is to improve the adhesion of the photothermal conversion coating.

[0059] The present invention also provides a solar evaporator prepared by the above preparation method.

[0060] Next, the technical solutions in the embodiments of the present invention will be described clearly and completely. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0061] Example 1

[0062] 1. Materials

[0063] Multi-walled carbon nanotubes (CNTs) were provided by Shanghai Yaotian New Materials Technology Co., Ltd. (length: 15 μm). Black titanium dioxide (TiO2) spraying powder (purity 99%, 15 - 45 μm, model HWG63172) was provided by Huawei Rike Co., Ltd. The dispersant (model: HH-2176W) and the defoaming agent (model: AD20) were provided by Guangzhou Houhuan Chemical Auxiliary Co., Ltd. Absolute ethanol was purchased from a chemical reagent co., Ltd. Waterborne polyurethane (WPU, model: AH-1502F) was provided by Anda Huatai New Materials Co., Ltd. The water-absorbing rod (7 mm × 140 mm, cotton material) was purchased from Zhejiang Deqing Huanyu Pen Industry Co., Ltd. The high-density foam board (30 cm × 20 cm, polystyrene material) was purchased from Lind Trade Co., Ltd.

[0064] 2. Preparation of the substrate material for the solar evaporator

[0065] The high-density foam board is processed by laser cutting technology to prepare a circular substrate with a diameter of 8 cm and a thickness of 1 cm. The surface of the substrate is designed with evenly distributed circular water delivery channels with a diameter of 0.6 cm and 8 holes to ensure uniform water delivery to the surface of the evaporator.

[0066] 3. Assemble the water-absorbing rod and the substrate material

[0067] The 2-cm-long water-absorbing rod after cutting and processing is placed in the hole of the 1-cm-thick circular substrate. The surface of the 7-mm-diameter water-absorbing rod is elastic, and filling it into the 6-mm-diameter hole helps to fix the water-absorbing rod, obtaining a circular substrate with the water-absorbing rod protruding 0.5 cm on each of the upper and lower surfaces.

[0068] 4. Preparation of the photothermal conversion coating

[0069] Carbon nanotubes, black titanium dioxide, dispersant, defoamer and absolute ethanol with a mass ratio of 1:2:0.5:0.5:96 are fully mixed, and then ultrasonic treatment is carried out for 20 minutes at 20 kHz using a high-frequency ultrasonic machine to ensure the full dispersion of carbon nanotubes and titanium dioxide particles, obtaining a mixed slurry.

[0070] First, a layer of waterborne polyurethane is sprayed on the circular substrate with the water-absorbing rod to increase the coating adhesion. When the waterborne polyurethane cures to a high viscosity (4000 mPas), the above-mentioned mixed slurry is sprayed onto the surface of the circular substrate with the water-absorbing rod by hot air-assisted air spraying method. After all the remaining small amount of absolute ethanol has evaporated, spraying is repeated to obtain a 43.3-μm photothermal conversion coating for the solar evaporator and the solar evaporator (the hot air spraying process is carried out using a DELIXI two-stage constant-temperature hot air gun (DHCHGW21600, Anhui) and a HOKIA spray gun (W-71E, Jiangsu). The temperature of each spraying is 150 °C, and the fixed spraying angle is independently 90°. The thickness of each spraying is 5-10 μm. Based on the spraying principle, it is difficult to accurately control the thickness of each spraying, and it can be ensured to be within the range of 5-10 μm).

[0071] Experimental Example 1

[0072] The infrared thermal imaging (HM300, Guangzhou) was used to explore the photothermal conversion performance of the solar evaporator. The coating surface optical profile (Contour GT-K 3D, Bruker, Germany) was used to explore the influence of the coating surface structure on the light absorption performance. The contact angle measuring instrument (JC2000D, Shanghai) was used to measure the contact angle of coatings with different thicknesses. The coating thickness gauge (TT230, Shanghai) was used to measure the coating thickness. The atomic force microscope (Dimension ICON, Bruker, USA) was used to observe the microscopic morphology and element distribution of the coating surface. The ultraviolet-visible near-infrared spectrophotometer (Lambda 1050, USA) was used to measure the reflectivity of coatings with different thicknesses. The cross-cut test instrument (BEVS2202, Guangzhou) was used to test the adhesion of the coating.

[0073] The spraying thickness of Example 1 was adjusted to obtain photothermal conversion coatings with thicknesses of 27.6μm, 32.9μm, 36.3μm, 38.0μm, 40.2μm, 56.3μm, and 76.8μm respectively.

[0074] The light reflectivities of the above-mentioned photothermal conversion coatings with different thicknesses in the range of 25 - 80μm were tested respectively. The results are as Figure 1 shown. It can be seen through Figure 1 that the thicker the coating, the lower the light reflectivity. Finally, the light reflectivity stabilizes at about 0.60%, which is converted to a light absorption rate of 99.40%, and the highest light absorption rate reaches 99.50%. The comparison diagram of the solar radiation energy density and the light absorption image of the photothermal conversion coating with a thickness of 43.30μm is as Figure 2 shown. It can be seen through Figure 2 that in the wavelength range with the highest solar radiation energy density, the coating has a higher light absorption ratio, reaching up to 99.90% at most, indicating that the photothermal conversion coating prepared by the present invention can absorb most of the solar irradiance.

[0075] Contact angle test:

[0076] Pure water droplets were successively dropped on the surfaces of photothermal conversion coatings with different thicknesses through the contact angle tester. The test results are as Figure 3 shown. It can be seen through Figure 3 that the larger the thickness of the photothermal conversion coating, the smaller the contact angle of the photothermal conversion coating, and the better the hydrophilicity. The contact angles of the tested coatings with different thicknesses in the thickness range of 25 - 80μm (27.6μm, 32.9μm, 36.3μm, 43.3μm, 56.3μm, 76.8μm) change from 123.83° to extremely small. Finally, the pure water droplets can quickly spread on the coating surface. The analysis results show that excellent hydrophilicity can be obtained when the coating thickness exceeds 40μm.

[0077] Interface bonding performance of the multi-coating system and the substrate:

[0078] The cross - hatch method was used to quantitatively characterize the coating adhesion. Aiming at the problem that the high - density foam substrate is too flexible and prone to introducing test errors, a rigid aluminum plate was selected as the alternative substrate and the same coating process parameters as in Example 1 were maintained. According to the ASTM D3359 - 09 standard, a precision cutting tool was used to prepare a vertical cross - grid (spacing 1 mm) on the surface of the photothermal conversion coating. Subsequently, a pressure - sensitive tape was applied and quickly peeled off, and a soft brush was used to remove the interface debris. As evaluated by the grading standard in Table S1, the test results are as follows: Figure 4 As described, no coating peeling was observed on the surface of the specimen, and the adhesion grade reached the highest level 0, indicating that there is excellent interfacial bonding strength between the photothermal conversion coating and the substrate. This result verifies that using the aluminum plate as a rigid substrate can effectively improve the adhesion test accuracy, and at the same time confirms that the current coating process can form a stable and reliable interlayer structure.

[0079] Effect of photothermal conversion coatings with different thicknesses on light absorption performance:

[0080] The surface microstructure of the photothermal conversion coating was characterized by scanning electron microscopy (SEM), and the results are as follows: Figure 5 As shown, among them, Figure 5 (a1), (a2), and (a3) in it are SEM images of the coating with a thickness of 27.65 μm at different magnifications, and (b1), (b2), and (b3) are SEM images and element distribution maps of the coating with a thickness of 43.3 μm at different magnifications. The SEM results show that with the increase in the number of spraying times, the thickness of the photothermal conversion coating gradually increases, and the porous light - absorbing microstructures formed on the surface become denser. This dense porous structure can effectively increase the light - scattering sites, thereby improving the light absorption efficiency on the surface of the photothermal conversion coating. In addition, a uniform distribution of black titanium dioxide particles was also observed on the surface of the photothermal conversion coating, and these particles further enhanced the photothermal conversion performance of the photothermal conversion coating.

[0081] Specifically, as in Figure 5 (a1), when the thickness of the photothermal conversion coating is 27.65 μm, the slurry fails to completely cover the resin substrate, resulting in some exposed areas on the surface of the photothermal conversion coating. As in Figure 5 (b1), when the thickness of the photothermal conversion coating is 43.3 μm, an obvious porous overlapping morphology is formed on the surface of the photothermal conversion coating. This structure not only increases the specific surface area but also provides more active sites for water transport and photothermal conversion. The experimental results show that the increase in the thickness of the photothermal conversion coating significantly improves its light absorption performance, providing an important basis for optimizing the performance of solar evaporators.

[0082] The elemental distribution on the surface of the photothermal conversion coating prepared in Example 1 was analyzed by EDS elemental scanning, and the results are as follows: Figure 5As shown in (b1), it can be obtained that titanium (Ti) and carbon (C) elements are evenly distributed on the surface of the photothermal conversion coating.

[0083] The cross-sectional microstructure of the photothermal conversion coating prepared in Example 1 is as Figure 6 shown, where Figure 6 (a) in is the SEM image of the cross-section of the 43.3 μm thick coating, Figure 6 (b) in is the high-magnification SEM image of the cross-section of the 43.3 μm thick coating, Figure 6 (c) in is the TEM image of the cross-section of the 43.3 μm thick coating, Figure 6 (d) in is the high-magnification TEM image of the cross-section of the 43.3 μm thick coating. Analysis shows that the structure of this photothermal conversion coating is formed by a mixture of carbon nanotubes and black titanium dioxide particles covering a resin film on a substrate material. Part of the carbon nanotubes is immersed in the resin, and the remaining CNTs are exposed to form a carbon nanotube network. The black titanium dioxide particles are enveloped in the CNTs network, thereby enhancing the adhesion and structural stability of CNTs and black titanium dioxide particles on the surface of the photothermal conversion coating. At the same time, the structure of the mixture of carbon nanotubes and black titanium dioxide particles is also beneficial for the absorption of sunlight and photothermal conversion. Sunlight enters the porous microstructure of the CNTs network on the surface of the photothermal conversion coating for multiple scattering and absorption. The photothermal conversion ability of the added black titanium dioxide particles is higher than that of CNTs, which assists in the light-to-heat conversion, thus taking into account both the light absorption and photothermal conversion efficiency of the photothermal conversion coating. The photothermal conversion coating prepared by the present invention can exhibit excellent effects in the application of photothermal conversion fields such as seawater desalination.

[0084] Water diffusion rate:

[0085] Place the material prepared in Example 1 on a horizontal tabletop. In a dry environment at room temperature and without wind, put it into a petri dish with a diameter of 9 cm filled with water, ensuring that the water does not overflow when the sample is immersed. Record the water surface diffusion time of the solar evaporator under normal light, as Figure 7 It can be seen that the solar evaporator obtained in Example 1 of the present invention has strong water absorption. At the 2nd second, the water absorption rod is full of water and diffuses to the sample platform. At the 6th second, the water can completely spread on the surface with a diameter of 8 cm.

[0086] Under the condition of simulated 1 sun light irradiation (100 mW*cm -2 ), irradiate the solar evaporator prepared in Example 1 and use an infrared thermal imager to monitor the change of its surface temperature in real time. The test results are as Figure 8As shown, it can be seen that the evaporator rapidly heats up to 83.1 °C within 40 s. As the illumination time further increases, the temperature change tends to stabilize and finally fluctuates within the range of 84.02 - 87.91 °C. The evaporator as a whole exhibits excellent photothermal conversion efficiency, providing a reliable technical basis for efficient solar-driven water evaporation.

[0087] Seawater desalination performance:

[0088] Comparative tests on the seawater desalination performance of evaporator samples with different surface material coatings were carried out. Under the light intensity condition of 100 mW·cm -2 , within 40 min, the evaporation rate of pure water (denoted as water) and the seawater desalination rates of evaporators with black titanium dioxide particles on the coating surface (Example 1, denoted as CNTs / black TiO2) and without black titanium dioxide particles (only different from Example 1 in that black titanium dioxide is not added, denoted as CNTs) were respectively tested. The test results are as Figure 9 shown. It can be found that the mass loss of pure water is the slowest, and its bulk water evaporation rate is 0.5 g·m -2 ·h -1 . For the evaporator with black titanium dioxide particles on the coating surface, the total mass of water is lost faster with time compared to the evaporator without black titanium dioxide particles. Finally, it is calculated that the evaporation rate of the evaporator without titanium dioxide particles on the surface is 1.31 g·m -2 ·h -1 , and the evaporation rate of the evaporator with titanium dioxide particles is 1.48 g·m -2 ·h -1 , which is 2.96 times the bulk water evaporation rate.

[0089] Example 2

[0090] 1. Materials

[0091] Multi-walled carbon nanotubes (CNTs) were provided by Shanghai Yaotian New Material Technology Co., Ltd. (length: 50 μm). Black titanium dioxide (TiO2) spraying powder (purity 99%, 15 μm) was provided by Huawei Ruike Co., Ltd. The dispersant (model: HH-2176W) and defoamer (model: AD20) were provided by Guangzhou Houhuan Chemical Auxiliary Co., Ltd. Absolute ethanol was purchased from the Chemical Reagent Co., Ltd. Waterborne polyurethane (WPU, model: AH-1502F) was provided by Anda Huatai New Material Co., Ltd. The water-absorbing rod (7 mm × 140 mm, cotton material) was purchased from Zhejiang Deqing Huanyu Pen Industry Co., Ltd. The high-density foam board (30 cm × 20 cm, polystyrene material) was purchased from Lind Trade Co., Ltd.

[0092] 2. Preparation of the substrate material of the solar evaporator

[0093] The high-density foam board is processed by laser cutting technology to prepare a circular substrate with a diameter of 8 cm and a thickness of 1 cm. The surface of the substrate is designed with uniformly distributed circular water channels with a diameter of 0.6 cm and 8 holes to ensure uniform water delivery to the surface of the evaporator.

[0094] 3. Assemble the water-absorbing rod and the substrate material

[0095] Place the 2-cm-long water-absorbing rod after cutting and processing into the holes of the 1-cm-thick circular substrate. The surface of the 7-mm-diameter water-absorbing rod is elastic, and filling it into the 6-mm-diameter hole helps to fix the water-absorbing rod, obtaining a circular substrate with the water-absorbing rod protruding 0.5 cm on each of the upper and lower surfaces.

[0096] 4. Preparation of the photothermal conversion coating

[0097] Carbon nanotubes, black titanium dioxide, dispersant, defoamer, and absolute ethanol with a mass ratio of 1.1:1.5:0.6:0.6:96.5 are fully mixed, and then ultrasonic treatment is carried out for 20 minutes at 20 kHz using a high-frequency ultrasonic machine to ensure the full dispersion of carbon nanotubes and titanium dioxide particles, obtaining a mixed slurry.

[0098] First, spray a layer of waterborne polyurethane on the circular substrate with the water-absorbing rod to increase the coating adhesion. When the waterborne polyurethane cures to a high viscosity (4000 mPas), the above-mentioned mixed slurry is sprayed onto the surface of the circular substrate with the water-absorbing rod by hot air-assisted air spraying method to obtain a 40.2-μm photothermal conversion coating for solar evaporator and a solar evaporator. (The hot air spraying process is carried out using a DELIXI two-gear constant-temperature hot air gun (DHCHGW21600, Anhui) and a HOKIA spray gun (W-71E, Jiangsu). The temperature for each spraying is 180 °C, and the fixed spraying angle is independently 70 °. The thickness of each spraying is 5-10 μm).

[0099] Example 3

[0100] 1. Materials

[0101] Multi-walled carbon nanotubes (CNTs) are provided by Shanghai Yaotian New Materials Technology Co., Ltd. (length: 5 μm). Black titanium dioxide (TiO2) spraying powder (purity 99%, 35 μm) is provided by Huawei Ruike Co., Ltd. The dispersant (model: HH-2176W) and the defoamer (model: AD20) are provided by Guangzhou Houhuan Chemical Auxiliary Co., Ltd. Absolute ethanol is purchased from the Chemical Reagent Co., Ltd. Waterborne polyurethane (WPU, model: AH-1502F) is provided by Anda Huatai New Materials Co., Ltd. The water-absorbing rod (7 mm × 140 mm, cotton material) is purchased from Zhejiang Deqing Huanyu Pen Industry Co., Ltd. The high-density foam board (30 cm × 20 cm, polystyrene material) is purchased from Lind Trade Co., Ltd.

[0102] 2. Preparation of Solar Evaporator Substrate Materials

[0103] Laser cutting technology is used to process high-density foam boards to prepare a circular base with a diameter of 8 cm and a thickness of 1 cm. The surface of the base is designed with evenly distributed circular water delivery channels with a diameter of 0.6 cm and 8 holes to ensure that water is evenly delivered to the evaporator surface.

[0104] 3. Assemble the water-absorbing rod and base material

[0105] Place the cut 2cm long absorbent rod into the hole of a 1cm thick circular base. The 7mm diameter absorbent rod has an elastic surface, and filling it into a 6mm diameter hole helps to fix the absorbent rod, resulting in a circular base with 0.5cm of absorbent rod protruding from the upper and lower surfaces.

[0106] 4. Preparation of Photothermal Conversion Coating

[0107] The carbon nanotubes, black titanium dioxide, dispersant, defoaming agent and anhydrous ethanol in a mass ratio of 0.9:3:0.4:0.4:95 were fully mixed, and then ultrasonically treated at 20 kHz for 20 minutes using a high-frequency ultrasonic machine to ensure that the carbon nanotubes and titanium dioxide particles were fully dispersed to obtain a mixed slurry.

[0108] First, a layer of water-based polyurethane is sprayed on a circular substrate with a water-absorbing rod to increase the adhesion of the coating. When the water-based polyurethane is cured to a high viscosity (4000mPas), the mixed slurry is sprayed onto the surface of the circular substrate with a water-absorbing rod by hot air assisted air spraying to obtain a 38.2μm photothermal conversion coating and a solar evaporator that can be used for a solar evaporator (a DELIXI dual-speed constant temperature hot air gun (DHCHGW21600, Anhui) and a HOKIA spray gun (W-71E, Jiangsu) are used for hot air spraying. The temperature of each spraying is 195°C, the fixed angle of the spraying is independently 45°, and the thickness of each spraying is 5 to 10μm).

[0109] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0110] The foregoing description of the disclosed embodiments enables those skilled in the art to practice or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a photothermal conversion coating that can be used for a solar evaporator, characterized in that: The steps include: 1) mixing carbon nanotubes, black titanium dioxide, a dispersant, a defoaming agent and a volatile solvent to obtain a mixed slurry; 2) spraying the mixed slurry multiple times using a hot air-assisted air spraying method to obtain a light-to-heat conversion coating that can be used in a solar evaporator; Wherein, the thickness of the photothermal conversion coating that can be used for the solar evaporator is 30 to 80 μm.

2. The method for preparing a photothermal conversion coating that can be used for a solar evaporator according to claim 1, characterized in that: The mass ratio of the carbon nanotube, black titanium dioxide, dispersant, defoamer and volatile solvent is 0.9-1.1:1.5-3:0.4-0.6:0.4-0.6:95-96.

5.

3. A method for preparing a photothermal conversion coating that can be used for a solar evaporator according to claim 1 or 2, characterized in that: The length of the carbon nanotubes is 1.5 to 50 μm; The particle size of the black titanium dioxide is 15 to 45 μm; The dispersant includes polyoxyethylene polyoxypropylene polymer and / or ethoxylated polyether; The defoaming agent includes one or more of polydimethylsiloxane, propylene glycol methyl ether and polyoxyethylene polyoxypropanolamine ether; The volatile solvent includes ethanol and / or isopropanol.

4. The method for preparing a photothermal conversion coating that can be used for a solar evaporator according to claim 3, characterized in that: The spraying temperature of the multiple spraying in step 2) is independently 150-200°C, the fixed angle of spraying is independently 40-90°, and the thickness of each spraying is 5-10 μm.

5. The method for preparing a photothermal conversion coating that can be used for a solar evaporator according to claim 4, characterized in that: Each spraying of the multiple sprayings ensures that the volatile solvent in the mixed slurry after the previous spraying is completely evaporated before spraying.

6. The photothermal conversion coating for solar evaporator prepared by the preparation method according to any one of claims 1 to 5.

7. A method for preparing a solar evaporator, characterized in that: The steps include: S1. Assembling a water-absorbing rod and a base material to obtain a base material with a water-absorbing rod; S2. sequentially spraying waterborne polyurethane and a photothermal conversion coating that can be used for a solar evaporator on a substrate material with a water-absorbing rod; The photothermal conversion coating that can be used for a solar evaporator is the photothermal conversion coating that can be used for a solar evaporator as claimed in claim 6.

8. The solar evaporator prepared by the preparation method according to claim 7.

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