A pentagonal star-shaped high-efficiency solar evaporator for treating high-salinity water and a preparation method thereof

Abstract

The application discloses a kind of processing high salt water five-star high-efficiency solar evaporator and preparation method thereof.The evaporator is with commercial melamine foam (MF) as initial skeleton, by directional spraying multi-walled carbon nanotube (MWCNTs) and polydimethylsiloxane (PDMS) composite functional coating on one side surface, construct top surface super-hydrophobic, bottom super-hydrophilic Janus asymmetric wetting structure.Evaporator is processed into specific five-star shape, utilize its unique edge ridge and tip effect, induce same direction enhanced marangoni convection in photo-thermal evaporation process.The design can drive salt ions from photo-thermal core area to edge ridge directionally crystallization, ensure that central region is long-term clean.The evaporator prepared by the application achieves 3.28 kg m ‑2 h ‑1 Evaporation rate and 0.27 kg m ‑2 h ‑1 High salt collection rate in 20 wt.% high salt brine, with low cost, simple process and other significant advantages, suitable for zero liquid discharge desalination and mineral resources recovery and other fields.

Description

Technical Field

[0001] This invention relates to the field of functional materials technology, specifically a pentagonal star-shaped high-efficiency solar evaporator with asymmetric wettability and its preparation method, which is particularly suitable for solar-driven water evaporation, water-salt dual production and salt collection applications. Background Technology

[0002] With the increasing prominence of global freshwater scarcity, solar-driven interfacial steam generation technology has shown great potential in seawater desalination and wastewater treatment due to its high efficiency and environmental friendliness. However, in the treatment of high-salinity water (e.g., 20 wt.% NaCl), rapid evaporation of water leads to a rapid saturation of the interfacial salt concentration, resulting in the formation of a dense salt crust in the photothermal layer. This not only weakens light absorption but also physically blocks water transport channels, leading to performance degradation. Existing salt rejection strategies mostly rely on diffusing salt back into the bulk phase, but this sacrifices the opportunity for mineral resource recovery. Therefore, developing a stable evaporator that can maintain efficient evaporation while enabling directional crystallization for convenient resource collection has significant scientific value and application prospects. Summary of the Invention

[0003] This invention aims to provide a pentagram-shaped high-efficiency solar evaporator for high-salt water treatment and its preparation method. The invention uses low-cost melamine foam as a substrate and constructs a Janus structure by single-sided spraying of a carbon nanotube / PDMS coating. The pentagram configuration has the largest effective evaporation perimeter and abundant apexes. During evaporation, strong assisted convection at the apexes generates significant radial thermal and solute gradients, inducing a unidirectional enhanced Marangoni flow from the center to the edges. This active transport mechanism causes salt to precipitate directionally at the edge ridges, maintaining the long-term stability of the central photothermal interface. This method, through specific biomimetic geometric design and single-sided spraying technology, achieves Janus asymmetric wettability construction, efficient photothermal conversion, and enhanced Marangoni flow-controlled salt management while maintaining the material's high porosity and porous structure. This effectively solves the problems of salt deposition and blockage, poor stability, and ineffective mineral resource recovery inherent in traditional evaporators in high-salt environments.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[0005] Substrate selection and formulation of composite photothermal coating: A substrate with a three-dimensional nanoscale network structure, a porosity of up to 99.5%, an average pore size of approximately 100 μm, and a density of 8–12 kg / m³ was selected. 3Melamine foam (MF) was used as the substrate. Multi-walled carbon nanotubes (MWCNTs) were added to an organic solvent and ultrasonically treated at 25–35°C for 30 min to form a uniform dispersion. Subsequently, polydimethylsiloxane (PDMS) and a corresponding curing agent (PDMS to curing agent mass ratio of 10:1) were added and magnetically stirred for 30 min to form a composite functional coating with high adhesion.

[0006] Construction of the Janus structure: Under a gas pressure of 0.5 MPa, the above dispersion was directionally sprayed onto one side of the MF substrate. The spraying distance was controlled at 10 cm, the single spraying time was 5 s, and two cycles were performed. The top surface after spraying achieved superhydrophobic properties, with a water contact angle (WCA) of 155°. ° The average roughness (Sa) increased significantly from 75.5 μm in the original MF to 87.2 μm.

[0007] Post-processing, molding, and material properties: The treated foam was cured at 80 °C for 1 h to obtain a superhydrophobic foam with a Janus structure. It was then processed into a specific pentagram geometry (5F) using a custom mold, with a micro / nano-roughened photothermal coating on the top surface and the original hydrophilic structure maintained on the sides and bottom. The modified pentagram-shaped evaporator (PCMF@STAR) achieved an average absorptivity of 97% across the entire 300–2400 nm spectrum and a thermal conductivity of only 0.216 W / m² in the wet state. -1 K -1 It exhibits excellent interface thermal localization capabilities.

[0008] This material is particularly suitable for solar-powered seawater desalination. Its Janus structure effectively inhibits salt deposition in the upper layer while maintaining efficient water transport. Surface wettability can be precisely controlled by adjusting the PDMS content (3–8 wt.%), adapting to the treatment needs of water bodies with varying salinity. All raw materials are commercially available products, and the preparation process requires no complex equipment, demonstrating significant potential for large-scale application.

[0009] The technical solution provided in this application embodiment has at least the following technical effects or advantages:

[0010] 1. Excellent Physicochemical Stability and Long-Term Durability: Due to the strong anchoring of MWCNTs by the PDMS binder, PCMF exhibits excellent mechanical toughness and coating adhesion. It does not collapse under 500 g of pressure, and its superhydrophobic properties show no significant degradation after 50 sandpaper abrasion cycles or 100 tape peeling cycles. Furthermore, the material demonstrates excellent chemical resistance, maintaining a stable self-floating state and exceeding 145°C even after immersion in strong acid (pH=1), strong alkali (pH=14), and 3.5 wt.% salt water for 48 hours. ° The contact angle.

[0011] 2. Highly efficient anti-scaling and directional salt collection capabilities: The unique pentagonal (5F) geometry significantly enhances the Marangoni effect at the edge edges. Through the synergistic effect of the radial thermal gradient and solute gradient from the photothermal center to the edge, strong active convection is induced, transporting salt ions from the photothermal core region to the edge edges for precipitation. This ensures no salt scale buildup even after 40 hours of continuous testing in 20 wt.% high-salt brine, achieving a scale buildup of 3.28 kg m³. -2 h -1 The ultra-high evaporation rate and 0.27 kg m -2 h -1 High salt collection efficiency.

[0012] 3. Significant Potential for Large-Scale Application and Multifunctionality: The single-sided spraying process employed in this invention is simple and efficient, enabling the construction of a single sample within seconds. Furthermore, it allows for the fabrication of large-area devices up to 1 m in length by adjusting the spraying area. The material supports patterned and irregularly shaped processing, adapting to various complex industrial scenarios. Outdoor experiments have validated its practical application value, achieving a thickness of 1.649 kg m² in a real-world environment. -2 h -1 The freshwater yield and 0.19 kg m -2 h -1 The salt yield is high, and the purified water quality fully meets drinking water standards. Attached Figure Description

[0013] Figure 1 shows microscopic SEM images of PCMF@STAR before and after coating;

[0014] Figure 2 shows a photograph of the contact angle test (WCA > 150) of the superhydrophobic top surface of the evaporator. ° );

[0015] Figure 3 shows a comparison of the surface crystallization patterns of PCMF@STAR and a conventional hydrophilic evaporator after 8 hours of operation in 20 wt.% brine;

[0016] Figure 4 shows the evaporation rate variation curves after 40 hours of continuous operation in a 20 wt.% NaCl solution;

[0017] Figure 5 These are surface temperature change curves and thermal images of the PCMF@STAR evaporator under simulated standard sunlight. Detailed Implementation

[0018] To better understand the above technical solutions, the following will provide a detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific implementation methods.

[0019] Example 1

[0020] This embodiment provides a method for preparing Janus foam material with asymmetric wetting properties, the specific steps of which are as follows:

[0021] S0101: Preparation of photothermal composite dispersion. 0.1 g of multi-walled carbon nanotubes (MWCNTs) were added to 20 mL of n-hexane solvent and ultrasonically treated at room temperature for 30 min to ensure complete dispersion of the nanotubes. Subsequently, 1 g of polydimethylsiloxane (PDMS) and 0.1 g of curing agent were added to the dispersion and magnetically stirred at 200 rpm for 30 min to obtain a photothermal composite functional coating with uniform composition.

[0022] S0102: Substrate Pretreatment. Commercially available open-cell melamine foam (MF) was selected as the skeleton substrate, with a porosity of 99.5% and an average pore size of approximately 100 μm. The MF was ultrasonically cleaned in anhydrous ethanol for 20 min to remove surface impurities, rinsed with deionized water, and then dried in an oven for later use.

[0023] S0103: Directional spraying modification. Using a spray gun with an air pressure of 0.5 MPa, the above-mentioned composite functional coating is directionally sprayed onto the upper surface of the MF substrate. The spraying distance is controlled at 10 cm, the single spraying time is 5 s, and two consecutive sprays are applied with a 10 min interval to ensure that the photothermal coating is distributed only in the top layer area of ​​the foam.

[0024] S0104: Thermosetting treatment. The sprayed foam material is placed in an oven at 80°C for 1 hour to cure, so that the photothermal coating is firmly anchored to the MF skeleton by PDMS adhesive, and finally Janus foam material with superhydrophobic upper layer and superhydrophilic lower layer is obtained.

[0025] Example 2

[0026] This embodiment constructs a biomimetic geometric configuration based on the material obtained in Embodiment 1. The specific steps are as follows:

[0027] S0201: Geometric Configuration Processing. Using a custom-made pentagonal star mold, the Janus foam obtained in Example 1 is cut into a specific pentagonal geometric shape to obtain the PCMF@STAR evaporator. Its structural features include: a rough coating of MWCNTs / PDMS on the top surface, exhibiting excellent blackbody absorption characteristics; and the retention of the original MF's open-cell structure and superhydrophilic properties on the sides and bottom.

[0028] S0202: Janus wettability verification. The water contact angle (WCA) of the evaporator top surface reached 155° according to the contact angle test. ° (Figure 2) It exhibits excellent superhydrophobic properties, which can effectively prevent the salt solution from directly wetting the evaporating surface; while the bottom remains superhydrophilic, relying on the strong capillary force generated by the high porosity to achieve rapid water replenishment.

[0029] Example 3

[0030] In this embodiment, the water evaporation performance and desalination performance of the prepared pentagonal star-shaped evaporator were tested, and the results are as follows:

[0031] S0301: High-salt desalination stability test. Please refer to the attached document. Figure 4 A continuous 40-hour test was conducted in a 20 wt.% NaCl high-salt solution under standard sunlight intensity. The results showed that PCMF@STAR consistently maintained a concentration of approximately 3.28 kg m³. -2 h -1 The high evaporation rate of the evaporator ensures that the top surface remains clean. In contrast, traditional cylindrical or square evaporators, under the same conditions, will quickly become covered with a salt crust, causing a sharp drop in evaporation rate (Figure 3).

[0032] S0302: Enhancement of Marangoni flow by shape effects. Please refer to the appendix. Figure 5 Infrared thermal imaging revealed that the pentagonal star-shaped evaporator exhibits a significant radial temperature gradient from the center (approximately 55.7°C) to the edge tips. This shape effect induces a change in surface tension at the liquid-gas interface, known as the Marangoni effect: the synergistic thermal gradient and solute gradient generate a strong thermocapillary force, driving the high-concentration salt flow to migrate from the photothermal center towards the edge edges.

[0033] S0303: Directional crystallization and salt production performance. Experiments showed (Figure 3) that salt crystals grew only directionally along the edges of the pentagram. The collected salt was dried and weighed, achieving a salt collection rate of 0.27 kg·m³. -2 ·h -1 This "water-salt dual-production" strategy, achieved by optimizing geometry and liquid transport pathways, not only ensures long-term anti-clogging of the interface but also makes it possible to recover high-value mineral resources.

[0034] The following performance evaluation was conducted on the pentagram-shaped high-efficiency solar evaporator (PCMF@STAR) prepared according to the embodiments of the present invention:

[0035] 1. Microscopic morphology observation after directional spraying modification

[0036] Test methods: The microstructure of the sponge before and after spraying the composite functional coating was observed using a scanning electron microscope (SEM, Hitachi, S4800); the changes in surface roughness of the substrate were detected using a laser confocal microscope (Zeiss LSM880). (See attached...) Figure 1 As can be seen, the original melamine foam (MF) has a typical three-dimensional highly open-cell structure with a very smooth skeleton surface. The modified PCMF still retains an open-cell structure of approximately 100 μm, indicating that the sprayed PDMS / MWCNTs coating did not block the foam pores, providing a smooth channel for subsequent vapor escape. High-magnification SEM images reveal that the carbon nanotube particles are tightly bonded to the foam framework, and the skeleton surface transforms from a smooth state to a micro-nano rough state. The average surface roughness of the foam significantly increases from 75.5 μm in MF to 87.2 μm in PCMF. This rich micro-nano structure lays the physical foundation for achieving superhydrophobic properties.

[0037] 2. Wetting performance test

[0038] Test Method: The static wettability and dynamic adhesion of droplets on the top surface of the pentagram evaporator were observed using a contact angle meter (LSA100). The droplet volume was 10 μL, and the test temperature was 25℃. The water droplets formed a perfect sphere on the PCMF@STAR surface. The water contact angle (WCA) on the top surface of the evaporator, measured by the contact angle meter, reached 155°. 。 It achieves superhydrophobicity, exhibiting excellent hydrophobic and salt-resistant properties. The untreated bottom remains superhydrophilic. This Janus structure ensures that the material can quickly absorb moisture from the bottom while effectively preventing surface salt crystallization and wetting.

[0039] 3. Evaluation of photothermal effect

[0040] Evaluation method: A xenon lamp with an AM1.5 filter (CEL-HXF300-T3) was used to simulate solar illumination at an intensity of 1 sun; an infrared thermal imager (FOTRIC 323Pro) was used to record the change in evaporator surface temperature over time in real time. (See attached image.) Figure 5The images show the surface temperature change curve and thermal imaging of the evaporator under simulated standard sunlight. Experimental results show that PCMF@STAR exhibits an extremely rapid thermal response after the onset of illumination, with the surface temperature rapidly rising from room temperature to 50.2℃ within 180 s, eventually stabilizing at approximately 55.7℃. At this time, the temperature of the bulk water at the bottom remains consistently around 25℃, indicating the low thermal conductivity (only 0.216 W / m² in the wet state) of the photothermal coating combined with the porous foam. -1 K -1 Significant interfacial thermal localization effects were achieved. Furthermore, the thermal imaging clearly shows that the temperature is highest in the central region of the pentagram, with a distinct temperature gradient extending to the five apexes. This radial gradient, induced by the shape effect, is the physical basis for the directional salt management of the Marangoni flow.

[0041] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0042] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A pentagonal star-shaped high-efficiency solar evaporator for treating high-salinity water and its preparation method, characterized in that... Includes the following steps: (1) Add multi-walled carbon nanotubes to n-hexane solvent and sonicate for 30 min to obtain a uniform dispersion; (2) Add polydimethylsiloxane (PDMS) and its curing agent to the above dispersion and stir magnetically for 30 min to form a composite functional coating; (3) Use directional spraying process to spray the composite functional coating onto one side of the melamine foam (MF) substrate under a pressure of 0.5 MPa, control the spraying distance to be 10 cm, the spraying time to be 5 s, and repeat twice; (4) Cure the sprayed foam at 80℃ for 2 h to obtain an asymmetric wettable foam with a Janus structure; (5) Use a custom mold to cut the foam into a pentagram geometric configuration to obtain the pentagram-shaped solar evaporator.

2. The preparation method according to claim 1, characterized in that: The mass concentration of the multi-walled carbon nanotubes is 4.5-5.5 mg / mL; the mass ratio of PDMS to curing agent is 10:

1.

3. The preparation method according to claim 1, characterized in that: The melamine foam has a porosity of up to 99.5%, an average pore size of 100 μm, and a density of 8-12 kg / m³. 3 .

4. A pentagonal star-shaped high-efficiency solar evaporator for high-salinity water treatment, manufactured by any one of the processes of claims 1-3, characterized in that: The evaporator has a Janus structure, in which the top surface of the loaded photothermal coating exhibits superhydrophobic properties, with a water contact angle (WCA) greater than 150°. ° The sides and bottom of the uncoated area retain superhydrophilic properties.

5. The pentagonal star-shaped high-efficiency solar evaporator according to claim 4, characterized in that: The evaporator has a regular polygonal star geometry with 3 to 8 angles, preferably 5 (5F configuration).

6. The pentagonal star-shaped high-efficiency solar evaporator according to claim 4, characterized in that: The evaporator has an average light absorption rate of 97% across the entire 300-2400 nm spectrum and a thermal conductivity of 0.216 W / m² in the wet state. -1 K -1 .

7. The pentagonal star-shaped high-efficiency solar evaporator according to claim 4, characterized in that: After being immersed in strong acid-base solutions with pH values ​​ranging from 1 to 14 for 48 hours, the water contact angle of the top surface of the evaporator remained at 145°. ° above.

8. The application of the pentagonal star-shaped high-efficiency solar evaporator according to any one of claims 4-7 in solar-driven interfacial evaporation desalination, characterized in that: The evaporator is floated on the surface of high-salt brine and operates under 1 solar intensity. This allows for efficient replenishment of water to the top surface while salt crystals are precipitated directionally at the edges and tips, and the top surface heat zone remains free of salt accumulation.

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