A hydrophilic-hydrophobic composite groove surface for enhancing dropwise condensation heat transfer under large supercooling and a preparation method thereof

Abstract

This application provides a hydrophilic-phobic composite trench surface for enhancing droplet condensation heat transfer under high subcooling and its preparation method. The surface is formed by processing a metal substrate and includes an array of superhydrophilic micro-grooves and hydrophobic ridges protruding between the grooves. Hydrophilic stripes are provided on both sides of the top of the hydrophobic ridges and are connected to the liquid film of the superhydrophilic micro-grooves. The preparation method includes four steps: metal substrate pretreatment, laser marking of the superhydrophilic micro-groove array, preparation of the hydrophobic ridges, and secondary laser marking of the hydrophilic stripes. This invention achieves directional and rapid transport of droplets on the ridges through the liquid film communication effect between the hydrophilic stripes and the superhydrophilic micro-grooves, significantly increasing the droplet renewal frequency, maintaining droplet condensation for a long time and avoiding flooding, thus significantly enhancing heat transfer efficiency. The preparation process is simple, has high processing precision, low cost, and is easy to achieve industrial mass production.

Description

Technical Field

[0001] This application relates to a hydrophilic and hydrophobic groove surface for enhancing droplet condensation heat transfer under high subcooling and its preparation method, belonging to the field of condensation and heat and mass transfer technology. Background Technology

[0002] Condensation, as a crucial phase change process, is widely used in many fields, from large power plants to micro-equipment, and improving its condensation efficiency is of great significance for energy conservation and emission reduction. This process mainly has two modes: film condensation formed on hydrophilic surfaces and droplet condensation on hydrophobic surfaces.

[0003] Film condensation occurs on hydrophilic surfaces, where the condensate forms a continuous liquid film covering the heat transfer surface. This liquid film significantly hinders heat transfer, resulting in low heat transfer efficiency. In contrast, droplet condensation occurs on hydrophobic surfaces, where the condensate grows in the form of discrete droplets. When the droplets reach a certain size, they spontaneously roll off or detach from the surface, keeping the heat transfer surface clean. Its heat transfer coefficient is usually an order of magnitude higher than that of film condensation, making it an ideal mode for enhancing condensation heat transfer.

[0004] However, at high subcooling (≥12K), condensate droplets become pinned to the hydrophobic surface, forming Wenzel droplets. These droplets grow to a sufficiently large size before detaching, resulting in a low droplet turnover rate and eventually flooding. This limits the condensation heat transfer performance of hydrophobic surfaces at high subcooling. Recently, researchers reported that hybrid hydrophilic-hydrophobic surfaces with wettability gradients can enhance condensation efficiency by delaying flooding. However, at high subcooling, the inability to expel bridging droplets leads to flooding again. Therefore, enhancing condensation heat transfer at high subcooling remains a challenge.

[0005] Three-dimensional hydrophilic-hydrophobic trench structures have attracted widespread attention from researchers due to their ability to maintain droplet detachment in hydrophobic regions under high subcooling, facilitate droplet self-disintegration, and limit the thickness of the liquid film in hydrophilic regions. In 2019, Lo et al. constructed a hybrid grooved surface combining superhydrophobic nanowires and hydrophilic microchannels to effectively control the behavior of condensate droplets, achieving a high heat flux density of 655±10 kW / m² and an improved heat transfer coefficient over a wide subcooling range, demonstrating its great potential for efficient condensation heat dissipation (Enhancing condensation heat transfer on three-dimensional hybrid surfaces[J]. Joule, 2019, 3(11): 2806-2823.). In 2023, Ren et al. designed a wettability-interval grooved surface, achieving directional and rapid removal of condensate through the spontaneous suction of the hydrophilic grooves and the repulsion of the hydrophobic ridges. At a subcooling of 12K, the heat flux density reached 1280 W / m², significantly better than a single wettability surface, proving its superior performance in enhancing condensation heat transfer (Experimental study on the condensation heat transfer on a wettability-interval grooved surface[J]. Applied Sciences-Basel, 2023, 13(18):). 10518.) Although the enhanced condensation effect of three-dimensional hydrophilic and hydrophobic groove surfaces is more advantageous under high subcooling, the ridges of current three-dimensional groove structures are all hydrophobic. Due to the pinning effect of droplets at the corners of the structure, it is difficult for condensed droplets on the ridges to enter the hydrophilic grooves, which restricts the rapid renewal of condensed droplets.

[0006] In summary, the key to achieving rapid removal of droplets on the ridge is the efficient droplet condensation of three-dimensional hydrophilic-reactive surfaces under high subcooling. Summary of the Invention

[0007] To address the aforementioned issues, a hydrophilic-phobic composite groove surface and its preparation method are provided for enhancing droplet condensation heat transfer under high subcooling conditions. The surface utilizes the liquid film connectivity effect between the hydrophilic stripes and the superhydrophilic micro-grooves to achieve directional and rapid droplet transport on the ridges, significantly increasing the droplet renewal frequency, maintaining a highly efficient droplet condensation mode over a long period, effectively avoiding flooding, and significantly enhancing condensation heat transfer efficiency. The preparation method is simple, has high processing precision, strong controllability, low processing cost, good repeatability, and is easy to implement for industrial mass production.

[0008] According to one aspect of this application, a hydrophilic-hydrophobic composite trench surface is provided for enhancing droplet condensation heat transfer under high subcooling, the hydrophilic-hydrophobic composite trench surface being formed by processing the surface of a metal substrate; The surface of the hydrophilic-hydrophobic composite trench includes an array of superhydrophilic micro-grooves and hydrophobic ridges formed by protrusions between the superhydrophilic micro-grooves. The top of the hydrophobic ridges is provided with hydrophilic stripes, and the hydrophilic stripes are in communication with the liquid film of the superhydrophilic micro-grooves.

[0009] Optionally, the top two sides of the hydrophobic ridge are provided with hydrophilic stripes that extend continuously along its length direction, and the width of the hydrophilic stripes is 10-50μm.

[0010] Specifically, by setting hydrophilic stripes on both sides of the top of the hydrophobic ridge and extending them continuously along their length, droplet capture and transport can be achieved in the entire area of ​​the hydrophobic ridge. By limiting the width of the hydrophilic stripes, it is not only conducive to stable processing, but also to matching the structural parameters of the superhydrophilic micro-groove. The liquid film connectivity effect is good, which improves the rate and stability of droplet transport.

[0011] If the width is too small, a continuous liquid film cannot be formed, making it difficult to achieve stable droplet transport. If the width is too large, it will encroach on the effective area of ​​droplet condensation of the hydrophobic ridge, reducing the overall heat transfer efficiency.

[0012] Optionally, the depth of the superhydrophilic microgroove is 50-300 μm, the width is 100-600 μm, and the center distance between two adjacent superhydrophilic microgrooves is 200-1500 μm.

[0013] Specifically, by limiting the size range of the composite trenches, a balance is ensured between condensate collection capacity, liquid film thickness control, and heat transfer area utilization. The trench depth ensures effective storage and smooth discharge of condensate, avoiding condensate overflow due to excessive shallowness and increased difficulty in laser processing and the formation of a thick liquid film that hinders heat transfer due to excessive depth. The trench width can form a stable capillary suction effect, enhancing the collection capacity of droplets transported by hydrophilic stripes. The limitation of the center distance can reasonably allocate the surface area ratio of hydrophobic ridges and superhydrophilic micron trenches, ensuring sufficient droplet condensation heat transfer area.

[0014] Optionally, the metal matrix is ​​any one of pure aluminum, aluminum alloy, or copper.

[0015] According to another aspect of this application, a method for preparing the hydrophilic-hydrophobic composite trench surface as described above is also provided, comprising the following steps: (1) Metal substrate pretreatment: sand the surface of the metal substrate with sandpaper, ultrasonically clean for 1-5 minutes, and dry after cleaning; (2) Fabrication of superhydrophilic micro-groove array: Laser marking is performed on the pretreated metal substrate to prepare an array of superhydrophilic micro-grooves; (3) Preparation of hydrophobic ridges: Polyimide tape is pasted onto the raised ridges obtained in step (2) and pressed tightly. The sample is immersed in deionized water, cooled to room temperature, and the tape is removed. After drying, the hydrophobic ridges are obtained. (4) Processing hydrophilic stripes: Position the sample after step (3) to the laser marking position of step (2), and perform secondary laser marking on both sides of the top of the hydrophobic ridge to form hydrophilic stripes.

[0016] Optionally, in step (1), the surface of the metal substrate is polished with 3000-grit sandpaper, and then ultrasonically cleaned with ethanol and deionized water for 1-5 minutes respectively, and then dried.

[0017] Specifically, using 3000-grit sandpaper to polish the metal substrate surface removes oxide layers, oil, and impurities while avoiding deep scratches caused by coarse sandpaper, resulting in a smooth and clean substrate surface. This eliminates the impact of surface defects on laser processing accuracy and ensures the regularity of the micro-nano structure after laser processing, thereby guaranteeing the effect of wettability control. The substrate is then ultrasonically cleaned stepwise with ethanol and deionized water. Ethanol quickly dissolves organic oil on the surface, while deionized water removes residual ethanol and inorganic impurities. The pretreated clean and smooth substrate provides a good foundation for subsequent laser processing and tight bonding with polyimide tape, effectively improving the structural and performance stability of the prepared hydrophilic-hydrophobic composite trench surface.

[0018] Optionally, the ultrasonic cleaning power in step (1) is 100-150W, and the drying method is natural air drying, drying at 40-60℃, or air gun drying.

[0019] Optionally, the parameters for laser marking in step (2) are: laser power 30%~70%, processing speed 500mm / s, laser frequency 30kHz, and processing times 50~150 times.

[0020] Optionally, in step (3), the sample is immersed in deionized water at 80℃-100℃ for 10-40 seconds, cooled to room temperature, the tape is removed, and the sample is dried with an air gun.

[0021] Preferably, the thickness of the polyimide tape is 55 μm.

[0022] Specifically, the presence of the grooves prevents the tape from adhering to the inside of the grooves without affecting the wettability inside the grooves; while the ridges with polyimide tape soften, flow and partially dissolve the adhesive layer due to hot water treatment, and the hydrophobic adhesive remains on the surface of the ridges during the subsequent cooling process, making them hydrophobic, thereby achieving precise zoning of wettability between the hydrophobic ridges and the superhydrophilic micro-grooves.

[0023] Optionally, the parameters for the secondary laser marking in step (4) are: laser power 1%, processing speed 4000mm / s, laser frequency 50kHz, and processing times 1.

[0024] In this application, room temperature refers to 25°C.

[0025] The beneficial effects of this application include, but are not limited to: 1. The top of traditional hydrophilic and hydrophobic channels is a hydrophobic region. The condensate droplets in the hydrophobic region are difficult to transport to the superhydrophilic channel due to the pinning effect of the ridge corners, which restricts the rapid renewal of condensate droplets on the hydrophobic ridge.

[0026] The hydrophilic-hydrophobic composite trench surface of this application solves the core technical problem of droplet pinning on the hydrophobic ridge and inability to be rapidly transported under high subcooling by setting hydrophilic stripes on the top of the hydrophobic ridge and forming a liquid film communication structure with the superhydrophilic micro-groove. It utilizes the Laplace force to achieve directional and rapid migration of droplets on the ridge, which greatly improves the droplet renewal frequency. It can maintain a highly efficient droplet condensation mode for a long time under high subcooling environment, effectively delaying or even avoiding flooding. The condensation heat transfer efficiency is significantly better than that of traditional hydrophilic-hydrophobic hybrid surfaces and three-dimensional hydrophilic-hydrophobic trench surfaces.

[0027] 2. This application achieves the synergistic effect of hydrophobic ridge droplet condensation, hydrophilic stripe directional transport, and superhydrophilic groove efficient drainage by specifically designing a hydrophilic-hydrophobic composite groove surface, thereby enhancing heat transfer while retaining sufficient effective heat transfer area.

[0028] 3. The preparation method of this application adopts processes such as secondary laser marking, which is simple and easy to operate. It does not require complex equipment or toxic and harmful reagents, has low processing cost, is environmentally friendly, and has strong controllability of laser processing, which can achieve precise control of surface structure and wettability. The prepared hydrophilic-hydrophobic composite trench has a regular surface structure and stable performance. Attached Figure Description

[0029] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a schematic diagram of the hydrophilic-hydrophobic composite trench structure prepared in this application.

[0030] Figure 2 This is a schematic diagram of a traditional hydrophobic channel structure.

[0031] Figure 3 This is a scanning electron microscope image of the hydrophilic-hydrophobic composite trench structure in Example 1.

[0032] Figure 4This is a scanning electron microscope image of the hydrophilic-hydrophobic groove structure in Comparative Example 1.

[0033] Figure 5 The image shows the wettability characteristics of various parts in Example 1.

[0034] Figure 6 The images show the surface morphology before condensation and the liquid film distribution after condensation for Example 1 and Comparative Example 1.

[0035] Figure 7 This is a diagram showing the steam condensation sequence on the surface of the hydrophilic-hydrophobic trench structure in Comparative Example 1. Figure 8 This is a diagram showing the vapor condensation sequence on the surface of the hydrophilic-hydrophobic composite trench structure in Example 1. Figure 9 To compare the update frequency of condensed droplets on the surface of hydrophilic and hydrophobic groove structures of two samples with dimensions of (15.6 mm * 20 mm) under steady state. Detailed Implementation

[0036] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of skill in the art. The reagents and raw materials used in this invention are readily available through conventional means, and unless otherwise specified, they shall be used in accordance with conventional methods in the art or as per the product instructions. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to the methods of this invention. The preferred embodiments and materials described in this patent are for illustrative purposes only.

[0038] Example 1 Commercially available 1060 pure aluminum was selected as the substrate, measuring 20mm × 40mm. A micron-scale superhydrophilic trench array structure was fabricated on its surface using nanosecond lasers. A low surface energy treatment was applied to the ridge surface to make it hydrophobic. Subsequently, a second nanosecond laser marking process was performed to create hydrophilic stripes at the boundaries of the hydrophobic ridges. A schematic diagram of the structure is shown below. Figure 1 The specific preparation method is as follows: (1) Pretreatment of 1060 pure aluminum substrate: 1060 pure aluminum was polished with 3000 grit sandpaper, and then ultrasonicated with ethanol and deionized water for 2 min each to remove surface contaminants. (2) Fabrication of superhydrophilic micron-scale trench array structure: A micron-scale trench array structure was prepared by laser marking on the 1060 pure aluminum substrate treated in step (1). The depth of the micron-scale trench was 150 μm, the width of the micron-scale trench was 300 μm, and the center distance between two adjacent micron-scale trench structures was 750 μm. The laser parameters were: laser power 30%, speed 500 mm / s, frequency 30 kHz, and processing times 50 times.

[0039] (3) Preparation of hydrophobic ridge structure: A 55 μm thick commercial polyimide tape was pasted onto the micron-sized ridge after step (2) and pressed tightly. The sample after the tape was pasted was immersed in deionized water at 80°C for 20 s. After the sample was taken out and cooled to room temperature, the tape was removed. Finally, the sample was dried with an air gun to obtain the hydrophobic ridge. (4) Preparation of hydrophilic stripes on hydrophobic ridges: The sample treated in step (3) is positioned at the laser marking location in step (2), and then laser marking is performed again near the two side boundaries of the hydrophobic ridge to obtain hydrophilic stripes. The hydrophilic stripes are located on both sides of the hydrophobic ridge, with a width of 20 μm, and are aligned with the edge of the superhydrophilic micro-groove. The laser parameters are: laser power of 1%, speed of 4000 mm / s, frequency of 50 kHz, and processing times of 1.

[0040] Example 2 The difference between this embodiment and Embodiment 1 is that the 1060 pure aluminum metal substrate is replaced with 6061 aluminum alloy.

[0041] In the preparation method: step (1) is the same; (2) Fabrication of superhydrophilic micron-level trench array structure: The depth of the micron-level trench is 50μm, the width of the micron-level trench is 100μm, and the center distance between two adjacent micron-level trench structures is 200μm.

[0042] Step (3) is the same; (4) Preparation of hydrophobic ridges and hydrophilic stripes: Position the sample treated in step (3) at the laser marking position in step (2), and then perform laser marking again near the two sides of the hydrophobic ridge to obtain hydrophilic stripes. The hydrophilic stripes are located on both sides of the hydrophobic ridge, with a width of 10 μm, and are aligned with the edge of the superhydrophilic micro-groove.

[0043] Example 3 The difference between this embodiment and Embodiment 1 is that the metal substrate 1060 pure aluminum is replaced with copper.

[0044] In the preparation method: step (1) is the same; (2) Fabrication of superhydrophilic micron-level trench array structure: The depth of the micron-level trench is 300μm, the width of the micron-level trench is 600μm, and the center distance between two adjacent micron-level trench structures is 1500μm.

[0045] Step (3) is the same; (4) Preparation of hydrophobic ridges and hydrophilic stripes: Position the sample treated in step (3) at the laser marking position in step (2), and then perform laser marking again near the two sides of the hydrophobic ridge to obtain hydrophilic stripes. The hydrophilic stripes are located on both sides of the hydrophobic ridge, with a width of 50 μm, and are aligned with the edge of the superhydrophilic micro-groove.

[0046] Comparative Example 1 Commercially available 1060 pure aluminum was selected as the substrate, measuring 20mm × 40mm. A micron-scale superhydrophilic trench array structure was fabricated on its surface using nanosecond lasers, and the ridge surface underwent a low surface energy treatment to impart hydrophobicity. A schematic diagram of the structure is shown below. Figure 2 The specific preparation method is as follows: (1) Pretreatment of 1060 pure aluminum substrate: 1060 pure aluminum was polished with 3000 grit sandpaper, and then ultrasonicated with ethanol and deionized water for 2 min each to remove surface contaminants. (2) Fabrication of superhydrophilic micron-level trench array structure: A micron-level trench array structure was prepared by laser marking on the 1060 pure aluminum substrate treated in step (1). The depth of the micron-level trench was 150 μm, the width of the micron-level trench was 300 μm, and the center distance between two adjacent micron-level trench structures was 750 μm. The laser parameters were: laser power 30%, speed 500 mm / s, frequency 30 kHz, and processing times 50 times.

[0047] (3) Preparation of hydrophobic ridge structure: A commercial polyimide tape with a thickness of 55 micrometers was pasted onto the micron-sized ridge after step (2) and pressed tightly. The sample after the tape was pasted was immersed in deionized water at 80°C for 20 seconds. After the sample was taken out and cooled to room temperature, the tape was removed. Finally, the sample was dried with an air gun to obtain the hydrophobic ridge.

[0048] Example 1: Morphology and Wettability Characterization of Hydrophilic and Hydrophobic Channel Structures The surface morphology of the hydrophilic-hydrophobic composite trenches prepared in Example 1 was characterized using scanning electron microscopy; the microstructures were observed at 100x and 5000x magnification as follows: Figure 3 As shown, a micrometer-scale trench array can be seen. Under low magnification, there is a clear water affinity boundary at the top. Under high magnification, nanometer-scale coral-like papillary structures are densely distributed inside the micrometer-scale trenches.

[0049] The surface morphology of the conventional hydrophilic-hydrophobic grooves prepared in Comparative Example 1 was characterized using scanning electron microscopy; the microstructures were observed at 100x and 5000x magnification as follows: Figure 4As shown, under low magnification, there is no water boundary between the top and the non-water content.

[0050] In addition, the contact angles of each region in Example 1 were characterized. Considering that the structure is too small, the same hydrophobic surface, hydrophilic surface and superhydrophilic surface as in Example 1 were prepared in three regions on a 1060 pure aluminum plane according to the wetted surface preparation process of each part in Example 1.

[0051] like Figure 5 As shown, the contact angle of the top hydrophobic region is 98 degrees, the contact angle of the top hydrophilic region (hydrophilic stripes) is 40 degrees, and the contact angle of the bottom superhydrophilic region is 5 degrees. The morphology of the surface before condensation and the distribution of droplets on the surface after condensation were characterized using an optical microscope, as shown below. Figure 6 As shown, in Example 1, there are obvious dividing lines between droplet condensation and film condensation on both sides of the ridge top surface, while in Comparative Example 1, the ridge top is entirely droplet condensation.

[0052] Experiment Example 2: Droplet Condensation Performance Test under High Subcooling Condensation was carried out in a water vapor environment with a supercooling degree of 11. The treated Example 1 and Comparative Example 1 were simultaneously placed vertically on a cold table with a base temperature of 1°C, and the water vapor condensation process was photographed by an SLR camera equipped with a macro lens.

[0053] like Figure 7 As shown, in Comparative Example 1, after the condensate droplets on the hydrophobic ridge grow to the width of the ridge, they cannot quickly enter the superhydrophilic groove due to the pinning effect of the ridge corners, until the condensate droplets are vertically elongated along the groove direction. However, as... Figure 8 As shown, in Embodiment 1 of this application, the condensate droplets on the hydrophobic ridge grow to the point that once they come into contact with the hydrophilic stripes, they quickly enter the superhydrophilic groove, thus achieving rapid renewal of the condensate droplets on the hydrophobic ridge.

[0054] Finally, the update frequency of condensate droplets on the surfaces of Example 1 and Comparative Example 1 under steady-state conditions with the same degree of subcooling was further statistically analyzed, such as... Figure 9 As shown, the refresh frequency of the condensed droplets on the ridge in Example 1 of this application is 3.6 times that on the ridge in Comparative Example 1, indicating that the structure prepared in Example 1 can achieve efficient droplet condensation.

[0055] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A hydrophilic-phobic composite trench surface for enhancing heat transfer during droplet condensation under high subcooling, characterized in that, The surface of the hydrophilic-hydrophobic composite trench is formed by processing the surface of a metal substrate; The surface of the hydrophilic-hydrophobic composite trench includes an array of superhydrophilic micro-grooves and hydrophobic ridges formed by protrusions between the superhydrophilic micro-grooves. The top of the hydrophobic ridges is provided with hydrophilic stripes, and the hydrophilic stripes are in communication with the liquid film of the superhydrophilic micro-grooves.

2. The hydrophilic-hydrophobic composite trench surface according to claim 1, characterized in that, The top two sides of the hydrophobic ridge are provided with hydrophilic stripes that extend continuously along its length direction, and the width of the hydrophilic stripes is 10-50μm.

3. The hydrophilic-hydrophobic composite trench surface according to claim 1, characterized in that, The superhydrophilic microgrooves have a depth of 50-300 μm, a width of 100-600 μm, and a center-to-center distance of 200-1500 μm between two adjacent superhydrophilic microgrooves.

4. The hydrophilic-hydrophobic composite trench surface according to claim 1, characterized in that, The metal matrix is ​​any one of pure aluminum, aluminum alloy, or copper.

5. A method for preparing a hydrophilic-hydrophobic composite trench surface as described in any one of claims 1-4, characterized in that, Includes the following steps: (1) Metal substrate pretreatment: sand the surface of the metal substrate with sandpaper, ultrasonically clean for 1-5 minutes, and dry after cleaning; (2) Fabrication of superhydrophilic micro-groove array: Laser marking is performed on the pretreated metal substrate to prepare an array of superhydrophilic micro-grooves; (3) Preparation of hydrophobic ridges: Polyimide tape is pasted onto the raised ridges obtained in step (2) and pressed tightly. The sample is immersed in deionized water, cooled to room temperature, and the tape is removed. After drying, the hydrophobic ridges are obtained. (4) Processing hydrophilic stripes: Position the sample after step (3) to the laser marking position of step (2), and perform secondary laser marking on both sides of the top of the hydrophobic ridge to form hydrophilic stripes.

6. The preparation method according to claim 5, characterized in that, In step (1), the surface of the metal substrate is polished with 3000-grit sandpaper, and then ultrasonically cleaned with ethanol and deionized water for 1-5 minutes respectively, and then dried.

7. The preparation method according to claim 5, characterized in that, In step (1), the ultrasonic cleaning power is 100-150W, and the drying method is air gun drying.

8. The preparation method according to claim 5, characterized in that, The parameters for laser marking in step (2) are: laser power 30%~70%, processing speed 500mm / s, laser frequency 30kHz, and processing times 50~150 times.

9. The preparation method according to claim 5, characterized in that, In step (3), the sample is immersed in deionized water at 80℃-100℃ for 10-40 seconds, cooled to room temperature, the tape is removed, and the sample is dried with an air gun.

10. The preparation method according to claim 5, characterized in that, The parameters for the secondary laser marking in step (4) are: laser power 1%, processing speed 4000mm / s, laser frequency 50kHz, and processing times 1.

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