Low-infrared-emissivity coating based on aluminum powder pretreatment and preparation method of low-infrared-emissivity coating

Abstract

The invention belongs to the technical field of functional coatings and composite materials thereof, and particularly relates to a low-infrared-emissivity coating based on aluminum powder pretreatment and a preparation method of the low-infrared-emissivity coating. Nanometer CeO2 with a porous structure and Al powder are calcined to form an interface heterojunction under the combined action of nanoparticles and nanowires, and then the interface heterojunction is used as a filler to be prepared into a low-infrared-emissivity coating with fluorocarbon resin and an auxiliary agent; after pretreatment, the aluminum powder with the interface heterojunction structure can greatly promote electron migration, so that the conductivity is improved by 2 orders of magnitude compared with that of a traditional metal oxide; and according to the Hagen-Rubens law, the conductivity is in negative correlation with the emissivity, so that the emissivity is reduced. The infrared emissivity of the final coating at the wave band of 8-14 microns is as low as 0.16, the coating has high stability, light weight, high temperature resistance and environmental erosion resistance, the preparation process is simple, the coating is suitable for the fields of aerospace thermal management, military stealth and industrial energy saving, and an innovation direction is provided for the next generation of infrared stealth coatings.

Description

Technical Field

[0001] The present invention belongs to the technical field of functional coatings and their composite materials, and particularly relates to a low infrared emissivity coating based on aluminum powder pretreatment and a preparation method thereof. Background Art

[0002] The core of infrared stealth technology lies in regulating the emissivity of the material surface in the infrared band to reduce the probability of the target being detected. The current mainstream low emissivity coating technology mainly relies on metal fillers (such as Al, Ag) or composite ceramic systems, but there are still significant bottlenecks in terms of high temperature resistance, environmental stability, and cost control.

[0003] Metallic aluminum has long been used as the main filler for low emissivity coatings due to its high reflectivity (>80%) in the infrared band and excellent electrical and thermal conductivity. For example, Patent CN105086527A uses bismuth oxide-coated flaky aluminum powder, but its emissivity in the 8-14μm band is still 0.42-0.44, and the aluminum powder is prone to oxidation, resulting in performance degradation. The platinum-doped enamel-metal composite coating proposed in Patent CN117049787A is heat-resistant, but the multi-layer structure has the defect of poor interfacial bonding force (adhesion <15MPa). Patent CN117070091A uses silver powder, which can reduce the emissivity below 0.25, but the raw material cost is high (the price of silver is about 50 times that of aluminum) and it is prone to sulfurization failure. The current low emissivity filler system has three major limitations: metal fillers (such as Al, Cu) are prone to oxidation, resulting in unstable emissivity; semiconductor fillers have limited performance in reducing infrared emissivity; and the organic-inorganic composite system has poor heat resistance. These technical bottlenecks drive the exploration of new low-cost materials. Summary of the Invention

[0004] Aiming at the problems of insufficient stability, weak high temperature adaptability, and complex preparation process of the existing low infrared emissivity coatings in complex environments, the present invention provides a low infrared emissivity coating based on aluminum powder pretreatment and a preparation method thereof; growing nano-phase CeO2 ceramics on the surface of aluminum powder to form a low infrared emissivity coating regulated by nano-ceramic particles and nano-wires, achieving a low emissivity in the 8-14μm band, while taking into account light weight, weather resistance, and scalable preparation.

[0005] A low infrared emissivity coating based on aluminum powder pretreatment, calculated by mass percentage, comprises the following raw material components:

[0006] Filler: 10-15wt% cerium nitrate, 30-40wt% aluminum powder, 30-50wt% absolute ethanol, 8-12wt% citric acid, 1-2wt% polyvinylpyrrolidone PVP, and the sum of each component is 100%.

[0007] Among them, after the cerium nitrate is calcined and decomposed into nano-ceria particles, a porous structure is formed on its surface through citric acid chelation regulation; then the interface with the aluminum powder is chemically bonded through Al-O-Ce bonding to form a heterojunction of nano-ceria-coated aluminum powder, and a filler is obtained.

[0008] Coating part: 17-20wt% fluorocarbon resin, 4-6wt% additives, 25-30wt% filler, 11-15wt% butyl acetate, 24-28wt% ethyl acetate, 11-15wt% fluorocarbon curing agent, and the sum of each component is 100%.

[0009] After dispersing the raw materials of the coating evenly, it is coated on the surface of the target carrier and dried to obtain a low infrared emissivity coating.

[0010] Furthermore, the aluminum powder is flaky, and the particle size of the flaky aluminum powder is 10-30μm. Due to its high metal reflectivity, aluminum has a low emissivity, and the flaky morphology allows light to enter at a larger incident angle compared to the spherical shape. Under the same proportion, the flaky filler has a higher reflectivity and a lower emissivity. In addition, the flakiness is prone to floating on the surface of the coating, which will further increase the reflection.

[0011] Furthermore, the specific preparation process of the filler is as follows:

[0012] Step 1: Weigh the raw materials according to the mass ratio: 0-15wt% cerium nitrate, 30-40wt% aluminum powder, 30-50wt% absolute ethanol, 8-12wt% citric acid, 1-2wt% polyvinylpyrrolidone PVP, and the sum of each component is 100%.

[0013] Step 2: Sol preparation: Dissolve the cerium nitrate fully in absolute ethanol; then add citric acid as a chelating agent, and add ammonia water to adjust the pH value to 2-3.5; add the dispersant PVP under stirring to obtain a mixed solution; stir and react the mixed solution in a water bath environment at 60°C for 2-4 hours to form a uniform and transparent sol.

[0014] Polyvinylpyrrolidone (PVP) is used as a dispersant. Its long-chain molecules are tightly coated on the surfaces of aluminum powder and ceria particles through physical adsorption and hydrogen bonding to form a steric hindrance, effectively inhibiting particle agglomeration; at the same time, the hydrophilic-hydrophobic balance characteristics of PVP can effectively match the ethanol solvent, making the Zeta potential of the dispersion stable above ±30mV, ensuring the uniform suspension of aluminum powder in the sol, and finally realizing the dense and uniform coating of ceria nanoparticles on the aluminum substrate. After calcination, PVP completely decomposes without residual pollution.

[0015] Step 3: Gel formation: Add the Al powder to the sol prepared in Step 2 and ultrasonically disperse it until it is uniform, ensuring that the sol fully wraps the Al powder particles; then let it stand and age at room temperature until the sol turns into a gel.

[0016] Step 4, Drying treatment: Dry the gel obtained in Step 3 completely at 60 - 80 °C to obtain a dry gel.

[0017] Step 5, Heat treatment: Heat the dry gel obtained in Step 4 to 500 °C at a heating rate of about 5 °C / min in an Ar gas atmosphere and hold for at least 2 hours to decompose the organic components in the dry gel.

[0018] Cerium nitrate is used as a precursor solution for generating cerium oxide. The cerium nitrate solution is weakly acidic (pH ≈ 2 - 3.5), which slightly etches the surface of the Al powder to form active sites, promotes the nucleation of CeO2, and at the same time avoids excessive corrosion of the Al powder by strong acids. Cerium nitrate completely decomposes into CeO2 when calcined at 300 - 500 °C (reaction formula: 4Ce(NO3)3 → 4CeO2 + 12NO2↑ + 3O2↑), with few residual impurities and no need for additional purification steps.

[0019] Citric acid acts as a chelating agent. Its carboxyl groups form stable complexes with Ce 3+ to effectively inhibit the rapid hydrolysis and aggregation of cerium nitrate, ensuring the uniform nucleation of cerium oxide nanoparticles on the surface of aluminum powder. At the same time, citric acid can decompose spontaneously during the calcination process. It not only acts as a fuel to promote the crystallization of CeO2 to produce high-purity nanoparticles but also avoids introducing impurities. The CO2 and H2O gases generated by the decomposition contribute to the formation of a porous structure, enhancing the specific surface area and catalytic activity of the coating, and ultimately realizing a low-temperature and controllable nano-coating process.

[0020] Step 6, Cooling and collection: After the heat treatment in Step 5, cool it naturally to room temperature to obtain the Al powder pretreated with nano-cerium oxide.

[0021] Furthermore, the proportion of C-F bonds in the molecular chain of the fluorocarbon resin is ≥ 30%. The high bond energy of the C-F bond endows the coating with excellent weather resistance and chemical inertness, and it can maintain stable adhesion in the wide temperature range from -50 °C to 200 °C. At the same time, the low surface energy of the fluorocarbon resin and the oriented arrangement of the flaky aluminum powder act synergistically to reduce the internal light scattering of the coating and enhance the infrared reflectivity, ultimately realizing the long-term infrared stealth of the coating in extreme environments to improve the anti-corrosion performance of the overall coating.

[0022] In the present invention, butyl acetate, as a solvent, has good solubility and volatility, which helps the coating to form rapidly, reduces the curing time and improves the production efficiency. Ethyl acetate has good wettability, which helps the coating to adhere evenly to the surface of the substrate. This is very important for forming a uniform coating and improving the adhesion. Ethyl acetate has a low viscosity and is easy to mix with other solvents. The fluorocarbon resin and the fluorocarbon curing agent work together to endow the coating with rapid curing and excellent weather resistance and abrasion resistance, and can resist the erosion of environmental factors such as physical wear, ultraviolet radiation, acid rain, humidity, etc. and chemical corrosion. The fluorocarbon curing agent also has a low surface energy, making the coating surface smoother and reducing the affinity for water and oil. This helps to improve the stain resistance of the coating and reduce the formation of surface attachments.

[0023] In summary, in the present invention, nano-CeO2 with a porous structure is calcined with Al powder to form an interfacial heterojunction in which nanoparticles and nanowires act together, and then it is used as a filler to make a low-infrared-emissivity coating with fluorocarbon resin and additives; the aluminum powder with an interfacial heterojunction structure formed after pretreatment will greatly promote electron migration, so that the conductivity is increased by two orders of magnitude compared with traditional metal oxides; according to the Hagen-Rubens law, the conductivity is negatively correlated with the emissivity, thus reducing the emissivity. The final coating of the present invention has an infrared emissivity as low as 0.16 in the 8-14 μm band, and has high stability, light weight, high temperature resistance and resistance to environmental erosion, and the preparation process is simple, which is applicable to the fields of aerospace thermal management, military stealth and industrial energy conservation, and provides an innovative direction for the next generation of infrared stealth coatings. Brief Description of the Drawings

[0024] Figure 1 It is a comparison chart of the conductivity of the coating samples obtained in each example and comparative example.

[0025] Figure 2 It is a FLIR comparison chart of the coating samples of the comparative example, Example 1 and Example 9.

[0026] Figure 3 It is a comparison chart of the average emissivity of the coating samples in each example and comparative example.

[0027] Figure 4 It is an image of the filler CeO2-Al prepared in Example 9 under a transmission microscope. Detailed Description of the Invention

[0028] The present invention will be further described below in conjunction with the examples.

[0029] Example:

[0030] A low-infrared-emissivity coating based on the pretreatment of aluminum powder, and its preparation method includes the following steps:

[0031] Step 1, Material Preparation: Weigh cerium nitrate according to the formula concentration ratio in Table 1, and weigh 10 g of flaky Al powder, 10 g of absolute ethanol, 2.5 g of citric acid, and 0.35 g of PVP. The particle size of the flaky aluminum powder is 10 - 30 μm.

[0032] Step 2, Sol Preparation: Dissolve the cerium nitrate weighed in Step 1 in absolute ethanol, and stir magnetically to fully dissolve it. Add citric acid as a chelating agent, and adjust the pH value to 2 - 3.5 by adding ammonia water according to Table 1; add the dispersant PVP under stirring to obtain a mixed solution (to improve the stability of the sol). Place the mixed solution in a constant temperature water bath, and stir and react at 60 °C for 2 hours to form a uniform and transparent sol.

[0033] Step 3, Gel Formation: Add the cleaned and dried flaky Al powder to the sol prepared in Step 2, and ultrasonically disperse for 1 hour to uniformly disperse the Al powder in the sol, ensuring that the sol fully wraps the Al powder particles; then let it stand and age at room temperature for 12 hours to gradually transform the sol into a gel.

[0034] Step 4, Drying Treatment: Put the gel obtained in Step 3 into a drying oven and dry at 60 °C for 12 hours until completely dry to obtain a dry gel.

[0035] Step 5, Heat Treatment: Put the dry gel obtained in Step 4 into a muffle furnace. Under an Ar gas atmosphere, heat it up to 400 - 600 °C at a rate of 5 °C / min and hold for 1 - 3 hours to decompose the organic components in the dry gel, decompose cerium nitrate and convert it into nano - cerium oxide, and grow a nano - cerium oxide coating on the surface of the flaky Al powder.

[0036] Step 6, Cooling and Collection: After the heat treatment is completed, let the muffle furnace cool naturally to room temperature, and take out to obtain the Al powder pretreated with nano - cerium oxide.

[0037] Step 7, Coating Preparation: Mix the filler prepared in Step 6 with 20 g of fluorocarbon resin (PTFE), 5 g of additives (including 2 g of leveling agent and 3 g of rheological agent), 15 g of butyl acetate, 24 g of ethyl acetate, and 11 g of fluorocarbon curing agent (B - 1530), and put them into a pneumatic disperser for complete dispersion to obtain the spraying raw material.

[0038] Step 8, Spraying and Curing: Put the spraying raw material prepared in Step 7 into a pressure spray gun, with a spraying pressure of 0.5 MPa, and coat it on the surface of the target carrier; then put it into an electro - thermal blast drying oven and cure at 60 °C for 24 hours to obtain a low - infrared emissivity coating based on the pretreatment of aluminum powder, with a coating thickness of 100 μm.

[0039] Table 1 shows the samples corresponding to different examples prepared by adjusting the cerium nitrate concentration, pH value, heat treatment temperature and time in the example process, and testing and characterization are carried out to illustrate the influence of process parameters on the growth morphology, structure of cerium oxide on the aluminum surface, and the emissivity of the coating.

[0040] Table 1:

[0041]

[0042]

[0043] Comparative example:

[0044] Directly mix 10 g of flaky Al powder without cerium nitrate pretreatment with 20 g of fluorocarbon resin, 5 g of additives, 15 g of butyl acetate, 24 g of ethyl acetate, and 11 g of fluorocarbon curing agent as the coating raw material. Then spray and cure using the same process as in the example to obtain a low infrared emissivity coating.

[0045] Figure 1 It is a conductivity comparison chart of the coating samples obtained in each example and the comparative example. Figure 2 It is a FLIR comparison chart of the coating samples of the comparative example, Example 1, and Example 9. Figure 3 It is an average emissivity comparison chart of the coating samples in each example and the comparative example. Figure 4 It is an image of the filler CeO2-Al prepared in Example 9 under a transmission microscope.

[0046] By comparing the experimental results of Examples 1, 2, and 9, it can be seen that the cerium nitrate concentration should be controlled at 0.2 mol / L to balance the coverage rate and particle size. When the cerium nitrate concentration is too low, the lack of precursors will lead to fewer nucleation sites and limited diffusion in the sol. When the concentration is too high, the hydrolysis rate is too fast, triggering homogeneous nucleation, and the particles grow self-grown in the solution and then deposit.

[0047] By comparing the experimental results of Examples 3, 4, and 9, it can be seen that the pH value is preferably controlled at 3.1 - 3.3. When the pH value is too low, the active dissolution of Al will damage the substrate flatness, resulting in inhibited hydrolysis. When the pH value is too high, rapid hydrolysis generates Ce(OH)3 colloid, resulting in sol instability.

[0048] By comparing the experimental results of Examples 5, 6, and 9, it can be seen that the heat treatment temperature should be controlled at 500 °C. When the temperature is too low, the carbonization residue of citric acid will cause lattice distortion of CeO2. When the temperature is too high, the thermal expansion coefficients of Al and CeO2 will mismatch, triggering microcracks.

[0049] By comparing the experimental results of Examples 7, 8, and 9, it can be seen that the heat treatment holding time should be controlled to 2 hours. Too short a time will lead to incomplete thermal decomposition and limited grain boundary migration.

[0050] It can be seen from the above embodiments and comparative examples that the low infrared emissivity coating designed by the present invention based on aluminum powder pretreatment shows significant technological breakthroughs and application potential; the present invention uses sol-gel assisted chemical conversion technology to utilize the wide bandgap semiconductor characteristics of CeO2 to form a heterojunction with an aluminum substrate, and through the surface plasmon resonance effect of nanoparticles and the synergistic effect of lattice oxygen vacancies, in situ grows nano cerium oxide CeO2 on the surface of aluminum powder and makes a low infrared emissivity coating, demonstrating the advancement of multi-scale composite optical regulation technology; the low infrared emissivity coating of the present invention achieves low infrared emissivity in the 8-14μm atmospheric window band, and also has good high temperature resistance and antioxidant ability, showing broad prospects in the fields of military stealth, spacecraft thermal management, and industrial energy conservation.

Claims

1. A low infrared emissivity coating based on aluminum powder pretreatment, characterized in that: Calculated by mass percentage, it includes the following raw material components: Filler: 10-15wt% cerium nitrate, 30-40wt% aluminum powder, 30-50wt% anhydrous ethanol, 8-12wt% citric acid, 1-2wt% polyvinylpyrrolidone PVP, the sum of the components is 100%; The cerium nitrate is calcined and decomposed into nano-cerium oxide particles, and the surface is regulated by citric acid chelation to form a porous structure; and then chemically bonded with the aluminum powder interface through Al-O-Ce bonding to form a heterojunction of nano-cerium oxide coated aluminum powder to obtain a filler; Coating part: 17-20wt% fluorocarbon resin, 4-6wt% additive, 25-30wt% filler, 11-15wt% butyl acetate, 24-28wt% ethyl acetate, 11-15wt% fluorocarbon curing agent, the sum of each component is 100%; After the raw materials of the coating are evenly dispersed, they are coated on the surface of the target carrier and dried to obtain a coating with low infrared emissivity.

2. The low infrared emissivity coating based on aluminum powder pretreatment according to claim 1, characterized in that: The aluminum powder is in the form of flakes, and the particle size of the flake aluminum powder is 10 to 30 μm.

3. The low infrared emissivity coating based on aluminum powder pretreatment as claimed in claim 1, characterized in that: The specific preparation process of the filler is: Step 1, weighing raw materials according to mass ratio: 0-15wt% cerium nitrate, 30-40wt% aluminum powder, 30-50wt% anhydrous ethanol, 8-12wt% citric acid, 1-2wt% polyvinylpyrrolidone PVP, the sum of each component is 100%; Step 2, sol preparation: fully dissolve cerium nitrate in anhydrous ethanol; then add citric acid as a chelating agent, add ammonia water to adjust the pH value to 2-3.5; add dispersant PVP in a stirring state to obtain a mixed solution; stir the mixed solution in a 60°C water bath environment for 2-4 hours to form a uniform and transparent sol; Step 3, gel formation: adding Al powder to the sol prepared in step 2 and ultrasonically dispersing the mixture until it is uniform, ensuring that the sol fully wraps the Al powder particles; and then standing and aging the mixture at room temperature until the sol is transformed into a gel; Step 4, drying treatment: drying the gel obtained in step 3 at 60-80°C to obtain a dry gel; Step 5, heat treatment: heating the dry gel obtained in step 4 to 300-500° C. in an Ar gas atmosphere and keeping the temperature for at least 2 hours to decompose the organic components in the dry gel; Step 6, cooling and collection: After the heat treatment in step 5 is completed, the mixture is naturally cooled to room temperature to obtain Al powder pretreated with nano-cerium oxide.

4. The low infrared emissivity coating based on aluminum powder pretreatment as claimed in claim 1, characterized in that: The CF bonds in the molecular chain of the fluorocarbon resin account for ≥30% to improve the corrosion resistance of the overall coating.

5. The low infrared emissivity coating based on aluminum powder pretreatment as claimed in claim 1, characterized in that: The pH value of the sol in step 2 is 3.

2.

6. The low infrared emissivity coating based on aluminum powder pretreatment according to claim 1, characterized in that: The concentration of the cerium nitrate is 0.2 mol / L.

7. The low infrared emissivity coating based on aluminum powder pretreatment as claimed in claim 1, characterized in that: The pH is 3.1-3.

3.

8. The low infrared emissivity coating based on aluminum powder pretreatment as claimed in claim 1, characterized in that: The heat treatment temperature should be 500°C and the heat treatment holding time should be 2 hours.

9. The low infrared emissivity coating based on aluminum powder pretreatment as claimed in claim 1, characterized in that: After the raw materials of the coating are evenly dispersed, a pressure spray gun is used with a spraying pressure of 0.3-0.6 MPa to coat the surface of the target carrier, and a low infrared emissivity coating with a thickness of 80-120 μm is obtained after drying.

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