A core-shell structured sheet-like silver-coated aluminum powder, its preparation method and application

Abstract

This application discloses a core-shell structured sheet-like silver-coated aluminum powder, its preparation method, and its applications, specifically relating to the field of electromagnetic shielding materials. The method includes: pretreatment of the sheet-like aluminum powder to remove the oxide film; dispersing the pretreated sheet-like aluminum powder in an aqueous solution of tris(hydroxymethyl)aminomethane hydrochloride, and adding dopamine hydrochloride to perform a polymerization reaction, obtaining sheet-like aluminum powder coated with a polydopamine layer; activating the polydopamine-coated sheet-like aluminum powder with silver nitrate to obtain activated sheet-like aluminum powder; mixing the activated sheet-like aluminum powder with a silver ammonia solution, and adding a reducing solution to perform a silver mirror reaction, obtaining sheet-like silver-coated aluminum powder. The structure of the sheet-like silver-coated aluminum powder is: the surface of the sheet-like aluminum powder is sequentially coated with a polydopamine layer and a silver shell layer from the inside out. This improves the environmental stability and corrosion resistance of the sheet-like silver-coated aluminum powder.

Description

Technical Field

[0001] This application relates to the field of electromagnetic shielding materials, and in particular to a core-shell structured sheet-like silver-coated aluminum powder, its preparation method, and its application. Background Technology

[0002] With the rapid development of science and technology, increasingly serious electromagnetic radiation poses a safety hazard to humankind. Electromagnetic shielding materials are the main materials for suppressing electromagnetic interference and reducing electromagnetic pollution. Currently, the fillers used for electromagnetic shielding in the domestic market are mainly silver-based or aluminum-based conductive metal materials. Although silver, as a precious metal, has good conductivity and conductivity stability, its high cost limits its application. Aluminum powder, due to its irregular surface and easy oxidation, weakens the electrical continuity between powder particles, resulting in insufficient shielding effectiveness. Etching treatment is usually required to remove the oxide film on the surface of aluminum powder.

[0003] Silver-coated aluminum powder is a novel electromagnetic shielding powder that organically combines the advantages of both materials. However, its preparation process is complex and its reliability is low, mainly due to the following reasons: First, aluminum powder is chemically reactive and generally has an aluminum oxide film on its surface, which has very poor conductivity. When the oxide film on the aluminum powder surface is not completely removed, the silver coating layer cannot directly contact the aluminum powder surface, thus weakening the conductivity. Second, the commonly used methods for removing the oxide film are alkaline washing or acid washing, which are prone to excessive corrosion, forming pores on the aluminum powder surface. This results in poor contact between the silver layer and the aluminum powder, easily causing the silver-coated aluminum powder to... Internal oxidation is prone to occur under high-temperature environments, and the direct contact between the surface silver coating layer and the aluminum core substrate under the influence of moisture-containing air can also cause electrochemical corrosion, thus affecting the environmental stability of the silver-plated aluminum powder. Furthermore, since the uniformity of the chemical silver plating process is very sensitive to the surface morphology of the coated powder, commercial silver-plated aluminum powder currently mainly uses spherical aluminum powder raw materials to ensure coating stability. However, spherical silver-plated aluminum powder has a high density and a low specific surface area, and the limited powder filler content makes it difficult to form an effective conductive network inside the coating, thus having limited improvement on electromagnetic shielding effectiveness. Summary of the Invention

[0004] The main objective of this application is to provide a core-shell structured sheet-like silver-coated aluminum powder, its preparation method, and its application, which solves the problem that existing electromagnetic shielding powders cannot simultaneously guarantee environmental stability and electromagnetic shielding effectiveness.

[0005] To achieve the above objectives, this application provides a method for preparing core-shell structured flake-shaped silver-coated aluminum powder, comprising: pretreatment of flake-shaped aluminum powder to remove oxide film; dispersing the pretreated flake-shaped aluminum powder in an aqueous solution of tris(hydroxymethyl)aminomethane hydrochloride, and adding dopamine hydrochloride to carry out a polymerization reaction to obtain flake-shaped aluminum powder coated with a polydopamine layer; activating the flake-shaped aluminum powder coated with the polydopamine layer with silver nitrate to obtain activated flake-shaped aluminum powder; mixing the activated flake-shaped aluminum powder with a silver ammonia solution, and adding a reducing solution to carry out a silver mirror reaction to obtain flake-shaped silver-coated aluminum powder.

[0006] Optionally, the aluminum flakes are pretreated with an oxide film using an ammonium fluoride solution; wherein the mass ratio of the aluminum flakes to the ammonium fluoride solution is 1:10-50, and the concentration of the ammonium fluoride is 5-10 g / L.

[0007] Optionally, in the reaction solution of the polymerization reaction, the concentration of tris(hydroxymethyl)aminomethane hydrochloride is 0.05-0.2 mol / L, and the concentration of dopamine hydrochloride is 0.5-2 g / L; the reaction time of the polymerization reaction is 16-48 h.

[0008] Optionally, the preparation method of activated flake aluminum powder includes: dispersing flake aluminum powder coated with a polydopamine layer in an aqueous solution of ethanol, adding silver nitrate solution to carry out an activation reaction, washing and drying the reaction solution to obtain activated flake aluminum powder; wherein, in the activation reaction, the reaction temperature is 20-60℃ and the reaction time is 4-10h.

[0009] Optionally, the mass ratio of the sheet aluminum powder coated with the polydopamine layer to silver nitrate is 20:1-4.

[0010] Optionally, the method for preparing the reducing solution includes: mixing a dispersant, a reducing agent, and deionized water to obtain the reducing solution; wherein the mass ratio of the dispersant to the reducing agent is 1:13-15.

[0011] Optionally, the dispersant includes one or more of gelatin, polymethylpyrrolidone, OP-10, and sodium dodecylbenzenesulfonate; the reducing agent includes one or more of glucose, formaldehyde solution, hydrazine hydrate, and potassium sodium tartrate.

[0012] Optionally, the particle size of the flake aluminum powder is 20-30 μm; before pretreatment to remove the oxide film from the flake aluminum powder, the method further includes: immersing the flake aluminum powder in anhydrous ethanol and subjecting it to ultrasonic treatment.

[0013] This application also provides a core-shell structured sheet-like silver-coated aluminum powder, obtained by the above preparation method, comprising sheet-like aluminum powder, wherein the surface of the sheet-like aluminum powder is sequentially coated with a polydopamine layer and a silver shell layer from the inside out.

[0014] This application also provides the application of core-shell structured flake silver-coated aluminum powder in electromagnetic shielding coatings.

[0015] Compared with the prior art, the beneficial effects of this application are as follows: The core-shell structure of the present invention uses flake-shaped silver-coated aluminum powder as raw material. When used as a conductive filler to form a conductive path, the flake-shaped silver powder has a relatively low resistance and stronger conductivity due to the line or surface contact between its particles compared to the point contact of spherical aluminum powder. At the same time, the specific surface area of ​​the flake-shaped silver powder is relatively larger than that of spherical silver powder, which can form a denser network structure in the material, thereby improving the conductivity of the material. The insulating polydopamine in the middle layer can slow down the galvanic corrosion between the silver shell layer and the aluminum powder core, improve the environmental stability of the flake-shaped silver-coated aluminum powder, and enhance its corrosion resistance. Thus, while ensuring environmental stability, the electromagnetic shielding effectiveness is improved.

[0016] The method for preparing core-shell structured sheet-like silver-coated aluminum powder of the present invention uses ammonium fluoride to pre-treat the aluminum powder to remove the oxide film, which not only ensures the complete removal of the oxide film, but also avoids the formation of pores caused by excessive acid and alkali corrosion, thus improving the uniformity of coating and the stability of the silver-coated aluminum powder. Using dopamine hydrochloride as a raw material, the weakly reducing polydopamine reacts with silver nitrate to complete the activation. Dopamine molecules contain active functional groups such as amino and hydroxyl groups, which can self-polymerize to form polydopamine under alkaline conditions. These active functional groups in polydopamine coordinate with silver ions, thereby playing a role in connecting and fixing silver ions during the silver deposition process, promoting uniform silver coating on the aluminum powder surface and forming a strong bonding force. The insulating properties of polydopamine slow down the galvanic corrosion between the silver shell layer and the aluminum powder core, improving the environmental stability and corrosion resistance of the sheet-like silver-coated aluminum powder. Attached Figure Description

[0017] Figure 1 This is a scanning electron microscope image of the pretreated flake-shaped silver-coated aluminum powder obtained in Example 1 of the preparation method of the core-shell structure of the present application; Figure 2 This refers to flaky aluminum powder pretreated using traditional methods. Figure 3 This is a scanning electron microscope image of the polydopamine-coated sheet silver-coated aluminum powder prepared in Example 1 of the preparation method of the core-shell structure of the present application. Figure 4 This is a scanning electron microscope (SEM) image of the flake-shaped silver-coated aluminum powder obtained in Example 1 of the preparation method of the core-shell structured flake-shaped silver-coated aluminum powder of this application. Figure 5 This is a structural diagram of the flake-shaped silver-coated aluminum powder obtained in Example 1 of the preparation method of a core-shell structured flake-shaped silver-coated aluminum powder according to this application; Figure 6 This image shows the salt spray test results of the coating prepared from the flake silver-coated aluminum powder obtained in Example 1 of the preparation method of the core-shell structure flake silver-coated aluminum powder of this application. Figure 7 The image shows the salt spray test results of the coating prepared from the flake-shaped silver-coated aluminum powder obtained in Comparative Example 2.

[0018] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] The first embodiment of the present invention provides a method for preparing the above-mentioned core-shell structured sheet-like silver-coated aluminum powder, specifically including the following steps: Step S1: Pretreatment to remove oxide film from flake aluminum powder; In this embodiment, the particle size of the flake aluminum powder is 20-30 μm. When using flake aluminum powder as a conductive filler to form a conductive path, because the particles have line or surface contact, compared to the point contact of spherical aluminum powder, the flake aluminum powder has a relatively low resistance and stronger conductivity. At the same time, the specific surface area of ​​flake aluminum powder is relatively larger than that of spherical aluminum powder, which can form a denser network structure in the material, thereby improving the conductivity of the material. In addition, the pretreatment of the flake aluminum powder with ammonium fluoride solution for oxide film not only ensures the complete removal of oxide film, but also avoids the formation of pores caused by excessive acid and alkali corrosion, improving the uniformity of coating and the stability of silver-coated aluminum powder. Specifically, the flake aluminum powder is dispersed in an aqueous solution of ammonium fluoride, stirred at room temperature for 30 min, and after removing the upper layer of foam, the remaining solution is washed with water and filtered to remove the oxide film on the surface of the flake aluminum powder. The mass ratio of flake aluminum powder to ammonium fluoride solution is 1:10-50, and the concentration of ammonium fluoride is 5-10 g / L.

[0021] Furthermore, before removing the oxide film, the flake aluminum powder needs to be treated to remove impurities. Specifically, the flake aluminum powder can be soaked in anhydrous ethanol and subjected to ultrasonic treatment, and then the flake aluminum powder can be separated from the solution by vacuum filtration.

[0022] Step S2: Disperse the pretreated flake aluminum powder in an aqueous solution of tris(hydroxymethyl)aminomethane hydrochloride, and add dopamine hydrochloride to carry out a polymerization reaction. Adjust the pH of the mixture to 8.4-8.6 to obtain flake aluminum powder coated with a polydopamine layer. Specifically, trimethylolaminomethane hydrochloride is added to deionized water to achieve a concentration of 0.05-0.2 mol / L, resulting in an aqueous solution of trimethylolaminomethane hydrochloride. Dopamine hydrochloride is added to the aqueous solution of trimethylolaminomethane hydrochloride, and the pH of the mixture is adjusted to 8.5 using ammonia and concentrated hydrochloric acid. Then, pretreated flake aluminum powder is added. The reaction system is wrapped in tin foil and stirred. After reacting for 16-48 hours, the reaction product is washed with water, filtered, and the solution and aluminum powder are separated. The product is then placed in a vacuum drying oven at 60-80℃ and dried for 8-12 hours to obtain flake aluminum powder coated with a polydopamine layer. The concentration of dopamine hydrochloride is 0.5-2 g / L, and the mass ratio of dopamine hydrochloride to flake aluminum powder is 6:25.

[0023] Step S3: Activate the sheet aluminum powder coated with a polydopamine layer using silver nitrate to obtain activated sheet aluminum powder. Specifically, aluminum flakes coated with a polydopamine layer are dispersed in an aqueous ethanol solution, and silver nitrate is added for activation. The reaction temperature is 20-60℃, and the reaction time is 4-10h. The reaction solution is then washed and dried to obtain activated aluminum flakes. The mass ratio of the aluminum flakes coated with the polydopamine layer to silver nitrate is 20:1-4.

[0024] In this embodiment, dopamine hydrochloride is used as a raw material. Taking advantage of its adsorption properties, a layer of polydopamine is first polymerized in situ on the surface of aluminum powder. On the one hand, the weakly reducing polydopamine reacts with silver nitrate to complete the activation. On the other hand, the insulating polydopamine slows down the galvanic corrosion between the silver shell and the aluminum powder core, thereby improving the environmental stability and corrosion resistance of the flake silver-coated aluminum powder.

[0025] Step S4: Mix activated flake aluminum powder and silver ammonia solution, and add reducing solution to carry out silver ammonia reaction to obtain flake silver-coated aluminum powder.

[0026] Specifically, the dispersant and reducing agent are mixed in deionized water at a mass ratio of 1:13-15 to obtain a reducing solution; activated flake aluminum powder is dispersed in silver ammonia solution, and ethanol solution is added and stirred thoroughly to obtain a mixture; the mixture is wrapped in tin foil and kept at 40℃ for 30 min; the reducing solution is injected into the tin foil using a syringe pump to carry out the silver ammonia reaction at a reaction temperature of 40℃ for 1 h; the pH of the reaction solution is kept at 11, and the presence of silver ions in the supernatant is detected using sodium chloride solution during the reaction until no silver ions remain in the supernatant; the supernatant is removed, and the mixture is washed with deionized water until neutral, and then vacuum dried to obtain flake silver-coated aluminum powder.

[0027] In this embodiment, maintaining the silver ammonia solution at 40°C allows the silver ions and other reactants in the solution to reach a relatively stable active state at this specific temperature, providing stable reaction conditions for the subsequent electroless plating process. If electroless plating begins directly without this heat preservation, the reaction rate may be very slow at the beginning due to the low temperature of the silver ammonia solution, affecting the plating effect and efficiency. The heat preservation process helps the various components in the silver ammonia solution to mix evenly. Specifically, at 40°C, molecular motion in the solution intensifies, allowing silver ions, ammonia molecules, and other components to be more evenly distributed in the solution. This allows silver ions to undergo a more uniform reduction reaction on the surface of the workpiece during electroless plating, resulting in a more uniform silver plating layer. Without the heat preservation process, the solution may exhibit localized temperature differences and uneven composition. During electroless plating, this unevenness can lead to uneven silver plating thickness, resulting in areas where the silver plating is too thick or too thin, affecting the quality and appearance of the silver-plated product.

[0028] The dispersant includes one or more of gelatin, polymethylpyrrolidone, OP-10, and sodium dodecylbenzenesulfonate; the reducing agent includes one or more of glucose, formaldehyde solution, hydrazine hydrate, and potassium sodium tartrate.

[0029] Example 1 Step S1: Soak 5g of flake aluminum powder in 200ml of anhydrous ethanol and let it stand for soaking. Then, sonicate for 30min. After that, separate the flake aluminum powder from the solution by suction filtration to obtain flake aluminum powder with surface impurities removed. Step S2: Disperse the flake aluminum powder obtained in step S1 in 400ml of deionized water, add 4g of ammonium fluoride, stir and react at room temperature for 30min, remove the upper foam, wash the remaining solution with water and filter to remove the oxide film on the surface of the flake aluminum powder. Step S3: Add tris(hydroxymethyl)aminomethane hydrochloride to deionized water to make the concentration of tris(hydroxymethyl)aminomethane hydrochloride 0.1 mol / L, thus obtaining an aqueous solution of tris(hydroxymethyl)aminomethane hydrochloride; add 1.2 g of dopamine hydrochloride to the aqueous solution of tris(hydroxymethyl)aminomethane hydrochloride, and adjust the pH of the mixture to 8.5 using ammonia and concentrated hydrochloric acid. Then add the flake aluminum powder obtained in step S2, wrap the reaction system with tin foil, stir at 350 rpm, and react for 24 h. After that, wash the reaction product with water, filter it, separate the solution and aluminum powder, and place it in a vacuum drying oven at 60°C for 12 h to obtain flake aluminum powder coated with a polydopamine layer. Step S4: Disperse the polydopamine-coated aluminum powder obtained in step S3 in a mixture of 400 ml deionized water and 20 ml anhydrous ethanol, and add 0.5 g silver nitrate for activation reaction. React in a water bath at 40 °C for 8 h, separate the upper solution, and then wash and dry the solid with deionized water to obtain activated aluminum powder. Step S5: Add 3.37g of silver nitrate to 674ml of deionized water, stir to dissolve the silver nitrate, and then add concentrated ammonia dropwise until the solution is just clear to obtain a silver ammonia solution. Add 0.5g of PVP, 6.74g of glucose, and 30.33g of potassium sodium tartrate to 202ml of deionized water and mix to obtain a reducing solution. Disperse the activated flake aluminum powder in the silver ammonia solution and add 20ml of ethanol solution and stir thoroughly to obtain a mixture. Wrap the mixture in tin foil and keep it at 40℃ for 30min. Use a syringe pump to inject the reducing solution into the tin foil at a rate of 200ml / h to carry out the silver ammonia reaction. After the injection is completed, let the reaction continue for 30min. Use a pH meter to check the pH and adjust the pH to 11 with ammonia to continue the reaction. During the reaction, use sodium chloride solution to check whether there are residual silver ions in the supernatant until there are no residual silver ions in the supernatant. Remove the supernatant and wash with deionized water until neutral. Vacuum dry to obtain flake silver-coated aluminum powder.

[0030] The scanning electron microscope image of the flake aluminum powder after step S2 in this embodiment is as follows: Figure 1 As shown, this invention treats flake aluminum powder with acid and alkali, and the resulting scanning electron microscope image is shown below. Figure 2 As shown, comparison Figure 1 and Figure 2 It can be seen that by observing the surface of aluminum powder treated with ammonium fluoride, acid and alkali using SEM, the oxide film on the surface of aluminum powder treated with ammonium fluoride is removed and there are no obvious pores on the surface of aluminum powder. However, the surface of aluminum powder treated with acid and alkali will show corrosion marks such as pores.

[0031] The scanning electron microscope image of the polydopamine-coated sheet aluminum powder obtained in step S3 of this embodiment is shown below. Figure 3As shown in the figure, the surface of the flake-shaped aluminum powder is coated with a polydopamine layer; the scanning electron microscope image of the flake-shaped silver-coated aluminum powder prepared in this embodiment is shown below. Figure 4 As shown in the image, it can be clearly seen that the surface of the aluminum powder is covered with a layer of material different from the aluminum substrate. This layer is deposited dopamine or its polymer product polydopamine. The aluminum powder originally has a relatively smooth surface, but after dopamine treatment, the surface becomes rough or exhibits a granular or film-like coating.

[0032] Example 2 Step S1: Soak 5g of flake aluminum powder in 200ml of anhydrous ethanol and let it stand for soaking. Then, sonicate for 30min. After that, separate the flake aluminum powder from the solution by suction filtration to obtain flake aluminum powder with surface impurities removed. Step S2: Disperse the flake aluminum powder obtained in step S1 in 400ml of deionized water, add 3g of ammonium fluoride, stir and react at room temperature for 30min, remove the upper foam, wash the remaining solution with water and filter to remove the oxide film on the surface of the flake aluminum powder. Step S3: Add 400 ml of deionized water to tris(hydroxymethyl)aminomethane hydrochloride to make the concentration of tris(hydroxymethyl)aminomethane hydrochloride 0.15 mol / L, thus obtaining an aqueous solution of tris(hydroxymethyl)aminomethane hydrochloride; add 1.6 g of dopamine hydrochloride to the aqueous solution of tris(hydroxymethyl)aminomethane hydrochloride, and adjust the pH of the mixture to 8.5 using ammonia and concentrated hydrochloric acid. Then add the flake aluminum powder obtained in step S2, wrap the reaction system with tin foil, stir at 350 rpm, and react for 20 h. After that, wash the reaction product with water, filter it, separate the solution and aluminum powder, and place it in a vacuum drying oven at 70 °C for 12 h to obtain flake aluminum powder coated with a polydopamine layer. Step S4: Disperse the polydopamine-coated aluminum powder obtained in step S3 in a mixture of 400 ml deionized water and 20 ml anhydrous ethanol, and add 0.5 g silver nitrate for activation reaction. React in a water bath at 40 °C for 12 h, separate the upper solution, and then wash and dry the solid with deionized water to obtain activated aluminum powder. Step S5: Add 3.37g of silver nitrate to 674ml of deionized water, stir to dissolve the silver nitrate, and then add concentrated ammonia dropwise until the solution is just clear to obtain a silver ammonia solution. Add 2g of gelatin and 40ml of formaldehyde solution to 400ml of deionized water and mix to obtain a reducing solution. Disperse the activated flake aluminum powder in the silver ammonia solution and add 20ml of ethanol solution and stir thoroughly to obtain a mixture. Wrap the mixture in tin foil and keep it at 40℃ for 30min. Use a syringe pump to inject the reducing solution into the tin foil at a rate of 200ml / h to carry out the silver ammonia reaction. After the injection is completed, let the reaction continue for 30min. Use a pH meter to check the pH and adjust the pH to 11 with ammonia to continue the reaction. During the reaction, use sodium chloride solution to check whether there are residual silver ions in the supernatant until there are no residual silver ions in the supernatant. Remove the supernatant and wash with deionized water until neutral. After vacuum drying, obtain flake silver-coated aluminum powder.

[0033] Example 3 Step S1: Soak 5g of flake aluminum powder in 200ml of anhydrous ethanol and let it stand for soaking. Then, sonicate for 30min. After that, separate the flake aluminum powder from the solution by suction filtration to obtain flake aluminum powder with surface impurities removed. Step S2: Disperse the flake aluminum powder obtained in step S1 in 300ml of deionized water, add 3g of ammonium fluoride, stir and react at 40℃ for 30min, remove the upper foam, wash the remaining solution with water and filter to remove the oxide film on the surface of the flake aluminum powder. Step S3: Add 300 ml of deionized water to tris(hydroxymethyl)aminomethane hydrochloride to make the concentration of tris(hydroxymethyl)aminomethane hydrochloride 0.5 mol / L, thus obtaining an aqueous solution of tris(hydroxymethyl)aminomethane hydrochloride; add 1.2 g of dopamine hydrochloride to the aqueous solution of tris(hydroxymethyl)aminomethane hydrochloride, and adjust the pH of the mixture to 8.5 using ammonia and concentrated hydrochloric acid. Then add the flake aluminum powder obtained in step S2, wrap the reaction system with tin foil, stir at 350 rpm, and react for 48 h. After that, wash the reaction product with water, filter it, separate the solution and aluminum powder, and place it in a vacuum drying oven at 80 °C for 12 h to obtain flake aluminum powder coated with a polydopamine layer. Step S4: Disperse the polydopamine-coated aluminum powder obtained in step S3 in a mixture of 120 ml deionized water and 20 ml anhydrous ethanol, and add 1.2 g silver nitrate for activation reaction. React in a water bath at 40 °C for 12 h, separate the upper solution, and then wash and dry the solid with deionized water to obtain activated aluminum powder. Step S5: Add 3.37g of silver nitrate to 450ml of deionized water, stir to dissolve the silver nitrate, and then add concentrated ammonia dropwise until the solution is just clear to obtain a silver ammonia solution; add 4g of OP-10 and 12g of hydrazine hydrate to 350ml of deionized water and mix to obtain a reducing solution; disperse the activated flake aluminum powder in the silver ammonia solution and add 20ml of ethanol solution and stir thoroughly to obtain a mixture; mix the reducing solution directly with the silver ammonia solution and continue the reaction for 30min, using sodium chloride solution to detect whether there are residual silver ions in the supernatant during the reaction until there are no residual silver ions in the supernatant; remove the supernatant and wash with deionized water until neutral, then vacuum dry to obtain flake silver-coated aluminum powder.

[0034] Example 4 Step S1: Soak 5g of flake aluminum powder in 200ml of anhydrous ethanol and let it stand for soaking. Then, sonicate for 30min. After that, separate the flake aluminum powder from the solution by suction filtration to obtain flake aluminum powder with surface impurities removed. Step S2: Disperse the flake aluminum powder obtained in step S1 in 300ml of deionized water, add 2g of ammonium fluoride, stir and react at room temperature for 30min, remove the upper foam, wash the remaining solution with water and filter to remove the oxide film on the surface of the flake aluminum powder. Step S3: Add 200 ml of deionized water to trimethylolaminomethane hydrochloride to make the concentration of trimethylolaminomethane hydrochloride 0.2 mol / L, thus obtaining an aqueous solution of trimethylolaminomethane hydrochloride; add 0.8 g of dopamine hydrochloride to the aqueous solution of trimethylolaminomethane hydrochloride, and adjust the pH of the mixture to 8.5 using ammonia and concentrated hydrochloric acid. Then add the flake aluminum powder obtained in step S2, wrap the reaction system with tin foil, stir at 350 rpm, and react for 24 h. After that, wash the reaction product with water, filter it, separate the solution and aluminum powder, and place it in a vacuum drying oven at 60°C for 12 h to obtain flake aluminum powder coated with a polydopamine layer. Step S4: Disperse the polydopamine-coated aluminum powder obtained in step S3 in a mixture of 150 ml deionized water and 20 ml anhydrous ethanol, and add 0.8 g silver nitrate for activation reaction. React in a water bath at 40 °C for 12 h, separate the upper solution, and then wash and dry the solid with deionized water to obtain activated aluminum powder. Step S5: Add 3.37g of silver nitrate to 450ml of deionized water, stir to dissolve the silver nitrate, and then add concentrated ammonia dropwise until the solution is just clear to obtain a silver ammonia solution; add 2g of SDBS and 9.53g of glucose to 350ml of deionized water and mix to obtain a reducing solution; disperse the activated flake aluminum powder in the silver ammonia solution and add 20ml of ethanol solution and stir thoroughly to obtain a mixture; inject the reducing solution into the mixture at a rate of 200ml / h using a syringe pump to continue the reaction for 30min, and use sodium chloride solution to detect whether there are residual silver ions in the supernatant during the reaction until there are no residual silver ions in the supernatant; remove the supernatant and wash with deionized water until neutral, and vacuum dry to obtain flake silver-coated aluminum powder.

[0035] Example 5 Step S1: Soak 5g of flake aluminum powder in 200ml of anhydrous ethanol and let it stand for soaking. Then, sonicate for 30min. After that, separate the flake aluminum powder from the solution by suction filtration to obtain flake aluminum powder with surface impurities removed. Step S2: Disperse the flake aluminum powder obtained in step S1 in 200ml of deionized water, add 2g of ammonium fluoride, stir and react at room temperature for 30min, remove the upper foam, wash the remaining solution with water and filter to remove the oxide film on the surface of the flake aluminum powder. Step S3: Add 150 ml of deionized water to trimethylolaminomethane hydrochloride to make the concentration of trimethylolaminomethane hydrochloride 0.2 mol / L, thus obtaining an aqueous solution of trimethylolaminomethane hydrochloride; add 0.5 g of dopamine hydrochloride to the aqueous solution of trimethylolaminomethane hydrochloride, and adjust the pH of the mixture to 8.5 using ammonia and concentrated hydrochloric acid. Then add the flake aluminum powder obtained in step S2, wrap the reaction system with tin foil, stir at 350 rpm, and react for 48 h. After that, wash the reaction product with water, filter it, separate the solution and aluminum powder, and place it in a vacuum drying oven at 60 °C for 10 h to obtain flake aluminum powder coated with a polydopamine layer. Step S4: Disperse the polydopamine-coated aluminum powder obtained in step S3 in a mixture of 150 ml deionized water and 20 ml anhydrous ethanol, and add 2 g silver nitrate for activation reaction. React in a water bath at 40 °C for 24 h, separate the upper solution, and then wash and dry the solid with deionized water to obtain activated aluminum powder. Step S5: Add 3.37g of silver nitrate to 300ml of deionized water, stir to dissolve the silver nitrate, and then add concentrated ammonia dropwise until the solution is just clear to obtain a silver ammonia solution; add 0.5g of PVP and 6.74g of glucose to 200ml of deionized water and mix to obtain a reducing solution; disperse the activated flake aluminum powder in the silver ammonia solution and add 20ml of ethanol solution and stir thoroughly to obtain a mixture; inject the reducing solution into the mixture at a rate of 200ml / h using a syringe pump to continue the reaction for 30min, and use sodium chloride solution to detect whether there are residual silver ions in the supernatant during the reaction until there are no residual silver ions in the supernatant; remove the supernatant and wash with deionized water until neutral, and vacuum dry to obtain flake silver-coated aluminum powder.

[0036] Comparative Example 1 The difference from Example 1 is that the flake aluminum powder is replaced with spherical aluminum powder.

[0037] Comparative Example 2 The difference from Example 1 is that step S3 is omitted, and the flake aluminum powder obtained in step S2 is directly activated in step S4.

[0038] A second embodiment of the present invention provides a core-shell structured sheet-like silver-coated aluminum powder, obtained by the above-described preparation method, such as... Figure 5 As shown, it includes flake aluminum powder, and the surface of the flake aluminum powder is coated with a polydopamine layer and a silver shell layer from the inside out.

[0039] The third embodiment of the present invention provides an application of the above-described core-shell structured flake silver-coated aluminum powder in coatings.

[0040] The silver-coated flake aluminum powders prepared in Examples 1-5 and Comparative Examples 1-2 were mixed with epoxy resin to prepare coatings, and the corresponding coatings were prepared using compressed air spraying technology. The specific methods are as follows: 75 parts of silver-coated flake aluminum powder, 20 parts of epoxy resin, and 5 parts of polyamide curing agent were mixed and dispersed at 3000 rpm for 15 minutes using a stirring disperser. Then, an industrial solvent was added to adjust the coating viscosity to 20-25 mPa·s, and stirring was continued at 3000 rpm for another 15 minutes to obtain the electromagnetic shielding coating. The epoxy resin is a transparent viscous liquid, the polyamide curing agent is a medium-viscosity brownish-yellow liquid, and the industrial solvent is xylene, a colorless and transparent liquid with a characteristic aromatic hydrocarbon odor.

[0041] The electromagnetic shielding coating with a thickness of 1 mm was prepared by cold spraying. The specific method is as follows: Pour the paint into the W-71 spray gun, adjust the spraying pressure to 0.5MPa, the spraying distance to 15-20cm, and the spray gun moving speed to 30-40cm / s. Spray evenly onto the substrate surface using a cross-shaped path. After each 0.2mm layer is sprayed, dry the sample at room temperature for 10 minutes until the resulting coating thickness reaches 1mm. Then place the sample in an oven and maintain the temperature at 30℃ for 30 minutes, followed by 80℃ for 12 hours to allow the coating to cure.

[0042] The average shielding effectiveness of the coatings prepared using the sheet-like silver-coated aluminum powder obtained in the above embodiments and comparative examples in the X-band was tested using an Agilent Technologies E8362B vector network analyzer via the waveguide method. The test results are shown in Table 1.

[0043] Table 1. Average shielding effectiveness of the X-band

[0044] As shown in Table 1, the sheet-like silver-coated aluminum powder composite coating prepared in the embodiments of the present invention has excellent shielding effect on electromagnetic waves in the X-band, and its electromagnetic shielding effectiveness value is higher than 80dB, which is much higher than that of the silver-coated spherical aluminum powder in Comparative Example 1 (70dB).

[0045] The corrosion resistance of the coatings prepared from the flake-shaped silver-coated aluminum powder obtained in Example 1 and Comparative Example 2 was tested by salt spray for 3000 hours, with alternating 24-hour spraying and 24-hour drying. The test results are as follows: Figure 6-7 As can be seen from the comparison in the figure, the coating prepared by the flake silver-coated aluminum powder obtained in Example 1 has excellent corrosion resistance. After long-term corrosion, there are no defects such as rust spots, blistering, or discoloration on the surface, which can meet the requirements for long-term use in corrosive environments.

[0046] The above are merely preferred embodiments of this application and do not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A method for preparing a core-shell structured, sheet-like silver-coated aluminum powder, characterized in that, include: Step S1: Soak 5g of flake aluminum powder in 200ml of anhydrous ethanol and let it stand for soaking. Then, sonicate for 30min. After that, separate the flake aluminum powder from the solution by suction filtration to obtain flake aluminum powder with surface impurities removed. Step S2: Disperse the flake aluminum powder obtained in step S1 in 400ml of deionized water, add 4g of ammonium fluoride, stir and react at room temperature for 30min, remove the upper foam, wash the remaining solution with water and filter to remove the oxide film on the surface of the flake aluminum powder. Step S3: Tris(hydroxymethyl)aminomethane hydrochloride was added to deionized water to make the concentration of tris(hydroxymethyl)aminomethane hydrochloride 0.1 mol / L, thus obtaining an aqueous solution of tris(hydroxymethyl)aminomethane hydrochloride; 1.2 g of dopamine hydrochloride was added to the aqueous solution of tris(hydroxymethyl)aminomethane hydrochloride, and the pH of the mixture was adjusted to 8.5 using ammonia and concentrated hydrochloric acid. Then, the flake aluminum powder obtained in step S2 was added. The reaction system was wrapped with tin foil and stirred at 350 rpm. After reacting for 24 h, the reaction product was washed with water and filtered to separate the solution and aluminum powder. The product was then placed in a vacuum drying oven at 60 °C and dried for 12 h to obtain flake aluminum powder coated with a polydopamine layer. Step S4: Disperse the polydopamine-coated aluminum powder obtained in step S3 in a mixture of 400 ml deionized water and 20 ml anhydrous ethanol, and add 0.5 g silver nitrate for activation reaction. React in a water bath at 40 °C for 8 h, separate the upper solution, and then wash and dry the solid with deionized water to obtain activated aluminum powder. Step S5: Add 3.37g of silver nitrate to 674ml of deionized water, stir to dissolve the silver nitrate, and then add concentrated ammonia dropwise until the solution is just clear to obtain a silver ammonia solution; add 0.5g of PVP, 6.74g of glucose and 30.33g of potassium sodium tartrate to 202ml of deionized water and mix to obtain a reducing solution; disperse activated flake aluminum powder in the silver ammonia solution and add 20ml of ethanol solution and stir thoroughly to obtain a mixture; wrap the mixture in tin foil and keep it at 40℃ for 30min; use a syringe pump to inject the reducing solution into the tin foil at a rate of 200ml / h to carry out the silver ammonia reaction; after the injection is completed, let the reaction continue for 30min; use a pH meter to check the pH and adjust the pH to 11 with ammonia to continue the reaction; during the reaction, use sodium chloride solution to check whether there are residual silver ions in the supernatant until there are no residual silver ions in the supernatant; Remove the supernatant and wash with deionized water until neutral. After vacuum drying, obtain core-shell structured flake-shaped silver-coated aluminum powder.

2. A core-shell structured, sheet-like silver-coated aluminum powder, characterized in that, It is obtained by the preparation method of claim 1.

3. The application of the core-shell structured flake silver-coated aluminum powder as described in claim 2 in electromagnetic shielding coatings.

Images