Compact silver coating completely-coated copper powder and preparation method and application thereof

Abstract

The invention belongs to the technical field of the electronic industry, and particularly relates to compact silver coating completely-coated copper powder and a preparation method and application thereof. The preparation method comprises the following steps: 1) removing an oxide layer of copper powder, then washing the copper powder, and preparing a copper powder suspension; 2) preparing an alkaline silver complex solution; (3) slowly dropwise adding the silver complex solution obtained in the step (2) into the copper powder suspension obtained in the step (1) under the condition of room temperature, and continuously stirring for reaction after dropwise adding is completed; and after the reaction is finished, washing and vacuum drying are carried out in sequence, and the compact silver coating completely-coated copper powder is obtained. By means of the technical scheme, the surface of the copper powder can be evenly coated with the compact silver plating layer, the use amount of silver is reduced, cost is reduced, copper is protected, and oxidation resistance is improved. The preparation method is simple in process, good in repeatability and mild in reaction condition. And the obtained silver plating layer is smooth, uniform and compact.

Description

Technical Field

[0001] This invention belongs to the field of electronic industry technology, specifically relating to a dense silver plating layer that completely coats copper powder, its preparation method, and its application. Background Technology

[0002] Silver (Ag) possesses excellent electrical and thermal conductivity, as well as good high-temperature oxidation resistance and stability, making it an important metal powder material in the modern electronics and information industry. In recent years, with the continuous rise in silver prices, increasing usage, and declining prices of electronic products, relevant enterprises and institutions have had to invest significant effort in researching ways to reduce the silver content of existing silver powder products and lower production costs while still meeting user performance requirements.

[0003] Researchers have proposed using copper (Cu), nickel, zinc, and aluminum powders to replace silver powder, but all have insurmountable drawbacks and unsatisfactory results. For example, nickel paste is easy to form a film, but has low bonding strength and is prone to oxidation under low-oxygen conditions; aluminum paste has low impact current resistance, which greatly limits its application range; zinc paste has a complex sintering process and high equipment requirements. Copper paste is the best alternative in terms of overall performance, with superior conductivity, less high migration than silver, and a cost less than one-twentieth of silver paste. However, the surface of copper particles is easily oxidized to form copper oxide and cuprous oxide, which gradually erode inward over time, leading to a decrease in electron conduction and loss of conductive filler function. Therefore, research on copper electronic pastes focuses on improving the oxidation resistance and conductivity stability of copper powder. One commonly used surface modification technology at home and abroad is to coat the surface of copper particles with a silver plating layer with anti-oxidation properties. Compared with silver powder, silver plating on the surface of copper powder can significantly reduce the amount of silver used, while simultaneously weakening the high migration of silver, improving the oxidation resistance of copper, and enhancing the overall performance of conductive paste. Ideally, silver-coated copper powder should have a dense, uniform silver coating on its surface. The main preparation methods include electrodeposition, high-vacuum thermal evaporation, ion exchange, and chemical methods. However, these methods suffer from cumbersome processes and low copper powder coating rates. Among these methods, the chemical method is the simplest, requiring no large equipment and suitable for industrial production.

[0004] Chemical methods for silver plating of copper powder include displacement and chemical reduction. In the displacement method, the copper powder itself acts as a reducing agent, and the coating process consumes surface atoms of the copper particles. The reaction equation is Cu + 2Ag. + →Cu 2+The +2Ag content indicates that this method is suitable for large copper powder particles. Smaller copper particles have a larger specific surface area and higher surface activity, resulting in a faster displacement reaction and easier complete oxidation of the copper. Furthermore, the copper ions that precipitate and the ammonium ions in the plating solution form copper ammonia ions, which preferentially adsorb onto the copper particle surface over silver ammonia ions, hindering the continued occurrence of the displacement reaction and resulting in speckled silver on the copper particle surface rather than a uniform and dense silver coating. Although copper ammonia ions can be removed by washing with dilute sulfuric acid, and a fully coated silver layer can be obtained through multiple coating processes, the process is cumbersome and inefficient. In summary, current silver coating technology is still immature, exhibiting problems such as uneven and discontinuous silver films on the copper powder surface, as well as thin silver coatings and difficulty in controlling their thickness. This leads to easy oxidation of silver-coated copper powder in practical applications, reducing its conductivity. Therefore, it remains essential to continue exploring and developing simple and convenient methods for preparing dense silver-coated copper powder.

[0005] Patent application CN102950283B discloses a method for preparing ultrafine silver-copper plating powder for electronic pastes. The steps include: (1) surface-treating ultrafine copper powder to obtain copper plating powder; (2) adding dispersant, additives, and reducing agent to deionized water, adding the copper plating powder, and adjusting the pH to 12-14 to obtain a plating mixture; (3) adding silver nitrate to deionized water and an amine complexing agent to obtain a silver amine solution; (4) adding the silver amine solution to the plating mixture to obtain silver-plated copper powder; (5) washing the silver-plated copper powder with deionized water, using it as copper plating powder, and repeating the silver plating cycle from (2) to (4) to obtain secondary silver-plated copper powder; (6) placing the secondary silver-plated copper powder in an organic solution, filtering, dehydrating, and drying to obtain ultrafine silver-plated copper powder. This method uses a separate reducing agent.

[0006] Patent application CN108176849A discloses a silver-coated copper powder, its preparation method, and its applications. The method involves first preparing a mixed solution of sodium alginate and tea polyphenols, then sequentially adding a copper ammonia solution and a silver ammonia solution, and finally centrifuging and drying to obtain silver-coated copper nanoparticles. This method uses sodium alginate as a stabilizer for the nano-copper and prepares it using copper ammonia solution and silver ammonia solution.

[0007] Patent application CN107737949B discloses a silver-coated copper powder and its preparation method. The method includes the following steps: a) mixing the copper powder to be treated, a complexing agent, a stabilizer, and water to obtain a plating solution; b) simultaneously adding a main salt solution and a reducing agent solution to the plating solution obtained in step a), and reacting to obtain a mixed solution; the main salt solution is a silver nitrate solution containing a complexing agent; c) filtering the mixed solution obtained in step b), and then washing and drying it sequentially to obtain the silver-coated copper powder. This preparation method uses a reducing agent solution. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a method for preparing a dense silver coating that completely coats copper powder, along with its applications. The technical solution of this invention enables the uniform coating of a dense silver layer onto the surface of copper powder, reducing silver usage and costs, while protecting the copper and improving its oxidation resistance. The preparation method is simple, reproducible, and uses mild reaction conditions. The resulting silver coating is smooth, uniform, and dense.

[0009] The technical solution provided by this invention is as follows:

[0010] A method for preparing a dense silver plating layer that completely coats copper powder includes the following steps:

[0011] 1) Remove the oxide layer from the copper powder, then wash the copper powder clean and prepare a copper powder suspension;

[0012] 2) Prepare an alkaline silver complex solution;

[0013] 3) At room temperature, the silver complex solution obtained in step 2) is slowly added dropwise to the copper powder suspension obtained in step 1). After the addition is complete, the reaction is stirred continuously. After the reaction is completed, the copper powder is washed and vacuum dried in sequence to obtain a dense silver coating that completely covers the copper powder.

[0014] In the above technical solution:

[0015] In step 2), an alkaline silver complex solution is used. This ensures that the silver replacement reaction proceeds slowly and uniformly, and also avoids the introduction of copper ammonia ions, thus preventing the copper ammonia ions from hindering the replacement reaction and resulting in a uniform and dense silver coating layer.

[0016] Specifically, step 1) includes the following steps:

[0017] 1a) At room temperature, a certain mass of ammonium sulfate and a certain volume of ammonia water are dispersed in deionized water to obtain a solution. Then, a certain mass of copper powder is dispersed in the solution. The suspension is stirred and ultrasonically treated to remove the oxide layer.

[0018] 1b) After removing the oxide layer, the copper powder is centrifuged and washed several times with deionized water, and then dispersed in deionized water, stirred and ultrasonically dispersed to obtain a copper powder suspension.

[0019] Based on the above technical solution:

[0020] Using a mixture of ammonium sulfate and ammonia as an activator can effectively remove the oxide layer of copper powder and protect the surface of copper powder from excessive hydrolysis.

[0021] After removing the oxide layer, the copper powder is washed to remove excess ammonium ions, thereby reducing the impact of ammonium ions on the silver plating growth process in subsequent steps.

[0022] Other reagents can also be used to remove the oxide layer of copper powder. After removal, the copper powder can be washed off.

[0023] Further:

[0024] In step 1a): the mass fraction of ammonia water is 25-28 wt%; the mass ratio of ammonium sulfate, ammonia water, deionized water and copper powder is (4-20):(0.1-2):(10-100):1.

[0025] In step 1b): the mass ratio of deionized water to copper powder is (10-100):1; the stirring time is 10-60 min; and the ultrasonic dispersion time is 5-25 min.

[0026] In the above technical solution:

[0027] By adjusting the ratio of ammonium sulfate and ammonia, as well as the oxide layer removal time, the degree of copper powder oxide layer removal can be controlled to ensure uniform and dense growth of the silver layer.

[0028] After centrifugation and washing, the copper powder agglomerates generated during the centrifugation process can be dispersed by stirring and ultrasonic dispersion. After the copper powder is completely dispersed and no agglomerates are present, the silver coating reaction process can be carried out to ensure good silver coating.

[0029] Specifically, in step 2): at room temperature, a certain mass of alkali and complexing agent is dissolved in deionized water and stirred thoroughly to dissolve. Then, a certain mass of silver salt is added and stirred in the dark until the silver salt is completely dissolved.

[0030] Further:

[0031] The alkali is selected from one or more of potassium hydroxide or sodium hydroxide, and its concentration in the silver complex solution is 1-10 mol / L;

[0032] The complexing agent includes one or more of ethylenediaminetetraacetic acid, tetrasodium ethylenediaminetetraacetic acid, ethylenediamine, triethylenetetraamine, or tetraethylenepentamine, and its concentration in the silver complex solution is 0.5–1.5 mol / L.

[0033] Silver salts are selected from silver nitrate;

[0034] The molar ratio of silver to copper ranges from (0.01 to 1):1.

[0035] In the above technical solution:

[0036] The concentration of the silver complexing solution, the dropping rate, the reaction time, and the stirring speed can control the coverage of the silver coating.

[0037] The thickness of the silver plating layer can be adjusted by changing the molar ratio of silver and copper.

[0038] Specifically, in step 3):

[0039] The silver complex solution was added at a rate of 2–20 mL / min, and the reaction continued for 0–20 min after the addition was complete.

[0040] Wash with deionized water and vacuum dry at a temperature of 50–70°C.

[0041] Specifically, in each step, the stirring speed is independently 500-1000 r / min.

[0042] Specifically, the copper powder has a particle size greater than 0.5 μm.

[0043] Preferably, the method for preparing a dense silver plating layer that completely coats copper powder includes the following steps:

[0044] Step 1: At room temperature, weigh 23.1g of ammonium sulfate and measure 1.8mL of ammonia water, dispersing it in 100mL of deionized water. Then, weigh 4g of copper powder with an average particle size of 3.5μm and disperse it in the above solution. Stir the suspension for 30min, then sonicate for 5min, and finally continue stirring for 20min to complete the oxide layer removal process. After removing the oxide layer, wash the copper powder three times with deionized water, then disperse it in 100mL of deionized water. After thorough dispersion by stirring and sonication, suspension A is obtained.

[0045] Step 2: Weigh 30g of sodium hydroxide and 35g of ethylenediaminetetraacetic acid complexing agent at room temperature and place them in 100mL of deionized water. Stir for 30min until the sodium hydroxide and ethylenediaminetetraacetic acid are completely dissolved. Then add 1.6g of silver nitrate and stir in the dark for 20min to ensure that the silver nitrate is completely dissolved, thus obtaining solution A.

[0046] Step 3: At room temperature, solution A is added dropwise to suspension A at a uniform and slow rate of 10 mL / min. After the addition is complete, the mixture is stirred for another 5 min. After the reaction is complete, the mixture is washed three times by centrifugation with deionized water and methanol, and then dried under vacuum at 60°C to obtain silver-coated copper powder.

[0047] The present invention also provides a dense silver plating layer that completely coats copper powder prepared according to the above preparation method.

[0048] The dense silver plating provided by this invention completely coats the copper powder, resulting in a fully coated, dense silver plating layer. The thickness of the silver plating layer can range from 0.05 to 0.3 μm, and can be further adjusted by changing the concentration of the silver solution.

[0049] This invention also provides an application where a dense silver plating completely coats copper powder for the preparation of conductive pastes.

[0050] The conductive paste prepared according to the present invention has good conductivity and high oxidation resistance.

[0051] The beneficial effects of this invention are:

[0052] This invention utilizes a mixed solution of ammonium sulfate and ammonia as an activation solution to remove the oxide layer on the surface of copper powder. The copper powder particles are completely dispersed by stirring and ultrasonic treatment, ensuring that the oxide layer on the surface of all copper powder particles is fully removed, so that the silver plating layer can grow through the displacement reaction.

[0053] In the process of silver plating growth, the present invention slowly and uniformly adds a silver complexing solution of appropriate concentration to ensure that the silver replacement reaction proceeds slowly and uniformly, thereby achieving uniform and dense growth of silver plating on the surface of copper powder particles, avoiding defects such as cracks or large-sized silver particles in silver plating grown by general methods.

[0054] The method for preparing a dense silver plating layer that completely coats copper powder provided by this invention is a simple and easy-to-operate wet chemical preparation method for silver-coated copper. The coverage and thickness of the silver plating layer can be adjusted, achieving uniform and dense coating. Attached Figure Description

[0055] Figure 1 This is the XRD pattern of the silver-coated copper powder (Cu@Ag-1) of the present invention.

[0056] Figure 2 These are scanning electron microscope (SEM) images of copper powder (Cu) and the silver-coated copper powder (Cu@Ag-1) of the present invention.

[0057] Figure 3 This is a distribution diagram of copper (Cu) and silver (Ag) elements and their overlay in the silver-coated copper powder (Cu@Ag-1) of the present invention.

[0058] Figure 4 These are the N2 adsorption-desorption isotherms of copper powder (Cu) and the silver-coated copper powder (Cu@Ag-1) of the present invention.

[0059] Figure 5 This is a scanning electron microscope image of the silver-coated copper powder (Cu@Ag-2) of the present invention.

[0060] Figure 6 This is a scanning electron microscope image of the silver-coated copper powder (Cu@Ag-3) of the present invention. Detailed Implementation

[0061] The principles and features of the present invention are described below. The embodiments given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0062] In the following embodiments, unless otherwise specified, all raw materials or reagents are from the prior art. The mass fraction of ammonia water is 26.5 wt%.

[0063] In a specific implementation, the method for preparing a uniform and dense silver plating layer coated with copper powder includes the following steps:

[0064] Step 1: Removal of the copper oxide layer. At room temperature, weigh a certain mass of ammonium sulfate and a certain volume of ammonia water and disperse them in deionized water. Then weigh a certain mass of copper powder and disperse it in the above solution. Stir and sonicate the suspension to remove the oxide layer. After removing the oxide layer, wash the copper powder three times with deionized water, then disperse it again in deionized water, stir and sonicate to obtain suspension A.

[0065] Step 2: Preparation of silver nitrate complexing solution. At room temperature, weigh a certain mass of alkali and complexing agent and dissolve them in deionized water. Stir thoroughly until the alkali and complexing agent are completely dissolved. Then add a certain mass of silver nitrate and stir in the dark until the silver nitrate is completely dissolved to obtain solution A.

[0066] Step 3: Copper powder coated with silver. At room temperature, solution A is added dropwise to suspension A at a uniform and slow rate. After the addition is complete, the mixture is stirred for a period of time. After the reaction is complete, the mixture is washed three times sequentially with deionized water and methanol, and then dried under vacuum at 60°C to obtain silver-coated copper powder.

[0067] In step 1, the molar ratio of ammonium sulfate to ammonia is 1–15, the mass of copper powder is 1–5 g, the mixture is stirred and dispersed for 10–60 min, and then ultrasonically treated for 5–25 min. Suspension A needs to be stirred and ultrasonically treated to disperse the copper powder agglomerates generated during centrifugation. After the copper powder is completely dispersed and no agglomerates are present, the silver coating reaction process can be carried out to avoid affecting the silver coating.

[0068] Implementation Case 1:

[0069] Step 1: At room temperature, weigh 23.1 g of ammonium sulfate and measure 1.8 mL of ammonia water, dispersing it in 100 mL of deionized water. Then, weigh 4 g of copper powder with an average particle size of 3.5 μm and disperse it in the above solution. Stir the suspension for 30 min, then sonicate for 5 min, and finally continue stirring for 20 min to complete the oxide layer removal process. After removing the oxide layer, wash the copper powder three times with deionized water, then disperse it in 100 mL of deionized water. After thorough dispersion by stirring and sonication, suspension A is obtained.

[0070] Step 2: Weigh 30g of sodium hydroxide and 35g of ethylenediaminetetraacetic acid complexing agent at room temperature and place them in 100mL of deionized water. Stir for 30min until the sodium hydroxide and ethylenediaminetetraacetic acid are completely dissolved. Then add 1.6g of silver nitrate and stir in the dark for 20min to ensure that the silver nitrate is completely dissolved, thus obtaining solution A.

[0071] Step 3: At room temperature, solution A is added dropwise to suspension A at a uniform and slow rate of 10 mL / min. After the addition is complete, the mixture is stirred for another 5 min. After the reaction is complete, the mixture is washed three times by centrifugation with deionized water and methanol, and then dried under vacuum at 60 °C to obtain silver-coated copper powder, Cu@Ag-1.

[0072] like Figure 1 The figure shows the XRD pattern of the obtained silver-coated copper powder (Cu@Ag-1). As can be seen from the figure, the silver-coated copper powder phase includes diffraction peaks of both copper and silver, with no other impurities.

[0073] like Figure 2 The image shown is a scanning electron microscope (SEM) image of the obtained silver-coated copper powder (Cu@Ag-1). It can be seen from the image that the copper powder particles have a smooth surface, while the surface roughness increases after the silver plating is deposited, resulting in a uniform and dense silver layer.

[0074] like Figure 3 The image shows the copper (Cu) and silver (Ag) elemental distribution and their overlay in the obtained silver-coated copper powder (Cu@Ag-1). The distribution of Cu and Ag elements indicates that the silver coating is uniformly applied to the surface of the copper particles.

[0075] Implementation Case 2:

[0076] Step 1: At room temperature, weigh 23.1 g of ammonium sulfate and measure 1.8 mL of ammonia water, dispersing it in 100 mL of deionized water. Then, weigh 4 g of copper powder with an average particle size of 3.5 μm and disperse it in the above solution. Stir the suspension for 30 min, then sonicate for 5 min, and finally continue stirring for 20 min to complete the oxide layer removal process. After removing the oxide layer, wash the copper powder three times with deionized water, then disperse it in 100 mL of deionized water. After thorough dispersion by stirring and sonication, suspension A is obtained.

[0077] Step 2: Weigh 30g of sodium hydroxide and 35g of ethylenediaminetetraacetic acid complexing agent at room temperature and place them in 100mL of deionized water. Stir for 30min until the sodium hydroxide and ethylenediaminetetraacetic acid are completely dissolved. Then add 3.2g of silver nitrate and stir in the dark for 20min to ensure that the silver nitrate is completely dissolved, thus obtaining solution A.

[0078] Step 3: At room temperature, solution A is added dropwise to suspension A at a uniform and slow rate of 10 mL / min. After the addition is complete, the mixture is stirred for another 5 min. After the reaction is complete, the mixture is washed three times by centrifugation with deionized water and methanol, and then dried under vacuum at 60°C to obtain silver-coated copper powder, Cu@Ag-2.

[0079] like Figure 5 The image shown is a scanning electron microscope (SEM) image of the obtained silver-coated copper powder (Cu@Ag-2). It can be seen that a short oxide layer removal time facilitates the formation of a loose silver particle coating layer.

[0080] Implementation Case 3:

[0081] Step 1: At room temperature, weigh 23.1 g of ammonium sulfate and measure 1.8 mL of ammonia water, dispersing it in 100 mL of deionized water. Then, weigh 4 g of copper powder with an average particle size of 3.5 μm and disperse it in the above solution. Stir the suspension for 5 min, then sonicate for 5 min, and finally continue stirring for 5 min to complete the oxide layer removal process. After removing the oxide layer, wash the copper powder three times with deionized water, then disperse it in 100 mL of deionized water. After thorough dispersion by stirring and sonication, suspension A is obtained.

[0082] Step 2: Weigh 30g of sodium hydroxide and 35g of ethylenediaminetetraacetic acid complexing agent at room temperature and place them in 100mL of deionized water. Stir for 30min until the sodium hydroxide and ethylenediaminetetraacetic acid are completely dissolved. Then add 1.6g of silver nitrate and stir in the dark for 20min to ensure that the silver nitrate is completely dissolved, thus obtaining solution A.

[0083] Step 3: At room temperature, solution A is added dropwise to suspension A at a uniform and slow rate of 10 mL / min. After the addition is complete, the mixture is stirred for another 5 min. After the reaction is complete, the mixture is washed three times by centrifugation with deionized water and methanol, and then dried under vacuum at 60 °C to obtain silver-coated copper powder, Cu@Ag-3.

[0084] like Figure 6 The image shown is a scanning electron microscope (SEM) image of the obtained silver-coated copper powder (Cu@Ag-3). It can be seen that when the concentration of the silver complex solution is higher, the silver coating can still uniformly coat the copper particles, but the outermost layer is composed of silver particles, resulting in a lower density.

[0085] Test example:

[0086] The content of copper and silver in silver-coated copper powder was determined by energy-dispersive X-ray spectroscopy.

[0087] The thickness of the silver coating was determined by backscattered electron imaging.

[0088] The results are shown in Table 1:

[0089] Table 1. Elemental ratios and silver plating thickness of silver-coated copper powder

[0090]

[0091] The mass and atomic ratio data of Cu and Ag are shown in Table 1. Since energy dispersive spectroscopy can detect more information about the surface of materials, the proportion of Ag is relatively high relative to the feed ratio.

[0092] The specific surface area of ​​copper powder and Cu@Ag-1, Cu@Ag-2, and Cu@Ag-3 obtained in each example was measured, and the results are as follows: Figure 4 As shown in Table 2:

[0093] Table 2 Specific surface area of ​​copper powder and silver-coated copper powder

[0094]

[0095] As shown in Table 2, the surface roughness of silver-coated copper powder increases and its specific surface area increases.

[0096] Application examples

[0097] The resistivity was tested using Cu@Ag-1 prepared in Example 1 as the raw material. The specific test steps were as follows:

[0098] Silver-coated copper powder was compacted inside a cylindrical plastic tube. The resistance of the silver-coated copper powder was measured using a digital multimeter, and the volume resistivity of the silver-coated copper powder was calculated using the following formula:

[0099] ρ=Rπr 2 / l

[0100] Where ρ is the volume resistivity, R is the measured resistance, and r and l are the radius and length of the plastic tube, respectively. The conductivity test result of silver-coated copper powder is: 2.5 x 10. -5 Ω·m.

[0101] Comparative Example 1

[0102] Step 1: At room temperature, weigh 4g of copper powder with an average particle size of 3.5μm and 23.1g of ammonium sulfate, disperse them in 100mL of deionized water, then add 1.8mL of ammonia water to the above solution. Stir the suspension for 30min, then sonicate for 5min, and finally continue stirring for 20min to complete the oxide layer removal process. After removing the oxide layer, wash the copper powder three times with deionized water, then disperse it again in 100mL of deionized water. After thorough dispersion by stirring and sonication, obtain suspension A.

[0103] Step 2: Weigh 30g of sodium hydroxide and 35g of ethylenediaminetetraacetic acid complexing agent at room temperature and place them in 100mL of deionized water. Stir for 30min until the sodium hydroxide and ethylenediaminetetraacetic acid are completely dissolved. Then add 0.16g of silver nitrate and stir for 20min in the dark to ensure that the silver nitrate is completely dissolved, thus obtaining solution A.

[0104] Step 3: At room temperature, solution A is added dropwise to suspension A at a uniform and slow rate of 10 mL / min. After the addition is complete, the mixture is stirred for another 5 min. After the reaction is complete, the mixture is washed three times by centrifugation with deionized water and methanol, and then dried under vacuum at 60°C to obtain silver-coated copper powder.

[0105] In this embodiment, due to the low amount of silver nitrate, only island-shaped silver plating is formed on the surface of the copper powder, and the silver plating cannot completely cover the copper powder particles.

[0106] Comparative Example 2

[0107] Step 1: At room temperature, weigh 4g of copper powder with an average particle size of 3.5μm and 23.1g of ammonium sulfate, disperse them in 100mL of deionized water, then add 1.8mL of ammonia water to the above solution. Stir the suspension for 30min, then sonicate for 5min, and finally continue stirring for 20min to complete the oxide layer removal process. After removing the oxide layer, wash the copper powder three times with deionized water, then disperse it again in 100mL of deionized water. After thorough dispersion by stirring and sonication, obtain suspension A.

[0108] Step 2: Weigh 30g of sodium hydroxide and 35g of ethylenediaminetetraacetic acid complexing agent at room temperature and place them in 100mL of deionized water. Stir for 30min until the sodium hydroxide and ethylenediaminetetraacetic acid are completely dissolved. Then add 1.6g of silver nitrate and stir in the dark for 20min to ensure that the silver nitrate is completely dissolved, thus obtaining solution A.

[0109] Step 3: At room temperature, solution A is added dropwise to suspension A at a uniform and slow rate of 30 mL / min. After the addition is complete, the mixture is stirred for another 5 min. After the reaction is complete, the mixture is washed three times by centrifugation with deionized water and methanol, and then dried under vacuum at 60°C to obtain silver-coated copper powder.

[0110] In this embodiment, the excessively high droplet speed resulted in uneven silver layer growth and the formation of silver particles.

[0111] Comparative Example 3

[0112] Step 1: At room temperature, weigh 4g of copper powder with an average particle size of 3.5μm and 23.1g of ammonium sulfate, disperse them in 100mL of deionized water, then add 1.8mL of ammonia water to the above solution. Stir the suspension for 30min, then sonicate for 5min, and finally continue stirring for 20min to complete the oxide layer removal process. After removing the oxide layer, wash the copper powder three times with deionized water, then disperse it again in 100mL of deionized water. After thorough dispersion by stirring and sonication, obtain suspension A.

[0113] Step 2: Weigh 30g of sodium hydroxide and 35g of polyvinylpyrrolidone K30 complexing agent at room temperature and place them in 100mL of deionized water. Stir for 30min until the sodium hydroxide and polyvinylpyrrolidone are completely dissolved. Then add 1.6g of silver nitrate and stir for 20min in the dark to ensure that the silver nitrate is completely dissolved, thus obtaining solution A.

[0114] Step 3: At room temperature, solution A is added dropwise to suspension A at a uniform and slow rate of 10 mL / min. After the addition is complete, the mixture is stirred for another 5 min. After the reaction is complete, the mixture is washed three times by centrifugation with deionized water and methanol, and then dried under vacuum at 60°C to obtain silver-coated copper powder.

[0115] In this embodiment, silver particles accumulate and cover the surface of copper particles to form a coating layer, but the uniformity and smoothness of the coating are low.

[0116] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a dense silver plating layer that completely coats copper powder, characterized in that, Includes the following steps: 1) Remove the oxide layer from the copper powder, then wash the copper powder clean and prepare a copper powder suspension; 2) Prepare an alkaline silver complex solution; 3) At room temperature, the silver complex solution obtained in step 2) is slowly added dropwise to the copper powder suspension obtained in step 1). After the addition is complete, the reaction is stirred continuously. After the reaction is completed, the copper powder is washed and vacuum dried in sequence to obtain a dense silver coating that completely covers the copper powder.

2. The method for preparing a dense silver plating layer that completely coats copper powder according to claim 1, characterized in that, Step 1) includes the following steps: 1a) At room temperature, a certain mass of ammonium sulfate and a certain volume of ammonia water are dispersed in deionized water to obtain a solution. Then, a certain mass of copper powder is dispersed in the solution. The suspension is stirred and ultrasonically treated to remove the oxide layer. 1b) After removing the oxide layer, the copper powder is centrifuged and washed several times with deionized water, and then dispersed in deionized water, stirred and ultrasonically dispersed to obtain a copper powder suspension.

3. The method for preparing a dense silver plating layer that completely coats copper powder according to claim 2, characterized in that: In step 1a): the mass fraction of ammonia water is 25-28 wt%; the mass ratio of ammonium sulfate, ammonia water, deionized water and copper powder is (4-20):(0.1-2):(10-100):1; In step 1b): the mass ratio of deionized water to copper powder is (10-100):1; the stirring time is 10-60 min; and the ultrasonic dispersion time is 5-25 min.

4. The method for preparing a dense silver plating layer that completely coats copper powder according to claim 1, characterized in that, In step 2): At room temperature, a certain mass of alkali and complexing agent is dissolved in deionized water and stirred thoroughly until dissolved. Then, a certain mass of silver salt is added and stirred in the dark until the silver salt is completely dissolved.

5. The method for preparing a dense silver plating layer that completely coats copper powder according to claim 4, characterized in that: The alkali is selected from one or more of potassium hydroxide or sodium hydroxide, and its concentration in the silver complex solution is 1-10 mol / L; The complexing agent includes one or more of ethylenediaminetetraacetic acid, tetrasodium ethylenediaminetetraacetic acid, ethylenediamine, triethylenetetraamine, or tetraethylenepentamine, and its concentration in the silver complex solution is 0.5–1.5 mol / L. Silver salts are selected from silver nitrate; The molar ratio of silver to copper ranges from (0.01 to 1):

1.

6. The method for preparing a dense silver plating layer that completely coats copper powder according to claim 1, characterized in that, In step 3): The silver complex solution was added at a rate of 2–20 mL / min, and the reaction continued for 0–20 min after the addition was complete. Wash with deionized water and vacuum dry at a temperature of 50–70°C.

7. The method for preparing a dense silver plating layer that completely coats copper powder according to any one of claims 1 to 6, characterized in that: In each step, the stirring speed is independently 500-1000 r / min.

8. The method for preparing a dense silver plating layer that completely coats copper powder according to claim 7, characterized in that: The copper powder has a particle size greater than 0.5 μm.

9. A dense silver plating layer completely coating copper powder prepared by the preparation method according to any one of claims 1 to 8.

10. An application of the dense silver plating layer that completely coats copper powder according to claim 9, characterized in that: Used to prepare conductive pastes.

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