Silver-coated nickel composite powder preparation method, silver-coated nickel composite powder and application thereof

Abstract

The application discloses a preparation method of silver-coated nickel composite powder, silver-coated nickel composite powder and application thereof. The preparation method comprises the following steps: S1, preparing spherical or quasi-spherical nickel powder with a length-diameter ratio of less than or equal to 1.5; S2, after removing the oxide layer of the nickel powder through acid pickling, sensitizing the nickel powder with an acidic stannous chloride solution; S3, mixing and dispersing the sensitized nickel powder with a dispersant, adding hydrazine hydrate as a reducing agent, and adding silver nitrate solution dropwise for reduction deposition under the condition that the temperature is less than 40 DEG C, and the molar ratio of Ag + to N2H4 is (1.7-4.5):1; S4, washing and drying, to obtain silver-coated nickel composite powder. Through the spherical nickel powder, sensitization treatment and accurate control of the reduction deposition condition, the application realizes continuous and dense coating of the silver layer and strong interface combination under the condition of low silver content, and the obtained powder is excellent in conductivity, oxidation resistance and process adaptability, and can be widely applied in the fields of electric contact materials, conductive paste, conductive adhesive, conductive paint, electromagnetic shielding coating, solar cell electrode and 3D printing conductive ink and the like.

Description

Technical Field

[0001] This invention relates to the field of metal-based composite powder materials technology, specifically to a method for preparing silver-coated nickel composite powder and the silver-coated nickel composite powder itself. Background Technology

[0002] Silver-coated nickel composite powders, by coating a nickel core with a silver layer, combine the high conductivity of silver with the low cost of nickel, representing an important direction for replacing pure silver and high-silver-content electrical contact materials. However, the preparation of silver-coated nickel powders in current technologies still faces many challenges.

[0003] Pure silver and silver alloys possess excellent electrical and thermal conductivity, but their low hardness, poor resistance to arc erosion, and high cost make them unsuitable for high-performance electrical contact materials. Mechanically hybrid AgNi materials, prepared through powder metallurgy, can reduce the amount of silver used, but they suffer from silver and nickel phase segregation. Under arc irradiation, the exposed nickel phase is easily oxidized, leading to a sharp increase in contact resistance and ultimately contact failure.

[0004] Currently, chemical plating is widely used in industry to prepare silver-coated nickel powder, but it has the following core defects when the silver content is low: (1) Insufficient uniformity and density of coating: the silver layer has pinholes, island growth or local exposed nickel cores, which affects the integrity of the conductive network; (2) Weak interface bonding strength: the silver-nickel interface is mainly based on physical adsorption and mechanical intercalation, lacking metallurgical bonding, and the silver layer is prone to peeling off; (3) Difficulty in constructing conductive network under ultra-low silver content: when the silver content drops to 5-8wt%, it is difficult to form a continuous conductive layer, and the resistivity increases significantly; (4) Poor process stability: the chemical plating solution is prone to spontaneous decomposition, and the quality fluctuates greatly between batches.

[0005] To address these issues, existing technologies have attempted to optimize both the morphology of the nickel powder and the process route. For example, Chinese patent CN108224A discloses a method for preparing silver-coated nickel powder using flake nickel powder as a substrate. However, the flake nickel powder substrate used in this method may exhibit anisotropic conductivity due to its orientational arrangement, and its sharp edges can easily cause stress concentration in the coating layer. Furthermore, this process involves steps such as palladium chloride activation and programmed temperature rise, increasing process complexity and cost.

[0006] Therefore, it is evident that achieving continuous and dense coating and strong interfacial bonding of silver layers with low silver content, while avoiding the use of precious metal activators and circumventing the inherent defects of flake nickel powder, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] This invention aims to overcome the aforementioned shortcomings in existing silver-coated nickel composite powder preparation technologies, and provides a method for preparing silver-coated nickel composite powder and the resulting silver-coated nickel composite powder. This method, while avoiding the use of precious metal activators and mitigating the inherent defects of flake nickel powder, achieves continuous and dense coating and strong interfacial bonding of the silver layer with low silver content, resulting in silver-coated nickel composite powder with excellent overall performance.

[0008] In a first aspect, the present invention provides a method for preparing silver-coated nickel composite powder, the specific technical solution of which is as follows: A method for preparing silver-coated nickel composite powder includes the following steps: S1. Prepare spherical or near-spherical nickel powder, wherein the aspect ratio of the nickel powder is ≤1.5; S2. After acid washing to remove the oxide layer of the nickel powder obtained in step S1, it is sensitized with an acidic stannous chloride solution. S3. The sensitized nickel powder is mixed and dispersed with a dispersant. Hydrazine hydrate is added as a reducing agent, and silver nitrate solution is added dropwise at a temperature <40℃ to carry out reduction deposition. Ag + The molar ratio of silver to nickel is (1.7-4.5):1, resulting in silver-coated nickel composite powder. S4. The obtained powder is washed and dried to obtain silver-coated nickel composite powder with a silver content of 3 wt% ~ 25 wt%.

[0009] In the context of this invention, the "aspect ratio" refers to the ratio of the longest axis to the shortest axis of a single nickel particle, measured by microscopic image analysis and averaged. The spherical or near-spherical nickel powder is preferably prepared by liquid-phase reduction, but other methods known in the art that can obtain spherical nickel powder with an aspect ratio ≤ 1.5 can also be used, such as hydrothermal methods or spray pyrolysis. Regardless of the preparation method used, as long as the aspect ratio of the obtained nickel powder is ≤ 1.5, the technical effects of this invention can be achieved.

[0010] This invention solves the problems of existing technologies through the synergistic effect of four steps: S1 uses spherical / quasi-spherical nickel powder (aspect ratio ≤ 1.5), fundamentally avoiding the orientation and corner stress defects of flake nickel powder; S2 replaces the activation of precious metal palladium with acid washing combined with acidic stannous chloride sensitization, reducing costs while constructing active sites for silver deposition; S3 ensures uniform dispersion of nickel powder through dispersants and ultrasound, and slowly adds silver nitrate solution at low temperature (<40℃) while precisely controlling Ag. +Compared with the reducing agent, the molar ratio inhibited the growth of silver islands, achieving continuous dense coating and high bonding strength at the interface. S4 ensured powder purity and coating integrity through rigorous washing and low-temperature drying. These four steps worked synergistically, successfully achieving continuous dense coating and strong interfacial bonding of silver layers with low silver content (3%~25%) while avoiding the use of precious metal activators and mitigating the inherent defects of flake nickel powder, resulting in silver-coated nickel composite powder with excellent comprehensive performance.

[0011] It should be noted that the addition of silver nitrate needs to be carried out at low temperatures (<40℃). If the temperature reaches or exceeds 40℃, the reduction reaction will be too vigorous, easily leading to island-like growth of silver layers and uneven coating. When the reaction temperature is below 15℃, the reaction rate is too low and not suitable for industrial production. Therefore, 15℃ to 40℃ is preferred. This invention preferably operates within the range of room temperature (25℃) to 35℃, requiring no additional heating or deep cooling, making it easy to implement industrially.

[0012] Furthermore, the D50 of the silver-coated nickel composite powder is 0.5μm~10μm, and the D90 / D50 ≤ 2.5.

[0013] Furthermore, the spherical or near-spherical nickel powder described in step S1 is prepared by a liquid-phase reduction method: using nickel sulfate as the nickel source and hydrazine hydrate as the reducing agent, the powder is reacted in an alkaline aqueous solution at 60-75°C in the presence of sodium citrate and polyvinylpyrrolidone. This preparation method is simple, easy to scale up, and allows for precise control of the morphology and particle size of the nickel powder.

[0014] Furthermore, the molar amount of hydrazine hydrate is 1.8 to 2.2 times that of nickel sulfate. This dosage range ensures sufficient reduction of nickel powder and regular morphology, which is beneficial for improving batch consistency.

[0015] Furthermore, the pickling in step S2 uses a 0.05~0.5 mol / L dilute acid, and the immersion time is 5-15 minutes. This parameter range can effectively remove the oxide layer on the surface of the nickel powder while avoiding excessive corrosion that would affect the morphology of the nickel powder. The dilute acid is selected from dilute sulfuric acid or dilute acetic acid.

[0016] Furthermore, the sensitization time in step S2 is 40-60 minutes. This time range allows Sn to achieve... 2+ Sufficient adsorption on the nickel powder surface provides ample active sites for subsequent silver deposition.

[0017] Further, the dispersant in step S3 is polyethylene glycol or sodium dodecyl sulfate, and the amount used is 2% to 5% of the mass of the aqueous solution.

[0018] Furthermore, the dispersion in step S3 includes ultrasonic dispersion, and the ultrasonic dispersion time is 20-40 minutes.

[0019] Furthermore, the dropping rate of the silver nitrate solution in step S3 is 0.5~2 L / h. This dropping rate ensures a uniform supply of silver ions, which is beneficial for the formation of a continuous and dense silver layer.

[0020] Furthermore, the reaction time for the reduction deposition in step S3 is 20-40 minutes. This reaction time ensures sufficient reduction of silver ions while preventing excessive migration of silver atoms that could affect the density of the coating layer.

[0021] Furthermore, the washing process in step S4 includes: first rinsing with deionized water until the conductivity is ≤10 μS / cm, and then rinsing twice with anhydrous ethanol. This washing method can effectively remove residual ions and organic matter, improving the purity of the powder.

[0022] Furthermore, the drying process in step S4 involves forced-air drying at 50-80°C for 8-16 hours. These drying conditions ensure thorough drying of the powder while preventing the silver layer from shrinking or agglomerating due to high temperatures.

[0023] Secondly, the present invention provides a silver-coated nickel composite powder, which is prepared by the aforementioned preparation method. The powder has a core-shell structure, with the nickel core being spherical or near-spherical and coated with a continuous and dense silver layer. A high-strength interface is formed between the silver layer and the nickel core, exhibiting excellent conductivity, oxidation resistance, and service stability even at low silver content.

[0024] Thirdly, this invention provides an application of a silver-coated nickel composite powder in the preparation of electrical contact materials, conductive pastes, conductive adhesives, conductive coatings, electromagnetic shielding coatings, solar cell electrodes, or 3D printing conductive inks. Applying this silver-coated nickel composite powder to the preparation of electrical contact materials, conductive pastes, conductive adhesives, conductive coatings, electromagnetic shielding coatings, solar cell electrodes, or 3D printing conductive inks can significantly reduce the material cost of downstream products, while ensuring or improving their conductivity and reliability, thanks to the powder's excellent conductivity, low silver content cost, and good process adaptability. Attached Figure Description

[0025] Figure 1 Scanning electron microscope (SEM) image of the nickel powder in the preparation example. Figure 2 SEM image (I) of the silver-coated nickel composite powder in Example 1; Figure 3 SEM image (II) of the silver-coated nickel composite powder in Example 1; Figure 4 The backscattered electron (BSE) diagram (I) of the silver-coated nickel composite powder in Example 1 is shown. Figure 5The second image shows the backscattered electron (BSE) pattern of the silver-coated nickel composite powder in Example 1. Figure 6 This is a graph showing the particle size analysis results of the silver-coated nickel composite powder in Example 1; Figure 7 The first image is a scanning electron microscope (SEM) image of the silver-coated nickel composite powder in Example 2. Figure 8 The second image is a scanning electron microscope (SEM) image of the silver-coated nickel composite powder in Example 2. Figure 9 This is a graph showing the particle size analysis results of the silver-coated nickel composite powder in Example 2; Figure 10 This is a scanning electron microscope (SEM) image of the silver-coated nickel composite powder in Example 3; Figure 11 The image shows the backscattered electron (BSE) pattern of the silver-coated nickel composite powder in Example 3. Figure 12 This is a graph showing the particle size analysis results of the silver-coated nickel composite powder in Example 3; Figure 13 The first image is a scanning electron microscope (SEM) image of the silver-coated nickel composite powder in Example 4. Figure 14 The second image is a scanning electron microscope (SEM) image of the silver-coated nickel composite powder in Example 4. Figure 15 The image shows the particle size analysis results of the silver-coated nickel composite powder in Example 4. Detailed Implementation

[0026] To enable those skilled in the art to better understand the technical solutions of this invention, the invention will be further described in detail below with reference to specific preparation examples, embodiments, and comparative examples. However, it should be understood that these examples are for illustrative purposes only and do not constitute any limitation on the scope of protection of this invention. All equivalent substitutions or improvements made based on the concept of this invention should be covered within the scope of protection of this invention.

[0027] Performance testing methods (1) Morphology and structure characterization: The surface morphology of the powder was observed by scanning electron microscopy (SEM), and the uniformity and density of the silver coating were observed by backscattered electron microscopy (BSE).

[0028] (2) Particle size distribution test: The particle size distribution of the powder was determined by a laser particle size analyzer, and the D50 and D90 / D50 values ​​were recorded.

[0029] (3) Determination of silver content: The mass fraction of silver in the powder was determined by inductively coupled plasma optical emission spectrometry (ICP-OES); (4) Volume resistivity test: Take 1g of silver-coated nickel composite powder and place it into a cylindrical mold with a diameter of 10mm. Hold the mold under 500MPa pressure for 30 seconds to form a compact with a thickness of approximately 2-3mm. Measure the resistance of the compact using a four-probe resistance meter (model: ST2742B automated powder resistivity meter), and measure the thickness and diameter of the compact using a micrometer. The volume resistivity is calculated using the formula ρ=R×(πd). 2 / 4) / L is calculated, where ρ is the volume resistivity (μΩ·cm), R is the measured resistance (μΩ), d is the compact diameter (cm), and L is the compact thickness (cm). It should be noted that, to ensure the comparability of test results between different embodiments and comparative examples, all powder samples were prepared using the same batch of molds, the same pressing pressure, and the same holding time, and the compact thickness was controlled within the range of 2.0-3.0 mm. Each group of samples was tested three times, and the arithmetic mean was taken as the final resistivity value to minimize systematic errors in sample preparation and measurement.

[0030] Preparation example: Dissolve 1075g sodium citrate and 54g polyvinylpyrrolidone in 8L of deionized water, stir until dissolved, then add 3200g nickel sulfate and stir until completely dissolved to obtain a reaction mother liquor. Dissolve 500g sodium hydroxide in 750g of deionized water to prepare a 40% sodium hydroxide solution. Dilute 1000g of 80% hydrazine hydrate with 2L of deionized water and set aside. Transfer the prepared reaction mother liquor to a reaction vessel, start stirring, slowly add sodium hydroxide solution, adjust the pH to 10, and add deionized water to bring the total volume to 12L. Turn on the heating system and heat to 75℃ at a rate of 1℃ / min. After holding at this temperature for 10min, start adding the diluted hydrazine hydrate solution dropwise. After the hydrazine hydrate is added, maintain the temperature at 75℃ and continue stirring for 90min. Stop heating, cool to room temperature, and then remove the reaction solution. The obtained product was washed with 5 L of deionized water until the conductivity of the washing solution was below 100 μS / cm, then washed with 2 L of anhydrous ethanol. This washing process was repeated twice to obtain spherical nickel powder. Figure 1 As shown, the nickel powder obtained in the preparation example is spherical. 100 particles were randomly selected for measurement. The average aspect ratio was 0.97, the maximum was 1.21, and all were ≤1.5. Example 1

[0031] The preparation steps of the silver-coated nickel composite powder in this embodiment are as follows: Step 1: Weigh 500g of nickel powder prepared by the preparation example and place it in a beaker. Add 6L of 2% dilute acetic acid, stir and mix, and let stand for 1 hour to remove impurities and oxide layer from the surface of the nickel powder. After the solid-liquid separation is obvious, pour off the upper clear liquid and wash the nickel powder with deionized water until the solution is neutral.

[0032] Step 2: Add 5L of 2% dilute acetic acid and 200g of SnCl2 to the nickel powder treated in Step 1, stir and disperse for 50min, let it stand and settle, then pour off the supernatant, and wash the powder with deionized water until the conductivity of the washing solution is less than 10μS / cm.

[0033] Step 3: Add 5 kg of deionized water and 150 g of polyethylene glycol (PEG800) to the nickel powder treated in Step 2, and stir in an ultrasonic machine until completely dissolved and evenly dispersed; then add 40 g of ammonia water and 2.3 g of hydrazine hydrate, and ultrasonically stir for 30 min to make it fully mixed and evenly mixed to obtain the nickel powder dispersion system.

[0034] Step 4: Dissolve 22.8g of AgNO3 in 500g of deionized water and stir for 30 minutes until completely dissolved. Add 105g of triethylenetetramine (TETA) and continue stirring for 30 minutes to obtain a silver nitrate solution.

[0035] Step 5: Slowly add the prepared silver nitrate solution dropwise to the above nickel powder dispersion system using a peristaltic pump. During the dropwise addition, control the reaction temperature below 40°C and the dropwise acceleration rate at 1.2 L / h. After the dropwise addition is complete, continue the reaction for 30 min to end the reaction.

[0036] Step Six: After the reaction is complete, first wash the powder with deionized water until the conductivity of the washing solution is below 10 μS / cm, then wash twice with anhydrous ethanol; place the washed powder in a forced-air drying oven and dry at 60℃ for 12 hours to obtain silver-coated nickel composite powder. (Refer to...) Figure 2-6 As shown in Table 2, the silver-coated nickel composite powder is spherical or nearly spherical, with a continuous and dense silver layer on the surface and no exposed nickel core; the silver content is 2.95% as measured by ICP-OES. Examples 2-3:

[0037] Examples 2-3 are based on Example 1, with the only difference being the amount of hydrazine hydrate added in step 3 and the amounts of silver nitrate, deionized water, and triethylenetetramine (TETA) added in step 4, as detailed in the table below.

[0038] Table 1 Material Usage Table for Examples 1-3

[0039] Reference Figure 7-9 As shown in Table 2, the silver-coated nickel composite powder is spherical or nearly spherical, with a continuous and dense silver layer on the surface and no exposed nickel core; the silver content was measured to be 14.83% by ICP-OES. Example 3 prepared a silver-coated nickel composite powder with a silver content of 25%. (Refer to...) Figure 10-12 As shown in Table 2, the silver-coated nickel composite powder is spherical or nearly spherical, with a continuous and dense silver layer on the surface and no exposed nickel core; the silver content is 24.72% as measured by ICP-OES. Example 4

[0040] This embodiment describes a method for mass production of silver-coated nickel composite powder with a 20% silver content, including a nickel powder preparation process and a silver-coated nickel composite powder preparation process. The specific steps are as follows: Nickel powder preparation process S1.1 Add 800 kg of deionized water to the reactor, start stirring at 80 rpm, add 53.8 kg of sodium citrate and 2.7 kg of polyvinylpyrrolidone in sequence, and stir for 15 min until completely dissolved; slowly add 160.0 kg of nickel sulfate, controlling the dissolution temperature to be below 40 ℃; slowly add 40% sodium hydroxide solution dropwise to adjust the pH to 10.

[0041] S1.2. Mix 50 kg of hydrazine hydrate with a mass fraction of 80% with 100 kg of deionized water to obtain 150 kg of hydrazine hydrate diluted solution with a mass fraction of approximately 26.7%, and add it dropwise to the reaction vessel using a metering pump. After the addition is complete, keep the mixture at 75 °C and stir for 2 h to complete the reaction.

[0042] S1.3 The reaction product is washed with deionized water until the conductivity of the washing solution is less than 10 μS / cm, and then washed twice with anhydrous ethanol to obtain nickel powder.

[0043] Preparation process of silver-coated nickel composite powder S2.1 Add 500 kg of 2% dilute acetic acid and 25 kg of nickel powder prepared by the nickel powder preparation process to the reactor, stir and mix, and let stand for 1 h to remove impurities and oxide layer on the surface of nickel powder; after the solid-liquid separation is obvious, pour off the upper clear liquid, and wash the nickel powder with deionized water until the solution is neutral.

[0044] S2.2 Add 500 kg of 2% dilute acetic acid and 25 kg of SnCl2 to the nickel powder treated in step S2.1, stir and disperse for 50 min, let it stand and settle, then pour off the supernatant, and wash the powder with deionized water until the conductivity of the washing solution is less than 10 μS / cm.

[0045] S2.3 Add 350 kg of deionized water and 10.5 kg of polyethylene glycol (PEG 800) to the washed nickel powder, and ultrasonically stir until completely dissolved and evenly dispersed; then add 2.8 kg of ammonia water and 1.37 kg of hydrazine hydrate, and continue ultrasonic stirring for 30 min to fully mix evenly to obtain a nickel powder dispersion system.

[0046] S2.4 In a separate reaction vessel, add 9.3 kg AgNO3 and 100 kg deionized water, stir for 30 min until completely dissolved, add 23.4 kg triethylenetetramine (TETA), and continue stirring for 30 min to obtain silver nitrate solution.

[0047] S2.5. The above silver nitrate solution is slowly added dropwise to the nickel powder dispersion system using a peristaltic pump. During the addition process, the reaction temperature is controlled to be below 40 °C. After the addition is complete, the reaction continues for 30 min to end the reaction.

[0048] S2.6 After the reaction is complete, first wash the powder with deionized water until the conductivity of the washing solution is less than 10 μS / cm, and then wash it twice with anhydrous ethanol.

[0049] S2.7. Place the washed powder in a forced-air drying oven and dry at 60 ℃ for 12 h to obtain silver-coated nickel composite powder. (Refer to...) Figure 13-15 As shown in Table 2, the silver-coated nickel composite powder is spherical or nearly spherical, with a continuous and dense silver layer on the surface and no exposed nickel core; the silver content is 19.97% as measured by ICP-OES.

[0050] Comparative Example Comparative Example 1: A silver-plated nickel powder sold in a certain city was tested and found to have a silver content of 56.29%, a D50 of 4.241 μm, and a D90 of 9.123 μm.

[0051] Comparative Example 2: Based on Example 1, the only difference is that commercially available flake nickel powder (Canada INCO 123 type flake nickel powder, aspect ratio 4.5~6.0, flake diameter 3~5μm) is used instead of the spherical nickel powder obtained in the preparation example. The rest of the steps are exactly the same as in Example 1.

[0052] Comparative Example 3: Based on Example 1, the difference lies in omitting the stannous chloride sensitization step and directly proceeding to the silver coating stage. A dense and uniform core-shell structure cannot be formed, resulting in excessively high resistivity (>1300 μΩ·cm), rendering it of no practical reference value.

[0053] The volume resistivity test results are shown in the table below: Table 2 Performance Test Results

[0054] As shown in Table 2, all embodiments of the present invention achieved excellent electrical conductivity at different silver contents: As the silver content gradually increased from 2.95% in Example 1 to 24.72% in Example 3, the volume resistivity continuously decreased from 172.7 μΩ·cm to 88.4 μΩ·cm, exhibiting a monotonically decreasing trend. This trend perfectly matches the laws of physics: the thicker the silver shell, the wider the electron transport path in the continuous silver layer, the less interface scattering, and the lower the resistivity. It is noteworthy that even with a silver content as low as 2.95% (Example 1), the present invention still achieved a volume resistivity of 172.7 μΩ·cm, demonstrating practical application value. This indicates that the silver layer constructed with ultra-low silver content possesses basic continuity and can form effective conductive channels.

[0055] Example 4 was prepared on a mass production scale, with a volume resistivity of 55.8 μΩ·cm. This is consistent with the resistivity variation trend of silver content in laboratory-scale Examples 2 (109.5 μΩ·cm) and 3 (88.4 μΩ·cm), verifying that the process of the present invention has good batch stability and repeatability in scale-up production.

[0056] Comparative Example 1 is a commercially available high-silver-content silver-coated nickel powder with a silver content of 56.29%, which is 2.28 times that of Example 3 (24.72%), but its volume resistivity is only slightly lower than that of Example 3. Although Comparative Example 1 has an extremely high silver content, its conductivity is severely weakened, suggesting that its silver layer has structural defects such as pinholes, island growth, and localized exposed nickel nuclei. In contrast, this invention achieves continuous and dense coating of the silver layer and high-strength interfacial adhesion through the synergistic effect of multiple methods, including eliminating stress concentration at sharp edges using a spherical nickel powder substrate, constructing uniform active nucleation sites through stannous chloride sensitization, and inhibiting island growth of the silver layer through slow low-temperature reduction (see reference). Figure 2-5 , Figure 7-8 and Figure 10-11 In this ideal core-shell structure, electrons travel along the intact silver shell with almost no interface barrier, maximizing the conductivity of silver. This comparison also fully demonstrates that the silver content is not the only factor determining the conductivity of powders; the structural integrity, continuity, and interfacial bonding state of the silver layer have a more critical impact on conductivity.

[0057] Comparative Example 2 (using flake nickel powder instead of spherical nickel powder) and Comparative Example 3 (omitting the sensitization step) had severe defects in the coating layer, making it impossible to measure the effective resistance value of the compact. This further confirms that the three technical means of "spherical nickel powder substrate," "stannous chloride sensitization," and "low-temperature precise reduction" in this invention are indispensable. Without any one of them, it is impossible to construct a continuous and dense silver layer, and the conductivity will deteriorate sharply or even be lost.

[0058] The above are all preferred embodiments of this application and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A method for preparing silver-coated nickel composite powder, characterized in that, Includes the following steps: S1. Prepare spherical or near-spherical nickel powder, wherein the aspect ratio of the nickel powder is ≤1.5; S2. After acid washing to remove the oxide layer of the nickel powder obtained in step S1, it is sensitized with an acidic stannous chloride solution. S3. The sensitized nickel powder is mixed and dispersed with a dispersant. Hydrazine hydrate is added as a reducing agent, and silver nitrate solution is added dropwise at a temperature <40℃ to carry out reduction deposition. Ag + The molar ratio of silver to nickel is (1.7-4.5):1, resulting in silver-coated nickel composite powder. S4. The obtained powder is washed and dried to obtain silver-coated nickel composite powder with a silver content of 3 wt% to 25 wt%.

2. The preparation method according to claim 1, characterized in that: The D50 of the silver-coated nickel composite powder is 0.5μm~10μm, and the D90 / D50 ≤ 2.

5.

3. The preparation method according to claim 1, characterized in that: The spherical or near-spherical nickel powder described in step S1 is prepared by liquid-phase reduction: using nickel sulfate as the nickel source and hydrazine hydrate as the reducing agent, it is reacted in an alkaline aqueous solution at 60-75℃ in the presence of sodium citrate and polyvinylpyrrolidone.

4. The preparation method according to claim 1, characterized in that: The pickling in step S2 uses 0.05~0.5mol / L dilute acid and the immersion time is 5-15 minutes.

5. The preparation method according to claim 1, characterized in that: The sensitization time in step S2 is 40-60 minutes.

6. The preparation method according to claim 1, characterized in that: The dispersant mentioned in step S3 is polyethylene glycol or sodium dodecyl sulfate, and the amount used is 2% to 5% of the mass of the aqueous solution.

7. The preparation method according to claim 1, characterized in that: The silver nitrate solution in step S3 is added at a rate of 0.5~2 L / h.

8. The preparation method according to claim 1, characterized in that: The reaction time for the reduction deposition in step S3 is 20 to 40 minutes.

9. A silver-coated nickel composite powder, characterized in that: It is prepared by the preparation method according to any one of claims 1-8.

10. The application of a silver-coated nickel composite powder prepared by any one of claims 1-8 or the silver-coated nickel composite powder of claim 9 in the preparation of electrical contact materials, conductive pastes, conductive adhesives, conductive coatings, electromagnetic shielding coatings, solar cell electrodes or 3D printing conductive inks.

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