Microcrystalline silver powder coated with nanosilver film and preparation method thereof

Abstract

This application discloses a microcrystalline silver powder with a surface-coated silver nanofilm and its preparation method. The preparation method includes: 1) mixing raw silver powder, dispersant, ethanol, and water, and then homogenizing and dispersing them to prepare silver powder system A; 2) preparing silver nitrate into a silver ammonia solution; 3) dissolving a reducing agent in water to obtain solution C; 4) under homogenized dispersion conditions, adding the silver ammonia solution to silver powder system A and mixing evenly, heating to 30-50℃, and adding a pH adjuster to adjust the pH to 9-11 to obtain silver powder system D; 5) adding solution C dropwise to silver powder system D; thus obtaining microcrystalline silver powder with a surface-coated silver film. By controlling the pH and reaction temperature, under homogenized dispersion conditions, the silver ammonia solution reacts with the reducing agent to generate silver nanocrystal nuclei, which grow uniformly and coat the surface of the microcrystalline silver powder. By adjusting the proportion of the silver ammonia solution, the amount of highly active silver film on the surface of the microcrystalline particles can be controlled to meet the requirements for preparing silver powder with different surface sintering activity.

Description

Technical Field

[0001] This application relates to microcrystalline silver powder with a surface-coated nano-silver film and its preparation method, belonging to the field of metal powder material preparation. Background Technology

[0002] Metallic silver possesses excellent physical and chemical properties, such as electrical conductivity, thermal conductivity, and antioxidant properties, and is widely used in antibacterial materials, medical materials, electronic pastes, decorative materials, catalysts, and other fields. Currently, its most widespread application is in the field of electronic pastes. As a conductive material, silver powder is an important component of electronic pastes, accounting for 50% to 90% of its mass, and determines the rheological properties, printing properties, and electrical properties of electronic pastes in subsequent applications.

[0003] With the development of my country's new energy strategic industries, silver powder for solar photovoltaic cell electrode materials has a very broad market application prospect. Currently, my country's solar energy industry is constantly developing and improving, with a huge existing scale, occupying an important position in the global solar energy industry. This presents a severe challenge for the iterative upgrading and industrialization of its upstream product—silver powder. Analyzing only the performance of silver powder itself, its morphology, particle size distribution, tap density, and specific surface area have a significant impact on the rheology and printability of electronic pastes. Among these, morphology and particle size distribution determine the tap density and specific surface area of ​​silver powder, which are key indicators for evaluating the usability of silver powder in subsequent processes. Particle size is a key indicator affecting the application performance of silver powder. Generally, micron-sized silver powder has the characteristics of high crystallinity, good dispersibility, and good conductivity, while nano-sized silver powder has the advantages of large specific surface area and high activity; however, large microcrystalline silver powder generally has low sintering activity, while nano-sized silver powder generally has poor dispersibility and high preparation cost. Therefore, pure micron-sized silver powder and pure nano-sized silver powder have defects in some application functions, which are among the limiting factors in related fields. Therefore, preparing high-performance silver powder is one of the key factors in preparing high-performance electronic pastes.

[0004] Currently, crystalline silicon solar cells generally employ a fast-sintering process, which places high sintering activity requirements on the silver powder that makes up the front-side silver paste. Furthermore, with the development of new high-efficiency cell technologies such as PERC and HJT, even higher sintering activity of the silver powder is required at lower temperatures. The introduction of nano-silver powder may be a solution to these challenges. By combining micron-sized silver powder with a small amount of nano-sized silver powder, it is possible to meet the comprehensive performance requirements of the front-side silver paste for crystalline silicon solar cells in terms of sintering activity and electrical performance. However, this also brings the problems of increased paste cost and quality risks arising from uncertainties in the uniformity of nano-silver powder dispersion in the paste and the stability of the process.

[0005] Therefore, uniformly coating the surface of microcrystalline silver powder with a highly active nanoscale silver film can simultaneously endow the silver powder with high conductivity and high sintering activity, while also solving the problems of increased cost, complex processes, and quality risks associated with the introduction of nanoscale silver powder. Over the past few decades, many scholars and research institutions both domestically and internationally have conducted research on the preparation, structure, and application of surface-coated composite particles. For example, Japanese Patent No. 63-240937 discloses a high-speed airflow impact-type HYB system for surface coating modification of powders; Patent No. 200910183400.X provides a method for surface modification of silver powder, which uses a coating machine to coat nanoparticles onto the surface of micron-sized silver powder, improving the application function of silver powder in conductor pastes. The aforementioned powder composite modification technologies all belong to physical coating methods. They require the separate preparation of two types of powders, one at the micron and the other at the nanometer scale. Then, through mechanical intervention in specific equipment, nanoparticles are coated onto the surface of the micron-sized powder. These two powders also need to meet specific properties, particularly dispersibility, which directly determines the uniformity of the coating. Smaller nanoparticles of silver powder are easier and more uniform to coat, but their dispersibility is relatively poor and their preparation cost is relatively high. Patent CN111922356 A describes the nano-coating modification of microcrystalline silver powder surface using an in-situ reaction system for preparing silver powder via wet chemical oxidation-reduction. Although this is a chemical in-situ synthesis on the silver powder surface, the addition of alkali and surfactants during the reaction causes soft agglomeration of the silver powder, resulting in incomplete coating modification and negatively impacting the printing performance of the silver paste. Summary of the Invention

[0006] To address the aforementioned issues, a method for preparing microcrystalline silver powder with a surface-coated nano-silver film is provided. This method involves adding a reducing agent solution dropwise to a mixture of dispersant, silver salt, and silver powder in a liquid-phase redox system. By controlling the reaction conditions, a highly active silver film is uniformly and completely coated onto the surface of the microcrystalline silver powder.

[0007] The technical solution provided in this application is as follows:

[0008] A method for preparing microcrystalline silver powder with a surface coated with a nano-silver film includes the following steps:

[0009] Step 1: Mix the raw material silver powder, dispersant, ethanol and water, and then homogenize and disperse the mixture to prepare silver powder system A;

[0010] Step 2: Prepare silver nitrate solution into silver ammonia solution to obtain solution B;

[0011] Step 3: Dissolve the reducing agent in deionized water to obtain solution C;

[0012] Step 4: Under homogeneous dispersion conditions, add solution B to the silver powder system A from step 1 and mix thoroughly. Heat to 30-50°C and add a pH adjuster to adjust the pH to 9-11 to obtain silver powder system D.

[0013] Step 5: Add solution C from step 3 dropwise to the silver powder system D from step 4;

[0014] Step 6: After surface treatment, solid-liquid separation, washing and drying of the reactants obtained in step (5), microcrystalline silver powder with a nano silver film coated on the surface is obtained.

[0015] Optionally, in step 4, solution B is added to the silver powder system A in step 1 and mixed evenly, heated to 35-40°C, and a pH adjuster is added to adjust the pH to 10-11 to obtain the silver powder system D.

[0016] Optionally, in step 4, the raw silver powder is spherical or near-spherical, and the particle size of the raw silver powder is 1-5 μm;

[0017] The homogeneous dispersion is performed using an emulsifier or a high-speed disperser;

[0018] The mass ratio of the raw material silver powder, dispersant, ethanol and water is 1:(0.1-0.6):(2-6):(0.5-2);

[0019] Furthermore, the mass ratio of the raw material silver powder, dispersant, ethanol and water is 1:(0.2-0.5):(3-5):1;

[0020] Furthermore, the mass ratio of the raw material silver powder, dispersant, ethanol and water is 1:(0.3-0.4):(4-5):1.

[0021] Optionally, the dispersant is selected from at least one of polyvinylpyrrolidone, polyethylene glycol, or polyvinyl alcohol;

[0022] Furthermore, the dispersant is polyvinylpyrrolidone.

[0023] Optionally, the preparation of the silver ammonia solution includes: dissolving the silver nitrate in deionized water, then adding ammonia water, and stirring evenly to obtain solution B;

[0024] The mass ratio of silver nitrate, deionized water, and ammonia is 1:(0.5-1.5):(0.8-1.5).

[0025] The mass of the silver nitrate is 10%-50% of the mass of the raw silver powder;

[0026] Further, the mass ratio of silver nitrate, deionized water, and ammonia is 1:(0.6-1.4):(0.8-1.4); the mass of silver nitrate is 12%-48% of the mass of the raw silver powder.

[0027] Further, the mass ratio of silver nitrate, deionized water and ammonia is 1:(0.8-1.2):(1-1.2); the mass of silver nitrate is 15%-48% of the mass of the raw silver powder.

[0028] Furthermore, the mass ratio of silver nitrate, deionized water, and ammonia is 1:(0.8-1.1):(1-1.1); the mass of silver nitrate is 20%-47% of the mass of the raw silver powder.

[0029] Optionally, the mass of silver in the silver nitrate is 10%-30% of the mass of the raw silver powder.

[0030] Optionally, the molar ratio of the reducing agent to silver nitrate is (0.4-1.1):1, and the reducing agent is selected from at least one of sodium borohydride, ascorbic acid, hydrazine hydrate, and hydrogen peroxide;

[0031] Furthermore, the molar ratio of the reducing agent to silver nitrate is (0.5-1):1, and the reducing agent is hydrazine hydrate.

[0032] Optionally, the pH adjuster is selected from at least one of ammonia, sodium hydroxide, potassium hydroxide, and sodium carbonate;

[0033] Furthermore, the pH adjuster is ammonia.

[0034] Optionally, the dropping rate of solution C is 10-150 ml / min, and the dropping time is controlled within 8-15 min;

[0035] Furthermore, the dropping rate of solution C is 11-140 ml / min, and the dropping time is controlled within 10-12 min.

[0036] Optionally, in step 6, the surface treatment agent used in the surface treatment is oleic acid and / or stearic acid, and the amount used is 0.5% to 3.5% of the mass of the raw silver powder;

[0037] Furthermore, the amount of the surface treatment agent used is 1% to 3% of the mass of the raw silver powder.

[0038] According to another aspect of this application, a microcrystalline silver powder with a surface coated with a nano-silver film is provided, wherein the microcrystalline silver powder is obtained by the preparation method described in any of the above claims;

[0039] Furthermore, the microcrystalline silver powder has a spherical or near-spherical morphology, and the microcrystalline silver powder includes a microcrystalline nucleus and a silver nanofilm coated on the surface of the microcrystalline nucleus, wherein the silver nanofilm is formed by the deposition of silver nanoparticles.

[0040] Furthermore, the microcrystalline silver powder has a particle size of 1.0–5.0 μm, a specific surface area of ​​0.3–1.0 m² / g, and a tap density of 5.5–6.4 g / cm³. 3 .

[0041] The beneficial effects of this application include, but are not limited to:

[0042] 1. The method for preparing microcrystalline silver powder with a surface-coated nano-silver film according to this application involves controlling the pH and reaction temperature under homogeneous dispersion conditions, allowing the silver ammonia solution to react with the reducing agent to generate nano-silver crystal nuclei, which then uniformly grow and coat the surface of the microcrystalline silver powder. This method offers good controllability; by adjusting the proportion of the silver ammonia solution, the amount of highly active silver film on the surface of the microcrystalline particles can be controlled, thus satisfying the preparation of silver powders with different surface sintering activity.

[0043] 2. The method for preparing microcrystalline silver powder with a surface-coated nano-silver film according to this application adopts a liquid-phase oxidation-reduction system. This method directly performs in-situ chemical growth coating on the surface of microcrystalline silver powder, and the reaction system selects an appropriate pH value, so that the surface nano-silver film has a smaller lattice and higher activity; it has broad application prospects in the field of thick film electronic pastes such as silver paste for solar cells and / or electrode paste for electronic components.

[0044] 3. The method for preparing microcrystalline silver powder with a surface coated with a nano-silver film according to this application has the following advantages: the equipment and process used are simple, the reaction conditions are mild, the production cycle is short, it is easy to operate, has good repeatability, is energy-saving and environmentally friendly, and is suitable for industrial scale-up and industrial application. Attached Figure Description

[0045] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0046] Figure 1 Here is a scanning electron microscope image of the silver powder used in Example 1;

[0047] Figure 2 A scanning electron microscope image of microcrystalline silver powder 1# prepared in Example 1;

[0048] Figure 3 Here is a scanning electron microscope image of the silver powder used in Example 2;

[0049] Figure 4 Scanning electron microscope image of microcrystalline silver powder 2# prepared in Example 2;

[0050] Figure 5 Here is a scanning electron microscope image of the silver powder used in Example 3;

[0051] Figure 6 Scanning electron microscope image of microcrystalline silver powder #3 prepared in Example 3;

[0052] Figure 7 Scanning electron microscope image of comparative microcrystalline silver powder 1'# prepared for Comparative Example 1;

[0053] Figure 8 Scanning electron microscope image of comparative microcrystalline silver powder 2'# prepared for Comparative Example 2;

[0054] Figure 9 Scanning electron microscope image of comparative microcrystalline silver powder 3'# prepared for comparison column 3. Detailed Implementation

[0055] The present invention will be described in detail below with reference to specific embodiments. The following embodiments are only for the purpose of enabling those skilled in the art to understand the technical solutions of the present invention, implement or use the present invention, and are not intended to limit the scope of protection of the present invention.

[0056] Unless otherwise specified, the materials and equipment used in the embodiments of this application are all commercially purchased or commonly used in the art. The methods in the embodiments, unless otherwise specified, are conventional methods in the art.

[0057] In this application, "raw material silver powder" refers to silver powder prepared using the preparation method in patent CN112475311A.

[0058] Example 1

[0059] Preparation of microcrystalline silver powder with a particle size of 1.6 μm and a surface coated with a silver nanofilm #1

[0060] (1) Solution preparation

[0061] Silver powder system A: 1200g ethanol, 267g deionized water, 80g dispersant, and 320g raw silver powder with a particle size of 1.6μm were added to the reaction vessel and dispersed using an emulsifier for 5 minutes to obtain silver powder system A. The ratio of ethanol to deionized water was 4.5:1; the ratio of raw silver powder to ethanol was 1:3.75; and the ratio of raw silver powder to dispersant was 1:0.25.

[0062] Solution B: Dissolve 75g of silver nitrate in 65g of deionized water, then add 80g of ammonia water, stir well to prepare a silver ammonia solution, and obtain solution B; the silver content in the silver ammonia solution is about 15% of the mass of the raw silver powder;

[0063] Reducing agent solution C: Add 56g of 40% hydrazine hydrate to 56g of deionized water and stir until homogeneous. The molar ratio of silver content to reducing agent in silver ammonia solution B is 1:1.

[0064] Silver powder system D: Add solution B to silver powder system A, add 102g of ammonia water, continue to disperse using an emulsifier, heat to 35℃ and keep warm for 5 minutes; at this time the solution pH is 10;

[0065] (2) Add the reducing agent solution C to the silver powder system D at a flow rate of 11 ml / min, control the temperature at 35℃, add the material for 10 min, and keep the reaction at 35℃ for 5 min.

[0066] (3) Add 6.4g of oleic acid to step (2), keep warm and disperse for 10min, and after solid-liquid separation, washing and drying, obtain 365g of microcrystalline silver powder 1# with a surface coated with a nano silver film. The average particle size of microcrystalline silver powder 1# is 1.6μm.

[0067] Example 2

[0068] Preparation of microcrystalline silver powder with a particle size of 2.5 μm and a surface coated with a silver nanofilm #2

[0069] (1) Solution preparation

[0070] Silver powder system A: Add 3000g ethanol, 666g deionized water, 150g dispersant, and 600g of raw material microcrystalline silver powder with a particle size of 2.5μm to the reaction vessel, and disperse using an emulsifier for 5min. The ratio of ethanol to deionized water is 4.5:1; the ratio of raw material silver powder to ethanol is 1:5; and the ratio of raw material silver powder to dispersant is 1:0.25.

[0071] Solution B: Dissolve 188g of silver nitrate in 163g of deionized water, then add 200.5g of ammonia water, stir well to prepare silver ammonia solution, and obtain solution B. The silver content in the silver ammonia solution is about 20% of the mass of the raw silver powder.

[0072] Reducing agent solution C: Add 140g of 40% hydrazine hydrate to 140g of deionized water and stir until homogeneous. The molar ratio of silver content to reducing agent in silver ammonia solution B is 1:1.

[0073] Silver powder system D: Add silver ammonia solution B to silver powder system A, add 267g of ammonia water, continue to disperse using an emulsifier, heat to 40℃ and keep warm for 5 minutes; at this time the solution pH is 10;

[0074] (2) Add the reducing agent solution C to the silver powder system D at a flow rate of 28 ml / min, control the temperature at 40℃, add the material for 10 min, and keep the reaction at 40℃ for 5 min.

[0075] (3) Add 12g of oleic acid to step (2), keep warm and disperse for 10min, and after solid-liquid-liquid separation, washing and drying, 720g of microcrystalline silver powder 2# with a highly active nano silver film on the surface is obtained. The average particle size of microcrystalline silver powder 2# is 2.5μm.

[0076] Example 3

[0077] Preparation of microcrystalline silver powder with a particle size of 3.0 μm and a surface coated with a nano-silver film #3

[0078] (1) Solution preparation

[0079] Silver powder system A: Add 15kg ethanol, 3500g deionized water, 500g dispersant, and 2000g of raw material microcrystalline silver powder with a particle size of 3.0 micrometers to the reactor. Disperse the mixture using an emulsifier for 5 minutes. The ratio of ethanol to deionized water is 4.5:1; the ratio of raw material silver powder to ethanol is 1:3.75; and the ratio of raw material silver powder to dispersant is 1:0.25.

[0080] Solution B: Dissolve 945g of silver nitrate in 820g of deionized water, then add 945g of ammonia water and stir until homogeneous. The silver content in the silver ammonia solution is approximately 30% of the mass of the raw silver powder.

[0081] Reducing agent solution C: Add 703g of 40% hydrazine hydrate to 750g of deionized water and stir until homogeneous. The molar ratio of silver content to reducing agent in silver ammonia solution B is 1:1.

[0082] Silver powder system D: Add solution B to silver powder system A, add 1543g of ammonia water, continue to disperse using an emulsifier, heat to 40℃ and keep warm for 5 minutes; at this time the solution pH is 11;

[0083] (2) Add the reducing agent solution C to the silver powder system D at a flow rate of 140 ml / min, control the temperature at 40℃, add the material for 10 min, and keep the reaction at 40℃ for 5 min.

[0084] (3) Add 40g of stearic acid to step (2), keep warm and disperse for 10min, and after solid-liquid separation, washing and drying, obtain 2600g of microcrystalline silver powder 3# with a highly active nano silver film on the surface. The average particle size of microcrystalline silver powder 3# is 3.0μm.

[0085] Comparative Example 1

[0086] Comparison of microcrystalline silver powder with surface coated with nano-silver film 1'#

[0087] (1) Solution preparation

[0088] Silver powder system A: Add 3000g ethanol, 666g deionized water, 150g dispersant, and 600g of raw material microcrystalline silver powder with a particle size of 2.5μm to the reaction vessel. Disperse the mixture using an emulsifier for 5 minutes. The ratio of ethanol to deionized water is 4.5:1; the ratio of raw material silver powder to ethanol is 1:5; and the ratio of raw material silver powder to dispersant is 1:0.25.

[0089] Silver ammonia solution B: Dissolve 188g of silver nitrate in 163g of deionized water, then add 200.5g of ammonia water, stir well to prepare silver ammonia solution, and obtain solution B; the silver content in the silver ammonia solution is about 20% of the mass of the raw silver powder;

[0090] Reducing agent solution C: Add 140g of 40% hydrazine hydrate to 140g of deionized water and stir until homogeneous. The molar ratio of silver content to reducing agent in silver ammonia solution B is 1:1.

[0091] Silver powder system D: Add silver ammonia solution B to silver powder system A, add 357g of ammonia water, and continue to disperse and heat to 40℃ using an emulsifier for 5 minutes; at this time, the solution pH is 12;

[0092] (2) Add the reducing agent solution C to the silver powder system D at a flow rate of 28 ml / min, control the temperature at 40℃, add the material for 10 min, and keep the reaction at 40℃ for 5 min.

[0093] (3) Add 12g of oleic acid to step (2), keep warm and disperse for 10min, and after solid-liquid-liquid separation, washing and drying, the finished product, comparative microcrystalline silver powder 1'#720g, is obtained.

[0094] (4) Scanning electron microscopy revealed that the particle size distribution of the microcrystalline silver powder 1'# was uneven, consisting of a mixture of 2.5μm silver powder and nano-silver powder. Compared to the raw silver powder, the surface was smooth and the morphology remained unchanged, indicating that a highly active silver film was not successfully coated. This was because the pH of the underlying solution in the silver powder system D was 12. The high pH caused the nucleation rate of newly formed silver to exceed the growth rate of crystal nuclei during the reaction process after the addition of the reducing agent solution C, which was far greater than the growth rate of the coating on the surface of the raw material, resulting in the formation of a large amount of nano-silver powder. Scanning electron microscopy tests are as follows: Figure 7 As shown.

[0095] Comparative Example 2

[0096] Comparison of microcrystalline silver powder with surface coated with nano-silver film 2'#

[0097] (1) The preparation method of microcrystalline silver powder 2'# is basically the same as that of Example 2, except that the reducing agent solution C is added to the silver powder system D at a flow rate of 28 ml / min, the temperature is controlled at 20℃, the feeding time is 10 min; and the reaction is kept at 20℃ for 5 min.

[0098] (2) Add 12g of oleic acid to step (2), keep warm and disperse for 10min, and after solid-liquid separation, washing and drying, obtain 2'#720g of finished microcrystalline silver powder;

[0099] (3) Scanning electron microscopy revealed that the particle size distribution of the microcrystalline silver powder 2'# was uneven, consisting of a mixture of 2.5μm silver powder, nano-silver powder, and submicron silver powder. Compared to the raw silver powder, the 2.5μm silver powder had a smooth surface and unchanged morphology, failing to successfully coat a highly active silver film. This was because the bottom solution temperature in silver powder system D was 20℃, and the reaction rate of the entire system slowed down during the addition of reducing agent solution C. The nucleation rate of newly formed silver was lower than the crystal growth rate, but still higher than the surface growth and coating rate of the raw silver powder. Scanning electron microscopy tests are as follows: Figure 8 As shown.

[0100] Comparative Example 3

[0101] Comparison of microcrystalline silver powder with surface coated with nano-silver film 3'#

[0102] (1) The preparation method of microcrystalline silver powder 3'# is basically the same as that in Example 2, except that the emulsifier in silver powder system A and silver powder system D is replaced by ordinary mechanical stirring.

[0103] (2) The obtained finished product is compared with microcrystalline silver powder 3'#. Scanning electron microscopy test results are as follows: Figure 9 As shown. From Figure 9 As can be seen from the data, compared with the microcrystalline silver powder 3'#, the dispersion is uneven and it is easy to clump together. The main reason is that ordinary mechanical stirring is not as uniform as the dispersion by an emulsifier or high-speed disperser.

[0104] The average particle size, specific surface area, tap density, weight loss on ignition, and TMA softening inflection point temperature of the microcrystalline silver powders 1# to 3# prepared in Examples 1 to 3 above, and the comparative microcrystalline silver powders 1'# to 3'# prepared in Comparative Examples 1 to 3 were tested. The test results are shown in Table 1.

[0105] Table 1. Test Results of Silver Powder Performance

[0106]

[0107] As can be seen from Table 1, after the raw material microcrystalline silver powder in the embodiments of the present invention is coated with a highly active nano-silver film (microcrystalline silver powder 1#~3#), compared with the corresponding raw material microcrystalline silver powder (raw material silver powder 1#~3#), the particle size is similar, the particle size distribution is narrower, the dispersibility is also better, the specific surface area and the weight loss on ignition are increased, but the tap density is basically the same, and the sintering softening inflection point temperature is significantly reduced, which can effectively increase the surface activity of microcrystalline silver powder.

[0108] The above description is merely an embodiment of this application, and the scope of protection of this application is not limited to these specific embodiments, but is determined by the claims of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the technical concept and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for preparing microcrystalline silver powder with a surface coated with a nano-silver film, characterized in that, The preparation method includes the following steps: Step 1: Mix the raw material silver powder, dispersant, ethanol and water, and then homogenize and disperse the mixture to prepare silver powder system A; Step 2: Prepare silver nitrate solution into silver ammonia solution to obtain solution B; Step 3: Dissolve the reducing agent in deionized water to obtain solution C, wherein the reducing agent is selected from at least one of sodium borohydride, ascorbic acid, hydrazine hydrate and hydrogen peroxide; Step 4: Under homogeneous dispersion conditions, add solution B to the silver powder system A from step 1 and mix thoroughly. Heat to 30-50°C and add a pH adjuster to adjust the pH to 9-11 to obtain silver powder system D. Step 5: Add solution C from step 3 dropwise to the silver powder system D from step 4; Step 6: After surface treatment, solid-liquid separation, washing and drying of the reactants obtained in step 5, microcrystalline silver powder with a silver film on the surface is obtained.

2. The preparation method according to claim 1, characterized in that, In step 4, solution B is added to the silver powder system A from step 1 and mixed evenly. The mixture is then heated to 35-40°C, and a pH adjuster is added to adjust the pH to 10-11 to obtain the silver powder system D.

3. The preparation method according to claim 1, characterized in that, In step 1, the raw silver powder is spherical or near-spherical, and the particle size of the raw silver powder is 1-5 μm; The homogeneous dispersion is performed using an emulsifier or a high-speed disperser; The mass ratio of the raw material silver powder, dispersant, ethanol and water is 1:(0.1-0.6):(2-6):(0.5-2).

4. The preparation method according to claim 3, characterized in that, The mass ratio of the raw material silver powder, dispersant, ethanol and water is 1:(0.2-0.5):(3-5):

1.

5. The preparation method according to claim 4, characterized in that, The mass ratio of the raw material silver powder, dispersant, ethanol and water is 1:(0.3-0.4):(4-5):

1.

6. The preparation method according to claim 1, characterized in that, The dispersant is selected from at least one of polyvinylpyrrolidone, polyethylene glycol, or polyvinyl alcohol.

7. The preparation method according to claim 6, characterized in that, The dispersant is polyvinylpyrrolidone.

8. The preparation method according to claim 1, characterized in that, In step 2, the preparation of the silver ammonia solution includes: dissolving the silver nitrate in deionized water, then adding ammonia water, and stirring evenly to obtain solution B; The mass ratio of silver nitrate, deionized water, and ammonia is 1:(0.5-1.5):(0.8-1.5). The mass of the silver nitrate is 10%-50% of the mass of the raw silver powder.

9. The preparation method according to claim 8, characterized in that, The mass ratio of silver nitrate, deionized water, and ammonia is 1:(0.8-1.2):(1-1.2). The mass of the silver nitrate is 15%-48% of the mass of the raw silver powder.

10. The preparation method according to claim 1, characterized in that, The molar ratio of the reducing agent to silver nitrate is (0.4-1.1):

1.

11. The preparation method according to claim 10, characterized in that, The molar ratio of the reducing agent to silver nitrate is (0.5-1):1, and the reducing agent is hydrazine hydrate.

12. The preparation method according to claim 1 or 2, characterized in that, The pH adjuster is selected from at least one of ammonia, sodium hydroxide, potassium hydroxide, and sodium carbonate.

13. The preparation method according to claim 12, characterized in that, The pH adjuster is ammonia.

14. The preparation method according to claim 1, characterized in that, In step 5, the dropping rate of solution C is 10-150 ml / min, and the dropping time is controlled between 8 and 15 min.

15. The preparation method according to claim 14, characterized in that, The dropping rate of solution C is 11-140 ml / min, and the dropping time is controlled within 10-12 min.

16. The preparation method according to claim 1, characterized in that, In step 6, the surface treatment agent used in the surface treatment is oleic acid and / or stearic acid, and the amount used is 0.5% to 3.5% of the mass of the raw silver powder.

17. The preparation method according to claim 16, characterized in that, The amount of the surface treatment agent used is 1% to 3% of the mass of the raw silver powder.

18. A microcrystalline silver powder with a surface coated with a nano-silver film, characterized in that, The microcrystalline silver powder is obtained by the preparation method according to any one of claims 1-17.

19. The microcrystalline silver powder with a surface coated with a nano-silver film according to claim 18, characterized in that, The microcrystalline silver powder has a spherical or near-spherical morphology. The microcrystalline silver powder includes a microcrystalline nucleus and a silver nanofilm covering the surface of the microcrystalline nucleus. The silver film is formed by the deposition of silver nanocrystal nuclei.

20. The microcrystalline silver powder with a surface coated with a nano-silver film according to claim 18, characterized in that, The microcrystalline silver powder has a particle size of 1.0–5.0 μm, a specific surface area of ​​0.3–1.0 m² / g, and a tap density of 5.5–6.4 g / cm³.

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