Micro-nano silver powder with controllable particle size and preparation method thereof

Abstract

The invention relates to the technical field of micro-nano particle preparation, and discloses particle size controllable micro-nano silver powder and a preparation method thereof.The preparation method comprises the steps that S1, micron-sized silver cores and nano-sized silver particles are prepared; s2, performing surface activation on the micron-sized silver core; s3, filling pore gaps of a porous template with the micron-sized silver cores; s4, spraying graphene dispersion liquid on the porous template, and forming graphene network coverage on the surface of each silver core; and S5, after the porous template is removed, nanoscale silver particles are deposited on the surface of the exposed silver core, and a silver shell structure wrapping the silver core and the graphene network is formed. Through gradient compounding of a micron silver core-graphene network-nano silver shell structure, the width difference between two sintering necks formed between the circuit board and connecting plate interfaces on the two sides during sintering is reduced by effectively utilizing the wettability of a nano silver shell and the stress dispersity of graphene, and meanwhile the excellent electrical conductivity and thermal conductivity of the material are utilized; and the sintering performance of the circuit board is optimized.

Description

Technical Field

[0001] The present invention relates to the technical field of micro-nano particle preparation, and particularly relates to a micro-nano silver powder with controllable particle size and a preparation method thereof. Background Art

[0002] In the field of electronic packaging, sintered silver powder is widely used for the connection between circuit boards. The silver sintering technology is a new connection technology that uses silver powder or silver paste as an intermediate layer material and realizes the tight bonding between the chip and the substrate through solid-state diffusion or liquid sintering under specific temperature, pressure, and time conditions. The connecting layer is composed of silver and has excellent electrical and thermal conductivity. The melting point of silver is as high as 961°C, and it will not produce the typical fatigue effect that appears in the soft soldering connection layer with a melting point less than 300°C. It has high reliability, and the sintering material does not contain lead, belonging to an environmentally friendly material.

[0003] When sintering silver powder, sintering necks are formed between silver powder particles and between the silver powder and the circuit board, which is the key part of the connection. The formation of sintering necks depends on atomic diffusion and usually occurs in the temperature range of 0.25 - 0.75 times the melting point of the material. However, there are significant problems in traditional silver powder sintering. During the sintering process, the size and distribution of the sintering necks between silver powder particles are affected by multiple factors such as sintering temperature, time, and the properties of silver powder. When the sintering temperature increases, the size of the sintering necks increases, but too high a temperature may cause excessive sintering of silver powder particles, resulting in problems such as grain growth and increased porosity, which instead affect the performance of the sintered layer, lead to uneven stress distribution inside the material, and are prone to crack generation and propagation, reducing the tensile strength and toughness of the material. After traditional silver powder sintering, the width difference of the sintering necks is large, resulting in stress concentration at the connection, easy fracture, poor bonding effect, and it is difficult to meet the heat dissipation and signal transmission requirements of high-power chips. Therefore, a new solution is proposed to solve these problems. Summary of the Invention

[0004] The present invention overcomes the deficiencies of the prior art and provides a micro-nano silver powder with controllable particle size and a preparation method thereof.

[0005] To achieve the above object, the technical solution adopted by the present invention is: a preparation method of a micro-nano silver powder with controllable particle size, including the following steps: S1, preparing a micron-sized silver core and nano-sized silver particles; S2, performing surface activation on the micron-sized silver core; S3, filling the micron-sized silver core into the pore gaps of a porous template; S4, spraying a graphene dispersion liquid on the porous template to form a graphene network coverage on the surface of each silver core; S5, after removing the porous template, depositing nano-sized silver particles on the exposed surface of the silver core to form a silver shell structure that wraps the silver core and the graphene network.

[0006] In a preferred embodiment of the present invention, the preparation of the micron-sized silver core in step S1 includes: mixing a silver nitrate solution with a concentration of 0.15–0.20 mol / L and an ascorbic acid solution with a concentration of 0.10–0.15 mol / L, adjusting the pH to 1-3 with nitric acid, adding a tannic acid solution with a concentration of 0.01–0.05 mol / L, reacting at 25-40 °C for 15-30 min, separating and drying to obtain a micron-sized silver core with a particle size between 0.5-2 μm.

[0007] In a preferred embodiment of the present invention, the preparation of the nano-sized silver particles in step S1 includes: mixing a silver nitrate solution with a concentration of 0.01–0.05 mol / L and an ascorbic acid solution with a concentration of 0.01–0.05 mol / L, adding polyvinylpyrrolidone with a mass concentration of 0.1–0.5% as a dispersant, adjusting the pH to 5-8, stirring and reacting at 30-50 °C for 10-30 minutes; after separating by centrifugal filtration, washing alternately with water and ethanol 3-5 times, and vacuum drying at 50-80 °C for 4-8 hours to obtain nano-sized silver particles with a particle size of 50-100 nm.

[0008] In a preferred embodiment of the present invention, the specific steps for surface activation of the micron-sized silver core include: immersing the micron-sized silver core in a mixed solution of 0.1-0.5 mol / L silver nitrate and 0.01 mol / L ascorbic acid, reacting at 30-50 °C for 5 min to form an activated surface, rinsing and drying for standby.

[0009] In a preferred embodiment of the present invention, the porous template is an anodic aluminum oxide template with a pore diameter of 0.8-2.5 μm.

[0010] In a preferred embodiment of the present invention, the deposition of the nano-sized silver particles in step S5 is achieved by an electrochemical method, and the specific steps include: mixing a silver nitrate solution with a concentration of 0.05–0.2 mol / L and sodium citrate with a concentration of 0.1–0.5 mol / L, adjusting the pH to 4-6 to prepare an electrolyte; using porous carbon paper to load the micron-sized silver core and immersing it in the electrolyte as a working electrode, using a platinum sheet as a counter electrode and inserting it into the electrolyte and then applying current to both electrodes, controlling the current density to be 5-15 mA / cm², and the deposition time to be 5-30 min; maintaining the electrolyte temperature at 25-40 °C during the deposition process and accompanied by magnetic stirring.

[0011] In a preferred embodiment of the present invention, the coverage of the silver shell is 40-80% of the surface of the micron-sized silver core.

[0012] In a preferred embodiment of the present invention, the concentration of the graphene dispersion liquid is 0.1–0.5 mg / mL, the spraying pressure is 0.2–0.8 MPa, the spraying times are 2-5 times, and each spraying volume is 50-100 mL.

[0013] In a preferred embodiment of the present invention, the graphene network covers 40-80% of the surface of the silver core.

[0014] The present invention solves the defects existing in the background technology and has the following beneficial effects: (1) By designing a gradient composite structure of micro silver core - graphene network - nano silver shell and optimizing processes such as activation treatment, template confinement, and electrochemical deposition, the present invention significantly improves the tensile resistance of silver powder sintering. During sintering, the graphene network and the nano silver shell cooperate to fill, achieving uniform stress transfer; at the same time, the nano silver shell will first wet the upper and lower circuit board surfaces, thus better enabling the silver material to bond with the interface. The wettability of the small particle nano silver shell is utilized to enhance the interface bonding, and at the same time, graphene is used as a stress buffer layer to improve the structural integrity of the material in the thermo-mechanical coupling environment, effectively solving the problem of poor tensile resistance caused by a large difference in neck width in the traditional process, and also improving the thermal conductivity and material strength by utilizing the material properties of graphene.

[0015] (2) The activation of the silver core surface in the present invention enhances the bonding strength of the silver core, graphene, and silver shell. The guiding effect of the anodic aluminum oxide template ensures the orderly bonding of graphene and the silver core, improves the uniformity of the bonding, and further enhances the bonding effect and tensile strength of the silver powder. The anodic aluminum oxide template and the graphene network cooperate to optimize the silver powder structure, reduce the porosity, and improve the thermal conductivity. This physical confinement effect enables the silver powder to be more evenly distributed and bonded, enhancing the thermal stability and mechanical properties, resulting in a small difference in neck width and higher tensile strength and better bonding effect when the silver powder of the present invention is sintered on the circuit board. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings; Figure 1 It is a flowchart of the steps of a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0018] In the following description, many specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may be practiced in other ways than those specifically described herein. Therefore, the scope of protection of the present invention is not limited by the specific embodiments disclosed below.

[0019] Exemplary material sources: All material sources mentioned in this application do not have special procurement source requirements, and commercially available products well-known to those skilled in the art can be used; For example: silver nitrate solution (product number BW05123), ascorbic acid (product number MS0625 - 500G), polyvinylpyrrolidone (product number MS4327A - 500G), and nitric acid (product number SJRY27) can all be purchased from Beijing Kezhan Biotechnology Co., Ltd.; It should be added that the above products are only exemplary procurement sources and do not represent that the product concentration is the solution concentration in the embodiments of this application; those skilled in the art can select commercially available standard solutions or prepare them by themselves according to actual needs. The concentrations marked in the embodiments of this application are achieved by conventional concentration adjustment means well-known to those skilled in the art.

[0020] Exemplary methods: As Figure 1 shown, a method for preparing micro-nano silver powder with controllable particle size includes the following steps: S1. Prepare micro-scale silver nuclei and nano-scale silver particles; S2. Activate the surface of the micro-scale silver nuclei; S3. Fill the pore gaps of the porous template with the micro-scale silver nuclei; S4. Spray a graphene dispersion liquid on the porous template to form a graphene network covering on the surface of each silver nucleus; S5. After removing the porous template, deposit nano-scale silver particles on the exposed surface of the silver nuclei to form a silver shell structure that wraps the silver nuclei and the graphene network.

[0021] Next, each step will be introduced in detail.

[0022] Specifically, the specific preparation steps of the micro-scale silver nuclei in step S1 include: mixing a silver nitrate solution with a concentration of 0.15 - 0.20 mol / L and an ascorbic acid solution with a concentration of 0.10 - 0.15 mol / L according to a certain mass ratio, adjusting the pH to 1 - 3 with nitric acid, adding a sufficient amount of tannic acid solution with a concentration of 0.01 - 0.05 mol / L, reacting at 25 - 40 °C for 15 - 30 min, separating and drying to obtain micro-scale silver nuclei with a particle size between 0.5 - 2 μm.

[0023] Specifically, the preparation of the nanoscale silver particles in step S1 includes: mixing a silver nitrate solution with a concentration of 0.01–0.05 mol / L and an ascorbic acid solution with a concentration of 0.01–0.05 mol / L in a certain mass ratio, adding polyvinylpyrrolidone with a mass concentration of 0.1–0.5% as a dispersant, adjusting the pH to 5-7, and stirring and reacting at 30-50 °C for 10-30 minutes; after separating the nanoscale silver particles by centrifugal filtration, washing them alternately with water and ethanol 3-5 times, and drying them under vacuum at 50-80 °C for 4-8 hours to obtain nanoscale silver particles with a particle size of 50-100 nm.

[0024] Specifically, the specific steps for surface activation of the micron-scale silver core in step S2 include: immersing the micron-scale silver core in a mixed solution of 0.1-0.5 mol / L silver nitrate and 0.01 mol / L ascorbic acid, reacting at 30-50 °C for 5 min to form an activated surface, rinsing and drying for standby.

[0025] Specifically, the porous template is an anodic aluminum oxide template with a pore diameter of 0.8-2.5 μm, and the arrangement of the silver cores is restricted by the adsorption of the nano micropores.

[0026] Specifically, in step S3, the specific steps for filling the activated micron-scale silver cores into the anodic aluminum oxide template with a pore diameter of 2.5 μm are as follows: S31. Horizontally fix the anodic aluminum oxide template on a vibration platform, evenly spread the silver core powder on the surface of the template, start the vibration platform, set the vibration frequency to 50-200 Hz, the amplitude to 0.5-2 mm, and the vibration time to 5-15 minutes; during the vibration, the silver cores enter the pore gaps of the template under the action of inertia force; S32. Control the filling amount by the weighing method, with the mass of the silver cores loaded per square centimeter of the template being 1.0-2.0 mg; S33. After filling, tilt the template and gently sweep the surface with a soft brush to remove the residual silver cores that have not entered the pores.

[0027] Specifically, the specific method for removing the porous template includes: S51. Place the template in ethanol or deionized water, perform ultrasonic treatment for 10-20 minutes, control the ultrasonic power at 100-200 W, and the frequency at 40 kHz, and use the cavitation effect to detach the silver cores from the pores.

[0028] S52. Filter using a filter membrane with a pore diameter of 0.1-0.2 μm to collect the silver cores.

[0029] Specifically, the deposition of the nanoscale silver particles in step S5 is achieved by an electrochemical method, and the specific steps include: Mix a silver nitrate solution with a concentration of 0.05–0.2 mol / L and sodium citrate with a concentration of 0.1–0.5 mol / L in proportion, adjust the pH to 4-6 with nitric acid, and prepare an electrolyte solution; use porous carbon paper loaded with micron-sized silver nuclei and immerse it in the electrolyte solution as the working electrode, and a platinum sheet as the counter electrode, control the current density to be 5-15 mA / cm², and the deposition time to be 5-30 min; keep the temperature of the electrolyte solution at 25-40 °C during the deposition process, and accompany it with magnetic stirring to ensure uniform precipitation. The thickness of the silver shell on the surface of the silver nuclei can be controlled by increasing or decreasing the precipitation time, and the particle size can be controlled.

[0030] Specifically, the method of using porous carbon paper to load micron-sized silver nuclei includes: S53. Select porous carbon paper with a pore size of 2-5 μm and a porosity of more than 80%, immerse it in a 0.1-0.5% polydopamine solution for 2-4 hours to form an adhesion layer on its surface; S54. Disperse the silver nuclei dispersed in step S52 in an ethanol solution with a concentration of 1-5 mg / mL, and ultrasonically stir for 10-20 min; S55. Immerse the porous carbon paper in the dispersion liquid, let it stand for 0.5-1 h, and continuously perform ultrasonic dispersion during this period. The silver nuclei are adsorbed on the surface of the porous carbon paper by van der Waals forces.

[0031] Specifically, the coverage of the silver shell is 40-80% of the surface of the micron-sized silver nuclei.

[0032] Specifically, the concentration of the graphene dispersion liquid is 0.1–0.5 mg / mL, the spraying pressure is 0.2–0.8 MPa, the number of spraying times is 2-5 times, and each spraying volume is 50-100 mL.

[0033] Specifically, the graphene network covers 40-80% of the surface of the silver nuclei.

[0034] Exemplary materials: A micro-nano silver powder is prepared by an exemplary method.

[0035] Example 1:

[0036] A preparation method of micro-nano silver powder with controllable particle size, the specific implementation steps are as follows: S1. Mix a 0.18 mol / L silver nitrate solution and a 0.12 mol / L ascorbic acid solution in a mass ratio of 2:3, adjust the pH to 2.5 with nitric acid, add a 0.03 mol / L tannic acid solution with a mass of one-fourth of the ascorbic acid solution, send it into a reaction kettle and react at 35 °C for 20 min. After separation by a high-speed centrifuge at a speed of 8000 rpm for 10 min, send it into a vacuum dryer and conduct vacuum drying at 30 °C to obtain micron-sized silver nuclei with a particle size of 0.6-2 μm; S2. Immerse the silver nuclei obtained in step S1 into a mixed solution of sufficient 0.3 mol / L silver nitrate and 0.01 mol / L ascorbic acid mixed at a mass ratio of 1:1, react in an environment of 30 °C for 10 min, rinse with clear water and then dry for standby; S3. Mix 0.03 mol / L silver nitrate solution and 0.03 mol / L ascorbic acid at a ratio of 1:1, add polyvinylpyrrolidone accounting for 0.3% of the total solution mass, adjust the pH to 6.5 with nitric acid, send it into a reaction kettle, react at 40 °C for 20 min and continuously stir during the reaction. After the reaction is completed, separate the solid silver particles by centrifugal filtration, wash them and then vacuum dry at 60 °C for 4 h to obtain nano silver particles with a particle size of 50 - 100 nm; S4. Fill the activated micron - sized silver nuclei into an anodic aluminum oxide template with a pore size of 2.5 μm. Load the 0.3 mg / mL graphene dispersion liquid into an electrostatic spraying machine, control the voltage at 35 kV, the pressure at 0.5 MPa, spray 3 times, 50 mL each time, and assist with vacuum for 10 min to form a graphene network coverage on the surface of the micron - sized silver nuclei; S5: Mix 0.1 mol / L silver nitrate and 0.3 mol / L sodium citrate at a mass ratio of 1:1 to prepare an electrolyte solution (pH = 5). Use porous carbon paper to load the micron - sized silver nuclei and immerse them in the electrolyte as the working electrode, and use a platinum sheet as the counter electrode to simultaneously extend into the electrolyte. Pass an electric current with a current density of 10 mA / cm² and deposit for 15 min to form a nano - sized silver shell on the surface of the silver nuclei covered with graphene. After rinsing with clear water and drying, obtain the target silver powder.

[0037] Example 2:

[0038] A preparation method of micro - nano silver powder with controllable particle size, and the specific implementation steps are as follows: S1. Mix 0.18 mol / L silver nitrate solution and 0.12 mol / L ascorbic acid solution at a mass ratio of 2:3, adjust the pH to 2.5 with nitric acid, add 0.03 mol / L tannic acid solution with a mass one - quarter of the ascorbic acid solution, send it into a reaction kettle, react at 35 °C for 20 min, separate by using a high - speed centrifuge at a rotation speed of 8000 rpm for 10 min, and then send it into a vacuum dryer for vacuum drying at 30 °C to obtain micron - sized silver nuclei with a particle size of 0.6 - 2 μm; S2. Immerse the silver nuclei obtained in step S1 into a mixed solution of sufficient 0.3 mol / L silver nitrate and 0.01 mol / L ascorbic acid mixed at a mass ratio of 1:1, react in an environment of 30 °C for 10 min, rinse with clear water and then dry for standby; S3. Mix 0.03 mol / L silver nitrate solution and 0.03 mol / L ascorbic acid in a 1:1 ratio, add polyvinylpyrrolidone accounting for 0.3% of the total solution mass, adjust the pH to 6.5 using nitric acid, transfer it into a reaction kettle, react at 40 °C for 20 min and continuously stir during the reaction. After the reaction is completed, separate the solid silver particles by centrifugal filtration, disperse and wash them, and then vacuum dry at 60 °C for 4 h to obtain nano-silver particles with a particle size of 50 - 100 nm; S4. Fill the activated micro-scale silver nuclei into an anodic aluminum oxide template with a pore size of 2.5 μm. Load the graphene dispersion liquid with a concentration of 0.3 mg / mL into an electrostatic spraying machine, control the voltage at 35 kV, the pressure at 0.3 MPa, spray twice, 50 mL each time, and assist with vacuum for 10 min to form a graphene network coverage on the surface of the micro-scale silver nuclei; S5: Mix 0.1 mol / L silver nitrate and 0.3 mol / L sodium citrate in a mass ratio of 1:1 to prepare an electrolyte solution (pH = 5). Use porous carbon paper to load the micro-scale silver nuclei and immerse it in the electrolyte solution as the working electrode. Use a platinum sheet as the counter electrode and insert it into the electrolyte solution at the same time. Pass an electric current with a current density of 10 mA / cm² and deposit for 15 min to cover a nano-scale silver shell on the surface of the silver nuclei covered with graphene. After rinsing with water and drying, obtain the target silver powder.

[0039] Example 3:

[0040] A preparation method of micro-nano silver powder with controllable particle size, and the specific implementation steps are as follows: S1. Mix 0.18 mol / L silver nitrate solution and 0.12 mol / L ascorbic acid solution in a mass ratio of 2:3, adjust the pH to 2.5 using nitric acid, add 0.03 mol / L tannic acid solution with a mass being one-fourth of the ascorbic acid solution, transfer it into a reaction kettle, react at 35 °C for 20 min. After the reaction is completed, separate the solid silver particles by centrifugal filtration, transfer them into a vacuum dryer, and conduct vacuum drying at 30 °C to obtain micro-scale silver nuclei with a particle size of 0.6 - 2 μm; S2. Immerse the silver nuclei obtained in step S1 into a mixed solution of 0.3 mol / L silver nitrate and 0.01 mol / L ascorbic acid mixed in a mass ratio of 1:1, react in an environment of 30 °C for 10 min, rinse with water and then dry for standby; S3. Mix 0.03 mol / L silver nitrate solution and 0.03 mol / L ascorbic acid in a 1:1 ratio, add polyvinylpyrrolidone accounting for 0.3% of the total solution mass, adjust the pH to 6.5 using nitric acid, transfer it into a reaction kettle, react at 40 °C for 20 min and continuously stir during the reaction. After the reaction is completed, centrifuge and disperse at a speed of 10000 rmp for 15 min using a high-speed centrifuge. After dispersion and washing, dry it in vacuum at 60 °C for 4 h to obtain silver nanoparticles with a particle size of 50 - 100 nm; S4. Fill the activated micron-sized silver nuclei into an anodic aluminum oxide template with a pore size of 2.5 μm. Load the 0.3 mg / mL graphene dispersion into an electrostatic spraying machine, control the voltage at 35 kV, the pressure at 0.7 MPa, spray 5 times, 50 mL each time, and perform vacuum assistance for 10 min to form a graphene network coverage on the surface of the micron-sized silver nuclei; S5: Mix 0.1 mol / L silver nitrate and 0.3 mol / L sodium citrate in a mass ratio of 1:1 to prepare an electrolyte solution (pH = 5). Use porous carbon paper to load the micron-sized silver nuclei and immerse it in the electrolyte as the working electrode. Use a platinum sheet as the counter electrode and insert it into the electrolyte at the same time. Pass an electric current with a current density of 10 mA / cm² and deposit for 15 min to cover a nano-sized silver shell on the surface of the silver nuclei covered with graphene. After rinsing with water and drying, obtain the target silver powder.

[0041] Example 4:

[0042] A preparation method of micro-nano silver powder with controllable particle size, and the specific implementation steps are as follows: S1. Mix 0.18 mol / L silver nitrate solution and 0.12 mol / L ascorbic acid solution in a mass ratio of 2:3, adjust the pH to 2.5 using nitric acid, add 0.03 mol / L tannic acid solution with a mass being one-fourth of the ascorbic acid solution, transfer it into a reaction kettle, react at 35 °C for 20 min. After the reaction is completed, separate the solid silver particles by centrifugal filtration, and transfer them into a vacuum dryer for vacuum drying at 30 °C to obtain micron-sized silver nuclei with a particle size of 0.6 - 2 μm; S2. Immerse the silver nuclei obtained in step S1 into a mixed solution of 0.3 mol / L silver nitrate and 0.01 mol / L ascorbic acid mixed in a mass ratio of 1:1, react in an environment of 30 °C for 10 min, rinse with water and dry for standby; S3. Mix 0.03 mol / L silver nitrate solution and 0.03 mol / L ascorbic acid in a 1:1 ratio, add polyvinylpyrrolidone accounting for 0.3% of the total solution mass, adjust the pH to 6.5 with nitric acid, transfer it into a reaction kettle, react at 40 °C for 20 min and continuously stir during the reaction. After the reaction is completed, centrifuge and disperse at a speed of 10000 rmp for 15 min with a high-speed centrifuge. After dispersion and washing, vacuum dry at 60 °C for 4 h to obtain silver nanoparticles with a particle size of 50 - 100 nm; S4. Fill the activated micron-sized silver nuclei into an anodic aluminum oxide template with a pore size of 2.5 μm. Load the graphene dispersion liquid with a concentration of 0.3 mg / mL into an electrostatic spraying machine, control the voltage at 35 kV, the pressure at 0.7 MPa, spray 5 times, 50 mL each time, and assist with vacuum for 10 min to form a graphene network coverage on the surface of the micron-sized silver nuclei; S5: Mix 0.1 mol / L silver nitrate and 0.3 mol / L sodium citrate in a mass ratio of 1:1 to prepare an electrolyte solution (pH = 5). Use porous carbon paper to load the micron-sized silver nuclei and immerse it in the electrolyte solution as the working electrode. Use a platinum sheet as the counter electrode and insert it into the electrolyte solution at the same time. Pass an electric current with a current density of 5 mA / cm² and deposit for 5 min to cover a nano-sized silver shell on the surface of the silver nuclei covered with graphene. After rinsing with water and drying, obtain the target silver powder.

[0043] Example 5:

[0044] A preparation method of micro-nano silver powder with controllable particle size, and the specific implementation steps are as follows: S1. Mix 0.18 mol / L silver nitrate solution and 0.12 mol / L ascorbic acid solution in a mass ratio of 2:3, adjust the pH to 2.5 with nitric acid, add 0.03 mol / L tannic acid solution with a mass being one-fourth of the ascorbic acid solution, transfer it into a reaction kettle, react at 35 °C for 20 min, separate by centrifuging at a speed of 8000 rpm for 10 min with a high-speed centrifuge, and then transfer it into a vacuum dryer for vacuum drying at 30 °C to obtain micron-sized silver nuclei with a particle size of 0.6 - 2 μm; S2. Immerse the silver nuclei obtained in step S1 into a mixed solution prepared by mixing 0.3 mol / L silver nitrate and 0.01 mol / L ascorbic acid in a mass ratio of 1:1, react in an environment at 30 °C for 10 min, rinse with water and dry for standby; S3. Mix 0.03 mol / L silver nitrate solution and 0.03 mol / L ascorbic acid in a 1:1 ratio, add polyvinylpyrrolidone accounting for 0.3% of the total solution mass, adjust the pH to 6.5 using nitric acid, transfer it into a reaction kettle, react at 40 °C for 20 min and continuously stir during the reaction. After the reaction is completed, centrifuge and disperse at a speed of 10000 rmp for 15 min using a high-speed centrifuge, wash after dispersion, and then vacuum dry at 60 °C for 4 h to obtain silver nanoparticles with a particle size of 50 - 100 nm; S4. Fill the activated micron-sized silver nuclei into an anodic aluminum oxide template with a pore size of 2.5 μm. Load the 0.3 mg / mL graphene dispersion into an electrostatic spraying machine, control the voltage at 35 kV, the pressure at 0.7 MPa, spray 5 times, 50 mL each time, and assist with vacuum for 10 min to form a graphene network coverage on the surface of the micron-sized silver nuclei; S5: Mix 0.1 mol / L silver nitrate and 0.3 mol / L sodium citrate in a mass ratio of 1:1 to prepare an electrolyte solution (pH = 5). Use porous carbon paper to load the micron-sized silver nuclei and immerse it in the electrolyte solution as the working electrode, and use a platinum sheet as the counter electrode and insert it into the electrolyte solution at the same time. Pass an electric current with a current density of 15 mA / cm² and deposit for 30 min to cover a nano-sized silver shell on the surface of the silver nuclei covered with graphene. After rinsing with water and drying, the target silver powder is obtained.

[0045] Comparative Example 1: Compared with Example 1, the difference is that: the activation step in S2 is omitted, and the silver nuclei prepared in S1 are directly used for subsequent graphene network coating and silver shell coating.

[0046] Comparative Example 2: Compared with Example 1, the difference is that: the graphene spraying step in S4 is omitted, and only a silver shell is deposited on the surface of the activated silver nuclei.

[0047] Comparative Example 3: S1. Mix 0.18 mol / L silver nitrate solution and 0.12 mol / L ascorbic acid solution in a mass ratio of 2:3, adjust the pH to 2.5 using nitric acid, add 0.03 mol / L tannic acid solution with a mass one-fourth of the ascorbic acid solution, transfer it into a reaction kettle, react at 35 °C for 20 min, separate by centrifuging at a speed of 8000 rpm for 10 min using a high-speed centrifuge, and then transfer it into a vacuum dryer for vacuum drying at 30 °C to obtain micron-sized silver nuclei with a particle size of 0.6 - 2 μm; S2. Immerse the silver nuclei obtained in step S1 into a mixed solution of 0.3 mol / L silver nitrate and 0.01 mol / L ascorbic acid mixed in a mass ratio of 1:1, react in a 30 °C environment for 10 min, rinse with water and dry for standby; S3. Mix 0.03 mol / L silver nitrate solution and 0.03 mol / L ascorbic acid in a 1:1 ratio, add polyvinylpyrrolidone accounting for 0.3% of the total solution mass, adjust the pH to 6.5 using nitric acid, feed it into a reaction kettle, react at 40 °C for 20 min and continuously stir during the reaction. After the reaction is completed, separate the solid silver particles by centrifugal filtration, wash them, and then vacuum dry at 60 °C for 4 h to obtain silver nanoparticles with a particle size of 50 - 100 nm; S4. Load the 0.3 mg / mL graphene dispersion into an electrostatic spraying machine, control the voltage at 35 kV, the pressure at 0.5 MPa, spray 3 times, 50 mL each time, and use vacuum assistance for 10 min. Directly coat the graphene dispersion on the surface of the micron-sized silver nuclei collected and piled up to form a graphene network coverage; S5: Mix 0.1 mol / L silver nitrate and 0.3 mol / L sodium citrate in a mass ratio of 1:1 to prepare an electrolyte solution (pH = 5). Use porous carbon paper to load the micron-sized silver nuclei and immerse it in the electrolyte as the working electrode. Use a platinum sheet as the counter electrode and insert it into the electrolyte at the same time. Pass an electric current with a current density of 10 mA / cm² and deposit for 15 min to coat a nano-sized silver shell on the surface of the silver nuclei covered with graphene. After rinsing with water and drying, obtain the target silver powder.

[0048] Comparative Example 4: Compared with Example 1, the difference is that directly mix the micron-sized silver powder with a particle size of 0.6 - 2 μm prepared in step S1 and the silver nanoparticles with a particle size of 50 - 100 nm prepared in step S3 in a mass ratio of 7:3 to obtain a mixed silver powder agglomerate.

[0049] Comparative Example 5: Compared with Example 1, the difference is that in step S4, control the voltage at 35 kV, the pressure at 0.7 MPa, spray 8 times, 50 mL each time, and use vacuum assistance for 15 min to form a graphene network coverage on the surface of the micron-sized silver nuclei; the remaining steps are the same.

[0050] Comparative Example 6: Compared with Example 1, the difference is that in step S5: mix 0.1 mol / L silver nitrate and 0.3 mol / L sodium citrate in a mass ratio of 1:1 to prepare an electrolyte solution (pH = 5), pass an electric current with a current density of 15 mA / cm² and deposit for 45 min to coat a nano-sized silver shell on the surface of the silver nuclei covered with graphene to obtain the target silver powder.

[0051] Performance test: Silver powder performance test: The silver powders in Examples 1-5, Comparative Example 5 and Comparative Example 6 were tested for resistivity according to ASTM B193-20 "Test Method for Resistivity of Conductive Materials"; the thermal conductivity of the silver powders was tested according to ASTM E1461-13 "Determination of Thermal Diffusivity by Laser Flash Method"; the coverage of the graphene network and silver shell was measured using the scanning electron microscope (SEM) image statistics method; the tapped density of the silver powder was measured according to GB / T 5162-2021 "Determination of Tapped Density of Metal Powders".

[0052] Performance test of sintered samples of SiC plates and Cu plates: Prepare several groups of SiC plates and Cu plates. Under nitrogen protection, the silver powders in Examples 1-5 and Comparative Examples 1-4 were placed between the SiC plates and Cu plates and sintered at 250 °C for 40 min. After sintering, it was cooled to room temperature to obtain sintered samples; According to ASTM D1002-10 "Test Method for Tensile Shear Strength of Bonded Joints", uniaxial tensile tests were carried out on the sintered samples, that is, tensile simulations were carried out on the SiC / Ag interface and the Cu / Ag interface respectively, and the tensile strength, fracture energy and fracture position were recorded; the width and morphology of the sintering necks formed between the silver layer and the upper and lower plates were observed using SEM, and the width difference between the two sintering necks was measured; 1000 thermal cycles were carried out in the temperature range of -50 °C to 150 °C, the tensile strength and fracture energy of the samples after thermal cycling were tested, the crack propagation of the samples was observed, and the tensile strength of the samples after thermal cycling was compared and recorded with the data before thermal cycling.

[0053] The silver powder performance tests were carried out on the silver powders in Examples 1-5, Comparative Example 5 and Comparative Example 6, and the sintered sample performance tests were carried out on the silver powders in Examples 1-5 and Comparative Examples 1-6. The test data tables are as follows: Table 1 Silver powder performance test data table

[0054] Table 2 Sintered sample performance test data table

[0055] According to the data analysis in Table 1: Examples 1-5 are silver powders with a silver core-graphene network-silver shell structure prepared by this invention application. They have the characteristics of low resistivity, high thermal conductivity and large tapped density. The reasons for the analysis are as follows: Graphene is a two-dimensional highly conductive material. The continuous conductive network formed by the high conductivity of graphene and the dense filling of the gaps between graphene by the nano silver shell significantly reduce the interface resistance; in addition, the activation treatment of the silver core enhances the surface activity, reduces interface defects, and further improves the conductive efficiency in combination with the nano silver shell; The high thermal conductivity benefits from the synergistic thermal conduction mechanism of graphene and the silver shell, where the graphene network provides a fast heat transfer channel, and the silver shell enhances heat conduction through both electron and phonon dual paths. In addition, the adsorption effect of the anodic aluminum oxide template ensures the ordered arrangement of silver nuclei. When spraying the graphene dispersion liquid onto the silver powder confined on the anodic aluminum oxide template, graphene is effectively filled into the uniformly arranged silver nuclei, filling the microscopic gaps, thereby greatly improving the bonding uniformity and bonding effect, reducing the porosity between the bonding interfaces, and resulting in a significant increase in thermal conductivity. The high tap density is achieved by the densification design of the composite structure. Graphene fills the gaps between silver nuclei as an intermediate layer to form a rigid skeleton. The nano-silver shell is uniformly coated on the surface of the micro-silver nuclei through electrochemical deposition, and the template confinement effect further reduces the voids between particles, significantly improving the tap density of the silver powder prepared in the embodiments of the present application.

[0056] Comparing Examples 1-3 with Comparative Example 5, it can be seen that the resistivity of the silver powder decreases with the increase in the graphene content, and the thermal conductivity increases with the increase in the graphene content. The coverage rate of the graphene network has a relatively small impact on the tap density. Therefore, it can be concluded that the increase in the coverage rate of the graphene network on the surface of the silver nuclei will improve the electrical conductivity and thermal conductivity of the silver powder to a certain extent. According to the data of Comparative Example 5, it can be seen that there is a critical value for the improvement of electrical conductivity and thermal conductivity in the later stage with the increase in the graphene content. And with the increase in the coverage rate of the graphene network, it will significantly affect the homogeneous bonding between the silver shell and the silver nuclei, manifested as an abnormal decrease in the tap density of the silver powder. Comparing Example 1, Example 4, Example 5 with Comparative Example 6, it can be concluded that with the increase in the coverage rate of the nano-silver shell in the silver shell, it will significantly improve the tap density, electrical conductivity, and thermal conductivity of the silver powder. However, there is a critical value for the tap density. Therefore, comparing Example 5 with Comparative Example 6, although the coverage rate of the silver shell has increased, the tap density of the silver powder has not increased. Therefore, when the coverage rate of the silver shell is about 80%, there is a critical value for the tap density.

[0057] According to the data analysis in Table 2: When sintering SiC plates and Cu plates using Examples 1-5, the connection performance is significantly higher than that of the comparative examples, which mainly benefits from the effect of the silver nucleus-graphene network-silver shell gradient composite structure in the embodiments of the present application. Specifically: The fracture energy is inversely proportional to the difference in sintering neck width (the larger the width difference, the lower the fracture energy). Therefore, materials with a larger difference in sintering neck width are more likely to fracture. When the width difference is large, the fracture energy is concentrated locally, leading to earlier failure of the material. In this embodiment, through the gradient structure design of the graphene network and the nano-silver shell, the difference in sintering neck width between Cu / Ag and SiC / Ag is effectively reduced. The smaller width difference increases the fracture energy, thereby improving the tensile strength. The reasons are as follows: During the sintering process, due to the high surface energy and low atomic migration activation energy of the nano silver shell, it preferentially wets the surfaces of the SiC and Cu substrates at the initial stage of sintering. As a support framework, the graphene network guides the uniform distribution of the nano silver shell along the interface through its two-dimensional continuous structure, avoiding local over-thickness or under-thickness. This gradient wetting effect reduces the difference in the contact area of silver particles between the SiC side and the Cu side, thereby synchronously promoting the initial growth of the sintering necks on both sides and reducing the width difference of the sintering necks on both sides during sintering. In addition, while the nano silver shell rapidly forms sintering necks through the surface diffusion-dominated mechanism, the micro silver nuclei provide a stable atomic source during the subsequent bulk diffusion stage. With the confinement effect of the graphene network, the silver nuclei are forced to release atoms directionally to the interface region, making the atomic migration rates on the SiC side and the Cu side tend to be consistent; a smaller sintering neck difference is formed between the silver powder and the upper and lower plates. Second, the dense filling and uniform coating of the silver shell enhance the bonding strength between the silver nuclei and graphene, and between the silver nuclei and the substrate, making the sintered layer less likely to break from the interface or internally when stressed. The mechanical flexibility of graphene can absorb the stress generated by the expansion difference between the two substrates on both sides, avoiding stress concentration and causing excessive coarsening of the unilateral sintering neck, and further eliminating the thermal hysteresis effect between SiC and Cu during the sintering process.

[0058] Comparing Example 1 with Comparative Example 1, the strength retention rate of Comparative Example 1 decreases after thermal cycling and the tensile strength is low. The reasons are as follows: In Example 1, the activation step treats the surface of the silver nuclei with a mixed solution of silver nitrate and ascorbic acid to generate active sites. The activation mechanism is that ascorbic acid, as a reducing agent, will preferentially reduce to , and the newly generated attaches to the surface of the silver nuclei in the form of islands or nanoparticles, forming a microscopically rough structure, increasing the specific surface area and active sites, thereby enhancing the subsequent bonding strength with graphene and the silver shell and reducing the hydroxyl content; while in Comparative Example 1, weak physical bonding is formed when the hydroxyl groups on the surface of the unactivated silver nuclei adsorb graphene, the interfacial bonding force decreases, the distribution of silver particles during sintering is uneven, and the porosity increases, affecting the sintering strength.

[0059] Comparing Example 1 with Comparative Example 2, due to the absence of the graphene network, the network structure of graphene-wrapped silver nuclei is not formed in Comparative Example 2. The silver shell is directly deposited on the surface of the bare silver nuclei, and the stress transfer function of graphene is lacking at the connection interface. Therefore, during sintering, the diffusion channel guided by graphene is not formed, the surface diffusion coefficient of silver atoms decreases, manifested as a decrease in thermal conductivity, an increase in the width difference of the sintering necks, and direct fracture occurs inside the Ag layer, indicating that there are quality defects in the silver powder as a connecting material itself.

[0060] Comparing Example 1 with Comparative Example 3, Comparative Example 3 lacks the restriction of the anodic aluminum oxide template. The random packing of silver nuclei after removing the anodic aluminum oxide template results in uneven binding between the silver nuclei and graphene. During sintering, there are significant differences in the contact areas between graphene or silver nuclei and the SiC plate and the Cu plate. The growth of sintering necks is uneven during sintering. The fracture in Comparative Example 3 occurs at the interface between the Ag layer and the Cu / Ag interface, proving that the uneven structure of the silver powder leads to uneven migration of silver powder particles during sintering, resulting in a large difference in the width of the sintering neck and deteriorating the bonding effect.

[0061] Comparing Example 1 with Comparative Example 4, Comparative Example 4 directly mixes micron-sized silver powder with a particle size of 0.6 - 2 μm and nano-sized silver powder with a particle size of 50 - 100 nm in a mass ratio of 7:3. Therefore, during sintering, both the nano-sized silver powder and the micron-sized silver powder will migrate to the surface of the circuit board, and the effect that the nano-sized silver powder preferentially wets the surface of the SiC / Cu plate will not be formed. At the same time, the stress buffering effect of graphene is lacking, which easily leads to agglomeration of nano / micron silver powder, forming stress concentration and prone to brittle fracture. The fracture occurs inside the Ag layer, proving that there are quality problems with the conductive paste itself.

[0062] Comparing Example 1 with Comparative Example 5, in Comparative Example 5, the graphene coverage rate is too large, which hinders the homogeneous binding of silver nuclei to the silver shell. At the same time, the overly dense graphene network inhibits the diffusion of silver atoms, resulting in an increase in the width difference of the sintering neck. At the same time, some cracks occur inside the Ag layer, proving that there is a probability of quality problems when sintering with the connecting material prepared in Comparative Example 5.

[0063] Comparing Example 5 with Comparative Example 6, the silver shell coverage rate of Comparative Example 6 is about 90%, but the width difference of the sintering neck is greater than that of Example 5. The reason is that when the overly thick silver shell wets the surface of the plate, a wetting layer with too high viscosity is formed, which affects the dispersibility of silver nuclei, affects the filling of local silver nuclei and graphene, resulting in a large width difference between the two sides of the sintering neck and a decrease in the tensile strength.

[0064] In summary, through multi-level structure design and precise process control, the present invention constructs a gradient composite system of "micron silver nuclei - graphene network - nano silver shell". Through the synergistic effect of key technologies such as activation treatment, template confinement, and electrochemical deposition, the problem of poor connection effect caused by a large width difference of the sintering neck formed between the silver powder as a connecting material and the connecting plates on both sides in the traditional sintering process is effectively solved. Through the synergistic filling effect of graphene and the nano silver shell, the present invention realizes the uniform transfer of stress during sintering by using the stress buffering effect of graphene; and improves the interfacial bonding performance by using the wettability of the nano silver shell.

[0065] Based on the inspiration of the ideal embodiments of the present invention, through the above description, relevant personnel can completely make various changes and modifications without departing from the technical idea of this invention. The technical scope of this invention is not limited to the content in the specification, and the technical scope must be determined according to the scope of the claims.

Claims

1. A method for preparing micro-nano silver powder with controllable particle size, characterized in that, It includes the following steps: S1. Prepare micron-sized silver nuclei and nano-sized silver particles; S2. Immerse the micron-sized silver nuclei in a mixed solution of 0.1 - 0.5 mol / L silver nitrate and 0.01 mol / L ascorbic acid, react at 30 - 50 °C for 5 min to form an activated surface, rinse and dry for standby; S3. Fill the pore gaps of the porous template with the said micron-sized silver nuclei; S4. Spray a graphene dispersion liquid on the said porous template, the concentration of the graphene dispersion liquid is 0.1–0.5 mg / mL, the spraying pressure is 0.2–0.8 MPa, the spraying times are 2 - 5 times, and 50 - 100 mL is sprayed each time to form a graphene network coverage on the surface of each silver nucleus; S5. Mix a silver nitrate solution with a concentration of 0.05–0.2 mol / L and sodium citrate with a concentration of 0.1–0.5 mol / L, adjust the pH to 4 - 6 to prepare an electrolyte solution. After removing the porous template, use porous carbon paper to load the micron-sized silver nuclei and immerse them in the electrolyte solution as the working electrode, use a platinum sheet as the counter electrode and insert it into the electrolyte solution, then apply electricity to both electrodes, control the current density to be 5 - 15 mA / cm², the deposition time is 5 - 30 min, keep the temperature of the electrolyte solution at 25 - 40 °C during the deposition process, and stir magnetically, deposit nano-sized silver particles on the exposed silver nucleus surface to form a silver shell structure that wraps the silver nucleus and the graphene network.

2. The method for preparing micro-nano silver powder with controllable particle size according to claim 1, wherein The preparation of the micron-sized silver nuclei in step S1 includes: mix a silver nitrate solution with a concentration of 0.15–0.20 mol / L and an ascorbic acid solution with a concentration of 0.10–0.15 mol / L, adjust the pH to 1 - 3 with nitric acid, add a tannic acid solution with a concentration of 0.01–0.05 mol / L, react at 25 - 40 °C for 15 - 30 min, separate and dry to obtain micron-sized silver nuclei with a particle size between 0.5 - 2 μm.

3. A method for preparing micro-nano silver powder with controllable particle size according to claim 1, characterized in that, The preparation of the nano-sized silver particles in step S1 includes: mix a silver nitrate solution with a concentration of 0.01–0.05 mol / L and an ascorbic acid solution with a concentration of 0.01–0.05 mol / L, add polyvinylpyrrolidone with a mass concentration of 0.1–0.5% as a dispersant, adjust the pH to 5 - 8, stir and react at 30 - 50 °C for 10 - 30 minutes; after centrifugal separation, wash alternately with water and ethanol for 3 - 5 times, and vacuum dry at 50 - 80 °C for 4 - 8 hours to obtain nano-sized silver particles with a particle size of 50 - 100 nm.

4. A method for preparing micro-nano silver powder with controllable particle size according to claim 1, characterized in that: The said porous template is an anodic aluminum oxide template with a pore diameter of 0.8 - 2.5 μm.

5. A method for preparing micro-nano silver powder with controllable particle size according to claim 1, characterized in that: The coverage of the said silver shell is 40 - 80% of the surface of the micron-sized silver nucleus.

6. A method for preparing micro-nano silver powder with controllable particle size according to claim 1, characterized in that: The graphene network covers 40 - 80% of the surface of the silver nucleus.

7. A micro-nano silver powder with controllable particle size, characterized in that, It is prepared by the method for preparing micro-nano silver powder with controllable particle size according to any one of claims 1 - 6.

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