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

Abstract

The present invention relates to the technical field of micro-nano particle preparation, and discloses a micro-nano silver powder with controllable particle size and a preparation method thereof, comprising: S1, preparing micron-sized silver cores and nano-sized silver particles; S2, surface activating the micron-sized silver cores; S3, filling the micron-sized silver cores into the pore gaps of a porous template; S4, spraying a graphene dispersion onto the porous template to form a graphene network covering the surface of each silver core; S5, after removing the porous template, depositing nano-sized silver particles on the exposed silver core surfaces to form a silver shell structure that encapsulates the silver cores and the graphene network. The present invention utilizes a gradient composite of a micron-sized silver core-graphene network-nano silver shell structure, effectively utilizing the wettability of the nano-silver shell and the stress dispersibility of graphene to reduce the width difference between two sintering necks formed at the interface with the connecting plates on both sides during sintering of the circuit board, while utilizing the excellent electrical and thermal conductivity of the material itself to optimize the sintering performance of the circuit board.

Description

Technical Field

[0001] The present invention relates to the technical field of micro-nano particle preparation, and in particular 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 connecting circuit boards. Silver sintering technology uses silver powder or silver paste as an intermediate layer material, achieving a tight bond 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, which has excellent electrical and thermal conductivity. Silver has a high melting point of 961°C and does not produce the typical fatigue effects seen in solder connection layers with melting points below 300°C. It has high reliability, and the sintered material is lead-free and environmentally friendly.

[0003] When silver powder is sintered, sintering necks form between the silver powder particles and between the silver powder and the circuit board, forming critical joints. The formation of sintering necks relies on atomic diffusion and typically occurs in a temperature range of 0.25-0.75 times the material's melting point. However, conventional silver powder sintering presents significant challenges. During the sintering process, the size and distribution of sintering necks between silver powder particles are affected by multiple factors, including sintering temperature, time, and the properties of the silver powder. While increasing the sintering temperature increases the size of the sintering necks, excessively high temperatures can lead to over-sintering of the silver powder particles, resulting in grain growth and increased porosity, which in turn affects the performance of the sintered layer and causes uneven stress distribution within the material, which can easily lead to crack initiation and propagation, reducing the material's tensile strength and toughness. After conventional silver powder sintering, the sintering necks vary greatly in width, causing stress concentration at the joints, making them prone to fracture and resulting in poor bonding. This makes it difficult to meet the heat dissipation and signal transmission requirements of high-power chips. Therefore, a new solution has been proposed to address these issues. 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 objectives, the technical solution adopted by the present invention is: a method for preparing micro-nano silver powder with controllable particle size, comprising the following steps: S1, preparing micron-sized silver cores and nano-sized silver particles; S2, surface activation of the micron-sized silver cores; S3, filling the micron-sized silver cores into the pore gaps of a porous template; S4, spraying a graphene dispersion onto the porous template to form a graphene network covering 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 encapsulates 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 minutes, separating and drying to obtain micron-sized silver cores with a particle size between 0.5-2 μm.

[0007] In a preferred embodiment of the present invention, the preparation of nano-scale 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 polyvinyl pyrrolidone with a mass concentration of 0.1-0.5% as a dispersant, adjusting the pH to 5-8, and stirring the mixture at 30-50°C for 10-30 minutes; after separation by centrifugal filtration, washing the mixture alternately with clean water and ethanol for 3-5 times, and vacuum drying the mixture at 50-80°C for 4-8 hours to obtain nano-scale silver particles with a particle size of 50-100 nm.

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

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

[0010] In a preferred embodiment of the present invention, the deposition of nano-sized silver particles in step S5 is achieved by an electrochemical method, specifically comprising the following steps: mixing a silver nitrate solution having a concentration of 0.05-0.2 mol / L with a sodium citrate solution having a concentration of 0.1-0.5 mol / L, adjusting the pH to 4-6, and preparing an electrolyte; using porous carbon paper to load micron-sized silver cores and immersing the porous carbon paper in the electrolyte as a working electrode, and a platinum sheet as a counter electrode. The platinum sheet is simultaneously inserted into the electrolyte and then energized to both electrodes, controlling the current density to 5-15 mA / cm², and the deposition time to 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 is 0.1-0.5 mg / mL, the spraying pressure is 0.2-0.8 MPa, the number of spraying is 2-5 times, and 50-100 mL is sprayed each time.

[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:

[0015] (1) The present invention significantly improves the tensile strength of silver powder sintering by designing a gradient composite structure of micron silver core-graphene network-nanosilver shell and optimizing the activation treatment, template confinement and electrochemical deposition processes. During sintering, the graphene network and the nanosilver shell are synergistically filled to achieve uniform stress transmission; at the same time, the nanosilver shell will first wet the upper and lower circuit board surfaces, thereby better allowing the silver material to bond with the interface. The wettability of the small-particle nanosilver shell is used to enhance the interface bonding. At the same time, graphene is used as a stress buffer layer to improve the structural integrity of the material in a thermal coupling environment. This effectively solves the problem of poor tensile strength caused by the large difference in neck width in traditional processes. The material properties of graphene are also used to improve thermal conductivity and material strength.

[0016] (2) The present invention activates the surface of the silver core, thereby enhancing the bonding strength of the silver core, graphene, and silver shell. The guiding effect of the anodized aluminum template ensures that the graphene and silver core are bonded in an orderly manner, improves the uniformity of the bonding, and further enhances the bonding effect and tensile strength of the silver powder. The anodized aluminum template and the graphene network synergistically optimize the silver powder structure, reduce porosity, and improve thermal conductivity. This physical confinement effect enables the silver powder to be more evenly distributed and bonded, enhances thermal stability and mechanical properties, and results in a smaller difference in the neck width formed by the silver powder of the present invention during sintering of the circuit board, a higher tensile strength, and a better bonding effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments described in the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive efforts.

[0018] Figure 1 is a flowchart of the steps of a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0019] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0020] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below.

[0021] Exemplary sources of material:

[0022] All materials mentioned in this application do not require special procurement sources, and commercially available products familiar to those skilled in the art can be used;

[0023] For example, silver nitrate solution (product number BW05123), ascorbic acid (product number MS0625-500G), polyvinyl pyrrolidone (product number MS4327A-500G), and nitric acid (product number SJRY27) can all be purchased from Beijing Kezhan Biotechnology Co., Ltd.

[0024] It should be noted that the above-mentioned products are only exemplary procurement sources and do not represent that the product concentration is the solution concentration in the examples of this application; those skilled in the art can choose commercially available standard solutions or prepare them themselves according to actual needs. The concentrations marked in the examples of this application are achieved by conventional concentration adjustment means familiar to those skilled in the art.

[0025] Exemplary methods:

[0026] like Figure 1 As shown, a method for preparing micro-nano silver powder with controllable particle size comprises the following steps:

[0027] S1. preparing micron-sized silver cores and nano-sized silver particles;

[0028] S2, surface activation of micron-sized silver cores;

[0029] S3, filling the micron-sized silver cores into the pore gaps of the porous template;

[0030] S4, spraying a graphene dispersion onto the porous template to form a graphene network covering the surface of each silver core;

[0031] S5. After removing the porous template, nanoscale silver particles are deposited on the exposed surface of the silver core to form a silver shell structure that wraps the silver core and the graphene network.

[0032] Below, each step will be described in detail.

[0033] Specifically, the specific preparation steps of the micron-sized silver core 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 in 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 minutes, separating and drying to obtain micron-sized silver cores with a particle size between 0.5-2 μm.

[0034] Specifically, the preparation of nano-scale 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 polyvinyl pyrrolidone with a mass concentration of 0.1-0.5% as a dispersant, adjusting the pH to 5-7, and stirring the mixture at 30-50°C for 10-30 minutes; after separating the nano-scale silver particles by centrifugal filtration, washing them alternately with clean water and ethanol for 3-5 times, and vacuum drying them at 50-80°C for 4-8 hours to obtain nano-scale silver particles with a particle size of 50-100 nm.

[0035] Specifically, the specific steps of surface activation of the micron-sized silver core in step S2 include: immersing the micron-sized silver core in a mixture of 0.1-0.5 mol / L silver nitrate and 0.01 mol / L ascorbic acid, reacting at 30-50° C. for 5 minutes to form an activated surface, rinsing and drying, and setting aside.

[0036] Specifically, the porous template is an anodic aluminum oxide template with a pore size of 0.8-2.5 μm, and the adsorption effect of nano-micropores is used to limit the arrangement of silver cores.

[0037] Specifically, in step S3, the specific steps of filling the activated micron-sized silver cores into the anodized aluminum oxide template with a pore size of 2.5 μm are as follows:

[0038] S31. Fix the anodized aluminum template horizontally on a vibration platform, evenly spread the silver core powder on the template surface, 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 process, the silver cores enter the template channel gaps under the action of inertial force;

[0039] S32, the filling amount is controlled by weighing method, and the mass of silver core loaded per square centimeter of template is 1.0-2.0 mg;

[0040] S33. After filling, tilt the template and gently sweep the surface with a soft brush to remove the residual silver core that has not entered the hole.

[0041] Specifically, the method for removing the porous template includes:

[0042] S51. Place the template in ethanol or deionized water and perform ultrasonic treatment for 10-20 minutes. Control the ultrasonic power to 100-200 W and the frequency to 40 kHz. Use the cavitation effect to separate the silver core from the pores.

[0043] S52. Filter using a filter membrane with a pore size of 0.1-0.2 μm to collect the silver nuclei.

[0044] Specifically, the deposition of nano-sized silver particles in step S5 is achieved by an electrochemical method, and the specific steps include:

[0045] A 0.05–0.2 mol / L silver nitrate solution and a 0.1–0.5 mol / L sodium citrate solution are mixed in proportion and the pH is adjusted to 4–6 using nitric acid to prepare an electrolyte. A micron-sized silver core is loaded onto porous carbon paper and immersed in the electrolyte as a working electrode, with a platinum sheet serving as the counter electrode. The current density is controlled at 5–15 mA / cm², and the deposition time is 5–30 minutes. During the deposition process, the electrolyte temperature is maintained at 25–40°C, accompanied by magnetic stirring to ensure uniform precipitation. The thickness of the silver shell on the surface of the silver core can be adjusted by increasing or decreasing the precipitation time, thereby achieving controllable particle size.

[0046] Specifically, the method of using porous carbon paper to load micron-sized silver cores includes:

[0047] S53, selecting porous carbon paper with a pore size of 2-5 μm and a porosity of more than 80%, and immersing the paper in a 0.1-0.5% polydopamine solution for 2-4 hours to form an adhesion layer on the surface;

[0048] S54, dispersing the silver cores dispersed in step S52 in a 1-5 mg / mL ethanol solution, and stirring ultrasonically for 10-20 minutes;

[0049] S55. Immerse the porous carbon paper in the dispersion and let it stand for 0.5-1 hour, during which ultrasonic dispersion is continuously performed, and the silver cores are adsorbed on the surface of the porous carbon paper through van der Waals forces.

[0050] Specifically, the coverage of the silver shell is 40-80% of the surface of the micron-sized silver core.

[0051] Specifically, the concentration of the graphene dispersion is 0.1-0.5 mg / mL, the spraying pressure is 0.2-0.8 MPa, the number of spraying is 2-5 times, and 50-100 mL is sprayed each time.

[0052] Specifically, the graphene network covers 40-80% of the surface of the silver core.

[0053] Example Materials:

[0054] A micro-nano silver powder is prepared by an exemplary method.

[0055] Example 1:

[0056] A method for preparing micro-nano silver powder with controllable particle size, the specific implementation steps are as follows:

[0057] S1, 0.18 mol / L silver nitrate solution and 0.12 mol / L ascorbic acid solution were mixed in a mass ratio of 2:3, the pH was adjusted to 2.5 with nitric acid, and a 0.03 mol / L tannic acid solution of one quarter the mass of the ascorbic acid solution was added, and the mixture was placed in a reactor and reacted at 35 ° C for 20 min. After separation by high-speed centrifugation at a speed of 8000 rpm for 10 min, the mixture was placed in a vacuum dryer and vacuum dried at 30 ° C to obtain micron-sized silver nuclei with a particle size of 0.6-2 μm;

[0058] S2, immersing the silver core obtained in step S1 in a mixture of 0.3 mol / L silver nitrate and 0.01 mol / L ascorbic acid in a mass ratio of 1:1, reacting at 30°C for 10 minutes, rinsing with clean water, and drying for later use;

[0059] S3, mixing 0.03 mol / L silver nitrate solution and 0.03 mol / L ascorbic acid in a ratio of 1:1, adding 0.3% polyvinyl pyrrolidone by weight of the total solution, adjusting the pH to 6.5 with nitric acid, placing the mixture into a reactor and reacting at 40° C. for 20 min with continuous stirring during the reaction, separating the solid silver particles by centrifugal filtration after completion of the reaction, washing, and vacuum drying at 60° C. for 4 h to obtain nanosilver particles with a particle size of 50-100 nm;

[0060] S4. Fill an anodized aluminum template with a pore size of 2.5 μm with activated micron-sized silver cores. Load a 0.3 mg / mL graphene dispersion into an electrostatic sprayer. Control the voltage to 35 kV and the pressure to 0.5 MPa. Spray 3 times, 50 mL each time, with vacuum assistance for 10 min to form a graphene network covering the surface of the micron-sized silver cores.

[0061] S5: Prepare an electrolyte (pH 5) by mixing 0.1 mol / L silver nitrate and 0.3 mol / L sodium citrate in a mass ratio of 1:1. Use porous carbon paper to load micron-sized silver cores and immerse them in the electrolyte as the working electrode. Use a platinum sheet as the counter electrode and extend it into the electrolyte at the same time. Pass a current with a current density of 10 mA / cm² and deposit for 15 minutes. Cover the surface of the silver core covered with graphene with a nano-sized silver shell. Rinse with clean water and dry it to obtain the target silver powder.

[0062] Example 2:

[0063] A method for preparing micro-nano silver powder with controllable particle size, the specific implementation steps are as follows:

[0064] S1, 0.18 mol / L silver nitrate solution and 0.12 mol / L ascorbic acid solution were mixed in a mass ratio of 2:3, the pH was adjusted to 2.5 with nitric acid, and a 0.03 mol / L tannic acid solution of one quarter the mass of the ascorbic acid solution was added, and the mixture was placed in a reactor and reacted at 35 ° C for 20 min. After separation by high-speed centrifugation at a speed of 8000 rpm for 10 min, the mixture was placed in a vacuum dryer and vacuum dried at 30 ° C to obtain micron-sized silver nuclei with a particle size of 0.6-2 μm;

[0065] S2, immersing the silver core obtained in step S1 in a mixture of 0.3 mol / L silver nitrate and 0.01 mol / L ascorbic acid in a mass ratio of 1:1, reacting at 30°C for 10 minutes, rinsing with clean water, and drying for later use;

[0066] S3, mixing 0.03 mol / L silver nitrate solution and 0.03 mol / L ascorbic acid in a ratio of 1:1, adding 0.3% polyvinyl pyrrolidone by weight of the total solution, adjusting the pH to 6.5 with nitric acid, placing the mixture into a reactor and reacting at 40° C. for 20 min with continuous stirring during the reaction, separating solid silver particles by centrifugal filtration after the reaction is completed, dispersing and washing, and then vacuum drying at 60° C. for 4 h to obtain nanosilver particles with a particle size of 50-100 nm;

[0067] S4. Fill an anodized aluminum oxide template with a pore size of 2.5 μm with activated micron-sized silver cores. Load a 0.3 mg / mL graphene dispersion into an electrostatic sprayer. Control the voltage to 35 kV and the pressure to 0.3 MPa. Spray twice, 50 mL each time, with vacuum assistance for 10 min to form a graphene network covering the surface of the micron-sized silver cores.

[0068] S5: Prepare an electrolyte (pH 5) by mixing 0.1 mol / L silver nitrate and 0.3 mol / L sodium citrate in a mass ratio of 1:1. Use porous carbon paper to load micron-sized silver cores and immerse them in the electrolyte as the working electrode. Use a platinum sheet as the counter electrode and extend it into the electrolyte at the same time. Pass a current with a current density of 10 mA / cm² and deposit for 15 minutes. Cover the surface of the silver core covered with graphene with a nano-sized silver shell. Rinse with clean water and dry it to obtain the target silver powder.

[0069] Example 3:

[0070] A method for preparing micro-nano silver powder with controllable particle size, the specific implementation steps are as follows:

[0071] S1, 0.18 mol / L silver nitrate solution and 0.12 mol / L ascorbic acid solution were mixed in a mass ratio of 2:3, the pH was adjusted to 2.5 with nitric acid, 0.03 mol / L tannic acid solution was added with a mass of one-fourth of the ascorbic acid solution, and the mixture was placed in a reactor and reacted at 35° C. for 20 min. After the reaction was completed, solid silver particles were separated by centrifugal filtration and placed in a vacuum dryer for vacuum drying at 30° C. to obtain micron-sized silver nuclei with a particle size of 0.6-2 μm;

[0072] S2, immersing the silver core obtained in step S1 in a mixture of 0.3 mol / L silver nitrate and 0.01 mol / L ascorbic acid in a mass ratio of 1:1, reacting at 30°C for 10 minutes, rinsing with clean water, and drying for later use;

[0073] S3, 0.03 mol / L silver nitrate solution and 0.03 mol / L ascorbic acid were mixed in a ratio of 1:1, 0.3% of the total solution weight of polyvinyl pyrrolidone was added, and the pH was adjusted to 6.5 with nitric acid. The mixture was placed in a reactor and reacted at 40° C. for 20 min with continuous stirring during the reaction. After the reaction was completed, the mixture was centrifuged at 10,000 rpm for 15 min in a high-speed centrifuge for centrifugal dispersion. After dispersion and washing, the mixture was vacuum dried at 60° C. for 4 h to obtain nanosilver particles with a particle size of 50-100 nm.

[0074] S4. Fill an anodized aluminum oxide template with a pore size of 2.5 μm with activated micron-sized silver cores. Load a 0.3 mg / mL graphene dispersion into an electrostatic sprayer. Control the voltage to 35 kV and the pressure to 0.7 MPa. Spray 5 times, 50 mL each time, with vacuum assistance for 10 min to form a graphene network covering the surface of the micron-sized silver cores.

[0075] S5: Prepare an electrolyte (pH 5) by mixing 0.1 mol / L silver nitrate and 0.3 mol / L sodium citrate in a mass ratio of 1:1. Use porous carbon paper to load micron-sized silver cores and immerse them in the electrolyte as the working electrode. Use a platinum sheet as the counter electrode and extend it into the electrolyte at the same time. Pass a current with a current density of 10 mA / cm² and deposit for 15 minutes. Cover the surface of the silver core covered with graphene with a nano-sized silver shell. Rinse with clean water and dry it to obtain the target silver powder.

[0076] Example 4:

[0077] A method for preparing micro-nano silver powder with controllable particle size, the specific implementation steps are as follows:

[0078] S1, 0.18 mol / L silver nitrate solution and 0.12 mol / L ascorbic acid solution were mixed in a mass ratio of 2:3, the pH was adjusted to 2.5 with nitric acid, 0.03 mol / L tannic acid solution was added with a mass of one-fourth of the ascorbic acid solution, and the mixture was placed in a reactor and reacted at 35° C. for 20 min. After the reaction was completed, solid silver particles were separated by centrifugal filtration and placed in a vacuum dryer for vacuum drying at 30° C. to obtain micron-sized silver nuclei with a particle size of 0.6-2 μm;

[0079] S2, immersing the silver core obtained in step S1 in a mixture of 0.3 mol / L silver nitrate and 0.01 mol / L ascorbic acid in a mass ratio of 1:1, reacting at 30°C for 10 minutes, rinsing with clean water, and drying for later use;

[0080] S3, 0.03 mol / L silver nitrate solution and 0.03 mol / L ascorbic acid were mixed in a ratio of 1:1, 0.3% of the total solution weight of polyvinyl pyrrolidone was added, and the pH was adjusted to 6.5 with nitric acid. The mixture was placed in a reactor and reacted at 40° C. for 20 min with continuous stirring during the reaction. After the reaction was completed, the mixture was centrifuged at 10,000 rpm for 15 min in a high-speed centrifuge for centrifugal dispersion. After dispersion and washing, the mixture was vacuum dried at 60° C. for 4 h to obtain nanosilver particles with a particle size of 50-100 nm.

[0081] S4. Fill an anodized aluminum oxide template with a pore size of 2.5 μm with activated micron-sized silver cores. Load a 0.3 mg / mL graphene dispersion into an electrostatic sprayer. Control the voltage to 35 kV and the pressure to 0.7 MPa. Spray 5 times, 50 mL each time, with vacuum assistance for 10 min to form a graphene network covering the surface of the micron-sized silver cores.

[0082] S5: Prepare an electrolyte (pH 5) by mixing 0.1 mol / L silver nitrate and 0.3 mol / L sodium citrate in a mass ratio of 1:1. Use porous carbon paper to load micron-sized silver cores and immerse them in the electrolyte as the working electrode. Use a platinum sheet as the counter electrode and extend it into the electrolyte at the same time. Pass a current with a current density of 5 mA / cm² and deposit for 5 minutes. Cover the surface of the silver core covered with graphene with a nano-sized silver shell. Rinse with clean water and dry it to obtain the target silver powder.

[0083] Example 5:

[0084] A method for preparing micro-nano silver powder with controllable particle size, the specific implementation steps are as follows:

[0085] S1, 0.18 mol / L silver nitrate solution and 0.12 mol / L ascorbic acid solution were mixed in a mass ratio of 2:3, the pH was adjusted to 2.5 with nitric acid, and a 0.03 mol / L tannic acid solution of one quarter the mass of the ascorbic acid solution was added, and the mixture was placed in a reactor and reacted at 35 ° C for 20 min. After separation by high-speed centrifugation at a speed of 8000 rpm for 10 min, the mixture was placed in a vacuum dryer and vacuum dried at 30 ° C to obtain micron-sized silver nuclei with a particle size of 0.6-2 μm;

[0086] S2, immersing the silver core obtained in step S1 in a mixture of 0.3 mol / L silver nitrate and 0.01 mol / L ascorbic acid in a mass ratio of 1:1, reacting at 30°C for 10 minutes, rinsing with clean water, and drying for later use;

[0087] S3, 0.03 mol / L silver nitrate solution and 0.03 mol / L ascorbic acid were mixed in a ratio of 1:1, 0.3% of the total solution weight of polyvinyl pyrrolidone was added, and the pH was adjusted to 6.5 with nitric acid. The mixture was placed in a reactor and reacted at 40° C. for 20 min with continuous stirring during the reaction. After the reaction was completed, the mixture was centrifuged at 10,000 rpm for 15 min in a high-speed centrifuge for centrifugal dispersion. After dispersion and washing, the mixture was vacuum dried at 60° C. for 4 h to obtain nanosilver particles with a particle size of 50-100 nm.

[0088] S4. Fill an anodized aluminum oxide template with a pore size of 2.5 μm with activated micron-sized silver cores. Load a 0.3 mg / mL graphene dispersion into an electrostatic sprayer. Control the voltage to 35 kV and the pressure to 0.7 MPa. Spray 5 times, 50 mL each time, with vacuum assistance for 10 min to form a graphene network covering the surface of the micron-sized silver cores.

[0089] S5: Prepare an electrolyte (pH 5) by mixing 0.1 mol / L silver nitrate and 0.3 mol / L sodium citrate in a mass ratio of 1:1. Use porous carbon paper to load micron-sized silver cores and immerse them in the electrolyte as the working electrode. Use a platinum sheet as the counter electrode and extend it into the electrolyte at the same time. Pass a current with a current density of 15 mA / cm² and deposit for 30 minutes. Cover the surface of the silver core covered with graphene with a nano-sized silver shell. Rinse with clean water and dry it to obtain the target silver powder.

[0090] Comparative Example 1:

[0091] Compared with Example 1, the difference is that the activation step of S2 is omitted, and the silver core prepared by S1 is directly used for subsequent graphene network coating and silver shell coating.

[0092] Comparative Example 2:

[0093] Compared with Example 1, the difference is that the S4 graphene spraying step is omitted, and only the silver shell is deposited on the surface of the activated silver core.

[0094] Comparative Example 3:

[0095] S1, 0.18 mol / L silver nitrate solution and 0.12 mol / L ascorbic acid solution were mixed in a mass ratio of 2:3, the pH was adjusted to 2.5 with nitric acid, and a 0.03 mol / L tannic acid solution of one quarter the mass of the ascorbic acid solution was added, and the mixture was placed in a reactor and reacted at 35 ° C for 20 min. After separation by high-speed centrifugation at a speed of 8000 rpm for 10 min, the mixture was placed in a vacuum dryer and vacuum dried at 30 ° C to obtain micron-sized silver nuclei with a particle size of 0.6-2 μm;

[0096] S2, immersing the silver core obtained in step S1 in a mixture of 0.3 mol / L silver nitrate and 0.01 mol / L ascorbic acid in a mass ratio of 1:1, reacting at 30°C for 10 minutes, rinsing with clean water, and drying for later use;

[0097] S3, mixing 0.03 mol / L silver nitrate solution and 0.03 mol / L ascorbic acid in a ratio of 1:1, adding 0.3% polyvinyl pyrrolidone by weight of the total solution, adjusting the pH to 6.5 with nitric acid, placing the mixture into a reactor and reacting at 40° C. for 20 min with continuous stirring during the reaction, separating the solid silver particles by centrifugal filtration after completion of the reaction, washing, and vacuum drying at 60° C. for 4 h to obtain nanosilver particles with a particle size of 50-100 nm;

[0098] S4, 0.3 mg / mL graphene dispersion was loaded into an electrostatic sprayer, the voltage was controlled to 35 kV, the pressure was 0.5 MPa, and the spraying was performed 3 times, 50 mL each time, with vacuum assistance for 10 min, and the graphene dispersion was directly applied to the surface of the collected micron-sized silver cores to form a graphene network coverage;

[0099] S5: Prepare an electrolyte (pH 5) by mixing 0.1 mol / L silver nitrate and 0.3 mol / L sodium citrate in a mass ratio of 1:1. Use porous carbon paper to load micron-sized silver cores and immerse them in the electrolyte as the working electrode. Use a platinum sheet as the counter electrode and extend it into the electrolyte at the same time. Pass a current with a current density of 10 mA / cm² and deposit for 15 minutes. Cover the surface of the silver core covered with graphene with a nano-sized silver shell. Rinse with clean water and dry it to obtain the target silver powder.

[0100] Comparative Example 4:

[0101] Compared with Example 1, the difference is that the micron-sized silver powder with a particle size of 0.6-2 μm prepared in step S1 is directly mixed with the nano-silver powder 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 mass.

[0102] Comparative Example 5:

[0103] Compared with Example 1, the difference is that: in step S4, the voltage is controlled to 35 kV, the pressure is 0.7 MPa, the spraying is performed 8 times, 50 mL is sprayed each time, and the vacuum assistance is performed for 15 minutes to form a graphene network covering on the surface of the micron-sized silver core; the other steps are the same.

[0104] Comparative Example 6:

[0105] Compared with Example 1, the difference is that in step S5: 0.1 mol / L silver nitrate and 0.3 mol / L sodium citrate are mixed in a mass ratio of 1:1 to prepare an electrolyte (pH 5), a current is passed at a current density of 15 mA / cm², and the deposition is carried out for 45 minutes. The surface of the silver core covered with graphene is then covered with a nano-scale silver shell to obtain the target silver powder.

[0106] Performance testing:

[0107] Silver powder performance test: The silver powder in Examples 1-5 and Comparative Examples 5 and 6 was tested for silver powder resistivity according to ASTM B193-20 "Test method for resistivity of conductive materials"; the thermal conductivity of the silver powder was tested according to ASTM E1461-13 "Determination of thermal diffusivity by laser flash method"; the graphene network and silver shell coverage were measured using a scanning electron microscope (SEM) image statistics method; the silver powder tap density was measured according to GB / T 5162-2021 "Determination of tap density of metal powders".

[0108] Performance test of sintered samples of SiC plate and Cu plate: prepare several groups SiC plate and Cu plate, under nitrogen protection, the silver powder of Example 1-5 and Comparative Example 1-4 was placed between the SiC plate and the Cu plate, sintered at 250 ° C for 40 minutes, and after sintering, cooled to room temperature to obtain a sintered sample;

[0109] The sintered samples were subjected to uniaxial tensile testing according to ASTM D1002-10, Test Method for Tensile Shear Strength of Adhesive Joints. This involved tensile simulations at the SiC / Ag interface and the Cu / Ag interface, respectively, and the tensile strength, fracture energy, and fracture location were recorded. The width and morphology of the sintered neck formed between the silver layer and the upper and lower plates were observed using a scanning electron microscope (SEM), and the width difference between the two sintered necks was measured. The samples were thermally cycled 1000 times within a temperature range of -50°C to 150°C, and the tensile strength and fracture energy of the samples after thermal cycling were tested. Crack propagation was observed, and the tensile strength of the samples after thermal cycling was compared with the data before thermal cycling.

[0110] Silver powder performance tests were conducted on the silver powders in Examples 1-5 and Comparative Examples 5 and 6, and sintered sample performance tests were conducted on the silver powders in Examples 1-5 and Comparative Examples 1-6. The test data are shown in the following table:

[0111] Table 1 Silver powder performance test data

[0112]

[0113] Table 2 Sintered sample performance test data

[0114]

[0115] According to the data analysis in Table 1:

[0116] Examples 1-5 are silver powders with a silver core-graphene network-silver shell structure prepared by the present invention, which have the characteristics of low resistivity, high thermal conductivity and high tap density. The reasons for this are analyzed as follows:

[0117] Graphene is a two-dimensional, highly conductive material. The continuous conductive network formed by graphene's high conductivity and the dense filling of the graphene gaps by the nano-silver shell significantly reduce interfacial resistance. Furthermore, the silver core activation treatment enhances surface activity and reduces interfacial defects, which, combined with the nano-scale silver shell, further improves conductivity.

[0118] The high thermal conductivity is attributed to the synergistic thermal conduction mechanism of graphene and the silver shell. The graphene network provides a rapid heat transfer channel, while the silver shell enhances thermal conductivity through dual pathways of electrons and phonons. Furthermore, the adsorption effect of the anodized aluminum template ensures the orderly arrangement of the silver cores. When the graphene dispersion is sprayed onto the silver powder confined on the anodized aluminum template, the graphene is effectively filled between the evenly arranged silver cores, filling the microscopic gaps. This greatly improves the bonding uniformity and effectiveness, reduces the porosity at the bonding interface, and manifests as a significant increase in thermal conductivity.

[0119] The high tap density is achieved by the densification design of the composite structure. Graphene serves as an intermediate layer to fill the gaps between the silver cores to form a rigid skeleton. The nano-silver shell is evenly coated on the surface of the micron silver core by electrochemical deposition, and the template confinement effect further reduces the gaps between the particles, significantly improving the tap density of the silver powder prepared in the embodiment of the present application.

[0120] Comparing Examples 1-3 and Comparative Example 5, it can be seen that the resistivity of the silver powder decreases with the increase of the graphene content, and the thermal conductivity increases with the increase of the graphene content. The coverage of the graphene network has little effect on the tap density. Therefore, it can be concluded that the increase in the coverage of the graphene network on the silver core surface 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 the increase in the graphene content has a critical value for the improvement of electrical conductivity and thermal conductivity in the later stage, and with the increase in the graphene network coverage, it will significantly affect the homogeneous combination of the silver shell and the silver core, which is manifested as an abnormal decrease in the tap density of the silver powder;

[0121] By comparing Example 1, Example 4, Example 5 and Comparative Example 6, it can be concluded that as the coverage of the nanosilver shell in the silver shell increases, the tap density, electrical conductivity and thermal conductivity of the silver powder are significantly improved; however, there is a critical value for the tap density. Therefore, when Example 5 is compared with Comparative Example 6, although the silver shell coverage is increased, the tap density of the silver powder is not increased. Therefore, when the silver shell coverage is about 80%, there is a critical value for the tap density.

[0122] According to the data analysis in Table 2:

[0123] When the SiC plate and the Cu plate were sintered using Examples 1-5, the connection performance was significantly higher than that of the comparative example, which was mainly due to the effect of the silver core-graphene network-silver shell gradient composite structure of the examples of the present application. Specifically:

[0124] 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 larger sintering neck width differences are more likely to fracture. Large width differences can also locally concentrate fracture energy, leading to earlier material failure. However, this embodiment effectively reduces the difference in sintering neck width between Cu / Ag and SiC / Ag through the gradient structure design of the graphene network and nanosilver shells. This smaller width difference increases fracture energy, thereby improving tensile strength.

[0125] The reason is that during the sintering process, the nanosilver shells preferentially wet the surfaces of the SiC and Cu substrates at the initial stage of sintering due to their high surface energy and low atomic migration activation energy. The graphene network, as a supporting skeleton, guides the nanosilver shells to be evenly distributed along the interface through its two-dimensional continuous structure, avoiding local excessive thickness or thinness. This gradient wetting effect reduces the difference in contact area between the silver particles on the SiC side and the Cu side, thereby simultaneously 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 nanosilver shells quickly form sintering necks through the surface diffusion-dominated mechanism, the micronized silver cores provide a stable atomic source in the subsequent bulk diffusion stage, and cooperate with the confinement effect of the graphene network to force the silver cores to release atoms in a directionally directed manner to the interface area, making the atomic migration rates on the SiC side and the Cu side tend to be consistent, forming a smaller sintering neck difference between the silver powder and the upper and lower plates.

[0126] Secondly, the dense filling and uniform coating of the silver shell enhance the bonding strength between the silver core and graphene, and between the silver core and the substrate, making it difficult for the sintered layer to break from the interface or inside when subjected to stress. The mechanical flexibility of graphene can absorb the stress generated by the expansion difference of the substrates on both sides, avoiding stress concentration causing excessive coarsening of the unilateral sintering neck, and further eliminating the thermal hysteresis effect between SiC and Cu during the sintering process.

[0127] Comparing Example 1 with Comparative Example 1, the strength retention rate of Comparative Example 1 after thermal cycling is reduced and the tensile strength is low. The reason is that in Example 1, the activation step is to treat the silver core surface with a mixture of silver nitrate and ascorbic acid to generate active sites. The activation mechanism is that ascorbic acid, as a reducing agent, will preferentially reduce the silver nitrate in the silver core. for , and the newly generated It is attached to the surface of the silver core in the form of islands or nanoparticles, forming a microscopic rough structure, increasing the specific surface area and active sites, thereby improving the subsequent bonding strength with graphene and the silver shell, and reducing the hydroxyl content; while in Comparative Example 1, the hydroxyl groups on the surface of the unactivated silver core adsorb on the graphene to form a weak physical bond, the interface bonding force is reduced, the silver particles are unevenly distributed during sintering, the porosity is increased, and the sintering strength is affected.

[0128] Comparing Example 1 with Comparative Example 2, due to the lack of a graphene network, a network structure of graphene-wrapped silver cores was not formed in Comparative Example 2, and the silver shell was directly deposited on the surface of the exposed silver core. The stress transfer function of graphene was lacking at the connection interface. Therefore, during sintering, no graphene-guided diffusion channel was formed, and the surface diffusion coefficient of silver atoms decreased, which was manifested as a decrease in thermal conductivity and an increase in the sintering neck width difference. Fracture occurred directly inside the Ag layer, indicating that the silver powder itself had quality defects as a connection material.

[0129] Comparing Example 1 with Comparative Example 3, Comparative Example 3 lacks the restriction of the anodized aluminum template. The random stacking of silver cores after removing the anodized aluminum template results in uneven bonding between the silver cores and graphene. During sintering, there is a significant difference in the contact area between the graphene or silver cores and the SiC plate and the Cu plate. The sintering neck grows unevenly during sintering. The fracture of Comparative Example 3 occurs inside the Ag layer and at the Cu / Ag interface, proving that the uneven structure of the silver powder leads to uneven offset of the silver powder particles during sintering, resulting in a large difference in the width of the sintering neck, which impairs the bonding effect.

[0130] Comparing Example 1 with Comparative Example 4, Comparative Example 4 directly uses 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 mixed 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 nano-sized silver powder will not preferentially wet the surface of the SiC / Cu board. At the same time, the stress buffering effect of graphene is also lacking, which easily leads to the agglomeration of nano-sized / micron-sized 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.

[0131] Comparing Example 1 with Comparative Example 5, in Comparative Example 5, the graphene coverage is too large, which hinders the homogeneous bonding of the silver core 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 sintering neck width difference; at the same time, some cracks are generated inside the Ag layer, proving that there is a probability of quality problems when sintering the connecting material prepared in Comparative Example 5.

[0132] Comparing Example 5 with Comparative Example 6, the silver shell coverage rate of Comparative Example 6 is about 90%, but the sintering neck width difference is greater than that of Example 5. The reason is that the excessively thick silver shell forms a wetting layer with excessive viscosity when wetting the surface of the plate, which affects the dispersion of the silver core and the filling of local silver cores and graphene, resulting in a large difference in the sintering neck width on both sides and a decrease in tensile strength.

[0133] In summary, the present invention, through multi-level structural design and precise process control, constructs a gradient composite system of "micronized silver core-graphene network-nanosilver shell." Through the synergistic effects of key technologies such as activation treatment, template confinement, and electrochemical deposition, it effectively solves the problem of poor connection quality caused by the large difference in sintering neck width between the silver powder used as the connecting material and the connecting plates on both sides in the traditional sintering process. The present invention utilizes the synergistic filling effect of graphene and the stress buffering effect of graphene to achieve uniform stress transfer during sintering; and utilizes the wettability of the nanosilver shell to improve the bonding performance of the interface.

[0134] The above description is based on the ideal embodiment of the present invention. Based on the above description, relevant personnel can make various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the content of the specification and 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: The following steps are involved: S1. preparing micron-sized silver cores and nano-sized silver particles; The preparation of the micron-sized silver cores comprises: 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 minutes, separating and drying to obtain micron-sized silver cores with a particle size between 0.5-2 μm; The preparation of nano-scale silver particles 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 polyvinyl pyrrolidone with a mass concentration of 0.1-0.5% as a dispersant, adjusting the pH to 5-8, and stirring the mixture at 30-50°C for 10-30 minutes; after centrifugation, washing the mixture with clean water and ethanol alternately for 3-5 times, and vacuum drying the mixture at 50-80°C for 4-8 hours to obtain nano-scale silver particles with a particle size of 50-100 nm; S2, immersing the micron-sized silver core in a mixture of 0.1-0.5 mol / L silver nitrate and 0.01 mol / L ascorbic acid, reacting at 30-50°C for 5 minutes to form an activated surface, rinsing, drying, and setting aside; S3, filling the micron-sized silver cores into the pore gaps of the porous template; S4, spraying a graphene dispersion onto the porous template, wherein the concentration of the graphene dispersion is 0.1-0.5 mg / mL, the spraying pressure is 0.2-0.8 MPa, the number of spraying is 2-5 times, and 50-100 mL is sprayed each time, forming a graphene network covering on the surface of each silver core; S5. Mix a 0.05–0.2 mol / L silver nitrate solution with a 0.1–0.5 mol / L sodium citrate solution, adjust the pH to 4–6, and prepare an electrolyte. After removing the porous template, use porous carbon paper to load micron-sized silver cores and immerse them in the electrolyte as the working electrode. The platinum sheet is used as the counter electrode. After simultaneously extending the platinum sheet into the electrolyte, power is applied to both electrodes. The current density is controlled to be 5–15 mA / cm², and the deposition time is 5–30 min. During the deposition process, the electrolyte temperature is maintained at 25–40°C, accompanied by magnetic stirring. Nanoscale silver particles are deposited on the surface of the exposed silver cores, forming a silver shell structure that wraps the silver cores and the graphene network.

2. The method for preparing micro-nano silver powder with controllable particle size according to claim 1, wherein: The porous template is an anodic aluminum oxide template with a pore size of 0.8-2.5 μm.

3. The method for preparing micro-nano silver powder with controllable particle size according to claim 1, wherein: The coverage of the silver shell is 40-80% of the surface of the micron-sized silver core.

4. The method for preparing micro-nano silver powder with controllable particle size according to claim 1, wherein: The graphene network covers 40-80% of the surface of the silver core.

5. A micro-nano silver powder with controllable particle size, characterized in that: The invention is prepared based on a method for preparing micro-nano silver powder with controllable particle size according to any one of claims 1 to 4.

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