Silver powder with coexisting submicron sheets and nano-particles and preparation method of silver powder

Abstract

The invention discloses silver powder with coexisting submicron sheets and nano-particles and a preparation method of the silver powder, and belongs to the technical field of silver powder preparation. A silver nitrate solution is added into a dispersing agent solution, then a directional end-capping reagent solution is added, and nucleation and growth of submicron sheets and nano-particles are controlled through the types of dispersing agents, the types of directional end-capping reagents, the temperature gradient and the like. The diameter of the prepared flaky silver powder ranges from 0.1 micrometer to 1.5 micrometers, the thickness of the flaky silver powder ranges from 30 nm to 120 nm, and the flaking rate ranges from 30% to 90%. And the particle size of the spherical silver powder is 30-150 nm. According to the preparation method of the silver powder, a silver nanocrystal seed solution does not need to be prepared independently, and the silver powder is different from common silver powder with single morphology, can be used for preparing the flake-shaped and particle-shaped coexisting silver powder in one pot, and is suitable for being used as a conductive functional phase of low-temperature curing type electronic paste.

Description

Technical Field

[0001] The invention belongs to the technical field of silver powder preparation, and particularly relates to silver powder containing coexisting submicron flakes and nanoparticles and a preparation method thereof. Background Art

[0002] With the rapid development of the new generation of information technology industry, electronic information products are developing towards high performance, precision, green environmental protection and miniaturization, which also puts higher requirements on the printability, sintering and conductivity of conductive pastes used in devices or products in the above fields.

[0003] Silver powder is one of the most widely used conductive phases in electronic pastes. In low-temperature sintering / curing conductive silver pastes, flake silver powder, which has a relatively low sintering temperature, is generally used as the conductive functional phase. This is because flake silver powder has a large specific surface area and, during the printing process, it accumulates radially, with surface or linear contact between particles, resulting in excellent conductivity. However, using only flake silver powder as the conductive phase still has drawbacks. For example, during the sintering process, the film shrinks, creating gaps between the flakes and reducing conductivity. If silver nanoparticles are mixed into the flake silver powder with uniform distribution, they can fill these gaps, providing a larger contact area for the electronic paste, forming a better conductive network, and further reducing the resistivity of the paste. Furthermore, the addition of silver nanoparticles can reduce the stacking thickness of the silver flakes, thereby maintaining excellent conductivity while reducing the overall silver content and cost, thus facilitating the miniaturization of electronic components. Therefore, silver powders containing both submicron flakes and nanoparticles hold significant market potential.

[0004] However, most silver powders used in electronic pastes currently produced both domestically and internationally consist of particles with a single morphology. For example, invention patent VN105290420A discloses a method for preparing flaky silver powder. This method uses silver nitrate as a silver precursor compound, PVP as a surfactant, and ferrous salt as a reducing agent to produce flaky silver powder under heating conditions. Invention patent CN106563814B discloses a method for preparing silver nanotriangular flakes using a photoinduction method. After irradiation for 3 to 5 hours, 50 to 80 nm silver nanotriangular flakes can be obtained. Invention patent CN104959625A discloses a method for preparing flaky silver powder. This method first produces spherical silver powder through chemical reduction, then adds a ball milling aid, solvent, and zirconium balls for 10 to 40 hours of ball milling. The sample is then filtered, washed, and dried to obtain the flaky silver powder.

[0005] However, the flaky silver powder prepared by the methods of existing public patents still has some shortcomings. For example, the polymer on the surface of some flaky silver powders prepared by chemical reduction methods is difficult to remove, and metal ions such as iron and copper will remain, which has a great impact on the conductivity and stability of the silver powder. For example, although the light induction method can produce high-purity silver submicron flakes with controllable size, it requires continuous irradiation of light of a specific frequency to induce a reaction during the preparation process, and then the frequency and irradiation time of the light source are adjusted to control the size of the silver submicron flakes. This method has high requirements for equipment, cumbersome operation, low yield, and long preparation cycle, which is not conducive to industrial production. The ball milling method for preparing flaky silver powder is complicated, time-consuming, and energy-intensive. The flaky silver powder prepared is easily contaminated by ball milling aids and introduced with impurities, and the size of the prepared flaky silver powder is not uniform. Therefore, the preparation method and structure control technology of silver powder composed of submicron flaky silver powder and nano-scale spherical silver powder are one of the key technologies that need to be broken through in China. Summary of the Invention

[0006] The present invention provides a silver powder containing submicron flakes and nanoparticles and a preparation method thereof, which can effectively solve the difficult problem of precisely controlling the structure, size and morphology of submicron flakes and nanoparticles, and the control method is simple and easy to control.

[0007] The present invention provides a method for preparing silver powder containing coexisting submicron flakes and nanoparticles, which specifically comprises the following steps: (1) The dispersant solution is kept at a specific temperature, and then its pH value is adjusted; then, a silver nitrate solution is added and kept warm, and then a directional capping agent solution is added and kept warm to obtain a mixed solution; (2) lowering the temperature of the mixed solution and keeping it warm for a period of time, then adding a reducing agent solution and keeping it warm for a period of time to obtain a silver powder stock solution in which submicron flakes and nanoparticles coexist; (3) The silver powder stock solution containing submicron flakes and nanoparticles is centrifuged, washed, and dried, and then a specific organic solvent is added for redispersion to obtain a silver powder paste containing submicron flakes and nanoparticles.

[0008] Preferably, the dispersant is at least one of gelatin, gum arabic, pectin, fish glue, bone glue, gelatin, peach gum, carrageenan, xanthan gum, gellan gum, guar gum, agar, locust bean gum, and konjac gum.

[0009] Preferably, the directional end-capping agent is at least one of glutaric acid, malic acid, adipic acid, pimelic acid, and pyrenetetracarboxylic acid.

[0010] Preferably, the reducing agent is one of ascorbic acid, hydrogen peroxide and formaldehyde.

[0011] Preferably, the mass concentration of the dispersant in the dispersant solution is 5~40 g / L; the mass concentration of silver nitrate in the silver nitrate solution is 50~200 g / L; the mass concentration of the directional capping agent in the directional capping agent solution is 10~50 g / L; the mass concentration of the reducing agent in the reducing agent solution is 50~200 g / L; the volume ratio of the dispersant solution, silver nitrate solution, directional capping agent solution and reducing agent solution is 40%-50%:10%-20%:10%-20%:20%-30%, and the sum of the volume fractions of the dispersant solution, silver nitrate solution, directional capping agent solution and reducing agent solution is 100%.

[0012] Preferably, the step (1) is specifically as follows: keeping the dispersant solution at 55-65°C for 10-30 min, adjusting its pH value to 1-3, adding silver nitrate solution and keeping it warm for 5-20 min, and then adding directional capping agent solution and keeping it warm for 5-20 min to obtain a mixed solution.

[0013] Preferably, the step (2) is specifically as follows: keeping the mixed solution at 35-50°C for 10-20 min, then adding a reducing agent solution and keeping it warm for 10-20 min, thereby obtaining a silver powder stock solution in which submicron flakes and nanoparticles coexist.

[0014] Preferably, the organic solvent is an ester, ether or alcohol organic solvent.

[0015] The ester organic solvent is tributyl citrate, dibutyl phthalate, and diethylene glycol ethyl ether acetate.

[0016] The ether organic solvent is diethylene glycol butyl ether, propylene glycol phenyl ether or carbitol methyl ether.

[0017] The alcohol organic solvent is terpineol, phenylethyl alcohol or trimethyl nonanol.

[0018] The present invention also claims a method for preparing the silver powder containing coexisting submicron flakes and nanoparticles. The prepared silver powder containing coexisting submicron flakes and nanoparticles has a submicron flake content of 30 to 90%, a submicron flake diameter ranging from 0.1 to 1.5 μm, a flake thickness of 30 to 120 nm, and a nanoparticle size of 30 to 150 nm. The nanoparticles include spherical and quasi-spherical particles.

[0019] The mechanisms involved in the present invention include: (1) The synthesis of silver powder mainly goes through two processes, namely the nucleation stage and the growth stage. In the nucleation stage, silver ions are rapidly reduced to silver atoms under the combined action of suitable dispersants and directional capping agents. When the Gibbs free energy of the reaction continues to decrease, the silver nucleus reaches stability, that is, a large number of free monomers are rapidly generated. As the reaction continues, the supersaturation of silver atoms in the solution gradually increases until it reaches the critical point. The silver atoms overcome the nucleation potential energy and form silver nuclei with a specific crystal system and morphology. Among them, the seeds required to grow into flaky silver powder are planar twin seeds. Therefore, the formation ratio of planar twin seeds will directly determine the proportion of subsequent flaky silver powder in the product. Generally, the reaction process in the nucleation stage is difficult to control. If it is not intervened, silver nuclei with different structures such as cubic system, single twin system and multi-twin system will be randomly generated; in the growth stage, there are two stages: silver ions diffuse to the surface of the planar twin nuclei and silver ions undergo surface chemical reactions on the surface of the nuclei. Considering the factors controlling silver particle growth, it can be categorized as either diffusion-controlled or surface-chemical reaction-controlled. Typically, during the growth phase, the reaction process is extremely rapid and difficult to control. Silver ions tend to be rapidly reduced and aggregated to form more planar twinned seeds and isotropic nanoparticles. This results in smaller synthesized flaky silver powders and an uncontrollable structure.

[0020] (2) The present invention first controls the formation of planar twin seeds through the combined effects of a dispersant and a directional capping agent. In the first stage of the reaction, a dispersant solution is first mixed with a silver nitrate solution at a specified temperature. Since polar carboxyl and amino groups are present in the structure of dispersants such as gelatin, and the amino group has weak reducing properties, the weak reducing effect of gelatin can induce the formation of twin seeds. Then, a directional capping agent solution is added. Since the carboxylate in the directional capping agent such as glutaric acid can be adsorbed on the (111) crystal plane of the twin to form a surface potential and steric hindrance, it promotes the adsorption and aggregation of free silver ions toward the (100) crystal plane of the twin, thereby facilitating the formation of planar twin seeds.

[0021] (3) The present invention also uses a gradient temperature to regulate the diffusion rate of free silver ions to the surface of the planar twin seed crystals and the rate of chemical reactions occurring on the surface, thereby controlling the size of the silver submicron flakes. In the second stage of the reaction, the mixed solution of the first stage is placed at a relatively low specific temperature, and then a reducing agent is added, which is conducive to the directional deposition of silver atoms on the (100) crystal plane of the planar twin crystals, thereby promoting the growth of silver submicron flakes. More importantly, the dispersant selected in the present invention, in addition to having weak reducing and dispersing properties, can increase the viscosity of its solution as the temperature decreases in the dissolved and dispersed state. Therefore, the diffusion rate of free silver ions to the surface of the planar twin seed crystals can be regulated by temperature regulation, thereby promoting the directional deposition of the remaining silver atoms on the (100) crystal plane of the planar twin crystals, rather than rapidly and independently aggregating and growing into new seed crystals. Therefore, the size of the silver submicron flakes can be controlled by gradient temperature regulation.

[0022] The technical solution provided by the present invention has the following beneficial effects: (1) The method for preparing the silver powder of the present invention does not require the separate preparation of a silver nanocrystal seed solution. Moreover, unlike ordinary silver powder with a single morphology, the method can prepare silver powder with both flakes and particles in one pot, which is suitable as a conductive functional phase for low-temperature curing electronic pastes.

[0023] (2) The present invention can achieve fine control of the flake rate and size of the prepared silver powder by regulating the dispersant, directional capping agent and reaction temperature. The overall preparation process of this method is simple, the preparation cost is low, it is safe and environmentally friendly, and it is easy to achieve mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The present invention is a process flow chart of the method for preparing silver powder with coexistence of submicron flakes and nanoparticles.

[0025] Figure 2 This is a scanning electron microscope (SEM) image of the silver powder prepared in Example 1 of the present invention.

[0026] Figure 3 This is a scanning electron microscope (SEM) image of the silver powder prepared in Example 2 of the present invention.

[0027] Figure 4 This is a scanning electron microscope (SEM) image of the silver powder prepared in Example 3 of the present invention.

[0028] Figure 5 This is a scanning electron microscope (SEM) image of the silver powder prepared in Example 4 of the present invention.

[0029] Figure 6 This is a scanning electron microscope (SEM) image of the silver powder prepared in Example 5 of the present invention.

[0030] Figure 7 This is a scanning electron microscope (SEM) image of the silver powder prepared in Example 6 of the present invention.

[0031] Figure 8 This is a scanning electron microscope (SEM) image of the silver powder prepared in Comparative Example 1 of the present invention.

[0032] Figure 9 This is a scanning electron microscope (SEM) image of the silver powder prepared in Comparative Example 2 of the present invention.

[0033] Figure 10 This is a scanning electron microscope (SEM) image of the silver powder prepared in Comparative Example 3 of the present invention.

[0034] Figure 11 This is a scanning electron microscope (SEM) image of the silver powder prepared in Comparative Example 4 of the present invention.

[0035] Figure 12 This is a scanning electron microscope (SEM) image of the silver powder prepared in Comparative Example 5 of the present invention.

[0036] Figure 13 This is a scanning electron microscope (SEM) image of the silver powder prepared in Comparative Example 6 of the present invention. DETAILED DESCRIPTION

[0037] Example 1 A method for preparing silver powder containing coexisting submicron flakes and nanoparticles, comprising the following steps: (1) Dissolve 15 g of gelatin in 0.5 L of deionized water to prepare solution a. The dissolution temperature is 55 °C, the stirring speed is 150 rpm, and the stirring time is 20 min. Set aside.

[0038] (2) Dissolve 100 g of silver nitrate in 1.0 L of deionized water to prepare solution b. The dissolution temperature is 55°C, the stirring speed is 150 rpm, and the stirring time is 10 min. Set aside.

[0039] (3) Dissolve 20 g of glutaric acid in 0.6 L of deionized water to prepare solution c. The dissolution temperature is 55°C, the stirring speed is 150 rpm, and the stirring time is 10 min. Set aside.

[0040] (4) Dissolve 90 g of ascorbic acid in 0.9 L of deionized water to prepare solution d. The dissolution temperature is 50°C, the stirring speed is 150 rpm, and the stirring time is 15 min. Set aside.

[0041] (5) First stage reaction: Solution a was transferred into a reaction vessel and kept at 55 °C for 10 min. The pH was adjusted to 2.5. Then, solution b was added at a rate of 30 mL / min at a stirring speed of 150 rpm and stirred for 15 min. Then, solution c was added at a rate of 30 mL / min and stirred for 10 min to obtain a mixed solution e for later use.

[0042] (6) Second stage reaction: The mixed solution e was placed at 50 °C and stirred for 20 min. Then, solution d was added at a rate of 20 mL / min at a stirring speed of 200 rpm and stirred for 20 min to obtain a silver powder stock solution in which submicron flakes and nanoparticles coexisted.

[0043] (7) The silver powder stock solution obtained in step (6) is centrifugally washed with deionized water and ethanol respectively, and the silver powder solution is concentrated by centrifugation and drying. Dibutyl phthalate is used to replace the original solvent to obtain the silver powder paste in which the submicron flakes and nanoparticles coexist.

[0044] like Figure 2 As shown in FIG, the silver powder prepared in this embodiment is composed of submicron flakes and nanoparticles, the submicron flake silver powder accounts for 50%, the diameter ranges from 100 to 200 nm, and the flake thickness is 30 to 50 nm; the particle size of the spherical nanoparticle silver powder is 30 to 100 nm.

[0045] Example 2 A method for preparing silver powder containing coexisting submicron flakes and nanoparticles, comprising the following steps: (1) Dissolve 15 g of gelatin in 0.5 L of deionized water to prepare solution a. The dissolution temperature is 55 °C, the stirring speed is 150 rpm, and the stirring time is 20 min. Set aside.

[0046] (2) Dissolve 100 g of silver nitrate in 1.0 L of deionized water to prepare solution b. The dissolution temperature is 55°C, the stirring speed is 150 rpm, and the stirring time is 10 min. Set aside.

[0047] (3) Dissolve 20 g of glutaric acid in 0.6 L of deionized water to prepare solution c. The dissolution temperature is 55°C, the stirring speed is 150 rpm, and the stirring time is 10 min. Set aside.

[0048] (4) Dissolve 90 g of ascorbic acid in 0.9 L of deionized water to prepare solution d. The dissolution temperature is 45°C, the stirring speed is 150 rpm, and the stirring time is 15 min. Set aside.

[0049] (5) First stage reaction: Solution a was transferred into a reaction vessel and kept at 55 °C for 10 min. The pH was adjusted to 2.5. Then, solution b was added at a rate of 30 mL / min at a stirring speed of 150 rpm and stirred for 15 min. Then, solution c was added at a rate of 30 mL / min and stirred for 10 min to obtain a mixed solution e for later use.

[0050] (6) Second stage reaction: The mixed solution e was placed at 45 °C and stirred for 20 min. Then, solution d was added at a rate of 20 mL / min at a stirring speed of 200 rpm and stirred for 20 min to obtain a silver powder stock solution in which submicron flakes and nanoparticles coexisted.

[0051] (7) The silver powder stock solution obtained in step (6) is centrifugally washed with deionized water and ethanol respectively, and the silver powder solution is concentrated by centrifugation and drying. Dibutyl phthalate is used to replace the original solvent to obtain the silver powder paste in which the submicron flakes and nanoparticles coexist.

[0052] like Figure 3 As shown in FIG, the silver powder prepared in this embodiment is composed of submicron flakes and nanoparticles, wherein the submicron flake silver powder accounts for 80%, the diameter ranges from 100 to 300 nm, and the flake thickness is 30 to 50 nm; the particle size of the spherical nanoparticle silver powder is 50 to 150 nm.

[0053] Example 3 A method for preparing silver powder containing coexisting submicron flakes and nanoparticles, comprising the following steps: (1) Dissolve 15 g of gelatin in 0.5 L of deionized water to prepare solution a. The dissolution temperature is 55 °C, the stirring speed is 150 rpm, and the stirring time is 20 min. Set aside.

[0054] (2) Dissolve 100 g of silver nitrate in 1.0 L of deionized water to prepare solution b. The dissolution temperature is 55°C, the stirring speed is 150 rpm, and the stirring time is 10 min. Set aside.

[0055] (3) Dissolve 20 g of glutaric acid in 0.6 L of deionized water to prepare solution c. The dissolution temperature is 55°C, the stirring speed is 150 rpm, and the stirring time is 10 min. Set aside.

[0056] (4) Dissolve 90 g of ascorbic acid in 0.9 L of deionized water to prepare solution d. The dissolution temperature is 40°C, the stirring speed is 150 rpm, and the stirring time is 15 min. Set aside.

[0057] (5) First stage reaction: Solution a was transferred into a reaction vessel and kept at 55 °C for 10 min. The pH was adjusted to 2.5. Then, solution b was added at a rate of 30 mL / min at a stirring speed of 150 rpm and stirred for 15 min. Then, solution c was added at a rate of 30 mL / min and stirred for 10 min to obtain a mixed solution e for later use.

[0058] (6) Second stage reaction: The mixed solution e was placed at 40 °C and stirred for 20 min. Then, solution d was added at a rate of 20 mL / min at a stirring speed of 200 rpm and stirred for 20 min to obtain a silver powder stock solution in which submicron flakes and nanoparticles coexisted.

[0059] (7) The silver powder stock solution obtained in step (6) is centrifugally washed with deionized water and ethanol respectively, and the silver powder solution is concentrated by centrifugation and drying. Dibutyl phthalate is used to replace the original solvent to obtain the silver powder paste in which the submicron flakes and nanoparticles coexist.

[0060] like Figure 4 As shown in FIG, the silver powder prepared in this embodiment is composed of submicron flakes and nanoparticles, wherein the submicron flake silver powder accounts for 85%, the diameter ranges from 200 to 500 nm, and the flake thickness is 40 to 70 nm; the particle size of the spherical nanoparticle silver powder is 50 to 100 nm.

[0061] Example 4 A method for preparing silver powder containing coexisting submicron flakes and nanoparticles, comprising the following steps: (1) Dissolve 15 g of gelatin in 0.5 L of deionized water to prepare solution a. The dissolution temperature is 55 °C, the stirring speed is 150 rpm, and the stirring time is 20 min. Set aside.

[0062] (2) Dissolve 100 g of silver nitrate in 1.0 L of deionized water to prepare solution b. The dissolution temperature is 55°C, the stirring speed is 150 rpm, and the stirring time is 10 min. Set aside.

[0063] (3) Dissolve 20 g of glutaric acid in 0.6 L of deionized water to prepare solution c. The dissolution temperature is 55°C, the stirring speed is 150 rpm, and the stirring time is 10 min. Set aside.

[0064] (4) Dissolve 90 g of ascorbic acid in 0.9 L of deionized water to prepare solution d. The dissolution temperature is 35°C, the stirring speed is 150 rpm, and the stirring time is 15 min. Set aside. (5) First stage reaction: Solution a was transferred into a reaction vessel and kept at 55 °C for 10 min. The pH was adjusted to 2.5. Then, solution b was added at a rate of 30 mL / min at a stirring speed of 150 rpm and stirred for 15 min. Then, solution c was added at a rate of 30 mL / min and stirred for 10 min to obtain a mixed solution e for later use.

[0065] (6) Second stage reaction: The mixed solution e was placed at 35 °C and stirred for 20 min. Then, solution d was added at a rate of 20 mL / min at a stirring speed of 200 rpm and stirred for 20 min to obtain a silver powder stock solution in which submicron flakes and nanoparticles coexisted.

[0066] (7) The silver powder stock solution obtained in step (6) is centrifugally washed with deionized water and ethanol respectively, and the silver powder solution is concentrated by centrifugation and drying. Dibutyl phthalate is used to replace the original solvent to obtain the silver powder paste in which the submicron flakes and nanoparticles coexist.

[0067] like Figure 5 As shown in FIG, the silver powder prepared in this embodiment is composed of submicron flakes and nanoparticles, wherein the submicron flake silver powder accounts for 90%, the diameter ranges from 0.5 to 1.5 μm, and the flake thickness is 80 to 120 nm; the particle size of the spherical nanoparticle silver powder is 50 to 100 nm.

[0068] Example 5 A method for preparing silver powder containing coexisting submicron flakes and nanoparticles, comprising the following steps: (1) Dissolve 2.5 g of pectin in 0.5 L of deionized water to prepare solution a. The dissolution temperature is 65°C, the stirring speed is 250 rpm, and the stirring time is 30 min. Set aside.

[0069] (2) Dissolve 50 g of silver nitrate in 1.0 L of deionized water to prepare solution b. The dissolution temperature is 65°C, the stirring speed is 250 rpm, and the stirring time is 5 min. Set aside.

[0070] (3) Dissolve 6 g of malic acid in 0.6 L of deionized water to prepare solution c. The dissolution temperature is 65°C, the stirring speed is 250 rpm, and the stirring time is 30 min. Set aside.

[0071] (4) Disperse 180 g of hydrogen peroxide in 0.9 L of deionized water to prepare solution d. The dispersion temperature is 50 °C, the stirring speed is 250 rpm, and the stirring time is 10 min. Set aside.

[0072] (5) First stage reaction: Solution a was transferred into a reaction vessel and kept at 65 °C for 20 min. The pH was adjusted to 1. Then, solution b was added at a rate of 80 mL / min at a stirring speed of 250 rpm and stirred for 5 min. Then, solution c was added at a rate of 80 mL / min and stirred for 20 min to obtain a mixed solution e for later use.

[0073] (6) Second stage reaction: The mixed solution e was placed at 50 °C and stirred for 10 min. Then, solution d was added at a rate of 50 mL / min at a stirring speed of 200 rpm and stirred for 10 min to obtain a silver powder stock solution in which submicron flakes and nanoparticles coexisted.

[0074] (7) The silver powder stock solution obtained in step (6) is centrifugally washed with deionized water and ethanol respectively, the silver powder solution is concentrated by centrifugation and drying, and tributyl citrate is used to replace the original solvent to obtain the silver powder paste in which the submicron flakes and nanoparticles coexist.

[0075] like Figure 6 As shown, the silver powder prepared in this embodiment consists of submicron flakes and nanoparticles, the submicron flake silver powder accounts for 30%, the diameter ranges from 0.2 to 0.5 μm, and the flake thickness is 30 to 50 nm; the particle size of the spherical nanoparticle silver powder is 50 to 150 nm.

[0076] Example 6 A method for preparing silver powder containing coexisting submicron flakes and nanoparticles, comprising the following steps: (1) Dissolve 20 g of pectin in 0.5 L of deionized water to prepare solution a. The dissolution temperature is 65°C, the stirring speed is 250 rpm, and the stirring time is 30 min. Set aside.

[0077] (2) Dissolve 200 g of silver nitrate in 1.0 L of deionized water to prepare solution b. The dissolution temperature is 65°C, the stirring speed is 250 rpm, and the stirring time is 5 min. Set aside.

[0078] (3) Dissolve 30 g of malic acid in 0.6 L of deionized water to prepare solution c. The dissolution temperature is 65°C, the stirring speed is 250 rpm, and the stirring time is 30 min. Set aside.

[0079] (4) Disperse 45 g of hydrogen peroxide in 0.9 L of deionized water to prepare solution d. The dispersion temperature is 50°C, the stirring speed is 250 rpm, and the stirring time is 10 min. Set aside.

[0080] (5) First stage reaction: Solution a was transferred into a reaction vessel and kept at 65 °C for 20 min. The pH was adjusted to 1. Then, solution b was added at a rate of 80 mL / min at a stirring speed of 250 rpm and stirred for 5 min. Then, solution c was added at a rate of 80 mL / min and stirred for 20 min to obtain a mixed solution e for later use.

[0081] (6) Second stage reaction: The mixed solution e was placed at 50 °C and stirred for 10 min. Then, solution d was added at a rate of 50 mL / min at a stirring speed of 200 rpm and stirred for 10 min to obtain a silver powder stock solution in which submicron flakes and nanoparticles coexisted.

[0082] (7) The silver powder stock solution obtained in step (6) is centrifugally washed with deionized water and ethanol respectively, the silver powder solution is concentrated by centrifugation and drying, and pineol is used to replace the original solvent to obtain the silver powder paste in which the submicron flakes and nanoparticles coexist.

[0083] like Figure 7 As shown, the silver powder prepared in this embodiment is composed of submicron flakes and nanoparticles, the submicron flake silver powder accounts for 40%, the diameter ranges from 0.3 to 0.5 μm, and the flake thickness is 30 to 50 nm; the particle size of the spherical nanoparticle silver powder is 80 to 150 nm.

[0084] Comparative Example 1 The only difference between the method for preparing silver powder with coexistence of submicron flakes and nanoparticles described in Comparative Example 1 and Example 2 is that in step (6), the reaction temperature in the second stage is 55°C.

[0085] like Figure 8 As shown, the silver powder prepared in this comparative example is composed of spherical silver powder and a small amount of irregular and very thin flaky silver powder, the proportion of submicron flaky silver powder is less than 10%, the diameter range is 150-300 nm, and the flake thickness is 20-30 nm; the particle size of the spherical silver powder is 50-100 nm.

[0086] Comparative Example 2 The only difference between the method for preparing silver powder with coexistence of submicron flakes and nanoparticles described in Comparative Example 1 and Example 2 is that in step (6), the reaction temperature in the second stage is 65°C.

[0087] like Figure 9 As shown, the silver powder prepared in this comparative example is mainly composed of quasi-spherical silver powder, which is partially agglomerated and has a particle size of 30 to 70 nm.

[0088] Comparative Example 3 The only difference between the method for preparing silver powder with coexistence of submicron flakes and nanoparticles described in this comparative example 3 and that in Example 2 is that in step (3), the capping agent glutaric acid is not added.

[0089] like Figure 10 As shown, the silver powders prepared in this comparative example are all spherical nano silver powders, and the particle size of the spherical silver powders is 50-150 nm.

[0090] Comparative Example 4 The only difference between the method for preparing silver powder with coexistence of submicron flakes and nanoparticles described in this comparative example 4 and that in Example 2 is that in step (1), the dispersant gelatin is not added.

[0091] like Figure 11 As shown, the silver powder prepared in this comparative example is composed of irregularly structured particles and some flaky silver powder, the silver powder is severely agglomerated, and the particle size of the granular silver powder is 50 to 250 nm.

[0092] Comparative Example 5 The only difference between the method for preparing the silver powder with coexistence of submicron flakes and nanoparticles described in Comparative Example 5 and Example 2 is that gelatin is replaced with PVP of the same mass.

[0093] like Figure 12 As shown, the silver powder prepared in this comparative example consists of silver powder with a quasi-spherical structure, and the particle size of the spherical silver powder is 0.05-3.5 μm.

[0094] Comparative Example 6 The only difference between the method for preparing silver powder with coexistence of submicron flakes and nanoparticles described in this comparative example 6 and that in Example 2 is that the two-stage reactions in steps (5) and (6) are both carried out at room temperature (20-35°C).

[0095] like Figure 13 As shown, the silver powder prepared in this comparative example is composed of silver particles with irregular structures. The particle size of the silver particles is 0.1 to 2.5 μm, and the silver particles are severely agglomerated.

[0096] From Example 1 (second stage reaction temperature 50 ° C, attached Figure 2 )、Example 2(Second stage reaction temperature 45℃,Attachment Figure 3 )、Example 3(Second stage reaction temperature 40℃,Attachment Figure 4 ) and Example 4 (second stage reaction temperature 35 ° C, attached Figure 5) It can be seen that as the reaction temperature of the second stage decreases from 50°C to 35°C, the yield of flaky silver powder in the prepared samples gradually increases and the size gradually increases. This shows that the control of the reaction temperature in the second stage of the present invention is crucial to the yield and size control of flaky silver powder. This is because in the second stage of the reaction, the mixed solution of the first stage is placed at a relatively low specific temperature, and then the reducing agent is added, which is conducive to the directional deposition of silver atoms on the (100) crystal plane of the planar twin, thereby promoting the growth of silver submicron flakes. More importantly, the dispersant selected in the present invention, in addition to having weak reducing and dispersing effects, can increase its solution viscosity as the temperature decreases in the dissolved and dispersed state. Therefore, by regulating the temperature, the diffusion rate of free silver ions to the surface of the planar twin seed can be controlled, thereby promoting the directional deposition of the remaining silver atoms on the (100) crystal plane of the planar twin, rather than rapid independent aggregation and growth into new seed crystals. Therefore, the yield and size of silver submicron flakes can be controlled by regulating the gradient temperature.

[0097] From Example 2 (second stage reaction temperature 45°C, attached Figure 3 )、Comparative Example 1 (second stage reaction temperature 55 ° C, attached Figure 8 ) and Comparative Example 2 (second stage reaction temperature 65 ° C, attached Figure 9 ) It can be seen that when the reaction temperature of the second stage is the same as that of the first stage, the flake rate of the prepared silver powder is only 10%. When the reaction temperature of the second stage is increased to 65°C, the prepared silver powder is composed of quasi-spherical silver powder and a small amount of irregular and very thin flaky silver powder. The silver powder is partially agglomerated, and the particle size of the quasi-spherical nano-silver powder is 20-70 nm. This shows that if the reaction temperature of the second reaction stage is the same as or higher than that of the first reaction stage, it is impossible to prepare a silver powder sample that meets the requirements. This is because too high a temperature in the second reaction stage will lead to an accelerated reaction rate and, more importantly, reduce the role of dispersion in regulating viscosity in the solvent. As a result, the prepared silver powder consists of a small amount of flaky silver powder and a large amount of nano-spherical particles.

[0098] By Example 2 (Attached Figure 3 ) and Comparative Example 3 (Attached Figure 10 ) By comparison, it can be seen that when no directional capping agent is added to the reaction system, the prepared silver powder sample is basically spherical nanoparticles. This shows that the addition of the directional capping agent in the present invention is crucial to the formation of submicron flaky silver powder, and the silver powder flake rate can be effectively controlled by regulating the type and amount of the directional capping agent. The main reason is that the carboxylate in the directional capping agent can be adsorbed on the (111) crystal plane of the silver twin to form surface potential and steric hindrance, promoting the adsorption and aggregation of free silver ions toward the (100) crystal plane of the twin, which is beneficial to the formation of planar twin seeds and ultimately promotes the formation of flaky silver powder. Therefore, the present invention can achieve controllable silver powder flake rate by the type and amount of the directional capping agent.

[0099] By Example 2 (Attached Figure 3 ) and Comparative Example 4 (Attached Figure 11 ) By comparison, it can be seen that when no dispersant is added to the reaction system, the prepared silver powder is composed of silver powder with irregular structure and some flaky silver powder. The silver powder agglomeration is relatively serious, and the particle size of the spherical silver powder is 50 to 250 nm. This shows that the addition of the dispersant selected in the present technical solution is crucial to the dispersibility and flake rate of the silver powder. The main reasons include: First, the attachment of the dispersant used in the present invention to the silver surface causes electrostatic repulsion and steric hindrance effect between the particles, which can effectively improve the dispersibility of the silver powder. Secondly, dispersants such as gelatin contain polar carboxyl and amino groups in their structure. The amino group has weak reducing properties. The weak reducing effect of gelatin can induce the formation of twin seeds, which is conducive to the formation of planar twin seeds. More importantly, gelatin can effectively control the viscosity of the solvent, thereby controlling the diffusion rate and reduction rate of silver ions.

[0100] By Example 2 (Attached Figure 3 ) and Comparative Example 5 (Attached Figure 12 ) By comparison, it can be seen that when gelatin is replaced with polyvinyl pyrrolidone (PVP) of the same mass in the reaction system, the prepared silver powder is composed of silver powder with a quasi-spherical structure, and its particle size is 0.05 to 3.5 μm, and no silver powder with a flaky structure appears in the product. This shows that the type of dispersant selected in the present technical solution is crucial to the structure and morphology of the prepared silver powder. The main reason is that although PVP has the effect of improving the dispersibility of silver powder, it cannot achieve efficient control of the diffusion rate and reduction rate of silver ions, and ultimately can only synthesize quasi-spherical silver particles. In the present invention, the control effect of the selected dispersant on the viscosity of the solvent is crucial to the morphology control of the synthesized silver powder.

[0101] By Example 2 (Attached Figure 3 ) and Comparative Example 6 (Appendix Figure 13 ) By comparison, it can be seen that when the two-stage reaction in steps (5) and (6) is carried out at room temperature, the prepared silver powder is composed of silver particles with irregular structure, and the silver particles are seriously agglomerated. The main reason is that the dissolution temperature of the dispersant selected in this specification is within the range of 55-65°C for complete dissolution and stable existence. If the first stage of step (5) is carried out at room temperature, the dispersant will exist in a partially dissolved state, which will result in the weak reducing property of the dispersant disappearing or greatly weakening in the first stage reaction, which is not conducive to the formation of silver planar twin crystal seeds. In addition, the partially dissolved dispersant is not conducive to the uniform distribution of silver ions in the solvent, and cannot play a good dispersing role. Therefore, the synthesized silver powder is seriously agglomerated.

[0102] In summary, in the present invention, the directional capping agent, dispersant, reaction temperature and various reaction parameters are crucial to the preparation of silver powder with coexistence of submicron flakes and nanoparticles, and the above technical parameters need to be coordinated and controlled to achieve the controllable preparation of the target silver powder.

[0103] While various embodiments of the present invention have been described above, the foregoing description is intended to be illustrative, non-exhaustive, and not limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is selected to best explain the principles of the embodiments, their practical applications, or improvements to existing technologies, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A method for preparing silver powder containing coexisting submicron flakes and nanoparticles, characterized in that: The steps include: (1) The dispersant solution is kept at a specific temperature, and then its pH value is adjusted; then, a silver nitrate solution is added and kept warm, and then a directional capping agent solution is added and kept warm to obtain a mixed solution; (2) lowering the temperature of the mixed solution and keeping it warm for a period of time, then adding a reducing agent solution and keeping it warm for a period of time to obtain a silver powder stock solution in which submicron flakes and nanoparticles coexist; (3) The silver powder stock solution containing submicron flakes and nanoparticles is centrifuged, washed, and dried, and then a specific organic solvent is added for redispersion to obtain a silver powder paste containing submicron flakes and nanoparticles.

2. The method for preparing the silver powder containing submicron flakes and nanoparticles according to claim 1, characterized in that: The dispersant is at least one of gelatin, gum arabic, pectin, fish glue, bone glue, gelatin, peach gum, carrageenan, xanthan gum, gellan gum, guar gum, agar, locust bean gum, and konjac gum.

3. The method for preparing the silver powder containing coexistence of submicron flakes and nanoparticles according to claim 1, characterized in that: The directional end-capping agent is at least one of glutaric acid, malic acid, adipic acid, pimelic acid, and pyrenetetracarboxylic acid.

4. The method for preparing the silver powder containing coexistence of submicron flakes and nanoparticles according to claim 1, characterized in that: The reducing agent is one of ascorbic acid, hydrogen peroxide and formaldehyde.

5. The method for preparing the silver powder containing coexisting submicron flakes and nanoparticles according to claim 1, characterized in that: The mass concentration of the dispersant in the dispersant solution is 5-40 g / L; the mass concentration of silver nitrate in the silver nitrate solution is 50-200 g / L; the mass concentration of the directional capping agent in the directional capping agent solution is 10-50 g / L; the mass concentration of the reducing agent in the reducing agent solution is 50-200 g / L; the volume fraction ratio of the dispersant solution, the silver nitrate solution, the directional capping agent solution and the reducing agent solution is 40%-50%: 10%-20%: 10%-20%: 20%-30%, and the sum of the volume fractions of the dispersant solution, the silver nitrate solution, the directional capping agent solution and the reducing agent solution is 100%.

6. The method for preparing the silver powder containing coexisting submicron flakes and nanoparticles according to claim 1, characterized in that: The step (1) is specifically as follows: the dispersant solution is kept at 55-65° C. for 10-30 min, the pH value thereof is adjusted to 1-3, the silver nitrate solution is added and kept warm for 5-20 min, and the directional capping agent solution is added and kept warm for 5-20 min to obtain a mixed solution.

7. The method for preparing the silver powder containing coexistence of submicron flakes and nanoparticles according to claim 1, characterized in that: The step (2) is specifically as follows: keeping the mixed solution at 35-50°C for 10-20 minutes, then adding a reducing agent solution and keeping it warm for 10-20 minutes, thereby obtaining a silver powder stock solution in which submicron flakes and nanoparticles coexist.

8. The method for preparing the silver powder containing coexistence of submicron flakes and nanoparticles according to claim 1, characterized in that: The specific organic solvent is an ester, ether or alcohol organic solvent.

9. The silver powder containing submicron flakes and nanoparticles prepared by the method for preparing silver powder containing submicron flakes and nanoparticles according to any one of claims 1 to 8, characterized in that: The content of submicron flakes is 30-90%, the diameter of submicron flakes ranges from 0.1 to 1.5 μm, the thickness of flakes is 30-120 nm, and the particle size of nanoparticles is 30-150 nm.

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