A method for preparing silver powder

Abstract

The present invention relates to a method for preparing silver powder, comprising adding a portion of a silver nitrate solution to a reducing solution at a rate of 200 to 500 ml / min and continuing to stir for 30 to 60 minutes; adding the remaining silver nitrate solution to the reducing solution at a rate of 50 to 200 ml / min and continuing to stir for 60 to 120 minutes, performing solid-liquid separation, washing, and drying to obtain silver powder; wherein the mass of the dispersant in the reducing solution is 5 to 15% of the mass of the silver nitrate in the added silver nitrate solution, and the mass of the surfactant in the reducing solution is 0.3 to 0.5% of the mass of the silver nitrate in the added silver nitrate solution. The silver powder obtained by the preparation method of the present invention has a high tap density and high sphericity, an average particle size of about 1.50 μm, and a tap density greater than 5.40 g / cm 3 ; With the help of dispersants and surfactants acting synergistically on the surface of silver powder crystal cores, the amount of dispersants used is effectively reduced, which also helps to save costs and reduce the burden of wastewater treatment.

Description

Technical Field

[0001] The invention relates to a method for preparing silver powder, in particular to a method for preparing silver powder with high tap density, and belongs to the field of powder preparation. Background Art

[0002] Currently, crystalline silicon solar cells are the most widely used solar cells. Their basic components are conductive silver paste, tempered glass, an anti-reflective coating, and a solar panel chip. Conductive silver paste is a crucial component of solar cells. After screen printing and high-temperature sintering, the paste serves as the medium for connecting the conductive electrodes to external circuits. Its performance directly impacts the conductivity and cost per kilowatt-hour of the solar cell. The main components of conductive silver paste are silver powder, an organic vehicle, and glass powder. Key technical indicators such as the conductivity and density of the resulting conductive silver paste film are primarily determined by the properties of the fine silver powder, which in turn depends on its morphology, tap density, particle size distribution, and burn-in loss rate. Studies have shown that if the silver powder particle size is too large, the paste will not fully pass through the screen during printing, resulting in a short sintering time and the formation of voids in the sintered film, which affects conductivity. If the silver powder particle size is too small, the paste will not fully wet the organic vehicle, resulting in poor screen printing results, large shrinkage of the silver film after sintering, numerous voids, and loose connections. Spherical silver powder with uniform particle size and high tap density can overcome the above shortcomings. The average particle size of silver powder usually required is about 1.50μm, the maximum particle size is less than 5.0μm, and the tap density is higher than 5.40g / cm 3 The conductive silver paste prepared with it has the advantages of high photoelectric conversion efficiency and stable conductive performance.

[0003] At present, the liquid phase reduction method is the main method for preparing spherical silver powder with high tap density. Usually, silver nitrate is used as the raw material, and formaldehyde, ascorbic acid, hydrazine hydrate, sodium borohydride, etc. are used as reducing agents to prepare silver powder. In addition, the type and amount of dispersant are also important parameters that need to be controlled in the preparation of spherical silver powder. When preparing silver powder, some researchers have explored the use of multiple dispersants to coordinately control various indicators such as the morphology, particle size, and tap density of the prepared silver powder. For example, by selecting PVP and Tween-80 as the binary dispersion system and controlling the pH value of the reaction system to 5, it is possible to prepare an average particle size of 1.25μm and a tap density of 4.6g / cm 3 Spherical silver powder (Gan Weiping, Lin Tao, Liu Xiaogang, et al. Weapon Materials Science and Engineering [J], 2014, 37(2): 50-54.).

[0004] Chinese invention patent application specification CN110434355A discloses a method for preparing spherical silver powder with high tap density and high dispersibility, comprising the following steps: (1) Preparation of silver nuclei: Mixing a first dispersant solution and a first reducing agent solution, stirring and heating to boiling, quickly adding a first silver nitrate solution, stirring and reacting for 30 to 60 minutes to obtain a silver nucleus solution; (2) Growth of nuclei: Adding a second dispersant solution and a second reducing agent solution to the silver nucleus solution prepared in step (1), stirring. Heating to 25 to 65°C, and adjusting the pH value to 2.5 to 5.5, adding a second silver nitrate solution, and then adding a coating agent, stirring for 20 to 40 minutes, solid-liquid separation, and obtaining the solid, thereby obtaining the silver powder. The silver powder prepared by the above method has a high tap density, but it requires the use of two types of dispersants, and the amount of dispersant required is large, which increases the difficulty of operation, cost, and wastewater treatment burden. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the object of the present invention is to provide a method for preparing silver powder with high tap density and high sphericity.

[0006] In order to solve the above technical problems, the technical solutions of the present invention are as follows:

[0007] A method for preparing silver powder comprises the following steps:

[0008] S1. Provide a silver nitrate solution with a temperature of 40-60°C and a concentration of 1.5-2.5 mol / L;

[0009] Providing a reducing solution with a temperature of 40-60°C and a reducing agent concentration of 0.5-1.5 mol / L;

[0010] The reducing agent is composed of ascorbic acid and rhamnose in a mass ratio of 25-50:1; the reducing solution contains a dispersant and a surfactant, the dispersant is composed of PVP, gum arabic, and Tween 80 in a mass ratio of 1:0.6-0.9:0.15-0.25, and the surfactant is composed of sodium lauryl sulfate and sodium hexametaphosphate in a mass ratio of 1:0.3-0.5; the pH value of the reducing solution is 2.5-4.0;

[0011] S2. Stirring the reducing solution at a rate of 400-800 rpm, adding part of the silver nitrate solution to the reducing solution at a rate of 200-500 ml / min, and continuing to stir for 30-60 min; adding the remaining silver nitrate solution to the reducing solution at a rate of 50-200 ml / min, and continuing to stir for 60-120 min, separating the solid and liquid, washing, and drying to obtain silver powder;

[0012] The molar amount of the reducing agent in the reducing solution is 0.55-0.65 times the molar amount of silver nitrate in the added silver nitrate solution; the mass of the dispersant in the reducing solution is 5-15% of the mass of the silver nitrate in the added silver nitrate solution; and the mass of the surfactant in the reducing solution is 0.3-0.5% of the mass of the silver nitrate in the added silver nitrate solution.

[0013] Optionally, the silver nitrate solution is prepared by weighing solid silver nitrate and adding it into pure water to prepare a silver nitrate solution with a concentration of 1.5 to 2.5 mol / L, and raising the temperature to 40 to 60°C.

[0014] Optionally, the reducing solution is prepared by adding ascorbic acid and rhamnose to pure water, stirring until the ascorbic acid and rhamnose are dissolved, and then preparing a reducing agent concentration of 0.5 to 1.5 mol / L. A dispersant and a surfactant are then added, stirring is continued until the solution becomes clear, and the temperature is raised to 40 to 60°C. The pH of the reducing solution is adjusted to 2.5 to 4.0 until the pH remains stable for at least 5 minutes. Optionally, a 35-45 g / L sodium hydroxide solution is used to adjust the pH.

[0015] Furthermore, the reducing agent is composed of ascorbic acid and rhamnose in a mass ratio of 30-45:1.

[0016] Furthermore, the dispersant is composed of PVP, gum arabic, and Tween 80 in a mass ratio of 1:0.6-0.9:0.15-0.25, and the surfactant is composed of sodium lauryl sulfate and sodium hexametaphosphate in a mass ratio of 1:0.3-0.5.

[0017] Furthermore, during stirring in S2, nitrogen gas is bubbled into the reducing solution at a partial pressure of 0.05 to 0.2 MPa and a flow rate of 200 to 400 ml / min. The applicants have discovered through in-depth research that bubbling nitrogen gas at a specific partial pressure and flow rate, in conjunction with mechanical stirring, creates a multi-dimensional disturbance, which causes the fine-grained silver powder to float in the reactor, promoting its growth and further improving the uniformity and sphericity of the silver powder particle size, resulting in a narrower particle size distribution.

[0018] Furthermore, in S2, the portion of silver nitrate solution accounts for 0.5-2% of the total volume of the added silver nitrate solution. The applicant has repeatedly discovered that adding a portion of the silver nitrate solution first and controlling the amount added to form an appropriate amount of regularly shaped silver nuclei before adding the remaining silver nitrate solution allows the silver nuclei to grow, which is beneficial for obtaining silver powder with better particle size uniformity and higher sphericity.

[0019] Furthermore, in S2, the reducing solution is stirred at a rate of 500-700 rpm, and part of the silver nitrate solution is added to the reducing solution at a rate of 300-400 ml / min, and the stirring is continued for 40-50 minutes; the remaining silver nitrate solution is added to the reducing solution at a rate of 80-180 ml / min, and the stirring is continued for 80-100 minutes, and the solid-liquid separation is carried out, and the silver powder is washed and dried to obtain silver powder.

[0020] Furthermore, in S2, during washing, the sample is first washed with pure water for 2-4 times, and then washed with ethanol until the conductivity of the washing solution is reduced to 10-20 μs / cm.

[0021] Furthermore, after S2, the method further includes: airflow-grinding the silver powder, mixing the mixture with anhydrous ethanol and a surface modifier in a ratio of 1 g: 1-3 ml: 0.001-0.003 g, stirring the mixture at a rate of 300-600 rpm for 1-2 hours, and drying the mixture to obtain modified silver powder;

[0022] Wherein, the surface modifier is at least one of stearic acid and isostearic acid.

[0023] Furthermore, the silver powder obtained in S2 is added to a jet mill at a speed of 10 to 30 g / min for jet milling; wherein the jet milling pressure is controlled to be 0.6 to 0.8 MPa.

[0024] Further, drying is carried out at 50-70°C for 6-8 hours.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The silver powder obtained by the preparation method of the present invention has high tap density and high sphericity, an average particle size of about 1.50 μm, and a tap density higher than 5.40 g / cm 3 The synergistic effect of dispersants and surfactants on the surface of the silver powder nuclei effectively reduces dispersant usage, saving costs and alleviating the burden of wastewater treatment. A combination of nitrogen stirring and mechanical agitation intensifies the reaction process, facilitating the floating of fine-grained silver powder. This allows the silver produced by the contact reaction between the reducing agent and silver nitrate to preferentially grow on the fine-grained silver powder, resulting in a more uniform silver powder particle size, a narrower particle size distribution, and a higher sphericity. Airflow pulverization and surface modification further increase the tap density of the spherical silver powder. The resulting silver powder has promising application prospects in fields such as solar cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a scanning electron microscope image of the silver powder prepared in Example 1 of the present invention, with a magnification of 5000 times.

[0028] Figure 2 This is a scanning electron microscope image of the silver powder prepared in Example 2 of the present invention, with a magnification of 5000 times. DETAILED DESCRIPTION

[0029] The present invention will be described in detail below with reference to the following examples. It should be noted that, unless conflicting, the embodiments and features of the embodiments may be combined. In the examples and comparative examples, the silver nitrate, sodium hydroxide, ascorbic acid, rhamnose, and ethanol used were all analytically pure reagents; the nitrogen used had a purity of no less than 99.99%.

[0030] Example 1

[0031] The preparation process of the high tap density spherical silver powder of this embodiment is as follows:

[0032] (1) First, add 34 g of silver nitrate to a beaker containing 100 ml of pure water. After stirring thoroughly to dissolve, place the beaker in a water bath and heat it to 50°C to obtain a silver nitrate solution. Then, add 21.14 g of ascorbic acid and 0.50 g of rhamnose to a reactor containing 200 ml of pure water. Set the stirrer speed to 400 rpm. After the ascorbic acid and rhamnose are completely dissolved, add 1.8 g of PVP (polyvinyl pyrrolidone), 1.27 g of gum arabic, 0.33 g of Tween 80, 0.1 g of sodium lauryl sulfate, and 0.039 g of sodium hexametaphosphate. Continue stirring until the solution is clear. Then, raise the reactor temperature to 50°C and adjust the pH of the reduction solution to 2.75 using 40 g / l of sodium hydroxide solution. Stir for more than 5 minutes.

[0033] (2) The reducing solution was then stirred at a rate of 600 rpm, and nitrogen was bubbled into the reducing solution from the inner bottom of the reaction vessel at a partial pressure of 0.1 MPa and a flow rate of 300 ml / min. Part of the silver nitrate solution was added to the reducing solution at a rate of 300 ml / min, and the stirring was continued for 60 minutes; the remaining silver nitrate solution was added to the reducing solution at a rate of 100 ml / min, and the stirring was continued for 90 minutes. The solution was filtered, washed twice with pure water and then with ethanol. After the conductivity of the washing solution was reduced to 10-20 μs / cm, the silver powder was placed in a drying oven at 60°C and dried for 8 hours to obtain spherical silver powder.

[0034] Wherein, the part of silver nitrate solution is 1% of the total volume of the added silver nitrate solution.

[0035] The silver powder obtained after drying was added to a jet mill at a speed of 10 g / min, and the air flow crushing pressure was controlled to 0.65 MPa. After the crushing was completed, the mass of the silver powder was weighed to be 20 g. The silver powder was mixed with ethanol at a solid-liquid ratio of 1 g:1 ml, and then 0.02 g of isostearic acid was added thereto. The stirring speed was controlled to 400 rpm. After stirring for 2 hours, the modified silver powder was placed in a drying oven at 60°C and dried for 8 hours to obtain spherical silver powder with high tap density.

[0036] Example 2

[0037] The preparation process of the high tap density spherical silver powder of this embodiment is as follows:

[0038] First, add 102g of silver nitrate to a beaker filled with 300ml of pure water. After thorough stirring to dissolve, place the beaker in a water bath and heat to 55°C. Then, add 63.40g of ascorbic acid and 2.0g of rhamnose to a reactor filled with 399ml of pure water. Set the stirrer speed to 400rpm. After the ascorbic acid and rhamnose are completely dissolved, add 7.65g of PVP, 6.12g of gum arabic, 1.53g of Tween 80, 0.33g of sodium lauryl sulfate, and 0.15g of sodium hexametaphosphate. Continue stirring until the solution is clear. Then raise the reactor temperature to 55°C and adjust the pH of the reduction solution to 2.5 using 40g / l sodium hydroxide solution. Stir for at least 5 minutes.

[0039] Subsequently, the reducing solution was stirred at a rate of 600 rpm, and nitrogen was bubbled into the reducing solution from the inner bottom of the reaction vessel at a partial pressure of 0.1 MPa and a flow rate of 300 ml / min. Part of the silver nitrate solution was added to the reducing solution at a rate of 300 ml / min, and stirring was continued for 50 minutes; the remaining silver nitrate solution was added to the reducing solution at a rate of 100 ml / min, and stirring was continued for 120 minutes. The solution was filtered and washed twice with pure water and then with ethanol. After the conductivity of the washing solution was reduced to 10-20 μs / cm, the silver powder was placed in a drying oven at 60°C and dried for 8 hours to obtain spherical silver powder.

[0040] Wherein, the portion of silver nitrate solution is 0.5% of the total volume of the added silver nitrate solution.

[0041] The silver powder obtained after drying was added to a jet mill at a speed of 20 g / min, and the air flow crushing pressure was controlled to be 0.6 MPa. After the crushing was completed, the mass of the silver powder was weighed to be 60 g. The silver powder was mixed with ethanol at a solid-liquid ratio of 1 g:1 ml, and then 0.09 g of isostearic acid was added thereto. The stirring speed was controlled to 400 rpm. After stirring for 2 hours, the modified silver powder was placed in a drying oven at 60°C and dried for 8 hours to obtain spherical silver powder with high tap density.

[0042] Comparative Example 1

[0043] Example 1 was repeated, except that all the silver nitrate solution was added to the reducing solution at a rate of 300 ml / min, stirring was continued for 150 min, and the solution was filtered and washed twice with pure water and then with ethanol. After the conductivity of the washing solution decreased to 10-20 μs / cm, the silver powder was placed in a drying oven at 60°C for 8 h to obtain spherical silver powder.

[0044] Comparative Example 2

[0045] Example 1 was repeated, except that the portion of silver nitrate solution was 5% of the total volume of the added silver nitrate solution.

[0046] Comparative Example 3

[0047] Example 1 was repeated, with the only difference that the reducing agent consisted of only 21.64 g of ascorbic acid.

[0048] Comparative Example 4

[0049] Example 1 was repeated, except that the reducing agent consisted of ascorbic acid and rhamnose in a ratio of 21.29 g:0.35 g.

[0050] Comparative Example 5

[0051] Example 1 was repeated, except that no surfactant was added to the ascorbic acid reducing solution.

[0052] Comparative Example 6

[0053] Example 1 was repeated, except that the pH of the ascorbic acid reducing solution was adjusted to 4.5 with sodium hydroxide solution.

[0054] Comparative Example 7

[0055] Example 1 was repeated, except that nitrogen was not bubbled in step (2).

[0056] Table 1 shows the particle size distribution and tap density test results of the silver powders described in Example 1, Example 2, and Comparative Examples 1 to 7. As can be seen from the figure, controlling the addition method of the silver nitrate solution, the composition of the reducing agent, and the nitrogen bubbling helps to obtain silver powders with more uniform and finer particle sizes and higher tap densities.

[0057] Table 1 Test results of silver powder particle size distribution and tap density in Example 1, Example 2 and Comparative Examples 1 to 7

[0058]

[0059]

[0060] Among them, the contents explained in the above embodiments should be understood as these embodiments are only used to more clearly illustrate the present invention, and are not used to limit the scope of the present invention. After reading the present invention, various equivalent forms of modifications to the present invention made by those skilled in the art fall within the scope defined by the claims attached to this application.

Claims

1. A method for preparing silver powder, characterized in that: The steps include: S1. Provide a silver nitrate solution with a temperature of 40-60°C and a concentration of 1.5-2.5 mol / L; Provide a reducing solution with a temperature of 40-60°C and a reducing agent concentration of 0.5-1.5 mol / L; The reducing agent is composed of ascorbic acid and rhamnose in a mass ratio of 25-50:1; the reducing solution contains a dispersant and a surfactant, the dispersant is composed of PVP polyvinyl pyrrolidone, gum arabic, and Tween 80 in a mass ratio of 1:0.6-0.9:0.15-0.25, and the surfactant is composed of sodium lauryl sulfate and sodium hexametaphosphate in a mass ratio of 1:0.3-0.5; the pH value of the reducing solution is 2.5-4.0; S2. Stirring the reducing solution at a rate of 400-800 rpm, adding part of the silver nitrate solution to the reducing solution at a rate of 200-500 ml / min, and continuing to stir for 30-60 min; adding the remaining silver nitrate solution to the reducing solution at a rate of 50-200 ml / min, and continuing to stir for 60-120 min, separating the solid and liquid, washing, and drying to obtain silver powder; The molar amount of the reducing agent in the reducing solution is 0.55-0.65 times the molar amount of silver nitrate in the added silver nitrate solution; the mass of the dispersant in the reducing solution is 5-15% of the mass of the silver nitrate in the added silver nitrate solution; and the mass of the surfactant in the reducing solution is 0.3-0.5% of the mass of the silver nitrate in the added silver nitrate solution.

2. The preparation method according to claim 1, characterized in that The reducing agent consists of ascorbic acid and rhamnose in a mass ratio of 30-45:

1.

3. The preparation method according to claim 1, characterized in that In S2 , during stirring, nitrogen gas is bubbled into the reducing solution at 0.05-0.2 MPa and a flow rate of 200-400 ml / min.

4. The preparation method according to claim 1, characterized in that In S2, the portion of silver nitrate solution is 0.5-2.0% of the total volume of the added silver nitrate solution.

5. The preparation method according to claim 1, characterized in that In S2, during washing, first wash with pure water 2-4 times, and then continue washing with ethanol until the conductivity of the washing solution is reduced to 10-20 μs / cm.

6. The preparation method according to any one of claims 1 to 5, characterized in that After S2, the method further includes: performing air flow pulverization on the silver powder, and then mixing the air flow pulverized silver powder with anhydrous ethanol and a surface modifier in a ratio of 1g:1~3ml:0.001~0.003g, stirring at a rate of 300-600rpm for 1-2h, and drying to obtain modified silver powder; wherein the surface modifier is at least one of stearic acid and isostearic acid.

7. The preparation method according to claim 6, characterized in that The silver powder obtained in S2 is added to the air flow mill at a speed of 10-30 g / min for air flow pulverization; wherein the air flow pulverization pressure is controlled to be 0.6-0.8 MPa.

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