Method for preparing flaky silver powder for low-temperature curing silver paste

Abstract

The present invention discloses a method for preparing flaky silver powder for low-temperature curing silver paste, and relates to the technical field of metal powder metallurgy. In the present invention, a dispersant is added when preparing a silver nitrate solution, and the speed at which the silver nitrate solution is dripped into a reducing agent solution is controlled. After the reaction is completed, a surfactant is added, and spherical silver powder is obtained after washing and drying. The spherical silver powder is then ground together with zirconium oxide grinding balls, and sieving is performed after the grinding is completed to obtain flaky silver powder. The present invention prepares spherical silver powder of different particle sizes and good particle size uniformity by regulating the reaction conditions, and adds a surfactant after synthesis to make the silver powder particles in a monodisperse state, and dry-ball mills the silver powder after drying. The surfactant free between the silver powder particles can be used as a grinding aid in the dry ball milling process to prevent the flake powder from sticking together. At the same time, a secondary coating is performed on the newly exposed surface of the flake powder to make the modification and coating of the silver flake powder more uniform.

Description

Technical Field

[0001] The present invention relates to the technical field of metal powder metallurgy, and in particular to a method for preparing flaky silver powder, and more specifically to a method for preparing flaky silver powder for low-temperature curing silver paste in the photovoltaic solar energy field. Background Art

[0002] Battery electrodes made from conductive silver paste play a crucial role in the photoelectric conversion efficiency of solar cells. Limited by the low-temperature silver paste curing process, the conductive function of low-temperature curing conductive silver paste relies primarily on the free carriers provided by the overlapping bonds between silver particles of specific morphology and size. Compared to the point-to-point contact of spherical silver powder, flake silver powders contact each other in line-to-face or face-to-face contact, resulting in a larger contact area, thus providing better conductivity for solar cell electrodes, especially those cured at low temperatures.

[0003] Currently, the main methods for preparing flaky silver powder are chemical reduction and mechanical ball milling. Chemical reduction methods include liquid-phase reduction, seed crystallization, template processing, photoinduction, and microwave radiation. While chemical reduction produces uniform flaky silver powder without further contamination from mechanical processing, this method is difficult to control, exhibits poor stability, and produces relatively thin flaky silver powder, which cannot meet product requirements. Currently, its application is limited to the laboratory stage.

[0004] Currently, the industry primarily uses mechanical ball milling to produce flaky silver powder. Mechanical ball milling, which can be divided into dry and wet ball milling, is characterized by high flatness, high yield, and ease of mass production. However, mechanical ball milling has many control factors, making it difficult to achieve consistent parameter consistency between batches of flaky silver powder produced using the same method. Furthermore, cold welding can lead to a low flake rate, wide particle size distribution, and low tap density. This results in poor flatness when printing conductive silver paste, uneven printed circuits, and increased silver electrode resistance. Summary of the Invention

[0005] In order to overcome the defects and shortcomings in the above-mentioned prior art, the present invention provides a method for preparing flaky silver powder for low-temperature curing silver paste, so as to prepare flaky silver powder with high flake rate, narrow particle size distribution and high tap density. The core of the present invention is to prepare spherical silver powder of different particle sizes and good particle size uniformity by regulating the reaction conditions, and add a surfactant after synthesis to make the silver powder particles monodisperse, and dry-ball mill the silver powder after drying. Among them, the surfactant free between the silver powder particles can be used as a grinding aid in the dry ball milling process to prevent the flake powder from sticking together, and at the same time, a secondary coating is performed on the newly exposed surface of the flake powder to make the modification and coating of the silver flake powder more uniform. This method overcomes the shortcomings of the traditional ball milling method, especially the wet ball milling method, in preparing flaky silver powder with a wide size distribution and insufficient degree of flakeness.

[0006] In order to solve the above problems in the prior art, the present invention is implemented through the following technical solutions.

[0007] The present invention provides a method for preparing flaky silver powder for low-temperature curing silver paste, the preparation method comprising the following steps:

[0008] S1. Add silver nitrate to deionized water, the concentration of the silver nitrate solution is 170 g / L; stir evenly and then add a dispersant; stir and dissolve at a constant temperature to obtain a silver nitrate solution;

[0009] S2. Add the reducing agent to deionized water, stir and dissolve at a constant temperature to obtain a reducing agent solution with a concentration of 97 g / L;

[0010] S3, adding the silver nitrate solution obtained in step S1 dropwise to the reducing agent solution obtained in step S2 at a dropping rate of 100-300 ml / min; the molar ratio of silver nitrate to reducing agent is 1:0.55;

[0011] S4, adding a surfactant to the mixed slurry of the silver nitrate solution and the reducing agent solution in step S3;

[0012] S5, washing the mixed slurry with the surfactant added in step S4, wherein the washing is performed by washing the mixed slurry with deionized water until the conductivity is below 100 μS / cm; and drying after washing to obtain spherical silver powder;

[0013] S6. Place the spherical silver powder obtained in step S5 and zirconia grinding balls in a planetary ball mill for ball milling, wherein the mass ratio of the spherical silver powder to the zirconia grinding balls is 1:3-9; the diameter of the zirconia grinding balls is 0.3-5.0 mm; the ball milling frequency is 15-45 Hz, and the ball milling time is 1-6 hours;

[0014] S7. After the ball milling is completed, the flaky silver powder and the zirconia grinding balls are sieved and separated to obtain flaky silver powder.

[0015] Further preferably, the dispersant is gum arabic or methyl cellulose, and the amount used is 0.5% to 1% of the mass of the silver powder.

[0016] More preferably, the reducing agent is ascorbic acid.

[0017] More preferably, the surfactant is any one of oleic acid and Tween-80 or a combination of the two, and the total amount used is 0.5% of the mass of the silver powder.

[0018] More preferably, in step S1 and step S2, the temperature for constant temperature stirring and dissolving is 40°C.

[0019] More preferably, in step S5, the mixed slurry to which the surfactant is added in step S4 is washed, and the drying temperature used for drying after washing is 70° C. and the drying time is 18 h.

[0020] In step S7, when the flaky silver powder and the zirconium oxide grinding balls are sieved and separated, the mesh size of the sieve used is 200 mesh.

[0021] Compared with the prior art, the beneficial technical effects brought about by the present invention are as follows:

[0022] The core of the present invention is to prepare spherical silver powders of varying particle sizes and good particle size uniformity by regulating reaction conditions. A surfactant is added after synthesis to render the silver powder particles monodisperse. After drying, the silver powder is dry-milled. The surfactant, free between the silver powder particles, acts as a grinding aid during the dry ball milling process, preventing flake adhesion and providing a secondary coating on the exposed flake surfaces, resulting in a more uniform modification and coating of the silver flakes.

[0023] 2. The present invention controls the amount of dispersant in step S1 and the reaction rate of the silver nitrate solution and the reducing agent solution in step S3, thereby preparing spherical silver powders of different particle sizes and narrow particle size distribution as required, thereby narrowing the particle size distribution of the final flaky silver powder.

[0024] 3. The present invention introduces a surfactant to make the silver powder particles monodisperse and improve the lubricity of the spherical silver powder and the grinding balls during the dry ball milling process, thereby avoiding the cold welding phenomenon and making the particle size distribution of the flaky silver powder narrower.

[0025] 4. The preparation method of flaky silver powder of the present invention overcomes the disadvantage of the wide particle size distribution of the commonly used industrial wet ball milling method, reduces the operation steps of washing the grinding balls and flaky silver powder, filtering, separating the ball powder and drying after ball milling, and saves time and cost.

[0026] 5. The present invention only adds the surfactant once, which overcomes the defect that the conventional ball milling method for preparing flaky silver powder requires adding the surfactant twice, resulting in a high burn-out rate and difficulty in applying to low-temperature curing silver paste.

[0027] 6. The particle size of the flaky silver powder prepared by the preparation method of the present invention is controllable and the distribution range is narrow, and the tap density is high, which can meet the use requirements of low-temperature curing silver paste for solar cell electrodes, and at the same time meet the silver powder requirements of different conductive silver paste application fields such as flexible display screens, 5G filter electrodes, and flexible circuits. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain examples of the present invention and should not be regarded as limiting the scope.

[0029] Figure 1 This is a SEM image of the monodisperse spherical silver powder prepared in the present invention;

[0030] Figure 2 This is the SEM image of the flaky silver powder with narrow particle size distribution prepared in the present invention. DETAILED DESCRIPTION

[0031] The technical solutions of the present invention are further described in detail below in conjunction with the accompanying drawings and specific embodiments. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. 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.

[0032] Example 1

[0033] This embodiment discloses a method for preparing flaky silver powder for low-temperature curing silver paste, the method comprising the following steps:

[0034] (1) Preparation of spherical silver powder

[0035] S1. Add 237.5 g of silver nitrate to 1397.5 g of deionized water, stir well, then add 1.5 g of gum arabic, stir well and maintain a constant temperature of 40°C;

[0036] S2. Add 145 g of ascorbic acid to 1485 g of deionized water, stir well, and maintain a constant temperature of 40°C.

[0037] S3, adding the silver nitrate solution dropwise to the ascorbic acid solution at a rate of 280 mL / min;

[0038] S4. After the reaction is completed, 0.37 g of oleic acid and 0.37 g of Tween-80 are added to the silver-containing slurry obtained in step S3, and the mixture is stirred for 20 min.

[0039] S5. Wash the silver-containing slurry obtained in step S4 with deionized water until the conductivity is below 100 μS / cm, and then dry it in a 70° C. oven for 18 hours to obtain spherical silver powder SS1.

[0040] (2) Preparation of flaky silver powder

[0041] S6. Put 70g of spherical silver powder SS1 and 420g of φ2mm zirconium oxide balls into a polytetrafluoroethylene ball milling jar, and place it in a planetary ball mill with the frequency set at 25Hz for 4h.

[0042] S7. Sieve the mixture of the grinding balls and flaky silver powder obtained in step S6 using a 200-mesh sieve to obtain flaky silver powder SF1.

[0043] Example 2

[0044] This embodiment discloses a method for preparing flaky silver powder for low-temperature curing silver paste, the method comprising the following steps:

[0045] (1) Preparation of spherical silver powder

[0046] The preparation method is the same as that of Example 1;

[0047] (2) Preparation of flaky silver powder

[0048] S6. Take 70g of the spherical silver powder SS1 obtained by the above method and 420g of φ3mm zirconium oxide balls, put them into a polytetrafluoroethylene ball mill, and put them into a planetary ball mill with the frequency set to 40Hz and ball milled for 4h;

[0049] S7. The obtained mixture of the grinding balls and the flaky silver powder is sieved with a 200-mesh sieve to obtain flaky silver powder SF2.

[0050] Example 3

[0051] This embodiment discloses a method for preparing flaky silver powder for low-temperature curing silver paste, the method comprising the following steps:

[0052] (1) Preparation of spherical silver powder

[0053] S1. Add 237.5 g of silver nitrate to 1397.5 g of deionized water, stir well, then add 0.75 g of gum arabic, stir well and keep the temperature at 40°C;

[0054] S2. Add 145 g of ascorbic acid to 1485 g of deionized water, stir well, and maintain a constant temperature of 40°C;

[0055] S3, adding the silver nitrate solution to the ascorbic acid solution at a rate of 140 mL / min;

[0056] S4. After the reaction is completed, 0.37 g of oleic acid and 0.37 g of Tween-80 are added to the silver-containing slurry obtained in step S3, and the mixture is stirred for 20 min.

[0057] S5. Wash the silver-containing slurry obtained in step S4 with deionized water until the conductivity is below 100 μS / cm, and then dry it in a 70° C. oven for 18 hours to obtain spherical silver powder SS2.

[0058] (2) Preparation of flaky silver powder

[0059] S6. Place 70 g of spherical silver powder SS2 and 420 g of φ2 mm zirconium oxide balls into a polytetrafluoroethylene ball milling jar and place them in a planetary ball mill at a frequency of 25 Hz for 4 h.

[0060] S7. Sieve the mixture of the grinding balls and flaky silver powder obtained in step S6 using a 200-mesh sieve to obtain flaky silver powder SF3.

[0061] Example 4

[0062] This embodiment discloses a method for preparing flaky silver powder for low-temperature curing silver paste, the method comprising the following steps:

[0063] (1) Preparation of spherical silver powder

[0064] The preparation method is the same as that of Example 3;

[0065] (2) Preparation of flaky silver powder

[0066] S6. Take 70g of the spherical silver powder SS2 obtained by the above method and 630g of φ3mm zirconium oxide balls, put them into a polytetrafluoroethylene ball mill, and put them into a planetary ball mill with the frequency set to 40Hz and ball mill for 4h;

[0067] S7. The obtained mixture of the grinding balls and the flaky silver powder is sieved with a 200-mesh sieve to obtain flaky silver powder SF4.

[0068] Example 5

[0069] This embodiment discloses a method for preparing flaky silver powder for low-temperature curing silver paste, the method comprising the following steps:

[0070] (1) Preparation of spherical silver powder

[0071] S1. Add 237.5 g of silver nitrate to 1397.5 g of deionized water, stir well, then add 0.75 g of methyl cellulose, stir well and keep the temperature at 40°C;

[0072] S2. Add 145 g of ascorbic acid to 1485 g of deionized water, stir well, and maintain a constant temperature of 40°C;

[0073] S3, adding the silver nitrate solution to the ascorbic acid solution at a rate of 140 mL / min;

[0074] S4. After the reaction is completed, 0.37 g of oleic acid and 0.37 g of Tween-80 are added to the silver-containing slurry obtained in step S3, and the mixture is stirred for 20 min.

[0075] S5. Wash the silver-containing slurry obtained in step S4 with deionized water until the conductivity is below 100 μS / cm, and then dry it in a 70° C. oven for 18 hours to obtain spherical silver powder SS3.

[0076] (2) Preparation of flaky silver powder

[0077] S6. Place 70 g of spherical silver powder SS3 and 420 g of φ2 mm zirconium oxide balls into a polytetrafluoroethylene ball milling jar and place them in a planetary ball mill at a frequency of 25 Hz for 4 h.

[0078] S7. Sieve the mixture of the grinding balls and flaky silver powder obtained in step S6 using a 200-mesh sieve to obtain flaky silver powder SF5.

[0079] Example 6

[0080] This embodiment discloses a method for preparing flaky silver powder for low-temperature curing silver paste, the method comprising the following steps:

[0081] (1) Preparation of spherical silver powder

[0082] The preparation method is the same as that of Example 5;

[0083] (2) Preparation of flaky silver powder

[0084] S6. Take 70g of the spherical silver powder SS3 obtained by the above method and 630g of φ3mm zirconium oxide balls, put them into a polytetrafluoroethylene ball mill, and put them into a planetary ball mill with the frequency set to 40Hz and ball mill for 4h;

[0085] S7. The obtained mixture of the grinding balls and the flaky silver powder is sieved with a 200-mesh sieve to obtain flaky silver powder SF6.

[0086] Example 7

[0087] This embodiment discloses a method for preparing flaky silver powder for low-temperature curing silver paste, the method comprising the following steps:

[0088] (1) Preparation of spherical silver powder

[0089] S1. Add 237.5 g of silver nitrate to 1397.5 g of deionized water, stir well, then add 0.5% of the theoretical weight of the reduced silver powder into gum arabic, stir well and maintain a constant temperature of 40°C.

[0090] S2. Add 145 g of ascorbic acid to 1485 g of deionized water, stir well, and maintain a constant temperature of 40°C;

[0091] S3, adding the silver nitrate solution to the ascorbic acid solution at a rate of 100 mL / min;

[0092] S4. After the reaction is completed, 0.25% of the theoretical weight of the silver powder after reduction by oleic acid and 0.25% of the theoretical weight of the silver powder after reduction by Tween-80 are added to the silver-containing slurry obtained in step S3, and the mixture is stirred for 20 minutes.

[0093] S5. Wash the silver-containing slurry obtained in step S4 with deionized water until the conductivity is less than 100 μS / cm, and then dry it in a 70° C. oven for 18 hours to obtain spherical silver powder;

[0094] (2) Preparation of flaky silver powder

[0095] S6. Place 70 g of spherical silver powder and 210 g of φ5 mm zirconium oxide balls into a polytetrafluoroethylene ball milling jar, place the jar in a planetary ball mill, set the frequency to 15 Hz, and mill for 4 h.

[0096] S7. Sieve the mixture of the grinding balls obtained in step S6 and the flaky silver powder using a 200-mesh sieve to obtain flaky silver powder.

[0097] Example 8

[0098] This embodiment discloses a method for preparing flaky silver powder for low-temperature curing silver paste, the method comprising the following steps:

[0099] (1) Preparation of spherical silver powder

[0100] S1. Add 237.5 g of silver nitrate to 1397.5 g of deionized water, stir well, then add 0.8% of the theoretical weight of the reduced silver powder into methyl cellulose, stir well and maintain a constant temperature of 40°C.

[0101] S2. Add 145 g of ascorbic acid to 1485 g of deionized water, stir well, and maintain a constant temperature of 40°C.

[0102] S3, adding the silver nitrate solution dropwise to the ascorbic acid solution at a rate of 300 mL / min;

[0103] S4. After the reaction is completed, 0.5% of the theoretical weight of the silver powder after reduction is added to the silver-containing slurry obtained in step S3, and stirring is continued for 20 minutes;

[0104] S5. Wash the silver-containing slurry obtained in step S4 with deionized water until the conductivity is less than 100 μS / cm, and then dry it in a 70° C. oven for 18 hours to obtain spherical silver powder;

[0105] (2) Preparation of flaky silver powder

[0106] S6. Put 70 g of spherical silver powder SS1 and 490 g of 1 mm zirconia balls into a polytetrafluoroethylene ball milling jar and place it in a planetary ball mill at a frequency of 30 Hz for 1 h.

[0107] S7. Sieve the mixture of the grinding balls obtained in step S6 and the flaky silver powder using a 200-mesh sieve to obtain flaky silver powder.

[0108] Example 9

[0109] This embodiment discloses a method for preparing flaky silver powder for low-temperature curing silver paste, the method comprising the following steps:

[0110] (1) Preparation of spherical silver powder

[0111] S1. Add 237.5 g of silver nitrate to 1397.5 g of deionized water, stir evenly, then add 1% of the theoretical weight of the reduced silver powder into gum arabic, stir evenly and maintain a constant temperature of 40°C.

[0112] S2. Add 145 g of ascorbic acid to 1485 g of deionized water, stir well, and maintain a constant temperature of 40°C;

[0113] S3, adding the silver nitrate solution dropwise to the ascorbic acid solution at a rate of 300 mL / min;

[0114] S4. After the reaction is completed, 0.5% of the theoretical weight of the silver powder after reduction by Tween-80 is added to the silver-containing slurry obtained in step S3, and the mixture is stirred for 20 minutes.

[0115] S5. Wash the silver-containing slurry obtained in step S4 with deionized water until the conductivity is less than 100 μS / cm, and then dry it in a 70° C. oven for 18 hours to obtain spherical silver powder;

[0116] (2) Preparation of flaky silver powder

[0117] S6. Place 70 g of spherical silver powder SS1 and 630 g of 0.3 mm zirconia balls in a polytetrafluoroethylene ball milling jar and place in a planetary ball mill at a frequency of 45 Hz for 6 h.

[0118] S7. Sieve the mixture of the grinding balls obtained in step S6 and the flaky silver powder using a 200-mesh sieve to obtain flaky silver powder.

[0119] The laser particle size data, burn-off data, and tap density data of the spherical silver powder and flaky silver powder obtained in Examples 1 to 6 are listed in Table 1.

[0120] Table 1 shows the laser particle size data, burnout data and tap density data of the spherical silver powder and flake silver powder obtained in Examples 1 to 6.

[0121]

[0122] SEM scanning results of spherical silver powder and flake silver powder are as follows Figure 1 and Figure 2 As shown in Table 1 and Figure 1 The obtained spherical silver powder is in a monodisperse state, with a narrow particle size distribution and controllable size; from Table 1 and Figure 2The obtained silver flake powder has no cold welding phenomenon, is monodispersed, has a narrow particle size distribution and is size controllable. As shown in Table 1, the obtained silver flake powder has a high tap density and can meet the use requirements of conductive silver paste, especially low-temperature curing conductive silver paste.

Claims

1. A method for preparing flaky silver powder for low-temperature curing silver paste, characterized in that: The preparation method comprises the following steps: S1. Add silver nitrate to deionized water to a concentration of 170 g / L; stir evenly and then add a dispersant; stir and dissolve at a constant temperature to obtain a silver nitrate solution; the dispersant is gum arabic or methyl cellulose, and the amount used is 0.5% to 1% of the mass of the silver powder; S2. Add the reducing agent to deionized water, stir and dissolve at a constant temperature to obtain a reducing agent solution with a concentration of 97 g / L; S3, adding the silver nitrate solution obtained in step S1 dropwise to the reducing agent solution obtained in step S2 at a dropping rate of 100-300 ml / min; the molar ratio of silver nitrate to reducing agent is 1:0.55; S4, adding a surfactant to the mixed slurry of the silver nitrate solution and the reducing agent solution in step S3; the surfactant is any one of oleic acid and Tween-80 or a combination of two, and the total amount is 0.5% of the mass of the silver powder; S5, washing the mixed slurry with the surfactant added in step S4, wherein the washing is performed by washing the mixed slurry with deionized water until the conductivity is below 100 μS / cm; and drying after washing to obtain spherical silver powder; S6. Place the spherical silver powder obtained in step S5 and zirconia grinding balls in a planetary ball mill for ball milling, wherein the mass ratio of the spherical silver powder to the zirconia grinding balls is 1:3-9; the diameter of the zirconia grinding balls is 0.3-5.0 mm; the ball milling frequency is 15-45 Hz, and the ball milling time is 1-6 hours; S7. After the ball milling is completed, the flaky silver powder and the zirconia grinding balls are sieved and separated to obtain flaky silver powder.

2. The method for preparing flaky silver powder for low-temperature curing silver paste according to claim 1, wherein: The reducing agent is ascorbic acid.

3. The method for preparing flaky silver powder for low-temperature curing silver paste according to claim 1 or 2, characterized in that: In step S1 and step S2, the temperature for constant temperature stirring and dissolution is 40°C.

4. The method for preparing flaky silver powder for low-temperature curing silver paste according to claim 1 or 2, characterized in that: In step S5, the mixed slurry with the surfactant added in step S4 is washed, and the drying temperature used for drying after washing is 70° C. and the drying time is 18 hours.

5. The method for preparing flaky silver powder for low-temperature curing silver paste according to claim 1 or 2, characterized in that: In step S7, when the flaky silver powder and the zirconium oxide grinding balls are sieved and separated, the mesh size of the sieve used is 200 mesh.

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