Preparation method of superfine silver powder for low-temperature silver paste

Abstract

The invention provides a preparation method of superfine silver powder for low-temperature silver paste, which comprises the following steps: respectively preparing a silver nitrate solution, a citric acid solution and a ferrous sulfate solution, and the molar ratio of ferrous sulfate to silver nitrate is (1-3): 1; synchronously dropwise adding the citric acid solution and the ferrous sulfate solution into the silver nitrate solution at normal temperature and normal pressure; the obtained mixed solution is innovatively and immediately placed in liquid nitrogen to be subjected to low-temperature freezing treatment. The photovoltaic silver paste prepared from the conductive silver powder shows good thixotropy and conductivity, and the photoelectric conversion efficiency can be effectively improved through the added liquid nitrogen freezing link. The process is simple, the operation process is easy to control, the cost is low, citric acid with high acidity is selected as an additive, and the silver powder can grow directionally by adjusting the molar ratio of ferrous sulfate to silver nitrate and controlling liquid nitrogen freezing, so that the particle size of the silver powder is effectively regulated and controlled, and micron silver powder particles narrow in particle size distribution and low in sintering temperature are obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic silver powder preparation, and in particular to a method for preparing ultrafine silver powder for low-temperature silver paste. Background Art

[0002] In the renewable energy sector, photovoltaic power generation has gradually gained public attention due to its green, environmentally friendly, and sustainable development. With increasingly fierce market competition, traditional photovoltaic cell technology must continue to innovate to meet diverse user needs. Conductive silver paste, as the current collection channel in photovoltaic cells, directly affects the battery's ultimate output. Conductive silver powder is an indispensable core material, accounting for over 80% of the total composition. This material is directly related to the silver paste's conductivity, stability, and sintering properties, making it crucial for cost reduction. Therefore, the efficient synthesis of conductive silver powder with excellent performance is a key factor in improving photovoltaic cell performance. Summary of the Invention

[0003] To address the problems of complex preparation processes, wide particle size distribution, and poor sintering performance in the prior art, the present invention provides a method for preparing ultrafine silver powder for low-temperature silver paste. This method uses dilute hydrochloric acid as a pH regulator, citric acid (which is relatively low-cost and water-soluble) as an additive, and ferrous sulfate as a reducing agent. By varying the low-temperature environment and the molar ratio of ferrous sulfate to silver nitrate, while leveraging the low-temperature properties of liquid nitrogen to rapidly freeze the silver particles, the reduction efficiency of the silver nitrate can be controlled, resulting in orderly growth of the silver powder, thereby producing silver powder particles with uniform particle size, good dispersibility, a low sintering temperature, and excellent conductivity.

[0004] In order to achieve the above objectives, the present invention adopts the following technical solutions:

[0005] As a further improvement of the present invention, in the solution preparation, the silver salt solution is a silver nitrate solution.

[0006] As a further improvement of the present invention, in the solution preparation, the pH regulator is any one of dilute hydrochloric acid, dilute sulfuric acid or dilute nitric acid.

[0007] As a further improvement of the present invention, in the solution preparation, the pH value of the silver nitrate solution is 4.5-6.

[0008] As a further improvement of the present invention, in the solution preparation, the additive is one or more of citric acid, lactic acid or ethylenediaminetetraacetic acid.

[0009] As a further improvement of the present invention, in the solution preparation, the reducing agent is any one of ferrous sulfate or ammonium ferrous sulfate.

[0010] As a further improvement of the present invention, in the solution preparation, the predetermined molar ratio between the reducing agent and silver nitrate is (1-3):1.

[0011] As a further improvement of the present invention, in the low-temperature freezing, the low-temperature environment is one of liquid nitrogen or an ice-water mixture.

[0012] As a further improvement of the present invention, in the low-temperature freezing, the freezing time is 2 to 15 minutes.

[0013] In the present invention, the "solutions" are all prepared with deionized water, and the reaction temperature in the entire silver powder preparation process is room temperature except for low-temperature freezing and drying.

[0014] The above-mentioned method for preparing conductive silver powder by efficient synthesis using liquid nitrogen freezing comprises the following steps: (1) Solution preparation: Weigh a certain amount of silver nitrate solid and dissolve it in deionized water to prepare a silver nitrate solution of the required concentration, and add an appropriate amount of any one of dilute hydrochloric acid, dilute sulfuric acid or dilute nitric acid to it; weigh one or more of citric acid, lactic acid or ethylenediaminetetraacetic acid and dissolve it in deionized water to prepare an additive solution; prepare a reducing agent solution according to the reaction molar ratio between the reducing agent and the silver nitrate; (2) Solution mixing: All the additive solution and reducing agent solution are added dropwise to the silver nitrate solution simultaneously, and the addition time is controlled within 10-15 minutes; (3) Low temperature freezing: After the addition is completed, immediately place the mixed solution in liquid nitrogen or ice-water mixture and freeze for 2-15 minutes; (4) Precipitation filtration: Add the thawed solution at room temperature into a funnel filled with Æ11 cm slow quantitative filter paper for filtration; (5) Cleaning and drying: The precipitate was washed with deionized water and then placed in a vacuum drying oven at 60-80 °C for 2 h.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] 1. The present invention provides a method for preparing ultrafine silver powder for low-temperature silver paste. By utilizing the low-temperature properties of liquid nitrogen itself, the generated silver particles are quickly frozen, effectively avoiding the occurrence of secondary agglomeration, retaining the original physical and chemical properties of the synthesized silver powder during the reaction process, and facilitating the acquisition of silver powder with a concentrated particle size distribution after grinding. The conductive silver paste prepared from the silver powder can effectively improve the photoelectric conversion efficiency of the battery.

[0017] 2. The present invention provides a method for preparing ultrafine silver powder for low-temperature silver paste. By adding an appropriate amount of acid solution to a silver nitrate solution in advance, on the one hand, small-particle silver chloride precipitate is generated, so that the subsequently reduced silver ions can grow on the basis of the silver chloride seed crystals, which is conducive to obtaining silver powder with uniform particle size; on the other hand, it helps to inhibit the hydrolysis of ferrous ions and maintain the reducing property of ferrous ions, so that the silver ions in the solution are fully reduced.

[0018] 3. The present invention provides a method for preparing ultrafine silver powder for low-temperature silver paste. By adjusting the molar ratio between ferrous sulfate and silver nitrate, on the one hand, the nucleation and growth processes of the silver powder can be separated to prevent the nucleation rate from being too fast and the particle size from being uneven; on the other hand, it helps to fully reduce silver ions and increase the silver powder yield.

[0019] 4. The present invention provides a method for preparing ultrafine silver powder for low-temperature silver paste, preferably using citric acid with good water solubility as an additive. On the one hand, its chemical structure is stable and not easily oxidized in the air. On the other hand, it can promote the dissolution of ferrous sulfate, making it fully and evenly dispersed in the solution, thereby improving the stability of ferrous ions and contributing to improving production efficiency.

[0020] 5. The preparation process of the present invention is simple, the reaction conditions are easy to control, the reaction is complete, the cost is low and the pollution is small, which meets the requirements of green and sustainable production. The silver powder for conductive silver paste produced in batches can effectively improve the photoelectric conversion efficiency and meet the needs of market users.

[0021] Preferably, in the solution preparation, the pH adjuster is dilute hydrochloric acid.

[0022] Preferably, in the solution preparation, the pH value of the silver nitrate solution is 5.1.

[0023] Preferably, in the solution preparation, the additive is citric acid.

[0024] Preferably, in the solution preparation, the reducing agent is ferrous sulfate.

[0025] Preferably, in the solution preparation, the predetermined molar ratio between the ferrous sulfate and the silver nitrate is (1-2.5):1.

[0026] Preferably, in cryogenic freezing, the low temperature environment is liquid nitrogen.

[0027] Preferably, in the low-temperature freezing, the freezing time is 5 to 10 minutes.

[0028] Preferably, during the cleaning and drying, the drying temperature is 70°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1This is the X-ray diffraction (XRD) pattern of the silver powder prepared in Comparative Example 2 of the present invention.

[0030] Figure 2 This is a scanning electron microscope (SEM) image of the silver powder prepared in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0031] The present invention is described in detail below with reference to specific implementation cases, but the present invention is not limited to these embodiments. The embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0032] Unless otherwise specified, the raw materials in the examples of the present invention were purchased from commercial sources, wherein silver nitrate, ferrous sulfate and citric acid were all of analytical grade.

[0033] In the present invention, a German ZEISS Sigma 300 scanning electron microscope was used to detect the micromorphology of the silver powder, a Malvern particle size analyzer was used to characterize the particle size of the silver powder, and a Smartlab SE X-ray diffractometer was used to perform phase analysis on the silver powder.

[0034] Example 1

[0035] A method for preparing ultrafine silver powder for low-temperature silver paste comprises the following steps:

[0036] (1) Solution preparation: Prepare 0.04 mol / L silver nitrate solution and 2.7 mg / mL citric acid solution. Add 0.12 mol / L dilute hydrochloric acid solution to the silver nitrate solution, stir for 10 seconds, and adjust the pH value to 5.1. Prepare ferrous sulfate solution at a molar ratio of 1:1 with silver nitrate.

[0037] (2) Solution mixing: All the citric acid solution and ferrous sulfate solution were simultaneously added dropwise to the silver nitrate solution. The citric acid solution was added for 10 minutes.

[0038] (3) Low temperature freezing: After the addition is completed, the mixed solution is immediately placed in an ice-water mixture at 0°C and frozen for 10 minutes;

[0039] (4) Precipitation filtration: Add the thawed solution at room temperature into a funnel filled with Æ11 cm slow quantitative filter paper for filtration;

[0040] (5) Cleaning and drying: The filtrate was cleaned with deionized water and then dried in a vacuum drying oven at 70°C for 2 h to obtain conductive silver powder after freezing the ice-water mixture.

[0041] Examples 2-4

[0042] A method for preparing ultrafine silver powder for low-temperature silver paste is different from Example 1 in that, in the solution preparation, the molar ratio of ferrous sulfate and silver nitrate is as shown in Table 1. The rest is substantially the same as Example 1 and will not be repeated here.

[0043] Table 1 Preparation conditions and silver powder parameters of Examples 1-4 sample The molar ratio of ferrous sulfate and silver nitrate Yield (g) Dispersibility Example 1 1：1 0.05 dispersion Example 2 1.5：1 0.05 dispersion Example 3 2：1 0.06 reunion Example 4 2.5：1 0.06 reunion

[0044] As can be seen from Table 1, the silver powder obtained when citric acid and ferrous sulfate are added dropwise at the same speed has weak dispersibility and low yield. As the molar ratio of ferrous sulfate to silver nitrate increases, the ferrous ion content added to the silver nitrate solution per unit time also increases. However, citric acid is also a complexing agent. When the addition rate is faster, it will react with the ferrous ions added dropwise at the same speed to form complex ions, resulting in a decrease in the concentration of free ferrous ions in the solution. Therefore, the silver powder yield does not change much. As the free ferrous ion content in the solution increases, the silver crystal nuclei are affected by it and grow rapidly in a directional manner. As a result, the newly formed crystal nuclei after reduction do not have time to be coated by the ferrous ion chelate and grow rapidly, thus reducing dispersibility.

[0045] Comparative Examples 1-3

[0046] A method for preparing ultrafine silver powder for low-temperature silver paste is different from that of Examples 2-4 in that the mixed solution is placed in a liquid nitrogen environment at -196°C for freezing. Other steps are substantially the same as those of Examples 2-4 and are not described here.

[0047] XRD analysis of the silver powder prepared in Comparative Example 2 revealed five diffraction peaks. These peak positions coincided with those of silver on the standard JCPDS card, with no other miscellaneous peaks present, indicating high purity. SEM observation at ×15k revealed smooth, flaky particles with excellent dispersibility. This is due to the citrate ions forming a diffuse double layer with the silver nuclei, effectively preventing agglomeration.

[0048] Table 2 Preparation conditions and silver powder parameters of Comparative Examples 1-3

[0049] Referring to Tables 1-2 and the particle size test results, it can be seen that the conductive silver powder prepared in Comparative Example 2 of the present invention has a narrow particle size distribution and good dispersibility. Its silver powder particle size distribution curve shows D10 = 1.319 μm and D50 = 1.819 μm. Compared with the scanned image of the silver powder, it is proved that the silver powder prepared under this condition is relatively uniform in size, has a smooth surface, and is of high purity, which helps to improve battery efficiency.

[0050] As can be seen from Table 2, as the ferrous sulfate content increases, the yield and dispersibility of the silver powder prepared in Comparative Examples 1-3 are improved. This is because the ferrous sulfate solution has a fast droplet acceleration rate, and the slowly added citric acid does not consume too much ferrous ions in a short period of time, resulting in a rapid increase in the ferrous ion content in the mixed solution per unit time, which increases the reduction rate of silver ions. At this time, citric acid has better controllability over the redox process; as the ferrous sulfate content continues to increase, the citric acid content remains unchanged at this time and is insufficient to form a complex coated on the surface of the newly formed crystal nucleus in a timely manner, thereby reducing dispersibility.

[0051] Comparing the performance of the products in the Examples and Comparative Examples, the silver powder preparation process in the Comparative Examples significantly outperforms the Examples. Rapidly freezing the mixed solution after the reaction with a large amount of liquid nitrogen effectively ensures the concentration of silver particle size, facilitating the production of silver powder with a narrow particle size distribution. Compared to silver powder frozen from an ice-water mixture, silver paste prepared using silver powder frozen from low-temperature liquid nitrogen as the conductive phase exhibits excellent thixotropy, reduced series resistance, improved battery efficiency, better conductivity, and a significantly lower thermal weight loss temperature.

[0052] In summary, by using low-temperature liquid nitrogen freezing treatment and reasonably controlling the molar ratio between ferrous sulfate and silver nitrate, micron-sized silver powder particles with uniform particle size, good dispersibility and controllable output were successfully prepared. The conductive silver paste produced in batches from this silver powder has good printing performance and photoelectric conversion efficiency, better linear preservation, and can meet the low-temperature conductive silver paste requirements of HJT.

[0053] The above contents are merely examples of the present invention and are intended only to illustrate the implementation scheme of the present invention. The scope of protection of this application is not limited by these specific examples but is determined by the claims of this application. Those skilled in the art may modify the technical solutions of the present invention. Any modification, equivalent substitution, improvement, etc. made within the technical concept and principle of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A method for preparing ultrafine silver powder for low-temperature silver paste, characterized in that: The preparation method of the silver powder comprises the following steps: (1) Solution preparation: weigh a certain amount of silver salt and add it to deionized water to dissolve to prepare a silver salt solution of required concentration, and add an appropriate amount of pH adjuster thereto; weigh a suitable additive and add it to deionized water to dissolve to prepare an additive solution; prepare a reducing agent solution according to the reaction molar ratio between the reducing agent and the silver salt; (2) Solution mixing: All the additive solution and reducing agent solution are simultaneously added dropwise to the silver salt solution, and the addition time is controlled within 10-15 minutes; (3) Low temperature freezing: After the addition is completed, the mixed solution is immediately placed in a low temperature environment and frozen for 2-15 minutes; (4) Precipitation filtration: Add the thawed solution at room temperature into a funnel filled with Æ11 cm slow quantitative filter paper for filtration; (5) Cleaning and drying: Clean the precipitate with deionized water and then place it in a vacuum drying oven at 60-80°C for 2 h.

2. The method for preparing ultrafine silver powder for low-temperature silver paste according to claim 1, characterized in that: The silver salt solution is a silver nitrate solution.

3. The method for preparing ultrafine silver powder for low-temperature silver paste according to claim 2, characterized in that: The pH adjuster is any one of dilute hydrochloric acid, dilute sulfuric acid or dilute nitric acid.

4. The method for preparing ultrafine silver powder for low-temperature silver paste according to claim 1, characterized in that: The additive is one or more of citric acid, lactic acid or ethylenediaminetetraacetic acid.

5. The method for preparing ultrafine silver powder for low-temperature silver paste according to claim 1, characterized in that: The reducing agent is any one of ferrous sulfate or ammonium ferrous sulfate, and the molar ratio between the reducing agent and silver nitrate is (1-3):

1.

6. The method for preparing ultrafine silver powder for low-temperature silver paste according to claim 1, characterized in that: The low temperature environment is one of liquid nitrogen or ice-water mixture and the freezing time is 2 to 15 minutes.

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