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

Abstract

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

Description

Technical Field

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

[0002] In the renewable energy sector, photovoltaic (PV) power generation has gradually gained public attention due to its green, environmentally friendly, and sustainable development characteristics. With increasingly fierce market competition, traditional PV cell technology needs continuous innovation to meet diverse user demands. Conductive silver paste, as the current collection channel in PV cells, directly affects the final output power of the cell. Conductive silver powder is an indispensable core material, accounting for over 80% of its composition, and directly relates to the conductivity, stability, and sintering performance of the silver paste, which is crucial for cost reduction. Therefore, efficiently synthesizing high-performance conductive silver powder is one of the key factors in improving PV cell performance. Summary of the Invention

[0003] To address the problems of complex preparation processes, wide particle size distribution, and poor sintering performance in existing technologies, this invention provides a method for preparing ultrafine silver powder for low-temperature silver paste. This method uses dilute hydrochloric acid as a pH adjuster, selects citric acid (which is low-cost and highly water-soluble) as an additive, and ferrous sulfate as a reducing agent. By changing the low-temperature environment and the molar ratio between citric acid and silver nitrate, and by utilizing the low-temperature properties of liquid nitrogen to rapidly freeze the silver particles, the reduction efficiency of silver nitrate can be controlled, allowing the silver powder to grow in an orderly manner. This results in silver powder particles with uniform particle size, good dispersibility, low sintering temperature, and good electrical conductivity.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[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 adjuster 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 either ferrous sulfate or ferrous ammonium sulfate.

[0010] As a further improvement of the present invention, in the preparation of the solution, 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 cryogenic freezing, the cryogenic environment is either liquid nitrogen or a mixture of ice and water.

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

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

[0014] The above-mentioned method for preparing conductive silver powder using liquid nitrogen freezing for high-efficiency synthesis includes the following steps:

[0015] (1) Solution preparation: Weigh a certain amount of silver nitrate solid and add it to deionized water to dissolve and prepare a silver nitrate solution of the required concentration. 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 add them to deionized water to dissolve and prepare an additive solution. Prepare a reducing agent solution according to the reaction molar ratio with silver nitrate.

[0016] (2) Solution mixing: Add all the additive solution and reducing agent solution to the silver nitrate solution simultaneously, and control the addition time to 10~15min;

[0017] (3) Low temperature freezing: After the addition is completed, immediately put the mixed solution into liquid nitrogen or ice water mixture and freeze for 2~15 minutes;

[0018] (4) Precipitation filtration: The thawed solution at room temperature is added to a funnel containing 11cm slow quantitative filter paper for filtration;

[0019] (5) Cleaning and drying: The precipitate is cleaned with deionized water and then placed in a vacuum drying oven and dried at 60~80℃ for 2 hours.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The present invention provides a method for preparing ultrafine silver powder for low-temperature silver paste. By utilizing the low-temperature characteristics of liquid nitrogen, the generated silver particles are rapidly frozen, effectively avoiding secondary agglomeration and preserving the original physicochemical properties of the synthesized silver powder during the reaction process. This facilitates grinding to obtain silver powder with a concentrated particle size distribution. The conductive silver paste prepared from this silver powder can effectively improve the photoelectric conversion efficiency of the battery.

[0022] 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 the silver nitrate solution in advance, small-particle silver chloride precipitate is generated, which allows the silver ions to be reduced subsequently to grow on the basis of silver chloride seed crystals, which is beneficial 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 can be fully reduced.

[0023] 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, the nucleation and growth processes of silver powder can be separated, preventing excessively fast nucleation rates and uneven particle size. On the other hand, it helps to fully reduce silver ions and increase silver powder yield.

[0024] 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 it is not easily oxidized in the air. On the other hand, it can promote the dissolution of ferrous sulfate, so that it is fully and uniformly dispersed in the solution, thereby improving the stability of ferrous ions and helping to improve production efficiency.

[0025] 5. The preparation process of this 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.

[0026] Preferably, in the solution preparation, the pH adjuster is dilute hydrochloric acid.

[0027] Preferably, in the solution preparation, the pH value of the silver nitrate solution is 5.1.

[0028] Preferably, in the solution preparation, the additive is citric acid.

[0029] Preferably, in the solution preparation, the reducing agent is ferrous sulfate.

[0030] Preferably, in the solution preparation, the predetermined molar ratio between ferrous sulfate and silver nitrate is (1~2.5):1.

[0031] Preferably, in cryogenic freezing, the cryogenic environment is liquid nitrogen.

[0032] Preferably, in low-temperature freezing, the freezing time is 5 to 10 minutes.

[0033] Preferably, the drying temperature during the washing and drying process is 70°C. Attached Figure Description

[0034] Figure 1This is the X-ray diffraction (XRD) pattern of the silver powder prepared in Comparative Example 2 of this invention.

[0035] Figure 2 This is a scanning electron microscope (SEM) image of the silver powder prepared in Comparative Example 2 of this invention. Detailed Implementation

[0036] The present invention will be described in detail below with reference to specific implementation examples. However, the present invention is not limited to these embodiments. The embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0037] Unless otherwise specified, the raw materials used in the embodiments of the present invention were all purchased commercially, and the silver nitrate, ferrous sulfate and citric acid were all of analytical grade.

[0038] In this invention, a German ZEISS Sigma 300 scanning electron microscope was used to detect the microstructure of silver powder, a Malvern particle size analyzer was used to characterize the particle size of silver powder, and a Smartlab SE X-ray diffractometer was used to perform phase analysis of silver powder.

[0039] Example 1

[0040] A method for preparing ultrafine silver powder for low-temperature silver paste includes the following steps:

[0041] (1) Solution preparation: Prepare a 0.04 mol / L silver nitrate solution and a 2.7 mg / mL citric acid solution. Add a 0.12 mol / L dilute hydrochloric acid solution to the silver nitrate solution, stir for ten seconds, and adjust its pH value to 5.1; prepare a ferrous sulfate solution with a molar ratio of 1:1 to silver nitrate.

[0042] (2) Solution mixing: Add all the citric acid solution and ferrous sulfate solution dropwise to the silver nitrate solution simultaneously. The citric acid solution is added over a period of 10 minutes.

[0043] (3) Low temperature freezing: After the addition is completed, immediately place the mixed solution in an ice-water mixture at 0°C and freeze for 10 minutes;

[0044] (4) Precipitation filtration: The thawed solution at room temperature is added to a funnel containing 11cm slow quantitative filter paper for filtration;

[0045] (5) Cleaning and drying: The filter material is cleaned with deionized water and then placed in a vacuum drying oven and dried at 70°C for 2 hours to obtain conductive silver powder after freezing the ice-water mixture.

[0046] Examples 2-4

[0047] A method for preparing ultrafine silver powder for low-temperature silver paste differs from Example 1 in that the molar ratios of ferrous sulfate and silver nitrate in the solution preparation are shown in Table 1. The other methods are largely the same as in Example 1 and will not be repeated here.

[0048] Table 1. Preparation conditions and silver powder parameters for Examples 1-4

[0049] sample molar ratio of ferrous sulfate and silver nitrate Yield (g) Dispersion 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

[0050] Table 1 shows that the silver powder obtained when citric acid and ferrous sulfate are added at the same rate has weak dispersibility and low yield. As the molar ratio of ferrous sulfate to silver nitrate increases, the content of ferrous ions added to the silver nitrate solution per unit time also increases. However, citric acid is also a complexing agent; when the dropping rate is fast, it reacts with the ferrous ions added at the same rate to form complex ions, leading to a decrease in the concentration of free ferrous ions in the solution. Therefore, the silver powder yield does not change significantly. With the increase of the free ferrous ion content in the solution, the silver crystal nuclei are affected by this and grow rapidly in a directional manner. This prevents the newly formed crystal nuclei after reduction from being coated with ferrous ion chelates and growing rapidly, thus reducing dispersibility.

[0051] Comparative Examples 1-3

[0052] A method for preparing ultrafine silver powder for low-temperature silver paste differs from Examples 2-4 in that the mixed solution is frozen in a liquid nitrogen environment at -196°C during the cryogenic freezing process. The rest is largely the same as Examples 2-4 and will not be repeated here.

[0053] The silver powder prepared in Comparative Example 2 showed five diffraction peaks under XRD analysis. These peaks corresponded to the diffraction peaks of silver on the standard JCPDS card, and no other impurity peaks were observed, indicating high purity of the prepared silver powder. Under SEM observation at 15k, the silver powder appeared as smooth, flaky particles with excellent dispersibility. This is because citrate ions can form a diffuse electric double layer structure with the silver crystal nuclei, effectively preventing agglomeration between the nuclei.

[0054] Table 2 Preparation conditions and silver powder parameters of Comparative Examples 1-3

[0055]

[0056] 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 this invention has a narrow particle size distribution and good dispersibility. Its silver powder particle size distribution curve shows D10=1.319μm and D50=1.819μm. Comparison with the scanning image of the silver powder proves that the silver powder prepared under this condition has a relatively uniform size, smooth surface, and high purity, which helps to improve battery efficiency.

[0057] As shown in Table 2, the yield and dispersibility of silver powder prepared in Comparative Examples 1-3 both improved with increasing ferrous sulfate content. This is because the ferrous sulfate solution has a faster dropping rate, and the slowly added citric acid does not consume too many ferrous ions in a short 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 good control over the redox process. As the ferrous sulfate content continues to increase, the citric acid content remains constant and is insufficient to form a complex coating on the surface of newly formed crystal nuclei in time, thus reducing the dispersibility.

[0058] By comparing the performance of the examples and the comparative examples, the silver powder preparation process of the comparative example is significantly superior to that of the examples. Rapidly freezing the reacted mixture with a large amount of liquid nitrogen effectively ensures the concentration of silver particle size, contributing to the acquisition of silver powder with a narrow particle size distribution. Compared to silver powder frozen with an ice-water mixture, the silver paste prepared using silver powder frozen at low temperature with liquid nitrogen as the conductive phase exhibits good thixotropy, reduced series resistance, improved battery efficiency, better conductivity, and a significantly lower thermal weight loss temperature.

[0059] In summary, by employing cryogenic liquid nitrogen freezing treatment and rationally controlling the molar ratio between ferrous sulfate and silver nitrate, micron-sized silver powder particles with uniform particle size, good dispersibility, and controllable yield 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 requirements of HJT low-temperature conductive silver paste.

[0060] The above description is merely an embodiment of the present invention, used only to illustrate the implementation of the present invention. The scope of protection of this application is not limited to these specific embodiments, but is determined by the claims of this application. Those skilled in the art can modify the technical solutions in the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the technical concept and principle of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for producing ultra-fine silver powder for low-temperature silver paste, characterized by, The preparation method of the silver powder comprises the following steps: (1) solution preparation: a certain amount of silver salt is weighed and added into deionized water to dissolve and prepare a silver salt solution with a required concentration, and an appropriate amount of pH adjuster is added into the solution; a suitable additive is weighed and added into deionized water to dissolve and prepare an additive solution; a reducing agent solution is prepared according to a reaction molar ratio with the silver salt; (2) solution mixing: all the additive solution and the reducing agent solution are synchronously added dropwise into the silver salt solution, and the feeding time is controlled within 10-15 min; (3) low-temperature freezing: after the dropwise addition is completed, the mixed solution is immediately placed into a low-temperature environment for freezing for 2-15 min; (4) precipitation and filtration: the solution after thawing at room temperature is added into a funnel with an Æ11 cm slow quantitative filter paper for filtration; (5) cleaning and drying: the precipitate is cleaned with deionized water, and then is placed into a vacuum drying box for drying 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 ferrous ammonium sulfate, and the molar ratio between the reducing agent and the 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 an ice-water mixture, and the freezing time is 2-15 min.

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