Silver-coated copper powder and preparation method and application thereof

Abstract

The invention discloses silver-coated copper powder and a preparation method and application thereof, and belongs to the technical field of metal powder. The silver-coated copper powder provided by the invention comprises copper particles and a silver layer wrapping the surfaces of the copper particles, no hole is formed in the copper particles; the silver-coated copper powder comprises complete particles; under the visual field of a scanning electron microscope, no hole exists between the silver layer of the complete particle and the copper particle; in the silver-coated copper powder, the number percentage of the complete particles is larger than or equal to 85%. According to the silver-coated copper powder provided by the invention, the cost is reduced, and meanwhile, the silver-coated copper powder has excellent oxidation resistance and relatively low resistivity. The invention further provides a preparation method and application of the silver-coated copper powder.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal powder, in particular to a silver-coated copper powder, a preparation method and application thereof. BACKGROUND

[0002] Heterojunction cells, HBC cells, and crystalline silicon perovskite stacked cells are the future development direction of photovoltaic cell technology, and the use of silver paste is more in unit power, and the high cost seriously limits its development. Silver paste, as the first non-silicon cost of photovoltaic cells, is the key to cost reduction. Metal copper powder has excellent electrical conductivity similar to silver (the volume resistivity of silver is 1.59 x 10 -6 Ω·cm, and the volume resistivity of copper is 1.72 x 10 -6 Ω·cm), and the price of copper is only 1 / 20 of that of silver. At the same time, using copper powder can also avoid many performance defects caused by silver migration under low-temperature sintering process of heterojunction cells. However, if only copper powder is used, its electrical conductivity and oxidation resistance are poor. Therefore, silver-coated copper powder is particularly suitable for the preparation of silver paste for low-temperature sintering of heterojunction solar cells. Using silver-coated copper powder instead of pure silver powder not only maintains the performance, but also greatly reduces the non-silicon cost of the cell, which is of great significance to the cost reduction and efficiency improvement of heterojunction cells, and has become the focus of the industry.

[0003] However, the silver-coated copper powder produced in the related industry has many coating defects, such as pores in the coating layer, many holes in the internal copper powder, and high porosity of the coating interface, which leads to high oxygen content, poor oxidation resistance, poor acid resistance, and low resistance, hindering the progress of silver-coated copper powder replacing silver powder in photovoltaic silver paste. Therefore, there is an urgent need for a silver-coated copper powder product and a preparation method that overcomes the above shortcomings. SUMMARY

[0004] The present application aims to at least solve one of the technical problems existing in the prior art. To this end, the present application provides a silver-coated copper powder, wherein the proportion of complete particles is high, so that the oxygen content of the powder is low, and the oxidation resistance, acid resistance and electrical conductivity are excellent.

[0005] The present application also provides a preparation method of the silver-coated copper powder.

[0006] The present application also provides a silver paste comprising the silver-coated copper powder.

[0007] The present application also provides the application of the above-mentioned silver paste.

[0008] According to an embodiment of the first aspect of the present application, a silver-coated copper powder is provided, which comprises copper particles, and a silver layer wrapped on the surface of the copper particles;

[0009] The copper particles are free of holes in the interior;

[0010] The silver-coated copper powder comprises complete particles; under a scanning electron microscope field of view, there is no gap between the silver layer and the copper particle of the complete particle;

[0011] The number percentage of the complete particles in the silver-coated copper powder is greater than or equal to 85%.

[0012] The silver-coated copper powder according to the embodiments of the present application has at least the following beneficial effects:

[0013] The silver-coated copper powder provided by the present application has more complete particles, and thus has good oxidation resistance and low resistivity, and is expected to replace pure silver powder and be applied to the production of silver paste.

[0014] According to some embodiments of the present application, when the magnification of the scanning electron microscope is 2000 times, the number percentage of the complete particles under a single scanning electron microscope field of view is greater than or equal to 85%. For example, it can be specifically 88%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100%, or a range value composed of any two of the above point values. When counting, only the area of a single particle that is displayed in the field of view by more than 80% is included in the count; for example, at the edge of the electron microscope field of view, only a small part of a particle is displayed, which is not included in the count.

[0015] According to some embodiments of the present application, when the magnification of the scanning electron microscope is 5000 times, the number percentage of the complete particles under a single scanning electron microscope field of view is greater than or equal to 90%. For example, it can be specifically 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100%, or a range value composed of any two of the above point values. However, because the number of particles of the silver-coated copper powder is small at this magnification, the statistical error is large, and therefore, in actual production, a 2000 times field of view is usually selected for the counting of the percentage of complete particles.

[0016] According to some embodiments of the present application, the silver-coated copper powder further comprises a defective particle; under a scanning electron microscope field of view, there is a gap between the silver layer and the copper particle of the defective particle.

[0017] According to some embodiments of the present application, under a scanning electron microscope field of view, in a single defective particle, the ratio l / d of the longest span range l of the gap to the circumference d of the defective particle is less than or equal to 5%. For example, it can be specifically 4%, 3%, 2%, 1%, 0%, or a range value composed of any two of the above point values.

[0018] Further, l is less than or equal to 500 nm. For example, it can be specifically 450 nm, 400 nm, 300 nm, 250 nm, 200 nm, 150 nm, 100 nm, 50 nm, or a range value composed of any two of the above point values.

[0019] It should be noted that if the defective particle has multiple pores, then l represents the sum of the span ranges of the multiple pores.

[0020] According to some embodiments of the present application, under the field of view of a scanning electron microscope, the ratio of the longest span range l of the pores in a single complete particle to the circumference d of the defective particle is l / d = 0.

[0021] According to some embodiments of the present application, the particle size distribution of the silver-coated copper powder is between 1.0 and 10.0 μm. For example, it can be specifically 1.5-10 μm or 2-10 μm. The particle size is mainly related to the particle size of the copper particles. The present application can theoretically obtain silver-coated copper powder of any particle size. The above particle size is only the range of mainstream products on the market at present. If needed, the preparation method provided by the present application can also be used to prepare silver-coated copper powder of other particle sizes. The subsequent explanation of D50 of silver-coated copper powder is the same.

[0022] According to some embodiments of the present application, the D50 particle size distribution of the silver-coated copper powder is between 3.2 and 4.0 μm. For example, it can be specifically 3.3 μm, 3.4 μm, 3.5 μm, 3.6 μm, 3.7 μm, 3.8 μm, 3.9 μm, 4.0 μm; or a range value composed of any two of the above point values.

[0023] According to some embodiments of the present application, the mass percentage content of silver in the silver-coated copper powder is 10-20%. For example, it can be specifically 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, 18%, 18.5%, 19%, 19.5%, 20%; or a range value composed of any two of the above point values.

[0024] According to the above statement, it can be known that when the particle size and silver content of the silver-coated copper powder provided by the present application are equivalent, with the increase of the proportion of complete particles and the increase of the l / d value in the defective particles, the acid resistance and oxidation resistance of the silver-coated copper powder are improved, and the resistivity is reduced.

[0025] According to the embodiments of the second aspect of the present application, a preparation method of silver-coated copper powder is provided, which comprises the following steps:

[0026] The silver complex solution is added to the copper dispersion liquid, and at the same time of adding the silver complex solution, the reducing solution and the second ammonia water are sequentially added to the copper dispersion liquid;

[0027] The raw materials for preparing the copper dispersion include copper powder and first ammonia water; the raw materials for preparing the silver complex solution include water-soluble silver salt and complexing agent; the complexing agent includes at least one of aminocarboxylic acid complexing agents, thiourea, thiosulfate, polyamine compounds and carboxylic acid complexing agents.

[0028] Since the preparation method employs all the technical solutions of the silver-coated copper powder described in the above embodiments, it possesses at least all the beneficial effects brought about by the technical solutions of the above embodiments. Furthermore:

[0029] In related technologies, it is usually necessary to remove the oxide layer or coating layer on the surface of copper powder to ensure that the displacement reaction between copper powder and silver ions proceeds fully. Therefore, neutral or acidic solvents (which may contain complexing agents) are often used to prepare copper dispersions with copper powder, or the solvent contains an alkanolamine protective agent (this method is not suitable for displacement reactions) to prevent copper powder oxidation. In the preparation method provided by this invention, ammonia water is added when preparing the copper dispersion, which can cause slight oxidation of copper powder and the formation of [Cu(NH3)4]2+ complex (the principle is that the copper surface will continue to be oxidized to Cu by oxygen in the oxygen and ammonia water environment). 2+ And Cu 2+ It will immediately combine with NH3 in ammonia water to form [Cu(NH3)4]2+ complex; this complex is tightly bound to the copper powder body, and at the same time better promotes the displacement reaction between the silver complex solution and the copper powder, so that the formed silver layer is tightly bound to the copper powder body, reducing the proportion of defective particles and the value of interfacial porosity in defective particles.

[0030] In the preparation method provided by this invention, a more stable silver complex (compared to silver ammonia ions) is formed by adding a specific type of complexing agent to the silver complexing solution, thereby freeing Ag. + The concentration decreased significantly, and the new Ag + The release rate of the complexing agent is slowed down, thus inhibiting the rate of silver salt participation in the substitution reaction to a certain extent, thereby improving the density of the resulting silver layer. Simultaneously, the complexing agent adsorbs on the silver crystal surface, inhibiting the growth of certain crystal planes and promoting the lateral expansion of the newly formed silver layer along the copper powder surface, forming a dense and continuous silver layer. Overall, by selecting a suitable complexing agent, the preparation method provided by this invention reduces the proportion of defective particles and the interfacial porosity value among the defective particles.

[0031] The preparation method provided by the present application actually has three reaction steps, ① the silver complex solution is added into the copper dispersion solution to perform a displacement reaction, so as to avoid the generation of interface pores and increase the binding tightness; ② the reducing solution is added to perform a preliminary reduction reaction, so as to repair defects and prevent the silver layer formed by the displacement from being discontinuous; and ③ the second ammonia water is added, and the silver complex solution is continuously added at the same time, so that the pH can be stabilized and supplemented under the balance control of the second ammonia water and the complexing agent, and through the competitive complexing relationship between the second ammonia water, the complexing agent and silver ions, the release rate of silver ions can be prevented from being seriously reduced by excessive free complexing agents, and in turn, the newly generated silver layer can be prevented from being biased to grow along specific grain boundaries to form a non-uniform coating, that is, a non-spherical silver-coated copper powder. That is, through the feeding sequence of the preparation raw materials, the preparation method provided by the present application can significantly improve the sphericity of the obtained silver-coated copper powder, the density of the silver layer, and the binding tightness of the silver layer and the copper powder.

[0032] The preparation method provided by the second aspect of the present application can be used to prepare the silver-coated copper powder provided by the first aspect of the present application.

[0033] According to some embodiments of the present application, the copper powder needs to be pretreated before use. Wherein,

[0034] The pretreatment includes acid washing and alkali washing performed in sequence.

[0035] The mass concentration of the acid solution used in the acid washing is 1-2%; for example, it can be 1.5% specifically.

[0036] The solute of the acid solution used in the acid washing includes at least one of nitric acid and sulfuric acid.

[0037] The duration of the acid washing is 0.5-1.5h; for example, it can be 1h specifically.

[0038] The mass concentration of the alkali solution used in the alkali washing is 5-10%; for example, it can be 8% specifically.

[0039] The duration of the alkali washing is 20-40min; for example, it can be 30min specifically.

[0040] Water washing is performed after the acid washing and the alkali washing respectively; so as to remove the acid and alkali residues; in the water washing, the solid-liquid mass ratio is 1:15-35; for example, it can be 1:20, 1:25 or 1:30 specifically; and the endpoint of the acid washing is near neutral (pH 6.5-7.5).

[0041] According to some embodiments of the present application, the mass ratio of the first ammonia water and the copper powder in the raw materials for preparing the copper dispersion solution is 1:5-10. For example, it can be specifically 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, or a range value formed by any two of the above point values. Excessive displacement reaction can lead to an increase in interfacial porosity (the interfacial porosity between the silver layer and the copper powder increases, because the difference in atomic weight between silver and copper, the difference in volume after displacement, causes the copper powder to be concave, and the silver layer to be convex). When the amount of ammonia water used in the copper dispersion solution is within the above range, the excessive displacement reaction can be significantly inhibited, and the quality of the obtained silver-coated copper powder can be further optimized.

[0042] According to some embodiments of the present application, the raw materials for preparing the copper dispersion solution further include disodium ethylenediaminetetraacetate, a dispersant, and water.

[0043] According to some embodiments of the present application, the dispersant includes at least one of polyvinylpyrrolidone, Tween 80, gum arabic gelatin, and Span.

[0044] According to some embodiments of the present application, the mass ratio of the disodium ethylenediaminetetraacetate and the first ammonia water in the raw materials for preparing the copper dispersion solution is 3-3.5:1. For example, it can be specifically 3:1, 3.2:1, 3.3:1, 3.4:1, 3.5:1, or a range value formed by any two of the above point values.

[0045] According to some embodiments of the present application, the mass ratio of the dispersant and the first ammonia water in the raw materials for preparing the copper dispersion solution is 2.5-3.5:1. For example, it can be specifically 2.5:2, 3:1, 3.5:1, or a range value formed by any two of the above point values.

[0046] According to some embodiments of the present application, the mass concentration of the copper powder in the raw materials for preparing the copper dispersion solution is 10-15%. For example, it can be specifically 10%, 11%, 12%, 13%, 14%, 15%, or a range value formed by any two of the above point values.

[0047] According to some embodiments of the present application, the preparation method of the copper dispersion solution includes mixing the raw materials except the copper powder, and then mixing the obtained mixture with the copper powder. The mixing time with the copper powder is 20-40 min. For example, it can be specifically 30 min. In actual production, the mass transfer rate can be improved by means such as stirring.

[0048] According to some embodiments of the present application, the mass ratio of the silver complex solution and the copper dispersion solution is 1:1.2-3. For example, it can be specifically 1:1.4, 1:1.5, 1:2, 1:2.5, 1:3, or a range value formed by any two of the above point values.

[0049] According to some embodiments of the present invention, in the raw materials for preparing the silver complexing solution, the mass ratio of the water-soluble silver salt to the complexing agent is 1:2 to 5. For example, it can be 1:2, 1:3, 1:4, 1:5; or a range of values ​​consisting of any two of the above points.

[0050] According to some embodiments of the present invention, the aminocarboxylic acid complexing agent includes at least one of EDTA and aminotriacetic acid.

[0051] According to some embodiments of the present invention, the polyamine compound includes at least one of diethylenetriamine, triethylenetetramine, and ethylenediamine.

[0052] According to some embodiments of the present invention, the carboxylic acid complexing agent includes at least one of citrate and thioglycolic acid.

[0053] According to some embodiments of the present invention, the complexing agent comprises ethylenediamine. This complexing agent does not introduce impurities and is relatively simple to control under certain conditions, thus it is the most preferred option, and the formed silver complex is [Ag(en)2]. + .

[0054] According to some embodiments of the present invention, the water-soluble silver salt includes silver nitrate.

[0055] According to some embodiments of the present invention, the mass concentration of the water-soluble silver salt in the raw materials for preparing the silver complex solution is 5-10%. For example, it can be 5%, 6%, 7%, 7.1%, 8%, 9%, 10%; or a range of values ​​composed of any two of the above points.

[0056] According to some embodiments of the present invention, the silver complex solution is added at a rate of 2–5 mL / min. Specifically, this could be 2 mL / min, 3 mL / min, 4 mL / min, or 5 mL / min; or a range consisting of any two of the above values. Controlling this addition rate effectively controls the total duration and rate of the displacement reaction. Within this range, uneven silver layer distribution caused by rapid displacement can be avoided.

[0057] According to some embodiments of the present invention, the solute in the reducing solution includes at least one of ascorbic acid, glucose, triethanolamine and sodium borohydride.

[0058] According to some embodiments of the present invention, the mass concentration of the solute in the reducing solution is 5% to 15%. For example, it can be 5%, 6%, 7%, 8%, 9%, 10%, 12%, 15%, or a range of values ​​composed of any two of the above points.

[0059] According to some embodiments of the present invention, the mass ratio of the reducing solution to the copper dispersion is 1:2 to 5. For example, it can be 1:2, 1:3, 1:4, 1:5, or a range of values ​​consisting of any two of the above points.

[0060] According to some embodiments of the present invention, the time difference between the initial addition of the silver complex solution and the reducing solution is 3 to 8 minutes; for example, it can be 5 minutes.

[0061] According to some embodiments of the present invention, the dropping rate of the reducing solution is 15 to 25 mL / min; for example, it can be 20 mL / min.

[0062] According to some embodiments of the present invention, the dripping time of the reducing solution is ≤12 min; for example, it can be 10 min. In actual production, this dripping time is used as the standard, and the dripping rate of the reducing solution can be adjusted if necessary.

[0063] According to some embodiments of the present invention, the mass ratio of the second ammonia solution to the copper dispersion is 1:20 to 60. For example, it can be 1:20, 1:25, 1:30, 1:32, 1:34, 1:36, 1:38, 1:40, 1:4, 1:50, 1:55, 1:60; or a range of values ​​consisting of any two of the above points.

[0064] According to some embodiments of the present invention, the second ammonia solution is added 8 to 12 minutes after the reducing solution is added.

[0065] The second ammonia solution is added by pouring, which is equivalent to adding it instantly.

[0066] According to some embodiments of the present invention, the addition time of the silver complexing solution is 70-100 minutes; for example, it can be 80 minutes, 85 minutes, 90 minutes; or a range of any two of the above values. Based on time calculations, it can be seen that the silver complexing solution should continue to be added after the second ammonia water addition is completed.

[0067] According to some embodiments of the present invention, the preparation method further includes continuing the reaction after the silver complex solution is added dropwise. The duration of the continued reaction is 35 to 50 minutes; specifically, it can be 35 minutes, 40 minutes, 45 minutes, 50 minutes; or a range of values ​​composed of any two of the above points.

[0068] According to some embodiments of the present invention, the temperature of the preparation reaction is 25–35°C. Within this temperature range, the properties of the resulting product are almost unaffected.

[0069] According to some embodiments of the present invention, the preparation method further includes a post-processing step after the continued reaction;

[0070] The post-processing steps include, sequentially, solid-liquid separation, water washing, alcohol washing, ball milling, and drying; wherein...

[0071] The solid-liquid separation method includes at least one of filtration, ion exchange, and static separation;

[0072] The alcohol washing is performed ≥ 2 times;

[0073] The ball milling is a wet ball milling process, and the solvent used includes ethanol; the mass ratio of solvent to solid material is 1:2 to 3; the milling medium is zirconium beads; the diameter of the zirconium beads is 2.5 to 3.5 mm; the ball-to-material ratio is 1:1 to 1.5; and the milling time is 90 to 120 min.

[0074] According to an embodiment of a third aspect of the present invention, a silver paste is provided, wherein the raw materials for preparing the silver paste include the silver-coated copper powder described in the first aspect of the present invention; or include the silver-coated copper powder prepared by the preparation method described in the second aspect of the present invention.

[0075] Since the silver paste adopts all the technical solutions of the silver-coated copper powder or preparation method of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments.

[0076] According to an embodiment of the fourth aspect of the present invention, an application of the silver paste provided in the third aspect of the present invention in the preparation of photovoltaic cells is provided.

[0077] Since the application adopts all the technical solutions of the silver paste in the above embodiments, it has at least all the beneficial effects brought about by the technical solutions in the above embodiments.

[0078] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. Attached Figure Description

[0079] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0080] Figure 1 This is a 2000x cross-sectional scanning electron microscope (SEM) image of the silver-coated copper powder obtained in Example 1 of the present invention;

[0081] Figure 2 This is a 2000x cross-sectional scanning electron microscope (SEM) image of the silver-coated copper powder obtained in Example 2 of the present invention;

[0082] Figure 3This is a 2000x cross-sectional scanning electron microscope (SEM) image of the silver-coated copper powder obtained in Example 3 of the present invention;

[0083] Figure 4 This is a 5000x cross-sectional scanning electron microscope (SEM) image of the silver-coated copper powder obtained in Example 1 of the present invention;

[0084] Figure 5 This is a 5000x cross-sectional scanning electron microscope (SEM) image of the silver-coated copper powder obtained in Example 2 of the present invention;

[0085] Figure 6 This is a 5000x cross-sectional scanning electron microscope (SEM) image of the silver-coated copper powder obtained in Example 3 of the present invention;

[0086] Figure 7 This is a cross-sectional scanning electron microscope (SEM) image of the silver-coated copper powder obtained in Comparative Example 1 of the present invention;

[0087] Figure 8 This is a cross-sectional scanning electron microscope (SEM) image of the silver-coated copper powder obtained in Comparative Example 2 of the present invention;

[0088] Figure 9 This is a cross-sectional scanning electron microscope (SEM) image of the silver-coated copper powder obtained in Comparative Example 3 of the present invention;

[0089] Figure 10 This is a cross-sectional scanning electron microscope (SEM) image of the silver-coated copper powder obtained in Comparative Example 4 of the present invention.

[0090] Figure 11 This is a cross-sectional scanning electron microscope (SEM) image of the defect particles in the silver-coated copper powder obtained in Example 2 of the present invention.

[0091] Figure 12 This is a cross-sectional scanning electron microscope (SEM) image of the defect particles in the silver-coated copper powder obtained in Example 1 of the present invention.

[0092] Figure 13 This is a cross-sectional scanning electron microscope (SEM) image of the defect particles in the silver-coated copper powder obtained in Example 3 of the present invention.

[0093] Figure 14 This is a cross-sectional scanning electron microscope (SEM) image of the defect particles in the silver-coated copper powder obtained in Comparative Example 1 of this invention.

[0094] Figure 15 This is a 2000x scanning electron microscope (SEM) image of the silver-coated copper powder obtained in Example 1 of the present invention;

[0095] Figure 16 This is a 5000x scanning electron microscope (SEM) image of the silver-coated copper powder obtained in Example 1 of this invention. Detailed Implementation

[0096] The following will describe the concept and technical effects of the present invention clearly and completely with reference to embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0097] Example 1

[0098] This example demonstrates the preparation of a silver-coated copper powder, the specific process of which is as follows:

[0099] S1. Material preparation, copper powder pretreatment:

[0100] 102g of self-produced copper powder with a particle size range of 1-10μm and a D50 of 3.4μm was sieved and solid inside.

[0101] Mix it with a 1.5% nitric acid aqueous solution, stir for 1 hour, and then filter. The amount of nitric acid aqueous solution used should be sufficient to submerge the copper powder.

[0102] Take 30 times the weight of copper powder in deionized water, stir and wash until near neutral (pH 6.5-7.5), then separate the solid and liquid components.

[0103] The copper powder obtained from solid-liquid separation is mixed with an 8% sodium hydroxide alkaline solution and stirred for 30 minutes to separate the solid and liquid. The amount of alkaline solution used must be sufficient to immerse the copper powder.

[0104] Take 15 times the weight of copper powder in deionized water, stir and wash until near neutral (pH 6.5-7.5), and filter to obtain copper powder filter cake;

[0105] Prepare copper dispersion:

[0106] Add 48.5g of disodium ethylenediaminetetraacetate (EDTA-2Na), 45g of polyvinylpyrrolidone (PVP), and 15g of ammonia (commercially available analytical grade ammonia, 1336-21-6) to 550g of deionized water to prepare a complexing solution. Take the above-treated copper powder filter cake (dried copper powder, about 100g in weight) and mix it thoroughly with the complexing solution for 30 minutes to ensure that the copper powder is evenly dispersed in the complexing solution, thus obtaining a copper dispersion.

[0107] Weigh out 16.7g of ammonia solution separately for later use (second ammonia solution, with the same composition as the first ammonia solution);

[0108] Prepare the reducing solution:

[0109] 12.5g of ascorbic acid was completely dissolved in 200mL of deionized water to prepare a reducing solution;

[0110] Prepare the silver complexing solution:

[0111] Prepare a silver nitrate solution by mixing 18.3g of silver nitrate with 165g of deionized water, and then mix it with 71.1g of ethylenediamine to prepare a silver complex solution.

[0112] S2. Reaction

[0113] The silver complex solution was slowly added dropwise to the copper dispersion at a rate of approximately 3.3 mL / min to induce a displacement reaction, and the silver layer quickly coated the surface of the copper powder particles.

[0114] Keeping the dropping rate of the silver complex solution constant, after 5 minutes of dropping, the reducing solution was added dropwise into the copper dispersion at a rate of 20 mL / min, and the dropping was completed in about 10 minutes.

[0115] Keeping the dropping rate of the silver complex solution constant, 10 minutes after the addition of the reducing solution, the second ammonia solution was added to the copper dispersion by pouring and flowing in parallel. At this time, the overall reaction proceeded for 25 to 30 minutes.

[0116] Maintain the dropping rate of the silver complex solution at a constant rate until the silver complex solution is completely consumed; then continue the reaction for about 45 minutes.

[0117] The reaction process is controlled at a temperature of 25–35°C throughout.

[0118] S3. Post-processing

[0119] The mixture obtained in step S2 was allowed to stand, the supernatant was discarded after sedimentation, the solid was repeatedly washed with deionized water, the solid was washed twice with anhydrous ethanol, and then filtered.

[0120] The cleaned solid product was added back into anhydrous ethanol at a mass ratio of 1:2.5 (anhydrous ethanol:solid product = 1:2.5), and then ball-milled for 105 min using 3 mm zirconium beads at a ball-to-solid mass ratio of 1:1.25. After ball milling, the product was filtered and dried to obtain silver-coated copper powder.

[0121] The prepared silver-coated copper powder has a particle size of 1-10 μm, a D50 of 3.41 μm, and a silver content of 10%. The silver content was measured using the fire assay method. The actual silver content was slightly lower than the theoretical value because the silver reaction could not reach 100%.

[0122] Example 2

[0123] This example demonstrates the preparation of a silver-coated copper powder, the specific process of which is as follows:

[0124] S1. Material preparation, copper powder pretreatment:

[0125] 102g of self-produced copper powder with a particle size range of 1-10μm and a D50 of 3.4um was sieved and solid inside.

[0126] Mix it with a 1.5% nitric acid aqueous solution, stir for 1 hour, and then filter. The amount of nitric acid aqueous solution used should be sufficient to submerge the copper powder.

[0127] Take 30 times the weight of copper powder in deionized water, stir and wash until near neutral (pH 6.5-7.5), then separate the solid and liquid components.

[0128] The copper powder obtained from solid-liquid separation is mixed with an 8% sodium hydroxide alkaline solution and stirred for 30 minutes to separate the solid and liquid. The amount of alkaline solution used must be sufficient to immerse the copper powder.

[0129] Take 15 times the weight of copper powder in deionized water, stir and wash until near neutral (pH 6.5-7.5), and filter to obtain copper powder filter cake;

[0130] Prepare copper dispersion:

[0131] Add 48.5g of disodium ethylenediaminetetraacetate (EDTA-2Na), 45g of polyvinylpyrrolidone (PVP), and 15g of ammonia (commercially available analytical grade ammonia, content (NH3) 25%-28%, 1336-21-6) (first ammonia) to 550g of deionized water to prepare a complexing solution. Take the above-treated copper powder filter cake (dried copper powder, mass about 100g) and mix it thoroughly with the complexing solution for 30 minutes to make the copper powder uniformly dispersed in the complexing solution, thus obtaining a copper dispersion.

[0132] Weigh out 20g of ammonia solution separately for later use (the second ammonia solution has the same composition as the first ammonia solution);

[0133] Prepare the reducing solution:

[0134] 15g of ascorbic acid was completely dissolved in 200mL of deionized water to prepare a reducing solution;

[0135] Prepare the silver complexing solution:

[0136] Prepare a silver nitrate solution by mixing 22g of silver nitrate with 198g of deionized water, and then mix it with 85.3g of ethylenediamine to prepare a silver complex solution.

[0137] S2. Reaction

[0138] The silver complex solution was slowly added dropwise to the copper dispersion at a rate of approximately 3.3 mL / min to induce a displacement reaction, and the silver layer quickly coated the surface of the copper powder particles.

[0139] Keeping the dropping rate of the silver complex solution constant, after 5 minutes of dropping, the reducing solution was added dropwise into the copper dispersion at a rate of 20 mL / min, and the dropping was completed in about 10 minutes.

[0140] Keeping the dropping rate of the silver complex solution constant, 10 minutes after the addition of the reducing solution, the second ammonia solution was added to the copper dispersion by pouring and flowing in parallel. At this time, the overall reaction proceeded for 25 to 30 minutes.

[0141] Maintain the dropping rate of the silver complex solution at a constant rate until the silver complex solution is completely consumed; then continue the reaction for about 45 minutes.

[0142] The reaction process is controlled at a temperature of 25–35°C throughout.

[0143] The reaction is carried out under stirring. Stirring is only to improve mass transfer, and the conditions are not important.

[0144] S3. Post-processing

[0145] The mixture obtained in step S2 was allowed to stand, the supernatant was discarded after sedimentation, the solid was repeatedly washed with deionized water, the solid was washed twice with anhydrous ethanol, and then filtered.

[0146] The cleaned solid product was added back into anhydrous ethanol at a mass ratio of 1:2.5, and then ball-milled for 105 min using 3 mm zirconium beads with a ball-to-material mass ratio of 1:1.25. After ball milling, the product was filtered and dried to obtain silver-coated copper powder.

[0147] The prepared silver-coated copper powder has a particle size of 1-10 μm, a D50 of 3.48 μm, and a silver content of 12%. The silver content was measured using the fire assay method. The actual silver content was slightly lower than the theoretical value because the silver reaction could not reach 100%.

[0148] Example 3

[0149] This example prepares a silver-coated copper powder, which differs from Example 1 in that:

[0150] In step S1, the amount of the second ammonia solution used is 33.3g;

[0151] The method for preparing the reducing solution is as follows:

[0152] 25g of ascorbic acid was completely dissolved in 200mL of deionized water to prepare a reducing solution;

[0153] The preparation method for silver complex solution is as follows:

[0154] Prepare a silver nitrate solution by mixing 36.7g of silver nitrate with 330g of deionized water, and then mix it with 142.2g of ethylenediamine to prepare a silver complex solution.

[0155] The silver-coated copper powder obtained in this example has a particle size of 1-10 μm, D50 = 3.53 μm, and a silver content of 20%.

[0156] Comparative Example 1

[0157] This example prepares a silver-coated copper powder, which differs from Example 2 in that:

[0158] In step S1, no first ammonia solution is added during the preparation of the copper dispersion.

[0159] Comparative Example 2

[0160] This example prepares a silver-coated copper powder, which differs from Example 2 in that:

[0161] In step S1, the preparation process of the silver complex solution is as follows: take 22g of silver nitrate and 198g of deionized water to prepare a silver nitrate solution, and mix it with 20g of ammonia water to prepare a silver complex solution.

[0162] Comparative Example 3

[0163] This example involves the purchase of copper powder for silver packaging.

[0164] Comparative Example 4

[0165] This example involves the purchase of copper powder for silver packaging.

[0166] Test case

[0167] The first aspect of this example tested the particle size, overall morphology, cross-sectional morphology, and silver content of the silver-coated copper powder obtained in the examples and comparative examples. The proportion of defective particles and the interfacial porosity (l / d) within the defective particles were statistically determined using the cross-sectional morphology.

[0168] The particle size was tested using a laser particle size analyzer.

[0169] The morphology was measured using SEM, specifically CP-SEM (section electron microscopy).

[0170] The statistical method for the proportion of defective particles is as follows: observe the silver-coated copper powder particles within the field of view, mark the defective particles with interface pores, and calculate the ratio of the number of defective particles to the total number of particles; if the magnification is 2000x, the number of silver-coated copper powder particles within the field of view must be >150; if the magnification is 5000x, the number of silver-coated copper powder particles within the field of view must be >10.

[0171] The statistical method for calculating the interfacial porosity in defective particles is as follows: The longest span *l* of the interfacial pores in a single defective particle is measured, and the circumference *d* of the equivalent circle of a single defective particle is measured; finally, 1 / d is calculated. In an electron microscopy test sample, a 3000x electron microscopy field of view is randomly selected, all defective particles are identified, and the *l* / d value is calculated. Furthermore, since the *l* of the defective particles in the silver-coated copper powder obtained in the example is very small, the value of *l* / d is very small, and even a slight error in the value of *d* has little impact on the final result. It should be noted that the overall morphology of defective particles is usually also defective; therefore, the morphology and particle size consistency of the selected defective particles for testing are relatively poor. Specific test examples are as follows... Figures 11-14 As shown. Figure 14 There are multiple particles within a single field of view, and Figures 11-13 Only one particle is shown in a single field of view. This is because the proportion of defective particles in the example is small, and the l / d value of the defective particles is small, requiring the search for defective particles and magnification for observation. In contrast, the comparative example typically shows many defective particles in a single field of view, and the l / d value is large, eliminating the need for magnification for measurement. Furthermore, due to the large number of defective particles in the comparative example, only a portion of the defective particles were selected for measurement.

[0172] The method for testing silver content is the fire assay method.

[0173] The results of the above tests are as follows Figures 1-16 And as shown in Tables 1 and 2, where, Figures 5-6 In sections 11-14, the dimensions are only for calculating l / d or statistical purposes; the specific results are summarized in Tables 1-2. Figures 5-6 The numbers marked in 11 to 14 do not affect the actual content of the invention.

[0174] Table 1. Statistics on particle size, silver content, proportion of defective particles, and porosity of defective particles of the silver-coated copper powder obtained in the examples and comparative examples.

[0175]

[0176] In Table 1, " / " indicates that no counting statistics were performed.

[0177] from Figures 1-6 The results show that the silver-coated copper powder obtained in Examples 1-3 contains almost no defective particles, and the copper particles have no pores inside.

[0178] from Figures 7-10The results of Comparative Example 1 and 13 show that the silver-coated copper powder provided by the present invention has a large number of pores, and the pores are relatively large. Even though the calculations are not indicated in Table 1, it is still easy to see that the proportion of defective particles and the interfacial porosity are significantly higher than those of the examples. Specifically, in the silver-coated copper powder prepared in Comparative Example 1, since the first ammonia water was not added, the bond between the silver coating layer and the copper powder is not tight, and interfacial pores are easily generated. In Comparative Example 2, ammonia water was used as the complexing agent of the silver complexing solution, which could not suppress the rate of silver salt participating in the substitution reaction, resulting in the inability to form a dense and continuous silver coating layer. Therefore, individual silver nuclei can be seen to form multiple fine silver powders under an electron microscope.

[0179] Table 2 shows the statistical results of the specific porosity of the silver-coated copper powder obtained in Examples 1-3 and Comparative Example 1.

[0180]

[0181] The second aspect of this example tested the acid resistance of the silver-coated copper powder obtained in the examples and comparative examples. Specifically, the silver-coated copper powder obtained in the examples or comparative examples was mixed with 90% acetic acid at a solid-liquid ratio of 1g:20mL. After stirring for 30 seconds, the color change of the supernatant after the powder settled was observed. The color change time for Examples 1-2 was over 20 minutes, and for Example 3 it reached over 40 minutes. In contrast, the supernatant of Comparative Examples 2-4 showed a clear blue color after only 30 seconds of stirring. This indicates that the number of intact particles after silver coating is closely related to the acid oxidation resistance of the silver-coated copper powder. Only when the number of intact particles reaches 85% or more can stable performance be guaranteed during application; and the stability of the silver-coated copper powder increases with the increase of the proportion of intact particles.

[0182] The third aspect of this example tested the oxygen content and resistivity of the silver-coated copper powder obtained in the embodiments and comparative examples, and the test method was as follows:

[0183] Oxygen content test: Automatic potentiometric titration method was used. Resistance test: A slurry was prepared using 89.6% silver-coated copper powder, 8.2% commercially available organic carrier, and 2.2% commercially available acrylic glass. This slurry was screen-printed onto the solar cell and sintered at 230°C. The resistance was then measured using a resistance meter. The organic carrier consisted of 50% acrylic resin, 30% diethylene glycol butyl ether, 10% ethyl cellulose, 5% epoxy-modified polyamide, and 5% hydrogenated castor oil. The acrylic glass consisted of 35% SiO2, 20% B2O3, 30% Bi2O3, and 15% Li2O.

[0184] The specific test results are shown in Table 3.

[0185] Table 3. Oxygen content and resistivity of the silver-coated copper powder obtained in the examples and comparative examples.

[0186] Powder oxygen content Internal self-resistivity μΩ-cm Example 1 0.059％ 33.40 Example 2 0.021％ 29.73 Example 3 0.018％ 25.61 Comparative Example 1 0.210％ 68.55 Comparative Example 2 0.350％ 76.17 Comparative Example 3 0.630％ 79.51 Comparative Example 4 0.740％ 83.32

[0187] In Table 3, low oxygen content indicates low oxidation level and strong oxidation resistance. This shows that in the silver-coated copper powder provided in the embodiments of the present invention, when the proportion of defective particles is relatively equal, the oxidation resistance decreases and the resistivity increases with the increase of the porosity of the defective particle interface.

[0188] In summary, the silver-coated copper powder provided by this invention, through reasonable preparation method adjustments, can significantly reduce the proportion of defective particles and lower the interfacial porosity. Therefore, it is undeniable that the silver-coated copper powder obtained by this invention possesses strong antioxidant properties and low resistivity. Due to these advantages, the silver-coated copper powder obtained by this invention can be used to prepare high-quality silver paste, and the resulting silver paste is expected to have wide applications in the photovoltaic field.

[0189] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention. Furthermore, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

Claims

1. A silver-coated copper powder, characterized in that, The silver-coated copper powder includes copper particles and a silver layer coated on the surface of the copper particles. The copper particles have no internal pores; The silver-coated copper powder comprises intact particles; under a scanning electron microscope, there are no pores between the silver layer and the copper particles of the intact particles. In the silver-coated copper powder, the percentage of intact particles is ≥85%.

2. The silver-coated copper powder according to claim 1, characterized in that, The silver-coated copper powder also includes defective particles; under a scanning electron microscope, pores exist between the silver layer and the copper particles of the defective particles.

3. The silver-coated copper powder according to claim 2, characterized in that, Under scanning electron microscopy, in a single defective particle, the ratio of the longest span l of the pore to the perimeter d of the defective particle is l / d ≤ 5%.

4. The silver-coated copper powder according to claim 3, characterized in that, Under scanning electron microscopy, the longest span of the pores in a single defective particle is l ≤ 500 nm.

5. The silver-coated copper powder according to any one of claims 1 to 4, characterized in that, The silver-coated copper powder contains 10-20% silver by mass.

6. A method for preparing silver-coated copper powder, characterized in that, The preparation method includes the following steps: A silver complexing solution was added to a copper dispersion, and simultaneously a reducing solution and a second ammonia solution were added to the copper dispersion. The raw materials for preparing the copper dispersion include copper powder and first ammonia water; the raw materials for preparing the silver complex solution include water-soluble silver salt and complexing agent; the complexing agent includes at least one of aminocarboxylic acid complexing agents, thiourea, thiosulfate, polyamine compounds and carboxylic acid complexing agents.

7. The preparation method according to claim 6, characterized in that, In the raw materials for preparing the copper dispersion, the mass ratio of the first ammonia water to the copper powder is 1:5 to 10. Preferably, the raw materials for preparing the copper dispersion further include disodium ethylenediaminetetraacetate, a dispersant, and water; Preferably, in the raw materials for preparing the copper dispersion, the mass concentration of the copper powder is 10-15%; Preferably, in the raw materials for preparing the silver complexing solution, the mass ratio of the water-soluble silver salt to the complexing agent is 1:2 to 5; Preferably, in the raw materials for preparing the silver complex solution, the mass concentration of the water-soluble silver salt is 5-10%; Preferably, the silver complex solution is added at a rate of 2–5 mL / min; Preferably, the mass concentration of the solute in the reducing solution is 5-15%; Preferably, the mass ratio of the reducing solution to the copper dispersion is 1:2 to 5.

8. The preparation method according to claim 6, characterized in that, The mass ratio of the second ammonia solution to the copper dispersion is 1:20 to 60.

9. A silver paste, characterized in that, The raw materials for preparing the silver paste include the silver-coated copper powder as described in any one of claims 1 to 5; or the silver-coated copper powder prepared by the preparation method described in any one of claims 6 to 8.

10. An application of the silver paste as described in claim 9 in the preparation of photovoltaic cells.

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