Nano silver powder and preparation method and application thereof

Abstract

The invention provides nano silver powder as well as a preparation method and application thereof. The preparation method of the nano silver powder comprises the steps that a reducing agent, a pH regulator, a first dispersing agent and a first part of water are subjected to first mixing, a reducing solution is obtained, and the temperature of the reducing solution is controlled to be 0-20 DEG C; silver nitrate, a second dispersing agent and a second part of water are subjected to second mixing, an oxidizing solution is obtained, the temperature is adjusted to-5 DEG C to-20 DEG C, and an oxidizing solid is obtained; and thirdly, under the conditions that the temperature is 0-20 DEG C and the speed is 10-200 rpm, the oxidizing solid and the reducing solution are subjected to third mixing, and the nanometer silver powder is obtained. By changing the state of the materials, solidifying the oxidation system containing the silver salt into the ice crystals and mixing the ice crystals with the reduction system at low temperature, the characteristic that the materials are converted from the solid state to the liquid state is utilized, the production cost of the nano-silver powder is remarkably reduced, the yield and purity of the nano-silver powder are improved, operation is easy, control is convenient, and the method is suitable for industrial production. The method is suitable for industrial large-scale production.

Description

Technical Field

[0001] The present invention relates to the technical field of nanomaterials, and in particular, to a nano silver powder, a preparation method thereof, and an application thereof. Background Art

[0002] Currently, due to its excellent antibacterial and conductive properties, nano silver powder has broad application prospects in the fields of biomedicine, electronic industry, catalytic materials, etc.

[0003] Traditional methods for preparing nano silver powder usually use ethylene glycol or polyethylene glycol as a reducing agent and a solvent. For example, Chinese application CN202011498607.9 discloses a method for preparing a nano silver powder with controllable morphology. This method consumes a large amount of ethylene glycol, which not only increases the preparation cost, but also the high boiling point and high viscosity characteristics of ethylene glycol lead to difficulties in subsequent processing, such as high energy consumption in the cleaning and drying processes, affecting the mass production efficiency of nano silver powder.

[0004] Another type of method, such as Chinese application CN202010848330.1, uses a diol as a solvent and polyvinylpyrrolidone (PVP) as a dispersant. The usage amount of PVP is as high as 15 times that of silver nitrate, and the mass ratio of nano silver to the dispersant reaches 1:24. This not only significantly increases the material cost, but also due to the residue of the polymer dispersant, the subsequent cleaning process is complex, and it is difficult to ensure the purity of the nano silver powder, thus affecting its performance and application. In addition, Chinese application CN202010604712.X proposes a method for preparing nano silver powder, in which the usage amount of PVP is 2.4 times that of silver nitrate. Although the particle size control has been improved, the mass ratio of nano silver to the dispersant is still as high as 1:3.8. The excessive use of the dispersant also increases the preparation cost and may cause agglomeration problems in the subsequent processing, reducing the dispersibility and stability of the silver powder. In Chinese application CN202011589085.3, the usage amount of the dispersant is 1 to 10 times, and further 2 to 5 times, of the silver content. Although the aggregation of nano silver powder is controlled to a certain extent, the large amount of dispersant used leads to a significant increase in cost, and the removal process of the dispersant is complex, affecting the yield and purity of nano silver powder.

[0005] There are also some methods, such as Chinese applications CN117300148A and CN117900504A, which, by using a combination of multiple dispersants and different organic solvent components, although improve the dispersibility and stability of nano silver powder to a certain extent, the total usage amount of the dispersant is still relatively high. This not only increases the cost burden, but also increases the difficulty of subsequent cleaning and separation, further affecting the yield and application performance of nano silver powder.

[0006] The common problems existing in the above prior art are as follows: 1) The use of relatively expensive and high-dose reducing agents and dispersants results in a relatively high preparation cost of nano-silver powder; 2) The large addition of organic substances such as dispersants, as well as the small particle size and difficult sedimentation characteristics of nano-silver powder itself, make the subsequent cleaning and separation processes complex and costly, affecting the purity, quality and yield of nano-silver powder. Summary of the Invention

[0007] The main object of the present invention is to provide a nano-silver powder, a preparation method thereof and an application, so as to solve the problems of high preparation cost, low purity, unstable quality and low yield of nano-silver powder in the prior art.

[0008] To achieve the above object, according to one aspect of the present invention, a preparation method of nano-silver powder is provided, including the following steps: Step S1, perform a first mixing of a reducing agent, a pH regulator, a first dispersant and a first portion of water to obtain a reducing solution, and control the temperature of the reducing solution to be 0-20°C; Step S2, perform a second mixing of silver nitrate, a second dispersant and a second portion of water to obtain an oxidizing solution, adjust the temperature to -5 to -20°C to obtain an oxidizing solid; Step S3, perform a third mixing of the oxidizing solid and the reducing solution at 0-20°C and 10-200 rpm to obtain nano-silver powder.

[0009] Further, in Step S1, the temperature of the reducing solution is controlled to be 0-5°C.

[0010] Further, in Step S2, the temperature of the oxidizing solution is adjusted to -5 to -10°C to obtain an oxidizing solid, and / or Step S2 further includes a step of maintaining the oxidizing solid at -5 to -20°C for 2-20 h.

[0011] Further, in Step S3, the temperature of the third mixing is 0-5°C, and / or the rotation speed of the third mixing is 50-100 rpm.

[0012] Further, the reducing agent includes one or more of ascorbic acid, glucose, hydrazine hydrate, formaldehyde, hydroxylamine hydrochloride, sodium hypophosphite and sodium borohydride, and / or the actual dosage of the reducing agent is 1.1-3 times of the theoretical dosage; and / or the pH regulator includes one or more of ammonia water, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate and potassium bicarbonate, and / or the pH of the reducing solution is controlled to be 4-11; and / or the first dispersant includes one or more of gelatin, gum arabic, polyvinylpyrrolidone and polyethylene glycol, and / or the weight ratio of the first dispersant to silver nitrate is (0.02-0.3):1; and / or the weight ratio of the first portion of water to silver nitrate is (10-20):1.

[0013] Further, in step S2, the second dispersant includes one or more of gelatin, gum arabic, polyvinylpyrrolidone, and polyethylene glycol, and / or the weight ratio of the second dispersant to silver nitrate is (0.02 - 0.3):1; and / or the weight ratio of the second part of water to silver nitrate is (5 - 15):1; and / or the reducing solution further includes a complexing agent; preferably, the complexing agent includes one or more of ammonia water, ethylenediaminetetraacetic acid, salicylic acid, sodium thiosulfate, and sodium cyanide; and / or the molar ratio of the complexing agent to silver element in silver nitrate is (3 - 6):1.

[0014] Further, step S3 further includes a step of coating the nano silver powder with a coating agent; preferably, the coating agent includes one or more of stearic acid, erucic acid, oleic acid, palmitic acid, and myristic acid; preferably, the weight ratio of the coating agent to silver nitrate is (0.003 - 0.03):1.

[0015] Further, step S3 further includes a step of washing the nano silver powder; preferably, water is used for washing; and / or step S3 further includes a step of drying the nano silver powder; preferably, freeze-drying is performed, the temperature is 15 - 30 °C, and the time is 5 - 15 h.

[0016] According to another aspect of the present invention, a nano silver powder obtained by using the above preparation method is provided.

[0017] According to another aspect of the present invention, the application of the above nano silver powder in the photovoltaic and semiconductor fields is provided.

[0018] By changing the state of the materials, the present invention solidifies the oxidation system including silver salt into ice crystals, mixes it with the reduction system at low temperature, and utilizes the characteristics of the materials transforming from solid state to liquid state. This process simulates the role of diol in the traditional process, that is, reducing the release rate of silver ions and controlling their reduction, while avoiding the problems brought by the high cost and high dosage of diol. The molecular kinetic energy weakens under low temperature conditions, and coupled with the natural barrier provided by the ice crystal solidification, it effectively simulates the spatial barrier effect of the polymer dispersant, which can greatly reduce the usage amount of the traditional dispersant, not only saving costs, but also simplifying the subsequent washing steps and avoiding the problems of the decline in the performance of silver powder and batch stability caused by the residual dispersant. The impregnation-reaction process starting from the surface of the ice crystal forms a nucleation-growth gradient of silver crystals. By controlling the melting rate of the ice crystal, the release rate of silver ions is precisely regulated, realizing the precise control and uniform dispersion of the particle size of the nano silver powder.

[0019] The preparation method of the present invention significantly reduces the production cost of nano silver powder, speeds up the precipitation speed of silver powder, and thus improves the yield and purity of nano silver powder. Moreover, the preparation method of the present invention is simple to operate, the process parameters are simple and easy to control, and it is suitable for large-scale industrial production. Brief Description of the Drawings

[0020] The accompanying drawings forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0021] Figure 1 shows a schematic diagram of the mechanism of Embodiment 1 of the present invention;

[0022] Figure 2 shows an SEM image of the silver nanoparticles of Embodiment 1 of the present invention;

[0023] Figure 3 shows an SEM image of the sintered silver nanoparticles of Embodiment 1 of the present invention;

[0024] Figure 4 shows an SEM image of the silver nanoparticles of Embodiment 2 of the present invention;

[0025] Figure 5 shows an SEM image of the sintered silver nanoparticles of Embodiment 2 of the present invention;

[0026] Figure 6 shows an SEM image of the silver nanoparticles of Embodiment 3 of the present invention;

[0027] Figure 7 shows an SEM image of the sintered silver nanoparticles of Embodiment 3 of the present invention;

[0028] Figure 8 shows an SEM image of the silver nanoparticles of Embodiment 4 of the present invention;

[0029] Figure 9 shows an SEM image of the sintered silver nanoparticles of Embodiment 4 of the present invention; and

[0030] Figure 10 shows an SEM image of the silver nanoparticles of Comparative Example 1 of the present invention;

[0031] Among them, the above-mentioned drawings include the following reference numerals:

[0032] A, silver ion; B, reducing agent; C, silver nanoparticles; D, dispersant; 1, infiltration reaction. Detailed Description of the Embodiments

[0033] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the drawings and in combination with the embodiments.

[0034] It should be noted that in the description and claims of the present invention, "first", "second", etc. are used to distinguish similar objects, and do not necessarily describe a specific order or sequence. It should be understood that such data used can be interchanged under appropriate circumstances to describe embodiments of the present invention.

[0035] As described in the background art of the present invention, in the prior art, there are problems such as high preparation cost, low purity, unstable quality, and low yield of nano silver powder. To solve the above problems, in a typical embodiment of the present invention, a method for preparing nano silver powder is provided, including the following steps: Step S1, perform a first mixing of a reducing agent, a pH regulator, a first dispersant, and a first portion of water to obtain a reducing solution, and control the temperature of the reducing solution to be 0 - 20°C; Step S2, perform a second mixing of silver nitrate, a second dispersant, and a first portion of water to obtain an oxidizing solution, adjust the temperature to -5 - -20°C to obtain an oxidizing solid; Step S3, under the conditions of 0 - 20°C and 10 - 200 rpm, perform a third mixing of the oxidizing solid and the reducing solution to obtain nano silver powder.

[0036] As mentioned in the background art, in the prior art, the preparation schemes of nano silver powder mainly revolve around the mode of slowing down the reduction rate of silver ions (reducing with diols with low reducing ability) or high-intensity spatial barrier (dispersants several times the amount of the finished nano silver), but there are problems such as high raw material cost and difficult cleaning of silver powder.

[0037] In step S1 of the preparation method of the present application, the reducing agent, the pH regulator, the first dispersant are mixed with water to form a reducing solution. By controlling at a low temperature, the occurrence of side reactions can be reduced, ensuring the stability and controllability of the reduction process.

[0038] In step S2, silver nitrate is mixed with the second dispersant and water to form an oxidizing solution, and then the solution is converted into a solid. This process can effectively fix silver ions. At the same time, by utilizing the characteristic that the molecular kinetic energy weakens at low temperature, a spatial barrier effect similar to that of a polymer dispersant can be provided, creating conditions for the controlled release and uniform reduction of silver ions in the subsequent process.

[0039] In step S3, a solid silver salt system and a liquid reduction system are combined to precisely synthesize silver nanoparticles at a specific temperature and rotation speed. During this process, the silver salt solidifies into ice crystals at low temperature, and silver ions A are uniformly locked inside the ice crystal structure, forming a natural and high-strength spatial barrier environment. When the reducing solution contacts the oxidizing solid, the reducing agent B gradually penetrates the surface of the ice crystal from the outside to the inside (i.e., wetting reaction 1), reacts with the silver ions, and forms particles of silver nanocrystals C. Based on the physical structure of the ice crystal, from the outer layer to the inner layer, the infiltration amount of the reducing agent gradually decreases and the concentration gradually drops. This gradient change prompts the reduction reaction rate of the silver ions to gradually decrease, thereby forming a nucleation-growth gradient of silver crystals from the surface to the inside of the ice crystal. The silver crystals in the inner layer grow more finely, while those in the outer layer are relatively coarser. This unique nucleation-growth process can not only limit the excessive growth of silver particles and avoid the occurrence of agglomeration, but also promote the uniform distribution of silver grains and ensure the controllability of the particle size of silver nanopowder. For ease of understanding, Figure 1 a schematic diagram of the mechanism is listed.

[0040] The key to step S3 lies in the weakening of molecular kinetic energy at low temperature and the solidification effect of the ice crystal lattice. The combined action of the two can simulate the spatial barrier effect of a polymer dispersant, significantly reduce the usage amount of dispersant D, and at the same time can effectively reduce the risk of secondary agglomeration of silver powder.

[0041] Furthermore, the present application specifically limits the temperature and rotation speed of the third mixing. Within this specific range, the dissolution rate of the solidified material can be finely regulated, that is, the ice crystal melting rate can be controlled, and then the release rate of silver ions can be precisely adjusted, effectively regulating the growth of silver crystal particles and realizing the preparation of silver powder with nanoscale size.

[0042] In addition, it should be noted that, on the one hand, compared with the reaction system of diol at 100 - 160 °C and the reaction system of a high-usage dispersant at 50 - 100 °C in the prior art solutions, the reaction system in the present application is in a low-temperature state throughout the process (such as -20 - 20 °C). Under this condition, it is not only beneficial to reduce molecular kinetic energy during reduction, but also beneficial to reduce the secondary agglomeration after the reduction of silver nanoparticles. On the other hand, the present application still needs to use a dispersant, but its more important role is to coat and block - prevent agglomeration after the formation of silver nanopowder, and the usage amount is relatively low.

[0043] In summary, by changing the state of the material, the present application solidifies the oxidation system including silver salt into ice crystals and mixes it with the reduction system at low temperature, innovatively utilizing the property of the material transforming from solid state to liquid state. This process simulates the role of diol in the traditional process, that is, reducing the release rate of silver ions and controlling their reduction, while avoiding the problems caused by the high cost and high dosage of diol. The molecular kinetic energy weakens under low-temperature conditions, and together with the natural barrier provided by the ice crystal solidification, it effectively simulates the steric hindrance effect of polymer dispersants, greatly reducing the usage amount of traditional dispersants. This not only saves costs but also simplifies the subsequent cleaning steps, avoiding the problems of silver powder performance degradation and batch stability caused by dispersant residues. The penetration-reaction process starting from the surface of the ice crystal forms a nucleation-growth gradient of silver crystals. By controlling the melting rate of the ice crystal, the release rate of silver ions can be precisely regulated, achieving precise control and uniform dispersion of the particle size of nano silver powder.

[0044] The preparation method of the present application can significantly reduce the production cost of nano silver powder, accelerate the precipitation speed of silver powder, and thus is beneficial to improving the yield and purity of nano silver powder. Moreover, the preparation method of the present application is simple to operate, with simple and easy-to-control process parameters, and is suitable for large-scale industrial production.

[0045] In addition, the present application mainly uses the oxidation system including silver salt as the frozen solid system and the reduction system as the liquid phase system, and slowly releases silver ions during the preparation process. Similarly, there are similar effects when using the reduction system as the frozen solid system and the oxidation system including silver salt as the liquid phase system for preparation, which will not be elaborated here. In a preferred embodiment, the oxidizing solid is a cylindrical whole and / or a crushed solid, preferably a cylindrical whole.

[0046] In a preferred embodiment, in step S1, the temperature of the reducing solution is controlled to be 0 - 5 °C. Under the above conditions, the molecular kinetic energy can be more effectively reduced, the stability and reaction selectivity of the reducing agent can be improved. Moreover, the reducing agent, in cooperation with the pH regulator and the dispersant, can further promote the fine nucleation of nano silver particles, thereby realizing the low-cost preparation of high-quality nano silver powder.

[0047] In a preferred embodiment, in step S2, the temperature of the oxidizing solution is adjusted to -5 - -10 °C to obtain the oxidizing solid. Under the above conditions, by using low-temperature solidification of the silver salt system and in cooperation with the dispersant, silver ions can be more effectively fixed and their activity can be reduced, realizing precise control of the particle size of nano silver powder, which is beneficial to greatly reducing the subsequent cleaning cost and improving the yield.

[0048] To further enhance the stability of the ice crystal structure, in a preferred embodiment, step S2 further includes a step of maintaining the oxidizing solid at -5 to -20°C for 2 to 20 hours, preferably 2 to 5 hours. Under the above conditions, the ice crystal structure acts synergistically with the reducing agent and the dispersant, enabling more precise control of silver ion release, with more uniform particle size of the silver nanopowder, lower cost, and higher preparation efficiency.

[0049] In a preferred embodiment, in step S3, the temperature of the third mixing is 0 to 5°C, and / or the rotation speed of the third mixing is 50 to 100 rpm. Under the above conditions, the slow release of silver ions can be controlled more precisely and effectively, which can further promote the uniform nucleation and growth of the silver nanopowder, and can also significantly reduce the production cost, improve the cleaning efficiency and yield. In a preferred embodiment, the temperature of the third mixing is the same as the temperature of the reducing solution in step S1.

[0050] The inventors further optimized the composition of the reducing solution to achieve the preparation of silver nanopowder with low cost and high quality. In a preferred embodiment, the reducing agent includes one or more of ascorbic acid, glucose, hydrazine hydrate, formaldehyde, hydroxylamine hydrochloride, sodium hypophosphite, and sodium borohydride, preferably hydrazine hydrate or ascorbic acid, and / or the actual amount of the reducing agent used is 1.1 to 3 times the theoretical amount. It should be noted that the aforementioned theoretical amount refers to the exact chemical amount of the reducing agent required to complete the expected chemical reaction under stoichiometric ratio, which is understandable to those skilled in the art; and / or the pH regulator includes one or more of ammonia water, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate, preferably ammonia water or sodium hydroxide, and / or the pH of the reducing solution is controlled to be 4 to 11, and / or the first dispersant includes one or more of gelatin, gum arabic, polyvinylpyrrolidone, and polyethylene glycol, preferably polyvinylpyrrolidone, and the weight ratio of the first dispersant to silver nitrate is (0.02 to 0.3):1, and / or the weight ratio of the first part of water to silver nitrate is (10 to 20):1.

[0051] In a preferred embodiment, in step S2, the second dispersant includes one or more of gelatin, gum arabic, polyvinylpyrrolidone, and polyethylene glycol, preferably polyvinylpyrrolidone, and the weight ratio of the second dispersant to silver nitrate is (0.02 - 0.3):1; preferably, the types of the first dispersant and the second dispersant are the same; preferably, the contents of the first dispersant and the second dispersant are the same; and / or, the weight ratio of the second part of water to silver nitrate is (5 - 15):1; and / or, the reducing solution further includes a complexing agent; preferably, the complexing agent includes one or more of ammonia water, ethylenediaminetetraacetic acid, salicylic acid, sodium thiosulfate, and sodium cyanide, preferably ammonia water; and / or, the molar ratio of the complexing agent to the silver element in silver nitrate is (3 - 6):1. Under the above conditions, silver ions can be more effectively fixed in the low-temperature ice crystals, coordinating the low-temperature curing and subsequent slow melting processes, which is beneficial to precisely control the particle size of the nano silver powder, reduce the dosage of the polymer dispersant, lower the cost, and improve the yield.

[0052] To more effectively prevent the agglomeration of particles and improve the stability and application performance of the nano silver powder, in a preferred embodiment, step S3 further includes a step of coating the nano silver powder with a coating agent. For the purpose of more effectively inhibiting the agglomeration of the nano silver powder, in a preferred embodiment, the coating agent includes one or more of stearic acid, erucic acid, oleic acid, palmitic acid, and myristic acid, preferably stearic acid and oleic acid; and / or, the weight ratio of the coating agent to silver nitrate is (0.003 - 0.03):1. Under the above conditions, the quality of the nano silver powder is more stable and the subsequent treatment is simpler.

[0053] In a preferred embodiment, step S3 further includes a step of washing the nano silver powder; preferably, water is used for washing, which has lower raw material costs and is more economical; and / or, step S3 further includes a step of drying the nano silver powder; preferably, the drying is freeze-drying, the temperature is 15 - 30 °C, and the time is 5 - 15 h. More preferably, step S3 further includes the steps of washing, coating, and drying the nano silver powder in sequence. Under the above conditions, impurities can be more effectively removed, avoiding secondary agglomeration at high temperatures, ensuring the high purity and stability of the silver powder, and further simplifying the post-treatment process and reducing energy consumption.

[0054] In another typical embodiment of the present invention, a nano silver powder obtained by using the above preparation method is also provided. As mentioned above, through the low-temperature curing of the solid-liquid mixing method in this application, coordinating the actions of components such as reducing agents, dispersants, and complexing agents, the particle size distribution of the nano silver powder can be effectively controlled, the particle stability can be enhanced, the production cost can be greatly reduced, and the obtained nano silver powder has a small and uniform particle size, stable quality, and high yield, purity, and low cost. In a preferred embodiment, the particle size of the nano silver powder is 10 - 500 nm, preferably 20 - 100 nm.

[0055] In another typical embodiment of the present invention, the above-mentioned nano silver powder is also provided for use in the photovoltaic and semiconductor fields. Using the nano silver powder of the present application in the photovoltaic field can effectively balance the raw material cost and performance.

[0056] The following further describes the present application in detail with specific embodiments, which should not be construed as limiting the scope claimed by the present application.

[0057] Example 1

[0058] The components of the reducing solution include: 1.4 kg of a reducing agent (hydrazine hydrate, with a weight percentage content of 50%), 1 kg of a pH regulator (ammonia water, with the weight percentage content controlled within 25 - 28%), 1 kg of a first dispersant (polyvinylpyrrolidone), and a first portion of water.

[0059] The components of the oxidizing solution include: 10 kg of a silver salt (silver nitrate), 15 kg of a complexing agent (ammonia water, with a weight percentage content of 26.5%), 2 kg of a second dispersant (polyvinylpyrrolidone), and a second portion of water.

[0060] The coating agent is 50 g of oleic acid.

[0061] Step S1: Add the reducing agent to a 250 L reaction kettle, dilute it with 70 L of water, add the pH regulator to adjust the pH to 8.5, add the pre-dissolved solution containing the first dispersant polyvinylpyrrolidone, and finally add water to prepare a 120 L reduction system. Adjust the temperature of the reaction kettle to control the temperature of the reducing solution at 2°C.

[0062] Step S2: Place the silver salt silver nitrate in a 150 L batching kettle, dissolve it with 50 L of water, add the complexing agent ammonia water to prepare a silver ammonia solution, add the pre-dissolved solution containing the second dispersant polyvinylpyrrolidone, add water to prepare a 100 L silver system and stir evenly; adjust the temperature to -10°C, cool down the system together with the stirring paddle and freeze it into a solid, and keep it for 5 h after freezing to make the ice crystals dense.

[0063] Step S3: Transfer the oxidizing solid (together with the stirring paddle) into the reaction kettle system, start stirring, slowly stir at a speed of 20 r / min, adjust the temperature of the reaction kettle to 2°C, and slowly react and melt the solidified material. After the solidified material has completely reacted and melted, wash the soft-aggregated nano silver powder with deionized water, then add the coating agent oleic acid to disperse and coat the nano silver powder, and freeze-dry it at 15°C for 15 h to obtain spherical nano silver powder with a size of 20 - 30 nm. The SEM image of the nano silver powder is shown in Figure 2 .

[0064] The nano silver powder is used in the photovoltaic HJT field. Specifically, the nano silver powder and the organic solvent with a weight ratio of 80:20 are mixed, and then evenly coated on a glass slide and sintered at 200 °C for 30 min. The SEM image of the nano silver powder after sintering is shown in Figure 3 .

[0065] Example 2

[0066] The difference from Example 1 is only that:

[0067] In step S1, the temperature of the reaction kettle is adjusted to control the temperature of the reducing solution at 6 °C;

[0068] In step S3, it is slowly stirred at a speed of 50 r / min, and the temperature of the reaction kettle is adjusted to 6 °C.

[0069] Spherical nano silver powder with a size of 30 - 50 nm is prepared.

[0070] The SEM image of the nano silver powder is shown in Figure 4 . The SEM image of the nano silver powder after sintering is shown in Figure 5 .

[0071] Analysis shows that by changing the temperature and stirring speed, especially increasing the temperature and accelerating the speed, the melting speed of the frozen solid can be increased, thereby increasing the particle size.

[0072] Example 3

[0073] The difference from Example 1 is only that:

[0074] The components of the reducing solution include: 5.5 kg of reducing agent (ascorbic acid), 1.2 kg of pH regulator (sodium hydroxide), 1 kg of first dispersant (polyvinylpyrrolidone) and the first part of water.

[0075] The components of the oxidizing solution include: 10 kg of silver salt (silver nitrate), 1 kg of second dispersant (polyvinylpyrrolidone) and the second part of water.

[0076] The coating agent is 32 g of oleic acid.

[0077] In step S1, the reducing agent is added to a 250 L reaction kettle, dissolved with 70 L of water, 1.2 kg is added to adjust the pH to 8.5, the pre-dissolved solution containing polyvinylpyrrolidone is added, and finally water is added to prepare a 120 L reduction system, and the temperature is adjusted to 6 °C.

[0078] In step S2, silver salt is added to a 150 L batching kettle, dissolved with 50 L of water, no complexing agent is added, a pre-dissolved solution containing polyvinylpyrrolidone is added, and water is added to prepare a 100 L silver system and stirred evenly; the temperature is adjusted to -10 °C, and the system together with the stirring paddle is cooled and frozen into a solid, and the freezing is maintained for 5 h to make the ice crystals dense.

[0079] In step S3, the oxidizing solid (together with the stirring paddle) is transferred into the reaction kettle system, stirring is started and slowly stirred at a speed of 20 r / min, the temperature of the reaction kettle is controlled at 6 °C, and the solidified material is slowly reacted and melted. After the solidified material is completely reacted and melted, the soft-aggregated nano-silver powder is washed with deionized water, and then the coating agent oleic acid is added to disperse and coat the nano-silver powder, and freeze-dried at 20 °C for 10 h to obtain 50 - 70 nm microcrystalline nano-silver powder.

[0080] The SEM image of the nano-silver powder is shown in Figure 6 The SEM image of the sintered nano-silver powder is shown in Figure 7 。

[0081] Analysis shows that different forms of nano-silver powder can be prepared by using different reducing agents and supporting processes.

[0082] Example 4

[0083] The difference from Example 3 is only that:

[0084] In step S1, 1 kg of pH regulator sodium hydroxide is added to adjust the pH, and a pre-dissolved solution containing 0.5 kg of the first dispersant polyvinylpyrrolidone is added;

[0085] In step S2, a pre-dissolved solution containing 0.5 kg of the second dispersant polyvinylpyrrolidone is added.

[0086] 70 - 150 nm microcrystalline nano-silver powder is prepared.

[0087] The SEM image of the nano-silver powder is shown in Figure 8 The SEM image of the sintered nano-silver powder is shown in Figure 9 。

[0088] Analysis shows that by changing the pH value of the system and using a lower dosage of the dispersant, nano-silver powder with a larger particle size can be prepared.

[0089] Example 5

[0090] The difference from Example 1 is only that:

[0091] In step S1, the temperature of the reaction kettle is adjusted, and the temperature of the reducing solution is controlled at 0 °C.

[0092] Step S2: Adjust the temperature to -10°C, cool down the system together with the stirring paddle and freeze it into a solid, and keep it for 2 hours after freezing to make the ice crystals dense.

[0093] Step S3: Stir slowly at a speed of 100 r / min, adjust the temperature of the reaction kettle to 0°C, and slowly melt the solidified material by reaction.

[0094] After cleaning, freeze-dry at 15°C for 15 hours to obtain silver nanoparticles with a size of 15 - 20 nm.

[0095] Analysis shows that within the effective process range, silver nanoparticles can be stably prepared.

[0096] Example 6

[0097] The difference from Example 1 is only that:

[0098] Step S1: Adjust the temperature of the reaction kettle and control the temperature of the reducing solution to 5°C.

[0099] Step S2: Adjust the temperature to -5°C, cool down the system together with the stirring paddle and freeze it into a solid, and keep it for 5 hours after freezing to make the ice crystals dense.

[0100] Step S3: Stir slowly at a speed of 50 r / min, adjust the temperature of the reaction kettle to 5°C, and slowly melt the solidified material by reaction.

[0101] After cleaning, freeze-dry at 20°C for 10 hours to obtain silver nanoparticles with a size of 40 - 60 nm.

[0102] Analysis shows that within the effective process range, silver nanoparticles can be stably prepared.

[0103] Example 7

[0104] The difference from Example 1 is only that:

[0105] Step S1: Adjust the temperature of the reaction kettle and control the temperature of the reducing solution to 0°C.

[0106] Step S2: Adjust the temperature to -20°C, cool down the system together with the stirring paddle and freeze it into a solid, and keep it for 2 hours after freezing to make the ice crystals dense.

[0107] Step S3: Stir slowly at a speed of 10 r / min, adjust the temperature of the reaction kettle to 0°C, and slowly melt the solidified material by reaction.

[0108] After cleaning, freeze-dry at 15°C for 15 hours to obtain silver nanoparticles with a size of 15 - 20 nm.

[0109] Analysis shows that within the effective process range, silver nanoparticles can be stably prepared.

[0110] Example 8

[0111] The difference from Example 1 is only that:

[0112] In step S1, adjust the temperature of the reaction kettle to control the temperature of the reducing solution at 20°C.

[0113] In step S2, adjust the temperature to -5°C, cool down the system together with the stirring paddle and freeze it into a solid, and keep it for 20 h after freezing to make the ice crystals dense.

[0114] In step S3, stir slowly at a speed of 200 r / min, adjust the temperature of the reaction kettle to 20°C, and slowly react and melt the solidified material.

[0115] After cleaning, freeze-dry at 30°C for 5 h to obtain 200 - 300 nm silver nanoparticles.

[0116] Analysis shows that within the effective process range, silver nanoparticles can be stably prepared.

[0117] Example 9

[0118] The components of the reducing solution include: 0.18 kg of reducing agent (sodium borohydride), no pH regulator, 0.25 kg of the first dispersant (gum arabic), and the first part of water.

[0119] The components of the oxidizing solution include: 5 kg of silver nitrate, 5 kg of complexing agent (ammonia water, with the weight percentage controlled within 25 - 28%), 0.25 kg of the second dispersant (gum arabic), and the second part of water.

[0120] The coating agent is 0.05 kg of hexadecanoic acid.

[0121] In step S1, add the reducing agent to a 250 L reaction kettle, add 70 L of water to dissolve it, the pH is about 10.0 after dissolution, add the pre-dissolved solution containing the first dispersant gum arabic, and finally add water to prepare a 120 L reduction system. Adjust the temperature of the reaction kettle to control the temperature of the reducing solution at 2°C.

[0122] In step S2, place silver salt silver nitrate in a 150 L batching kettle, add 50 L of water to dissolve it, add the complexing agent ammonia water to prepare a silver ammonia solution, add the pre-dissolved solution containing the second dispersant gum arabic, add water to prepare a 100 L silver system and stir evenly; adjust the temperature to -10°C, cool down the system together with the stirring paddle and freeze it into a solid, and keep it for 10 h after freezing to make the ice crystals dense.

[0123] Step S3: Transfer the oxidizing solid (along with the stirring paddle) into the reaction kettle system, start stirring, and stir slowly at a speed of 10 r / min. Adjust the temperature of the reaction kettle to 2 °C to allow the solidified material to react and melt slowly. After the solidified material has completely reacted and melted, wash the soft-aggregated silver nanoparticles with deionized water, then add the coating agent hexadecanoic acid to disperse and coat the silver nanoparticles, and freeze-dry at 15 °C for 15 h to obtain flaky silver nanoparticles with a size of 50-100 nm.

[0124] Analysis shows that silver nanoparticles can also be prepared by adjusting conditions such as the pH value and concentration under different reduction systems.

[0125] Example 10

[0126] The components of the reducing solution include: 10 kg of reducing agent (formaldehyde, with a weight percentage of 37-40%), 0.5 kg of pH regulator (sodium carbonate), 0.5 kg of the first dispersant (a combined dispersant of 50% gelatin + 50% polyethylene glycol), and the first part of water.

[0127] The components of the oxidizing solution include: 5 kg of silver salt (silver nitrate), 3.5 kg of complexing agent (ammonia water, with the weight percentage controlled within 25-28%), 0.5 kg of the second dispersant (a combined dispersant of 50% gelatin + 50% polyethylene glycol), and the second part of water.

[0128] The coating agent is 30 g of stearic acid.

[0129] Step S1: Add the reducing agent to a 250 L reaction kettle, dilute it with 70 L of water, add the pH regulator to adjust the pH to 8.0, add the pre-dissolved solution containing the first dispersant, and finally add water to prepare a 120 L reduction system. Adjust the temperature of the reaction kettle to control the temperature of the reducing solution at 2 °C.

[0130] Step S2: Place silver nitrate, the silver salt, in a 150 L batching kettle, dissolve it with 50 L of water, add the complexing agent ammonia water to prepare a silver ammonia solution, add the pre-dissolved solution containing the second dispersant, add water to prepare a 100 L silver system and stir evenly; adjust the temperature to -10 °C, cool down the system together with the stirring paddle and freeze it into a solid, and keep it for 5 h after freezing to make the ice crystals dense.

[0131] Step S3: Transfer the oxidizing solid (along with the stirring paddle) into the reaction kettle system, start stirring, and stir slowly at a speed of 50 r / min. Adjust the temperature of the reaction kettle to 2 °C to allow the solidified material to react and melt slowly. After the solidified material has completely reacted and melted, wash the soft-aggregated silver nanoparticles with deionized water, then add the coating agent to disperse and coat the silver nanoparticles, and freeze-dry at 30 °C for 10 h to obtain spherical silver nanoparticles with a size of 200-400 nm.

[0132] Analysis shows that nano - silver powder can also be prepared under different reduction systems and dispersant combinations.

[0133] Comparative Example 1

[0134] The difference from Example 3 is only that: the process of freezing into a solid was not adopted during the preparation process, and the reaction was completed using a conventional feeding method;

[0135] Specifically, in step S1, the temperature of the reduction system was 25°C.

[0136] In step S2, the temperature of the oxidation system was 25°C.

[0137] In step S3, start the stirring of the reaction kettle at a speed of 200 r / min, control the temperature of the reaction kettle at 25°C, add the materials in the batching kettle to the reaction kettle system at a speed of 5 L / min, and let the materials react completely. After the materials react completely, add 20 g of oleic acid to conduct hydrophobic dispersion coating on the nano - silver powder, then use deionized water to wash the soft - agglomerated nano - silver powder, and freeze - dry to obtain microcrystalline nano - silver powder.

[0138] The SEM image of the nano - silver powder is shown in Figure 10 .

[0139] Analysis shows that compared with this comparative example, the low - temperature freezing - solid slow - release process in Example 3 can, under the same dosage of dispersant, more precisely control the release of silver ions, making the particle size of the prepared nano - silver powder smaller and achieving the preparation of nano - scale silver powder.

[0140] The test results of the nano - silver powder prepared in the above examples and comparative examples are shown in Table 1.

[0141] Test method:

[0142] Yield test: The yield of silver powder is the ratio of the mass M1 of the actually prepared nano - silver powder to the theoretical amount M0 of silver powder obtained by reducing silver salt.

[0143] Purity test: Weigh the mass M1 of the nano - silver powder, sinter it at 530°C for 30 min to burn off the organic components on the surface of the nano - silver powder, and obtain the mass M2 of the sintered silver powder; the purity of silver powder = M2 / M1.

[0144] Sintering test: Use the prepared nano - silver powder, mix it with an organic solvent in a ratio of 80:20, then evenly coat it on a glass slide, and sinter it at 200°C for 30 min. Analyze the SEM of the nano - silver powder after sintering.

[0145] Table 1

[0146]

[0147] As can be seen from the above, in each embodiment of the present invention, by changing the state of the material, the oxidation system including silver salt is solidified into ice crystals and mixed with the reduction system at low temperature, innovatively utilizing the property of the material transforming from solid state to liquid state. This process simulates the role of diol in the traditional process, that is, reducing the release rate of silver ions and controlling their reduction, while avoiding the problems brought by the high cost and high dosage of diol. The molecular kinetic energy weakens under low temperature conditions, and coupled with the natural barrier provided by the ice crystal solidification, it effectively simulates the steric hindrance effect of polymer dispersants, greatly reducing the usage amount of traditional dispersants. This not only saves costs but also simplifies the subsequent cleaning steps, avoiding the problems of silver powder performance degradation and batch stability caused by the residue of dispersants. The infiltration-reaction process starting from the surface of the ice crystal forms a nucleation-growth gradient of silver crystals. By controlling the melting rate of the ice crystal, the release rate of silver ions is precisely regulated, achieving precise control and uniform dispersion of the particle size of nano silver powder.

[0148] The preparation method of the present invention significantly reduces the production cost of nano silver powder, speeds up the precipitation rate of silver powder, and thus improves the yield and purity of nano silver powder. Moreover, the preparation method of the present invention is simple to operate, with simple process parameters and easy to control, and is suitable for large-scale industrial production.

[0149] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of nano silver powder, characterized in that, It includes the following steps: Step S1: First, mix a reducing agent, a pH regulator, a first dispersant, and a first portion of water to obtain a reducing solution, and control the temperature of the reducing solution to be 0 - 20°C; Step S2: Second, mix silver nitrate, a second dispersant, and a second portion of water to obtain an oxidizing solution, adjust the temperature to -5 - -20°C to obtain an oxidizing solid; Step S3: Third, under the conditions of 0 - 20°C and 10 - 200 rpm, mix the oxidizing solid and the reducing solution to obtain the nano - silver powder.

2. The preparation method according to claim 1, wherein In step S1, control the temperature of the reducing solution to be 0 - 5°C.

3. The preparation method according to claim 1 or 2, wherein In step S2, adjust the temperature of the oxidizing solution to -5 - -10°C to obtain the oxidizing solid, and / or Step S2 further includes the step of maintaining the oxidizing solid at -5 - -20°C for 2 - 20 h.

4. The preparation method according to any one of claims 1 to 3, characterized in that, In step S3, the temperature of the third mixing is 0 - 5°C, and / or the rotation speed of the third mixing is 50 - 100 rpm.

5. The preparation method according to any one of claims 1 to 4, wherein The reducing agent includes one or more of ascorbic acid, glucose, hydrazine hydrate, formaldehyde, hydroxylamine hydrochloride, sodium hypophosphite, and sodium borohydride, and / or the actual dosage of the reducing agent is 1.1 - 3 times the theoretical dosage; and / or The pH regulator includes one or more of ammonia water, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate, and / or control the pH of the reducing solution to be 4 - 11; and / or The first dispersant includes one or more of gelatin, gum arabic, polyvinylpyrrolidone, and polyethylene glycol, and / or the weight ratio of the first dispersant to silver nitrate is (0.02 - 0.3):1; and / or The weight ratio of the first portion of water to silver nitrate is (10 - 20):

1.

6. The preparation method according to any one of claims 1 to 5, characterized in that, In step S2, The second dispersant includes one or more of gelatin, gum arabic, polyvinylpyrrolidone, and polyethylene glycol, and / or the weight ratio of the second dispersant to silver nitrate is (0.02 - 0.3):1; and / or The weight ratio of the second portion of water to silver nitrate is (5 - 15):1; and / or The reducing solution further includes a complexing agent; preferably, the complexing agent includes one or more of ammonia water, ethylenediaminetetraacetic acid, salicylic acid, sodium thiosulfate, and sodium cyanide; and / or the molar ratio of the complexing agent to the silver element in silver nitrate is (3 - 6):

1.

7. The preparation method according to any one of claims 1 to 6, characterized in that, Step S3 further includes the step of coating the nano - silver powder with a coating agent; Preferably, the coating agent includes one or more of stearic acid, erucic acid, oleic acid, palmitic acid, and myristic acid; Preferably, the weight ratio of the coating agent to silver nitrate is (0.003 - 0.03):

1.

8. The preparation method according to any one of claims 1 to 7, wherein The step S3 further includes a step of washing the nano silver powder; preferably, water is used for the washing; and / or, The step S3 further includes a step of drying the nano silver powder; preferably, the drying is freeze-drying, the temperature is 15-30 °C, and the time is 5-15 h.

9. A nano silver powder, characterized in that, Obtained by using the preparation method according to any one of claims 1 to 8.

10. Application of the nano silver powder according to claim 9 in the fields of photovoltaics and semiconductors.

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