Preparation methods, applications, and properties of ultrafine nickel powder

Abstract

This invention provides a method for preparing ultrafine nickel powder, the ultrafine nickel powder itself, and its applications, specifically relating to the field of metal powder preparation technology. The preparation method includes the following steps: A) adding a soluble nickel source, a chelating agent, and a coating agent to a mixed solvent to obtain a nickel source solution; adding a reducing agent to another mixed solvent to obtain a reducing agent solution; B) adding the reducing agent solution to the nickel source solution to carry out a reaction, and centrifuging the reaction mother liquor after the reaction is complete; performing a first wash on the precipitate obtained by centrifugation; continuing to centrifuge the mother liquor obtained from the first wash, and performing a second and third wash; finally, washing the precipitate obtained from the third wash with deionized water to obtain a solid-liquid mixture; C) filtering and vacuum drying the solid-liquid mixture to obtain the ultrafine nickel powder.

Description

Technical Field

[0001] This invention relates to the field of metal powder preparation technology, and in particular to a method for preparing ultrafine nickel powder, the ultrafine nickel powder itself, and its applications. Background Technology

[0002] Ultrafine nickel powder is a type of metallic nickel powder with diameters reaching the micrometer or nanometer scale. Its small size, high specific surface energy, and large specific surface area give it excellent properties in mechanics, electricity, and magnetism, leading to its wide applications in metallurgy, electronics, chemical engineering, aerospace, and defense. Particularly in the field of conductive pastes, metallic nickel paste prepared using ultrafine nickel powder as the main raw material exhibits excellent conductivity, high chemical stability, low electromigration, good solderability, and good printability. It is widely used in devices such as multilayer ceramic chip capacitors (MLCCs), cathodes for DC plasma flat panel displays, grids for monocrystalline silicon solar cells, thermistors / humidity-sensitive thick-film resistors, and sensors.

[0003] The preparation methods of ultrafine nickel powder can be classified into three categories according to the reaction system state: solid-phase method, liquid-phase method, and gas-phase method. The solvent / solute reduction method in the liquid phase has advantages over other methods, including mild conditions, wide availability of nickel, simple equipment, easy operation, and fine particle size, uniform distribution, and controllable morphology. However, single-solvent reactions have some drawbacks. For example, polyol systems (ethylene glycol, glycerol, etc.) react relatively slowly, the low solubility of the nickel source leads to low yield, and the resulting products are difficult to separate. Furthermore, particle size cannot be effectively controlled due to solution viscosity and polarity. In aqueous systems, the reduction reaction has a high energy barrier, requiring highly reactive reducing agents (expensive sodium borohydride, hydrazine hydrate, etc., which are toxic to humans). When choosing non-toxic reducing agents (L-ascorbic acid, glucose, sodium hypophosphite, etc.), the energy barrier is too high to initiate the reduction reaction.

[0004] In actual industrial production, the post-processing methods for ultrafine nickel powder obtained by the liquid phase method are complicated and ineffective. The surface of ultrafine nickel powder is easily coated with viscous complex salts (such as xNiSO4·yNi(OH)2·zH2O) and adsorbs a large number of hydroxyl groups. This causes nickel hydroxide / nickel oxide to be mixed in during subsequent solid-liquid separation and vacuum drying. Alternatively, the hydroxyl groups adsorbed on the surface may undergo thermal condensation and dehydration to form bridging oxygen between nickel atoms. This not only significantly increases the oxygen content of the ultrafine nickel powder but also produces a relatively loose oxide layer, causing the nickel powder surface to crack and affecting its performance.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] One of the objectives of this invention is to provide a method for preparing ultrafine nickel powder, which aims to solve at least one of the above-mentioned technical problems.

[0007] The second objective of this invention is to provide an ultrafine nickel powder.

[0008] The third objective of this invention is to provide an application of ultrafine nickel powder in conductive paste.

[0009] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0010] The first aspect of the present invention provides a method for preparing ultrafine nickel powder, comprising the following steps:

[0011] A. A nickel source solution is obtained by adding a soluble nickel source, a chelating agent, and a coating agent to a mixed solvent; a reducing agent is obtained by adding a reducing agent to another mixed solvent.

[0012] B. Add a reducing agent solution to the nickel source solution, heat to carry out the reaction, and centrifuge the reaction mother liquor after the reaction is completed; perform a first wash on the precipitate obtained by centrifugation; continue to centrifuge the mother liquor obtained from the first wash, and perform a second and third wash; finally, wash the precipitate obtained from the third wash with deionized water to obtain a solid-liquid mixture.

[0013] C. The solid-liquid mixture is filtered and vacuum dried to obtain the ultrafine nickel powder.

[0014] The mixed solvent is a mixture of deionized water and polyol.

[0015] The chelating agent includes at least one of sodium citrate, sodium malate, and sodium succinate.

[0016] Preferably, the reducing agent includes at least one of L-ascorbic acid, glucose, cellulose, and sodium hypophosphite.

[0017] Further, the first cleaning is performed using a first cleaning agent.

[0018] The first cleaning agent comprises anhydrous ethanol, deionized water and 3 wt.% hydrogen peroxide solution in a volume ratio of (100-300):(100-300):(1-10).

[0019] Preferably, the second cleaning agent is used for the second cleaning.

[0020] The second cleaning agent comprises anhydrous ethanol, deionized water, and acid in a volume ratio of (100-300):(100-300):(0-10);

[0021] The acid includes carboxylic acids or non-oxidizing inorganic acids.

[0022] Preferably, the third cleaning is performed using a third cleaning agent.

[0023] The third cleaning agent comprises anhydrous ethanol and deionized water in a volume ratio of (1-3):1.

[0024] Furthermore, the first cleaning, the second cleaning, and the third cleaning are performed using ultrasonic cleaning.

[0025] Preferably, the ultrasonic cleaning time is 5 min to 10 min, and the temperature is <50℃.

[0026] Furthermore, in the mixed solvent, the volume ratio of deionized water to polyol is 1:1 to 5, preferably 1:2.

[0027] Preferably, the polyol includes at least one selected from ethylene glycol, 1,2-propanediol, 1,3-propanediol, and glycerol.

[0028] Preferably, the soluble nickel source includes at least one of nickel acetate, nickel chloride, nickel nitrate, and nickel sulfate.

[0029] Preferably, the coating agent comprises polyvinylpyrrolidone.

[0030] Furthermore, in the nickel source solution, the concentration of the soluble nickel source is 0.1 mol / L to 1 mol / L, preferably 0.2 mol / L to 0.4 mol / L.

[0031] Preferably, the concentration of the chelating agent in the nickel source solution is 0.002 mol / L to 0.05 mol / L, and more preferably 0.02 mol / L to 0.04 mol / L.

[0032] Preferably, in the nickel source solution, the concentration of the coating agent is 0.1 g / L to 20 g / L, more preferably 5 g / L to 10 g / L.

[0033] Preferably, the concentration of the reducing agent in the reducing agent solution is 0.3 mol / L to 3 mol / L, and more preferably 1.2 mol / L to 2.4 mol / L.

[0034] Preferably, in step B, the volume ratio of the nickel source solution to the reducing agent solution is 1:1.

[0035] Furthermore, in step B, the reaction temperature is 40℃~200℃, and the time is 0.5h~6h.

[0036] Preferably, in step B, the reaction temperature is 120℃~140℃ and the time is 2h~4h.

[0037] Furthermore, in step C, the vacuum drying temperature is 30℃~70℃, and the time is 4h~16h.

[0038] Preferably, the vacuum drying temperature is 40℃~50℃ and the time is 6h~8h.

[0039] The second aspect of the present invention provides ultrafine nickel powder prepared by the preparation method described above.

[0040] Furthermore, the particle size of the ultrafine nickel powder is 2μm to 5μm.

[0041] Preferably, the oxygen content of the ultrafine nickel powder is 1.5 wt.% to 2.5 wt.%.

[0042] The third aspect of this invention provides the application of the aforementioned ultrafine nickel powder in the preparation of conductive pastes.

[0043] Compared with the prior art, the present invention has at least the following beneficial effects:

[0044] The method for preparing ultrafine nickel powder provided by this invention uses a non-toxic reducing agent, avoiding potential negative impacts on the environment and operators, and solving the problem that non-toxic reducing agents cannot initiate reactions. It utilizes a mixed solvent system of polyol and water, combining the advantages of polyol systems (low surface tension, low nucleation barrier, high boiling point) and water systems (high solubility, moderate viscosity, and rapid growth) to prepare ultrafine nickel powder. Simultaneously, a chelating agent alters the electron cloud dispersion of nickel ions, lowering the reduction barrier and making it easier to reduce nickel ions to obtain ultrafine nickel powder. The post-treatment process thoroughly removes impurities from the nickel powder and performs a certain amount of pre-oxidation, maintaining a low oxygen content and good particle morphology, resulting in higher quality ultrafine nickel powder. This preparation method is simple, has a large batch capacity, good process controllability, and is suitable for industrial production.

[0045] The ultrafine nickel powder provided by this invention has a particle size of 2μm to 5μm and an oxygen content of 1.5wt.% to 2.5wt.%, which can better meet different application needs.

[0046] The application of ultrafine nickel powder in conductive pastes provided by this invention, given the advantages of the aforementioned ultrafine nickel powder, is beneficial for preparing conductive pastes with excellent conductivity, high chemical stability, low electromigration, good solderability, and good printability, which is conducive to the development of downstream industries. Attached Figure Description

[0047] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0048] Figure 1 Here is a SEM image of the ultrafine nickel powder obtained in Example 1;

[0049] Figure 2 This is a SEM image of the ultrafine nickel powder obtained in Comparative Example 4. Detailed Implementation

[0050] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown herein can generally be arranged and designed in various different configurations.

[0051] The first aspect of the present invention provides a method for preparing ultrafine nickel powder, comprising the following steps:

[0052] A. A nickel source solution is obtained by adding a soluble nickel source, a chelating agent, and a coating agent to a mixed solvent; a reducing agent is obtained by adding a reducing agent to another mixed solvent.

[0053] B. Add a reducing agent solution to the nickel source solution, heat to carry out the reaction, and centrifuge the reaction mother liquor after the reaction is completed; perform a first wash on the precipitate obtained by centrifugation; continue to centrifuge the mother liquor obtained from the first wash, and perform a second and third wash; finally, wash the precipitate obtained from the third wash with deionized water to obtain a solid-liquid mixture.

[0054] C. The solid-liquid mixture is filtered and vacuum dried to obtain the ultrafine nickel powder.

[0055] The mixed solvent is a mixture of deionized water and polyol.

[0056] The chelating agent includes at least one of sodium citrate, sodium malate, and sodium succinate.

[0057] The reducing agent includes at least one of L-ascorbic acid, glucose, cellulose, and sodium hypophosphite.

[0058] The method for preparing ultrafine nickel powder provided by this invention uses a non-toxic reducing agent, avoiding potential negative impacts on the environment and operators, and solving the problem that non-toxic reducing agents cannot initiate reactions. It utilizes a mixed solvent system of polyol and water, combining the advantages of polyol systems (low surface tension, low nucleation barrier, high boiling point) and water systems (high solubility, moderate viscosity, and rapid growth) to prepare ultrafine nickel powder. Simultaneously, a chelating agent alters the electron cloud dispersion of nickel ions, lowering the reduction barrier and making it easier to reduce nickel ions to obtain ultrafine nickel powder. The post-treatment process thoroughly removes impurities from the nickel powder and performs a certain amount of pre-oxidation, maintaining a low oxygen content and good particle morphology, resulting in higher quality ultrafine nickel powder. This preparation method is simple, has a large batch capacity, good process controllability, and is suitable for industrial production.

[0059] Further, the first cleaning is performed using a first cleaning agent.

[0060] The first cleaning agent comprises anhydrous ethanol, deionized water, and 3 wt.% hydrogen peroxide solution in a volume ratio of (100-300):(100-300):(1-10). The primary function of the first cleaning is pre-oxidation, forming a relatively dense thin oxide layer on the surface of the nickel powder.

[0061] Typical, but not limiting, volume ratios of anhydrous ethanol, deionized water, and 3 wt.% hydrogen peroxide solution can be, for example, 100:100:1, 100:100:5, 100:100:10, 300:100:1, 300:100:5, 300:100:10, 100:300:1, 100:300:5, or 100:300:10.

[0062] Preferably, the second cleaning agent is used for the second cleaning.

[0063] The second cleaning agent comprises anhydrous ethanol, deionized water, and an acid in a volume ratio of (100–300):(100–300):(0–10); the acid includes carboxyl acids or non-oxidizing inorganic acids. The carboxyl acids include formic acid, acetic acid, oxalic acid, or propionic acid; the non-oxidizing inorganic acids include dilute sulfuric acid or dilute hydrochloric acid.

[0064] Typical, but not limiting, volume ratios of anhydrous ethanol, deionized water, and acid can be, for example, 100:100:1, 100:100:5, 100:100:10, 300:100:1, 300:100:5, 300:100:10, 100:300:1, 100:300:5, or 100:300:10.

[0065] The second cleaning is mainly for removing impurities, such as excess coating agent and basic double salt.

[0066] Preferably, the third cleaning is performed using a third cleaning agent.

[0067] The third cleaning agent comprises anhydrous ethanol and deionized water in a volume ratio of (1-3):1. The main function of the third cleaning is to remove impurities, including some excess coating agent and acid.

[0068] In step B, the purpose of using deionized water for the third cleaning is to remove acid and prevent the ultrafine nickel powder from being exposed to air and oxidizing.

[0069] Furthermore, the first cleaning, the second cleaning, and the third cleaning are performed using ultrasonic cleaning.

[0070] Preferably, the ultrasonic cleaning time is 5 min to 10 min, and the temperature is <50℃.

[0071] During the filtration process, when approximately 25–50 mL of the filtrate remains, carefully pour in anhydrous ethanol to wash the nickel powder cake as much as possible, avoiding the possibility of the liquid being completely evaporated. The purpose of using anhydrous ethanol for washing is to minimize the adhesion of hydroxide ions in the water to the surface of the ultrafine nickel powder and prevent the formation of surface hydroxyl groups.

[0072] The overall idea of ​​the above cleaning process is pre-oxidation - cleaning agent to remove complex salts - removal of cleaning agent - reduction of surface hydroxyl groups. The present invention can adjust the cleaning agent and the number of cleaning times for each part within a certain range.

[0073] Furthermore, in the mixed solvent, the volume ratio of deionized water to polyol is 1:1 to 5, preferably 1:2.

[0074] Preferably, the polyol includes at least one selected from ethylene glycol, 1,2-propanediol, 1,3-propanediol, and glycerol.

[0075] Preferably, the soluble nickel source includes at least one of nickel acetate, nickel chloride, nickel nitrate, and nickel sulfate.

[0076] Preferably, the coating agent comprises polyvinylpyrrolidone.

[0077] Furthermore, in the nickel source solution, the concentration of the soluble nickel source is 0.1 mol / L to 1 mol / L, preferably 0.2 mol / L to 0.4 mol / L.

[0078] Typical, but not limiting, concentrations of soluble nickel sources can be, for example, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, or 1 mol / L.

[0079] Preferably, the concentration of the chelating agent in the nickel source solution is 0.002 mol / L to 0.05 mol / L, and more preferably 0.02 mol / L to 0.04 mol / L.

[0080] Chelating agents can alter the charge state, geometry, and reactivity of metal ions. In the reduction reaction involved in this invention, the chelating agent not only lowers the reaction energy barrier and promotes the reaction, but also stabilizes the reaction rate and prevents the explosive reduction of nickel ions from causing a decrease in the sphericity of nickel powder and the agglomeration of nickel powder particles. When the concentration of the chelating agent is below 0.002 mol / L, insufficient chelating agent dispersed on the nickel ions easily leads to a decrease in the sphericity of the nickel powder or large-scale agglomeration, or even failure to initiate the reaction. When the concentration of the chelating agent is above 0.05 mol / L, excessive chelating agent on the surface of the nickel ions reduces the effective concentration of nickel ions, also preventing the reaction from initiating.

[0081] Typical, but not limiting, concentrations of chelating agents can be, for example, 0.002 mol / L, 0.01 mol / L, 0.02 mol / L, 0.03 mol / L, 0.04 mol / L, or 0.05 mol / L.

[0082] Preferably, in the nickel source solution, the concentration of the coating agent is 0.1 g / L to 20 g / L, more preferably 5 g / L to 10 g / L.

[0083] Typical, but not limiting, concentrations of the coating agent may be, for example, 0.1 g / L, 0.5 g / L, 1 g / L, 5 g / L, 10 g / L, 15 g / L, or 20 g / L.

[0084] Preferably, the concentration of the reducing agent in the reducing agent solution is 0.3 mol / L to 3 mol / L, and more preferably 1.2 mol / L to 2.4 mol / L.

[0085] Typical, but not limiting, concentrations of the reducing agent can be, for example, 0.3 mol / L, 0.6 mol / L, 0.9 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, 2.4 mol / L, or 3 mol / L.

[0086] Preferably, in step B, the volume ratio of the nickel source solution to the reducing agent solution is 1:1. Further, in the centrifugal precipitation of this invention, the centrifugation speed is (500–5000) r / min, preferably 2000 r / min. The centrifugation time is (0.5–5) min, preferably 1 min.

[0087] The speed and time of centrifugation should be controlled. The speed should not be too fast and the time should not be too long to avoid the liquid from heating up or the nickel powder from agglomerating under the action of centrifugal force. It is sufficient to basically separate the solid and liquid phases.

[0088] The temperature for ultrasonic cleaning should be 50℃ or below, preferably 30℃ or below. The ultrasonic cleaning time and power should be adjusted to ensure the cleaning solution thoroughly washes the solid material and that the cleaning process is uniform.

[0089] Furthermore, in step B, the reaction temperature is 40℃~200℃, and the time is 0.5h~6h.

[0090] Preferably, in step B, the reaction temperature is 120℃~140℃ and the time is 2h~4h.

[0091] Furthermore, in step C, the vacuum drying temperature is 30℃~70℃, and the time is 4h~16h. For larger-scale nickel powder preparation, the drying temperature range remains unchanged, but more attention should be paid to the uniformity of heating of the wet nickel powder. This requirement can be ensured by appropriately extending the drying time, spreading the nickel powder as evenly as possible, or applying a certain amount of stirring.

[0092] Preferably, the vacuum drying temperature is 40℃~50℃ and the time is 6h~8h.

[0093] The second aspect of the present invention provides ultrafine nickel powder prepared by the preparation method described above.

[0094] The ultrafine nickel powder provided by this invention has a particle size of 2μm to 5μm and an oxygen content of 1.5wt.% to 2.5wt.%, which can better meet different application needs.

[0095] Furthermore, the particle size of the ultrafine nickel powder is 2μm to 5μm.

[0096] Preferably, the oxygen content of the ultrafine nickel powder is 1.5 wt.% to 2.5 wt.%.

[0097] The third aspect of this invention provides the application of the aforementioned ultrafine nickel powder in the preparation of conductive pastes.

[0098] The application of ultrafine nickel powder in conductive pastes provided by this invention, given the advantages of the aforementioned ultrafine nickel powder, is beneficial for preparing conductive pastes with excellent conductivity, high chemical stability, low electromigration, good solderability, and good printability, which is conducive to the development of downstream industries.

[0099] The following detailed description of some embodiments of the present invention is provided in conjunction with examples. Unless otherwise specified, the following embodiments and features can be combined with each other. Unless otherwise specified, the raw materials used in the present invention are commercially available.

[0100] Example 1

[0101] This embodiment provides an ultrafine nickel powder, the preparation process of which is as follows:

[0102] 1. Prepare a mixed solvent by mixing 100 mL of deionized water and 200 mL of ethylene glycol. Add 25.3 g of nickel chloride hexahydrate, 2 g of sodium citrate, and 2 g of PVP to 200 mL of the mixed solvent and stir until homogeneous to obtain a nickel source solution.

[0103] 2. Add 26.4g of sodium hypophosphite to the remaining 100mL of mixed solvent and stir until homogeneous to obtain a reducing agent solution.

[0104] 3. Under stirring, add the reducing agent solution to the nickel source solution, and heat the container to 130°C using an oil bath to react for 2 hours to obtain the reaction mother liquor.

[0105] 4. Centrifuge the mother liquor to collect the precipitate. Add the first cleaning agent to the precipitate. The volume ratio of anhydrous ethanol, deionized water, and 3% hydrogen peroxide solution in the first cleaning agent is 150:150:5. Sonicate for 8 minutes. Centrifuge to collect the precipitate. Add the second cleaning agent to the precipitate. The volume ratio of anhydrous ethanol, deionized water, and glacial acetic acid in the second cleaning agent is 150:150:5. Sonicate for 8 minutes. Centrifuge to collect the precipitate. Add the third cleaning agent to the precipitate. The volume ratio of anhydrous ethanol and deionized water in the third cleaning agent is 1:1. Sonicate for 8 minutes. Continue centrifuging to collect the precipitate for a fourth cleaning. The fourth cleaning uses deionized water.

[0106] 5. Continue to centrifuge the solid-liquid mixture obtained from the fourth washing to precipitate. Add deionized water to the precipitate and filter it. When about 30 mL of the mixture remains after filtration, slowly pour in 800 mL of anhydrous ethanol to rinse the nickel powder cake. The rinsing should be continuous to avoid the liquid being dried out.

[0107] 6. Interrupt the vacuum pump within 10 seconds of complete liquid filtration to prevent the nickel powder from being oxidized by contact with a large flow of air; send the filtered ultrafine nickel powder into a vacuum oven and dry it at 50°C for 7 hours to obtain ultrafine nickel powder.

[0108] During the preparation process, it was found that the mother liquor after the reaction was yellowish-brown or black, with a black precipitate. After washing and drying, a black powder was obtained. The mixed solvent method increased the boiling point, allowing the reduction reaction to occur at a higher temperature.

[0109] Example 2

[0110] This embodiment provides an ultrafine nickel powder. The difference from Embodiment 1 is that sodium succinate is used instead of sodium citrate. All other raw materials and methods are the same as in Embodiment 1, and will not be repeated here.

[0111] During the preparation process, it was found that the mother liquor after the reaction was yellow-green or black, with black precipitate. After washing and drying, black powder was obtained.

[0112] Example 3

[0113] This embodiment provides an ultrafine nickel powder. The difference from Embodiment 1 is that 54.0g of L-ascorbic acid is used instead of 26.4g of sodium hypophosphite. The other raw materials and methods are the same as in Embodiment 1, and will not be repeated here.

[0114] During the preparation process, it was found that the mother liquor after the reaction was black, and there was black precipitate, but it was significantly less than in Examples 1 and 2. After washing and drying, a small amount of black powder was obtained because the reducing power of L-ascorbic acid is slightly weaker than that of sodium hypophosphite.

[0115] Example 4

[0116] This embodiment provides an ultrafine nickel powder. The difference from Example 1 is that the mixed solvent contains 200 mL of deionized water and 100 mL of ethylene glycol. In step 3, the container is heated to 95°C and reacted for 2 hours using an oil bath. The remaining raw materials and methods are the same as in Example 1, and will not be repeated here.

[0117] During the preparation process, it was found that increasing the proportion of water resulted in more water vapor escaping, and the time it took for the solution to change from green to yellowish-brown increased. The final product agglomerated, and the yield decreased. This is because the increase in water volume leads to a decrease in the overall viscosity of the solution, which affects the dispersibility of the nickel powder. Furthermore, with the increase in water volume, the solution can only react at a lower temperature, which reduces the reducing power of the reducing agent, resulting in a decrease in yield.

[0118] Example 5

[0119] This embodiment provides an ultrafine nickel powder, the preparation process of which is as follows:

[0120] 1. Prepare a mixed solvent by mixing 100 mL of deionized water and 200 mL of ethylene glycol. Add 28.52 g of nickel chloride hexahydrate, 3.1 g of sodium citrate and 6 g of PVP to 200 mL of the mixed solvent, and stir until homogeneous to obtain a nickel source solution.

[0121] 2. Add 79.2g of sodium hypophosphite to the remaining 100mL of mixed solvent and stir until homogeneous to obtain a reducing agent solution.

[0122] The remaining raw materials and steps are the same as in Example 1, and will not be repeated here.

[0123] During the preparation process, it was found that the dissolution time of the material was longer than that of Example 1, a small amount of yellow-green or yellow-brown substances appeared on the wall of the container during the reaction, the color of the waste liquid after the reaction was darker than that of Example 1, the yield was slightly reduced, the particle size of the product increased and the agglomeration phenomenon increased.

[0124] Example 6

[0125] This embodiment provides an ultrafine nickel powder. Unlike embodiment 1, the first cleaning is not performed; instead, the second cleaning is started directly. The remaining raw materials and steps are the same as in embodiment 1, and will not be repeated here.

[0126] Example 7

[0127] This embodiment provides an ultrafine nickel powder. Unlike embodiment 1, the second cleaning is omitted, while the remaining cleaning methods are performed in sequence. The remaining raw materials and steps are the same as in embodiment 1, and will not be repeated here.

[0128] Example 8

[0129] This embodiment provides an ultrafine nickel powder. Unlike embodiment 1, the third cleaning is omitted, while the remaining cleaning methods are performed in sequence. The remaining raw materials and steps are the same as in embodiment 1, and will not be repeated here.

[0130] Comparative Example 1

[0131] This comparative example provides an ultrafine nickel powder. Unlike Example 1, it does not use PVP. Other raw materials and methods are the same as in Example 1, and will not be repeated here.

[0132] During the preparation process, it was found that the mother liquor after the reaction was yellowish-brown or black with black precipitate. After washing and drying, black powder was obtained, but there was a certain degree of caking. Because of the lack of PVP, the ultrafine nickel powder was prone to agglomeration.

[0133] Comparative Example 2

[0134] This comparative example provides an ultrafine nickel powder. Unlike Example 1, the solvent for the nickel source solution and reducing agent solution is deionized water, and the reaction temperature is 90°C for 2 hours. The other raw materials and methods are the same as in Example 1, and will not be repeated here.

[0135] During the preparation process, it was found that the mother liquor after the reaction was bluish-green or dark green, with almost no precipitation, or only a small amount of grayish-green / grayish-white precipitate. X-ray energy dispersive spectroscopy analysis revealed that the grayish-green / grayish-white substance was mainly nickel hydroxide hydrate. The boiling point of the aqueous solution system is limited to below 100℃. At this temperature, the reducing power of the non-toxic reducing agent sodium hypophosphite is insufficient, thus failing to reduce the nickel ions. The nickel ions remain in free form in the solution, or combine with hydroxide ions in the water to form nickel hydroxide hydrate.

[0136] Comparative Example 3

[0137] This comparative example provides an ultrafine nickel powder. Unlike Example 1, the solvent for the nickel source solution and reducing agent solution is ethylene glycol, and the reaction temperature is 150°C for 2 hours. The other raw materials and methods are the same as in Example 1, and will not be repeated here.

[0138] During the preparation process, it was found that the solute required a relatively long time to dissolve before the reaction (requiring a temperature of around 50℃ and sonication for 0.5–1 hour). This is because the solubility of nickel salts and reducing agents in the ethylene glycol system is much lower than that in water. After the reaction, the mother liquor was yellowish-brown or black, with virtually no precipitate, and the product could not be effectively separated. This was because the reaction in the ethylene glycol system was slow, and crystal growth required a long maturation time. UV-Vis spectroscopy analysis revealed colloidal nickel in the mother liquor (with absorption peaks at 250–300 nm), and the content of divalent nickel decreased (with weakened absorption peaks at 390–400 nm).

[0139] Comparative Example 4

[0140] This comparative example provides an ultrafine nickel powder. The difference from Example 1 is that in step 4, the reaction mother liquor is centrifuged to obtain the precipitate, and deionized water is added to the precipitate to wash it three times. During the filtration, the precipitate is directly filtered without rinsing with anhydrous ethanol. The remaining steps are the same as in Example 1 and will not be repeated here.

[0141] Test Example 1

[0142] The particle size of the ultrafine nickel powders obtained in Examples 1-8 and Comparative Examples 1-4 was measured using a scanning electron microscope; the oxygen content was measured using an X-ray energy dispersive spectroscopy (EDS) instrument.

[0143] The obtained data is shown in Table 1 below.

[0144] Table 1

[0145]

[0146]

[0147] As shown in Table 1, in Example 2, replacing the chelating agent alters the particle size of the final nickel powder compared to Example 1, without significantly affecting the oxygen content. In Example 3, compared to Example 1, it slightly alters the particle size with little effect on the oxygen content, but reduces reducibility, leading to a decrease in the yield of the final product. Increasing the yield can prolong the reaction time, but it also causes particle growth, increasing the nickel powder particle size or causing particle agglomeration. In Example 4, compared to Example 1, changing the solvent ratio and increasing the water ratio leads to a decrease in the preset reaction temperature (for safety reasons), which affects the nickel powder yield and the start of the reaction time. Changes in the viscosity of the mixed solvent also alter the nickel powder particle size or result in more agglomerated particles. In Example 5, compared to Example 1, increasing the material concentration prolongs the dissolution time and leads to incomplete reaction, increasing the difficulty of impurity removal. Furthermore, higher concentrations also increase the particle size, which is detrimental to refining the nickel powder.

[0148] Compared to Example 1, the lack of a cleaning step in Examples 7-9 affects the particle size, morphology, and oxygen content of the nickel powder. Specifically, Example 7, lacking pre-oxidation, causes some particles to oxidize during the cleaning process. However, overall, this significantly impacts the anti-oxidation time of the nickel powder, i.e., its shelf life. During testing, the short intervals made the effect less noticeable than other steps. Example 8, lacking acid washing to remove double salts, results in ineffective removal of surface hydroxyl groups during filtration, leading to oxidation. Similarly, Example 9, lacking a deacidification step, results in high surface activity of the nickel powder, leading to oxidation. The macroscopic phenomenon of oxidation is the rapid discoloration and agglomeration of the nickel powder cake into nickel hydroxide / nickel oxide. Comparative Example 4, where the reaction mother liquor was directly washed three times with deionized water and filtration was performed without rinsing with anhydrous ethanol, resulted in particles that were essentially transformed into nickel hydroxide / nickel oxide, with an oxygen content reaching 12.96 wt%.

[0149] Overall, the ultrafine nickel powder prepared by the method of this invention has significant advantages in terms of nickel powder particle size, nickel powder morphology, and nickel powder oxygen content. Moreover, the steps are closely linked, and the material ratio is properly arranged, which maintains good performance and ensures the production efficiency of reaction products, meeting the needs of large-scale industrial production.

[0150] Experimental Example 2

[0151] The ultrafine nickel powders obtained in Example 1 and Comparative Example 4 were subjected to SEM, and the corresponding SEM images are shown below. Figure 1 and Figure 2 As shown.

[0152] from Figure 1 It can be seen that the nickel powder particles have good sphericity and low oxygen content. The post-processing process of this invention can maintain the basic morphology of the nickel powder and has a significant antioxidant effect.

[0153] from Figure 2 It can be seen that during the washing and filtration steps, ultrafine nickel powder that has not undergone pre-oxidation and surface hydroxyl removal treatment will be rapidly oxidized, forming plate-like / columnar nickel hydroxide / nickel oxide particles. Moreover, due to the lack of treatment to remove double salts, direct filtration will cause the ultrafine nickel powder to agglomerate, resulting in nickel hydroxide / nickel oxide particles with uneven morphology, wide particle size distribution, and low practical application value.

[0154] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing ultrafine nickel powder, characterized in that, Includes the following steps: A. A soluble nickel source, chelating agent, and coating agent are added to a mixed solvent to obtain a nickel source solution; a reducing agent is added to another mixed solvent to obtain a reducing agent solution. B. Add a reducing agent solution to the nickel source solution, heat to carry out the reaction, and centrifuge the reaction mother liquor after the reaction is completed; perform a first wash on the precipitate obtained by centrifugation; continue to centrifuge the mother liquor obtained from the first wash, and perform a second and third wash; finally, wash the precipitate obtained from the third wash with deionized water to obtain a solid-liquid mixture. C. The solid-liquid mixture is filtered and vacuum dried to obtain the ultrafine nickel powder; The mixed solvent is a mixture of deionized water and polyol; The polyol includes at least one selected from ethylene glycol, 1,2-propanediol, 1,3-propanediol, and glycerol; the coating agent includes polyvinylpyrrolidone. The chelating agent includes at least one of sodium citrate, sodium malate, and sodium succinate; The reducing agent includes at least one of L-ascorbic acid, glucose, cellulose, and sodium hypophosphite; In the nickel source solution, the concentration of the soluble nickel source is 0.1 mol / L to 1 mol / L, the concentration of the chelating agent is 0.002 mol / L to 0.05 mol / L, and the concentration of the coating agent is 0.1 g / L to 20 g / L; In the reducing agent solution, the concentration of the reducing agent is 0.3 mol / L to 3 mol / L.

2. The preparation method according to claim 1, characterized in that, The first cleaning is performed using the first cleaning agent; The first cleaning agent comprises anhydrous ethanol, deionized water and 3 wt.% hydrogen peroxide solution in a volume ratio of (100~300):(100~300):(1~10).

3. The preparation method according to claim 1, characterized in that, The second cleaning is performed using a second cleaning agent; The second cleaning agent comprises anhydrous ethanol, deionized water, and acid in a volume ratio of (100~300):(100~300):(0~10); The acid includes carboxylic acids or non-oxidizing inorganic acids.

4. The preparation method according to claim 1, characterized in that, The third cleaning is performed using a third cleaning agent; The third cleaning agent comprises anhydrous ethanol and deionized water in a volume ratio of (1~3):

1.

5. The preparation method according to claim 1, characterized in that, The first, second, and third cleaning processes are ultrasonic cleaning.

6. The preparation method according to claim 5, characterized in that, The ultrasonic cleaning time is 5 min to 10 min, and the temperature is <50℃.

7. The preparation method according to any one of claims 1 to 6, characterized in that, In the mixed solvent, the volume ratio of deionized water to polyol is 1:1 to 5.

8. The preparation method according to any one of claims 1 to 6, characterized in that, In the mixed solvent, the volume ratio of deionized water to polyol is 1:

2.

9. The preparation method according to any one of claims 1 to 6, characterized in that, The soluble nickel source includes at least one of nickel acetate, nickel chloride, nickel nitrate, and nickel sulfate.

10. The preparation method according to any one of claims 1 to 6, characterized in that, In the nickel source solution, the concentration of the soluble nickel source is 0.2 mol / L to 0.4 mol / L.

11. The preparation method according to any one of claims 1 to 6, characterized in that, In the nickel source solution, the concentration of the chelating agent is 0.02 mol / L to 0.04 mol / L.

12. The preparation method according to any one of claims 1 to 6, characterized in that, In the nickel source solution, the concentration of the coating agent is 5 g / L to 10 g / L.

13. The preparation method according to any one of claims 1 to 6, characterized in that, In the reducing agent solution, the concentration of the reducing agent is 1.2 mol / L to 2.4 mol / L.

14. The preparation method according to any one of claims 1 to 6, characterized in that, In step B, the volume ratio of the nickel source solution to the reducing agent solution is 1:

1.

15. The preparation method according to any one of claims 1 to 6, characterized in that, In step B, the reaction temperature is 40℃~200℃ and the time is 0.5h~6h.

16. The preparation method according to any one of claims 1 to 6, characterized in that, In step B, the reaction temperature is 120℃~140℃ and the time is 2h~4h.

17. The preparation method according to any one of claims 1 to 6, characterized in that, In step C, the vacuum drying temperature is 30℃~70℃, and the time is 4h~16h.

18. The preparation method according to any one of claims 1 to 6, characterized in that, The vacuum drying temperature is 40℃~50℃, and the time is 6h~8h.

19. An ultrafine nickel powder prepared by the preparation method according to any one of claims 1 to 18.

20. The ultrafine nickel powder according to claim 19, characterized in that, The particle size of the ultrafine nickel powder is 2μm~5μm.

21. The ultrafine nickel powder according to claim 19, characterized in that, The oxygen content of the ultrafine nickel powder is 1.5 wt.% to 2.5 wt.%.

22. The use of the ultrafine nickel powder according to any one of claims 19-21 in the preparation of conductive paste.

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