Highly dispersed, low burning loss ultrafine silver powder and preparation method thereof

Abstract

The invention discloses an ultrafine silver powder with high dispersion and low burning loss and a preparation method thereof. The method comprises the following steps: firstly preparing a silver nitrate solution A, a reducing solution B, and a reducing base liquid C, and controlling the temperature of each solution to be 18-28°C; then simultaneously adding the silver nitrate solution A and the reducing agent solution B to the reducing base liquid C at a rate of 0.6 L / s for reaction, and continuously stirring the entire process; after the silver nitrate solution A and the reducing solution B are completely added to the reducing base liquid C, immediately adding an appropriate amount of a precipitant, continuing stirring for 5 minutes, terminating the reaction, and obtaining a silver powder suspension; then obtaining modified silver powder through centrifugation, washing, emulsification modification, drying, powder breaking, and sieving processes; and finally obtaining the target ultrafine silver powder through airflow pulverization. The ultrafine silver powder is suitable for various electronic paste products such as 5G communications, semiconductors, and crystalline silicon solar cells.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparation of precious metal ultrafine silver powder and application of electronic paste, and particularly relates to ultrafine silver powder with high dispersion and low burning loss and a preparation method thereof. Background Art

[0002] Silver powder is a common metal powder used in the electronics industry. It has excellent thermal and electrical conductivity and is widely used in electronic pastes such as automotive glass hot wire silver paste, solid tantalum capacitor silver paste, vacuum fluorescent display silver paste, semiconductor ceramic capacitor silver paste, conductive adhesive, and electromagnetic shielding.

[0003] Ultrafine silver powder is a type of silver powder with a particle size ranging from nanometer to submicron. It has the characteristics of high sintering activity, dense sintered silver layer, controllable specific surface area and morphology. It is widely used in electronic industries such as 5G communications, semiconductors, and crystalline silicon solar cells.

[0004] Currently, the main methods for producing ultrafine silver powder include vapor phase, liquid phase chemical reduction, and solid phase methods. The vapor phase method consumes a lot of energy, requires a lot of investment, and has low yields. The solid phase method produces ultrafine silver powder with a relatively large particle size and a wide particle size distribution. The liquid phase chemical reduction method is currently the most widely used method due to its simple operation, low investment, high output, minimal losses, and excellent performance. The principle of the liquid phase chemical reduction method for producing ultrafine silver powder is to use a reducing agent to deposit silver from an aqueous solution of its salt or complex or an organic system in powder form.

[0005] Related research has been done so far:

[0006] The Chinese invention patent application, publication number CN107661986A, discloses a method for mass-producing highly dispersed and highly spherical ultrafine silver powder. The method uses vitamin C as a reducing agent at a temperature of 20-30°C and adjusts the pH of the reaction solution to 8.0-14.0 by adding alkali solution. The resulting particles have a diameter of 0.3-3.0 μm and a tap density greater than 4.0 g / cm 3 , ultra-fine silver powder with burnout less than 0.7%.

[0007] The Chinese invention patent application with publication number CN112264629A discloses a method for preparing low-cost, highly dispersed silver powder and its application. The method uses vitamin C as a reducing agent at a temperature of 10-40°C and adjusts the pH value of the silver-containing solution to 8.5-10.0 by adding ammonia water. The resulting silver particles have uniform particle size and good dispersion, with a particle size of 0.4-0.8μm and a specific surface area of 1.1-1.7m 2 / g of ultrafine silver powder.

[0008] The Chinese invention patent with the announcement number CN113399678B discloses a method for preparing low-cost, highly dispersed ultrafine silver powder. The method uses ascorbic acid, glucose, or formaldehyde as a reducing agent at a temperature of 30-60°C and a special washing process to obtain a particle size of 0.2-1.2 μm and a tap density of 4.5-5.5 g / cm 3 Highly dispersed ultrafine silver powder.

[0009] The aforementioned methods for preparing super silver powder have several drawbacks. These include the need for pH adjustment, high reaction temperatures and energy consumption, cumbersome processes, and long reaction times. The resulting ultrafine silver powder has a wide particle size distribution. Excessive use of dispersants can make silver powder cleaning difficult, leading to high silver powder burn-off rates and poor performance. Furthermore, solid-liquid separation of the silver powder after the reaction is difficult, and the washing and filtration processes for ultrafine powders remain the primary factors limiting the efficiency of liquid-phase reduction methods for producing ultrafine powders. Summary of the Invention

[0010] To solve the above technical problems, the present invention provides an ultrafine silver powder with high dispersion and low burning loss and a preparation method thereof. The prepared ultrafine silver powder has uniform particle size and good dispersibility, and is suitable for use in high-temperature sintering silver pastes such as 5G communication and filter silver pastes, and crystalline silicon solar cell silver pastes, and can exhibit excellent performance. This method adds an alkaline substance to a silver salt solution to generate Ag2O crystal nuclei. At the same time, an alkaline substance is added to a reducing agent to improve the reducing property of the reducing agent, so that the growth of the silver powder generated by the reaction of the silver salt solution and the reducing agent conforms to the Lamer model of "explosive nucleation-slow growth", resulting in a higher particle size concentration of the prepared silver powder. In addition, by adding a precipitant to the reaction solution after the reaction is completed, the silver powder forms soft agglomerates, which are more easily precipitated and less likely to pass through the filter cloth, thereby solving the problem of difficult solid-liquid separation of the silver powder during washing and improving production efficiency. The process used has a high degree of mechanical automation and can continuously produce ultrafine silver powder in batches and in large quantities, with minimal environmental pollution.

[0011] The purpose of the present invention is achieved through the following technical solutions:

[0012] A method for preparing highly dispersed, low-burning-loss ultrafine silver powder comprises the following steps:

[0013] 1) Prepare silver nitrate solution A, reducing solution B, and reducing base solution C in three stainless steel reactors respectively:

[0014] Preparation of silver nitrate solution A: Prepare a silver nitrate solution with a mass concentration of 320g / L-640g / L, then quickly add an alkaline substance to the silver nitrate solution, stir and mix evenly, and control the temperature of the silver nitrate solution to 18-28°C; wherein, the amount of alkaline substance added is 0.4%-1.7% of the mass of the silver nitrate;

[0015] Preparation of reducing solution B: Weigh 60% by weight of reducing agent and 60% by weight of dispersant to prepare reducing agent solution B with a reducing agent concentration of 10g / L-248g / L. Then quickly add the alkaline substance to reducing agent solution B and stir to mix evenly. Control the reducing agent solution temperature at 18-28°C. The amount of alkaline substance added is 6%-11% of the mass of silver nitrate.

[0016] Preparation of reducing base solution C: Weigh 40% by weight of reducing agent and 40% by weight of dispersant to prepare reducing base solution C with a reducing agent concentration of 3.5g / L-41g / L, stir and mix evenly, and control the reducing agent solution temperature at 18-28°C; the dispersant is a mixture of one or more of gum arabic, sodium lauryl sulfate, gelatin, citric acid, polyethylene glycol, polyvinyl pyrrolidone, guar gum, benzotriazole, sorbitol, and sodium polyacrylate, mixed in any proportion, and added in an amount of 1-5% by weight of the silver nitrate;

[0017] The alkaline substance is one of 25% by mass ammonia water, sodium hydroxide, potassium hydroxide, and sodium carbonate; the reducing agent in the reducing solution B or the reducing base solution C is one of vitamin C and formaldehyde solution, the amount used is 1.23 times the theoretical molar amount, and the reducing solution B and the silver nitrate solution A are added in equal volumes and at the same flow rate;

[0018] 2) Reaction stage: Simultaneously open the valves of the silver nitrate solution A and the reducing solution B reactors, add the silver nitrate solution A and the reducing solution B to the reducing base solution C at a rate of 0.6 L / s for reaction, and continuously stir throughout the entire process. After the silver nitrate solution A and the reducing solution B are completely added to the reducing base solution C, immediately add a precipitant, continue stirring for 5 minutes, terminate the reaction, and obtain a silver powder suspension; wherein, the amount of the precipitant added is 0.2%-0.5% of the mass of the silver nitrate, and the precipitant is an amine substance or adipic acid;

[0019] 3) placing the silver powder suspension obtained in step 2) into a centrifuge for solid-liquid separation to obtain soft agglomerated silver powder, then pouring the soft agglomerated silver powder into a washing kettle, adding 45° C. warm water equivalent to 1-2 times the mass of the soft agglomerated silver powder, stirring and washing for 30 minutes, and then performing solid-liquid separation by centrifugation;

[0020] 4) continuously passing deionized water into the centrifuge to centrifuge and wash the silver powder obtained in step 3) until the conductivity of the washed water is less than 30 μS / cm, stopping the passing of deionized water and centrifuging for another 10 minutes to obtain silver powder containing a small amount of water;

[0021] 5) Emulsification process: The silver powder containing a small amount of water obtained in step 4) is transferred to an emulsifier, and a modification agent (0.1%-0.5% by weight of the silver powder) is weighed and dissolved in alcohol (5%-10% by weight of the silver powder). The modified agent is then poured into the emulsifier to emulsify and modify the silver powder, and the mixture is thoroughly stirred and dispersed into a uniform slurry. The modification agent is oleic acid or stearic acid, or one selected from amides or alcohols.

[0022] 6) Transfer the silver powder paste obtained in step 5) to a drying tray and place in a 60°C forced air drying oven to dry for 15 hours, then crush for 10 seconds and pass through a 300 mesh sieve;

[0023] 7) The silver powder obtained in step 6) is subjected to air flow milling to obtain highly dispersed, low-burning ultrafine silver powder. The electron microscope image shows a uniformly dispersed spherical shape, a D50 particle size controlled between 0.4-1.2 μm, and a tap density of 4.0-5.0 g / cm 3 , burning rate ≤0.5%, specific surface area 1.100-2.100m 2 / g.

[0024] In the present invention:

[0025] In step 1), the alkaline substance is preferably ammonia water or sodium hydroxide with a mass fraction of 25%; the reducing agent in the reducing solution B or the reducing base solution C is one of vitamin C and formaldehyde solution, the amount of which is 1.23 times the theoretical molar amount, and the reducing solution B and the silver nitrate solution A are added at equal volumes and the same flow rate; the design of adding an alkaline substance to the silver salt solution to generate Ag2O crystal nuclei, the design of adding an alkaline substance to the reducing agent to improve the reducing property of the reducing agent, and the design of adding equal volumes and the same flow rate of the reducing solution B and the silver nitrate solution A make the growth of the silver powder generated by the reaction of the silver salt solution and the reducing agent conform to the Lamer model of "explosive nucleation-slow growth", effectively ensuring the consistency of the concentration of each part in the reaction system, making the prepared silver powder particle size concentration higher, and reducing production quality problems.

[0026] In step 1), the dispersant is preferably gum arabic, sodium lauryl sulfate, gelatin, citric acid, polyethylene glycol, polyvinyl pyrrolidone, or guar gum. The dispersant can effectively control the growth morphology and particle size of the silver powder particles and prevent hard agglomeration between the silver powder particles.

[0027] In step 2), the precipitant is an amine substance or adipic acid. The precipitant can form soft agglomerates between multiple small silver powder particles to form coarse silver powder particles, making the silver powder particles more easily precipitated in the reaction solution and less likely to pass through the centrifugal filter cloth, thereby solving the problem of difficult solid-liquid separation and low production efficiency of silver powder during the reaction and washing process.

[0028] In step 5), the modifier is preferably oleic acid or stearic acid. The modifier can be adsorbed on the surface of the silver powder particles to modify the surface of the silver powder particles, enhance the compatibility between the silver powder and the silver paste carrier, and improve the performance of the silver paste; the sufficient stirring is stirring for 5 minutes.

[0029] In step 7), air flow milling is performed in the air flow mill, wherein the air flow milling process feed rate is 45 kg / h, and the air grinding pressure adopts dry compressed air of 0.7-0.8 MPa. The air flow milling process can disperse the microscopic soft agglomerates between the silver powder particles and perform air grinding modification on the surface of the silver powder particles to make the silver powder particles more spherical.

[0030] The present invention also relates to an ultrafine silver powder with high dispersion and low burning loss, which is obtained by using the above-mentioned method for preparing an ultrafine silver powder with high dispersion and low burning loss. The ultrafine silver powder has a uniformly dispersed spherical shape in an electron microscope image, a D50 particle size controlled between 0.4-1.2 μm, and a tap density of 4.0-5.0 g / cm 3 , burning rate ≤0.5%, specific surface area 1.100-2.100m 2 / g, suitable for various electronic paste products such as 5G communications, semiconductors, crystalline silicon solar cells, etc.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] 1. The method for preparing a highly dispersed, low-burning-loss ultrafine silver powder described in the present invention does not require pH and temperature adjustment during the production process, and the reaction can be completed at room temperature, thereby reducing reaction energy consumption. In addition, compared with traditional processes, the preparation method of the present invention has a relatively simple process and a shorter reaction time, which greatly improves production efficiency and saves production costs.

[0033] 2. The method for preparing highly dispersed, low-burning-loss ultrafine silver powder disclosed herein optimizes the addition of the various reaction raw materials during the reaction process by preparing AgO nuclei, enhancing the reducibility of the reducing agent by adding an alkaline substance, and designing equal volumes and flow rates for the addition of reducing solution B and silver nitrate solution A. This ensures that the growth of the silver powder generated by the reaction of the silver salt solution and the reducing agent conforms to the Lamer model of "explosive nucleation-slow growth," effectively ensuring the consistency of the concentrations of various components in the reaction system, resulting in a higher particle size concentration of the prepared silver powder and reducing production quality issues.

[0034] 3. The method for preparing a highly dispersed, low-burning-loss ultrafine silver powder described in the present invention uses a water-soluble dispersant that is easily soluble and used in a small amount. It can be easily cleaned and removed by washing with warm water at 45°C. Compared with traditional ultrafine silver powder, the ultrafine silver powder prepared by the present invention has a lower burn-loss rate, and warm water is used instead of traditional methanol or ethanol to wash the silver powder, which lowers the production cost and is more environmentally friendly.

[0035] 4. The highly dispersed, low-burning-loss ultrafine silver powder described in the present invention adopts a sedimentation agent to form soft agglomerates between multiple small silver powder particles to form coarse silver powder particles, thereby making the silver powder particles more easily precipitated in the reaction solution and less likely to pass through the centrifugal filter cloth, thereby solving the problems of difficult solid-liquid separation and low production efficiency of silver powder during the reaction and washing process. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG1 is an electron microscope image of the ultrafine silver powder prepared in Example 3 of the present invention ( FIG1( a ) is an electron microscope image of the ultrafine silver powder prepared in Example 3 at a magnification of 5000 times; FIG1( b ) is an electron microscope image of the ultrafine silver powder prepared in Example 3 at a magnification of 20,000 times). DETAILED DESCRIPTION

[0037] The present invention is further described in detail below by way of examples, but these examples should not be considered as limiting the present invention. Unless otherwise specified, the raw materials and equipment in the examples of this application were purchased from commercial sources, and the methods in the examples, unless otherwise specified, are conventional methods in the art.

[0038] Example 1:

[0039] A method for preparing highly dispersed, low-burning-loss ultrafine silver powder comprises the following steps:

[0040] 1) Prepare silver nitrate solution A, reducing solution B, and reducing base solution C in three stainless steel reactors respectively:

[0041] Preparation of silver nitrate solution A: Weigh 48 kg of silver nitrate and add it to reactor A. Then add 75 L of deionized water and stir to dissolve. Then quickly add 650 g of ammonia water (25%) to the silver nitrate solution and stir to mix evenly. Control the temperature of the silver nitrate solution to 26 ± 2 °C.

[0042] Preparation of reducing solution B: Add 75 L of deionized water to reactor B, then weigh 4.27 kg of 37% formaldehyde solution, 0.288 kg of gelatin, and 1.008 kg of benzotriazole into reactor B and stir to dissolve. Then weigh 5 kg of sodium carbonate and add it to reactor B, stir to dissolve and mix evenly. Control the reducing agent solution temperature to 26 ± 1 °C.

[0043] Preparation of reducing solution C: Add 300 L of deionized water to reactor B. Then weigh 2.85 kg of 37% formaldehyde solution, 0.192 g of gelatin, and 0.672 kg of benzotriazole and add them to reactor B and stir to dissolve. Control the reducing agent solution temperature at 26 ± 2°C.

[0044] 2) Reaction stage: Open the valves of the silver nitrate solution A and the reducing solution B reactor at the same time, add the silver nitrate solution A and the reducing solution B to the reducing base solution C at a rate of 0.6 L / s for reaction, and continue stirring throughout the process. After the silver nitrate solution A and the reducing solution B are completely added to the reducing base solution C, immediately add 240 g of adipic acid (previously dissolved in 300 g of alcohol), continue stirring for 5 minutes, and terminate the reaction to obtain a silver powder suspension;

[0045] 3) placing the silver powder suspension obtained in step 2) into a centrifuge for solid-liquid separation to obtain soft agglomerated silver powder, then pouring the silver powder into a washing kettle, adding 50 L of 45° C. warm water, stirring and washing for 30 minutes, and then centrifuging for solid-liquid separation;

[0046] 4) continuously passing deionized water into a centrifugal apparatus to centrifuge and wash the silver powder obtained in step 3) until the conductivity of the washed water is less than 30 μS / cm, stopping the passing of deionized water, and centrifuging for another 10 minutes to obtain a silver powder containing a small amount of water;

[0047] 5) Emulsification process: The silver powder obtained in step 4) was transferred to an emulsifier, 90.9 g of stearamide was weighed and dissolved in 2.5 kg of alcohol, and then poured into the emulsifier to emulsify and modify the silver powder. The mixture was stirred thoroughly (about 5 minutes) to disperse into a uniform slurry;

[0048] 6) Transferring the silver powder obtained in step 5) to a drying tray and drying in a 60° C. forced air drying oven for 15 h, then crushing the powder for 10 s and passing through a 300 mesh sieve;

[0049] 7) The silver powder obtained in step 6) is subjected to air flow grinding in an air flow mill to obtain the desired ultrafine silver powder.

[0050] Example 1: 30.345 kg of ultrafine silver powder was prepared with a yield of 99.56%. The particle sizes were: D10-0.254 μm, D50-0.413 μm, D90-0.701 μm, and the tap density was 4.18 g / cm 3 Specific surface area: 2.096m 2 / g, and the burning loss rate at 538℃ is 0.48%.

[0051] Example 2:

[0052] A method for preparing highly dispersed, low-burning-loss ultrafine silver powder comprises the following steps:

[0053] 1) Prepare silver nitrate solution A, reducing solution B, and reducing base solution C in three stainless steel reactors respectively:

[0054] Preparation of silver nitrate solution A: Weigh 48 kg of silver nitrate and add it to reactor A. Then add 100 L of deionized water and stir to dissolve. Then quickly add 540 g of ammonia water (25%) to the silver nitrate solution and stir to mix evenly. Control the temperature of the silver nitrate solution to 25 ± 2 °C.

[0055] Preparation of reducing solution B: Weigh 18.36 kg of vitamin C, 0.288 kg of gum arabic, and 0.864 kg of sodium lauryl sulfate into reactor B, then add 100 L of deionized water and stir to dissolve. Then weigh 4.5 kg of sodium hydroxide and add it into reactor B, stir to dissolve and mix evenly, and control the reducing agent solution temperature to 25 ± 2 °C.

[0056] Preparation of reducing agent base solution C: Weigh 12.24 kg of vitamin C, 0.192 kg of gum arabic, and 0.576 kg of sodium lauryl sulfate into reactor B, then add 300 L of deionized water and stir to dissolve. Control the reducing agent solution temperature at 25 ± 2 °C.

[0057] 2) Reaction stage: Open the valves of the silver nitrate solution A and the reducing solution B reactor at the same time, add the silver nitrate solution A and the reducing solution B to the reducing base solution C at a rate of 0.6 L / s for reaction, and continue stirring throughout the process. After the silver nitrate solution A and the reducing solution B are completely added to the reducing base solution C, immediately add 192 g of hexadecylamine (dissolved in 300 g of alcohol in advance), continue stirring for 5 minutes, and terminate the reaction to obtain a silver powder suspension;

[0058] 3) placing the silver powder suspension obtained in step 2) into a centrifuge for solid-liquid separation to obtain soft agglomerated silver powder, then pouring the silver powder into a washing kettle, adding 50 L of 45° C. warm water, stirring and washing for 30 minutes, and then centrifuging for solid-liquid separation;

[0059] 4) continuously passing deionized water into a centrifugal apparatus to centrifuge and wash the silver powder obtained in step 3) until the conductivity of the washed water is less than 30 μS / cm, stopping the passing of deionized water, and centrifuging for another 10 minutes to obtain a silver powder containing a small amount of water;

[0060] 5) Emulsification process: The silver powder obtained in step 4) was transferred to an emulsifier, 90.9 g of oleic acid was weighed and dissolved in 2.0 kg of alcohol, and then poured into the emulsifier to emulsify and modify the silver powder. The mixture was stirred thoroughly (about 5 minutes) to disperse into a uniform slurry;

[0061] 6) Transferring the silver powder obtained in step 5) to a drying tray and drying in a 60° C. forced air drying oven for 15 h, then crushing the powder for 10 s and passing through a 300 mesh sieve;

[0062] 7) The silver powder obtained in step 6) is subjected to air flow grinding in an air flow mill to obtain the desired ultrafine silver powder.

[0063] Example 2: 30.370 kg of ultrafine silver powder was prepared with a yield of 99.64%. The particle sizes were: D10-0.346 μm, D50-0.642 μm, D90-0.927 μm, and the tap density was 4.32 g / cm 3 Specific surface area: 1.836m 2 / g, and the burning loss rate at 538℃ is 0.41%.

[0064] Example 3:

[0065] A method for preparing highly dispersed, low-burning-loss ultrafine silver powder comprises the following steps:

[0066] 1) Prepare silver nitrate solution A, reducing solution B, and reducing base solution C in three stainless steel reactors respectively:

[0067] Preparation of silver nitrate solution A: Weigh 48 kg of silver nitrate and add it to reactor A. Then add 120 L of deionized water and stir to dissolve. Then quickly add 413 g of ammonia water (25%) to the silver nitrate solution and stir to mix evenly. Control the temperature of the silver nitrate solution to 24 ± 2 °C.

[0068] Preparation of reducing solution B: Weigh 18.36 kg of vitamin C, 0.432 kg of polyvinylpyrrolidone, and 0.432 kg of benzotriazole and add them to reactor B. Then add 100 L of deionized water and stir to dissolve. Then weigh 4.5 kg of sodium hydroxide and add it to reactor B. Stir to dissolve and mix evenly. Control the reducing agent solution temperature at 24 ± 2 °C.

[0069] Preparation of reducing agent base solution C: Weigh 12.24 kg of vitamin C, 0.288 kg of polyvinylpyrrolidone, and 0.288 kg of benzotriazole into reactor B, then add 300 L of deionized water and stir to dissolve. Control the reducing agent solution temperature at 24 ± 2 °C.

[0070] 2) Reaction stage: Open the valves of the silver nitrate solution A and the reducing solution B reactor at the same time, add the silver nitrate solution A and the reducing solution B to the reducing base solution C at a rate of 0.6 L / s for reaction, and continue stirring throughout the process. After the silver nitrate solution A and the reducing solution B are completely added to the reducing base solution C, immediately add 168 g of adipic acid (previously dissolved in 300 g of alcohol), continue stirring for 5 minutes, and terminate the reaction to obtain a silver powder suspension;

[0071] 3) placing the silver powder suspension obtained in step 2) into a centrifuge for solid-liquid separation to obtain soft agglomerated silver powder, then pouring the silver powder into a washing kettle, adding 60 L of 45° C. warm water, stirring and washing for 30 minutes, and then centrifuging for solid-liquid separation;

[0072] 4) continuously passing deionized water into a centrifugal apparatus to centrifuge and wash the silver powder obtained in step 3) until the conductivity of the washed water is less than 30 μS / cm, stopping the passing of deionized water, and centrifuging for another 10 minutes to obtain a silver powder containing a small amount of water;

[0073] 5) Emulsification process: The silver powder obtained in step 4) was transferred to an emulsifier, 30.48 g of oleic acid and 30.48 g of stearic acid were weighed and dissolved in 2.0 kg of alcohol, and then poured into the emulsifier to emulsify and modify the silver powder. The mixture was stirred thoroughly (about 5 minutes) to disperse into a uniform slurry;

[0074] 6) Transferring the silver powder obtained in step 5) to a drying tray and drying in a 60° C. forced air drying oven for 15 h, then crushing the powder for 10 s and passing through a 300 mesh sieve;

[0075] 7) The silver powder obtained in step 6) is subjected to air flow grinding in an air flow mill to obtain the desired ultrafine silver powder.

[0076] Example 3: 30.437 kg of ultrafine silver powder was prepared with a yield of 99.86%. The particle sizes were: D10-0.513 μm, D50-0.855 μm, D90-1.217 μm, and the tap density was 4.62 g / cm 3 Specific surface area: 1.487m 2 / g, and the burning loss rate at 538℃ is 0.37%.

[0077] Figure 1 is an electron microscope image of the ultrafine silver powder prepared in Example 3 ( Figure 1a This is an electron microscope image of the prepared ultrafine silver powder magnified 5000 times; Figure 1b (This is an electron microscope image of the prepared ultrafine silver powder at 20,000 times magnification).

[0078] Example 4:

[0079] A method for preparing highly dispersed, low-burning-loss ultrafine silver powder comprises the following steps:

[0080] 1) Prepare silver nitrate solution A, reducing solution B, and reducing base solution C in three stainless steel reactors respectively:

[0081] Preparation of silver nitrate solution A: Weigh 48 kg of silver nitrate and add it to reactor A. Then add 150 L of deionized water and stir to dissolve. Then quickly add 289 g of sodium hydroxide to the silver nitrate solution and stir to dissolve and mix evenly. Control the temperature of the silver nitrate solution to 22 ± 2 °C.

[0082] Preparation of reducing solution B: Weigh 18.36 kg of vitamin C, 0.144 kg of guar gum, and 0.432 kg of sodium polyacrylate into reactor B, then add 100 L of deionized water and stir to dissolve. Then weigh 4.0 kg of sodium hydroxide and add it into reactor B, stir to dissolve and mix evenly, and control the reducing agent solution temperature to 22 ± 2 °C.

[0083] Preparation of reducing agent base solution C: Weigh 12.24 kg of vitamin C, 0.096 kg of guar gum, and 0.288 kg of sodium polyacrylate into reactor B, then add 300 L of deionized water and stir to dissolve. Control the reducing agent solution temperature at 22 ± 2 °C.

[0084] 2) Reaction stage: Open the valves of the silver nitrate solution A and the reducing solution B reactor at the same time, add the silver nitrate solution A and the reducing solution B to the reducing base solution C at a rate of 0.6 L / s for reaction, and continue stirring throughout the process. After the silver nitrate solution A and the reducing solution B are completely added to the reducing base solution C, immediately add 150 g of octadecylamine (dissolved in 300 g of alcohol in advance), continue stirring for 5 minutes, terminate the reaction, and obtain a silver powder suspension;

[0085] 3) placing the silver powder suspension obtained in step 2) into a centrifuge for solid-liquid separation to obtain soft agglomerated silver powder, then pouring the silver powder into a washing kettle, adding 45 L of 45° C. warm water, stirring and washing for 30 minutes, and then centrifuging for solid-liquid separation;

[0086] 4) continuously passing deionized water into a centrifugal apparatus to centrifuge and wash the silver powder obtained in step 3) until the conductivity of the washed water is less than 30 μS / cm, stopping the passing of deionized water, and centrifuging for another 10 minutes to obtain a silver powder containing a small amount of water;

[0087] 5) Emulsification process: The silver powder obtained in step 4) was transferred to an emulsifier, 30.48 g of oleic acid and 30.48 g of stearic acid were weighed and dissolved in 2.5 kg of alcohol, and then poured into the emulsifier to emulsify and modify the silver powder. The mixture was stirred thoroughly (about 5 minutes) to disperse into a uniform slurry;

[0088] 6) Transferring the silver powder obtained in step 5) to a drying tray and drying in a 60° C. forced air drying oven for 15 h, then crushing the powder for 10 s and passing through a 300 mesh sieve;

[0089] 7) The silver powder obtained in step 6) is subjected to air flow grinding in an air flow mill to obtain the desired ultrafine silver powder.

[0090] Example 4: 30.336 kg of ultrafine silver powder was prepared with a yield of 99.53%. The particle sizes were: D10-0.763 μm, D50-1.130 μm, D90-1.651 μm, and the tap density was 4.91 g / cm 3Specific surface area: 1.151m 2 / g, and the burning loss rate at 538℃ is 0.32%.

[0091] Comparative Example 1:

[0092] Compared with Example 3, no sedimentation agent was added in Comparative Example 3, and the other steps were the same as in Example 3.

[0093] Comparative Example 1 prepared 29.963 kg of ultrafine silver powder with a yield of 98.30%. The particle sizes were: D10-0.491 μm, D50-0.837 μm, D90-1.183 μm, and the tap density was 4.29 g / cm 3 Specific surface area: 1.516m 2 / g, and the burning loss rate at 538℃ is 0.72%.

[0094] The comparison of various indicators of the ultrafine silver powder obtained in Examples 1-4 and Comparative Example 1 is shown in Table 1:

[0095] Table 1 Main performance indicators of ultrafine silver powder obtained in Examples and Comparative Examples

[0096]

[0097] Result analysis:

[0098] 1. From the results of Examples 1-4 in Table 1, it can be seen that the present invention provides a highly dispersed, low burning loss ultrafine silver powder and a preparation method thereof, and prepares a tap density of 4.0-5.0 g / cm 3 , burning rate ≤0.5%, specific surface area 1.100-2.100m 2 / g ultrafine silver powder was prepared by adding alkaline substances to silver nitrate solution to prepare Ag2O crystal nuclei, improving the reducibility of the reducing agent by adding alkaline substances, and designing the addition flow rate of reducing solution B and silver nitrate solution A with equal volumes. This optimized the addition method of each reaction raw material in the reaction process, so that the growth of the silver powder generated by the reaction of silver nitrate solution and reducing agent conforms to the Lamer model of "explosive nucleation-slow growth", effectively ensuring the consistency of the concentration of each component in the reaction system, and making the prepared silver powder particle size more concentrated, so that the D50 particle size of the ultrafine silver powder can be regulated between 0.4-1.2μm.

[0099] 2. Comparison between Example 3 and Comparative Example 1 shows that the addition of the precipitant causes multiple small silver powder particles to form soft agglomerates into coarse silver powder particles, making the silver powder particles more easily precipitated in the reaction solution and less likely to pass through the centrifugal filter cloth, thereby reducing the loss of ultrafine silver powder during the production process, thereby improving the final yield of ultrafine silver powder, and at the same time solving the problems of difficult solid-liquid separation and low production efficiency of silver powder during the reaction and washing processes, and avoiding the ultrafine silver powder from being retained in the reaction solution for too long, resulting in the ultrafine silver powder being difficult to clean and having high burn loss.

[0100] By comparing the basic properties of the embodiment and the comparative example, it can be seen that the formula and silver powder preparation process of the embodiment are obviously better than those of the comparative example.

[0101] The above embodiments of the present invention are merely examples for the purpose of clearly illustrating the present invention and are not intended to limit the embodiments of the present invention. Those skilled in the art will appreciate that other variations or modifications may be made based on the above description. It is not necessary and impossible to enumerate all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention are intended to be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing highly dispersed, low-burning-loss ultrafine silver powder, characterized by: The steps include: 1) Prepare silver nitrate solution A, reducing solution B, and reducing base solution C in three stainless steel reactors respectively: Preparation of silver nitrate solution A: Prepare a silver nitrate solution with a mass concentration of 320g / L-640g / L, then quickly add an alkaline substance to the silver nitrate solution, stir and mix evenly, and control the temperature of the silver nitrate solution to 18-28°C; wherein, the amount of alkaline substance added is 0.4%-1.7% of the mass of the silver nitrate; Preparation of reducing solution B: Weigh 60% by mass of reducing agent and 60% by mass of dispersant to prepare reducing agent solution B with a reducing agent concentration of 10g / L-248g / L. Then quickly add the alkaline substance to reducing agent solution B and stir to mix evenly. Control the reducing agent solution temperature at 18-28°C. The amount of alkaline substance added is 6%-11% of the mass of silver nitrate. Preparation of reducing base solution C: Weigh 40% by weight of reducing agent and 40% by weight of dispersant to prepare reducing base solution C with a reducing agent concentration of 3.5g / L-41g / L, stir and mix evenly, and control the reducing agent solution temperature at 18-28°C; the dispersant is a mixture of one or more of gum arabic, sodium lauryl sulfate, gelatin, citric acid, polyethylene glycol, polyvinyl pyrrolidone, guar gum, benzotriazole, sorbitol, and sodium polyacrylate, mixed in any proportion, and added in an amount of 1-5% by weight of the silver nitrate; The alkaline substance is one of 25% by mass ammonia water, sodium hydroxide, potassium hydroxide, and sodium carbonate; the reducing agent in the reducing solution B or the reducing base solution C is one of vitamin C and formaldehyde solution, the amount used is 1.23 times the theoretical molar amount, and the reducing solution B and the silver nitrate solution A are added in equal volumes and at the same flow rate; 2) Reaction stage: Simultaneously open the valves of the silver nitrate solution A and the reducing solution B reactors, add the silver nitrate solution A and the reducing solution B to the reducing base solution C at a rate of 0.6 L / s for reaction, and continuously stir throughout the entire process. After the silver nitrate solution A and the reducing solution B are completely added to the reducing base solution C, immediately add a precipitant, continue stirring for 5 minutes, terminate the reaction, and obtain a silver powder suspension; wherein, the amount of the precipitant added is 0.2%-0.5% of the mass of the silver nitrate, and the precipitant is an amine substance or adipic acid; 3) placing the silver powder suspension obtained in step 2) into a centrifuge for solid-liquid separation to obtain soft agglomerated silver powder, then pouring the soft agglomerated silver powder into a washing kettle, adding 45° C. warm water equivalent to 1-2 times the mass of the soft agglomerated silver powder, stirring and washing for 30 minutes, and then performing solid-liquid separation by centrifugation; 4) continuously passing deionized water into the centrifuge to centrifuge and wash the silver powder obtained in step 3) until the conductivity of the washed water is less than 30 μS / cm, stopping the passing of deionized water and centrifuging for another 10 minutes to obtain silver powder containing a small amount of water; 5) Emulsification process: The silver powder containing a small amount of water obtained in step 4) is transferred to an emulsifier, and a modifier (0.1%-0.5% by weight of the silver powder) is weighed and dissolved in alcohol (5%-10% by weight of the silver powder). The silver powder is then poured into the emulsifier for emulsification and modification, and the mixture is thoroughly stirred and dispersed into a uniform slurry. The modifier is oleic acid or stearic acid, or one selected from amides or alcohols. 6) Transfer the silver powder paste obtained in step 5) to a drying tray and place in a 60°C forced air drying oven to dry for 15 hours, then crush for 10 seconds and pass through a 300 mesh sieve; 7) The silver powder obtained in step 6) is subjected to air flow milling to obtain highly dispersed, low-burning ultrafine silver powder. The electron microscope image shows a uniformly dispersed spherical shape, a D50 particle size controlled between 0.4-1.2 μm, and a tap density of 4.0-5.0 g / cm 3 , burning rate ≤0.5%, specific surface area 1.100-2.100m 2 / g.

2. The method for preparing a highly dispersed, low burning loss ultrafine silver powder according to claim 1, characterized in that: In step 1), the alkaline substance is selected from 25% by mass ammonia water or sodium hydroxide.

3. The method for preparing a highly dispersed, low burning loss ultrafine silver powder according to claim 1, characterized in that: In step 1), the dispersant is selected from one of gum arabic, sodium lauryl sulfate, gelatin, citric acid, polyethylene glycol, polyvinyl pyrrolidone, and guar gum.

4. The method for preparing a highly dispersed, low burning loss ultrafine silver powder according to claim 1, characterized in that: In step 5), the modifier is selected from oleic acid or stearic acid; and the sufficient stirring is stirring for 5 minutes.

5. The method for preparing a highly dispersed, low burning loss ultrafine silver powder according to claim 1, characterized in that: In step 7), the air flow milling is carried out in an air flow mill, wherein the air flow milling process feed rate is 45 kg / h, and the air grinding pressure adopts dry compressed air of 0.7-0.8 MPa.

6. A highly dispersed, low burning loss ultrafine silver powder, characterized by: The method for preparing a highly dispersed and low-burning-loss ultrafine silver powder according to any one of claims 1 to 5 is used to obtain the ultrafine silver powder, wherein the electron microscope image of the ultrafine silver powder is a uniformly dispersed spherical shape, the D50 particle size is regulated between 0.4 and 1.2 μm, and the tap density is 4.0 to 5.0 g / cm 3 , burning rate ≤0.5%, specific surface area 1.100-2.100m 2 / g.

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