Preparation Method of Nano-Silver Powder for Pressure-Sensitive Thermistor and Its Electronic Paste

Abstract

The invention discloses a preparation method of nano silver powder for varistor and thermistor and an electronic paste thereof, belonging to the technical field of nano silver. The preparation method of the invention is that under the condition of constant temperature stirring, a silver-containing oxidant solution and a complexing agent solution are simultaneously added into a dispersant solution within 5 - 10 minutes, after stirring for 5 - 20 minutes, a reducing agent solution is added into the mixed solution within 1 - 20 minutes, and stirring continues for 5 - 30 minutes. The solution is washed until the pH of the solution is neutral, and then a surface modifier solution is added thereto. After constant temperature stirring for 10 - 30 minutes, solid-liquid separation is carried out when the conductivity of the solution ≤ 100 μs / cm. The obtained silver powder is dried and pulverized to obtain nano silver powder with a particle size of 20 - 400 nm and a specific surface area of 5.0 - 14 m<supgt;2< / supgt> / g. The nano silver powder has excellent particle size uniformity, and its synthesis process is simple, with low production cost and short cycle. The maximum single batch can reach 80 kg, and it can be widely used in electronic pastes for varistors and thermistors.

Description

Technical Field

[0001] The present invention relates to a preparation method of nano silver powder for varistor and thermistor and an electronic paste thereof, specifically to a preparation method of nano silver powder suitable for varistor and thermistor, and an electronic paste prepared from the nano silver powder, belonging to the technical field of nano silver technology. Background Art

[0002] Micro-nano silver powder has good electrical and thermal conductivity and is widely used in the electronics industry, mainly as raw materials for producing components such as piezoelectric ceramics, capacitors, potentiometers, etc., and conductive pastes, conductive inks, etc. For silver powder for high-temperature varistor and thermistor, the surface brightness of the silver electrode is a key index, which has strict requirements for the specific surface area and surface activity of the silver powder required for preparing this kind of silver paste. Generally speaking, the specific surface area of such silver micro-powder should be greater than 6m 2 / g, and has high dispersibility and uniformity, and the formed silver paste has good leveling property, so as to ensure good printing performance of the silver paste, higher surface density of the silver layer after sintering, and brighter silver layer.

[0003] In the prior art, Chinese Patent CN103406550A discloses a method for preparing silver micro-powder for varistor electronic paste. By continuously and uniformly putting the mixed solution of gelatin solution, ascorbic acid solution and sodium carbonate solution into silver nitrate solution, and adopting the washing method of precipitating and extracting the supernatant, silver micro-powder with a loose net-like flocculent structure and agglomeration is finally prepared. This kind of silver powder has a relatively concentrated particle size distribution and high surface activity, but the highest specific surface area of the powder can only reach 3.5m 2 / g. In addition, Chinese Patent CN105880627A also discloses a preparation method of silver micro-powder with precisely controllable specific surface area. Using a mixture of polyvinyl alcohol and gelatin as a dispersant, an excessive amount of triethanolamine solution is slowly dropped into silver nitrate solution adjusted to pH 10-13 with sodium carbonate, and silver powder with a specific surface area of 1.0-8.0m 2 / g is prepared at 25-40°C. The silver micro-powder prepared by this patent is mixed evenly in a certain proportion to achieve an ideal sintering effect, but its reaction is slow, resulting in a long production cycle, which is not conducive to actual industrial production. In addition, Chinese Patent CN116329540A also discloses a nano silver powder for conductive paste, its preparation process and application. By sequentially and slowly dropping hydrazine hydrate and pH regulator into the mixed solution of silver nitrate and dispersant, and heating to 30-60°C for reaction for 2-4h, and then obtaining nano silver powder through centrifugal separation and filtration. This method uses a mixture of polyvinylpyrrolidone, gum arabic and gelatin to reduce the particle size of nano silver powder and prevent powder agglomeration, but its process operation is complex and it is difficult to mass-produce industrially.

[0004] In summary, based on the performance requirements of existing silver powders for pressure-sensitive and thermosensitive resistors at high temperatures (specific surface area greater than 6 m 2 / g, high dispersibility, and uniformity), no nano-silver powders and their conductive pastes with industrial application prospects have been reported so far. Therefore, the present invention came into being. SUMMARY OF THE INVENTION

[0005] The present invention aims to provide a method for preparing nano-silver powder for pressure-sensitive and thermosensitive resistors. During the preparation process, a complexing agent is introduced to control the release rate of silver ions. At the same time, through the adsorption of the ionic group of the polymer graft dispersant on the surface of the silver powder and the steric repulsion effect of the graft chain on the silver powder, the solubility of the silver powder in the solution is controlled, simplifying the washing process, saving costs, and achieving the controllable and precise synthesis of nano-silver powder with high dispersibility and high specific surface area. The use of a surface modifier is also utilized to further reduce the hydrophobicity of the nano-silver powder and enhance its compatibility in the electronic paste. Moreover, the nano-silver powder prepared by this method has excellent particle size uniformity, and its synthesis process is simple, with low production cost and short cycle. The maximum single batch can reach 80 kg, and it can be widely used in electronic pastes for pressure-sensitive and thermosensitive resistors. At the same time, the present invention also provides an electronic paste containing the nano-silver powder for pressure-sensitive and thermosensitive resistors obtained by this preparation method.

[0006] The present invention is achieved through the following technical solutions: A method for preparing nano-silver powder for pressure-sensitive and thermosensitive resistors, comprising the following steps:

[0007] S1. Solution preparation: Dissolve a silver-containing oxidant in pure water to prepare a silver-containing oxidant solution with a silver ion concentration of 0.1 - 3 mol / L, dissolve a complexing agent in pure water to prepare a complexing agent solution with a mass-volume concentration of 12 - 300 g / L, dissolve a dispersant in pure water, methanol, or ethanol to prepare a dispersant solution with a mass fraction of 1 - 10%, dissolve a reducing agent in pure water to prepare a reducing agent solution with a mass-volume concentration of 100 - 900 g / L, and dissolve a surface modifier in ethanol to prepare a surface modifier solution with a mass fraction of 3 - 10%.

[0008] S2. Reaction: Under the condition of constant-temperature stirring, simultaneously add the silver-containing oxidant solution and the complexing agent solution to the dispersant solution within 5 - 10 min, stir for 5 - 20 min, then add the reducing agent solution to the mixed solution within 1 - 20 min, and continue to stir for 5 - 30 min.

[0009] S3. Washing: Add deionized water to the mixed solution obtained in step S2, then let it stand for sedimentation and extract the supernatant for washing until the pH of the solution is neutral.

[0010] S4. Modified washing: Add a surface modifier solution to the solution obtained in step S3. After stirring at a constant temperature for 10 - 30 min, add deionized water to the solution, then let it stand for sedimentation and draw the supernatant for washing until the conductivity of the solution ≤ 100 μs / cm, and then perform solid-liquid separation;

[0011] S5. Drying and pulverization: After drying and pulverizing the silver powder obtained by solid-liquid separation in step S4, nano silver powder with a particle size of 20 - 400 nm and a specific surface area of 5.0 - 14 m 2 / g is obtained.

[0012] The silver-containing oxidant is silver nitrate.

[0013] The complexing agent is selected from at least one of sodium hydroxide, potassium hydroxide, sodium chloride, potassium chloride, sodium carbonate, potassium carbonate, and ammonia water.

[0014] The dispersant is a polymer graft polymer, and its dosage is 0.1 - 5% of the silver content in the silver-containing oxidant.

[0015] The reducing agent is selected from at least one of formaldehyde, sodium borohydride, ascorbic acid, hydrazine hydrate, triethanolamine, and glucose.

[0016] The surface modifier is selected from one of lauric acid, oleic acid, ricinoleic acid, palmitic acid, stearic acid, palmitoleic acid, and arachidic acid, and its dosage is 0.5 - 3% of the silver content in the silver-containing oxidant.

[0017] The conditions for constant-temperature stirring are a water bath temperature of 20 - 80 °C and a mechanical stirring speed of 200 - 350 rpm.

[0018] The drying and pulverization is to dry in a blast drying oven at 80 °C for 24 - 36 h and then pulverize with a pulverization equipment.

[0019] An electronic paste for a varistor and thermistor contains the nano silver powder prepared by the above method.

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

[0021] (1) The present invention is completely different from the existing preparation principle of nano silver powder. During the liquid-phase synthesis process, a complexing agent is introduced to control the release rate of silver ions, thereby realizing the regulation of silver atoms from nucleation to growth; at the same time, a polymer graft polymer dispersant is introduced. Utilizing the strong adsorption effect of its ionic main chain on the surface of silver powder and the steric repulsion effect given by different graft chains to silver powder, the rapid coating of silver powder and the control of its solubility in the solution are achieved, ensuring the high dispersibility of silver powder and simplifying the washing process of the powder.

[0022] (2) In the method of the present invention, through the use of a surface modifier, on the one hand, the hydrophobicity of the silver powder is further enhanced, making the powder easier to settle, improving the silver powder collection rate while reducing the equipment cost required for the washing section; on the other hand, the use of the surface modifier enhances the compatibility of the nano-silver powder in the electronic paste, so that a single powder still has good application effects in a paste system with a silver content of 30-80%.

[0023] (3) The preparation process flow of the method of the present invention is simple, the production cycle is short, the cost is low, the production of 80 kg of nano-silver powder in a single batch can be realized, the repeatability is strong, and the industrial-scale production can be realized.

[0024] In summary, the present invention discloses a new preparation method of nano-silver powder for pressure-sensitive and thermosensitive resistors. During the reaction process, by reasonably selecting the types and dosages of complexing agents and dispersants, the controllable and precise synthesis of nano-silver powder with high dispersibility and high specific surface area can be achieved. The use of a surface modifier can simplify the washing process, reduce the production cost, and enhance the application performance of nano-silver powder in electronic paste. The preparation process of this method is simple and easy to control, and large-scale stable production can be realized. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is the electron microscope image of the nano-silver powder in Example 1 of the present invention.

[0026] Figure 2 It is the electron microscope image of the nano-silver powder in Example 2 of the present invention.

[0027] Figure 3 It is the electron microscope image of the nano-silver powder in Example 3 of the present invention.

[0028] Figure 4 It is the electron microscope image of the nano-silver powder in Example 4 of the present invention.

[0029] Figure 5 It is the electron microscope image of the nano-silver powder in Comparative Example 1 of the present invention.

[0030] Figure 6 It is the electron microscope image of the nano-silver powder in Comparative Example 2 of the present invention.

[0031] Figure 7 It is the electron microscope image of the nano-silver powder in Comparative Example 3 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0032] Hereinafter, the object of the invention, technical solutions and beneficial effects of the present invention will be further described in detail.

[0033] It should be noted that the following detailed description is exemplary and is intended to provide further illustration of the claimed invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the technical field to which the present invention pertains.

[0034] The present invention aims to meet the relevant performance requirements of nano - silver powder for existing pressure - sensitive and thermosensitive resistors, and provides a new preparation method for nano - silver powder. This nano - silver powder is mainly prepared from a silver - containing oxidant, a complexing agent, a dispersant, a reducing agent, and a surface modifier through special reaction steps. Among them, by appropriately selecting the complexing agent, the release rate of silver atoms in the reaction system can be controlled, and in combination with a dispersant of a polymer - grafted polymer, through ionic groups adsorbed on the powder surface, controlling the solubility in the solvent, and grafted chains providing steric repulsion effects, the controllable and precise synthesis of nano - silver powder with high dispersibility and high specific surface area can be achieved. The use of the surface modifier can further reduce the hydrophobicity of the nano - silver powder and enhance its compatibility in the electronic paste. Therefore, the nano - silver powder prepared by this method has excellent particle size uniformity, enabling a single powder to still have good application effects in a paste system with a silver content of 30 - 80%.

[0035] The technical solution of the present invention can be further summarized as follows:

[0036] First, prepare the following material solutions:

[0037] Silver nitrate solution: Dissolve silver nitrate in pure water to prepare a silver nitrate solution with a silver ion concentration of 0.1 - 3 mol / L.

[0038] Complexing agent solution: Dissolve the complexing agent in pure water to prepare an alkaline solution with a mass - volume concentration of 12 - 300 g / L. The complexing agent can be selected from at least one of sodium hydroxide, potassium hydroxide, sodium chloride, potassium chloride, sodium carbonate, potassium carbonate, and ammonia water.

[0039] Dispersant solution: Dissolve the polymer - grafted polymer in pure water, methanol, or ethanol to prepare a dispersant solution with a mass fraction of 1 - 10%. The dosage of the polymer - grafted polymer is 0.1 - 5% of the silver content in the silver - containing oxidant. Optionally, the polymer - grafted polymer can be, for example, poly(ethyleneimine - graft - polyhydroxy acid ester), sodium polyacrylate - grafted starch, cellulose - grafted polyacrylic acid, cellulose - polyacrylamide graft copolymer, starch - acrylic acid graft copolymer, starch - acrylonitrile graft copolymer, carboxymethyl cellulose - acrylic acid graft copolymer, and other polymer - grafted polymers.

[0040] Reducing agent solution: Dissolve the reducing agent in pure water to prepare a reducing agent solution with a mass - volume concentration of 100 - 900 g / L. The reducing agent can be selected from at least one of formaldehyde, sodium borohydride, ascorbic acid, hydrazine hydrate, triethanolamine, and glucose.

[0041] Surface modifier solution: dissolve the surface modifier in ethanol to prepare a surface modifier solution with a mass fraction of 3% to 10%. The surface modifier can be selected from one of lauric acid, oleic acid, ricinoleic acid, palmitic acid, stearic acid, palmitic acid, and arachidic acid, and the amount of the surface modifier used is 0.5 to 3% of the silver content in the silver-containing oxidant.

[0042] Secondly, reaction and washing: under constant temperature stirring conditions, add the above silver nitrate solution and complexing agent solution to the dispersant solution at the same time within 5 to 10 minutes, stir for 5 to 20 minutes, add the reducing agent solution to the mixed solution within 1 to 20 minutes, continue stirring for 5 to 30 minutes, add deionized water and wash several times by static sedimentation to extract the clear liquid until the pH of the supernatant is neutral.

[0043] Then, modification and washing: add the surface modifier solution to the solution obtained by washing, stir at a constant temperature for 10 to 30 minutes, add deionized water and wash several times by static sedimentation and extraction of clear liquid, until the conductivity of the supernatant is ≤100μs / cm to separate the solid and liquid.

[0044] Finally, drying and pulverizing: the silver powder obtained by solid-liquid separation is placed in an oven at 80°C for 24 to 36 hours and pulverized by crushing and pulverizing equipment to obtain highly dispersed nano silver powder with a particle size range of 20 to 400 nm and a specific surface area of 5.0 to 14 m 2 / g, and the silver powder has a good application effect in electronic paste with a silver content of 30-80%, meeting the actual application needs of varistors.

[0045] In order to achieve effective control of the specific surface area and dispersibility of the silver powder in the present invention, during the preparation process, it is necessary to adjust factors such as the type and amount of the complexing agent and the high molecular graft polymer, the type and addition time of the reducing agent, and the type and amount of the modifier. Different combinations of factors can obtain silver powders with different specific surface areas and dispersities.

[0046] The drying time of the powder in the present invention is 24 to 36 hours. In specific operation, the drying time can be determined according to the water content of the powder. The silver powder obtained in the present invention has a small size and a high surface energy. Therefore, the powder is in a block shape macroscopically after drying and needs to be broken up by a special crushing and pulverizing device to restore its original dispersion state. This process does not affect the dispersibility of the powder itself.

[0047] The specific implementation modes of the present invention are described below in conjunction with the embodiments. Of course, the protection scope of the present invention is not limited to the following embodiments.

[0048] It should be noted that the following examples and comparative examples are all based on the production of 20 kg of silver powder in a single batch. In actual implementation, it can be scaled up or down proportionally according to different conditions such as actual production equipment. For example, 1 kg, 10 kg or 80 kg of silver powder can be produced in a single batch. In addition, the 20 kg of silver powder produced in a single batch in the present invention refers to the theoretical content of silver in the silver nitrate used. In the actual production process, due to the presence of dispersants and modifiers, the collected amount of silver powder will be slightly more than 20 kg.

[0049] Example 1: Preparation of nano silver powder (20 kg)

[0050] Accurately weigh 31.53 kg of silver nitrate solid and dissolve it in 200 kg of deionized water, stir and keep the temperature at 40 °C to obtain a silver nitrate solution for standby;

[0051] Accurately weigh 15 kg of sodium hydroxide solid and dissolve it in 85 kg of deionized water, stir and keep the temperature at 40 °C to obtain a sodium hydroxide solution for standby;

[0052] Accurately weigh 400 g of polymer grafted polymer and dissolve it in 20 kg of deionized water, stir and keep the temperature at 40 °C to obtain an aqueous dispersant solution for standby;

[0053] Accurately weigh 33.44 kg of anhydrous glucose solid and dissolve it in 100 kg of deionized water, stir and keep the temperature at 40 °C to obtain a glucose solution for standby;

[0054] Dissolve 200 g of lauric acid in 1 kg of ethanol, stir until colorless and transparent to obtain a lauric acid solution for standby.

[0055] At a water bath temperature of 40 °C and a stirring speed of 250 rpm, add the sodium hydroxide solution and the silver nitrate solution to the dispersant solution within 9 min. After stirring at a constant temperature for 10 min, add the glucose solution to the mixed solution within 15 min. After stirring at a constant temperature for 20 min, add deionized water, let it stand and settle for 20 min, then extract the supernatant. Repeat the washing process multiple times until the pH of the supernatant is neutral. Then add the lauric acid solution to it, stir at a constant temperature for 30 min, add deionized water, let it stand and settle for 20 min, then extract the supernatant. Repeat the washing process multiple times until the conductivity of the supernatant < 100 us / cm, and perform solid-liquid separation.

[0056] Place the silver powder obtained by solid-liquid separation in an oven at 80 °C and dry it for 28 h, then pulverize it with a pulverizing equipment to obtain nano silver powder with a particle size range of 50 - 200 nm and a specific surface area of 9.8 m 2 / g (see Figure 1 ).

[0057] Example 2: Preparation of nano silver powder (20 kg)

[0058] Accurately weigh 31.53 kg of silver nitrate solid, dissolve it in 200 kg of deionized water, stir and keep the temperature constant at 30 °C to prepare a silver nitrate solution for standby;

[0059] Accurately weigh 7.87 kg of sodium hydroxide solid, dissolve it in 98 kg of deionized water, stir and keep the temperature constant at 30 °C to prepare a sodium hydroxide solution for standby;

[0060] Accurately weigh 600 g of polymer grafted polymer, dissolve it in 50 kg of deionized water, stir and keep the temperature constant at 30 °C to prepare an aqueous dispersant solution for standby;

[0061] Accurately weigh 11.6 kg of 80% hydrazine hydrate, dissolve it in 11.6 kg of deionized water, stir and keep the temperature constant at 30 °C to prepare a hydrazine hydrate solution for standby;

[0062] Dissolve 160 g of oleic acid in 0.5 kg of ethanol, stir until colorless and transparent to prepare an oleic acid solution for standby.

[0063] At a water bath temperature of 30 °C and a stirring speed of 250 rpm, add the sodium hydroxide solution and the silver nitrate solution to the dispersant solution within 10 min. After stirring at a constant temperature for 5 min, add the glucose solution to the mixed solution within 5 min. After stirring at a constant temperature for 5 min, add deionized water, let it stand and settle for 10 min, then draw the supernatant. Repeat the washing process several times until the pH of the supernatant is neutral. Then add the oleic acid solution to it, stir at a constant temperature for 30 min, add deionized water, let it stand and settle for 30 min, then draw the supernatant. Repeat the washing process several times until the conductivity of the supernatant < 100 μS / cm, and perform solid-liquid separation.

[0064] Place the silver powder obtained by solid-liquid separation in an oven at 80 °C and dry it for 24 h, then pulverize it with a pulverizing equipment to obtain nano-silver powder with a particle size range of 200 - 400 nm and a specific surface area of 5.5 m 2 / g (see Figure 2 ).

[0065] Example 3: Preparation of nano-silver powder (20 kg)

[0066] Accurately weigh 31.53 kg of silver nitrate solid, dissolve it in 200 kg of deionized water, stir and keep the temperature constant at 80 °C to prepare a silver nitrate solution for standby;

[0067] Accurately weigh 10.12 kg of sodium hydroxide solid, dissolve it in 138 kg of deionized water, stir and keep the temperature constant at 80 °C to prepare a sodium hydroxide solution for standby;

[0068] Accurately weigh 1000 g of polymer grafted polymer, dissolve it in 60 kg of deionized water, stir and keep the temperature constant at 80 °C to prepare an aqueous dispersant solution for standby;

[0069] Accurately weigh 13.83 kg of triethanolamine and dissolve it in 20 kg of deionized water. Stir and keep the temperature constant at 80 °C to prepare a triethanolamine solution for standby.

[0070] Dissolve 400 g of arachidic acid in 1 kg of ethanol and stir until it becomes colorless and transparent to prepare an arachidic acid solution for standby.

[0071] At a water bath temperature of 80 °C and a stirring speed of 200 rpm, add the sodium hydroxide solution and silver nitrate solution to the dispersant solution within 10 min. After stirring at a constant temperature for 20 min, add the triethanolamine solution to the mixed solution within 3 min. After stirring at a constant temperature for 30 min, add deionized water. Let it stand and settle for 30 min, then draw the supernatant. Repeat the washing process multiple times until the pH of the supernatant is neutral. Then add the arachidic acid solution to it. After stirring at a constant temperature for 30 min, add deionized water. Let it stand and settle for 30 min, then draw the supernatant. Repeat the washing process multiple times until the conductivity of the supernatant < 100 μS / cm, and then perform solid-liquid separation.

[0072] Place the silver powder obtained by solid-liquid separation in an oven at 80 °C and dry it for 36 h. Then pulverize it with a crushing and pulverizing device to obtain nano-silver powder with a particle size range of 90 - 370 nm and a specific surface area of 7.1 m 2 / g (see Figure 3 ).

[0073] Example 4: Preparation of nano-silver powder (20 kg)

[0074] Accurately weigh 31.53 kg of silver nitrate solid and dissolve it in 150 kg of deionized water. Stir and keep the temperature constant at 45 °C to prepare a silver nitrate solution for standby.

[0075] Accurately weigh 7.87 kg of sodium hydroxide solid and dissolve it in 85 kg of deionized water. Stir and keep the temperature constant at 45 °C to prepare a sodium hydroxide solution for standby.

[0076] Accurately weigh 1000 g of polymer grafted polymer and dissolve it in 40 kg of deionized water. Stir and keep the temperature constant at 45 °C to prepare an aqueous dispersant solution for standby.

[0077] Accurately weigh 20 kg of ascorbic acid solid and dissolve it in 110 kg of deionized water. Stir and keep the temperature constant at 45 °C to prepare an ascorbic acid solution for standby.

[0078] Dissolve 200 g of palmitic acid in 1 kg of ethanol and stir until it becomes colorless and transparent to prepare a palmitic acid solution for standby.

[0079] At a water bath temperature of 45 °C and a stirring speed of 200 rpm, the sodium hydroxide solution and silver nitrate solution were added to the dispersant solution within 10 min. After stirring at a constant temperature for 15 min, the ascorbic acid solution was added to the mixed solution within 5 min. After stirring at a constant temperature for 5 min, deionized water was added. After standing and settling for 30 min, the supernatant was drawn, and the washing was repeated multiple times until the pH of the supernatant was neutral. Then, the palmitic acid solution was added thereto. After stirring at a constant temperature for 30 min, deionized water was added. After standing and settling for 30 min, the supernatant was drawn, and the washing was repeated multiple times until the conductivity of the supernatant was <100 μS / cm, and solid-liquid separation was performed.

[0080] The silver powder obtained by solid-liquid separation was placed in an oven at 80 °C and dried for 32 h, and then pulverized by a pulverizing equipment to obtain nano-silver powder with a particle size range of 50 - 330 nm and a specific surface area of 8.2 m 2 / g (see Figure 4 ).

[0081] Comparative Example 1:

[0082] This comparative example used the same solutions as in Example 1, including silver nitrate solution, sodium hydroxide solution, aqueous dispersant solution, glucose solution, and lauric acid solution. The following reaction process was adopted during preparation:

[0083] At a water bath temperature of 40 °C and a stirring speed of 250 rpm, the glucose solution and silver nitrate solution were added to the dispersant solution within 9 min. After stirring at a constant temperature for 10 min, the sodium hydroxide solution was added to the mixed solution within 15 min. After stirring at a constant temperature for 20 min, deionized water was added. After standing and settling for 20 min, the supernatant was drawn, and the washing was repeated multiple times until the pH of the supernatant was neutral. Then, the lauric acid solution was added thereto. After stirring at a constant temperature for 30 min, deionized water was added. After standing and settling for 20 min, the supernatant was drawn, and the washing was repeated multiple times until the conductivity of the supernatant was <100 μS / cm, and solid-liquid separation was performed.

[0084] The silver powder obtained by solid-liquid separation was placed in an oven at 80 °C and dried for 28 h, and then pulverized by a pulverizing equipment to obtain nano-silver powder with a particle size range of 110 - 5400 nm and a specific surface area of 5.1 m 2 / g (see Figure 5 ).

[0085] Comparative Example 2:

[0086] The difference between this comparative example and Example 1 was that no dispersant solution was used, and the remaining solutions and preparation processes were exactly the same, as follows:

[0087] Under a water bath temperature of 40 °C and a stirring speed of 250 rpm, the sodium hydroxide solution was added to the silver nitrate solution within 9 min. After stirring at a constant temperature for 10 min, the glucose solution was added to the mixed solution within 15 min. After stirring at a constant temperature for 20 min, deionized water was added. After standing and settling for 20 min, the supernatant was drawn. The washing was repeated multiple times until the pH of the supernatant was neutral. Then, the lauric acid solution was added thereto. After stirring at a constant temperature for 30 min, deionized water was added. After standing and settling for 20 min, the supernatant was drawn. The washing was repeated multiple times until the conductivity of the supernatant < 100 μS / cm, and solid-liquid separation was performed.

[0088] The silver powder obtained by solid-liquid separation was placed in an oven at 80 °C and dried for 28 h, and then pulverized by a pulverizing equipment to obtain nano-silver powder with a particle size range of 70 - 4250 nm and a specific surface area of 6.3 m 2 / g (see Figure 6 ).

[0089] Comparative Example 3:

[0090] The difference between this comparative example and Example 1 is that the lauric acid solution was replaced with KH550 solution, and the rest of the solutions and the preparation process were exactly the same, as follows:

[0091] Preparation of KH550 solution: 200 g of KH550 was dissolved in 1 kg of ethanol and stirred until colorless and transparent to obtain KH550 solution.

[0092] Under a water bath temperature of 40 °C and a stirring speed of 250 rpm, the sodium hydroxide solution was added to the silver nitrate solution within 9 min. After stirring at a constant temperature for 10 min, the glucose solution was added to the mixed solution within 15 min. After stirring at a constant temperature for 20 min, deionized water was added. After standing and settling for 20 min, the supernatant was drawn. The washing was repeated multiple times until the pH of the supernatant was neutral. Then, the KH550 solution was added thereto. After stirring at a constant temperature for 30 min, deionized water was added. After standing and settling for 20 min, the supernatant was drawn. The washing was repeated multiple times until the conductivity of the supernatant < 100 μS / cm, and solid-liquid separation was performed.

[0093] The silver powder obtained by solid-liquid separation was placed in an oven at 80 °C and dried for 28 h, and then pulverized by a pulverizing equipment to obtain nano-silver powder with a particle size range of 140 - 17930 nm and a specific surface area of 4.5 m 2 / g (see Figure 7 ).

[0094] As can be seen from Examples 1 to 4, through the cooperation of a complexing agent and a polymer grafted polymer, the present invention can form nano silver powder with high dispersibility and high specific surface area. The introduction of the complexing agent can control the release rate of silver ions. When the addition sequence of the complexing agent is changed (see Comparative Example 1), even if a polymer grafted polymer is used, it is impossible to achieve rapid coating and solubility of silver powder in the system, and it is impossible to ensure the control of the high dispersibility of silver powder; when the polymer grafted polymer is omitted (see Comparative Example 2), even if the release rate of silver ions can be controlled, the dispersion performance of silver powder in the system cannot be controlled, resulting in adhesion and agglomeration of the powder due to high surface energy; through the use of a surface modifier solution, the present invention can further reduce the hydrophobicity of nano silver powder and enhance the compatibility of nano silver powder in electronic pastes. If it is replaced with other solutions (see Comparative Example 3), it may cause serious adhesion and agglomeration of the powder, the powder still shows hydrophilicity, which is not conducive to the subsequent washing process, and affects the compatibility and application performance of the powder in electronic pastes.

[0095] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Any simple modification and equivalent change made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A preparation method of nano silver powder for pressure-sensitive and thermosensitive resistors, characterized in that: It includes the following steps: S1. Solution preparation: Dissolve silver nitrate in pure water to prepare a silver nitrate solution with a silver ion concentration of 0.1 - 3 mol / L, dissolve the complexing agent in pure water to prepare a complexing agent solution with a mass - volume concentration of 12 - 300 g / L, dissolve the dispersant in pure water, methanol or ethanol to prepare a dispersant solution with a mass fraction of 1 - 10%, dissolve the reducing agent in pure water to prepare a reducing agent solution with a mass - volume concentration of 100 - 900 g / L, and dissolve the surface modifier in ethanol to prepare a surface modifier solution with a mass fraction of 3 - 10%. The dispersant is a polymer - grafted polymer, and its dosage is 0.1 - 5% of the silver content in the silver - containing oxidant. The polymer - grafted polymer is one of polyethyleneimine - graft - polyhydroxy acid ester, sodium polyacrylate - grafted starch, cellulose - grafted polyacrylic acid, cellulose - polyacrylamide graft copolymer, starch - acrylic acid graft copolymer, starch - acrylonitrile graft copolymer, or carboxymethyl cellulose - acrylic acid graft copolymer. S2. Reaction: Under the condition of constant - temperature stirring, add the silver nitrate solution and the complexing agent solution into the dispersant solution simultaneously within 5 - 10 min. After stirring for 5 - 20 min, add the reducing agent solution into the mixed solution within 1 - 20 min, and continue to stir for 5 - 30 min. S3. Washing: Add deionized water to the mixed solution obtained in step S2, then let it stand for sedimentation and draw out the supernatant for washing until the pH of the solution is neutral. S4. Modifying washing: Add the surface modifier solution to the solution obtained in step S3, stir at a constant temperature for 10 - 30 min, then add deionized water to the solution. Then let it stand for sedimentation and draw out the supernatant for washing until the conductivity of the solution ≤ 100 μs / cm, and then perform solid - liquid separation. S5. Drying and pulverizing: After drying and pulverizing the silver powder obtained by solid-liquid separation in step S4, nano silver powder with a particle size of 20 - 400 nm and a specific surface area of 5.0 - 14 m 2 / g can be obtained.

2. The preparation method according to claim 1, wherein: The complexing agent is selected from at least one of sodium hydroxide, potassium hydroxide, sodium chloride, potassium chloride, sodium carbonate, potassium carbonate, and ammonia water.

3. The preparation method according to claim 1, wherein: The reducing agent is selected from at least one of formaldehyde, sodium borohydride, ascorbic acid, hydrazine hydrate, triethanolamine, and glucose.

4. The preparation method according to claim 1, wherein: The surface modifier is selected from one of lauric acid, oleic acid, ricinoleic acid, palmitic acid, stearic acid, palmitoleic acid, and arachidic acid, and its dosage is 0.5 - 3% of the silver content in the silver - containing oxidant.

5. The preparation method according to claim 1, characterized in that: The condition of constant - temperature stirring is a water - bath temperature of 20 - 80 °C and a mechanical stirring speed of 200 - 350 rpm.

6. The preparation method according to claim 1, characterized in that: The drying and pulverizing is to dry in a blast drying oven at 80 °C for 24 - 36 h and then pulverize with a pulverizing equipment.

7. An electronic paste for a pressure-sensitive thermistor, characterized in that: This electronic paste contains the nano - silver powder obtained by the preparation method described in any one of claims 1 - 6.

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