Preparation method of high-tap-density high-purity silver powder

Abstract

The invention relates to the technical field of nano silver powder preparation, in particular to a preparation method of high-tap-density and high-purity silver powder. The problems that in the prior art, silver powder is insufficient in purity and low in tap density are solved. The method comprises the following steps: preparing silver nitrate into a solution, carrying out electrolytic pretreatment, and carrying out chemical reduction, coprecipitation and dissolution reduction to obtain a silver powder suspension; the silver powder suspension is subjected to physical vapor deposition, silver vapor is deposited on the surface of the silver powder under the action of argon, and silver powder particles are tightly arranged; preparing an electrolyte from silver nitrate and bismuth nitrate, and carrying out electrolytic refining; and finally, the high-tap-density high-purity silver powder is obtained through ultrasonic dispersion and supercritical fluid drying treatment. By strictly controlling the process sequence and parameters and utilizing the synergistic effect among the processes, impurities are effectively removed, the particle size distribution of nano silver powder particles is optimized, the purity and tap density of the nano silver powder are improved, and the application of the silver powder in conductive materials is optimized.

Description

Technical Field

[0001] The invention relates to the technical field of nano silver powder preparation, in particular to a method for preparing high-purity silver powder with high tap density. Background Art

[0002] Nano silver powder has become an important material in the electrical and electronic industries due to its excellent conductivity and application performance. It is the most widely used precious metal powder in the electronics industry and a basic functional material for conductive pastes such as thick films, resistors, ceramics, and dielectrics. The tap density and purity of nano silver powder have a significant impact on the performance of conductive pastes.

[0003] Nano silver powder with high tap density can be more densely packed in electronic material systems, significantly increasing the silver content per unit volume, thereby greatly enhancing the conductive properties of the material; increasing the purity of nano silver powder is conducive to improving the uniformity of conductivity, avoiding the generation and accumulation of heat, and extending the service life of the material.

[0004] However, existing nano silver powder preparation technologies, including chemical reduction, physical vapor deposition and electrolysis, have exposed problems of low tap density and insufficient purity in practical applications. In the chemical reduction process, the residual reducing agent and the dissolved materials of the reaction container are easily mixed into the generated silver powder, which greatly reduces the purity of the silver powder. In addition, the prepared nano silver powder particles are prone to agglomeration due to their own characteristics and reaction conditions, resulting in loose particle stacking and difficulty in improving the tap density, further damaging the material performance.

[0005] The physical vapor deposition method has a slow deposition rate and low production efficiency, which limits large-scale production. At the same time, during the silver powder collection process, even a tiny external force may destroy the original stacking state of the silver powder, resulting in a decrease in tap density. During the electrolysis process, impurity ions are very likely to co-deposit with silver ions at the cathode under the action of direct current, seriously affecting the purity of the silver powder.

[0006] In summary, the preparation of existing silver powder can meet the basic application of electronic materials to a certain extent. However, the prepared silver powder still has the problems of low tap density and insufficient purity, which limits the performance improvement of electronic conductive materials and affects the service life of the materials.

[0007] Therefore, a method for preparing high-purity silver powder with high tap density is proposed. Summary of the invention

[0008] The object of the present invention is to provide a method for preparing high-purity silver powder with high tap density. The present invention prepares silver nitrate into a solution, performs electrolytic pretreatment, and then obtains a silver powder suspension through chemical reduction, coprecipitation and dissolution reduction processes; the silver powder suspension is subjected to physical vapor deposition, and silver vapor is deposited on the surface of the silver powder under the action of argon carrier gas, so that the silver powder particles are arranged closely; then, an electrolyte is prepared with silver nitrate and bismuth nitrate for electrolytic refining; finally, high-purity silver powder with high tap density is obtained through ultrasonic dispersion and supercritical fluid drying. By strictly controlling the process sequence and parameters and utilizing the synergistic effect between the processes, impurities can be effectively removed, the particle size distribution of nano silver powder particles can be optimized, the purity and tap density of the silver powder can be improved, and the application of the silver powder in conductive materials can be optimized.

[0009] To achieve the above object, the present invention provides the following technical solutions:

[0010] In one aspect, the present invention provides a method for preparing high-purity silver powder with high tap density. The preparation of the silver powder comprises the following steps:

[0011] S1: adding silver nitrate powder to deionized water and performing pre-electrolysis to obtain a pre-electrolyte;

[0012] S2 chemically reducing the pre-electrolyte with ascorbic acid and dispersing with polyvinyl pyrrolidone to obtain a silver powder dispersion;

[0013] S3: the silver powder dispersion and ethylenediaminetetraacetic acid are subjected to complexation coprecipitation to obtain a precipitate; and then a silver powder suspension is obtained through a dissolution reduction process;

[0014] S5 obtains treated silver powder by physical vapor deposition of the silver powder suspension; obtains refined silver powder by electrolytic refining of the treated silver powder;

[0015] S6 disperses the refined silver powder by ultrasonication and dries it by supercritical fluid to obtain high-purity silver powder with high tap density;

[0016] Among them, the temperature of pre-electrolysis treatment is 20-30℃; the temperature of electrolytic refining is 30-40℃;

[0017] The temperature of dissolution reduction is 40-45°C; the time of dissolution reduction is 30-60min;

[0018] The argon gas flow rate for physical vapor deposition is 5-15 sccm.

[0019] Preferably, the pre-electrolysis treatment comprises the following steps:

[0020] Silver nitrate powder with a purity greater than 99% was added to deionized water, and stirred at 300 rpm at 30°C to obtain a 0.8 mol / L uniform solution; the uniform solution was added to an electrolytic cell with a platinum electrode, and the power was turned on to maintain a current density of 5-10 mA / cm2 The temperature of the electrolyte is 20-30°C, the pH value is maintained at 3, and the pre-electrolyte is obtained by electrolysis for 10-30 minutes; the average particle size of the silver nitrate powder is 1-10 μm.

[0021] Preferably, the chemical reduction comprises the following steps:

[0022] Ascorbic acid is dissolved in deionized water to prepare a reducing solution with a concentration of 0.3-0.5 mol / L; the pre-electrolyte is transferred to a reactor, and a mixed solvent of ethanol and deionized water in a volume ratio of 3:1 is added, and the mixture is stirred evenly to form a mixed system; polyvinyl pyrrolidone with a mass concentration of 1.5 g / L is added to the mixed system, and the mixture is stirred at 500 rpm for 30 minutes at 25°C to obtain a dispersed system; the dispersed system is heated to 40-60°C with the assistance of microwaves, a dilute nitric acid solution is added to adjust the pH value to 5, and the reducing solution is slowly added through a dropping funnel, and the reaction is performed for 2-5 hours to obtain a silver powder dispersion.

[0023] Preferably, the coprecipitation and dissolution reduction process comprises the following steps:

[0024] Adding sodium hydroxide solution to the silver powder dispersion to adjust the pH value to 10, adding ethylenediaminetetraacetic acid under stirring, and continuously stirring for 2 hours after the addition to obtain a reaction system; placing the reaction system in a centrifuge, separating and centrifuging, removing the supernatant, and washing with deionized water to obtain a precipitate;

[0025] The precipitate was placed in a constant temperature water bath and heated to 40°C, dilute nitric acid with a molar concentration of 1 mol / L was added, and the solution was stirred at 200 rpm for 30 minutes to obtain a uniform solution; a sodium hydroxide solution with a molar concentration of 2 mol / L was slowly added to the uniform solution, the pH value was adjusted to 8, the dissolution reduction temperature was 40-42°C, polyvinyl pyrrolidone was added, and the solution was stirred at 400 rpm for 30-45 minutes to obtain a silver powder suspension.

[0026] Preferably, the molar ratio of silver ions to ethylenediaminetetraacetic acid in the silver nitrate powder is 1:1.2-1.5; the mass concentration of polyvinyl pyrrolidone is 1.5-2.0 g / L; and the polyvinyl pyrrolidone is K30.

[0027] Preferably, physical vapor deposition comprises the following steps:

[0028] The silver powder suspension is transferred to a sample placement table of a physical vapor deposition device, and a high vacuum system is turned on to evacuate the physical vapor deposition device; a silver block with an average particle size of 2 mm is used as an evaporation source, and an electron beam evaporator is turned on to gasify and obtain silver vapor; an appropriate amount of argon gas is introduced into the device through a gas flow control center, and the flow rate is controlled at 5-12 sccm to perform a deposition reaction, and the deposition reaction time is 30-60 minutes to obtain treated silver powder; the mass ratio of the silver amount in the silver block to the silver nitrate powder is 1:5-10.

[0029] Preferably, electrolytic refining comprises the following steps:

[0030] Silver nitrate and bismuth nitrate are dissolved in deionized water with a molar ratio of silver nitrate to bismuth nitrate of 2:1 to obtain an electrolyte, and the pH value of the electrolyte is adjusted to 4 with dilute nitric acid; treated silver powder is used as an anode and a high-purity silver plate is used as a cathode to add into the electrolyte, and the temperature of the electrolyte is controlled at 30-40°C to obtain an electrolytic system; a voltage is applied to the electrolytic system, and the voltage is maintained at 1.5-2.0V, and the refining time is 2-4h to obtain refined silver powder.

[0031] Preferably, supercritical fluid drying comprises the following steps:

[0032] The refined silver powder is maintained at an ultrasonic power of 200 W and an ultrasonic time of 10 minutes, and dispersed silver powder is obtained by ultrasonic treatment; the dispersed silver powder is slowly transferred to a reactor of a supercritical fluid device, and a high-pressure pump is turned on to pump liquid carbon dioxide into the reactor, and the temperature of the reactor is raised to 35°C; when the carbon dioxide reaches a supercritical state, it is stirred at 100-200 r / min for 45-90 minutes; the pressure is reduced at a rate of 0.2 MPa / min by controlling a pressure reducing valve, and the temperature in the reactor is slowly reduced to room temperature at a cooling rate of 2°C / min, and high-tapped density and high-purity silver powder are collected.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. The present invention can effectively remove impurities in the silver nitrate raw material through electrolytic pretreatment and chemical reduction. During co-precipitation, ethylenediaminetetraacetic acid and silver ions are complexed, which can accurately regulate the precipitation and redissolution of silver ions, thereby controlling the nucleation and growth of silver powder; by controlling parameter changes and utilizing the synergistic effect between processes, the distribution uniformity of the particle size of the silver powder particles is improved, which is beneficial to improving the conductive performance in the conductive material.

[0035] 2. The present invention can remove a large amount of impurities and reduce the burden of subsequent processes by controlling the temperature, current density and time during the electrolytic pretreatment process; the synergistic use of co-precipitation, electrolytic refining and supercritical fluid drying processes allows the silver powder crystals to grow uniformly, forming a more dense and regular crystal structure, further improving the purity of the silver powder.

[0036] 3. The present invention adjusts the process sequence and utilizes physical vapor deposition to fill the gaps between silver powder particles; during the electrolytic refining process, the deposition behavior of silver ions is improved to promote the silver powder to form a denser crystal structure; finally, supercritical fluid drying is used to remove liquid impurities and avoid agglomeration of silver powder. The tap density of silver powder is increased through the synergistic relationship between the various process steps. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 The present invention is a process flow chart of high-purity silver powder with high tap density. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] See also Figure 1 , Figure 1 The process flow chart of the high-tap density high-purity silver powder prepared by the present invention is as follows: The present invention provides a method for preparing high-tap density high-purity silver powder, and the technical scheme is as follows:

[0040] Example 1

[0041] Silver nitrate powder with a purity greater than 99% is added to deionized water, and stirred at 300 rpm at 30°C to obtain a 0.8 mol / L uniform solution; the uniform solution is added to an electrolytic cell of a platinum electrode, the power is turned on, the current density is maintained at 8 mA / cm2, the temperature of the electrolyte is maintained at 20°C, the pH value is maintained at 3, and the pre-electrolyte is obtained by electrolysis for 20 minutes;

[0042] Ascorbic acid was dissolved in deionized water to prepare a reducing solution with a concentration of 0.4 mol / L; the pre-electrolyte was transferred to a reactor, and a mixed solvent was added, wherein the mixed solvent was prepared by ethanol and deionized water in a volume ratio of 3:1, and the mixture was stirred at 200 rpm to form a mixed system; polyvinyl pyrrolidone with a mass concentration of 1.5 g / L was added to the mixed system, and the mixture was stirred at 500 rpm for 30 min at 25°C to obtain a dispersed system; the dispersed system was heated to 50°C with the aid of microwaves, a dilute nitric acid solution was added to adjust the pH value to 5, and the reducing solution was slowly added through a dropping funnel, the dropping speed was controlled to 2 drops / second, the temperature and pH value were maintained, and the reaction was performed for 3 hours to obtain a silver powder dispersion;

[0043] Adding sodium hydroxide solution to the silver powder dispersion to adjust the pH value to 10, adding analytically pure ethylenediaminetetraacetic acid under a constant temperature water bath of 50° C. and stirring at 300 r / min, with the molar ratio of silver ion to EDTA being 1:1.5, and stirring continuously for 2 h after the addition to obtain a reaction system; placing the reaction system in a centrifuge, separating and centrifuging for 10 min at a centrifuge speed of 5000 r / min, removing the supernatant, and washing with deionized water to obtain a precipitate;

[0044] The precipitate was added to a flask, placed in a constant temperature water bath and heated to 40°C, diluted nitric acid with a molar concentration of 1 mol / L was added, and stirred at 200 rpm for 30 min to obtain a uniform solution; a sodium hydroxide solution with a molar concentration of 2 mol / L was slowly added to the uniform solution, the pH value was adjusted to 8, the temperature was controlled at 40°C, and polyvinyl pyrrolidone with a mass concentration of 1.8 g / L was added, and stirred at 400 rpm for 35 min to obtain a silver powder suspension;

[0045] The silver powder suspension was carefully transferred to the sample placement table of the physical vapor deposition device, and the high vacuum system was turned on to evacuate the physical vapor deposition device for 40 minutes. The vacuum degree was controlled at 10 -4 Pa; a silver block with an average particle size of 2 mm is used as an evaporation source, and the mass ratio of the silver block to the silver in silver nitrate is 1:8, and an electron beam evaporator is turned on to bombard the silver block with an electron beam to gradually increase its temperature to 1600°C, and to gasify and obtain silver vapor; an appropriate amount of argon gas is introduced into the device through a gas flow control center, and the flow rate is controlled at 10 sccm, and the silver vapor is transported to the surface of the silver powder as a carrier gas to perform a deposition reaction, and the reaction time is 45 minutes to obtain the treated silver powder, thereby increasing the tap density of the silver powder;

[0046] Silver nitrate and bismuth nitrate are dissolved in deionized water, the molar ratio of silver nitrate to bismuth nitrate is 2:1, the mass concentration is controlled to be 0.05 g / L to obtain an electrolyte, and the pH value of the electrolyte is adjusted to 4 with dilute nitric acid; treated silver powder is used as an anode, a high-purity silver plate is used as a cathode, the area ratio of the cathode and the cathode is 1:3, and the temperature of the electrolyte is controlled at 30°C to obtain an electrolytic system; a voltage is applied to the electrolytic system, and the voltage is maintained at 1.8 V for electrolytic refining, and the refining time is 3 hours to obtain refined silver powder;

[0047] The refined silver powder was placed in an ultrasonic cleaner, the ultrasonic power was set to 200W, and the ultrasonic time was 10min to obtain dispersed silver powder; the dispersed silver powder was slowly transferred to the reactor of the supercritical fluid device, the feed valve was closed, the reactor was sealed, and the vacuum degree was controlled at 10 -3Pa or less; start the high-pressure pump to maintain the pressure in the autoclave at 7.5MPa, pump liquid carbon dioxide into the reactor at a flow rate of 8mL / min, start the heating device, and increase the temperature in the reactor to 35°C at a heating rate of 1°C / min; when the carbon dioxide reaches a supercritical state, stir at 150r / min for 60min; control the pressure reducing valve to reduce the pressure to normal pressure at a rate of 0.2MPa / min, slowly reduce the temperature in the reactor to room temperature at a cooling rate of 2°C / min, open the reactor, and use a drying tool to carefully collect the silver powder with high tap density and high purity.

[0048] Examples 2-6 refer to the preparation methods and parameter conditions of Example 1, with the differences shown in Table 1.

[0049] Table 1 Parameter changes of Examples 1-6

[0050]

[0051] Comparative Example 1 refers to Example 1, except that no chemical reduction treatment is performed to obtain the silver powder dispersion, and the silver powder suspension is obtained by direct dissolution and reduction.

[0052] Comparative Example 2 refers to Example 1, except that the silver powder dispersion is not subjected to a coprecipitation and re-dissolution reduction process, but is directly subjected to physical vapor deposition.

[0053] Comparative Example 3 refers to Example 1, except that the pre-electrolytic solution is co-precipitated to obtain a precipitate, which is then chemically reduced to obtain a silver powder suspension.

[0054] Comparative Example 4 refers to Example 1, except that polyvinyl pyrrolidone is not added during the dissolution reduction process.

[0055] Comparative Example 5 refers to Example 1, except that microwave-assisted heating is not used in the chemical reduction process to carry out the reduction reaction.

[0056] Comparative Example 6 refers to Example 1, except that the temperature during the dissolution reduction and coprecipitation stages is 50°C.

[0057] Comparative Example 7 refers to Example 1, except that the temperature during the dissolution reduction and coprecipitation stages is 30°C.

[0058] Comparative Example 8 refers to Example 1, except that the stirring time in the precipitation stage is 60 min.

[0059] Comparative Example 9 refers to Example 1, except that the pH value of the precipitation stage is 10.

[0060] Experimental Example 1 Average Particle Size Measurement

[0061] The silver powder suspensions prepared in Examples 1-6 and Comparative Examples 1-9 were diluted to ensure that the particles were in a monodisperse state in the suspension to prevent particle agglomeration from affecting the measurement results; the diluted suspension was injected into the sample pool of the laser particle size analyzer. Before measurement, the instrument was calibrated and tested using standard samples with known particle sizes to ensure the accuracy of the instrument measurement. During the measurement, the uniformity of the suspension was ensured by stirring at 200 rpm, and the scattered light data was automatically measured and analyzed. The test results are shown in Table 2.

[0062] Table 2 Average particle size test of Examples 1-6 and Comparative Examples 1-9

[0063] Example Average particle size / nm Example 1 80-90 Example 2 75-110 Example 3 80-110 Example 4 80-100 Example 5 90-110 Example 6 75-95 Comparative Example 1 150nm-1μm Comparative Example 2 200-300 Comparative Example 3 300-400 Comparative Example 4 100-600 Comparative Example 5 400-500 Comparative Example 6 200-400 Comparative Example 7 80-180 Comparative Example 8 400-600 Comparative Example 9 150-300

[0064] From the results in Table 2, it can be seen that the change of the process method and the adjustment of the parameters have a significant effect on the average particle size of the prepared silver powder; in Comparative Example 1, no chemical reduction treatment is performed, and the average particle size is significantly increased. First, the chemical treatment steps such as electrolytic pretreatment and reduction coprecipitation can effectively remove impurities in the silver nitrate raw material. Without this step, some impurity ions introduced will inhibit the growth of silver ions on certain crystal faces, resulting in uneven growth of silver powder particles, a wider average particle size distribution, and failure to form uniform and fine silver powder particles; secondly, during coprecipitation, ethylenediaminetetraacetic acid complexes with silver ions, which can accurately regulate the precipitation and redissolution of silver ions, thereby controlling the nucleation and growth of silver powder. Without pre-regulation of the coprecipitation process, silver ions may be localized in the local Some areas quickly aggregate to form large particles, while other areas have insufficient nucleation, which ultimately makes the average particle size of the silver powder larger, and the silver powder is difficult to stabilize within a smaller particle size range; finally, when dissolution and reduction are directly carried out, impurities in the silver nitrate raw material and other substances that may be introduced during the dissolution process will make the dissolution and reduction environment unstable, resulting in different activation energies for reduction and nucleation, chaotic and disordered nucleation process, and different sizes of crystal nuclei formed, which aggravates the difference in silver powder particle size and makes it difficult to obtain a silver powder suspension with uniform particle size; combined with the results of comparative examples 2-3, it can be seen that coprecipitation can remove impurities, and then dissolution and reduction can optimize the silver powder particle size. If this process is skipped, impurities will affect the agglomeration and growth of silver powder particles, making the average particle size larger and unevenly distributed; coprecipitation first and then chemical reduction Impurities may be precipitated together with silver ions. In the chemical reduction stage, impurities will hinder the orderly combination of silver atoms to form regular particles, so that the silver powder cannot form uniform particle size according to the ideal state during the growth process; in Comparative Example 4, polyvinyl pyrrolidone can be adsorbed on the surface of silver powder particles, inhibiting the rapid deposition of silver ions in certain directions, making the growth of silver powder particles more uniform. Compared with not adding PVP, the deposition of silver ions is uncontrolled, the growth rate of particles varies greatly, and the increase in agglomeration leads to a larger average particle size and a wider distribution; combined with Comparative Example 5, microwave-assisted heating can heat the reaction system quickly and evenly, accelerate the reaction rate, promote the rapid reduction of silver ions into nuclei, and form silver powder particles with uniform particle size; the results of Comparative Examples 6-7 show that lower temperature is conducive to Improve the nucleation rate. According to the classical nucleation theory, at a lower temperature, the movement speed of silver ions in the solution is relatively slow, the collision frequency between molecules is reduced, and silver ions are more likely to form crystal nuclei on multiple dispersed active sites. The number of crystal nuclei is relatively large, and the silver ions available to many crystal nuclei in the subsequent growth process are relatively small, and then grow to form silver powder particles with smaller particle size. When the temperature is too high, the movement speed of silver ions is accelerated, which may lead to too fast and uneven crystal nucleation speed, and some crystal nuclei grow rapidly, and finally the silver powder particle size distribution becomes wider and the particle size distribution is uneven. In addition, when the temperature is too low, the deposition rate slows down, the particle growth stagnates, and even secondary nucleation may occur, which affects the consistency of the silver powder particle size, the particle size distribution range becomes wider, and the uniformity decreases.The results of Comparative Examples 8-9 show that the reaction time during the reprecipitation process is short, the crystal nucleus does not have enough time to grow, and the particle size of the silver powder particles will be relatively small. When the reaction time is too long, the crystal nucleus continues to grow, and the particle size of the silver powder particles will increase; the appropriate pH value can adjust the existence form and reaction activity of silver ions. In a weakly alkaline environment, silver ions may exist in the form of silver hydroxide complexes, and their reaction activity is moderate, which is conducive to the formation of silver powder particles with uniform particle size; in summary, by pre-treating the silver nitrate solution, chemical reduction, co-precipitation and dissolution reduction process, electrolytic pre-treatment and chemical reduction can effectively remove impurities in the silver nitrate raw material, and ethylenediaminetetraacetic acid and silver ions are complexed during co-precipitation, which can accurately control the precipitation and re-dissolution of silver ions, and then control the nucleation and growth of silver powder; by controlling the temperature, pH and time parameters in the precipitation and dissolution process, the synergistic effect between processes is used to improve the distribution uniformity of the particle size of the silver powder particles, which is conducive to improving the conductive performance in the conductive material. ;

[0065] Examples 7-11 refer to the preparation methods and parameter conditions of Example 1, with the differences shown in Table 3.

[0066] Comparative Example 10 refers to Example 1, except that no electrolytic pretreatment is performed.

[0067] Comparative Example 11 refers to Example 1, except that the coprecipitation and re-dissolution process is not performed.

[0068] Comparative Example 12 refers to Example 1, except that no electrolytic refining treatment is performed.

[0069] Comparative Example 13 refers to Example 1, except that the supercritical fluid drying process is not performed.

[0070] Comparative Example 14 refers to Example 1, except that bismuth nitrate is not added during the electrolytic refining process.

[0071] Comparative Example 15 refers to Example 1, except that the pre-electrolysis temperature is 40°C

[0072] Comparative Example 16 refers to Example 1, except that the electrolyte temperature during the electrolytic refining process is 50°C.

[0073] Experimental Example 2 Purity Determination

[0074] The silver powder prepared in Example 1, Example 7-11, and Comparative Examples 10-16 was tested for purity. First, a series of silver standard solutions of different concentrations (for example, concentrations of 0.5 mg / L, 1.0 mg / L, 2.0 mg / L, 4.0 mg / L, and 8.0 mg / L) were prepared for drawing a standard curve. Weigh 0.1 mass of silver powder sample, dissolve it in a nitric acid aqueous solution with a volume ratio of 1:1, heat it to completely dissolve it, then cool it and dilute it to a certain volume (such as 100 mL) to obtain a sample solution. Preheat and debug the atomic absorption spectrometer according to the operating procedures, and set the wavelength to 328.1 nm, lamp current, and slit width parameters. The standard solution and sample solution are injected into the atomic absorption spectrometer in turn, and their absorbances are measured; a standard curve is drawn according to the absorbance of the standard solution, and then the corresponding silver concentration is checked from the standard curve according to the absorbance of the sample solution, and then the mass fraction of silver in the silver powder, that is, the purity, is calculated; the test results are shown in Table 3.

[0075] Table 3 Test results of Example 1, Examples 7-11, and Comparative Examples 10-16

[0076]

[0077] From the results in Table 3, it can be seen that in Comparative Example 10, the purity of the silver powder obtained is significantly reduced without electrolytic pretreatment. In the electrolytic pretreatment, the current density and the electrolysis temperature are controlled to ensure the ion activity and reaction rate. Combined with Comparative Example 14, the temperature is prevented from being too high so that the impurity ions participate in the cathode reaction, and the silver ions in the electrolyte are also prevented from being reduced due to excessive temperature, thereby reducing the silver ion concentration in the electrolyte. Under this condition, metal ions that are more active than silver (such as copper, lead, etc.) are preferentially oxidized at the anode to enter the solution and are reduced at the cathode. This process accurately separates impurities and silver ions, greatly reduces the content of impurity metal ions in the raw materials, and improves the purity of the silver powder. Combined with Comparative Example 11, it works synergistically with the coprecipitation process to further improve the purity of the silver ions and reduce impurities. Interference; a large amount of impurities are removed through the pre-electrolysis process to reduce the burden of subsequent processes. Subsequent co-precipitation, electrolytic refining and other steps are performed in sequence to deeply purify the silver powder, and the purity of the silver powder is synergistically improved; the results of comparative example 12 show that the silver powder has been initially synthesized, but still contains certain impurities. During electrolytic refining, the low concentration environment is conducive to the orderly migration of silver ions between electrodes. The adjustment of pH value ensures the stability of the acidic environment of the solution and inhibits the hydrolysis of silver ions. At the same time, it also affects the existence form and reaction activity of impurity ions. The silver powder at the anode undergoes an oxidation reaction with impurities and enters the solution. At the cathode, according to the principle of electrochemistry, the standard electrode potential of silver ions determines that it obtains electrons before most impurity ions and is reduced to silver. Under such conditions, the impurity ions remain in the electrolyte. , achieving effective separation of silver and impurities, and improving the purity of silver powder; in addition, the electrolytic refining process can not only remove impurities, but also optimize the crystal structure of silver powder. During the electrolysis process, silver ions are uniformly deposited on the cathode surface to form a regular crystal structure, ensuring uniform distribution of the electric field on the electrode surface. The uniform crystal growth process reduces crystal defects and gaps, making it difficult for impurities to be mixed in. The silver powder crystal structure is more compact and regular, further improving the purity of silver powder; in Comparative Example 13, the high diffusivity and solubility of supercritical carbon dioxide enable it to penetrate into the silver powder particles, dissolve and bring out impurities on the particle surface and gaps, such as organic matter and moisture. Compared with the traditional drying method, it avoids the impurities in the silver powder due to the agglomeration of silver powder during the treatment process. The purity of the silver powder is improved by making it difficult to remove the impurities. The results of Comparative Examples 14 and 15 show that bismuth nitrate can be used as an additive in electrolytic refining to optimize the crystallization process of the cathode silver powder. Without the addition of bismuth nitrate, the crystallization process of silver ions at the cathode lacks effective regulation, and the crystal growth may be irregular, which may easily form gaps or voids, making it easier for impurities to be mixed in and reduce the purity of the silver powder. Excessively high electrolytic refining temperature will accelerate the ion diffusion rate, making it easier for impurity ions to reach the cathode to participate in the reaction, and may change the selectivity of the reduction reaction of silver ions at the cathode, causing impurities to be deposited at the cathode together with silver ions, affecting the purity of the silver powder. In summary, a large amount of impurities can be removed by controlling the temperature, current density and time during the electrolytic pretreatment process, thereby reducing the burden of subsequent processes.The synergistic co-precipitation, electrolytic refining and supercritical fluid drying process make the silver powder crystals grow evenly, forming a more dense and regular crystal structure, further improving the purity of the silver powder. ;

[0078] Examples 12-16 refer to the preparation methods and parameter conditions of Example 1, with the differences shown in Table 4.

[0079] Comparative Example 12 refers to Example 1, except that no electrolytic refining treatment is performed.

[0080] Comparative Example 13 refers to Example 1, except that the supercritical fluid drying process is not performed.

[0081] Comparative Example 17 refers to Example 1, except that no physical vapor deposition treatment is performed.

[0082] Comparative Example 18 refers to Example 1, except that electrolytic refining is performed first and then physical vapor deposition treatment is performed.

[0083] Comparative Example 19 refers to Example 1, except that the argon gas flow rate during the physical vapor deposition process is 20 sccm.

[0084] Comparative Example 20 refers to Example 1, except that the pressure reduction rate is 0.5 MPa / min.

[0085] Comparative Example 21 refers to Example 1, except that the cooling rate is 5°C / min.

[0086] Experimental Example 3: Tap density test

[0087] The high-tapped-density high-purity silver powder prepared in Example 1, Examples 12-16, Comparative Examples 12-13, and Comparative Examples 17-21 was subjected to a tap density test. The tap density was measured by an automatic tap density instrument. 10 g of the silver powder sample was weighed and placed in a sample container provided with the instrument. The frequency was set at 300 times / minute and the amplitude was about 6 mm. The instrument was started and the volume change of the silver powder was monitored in real time during the vibration process. When the set number of vibrations was reached, the tap density of the silver powder was automatically calculated and displayed. The test results are shown in Table 4.

[0088] Table 4 Tap density test of Example 1, Examples 12-16, Comparative Examples 12-13, Comparative Examples 17-21

[0089]

[0090] It can be seen from the results in Table 4 that the tap density of silver powder is significantly affected by the changes in the process sequence and the adjustment of the process parameters in the comparative example. The results of Comparative Example 12 show that at the level of process principle, during the electrolytic refining process, the silver powder at the anode is dissolved, and silver ions are electrolytically deposited on the high-purity silver plate at the cathode. On the one hand, impurities in the silver powder can be removed, and on the other hand, bismuth nitrate can improve the deposition behavior of silver ions and promote the silver powder to form a denser crystal structure. Without electrolytic refining, impurities in the silver powder cannot be effectively separated, affecting the close stacking of the silver powder particles, resulting in an increase in internal voids, thereby reducing the tap density. From the level of process sequence, electrolytic refining is a key purification and structural optimization step after the initial synthesis of silver powder. Without electrolytic refining, impurities The presence of will interfere with the improvement of the quality of silver powder in the entire process; the results of comparative example 13 show that supercritical fluid drying utilizes the high diffusivity and solubility of supercritical carbon dioxide to enable it to penetrate between the silver powder particles and replace the liquid medium therein, and dries under a supercritical state without a gas-liquid interface, thereby avoiding the agglomeration of particles caused by capillary forces in the traditional drying process; using the traditional drying method, the silver powder particles are very easy to agglomerate, and there are a large number of gaps between the agglomerated particles, which cannot be tightly stacked, significantly reducing the tap density of the silver powder; the results of comparative example 17 show that in the physical vapor deposition process, the silver vapor generated by the gasification of the silver block is deposited on the surface of the silver powder particles under the drive of the argon carrier gas, which can fill the gaps between the particles and make the silver powder particles discharge. The arrangement is closer. Combined with the results of Comparative Example 19, it can be seen that when the argon flow rate is too high and the carrier gas speed is too fast, the distribution of silver vapor on the surface of the silver powder will become uneven, and too much silver vapor will be deposited in some areas, while insufficient deposition will occur in some areas. This uneven deposition will cause the silver powder particles to stack irregularly, and the gaps between the particles cannot be effectively filled, thereby reducing the tap density of the silver powder; the results of Comparative Example 18 show that physical vapor deposition is performed first and then electrolytic refining is used. Physical vapor deposition covers the surface of the silver powder particles with a uniform layer of silver, optimizes the surface morphology and stacking structure of the particles, and provides a more favorable basis for subsequent electrolytic refining. By changing the process sequence, electrolytic refining will change the structure and surface state of the silver powder, so that during subsequent physical vapor deposition, silver vapor The deposition effect of the gas becomes worse, and it cannot be evenly deposited on the surface of the silver powder particles, the particles are irregularly stacked, the gaps are enlarged, and the tap density is reduced; the results of comparative examples 20-21 show that slow pressure reduction can make the supercritical carbon dioxide smoothly transform into a gaseous state and discharge, ensure the stable state of the silver powder particles, avoid the agglomeration and deformation of the silver powder particles, and the silver powder particles are tightly stacked, reducing the generation of gaps, thereby reducing the tap density of the silver powder; similarly, slow cooling can make the silver powder particles cool smoothly, avoid excessive internal stress, and avoid the reduction of tap density; in summary, by first performing physical vapor deposition, then performing electrolytic refining treatment, and then drying through supercritical fluid process sequence, physical vapor deposition is used to fill the gaps between the silver powder particles;During the electrolytic refining process, the deposition behavior of silver ions is improved to promote the formation of a denser crystal structure of silver powder; supercritical fluid drying is then used to remove liquid impurities and avoid the agglomeration of silver powder. Through the synergistic relationship between the various steps of the process, the tap density of silver powder is increased. ;

[0091] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing high-purity silver powder with high tap density, characterized in that: The preparation of the silver powder comprises the following steps: S1: adding silver nitrate powder to deionized water and performing pre-electrolysis to obtain a pre-electrolyte; S2 chemically reducing the pre-electrolyte with ascorbic acid and dispersing with polyvinyl pyrrolidone to obtain a silver powder dispersion; S3 co-precipitates the silver powder dispersion with ethylenediaminetetraacetic acid to obtain a precipitate; and then dissolves and reduces the precipitate to obtain a silver powder suspension; S5: subjecting the silver powder suspension to physical vapor deposition to obtain treated silver powder; subjecting the treated silver powder to electrolytic refining to obtain refined silver powder; S6: dispersing the refined silver powder by ultrasonic method and drying with supercritical fluid to obtain the high-tap density high-purity silver powder; Wherein, the temperature of the pre-electrolysis treatment is 20-30°C; the temperature of the electrolytic refining is 30-40°C.

2. The method for preparing a high-tap density high-purity silver powder according to claim 1, characterized in that: The pre-electrolysis treatment comprises the following steps: The silver nitrate powder with a purity greater than 99% was added to deionized water, and stirred at 300 rpm at 30°C to obtain a 0.8 mol / L uniform solution; the uniform solution was added to an electrolytic cell with a platinum electrode, and the power was turned on to maintain a current density of 5-10 mA / cm 2 The temperature of the electrolyte is 20-30° C., the pH value is maintained at 3, and the electrolysis is performed for 10-30 minutes to obtain the pre-electrolyte.

3. The method for preparing a high-purity silver powder with high tap density according to claim 1, characterized in that: The chemical reduction comprises the following steps: The ascorbic acid is dissolved in deionized water to prepare a reducing solution with a concentration of 0.3-0.5 mol / L; the pre-electrolyte is transferred to a reactor, and a mixed solvent of ethanol and deionized water in a volume ratio of 3:1 is added, and the mixture is stirred evenly to form a mixed system; polyvinyl pyrrolidone with a mass concentration of 1.5 g / L is added to the mixed system, and the mixture is stirred at 500 rpm for 30 min at 25° C. to obtain a dispersed system; the dispersed system is heated to 40-60° C. by microwave-assisted heating, a dilute nitric acid solution is added to adjust the pH value to 5, and the reducing solution is slowly added through a dropping funnel, and the reaction is performed for 2-5 hours to obtain the silver powder dispersion.

4. The method for preparing a high-purity silver powder with high tap density according to claim 1, characterized in that: The coprecipitation and dissolution reduction process comprises the following steps: Adding sodium hydroxide solution to the silver powder dispersion to adjust the pH value to 10, adding the ethylenediaminetetraacetic acid under stirring, and continuously stirring for 2 hours after the addition to obtain a reaction system; placing the reaction system in a centrifuge, separating and centrifuging, removing the supernatant, and washing with deionized water to obtain the precipitate; The precipitate is placed in a constant temperature water bath and heated to 40°C, diluted nitric acid with a molar concentration of 1 mol / L is added, and the solution is stirred at 200 rpm for 30 minutes to obtain a uniform solution; a sodium hydroxide solution with a molar concentration of 2 mol / L is slowly added to the uniform solution, the pH value is adjusted to 8, the dissolution reduction temperature is 40-42°C, polyvinyl pyrrolidone is added, and the solution is stirred at 400 rpm for 30-45 minutes to obtain the silver powder suspension.

5. The method for preparing high-purity silver powder with high tap density according to claim 4, characterized in that: The molar ratio of silver ions in the silver nitrate powder to the ethylenediaminetetraacetic acid is 1:1.2-1.5; and the mass concentration of the polyvinyl pyrrolidone is 1.5-2.0 g / L.

6. The method for preparing high-purity silver powder with high tap density according to claim 1, characterized in that: The physical vapor deposition comprises the following steps: The silver powder suspension is transferred to a sample placement table of a physical vapor deposition device, and a high vacuum system is turned on to evacuate the physical vapor deposition device; a silver block with an average particle size of 2 mm is used as an evaporation source, and an electron beam evaporator is turned on to gasify and obtain silver vapor; an appropriate amount of argon gas is introduced into the device through a gas flow control center, and the flow rate is controlled at 5-12 sccm to carry out a deposition reaction, and the deposition reaction time is 30-60 min to obtain the treated silver powder; the mass ratio of the silver amount in the silver block to the silver nitrate powder is 1:5-10.

7. The method for preparing high-purity silver powder with high tap density according to claim 1, characterized in that: The electrolytic refining comprises the following steps: Dissolve silver nitrate and bismuth nitrate in deionized water, wherein the molar ratio of the silver nitrate to the bismuth nitrate is 2:1, to obtain an electrolyte, and adjust the pH value of the electrolyte to 4 with dilute nitric acid; add the treated silver powder as an anode and a high-purity silver plate as a cathode into the electrolyte, and control the temperature of the electrolyte at 30-40°C to obtain an electrolytic system; apply voltage to the electrolytic system, maintain the voltage at 1.5-2.0V, and perform refining for 2-4h to obtain the refined silver powder.

8. The method for preparing high-purity silver powder with high tap density according to claim 1, characterized in that: The supercritical fluid drying comprises the following steps: The refined silver powder is maintained at an ultrasonic power of 200 W and an ultrasonic time of 10 minutes, and the dispersed silver powder is obtained by ultrasonic treatment; the dispersed silver powder is slowly transferred to a reactor of a supercritical fluid device, and liquid carbon dioxide is pumped into the reactor, and the temperature and pressure are increased. When the carbon dioxide reaches a supercritical state, the reaction is stirred at 100-200 r / min for 45-90 minutes; the pressure is reduced at a rate of 0.2 MPa / min by controlling a pressure reducing valve, and the temperature in the reactor is slowly reduced to room temperature at a cooling rate of 2°C / min, and the high-tap density high-purity silver powder is collected.

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