A nano-silver sol with a low surface coating agent and a preparation method thereof

Abstract

The present invention discloses a nano silver sol with a low coating agent on the surface and a preparation method thereof, belonging to the technical field of preparation of nano metal sols. A dispersant solution is respectively added to an oxidant solution and a reducing agent solution, and combined with staged heating to realize the control of nucleation and growth, better play the steric hindrance effect of the dispersant, and finally wash away the dispersant with weak adsorption and free dispersant through the agglomeration and net capture effects of a precipitant, so as to obtain a nano silver sol with low heat loss, excellent dispersibility and high stability. This method has low cost and high collection rate and is suitable for industrial production.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparation of nano metal sols, and particularly relates to a nano silver sol with a low surface coating agent and a preparation method thereof. Background Art

[0002] Due to its good electrical conductivity, nano silver particles play an extremely important role in the microelectronics field, such as being applied to chip packaging, conductive inks for printed electronic devices, etc. In addition, due to the surface effect, quantum size effect, etc. of nano silver particles, they also have some special uses, such as surface-enhanced Raman applications, medical applications, etc.

[0003] With the development of chip packaging towards low-temperature sintering and printed electronic devices and circuits towards thinner lines, it is required that the nano silver sol has good dispersibility, stability, and low-temperature sinterability. However, the initial temperature of the existing nano silver sol preparation methods is mainly high temperature. In order to ensure the dispersibility and stability of the nano silver sol, a large amount of dispersant is mostly used for dispersion. Finally, a large amount of coating agent remains on the surface of the nano silver particles in the obtained nano silver sol, and it is very difficult to completely remove them under low-temperature sintering, resulting in poor electrical properties and appearance morphology. In addition, the existing nano silver sol preparation methods mainly use high-speed centrifugation or reverse osmosis to wash the silver sol, which has problems such as complex process, incomplete collection of silver particles, high cost, and difficult redispersion.

[0004] Therefore, it is of great significance to prepare a silver sol with a low surface coating agent and uniform dispersion for low-temperature sintering and fine-line printing. In addition, finding a preparation method with simple process and low cost is of great practical significance for the preparation of nano powders and nano silver sols. Summary of the Invention

[0005] To solve the problems of high surface coating agent content, high cost, and complex process in the prior art, the present invention provides a nano silver sol with a low surface coating agent and a preparation method thereof. The coating agent solution is respectively added to the oxidant solution and the reductant solution, and combined with stage heating to control nucleation and growth, better exert the steric hindrance effect of the coating agent. Finally, the weakly adsorbed and free coating agents are washed off through the coiling and net-trapping effects of the precipitating agent, and a nano silver sol with low heat loss, excellent dispersibility, and high stability is obtained. This method has low cost and high collection rate and is suitable for industrial production.

[0006] The object of the present invention is achieved through the following technical solutions:

[0007] A nano silver sol with a low surface coating agent, comprising silver nanoparticles, a coating agent, and a solvent; wherein: the mass fraction of silver nanoparticles is 0.5wt%-50wt%, and the mass fraction of the coating agent is <3wt%.

[0008] Preferably, the coating agent is polyvinylpyrrolidone or polyethylene glycol.

[0009] A method for preparing a nano silver sol with a low surface coating agent, comprising the following steps:

[0010] Step 1: Under high-speed stirring, add the regulator solution C to the reducing agent solution B, and then add one portion of the coating agent solution D to the oxidizing agent solution A, stir for 5 - 10 min to obtain solution E; add another portion of the coating agent solution D to the reducing agent solution B, stir for 5 - 10 min to obtain solution F;

[0011] Step 2: Under high-speed stirring, add solution E to solution F through a flow meter, control the addition time of solution E to be 1 - 30 min, and keep the temperature constant during the addition process; after solution E is added, start heating and aging the reaction solution to obtain a silver solution;

[0012] Step 3: Add a precipitating agent and deionized water to the silver solution obtained in Step 2, wash it repeatedly 3 - 5 times, pour off the supernatant for the last time, and collect the silver powder for use;

[0013] Step 4: Disperse the silver powder collected in Step 3 in solution H by ultrasonic wave and stirring with a dual-frequency ultrasonic cleaner, and stir at the same time at a rotation speed of 250 r / min. The total time of ultrasonic wave and stirring is 10 - 30 min to form a silver sol, and then conduct tests.

[0014] Preferably, in Step 1, the preparation of the oxidizing agent solution A: Dissolve the oxidizing agent in solution G, configure it into a concentration of 0.01M - 0.5M, and control the solution temperature to be 10 - 30°C to obtain the oxidizing agent solution A; the oxidizing agent is one or more of silver nitrate, silver ammonia, and silver carbonate; the solution G is one or more of deionized water, glycerol, and ethylene glycol.

[0015] Preferably, in Step 1, the preparation of the reducing agent solution B: Dissolve the reducing agent in solution G, where the amount of the reducing agent is 0.25 - 4 times the mass of the oxidizing agent, configure it into a concentration of 0.04M - 0.5M, and control the solution temperature to be 10 - 30°C to obtain the reducing agent solution B; the solution G is one or more of deionized water, glycerol, and ethylene glycol; the reducing agent is one or more of hydrazine hydrate, triethanolamine, and sodium borohydride.

[0016] Preferably, in Step 1, the preparation of the regulator solution C: Dissolve the regulator in solution G, the concentration of the regulator is 0.01 - 0.05M, and control the solution temperature to be 10 - 30°C; the solution G is one or more of deionized water, glycerol, and ethylene glycol; the regulator is one or more of sodium hydroxide, ammonia water, and sodium carbonate.

[0017] Preferably, in Step 1, the preparation of the coating agent solution D: Dissolve the coating agent in solution G, where the mass of the coating agent is 1 - 5 times that of the oxidizing agent, and configure it into a concentration of 50 - 500 g / L, controlling the solution temperature at 10 - 30°C; the solution G is one or several of deionized water, glycerol, and ethylene glycol; the coating agent is polyvinylpyrrolidone or polyethylene glycol.

[0018] Preferably, the oxidizing agent is one or several of silver nitrate, silver ammonia, and silver carbonate; the solution G is one or several of deionized water, glycerol, and ethylene glycol; the reducing agent is one or several of hydrazine hydrate, triethanolamine, and sodium borohydride; the regulator is one or several of sodium hydroxide, ammonia water, and sodium carbonate; the coating agent is one of polyvinylpyrrolidone and polyethylene glycol.

[0019] Preferably, in Step 1, the coating agent solution D is divided into two parts according to the ratio of 1 - 9:9 - 1, and added to the oxidizing agent solution A and the reducing agent solution B respectively.

[0020] Preferably, in Step 2, the temperature during the addition of solution E is kept constant at 10 - 30°C. After the addition of solution E is completed, it is heated to 130 - 160°C at a heating rate of 2°C / min - 15°C / min, and the reaction aging time is 1 - 3 h.

[0021] Preferably, in Step 3, the addition amount of the precipitating agent is 50% - 1000% of the mass of silver, and it is configured into a concentration of 100 g / L. The addition amount of deionized water is 10% - 1000% of the mass of the silver solution; the precipitating agent is one or several of polyferric sulfate, alum, and polyacrylamide.

[0022] Preferably, in Step 4, the solution H is one of deionized water, ethanol, isopropanol, ethylene glycol, and glycerol.

[0023] The beneficial effects of this technical solution are as follows:

[0024] First, the present invention provides a nano - silver sol with a low surface coating agent. The average particle size of silver particles in the nano - silver sol is 20 - 200 nm, with good overall uniformity, good stability, capable of long - term storage without sedimentation, and there will be no problem of screen clogging during fine - line printing.

[0025] Second, the present invention provides a nano - silver sol with a low surface coating agent. The amount of organic matter coated on the surface of silver particles is < 3 wt%, which can be completely removed at a curing temperature lower than 150°C, and is suitable for low - temperature sintering such as chip packaging.

[0026] III. A method for preparing a nano silver sol with a low surface coating agent. The coating agent is added to the oxidant and the reductant respectively, and combined with staged heating to well control the nucleation and growth rates, so that the coating agent has sufficient time to combine with silver particles during the nucleation stage, better exert the steric hindrance effect of the coating agent, and avoid agglomeration caused by too high initial reaction temperature.

[0027] IV. A method for preparing a nano silver sol with a low surface coating agent. A precipitating agent is added to the silver solution, and by using the collecting and netting effects of the precipitating agent, the weakly adsorbed and free organic substances are removed. The settled silver particles do not form agglomerates and are easily redispersed. In addition, by using sedimentation washing, the particle sedimentation and separation in the silver sol are well realized, solving the problem of difficult washing and separation of the silver sol, improving the collection rate of the silver sol, and reducing the production cost.

[0028] V. A method for preparing a nano silver sol with a low surface coating agent. The silver particles in the silver sol are separated by the sedimentation method and dispersed in a low-boiling organic solution by means such as ultrasonic waves, and a silver sol with a high silver content of 50% can be obtained.

[0029] VI. A method for preparing a nano silver sol with a low surface coating agent. The finally prepared silver sol has good dispersion, low burn loss, can be stored for a long time, and can achieve low-temperature curing. The synthesized silver sol has a particle size D10 of 0.1 - 0.15 μm, D50 of 0.1 - 0.2 μm, D90 of 0.2 - 0.5 μm, D95 of 0.3 - 1 μm, the average size in the ZETA test is 20 - 200 nm, the absolute value of the ZETA test potential is 8 - 30 mV, and the burn loss rate at 550 °C is 0.5% - 3%. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is the SEM image of the silver sol in Example 1 of the present invention;

[0031] Figure 2 It is the SEM image of the silver sol in Example 2 of the present invention;

[0032] Figure 3 It is the SEM image of the silver sol in Example 3 of the present invention;

[0033] Figure 4 It is the SEM image of the silver sol in Example 4 of the present invention;

[0034] Figure 5 It is the SEM image of the silver sol in Example 5 of the present invention;

[0035] Figure 6 It is the SEM image of the silver sol in Example 6 of the present invention;

[0036] Figure 7 It is the SEM image of the silver sol in Example 7 of the present invention;

[0037] Figure 8 It is the SEM image of the silver sol in Comparative Example 3 of the present invention;

[0038] Figure 9 It is the SEM image of the silver sol in Comparative Example 4 of the present invention. Detailed implementation manners

[0039] The present invention will be further described in detail below in conjunction with the embodiments, but the implementation manners of the present invention are not limited thereto.

[0040] In Examples 1 - 7 and Comparative Examples 1 - 4, D10, D50, D90, and D95 were measured using a laser diffraction particle size distribution tester, the particle size and potential were measured using a Malvern ZETA potentiometer, and the mass difference percentage before and after the silver powder obtained by washing and drying the silver sol in a muffle furnace at 550°C for 1 h was measured to obtain the content of the surface coating agent of the silver particles.

[0041] Example 1

[0042] The preparation of a nano - silver sol with a low surface coating agent includes the following steps:

[0043] Step S1: Solution preparation

[0044] Preparation of oxidant solution A

[0045] Accurately weigh 15.77 g of silver nitrate and dissolve it in 9.18 L of ethylene glycol to prepare a 0.01 M silver nitrate solution. Control the solution temperature at 10°C and stir for 30 min to obtain oxidant solution A.

[0046] Preparation of reducing agent solution B

[0047] Accurately weigh 0.878 g of sodium borohydride and dissolve it in 568 mL of ethylene glycol to prepare a 0.04 M sodium borohydride solution. Control the solution temperature at 10°C and stir for 30 min to obtain reducing agent solution B.

[0048] Preparation of regulator solution C

[0049] Accurately weigh 0.99 g of sodium hydroxide and dissolve it in 50 mL of ethylene glycol. Control the solution temperature at 10°C and stir for 30 min to obtain regulator solution C.

[0050] Preparation of coating agent solution D

[0051] Accurately weigh 10 g of PVP (K30) and dissolve it in 100 mL of ethylene glycol. Control the solution temperature at 10°C and stir for 1 h to obtain coating agent solution D.

[0052] Step S2: Adding coating agent and regulator

[0053] Under a stirring speed of 300 rpm, the regulator solution C was quickly added to the reducing agent solution B, and stirred for 10 min. Then the coating agent solution D was divided into two parts according to a volume ratio of 9:1 and added to the reducing agent solution B and the oxidizing agent solution A respectively. The solution temperature was controlled at 10 °C and stirred for 5 min respectively to obtain solution E and solution F.

[0054] Step S3: Reduction reaction

[0055] Under a stirring speed of 300 rpm, solution E was added to solution F at a rate of 306 mL / min for 30 min. The temperature during the addition process was controlled at 10 - 12 °C. Immediately after the addition, the temperature was raised to 150 °C at a heating rate of 2 °C / min, and the reaction ended after 2 h starting from the start of dropping.

[0056] Step S4: Sedimentation and washing

[0057] Accurately weigh 5 g of polyferric sulfate and add it to 200 mL of deionized water, stir and dissolve it to prepare a solution with a concentration of 100 g / L, then add it to the final solution of step S3. Then add 990 mL of deionized water to it, stir for 10 min, stop stirring and sediment for 30 min, pour out the supernatant, then add 990 mL of deionized water to it, stir for 10 min, stop stirring and sediment for 30 min, and this process was repeated for washing 3 times. (In this example, the proportions of the precipitant and deionized water are 50% and 10% respectively.)

[0058] Step S5: Preparation of silver sol

[0059] The silver powder after washing in step S4 was added to 100 g of ethylene glycol, ultrasonicated and stirred for 30 min to obtain a silver sol with a solid content of 10% and tested.

[0060] The test results in the above Example 1 are as follows. The electron micrograph of the silver sol is as Figure 1 shown, D10 = 0.13 um, D50 = 0.16 um, D90 = 0.21 um, D95 = 0.27 um, the ZETA test size is 37 nm, the potential is -12 mv, and the coating agent content measured at 550 °C is 2.48%. From Figure 1 it can be seen that the synthesized silver powder has good dispersibility and uniformity.

[0061] Example 2

[0062] The preparation of a nano - silver sol with a low coating agent on the surface includes the following steps:

[0063] Step S1: Solution preparation

[0064] Preparation of oxidizing agent solution A

[0065] Accurately weigh 15.77 g of silver nitrate and dissolve it in 1.83 L of ethylene glycol to prepare a 0.05 M silver nitrate solution. Control the solution temperature at 10 °C and stir for 30 min to obtain oxidant solution A.

[0066] Preparation of reducing agent solution B

[0067] Accurately weigh 1.756 g of sodium borohydride and dissolve it in 568 mL of ethylene glycol to prepare a 0.08 M sodium borohydride solution. Control the solution temperature at 10 °C and stir for 30 min to obtain reducing agent solution B.

[0068] Preparation of regulator solution C

[0069] Accurately weigh 0.99 g of sodium hydroxide and dissolve it in 50 mL of ethylene glycol. Control the solution temperature at 10 °C and stir for 30 min to obtain regulator solution C.

[0070] Preparation of coating agent solution D

[0071] Accurately weigh 20 g of PVP (K30) and dissolve it in 100 mL of ethylene glycol. Control the solution temperature at 10 °C and stir for 1 h to obtain coating agent solution D.

[0072] Step S2: Add coating agent and regulator

[0073] Under a stirring speed of 300 rpm, quickly add regulator solution C to reducing agent solution B and stir for 10 min. Then divide coating agent solution D into two parts according to a volume ratio of 7:3 and add them to reducing agent solution B and oxidant solution A respectively. Control the solution temperature at 10 °C and stir for 10 min respectively to obtain solution E and solution F.

[0074] Step S3: Reduction reaction

[0075] Under a stirring speed of 300 rpm, add solution E to solution F at a rate of 92 mL / min for 20 min. Control the temperature during the addition process at 10 - 12 °C. Immediately after the addition, raise the temperature at a rate of 5 °C / min to 150 °C and end the reaction 2 h after the start of dropping.

[0076] Step S4: Sedimentation and washing

[0077] Accurately weigh 10 g of polymeric ferric sulfate and add it to 100 mL of deionized water. Stir to dissolve and prepare a solution with a concentration of 100 g / L. Then add it to the final solution in step S3, and further add 2.54 L of deionized water. Stir for 10 min, stop stirring, and let it settle for 30 min. Pour out the supernatant, then add 2.54 L of deionized water again, stir for 10 min, stop stirring, and let it settle for 30 min. This process is repeated for washing 3 times. (In this example, the proportions of the precipitant and deionized water are 100% and 100% respectively.)

[0078] Step S5: Preparation of silver sol

[0079] Add the silver powder washed in step S4 to 50 g of ethylene glycol, ultrasonically stir for 30 min to prepare a silver sol with a solid content of 20% and conduct tests.

[0080] The test results in the above Example 2 are as follows. The electron microscopy image of the silver sol is as Figure 2 shown, D10 = 0.12 μm, D50 = 0.15 μm, D90 = 0.25 μm, D95 = 0.29 μm, the size measured by ZETA test is 68 nm, the potential is -20.2 mV, and the content of the coating agent measured at 550 °C is 2.72%. It can be seen Figure 2 that the synthesized silver powder has good dispersibility and uniformity.

[0081] Example 3

[0082] Preparation of a nano silver sol with a low coating agent on the surface, including the following steps:

[0083] Step S1: Solution preparation

[0084] Preparation of oxidant solution A

[0085] Accurately weigh 15.77 g of silver nitrate and dissolve it in 0.93 L of ethylene glycol to prepare a 0.1 M silver nitrate solution. Control the solution temperature at 10 °C and stir for 30 min to obtain oxidant solution A.

[0086] Preparation of reducing agent solution B

[0087] Accurately weigh 3.512 g of sodium borohydride and dissolve it in 568 mL of ethylene glycol to prepare a 0.16 M sodium borohydride solution. Control the solution temperature at 10 °C and stir for 30 min to obtain reducing agent solution B.

[0088] Preparation of regulator solution C

[0089] Accurately weigh 0.99 g of sodium hydroxide and dissolve it in 50 mL of ethylene glycol. Control the solution temperature at 10 °C and stir for 30 min to obtain regulator solution C.

[0090] Preparation of coating agent solution D

[0091] Accurately weigh 30 g of PVP (K30) and dissolve it in 100 mL of ethylene glycol. Control the solution temperature at 10 °C and stir for 1 h to obtain the coating agent solution D.

[0092] Step S2: Add the coating agent and the regulator

[0093] Under a stirring speed of 500 rpm, quickly add the regulator solution C to the reducing agent solution B and stir for 10 min. Then divide the coating agent solution D into two equal parts by volume (5:5) and add them to the reducing agent solution B and the oxidizing agent solution A respectively. Control the solution temperature at 10 °C and stir for 5 min respectively to obtain solution E and solution F.

[0094] Step S3: Reduction reaction

[0095] Under a stirring speed of 500 rpm, add solution E to solution F at a rate of 93 mL / min for 10 min. Control the temperature during the addition process at 10 - 12 °C. Immediately after the addition, increase the temperature to 150 °C at a heating rate of 5 °C / min and start timing the reaction from the start of dropping. The reaction ends after 2 h.

[0096] Step S4: Sedimentation and washing

[0097] Accurately weigh 20 g of polyferric sulfate and add it to 200 mL of deionized water. Stir to dissolve and prepare a solution with a concentration of 100 g / L. Then add it to the final solution of step S3. Add 3.29 L of deionized water to it, stir for 10 min, stop stirring and let it settle for 30 min, pour off the supernatant. Then add 3.29 L of deionized water to it, stir for 10 min, stop stirring and let it settle for 30 min. Repeat this washing process 3 times. (In this example, the ratios of the precipitant and deionized water are 200% and 200% respectively.)

[0098] Step S5: Preparation of silver sol

[0099] Add the silver powder washed in step S4 to 40 g of ethylene glycol, ultrasonically stir for 30 min to obtain a silver sol with a solid content of 25% and conduct tests.

[0100] The test results in the above Example 3 are as follows. The electron micrograph of the silver sol is as Figure 3 shown. D10 = 0.14 μm, D50 = 0.16 μm, D90 = 0.22 μm, D95 = 0.26 μm. The size measured by ZETA test is 97 nm, the potential is -30 mV, and the content of the coating agent measured at 550 °C is 2.10%. It can be Figure 3 seen that the synthesized silver powder has good dispersibility and there are particles of different sizes.

[0101] Example 4

[0102] Preparation of nano - silver sol with low coating agent on the surface, including the following steps:

[0103] Step S1: Solution preparation

[0104] Preparation of oxidant solution A

[0105] Accurately weigh 15.77 g of silver nitrate and dissolve it in 309 mL of ethylene glycol to prepare a 0.3 M silver nitrate solution. Control the solution temperature at 10 °C and stir for 30 min to obtain oxidant solution A.

[0106] Preparation of reductant solution B

[0107] Accurately weigh 7.024 g of sodium borohydride and dissolve it in 568 mL of ethylene glycol to prepare a 0.32 M sodium borohydride solution. Control the solution temperature at 10 °C and stir for 30 min to obtain reductant solution B.

[0108] Preparation of regulator solution C

[0109] Accurately weigh 0.99 g of sodium hydroxide and dissolve it in 50 mL of ethylene glycol. Control the solution temperature at 10 °C and stir for 30 min to obtain regulator solution C.

[0110] Preparation of coating agent solution D

[0111] Accurately weigh 40 g of PVP (K30) and dissolve it in 100 mL of ethylene glycol. Control the solution temperature at 10 °C and stir for 1 h to obtain coating agent solution D.

[0112] Step S2: Adding coating agent and regulator

[0113] Under a stirring speed of 700 rpm, quickly add regulator solution C to reductant solution B and stir for 10 min. Then divide coating agent solution D into two parts according to a volume ratio of 3:7 and add them to reductant solution B and oxidant solution A respectively. Control the solution temperature at 10 °C and stir for 10 min respectively to obtain solution E and solution F.

[0114] Step S3: Reduction reaction

[0115] Under a stirring speed of 700 rpm, add solution E to solution F at a speed of 62 mL / min for 5 min. Control the temperature during the addition process at 10 - 12 °C. Immediately after the addition, raise the temperature at a rate of 10 °C / min to 150 °C and end the reaction 2 h after the start of dropping.

[0116] Step S4: Sedimentation and washing

[0117] Accurately weigh 50 g of polyferric sulfate and add it to 500 mL of deionized water. Stir to dissolve and prepare a solution with a concentration of 100 g / L, then add it to the final solution in step S3. Then add 5.13 L of deionized water thereto, stir for 10 min, stop stirring and let it settle for 30 min, pour off the supernatant, then add 5.13 L of deionized water thereto, stir for 10 min, stop stirring and let it settle for 30 min. This process is repeated for washing 3 times. (In this example, the proportions of the precipitant and deionized water are 500% and 500% respectively.)

[0118] Step S5: Preparation of silver sol

[0119] Add the silver powder washed in step S4 to 25 g of ethylene glycol, ultrasonically stir for 30 min to prepare a silver sol with a solid content of 40% and conduct tests.

[0120] The test results in the above Example 4 are as follows. The electron microscope image of the silver sol is as Figure 4 shown, D10 = 0.13 μm, D50 = 0.17 μm, D90 = 0.26 μm, D95 = 0.37 μm, the size measured by ZETA test is 121 nm, the potential is -20 mV, and the content of the coating agent measured at 550 °C is 1.89%. It can be seen from Figure 4 this that the synthesized silver powder has good dispersibility and good consistency.

[0121] Example 5

[0122] Preparation of a nano silver sol with a low coating agent on the surface, including the following steps:

[0123] Step S1: Solution preparation

[0124] Preparation of oxidant solution A

[0125] Accurately weigh 15.77 g of silver nitrate and dissolve it in 186 mL of ethylene glycol to prepare a 0.5 M silver nitrate solution. Control the solution temperature at 10 °C and stir for 30 min to obtain oxidant solution A.

[0126] Preparation of reducing agent solution B

[0127] Accurately weigh 14.047 g of sodium borohydride and dissolve it in 743 mL of ethylene glycol to prepare a 0.5 M sodium borohydride solution. Control the solution temperature at 10 °C and stir for 30 min to obtain reducing agent solution B.

[0128] Preparation of regulator solution C

[0129] Accurately weigh 0.99 g of sodium hydroxide and dissolve it in 50 mL of ethylene glycol. Control the solution temperature at 10 °C and stir for 30 min to obtain regulator solution C.

[0130] Preparation of coating agent solution D

[0131] Accurately weigh 50 g of PVP (K30) and dissolve it in 100 mL of ethylene glycol. Control the solution temperature at 10 °C and stir for 1 h to prepare the coating agent solution D.

[0132] Step S2: Add the coating agent and the regulator

[0133] Under a stirring speed of 250 rpm, quickly add the regulator solution C to the reducing agent solution B and stir for 7 min. Then divide the coating agent solution D into two parts in a volume ratio of 1:9 and add them to the reducing agent solution B and the oxidizing agent solution A respectively. Control the solution temperature at 10 °C and stir for 10 min respectively to obtain solution E and solution F.

[0134] Step S3: Reduction reaction

[0135] Under a stirring speed of 300 rpm, add solution E to solution F at a speed of 186 mL / min for 1 min. Control the temperature during the addition process at 10 - 12 °C. Immediately after the addition, increase the temperature at a rate of 15 °C / min to 150 °C and start timing from the start of dropping. The reaction ends after 2 h.

[0136] Step S4: Sedimentation and washing

[0137] Accurately weigh 100 g of polyferric sulfate and add it to 1 L of deionized water. Stir and dissolve it to prepare a solution with a concentration of 100 g / L. Add it to the final solution of step S3, and then add 10.79 L of deionized water. Stir for 10 min, stop stirring and sediment for 30 min, pour out the supernatant, then add 10.79 L of deionized water, stir for 10 min, stop stirring and sediment for 30 min. This process is repeated for washing 4 times. (In this example, the proportions of the precipitant and deionized water are 1000% and 1000% respectively.)

[0138] Step S5: Preparation of silver sol

[0139] Add the silver powder washed in step S4 to 20 g of ethylene glycol, ultrasonically stir for 30 min to prepare a silver sol with a solid content of 50% and conduct tests.

[0140] The test results in the above Example 5 are as follows. The electron micrograph of the silver sol is as Figure 5 shown. D10 = 0.13 μm, D50 = 0.15 μm, D90 = 0.39 μm, D95 = 0.87 μm. The size measured by ZETA test is 187 nm, the potential is -16 mV, and the content of the coating agent measured at 550 °C is 2.56%. It can be seen Figure 5 that the particle size distribution of the synthesized silver powder is relatively wide and the consistency is slightly poor.

[0141] Example 6

[0142] Preparation of a nano - silver sol with a low coating agent on the surface, comprising the following steps:

[0143] Step S1: Solution preparation

[0144] Preparation of oxidant solution A

[0145] Weigh silver ammonia and dissolve it in deionized water to prepare a solution with a concentration of 0.1 M, control the solution temperature at 20 °C, and stir for 30 min to obtain oxidant solution A.

[0146] Preparation of reductant solution B

[0147] Weigh hydrazine hydrate and dissolve it in deionized water to prepare a solution with a concentration of 0.1 M, control the solution temperature at 20 °C, and stir for 30 min to obtain reductant solution B.

[0148] Preparation of regulator solution C

[0149] Weigh ammonia water and dissolve it in deionized water to prepare a solution with a concentration of 0.05 M, control the solution temperature at 20 °C, and stir for 30 min to obtain regulator solution C.

[0150] Preparation of coating agent solution D

[0151] Weigh polyethylene glycol and dissolve it in deionized water to prepare a solution with a concentration of 100 g / L, control the solution temperature at 20 °C, and stir for 1 h to obtain coating agent solution D.

[0152] Step S2: Adding coating agent and regulator

[0153] Under a stirring speed of 300 rpm, quickly add regulator solution C to reductant solution B, stir for 5 min, then divide coating agent solution D into two parts according to a volume ratio of 1:9, and add them to reductant solution B and oxidant solution A respectively. Control the solution temperature at 20 °C and stir for 7 min respectively to obtain solution E and solution F.

[0154] Step S3: Reduction reaction

[0155] Under a stirring speed of 300 rpm, add solution E to solution F in about 1 min, control the temperature during the addition process at 20 - 22 °C, and immediately increase the temperature to 160 °C at a heating rate of 8 °C / min after the addition is completed. The reaction ends 1 h after the start of dropping.

[0156] Step S4: Sedimentation and washing

[0157] Weigh polyacrylamide and add it to deionized water. The polyacrylamide is 1000% of the mass of silver. Stir and dissolve it to prepare a solution with a concentration of 100 g / L, and add it to the final solution in step S3. Then add deionized water with a volume 1000% of the total volume of the silver solution, stir for 10 min, stop stirring and let it settle for 30 min, and pour out the supernatant. This process is repeated 5 times for washing.

[0158] Step S5: Preparation of silver sol

[0159] Add the silver powder washed in step S4 to 2000 g of ethylene glycol, ultrasonically stir for 30 min to prepare a silver sol with a solid content of 0.5% and conduct tests.

[0160] The test results in the above Example 6 are as follows. The electron microscopy image of the silver sol is as Figure 6 shown, D10 = 0.13 um, D50 = 0.16 um, D90 = 0.25 um, D95 = 0.32 um, the ZETA test size is 151 nm, the potential is -18.7 mv, and the content of the coating agent measured at 550 °C is 1.56%. It can be seen from Figure 6 this that the particle size distribution of the synthesized silver powder is relatively wide and there are large and small powders.

[0161] Example 7

[0162] Preparation of a nano silver sol with a low coating agent on the surface, including the following steps:

[0163] Step S1: Solution preparation

[0164] Preparation of oxidant solution A

[0165] Weigh silver nitrate and silver carbonate and dissolve them in glycerol and ethylene glycol to prepare a solution with a concentration of 0.1 M. Control the solution temperature at 30 °C and stir for 30 min to obtain oxidant solution A.

[0166] Preparation of reducing agent solution B

[0167] Weigh triethanolamine and hydrazine hydrate and dissolve them in glycerol and ethylene glycol to prepare a solution with a concentration of 0.1 M. Control the solution temperature at 30 °C and stir for 30 min to obtain reducing agent solution B.

[0168] Preparation of regulator solution C

[0169] Weigh sodium hydroxide and sodium carbonate and dissolve them in glycerol and ethylene glycol to prepare a solution with a concentration of 0.05 M. Control the solution temperature at 30 °C and stir for 30 min to obtain regulator solution C.

[0170] Preparation of coating agent solution D

[0171] Weigh polyvinylpyrrolidone and polyethylene glycol and dissolve them in glycerol and ethylene glycol to prepare a coating agent solution D with a concentration of 100 g / L. Control the solution temperature at 30 °C and stir for 1 h.

[0172] Step S2: Add the coating agent and the regulator

[0173] Under a stirring speed of 300 rpm, quickly add the regulator solution C to the reducing agent solution B and stir for 5 min. Then divide the coating agent solution D into two parts according to a volume ratio of 1:9 and add them to the reducing agent solution B and the oxidizing agent solution A respectively. Control the solution temperature at 30 °C and stir for 7 min respectively to obtain solution E and solution F.

[0174] Step S3: Reduction reaction

[0175] Under a stirring speed of 300 rpm, add solution E to solution F in about 1 min, control the temperature during the addition process at 28 - 30 °C, and immediately increase the temperature to 130 °C at a heating rate of 2 °C / min after the addition. Start timing from the dropwise addition and end the reaction after 3 h.

[0176] Step S4: Sedimentation and washing

[0177] Weigh polyacrylamide and add it to deionized water. The polyacrylamide is 1000% of the mass of silver. Stir to dissolve and prepare a solution with a concentration of 100 g / L. Add it to the solution obtained in the final step S3, and then add deionized water with a volume 1000% of the total volume of the silver solution. Stir for 10 min, stop stirring and let it settle for 30 min, and pour out the supernatant. Repeat this washing process 5 times.

[0178] Step S5: Preparation of silver sol

[0179] Add the silver powder washed in step S4 to 2000 g of ethylene glycol, ultrasonically stir for 30 min to prepare a silver sol with a solid content of 0.5% and conduct tests.

[0180] The test results in the above Example 7 are as follows. The electron microscopy image of the silver sol is as Figure 7 shown. D10 = 0.14 μm, D50 = 0.23 μm, D90 = 0.36 μm, D95 = 0.65 μm. The size measured by ZETA test is 62 nm, the potential is -23.1 mV, and the content of the coating agent measured at 550 °C is 1.76%. It can be seen that the particle size distribution of the synthesized silver powder is concentrated and the morphology of the silver powder is non-uniform. Figure 7

[0181] Comparative Example 1

[0182] The difference between this comparative example and Example 1 is that in step S4, the washing is repeated 1 time.

[0183] ​The test results in Comparative Example 1 above are as follows. The electron microscopy image of the silver sol is consistent with Figure 1 . D10 = 0.13 μm, D50 = 0.16 μm, D90 = 0.22 μm, D95 = 0.23 μm. The size measured by ZETA test is 37 nm, and the potential is -16 mV. The content of the coating agent measured at 550 °C is 16.7%. It can be seen from the test data that too few washing times result in incomplete washing of the free organic matter in the silver sol.

[0184] Comparative Example 2

[0185] The difference between this comparative example and Example 1 is that: instead of sedimentation washing, the silver powder is directly collected by centrifugation and enters step S5 for the preparation of silver sol.

[0186] The test results in Comparative Example 2 above are as follows. The electron microscopy image of the silver sol is consistent with Figure 1 . D10 = 0.13 μm, D50 = 0.16 μm, D90 = 0.22 μm, D95 = 0.24 μm. The size measured by ZETA test is 37 nm, and the potential is -25 mV. The content of the coating agent measured at 550 °C is 78.6%. It can be seen from the test data that the content of the organic matter on the surface of the unwashed silver sol is relatively high and it is difficult to completely remove during low-temperature sintering.

[0187] Comparative Example 3

[0188] The difference between this comparative example and Example 2 is that: in step S2: adding the coating agent and the regulator

[0189] Under a stirring speed of 300 rpm, the regulator solution C is quickly added to the reducing agent solution B and stirred for 5 min. Then the coating agent solution D is added to the reducing agent solution B, and the solution temperature is controlled at 30 °C and stirred for 7 min to obtain solution E.

[0190] Under a stirring speed of 300 rpm, solution E is added to solution A in about 20 min, and the temperature during the addition process is controlled at 10 - 12 °C. Immediately after the addition, the temperature is raised to 150 °C at a heating rate of 5 °C / min, and the reaction ends 2 h after the start of dropping.

[0191] The test results in Comparative Example 3 above are as follows. The electron microscopy image of the silver sol is as shown in Figure 8 . D10 = 0.13 μm, D50 = 0.21 μm, D90 = 0.37 μm, D95 = 0.53 μm. The size measured by ZETA test is 83 nm, and the potential is -10.3 mV. The content of the coating agent measured at 550 °C is 2.93%. It can be seen from Figure 8 this that the synthesized silver powder has poor consistency.

[0192] Comparative Example 4

[0193] The difference between this comparative example and Example 2 lies in: Step S2: adding a coating agent and a regulator

[0194] Under a stirring speed of 300 rpm, the regulator solution C was quickly added to the reducing agent solution B, and stirred for 5 min. The solution temperature was controlled at 30°C, and stirred for 7 min to obtain solution E; the coating agent solution D was added to the oxidizing agent solution A, and the solution temperature was controlled at 30°C, and stirred for 7 min to obtain solution F.

[0195] Under a stirring speed of 300 rpm, solution E was added to solution F in about 20 min, and the temperature during the addition process was controlled at 10 - 12°C. Immediately after the addition, the temperature was raised to 150°C at a heating rate of 5°C / min, and the reaction ended after 2 h from the start of dropping.

[0196] The test results in the above Comparative Example 4 are as follows. The electron microscope image of the silver sol is as Figure 9 shown. D10 = 0.14 um, D50 = 0.28 um, D90 = 0.52 um, D95 = 0.75 um. The ZETA test size is 212 nm, the potential is -5.6 mv, and the coating agent content measured at 550°C is 2.63%. It can be seen from Figure 9 this that the size of the synthesized silver powder is relatively large.

[0197] 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 or 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 a nano silver sol with a low coating agent on the surface, characterized in that, It includes the following steps: Step 1: Under high-speed stirring, add the regulator solution C to the reducing agent solution B, and then add one portion of the coating agent solution D to the oxidizing agent solution A, stir for 5 - 10 min to obtain solution E; add the other portion of the coating agent solution D to the reducing agent solution B, stir for 5 - 10 min to obtain solution F; Preparation of the oxidizing agent solution A: Dissolve the oxidizing agent in solution G to prepare a solution with a concentration of 0.01M - 0.5M, control the solution temperature at 10 - 30°C to obtain the oxidizing agent solution A; The oxidizing agent is one or more of silver nitrate, silver ammonia, and silver carbonate; Preparation of the reducing agent solution B: Dissolve the reducing agent in solution G, where the amount of the reducing agent is 0.25 - 4 times the mass of the oxidizing agent, prepare a solution with a concentration of 0.04M - 0.5M, control the solution temperature at 10 - 30°C to obtain the reducing agent solution B; The reducing agent is one or more of hydrazine hydrate, triethanolamine, and sodium borohydride; Preparation of the regulator solution C: Dissolve the regulator in solution G, the concentration of the regulator is 0.01 - 0.05M, control the solution temperature at 10 - 30°C; The regulator is one or more of sodium hydroxide, ammonia water, and sodium carbonate; Preparation of the coating agent solution D: Dissolve the coating agent in solution G, the mass of the coating agent is 1 - 5 times that of the oxidizing agent, prepare a solution with a concentration of 50 - 500g / L, control the solution temperature at 10 - 30°C; The coating agent is polyvinylpyrrolidone or polyethylene glycol; The coating agent solution D is divided into two portions in a ratio of 1 - 9:9 - 1 and added to the oxidizing agent solution A and the reducing agent solution B respectively; Step 2: Under high-speed stirring, add solution E to solution F through a flowmeter, control the addition time of solution E to be 1 - 30 min, and keep the temperature constant during the addition process; After solution E is added completely, start to heat up and age the reaction solution to obtain a silver solution; The constant temperature during the addition of solution E is 10 - 30°C. After solution E is added completely, raise the temperature to 130 - 160°C at a heating rate of 2°C / min - 15°C / min, and the reaction aging time is 1 - 3 h; Step 3: Add the precipitant and deionized water to the silver solution obtained in Step 2, wash it repeatedly 3 - 5 times, pour off the supernatant for the last time, and collect the silver powder for use; The addition amount of the precipitant is 50% - 1000% of the theoretical mass of silver particles, and it is prepared into a solution with a concentration of 100g / L. The addition amount of the deionized water is 10% - 1000% of the mass of the silver solution; The precipitant is one or more of polyferric sulfate, alum, and polyacrylamide; Step 4: Disperse the silver powder collected in Step 3 in solution H by ultrasonic wave and stirring using a dual-frequency ultrasonic cleaning machine, and stir at the same time at a rotation speed of 250r / min. The total time of ultrasonic wave and stirring is 10 - 30 min to form a silver sol and conduct tests.

2. The preparation method of the nano-silver sol with a low coating agent on the surface according to claim 1, wherein: In Step 1, solution G is one or more of deionized water, glycerol, and ethylene glycol.

3. The preparation method of the nano-silver sol with a low coating agent on the surface according to claim 2, wherein: In Step 4, solution H is one of deionized water, ethanol, isopropanol, ethylene glycol, and glycerol.

4. A nano-silver sol obtained by the preparation method of a nano-silver sol with a low surface coating agent according to any one of claims 1-3, characterized in that: It includes silver nanoparticles, a coating agent and a solvent; wherein: the mass fraction of the silver nanoparticles is 0.5 wt% - 50 wt%, and the mass fraction of the coating agent is < 3 wt%.

5. The silver nano-sol according to claim 4, characterized in that: The coating agent is polyvinylpyrrolidone or polyethylene glycol.

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