Flaky nano-silver flocculation separation and surface coating layer thinning method and application

Abstract

A flaky nano-silver flocculation separation and surface coating layer thinning method is characterized in that after inorganic base is added into nano-silver dispersion liquid, double electric layers among flaky nano-silver particles are compressed to flocculate and naturally settle at the bottom of the dispersion liquid, and the flocculated nano-silver is cleaned repeatedly to remove the inorganic base to obtain the flaky nano-silver with the coating layer thinned. An analysis result shows that the surface coating layer of the cleaned flaky nano-silver is thinned through inorganic base flocculation, and low-temperature sintering of the flaky nano-silver is facilitated.

Description

Technical Field

[0001] The present invention relates to the field of preparation of nanomaterials, and specifically to a method for flocculation separation, surface coating layer degradation and thinning of flaky nano silver and its application. Background Art

[0002] Among precious metals, silver has the highest electrical conductivity and thermal conductivity, and its price is relatively low; the morphological parameters of silver powder have a great influence on its performance. Spherical nano silver powder particles form a conductive network through point-to-point contact, and flaky nano silver powder particles form a conductive network through surface-to-surface contact. Since flaky particles have a larger contact area than silver powder particles, it is easier to form sintering necks between particles, facilitating the formation of a dense conductive network.

[0003] In the prior art, flaky nano silver is mainly prepared by the liquid-phase reduction method. At present, there are many literatures and patents reporting the preparation and size control of flaky nano silver: on the basis of realizing the controllable preparation of flaky nano silver, efficiently separating the flaky nano silver from the liquid-phase reduction dispersion is the key to its industrial application. However, due to the steric hindrance and / or potential of surfactants, the natural sedimentation rate of flaky nano silver in the dispersion is relatively low. For example, the invention patent discloses a method of membrane separation (see Patent Document 1: CN103769608B, Patent Document 2: CN114377471B) to achieve the solid-liquid separation of metal nanoparticles, but this separation method limits the size of metal nanoparticles and only provides separation cases of spherical nano metal particles. In addition, in the current non-patent literature reports, flaky nano silver is mainly separated by centrifugation (see Non-Patent Document 1: ACS Appl. Electron. Mater. 2021, 3, 5365−5373; Non-Patent Document 2: CrystalGrowth&Design, Vol. 6, No. 9, 2006; Non-Patent Document 3: Nano Res. 2010, 3(12): 843-851). However, in order to achieve good morphology control, the concentration of flaky nano silver in the dispersion is often low, and the efficiency of direct centrifugation separation is even lower, making it difficult to meet the needs of batch production.

[0004] The low-temperature sintering activity of flaky silver nanoparticles is crucial for their applications in the fields of thermal and electrical conductivity. However, due to current preparation conditions, the surface of flaky silver nanoparticles adsorbs the surfactant polyvinylpyrrolidone. Although the coating of the surfactant on the surface of flaky silver nanoparticles endows them with dispersibility and stability, it also inhibits the low-temperature sintering performance of flaky silver nanoparticles. Reducing the coating amount of polyvinylpyrrolidone on the surface of flaky silver nanoparticles and decreasing the decomposition temperature of the polyvinylpyrrolidone coating layer are both beneficial to improving the low-temperature sintering activity of flaky silver nanoparticles. However, in current reports, multiple washing methods are used to remove the excess polyvinylpyrrolidone on the surface of flaky silver nanoparticles. In this regard, it can be expected that such physical washing cannot significantly change the chemical properties of polyvinylpyrrolidone on the surface of flaky silver nanoparticles, especially the decomposition temperature. There is a literature (Non-patent Document 4: ACS Nano 2011, 5, 4, 3354-3359) reporting a method for desorbing polyacrylic acid on the surface of spherical silver nanoparticles using sodium chloride. However, this method directly causes the sintering of spherical silver nanoparticles and is difficult to apply to multiple scenarios of silver nanoparticles. Summary of the Invention

[0005] In the prior art, the separation of flaky silver nanoparticles mainly relies on centrifugal concentration. However, due to the low concentration of silver nanoparticles in the flaky silver nanoparticle dispersion, this centrifugation method has low efficiency and is thus not conducive to its industrial application. On the other hand, the coating layer on the surface of the flaky silver nanoparticles obtained in this way inhibits their low-temperature sintering activity, and in the prior art, such a surface coating layer only relies on multiple washing to remove the excess surface coating layer, but it is difficult to regulate the chemical properties of the coating layer. Additionally, although some methods can remove the coating layer, they directly cause the aggregation and sintering of silver nanoparticles, so it is not easy to apply silver nanoparticles to multiple scenarios.

[0006] In view of the above problems in this field, the inventors have found through repeated experimental studies that after adding an inorganic base to the dispersion of flaky silver nanoparticles, the flaky silver nanoparticles are flocculated to form aggregates under the action of ionic strength. The formation of large-sized aggregates accelerates the natural sedimentation rate of flaky silver nanoparticles in the dispersion and naturally deposits to the bottom of the container in a short time. The convenient separation of flaky silver nanoparticles in a low-concentration flaky silver nanoparticle dispersion is achieved by removing the supernatant. Moreover, the inventors have also found that this flocculation can be well dispersed in the solvent, and after analysis by scanning electron microscopy, no phenomena such as aggregation, deformation, and sintering are found. Further, the inventors characterized the surface coating of the washed flaky silver nanoparticle flocs by thermogravimetry-differential scanning calorimetry and transmission electron microscopy, and found that both the coating amount and the coating layer thickness of the flaky silver nanoparticles are significantly reduced, so the decomposition temperature of the coating layer is also significantly decreased. The flaky silver nanoparticles processed by the technical solution of the present invention can be better applied in the field of thermal and electrical conductive materials.

[0007] Specifically, the first aspect of the present invention provides a method for flocculating and separating sheet-shaped silver nanoparticles and thinning the surface coating layer, and the specific steps are as follows: Under stirring, inorganic base particles or an inorganic base solution are added to the sheet-shaped silver nanoparticle dispersion liquid. After the inorganic base is completely dissolved, stirring is continued for 5 to 60 minutes, and then stirring is stopped; The liquid after stirring in step (1) is allowed to stand for 1 to 24 hours. After the supernatant of the sheet-shaped silver nanoparticle dispersion liquid becomes clear, sheet-shaped silver nanoparticle flocs are obtained at the bottom of the container; The supernatant of the silver nanoparticle flocs in step (2) is removed, thereby achieving the separation of the sheet-shaped silver nanoparticles containing the coating layer; The silver nanoparticle flocs containing the coating layer obtained in step (3) are fully dispersed in a cleaning solvent, the cleaning solvent is removed by centrifugation, and this step is repeated 2 to 6 times to collect the thinned sheet-shaped silver nanoparticles.

[0008] In a preferred embodiment: In the sheet-shaped silver nanoparticle dispersion liquid in step (1), it contains a dispersion medium, a surfactant, and sheet-shaped silver nanoparticles; in addition, the content of each of the above components relative to the total content of the dispersion liquid, the organic solvent as the dispersion medium is N,N-dimethylformamide or diethylene glycol, and the content of N,N-dimethylformamide or diethylene glycol is 0 to 5 wt%; the surfactant is polyvinylpyrrolidone, and the content of polyvinylpyrrolidone is 0.00001 to 0.5 wt%; the concentration of the sheet-shaped silver nanoparticles is 0.1 to 15 wt%; the remaining content is water; In step (1), in the sheet-shaped silver nanoparticle dispersion liquid, the sheet-shaped silver nanoparticles are in a structure of sheet-shaped silver nanoparticles with a polyvinylpyrrolidone coating layer. In addition, the morphology of the sheet-shaped silver nanoparticles is one or more of triangle, truncated triangle, hexagon, circle, and ellipse; In the sheet-shaped silver nanoparticle dispersion liquid, the thickness of the sheet-shaped silver nanoparticles is 10 to 250 nm, and the sheet diameter is 200 to 5000 nm; the preferred thickness is 20 to 200 nm, and the preferred sheet diameter is 250 to 4500 nm.

[0009] In a preferred embodiment, the inorganic base is one or more of ammonium hydroxide, barium hydroxide, calcium hydroxide, aluminum hydroxide, sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium bicarbonate, sodium carbonate, potassium carbonate, and potassium bicarbonate. Among them, sodium hydroxide, potassium hydroxide, and ammonium hydroxide are preferred.

[0010] In a preferred embodiment, the concentration of the inorganic base added to the silver nanoparticle dispersion liquid is 0.2 to 2.5 mol / L.

[0011] In step (2), the preferred standing time of the liquid is 2 to 20 hours.

[0012] In step (3), the flocculated supernatant is removed by pouring or extraction; In step (4), the cleaning solvent for the flocculated sheet-like silver nanoparticles can be selected from one or more of water, ethanol, methanol, isopropanol, and n-propanol; water and ethanol are preferred solvents.

[0013] The mass ratio of the cleaning solvent to the sheet-like silver nanoparticles is 1000:1 to 20:1.

[0014] The sheet-like silver nanoparticles obtained by using the method for separating and thinning the surface coating layer of the sheet-like silver nanoparticles provided in the first aspect of the present invention have a reduced coating layer thickness and coating amount compared with the sheet-like silver nanoparticles directly centrifuged and washed the same number of times, so that the decomposition temperature of the coating layer is reduced.

[0015] The second aspect of the present invention provides an application of the flocculated and separated and surface coating layer-thinned sheet-like silver nanoparticles obtained in the first aspect of the present invention in conductive and heat-conductive materials.

[0016] In this regard, the present inventors believe that on the one hand, the inorganic base provides ionic strength to compress the double electric layer on the surface of the sheet-like silver nanoparticles, causing the sheet-like silver nanoparticles to flocculate. On the other hand, the inorganic base reacts chemically with the coating layer on the surface of the sheet-like silver nanoparticles, including the breakage of the main chain and the ring-opening of pyrrolidone. In particular, under the action of the inorganic base, pyrrolidone undergoes ring-opening to generate carboxyl groups, and the carboxyl groups are converted into carboxylate ions under the action of the base. The potential action of the carboxylate ions enables the flocculated sheet-like silver nanoparticles to be further dispersed, thereby achieving the effect of thinning the coating layer. In this way, the decomposition temperature of the coating layer is reduced, and the low-temperature sintering activity of the sheet-like silver nanoparticles is improved, thereby realizing a broader application of the sheet-like silver nanoparticles with a surface coating layer in conductive and heat-conductive materials.

[0017] Advantages of the present invention The present invention can achieve the convenient and efficient separation of the sheet-like silver nanoparticles in the sheet-like silver nanoparticle dispersion; The sheet-like silver nanoparticles treated by the method provided by the present invention are thinned by the surface coating layer, so that the decomposition temperature of the coating layer is reduced, and the low-temperature sintering activity of the sheet-like silver nanoparticles is improved; The sheet-like silver nanoparticles treated by the method provided by the present invention can be better used in heat-conductive and conductive materials. Description of the drawings

[0018] Figure 1 (A) is a photograph of the sedimentation of the sheet-like silver nanoparticle dispersion in Example 1, Figure 1 (B) is a scanning electron microscope photograph of the sheet-like silver nanoparticle flocs, Figure 1 (C) is a scanning electron microscope photograph of the sheet-like silver nanoparticles after cleaning the sheet-like silver nanoparticle flocs with the cleaning solvent, Figure 1(D) is the transmission electron microscope image of the flaky silver nanoparticles after cleaning the flaky silver nanoparticle flocs with a cleaning solvent. Figure 1 (E) is a partially enlarged view of the transmission electron microscope image of the flaky silver nanoparticles after cleaning the flaky silver nanoparticle flocs with a cleaning solvent, and the thickness marked in the figure is the thickness of the surface coating layer. Figure 2 (A) is the state photograph of the flaky silver nanoparticles in Comparative Example 1 after standing for 2 h. Figure 2 (B) is the transmission electron microscope image of the flaky silver nanoparticles in Comparative Example 1 after centrifugal cleaning. Figure 2 (C) is a partially enlarged view of the transmission electron microscope image of the flaky silver nanoparticles in Comparative Example 1 after centrifugal cleaning, and the thickness of the surface coating layer is marked in the figure. Figure 3 is the scanning electron microscope image of the aggregated flaky silver nanoparticles obtained in Comparative Example 6. The left figure is the photograph at 10,000 times magnification, and the right figure is the photograph at 2,000 times magnification. Detailed implementation manners

[0019] The technical solutions of the present disclosure will be described in detail below with specific examples. These specific examples can be combined with each other, and the same or similar concepts or processes may not be repeated in some examples. The flaky silver nanoparticle dispersion in the examples was prepared by referring to the methods disclosed in the applicant's previous patent (CN119076961A) and literature.

[0020] Example 1: 10 mL of flaky silver nanoparticle dispersion: 90 wt% water, 5 wt% N,N-dimethylformamide, 4.99 wt% flaky silver nanoparticles, 0.01 wt% polyvinylpyrrolidone. The median flake diameter of the flaky silver nanoparticles is about 600 nm, the flake thickness is about 40 nm, and the morphology is hexagonal. The flaky silver nanoparticles in this dispersion are flaky silver nanoparticles containing a polyvinylpyrrolidone coating layer (hereinafter sometimes abbreviated as flaky silver nanoparticles, the same below).

[0021] Steps of flocculation separation and surface coating layer thinning:

[0022] (1) Ammonium hydroxide was added to the above-mentioned flaky silver nanoparticle dispersion to make its concentration reach 1.5 M, and after stirring for 30 min, it was left to stand. (2) After standing for 2 h, the flaky silver nanoparticles flocculated to the bottom, and the supernatant of the dispersion became clear. (3) The supernatant of the dispersion was removed by pouring to obtain the flaky silver nanoparticle flocs with a coating layer. (4) 10 mL of ethanol was added to the silver nanoparticle flocs obtained at the bottom in the above step, ultrasonically dispersed, and then centrifuged at 3000 rpm; the washing with ethanol was repeated twice to obtain the flaky silver nanoparticles with a thinned surface coating layer.

[0023] Characterization of flaky silver nanoparticles:

[0024] (1) The morphology of the flocculated and redispersed flaky silver nanoparticles in ethanol was characterized by scanning electron microscopy; (2) The surface coating of the flaky silver nanoparticles was studied by transmission electron microscopy and thermogravimetric-differential thermal analysis.

[0025] As Figure 1 the characterization results show that Figure 1 (A) shows that the flaky silver nanoparticles flocculated and deposited at the bottom, the supernatant was clear, and the flaky silver nanoparticles were separated from the mother liquor and closely packed; Figure 1 (B) shows that the flocculated flaky silver nanoparticles are composed of multiple flaky silver nanoparticles disorderly and closely packed together; Figure 1 (C, D) show that the flaky silver nanoparticles with a coating layer have good dispersion performance after washing and still maintain a good flaky shape. Figure 1 (E) shows that the average thickness of the surface coating layer of the obtained flaky silver nanoparticles is about 1.09 nm, the coating amount is 1.16 wt%, and the thermogravimetric-differential thermal analysis shows that the maximum exothermic peak of the coating layer is 250 °C.

[0026] Comparative Example 1: 10 mL of flaky silver nanoparticle dispersion: 90 wt% water, 5 wt% N,N-dimethylformamide, 4.99 wt% flaky silver nanoparticles, 0.01 wt% polyvinylpyrrolidone. The median flake diameter of the flaky silver nanoparticles is about 600 nm, the flake thickness is about 40 nm, and the morphology is hexagonal. The flaky silver nanoparticles in this dispersion are flaky silver nanoparticles with a coating layer (sometimes abbreviated as flaky silver nanoparticles).

[0027] Separation step: After the dispersion was allowed to stand for 2 h, the flaky silver nanoparticle dispersion was centrifuged at 3000 rpm and washed twice with ethanol to obtain flaky silver nanoparticles; Characterization of flaky silver nanoparticles: The surface coating of the flaky silver nanoparticles was studied by transmission electron microscopy and thermogravimetric-differential thermal analysis.

[0028] Figure 2 (A) shows that after standing for 2 h, the silver nanoparticle dispersion hardly settled. Figure 2 (B, C) show that the flaky characteristics of the silver nanoparticles are good after centrifugal washing. The average thickness of the surface coating layer is about 2.06 nm, the coating amount is 1.78 wt%, and the maximum exothermic peak of the coating layer is 285 °C.

[0029] Combining Example 1 and Comparative Example 1, it can be found that: First, for the same flaky silver nanodispersion, after adding inorganic base to the dispersion of the present invention in Example 1, the flaky silver nanoparticles are affected by the ionic strength and flocculate to form an assembly. The formation of large-sized assemblies accelerates the natural sedimentation rate of the flaky silver nanoparticles in the dispersion and naturally deposits at the bottom of the container in a relatively short time. The separation from the mother liquor is thus achieved through the flocculation of large-sized assemblies. In contrast, in Comparative Example 1, no inorganic base was added, so no flocculated assembly could be formed, and separation was achieved only by centrifugation. Second, the surface coating layer thickness of the flaky silver nanoparticles obtained in Example 1 is 1.09 nm, the coating amount is 1.16 wt%, and the maximum exothermic peak of the coating layer is 250 °C. While for the flaky silver nanoparticles obtained in Comparative Example 1 using the technical scheme of direct centrifugation and washing the same number of times, the average thickness of the surface coating layer is about 2.06 nm, the coating amount is 1.78 wt%, and the maximum exothermic peak of the coating layer is 285 °C. The above comparison shows that the technical scheme of the present invention can achieve the flocculation separation of flaky silver nanoparticles and the thinning of the surface coating layer, and the decomposition temperature of the coating layer is reduced, which provides convenience and prospects for the better use of flaky silver nanoparticles in heat-conducting and conductive materials.

[0030] Example 2: The flaky silver nanoparticles obtained in Example 1 were dispersed in terpineol to prepare a slurry with a silver content of 80 wt%. The slurry was printed into a film with a thickness of 100 microns and sintered at 200 °C. After testing, the volume resistivity was 3.8 μΩ. The silver paste was prepared into a standard sample for thermal conductivity testing and sintered at 200 °C. After testing, the thermal conductivity was 280 W / m·K.

[0031] Comparative Example 2: The flaky silver nanoparticles obtained in Comparative Example 1 were dispersed in terpineol to prepare a slurry with a silver content of 80 wt%. The slurry was printed into a film with a thickness of 100 microns and sintered at 200 °C. After testing, the volume resistivity was 9.6 μΩ. The silver paste was prepared into a standard sample for thermal conductivity testing and sintered at 200 °C. After testing, the thermal conductivity was 176 W / m·K.

[0032] Combining the results of Example 2 and Comparative Example 2, it can be found that: Compared with Comparative Example 2, the volume resistivity of the flaky silver nanoparticles obtained by the present invention is reduced by 5.8 μΩ, and the thermal and electrical conductivity performance is increased by 104 W / m·K. Thus, it can be seen that the technical scheme of the present invention greatly improves the low-temperature sintering activity of the obtained flaky silver nanoparticles.

[0033] Example 3: 10 L of flaky silver nanodispersion: 89.5 wt% water, 0.5 wt% N,N-dimethylformamide, 9.95 wt% flaky silver nanowires, 0.05 wt% polyvinylpyrrolidone. The median flake diameter of the flaky silver nanowires is about 4500 nm, the flake thickness is about 200 nm, and the morphology is triangular and hexagonal. The flaky silver nanowires in this dispersion are flaky silver nanowires with a coating layer (sometimes abbreviated as flaky silver nanowires).

[0034] Steps of flocculation separation and surface coating layer thinning: (1) Add potassium hydroxide to the above-mentioned flaky silver nanodispersion to make the concentration 2.5 M, stir for 30 min and then let it stand. (2) After standing for 24 h, the flaky silver nanowires flocculate to the bottom and the supernatant of the dispersion becomes clear.

[0035] (3) Remove the supernatant of the dispersion by pouring to obtain flocculated silver nanowires with a coating layer. (4) Add 10 L of ethanol to the flocculated silver nanowires with a coating layer obtained at the bottom in the above step, ultrasonically disperse and then centrifuge at 3000 rpm; repeat the process of washing 5 times with 10 L of deionized water to obtain flaky silver nanowires with a thinned surface coating layer.

[0036] After characterization, the average thickness of the surface coating layer of the flaky silver nanowires is about 0.51 nm, the coating amount is 0.64 wt%, and the maximum exothermic peak of the coating layer is 251 °C.

[0037] Comparative Example 3: 10 L of flaky silver nanodispersion: 89.5 wt% water, 0.5 wt% N,N-dimethylformamide, 9.95 wt% flaky silver nanowires, 0.05 wt% polyvinylpyrrolidone. The median flake diameter of the flaky silver nanowires is about 4500 nm, the flake thickness is about 200 nm, and the morphology is triangular and hexagonal. The flaky silver nanowires in this dispersion are flaky silver nanowires with a coating layer (sometimes abbreviated as flaky silver nanowires).

[0038] Centrifuge the above-mentioned flaky silver nanodispersion by batchwise centrifugation multiple times. The centrifugation speed and cleaning solvent are the same as those in Example 3.

[0039] The characterization results show that the average thickness of the surface coating layer of the flaky silver nanowires is about 1.5 nm, the coating amount is 1.54 wt%, and the maximum exothermic peak of the coating layer is 289 °C.

[0040] From the characterization results of Example 3 and Comparative Example 3, it can be seen that the coating layer of the flaky nano-silver dispersible tablets obtained in the present invention is thinned, the thickness and coating amount of the coating layer are reduced by about 2 / 3, and the maximum exothermic peak of the coating layer is reduced by about 40 °C. It can be seen that the surface coating layer of the flaky nano-silver dispersible tablets obtained in the present invention is thinned, and the decomposition temperature of the coating layer is reduced, which can be better applied to thermal conductive and conductive materials.

[0041] Example 4: 100 L of flaky nano-silver dispersion: 95 wt% water, 4.8 wt% N,N-dimethylformamide, 0.199 wt% flaky nano-silver, 0.001 wt% polyvinylpyrrolidone. The median flake diameter of the flaky nano-silver is about 250 nm, the flake thickness is about 20 nm, and the morphology is triangular and hexagonal. The flaky nano-silver in this dispersion is flaky nano-silver with a coating layer (sometimes abbreviated as flaky nano-silver).

[0042] Steps of flocculation separation and reduction of the surface coating layer: (1) Add sodium hydroxide to the above-obtained flaky nano-silver dispersion to make its concentration 0.25 M, stir for 60 min and then let it stand. (2) After standing for 15 h, the flaky nano-silver flocs to the bottom, and the supernatant of the dispersion becomes clear.

[0043] (3) Remove the supernatant of the dispersion by pouring to obtain flocs of flaky nano-silver with a coating layer. (4) Add 5 L of ethanol to the flocs of flaky nano-silver with a coating layer obtained at the bottom above, ultrasonically disperse and then centrifuge at 3000 rpm; repeat washing 3 times with 5 L of deionized water to obtain flaky nano-silver with a thinned surface coating layer. After characterization, the flaky characteristics and dispersibility of the flaky nano-silver are good. The average thickness of the surface coating layer is about 1.3 nm, the coating amount is 1.35 wt%, and the maximum exothermic peak of the coating layer is 253 °C.

[0044] Comparative Example 4: 100 L of flaky nano-silver dispersion: 95 wt% water, 4.8 wt% N,N-dimethylformamide, 0.199 wt% flaky nano-silver, 0.001 wt% polyvinylpyrrolidone. The median flake diameter of the flaky nano-silver is about 250 nm, the flake thickness is about 20 nm, and the morphology is triangular and hexagonal. The flaky nano-silver in this dispersion is flaky nano-silver with a coating layer (sometimes abbreviated as flaky nano-silver).

[0045] The flaky nano-silver dispersion prepared in the above ratio is centrifuged by the method of batchwise centrifugation multiple times. The centrifugation speed and cleaning solvent are the same as those in Example 4.

[0046] The characterization results show that the average thickness of the surface coating layer of the flaky nano - silver is about 3.0 nm, the coating amount is 2.36 wt%, and the maximum exothermic peak of the coating layer is 281 °C.

[0047] From the characterization results of Example 4 and Comparative Example 4, it can be seen that the coating layer of the flaky nano - silver obtained by the present invention is thinned. The thickness of its coating layer is reduced by 1.7 nm, the coating amount is reduced by 1.01 wt%, and the maximum exothermic peak of the coating layer is reduced by 28 °C. It can be seen that the surface coating layer of the flaky nano - silver obtained by the present invention is thinned, and the decomposition temperature of the coating layer is reduced. Therefore, it can be better applied to thermal - conductive and conductive materials. In addition, in Example 4, the separation of flaky nano - silver flocculation and supernatant was achieved through flocculation. Only 5 L of the flaky nano - silver dispersion liquid needs to be processed during the subsequent cleaning and centrifugation processes. While in Comparative Example 1, 100 L of the dispersion liquid needs to be centrifuged to separate the flaky nano - silver from the dispersion liquid, which takes a lot of time and cost. It can be seen that the technical solution provided by the present invention can achieve the rapid and convenient separation of flaky nano - silver from the dispersion liquid.

[0048] Comparative Example 5: 100 L of flaky nano - silver dispersion liquid: 95 wt% water, 4.8 wt% N,N - dimethylformamide, 0.199 wt% flaky nano - silver, 0.001 wt% polyvinylpyrrolidone. The median flake diameter of the flaky nano - silver is about 250 nm, the flake thickness is about 20 nm, and the morphology is triangular and hexagonal. The flaky nano - silver in this dispersion liquid is the flaky nano - silver with a coating layer (sometimes abbreviated as flaky nano - silver).

[0049] Flocculation separation step: (1) Add sodium hydroxide to the flaky nano - silver dispersion liquid prepared in the above proportion to make its concentration 0.15 M, stir for 60 min and then let it stand. (2) After standing for 24 h, the flaky nano - silver does not flocculate, and the flaky nano - silver and the supernatant cannot be separated. The subsequent steps cannot be carried out.

[0050] It can be seen that when the concentration of the inorganic base is less than 0.2 M, due to the too - small concentration, the flocculation of flaky nano - silver cannot be effectively formed.

[0051] Comparative Example 6: 100 L of flaky nano - silver dispersion liquid: 95 wt% water, 4.8 wt% N,N - dimethylformamide, 0.199 wt% flaky nano - silver, 0.001 wt% polyvinylpyrrolidone. The median flake diameter of the flaky nano - silver is about 250 nm, the flake thickness is about 20 nm, and the morphology is triangular and hexagonal. The flaky nano - silver in this dispersion liquid is the flaky nano - silver with a coating layer (sometimes abbreviated as flaky nano - silver).

[0052] Flocculation separation step: (1) Add sodium hydroxide to the above-mentioned flaky silver nanodispersion to make its concentration 2.7 M, stir for 60 min and then let it stand. (2) After standing for 12 h, the flaky silver nanoparticles flocculate to the bottom and the supernatant of the dispersion becomes clear.

[0053] (3) Remove the supernatant of the dispersion by pouring to obtain silver nanoparticle flocs. (4) Add 5 L of ethanol to the silver nanoparticle flocs at the bottom in the previous step, ultrasonically disperse and then centrifuge at 3000 rpm; repeat the process of washing 3 times with 5 L of deionized water to obtain flaky silver nanoparticles. Figure 3 SEM images of the aggregated flaky silver nanoparticles obtained in Comparative Example 6. The left figure is the image at 10,000 times magnification, and the right figure is the image at 2,000 times magnification. As shown in the figure, the flaky silver nanoparticles obtained in Comparative Example 6 are overlapped into spherical shapes, indicating that atomic diffusion has occurred between the flaky silver nanoparticles and the flaky silver nanoparticles have aggregated.

[0054] Combining Examples 4 and Comparative Example 6 Figure 3 It can be seen that when the concentration of the inorganic base is too high, it will cause the aggregation of flaky silver nanoparticles during sedimentation. The inventor believes that when the concentration of the inorganic base is too high, the degree of compression of the electric double layer is too high, so diffusion occurs between the flaky silver nanoparticles in the flaky silver nanoparticle flocs, resulting in the flocs being unable to be redispersed and forming aggregates. Therefore, the beneficial effects of the present invention cannot be achieved.

Claims

1. A method for flocculation separation of flaky nanosilver and thinning of the surface coating layer, the specific steps are as follows: Adding inorganic base particles or inorganic base solution to the flaky nanosilver dispersion under stirring, and continuing stirring for 5 to 60 minutes after the inorganic base is completely dissolved, and then stopping stirring; The liquid stirred in step (1) is allowed to stand for 1 to 24 hours, and after the supernatant of the flaky nanosilver dispersion is clarified, flaky nanosilver flocculation is obtained at the bottom of the container; The nanosilver flocculation in step (2) is removed from the supernatant, thereby achieving the flocculation and separation of the above-mentioned nanosilver flakes containing the coating layer; The nanosilver flocs containing the coating layer obtained in step (3) are fully dispersed in a cleaning solvent, and the cleaning solvent is removed by centrifugation. This step is repeated 2 to 6 times to collect the thinned nanosilver flakes.

2. The method for flocculation separation of flaky nanosilver and thinning of the surface coating layer according to claim 1, characterized in that: The flaky nano-silver dispersion contains a dispersion medium, a surfactant and flaky nano-silver; Wherein, the dispersion medium is an organic solvent of diethylene glycol or N,N-dimethylformamide, and its content is 0~5wt%; The concentration of flaky nanosilver is 0.1~15wt%; The surfactant is polyvinyl pyrrolidone, and its content is 0.00001~0.5wt%; In the dispersion, the rest is water.

3. The method for flocculation separation of flaky nanosilver and thinning of the surface coating layer according to claim 1 or 2, characterized in that: The morphology of the flaky nanosilver is one or more of a triangle, a truncated triangle, a hexagon, a circle, and an ellipse.

4. The method for flocculation separation of flaky nanosilver and thinning of surface coating layer according to claim 1 or 2, characterized in that: The thickness of the flaky nanosilver is 10-250 nm, and the diameter of the flaky nanosilver is 200-5000 nm.

5. The method for flocculation separation of flaky nanosilver and thinning of surface coating layer according to claim 1, characterized in that: The inorganic base in step (1) is one or more of ammonium hydroxide, barium hydroxide, calcium hydroxide, aluminum hydroxide, sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium bicarbonate, sodium carbonate, potassium carbonate, and potassium bicarbonate.

6. The method for flocculation separation of flaky nanosilver and thinning of surface coating layer according to claim 1, characterized in that: In step (1), the concentration of the inorganic base in the nanosilver dispersion is 0.2-2.5 mol / L.

7. The method for flocculation separation of flaky nanosilver and thinning of surface coating layer according to claim 1, characterized in that: In step (4), the cleaning solvent for the flaky nanosilver flocculation is selected from one or more of water, ethanol, methanol, isopropanol, and n-propanol.

8. The method for flocculation separation of flaky nanosilver and thinning of surface coating layer according to claim 1, characterized in that: In step (4), the mass ratio of the cleaning solvent to the flaky nanosilver is 1000:1 to 20:

1.

9. An application, characterized in that, The flaky nanosilver with a thinned surface coating layer obtained by the method for flocculation separation and thinning of the surface coating layer of flaky nanosilver according to any one of claims 1 to 8 is applied to the preparation of conductive and thermal conductive materials.

Images