Radial-thickness-controllable flaky nano-silver as well as preparation method and application of radial-thickness-controllable flaky nano-silver

Abstract

The preparation method of the diameter-thickness-controllable flaky nano-silver particles is characterized in that seed crystals are prepared firstly, and then the seed crystals are used for inducing growth to obtain flaky nano-silver. By regulating and controlling the seed crystal concentration and the molar ratio of the solution silver salt to the reducing agent, the sheet diameter and the sheet thickness can be controlled. Along with the increase of the seed crystal concentration, the sheet diameter and the thickness of the sheet-shaped nano-silver are synchronously reduced. Within a certain range, the thickness of the flaky nano-silver is increased and the diameter of the flaky nano-silver is reduced along with the increase of the use amount of the reducing agent hydrazine hydrate. The flaky nano-silver is triangular or hexagonal, and the content of the flaky nano-silver is larger than or equal to 90%. The sheet diameter of the sheet-shaped nano silver ranges from 300 nm to 2000 nm, and the thickness of the sheet-shaped nano silver ranges from 10 nm to 200 nm. The diameter-thickness-controllable flaky nano-silver can be applied to preparation of heat-conducting and electric-conducting materials.

Description

Technical field:

[0001] The invention relates to the preparation of nano metal powder materials, and in particular to a diameter-thickness controllable flaky nano silver and a preparation method and application thereof. Background technology:

[0002] Among the precious metals, silver has the highest electrical and thermal conductivity and is relatively cheap. The morphological parameters of silver powder have a great influence on its performance. Spherical nanosilver powder particles form a conductive network through point-to-point contact, and flaky nanosilver particles form a conductive network through surface-to-surface contact. In this way, since flaky particles have a larger contact area than spherical silver powder particles, it is easier to form sintering necks between particles, so it is easier to form a dense conductive network. In addition, the preparation of flaky nanosilver with controllable sheet diameter and thickness is conducive to establishing the structure-activity relationship between the size and performance of flaky nanosilver, providing a theoretical basis and material foundation for its large-scale application.

[0003] The most suitable route for the industrial preparation of flaky nanosilver is to use liquid phase reduction and seed induction methods. However, the currently reported preparation methods are difficult to achieve fine control of the diameter and thickness of flaky nanosilver. Especially in the thickness direction, halogen capping is mainly used to achieve thickness control. As we all know, the electronics industry has strict requirements on the halogen content of materials. After using halogen to control the thickness of flaky nanosilver, halogen atoms remain on the surface of flaky nanosilver, which will seriously affect the application performance of the prepared flaky nanosilver.

[0004] Therefore, based on the current situation, it is imperative to develop a method for preparing flaky nanosilver that does not require the use of halogens and can precisely control the diameter and thickness. Summary of the invention:

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art. The inventors have found through repeated and careful experimental studies that hydrazine hydrate can not only be used as a reducing agent, but also as a capping agent to selectively adsorb the crystal surface of silver to achieve thickness direction regulation. In other words, the present invention provides a liquid phase reduction preparation method that does not require the use of halogen as a capping agent and can finely regulate the diameter and thickness of flaky nanosilver particles. The flaky nanosilver with controllable diameter and thickness prepared by the present invention can be applied to thermal and electrical conductive materials.

[0006] Specifically,

[0007] The first aspect of the present invention provides a method for preparing the above-mentioned flaky nanosilver with controllable diameter and thickness, which is a liquid phase reduction method for preparing flaky nanosilver with controllable diameter and thickness, and the specific steps are as follows:

[0008] (1) Preparation of seed crystals

[0009] Under stirring conditions of 20-30°C and 300-500rpm, add organic solvent A and surfactant to a beaker and stir evenly to obtain solution B. Then add silver salt to solution B and fully dissolve to obtain solution C. Solution C is heated to 100-160°C under stirring at 300-500rpm for reaction for 2-8h, and naturally cooled to room temperature to obtain seed crystal D;

[0010] (2) Preparation of flake-shaped silver nanoparticles with controllable diameter and thickness

[0011] Prepare a silver salt aqueous solution E and a reducing agent aqueous solution F; prepare an aqueous solution G of a seed crystal D, and use concentrated nitric acid to adjust the pH of the solution G to 1-7; lowering the pH of the solution G to 1-7 can allow the reduction reaction to obtain a kinetically controlled product. This is because hydrazine hydrate has a very strong reducing property at high pH, ​​so what is obtained is thermodynamically controlled spherical particles rather than flaky particles, so the pH of the solution G of the present invention is lower than 7.

[0012] Under mechanical stirring at 5-40° C. and 100-500 rpm, silver salt aqueous solution E and reducing agent aqueous solution F are simultaneously added dropwise to the seed solution G at the same dropping rate of 0.2-20 mL / min. After the addition is completed, the reaction is continued for 2-10 hours to obtain a flaky nanosilver dispersion.

[0013] The dispersion is centrifuged to obtain sheet-like nanosilver.

[0014] For the above preparation method, seed crystals are first prepared, and then the seed crystals are used to induce growth to obtain flaky nanosilver.

[0015] Secondly, the seed concentration in solution G and the molar ratio of silver salt to reducing agent in solutions E and F are adjusted to control the sheet diameter and sheet thickness.

[0016] In a preferred embodiment, in step (1), the organic solvent A is selected from one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and N-ethylpyrrolidone; the surfactant is selected from one or more of polyvinyl alcohol, polyvinylpyrrolidone, sodium citrate, hexadecyltrimethylammonium bromide, gelatin, and citric acid; the silver salt is selected from one or more of silver nitrate, silver formate, silver acetate, silver butyrate, and silver benzoate; based on 100 parts by mass of the organic solvent, the amount of the surfactant is 5 to 15 parts, and the amount of the silver salt is 0.5 to 5 parts;

[0017] In a preferred embodiment, in step (2), the reducing agent is hydrazine hydrate; the volumes of solution E and solution F are equal, the concentration of solution E is 0.05-0.5 mol / L, the concentration of solution F is 0.025-1.25 mol / L, the concentration of solution G is 5-100 g / L, the molar ratio of hydrazine hydrate to silver salt is 0.4-2, the volume ratio of solution G to solution E or solution F is 1-4, the centrifugal force is 500-8000 G, and the centrifugation time is 5-30 min;

[0018] In a preferred solution, when other conditions are fixed, as the concentration of the seed crystal D in the solution G increases, the diameter and thickness of the flaky nanosilver decrease simultaneously. This is because the size of the seed crystal is fixed, and the higher the seed crystal concentration, the fewer silver atoms can be obtained from a single seed crystal, so the diameter and thickness of the flaky nanosilver decrease simultaneously.

[0019] In a preferred embodiment, when other conditions are fixed, the molar ratio of hydrazine hydrate to silver salt in solution E and solution F is between 0.4 and 2, and as the amount of reducing agent hydrazine hydrate increases, the thickness of the sheet of flaky nanosilver increases and the diameter of the sheet decreases. This is because: hydrazine hydrate selectively adsorbs different crystal faces of silver, and the adsorption energy of hydrazine hydrate on the {100} and {110} crystal faces of silver is greater than that on the {111} crystal face. When other conditions are fixed, as the concentration of hydrazine hydrate increases, the degree of end-capping of the {100} and {110} crystal faces also increases, resulting in a decrease in the diameter of the sheet of flaky nanosilver and an increase in thickness. Therefore, the concentration of hydrazine hydrate needs to be controlled within an appropriate range. When the concentration of hydrazine hydrate is too low, the silver salt cannot be completely reduced; when the concentration of hydrazine hydrate is too high, the reduction rate is too fast, the growth of silver atoms is controlled by thermodynamics, and the products are mostly spherical particles.

[0020] In a preferred embodiment, the volumes of solution E and solution F are equal, and the volume ratio of solution G to solution E or solution F is 1-4.

[0021] The second aspect of the present invention provides a series of flaky nanosilver with controllable diameter and thickness, which are flaky nanosilver with controllable diameter and thickness obtained by the above-mentioned preparation method, characterized in that the morphology of the flaky nanosilver is triangular and / or hexagonal, and the content of the flaky nanosilver is greater than or equal to 90%, the diameter of the flaky nanosilver is in the range of 300 to 2000 nm, and the thickness is in the range of 10 to 200 nm.

[0022] In a preferred embodiment, the diameter of the flaky nanosilver is in the range of 400 to 1500 nm, and the thickness is in the range of 20 to 160 nm.

[0023] The third aspect of the present invention provides the use of the diameter-thickness controllable flaky nanosilver obtained by the above preparation method in thermally conductive and electrically conductive materials.

[0024] Beneficial effects of the present invention:

[0025] According to the present invention, a preparation method of flaky nanosilver with controllable diameter and thickness by liquid phase reduction is provided. In this case, hydrazine hydrate can not only be used as a reducing agent, but also can be used as a capping agent to selectively adsorb the crystal surface of silver to achieve flaky nanosilver with thickness control. The diameter and thickness of flaky nanosilver can be finely controlled by adjusting the molar ratio of hydrazine hydrate:silver salt. In addition, in the technical solution provided by the present invention, the regulation in the thickness direction does not involve the introduction of halogen ions, so it has no effect on the use performance of flaky nanosilver. Finally, the series of flaky nanosilver provided by the present invention provide basic materials for establishing the structure-activity relationship between the diameter and thickness parameters of flaky nanosilver and the use performance. Description of the drawings:

[0026] Figure 1 The SEM image and size statistics of the 1# flaky nanosilver obtained in Example 3

[0027] Figure 2 The SEM image and size statistics of 2# flake nanosilver obtained in Example 4

[0028] Figure 3 The SEM image and size statistics of 3# flaky nanosilver obtained in Example 5

[0029] Figure 4 AFM, SEM and size statistics of 4# flaky nanosilver obtained in Example 6

[0030] Figure 5 The SEM image and size statistics of 5# flaky nanosilver obtained in Example 7

[0031] Figure 6 The SEM image and size statistics of 6# flake nanosilver obtained in Example 8

[0032] Figure 7 The SEM image and size statistics of 7# flake nanosilver obtained in Comparative Example 1 Specific implementation method:

[0033] The technical solution of the present invention is described in detail with specific embodiments below. The following specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described in detail in some embodiments.

[0034] To achieve the above purpose, the technical route of the present invention is to first prepare seed crystals, and then use the seed crystals to induce growth to obtain flaky nanosilver. The preparation of flaky nanosilver with different diameters and thicknesses is achieved by adjusting the experimental parameters. The application performance of the flaky nanosilver is proved by the thermal conductivity and electrical conductivity of the flaky nanosilver after sintering.

[0035] Example 1: Preparation of 1# Seed Crystal

[0036] Under stirring at 30°C and 500 rpm, 500 g of N,N-dimethylacetamide and 25 g of sodium citrate were added to a beaker and stirred evenly to obtain solution B. Subsequently, 2.5 g of silver nitrate was added to solution B and fully dissolved to obtain solution C. Solution C was heated to 160°C under stirring at 500 rpm for reaction for 2 h, and naturally cooled to room temperature to obtain seed crystal 1#;

[0037] Example 2: Preparation of 2# Seed Crystal

[0038] Under stirring at 20°C and 300 rpm, 500 g of N,N-dimethylformamide and 75 g of polyvinyl pyrrolidone were added to a beaker and stirred to obtain solution B. Subsequently, 25 g of silver nitrate was added to solution B and fully dissolved to obtain solution C. Solution C was heated to 100°C and reacted for 8 h under stirring at 300 rpm, and naturally cooled to room temperature to obtain seed crystal 2#;

[0039] Example 3: Preparation of 1# flake nanosilver

[0040] Prepare 2 kg of 0.1 M silver nitrate aqueous solution E and 0.05 M hydrazine hydrate aqueous solution F respectively; prepare 8 kg of 1# seed crystal aqueous solution G (7.9 kg of deionized water, 0.1 kg of 1# seed crystal), and adjust the pH of G solution to 3 with concentrated nitric acid;

[0041] Under mechanical stirring at 5°C and 500 rpm, silver salt aqueous solution E and reducing agent aqueous solution F were added dropwise to the seed solution G at the same dropping rate of 2.0 mL / min. After the addition was completed, the reaction was continued for 10 h to obtain a flaky nanosilver dispersion.

[0042] The dispersion was centrifuged at 500G for 30 min to obtain flaky nanosilver. Figure 1 As shown, the average flake diameter is 740nm, the average flake thickness is 50nm, and the proportion of flaky nanosilver is greater than or equal to 95%.

[0043] Example 4: Preparation of 2# flake nanosilver

[0044] Prepare 4 kg of 0.1 M silver nitrate aqueous solution E and 0.05 M hydrazine hydrate aqueous solution F respectively; prepare 16 kg of 1# seed crystal aqueous solution G (15.6 kg of deionized water, 0.4 kg of 1# seed crystal), and adjust the pH of G solution to 3 with concentrated nitric acid;

[0045] Under mechanical stirring at 15°C and 500rpm, silver salt aqueous solution E and reducing agent aqueous solution F were simultaneously added to the seed solution G at the same dropping rate of 4.0mL / min. After the addition was completed, the reaction was continued for 4h to obtain a flaky nanosilver dispersion.

[0046] The dispersion was centrifuged at 2000G for 25 min to obtain flaky nanosilver. Figure 2 As shown, the average flake diameter is 600nm, the average flake thickness is 40nm, and the proportion of flaky nanosilver is greater than or equal to 95%.

[0047] Example 5: Preparation of 3# flake nanosilver

[0048] Prepare 8 kg of 0.1 M silver nitrate aqueous solution E and 0.05 M hydrazine hydrate aqueous solution F respectively; prepare 32 kg of 1# seed crystal aqueous solution G (30.4 kg of deionized water, 1.6 kg of 1# seed crystal), and adjust the pH of G solution to 3 with concentrated nitric acid;

[0049] Under mechanical stirring at 15°C and 500rpm, silver salt aqueous solution E and reducing agent aqueous solution F were added dropwise to the seed solution G at the same dropping rate of 8.0mL / min. After the addition was completed, the reaction was continued for 4h to obtain a flaky nanosilver dispersion.

[0050] The dispersion was centrifuged at 4000G for 20 min to obtain flaky nanosilver. Figure 3 As shown, the average flake diameter is 530nm, the average flake thickness is 30nm, and the proportion of flake nanosilver is greater than or equal to 95%.

[0051] It can be concluded from Examples 3-5 that as the concentration of the seed solution increases, the diameter and thickness of the flaky nanosilver decrease simultaneously.

[0052] Example 6: Preparation of 4# flake nanosilver

[0053] Prepare 8 kg of 0.4 M silver nitrate aqueous solution E and 0.36 M hydrazine hydrate aqueous solution F respectively; prepare 32 kg of 2# seed crystal aqueous solution G (30.8 kg of deionized water, 0.2 kg of 2# seed crystal), and adjust the pH of G solution to 3 with concentrated nitric acid;

[0054] Under mechanical stirring at 25°C and 500rpm, silver salt aqueous solution E and reducing agent aqueous solution F were added dropwise to the seed solution G at the same dropping rate of 8.0mL / min. After the addition was completed, the reaction was continued for 4h to obtain a flaky nanosilver dispersion.

[0055] The dispersion was centrifuged at 4000G for 20 min to obtain flaky nanosilver. Figure 4 As shown, the average flake diameter is 1450nm, the average flake thickness is 20nm, and the proportion of flake nanosilver is greater than or equal to 80%.

[0056] Example 7: Preparation of 5# flake nanosilver

[0057] Prepare 8 kg of 0.4 M silver nitrate aqueous solution E and 0.6 M hydrazine hydrate aqueous solution F respectively; prepare 32 kg of 2# seed crystal aqueous solution G (30.8 kg of deionized water, 0.2 kg of 2# seed crystal), and adjust the pH of G solution to 2.5 with concentrated nitric acid;

[0058] Under mechanical stirring at 25°C and 500rpm, silver salt aqueous solution E and reducing agent aqueous solution F were added dropwise to the seed solution G at the same dropping rate of 8.0mL / min. After the addition was completed, the reaction was continued for 4h to obtain a flaky nanosilver dispersion.

[0059] The dispersion was centrifuged at 4000G for 20 min to obtain flaky nanosilver. Figure 5 As shown, the average flake diameter is 875nm, the average flake thickness is 60nm, and the proportion of flaky nanosilver is greater than or equal to 95%.

[0060] Example 8: Preparation of 6# flake nanosilver

[0061] Prepare 8 kg of 0.4 M silver nitrate aqueous solution E and 0.72 M hydrazine hydrate aqueous solution F respectively; prepare 32 kg of 2# seed crystal aqueous solution G (30.8 kg of deionized water, 0.2 kg of 2# seed crystal), and adjust the pH of G solution to 2.0 with concentrated nitric acid;

[0062] Under mechanical stirring at 25°C and 500rpm, silver salt aqueous solution E and reducing agent aqueous solution F were added dropwise to the seed solution G at the same dropping rate of 8.0mL / min. After the addition was completed, the reaction was continued for 4h to obtain a flaky nanosilver dispersion.

[0063] The dispersion was centrifuged at 4000G for 20 min to obtain flaky nanosilver. Figure 6 As shown, the average flake diameter is 460nm, the average flake thickness is 150nm, and the proportion of flake nanosilver is greater than or equal to 95%.

[0064] It can be concluded from Examples 6-8 that as the molar ratio of hydrazine hydrate to silver salt increases, the diameter of the flaky nanosilver decreases and the thickness increases.

[0065] Example 9

[0066] 4# flake nanosilver was dispersed in ethanol and sprayed on the PET film several times to form a film with a thickness of 0.1mm, a length of 20mm and a width of 20mm. The film was sintered at 200℃ for 60min and the volume resistivity was 5.1μΩ·cm.

[0067] Example 10

[0068] The 2# and 6# nanosilver flakes were dispersed in diethylene glycol to obtain nanosilver paste, and the standard samples were prepared. After sintering at 200°C, the thermal conductivity at 25°C was tested. The thermal conductivity of the 2# sample was 210W / mk, and the thermal conductivity of the 6# sample was 180W / mk. It can be seen that the performance of the thick (6#) nanosilver after sintering is inferior to that of the thin (2#) nanosilver flakes.

[0069] Comparative Example 1: Preparation of 7# flake nanosilver

[0070] Prepare 8 kg of 0.4 M silver nitrate aqueous solution E and 1.05 M hydrazine hydrate aqueous solution F respectively; prepare 32 kg of 2# seed crystal aqueous solution G (30.8 kg of deionized water, 0.2 kg of 2# seed crystal), and adjust the pH of G solution to 2.0 with concentrated nitric acid;

[0071] Under mechanical stirring at 25°C and 500rpm, silver salt aqueous solution E and reducing agent aqueous solution F were added dropwise to the seed solution G at the same dropping rate of 8.0mL / min. After the addition was completed, the reaction was continued for 4h to obtain a flaky nanosilver dispersion.

[0072] Dispersion liquid 4000G After 20 minutes of centrifugation, the nanosilver flakes were obtained. Figure 7 As shown, the average flake diameter is 650nm and the average flake thickness is 60nm , the proportion of flaky nanosilver is about 40% It can be seen that when the molar ratio of hydrazine hydrate to silver nitrate is 2.625, which is beyond the range of 0.5 to 2, the product contains a large portion of thermodynamically controlled spherical particles, and it is difficult to obtain a high proportion of flaky nanosilver, and the thickness of the flaky nanosilver is reduced.

[0073] This is because: the adsorption energy of hydrazine hydrate on the {100} and {110} crystal planes of silver is greater than that on the {111} crystal plane. When other conditions are fixed, as the concentration of hydrazine hydrate increases, the degree to which the {100} and {110} crystal planes are capped increases, resulting in a decrease in the diameter of the sheet-like nanosilver and an increase in thickness. Therefore, the concentration of hydrazine hydrate needs to be controlled within an appropriate range. When the concentration of hydrazine hydrate is too high, the reduction rate is too fast, and the growth of silver atoms becomes controlled by thermodynamics. At this time, the product is mostly spherical particles, so the purpose of the present invention cannot be achieved.

Claims

1. A method for preparing flaky nanosilver with controllable diameter and thickness, the specific steps are as follows: (1) Preparation of seed crystals Under stirring conditions of 20-30°C and 300-500rpm, add organic solvent A and surfactant to a beaker, stir evenly to obtain solution B, then add silver salt to solution B, fully dissolve to obtain solution C, heat solution C to 100-160°C under stirring at 300-500rpm for reaction for 2-8h, and cool naturally to room temperature to obtain seed crystal D; (2) Preparation of flake-shaped silver nanoparticles with controllable diameter and thickness Prepare a silver salt aqueous solution E and a reducing agent aqueous solution F; prepare an aqueous solution G of seed crystal D, and use concentrated nitric acid to adjust the pH of the G solution to 1-7; Under mechanical stirring at 5-40° C. and 100-500 rpm, silver salt aqueous solution E and reducing agent aqueous solution F are simultaneously added dropwise to the seed solution G at the same dropping rate of 0.2-20 mL / min. After the addition is completed, the reaction is continued for 2-10 hours to obtain a flaky nanosilver dispersion. The dispersion was centrifuged to obtain flaky nanosilver.

2. The method for preparing the flaky nanosilver with controllable diameter and thickness as claimed in claim 1, wherein: The organic solvent A is selected from one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone and N-ethylpyrrolidone.

3. The method for preparing the flaky nanosilver with controllable diameter and thickness as claimed in claim 1, wherein: The surfactant is selected from one or more of polyvinyl alcohol, polyvinyl pyrrolidone, sodium citrate, hexadecyltrimethylammonium bromide, gelatin and citric acid.

4. The method for preparing the flaky nanosilver with controllable diameter and thickness as claimed in claim 1, wherein: The silver salt is selected from one or more of silver nitrate, silver formate, silver acetate, silver butyrate and silver benzoate.

5. The method for preparing the flaky nanosilver with controllable diameter and thickness as claimed in claim 1, wherein: Based on 100 parts by mass of the organic solvent, the amount of the surfactant used is 5 to 15 parts, and the amount of the silver salt used is 0.5 to 5 parts.

6. The method for preparing the flaky nanosilver with controllable diameter and thickness as claimed in claim 1, wherein: The reducing agent is hydrazine hydrate.

7. The method for preparing the flaky nanosilver with controllable diameter and thickness as claimed in claim 1, wherein: The volumes of the E and F solutions are equal, the concentration of the E solution is 0.05-0.5 mol / L, the concentration of the F solution is 0.025-1.25 mol / L, the concentration of the G solution is 5-100 g / L, and the molar ratio of hydrazine hydrate to silver salt is 0.4-2.

8. The method for preparing the flaky nanosilver with controllable diameter and thickness as claimed in claim 1, wherein: The volumes of solutions E and F are equal, and the volume ratio of solution G to solution E or solution F is 1-4.

9. A nanosilver flake with controllable diameter and thickness obtained by the preparation method according to claim 1, characterized in that: The morphology of the flaky nanosilver is triangular or hexagonal, and the content of the flaky nanosilver is greater than or equal to 90%. The diameter of the flaky nanosilver is in the range of 300-2000nm, and the thickness is in the range of 10-200nm.

10. An application, wherein the flaky nanosilver with controllable diameter and thickness obtained by the preparation method according to claims 1 to 8 is applied to conductive and thermal conductive materials.

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