CN117862520A - A method for preparing flaky silver powder using shellac - Google Patents

Abstract

The invention relates to the technical field of metal powder preparation, and discloses a method for preparing flake silver powder by using shellac, which comprises the following steps: step S1: preparing a natural resin sacrificial template: coating the prepared lac solution on a flexible substrate, and curing; step S2: hydrophilic and sensitization treatment; step S3: preparing a silver film: spraying silver-ammonia solution and reducing solution on a natural resin sacrificial template for reaction, purging, flushing a silver film with ultrapure water, and purging at high temperature to obtain a bright silver film; step S4: immersing a natural resin sacrificial template plated with a silver film into an alcohol solution, a ketone solution or an alkaline solution, and stirring to obtain a flaky silver powder mother solution; step S5: crushing the flake silver powder mother liquor by adopting ultrasonic waves, and soaking and washing by ethanol, centrifuging and drying to obtain the flake silver powder. The prepared flake silver powder has uniform morphology and is flake; the surface is flat; the residue is little, and the conductive filler can produce very good conductive effect.

Description

A method for preparing flaky silver powder using shellac

Technical Field

The invention relates to the technical field of metal powder preparation, and in particular to a method for preparing flaky silver powder by using shellac.

Background technique

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

Silver powder plays an important role in conductive pastes as a conductive phase. Flake silver powder can form surface-to-surface contact in the process of forming a conductive path and has better conductive properties than silver powders of other morphologies.

In recent years, the research on the preparation of nanosilver powder using green resources has gradually increased. Currently reported green synthesis methods of nanosilver mostly extract substances with reducing properties from plant resources to reduce silver ions. However, due to the difficulty in controlling the composition and dosage of the extract, the morphology and particle size of the prepared nanosilver powder are difficult to control, and the yield is low, so it cannot be used as a conductive filler.

At present, the preparation of commercial flaky silver powder is mainly based on mechanical ball milling, but the energy consumption of mechanical ball milling is high. When preparing flaky silver powder by mechanical ball milling, stearic acid is usually used as the ball milling medium, resulting in the surface of the silver powder prepared by ball milling being covered with a stearic acid film. This type of silver powder is suitable for use in traditional conductive slurries. With the development of wearable electronic materials, the matrix used in flexible conductive composite materials is no longer an oily system, but mostly a hydrophilic system such as hydrogel. Silver powder prepared by ball milling usually needs to be processed before it can be used in flexible conductive composite materials, and the conductive performance cannot be well exerted.

There are also many reports on the preparation of flaky silver powder by chemical reduction method and template method. However, the existing chemical reduction method has problems such as difficult to control the morphology of silver powder, more additives, complex types of residual substances on the surface of silver powder, and inability to mass produce. The template method introduces a large amount of synthetic resin in the preparation process, which leads to complex post-processing, large amount of waste liquid discharge, and high overall production power consumption cost, which is not conducive to the green development of the industry.

Commonly used soluble resins in the preparation process of silver powder include water-soluble epoxy resin, water-soluble polyurethane resin, water-soluble acrylic resin or polyvinyl alcohol. However, water-soluble resins need to introduce additional hydrophilic groups such as carboxyl, hydroxyl, amino and amide groups during the synthesis process, and their preparation process is complicated, making their cost higher than that of non-water-soluble resins; taking acrylic resin as an example, its selling price is 40,000-50,000 yuan/ton. For example, the prior art CN104148655A discloses a green preparation method for flaky silver powder, which uses a water-soluble resin to prepare flaky silver powder using a vacuum evaporation coating method, and then dissolves the water-soluble resin in water to separate the silver powder. However, according to common knowledge in the art, even water-soluble resins have good water resistance after self-crosslinking and film formation, and thus cannot be dissolved by pure water, and still need to be dissolved using organic solvents, which poses a threat to the environment; in addition, there is a high silver loss rate during the vacuum evaporation coating process, which increases the preparation cost of its silver powder again.

Shellac, also known as lac, is a natural resin secreted by lac insects after transforming the plant juice they absorb. It has the characteristics of strong adhesion, insulation, moisture resistance, good film-forming properties, and is non-toxic and odorless. It is often used in coatings, medicine, food, fruit preservation and other industries. "Green industry" is also more and more popular with people, just like "green products". In addition, my country has abundant lac resources and low prices. Compared with synthetic resins, it has the advantages of lower cost and more convenient acquisition.

Summary of the invention

The purpose of the present invention is to provide a method for preparing flaky silver powder by using shellac in view of the problems of high cost, long degradation period and increased environmental pressure in the current preparation of flaky silver powder by synthetic resin. The method has the advantages of sustainable and renewable green resources, green and pollution-free preparation process, lower cost, and uniform thickness and surface of the prepared flaky silver powder.

The technical solution of the present invention is as follows:

A method for preparing flaky silver powder using shellac comprises the following steps:

Step S1: preparing a natural resin sacrificial template: dissolving shellac in ethanol to prepare a shellac solution; then coating the shellac solution on a flexible PET substrate and curing it at a curing temperature of 25-50° C. for a curing time of 5-20 min;

Step S2: hydrophilizing the natural resin sacrificial template with sodium dodecyl sulfate (SDS) solution. Sensitizing the sacrificial template with _SnCl2 solution;

Step S3: preparing a silver film: preparing a silver ammonia solution, spraying the silver ammonia solution and a reducing solution onto a natural resin sacrificial template simultaneously for reaction, and evenly blowing with an air compressor, washing the silver film with ultrapure water, and blowing at a high temperature of 60-90° C. until the liquid on the surface of the silver film disappears, thereby obtaining a bright silver film on the natural resin sacrificial template;

Step S4: immersing the natural resin sacrificial template coated with the silver film in an alcohol solution, a ketone solution or an alkaline solution, stirring, and obtaining a flaky silver powder mother solution;

Step S5: using ultrasound to crush the flaky silver powder mother liquor, and then washing with ethanol, centrifuging, and drying to obtain nano-sized flaky silver powder.

The coating method in step S1 is screen printing or scraping.

Preferably, the curing temperature is 50° C. and the curing time is 10 min.

Preferably, the alcohol solution is such as ethanol or methanol.

The use of shellac to produce flake silver powder has the following advantages over synthetic resins:

(1) Shellac is a natural biological resource that is sustainable and renewable. It has lower costs, and the production process produces almost no emissions and waste, making it more environmentally friendly.

(2) Shellac can be naturally degraded by breaking the carbon chain, while the synthetic resins currently reported for the preparation of flaky silver powder take a considerable amount of time to degrade.

(3) Shellac is widely used as a coating agent in coatings, medicines and foods due to its good film-forming property. In the present invention, the good film-forming property of shellac is utilized to prepare a smooth and flat natural resin sacrificial template, guide the orderly growth of silver nanofilm thereon, and realize the preparation of flaky silver powder with uniform thickness and uniform surface.

In addition, after adding the high-temperature purging step, the silver film obtained on the natural resin sacrificial template is brighter, effectively reducing the generation of spotty or strip-shaped defects on the surface of the silver film, and preventing the silver film from subsequently turning black or yellow.

According to a preferred embodiment, the shellac solution in step S1 is prepared by dissolving shellac and ethyl cellulose in ethanol, the solid content of the shellac solution is 15 wt %, and the mass ratio of shellac to ethyl cellulose is in the range of 9:1-7:3.

The shellac solution coated on the PET substrate forms a shellac film with a smooth surface after solidification, but the shellac film slowly dissolves under the action of silver ammonia solution and reducing solution, which leads to slight unevenness in the plated silver film layer and reduces the quality of the prepared flaky silver powder. After adding ethyl cellulose to the shellac solution, the dissolution of the shellac film is inhibited, and the flatness of the shellac film surface is further maintained, ensuring that the thickness of the prepared flaky silver powder is uniform.

Preferably, the mass ratio of shellac to ethyl cellulose is 7:3.

According to a preferred embodiment, in step S2, the concentration of the SDS solution is 0.35 mM; the _SnCl2 solution is prepared by mixing 10 mL/L of hydrochloric acid with 44.3 mM _SnCl2 .

According to a preferred embodiment, in step S2, the hydrophilic treatment time is 5 minutes, and the sensitization treatment time is 5 minutes.

According to a preferred embodiment, in step S3, the reducing solution is glucose, sodium potassium tartrate or a mixture of both.

In step S3, when the reducing solution is a mixture of sodium potassium tartrate and glucose, the mass ratio thereof is 5:8-1:8, preferably 1:7.

When the mass ratio of sodium potassium tartrate to glucose in the reducing solution is 1:8, the deposition speed of the silver film is too fast and the flatness of the silver film will be reduced. When the mass ratio is 5:8, the deposition speed of the silver film is too slow, impurities will be generated on the surface of the silver film, and the silver film will turn yellow and black, affecting the quality of the prepared flaky silver powder. When the mass ratio of sodium potassium tartrate to glucose is 1:7, the deposition speed of the silver film is optimal, which can ensure both the flatness of the silver film surface and the quality of the flaky silver powder.

According to a preferred embodiment, in step S3, the reaction time of spraying onto the natural resin sacrificial template is 5-8 minutes, preferably 6 minutes.

According to a preferred embodiment, in step S3, the silver ammonia solution is prepared by dropping 28 wt% ammonia water into a 58.8 mM AgNO ₃ solution, and observing that the solution changes from transparent to turbid, and continuing to drop until the solution becomes transparent. At this time, 1 M NaOH solution is added to make the solution turbid again, and then 28 wt% ammonia water is added to obtain a transparent silver ammonia solution.

According to a preferred embodiment, in step S4, the natural resin sacrificial template coated with a silver film is immersed in an alcohol solution, wherein the alcohol solution is ethanol, the immersion time is 1-5 minutes, preferably 2 minutes, and the stirring speed is 500-1000 r/min, preferably 700 r/min.

Controlling the immersion time and stirring speed can control the dissolution rate of the shellac film. If the shellac film dissolves slowly, the dissolution time of the shellac film will be prolonged, and there may be a problem that part of the shellac film adheres to the surface of the flaky silver powder again during the dissolution process. After experiments, the applicant of this application found that when the stirring speed is 700 r/min and the immersion time is 2 minutes, the dissolution rate of the shellac film is most suitable, and there is almost no residue on the surface of the prepared flaky silver film.

Ethanol has a good elution effect and less impurities remain, as shown in Figure 5. Ethanol is environmentally friendly and can be recycled.

According to a preferred embodiment, in step S3, the temperature of the high temperature purge is preferably 80-85°C, more preferably 85°C.

According to a preferred embodiment, in step S5, the ultrasonic power is 480 W, and the ultrasonic time is 10-50 minutes. The ultrasonic time can be adjusted according to the requirements for the particle size of the silver powder in subsequent applications.

On the other hand, the present application also provides the use of shellac in the preparation of flaky silver powder.

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

1. A method for preparing flaky silver powder using shellac. The surface of the flaky silver powder prepared using shellac is not covered with stearic acid, can be well dispersed in a flexible matrix, and has excellent conductive properties. In the process of preparing flaky silver powder using natural resin, shellac material is used as a plating base material, which provides a strong guarantee for the subsequent large-scale roll-to-roll continuous mass production. The mature chemical deposition technology used in the process also guarantees the large-scale production of silver nanofilms and reduces silver waste;

2. A method for preparing flaky silver powder using shellac, utilizing the good film-forming property of shellac to prepare a smooth and flat natural resin sacrificial template, and preparing flaky silver powder with uniform thickness and uniform surface. The preparation process can control the size of the prepared flaky silver powder, and the surface is uniform, which is suitable for use in conductive fillers to play a good conductive role;

3. A method for preparing flaky silver powder using shellac. Using shellac instead of synthetic resin not only saves costs in industry, but also improves the overall surface quality of the prepared silver powder and broadens the application scenarios of the silver powder. Environmentally, it utilizes natural renewable resources, can be naturally and quickly degraded, and the preparation process is simple to operate. It does not require the use of organic solvents such as acetone and will not cause pressure on the environment. It is a truly green process for preparing flaky silver powder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a SEM image comparing the flaky silver powder (a), (b) prepared in Example 1 of the present invention and the commercial flaky silver powder (c), (d); FIG1 (a) is a general morphology image of the flaky silver powder prepared in the present invention, and FIG1 (b) is a side cross-sectional view of the flaky silver powder prepared in the present invention; FIG1 (c) is a general morphology image of the commercial flaky silver powder, and FIG1 (d) is a side interface image of the commercial flaky silver powder;

FIG2 is a comparison diagram of the particle size distribution of flaky silver powder prepared under different ultrasonic durations in Examples 1-5 of the present invention;

FIG3 is an XRD diagram of the flaky silver powder prepared in Example 1 of the present invention;

FIG4 is an AFM image of the flaky silver powder prepared in Example 1 of the present invention;

FIG5 is a TGA graph of the flaky silver powder prepared in Example 1 of the present invention;

FIG6 is a comparison diagram of the silver plating effects of natural resin sacrificial templates prepared using different shellacs and ethyl celluloses in Examples 1, 2 and 5 of the present invention;

FIG7 is a metallographic image of flaky silver powder prepared at different purge temperatures in the present invention;

FIG8 is a graph showing the surface resistance of flaky silver powder prepared at different purge temperatures in the present invention;

FIG9 is a SEM image of flaky silver powder prepared in Example 7 and Comparative Example 1 of the present invention;

FIG10 is a SEM image of the spherical silver powder prepared in Comparative Example 2 of the present invention;

FIG. 11 is a SEM image of the flaky silver powder prepared in Comparative Example 2 of the present invention.

Detailed ways

The following examples are only basic descriptions of the present invention, so that those skilled in the art can further understand the present invention, and are not intended to limit the scope of the present invention. Unless otherwise specified, the methods used in this application are all conventional methods. Unless otherwise specified, the experimental materials used in this application are all commercially available. Commercial flaky silver powder was purchased from MCC Xindun Alloy Co., Ltd.

The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

Example 1

A method for preparing flaky silver powder using shellac comprises the following steps:

Step S1: Preparation of natural resin sacrificial template:

Step S1.1: preparing a shellac solution, dissolving shellac and ethyl cellulose in an ethanol solution at a mass ratio of 7:3 to prepare a shellac solution with a solid content of 15 wt %;

Step S1.2: Print the shellac solution on the flexible PET substrate by screen printing, and place it in a 50°C oven for curing for 10 min to form a natural resin sacrificial template.

Step S2:

Step S2.1: Hydrophilic treatment: Place the natural resin sacrificial template in a 0.35 mM SDS solution, take it out after 5 min, and rinse it with ultrapure water;

Step S2.2: Sensitization treatment: Place the natural resin sacrificial template treated in step S2.1 into an acidic SnCl ₂ solution, take it out after 5 minutes, and rinse it with ultrapure water; the SnCl ₂ solution is a mixture of 10 mL/L hydrochloric acid and 44.3 mM SnCl ₂ .

Step S3.1: Prepare a silver ammonia solution according to the following preparation method: add 28 wt% ammonia water to 58.8 mM AgNO ₃ solution, observe that the solution changes from transparent to turbid, and continue to add until the solution becomes transparent. At this time, add 1 M NaOH solution to make the solution turbid again, and then add 28 wt% ammonia water to obtain a transparent silver ammonia solution;

Step S3.2: Dissolve potassium sodium tartrate and glucose in ultrapure water at a mass ratio of 1:7, and spray them onto the natural resin sacrificial template simultaneously with the silver ammonia solution at a volume ratio of 1:1, and at the same time, use an air compressor to evenly blow it. After reacting for 6 minutes, rinse with ultrapure water 3 times, and blow it with hot air at 85°C to obtain a bright silver film;

Step S4: immerse the silver film obtained in step S3.2 in an ethanol solution for 2 minutes and stir at a speed of 700 r/min to obtain a flaky silver powder mother solution;

Step S5: The flaky silver powder mother liquor is placed in a high-energy ultrasonic pulverizer for pulverization, the ultrasonic power is set to 480 W, and the ultrasonic time is set to 20 minutes to obtain flaky silver powder with an average particle size of 12.5 µm.

The SEM images of the flaky silver powder prepared in this embodiment and the SEM images of the commercial flaky silver powder (purchased from Zhongye Xindun Alloy Co., Ltd.) are shown in Figure 1. By comparing Figures 1 (a) and (c), it can be seen that the flaky silver powder prepared in this embodiment has a uniform morphology, a thin flaky structure, a relatively uniform particle size, and no lubricating layer covering the surface, while the silver powder has an uneven morphology, with other morphologies of silver powder mixed in. By comparing Figures 1 (b) and (d), it can be seen that the flaky silver powder prepared in the present invention has a uniform thickness of about 55nm; while the commercial flaky silver powder has unclear sides and uneven thickness.

The XRD pattern of the flaky silver powder prepared in this embodiment is shown in FIG3 . It can be seen from this figure that the flaky silver powder has a strong peak at 38.12° and four weak peaks at 44.40°, 64.5°, 77.40° and 81.6°. These five diffraction peaks represent the (111), (200), (220), (311) and (222) crystal planes of the silver face-centered cubic structure. In addition to the above five diffraction peaks, there are no other diffraction peaks on the spectrum, which is completely consistent with the diffraction peaks on the silver standard card, indicating that the powder prepared in this article is a very pure silver powder. The strong diffraction peak of the sample at 38.12° shows from another aspect that the silver powder prepared in this article has an ideal flaky structure.

The AFM image of the flaky silver powder prepared in this embodiment is shown in FIG4 , from which it can be seen that the surface of the flaky silver powder prepared in the present invention is relatively flat and has low roughness. This is conducive to the subsequent preparation of the conductive composite material, and when constructing the conductive channel, a good surface-to-surface contact can be formed, thereby improving the transmission rate of electrons.

The TGA graph of the flaky silver powder prepared in this example is shown in FIG5 , from which it can be seen that during the entire heating process, the mass loss of the silver flakes is very small, only 1.52 wt %, indicating that there is almost no other substance remaining on the surface of the flaky silver powder prepared by this method.

Example 2

A method for preparing flaky silver powder using shellac comprises the following steps:

Step S1: Preparation of natural resin sacrificial template:

Step S1.1: preparing a shellac solution, dissolving shellac and ethyl cellulose in an ethanol solution at a mass ratio of 8:2 to prepare a shellac solution with a solid content of 15 wt %;

Step S1.2: Print the shellac solution on the flexible PET substrate by screen printing, and place it in a 50°C oven for curing for 10 min to form a natural resin sacrificial template.

Step S2.1: Hydrophilic treatment: Place the natural resin sacrificial template in a 0.35 mM SDS solution, take it out after 5 min, and rinse it with ultrapure water;

Step S2.2: Sensitization treatment: Place the natural resin sacrificial template treated in step S2.1 into an acidic SnCl ₂ solution, take it out after 5 minutes, and rinse it with ultrapure water; the SnCl ₂ solution is a mixture of 10 mL/L hydrochloric acid and 44.3 mM SnCl ₂ .

Step S3.1: Prepare a silver ammonia solution according to the following preparation method: add 28 wt% ammonia water to 58.8 mM AgNO ₃ solution, observe that the solution changes from transparent to turbid, and continue to add until the solution becomes transparent. At this time, add 1 M NaOH solution to make the solution turbid again, and then add 28 wt% ammonia water to obtain a transparent silver ammonia solution;

In step S3.2, potassium sodium tartrate and glucose in a mass ratio of 1:4 are dissolved in ultrapure water, and sprayed onto the natural resin sacrificial template simultaneously with the silver ammonia solution in a volume ratio of 1:1, and at the same time, the template is evenly purged with an air compressor. After reacting for 7 minutes, the template is rinsed with ultrapure water 3 times, and then purged with hot air at 90°C to obtain a bright silver film.

Step S4: The bright silver film obtained in step 3.2 is immersed in a 1 M NaOH solution at 50°C for 10 minutes and stirred at a speed of 500 r/min to obtain a flaky silver powder mother solution.

Step S5: The flaky silver powder mother liquor is placed in a high-energy ultrasonic pulverizer for pulverization, the ultrasonic power is set to 480 W, and the ultrasonic time is set to 40 minutes to obtain flaky silver powder with an average particle size of 9 µm.

The SEM image, XRD image, AFM image and TGA image of the flaky silver powder prepared in this example are similar to those in Example 1.

Example 3

Example 3 is a further improvement on Example 1; a method for preparing flaky silver powder using shellac comprises the following steps:

Step S1: Preparation of natural resin sacrificial template:

Step S1.1: preparing a shellac solution, dissolving shellac and ethyl cellulose in an ethanol solution at a mass ratio of 7:3 to prepare a shellac solution with a solid content of 15 wt %;

Step S1.2: Print the shellac solution on the flexible PET substrate by screen printing, and place it in a 50°C oven for curing for 10 min to form a natural resin sacrificial template.

Step S2.1: Hydrophilic treatment: Place the natural resin sacrificial template in a 0.35 mM SDS solution, take it out after 5 min, and rinse it with ultrapure water;

Step S2.2: Sensitization treatment: Place the natural resin sacrificial template treated in step S2.1 into an acidic SnCl ₂ solution, take it out after 5 minutes, and rinse it with ultrapure water; the SnCl ₂ solution is a mixture of 10 mL/L hydrochloric acid and 44.3 mM SnCl ₂ .

Step S3.1: Prepare a silver ammonia solution according to the following preparation method: add 28 wt% ammonia water to 58.8 mM AgNO ₃ solution, observe that the solution changes from transparent to turbid, and continue to add until the solution becomes transparent. At this time, add 1 M NaOH solution to make the solution turbid again, and then add 28 wt% ammonia water to obtain a transparent silver ammonia solution;

Step S3.2: Dissolve potassium sodium tartrate and glucose in ultrapure water at a mass ratio of 1:6.5, and spray them onto the natural resin sacrificial template simultaneously with the silver ammonia solution at a volume ratio of 1:1, and at the same time, use an air compressor to evenly blow it. After reacting for 7 minutes, rinse with ultrapure water 3 times, and blow it with hot air at 80°C to obtain a bright silver film;

Step S4: immerse the silver film obtained in step S3.2 in an ethanol solution for 2 minutes and stir at a speed of 600 r/min to obtain a flaky silver powder mother solution;

Step S5: The flaky silver powder mother liquor is placed in a high-energy ultrasonic pulverizer for pulverization, the ultrasonic power is set to 480 W, and the ultrasonic time is set to 30 minutes to obtain flaky silver powder with an average particle size of 10 µm.

The SEM image, XRD image and AFM image of the flaky silver powder prepared in this example are similar to those in Example 1.

Example 4

Example 4 is a further improvement on Example 3; a method for preparing flaky silver powder using shellac comprises the following steps:

Step S1: Preparation of natural resin sacrificial template:

Step S1.1: preparing a shellac solution, dissolving shellac and ethyl cellulose in an ethanol solution at a mass ratio of 7:3 to prepare a shellac solution with a solid content of 15 wt %;

Step S1.2: Print the shellac solution on the flexible PET substrate by blade coating, and place it in a 50 °C oven for curing for 10 min to form a natural resin sacrificial template.

Step S2.1: Hydrophilic treatment: Place the natural resin sacrificial template in a 0.35 mM SDS solution, take it out after 5 min, and rinse it with ultrapure water;

Step S2.2: Sensitization treatment: Place the natural resin sacrificial template treated in step S2.1 into an acidic SnCl ₂ solution, take it out after 5 minutes, and rinse it with ultrapure water; the SnCl ₂ solution is a mixture of 10 mL/L hydrochloric acid and 44.3 mM SnCl ₂ .

Step S3.1: Prepare a silver ammonia solution according to the following preparation method: add 28 wt% ammonia water to 58.8 mM AgNO ₃ solution, observe that the solution changes from transparent to turbid, and continue to add until the solution becomes transparent. At this time, add 1 M NaOH solution to make the solution turbid again, and then add 28 wt% ammonia water to obtain a transparent silver ammonia solution;

Step S3.2 is: dissolving potassium sodium tartrate and glucose in a mass ratio of 5:8 in ultrapure water, spraying them onto the natural resin sacrificial template simultaneously with the silver ammonia solution in a volume ratio of 1:1, and evenly blowing them with an air compressor. After reacting for 6 minutes, rinse with ultrapure water 3 times, and blow with hot air at 85°C to obtain a bright silver film.

Step S4: immersing the silver film obtained in step S3.2 in an ethyl acetate solution for 2 minutes, stirring at a speed of 900 r/min to obtain a flaky silver powder mother solution;

Step S5 is: placing the flaky silver powder mother liquor into a high-energy ultrasonic pulverizer for pulverization, setting the ultrasonic power to 480 W and the ultrasonic time to 50 minutes, to obtain flaky silver powder with an average particle size of 6 µm.

The SEM image, XRD image, AFM image and TGA image of the flaky silver powder prepared in this example are similar to those in Example 1.

Example 5

A method for preparing flaky silver powder using shellac comprises the following steps:

Step S1: Preparation of natural resin sacrificial template:

Step S1.1: preparing a shellac solution, dissolving shellac and ethyl cellulose in an ethanol solution at a mass ratio of 9:1 to prepare a shellac solution with a solid content of 15 wt %;

Step S1.2: Print the shellac solution on the flexible PET substrate by blade coating, and place it in a 40°C oven for curing for 15 min to form a natural resin sacrificial template.

Step S2.1: Hydrophilic treatment: Place the natural resin sacrificial template in a 0.35 mM SDS solution, take it out after 5 min, and rinse it with ultrapure water;

Step S2.2: Sensitization treatment: Place the natural resin sacrificial template treated in step S2.1 into an acidic SnCl ₂ solution, take it out after 5 minutes, and rinse it with ultrapure water; the SnCl ₂ solution is a mixture of 10 mL/L hydrochloric acid and 44.3 mM SnCl ₂ .

Step S3.1: Prepare a silver ammonia solution according to the following preparation method: add 28 wt% ammonia water to 58.8 mM AgNO ₃ solution, observe that the solution changes from transparent to turbid, and continue to add until the solution becomes transparent. At this time, add 1 M NaOH solution to make the solution turbid again, and then add 28 wt% ammonia water to obtain a transparent silver ammonia solution;

Step S3.2 is: dissolving potassium sodium tartrate and glucose in a mass ratio of 1:7 in ultrapure water, spraying them onto the natural resin sacrificial template simultaneously with the silver ammonia solution in a volume ratio of 1:1, and evenly blowing them with an air compressor. After reacting for 6 minutes, rinse with ultrapure water 3 times, and blow with hot air at 60°C to obtain a bright silver film.

Step S4: immersing the silver film obtained in step S3.2 in an ethanol solution for 4 minutes, stirring at a speed of 500 r/min to obtain a flaky silver powder mother solution;

Step S5 is: placing the flaky silver powder mother liquor into a high-energy ultrasonic pulverizer for pulverization, setting the ultrasonic power to 480 W and the ultrasonic time to 10 minutes, to obtain flaky silver powder with an average particle size of 13 μm.

The SEM image, XRD image, AFM image and TGA image of the flaky silver powder prepared in this example are similar to those in Example 1.

Example 6

A method for preparing flaky silver powder using shellac comprises the following steps:

Step S1: Preparation of natural resin sacrificial template:

Step S1.1: preparing a shellac solution, dissolving shellac and ethyl cellulose in an ethanol solution at a mass ratio of 7:3 to prepare a shellac solution with a solid content of 15 wt %;

Step S1.2: Print the shellac solution on the flexible PET substrate by blade coating, and place it in a 25 °C oven for curing for 20 min to form a natural resin sacrificial template.

Step S2.1: Hydrophilic treatment: Place the natural resin sacrificial template in a 0.35 mM SDS solution, take it out after 5 min, and rinse it with ultrapure water;

Step S2.2: Sensitization treatment: Place the natural resin sacrificial template treated in step S2.1 into an acidic SnCl ₂ solution, take it out after 5 minutes, and rinse it with ultrapure water; the SnCl ₂ solution is a mixture of 10 mL/L hydrochloric acid and 44.3 mM SnCl ₂ .

Step S3.1: Prepare a silver ammonia solution according to the following preparation method: add 28 wt% ammonia water to 58.8 mM AgNO ₃ solution, observe that the solution changes from transparent to turbid, and continue to add until the solution becomes transparent. At this time, add 1 M NaOH solution to make the solution turbid again, and then add 28 wt% ammonia water to obtain a transparent silver ammonia solution;

Step S3.2 is: dissolving potassium sodium tartrate and glucose in a mass ratio of 1:3 in ultrapure water, spraying them onto the natural resin sacrificial template simultaneously with the silver ammonia solution in a volume ratio of 1:1, and evenly blowing them with an air compressor. After reacting for 6 minutes, rinse with ultrapure water 3 times, and blow with hot air at 85°C to obtain a bright silver film.

Step S4: immersing the silver film obtained in step S3.2 in an acetone solution for 2 minutes, stirring at a speed of 1000 r/min to obtain a flaky silver powder mother solution;

Step S5 is: placing the flaky silver powder mother liquor into a high-energy ultrasonic pulverizer for pulverization, setting the ultrasonic power to 480 W and the ultrasonic time to 40 minutes, to obtain flaky silver powder with an average particle size of 9 μm.

The SEM image, XRD image, AFM image and TGA image of the flaky silver powder prepared in this example are similar to those in Example 1.

FIG2 is a particle size distribution diagram of the flaky silver powder prepared in Examples 1-5 of the present invention. It can be seen from FIG2 that the particle size of the silver flakes can be controlled by the ultrasonic time. As the ultrasonic treatment time increases, the particle size distribution of the silver flakes shows a trend of gradually decreasing. When the ultrasonic treatment time is 50 minutes, the average particle size of the silver flakes is about 6 μm.

As shown in FIG6, the effect of silver plating of the natural resin sacrificial template made of shellac solution with different ratios of shellac and ethyl cellulose in Examples 5, 2 and 1 of the present application; as shown in FIG6, when the ratio of shellac to ethyl cellulose is 9:1, the coating is not uniform enough, and there is almost no reflection effect on the surface; and when the ratio of shellac to ethyl cellulose is 8:2, the uniformity of the coating is improved, and the surface has a certain reflection effect, but the surface of the coating has defects. When the ratio of shellac to ethyl cellulose is 7:3, the coating is very uniform, the silver film on the surface is very bright, the whole surface has a reflection effect, and there are no visible defects and bumps on the surface, indicating that when the ratio of shellac to ethyl cellulose is 7:3, the prepared natural resin sacrificial template is most suitable for the uniform growth of the silver film thereon, and this natural resin sacrificial template can be used to prepare a flat and bright silver film.

As shown in Figure 7, it is the metallographic image of the silver film obtained at the purge temperatures of 60°C, 70°C, 80°C, 85°C and 90°C in this application. It can be seen from Figure 7 that there will be defects on the un-purged surface. After purging, the surface defects of the silver film are reduced. When the temperature is 85°C and 90°C, the surface condition of the silver film is the best. The surface resistance of the silver film is tested using a four-probe tester, and the results are shown in Figure 8. It can be seen from Figure 8 that the surface resistance of the un-purged silver film is relatively high. As the purge temperature increases, the surface resistance of the silver film gradually decreases. After the temperature reaches 80°C, the surface resistance of the silver film reaches the lowest value and remains basically unchanged. It can be seen that the optimal purge temperature is 85°C.

Example 7

A method for preparing flaky silver powder using shellac comprises the following steps:

Step S1: Preparation of natural resin sacrificial template:

Step S1.1: preparing a shellac solution, dissolving shellac and ethyl cellulose in an ethanol solution at a mass ratio of 9:1 to prepare a shellac solution with a solid content of 15 wt %;

Step S1.2: Print the shellac solution on the flexible PET substrate by screen printing, and place it in a 25°C oven for curing for 20 min to form a natural resin sacrificial template.

Step S2.1: Hydrophilic treatment: Place the natural resin sacrificial template in a 0.35 mM SDS solution, take it out after 5 min, and rinse it with ultrapure water;

Step S2.2: Sensitization treatment: Place the natural resin sacrificial template treated in step S2.1 into an acidic _SnCl2 solution, take it out after 5 minutes, and rinse it with ultrapure water; the _SnCl2 solution is a mixture of 10 mL/L hydrochloric acid and 44.3 mM _SnCl2 .

Step S3.1: Prepare a silver ammonia solution according to the following preparation method: add 28 wt% ammonia water to 58.8 mM AgNO ₃ solution, observe that the solution changes from transparent to turbid, and continue to add until the solution becomes transparent. At this time, add 1 M NaOH solution to make the solution turbid again, and then add 28 wt% ammonia water to obtain a transparent silver ammonia solution;

Step S3.2: Sodium potassium tartrate and glucose in a mass ratio of 1:4 are dissolved in ultrapure water, and sprayed onto the natural resin sacrificial template simultaneously with the silver ammonia solution in a volume ratio of 1:1, and at the same time, the template is evenly purged with an air compressor. After reacting for 6 minutes, the template is rinsed with ultrapure water 3 times, and then purged with hot air at 80°C to obtain a bright silver film.

Step S4: immersing the silver film obtained in step S3.2 in an ethanol solution for 5 minutes, stirring at a speed of 500 r/min, to obtain a flaky silver powder mother solution;

Step S5: The flaky silver powder mother liquor is placed in a high-energy ultrasonic pulverizer for pulverization, the ultrasonic power is set to 480 W, and the ultrasonic time is set to 10 minutes to obtain flaky silver powder with an average particle size of 13 µm.

Example 8

A method for preparing flaky silver powder using shellac comprises the following steps:

Step S1: Preparation of natural resin sacrificial template:

Step S1.1: preparing a shellac solution, dissolving shellac and ethyl cellulose in an ethanol solution at a mass ratio of 8:2 to prepare a shellac solution with a solid content of 15 wt %;

Step S1.2: Print the shellac solution on the flexible PET substrate by screen printing, and place it in a 30°C oven for curing for 10 min to form a natural resin sacrificial template.

Step S2.1: Hydrophilic treatment: Place the natural resin sacrificial template in a 0.35 mM SDS solution, take it out after 5 min, and rinse it with ultrapure water;

Step S2.2: Sensitization treatment: Place the natural resin sacrificial template treated in step S2.1 into an acidic _SnCl2 solution, take it out after 5 minutes, and rinse it with ultrapure water; the _SnCl2 solution is a mixture of 10 mL/L hydrochloric acid and 44.3 mM _SnCl2 .

Step S3.1: Prepare a silver ammonia solution according to the following preparation method: add 28 wt% ammonia water to 58.8 mM AgNO ₃ solution, observe that the solution changes from transparent to turbid, and continue to add until the solution becomes transparent. At this time, add 1 M NaOH solution to make the solution turbid again, and then add 28 wt% ammonia water to obtain a transparent silver ammonia solution;

Step S3.2: Sodium potassium tartrate and glucose in a mass ratio of 1:3 are dissolved in ultrapure water, and sprayed onto the natural resin sacrificial template simultaneously with the silver ammonia solution in a volume ratio of 1:1, and at the same time, the template is evenly purged with an air compressor. After reacting for 5 minutes, the template is rinsed with ultrapure water 3 times, and then purged with hot air at 85°C to obtain a bright silver film.

Step S4: The bright silver film obtained in step 3.2 is immersed in a 1 M NaOH solution at 50°C for 10 minutes and stirred at a speed of 500 r/min to obtain a flaky silver powder mother solution.

Step S5: The flaky silver powder mother liquor is placed in a high-energy ultrasonic pulverizer for pulverization, the ultrasonic power is set to 480 W, and the ultrasonic time is set to 40 minutes to obtain flaky silver powder with an average particle size of 9.1 µm.

Step S6.1: Take 0.162 g of the silver powder in S5, 0.164 g of polydimethylsiloxane (main agent: curing agent = 10:1) and 0.09 g of 4-methyl-2-pentanone and mechanically mix them for 30 min.

Step S6.2: A conductive elastomer film was deposited on a polydimethylsiloxane-based substrate through a mask, and vacuumed at -8 kPa for 20 min to eliminate bubbles, and cured at 145 °C for 4 h to obtain a conductive composite material with a solid content of 49.7 wt %.

Step S6.3: The conductive composite material obtained in step S6.2 was tested by a four-probe tester, and its conductivity was found to be 9353.1 S/cm.

Comparative Example 1

The difference between this comparative example and Example 7 is that:

Step S1: Preparation of acrylic resin sacrificial template:

Step S1.1: dissolving the heat-shrinkable acrylic resin in a cyclohexanone solution and diluting it to 15 wt %;

Step S1.2: Print the diluted acrylic resin on the flexible PET substrate by screen printing, and place it in an 80°C oven for curing for 1 h to form an acrylic resin sacrificial template.

Step S4: immersing the silver film obtained in step S3.2 in an acetone solution for 15 minutes to obtain a mother solution of flaky silver powder;

Step S5: The flaky silver powder mother liquor is placed in a high-energy ultrasonic pulverizer for pulverization, the ultrasonic power is set to 480 W, and the ultrasonic time is set to 50 minutes to obtain flaky silver powder with an average particle size of 7.5 µm.

The remaining steps are the same as those in Example 7.

The silver powder prepared in Example 7 and Comparative Example 1 was subjected to SEM test, and the results are shown in FIG9 , where FIG9a and FIG9b are the silver powder prepared in Example 7, and FIG9c and FIG9d are the silver powder prepared in Comparative Example 1. By comparing FIG9a and FIG9b with FIG9c and FIG9d, it can be seen that when the magnification is the same, the surface of the silver powder prepared in Example 7 is smooth and flat, while the surface of the silver powder prepared in Comparative Example 1 is relatively rough, and the surface of the silver powder is uneven, which is more obvious after comparison and magnification. The large roughness and unevenness of the silver powder surface will inevitably lead to fluctuations in the thickness of the silver powder and reduced uniformity.

From the perspective of the materials used, it can be found that the shellac, ethyl cellulose and ethanol used in Example 7 are all environmentally friendly materials or reagents; while in the corresponding process, acrylic resin, acetone and other reagents are used in Comparative Example 1. In Example 7, the curing temperature is 25°C and the curing time is 20 min; in Comparative Example 1, the curing temperature is 80°C and the curing time is 1 h. In Example 7, the curing temperature and time of the oven used for curing are lower, and the energy consumption is lower.

Comparative Example 2

In this comparative example, the flaky silver powder was prepared by mechanical ball milling.

Step S1: preparing spherical silver powder as a precursor:

Step S1.1: Weigh 4 g of polyvinyl pyrrolidone and dissolve it in 200 ml of ethylene glycol, add 1 g of AgNO ₃ and stir thoroughly to dissolve to prepare a precursor solution;

Step S1.2: Transfer the precursor solution to a 500 ml flask, add a magnetic bar, place it in a 160°C oil bath, set the stirring speed to 300 r/min, and react at a constant temperature for 60 minutes.

Step S1.3: The silver powder obtained after the reaction in step S1.2 was washed three times with ethanol, centrifuged at 8000 r/min, and vacuum dried at 40 °C for 3 h to obtain spherical silver powder. The morphology was tested by SEM. The results are shown in Figure 10.

Step S2.1: 25 g of stainless steel grinding balls were placed in a 100 ml ball mill, and 2.5 g of spherical silver powder, 0.05 g of calcium stearate, 0.05 ml of oleic acid and 3 ml of ethanol were placed in the ball mill in sequence as ball milling media. The ball mill was then sealed;

Step S2.2: Fix two ball milling jars on the ball mill facing each other, and pre-mill at a speed of 50 r/min for 0.5 h;

Step S2.3: The ball mill speed was reset to 250 r/min and the ball milling was continued for 24 h.

Step S3: The ball-milled material in step 2.3 was washed with ethanol three times and vacuum dried at 50°C for 2 h to obtain flaky silver powder with a particle size of 2-6 µm. The morphology was tested by SEM, and the results are shown in FIG11 .

Step S4.1: Take 0.651 g of the silver powder in S3, 0.202 g of polydimethylsiloxane (main agent: curing agent = 10:1) and 0.20 g of 4-methyl-2-pentanone and mechanically mix them for 30 min.

Step S4.2: A conductive elastomer film was deposited on a polydimethylsiloxane-based substrate through a mask, and vacuum was drawn at -8 kPa for 20 min to eliminate bubbles. The film was cured at 145 °C for 4 h to obtain a conductive composite material with a solid content of 76.2 wt %.

Step S4.3: The conductive composite material obtained in step S4.2 was tested by a four-probe tester, and its conductivity was found to be 7486.3 S/cm.

By comparing the conductive properties, it can be seen that the conductivity of the conductive composite material prepared in Example 8 is as high as 9353.1 S/cm when the silver powder solid content is 49.7 wt %, and the conductivity of the conductive composite material prepared in Comparative Example 2 is 7486.3 S/cm when the silver powder solid content is 76.2 wt %. It can be seen that when the solid content of the silver powder prepared in Example 8 of the present invention is reduced by about 34wt %, the conductivity of the conductive composite material is greatly improved. As is known to all, the cost of the conductive composite material mainly comes from the price of silver powder. Using the flaky silver powder prepared by the present invention as a conductive filler can reduce the production cost by about 34% while improving the conductive properties.

The above-mentioned embodiments only express the specific implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the protection scope of the present application. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the technical solution concept of the present application, and these all belong to the protection scope of the present application.

Claims (10)

1. A method for preparing flake silver powder by using lac, which is characterized by comprising the following steps:

step S1: preparing a natural resin sacrificial template: dissolving lac in ethanol to prepare a lac solution; coating the lac solution on a flexible substrate, and curing;

step S2: carrying out hydrophilic treatment and sensitization treatment on the natural resin sacrificial template;

step S3: preparing a silver film: preparing silver-ammonia solution, spraying the silver-ammonia solution and the reducing solution on a natural resin sacrificial template simultaneously for reaction, uniformly purging, flushing a silver film with ultrapure water, and purging at a high temperature of 60-90 ℃ to obtain a bright silver film on the natural resin sacrificial template;

step S4: immersing a natural resin sacrificial template plated with a silver film into an alcohol solution, a ketone solution or an alkaline solution, and stirring to obtain a flaky silver powder mother solution;

step S5: crushing the silver flake mother solution by adopting ultrasonic, and soaking in ethanol, centrifuging and drying to obtain the silver flake.

2. The method for preparing flake silver powder by using shellac according to claim 1, wherein the shellac solution in the step S1 is prepared by dissolving shellac and ethyl cellulose in ethanol, the solid content of the shellac solution is 15 wt%, and the mass ratio of shellac to ethyl cellulose is 9:1-7:3.

3. The method for preparing plate-like silver powder using shellac according to claim 2, wherein the mass ratio of shellac to ethylcellulose is 7:3.

4. The method for preparing flake silver powder by using shellac according to claim 1, wherein in the step S3, the reducing solution is one or two of glucose and sodium potassium tartrate, and when the reducing solution is a mixture of sodium potassium tartrate and glucose, the mass ratio of sodium potassium tartrate to glucose is: 5:8-1:8.

5. The method for preparing flake silver powder by using shellac according to claim 4, wherein in the step S3, when the reducing solution is a mixture of potassium sodium tartrate and glucose, the mass ratio of potassium sodium tartrate to glucose is: 1:7.

6. The method for preparing plate-like silver powder using shellac according to claim 1, wherein the reaction time for spraying onto the natural resin sacrificial template for the reaction in step S3 is 5 to 8 minutes.

7. The method for preparing plate-like silver powder using shellac according to claim 1, wherein in step S4, the natural resin sacrificial template coated with the silver film is immersed in an ethanol solution for 1 to 5 minutes with a stirring speed of 500 to 1000 r/min.

8. The method for preparing plate-like silver powder using shellac according to claim 7, wherein the immersion time of the natural resin sacrificial template coated with the silver film in the ethanol solution is 2 minutes and the stirring speed is 700 r/min in step S4.

9. The method for preparing plate-like silver powder using shellac according to claim 1, wherein the high-temperature purging is performed at 80 to 85 ℃ in step S3.

10. The application of lac in preparing flake silver powder.