China Powder Network News "The Luoshan jade belt is the most romantic, and the silver grate is inserted obliquely to wrap the head slowly." Silver usually appears in everyone's sight in the form of jewelry. But in industry, silver powder can be used in electrical and electronic equipment, solder, catalyst, medicine and health, packaging, wood preservation, water purification, solar cell silver paste and other fields. Especially in the electronics industry, silver powder is the most widely used precious metal powder, mainly used in electronic paste.
Image source: Xi'an Hongxing Electronic Paste
Electronic paste is a kind of viscous paste prepared from conductive phase powder, binder, solvent and auxiliary agent in a certain proportion. It is an electronic functional material integrating material, metallurgy, chemical industry and electronic technology. Due to its good electrical conductivity, silver powder plays a central role as a conductive phase in electronic paste, and its morphology, structure and particle size characteristics will affect the performance of the paste. Therefore, the preparation technology of silver powder is particularly important.
Silver powder classification
Silver powder is mainly divided into flake silver powder, spherical silver powder and dendritic silver powder. The SEM images are as follows:
From top to bottom are the SEM images of flake silver powder, spherical silver powder and dendritic silver powder
◆The spherical silver powder has high sphericity, and the silver paste prepared from the spherical silver powder has good fluidity and can pass through the fine grid lines of the positive electrode well. The properties of spherical silver powder can meet the demand of front silver paste for silver powder. The study found that different production methods of spherical silver powder and surface treatment can affect the performance of silver paste. In order to better meet the needs of front silver paste, spherical silver powder is currently developing towards a high degree of spheroidization and controllable smoothness.
◆Flake silver powder is mainly produced by processing spherical silver powder. Due to its unique two-dimensional structure, the contact area of this type of silver powder in the silver paste is larger than that of the silver powder with other morphologies, and the obtained silver paste has lower resistance and better electrical conductivity. At the same time, the flake silver powder has a flake structure in the silver paste, which can improve the compactness of the sintering of the silver paste. At the same time, the surface area of the flake silver powder is larger than other silver powders, which means that the silver paste made of the same quality flake silver powder has a larger coating area, so that it can reduce the silver content in the silver paste and the coating thickness at the same time. maintains good electrical conductivity. As the raw material of back silver paste, flake silver powder can keep low resistivity while reducing the cost of silver paste. However, due to the poor fluidity of the prepared silver paste, it cannot be used for positive silver electrodes with extremely thin grid lines.
◆Dendrite silver powder is formed by the spontaneous aggregation of silver powder particles into a highly ordered dendritic structure. Some scholars have studied the application of dendritic silver powder in silver paste and found that dendritic silver powder is not suitable for conductive silver paste. According to research, the thick film formed by sintering the silver paste prepared from dendritic silver powder is too loose and has poor electrical conductivity. This is because the surface energy of the dendritic silver powder is too large and it is easy to agglomerate, which leads to the poor ability of the silver paste made from the dendritic silver powder to pass through the screen during printing, and the silver paste shrinks seriously during sintering. The performance is also very poor, and dendritic silver powder is generally not used in the silver paste of solar cells.
Silver powder for electronic silver paste
At present, the silver powder used in electronic paste is mostly ultrafine silver powder, and its morphology is generally spherical silver powder and flake silver powder. If the particle size of the silver powder is too large or too small, the properties and conductivity of the silver paste will be affected, so it is necessary to control the particle size of the silver powder within an appropriate range. In order to achieve a better effect, a single particle size of silver powder cannot be used in the preparation of silver powder, and large particle size and small particle size silver particles are required to be filled with each other to ensure sufficient contact area. When formulating silver paste, mixed silver powder with a particle size ranging from 0.2 to 4 μm is generally used to ensure that small particles of silver powder can fill the gaps between large particles of silver powder, so that the formed silver paste has better compactness.
Preparation technology of silver powder for electronic silver paste
According to the essence of the milling process, the preparation methods of silver powder can be roughly divided into physical methods and chemical methods.
1 Physics
The physical method only produces physical changes to the silver powder, and the trace chemical reaction is caused by local oxidation. The physical method overcomes the metal bond force between the silver crystals and the van der Waals force between the particles, does not introduce other impurities, and divides the silver crystal into fine particles infinitely with the help of the energy provided by the external force. In the preparation of ultrafine silver powder, the commonly used physical methods include laser ablation, atomization, and high-energy ball milling.
1.1 Laser ablation method
A pulsed laser is used to ablate a metallic silver target to obtain nano-scale particles dispersed in a liquid. The laser ablation technology is simple and fast, the prepared particles are pure, and the stability and controllability are high. Figure 1 is a schematic diagram of a nanosecond laser ablation device. A laser generator is used to hit a continuous focused laser on a pure silver plate. The pure silver plate is placed in a liquid environment containing a dispersant, and the laser bombards the target. The nanoparticle is sputtered out of the material, and finally dispersed uniformly in the liquid environment.
Figure 1 Schematic diagram of a nanosecond laser ablation
1.2 High energy ball milling
High-energy ball milling is a mechanical pulverization method. Small pieces of material or pre-treated coarse powder are crushed, crushed and ground by a ball mill to obtain finer metal powder or alloy powder, which is the main method for preparing flake silver powder. When the ball mill rotates, under the action of centrifugal force, the ball falls naturally under the gravity as the cylinder rises to the high point, and the material is crushed by the friction between the ball and the ball and the impact of the falling ball.
1.3 Atomization method
The atomization method is to crush the molten metal liquid in the atomization device through the high-pressure air flow, and the crushed metal liquid splashes into countless small spherical particles, and finally the powder is collected by the cooling medium. The metal powder obtained by the ordinary atomization method is relatively coarse, generally 0.5~1mm. In order to obtain finer ultra-fine powder, the molten metal flows from the liquid container into the chute, and then the liquid is sent to the flowing conveyor belt from the chute. , the flow liquid is crushed and falls into the cooling medium. The methods of atomization include gas atomization, water atomization, centrifugal atomization, and vacuum atomization.
2 Chemical method
The agglomeration of powder is the biggest problem to be solved in the milling process. The superfine powder has a large surface area and will spontaneously agglomerate to reduce the surface area and achieve a stable state. The cost of preparing metal powders by physical methods is relatively high. The ball milling method is easy to introduce new impurities and the particle size of the ball milling is limited. The laser ablation method cannot be used for large-scale production, and the process is complicated and costly. The atomization method is used for mass production of coarse powder. Not suitable for fine powders. Silver powder for electronic paste has strict performance requirements, requiring regular morphology, small particle size, uniform particle size distribution, high dispersibility and high tap density.
There are many controllable factors in the chemical method, and silver powder with different properties can be produced by controlling the reaction conditions. Nucleation and nucleation growth are required in the phase transition process. Currently, there are various methods and theories to control the nucleation and growth processes in the phase transition. The main chemical methods for preparing ultrafine silver powder are: liquid-phase reduction method, liquid-phase precipitation conversion method, and microemulsion method. silver powder.
2.1 Liquid phase reduction method
The method is that a reducing agent is added to the silver salt solution, and the silver ions are reduced to silver element by controlling the conditions of the reduction reaction. The silver salt is completely dissolved in the liquid to form a homogeneous liquid phase, and the reducing agent can be selected from inorganic or organic reducing agents. In order to prepare ultrafine silver powder, a certain dispersant or protective agent is often added to the reduction system to reduce the agglomeration of silver particles. The liquid-phase chemical reduction method uses silver nitrate solution or silver ammonia solution as the oxidant precursor, and reduces the silver particles by controlling the temperature and pH of the reaction process. After the reduction, the obtained silver powder was filtered, washed and dried to obtain silver powder.
2.2 Microemulsion method
Microemulsion is formed by two immiscible liquids, and is a thermodynamically stable, isotropic, transparent or translucent dispersion system, one or two microscopically stabilized by a surfactant interface film. Liquid droplets. These stable droplets constitute a tiny reactor with a diameter of between tens of nanometers. When the reducing agent solution and the silver nitrate solution are mixed in these tiny reactors at the same time, the nano-silver powder can be spontaneously reduced, and the micro-droplet film is surrounded by the surfactant, so the formed nano-silver nuclei will not diffuse outward and agglomerate. The microemulsion method is a special method in the liquid phase reduction method. Both of them are reactions that occur in a pure liquid phase system, but the microemulsion method is a combination of two immiscible solvents. The silver powder prepared is as follows. Nano-scale, the particle size is more fine and uniform.
2.3 Liquid-phase precipitation conversion method
In the method, silver nitrate is converted into silver precipitates such as silver chloride, silver carbonate, silver acetate and silver oxide, and the precipitates are reduced in a liquid phase environment by adding a reducing agent to obtain silver powder. Silver powder can also be directly decomposed by thermal decomposition method. For example, silver chloride begins to decompose at 200 °C, and the decomposition is the most violent at 400 °C. After more than 500 °C, there is basically no silver chloride remaining, and the temperature rises to cause volatilization. In order to prevent high temperature volatilization Smelting agent sodium carbonate can be added.
Compared with the pure liquid-phase reduction method, the liquid-phase precipitation conversion method can reduce the reduction electrode potential of silver, so that most reducing agents can reduce silver powder. The precipitation phase as the parent phase provides the nucleation site and nucleation energy, and it is easier to precipitate silver powder. The precipitation transformation method can add a protective agent during the preparation of the precipitation, and strictly control the particle size and morphology of the precipitation. The binding ability of silver ions and other anions is very strong, and the formed precipitates agglomerate into flocs, which will disperse when reducing silver powder.
summary
So far, silver powder has achieved many achievements in the research of conductive silver paste, but there are still many problems to be explored. In addition, the silver powder prepared by different process methods has its own advantages and disadvantages, so researchers need to pay attention to the process optimization of silver powder, in order to prepare silver powder with better performance to meet the growing demand of electronic silver paste.
Reference source:
[1] Liu Zhongqi, Liu Chunsong, et al. Review of the preparation process of silver powder for electronic paste. 2014.
【2】Dong Ge, et al. Effect of silver powder properties on solar cell paste. Functional Materials. 2021.
【3】Su Shaojing. Preparation of silver powder for electronic paste by chemical reduction method and research on its properties. 2018.
(Edited by China Powder Network/Xingyao)
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