Application & preparation method of superfine silver powder
Silver is a chemical element and a transition metal. In nature, it mainly exists as silver compound ore. In industry, according to the particle size classification, silver powder can be divided into the following categories: fine silver powder, ultra-fine silver powder, ultra-fine silver powder, and nano-silver powder. According to the morphology of superfine silver powder, it can be divided into spherical silver powder and flake silver powder.
The physical properties of silver
| Physical properties | Numerical value | Physical properties | Numerical value |
| Chemical formula | Ag | Heat of vaporization | 150.58KJ/mol |
| Atomic number | 47 | Heat of fusion | 11.3KJ/mol |
| Crystal structure | Face Centered Cubic (fcc) | Specific heat capacity | 232KJ/(Kg·K) |
| Lattice constant a | 0.40362nm | Reflectivity | 0.91 |
| Relative atomic mass | 107.88 | Conductivity | 6.301x107S/m |
| Atomic radius | 0.144nm | Thermal conductivity | 429W/(m·K) |
| Outer electronic structure | 4d105s1 | Moh's hardness | 2.5 |
| Main oxidation state | +1,+2,+3 | Vickers hardness | 251MPa |
| First ionization energy | 7.567 eV | Brinell hardness | 24.SHB Mpa |
| Electronegativity | 1.93 | Expansion coefficient (25℃) | 18.9μm/(m-K) |
| Water soluble | Insoluble in water | Young's modulus | 83Gpa |
| Relative density (water = 1) | 10.49 | Shear modulus | 30Gpa |
| Melting point | 961.93 ℃ | Bulk modulus | 100Gpa |
| Boiling point | 222.12℃ | Poisson's ratio | 0.37 |
Silver also has good electrical conductivity and chemical stability. Due to the difference in morphology and particle size of ultrafine silver powder, the surface atomic arrangement of its crystal structure changes accordingly, resulting in a large number of surface defects, making the material unsaturated and chemically active, and possessing: small size effect, quantum effect, and macroscopic quantum Tunnel effect, surface effect.
As a conductive phase, silver powder is used in electronic pastes, and its properties will have a great impact on the performance of conductive pastes, especially the front silver paste of solar cells. Its application performance depends largely on the silver powder used. Nature.
The dispersibility of silver powder has an important influence on the printing and sintering of the front silver paste and the conductivity of the battery. The particle size of the silver powder will affect its tap density, thus affecting the compactness of the silver paste after sintering. The morphology of silver powder will affect its specific surface area. Particles with a large specific surface area have large surface free energy and are in an unstable state. They tend to shrink during sintering, thereby affecting the performance of the conductive paste.
Application of superfine silver powder
- Application in optics
The photosensitive paste prepared by mixing photosensitive resin with ultrafine silver powder as conductive function is printed on the master plate. After exposure and etching, the electrode pattern is continuous, the line width is uniform, and the edge is straight. It has been used as the electrode material of plasma display. In preparation.
- Application in the field of electromagnetic shielding
Ultra-fine silver powder has high conductivity. In the electromagnetic field, it can reflect the electromagnetic waves that are propagating back to the original space, thereby playing the role of electromagnetic shielding. At the same time, due to the high conductivity of ultra-fine silver powder, the magnetic permeability is relatively low. Therefore, the electromagnetic shielding effect of ultrafine silver powder is more suitable for high-frequency magnetic fields, but not for low-frequency magnetic fields whose main shielding effect is absorption loss.
- Application in the biomedical field
Ultrafine silver powder has the ability to kill bacteria, which is largely caused by the small size effect of Ag+ in the solution and nanometer ultrafine silver powder. The high chemical activity can destroy the cell membrane of the virus and make some groups on the virus DNA Loss of activity, inhibits the reproduction of the virus to achieve a sterilization effect.
- Application in the field of catalysis
For nano-silver, the essence of its catalytic process is the chemical adsorption and desorption of oxygen by silver, which can be widely used in the field of drugs and chemicals for the epoxidation of olefins, and the field of supported silver catalysts for the selective oxidation of alcohols. The field of catalysts for reducing the NOX emitted from automobile exhaust to generate nitrogen; the field of fuel cells for selective oxidation of carbon monoxide and environmental pollution treatment fields.
- Application in the field of photovoltaic power generation
The cathode material of solar cells is usually composed of conductive silver paste prepared from micron-sized spherical silver powder. The conductive silver paste is screen-printed and attached to the solar crystalline silicon wafer to form a grid (anode) through high-humidity sintering, which can convert light energy into Electrical energy.
- Applications in the microelectronics industry
Because of its high electrical conductivity and excellent heat transfer, ultrafine silver powder has been widely used in the field of microelectronics industry, such as used as conductive connection and transmission media and various electronic pastes, etc., for the development of a new generation High-performance electronic components. Utilizing the quantum properties of silver nanowires, it can be used as a connecting wire for nanoscale devices, so as to meet the requirements of the connecting wire for large specific surface area, small diameter, and uniform orientation.
- Applications in other fields
Because of its excellent thermal and electrical conductivity, ultra-fine silver powder is used in automotive rear windshield defrosting heating resistance wires, etc.; nano-silver powder can promote cell repair and is often used in the field of rehabilitation after medical operations.
Preparation method of superfine silver powder
The preparation methods of ultra-fine silver powder can be divided into physical preparation methods and chemical preparation methods. The physical methods include mechanical ball milling, evaporation and condensation, DC arc plasma, laser ablation, and atomization. Chemical methods include sonochemical method, electrolysis method, liquid phase chemical reduction method, spray thermal decomposition method, and liquid phase precipitation conversion method.
Advantages and disadvantages of different physical methods to produce ultra-fine silver powder
| Physical preparation method | Advantages | Disadvantages |
| Mechanical ball milling | Simple process, low cost, suitable for large-scale production | Wide particle size distribution, uneven performance, low efficiency |
| Evaporative condensation method | Silver powder has high purity, uniform particle size and good crystallinity | High equipment requirements, difficult for industrial production |
| Laser ablation | The process is simple, the purity of silver powder is high, and the stability is good | High cost |
| Atomization | Silver powder has high purity and good crystallinity | Limited by equipment, only micron-level silver powder can be produced |
| DC arc plasma method | High purity of silver powder, high purity of silver powder | Wide particle size distribution, high requirements for equipment, high investment |
Advantages and disadvantages of different chemical methods for preparing ultrafine silver powder
| Chemical preparation method | Advantages | Disadvantages |
| Liquid phase chemical reduction | The process is simple, the price of raw materials is low, the energy consumption is small, the parameters are easy to control, and it is suitable for large-scale production | Difficulty in process improvement |
| Spray pyrolysis | Simple process, high production efficiency, environmentally friendly | Wide particle size distribution |
| Electrolysis | The process equipment is simple, the purity of silver powder is high, and the requirement for the silver content of the raw material is low | Process energy consumption is high, production cost is high |
| Microemulsion method | Silver powder has good dispersibility and particle size can be precisely controlled | Difficulty in solid-liquid separation |
Because the liquid-phase chemical reduction method has the advantages of simple process, low raw material price, low energy consumption, easy control of parameters, and suitable for large-scale production, the current industrial ultrafine silver powder is mainly prepared by the liquid-phase chemical reduction method.
In the preparation process of ultrafine silver powder by liquid-phase chemical reduction method, the main factors affecting the performance of ultrafine silver powder are the concentration of reactants, the type of reducing agent, the reaction temperature, the type of dispersant, and the pH value of the reaction system.
As the application of silver powder in solar energy, Internet of Things and other industries continues to increase, the position and role of silver powder as a supporting material for strategic emerging industries will continue to increase, and the consumption prospects are broad.
Article source: China Powder Network