Silicon nitride ceramic is a kind of inorganic nonmetallic strong covalent bond compound with hexagonal crystal structure. There are two kinds of isomers, α-silicon nitride and β-silicon nitride.

Because of its strong binding force between N atoms and Si atoms, silicon nitride has excellent mechanical properties, such as: high strength, good toughness, good stability, stable chemical properties, good insulation, high hardness, etc., has been widely used It is used in high-temperature engine, high-speed cutting and other fields.

However, the sintering of silicon nitride is more difficult, often requiring a higher sintering temperature, and even sintering under high-pressure conditions, which wastes resources and increases costs. In order to reduce the sintering temperature and improve the sintering activity, high-purity, ultra-fine silicon nitride powder is required.

This article takes the preparation of ultra-fine silicon nitride powder as the starting point, summarizes the main preparation methods of current silicon nitride powder, and analyzes and compares the advantages and disadvantages of various preparation methods and the application status in detail.

There are many preparation methods for silicon nitride powder, which can be divided into three categories: solid phase reaction method, liquid phase reaction method, and gas phase reaction method.

Solid phase reaction

01 Direct Nitriding

This method uses nitrogen-containing atmospheres such as nitrogen and ammonia to directly react with silicon powder at high temperature (above 1400°C) to form silicon nitride. This method is the earliest method for preparing silicon nitride, and the preparation process is relatively mature. It is currently the main method for industrial production of silicon nitride.

For example, the domestic Ziguang Fangda High-tech Ceramics Co., Ltd. and the German Starck company use this method.

02 Thermal Reduction Method

This method is a method of mixing carbon and silicon dioxide, introducing nitrogen gas in a vacuum environment, and reacting to form silicon nitride under the action of high temperature.

However, excess carbon may cause carbon or silicon carbide inclusions in the product. Therefore, the amount of carbon added is a crucial factor in the use of carbon thermal reduction method.

03Self -spreading method

The self-propagating method is also called the self-propagating high-temperature synthesis method (SHS). The United States, Japan, etc. are also called combustion synthesis technology, which is an emerging preparation method in recent years.

The principle of SHS is that after the raw materials are ignited, the reaction diffuses to the unreacted area until the reaction is complete. The characteristic of this method is that the reaction time is extremely short (usually in seconds), no pollution, no other impurities are produced, and the synthesized product has high purity. 

Liquid phase reaction

01 Thermal decomposition method

Thermal decomposition method, also known as silicon tetrachloride (SiCl ₄ ) method. This method is mainly used to prepare silicon nitride whiskers.

The characteristic of the thermal decomposition method is that the reaction is intense, the production is rapid, and high-purity silicon nitride can be obtained, which is the main method for producing silicon nitride whiskers. The earliest silicon nitride whiskers that can be truly scaled and industrially produced with excellent performance and stable quality are the amino silicon thermal decomposition method invented by UBE Japan.

02 Sol-gel method

The sol-gel method (sol-gel) is a new type of material preparation method.

However, the raw materials of this method are all organic compounds, and the cost is relatively high, which is not suitable for large-scale industrial production. At present, it is only suitable for laboratory preparation.

01High temperature gas phase reaction method

The high-temperature gas-phase reaction method (CVD) is a method of exciting a gas-phase reaction at high temperature to synthesize high-purity, ultra-fine silicon nitride.

Although the CVD method can produce high-purity, ultra-fine silicon nitride, it is currently limited to the laboratory stage. The main reason is that the silicon nitride obtained by this method has less alpha phase, poor powder sintering performance, and low production efficiency. Not suitable for the production of silicon nitride ceramic materials.

02 Laser gas phase reaction method

The laser gas phase reaction method is a method of irradiating the reaction gas with a laser beam to induce a chemical reaction.

The principle of using this method to prepare silicon nitride is to use the strong absorption effect of silicon tetrahydride gas on the carbon dioxide laser beam, and use a continuous carbon dioxide laser beam to irradiate the mixed gas of silicon tetrahydride and ammonia gas to cause laser pyrolysis and synthesis reaction. Thus, a spherical amorphous silicon nitride powder with ultrafine and uniform particle size distribution is obtained.

03 Plasma gas phase reaction method

The plasma gas phase reaction method is a method for preparing silicon nitride powder by using high-frequency induction plasma as a heat source to excite the reaction gas.

The plasma used to prepare silicon nitride mainly includes direct current plasma, high frequency plasma and mixed plasma. One of the most commonly used is high-frequency pure nitrogen plasma as a heat source.

Compared with other preparation methods, the plasma gas phase reaction method has a rapid reaction, and the silicon nitride powder prepared by the method has high purity, small particle size, narrow distribution, and loose powder. It is an excellent laboratory method for preparing ultrafine nitrides. However, the raw material problem is still relatively difficult to solve, and the raw material is a toxic chemical substance. It is still difficult to achieve industrialization and the output is still insufficient.

Application of silicon nitride ceramics

1 Refractory

Si ₃ N ₄ ceramics have excellent mechanical properties and high temperature properties, and are often used as refractory materials. For example, in the metallurgical industry, it is used to prepare ceramic boats, crucibles, evaporating dishes, or high-temperature furnace shafts; in the aerospace industry, it is used to manufacture rocket nozzles, throat liners, and other high-temperature structural heat-resistant components.

2 ceramic cutter

Because of its high hardness, high strength, high heat resistance, high thermal shock resistance, and high chemical stability, silicon nitride ceramics make it the best material for cutting tools. At present, silicon nitride ceramic tools are widely used in industrial production.

3 ceramic engine

Si ₃ N ₄ ceramic material has the advantages of low density, high hardness, low thermal expansion coefficient, high temperature resistance, wear resistance, good thermal stability and chemical stability, etc. It is an ideal material for preparing engines. The ceramic engine made of silicon nitride has the advantages of high thermal efficiency, simple structure, high working temperature and low noise.

At present, Si ₃ N ₄ ceramic materials are used to manufacture components on automobile engines, such as glow plug combustion chambers, nozzles, connecting rods, etc.

Conclusion

For the preparation of silicon nitride powder, the current main problem is how to mass-produce high-purity and ultra-fine nano-sized silicon nitride powder on an industrial scale.  
The key points of future research include: on the one hand, for the existing industrial preparation methods, it is necessary to optimize the preparation process, simplify the preparation process, and strictly control the quality of the raw materials, thereby improving the quality and production efficiency of the powder; on the other hand, to increase the The laboratory uses new methods of research and strives to promote its industrial application.

Reference source

Xu Chenhui et al. Research progress in preparation of silicon nitride ceramic powder

Chen Li, Feng Jian. Research status and application of silicon nitride ceramic materials

Liu Xiongzhang. Preparation of Nanometer Silicon Nitride Powder and Analysis of Its Micromorphology

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