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The production process of silicon nitride products

wallpapers News 2021-12-08
Silicon nitride products can be divided into reaction sintered products, hot-pressed products, atmospheric sintered products, isostatic pressure sintered products, and reactive re-fired products according to the process. Among them, reaction sintering is a commonly used method for producing silicon nitride refractory products.
The reaction sintering method to produce silicon nitride products is to use finely ground silicon powder (with a particle size generally less than 80μm) to be molded by machine pressing or isostatic pressing. After the green body is dried, it is heated to 1350~1400℃ in nitrogen. During the sintering process. It is made by nitriding at the same time. With this production method, the raw material conditions, firing process, and atmosphere conditions have a great influence on the performance of the product.
Silicon powder contains many impurities, such as Fe, Ca, Aì, Ti, etc. Fe is considered to be the catalyst in the reaction process. It can promote the diffusion of silicon, but at the same time, it will also cause defects such as pores. The main function of Fe as an additive: It can be used as a catalyst in the reaction process to promote the formation of SiO2 oxide film on the surface of the product; the formation of iron-silicon fusion system, nitrogen dissolves in liquid FeSi2 and promotes the formation of β-Si3N4. However, if the iron particles are too large or the content is too high, defects such as pores will also appear in the product, which will reduce the performance. Generally, the amount of iron added is 0-5%. Impurities such as Al, Ca, and Ti can easily form eutectic with silicon. Appropriate addition can promote sintering and improve the performance of products.
The finer the particle size of the silicon powder and the larger the specific surface area, the firing temperature can be lowered. Compared with silica fume with coarser particle size, the content of α-Si3N4 in the product is higher for the finer particle size silicon powder. Reducing the particle size of silicon powder can reduce the size of the microscopic pores of the product. Proper particle size ratio can increase product density.
The temperature has a great influence on the nitriding rate. The nitridation reaction starts at 970-1000°C, and the reaction rate increases at around 1250°C. In the high-temperature stage, due to the exothermic reaction, if the temperature quickly exceeds the melting point of silicon (1420°C), the flow of silicon will easily occur, which will cause the silicon powder body to melt and collapse.

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