Silicon nitride ceramic technology
Silicon nitride ceramic is a kind of inorganic material ceramic which does not shrink during sintering. The strength of silicon nitride is very high, especially the hot pressed silicon nitride, which is one of the hardest materials in the world. It has the properties of high strength, low density and high temperature resistance.
Si3N4 ceramic is a kind of covalent bond compound. Its basic structural unit is [SiN4] tetrahedron. The silicon atom is located in the center of the tetrahedron. There are four nitrogen atoms around the tetrahedron, which are located at four vertices of the tetrahedron. Then, in the form of three tetrahedrons sharing one atom, a continuous and solid network structure is formed in three-dimensional space.
The preparation technology of silicon nitride ceramics has developed rapidly in the past few years. The preparation technology mainly focuses on reaction sintering, hot pressing sintering, atmospheric pressure sintering and air pressure sintering. Due to different preparation technology, each type of silicon nitride ceramics has different microstructure (such as porosity and pore morphology, grain morphology, intergranular morphology and intergranular second phase content). Therefore, each type of silicon nitride ceramics has its own characteristics In order to obtain Si3N4 ceramic materials with excellent properties, high-quality Si3N4 powders should be prepared first. The quality of Si3N4 powders prepared by different methods is not the same, which leads to the difference in application. The failure of application of many ceramic materials is often attributed to the developer's lack of understanding of the differences between various ceramic powders and their properties It is said that high-quality Si3N4 Powder should have the characteristics of high content of α phase, uniform composition, few impurities, uniform distribution in ceramics, small particle size, narrow particle size distribution and good dispersion. The α phase in good Si3N4 Powder should account for at least 90%, which is because part of α phase will turn into β phase in the sintering process of Si3N4, but not enough α phase content, which will reduce the strength of ceramics.
Silicon nitride ceramic process:
It uses silicon powder as the raw material, first uses the usual forming method to make the required shape, carries on the preliminary nitriding in the nitrogen and under the high temperature of 1200 ℃, causes one part of silicon powder to react with nitrogen to form silicon nitride, at this time, the whole body has certain strength. Then the second nitriding was carried out in a high temperature furnace at 1350 ℃ ~ 1450 ℃ to form silicon nitride. Silicon nitride with a theoretical density of 99% can be prepared by hot pressing sintering.