Study On High-Quality Silicon Nitride Ceramic Powder Production

Silicon nitride（Si₃N₄）has excellent properties such as high temperature and wear resistance,high hardness and strength,small friction coefficient,good chemical stability and high resistivity.High-performance silicon nitride ceramics have been widely used in new energy vehicles,aerospace and other industries.However,the key production technologies are still controlled by foreign countries,and high qualified powders rely on imports.So it is of great significance to carry out research on high-quality silicon nitride ceramic powders.In this thesis,silica fume and silica were used as raw materials to prepare high-quality Si₃N₄powder by direct nitriding method and carbon-thermal reduction method.Aiming at the problems existing in industrial production such as uneven particle size,low mass fraction ofαphase and poor dispersion,the effects of reaction raw materials,nitriding conditions and diluent addition amount on silicon nitride ceramic powder were discussed,and the preparation conditions were optimized to prepare high-quality silicon nitride powder with uniform particle size,high mass fraction ofαphase and good dispersibility.Direct nitriding method was applied by using silica fume as raw materials.The effects of silica fume particle size,nitriding temperature,nitriding time,nitrogen flow,heating rate,and diluent dosage on silicon nitride ceramic powders were explored.Laser particle size analyzer,X-ray diffraction,nitrogen isothermal adsorption and desorption,scanning electron microscopy and other test technologies were used to characterize and analyze the prepared materials in order to optimize the preparation conditions.The results showed that the silica fume raw materials with a small D50（275.36 nm）were obtained by using a ball-to-material ratio of 50:1,a ball milling speed of 500 r/min,and a ball milling time of 8 h,and when the reaction temperature was 1400℃,the heating rate was 5℃/min（between 1100-1400℃）,the nitriding time was 3 h,and the nitrogen flow rate was 0.8L/min.The produced silicon nitride material has spherical particles,αphase mass fraction of 95.02%,good dispersion,a particle size range of 396-458 nm,and a uniform particle size.Carbon-thermal reduction method was applied by using silica as raw materials.The effects of silica particle size,carbon-silicon ratio,nitriding temperature,nitrogen flow,heating rate,and diluent dosage on silicon nitride ceramic powder were explored.Laser particle size analyzer,X-ray diffraction,scanning electron microscopy,and other test technologies were used to characterize and analyze the prepared materials in order to optimize the preparation conditions.The results showed that the spherical Si O₂powder with a particle size of 500 nm,uniform particle size,and good dispersibility were prepared by adjusting the amount of ammonia by sol-gel method.Then the Si O₂mixture completely wrapped by carbon particles were obtained by ball milling.Due to the contact area between Si O₂and carbon particles,the nitriding efficiency of Si O（g）is improved.When C/Si O₂=3,reaction temperature 1400℃,heating rate 5℃/min（between 1100-1400℃）,nitriding time 6 h,nitrogen flow rate 0.8 L/min.The produced silicon nitride powder had prismatic particles,αphase mass fraction of 96.38%,good dispersion,complete nitriding,and a particle size range of 531.2-712.4 nm.The findings demonstrated that the inclusion of a Si₃N₄diluent can lower the nitriding temperature,encourage an early interface response,promote the nitriding reaction,and regulate grain morphology and size.The heating rate is the key factor to control the reaction process,when the heating rate is too fast or too slow,the conversion degree of silicon nitrideαphase toβphase exceeds the internal reaction degree,and the silica fume reaction is incomplete,therefore,the appropriate nitriding temperature and heating rate are the key to optimizing the process parameters.The research results provide a technical basis for process optimization in industrial production.