Study On Preparation And Properties Of High Performance Silicon Nitirde

Silicon nitride ceramics possessing good wear and corrosive resistance, hightemperature stability and thermal shock resistance, are considered as structuralmaterials for wide applications in many fields such as aerospace, mechanicalindustry, electronic power industry and chemical industry. The materialperformance of silicon nitride ceramics can be enhanced effectively by usingsuitable sintering additives, which can improve the thermal conductivity and thefracture toughness of Silicon nitride ceramics.In this thesis, dense silicon nitride ceramics with different amounts ofsintering additives, which uses α-Si₃N₄powders as raw materials and CeO₂assintering additives, were prepared by hot pressed sintering at maximumtemperature of1800℃for an hour under uniaxial pressure of30MPa. The effect ofsintering additives content on grain boundary phase composition, phasetransformation and the grain size of silicon nitride ceramics were studied throughcharacterization of phase identification and microstructure analysis. The influenceof sintering additives content on mechanical properties and the thermalconductivity of silicon nitride ceramics were investigated through systematicalmaterial property tests.The results show that Si₃N₄ceramics sintered with CeO₂additive can achievea complete densification. Ce_4.67（SiO₄）₃O, Si₂ON₂and Ce₂Si₂O₇were identified asgrain boundary phase of Si₃N₄ceramics when the content of CeO₂is less than8mol%, and the grain boundary phase composition varies regularly with increasingCeO₂content. The grain size of Si₃N₄ceramics changes slightly with increasingCeO₂content, while the number of elongated grains increased by the addition ofCeO₂. The effect of microstructure and grain boundary phase on the relativedensity, strength, hardness and fracture toughness of Si₃N₄ceramics depends onthe content of CeO₂, and the content variation of CeO₂has a significant influenceon mechanical properties of Si₃N₄ceramics. The thermal diffusion coefficient andthermal conductivity of Si₃N₄ceramics increase with CeO₂content when it’sbelow7mol%. The thermal diffusion coefficient increases by50%, and the thermalconductivity increases by38.7%when the amount of CeO₂increased from1mol%to7mol%. Si₃N₄transports heat primarily by phonons, the thermal conductivity ofSi₃N₄ceramics at room temperature relies on the purification of grains.