| In this study, Spark Plasma Sintering （SPS） and following heat treatment wasadopted to fabricate silicon nitride ceramics with different kind and amount of rareearth fluoride additives. Phase composition, sintering properties, microstructure,thermal conductivity and mechanical properties of the samples were characterized.The influence of these factors on bulk density, thermal conductivity and flexurestrength of Si3N4ceramic were analyzed. Comparing with the properties of Si3N4ceramic sintered with rare earth oxides, we can find a better way to achieve higherthermal conductivity of Si3N4ceramic.Firstly, the sintering properties of Si3N4ceramics with different rare earthfluoride （ReF3, Re=La, Ce, Nd, Gd, Yb） additives were studied in this research.During SPS sintering of Si3N4ceramic with rare earth fluorides, the first stage isliquid formation of rare earth fluoride with MgO and SiO2for the particlere-arrangement. The later stage is solution-reprecipitation phase transition of Si3N4for high density. The shrinkage rates between two stages show a complementaryrelationship. The excess of rare earth compound will make the liquid viscosityincreased, resulting in the shrinkage rate of Si3N4ceramic reduced and low density.Thermal conductivity, strength, composition and microstructure of Si3N4ceramics sintered with different rare earth fluorides （ReF3, Re=La, Ce, Nd, Gd, Yb）and rare earth oxides （Re2O3, Re=La, Ce, Nd, Gd） were studied in this work.Rare-earth fluorides can react with the oxide in Si3N4powder twice to reduce oxygenamount for promoting thermal conductivity. The combination ability with oxygen ofdifferent rare earth cations determines the thermal conductivity and leads the resultof the thermal conductivity increase with the cation radius increase from Gd to La.Flexural strength of Si3N4ceramics sintered with rare earth fluorides are superior tothat of rare earth oxide sintering aids specimen, and the maximum can reach960MPa. |
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