Study On Sintering Behavior And Properties Of Silicon Nitride Ceramics With Rare-Earth Oxychloride Additives

In this study, Spark Plasma Sintering （SPS） and a following heat treatment wasapplied to fabricate silicon nitride ceramics with MgO and different rare earthoxychloride （ReOCl, Re=Y, La, Nd, Gd, Yb） additives. The effect of SPS and heattreatment conditions, YOCl/MgO ratio, different rare earth oxychloride additives onsintering behavior, thermal conductivity and flexure strength of Si₃N₄ceramics werestudied.Firstly, the effect of sintering conditions, including SPS and the following heattreatment and YOCl addition on sintering behavior and properties of Si₃N₄ceramicswere studied. Results show that two shrinkage rate peaks in densification process appear,at1400°C and1550°C, representing particle arrangement and dissolution-recrystallization process, respectively. An obvious decrease in density occurrs after heattreatment, which is resulted by the evaporation of MgO and the escape of SiClxby thereaction of YOCl with SiO2. However, YOCl additive helps to form liquid duringsintering and hence, promote densification of Si₃N₄ceramics. Eventually, sampledensity first decreases and then increases with YOCl addition. The addition of YOClcould remove the oxygen in Si₃N₄crystals by the reaction of YOCl with SiO2impurityand hence, improve thermal conductivity. However, excess YOCl additive could lead tomore glass phase residues in grain boundary and prevent a further improve of thermalconductivity.Then, Si₃N₄with a total amount of9wt%of YOCl and MgO additives,YOCl/MgO ratio changing from0:9to9:0, were studied in this work. Results show thatcompact Si₃N₄ceramics can be obtained when YOCl/MgO ratio <4:5and thermalconductivity and bending strength increases with YOCl/MgO ratio decreasing. WhileYOCl/MgO ratio>4:5, relative density, thermal conductivity and bending strengthdecreases sharply with YOCl/MgO ratio increasing. The sample with YOCl/MgO=4:5obtains the maximum thermal conductivity of90W/m·K and the maximum bendingstrength of1063MPa.Lastly, Si₃N₄ceramics with different rare earth oxychloride （ReOCl, Re=La, Nd,Gd, Yb） additives were studied in this research. The results show that YbOCl-added sample fails to be compact and shows poor properties. LaOCl-added, NdOCl-added andGdOCl-added samples all obtain compact Si₃N₄ceramics with bending strengthexceeding1000MPa. Thermal conductivity sharply increases as cation radiusdecreasing from La to Nd to Gd. The GdOCl-added sample owns the maximum thermalconductivity of89W/m·K and maximum bending strength of1126MPa.