Study On Mechanical-Thermal Properties Of Liquid Phase Sintered Silicon Carbide Ceramics

Silicon carbide ceramics are widely used as structural materials in harsh working conditions due to excellent mechanical properties,high temperature resistance property and other physical/chemical properties.With development of semiconductor industry and LED industry,some key components require rapid heating and cooling which need higher thermal conductivity ceramic material.The coordination of mechanical and thermal properties of materials is the most basic requirement.In this paper,dense silicon carbide ceramics were prepared by liquid phase sintering.The generation of liquid phase in the material and the influence of mechanical properties and thermal properties of the material were studied.The work is includes into three parts:1.Liquid phase sintering silicon carbide ceramics were prepared by spark plasma sintering（SPS）.The sintering process curve of silicon carbide was studied when liquid phase additives with the molar ratio of Al₂O₃:RE₂O₃（RE=La,Sm,Sc,Gd,Y）=5:3 were used as sintering additive and the samples were sinteried at temperature 2050°C.The temperature range of liquid phase produced by different rare earth oxide systems during SPS sintering was analyzed.The mechanical properties,electrical resistivity and thermal properties of silicon carbide ceramics were analyzed.The effect of liquid phase on the phase composition and microstructure of silicon carbide ceramics,and then on the mechanical-thermal properties was studied.The results show:（1）The densification degree of silicon carbide ceramics varies with rare earth oxides,and the highest density is above 98%.The process curve shows that four stages:physical shrinkage,gas expansion,displacement stagnation and liquid phase formation are observed.In the first three stages,a similar trend curve was observed in all samples.In the fourth stage,shrinkage temperature varies with liquid phase,and the final shrinkage displacement change range is:RSc>RY>RGd>RLa>RSm.（2）The hardness of un-densed samples RLa and RSm is only about 10 GPa,and the hardness of densed samples RSc,RY and RGd is more than 20 GPa.The fracture toughness of the samples RSc,RY and RGd is more than 4.5 MPa·m^1/2.The average resistivity of RSc,RY and RGd samples with higher density is not significantly different from that of RLa and RSm samples with lower density.The resistivity of all samples is in the range of 10⁹-10¹¹Ω·cm.The sintered samples have lower thermal diffusivity and the thermal conductivity,in which the sample with Al₂O₃-Gd₂O₃ as additive had a maximum thermal conductivity of63.6 W/m·K at room temperature.2.Aluminium-free liquid phase sintered silicon carbide ceramics were prepared by hot pressing sintering.The sintering additives with a molar ratio of Y₂O₃:RE₂O₃（RE=Sc,Gd,Ho,La,Sm,Yb）=1:1 were used to prepare the silicon carbide ceramics with a sintering temperature of 1900°C.The phase composition of the samples after hot pressing sintering was analyzed.The microstructure of the samples was analyzed by SEM,TEM,EDS and SAED,and the influence of the mechanical and thermal properties of the samples was discussed.The results show:（1）High-density ceramics were prepared by hot pressing sintering,with the relative density above 95%.The thermal conductivity of the hot-pressed sample is in range of 74.8-85.6 W/m·K which is greatly improved compared with the SPS sintered sample（61.2-63.6W/m·K）.The grain size of samples not show abnormally growth during the hot-pressing sintering process,and the average grain size is in range of 0.6-0.73μm.（2）The phase component and micro structure of RY-Sc and RY-Ho samples with relative density of more than 98%were analyzed by XRD,SEM and TEM.It is showed that the liquid additive of the sample RY-Ho formed Y_xHo_yO₃,Y₂Si₂O₇ and Ho₂Si₂O₇ after sintering,while,incase of sample RY-Sc,Sc₂O₃ and Y₂Si O₅formed.The thermal conductivity of RY-Ho and RY-Sc sample is 85.6 W/m·K and 74.7 W/m·K respectively.The lattice oxygen in Si C grains of RY-Ho sample is less than that of RY-Sc sample.The decrease of lattice oxygen in Si C lattice increase the thermal conductivity of the sample.3.The sintering additive（Al₂O₃:Y₂O₃=5:3）that may densify the sample dense by adjusting the process was selected through pressureless sintering.The influence of content of binder,dispersant and lubricant on slurry viscosity were studied to obtain low viscosity and high solid content slurry.After optimization,the viscosity of the slurry was 3737 m Pa·s when the solid content was 45.4%,the ratio of oleic acid:triethanolamine:PVB:polyethylene glycol was 2:1:5:2 respectively.The morphology of grain produced by spray granulation is spherical,and the green body structure after dry pressing is uniform.The sample obtained by pressureless sintering are not ideal which indicate the presureless sintering process need to be re-optimized.