Research Of Fabrication And Thermal Conductivity Of Silicon Carbide Ceramic

SiC ceramics have also been applied in the engine,spray nozzle,heat exchanger,reflecting mirror except microelectronics industry,due to its lower density,high mechanical strength,low coefficient of thermal expansion and high thermal conductivity.These applications are closely related to the excellent thermal conductivity of SiC ceramics.Therefore,it need to be further studied that how to improve the thermal conductivity of SiC.In this dissertation,SiC ceramics were prepared by liquid phase sintering method.Research is conducted to understand the effects of composition and content of sintering additives on the density,phase composition,microstructure,mechanical properties and thermal conductivity of SiC ceramics were studied.It is demonstrated that the complex sintering additives Y2O3 + AlN result in the higher density of SiC ceramic compared with the single phase sintering additives.The porosity of ceramic increases at first and then decreased with the increasing of Y2O3 + AlN content from 1.0 wt.% to 11.6 wt.%.The SiC ceramic with 3 wt.% sintering additives of Y2O3 and AlN exhibits the highest density of 99.72% and highest thermal conductivity of 73.53 W/(m·K).Moreover,thermal conductivity increased to 102.78 W/(m·K)after further heat treatment.The grain boundary,the second phase in the grain boundary,and the intragranular impurity atoms in ceramic are determined as the main influence factors for the phonon scattering in the SiC.GNPs/SiC ceramic matrix composites were fabricated by adding multylayered graphene nanoplatets.The dispersion behavior and bonding mode of GNPs in SiC matrix were studied.The effect of GNPs on the microstructure,thermal conduction and mechanical properties of the composites were investigated.The distribution direction of most GNPs is mainly perpendicular to the direction of pressure.The significant difference of thermal conductivity of GNPs/SiC composites between the vertical and parallel directions of axial pressure is attributed to the distribution and anisotropy of GNPs.The thermal conductivity in vertical direction increases at first and then decreases with the increasing of GNPs addition,and the highest thermal conductivity of 84.98 W/(m·K)has been obtained with addition of 3 wt.%.By contrast,the thermal conductivity in the other direction decreases with the increasing of GNPs addition.The SiC ceramics were joined by Zn-Al brazing by ultrasonic assisted brazing.The shear strength of the joint is 21.08 MPa,and the fracture happens at the bonding surface of Zn-Al and SiC.It is established the thermal resistance model of SiC/Zn-Al/SiC joint,and the thermal resistivity of the interface between SiC and Zn-Al is calculated as r = 0.13 K/W.