Synergistic Modification Of Thermal Conductivity And Flexural Strength For SiC Ceramics

Silicon carbide(SiC),a promising semi-conductor material that possesses promising thermal and mechanical properties as well as excellent heat and corrosion resistance,which is widely applied as substrate,optical and electronic packaging materials.In this study,α-SiC was employed as the maj or research target;Rare earth additives and aluminum nitride(AlN)were selected as main sintering additives for the hot-pressing of SiC ceramics.Effects of additive combinations,sintering method optimization,post sintering annealing and reinforcement phases on the modification of phase constitution,microstructure and defect concentration of SiC ceramics were systematically studied.The aim of this study is to enhance the thermal conductivity and flexural strength of SiC ceramics,simultaneously.Effects of different rare earth oxides(Re2O3)and fluorides(ReF3)as well as their combinations on the properties of SiC ceramics are investigated.Re2O3 remarkably improved the sinterability of SiC ceramics.Decent thermal conductivity(175.7 W/m·K)and flexural strength(566.9 MPa)were yielded by addition of 2 wt%Nd2O3/Gd2O3,respectively.ReF3 possess better ability of extracting oxygen impurity and cleaning the grain boundary of SiC ceramics.However,the resultant volatile SiF4 leads to higher porosity and neutralizes the advantages of ReF3.3 wt%NdF3/YF3 yielded decent thermal conductivity(165.9 W/m·K)and flexural strength(594.8 MPa),respectively.The combination of Re2O3 and ReF3 can firmly offset each others’deficiencies.The addition of 1 wt%Nd2O3 and 2 wt%NdF3 helped to acquire a decent thermal conductivity of 187.8 W/m·K;1 wt%Gd2O3 and 2 wt%GdF3 yielded a proper flexural strength of 607.6 MPa;1 wt%La2O3 and 2 wt%LaF3 obtained both good thermal conductivity(178.4 W/m·K)and flexural strength(584.2 MPa).Impedance spectroscopy analyses show obvious VSi"" concentration difference among samples,lower VSi"" concentration(higher resistance)leads to higher thermal conductivity.The ability of reducing VSi"" concentration(extracting oxygen impurity)is mainly based on the chemical nature of additive themselves.AlN was employed to strengthen SiC ceramics.When the content of AlN exceeded 4 wt%,the formation of thermally detrimental 2Hss phase(between SiC and AlN)started to thriving,4 wt%AlN(4A sample)resulted in an acceptable thermal conductivity of 121.7 W/m·K.When 10 wt%AlN was added,a high flexural strength of 736.3 MPa was bred from the strengthen effect of 2Hss phase.Based on aforementioned studies,AlN can strengthen grain boundary while rare earth additives can effectively reduce the concentration of VSi"" defect and hence weakened the phonon scattering inside SiC ceramics.Their complex assembling can theoretically promote the thermal conductivity and flexural strength,simultaneously.When a combination of 4 wt%AlN,1 wt%La2O3 and 2 wt%LaF3 was employed(4AlLa2La sample),balanced thermal conductivity(151.3 W/m·K)and flexural strength(570.3 MPa)were obtained,Which are evidently higher than that of 4A sample,but slightly lower than the sample sintered with 1 wt%La2O3 and 2 wt%LaF3.Even so,4AlLa2La sample possesses both oxygen extraction and grain boundary strengthen mechanism,given a higher potential of subsequent property improvement.Sintering atmospheres(Ar/N2/Vacuum)remarkably affected the properties of 4A1La2La sample,results suggest that Ar is still the optimal atmosphere for all sintering systems.Based on the study of sintering parameter modifications,a novel two-step hot-pressing method was designed.Through this method,the grain growth of SiC was obviously inhibited and the concentration of VSi"" defect was further reduced,which helped 4A1La2La sample to obtain better thermal conductivity(165.3 W/m·K)and flexural strength(646.8 MPa).Accordingly,such two-step hot-pressing method is a general solution to achieve the synchronous improvement of thermal conductivity and flexural strength.To further consolidate these improvements on properties,effects of annealing treatment was systematically studied.Thermal conductivity and flexural strength of SiC ceramics were both slightly improved after annealing in Ar at 1500℃.When annealled in air at 1300℃,continuous oxidation layers were generated,which enhanced the residual compressive stress among SiC grains and hence reduced their interfacial thermal resistance and can also inhibited the propagation of inner cracks.Moreover,the concentration of VSi"" defect also decreased after air annealing.These factors added together led to the promotion of the properties in 4A1La2La sample,yielded a thermal conductivity of 186.5 W/m K and flexural strength of 753.6 MPa(after air annealled at 1300℃ for 1h).Reinforcement phases were incorporated to further ameliorate the properties of 4A1La2La sample.When 1 wt%flat-like BNp was added,it stayed at the grain boundary of SiC and was beneficial to the bridging and deflection of inner cracks,led to the improvement of flexural strength.Whereas,overdosed BNp addition will also form a thermally detrimental passageway throughout the interface of SiC ceramics and bring down thermal conductivity.When Si3N4 whiskers were added,the high hardness/strength β-Sialon phase was formed and thereby remarkably reinforced the SiC grain boundaries.Additionally,in line with impedance spectroscopy analysis results,when 0.25 wt%carbon was added along with 0.5 wt%Si3N4 whiskers,the concentration of VSi"" defect inside SiC ceramics was further reduced,the thermal conductivity and flexural strength of 4A1La2La sample reached 175.4 W/m·K and 717.4 MPa,which were finally improved to 192.7 W/m·K and 805.3 MPa after air annealing at 1300℃ for 1h.Additionally,the anisotropy character caused by uniaxial pressure from hot-pressing was also briefly studied.The results suggest that this character could be alleviated by the addition of BNp or annealing treatment(either in Ar or air atmosphere).When BNp and annealing were both utilized,the anisotropy in SiC ceramics can be significantly reduced.