Research On Powder Preparation And Microstructure And Properties Of Silicon Nitride Ceramics With High Thermal Conductivity

Silicon nitride(Si3N4)has attracted wide attention in the field of power semiconductor heat dissipation,especially in the application of IGBT modules in new energy vehicles,due to its high strength,high thermal conductivity,excellent mechanical properties and thermal shock resistance.At present,the thermal conductivity of Si3N4 ceramics in industry is far lower than the theoretical thermal conductivity.Therefore,it is important to study high quality powder and high performance Si3N4 ceramics.In this study,high quality α-Si3N4 powders were synthesized by carbothermal reduction nitridation,and Si3N4 ceramics with high thermal conductivity and excellent mechanical properties were prepared using twostep sintering method.The main research contents are listed as follows:(1)The nucleation and growth mechanism of Si3N4 in carbothermal reduction had been studied.High quality α-Si3N4 powders were synthesized.Using spherical,monodisperse and amorphous SiO2(D50=500 nm)as raw material,the mixture of SiO2 completely coated with carbon particles can not only increase the contact area between SiO2 and carbon particles,but also facilitate α-Si3N4 nucleation and inhibit grain growth and agglomeration.High-purity α-Si3N4(>99%)powder is achieved from C/SiO2=3:1 at 1400℃ under N2 flow rate of 800 ml/min.The powder,with an average size of 0.5 μm,showes good dispersity and regular morphology.The oxygen content is 0.8 wt%and metal impurity contents are less than 0.01 wt%.The number and rate of nucleation can be changed by controlling the reaction temperature,nitrogen flow rate and "Si3N4 seed" content.The morphology of Si3N4 powder is adjustable and particle size is controllable.When 5 wt%seeds are added,most products are spherical particles with an average size of 0.3 μm.The oxygen content is 0.92 wt%.With further increase the seeds content to 10 wt%,nitridation products are all spherical particles with an average particle size of 0.1 μm.The oxygen content is 1.21 wt%.(2)Si3N4-5wt%Y2O3-3wt%MgO composites were prepared by SPS combined with heat treatment.The effect of oxygen content on grain size,secondary phase content and thermal conductivity of the sintered samples had been studied in detail.The sample sintered at 1650℃ under 40 MPa for 4 min has high relative density(>99%).The average grain size is 300 nm and the secondary phase is amorphous.Thus,the thermal conductivity and flexural strength of the sintered sample are 41.8 W·m-1·K-1 and 735 MPa,respectively.After heat treatment at 1850℃ for 5 h under a N2 pressure of 1.2 MPa,the average grain size of the sample is 930 nm because the number of large elongated grains(>1 μm)increases,and the secondary phase changes from amorphous to crystallized(Y2Si3O3N4).The thermal conductivity and flexural strength of the sintered samples increase to 73.9 W·m1·K-1 and 859 MPa,respectively.The oxygen content of the raw powder decreases from 1.21 wt%to 0.73 wt%by adjusting the oxide layer thickness on the surface of Si3N4 particles through secondary nitriding process.The content of secondary phase of the sintered sample after heat treatment decreases from 11.3%to 8.2%,and the average grain size is 960 nm.Thus,the thermal conductivity and flexural strength of the samples increase to 80.2 W·m-1·K-1 and 847 MPa,respectively.(3)High strength and high thermal conductivity Si3N4 ceramics prepared by two-step sintering method was studied.After first-step sintering at 1400℃ for 2 h,using 5 wt%Y2O3-3 wt%MgO as sintering aids,the O content decreases from 3.21 to 2.07 wt%and the Mg content decreases from 1.81 to 0.57 wt%because MgO reacts with SiO2 on Si3N4 surface to form low-melting-point MgSiO3 compound which is prone to volatilize at high temperature(<1500℃).After final sintering,Si3N4 ceramics with high density are prepared at 1850℃ for 5 h under an N2 pressure of 1.2 MPa.The thermal conductivity and flexural strength of the sintered samples are 92.1 W·m-1·K-1 and 812 MPa,respectively.The secondary phase with less content of SiO2 is obtained due to pre-deoxidation at low temperature before final sintering.Thus,total oxygen content of the sintered samples decreases from 2.51 to 1.85 wt%,and the secondary phase changes from Y4Si2O7N2 to Y2Si3O3N4.Compared with the one-step sintering method,the flexural strength,thermal conductivity,and fracture toughness of the high-density ceramics prepared by twostep sintering are increased by 28.7%,16.9%,and 31.6%,respectively.With the extension of sintering time,the average grain size increases and secondary phase transfers to triangular grain boundaries which further improves thermal conductivity.The thermal conductivity and flexural strength of the samples sintered at 1850℃ for 15 h are 110.3 W·m-1·K-1 and 714 MPa,respectively.