Study On Preparation And Properties Of High Thermal Conductivity Si₃N₄ Matrix Composites

With the development of ultra-large-scale and high-power integrated circuits,the problem of heat dissipation needs to be solved in the design of devices.Ceramic materials with high strength and high thermal conductivity,have been considered as ideal packaging materials of electronic devices.Silicon nitride ceramics present high strength and toughness due to the specific interlocking microstructure formed by large elongate β-Si₃N₄ grains,and the theoretical thermal conductivity of Si₃N₄ crystal can be 200 W/（m·K）or higher.As a candidate material,silicon nitride has been widely studied for electronic packaging.So far,the major approach for enhancing thermal conductivity of silicon nitride ceramics is the pure large elongate β-Si₃N₄ grains obtained by the solution-reprecipitation mechanism during a long time sintering at a temperature higher than 1850 oC.However,the coarse microstructure formed by the large elongate β-Si₃N₄ grains has inevitably reduced mechanical properties of silicon nitride,and the sintering of silicon nitride with a high thermal conductivity consumes time and energy.Furthermore,a rapid sintering technique such as SPS has hindered the α→β phase transformation of Si₃N₄,which leads to the reduction of the mechanical properties and the thermal conductivity of Si₃N₄ composites.This paper is divided into the following three parts for the above problems:Dense silicon nitride ceramic was prepared by hot-pressing sintering with two kinds of lanthanide oxides（CeO₂+Yb₂O₃）as sintering additives.The composition of grain boundary phase and the grain size of β-Si₃N₄ can be controlled by the content of CeO₂ and Yb₂O₃.The effect of CeO₂ and Yb₂O₃ addition on the formation temperature of liquid phase,the densification,the microstructure and the properties of Si₃N₄ ceramics are independent of each other.In this paper,the bending strength of Si₃N₄ samples sintered with CeO₂ addition and Yb₂O₃ addition ranged from 630.78 MPa to 786.30 MPa.The fracture toughness of Si₃N₄ samples increased with the average grain diameter and was in the interval from 7.61 MPa·m1/2 to 8.89 MPa·m1/2.HP sintered Si₃N₄ samples exhibited high mechanical properties and thermal diffusivity in the case of the amount of additions =7 mol%.In the case of HP sintered Si₃N₄ with 7 mol% CeO₂ addition,the bending strength,the fracture toughness and the thermal conductivity were 718.16 MPa,8.29MPa·m1/2,80.12W·m^-1·K^-1,respectively.And Si₃N₄ samples sintered with 5mol% CeO₂ and 2mol% Yb₂O₃ had the bending strength of 766.09 MPa,the fracture toughness of 8.30MPa·m1/2 and the thermal conductivity of 78.91W·m^-1·K^-1.As the conductive second-phase,Cu addition and Mo addition can promote the transformation of α to β-Si₃N₄ effectively during the SPS process of silicon nitride.The former was attributed primarily to the effect of Joule heating on the solution re-precipitation mechanism,while the latter was associated with the pure β-Si₃N₄ seeds formed by the in-situ reaction between Mo and α-Si₃N₄.The transformation of α to β-Si₃N₄ can be promoted by conductive Cu addition,however,be suppressed with increasing Cu concent.Cu/Si₃N₄ ceramic composites had a significantly higher α→β transformation ratio in comparison to the monolithic Si₃N₄（β: α ratio is 52.6:47.4）,with the highest β: α ratio of 84.3:15.7 when the content of Cu is 3wt%.The addition of Mo particles can promote the transformation of α to β-Si₃N₄ competely,and the content of Mo has no effect on the transformation ratio of α to β-Si₃N₄.The phase transformation during SPS of Si₃N₄ can effectively promote the abnormal growth of β-Si₃N₄ grain,and the pure β-Si₃N₄ seeds formed by the in-situ reaction between Mo and α-Si₃N₄ can promote the transformation of α to β-Si₃N₄ and enhance the growth of elongate β-Si₃N₄ grains.The number of large elongate β-Si₃N₄ grains for Si₃N₄ samples decreased with grain size of Mo particles and increased with the purity of Mo particles.The multi stage sintering can significantly enhance the grain size of β-Si₃N₄,which was attributed primarily to the fully phase transformatio and the anisotropic growth of β-Si₃N₄ grains,resulted from longer time of solution-reprecipitation process during SPS.In the case of Si₃N₄ samples sintered with 6wt% Mo by the two stage SPS,the bending strength of samples were 710.02 MPa and 683.27 MPa,respectively,the fracture toughness of samples were 8.29MPa·m1/2 and 7.94MPa·m1/2,respectively,and the thermal conductivity of samples were 83.95W·m^-1·K^-1 and 92.55W·m^-1·K^-1,respectively.The thermal conductivity of Si₃N₄ samples sintered by SPS is higher than that for hot pressing sintering by 8 times longer.