Fabrication,Microstructure Tailoring and Performances of Silicon Nitride Thermal Conductive Ceramics with High Strength and Toughness

Author:Yang Chun Ping

Supervisor:wu yi yong ye feng


Degree Year:2019





Si3N4 ceramic is one of the excellent structure materials with high strength,high toughness and high hardness.More importantly,its theoretical thermal conductivity is up to 200320W·m-1K-1 at room temperature,which is expected to replace low-intensity and deliquescent AlN ceraimics to become a new generation of ceramic substrate,so as to further improve heat dissipation system of large-scale integrated circuits.However,the experimental value of thermal conductivity of Si3N4 ceramics is quite lower than the theoretical one.It seems difficult to further optimize the mechanical/thermal properties of Si3N4 ceramics via traditional sintering technology and sintering additives.For example,the method which fabricated Si3N4 ceramics with thermal conductivity exceeding 100W·m-1K-1 through prolonging sintering time is a high-energy consuming technique at the cost of material mechanical properties.Therefore,preparation and investigation of high-strength and high thermal conductive Si3N4 ceramics via high efficient preparation techniques has become a hotspot of application research.In this work,the Si3N4 ceramics were prepared using Gas Pressure Sintering(GPS)and Spark Plasma Sintering(SPS)combined post-sintering heat treatment.The effects of sintering additives and sintering process on the microstructure and properties were studied.And the basic laws of microstructure evolutions were summarized.The influence mechanisms of microstructure on thermal conductivity and mechanical properties were discussed.The results showed that rare earth elements with different ionic radius can influence viscosity of liquid phase,wettability and interface energy during sintering of Si3N4 ceramics.With the increase of ionic radius,density,mean grain size and frequency of large grain decreased.During the fabrication of Si3N4 ceramics by two-steps GPS process,the ion diffusion rates and exclusion of closed pores can be accelerated.The relative density of Si3N4 ceramics prepared by two-steps GPS process was increased to 99.4%,and mean grain size and frequency of large grains were 7.3μm and 68.1%.The thermal conductivity of Si3N4 ceramics sintered by two-steps GPS is60W·m-1K-1.Si3N4 ceramics were fabricated with Y2O3 and low-melting-point MgF2 as sintering additives to replace MgO and MgSiN2 by SPS and post-sintering heat treatment.MgF2as sintering aid can effectvely reduce initial liquid phase temperature and viscosity of liquid phase,and improve the solubility of N atoms effectively.The densification,phase transformation,grain growth and thermal conductivity of Si3N4 ceramics using Y2O3-MgF2 as sintering aids were improved significantly.The volatile phase SiF4 which formed by Si and F is beneficial to improve thermal conductivity due to decrease of intergranular phase content After post-sintering heat treatment,the thermal conductivity value of the sample with 3mol%Y2O3-2mol%MgF2 is 76W·m-1K-1.Microstructure tailoring and performance optimization of Si3N4 ceramics were achieved withβ-Si3N4 powder as seeds andα-Si3N4 as raw material via SPS process and post-sintering heat treatment.As the content ofβ-Si3N4 seeds increased,the size and frequency of elongatedβ-Si3N4 grains increased first and then decreased.The thermal conductivity of Si3N4 ceramics related to frequency of elongated large grain.The purified large sizeβ-Si3N4 grains can lengthen the phonon mean free path,decrease the phonon scattering and improve the thermal conductivity of Si3N4 ceramics.The sample b10 with 10mol%β-Si3N4 seeds had the best comprehensive performances,thermal conductivity of 84.6W·m-1K-1,the thermal conductivity at 500℃of 26.8W·m-1K-1,flexural strength of 893MPa,fractrue toughness of 7.8MPa·m1/2 and Vicker hardness of16.8GPa.The ion diffusion rates increased with low-viscosity liquid phase,so that the grain growth under perpendicular direction of SPS pressure accelerated.The differences of the thermal conductivity between in parallel and perpendicular direction of SPS pressure become large.The anisotropies of grain growth and thermal conductivity become weake with the increase of nucleation amounts due to obvious steric effect to make a limited growth space.The Si3N4 ceramics with addition of seeds had a core-shell structure which core and shell had the same cystal structure and orientation,and epitaxy begins withβ-Si3N4seed.The thickness of equilibrium amorphous film of Si3N4 ceramics with Y2O3-MgF2as sintering aids is 1.05nm,and there is a step structure at the grain boundary to reduce energy.Oxgen content inβ-Si3N4 grain is less than 0.7%which tested by EDS analysis.Low lattice content can reduce point defects formed by Si vacancies to reduce the phonon scattering and enhance thermal conductivity.The content of fluorine is less than0.1%in both grains and grain boundaries.High thermal conductive Si3N4 ceramics with fluoride as sintering additive prepared by SPS and post-sintering heat treatment have an important application prospect in the field of high-performance ceramic substrates,and their microstructure and properties can optimize via addingβ-Si3N4 seeds.