Study On The Preparation Of High Strength,Toughness And Low Dielectric Loss Silicon Nitride Ceramics

For the semiconductor field,microwave communication field,and aerospace field,low dielectric loss,excellent mechanical properties and good thermal shock resistance of materials are important conditions.Silicon nitride（Si₃N₄）ceramics have received widespread attention due to its excellent mechanical properties,good potential thermal conductivity,and excellent dielectric properties.Therefore,silicon nitride（Si₃N₄）has become an important ceramic substrate material for a wide range of applications.In view of the high oxygen content and high cost of high-purity Si₃N₄ powder,the use of silicon（Si）powder to prepare high-performance silicon nitride ceramics has become a research focus in recent years.To meet broaden application field of silicon nitride ceramic substrate,the low-cost preparation is required.This article mainly studies on the influence of sintering aids on the mechanical properties,thermal properties,dielectric properties and microstructure of Si₃N₄ ceramics,using gas pressure sintering and pressureless sintering.And annealing process also has been employed to improve properties of the sintered samples.Si₃N₄ powder was raw material.Gd₂O₃ contents were fixed at 4mol%.As for Mg O+Gd₂O₃ sintering aid systems,Mg O contents were 3mol%,4mol%,5mol%,6mol%,respectively.After ball milling,rotary evaporation,drying,dry pressing,cold isostatic pressing,samples were sintered by gas pressure sintering（GPS）.The influence of Mg O contents on the properties of Si₃N₄ ceramics were studied.The results showed that there is noα-Si₃N₄ phase detected in the samples after sintering,suggesting that a complete transformation fromα-Si₃N₄ phase toβ-Si₃N₄ phase is realized.With the increase of Mg O content,the porosity of Si₃N₄ ceramics gradually decreased.When the content of Mg O in the raw material was 4mol%,the Si₃N₄ ceramics showed a best performance with 899±15.2MPa of three-point bending strength,7.8±0.17MPa·m^1/2 of fracture toughness is,and 77.8W/m·k of thermal conductivity,9.28×10^-5 of dielectric loss tangent.As for Mg Si N₂+Gd₂O₃ sintering system,the contents of Mg Si N₂ were 4mol%,5mol%,6mol%,7mol%,respectively.The influence of Mg Si N₂ contents on the properties of Si₃N₄ ceramics was studied.The results showed that with the increase of Mg Si N₂ content,the porosity of Si₃N₄ ceramics also showed a decreasing trend.When the content of Mg Si N₂ in the raw material is 6mol%,the performance of Si₃N₄ ceramics showed good properties.The three-point bending strength was 898.2±58.8MPa.The fracture toughness was 10.6±0.4MPa·m^1/2,and the thermal conductivity was 61.9W/m·k.The dielectric loss tangent was 9.10×10^-5.The results showed that the Si₃N₄ceramics with good properties could be prepared by adding proper contents of Mg O+Gd₂O₃ and Mg Si N₂+Gd₂O₃,using gas pressure sintering.On the basis of the research results of gas pressure sintering,in order to further explore the low-cost preparation process,pressureless sintering was employed as heat treatment.To densified the pressureless sintered Mg O+Gd₂O₃+Si₃N₄ ceramics,Zr O₂ and Zr O₂+Si powders were added to the raw materials as sintering aids.After shaping,nitriding and pressureless sintering,the influence of Mg O contents on the properties of Si₃N₄ ceramics was studied.The results showed that as the Mg O content increases,the porosity of the sample decreases.When the content of Mg O in the raw material is 6mol%,the Si₃N₄ceramics exhibited good properties with 932.8±46MPa of bending strength,10.6±0.18MPa·m^1/2 of fracture toughness,64.4W/m·k of thermal conductivity,and 1.62×10^-2dielectric loss tangent.Finally,the sintered Mg O+Si+Gd₂O₃+Zr O₂+Si₃N₄ samples were annealed at 1300℃for2 hours.XRD results showed that most of the conductive phases（Zr N and Zr₃N₄）changed to insolation phases（Gd₂Zr₂O₇ and Zr O₂）after annealing.The dielectric loss of all samples was reduced by one order of magnitude compared to the unannealed samples.The Si₃N₄ ceramics with 3mol%Mg O contents showed excellent performance.The thermal conductivity was70.5W/mk.The three-point bending strength was 910.9MPa.And the fracture toughness was10.7±0.49MPa·m^1/2.In this research,based on Mg O+Si+Gd₂O₃+Zr O₂+Si₃N₄ powders,the Si₃N₄ceramics with excellent comprehensive properties could be successfully prepared by pressureless sintering.After further research and development,Si₃N₄ ceramic substrate is expected to achieve low-cost preparation through this way.And it also will to be applied on the fields of aviation,rail transit and microwave communications.