Study On Low Temperature Preparation And Properties Of Silicon Nitride Ceramics With High Thermal Conductivity

High thermal conductivity of Si₃N₄ ceramics with high strength,hardness,excellent oxidation resistance and corrosion resistance and high thermal shock resistance,is a kind of excellent comprehensive properties of structural ceramics material,and the theory of thermal conductivity and thermal expansion coefficient between excellent device matching nature make it expected to become a new generation of insulation substrate material,it also became a hot issue in the research of scholars at home and abroad in recent years.In the existing high thermal conductivity silicon nitride preparation process,on the basis of the selection of high-purity raw material powder,the high thermal conductivity of Si₃N₄ ceramics usually needs to be sintered for a long time under high temperature and high nitrogen pressure environment to promote the purification within and between crystals,which requires a high requirement on the equipment and also brings the disadvantage of low production efficiency and high production cost.For this reason,this paper proposes to adopt two composite auxiliaries LiF-Y₂O₃ and Li₂O-Y₂O₃ to achieve densification at low temperature,at the same time,promote the removal of reactive volatilization of oxygen and intercrystalline liquid phase,and accelerate the purification of Intracrystalline intercrystalline,so as to achieve the purpose of low temperature preparation of silicon nitride ceramics with high thermal conductivity.This article first LiF/Li₂O in compound fertilizer and the proportion of Y₂O₃ study,in the air environment,through different buried powder formulation and buried powder form,determine the air atmosphere sintering process of Si₃N₄ ceramics,under this process,the sintering comparison of Si₃N₄ ceramics with different LiF-Y₂O₃ and Li₂O-Y₂O₃ auxiliary-agent ratios was conducted,find out the best Li-Y ratio which is beneficial to the densification process.Subsequently,under the Li-Y ratio condition,the unidirectional pressure of 30MPa and nitrogen pressure of 12KPa were used for hot-pressed sintering at different sintering temperatures,and the effects of Li₂O and LiF on the densification process,phase composition,microstructure,mechanical properties and thermal conductivity of Si₃N₄ ceramics were compared.Finally,the effects of holding time on phase composition,microstructure and thermal/mechanical properties of Si₃N₄ ceramics were studied.The results show that the air atmosphere sintering of Si₃N₄ ceramics can be realized by using the inner layer of Si₃N₄ combined with the outer SiO₂ powder burying method,which can prevent the high-temperature oxidation of Si₃N₄ ceramics.When the MgO-LiF-Y₂O₃ compound additive system is used,silicon nitride ceramics with a relative density of more than 95%can be obtained by pressureless sintering in air atmosphere at 1600?.In addition,when the ratio of Li:Y is 1:2,the ratio of the optimal auxiliary agent is determined.Li-Y compound additive system,LiF and Li₂O can achieve good densification effect at low temperature,in the process of densification,LiF additives sintering densification starting temperature is 1312?,1280?later than Li₂O,but compared with the addition of Li₂O,containing F liquid have stronger ability of densification and reaction volatile in densification before final stage,makes the LiF-Y₂O₃ as additives of silicon nitride ceramic eventually relative density can reach above 99%;The reactive volatilization process between LiF and SiO₂ continues in the sintering process,so that Si and O are continuously removed from the liquid phase,thereby increasing the O content in the liquid phase and reducing the O content in the system.In terms of microstructure,the F containing liquid phase is more favorable for grain growth.Compared with the addition of Li₂O,the thermal conductivity of Si₃N₄ceramics obtained by using LiF-Y₂O₃ composite additive is higher due to the decrease of O content in the system and the larger grain size.The thermal conductivity of Si₃N₄ceramics can be significantly improved by extending the sintering holding time.By using LiF-Y₂O₃ composite additive,the thermal conductivity of Si₃N₄ ceramics can reach 59W/?m·K?after 8 hours of hot pressing at 1750?.For Li-Y low temperature compound additive system,LiF is more advantageous to obtain high thermal conductivity than Li₂O.