Research On Sintered By Pressureless And Properties Of Porous Silicon Nitride Ceramics

Si₃N₄ ceramics are widely used as important structural functional materials,owing to their excellent properties,for example low density,high strength,low dielectric loss,high thermal shock resistance and so on,they have important applications in the industrial field,but they are troubled by many problems such as complex preparation process,large sintering energy consumption and high production cost.This paper selects YbF₃ as sintering aids to provide sintering liquid phase at lower temperatures and promote Si₃N₄dissolution and phase transition by Si O₂ reaction with the Si₃N₄ surface.The infulence of sintering temperature,holding time,sintering atmosphere and the type and content of sintering aids on the preparation and properties of Si₃N₄ ceramics were studied in this paper.Experimental results,if the sintering temperature is below 1400℃,Si₃N₄ ceramics are hard to phase,its density and mechanical properties are poor.At temperatures above1400℃,Si₃N₄ ceramics began to change fromα-Si₃N₄ toβ-Si₃N₄.Content of sintering auxiliaries has a significant effect on porosity and transformation of Si₃N₄ ceramics,when the YbF₃ content of sintering aids is 6 wt.%,under 1500℃conditions and N₂ atmosphere,heat preservation time 2 hours,can complete certain densification and phase transition.YbF₃ single sintering assistant was used,when the sintering temperature reached the YbF₃ melting point,liquid phase began to appear inside the Si₃N₄ ceramics,which promoted the dissolution,diffusion,precipitation and precipitation,thus realizing the crystal transformation and phase transformation.While the reaction and volatilization of the sintering assistant formed pores during sintering,which increased the porosity of Si₃N₄ ceramics.The experimental results show that when the YbF₃ content reaches 9wt.%,the densification and phase transition can not be completed under the condition of up to 1500℃pressureles sintering.Only the optimum mechanical properties of the specim en ar e 1 62.2 MP a,and th e mini mum diel ect ric cons tant is 4.49.When YbF₃ and MgF₂ and CaF₂ are used to form composite auxiliaries,at 1500℃sintering,with a YbF₃ content of 3 wt.%,the liquid phase formation temperature of the added Mg F₂ is higher than that of the added YbF₃.When the Mg F₂ content reaches 6wt.%,to get the best performance of the sample,the bending strength is 217.8 MPa,the dielectric constant is 8.17.When the YbF₃ and Ca F₂ composite sintering aids were sintered at 1500℃,and the pattern of change is similar to that of adding Mg F₂,the flexural strength is 241.2 MPa,the dielectric constant is 2.89.