Preparation And Properties Of Porous Silicon Nitride Ceramics By Gel Casting

Porous Si₃N₄ ceramic（Si₃N₄-PC）has been widely applied in the fields of broadband wave transparent radome,fluid filter,catalyst carrier,separation membrane and human tissue components due to its outstanding structural,mechanical and dielectric properties.Currently,the researches of Si3N4-PC focus on high porosity and pore building methods while the strength and toughness urgently need promotion.In this study,Si3N4-PC is prepared by gel casting combined with reaction sintering and reaction pressureless sintering.It aims at modulating the microstructure and properties of Si3N4-PC via controlling the gel casting system,particle size and sintering process.The results are as follows:（1）EGDGE-TEPA is a new type of ceramic gel casting system with excellent performance.The Si green bodies are uniform and dense,without defects such as bubbles,cracks and warpage.When the EGDGE addition amount is 11%,the density of the green body reaches 1.03 g·cm^-3,bending strength up to 20.56 MPa.（2）For reaction sintering Si3N4-PC,when using raw Si powder with a single particle size of 1μm,5μm,and 45μm,as the particle size of the Si powder increases,the P_a of Si₃N₄-PC decreases,r_b,σ_F andγ_wofof increases,which can be adjusted in the range of 39～47%,1.73～1.96 g·cm^-3,49.16～75.02 MPa and 691.89～822.49 J·m^-2;When using raw Si powder with double particle size composition,the Si powder with 5μm&45μm double particle size composition can produce a close-packing effect,thereby expanding the control range of Si₃N₄-PC,so the lower limit of its P_a can be reduced to33%,the upper limits of itsr_b,σ_F andγ_wofof are increased to 2.17 g·cm^-3,109.94 MPa and 990.74 J·m^-2;（3）For reaction pressureless sintering Si3N4-PC,when using a 5μm single particle size raw Si powder,the minimum P_a is 38%,the maximumr_b andσ_F are 2.03 g·cm^-3and 112.44 MPa respectively.When using 5μm&45μm double particle size raw Si powder,the maximum values ofσ_F andγ_wofof are 119.56 MPa and 1167.88 J·m^-2.The control ranges of P_a,r_b,σ_F andγ_wofof of the reaction pressureless sintering Si₃N₄-PC are38～51%,1.61～2.03 g·cm^-3,29.55～119.56 MPa and 252.01～1167.88 J·m^-2;（4）The sintering process and the raw material particle size have a significant effect on the phase composition of Si₃N₄-PC.Theα-Si₃N₄ andβ-Si₃N₄ phases exist in both reaction sintering Si3N4-PC and reaction pressureless sintering Si3N4-PC.A large amount of Si2N2O phase are formed in the 1μm raw Si powder body after reaction sintering,whileβ-Si₃N₄ are generated during the reaction pressureless sintering.The reaction pressureless sintering can be regarded as two successive procedures of reaction sintering and pressureless sintering,the former temperature is lower than 1400℃,theα-Si₃N₄ phase is mainly form,the latter temperature reaches 1650℃,and the phase transition ofα-Si₃N₄ toβ-Si₃N₄ mainly occurs,that is,andβ-Si₃N₄ phase is mainly formed.（5）The sintering process and the particle size of the raw materials play a strong role in controlling microstructure of Si3N4-PC.After the reaction sintering,a particle-bonded structure is formed in the 1μm raw Si powder body,the pore size is much smaller than 1μm,and its size and distribution are uniform.A large number of Si3N4 whiskers are formed in the 5μm and 45μm raw Si powder bodies and interspersed with each other to form a network structure,the pore size is about 1μm and its size and distribution are relatively uniform.After reaction pressureless sintering,a frame structure formed by short columnarβ-Si₃N₄ particle sintered with each other and pores with a pore size of about 1μm are formed in the 1μm raw Si powder body.A dense structure formed by long columnarβ-Si₃N₄ densely interspersed cement and a few submicron pores are formed in the body of 5μm raw Si powder.A hybrid structure of small rod-shapedβ-Si₃N₄ and granular Si₃N₄ are formed in the body of 45μm raw Si powder,and the pores with a wide size distribution from micron to submicron level.