Study On Preparation And Properties Of Porous Silicon Nitride Wave-Transmitting Ceramic Composites

In this paper, porous Si₃N₄-SiO₂-BN wave-transmitting ceramiccomposites were prepared by gelcasting and cold isostatic pressing. In order toacquire the ceramic composites with dielectric constant less than3.0,dielectric loss less than0.01and flexural strength higher than50.0MPa, theinfluence of processing parameters, material composition and structure onperformance of wave-transmitting ceramic composites were researched, themain contents of this paper were summarized as follows:Porous Si₃N₄-SiO₂-BN ceramic composites were prepared by coldisostatic pressing, and PMMA was added as pore-forming agent, the factorsthat affect the dielectric properties and mechanical properties were studied.And found that ceramic composites were manufactured by pressurelesssintering, experimental results show that the flexural strength of ceramiccomposites was as high as65.1MPa by adding4wt%Yb₂O₃,10.6wt%BN andsintering at1750℃for2h. Using scanning electron microscope, the resultswere obtained that Yb₂O₃was a better sintering aid and more β-Si₃N₄was produced in the ceramic. Ceramic composites with50%porosity have beenprepared, the flexural strength increased from27.6MPa to57.1MPa withchanging of PMMA size, the reason was that the size of pore was smaller, thestructure was stronger. BN greatly improved the dielectric properties, whenBN content increased from4.6wt%to12.5wt%, the dielectric constant ofceramic composites reduced from3.9to3.0, but also the flexural strengthwere more than50.0MPa. Finally, the ceramic composites with4wt%Yb₂O₃,12.5wt%BN were sintered at1750℃for2h, the diameter of pore was about1.8μm, dielectric constant was3.0, dielectric loss less than0.002and flexuralstrength was54.5MPa, meet the requirement of wave-transmitting radome.Porous Si₃N₄-SiO₂-BN ceramic composites were prepared by gelcasting.In the preparation process, crack and deformation has been found on the greenbody. The process was improved in this paper: Firstly, green bodies weresteeped in polyethylene glycol to dry instead of drying in air; Secondly,demould process was improved, demoulding follow a particular order insteadof demoulding directly. And these improvements have greatly improved thequality of green body. The influences of monomer content and rate ofmonomers on quality of green bodies were studied, and found that whenmonomer content was3.5mol/L and rate of monomers was20:1, green bodieshad low shrinkage rate and warpage rate （all less than6%）, but also had veryhigh flexural strength （more than30MPa）. Ceramic composites with6wt%BNwere manufactured by pressureless sintering, the dielectric constant of ceramic composites was2.8, but also the flexural strength was about50.5MPa, meetthe requirement of wave-transmitting radome.