Influence of pore-forming agents on permeability coefficient of porous Si3N4 ceramics
To improve their permeability,porous Si3N4 ceramics were prepared using Si powder as the raw material,Y2O3 and CaF2 as sintering aids,and polymethyl methacrylate microspheres(5 and 20 μm)as pore-forming agents,gel-casting,binder-removing,and finally reaction sintering at 1 650 ℃ for 3 h in a ni-trogen atmosphere.The effects of the particle sizes and additions(volume fractions of 0,15％,30％,45％,and 60％)of the pore-forming agents on the apparent porosity,pore size distribution,pore structure and permeability coefficient of the porous Si3N4 ceramics were studied.The results show that:(1)the samples with pore agents exhibit dual pore structures,which are composed of stacking-derived pores and spherical evolution pores,with bimodal pore size distribution;(2)with the same particle size,as the pore-forming agent addition increases,the apparent porosity and permeability coefficient of the samples increase,while the bulk density,bending strength,and fracture toughness decrease;(3)at the same addition,the samples with the smaller-sized agents(5 μm)have higher apparent porosity,permeability coefficient,and bending strength;using smaller-sized pore-forming agents can reduce the thickness of the pore walls and enlarge the pore sizes of the stacking-derived pores within the pore walls,thus enhancing the permeability of the pore walls;(4)the sample with 60％pore-forming agents of 5 μm has the best comprehensive perform-ance,with the apparent porosity of 55.1％,permeability coefficient of 11.62 x 10-14 m2 and bending strength of 35.3 MPa.
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