Effects of pore size on microstructure, mechanical and dielectric properties of gel casting BN/Si₃N₄ ceramics with spherical-shaped pore structures

Porous BN/Si3N4 ceramics with spherical-shaped pore structures were prepared by gel casting, in which polymethylmethacrylate microsphere (PMMA) was used as pore-forming agent. Effect of PMMA size on microstructure, phase composition, mechanical and dielectric properties of the porous ceramics were investigated in detail. Pore structures in the fabricated BN/Si3N4 ceramics are composed of both spherical- shaped micro pores depending on the diameter of starting PMMA and irregular sub-micro pores formed by stacking ceramic particles. The micro pores are homogeneously distributed in ceramic matrix, without ''pore overlapping'', and the sub-micro pores are acted as the channels between micro pores. Phase composition, mechanical and dielectric properties of the obtained ceramics can be well controlled by the diameter of starting PMMA because of the different porosity and pore size. The composite with 10 wt.% starting PMMA (with the diameter of 10 lm) show flexural strength of 71.6 ± 4.5 MPa, and the dielectric constant is about 3.35 under different frequencies. The excellent mechanical and dielectric properties of the porous BN/Si3N4 ceramics may attribute to the spherical-shaped pore structure.