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

•Porous BN/Si3N4 with spherical-shaped pore structure was prepared by gel casting.•Micro spherical pores and irregular sub-micro pores composed the pore structure.•Increasing pore size has an increased inhibition on α- to β-Si3N4 transformation.•Mechanical and dielectric properties depending on the pore size were revealed.

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 μm) 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.

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