In order to develop porous silicon nitride ceramics with tailor-made pore structure for "Bio-filter", suitable sintering aids were investigated. They must inhibit densification while promoting the α-β transformation to develop the fibrous β-Si3N4 crystals that would be essential for high toughness. The conventional technique for developing porous Si3N4 using a removable substance such as starch gave much lower porosity than the volume fraction of starch when MgO was used as a sintering aid. It was found that BaCO3 met the necessary conditions, probably because the decomposition temperature was as high as the liquidus temperature, and the internal CO2 pressure effectively inhibited the densification while promoting the phase transformation. The addition of 7 mol% as BaO gave about 44% of porosity at a sintering temperature as high as 1850°C. It was observed that the porous body consisted of interlocked fibrous β-Si3N4 crystals with sub-micrometer pore sizes.
