Structure and performance control of porous Si₃N₄ ceramics fabricated by freeze-drying process

Effects of glycerin and silica sol on the structure and performance of porous Si₃N₄ ceramics that were prepared via freeze-drying were studied. Adding glycerol affected growth process of ice crystals, changed pore structural from bimodal peaks to triple peaks distribution, and formed regular honeycomb structure. This improved mechanical properties and electromagnetic wave transmission properties of Si₃N₄ ceramics. Therefore, adding glycerol changed mesoscopic morphology of the material. For porous Si₃N₄ ceramics with 72.5% porosity, flexural strength was 38 MPa, compressive strength was 42.6 MPa, and dielectric constant was 1.75 in the frequency range of 12.4–18 Hz as the amount of added glycerol increasing to 5 wt%. By reducing the sintering eutectic point of the system, the addition of silica sol promoted sintering shrinkage and formed new phases. Thus, pore size distribution was changed from bimodal to unimodal. Moreover, dielectric constant of samples increased from 2.25 to 3.5. Finally, structure and performance of porous Si₃N₄ ceramics were controlled via addition of glycerol and silica sol.