The effect of the crystallization of oxidation-derived SiO2 on the properties of porous Si3N4–SiO2 ceramics synthesized by oxidation

The effect of the crystallization of oxidation-derived SiO2 on the properties of porous Si3N4–SiO2 ceramics synthesized by oxidation was studied over the range of oxidation temperature and time. The SiO2 remains amorphous at the temperature lower than 1250 °C and starts to crystallize as the temperature rises to 1300 °C. At the temperature higher than 1300 °C, the crystallization of SiO2 impacts greatly the flexural strength of porous Si3N4–SiO2 ceramics. A rapid crystallization of SiO2 at 1350 °C decreases the bonding strength of the necks between Si3N4 particles, leading to a serious decrease of flexural strength. An appropriate crystallization of SiO2 at 1300 °C strengthens the bonding necks between Si3N4 particles. The porous Si3N4–SiO2 ceramics oxidized at 1300 °C for 3 h show the highest flexural strength value of 82 MPa. The crystallization of SiO2 has no effect on the dielectric property of porous Si3N4–SiO2 ceramics. Porous Si3N4–SiO2 ceramics demonstrate a low dielectric constant of 3.0–4.0 and dielectric loss of 3.0–4.0×10–3 due to their high total porosity and SiO2 content.