Effect of Si3N4 solid contents on mechanical and dielectric properties of porous Si3N4 ceramics through freeze-drying

Porous Si3N4 ceramics with uniform pore distributions were fabricated by freeze drying with liquid N2 as refrigerant, and then sintered at 1700 °C under an N2 pressure of 0.3 MPa. The Si3N4 dispersant slurries were prepared by ball milling for 12 h and casting into a mold using liquid nitrogen as a freezing medium with the top surface of the sample exposed to air at room temperature. The temperature gradient contributed to bimodal peaks of pore size distribution. Effects of solid concentration on the porosity, mechanical, and dielectric properties of the Si3N4 sample were systematically investigated. As the solid content increased from 15 vol.% to 40 vol.%, the porosity decreased from 83.9% to 40.58%. Therefore, flexural strength and compressive strength increased from 0.1 MPa to 94.7 MPa, and 1.3 MPa-314 MPa, respectively. Moreover, the dielectric constant increased from 1.4 to 4.0. This study indicated that the freeze-drying process is a useful method for fabricating novel porous Si3N4 ceramics with unidirectional channels.