Porous Si3N4 ceramics with uniform microstructure fabricated by in-situ pre-gel casting combined with non-directional freeze-drying

Porous Si3N4 ceramics hold great promise for high-temperature wave-transparent materials, but it is a significant challenge to achieve both a low dielectric constant and high mechanical strength. This study explores a method combining in-situ pre-gel casting and non-directional freeze-drying to produce high-porosity, uniform microstructure Si3N4 ceramics. By adjusting gel concentration and pre-gelation time, ice crystal growth effectively restricted, resulting in a uniform pore structure. The resulting porous Si3N4 ceramics exhibited low drying shrinkage (1.50 %) and high porosity (59.13 %), with a flexural strength of 119.48 MPa, surpassing porous ones produced by other wet forming processes. At 30 GHz, the dielectric constant was 2.45, and the dielectric loss was 5.65 x 10-3.