Preparation and properties of ultralight fibrous β-Si3N4 3D scaffolds with honeycomb-like structure by directional freeze-casting

The directional freezing of β-Si 3 N 4 whiskers suspensions followed by high-temperature sintering was employed for fabricating novel highly porous fibrous Si 3 N 4 three-dimensional (3D) scaffolds. A honeycomb-like structure was achieved, in which the directionally aligned lamellar walls were composed of the oriented fibrous Si 3 N 4 grains and bridged by the transverse grains. Ultrahigh porosities ranging from 97.8% to 90.2% and rather low densities from 0.073 to 0.320 g cm −3 could be obtained by controlling the Si 3 N 4 contents from 1.5 to 7.5 vol%. The longitudinal compressive strength was superior to the transverse and increased obviously from 0.19 to 3.7 MPa as the porosity decreased. The superior compressive strength was due to the excellent resistance to bucking-induced elastic instability for the lamellar fibrous Si 3 N 4 walls. Meanwhile, the dielectric constant and loss were decreased to 1.08 and 6.6 × 10 −4 , respectively. This study provides a strategy for fabricating porous Si 3 N 4 ceramics with ultrahigh porosities and improved strength.