Abstract: Highly porous Si₃N₄ ceramics with unidirectionally aligned pore channels are gaining significant attention across various fields due to their outstanding functional capabilities. However, achieving high strength in such unidirectional highly porous Si₃N₄ ceramics remains challenging. Herein, we design and fabricate a novel β-Si₃N₄ scaffold composed of directionally aligned lamellar walls with a textured microstructure by directionally freeze casting of α-Si₃N₄ suspensions with fine elongated β-Si₃N₄ seeds addition, followed by liquid phase sintering. During the sintering, the scaffold exhibited anisotropic shrinkage, and fibrous β-Si₃N₄ grains were synthesized through epitaxial growth on the seeds preferentially oriented or the nuclei originated from α-Si₃N₄ powders, resulting in the grains aligned parallel to lamellar walls and bridged the walls. Seed additions of 7 to 15 wt% were beneficial for the optimized distribution of the two types of β-Si₃N₄ grains, which contributed to the excellent resistance to bucking-induced fracture for the walls. Compared with other unidirectional porous Si₃N₄ prepared by freeze-casting in the literature, the Si₃N₄ scaffold exhibited outstanding compressive strength, ranging from 2.8 to 22.0 MPa, as the porosity decreased from 94.4 % to 88.0 % and the density increased from 175 to 365 mg/cm³. The lightweight and strong Si₃N₄ scaffolds are promising candidates for engineering applications in harsh environments.

Key words: Lightweight, Fibrous Si3 N4 scaffold, Seeds, Directional freeze-casting, High strength