Multi-directional freeze casting of porous ceramics with bone-inspired microstructure

Porous ceramics are favored in a multitude of applications, such as filters, catalyst supports, and tissue engineering scaffolds. However, conventional fabrication techniques find it particularly challenging to preserve sufficient mechanical strength in highly porous ceramics. Although unidirectional freeze casting can fabricate porous ceramics with high strength vertically, the strength in other directions is inadequate due to a lack of lateral structural control. Herein, inspired by the cancellous bone, we propose a novel multi-directional freeze casting technique to prepare highly mechanically efficient porous ceramics. A multi-directional temperature field is ingeniously designed to mimic the stress-responsive growth pattern of the cancellous bone. To further the lateral structural control, ceramic fibers are incorporated to form mineral bridging. In this process, alumina-mullite composite ceramics are prepared with hierarchical structures, including micro-level multi-oriented struts, sub-micro-level interlamellar bridges and nano-level eutectic phases. They endow the ceramics with high porosity (∼75%) and high strength in all 3D spatial directions (8.4–20.1 MPa), while effectively preventing the catastrophic brittle failure. Therefore, the mechanically enhanced porous ceramics demonstrate the remarkable controllability of multi-directional freeze casting in hierarchical structures. Also, our work opens up a new horizon for fabricating highly mechanically efficient porous materials, including hierarchically structured biomimetic ceramics.