Multi-Directional Freeze Casting of Porous Ceramics with Bone-Inspired Microstructure

Abstract

Porous ceramics are favored in a multitude of applications, such as filters, catalyst supports, and tissue engineering scaffolds. However, conventional fabrication techniques find it challenging to preserve sufficient mechanical strength. Although unidirectional freeze casting can fabricate porous ceramics with high strength in the vertical direction, the strength in other directions is inadequate due to the lack of lateral structural control. Herein, inspired by the cancellous bone that is lightweight and strong, we report 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. Alumina-mullite composite ceramics are prepared with hierarchical structures across multiple scales, including the micro-level multi-oriented struts as compressive groups, and the sub-micro level interlamellar bridges and nano-level eutectic phases as tensile groups. The multi-oriented struts endow the porous ceramics with high strength in all 3D spatial directions (8.4–20.1 MPa), while simultaneously, the tensile groups effectively prevent the catastrophic brittle failure. Therefore, the porous ceramics with enhanced mechanical properties demonstrate the remarkable controllability of the proposed multi-directional freeze casting process in hierarchical structures.

Keywords: freeze casting, Bioinspired Materials, Porous Ceramics, mechanical properties