Robust Cellular Shape‐Memory Ceramics via Gradient‐Controlled Freeze Casting

• Published in: Advanced engineering materials Vol. 21; no. 12
• Main Authors: Zeng, Xiaomei, Arai, Noriaki, Faber, Katherine T.
• Format: Journal Article
• Language: English
• Published: 01.12.2019
• Subjects: cellular structures; ice templating; martensitic transformations; porous ceramics; shape-memory ceramics
• ISSN: 1438-1656; 1527-2648
• DOI: 10.1002/adem.201900398

Shape‐memory ceramics offer promise for applications like actuation and energy damping, due to their unique properties of high specific strength, high ductility, and inertness in harsh environments. To date, shape‐memory behavior in ceramics is limited to micro‐/submicro‐scale pillars and particles to circumvent the longstanding problem of transformation‐induced fracture which occurs readily in bulk polycrystalline specimens. The challenge, therefore, lies in the realization of shape‐memory properties in bulk ceramics, which requires careful design of 3D structures that locally mimic pillar structures. Herein, it is demonstrated that with a gradient‐controlled freeze‐casting approach, honeycomb‐like cellular structures can be fabricated with thin and directionally aligned walls to facilitate martensitic transformation under compression without fracture. With this approach, robust bulk shape‐memory ceramics are demonstrated in a highly porous structure under compressive stresses of 25 MPa and strains up to 7.5%. Directional cellular structures of shape‐memory zirconia are achieved through gradient‐controlled freeze casting by exploiting constitutional supercooling. The well‐aligned oligocrystalline walls can accommodate significant shape deformation induced by martensitic transformation without fracture. The shape‐memory effect is realized at the bulk scale with a significant strain up to 7.5%, having at a high mechanical strength stress relative to other porous ceramics.