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

Abstract

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%.

Freeze‐cast honeycomb structures via gravity‐enhanced convection

“…With the slower freezing front velocity, one would advance to the left in the stability‐microstructure map in which cellular growth is expected (indicated by an orange arrow in Figure 4D). This is demonstrated by Zeng et al who observed cellular pores by decreasing the freezing front velocity at constant temperature gradient 19 . However, this could not be the sole factor for the formation of the cells because the sample frozen under 0.7 µm/s and 4.9 K/mm with conventional freezing still exhibits dendritic pores with a bimodal pore‐size distribution (Figure 4E).…”

“…The polymer solution was degassed for 10 min to avoid air bubble formation during freezing. Directional freezing was conducted using a gradient‐controlled freeze‐casting setup 19 . The polymer solution was poured into a cylindrical mold placed on a thermoelectric plate.…”

Latest Advances in Development of Smart Phase Change Material for Soft Actuators

“…Generally, transition temperature is in the wide range that can be adjusted between 0 and 1200 C. [107] Apart from fine structure, Zeng et al presented a new method that aligns honeycomb-shaped cellular porous structure directionally to develop zirconia-based ceramics by gradient-controlled freezing casting. [108] In addition, the comparison of 1.5 and 2.0 mol% yttria stabilized zirconia between single crystal and polycrystalline structure has been analyzed by Crystal et al. [109] Owing to high ductility, high specific strength, and inertness in harsh environments, SMC is expected to offer promising solutions to the soft actuation applications.…”

Realizing reversible phase transformation of shape memory ceramics constrained in aluminum

“…In the past decade, the brittle nature of ZrO 2 has been mitigated by constructing miniature specimens (e.g., micro-/nano-pillars 8 , single/oligo-crystals 9 – 12 , nano-fibers 13 , 14 ), and structures formed thereof (e.g., honeycombs via 3D printing 15 , cellular foams via freeze casting 16 , 17 , and granular packings 3 , 18 ), where the ceramic is relatively free to expand/contract because of the high specific surface area and/or the removal of grain boundaries. Nevertheless, it remains a challenge to integrate these small-volume shape memory ceramics into a matrix material and realize their reversible phase transformation in a confined state without causing fracture.…”