A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 1 5 6 DIRECT ELECTRICAL ACTIVATIONOF THE SHAPEMEMORY EFFECT New research explores a zirconia-based shape memory ceramic that exhibits a unique type of electrically triggered martensitic transformation. Christopher A. Schuh, FASM,* and Alan Lai, Massachusetts Institute of Technology Shape memory ceramics (SMCs) exhibit a number of significant advantages over the more common shape memory metals. Zirconia-based SMCs, for example, can achieve much higher transformation stresses (~GPa) and be used at much higher temperatures (> 500°C) than metal-based versions. Recent work exploring SMCs in small volumes[1] and single crystal forms[2] has discovered significant capacity for sustained cyclic transformation, avoiding the cracking that characterized earlier work on ceramics undergoing a martensitic transformation[3]. One particularly unique advantage of SMCs over shape memory metals is that they do not conduct electricity; as dielectrics, SMCs can sustain electric fields. In the authors’ recent work, they have shown that such applied electric fields couple to the martensitic transformation[4], opening the door to a new class of what they term paraelectroactive shape memory ceramics. *Member of ASM International MODES OF SHAPE MEMORY ACTUATION As actuators, shape memory alloys are valued for their very high specific work output. For their size and weight, they can actuate over relatively long stroke distances with high forces. This is shown in the figure of merit map where contours of specific work increase toward the upper righthand corner (Fig. 1). However, a perennial challenge for shape memory alloys as actuators is that they are activated by heating, either directly with heaters or indirectly via electrical resistance heat. In either case, the kinetics of heat flow limit the speed of two-way actuation. It is highly desirable to trigger shape memory transformations directly with fields instead of temperature, avoiding the kinetics of heat transfer and permitting actuation on demand through control electronics. The field-driven shape memory effect, through martensite domain flipping, has been demonstrated in magnetic shape memory alloys and ferroelectrics. In these cases, the martensite domains carry an orientation of magnetization or electrical polarization, respectively, and an applied field can bias the formation of domains that are aligned with it[5,6]. In order to achieve actuation through a martensitic transformation, the austenite and mar- tensite phases must have a difference in some physical property, as highlighted in Table 1 for various different thermodynamic cases. In thermal transformations, this is simply the entropy difference between the phases. If there is a strain/shape difference between the phases, this results in shape memory and superelastic properties. In each case, the work input favors transformation to the phase more responsive to it. For magnetic shape memory and ferroelectric materials, the applied field couples with the inherent magnetic or electric axes, i.e., the magnetization or the polarization, respectively, and favors domains that are aligned. ELECTRICALLY DRIVEN TRANSFORMATION IN ZIRCONIA In this landscape, shape memory ceramics such as zirconia offer a unique set of properties. FEATURE Fig. 1 — An Ashby map compares actuation technologies on the basis of stress and strain, with the product of those quantities leading to the figure of merit (specific work) captured by the dashed lines. Shape memory materials are solid state options with the highest combination of stress and strain[1]. 8
RkJQdWJsaXNoZXIy MTYyMzk3NQ==