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 8 FEATURE PARAELECTRIC SHAPE MEMORY What is perhaps most unexpected about the electrical activation result in Fig. 2 is the fact that although zirconia is dielectric, it is not ferroelectric. In Table 1, the ferroelectrics occupy a special position due to their inherent electrical polarization; their crystal structures have built-in asymmetries that allow them to carry a permanent or spontaneous polarization and when a field is applied, it interacts directly with the polarization field inherent to the material. In contrast, zirconia is paraelectric with no spontaneous polarization to drive the transformation, because both its austenitic tetragonal and martensitic monoclinic phases are centrosymmetric and have no such intrinsic polarization[7]. The authors suggest that in zirconia, the transformation seen in Fig. 2 is driven by a second-order property—electrical susceptibility—which is dependent on the field itself. When a field is applied, the structure of zirconia can distort slightly and develop a polarization; this induced polarization increases as the field does. Because the two phases involved have different susceptibilities, they can develop a significant polarization difference between them, which can drive the transformation to the more polarizable phase, in this case to tetragonal austenite. The contrast between this mode of electrical activation and that widely known in ferroelectrics is important, because it could foreshadow a new class of electroactive shape memory materials outside of the ferroelectrics family. These results encourage a search for other materials that, like zirconia, exhibit a martensitic transformation between paraelectric phases, but also have enough of an electrical susceptibility mismatch to drive the transformation. Such paraelectroactive shape memory materials could present an interesting new frontier for actuation technology. This susceptibility-mismatch shape memory transformation could foreshadow other couplings among combinations of thermal, mechanical, magnetic, or electrical properties. Table 2 lists a number of such second-order properties, many of which are named and tabulated, but not yet recognized as driving phase transformations by themselves. Some are other simple susceptibilities like elastic compliance or magnetic susceptibility that only involve a single work type (the diagonal terms), while others are properties that link multiple work inputs (off-diagonal terms) such as piezoelectric and magnetoelectric constants. An interesting direction for future research is to identify materials that have no spontaneous difference in a first-order property (entropy, strain, magnetization, polarization), but which can develop a large phase difference by virtue of these second-order properties, triggering the martensitic transformation and shape memory effects. As indicated by the number of material properties listed in Table 2, the potential material search space could be very large. THERMODYNAMIC IMPLICATIONS One intriguing aspect of this susceptibility-mismatch TABLE 2 − PROPERTY MISMATCH AND SHAPE-MEMORY TRANSFORMATIONS For each type of property mismatch that can trigger a shape-memory transformation, there are second-order properties, or susceptibilities, which can also trigger the transformation. These susceptibilities imply that upon application of a driving force (temperature, stress, magnetic field, or electric field), the phase properties change; if this change leads to a large mismatch between austenite and martensite phases, the transformation will be favored. The results in Fig. 2 speak to the electrical susceptibility mismatch transformation in the lower-right corner of this table. 1 0
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