Bringing out the flex in flexoelectrics

Bend a dielectric and it will become electrically polarized. Conversely, an electric field gradient in the dielectric will induce a strain. The effect is called flexoelectricity, and unlike the better-known piezoelectric effect, it can arise in crystals with any symmetry. But it’s typically so small that it’s hard to disentangle from the larger effects of piezoelectricity and electrostriction, constriction caused by field-induced alignment of electrical domains. Now, North Carolina State University’s Xiaoning Jiang and his colleagues have devised a way to isolate the converse flexoelectric effect. They attached electrodes on the slanted faces of trapezoidal blocks of barium strontium titanate, a ceramic known to exhibit an anomalously large flexoelectric effect. Each sample essentially acted as a nonparallel-plate capacitor so that in the z direction, the field component was very small but the field gradient was large, as shown in this image from a simulation. The researchers then applied AC voltages to the electrodes and measured the shear deformation along the x direction. In principle, their setup should eliminate electrostrictive and piezoelectric deformation. In practice, the experiment showed some contributions from each effect, but they could be readily accounted for. Based on their analysis, the researchers predict that the flexoelectric effect should strengthen relative to the piezoelectric effect, and may even become dominant, as the trapezoid becomes smaller. If true, that could be interesting to makers of micro- or nanoelectromechanical devices. (L. Shu et al., Appl. Phys. Lett. 104, 232902, 2014 doi:10.1063/1.4882060 .)