Flexoelectret: An Electret with a Tunable Flexoelectriclike Response

Because of the flexoelectric effect, dielectric materials usually polarize in response to a strain gradient. Soft materials are good candidates for developing a large strain gradient because of their good deformability. However, they always suffer from lower flexoelectric coefficients compared to ceramics. In this work, a flexoelectriclike effect is introduced to enhance the effective flexoelectricity of a polydimethylsiloxane bar. The flexoelectriclike effect is realized by depositing a layer of net charges on the middle plane of the bar to form an electret. Experiments show that the enhancement of flexoelectricity depends on the density of inserted net charges. It is found that a charged layer with surface potential of $-5723\mathrm{V}$ results in a 100 times increase of the material’s flexoelectric coefficient. We also show that the enhancement is proportional to the thickness of electrets. This work provides a new way of enhancing flexoelectricity in soft materials and further prompts the application of soft materials in electromechanical transducers.

Figure 1

Schematic illustration of the mechanism of the flexoelectret. Arrows in this figure represent polarization. Longer arrows correspond to larger polarization. (a) Structure of the flexoelectret. A layer of charges is deposited on the middle plane of a PDMS bar. (b) Initial state of the flexoelectret. The polarizations above and below the charge layer have the same magnitude $P_0$ but opposite directions. (c) Under uniform compression, the shape of the bar has changed but is still symmetric about the middle plane. The polarizations of two parts still have the same magnitude $P_1$ and opposite directions. (d) Under bending the symmetry of the bar is broken. The magnitudes of polarization $P_2^{\prime }$ and $P_2^{\prime \prime }$ are no longer the same, which generates net polarization along the thickness direction.

Figure 2

Structure of a lipid bilayer membrane. Arrows represent the average polarization of the lipid molecules. Longer arrows correspond to larger polarization. (a) A flat lipid bilayer membrane, which is symmetric about its middle plane. (b) A curved lipid bilayer membrane. The symmetry of membrane is broken, which leads to net polarization.

Figure 3

Experimental setup and results. (a) Experimental setup for flexoelectric measurement. (b) Output charge $Q$ from the upper electrode and vertical displacement of the center point of the sample, corresponding to the red and blue lines, respectively. (c) Output charge $Q$ and the effective flexoelectric coefficient ${\mu }_{13}^{\text{eff}}$ as a function of the surface potential of the charge layer. The inset shows output charge of pure PDMS and flexoelectret ($-33V$).

Figure 4

Schematic illustration of the FEM model of flexoelectrets and calculated results. The polarization and electric displacement distribution are all absolute values. Dashed lines represent the deformed model. (a) Model under uniform pressure. (b) Preexisting polarization distribution in the initial state, which is uniform. (c) Polarization distribution under uniform compression, which is still uniform. (d) Model under three-pointing bending. (e),(f) Polarization and electric displacement distribution under bending, respectively, showing a strong asymmetry. (g) Calculated effective flexoelectric coefficient as a function of charge density. The inset shows the dependence of the electronic flow on the strain gradient and charge density.

Figure 5

Normalized effective flexoelectricity as a function of sample thickness. The results show a linear relationship between flexoelectricity and thickness.