Space Charge Induced Flexoelectric Transducers for Energy Harvesting

Lead zirconate titanate (PZT) is widely used in energy harvesting because of its excellent material properties. However, as the material contains lead, there are significant environmental concerns with its production and use. Flexoelectricity refers to the coupling between strain gradient and electric polarization that exists, in principle, in all dielectric materials and would allow for energy harvesting without using piezoelectric materials. However, the effect is very weak in most materials. Promisingly, it has recently been shown that space charge polarized materials (i.e., semiconducting materials with insulating barrier layers) can exhibit enhanced flexoelectricity. This space charge induced flexoelectric effect opens up the possibility of a non-toxic replacement for PZT in energy harvesting applications. In this paper we investigate the use of doped silicon with hafnium oxide insulating layers as flexoelectric transducers that could replace PZT in many applications including energy harvesting. Specifically, we experimentally demonstrate flexoelectricity in a bending beam and show an effective flexoelectric coefficient of 4.9 μC/m. Finally, we develop and demonstrate a finite element model for flexoelectricity.