Possible piezoelectric composites based on the flexoelectric effect

Current piezoelectric composite materials contain two or more phases out of which at least one reveals piezoelectric properties in itself. We show that this is in fact not a necessary condition. The mechanism of the linear stress-polarization response averaged over a composite sample can be also based on flexoelectric properties of one or more constituents. Proper shaping of the composite constituents is required, such that the system as a whole acquires a symmetry allowing for nonzero piezoelectric coefficients even if none of the components is piezoelectric. Externally applied stress is transformed, due to proper geometry of the constituents with different elastic properties, into a strongly nonhomogeneous distribution of induced strain. Flexoelectric properties which are, by symmetry, allowed in all materials, transform the strain gradient into polarization. The proposed piezoelectric composite falls into the category of composites with product properties since it involves different assets of the phases (elastic, flexoelectric and dielectric) and the interaction between the phases, determining the inhomogeneous distribution of stress, is essential.