Comprehensive Review on Flexoelectric Energy Harvesting Technology: Mechanisms, Device Configurations, and Potential Applications

Abstract

Flexoelectricity is the electromechanical coupling between mechanical strain gradient and electric polarization or vice versa. This unique effect becomes considerable as the material dimension decreases and is found in a wide range of materials. In this review, flexoelectric effects in different dielectric materials and their characterization methods have been summarized. Also, the different mechanical structures for the measurement of electric outputs corresponding to different components of strain gradient are discussed. We have also discussed in detail the recent theoretical models developed in the field of flexoelectricity. Furthermore, the factors contributing to the effect such as various mechanisms of the flexoelectric response in ferroelectrics and affecting the effective flexoelectric coefficient have been highlighted. Along with providing a broad material choice, it has found vast potential applications in sensors, actuators, transducers, etc., and thus, a flexoelectric nanogenerator has proven to be an energy harvesting technique. The recent advances in flexoelectric energy harvesting techniques have been reviewed. Apart from these applications, we have given a brief report on the recent developments and other applications of the flexoelectric effect. Moreover, the recent discoveries in flexoelectricity that claim that this effect may promote the development of nanoelectronics have been summarized. At the end of the review, the future perspective of flexoelectric-based applications has been discussed.

Multilayered MoS2 Sphere-Based Triboelectric–Flexoelectric Nanogenerators as Self-Powered Mechanical Sensors for Human Motion Detection

“…Continuously, upon application of completely external pressure, vertical strain gradients occur within the multilayered MoS 2 spheres owing to the stacked sphere structure. The strain gradient, as shown in the following eq , is proportional to the electric polarization induced by the flexoelectric effect where P l is the induced flexoelectric polarization, S ij is the elastic strain, μ ijkl is the flexoelectric coefficient, and x k is the position coordinate.…”

“…28,29 Therefore, unlike the piezoelectric effect, the electric polarization of the flexoelectric effect can be improved by maximizing the strain/stress gradient based on the control of various nano-or microscale materials and devices, which can be applied to all-dielectric materials to effectively generate electrical signals from mechanical stimuli. 30,31 Recently, many researchers have been interested in flexoelectric devices and the operating mechanism. For instance, Kim et al reported flexoelectric-boosted electromechanical properties of piezoelectric nanoparticles using an induced built-in strain gradient in a heterogeneous core− sphere nanostructure of BaTiO 3 @SrTiO 3 .…”

“…The flexoelectric effect can be considered a new alternative for the development of effective high-performance self-powered mechanical sensors. The flexoelectric effect results from the interactions between electrical polarization and a strain/stress gradient and essentially involves the generation of an electric polarization response under a mechanical strain/stress gradient or a mechanical response under an electric field gradient. , In addition, the flexoelectric effect can occur even in dielectric materials with a symmetric crystal structure owing to induction by mechanical gradients, unlike the piezoelectric effect that is observed in specific materials with a structural asymmetric crystal structure. , Therefore, unlike the piezoelectric effect, the electric polarization of the flexoelectric effect can be improved by maximizing the strain/stress gradient based on the control of various nano- or microscale materials and devices, which can be applied to all-dielectric materials to effectively generate electrical signals from mechanical stimuli. , Recently, many researchers have been interested in flexoelectric devices and the operating mechanism. For instance, Kim et al reported flexoelectric-boosted electromechanical properties of piezoelectric nanoparticles using an induced built-in strain gradient in a heterogeneous core–sphere nanostructure of BaTiO 3 @SrTiO 3 .…”

Electrospinning Triboelectric Laminates: A Pathway for Scaling Energy Harvesters

“…Small scale mechanical motion, including human or animal motion, vibrations and acoustic waves, are of great importance for powering wearable, implantable, and remote microelectronics. [ 1 ] Electromechanical energy harvesters, including piezoelectric, [ 2 ] triboelectric, [ 3 ] and flexoelectric devices, [ 4 ] are able to capture this mechanical energy and generate electricity. Of these, only triboelectric devices are able to be produced from a wide range of generally available materials.…”

Theoretical estimation of longitudinal flexoelectric response in polar perovskite oxides

“…The precise flexoelectric coefficients will be equally significant from basic concepts and application viewpoints. The uncertainty in flexoelectric coefficients will enormously affect the accuracy of devices because they act as input parameters in developing flexoelectric-based devices such as flexoelectric-energy-harvesters [18], sensors [19], and actuators [20]. Therefore, various experimental designs, such as threeand four-point bending [21,22], pyramid compression [23], shock wave [24], and converse flexoelectric method [25], are proposed for measuring the pure flexoelectric coefficients.…”