Advancements and Prospects of Flexoelectricity.

The flexoelectric effect, as a novel form of the electromechanical coupling phenomenon, has attracted significant attention in the fields of materials science and electronic devices. It refers to the interaction between strain gradients and electric dipole moments or electric field intensity gradients and strain. In contrast to the traditional piezoelectric effect, the flexoelectric effect is not limited by material symmetry or the Curie temperature and exhibits a stronger effect at the nanoscale. The flexoelectric effect finds widespread applications ranging from energy harvesting to electronic device design. Utilizing the flexoelectric effect, enhanced energy harvesters, sensitive sensors, and high-performance wearable electronic devices can be developed. Additionally, the flexoelectric effect can be utilized to modulate the optoelectronic properties and physical characteristics of materials, holding the potential for significant applications in areas such as optoelectronic devices, energy storage devices, and flexible electronics. This review provides a comprehensive overview of the historical development, measurement of flexoelectric coefficients, enhancement mechanisms, and current research progress of the flexoelectric effect. Additionally, it offers a perspective on future prospects of the flexoelectric effect.

Flexible BaTiO3 Thin Film-Based Coupled Nanogenerator for Simultaneously Scavenging Light and Vibration Energies.

Ferroelectric materials have a variety of properties, such as piezoelectricity, pyroelectricity, and the ferroelectric photovoltaic effect, which enable them to obtain electrical energy from various external stimuli. Here, we report a coupled nanogenerator based on flexible BTO ferroelectric films with a cantilevered beam structure. It combines the photovoltaic and flexoelectric effects in a ferroelectric materials-based coupled nanogenerator for simultaneously scavenging vibration energy and light energy, thus improving energy scavenging performance. As compared with the photovoltaic effect individually, simultaneous vibration and light illumination under a light intensity of 57 mW/cm2 at 405 nm can produce a photo-flexoelectric coupling current of 85 nA, where the current peak has been enhanced by 121%. Due to the photo-flexoelectric coupling effect, the device has outstanding charging performance, where a 4.7 µF capacitor can be charged to 60 mV in 150 s. These devices have potential applications in multi-energy scavenging and self-powered sensors.

Coupling Enhancement of a Flexible BiFeO3 Film-Based Nanogenerator for Simultaneously Scavenging Light and Vibration Energies.

Coupled nanogenerators have been a research hotspot due to their ability to harvest a variety of forms of energy such as light, mechanical and thermal energy and achieve a stable direct current output. Ferroelectric films are frequently investigated for photovoltaic applications due to their unique photovoltaic properties and bandgap-independent photovoltage, while the flexoelectric effect is an electromechanical property commonly found in solid dielectrics. Here, we effectively construct a new form of coupled nanogenerator based on a flexible BiFeO3 ferroelectric film that combines both flexoelectric and photovoltaic effects to successfully harvest both light and vibration energies. This device converts an alternating current into a direct current and achieves a 6.2% charge enhancement and a 19.3% energy enhancement to achieve a multi-dimensional "1 + 1 > 2" coupling enhancement in terms of current, charge and energy. This work proposes a new approach to the coupling of multiple energy harvesting mechanisms in ferroelectric nanogenerators and provides a new strategy to enhance the transduction efficiency of flexible functional devices.

Controllable Piezo-flexoelectric Effect in Ferroelectric Ba0.7Sr0.3TiO3 Materials for Harvesting Vibration Energy.

The rapid development of the automotive and aerospace industries has led to an increasingly urgent need for electromechanical coupling materials and devices. Here, we have demonstrated the tunable piezo-flexoelectric effect in ferroelectric Ba0.7Sr0.3TiO3 materials for scavenging vibration energy. The positive peak output current of an ITO/Ba0.7Sr0.3TiO3/Ag cantilever device based on the flexoelectric effect is only 45 nA at room temperature, which is promoted to 90 nA by the piezo-flexoelectric effect. In addition, the piezo-flexoelectric current of the device can be further boosted to 270 nA by increasing the working temperature to 41.0 °C with a corresponding enhancement ratio of 348.28%. The significantly improved piezo-flexoelectric current is ascribed to the ultrahigh dielectric constant, which is related to the tetragonal-cubic phase transition of the Ba0.7Sr0.3TiO3 materials. This work reveals the temperature-modulated piezo-flexoelectric effect in ferroelectric Ba0.7Sr0.3TiO3 materials, providing a convenient route for scavenging and sensing of vibration energy.