Converse flexoelectricity is a mechanical stress induced by an electric polarization gradient. It can appear in any material, irrespective of symmetry, whenever there is an inhomogeneous electric ...field distribution. This situation invariably happens in piezoresponse force microscopy (PFM), which is a technique whereby a voltage is delivered to the tip of an atomic force microscope in order to stimulate and probe piezoelectricity at the nanoscale. While PFM is the premier technique for studying ferroelectricity and piezoelectricity at the nanoscale, here we show, theoretically and experimentally, that large effective piezoelectric coefficients can be measured in non-piezoelectric dielectrics due to converse flexoelectricity.
BiFeO3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of ...yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically.
Despite the attention devoted to multiferroic BiFeO3 in recent years, many fundamental aspects, such as the phase diagram, are still poorly understood. The phase diagram in the figure, for example, includes several confirmed phase transitions, but does not account for other observed anomalies both above and below room temperature. These unresolved issues, as well as progress made towards applications, are discussed in this Review.
Bones generate electricity under pressure, and this electromechanical behavior is thought to be essential for bone's self‐repair and remodeling properties. The origin of this response is attributed ...to the piezoelectricity of collagen, which is the main structural protein of bones. In theory, however, any material can also generate voltages in response to strain gradients, thanks to the property known as flexoelectricity. In this work, the flexoelectricity of bone and pure bone mineral (hydroxyapatite) are measured and found to be of the same order of magnitude; the quantitative similarity suggests that hydroxyapatite flexoelectricity is the main source of bending‐induced polarization in cortical bone. In addition, the measured flexoelectric coefficients are used to calculate the (flexo)electric fields generated by cracks in bone mineral. The results indicate that crack‐generated flexoelectricity is theoretically large enough to induce osteocyte apoptosis and thus initiate the crack‐healing process, suggesting a central role of flexoelectricity in bone repair and remodeling.
The generation of electricity by bending (flexoelectricity) is measured in bones and bone mineral, hydroxyapatite. The measured flexoelectric coefficients are then used to calculate the flexoelectric fields around cracks in hydroxyapatite. The results indicate that crack‐induced flexoelectricity is sufficiently large to have an important physiological effect as the signaling mechanism for the healing of bone fractures.
The bending-induced polarization of barium titanate single crystals has been measured with an aim to elucidate the origin of the large difference between theoretically predicted and experimentally ...measured flexoelectricity in this material. The results indicate that part of the difference is due to polar regions (short-range order) that exist above T(C) and up to T*≈200-225 °C. Above T*, however, the flexovoltage coefficient still shows an unexpectedly large anisotropy for a cubic material, with (001)-oriented crystals displaying 10 times more flexoelectricity than (111)-oriented crystals. Theoretical analysis shows that this anisotropy cannot be a bulk property, and we therefore interpret it as indirect evidence for the theoretically predicted but experimentally elusive contribution of surface piezoelectricity to macroscopic bending-induced polarization.
Harvesting environmental energy to generate electricity is a key scientific and technological endeavour of our time. Photovoltaic conversion and electromechanical transduction are two common ...energy-harvesting mechanisms based on, respectively, semiconducting junctions and piezoelectric insulators. However, the different material families on which these transduction phenomena are based complicate their integration into single devices. Here we demonstrate that halide perovskites, a family of highly efficient photovoltaic materials
, display a photoflexoelectric effect whereby, under a combination of illumination and oscillation driven by a piezoelectric actuator, they generate orders of magnitude higher flexoelectricity than in the dark. We also show that photoflexoelectricity is not exclusive to halides but a general property of semiconductors that potentially enables simultaneous electromechanical and photovoltaic transduction and harvesting in unison from multiple energy inputs.
The recent discovery of bone flexoelectricity (strain-gradient-induced electrical polarization) suggests that flexoelectricity could have physiological effects in bones, and specifically near bone ...fractures, where flexoelectricity is theoretically highest. Here, we report a cytological study of the interaction between crack stress and bone cells. We have cultured MC3T3-E1 mouse osteoblastic cells in biomimetic microcracked hydroxyapatite substrates, differentiated into osteocytes and applied a strain gradient to the samples. The results show a strong apoptotic cellular response, whereby mechanical stimulation causes those cells near the crack to die, as indicated by live-dead and caspase staining. In addition, analysis two weeks post-stimulation shows increased cell attachment and mineralization around microcracks and a higher expression of osteocalcin -an osteogenic protein known to be promoted by physical exercise. The results are consistent with flexoelectricity playing at least two different roles in bone remodelling: apoptotic trigger of the repair protocol, and electro-stimulant of the bone-building activity of osteoblasts.
The proximity of a thermodynamic triple point and the formation of transient metastable phases may result in complex phase and microstructural trajectories across the metal-insulator transition in ...strained VO2 films. A detailed analysis using in-situ synchrotron X-ray diffraction unveils subtle fingerprints of this complexity in the structure of epitaxial films. During phase transition the low-temperature monoclinic M1 phase is constrained along the {111}R planes by the coexisting high-temperature R phase domains, which remain epitaxially clamped to the substrate. This geometrical constraint induces counteracting local stresses that result in a combined tilt and uniaxial in-plane compression of M1 domains, and a concomitant anomalous cR-axis elongation. This mechanism progressively transforms the M1 phase into the transitional triclinic phase (T), and ultimately into the monoclinic M2 phase, generating strong strain and tilt gradients that remain frozen after the complete transformation of the R phase upon cooling to RT. The transformation path of VO2 films, the complex competition between stable and metastable VO2 polymorphs and its impact on the structure of the low temperature monoclinic state, provide essential insights for understanding the electronic and mechanical properties of the films at the nanoscale, as well as to control their use in functional devices.
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The mechanical properties of materials are insensitive to space inversion, even when they are crystallographically asymmetric. In practice, this means that turning a piezoelectric crystal upside down ...or switching the polarization of a ferroelectric should not change its mechanical response. Strain gradients, however, introduce an additional source of asymmetry that has mechanical consequences. Using nanoindentation and contact‐resonance force microscopy, this study demonstrates that the mechanical response to indentation of a uniaxial ferroelectric (LiNbO3) does change when its polarity is switched, and use this mechanical asymmetry both to quantify its flexoelectricity and to mechanically read the sign of its ferroelectric domains.
Ferroelectrics are smart functional materials. Their polarization responds to external stimuli (stress, voltage, and temperature) for transductors and can be switched for memories. Thanks to the flexoelectricity, their mechanical response to indentation is also found to be switchable, so that they may be regarded as mechanically smart. A practical consequence, demonstrated here, is the mechanical reading of ferroelectric polarity.
Reversible electric‐field control of the metal‐insulator transition in the rare earth nickelate compound NdNiO3 is demonstrated. Using the electric double layer technique, giant modulations in the ...conductance as well as large shifts in the transition temperature are obtained.
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