Final Report Summary - FLEXOELECTRICITY (Flexoelectricity)
Flexoelectricity is the ability of materials to generate a voltage when they are bent or, conversely, to bend under voltage. It can be present in all dielectric materials, but in practice the flexoelectric coefficients are so small that flexoelectricity is virtually imperceptible at the human scale. At the micro and the nanoscales, however, it is possible bend materials much more, so bending-induced electricity can become large. That was the basic premise of our project. Before before getting down to the nanoscale, however it has been necessary to even resolve some fundamental aspects. Questions as basic as the intrinsic magnitude of flexoelectricity, for example, presented disagreements of orders of magnitude between theoretical estimates and experimental mesurements. Our research has resolved many of these questions and gone on to make significant new discoveries with profound implications that reach beyond flexoelectricity itself, in such diverse areas as mechanical properties of materials, MEMS devices or bone biology. Below we list summarize in chronological order the main outcomes of the project.
1. Intrinsic magnitude of flexoelectricity. We have resolved the long-standing controversy about this problem, showing that the flexocoupling coefficient (flexoelectricity divided by permittivity) of oxides is of the order of 10V, with higher values being generated by (i) polar regions and/or (ii) surface piezoelectricity. The latter had been predicted but never before evidenced.
2. Semiconductor flexoelectricity. We have discovered that flexoelectricity is not only a property of dielectric insulators, but also of semiconductors. Moreover, the effective flexoelectric coefficient can be bigger for semiconductors than for dielectrics.
3. Flexoelectric microelectromechanical systems (MEMS). We exploited converse flexoelectricity (bending induced by voltage) to make the first-ever flexoelectric MEMS actuators: cantilevers that oscillate in response to an alternating voltage. Our devices had a performance comparable to that of state-of-the-art piezoelectric cantilevers. Additionally, we have invented a new type of device, a “strain diode” that bends for one voltage polarity but not for the opposite.
4. Flexoelectricity and mechanical properties. Flexoelectricity introduces asymmetry in the mechanical response of ferroelectrics and piezoelectrics, whereby their local mechanical response (hardness, stiffness and so on) can be changed just by turning them upside down or switching their polarization. This is a profound discovery, as it changes the symmetry restrictions on what is mechanically possible in solids. On a practical level, our discovery enables new functionalities: smart coatings whose mechanical properties can be switched with a voltage, “crack valves” where crack propagation can be switched on or off with a voltage, or ferroelectric memories that can be read “by touch” without the need for electrodes.
5. Bioflexoelectricity. We have discovered that bones are flexoelectric. According to our measurements and calculations, flexoelectricity plays an important role in bone self-repair: flexoelectric fields generated around cracks are sufficiently large to stimulate and guide the repair cells to the fracture front. We believe this discovery can have a disruptive effect by enhancing osteogenesis in prosthetic implants and bone grafts.
The above results have been published in high impact journals (Nature, Nature Nanotechnology, Advanced Materials, Physical Review Letters) and some of them, such as the invention of flexoelectric MEMS, the discovery of semiconductor flexoelectricity or the discovery of bone flexoelectricity have received attention from the press. There have also been recognitions: the discovery of semiconductor flexoelectricity was nominated among the top-8 scientific highlights of the year 2016 in Spain by the national newspaper La Vanguardia, and the PhD thesis where this discovery was described (by Dr Jackeline Narvaez) received the prize for the best PhD thesis in experimental condensed matter physics in Spain.
1. Intrinsic magnitude of flexoelectricity. We have resolved the long-standing controversy about this problem, showing that the flexocoupling coefficient (flexoelectricity divided by permittivity) of oxides is of the order of 10V, with higher values being generated by (i) polar regions and/or (ii) surface piezoelectricity. The latter had been predicted but never before evidenced.
2. Semiconductor flexoelectricity. We have discovered that flexoelectricity is not only a property of dielectric insulators, but also of semiconductors. Moreover, the effective flexoelectric coefficient can be bigger for semiconductors than for dielectrics.
3. Flexoelectric microelectromechanical systems (MEMS). We exploited converse flexoelectricity (bending induced by voltage) to make the first-ever flexoelectric MEMS actuators: cantilevers that oscillate in response to an alternating voltage. Our devices had a performance comparable to that of state-of-the-art piezoelectric cantilevers. Additionally, we have invented a new type of device, a “strain diode” that bends for one voltage polarity but not for the opposite.
4. Flexoelectricity and mechanical properties. Flexoelectricity introduces asymmetry in the mechanical response of ferroelectrics and piezoelectrics, whereby their local mechanical response (hardness, stiffness and so on) can be changed just by turning them upside down or switching their polarization. This is a profound discovery, as it changes the symmetry restrictions on what is mechanically possible in solids. On a practical level, our discovery enables new functionalities: smart coatings whose mechanical properties can be switched with a voltage, “crack valves” where crack propagation can be switched on or off with a voltage, or ferroelectric memories that can be read “by touch” without the need for electrodes.
5. Bioflexoelectricity. We have discovered that bones are flexoelectric. According to our measurements and calculations, flexoelectricity plays an important role in bone self-repair: flexoelectric fields generated around cracks are sufficiently large to stimulate and guide the repair cells to the fracture front. We believe this discovery can have a disruptive effect by enhancing osteogenesis in prosthetic implants and bone grafts.
The above results have been published in high impact journals (Nature, Nature Nanotechnology, Advanced Materials, Physical Review Letters) and some of them, such as the invention of flexoelectric MEMS, the discovery of semiconductor flexoelectricity or the discovery of bone flexoelectricity have received attention from the press. There have also been recognitions: the discovery of semiconductor flexoelectricity was nominated among the top-8 scientific highlights of the year 2016 in Spain by the national newspaper La Vanguardia, and the PhD thesis where this discovery was described (by Dr Jackeline Narvaez) received the prize for the best PhD thesis in experimental condensed matter physics in Spain.