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On the mechanisms of tip-force induced switching in ferroelectric thin films: The crossover of depolarization, shear strain and flexoelectricity
摘要： The recent observation of mechanical switching of ferroelectric polarization has placed the mechanical manipulation of ferroelectrics on an equal footing with the conventional electrical manipulation. However, discussions on the exact switching mechanisms due to mechanical loads are ongoing for the complexity in experimental situations. In this work, based on continuum mechanical and thermodynamic modeling and simulation, we analyze the mechanisms of tip-force induced switching in ferroelectric thin films. The roles of depolarization, shear strain and flexoelectricity in mechanical switching, both in normal and sliding loading modes, are separated out and the switching characteristics are analyzed. The depolarization field in the film is demonstrated to enable bidirectional switching. The coupling between shear strain and polarization components is shown to be important in the sliding loading mode. A great influence of flexoelectricity-modified polarization boundary condition on the switching process is revealed. The previous speculation that the switching process experiences an intermediate paraelectric phase is proved. The regulation of loading force, misfit strain, temperature and film thickness on the switching are further given for each mechanism. Taking all of the three mechanisms into account, we present the phase diagrams of mechanical switching for films in an initial upward or downward polarization state. The revealed characteristics of various switching mechanisms should provide useful guidelines for their verification in experiments, and the tunability of the switching by various influencing factors is instructive for the design and optimization of ferroelectric devices via mechanical engineering.
关键词： shear strain,mechanical switching,ferroelectrics,flexoelectricity,depolarization
更新于2025-09-23 15:23:52
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Geometrically nonlinear analysis of Timoshenko piezoelectric nanobeams with flexoelectricity effect based on Eringen's differential model
摘要： This study analyzes the nonlinear free vibration and post-buckling of nanobeams with flexoelectric effect based on Eringen’s differential model. The nanobeam is modeled based on Timoshenko beam's theory. The von-Kármán strain-displacement relation together with the electrical Gibbs free energy and Hamilton's principle are employed to derive equations of motion. The nonlinear free vibration frequencies are obtained for pinned-pinned (P-P) and clamped-clamped(C-C) boundary conditions. Multiple scales method is employed to obtain the closed-form solution for the nonlinear governing equations. By employing this methodology, the natural frequencies of nanobeams are obtained and their post-buckling behavior is examined. The influence of nonlocal parameter, amplitude ratio, and input voltage on the top surface and flexoelectricity constant on nonlinear free vibration and post-buckling characteristics of nanobeamis investigated. In this paper, it is concluded that the flexoelectricity has a significant effect on free vibration of the beams in nano scale and its effect has to be considered in designing nano-electro-mechanical systems (NEMS) such as nano- generators and nano-sensors.
关键词： Nonlinear vibration,Flexoelectricity,Nonlocal elasticity,Postbuckling
更新于2025-09-23 15:22:29
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Flexoelectricity in Monolayer Transition Metal Dichalcogenides
摘要： Flexoelectricity, the coupling effect of the strain gradient and charge polarization, is an important route to tune electronic properties of low-dimensional materials. Here our extensive first-principles calculations reveal that structural wrinkling and corrugation will cause significant flexoelectricity in transition metal dichalcogenide (TMD) monolayers. The flexoelectricity is induced by the strain gradients created along the finite thickness of the wrinkled TMD monolayers and becomes more dominant in determining out-of-plane polarizations with decreasing wavelengths of the TMD wrinkles. According to the first-principles calculations and whole structural symmetry, a theoretical model is developed to describe the total out-of-plane polarizations and flexoelectric effect of the wrinkled TMD monolayers. The unveiled flexoelectricity in monolayer TMDs highlights a potential for their application in energy conversion devices.
关键词： energy conversion,monolayer,strain gradient,flexoelectricity,polarization,transition metal dichalcogenides
更新于2025-09-23 15:21:21
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Mechanical writing of in-plane ferroelectric vortices by tip-force and their coupled chirality
摘要： Recent experiments have demonstrated the existence of vortex or flux-closure domains in ferroelectric nanostructures, which are attractive to develop high-density data storage and novel configurable electronic devices. However, it remains challenging to stabilize in-plane vortex or flux-closure domains in ferroelectric film for the absence of a lateral geometry confinement. Based on a 3D phase field model, here we show that stabilization of isolated or interacting in-plane vortices in ferroelectric film can be achieved via applying a mechanical tip-force. The formation of such dipole vortices is caused by a conjoint effect of the tip-force-induced depolarization effect and in-plane strain. The effects of factors like film thickness, misfit strain, tip force and temperature on the vortex formation are systematically revealed and summarized as phase diagrams. The interaction between tip-induced vortices is also investigated. It is found that as the two tips get closer than the critical distance, the two initially isolated vortices become coupled, with identical or opposite chirality, depending on the distance between the two tips. A maximum data storage density of isolated in-plane vortices in ferroelectric thin film is estimated to be ~1 Tb in?2. Our work thus demonstrates a mechanical strategy to stabilize dipole vortices, and provides a comprehensive insight into the characteristics of ferroelectric film under a mechanical tip force.
关键词： flexoelectricity,ferroelectric,mechanical force,domain structure,vortex
更新于2025-09-23 15:19:57
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An isogeometric approach to flexoelectric effect in ferroelectric materials
摘要： Flexoelectricity is an electromechanical coupling effect between the polarization and strain gradient in all dielectrics regardless of point group symmetry. Due to its significant influence on material behavior at the nanoscale, the flexoelectric effect has attracted more and more attention in recent years. In this paper, a real space phase field model for the flexoelectric effect in ferroelectric materials is developed by using isogeometric analysis (IGA). The IGA employs the same smooth and high-order basis functions to describe both the geometry of material and the solution of phase field, which is able to give an accurate and efficient description of the flexoelectric effect in ferroelectrics with arbitrary geometrical shapes and boundary conditions. To this end, phase field simulations on the effect of flexoelectricity are conducted for nanoscale ferroelectrics with different geometrical shapes and boundary conditions. The simulation results show that the flexoelectric effect has significant influence on the domain structures and domain switching of ferroelectric materials at the nanoscale. For ferroelectric nanobeam under bending load, due to the flexoelectric effect, the mechanical bending can break the symmetry of hysteresis loop between electric field and polarization. As for ferroelectric nanodots, the flexoelectric effect increases the magnitude of spontaneous polarizations and results in the tilting of polarization vortex. In addition, out-of-plane components appear in the polarization vortex of ferroelectric nanodots due to the flexoelectric effect, which increases the coercive field for the switching of polarization vortex and changes the switching process significantly. The present work not only presents an effective nonlocal model for the domain evolution in ferroelectric materials with the consideration of the flexoelectric effect, but also predicts new switching behavior of the polarization vortex due to the flexoelectric effect in ferroelectric nanodots.
关键词： Ferroelectrics,Isogeometric analysis,Flexoelectricity,Phase field model
更新于2025-09-09 09:28:46