Nano 180° domains written by local mechanical force via the flexoelectric effect have recently attracted great attention since they may enable applications in which memory bits are written mechanically.

With the development of integrated circuit technology and the decreasing size of devices, ferroelectric films used in nano ferroelectric devices must become thinner and thinner. Along with the downscaling of the ferroelectric film is the increasing serious leakage current which has seriously hindered the broad application of ferroelectric devices. Here we tuned the leakage currents in Pb(Zr 0.1 Ti 0.9 )O 3 ferroelectric thin films through flexoelectricity by means of the phase field method with diffusion equations for the electron/hole. It is shown that the strain gradient generated by the local compressive force can raise the hole current but reduce the electron current in ferroelectric film. Pure mechanical force can therefore be used to diminish the leakage current. With the further study of the effects of different flexoelectric coupling types on leakage current, we demonstrate that the flexocoupling type described by the longitudinal flexoelectric coefficient promotes the increase of the hole current but has a side-effect on the increase of the electron current. In contrast, the role of the flexocoupling type described by the transverse flexoelectric coefficient is just the opposite.

The size of plastic deformation zone during fastener hole strengthening is a critical indicator of the strengthening effect. In this study, a considerable plastic deformation zone in 1.5 mm aluminum alloy plate with a hole was produced via electromagnetic strengthening. The finite element analysis results showed that the electromagnetic strengthening process could achieve high compressive hoop residual stress around the fastener hole in thin plate without serious axial deformation compared with conventional cold hole expansion process. The simulation results were experimentally validated by the grid method. Furthermore, for the same discharge energy, the size of plastic deformation zone varies with the discharge capacitance, and there was an optimal combination of the discharge capacitance and discharge voltage.What's more, even the plastic deformation zone was the same at the maxed load, different unloading process during the electromagnetic hole expansion process also had a great influence on the strengthening effect.

With the development of the integrated circuit technology and decreasing of the device size, ferroelectric films used in nano ferroelectric devices become thinner and thinner. Along with the downscaling of the ferroelectric film, there is an increasing influence of two strain gradient related terms. One is the strain gradient elasticity and the other one is flexoelectricity. To investigate the interrelationship between flexoelectricity and strain gradient elasticity and their combined effect on the domain structure in ferroelectric nanofilms, a phase field model of flexoelectricity and strain gradient elasticity on the ferroelectric domain evolution is developed based on Mindlin's theory of strain-gradient elasticity. Weak form is derived and implemented in finite element formulations for numerically solving the model equations. The simulation results show that upper bounds for flexoelectric coefficients can be enhanced by increasing strain gradient elasticity coefficients. While a large flexoelectricity that exceeds the upper bound can induce a transition from a ferroelectric state to a modulated/incommensurate state, a large enough strain gradient elasticity may lead to a conversion from an incommensurate state to a ferroelectric state. Strain gradient elasticity and the flexoelectricity have entirely opposite effects on polarization. The observed interrelationship between the strain gradient elasticity and flexoelectricity is rationalized by an analytical solution of the proposed theoretical model. The model proposed in this paper could help us understand the mechanism of phenomena observed in ferroelectric nanofilms under complex electromechanical loads and provide some guides on the practical application of ferroelectric nanofilms.

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