Investigation Of Electromechanical Coupling Properties And Nonlinear Behaviors Of Flexoelectricity-based Miniature Smart Devices

Flexoelectricity,which refers to the spontaneous electric polarization induced by inhomogeneous deformation or the strain gradient,is a unique electromechanical coupling phenomenon of the solid dielectrics at micro-/nano-scales.Exploiting the potential of flexoelectric materials is of great significance to the development of new smart devices,and thus is expected to influence the fields of human-computer interaction,soft robot,biomedicine and aerospace.Based on the theory of flexoelectricity,the main research works are as follows:1.By considering the flexoelectric effect of the piezoelectric layer,the surface effect as well as the Casimir effect,an electrostatic nanoswitch model based on piezo-elastic laminated beam is developed.Based on the Euler-Bernoulli beam's assumption and the Hamilton's principle,the nonlinear governing equation of the nanoswitch is derived,which is then discreted by the Galerkin method and numerically solved by the Newton's iteration and Runge-Kutta methods.By choosing lead zirconate titanate(PZT-5H)and polyvinylidene fluoride(PVDF)as the material of the piezoelectric layer,the influences of the geometric size,initial gap and flexoelectricity on the static and dynamic pull-in behaviors of nanoswitches are investigated.Numerical results indicate that the flexoelectric effect significantly influences the nonlinear pull-in behaviors of the nanoswitch and such an influence is size-dependent at the nanoscale.2.The electrical outputs and the nonlinear dynamics of axially preloaded flexoelectric energy harvesters are investigated.With the consideration of the von-Kármán geometric nonlinearity,the discreted electromechanical coupling equations of the prebuckling and postbuckling energy harvesters are derived based on the energy method and Galerkin method,which are numerically solved by the Runge-Kutta method.The influences of the external resistance load,damping ratio and amplitude of the base excitation on the performance and nonlinear behaviors of both energy harvesters are examined.Results indicate that the frequency response curves show a typical nonlinear hardening behavior and a softening behavior for the prebuckling and postbuckling energy harvesters,respectively.As compared with the prebuckling energy harvesters,the nonlinear resonant behavior of the postbuckling energy harvesters is more easier to be triggered,and its dynamic responses vary from intrawell to chaotic and interwell oscillations.It is also found that the output voltage and power of the energy harvesters are significantly enhanced due to flexoelectricity and such an enhancement is size-dependent.3.The nonlinear behaviors and performance of composite flexoelectric energy harvesters are investigated.By considering geometric nonlinearity,inertial nonlinearity and flexoelectric effect,unimorph and bimorph energy harvesters based on cantilevered beam with a tip mass are developed.The nonlinear electromechanical coupling equations of the energy harvesters are derived based on Hamilton's principle,which are then numerically solved by the assumed-modes method and Runge-Kutta method.Results show that the geometric nonlinearity leads to a hardening behavior for the frequency response of the energy harvester while the inertial nonlinearity leads to a softening behavior.The tip mass,acceleration of the base excitation and flexoelectric effect significantly influence the performance and nonlinear behaviors of the energy harvesters.By comparing the performance of the composite beam energy harvesters with different configurations and materials,it is found that the nonlinear resonant behavior of the unimorph energy harvesters is more easier to be triggered than bimorph ones with a wider frequency operation bandwidth.The influence of flexoelectricity on the performance of PVDF energy harvesters is more prominent than that of the PZT-5H ones.4.The static bending and free vibration behaviors of flexoelectric cylindrical shells are studied.With the consideration of both piezoelectric and flexoelectric effects,the cylindrical shell under radially distributed pressure load and electrical open-circuit condition is presented.Based on the Love's thin shell assumption and the energy method,the governing equations of the cylindrical shells are derived.For simply-supported shells,the displacements and natural frequencies are obtained by using Navier's method.Results show that both piezoelectricity and flexoelectricity increase the bending stiffness of the cylindrical nanoshells,and thus reduce its radial displacement and enlarge its natural frequency.When the open angle is relatively small,the electric potential of purely flexoelectric shell is larger than that of piezoelectric or piezoelectric-flexoelectric ones,indicating that the flexoelectric sensing performance is better than the piezoelectric sensing.With the decrease of the length,radius and thickness,the fundamental frequency of the cylindrical shells increases significantly and the flexoelectric effect becomes more prominent.In conclusion,by theoretical modeling and numerical computation,this work qualitatively and quantitatively investigates the influence of flexoelectricity on the properties of the micro-/nano-structures-based smart devices,and discuss the nonlinear behaviors of micro-/nano-switches and energy harvesters.The results obtained could provide guidance for the design and application of the flexoelectricity-based smart devices with better performance.