Investigation Of The Electromechanical And Electrocaloric Properties Of Ferroelectric Nanostructures

In recent years,ferroelectric nanostructures,including films,nanowires,nanocylinders and nanoscale metamaterials etc.,have drawn much attention due to their distinctive material properties as compared to bulk ferroelectric materials.By virtue of their excellent dielectric,electromechanical and electrocaloric properties,these ferroelectric nanostructures have prospective applications in ferroelectric non-volatile random access memory,micro-electromechanical system,high frequency capacitors and solid-state refrigeration.The focus of this thesis is on the research of energy conversion in ferroelectric nanostructures(such as the conversions of electrical energy into mechanical energy and of electrical energy into thermal energy).The contents of this thesis include:(1)Aiming at dealing with the huge amount of computation that stems from using nonlinear finite element method to achieve the ferroelectric phase-field method,a scheme of accelerating numerical simulations by GPU computing was proposed.Numerical experiments show that the GPU implementation has a big advantage of speed over the CPU implementation when large simulation models are involved.This scheme greatly speeded up the numerical simulations in other chapters of this thesis.(2)Using the 2D phase-field modelling,the inverse piezoelectric properties in nanoscale ferroelectric metamaterials based on Archimedean lattices were investigated.Due to the intrinsic anisotropy of ferroelectric materials and the complex geometric structures of Archimedean lattices,these nanoscale ferroelectric metamaterials exhibit much better inverse piezoelectric properties than their bulk counterparts.In addition,by analogy with the cutting theory of quartz crystal,we investigated the feasibility of tuning such inverse piezoelectric properties by rotating the lattices.Furthermore,the simulation results show that,at certain rotating angles,the lattices exhibit considerable longitudinal and transverse inverse piezoelectric coefficients that are positive or negative simultaneously,which is rare to be seen in conventional bulk ferroelectric materials.(3)Using the 3D phase-field modelling,the giant electrocaloric effect driven by domain transition in ferroelectric nanocylinders was investigated with consideration of the influence of the surface polarization effect.The simulation results show that the surface polarization effect,characterized by the extrapolation length,can change the polarization near surfaces and significantly influence the polarization switching behavior.Further investigation shows that the giant electrocaloric effect driven by domain transition in the nanocylinders near room temperature can also be influenced by this surface effect,with the value and temperature location of its peak changed.The innovation points of the current work are:(1)The use of the finite element method to achieve the phase field model has brought many computational advantages,however it has also brought a huge amount of computation.For the first time,this thesis attempted to apply GPU computing to the phase-field model and successfully accelerated the numerical simulations.(2)With the help of the concept of metamaterials,excellent inverse piezoelectric properties were predicted in nanoscale ferroelectric Archimedes lattices.In addition,tuning such properties was achieved by rotating the lattices.(3)For the first time,the influence of the surface polarization effect was considered in the electrocaloric effect research of nanoscale multi-domain ferroelectrics.As a form of size effect,such surface effect is a factor which has to be considered in the design of nanoscale ferroelectric refrigeration devices.