Study On Domain Structures And Energy Storage Efficiency Of Ferroelectric Nanocomposites

Composite materials comprised of ferroelectric nanoparticles in a dielectric matrix have a variety of functional properties attractive for a wide range of novel electronic and energy storage devices.However,the dependence of these functionalities on shapes,sizes and orientation of ferroelectric particles is currently not fully understood.In this disser-tation,a time-dependent Ginzburg–Landau approach combined with coupled-physics finite-element-method are utilized to simulate the behavior of polarization in isolated Pb Ti O₃or Ba Ti O₃nanoparticles embedded in a dielectric medium.The equilibrium polarization topology is strongly affected by particle size,as well as the choice of inclu-sion and matrix materials,with monodomain,vortex-like and multidomain structures emerging for various combinations of size and materials parameters.The calculations results show that there is a critical particle size below which ferroelectricity vanishes.For the Pb Ti O₃and Ba Ti O₃particle,this size is 2 and 3.4 nm,respectively.Particle shapes belonging to the superellipsoidal family were probed,including oc-tahedral,near octahedral,sphere,near cubic and cubic geometries.For each shape,a parametric sweep of particle sizes ranging from 2 to 18 nm was conducted,revealing a trend for the texture transformations from a monodomain,through a vortex-like,to a multidomain structures,as the size increases.Critical particle sizes for the domain structures instabilities were found to be strongly dependent on the particle shape,with octahedral particles undergoing transitions at much larger volumes,compared to the cubic particles.Furthermore,for each of the considered non-spherical shapes of appro-priate size,it was possible to obtain multiple vortex-like structures whose paraelectric cores are aligned with every rotational axis of the particle point symmetry group.The shape-dependent metastability of the vortex-like structures opens up new avenues for controlling polarization at the nanoscale in a variety of technological applications.Then we investigated the electric field driven response of isolated ferroelectric nanoparticles embedded in a dielectric matrix and its dependence on particle size,shape,and orientation.Particle shapes belonging to the superellipsoidal family were consid-ered,including octahedral,spherical,and cuboidal structures,as well as a number of intermediate geometries.Perovskite Pb Ti O₃and Sr Ti O₃,respectively,were chosen as the inclusion ferroelectric particle and surrounding dielectric materials.In particles of all shapes that are large enough to support domain walls at zero applied field we ob-served polarization switching by a formation of intermediate phases,which possess an appreciable amount of vorticity stemming from the domain wall motion through the ferroelectric inclusion volume.The system coercive field E_cand energy storage effi-ciency were found to be strongly dependent on the particle shape and orientation,but not on its size.In near spherical particles with easy polarization axis pointing away from the direction of E,smallest E_cand highest storage efficiencies were obtained,while non-spherical particles with aligned easy polarization and E directions exhibited high-est E_cand relatively low energy storage efficiencies.The results of these investigations show the great promise of ferroelectric nanocomposites for a variety of advanced en-gineering applications,including energy storage,non-volatile multibit memories,opto and low-power electronics,as well as metamaterials.