Experimental Study And Phase Field Simulation On The Multi-field Properties Of Electromagnetics

As the core components of functional devices,electromagnetic materials show great promising applications in the fields of biological medicine,information storage,aeronautics and astronautics and power electronics,owing to their intrinsic magneto-electro-mechanical coupling properties.Electromagnetic functional devices are often served in magneto-electro-mechanical coupling environment.Therefore,test and characterization of the multi-field deformation and fracture behavior of electromagnetics,and regulation of their physical and mechanical properties are meaningful for the design and the quality assessment of functional devices.In this thesis,we focus on the e xperimental study,regulation and improvement on the multi-field coupling properties of electromagnetics.This dissertation consists of the following research work from the aspects of instrument development,experimental study,testing method and domain engineering:(1)The electric,magnetic and thermal modules of the multi-field and multi-load experimental instruments are developed based on the modular design.There are no field and space interference in the de veloped instruments.Furthermore,the constitutive curves of electromagnetic materials are investigated,which verify the feasibility testing and functional completeness of the instruments.(2)The elastic indentation size effect of ferroelectrics caused b y flexoelectric effect is investigated experimentally.It is found that the positive electric field can weaken the elastic indentation size effect while the negative electric field can enhance it,which can be explained by the interaction between the equivalent flexoelectric electric field and the external electric field.We also find that with the increase of the magnetic field,the modulus increases while the hardness and fracture toughness decrease in the ferroelectric of layered magnetoelectric materials,and reveal that the interfacial strain-mediated effect is the tunable mechanism.(3)Based on the micro-and nanoindentation method,the testing method of flexoelectric coefficient has been established.The new testing method can extract the flexoelectr ic coefficient by combining an indentation experiment with finite element analysis,which avoid the interference of dynamic flexoelectric effect,the inaccuracy of static charge's measurement,the calculation error of stress/strain,and the complicated building process of experimental platform.(4)The finite elements based on real-space phase field model for ferroelectric/ferromagnetic materials have been developed.The regulation of flexoelectricity on the mechanical properties of a ferroelectric nano-cone-frustum has been investigation.The domain switching strain can be further increased by reasonably designing the structure configuration to control the formation of vortex state under high loads.The butterfly change in electric field-dependent biaxial modulus of ferroelectrics has been found.The electromechanical coupling properties of ferroelectrics can be regulated by the internal defects which change the local stress and electric field distributions,and the solution by introducing the ion doping with opposite polarization to the initial polarization is the optimum material modification solution.Besides,the electric field induced magnetization for the magnetoelectric composites has been studied,and it is found that an one-way reversible field induced magnetic behavior can be realized under a smaller electric field by applying an appropriate compressive strain.