| Ferroic-materials,which include ferroelectric materials,ferromagnetic materials,antiferroelectric materials and antiferromagnetic materials,are widely used as functional materials in the research of capacitors,nonvolatile memories,energy storage components and magnetoelectric coupling.Based on the spherical aberration corrected electron microscopy,this thesis mainly focuses on the measurement of order parameters of ferroic-materials.Firstly,the influence of Wiener filter on the measurement of ferroelectric polarization is analyzed,then the magnetic parameters of ferromagnetic La1-xSrxMn O3/Sr Ti O3 thin film system is determined on the nanoscale.Next,the atomic-structure of antiferroelectric silver niobate is analyzed,the definition of reduced antiferroelectric parameters is given on the basis of it,and the energy storage performance of tantalum-doped silver niobate system is studied.Modern electron microscopy can quantitatively measure the local ionic ferroelectric displacement in ferroelectric materials,but in which extent will Wiener Filter affects the measurement accuracy has not yet been determined.Combining qualitative analysis and simulation experiments,it is found that Wiener Filter does not reduce the measurement accuracy of non-periodic displacement when using appropriate parameters.As shown in our synthetic experiment,when the thickness of the amorphous layer is not more than 4nm,the average relative length deviation of the displacement measurement is less than12%,and the average angular deviation is within 5°.EMCD is a new magnetic measurement technique based on electron microscopy,which has many advantages but the magnetic moment resolution needs to be improved.By fully considering the dynamical diffraction effect and optimizing the data processing process,La1-xSrxMn O3/Sr Ti O3 thin film with weak ferromagnetism was studied on the nanoscale with EMCD technique,and corresponding magnetic parameters were obtained.Such results proved that it is feasible and effective to analyze weak magnetic system with EMCD technique.Researchers have already had a good understanding of the structure of antiferroelectric materials on the mesoscale,but the knowledge of structures in the microscopic view is still absent.Using spherical aberration-corrected STEM technique,HAADF images of antiferroelectric silver niobate were obtained.It is found that each kind of cation deviates along two opposite±[11—0]c directions and such an atomic arrangement eventually forms wavy?11—0?c atomic planes,and two main defects,namely90°antiferroelectric domain boundary and antiphase domain wall were also found.Combined with diffuse scattering lines,we found that the B-site niobium ions also have displacements along±[001]c directions and form a series of displacement nanoregions with a typical scale 10 nm.All the niobium ions in each nanoregion move along one identical direction,which can be either one of the two directions of±[001]c,and the maximum displacement value is about 10 pm.Furthermore,we defined the reduced antiferroelectric order parameter based on experimental images,which is suitable for a series of complex oxide systems represented by silver niobate.With the help of peak finding and Gaussian fitting technique,the antiferroelectric order parameters of silver niobate before and after tantalum doping were obtained.It was found that the reduced antiferroelectric order parameter decreased slightly after doping.Next,in the framework of phenomenological theory,it is demonstrated that both the increase of antiferroelectric order parameter and the decrease of the permittivity of paraelectric phase can effectively increase the energy storage density.The main reason for the improvement of energy storage density after tantalum doping is that the permittivity of the high-temperature paraelectric phase decreases significantly.Finally,based on the analysis above,we gave a strategy to design new electrostatic energy storage materials with high performance. |
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