Fabrication Of Twinned BaTiO₃ And Characterization Of Its Electromechanical Coupling Properties

Ferroelectric materials with excellent electromechanical coupling properties have been widely used in sensors and actuators.At present,the method of improve the electromechanical coupling properties of ferroelectrics is mainly through the morphotropic phase boundary（MPB）,the essence of which is the coexistence of ferroelectric multi-variables to cause the excellent electromechanical coupling properties.However,there are two disadvantages of this method.First,Lead-based ferroelectrics with MPB are harmful to the environment.Second,lead-free ferroelectrics with MPB are difficult to be fabricated due to their complex components.Therefore,this requires to realize the coexistence of multiple ferroelectric variants in one simple component lead-free ferroelectric material in order to achieve excellent electromechanical coupling properties.The variants of general ferroelectrics will be multiplied when the structure is twined.Thus,we try to fabricate the general ferroelectrics with twined structures to realize multi-variables,and expect a greatly enhancement in electromechanical coupling properties.In this thesis,we choose the classical ferroelectric material BaTiO₃ as example.We have explored and fabricated twinned BaTiO₃ powders,twinned BaTiO₃/PVDF composite films and twinned BaTiO₃ thin films.Taking the twin structures of BaTiO₃as the research core,we study the effect of twin structure on the electromechanical coupling properties from powders,composite films and thin films.The main contents and conclusions are as follows:（1）we successfully fabricated the twinned BaTiO₃ powders by two-step alkali heat method based on titanium dioxide.The phase analysis of twinned BaTiO₃powders was determined by XRD and Raman spectroscopy,and found that BaTiO₃powders show a tetragonal phase.SEM observations show that the morphological structure of twinned BaTiO₃ powders are stacked interspersed with multiple cubic blocks,and the surface is smooth.The formation mechanism of twinned BaTiO₃powders is:in the TiO₂,Ti-O octahedrons are connected by dots,and the addition of Ba ions form ordinary BaTiO₃ crystals;however,the Ti-O octahedrons become surface-connected after the alkali-thermal reaction,{111}twin crystals will be formed by Ba ions adding.（2）The twinned BaTiO₃/PVDF composite films,pure PVDF films and common BaTiO₃/PVDF composite films with different doping mass fractions are prepared by the blending method.Comparing with common BaTiO₃/PVDF films,twinned BaTiO₃/PVDF films have better dielectric,ferroelectric and electromechanical coupling properties.When the doping mass fraction is 30%,the piezoelectric coefficient d₃₃ of the twinned BaTiO₃/PVDF composite films have nearly 300%improvement than that of common BaTiO3/PVDF composite films.（3）We also fabricated the BaTiO₃ thin film by Sol-gel method,and then the BaTiO₃ thin film was hot-pressed by three-point bend loading model to produce the twin structures in BaTiO₃ thin films.The twin structures in BaTiO₃ thin films can be observed and analyzed by transmission electron microscope（TEM）.The BaTiO₃ thin film are tested by semiconductor analyzer,ferroelectric analyzer and piezoelectric force microscope（PFM）,the results show that the region of the BaTiO₃ thin film which are subjected to hot pressing treatment with higher tensile stress have more twin structures,and the dielectric properties,ferroelectric properties,and electromechanical coupling properties also have great enhancement.Overall,the twin structures in BaTiO₃ thin films by hot-pressing treatment can greatly improve the electromechanical coupling properties.