Study On The Electro-thermo-elastic Properties Of Ferroelectric Thin Films With Multi-variants Coexistence

Ferroelectric materials have rich electro-thermo-elastic properties,showing application potential.The key to promoting their wide application is to obtain lead-free ferroelectric materials with excellent properties.Presently,it is a common way to enhance the performance of ferroelectric materials by searching for ferroelectric materials with morphotropic phase boundary so as to obtain the coexistence of many ferroelectric variants and enhance their electro-thermo-elastic properties.However,it is difficult to produce morphotropic phase boundary in ordinary ferroelectric thin film materials.It is urgent to obtain ferroelectric thin films with the coexistence of many ferroelectric variants by other means to improve the performance of ferroelectric thin films.Therefore,in this thesis combines theory and experiment methods to study different ways to increase the types of ferroelectric variants in ferroelectric thin films,including:inducing the coexistence of multi-variants by different orientation growth methods and misfit strain of thin films,inducing the coexistence of multi-variants based on tetragonal phase twinned structure,and inducing the coexistence of multi-variants based on the coexistence of tetragonal and hexagonal phases.The results show that the coexistence of multi-variants generated by these methods can enhance the electro-thermo-elastic properties of ferroelectric thin films.The research content and main conclusions of the thesis are:（1）A framework for free energy and electro-thermo-elastic properties of ferroelectric thin films with multi-variant coexistence is deduced and established.Taking（1-x）PMN-x PT thin films as an example,the effect of orientation growth on the electrocaloric properties of ferroelectric thin films is studied.It is found that the multi-variant coexistence phase boundary has excellent electrocaloric performance.The results show that the phase structure of（111）-oriented ferroelectric films is simple,while the phase structure of（110）-oriented ferroelectric films is very complex,and the（001）,（110）and（111）-oriented films show excellent electrocaloric response at the tetragonal（Te_c）-paraelectric（PE）phase boundary,orthorhombic(O_aa)-paraelectric（PE）phase boundary and rhombohedral(R_aaa)-paraelectric（PE）phase boundary,respectively,which provides theoretical guidance for enhancing the electrocaloric effect performance of ferroelectric films.（2）The thermodynamic model of ferroelectric thin films with twin multi-variant structure is established,and the electromechanical properties of tetragonal twin multi-variant Ba Ti O₃ferroelectric thin films are calculated.It is found that the tetragonal twin multi-variant aa_twphase could improve the electromechanical properties of the thin films.The phase diagrams of Ba Ti O₃ thin films under in-plane shear strain S₆,in-plane normal strain S_m and out-of-plane normal stressσ₃ is obtained by combining the compatibility boundary conditions analysis and the minimum energy principle.The results showed that:S₆<0 is conducive to the formation of tetragonal twinning multi-variant aa_tw structure,S₆>0 is conducive to the formation of tetragonal twinning multi-variant cc_twstructure,the out-of-plane compression stressσ₃ is conducive to the formation of tetragonal twinned structure.The thin film formed tetragonal twinning multi-variant aa_tw structure that exhibits excellent dielectric and piezoelectric properties.This provides theoretical guidance for enhancing the mechanical and electrical properties of ferroelectric thin films.（3）In the experiment,the coexistence of tetragonal twin multi-variants in Ba Ti O₃ferroelectric thin films is realized by mechanical loading,and its influence on the electromechanical properties of Ba Ti O₃thin films is studied.It is found that the tetragonal twin structure can improve the electromechanical properties of the films.Combined with different mechanical loading methods,ferroelectric thin films with different content of tetragonal twin multi-variants are prepared.The results show that Ba Ti O₃thin film samples with more tetragonal twin multi-variants content have higher polarization strength,larger dielectric constant,and higher piezoelectric response,compared with ordinary Ba Ti O₃ thin films.The out-of-plane piezoelectric response of ferroelectric thin films with high twin content is increased by 26%,indicating that the formation of twinned structures is a new way to improve the electromechanical properties of ferroelectric thin films.（4）Zr doping and misfit strain is used to induce the coexistence of tetragonal and hexagonal variants in Ba Ti O₃ ferroelectric thin films.It is found that the electromechanical properties of the films can be doubled.The results show that compared with Ba Ti O₃ thin films,the Ba Zr_0.15Ti_0.85O₃ film formed by Zr doping has obvious hexagonal phase Raman diffraction peaks,with more obvious coexistence of tetragonal and hexagonal variants,the phase characterization results show that the Ba Zr_0.15Ti_0.85O₃ thin films have obvious coexistence of tetragonal phase and hexagonal phase variants compared with Ba Ti O₃ thin films.The Ba Zr_0.15Ti_0.85O₃ thin film has higher remanent polarization and maximum polarization,lower coercive field and higher dielectric and piezoelectric response.Its out-of-plane average piezoelectric response amplitude is 2.3 times that of Ba Ti O₃ thin film,which provides another way to improve the electromechanical properties of ferroelectric thin films.