Phenomenological Model And Dynamic Analysis Of The Ferroelectric-Antiferroelectric Phase Transition Of PZT95/5 In A Mechanic-Thermo-Electric Multi-Field Environment

Antiferroelectric materials have unique characteristics of double hysteresis loops,high energy storage density,high breakdown voltage and other excellent properties.High-power devices made of antiferroelectric materials have the advantages of small size,light weight and high energy storage density.They have become indispensable materials in aerospace,precision instruments and high-end equipment manufacturing industries such as high-precision sensors,transducers,powerful energy storage capacitors,new generations of high-performance pulse power,infrared pyroelectric detection and electrocaloric refrigeration and other high-tech applications.Pb Zr_0.95Ti_0.05O₃（PZT95/5）is near the boundary of ferroelectric（FE）and antiferroelectric（AFE）phases.Due to its abundant microscopic domain and phase structures,as well as strong mechanic-electric coupling effect,it is easy to translate between different ferroelectric,antiferroelectric and paraelectric phases under external fields（such as temperature,pressure,electric field,etc.）.Accompanied by significant changes in the mechanical and electrical properties,especially high current outputs resulting from the release of bound charges under pressure,it has very important applications in high power explosive power supplies,energy exchange equipments and high-tech devices.The internal structure of PZT95/5 ceramics exhibits complex domain and phase transitions with the change of external temperature,pressure and electric field.However,existing experiments are difficult to fully characterize the internal structure changes of the material under complex multi-field environment,the relevant theoretical work cannot reflect the dynamic response mechanism of the domain structure of the antiferroelectric material under different temperature,stress and electric field,which makes it difficult to carry out the simulation work and cannot provide strong support for the applications.Therefore,to carry out theoretical research and develop dynamic method on AFE-FE phase transition in a multi-field environment is of great significance,and also the basic scientific issues and hot topic of antiferroelectric materials questions in engineering and application research.In this thesis,the thermodynamic theory and phase translation dynamic analysis of PZT95/5 material in multi-field environment are studied.Firstly,based on the micro experimental characterizations,a thermodynamic model which can describe the phase transitions of antiferroelectric materials with special polar microstructure under multi-field is established by considering the oxygen octahedron tilt in antiferroelectric orthogonal phase and the effect of stress on the polarization and tilt angle.The thermodynamic model systematically considered all phases in PZT95/5 material for the first time.The simulated temperatures-pressure phase diagram is consistent with the experimental phase diagram,and the ferroelectric-antiferroelectric phase translation of the material under temperature,hydrostatic pressure and electric field is studied.The free energy,polarization and mechanical properties of PZT95/5 under different environments characterized by simulation are in good agreement with experimental results.Combined with oxygen octahedral tilt angle,the special polar microstructure of the intermediate phase between ferroelectric and antiferroelectric phases,which is named as ferrielectric phase（Fi E phase）,is further revealed,and the relationship between the incommensurate modulations in Fi E phase and the external fields is given.Secondly,combined with experiments,the thermodynamic model of PZT95/5under uniaxial compression was established,and then extended to arbitrary compression state.It can reveal the phase transition characteristics of the material under different loading stress,and so as to make a more comprehensive study.The results show that the FE-AFE phase transition depends not only on the compressive stress,but also on the direction of the pressure relative to the polarization.The FE-AFE phase transition is more likely to occur when the polarization is parallel to the stress direction.Furthermore,the triaxial constant-stress-difference（CSD）tests of Sandia National Laboratory was simulated.The results show that increasing the stress difference decreases the mean stress of phase translation,and the applied electric field increases the phase translation pressure of polarized ceramics,which is consistent with the experimental results of Sandia Laboratory.Finally,based on the thermodynamic model of PZT95/5 in multi-field environment,a three-order parameters phase-field model of FE-AFE multiphase coexistence under thermal-mechanic-electric multi-field was developed.The domain structure evolution and phase transition behavior of PZT95/5 in different external field were investigated numerically.The results show that the material evolution under temperature field is consistent with the experimental phase diagram.In the process of FE-AFE translation induced by compressive stress,oxygen octahedral rotation in ferroelectric phase promotes the translation.The electric field induces antiferroelectric to ferroelectric phase transition,during which the Fi E phase is formed.