Growth,Piezoelectric Properties And Domain Structure Of Potassium Niobate Based Crystals

Piezoelectric materials,which can convert mechanical and electrical energy,are one of the most widely used functional materials.At present,piezoelectric materials have been widely used in many applied fields,such as sensors,actuators,ultrasound transducers,etc.Lead zirconate titanate??Pb,Zr?Ti O₃,PZT?based materials occupy an important position in the equipment field because of their excellent piezoelectric performance.However,the high lead content in PZT does significant harm to the environment and human health.Therefore,it is necessary to find lead-free piezoelectric materials with good performance and environmental friendliness to replace lead-based piezoelectric materials.It has become one of the main topics in the field of piezoelectric materials.In the past 15 years,potassium niobate?KNb O₃,KN?based materials have received great attention,due to their high Curie temperature and excellent piezoelectric and ferroelectric properties.In the last decades,the study on KN-based ceramics is mainly focused on the phase boundary,which increases the activity of domains in the material to improve their piezoelectric performance.Compared with ceramics,the advantages of single crystals are mainly reflected in the consistency of the grain orientation and domain engineering.Hence,the growth of the single crystal is also considered as a method to improve the piezoelectric performance.In addition,the research on the relationship between the mesoscopic domain structure and macroscopic properties of KN-based single crystals is still in its infancy.Single crystals are also able to investigate inherent characteristics of materials,which is conducive to exploring the inherent relationship between the composition,structure and performance.As a result,exploring the single crystal growth method based on KN and studying the intrinsic relationship between the mesoscopic structure and macroscopic properties are of great significance to promote the research process of this material system.Firstly,the growth methods and conditions of KN-based single crystals grown by the top-seed solution growth method are investigated.The segregation of the components during the crystal growth process was studied,which enables controlled the composition of the potassium sodium tantalum niobate(K_1-xNa_xTa_1-yNb_yO₃,KNTN)single crystal.The relationship between single crystal phase structure and spontaneous polarization and structure of the ferroelectric domain wall is summarized.In addition,methods for characterizing the dielectric,ferroelectric,and piezoelectric performance were explored,and the temperature and electric field dependence of the dielectric constant,ferroelectric and piezoelectric coefficients of the materials were characterized.Secondly,the growth conditions and methods of potassium tantalum niobate?KTa1-xNbx O₃,KTN?single crystals and the segregation law of the components during the growth process were systematically studied.A KTN single crystal with a Ta/Nb composition gradient was designed and grown.The process of establishing the internal electric field by the macro-polarization caused by the component gradient was analyzed,in principle.The structure and evolution of domains under applied electric field and temperature verified the presence of internal electric field.The physical processes that affect the structure and movement of electric domain was also studied.What is more,with the relationship between the domain evolution and the macroscopic properties,the connection between the composition gradients in all directions of the sample and the macroscopic dielectric,ferroelectric,and piezoelectric properties are revealed.Thirdly,the method of characterizing the full matrix material parameters of piezoelectric materials was explored,and the relationships between the elastic modulus,dielectric constants,piezoelectric coefficients,and impedance spectra of the samples were calculated.The connnections between the polarization and the speed of sound waves in landscape mode and portrait mode were investigated.And then the full matrix material parameters poled along the [001]PC for the tetragonal KTN and orthogonal KNTN single crystals were measured via resonance and sound velocity coupling methods,which can be verified with the equation between them.Based on second-,third-,and fourth-order tensors of rotation and the transformation method between bisubscript and unisubscript in the dielectric,elastic,piezoelectric,and electromechanical coupling constants,the orientation dependence of these performance is theoretically analyzed.In addition,the contribution of the domain walls to the material parameters in the single crystal is determined with measurement values.Finally,the temperature dependence of the dielectric constants for the KNTN single crystals with various compositions were measured.And the phase structure and piezoelectric coefficients at room temperature were determined.The domain structures were observed with the polarized light microscope and piezoresponse force microscope.The connection between the domain structure of single crystal and the phase transition temperature was obtained.As the phase transition temperature increases,the domain density decreases and the driving voltage increases.In addition,the domain structure gradually changed from a labyrinthine structure to a strip structure.The domain structure and evolution,which influence the dielectric properties of the sample during the temperature-induced phase transition were studied.Thermal hysteresis and metastable structures under an applied electric field were observed during the single crystal phase transition.In addition,the large strain,which originated from a lattice distortion caused by the electric-field-induced phase transition was studied,and a large inverse piezoelectric coefficient was obtained,approximately 1400 pm/V.