Crystal Growth And Gigantic Piezoelectric Properties Of Potassium Sodium Niobate Crystals

In the past few decades,the traditional lead-based piezoelectric materials has dominated application area because of its good temperature stability and excellent piezoelectric properties.However,lead-based materials will cause great harm on the environment and the human body,and many countries have imposed restrictions on the use of lead-containing materials.Therefore the development of alternative,excellent and environmentally friendly lead-free piezoelectric materials has become emergency in the field of materials science.At present,potassium sodium niobate(K1-xNaxNbO3,referred to as KNN)based lead-free piezoelectric materials because of its excellent piezoelectric,ferroelectric and dielectric properties,and high C urie temperature,are considered as one of the most competive substitute of the lead-based materials.The research on KNN-based lead-free piezoelectric materials is mainly focused on the ceramic,and the method to improve the properties of KNN-based lead-free material by chemical doping has been caught in the contradictions between the performance and temperature stability.The main factors,in addition to the ceramic sintering process and optimize the selection,is that the research on the physical mechanism is not suitablefor ceramic,but suitablefor single crystal with high orientation and simple structure.Therefore,the growth,electrical properties,piezoelectric propert ies,large strain and gigiantic piezoelectric constant of pure KNN crystal are studied in this paper.The KNN single crystals with the composition x = 0.11 – 0.70 were first grown by the top seed solvate method.The optimized growth conditions have been explored through observing the temperature field and process conditions of crystal growth.By continuously optimizing the growth parameters,KNN crystals with high quality,large size and no cracks are grown.The influence of the K and Na elements and the temperature on the composition in the single crystal were discussed by analyzing the relationship between the growth temperature,the composition and the excess solvent.The structure of KNN crystal was systematically studied.The structure of different components at room temperature was analyzed by XRD.The change of lattice constant with the composition was studied.The growth morphology of KNN crystal was calculated and analyzed by the first principles principle.It was confirmed that the crystal fillet existed in the single crystal.The electrical properties of KNN single crystal were systematically studied.The Curie temperature and the quadratic phase transition temperature of the crystal were determined from the dielectric properties of KNN crystals.In the study of ferroelectric properties,the low I / V curve shows that KNN crystal has good quality although there is a phenomenon of leakage in the hysteresis loop of low frequency.A complete sets of full matrix date of K0.47Na0.53NbO3 and K0.8Na0.2NbO3 were obtained.It is found that K0.47Na0.53NbO3 and K0.8Na0.2NbO3 crystals have relatively large thickness electromechanical coefficients and have considerable application prospects in thickness electromechanical devices.And the orientation effect of the ferroelectric crystal is studied by using the coordinate change method.It is found that the orientation of the ferroelectric domain plays a major role in the macroscopic electromechanical performance of the crystal.The evolution of piezoelectric properties of KNN crystals was studied systematically.The piezoelectric constants of different crystal components before and after polarization were studied by quasi-static method,and the relationship between spontaneous polarization direction and composition was recorded.The relationship between the components,the direction of spontaneous polarization and the free energy is determined,and the free energy can affected the spontaneous polarization direction and eventually the macroscopic piezoelectric propert ies.It is found that only when x = 0.53 and 0.57,there is a huge electrostrictive behavior,which further proves that the free energy affects spontaneous polarization.The effect of different temperature and frequency on the electrostrain is studied systematically,which shows the good working temperature and stability,and has good recoverability.The cyclic strain S reaches 0.55% at 0.1 Hz test frequency.Through the improved growth method,the KNN crystal with huge piezoelectric coefficient was grown with a piezoelectric performance of 1500 pC /N.By analyzing the crystal domain structure,it can be found that the density of the domain is larger than that of the normal crystal.At the same time,the curved domain and the ladder-like domain are observed under the PLM microscope.The twist domain even a monoclinic phase is observed by PFM,and the the changing and complexity domain is observed by TEM.The XRD rocking curve shows more intuitive response to the stress into the crystal as the lattice causes a huge change.Through the analysis of Landau theory,it can be predicted that when the internal stress of KNN crystal is larger,the piezoelectric constant can reach more than 3000 pC/N.The improvement of piezoelectricity is determined because of the reduced free energy,resulting in spontaneous polarization caused by easy deflection.