Study On Growth And Electromechanical Properties Of Potassium Sodium Tantalate Niobate

Piezoelectrics and ferroelectrics are important functional materials, whoseapplications have been throughout every corner of our daily lives and the production.They can convert mechanical energy into electrical energy and vice versa, and havebeen widely used in many electromechanical devices. Recently, Pb（Zr1-xTix）O3（PZT）ceramics have been the mainstay in the field of piezoelectric materials, whoseproduction process and applied technology have been very mature. However,Lead-based ceramics have a high lead content （around60%in weight）. lead can causegreat pollution to the environment during the production process as a result of itsvolatility and toxicity. With our enhanced awareness of environmental protection,electronic products containing lead are prohibited by the international communitythrough laws, regulations and government directives. Thus，it is significant to developenvironmental friendly lead-free piezoelectric materials. In recent decades,Domestic and foreign researchers have undertaken extensive research on lead-freepiezoelectric materials. Among the varieties of lead-free piezoelectric ceramics thatare extensively investigated, researchers have achieved a major breakthrough onpotassium sodium niobate （KNN） ceramics. In2004, Japanese scientists reported thatKNN ceramics doped with lithium and tantalum have good piezoelectric propertieswhich are comparable to that of PZT ceramics. The report attracted considerableattentions to KNN-based piezoelectrics which are lead-free. In the future, KNN-basedpiezoelectrics will be a promising alternative candidate for alternative PZT systems.First, we used the top seed solution growth method to grow K1-xNaxTa1-xNbxO3（KNTN） piezoelectric single crystals. Excess K₂O is used as a cosolvent. Thepreparation of single crystals includes a series of process. The optimal growthprocedure was investigated, and high-quality crystals were obtained. The size of theas-grown crystal is up to10mm×10mm×25mm. The physical properties of KNTNwere investigated in detail. The structures of KNTN crystals were studied by usingX-ray powder diffraction method（XRD）and the compositions of KNTN crystals werecharacterized by Energy Dispersive Spectrometry（EDS）.The measurement resultsindicate that the as-grown crystals are with high composition uniformity and they arein pure orthorhombic phase at room temperature.The temperature dependence and the poling electric field dependence of themacroscopic electromechanical properties of the [001] poled KNTN single crystalshave been determined experimentally. KNTN crystals are poled easy due to their lowcoercive and remanent polarization. With the increase of temperature, the structure ofKNTN changes from orthorhombic phase to tetragonal phase at103degreesCelsius,and changes from tetragonal phase to paraelectric phase at260degreesCelsius. The dielectric loss of KNTN crystals is quit low. For [001] oriented KNTN single crystals, an optimal poling electric field is1000V/mm at room temperature.After poling, engineered domain configuration are formed, and the superiorlongitudinal electromechanical coupling factors k33being0.56, longitudinalpiezoelectric coefficient d33being140×10-12C/N can be obtained in KNTN singlecrystals. The piezoelectric properties of KNTN crystals remain stable in widetemperature range（from room temperature to90degrees Celsius）. The structure ofKNTN changes from pure orthorhombic phase to the coexistence of orthorhombic andtetragonal near the curie point（103degrees Celsius）. The superior longitudinalelectromechanical coupling factors k33and longitudinal piezoelectric coefficient d33reach their maximum at this temperature. Orthorhombic phase transforms completelyto tetragonal phase above120degrees Celsius, resulting in the longitudinalpiezoelectric coefficient d33decreased sharply. These results are significant forunderstanding the characteristics of KNTN single crystals, finding suitable workenvironment, and improving the application dependability of this material.By using combined ultrasonic and resonance method, we accurately measured acomplete set of material properties of the [001] poled KNTN single crystals. Thecombined ultrasonic and resonance method offers much better accuracy andconvenience than the resonance method in the full matrix property characterization ofthese multidomain single crystals. We prepared several resonators with geometriesdefned in the IEEE standards and prepared three standard samples in addition tomesure the phase velocities of ultrasonic waves propagating along certaincrystallographic directions. the crystal symmetry is rhombohedral3m according to theXRD pattern. After being poled along [001], the effective symmetry of themultidomain system becomes tetragonal4mm, which has11independentelectromechanical constants: six elastic, three piezoelectric, and two dielectricconstants. we made18independent measurements, which provided seven controlchecks to guarantee self-consistency of the11independent constants beingdetermined. KNTN crystals posess good piezoelectric properties: d33¹40×10-12C/N,d15⁶9×10-12C/N,d31^-56×10-12C/N,k33⁰.56，kt⁰.44.The complete set of fullmatrix data provides reference for theoretical research on KNTN crystals andceramics.