Polar Fatigue And Dielectric Relaxation In Ferroelectric Single Crystal Or Ceramic

Ferroelectric materials are used widely in ultrasonic transducers,hydrophones,memories,actuators,and sensors due to their excellent properties such as high piezoelectric coefficient,high dielectric constants,large strain,and high energy storage density.Ultrahigh electrical and acoustic performances have been much attraction.However,the piezoelectric properties are degraded,and the polar fatigue occurs during cyclic electric loading because the charged defects pin the domain switching,which is disadvantage to improve sensitivity of electronic device.The movement of charged defects impelled by an electric field has a contribution to the dielectric relaxation,and the physical mechanism is obtained by the dielectric analysis.In this paper,the polar fatigues of binary PMN-32PT and ternary 6PSN-61PMN-33PT ferroelectric single crystal with different orientation as a function of cyclic electric fields(E>Ed E<Ec)and frequency(10 Hz and 300 Hz)were investigated at different cycle numbers.Compared with the high frequency 300 Hz,the fatigue resistance of the two single crystals was decreased at low frequency 10 Hz,which can be attributed to the larger domain pinning effect and internal stress due to the presence of multiple polarization mechanisms for the low frequency.The binary PMN-32PT crystal exhibited better fatigue resistance than that of ternary 6PSN-61PMN-33PT.Moreover,the 88.6%of piezoelectric constant in the[100]-oriented PMN-32PT was remained when the cycles numbers is 106 at the low electric field(E<Ec)and high frequency 300 Hz,demonstrating a good fatigue resistance.The similar electrical fatigue for the[100]and[111]oriented 6PSN-61PMN-33PT indicated that the fatigue properties are not sensitive to crystal orientation.After annealing at 500? for fatigued crystals,the remnant polarization Pr and the piezoelectric constant were returned respectively to 91%and 81.8%for PMN-32PT.However,the piezoelectric and dielectric properties of the 6PSN-61PMN-33PT did not returned by annealing process,indicating the irreversible damage.The dielectric relaxation and electrical conduction mechanism of the crystals were investigated by broadband impedance spectroscopy.The influences of charge defects on polar fatigue were analyzed.The relaxation activation energy showed that the main charge defects in relaxor PT-based ferroelectric single crystals were electrons and oxygen vacancies.The dielectric relaxation mainly came from the short-range transition of oxygen vacancies of the defect dipoles.However,the electrical conduction originated from long-range migration of oxygen vacancies.The defect charges moved to the domain walls,the defect interface,and the sample/electrode interface under the electric field,pinning the domain wall motion or causing the interface stress,which resulted in a decrease of polarization and low electrical properties.The motion of charge defect was not obvious under electric field E<Ec,the domain wall pinning caused by space charge resulted in the small fatigue,and electrical properties of the crystals can recover after annealing.When the fatigue electric field E>Ec was applied,the charge defect movement was remarkable,the pinning of the domain wall was enhanced,the stress in the sample was increased,leading to a significant electrical fatigue.Compared with the ternary single crystal,the binary single crystal had better fatigue resistance,which can be attributed to low oxygen vacancy concentration.In order to compare the fatigue resistance of porous piezoelectric ceramics and single crystals,lead-free porous barium titanate(BaTiO3)ceramics with 58%porosity were prepared by solid phase sintering with dextrin as the pore-forming agent.The electrical properties,acoustic properties and polar fatigue were tested.The ceramics sintered at 1050? with a 10%dextrin content exhibited a high hydrostatic pressure(about 8376×10-15 Pa-1)and a low acoustic impedance(about 2.84 Mrayls).The bonding force between the ceramic grains sintered at 1280? was enhanced significantly.The polar fatigue test showed that the porous titanate ceramic maintained 97%dielectric constant of the original value after 106 cycles,and the piezoelectric constant remained 90%.The fatigue resistance of porous BaTiO3 ceramics was significantly higher than that of ferroelectric single crystals.These excellent performances indicated its potential application in the field of sensors and hydrophones.