Construction Of Local Stress Field And Piezoelectric Properties Of Bismuth Titanate-Based Lead-Free Piezoelectric Ceramics

Bi_0.5Na_0.5TiO₃（BNT）is an A-site complex perovskite ferroelectric material with R3c phase structure at room temperature.It has high Curie temperature,large electric field-induced strain,special relaxor properties and large thickness mode electromechanical coupling factors.Thus,BNT based ceramics are widely used in actuators,filters,ultrasonic transducers and other fields.However,they still have some problems,such as large coercive field and large strain hysteresis,low depolarization temperature（Td）,making the working temperature zone of ceramic devices lower,which limits the practical application of BNT-based materials.Therefore,in this paper,BNT ceramics were used as the matrix to improve the depolarization temperature and piezoelectric properties of BNT-based ceramics through heat treatment processes and composites construction,and explores the mechanism of action.（1）0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO3（BNT-6BT）ceramics were prepared using a solid-state reaction method.The effects of quenching on the depolarization temperature of BNT-6BT ceramics and mechanism were investigated.The dielectric temperature spectrum shows that the samples after air quenching and liquid nitrogen quenching both exhibit a higher relaxation-ferroelectric transition temperature（TF-R）,the temperature-dependence of the piezoelectric-coefficient（d33）shows that quenching can increase the depolarization temperature of BNT-6BT ceramics.After liquid nitrogen quenching oxygen annealing and liquid nitrogen quenching nitrogen annealing,the TF-R of the sample returns to its original temperature.Combined with the impedance test results,it is inferred that the main reason for the increase in the depolarization temperature of BNT-6BT ceramics is not the oxygen vacancies,but the internal stress introduced after quenching.There is an internal stress field in the quenched ceramic,which induces a stable rhombohedral ferroelectric phase,thereby increasing the thermal depolarization temperature of BNT-6BT.（2）0.97(0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO3)-0.03AgNbO3（BNT-6BT-3AN）ceramics were prepared using a solid-state reaction method.The samples were quenched at different quenching temperatures in air.The effects of quenching on the ferroelectric and piezoelectric properties of BNT-6BT-3AN ceramics were studied.Ferroelectric and dielectric test have proved that the ergodic relaxor ferroelectric BNT-6BT-3AN ceramics can be transformed into non-ergodic relaxor ferroelectrics after quenching above 1000℃.After annealing at 600℃,the sample quenched at 1000℃ returned to ergodicity,while the sample quenched at 1100℃ was still non-ergodic.XRD refinement and Raman test results show that quenching can increase the content of the tetragonal phase and lattice distortion,meanwhile increases the deviation of the Bi ion position after quenching and the order of the surrounding ions,inducing ergodicity to nonergodic transformation without changing the composition of materials.（3）0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO3:xZnO（x=0.1,0.2;the particle size of ZnO is nanometer,micrometer and nanorod）composite ceramics were prepared using a two-step solid state reaction.The effects of different particle size and morphology of ZnO on the piezoelectric properties of BNT-6BT ceramics were studied.Td of the composites with different particle sizes and morphologies of ZnO do not show obvious differences.The dielectric temperature spectrum and the temperature-dependence of the piezoelectric-coefficient（d33）show that the depolarization temperature of BNT-6BT ceramics can be increased by composited ZnO with different particle sizes and morphologies.d33 of the composites with 0.1ZnO is higher than 0.2ZnO samples.The second phase Zn2TiO4 appears in the ceramics,and the second phase is not ferroelectric,which leads to the degradation of the ferroelectric properties of BNT-6BT ceramics.（4）0.97(0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO3)-0.03AgNbO3:ZnO composite ceramics were prepared using a two-step solid state reaction method.The effects of ZnO addition on electromechanical properties of the composites were studied.Combining XRD and EDS,it is found that ZnO reacts with the matrix and formed the second phase Zn2TiO4,and Zn2+is dissolved into the BNT-6BT-3AN matrix.With the increase of ZnO content,the composites change from ergodicity to non-ergodicity.For x=0.05,a strain loop with smaller negative strain is obtained.After addition of ZnO,the grains near the second phase show a core-shell（P4bm/R3c-R3c）structure.The shell near the second phase is the ferroelectric R3c phase,while the core far away from the second phase is at a relaxor state.The characteristic P4bm phase and a small amount of R3c phase are coupled between the two phases,which causes the deformation of BNT-6BT-3AN to be restrained when the electric field is applied to induce the phase change,which reduces the hysteresis od electrostrain,and the electrostrain has good temperature stability.