Piezoelectric ceramics for the exchange of mechanical energy and electric energy are widely used in the production of piezoelectric sensors, piezoelectric energy harvesters, piezoelectric transducers and other functional devices. Among the piezoelectric ceramics, Pb(Zr, Ti)O₃ (PZT) based perovskite piezoelectric ceramic with high piezoelectric properties is popular. However, the serious thermal depolarization behavior of PZT at a high temperature makes it difficult to meet the application requirements of high-temperature piezoelectric applications in aerospace, nuclear industry and other fields. The Curie temperature of BiScO₃-PbTiO₃ (BS-PT) perovskite piezoelectric ceramic, located at the morphotropic phase boundary (MPB), is nearly 100 ℃ higher than that of PZT-based piezoelectric ceramics when their piezoelectric coefficient (d₃₃) values are similar. Therefore, BS-PT piezoelectric system is considered as a matrix with a great potential for high-temperature piezoelectric applications. Obtaining BS-PT-based high-temperature piezoelectric ceramics with both high temperature (d₃₃) and high thermal depolarization temperature (T_d) remains a key challenge. For perovskite-type piezoelectric ceramics, the Curie temperature is like-inversely proportional to the tolerance factor, and ferroelectric active ions are usually attributed to promoting the polarization coupling. These indicate that introducing the small tolerance factor and ferroelectric active ions into BS-PT matrix could break through the bottleneck above. In this paper, Bi(Zn_2/3Nb_1/3)O₃ (BZN), with ferroelectric active Bi/Zn ions and a tolerance factor similar to BS was introduced into BS-PT matrix, and the related microstructure, electrical properties and their correlation mechanism were investigated.

For the preparation of BS-xPT-BZN (0.60≤x≤0.63) piezoelectric ceramics by a high-temperature solid-state reaction method, the raw materials were mixed and ground in a ball mill, and then sintered at 850 ℃ for 2 h. Afterwards, the sintered materials were pressed, and further sintered at 1 140 ℃ for 2 h. The BS-xPT-BZN piezoelectric ceramic with silver electrodes was placed in a silicone oil bath at 120 ℃ and loaded in a DC electric field of 50 kV/cm for 30 min for artificial polarization. The phase composition, grain morphology, ferroelectric, piezoelectric and dielectric properties of the BS-xPT-BZN ceramics were characterized by a model D8-Advance type X-ray diffractometer, a model Sigma-300 type field emission scanning electron microscope, a model ZJ-6A type quasi-static d₃₃ meter, and a model TZDM-RT-1000 high-temperature dielectric temperature spectrum test system.

All the BS-xPT-BZN ceramic samples have a dense microstructure, and the average grain sizes are 2.29-3.91 μm. Meanwhile, the BS-xPT-BZN piezoelectric ceramics sintered have a perovskite phase structure. The tetragonal phase content gradually increases and all the ceramic samples maintain the characteristics of two phases coexist as the PT content increases. As a result, the composition with a high Curie temperature and located at the morphotropic phase boundary (MPB) is obtained. Among all the compositions studied, the ceramic sample with x of 0.62 has a maximum room temperature d₃₃ value, and thus is considered as MPB composition. This is because the maximum d₃₃ value is usually obtained in the MPB composition with a flatter Gibbs free energy profile. In addition, the Curie temperature of the MPB is as high as 438 ℃, which is conducive to the acquisition of T_d and high-temperature application. Also, the in-situ variable temperature d₃₃ test results show that the depolarization temperature of MPB sample with x of 0.62 is up to 409 ℃ and accompanied by a large high temperature d₃₃ (709 pC·N^-1), which is much better than that of reported Pb-based perovskite piezoelectric ceramics. The excellent high-temperature piezoelectric performance is due to the vertical MPB phase boundary characteristics of BS-PT matrix itself, and the increase of tetragonal phase content of BS-PT matrix results from the introduction of BZN, which has a small tolerance factor and contains ferroelectric active ions. This study demonstrates that BS-xPT-BZN piezoelectric ceramics are advanced high-temperature piezoelectric materials suitable for electromechanical applications at elevated temperatures.

BS-xPT-BZN (0.60≤x≤0.63) ternary system high-temperature piezoelectric ceramics were prepared by a high-temperature solid-phase reaction method. The results showed that the ceramic samples had a good grain development and compact structure. The ceramics with x of 0.62 located at the morphotropic phase boundary had the optimal comprehensive high-temperature performance (i.e., the Curie temperature T_C of 438 ℃, thermal depolarization temperature T_d of 409 ℃, and the maximum in-situ high-temperature quasi-static piezoelectric coefficient d₃₃ of 709 pC·N^-1). The excellent high temperature piezoelectric properties could be mainly related to the MPB characteristics that maintained multi-phase coexistence in a wide temperature region. The large high-temperature piezoelectric properties and high thermal depolarization temperature indicate that BS-xPT-BZN piezoelectric ceramics could be a promising candidate for the preparation of advanced high-temperature piezoelectric devices.