Electromechanical Properties And Thermal Stability Of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ And Pb(Sc_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ Single Crys

In recent years, Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃（PMN-PT） a kind of the relaxor–based ferroelectric single crystals, are paid much attention because of the excellent electromechanical properties. However, people always focus on the rhombohedral phase or the MPB phase. And the rhombohedral phase or the MPB phase PMN-PT single crystals have low phase transition temperature and low coercive field, which limits their application. The tetragonal PMN-PT single crystals have only one phase transition temperature above room temperature, and then they can be used in a wider temperature range. It was reported Pb（Sc1/2Nb1/2）O3-Pb（Mg1/3Nb2/3）O3-Pb Ti O3（PSN-PMN-PT） ceramics have high transition temperatures. It means the PSN-PMN-PT single crystals would have a wider temperature range. Consider the inevitableheat production during piezoelectric device application, the electromechanical properties and thermal stability of tetragonal PMN-PT and PSN-PMN-PT single crystals are studied to find the material with a wider temperature range.Firstly, [001]C poled tetragonal PMN-0.37 PT single crystals have been studied.The Cutie temperature TC is 174℃, the coercive field is 7.4 k V/cm. The full tensor properties shows the [001]C poled PMN-0.37 PT single crystal possesses strong anisotropy, high transverse dielectric constant and shear piezoelectric constant. It can calculated that the best properties can be got in [011]C poled crystals. When the temperature rises, the anisotropy becomes lower and the direction of the maximum longitudinal piezoelectric constant turns to [001]C. The single crystals have better terminal stability of piezoelectric coefficient and transverse dielectric constant below 140℃.Secondly, we studied the [011]C poled tetragonal PMN-PT. The increasing electric field caused the transition phase from tetragonal phase to monoclinic MC phase and then to single doamain orthorhombic phase. And when the electric field is decreased it would transform to monoclinic MC phase then to the coexistence of MC phase and tetragonal phases. The phase coexistence would be stable after 48 hours.The existence of MC phase results in excellent electromechanical properties. Whentemperature is rising, the volume fraction of MC phase wouldn’t change, but the longitudinal electromechanical factor is sensitive to temperature. So the single crystals with MC phase should not be used in electric machine but can be applied in thermal-sensitive device.Lastly, [001]C poled 0.06PSN-0.61PMN-0.33 PT single crystals are studied. The useage temperature range is higher than PMN-0.33 PT, also with higher coercive field and strain. The full tensor properties show that the elastic constants are higher,and both the longitudinal piezoelectric constant and electromechanical effective are ultrahigh. The piezoelectric property show high anisotropy. The analysis on the thermal stability of the single crystals show that: The longitudinal and transverse piezoelectric constants are good and stable between 80℃ and 130℃.And the shear piezoelectric constant is also stable and good blow 80℃.