Studies On The Composite Modification Of Lead-free(NA_1/2BI_1/2)TIO₃-Based Ferroelectric Ceramics

Na_0.5Bi_0.5TiO₃（NBT for short）is an A-site complex perovskite material with a rhombohedral symmetry at room temperature,and possesses high Curie temperature,relatively large ferroelectric properties and large thickness mode electromechanical coupling factors,which is suitable in a variety of applications such as piezoelectric sensor,motor and ultrasonic transducer.However,NBT ceramics possesses relatively high coercive field（E_C≈70 kV/cm）,which makes the poling of NBT difficult.The depolarization of NBT based solid solutions at around 200°C（T_d）leads to the disappearance of macroscopic ferro-/piezoelectric behaviors due to the ferroelectric to relaxor transition,and limits the up-limit working temperature.In other aspect,when the ferroelectric to relaxor transition temperature lowers to around room temperature,the ferroelectric hysteresis loops become into pinched ones,which will induce a large electrostrictive effect due to the coexistence of ferroelectric and relaxor phases.Therefore,in this paper,the thermal depolarization and electrostrictive behaviours of NBT based ferroelectric materials were studied.Main works are as follows:1.We successfully prepared 0.8(Na_0.5Bi_0.5)TiO₃:0.2ZnO composite ceramics.The research shows that the ceramics have a composite structure coexisting of three phases,and the third phase except of ZnO and(Na_0.5Bi_0.5)TiO₃ is the Zn₂TiO₄,which is generated by the strong reaction between NBT and ZnO.Temperature dependent dielectric and ferroelectric responses showed that,with the addition of Zn²⁺doping,the dielectric anomaly peak around 100°C disappears,and the saturated ferroelectric hysteresis keep up to 135°C,which confirms the deferred thermal depolarization in the composites due to the composite structure.2.We successfully prepared(Na_0.5Bi_0.5)_0.94Ba_0.06Ti_1-xZn_x O₃（0≤x≤0.06）ceramics.The research shows that the compositions with x≤0.04 show a pure perovskite structure;however,the doping amount of ZnO exceeding over4 mol.%brings in the occurrence of the impurity phase Zn₂TiO₄.The temperature dependent ferroelectric and dielectric tests as well as high temperature XRD analysis show that the thermal depolarization temperature T_d of Zn²⁺-doped NBT-6BT increases compared with the pure sample.we believe that the deferred thermal depolarization by Zn²⁺doping could be ascribed to the large ionic polarizability and ionic radii of Zn²⁺,which increases dipole moment of BO₆ octahedra,and strengthens the coherence of neighboring BO₆,thus suppressing the ferroelectric phase to relaxor phase transition,and increasing the depolarization temperature.3.We successfully prepared（1–x）(0.875Bi_0.5Na_0.5TiO₃–0.125BaTiO₃)–xBi(Mg_0.5Ti_0.5)O₃（0≤x≤0.08）.The research shows that,as the addition of BMT,the phase structure of 0.875NBT-0.125BT transforms from tetragonal ferroelectric phase P4mm to relaxor phase P4bm,and the phase transition temperature from ferroelectric phase to relaxor phase lowers.When the amount of BMT added arrives to 4 mol.%,the phase transition temperature lowers to around room temperature,and thus the system reached a state where the ferroelectric P4mm phase and the relaxor P4bm phase coexist.The coexistence of ferroelectric and relaxor phases has a significant effect on the electrostrictive properties of the ceramics.The electrostriction of the ceramics reach to the maximum strain S_max of 0.30%at BMT content of 4 mol.%and the electrostrictive coefficient Q₃₃ is 0.0254 m⁴/C²;meanwhile,the electrostriction of the ceramic presents a good stability in a wide temperature range.