Preparation And Electrical Properties Of BNT-based Lead-free Piezoelectric Ceramics

Piezoelectric ceramics is a kind of high technical function materials, which can realize the transformation between mechanical energy and electric energy. It is widely applied to electronic and micro-electronic equipments. The traditional piezoelectric ceramics can bring many problems in applied process. So it is urgent to employ new-type lead-free piezoelectric ceramics. Bi_0.5Na_0.5TiO₃ with its better ferroelectric and piezoelectric properties is a promising lead-free piezoelectric ceramics.In this paper, (1-x)Bi_0.5Na_0.5TiO₃-xBi_0.5K_0.5TiO₃ lead-free ceramics were prepared by conventional mixed-oxide method(CMO). Preparation technique, design of composition, adding doped element and forming ternary piezoelectric ceramics were also studied. The major content was as follows:The influence of BKT content near morphotropic phase boundary (MPB) on the phase structure, density and electrical properties were investigated in details. The results showed that the phase structure transferred from rhombohedral to tetragonal phase with increasing BKT content. The MPB was existed when BKT content is between 0.16 and 0.20 mol. The relax ferroelectrics within BKT content was studied in this paper. All the ceramics were ferroelectric relaxors proved by Curie-Weiss law. The better compact and homogeneous ceramics were obtained at x=0.18. Considering the sintered temperature and electrical properties factors, ceramics sintered at 1170℃with 0.18 mol BKT achieved better properties, which were as follows: d₃₃=144 pC/N, K_p=0.29,ε_r=893 and tanδ=0.37.In order to improve the properties of piezoelectric ceramics, we selected Nd₂O₃ and CeO₂ as the doping agents. The result showed:All the ceramics are pure perovskite phase within Nd₂O₃ changing content. The diffractive peaks moved a little with different content. The grain size first decreased and increased with increasing Nd₂O₃ content. The better homogenous microstructure was achieved at x=0.0125 wt.%.ε_r peak of BNT-BKT ceramics doped with Nd₂O₃ became wider than the basic ceramics without Nd₂O₃. All the ceramics are relaxor ferroelectrics proved by Curie-Weiss law measured at 10 kHz. Otherwise, d₃₃ and K_p took on increasing trend and then decreasing trend. The 0.82Bi_0.5Na_0.5TiO₃-0.18Bi_0.5K_0.5TiO₃ doped with 0.0125wt.% Nd₂O₃ sintered at 1160℃ showed optimum properties: d₃₃=134 pC/N, K_p=0.27,ε_r=925 and tanδ=0.041.All the ceramics are pure perovskite phase within CeO₂ changing content. The grain size first increased and then decreased with increasing CeO₂ content. The better homogenous microstructure was achieved at x=0.1 wt%. The twoε_r peak was also existed in BNT-BKT ceramics doped with CeO₂ and the peak moved to the low temperature regions with increasing CeO₂ content. All the CeO₂ doped ceramics were relaxor ferroelectrics proved by Curie-Weiss law measured at 1 kHz, however, the relaxor ferroelctric properties of ceramics decresed with CeO₂ content increasing. Both d₃₃ and K_p showed the trend which was first decreasing and then increasing. The 0.92Bi_0.5Na_0.5TiO₃-0.18Bi_0.5K_0.5TiO₃ doped with 0.1wt.% CeO₂ sintered at 1180℃showed optimum properties: d₃₃=138 pC/N, K_p=0.30,εr=879 and tanδ=0.032.In the end, we modified the 0.92Bi_0.5Na_0.5TiO₃-0.18Bi_0.5K_0.5TiO₃+0.1wt.% CeO₂ with the third composition BaTiO₃. The phase structure, microstructure and electrical properties were stuied in details. The results showed e_r and tan d first increased and then decreased while d₃₃ and K_p showed the contrary trend with increasing BT content. Taking factors of microstructure and electrical properties under consideration, the 0.82BNT-0.18BKT+0.06 molBT+0.1wt.%CeO₂ ceramics sintered at 1180℃was chose for its better properties which are as follows: d₃₃=144 pC/N, K_p=0.23,ε_r=1216 and tanδ=0.041.