IMF2017

Bismuth sodium titanate (Bi_0.5Na_0.5)TiO₃ (BNT) based ceramics which are near the morphotropic phase boundary (MPB) have piezoelectric properties comparable to those of lead‑containing Pb(Zr,Ti)O₃ (PZT). However, the lower depolarization temperature Td for compositions near the MPB causes concern regarding thermal stability under long-time operation. In our previous study of (Bi_0.5Na_0.5)_0.925Ba_0.075(Ti_1-xMn_x)O₃ for x = 0, 0.2, 1.0, and 2.0 mol% ceramics we found that the depolarization temperature (Td) was enhanced remarkably to ~152 °C for the 0.2% Mn composition. We concluded that Mn doping can enhance structural thermal stability in BN7.5BT ceramics as temperature increases. Three ceramic disks of each composition, with 1 mm thickness and 8.3 mm diameter, were sintered for 2 hours at 1170 oC. The disks were coated with silver paste and placed inside a small home-built water-cooled oven and connected to a Solartron SI 1260 Impedance Gain-Phase Analyzer. Runs were made at 100 to 600 ^oC in 50 ^oC steps. The frequencies used were 0.1 Hz to 1 MHz in 71 logarithmically spaced steps. From the results for ac resistivity ρ’+jρ” we used the relations ϵ’= -ρ”/ωϵo(ρ’2+ρ”2) and ϵ”=-ρ’/ωϵo(ρ’2+ρ”2) to find the ac permittivity ϵ’+jϵ”. The permittivity found using these relations showed intertwined dielectric and conductivity behavior, including effects of phase-shifted conductivity. The dielectric behavior is typical of relaxor ferroelectrics, obeying a power law relation above the dielectric peak and then the Curie-Weiss law above the Burns temperature.