Dielectric and ferroelectric properties of (Bi_0.5Na_0.5)_0.94-δBa_0.06Ti_1−xNb_xO₃ lead-free ceramics

Lead-free ceramics (Bi_0.5Na_0.5)_0.94-δBa_0.06Ti_1−xNb_xO₃ (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05) were prepared via a solid-state sintering method. The ceramics exhibit pure perovskite structure as x ≤ 0.04, while the ceramic with x = 0.05 has a secondary phase. The rhombohedral (R)–tetragonal (T) morphotropic phase boundary exists in all the samples. The relative content of the T phase increases due to the Nb⁵⁺ doping. The ceramics show dense microstructures with mean grain sizes around 1–2 μm. The changes of dielectric constant and dielectric loss as a function of temperature for the unpoled and poled samples were compared. Two dielectric anomalies appear on the dielectric curves around the temperatures denoted as T_RE and T_m. With the increase of Nb⁵⁺ amount, the values of T_RE decrease and T_m increase. The Nb⁵⁺-doped ceramics exhibit better temperature stability of dielectric constant during a wide high-temperature window. The polarization hysteresis (P–E) loops change from typical ferroelectric loops for the sample with x = 0 to constricted P–E loops with increasing Nb⁵⁺ amount. The ceramic with x = 0.03 shows a maximum strain (S_max) of 0.55% and piezoelectric strain constant (d₃₃^*) of 675 pm/V.