Microstructure, dielectric and ferroelectric properties of 0.94Bi0.5Na0.5TiO3–0.06BaTiO3 (NBTB) and 0.05BiFeO3–0.95NBTB ceramics: Effect of sintering atmosphere

0.94Bi0.5Na0.5TiO3–0.06BaTiO3 (NBTB) and 0.05BiFeO3–0.95NBTB (BF-doped NBTB) lead-free ceramics were prepared by solid state reaction method. The ceramics were sintered at 1180 °C for 2 h in O2 and N2. All ceramics exhibited a single phase of perovskite structure. Relative amount of tetragonal phase was related to the sintering atmospheres. Both grain size and shape were influenced by the sintering atmospheres. Sintering the ceramics in N2 weakened their dielectric anomalies corresponding to the transition from ferroelectric phase to the so-called “intermediate phase”. When the NBTB and BF-doped NBTB ceramics were sintered in N2, their maximum dielectric constant and the degree of diffuseness of the transition from the “intermediate phase” to paraelectric phase increased, but their Curie temperatures decreased. The difference in dielectric properties of the ceramics sintered in different atmospheres was closely related to the difference in oxygen vacancy concentration. The correlation between ferroelectric properties and sintering atmospheres is associated with a competing effect among oxygen vacancy concentration, A-site vacancy concentration and percentage of tetragonal phase.

Research highlights▶ The 0.94Bi0.5Na0.5TiO3–0.06BaTiO3 (BNBT) and 0.05BiFeO3–0.95BNBT (BF-doped BNBT) lead free ceramics were sintered in atmospheres of O2 and N2. The sintering atmospheres had an important effect on the crystalline structure, microstructure, dielectric and ferroelectric properties of the ceramics. ▶ A rhombohedral–tetragonal morphotropic phase boundary exists in the BNBT and BF-doped BNBT ceramics sintered in both atmospheres. For the BNBT sintered in O2, BNBT in N2, BF-doped BNBT in O2, and BF-doped BNBT in N2, the relative amounts of tetragonal phase are 49.8%, 42.5%, 42.9% and 36.1%, respectively; and the mean sizes of grains are 2.41, 3.76, 3.68 and 5.06 μm, respectively. The BF-doped BNBT ceramics sintered in N2 consist of grains with round edges and corners, while the ceramics sintered in O2 consist of cube-shaped grains with sharp edges and corners. Sintering the BNBT and BF-doped BNBT ceramics in N2 can weaken the dielectric anomalies corresponding to the transition between ferroelectric phase and anti-ferroelectric phase. The maximum dielectric constant and the degree of diffuseness of the transition between anti-ferroelectric phase and paraelectric phase increased, but the Curie temperature decreased when the BNBT and BF-doped BNBT ceramics were sintered in N2. The change in dielectric properties of the ceramics sintered in different atmospheres is close related to the variation in oxygen vacancy concentration. The ferroelectric properties of the ceramics changed with changing sintering atmospheres. The correlation between ferroelectric properties of the BNBT and BF-doped BNBT ceramics and sintering atmospheres is associated with a competing effect among oxygen vacancy concentration, A-site vacancy concentration and percentage of tetragonal phase.