Boosting the High Performance of BiFeO 3 -BaTiO 3 Lead-Free Piezoelectric Ceramics: One-Step Preparation and Reaction Mechanisms.

One of the notorious problems in BiFeO ₃ -based piezoelectric ceramics is how to limit the formation of Bi ₂₅ FeO ₃₉ and Bi ₂ Fe ₄ O ₉ impurities to achieve excellent piezoelectric performance. In this study, a one-step preparation technology, namely, excluding PVA, calcining, and sintering are completed in one step, instead of three steps in the ordinary sintering method, is developed to prepare BiFeO ₃ - x BaTiO ₃ (BF- x BT) ceramics. The significance of this one-step method is that the thermodynamically unstable region of BiFeO ₃ is successfully avoided based on the Gibbs free energy of BiFeO ₃ , Bi ₂₅ FeO ₃₉ , and Bi ₂ Fe ₄ O ₉ . Benefiting from preventing the formation of Bi ₂₅ FeO ₃₉ and Bi ₂ Fe ₄ O ₉ impurities, the resultant ceramics show dense structures, macroscopic stripe domains, and a small number of island domains and display saturated P - E curves, sharp I - V characteristics, butterfly-shape S - E loops, and good piezoelectric properties ( d ₃₃ = 174-199 pC/N; T _C = 494-513 °C). By analyzing X-ray diffraction patterns of BF- x BT (0 ≤ x ≤ 1) powders at different calcination temperatures ( T _cal ), the different reaction mechanisms between 750 °C ≤ T _cal ≤ 900 °C and 950 °C ≤ T _cal ≤ 1000 °C are revealed. When 750 °C ≤ T _cal ≤ 900 °C, Bi ³⁺ diffuses into Fe ₂ O ₃ particles to form BiFeO ₃ and Bi ₂₅ FeO ₃₉ and then reacts with BaTiO ₃ ; in this temperature range, the formed Bi ₂₅ FeO ₃₉ is hard to eliminate. At 950 °C ≤ T _cal ≤ 1000 °C, Bi ³⁺ and Fe ions simultaneously diffuse into BaTiO ₃ to form BF- x BT, which is beneficial to preventing the formation of Bi ₂₅ FeO ₃₉ and the improvement of performance.