Boosting the High Performance of BiFeO₃-BaTiO₃ Lead-Free Piezoelectric Ceramics: One-Step Preparation and Reaction...

One of the notorious problems in BiFeO₃-based piezoelectric ceramics is how to limit the formation of Bi₂₅FeO₃₉ and Bi₂Fe₄O₉ 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 BiFeO₃-xBaTiO₃ (BF-xBT) ceramics. The significance of this one-step method is that the thermodynamically unstable region of BiFeO₃ is successfully avoided based on the Gibbs free energy of BiFeO₃, Bi₂₅FeO₃₉, and Bi₂Fe₄O₉. Benefiting from preventing the formation of Bi₂₅FeO₃₉ and Bi₂Fe₄O₉ 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 (d₃₃ = 174-199 pC/N; T_C = 494-513 °C). By analyzing X-ray diffraction patterns of BF-xBT (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, Bi³⁺ diffuses into Fe₂O₃ particles to form BiFeO₃ and Bi₂₅FeO₃₉ and then reacts with BaTiO₃; in this temperature range, the formed Bi₂₅FeO₃₉ is hard to eliminate. At 950 °C ≤ T_cal ≤ 1000 °C, Bi³⁺ and Fe ions simultaneously diffuse into BaTiO₃ to form BF-xBT, which is beneficial to preventing the formation of Bi₂₅FeO₃₉ and the improvement of performance.