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

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; TC = 494–513 °C). By analyzing X-ray diffraction patterns of BF-xBT (0 ≤ x ≤ 1) powders at different calcination temperatures (Tcal), the different reaction mechanisms between 750 °C ≤ Tcal ≤ 900 °C and 950 °C ≤ Tcal ≤ 1000 °C are revealed. When 750 °C ≤ Tcal ≤ 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 ≤ Tcal ≤ 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.