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.