RESUMO
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.
RESUMO
BiFeO3-BaTiO3 is a promising high-temperature piezoelectric ceramic that possesses both good electromechanical properties and a Curie temperature (TC). Here, the piezoelectric charge constants (d33) and strain coefficients (d*33) of (1 - x)BiFeO3-xBaTiO3 (BF-xBT; 0.20 ≤ x ≤ 0.50) lead-free piezoelectrics were investigated at room temperature. The results showed a maximum d33 of 225 pC/N in the BF-0.30BT ceramic and a maximum d*33 of 405 pm/V in the BF-0.35BT ceramic, with TCs of 503 and 415 °C, respectively. To better understand the performance enhancement mechanisms, a phase diagram was established using the results of XRD, piezoresponse force microscopy, TEM, and electrical property measurements. The superb d33 of the BF-0.30BT ceramic arose because of its location in the optimum point in the morphotropic phase boundary, low oxygen vacancy (VO··) concentration, and domain heterogeneity. The superior d*33 of the BF-0.35BT ceramic was attributed to a weak relaxor behavior between coexisting macrodomains and polar nanoregions. The presented strategy provides guidelines for designing high-temperature BF-BT ceramics for different applications.