Thermal Quenching Effects on the Ferroelectric and Piezoelectric Properties of BiFeO3–BaTiO3 Ceramics

By modification of the heat-treatment process to avoid the intermediate temperature range where secondary phases are easily formed in BiFeO3, piezoelectric properties were improved; the direct piezoelectric sensor coefficient d33 increased from 240 to 352 pC/N in 0.67BiFeO3–0.33BaTiO3 (BF33BT) ceramic and from 402 to 454 pC/N in 1 mol % BiGaO3-doped BF33BT (BG) ceramic with almost unchanged high Curie temperatures around 454 °C. To understand the effects of water quenching after sintering, eight different BF33BT ceramics were prepared by using different cooling routes together with furnace-cooled and water-quenched BG ceramic. Structural, ferroelectric, and piezoelectric properties of these ceramics were compared. All after-annealed BF33BT ceramics were compared with quenched ceramics. Physical properties of the after-annealed ceramics were similar to those of the furnace-cooled ceramic. It was confirmed that the overall duration of heat treatment in the second-phase-forming intermediate temperature range plays an important role in processing BF33BT ceramics without secondary phases or defects.