By modification of the heat-treatment process to avoid the intermediate temperature range where secondary phases are easily formed in BiFeO 3 , piezoelectric properties were improved; the direct piezoelectric sensor coefficient d 33 increased from 240 to 352 pC/N in 0.67BiFeO 3 −0.33BaTiO 3 (BF33BT) ceramic and from 402 to 454 pC/N in 1 mol % BiGaO 3 -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 afterannealed 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.

Through modification of the heat-treatment process using a higher heating rate and a lower binder burnout temperature, the piezoelectric performance of waterquenched 0.67Bi 1.05 FeO 3 -0.33BaTiO 3 (BF33BT) lead-free piezoelectric ceramics was improved. The observed physical properties of BF33BT ceramics were very sensitive to the process temperatures. The sintering temperature (T S ) was changed within a narrow temperature range, and its effects were investigated. The largest rhombohedral distortion (90°-α R = 0.14°) and tetragonality (c T /a T = 1.022) were observed for the ceramic sintered at 980°C, and its Curie temperature was 476°C.This ceramic showed good piezoelectric properties and large grains; the piezoelectric sensor charge coefficient (d 33 ) was 352 pC/N, and the piezoelectric actuator charge coefficient (d à 33 ) was 270 pm/V. The high piezoelectric performance and low T S of BF33BT ceramics indicate their potential as new low-cost eco-friendly lead-free piezoceramics. K E Y W O R D Sgrain size, lead-free, piezoelectric, sintering temperature, structural distortion

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