Current status and prospects of lead-free piezoelectric ceramics

Dielectric, ferroelectric and piezoelectric properties of perovskite ferroelectric and bismuth layer-structured ferroelectric (BLSF) ceramics are described as superior candidates for lead-free piezoelectric materials to reduce environmental damages. Perovskite type ceramics seem to be suitable for actuator and high power applications that are required a large piezoelectric constant, d33 (>300 pC/N) and a high Curie temperature, Tc (>200 °C). For BaTiO3 (BT)-based solid solutions, that is, (1 − x)BaTiO3 − x(Bi0.5K0.5)TiO3 [BTBK − 100x] ceramics, the Tc increases with increasing the amount of x. BTBK-20 + MnCO3 0.1 wt.% ceramic shows the high Tc than 200 °C and the electromechanical coupling factor, k33 = 0.35. In the case of a(Bi1/2Na1/2)TiO3 − bBaTiO3 − c(Bi1/2K1/2)TiO3 [BNBK (100a/100b/100c)] solid solution ceramics, the d33 and Tc are 191 pC/N and 301 °C for the BNBK (85.2/2.8/12), respectively. On the other hand, BLSF ceramics seem to be excellent candidates as piezoelectric sensors for high temperatures and ceramic resonators with high mechanical quality factor (Qm), and low temperature coefficient of resonance frequency (TC-fr). Donor-doped Bi4Ti3O12 ceramics such as Bi4Ti3−xNbxO12 [BITN-x] and Bi4Ti3−xVxO12 [BITV-x] show high Tc than 650 °C. The k33 value of the grain-oriented (HF) BITN-0.08 ceramic is 0.39 and is able to keep the same value up to 350 °C. Bi3TiTaO9 (BTT)-based solid solution system, Srx−1Bi4−xTi2−xTaxO9 [SBTT2(x)] (1 ≦ x ≦ 2), displays the high Qm value (=13500) in (p)-mode at the x = 1.25 composition.