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, d 33 (>300 pC/N) and a high Curie temperature, T c (>200 °C). For BaTiO 3 (BT)-based solid solutions, that is, (1 − x)BaTiO 3 − x(Bi 0.5K 0.5)TiO 3 [BTBK − 100 x] ceramics, the T c increases with increasing the amount of x. BTBK-20 + MnCO 3 0.1 wt.% ceramic shows the high T c than 200 °C and the electromechanical coupling factor, k 33 = 0.35. In the case of a(Bi 1/2Na 1/2)TiO 3 − bBaTiO 3 − c(Bi 1/2K 1/2)TiO 3 [BNBK (100 a/100 b/100 c)] solid solution ceramics, the d 33 and T c 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 ( Q m), and low temperature coefficient of resonance frequency (TC- f r). Donor-doped Bi 4Ti 3O 12 ceramics such as Bi 4Ti 3− x Nb x O 12 [BITN- x] and Bi 4Ti 3− x V x O 12 [BITV- x] show high T c than 650 °C. The k 33 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. Bi 3TiTaO 9 (BTT)-based solid solution system, Sr x−1 Bi 4− x Ti 2− x Ta x O 9 [SBTT2( x)] (1 ≦ x ≦ 2), displays the high Q m value (=13500) in (p)-mode at the x = 1.25 composition.