Current Developments and Prospective of Lead-Free Piezoelectric Ceramics

The dielectric, ferroelectric and piezoelectric properties of perovskite ferroelectric and bismuth layered-structured ferroelectric (BLSF) ceramics are described being superior candidates for lead-free piezoelectric materials to reduce environmental damage. Perovskite-type ceramics seem to be suitable for actuator and high-power applications that require a large piezoelectric constant, d 33 , and a high Curie temperature, T c , or a depolarization temperature, T d (>200 °C). For BaTiO 3 -based solid solutions, (1- x )BaTiO 3 – x (Bi 0.5 K 0.5 )TiO 3 (BT–BKT100 x ) ceramics, T c increases with increasing amount of x . The BT–BKT20 + MnCO 3 (0.1 wt %) ceramic shows a high T c greater than 200 °C and an electromechanical coupling factor of k 33 =0.35. In the case of a (Bi 1/2 Na 1/2 )TiO 3 – b (Bi 1/2 K 1/2 )TiO 3 – c BaTiO 3 [BNBK (100 a /100 b /100 c )] solid solution ceramics, d 33 is 191 pC/N for BNBK (85.2/2.8/12). KNbO 3 (KN)-based ceramics are also a candidate for lead-free piezoelectrics. In Mn-doped KN ceramics, a higher k 33 of 0.507 is obtained for KN + MnCO 3 (0.1 wt %). On the other hand, BLSF ceramics seem to be excellent candidates as piezoelectric sensors for high temperatures and ceramic resonators with a high mechanical quality factor, Q m , and a low temperature coefficient of resonance frequency, TC-f . The k 33 value of the donor (Nb)-doped and grain-oriented (HF) Bi 4 Ti 3- x Nb x O 12 (BITN- x ) ceramic is 0.39 for x =0.08 and is able to keep the same stable value up to 350 °C. Nd(0.01) and V(0.75) co-doped Bi 4 Ti 3 O 12 ceramics, BNTV(0.01, 0.75), show a relatively low TC-f . Bi 3 TiTaO 9 (BTT)-based solid solution, 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 x =1.25. For resonator applications, (Sr 1- x Ca x ) 2 Bi 4 Ti 5 O 18 (SCBT) (0≦ x ≦0.5) ceramics are suitable.