Developments of Piezoelectric Actuators by Using Environmentally Gentle Lead-Free Piezoelectric Ceramics

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Recently lead-free piezoelectric materials are strongly demanded from the viewpoint of environmental protection by the registration such as the draft Directives on Waste form Electrical and Electronic Equipment (WEEE), Restriction of Hazardous Substances (RoHS) and End-of Life Vehicles (ELV) in EU. Candidate materials with the perovskite structure for lead-free piezoelectric ceramics in place of Pb-based Pb (Zr, Ti)O_3 (PZT) seem to be BaTiO_3 (BT), (Bi_<1/2>Na_<1/2>)TiO_3 (BNT), (Bi_<1/2>K_<1/2>)TiO_3 (BKT) and these solid solutions, and more KNbO_3 (KN)-based solid solutions etc. The desirable properties for piezoelectric actuators should show at least the piezoelectric strain constant, d_<33>>300 pC/N and the Curie temperature, Tc, or the depolarization temperature, T_d, >200℃. In this project, dielectric, ferroelectric and piezoelectric properties of grain-oriented and domain controlled textured ferroelectric ceramics with perovskite structures and bismuth layered-structures (BLSF) … More to apply to piezoelectric actuators and piezoelectric ceramic resonators.1. Grain oriented 0.9(Bi_<1/2>K_<1/2>)TiO_3-0.1BaTiO_3 (BKT-BT10) with perovskite structure was fabricated by a reactive template grain growth (RTGG) method using the template of Bi4Ti3O12. And more, multilayer type piezoelectric actuators using the textured composition were fabricated and the actuator properties were evaluated. The depolarization temperature, Td, of BKT-BT10 is higher than 250℃. The electric field induced strain, S, of textured and domain controlled BKT-BT10 is 1.5 times higher than that of non-oriented, and the dynamic piezoelectric strain constant, d33, is 168 pC/N. Textured and domain controlled this solid solution ceramics seem to be a superior candidate material for lead-free piezoelectric multilayer type ceramic actuators.2. Domain controlled bismuth layer-structured ferroelectric (BLSF) SrBi2Nb_<2-x>V_xO_9(SBNV-x) ceramics were fabricated by the hot-forging (HF) method. Temperature dependences in the various vibration modes of piezoelectric properties of SBNV-x were evaluated for ceramic resonator applications, especially, temperature coefficient of resonant and antiresonant frequencies (TC-f) were measured as a function of the crystal orientation axis. The TC-f was lowered in the a-b axis because of the origin of an anisotropy in BLSF. Electrical quality factor, Q_e, the mechanical quality factor, Q_m, and TC-f of grain-oriented (HF) SrBi_2Nb_<1.95>V_<0.05>O_9 (SBNV-0.05) were Q_c=66, Q_m=2200 and TC-f=16.5 ppm/℃ for (33)-mode, and Q_c=21.6, Q_m=4600 and TC- f=27.7 ppm/℃ for (15)-mode, respectively. These results indicate that grain oriented SBNV ceramics can be commercialized in the form of lead-free piezoelectric resonators. Less