Developments of Piezoelectric Actuators by Using Envirnmentally Gentle Lead-Free Piezoelectric

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Lead-free materials are recently demanded from the viewpoint of environmental protection. For example, the registration will be enforced in the EU as the draft Directives on Waste form Electrical and Electronic Equipment(WEEE), Restriction of Hazardous Substances(RoHS) and End-of Life Vehicles(ELV). Therefore, lead-free piezoelectric materials have been widely attracting attention as new materials in place of Pb-based Pb(Zr, Ti)O3 (PZT) ceramics. To replace PZT systems, it is necessary that required piezoelectric properties for various applications were divided and were developed the corresponding each application. For example, the perovskite type ceramics seem to be a suitable for actuators and high power applications. In this project, we try to develop piezoelectric actuators by using envirnmentally gentle lead-free piezoelectric ceramics. Candidate materials with the perovskite structure for lead-free piezoelectric ceramics are BaTiO3(BT), (Bi1/2Na1/2)TiO3(BNT), (Bi1/2K1/2)TiO3(BKT) … More and KNbO3(KN) etc. The desiable properties for piezoelectric actuators should show at least the piezoelectric strain constant, d33>300 pC/N and the Curie tempearature, Tc>200℃.Dielecric, ferroelectric and piezoelectric properties of a solid solution based on three components of BNT, BKT and BT, that is, x(Bi_<1/2>Na_<1/2>)TiO_3-y(Bi_<1/2>K_<1/2>)TiO_3-z BaTiO_3(x+y+z=1)[BNBKy:z(x)] are studied as a candidate for the actuator applications. BNT-based ferroelectrics usually have the second phase transition below 200℃ and show no piezoelectric activities above this temperature. In first year of this project, we had the best composition BNBK4:1(0.852) with d_<33>=190 pC/N and T_c=300℃ near the morphotropic phase boundary(MPB). However, the second phase transition temperature (T_2=120℃) is lower than that of our goal. In second year, we again tried to study the three component system. Finally the k_<33> and d_<33> are 0.474 and 140.2 pC/N for BNBK2:1(0.78) and more 0.462 and 127.9 pC/N for BNBK2:1(0.72), respectively. Temperature dependences of piezoelectric properties of the three-component solid solution ceramics display that piezoelectric activities keep up to higher than 200℃ with the higher second phase transition. This solid solution seems to be a superior candidate material for lead-free piezoelectric multilayer ceramic actuators. Less