Strain Mechanisms in Lead-Free Ferroelectrics for Actuators

Abstract

The perovskite solid solution family of Lead Zirconate Titanate has traditionally been the focus of developmental efforts in the piezoceramics community. This is due to the possibility of engineering its electromechanical properties with dopants (≤ 3 at. %) or modifiers (≥ 5 at. %) leading to desirable and relatively easily addressable functional properties for a broad range of applications. As a result of more than seven decades of continuous developmental efforts, PZT-based solid solutions are the preferred piezoelectrics in present day applications, comprising 95 % of the whole piezoelectric materials implemented in the international production of actuators. Global awareness for well-being of the environment has increased in the past decades. In particular, Pb and PbO were identified as toxic for human health and the environment. On these grounds, directives against the use of Pb and other toxic elements in consumer products have been globally introduced. These regulations have stimulated an increased research in lead-free piezoceramics. Three main solid solution families have been recognized as technologically relevant replacements: barium titanate-based (BT-based), bismuth alkali-based (BNT- or BKT-based), and alkali niobate-based (KNN-based) materials. In this work, two systems were selected as representative candidates, one from the BT-based and the other from the BNT-based family of materials. Namely, (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-BCT) and (Bi1/2Na1/2)TiO3(BNT) -SrTiO3 (ST) were selected. These model materials were chosen to expand the state-of-the-art knowledge of strain mechanisms of lead-free piezoceramics in the small and large signal regimes, and thus promote their technological implementation. This work focuses in the relationship between the dielectric and electromechanical properties and phase transitions, as well as microstructure of these systems. The research performed combines both fundamental understanding on the functionality of these materials, as well as application oriented knowledge.