Lead Free Ferroelectrics

Pb(Zr1-xTix)O3 (PZT) based compositions lying near the morphotropic phase boundary (MPB) have been the leader materials for the modern sensors, actuators and transducers applications due to their superior dielectric, piezoelectric and electromechanical coupling coefficients. However the recent EU environmental regulations forecast the end of this hegemonic position due to lead toxicity. The awareness of PZT toxicity concerns has driven the search for novel lead free piezoelectrics able to respond to an endless list of demanding applications, as piezoelectric motors, printing machines, piezoelectrically controlled thread guides, micropositioning systems, microelectromechanical systems (MEMs) and more recently to nanoelectromechanical (NEMs), among others.

Currently, lead free piezoelectrics can be grouped into two main perovskite families: K0.5Na0.5NbO3 (KNN) and Na0.5Bi0.5TiO3 (NBT). However both families still present inferior properties to those of PZT. KNN is one of the leading lead-free ferro- and piezoelectric material, owing to the relatively high TC ≈ 420 °C, good piezoelectric properties (piezoelectric constant; d33 = 80 pC/N) and electromechanical coupling coefficient (48 %) 13. Although, the low d33 of KNN might be unsuitable for practical applications, d33 > 400 pC/N and comparable to those of PZT at room temperature have been reported for modified KNN ceramics.

In this project K0.5Na0.5NbO3 (KNN)-based compositions will be engineered with high density and attractive electromechanical performance for replacing current PZT-based piezoelectric devices. Electromechanical properties enhancement will be attempted by (a) texturing with KNN crystalline templates and (b) shifting O-T phase transition with selected dopants. The extrinsic contributions to polarizabilty from differentiated sized grain, the origin of properties degradation under thermal cycling between distinct ferroelectric phases and the viability of “soft” and “hard” KNN piezoelectrics through acceptor and donor doping, amongst other topics are understudy within the project.