Scanning probe microscopy studies of ferroelectric domains in lead-free piezoelectrics

Vladimir Shvartsman, Institute for Materials Science/University of Duisburg-Essen, Essen, Germany
Danka Gobeljic, Institute for Materials Science/University of Duisburg-Essen, Essen, Germany
Doru Lupascu, Institute for Materials Science/University of Duisburg-Essen, Essen, Germany
Jürgen Rödel, Technical University Darmstadt, Darmstadt, Germany

Health and environmental concerns have encouraged development of non-toxic lead-free piezoelectric materials to replace ubiquitous Pb(Zr,Ti)O3 (PZT). Among the most promising compounds are those based on K0.5Na0.5NbO3 (KNN) and Bi0.5Na0.5TiO3 – BaTiO3.1 These materials exhibit excellent electromechanical characteristics comparable to and even surpassing those of PZT. It is well known that electromechanical properties of ferroelectrics strongly depend on their domain structure. Therefore, studies of ferroelectric domains may shed light on mechanisms controlling the enhanced electromechanical response in lead-free piezoelectrics. In this presentation, we report on results of piezoresponse force microscopy (PFM) studies of some lead-free piezoelectric ceramics exhibiting a remarkable electromechanical behaviour.

PFM is a modification of atomic force microscopy based on probing of local piezoelectric deformations induced by an ac electric field applied between the sharp AFM tip and the sample surface. Due to the strong inhomogeneity of the field, the measured response corresponds to a small volume around the contact point providing very high spatial resolution of the method.

Performing temperature PFM studies, we have shown that the enhanced piezoelectric response of KNN-based ceramics is related to a reconstruction of the domain structure at the transition between orthorhombic and tetragonal ferroelectric phases. The temperature dependence of the local piezoresponse shows a good qualitative agreement with macroscopic data.

For Bi0.5Na0.5TiO3-based ceramics we have addressed effect of chemical composition and microstructure on morphology of domains. A correlation between the type of domain pattern and macroscopic behaviour was established. Field-induced transformation of domains was studied. Both local polarization switching and relaxation were measured and compared to macroscopic behaviour, ferroelectric or relaxor-like.

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