Local Electromechanical Properties of CaCu 3 Ti 4 O 12 Ceramics
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1255-M03-19
Local electromethanical properties of the CaCu3Ti4O12 ceramics Ronald Tararam1, Igor Bdikin2,3, José Arana Varela1, Paulo Roberto Bueno1, José Gracio2, Andrei Kholkin3 1
Departamento de Físico-Química, Instituto de Química, Universidade Estadual Paulista (UNESP), 14800-900, Araraquara, SP, Brasil 2 Department of Mechanical Engineering & TEMA, University of Aveiro, 3810-193 Aveiro, Portugal 3 Department of Ceramics and Glass Engineering & CICECO, University of Aveiro, 3810-193 Aveiro, Portugal ABSTRACT Scanning probe microscopy (SPM) was used to probe piezoelectric vibrations and local conductivity in CaCu3Ti4O12 (CCTO) ceramics at room temperature. Piezoelectric contrast was observed on the polished surfaces of CCTO in both vertical (out-of-plane) and lateral (in-plane) modes and depended on the grain orientation varying in sign and amplitude. The piezoelectric contrast is shown to be controlled by the electrical bias (local poling) and displayed a ferroelectric-like reversible hysteresis accompanied with a change of the phase of piezoelectric signal. Flexoelectric effect (strain-gradient-induced polarization) due to surface relaxation was invoked to explain the observed contrast inside the grains. INTRODUCTION Recently, there has been a considerable interest in the cubic perovskite-like compound CaCu3Ti4O12 (CCTO) [1-13], because its giant dielectric constant is promising for many potential applications and may help miniaturize capacitor-based devices. A number of theoretical studies and experimental observations have attempted to elucidate remarkable nature of the exceptional dielectric property of CCTO. Some groups tend to believe that this phenomenon is attributed to extrinsic effects, such as Maxwell-Wagner-type relaxation resulting from spatial inhomogeneity inside grains, electrode effect, or internal barrier layer capacitor model. However, others believe that the high dielectric constant (greater than 1000) may come from the local dipole moments associated with the displacement of B ions or relaxor (dipole glass) behavior. Subramanian et al. [1] explained unusually high dielectric constant of CCTO in terms of its crystal structure where local dipole moments associated with off-center displacement of Ti ions give high polarizability and the transition to ferroelectric state is frustrated by the TiO6 octahedral tilt which is required to accommodate the Cu2+ square planar coordination. It has been found that the temperature at which the step-like decrease in dielectric constant takes place strongly depends on the measuring frequency and roughly follows an Arrhenius behaviour and Vogel-Fulcher relationship. This ferroelectric-like relaxor behavior [14-16] may be responsible for the giant dielectric constant. The aim of the present work is to study the local piezoelectric properties by piezoresponse force microscopy (PFM) that has been successfully applied to relaxor materials [17]. EXPERIMENTAL PROCEDURE CCTO ceramics with different grain sizes were prepared using conventional ceramic processing. H
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