Frequency-dependent electromechanical properties for sol-gel deposited ferroelectric lead zirconate titanate thin layers
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The electromechanical properties of sol-gel-derived ferroelectric Pb(Zro.53Tio.47)03 (PZT 53/47) thin layers deposited on silicon were determined as a function of field strength, measurement frequency, and total thickness. Both electrically induced strains (e) and piezoelectric properties (J 33 ) were characterized by interferometry. Dielectric spectroscopy and polarization switching (P-E) measurements were determined for comparative purposes. An asymmetry between forward and the reverse bias conditions in the e-E displacements was found for both five-layer deposited and nine-layer deposited structures. However, no asymmetry was observed in the P-E hysteresis characteristics. In addition, the electrically induced strains and the piezoelectric response were found to be dependent on measurement frequency. No significant frequency dependence was observed in the polarization or dielectric responses. The results are discussed in terms of a possible clamping effect on polarization switching. I. INTRODUCTION Sol-gel-derived ferroelectric and dielectric thin layers have attracted considerable attention for potential electromechanical applications.1-2 Miniature, high-power transducers and actuators are needed for various applications ranging from micromotors, to micropositioning, and to high-frequency medical ultrasonic imaging. Three basic electromechanical distortion mechanisms for ferroelectric and antiferroelectric materials are 34 (i) piezoelectricity, (ii) electrostriction, and (iii) induced phase transformations. Applications require materials that are uniform in chemical composition and physical structure. Fabrication of various ferroelectric and antiferroelectric thin layers by sol-gel methods can yield physical properties which match those of their bulk ceramic counterparts in terms of dielectric constant (K), and saturation (Ps), and remanent (Pr) polarizations for all three basic types of electromechanical coupling mechanisms.5^7 Lead zirconate titanate (PZT) with compositions near the morphotropic phase boundary are representative of typical piezoelectric materials.8 Field-induced strains occur in these materials on polarization reversal, which are characterized by butterfly-like hysteresis loops. Polarization reversal is known to occur in this phase region by the creation and subsequent motion of 90° domains, resulting in field-induced strains of —0.2 to 0.3%. 9 Polarization reversal can also occur by 180° domains, which, however, results in significantly lower electrically induced strains. Lead magnesium niobate (PMN) and La-modified PZT (PLZT) are relaxor ferroelectrics and are representative of the electrostrictive J. Mater. Res., Vol. 10, No. 6, Jun 1995 http://journals.cambridge.org
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materials.10 Field-induced strains of —0.1 to 0.2% occur in these materials due to an alignment of nanopolar domains. Lead zirconate (PZ) and Sn-modified PZT (PZST) are representative of the induced-phase transformation materials. Large field-induced strains of — 1% can occur in these materials due to
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