Crystallographic texturing of polycrystalline ferroelectric ceramics offers a means of achieving significant enhancements in the piezoelectric response. Templated grain growth (TGG) enables the fabrication of textured ceramics with single crystal-like properties, as well as single crystals. In TGG, nucleation and growth of the desired crystal on aligned single crystal template particles results in an increased fraction of oriented material with heating. To facilitate alignment during forming, template particles must be anisometric in shape. To serve as the preferred sites for epitaxy and subsequent oriented growth of the matrix, the template particles need to be single crystal and chemically stable up to the growth temperature. Besides templating the growth process, the template particles may also serve as seed sites for phase formation of a reactive matrix. This process, referred to as Reactive TGG (RTGG), has been used to obtain highly oriented Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃, Sr_0.53Ba_0.47Nb₂O₆, and (Na_1/2Bi_1/2)TiO₃-BaTiO₃. Highly oriented Bi₄Ti₃O₁₂, Sr₂Nb₂O₇, CaBi₄Ti₄O₁₅, Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃, Sr_0.53Ba_0.47Nb₂O₆ and (Na_1/2Bi_1/2)TiO₃-BaTiO₃ ceramics have been produced by TGG. The resulting ceramics show texture levels up to 90%, and significant enhancements in the piezoelectric properties relative to randomly oriented ceramics with comparable densities. For example, piezoelectric coefficients of textured piezoelectrics are from 2 to 3 times higher than polycrystalline ceramics and as high as 90% of the single crystal values. In textured PMN-PT, a low field (< 5 kV/cm) piezoelectric coefficient (d ₃₃) of ∼1600 pC/N was obtained with > 0.3% strain (at 50 kV/cm). The high field dielectric and electromechanical properties of textured perovskites are more hysteretic than those of single crystals, probably as a result of clamping by the residual template particles, residual random grains, the presence of non-ferroelectric second phases, and a wide orientation distribution. Lateral clamping of one grain by another may also be an important factor in fiber-textured samples. Means to further improve the quality of texture and thus properties of textured piezoelectric ceramics by TGG are presented.

In this article, domain wall motion and the extrinsic contributions to the dielectric and piezoelectric responses in sol–gel derived lead zirconate titanate (PZT) films with compositions near the morphotropic phase boundary were investigated. It was found that although the films had different thicknesses, grain sizes, and preferred orientations, similar intrinsic dielectric constants were obtained for all films between 0.5 and 3.4 μm thick. It was estimated that about 25%–50% of the dielectric response at room temperature was from extrinsic sources. The extrinsic contribution to the dielectric constant of PZT films was mainly attributed to 180° domain wall motion, which increased with both film thickness and grain size. In studies on the direct and converse longitudinal piezoelectric coefficients of PZT films as a function of either stress or electric driving field, it was found that the ferroelastic non-180° domain wall motion was limited. Thus extrinsic contributions to the piezoelectric response were small in fine grain PZT films (especially those under 1.5 μm in thickness). However, as the films became thicker (>5μm), nonlinear behavior between the converse piezoelectric coefficient and the electric driving field was observed. This indicated that there was significant ferroelectric non-180° domain wall motion under high external excitation in thicker films. The activity of the non-180° domain walls was studied through non-180° domain switching. For fine grain films with film thicknesses less than 2 μm, non-180° switching was negligible. Transmission electron microscopy plan-view micrographs evidenced non-180° domain fringes in these films, where the vast majority of grains were 50–100 nm in diameter and showed a single set of domain fringes. Taken together, these measurements suggest that the pinning of non-180° domain walls is very strong in films with thickness less than 2 μm. In thicker films, non-180° domain switching was evidenced when the poling field exceeded a threshold field. The threshold field decreased with an increase in film thickness, suggesting more non-180° domain wall mobility in thicker films. Non-180° domain switching in large grained PZT films was found to be much easier and more significant than in the fine grained PZT films.

▪ Abstract This paper reviews the literature on size effects in ferroelectric materials, with an emphasis on thin film perovskite ferroelectrics. The roles of boundary conditions, defect chemistry, electrode interfaces, surface layers, and microstructure in controlling the measured properties of ferroelectric films, as well as the observed deviation from bulk properties are discussed. Examples of the manifestation of size effects in terms of the low and high field dielectric properties, the piezoelectric effect, and the leakage behavior of films are given.

Microelectromechanical systems (MEMS) incorporating active piezoelectric layers offer integrated actuation, sensing, and transduction. The broad implementation of such active MEMS has long been constrained by the inability to integrate materials with giant piezoelectric response, such as Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT). We synthesized high-quality PMN-PT epitaxial thin films on vicinal (001) Si wafers with the use of an epitaxial (001) SrTiO₃ template layer with superior piezoelectric coefficients (e_31,f = –27 ± 3 coulombs per square meter) and figures of merit for piezoelectric energy-harvesting systems. We have incorporated these heterostructures into microcantilevers that are actuated with extremely low drive voltage due to thin-film piezoelectric properties that rival bulk PMN-PT single crystals. These epitaxial heterostructures exhibit very large electromechanical coupling for ultrasound medical imaging, microfluidic control, mechanical sensing, and energy harvesting.