A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Perspective on the Development of Lead-free Piezoceramics

Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (T(c)) is traditionally accomplished by chemical substitution-as in Ba(x)Sr(1-x)TiO(3), the material widely investigated for microwave devices in which the dielectric constant (epsilon(r)) at GHz frequencies is tuned by applying a quasi-static electric field. Heterogeneity associated with chemical substitution in such films, however, can broaden this phase transition by hundreds of degrees, which is detrimental to tunability and microwave device performance. An alternative way to adjust T(c) in ferroelectric films is strain. Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase T(c) by hundreds of degrees and produce room-temperature ferroelectricity in strontium titanate, a material that is not normally ferroelectric at any temperature. This strain-induced enhancement in T(c) is the largest ever reported. Spatially resolved images of the local polarization state reveal a uniformity that far exceeds films tailored by chemical substitution. The high epsilon(r) at room temperature in these films (nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications.

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Room-temperature ferroelectricity in strained SrTiO3.

Fundamentals, processes and applications of high-permittivity polymer–matrix composites

Perovskite lead-free dielectrics for energy storage applications

Piezoelectric actuators convert electrical into mechanical energy and are implemented for many large-scale applications such as piezoinjectors and ink jet printers. The performance of these devices is governed by the electric-field-induced strain. Here, the authors describe the development of a class of lead-free (0.94−x)Bi0.5Na0.5TiO3–0.06BaTiO3–xK0.5Na0.5NbO3 ceramics. These can deliver a giant strain (0.45%) under both unipolar and bipolar field loadings, which is even higher than the strain obtained with established ferroelectric Pb(Zr,Ti)O3 ceramics and is comparable to strains obtained in Pb-based antiferroelectrics.

Giant strain in lead-free piezoceramics Bi0.5Na0.5TiO3–BaTiO3–K0.5Na0.5NbO3 system

The temperature dependence of dielectric properties and electrical conduction of $({\mathrm{Sr}}_{1\ensuremath{-}1.5x}{\mathrm{Bi}}_{x}){\mathrm{TiO}}_{3} (0.0133l~xl~0.133)$ was measured from 10 to 800 K. Three sets of oxygen vacancies related dielectric peaks (peaks A, B, and $C)$ were observed. These peaks could be greatly suppressed or eliminated by annealing the samples in an oxidizing atmosphere, and enhanced or recreated by annealing in a reducing atmosphere. The results show that the Maxwell-Wagner polarization is not the main mechanism, and the Skanavi's model also cannot be directly applied. A tentative explanation was suggested. Peak A, observed in the temperature range of 100--350 K with the activation energy for dielectric relaxation ${E}_{\mathrm{relaxA}}=0.32--0.49$ eV, is attributed to the coupling effect of the conduction electrons with the motion of the off-centered Bi and Ti ions; the conduction carriers in this temperature range are from the first ionization of oxygen vacancies ${(V}_{o}).$ Peaks B and C are also discussed.

Oxygen-vacancy-related low-frequency dielectric relaxation and electrical conduction in B i : S r T i O 3

The polarization and strain behavior of Ba(Ti1−xZrx)O3 (x=0–03) ceramics are studied The unipolar strain levels of ∼018% at 40 kV/cm and of ∼025% at ∼120 kV/cm with small hysteresis were obtained for the ceramics with 003⩽x⩽008 The remnant polarization is ∼13–18 μC/cm2 for 003⩽x⩽008 and below 2 μC/cm2 for 015⩽x⩽03 at room temperature The electromechanical coupling coefficient K33=565% and the piezoelectric stain coefficient d33=236 pC/N were obtained for the ceramics with x=005 These results indicated that the Ba(Ti1−xZrx)O3 system is a promising lead-free high strain material for applications

Piezoelectric and strain properties of Ba(Ti1−xZrx)O3 ceramics

In this study, the phase transition behavior of the BaTiO3–BaZrO3 system was studied using micro-Raman scattering and dielectric measurement techniques. BaZrxTi1−xO3 ceramics were prepared for x=0.00, 0.05, 0.08, 0.15, 0.20, and 1.00 compositions using a solid-state reaction technique. A single-phase perovskite structure of the ceramics was identified by the x-ray diffraction technique. The basic phase transition temperatures in these compositions were studied in the temperature range of 70–575 K. The tetragonal to cubic transition temperature was found to decrease with increasing Zr content. The orthorhombic to tetragonal transition temperature that increases with an initial increase in Zr content merges with the tetragonal–cubic transition for x⩾0.15 compositions. Raman spectra of rhombohedral and orthorhombic phases could not be distinguished. Excellent agreement between the crystallographic transition temperatures obtained by both techniques suggested that Zr substituted octahedra were uniformly distr...

Micro-Raman scattering and dielectric investigations of phase transition behavior in the BaTiO3–BaZrO3 system

In this article, we report the successful growth of Ba(Ti1−xZrx)O3 (x=0.05–0.2) single-crystal fibers by the laser-heated pedestal growth technique. A single-phase perovskite structure of the materials has been identified by the x-ray diffraction technique. The phase diagram for Ba(Ti1−xZrx)O3 single crystals is established for x⩽0.2. Dielectric properties as function of temperature and frequency and room-temperature hysteresis loops are measured. The remnant polarization (Pr) and coercive fields (Ec) are obtained and compared for both single crystals and ceramics. The small dielectric relaxation has been observed for the lower-temperature phase transition (around 40 °C at 1 kHz) of the sample with x=0.08. The relaxation times follow the Arrhenius law with τ0=0.4×10−13 s and Erelax=0.53 eV. A common feature of the low-temperature relaxation mode in the sample is also observed, which follows the Arrhenius law with the τ0=0.8×10−10 s and Erelax=0.46 eV.

Dielectric behavior of Ba(Ti1-xZrx)O3 single crystals

Ferroelectric-relaxor behavior of Ba(Ti0.7Zr0.3)O3 ceramics is studied in the temperature range from 150 to 450 K. A broad dielectric peak with a high-dielectric maximum exhibits frequency dispersion. The dielectric relaxation rate follows the Vogel–Fulcher relation with E=0.21 eV and TVF=199.6 K. The polarization hysteresis loops were observed with a remnant polarization of 10 μC/cm2 at 175 K. The high-dielectric constant and high-polarization properties are expected to find practical applications at low temperatures.

Zhi Yu

Papers

Oxygen-vacancy-related low-frequency dielectric relaxation and electrical conduction in B i : S r T i O 3

Piezoelectric and strain properties of Ba(Ti1−xZrx)O3 ceramics

Micro-Raman scattering and dielectric investigations of phase transition behavior in the BaTiO3–BaZrO3 system

Dielectric behavior of Ba(Ti1-xZrx)O3 single crystals

Ferroelectric-relaxor behavior of Ba(Ti0.7Zr0.3)O3 ceramics