Lead has recently been expelled from many commercial applications and materials (for example, from solder, glass and pottery glaze) owing to concerns regarding its toxicity. Lead zirconium titanate (PZT) ceramics are high-performance piezoelectric materials, which are widely used in sensors, actuators and other electronic devices; they contain more than 60 weight per cent lead. Although there has been a concerted effort to develop lead-free piezoelectric ceramics, no effective alternative to PZT has yet been found. Here we report a lead-free piezoelectric ceramic with an electric-field-induced strain comparable to typical actuator-grade PZT. We achieved this through the combination of the discovery of a morphotropic phase boundary in an alkaline niobate-based perovskite solid solution, and the development of a processing route leading to highly textured polycrystals. The ceramic exhibits a piezoelectric constant d33 (the induced charge per unit force applied in the same direction) of above 300 picocoulombs per newton (pC N(-1)), and texturing the material leads to a peak d33 of 416 pC N(-1). The textured material also exhibits temperature-independent field-induced strain characteristics.

Lead-free piezoceramics

Ferroelectric ceramics were born in the early 1940s with the discovery of the phenomenon of ferroelectricity as the source of the unusually high dielectric constant in ceramic barium titanate capacitors. Since that time, they have been the heart and soul of several multibillion dollar industries, ranging from high-dielectric-constant capacitors to later developments in piezoelectric transducers, positive temperature coefficient devices, and electrooptic light valves. Materials based on two compositional systems, barium titanate and lead zirconate titanate, have dominated the field throughout their history. The more recent developments in the field of ferroelectric ceramics, such as medical ultrasonic composites, high-displacement piezoelectric actuators (Moonies, RAINBOWS), photostrictors, and thin and thick films for piezoelectric and integrated-circuit applications have served to keep the industry young amidst its growing maturity. Various ceramic formulations, their form (bulk, films), fabrication, function (properties), and future are described in relation to their ferroelectric nature and specific areas of application.

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Ferroelectric ceramics : History and technology

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

An exceptionally high electrostrictive response ( approximately 4 percent) was observed in electron-irradiated poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer. The material exhibits typical relaxor ferroelectric behavior, suggesting that the electron irradiation breaks up the coherent polarization domain (all-trans chains) in normal ferroelectric P(VDF-TrFE) copolymer into nanopolar regions (nanometer-size, all-trans chains interrupted by trans and gauche bonds) that transform the material into a relaxor ferroelectric. The expanding and contracting of these polar regions under external fields, coupled with a large difference in the lattice strain between the polar and nonpolar phases, generate an ultrahigh strain response.

https://science.sciencemag.org/content/sci/280/5372/2101.full.pdf

Giant Electrostriction and Relaxor Ferroelectric Behavior in Electron-Irradiated Poly(vinylidene fluoride-trifluoroethylene) Copolymer

Friction, wear and lubrication between materials in contact are of fundamental importance in many pure and applied sciences. Owing to the development of experimental and computer-simulation techniques for studying these phenomena at the atomic scale, an understanding is beginning to emerge of the molecular mechanisms of tribology in thin films and at surfaces.

Nanotribology: friction, wear and lubrication at the atomic scale

This review of the PLZT materials and their applications describes the present state of the technology and emphasizes the latest developments in the field of electrooptic ceramics. Despite the reported development of new, transparent materials in other compositional systems, the PLZT's still remain the standard of the industry and continue to find an ever increasing number of applications in the areas of shutters, filters, displays, and spatial light modulators. Their fast response, high resolution and wide operating temperature range make them viable candidates for several emerging technologies including high resolution, non-impact, facsimile printing, electrophotographic copying, color reproduction and integrated optical processing. With the presently successful development of sputtered PLZT thin films, both bulk and thin film PLZT materials can now be considered for future electrooptic devices.

PLZT electrooptic materials and applications: a review

This is a review paper on the history of ferroelectric thin films, current practices in their fabrication, characteristics of interest which relate to their use, and the application of these films to memory and electrooptic devices.

Ferroelectric thin films for electronic applications

Ferroelectric thin film compositions in the PLZT system were prepared from water-soluble acetate precursors. Dip coated, multilayer thin films were applied to various substrates, pyrolyzed at 700°C and characterized in regard to dielectric and ferroelectric properties.

Plzt thin films prepared from acetate precursors

The newly developed Rainbow actuator features high displacement and good load-bearing capability, which are considered to be associated with Rainbow's unique dome-shaped configuration and high internal stress created during processing. The effects of the internal stress on the field-induced displacements of a series of PLZT Rainbow samples were investigated. It was found that the displacements were significantly enhanced for the samples having a reduced/total thickness ratio around 0.3 due to the interaction of the internal stress with ferroelectric domains. Finite element modeling was used to analyze the internal stress of the Rainbow actuators. The stress distribution revealed by the X-ray diffraction (XRD) technique agreed well with the modeling. Enhancement of domain reorientation by the internal stress was observed from the behavior of the XRD peaks in the presence of applied electric fields. The Rainbow samples with maximum stress-enhanced domain reorientation were found to exhibit maximum field-induced displacements. A finite element model incorporating the stress-enhancing mechanism with the displacement calculation was developed. This model yielded displacement values which were in good agreement with the experimental data.

Gene H. Haertling

Papers

Ferroelectric ceramics : History and technology

PLZT electrooptic materials and applications: a review

Ferroelectric thin films for electronic applications

Plzt thin films prepared from acetate precursors

Stress‐Enhanced Displacements in PLZT Rainbow Actuators