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

The piezoelectric properties of relaxor based ferroelectric single crystals, such as Pb(Zn1/3Nb2/3)O3–PbTiO3 and Pb(Mg1/3Nb2/3)O3–PbTiO3 were investigated for electromechanical actuators. In contrast to polycrystalline materials such as Pb(Zr,Ti)O3, morphotropic phase boundary compositions were not essential for high piezoelectric strain. Piezoelectric coefficients (d33’s)>2500 pC/N and subsequent strain levels up to >0.6% with minimal hysteresis were observed. Crystallographically, high strains are achieved for 〈001〉 oriented rhombohedral crystals, although 〈111〉 is the polar direction. Ultrahigh strain levels up to 1.7%, an order of magnitude larger than those available from conventional piezoelectric and electrostrictive ceramics, could be achieved being related to an E-field induced phase transformation. High electromechanical coupling (k33)>90% and low dielectric loss <1%, along with large strain make these crystals promising candidates for high performance solid state actuators.

Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals

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

Electrical actuators were made from films of dielectric elastomers (such as silicones) coated on both sides with compliant electrode material. When voltage was applied, the resulting electrostatic forces compressed the film in thickness and expanded it in area, producing strains up to 30 to 40%. It is now shown that prestraining the film further improves the performance of these devices. Actuated strains up to 117% were demonstrated with silicone elastomers, and up to 215% with acrylic elastomers using biaxially and uniaxially prestrained films. The strain, pressure, and response time of silicone exceeded those of natural muscle; specific energy densities greatly exceeded those of other field-actuated materials. Because the actuation mechanism is faster than in other high-strain electroactive polymers, this technology may be suitable for diverse applications.

http://science.sciencemag.org/content/sci/287/5454/836.full.pdf

High-Speed Electrically Actuated Elastomers with Strain Greater Than 100%

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

Investigations in the development of lead-free piezoelectric ceramics have recently claimed comparable properties to the lead-based ferroelectric perovskites, represented by Pb(Zr,Ti)O3, or PZT In this work, the scientific and technical impact of these materials is contrasted with the various families of “soft” and “hard” PZTs On the scientific front, the intrinsic nature of the dielectric and piezoelectric properties are presented in relation to their respective Curie temperatures (T
 C) and the existence of a morphotropic phase boundary (MPB) Analogous to PZT, enhanced properties are noted for MPB compositions in the (Na,Bi)TiO3-BaTiO3 and ternary system with (K,Bi)TiO3, but offer properties significantly lower The consequences of a ferroelectric to antiferroelectric transition well below T
 C further limits their usefulness Though comparable with respect to T
 C, the high levels of piezoelectricity reported in the (K,Na)NbO3 family are the result of enhanced polarizability associated with the orthorhombic-tetragonal polymorphic phase transition being compositionally shifted downward As expected, the properties are strongly temperature dependent, while degradation occurs through the thermal cycling between the two distinct ferroelectric domain states Extrinsic contributions arising from domains and domain wall mobility were determined using high field strain and polarization measurements The concept of “soft” and “hard” lead-free piezoelectrics were discussed in relation to donor and acceptor modified PZTs, respectively Technologically, the lead-free materials are discussed in relation to general applications, including sensors, actuators and ultrasound transducers

Lead-free piezoelectric ceramics: Alternatives for PZT?

Fabrication of perovskite lead magnesium niobate

The processing, electromechanical properties, and microstructure of lead zirconate titanate (PZT) ceramics over the grain-size range of 0.1-10 μm were studied. Using measurements over a large temperature range (15-600 K), the relative role of extrinsic contribution (i.e., domain-wall motion) was deduced to be influenced strongly by the grain size, particularly for donor-doped PZT. Analytical transmission electron microscopy studies were conducted to investigate the trend in domain configurations with the reduction of grain size. The correlations between domain density, domain variants, domain configurations (before and after poling), spontaneous deformation, and the elastodielectric properties were qualitatively discussed, leading to new insights into the intrinsic and extrinsic effects and relevant size effects in ferroelectric polycrystalline materials.

Intrinsic and Extrinsic Size Effects in Fine-Grained Morphotropic-Phase-Boundary Lead Zirconate Titanate Ceramics

New morphotropic phase boundary (MPB) piezoelectrics, with ferroelectric phase transition (Tc) exceeding that of PbZrO3–PbTiO3 (PZT), were investigated. Based on a perovskite tolerance factor-Tc relationship, new high Tc MPB systems were projected in the Bi(Me)O3–PbTiO3 system, where Me is a relatively large B+3-site cation. For the (1-x)BiScO3–(x)PbTiO3 solid solution, a MPB was found at x-0.64 separating the rhombohedral and tetragonal phases, with correspondingly enhanced dielectric and piezoelectric properties. A transition temperature Tc of ~ 450°C was determined with evidence of Tc's on the order of ≥ 600°C in the BiInO3 and BiYbO3 analogues, though issues of perovskite stability remain for the smaller tolerance end-member systems.

https://iopscience.iop.org/article/10.1143/JJAP.40.5999/pdf

Thomas R. Shrout

Papers

Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals

Lead-free piezoelectric ceramics: Alternatives for PZT?

Fabrication of perovskite lead magnesium niobate

Intrinsic and Extrinsic Size Effects in Fine-Grained Morphotropic-Phase-Boundary Lead Zirconate Titanate Ceramics

New High Temperature Morphotropic Phase Boundary Piezoelectrics Based on Bi(Me)O3?PbTiO3 Ceramics