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

Lead-free piezoelectric ceramics (1−x)(Na0.5K0.5)NbO3–xLiNbO3 {[Lix(Na0.5K0.5)1−x]NbO3} (x=0.04–0.20) have been synthesized by an ordinary sintering technique. The materials with perovskite structure is orthorhombic phase at x⩽0.05 and becomes tetragonal phase at x⩾0.07, a phase K3Li2Nb5O15 with tetragonal tungsten bronze structure begins to appear at x=0.08 and becomes dominant with increasing the content of LiNbO3. A morphotropic phase boundary between orthorhombic and tetragonal phases is found in the composition range 0.05<x<0.07. Analogous to Pb(Zr,Ti)O3, the piezoelectric and electromechanical properties are enhanced for compositions near the morphotropic phase boundary. Piezoelectric constant d33 values reach 200–235pC∕N. Electromechanical coefficients of the planar mode and the thickness mode reach 38%–44% and 44%–48%, respectively. The Curie temperatures (TC) of [Lix(Na0.5K0.5)1−x]NbO3 (x=0.04–0.20) are in the range of 452–510°C, at least 100°C higher than that of conventional Pb(Zr,Ti)O3. Our re...

Phase transitional behavior and piezoelectric properties of (Na0.5K0.5)NbO3–LiNbO3 ceramics

Potassium-sodium niobate lead-free piezoelectric materials: past, present, and future of phase boundaries.

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

In response to the current environmental regulations against the use of lead in daily electronic devices, a number of investigations have been performed worldwide in search for alternative piezoelectric ceramics that can replace the market-dominating lead-based ones, representatively Pb(Zr
 x
 Ti1-x
 )O3 (PZT)-based solid solutions. Selected systems of potential importance such as chemically modified and/or crystallographically textured (K, Na)NbO3 and (Bi1/2Na1/2)TiO3-based solid solutions have been developed. Nevertheless, only few achievements have so far been introduced to the marketplace. A recent discovery has greatly extended our tool box for material design by furnishing (Bi1/2Na1/2)TiO3-based ceramics with a reversible phase transition between an ergodic relaxor state and a ferroelectric with the application of electric field. This paired the piezoelectric effect with a strain-generating phase transition and extended opportunities for actuator applications in a completely new manner. In this contribution, we will present the status and perspectives of this new class of actuator ceramics, aiming at covering a wide spectrum of topics, i.e., from fundamentals to practice.

Giant electric-field-induced strains in lead-free ceramics for actuator applications – status and perspective

Doped alkaline–bismuth–titanate perovskite single crystals have been grown in ferroelectric phases with high piezoelectric actuation. Rhombohedral-phase Na1/2Bi1/2TiO3–BaTiO3 crystals exhibit up to 0.25% free strain with low hysteresis along the cubic 〈001〉 direction (d33∼450 pC/N). Tetragonal phase crystals exhibit free strains as high as 0.85% with greater hysteresis characteristic of domain switching; low field d33 exceeds 500 pC/N. Strain energy densities exceed those of optimized polycrystalline lead perovskites, and actuation capability is retained at compressive stresses >100 MPa.

Lead-free high-strain single-crystal piezoelectrics in the alkaline–bismuth–titanate perovskite family

A single crystal perovskite material is provided having the formula, MαBiβM′γM″δO3±z, where M is one or more of Na, K, Rb and Cs; M′ is one or more of Ca, Sr, Ba, Pb, Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; and M″ is one or more of Ti, Zr, Hf, Sn, Ge, Mg, Zn, Al, Sc, Ga, Nb, Mo, Sb, Ta and W; where z≦0.1; 0.9≦δ≦1.1; α, β and γ are greater than zero; and (α+β+γ) is in the range of about 0.75 to 1.1. A perovskite material of the formula, NaωMαBiβM′γM″δO3±z, is provided where M is one or more of K, Rb and Cs; M′ is one or more of Ca, Sr, Ba, Pb, Y, La, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lu; M″ is one or more of Ti, Zr, Hf, Sn, Ge, Mg, Zn, Al, Sc, Ga, Nb, Mo, Sb, Ta and W; where z≦0.1; 0.9≦δ≦1.1; α, β and γ are greater than zero; and (α+β+γ) is in the range of about 0.75 to 1.1. The materials exhibit improved d33 and piezoelectric strain values, as well as low hysteresis.

Piezoelectric actuators and method of making same

Single crystals of sodium bismuth titanate, (Na, Bi)TiO3 (NBT), and its solid solutions were grown by a liquid phase epitaxy (LPE) using strontium titanate, SrTiO3 (ST), single crystals as substrates. X-ray diffraction measurements and polarized optical images revealed that the crystals could be epitaxially grown on both the (001) face and the (111) face of the ST substrate. However, due to lattice mismatch, the grown crystals had a multi-domain structure with domain boundaries at both 90 and 180°. The NBT crystals grown by using unsealed platinum pan had an inhomogeneous chemical composition, while homogeneous crystals were grown by using sealed platinum capsules for the growth cell. It is concluded from the results of several experiments on inter-diffusion between the substrate and the grown crystal that liquid phase epitaxy is appropriate for the growth of NBT.

Liquid-Phase Epitaxial Growth of BaTiO3 Doped (Na0.5Bi0.5)TiO3 Single Crystals on a SrTiO3 Single Crystal Substrate

While lead oxide-based piezoelectrics exhibit some of the highest field-induced strains and most efficient electrical-mechanical energy conversion of known compounds, the toxicity and high vapor pressure during processing of lead oxide ceramics have resulted in an increasing demand for environmentally benign alternative materials with comparable properties. In this work Na/sub 1/2/Bi/sub 1/2/TiO/sub 3/-BaTiO/sub 3/ solid solutions have been grown in single crystal form by a flux method. Rhombohedral phase crystals, oriented in the pseudocubic directions, exhibit low-hysteresis actuation with up to 0.24% free strain near the morphotropic phase boundary. Tetragonal phase crystals, oriented in the pseudocubic direction, exhibit free strains as high as 0.85% with greater hysteresis. Actuation energy densities exceed those of optimized polycrystalline lead-perovskites such as Pb(Zr/sub x/Ti/sub 1-x/)O/sub 3/ and Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/. Significant actuation capability is retained at stresses in excess of 100 MPa.

Growth and characterization of Na/sub 1/2/Bi/sub 1/2/TiO/sub 3/-K/sub 1/2/Bi/sub 1/2/TiO/sub 3/BaTiO/sub 3/ single crystal piezoelectrics

Perovskite oxides of (Bi,Na)TiO/sub 3/ (BNT) and its solid solutions are possible substitutes for lead oxide based actuator materials. In this study, pure and doped BNT single crystals were grown on the [001] face of SrTiO/sub 3/ (ST) single crystal substrates in order to optimize the conditions for liquid phase epitaxial (LPE) growth. X-ray diffraction (XRD) indicated an epitaxial relationship between the grown NBT and ST substrates, with the a-axis of BNT (both tetragonal and rhombohedral) along a-axis of the ST substrates. Line broadening of the XRD pattern due to lattice mismatch was also observed. Polarized optical microscopy revealed that 90/spl deg/ domains were formed in the as-grown crystals. EDS mapping analyses indicated that interface between grown NBT and ST was compositionally sharp. After removing the substrates, electromechanical constants were evaluated.

Gregory W. Farrey

Papers

Lead-free high-strain single-crystal piezoelectrics in the alkaline–bismuth–titanate perovskite family

Piezoelectric actuators and method of making same

Liquid-Phase Epitaxial Growth of BaTiO3 Doped (Na0.5Bi0.5)TiO3 Single Crystals on a SrTiO3 Single Crystal Substrate

Growth and characterization of Na/sub 1/2/Bi/sub 1/2/TiO/sub 3/-K/sub 1/2/Bi/sub 1/2/TiO/sub 3/BaTiO/sub 3/ single crystal piezoelectrics

Liquid phase epitaxial growth of perovskite (Bi,Na)TiO/sub 3/ and solid solutions on SrTiO/sub 3/ [001] substrates