There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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 zirconate titanate (PZT) based piezoelectric materials are well known for their excellent piezoelectric properties. However, considering the toxicity of lead and its compounds, there is a general awareness for the development of environmental friendly lead-free materials as evidenced from the legislation passed by the European Union in this effect. Several classes of materials are now being considered as potentially attractive alternatives to PZTs for specific applications. In this paper, attempts have been made to review the recent developments on lead-free piezo materials emphasizing on their preparation, structure–property correlation, etc. In this context, perovskite systems such as bismuth sodium titanate, alkali niobates (ANbO3), etc. and non-perovskites such as bismuth layer-structured ferroelectrics are reviewed in detail. From the above study, it is concluded that some lead-free compositions show stable piezoelectric responses even though they do not match the overall performance of PZT. This has been the stimulant for growing research on this subject. This topic is of current interest to the researchers worldwide as evidenced from the large number of research publications. This has motivated us to come out with a review article with a view that it would give further impetus to the researchers already working in this area and also draw the attention of the others.

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Review: environmental friendly lead-free piezoelectric materials

Feature size is a natural determinant of material properties. Its design offers the technological perspectives for material improvement. Grain size, crystallite size, domain width, and structural defects of different nature constitute the classical design elements. Common ferroelectric ceramics contain micrometer grain sizes and submicrometer domain widths. Domain wall mobility is a major contribution to their macroscopic material properties providing approximately half of the macroscopic output in optimized materials. The extension into the dynamic nanoworld is provided by relaxor ferroelectrics. Ionic and nanoscale field disorders form the base to a state with natural nanometer-size polar structures even in bulk materials. These polar structures are highly mobile and can dynamically change over several orders of magnitude in time and space being extremely sensitive to external stimuli. This yields among the largest dielectric and piezoelectric constants known. In this feature article, we want to outline how lead-free relaxors will offer a route to an environmentally safer option in this outstanding material class. Properties of uniaxial, planar, and volumetric relaxor compositions will be discussed. They provide a broader and more interesting scope of physical properties and features than the classical lead-containing relaxor compositions.

Lead-Free Relaxor Ferroelectrics

The utilization of antiferroelectric (AFE) materials is thought to be an effective approach to enhance the energy density of dielectric capacitors. However, the high energy dissipation and inferior reliability that are associated with the antiferroelectric-ferroelectric phase transition are the main issues that restrict the applications of antiferroelectric ceramics. Here, simultaneously achieving high energy density and efficiency in a dielectric ceramic is proposed by combining antiferroelectric and relaxor features. Based on this concept, a lead-free dielectric (Na0.5 Bi0.5 )TiO3 -x(Sr0.7 Bi0.2 )TiO3 (NBT-xSBT) system is investigated and the corresponding multilayer ceramic capacitors (MLCCs) are fabricated. A record-high energy density of 9.5 J cm-3 , together with a high energy efficiency of 92%, is achieved in NBT-0.45SBT multilayer ceramic capacitors, which consist of ten dielectric layers with the single-layer thickness of 20 µm and the internal electrode area of 6.25 mm2 . Furthermore, the newly developed capacitor exhibits a wide temperature usage range of -60 to 120 °C, with an energy-density variation of less than 10%, and satisfactory cycling reliability, with degradation of less than 8% over 106 cycles. These characteristics demonstrate that the NBT-0.45SBT multilayer ceramic is a promising candidate for high-power energy storage applications.

Multilayer Lead-Free Ceramic Capacitors with Ultrahigh Energy Density and Efficiency.

In search of lead-free ferroelectric ceramics with improved properties, an investigation was carried out on barium and strontium-modified Na0.5Bi0.5TiO3 (NBT) ceramics. Many compositions among the Ba and Sr-modified ceramic materials exhibit diffuse phase transition and are characterised by a strong temperature and frequency dispersion of the permittivity which would be connected with the cation disorder in the A site of the perovskite unit cell.

Sodium-bismuth titanate based lead-free ferroelectric materials

Ceramics with compositions belonging to the Na0.5Bi0.5TiO3–PbTiO3 and Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 systems were fabricated by natural sintering of powders prepared by thermal decomposition of adequate precursor solutions. The ferroelectric to paraelectric phase transitions were studied by variable temperature X-ray diffractometry, differential scanning calorimetry and impedance measurement in a wide range of temperature and frequency. In contrast with pure NBT, both the permittivity and dielectric loss of the NBT-rich solid solutions show a strongly temperature and frequency dependent behaviour. The permittivity decreases and its maximum is shifted towards high temperatures as the frequency increases. In the high temperature range, the thermal variation of the permittivity is well described by a law 1 e − 1 e m =C T−T m γ with γ close to 1.5. Such a relaxor-like behaviour is interpreted in terms of cation disorder due to the statistical repartition of (Na,Bi) and Pb (or Na and K). This would be one very rare case of relaxor phenomena correlated with the A-site occupancy in perovskite-like materials.

Relaxor behaviour of low lead and lead free ferroelectric ceramics of the Na0.5Bi0.5TiO3–PbTiO3 and Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 systems

A (Na0.5Bi0.5)0.70Ba0.30TiO3 ceramic has been studied by X-ray diffraction and by measurements of dielectric and ferroelectric properties between room temperature and 450 °C. A sharp increase in the electric permittivity and dielectric loss near 200 °C has been observed. This sharp increase in dielectric responses indicates a transformation between normal and relaxor ferroelectric states. It is found that polar regions can exist at higher temperatures. The X-ray diffraction study shows that the transformation corresponds to the first order phase transition from tetragonal to cubic. The use of the (Na0.5Bi0.5)0.70Ba0.30TiO3 ceramic for device application has also been indicated.

J.P. Mercurio

Papers

Sodium-bismuth titanate based lead-free ferroelectric materials

Relaxor behaviour of low lead and lead free ferroelectric ceramics of the Na0.5Bi0.5TiO3–PbTiO3 and Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3 systems

Structural and dielectric properties of (Na0.5Bi0.5)0.70Ba0.30TiO3 ceramics

Raman spectroscopy study of the Na0.5Bi0.5TiO3–PbTiO3 system

Temperature behaviour of structural, dielectric and piezoelectric properties of sol-gel processed ceramics of the system LiNbO3-NaNbO3