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.

Electromechanical coupling in actuators provides high strain with high force, e.g. to drive motors, control fuel injection, etc.[1] This strain is provided through either piezoelectricity or electrostriction. Most piezoelectric and electrostrictive devices use lead-based materials (e.g., ferroelectric Pb(Zr,Ti)O 3 (PZT) for piezoelectrics and relaxor Pb(Mg 1/3 Nb 2/3 )O 3 (PMN) for electrostrictors). Environmental legislation in the European Union, [2] in parts of Asia, and the US demands elimination of toxic lead for these materials systems. Recently, this spurred a large effort in the research of lead-free actuator materials, focusing development on piezoelectric lead-free materials, [3][4][5][6] with rare examples on lead-free relaxor ferroelectrics. [7] The new materials still suffer a range of problems, for example the strong temperaturedependence of obtainable strain. [8] In this paper we demonstrate a new concept of using lead-free antiferroelectrics as electrostrictors, providing high strain and minimal losses at room temperature combined with minimal temperature dependence.Piezoelectric strain is possible only in materials with sufficiently low symmetry (most noncentrosymmetric materials) while the electrostrictive effect is present in all materials. [9] In tensor notation the electric-field induced strain, S ij , can be written either as a power series in electric field, E k , or in polarization P k :ði; j; k; lÞ ¼ 1; 2; 3 (1)The first term in either equation represents the contribution of the converse piezoelectric effect, the second term electrostriction. The piezoelectric coefficients d kij and g kij are collected in a third rank tensor, while the electrostriction coefficients M ijkl and Q ijkl constitute a fourth rank tensor. The first equation only holds true for small electric fields, whereas the second equation is more fundamental. [10] In order to make the fourth-rank tensor in Equation 2 more manageable, it is reduced to a second rank 6 Â 6 matrix. [10] If only non-shear strain components are considered the strain contribution by electrostriction is given by: [10]

BNT-BKT) lead-free piezoceramics was investigated. Polarization and strain hysteresis loops indicate that the ferroelectric order is disrupted significantly with the addition of KNN as a replacement for BNT and the destabilization of the ferroelectric order is accompanied by an enhancement of the unipolar strain, which peaks at a value of B0.48% (corresponding to a large signal d 33 of B600 pm/V) at 1 mol% KNN content. This strain was analyzed as derived from an electrostrictive effect at lower electric fields and a converse piezoelectric effect at higher electric fields. By limiting the electric driving field to exclude the contribution from the converse piezoelectric effect, a temperature-insensitive large-field d 33 of B250 pm/V up to 2001C was achieved.