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.

Lead-free piezoelectric ceramics, ͑1−x − y͒Bi 0.5 Na 0.5 TiO 3 -xBaTiO 3 -yK 0.5 Na 0.5 NbO 3 ͑0.05ഛ x ഛ 0.07 and 0.01ഛ y ഛ 0.03͒, have been synthesized by a conventional solid state sintering method. The room temperature ferroelectric and piezoelectric properties of these ceramics were studied. Based on the measured properties, the ceramics were categorized into two groups: group I compositions having dominant ferroelectric order and group II compositions displaying mixed ferroelectric and antiferroelectric properties at room temperature. A composition from group II near the boundary between these two groups exhibited a strain as large as ϳ0.45% at an electric field of 8 kV/ mm. Polarization in this composition was not stable in that the piezoelectric coefficient d 33 at zero electric field was only about 30 pm/ V. The converse piezoelectric response becomes weaker when the composition deviated from the boundary between the groups toward either the ferroelectric or antiferroelectric compositions. These results were rationalized based on a field induced antiferroelectric-ferroelectric phase transition.

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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]