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.

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]

Articles you may be interested inLarge strain response based on relaxor-antiferroelectric coherence in Bi0.5Na0.5TiO3-SrTiO3-(K0.5Na0. The temperature dependence of the dielectric and ferroelectric properties of lead-free piezoceramics of the composition ͑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, 0.01ഛ y ഛ 0.03͒ was investigated. Measurements of the polarization and strain hystereses indicate a transition to predominantly antiferroelectric order when heating from room temperature to 150°C, while for 150Ͻ T Ͻ 200°C both remnant polarization and coercive field increase. Frequency-dependent susceptibility measurements show that the transition is relaxorlike. For some samples, the transition temperature T d is high enough to allow mostly ferroelectric ordering at room temperature. These samples show a drastic increase of the usable strain under an external electric field just after the transition into the antiferroelectric state at high temperatures. For the other samples, T d is so low that they display significant antiferroelectric ordering already at room temperature. In these samples, the usable strain is relatively stable over a wide temperature range. In contrast to T d , the temperature T m of the transition into the paraelectric high-temperature phase depends far less on the sample composition. These results confirm that the high strain in this lead-free system is due to a field-induced antiferroelectric-ferroelectric phase transition and that this effect can be utilized in a wide temperature range.

The electromechanical behavior of (1−x)Bi0.5Na0.5TiO3–xK0.5Na0.5NbO3 (BNT-KNN) lead free piezoelectric ceramics is investigated for 0⩽x⩽0.12 to gain insight into the antiferroelectric-ferroelectric (AFE-FE) phase transition on the basis of the giant strain recently observed in BNT-based systems. At x≈0.07, a morphotropic phase boundary (MPB) between a rhombohedral FE phase and a tetragonal AFE phase is found. While the piezoelectric coefficient is largest at this MPB, the total strain further increases with increasing KNN content, indicating the field-induced AFE-FE transition as the main reason for the large strain.