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.

Origin of the large strain response in "K 0.5 Na 0.5 …NbO 3 -modified "Bi 0.5 Na 0.5 …TiO 3 -BaTiO 3 lead-free piezoceramics The mechanism of the giant unipolar strain recently observed in a lead-free piezoceramic, 0.92͑Bi 0.5 Na 0.5 ͒TiO 3 − 0.06BaTiO 3 − 0.02͑K 0.5 Na 0.5 ͒NbO 3 ͓S.-T. Zhang, A. B. Kounga, E. Aulbach, H. Ehrenberg, and J. Rödel, Appl. Phys. Lett. 91, 112906 ͑2007͒ was investigated. The validity of the previously proposed mechanism that the high strain comes both from a significant volume change during the field-induced phase transition, from an antiferroelectric to a ferroelectric phase and the domain contribution from the induced ferroelectric phase was examined. Monitoring the volume changes from the simultaneously measured longitudinal and transverse strains on disk-shaped samples showed that the phase transition in this specific material does not involve any notable volume change, which indicates that there is little contribution from a volume change due to the phase transition to the total strain response. Temperature dependent hysteresis measurements on unpoled samples of a nearby ferroelectric composition, 0.93͑Bi 0.5 Na 0.5 ͒TiO 3 − 0.06BaTiO 3 − 0.01͑K 0.5 Na 0.5 ͒NbO 3 demonstrated that the origin of the large strain is due to the presence of a nonpolar phase that brings the system back to its unpoled state once the applied electric field is removed, which leads to a large unipolar strain.

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.

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.

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Temperature-dependent electrical properties of lead-free 0.94Bi 0.5 Na 0.5 TiO 3 -0.06BaTiO 3 (BNT-BT) ceramics were investigated. Below 1001C, this rhombohedral-tetragonal morphotropic phase-boundary composition shows dominant ferroelectric order with typical ferroelectric polarizationelectric field (P(E)) loop and butterfly bipolar strain-electric (S(E)) curve. Antiferroelectric order tends to prevail when temperature reaches about 1001C, characterized by a pinched P(E) loop and altered bipolar S(E) butterfly. Near the ferroelectricantiferroelectric transition temperature, the composition shows a giant bipolar and unipolar strain of 0.40% and 0.42%, respectively. The highest value of maximum strain divided by the applied field (i.e., S max /E max ) reaches 700 pm/V at 1001C. With a further increase of temperature to 2001C, a slight decrease of the strain is observed. Especially, it is found that the hysteresis of the unipolar S(E) curve decreases with increasing temperature. These results may be helpful for further understanding and thus designing new BNT-based lead-free piezoelectric systems. K. Bowman-contributing editorThis work was supported by the Deutsche Forschungsgemeinschaft (DFG) under SFB 595. The Raman spectra measurements were supported by the Nature Science Foundation of Jiangsu province (BK2006514). S. T. Z thanks the Alexander von Humboldt foundation for financial support.