Potassium-sodium niobate lead-free piezoelectric materials: past, present, and future of phase boundaries.

After twenty years of partly quiet and ten years of partly enthusiastic research into lead-free piezoceramics there are now clear prospects for transfer into applications in some areas. This mimics prior research into eliminating lead from other technologies that resulted in restricted lead use in batteries and dwindling use in other applications. A figure of merit analysis for key devices is presented and used to contrast lead-containing and lead-free piezoceramics. A number of existing applications emerge, where the usage of lead-free piezoceramics may be envisaged in the near future. A sufficient transition period to ensure reliability, however, is required. The use of lead-free piezoceramics for demanding applications with high reliability, displacements and frequency as well as a wide temperature range appears to remain in the distant future. New devices are outlined, where the figure of merit suggests skipping lead-containing piezoceramics altogether. Suggestions for the next pertinent research requirements are provided.

Transferring lead-free piezoelectric ceramics into application

Perovskite lead-free dielectrics for energy storage applications

In response to the current environmental regulations against the use of lead in daily electronic devices, a number of investigations have been performed worldwide in search for alternative piezoelectric ceramics that can replace the market-dominating lead-based ones, representatively Pb(Zr
 x
 Ti1-x
 )O3 (PZT)-based solid solutions. Selected systems of potential importance such as chemically modified and/or crystallographically textured (K, Na)NbO3 and (Bi1/2Na1/2)TiO3-based solid solutions have been developed. Nevertheless, only few achievements have so far been introduced to the marketplace. A recent discovery has greatly extended our tool box for material design by furnishing (Bi1/2Na1/2)TiO3-based ceramics with a reversible phase transition between an ergodic relaxor state and a ferroelectric with the application of electric field. This paired the piezoelectric effect with a strain-generating phase transition and extended opportunities for actuator applications in a completely new manner. In this contribution, we will present the status and perspectives of this new class of actuator ceramics, aiming at covering a wide spectrum of topics, i.e., from fundamentals to practice.

Giant electric-field-induced strains in lead-free ceramics for actuator applications – status and perspective

Temperature-dependent dielectric permittivity of 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO3 (BNT-6BT) lead-free piezoceramics was studied to disentangle the existing unclear issues over the crystallographic aspects and phase stability of the system. Application of existing phenomenological relaxor models enabled the relaxor contribution to the entire dielectric permittivity spectra to be deconvoluted. The deconvoluted data in comparison with the temperature-dependent dielectric permittivity of a classical perovskite relaxor, La-modified lead zirconate titanate, clearly suggest that BNT-6BT belongs to the same relaxor category, which was also confirmed by a comparative study on the temperature- dependent polarization hysteresis loops of both materials. Based on these results, we propose that the low-temperature dielectric anomaly does not involve any phase transition such as ferroelectric- to-antiferroelectric. Supported by transmission electron microscopy and X-ray diffraction experiments at ambient temperature, we propose that the commonly observed two dielectric anomalies are attributed to thermal evolution of ferroelectric polar nanoregions of R3c and P4bm symmetry, which coexist nearly throughout the entire temperature range and reversibly transform into each other with temperature.

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On the phase identity and its thermal evolution of lead free (Bi1/2Na1/2)TiO3-6 mol% BaTiO3

A comprehensive review of aging and fatigue phenomena in bulk polycrystalline ferroelectrics is presented. Three material classes are covered, namely the most widely used Pb[Zr1−xTix]O3 (PZT) ceramics and lead-free materials, including those based on bismuth sodium titanate Bi1/2Na1/2TiO3 (BNT) and alkali niobate [KxNa1−x]NbO3 (KNN). Aging is studied in poled and unpoled states both experimentally and theoretically. The variety of different loading regimes for fatigue includes DC electric field, unipolar, sesquipolar and bipolar cycling and all these differently combined with mechanical loading at different frequencies and temperatures. The role of device geometries and electrode materials is addressed and models describing charge migration and defect dipole re-orientation are discussed in the context of recent experimental studies.

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Mechanisms of aging and fatigue in ferroelectrics

An in situ structural description of the origin of the ferroelectric properties as a function of the applied electric field E was obtained by synchrotron x-ray diffraction. A setup was used to average the effects of the preferred orientation induced by the strong piezoelectric strain and solve in situ the crystal structure as a function of the applied electric field. Hence, we were able to describe the microscopic origin of the macroscopic ferro- and piezoelectric properties of the most widely used ferroelectric material, lead zirconate titanate.

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Structural description of the macroscopic piezo- and ferroelectric properties of lead zirconate titanate.

A considerable body of knowledge now exists from studies involving the development of lead-free piezoelectric ceramics and a number of high potential alternatives to current lead-based materials have been identified. Stability under cyclic electric fields is an important property of piezoelectric materials. Here, we review the research to date which shows that fatigue under cyclic electrical loading is prevalent in many lead-free piezoelectric ceramic compositions. However, the variety of compositions and mechanisms for piezoelectric behavior in these materials corresponds to significant variances in the nature of fatigue degradation and the likely mechanisms thereof, which do not directly parallel those of well-studied lead-based materials. In particular, the use of field-induced phase changes as an actuation mechanism provides distinctive fatigue behaviors. Particular attention is given to fatigue of ferroelectric and relaxor (ergodic and nonergodic) structures and their dependence upon temperature and electric field and the potential design of materials with high fatigue resistance.

Electric Fatigue of Lead-Free Piezoelectric Materials

The bipolar and unipolar fatigue behavior of the lead-free piezoand ferroelectric 0.94Bi1/2Na1/2TiO3–0.06BaTiO3 (94BNT–6BT) was investigated. To obtain a complete profile of the fatigue behavior, both large-signal (polarization, strain) and small-signal (d33, e33) properties were measured at room temperature. The results indicate that the material suffers a significant degradation in both large- and small-signal properties after cycling, with most of the fatigue occurring in the first 10 4 cycles of the fatigue test.

Bipolar and Unipolar Fatigue of Ferroelectric BNT-Based

The fatigue behavior of the lead-free piezoelectric Bi1/2Na1/2TiO3–BaTiO3−K0.5Na0.5NbO3 (BNT-BT-KNN) under bipolar and unipolar electrical loading is investigated. Special attention is paid to the effect of a decreasing ferroelectric order which was affected by an increase in KNN-content. It is found that, in mainly ferroelectric systems with low KNN-content, bipolar cycling leads to domain wall pinning and formation of microcracks, while in mainly non-ferroelectric systems with high KNN-content fatigue can be rationalized by the repeated field-induced phase transition occurring during each field cycle. Under unipolar load, fatigue in low-KNN systems is dominated by domain wall pinning and the formation of a space charge field. Only the latter effect is present in non-ferroelectric high-KNN systems, which fatigue to a smaller degree.

Julia Glaum

Papers

Mechanisms of aging and fatigue in ferroelectrics

Structural description of the macroscopic piezo- and ferroelectric properties of lead zirconate titanate.

Electric Fatigue of Lead-Free Piezoelectric Materials

Bipolar and Unipolar Fatigue of Ferroelectric BNT-Based

Effect of Ferroelectric Long-Range Order on the Unipolar and Bipolar Electric Fatigue in Bi1/2Na1/2TiO3-Based Lead-Free Piezoceramics