Implantable active devices such as pacemakers are facing rigorous requirements. Because they reside within the body for years, materials applied in this surrounding must exhibit biocompatibility and extraordinary reliability. They also have to provide a number of functional properties. In this work we present a method that enables the realization of a highly complex profile of properties by means of a dual composite approach. Using multilayer technology, an electrical conductor is embedded into a ceramic matrix, thus, creating conductive paths that are insulated from each other. In addition to this macroscopically hybrid architecture, this approach features a second composite aspect: the conductor is not composed of a single metallic phase, but is a ceramic-metal mixture. Owing to its interpenetrating microstructure, this CerMet allows for a strong and hermetic integration of the conductor into the ceramic matrix otherwise impossible due to mismatch in thermal expansion. In fact, the CerMet ceramic composite exhibits a higher strength than the pure ceramic as revealed by a three-point bending test study. At the same time, the CerMet offers high and virtually metal-like conductor properties, enabling a down-scaling of the conductive paths to 150µm diameter and smaller. Furthermore, the described composite is biocompatible, non-magnetic, and chemically inert, which is vital for the application in active, implantable, medical devices. Beside the general fabrication route, we present the microstructural, functional, and mechanical properties of this newly developed class of dual composites.

Statistical distribution of switching times is a key information necessary to describe the dynamic response of a polycrystalline bulk ferroelectric to an applied electric field. The Inhomogeneous Field Mechanism (IFM) model offers a useful tool which allows extraction of this information from polarization switching measurements over a large time window. In this paper, the model was further developed to account for the presence of non-switchable regions in fatigued materials. Application of the IFM-analysis to bipolar electric cycling induced fatigue process of various lead-based and lead-free ferroelectric ceramics reveals different scenarios of property degradation. Insight is gained into different underlying fatigue mechanisms inherent to the investigated systems.

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.

Polar nanoregions (PNRs) play a key role in the functionality of relaxor ferroelectrics; however, visualizing them in lead-free relaxor ferroelectrics with high lateral resolution is still challenging. Thus, we studied herein the local ferroelectric domain distribution of the lead-free bismuth-based (1 - x)(Bi1/2Na1/2TiO3Bi1/2K1/2TiO3) - x(Bi1/2Mg1/2TiO3) piezoceramics which show a relaxor behavior using dual AC resonance tracking (DART) piezoresponse force microscopy (PFM). By using excitation frequencies at either side of the contact resonance peak of the torsional cantilever vibration, an enhanced contrast in the amplitude and phase images of the piezoresponse can be achieved. Additionally, this tracking technique reduces the topographical crosstalk while mapping the local electromechanical properties. The true drive amplitude, drive phase, contact resonant frequency and quality factor can be estimated from DART-PFM data obtained with vertically or torsionally vibrating cantilevers. This procedure yields a three-dimensional quantitative map of the local piezoelectric properties of the relaxor ferroelectric samples. With this approach, torsional DART allowed for the visualization of fine substructures within the monodomains, suggesting the existence of PNRs in relaxor ferroelectrics. The domain structures of the PNRs were visualized with high precision, and the local electromechanical characteristics of the lead-free relaxor ferroelectrics were quantitatively mapped.

The stability of the field-induced ferroelectric (FE) state was studied in relaxor lead-free ceramics (1-y)[0.81Bi(1/2)Na(1/2)TiO(3)-0.19Bi(1/2)K(1/2)TiO(3)]-yBiZn(1/2)Ti(1/2)O(3) both macroscopically and microscopically. A strong dc electric field results in the formation of a stable FE state with a large piezoelectric coefficient for compositions with a small amount of Bi(Zn1/2Ti1/2)O-3, which are in the non-ergodic relaxor state at room temperature. Increasing temperature promotes ergodic relaxor behavior, which is accompanied by the rapid destabilization of the induced state, that is, small relaxation times. Based on the obtained data, it is proposed that the depolarization is a two-step process consisting of an initial realignment of the FE domains and their subsequent breakup into polar nanoregions. The ergodic relaxor behavior is also promoted by increasing the Bi(Zn1/2Ti1/2)O-3 content. The related charge disorder results in an enhancement of random electric fields and consequently a stable FE state cannot be induced even at room temperature.

We address the unsolved question on the structure of relaxor ferroelectrics at the atomic level by characterizing lead-free piezoceramic solid solutions (100 - x)(Bi1/2Na1/2)TiO3-xBaTiO(3) (BNT-xBT) (for x = 1, 4, 6, and 15). Based on the relative intensity between spectral components in quadrupolar perturbed Na-23 nuclear magnetic resonance, we present direct evidence of the coexistence of cubic and polar local symmetries in these relaxor ferroelectrics. In addition, we demonstrate how the cubic phase vanishes whenever a ferroelectric state is induced, either by field cooling or changing the dopant amount, supporting the relation between this cubic phase and the relaxor state.

Dynamics of the main dielectric anomaly in Na1/2Bi1/2TiO3 (NBT) was studied by time-domain THz and microwave spectroscopy, using also previously published data and their new overall fits. Above the dielectric maximum temperature Tm 600 K, the response consists of coupled sub-THz oscillator and a relaxation mode, assigned to strongly anharmonic Bi-ion vibrations and hopping, whose slowing down explains the paraelectric-like permittivity increase to Tm. Below Tm, the main relaxation continues slowing down and additional relaxation, assigned to quasi-Debye losses, appears in the 10^11 Hz range. The oscillator hardens on cooling and takes over the whole oscillator strength. The permittivity decrease below Tm is caused by the reduced strength of the relaxations due to dominance of the rhombohedral phase within the coexistence region with the tetragonal phase. The anharmonic dynamics of Bi is supported by previous structural studies. NBT represents a hybrid between standard and relaxor ferroelectric behaviour.

The effect of heterovalent B-site doping on ergodicity of relaxor ferroelectrics is studied using (1 − y)(0.81Bi1/2Na1/2TiO3-0.19Bi1/2K1/2TiO3)-yBiZn1/2Ti1/2O3 (BNT-BKT-BZT) with y = {0.02;0.03;0.04} as a model system. Both the large- and small-signal parameters are studied as a function of electric field. The crystal structure is assessed by means of neutron diffraction in the initial state and after exposure to a high electric field. In order to measure ferroelastic domain textures, diffraction patterns of the poled samples are collected as a function of sample rotation angle. Piezoresponse force microscopy (PFM) is employed to probe the microstructure for polar regions at a nanoscopic scale. For low electric fields E < 2 kV·mm−1, large- and small-signal constitutive behavior do not change with composition. At high electric fields, however, drastic differences are observed due to a field-induced phase transition into a long-range ordered state. It is hypothesized that increasing BZT content decreases the degree of non-ergodicity; thus, the formation of long-range order is impeded. It is suggested that frozen and dynamic polar nano regions exist to a different degree, depending on the BZT content. This image is supported by PFM measurements. Moreover, PFM measurements suggest that the relaxation mechanism after removal of the bias field is influenced by surface charges.

The loss of macroscopic polarisation in relaxor ferroelectric (Na0.8K0.2)½Bi½TiO3 ceramics doped with BiZn½Ti½O3 has been studied by electrical and structural methods. These indicate that the phenomena that are coupled in a displacive phase transition are not necessarily coupled in the depolarisation of Na½Bi½TiO3-based relaxors and a concept of correlated and uncorrelated switching of dipoles within adjacent unit cells is used to explain this. Second harmonic generation performed on poled ceramics during heating yields values of the freezing temperature and shows a broad temperature range of ∼100 °C across which the structure changes from field-induced ferroelectric to an equilibrium-state ergodic relaxor. Electrical poling at room temperature causes poled regions to increase in size by ∼2 orders of magnitude. A model illustrating the main steps in thermal depolarisation is described that does not require a phase transition to take place on a unit cell level.

The temperature-dependent Young's modulus Y(T) of the lead-free piezoceramics of 0.8Bi1/2Na1/2TiO3-0.2Bi1/2K1/2TiO3 (20BKT) and 0.96(0.8Bi1/2Na1/2TiO3-0.2Bi1/2K1/2TiO3)-0.04BiZn1/2Ti1/2O3 (4BZT) is measured with the impulse excitation technique and contrasted with corresponding dielectric and structural data. While the dielectric properties suggest a phase transition, the high resolution XRD patterns remain virtually unchanged from room temperature up to high temperatures, confirming no change in their long-range order. In contrast, the elastic properties indicate a broad and diffuse ferroelastic transition denoted by a minimum in Y(T). By analogy to the elastic and dielectric data of PbZrxTi1−xO3 and PLZT, it is concluded that 20BKT and 4BZT are relaxors with polar nanoregions embedded in a metrically cubic matrix. Interestingly, no indication for the freezing temperature was reflected in any of the employed measurement techniques. From the saturation of Y(T), it is suggested that the Burns temperature may be approximated as 700 °C. Moreover, it is found that the modification with the ternary end-member BiZn1/2Ti1/2O3 results in an increase in Young's modulus. A comparison with the Bi1/2Na1/2TiO3-BaTiO3-K0.5Na0.5NbO3 yields the same results.

Aging of piezoelectric properties was investigated in lead-free (1 − x)(Bi1/2Na1/2)TiO3-xBaTiO3 doped with 1at. % Fe. The relaxor character of the un-poled material prevents macroscopic aging effects, while in the field-induced ferroelectric phase aging phenomena are similar to those found in lead zirconate titanate or barium titanate. Most prominent aging effects are the development of an internal bias field and the decrease of switchable polarization. These effects are temperature activated, and can be explained in the framework of defect complex reorientation. This picture is further supported by electron paramagnetic resonance spectra indicating the existence of (FeTi′−VO••)• defect complexes in the Fe-doped material.

The large-signal unipolar behavior of PZT is characterized under combined electrical, thermal, and mechanical loading. Maximum strain S-max and polarization P-max feature a pronounced sensitivity on stress with a field-dependent peak evolving at around -50 MPa that is associated with enhanced non-180 degrees domain switching. As notable strains are achieved in excess of the quasi-statically measured blocking stress, it is suggested that the testing procedure presented within this work is suited to supplement blocking force measurements in order to comprehensively evaluate the electromechanical performance of a piezoceramic. With the suppression of non-180 degrees domain switching at high stress levels, S-max(sigma) decreases at a faster rate than P-max(sigma). Accordingly, the electrostrictive coefficient Q(11) is shown to be stress-dependent. This observation is rationalized with the stress-dependent change of domain processes. It is furthermore found that Q(11) features a notable dependence on temperature, increasing from 0.018 m(4) C-2 at 25 degrees C to 0.028 m(4) C-2 at 150 degrees C under zero-stress. To assess the actuatoric efficiency, a novel figure of merit eta* is defined to quantify the fraction of input energy utilized for mechanical work. (C) 2012 Elsevier B.V. All rights reserved.

A non-contacting, non-destructive method for the characterization of small signal properties on piezoelectric single fibers is presented. The technique comprises ultrasound generation, propagation and detection within the fiber. Two electromechanically active segments are prepared by applying two pairs of ring electrodes and subsequent poling of the interspace material. Measurements are carried out in pulse-echo-mode by transmitting and receiving ultrasound with one of the two poled segments. The other segment operates under either closed or open circuit conditions causing a small time difference during runtime measurement. Applying the piezoelectric equations, the electromechanical data k(33) and d(33) and the elastic compliance s(33) can be calculated. The practicability of the method was tested and verified for PZT fibers derived from the polysulphone process. The validity of the ultrasound runtime measurements is proved by simulation using an electromechanical model.

Lead‐free piezoelectric (1 – x – y)(Bi1/2Na1/2)TiO3–xBaTiO3–y(K0.5Na0.5)NbO3 (BNT–BT–KNN) ceramics were examined in situ under increasing temperature in the transmission electron microscope. Changing superstructure reflections indicate a transition from rhombohedral to tetragonal to cubic phase with broad coexistence regions. The additional evolution of the microstructure in combination with dielectric measurements leads to a model of two relaxor‐type phase evolutions with temperature.

The strain and polarization hystereses of lead-free 0.94Bi1/2Na1/2TiO3–0.06BaTiO3 during unipolar electric field loading are obtained from room temperature to 150°C under uniaxial compressive stress up to 446MPa. At intermediate temperatures a stress-dependent peak evolves in both the maximum strain and polarization. At 125°C a large strain with a large-signal piezoelectric coefficient d33∗ of 884pmV−1 is observed, which decays upon the application of stress. This behavior is rationalized with a change in the primary strain mechanism from domain switching at low temperatures to a reversible electric field-induced transition from an ergodic relaxor state to a long-range order at high temperatures. Moreover, the energy terms w (the output mechanical work) and eP (the charged electrical energy density) that are related to the deformation and the polarization, respectively, are analyzed and used to define a large-signal efficiency η*=w(w+eP)−1. It is found that η* saturates at ∼150MPa but decreases with increasing temperature and electric field. It is furthermore observed that notable strains are achieved at stress levels even far beyond the quasi-statically determined blocking force. Therefore, it is proposed that the presented testing procedure is suited to assess the dynamic actuatoric performance of a piezoceramic.

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 )O-3 (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.