Robert Dittmer

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

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(ZrxTi1-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.

Lead-free high-temperature dielectrics with wide operational range

The dielectric, electrical and structural properties of (1–x)(0.94Bi1/2Na1/2TiO3–0.06BaTiO3)–xK0.5Na0.5NbO3 (BNT–BT–xKNN) with x=0.09, 0.12, 0.15, and 0.18 were investigated as potential candidates for high-temperature capacitors with a working temperature far beyond 200 °C. Temperature dependent dielectric permittivity (e) showed two local broad maxima that at the optimal composition of KNN (x=0.18) are combined to form a plateau. This then results in a highly temperature-insensitive permittivity up to ∼300 °C at the expense of a small reduction in absolute permittivity values. High-temperature in situ x-ray diffraction study showed pseudocubic symmetry without obvious structural changes, which implies that the dielectric anomalies observed could only be a consequence of a slight change in space group. BNT–BT–0.18KNN showed a permittivity of ∼2150 at the frequency of 1 kHz at 150 °C with a normalized permittivity e/e150 °C varying no more than ±10% from 43 to 319 °C. With very good electrical properties ...

Electric-field-induced strain mechanisms in lead-free 94%(Bi1/2Na1/2)TiO3–6%BaTiO3

High resolution neutron diffraction has been used to investigate the structural origin of the large electric-field-induced remanent strain in 94(Bi1/2Na1/2)TiO3–6BaTiO3 ceramics. The virgin material was found to be a mixture of near-cubic phases with slight tetragonal and rhombohedral distortions of a0a0c+ and a−a−a− octahedral tilt type, respectively. Application of an electric field of 4.57 kV/mm transformed the sample to a predominantly rhombohedral a−a−a− modification with a significantly higher degree of structural distortion and a pronounced preferred orientation of the c-axis along the field direction. These electric field-induced structural effects contribute significantly to the macroscopic strain and polarization of this system.

Ergodicity reflected in macroscopic and microscopic field-dependent behavior of BNT-based relaxors

The effect of heterovalent B-site doping on ergodicity of relaxor ferroelectrics is studied using (1 − y)(0.81Bi1/2 Na 1/2TiO3-0.19Bi1/2K1/2TiO3)-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.

Structure and properties of La-modified Na0.5Bi0.5TiO3 at ambient and elevated temperatures

The crystal structure and property changes of sodium bismuth titanate (Na0.5Bi0.5TiO3, NBT) piezoelectric ceramics are reported as a function of La modification (0.5–2.0 at. %) and increasing temperature using high resolution x-ray diffraction, permittivity, depolarization, and polarization and strain hysteresis measurements. La substitution is found to decrease the depolarization temperature of NBT (e.g., 1.5 at. % La substitution lowers the depolarization temperature by 60 °C relative to the unmodified composition) with little impact on the room temperature polarization and strain hysteresis. The room temperature structures of the various NBT compositions were modeled using a mixture of the monoclinic Cc space group and the cubic Pm3¯m phase, where the Pm3¯m phase is used to model local regions in the material which do not obey the long range Cc space group. With increasing La substitution, the lattice parameter distortions associated with the Cc phase approached that of the prototypical cubic unit cell...

Aging in the relaxor and ferroelectric state of Fe-doped (1-x)(Bi1/2Na1/2)TiO3-xBaTiO3 piezoelectric ceramics

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.

Frequency-dependence of large-signal properties in lead-free piezoceramics

The dependence of large signal properties of (1−x)(0.81Bi1/2Na1/2TiO3-0.19Bi1/2K1/2TiO3)-xBi(Zn1/2Ti1/2)O3 with x = 0.02, 0.03, and 0.04 on the measurement frequency was investigated for a wide range of frequencies from 0.1 Hz to 100 Hz. A significant frequency dispersion in the characteristic parameters representatively maximum and coercive values was denoted. On extension with the temperature dependent dielectric permittivity measurement, it was shown that the observed frequency dependence is primarily correlated with the dynamics of field-induced phase transition from a relaxor state to a long-range ferroelectric state. Increasing the substitutional disorder introduced by Bi(Zn1/2Ti1/2)O3 addition was demonstrated to pronounce the frequency dependence. It was proposed that the change be due to the increase in random fields and consequent dominance of ergodicity, based on the frequency-dependent hysteresis measurements at an elevated temperature above so-called induced-ferroelectric-to-relaxor transition temperature.

Peculiar Bi-ion dynamics in Na1/2Bi1/2TiO3from terahertz and microwave dielectric spectroscopy

Dynamics of the main dielectric anomaly in Na1/2Bi1/2TiO3 (NBT) were 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 1011 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 are supported by previous structural studies. NBT represents a hybrid between standard and relaxor ferroelectric behaviour.

Investigation of the depolarisation transition in Bi-based relaxor ferroelectrics

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.

Local structure change evidenced by temperature-dependent elastic measurements: Case study on Bi1/2Na1/2TiO3-based lead-free relaxor piezoceramics

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)-0.04 BiZn1/2Ti1/2O3 (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. V C 2014 AIP Publishing LLC.