Hans Kungl

Nanodomains in morphotropic lead zirconate titanate ceramics : On the origin of the strong piezoelectric effect

The outstanding piezoelectric properties of lead zirconate titanate (PZT) ceramics with compositions close to the morphotropic phase boundary of the quasibinary phase diagram of lead zirconate and lead titanate are still under debate. A combination of ex situ and in situ transmission electron microscopy and high resolution x-ray diffraction revealed that the extrinsic piezoelectric effect in morphotropic PZT is closely connected to the existence of nanodomains. The in situ transmission electron microscopy investigations with applied electric field show that mainly the nanodomains respond to the electric field while the microdomain structure does not change noticeably in our experiments.

Composition dependence of the domain configuration and size in Pb(Zr1−xTix)O3 ceramics

The composition dependent variation of domain configuration and size in Pb(Zr1−xTix)O3 (PZT) has been investigated in a detailed transmission electron microscopy study in the range of 0.40⩽x⩽0.55. Single phase composition, Pb(Zr0.45Ti0.55)O3 and Pb(Zr0.60Ti0.40)O3, the former belonging to the tetragonal, the latter to the rhombohedral phase, feature small microdomain widths coupled with a pronounced bimodal domain distribution. Samples with compositions around the morphotropic phase boundary exhibit a decrease of bimodal distribution and an increase in microdomain width associated with nanodomain formation. The investigation of micro- and nanodomains, as well as the bimodal distribution of microdomains in undoped PZT ceramics, with respect to composition, is reported. We define nanodomains as “domains arranged within microdomains possessing a width of a few nanometers.” The strict alternation of the two orientation variants of microdomains is denoted as “bimodal domain distribution,” and is characterized ...

Nonlinearity of strain and strain hysteresis in morphotropic LaSr-doped lead zirconate titanate under unipolar cycling with high electric fields

The strain behavior of morphotropic, 1La2Sr-doped lead zirconate titanate was investigated. Particular attention was paid to the electric field induced strain (S-E characteristic) in the high field range for morphotropic materials having different Zr∕Ti ratios. Unipolar electrical cycles with maximum fields between 1 and 3kV∕mm were applied to the poled materials under load-free conditions. Strain curves were recorded and the data were evaluated with respect to field dependence of strain and strain hysteresis. Nonlinearity of strain was detected for all materials. Depending on the composition, marked differences were found with respect to the types of nonlinearity. The corresponding data on strain hysteresis indicate that the nonlinearity was due to a loss generating strain mechanism. Therefore the nonlinearity was attributed to domain switching. A phenomenological model is suggested, which is based on the assumptions of a distribution of domain switching over a field range. This field range is characteri...

Local variations in defect polarization and covalent bonding in ferroelectric Cu2+-doped PZT and KNN functional ceramics at the morphotropic phase boundary

Cu(2+)-doped Pb[Zr(0.54)Ti(0.46)]O(3) (PZT) and Cu(2+)-doped [K(0.5)Na(0.5)]NbO(3) (KNN) ferroelectrics with a dopant concentration of 0.25 mol% were investigated by means of multi-frequency and multi-pulse electron paramagnetic resonance (EPR) spectroscopy. Through the use of high magnetic fields and pulsed microwave fields an enhanced resolution was achieved yielding valuable information about the structural distortion at the dopant site. The results obtained suggest that Cu(2+) substitutes for both systems as an acceptor centre for the perovskite B-site. For reasons of local charge compensation, different kinds of defect associates invoking one and two oxygen vacancies are formed. These two kinds of extended defects differ in their electric and elastic properties. The results obtained are analyzed in order to characterize differences of the local structure in the Cu(2+)-defect center for morphotropic phase boundary compositions between the two systems. In particular, it is found that Cu(2+)-doping in KNN creates 50% more oxygen vacancies than the same amount of copper in PZT. Furthermore, local differences in covalent and ionic bonding are monitored.

Aging of poled ferroelectric ceramics due to relaxation of random depolarization fields by space-charge accumulation near grain boundaries

Migration of charged point defects triggered by the local random depolarization field is shown to plausibly explain aging of poled ferroelectric ceramics providing reasonable time and acceptor concentration dependences of the emerging internal bias field. The theory is based on the evaluation of the energy of the local depolarization field caused by mismatch of the polarizations of neighbor grains. The kinetics of charge migration assumes presence of mobile oxygen vacancies in the material due to the intentional or unintentional acceptor doping. Satisfactory agreement of the theory with experiment on the Fe-doped lead zirconate titanate is demonstrated.

Interactions of defect complexes and domain walls in CuO-doped ferroelectric (K,Na)NbO3

“Lead-free” piezoelectric sodium potassium niobate has been studied with respect to its defect structure when doping with CuO. The results indicate that two kinds of mutually compensating charged defect complexes are formed, ( Cu ′ ′ ′ Nb − V O • • ) ′ and ( V O • • − Cu ′ ′ ′ Nb − V O • • ) • . Concerning the interplay of these defect complexes with the piezoelectric materials properties, the trimeric ( V O • • − Cu ′ ′ ′ Nb − V O • • ) • defect complex primarily has an elastic dipole moment and thus is proposed to impact the electromechanical properties, whereas the dimeric ( Cu ′ ′ ′ Nb − V O • • ) ′ defect possesses an electric dipole moment in addition to an elastic distortion. Both types of defect complexes can impede domain-wall motion and may contribute to ferroelectric “hardening.”

RECENT DEVELOPMENTS AND FUTURE PERSPECTIVES OF LEAD-FREE FERROELECTRICS

Lead-free ferroelectric materials are currently subject to extensive research activities owing to environmental concerns and corresponding legislation which point to substitute lead-based ferroelectric materials. The paper provides an overview on recent research activities and trends in the field of lead-free ferroelectrics. The current work is grouped by contributions to atomistic-scale analysis of lead-free materials, structure and phase formation, micro structure and dopant effects on piezoelectric properties. The results and challenges to research in these fields are briefly sketched; so a framework is provided in which the contributions of the present topical issue to the major topics in lead-free ferroelectrics are related. The contributions in this issue are listed in Refs. 63–77. Finally, the current status of research and technology on lead-free materials is summarized.

Formation of magnetic grains in ferroelectric Pb[Zr0.6Ti0.4]O3 ceramics doped with Fe3+ above the solubility limit

The effect of excess iron-doping in piezoelectric lead-zirconate-titanate (PZT) on the microstructure was investigated by means of transmission electron microscopy in conjunction with electron diffraction and energy dispersive x-ray spectroscopy analysis. Electron paramagnetic resonance (EPR) spectroscopy has been used to characterize the site of the iron functional centers. A crystalline secondary magnetic phase formed upon sintering, magnetoplumbite (PbFe12O19), has been unequivocally identified. Furthermore, the phase assemblage observed in the Fe3+-doped PZT 60/40 sample could be correlated with an exchange-coupled resonance in the EPR spectra.

Multifrequency electron paramagnetic resonance analysis of polycrystalline gadolinium-doped PbTiO3—Charge compensation and site of incorporation

Gadolinium(III)-modified polycrystalline lead titanate (PbTiO3) of 1.0mol% dopant level was investigated by means of multifrequency electron paramagnetic resonance spectroscopy in order to determine sign and size of the local-environment sensitive finestructure parameter B20. The Gd3+ ions were assigned to substitute for Pb2+ at the A site of the perovskite ABO3 lattice, providing donor centers. The obtained value amounts to B20=0.776(2)GHz at ambient temperature and 0.860(2)GHz at 5K, thus showing a considerable temperature dependence. A charge-compensation mechanism is proposed that is based solely on the creation of lead and oxygen vacancies.

Defect structure and formation of defect complexes in Cu2+-modified metal oxides derived from a spin-Hamiltonian parameter analysis

The nearest neighbour oxygen octahedron about copper functional centres in metal oxides has been systematically studied by means of electron paramagnetic resonance (EPR) spectroscopy. In particular, the determined g ∥,zz and spin-Hamiltonian parameters were analysed, finding linear dependences as a function of chemical bonding and local distortion of the oxygen octahedron. Moreover, through the introduction of a dimensionless coordination parameter ξ, different defect structures with respect to the number of coordinated oxygen vacancies may be distinguished. This allows for a distinct assignment of defect complexes between the copper functional centre with one or two oxygen vacancies.