Small-signal Permittivity Research Articles

Robust CaZrO3-modified (K, Na)NbO3-based lead-free piezoceramics: High fatigue resistance insensitive to temperature and electric field

Robust resistivity against electrical cycling at not only ambient conditions but also enhanced temperatures is an essential requirement for high-end actuator applications. In this study, the temperature and electric field dependence of unipolar fatigue behaviors of CaZrO3-modfied (K, Na)NbO3 lead-free piezoceramics were investigated. The space charge accumulation during unipolar cycling is responsible for the build-up of internal bias field, the dynamics of which with respect to temperature and driving field can be described using a model based on Maxwell-Wagner relaxation process. Besides, clamping of domain walls can be inferred by comparing the large and small signal permittivity. Most intriguingly, the unipolar strain exhibits a fatigue-free behavior even at elevated temperatures, rendering the material exceptionally suitable for actuator applications.

The Effect of Electric Poling on the Performance of Lead‐Free (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 Piezoceramics

The effect of increasing poling fields on the properties of (1−x)BZT–xBCT compositions across the morphotropic phase boundary (MPB) is studied using large signal polarization and strain, small signal permittivity and piezoelectric coefficient, and XRD measurements. Successive poling causes charge carrier migration inducing an internal bias field, which becomes large with respect to the coercive field resulting in biased ferroelectric and ferroelastic switching. Improvements in piezoelectric coefficient of 9% are significantly smaller in the tetragonal 60BCT composition compared with the improvement of approximately 50% in the rhombohedral 40BCT and MPB 50BCT compositions. While the properties continue to change with increased poling fields, the remnant ferroelastic domain texture parallel to the field direction, as observed from XRD, stays approximately constant. The improvement in overall domain alignment leading to largely enhanced intrinsic piezoelectricity originates from the alignment of 180° domains and possibly non‐180° domains in grains with orientations inclined to the electric field. As a result, poling is most effective in BZT–BCT materials that have low coercive fields, show low distortions and possess more polarization orientations, such as compositions in the rhombohedral phase field or near the MPB.

In Situ X-ray Diffraction of Biased Ferroelastic Switching in Tetragonal Lead-free (1−x )Ba(Zr0.2 Ti0.8 )O3 -x (Ba0.7 Ca0.3 )TiO3 Piezoelectrics

The effect of electric poling on the bipolar switching for tetragonal BZT–BCT materials is studied using large signal polarization and strain, small signal permittivity, and piezoelectric coefficient, as well as electric field–dependent in situ XRD experiments. Charge carrier agglomeration at domain and grain boundaries with increasing poling fields gives rise to an internal bias field that gradually biases domain switching behavior. The biased switching after electric poling is quantified during a bipolar measurement cycle from analysis of the electric and structural data. For a fresh sample the ferroelastic domain texture induced by a positive and negative electric measurement field is of the same magnitude. After poling, the induced ferroelastic domain texture is larger under a positive measurement field and smaller under a negative measurement field. A very large domain texture is achieved during poling, corresponding to 85% of the domains becoming aligned with their 002 pole in field direction. While the domain texture is significantly improved at higher poling fields, relaxation upon removal of the electric field appears independent of the poling history. This suggests a large extrinsic contribution to the macroscopic strain. It also facilitates the biased ferroelastic switching arising from the internal bias field developed during poling.

Neutron irradiation effects on domain wall mobility and reversibility in lead zirconate titanate thin films

The effects of neutron-induced damage on the ferroelectric properties of thin film lead zirconate titanate (PZT) were investigated. Two sets of PbZr0.52Ti0.48O3 films of varying initial quality were irradiated in a research nuclear reactor up to a maximum 1 MeV equivalent neutron fluence of (5.16 ± 0.03) × 1015 cm−2. Changes in domain wall mobility and reversibility were characterized by polarization-electric field measurements, Rayleigh analysis, and analysis of first order reversal curves (FORC). With increasing fluence, extrinsic contributions to the small-signal permittivity diminished. Additionally, redistribution of irreversible hysterons towards higher coercive fields was observed accompanied by the formation of a secondary hysteron peak following exposure to high fluence levels. The changes are attributed to the radiation-induced formation of defect dipoles and other charged defects, which serve as effective domain wall pinning sites. Differences in damage accumulation rates with initial film quality were observed between the film sets suggesting a dominance of pre-irradiation microstructure on changes in macroscopic switching behavior.

Temperature and driving field dependence of fatigue processes in PZT bulk ceramics

The temperature- and field-dependent degradation properties of bulk Pb(Zr,Ti)O 3 material (PZT) under a unipolar electric field were investigated. Unipolar cycling leads to the build-up of an internal bias field based on the agglomeration of charges at grain boundaries. A simple model was developed which describes the general dynamics of unipolar fatigue and its dependence on temperature and driving field. Comparing the large and small signal permittivity before and after fatigue led to the conclusion that domain walls became clamped by the agglomerated charges. This clamping effect could be visualized by transmission electron microscopy (TEM). Additionally, the TEM investigations revealed that unipolar fatigue leads to a weakening of the microstructure and to the development of microcracks.

Evaluation of domain wall motion in bipolar fatigued lead-zirconate-titanate: A study on reversible and irreversible contributions

Mobility of ferroelectric domain walls is a critical factor in the fatigue of piezoelectric ceramics. Here, reversible and irreversible domain wall motion is evaluated for lead-zirconate-titanate both before and after fatigue cycling. To this end, the small-signal permittivity at different levels of bias field is compared to the large-signal permittivity, i.e., the first derivative of the polarization hysteresis loop. While the small-signal permittivity is just determined by the reversible processes due to the small electric excitation field, the large-signal permittivity reflects both reversible and irreversible contributions. The ratio of large- and small-signal permittivity is suggested as measure for the reversible contribution to the overall polarization change. Fatigue leads to a decrease in the small-signal permittivity and hence a general suppression of the reversible processes. Furthermore it causes a shift in the irreversible contributions to higher electric fields and a retarded backswitching when the external electric field is reduced after the maximum field value was reached. This reinforces the notion of bipolar electric fatigue caused by pinned domain walls due to agglomeration of charged defects in the sample bulk.

Epitaxial Pb(Mg1/3Nb2/3)O3-PbTiO3 thin films grown by MOCVD

Epitaxial Pb(Mg1/3Nb2/3)O3 (PMN) and (l-x)(Pb(Mg1/3Nb2/3)O3)-x(PbTiO3) (PMN-PT) thin films have been deposited by metalorganic chemical vapor deposition at 700 – 780°C on (100) SrTiO3 and SrRuO3/SrTiO3 substrates. Room temperature values of 900 and 1.5%, were measured for the zero-bias permittivity and loss respectively, at 10 kHz for 220 nm thick pure PMN films. For PMN-PT films, the small-signal permittivity ranged from 1000 to 1500 depending on deposition conditions and Ti content; correspondingly low values for the zero-bias dielectric loss between 1 and 5% were determined for all samples. For PMN-PT with x of approximately 0.30–0.35, polarization hysteresis with Prμ18μC/cm2 was obtained. Initial piezoresponse results are discussed.

Extrinsic contributions in a nonuniform ferroic sample: Dielectric, piezoelectric and elastic

The contribution Δε of extremely small motions of domain walls to small-signal permittivity of a multidomain ferroelectric sample has been a research issue for many years. In ferroelastic ferroelectrics such motions contribute also to their piezoelectric (by Δd) and elastic (by Δs) properties. Data about their simultaneous existence are scarce but those available point to mutual proportionality of Δ, Δd and Δs, as expected. To understand the magnitude of extrinsic contributions, the origin of the restoring force acting on domain walls must be understood. In the present contribution the theory has been developed based on the model of a plate-like sample in which the ferroelectric- ferroelastic bulk is provided with a nonferroic surface layer. Motion of domain walls in the bulk results in a change of electric and elastic energy both in the bulk and in the layer, which provides the source of restoring force. This makes it possible to determine all mentioned extrinsic contributions. We discuss the applicability of the model to available data for single crystals and also for ceramic grains.

Ferroelectric and Piezoelectric Properties of MOCVD Pb(Mg1/3Nb2/3)O3-PbTiO3 Epitaxial Thin Films

AbstractWe have grown epitaxial Pb(Mg1/3Nb2/3)O3 (PMN) and (1-x)(Pb(Mg1/3Nb2/3)O3)- x(PbTiO3) (PMN-PT) thin films by metalorganic chemical vapor deposition at 700 - 780°C on (100) SrTiO3 and SrRuO3/SrTiO3 substrates. The zero-bias permittivity and loss measured at room temperature and 10 kHz for 220 nm thick pure PMN films were 900 and 1.5%, respectively. For PMN-PT films, the small-signal permittivity ranged from 1000 to 1500 depending on deposition conditions and Ti content; correspondingly low values for the zero-bias dielectric loss between 1 and 5% were determined for all specimens. For PMN-PT with x of approximately 0.30–0.35, polarization hysteresis with Pr,≈18μC/cm2 was obtained. Initial piezoresponse data are discussed.

Domain wall dynamics and small-signal permittivity of TGS

Measurements are reported on the small signal complex dielectric constants ε' and ε'' (or ε' and tan δ) for thin samples of single crystal TGS, as functions of frequency, degree of poling, dc and ac field strengths and temperatures. The data strongly suggest that most of the effects are caused by dynamic domain wall effects. At low ac fields there is an enormous dip in tan 6 between 10-20 kHz that indicates an almost complete absence of any loss mechanism at that frequency.

Small-Signal Permittivity of the Stationary (100)-180° Domain Wall in BaTiO3

The small-signal permittivity of the (100)-type 180° domain wall in BaTiO 3 is calculated relative to the signal-domain permittivity according to a previously-reported microscopic model of 180° domain walls in BaTiO 3 . A perturbation approach is developed which makes use of the known zero-field solutions of this domain wall and the single domain. It is found that the (100) domain-wall permittivity is about 6\(\frac{1}{2}\)-times larger than the single-domain permittivity, and there are two reasons for this effect; First, the internal fields and polarizabilities of the ions in the wall region differ from the single-domain values; and second, the presence of a field lowers the domain-wall symmetry promoting the polarization of the Ba and O b ions on the (100) domain-wall plane. The dominant contribution to the enhanced domain-wall permittivity is the induced Ba and O b polarizations.

The Contribution of Domain‐Wall Oscillations to the Small‐Signal Permittivity of Triglycine Sulphate

AbstractThe complex permittivity ϵ* of ferroelectric triglycine sulphate is measured in a weak ac field in the frequency range 5 × 102 to 5 × 107 Hz as a function of the net polarization Pa of the sample. The value of Pa is adjusted in steps by charge supplied from a parallel capacitor. Throughout the frequency range both components of ϵ* are larger for crystals with |Pa|≠|Ps| than for a single‐domain crystal with |Pa| = |Ps|. This indicates a contribution Δϵw* to the permittivity by the domain walls. The imaginary component Δϵw″ is an order of magnitude greater than the value of ϵ″ of a singel‐domain crystal; Δϵw′ is up to five times larger than the ϵ′ of a single‐domain crystal. The frequency dependence of Δw* suggests contributions from two different mechanisms. The frequency dependence of one of them is given approximately by the Debye equations with τ ≈ 10−7 s. Both components of the second contribution to Δϵw* decrease monotonically with frequency. The probable mechanisms of these contributions are discussed.