Doru C. Lupascu

Correlation between microstructure, strain behavior, and acoustic emission of soft PZT ceramics

Abstract The influence of grain size and domain configuration on the microscopic and macroscopic electromechanical properties of soft lead zirconate titanate (PZT) ceramics were studied. Fine- and coarse-grained PZT ceramics with an average grain size of 1.5 and 3.5 μm were prepared using the conventional mixed oxide route and different sintering conditions (1225°C/2 h and 1300°C/10 h). The experiments were performed on PZT with different Zr/Ti-ratios in order to obtain pure tetragonal (45/55), pure rhombohedral (60/40), and morphotropic (54/46) compositions. To distinguish between the intrinsic and extrinsic effect microscopic strain measurements (Synchrotron X-ray diffraction) were compared with macroscopic measurements of the mechanical hysteresis loop. The results were correlated with acoustic emission measurements which microscopically reflected the intrinsic and extrinsic contributions to the macroscopic strain of the different crystallographic modifications.

Damage evolution in ferroelectric PZT induced by bipolar electric cycling

Abstract The fatigue behaviour of commercial bulk Lead Zirconate Titanate (PZT) induced by bipolar cycling was investigated. Polarisation and strain hysteresis loops as well as acoustic emissions (AE) were monitored. Higher cycling fields (2× E c ) yield stronger fatigue, higher AE energy values and lower threshold values for the onset of AE events at high cycle numbers. AE amplitudes and energies were found to be a measure for the degree of domain clamping. Cycling at 2 E c leads to an increasing asymmetry of the strain hysteresis, which is explained by an offset polarisation. Corrosion paths were observed by SEM, most numerously in the centre of a strongly fatigued sample. A damage scenario based on domain wall clamping due to the coalescence of point defects is presented.

Fatigue in ferroelectric ceramics and related issues

1 Introduction.- 2 Macroscopic Phenomenology.- 3 Agglomeration and Microstructural Effects.- 4 Acoustic Emission and Barkhausen Pulses.- 5 Models and Mechanisms.- 6 Recent Developments.- 7 Summary.- A Solutions to Integrals.- References.

Constraint-induced crack initiation at electrode edges in piezoelectric ceramics

A strain incompatibility arises between the electrically active and inactive parts of a partially electroded piezoelectric material. This leads to cracking perpendicular to the electrode edge. Symmetric partial electrodes of different widths were applied to lead zirconate titanate (PZT) plates to study the development of these cracks. Different cracking patterns appeared upon application of an electric field, depending on the degree of coverage and on the thickness of the plates. The apparent coercive fields are correlated to the degree of clamping and the amount of cracking in the specimens. Non-linear finite element modelling is used to analyse the material response.

Microstructural modifications of ferroelectric lead zirconate titanate ceramics due to bipolar electric fatigue

Fatigue in ferroelectric ceramics is the result of an intricate interplay of electrical, mechanical and electrochemical processes, each of which has been claimed responsible for fatigue. We present experimental results interrelating the different scales of the fatigue mechanism showing that all forms of fatigue occur. Electrochemical mechanisms lead to point defect agglomeration. The agglomerates are rendered visible as etch grooves in strongly etched surfaces by scanning electron microscopy. The overall length of the etch grooves strongly increases during cycling. Micro- and macrocracking were both observed and quantified using electron and optical microscopy. Their number similarly increases as a result of fatigue. Furthermore, a shrinkage of the unit cell volume was found by XRD.

Fatigue studies in compensated bulk lead zirconate titanate.

Impedance analysis studies were carried out on compensated bulk lead zirconate titanate samples. Fatigue is concomitant with the onset of dielectric loss. This is shown to be dominantly due to an irreversibly modified near-surface layer that can be polished off. The highly compensated nature of these samples minimizes the role of oxygen vacancies.

Strong electrocaloric effect in lead-free 0.65Ba(Zr0.2Ti0.8)O3-0.35(Ba0.7Ca0.3)TiO3 ceramics obtained by direct measurements

Solid solutions of (1 − x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 promise to exhibit a large electrocaloric effect (ECE), because their Curie temperature and a multiphase coexistence region lie near room temperature. We report on direct measurements of the electrocaloric effect in bulk ceramics 0.65Ba(Zr0.2Ti0.8)O3-0.35(Ba0.7Ca0.3)TiO3 using a modified differential scanning calorimeter. The adiabatic temperature change reaches a value of ΔTEC = 0.33 K at ∼65 °C under an electric field of 20 kV/cm. It remains sizeable in a broad temperature interval above this temperature. Direct measurements of the ECE proved that the temperature change exceeds the indirect estimates derived from Maxwell relations by about ∼50%. The discrepancy is attributed to the relaxor character of this material.

Unipolar fatigue of ferroelectric lead–zirconate–titanate

In multilayer actuators the ferroelectric material is known to degrade under bipolar and to some degree under unipolar electric loading. Fatigue effects due to unipolar cycling of a commercial bulk lead–zirconate–titanate up to 4·108 cycles at 2 Ec are demonstrated. Unipolar as well as bipolar polarisation and strain hysteresis loops are measured after fatigue. Bipolar measurements after cycling show a small decrease of the remnant polarisation and an increasing asymmetry of the strain hysteresis with cycle number, which is explained by an offset polarisation. This offset polarisation is reduced under application of bipolar fields. A small offset-field is also observed. Etch grooves known to occur after bipolar fatigue are not found after unipolar fatigue. The results reveal that different mechanisms are responsible for bipolar and unipolar fatigue.

Stretched exponential relaxation in perovskite ferroelectrics after cyclic loading

The switching kinetics in fatigued polycrystalline bulk ferroelectrics do not exhibit relaxation with a singular time constant but rather stretched exponential behavior. Several microscopic mechanisms are discussed to explain this effect for a polycrystalline material. Some of the mechanisms may apply equally well to thin films, others are characteristic of a macroscopic sample or a thick film.

Converse magnetoelectric effect in CoFe2O4–BaTiO3 composites with a core–shell structure

Multiferroic composites were prepared by covering CoFe2O4 nanoparticles with a shell of BaTiO3 using a sol–gel technique. Scanning probe microscopy confirmed the formation of a core–shell structure with a magnetic core and a piezoelectric shell. The converse magnetoelectric effect was studied at different temperatures and bias fields. The magnetoelectric coefficient peaks at approximately 270 K and reaches the value αC≈(2.2 ± 0.1)10 − 11 s m − 1, which surpasses those reported previously for similar structures. A change of the sign of the magnetoelectric coefficient observed for an increasing magnetic bias field is related to the non-monotonic field dependence of magnetostriction in polycrystalline CoFe2O4.