A. Albareda

Contribution of reversible processes to the non-linear dielectric response in hard lead zirconate titanate ceramics

The dependence of the dielectric constant and its losses on the applied field is studied, as well as the correlation between the dielectric constant and dielectric losses. The results show that hard lead zirconate titanate cannot be described by the Rayleigh model, so the hysteretic response of those materials cannot be attributed exclusively to the contribution of the irreversible processes. The experiments reveal the existence of reversible processes that could be related to the bending of the domain walls, which contribute to the dielectric constant of those materials without producing dielectric losses. The dependence of the dielectric constant on the poling state and on ageing are also studied. It can be reasserted that there are two distinct mechanisms that contribute to the dielectric response. The results show that the reversible contribution is independent of the ageing processes suffered by the sample.

Evaluation of domain wall motion in lead zirconate titanate ceramics by nonlinear response measurements

Nonlinear response of pure and doped Pb(Zr1−xTix)O3 ceramics, with different compositions, has been analyzed in order to evaluate the domain wall motion in these materials. The study of dielectric and converse piezoelectric response shows a clear dependence of the domain wall mobility on the ferroelectric phase. Large lattice distortion in tetragonal samples produces a low mobility of the ferroelectric-ferroelastic domain walls. The influence of the type of defects on the nonlinear response has been explored. The results show that the relative increase of the domain walls mobility in donor-doped materials is greater than the decrease in acceptor-doped materials due to the pinning produced by complex defects. Rayleigh law has been used to evaluate the irreversible contribution of the domain walls movement to the nonlinear dielectric response. The analysis reveals that in presence of oxygen vacancies, the dielectric response cannot be attributed exclusively to the contribution of the irreversible domain wal...

Extrinsic contribution and non-linear response in lead-free KNN-modified piezoceramics

Finding lead-free ceramics with good piezoelectric properties is nowadays one of the most important challenges in materials science. The (K, Na, Li)(Nb, Ta, Sb)O3 system is one of the most promising candidates as a lead-free ceramic for transducer applications and is currently the object of important research work. In this paper, (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3 was prepared by a conventional ceramic processing route. For this composition, orthorhombic-to-tetragonal phase transition was observed at temperatures very close to room temperature. As a consequence, good room temperature electromechanical properties were observed, displaying good thermal stability. We show that the most important contribution to dielectric, piezoelectric and elastic response comes from extrinsic effects, as was observed in other perovskite based materials. Nonlinearities in electromechanical properties induced by high electric field or mechanical stress were studied. Non-linear dielectric response was found to be less important than for soft PZT ceramics and was analysed within the Rayleigh framework. The results reveal that the non-linear response at room temperature in this material is mainly due to the irreversible wall domain movement.

High electric field measurement of dielectric constant and losses of ferroelectric ceramics

Nonlinear behaviour of power piezoceramic materials has to be studied under strong electric fields. A bridge designed for measuring it at low frequency is described. It consists basically of a capacitance comparison bridge that can be used in two different modes: balanced and unbalanced. The nonlinear electric displacement is split into dissipative and nondissipative terms. Dielectric constant is computed from the last term and related to the instantaneous field. The behaviour of soft and hard PZT ceramic materials has been tested according to this model. In both materials, the dielectric constant not only depends on the instantaneous value of the field but also on its amplitude. While the dependence of the real and imaginary parts of the dielectric constant in soft ceramics is large and practically linear to the field amplitude, in hard PZT it is low and nearly quadratic. The rate between both parts depends on the type of ceramic. Dielectric constant can be divided into two terms. One term is allied to the irreversible motion of domain walls, while the other is related to reversible motion. Balanced and unbalanced modes have been tested for both materials, and results are coherent. Accuracy and limitations of the method are discussed.

Unexpected dielectric response in lead zirconate titanate ceramics: The role of ferroelectric domain wall pinning effects

Temperature dependent dielectric response has been measured in Pb(Zr1-xTix)O-3 ceramics. Samples of different compositions (x=0.40, 0.47, and 0.60), pure and doped with Nb-or Fe, were studied at temperatures between 15 and 700 K and in the frequency range from 100 Hz to 1 MHz. Unexpected dielectric behavior has been found around room temperature. Anomalous temperature dependent permittivity is observed in pure and Fe-doped samples but not in Nb-doped samples. The anomaly appears related to the presence of oxygen vacancies but not on the sample crystallographic phase. The authors suggest that the anomaly may be a manifestation of the domain wall pinning effect

Optimization of Elastic Nonlinear Behavior Measurements of Ceramic Piezoelectric Resonators with Burst Excitation

A system of nonlinear measurement and nonlinear elastic characterization of resonators is presented, which increases the possibilities and characteristics of the other classic nonlinear characterization methods. This characterization has been necessary due to the use of resonators in power devices, where their behavior departs from the linear characteristics. The use of burst signals and a system of acquisition and data processing is proposed instead of impedance analyzers, thus avoiding the thermal effects associated with the high-signal measures, which are necessary for this characterization. The measures are repeated for different amplitudes and at the same frequency near the resonance by a single amplitude sweep, which is simpler and faster to carry out than the multiple frequency sweepings used in other methods. As a last resort, a variation on the proposed method, closer to the classical measures, is put forward, in which the resonance is ensured in all the measures. Special emphasis is placed on obtaining nonlinear characterization of the piezoceramic material in order to increase its optimization in the transducers in terms of both its use and its composition and structure.

Extrinsic contribution to the non-linearity in a PZT disc

Non-linear increases in elastic, piezoelectric (direct and reverse) and dielectric coefficients have been measured under a high electrical field or under high mechanical stress. The permittivity and reverse piezoelectric coefficient can be measured by applying a high voltage at a low frequency, while the elastic compliance and direct piezoelectric coefficient can be measured at the first radial resonance frequency in order to apply a high stress. The non-linear behaviour has been analysed at the radial resonance of a disc.In all the materials tested, the results show that there is a close relation between the non-linear increments of the different coefficients. An empirical model has been proposed in order to describe and understand these relations. It is assumed that either the strain or the electrical displacement is produced by intrinsic and extrinsic processes, but only the latter, which consist mainly in the motion of domain walls, contribute to the non-linearity. The model enables us to find the domain wall contribution to elastic, piezoelectric and dielectric non-linearities, and allows us to compare the amplitudes of the fields and stresses that produce the same displacement of domain walls.

Influence of extrinsic contribution on the macroscopic properties of hard and soft lead zirconate titanate ceramics

In this work, the contribution of the extrinsic effect to the macroscopic properties in soft and hard lead zirconate titanate ceramics is directly evaluated. Close to the room temperature, poled hard ceramics show an anomalous behavior, which is notably different from that of soft ceramics, not only in dielectric but also in piezoelectric and elastic responses. Hence, at room temperature their properties are thermally stable and the losses are unusually low. It is suggested that two mechanisms are present, with one mechanism inhibiting the other.

Nonlinear characterization with burst excitation of 1-3 piezocomposite transducers.

Ultrasonic transducers made with 1-3 connectivity piezocomposites are frequently used in Medical applications and nondestructive testing. When the transducer is used for special applications as, for instance air-coupled transmission, it is necessary to compensate for the high difference of acoustic impedance between transducer and medium using high amplitude pulses to generate high acoustic signal. Thus, the nonlinear behavior of the transducer must be taken into account in similar application conditions. The newly developed method, which performs the nonlinear characterization with burst signal excitation near the thickness resonance frequency, is based on the measure of the current as well as the vibration velocity of the piezocomposite transducer. The current of the stationary response is measured before the end of the burst signal excitation. Burst excitation enables us to measure the nonlinear characterization without producing overheating in the transducers. The amplitude level dependence of mechanical losses tandelta(m) and the stiffness increases |Deltac/c(0)| have been studied, as well as the velocity dependence of a point of the transducer, measured with a laser vibrometer. In this method, the power level applied to the transducers can be higher than other nonlinear measurement methods, providing measurements of high accuracy.

Nonlinear mechanical behavior of piezocomposites for ultrasonic transducers

A comparative study is carried out between the nonlinear behavior of a composite and the piezoceramic used to obtain it. This characterization is necessary for using the composite in power transducer applications. A study of the losses and the resonator stiffness variations has also been done. Both these effects, as well as the possibility of the frequency hysteresis, show different behavior in the composites, since the increases in the ceramics are different from those in the composites. In this study two measurement methods are used: principally the motional impedance increase with the motional current measurements. The results obtained are normalized in order to make them independent of the resonator size, and thus make the comparison between the composite and the ceramic easier. The figure of the mechanical loss tangent tan delta m versus the mean strain shows that the losses can be greater in the ceramic than in the composite for soft ceramics. The dependence behavior of the losses and stiffness variations versus the mean strain is studied for both resonators.