J. E. García

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...

High-power ultrasonic system for the enhancement of mass transfer in supercritical CO2 extraction processes

Oil is an important component of almonds and other vegetable substrates that can show an influence on human health. In this work the development and validation of an innovative, robust, stable, reliable and efficient ultrasonic system at pilot scale to assist supercritical CO(2) extraction of oils from different substrates is presented. In the extraction procedure ultrasonic energy represents an efficient way of producing deep agitation enhancing mass transfer processes because of some mechanisms (radiation pressure, streaming, agitation, high amplitude vibrations, etc.). A previous work to this research pointed out the feasibility of integrating an ultrasonic field inside a supercritical extractor without losing a significant volume fraction. This pioneer method enabled to accelerate mass transfer and then, improving supercritical extraction times. To commercially develop the new procedure fulfilling industrial requirements, a new configuration device has been designed, implemented, tested and successfully validated for supercritical fluid extraction of oil from different vegetable substrates.

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.

Evidence of temperature dependent domain wall dynamics in hard lead zirconate titanate piezoceramics

This work presents a study of the domain wall dynamics in Pb(Zr1−xTix)O3 (PZT)-based piezoceramics by means of the temperature dependence non-linear dielectric response and hysteresis loop measurements. In soft PZT, non-linear response gradually increases as the temperature is raised. A similar response is displayed by hard PZT at low temperatures. However, rather more complex behavior is detected at temperatures above 200 K. The anomalous response, which is very marked at room temperature, becomes even greater when the electric field is increased. The non-linear dielectric response is analyzed in the framework of the Rayleigh model. The results suggest a clear change in the domain wall dynamics in hard PZT, which is not observed in soft PZT. Observation of the hysteresis loops confirms that a strong effect of domain wall pinning emerges near room temperature. The change in domain wall dynamics appears as the main cause of the dielectric response difference between both kinds of materials at room temperature.

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

Accuracy of photocarrier radiometric measurement of electronic transport properties of ion-implanted silicon wafers

The determination of the electronic transport properties of ion-implanted silicon wafers with the photocarrier radiometry (PCR) technique by fitting frequency scan data to a single layer model via a multiparameter fitting procedure is presented. A three-layer model is used to simulate the inhomogeneous structure of the ion-implanted wafers. The effects of the structural, electronic, and optical properties of the implanted layer, which are affected significantly by ion implantation, on the frequency behavior of the PCR signal of implanted wafers are discussed. Data simulated with the three-layer model are fitted to a single-layer model to extract the electronic transport properties of implanted wafers. The fitted carrier lifetime and diffusion coefficient are found to be close to that of the substrate layer which is assumed to remain intact after the ion implantation process. When self-normalized relative amplitude is used in the multiparameter fitting, the fitted surface recombination velocity is determin...

Three-layer photocarrier radiometry model of ion-implanted silicon wafers

A three-dimensional three-layer model is presented for the quantitative understanding of the infrared photocarrier radiometry (PCR) response of ion-implanted semiconductors, specifically Si. In addition to the implanted layer and intact substrate normally assumed in all existing two-layer theoretical models to describe the photothermal response of ion-implanted semiconductors, a surface layer is considered in this three-layer model to represent a thin, less severally damaged region close to the surface. The effects on the PCR signal of several structural, transport, and optical properties of ion-implanted silicon wafers affected significantly by the ion implantation process (minority carrier lifetime, diffusion coefficient, optical absorption coefficient, thickness of the implanted layer, and front surface recombination velocity) are discussed. The dependence of the PCR signal on the ion implantation dose is theoretically calculated and compared to experimental results. Good agreement between experimental...

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.