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This work presents an experimental method for obtaining the best excitation pulse type and width for optimal driving of an industrial non-destructive testing (NDT) ultrasound transducer by using its electrical impedance parameters as input. Within the optimization method, a classic low-voltage frequency sweeping signal and two different types of high-voltage impulse excitation signals (unipolar and bipolar), with different pulse widths, are used for the electromechanical characterization of the transducer. The optimization of the excitation signal is verified by measuring the voltage, current and electrical power of the excitation and received signals obtained by ultrasound pulse reflection from two cracks (2 mm and 3 mm) in a stainless-steel block. Additionally, the influence of the output electrical impedance of the power amplifier on the optimal transfer of electrical power from the amplifier to the loading (the NDT transducer on the block) and vice versa is also discussed in this work. The optimum working point for the ultrasound NDT transducer, regarding the impulse excitation type and its pulse width, is a bipolar impulse excitation with the width and amplitude calculated from maximum values of the measured voltage, electric current and power generated in the transducer receive mode by the reflected ultrasound signals.

Electromechanical and acoustical characterization of PZT piezoceramic elements and high-power ultrasound transducers has been done around resonance frequencies in order to determine the features of the models that describe the considered devices, both as equivalent circuits and as nonlinear oscillators. Several measurement methods have been compared, including the frequency-sweeping method with constant excitation voltage, the magnitude-sweeping method at a constant excitation frequency and the method that uses impulse excitation. The parameters that describe the losses in a coupled nonlinear electro-mechanical-acoustical system greatly depend on the type of excitation and its level. A novel algorithm is tested, designed for tracking the changes of the series resonance frequency and impedance magnitude and phase with the excitation level. It enables a more precise determination of the resonance frequency and impedance changes over different levels of excitation, which is especially important in high-power resonant applications. The characterization based on frequency sweeping at different excitation levels is influenced by changing electrical parameters (current, voltage, power) due to different complex loading of the excitation amplifier and the changes of thermodynamic conditions. The characterization of transducers in loaded condition through frequency sweeping at higher signal levels gives no results when cavitation in front of the horn appears, because the impedance magnitude changes at higher excitation voltages. Using the tracking algorithm, the impedance magnitude at resonance frequency has been determined in a range of different excitation levels up to 35 V RMS . Before the onset of nonlinear effects such as cavitation or acoustic streaming, the impedance magnitude increases due to viscosity effects.

Determination of electromechanical piezoceramic material parameters is usually done by fitting the measured input electrical impedance of the piezoceramic sample to the theoretical modelling equation for the input electrical impedance of the unloaded free piezoceramic resonator. The input electrical impedance of the sample is usually measured by using low voltage or current magnitude frequency sweeping signals. In this work, the complex material parameters of piezoceramic samples are determined in the real operating conditions by using the high voltage short impulse excitation signals. The input electrical impedance determined in the impulse mode around thickness extensional vibration mode (TE) and calculated piezoceramic parameters (clamped dielectric permittivity, electromechanical coupling factor, elastic stiffness and piezoelectric constant) are compared to the results obtained by using the low voltage magnitude frequency sweeping signals. When impulse excitation is used, the series resonance frequency is decreased and the input electrical impedance magnitude at series resonance is increased, which means that overall losses included in the piezoceramic parameters are increased. The complex material parameters obtained from the input electrical impedances determined by using the low voltage magnitude sweeping signal and high level short impulse signals are included in the KLM theoretical model describing the piezoceramic sample behaviour around TE mode. Better agreement between measured and theoretically determined current magnitude response around TE mode has been obtained, in the KLM model, when piezoceramic parameters determined by using the impulse signal excitations are included in the modelling. The physical reason for increase of the losses in piezoceramic material could lie in the fact that the ferroelectric domains in the piezoceramic respond harder on very short impulse excitation signals than on continuous frequency sweeping signals which are usually used in determination of piezoelectric material parameters.

Electromechanical characterization of piezoceramic bulk elements around resonance is usually done with low-level continuous excitation signals at room temperature, but in real applications such elements are driven with different types of electrical signals, usually at higher levels and at different ambient temperatures. Both homemade and commercial soft and hard PZT piezoceramic elements were characterized using the established characterization methods that include the measurements of electrical admittance and surface displacement of the piezoceramic elements around the series resonance frequencies of two modes of vibration (radial and thickness extensional). The measurements included fast frequency sweeps at constant voltage excitation levels, burst measurements, at different temperatures and at different levels of excitation. A novel method for electromechanical characterization of piezoceramic elements that utilizes the resonance frequency tracking at different excitation levels (electric fields up to 5 kV/m, currents up to 1.3 A at resonance) and temperature conditions (up to 150 °C) has been proposed. The main idea is to keep the investigated element in resonance as the excitation level changes by constant tracking of its resonance frequency. The electromechanical parameters of the considered elements change mostly due to the nonlinear effects and the changes due to different thermodynamic conditions can be neglected when fast algorithm is applied. The decrease of the input electrical admittance magnitude is more expressed than the change of the resonance frequency when algorithm is applied.

The orientation dependence of the longitudinal piezoelectric coefficient, d(33)(*), is investigated as a function of temperature in BaTiO3 and PbTiO3 crystals using the Landau-Ginsburg-Devonshire theory. We show that a presence of the ferroelectric-ferroelectric phase transitions in BaTiO3 leads to enhanced d(33)(*) along nonpolar directions. The reason for this is that in the vicinity of a phase transition temperature at which a polarization vector changes its direction (tetragonal-orthorhombic/monoclinic, orthorhombic/monoclinic-rhombohedral), the shear piezoelectric coefficients become high. It is shown for all ferroelectric phases of BaTiO3 that the shear stress deforms the crystal cell and changes the polarization direction in a similar way as the corresponding temperature-induced phase transition. The influence of the piezoelectric shear effect on the anisotropy of d(33)(*) is particularly pronounced in the orthorhombic/monoclinic phase where the piezoelectric shear coefficients are determined by the presence of both the high-temperature tetragonal and the low-temperature rhombohedral phases. In PbTiO3, which does not exhibit ferroelectric-ferroelectric phase transitions, the shear piezoelectric effect is weak and d(33)(*) has its maximum along the polar axis at all temperatures. These results can be generalized to include phase transitions induced by electric-field and composition variations and are valid for all perovskite materials, including complex relaxor-ferroelectric perovskites that have recently attracted attention for their exceptionally large piezoelectric properties. (C) 2003 American Institute of Physics.

Using recently published experimental data we calculated the piezoelectric response of rhombohedral 0.67Pb(Mg1/3Nb2/3)O-3-0.33PbTiO(3) monodomain single crystals along arbitrary directions. The calculations indicate that the value of the longitudinal piezoelectric coefficient along the [001] axis of the pseudocubic system ([111] axis of the rhombohedral system) of a monodomain crystal is close to the value determined experimentally along the same direction of a multidomain crystal with a so-called "engineered domain state." The results thus show that the large piezoelectric response in this material is dominated by intrinsic crystal lattice effects and that the multidomain state has a relatively minor effect (1900 pC/N for certain directions of the monodomain crystal. (C) 2003 American Institute of Physics.

This paper discusses the mechanisms that can contribute to the enhanced longitudinal piezoelectric effect along nonpolar directions in perovskite crystals, such as BaTiO3, PbTiO3, KNbO3, Pb(Mg1/3Nb2/3)O-3-PbTiO3 and Pb(Zn1/2Nb2/3)O-3-PbTiO3. Piezoelectric anisotropy is discussed in relation to temperature induced phase transitions, compositional variation in solid solutions with morphotropic phase boundaries, applied electric fields, the domain wall structure and domain wall displacement. (c) 2006 Springer Science + Business Media, Inc.

The question of the origin of the piezoelectric properties enhancement in perovskite ferroelectrics is approached by analyzing the Gibbs free energy of tetragonal $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$, $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$, and $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_{3}$ in the framework of the Landau-Ginzburg-Devonshire theory. The flattening of the Gibbs free-energy profile appears as a fundamental thermodynamic process behind the piezoelectric enhancement. The generality of the approach is demonstrated by examining the free-energy flattening and piezoelectric enhancement as a function of composition, temperature, electric field, and mechanical stress. It is shown that the anisotropy of the free-energy flattening is the origin of the anisotropic enhancement of the piezoelectric response, which can occur either by polarization rotation or by polarization contraction. Giant enhancement of the longitudinal piezoelectric response $({d}_{33}\ensuremath{\propto}{10}^{3}\phantom{\rule{0.3em}{0ex}}\mathrm{pC}∕\mathrm{N})$ is predicted in $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$ under uniaxial compression.

The influence of the uniaxial bias stress on the piezoelectric properties of tetragonal BaTiO3 and PbTiO3 monodomain crystals is modeled in the framework of the phenomenological Landau-Ginzburg-Devonshire theory. It is shown that tensile and compressive stresses, both applied along the spontaneous polarization direction, reduce and enhance the piezoelectric response, respectively. The enhancement effect is due to the flattening of the free-energy profile and the corresponding dielectric softening of crystals, caused by the compressive stress. In BaTiO3 crystals, at temperatures close to the tetragonal-orthorhombic phase transition temperature, the free-energy profile flattening and dielectric softening are the largest along axes perpendicular to the polarization direction, facilitating thus the polarization rotation away from the [001](c) polar axis. The resulting enhancement of the shear piezoelectric coefficient is directly responsible for the increase of the longitudinal piezoelectric coefficient along the [111](c) axis. At temperatures deep within the tetragonal phase in BaTiO3, and over the whole ferroelectric region of PbTiO3, the flattening of the free-energy profile and the dielectric softening by compressive stress are the strongest along the polar axis. The resulting enhancement of the longitudinal piezoelectric coefficient is thus the largest along the polar [001](c) direction. These results, which can be applied to other perovskite crystals, have broad implications.

The orientation dependence of the longitudinal piezoelectric coefficient is investigated as a function of temperature in BaTiO/sub 3/ and PbTiO/sub 3/ crystals using Landau-Ginsburg-Devonshire theory. We show that a presence of the ferroelectric - ferroelectric phase transitions in BaTiO/sub 3/ leads to an enhanced piezoelectric longitudinal response along nonpolar directions. The reason for this is the abrupt increase of the shear piezoelectric coefficients in the vicinity of a FE - FE phase transition temperature. In PbTiO/sub 3/, which does not exhibit such phase transitions, the shear piezoelectric effect is weak and the longitudinal piezoelectric coefficient has its maximum along the polar axis at all temperatures. These results can be generalized to include phase transitions induced by electric-field and composition variation and are valid for all perovskite materials. Further, using Landau-Ginzburg-Devonshire theory and tetragonal BaTiO/sub 3/ as an example, we demonstrate that a strong electric bias fields applied anti-parallel to the spontaneous polarization may lead to a large enhancement of piezoelectric properties in monodomain perovskite crystals. The enhancement is due to the field-induced dielectric softening of the crystal.

We have measured the thermal conductivity of (TaSe4)2I in the chain and perpendicular to chain directions. A strong anisotropy is observed in the temperature range of 80–320 K. The detailed analysis of the temperature dependence in both of directions confirms that in addition to the lattice and free carriers there are very anisotropic contributions to the thermal conductivity coming from the phase fluctuations.

Piezoelectric properties of ferroelectric crystals can be greatly enhanced in the proximity of electric and mechanical coercive fields. While the thermodynamic origin of the enhancement in both cases can be traced to the flattening of a free energy profile, we show using the Landau–Ginzburg–Devonshire theory that there is a qualitative difference in how the proximity of 90° and 180° polarization switching affects the piezoelectric response and its anisotropy. Taking as an example PbTiO3 monodomain single crystals biased by near-coercive electric and stress fields applied along the polar axis, it is shown that the antiparallel electric field softens the material along the polar direction while the compressive stress softens the crystal along nonpolar directions. In the former cases the piezoelectric enhancement is dominated by the polarization contraction along the polar axis while in the latter case it is dominated by the polarization rotation away from the polar axis. An analogy is made between the effec...

Note: Damjanovic, D Swiss Fed Inst Technol, EPFL, Dept Mat, Ceram Lab, CH-1015 Lausanne, Switzerland Swiss Fed Inst Technol, EPFL, Dept Mat, Ceram Lab, CH-1015 Lausanne, Switzerland721TJTimes Cited:6Cited References Count:3 Reference LC-ARTICLE-2003-009View record in Web of Science Record created on 2006-08-21, modified on 2017-05-10

The importance of a high shear coefficient d15 (or d24) to the piezoelectric properties of domain-engineered and polycrystalline ferroelectrics is discussed. The extent of polarization rotation, as a mechanism of piezoelectric response, is directly correlated to the shear coefficient. The terms "rotator" and "extender" are introduced to distinguish the contrasting behaviors of crystals such as 4mm BaTiO3 and PbTiO3. In "rotator" ferroelectrics, where d15 is high relative to the longitudinal coefficient d33, polarization rotation is the dominant mechanism of piezoelectric response; the maximum longitudinal piezoelectric response is found away from the polar axis. In "extender" ferroelectrics, d15 is low and the collinear effect dominates; the maximum piezoelectric response is found along the polar axis. A variety of 3m, mm2 and 4mm ferroelectrics, with various crystal structures based on oxygen octahedra, are classified in this way. It is shown that the largest piezoelectric anisotropies d15/d33 are always found in 3m crystals; this is a result of the intrinsic electrostrictive anisotropy of the constituent oxygen octahedra. Finally, for a given symmetry, the piezoelectric anisotropy increases close to ferroelectric-ferroelectric phase transitions; this includes morphotropic phase boundaries and temperature induced polymorphic transitions., Comment: accepted in J. Appl. Phys

Using Landau-Ginzburg-Devonshire theory and tetragonal BaTiO3 as an example, this letter demonstrates that strong electric bias fields applied antiparallel to spontaneous polarization may lead to a large enhancement of piezoelectric properties in monodomain perovskite crystals with multiple phase transitions. At temperatures close to tetragonal-orthorhombic phase transition, the largest enhancement is approximately along the [111] axis. The enhancement is due to the field induced dielectric softening of the crystal. (C) 2004 American Institute of Physics.

Today, high crystal quality in sufficient size with high crystal growth yield appears to be a key point for a wide dissemination and use in ferroelectrics applications. So, this paper is related to the growth of PZN-PT solid solution compound like (1-x)(PbZn/sub 1/3/Nb/sub 2/3/O/sub 3/)-xPbTiO/sub 3/ and PMN-PT compound like (1-y)(PbMg/sub 1/3/Nb/sub 2/3/O/sub 3/)-yPbTiO/sub 3/. For these two relaxor ferroelectric single crystals promising properties are obtained near the Morphotropic Phase Boundary (MPB) transition which occurred at values closed to x=0.09 for PZN-PT and y=0.34 for PMN-PT. The complex unit cell phase transition combined with an adequate applied electric field, oriented in the pseudo /sub c/ direction; confer to these single crystals some interesting piezoelectric properties. We present some crystal growth results obtained by the Vertical Gradient Freeze-Solution Growth method (VGF-SG) for PZN-PT and by a pseudo Kyropoulos method developed for PMN-PT. The obtained single crystals, up to 120 g for PZN-PT and 160 g for PMN-PT are then analysed in term of composition and structure. Both PMN-PT and PZN-PT perovskite crystals are rhombohedral (@RT), they show a good global homogeneity. Samples demonstrate ferroelectric behaviour, and after polarisation process, dielectric constants up to 48000 near the Curie temperature (Tc /spl sim/174/spl deg/C @ 1kHz) are obtained for PZN-PT and respectively 74000 for PMN-PT (Tc /spl sim/ 142/spl deg/C @ 100 Hz). The piezoelectric coefficient d/sub 33/ gives values up to /spl sim/2200 pC/N (@RT 9 kV/cm) for PZN-PT and 1700 pC/N(@RT) for PMN-PT. A value up to 94% is measured for the electromechanical coupling coefficient k/sub 33/.

While in the paralectric phase of ferroelectrics, and in centrosymmetric materials in general, the dielectric tunability is symmetrical for positive and negative electric bias fields, in noncentrosymmetric materials this is not the case. By using the thermodynamic theory, and tetragonal phase of monodomain single-crystal perovskites BaTiO3 and Pb(Zr,Ti)O-3, as examples, we show that the theoretical tunability region of ferroelectrics may, in principle, be considerably broadened and tunability greatly enhanced by applying electric-bias field antiparallel to polarization. Available experimental data are in qualitative agreement with the theoretical predictions. (c) 2006 American Institute of Physics.