Louis Pascal Tran-Huu-Hue

Measurement of losses in five piezoelectric ceramics between 2 and 50 MHz

Approximate formulas including losses to predict the electrical impedance of a thin unloaded piezoelectric plate around antiresonant frequencies of the thickness modes have been derived. To do so, a total loss factor is defined that includes both mechanical and electrical losses. Complex electrical impedance measurements on a lead metaniobate and four PZT-type materials between 2 and 50 MHz have been performed. The total loss factors were deduced from both the peak real impedance and from the -6 dB bandwidth of the real impedance peak. Results for fundamental and harmonic thickness modes on thin plates are discussed and the five materials are compared. It is found that for these piezoceramics the total loss factor is well approximated by a linear function of frequency. Finally, a frequency-dependent loss factor is included in the KLM equivalent circuit and it is shown that the theoretical impedance curves obtained with this model are in good agreement with measurements.<<ETX>>

Piezoelectric Transducer Design for Medical Diagnosis and NDE

The number one technique for medical imaging and non-destructive evaluation (NDE) is ultrasound. This is due to its non-ionizing character, low cost and to the fact that images and measurements contain data linked to several physical and structural parameters of the explored media. The overall performance of an ultrasonic system is mainly determined by the transducer characteristics. Consequently, each application having its specific requirements, very different transducers need to be designed. Furthermore, the measurement techniques and imaging modalities are in constant evolution, requiring higher performance and versatility of the transducers. Not only must frequency bandwidth and sensitivity be increased, but transducers must also be able to operate in various modes such as pulse-echo (classical A, B or C modes), burst (Doppler or other velocity measurements) or harmonic reception (non-linear acoustics). Innovations such as ultrasound stimulated elastography and combination of different techniques such as ultrasound and MRI or ultrasound therapy and imaging are only possible if specific transducers are developed. The structure of a single-element ultrasonic transducer based on piezoelectric effect is first described, with particular focus on the influence of the constitutive materials on transducer performance. More complex transducers such as annular, linear, curved, phased, and 2D arrays are described and most important design issues discussed. Piezoelectric material issues relative to transducer applications are then addressed. Methods to characterize and models to predict transducer behaviour are presented, and several comparisons are shown to illustrate achieved performance. Finally, transducers designed for high-resolution imaging are presented, and other current developments briefly described. Some particularly interesting future trends are highlighted in the conclusion.

Non-planar pad-printed thick-film focused high-frequency ultrasonic transducers for imaging and therapeutic applications

Pad-printed thick-film transducers have been shown to be an interesting alternative to lapped bulk piezoceramics, because the film is deposited with the required thickness, size, and geometry, thus avoiding any subsequent machining to achieve geometrical focusing. Their electromechanical properties are close to those of bulk ceramics with similar composition despite having a higher porosity. In this paper, pad-printed high-frequency transducers based on a low-loss piezoceramic composition are designed and fabricated. High-porosity ceramic cylinders with a spherical top surface are used as the backing substrate. The transducers are characterized in view of imaging applications and their imaging capabilities are evaluated with phantoms containing spherical inclusions and in different biological tissues. In addition, the transducers are evaluated for their capability to produce high-acoustic intensities at frequencies around 20 MHz. High-intensity measurements, obtained with a calibrated hydrophone, show that transducer performance is promising for applications that would require the same device to be used for imaging and for therapy. Nevertheless, the transducer design can be improved, and simulation studies are performed to find a better compromise between low-power and high-power performance. The size, geometry, and constitutive materials of optimized configurations are proposed and their feasibility is discussed.

Prediction of Electroacoustic Performances of High Frequency P(VDF-TrFE) Transducers.

Three transducers were made with a 25 μm P(VDF-TrFE) film at a resonant frequency of 50 MHz using three kinds of light backings (Z backing < Z copolymers, half wave length resonance). The electroacoustic behaviour of these transducers was predicted by a numerical model based on the KLM equivalent circuit which includes mechanical and dielectric losses and which take into account the fact that, due to construction, there is two vibration modes. Experimental impedances, 50 Ω loop sensitivities and impulse responses were measured and compared to simulations. A good agreement was found and the results are discussed in terms of compromise between sensitivity and bandwidth.

Non-planar pad-printed thick-film focused high-frequency ultrasonic transducers for imaging and HIFU applications

Most high-frequency ultrasound transducers for imaging applications are based either on piezopolymers or on lapped bulk piezoceramics. The latter are planar, so focusing can be obtained by adding a lens (at the price of lower sensitivity) or by fracturing into a curved geometry. Pad printed thick-film transducers have been shown to be an interesting alternative, namely because the film is deposited with the required thickness, size and geometry, thus avoiding any subsequent machining and allowing geometrical focusing to be achieved. Film electromechanical properties are close to those of bulk ceramics with similar composition despite higher porosity. In this paper, pad-printed high-frequency transducers based on a low-loss piezoceramic composition are designed and fabricated. High-porosity ceramic cylinders with a spherical top surface are used as substrates and serve as backing material. The transducers are first characterized in view of imaging applications using a short pulse excitation, and their imaging capabilities are evaluated. Secondly, the transducers are excited by a one-period sine wave using several power levels to evaluate their capability to produce high-intensity focused ultrasound (HIFU) at frequencies around 20 MHz. The results, obtained via hydrophone measurements, are discussed showing that transducer performance is promising for applications that would require the same device to be used for imaging and for therapy. Nevertheless, the transducer design can be improved, and simulation studies are performed in order to find a better compromise between low-power and high-power performance. The size, geometry and constitutive materials of optimized configurations are proposed and their feasibility is discussed.

Electroacoustic performance of high frequency PZT based transducer fabricated by electrophoretic deposition: comparison with screen printing technique

The goal of this work is to compare the functional properties of piezoelectric PZT thick films fabricated by electrophoretic deposition process with those of films obtained by screen printing technique. Thick film fabrication, structural and electromechanical characterizations are reported. The density of the film is 82 % of the theoretical density and coupling coefficient is 47 %. The simulations of the electroacoustic performance of a 40MHz integrated ultrasonic transducer shows that such fabrication process lead to transducer performance suitable for high resolution medical imaging.

A Simplified Numerical Method To Predict The Impulse Radiation Pattern Of Linear And Curved Array Transducers.

Linear and curved array transducers are still frequently used in echographic exploration. We have developed a quick and simplified 1-dimensional model to predict the radiation pattern of such transducers. In this paper we present the validations and the limitations of this model. This simplified model can be currently used to design array transducers.

Nonlinear characterisation of piezoelectric samples using crossed (electrical/acoustical) measurements

Nonlinear phenomena in piezoelectric materials such as a change of resonance frequency with the amplitude of excitation or harmonic generation have been observed by several authors. Methods have been developed to measure nonlinear parameters on samples for various geometries [1] [2]. In this study, measurement of the displacement at one end of a piezoceramic rod driven by high sinusoidal voltages is performed in order to detect harmonic generation. A burst excitation of 500 periods with an on/off time ratio of one per cent is used to ensure the establishment of a steady state response while avoiding overheating. The frequency range between resonance and antiresonance is explored. External nonlinear sources having been eliminated or taken into account, distortion analysis has allowed two types of nonlinearities, electromechanical and mechanical, to be identified and the respective values of third order constants have been deduced.

Homogenization of piezocomposites containing both 0-3 and 3-3 connectivities

For high volume fraction piezoelectric particle loaded materials, two connectivities can be differentiated: the 0-3 connectivity which corresponds to isolated ferroelectric particles in the polymer matrix and the 3-3 connectivity which corresponds to particles in contact. A new model based on a matrix method previously developed for pure 0-3 and 3-3 connectivities is implemented to calculate all the effective parameters of such a composite which possesses both 0-3 and 3-3 connectivities. For this, a proportion of 0-3 connectivity in the composite is introduced. The effective properties are studied as a function of this proportion. Comparisons with experimental results show that this model allows an evaluation of the proportion of each connectivity through the analysis of the electromechanical performances of composite samples.