M. Lethiecq

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>>

Modeling of highly loaded 0-3 piezoelectric composites using a matrix method

A model previously developed for pure 0-3 connectivity piezocomposites has been extended to 3-3 connectivity. This matrix method allows the prediction of the effective electroelastic moduli of a piezocomposite according to its connectivity. It is used to optimize composite performance by choosing the optimal constituents for each phase. A simple combination of the results for 0-3 and 3-3 connectivities allows the effective proportion of 3-3 connectivity to be defined in highly loaded 0-3 piezocomposites. This theoretical analysis has been used to evaluate effective proportions of 3-3 connectivity in five composite samples. The values obtained are shown to be a function of the ceramic volume fraction and fabrication process. The results of this study were used to optimize the fabrication process.

Effective electroelastic moduli of 3-3(0-3) piezocomposites

In a high volume fraction particle-loaded piezocomposite, some ferroelectric particles appear to be in contact, as in a 3-3 connectivity material; others are isolated in the polymer matrix, as in a pure 0-3 connectivity material. Such a material can be considered as a composite of composites characterized by a 3-3(0-3) connectivity. This paper follows two others that have described a matrix method to calculate all of the effective parameters of pure 0-3 and 3-3 connectivity piezocomposites. These previous models are used to obtain the effective properties of a 3-3(0-3) composite. A proportion of 0-3 connectivity in the composite is introduced, and the effective properties are studied as a function of this proportion. Experimental results compared with these predictions show that the model allows an evaluation of the proportion of 0-3 connectivity through the analysis of the electromechanical performance of the composite samples.

Modeling of ultrasonic wave propagation in integrated piezoelectric structures under residual stress

The objective of this study is to understand the role of residual stress in piezoelectric layers in order to predict the performance of integrated structures. This is of particular importance in thick or thin film technology. Considering a bulk piezoelectric material, the Christoffel equation for a piezoelectric material is modified to take into account a uniform residual stress on a given cross section. A numerical study of its influence is carried out on the slowness curves and coupling coefficients of a lithium niobate material. In a second part, modified Christoffel tensor is used to calculate the dispersion curves of Lamb waves in a piezoelectric plate. The Lamb modes are found to be sensitive to the residual stress. In particular, it is shown how the behavior of the first Lamb modes is modified with residual stress. In a third part, these results are extended to a piezoelectric film laid down on a substrate in order to model the importance of these phenomena on the behavior of an integrated structure. The numerical study of guided waves in a lithium niobate plate is performed first, then the case of a lithium niobate film laid down on a silicon substrate is considered.

Mass sensitivity of acoustic plate mode in liquids

In this paper, the concept of electrical effective permittivity function is used to calculate the eigen-frequencies and the particle displacements of piezoelectric acoustic plate modes (APM). These results allowed us to determine the mass sensitivities of the first order vibration modes using a first order perturbation theory. Theoretical results are discussed and compared to those of a variational method and isotropic two-layer composite analysis in the case of a shear horizontal APM sensor on a singly rotated cut quartz substrate. Experimental measurements by a copper electrodeposition are carried out and show that the perturbation method leads to a better understanding of the APM behavior.

1D ultrasound array: performance evaluation and characterization by laser interferometry

Nowadays, ultrasound medical imaging is more and more demanding with respect to transducer performance evaluation and optimization. Conventional tools and setups are limited in the complete evaluation and characterization of ultrasound imaging transducer, for example in the analysis of cross coupling (radiating and nonradiating). A vibration measurement method for ultrasonic array transducer using laser interferometry is presented. This method allows array designers to perfectly characterize the acoustic transducer, and contributes to the advanced optimization of bandwidth, cross coupling and sensitivity of ultrasound imaging array transducers.

High frequency properties of new fine-grained modified lead titanate ceramics [transducers]

Fabrication process is described for transducer applications. The electro-mechanical characterization is performed on the fundamental resonance and the overtones up to 100 MHz. The frequency dependence of parameters is shown. Two transducers are simulated (center frequency of 65 MHz), with a K.L.M. model, to compare the performances obtained when using a piezoelectric copolymer and this MPT ceramic as active elements.

Calculation of dissipation resistances in a single-element transducer

In this paper, a new formulation of the electrical input impedance of a single element transducer is presented. The resistive part of the electrical impedance that takes into account acoustic radiation in the front medium and losses in the transducer is split into a radiation resistance on one hand and into dissipation resistances related to each transducer component on the other hand. To confirm these theoretical results, characterization methods based on temperature measurements and pulse-echo response are presented. Measurements have been conducted on 1 MHz transducers, which consist of a piezoelectric ceramic glued on a backing. The results show a good agreement between experience and theory for dissipation resistance and radiation resistance values, which confirms the theoretical approach.

Analytical 2D model of transducer arrays for predicting elementary electroacoustic response and directivity pattern

The purpose of this paper is to provide a new analytical model from which the elementary electroacoustic response and directivity pattern of 1D piezocomposite arrays are simultaneously predicted. The approach is based on the propagation theory in multi-layers structures. The model has been used for simulating a complete 1D array. A test device has been constructed and characterized through measurements of displacement and angular directivity. Experimental results are presented and compared with theory.

New piezoceramic films for high resolution medical imaging applications

The fabrication process of new PZT-PNN piezoelectric ceramics (Ferroperm Pz21) is first described. Thick films are obtained through a tape casting route with a thickness between 60 and 90 /spl mu/m. This material is designed essentially to be integrated in high frequency medical transducers where properties such as high permittivity and high coupling factor are required. The dielectric, elastic, piezoelectric and microstructure properties of this ceramic are characterized as a function of frequency and used to simulate the behavior of an imaging transducer operating around 30 MHz. The performance obtained shows that Pz21 ceramics are well adapted for high resolution medical imaging applications.