Investigation of cross-coupling in 1-3 piezocomposite arrays

Plate waves inside the piezoelectric layer are much involved in the element cross-coupling in transducer arrays for medical imaging. In this work, such waves are analyzed in 1-3 piezocomposite materials on the basis of conventional guided modes formalism in which the piezocomposite is considered as a homogeneous medium. Cross-coupling measurements have been made on two different transducer arrays using a network analyzer and a laser interferometric probe. It is shown how the analysis in terms of symmetrical Lamb waves gives an interesting qualitative interpretation, explaining most of the cross-coupling amplitude variations with frequency. Results show that the 0th and 3rd symmetrical Lamb waves are mainly involved in coupling inside composite plates. The S₀ mode is responsible for the inter-element coupling, whereas the S₃ mode widens the effective width of the excited element     

Theoretical and experimental investigations of lateral modes in 1-3piezocomposites

Materials with a periodic microstructure show resonances caused by the elastic wave Bragg diffraction. This paper presents a simple approach to describe these resonances (called lateral resonances) in 1-3 piezoelectric composite materials which have a 2-D periodicity. Our model is based on the analysis of the propagation of transverse waves in a 2-D periodic medium of infinite thickness and takes into account the periodic and interfacial boundary conditions. This model predicts the displacement field vectors and frequencies of lateral resonances from which the phase velocity of lateral waves is determined. The theoretical and experimental variations of this velocity versus the ceramic rod width to pitch ratio are compared. It is shown that the first lateral mode frequency is maximum when the ceramic volume fraction is around 0.65. Theoretical predictions of the mechanical displacement at the composite surface are compared with measurements obtained by an interferometric laser technique. A good agreement is observed, showing that lateral waves are mainly vertically polarized     

Two-dimensional electroacoustic model of transducer array based on 1-3 piezocomposite materials

An analytical model is presented to achieve simultaneous prediction of the elementary electroacoustic response and directivity pattern of a one-dimensional (1-D) piezocomposite array. The theoretical approach was based on guided wave theory in a multilayered structure in which the 1-3 piezocomposite material is considered as a homogeneous piezoelectric plate. A matrix method was applied to simulate the displacement fields generated at the surface of the array when one element was excited with an electrical pulse. A test device was manufactured, then characterized through measurements of displacement performed with an interferometric laser probe when the array vibrated in air and in water. The experimental results are presented and compared with theory.     

A matrix method for modeling electroelastic moduli of 0-3 piezo-composites

A model is proposed to predict the electroelastic moduli of 0-3 connectivity piezo-composites from which parameters such as longitudinal wave velocity and thickness mode coupling factor can be deduced. The composite, a polymer loaded with ceramic particles, is represented by a unit cell, and a matrix manipulation is shown to be a practical way to perform a generalization of the series and parallel analysis used for 2-2 connectivity composites. The anisotropy of the ceramic phase is taken into account, and its effect on the properties of the composite is shown. The model is then used to optimize composite performance and to choose the two constituents through comparison of results obtained using several commercial polymers and ceramics.     

A finite difference model for cMUT devices

A finite difference method was implemented to simulate capacitive micromachined ultrasonic transducers (cMUTs) and compared to models described in the literature such as finite element methods. Similar results were obtained. It was found that one master curve described the clamped capacitance. We introduced normalized capacitance versus normalized bias voltage and metallization rate, independent of layer thickness, gap height, and size membrane, leading to the determination of a coupling factor master curve. We present here calculations and measurements of electrical impedance for cMUTs. An electromechanical equivalent circuit was used to perform simulations. Our experimental measurements confirmed the theoretical results in terms of resonance, anti-resonance frequencies, clamped capacitance, and electromechanical coupling factor. Due to inhomogeneity of the tested element array and strong parasitic capacitance between cells, the maximum coupling coefficient value achieved was 0.27. Good agreement with theory was obtained for all findings.