Broad bandwidth air-coupled micromachined ultrasonic transducers for gas sensing.

The present work aims to develop ultra-wide bandwidth air-coupled capacitive micromachined ultrasonic transducers (CMUTs) for binary gas mixture analysis. The detection principle is based on time-of-flight (ToF) measurements, in order to monitor gas ultrasound velocity variations. To perform such measurements, CMUTs were especially designed to work out of resonance mode, like a microphone. The chosen membrane size is 32 × 32 µm2 and gap height is 250 nm. The resonance frequency and collapse voltage were found at 8 MHz and 58 V respectively. As mentioned, the CMUTs were exploited in quasi-static operating mode, in a very low frequency band, from 1 MHz to 1.5 MHz frequencies. The transducer impulse response was characterised, and a -6 dB relative fractional frequency bandwidth (FBW) higher than 100% was measured, enabling to use CMUT for the targeted application. Additionally, a measuring cell has been designed to hold the fabricated CMUT emitter and receiver prototypes facing each other. The volume inside the cell was kept lower than 3 mL and the surface of emitter/receiver was 1.6 × 8 mm2. To validate the general principle of the proposed technique, two binary gas mixtures of CO2/N2 and H2/N2, with varying concentrations, have been tested. The results are very promising with a measured limit of detection (LOD) of 0.3% for CO2 in N2 and 0.15% for H2 in N2.

Investigation of ultrasonic absorption in the MHz frequency range by silicon substrates with a built-in porous silicon layer.

The present study is focused on the development and characterization of an innovative substrate to optimize the axial resolution of ultrasonic transducers on a silicon substrate that is dedicated to ultrasound imaging. The substrate must efficiently dampen wave propagation to avoid degradation of the axial resolution of ultrasound images. In this study, the proposed approach implements a silicon substrate with a built-in acoustic damping layer composed of porous silicon. Porous silicon layers with thicknesses of less than 100 µm and porosities varying from 27% to 62% were fabricated, and their substrate resonances were characterized. The experimental results obtained in the frequency range from 6 MHz to 10 MHz show that the substrate acoustic damping is controlled by adjusting the characteristics of the porous silicon layer; a significant damping of 70% is demonstrated with only 70 µm of porous silicon.

Modeling of piezoelectric transducers with combined pseudospectral and finite-difference methods.

A new hybrid finite-difference (FD) and pseudospectral (PS) method adapted to the modeling of piezoelectric transducers (PZTs) is presented. The time-dependent equations of propagation are solved using the PS method while the electric field induced in the piezoelectric material is determined through a FD representation. The purpose of this combination is to keep the advantages of both methods in one model: the adaptability of FD representation to model piezoelectric elements with various geometries and materials, and the low number of nodes per wavelength required by the PS method. This approach is implemented to obtain an accurate algorithm to simulate the propagation of acoustic waves over large distances, directly coupled to the calculation of the electric field created inside the piezoelectric material, which is difficult with classical algorithms. These operations are computed using variables located on spatially and temporally staggered grids, which attenuate Gibbs phenomenon and increase the algorithm's accuracy. The two-dimensional modeling of a PZT plate excited by a 50 MHz sinusoidal electrical signal is performed. The results are successfully compared to those obtained using the finite-element (FE) algorithm of ATILA software with configurations spatially and temporally adapted to the FE requirements. The cost efficiency of the FD-PS time-domain method is quantified and verified.

Influence of acousto-optic interactions on the determination of the diffracted field by an array obtained from displacement measurements.

This paper deals with the influence of acousto-optic interactions on the displacement measurements performed over transducer array and their effects on the predicted diffraction field. Changes on the temporal/spatial responses and the plane wave decomposition of the displacement are discussed. Modifications made on the directivity pattern are shown. A theoretical analysis of acousto-optic phenomenon, based on the plane wave decomposition of radiated field by the array is developed. Theoretical and experimental results are compared, showing first that waves with phase velocity near the one of the fluid are greatly amplified. Second, the interaction of laser beam with edge wave produced by the vertical size of elements induces a parasitic temporal pulse on the x-t diagram and so an interference pattern in the omega-k diagram. Corrections are proposed to eliminate errors induced by acousto-optic interactions and validated by comparing predicted diffraction field with measurements.

Investigation of cross-coupling in 1-3 piezocomposite arrays.

Plate waves inside the piezoelectric layer are much involved in the elements 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 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.

Characterization of airborne transducers by optical tomography

This paper describes the application of an acousto-optic method to the measurement of airborne ultrasound. The method consists of a heterodyne interferometric probing of the pressure emitted by the transducer combined with a tomographic algorithm. The heterodyne interferometer measures the optical phase shift of the probe laser beam, proportional to the acoustic pressure integrated along the light path. A number of projections of the sound field, e.g. a set of ray integrals obtained along parallel paths, are made in moving the transducer to be tested. The main advantage of the method is its very high sensitivity in air (2 x 10(-4) Pa Hz-1/2), combined with a large bandwidth. Using the same principle as X-ray tomography the ultrasonic pressure in a plane perpendicular to the transducer axis can be reconstructed. Several ultrasonic fields emitted by wide-band home made electrostatic transducers, with operating frequencies between 200 and 700 kHz, have been measured. The sensitivities compared favorably with those of commercial airborne transducers.

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.