A theoretical and experimental investigation into multilayered piezoelectric composite transducers

The behaviour of a number of 3-1 connectivity piezoelectric composite plate transducers is presented. The fundamental thickness mode resonance of such devices is found to be contaminated by lateral resonant activity; this is evidenced in the measured and predicted electrical impedance profile and the surface displacement data at the,fundamental thickness mode. Measurements taken on the 3-1 devices infer that they are not acting as true composites. In addition to this the finite element technique is applied to a number of stacked 3-1 and 1-3 connectivity devices to predict the mechanical Q-factor, and hence bandwidth, as a function of polymer filler properties

Simulation of the influence of hydrophones used for the characterization of pressure field distribution in low frequency, high power ultrasonic reactor vessels

This paper describes the use of a finite element (FE) modeling approach to investigate the influence of different hydrophone designs in laboratory scale reactor vessels. In addition to conventional PVDF membrane and piezoceramic hydrophone, the performance of a conceptual array hydrophone, comprising a 2D matrix of PVDF array elements, will be simulated. The FE modeling concentrates on two issues: the disturbance to the field through the introduction of each hydrophone configuration; and their suitability and response to measuring non-linear effects. To simplify the model the ultrasonic transducer is not directly represented. Here, a pressure loading function is used as the excitation technique, with a sawtooth waveform applied for the simulation of the non-linear detection capability of each hydrophone configuration. The results from the simulation programme demonstrate that the dynamics of the reactor vessel are critical to optimize the performance of the ultrasonic system. In addition, the introduction of a hydrophone alters the wave propagation, and hence the field distribution beyond a given probe location. Nevertheless, the spatial pressure distribution at the active area remains reasonably accurate if within the useable bandwidth of the device. Accordingly, the broadband nature of the membrane device is suited to operation in both the linear and non-linear regimes, with the PVDF array membrane device offering a fast, convenient measurement of the pressure field distribution for industrial applications

Multi-layered piezoelectric composite transducers

Multilayered piezoelectric materials present themselves as a suitable technology for the development of sub 100kHz transducers. A variety of different configurations have been proposed, including stacked 2-2, 1-3 and 3-1 connectivity configurations. Historically multilayer devices designed for low frequency of operation have comprised uniform layer thickness through the height of the device. The potential for extended bandwidth through the use of non-uniform layers through the thickness dimension has been investigated. In addition commercially available stacked ceramic mechanical actuators have been investigated. A combination of theoretical and experimental assessment has been employed to evaluate each transducer technology. Selection of the passive phase for these multilayer devices is critical. Typically, these devices operate in the high power regime and as such selection of the passive polymer material is crucial - thermal stability coupled with thermal conductivity would be a virtue. To this end a number of polymer materials possessing the appropriate thermal properties have been investigated

The causal differential scattering approach to calculating the effective properties of random composite materials with a particle size distribution

An implementation of the Causal Differential Method (CDM) for modelling the effective properties of a random two-phase composite material is presented. Such materials are commonly used as ultrasonic transducer matching layersor backing layers. The method is extended to incorporate a particle size distribution in the inclusion phase. Numerical issues regarding the implementation and convergence of the method are discussed. It is found that, for a given frequency of excitation, the calculated velocity for the composite has a distribution whose variance increases as the volume fraction of inclusions increases. The model predictions would suggest that to reliably and repeatedly manufacture these composites, with a desired mechanical impedance, a low volume fraction of inclusions should be used

A theoretical investigation of 2-2 composite transducers with high shear attenuation in the passive phase

This paper is about a theoretical investigation of 2-2 composite transducers with high shear attenuation in the passive phase. It was presented at the Étude de la propagation ultrasonore en milieux non homogènes en vue du contrôle non-destructif in 2007

Ultrasonic wave propagation in cylindrical vessels and implications for ultrasonic reactor design

Reactors in which processes are enhanced by ultrasound are hampered by the lack of a theoretical framework on their design. Simulation results of ultrasonic wave propagation in a cylindrical geometry are presented in this work, which are then used to develop guidelines for the design of ultrasonic reactors. These guidelines are used to design a new type of reactor with a novel geometry, operating at a frequency of 27kHz, 39kHz and 82kHz. This reactor is characterized using Weissler's reaction dosimetr

Improving the thermal stability of 1-3 piezoelectric composite transducers manufactured using thermally conductive polymeric fillers

With a view to improving the thermal stability of ultrasonic transducers prepared using 1-3 piezoelectric composites, the use of front face layers manufactured from thermally insulating and partially thermally conductive polymeric materials has been investigated. Experimentally, heat dissipation was investigated, in air and in water, using different transducer configurations and the advantage of including a front face layer manufactured from thermally conductive polymeric material is demonstrated. The PZFlex finite element modelling package was utilised to assess the thermal diffusivity of each polymer in the different transducer configurations and was found to compare well with experiment

Investigating the influence of the constituent materials on the performance of periodic piezoelectric composite arrays

This paper describes a theoretical investigation into the influence of the constituent materials on periodic composite array transducer performance. A finite element (FE) model, configured in PZFlex, is used to analyze the performance of a wedge coupled array transducer operating into a steel component. Here, the improvements offered by new single crystal piezoelectric materials are compared to standard PZT‐based configurations. In addition, new passive polymer materials, possessing low longitudinal loss and high shear loss, are evaluated for their potential to significantly reduce inter‐element mechanical cross talk. The FE results illustrate the potential for the next generation of array transducers incorporating these new materials and this is highlighted in the A‐scan predictions from simulated defects

Incorporation of viscoelastic loss into the plane wave expansion approach to modelling composite transducers

The plane wave expansion (PWE) method has been proposed as a theoretical model for periodic composite ultrasonic transducers. This paper extends previous work by importantly including viscoelastic loss in the material parameters. Some of the issues with model formulation, such as ill-conditioning in the large matrices, have been addressed through parameter scaling and Tikhonov regularisation. Identification of each mode of vibration has been carried out by visualising the spatial and temporal profiles of the displacement, electrical potential and Poynting vector. A comparison between the theoretical predictions and experimental data from a piezoelectric composite device is presented. The effect that the elastic properties of the passive phase have on device performance is also investigated. It is found that high shear attenuation in the passive phase gives rise to a large frequency stop band gap around the fundamental thickness mode

Experimental assessment of periodic piezoelectric composite arrays incorporating an anisotropic passive phase

This paper discusses the experimental assessment of a number of piezoelectric composite array structures incorporating a novel passive phase exhibiting anisotropic elastic properties. The passive polymer phase has been designed to limit inter-element crosstalk by attenuating lateral propagation across the array aperture. A selection of water coupled linear array coupons, operating with a nominal 400 kHz fundamental thickness mode frequency, has been prepared comprising the novel anisotropic passive phase. As a control, comparisons are made to similarly configured devices employing isotropic filler materials. Scanning laser vibrometry and measurements of electrical impedance characteristic on the array substrate demonstrate that the fundamental thickness mode of the devices configured with anisotropic polymer fillers is not contaminated by parasitic modes of vibration. The reasons for this are explained by considering the dispersion characteristics of the substrate. Water coupled hydrophone measurements of array element directivity; transmit voltage response and subsequently efficiency calculations illustrate that the observed reduction in mechanical cross talk has not been achieved at the expense of element sensitivity. Finally, comparisons between the experimental data and the PZFlex derived array responses are made, with good corroboration demonstrate