Richard O'Leary

Tapered transmission line technique based graded matching layers for thickness mode piezoelectric transducers

Conventionally, in order to acoustically match thickness mode piezoelectric transducers to a low acoustic impedance load medium, multiple quarter wavelength (QW) matching layers are employed at the front face of the device. Typically a number of layers, 2–4 in number, are employed resulting in discrete impedance steps within the acoustic matching scheme. This can result in impedance matching with limited bandwidth characteristics. This paper investigates the application of tapered transmission line filter theory to implement a graded impedance profile, through the thickness of the matching layer scheme, to solve the impedance mismatch problem whilst accounting for enhanced transducer sensitivity and bandwidth.

Improving the thermal stability of 1-3 piezoelectric composite transducers

The effect of temperature on the behavior of 1-3 piezoelectric composites manufactured using various polymeric materials was assessed experimentally through electrical impedance analysis and laser vibrometry. Device behavior varied with temperature irrespective of the polymer filler. Most significant changes in the piezoelectric composites were recorded around the glass transition temperature (T/sub g/) of the polymer; movement to lower fundamental resonant frequencies and higher values of electrical impedance minima were observed at higher temperatures. Decoupling of the pillars from the polymer matrix was observed by laser vibrometry at high temperatures. The use of high T/sub g/ polymer extended the operational temperature range of a piezoelectric composite, and a high T/sub g/ polymer with improved thermal conductivity also proved beneficial. For all devices, at temperatures very close to room temperature, subtle changes in device performance, linked to polymer softening were observed. Particulate-filled materials have been investigated, and it is recognized that the high viscosities and low mechanical damping of such materials could be problematic for piezoelectric composite manufacture. The thermal solver of the PZFlex finite element code has been used to predict the temporal and spatial temperature response of a selection of the devices presented. The simulated and experimental data compare favorably.

Experimental and theoretical evaluation of the thermal behaviour of 1-3 piezoelectric composite transducers

This paper describes progress towards the understanding of temperature effects in 1-3 piezoelectric composite transducers, carried out via a combination of experimental investigation and finite element (FE) analysis using the commercially available PZFlex code. The elastic properties and internal absorption of different passive materials are measured using a through transmission ultrasonic technique, with dynamic mechanical thermal analysis and differential scanning calorimetry being employed to evaluate the glass transition behaviour and specific heat capacities, respectively. The fillers are then incorporated into piezoelectric composite devices and the transducer performance measured over a wide temperature range by means of electrical impedance analysis and laser scanning of the active surface. The FE models are employed to predict the temperature distribution within such transducers as a function of constituent material properties and the data is correlated with the experimentally measured characteristics. The influence of glass transition temperature on viscoelastic properties is highlighted, along with the design compromises necessary to ensure effective high power performance.

Investigating the thermal stability of 1-3 piezoelectric composite transducers by varying the thermal conductivity and glass transition temperature of the polymeric filler material

The thermal behaviour of a number of 1-3 piezoelectric composite transducers is discussed. In particular, devices manufactured from a polymer filler with a relatively high glass to rubber transition temperature (T/sub g/), and from polymer systems with increased thermal conductivity, are evaluated. The mechanical properties of the various filler materials were obtained via ultrasonic measurements, with the thermal properties extracted using dynamic mechanical thermal analysis (dmta), differential scanning calorimetry (dsc) and laserflash studies. A range of ultrasonic transducers were then constructed and their thermal stability studied using a combination of impedance analysis and laser surface displacement measurement.

Analysis of ultrasonic transducers with fractal architecture

Ultrasonic transducers composed of a periodic piezoelectric composite are generally accepted as the design of choice in many applications. Their architecture is normally very regular and this is due to manufacturing constraints rather than performance optimization. Many of these manufacturing restrictions no longer hold due to new production methods such as computer controlled, laser cutting, and so there is now freedom to investigate new types of geometry. In this paper, the plane wave expansion model is utilized to investigate the behavior of a transducer with a self-similar architecture. The Cantor set is utilized to design a 2-2 configuration, and a 1-3 configuration is investigated with a Sierpinski carpet geometry. Ideally a single longitudinal mode in the thickness direction will drive the transducer in a piston-like fashion. In this paper it was found that by increasing the fractal generation level, the bandwidth surrounding the main thickness mode will increase, but there will be a corresponding reduction in the amplitude of the electrical conductance. It is also shown that a shift in the frequency of operation of the device can be achieved by altering the spatial periodicity of the electrical excitation.

Performance of periodic piezoelectric composite arrays incorporating a passive phase exhibiting anisotropic properties

This paper explores the minimisation of interelement cross talk in 1-D and 2-D periodic composite array structures through the incorporation of a passive phase exhibiting anisotropic elastic properties. Initially the PZFlex finite element code was used to monitor array aperture response as a function of material properties. It is shown that in array structures comprising passive polymer materials possessing low longitudinal loss and high shear loss, inter-element mechanical cross talk is reduced, without a concomitant reduction in element sensitivity. A number of polymer materials with the desired properties were synthesised and their elastic character confirmed through a program of materials characterisation. Finally, a range of experimental devices exhibiting improved directional response, as a result of a significant reduction in interelement cross talk, are presented and the predicted array characteristics are shown to compare favourably in each case.

Application of frequency compounding to ultrasonic signals for the NDE of concrete

In ultrasonic NDT of heterogeneous materials the internal microstructure of the material produces backscattered noise that can make the detection of true defects difficult. The noise is caused by the stationary scatterers that cause constructive and destructive interference to the propagating wavefront. Morevoer, in situations where the defects are significantly larger than these random scatterers, defects detection is significantly limited due to the presence of the additive noise caused by the interfering scatterers. This interference causes the presence of speckle noise in ultrasound imaging, thereby limiting the detectability, and making images generally difficult to interpret. Speckle also limits automated computer-aided analysis, such as edge detection, and 3D display. In this study, Frequency Compounding (FC) method has been applied to the ultrasonic signals in concrete. Due to the heterogeneous nature of the material, the received echoes suffer from clutter arising from the material microstructure. In this study, it was shown in simulations that the signal-to-noise ratio (SNR) of the A-scan signals can be improved by splitting the spectrum into narrower subbands and sum. Experiments were performed to validate the simulated results. The possibility of using this type of processing in such material and merits are discussed.

Harmonic analysis of lossy piezoelectric composite transducers using the plane wave expansion method

Periodic composite ultrasonic transducers offer many advantages but the periodic pillar architecture can give rise to unwanted modes of vibration which interfere with the piston like motion of the fundamental thickness mode. In this paper, viscoelastic loss is incorporated into a three-dimensional plane wave expansion model (PWE) of these transducers. A comparison with experimental and finite element data is conducted and a design to damp out these lateral modes is investigated. Scaling and regularisation techniques are introduced to the PWE method to reduce ill-conditioning in the large matrices which can arise. The identification of the modes of vibration is aided by examining profiles of the displacements, electrical potential and Poynting vector. The dispersive behaviour of a 2-2 composite transducer with high shear attenuation in the passive phase is examined. The model shows that the use of a high shear attenuation filler material improves the frequency band gap surrounding the fundamental thickness mode.

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 Weisslers reaction dosimetry.

The use of fractal geometry in the design of piezoelectric ultrasonic transducers

The geometry of composite piezoelectric ultrasonic transducers is typically regular and periodic with one dominant length scale. In many applications there is motivation to design transducers that operate over a wide bandwidth so that, for example, signals containing a broad frequency content can be received. The devices length scale will dictate the central operating frequency of the device and so, in order to construct a wide bandwidth device, it would seem natural to design a device that contains a range of length scales. The objective of this article therefore is to consider one such transducer design and build a theoretical model to assess its performance. For the composite geometry a fractal medium is chosen as this contains a wide range of length scales. Numerical results of a theoretical model are presented. They suggest that this device would have a three-fold improvement in the reception sensitivity bandwidth as compared to a conventional composite design. Finite-element analysis provides information on the effect of poling on the devices performance. A preliminary experimental investigation was undertaken, with a Sierpinski gasket fractal transducer design, and good correlation between the simulated and experimentally measured operation was observed.