D. MacLennan

2F-6 Properties and Application-Oriented Performance of High Frequency Piezocomposite Ultrasonic Transducers

Routine high frequency piezocomposite fabrication is now becoming possible. However, specialized characterisation techniques and reference results are absent from the literature and there is an urgent demand for characterisation to support the use of this material in transducers for biomedical diagnosis and non-destructive evaluation. In this paper, we report on a novel procedure for high frequency piezocomposite fabrication and results of intensive characterisation based on electrical impedance spectroscopy and other techniques. 1-3 composite structures have been made with circular PZT pillars with diameter and kerf down to 12 mum and 6 mum respectively, aspect ratio > 3, volume fractions around 40% and active area of around 4 mm using a process which is capable of scalable commercial production. Experimental electrical impedance measurements have proved to be closely matched to one-dimensional models and piezocomposite material performance greatly exceeds that of other materials such as PVDF and LiNbO3. Comprehensive characterization of practical performance has been carried out including both through-transmission and pulse-echo measurements, generating single point results and high resolution 1D and 2D scans to produce 2D and 3D data sets, including tungsten wires test objects to determine axial and lateral resolution and transducer sensitivity. A typical prototype focused transducer is described here, with f- number of 4 and centre frequency of 37 MHz. Performance investigation yielded a through-transmission -3 dB bandwidth of 24 MHz, pulse-echo -6 dB bandwidth of 22 MHz, and lateral and axial resolutions of 250 mum and 150 mum respectively.

Net-shape ceramic manufacturing as an aid to realize ultrasonic transducers for high-resolution medical imaging

High frequency ultrasonic transducers are in demand for medical imaging procedures requiring high spatial resolution. However, cost-effective fabrication for frequencies above approximately 20 MHz is challenging. One of the problems is the need for a thin layer of piezoelectric material. This is difficult because typical thick and thin film processes produce material which is too thin and lapping bulk materials is expensive and inefficient. In this paper, net shape ceramic processing is reported as an alternative. This can be achieved with viscous polymer processing followed by calendering of the green state material to produce thin sheets which can be dried, sintered and then cut to shape. Although such thin specimens are fragile, the addition of a supportive acoustic backing material allows straightforward processing into the final ultrasonic transducer. Here, piezoceramic made with TRS600FG material is reported, finished to thicknesses of 50 µm and 110 µm. The behaviour of these samples has been found to be similar to bulk material, for example with a thickness mode coupling coefficient, kT, of 0.52 and relative permittivity, eR S , of 1540. Prototype ultrasonic transducers with element diameters of a few millimetres have been made and operated at frequencies approaching 50 MHz. Testing has been performed underwater and the successful results suggest that net shape ceramic manufacturing is compatible with the fabrication of high frequency ultrasonic transducers.

P3K-5 Passive Materials for High Frequency Ultrasound Components

This study reports the design of materials suitable for backing in piezocomposite devices specifically operating at frequencies above 30MHz. Tungsten loaded epoxy backing samples were made with varying volume fractions with l-5 mum tungsten powder and RX771/HY1300 epoxy. Higher volume fractions were achieved with high shear milling exploiting a new material fabrication technology using PVB polymer, without the need for pressing. Acoustic measurements were carried out at 36MHz to characterise the samples for use in both single element transducers and arrays. The variation in longitudinal and shear wave velocities and acoustic impedance follow the Devaney scattering model with the acoustic impedance ranging from 3-15MRayls. The results show that this material is suitable for use as backing in high frequency piezocomposite devices and acoustic properties can be tailored by adjusting the volume fraction of tungsten.

P3Q-1 Ultra Precision Grinding in the Fabrication of High Frequency Piezocomposite Ultrasonic Transducers

High frequency ultrasonic transducers are needed for high spatial resolution measurements in applications such as medical diagnosis and nondestructive testing. However, cost-effective fabrication of high performance transducers with frequencies above 20 MHz is challenging because of the need for a thin layer of active material. Piezocomposites are the material of choice in such transducers at lower frequencies, but current fabrication methods cannot easily achieve sufficiently thin active layers. Commercially, piezocomposite is usually finished to thickness by grinding, providing surface finish acceptable for most applications. However, conventional grinding is insufficiently precise for high frequency operation and is subject to undesirable intra-process variation. The most widely used alternative is precision lapping and polishing, but this is slow and therefore expensive. In the work reported here, an alternative process of ultra precision grinding was studied, using the Loadpoint PicoAce machine operating in the ductile machining mode. To determine the capabilities of this machine for piezocomposite processing, 1-3 connectivity material was fabricated using both a standard commercial process and a novel approach based on viscous polymer processing. Unsupported layers of piezocomposite of thickness much less than 100 mum have been achieved with surface roughnesses of less than 1 mun and minimal discontinuity between the ceramic and polymer phases. These results suggest that ultra precision grinding may have a role to play in practical implementation of high frequency piezocomposites for ultrasonic transducers

Fundamental performance characterisation of high frequency piezocomposites made with net-shape viscous polymer processing for medical ultrasound transducers

High frequency ultrasound (HFUS) transducers are in demand for medical diagnoses requiring high spatial resolution. Compared with bulk piezoceramics, conventional crystals, and piezopolymers, piezoceramic-polymer composites have highly desirable properties such as improved piezoelectric coupling and acoustic matching to tissue. However, for 30 MHz operation, a typical 1-3 piezocomposite is approximately only 50 mum thick, requiring ceramic pillar widths of around 15 mum or less. Fabrication is thus challenging with dice-and-fill technology. This may be overcome using micromoulding of ceramic paste produced by viscous polymer processing (VPP). This brings a need for material characterisation to support the new approach but conventional techniques cannot be used due to the small structures involved. This paper therefore reports characterisation using electrical impedance spectroscopy followed by data fitting to a theoretical model based on the 1D piezoelectric wave equation and homogenized piezocomposite model of Smith and Auld. Results are presented from multiple VPP and HFUS piezocomposites spanning a development program of several years. The piezocomposites investigated showed effective piezoelectric properties in the following ranges: c33: 1.76-6.77 (times 1010 Nm2, e33: 2.04-8.50 (Cm-1), eR S: 70.6-460, d33: 7.82-12.7 (times 10-11 mV-1), h33: 2.01-2.09 (times 109 Vm-1) and kT: 0.45-0.51. These data confirm that the VPP fabrication method has good potential for HFUS piezocomposites.