Single element and linear array high-frequency ultrasonic transducers using PZT sol-gel composites

Sol gel composite processing, invented at Queen's University, is a simple and inexpensive method for fabricating ceramic layers 5--100 mum thick. The composite technology has been refined and developed for the specific application of small area single element and linear array ultrasonic transducers for medical imaging.;A one-dimensional impedance method for characterizing the coatings has been developed. The thickness mode elastic, dielectric and piezoelectric parameters of the piezoelectric coating and the thickness mode elastic parameter of the substrate are determined. All parameters are complex to model the losses within the composite and the substrate. The parameters are determined using a modified Levenberg-Marquardt non-linear regression routine. From the results, the thickness extensional electromechanical coupling coefficient can be calculated. The real components of the properties of PZT sol gel composites are: an elastic stiffness of 2.5--9.7 x 1010 N/m3, a relative dielectric constant of 200--400 and a thickness extensional electromechanical coupling coefficient of 0.2--0.35. These parameters depend on the type of sol gel composite recipe used and depend on the substrate on which the coating is grown.;Single element devices in the frequency range of 70--160 MHz have been fabricated. They have fractional bandwidths of up to 52% and minimum insertion losses of up to -46 dB. Real time grayscale images show good detail of microscopic tissue structures.;Laser micromachining techniques have been developed to ablate >5 mum wide features, with an aspect ratio >3, into ceramics and provides the versatility to micromachine complex arbitrary structures. A KrF excimer laser (248 nm) has been used to ablate 40 and 60 MHz linear array structures in PZT sol gel composites and bulk hot isostatic pressed (HIP) PZT with up to 192 and 17 elements respectively. Impedance and pulse-echo experiments demonstrate that the sol gel composite ceramic arrays are still piezoelectric after laser machining. This represents a technological first for bulk wave high frequency ultrasonic arrays.