Composite Monolithic Transducers
During the first few decades of ultrasonic NDT, transducers were based on solid disks of quartz or piezoelectric ceramics. More recently, composite transducers have been introduced as an alternative. While increased manufacturing costs make them more expensive, they have the advantage of significantly increasing sensitivity by as much as 12 dB over comparable conventional elements while maintaining broad bandwidth and a relatively fast pulse recovery time.
Composite transducers are made by dicing standard piezoelectric material into a grid. The spaces in the diced element are filled with epoxy, and the bottom is ground away to leave a series of tiny piezoelectric blocks in an epoxy matrix. Both sides are then plated for electrical contact. The raw material is cut to size in a square, rectangular, or circular shape depending on the model of transducer it will become.
In the resulting transducer element, the many piezoelectric blocks each act as point sources for spherical wave fronts that combine into a single wave in accordance with Huygens' Principle. Transducer sensitivity is increased because the individual piezoelectric blocks can more freely expand and contract as compared to a given point in the middle of a solid disk or plate. Additionally, the presence of the epoxy lowers the acoustic impedance of the transducer, creating more efficient sound coupling into wedges, delay lines, and water, as well as into nonmetallic test materials like composites and polymers. One potential disadvantage is in near surface resolution from the more freely resonating transducer in direct contact testing. This particular transducer manufacturing technique is of special relevance as it establishes the base line for phased array transducer construction and wavefront equivalencies.
