EVOLUTION OF PIEZOELECTRIC TRANSDUCERS TO FULL SCALE NON-CONTACT ULTRASONIC ANALYSIS MODE*

Abstract: While non-contact – air/gas coupled – uses of sound waves are buried in the antiquities of our civilization, in modern times, presumably one of the first applications is related to Antarctic ice thickness measurement by sending high intensity sound waves from an airplane in the 1920s. Initial industrial Non-Contact Ultrasound (NCU) was confined between 20 kHz to <100 kHz. The primary hurdle to NCU is the transducer inefficiency in air/gases. This complication is further exacerbated in MHz regime. Consequently, NCU was not taken seriously. During the last 25 years a handful of laboratories have endeavored to develop NCU transducers by applying a variety of plastics, elastomers, and density gradient Z-matching layers on piezoelectric materials. While the transducer efficiency is increased, yet it is not significant for practical NCU. Recent transducer advances have phenomenally enhanced the efficiency between <60 kHz to ~10 MHz. Further increment of efficiency will depend on the piezoelectric material. To this effect, a new piezoelectric composite characterized by the highest possible coupling, has elevated NCU transducers to new heights. As a result of these unusual developments, NCU is not only a reality, but it has also opened doors to ultrasound, hitherto, closed to conventional contact or liquid coupled modes. In this paper we review the significance of materials characterization in light of non-contact ultrasound. Also provided are NCU modus operandi, applications, limitations, and recommendations