Aging Aircraft NDE With Micromachined Ultrasonic Air Transducers

Due to the large impedance mismatch between common piezoelectric materials and air, conventional piezoelectric transducers are not very efficient sources of ultrasound in air. Therefore, piezoelectric nondestructive evaluation NDE systems must use a coupling fluid to improve the power transfer to the sample. Use of a coupling fluid or immersion of the sample complicates inspection, and in some cases is undesirable. Air-coupled ultrasonic systems are preferable as long as efficient transducers are available for transferring ultrasound into air. This report discusses some capacitive micro machined ultrasonic transducers CMUTs that have more than 100 dB 113 dynamic range in a bistatic transmission system. These transducers consist of thousands of 1 m-thick silicon-nitride membranes, resonating at 2-3 MHz electrically connected in parallel. Equivalent circuit modeling of the transducers provides insight into the design of the devices and enables accurate predictions of CMUTs behavior in NDE systems. Finally, the report concludes with some discussion of current research in developing efficient, wide-band CMUTs.