Progress in Non-Contact Acoustic Methods

Four non-contact acoustic systems have been developed for the purpose of materials characterization. These systems rely upon capacitance and photoacoustic transducers, which are capable of generating and receiving acoustic waves in materials without contact. The first non-contact system employs capacitance transducers to study the vibrations of miniaturized tuning forks, whose resonant frequencies alter with changing temperature and material properties. Both single-crystal silicon and Zr-2.5wt%Nb tuning forks are investigated at temperatures up to 700^ circC, where the system's advantages for quantitative measurements of phase transformations in solids are clearly demonstrated. The second non-contact system, a photoacoustic (laser) system, also investigates materials at high temperature ({<=}800^circC), by measuring absolute velocities of ultrasonic surface waves as a function of temperature. This is accomplished with <0.5% accuracy and without the need for thermal expansion corrections. The third non-contact system employs an air-gap capacitance transducer with guard -ring to generate wideband ultrasound in solid media. The measured characteristics of this transducer agree so closely with established theory that the device approaches the first true standard source for ultrasound in solids. In the fourth and final system, an air-coupled capacitance transducer is developed using micromachining techniques in order to generate and detect ultrasonic waves in materials through an intermediary air gap. Its high sensitivity and wideband frequency response have allowed a wide range of material applications to be investigated. In general, the fact that the four systems do not contact the materials gives them significant advantages in accurate quantitative measurements, standards developments, and demanding material environments. Of particular interest are the demonstrated advantages of using micromachining and single-crystal silicon for the accurate fabrication of transducers and for the calibration of acoustic measurement systems.