Air-coupled ultrasonic transducers for measurement of green-state ceramics at elevated temperatures

Air-coupled ultrasonic transducers have been used to study the properties of green-state ceramic components. The work used electrostatic (capacitance) transducers. In a first set of experiments, the transducers were C-scanned over the entire area of selected disc-shaped samples to locate changes in structure that could lead to problems in the firing stage. The transducers were modified subsequently to operate at elevated temperatures for monitoring the manufacturing process, and it was shown that good sensitivity could be obtained in silicon nitride samples at temperatures in excess of 200°C. It was shown that ultrasonic velocity was related to both softening of the polymer binder and the change in weight of the samples as they were heated, the latter giving an indication of polymer burn-out rates