AIR-COUPLED ULTRASONIC TRANSDUCERS FOR MEASUREMENT OF GREEN-STATE CERAMICS AT ELEVATED-TEMPERATURES

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