Non-destructive evaluation of green-state ceramics using micromachined air-coupled capacitance transducers

Air-coupled ultrasound has been used to study the manufacture of ceramic components by both slip casting and injection moulding. A capacitance transducer, designed to operate at high temperatures, was used as both a source and receiver with a 500 kHz bandwidth to pass ultrasound through 3 mm thick green-state injection-moulded and slip-cast samples of silicon nitride. The transducers were C-scanned over the entire area of the sample in order to locate density changes which could lead to problems in the firing stage. Additionally: through-transmitted waveforms at 150 degrees C were obtained, indicating that the technique may be useful for performing measurements at high temperatures. (C) 1998 Elsevier Science B.V.