PMN–PT single-crystal transducer for non-destructive evaluation

A lead magnesium niobate–lead titanate (PMN–PT) single-crystal disc was used to construct an ultrasonic transducer for non-destructive evaluation (NDE). The surface displacements (at the thickness mode resonance) of the PMN–PT single crystal and a PZT disc of similar dimensions were measured using a laser scanning technique. It was observed that the PMN–PT disc has rather uniform ultrasonic displacements across its surface while the PZT exhibited an interference pattern indicative of coupling of the radial mode vibration with the thickness mode. This is consistent with the electrical impedance versus frequency measurements showing that for the PZT disc, a strong radial mode with many harmonics were clearly observed, and unwanted peaks existed near the thickness mode resonance. However, for the PMN–PT single-crystal disc, many resonance peaks were observed near the radial mode resonance but the harmonics of the radial mode cannot be identified. The PMN–PT disc has a clean thickness mode resonance peak and a high thickness electromechanical coupling coefficient. The discs were mounted in stainless steel housings with appropriate electrical connections to form transducers. Tungsten/epoxy backing and aluminum/epoxy front-face matching were incorporated to provide the necessary performance of very short ringdown times required for NDE applications. The characteristics of the PMN–PT transducer were compared with that of a PZT transducer of similar construction.