High-temperature piezoelectric film ultrasonic transducers by a sol-gel spray technique and their application to process monitoring of polymer injection molding

Thick-film (90 μm) piezoelectric ceramic high-temperature ultrasonic transducers (HTUTs) have been successfully deposited on metallic substrates by a sol-gel spray technique. The gel is composed of fine powders of bismuth titanate dispersed in a lead-zirconate-titanate solution. The films with desired thickness have been obtained through multilayer coating approach. Piezoelectricity is achieved using the corona discharge poling method. The center frequencies of ultrasonic signals generated by these HTUTs are around 10 MHz and their signal-to-noise ratio (SNR) is more than 30 dB in pulse-echo mode at 500°C. The main advantages of these new HTUTs are that they 1) are applicable at temperatures higher than 500°C, 2) are miniature, 3) can be coated on flat and curved surfaces, 4) do not need ultrasonic couplant, 5) can be operated at low and medium megahertz frequency range with sufficient frequency bandwidth, and 6) have sufficient piezoelectric strength and SNR. The ability of the HTUTs to monitor the polymer injection molding process in real-time at the mold insert of the machine is demonstrated.