Directly mounted sol-gel composite piezoelectric devices by spray coating technique for high temperature ultrasonic transducers

In this paper, we propose a fabrication method of a high temperature-resistant ultrasonic transducer for non-destructive tests. In typical pipe non-destructive wall thickness measurements, couplant material and backing material which do not have heat resistance are necessary. Therefore, there are restrictions for the practical online pipe thickness monitoring in chemical and power plants. To avoid a limitation of these non-heat-resistant material, we propose a couplant-less transducer fabrication method by attaching a piezoelectric film directly on the tested object with the spray coating method. Our method is based on the sol-gel spray technique to fabricate a piezoelectric ceramic film. With the sensor fabricated with this method, we measured the wall thickness under high temperature (100, 200, 300, 400, 500, 600°C) conditions. From the results of the high-temperature experiment, it was found that the sensitivity decrease due to temperature rise was limited to 3dB and the difference of frequency response was also limited to 2 MHz at the second peak frequency at the maximum. According to the results, the performance deterioration due to high temperature was within an acceptable range. Additionally, the wall thickness could be calculated from reflected pulse waves. We concluded that sufficient availability of the piezoelectric film under high temperature were confirmed. This paper describes the effectiveness of piezoelectric film attached directly on the tested object.