Investigation of high-temperature ultrasonic transducer design using lithium niobate piezocomposite

The design and fabrication of lithium niobate piezocomposite transducers aims to achieve high-temperature ultrasonic NDT measurements at 400ºC. In this paper, three aspects are investigated: exploring design parameters at room temperature by a comparison of experimental and modelled results for lithium niobate piezocomposites with an epoxy matrix; high-temperature electromechanical testing of lithium niobate piezocomposites with a cement matrix; and fabrication and testing of a high-temperature transducer operating at 400ºC for defect detection. The piezocomposites were made with a 1-3 structure using a y/36º-cut lithium niobate single-crystal material. The typical piezocomposite thickness was 1 mm, with a pillar width of 0.4-0.8 mm, a kerf width of 0.5 mm, a volume fraction of lithium niobate of 30-40% and a pillar aspect ratio (pillar height to pillar width) ranging from 1 to 6. The operating frequency of the samples was between 1 MHz and 4 MHz. The results of the lithium niobate piezocomposite with an epoxy matrix and pillar aspect ratios of 3, 4 and 6 indicated that a high coupling coefficient kt should be achievable, even with a low pillar aspect ratio. The piezocomposite material was heated in air to 400-500ºC over several thermal cycles and showed good stability of the electromechanical impedance spectrum. A 13 x 13 mm2 transducer with an operating frequency of 3 MHz was used to demonstrate the detection of an artificial defect in a steel block using a high-temperature couplant on a hot plate at 400ºC.