Investigation of High-Temperature Ultrasonic Transducer Design Using Lithium Niobate Piezocomposite

Design and fabrication of lithium niobate piezocomposite transducers is aimed at achieving high temperature ultrasonic NDT measurements at 400°C. We investigated three aspects: exploring design parameters at room temperature by comparison of experimental and modelled results for lithium niobate piezocomposites with epoxy matrix; high temperature electromechanical testing of lithium niobate piezocomposites with cement matrix; and fabrication and testing of a high temperature transducer operating at 400°C for defect detection. The piezocomposites were made with a 1-3 structure using y/36°-cut lithium niobate single crystal material. Typical piezocomposite thickness was 1 mm, pillar width 0.4-0.8 mm, kerf width 0.5 mm, volume fraction of lithium niobate 30-40%, and pillar aspect ratio (pillar height to pillar width) range from 1 to 6. Operating frequency of the samples was between 1 and 4 MHz. Results on lithium niobate piezocomposite with an epoxy matrix with pillar aspect ratio 3, 4, and 6 indicated that high coupling coefficient kt should be achievable even with low pillar aspect ratio. Piezocomposite material was heated in air to 400-500°C over several thermal cycles and showed good stability of the electromechanical impedance spectrum. A 13 x 13 mm 2 transducer with operating frequency 3 MHz was used to demonstrate detection of an artificial defect in a steel block using high temperature couplant on a hot plate at 400°C.