1 Mn - doped CaBi 4 Ti 4 O 15 / Pb ( Zr , Ti ) O 3 Ultrasonic 2 Transducers for Continuous Monitoring at Elevated 3 Temperatures 4

Continuous ultrasonic in-situ monitoring for industrial applications is difficult owing to 11 the high operating temperatures in industrial fields. It is expected that ultrasonic transducers 12 consisting of CaBi4Ti4O15(CBT)/Pb(Zr,Ti)O3(PZT) sol-gel composite could be one solution for 13 ultrasonic nondestructive testing (NDT) above 500 °C because no couplant is required and CBT has 14 a high Curie temperature. To verify the high temperature durability, CBT/PZT sol-gel composite 15 films were fabricated on titanium substrates by spray coating, and the CBT/PZT samples were tested 16 in a furnace at various temperatures. Reflected echoes with a high signal-to-noise ratio were 17 observed up to 600 °C. A thermal cycle test was conducted from room temperature to 600 °C, and 18 no significant deterioration was found after the second thermal cycle. To investigate the long-term 19 high-temperature durability, a CBT/PZT ultrasonic transducer was tested in the furnace at 600 °C 20 for 36 h. Ultrasonic responses were recorded every 3 h, and the sensitivity and signal-to-noise ratio 21 were stable throughout the experiment. 22