LiNbO3 (LN)-Based sol-gel composites are suitable for high-temperature ultrasonic transducer applications at 700 °C; however, poling requires high temperatures (>550 °C) and shows relatively low signal strength. To realize the polarization temperature reduction of the LN-based sol-gel composite ultrasonic transducer, alumina (AO) and strontium-doped titanium oxide (TO) were chosen as the sol-gel phase material, and LN/AO and LN/TO films were fabricated on an Inconel substrate. In each poling process, corona discharge was executed for LN/AO and LN/TO after heat treatment in the furnace at 400 °C and 200 °C, respectively. Ultrasonic measurements up to 700 °C were performed, and both transducers confirmed the reflected echoes and ultrasonic performance stability at 700 °C. In particular, LN/TO showed the highest signal strength compared to the previously developed high-temperature lead-free sol-gel composite materials, LN/BiT, Bi4Ti 3O12(BiT)/BiT, and CaBi2Ta2O9 (CBTa)/BiT.

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