High-temperature piezoelectric crystals, including YCa4O(BO 3)3, LiNbO3, and AlN, have been studied for use in ultrasonic transducers under continuous operation for 55 h at 550 °C. Additionally, thermal ratcheting tests were performed on the transducers by subjecting the crystals to heat treatments followed by ultrasonic performance testing at room temperature and 500°C. The changes resulting from the heat treatments were less than the statistical spread obtained in repeated experiments and were thus considered negligible. Finally, in situ measurements of the pulse-echo response of YCa4O(BO 3)3 were performed at temperatures up to 950 °C for the first time, showing stable characteristics up to these high temperatures. © 1986-2012 IEEE.