This ultrasonic, pulse-echo probe can sustain as high as 250 ℃, and uses a piezoelectric transducer to generate and receive the ultrasonic pulses. The transducer is made of a piezoelectric material with high Curie temperature, and the probe is configured such that it is operated as air-backed and, thus, has minimum losses of power. For use of the piezoelectric transducers at high temperature, other aspects need to be considered, such as phase transition, thermal aging, electrical resistivity, chemical stability (decomposition and defect creation), and the stability of properties at elevated temperatures. Among them, the phase transition at elevated temperature is the critical limitation as the transducer is permanently depolarized at a certain temperature, known as the Curie point, and cannot be used for transducer applications. Although the piezoelectric materials that possess high Curie points ＞500 ℃ are available, such as bismuth layer (BLSF), LiNbO_3, and quartz, the issues associated with these materials are that the transducer properties are considerably lower than conventional piezoelectric material such as lead zirconate titanate (PZT).
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