This ultrasonic, pulse-echo probe can sustain as high as 250 °C, 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.

An illustration of the health monitoring system and the pulse echo method of measuring condensed water height using time-of-flight of reflected ultrasonic pulses.
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 °C are available, such as bismuth layer (BLSF), LiNbO3, 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).

The health monitoring system working at high temperatures >200 °C requires high-performance piezoelectric transducers as the steam pipes involve several issues such as the effect of the pipe curvature that causes ultrasonic wave losses and increased attenuation at high temperatures. These effects greatly reduce the sensitivity, preventing the ultrasound wave from propagating through material media in steam pipe systems.

In order to meet the requirements of high Curie point and high piezoelectric properties, a modified Navy type II (known as PZT5A) was selected because this material family offers a combination of high piezoelectric properties and high Curie temperatures. Based on the preliminary results of tests up to 250 °C, a probe that was developed using a Type II piezoelectric transducer yielded satisfactory bandwidth and sensitivity with high thermal stability.

Although both ceramics showed similar transducer performance below 250 °C, TRS203 ceramics possess higher transition temperature compared to conventional type II ceramics, allowing for sensing over a broader temperature range.

This work was done by Yoseph Bar-Cohen, Xiaoqi Bao, and Stewart Sherrit of Caltech; and Hyeong Jae Lee, Post Doc., for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
JPL
Mail Stop 321-123
4800 Oak Grove Drive
Pasadena, CA 91109-8099
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Refer to NPO-49045.