Abstract
Long-term installation of ultrasonic transducers in high temperature environments allows for continuous monitoring of critical components and processes without the need to halt industrial operations. Transducer designs based on the high-Curie-point piezoelectric material lithium niobate have been shown to both be effective and stable at extreme temperatures for long-term installation. In this study, several brazing techniques are evaluated, all of which aim to provide both mechanical bonding and acoustic coupling directly to a bare lithium niobate piezoelectric element. Two brazing materials—a novel silver-copper braze applied in a reactive air environment and an aluminum-based braze applied in a vacuum environment—are found to be suitable for ultrasound transmission at elevated temperatures. Reliable wide-bandwidth and low-noise ultrasound transmission is achieved between room temperature and 800 °C.
Highlights
Contact ultrasound inspection is a common technique for industrial non-destructive evaluation (NDE)
This study aims to identify suitable and reliable brazing techniques for mechanically and acoustically bonding to bare, non-metallized lithium niobate piezoelectric elements
Zirconia is an oxide-containing ceramic material with several material characteristics in common with lithium niobate, including coefficient of thermal expansion (CTE), such that it is a suitable mate for brazing trials with LiNbO3 [11]
Summary
Contact ultrasound inspection is a common technique for industrial non-destructive evaluation (NDE). Example applications are monitoring the integrity of critical components and process parameters in electric power generation stations and petrochemical plants. It is often desirable for long-term installation of ultrasonic transducers in elevated temperature environments to allow continuous on-line monitoring. This eliminates the need for costly plant shutdowns in order to carry out NDE tests, and enables earlier warning of serious problems. A common restriction on this technology is that specimens being evaluated must have polished or otherwise smooth surfaces, successful ultrasound detection of cracks at temperatures up to 1000 ◦ C has been demonstrated with reasonable sensitivity in materials with rough surfaces [1]
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