Brazing Strategies for High Temperature Ultrasonic Transducers Based on LiNbO3 Piezoelectric Elements

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.

Dry Coupling of Ultrasonic Transducer Components for High Temperature Applications

“…This optimal bandwidth figure was consistent with previous studies where a lithium niobate crystal was brazed onto the porous zirconia backing with AgCu braze foil [7,8].…”

“…The suitable transducer components identified for high-temperature application were a rhombohedral 36 • Y-cut lithium niobate piezoelectric crystal, a porous zirconia mechanical backing, and a stainless-steel 321 protective layer [5,7]. Methods for acoustically coupling the backing to the piezoelectric element were investigated in those earlier studies; a brazing system based on a silver-copper 72-28 braze (AgCu) was developed, but led to high reject rates of transducers due to cracking of the piezoelectric element [7,8].…”

High Temperature Ultrasonic Transducers: A Review

“…Bosyj et al tested brazing techniques for ultrasonic transducers with LiNbO 3 for high temperatures applications [ 119 ]. There are two types of brazing materials: active and passive.…”

“…Comparing TiBraze ® Al-665 (Hilliard, OH, USA), Morgan Gold-ABA ® (Hayward, CA, USA), Gold, and AgCu brazes and brazing techniques (vacuum brazing and reactive air brazing), the best results were obtained using TiBraze ® Al-665 and AgCu. The transducers, made using those brazes, were able to transmit and receive signals up to 800 °C [ 119 ].…”