There are many fields such as online monitoring of manufacturing processes, non-destructive testing in nuclear plants, or corrosion rate monitoring techniques of steel pipes in which measurements must be performed at elevated temperatures. For that high temperature ultrasonic transducers are necessary. In the presented paper, a literature review on the main types of such transducers, piezoelectric materials, backings, and the bonding techniques of transducers elements suitable for high temperatures, is presented. In this review, the main focus is on ultrasonic transducers with piezoelectric elements suitable for operation at temperatures higher than of the most commercially available transducers, i.e., 150 °C. The main types of the ultrasonic transducers that are discussed are the transducers with thin protectors, which may serve as matching layers, transducers with high temperature delay lines, wedges, and waveguide type transducers. The piezoelectric materials suitable for high temperature applications such as aluminum nitride, lithium niobate, gallium orthophosphate, bismuth titanate, oxyborate crystals, lead metaniobate, and other piezoceramics are analyzed. Bonding techniques used for joining of the transducer elements such as joining with glue, soldering, brazing, dry contact, and diffusion bonding are discussed. Special attention is paid to efficient diffusion and thermo-sonic diffusion bonding techniques. Various types of backings necessary for improving a bandwidth and to obtain a short pulse response are described.

Usually for non-destructive testing at high temperatures, ultrasonic transducers made of PZT and silver electrodes are used, but this could lead to damage to or malfunction of the ultrasonic transducer due to poor adhesion between PZT and silver. Soldering is one of the most common types of bonding used for individual parts of ultrasonic transducers (protector, backing, matching layer, etc.), but silver should be protected using additional metal layers (copper) due to its solubility in solder. A mathematical modelling could help to predict if an ultrasonic transducer was manufactured well and if it could operate up to 225 °C. The observed von Mises stresses were very high and concentrated in metal layers (silver and copper), which could lead to disbonding under long-term cyclic temperature loads. This paper presents a multilayer ultrasonic transducer (PZT, silver electrodes, copper layers, backing), which was heated evenly from room temperature to 225 °C and then cooled down. In the B-scan, it was observed that the amplitude of the reflected signal from the bottom of the sample decreased with an increase in temperature. However, after six heating–cooling cycles, the results repeated themselves and no signs of fatigue were noticed. This ultrasonic transducer was well manufactured and could be used for non-destructive testing when the environment temperature changes in cycles up to 225 °C.

Non-destructive monitoring of reactor vessel walls in heavy-liquid-metal-cooled systems requires ultrasonic probes capable of stable long-term operation at elevated temperature and under challenging acoustic coupling conditions. In lead–bismuth eutectic (LBE) cooled reactors, defect detectability can be affected by temperature gradients, oxide-layer formation, wetting behaviour and changes in the coupling interface. This study evaluates a custom-developed high-temperature ultrasonic transducer for in-situ detection of notch-type defects in a stainless-steel reactor-vessel mock-up under laboratory and LBE conditions. The methodology combines cyclic laboratory testing in air, thermomechanical modelling of the mock-up during immersion in the Helios 3 LBE facility, in-situ ultrasonic measurements and analytical/experimental assessment of coupling-layer thickness effects. Laboratory tests confirmed repeatable defect detection over multiple heating–cooling cycles up to 200 °C, with amplitude reduction during heating followed by near recovery during cooling, indicating no irreversible transducer degradation. In the Helios 3 LBE facility, the defect remained detectable during two heating–cooling cycles, although the reflected amplitude recovered only partially after cooling. Inspection of the probe showed no evidence of permanent degradation, suggesting that the incomplete recovery was mainly caused by changes in acoustic coupling rather than transducer failure. Analytical modelling and validation experiments showed that an increase of only 18 µm in the coupling gap can produce an approximately −6 dB signal loss. These results demonstrated that the developed transducer is suitable for high-temperature defect detection in representative LBE environments and that coupling-layer stability is a key factor for reliable ultrasonic monitoring of LBE-cooled reactor components.