Abstract
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.
Highlights
Ultrasonic measurements and non-destructive testing (NDT) are already used in extreme conditions such as high temperature, pressure, corrosive environments, radioactive radiation, etc. Such conditions are met during online monitoring of manufacturing processes at elevated temperatures, non-destructive testing in nuclear plants, measurements and inspections in the aerospace field, etc. [1]
Ultrasonic transducers used in hazardous conditions such as a high temperature, a corrosive environment and a radioactive radiation can be classified into three main types: 1. Ultrasonic transducers with a thin protective layer; 2
It allows getting a stable connection among layers in a wide temperature range and to achieve a reliable operation of the transducer. This transducer was able to produce clear echo signals up to 800 ◦C and to maintain full functionality after cooling to room temperature [33]. As it was mentioned above, one of the most important requirements for the high temperature ultrasonic transducers used in hot liquid sodium is wettability of the protective layer
Summary
Ultrasonic measurements and non-destructive testing (NDT) are already used in extreme conditions such as high temperature, pressure, corrosive environments, radioactive radiation, etc. In Russia the Industrial and Nuclear Supervision Service Rostechnadzor in the year 2021 issued a license for construction of the 300 MWe fast neutron reactor BREST-OD-300 cooled with a lead-coolant [17,18]. They claim that the necessary regulations for safe operation of the reactor and pipelines were developed. The European project PASCAL can be mentioned because it is devoted to support the ongoing pre-licensing processes of fourth generation heavy liquid metal cooled nuclear reactors ALFRED and MYRRHA [22].
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