Interest of optimizing the response of a high-temperature ultrasonic transducer by integrating a porous aluminium backing element for the detection of structures immersed in liquid sodium

With the emergence of major applications in harsh environments (such as the new generation of nuclear reactors and power plants, under sodium visualization reactors...), the scientific and industrial community expresses a real need for developing a range of instrumentation dedicated to monitor, control, and check resistance to ageing and damage of a large panel of structures. Ultrasonic transducers are particularly well adapted to address this issue. Up to date, there are no commercial solutions for this type of application and only a few prototypes of ultrasonic transducers exists. Thanks to the use of a specific backing material produced by uniaxial pressing of an aluminium powder containing a small amount of wax followed by sintering, we designed two ultrasonic prototypes transducers dedicated to continuous high temperature experiments. The first one is a PZ27 based 2 MHz piezo-element and the second one a Cerium-modified NBT based 10 MHz. These sensors, with a thickness of 5 mm, tested on a steel sample from room temperature to 140°C (for PZ27) and 335°C (for NBT), turned out to be entirely consistent with what one might expect for echographic time-resolved measurements. Furthermore, temperature-dependent velocity propagation relationships obtained on steel were consistent with the literature. Given their small thickness, they might be easily integrated into various experiments in harsh environments, where space is limited: measurements in sodium reactors, nuclear fuel assemblies, narrow zones in aeronautics or petrochemical industry etc.