"We have already implemented our sensors at temperatures of up to 600°C. Generally speaking, temperatures of up to 900°C are possible," said Dr Bernhard Brunner.
Up to now, piezo sensors measuring pressure, force or voltage have been used to detect faults, such as cracks, in technical systems – but only to around 200°C.
According to the team, the ultrasonic sensors remain stable over long periods – at least two years in any use case – and for many applications, researchers expect a service life of several decades.
The challenge lies in constructing standard piezoelectric crystals that can withstand long-term use as sound transducers on hot components. Especially problematic is the adhesive that coats the sensors and attaches them to the component.
"That’s why we use glass solder as both a glue and a housing material," explained Dr Brunner.
This means the glass belonging to this group of adhesives must withstand not only heat, but also the several hundred degrees’ difference between the ambient temperature in the room and the operating temperature of the component.
While the steel in the component expands significantly when it is heated, the dimensions of the crystal change only marginally. The glass solder in which the sensor is embedded must endure these deformations without shattering.
To this end, the researchers coated the sensor with multiple layers consisting of different glass solders. To ensure that the electric signalling lines do not corrode in high temperatures, the feed lines are made of precious metals, such as platinum.
Possible applications for the high temperature ultrasonic transducer include sensors for contactless measurements of hot liquids – such as oil – flowing through pipes or of the temperature of a gas or a liquid by measuring the sound velocity.
