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Phoenix666 [soylentnews.org] writes:

Technical systems must be regularly checked for defects, such as cracks. Up to now, piezo sensors measuring pressure, force or voltage have been used to reliably detect such faults – but only to around 200 degrees Celsius. Now, special high-temperature piezo sensors can continuously monitor components that are as hot as 900 degrees Celsius. Fraunhofer researchers will present their development at the SENSOR+TEST measurement fair in Nürnberg, May 30 to June 1, 2017.

If a component such as a steam pipe in a coal-fired power station has a crack, corrosion or other flaw, repairing it is imperative. Ultrasonic sensors mounted externally can detect flaws like these, but only when the component does not heat up to more than around 200 degrees Celsius. Above that temperature, conventional piezoelectric materials can no longer determine pressure, force, voltage or acceleration or act as a gas sensor. Furthermore, at these temperatures any plastic encapsulations that are not heat-resistant will fail.

First sensors for high-temperature applications

Researchers at the Fraunhofer Institute for Silicate Research ISC have now successfully realized piezo sensors for high-temperature applications [phys.org]. "We have already implemented our sensors at temperatures of up to 600 degrees Celsius. Generally speaking, temperatures of up to 900 degrees Celsius are possible," says Dr. Bernhard Brunner, head of the Application Technology department at Fraunhofer ISC's Center Smart Materials. Additionally, 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 principle is the same as for other piezo sensors: they are mounted externally on the component, for instance on a hot steel pipe. When an alternating voltage is applied to the piezoelectric crystal, it mechanically deforms and sends an ultrasonic wave into the material. After the sound wave, the sensor switches to receive and detects the signal reflected by the component. In most cases, it receives the same original signal it sent. However, if the component is cracked or has a corroded spot, the defect alters the reflected signal and indicates the defect's location. When several transducers are used that serve as transmitter and receiver, the location of the flaw can be pinpointed exactly to within a few millimeters. Depending on the component's material, the sensor's range covers a few meters.

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