Heat-Resistant Ultrasonic Transducers

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Ultrasonic sensors can be used to detect cracks, corrosion or other flaws in a component. However, the components must not be hotter than 200 °C since, above this temperature, conventional piezoelectric materials can no longer determine pressure, force, voltage or acceleration, or act as a gas sensor. Furthermore, any plastic encapsulations that are not heat-resistant will fail. Researchers at the Fraunhofer Institute for Silicate Research ISC have therefore developed piezo sensors for high-temperature applications, which are expected to be suitable for components with temperatures up to 900 °C. Furthermore, the ultrasonic sensors should remain stable over long periods and for at least two years, and for many applications, researchers expect a service life of several decades.

The sensor is mounted externally on the component. When an alternating voltage is applied to the piezoelectric crystal, it mechanically deforms and sends an ultrasonic wave into the material. The sensor then 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 thereby indicates the defect’s location. When several transducers are used that serve as transmitters and receivers, they should even be able to pinpoint the flaw to within a few millimetres. Depending on the component material, the sensor range covers a few metres according to the researchers.

The researchers can also use the high-temperature ultrasonic transducer, for example, to measure how much of a hot liquid flows through a pipe. The sensors could also determine contactlessly the temperature of a gas or a liquid within a few milliseconds. The researchers will present their prototypes at the SENSOR+TEST 2017 measurement fair in Nuremburg, Germany, from 30 May to 1 June 2017.