In extremely harsh environments, conventional sensors can be destroyed within a short space of time - this includes the inside of power plant or aircraft turbines or boreholes in the ground where high temperatures and pressures prevail. Aggressive gases, liquids and dust also damage sensors. Eight Fraunhofer Institutes have therefore joined forces in the eHarsh project to develop particularly robust sensors for extremely harsh environments for the first time. "We have a lot of detailed knowledge in the various institutes," says eHarsh coordinator Holger Kappert from the Fraunhofer Institute for Microelectronic Circuits and Systems IMS. "We are familiar with heat-resistant ceramics, can test material properties and produce robust microelectronic circuits. However, none of us were able to produce such a sensor on our own. It was only through the interaction and combination of many individual technologies that we have now succeeded." The team initially focused on applications with high temperatures and pressures - the aforementioned turbines and boreholes. The aim was not only to install robust pressure and thermocouples in the turbines and boreholes, but also the electronics for evaluating the measured values. "The advantage of on-site electronics and signal processing in the sensor is the higher quality of the sensor signals," says Holger Kappert. "It would also make it possible to network sensors better in future and save on expensive cabling."

Sensor head with ceramic sensor element.

© Fraunhofer ILT

This would be particularly interesting in aircraft engines because it would reduce the weight. Such engines are complex. Air flows, electrical voltages and power must be precisely regulated depending on the flight maneuver. With the help of small, robust sensors directly in the engine, measuring the condition of the engine and controlling the combustion process could become even more precise in the future - for example, in order to use fuel more efficiently.

The sensor housing is made of metal, while the sensor elements are made of ceramic that can withstand temperatures of up to 500 °C. The electronic inner workings can withstand temperatures of around 300 °C. One challenge was to join the various components together in such a way that they do not separate from each other even when repeatedly heated and cooled, when the materials expand and contract to different degrees. Among other things, circuit boards made of heat-resistant ceramic and conductor tracks with an admixture of tungsten, which is also used for the filaments of light bulbs, are used.

The sensors are not only heat-resistant, but can also withstand pressures of up to 200 bar. This means that such sensors can be used in pumps for geothermal energy in the future. In geothermal energy, buildings are heated with hot water from the ground. The pumps are located deep down in the borehole and must be able to withstand both the heat and the pressures. Thanks to the new sensors, simple, permanent monitoring is now possible.