Broadband Ultrasonic Transducer for High Temperatures

Silicon processing techniques have developed over the last few decades enabling precise control over structures on a nanometer scale. Over the last decade, this progress was successfully employed to realize both backplates for capacitance ultrasonic transducers (CUTs) and capacitive micromachined ultrasonic transducers (cMUTs) with high accuracy and reproducibility. Currently, this new technology matches and even outperforms the piezoelectric transducer technology in terms of efficiency and bandwidth, especially in air-coupled applications. However, there is a significant lack of an ultrasonic transducer technology which is able to generate and receive broadband ultrasound in fluids at elevated temperatures. Due to this fact many applications remained unrealized up to now, such as nondestructive evaluation of hot surfaces and materials, and acoustic transit-time flowmetering (UFM) of hot and pulsating gas flows, e.g. the exhaust gas mass flow of automotive combustion engines. The latter example shows the ecological significance of such a transducer technology. Such a measurement system, which is the basis of control and optimization (and therefore also for the reduction) of exhaust emissions from automotive combustion engines and will play a major role for the improvement of our environmental conditions. The major goal of the proposed research is to provide a new capacitive transducer technology that can generate and receive efficient broadband ultrasound in high-temperature gaseous media. The research is two-fold: in a first step, I would like to continue my research on a novel CUT which makes use of a patterned, thermally oxidized, silicon backplate, covered with a thin bulk-conducting titanium foil as the moving membrane. This approach has already enabled the realization of a CUT that demonstrates a remarkable temperature range (up to 500-600 degree Celsius) in comparison to the state-of-the-art. However, a few significant problems, e.g. a polarization effect, have been observed during the first tests, which require further research; in a second step, I would like to focus my research on the extension of the temperature range of cMUTs.