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Demonstration of SiC Pressure Sensors at 750 ºC.
Keywords: Silicon Carbide, 750 C , Pressure Sensor
We report the first demonstration of MEMS-based 4H-SiC piezoresistive pressure sensors tested at 750 ºC and the discovery of strain sensitivity recovery with increasing temperature above 400 ºC, eventually achieving up to 100 % recovery of the room temperature values at 750 ºC. This strain recovery phenomenon in 4H-SiC is uncharacteristic of the monotonic decrease in strain sensitivity with increasing temperature that is well known in silicon piezoresistors. The room temperature full-scale output (FSO) at 200 psi ranges between 29 and 36 mV for a bridge input voltage of 10 V. Although the FSO at 400 oC dropped by 60 %, full strain sensitivity recovery was achieved at 750 ºC. This result will allow the insertion of pressure sensors further into the combustion chamber to improve the accurate quantification of combustor dynamics. Efforts are ongoing to lower greenhouse gas emissions from air vehicles by 50 %, relative to 2005, by 2050 and improve fuel combustion efficiency by 2 %/year to 2020 as part of the ICAO (International Civil Aviation Organization) goals [1]. Lean burning (LB) (i.e., lower fuel/air ratio), has been demonstrated to further reduce undesirable emissions and increase combustion efficiency. However, LB increases the potential for thermoacoustic instability, which is the precursor to flame-out or possible engine damage [2]. This instability must be detected early, preferably at point of nucleation, and mitigated. Pressure sensors are used to measure both static and dynamic pressures during engine ground tests. State-of-the-art (SOA) pressure sensors can operate reliably up to about 350 ºC, albeit with water cooling jackets to ensure measurement reliability. Even with the cooling jackets, these SOA sensors are placed several inches or more away from the combustion chamber via an infinite loop tube. Such measurement strategies cause the attenuation of key frequency components and a delay in pressure propagation through the sampling tube, thus affecting signal fidelity and diminishing the opportunity for real time active control of combustion instabilities. Currently, there is no commercially available pressure sensor that can operate simultaneously in the static and dynamic mode at temperatures as high as 600 ºC. The only known commercially available dynamic pressure sensor that is reported to operate up to 780 ºC is Meggitt Corporation’s CP-215 piezoelectric pressure sensor made from GaPO4 piezoelectric crystal [3]. This material does not exist naturally, hence is synthesized by few manufacturers. In this work, we have utilized the strain sensitivity recovery observed in 4H-SiC piezoresistors, combined with an advanced high temperature metallization scheme to extend the operation of MEMS fabricated pressure sensors to 750 ºC, initially for one hour duration. The room temperature FSOs at 200 psi of the three sensors tested ranged between 29 and 36 mV. At 400 ºC, the FSOs were about 40% of the room temperature values. However, at 750 ºC, the FSOs have practically recovered to or nearer their respective room temperature values. To the best of our knowledge, this is the first reported case of a semiconductor piezoresistive sensor operation at 750 ºC, and also the first reported case of strain sensitivity recovery in SiC.
Robert Okojie,
NASA Glenn Research Center
Cleveland, OH
USA


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