High temperature static strain microwave acoustic sensor

Strain sensors are used to guarantee integrity, safety, and appropriate maintenance in a multitude of applications such as civil structures, industrial equipment, and aircrafts. In harsh environments associated with power plants, aerospace, oil drilling, and industrial processes, there is a pressing need for robust, reliable, small footprint strain sensors, which can operate wirelessly, battery-free, and require minimal or no maintenance. The University of Maine group has been reporting on successful harsh-environment surface acoustic wave resonator (SAWR) temperature sensors fabricated on langasite (LGS) substrates and employing high temperature Pt-based electrode alloys. The usage of this platform for strain sensor application poses technological challenges, such as packaging and sensor assembly for the temperature span; packaged sensor stability and longevity over multiple cycles; and finally temperature cross-sensitivity. This paper discusses the design, fabrication, calibration, and testing up to 400°C of LGS SAWR static strain sensors using co-deposited Pt-Al
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and PtNi-PtZr multi-layered electrodes operating at 195 MHz and 338 MHz. The LGS SAWR sensors were mounted on a constant stress beam and characterized up to 200 microstrains, με. At room temperature, a commercial strain gauge was used for calibration. At higher temperatures (up to 400°C), the sensors were calibrated against a finite element analysis (FEA) model, due to the inexistency of reliable static commercial sensors at those temperatures. Linear regression of the SAWR frequency shift versus strain resulted in correlations of 0.997 and 0.976 at 300°C and 400°C, respectively. The associated extracted sensor sensitivities to strain at 300°C and 400°C were 42Hz/με and 37Hz/με. The reported results confirmed the viability of using LGS SAWR as high temperature static strain sensors.