Dual configuration high temperature hydrogen sensor on LGS SAW devices

In the past few years there has been an increasing interest in the langasite family of crystals (LGX) for surface acoustic wave (SAW) applications in communications, frequency control, and sensors. LGX has several interesting properties including: up to about six times higher electromechanical coupling than quartz ST-X; existence of temperature compensated cuts with zero power flow angle and minimal diffraction; up to 26% reduction in phase velocities with respect to quartz ST-X, which allows the fabrication of smaller devices; and the absence of a crystal phase transition up to the crystal melting point (around 1177K). Due to this absence of crystal phase transition, bulk acoustic wave (BAW) and SAW devices have been explored as temperature sensors at temperatures up to several hundred °C. In addition to temperature and pressure sensors, a need exists for high temperature sensors capable of detecting target gases. Hydrogen (H₂) detection, in particular, is of paramount importance in applications such as hydrogen fuel cells, fuel leakage from jet engines, and power plants. This paper reports on a dual LGS SAW device configuration for the detection of H₂ gas at 250°C using original all palladium (Pd) electrodes. The Pd electrodes are used for the SAW transduction and reflection functions and to detect H₂. The frequency differences between two identical all Pd SAW resonators have been tracked. The dual configuration scheme has been used to minimize temperature cross interference, since the LGS (0°, 138.5°, 26.6°) orientation selected in this work is not temperature compensated at 250°C. The detection of H₂ gas produces a 6-8 KHz differential frequency shift with respect to the reference for H₂ gas concentrations of 100 ppm, 250 ppm, 500 ppm, and 1000 ppm. The SAW resonators respond to the presence of H₂ in a matter of seconds and become stable between 25 to 75 minutes later. The devices have been continuously operated at 250°C for a period of sixteen weeks, with less than three dB of degradation in the |S21| response. The dual configuration high temperature LGS SAW devices and experiments reported in this work prove the capability of these crystal- s to withstand prolonged exposure to high temperatures (250°C) and to perform as appropriate high temperature H₂ gas sensors.