Two Surface acoustic wave (SAW) resonators were fabricated on langasite substrates with Euler angle of (0°, 138.5°, 117°) and (0°, 138.5°, 27°). A dipole antenna was bonded to the prepared SAW resonator to form a wireless sensor. The characteristics of the SAW sensors were measured by wireless frequency domain interrogation methods from 20 °C to 600 °C. Different temperature behaviors of the sensors were observed. Strain sensing was achieved using a cantilever configuration. The sensors were measured under applied strain from 20 °C to 500 °C. The shift of the resonance frequency contributed merely by strain is extracted from the combined effects of temperature and strain. Both the strain factors of the two SAW sensors increase with rising ambient temperature, and the SAW sensor deposited on (0°, 138.5°, 117°) cut is more sensitive to applied strain. The measurement errors of the two sensors are also discussed. The relative errors of the two sensors are between 0.63% and 2.09%. Even at 500 °C, the hysteresis errors of the two sensors are less than 5%.

We report in this paper on the study of surface acoustic wave (SAW) resonators based on an AlN/titanium alloy (TC4) structure. The AlN/TC4 structure with different thicknesses of AlN films was simulated, and the acoustic propagating modes were discussed. Based on the simulation results, interdigital transducers with a periodic length of 24 μm were patterned by lift-off photolithography techniques on the AlN films/TC4 structure, while the AlN film thickness was in the range 1.5–3.5 μm. The device performances in terms of quality factor (Q-factor) and electromechanical coupling coefficient (k²) were determined from the measure S₁₁ parameters. The Q-factor and k² were strongly dependent not only on the normalized AlN film thickness but also on the full-width at half-maximum (FWHM) of AlN (002) peak. The dispersion curve of the SAW phase velocity was analyzed, and the experimental results showed a good agreement with simulations. The temperature behaviors of the devices were also presented and discussed. The prepared SAW resonators based on AlN/TC4 structure have potential applications in integrated micromechanical sensing systems.

High temperature characteristics of langasite surface acoustic wave (SAW) devices coated with an AlN thin film have been investigated in this work. The AlN films were deposited on the prepared SAW devices by mid-frequency magnetron sputtering. The SAW devices coated with AlN films were measured from room temperature to 600 °C. The results show that the SAW devices can work up to 600 °C. The AlN coating layer can protect and improve the performance of the SAW devices at high temperature. The SAW velocity increases with increasing AlN coating layer thickness. The temperature coefficients of frequency (TCF) of the prepared SAW devices decrease with increasing thickness of AlN coating layers, while the electromechanical coupling coefficient (K²) of the SAW devices increases with increasing AlN film thickness. The K² of the SAW devices increases by about 20% from room temperature to 600 °C. The results suggest that AlN coating layer can not only protect the SAW devices from environmental contamination, but also improve the K² of the SAW devices.

In this paper, we report a wireless gas sensor based on surface acoustic waves (SAW). For room temperature detection of oxygen gas, a novel nanostructured Zn_xFe_yO gas-sensitive film was deposited on the surface of a SAW resonator by an oblique magnetron co-sputtering method. The measurements of X-ray diffraction (XRD) and a scanning electron microscope (SEM) showed that the crystal phase composition and the microstructures of Zn_xFe_yO films were significantly affected by the content of Fe. The experimental results showed that the sensors had a good response to O₂ at room temperature. The max frequency shift of the sensors reached 258 kHz as the O₂ partial pressure was 20%. Moreover, X-ray photoelectron spectroscopy (XPS) was performed to analyze the role of Fe in the sensitization process of the Zn_xFe_yO film. In addition, the internal relationship between the Fe content of the film and the sensitivity of the sensor was presented and discussed. The research indicates that the nanostructured Zn_xFe_yO film has a good potential for room temperature O₂ gas detection applications.

In this paper, a wireless oxygen sensor based on a surface acoustic wave (SAW) was reported. For high-temperature applications, novel Al₂O₃/ZnO/Pt multilayered conductive film was deposited on langasite substrate as the electrodes, and ZnO film obtained by the pulse laser deposition (PLD) method was used as the sensitive film. The measurements of X-ray diffraction (XRD) and a scanning electron microscope (SEM) showed that the c-axis orientation of the ZnO grains and the surface morphology of the films were regulated by the deposition temperature. Meanwhile, the gas response of the sensor was strongly dependent on the surface morphology of the ZnO film. The experimental results showed that the oxygen gas sensor could operate at a high-temperature environment up to 850 °C with good stability for a long period. The max frequency shift of the sensors reaches 310 kHz, when exposed to 40% O₂ gas at 850 °C. The calculated standard error of the sensors in a high-temperature measurement process is within 3%. Additionally, no significant signal degradation could be observed in the long-term experimental period. The prepared SAW oxygen gas sensor has potential applications in high-temperature sensing systems.