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A novel SAW temperature-humidity-pressure (THP) sensor based on LiNbO3 for environment monitoring

In this paper, a temperature-humidity-pressure (THP) sensor based on surface acoustic wave (SAW) devices is proposed for the simultaneous monitoring of temperature, humidity and pressure. The developed THP sensor consists of three SAW one-port resonators fabricated onto Y-cut LiNbO3 with frequencies of 81, 85.2, and 89.92 MHz for temperature, humidity and pressure sensing respectively. Graphene oxide was used as a sensitive layer material for the SAW humidity sensor. A flexible cantilever beam was used as a sensitive structure for the pressure sensor. Prior to fabrication, both the temperature sensitivity of the substrate and the deformation of the cantilever beam were simulated using the COMSOL Multiphysics. The frequency drift of the humidity sensor is obtained through analysis. According to the simulation results and analysis, the resonant frequency and frequency distribution of each sensor were reasonably set to avoid mutual interference. Experimental tests demonstrate that the proposed THP sensor can operate in an ambient environment with a temperature range of 25 °C–200 °C, pressure range of 0–42 kPa, and humidity range of 10%–90% relative humidity (RH) with good repeatability and stability, which can be attributed to simulation optimization and the use of strongly hydrophilic nanomaterials. The humidity sensor exhibits relatively high sensitivity (absolute sensitivity of 5.1 kHz/%RH and relative sensitivity of 59.97 ppm/%RH) over a wide test range from 10% to 90% RH with very little hysteresis at 35 °C and 0 kPa. Additionally, the humidity sensor provides good reversibility, excellent short-term repeatability and stability with a fast response time of approximately 8.3 s and recovery time of approximately 4.8 s.