Pressure self-compensation for humidity sensing using graphene-oxide-modified dual-frequency CMUT

Abstract Capacitive micro-machined ultrasonic transducer (CMUT) has emerged as a promising candidate for gravimetric humidity sensing applications for its unique advantages. However, the measurement accuracy is badly influenced by the ambient pressure especially in the occasions of large pressure change such as meteorological observation. Therefore, a novel dual-frequency CMUT was proposed for pressure self-compensation during humidity measurement for the first time. The pressure self-compensation was carried out by obtaining the humidity measurement curves under different pressures. The CMUT sensor was designed with two resonant frequencies of 180 kHz and 1.8 MHz. Firstly, the pressure was detected by measuring the frequency shift around 180 kHz and this frequency was almost not affected by the humidity conditions. Then the relative humidity (RH) could be acquired through the resonant frequency shift around 1.8 MHz in RH of 30 %–90 % at this pressure. The cells of 1.8 MHz were functionalized by graphene oxide (GO) to improve the sensitivity of humidity sensing. The results demonstrated that the dual-frequency sensor showed high sensitivity for pressure detection and exhibited good sensitivity for RH sensing under different pressures, for example the sensitivity at 1 atm is 1.96 kHz/%RH for RH of 60 %–90 % and 663 Hz/%RH for RH of 30 %–60 %. Decent repeatability was also achieved by the CMUT sensor. What’s more, this dual-frequency sensor can be used in the cases where simultaneous detection of pressure and humidity is necessary. Compared with detection of pressure and humidity simultaneously by two sensors, this CMUT sensor owns many advantages such as small volume, low cost, high consistency as well as easy operation.