Drift Phenomenon of Capacitive-Type Relative Humidity Sensors in a Hot and Humid Atmosphere

Abstract

Capacitive-type relative humidity sensors have been widely used. However, it is known that the output of the sensor gradually increases to a greater or lesser degree by aging of the sensor in a hot and humid atmosphere. [1][2][3][4][5] While the magnitude of such drift depends on the type of sensing polymers used, the origin of the drift has not yet been made clear. An understanding of this drift phenomenon and its implications is essential for preparing a stable and reliable capacitive-type humidity sensor.This type of sensor employs a simple sensing mechanism. The water sorption and desorption of the sensing polymer changes its effective permittivity of dielectrics. This phenomenon causes a sensor-capacitance change. Many factors are thought to contribute to a sensor's long-term lack of stability. Among them, in terms of the sensing mechanism, changes in the sensing polymer's water sorption characteristics over time can lead to changes in the sensor's dielectric properties. In the case of some linear polyimides, it has been suggested that the lack of long-term stability arises from a hydrolysis of the film in the presence of moisture and heat, which changes the polarizability and water-sorption properties of the polyimide. 4 These changes modify the humidity-sensing characteristics of the film. The chemical stability of the materials in a hot and humid atmosphere is important. In addition, the swelling and/or deformation of the film in such an atmosphere may also lead to changes in the water-sorption ability of the sensing films. An exact understanding of this phenomenon makes it easier to choose the appropriate sensing materials and to prepare reliable sensors. In this study, several polymers with different chemical structures and hydrophilicity were chosen for the model sensing materials. The drift phenomenon in a hot and humid atmosphere was analyzed based on dielectric, water sorption ability, and X-ray photoelectron spectroscopy (XPS) measurements as well as infrared (IR) analysis.

A capacitive humidity sensor using cross-linked cellulose acetate butyrate

“…Indeed, one of problems of CAB sensors is commonly instabilities in the sensor RH characteristics if the sensor is left in a high temperature and high humidity environment for extended periods [16].…”

Highly efficient cw chemical oxygen—iodine laser with transsonic iodine injection and a nitrogen buffer gas

“…Polyimide is one of the most widely used polymers in various fields, such as the aerospace, 7) automotive, 8) and electronics industries, and is used in numerous applications including humidity sensors, 9) thin-film transistors, 10) and long-term storage, 11) because of its significant mechanical, thermal, electrical, and chemical properties. Recently, the use of polyimide films in the field of biotechnology for tissue engineering has been proposed.…”

Effect of Sensing Film Thickness on Drift Phenomenon of Capacitive-Type Humidity Sensors

“…As mentioned before, one proved mechanism of change in water sorption ability induced by aging was the introduction of the hydrophilic oxygen-containing group in the surface region of the sensing polymers. 6 If this is the only mechanism, then we would have observed a larger drift for the thinner films. In fact, the opposite result was obtained in this study as shown in Fig.…”

“…Among many possible factors that may contribute to the sensor's drift, we revealed that the drift of the sensor capacitance was closely related to the increase in the water sorption ability of the sensing polymers. 6 In that work, the increase in water sorption ability was attributed to the introduction of the oxygen-containing group at the surface of the sensing polymers. In addition, volumetric change by the swelling of the film in such an atmosphere may also be suggested.…”