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

Abstract

The long‐term aging effects on capacitive‐type relative humidity sensors in a hot and humid atmosphere were investigated to better understand the mechanism of the drift phenomenon. For this purpose, four kinds of polymers, cellulose acetate (CA), cellulose acetate butyrate (CAB), poly(methyl methacrylate) (PMMA), and cross‐linked polyimide (PI) were used as sensing materials. For all sensors, the electrical capacitance increased with aging in a 40°C and 90% relative humidity atmosphere. The QCM measurement showed that the drift of the sensor capacitance was caused by the increase in the water sorption ability of the sensing polymers. From XPS data, the increase in the water sorption ability could be attributed to the introduction of the oxygen‐containing group at the surface of the sensing polymers. Morphological changes were also suggested by the film thickness measurement. Furthermore, it was confirmed that the magnitude of the drift was affected not only by the increment but also by the state of the sorbed water. Fourier transform infrared data indicated that the sorbed water in the hydrophobic polymers such as PMMA and PI are close to the isolated state and that the state is hardly affected by aging. Because the change in the isolated water does not significantly affect the electrical capacitance. PMMA and PI sensors show little drift even after 250 days aging. © 2000 The Electrochemical Society. All rights reserved.