A study of the water vapor absorption for integrating humidity sensor

Humidity can damage components of electronic systems, such as missiles, so such systems must be stored in containers, which are impermeable to moisture. Conventional humidity sensors measure the current humidity level and thus can not detect if a transient high humidity level had occurred prior to the measurement. However, the output of an integrating humidity sensor reflects a past high humidity level, even if the current humidity level is low, and can be used to indicate if damage may have occurred during storage or transport.; In this study, an integrating humidity sensor consisting of hygroscopic salt particles (magnesium sulfate) dispersed in a polybutadiene polymer film has been fabricated and investigated. Upon exposure to moisture, the water vapor reacts with the hygroscopic salt and results in a change of the electrical properties of the film. The change in capacitance is related to the amount of water vapor absorbed by the hybrid polymer film. Even after the humidity level is reduced, the sensor output is still different from the initial value. This reaction is irreversible so the sensor response is related to the total amount of water vapor during exposure.; Electrochemical impedance spectroscopy is used to study the mechanism of water vapor absorption during sensor exposure to the humidity environment. The experimental data analyzed with complex plots and Bode plots indicate that the integrating humidity sensor exhibits a change from capacitive behavior to resistive behavior after exposure to humidity environment for 35 days later. Analysis of these impedance plots indicates that both electron charge transfer processes and diffusion processes occurred during the sensor exposure. Based on this physical understanding, the proposed equivalent circuit model was constructed using a ladder network with a transmission line model, which is called a Warburg impedance, and employed in a computer simulation to analyze the electrochemical impedance behavior of the integrating humidity sensor. The computer simulation results indicate that mass transfer diffusion processes dominate the water vapor absorption mechanism in the early exposure. However, electron charge transfer processes prevail over the diffusion process after the hygroscopic salts are saturated.