Experimental Measurement of Diffusion Coefficient of Polyimide Film for Capacitive Humidity Sensors

Polyimide (PI) film is widely used as the key component of the capacitive humidity sensor, whose diffusion coefficient has a significant impact on the sensor's dynamic characteristics, but is rarely discussed. This paper provides a test method and processes for effective diffusion coefficients of water molecules in self-synthesis PI films. The films were formed by four ingredients (PMDA-ODA, BPDA-ODA and BPDA-BAPP, PMDA-BAPP) with PI acid concentrations of 23%, 20%, 17% and 15%, and tested in temperatures of 20 °C, 35 °C and 50 °C, respectively. The results indicated that BPDA-BAPP film was good as a moisture sensitive film, whose average effective diffusion coefficient was 2.709 × 10^-14 m²/s. The temperature of the environment had a significant effect on the humidity-sensitive properties, but the PI acid concentration effect could be indirect.