Experimental Investigation on Humidity-Sensitive Properties of Polyimide Film for Humidity Sensors

Due to excellent properties, polyimide (PI) film is widely used in the capacitive humidity sensor as the humidity-sensitive medium. However, some key properties of this film are uninvestigated and need further research, especially those related to sensor characteristics. So, samples of PI films were self-synthesized in a laboratory. In order to test their effective diffusion coefficients using the self-build test system, films were created from different chemical reagents (PMDA-ODA, BPDA-ODA, BPDA-BAPP and PMDA-BAPP) with four varying thicknesses and four separate concentrations. They were tested in three disparate temperatures, respectively. The effective diffusion coefficients of water molecules in these PI films were 4.25 × 10−13, 5.49 × 10−13, 6.34 × 10−13 and 6.95 × 10−13 m2/s ordered by decreasing thickness, and 6.08 × 10−13, 5.86 × 10−13, 4.25 × 10−13 and 3.76 × 10−13 m2/s ordered by decreasing concentration of PI film. The effective diffusion coefficients of water molecules in these four films were 6.11 × 10−13m2/s for PMDA-ODA, 6.38 × 10−13m2/s for BPDA-ODA, 6.14 × 10−13m2/s for BPDA-BAPP and 5.86 × 10−13m2/s for PMDA-BAPP. When the environment temperature rises from 20 to 50 °C, the effective diffusion coefficient rises from 6.38 × 10−13 to 1.67 × 10−12 m2/s. The results should greatly enhance the properties’ improvement of the capacitive humidity sensor.