Capacitive humidity sensors based on large diameter porous alumina prepared by high current anodization

Humidity sensors based on nanoporous aluminum oxide are described. Accelerated mild anodization technique was employed to prepare ordered porous aluminum oxide templates in phosphoric acid electrolyte at 165 and 185 V, anodization voltages. Impedance spectroscopy was used to characterize sensitivity, response and recovery time of prepared sensors with pore diameters changing from 67 to 190 nm. Increasing the pores diameter to an optimum limit improved the sensing parameters of the prepared sensors. Sensitivity enhancement was accompanied with reduction of humidity sensing threshold to 30% RH at optimum condition. Their low response and recovery time, less than 5 and 0.5 s, respectively, categorize them as ultrafast or rapid sensors. Sensing properties is seen to reduce with reduce in aluminum purity while it improves with increase in the self ordering of alumina template. It is also found that increasing the anions percentage in the pores walls improves the sensing behavior.

► Increasing the pores diameter to a critical value improved the sensing properties to an optimum limit.
► Maximum sensitivity was seen in more than 100 nm pores diameter and humidity sensing threshold reduced to 30% RH.
► Reducing the substrate purity decreases the sensing properties.
► Increasing the anions percentage in the pores wall increases the sensing behavior of the sensors directly.