Rapid response relative humidity sensor using anodic aluminum oxide film

A capacitive humidity sensor having a substantially linear response and capable of detecting relative humidity in a full range of 0 to 100% relative humidity, the humidity sensor comprising:

• an aluminum substrate forming one electrode of the capacitive humidity sensor;
• an anodic aluminum oxide thin film formed from at least a portion of the aluminum substrate and having a thickness of 2-20 μm formed on a portion of the aluminum substrate, the anodic aluminum oxide film having channels with a channel diameter from 10 nm to 100 nm configured to adsorb water vapor from an ambient environment;
• and
• a porous metal layer formed on a portion of the anodic aluminum oxide film formed on the aluminum substrate, the porous metal layer forming a second electrode of the capacitive humidity sensor, the aluminum substrate, anodic aluminum oxide thin film and porous metal layer having undergone heat treatment to produce a substantially linear humidity versus capacitance sensor response,
• wherein said heat treatment is selected from one of the following conditions:
• a) annealing at a temperature of 85° C. for 5-50 hours at 90% relative humidity;
• b) annealing at a temperature of 90° C. for 5-50 hours in dry air;
• or
• c) immersion in boiling water at a temperature of 100° C. for 10-100 minutes;
• and
• wherein a response time of said capacitive humidity sensor is less than 6 seconds when the channel diameter is larger than 10 nm.