Silica Aerogel Platforms as Conductimetric Sensors

Wang et al. [22] and Wang and Wu [23] prepared mesoporous silica aerogel thin films for use as humidity sensors. They either spin-coated [22] or dip-coated [23] sol-gel-derived silica colloids onto gold electrode-patterned alumina substrates, then used CSCE extraction to obtain aerogel films. The response of the aerogel films to applied electric fields was evaluated over a range of plausible ambient temperamres and relative humidities. When water adsorbed onto the sensor surface, the conductivity of the material increased. 

The average pore diameter and surface area of the spin-coated aerogel films were comparable to those achieved for silica aerogel monoliths [22]. Under conditions of relative humidity of 40% or higher, the electrical resistance decreased as humidity increased. Sensor performance depended on the uniformity and the porosity of the film. 

The dip-coated materials [23] were suited for operation over a wider range of relative humidity (20-90%) and ambient temperature (15-35°C). Unsurprisingly, as the film thickness decreased, surface conductivity increased and the response time increased. However, thicker films recovered more rapidly. The response could be tailored by controlling the film thickness and pore structure of the materials. 

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