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Humidity sensing by glass-ceramic nanocomposites containing silver
(Nanowerk Spotlight) Humidity is a measure of the moisture content of an environment. Control of humidity is thus essential for maintaining the desired level of moisture in an enclosure be it in a hospital or in a semiconductor-processing unit or in a laboratory. For humidity control an efficient sensor is an absolute necessity.
The conventional humidity sensors are polymer films and porous ceramics. In our present work nanometer-sized metal particles grown at the interface of glass and a ceramic have been exploited to act as sensing elements. These ultra small particles have large surface areas compared to their volumes. The former provides a rather large number of sites, which get affected in a humid environment.
As a result the effect of water molecule adsorption gets amplified as compared to that of a conventional sensor. The property used to assess the sensitivity to relative humidity is dc electrical resistance. We show in our paper that the electrical resistance decreases by four orders of magnitude as the relative humidity is increased from 35% to 95%. Conventional humidity sensors exhibit change of resistance by one or two order of magnitude only.
The scientific core of our finding concerns the physical mechanism involved in the above-mentioned electrical resistance change. The large number of nanoparticles of silver present within the glass-ceramic sample can be described as an amorphous system. The adsorption of water molecules at the surfaces of the nanoparticles creates localized states for electrons. The latter can jump from one particle to another on application of electric field, which causes a decrease of electrical resistance.
The detailed analyses of data show that there is a variable range hopping mechanism operative in this system, the model for which was developed for amorphous systems by Mott in 1972. Our findings are therefore new both with respect to the magnitude of sensitivity and the physical mechanism for the sensing behavior.
The society might benefit from the use of these efficient sensors in controlling the moisture content of enclosed spaces which are critical for medical treatment, office work, laboratory activities etc. The industrial applications can be visualized in these areas of human activity. The strategy used in this investigation could be exploited to miniaturize humidity sensing devices.
Our recent paper, titled "Humidity sensing by composites of glass ceramics containing silver nanoparticles and their conduction mechanism" was published in the April 26, 2006 issue of Sensors and Actuators B: Chemical.
By Dipankar Chakravorty, Indian Association for the Cultivation of Science, Kolkata, India

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