1. Introduction
The humidity control is of utmost importance as it affects both
living organisms and materials. Therefore, humidity sensors are
currently used in many fields such as manufacturing processes,
environmental and health monitoring or domestic applications [1].
Porous ceramics form a common group of materials to sense
humidity [2,3]. The adsorption mechanism of the water molecules
on the surface of the ceramic oxides is well known (Fig. 1). First,
a chemisorbed layer is formed on the available sites of the oxide
surface, causing dissociation of water molecules to surface hydroxyls.
After the first adsorbed layer is formed, subsequent layers
are physisorbed. The molecules in the first physisorbed layer
are bounded through double hydrogen bonds and cannot move.
At higher humidity levels, water molecules are singly bounded
and form a network with a liquid-like behavior. Thus, the water
molecules from the second layer can move or rotate freely (like in bulk liquid water) and the Grotthuss mechanism i.e. the proton transport becomes dominant [2].
1. IntroductionThe humidity control is of utmost importance as it affects bothliving organisms and materials. Therefore, humidity sensors arecurrently used in many fields such as manufacturing processes,environmental and health monitoring or domestic applications [1].Porous ceramics form a common group of materials to sensehumidity [2,3]. The adsorption mechanism of the water moleculeson the surface of the ceramic oxides is well known (Fig. 1). First,a chemisorbed layer is formed on the available sites of the oxidesurface, causing dissociation of water molecules to surface hydroxyls.After the first adsorbed layer is formed, subsequent layersare physisorbed. The molecules in the first physisorbed layerare bounded through double hydrogen bonds and cannot move.At higher humidity levels, water molecules are singly boundedand form a network with a liquid-like behavior. Thus, the watermolecules from the second layer can move or rotate freely (like in bulk liquid water) and the Grotthuss mechanism i.e. the proton transport becomes dominant [2].
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