Humidity sensing characteristics of NiO/Al2O3 nanocomposites, prepared by sol-gel method, are
studied by impedance spectroscopy. Modeling of the obtained impedance spectra with an appropriate equivalent circuit enables us to separate the electrical responses of the tightly bound
chemisorbed water molecules on the grain surfaces and the loosely associated physisorbed water
layers. Dependence of the dielectric properties and ac conductivity of the nanocomposites on relative humidity RH were studied as a function of the frequency of the applied ac signal in the
frequency range of 0.1–105 Hz. The electrical relaxation behavior of the investigated materials is
presented in the conductivity formalism, where the conductivity spectra at different RHs are
analyzed by the Almond-West formalism D. P. Almond et al., Solid State Ionics 8, 159 1983.
The dc conductivity and the hopping rate of charge carriers, determined from this analysis, show
similar dependences on RH, indicating that the concentration of mobile ions is independent of RH
and is primarily determined by the chemisorption process of water molecules. Finally, the results are discussed in view of a percolation-type conduction mechanism, where mobile ions are provided by the chemisorbed water molecules and the percolation network is formed by the physisorbed water layers.