The dielectric properties of humid cellulose

Abstract

The electrical characteristics of cellophane have been measured at variable moisture contents and temperatures in the frequency range 10 −3–10 5 Hz. The data show very strong dependence on moisture content, which is typical of hydrophilic solids, and exhibit constant phase angles over wide frequency ranges. Complex admittance is proportional to ( jω) −( β−1) where ω is the frequency and β ≈ 0.95. We use a model of hopping charge transport to explain the frequency and moisture dependence of the measured spectra. At low frequencies conduction occurs predominantly by phonon-assisted proton hopping between absorbed water molecules, with β dependent on the energetic disorder of the protonic states. The moisture dependence of the conductivity σ is shown to arise from its exponential dependence on the critical percolation distance R C and to have the form exp{− BN −1/3}, where N is the water concentration and B is a constant. This result explains the universally observed logarithmic dependence on N of the low frequency conductivity in hydrophilic solids. The model also explains the characteristic low frequency dispersion observed in solid ionic conductors. In hydrophilic solids in-phase charge transport due to rotating water molecule dipoles increases with frequency and eventually becomes the dominant contribution to the total conductivity.