Electric conductivity percolation in naturally dehydrating, lightly wetted, hydrophilic fumed silica powder

Abstract

In studying the dehydration of surface-moistened fumed silica Aerosil powders, we found a conductivity percolation transition at low hydration levels. Both the percolation exponent and the threshold are typical for correlated site-bond transitions in complex two-dimensional (2D) systems. The exponent values, 0.94-1.10, are indicative of severe heterogeneity in the conducting medium. The surface moisture at the percolation threshold takes on a universal value of 0.65 mg([H2O])/m(2)([silica]), independent of the silica grain size, and equivalent to twice the first hydration monolayer. This level is just sufficient to sustain a quasi-2D, hydrogen-bonded water network spanning the silica surface.

Two-dimensional percolation transition at finite temperature: Phase boundary for in-plane magnetism in films with two atomic layers of Fe on W(110)

“…The results of measurements on a large number of films is summarized in fig. (3). Each symbol marks the temperature and deposition at which the real part of the susceptibility has a maximum for the individual samples.…”

“…This yields the parameters ln(y c ) = 5.4 ± 0.6 and 2J/k B = 2670 ± 270 K. The two bold lines in fig. (3) show the limiting cases for the phase boundary using the fitted parameters at their error bounds. Using the measured value of the surface anisotropy 36 K = 0.61 mJ/m 2 gives K/k B ≈ 1.4 K/atom, so that K/J ≈ 0.0005 and the estimate just below eq.…”

“…( 13) have already been determined from fitting the phase boundary in fig. (3). Beginning with the data for θ= 1.20 ML, one expects θ c − θ ≈ 0.024 ML, but it is difficult to estimate an error bound on this value.…”

“…As it is often the case that connectivity permits long-range order or non-equilibrium transport to exist, percolation is a central issue in condensed matter physics and materials science. A few recent examples include the performance of field-effect transistors 1,2 , the conductivity of silica powder 3 , and the metal-insulator transition 4 . There is, in addition, an enormous literature in computational simulations of percolation.…”

Note: Optimization of the numerical data analysis for conductivity percolation studies of drying moist porous systems

“…For example, for Aerosil samples, where dielectric loss spectra due to DC contribution continues over 2-3 decades of the frequency, 3 it is possible to follow ε f scaling with h at frequencies separated from each other even by a factor of 10, cf. Fig.…”