Low-frequency Conductivity Measurements Research Articles

Low-Frequency Electrical Conductivity of Aqueous Kaolinite Suspensions II: Counterion Effects and Estimating Stern Layer Mobilities of Counterions

AbstractThe electrical state of the interface between a kaolinite-dominated clay sample and aqueous electrolyte solutions was characterized using low-frequency conductance measurements. From these measurements, the ζ-potential and surface conductivity contributions from the diffuse and non-diffuse parts of the electrical double layer were obtained. The suspensions were studied as a function of volume fraction, electrolyte concentration, and electrolyte type (LiCl, NaCl, KCl, CsCl, CaCl2, SrCl2, and BaCl2). Interpretation in terms of the surface conductance revealed that a substantial part of the surface conductivity originates in the inner part of the double layer. Electrokinetic potentials and related diffuse double layer properties are highly dependent on the nature of monovalent counterions, whereas divalent counterions do not show such clear dependencies. Further presented was a simple way to estimate the order of magnitude of counterion mobilities in the inner part of the electrical double layer. All counterions were shown to have a substantial mobility in the inner part of the double layer. Finally, we suggest that the apparent ion-specific effects observed in the diffuse part of the double layer are at least in part related to the finite size of the counterions. Our findings are relevant to scenarios where fluid flow in porous media is accompanied by charged species transport, e.g., in electro-osmotic remediation, spectral-induced polarization, or permeability measurements.

Atomic layer deposition of Al2O3 for single electron transistors utilizing Pt oxidation and reduction

This work describes the fabrication of single electron transistors using electron beam lithography and atomic layer deposition to form nanoscale tunnel transparent junctions of alumina (Al2O3) on platinum nanowires using either water or ozone as the oxygen precursor and trimethylaluminum as the aluminum precursor. Using room temperature, low frequency conductance measurements between the source and drain, it was found that devices fabricated using water had higher conductance than devices fabricated with ozone. Subsequent annealing caused both water- and ozone-based devices to increase in conductance by more than 2 orders of magnitude. Furthermore, comparison of devices at low temperatures (∼4 K) showed that annealed devices displayed much closer to the ideal behavior (i.e., constant differential conductance) outside of the Coulomb blockade region and that untreated devices showed nonlinear behavior outside of the Coulomb blockade region (i.e., an increase in differential conductance with source-drain voltage bias). Transmission electron microscopy cross-sectional images showed that annealing did not significantly change device geometry, but energy dispersive x-ray spectroscopy showed an unusually large amount of oxygen in the bottom platinum layer. This suggests that the atomic layer deposition process results in the formation of a thin platinum surface oxide, which either decomposes or is reduced during the anneal step, resulting in a tunnel barrier without the in-series native oxide contribution. Furthermore, the difference between ozone- and water-based devices suggests that ozone promotes atomic layer deposition nucleation by oxidizing the surface but that water relies on physisorption of the precursors. To test this theory, devices were exposed to forming gas at room temperature, which also reduces platinum oxide, and a decrease in resistance was observed, as expected.

Electrical conductivity relaxation in PVOH+LiH2PO4+Al2O3 polymer composites

Low-frequency conductivity measurements have been performed in solid polymer electrolyte composites based on the anhydrous PVOH–LiH2PO4–Al2O3 system. A typical power law dependency in the real part of the conductivity, at higher frequencies, of the form ωn is observed, with an exponent n that depends on the alumina content and nearly independent of temperature. An analysis of the frequency dependence of the electrical susceptibility is conducted to obtain relaxation functions of the form exp[−(t/τ)β], with an exponent β ≈ n − 1. Correlation times, τ, and parameters characterizing the electrical relaxation in time and frequency domains are compared to show the equivalence of these representations. The anhydrous dc conductivity of the electrolytes increases with increasing lithium salt content, becoming of the order of 10−5 S/cm for a salt molar fraction of x = 0.14. This conductivity value increased by about one order of magnitude by addition of nanoporous particles of Al2O3. The temperature dependence of the samples conductivity was well described by the Vogel–Tammann–Fulcher’s equation indicating the effect of the polymer chains flexibility on ion migration. Although all membranes exhibited a “universal dynamic response” associated to the random hopping of the mobile carriers, variations in the measured relaxation parameters with alumina content indicate an increase of ionic correlations when adding the nonporous particles to the polyelectrolyte.

Surface acoustic wave propagation and inhomogeneities in low-density two-dimensional electron systems near the metal–insulator transition

We have measured the surface acoustic wave velocity shift in a GaAs/AlGaAs heterostructure containing a two-dimensional electron system (2DES) in a low-density regime (<10 10 cm −2) at zero magnetic field. The interaction of the surface acoustic wave with the 2DES is not well described by a simple model using low-frequency conductivity measurements. We speculate that this conflict is a result of inhomogeneities in the 2DES, which become very important at low density. This has implications for the putative metal–insulator transition in two dimensions.

SrTiO3-BiCrO3 solid solutions: Synthesis, x-ray diffraction study, and dielectric properties in the microwave range

SrTiO3-BiCrO3 ceramics were prepared by solid-state reactions. The materials were single-phase by x-ray diffraction and had a cubic perovskite structure up to 70 mol % BiCrO3. Low-frequency conductivity measurements revealed semiconducting behavior of the solid solutions. Dielectric measurements in the microwave range revealed flat maxima of a relaxation nature in the temperature dependences of the dielectric permittivity and tan δ for the samples containing 30 and 40 mol % BiCrO3. The solid solutions were found to exhibit relatively high dielectric losses in the microwave range.

Optical and low frequency conductivity measurements on pure and mixed stages of graphite-ferric chloride intercalation compound

Room temperature measurements of the 0.5-2.0 eV optical reflectance of carefully prepared pure stages 3,4,6 and the mixed stages (2,3) and (3,4) graphite-ferric chloride intercalation compound have been performed. The stages show metallic nature with a large contribution from the imaginary part of the dielectric constant. The higher value of the dielectric constant shows the charge transfer from graphite to ferric chloride to be ionic in nature.

Theory and experiment of electrical conductivity and percolation locus in water-in-oil microemulsions.

An extensive set of measurements of low-frequency conductivity \ensuremath{\sigma} of a three-component microemulsion system, sodium-di-2-ethyl-hexylsulfosuccinate (AOT)-water-decane, as a function of temperature (T from 10 to 50 \ifmmode^\circ\else\textdegree\fi{}C) and volume fraction of the dispersed phase (\ensuremath{\varphi} from 0.098 to 0.65) has been made. The measurements were made at a fixed value of the water-to-surfactant molar ratio equal to 40.8, where the microemulsion consists of AOT-coated water droplets, 85 \AA{} in radius, dispersed in oil. \ensuremath{\sigma} can be quantitatively calculated at low \ensuremath{\varphi} by means of a charge fluctuation model of Eicke et al., while it can be interpreted for higher vlaues of \ensuremath{\varphi} in terms of power-law behavior with indices related to the dynamic percolation picture below the threshold and the static one above it. The percolation locus in \ensuremath{\varphi}-T plane has been determined starting from the vicinity of the lower consolute point up to \ensuremath{\varphi}=0.65. This line can be successfully interpreted in terms of a modified version of the analytical theory of percolation given by Xu and Stell.