Showing papers on "Ionic conductivity published in 1992"

Cation microsegregation and ionic mobility in mixed alkali glasses

Abstract: MUCH is known about short-range (<5A) structural order in oxide glasses from experimental probes of local structure such as X-ray absorption fine structure (XAFS)1, but over the medium range (5-20 A) their structures are poorly understood. Computer simulations based on measured parameters for local atomic environments, however, can provide structural models on the nanometre scale, which enable dynamic properties such as ionic transport to be considered. Here we describe a molecular dynamics simulation of the effects of mixed alkali cations on the structure of binary silicate glasses. It is well known that the ionic conductivity of alkali glasses falls markedly when more than one alkali is present2. We demonstrate that the alkalis segregate from the silicate network over distances of a few angstroms. Although ionic mobility is expected to be higher in these microsegregated regions than in the surrounding silicate network, we suggest that stochastic mixing of alkalis nevertheless impedes the hopping process of a given alkali ion. This is manifest as an increase in the average activation energy for hopping and results in a lowering of the total ionic conductivity.

Nafion Distribution in Gas Diffusion Electrodes for Solid‐Polymer‐Electrolyte‐Fuel‐Cell Applications

Abstract: The location of Nafion R in solid-polymer-electrolyte-fuel-cell electrodes was investigated by porosimetry and scanning microprobe analysis. At low Nafion loadings, the polymeric electrolyte uniformly fills the micro- and macropores of the electrode structure and increases its ionic conductivity. The pores are completely filled in the range from 0.8 to 1.0 mg of Nafio/cm 2 of electrode, and further addition of polymer results in the formation of a film on the external surface of the electrode. This film causes an additional resistance in series with the ionic resistance in the active layer

Conductivity and the electric modulus in polymers

Abstract: At relatively high temperatures and low frequencies the electrical properties of many polymers are dominated by ionic conductivity. In cases where the glass transition is obscured by conductivity, it can be revealed by treating the data din terms of the elctric modulus. The magnitude of the activation energy for conductivity (24-67 kcal/mol) for nylons 6, 66, and 6I, poly(butylene terephtalate) and polycarbonate indicates that the transport of charged particles requires the motion of polymer segments

Ionic conductivity in poly(propylene glycol) complexed with lithium and sodium triflate

Abstract: Conductivity and viscosity measurements have been made for poly(propylene glycol)‐MCF3SO3 (M=Li, Na) complexes in order to examine more closely the Vogel–Tammann–Fulcher (VTF) empirical relationship which has been found in previous reports to provide a good fit to the experimental data. Further, a dynamic bond percolation model of ion conduction in polymer electrolytes has predicted VTF behavior and an inverse relationship between molar conductivity and viscosity or Walden ‘‘rule’’ behavior. We find that deviations occur from both the VTF and Walden empirical relationships and propose a modest alteration in the form of the dynamic percolation model for ions moving in polyether systems.

Ionic and Electronic Conductivity of Poly‐(3‐methylpyrrole‐4‐carboxylic Acid)

Abstract: The electronic and ionic conductivities of poly-(3-methylpyrrole-4-carboxylic acid) have been measured in situ by twin electrode voltammetry and impedance spectroscopy, respectively. The electronic conductivity of this polymer, ca. 10 -2 (Ω cm) -1 , is about 1000 times higher than its ionic conductivity, allowing application of a simple transmission line circuit to model ion transport within the polymer. Under almost all conditions investigated the impedance response was similar to that of a transmission line and unambiguous measurements of the polymer's ionic conductivity could be obtained

Enhanced ionic conductivity of polypyrrole due to incorporation of excess electrolyte during potential cycling

Abstract: The anomalous initial reduction peak in cyclic voltammograms of polypyrole in acetonitrile has been shown, by impedance spectroscopy and X-ray emission analysis, to be due to the uptake of cations. Reoxidation is accompanied by anion uptake. As a result of these two processes, polypyrrole that has been electrochemically cycled in acetonitrile contains a large non-equilibrium concentration (several mol dm–3) of electrolyte and exhibits a greatly enhanced ionic conductivity. Contrary to a previous report, the ionic conductivity of (cycled) polypyrrole was found to be independent of the electrolyte solution concentration. At equilibrium, polypyrrole appears to be permselective in dilute acetonitrile and aqueous electrolyte solutions. Its ionic conductivity is much higher in water and is much less influenced by potential cycling. Electrolyte uptake during potential cycling in acetonitrile therefore appears to be due to poor solvation of the polymer and the consequent low mobility of its counterions.

Defect‐property correlations in garnet crystals. VI. The electrical conductivity, defect structure, and optical properties of luminescent calcium and cerium‐doped yttrium aluminum garnet

Abstract: The electrical and optical properties of calcium and cerium‐doped yttrium aluminum garnet (Ca,Ce:YAG) have been studied. Ca,Ce:YAG is a mixed ionic and electronic conductor with an ionic conductivity activation energy of 4.3 eV. Evidence of cluster formation with a consequent higher‐than‐expected activation energy is presented. The cerium normally enters the crystal as Ce+4, but it may be converted to Ce+3 under reducing atmospheres at elevated temperatures.

Electrical Properties and Sinterability for Lithium Germanium Phosphate Li1+xMxGe2-x(PO4)3, M=Al, Cr, Ga, Fe, Sc, and In Systems.

Abstract: The electrical properties and sinterability were studied for Li1+xMxGe2−x(PO4)3, M = Al3+, Cr3+, Ga3+, Fe3+, Sc3+, and In3+ systems. Due to the closer ionic radius of Al3+ and Cr3+ compared to that of Ge4+, those M3+ ions easily substitute the Ge4+ site. Larger cations, such as Ga3+, Fe3+, Sc3+, and In3+, were difficult to substitute the Ge4+ site. The ionic conductivity and sinterability improved with an increase in x for all of the M3+-substituted systems. In particular, an Al3+- or Cr3+-substituted system shows higher conductivity; the maximum conductivity is 2.4 × 10−4 S cm−1 at 298 K for Li1.5Al0.5Ge1.5(PO4)3. The enhancement in the conductivity is attributed to a decrease in the porosity and a lowering of the activation energy in the grain boundaries. The activation energy for Li+ ion conduction of the bulk component was 0.38 eV for Li1+xMxGe2−x(PO4)3 electrolytes, and was almost independent of M3+ substitution.

Effect of anion polarization on conductivity behavior of poly(ethylene oxide) complexed with alkali salts

Abstract: Conductivity behavior of poly(ethylene oxide) (PEO) amorphous electrolytes containing MCF 3 SO 3 (M=Li, K, Rb, or Cs) in a molar ratio EO/M=9 agrees with anterior data reported on PEO-MSCN (M=Li, K, or Cs) amorphous electrolytes. At any reduced temperature T-T g (T g =glass transition temperature) over the range 30

Ion migration in chalcopyrite semiconductors

Abstract: Quantitative data are presented that show partial ionic conductivity of Cu or Ag in ternary and quaternary electronic semiconductors, with idealized stoichiometry Cu x Ag 1-x InSez. A trend of increasing facility of ionic motion with increasing Ag content was observed. Ionic transference numbers up to 0.13 and 0.55 were measured for CuInSe 2 and AgInSe 2 , respectively. This trend can be correlated with the degree of compactness of the structure. It is supported by results from measurements of effective values of chemical diffusion coefficients, obtained by a potentiostatic current decay technique

Ionic conductivity in dehydrated zeolites

Abstract: The ionic conductivity of pressed pellets of dehydrated synthetic offretite, cancrinite and zeolite A, with various alkali metal ions, was determined by low-frequency impedance spectroscopy. Experiments were carried out in the frequency range 10 Hz–10 MHz at temperatures from 100–600°C. The conduction activation energies range between 55 kJ mol−1 (Na-zeolite A) and 108 kJ mol−1 (Li-cancrinite). The best conductivity value obtained was that of Na-zeolite A with 2.9×10−3Ω−1cm−1 at 600°C.

Framework structure, phase transition, and transport properties in MIIZr4(PO4)6 compounds (MII=Mg, Ca, Sr, Ba, Mn, Co, Ni, Zn, Cd, and Pb)

Abstract: The crystal structure, phase transition, electrical conduction behavior of various kinds of MIIZr4(PO4)6 (MIIZP) compounds (MII = Mg, Ca, Sr, Ba, Mn, Co, Ni, Zn, Cd, and Pb) have been investigated. The Mg, Co, Ni, and Zn compounds are of the β-Fe2(SO4)3-type structure and show an order-disorder transition between 600 and 720 °C; the Ca, Sr, Ba, Cd, and Pb compounds, however, are of the NASICON-type structure and do not show any phase transition between r.t. and 1000 °C. The Mn compound sintered at 900 °C shows the former characteristics (the phase transition temperature is around 560 °C), and that sintered above 900 °C the latter. The ZnZP and MnZP show the highest conductivity, ca. 2 × 10−3 and ca. 8 × 10−4 S cm−1 at 800 °C, in the β-Fe2(SO4)3- and the NASICON-type structure, respectively. The ionic size of the mobile cation is the predominant factor regarding ionic conduction in the zirconium phosphate framework.

Optical properties and ionic conductivity of KTiOAsO4 crystals

Abstract: Crystals of KTiOAsO4 were grown by the high temperature solution method and their optical and ionic conductivity properties evaluated. The band edge and IR cutoff were 370 and 4850 nm, respectively. A domain structure prevented efficient frequency conversion in as‐grown crystals. The ionic conductivity at 22 °C and 120 kHz was 1.5×10−8, 7.3×10−9, and 1.7×10−6 S/cm for the [100], [010], and [001], respectively. The alleged ferroelectric Curie temperature was 852 ±2 °C.