Showing papers on "Conductivity published in 1999"

Pyrrolidinium Imides: A New Family of Molten Salts and Conductive Plastic Crystal Phases

Abstract: A new family of molten salts is reported, based on the N-alkyl, N-alkyl pyrrolidinium cation and the bis(trifluoromethane sulfonyl)imide anion. Some of the members of the family are molten at room temperature, while the smaller and more symmetrical members have melting points around 100 °C. Of the room-temperature molten salt examples, the methyl butyl derivative exhibits the highest conductivity; at 2 × 10-3 S/cm this is the highest molten salt conductivity observed to date at room temperature among the ammonium salts. This highly conductive behavior is rationalized in terms of the role of cation planarity. The salts also exhibit multiple crystalline phase behavior below their melting points and exhibit significant conductivity in at least their higher temperature crystal phase. For example, the methyl propyl derivative (mp = 12 °C) shows ion conductivity of 1 × 10-6 S/cm at 0 °C in its higher temperature crystalline phase.

Electrochemical properties of imidazolium salt electrolytes for electrochemical capacitor applications

Abstract: The specific ionic conductivity, dynamic viscosity, and electrochemical stability of several imidazolium salts are reported as neationic liquids and their solutions in several organic solvents. The temperature dependence of conductivity and viscosity are analyzed for 1‐ethyl‐3‐methylimidazolium and 1,2‐dimethyl‐3‐n‐propylimidazolium salts, and the influence of theanions bis(trifluoromethylsulfonyl)imide , bis(perfluoroethylsulfonyl)imide , hexafluoroarsenate , hexafluorophosphate , and tetrafluoroborate on these properties are discussed. These imidazolium salts make possible electrolytes with high concentration (>3 M), high room temperature conductivity (up to 60 mS/cm), and a wide window of stability . Differential scanning calorimetric results confirm a large glass phase for the ionic liquids, with substantial (>80°C) supercooling. Thermal gravimetric results indicate the imidazolium salts with and anions to be thermally more stable than the lithium salt analogs. The Vogel‐Tammann‐Fulcher interpretation accurately describes the conductivity temperature dependence. © 1999 The Electrochemical Society. All rights reserved.

Bismuth based oxide electrolytes— structure and ionic conductivity

Abstract: Bismuth oxide systems exhibit high oxide ion conductivity and have been proposed as good electrolyte materials for applications such as solid oxide fuel cells and oxygen sensors. However, due to their instability under conditions of low oxygen partial pressures there has been difficulty in developing these materials as alternative electrolyte materials compared to the state-of-the-art cubic stabilised zirconia electrolyte. Bismuth oxide and doped bismuth oxide systems exhibit a complex array of structures and properties depending upon the dopant concentration, temperature and atmosphere. In this paper we comprehensively review the structures, thermal expansion, phase transitions, electrical conductivity and stability of bismuth oxide and doped bismuth oxide systems. ©

Generation of chemically active species by electrical discharges in water

Abstract: Pulse positive streamer corona discharges in water solution with a different conductivity have been investigated in reactors with the needle-plate and coaxial electrode geometry. A special composite anode was used in the coaxial geometry. With such an anode hundreds of streamers were generated at each voltage pulse. Production of H, O and OH radicals by the discharge was proved by emission spectroscopy and formation of H2O2 and degradation of phenol was demonstrated by chemical methods. Assuming that the broadening of the line profile was caused by the dynamic Stark effect, plasma with an electron density over 1018 cm-3 was generated during the initial phase of voltage pulse in the both reactors in spite of the very different electrode geometry and wave-forms of voltage pulses. Production of OH radicals was most effective at solution conductivity below .

Structure of the polymer electrolyte poly(ethylene oxide)6:LiAsF6

Abstract: Polymer electrolytes—salts (such as LiCF3SO3) dissolved in solid, high-molar-mass polymers (for example, poly(ethylene oxide), PEO)1,2,3—hold the key to the development of all-solid-state rechargeable lithium batteries4. They also represent an unusual class of coordination compounds in the solid state5. Conductivities of up to 10−4 S cm−1 may be obtained, but higher levels are needed for applications in batteries5,6,7. To achieve such levels requires a better understanding of the conduction mechanism, and crucial to this is a knowledge of polymer-electrolyte structure. Crystalline forms of polymer electrolytes are obtained at only a few discrete compositions. The structures of 3 : 1 and 4 : 1 complexes (denoting the ratio of ether oxygens to cations) have been determined5,8,9. But the 6 : 1 complex is of greater interest as the conductivity of polymer electrolytes increases significantly on raising the polymer content from 3 : 1 to 6 : 1 (refs 10, 11). Furthermore, many highly conducting polymer-electrolyte systems form crystalline 6 : 1 complexes whereas those with lower conductivities do not. Here we report the structure of the PEO:LiAsF6 complex with a 6 : 1 composition. Determination of the structure was carried out abinitio by employing a method for flexible molecular structures, involving full profile fitting to the X-ray powder diffraction data by simulated annealing12. Whereas in the 3 : 1 complexes the polymer chains form helices, those in the 6 : 1 complex form double non-helical chains which interlock to form a cylinder. The lithium ions reside inside these cylinders and, in contrast to other complexes, are not coordinated by the anions.

Effect of Electrolyte Concentration and Nature on the Morphology and the Electrical Properties of Electropolymerized Polypyrrole Nanotubules

Abstract: Polypyrrole (PPy) with four different counterions was electrochemically synthesized in homemade polycarbonate particle track-etched membranes with different pore sizes. A dependence of the PPy growing rate on the pore size and on the electrolyte nature and concentration has been observed. The morphology of the different template-synthesized nanostructures has been carefully analyzed by field-emission scanning electron microscopy. In all cases, nanotubules were observed. However, their thickness depends on the pore diameter of the template membrane and on the nature of the supporting electrolyte. The electrical conductivity of PPy in such a structure is known to increase compared to the bulk conductivity, which is also shown here. In addition Raman spectroscopy showed that the relative conjugation length increases with decreasing pore size.

The conductivity of dense molecular gas

Abstract: We evaluate the conductivity tensor for molecular gas at densities ranging from 104 to 1015 cm-3 for a variety of grain models. The Hall contribution to the conductivity has generally been neglected in treatments of the dynamics of molecular gas. We find that it is not important if only 0.1-μm grains are considered, but for a Mathis--Rumpl--Nordsieck grain-size distribution (with or without PAHs) it becomes important for densities between 107 and 1011 cm-3. If PAHs are included, this range is reduced to 109-- 1010 m cm-3. The consequences for the magnetic field evolution and dynamics of dense molecular gas are profound. To illustrate this, we consider the propagation of Alfven waves under these conditions. A linear analysis yields a dispersion relation valid for frequencies below the neutral collision frequencies of the charged species. The dispersion relation shows that there is a pair of circularly polarized modes with distinct propagation speeds and damping rates. We note that the gravitational collapse of dense cloud cores may be substantially modified by the Hall term.

Electrical spectroscopy of porous rocks: a review—I. Theoretical models

Abstract: The complex dielectric permittivity e* of porous water-bearing rocks in the frequency range from a few to hundreds of megahertz reveals several intensive relaxation effects and a non-trivial dependence on the water content. At high frequencies, f > 10 MHz, both the real part of the complex dielectric permittivity e′ and the conductivity σ of water-bearing rocks are correctly predicted by the Maxwell–Wagner–Bruggeman–Hanai (MWBH) theory of composite dielectrics. This theory takes into account only the bulk properties of components, their partial volumes and the configuration of particles. The theory ignores two important factors: the surface contribution to polarization and the effect of clustering of components. At frequencies f < 10 MHz there are certain frequency domains which exhibit relaxation processes not predicted by MWBH theory. The characteristic times of these processes range from 10−6 to 10 s. These relaxation effects are related to different surface polarization processes which are, in order of increasing water content, (i) orientational polarization of bound water, (ii) polarization of liquid films or pockets, producing a polarization catastrophe effect, (iii) polarization of rough fractal surfaces, (iv) polarization of the ‘closed’ electrical double layer (EDL), when the displacement of the excess surface charges is limited by the external boundary of the EDL, and (v) polarization of the ‘open’ double layer, implying free exchange of excess ions with the bulk electrolyte and generation of transient diffusional potentials, which lag behind the applied field. Some theoretical models predict large effective values of relative dielectric constants in the range 105–106 at low frequencies. Knowledge of the characteristic signatures of these physical mechanisms is important for the correct interpretation of experimental data. Analysis of existing theories of polarization of heterogeneous media shows that electrical spectroscopy can be useful for the interpretation of frequency spectra of complex dielectric permittivity or conductivity of water-bearing rocks and porous materials in general, and for the determination of water content, its thermodynamic state, the connectivity of water-bearing channels and their correlation lengths and the surface to volume ratio and surface charge in particular, in addition to the traditional formation factor, which is obtained from ohmic conductivity measurements.

Dimensionality Dependence of the Conductivity Dispersion in Ionic Materials

Abstract: The dielectric response of many materials exhibits universal behavior in the form of a power law frequency dependence of the ac conductivity. This response is seen in all types of structures both crystalline and amorphous and for all types of polarizing species including dipoles and ions. Here I demonstrate that for ionic materials the power law exponent decreases with decreasing dimensionality of the ion conduction pathways. Although percolation concepts such as random walks on a self-similar fractal lattice provide a qualitative explanation, experimental findings instead indicate that the dispersion is the result of localized ion motion occurring on an atomic length scale. {copyright} {ital 1999} {ital The American Physical Society }

Sum Rules and Interlayer Conductivity of High-Tc Cuprates

Abstract: Analysis of the interlayer infrared conductivity of cuprate high–transition temperature superconductors reveals an anomalously large energy scale extending up to midinfrared frequencies that can be attributed to formation of the superconducting condensate. This unusual effect is observed in a va- riety of materials, including Tl2Ba2CuO6+ x , La2− x Sr x CuO4, and YBa2Cu3O6.6, which show an incoherent interlayer response in the normal state. Midinfrared range condensation was examined in the context of sum rules that can be formulated for the complex conductivity. One possible interpretation of these experiments is in terms of a kinetic energy change associated with the superconducting transition.

High temperature oxygen sensors based on doped SrTiO3

Abstract: The sensor properties of semiconducting strontium titanate can be modified by dopants. On the one hand, the ambiguous dependence of the electrical conductivity on the oxygen partial pressure can be changed by a small lanthanum (donor) concentration to an unambiguous one over a wide oxygen partial pressure range. Polycrystalline Sr 0.995 La 0.005 TiO 3 and/or thick films have response times of few milliseconds, caused by a surface type mechanism of the conductivity. The bulk type mechanism is very slow and is responsible for the long-term behaviour. On the other hand, the temperature dependence of strontium titanate can almost be suppressed for a small oxygen partial pressure range by a high iron (acceptor) content. On the basis of SrTi 0.65 Fe 0.35 O 3 a fast thick film oxygen sensor is proposed, which can be applied to control of lean-burn engines.

Partial‐melt electrical conductivity: Influence of melt composition

Abstract: The electrical conductivity of a partial melt is influenced by many factors, including melt conductivity, crystalline conductivity, and melt fraction, each of which is influenced by temperature. We have performed measurements of bulk conductivity as a function of temperature of an Fo80-basalt partial melt between 684° and 1244°C at controlled oxygen fugacity. Melt fraction and composition variations with temperature calculated using MELTS [Ghiorso and Sack, 1995] indicate that the effect on melt conductivity of changing melt composition is balanced by changes in temperature (T). Thus bulk conductivity as a function of T or melt fraction in this system can be calculated assuming a constant melt conductivity. The bulk conductivity is well modeled by simple parallel calculations, by the Hashin-Shtrikman upper bound, or by Archie's law (σpartial melt/σmelt = C1Xmn). We estimate apparent values of the Archie's law parameters between 1150° and 1244°C as C1 = 0.73 ± 0.02 and n = 0.98 ±0.01. Estimates of the permeability of the system are obtained by using an electrical conductivity-critical scale length relationship and range from ∼10−14 to 10−18 m2, comparing favorably with previously published values.

Enhanced Lithium‐Ion Transport in PEO‐Based Composite Polymer Electrolytes with Ferroelectric BaTiO3

Abstract: The ion-conduction properties of a polyethylene oxide (PEO)-based composite polymer electrolyte comprised of PEO, LiClO{sub 4}, and the ferroelectric material BaTiO{sub 3} were studied. The addition of BaTiO{sub 3} resulted in an increase in conductivity over the temperature range 25--115 C. The optimum amount of BaTiO{sub 3} (purity 99.9%, particle size 0.6--1.2 {micro}m) was 1.4 wt %, which is very low in comparison with previously reported composite polymer electrolytes. The ionic conductivity of a composite polymer electrolyte containing 1.4 wt % BaTiO{sub 3} was 1 {times} 10{sup {minus}5} S/cm at 25 C, which is at least one order of magnitude higher than that of the pristine polymer electrolyte (4 {times} 10{sup {minus}7} S/cm). The transport number of the lithium ion in this composite polymer electrolyte was higher than that of the pristine polymer electrolyte. The increase in the conductivity and the lithium-ion transport number is explained on the basis of the spontaneous polarization of the ferroelectric material due to its particular crystal structure. The addition of BaTiO{sub 3} powder greatly enhanced the lithium/electrolyte interface stability.

Research on the electrochemistry of oxygen ion conductors in the former Soviet Union. II. Perovskite-related oxides

Abstract: The review is devoted to the analysis of experimental results on electrochemical and physicochemical properties of the perovskite-related oxide phases obtained at scientific centers of the former Soviet Union. The main attention is focused on oxides with high electronic conductivity, which are potentially useful as electrodes for high-temperature electrochemical cells with oxygen-ion conducting solid electrolytes and interconnectors of solid oxide fuel cells, and on mixed ionic-electronic conductors for oxygen separation membranes. Along with perovskite-like solid solutions based on LnMO3−δ (Ln is a rare-earth element, M = Cr, Mn, Fe, Co, Ni) and SrCoO3−δ, properties of the oxide phases Ln2MO4±δ (M = Cu, Ni, Co) with the K2NiF4-type structure are briefly reviewed.

Comparative study of lithium ion conductors in the system Li1+xAlxA2−xIV (PO4)3 with AIV=Ti or Ge and 0≤x≤0·7 for use as Li+ sensitive membranes

Abstract: Preparations and physico-chemical characterizations of NASICON-type compounds in the system Li1+xAlxA2−xIV(PO4)3 (AIV=Ti or Ge) are described. Ceramics have been fabricated by sol-gel and co-grinding processes for use as ionosensitive membrane for Li+ selective electrodes. The structural and electrical characteristics of the pellets have been examined. Solid solutions are obtained with Al/Ti and Al/Ge substitutions in the range 0≤x≤0·6. A minimum of the rhombohedral c parameter appears for x about 0·1 for both solutions. The grain ionic conductivity has been characterized only in the case of Ge-based compounds. It is related to the carrier concentration and the structural properties of the NASICON covalent skeleton. The results confirm that the Ti-based framework is more calibrated to Li+ migration than the Ge-based one. A grain conductivity of 10−3 S cm−1 is obtained at 25°C in the case of Li1·3Al0·3Ti1·7(PO4)3. A total conductivity of about 6×10−5 S cm−1 is measured on sintered pellets because of grain boundary effects. The use of such ceramics in ISE devices has shown that the most confined unit cell (i.e. in Ge-based materials) is more appropriate for selectivity effect, although it is less conductive.©

Oxide ionic conductivity of apatite type Nd9·33(SiO4)6O2 single crystal

Abstract: Single crystal of hexagonal apatite type Nd 9·33 (SiO 4 ) 6 O 2 which is an oxide ionic conductor was prepared by the FZ method and an anisotropy of its conductivity was investigated. The conductivity of a parallel component to a c-axis (2·1×10 −8 S cm −1 at 30°C) was higher about one order of magnitude, compared with that of perpendicular component.

Determination of rock properties by low‐frequency AC electrokinetics

Abstract: In brine-saturated rock the existence of a mobile space charge at the fluid/solid interface leads to the electrokinetic phenomena of electroosmotic pressure and streaming potential. The coupling coefficients of these electrokinetic effects, when combined with the conductivity of the brine-saturated rock, determine the brine permeability of rock exactly. A sensitive low-frequency AC technique has been used to measure electrokinetic response of a collection of eight rock and four glass bead samples saturated with NaCl brine as a function of salt concentration (fluid conductivity of 0.5 to 6.38 S/m); the response of four of the original 12 samples has also been measured as a function of temperature from 0° to 50°C. All data verify the predicted permeability relationship. Additionally, the frequency response of the electroosmotic pressure signal alone can also be used to determine the permeability, given knowledge of experimental parameters. The concentration and temperature dependence of electroosmosis and streaming potential is found to mostly conform to the predictions of a simple model based on the Helmholtz-Smoluchowski equation, the Stern model of the electrochemical double layer, and an elementary theory of ionic conduction.