Calculation of ionic conductivity activation energies in ionic oxide glasses from spectroscopic data

Activation energies for ionic conduction in inorganic oxide glasses are related to cation-motion vibrational band frequencies from the far infrared spectra of the glasses, and the calculated and observed values are compared.     

Nonlinear ionic pulses along microtubules

Microtubules are cylindrically shaped cytoskeletal biopolymers that are essential for cell motility, cell division and intracellular trafficking. Here, we investigate their polyelectrolyte character that plays a very important role in ionic transport throughout the intra-cellular environment. The model we propose demonstrates an essentially nonlinear behavior of ionic currents which are guided by microtubules. These features are primarily due to the dynamics of tubulin C-terminal tails which are extended out of the surface of the microtubule cylinder. We also demonstrate that the origin of nonlinearity stems from the nonlinear capacitance of each tubulin dimer. This brings about conditions required for the creation and propagation of solitonic ionic waves along the microtubule axis. We conclude that a microtubule plays the role of a biological nonlinear transmission line for ionic currents. These currents might be of particular significance in cell division and possibly also in cognitive processes taking place in nerve cells.     

Interfaces in ionic devices

The technology of Ionics is based on the availability of materials with fast ion transport. Individual materials are, however, meaningless from a practical point of view; all applications require combinations of materials with appropriate ionic and electronic properties. This situation is similar to Electronics which requires combinations of semi-conducting or metallic conducting materials with differences in the chemical potentials of the electrons. The technology of Ionics requires interfaces between ionic and electronic conductors which generate strong electrical fields or allow to modify the field by the application of external voltages. Ions and electrons equilibrate both at these “ionic junctions”. While semi-conductor junctions have commonly a width in the μm-range, the space charge region is several orders of magnitude smaller in the case of ionic junctions, i.e. in the nm or even sub-nm-range. The interfaces have to be chemically stable for the lifetime of the device which is difficult to achieve in view of the commonly large number of components present in both phases and the existence of mobile species with sometimes large variations in the activity of the electroactive component. Furthermore, the kinetics of transfer of ions across the interface has to be fast to allow high current densities which are required in many cases. In addition, two such interfaces are required to convert the electronic current into an ionic one and again back into an electronic current at the opposite side of the electrolyte. The development of ionic devices depends to the strongest extent on the engineering of appropriate interfaces. Examples of the role and engineering of interfaces will be presented for applicationes such as chemical sensors, electrochromic devices, fuel cells, batteries and photogalvanic solar cells.     

A short review on stable metal nanoparticles using ionic liquids,supported ionic liquids,and poly(ionic liquids)

Metal nanoparticles (NPs) are a subject of global interest in research community due to their diverse applications in various fields of science. The stabilization of these metal NPs is of great concern in order to avoid their agglomerization during their applications. There is a huge pool of cations and anions available for the selection of ionic liquids (ILs) as stabilizers for the synthesis of metal NPs. ILs are known for their tunable nature allowing the fine tuning of NPs size and solubility by varying the substitutions on the heteroatom as well as the counter anions. However, there has been a debate over the stability of metal NPs stabilized by ILs over a long period of time and also upon their recycling and reuse in organocatalytic reactions. ILs covalently attached to solid supports (SILLPs) have given a new dimension for the stabilization of metal NPs as well as their separation, recovery, and reuse in organocatalytic reactions. Poly(ILs) (PILs) or polyelectrolytes have created a significant revolution in the polymer science owing to their characteristic properties of polymers as well as ILs. This dual behavior of PILs has facilitated the stabilization of PIL-stabilized metal NPs over a long period of time with negligible or no change in particle size, stability, and size distribution upon recycling in catalysis. This review provides an insight into the different types of imidazolium-based ILs, supported ILs, and PILs used so far for the stabilization of metal NPs and their applications as a function of their cations and counter anions.     

Positron-electron doublet in ionic crystals

The states of a positron-electron doublet in ideal ionic crystals are analyzed, taking into account the Coulomb interaction of the particles, with their localization both on the immediately nearest ions of different sign, and on the ions next after them. The exactly-solvable quasi-one-dimensional model of a crystal is considered in detail. The wave functions and the characteristic energies of the doublet are obtained. It is shown that states of two types are realized, one of which has a threshold nature, and arises only for definite magnitudes of the Coulomb integrals. For the ground state of the doublet, one can with good precision take the state corresponding to the computation of the Coulomb interaction for localization of the particles on the nearest ions. The role of the Coulomb interactions of higher order are discussed.Translated from Izvestiya Vysshikh Uchebnyk Zavedenii, Fizika, No. 12, pp. 56–61, December, 1989.     

Structural properties of ionic liquids

Two types of ionic liquids are considered: molten salts and aqueous solutions of strong electrolytes. It is shown that in both cases, modern theoretical methods are capable of making definite predictions about the spatial correlations between the ions themselves and between the ions and water molecules. It is further shown that the technique of neutron diffraction as applied to isotopically enriched samples allows, for the first time, detailed comparisons to be made between theory and experiment. The results of these comparisons are discussed and areas in which the theory is in urgent need of revision are identified.     

Charged dislocations in ionic crystals

Experiments and theories concerning charged dislocations in alkali halide crystals are reviewed in detail, with particular attention to the way in which the experiments should be interpreted and to the range of applicability of the sweep-up and various forms of thermal-equilibrium models. Possible effects on mechanical properties and internal friction are analysed. The transient and steady-state effects of plastic deformation on ionic conductivity are described and new interpretations involving charged dislocations are proposed. A survey of results on the scattering of light by alkali halide single crystals leads to the conclusion that charged dislocations do not play an important role.

Evidence about charges on surfaces and on dislocations in AgCl and AgBr is reviewed and compared with that for alkali halides. Comparisons are also made with MgO, the CsCl and CaF₂ structures, semiconductors and ice.     

Sensitivities of ionic explosives

ABSTRACT

We have investigated the relevance for ionic explosive sensitivity of three factors that have been demonstrated to be related to the sensitivities of molecular explosives. These are (1) the maximum available heat of detonation, (2) the amount of free space per molecule (or per formula unit) in the crystal lattice and (3) specific features of the electrostatic potential on the molecular or ionic surface. We find that for ionic explosives, just as for molecular ones, there is an overall tendency for impact sensitivity to increase as the maximum detonation heat release is greater. This means that the usual emphasis upon designing explosives with large heats of detonation needs to be tempered somewhat. We also show that a moderate detonation heat release does not preclude a high level of detonation performance for ionic explosives, as was already demonstrated for molecular ones. Relating the free space per formula unit to sensitivity may require a modified procedure for ionic explosives; this will continue to be investigated. Finally, an encouraging start has been made in linking impact sensitivities to the electrostatic potentials on ionic surfaces, although limited so far to ammonium salts.     

Surface excitons on ionic crystals

We have recorded electronic EEL spectra of (001) faces of a number of simple ionic solids, with higher resolution and much lower beam currents than in previous work. We are able to confirm reports that some, but not all, solids show surface excitons at lower energy than the bulk values. We propose a new model for surface excitons, involving the excitation of an electron to a Rydberg-like state outside the crystal surface.     

The haven ratio in fast ionic conductors

Measurements of the Haven Ratio are conventionally performed in order to extract unique information about the diffusion mechanism in ionic conductors. In this review article, the historical background for such measurements, their present status, results, and the current theories of interpretation are discussed in detail with emphasis on the special problems found in fast ionic conductors. Likely directions for future work are also pointed out.     

How multivalency controls ionic criticality

To understand how multivalency affects criticality in z:1 ionic fluids, we report an ion-cluster association theory embodying ionic solvation and excluded volume for equisized hard-sphere models with z=1-3. In accord with simulation but contradicting integral equation and field theories, the reduced critical temperature falls when z increases while the density rho(c) rises steeply. These trends can be explained semiquantitatively by noting that 80%-90% of the ions near T(c) are bound in neutral or charged clusters, depleting the ionic strength. For z not equal 1, predicted interphase Galvani potentials vanish at T(c).     

Engineering of solid state ionic devices

Solid state ionic devices such as high performance batteries, fuel and electrolysis cells, electrochromic devices, chemical sensors, thermoelectric converters or photogalvanic solar cells are of tremendous practical interest in view of our energy and environmental needs. The challenges are the achievement of higher energy and power densities, longer lifetimes, cheaper materials, lower cost, improved sensitivity and higher stability. The engineering of new devices is based on the better fundamental understanding of materials for galvanic cells and their interaction in order to approach solutions more systematically than in the past. The fundamental aspects of the generation of voltages and electrical currents are compiled and analysed in view of the materials requirements. Conflicts exist in forming chemically stable interfaces of functionally different electrolyte and electrode materials, achieving simultaneously high energy and power densities in view of low conductivities of chemically stable materials, fast chemical diffusion in electrodes which should have a wide range of non-stoichiometry for delivering and absorbing the mobile ionic species, practical problems of using less expensive polycrystalline materials which have high intergranular resistances and finally reaching both ionic and electronic equilibria at the electrolyte - electrode interfaces at low temperatures. The engineering of new or improved solid state ionic devices is commonly based on individual materials considerations and their interaction in galvanic cells. Simultaneously high ionic conductivity and chemical stability may be reached by designing structures of poly-ions of the non-conducting components with the conducting species in-between. The chemical stability may be based on kinetic restrictions for sufficiently long periods of time of operation of the devices. Electrodes should not be made of metallic conductors but of electronic semi-conductors with fast enhancement of the diffusion of ions by internal electrical fields. Device considerations are based on the development of single element arrangements (SEAs) which incorporate the electrodes into the electrolyte in the case of fuel and electrolysis cells. The electronic conductivity is generated by the applied gas partial pressures or the applied voltage. The same simplification may be applied for electrochromic systems which consist of a single active layer instead of the conventional three galvanic cell materials. A new design of active chemical sensors probing the environment by the magnitude of the applied voltage or current may overcome the limitations of cross sensitivities and interfacial reactions which allows the simultaneous detection of several species by a single galvanic cell. The paper has been prepared for presentation at the International Conference on Ionic Devices — 2003, Anna University, Nov. 28–30, 2003, Chennai, India.     

Theory of ionic transport in crystallographic tunnels

A model has been developed for the treatment of the motion of ions through crystallographic tunnels, as are found in materials that are interesting solid electrolytes. Consideration of both point charge and higher order attractive terms as well as overlap repulsion effects allows the calculation of the minimum energy positions of mobile ions and the activation energy barrier that they must surmount to move through the tunnel in the lattice. Calculations have been made for ions of different sizes in the AgI lattice which show that there is a set of minimum energy paths which do not follow the centerline of the tunnel, but deviate, periodically, with both direction and magnitude depending upon the cationic size. Also, in accordance with experimental observations, the activation energy for motion is smallest for cations of intermediate size, where the Coulombic, polarization, and repulsive contributions to the total energy are best balanced.     

Determining ionic conductivity from structural models of fast ionic conductors

Reverse Monte Carlo produced structural models of silver ion conducting glasses and crystals have been investigated by the bond-valence technique. Both absolute ionic conductivity and activation energy can be determined directly from the "pathway volume" of the structural models, i.e., from the volume fraction of the percolating pathway cluster. This pathway volume-conductivity relation was found to hold for glassy and crystalline systems with silver ion conductivities differing by more than 11 orders of magnitude. Earlier, less universal rules were rationalized and unified by means of this approach.     

Radiation effects in ionic solids

Current development of the research of radiation damage in ionic solids is reviewed. Emphasis is placed on the correlation between elementary radiation damage processes and the atomic and electronic structures of the materials. Both the radiation damage induced by electronic excitation and by elastic collision are treated. For the former two crucial processes, the self-trapping of excitons and the formation of stable Frenkel pairs from the self-trapped excitons in several materials, is discussed in terms of the structures of materials. Deficiency in the available data on the knock-on threshold energies are pointed out. Available information of Frenkel pairs produced by electronic and elastic encounters is surveyed. Possible models of defect clustering are summarized and existing information on clustering is discussed on their basis.     

Phase behavior of ionic microgels

We employ effective interaction potentials between spherical polyelectrolyte microgels in order to investigate theoretically the structure, thermodynamics, and phase behavior of ionic microgel solutions. Combining a genetic algorithm with accurate free energy calculations we are able to perform an unrestricted search of candidate crystal structures. Hexagonal, body-centered orthogonal, and trigonal crystals are found to be stable at high concentrations and charges of the microgels, accompanied by reentrant melting behavior and fluid-fcc-bcc transitions below the overlap concentration.     

Nonextensive statistical mechanics of ionic solutions

Classical mean-field Poisson–Boltzmann theory of ionic solutions is revisited in the theoretical framework of nonextensive Tsallis statistics. The nonextensive equivalent of Poisson–Boltzmann equation is formulated revisiting the statistical mechanics of liquids and the Debye–Hückel framework is shown to be valid for highly diluted solutions even under circumstances where nonextensive thermostatistics must be applied. The lowest order corrections associated to nonadditive effects are identified for both symmetric and asymmetric electrolytes and the behavior of the average electrostatic potential in a homogeneous system is analytically and numerically analyzed for various values of the complexity measurement nonextensive parameter q.     

About the dissociation rate of ionic crystals

A new method of calculation of dissociation rates of ionic crystals is developed. Crystals of ideal composition and those with a homogeneity range are considered. Using available data from the literature, the obtained equations are tested for the dissociation rate of copper iodide in vacuum at 200–300 °C. The agreement between the calculated and the experimental values is found to be good.