The role of an insulating surface layer on the relaxation time of the ionic redistribution in an electrolytic cell

We analyze the influence of a surface dielectric layer on the transient phenomena related to the ionic redistribution in an electrolytic cell submitted to a step-like external voltage. The adsorption-desorption phenomenon is taken into account in the famework of the Gouy-Chapman approximation, where the ions are assumed dimensionless. In the limit of small amplitude of the applied voltage, where the equations of the problem can be linearized, we obtain an analytical solution for the surface densities of ions, for the electrical potential and for the relaxation time for the transient phenomena. In the general case, when the linearized analysis is no longer valid, the solution of the problem is obtained numerically. The role of the thickness of the dielectric layer on the relaxation time is also discussed.

Mechanism for interplay between electron and ionic fluxes in KhFek[Fe(CN)(6)](l)center dot mH(2)O compounds

This paper develops a framework for the interpretation of ionic insertion/deinsertion reactions in an aqueous environment taking place in transition-metal hexacyanoferrates of the general formula KhFek3+ [Fe2+ (CN)(6)](l)center dot mH(2)O, also called Prussian Blue. Three different processes were fully separated in the electrochemistry of these films. It was clearly identified that one of these electrochemical processes involves the insertion/deinsertion of H3O+ (hydrated protons) through the channels of the KhFek3+ [Fe2+ (CN)(6)](l) center dot mH(2)O structure to reach the film electroneutrality during the electron transfer between Everitt's Salt and Prussian Blue. The other electrochemical processes involve K+ or H+ (proton) exchange through the water crystalline structure existing in the channels of the KhFek3+ [Fe2+(CN)(6)](l)center dot mH(2)O structure.

Determination of solubility product of mercurous acetate at various ionic strengths in aqueous medium

The conditionals (K-ps) and thermodynamics (K-ps(o)) solubility products of mercurous acetate have been determined, in aqueous solution, at 25 degrees C and ionic strength (I) comprised between 0.300 and 3.000 mol/L (NaClO4). The investigation was carried out potentiometrically by using a second class electrodes which responds to acetate ions. The values obtained for [pK(ps) (I)] were: 9.49+/-0.08 (0.000); 9.51+/-0.08 (0.300); 9.53+/-0.08 (0.500); 9.54+/-0.08 (0.700); 9.55+/-0.09 (0.900), 9.57+/-0.09 (1.200); 9.59+/-0.10(1.500); 9.61+/-0.10 (1.800); 9.63+/-0.10 (2.100); 9.65+/-0.11 (2.400); 9.67+/-0.11 (2.700) e 9.69+/-0.12 (3.000).

An implicit BEM formulation to model strong discontinuities in solids

A boundary element method (BEM) formulation to predict the behavior of solids exhibiting displacement (strong) discontinuity is presented. In this formulation, the effects of the displacement jump of a discontinuity interface embedded in an internal cell are reproduced by an equivalent strain field over the cell. To compute the stresses, this equivalent strain field is assumed as the inelastic part of the total strain. As a consequence, the non-linear BEM integral equations that result from the proposed approach are similar to those of the implicit BEM based on initial strains. Since discontinuity interfaces can be introduced inside the cell independently on the cell boundaries, the proposed BEM formulation, combined with a tracking scheme to trace the discontinuity path during the analysis, allows for arbitrary discontinuity propagation using a fixed mesh. A simple technique to track the crack path is outlined. This technique is based on the construction of a polygonal line formed by segments inside the cells, in which the assumed failure criterion is reached. Two experimental concrete fracture tests were analyzed to assess the performance of the proposed formulation.

Mechanism of conduction in doped polyaniline

The conductivity of H2SiF6-doped emeraldine polymers is studied as a function of temperature in the range 50 less than or equal to T less than or equal to 180 K. The dopant concentration of the samples varies between 0.1 M and 1.0 M. The temperature dependence of the do electrical conductivity gives evidence for a transport mechanism based on variable-range hopping in three dimensions. Using Mott's formula for the de conductivity, physically meaningful values of the density of states at the Fermi energy, the hopping energy and hopping distance are calculated.

Early Pancreas Transplant Improves Motor Nerve Conduction in Alloxan-Induced Diabetic Rats

The purpose of this study was to assess the temporal relationship between pancreas transplant and the development of electrophysiological changes in the sciatic and caudal nerves of alloxan-induced diabetic rats. Nerve conduction studies were performed in diabetic rats subjected to pancreas transplantation at 4, 12, and 24 weeks after diabetes onset, using nondiabetic and untreated diabetic rats as controls. Nerve conduction data were significantly altered in untreated diabetic control rats up to 48 weeks of follow-up in all time points. Rats subjected to pancreas transplantation up to 4 and 12 weeks after diabetes onset had significantly increased motor nerve conduction velocity with improvement of wave amplitude, distal latency, and temporal dispersion of compound muscle action potential in all follow-up periods (P<0.05); these parameters remained abnormal when pancreas transplantation were performed late at 24 weeks. Our results suggest that early pancreas transplant (at 4-12 weeks) may be effective in controlling diabetic neuropathy in this in vivo model.

Strongly luminescent, highly ionic europium in a lanthanum diphenylphosphinate matrix

A series of insoluble, chemically inert and thermally stable compounds La1-xEux(DPP)3 (x = 0.50, 0.20, 0.10, and 0.050; DPP = diphenylphosphinate) was synthesized and characterized by elemental and thermogravimetric analysis, FT Infrared spectroscopy and X-ray powder diffraction. Luminescence spectroscopy at both 77 and 298 K showed changes in the intensity of the hypersensitive transition 5D 0 → 7F2 of Eu3+ which are dependent of the excitation wavelength, suggesting that the europium occupies two different sites in the compounds. The large quantum efficiency and quantum yield, as well as the long radiative lifetime of the 5D0 Eu3+ level of the series of compounds, which are desirable qualities for light-conversion molecular devices, are discussed in terms of the interactions and the energy transfer process between the ligands and the metal ion. © 2012 Elsevier B.V. All rights reserved.

Structure and Ionic Conductivity in the Mixed-Network Former Chalcogenide Glass System [Na2S](2/3)[(B2S3)(x)(P2S5)(1-x)](1/3)

Glasses in the system [Na2S](2/3)[(B2S3)(x)(P2S5)(1-x)](1/3) (0.0 <= x <= 1.0) were prepared by the melt quenching technique, and their properties were characterized by thermal analysis and impedance spectroscopy. Their atomic-level structures were comprehensively characterized by Raman spectroscopy and B-11, P-31, and Na-23 high resolution solid state magic-angle spinning (MAS) NMR techniques. P-31 MAS NMR peak assignments were made by the presence or absence of homonuclear indirect P-31-P-31 spin-spin interactions as detected using homonuclear J-resolved and refocused INADEQUATE techniques. The extent of B-S-P connectivity in the glassy network was quantified by P-31{B-11} and B-11{P-31} rotational echo double resonance spectroscopy. The results clearly illustrate that the network modifier alkali sulfide, Na2S, is not proportionally shared between the two network former components, B and P. Rather, the thiophosphate (P) component tends to attract a larger concentration of network modifier species than predicted by the bulk composition, and this results in the conversion of P2S74-, pyrothiophosphate, Na/P = 2:1, units into PS43-, orthothiophosphate, Na/P = 3:1, groups. Charge balance is maintained by increasing the net degree of polymerization of the thioborate (B) units through the formation of covalent bridging sulfur (BS) units, B S B. Detailed inspection of the B-11 MAS NMR spectra reveals that multiple thioborate units are formed, ranging from neutral BS3/2 groups all the way to the fully depolymerized orthothioborate (BS33-) species. On the basis of these results, a comprehensive and quantitative structural model is developed for these glasses, on the basis of which the compositional trends in the glass transition temperatures (T-g) and ionic conductivities can be rationalized. Up to x = 0.4, the dominant process can be described in a simplified way by the net reaction equation P-1 + B-1 reversible arrow P-0 + B-4, where the superscripts denote the number of BS atoms for the respective network former species. Above x = 0.4, all of the thiophosphate units are of the P-0 type and both pyro-(B-1) and orthothioborate (B-0) species make increasing contributions to the network structure with increasing x. In sharp contrast to the situation in sodium borophosphate glasses, four-coordinated thioborate species are generally less abundant and heteroatomic B-S-P linkages appear to not exist. On the basis of this structural information, compositional trends in the ionic conductivities are discussed in relation to the nature of the charge-compensating anionic species and the spatial distribution of the charge carriers.

Heat conduction in a chain of harmonic and anharmonic oscillators under the presence of an energy conserving noise

Reproducing Fourier's law of heat conduction from a microscopic stochastic model is a long standing challenge in statistical physics. As was shown by Rieder, Lebowitz and Lieb many years ago, a chain of harmonically coupled oscillators connected to two heat baths at different temperatures does not reproduce the diffusive behaviour of Fourier's law, but instead a ballistic one with an infinite thermal conductivity. Since then, there has been a substantial effort from the scientific community in identifying the key mechanism necessary to reproduce such diffusivity, which usually revolved around anharmonicity and the effect of impurities. Recently, it was shown by Dhar, Venkateshan and Lebowitz that Fourier's law can be recovered by introducing an energy conserving noise, whose role is to simulate the elastic collisions between the atoms and other microscopic degrees of freedom, which one would expect to be present in a real solid. For a one-dimensional chain this is accomplished numerically by randomly flipping - under the framework of a Poisson process with a variable “rate of collisions" - the sign of the velocity of an oscillator. In this poster we present Langevin simulations of a one-dimensional chain of oscillators coupled to two heat baths at different temperatures. We consider both harmonic and anharmonic (quartic) interactions, which are studied with and without the energy conserving noise. With these results we are able to map in detail how the heat conductivity k is influenced by both anharmonicity and the energy conserving noise. We also present a detailed analysis of the behaviour of k as a function of the size of the system and the rate of collisions, which includes a finite-size scaling method that enables us to extract the relevant critical exponents. Finally, we show that for harmonic chains, k is independent of temperature, both with and without the noise. Conversely, for anharmonic chains we find that k increases roughly linearly with the temperature of a given reservoir, while keeping the temperature difference fixed.

Molekulardynamiksimulationen zur Untersuchung des Mischalkali-Effektes in silikatischen Gläsern

In der vorliegenden Arbeit wird mittels Molekulardynamik(MD)-Computersimulationen die Dynamik von verschiedenen Alkalisilikaten in der Schmelze und im Glas untersucht. Es ist bekannt, daß diese Systeme ionenleitend sind, was auf eine hohe Mobilität der Alkaliionen im Vergleich zu den glasbildenden Komponenten Si und O zurückzuführen ist. Im Mittelpunkt des Interesses steht der sog. Mischalkalieffekt (MAE), der in ternären Mischungen aus Siliziumdioxid mit zwei Alkalioxiden auftritt. Gegenüber Mischungen mit nur einer Alkaliionensorte weisen letztere Systeme eine signifikante Verlangsamung der Alkaliionendiffusion auf. Zunächst werden zwei binäre Alkalisilikate simuliert, nämlich Lithiumdisilikat (LS2) und Kaliumdisilikat (KS2). Die Simulationen zeigen, daß der Ursprung der hohen Mobilität der Alkaliionen in der Struktur begründet ist. KS2 und LS2 weisen auf intermediären Längenskalen Ordnung auf, die in partiellen statischen Strukturfaktoren durch Prepeaks reflektiert ist. Die den Prepeaks zugrundeliegende Struktur erklärt sich durch perkolierende Netzwerke aus alkalioxidreichen Kanälen, die als Diffusionskanäle für die mobilen Alkaliionen fungieren. In diesen Kanälen bewegen sich die Ionen mittels Sprüngen (Hopping) zwischen ausgezeichneten Plätzen. In der Simulation beobachtet man für die hohen Temperaturen (4000K>=1500K) eine ähnliche Aktivierungsenergie wie im Experiment. Im Experiment findet allerdings unterhalb von ca.1200K ein Crossover in ein Arrheniusverhalten mit höherer Aktivierungsenergie statt, welches von der Simulation nicht nachvollzogen wird. Das kann mit der in der Simulation nicht im Gleichgewicht befindlichen Si-O-Matrix erklärt werden, bei der Alterungseffekte beobachtet werden. Am stärksten ist der MAE für eine Alkalikomponente, wenn deren Konzentrationsanteil in einem ternären Mischalkalisystem gegen 0 geht. Daher wird ein LS2-System untersucht, in dem ein Li-Ion gegen ein K-Ion getauscht wird. Der Einfluß des K-Ions ist sowohl lokal in den charakteristischen Abständen zu den ersten nächsten Nachbarn (NN) zu sehen, als auch in der ortsaufgelösten Koordinationszahlverteilung bis zu Längenskalen von ca. 8,5 Angstrom. Die Untersuchung der Dynamik des eingesetzten K-Ions zeigt, daß die Sprungwahrscheinlichkeit nicht mit der Lokalisierung, einem Maß für die Bewegung eines Teilchens um seine Ruheposition, korreliert ist, aber daß eine chemische Umgebung mit wenig Li- und vielen O-NN oder vielen Li- und wenig O-NN ein Sprungereignis begünstigt. Zuletzt wird ein ternäres Alkalisilikat (LKS2) untersucht, dessen Struktur alle charakteristischen Längenskalen von LS2 und KS2 aufweist. Es stellt sich also eine komplexe Struktur mit zwei perkolierenden Subnetzwerken für Alkaliionen ein. Die Untersuchung der Dynamik zeigt eine geringe Wahrscheinlichkeit dafür auf, daß Ionen in ein Subnetzwerk andersnamiger Ionen springen. Auch kann gezeigt werden, daß das Modellpotential den MAE reproduzieren kann, daß also die Diffusionskonstanten in LKS2 bei bis zu einer Größenordnung langsamer sind als in KS2 bzw. LS2. Der beobachtete Effekt stellt sich zudem vom funktionalen Verlauf her so dar, wie er beim MAE erwartet wird. Es wurde auch festgestellt, daß trotz der zeitlichen Verzögerung in den dynamischen Größen die Anzahl der Sprünge pro Zeit nicht geringer ist und daß für niedrige Temperaturen (d.h.im Glas) Sprünge auf den Nachbarplatz mit anschließendem Rücksprung auf die vorherige Position deutlich wahrscheinlicher sind als bei hohen Temperaturen (also in der Schmelze). Die vorliegenden Resultate geben Aufschluß über die Details der Mechanismen mikroskopischer Ionenleitung in binären und ternären Alkalisilikaten sowie dem MAE.

Ionic liquids in a multiscale study

Trotz des hohen Interesse an Ionischen Flüssigkeiten wird das zielgerichtete Design und die Anwendung Ionischer Flüssigkeiten durch fehlendes grundlegendes Verständnis erschwert. Deshalb wurde die Balance der molekularen Wechselwirkungen in Ionischen Flüssigkeiten studiert, um die Eigenschaften dieser zu verstehen und die Kraftfeldentwicklung im Rahmen des Multiskalenansatzes zu systematisieren. Es wurden reine Imidazolium-basierte Ionische Flüssigkeiten, Mischungen mit kleinen Molekülen und eine protische Ionische Flüssigkeit mit ab-initio-Methoden, hauptsächlich Car-Parrinello-Molekulardynamik, untersucht. Weiterhin wurden Eigenschaften der Flüssigphase mit denen von Ionenpaaren verglichen.rnIm Fokus standen die molekularen elektrostatischen Eigenschaften und es wurde gezeigt, dass Coulomb-Wechselwirkungen zu einzigartigen Charakteristika führten. So waren die Ionen-Nettoladungen stets reduziert, die molekularen Dipolmomentverteilungen sehr breit, elektronische Polarisation war entscheidend. Die elektrostatischen Eigenschaften waren allgemein lokal auf molekularen Größen- und Zeitskalen und hingen stark von Phasenzustand und Zusammensetzung ab. Für andere molekulare Eigenschaften, wie der Neigung zu dispersiven Kontakten oder Wasserstoffbrücken, wurde gezeigt, dass sie einen entscheidenden Einfluss auf die Feinstruktur Ionischer Flüssigkeiten hatten. Das Gleichgewicht der Wechselwirkungen zeigte sich auch in Leistungsspektren, die sich aus den ab-initio-Molekulardynamiksimulationen ergaben. Diese boten einen neuen Weg für den Vergleich zum Experiment und für einen Einblick in die schnelle Dynamik Ionischer Flüssigkeiten.

Influence of a totally open oval window on bone conduction in otosclerosis

The aim of this study was to evaluate changes in bone conduction thresholds before, during and after total stapedectomy.