Energy Barrier Model Research Articles

Alkali ion exchange in γ-irradiated glasses

The alkali ion exchange of γ-irradiated glasses was investigated using the structural energy barrier model for ion exchange of glasses. It is shown that rates of alkali ion exchange have a non-Arrhenius behaviour depending both on irradiation dose and dose rate. For this reason some effects cannot be simulated by external irradiation and require in situ measurements. Higher doses and dose rates of γ-radiation lead to increased ion exchange rates. Significant changes occur in the activation energies demonstrating a many times decrease depending on glass composition. Radiation-induced changes are higher at relatively low temperatures being diminished by increased glass temperature. Numerical estimations show that changes in alkali ion exchange kinetics occur below the glass network damaging doses.

Effect of Surface Pressure on Oxygen Transfer across Molecular Monolayers at the Air/Water Interface: Scanning Electrochemical Microscopy Investigations Using a Mercury Hemispherical Microelectrode Probe

Investigations of the kinetics of molecular transfer across the liquid/gas interface and the effect of a molecular monolayer are of considerable interest as a model for certain biological and environmental processes. In this work, a combined scanning electrochemical microscopy (SECM)−Langmuir trough technique has been used to investigate the effect of the chemical character and mechanical compression of molecular monolayers on the rate of oxygen transfer across the air/water (A/W) interface. Specifically, monolayers comprising the fatty alcohol 1-octadecanol and the phospholipid l-α-dipalmitoyl phosphatidic acid were considered. A mercury hemispherical microelectrode probe has been used to measure interfacial kinetics in SECM, and a numerical model has been developed for mass transport in this configuration to allow quantitative analysis of experimental data. The results obtained suggest that, for both monolayers, the oxygen-transfer rate across the interface decreased compared to that across the clean interface, with the blocking effect becoming more pronounced as the surface pressure of the monolayer increased. A simple energy-barrier model was used successfully to interpret the dependence of the rate constant of oxygen transfer on the surface pressure. The experimental data also provide evidence for the effect of the SECM probe on the deformation of the water surface at very close distances to the A/W interface.

Complex conductance of carnauba wax/polyaniline composites

Complex conductance,ae ⁄ (f) = ae0(f)+iae(f), measurements of polyaniline/carnauba wax composites were carried out at room temperature in samples containing different amounts of PAni. While ae(f) ever grows as a function of the frequency f;ae0(f) was observed to vary as f n (n … 1) in the high frequency domain, and to be frequency-independent for lower frequencies. This is the quasi-universal behavior characteristic of conduction in a disordered medium. Superposed on this, another signal was observed to occur, and the whole impedance response was then analysed in an Argand Diagram, disclosing an electrode process in series with the bulk one. An equivalent conductance expression was used to explain such electrical behavior, in which the bulk properties of the composite were represented through the Random Free Energy Barrier model, while the composite-electrode interface by a simple parallel combination of a capacitor and a resistor. From the theoretical fittings it was concluded that the increase of PAni concentration acts in the sense of increasing not only the sample conductance, but also the bulk capacitance. The separation of the bulk and the interface contributions was then achieved.

Kinetics of alkali ion exchange of irradiated glasses

ABSTRACTThe kinetics of alkali ion exchange of irradiated glasses were investigated using the structural energy barrier model for ion exchange of glasses. Derived rates of alkali ion exchange depend both on irradiation dose D(Gy) and dose rate q(Gy/s) illustrating that some effects cannot be simulated by external irradiation and require in-situ measurements. Higher D and q lead to increased ion exchange rates. Significant changes occur in the activation energies demonstrating a 4 – 6 times decrease depending on glass composition. Radiation-induced changes are higher at relatively low temperatures and are diminished by increased glass temperature. Numerical estimations show that changes in alkali ion exchange kinetics occur at D far below damaging doses.

Kinetics of bromine transfer across Langmuir monolayers of phosphatidylethanolamines at the water/air interface

The kinetics of Br2 transfer across phosphatidylethanolamine monolayers at the water/air (W/A) interface have been investigated using scanning electrochemical microscopy–double potential step chronoamperometry (SECM-DPSC). An homologous series of three phosphatidylethanolamines was considered: L-α-phosphatidylethanolamine dilauroyl (DLPE), L-α-phosphatidylethanolamine dipalmitoyl (DPPE) and L-α-phosphatidylethanolamine distearoyl (DSPE). The SECM-DPSC approach involved positioning a submarine ultramicroelectrode close to a monolayer assembled at the W/A interface at a defined and controllable surface pressure. Br2 was produced in an initial (forward) potential step, by the diffusion-controlled oxidation of Br−, and then collected by diffusion-controlled reduction in a second (reverse) potential step. The resulting current–time behaviour provided information on both the tip–interface separation (forward step response) and the kinetics of Br2 transfer (reverse step response). DSPE monolayers were found to diminish the rate of Br2 transfer across the W/A interface, with the rate constant decreasing as the surface pressure increased. These experimental data were interpreted using a simple energy barrier model. In contrast, monolayers of DLPE and DPPE had no detectable effect on Br2 transfer kinetics on the SECM time scale.

The voltage-independent cation channel in the plasma membrane of wheat roots is permeable to divalent cations and may be involved in cytosolic Ca2+ homeostasis.

A voltage-independent cation (VIC) channel has been identified in the plasma membrane of wheat (Triticum aestivum) root cells (P.J. White [1999] Trends Plant Sci 4: 245-246). Several physiological functions have been proposed for this channel, including roles in cation nutrition, osmotic adjustment, and charge compensation. Here, we observe that Ca(2+) permeates this VIC channel when assayed in artificial, planar lipid bilayers, and, using an energy barrier model to describe cation fluxes, predict that it catalyzes Ca(2+) influx under physiological ionic conditions. Thus, this channel could participate in Ca(2+) signaling or cytosolic Ca(2+) homeostasis. The pharmacology of (45)Ca(2+) influx to excised wheat roots and inward cation currents through the VIC channel are similar: Both are insensitive to 20 microM verapamil or 1 mM tetraethylammonium, but inhibited by 0.5 mM Ba(2+) or 0.5 mM Gd(3+). The weak voltage dependency of the VIC channel (and its lack of modulation by physiological effectors) suggest that it will provide perpetual Ca(2+) influx to root cells. Thus, it may effect cytosolic Ca(2+) homeostasis by contributing to the basal Ca(2+) influx required to balance Ca(2+) efflux from the cytoplasm through ATP- and proton-coupled Ca(2+) transporters under steady-state conditions.

AC conductivity and its scaling behavior in lithium and sodium bismuthate glasses

Ac conductivity measurements and its scaling analysis is studied on the new sodium and lithium bismuthate systems xNa 2O+(1− x)Bi 2O 3 and xLi 2O+(1− x)Bi 2O 3 for various compositions in the frequency range from 20 Hz to 1 MHz and in the temperature range from 467 to 750 K. The measured electrical relaxation data of these glasses are analysed using Jonscher's universal power law (UPL) and random free energy barrier model (RFEBM). All experimentally known features of the ac conductivity, including its scaling behavior, are explained satisfactorily based on the power law and random free energy barrier model.

Ac conductivity relaxation processes and its scaling behavior in sodium bismuthate glasses

Ac conductivity measurements and its analysis has been performed on the glassy ionic conductingx Na2O + (1 −x) Bi2O3 system in the frequency range from 20 Hz to 1 MHz and in the temperature range 450 K to 750 K. The composition dependence of the dc electrical conductivity and activation energy has been obtained. The electrical relaxation analysis has been inquired on these glasses with universal power law and the random free energy barrier model. The relaxation parameters are obtained from the analysis has been compared. All experimentally known features of the σ′(ω) including its scaling behavior are explained satisfactorily. The results show an excellent agreement of scaling behavior based on random free energy barrier model and universal power law.

Effect of phospholipids on the kinetics of dioxygen transfer across a 1,2-dichloroethane/water interface

The effect of a series of 1,2-diacyl-sn-glycero-3-phosphocholines (C14 : 0, C16 : 0, C18 : 0) adsorbed at the interface between a buffered aqueous phase and 1,2-dichloroethane on the transfer of molecular oxygen between the two phases has been investigated by scanning electrochemical microscopy (SECM) in the induced transfer mode. A monolayer of C18 : 0 forms a barrier for dioxygen transfer from the organic to the aqueous phase. An increase in surface concentration of C18 : 0 results in a decrease in the interfacial rate constant characterising the transfer. This behaviour is analysed in terms of a simple energy barrier model. In contrast, C14 : 0 and C16 : 0 have only a small effect on dioxygen transfer on the SECM timescale, although surface tension measurements show similar adsorption behaviour and excess surface concentrations for all three phospholipids. Reasons for these observations are discussed.

Solid-State NMR, Ionic Conductivity, and Thermal Studies of Lithium-doped Siloxane−Poly(propylene glycol) Organic−Inorganic Nanocomposites

Hybrid organic−inorganic ionic conductors, also called ormolytes (organically modified electrolytes), were obtained by dissolution of LiClO4 in siloxane−poly(propylene glycol) matrixes. The dynamic features of these nanocomposites were studied and correlated to their electrical properties. Solid-state nuclear magnetic resonance (NMR) spectroscopy was used to probe the effects of the temperature and nanocomposite composition on the dynamic behaviors of both the ionic species (7Li) and the polymer chains (13C). NMR, dc ionic conductivity, and DSC results demonstrate that the Li+ mobility is strongly assisted by the segmental motion of the polymer chain above its glass transition temperature. The ac ionic conductivity in such composites is explained by use of the random free energy barrier (RFEB) model, which is in agreement with their disordered and heterogeneous structures. These solid ormolytes are transparent and flexible, and they exhibit good ionic conductivity at room temperature (up to 10-4 S/cm). Co...

Electrical conductivity ofSrTi1−xRuxO3

${\mathrm{SrTi}}_{(1\ensuremath{-}x)}{\mathrm{Ru}}_{x}{\mathrm{O}}_{3}$ perovskite oxide compound exhibits different physical properties depending on the x value and the temperature range. High electrical conductivity, ferromagnetism phenomena, piezoelectricity, and superconductivity are currently properties found in this perovskite family. In this paper we present a study of electronic conduction of this mixed perovskite in function of x and temperature by alternating conductivity technique. Pure ${\mathrm{SrTiO}}_{3}$ is an insulating material that progresses to a conductive one as x increases. The model used to explain this conductivity evolution is the random free energy barrier model developed to explain transport phenomena in disordered materials. Disorder in this perovskite is supported by diffraction studies that evidenced slight delocalizations of oxygen atoms in the lattice, added due to random substitutions of ${\mathrm{Ti}}^{4+}$ by ${\mathrm{Ru}}^{4+}.$

Ionic Conduction and Structural Properties of Organic-Inorganic Composite Based on Poly(propileneglycol)

Correlation between ionic conduction and structural transition of poly(propileneglycol)-silica composite, complexed with lithium salt, was performed by dc and ac electrical measurements, thermal analysis (DSC) and nuclear magnetic resonance spectroscopy in the 200 to 300 K temperature range. At about 285 K an abrupt increase in the dc conductivity and a change of ac conductivity behaviour were observed. Around this temperature it was also observed a linewidth narrowing 7Li NMR while DSC thermograms shows a glass transition T g immediately followed by a melt point T m. The dc conductivity-temperature behaviour was explained by the Vogel-Tamman-Fülcher model and ac by the Random Free Energy Barrier Model.

Random Free Energy Barrier Model applied to ac conductivity in polyaniline films

Alternating conductivity measurements of polyaniline, in the 1–105 Hz range, were carried out in samples under different temperatures. It was observed that for low frequencies the real component, σ'(ƒ), of ac conductivity is frequency-independent, however obeying the relation σ'(ƒ) ∝ ƒ s, beyond a critical value; the imaginary component increases with frequency in the whole measurements frequency range. The Random Free Energy Barrier Model approach was used to explain the experimental results, and from the theoretical-experimental fittings, it was concluded that the temperature acts in the sense of increasing the carrier mobility.

Alternating electrical conductivity of polyaniline

The ac complex conductivity σ*(f) of polyaniline (PAN) films at different doping levels and different temperatures, in the 1–100 KHz frequency range, are reported. The results are typical of a disordered medium where the real component of ac conductivity is frequency independent at low frequencies, rising for higher values of frequencies. In order to interpret both the real and the imaginary components of σ*(f), we developed a model which considers the doped PAN as a disordered insulating matrix, sprinkled with conductive islands generated by doping, as indicated by energy dispersed x-ray microanalysis. The conduction through the insulating matrix obeys the random free energy barrier model, while in the conductive islands a metallic frequency-independent conductivity is considered. From the fittings we obtained the activation energy value of the maximum energy barrier of the doping mechanism and estimated the concentration of hopping sites.

Fly/stiction: mechanical instability of a head-disc interface

The fly/stiction phenomenon is described in detail. Unlike the normal stiction values, fly/stiction values exhibit a unique bi-/tri-modal distribution. Fundamentally, the fly/stiction phenomenon is related to the mechanical instability of a head-disc interface, triggered by the excessive accumulation of liquids at the interface. An energy barrier model is presented to explain various observed aspects of the fly/stiction phenomenon, and to provide directions to reduce the risk of fly/stiction failure.

Stress and time dependence of relaxation and the Kohlrausch stretched exponent formula

A general non-linear description of the kinetics of relaxation is derived. Existing phenomenological approaches are extended introducing model considerations of the flow–stress behavior of a system. Particular attention is given to the stress and time dependence in the kinetics of relaxation. It is shown that the empirical Kohlrausch stretched exponent formula is an approximation to dependences following from the Prandtl–Eyring potential energy barrier model of viscous flow of matter.

A rate theory model for Mg2+ block of ATP-dependent potassium channels of rat skeletal muscle.

1. We have studied the block by intracellular Mg2+ (0.08-4mM) of ATP-dependent potassium channels (KATP channels) from rat skeletal muscle using inside-out excised sarcolemmal patches. The block is voltage dependent, is relieved by extracellular potassium and has rapid kinetics, allowing the use of amplitude distribution analysis to estimate on and off rates. 2. To gain insight into the pore properties necessary to produce such a block, we have used an energy barrier model based on Eyring rate theory. The model has two energy wells and three barriers for K+ within the pore, while intracellular Mg2+ has access only to the inner well. We fitted the model to unitary current-voltage relations in different [Mg2+], to on and off rates, and to dissociation constants for Mg2+ block. 3. The voltage dependence of block was almost entirely due to the rate constant for unblocking. This implies that the inner energy barrier is asymmetrical, so that Mg2+ entry senses little of the voltage field, but Mg2+ exit senses about 20% of the voltage field. Best fits were obtained by placing the barrier and binding site 0.01 and 0.22, respectively, of the electrical distance through the pore from the inside. 4. The relief of block by [K+]o resulted from an increase in the unblocking rate for Mg2+, implying ionic repulsion between ions in the pore.

Enhanced coercivity due to a local anisotropy increase

In most of the cases, a local wall energy decrease, usually associated with defects, leads to domain wall pinning. Another mechanism for coercivity is to create local maxima in the anisotropy energy. It is observed that in epitaxial garnet crystals, locally enhanced anisotropy due to stress arising from iridium inclusions embedded in the substrate is the source of coercivity. By modifying our previous energy barrier model, the coercivity has been calculated as a function of the size and position of the iridium inclusions in the substrate. It is shown that the coercivity varies nonlinearly with inclusion size and depth in the substrate material, in agreement with experimental observation.

An energy-barrier model of biased transport in disordered systems

Biased diffusion in disordered media is analysed by means of the exit-time equation. Numerical results on percolation lattices show that a disordered medium behaves in the presence of a biasing field as an energy barrier whose intensity is monotonically increasing with field strength. This result is confirmed by a theoretical model.

One-dimensional energy barrier model for coercivity

The one-dimensional energy well model used to compute the coercivity of magneto-optic media based on domain-wall motion has been extended to include energy barriers. It is found that a local increase in either the anisotropy or exchange energy causes local wall pinning. In this case the wall is pinned in front of the energy barrier. When an applied field is gradually increased, the wall is pushed closer to the energy barrier, until the critical field is exceeded and a Barkhausen jump occurs. Variation of coercivity with barrier height was studied, and the results show that the coercivity increases essentially linearly with increase in height of the energy barrier. The effect of well shape on the coercivity was studied using several types of barriers of varying widths. Finally, the coercivity as a function of temperature for exchange and anisotropy barriers was computed and compared to the measured values. The results obtained in these calculations are compared with those for the well model, and it is found that by a proper choice of the dimensions of the energy barrier the coercivity obtained can approximate measured values, as in the case for the energy well model.