Conductivity Of Na Research Articles

Ionic conductivity of sodium silicates with lovozerite-type structure

The ionic conductivity of Na,Zr and Na,Sn silicates of the lovozerite family (Na8 − x H x ZrSi6O18 structural type, space group R \( \bar 3 \) m) was studied in the temperature range of 293–800 K using the impedance spectroscopy method (5−5 × 105 Hz). The compositions of the studied compounds were obtained using the method of hydrothermal synthesis in the MO2-SiO2-NaOH-H2O and MO2-SiO2-CaO-NaOH-H2O (M = Zr, Sn) systems at 573–823 K. The samples for electrophysical studies were prepared according to the ceramic technology. It was found that isovalent cation substitutions of Sn4+ → Zr4+ in Na8M4+Si6O18 and Na6CaM4+Si6O18 and H+ → Na+ in Na8 − x H x ZrSi6O18 result in an increase in the ionic conductivity by 2–3 orders of magnitude, without affecting the ionic transport activation energy (0.6–0.7 eV). The best electrolytic characteristics are typical for the Na5H3ZrSi6O18 compound, for which the ionic conductivity value is 5 × 10−4 S/cm at 573 K.

Changes in Na+ channel expression and nodal persistent Na+ currents associated with peripheral nerve regeneration in mice

Patients with peripheral neuropathy frequently suffer from positive sensory (pain and paresthesias) and motor (muscle cramping) symptoms even in the recovery phase of the disease. To investigate the pathophysiology of increased axonal excitability in peripheral nerve regeneration, we assessed the temporal and spatial expression of voltage-gated Na(+) channels as well as nodal persistent Na(+) currents in a mouse model of Wallerian degeneration. Crushed sciatic nerves of 8-week-old C57/BL6J male mice underwent complete Wallerian degeneration at 1 week. Two weeks after crush, there was a prominent increase in the number of Na(+) channel clusters per unit area, and binary or broad Na(+) channel clusters were frequently found. Excess Na(+) channel clusters were retained up to 20 weeks post-injury. Excitability testing using latent addition suggested that nodal persistent Na(+) currents markedly increased beginning at week 3, and remained through week 10. These results suggest that axonal regeneration is associated with persistently increased axonal excitability resulting from increases in the number and conductance of Na(+) channels.

Magnetron sputtering of NASICON (Na 3Zr 2Si 2PO 12) thin films : Part II: A novel approach

Despite the discovery of sodium superionic conductivity in Na 1+ x Zr 2Si x P 3− x O 12 (0.4 < x < 0.8) compounds in 1976, up to now their synthesis in homogeneous thin films has not been achieved on wide surfaces. In Part I, we have shown the limitations of the classical radiofrequency sputtering methods for depositing this kind of films with high microscopic and macroscopic chemical homogeneities. The present paper (Part II) proposes a new sputtering method based on the reactive co-sputtering of ZrSi alloy and Na 3PO 4 targets. The last one is powered by a high frequency pulsed direct current (DC) generator. It is possible to tune the discharge voltage as the current is regulated thanks to the modifications of the discharge frequency and/or the pulse off-time. The deposition rate of the Na 3PO 4 target is strongly influenced by the discharge voltage. Thin films were deposited in the Na 2O–ZrO 2–SiO 2–P 2O 5 system with a high physical quality. The experimental configuration allows to adjust the Na/Si atomic ratio by varying the intensity dissipated by the ZrSi target and the target–substrate distance. Hence it is possible to reach on a large area the ratio 1.5 characteristic for Na 3Zr 2Si 2PO 12 compound. The structural analysis of a selected film showed an amorphous film in the as deposited condition. After annealing at 700 °C the monoclinic structure, which corresponds to the superionic conducting phase Na 3Zr 2Si 2PO 12 was obtained.

Magnetron sputtering of NASICON (Na 3Zr 2Si 2PO 12) thin films Part I: Limitations of the classical methods

Despite the discovery of sodium superionic conductivity in Na 1+ x Zr 2Si x P 3− x O 12 (0.4 < x < 0.8) compounds in 1976, up to now their synthesis in homogeneous thin films has not been achieved on wide surfaces. In order to study the efficiency of classical sputtering methods for the synthesis of such compounds, thin films have been prepared by sputter deposition from two types of targets with the Na 3Zr 2Si 2PO 12 stoichiometry. The first one, associated with a NASICON (Sodium (Na) SuperIonic CONductor) structure, presented strong mechanical and chemical instabilities during sputtering that prevented a good experimental reproducibility. On the other hand the second one, a composite mixture of Na 3PO 4 and ZrSiO 4, was stable. Nevertheless, strong macroscopic and microscopic chemical heterogeneities were observed for the corresponding deposited films. These features are interpreted in terms of sodium mobility in the coating and re-sputtering of light elements during the deposition process.

Na-, K-, 及びH-ケニヤアイト中におけるプロトン伝導

Na-, K-, 及びH-ケニヤアイト中におけるプロトン伝導

Tight-binding calculation of the optical conductivity in Na xCoO 2

Optical conductivity in Na x CoO 2 is calculated with tight binding approximation. The calculated gross features agree with the experimental findings. Interband transitions between nearly degenerate t 2g bands bring about absorption in the mid-infrared and near infrared regions. Structures observed at higher energies are also quite well reproduced with the present calculation. Possible effects of electron correlation are also briefly discussed.

Determination of the p-electronic conduction parameter of NASICON by potentiometric measurements

Using the galvanic cell: Pt |Au,CO2,O2|aNa‴Na2CO3(Au)|aNa″NASICON|aNa′FeO+NiO(borateglass)|FeNi(48)|Au |Pt the p-electronic conduction parameter a⊕ of NASICON was characterized as a function of the sodium activity aNa″ and the temperature. For that purpose, the isothermal voltage response to successive variations of aNa″ by changing the composition of the CO2–O2 gas mixture was evaluated by a non-linear regression procedure. Within the temperature range under investigation (300–620°C) it holds that: loga⊕=11.076±0.1−(18278.0±1100)KT(T≤500°C)loga⊕=−3.519±1.7−(7904±1400)KT(500°C≤T≤620°C) Thus, in a CO2 sensor with NASICON as solid electrolyte the sodium activity prevailing at the measuring electrode is outside the ionic domain of the electrolyte. This is in apparent contradiction to most of the literature reports on the behavior of such gas sensors, but confirms previous findings on the extent of the electronic conductivity in Na–beta-alumina under comparable conditions.

Conditions for the Triggering of Spreading Depression Studied With Computer Simulations

In spite of five decades of study, the biophysics of spreading depression (SD) is incompletely understood. Earlier we have modeled seizures and SD, and we have shown that currents through ion channels normally present in neuron membranes can generate SD-like depolarization. In the present study, we define the conditions for triggering SD and the parameters that influence its course in a model of a hippocampal pyramidal cell with more complete representation of ions and channels than the previous version. "Leak" conductances for Na(+), K(+), and Cl(-) and an ion pump were present in the membrane of the entire cell; fast inactivating voltage dependent conductances for sodium and potassium in the soma; "persistent" conductances in soma and apical dendrite, and K(+)- and voltage-dependent N-methyl-D-aspartate (NMDA)-controlled conductance in the apical dendrite. The neuron was surrounded by restricted interstitial space and by a "glia-endothelium" system of extracellular ion regulation bounded by a membrane having leak conductances and an ion pump. Ion fluxes and concentration changes were continuously computed as well as osmotic cell volume changes. As long as reuptake into the neuron and "buffering" by glia kept pace with K(+) released from the neuron, stimulating current applied to the soma evoked repetitive firing that stopped when stimulation ceased. When glial uptake was reduced, K(+) released from neurons could accumulate in the interstitium and keep the neuron depolarized so that strong depolarizing pulses injected into the soma were followed either by afterdischarge or SD. SD-like depolarization was ignited when depolarization spreading into the apical dendrite, activated persistent Na(+) current and NMDA-controlled current. With membrane parameters constant, varying the injected stimulating current influenced SD onset but neither the depolarization nor the increase in extracellular K(+). Glial "leak" conductance influenced SD duration and SD ignition point. Varying maximal conductances (representing channel density) also influenced SD onset time but not the amplitude of the depolarization. Hypoxia was simulated by turning off the Na-K exchange pump, and this resulted in SD-like depolarization. The results confirm that, once ignited, SD runs an all-or-none trajectory, the level of depolarization is governed by feedback involving ion shifts and glutamate acting on ion channels and not by the number of channels open, and SD is ignited if the net persistent membrane current in the apical dendrites turns inward.

Internal Mobility, Phase Transitions, and Ionic Conductivity in Na(NH4)6Zr4F23 and Li(NH4)6Zr4F23

The dynamics of fluoride and ammonium ions in Na(NH4)6Zr4F23 (I) and Li(NH4)6Zr4F23 (II) was studied by 1H and 19F NMR in the temperature range 170-440 K. Types of ionic motion were determined, and their activation energies were evaluated. In I, phase transitions were found in the temperature ranges 360-370 and 410-415 K. The experimental values of conductivity σ of Na(NH4)6Zr4F23 and Li(NH4)6Zr4F23 (σ ≈ 4 × 10-3 S/cm at T = 420 K) permit one to attribute these fluorides to the class of superionic conductors.

Phonon Band Structures and Resonant Scattering in Na&lt;SUB&gt;8&lt;/SUB&gt;Si&lt;SUB&gt;46&lt;/SUB&gt; and Cs&lt;SUB&gt;8&lt;/SUB&gt;Sn&lt;SUB&gt;46&lt;/SUB&gt; Clathrates

The low and glasslike thermal conductivity of metal doped semiconductor clathrate compounds makes them potentially high efficiency thermoelectric materials. The cause of this unique and remarkable property has been postulated to be due to resonant scattering of lattice phonons by localized vibrations of the dopants. We present theoretical evidence in support of this hypothesis through the analysis of electronic and vibrational interactions between dopant atoms with the host framework. In particular, the contrasting behavior of two clathrates: the glasslike thermal conductivity in Na 8 Si 46 and the normal behavior in Cs 8 Sn 44 can be rationalized.

Molecular Dynamics Simulation of Limiting Conductance for Na2+, Cl2−, Na°, and Cl° in Supercritical Water

We report results of molecular dynamics simulations of the limiting conductance of Na2+, Cl2−, Na°, and Cl° in supercritical water using the SPC/E model for water in conjuction with our previous study (Lee et al., Chem. Phys. Lett. 293, 289 (1998)). The behavior of the limiting conductances of Na2+ and Cl2− in the whole range of water density shows almost the same trend as those of Na+ and Cl−, but the deviation from the assumed linear dependence of limiting conductances of Na2+ and Cl2− on the water density is smaller than that of Na+ and Cl−. The ratio of the limiting conductance of the divalentions to that of the corresponding monovalentions over the whole range of water density is almost constant. In the cases of Na2+ and Cl2−, the dominating factor of the number of hydration water molecules around ions in the higher-density region and the dominating factor of the interaction strength between the ions and the hydration water molecules in the lower-density region are also found as was the cases for Na+ and Cl−. These factors, however, are not so strong as for the corresponding monovalent ions because the change in the energetics, structure, and dynamics are very small mainly due to the strong Coulomb interaction of the divalent ions with the hydration water molecules. The diffusion coefficient of Na° and Cl° monotonically increases with decreasing water density over the whole range of water density. The increase of the diffusion coefficient with decreasing water density is attributed only to the dramatic decrease of the hydration number of water in the first solvation shell around the uncharged species. Among the two important competing factors in the limiting conductance of Na+ and Cl−, the effect of the number of hydration water molecules around the uncharged species is the only existing factor over the whole range of water density since the interaction strength between the uncharged species and the hydration water molecules very small through the LJ interaction. This result has confirmed the dominating factor of the number of hydration water molecules around ions in the higher-density region in the explanation of the limiting conductance of Na+ and Cl− in supercritical water at 673 K.

L-type Ca2+ current as the predominant pathway of Ca2+ entry during I(Na) activation in beta-stimulated cardiac myocytes.

In the present study Ca2+ entry via different voltage-dependent membrane channels was examined with a fluorescent Ca2+ indicator before and after beta-adrenergic stimulation. To clearly distinguish between Ca2+ influx and Ca2+ release from the sarcoplasmic reticulum the Ca2+ store was blocked with 0.1 microM thapsigargin and 10 microM ryanodine. Omitting Na+ from the pipette filling solution minimized Ca2+ entry via Na+-Ca2+ exchange. Individual guinea-pig ventricular myocytes were voltage clamped in the whole-cell configuration of the patch-clamp technique and different membrane currents were activated using specific voltage protocols. The intracellular Ca2+ concentration was simultaneously recorded with a laser-scanning confocal microscope using fluo-3 as a Ca2+ indicator. Ca2+ entry pathways were discriminated using pharmacological blockers under control conditions and during beta-adrenergic stimulation with 1 microM isoproterenol (isoprenaline) in the bathing solution or 100 microM cAMP in the patch-clamp pipette. Isoproterenol or cAMP potentiated the Ca2+ influx signals recorded during L-type Ca2+ current activation but, more interestingly, also during Na+ current (INa) activation. The Ca2+ influx signal arising from L-type Ca2+ current activation was usually blocked by 50 microM Cd2+. However, the Ca2+ influx signal elicited by the Na+ current activation protocol was only curtailed to 56.4 +/- 28.2 % by 100 microM Ni2+ but was reduced to 17.9 +/- 15.1 % by 50 microM Cd2+ and consistently eliminated by 5 mM Ni2+. The pronounced Cd2+ and moderate Ni2+ sensitivity of the Ca2+ influx signals suggested that the predominant source of Ca2+ influx during the Na+ current activation - before and during beta-adrenergic stimulation - was a spurious activation of the L-type Ca2+ current, presumably due to voltage escape during Na+ current activation. Calculations based on the relationship between Ca2+ current and fluorescence change revealed that, on average, we could reliably detect rapid Ca2+ concentration changes as small as 5.4 +/- 0.7 nM. Thus, we can estimate an upper limit for the Ca2+ permeability of the phosphorylated TTX-sensitive Na+ channels which is less than 0.04:1 for Ca2+ ions flowing through Na+ channels via the proposed 'slip-mode' Ca2+ conductance. Therefore the slip-mode Ca2+ conductance of Na+ channels does not contribute noticeably to the Ca2+ signals observed in our experiments.

Ionic conduction in Na +-β-alumina studied by molecular dynamics simulation

The structure and dynamics of the fast-ionic conductor Na +-β-alumina are studied by classical molecular dynamics simulation. Four different compounds are simulated: stoichiometric-, non-stoichiometric- and two Mg-stabilised β-aluminas. The Beevers–Ross (BR)-site is the most stable position for stoichiometric Na + ions in the conduction plane. Excess Na + ions are found to occupy positions that are slightly off-center from the aBR lattice site. This shifted aBR, or A-site, occupation is associated with a reconstruction of three ions and a neighbouring BR vacancy. Interstitial oxygens in the conduction plane stabilise the A-site by creating permanent BR vacancies. The diffusion coefficients and conductivities as a function of temperature display close to Arrhenius behaviour. Results are in agreement with experiment for the 300–900 K regime. We find a tendency for an increasing apparent activation energy at higher temperatures.

Relations Between Transport Coefficients in Lennard–Jones Fluids and in Liquid Metals

Several simple approximate hard-sphere relations for transport coefficients are compared with the results of molecular dynamics (MD) simulations performed on Lennard–Jones (LJ) fluids. Typically the individual transport coefficients: self-diffusion coefficients, D, shear viscosity, ηs, bulk viscosity, ηB, and thermal conductivity, λ, agree within a factor of two of the exact results over the fluid and liquid parts of the phase diagram, which seems reasonable in view of the approximations involved in the models. We have also considered the ratio, λ/ηs, and the product, Dηs, for which simple analytic expressions exist in the hardsphere models. These two quantities also agree within a factor of two of the simulation values and hard sphere analytic expressions. Using time correlation functions, Tankeshwar has recently related the ratio λ/D to thermodynamic quantities, in particular, to the differences in specific heats, C p − C V, and to the isothermal compressibility, κT. Using D and thermodynamic values taken solely from LJ MD simulations, his relation was tested and found to give typically better than ~20% agreement at liquid densities, deteriorating somewhat as density decreases into the gas phase. Finally liquid metals are considered. In this case, λ is dominated by its electronic contribution, which is related approximately to the electrical conductivity by the Wiedemann–Franz Law. Some theoretical results for the electrical conductivity of Na are referenced, which allow a semiquantitative understanding of the measured thermal conductivity of the liquid metal. Shear viscosity is also discussed and, following the work of Tosi, is found to be dominated by ionic contributions; Nevertheless, at the melting temperature of Na, a relation emerges between thermal conductivity, electrical resistivity and shear viscosity.

Solid phase growth and characterization of β-aluminates films on α-Al 2O 3 and relative supports

Na, K and Cs-β-alumina films have been grown by solid state reactions of MAlO 2 powders with α-Al 2O 3 single crystal and ceramic supports or with MgAl 2O 4 and Y 3Al 5O 1 2 single crystal supports. It has been shown that the extent of perfection and the conductivity of the films reduces as the film thickness increases. The conductivity of Na–β,β″-Al 2O 3 thin films is comparable with the conductivity of single crystals.

Doping effects in AV 2O 5 ( A = Li, Na and Ca)

The doping effects on the electric and magnetic properties in AV 2O 5 ( A = Li, Na and Ca) have been investigated. In all these compounds, the Curie-like increase of magnetic susceptibility at low temperature was observed on increasing the Na(Li)-deficiency in NaV 2O 5 (LiV 2O 5) or Ti-substitution in CaV 2O 5, although no long-range magnetic order has ever appeared. In Na x V 2O 5, the spin-Peierls transition is suppressed by Na-deficiency and vanishes around x = 0.97. The conductivity in Na x V 2O 5 and Li x V 2O 5 increased with doping level. The temperature dependence of conductivity in Na x V 2O 5 is consistent with variable range hopping in quasi-one-dimensional system, while LiV 2O 5 shows an activation type behavior with E a ≈ 0.3eV in the temperature dependence of resistivity.

Ionic Conductivity in Na+/Pr3+–β″–Al2O3Crystals

The conduction behaviour of Pr3+ion-exchanged Na+–β″-Al2O3crystals varying in degree of exchange (ξ) has been investigated by impedance spectroscopy in the temperature range 300–1000K. The Arrhenius plots show a curvature at about 560K to higher activation energies explainable by the beginning of lanthanide ion motion. With growing Pr3+exchange, the ionic conductivity (σ) is lowered combined with an increasing activation energy (Ea). For a given degree of exchange, σ scatters about one to two orders of magnitude andEawithin 0.15–0.2 eV. These deviations can be understood by the different crystal compositions influencing the thickness of the conduction slabs and the ionic distributions. A lower in-plane cation concentration (for a given ξ) induces widened conduction channels, resulting in an increased conductivity and reduced activation energies.

Conductivity in Na +- and Li +-montmorillonite as a function of equilibration humidity

A considerable influence of relative humidity on the conductivity of sodium and lithium-montmorillonite was found. At high humidity the conductivity values are similar for both type of samples and higher than those measured at low humidity, whereas the activation energies show the opposite relationship. In the low humidity zone the lithium-containing samples exhibit higher conductivity than that of the sodium ones. An explanation is given based on the different nature of the sorbed water and on the hydration power of the Li + and Na + ions.

Monovalent cation transport: lack of structural deformation upon cation binding.

Cations often deform the structure of regulatory proteins to affect a functional response, but for other protein functions a more passive effect is desired. For instance, it is shown here that in the conductance of Na+ by the gramicidin channel there appears to be no significant structural deformation of either the side chains or backbone upon Na+ binding in the channel. This is based on 15N and 13C chemical shifts, 2H quadrupolar interactions, and 15N-2H dipolar interactions obtained by solid-state NMR spectroscopy of uniformly aligned lipid bilayer preparations of the gramicidin channel in the presence and absence of Na+. This conclusion is despite some significant changes in the 15N alpha and 13C1 chemical shift values which are argued here to be the result of indirect polarization effects upon cation binding rather than reflections of structural and dynamic changes. The lack of structural deformation implies that Na+ moves to the carbonyl oxygens lining the pore of this channel for solvation rather than the carbonyl groups moving in toward the channel axis. This forces the cations onto a helical path following the positions of the carbonyl oxygens around the channel pore. Furthermore, an ideal binding site geometry for Na+ in the channel is avoided. Instead, adequate binding energy is provided by the channel to compensate for the loss of hydration energy when the cations enter the channel. The avoidance of strong binding ensures that efficient transport of the cations through the channel can be realized.

Unitary conductance of Na+ channel isoforms in cardiac and NB2a neuroblastoma cells.

Unitary conductances of native Na+ channel isoforms (gamma Na) have been determined under a variety of conditions, making comparisons of gamma Na difficult. To allow direct comparison, we measured gamma Na in cell-attached patches on NB2a neuroblastoma cells and rabbit ventricular myocytes under identical conditions [pipette solution (in mM): 280 Na+ and 2 Ca2+, pH 7.4; 10 degrees C]. gamma Na of NB2a channels, 22.9 +/- 0.9 pS, was 21% greater than that of cardiac channels, 18.9 +/- 0.9 pS. In contrast, respective extrapolated reversal potentials, +62.4 +/- 4.6 and +57.9 +/- 5.1 mV, were not significantly different. Several kinetic differences between the channel types were also noted. Negative to -20 mV, mean open time (MOT) of the NB2a isoform was significantly less than that of cardiac channels, and, near threshold, latency to channel opening decayed more rapidly in NB2a. On the basis of analysis of MOT between -60 and 0 mV, the rate constants at 0 mV for the open-to-closed (O-->C) and open-to-inactivated (O-->I) transitions were 0.42 +/- 0.11 and 0.47 +/- 0.11 ms-1 in NB2a and 0.10 +/- 0.06 and 1.19 +/- 0.07 ms-1 in myocytes. The slope factors were -38.9 +/- 8.7 and +10.7 +/- 6.1 mV in NB2a and -27.3 +/- 7.1 and +23.7 +/- 4.9 mV in myocytes. Transition rate constants were significantly different in NB2a and cardiac cells, but voltage dependence was not.