Dc Ionic Conductivity Research Articles

DC ionic conductivity of NaNO 3: γ-Al 2O 3 composite solid electrolyte system

We present DC ionic conductivity measurements on composites formed between Na + ion conductor (NaNO 3) and dispersed insulating oxide (alumina). Enhancement of conductivity is noticed to increase with mole percent (m/o) of the dispersoid. The maximum enhancement observed is more than two orders of magnitude with respect to the host material. X-ray diffraction and differential scanning calorimetry studies ruled out the formation of solid solutions between the host material and the dispersoid. The experimental data indicating higher conductivity in dispersed system is interpreted in terms of the formation of space charge layer between the host material and the dispersoid in which defect concentration increases and that is thought to be the possible mechanism of conductivity enhancement. Activation energies obtained from the conductivity data in the extrinsic conduction region indicated least value for the systems at threshold mole percentage.

Spectral response from modulus time domain data of disordered materials

The electric response of the glass-forming glycerol and crystalline ionic conductor Li0.18La0.61TiO3 is probed by modulus time domain measurements. A capacity correction algorithm is proposed to overcome the low capacitance limit of the technique. This method allows to Fourier-transform time domain data yielding undisturbed permittivity spectra. The algorithm is checked first in glycerol, where the dielectric data recorded in frequency and time domain show an overlap of several decades. It is also applied to match the dielectric data of the crystalline ionic conductor Li0.18La0.61TiO3 from modulus time domain with overlapping frequency domain data, forming data sets covering 11 decades in frequency. The extension of the electrical characterization to low frequencies allows the detection of an Arrhenius behavior for the dc ionic conductivity at low temperatures, in disagreement with previous analysis in terms of Vogel-Fulcher-Tammann laws.

Predictability of ion transport properties from the structure of solid electrolytes

The concept of bond valence (BV) is widely used in crystal chemical considerations, e.g. to assess equilibrium positions of atoms in crystal structures from an empirical relationship between bond lengthR M−X and bond valenceS A−X =exp [(R 0 −R M−X ) /b] as sites where the BV sumV(A)=∑ s M−X equals the formal valenceV id of the cationM + . Our modified BV approach that systematically accounts for the softness of the bond may then be effectively used to study the interplay between structure and properties of solid electrolytes. This is exemplified for correlations to experimental data from IR, NMR, and impedance spectroscopy. Combining the bond valence approach with reverse Monte Carlo (RMC) modeling or molecular dynamics (MD) simulations provides a deeper understanding of ion transport mechanisms, especially in highly disordered or amorphous solids. Local structure models for crystalline electrolytes are derived by combining crystallographic structure information with simulations. A method for the prediction of the activation energy of the ionic conductivity from the bond valence analysis of the crystal structure is proposed. Taking into account the mass dependence of the conversion factor from bond valence mismatch into an activation energy scale, we could establish a correlation that holds for different types of mobile ions. The strong coupling of the H+ transfer to the anion motion in proton conductors requires a special treatment. For glassy solid electrolytes RMC structure models are BV-analyzed to assess the total number of equilibrium sites and to identify transport pathways for the mobile ions. Recently, we have reported a correlation between the pathway volume fraction and the transport properties that permits to predict both absolute value and activation energy of the dc ionic conductivities of disordered solids (including mixed alkali glasses) directly from their structural models. Here we discuss a corresponding BV analysis of molecular dynamics simulation trajectories that allows quantifying the evolution of pathways in time and the influence of temperature on the transport pathways.

Two dimensional fluoride ion conductor RbSn$_\mathsf{2}$F$_\mathsf{5}$ studied by impedance spectroscopy and $^\mathsf{19}$F, $^\mathsf{119}$Sn, and $^\mathsf{87}$Rb NMR

RbSn2F5 is a two-dimensional fluoride ion conductor. It undergoes a first-order phase transition to a superionic state at 368 K. The structure of the low temperature phase has been determined from the Rietveld analysis of the X-ray powder diffraction. The dynamic properties of the fluoride ions in RbSn2F5 have been studied by impedance spectroscopy and solid state NMR. The dc ionic conductivity of this sample shows an abrupt increase at the phase transition temperature. We have obtained the hopping frequency and the concentration of the charge carriers (F- ions) at different temperatures from the analysis of the conductivity spectra using Almond-West formalism. The estimated values of the charge carriers’ concentration agree well with that determined from the structure and were found to be independent of temperature. The relatively small value of the power-law exponent, \(n \approx 0.55\), supports the two-dimensional property of the investigated material. Furthermore, 19F NMR with simulation has suggested the diffusive motions of the fluoride ions between different sites. In contrast, 119Sn and 87Rb NMR spectra below 250 K supported the intrinsic disordered nature due to the random distribution of the fluoride ion vacancies.

Effect of alumina on dc ionic conductivity of barium nitrate solid electrolyte

The dispersion of γ-Al 2O 3 on dc ionic conductivity of a Frenkel type defect solid, Ba(NO 3) 2, showed an enhancement of ionic conductivity of nearly two orders of magnitude with respect to pure host material in the extrinsic region. This has been explained on the basis of the formation of large number of grain boundaries and also attributed to formation of space charge layers between host ionic material and insulating particles. The activation energy in the extrinsic motion region was found to decrease due to the formation of vacancies in the space charge region, which is thought to be more probable than the formation of interstitials.

Modulation of iono-covalency in Ag–O bond of layer-type oxides Ag x[Ni x/2 IITi 1− x/2 IV]O 2 ( x=0.85, 0.70)

Layer-type oxides Ag x [Ni x/2 IITi 1− x/2 IV]O 2 ( x=0.85, 0.70) have been prepared by cation-exchange reactions using corresponding Na precursors. For x=0.85, a 3R-delafossite phase was obtained as expected, with Ag + ions at linear sites between [Ni,Ti]O 2 sheets. However, x=0.70 was found to exhibit a six-layered rhombohedral structure, which furnishes the rock-salt layer for Ag + ions between [Ni,Ti]O 2 sheets. This structure change has been considered as a consequence of bond transition of Ag–O from covalent to ionic, due to the enhanced covalency of its competing bond [Ni,Ti]–O. DC polarization and ionic conductivity results confirmed that the interaction of Ag + ions with oxides at rock-salt layer are highly ionic enough to exhibit a conductivity of 3.6×10 −5 S cm −1 at 300 K.

Modeling Li-ion conductivity in fast ionic conductor La 2/3− xLi 3 xTiO 3

Monophasic samples of fast ionic conductors La 2/3− x Li 3 x TiO 3 (LLTO), with x varying from 0.06 to 0.13, are prepared by solid-state reaction. The total dc-conductivity is measured by complex impedance spectroscopy in the 10 MHz –1 Hz frequency range. Considering only the resulting location of oxygen atoms and employing bond valence equations, the conduction geometry and dc ionic conductivity are modeled. An averaged pathway for the Li + conduction is proposed in this paper, assuming that the time-averaged position for Li + is the geometrical centre of the A-cage. The saddle point of this pathway (Vumax) can be related to the activation energy for the ionic jump. Moreover, in order to model dc-ionic conductivity, not only activation energy, but also number of carriers and site occupancies have been considered. We propose three possibilities for the Li + location in the structure in order to predict bulk conductivity in LLTO phase. Experimental evidence allows the exclusion of one of the three possibilities, while the other two are both in agreement with experimental values.

Bond valence analysis of ion transport in reverse Monte Carlo models of mixed alkali glasses

ABSTRACTAn analysis of RMC structure models of ion conducting glasses in terms of our bond softness sensitive bond-valence method enables us to identify the conduction pathways for a mobile ion as regions of sufficiently low valence mismatch. The strong correlation between the volume fraction F of the “infinite pathway cluster” and the transport properties yields a prediction of both the absolute value and activation energy of the dc ionic conductivities directly from the structural models. Separate correlations for various types of mobile cations can be unified by employing the square root of the cation mass as a scaling factor. From the application of this procedure to RMC models of mixed alkali glasses, the mixed alkali effect, i.e. the extreme drop of the ionic conductivity when a fraction of the mobile ions is substituted by another type of mobile ions may be attributed mainly to the blocking of conduction pathways by unlike cations. The high efficiency of the blocking can be explained by the reduced fractal dimension of the pathways on the length scale of individual ion transport steps.

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...

Test of the fractional Debye-Stokes-Einstein equation in low-molecular-weight glass-forming liquids under condition of high compression.

From temperature studies at ambient pressure, it was pointed out for several glass-forming liquids that the alpha-relaxation time (tau) can be related to the dc-ionic conductivity (sigma) through the phenomenological fractional Debye-Stokes-Einstein (DSE) equation. In the present paper we test the validity of fractional DSE equation for relaxation data obtained from pressure variable experiments. To this end we carried out broadband dielectric measurements (10 mHz-10 MHz) in a wide range of pressures (0.1-300 MPa). The material under study were N,N-diglycidyl-4-glycidyloxyaniline and N,N-diglycidylaniline. As a result we found that the fractional DSE equation is also obeyed for pressure pathways.

Relaxation phenomena in lithium iodate crystals

Ionic conductivity of lithium iodate crystals grown in various conditions has been studied in the polar c-axis direction. The dielectric response at room temperature is associated with relaxation of space charges. The temperature dependence of the dc ionic conductivity shows quite different activation energies in the range 35–470 K which are well related to the doping and acidity of the growth solution.

Kinetics of silver structures growth by electrodiffusion in quartz crystals

Silver layers were grown in quartz crystals by electrodiffusion from an anodic layer of AgNO 3. Different experimental arrangements were employed in order to obtain such layers in a reproducible way. We performed DC ionic conductivity measurements on Z-cut and X-cut samples as a function of temperature in the heating up phase and as a function of time at the electromigration temperature. The metal structures and the quartz samples were analyzed by electron microscopy and electron diffraction. Based on the experimental results and on the observations a model for the metal structure formation is proposed.

Ionic charge transport in strongly structured molten salts

Data on the DC ionic conductivity for strongly structured molten halides of divalent and trivalent metals near freezing are interpreted as mainly reflecting charge transport by the halogen ions. On this assumption the Nernst–Einstein relation allows an estimate of the translational diffusion coefficient D tr of the halogen. In at least one case (molten ZnCl 2) D tr is much smaller than the measured diffusion coefficient, pointing to substantial diffusion via neutral units. The values of D tr estimated from the Nernst–Einstein relation are analyzed on the basis of a model involving two parameters, i.e. a bond-stretching frequency ω and an average waiting time τ. With the help of Raman scattering data for ω, the values of τ are evaluated and found to mostly lie in the range 0.02–0.3 ps for a vast class of materials.

Mechanical loss, creep, diffusion and ionic conductivity of ZrO 2-8 mol%Y 2O 3 polycrystals

Polycrystalline ZrO 2-8 mol%Y 2O 3 was investigated by combining several experimental techniques on identical materials sintered out of the same high purity powder. The mechanical loss spectrum (damping and elastic modulus) was measured in a large frequency and temperature range (10 −2Hz–1.5kHz; −150 to 1400°C). Damping due to point defect relaxation at low temperature and to viscoelastic relaxation at high temperature was revealed. The creep resistance was investigated with four-point bending tests (stress and temperature ranges: 20–75 MPa, 1100–1290°C), indicating Nabarro-Herring creep as the main rate-controlling mechanism. Both viscoelastic deformation and creep seem to be controlled by cation diffusion. Measurements of the 96Zr tracer diffusivity by secondary ion mass spectrometry at 1125–1460°C yielded an activation enthalpy of 460 kJ/mol. Close values were obtained for creep (440 kJ/mol) and viscoelastic relaxation (530 kJ/mol). Finally, the ionic DC-conductivity of these electrolytes was measured with high accuracy in the range 300–1250°C.

An evaluation of random R-C networks for modelling the bulk ac electrical response of ionic conductors

The ac electrical characteristics of a 2-D square randomly filled RC network are shown to exhibit many features in common with those of a typical ionic conductor. These include the ‘universal’ power law frequency responses of ac conductivity and permittivity. It is suggested that a random network of ion conduction channels and insulating islands may provide a realistic model of the conduction paths within a bulk ionic conductor. DC ionic conductivity is identified with a resistive percolation path through the network and the higher frequency power law component with that of the random network of ac conducting linked capacitive and conductive regions. The dielectric loss peak associated with ionic conductors is also found to be a random R-C network characteristic. The effect of the insulating islands having a Debye-like capacitance is modelled and ZrO2 12 mol% Y2O3 data are presented which exhibit the effects predicted.

A parameter study on the impedance of poly(3-methylthiophene) film electrodes

Poly(3-methylthiophene) films (PMT), polymerised galvanostatically ( i=1.0 mAcm −2) in propylene carbonate +0.03 M TBAPF 6+0.2 M 3-methylthiophene solution, were characterised by cyclic voltammetry and coulometry combined with gravimetrical measurements and Electrochemical Impedance Spectroscopy (EIS). The dc electronic and ionic conductivities of PMT-films were measured on free-standing membranes as function of the electrode potential. From these measurements, the mobilities and diffusivities of electrons and ions were determined at T=298 K ( μ e=0.21 cm 2 V −1 s −1, μ i=6.3×10 −8 cm 2 V −1 s −1 and D e=5.6×10 −3 cm 2 s −1, D i=1.6×10 −9 cm 2 s −1). EIS measurements on Pt/PMT-film electrodes were carried out in the monomere free propylene carbonate solution in dependence of the following parameters: PMT-film thickness l, supporting electrolyte concentration c, temperature T and polarisation potential E. The low frequency part of the impedance spectra could be fitted by a simple cylindrical pore model. The dependence of the fit parameters on the system parameters favours the assumption that the PMT-film is porous and the pores are filled with the bathing electrolyte.

Enhanced specific grain boundary conductivity in nanocrystalline Y 2O 3-stabilized zirconia

Stabilized zirconia samples containing 1.7 and 2.9 mol% Y 2O 3 with average grain sizes of 25–50 nm were prepared by pressureless sintering and hot-pressing. Phase content, microstructure and average grain size of the samples were examined using X-Ray Diffraction (XRD) and High Resolution Scanning Electron Microscopy (HRSEM). The density of the samples measured based on the Archimedes Principle was in the range of 93–96% of the theoretical density. The samples were also investigated using X-ray Photoelectron Spectroscopy (XPS) for traces of Si. Impedance Spectroscopy was used to determine the dc ionic conductivities of the grain interior (bulk) and of the grain boundaries. The activation energies of both processes were slightly lower as for comparable microcrystalline samples as reported in the literature. This result is attributed to the complete tetragonal structure and the low Si-content of the samples. The conductivities of the bulk and the grain boundary process were in the same range as for microcrystalline samples. Therefore, due to the much smaller grain size the specific grain boundary conductivities of the nanocrystalline samples is 1–2 orders of magnitude higher than that of the microcrystalline samples. This is also attributed to the low Si-content and its grain size-dependent segregation in the nanocrystalline samples.

Aggregation of the doping salt in B 2S 3–Li 2S–LiI glasses, effect on the dynamical properties

LiI aggregates have been detected by 7 Li NMR in LiI-rich (1− y) [0.4B 2S 3 0.6Li 2S] yLiI thioborate glasses. Such aggregates are not detectable by XRD because of their size. The amount of actually dissolved doping salt in the LiI-rich glasses is smaller than for glasses with lower y, for which no aggregates exist. The formation of aggregates leads to a decrease in the ionic DC conductivity. In addition, the DC behaviour of the dissolved lithium ions is perturbed by the presence of the nearby aggregates, but this seems not to be the case for the high frequency ion hops, as probed by NMR.

Curing kinetics of phase separating epoxy thermosets studied by dielectric and calorimetric investigations: A simple model for the complex dielectric permittivity

Dielectric relaxation spectroscopy (3 kHz ≤ ƒ ≤ 3 MHz), differential scanning calorimetry, and temperature-modulated calorimetry have been performed during isothermal curing of an epoxy network (diglycidylether of bisphenol A crosslinked with diaminodiphenyl methane), and of two thermoplast modified epoxy resins (semi-interpenetrating polymer networks) consisting of the epoxy network component and different amounts (10 and 20 wt %) of a linear high Tg polymer (polysulfone or polyethersulfone). During reaction, the homogeneous-mixtures phase separate into an epoxy-rich and a linear polymer-rich phase with different mobilities of the electrical dipoles. The complex dielectric permittivity is composed of a contribution from the ionic dc-conductivity and a contribution from relaxations of the permanent electrical dipoles in the two phases. The decrease of the dc-conductivity in the initial stage of cure is related to the time for gelation or vitrification. The contribution of the dipole relaxations to the dielectric permittivity reflects an increase of the relaxation times with curing time for both phases. The time-dependent changes in the complex dielectric permittivity are described by a simple two-phase model based on two Havriliak–Negami functions combined with Vogel–Fulcher equations for the description of the curing-time dependence of the relaxation times. The increase of the relaxation times in the phases during isothermal curing is incorporated by time-dependent Vogel temperatures. The latter are related to the time evolution of the glass-transition temperatures in the two phases measured independently by calorimetry. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2461–2470, 1998

Effect of Structural Changes on DC Ionic Conductivity of Rubidium Nitrate Single Crystals

The effect of structural changes on dc ionic conductivity - from room temperature to the melting point at atmospheric pressure - of rubidium nitrate single crystals grown by the slow evaporation method is presented. The anomalous variation of conductivity at structural phase transition points is interpreted on the basis of the unit cell dimensions. Also an attempt is made to remove the ambiguity of room temperature structure.