Proton conduction in the binary system (1 − x)CsHSeO4–xKHSeO4 studied with a free-energy model

Electrical Impedance Spectroscopy (EIS) was used to study the electrical properties of the (1???x)CsHSeO₄–xKHSeO₄ binary system with concentrations x?=?0.0 and 0.1. The results show a higher proton-conduction phase above 80°C for both concentrations, however, while DC conductivity of CsHSeO₄ shows a gradual change to higher values in the 80–118°C temperature range, the 0.9CsHSeO₄–0.1KHSeO₄ concentration reveals an abrupt change at about 80°C to an intermediate temperature phase. The observed behavior for the doped sample was modeled using a trial free-energy density, based on the concentration of mobile ions, that takes into account the formation of defects, configurational and phonon entropies, and defect-defect interactions. By minimising the free-energy density one obtains two roots for the carrier concentration at a given temperature, which corresponds to a stable and metastable configuration. It is possible to characterise the phase behavior of the system by means of temperature and two model parameters, which depend on the crystalline properties of the system, but not on temperature. One can successfully explain the conductivity behavior of the system by changing the model parameters if it is assumed that its variations are due to the carriers density.     

Enhanced conductivity in ionic conductor-insulator composites: numerical models in two and three dimensions

We describe a two-dimensional (2D) and a three-dimensional (3D) percolation model for ionic conductor-insulator composites such as copper(I) bromide-titanium dioxide (CuBr-TiO₂) or lithium iodide-alumina (LiI-Al₂O₃). These composites present an enhanced conductivity closely related to the insulator concentration. This effect is explained by the formation of highly conducting space charge regions near the phase boundaries which are represented by good conductor bonds. Our numerical model takes into account grain size and correlation effects. The dimension has a leading role for the conduction properties. In the 2D case, the good conductor bonds do not percolate, whatever the insulator concentration, and the maximum conductivity of the composite samples is of the same order as that of the ionic conductor grains. The behavior of the system is very different in the 3D case where, for a large domain of composition, the good conductors percolate through the regions between the conductor grains. For the CuBr-TiO₂ composites the conductivity versus composition curve is bell-shaped. Conversely, in the LiI-Al₂O₃ system, a linear relation between the conductivity and the insulator volume fraction is obtained in the experiments. Our model gives a plausible interpretation of the conductivity in both systems. Received 10 April 2001     

The conductivity of a 2D system with a doubly periodic arrangement of circular inclusions

We present a scheme for the evaluation of the conductivity and other effective properties of a model composite with a regular anisotropic structure, namely, a 2D system with circular inclusions forming a rectangular array. Exact expressions for the electric potential and the effective conductivity tensor [^(s)] _e\hat \sigma _e were obtained in the form of infinite series. For small inclusion densities, a virial expansion for [^(s)] _e\hat \sigma _e was derived from the general formulas and its applicability conditions were found. The first terms of this expansion yield the well-known Rayleigh result for the isotropic model (square array).     

Transport in dispersed ionic conductors: Effect of insulating particle size

Dispersed ionic conductors are random mixtures of a solid salt, e.g. AgI, LiI, with fine particles of an insulating second phase, like Al₂O₃ or SiO₂. These composites can show a dramatic increase in ionic conductivity compared to the pure homogeneous system. Generally, this observation is attributed to an increased conductivity along the internal interface between the conducting salt and the insulating material. In this work a three-component random resistor network (RRN) model for dispersed ionic conductors is reviewed. In the model, the ionic conductor is represented by normally conducting bonds, the insulating material by non-conducting bonds and the interface between the two phases by highly conducting bonds. A special feature of the model is the existence of two critical concentrations of the insulating phase, p′_c and p″_c , for interface percolation and bulk conduction, respectively, where critical transport properties corresponding to conductor/superconductor and conductor/insulator networks are predicted. The model describes satisfactorily the dependence on composition of the conductivity and activation energy of dispersed ionic conductors. Furthermore, the observed effect on the conductivity of the size of dispersed particles can be described qualitatively well by a generalized version of the RRN model, which in addition predicts a sensitive dependence of the critical thresholds on particle size. Non-universality features in the critical exponents for the conductivity are also discussed within a continuum percolation analog of the model.     

Electrical conductivity of germanium with dislocation grids

Samples of n-type germanium with a donor concentration N _d=2.4×10¹⁶ cm⁻³ are plastically deformed to a degree of strain equal to 18–40% to detect static conduction by electrons trapped on dislocations in a system of dislocation grids. In samples with 20%<δ<31%, which retain an electronic type of conductivity, the conductivity for T<8 K, which is weakly temperature-dependent, is associated with conduction by electrons trapped on dislocations. The nonmonotonic dependence of the conductivity at 4.2 K on the degree of strain as the latter increases from 18% to 40% attests to the existence of an energy gap between the donor and acceptor dislocation states in strongly plastically deformed germanium. Zh. éksp. Teor. Fiz. 115, 115–125 (January 1999)     

Anomalous thermal conductivity of single crystal Cu2Te2O5Cl2

A strong increase of the thermal conductivity is observed at the phase transition (T _c=18.2 K) in Cu₂Te₂O₅Cl₂ single crystal. This behavior is compared with that of the spin-Peierls system NaV₂O₅, where a similar experimental observation has been made, and the conventional spin-Peierls system CuGeO₃, where a modest kink in the thermal conductivity curve has been observed. The strong increase of the thermal conductivity atT _c in Cu₂Te₂O₅Cl₂ could be partially attributed to the opening of the energy gap in the magnetic excitation spectrum evident from the magnetic susceptibility measurements. However, the main reason for the anomaly of the thermal conductivity could be explained by a strong spin-lattice coupling in this system, which what is in agreement with the preliminary X-band electron spin resonance measurement.     

Effects of various LiPF<Subscript>6</Subscript> salt concentrations on PEO-based solid polymer electrolytes

In this research, various weight percents of LiPF₆ are incorporated into PEO-based polymer electrolyte system. Thin film electrolytes are prepared via solution casting technique and characterized by FTIR, XRD and DSC analyses in order to study their complex behaviour. The amorphicity of the electrolytes are measured by DC impedance. The results reveal that the conductivity increases with increasing temperature when the salt concentration increases to 20 wt.%. The conductivity for 20 wt.% of salt remains similar to the conductivity of 15 wt.% of salt at 318 K. Impedance studies show that the conductivity increases with increasing LiPF₆ concentration, whereas XRD studies reveal that the phase changes from crystalline to amorphous when LiPF₆ concentration increases. DSC studies indicate a decrease in T _m with increasing LiPF₆ concentration. Finally, the complexation process is examined using FTIR.     

Ionic conductivity study of CaF2: Nd single crystals

The electrical ionic conductivity of unirradiated and irradiated CaF₂: Nd crystals in the range of 60 to 800°C has been measured. The conductivity plot is basically divided into four parts, i.e., intrinsic and extrinsic unassociated, extrinsic associated, and extrinsic segregated regions. Activation energy (for unirradiated samples) in the extrinsic unassociated region is in the range of 0.69 to 1.20 eV depending on the doping concentration while for the intrinsic region, it is of the order of 1.89 eV. The conductivity in the extrinsic unassociated region increases with increase of Nd content in the sample. Also, the conductivity in the extrinsic region forγ-irradiated sample is higher than that for unirradiated one. In the intrinsic region, however, the conductivity is independent of dopant concentration orγ-irradiation. From these results it is surmised thatF ⁻ interstitials are the charge carriers in this region for CaF₂: Nd³⁺ system.     

Quasi-one dimensional electrical conductivity and thermoelectric power studies on a discotic liquid crystal

We have studied the electrical conductivity of well aligned samples of hexahexylthiotriphenylene (HHTT) in the pure as well as doped states. The dopant used was a small concentration (0.62 mole %) of the electron acceptor trinitrofluorenone (TNF). In the columnar phases, doping causes the AC(1 kHz) conductivity along the columnar axis (σ _‖) to increase by a factor of 10⁷ or more relative to that in undoped samples; σ _‖ attains a value of 10⁻²S/m, which was the maximum measurable limit of our experimental set up. On the other hand, in the isotropic phase doping makes hardly any difference to the conductivity. The frequency dependence of the conductivity has been investigated. The DC conductivity of doped samples exhibits an enormous anisotropy, σ _‖/σ _⊥ ≥ 10¹⁰, which is 7 orders higher than that reported for any liquid crystalline system, and, to our knowledge, the largest observed in an organic conductor. We also report the first thermoelectric power studies on these ‘molecular wires’. The sign of the thermoelectric power is in conformity with the expected nature of the charge carriers, namely, holes.     

Mechanism for Conversion of the Conductivity Type in Arsenic-Doped p-CdxHg1–xTe Subject to Ionic Etching

Based on an analysis of chemical diffusion of mercury in p-Cd _x Hg_1–x Te:As narrow-band solid solutions, a mechanism for conversion of the conductivity type upon ionic etching is suggested. It is shown that the n–p conversion of the conductivity in this case is due to the formation of a donor complex between arsenic in the Te sublattice and an interstitial Hg atom. Moreover, the electron concentration in the converted layer corresponds to the concentration of the implanted arsenic impurity. The theoretical results are confirmed by the experimental investigation of the electron concentration distribution over the n-layer of a p-Cd _x Hg_1–x Te:As epistructure converted upon ionic etching.     

Ion conductivity and dielectric relaxation behavior in FIC silver based phospho-molybdate glasses

Electrical conductivity and dielectric relaxation studies of silver ion-conducting glasses have been prepared using xAg₂SO₄-15Ag₂O-(90-x)(90P₂O₅-10MoO₃) glass system over a temperature range of 298–353 K and frequencies of 10 Hz to 10 MHz. DC conductivities exhibit Arrhenius behavior over the entire temperature range with a single activation barrier. The ac conductivity behavior of these glasses has been analyzed using single power law; conductivity increases linearly in logarithmic scale with Ag₂SO₄ concentration. The power law exponent (s) decreases, while stretched exponent (β) is insensitive to increase of temperature. Scaling behavior has also been carried out using the reduced plots of conductivity and frequency, which suggest that ion transport mechanism remains unaffected at all temperatures and compositions.     

Quenching of point-defects in a plastic crystal

A new method of producing and studying a non-equilibrium point-defect concentration in small bulk samples of plastically deformable material is described. The technique involves rapid compression of the specimen and is therefore particularly useful for materials of low thermal conductivity. This type of quenching has been applied to the plastic crystal hexamethylethane and the subsequent changes in the concentration of point-defects with time monitored by measurement of the proton NMR relaxation times T ₁ and T _1ρ. Analysis of these experiments allows evaluation of the motional volume associated with the translation of the defects. Comparison with the total activation volume, ΔV _d*, found for molecular self-diffusion in polycrystalline hexamethylethane shows that the motional contribution to ΔV _d* is approximately 17 per cent, an observation which confirms that self-diffusion proceeds by a vacancy mechanism in this plastic crystal. Possible further development of the basic pressure-quenching experiments is also discussed.     

Transport and dielectric properties of V2O5-MnO-TeO2 glasses

The frequency (1-10 kHz) and temperature (80-350 K) dependences of the ac conductivity and dielectric constant of the V₂O₅-MnO-TeO₂ system, containing two transition-metal ions, have been measured. The dc conductivity _dc measured in the high-temperature range (200-450 K) decreases with addition of the oxide MnO. This is considered to be due to the formation of bonds such as V--O--Mn and Mn--O--Mn in the glass. The conductivity arises mainly from polaron hopping between V^4+M and V⁵⁺ ions. It is found that a mechanism of adiabatic small-polaron hopping is the most appropriate conduction model for these glasses. This is in sharp contrast with the behaviour of the Mn-free V₂O₅-TeO₂ glass, in which non-adiabatic hopping takes place. High-temperature conductivity data satisfy Mott's small-polaron hopping model and also a model proposed by Schnakenberg in 1968. A power-law behaviour ( _ac = Aω^s , with s < 1) is well exhibited by the ac conductivity σ_ac data of these glasses. Analysis of dielectric data indicates a Debye-type relaxation behaviour with a distribution of relaxation times. The MnO-concentration-dependent σ_ac data follow an overlapping large-polaron tunnelling model over the entire range of temperatures studied. The estimated model parameters are reasonable and consistent with changes in composition.     

Investigation of conductivity in the Sr(Fe1−xTix)Oy system by iron-57 Mössbauer spectroscopy

On the basis of our previous work electrical conductivity in the Sr(Fe_1–xTi_x)O_y system (0.0x0.9,y3) has been further studied by means of Mössbauer spectroscopy. When 0.0x0.6, the concentration of Fe³⁺ (II) doublet relates to the final firing temperature and electrical conductivity of the materials is sensitive to the concentration of Fe³⁺ (II). Atx=0.25, the curve of the resistivity versus Ti contentx shows a local minimum which is observed for the first time. The results indicate that the coexistence of Fe⁴⁺ and Fe³⁺ in the same lattice leads to high conductivity; the conductivity increases when the Fe⁴ concentration approaches to that of the Fe³⁺ one. When the temperature is at 260 K and 230 K, the presence of the intermediate state showing quadrupole splitting has an effect on the conductivity of the materials.     

Electrical and thermal conductivities and Seebeck coefficient of liquid copper–bismuth alloys

The electrical resistivity and absolute thermoelectric power of the liquid Cu _x –Bi_{(1? x )} system have been measured over the whole composition range from the liquidus to 1100°C. The thermal conductivity is deduced from measurements of these two properties. The experimental results are interpreted and discussed in term of the extended Faber–Ziman formula using the t-matrix formalism with hard-sphere structure factors. The concentration and energy dependence of the phase shifts have been taken into account for a complete conductivity and thermopower calculation.     

Study of phase transition in a [CdHgI4 : 0.2AgI] mixed conducting composite system doped with KI and K2SO4

New composite superionic systems, [CdHgI₄?:?0.2AgI]?:?0.xKI and [CdHgI₄?:?0.2AgI]?:?0.xK₂SO₄ (x?=?0.2, 0.4, 0.6?mol. wt%), were prepared, using [CdHgI₄?:?0.2AgI] mixed composite system as the host. Electrical conductivity was measured to study the transition behavior at frequencies of 100?Hz, 120?Hz, 1?kHz, and 10?kHz in the temperature range from 150°C to 250°C using a GENRAD 1659 RLC Digibridge. A sharp increase in conductivity was observed during β?→?α phase transition. Upon increasing the dopant-to-host ratio, the conductivity of the superionic systems exhibited Arrhenius (thermally activated)-type behavior. Differential thermal analysis, differential scanning calorimetry, thermogravimetric analysis, and X-ray powder diffraction were performed to confirm the doping effect and transition in the host. The phase transition temperature increased with an increase in the dopant concentration. Activation energies in eV for pre- and post-transition phase behavior are reported.     

Enhanced long-term stability of bismuth oxide-based electrolytes for operation at 500 °C

Cubic-stabilized ((DyO_1.5) _x –(WO₃) _y –(BiO_1.5)_{1 − x − y} ) electrolytes (DWSB) with much higher conductivity than (ErO_1.5)_0.2(BiO_1.5)_0.8, 20ESB, were developed through a double-doping strategy. (DyO_1.5)_0.08–(WO₃)_0.04–(BiO_1.5)_0.88, 8D4WSB, is the highest conductivity composition but underwent the greatest conductivity degradation at 500 °C due to its low total dopant concentration. The effect of dopant composition on conductivity behavior with time at 500 °C demonstrates that there is a trade-off between initial conductivity and long-term stability at this temperature. Therefore, it is necessary to find an optimal total and relative concentration of dopants to provide the enhanced long-term stability needed to make this DWSB electrolyte system feasible for 500 °C operation. To this end, it was found that (DyO_1.5)_0.25–(WO₃)_0.05–(BiO_1.5)_0.70, 25D5WSB, maintained a conductivity of 0.0068 S/cm without appreciable degradation after annealing at 500 °C for 500 h. Moreover, since bismuth oxide-based electrolytes do not exhibit any grain boundary impedance, the total conductivity of 25D5WSB is significantly higher than that of alternate electrolytes (e.g., GDC: Gd_0.1Ce_0.9O_1.95) at this temperature.     

Heat conductivity of a nonlinear β-FPU lattice

In this work, we propose a new approach to the computation of heat conductivity in nonlinear systems. The total heat conductivity process is decomposed into two parts: one part is an equilibrium process at the same temperature T of either end of the lattice, which does not transfer energy and the other is a nonequilibrium process at temperature ΔT of one end and a zero temperature of the opposite end of the lattice. This approach makes it possible to somewhat reduce the time of computation of heat conductivity at ΔT → 0. The threshold temperature T _thr is found to behave as T _thr ∼ N ⁻³, where N is the lattice length. The threshold temperature conventionally separates two mechanisms of heat conductivity: at T < T _thr, phonon heat conductivity is dominant; at T > T _thr, the contribution of soliton heat conductivity increases with increasing temperature. The problem of the computation of heat conductivity in the limit ΔT → 0 reduces to the heat conductivity of a harmonic lattice with time-dependent bond rigidities determined by an equilibrium process at temperature T. An exact expression for the temperature dependence of sound velocity in a lattice with a β-FPU potential at T < 10 is derived. A numerical experiment confirmed the existence of solitons and breathers that correspond to a modified Korteweg-de Vries (KdV) equation. The problem of the quantitative contribution of solitons and breathers to heat conductivity requires further study.     

Thermoelectric power and magnetic properties of Cu doped NiZn ferrite for technological application

A series of samples in the system Ni_0.65Zn_0.35Cu_xFe_2-xO₄ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by the usual ceramic technique. The thermoelectric power and the magnetic susceptibility were measured. The transition from the ferrimagnetic to the paramagnetic state is accompanied by an increase in the thermo EMF. NiZn ferrite shows n-type conductivity due to the presence of Fe²⁺ ions. The addition of Cu²⁺ ions creates lattice vacancies which give rise to p-type conductivity.

The Tawfik coefficient was determined for NiZn ferrite in the paramagnetic state. This coefficient was reduced by addition of Cu up to x < 0.5.     

The effect of composition,electron irradiation and quenching on ionic conductivity in a new solid polymer electrolyte: (PEG)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>NH<Subscript>4</Subscript>I

We have prepared, characterized and investigated a new PEG-2000 based solid polymer electrolyte (PEG) _x NH₄I. Ionic conductivity measurements have been made as a function of salt concentration as well as temperature in the range 265–330 K. Selected compositions of the electrolyte were exposed to a beam of 8 MeV electrons to an accumulated dose of 10 kGy to study the effect on ionic conductivity. The electrolyte samples were also quenched at liquid nitrogen temperature and conductivity measurements were made. The ionic conductivity at room temperature exhibits a characteristic double peak for the composition x = 20 and 70. Both electron beam irradiation and quenching at low temperature have resulted in an increase in conductivity by 1–2 orders of magnitude. The enhancement of conductivity upon irradiation and quenching is interpreted as due to an increase in amorphous region and decrease in crystallinity of the electrolyte. DSC and proton NMR measurements also support this conclusion.