Electrical conductivity and dielectric relaxation in non-crystalline films of tungsten trioxide

Amorphous tungsten trioxide (a-WO₃) thin films were prepared by thermal evaporation technique. The electrical conductivity and dielectric properties of the prepared films have been investigated in the frequency range from 100 Hz to 100 kHz and in the temperature range 293–393 K. In spite of the absence of the dielectric loss peaks, application of the dielectric modulus formulism gives a simple method for evaluating the activation energy of the dielectric relaxation. The frequency dependence of σ(ω) follows the Jonscher’s universal dynamic law with the relation σ(ω) = σ_dc + Aω^s, where s is the frequency exponent. The conductivity in the direct regime, σ_dc, is described by the small polaron model. The electrical conductivity and dielectric properties show that Hunt’s model is well adapted to a-WO₃ films.     

Novel feature of the universal power law dispersion of the ac conductivity in disordered matter

The ac conductivity σ_ac of different types of materials as a function of angular frequency ω is approximated by a ‘universal’ power law σ_ac = Aωⁿ, A and n are fitting parameters .The exponent and the pre-exponential factor depend on temperature, in general. In the present Letter, we suggest an empirical law that states that the temperature evolution of log A is proportional to the temperature evolution of n. Data reported on different materials ascertain that the ratio −log A/n is constant, regardless the nature of the material and the type of conductivity.     

Effect of proton irradiation on electrical properties of a-As2S3

This paper reports the effect of proton irradiation on the electrical properties of a-As₂S₃ in the temperature range of 323–418 K and frequency range 0.1–100 kHz. The variation of transport property is studied with proton irradiation dose (1 × 10¹³ ions/cm² and 1 × 10¹⁵ ions/cm²). It has been observed that proton irradiation changes the dc conductivity (σ_dc), dc activation energy (ΔE_dc) and ac conductivity (σ_ac(ω)). The σ_dc and σ_ac(ω) increases with dose of proton irradiation. The value of frequency exponent (s) decreases with the temperature and irradiation dose. These results are explained in terms of change in density of defect states in these glasses.     

Asymptotic description of schematic models for CKN

The fits of 0.4Ca(NO₃)₂0.6 K(NO₃) (CKN) by schematic mode-coupling models [V. Krakoviack, C. Alba-Simionesco, J. Chem. Phys. 117 (2002), 2161] are analyzed by asymptotic expansions. The validity of both the power-law and the Cole–Cole-peak solutions for the critical spectrum are investigated. It is found that the critical spectrum derived from the fits is described by both expansions equally well when both expansions are carried out up to next-to-leading order. The expansions up to this order describe the data for 373 K over two orders of magnitude in frequency. In this regime an effective power-law ω^a can be identified where the observed exponent a is smaller than its calculated value by about 15%; this finding can be explained by corrections to the leading-order terms in the asymptotic expansions. For higher temperatures, even smaller effective exponents are caused by a crossover to the alpha-peak spectrum.     

Simulation and scaling of ac conductivities in ion-conducting glasses

Simulation is made about ac conductivity, σ(ω), in ion-conducting glass by recurring to one of the Maxwell’s equations. It is concluded that (1) σ(ω) is made of conduction and conduction-related polarization of the mobile ions, both governed by the same relaxation time, τ, and that (2) high frequency dispersion in σ(ω), high frequency dispersion in the electric modulus peak, and distortion of a semicircle in the complex impedance plane, all arise from a distribution in τ, caused by the disordered structure of glass. Finally, a procedure of scaling ac data measured at various temperatures into a single master curve is given.     

Conductivity percolation transition of Agx(Ge0.25Se0.75)100−x glasses

The ionic conductivity of several chalcogenide glasses increases abruptly with mobile ion addition from values typical of insulating materials (10⁻¹⁶–10⁻¹⁴ Ω⁻¹ cm⁻¹) to values of fast ionic conductors (10⁻⁷–10⁻¹ Ω⁻¹ cm⁻¹). This change is produced in a limited concentration range pointing to a percolation process. In a previous work [M. Kawasaki, J. Kawamura, Y. Nakamura, M. Aniya, Solid State Ionics 123 (1999) 259] the transition from semiconductor to fast ionic conductor of Ag_x(Ge_0.25Se_0.75)_100−x glasses was detected at x¹ ≅ 10 at.% in the form of a steep change in the conductivity. Ag_x(Ge_0.25Se_0.75)_100−x glasses with x ⩽ 25 at.%, prepared by a melt quenching method, are investigated by impedance spectroscopy in the frequency range 5 Hz–2 MHz at different temperatures, T, from room temperature to 363 K and by DC measurements at room temperature. The conductivity of the glasses, σ, was obtained as a function of silver concentration and temperature. For x ⩾ 10 at.% our results are in agreement with those reported by Kawasaki et al. [M. Kawasaki, J. Kawamura, Y. Nakamura, M. Aniya, Solid State Ionics 123 (1999) 259]. The percolation transition was observed in the range 7 ⩽ x ⩽ 8. The temperature dependence of the ionic conductivity follows an Arrhenius type equation σ = (σ_o/T) · exp(−E_σ/kT). The activation energy of the ionic conductivity, E_σ, and the pre-exponential term, σ_o, are calculated. The results are discussed in connection with other chalcogenide and chalcohalide systems and linked with the glass structures.     

Transition from a single-ion to a collective diffusion mechanism in alkali borate glasses

Two distinct regions of the dc conductivity and its temperature dependence as a function of the mol fraction of alkali oxides, X, are observed in Na- and Li-borate glasses. At low alkali contents the dc conductivity σ_dc increases only moderately with X. However, at higher alkali contents, log σ_dc increases linearly and the activation enthalpy ΔH of σ_dc × T decreases linearly with log X, i.e. the dc conductivity reveals an effective power-law behavior. The transition between low alkali and high alkali behavior takes place at X ≈ 0.08 for Na-borate and at X ≈ 0.09 for Li-borate glasses. This behavior suggests that the diffusion mechanism changes at these alkali contents. The results are discussed in terms of ion separations and the transition from a single-ion jump to a collective diffusion mechanism. The vanishing of the mixed-alkali peak in Na–Rb borate and alumino-germanate glasses at sodium contents similar to that observed for the change in slope of σ_dc(X) in this work suggests that both phenomena share the same origin.     

Hopping conduction in amorphous Nb2O5 thin films

The low field conduction mechanism in amorphous Nb₂O₅ doped with Nb is investigated by measurements of the ac conductivity as a function of frequency (3 Hz?6 × 10⁶ Hz), dc conductivity as a function of temperature (100–400 K), capacitance as a function of frequency (3 Hz?6 × 10⁶ Hz) and conductance G as a function of voltage at 10³ Hz. Loss tangent and quality factor data are also given because of their technical and scientific relevance. Evidence for hopping conduction at low applied fields is presented by the following results: (1) a monotonic increase in ac conductivity σ(ω)αωⁿ where 0.5 < n < 1.0 in the range 3 Hz?6 × 10⁶ Hz; (2) a linear dependence of current on voltage at low fields; and (3) low activation energy for dc conduction with a transition at 210 K to a still lower activation energy; and (4) a decrease in polarizability with frequency. At high fields, E > 10⁵ V/cm, dc conductivity is dominated by the field emission mechanism of the Poole-Frenkel or Poole type.     

AC conductivity in amorphous germanium

The ac conductivity in evaporated amorphous germanium films has been measured as a function of annealing and has been found to obey the ω^0.8 law, in accordance with the hopping model. The dc conductivity measurements on the same samples show a $\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ law behaviour. The densities of localized states near the Fermi level g(E_F), obtained from both experiments are in reasonable agreement with each other. Both the measurements show a reduction by about a factor of 2 in g(E_F) when a freshly prepared film is fully annealed. High-temperature substrate films also show the ω^0.8 behaviour. This suggests that the frequency dependence of the ac conductivity is not caused by voids alone. Other possible explanations of our results are also discussed.     

Ion transport behavior in a new fast Ag ion conducting composite electrolyte system: 0.85[0.75AgI:0.25AgCl]:0.15CeO2

Investigations on ion transport behavior of a new fast Ag⁺ ion conducting composite electrolyte system: 0.85[0.75AgI:0.25AgCl]: 0.1CeO₂ are reported. An alternate host: ‘[0.75AgI:0.25AgCl] mixed system/solid solution’ has been used as first-phase host matrix salt, in place of the traditional host AgI, while the micron-size particles of an insulating and chemically inert CeO₂ as second-phase dispersoid. The soaking time, plays important role in determining the conductivity enhancement in the composite system. The system: 0.85[0.75AgI:0.25AgCl]:0.1CeO₂ prepared at soaking time ∼10 min. exhibited optimum conductivity:σ_rt ∼ 1.2 × 10⁻³ S cm⁻¹ at room temperature, which is an order of magnitude higher than that of the pure host. Structural characterization studies were performed by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Temperature dependent measurement on the basic ionic parameters viz. conductivity (σ), ionic mobility (μ), mobile ion concentration (n), room temperature ionic transference number (t_ion) and ionic drift velocity (v_d) have been carried out on the system.     

ac Conductivity of 4.5 TiO2−x · 2 P2O5

The ac conductivity of a member of the family of glasses 4.5 TiO_2?x · 2 P₂O₅ has been measured between 77 and 300 K, and up to 100 kHz. The dc conductivity was measured over only part of this temperature range. The measured ac conductivity can be represented by σ_ac = σ₀ + σ₁ω^s, with s < 1, and temperature dependent. A similar equation describes the ac dielectric constant, ?_ac = ?₀ + ?₁ω^s?1, where $\text{?}{}_1\text{= σ}{}_1\text{tan}\text{1}\text{2}\text{sπ}$. A simple proportionality of s to temperature holds at low temperature; at the higher temperatures, the T-dependence of s is no longer simple. The observed behaviour of the ac properties of this glass is in general accordance with a recently proposed model for systems where transport occurs by hopping. The over-all behaviour is comparable to other transition metal glasses.Using the model and treating the carriers as polarons yields an expression for s in terms of temperature. Values for the polaron radius and the effective dielectric constant are then extracted from the measurements. These values are in good agreement with values for similar systems obtained by other means.     

Ionic conduction, diffusion and glass transition in 0.2[XNa2O · (1−X)Rb2O] · 0.8B2O3

We have investigated the ionic transport in the 0.2[XNa₂O · (1−X)Rb₂O] · 0.8B₂O₃ mixed-alkali system with X=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 in the glassy and the undercooled-liquid state by means of impedance spectroscopy and tracer diffusion experiments. The calorimetric glass-transition temperature T_g obtained by differential scanning calorimetry shows a minimum with composition. The composition dependence of the electrical conductivity below T_g exhibits a minimum, as well. These deviations from an ‘ideal’ linear mixing rule are usually denoted as mixed-alkali effect. The dc conductivities times temperature σ_dc×T follow the Arrhenius law in the range below and above T_g, respectively. The glass transition appears as a kink in the Arrhenius presentation of σ_dc×T. Below the glass-transition temperature the onset frequency ν_on of the conductivity dispersion has an Arrhenius-like temperature dependence. According to ‘Summerfield scaling’ the activation enthalpies of σ_dc×T and ν_on are expected to be the same. This is indeed observed but only for the single-alkali compositions. The activation enthalpies of σ_dc×T as a function of composition show a classical mixed-alkali maximum, however the activation enthalpies of the onset frequencies as a function of composition exhibit a nearly constant behavior in contrast to the expectation from Summerfield scaling. The tracer diffusion measurements reveal a major difference in diffusion of ⁸⁶Rb and ²²Na in mixed-alkali glasses. A diffusivity crossover of tracer diffusion coefficients of ²²Na and ⁸⁶Rb occurs near X=0.2. By comparison of tracer and conductivity diffusivities the Haven ratio is deduced which shows a maximum near the conductivity minimum composition.     

Electrical properties of glassy AsxGe10Te90−x alloys

This paper analyses the electrical properties of glassy alloys of As_xGe₁₀Te_90?x, while reporting the conductivity and dielectric constant of As₅Ge₁₀Te₈₅ and As₁₅Ge₁₀Te₇₅ compositions in the temperature range 77–383 K and the frequency range from dc to 5 MHz. The dc conductivity has been shown to be of the form

The ratio σ₀₁/σ₀₂ is of the order of 10⁶. ΔE₁, the higher temperature activation energy, is dependent on the composition, while ΔE₂, the lower temperature activation energy, is less dependent on the composition. The dielectric constant has been found to be independent of temperature and frequency up to about 253 K. However, at higher temperatures, it becomes activated and proportional to log ω.Some common features of As_xGe₁₀Te_90?x are a kink in dc conductivity, a ω^0.8 relationship for ac conductivity, no evidence of variable-range hopping at low temperatures, field-dependent conductivity and memory switching. The data can be interpreted in terms of the dangling-bond theory of Mott and his collaborators. A high density of states of the order of 10²⁰eV^?1 cm^?3 near the Fermi level may be expected.     

ac conductivity in a-Ge–Se–Bi glasses

In the present paper the effect of Bi impurity (low ∼4 at.% and high ∼10 at.%) on the ac conductivity (σ_ac) of a-Ge₂₀Se₈₀ glassy alloy is studied and the experimentally deduced values are fitted with theoretically deduced values by using correlated barrier hopping model (CBH). Frequency dependent ac conductance of the samples over a frequency range of 100–50 kHz has been taken in the temperature range (268–360 K). At frequency 2 kHz and temperature 298 K, the value of ac conductivity (σ_ac) decreases at low concentration of Bi (4 at.%). However, the value of σ_ac increases at higher concentration of Bi (10 at.%). The ac conductivity is proportional to ω^s for undoped and doped samples. The value of frequency exponent (s) decreases as the temperature increases. These results have been explained on the basis of some structural changes at low and high concentration of Bi impurity.     

Comparison of analysis of dielectric spectra of PCL in the ε1 and the M1 formalism

The complex dielectric modulus, M¹(ω, T), for poly(ε-caprolactone) has been obtained by inverting the complex dielectric permittivity data, ε¹(ω, T) recorded in a wide temperature and frequency range. The analysis of the dielectric loss modulus spectra M″(ω, T) have been performed by using a simulated annealing Monte Carlo procedure in order to extract the relaxation parameters for each relaxation mode. Besides the local modes γ and β, the segmental α mode associated to the glass transition, the conductivity and the interfacial polarization peaks could be analyzed with a better precision than by using the conventional non-linear least squares method in ε″ domain. The merging of the α and β modes at high temperatures is much better defined due to the less significant contribution of the ionic conductivity in M¹ formalism.     

Dielectric study and ac conductivity of iron sodium silicate glasses

A series of glasses with formula (SiO₂)_0.7−x(Na₂O)_0.3(Fe₂O₃)_x with ( 0.0 ≤ x ≤ 0.20) were prepared and studied by means of AC measurements in the frequency range 20 kHz to 13 MHz at room temperature. The study of frequency dependence of both dielectric constant ε' and dielectric loss ε" showed a decrease of both quantities with increasing frequency. The results have been explained on the basis of frequency assistance of electron hopping besides electron polarization. From the Cole-Cole diagram the values of the static dielectric constant ε_s, infinity dielectric constant ε_∞, macroscopic time constant τ, and molecular time constant τ_m are calculated for the studied amorphous samples. The frequency dependence of the ac conductivity obeys a power relation, that is σ_ac (ω) = Α ω^s. The obtained values of the constant s lie in the range of 0.7 ≤ s ≤ 1 in agreement with the theoretical value which confirms the simple quantum mechanical tunneling (QMT) model. The increase in ac conductivity with iron concentration is likely to arise due to structural changes occurring in the glass network. The structure of a glass with similar composition was published and showed clustering of Fe²⁺ and Fe³⁺ ions which favor electron hopping and provide pathways for charge transport.     

Bismuth germanate and bismuth silicate glasses in cryogenic detectors

We present and discuss the results of measurements of conductivity and secondary electron emission coefficient of Bi_xGe_1−xO_2−0.5x (where x=0.13, 0.23, 0.33, 0.47) and Bi_xSi_1−xO_2−0.5x (where x=0.47, 0.57, 0.67) glasses reduced in hydrogen. The surface conductivity of reduced glasses strongly depends on reduction temperature, reduction time and bismuth content. The temperature dependence of the surface conductivity in a high temperature regime is well described by σ∼exp[−(T₀/T)ⁿ] law where n=1/4. The secondary electron emission coefficient of reduced glasses is practically independent of the degree of reduction but increases when binary glasses are modified by alkali ions. The research results confirmed that bismuth silicate and bismuth germanate glasses modified by alkali ions may be good materials for detectors working in cryogenic temperature.     

Free induction decays in entangled polymer melts

The relaxation of the transverse magnetization components caused by both dipolar interactions between the spins of different polymer chains and the dipolar coupling between CH-protons on an isolated Kuhn segment along a single polymer chain have been calculated. Explicit expressions for the transverse relaxation function are given in terms of the absolute mean squared displacement of the Kuhn segment during melt g_r(t), the tangent vector dynamical correlation function 〈b_n(t)b(0)〉, the segmental relaxation time τ_s, the Kuhn segment length b, the bond length a₀, the internuclear distance d, and the spin number density ρ_s. It is shown that the functional dependence of the intramolecular relaxation function on 〈b_n(t)b(0)〉 is fairly weak. The time-dependence of the intramolecular contribution to the transverse relaxation function is dominated by the probability density distribution function of the end-to-end vector of the Kuhn segment. The long-time decay of the intramolecular contribution to the transverse relaxation function is found to scale as t^? 3/2 for τ_s < < t < < τ_max, where τ_maxis the maximum relaxation time of polymer chains in melts. For times much less than the spin–spin relaxation time, T₂ ≈ 10^? 3 ? 10^? 2s, we show that the intermolecular contribution to the relaxation function is given by the following expression: exp(? λ₁(b, τ_s, ρ_s)t²/g_r^3/2(t)). Both the numerical coefficient and the functional dependence of λ₁on b, τ_s and ρ_s reproduce the expression obtained from the frequently used second cumulant approximation. For longer times (T₂ ≤ t < < τ_max), the intermolecular contribution is determined by the following relation: exp(? λ₂(b, τ_s, ρ_s, t)g_r(t)). We show that λ₂ increases logarithmically with t. The molecular mass independence of λ₁and λ₂ shows that, in polymer melts with molecular masses M_w far above the critical value M_c, the relevant experimental window for the decay of the intermolecular relaxation function is connected with the anomalous diffusion regime. Comparison with the experimental data suggests that the intermolecular contribution plays a significant role in the NMR relaxation process in polymer systems close to the melting point.     

Frequency dependence of conduction and polarization in conductive polymers

We report the results of the measurement and analysis of the complex conductivities of two high polymers over the frequency range 10²–10⁶ Hz, and temperature range 70–300 K. Giant polarization of the nomadic type is observed, with dielectric constants ranging from about 50 to 6000 in these aromatic hydrocarbon polymers. The complex conductivities resemble power law behavior, σ_ac = Aω^s (with s in the range 0.7–1.0) in some temperature ranges, and deviates from this in others. The dc conductivity and the real part of the ac conductivity at various frequencies follow a $\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}$ law. The dielectric constant varies as expected for nomadic polarization in long-chain molecules. An attempt is made to develop an understanding of the observed dependences of the complex conduction or polarization on temperature and frequency in terms of interchain and intrachain transport processes.     

Coordination structure of Li+ in concentrated aqueous lithium alaninate solutions

Time-of-Flight (TOF) neutron diffraction measurements with the ⁶Li/⁷Li isotopic substitution technique were carried out for aqueous 18 mol% lithium alaninate solutions in D₂O in order to obtain structural information on the interaction between Li⁺ and the amino acid molecule in the concentrated aqueous solution. The first-order difference function, Δ_Li(Q), was obtained from the difference between scattering cross sections observed for solutions involving dl-CH₃CH(ND₂)COO⁶Li and dl-CH₃CH(ND₂)COO⁷Li. The distribution function around Li⁺, G_Li(r), exhibits well resolved first and second peaks that are tentatively attributable to the nearest neighbor Li⁺ ⋯ O and Li⁺ ⋯ D interactions. However, the coordination number, n_LiD, estimated from the area under the second peak is found to be much larger than the value, 2n_LiO, evaluated from the first peak of the present G_Li(r). This implies that the nearest neighbor Li⁺ ⋯ alaninate ion interaction may contribute to the second peak. Structural parameters concerning the first coordination shell of the Li⁺ has been determined through the least squares fitting analysis of the observed Δ_Li(Q). It has been revealed that the Li⁺ is surrounded by 2.4(1) D₂O molecules and 2.3(2) alaninate ions with interatomic distances of r(Li⁺ ⋯ O_D2O) = 1.97(1) Å, r(Li⁺ ⋯ D_D2O) = 2.62(1) Å and r(Li⁺ ⋯ O_alaninate) = 2.38(1) Å, respectively.