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.     

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.     

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.     

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.     

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.     

Effect of devitrification on ion motion in lithium-disilicate glass

Li-disilicate glass-ceramics consist of microcrystallites imbedded in the glassy Li₂O · 2SiO₂ matrix where the number and size of the crystallites depend on the devitrification heat treatment. To assess ion motion in these model glass-ceramics, we have measured the temperature dependence of the dc conductivity, σ_dc, and the ⁷Li nuclear spin relaxation (NSR) rate, 1/T₁, in samples with various crystalline fraction, c, ranging from c = 0 (pure glass) to c = 1 (fully devitrified polycrystalline ceramic). The Cole–Cole presentation of the complex impedance shows two separate arcs caused by the remarkable difference of the ionic motion in the glassy and crystalline phase. These two arcs correspond to a bi-exponential decay of the ⁷Li nuclear spin magnetization where the resulting two NSR rates are induced by the ionic motion in the two phases. Thus the NSR and σ_dc data provide a comprehensive picture of the ionic motion in the glassy and crystalline phases. In particular, the ionic motion is the fastest in the glass; then at lower values of c we observe a metastable crystalline phase with ionic motion much greater than in the stable (LS₂) crystalline phase existing at large c-values.     

Bond character,optical properties and ionic conductivity of Li2O/B2O3/SiO2/Al2O3 glass: Effect of structural substitution of Li2O for LiCl

A glass of composition (20 ? x)Li₂O–xLiCl–65B₂O₃–10SiO₂–5Al₂O₃ where 0 ? x ? 12.5 wt% is prepared using the normal melt-quenching technique. The optical constants and electrical conductivity and their correlation are investigated, furnished and discussed with the substitution of Li₂O for LiCl. The mechanism of the optical absorption and the calculated Urbach energy follow the rule of phonon-assisted transitions. The ionic conduction mechanism is determined by activation energy process. Substitution up to 10 wt% LiCl provides high ionic conductivity (1.9 × 10^?2 Ω^?1 m^?1) due to the high average electronegativity of LiCl which increases the polarizability of lithium ions. The small cation–anion distance approach confirmed the enhancement in ionic conductivity of LiCl containing glass compared to that of Li₂O. Due to the large size of Cl^? ions, there is an expansion of the lattice which in turn broadens the available path windows. For 12.5 wt% LiCl, anomalous density behavior is observed and a reduction in conductivity is occurred, σ = 5.4 × 10^?3 Ω^?1 m^?1. Owing to the model of bond fluctuation, the reduction is attributed to the increase in the alkali halide concentration which creates bottlenecks that hinder the motion of Li⁺ ions. The ionic conductivity character is strongly supported by the behavior of the glass ionicity factor, density, molar volume, refractive index, average boron–boron separation, molar refraction, metallization criterion and non-bridging oxygen concentration of the studied glass.     

Effect of silver doping on the electrical properties of a-Sb2Se3

This paper reports the effect of Ag-doping on electrical properties of a-Sb₂Se₃ in the temperature range 230–340 K and frequency range 5–100 kHz. The variation of transport properties with thermal doping has been studied. Ag-doping produces two homogeneous phases in the sample, which are found to be voltage dependent in the temperature range studied and frequency dependent in lower frequency region (0.1–10 kHz). Activation energy E_g and C′ [= σ₀ exp (γ/k), where γ, is the temperature coefficient of the band gap] calculated from dc conductivity has been found to vary from (0.42 ± 0.01) eV to (0.26 ± 0.01) eV and (4.11 ± 0.01) × 10⁻⁵ to (2.90 ± 0.02) × 10⁻⁶ Ω⁻¹ cm⁻¹ respectively. Ag-doping can be used to make the sample useful in device applications.     

Acid catalyst concentration effect on structure and ion relaxation studies of Li2O–P2O5–B2O3–SiO2 glasses synthesized by sol–gel process

Lithium phosphoborosilicate (LPBS) glasses were synthesized through the sol–gel process by varying nitric acid concentrations as a catalyst. The sol–gel process was monitored through XRD and DSC to optimize the LPBS glass forming treatment. Characterization of LPBS glasses was conducted using XRD, FTIR and DSC techniques. Impedance measurements were carried out at different temperatures on LPBS samples synthesized by sol–gel process with various nitric acid concentrations and impedance data were analyzed using Boukamp equivalent circuit software. The conductivity of LPBS samples was calculated from analyzed impedance data and it was found that sample synthesized with 2.5 N nitric acid concentration showed the high conductivity σ = 2.28(±0.02) × 10⁻⁷ S cm⁻¹ at 443 K. Activation energy (E_a) is obtained from Arrhenius plots of dc conductivity and it is found to be 0.39 (±0.02) eV for the high conductance sample. Ac conductivity data were analyzed using Jonscher’s power law (JPL) and the power law exponent (s) exhibits a low s value for high conducting LPBS sample and a non-linear behavior with temperature. The electric modulus data were fitted with Kohlraush–William–Watts (KWW) stretched exponential function and modulus formalism is used to study the ionic relaxation behavior at different temperatures in LPBS glasses synthesized with varying nitric acid concentrations.     

Thermo-optical properties and nonradiative quantum efficiency of Er3+-doped and Er3+/Tm3+-co-doped tellurite glasses

Thermal lens (TL) measurements were performed in tellurite glasses, 70TeO₂–19WO₃–7Na₂O–4Nb₂O₅ (mol%), undoped, doped with Er³⁺ (1.19 × 10²⁰ ions/cm³) and co-doped with Er³⁺ (1.19 × 10²⁰ ions/cm³)/Tm³⁺ (1.56 × 10²⁰ ions/cm³). The absolute nonradiative quantum efficiency (ϕ) was determined by the TL method. The ϕ values for Er³⁺/Tm³⁺-co-doped and Er³⁺-doped tellurite glasses were 0.98 and 0.74, respectively. Fluorescence spectra were performed at λ_e = 488 nm and used to estimate the fluorescence quantum efficiency (η) using the TL results. These values were compared with results obtained by Judd–Ofelt calculations.     

InOx semiconductor films deposited on glass substrates for transparent electronics

Transparent undoped semiconductor indium oxide films were deposited by radio frequency (rf) plasma enhanced reactive thermal evaporation (rf-PERTE) of indium at low substrate temperature. It was experimentally verified that the variation of rf power density has a strong influence on the electrical and structural properties of the films. The thickness of the InO_x films is of about 100 nm. Results show that InO_x films show an average visible transmittance of about 85% and energy gap of about 2.6 eV. Structural and electrical conductivity measurements show that films are polycrystalline and there exists a linear variation of conductivity logarithm vs reciprocal of temperature. Electrical conductivity variation of 17.6 to 5.8 × 10⁻³ (Ω cm)⁻¹ for films produced at rf power densities ranging from 3.9 to 78.1 mW cm⁻³ was obtained. This controllable semiconductor behavior can therefore satisfy the requirement of a particular application for these type of films.     

Low temperature dielectric dispersion and electrical conductivity studies on Fe2O3 mixed lithium yttrium silicate glasses

Lithium yttrium silicate glasses mixed with different concentrations of Fe₂O₃ of the composition (40 ? x) Li₂O–10Y₂O₃–50SiO₂: x Fe₂O₃, with x = 0.3, 0.5, 0.8, 1.0, 1.2 and 1.5 (all in mol%) were synthesized. Electrical and dielectric properties including dielectric constant, ε′(ω), loss, tan δ, ac conductivity, σ_ac, impedance spectra as well as electric moduli, M(ω), over a wide continuous frequency range of 40 Hz to 10⁶ Hz and in the low temperature range 100 to 360 K were measured as a function of the concentration of Fe₂O₃. The dc conductivity is also evaluated in the temperature range 100 … 360 K. The temperature and frequency dispersions of dielectric constant as well as dielectric loss have been analyzed using space charge polarization model. The ac and dc conductivities have exhibited increasing trend with increasing Fe₂O₃ content beyond 0.5 mol%, whereas the activation energy for the conductivity demonstrated decreasing tendency in this dopant concentration range. Both quantum mechanical tunneling (QMT) and correlated barrier hopping models (CBH) were used for clarification of ac conductivity origin and the corresponding analysis has indicated that CBH model is more appropriate for this glass system. For the better understanding of relaxation dynamics of the electrical properties we have drawn the scaling plots for ac conductivity and also electric moduli. The plots indicated that the relaxation dynamics is independent on temperature but depends on concentration of Fe₂O₃. The dc conductivity is analyzed using small polaron hoping model. The increase of conductivity with the concentration of Fe₂O₃ beyond 0.5 mol% is explained in terms of variations in the redox ratio of iron ions in the glass network. The results were further analyzed quantitatively with the support of experimental data from IR, optical absorption and ESR spectral studies. The overall analysis has indicated that Li₂O–Y₂O₃–SiO₂ glasses containing more than 0.5 mol% of Fe₂O₃ are more suitable for achieving good electrical conductivity in these glasses.     

Magnetic properties and low frequency magnetoimpedance of novel amorphous CoFeTaBCr ribbons

In this report, we study the effect of Cr addition on the dc an ac magnetic properties of novel melt spun amorphous Co_43−x/2Fe_20−x/2Cr_xB_31.5Ta_5.5 (x = 0, 1, 2, 4) alloy series. The saturation magnetization decreases with increasing Cr content as well as the ac relative initial permeability, which showed a high relaxation frequency of 3 MHz. In addition, the magnetoimpedance effect was detected in this alloy series, with a maximum variation ratio of 3% at x = 0.     

Influence of thermal and thermoelectric treatments on structure and electric properties of B2O3–Li2O–Nb2O5 glasses

A transparent glass with the composition 60B₂O₃–30Li₂O–10Nb₂O₅ (mol%) was prepared by the melt quenching technique. The glass was heat-treated with and without the application of an external electric field. The as-prepared sample was heat-treated (HT) at 450, 500 and 550 °C and thermoelectric treated (TET) at 500 °C. The following electric fields were used: 50 kV/m and 100 kV/m. Differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman, dc and ac conductivity, as a function of temperature, were used to investigate the glass and glass-ceramics properties. LiNbO₃ crystals were detected, by XRD, in the 500 °C HT, 550 °C HT and 500 °C TET samples. The presence of an external electric field, during the heat-treatment process, improves the formation of LiNbO₃ nanocrystals at lower temperatures. However, in the 550 °C HT and in the TET samples, Li₂B₄O₇ was also detected. The value of the σ_dc decreases with the rise of the applied field, during the heat-treatment. This behavior can indicate an increase in the fraction of the LiNbO₃ crystallites present in these glass samples. The dc and ac conduction processes show dependence on the number of the ions inserted in the glass as network modifiers.The Raman analysis suggests that the niobium ions are, probably, inserted in the glass matrix as network formers.These results reflect the decisive effect of temperature and electric field applied during the thermoelectric treatment in the structure and electric properties of glass-ceramics.     

Electrical properties of annealed a-SiC:H thin films

In the present work the dependence of electrical properties of a-SiC:H thin films on annealing temperature, T_a, has been extensively studied. From the measurements of dark dc electrical conductivity, σ_D, in the high temperature range (from 283 up to 493 K), was found that the conductivity activation energy, E_a, is invariant for T_a ⩽ 673 K and equal to 0.64 eV, whereas for T_a from 673 up to 873 K, E_a increases at about 0.2 eV reaching to a maximum value 0.85 eV at T_a = 873 K, suggesting the optimum material quality. This behavior of E_a as a function of T_a is mainly attributed to relaxation of the strain in the amorphous network, which is possibly combined with weak hydrogen emission for temperatures up to 873 K. For further increase of T_a (>873 K) the phenomenon of hydrogen emission, causes rapid decrease of E_a down to 0.24 eV at T_a = 998 K, deteriorating the material quality. These results are also supported by the measurements of dark dc electrical conductivity in the low temperature range (from 133 up to 283 K), where the dependence of the density of gap states at the Fermi level, N(E_F), on annealing temperature presents the minimum value at T_a = 873 K. The Meyer–Nelder rule was found to hold for the a-SiC:H thin films for annealing temperatures up to 873 K. Finally, the dependence of dark dc electrical conductivity at room temperature, σ_DRT, on T_a showed to reflect directly the dependence of E_a on T_a.     

Optical properties and valence state of Sm ions in aluminoborosilicate glass under β-irradiation

Sm-doped borosilicate glasses exposed to β-irradiation with doses from 8 × 10⁵ up to 4 × 10⁹ Gy have been studied by luminescence, Raman and electron paramagnetic resonance (EPR) spectroscopies. The luminescence spectra for pristine and irradiated glasses reveal that the β-irradiation process affects valence state of samarium ions. Intense emission at 684 and 727 nm excited by Ar+ laser (514.5 nm) due to the transition of Sm²⁺ ion was observed after irradiation. Relative proportion of Sm²⁺ ions estimated as a function of both Sm₂O₃ content and irradiation dose has the tendency to increase with increasing irradiation dose. In contrast, the EPR spectra of the studied samples reveal a decrease of the defect content, which are mostly hole defects, produced during irradiation, as a function of Sm₂O₃ content. Finally, the addition of Sm₂O₃ leads to a decrease of the Si–O–Si bending vibration modes shift and polymerisation changes under irradiation.     

Spectroscopic properties and energy transfer in Yb3+–Ho3+ co-doped germanate glass emitting at 2.0 μm

A new kind of germanate glass co-doped with Yb³⁺–Ho3⁺ was prepared. The J-O parameters were calculated to be Ω₂ = (6.59 ± 0.21) × 10^? 20 cm², Ω₄ = (2.77 ± 0.36) × 10^? 20 cm², and Ω₆ = (1.90 ± 0.25) × 10^? 20 cm². The little overlap between the absorption cross section and stimulated emission cross section indicates a non-resonant energy transfer process. The calculation demonstrates that the energy transfer between Yb³⁺ and Ho³⁺ is one-phonon assisted in a great measure. The gain coefficient of Ho³⁺ at 2.0 μm was also calculated. The fluorescence measurement shows the Yb³⁺ co-doping enhances the 2.0 μm emission remarkably.     

Characterization of chitosan/PVA blended electrolyte doped with NH4I

The (chitosan–PVA)–NH₄I electrolytes have been prepared by the solution casting method. The prepared electrolytes are analyzed using Fourier transform infrared (FTIR) spectroscopy in order to determine the interaction between salt and the polymer blend hosts which can be deduced from the band shifting. From infrared spectra, shifts are observed at the amine, carboxamide, carbonyl and hydroxyl bands of chitosan and PVA. These shifts indicate that complexation has occurred. The crystallinity/amorphousness of the blended electrolytes has been examined by X-ray diffraction (XRD). XRD pattern shows that the crystallinity of chitosan–NH₄I electrolyte increases with PVA concentration. Impedance of the electrolytes has been measured using electrochemical impedance spectroscopy (EIS) over the frequency range from 50 Hz to 1 MHz. The highest conducting sample 55 wt.% (chitosan–PVA)–45 wt.% NH₄I has conductivity of 1.77 × 10^? 6 S cm^? 1. The chitosan:PVA ratio is 1:1. This is higher than the conductivity for the unblended electrolyte 55 wt.% chitosan–45 wt.% NH₄I which is 3.73 × 10^? 7 S cm^? 1. From ln τ versus 10³/T plot, the activation energy for relaxation process is 0.87 eV. This is different from activation energy for dc conductivity which is 0.38 eV. Ion conduction is by hopping.     

Thermal,vibrational and Ac impedance studies on proton conducting polymer electrolytes based on poly(vinyl acetate)

Proton conducting polymer electrolytes based on poly(vinyl acetate) (PVAc) and perchloric acid (HClO₄) have been prepared by solution casting technique with various compositions. The X-ray diffraction analysis confirms the polymer–HClO₄ complex formation. FTIR spectra analysis reveals the interaction between proton and ester oxygen of poly(vinyl acetate) (PVAc). The shift in T_g towards the lower temperature indicates that the polymer salt interaction takes places in the amorphous phase of the polymer matrix. Ac impedance spectroscopy reveals that 75 mol% PVAc:25 mol% HClO₄ exhibits maximum conductivity, 3.75 × 10^? 3 S cm^? 1 at room temperature (303 K). The increase in conductivity with increase in dopant concentration and temperature may be attributed to the enhanced mobility of the polymer chains, number of charge carriers and rotations of side chains. The temperature dependence of conductivity shows non-Arrhenius behavior at higher temperatures.