Effect of isothermal annealing and visible-light illumination on the AC-impedance behavior of undoped selenium thin films

The effect of post-deposition isothermal annealing (30 °C ? T_A ? 70 °C) and visible-light illumination on the complex AC-impedance of undoped selenium thin films deposited at the substrate temperatures T_S = 30, 50, 70 °C has been studied in the frequency range 0.2–12 kHz. The AC-impedance of amorphous selenium (a-Se) films (T_S, T_A < 50 °C) was mainly capacitive, with no loss peaks being observed in their Z″(ω)–ω curves, irrespective of illumination. This behavior was ascribed to a dominant charge-carrier trapping effect of bulk/surface charged defects usually present in a-Se. On the other hand, the measured Z″(ω)–Z′(ω) diagrams of illuminated polycrystalline Se samples (50 °C ? T_S, T_A ? 70 °C) exhibited almost full semicircles, whereas their Z″(ω)–ω curves revealed prominent loss peaks at well-defined frequencies. As the annealing temperature or light intensity is increased the loci of the points determined by intersections of these semicircles with the Z′-axis at the low-frequency side shift greatly towards the origin, while the loss-peak positions shift to higher frequencies. These experimental findings were explained in terms of a significant increase in electrical conductivity of selenium films due to thermally-induced crystallization at temperatures beyond glass-transformation region of undoped selenium and to creation of electron–hole pairs by visible-light illumination.     

Semi-interpenetrating polymer networks based on polyurethane and poly(2-hydroxyethyl methacrylate): Dielectric study of relaxation behavior

The study of molecular dynamics by broadband dielectric spectroscopy (BDS) is presented for polyurethane (PU), poly(2-hydroxyethyl methacrylate) (PHEMA) and for semi-IPNs based on PU and PHEMA synthesized by photopolymerization. The dielectric properties were performed in wide range of frequencies and temperatures with the goal to establish the relation between the relaxations and the structure. Five relaxation phenomena were finally detected for PHEMA : γ-, βsw-, β-relaxations at low temperatures and α-relaxation at 150 °C at high frequencies plus ionic conductivity relaxation which starts at 0 °C. For semi-IPNs the overlapping of γ- and βsw-relaxations of PHEMA (?125/?75 °C), then with increasing the temperature α-relaxation in PU (?75/0 °C), next ionic conductivity relaxation which starts at 0 °C, and finally the α-relaxation of PHEMA (+125/+170 °C) were detected. The α-relaxation of PHEMA in semi-IPNs shifts to lower temperatures and became broader with increasing amount of PU due to incomplete phase separation in the system and formation of interphases. The dielectric relaxation phenomena were fitted with Havriliak–Negami equation. Activation energy, τ_o and α parameters were calculated. For α-relaxations corresponding dielectric characteristics have been determined from Vogel–Fulcher–Tammann equation. The relaxation map for investigated PU, PHEMA and semi-IPNs was built.     

Relaxation of polaronic charge carriers in lithium manganese spinels

Lithium manganese spinels Li_1+xMn_2−xO₄, 0 ⩽ x ⩽ 0.33, were prepared by wet chemistry technique followed by heat-treatment at 750 °C or 800 °C. Differential scanning calorimetry was used to reveal phase transitions. Electrical properties were studied by impedance spectroscopy. LiMn₂O₄ exhibited phase transition below room temperature. The transition, seen as an exothermic event in DSC and a steep decrease of conductivity upon cooling, was sharp in sample sintered at 800 °C and broadened over a range of temperature in sample sintered at 750 °C. In the low temperature phase of LiMn₂O₄, two relaxations of similar strength were observed in the frequency dependent permittivity. The low frequency process was identified as relaxation of charge carriers since the relaxation frequency followed the same temperature dependence as the dc conductivity. The high frequency process exhibited milder temperature dependence and was attributed to dipolar relaxation in the charge-ordered structure. The dipolar relaxation was barely visible in Li substituted samples, x ⩾ 0.05, which did not undergo structural phases transition. Measurements extended to liquid nitrogen temperature showed gradual lowering of the activation energy of conductivity and relaxation frequencies, behavior typical for phonon-assisted hopping of small polarons.     

The electrical conductivity and Hall effect of glassy carbon

The electrical conductivity and Hall effect of glassy carbon heat treated for three hours between 1200 and 2700°C was measured at temperatures from 3 to 300 K in magnetic fields up to 5 tesla.The electrical conductivity, of the order of 200 (ohm cm)^?1 at room temperature, can be empirically written $\text{σ = A + B}\text{exp}\text{(?CT}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>4</mtext>}}\text{)-DT}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$, where the first term is a strongly scattering metallic component, the second term is attributed to variable range hopping, and the third and new term is negative correction to the metallic conductivity associated with one-dimensionality. All of the constants A, B and C were insensitive to heat-treatment temperature; the constant D decreased with increasing temperature until it disappeared at about 2200°C.The Hall coefficient was independent of magnetic field, insensitive to temperature, but was a strong function of heat-treatment temperature, crossing over from negative to positive at about 1700°C and ranging from ?0.048 to 0.126 cm³/coul.The idea of one-dimensional filaments in glassy carbon suggested by the electrical conductivity is compatible with the present consensus view of the microstructure constructed through such means as lattice imaging in transmission electron microscopy, and X-ray diffraction and small angle scattering.     

Electrical conductivity of low-temperature carbons as a function of frequency

The electrical conductivity of anthracene carbon samples, heat treated to temperatures of 600, 650, 730 and 800°C is measured at variable frequency up to 100 MHz. The carbons heat-treated to 600 and 650°C have a specific behaviour of amorphous solids, whereas the carbons HTT 730° and 800°C have an electrical conductivity independent of the frequency. The experimental results are correctly explained by means of the equations developed by Pollak and Geballe. Analysis of the results points to the conclusion that a hopping conduction between localized states could be a possible process of conduction in such solids.     

Optical properties of fluid sulfur up to supercritical conditions

The optical constants ?₁(ω) and ?₂(ω) of fluid sulfur have been derived for the first time over the entire liquid-vapour range at temperatures between 120°C and 1100°C and at pressures between 20 bar and 500 bar. The data have been obtained from reflectivity measurements by use of a modified Kramers-Kronig analysis.     

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.     

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.     

Electrical properties of A2.6+xTi1.4−xCd(PO4)3.4−x (A = Li,K; x = 0.0–1.0) phosphate glasses

Electrical properties of A_2.6+xTi_1.4−xCd(PO₄)_3.4−x (A = Li, K; x = 0.0–1.0) phosphate glasses are investigated over a frequency range from 42 Hz to 1 MHz at different temperatures. Impedance spectroscopy is used to separate the bulk conductivity from electrode effect of electrical conductivity data. The bulk dc conductivity is Arrhenius activated, with activation energies and pre-exponential factors following the Meyer–Neldel rule. The real part of ac conductivity shows universal power law feature. The variation of dielectric constant with frequency is attributed to ion diffusion and polarization occurring in the phosphate glasses. The frequency dependent imaginary part of electric modulus M″(ω) plot shows non-Debye feature in conductivity relaxation. The Kohlrausch–Williams–Watts stretched exponential function was used to describe the modulus spectra and the stretching exponent β is found to be temperature independent. Scaling in M″(ω) shows that the electrical relaxation mechanisms are independent of temperature for given composition at different temperatures.     

Temperature dependency of impedance spectroscopy behaviors in side-chain liquid crystalline polymer

In this paper, the electrical properties of side-chain liquid crystalline polymer (SLCP) are investigated by impedance spectroscopy technique. We report the measurement of dielectric and conductivity for SLCP from 1 kHz to 10 MHz within the temperature range from 300 to 370 K. The DC conductivity obeys Arrhenius law and it gives a small deviation at 315 K. The activation energies are equal to 0.20 eV and 0.75 eV for high and low temperatures, respectively. The frequency dependence of conductivity satisfies the power law, σ_AC = Aw^s, with s = 0.50–0.57. The evaluated power law exponent s exhibits nearly linear decreasing behavior with temperature. This suggests that the Correlated Barrier Hopping (CBH) model is the operating mechanism.     

AC conductivity of (As2Te3)95Ge5

The elctrical conductivity of amorphous chalcogenide (As₂Te₃)₉₅Ge₅ is investigated at variable frequency, from 1 kHz up to 35 GHz, and a variable temperature, from 77 to 300 K. The low-temperature conductivity is constant with temperature at a fixed frequency. At a fixed temperature, it obeys a ω^0.8 law only at low frequencies. The results are analysed with respect to the polaron problem, but also with a simple model of several dilute localized levels having a wide energy distribution in the forbidden band.     

Dielectric studies on pure and doped sr (NO3)2

Single crystals of pure and Na⁺-doped strontium nitrate were grown by slow evaporation of aqueous solution. Systematic measurement of dielectric constant (ϵ) and loss (tan δ) have been carried out in the frequency range of 100 Hz to 100 kHz at temperatures ranging from room temperature to 420 °C. A.C. conductivity (σ) is obtained from the data on ϵ and tan δ, ϵ, and σ were found to show anomalies around 300 °C. The results are discussed in the context of order-disorder phase transition.     

Experimental confirmation of oscillating properties of the complex conductivity: Dielectric study of polymerization/vitrification reaction

Clear evidence of the existence of fractional kinetics containing the complex power-law exponents were obtained by conductivity measurements of polymerization reaction of polyvinylpyrrolidone (PVP) performed inside a dielectric cell. We established the relationship between the Fourier image R(jω) of the complex memory function K(t) and the time-dependent mean square displacement 〈r²(t)〉. This relationship helps to understand the origin of the different power-law exponents appearing in the real part of complex conductivity Re[σ(ω)] and find a physical/geometrical meaning of the power-law exponents that can form the complex-conjugated values. The complex-conjugated values of the power-law exponents leading to oscillating behavior of conductivity follows from the fractional kinetics suggested by one of the authors (R.R.N.). The relationships [R(jω)⇔Re[σ(ω)]⇔〈r²(t)〉] are becoming very efficient in classification of different types of collective motions belonging to light and heavy carriers involved in the relaxation/transfer process. The conductivity data obtained for Re[σ(ω)] during the whole polymerization process of the PVP at different temperatures (80, 90, 100 °C) are very well described by the fitting function that follows from the suggested theory. Original fitting procedure based on the application of the eigen-coordinates (ECs) method helps to provide a reliable fitting procedure in two stages and use the well-developed and statistically stable linear least square method (LLSM) for obtaining the correct values of the fitting parameters that describe the behavior of Re[σ(ω, T_r)] in the available frequency range for the current time of the chemical reaction T_r measured during the whole process of polymerization. The suggested theory gives a unique possibility to classify the basic types of motions that take place during the whole polymerization process.     

Ionic conductivity of binary fluorides of potassium and rare earth elements

The ionic conductivity s of KYF₄ and K₂RF₅ single crystals (R = Gd, Ho, Er) and KNdF₄ and K₂RF₅ ceramic samples (R = Dy, Er) has been studied in the temperature range of 340–500°C. A comparative analysis of the σ values for these objects has been performed. Binary fluorides of potassium and rare earth elements were synthesized by the hydrothermal method (temperature 480°C, pressure 100–150 MPa) in the R₂O₃–KF–H₂O systems. The σ values of tetraf luorides are 3 × 10^–5 S/cm (KYF₄ single crystal) and 3 × 10^–6 S/cm (KNdF₄ ceramics) at 435°C. A K₂ErF₅ single crystal with σ = 1.2 × 10^–4 S/cm at 435°C has the maximum value of ionic conductivity among pentafluorides. The anisotropy of ionic transport was found in K₂HoF₅ single crystals, σ_∥c/σ_⊥c = 2.5, where σ_∥c and σ_⊥c are, respectively, the conductivities along the crystallographic c axis and in the perpendicular direction.     

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.     

Studies on electrical and the dielectric properties in MS structures

In this study, we investigated frequency dependent electrical and dielectric properties of metal-semiconductor (MS) structures using capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics in the frequency range 100 kHz-10 MHz in the room temperature. The dielectric constant (ε′), dielectric loss (ε″), dielectric loss tangent (tan δ) and ac electrical conductivity (σ_ac) were calculated from the C-V and G/ω-V measurements and plotted as a function of frequency. In general, ε′, ε″ and tan δ values decreased with increasing the frequency, while σ_ac increased with increasing frequency. Furthermore, the voltage and frequency dependence of series resistance were calculated from the C-V and G/ω-V measurements and plotted as functions of voltage and frequency. The distribution profile of R_S-V gives a peak in the depletion region at low frequencies and disappears with increasing frequencies. Also, series resistance values decreased with increasing frequency. The experimental results show that both frequency dependent electrical and dielectric parameters were strongly frequency and voltage dependent.     

Dielectric behaviour in a vanadium phosphate glass

The dielectric constant and conductivity of 80% V₂O₅: 20% P₂O₅ glass has been measured in the frequency range 10² to 10⁹Hz and in the temperature range 80 to 350°K. It is shown that the dielectric behaviour over these ranges is described by a Debye type relaxation process with distribution of relaxation times. A method is proposed to determine the width of distribution from the data at fixed frequencies and different temperatures. The width of distribution increases at frequencies ω > 10/τ, which leads to an a.c. conductivity at these frequencies almost linearly proportional to frequency and independent of temperature. The estimated value of the static dielectric constant of about 30 was found to decrease with temperature while the infinite frequency dielectric constant of 10 was independent of temperature. The carrier concentration calculated from the dielectric relaxation time and the d.c. conductivity through a thermal diffusion model shows reasonable agreement with direct measurement using electron paramagnetic resonance.     

Dielectric Studies of Three Liquid Crystals HBT,E8 and PCH-1132

The dielectric behavior of a Schiff base N-(P-Hexyloxybenzylidene)-P-toluidine (HBT) and two technologically important mixtures, E₈ and PCH-1132, are reported in the radio frequency region. The value of the mean dielectric constant increases by ～5% on going from the nematic to the isotropic phase in the case of E₈ and PCH-1132. This may perhaps be due to antiferroelectric arrangement of the molecules in the nematic phase. These mesogens show unusual dielectric behavior and exhibit positive anisotropy at lower frequencies and negative dielectric anisotropy above a critical frequency termed as isotropic frequency. The isotropic frequency is found to be temperature dependent. All these nematogens exhibit single relaxation in the radio frequency region which is due to hindered rotation of the molecules under nematic potential. The relaxation frequencies are 3.98 MHz (at 61.4°C), 2.27 MHz (at 34.3°C), 1.87 MHz (at 33°C) in case of HBT, E₈ and PCH-1132 respectively. The activation energy is found to be 16.7 KCal/mole, 14.6 KCal/mole and 16.1 KCal/mole for HBT, Ea and PCH-1132 respectively. In smectic B phase of HBT, the dipolar orientations are found to be somewhat “locked” similar to that in solids. The inversion of the sign of dielectric anisotropy in S_B phase of HBT is also discussed.     

The dielectric behavior of a thermoelectric treated B2O3-Li2O-Nb2O5 glass

A transparent glass with the composition 60B₂O₃-30Li₂O-10Nb₂O₅ (mol%) was prepared by the melt-quenching technique. Glass-ceramics, containing LiNbO₃ ferroelectric crystallites, were obtained by heat-treatment (HT) above 500 °C, with and without the presence of an external electric field. The dielectric properties of the glass and glass-ceramic were investigated, as a function of temperature (270-315 K), in the 10 mHz-32 MHz frequency range. The presence of an external electric field, during the heating process, improves the formation of LiNbO₃ crystallites. The rise of the treatment temperature and the applied field, during the heat-treatment, leads to a decrease in the dc electric conductivity (σ_dc), indicating a decrease of the charge carriers number. The dielectric permittivity (ε′) values (300 K;1 kHz) are between 16.25 and 18.83, with the exception of the 550 °C HT sample that presents a ε′ value of 11.25. An electric equivalent circuit composed by an R in parallel with a CPE element was used to adjust the dielectric data. The results reflect the important role carried out by the heat-treatment and the electric field during the HT in the electric properties of glass-ceramics.     

Crystallization of Li2OSiO2 glass studied by positron annihilation

The crystallization of 34 Li₂O65 SiO₂1 P₂O₅ glass has been studied by positron annihilation. The changes in the positron lifetime spectra have been correlated to the magnitude of the amorphous X-ray scattering. A linear relationship has been found for heat-treatment temperatures less than 700°C, above which non-linear phenomena in the lifetime spectra set in. The volume crystallinity reaches a saturation value, which varies from 70% at the heat-treatment temperature of 535°C to 80% at 700°C because of the phase separation. The dimensionality of the crystal growth in this glass is found to be n = 1.5 ± 0.1. The effect of the phase separation on the rate and dimensionality of the crystallization is discussed. The results show that positron annihilation is a sensitive and accurate method to follow crystallization in vitreous materials.