Dielectric and conductivity relaxations in PVAc based polymer electrolytes

The polymer electrolytes composed of a blend of poly (vinyl acetate) (PVAc) and poly (methylmethacrylate) (PMMA) as a host polymer and LiClO₄ as a salt are prepared by a solution casting technique. The formation of blend polymer- salt complex has been confirmed by FT-IR spectral studies. The conductivity- temperature plots are found to follow an Arrhenius nature. Arrhenius plot shows the decrease in activation energy with the increase in salt concentration. The dielectric behaviour of the sample is analysed using dielectric permittivity (ε′), dielectric loss (ε″) and electric modulus (M″) of the samples. The impedance cole- cole plot shows the high frequency semi- circle is due to the bulk effect of the material and the depression in the semicircle shows the non-Debye nature of the material. The bulk conductivity is found to vary between 2.5×10⁻⁵ Scm⁻¹ to 1.7×10⁻³ Scm⁻¹ with the increase of salt concentration of blend polymer samples. The migration energy derived from the dissipation factor is almost equal to the activation energy calculated from conductivity. The modulus spectrum of the samples shows the non-Debye behaviour of the polymer electrolyte films. The low frequency dispersion of the dielectric constant implies the space charge effects arising from the electrodes. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Electrical properties of cerium fluoride thin films

Thin films of ionic conductors have low internal resistance. Hence, it could be used as an electrolyte material in sensors to operate at ambient temperatures. Cerium fluoride, a unipolar fluoride ion conductor, has got a different application in electrochemical sensor. In the present work, cerium fluoride thin films have been prepared by physical vapor deposition method and their electrical properties are studied. X-ray diffraction studies reveal the polycrystalline nature of the prepared thin films and the structure of the material. Scanning electron microscopy (SEM) images show grain-like structures. Conductivity analysis of the thin films has been studied by ac impedance analysis and the maximum conductivity value is found to be 1.04 × 10⁻⁶ S cm⁻¹. The impedance spectra emphasize intergranular conduction in the prepared thin films.     

Synthesis and impedance analysis of proton-conducting polymer electrolyte PVA:NH4F

An attempt has been made to prepare a new proton-conducting polymer electrolyte based on poly(vinyl alcohol) doped with ammonium fluoride (NH₄F) by solution casting technique. The complex formation between polymer and dissociated salt has been confirmed by X-ray diffraction and Fourier transform infrared spectroscopy studies. The highest ionic conductivity has been found to be 6.9?×?10^?6?Scm^?1 at ambient temperature (303 K) for 85PVA:15NH₄F polymer electrolyte. The conductance spectra contain a low frequency plateau region and high frequency dispersion region. The dielectric spectra exhibit the low frequency dispersion, which is due to space charge accumulation at the electrode–electrolyte interface. The modulus spectra indicate non-Debye nature of the material. The highest ionic conductivity polymer electrolyte 85PVA:15NH₄F has low activation energy 0.2 eV among the prepared polymer electrolytes.     

Optical properties of BiFeO<Subscript>3</Subscript> ceramics in the frequency range 0.3–30.0 THz

Reflection and transmission infrared spectra of BiFeO₃ ceramic samples have been measured using submillimeter spectroscopy (on a backward-wave tube spectrometer) and Fourier-transform infrared spectroscopy in the frequency range from 5 to 1000 cm⁻¹ at temperatures in the range from 10 to 500 K. New resonant modes (probably, magnetic in nature) with the eigenfrequencies decreasing with an increase in the temperature have been recorded in the range 10–30 cm⁻¹ by IR spectroscopy for the first time. An additional absorption with a fairly large dielectric contribution has been revealed in the range 30–60 cm⁻¹. It has been demonstrated that the corresponding oscillators couple with both the lowest frequency phonon mode and the magnetic subsystem.     

Nature of low-energy excitations in La<Subscript>1.87</Subscript>Sr<Subscript>0.13</Subscript>CuO<Subscript>4</Subscript> superconducting cuprate

The spectra of the conductivity and dielectric constant of La_1.87Sr_0.13CuO₄ cuprate have been directly measured in the frequency range of 0.3 to 1.2 THz (10–40 cm⁻¹) and the temperature range of 5 to 300 K in the E | c polarization (the electric field vector of radiation is perpendicular to the copper-oxygen planes). Excitation has been observed in the superconducting phase, and its nature has been attributed to the transverse optical excitation of the condensate of Cooper pairs, which appears because Josephson junctions between CuO planes are modulated due to in-plane magnetic and charge stripes. Additional quasiparticle absorption of unknown origin has been detected at frequencies below ≈15 cm⁻¹ at liquid helium temperatures.     

Conductivity studies of starch-based polymer electrolytes

A proton-conducting polymer electrolyte based on starch and ammonium nitrate (NH₄NO₃) has been prepared through solution casting method. Ionic conductivity for the system was conducted over a wide range of frequency between 50 Hz and 1 MHz and at temperatures between 303 K and 373 K. Impedance analysis shows that sample with 25 wt.% NH₄NO₃ has a smaller bulk resistance (R _b) compared to that of the pure sample. The amount of NH₄NO₃ was found to influence the proton conduction; the highest obtainable room temperature conductivity was 2.83 × 10⁻⁵ S cm⁻¹, while at 100 °C, the conductivity in found to be 2.09 × 10⁻⁴ S cm⁻¹. The dielectric analysis demonstrates a non-Debye behavior. Transport parameters of the samples were calculated using the Rice and Roth model and thus shows that the increase in conductivity is due to the increase in the number of mobile ions.     

Kinetics of lithium intercalation into carbon anodes: in situ impedance investigation of thickness and potential dependence

Electrochemical impedance spectroscopy (EIS) in three-electrode sandwich configuration was applied to investigate the kinetics of Li intercalation into mesocarbon-microbeads (MCMB) based anodes. A new frequency domain model, considering porous macroscopic structure of the active material and spherical diffusion inside the particles, has been applied to analyze the potential and thickness dependence of impedance spectra. Values of several kinetically relevant parameters like specific conductivity of the layer of intercalation material (2.6×10⁻⁴ S cm⁻¹), chemical diffusion coefficient of Li-ions in MCMB (2.2×10⁻⁹ cm² s), charge transfer resistance (184 Ω cm²), are obtained from the analysis. Applicability of the proposed model to prediction of time-domain charge curves has been tested using numerical inversion of Laplace transformation (NILT). The time domain modeling led to the conclusion that phase nucleation at the boundaries between Li-rich and Li-poor phases coexistent during intercalation is the rate-limiting step at the initial stage of the impulse-discharge.     

An electrochemical study of copper diffusion in non-stoichiometric copper sulphide

The diffusion of copper in hexagonal chalcocite (Cu₂S) is not very well-known. In this work electrochemical cells with natural polycrystalline chalcocite working electrodes have been studied by electrochemical impedance spectroscopy (EIS) over a large frequency range at equilibrium potential. To study this phenomenon between 120 and 160°C a solid electrolyte RbCu₄Cl₅ has been used. The impedance spectra present two distinct regions. At high frequencies the general shape of the diagram in the Nyquist plane is a depleted are of a circle. Changes in electrolyte resistance, interfacial capacitance and transfer resistance have been studied as a function of temperature. At low frequencies, a diffusion impedance is observed attributed to the mobility of copper vacancies. Diffusion coefficients, with an activation energy of 1.7 eV, have been deduced from the impedance diagrams (4.7·10⁻⁵cm² s⁻¹ at 130°C). These results are compared with those obtained with orthorhombic chalcocite between 30 and 60 °C by EIS and using an electrochemical cell with a cupric liquid electrolyte. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Metallic/semiconducting ratio of carbon nanotubes in a bundle prepared using CVD technique

We present an investigation of the nature of single-walled carbon nanotubes (SWCNTs) in a bundle by resonant Raman spectroscopy. The calculation has been done for the three peak positions in radial breathing mode (RBM) spectra obtained by using a laser excitation wavelength (γ) of 633 nm from He-Ne laser on SWNT bundle sample prepared by chemical vapor deposition (CVD) technique using iron catalyst at 800°C. The detailed analysis in the present study is focused on peak positions 162 cm⁻¹, 186 cm⁻¹, and 216 cm⁻¹. The firs step of the analysis is to construct a list of possible (n, m) pairs from the diameters calculated from the RBM peak positions. The parameters of SWNTs studied gives in-depth understanding of many symmetry, resonance and characteristic properties of SWNT bundles. Our results indicate that the contribution of metallic SWNTs in the bundle is small at RBM peak positions 162 cm⁻¹, 186 cm⁻¹ and in agreement with pervious results at peak position 216 cm⁻¹.     

Transmission spectra of epitaxial layers of Pb1 ? xEuxTe (0 ≤ x ≤ 0.37) solid solutions in the frequency range 7–4000 cm?1

The spectra of epitaxial Pb_{1 − x} Eu _x Te (0 ≤ x ≤ 0.37) solid solution layers grown on BaF₂ and Si substrates have been investigated over a wide frequency range 7–4000 cm⁻¹ at temperatures of 5–300 K. Apart from the phonon and impurity absorption lines, the absorption in a local mode in PbEuTe layers of substrates and buffer layers has been observed in the frequency range 110–114 cm⁻¹. As the temperature decreases from 300 to 5 K, the transverse phonon mode softens from 33.0 to 19.5 cm⁻¹.     

Impedance spectroscopy study of BNKLT polycrystalline ceramic

Complex impedance analysis of perovskite structured polycrystalline, [Bi_0.5(Na_1−x−y K _x Li _y )_0.5]TiO₃, at x=0.2, y=0.1 ceramic was synthesized by a mixed oxide method. The formation of single-phase material was confirmed by X-ray studies, and it was found to be rhombohedral structure at room temperature. Under scanning electron microscope, grains separated by well-defined boundaries are visible, which is in good agreement with impedance analysis. The BNKLT ceramic shows excellent piezoelectric properties and the optimum properties measured are: d ₃₃=251 pC/N, g ₃₃=24×10⁻³ mV/N, k _p =30.5% and k _t =28.1%. A complex impedance spectroscopy (CIS) study has been carried out to investigate the electrical properties. Impedance and modulus plots helped to separate the grain and grain boundary to the overall polarization or electrical behavior. CIS analysis suggests the presence of temperature-dependent relaxation process in the material. A possible hopping mechanism for electrical transport processes in the studied material is evident from the modulus analysis. The modulus mechanism indicates the non-Debye type of conductivity relaxation in the materials, which is supported by impedance data. The activation energies have been calculated from impedance (E _τ =0.58 eV) and electric modulus (E _τ =0.40 eV) studies, which suggests that the conduction is ionic in nature. The variation in width of the curves, M"/M"_maxM'/M'_{\max} and Z"/Z"_maxZ'/Z'_{\max} at FWHM, allows to conform that the relaxation process involved is of non-Debye type.     

Electrical transport study of potato starch-based electrolyte system

A biopolymer electrolyte system having conductivity ∼1.3 × 10⁻⁴ S cm⁻¹ has been prepared using potato starch, NaI, glutaraldehyde and poly(ethylene glycol) (PEG; molecular weight = 300). High ionic transference numbers (∼0.99) of the material confirmed its electrolytic behaviour. Conductivity and dielectric behaviour as a function of frequency has been studied. Conductivity follows ‘universal power law’ (σ = σ ₀ + Aω ⁿ ) with exponent ‘n’ varying from 0.94 to 1.18. Cross-linking and plasticization increases long pathways motion of charge carriers, comparable to sample dimension. Humidity-independent behaviour (up to 80% relative humidity), of impedance and water intake by the system, indicates the system’s potentiality as a promising candidate for humidity immune device fabrication. The addition of PEG has a twofold effect on the material’s conductivity. It not only increases conductivity but also improves the material’s immunity towards humid atmosphere.     

Specific features of the EPR spectra of KTaO<Subscript>3</Subscript>: Mn nanopowders

The electron paramagnetic resonance spectra of KTaO₃: Mn nanocrystalline powders in the temperature range from 77 to 620 K have been measured and studied for the first time. The change observed in the spectra has been investigated as a function of the doping level. The doping regions in which Mn²⁺ ions are individual paramagnetic impurities have been established, as well as the regions where the dipole-dipole and exchange interactions of these ions begin to occur. The spin-Hamiltonian constants for the spectrum of non-interacting individual Mn²⁺ ions have been determined as follows: g = 2.0022, D = 0.0170 cm⁻¹, and A = 85 × 10⁻⁴ cm⁻¹. A significant decrease in the axial constant D in the KTaO₃: Mn nanopowder, as compared to the single crystal, has been explained by the remoteness of the charge compensator from the paramagnetic ion and by the influence of the surface of the nanoparticle. It has been assumed that the Mn²⁺ ions are located near the surface and do not penetrate deep into the crystallites.     

Free carrier concentration and conductivity in chlorine doped lead telluride

Plasma resonance in the IR reflection spectra is used to measure the concentration and relaxation time of free charge carriers along with the conductivity in PbTe_{1 − x} Cl _x solid solutions. It is found that with increasing the chlorine concentration, the electron concentration and conductivity increase and reach saturation at x = 0.03 (n = 5.5 × 10¹⁹ cm⁻³, σ = 3750 Ohm⁻¹ · cm⁻¹). The relaxation time decreases with increasing the chlorine concentration and reaches the minimum value of 2.2 × 10⁻¹⁴ s at x = 0.03; then, it almost does not change.     

Structural and electrical properties of KCa<Subscript>2</Subscript>Nb<Subscript>5</Subscript>O<Subscript>15</Subscript> ceramics

A polycrystalline sample of KCa₂Nb₅O₁₅ with tungsten bronze structure was prepared by a mixed oxide method at high temperature. A preliminary structural analysis of the compound showed an orthorhombic crystal structure at room temperature. Surface morphology of the compound shows a uniform grain distribution throughout the surface of the sample. Studies of temperature variation on dielectric response at various frequencies show that the compound has a transition temperature well above the room temperature (i.e., 105°C), which was confirmed by the polarization measurement. Electrical properties of the material have been studied using a complex impedance spectroscopy (CIS) technique in a wide temperature (31–500°C) and frequency (10²–10⁶ Hz) range that showed only bulk contribution and non-Debye type relaxation processes in the material. The activation energy of the compound (calculated from both the loss and modulus spectrum) is same, and hence the relaxation process may be attributed to the same type of charge carriers. A possible ‘hopping’ mechanism for electrical transport processes in the system is evident from the modulus analysis. A plot of dc conductivity (bulk) with temperature variation demonstrates that the compound exhibits Arrhenius type of electrical conductivity.      

Hole mobility and trapping in PVK films doped with CdSe/CdS and CdSe quantum dots

Experimental study of the hole mobility in polyvinylcarbazole (PVK) films doped with two kinds of nanocrystals, on bare core CdSe and core-shell CdSe/CdS quantum dots, with concentrations ranging from 3 · 10¹⁰ to 3 · 10¹⁵ cm⁻³, is presented. The quantum dots investigated were made using colloidal chemistry. The hole mobility was measured using the time-of-flight technique as a function of the applied electrical field in the range 10⁵–10⁶ V/cm and for temperatures from 20°C to 50°C. The transient curves, being featureless on a linear plot, show on a double logarithmic scale a sharp inflection point indicating a dispersive carrier drift process. The recovered values of the mobility are in the range 3 · 10⁻⁸–10⁻⁶ cm²·V⁻¹·s⁻¹ and their field and temperature dependences can be analyzed formally within the framework of the Gaussian disorder model proposed by B?ssler. The energetic disorder is, within the experimental accuracy, independent of the concentration and type of quantum dots for the CdSe quantum dots at all concentrations and for the CdS/CdSe quantum dots up to 10¹⁴ cm⁻³. The spatial disorder factors are very large (from 5.3 to 8.7) and do not depend in a systematic way upon the type and concentration of quantum dots (QDs). The experiments show that the apparent mobility does not change considerably with concentration, but it was found that the samples with CdSe/CdS quantum dots at concentrations from 10¹⁵ to 3 · 10¹⁵ cm⁻³ show a decreased photocurrent response. The dependence of the time-integrated transients (corresponding to the full charge value) upon the quantum-dot concentration has been determined. Differences in total photogenerated charge for pure and doped polymer films imply that the quantum dots of that type are the hole traps with capture times much more smaller than the transit time and with emission times a few orders longer than the transit time. CdSe quantum dots without a shell do not seem to exhibit the same properties as core shells and do not produce considerable changes in the charge transfer, even at a density of 10¹⁵ cm⁻³.     

Raman scattering of light and IR absorption in carbon nanotubes

Raman light scattering and IR absorption spectra of samples containing multilayer carbon nanotubes in different stages of purification by the selective oxidation technique have been investigated. It was found that the Raman spectra of carbon nanotubes exhibit softening of the mode at 1582 cm⁻¹ corresponding to E _2g vibrations of graphite hexagons and a line at 120 cm⁻¹ due to the radial vibrations of nanotubes. In IR absorption spectra measured in the region of 0.07–0.3 eV, several sets of lines with a spacing of 15 meV (120 cm⁻¹) between lines of each group have been detected. We suggest that each group corresponds to electron transitions generating electron-hole pairs in semiconducting nanotubes and contains a phononless 00-line and its phonon replicas with spacing between them equal to the “breathing” mode energy of 120 cm⁻¹. Measurements of electric conductivity at a frequency of 9300 MHz indicate that, in addition to semiconducting nanotubes, the samples contain nanotubes with properties of a highly disordered semimetal. Zh. éksp. Teor. Fiz. 113, 1883–1891 (May 1998)     

Spectral components that form UV absorption spectrum of Ce3+ and Ce(IV) valence states in matrix of photothermorefractive glasses

We have measured the UV absorption spectra of photothermorefractive glasses of the system Na₂O-ZnO-Al₂O₃-NaF-SiO₂ doped by cerium oxide in the range of (2.8–5.0) × 10⁴ cm⁻¹ (360–200 nm). The spectra have been processed by the method of dispersion analysis based on the analytical convolution model for the complex dielectric function of glasses. We show that the absorption band centered at 3.3 × 10⁴ cm⁻¹ (∼303 nm) that is attributed to the transition 2F _5/2 → 5d in the Ce³⁺ ion, is an envelope of three spectral components. The broad absorption range (3.5–4.7) × 10⁴ cm⁻¹ (200–270 nm) that is commonly interpreted as a charge transfer band of the Ce(IV) valence state, is an envelope of at least three spectral components.     

Application of abrupt cut-off models in the analysis of the capacitance spectra of conjugated polymer devices

In this paper, a detailed study of the capacitance spectra obtained from Au/doped-polyaniline/Al structures in the frequency domain (0.05 Hz–10 MHz), and at different temperatures (150–340 K) is carried out. The capacitance spectra behavior in semiconductors can be appropriately described by using abrupt cut-off models, since they assume that the electronic gap states that can follow the ac modulation have response times varying rapidly with a certain abscissa, which is dependent on both temperature and frequency. Two models based on the abrupt cut-off concept, formerly developed to describe inorganic semiconductor devices, have been used to analyze the capacitance spectra of devices based on doped polyaniline (PANI), which is a well-known polymeric semiconductor with innumerous potential technological applications. The application of these models allowed the determination of significant parameters, such as Debye length (≈20 nm), position of bulk Fermi level (≈320 meV) and associated density of states (≈2×10¹⁸ eV⁻¹ cm⁻³), width of the space charge region (≈70 nm), built-in potential (≈780 meV), and the gap states’ distribution.     

Measurements of the temperature and water vapor concentration in a hot zone by tunable diode laser absorption spectrometry

A tunable diode laser absorption spectroscopy (TDLAS) technique and appropriate instrumentation was developed for the measurement of temperature and water vapor concentrations in heated gases. The technique is based on the detection of the spectra of H₂O absorption lines with different energies of low levels. The following absorption lines of H₂O were used: 7189.344 cm⁻¹ (E″=142 cm⁻¹), 7189.541 cm⁻¹ (E″=1255 cm⁻¹), 7189.715 cm⁻¹ (E″=2005 cm⁻¹). Spectra were recorded using fast frequency scanning of a single distributed feedback (DFB) laser. A unique differential scheme for the recording of the absorption spectra was developed. An optimal technique for fitting the experimental spectra was developed.