Interfacial polarization in piezoelectric fibre–polymer composites

Polymer composites of an epoxy resin matrix filled with PZT fibres were studied by means of dielectric spectroscopy in the frequency range 0.1 Hz to 100 kHz and temperature interval from 80 °C to 170 °C. An interfacial or a Maxwell–Wagner–Sillars relaxation process was revealed in the frequency range between 0.1 Hz and 10 Hz and temperatures above the glass transition. This interfacial relaxation was found to follow the Debye law for the distribution of relaxation times.     

Molecular dynamics in glass-forming poly(phenyl methyl siloxane) as investigated by broadband thermal,dielectric and neutron spectroscopy

The molecular dynamics of glass-forming poly(methyl phenyl siloxane) (PMPS) is studied by thermal (10⁻³–5 × 10² Hz), dielectric (10⁻³–10⁹ Hz) and neutron (5 × 10⁸–10¹² Hz) spectroscopy. Because of the broad frequency range of 15 orders of magnitude the study provides a precise determination of glassy dynamics in a wide temperature range using different probes. The relaxation rates extracted from the different methods agree quantitatively in both their absolute values and in their temperature dependencies. A detailed analysis of the temperature dependence of the relaxation rate f_p by a derivative technique shows that the α-relaxation of PMPS has to be characterized by a high and a low temperature branch separated by a crossover temperature T_B = 250 K. In both temperature ranges the temperature dependence of f_p has to be described by Vogel/Fulcher/Tammann laws with different Vogel temperatures. Also the analysis of the dielectric strength in its temperature dependence gives a crossover behavior from a low to a high temperature region with a similar value of T_B. T_B can be interpreted as onset of cooperative fluctuations and the formation of dynamical heterogeneities. The dependence of the relaxation rate on the scattering vector Q extracted from neutron scattering obeys a power law τ ∼ Q^−Slope, where the power Slope varies between Slope = 2 and Slope = 3.5 with increasing temperature. This anomalous dependence of the relaxation time on the momentum transfer is discussed in terms of dynamic heterogeneities in the underlying motional processes even at temperatures above T_B. Besides the segmental dynamics the fast Methyl group rotation is considered as well. The relaxation rates of this process have an activated temperature dependence with an activation energy of 8.3 kJ/mol. The data were discussed in the framework of the threefold jump model were the incoherent elastic scattering from ‘fixed’ atoms which are frozen on the time scale of the Methyl group rotation was taken into account.     

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.     

Slow and fast dynamics in glycerol–water mixtures

We discuss the relaxation dynamics of glycerol–water mixtures as studied by broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). BDS studies were performed in a frequency range of 1 Hz to 250 MHz at temperatures ranging from 173 to 323 K and DSC measurements were made between 138 and 313 K. The experimental results obtained for the glycerol-rich mixtures in terms of dielectric loss ‘master plots’ suggest that the main dielectric relaxation process, as well as the high frequency ‘excess wing’ and dc-conductivity, follow the same temperature dependence. This result indicates that all of these processes are based on the same physical origin. A new phenomenological relationship for the complex dielectric permittivity is proposed. The structural and dynamic properties of glycerol–water mixtures are discussed. The experimental data for water-rich mixtures show the existence of a critical concentration of 40 mol% of glycerol that relates to the numbers of hydrogen bonds of glycerol and of water molecules. Below this concentration, water cooperative domains appear, coexisting with the glycerol cooperative domains. Below 20 mol% of glycerol, ice nanocrystals appeared that led to three relaxation processes; result from ice, mesoscopic glycerol–water domains, and water on the interface between former two.     

Exploration on nano-composite fumed silica-based composite polymer electrolytes with doping of ionic liquid

Fumed silica (SiO₂)-based composite polymer electrolytes were prepared by means of solution casing technique. Horizontal attenuated total reflectance-Fourier Transform Infrared (HATR-FTIR) study shows the complexation between polymer matrix and SiO₂. The highest ionic conductivity of (4.11 ± 0.01) × 10^? 3 Scm^? 1 is achieved upon inclusion of 8 wt.% of SiO₂. Three different regions have been observed in the frequency dependence–ionic conductivity study. The conductivity rises sharply with frequency at low frequency regime. It is followed by a frequency independent plateau region and sharp decrease in the conductivity at high frequency range. The dielectric permittivity (ε^') and dielectric loss (ε^") are decreased with increasing the frequency. This phenomenon is mainly attributed to the electrode polarization effect. The formation of electrical double layer has been proven in these dielectric permittivity studies. This indicates the non-Debye properties of the nano-composite polymer electrolytes.     

Dielectric spectroscopy of low-loss sugar lyophiles: II. Relaxation mechanisms in freeze-dried lactose and lactose monohydrate

This article presents an analysis of the dielectric relaxation mechanisms of the disaccharide lactose, in both the freeze-dried state and the crystalline monohydrate. Complex dielectric permittivity spectra for both forms of lactose were measured over wide ranges of frequency (0.1 Hz to 1 MHz) and temperature (−120 °C to +100 °C). The crystalline monohydrate displays a sharp peak in the temperature domain which was ascribed to the percolation of migrating protons over the surfaces of the crystals. The freeze-dried material displayed three relaxation processes. The first (γ) and second (β) processes were observed at low temperature and present as very broad relaxations with low dielectric strength. The relaxation times for both processes showed Arrhenius temperature-dependencies, with activation energies of 52 kJ/mol and 72 kJ/mol, respectively. The mechanisms of dielectric relaxation were ascribed to the motion of the pendant hydroxymethyl group and the relative motion of the pair of saccharide rings, respectively. Analysis and discussion of the third relaxation process is beyond of the scope of this article.     

Dielectric properties of bis(2,4 pentanedionato)copper(II) crystalline films grown on Si substrate for low-k applications

Bis(acetylacetonato)copper(II) thin films were prepared by sublimation at about 245 °C in vacuum on p-Si and glass substrates for dielectric and optical investigations. They were characterized by the X-ray diffraction (XRD) and energy-dispersion X-ray fluorescence (EDXRF) methods. The XRD pattern reveals that the prepared films were polycrystalline of monoclinic P2₁/n structure. The optical absorption spectrum of the prepared film was not identical to that of the molecular one, which identified by a strong absorption peak at 635 nm. The onset energy of the optical absorption of the complex was calculated by using Hamberg et al. method, which is usually used for common solid-state semiconductors and insulators. The dielectric properties for the complex as insulator were investigated on samples made in form of a metal-insulator-semiconductor (MIS) structure. The dielectric properties were studied in frequency range 1–1000 kHz and temperature range 298–333 K. The dielectric relaxation was analyzed in-terms of dielectric modulus M¹(ω). Generally, the present study shows that films of the complex grown on Si substrate are a promising candidate for low-k dielectric applications; it displays low-k value around 1.7 ± 0.1 at high frequencies.     

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.     

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.     

Dielectric relaxation and morphologic properties of CaCu3Ti4O12 doped with GeO2

CaCu₃Ti₄O₁₂ (CCTO) is a material with giant dielectric constant, presenting good stability over a wide temperature and frequency ranges. The preparation method and doping has a great influence on the microstructure and dielectric properties of this material. In this work, doping CCTO with 2–10 wt% GeO₂ has been shown to increase the dielectric constant. We studied the prepared samples by X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance spectroscopy. X-ray diffraction shows the presence of nanocristals. Grains and grain boundaries compositions have been observed by scanning electron microscopy with energy dispersive X-ray spectrometry mapping. Impedance spectroscopy measurements, in the frequency range from 75 kHz to 30 MHz, and temperature from 250 to 325 K, have been performed. The data were analyzed using the Cole–Cole model of dielectric relaxation.     

Molecular relaxations in radiationally crosslinked poly(vinyl methyl ether) hydrogels

Molecular relaxations in poly(vinyl methyl ether) hydrogels obtained by electron beam irradiation (range of doses: 13–50 kGy) of 10% polymer solution in water were investigated. Broadband dielectric spectroscopy (10⁻¹–10⁶ Hz) and differential scanning calorimetry results shown two relaxation processes of the polymer network origin – glass transition and polymer chains collapsing. Also detected were two relaxations connected with frozen water motions. At the lowest temperature there is a relaxation connected with the ice strongly defected by the polymer chains and at higher temperatures there is a relaxation connected with the hexagonal ice I.     

The role of vanadium valence states and coordination on electrical conduction in lithium iodide borate glasses mixed with small concentration of silver iodide

LiI–AgI–B₂O₃ glasses mixed with different concentrations of V₂O₅ (ranging from 0 to 1.0 mol%) were prepared. Electrical and dielectric properties over wide ranges of frequency (10^?2–10⁷ Hz) and temperature (173–523 K) have been studied. Additionally spectroscopic properties viz., optical absorption and ESR spectra have been investigated. The optical absorption and ESR studies have revealed that vanadium ions do exist in both V⁴⁺ and V⁵⁺ states and the redox ratio is the highest in the glasses containing 0.8 mol% of V₂O₅. The results of conductivity measurements have indicated that there is a mixed conduction (both ionic and electronic). The ionic conduction seems to be dominant over polaron hopping only in the glasses containing V₂O₅ more than 0.8 mol% of V₂O₅. The impedance spectra have also indicated that the conduction is predominantly polaronic in nature. The frequency and temperature dependence of the electrical moduli as well as dielectric loss parameters have exhibited relaxation character attributed to the vanadyl complexes. The relaxation effects have been analyzed by the graphical method and from this analysis it has been established that there is a spreading of relaxation times. The results have been further discussed quantitatively in the light of different valance states of vanadium ions with the aid of the data on spectroscopic properties.     

Dielectric properties of non-swelling bentonite: The effect of temperature and water saturation

In the present work, dielectric and conductivity measurements in the frequency range of 10 mHz–1 MHz were carried out by means of a high-resolution broadband spectrometer on non-swelling bentonite samples, at various low levels of hydration. The evolution of dielectric response was also examined after annealing of the samples up to 260 °C. Conductivity results in conjunction with the electric modulus representation (M1) is proved to be the most suitable for the analysis of the dielectric data. Dielectric response is dominated by ionic (surface and bulk) conductivity as well as by grain polarization due to migration of ions along the clay layers. Both mechanisms are strongly affected by the concentration of bound water and the thermodielectric effect. Havrilliak–Negami dielectric relaxation functions were used to model the observed relaxations mechanisms in modulus formalism and the fitting parameters were evaluated at various water contents of the measured samples.     

Dynamics of alcohols dispersed with aerosils

We investigated the dynamics of three alcohols dispersed with aerosils in the frequency range from 1 MHz to 1 GHz with dielectric relaxation spectroscopy. In comparison with the pure systems, no new relaxation processes were observed in this frequency range. The main relaxation time in octanol decreased with increasing aerosil concentration. In ethanol and glycerol there was no significant change in the relaxation times found. These results are different from those well-known for liquid crystal–aerosil dispersions. We think that the absence of the retarded relaxation at the surface of the aerosils can be explained by the fact that the character of the hydrogen bond is not too much different for an intermolecular one as compared to an alcohol–aerosil bond. Also it seems that the (shorter) alcohols are already disordered to an extent that the addition of an extra disordering agent does not change the dynamics.     

Synthesis and characterizations of phthaloyl chitosan-based polymer electrolytes

Phthaloyl chitosan (PhCh) has been synthesized by reacting excess phthalic anhydride with chitosan in the presence of nitrogen gas (N₂). Confirmation of phthaloyl chitosan structure by Fourier-transform infrared (FTIR) spectroscopy shows bands at 1773 and 1713 cm^? 1 attributable to the pthalimido group. From X-ray diffraction (XRD), the samples are largely amorphous. Thermal stability of chitosan is increased on phthaloylation. Ammonium thiocyanate (NH₄SCN) salt has been added to the solution of phthaloyl chitosan in N,N-dimethylformamide (DMF) before casting to form films. The amount of NH₄SCN salt added ranges from 5 to 50 wt. % concentration. The highest conductivity at 298 K is (2.42 ± 0.01) × 10^? 5 S cm^? 1 for the sample 70 wt. % PhCh-30 wt. % NH₄SCN. Impedance of the films has been measured at temperatures between 298 K and 373 K and in the frequency range from 0.1 Hz to 1 × 10⁷ Hz. Relaxation peaks are observed from dielectric loss and tangent delta variation with frequency at ambient and elevated temperatures in the frequency range investigated for the highest conducting sample. Decomposition voltage is ~ 2.07 V and transference number measurements show that charge conduction is mainly by ions.     

Ion beam synthesis of C-based optically-active nanoclusters in silica

Carbon nanoclusters formed using ion implantation and thermal annealing are shown to photoluminescence in the visible range. Silica samples were implanted with a fluence of 2 × 10¹⁷ atoms/cm², 70 keV carbon ions and thermally annealed for 4 h at 1100 °C. Photoluminescence measurement made at select intervals during the anneal process show continued growth of the nanoclusters within the silica throughout the process. However, Rutherford backscattering showed a rapid loss of carbon during the initial 15 min of annealing indicating a competition between the growth of the second-phase nanoparticles and the formation of CO, a volatile form of carbon.     

Dielectric relaxation of Al/Lu2O3/Al thin film structures from 10 μHz to 10 MHz

Results of dielectric studies of lutetium sesquioxide layers examined in Al/Lu₂O₃/Al thin film sandwiches are reported. The dielectric measurements were carried out in the frequency range 10⁻⁵–10⁷ Hz and for temperatures from 292 K to 500 K. Results are presented as plots of frequency functions: the capacitance, the dielectric loss factor, tan δ(f) and also on the complex plane as Cole–Cole plots and Nyquist plots. The influence of the external voltage on C(U) and tan δ(U) was examined. Experimental data were analyzed taking into account thin insulating Lu₂O₃ film, near-electrode regions of Al/Lu₂O₃ and Lu₂O₃/Al interfaces and series resistance of electrodes and leads. The parameters of Lu₂O₃ film, near-electrode regions and resistance of contacts and leads were determined.     

Conductivity and dielectric properties of polyvinyl alcohol–polyvinylpyrrolidone poly blend film using non-aqueous medium

Several methods such as copolymerization, plasticization and blending etc., have been used to modulate the conductivity of polymer electrolytes. Polymer blending is one of the most important contemporary ways for the development of new polymeric materials and it is a useful technique for designing materials with a wide variety of properties. Polymer blend electrolyte has been prepared with different concentrations of PVA and PVP by solution casting technique using DMSO as solvent. The prepared films have been investigated by different techniques. The increase in amorphous nature of the polymer electrolytes has been confirmed by XRD analysis. The FTIR analysis reveals that the interchain hydrogen bonding within a PVA–PVP blends. The dielectric permittivity (ε*) and modulus (M*) have been calculated from the ac impedance spectroscopy in the frequency range 42 Hz– 1 MHz and the temperature range 308–373 K. The maximum conductivity has been found to be 1.58 × 10^? 6 S cm^? 1 at room temperature for 70PVA:30PVP concentration. The conductivity has been increased to 5.49 × 10^? 5 S cm^? 1 when the temperature is increased to 373 K. The activation energy of all samples was calculated using the Arrhenius plot and it has been found to be 0.53 eV to 0.78 eV.     

Dynamic properties of solvent confined in silica gels studied by broadband dielectric spectroscopy

We report the results of a broadband (10⁻²–10⁷ Hz) dielectric spectroscopy study on a solvent system (glycerol–water solution) confined in a porous silica matrix. The dielectric relaxation of the system is studied as a function of both temperature (120–280 K) and solvent composition (0–36 glycerol molar percentage), at constant matrix composition. Our data show that glycerol–water systems confined inside silica gel are characterized by a very complex dynamics quite different from that observed in solution, thus indicating that confinement may deeply modify solvent dynamics. Indeed in addition to the relaxation processes similar to those occurring in bulk samples, new dielectric relaxations are detected: two non-collective relaxations, attributed to water molecules strongly interacting with pore surfaces and to the glycerol trapped within the matrix structure, respectively; a relaxation in the glycerol free sample (and in samples at very low glycerol content) almost coincident with that observed in other different confinement conditions and governed by geometrical confinement per se. Moreover, at high glycerol content we observe two non-Arrhenius processes at least 4 order of magnitude slower than solution-like main relaxation; at low glycerol content the two above relaxations merge and show a fragile to strong transition at about 200 K.     

Preparation,vibrational structure and dielectric properties studies of cotton linter and its derivatives

Preparation of unhydrolyzed and hydrolyzed cotton linters as well as their derivatives (carboxylate, phosphate and phosphosulphonate), vibrational structure and dielectric properties were studied in this work. The vibrational modes were analyzed by FTIR spectroscopy and the band at 1730 cm^?1 in the carboxylation of unhydrolyzed and hydrolyzed was appeared due to introduce of carboxylic group. The new bands appeared at 1208 and 1400 cm^?1 in the phosphorylation and phosphosulphonation assigned to C–O–P and C–O–S, respectively. The data indicated that, new chemical structure for the derivatives of unhydrolyzed and hydrolyzed cotton linter was formed. The crystallinity index (CrI) was measured from absorbance ratio of the band 1428 and 900 cm^?1. The real part of dielectric constant (ε′) and AC electrical conductivity was studied in the temperature range (295–375 K) at two different frequencies, 800 and 2000 kHz. The ε′ was decreased with increasing frequency due to dispersion. The frequency–temperature dependent conductivity and mechanisms of the electronic conduction were discussed.