Evaluation of a.c. conductivity behaviour of graphite filled polysulphide modified epoxy composites

Composites of epoxy resin having different amounts of graphite particles have been prepared by solution casting method. Temperature dependence of dielectric constant, tan δ and a.c. conductivity was measured in the frequency range, 1–20 kHz, temperature range, 40–180°C for 0.99, 1.96 and 2.91 wt% graphite filled and unfilled epoxy composites. It was observed that the dielectric constant, tanδ and a.c. conductivity increase with increasing temperature. Near the transition temperature the materials show anomalous behaviour for the observed properties. Peaks of dielectric constant, tan δ and a.c. conductivity were observed to shift towards lower temperature with increasing frequency. Clear relaxation (tan δ) peaks around 169°C were observed in epoxy resin, which shifted to lower temperature side on increasing the frequency. Addition of 2.91 wt% graphite shifted the tan δ peaks towards higher temperature side by creating hindrances to the rotation of polymer dipoles. Addition of 2–91 wt% graphite leads to an increased relaxation time τ of dipoles in polysulphide epoxy from 1.44 × 10⁻⁵− 3.92 × 10⁻⁵ (s) at 90°C by creating the hindrance to the rotation of dipoles.     

Impedance and modulus spectroscopic studies on (CuI)100-x-(Ag2SO4)x (0≤x≤60) mixed system

The frequency-dependent conductivity, σω measurements on (CuI)100-x-(Ag2SO4)x (0≤x≤60) mixed system in the frequency range 1 Hz–65.5 kHz and over the temperature range 293–403 K have been carried out. These studies have illustrated similarities in the behaviour of the present system and the other fast ionic solid systems which are generally found to obey the Jonscher's universal power law, σa.c.(ω)=σ0+Aωn, where σa.c.(ω) is the conductivity at frequency ω, σ0 is the limiting zero frequency conductivity or d.c. conductivity and A and n are fitting parameters. The value of n decreases with increasing temperature and A and σ0 increase with temperature (n is a temperature-dependent frequency exponent, A is a frequency-independent and temperature-dependent parameter). These results appear to suggest a mechanism of fast ion conduction due to the presence of well-defined pathways. The strong low-frequency dispersion observed in the case of high conductivity compositions is attributed to the electrode polarization effects. The observed impedance and modulus spectra in correlation with the Arrhenius plots obtained at different frequencies have clearly indicated the frequency dispersion of conduction due to many-body effects and the formation of a large capacitance associated with the electrodes. Thus, the present analysis has suggested a non-Debye type of relaxation process arising due to many-body effects and a distribution of relaxation times, which is a temperature-independent phenomenon exhibited by the heterogeneous electrical structure of the mixed system. This revised version was published online in November 2006 with corrections to the Cover Date.     

Mechanical and electrical behaviour of “Magnex DC” conductive polymer composite

Mechanical and electrical behaviour of the “Magnex DC” conductive polymer composite is reported. X-ray diffraction patterns and scanning electron microscopy (SEM) show that the composite has a semicrystalline, isotropic, and defected structure. It was found that both the elastic modulus and yield stress increase with decreasing temperature; the yield stress increases nonlinearly with increasing strain rate due to the voids in the composite. In addition, the activation energy and the activation volume of a single rate-activated process observed at relatively high strain rates are determined. The dependence of the impedance behaviour on frequency is investigated. It was observed that the a.c. conductivity is almost independent of frequency below 10 kHz and increases with frequency above this range. The values obtained for the conductivity indicate that the composite has a relatively small electrical conduction which is rather less than that for the pure semiconductors.     

Evaluation of a.c. conductivity of rubber ferrite composites from dielectric measurements

The effect of frequency, composition and temperature on the a.c. electrical conductivity were studied for the ceramic, Ni_1−xZn_xFe₂O₄, as well as the filler (Ni_1−xZn_xFe₂O₄) incorporated rubber ferrite composites (RFCs). Ni_1−xZn_xFe₂O₄ (where) (bix)varies from 0 to 1 in steps of 0.2 were prepared by usual ceramic techniques. They were then incorporated into a butyl rubber matrix according to a specific recipe. The a.c. electrical conductivity (σ_a.c) calculations were carried out by using the data available from dielectric measurements and by employing a simple relationship. The a.c. conductivity values were found to be of the order of 10⁻³ S/m. Analysis of the results shows that σ_a.c. increases with increase of frequency and the change is same for both ceramic Ni_1−xZn_xFe₂O₄ and RFCs. σ_a.c increases initially with the increase of zinc content and then decreases with increase of zinc. Same behaviour is observed for RFCs too. The dependence of σ_a.c on the volume fraction of the magnetic filler was also studied and it was found that the a.c. conductivity of RFCs increases with increase of volume fraction of the magnetic filler. Temperature dependence of conductivity was studied for both ceramic and rubber ferrite composites. Conductivity shows a linear dependence with temperature in the case of ceramic samples.     

Dielectric studies of tin based chalcogenide glasses

The dc conductivity and dielectric parameters of glassy system of a-Se₇₀Te_30−x Sn _x (x = 0, 2, 4, 6, 8 and 10) glasses have been investigated. Frequency and temperature dependence of dielectric constants (ε′) and dielectric loss (ε′′) are studied in the frequency range 120–100 kHz and temperature range 300–390 K. Dielectric dispersion is observed when Tin (Sn) is incorporated to a-Se–Te system in the entire temperature range. These results explain that the dc conduction loss is dominated in the present system. From dc conductivity studies it is observed that the dc conductivity and activation energy increases with increasing tin concentration in the present system.     

AC electric conductivity of MnGa2S4 single crystals

The frequency spectra and temperature dependences of the ac electric conductivity of MnGa₂S₄ single crystals have been studied. It is established that the conductivity increases with the frequency and exhibits a resonance at 435–500 kHz. The activation energies of current carriers are determined. The activation energy exhibits a frequency dependence, which is related to a decrease in the relaxation time of blocking barrier layers with increasing frequency.     

The temperature dependences of permittivity and a.c. conductivity in Ca-Al-Si oxynitride glasses

The temperature dependence of the permittivity and conductivity of a Ca-Al-Si oxynitride glass has been measured between 200 and 500° C at frequencies covering the range 0.5 to 10 kHz. A d.c. component of the conductivity has been extracted which has an activation energy of 1.6 ± 0.2 eV for the associated charge carriers, and the remaining dielectric conductivity is shown to vary with frequency in a manner consistent with the presence of loss peaks, the loss-peak frequency having a thermal activation of 0.9 ± 0.1 eV. Below and above the resonant loss frequencies, the dielectric loss shows power-law behaviour. Comparisons are presented with other results on oxynitride and normal oxide glass and with silicon nitride.     

Phase transition and dielectric study in Ba<Subscript>0·95</Subscript>Dy<Subscript>0·05</Subscript>TiO<Subscript>3</Subscript> ceramic

Temperature and frequency dependence dielectric permittivity of Ba0·95Dy0·05TiO₃ ceramic has been studied in the temperature range of 100–350 K at the frequencies, 1 kHz, 10 kHz, 100 kHz and 1 mHz. Diffuse phase transition and frequency dispersion is observed in the permittivity-vs-temperature plots. This has been attributed to the occurrence of relaxor ferroelectric behaviour. The observed relaxor behaviour has been quantitatively characterized based on phenomenological parameters. A comparison with the Zr doped BaTiO₃ has also been presented. The microstructure of as-sintered samples shows a dense and almost uniform micrograph without any impurity phases; the grains are almost spherical with random orientation.     

Dielectric and thermal studies on gel grown strontium tartrate pentahydrate crystals

Results of dielectric and thermal studies on strontium tartrate pentahydrate crystals are described. The value of dielectric constant is shown to be independent of temperature till 360 K at all the frequencies (110–700 kHz) of the applied a.c. field. It increases abruptly achieving a peak value of 25.5 at 100 kHz; the peak value being strongly dependent on frequency. In the temperature range, 87 < T < 117°C, the value of ɛ′ falls suggesting a transition at around 100°C or so. The dielectric constant, ɛ′, of the material is shown to be frequency dependent but temperature independent in the pre- or post-T _c range 87 < T < 117°C, suggesting that the contribution towards polarization may be due to ionic or space charge polarization which gets eliminated at higher frequencies. The ferroelectric transition is supported by the results of thermoanalytical studies. It is explained that crystallographic change due to polymorphic phase transition may be occurring in the material, besides the change due to loss of water molecules, which leads to the dielectric anomaly at around 100°C. Coats-Redfern approximation method is applied for obtaining non-isothermal kinetic parameters leading to calculation of activation energies corresponding to three decomposition stages of material in the temperature ranging from 379–1113 K.     

Study of ac electrical conductivity and dielectric properties of ortho-hydroxy acetophenone azine thin films

AC electrical conductivity measurements were carried out in the temperature range 290–473 K over the frequency range 0.1–20 kHz of vacuum deposited Ortho-hydroxy acetophenone azine films. It was found that the ac electrical conductivity increases with frequency according to the relation σ_ac(ω) = A ω^s. The values of the dielectric constant, , slightly changed in higher frequencies irrespective of temperature change, whereas its value increases in higher temperature with the decrease in frequency. Also, the dielectric loss, ɛ′′, and tan δ, has been found to increase with raise in temperature and decrease in frequency. The obtained experimental data has been analyzed with reference to various theoretical models. The analysis shows that the correlated barrier hopping (CBH) model is the most appropriate mechanism for the ac electrical conduction in these films.     

On structural,optical and dielectric properties of zinc aluminate nanoparticles

Zinc aluminate nanoparticles with average particle size of 40 nm were synthesized using a sol–gel combustion method. X-ray diffractometry result was analysed by Rietveld refinement method to establish the phase purity of the material. Different stages of phase formation of the material during the synthesis were investigated using differential scanning calorimetry and differential thermogravimetric analysis. Particle size was determined with transmission electron microscopy and the optical bandgap of the nanoparticles was determined by absorption spectroscopy in the ultraviolet-visible range. Dielectric permittivity and a.c. conductivity of the material were measured for frequencies from 100 kHz to 8 MHz in the temperature range of 30–120°C. The presence of Maxwell–Wagner type interfacial polarization was found to exist in the material and hopping of electron by means of quantum mechanical tunneling is attributed as the reason for the observed a.c. conductivity.     

A.c. electrical properties of Au/CIAIPc/Au device

The a.c. electrical properties of chloro aluminum phthalocyanine (CIAIPc) thin film sandwiched between two gold (Au) electrodes have been studied in the frequency range 10 Hz to 100 kHz and the temperature range 293 K to 400 K. The a.c. conductivity Σ(Ω) was found to vary as Ω ^s , with the index s < 1, indicating that a hopping process of conduction is dominant at low temperatures and higher frequencies. At higher temperatures and lower frequencies a free carrier conduction mechanism is likely to dominate. At low temperatures the capacitance of the Au-CIAIPc-Au device is temperature-insensitive and increases rapidly above 300 K. The capacitance and loss tangent decreased with frequency and increased with temperature. Such characteristics were found to be in good agreement with the devices Au/CuPc/Au and Au/PbPc/Au.     

QCM Measurements of Superfluid Response in 4He Films up to 180 MHz

At finite frequencies, a dynamic Kosterlitz–Thouless (KT) theory predicts a frequency dependence of the superfluid transition in ⁴He films on planar surfaces. We report results of QCM measurements to study the superfluid response on planar gold surfaces for very high frequencies up to 180 MHz in the temperature range of 0.6–1.0 K. As the frequency is increased, we observed the expected KT behavior that the superfluid transition shifts to a higher temperature from the static transition temperature T _KT and the transition temperature region broadens. The frequency dependence of the dissipation peak temperature at the transition agrees with a simple equation of the frequency dependence based on the dynamic KT theory. The microscopic parameter for the dynamic transition, the ratio of the diffusion constant to the square of the vortex core radius D/r ₀ ², is estimated to be on the order of 10¹⁰ s⁻¹.     

Dielectric behaviour and a.c. conductivity in Cu x Fe3−x O4 ferrite

The dielectric properties (dielectric constant and loss) for the system Cu _x Fe_3−x O₄ with x = 1.0, 0.8, 0.6, 0.4 and 0.2, were studied in the temperature range 300 ∼ 800 K and also in the frequency range 1 kHz ∼ 1 MHz. A.c. conductivity was derived from dielectric constant and loss tangent data. The conduction in this system is interpreted as due to small polaron hopping. The dielectric relaxation was observed for the compositions with tetragonal structure whereas normal behaviour was observed for cubic structure.     

Investigations on d.c. conductivity behaviour of milled carbon fibre reinforced epoxy graded composites

This paper reports the d.c. conductivity behaviour of milled carbon fibre reinforced polysulphide modified epoxy gradient composites. Milled carbon fibre reinforced composites having 3 vol. % of milled carbon fibre and poly sulphide modified epoxy resin have been developed. D.C. conductivity measurements are conducted on the graded composites by using an Electrometer in the temperature range from 26°C to 150°C. D.C. conductivity increases with the increase of distance in the direction of centrifugal force, which shows the formation of graded structure with the composites. D.C. conductivity increases on increase of milled carbon fibre content from 0·45 to 1·66 vol.%. At 50°C, d.c. conductivity values were 1·85 × 10⁻¹¹, 1·08 × 10⁻¹¹ and 2·16 × 10⁻¹² for samples 1, 2 and 3, respectively. The activation energy values for different composite samples 1, 2 and 3 are 0·489, 0·565 and 0·654 eV, respectively which shows decrease in activation energy with increase of fibre content.     

Dielectric properties of Na<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>NbO<Subscript>3</Subscript> in orthorhombic phase

Pellets of ceramic Na_1−xK_xNbO₃ (x = 0, 0.2 and 0.5), were prepared by conventional solid-state reaction method. Prepared samples were characterized using XRD and SEM. The frequency and temperature variation of dielectric constant, loss tangent and dielectric conductivity of prepared samples were measured in the frequency range from 10 KHz-1 MHz, and in the temperature range from 50–250°C for x = 0.2 and 0.5, and between 50 and 480°C for x = 0 compositions. It was observed that the dielectric constant and loss tangent decrease, and conductivity increases with increasing frequency. Near the transition temperature the material shows anomalous behaviour for the observed properties, and the peaks of dielectric constant and loss tangent were observed shifting towards lower temperature with increasing frequency.     

Anomalous d.c. conductance in Nd+3-doped ferroelectric lead germanate single crystal

Pure lead germanate (LG), 0·1 wt% Nd⁺³-doped lead germanate (LG.1Nd⁺³), 0·2wt% Nd⁺³-doped lead germanate (LG.2Nd⁺³) were grown by controlled cooling of melt. Variation of d.c. conductivity was studied in the temperature range 293°K–523°K. Conductivity increases with increasing temperature, showing semiconducting behaviour for all the compositions under study. The activation energy in ferroelectric phase is larger than in paraelectric phase for all the compositions. The result suggests polaron hopping to be a significant contributor to conduction mechanism.     

Studies on ac response of zinc oxide pellets

The ac responses of the ZnO pellets have been studied by ac measurements (impedance, capacitance and phase angle) over the temperature range 300–435 K. The ac conductivity of the ZnO pellets is observed to be proportional to ω ^s , where ω is the angular frequency and the exponent s is a temperature- and frequency-dependent parameter. Based on the existing theories of ac conduction, it has been concluded that for low frequency region (20 Hz–2 kHz), the dominant conduction mechanism in the ZnO pellets is multihopping at all temperatures, whereas for high frequency region (500 kHz–2 MHz), the small polaron tunneling model is the dominant mechanism in the pellets. Activation energies for ac conduction processes are estimated to be in the range of 0.028–0.277 eV which are found to vary with the frequency of the ac signal. These results are found to be consistent with the hopping model. The ac capacitance and the dielectric loss tangent are found to be dependent on both frequency and temperature. Such dependences have been explained taking into account the equivalent circuit model comprising a frequency-independent capacitive element in parallel with a temperature-dependent resistive element, both in series with a low value resistance. Impedance spectroscopy studies show single semicircular arcs in the complex impedance spectra at all temperatures in the range 300–435 K, with their centres lying below the real axis at a particular angle of depression indicating a multirelaxation behaviour in the pellets.     

Effect of Ag on the electrical properties of a-Ge<Subscript>20</Subscript>Se<Subscript>80</Subscript> glasses

This article reports the effect of Ag impurity (low ~4 and ~6 at.% and high ~10 at.%) on the ac conductivity (σ _ac) of a-Ge₂₀Se₈₀ glass. Frequency (ω)-dependent ac conductance and capacitance of the samples over a frequency range ~100 Hz to 50 kHz was taken in the temperature range ~268–358 K. At frequency 2 kHz and temperature 298 K, the value of σ _ac increases at low as well as at higher concentration of Ag. The σ _ac is proportional to ω ^s for undoped and doped samples. The value of frequency exponent (s) decreases as the temperature increases.     

Conductivity and dielectric studies on (Na0·4Ag0·6)2PbP2O7 compound

Electrical properties of the (Na_0·4Ag_0·6)₂PbP₂O₇ compound were studied using complex impedance spectroscopy in the frequency range 200 Hz–5 MHz and temperature range (484–593 K). Combined impedance and modulus plots were used to analyse the sample behaviour as a function of frequency at different temperatures. Temperature dependence of d.c. and a.c. conductivity indicates that electrical conduction in the material is a thermally activated process. The frequency dependence of the a.c. conduction activation energy was found to obey a mathematical formula.