A.C. Conductivity and Dielectric Properties of Polycarbonate Sheet

ABSTRACT

The a.c. conductivity σ_a.c(ω), the dielectric constant ?′ and dielectric loss ?″ of polycarbonate sheet have been studied in the frequency range 1.5 × 10²–5 × 10⁶ Hz and temperature range 294–473 K. A.c. conductivity σ_a.c(ω) results show that it depends strongly on the frequency and slightly on the temperature, with activation energy between 0.0003 and 0.085 e.V. The a.c. conductivity σ_a.c(ω) as a function of frequency is well described by a power law Aω^S where S is the frequency exponent. The obtained values of S < 1 in the first region over the frequency range 1.5 × 10²–4 × 10³ Hz, the temperature dependence of both a.c. conductivity σ_a.c(ω) and the parameter S are reasonably well interpreted by the correlated barrier hopping (CBH) model. The second region, where S > 1 over the frequency range 4 × 10³–5 × 10⁶ Hz, has a strong dependence on the frequency but near independence of the temperature, suggesting that the conductivity may be interpreted by the Maxwell–Wagner (M–W) dispersion. In the first region, the optical band gap of the material, E_g, and the maximum barrier height, W_m, are calculated, suggesting that there is agreement with that proposed by the theory of hopping of charge carriers over potential barrier as suggested. Spatial inhomogeneity may affect the conduction mechanisms in polycarbonate. The experimental values of dielectric constant, ?′ and dielectric loss, ?″, show their dependence on temperature and frequency.     

Electrical and structural properties of polyaniline/cellulose triacetate blend films

Polyaniline (PANI) has been blended with cellulose triacetate (CTA) to obtain both good mechanical properties and good electrical properties. The effects of PANI weight percentage on the optical, structural, morphological and electrical properties in the blend films of polyaniline and cellulose triacetate (PANI/CTA) have been investigated. The phenomenon of percolation was observed in these blend films. It is found that the electrical conductivity of the blend films increases with the increase of polyaniline content up to a value of 10⁻⁴ S cm⁻¹ at 84 weight percentage of PANI. The experimental percolation threshold of the dried blend films is obtained at 9.5 wt% of polyaniline. The values of Mott’s temperature, density of states at the Fermi level [N (E _f)], hopping distance (R _hop), and barrier height (W _hop) for PANI/CTA blend films are calculated. By applying Mott’s theory, it is found that the PANI/CTA blend films obey the three dimensional variable range hopping mechanism.     

Temperature dependence of the dynamic electrical properties of Cu1+xMn1-xO2 (x = 0 and 0.06) crednerite materials

Polycrystalline samples of Cu_1+xMn_1-xO₂ (x?=?0 and 0.06) have been obtained by solid state reaction in silica tubes. Measurements of complex impedance (Z?=?Z′?+?iZ′′) at various temperatures T, between 30 °C and 120 °C and over the frequency range 100?Hz–2?MHz were performed. The frequency dependence of Z′′(f) exhibits a maximum which moves towards higher frequencies by increasing the temperature, proving thus the hopping of the charge carriers between the localized states is the dominant mechanism for the electrical conduction in the investigated samples. The barrier energy values were: 0.287?eV for CuMnO₂ and 0.208?eV for Cu_1.06Mn_0.94O₂.The conductivity spectrum, σ(f) follows the Jonscher universal law at each constant temperature. Based on the temperature and frequency dependencies of the electrical conductivity and using the variable-range-hopping (VRH) model, the frequency and temperature dependencies of the density of localized states near the Fermi level, N(E_F), the hopping distance, R and the hopping energy, W were computed. The results show that at constant frequency, N(E_F) does not depend on temperature for both samples. At constant temperature and frequencies up to 30?kHz, increasing the concentration of Cu ions leads to the decrease of R and W, whilst at high frequencies (over 100?kHz), R and W increase with the increase in the concentration of Cu ions.     

Electrical conductivity of Ca-substituted lanthanum manganites

The physico-chemical properties of substituted perovskites materials have been analyzed with the aim of studying the relationships between structure and properties in this class of materials. Investigations were carried out into the effect of substitution in lanthanum manganites La_1–xCa_xMnO₃ (x = 0; 0.05; 0.1; 0.15) materials obtained through sol-gel method followed by heat treatment at low temperatures and X-ray diffraction, into the surface area thereof, as well as into transmission electron microscopy for purposes of morpho-structural characterization. The results indicated a well-crystallized Pm-3m perovskite-type structure, and 20 nm average crystallite sizes for all samples. By means of complex impedance measurements in the 20 Hz–2 MHz frequency range, the electrical conductivity was determined at temperatures between 30 °C and 120 °C; the results showed that the conductivity obeys a Jonscher's universal law. It was found that below 10 kHz, the dc component of the conductivity increases with temperature for all samples, indicating that electrical conduction processes are activated thermally, in agreement with Mott's variable-range hopping (VRH) model; the model parameters (hopping distance and hopping energy) were also determined. In the high frequency range (f > 200 kHz) the ac conductivity is attributed to charge carriers hopping between the nearest neighboring states, in agreement with the correlated barrier hopping (CBH) model. Using this model, the energy band gap values: 0.364 eV, 0.372 eV and 0.424 eV of the substituted samples were found.     

Magnetoresistive conductive polymer-tungsten trioxide nanocomposites with ultrahigh sensitivity at low magnetic field

The effect of conductive polymer matrix including polyaniline (PANI) and polypyrrole (PPy) on the magnetoresistance (MR) behaviors in the variable range hopping (VRH) regime has been investigated in the disordered polymer nanocomposites containing tungsten trioxide (WO₃) nanoparticles. These nanocomposites have demonstrated ultrahigh MR sensitivity at low magnetic field regime. The observed positive MR has been well explained by the wave-function shrinkage model. The conductive polymer matrix has shown different effects on the MR behaviors of the nanocomposites. The WO₃/PANI nanocomposites have a lower localization length (a₀) and density of states at the Fermi level (N(E_F)), and higher average hopping distance (R_hop) and average hopping energy (W) compared with those of the WO₃/PPy nanocomposites.     

Hopping Mechanism of Charge Transfer in the Thin Layers of a Ge<Subscript>28.5</Subscript>Рb<Subscript>15</Subscript>S<Subscript>56.5</Subscript> Vitreous System

The results of the study of the charge transfer processes in the thin layers of a Ge_28.5Рb₁₅S_56.5 vitreous system are presented. The power-law dependence of the conductivity on frequency and a decrease in the value of the exponent s with increasing temperature are found. The charge transfer is a thermally activated process and two areas on the temperature dependence of conductivity are present. The activation energies of these areas are Е₁ = 0.20 ± 0.01 and Е₂ = 0.50 ± 0.01 eV, respectively. The obtained results are explained within the scope of the correlated barrier hopping (CBH) model of the hopping conductivity in disordered systems. The main microparameters of the system, namely, the density of the localized state (N), hopping length (R_ω), and the maximal value of the height of the potential barrier (W_M), are calculated.     

Thermal and dc Conductivity Behavior of Fly Ash Filled Polyaniline Composites

In this paper temperature dependence of dc conductivity (σ_dc) of emeraldine base form of polyaniline (PANI) and fly ash filled PANI are presented. Samples were prepared by in situ polymerization of aniline using ammonium persulphate as an oxidant and hydrochloric acid as dopant. Fly ash filled PANI composites were prepared by adding 3 gms of fly ash. Thermal characteristics of samples were measured using differential scanning calorimetry. The dc conductivity (σ_dc) of fly ash filled PANI was found to be on the order of 1.63 × 10^?11 s/cm at room temperature, which was lower than that of pure PANI. The activation energies calculated from σ_dc for PANI and the PANI 3 fly ash system were 1.35 and 1.16 eV, respectively. It was found that addition of fly ash to PANI drastically decreased the enthalpy from 2259.2 to 196.6 mJ. the endothermic peak due to the glass transition temperature shifted from 99.8 to 94.6°C. This was attributed to the change in the morphology of the composites on adding fly ash, as observed in the scanning electron micrographs.     

Study of polaron transport mechanisms in two transition metal ions doped borophosphate glasses

Borophosphate glasses in the compositions, (B₂O₃)_0.2 · (P₂O₅)_0.3 · (V₂O₅) _x · (CoO)_0.5x , where x = 0.05, 0.1, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50 were synthesized at 1500 K by standard melt quenching method. Non-crystalline nature of the samples was confirmed by XRD studies. Room temperature density and dc electrical conductivity in the temperature range from 350 K to 625 K have been measured. Density decreased up to about 0.25 mole fractions of V₂O₅ and increased thereafter. Conductivity was almost constant up to 0.25 mole fractions of V₂O₅ and increased for higher amount of V₂O₅. Temperature variation of conductivity data has been analyzed using Mott’s small polaron hopping (SPH) model and, activation energy and Debye’s temperature were determined. Activation energy decreased with increase of V₂O₅ content. The data deviated from the Mott’s SPH model has been analyzed in view of variable range hopping models of Mott and Greaves and the density of states at Fermi level was determined. It is for the first time that borophosphate glasses doped with V₂O₅ and CoO were synthesized to study conductivity variation with temperature. The data analyzed using small polaron hopping and variable range hopping models.     

Effect of local structure of MnCl2-filled PVDF films on their optical,electrical, electron spin resonance,and magnetic properties

The filling level (W) dependence of the local structure of MnCl₂ through the PVDF matrix was explored. The presence of α- and β-PVDF crystalline phases were detected by X-ray diffraction, and it was confirmed by the infrared (IR) absorption spectra. A significant head-to-head content was implied by IR spectroscopy. Two filling, level-dependent optical energy gaps were found through the UV–VIS spectral investigation. An intrachain one-dimensional interpolaron hopping mechanism was assumed to proceed in the temperature range of 350–375 K. The calculated values of the charge carrier hopping distance were in the range of 6.5–9.7 nm. The temperature dependence of the direct current (dc) magnetic susceptibility exhibited a Curie–Weiss behavior. Positive values of the paramagnetic Curie temperature (θ_p) for W up to 14.5% indicated the presence of a ferromagnetic interaction, while negative θ_p obtained for higher W values suggested an antiferromagnetic interaction at lower temperature. The electron spin resonance (ESR) analysis revealed the existance of both isolated and aggregated Mn²⁺ ions within the PVDF matrix. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1437–1445, 1998     

Dielectric and conductivity studies on cobalt phthalocyanine tetramers

Electrically conductive organic and metalloorganic polymers are of great interest and they have applications in electronic, optical, photonic, photoelectric, electrochemical, and dielectric devices. Tetrameric cobalt phthalocyanine was prepared by conventional chemical method. The dielectric permittivity of the tetrameric cobalt phthalocyanine sample was evaluated from the observed capacitance values in the frequency range 100 KHz to 5 MHz and in the temperature range of 300 to 383°K. It is found that the system obeys the Maxwell Wagner relaxation of space charge phenomenon. Further, from the permittivity studies AC conductivity was evaluated. The values of AC conductivity and DC conductivity were compared. Activation energy was calculated. To understand the conduction mechanism Mott's variable range hopping model was applied to the system. The T^?1/4 behavior of the DC conductivity along with the values of Mott's Temperature (T₀), density of states at the Fermi energy N (E_F), and range of hopping R and hopping energy W indicate that the transport of charge carriers are by three‐dimensional variable range hopping. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2529–2535, 2004     

Dielectric relaxation and conduction mechanism of complex perovskite Ca0.90Sr0.10Cu3Ti3.95Zn0.05O12 ceramic

In this work, Sr and Zn co-doped calcium copper titanate having nominal formula Ca_0.90Sr_0.10Cu₃Ti_3.95Zn_0.05O₁₂ has been synthesized and systematically studied for structure, dielectric and electrical properties. The phase purity of the prepared composition has been confirmed by X-ray diffraction (XRD) pattern and various structural parameters are presented here with Rietveld refinement data. The Scanning electron microscopy (SEM) image revealed the presence of homogeneous fine-grained microstructure investigated in the prepared ceramic. The impedance data well fitted with 2 R-CPE model confirmed the non-ideality of the system. Dielectric studies have shown the existence of two relaxation processes in low and high frequency domains. Low frequency dielectric response proposed as a combination of Maxwell-Wagner relaxation and intrinsic defect (V_o⁺⁺) while high frequency behavior has been interpreted in terms of polaronic relaxation caused by hopping process of an electron between Ti³⁺ and Ti⁴⁺ states. The frequency dependent behavior of conductivity has been well fitted to Dong’s model and interpreted in term of Overlapping Large Polaron Tunneling (OLPT) mechanism. The various parameters; relaxation angular frequency (ω₁) of Debye process, ionic hopping frequency (ω₂₎, dc conductivity (σ_o), cole- cole parameter (α) and frequency exponent (s) have been determined by Dong’s Model fitting.     

Electric and dielectric properties of some luminescent solar collectors based on phthalocyanines and hematoporphyrin doped PMMA

The electrical properties: DC-electrical conductivity (σ_DC), AC-electrical conductivity (σ_AC), dielectric permittivity (ɛ′) and dielectric loss (ɛ″) for pure polymethylmethacrylate (PMMA) and mixed with phthalocyanine, Ni-phthalocyanine and hematoporphyrin IX chloride laser dyes have been investigated. The conductivity values for all polymer–dye samples are higher than that of the pure PMMA. For all investigated samples, at the constant temperature, the frequency dependence of (σ_AC) was found to obey the relation (σ_AC) = Aω^s. The results are discussed according to the correlated barrier hopping CBH mechanism from which the Coulomb well barrier of charge carriers (W_M) was calculated. For the dye-doped samples, the hematoporphyrin IX chloride mixed sample showed the larger value of (W_M) compared with the others. At all frequencies, ɛ′ values for doped samples are higher than that of pure one. This is due to a free volume enhancement offered by the large size of the dye molecule. Also, the addition of dyes to PMMA causes an increase in the intensity of the loss peak (ɛ″) accompanied by a shift in its position. The effect of chemical structure of investigated dyes on the electrical properties is discussed.     

Preparation,Infra-red,MAS-NMR and Structural Characterization of a New Copper Based Inorganic–Organic Hybrid Compound: [C<Subscript>5</Subscript>H<Subscript>6</Subscript>N<Subscript>2</Subscript>Cl]<Subscript>2</Subscript>CuCl<Subscript>4</Subscript>

The ionic salt [2(C₅H₆N₂Cl)⁺], [CuCl₄]^2? complex of copper(II) has been synthesized and characterized. The X-ray diffraction analysis with a single crystal of this compound showed that the title compound (4-amino-2-chloropyridinium)₂CuCl₄ [(CAP)₂CuCl₄], crystallized at room temperature in the monoclinic system, space group C_2/c (N°.15) and the following : a = 16.0064 (2) Å; b = 7.7964 (10) Å; c = 14.7240 (2) Å; β = 102.497 (10)°; V = 1793.91 (4) ų and Z = 4. The structure was solved by using 1,589 independent reflections down to R value of 0.021. The unit cell is made up of tetrachlorocuprate(II) anions and 4-amino-2-chloropyridinium cations linked together by an extensive hydrogen bond network of types N–H···Cl (N: pyridinium) and N–H···Cl (N: amine), and cation-lone pair of nitrogen element interactions. Solid state NMR spectra showed one and five isotropic resonances, ⁶³Cu and ¹³C, respectively, confirming the solid state structure determined by X-ray diffraction. Impedance spectroscopy study, reported for single crystal, revealed that the conduction in the material was due to a hopping process. This work aims to reveal the thermal properties of a new copper(II) based organic–inorganic hybrid and the conductivity properties that these compounds exhibit.     

Controlling the dielectric and optical properties of PVA/PEG polymer blend via e-beam irradiation

Polyvinyl alcohol (PVA)/polyethylene glycol (PEG) blend films were prepared by the solution casting method. The films were irradiated using 1.5 MeV of the electron beam at varying doses over the range 0–70 kGy to investigate the modifications induced in the dielectric and optical properties. The dielectric constant (ε′) and dielectric loss (ε″) were measured in the temperature range 308–408 K and in the frequency range 30 kHz - 3 MHz. According to the frequency and temperature dependence of ε′; an α- relaxation peaks were observed in all samples and assigned to the micro-Brownian motion of the polymer blend chains. Both ε′ and ε″ showed a decrease with 5 and 10 kGy doses and an increase in the dose rang 10–70 kGy. The temperature dependence of the ac conductivity (σ _ac (T)) shows an Arrhenius type behavior separated into three distinct regions. The frequency dependence of the ac conductivity (σ _ac (f)) indicates that the correlated barrier hopping (CBH) is the most suitable mechanism for conduction. Also, the influence of e-beam irradiation on the absorption coefficient (α), the indirect optical band gap (E _g ) and the refractive index (n) of PVA/PEG copolymer films were investigated. The results of the present system are compared with those of similar materials.     

Characterization of coals by the study of complex permittivity

In a study aimed at quantitative characterization of coals, the complex permittivity of non-demineralized coals was measured at variable frequency (5–10⁷ Hz) and temperature (220–380 K). The results show that the polarization conductivity follows a law of the type σ′(ω) = Aω^s. This shows that there is a hopping conductivity process between localized sites. These sites could be basic structural units (BSU) forming organized microdomains. This work shows that the parameters calculated from the experimental results differ according to the origin of the coal.     

A potential electrolyte (Ce1-x CaxO2-δ) for fuel cells:Theoretical and experimental study

First-principles calculations are performed using density function theory to explore the effects of dopant Ca in ceria (Ce_1-xCa_xO_2-δ). The impact of oxygen vacancy on band gap and density of states is examined in doped ceria using generalized gradient approximations. Vacancy association and vacancy formation energies of the doped ceria are calculated to reveal the effect of dopant on ion conduction. The experimental study of the sample (Ce_0.875Ca_0.125O_2-δ) was performed to compare with the theoretical results. The obtained results from theoretical calculation and experimental techniques show that oxygen vacancy increases the volume, lattice constant (5.47315?Å) but decrease the band gap (1.72?eV) and bulk modulus. The dopant radius (1.173?Å) and lattice constant (5.4718?Å) are also calculated by equations which is close to the DFT lattice parameter. The result shows that oxygen vacancy shifts the density of states to lower energy region. Band gap is decreased due to shifting of valence states to conduction band. Vacancy formation shows a significance increase in density of states near the Fermi level. Density of states at Fermi level is proportional to the conductivity, so an increase in density of states near the Fermi level increases the conductivity. The experimental measured ionic conductivity is found to 0.095?S?cm^?1 at 600?°C. The microstructural studies is also reported in this work.     

Electrical properties of diphenyldiacetylene annealed under elevated pressure

A unique class of conjugated compounds composed of the derivative of condensed polycyclic aromatic compound with the phenyl group and diphenyldiacetylene oligomer was synthesized by annealing of diphenyldiacetylene under elevated pressure. The effect of annealing pressure on the conductivity of the compounds was studied. The total conductivity of the compound decreased with a decrease of frequency, approaching a constant value (dc conductivity: C_dc). The dc conductivity of the compound increased from below 10^?15 to 10 S cm^?1 with increasing annealing pressure. The dc conductivity of the oligomer was below 10^?15 S cm^?1 and that of the derivative increased from 10^?8 to 10 S cm^?1 with decreasing H/C (H/C:0.45–0.04). The conduction of the conjugated compound was electronic. The temperature coefficient of those dc conductivities was positive, with an approximately linear relation between In (C_dcT^0.5) and (1/T)^0.25, where T is the temperature. The ac conductivities C_ac were proportional to temperature and frequency f and had the following equation C_ac = Tf^S, S = 0.67–0.75. These results showed that the conduction mechanism can be explained by the hopping in a manifold of states at the Fermi level. © 1994 John Wiley & Sons, Inc.     

Studies on Fourier transform infrared spectroscopy,scanning electron microscope,and direct current conductivity of polyaniline doped zinc ferrite

In situ polymerization of aniline was carried out in the presence of zinc ferrite to synthesize polyaniline/ZnFe₂O₄ composites (PANI/ZnFe₂O₄) by chemical oxidation method. The composites have been synthesized with various compositions (10, 20, 30, 40, and 50 wt %) of zinc ferrite in PANI. From the Fourier transform infrared spectroscopy (FTIR) studies on polyaniline/ZnFe₂O₄ composites, the peak at 1140 cm⁻¹ is considered to be measure of the degree of electron delocalization. The surface morphology of these composites was studied with scanning electron micrograph (SEM). The dc conductivity has been studied in the temperature range from 40–160°C and supports the one‐dimensional variable range hopping (1DVRH) model proposed by Mott. The results obtained for these composites are of scientific and technological interest. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Enhancement of dielectric characteristics in donor doped Aurivillius SrBi2Ta2O9 ferroelectric ceramics

In this study, investigations have been made on the crystal structure, surface morphology, dielectric and electrical properties of tungsten doped SrBi₂(W_xTa_1−x)₂O₉ (0.0 ≤ x ≤ 0.20) ferroelectric ceramics. Dielectric measurements performed as a function of temperature at 1, 10 and 100 kHz show an increase in Curie temperature (T_c) over the composition range of x = 0.05–0.20. W⁶⁺ substitution in perovskite-like units results in a sharp dielectric transition at the ferroelectric Curie temperature with the dielectric constant at their respective Curie temperature increasing with tungsten doping. The dielectric loss reduces significantly with tungsten addition. The temperature dependence of ac and dc conductivity vis-à-vis tungsten content shows a decrease in conductivity, which is attributed to the suppression of oxygen vacancies. The activation energy calculated from the Arrhenius plots is found to increase with tungsten content.     

Frequency scaling of ac hopping transport in amorphous carbon nitride

The dynamics of hopping transport in amorphous carbon nitride is investigated in both Ohmic and non-linear regimes. Dc current and ac admittance were measured in a wide range of temperatures (90 K < T < 300 K), electric fields (F < 2 × 10⁵ V cm^− 1) and frequencies (10² < f < 10⁶ Hz).The dc Ohmic conductivity is described by a Mott law, i.e. a linear ln(σ_OHMIC) vs T^− 1/4 dependence. The scaling of field-enhanced conductivity as ln(σ / σ_OHMIC) = ϕ[F^S / T] with S ≈ 2/3, observed for F > 3 × 10⁴ V cm^− 1 over 5 decades in σ(T,F), is explained by band tail hopping transport; the filling rate, Γ_F(E_DL), of empty states at the transport energy is obtained with a “filling rate” method which incorporates an exponential distribution of localized states, with a non-equilibrium band tail occupation probability f(E) parametrized by an electronic temperature T_EFF (F).As the ac frequency and temperature increase, the increase in conductance G is accurately described by Dyre's model for hopping transport within a random spatial distribution of energy barriers. This model predicts a universal dependence of the complex ac conductivity of the form σ_ac = σ(0)[iωτ / ln(1 + iωτ)], where σ(0) is the zero frequency ac conductivity and τ(T,F) is a characteristic relaxation time. We find that the inverse characteristic time 1 / τ can also be described by a Mott law. It is compatible with the filling rate Γ_F(E_DL) at the transport energy, which governs the dc conductivity; this rate increases with increasing dc field, as more empty states become available in the band tail for hopping transitions. This “universal” scaling law for the ac conductance provides a scaling parameter K(T,F) = τ(T,F) σ(T,F,ω = 0) / ɛ which is found to decrease with increasing electric field from 5 to 0.5, depending weakly on temperature. Our band tail hopping model predicts a high-field value of K(T,F) smaller than the Ohmic value, under the condition (eFγ^− 1 / E°) ≤ (kT / E°)^1/4, where γ^− 1 is the localization radius and E° the disorder energy of the band tail distribution.