Electrical properties and conduction mechanism of [N(C2H5)4][N(CH3)4]CuCl4 compound

The [N(CH₃)₄][N(C₂H₅)₄]CuCl₄ single crystal has been synthetized in order to determinate the temperatures transition and to study the electrical properties and the conduction mechanism. At room temperature, this compound crystallizes in the tetragonal system with P-42₁m space group. The calorimetric study shows three anomalies at 248, 284 and 326 K. Electrical conduction and dielectrical relaxation mechanisms at various frequencies and temperatures were analyzed by impedance spectroscopy and the equivalent circuit based on the Z-View-software was proposed. The variation of f_p relaxation determinate by the modulus study and σ_dc specific to the AC conductivity as a function of temperature and confirm the all transitions for our sample. The values of the activation energy are determined and compared by those, which are found in the similar compound. Frequencies dependence of alternative current (AC) conductivity is interpreted in terms of Jonscher's law and the conduction mechanisms for each phase are determined with the Elliot's theory.     

Suppression of the fractal conductivity channel and superlocalization effects in porous a-Si:H

The temperature dependence of the hopping conductivity and the relaxation kinetics of the transient current in porous amorphous silicon are investigated after treatment in a hydrogen plasma at 200 °C. It is discovered that posthydrogenation of the material increases the dimension of the conducting channel from 2.5 to 3, while suppressing and slowing the relaxation of the transient current. The results obtained are attributed to passivation of the electrically active dangling bonds on the pore surface by hydrogen. It is concluded that electron transport in porous amorphous silicon in the temperature range T>T*, where T* lies in the range 130–270 K and depends on the density of states, takes place between superlocalized states of the internal surface, which is enriched with dangling bonds and acts as a fractal percolation system. When the temperature is lowered below T*, a transition to one-dimensional hopping conduction in the bulk silicon regions occurs. Zh. éksp. Teor. Fiz. 112, 926–935 (September 1997)     

A Fractal Model for the Effective Thermal Conductivity of Granular Flow with Non—uniform Particles

The equipartition of energy applied in binary mixture of granular flow is extended to granular flow with non-uniform particles.Based on the fractal characteristic of granular flow with non-uniform particles as well as energy equipartition,a fractal velocity distribution function and a fractal model of effective thermal conductivity are derived.Thermal conduction resulted from motions of particles in the granular flow,as well as the effect of fractal dimension on effective thermal conductivity,is discussed.     

Relaxation processes of water confined to AlMCM-41 molecular sieves. Influence of the hydroxyl groups of the pore surface

A series of AlMCM-41 molecular sieves was prepared with constant composition (Si/Al = 14.7) and presumably same pore structure but different pore diameters (from 2.3 to 4.6 nm). The pore size distribution is narrow for each sample. The rotational fluctuations of water molecules confined inside the pores were investigated applying broadband dielectric spectroscopy (10⁻²–10⁷ Hz) over a large temperature interval (213–333K). A relaxation process, slower than that expected for bulk water, was observed which is assigned to water molecules forming a surface layer on the pore walls. The estimated relaxation time has an unusual non-monotonic temperature dependence, which is rationalized and modeled assuming two competing processes: rotational fluctuations of constrained water molecules and defect formation (Ryabov model). This paper focuses on the defects and notably the influence of the hydroxyl groups of the pore walls. The Ryabov model is fitted to the data and characteristic parameters are obtained. Their dependence on pore diameter is considered for the first time. The found results are compared with those obtained for other types of molecular sieves and related materials.     

Alternating current characterization of nano-Pt(Ⅱ)octaethylporphyrin(PtOEP) thin film as a new organic semiconductor

Alternating current(AC) conductivity and dielectric properties of thermally evaporated Au/Pt OEP/Au thin films are investigated each as a function of temperature(303 K–473 K) and frequency(50 Hz–5 MHz).The frequency dependence of AC conductivity follows the Jonscher universal dynamic law.The AC-activation energies are determined at different frequencies.It is found that the correlated barrier hopping(CBH) model is the dominant conduction mechanism.The variation of the frequency exponent s with temperature is analyzed in terms of the CBH model.Coulombic barrier height Wm,hopping distance Rω,and the density of localized states N(EF) are valued at different frequencies.Dielectric constant ε_1(ω,T) and dielectric loss ε_2(ω,T) are discussed in terms of the dielectric polarization process.The dielectric modulus shows the non-Debye relaxation in the material.The extracted relaxation time by using the imaginary part of modulus(M')is found to follow the Arrhenius law.     

Dielectric study of polyaniline/poly (methylmethacrylate) composite films below the percolation threshold

We report results of dielectric relaxation studies of polyaniline/poly(methylmethacrylate) composites with polyaniline amount less than the percolation threshold in the frequency range of 0.1 Hz to 1 MHz and temperature range of 10 °C–170 °C. We find a significant dependence of the glass transition temperature Tg on the polyaniline amount in the composite. α and β relaxation processes relative to the PMMA matrix are also affected by the presence of polyaniline inclusion. We identify a relaxation process due to ionic conductivity and another process attributed to residual solvent. The characteristic relaxation frequency of each process and the activation energy depend on the polyaniline amount in the composite. The ac conductivity in the high frequency range is fitted to the universal power law of Jonscher characteristic of disordered materials.     

Surface fractal analysis of pore structure of high-volume fly-ash cement pastes

The surface fractal dimensions of high-volume fly-ash cement pastes are evaluated for their hardening processes on the basis of mercury intrusion porosimetry (MIP) data. Two surface fractal models are retained: Neimark's model with cylindrical pore hypothesis and Zhang's model without pore geometry assumption. From both models, the logarithm plots exhibit the scale-dependent fractal properties and three distinct fractal regions (I, II, III) are identified for the pore structures. For regions I and III, corresponding to the large (capillary) and small (C-S-H inter-granular) pore ranges respectively, the pore structure shows strong fractal property and the fractal dimensions are evaluated as 2.592-2.965 by Neimark's model and 2.487-2.695 by Zhang's model. The fractal dimension of region I increases with w/b ratio and hardening age but decreases with fly-ash content by its physical filling effect; the fractal dimension of region III does not evolve much with these factors. The region II of pore size range, corresponding to small capillary pores, turns out to be a transition region and show no clear fractal properties. The range of this region is much influenced by fly-ash content in the pastes. Finally, the correlation between the obtained fractal dimensions and pore structure evolution is discussed in depth.     

Crystal structure,NMR study,dielectric relaxation and AC conductivity of a new compound [Cd3(SCN)2Br6(C2H9N2)2]n

The crystal structure, the ¹³C NMR spectroscopy and the complex impedance have been carried out on [Cd₃(SCN)₂Br₆(C₂H₉N₂)₂]_n. Crystal structure shows a 2D polymeric network built up of two crystallographically independent cadmium atoms with two different octahedral coordinations. This compound exhibits a phase transition at (T=355±2 K) which has been characterized by differential scanning calorimetry (DSC), X-rays powder diffraction, AC conductivity and dielectric measurements. Examination of ¹³C CP/MAS line shapes shows indirect spin–spin coupling (¹⁴N and ¹³C) with a dipolar coupling constant of 1339 Hz. The AC conductivity of this compound has been carried out in the temperature range 325–376 K and the frequency range from 10⁻² Hz to 10 MHz. The impedance data were well fitted to two equivalent electrical circuits. The results of the modulus study reveal the presence of two distinct relaxation processes. One, at low frequency side, is thermally activated due to the ionic conduction of the crystal and the other, at higher frequency side, gradually disappears when temperature reaches 355 K which is attributed to the localized dipoles in the crystal. Moreover, the temperature dependence of DC-conductivity in both phases follows the Arrhenius law and the frequency dependence of σ(ω,T) follows Jonscher's universal law. The near values of activation energies obtained from the conductivity data and impedance confirm that the transport is through the ion hopping mechanism.     

AC conductivity behaviour and charge carrier concentrations of some vanadate glassy system

The investigation of wide range of temperature and frequency dependent conductivity of some semiconducting glassy system reveals DC conductivity, crossover frequency and frequency exponent. The composition dependence of AC conduction activation energy and the permissible energy of polaron migration have also been computed. The Nernst-Einstein relation proves that the concentration of mobile charge carriers does not undertake a substantial part in electrical conduction.     

Electrical properties and dielectric relaxation of thermally evaporated zinc phthalocyanine thin films

The electrical transport properties and dielectric relaxation of Au/zinc phthalocyanine, ZnPC/Au devices have been investigated. The DC thermal activation energy at temperature region 400-500 K is 0.78 eV. The dominant conduction mechanisms in the device are ohmic conduction below 1 V and space charge limited conduction dominated by exponential trap distribution in potentials >1 V. Some parameters, such as concentration of thermally generated holes in valence band, the trap concentration per unit energy range at the valence band edge, the total concentration of traps and the temperature parameter characterizing the exponential trap distribution and their relation with temperatures have been determined. The AC electrical conductivity, σ_ac, as a function of temperature and frequency has been investigated. It showed a frequency and temperature dependence of AC conductivity for films in the temperature range 300-400 K. The films conductivity in the temperature range 400-435 K increased with increasing temperature and it shows no response for frequency change. The dominant conduction mechanism is the correlated barrier hopping. The temperature and frequency dependence of real and imaginary dielectric constants and loss tangent were investigated.     

Structural,dielectric, electrical and magnetic properties of chemicothermally synthesized material: BiCaFeCeO6

In this communication, detailed studies of structural, micro-structural, dielectric, electrical (impedance, modulus and conductivity) and magneto-electric characteristics of a chemico-thermally synthesized sample of a double perovskite bismuth calcium iron cerate (BiCaFeCeO₆) have been reported. Preliminary structural analysis of room temperature X-ray diffraction data shows orthorhombic structure of the material. The homogeneous distribution of the grains of different dimensions (shape, size, etc) with a small number of voids observed in the scanning electron micrograph suggests the formation of high-density sample. Detailed analysis of dielectric and impedance experimental data, collected at different frequency and temperatures, have provided many important characteristics of the material, such as (a) grains, grain boundaries, and electrode dependent capacitive and impedance parameters, (b) co-relation between the structure, micro-structure and physical properties and (c) the relaxation characteristics of the tested samples. The nature of frequency dependence of AC conductivity of the material obeys the Jonscher's universal power law. The temperature dependence of conductivity provides the conduction mechanism in the material. Detailed studies of field dependence of electric polarization, magnetization and magneto-electric coefficient at room temperature exhibit the multiferroic characteristics of the material.     

Structural and electrical performance of epitaxial InP based heterojunctions prepared by liquid phase epitaxy

Thin films of InP was grown on single crystalline substrates of Si to form InP/Si heterojunctions by liquid phase epitaxy (LPE) and its morphology and crystalline characteristics were achieved. The essential electrical properties and its main parameters were extracted using the current density-voltage. The analysis was done to obtain the rectification characteristics which has its maximum value at a certain voltage of 0.7 V. Moreover, the heterojunction obeys ohmic behavior followed by quadratic space charge limited conduction at lower and higher voltage regions, respectively. The conductivity under AC bias as well as the dielectric behaviors of the heterojunction was explored in the frequency range 100 kHz–5 MHz and in the temperature range 298–623 K. The AC conductivity is interpreted by the correlated barrier hopping model via single polaron with activation energy dependent on the applied frequency. The response of the dielectric constants confirms its remarkable dependence on both frequency and temperature.     

Frequency dependence of ionic conductivity and dielectric relaxation studies in Na3H(SO4)2 single crystal

Electrical impedance measurements of Na₃H(SO₄)₂ were performed as a function of both temperature and frequency. The electrical conductivity and dielectric relaxation have been evaluated. The temperature dependence of electrical conductivity reveals that the sample crystals transformed to the fast ionic state in the high temperature phase. The dynamical disordering of hydrogen and sodium atoms and the orientation of SO₄ tetrahedra results in fast ionic conductivity. In addition to the proton conduction, the possibility of a Na⁺ contribution to the conductivity in the high temperature phase is proposed. The frequency dependence of AC conductivity is proportional to ω^s. The value of the exponent, s, lies between 0.85 and 0.46 in the room temperature phase, whereas it remains almost constant, 0.6, in the high-temperature phase. The dielectric dispersion is examined using the modulus formalism. An Arrhenius-type behavior is observed when the crystal undergoes the structural phase transition.     

Dielectric and AC conductivity behavior of BaBi2Nb2O9 ceramics

The complex dielectric and AC conductivity response of BaBi₂Nb₂O₉ relaxor ferroelectric ceramics were studied as a function of frequency (100 Hz-10 MHz) at various temperatures. The observed dielectric behavior was characterized by two types of relaxation processes which were described by the ‘universal relaxation law’. The frequency dependence of conductivity which showed a classical relaxor behavior followed the Jonscher's universal law σ(ω)=σ₀+Aωⁿ. The exponent n exhibited a minimum in the vicinity of temperatures of dielectric anomaly while the pre-factor A showed a maximum. The temperature dependence of n followed the Vogel-Fulcher relation with activation energy of about 0.14 eV.     

Roles of nonequilibrium conduction electrons on the magnetization dynamics of ferromagnets

The mutual dependence of spin-dependent conduction and magnetization dynamics of ferromagnets provides the key mechanisms in various spin-dependent phenomena. We compute the response of the conduction electron spins in a spatial and time varying magnetization M(r,t) in the time-dependent semiclassical transport theory. We show that the induced nonequilibrium conduction spin density in turn generates four spin torques acting on the magnetization-with each torque playing a different role in magnetization dynamics. By comparing with recent theoretical models, we find that one of these torques which has not been previously identified is crucial to consistently interpreting experimental data on domain wall motion.     

分形多孔介质中的热传导

本文将多孔介质视为由骨架和空隙组成的二元混合介质,研究了多孔介质中的热传导过程,发现分形结构中的导热规律与孔隙的分布有关,存在着与实体导热完全不同的特征。计算表明,分形介质中的导热过程除了与基质(骨架)的分形维数有关外,还与基质率以及反映介质中热量传递动态过程有关。     

Determining the relaxation polarization parameters in dielectrics with high electrical conductivity

In dielectrics with high steady leakage of conductivity, the frequency–temperature dependences of the dielectric loss tangent generally allow us to determine the relaxation time and activation energy of relaxation process only when they are strong. With weak relaxation processes, there are no extrema in the frequency dependence of the dielectric loss tangent. In such cases, the parameters of the relaxation processes are initially determined from the frequency behavior of the imaginary parts of the electrical module or impedance. However, the frequency dependences of these quantities when there is electrical conduction can contain three extrema. Identifying the maxima associated with relaxation polarization therefore requires additional research.     

Dielectric property of CaCu3Ti4O12 thin film grown on Nb-doped SrTiO3(100) single crystal

Thin film of CaCu₃Ti₄O₁₂ (CCTO) has been deposited on Nb-doped SrTiO₃(100) single crystal using pulsed laser deposition. The dielectric constant and AC conductivity of CCTO film in the metal–insulator–metal capacitor configuration over a wide temperature (80 to 500 K) and frequency (100 Hz to 1 MHz) range have been measured. The small dielectric dispersion with frequency observed in the lower temperature region (<300 K) indicates the presence of small defects in the deposited CCTO thin film. The frequency-dependent AC conductivity at lower temperature indicates the hopping conduction. The dielectric dispersion data has been analyzed in the light of both conductivity relaxation and Debye type relaxation with a distribution of relaxation times. Origin of dielectric dispersion is attributed to the distribution of barrier heights such that some charge carriers are confined between long-range potential wells associated with defects and give rise to dipolar polarization, while those carriers which do not encounter long-range potential well give rise to DC conductivity.     

Dielectric relaxation spectroscopy of phlogopite mica

An in-depth investigation of the dielectric characteristics of annealed phlogopite mica has been conducted in the frequency range 0.1 Hz–10 MHz and over the temperature range 653–873 K through the framework of dielectric permittivity, electric modulus and conductivity formalisms. These formalisms show qualitative similarities in relaxation processes. The frequency dependence of the M″ and dc conductivity is found to obey an Arrhenius law and the activation energy of the phlogopite mica calculated both from dc conductivity and the modulus spectrum is similar, indicating that same type of charge carriers are involved in the relaxation phenomena. The electric modulus and conductivity data have been fitted with the Havriliak–Negami function. Scaling of M′, M″, ac conductivity has also been performed in order to obtain insight into the relaxation mechanisms. The scaling behaviour indicates that the relaxation describes the same mechanism at different temperatures. The relaxation mechanism was also examined using the Cole–Cole approach. The study elaborates that the investigation regarding the temperature and frequency dependence of dielectric relaxation in the phlogopite mica will be helpful for various cutting edge applications of this material in electrical engineering.