Electrical Relaxation Research Articles

Unusual resistive states of multiband superconductors in the effective field theory approach

Starting from the microscopic approach based on multiband Keldysh-Usadel kinetic theory we derive the minimal field-theoretical model equivalent to the time-dependent Ginzburg-Landau theory. We discuss the properties of resistive states determined by the ratio of electric field relaxation length to the superconducting coherence length. In contrast to the well-studied single-band systems we find that this ratio can vary in wide limits in multiband superconductors. As a result, the properties of resistive states in multiband superconductors can be tuned by the microscopic parameters such as the ratio of diffusion coefficients and pairing constants in different bands. As an example we consider moving Abrikosov vortices in two-band superconductor and show that the flux-flow magneto-resistance can strongly deviate from the single-component Bardeen-Stephen estimation in agreement with recent experimental results.

Sintering temperature effects on the impendence spectroscopy properties of Nd0.67Pb0.33Mn0.9Al0.1O3 perovskites

ABSTRACTThe impendence spectroscopy properties of polycrystalline Nd0.67Pb0.33Mn0.9Al0.1O3 perovskites prepared by sol–gel method at two different sintering temperatures (900°C and 1100°C), have been investigated by performing the measurements as a function of temperature and frequency of electrical conductance and impedance. In the whole explored temperature range, conductance measurements show that both samples present semiconductor-like behaviors. The non-overlapping small polaron tunneling (NSPT) model dominates the conduction process for the studied samples. An electrical relaxation phenomenon was observed from the variations of imaginary parts of impedance. Electrical parameters deduced from the Nyquist plots analysis using an equivalent circuit show that the conduction process for the samples is due to the grain boundaries contribution.

Improving oxygen incorporation rate on (La0.6Sr0.4)0.98FeO3-δ via Pr2Ni1-xCuxO4+δ surface decoration

Insufficient electrocatalytic activity for oxygen reduction/evolution in lanthanum strontium ferrite (LSF) perovskites at temperatures below 700 °C, limits the utilization of these Co-free oxides as oxygen electrodes for intermediate-temperature solid oxide fuel/electrolysis cells (SOFC/SOEC). Marked performance improvement is here reported by decorating the surface with Pr2Ni1-xCuxO4 catalyst particles. Infiltrating porous LSF electrodes with aqueous nitrates solutions containing Pr, Ni and Cu, targeting the compositions Pr2Ni0.7Cu0.3O4 and Pr2Ni0.6Cu0.4O4, significantly reduces the polarization resistance at 650 °C from 0.98 Ω cm2 to 0.16 Ω cm2 and 0.13 Ω cm2 respectively. Electrical Conductivity Relaxation (ECR) measurements reveal that surface exchange rates, expressed by (kchem), are much more stable in week long aging experiments for Pr2Ni0.7Cu0.3O4 surface decorated LSF bars compared to the bare LSF. This shows that a Pr2Ni1-xCuxO4 infiltrated porous LSF backbone can be an efficient Co-free oxygen electrode for intermediate-temperature SOFC/SOEC.

Dielectric relaxation and electrical properties of Na0.5Bi4La0.5Ti4O15 electroceramics

Aurivillius single-phase Na0.5Bi4La0.5Ti4O15 ceramics was prepared via solid state mixed oxide sintering technique. The ac data in terms of dielectric and impedance were collected to investigate the electrical properties and dielectric relaxation behaviors in the ceramics. The detailed frequency spectrum analysis demonstrates the existence of single dielectric relaxation behavior in the material, which is due to the grains contribution. A phase transition was observed to take place at ~570 °C. Ar-annealing study showed that above ∼280 °C, relaxation and conduction of the ceramics was assigned to the motion of ionized oxygen vacancies. The kinetic analysis was performed to probe the conduction-relaxation mechanisms in the ceramics. The present results could be significant for deep understanding of electrical properties and defect-relaxation behaviors in Na0.5Bi4La0.5Ti4O15 related ceramics.

Oxygen transport kinetics affected by grain size – A permeation model study

Grain size has obvious influence on the transport kinetics of oxygen species for mixed ionic-electronic conducting materials. Electrical conductivity relaxation and oxygen isotopic exchange with secondary ion mass spectrometry measurements cannot be used to determine the kinetic parameters at a steady state, while the permeation model study can give the kinetic parameters under real experimental steady states. Cobalt-free BaCe0.1Fe0.9O3-δ (BCF) membranes selected as an example with different grain sizes are studied by Zhu's model to disclose the influence of grain size on oxygen transport kinetics. The model is available for analyzing data of BCF membranes to obtain reasonable and exclusive oxygen diffusion and oxygen exchange kinetic parameters. The permeation flux of BCF membranes decreases with the increase of grain size. Model studies reveal that interfacial oxygen exchange resistances on both sides increase with the increase of grain size, while the bulk diffusion resistance changes slightly. It indicates that grain boundaries are active sites for oxygen exchange reactions and have little effect on the bulk diffusion.

Study of Effect of Y substitution on structural, dielectric, impedance and magnetic properties of Bismuth Ferrite

Yttrium substituted Bismuth Ferrite of a composition, Bi(Fe0.95Y0.05)O3 (abbreviated as BFYO5) has been synthesized through a conventional mixed oxide route. The orthorhombic crystal structure and a pure-phase formation of the polycrystalline material was verified through X-ray diffraction pattern. The composition of its constituent elements and their concentration ratio were confirmed by using energy dispersive X-ray spectroscopy technique. Scherrer's equation was used to estimate the average crystallite size of the examined sample specimen and found to be 39 nm. Analysis of the BFYO5 compound's surface morphology was conducted using field emission scanning electron microscope, which clearly shows a densely packed specimen with a homogeneous distribution of various grain dimensions. The interrelationship between frequency and temperature in dielectric, impedance and conductivity parameters with the structure of a sample has also been reported. The impedance spectroscopy analysis shows the nature of negative temperature coefficient of resistance, grain and its boundary effect on the resistive and capacitive features and the presence of non-Debye type of electrical relaxation process in the studied material. The analysis of polarization versus electric field shows the improvement in the value of remnant polarization comparing with the parent compound bismuth ferrite. The substantial enhancement in the magnetization value of the prepared material was also confirmed by its magnetic hysteresis loop study. Based on an important enhancement in the electrical properties, remnant polarization and its magnetic properties, BFYO5 material could be considered as a promising candidate for some of the advance device applications.

The effect of group IIIA oxides on the oxygen reduction reaction at cathodes for intermediate-temperature solid oxide fuel cells

This study investigates the effect of B2O3, Al2O3, Ga2O3 and In2O3, belonging to group IIIA, on the oxygen reduction reaction (ORR) at typical electrocatalysts, lanthanum strontium cobalt ferrite (LSCF) and lanthanum strontium ferrite (LSF), for intermediate temperature solid oxide fuel cells (SOFCs). Amongst these oxides, In2O3 significantly enhances the electrochemical performance by improving ORR as determined by AC impedance spectroscopy using symmetrical cells comprising samaria-doped ceria as the electrolyte. When 13.73 and 7.88 wt % In2O3 is impregnated into the porous LSCF and LSF electrodes, at 650 °C, the area specific interfacial polarization resistance (ASR) decreases from 0.41 to 0.27 Ω cm2 and from 0.99 to 0.39 Ω cm2, respectively. The distribution of relaxation time analysis suggests that In2O3 efficiently accelerates the charge transfer and oxygen incorporation processes. The chemical oxygen surface exchange coefficients are greatly increased by In2O3 deposition as demonstrated with the electrical conductivity relaxation technique. For example, at 800 °C, the deposition of 0.202 mg cm−2 In2O3 particles increases the coefficient from 4.53 × 10−5 to 2.81 × 10−4 cm s−1 and from 2.39 × 10−5 to 9.3 × 10−5 cm s−1 for LSCF and LSF, respectively.

Shape anisotropic Fe3O4 nanotubes for efficient microwave absorption

Although Fe3O4 particles have exhibited excellent microwave absorbing capacity and widely used in practical application due to the synergistic effect of magnetic loss and dielectric loss, their applications are still limited for the required high mass fraction in absorbers. To overcome this problem, the development of Fe3O4 materials with low dimensional structures is necessary. In this study, the shape anisotropic Fe3O4 nanotubes (NTs) with low mass ratios were applied to realize efficient microwave absorption. The NTs with different aspect ratios were prepared through facile electrospinning followed by two-step thermal treatments and mechanical shearing. The cross-linked nanotubular structure enabled the absorbers to have much higher electrical conductivity, multiple scattering, polarization relaxation and better anti-reflection surface, while the shape anisotropic NTs maintained significant multiple resonances with stronger coercivity. These all were beneficial to microwave absorption with enhanced dielectric loss, magnetic loss and sterling impedance matching. Results showed that the absorber with 33.3 wt.% of short Fe3O4 NTs had minimum reflection loss of -58.36 dB at 17.32 GHz with a thickness of 1.27 mm, and had the maximum effective absorbing bandwidth (EAB) of 5.27 GHz when the thickness was 1.53 mm. The absorber with 14.3 wt.% of long Fe3O4 NTs presented the widest EAB in certain radar band with attenuated 80.75% X band and 85% Ku band energy bellow -10 dB at the thickness of 2.65 and 1.53 mm, respectively. This study provided an approach for the development of shape anisotropic magnetic absorbing materials, and broadened their practical applications as magnetic absorbers.

Hydrostatic pressure influence on electric relaxation response of bismuth manganite ceramics

AbstractElectrical impedance of bismuth manganite ceramics was studied under ambient and high hydrostatic pressure. Local disorder of crystal lattice was confirmed using XRD and DSC. Two relaxation processes were discerned. One was attributed to small polarons, which showed a changeover from variable range hopping features related to Fermi glass and structural disorder to the nearest neighbor hopping behavior in higher temperature range related to uniform distribution of energy levels. Hydrostatic pressure shortened relaxation times that would be interesting for applications. The second process, which was assigned to the nearest neighbor hopping of polarons, also exhibited a changeover to glassy features in a high‐temperature range.

FREQUENCY DISPERSION DEPENDENCE OF DIELECTRIC PERMITTIVITY OF CRYOGENIC ROCKS AT THE MACRO LEVEL

Purpose. The purpose of this article is to establish the analytical expression of the frequency dispersion dependence of the real part of the dielectric permittivity of cryogenic rocks according to experimental studies. Methodology. Analytical and experimental investigations of the dependence of dielectric permittivity of cryogenic rocks on the field frequency of applied loads at the macro level have been carried out. An empirical analysis of the obtained results was carried out and a comparison of these data with experimental data was performed to prove the adequacy of the proposed mathematical models. Findings. The dielectric permittivity decreases exponentially with increasing frequency at all temperature values. The dependence parameter that corresponds to the frequency at the moment of electrical relaxation (anomalous dispersion zone) shifts toward low frequencies as the temperature decreases. The maximum and minimum values of the dielectric permittivity of cryogenic rocks are constant and independent of temperature change for a particular cryogenic rock and are dependent on humidity, mineralization and morphological structure. Originality. In this article, mathematical expressions are developed to describe the frequency dependence of dielectric permittivity under low frequency and high frequency loading, which is described by a single analytical dependence. Practical implications. The practical significance of the results obtained is the ability to establish seismic-acoustic, elastic-deformation and strength properties of rocks through the study of the characteristics of the electromagnetic properties of cryogenic rocks. Conclusions. The obtained analytical dependence of the dielectric permittivity of mountain cryogenic rocks in the form of a double exponent allows to adequately describe the influence of an external electric field, taking into account the humidity and temperature of the rocks.

Effect of Structural Water on the Dielectric Properties of Hydrated Tungsten Trioxide

Layered hydrated tungsten trioxide with orthorhombic phase is prepared by a hydrothermal method in an acidic environment. Dielectric studies reveal the presence of space charge and ionic polarizations in the as-prepared samples at different temperatures and frequencies. The scaling behavior shows the gradual shifting of space charge to ionic polarization at 343 K. Impedance along with phase measures a shift from purely resistive to capacitive for the sample with an increase in frequency above 343 K. Modular dispersion spectra suggest that electrical transport and conductivity relaxation occur in the material. Nyquist plots are drawn from impedance and modulus data and the activation energies calculated show a transition from metallic to insulating around 343 K. Comparison of variation of imaginary parts of impedance and modulus with frequency shows the movement of grain boundaries to grain, which supports the transition.

Effects of Sintering Temperature on Microstructural, Magnetic, and Impedance Spectroscopic Properties of Ni0.4Cd0.3Zn0.3Fe2O4 Ferrites

Ni0.4Cd0.3Zn0.3Fe2O4 ferrites have been prepared through the use of sol-gel method at 900 °C and 1100 °C. Rietveld refinements of XRD patterns indicate that the prepared samples crystallize in the cubic spinel structure. Lattice constant and grain size are found to increase with sintering temperature. Magnetic measurements show that the maximum magnetization (Ms) rises, whereas both coercivity (Hc) and remanence (Mr) decrease when increasing the sintering temperature. Frequency and temperature dependence of electrical conductivity, electrical modulus, and electrical impedance have been studied using impedance spectroscopy technique. As the sintering temperature increases, the conductivity of the samples increases. The variation of imaginary part of modulus (M″) displays the presence of an electrical relaxation phenomenon and non-Debye nature. Nyquist representations have been analyzed using an electrical equivalent circuit. The obtained results reveal that the conduction mechanism of the samples is achieved basically of the grain boundary contribution.

In Situ Exsolved Nanoparticles on La0.5Sr1.5Fe1.5Mo0.5O6-δ Anode Enhance the Hydrogen Oxidation Reaction in SOFCs

In situ exsolution of nanoparticles is widely considered as an efficient and cost-effective method for increasing the number of active sites and consequently the catalytic activity on ceramic anodes in solid oxide fuel cells (SOFCs). In this study, by doping on the A-site of Sr2Fe1.5Mo0.5O6-δ (SF1.5 M), evenly distributed Fe nanoparticles (∼100 nm) were exsolved on the La0.5Sr1.5Fe1.5Mo0.5O6-δ (LSFM) surface under a typical anode operating environment (humidified H2, 800 °C). In addition, the exsolution–dissolution reversibility of the exsolved Fe nanoparticles was observed during a redox cycle. Electrical conductivity relaxation (ECR) analysis demonstrated that the surface reaction kinetics on the LSFM anode is enhanced by in situ exsolution. Based on electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) analysis, the perovskite structure was not damaged by the exsolution or the surface phase transition. During exsolution, the ionic conductivity increased. The higher surface catalytic activity and faster oxygen transportation led to enhanced electrochemical performance.

Passivation and activation of La0.6Sr0.4FeO3 thin film electrodes.

The oxygen exchange activity of thin dense La0.6Sr0.4FeO3 electrodes prepared by pulsed laser deposition was investigated by electrochemical impedance spectroscopy and electrical conductivity relaxation below 600 °C. The value of the surface exchange coefficient (kchem) measured at 491 °C decreased from an initial 4.4 × 10-6 cm s-1 to 1.7 × 10-7 cm s-1 after aging for 7 days. The rapid decrease in the oxygen exchange rate was accompanied by the increase in the surface concentration of 'non-lattice strontium' detected by X-ray photoelectron spectroscopy. Subsequent electrochemical tests over 40 days showed that the electrode performance could be recovered by rinsing the passivated electrode in deionized water. Repeated treatments eventually also led to improved stability of electrochemical performance.

Electrochemical Kinetic Property Determination of MIECs by Comprehensive Multi-physical Coupling Model

Surface exchange and bulk diffusion coefficients are important electrochemical kinetic properties of mixed ionic-electronic conductors (MIECs), which are usually obtained through electrical conductivity relaxation (ECR) experiments. However, the extensively employed method is built upon the ideal diffusion described by Fick’s second law with linear absorbing boundary, which is almost impossible to achieve in actual experiments. To simulate the actual experiment system and improve the estiamtion accuracy, a comprehensive mathematical model is established to describe the coupled multi-physical processes involved, such as convection-diffusion in the surrounding atmosphere, surface exchange, bulk diffusion of MIECs, and heat transfer. A comparison between different models is carried out, and the results show that the new model established is more accurate and compatible with more complex conductivity curves.

Influence of alkaline-earth metal substitution on structure, electrical conductivity and oxygen transport properties of perovskite-type oxides La0.6A0.4FeO3-δ (A = Ca, Sr and Ba).

Structural evolution, electrical conductivity, oxygen nonstoichiometry and oxygen transport properties of perovskite-type oxides La0.6A0.4FeO3-δ (A = Ca, Sr, and Ba) were investigated. La0.6Ca0.4FeO3-δ (LCF64) and La0.6Sr0.4FeO3-δ (LSF64) show a phase transformation in air at elevated temperature, i.e., from orthorhombic (Pnma) to rhombohedral (R3[combining macron]c) and from rhombohedral to cubic (Pm3[combining macron]m), respectively, while La0.6Ba0.4FeO3-δ (LBF64) remains cubic over the entire temperature range from room temperature to 1000 °C. The different phase behaviour of the solids is interpreted to reflect the decreased tendency for octahedral tilting with increasing alkaline-earth-metal dopant ion radius. The electrical conductivity of LSF64 is 191 S cm-1 in air at 800 °C, decreasing to a value of 114 S cm-1 at a pO2 of 0.01 atm, and found over this pO2 range roughly twice as high as those of LCF64 and LBF64. Failure to describe the data of electrical conductivity using Holstein's small polaron theory is briefly discussed. Chemical diffusion coefficients and surface exchange coefficients of the materials in the range 650-900 °C were extracted from data of electrical conductivity relaxation. Data of oxygen nonstoichiometry was used to calculate the vacancy diffusion coefficients from the measured chemical diffusion coefficients. The calculated migration enthalpies are found to decrease in the order LCF64 (1.08 ± 0.04 eV) > LSF64 (0.95 ± 0.01 eV) > LBF64 (0.81 ± 0.01 eV). The estimated ionic conductivities of the materials, at 900°C, are within a factor of 1.4.

A quantitative analysis of two-fold electrical conductivity relaxation behaviour in mixed proton–oxide-ion–electron conductors upon hydration

Electrical conductivity relaxation experiments on oxides with three mobile charge carriers, H+, O2- and e-, yield in (de-)hydration experiments kinetic parameters (diffusion coefficients and surface reaction constants). In addition, three amplitude factors are obtained, but they have not been given further consideration because quantitative expressions for their forms are lacking. In this study, the forms of the amplitude factors are derived for a diffusion-limited and a surface-reaction-limited case and a mixed case. In order to demonstrate the benefits of the approach, the electrical conductivity relaxation behaviour of lanthanum tungstate (La5.4WO11.1, LaWO54) was investigated experimentally over the temperature range 923 ≤ T/K ≤ 1223. A switch from two-fold non-monotonic relaxation behaviour at high temperatures to two-fold monotonic behaviour at low temperatures upon hydration was observed. The switch in sign of the fast kinetics' amplitude factor can be assigned to the electrochemical mobility of protons surpassing the electron-hole mobility with decreasing temperature.

Transport phenomena of Cu-S-Te chalcogenide nanocomposites: frequency response and AC conductivity.

In this work, the development and electrical characterization of several chalcogenide nanocomposites have been reported. X-ray diffraction (XRD) has been used to reveal their microstructures. Mott's variable range hopping model has been used to interpret the DC conductivity data of the nanocomposites at lower temperatures. The DC conductivity data at higher temperatures has been explained well using Greave's model. To explain the AC conductivity data, the Meyer-Neldel (MN) conduction rule has been employed. The AC conductivity spectra at different temperatures have been analyzed using Almond-West formalism. Different conduction models, namely, correlated barrier hopping (CBH) and modified non-overlapping small polaron tunneling (NSPT), have been used to interpret the conduction mechanism of the nanocomposites. Scaling of the AC conductivity spectra reveals that the electrical relaxation process is independent of temperature, but depends on the nanocomposite composition. The conductivity mechanism is explained using a schematic structural model.

Effect of Fe doping on the electrical properties of BaTiO3 crystalline materials at room temperature

In this study, we report the room temperature electrical properties of BaFexTi1-x O3 (x = 0.4, 0.5, and 0.6) crystalline materials. The materials were synthesized via a solid-state reaction. The structural properties were investigated via X-ray diffraction, showing a hexagonal perovskite structure with the P63/m space group. The room temperature electrical properties were characterized using impedance spectroscopy. The impedance data indicate a decreasing of ferroelectric behavior and the appearance of ferromagnetic behavior. The frequency spectra of the impedance show the presence of electrical relaxation in the materials, the extent of which depends on the Fe content. Doping BaTiO3 with Fe reduces the relaxation time in the system.

Magnetic, Thermal and Electrical Transport Properties of o-Substituted Polyanilines Encapsulated with Fe2O3 Nanoparticles

Poly(2-nitroaniline), poly(2-nitroaniline)-Fe2O3 and poly(2-methylaniline)-Fe2O3 nanocomposites synthesized by in situ chemical oxidative polymerization technique were characterized by XRD, thermal (TGA and DTA) and FTIR & UV-visible spectroscopic techniques. The thermal stability was confirmed by the integral procedural decomposition temperature (IPDT) and oxidation index (OI) calculations. The electrical conductivity and dielectric properties were also investigated. At low frequency region, the dielectric constant decreases with increase in frequency due to electrical relaxation process. At high frequencies, dielectric constant is independent of frequency. At low frequency, there was strong frequency dispersion of permittivity and above 3 Hz, a frequency independent behaviour in permittivity was observed. The materials exhibit ferromagnetic behaviour.