Depressed Arc Research Articles

Impedance Study of Temperature-Stable Relaxor Dielectrics in the System of BCT-BMT Ceramics.

Preparation of a lead-free system (Ba0.8Ca0.2)TiO3-xBi(Mg0.5Ti0.5)O3 (BCT-BMT) with x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5 was carried out using a solid-state reaction technique. X-ray (XRD) diffraction analysis confirmed a tetragonal structure for x = 0, which shifted to cubic (pseudocubic) at x ≥ 0.1. From Rietveld refinement, a single phase with a tetragonal symmetry model (P4mm) was observed for x = 0, and however, for sample x = 0.1 and sample x = 0.5, the data are modeled to cubic (Pm3m). Composition x = 0 showed a prominent Curie peak, typical of ordinary ferroelectrics with a Curie temperature (Tc) ∼130 °C, modified to a typical relaxor dielectric at x ≥ 0.1. However, samples at x = 0.2-0.5 displayed a single semicircle attributed to the bulk response of the material, whereas a slightly depressed second arc appeared for x = 0.5 at 600 °C, indicating a slight contribution to the electrical properties, ascribed to the grain boundary of the material. Finally, the dc resistivity increased with the increase of the BMT content and the solid solution increased the activation energy from 0.58 eV at x = 0 to 0.99 eV for x = 0.5. Adding the BMT content eliminated the ferroelectric behavior at compositions x ≥ 0.1 and led to a linear dielectric response and electrostrictive behavior with a maximum strain of 0.12% for x = 0.2.

Enhanced photocatalytic reduction of Cr(VI) by the novel hetero-system BaFe2O4/SnO2

In this study, BaFe2O4 nanoparticles with a crystallite size of 86 nm were produced using the sol–gel method and applied in the photoreduction of chromate. The spinel phase was identified by X-ray diffraction and scanning electron microscopy-energy-dispersive X-ray analysis. The structure was refined in orthorhombic symmetry by Rietveld analysis. A forbidden band (Eg) of 2.03 eV was extracted from the diffuse reflectance data and the optical transition was directly permitted. The thermal dependence of the electrical conductivity (σ) followed an exponential law with an activation energy of 0.23 eV. In addition, photo-electrochemical characterization of the spinel was conducted to establish the energy diagram for the hetero-system BaFe2O4/SnO2/HCrO4− solution in order to assess the interfacial reaction mechanism. Similar to most spinels, BaFe2O4 exhibits p-type behavior with a flat band potential (Efb) of 0.52 VSCE. Therefore, the potential of the conduction band (ECB) is sufficiently negative to be involved in an electronic injection with SnO2. The photoluminescence spectrum obtained for the rutile contained an intense peak at 590 nm and its intensity was substantially reduced in the hetero-system. Electrochemical impedance spectroscopy detected a depressed arc assigned to a constant phase element and its resistance (1.64 kΩ cm2) decreased to 1.05 kΩ cm2 under visible light, thereby confirming the semiconductor behavior. The reduction of Cr(VI) was successfully achieved with the BaFe2O4/SnO2 hetero-system under visible light as an application. Photocorrosion was prevented by using oxalate (C2O42−) as a scavenger to captures the holes and improve the photo-activity. The quasi-total chromate reduction had a reduction efficiency of 99% with a concentration of 30 mg L−1 under a light flux of 16 mW cm−2. The kinetics followed a first-order model with a photocatalytic half-life of 38 min and the results were fitted in an appropriate manner to the Langmuir–Hinshelwood model. Moreover, BaFe2O4/SnO2 exhibited photocatalytic efficacy for five successive runs and the final reduction efficiency was still 92%.

Kaolinite under pressure at audio frequency range and its electrical features

ABSTRACT Electrical features of a humid kaolinite specimens was measured at various pressures and at various frequencies . Surface impedance is supposed on kaolinite particle surfaces (from humidity), chemical reaction and charge transport along surface hydroxyls of kaolinite edges besides the permanent negative charges. Classic mixture laws do not support the interpretation of electrical properties, especially, at high pressures. Accordingly, interpretation was made with percolation threshold theories. Pressure increase causes decrease in pore spaces and accordingly increases possible connections of pore distances filled with humidity to total pore spaces after applying pressure. With pressure increase, the pore voids between particles decrease and water conducting paths increase. Also, dielectric constant and conductance increase with increase of pressure. On the other hand, at a certain pressure, at low frequencies, a noticeable increase of capacitance with pressure increase, that tends to reach constant value, for relatively high pressures. The diffusion and interfacial mechanisms was suggested to interpret samples with increase of pressure and frequency. The latter mechanism acts simultaneously with surface chemical reaction mechanism (Debye behavior) to give a depressed arc (relaxation time distribution). Finally, the pressure and frequency are effective factors on electrical properties.

A new implementation of electrochemical impedance spectroscopy (EIS) and other methods to monitor the progress of hydration of strontium monoaluminate (SrAl2O4) cement

Electrochemical impedance spectroscopy has been employed to monitor hydration of strontium monoaluminate (SrAl2O4) cement. Other supported techniques such as X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, thermal analysis (DSC–TG–EGA) and microcalorimetry were also used. In the impedance spectrum at 102 day on a 0.5 water/cement ratio paste, a large double depressed low-frequency arc, a single depressed arc at middle-frequency region and a small part of a large depressed arc at high-frequency region were discriminated. It was due to the specific phase composition and crystal phase content in the fully hardened cement paste. Hence, the new electrochemical equivalent model R1(C1(R2W1))(C2(R3W2))(C3(R4W3))(C4(R5W4)) was implemented and fitted to the experimental results of the fully hydrated and hardened SrAH cement paste. Various hydration products including crystalline Sr3AH6, and amorphous phases SrAH7 and AH3-gel were formed at an early age of hydration. At final hydration process, the main reaction products detected are the ones most thermodynamically stable, i.e. crystalline Sr3AH6 and Al(OH)3. The heat evolution of SrAl2O4 cement under different temperatures (20 °C and 40 °C) was examined by isothermal calorimetry. The curing temperature was found to have a visible effect on cement hydration kinetics.

Three and Four-Electrode Electrochemical Impedance Spectroscopy Studies Using Embedded Composite Thin Film Pseudo-Reference Electrodes in Proton Exchange Membrane Fuel Cells

One and two pseudo-reference electrode proton exchange membrane fuel cell devices were fabricated using composite thin film composed of Nafion and Ag paste as the reference electrode. Three and four-electrode electrochemical studies were performed isolating different parts of the cell. Summing the scans utilizing the reference electrode(s) resulted in good agreement with the two-electrode scan across the entire cell. A DC current was passed through the cell under constant resistive load conditions. The equations necessary to determine the cell and the anode/cathode-reference impedance under constant resistive load are presented. A high frequency depressed arc present in the spectra of the entire cell had contributions from both the anode-reference and the cathode-reference scans. Reactance present in the four-electrode measurements isolating a section of the membrane further suggests that the high frequency depressed semicircle in the cell scan can, at least in part, be attributed to the membrane. The present work shows that by using embedded pseudo-reference electrodes, the polarization behavior of cathode and anode can be investigated independently and unambiguously without the need for de-convolution of spectra across the entire cell.

Impedance spectroscopy studies of Gd-CeO2-(LiNa)CO3 nano composite electrolytes for low temperature SOFC applications

Electrical properties of 20 mol % Gd doped CeO2 with varying amounts of (LiNa)CO3 have been investigated by employing AC-impedance spectroscopic technique. The impedance spectra show a high frequency depressed arc, represents the bulk composite and low frequency incomplete semicircle representing electrode contribution. The bulk resistance of the composites decreases with increasing carbonate content up to 30 wt% (LiNa)CO3, thereafter the resistance increases, whereas all the compositions show a decrease in resistance with increasing temperature. The typical nature of the impedance spectra of the composite shows the possibility of coexistence of multi ionic transport or existence of space charge effect at the interface of Gd-CeO2 and carbonate phase. The composite containing 25 wt% (LiNa)CO3 shows the highest ionic conductivity of 0.1757 S cm−1 at 550 °C and lowest activation energy of 0.127 eV in the temperature range 550–800 °C. A symmetric cell is fabricated with GDC-25 wt% (LiNa)CO3 electrolyte, NiO-GDC(LiNa)CO3 anode and lithiated NiO-GDC(LiNa)CO3 cathode. Pure H2 and air are used as fuel and oxidant. The cell delivers a maximum power density of 45 mW/cm2, 58 mW/cm2 and 92 mW/cm2 at 450, 500 and 550 °C, respectively.

The influence of the solid electrolyte on the impedance of hydrogen oxidation at patterned Ni electrodes

In order to investigate the role played by the solid electrolyte in the H 2 oxidation reaction, patterned Ni electrodes on YSZ (yttria stabilized zirconia) single crystals with different crystallographic orientations, on polycrystalline YSZ and on polycrystalline LSGM ((La,Sr)(Ga,Mg)O 3 − x) were studied in H 2 + H 2O gas as a function of partial pressure of H 2O in the temperature range of 673–823 K. Symmetrical cells using YSZ single crystals exhibit a single almost perfect electrode semicircle in the Nyquist plot. The corresponding polarisation resistance (R p) depends on the surface orientation of the YSZ single crystals with a maximum of R p for < 110> and a minimum for < 100>. The capacitance obtained from the impedance arc is almost temperature independent but different for the three surfaces. In the case of polycrystalline YSZ a depressed impedance arc results with the capacitance being between the extreme values measured for single crystals of different orientation. A modified situation is found for patterned Ni electrode on LSGM: the capacitance of the single arc depends on temperature and is much larger than in the case of YSZ cells. The polarisation resistance on polycrystalline LSGM is much smaller than on polycrystalline YSZ while activation energies of the polarization resistance are still similar. Capacitances and resistances are discussed in terms of possible mechanisms.

Interpretation of impedance spectra based on local minima of imaginary Z vs frequency

Impedance spectra have been analysed based on interpretation of minima of the imaginary part of impedance vs frequency. One derived useful relations and used simulated examples to illustrate how this method can be applied. The bulk relaxation frequency, bulk capacitance and grain boundary capacitance can be extracted from the values of imaginary and real impedance parts at the minimum of Z″, and corresponding frequency. The proposed relations are also used to analyse impedance spectra obtained for ceramic samples of Yb 2Ti 2O 7-based materials. The consistency of the analysis is also examined for cases when the representation of imaginary part of impedance vs real impedance shows a significantly depressed arc. This can be interpreted as resulting from distribution of relaxation frequency values.

Effect of annealing temperature on mixed proton transport and charge transfer-controlled oxygen reduction in gas diffusion electrode

Abstract The effect of annealing temperature Tann on mixed proton transport and charge transfer-controlled oxygen reduction in gas diffusion electrodes used in polymer electrolyte membrane fuel cells (PEMFCs) was investigated in 1 M H2SO4 solution using AC-impedance spectroscopy and potentiostatic current transient technique. For this purpose, the gas diffusion electrodes were annealed at different temperatures ranging from 140° to 180 °C in order to control the proton transport resistance distribution across the active catalyst layer (ACL). For the annealed gas diffusion electrodes with different proton transport resistance distributions, the measured impedance spectra exhibited a straight line inclined at a constant angle higher in absolute value than 45° to the real axis at high frequencies, followed by a depressed arc at low frequencies. From the quantitative analysis of the measured impedance spectra based upon the transmission line model (TLM) modified with the proton transport resistance distribution, it was found that as Tann increased, the average proton transport resistance Rave and the standard deviation σ of the proton transport resistance distribution increased as well. Furthermore, as Tann rose, the charge transfer resistance Rct increased and simultaneously the double layer capacitance Cdl decreased due to the smaller electrochemical active area Aea. From the analysis of the cathodic current transients measured during nitrogen blowing, it was noted that as Tann increased, the current decayed more rapidly with time, suggesting that the larger values of Rave and σ kinetically impede proton transport through the Nafion membrane within the ACL due to the wider RC time constant distribution.

The current detour effect observed on materials with random microstucture: experimental evidence from Li 3 xLa 2/3− xTiO 3 studied by impedance spectroscopy

Impedance spectroscopy (IS) has been used to study the influence on the low frequency part of the impedance diagrams of the microstructure of a fast ionic conductor, Li 3 x La 2/3− x TiO 3 with x = 0.10 (named hereafter LLTO). This oxide has been synthesised by sol–gel method. After synthesis, the powder of LLTO displays a large distribution of grain size and agglomerates. The grain size distribution and the porosity of the ceramic have been changed by heat-treatment from 600 °C to 1200 °C in air. The impedance spectra of these ceramics, recorded at different temperatures from room temperature (RT) to 400 °C, show a low-frequency depressed arc, which is characteristic of the grain boundary response of the ceramic. Its shape depends strongly on the heat-treatment of the ceramic, and therefore, on its microstructure. It is a simple arc when the pellet is well sintered but becomes very complex for non-sintered ceramics with high resistive grain boundary and pores. The observed “fish” shape indicates the presence of current “detours effect” in the material. This effect means that current detours around blocking grain boundary and/or pores occur to lower the impedance. Consequently, the brick layer model (BLM), which assumes an ideal microstructure, and then no current “detours effect”, can not be used to analyse these impedance data.

The ac impedance study of electrochemical lithium intercalation into porous vanadium oxide electrode

The electrochemical lithium intercalation into vanadium oxide in LiClO 4 propylene carbonate solution has been investigated by using electrochemical impedance spectroscopy as a function of lithium content. Impedance spectra measured in the lithium content range investigated ( δ = 0–0.8 in Li δ V 2 O 5) consist of a high frequency depressed arc and a low frequency straight line. The high frequency arc is attributed to the charge transfer reaction at the vanadium oxide/electrolyte interface and the low frequency line is associated with the diffusion of lithium ion through single vanadium oxide phase or with the movement of the boundary separating two phases. The chemical and component diffusivities of lithium ion in vanadium oxide were determined from the low frequency line combined with coulometric titration curve. The variation of the chemical and component diffusivities with lithium content has been discussed in relation to the coulombic interaction between the intercalated lithium ion and the oxide lattice.

Electrical properties of cement paste obtained from impedance spectroscopy

The impedance spectra of hydrating Portland cement paste showed a small part of a large depressed arc and a single depressed arc at low- and high-frequency regions, respectively. A new equivalent circuit was proposed for hydrating cement paste, which consists of a serial combination of a resistor and a constant phase element (CPE) with a resistor in parallel for the bulk effects, and a parallel combination of a resistor and a CPE for the electrode effects. The microstructural evolution in hydrating cement paste was associated with the changes in the resistances, capacitance and the critical relaxation time.

Complex admittance study of potassium yttrium fluoride

Abstract The complex admittance of pure and 0.5 mol% sodium doped potassium yttrium fluoride polycrystalline pellets, between two inert blocking electrodes, was studied from 300 to 950 K in the frequency range 50–100 kHz. In most cases the admittance plot exhibited a depressed circular arc in the low frequency region and a straight line in the high frequency region. The bulk conductivity σ, the dielectric constant e′ and the loss angle tan δ were determined from the admittance plots. The high values of e′ observed at high temperatures were attributed to space charge polarization effects. The a.c. conductivity was found to increase with increasing frequency.

The shape of complex impedance arcs for alkali ion conducting glasses

A method of using complex impedance plots to determine dc resistances is investigated in which the shape of the arc is assumed to be the same at all temperatures. Using a reduced impedance plot on which measurements made at several different temperatures are all plotted together, least squares calculations are used to determine the best-fitting depressed circular arc (DCA) and elliptic arc (EA). The technique is applied to a number of single alkali glasses in the system Li 2O-B 2O 3-SiO 2. It is shown that there are systematic departures from both DCA and EA but that the EA is the most accurate at very low frequency end. The EA gives extrapolated dc resistivities about 1 1 2 % lower tahn those the DCA. However the scatter of dc resistivities from straight line Arrhenius plots was higher than this. The impedance plot for a mixed alkali glass is not well fitted by either a DCA or an EA and its anomalous shape is noted.

Ac polarization phenomena of the YSZ/Rh and YSZ/Pd electrode systems

Electrode characteristics of YSZ/Rh and YSZ/Pd were studied as a function of oxygen partial pressure and temperature by ac impedance spectroscopy. Similar impedance spectra were observed in both systems. Below the decomposition pressure of Rh 2O 3 and PdO the impedance spectrum consisted of two arcs. Both arcs, especially the low frequency one, decreased in size with increasing oxygen partial pressure. In the case of YSZ/Rh, the high frequency arc gradually grew again with increasing oxygen pressure after the low frequency arc disappeared. Just above the decomposition pressure of the oxides a large depressed impedance arc was observed. For each system, the bulk and the electrode resistances were abruptly changed at the decomposition pressure. The activation energy of electrode resistance for each arc was obtained.

The shape of complex impedance plane arcs for ionically conducting glasses

The complex resistivity (ϱ ∗) plots have been calculated for a conducting Cole-Cole dielectric, for the Fourier transform of the fractional power decay law, exp(− t b ), and for Dyre's uniform distribution law. It is shown that a depressed circular arc in the ϱ ∗ plane can never truly represent the properties of a real material, since the high frequency permittivity ϵ ∞ is not included. Nevertheless the calculated arcs are mostly not far from circular. For the Cole-Cole dielectric the shape is found to depend as much on the ratio ϵ ∞/( ϵ s − ϵ ∞) and on Namikawa's p = ( ϵ s − ϵ ∞) ϵ 02 πf m / σ 0 as on the Cole-Cole α. For the exp(− t b ) law, the ϱ ∗ arc becomes increasingly depressed as b decreases. It is shown that a more accurate extrapolation for a limited range of frequency points to the real axis at low frequencies maybe obtained by fitting an on-axis ellipse rather than a depressed circular arc.

Cathodic and Anodic Polarization Phenomena at Platinum Electrodes with Doped CeO2 as Electrolyte: II . Transient Overpotential and A‐C Impedance

The preceding paper (Part I) dealt with the study of electrode polarization phenomena by means of steady‐state overpotential measurements. In this paper, two other manifestations of polarization are explored simultaneously with the steady‐state measurements. The first is the transient decay of overpotential with time following current interruption, while the second is the a‐c response as a function of frequency, best described in terms of complex impedance plots. It is shown that at high where the charge‐transfer mechanism dominates, the overpotential decay curves can be quantitatively interpreted, while the contribution of the electrode behavior to the complex impedance is a single depressed circular arc. Both measurements give a double layer capacitance at each electrode ∼100 μF/cm2. At low , where concentration polarization develops at the cathode, the transient curves show a variable effective exchange current which can be calculated from the limiting current of the steady‐state data. Correspondingly, the a‐c impedance plots show the beginning of an additional arc. These results are completely consistent with the mechanisms developed in Part I .