Constant Equivalent Circuit Research Articles

A distributed constant equivalent circuit model and a method for estimating parameters for paper-based lactate biofuel cells

In this study, a distributed constant-type equivalent circuit model for paper-based lactate biofuel cells (PBFCs) where the anode and cathode are formed on the same plane was constructed, and a method of determining parameter values in the equivalent circuit model was proposed. When PBFC were prepared and the electrochemical impedance of the entire cell was measured, the Nyquist diagram produced a spectrum due to the current line distribution. Parameters, such as charge transfer and solution resistances, were calculated from the obtained spectrum using theoretical impedance equations derived from the proposed equivalent circuit model proposed. In addition, the behavior of the current distribution in the battery was evaluated in detail by moving the reference electrode on and performing local electrochemical impedance spectroscopy at multiple points on the electrode. The models and methods proposed in this study are good for evaluating systems affected by distributed current lines, such as flat batteries, and for determining cell design, electrode areas, and arrangements.

A Distributed Constant Equivalent Circuit Model and a Method for Estimating Parameters for Paper-Based Lactate Biofuel Cells

A Distributed Constant Equivalent Circuit Model and a Method for Estimating Parameters for Paper-Based Lactate Biofuel Cells

A Distributed Constant Equivalent Circuit Model and a Method for Estimating Parameters for Paper-Based Lactate Biofuel Cells

A Distributed Constant Equivalent Circuit Model and a Method for Estimating Parameters for Paper-Based Lactate Biofuel Cells

Impedimetric detection of Pseudomonas aeruginosa attachment on flexible ITO-coated polyethylene terephthalate substrates

Biofilm monitoring in environmental and biomedical applications remains a challenge. Currently, conventional biochemical methods do not provide a quick quantitative measure of attached biomass. Thus, there is a need for rapid in situ detection tools for routine biofilm characterization. Electrochemical impedance spectroscopy (EIS) characterizes the electroactivity of bacteria within a biofilm and has been extensively used to monitor strong electroactive biofilms. Yet, studies on weak electricigens such as Pseudomonas aeruginosa remain underrepresented. Here, conductive indium tin oxide coated polyethylene terephthalate (ITO:PET) sheets were used as flexible growth substrates instead of more conventional carbonaceous or gold materials. EIS was compared with standard optical methods for the detection of P. aeruginosa biofilms formed on ITO:PET under static growth conditions. Relaxation time analysis showed a dominant time constant at approximately 1 s and confirmed the validity of a two time constant equivalent circuit model for the biofilm impedance. The interfacial resistance calculated from the equivalent circuit analysis showed a rapid drop after bacterial attachment whereas capacitance of the biofilm was masked by the capacitance of ITO:PET. The trends for interfacial resistance and capacitance were independent to the geometry of the ITO:PET working electrode. Moreover, most robust behaviour was observed for rectangular electrodes. EIS across a broad range of potentials with and without inhibitors showed a marked difference between the interfacial resistance of viable and energy inhibited biofilms. Moreover, EIS of exopolysaccharide Δpsl mutant showed a substantial drop in current. Overall, our results indicated that EIS enabled the detection of biofilm formation across large surface areas as early as 24 h after inoculation for the weak electroactive species P. aeruginosa using a flexible polymeric substrate.

Self-organized TiO2 nanotube layer on Ti–Nb–Zr alloys: growth, characterization, and effect on corrosion behavior

Ti alloys are widely applied in implanted biomedical devices due to their unique mechanical and biological performances. A strategy employed to improve bone integration on orthopedic and dental implants is to grow a self-organized TiO2 nanotube layer on the surface of titanium alloy implants. This paper describes the formation of self-organized TiO2 nanotubes on Ti–35Nb–2Zr and Ti–35Nb–4Zr alloys by the anodization process, as well as the effects of Zr content on TiO2 phase stability. The morphological and chemical characteristics of the nanotubes were analyzed by scanning electron microscopy, X-ray diffraction, and X-ray photoelectron microscopy. In addition, a comparison was made of the electrochemical stabilities of TiO2 nanotube-coated surfaces and surfaces without nanotubes, which revealed higher corrosion resistance for the nanotube-modified surface. The electrochemical impedance spectroscopy results were fitted with two-time constant equivalent circuit representing the barrier layer (nanotube bottom) and the porous layer (nanotube wall). The addition of Zr suppressed ω-phase formation, preserving the alloy’s low elastic modulus (64 GPa). This Zr addition also delayed the anatase-to-rutile transformation and slightly increased the nanotubes’ length to 1.14 µm. These features make the Ti–35Nb–4Zr alloy a very good candidate for use in the biomedical field, especially for applications that require low elastic modulus with enhanced corrosion resistance.

Ellipsometry and Raman Investigations into the Influence of Chloride on Steel Corrosion Rates in Alkaline Environments

The contribution to pitting corrosion from chloride is a well-accepted agreement, however, the mechanism and the relationship between the presence of chloride and the generation of pitting corrosion in different cases is still remains to be studied. Likewise, the study of the inhibition mechanism of corrosion inhibitors is also deficient. Spectroscopic methods such as Raman and ellipsometry have the advantage of being surface sensitive and non-destructive characterization processes. It gives one a chance to investigate the dynamics of localized corrosion on steel in high alkaline solution (pH=12.5) in the presence of chloride ions. An industrial example of such an environment is reinforced concrete. In this study, these spectroscopic techniques are combined with electrochemical measurements such as open circuit potential (OCP; Figure 1) and electrochemical impedance spectroscopy (EIS; Figure 2) to analyze the corrosion tendency of attack by chloride at different concentrations at pH12.5. It is found that for chloride concentrations higher than 50mM the OCP spontaneously decreases, indicating an irreversible destruction of the passive film formed in the alkaline solution. The EIS data could be fitted to a simple one time constant equivalent circuit in either the Bode or Nyquist formats (Figure 3). The constant phase element CPE is linked the capacitance by: ZCPE=[Cox(jω)n]-1 It is shown that the capacitance, Cox, increased with the increasing chloride concentration, possibly indicating a thicker oxide layer on the carbon steel sample surface. EIS also revealed that the corrosion rate increased with increasing chloride content, as indicated by a reduction in the charge transfer resistance, R ct (Figure 3). SEM, Raman spectroscopy and ellipsommetry were applied for surface characterization, with the in-situ Raman and ellipsometry cells shown in the Figure 4. SEM revealed that when NaCl was introduced to the alkaline solution, the sample surface obtained a higher roughness after 24 h immersion test. For Raman, because of the thinness of the corrosion product layer, it was first necessary to electrochemically deposit to induce the surface enhanced Raman (SERS) in order to collect spectra from the carbon steel in pH=12.5 alkaline solution with or without NaCl (Figures 5 & 6). The locations of Raman peaks i of selected iron oxides, hydroxides and oxyhydroxides are listed in the table (Ref 1 & 2), with the most intense peak being bold. ComponentRaman shift (cm-1)α-FeOOH 290 β-FeOOH400 720 γ-FeOOH 252 380526650δ-FeOOH663(braod)Fe(OH)2 460 545 Fe(OH)3 692(broad)Fe3O4 550 670 It was found that in the absence of chloride the main components of the passive are Fe(OH)2, Fe3O4, γ & δ – FeOOH. However, on addition of chloride, the γ - FeOOH in the oxide layer is converted to β-FeOOH. The in-situ ellipsometry data allowed the thicknesses of the films to be determined and it was found that the oxide layer thicken by around 20% on the addition of chloride to the alkaline solution. It those concluded that as the chloride ions convert γ - FeOOH converted to β-FeOOH this both thickens and destabilizes the passive film. Figure 1

Correlation of Impedance and Effective Electrode Area of Iridium Oxide Neural Electrodes

Iridium oxide is routinely used for bionic applications owing to its high charge injection capacity. The electrode impedance at 1 kHz is typically reported to predict neural recording performance. In this article, the impedance of activated iridium oxide films (AIROFs) has been examined. The impedance of unactivated iridium electrodes was half that of platinum electrodes of similar geometry, indicating some iridium oxide was present on the electrode surface. A two time constant equivalent circuit was used to model the impedance of activated iridium. The impedance at low and intermediate frequencies decreased with increasing number of activation pulses and total activation charge. The impedance at 12 Hz correlated with the steady-state diffusion electroactive area. The impedance at 12 Hz also correlated with the charge density of the electrode. The high charge density and low impedance of AIROFs may provide improved neural stimulation and recording properties compared with typically used platinum electrodes.

Correlation of impedance and effective electrode area of chondroitin sulphate doped PEDOT modified electrodes

Neural electrode arrays were coated with PEDOT doped with chondroitin sulphate. The impedance of the individually addressable microelectrodes were measured and fit with a 2 time constant equivalent circuit. The typically reported impedance at 1kHz was significantly reduced with very thin coatings, but showed no correlation with effective electrode area. The impedance and phase angle at low frequencies was correlated with the effective electrode area. The charge density of the electrode was also correlated with the linear diffusion electroactive area.

Correlation of Impedance and Effective Electrode Area of Dextran Sulfate Doped PEDOT Modified Electrodes

The impedance of electrodes at 1 kHz is typically reported to assess the signal-to-noise ratio of neural recording electrodes. The impedance response of platinum electrodes modified by poly-3,4-ethylenedioxythiophene doped with dextran sulfate has been examined. The modified electrodes have lower impedance at low and intermediate frequencies compared to unmodified electrodes. The impedance and phase angle at low frequencies is strongly correlated with the electrode area. The geometric and linear diffusion charge densities of the modified electrodes are also dependent on the electrode area and impedance at low frequencies. A 3 time constant equivalent circuit provided a better fit to the impedance than a 2 time constant model. The decrease in impedance at low frequencies indicates PEDOT-DS will be suitable for reducing the thermal noise and increasing the signal-to-noise ratio for neural recording electrodes.

Electrochemical behavior and effect of heat treatment on morphology, crystalline structure of self-organized TiO2 nanotube arrays on Ti-6Al-7Nb for biomedical applications.

In the present work, we investigate the formation of self-organized titanium oxide nanotube layers by anodic oxidation on titanium alloy Ti-6Al-7Nb in electrolyte solution containing sulfuric acid and hydrofluoric acid. The anodized surface was characterized by micro-Raman, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDS). The corrosion behavior of the treated and untreated samples was investigated through electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization studies in simulated body fluid (Hanks' solution). The investigations show that the native oxide on the sample is replaced by self-assembled nanoarray by anodization. FESEM of samples annealed at 450 to 800 °C show tubular morphology whereas those annealed at 850 °C show collapse of nanotubes. Electrochemical impedance data of the substrate and 10 V anodized samples were fitted with a two-time constant equivalent circuit and that of anodized samples (20, 30 V) with a three-time constant equivalent circuit.

Electrochemical behaviour and bioactivity of self-organized TiO2 nanotube arrays on Ti-6Al-4V in Hanks’ solution for biomedical applications

In the present work, the formation of self-organized titanium oxide nanotube layer by anodic oxidation on titanium alloy Ti-6Al-4V and its influence on growth of apatite on Ti-6Al-4V is reported. Anodization was carried out in sulfuric acid and hydrofluoric acid containing electrolyte and the anodized surface was characterized by micro-Raman, XRD and FESEM. The corrosion behavior of the anodized sample was investigated through electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization studies in simulated body fluid (Hanks’ solution). The influence of TiO2 nano tubes on the growth of apatite on the anodized sample was investigated by immersion in Hanks’ solution for 1 and 7 days. The investigations show that the native oxide on the sample is replaced by self assembled nanotube array by anodization. The anodic oxide consist both anatase and rutile. After vacuum annealing at 600°C, intensity of rutile peak increased. FESEM of samples annealed at 450°C show tubular morphology whereas those annealed at 600°C show collapse of nano tubes. Corrosion resistance of the anodized sample was comparable with that of the untreated samples. Electrochemical impedance data of the substrate was fitted with two time constant equivalent circuit and that of anodized samples with three time constant equivalent circuit. FESEM images of immersed samples show that the growth of apatite is more with larger sized deposits on anodized surface as compared to that on the untreated substrate. XPS investigation of immersed sample shows presences of calcium, phosphorous and oxygen in hydroxide/phosphate form on the anodized sample.

Distributed-Type Equivalent Circuit to Determine Charge Transfer Resistance of Metals Under a Thin Solution Film

Background Atmospheric corrosion proceeds under an extremely thin solution film. In actual environment, the thickness or concentration of thin solution film increases and decreases continuously with the change in temperature and humidity. These changes act as a very important role in the atmospheric corrosion process. However, it is quite difficult to monitor corrosion rate of metals under a thin solution film with conventional electrochemical methods which are suitable for studying corrosion behavior in bulk solution. The largest problem is an uneven current distribution over working electrode which leads to serious errors in the measurements of the corrosion rate because an extremely high solution resistance. By using Electrochemical Impedance Spectroscopy (EIS), current distribution over electrode surface can be evaluated from frequency dependency of the phase shift1. The current distribution becomes uneven if the phase shift goes no further than -45° on Bode plot when constant phase element α=1. In such a condition, a correct charge transfer resistance R ct cannot be obtained. In the present study, some approaches were proposed to measure the R ct under a thin solution film with uneven current distribution. Experimental A two-electrode cell composed of a pair of identical carbon steel plates (1 cm × 1 cm) in parallel with a gap of 0.2 mm was used. NaCl or NaNO3 solution film with different thickness was covered over cell surface. The thickness of thin film was controlled using a micropipette to render a pre-determined volume, which was assumed to spread over cell surface uniformly. When thickness was smaller than 25 μm, ethanol was mixed into initial solution to guarantee a uniform spread of thin film. The EIS measurements were performed at OCP with a 10 mV amplitude. The frequency range was from 10 kHz to 10 mHz, with 5 points per decade. Transmission line (TML) model of a metal covered with a thin electrolyte layer With EIS measurement, the metal-thin film interface can be presented by a one-dimensionally distributed constant equivalent circuit, i.e. Transmission line (TML) circuit, as shown in Fig. 1. The total impedance Ztotal of single electrode is given byZtotal =Zw coth γ Xw + Rsg /2 (1)γ = (Rs */ Z*)1/2 (2)Zw =L-1(Rs *Z*)1/2 (3)where Xw and L are width and length of electrode respectively, Rs* and Z* are a solution resistance over electrode and a metal-thin electrolyte interfacial impedance per unit length, respectively, and Rsg is a solution resistance over gap between two electrodes. When the value of (γ Xw) is very large, which means current distribution tends to be uneven, Eq. (1) can be converted toZtotal =Zw + Rsg /2 = (RsRct)1/2 + Rsg /2 (4)by eliminating the term of coth γXw. Results and discussion Current distribution over working electrode tends to be uneven when thickness or concentration of electrolyte film decreases. Fig. 2 shows the EIS data of carbon steel covered with a 500μm NaCl solution film with concentration from 0.002M to 2M. Current distribution was evaluated from frequency dependency of phase shift. Uneven current distribution established over electrode surface in whole frequency range when concentration is smaller than 0.02M. Hence, TML model was applied to determine the accurate corrosion rate. The fitting results show quite good consistency with the original data. In the present study, the TML model was applied to eliminate the effect of uneven current distribution during monitoring of atmospheric corrosion rate. Furthermore, the rate-determining step of a metal covered with an extremely thin solution film is discussed.

AC charging/discharging of de-alloyed Si–Al–V alloy ribbons

Abstract In anticipation of prompt ac charging/discharging characteristics, the capacitance of de-alloyed Si 1− x (Al 0.5 ,V 0.5 ) x ( x = 0.15, 0.2, 0.3, and 0.4) alloy ribbons with resistivities of 0.01–4300 Ωcm was measured as a function of frequency between 1 mHz and 1 MHz, using exponential transient analysis for electric charging/discharging. The capacitance of de-alloyed Si–7.5 at.%.Al–7.5 at.%. V obtained by prompt (100 ms–25 ns) charging/discharging decreased logarithmically from 105 μF to 18 pF as frequency increased from 1 mHz to 10 kHz, and then saturated between 10 kHz and 1 MHz. From the observed electrode distance dependence on capacitance, we deduced that the alloy consisted of a distributed constant equivalent circuit (series with 1.2% parallel), analogous to electric double layer capacitors.

Electric storage in de-alloyed Si-Al alloy ribbons

The capacitance of de-alloyed Si1−xAlx (x = 0.2, 0.3, and 0.4) alloy ribbons with resistivities of 1–1000 Ωcm was measured as a function of frequency between 1 mHz and 1 MHz from electric charge/discharge pulse curves of 10 V applied at 100 ms–25 ns intervals, respectively, using an exponential transient analysis for electric charging/discharging. The capacitance of the de-alloyed Si-20 at.% Al specimen obtained by prompt charging/discharging decreased logarithmically from 102 μF (0.55 F/cm3) to 1.2 pF (53 nF/cm3) as frequency increased from 1 mHz to 1 MHz. From the observed electrode distance dependence on capacitance and nonlinear electronic transport behaviour, we deduced that the alloy consisted in a distributed constant equivalent circuit (series with 1.7% parallel), analogous to electric double-layer capacitors.

Improvement of the corrosion resistance and structural and mechanical properties of a titanium base alloy by thermo‐mechanical processing

AbstractIn this work, a new ternary Ti–25Ta–5Zr alloy (with nontoxic alloying elements) was obtained and used to develop a thermo‐mechanical procedure in order to optimize the balance strength – elastic modulus. Data about structural (by SEM) and mechanical properties are investigated. Also, its electrochemical behaviour in Ringer–Brown and Ringer solutions of different pH values (simulating severe functional conditions of an implant) was studied by cyclic potentiodynamic and linear polarization and electrochemical impedance spectroscopy (EIS). From cyclic potentiodynamic polarization curves it resulted the nobler behaviour of the thermo‐mechanical treated alloy than as‐cast alloy due to the favourable influence of the applied processing. Impedance spectra were fitted with one time constant equivalent circuit characterizing a very stable, resistant oxide passive film. The values of the open circuit potentials for the treated alloy are nobler than of the as‐cast alloy and tended to more positive values in time, proving that the passive film on its surface is more compact and thickened in time.

Capacitance Distribution of Ni-Nb-Zr-H Glassy Alloys

Capacitance distribution of {(Ni(0.6)Nb(0.4)(1-x)Zrx}(100-y)-Hy (x = 0.30, 0.35, 0.40, 0.45 and 0.50, 0 < or = y < or = 20) glassy alloy ribbons was carried out by ac impedance analysis at frequency of 1 kHz, in terms of a distributed constant equivalent circuit. The capacitance can be represented by oblique contour lines. The highest capacitance (1-11 microF) could be found near the point when x = 0.40, y = 10, which is a composition occurring room-temperature Coulomb oscillation, while capacitance of the composition (x = 0.35, y = 4) occurring ballistic transport was around 0.8 microF. The capacitance difference would be explained by an effect of hydrogen localization derived from morphology of distorted Zr-centered icosahedral Zr5Ni5Nb3 clusters and ideal Ni-centered clusters. The electrocapillarity equation showed that the specific capacitance between two electrodes increases parabolic with decreasing the distance, as a polarized glutinous liquid.

Multispectral impedance quality testing of coil-coating system using principal component analysis

The task of quality checking of the coil-coated system was undertaken using impedance spectroscopy. In order to properly characterize the system the new impedance spectrum was calculated as an averaged result of 12 experimental spectra obtained from 12 different places of the sample at the same time of immersion in 3% of NaCl. The experimental spectra were recorded for a 24-h period at 1h intervals between starting points of each measurement. Averaged impedance at each frequency point of the calculated spectrum was characterized by standard deviation. Impedance analysis using two-time constant equivalent electrical circuit was conducted showing the permissible parameter scatter for the checked system. A new classification method of obtained parameters using principal component analysis was presented. This approach enables to identify samples of low quality.

Corrosion susceptibility of implant materials Ti–5Al–4V and Ti–6Al–4Fe in artificial extra-cellular fluids

The paper is aimed at detecting the corrosion susceptibility (generated by the potential gradients, due to the pH changes) of implant materials Ti–5Al–4V and Ti–6Al–4Fe alloys exposed in extra-cellular fluids for long term (10 000 exposure hours) in comparison with pure titanium. The titanium and its ternary alloys exhibited spontaneous passivity in Ringer’s solutions of pH=6.98, 4.35 and 2.5, simulating the real situations that can arise in surgical applications. Potential gradients determined in presumptive extreme pH conditions indicate that there is no probability for local corrosion. Impedance spectra were fitted with one time constant equivalent circuit, typically for protective, compact oxide film. The fitting parameters indicate long-term stability of the passive layers in surgical implant conditions. Atomic force microscopy (AFM) and spectral infrared (IR) data are important arguments in supporting the conclusion that titanium and its ternary alloys have a very good corrosion resistance in long-term functional conditions.

Correlation between the Susceptibility to Breakdown and Surface Properties of Thin Films on Titanium Alloys in Physiological Fluids

AbstractCorrelation between the susceptibility to breakdown and surface properties of thin films on implant titanium alloys Ti-5Al-4V and Ti-6Al-4Fe in Ringer 1 and Ringer 2 solutions of different pH values (6.98, 4.35, 2.5) which simulate the conditions that can appear at the longterm (13,000 hours) contact between implant and tissues were studied in this paper. The passive films are very stable up to +1.8 V; Ti-6Al-4Fe alloy presents pitting attack at a pitting initiation potential of +3.5 V in Ringer 1 solution and at +3.9 V in Ringer 2 solution. The pitting protection potential is about +1.75 V in both solutions; this is a very noble potential. Impedance spectra were modeled with one time constant equivalent electrical circuit, typically for a compact oxide film, for all materials, in all tested solutions. The fitting parameters indicate a long-term stability.

A comparative study, by EIS and Tafel experiments, of the electrochemical behavior of FINEMET alloys modified with chromium

The electrochemical corrosion behavior of Fe (73.5− X) Cu 1Nb 3Si 13.5B 9Cr X(0–5) FINEMET type alloys have been investigated in 0.5 N KOH solutions using potentiodynamic polarization and AC impedance spectroscopic techniques. Kinetic parameters were obtained after holding the samples at open-circuit potential for 20 min. The open-circuit potential of all specimens decreases with inmersion time, and a shift of stationary potential in the anodic direction was observed with the Cr addition. The present study attempts to correlate impedance data with polarization data and the results indicate that both values are comparable for passivated alloys. In the case of FINEMET type alloys, the addition of chromium results in the formation of a passive film which prevents the corrosion of the alloy and which can be represented by a single time constant equivalent circuit.