Time Constant Equivalent Circuit Research Articles

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.

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 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.

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.

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.

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.

A comprehensive EMF behind transient reactance (EBTR) model for power system stability study

Abstract EMF behind transient reactance (EBTR) model of a synchronous generator has widely been used in the studies of power system stability. The damping in the quadrature axis of a synchronous machine is normally represented by a single time constant equivalent circuit. However, the presence of field in addition to the damper circuit in the direct axis of the machine makes modelling difficult in the direct axis based on the emf behind transient reactance. As a result approximate representations are normally employed. The models based on appropriate representations do not produce accurate results. In this paper, an accurate comprehensive model for the direct axis is presented. The development emerges from the detailed representation of the machine with fluxes as the state variable. The emfs behind field and d-axis damper transient reactances are considered in the model. The validity of the model is tested by comparison with the responses obtained from a detailed model [1] .

Some relations between changes in the linear electrical properties of striated muscle fibers and changes in ultrastructure.

Some of the linear electrical properties of frog sartorius muscle have been investigated in Ringer's fluid and in a Ringer fluid made hypertonic by the addition of sucrose or NaCl. Electrical constants were determined from measurements of the phase angle of the admittance of a fiber for an applied alternating current, from measurements of the voltage induced by an inward pulse of current, and from measurements of the conduction velocity of the action potential and the time constant of its foot. The dilation of the transverse tubular system induced by the sucrose hypertonic Ringer fluid was correlated with the change in the electrical constants. From this it is concluded that a two time constant equivalent circuit for the membrane, as proposed by Falk and Fatt, is in good agreement with our results. Both the area of the membrane of the transverse tubular system, and the capacity (c(e)) attributed to it, increased in the sucrose hypertonic Ringer fluid. The resistance r(e), which is in series with c(e), did not fall when the transverse tubular system was dilated and probably is not located in that system.