Frequency Time Constant Research Articles

Insight into physical interpretation of high frequency time constant in electrochemical impedance spectra of Mg

Electrochemical impedance spectroscopy measurements were performed to study the evolution of surface condition, which related to the high frequency time constant of Mg, in 0.05 M NaCl solution at three different pH. Based on the assumption that the high frequency time constant originates solely from the surface film, the thickness of MgO film was calculated via fitting EIS results. The deduced results were compared with the actual surface condition, characterized by transmission electron microscopy and local measurements. The results verified that the time constant at the high frequency range originates from the barrier properties of the surface film.

A Technique to Determine the Designable Inductance Value(s) in Wide-Band CMOS-Amplifier System Employing Shunt-Peaking

This paper presents a technique towards obtaining an estimate of the value of inductor(s) to expand the bandwidth of operation in a complementary metal-oxide semiconductor (CMOS) amplifier system which exploits shunt peaking principle. The basic principle is placement of the zeros of the transfer function in an interleaved manner relative to the uncompensated RC time-constant frequency (TCF) and the band-edge frequency (BEF) (i.e., product of the poles) of the transfer function. Application of the analytical results has been demonstrated for (i) a common-gate (CG) amplifier stage in a 0.18-μm CMOS process and (ii) an inter-stage inductor coupling network which serves as an interface between two amplifier stages. MATLAB simulation has been used to obtain the range of design inductance values. The TSMC 180-nm CMOS process has been used in Cadence CAD environment to validate the theoretical predictions. The inductors laid out have been modeled using the ASITIC program to obtain more realistic results. The proposed technique provides a bandwidth extension of the CMOS common-gate amplifier from 6.68 GHz to 10.4 GHz with 1 dB peaking using only a 1.85-nH inductor. For the inter-stage coupling network, the suggested design procedure leads to a bandwidth extension ratio (BWER) exceeding three, with less than 3-dB ripple.

Inhibitive effect of Ce(III) and La(III) cations for AA2219 aluminium alloy corrosion in sodium chloride medium

In this paper, the rare earth chlorides were used to mitigate the dealloying of Al2Cu intermetallic particles in aluminium alloy 2219-T87 plate exposed to 0.6 M NaCl solution. The negative shift of cathodic branches of potentiodynamic polarisation curves revealed the cathodic inhibition towards intermetallic dealloying. The low and high frequency time constants obtained from electrochemical impedance spectra revealed an increase in the corrosion resistance due to precipitation of rare earth oxides/hydroxides on the porous surface of native oxide film of aluminium alloy. Scanning electron microscopy coupled with energy dispersive X-ray analysis and atomic force microscopy revealed that intermetallic inclusions are the sites for the formation of cerium and lanthanum deposits which suppress copper redeposition and minimise galvanic interactions between the matrix and Al2Cu particles.

Carbon Dioxide and Fruit Odor Transduction in Drosophila Olfactory Neurons. What Controls their Dynamic Properties?

We measured frequency response functions between odorants and action potentials in two types of neurons in Drosophila antennal basiconic sensilla. CO2 was used to stimulate ab1C neurons, and the fruit odor ethyl butyrate was used to stimulate ab3A neurons. We also measured frequency response functions for light-induced action potential responses from transgenic flies expressing H134R-channelrhodopsin-2 (ChR2) in the ab1C and ab3A neurons. Frequency response functions for all stimulation methods were well-fitted by a band-pass filter function with two time constants that determined the lower and upper frequency limits of the response. Low frequency time constants were the same in each type of neuron, independent of stimulus method, but varied between neuron types. High frequency time constants were significantly slower with ethyl butyrate stimulation than light or CO2 stimulation. In spite of these quantitative differences, there were strong similarities in the form and frequency ranges of all responses. Since light-activated ChR2 depolarizes neurons directly, rather than through a chemoreceptor mechanism, these data suggest that low frequency dynamic properties of Drosophila olfactory sensilla are dominated by neuron-specific ionic processes during action potential production. In contrast, high frequency dynamics are limited by processes associated with earlier steps in odor transduction, and CO2 is detected more rapidly than fruit odor.

An in situ electrochemical impedance spectroscopy/synchrotron radiation grazing incidence X-ray diffraction study of the influence of acetate on the carbon dioxide corrosion of mild steel

A combination of electrochemical impedance spectroscopy (EIS) and in situ synchrotron radiation grazing incidence X-ray diffraction (SR-GIXRD) has been used to study the influence of acetate on the carbon dioxide corrosion of mild steel. The SR-GIXRD data demonstrated that normal corrosion – in a carbon dioxide saturated brine – induced the formation of a thick corrosion scale of Fe 2(OH) 2CO 3 and Fe 2O 2CO 3, and this totally obscured the α-Fe diffraction peaks of the underlying steel substrate after 24 h. On the other hand, the carbon dioxide corrosion of mild steel in the presence of acetate also detected the Bragg diffraction peaks for Fe 2(OH) 2CO 3 and Fe 2O 2CO 3; however, the α-Fe diffraction peaks of the underlying steel substrate were not extinguished with time, and there was a reversal in the pattern of evolution of the intensities of the Fe 2(OH) 2CO 3 and Fe 2O 2CO 3 phases in acetate. Accordingly, the EIS data showed a poorly defined medium frequency time constant for the corroded steel specimen in brine spiked with acetate, and this medium frequency time constant was extinguished as a function of time. Alternatively, EIS of the corroded specimen also revealed a medium frequency time constant after 24 h. In addition, EIS complex–plane impedance plots showed that the corroded electrode had become passivated in an acetate-spiked brine, as evidenced by a three-fold enhancement in the charge transfer resistance at low frequency. These EIS/SR-GIXRD outcomes suggest that acetate affects the crystallization chemistry of the Fe 2(OH) 2CO 3/Fe 2O 2CO 3 corrosion scale, and this causes a mild passivation of the corroded steel surface.

Microstructural and electrochemical characterization of environmentally friendly conversion layers on aluminium alloys

Cerium conversion layers (CeCL) have been investigated as a replacement for chromium conversion layers to protect Al alloys against corrosion. In this work the microstructure and the electrochemical behaviour of aluminium alloy 2024 with and without CeCL were investigated using, respectively, SEM-EDX and EIS. EDX results have shown that the presence of dispersed plated Cu particles on the alloy surface enhances the formation of the CeCL increasing the intensity of Ce peaks in the EDX spectra. EIS measurements on conversion-coated samples have shown that the presence of the layer increases the impedance, and that its presence is detected by the presence of a high frequency time constant. Results of potentiodynamic experiments have shown that the corrosion protection afforded by the conversion layer is due to the hindrance of the oxygen reduction reaction and that the pitting potential of the alloy is not changed.

An in vitro study of Ti and Ti-alloys coated with sol–gel derived hydroxyapatite coatings

Electrochemical–corrosion properties of Ti, Ti6Al6Nb and Ti6Al4V implant materials coated with calcium phosphate (CaP) coatings were investigated in vitro at the open circuit potential using non-destructive electrochemical impedance spectroscopy (EIS) technique in simulated physiological-Hank's Balanced Salt Solution (HBSS). Characterization of the structure and composition of CaP coatings obtained by the sol–gel method was performed using X-ray diffraction analysis. The interpretation of the EIS results was based upon a two-layer model of the surface film consisting of an inner barrier TiO 2 (CaTiO 3) layer and the outer porous hydroxyapatite (HAP)+β-tricalcium phosphate (β-TCP) layer, described by using the high frequency time constant (τ 1 HF) and the low frequency time constant (τ 2 LF), respectively. Spontaneously passivated surfaces of naturally grown surface films on all three substrates in HBSS were used for reference. The chemical composition of the substrate as well as the composition and crystallinity of the CaP coating have a direct influence on electrochemical–corrosion properties of the implant materials under in vitro conditions. Well-crystallized HAP and β-TCP exhibited a beneficial corrosion protection effect on the substrate during prolonged exposure to HBSS. In the case of poorly crystallized coatings containing a greater amount of carbonate incorporated in the structure of HAP, the total impedance of the system markedly changed with prolonged exposure to physiological solution due to degradation of the coating.

Investigation of the electrical properties of SnO2 varistor system using impedance spectroscopy

The electrical properties of tin oxide varistors doped with CoO, Nb2O5, and Cr2O3, were investigated using the impedance spectroscopy technique with the temperature ranging from 25 to 400 °C. The impedance data, represented by means of Nyquist diagrams, show two time constants with different activation energies, one at low frequencies and the other at high frequencies. These activation energies were associated with the adsorption and reaction of O2 species at the grain boundary interface. The Arrhenius plots show two slopes with a turnover at 200 °C for both the higher and lower frequency time constants. This behavior can be related with the decrease of minor charge carrier density. The barrier formation mechanism was associated with the presence of CrSn⋅ at the surface, which promotes the adsorption of the O′ and O″ species which are in turn proposed as being responsible for the barrier formation.

Impedance analysis of surface modified Ga 0.5In 0.5P—aqueous electrolyte interface

In this work electrochemical impedance spectroscopy was carried out on pre- and post-etched Ga 0.5In 0.5P (1.8–1.9 eV) as well as quinolinol-modified surfaces, over seven decades of frequencies. The spectra were examined in the Bode as well as complex impedance planes and deconvoluted using nonlinear-least-squares fitting to obtain equivalent circuits that isolated the space charge from the surface states. Chemical wet-etching of Ga 0.5In 0.5P was found to remove surface states as evidenced in the removal of the low frequency time constant from the spectra. Treatment with 8-quinolinol was found to shift the flat-band potential in the positive direction vs. she by about 300mV in near neutral electrolytes.

Underpotential Deposition of Copper on Gold and the Effects of Thiourea Studied by AC Impedance

This paper reports ac impedance and cyclic voltammetry studies of underpotential deposition (UPD) of Cu on Au in an electrolyte composed of and . A theoretical kinetic model was developed which fits the experimental impedance spectra over frequencies ranging from 0.001 to 20,000 Hz. Spectra are characterized by a two‐time constant process; two capacitive loops were observed in the Nyquist diagrams corresponding to two distinctive peaks in the Bode phase plot. The high frequency time constant is associated with the double‐layer capacitance, while the low frequency process is due to relaxation phenomena associated with the charge transfer involved in Cu2+ electrosorption. Thiourea is one of the most important organic additives for commercial plating baths. Therefore, the coadsorption of thiourea and UPD Cu on Au was also studied by ac impedance. Adsorbed thiourea was found to have a dramatic effect on the high frequency peak in the Bode phase plot. Measurements such as these are proposed as a means of determining organic additive concentrations in Cu plating and electroforming baths.

Distortion of non-invasive cardiac pulse curves. A capillary-damped pick-up and a calibration unit forapex cardiograms and other pulse curves.

A calibration unit and a capillary-damped pick-up, of funnel type, intended for apex cardiograms and other non-invasive cardiac pulse curves are presented. The physical properties of a recording system, suitable for non-invasive cardiac diagnostic techniques, are reviewed. In 4 identical recording units consisting of a funnel with and without capillary damping, connected via air transmission to a transducer, the overamplification in the underdamped systems varied between 2-4 and 4-3, and in the capillary-damped systems between 1-1 and 1-2. The capillary damping gave an approximately linear relation between phase shift and frequency in the high frequency range with a calculated delay of about 4ms. The low frequency time constant in the systems varied between 1-9 and 4-6 s, depending on the amplifier setting used, which for sine curves of frequency 0-5 and 2 Hz means an apparent prematurity for their maxima and minima varying between 53 and 1 ms, this being larger with lower frequency time constants and lower frequency curves.

A simple room temperature detector for use with carbon dioxide lasers

It is shown that pyroelectric thermal receivers may be useful for laboratory studies involving CO2 lasers. A detector of this type using a commercially available ceramic disk capacitor has been constructed and has been found to have a high frequency time constant of better than 5 ?sec. This detector may be used to measure (i) the energy in a Q-switched pulse, (ii) the time-dependent intensity of radiation from a pulsed laser discharge and (iii) the intensity of radiation from a continuous wave laser. In the last case, this detector is linear with respect to incident radiation flux over the range 0-60 w cm-2 and gives an output of 1.3MVW-1 cm-2 when shunted with a 1 M? resistor.

A fast-response pyroelectric thermal detector

A thermal radiation detector having a high frequency time constant of 5 μsec (which can easily be improved to better than 1 μsec) is described. The instrument has been used to measure the radiation from a high temperature gas discharge. In the detector a pyroelectric output is obtained when radiation is absorbed on to a piece of ferroelectric material (barium titanate) whose thickness is greater than the thermal diffusion depth for the length in time of the radiation pulse being measured. The electrical signal is examined theoretically and experimentally and is shown to be directly related to either the incident energy or flux, and it is also shown that the upper frequency response is limited by mechanical resonances of the ferroelectric (engendered by thermal expansion) rather than by a thermal time constant. This device is very convenient since it is simple to construct and requires no auxiliary current supply.