Complex Impedance Measurements Research Articles

Experimental Investigations on the Electrical Conductivity and Complex Dielectric Permittivity of ZnxMn1−xFe2O4 (x = 0 and 0.4) Ferrites in a Low-Frequency Field

Two samples of ZnxMn1−xFe2O4 (x = 0, sample A; and x = 0.4, sample B) were synthesized by the hydrothermal method. From complex impedance measurements in the range 100 Hz–2 MHz and for temperatures T between 30 and 130 °C, the barrier energy between localized states ΔErelax was determined for the first time in these samples. For sample B, a single value of ΔErelax was highlighted (0.221 eV), whilst, for sample A, two values were obtained (0.012 eV and 0.283 eV, below 85 °C and above 85 °C, respectively), associated with two zones of different conductivities. Using the Mott’s VRH model and the CBH model, we determined for the first time both the bandgap energy barrier (Wm) and the hopping (crossover) frequency (ωh), at various temperatures. The results show that, for sample A, Wm has a maximum equal to 0.72 eV at a temperature between 70 and 80 °C, whilst, for sample B, Wm has a minimum equal to 0.28 eV at a temperature of 60 °C, the results being in good agreement with the temperature dependence of the static conductivity σDC(T) of the samples. By evaluating σDC and eliminating the conduction losses, we identified, using a novel approach, a dielectric relaxation phenomenon in the samples, characterized by the activation energy EA,rel. At various temperatures, we determined EA,rel, which ranged from 0.195 eV to 0.77 eV. These results are important, as understanding these electrical properties is crucial to various applications, especially in technologies where temperature variation is significant.

The X-ray peak profiling, optical and dielectric properties of Ag@ZnFe2O4/rGO ternary nanocomposites: LED assisted photocatalysis and humidity sensing

Ag@ZnFe2O4/reduced graphene oxide (Ag@ZnFe2O4/rGO) ternary nanocomposites were successfully synthesized (NCs) via hydrothermal method and explored the LED assisted photocatalytic performance for the degradation of organic dyes as well as humidity sensing properties of the material. For comparison, bare ZnFe2O4, and ZnFe2O4/rGO also synthesized. The freshly prepared samples were investigated using XRD, TEM, UV–vis–NIR spectroscopy, VSM, Impedance analyzer, and particle size analyzer. The detailed structural parameters were calculated through X-ray peak profiling using Classical Scherrer, Modified Scherrer, Williumson-Hall, Halder-Wagner, Size Strain Plot methods along with Rietveld refinement. From UV–vis–DRS analysis, optical energy gaps were obtained in the range of 1.58–1.92 eV. Ag@ZnFe2O4/rGO NCs showed significant photocatalytic activity and exceptional adsorption capacities for the breakdown of methylene blue (MB) 95.7% and rhodamine B (RhB) 77.7% after 100 min. The synergistic interactions of Ag, ZnFe2O4, and rGO minimized NCs agglomeration and small particle size with higher surface area, resulting in a higher absorption in both UV and visible light. The photocatalysis mechanism was explored by scavenger test using ascorbic Acid, NaHCO3, and isopropyl alcohol. The magnetic studies revealed the paramagnetic nature of NCs. Frequency dependent dielectric data obey the modified Debye model with relaxation time of ∼10−4 s and spreading factor of 0.534 (±0.08). Complex impedance measurements support the grain-interior process that contributes to dielectric properties. The impedance-type humidity sensing characteristics of the fabricated sensor were investigated by exposing in a wide humidity range of 11–98% RH at ambient temperature. The Ag@ZnFe2O4/rGO sensor displayed an ultrahigh sensitivity when compared to traditional humidity sensors. On the basis of, Ag@ZnFe2O4/rGO NCs are exciting nanomaterials for outstanding photocatalytic activity for the degradation of organic dyes along with potent humidity sensor.

The Effect of Magnetically Induced Local Structure and Volume Fraction on the Electromagnetic Properties of Elastomer Samples with Ferrofluid Droplet Inserts

The magnetic permeability (μ), dielectric permittivity (ε) and electrical conductivity (σ) of six elastomer samples obtained by mixing silicone rubber (RTV-530) with a kerosene-based ferrofluid in different volume fractions (φ), 1.31%, 2.59% and 3.84%, were determined using complex impedance measurements over a frequency range of 500 Hz–2 MHz. Three samples (A0, B0 and C0) were manufactured in the absence of a magnetic field, and the other three samples (Ah, Bh and Ch) were manufactured in the presence of a magnetic field, H = 43 kA/m. The component μ″ of the complex effective magnetic permeability of all samples presents a maximum at a frequency, fmax, that moves to higher values by increasing φ, with this maximum being attributed to Brownian relaxation processes. The conductivity spectrum, σ (f), of all samples follows the Jonscher universal law, which allows for both the determination of the static conductivity, σDC, and the barrier energy of the electrical conduction process, Wm. For the same φ, Wm is lower, and σDC is higher in the samples Ah, Bh and Ch than in the samples A0, B0 and C0. The performed study is useful in manufacturing elastomers with predetermined properties and for possible applications such as magneto-dielectric flexible electronic devices, which can be controlled by the volume fraction of particles or by an external magnetic field.

The effect of heterogeneous contact impedances on complex resistivity measurements

SUMMARY Spectral induced polarization (SIP) and electrical impedance tomography (EIT) are advanced approaches for structural characterisation of soils and rocks based on measurements of the frequency-dependent complex electrical resistivity. Accurate SIP and EIT measurements require the reduction of electrode effects, which have often been attributed to the polarization of metal electrodes in the current pathway. In this study, we demonstrate an alternative electrode effect that influences the accuracy of complex impedance measurements. This alternative electrode effect is caused by a heterogeneous distribution of the contact impedance, which can occur due to ageing or corrosion of the electrode. First, theoretical considerations are given to explain how this electrode effect influences the accuracy of complex impedance measurements. Next, we demonstrate using SIP and EIT measurements that this alternative mechanism better explains the observed electrode effects than previous explanations based only on the presence of metal. The SIP measurements were made on a water-filled container with a metal sphere between two non-polarizing electrodes, which showed that the mere presence of metal in the current pathway only leads to small electrode effects that are typically well below 1 mrad even for small electrode spacings. When the same metal sphere was used as a potential electrode, the observed electrode effect was about 2.5 times larger and varied with rotation. These changes can be explained by a heterogeneous contact impedance resulting in a varying electrode voltage. The EIT measurements were performed with a cylindrical sample holder filled with water. They are used to illustrate how the presented theoretical considerations can be used to investigate the magnitude of the electrode effects for different electrode configurations. We conclude that electrode effects are important to consider for laboratory SIP and EIT measurements and field measurement with closely spaced electrodes.

Self-Powered, Highly Sensitive, and Flexible Humidity Sensor Based on Carboxymethyl Cellulose for Multifunctional Applications.

Self-powered humidity sensors based on friction electric and piezoelectric principles have been proposed in recent years. However, the complicated structure and preparation processes usually involved in these power generation humidity sensors limit their wider application. Herein, we developed a self-powered flexible humidity sensor with a simple structure and facile preparation process based on the primary battery principle. The self-powered humidity sensor consists of copper conductive tape as the positive electrode, nickel conductive tape as the negative electrode, and a carboxymethyl cellulose film dissolved with lithium chloride and sodium chloride as the sensing layer. The sensor exhibits good sensing linearity (R2 = 0.99791) in a wide relative humidity range (11-95%) with a satisfying response voltage of 41 mV (RH 95%) and excellent flexibility. Furthermore, the conduction mechanism of the sensing film was investigated by the complex impedance and phase angle-frequency spectral measurement and analysis. Multifunctional applications of human respiration monitoring to determine the respiratory rate and type, noncontact sensing, diaper and soil moisture detection, and power generation were demonstrated. The low-cost and facile preparation method in this work could provide a useful strategy for developing a self-powered, flexible, and multifunctional humidity sensor.

Fuel cell type SO2 sensor based on Cu/CF-N sensing electrode and Nafion proton conductor at room temperature

In this work, nitrogen doped carbon fiber loaded copper (Cu/CF-N) sensing electrode and Nafion proton conductor are used to fabricate fuel cell type gas sensors that realize fast response/recovery for SO2 detection at room temperature. This is the first time that this type of sensor has been used to detect SO2. We investigated the effect of Cu content on sensors’ performance, and found that the Cu (20 wt%)/CF based sensor exhibited the highest response. This can be attributed to the optimal Cu content, which exposes the most active sites. Moreover, nitrogen was doped into electrode material to further improve the performance. Nitrogen atoms enhance the conductivity of the material and its ability to adsorb SO2. These are confirmed by the complex impedance measurement and TPD analysis. The upgraded Cu/CF-N fuel cell type gas sensor showed a response of − 435.6 nA to 50 ppm SO2 and a low detection limit of 500 ppb. Significantly, the Cu/CF-N sensor demonstrated short response time (44 s) and recovery time (87 s) at room temperature. Furthermore, with the advantages of no power consumption, excellent selectivity and good repeatability, the fuel cell type gas sensors have broad prospects in the field of room temperature gas detection.

Carrier transport and Negative differential resistance of electrically bipolar devices based on poly(3,4-ethylene-dioxythiophene): Poly (styrene sulfonate) film

A poly (3,4-ethylene-dioxythiophene): poly (styrene sulfonate) (PEDOT: PSS) film sandwiched between copper electrodes was used to create an electrically bipolar device using a conventional casting technique. X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy-Dispersive X-ray spectroscopy (EDX) were used to investigate the structure, morphological behaviour, and homogeneous distribution of PEDOT conducting grains in the insulator matrix PSS. The device exhibits normal hysteretic electrical behaviour and a negative differential resistance (NDR) effect in the swept current-voltage (I–V) curves. By varying the PEDOT:PSS films' positive charge voltage and charging rate, the NDR effect may be tuned. A maximum current ratio of 103 can exist between the High Conducting State (HCS) and the Low-Conducting State (LCS). Complex impedance measurements of the Cu/PEDOT:PSS/Cu device were carried out at applied ac-voltage amplitude, Vac, range from 0.1 V to 2 V and over the frequency range from 100 Hz to 10 MHz at room temperature. Different models based on ac and dc experimental data are used to characterize and confirmed the carrier transport processes in the HC, LC-States and NDR.

High-temperature impedance properties of Sr1-xCaxTiO3 ceramics for lead-free capacitor applications

ABSTRACT Lead-free Sr0.6Ca0.4TiO3 ceramic was prepared by solid-state reaction route. X-ray diffraction technique showed the phase purity and identified the orthorhombic perovskite structure. Scanning Electronic Microscopy observation evidenced homogeneous morphology and dense microstructure. Electrical conduction and dielectric relaxation mechanisms at high temperature have been studied by complex impedance measurement techniques over a wide range of frequencies and temperatures, from 100 Hz to 1 MHz and from 573 K to 783 K. Fitting of impedance measurements allowed us to study grains and grain boundaries electrical characteristics and their contribution to the conduction and dielectric relaxation processes in the ceramic. Electrical conduction in grains is due to oxygen vacancy jumps for temperatures below 733 K and to polarons above 763 K. Electrical DC conductivity in grains and grain boundaries is mainly due to oxygen vacancies. Impedance measurements and relaxation time distribution function calculations show non-Debye-type relaxation effects for both grains and grain boundaries.

Atomically Thin 2D TiO2 Nanosheets with Ligand Modified Surface for Ultra-sensitive Humidity Sensor

As a kind of two-dimensional (2D) nanostructured materials, metal oxide nanosheets (MONS) are attractive and promising humidity sensing materials due to their considerable surface area, good charge carrier transportation, and designable surface functional groups properties. Nevertheless, the ultra-thin MONS modified with active functional groups for humidity sensing are still rare. As a proof of concept, the atomically thin TiO2 nanosheets with high surface area and electron-donating amino groups are prepared by a structure-maintained post-ligand modification strategy. The fabricated TiO2-based sensors demonstrate superior humidity sensing performance with high response, short response time, narrow hysteresis, and ultra-low theoretical limit of detection of about 15 ppm. Additionally, the possible mechanism is proposed from the AC complex impedance measurements and DC instantaneous reverse polarity experiments. This work provides a possible path for developing the high-performance 2D nanostructured metal oxides-based humidity materials through the surface chemical method.

Frequency response analysis of a toroidal field coil of the divertor tokamak test facility

The magnet system of the Divertor Tokamak Test (DTT) facility is mainly made up with superconducting coils. During the operation of the DTT machine, high voltages will appear across each Toroidal Field Coil (TFC) when fast current will ramp down; the reason why, a reliable electrical insulation is required for the operation of the TFC system. With the aim of checking the correct sizing and implementation of the electrical insulation, different DC and AC electrical tests will be performed during all the coil manufacturing process. Among these, the impedance spectrum test, or rather the complex impedance measurement over several decades of frequency range, can be used to analyze the frequency response of the coil. The variations of the resonance frequency (f0), during its electrical acceptance test, can be useful to make predictions about the detectability of internal failure conditions in a coil. This paper focuses on analysis of the frequency response of a TFC, using a numerical simulation approach. The goal is to assess the impedance spectrum of a TFC within a fixed frequency band by a frequency-dependent lumped network. Starting from the TFC layout, a WP with the casing has been modeled by a complex network of lumped parameters in Ansys 2021/R2 environment. The data (amplitude and phase angle vs frequency) obtained have been used to study the f0 variation during the different manufacturing stage of the WP. This model will be able to make predictions about the detectability of internal short-circuit in a TFC.

Investigations on the electrical conductivity and complex dielectric permittivity of a ferrofluid subjected to the action of a polarizing magnetic field

From complex impedance measurements of a kerosene-based ferrofluid with magnetite particles, in the range 20 Hz - 2 MHz and at different values of a polarizing magnetic field, H in the range 0 - 90 kA/m, electrical conductivity and complex dielectric permittivity were determined. The conductivity mechanism in ferrofluid was explained using both the Mott's VRH model and CBH model, thus allowing us to determine for the first time the effect of H on the hopping distance of charge carriers, on maximum energy barrier height and on the hopping frequency. The dielectric behavior is explained in terms of Schwarz relaxation process, determining that the mechanical mobility of ions on particle surface increases by increasing of H. Also, an original method was proposed to evaluate the effect of H on the dielectric activation energy, EA(H) finding a decrease of it by approximately 30 meV by increasing of H by 86.37 kA/m.

Microwave volt-impedance spectroscopy of semiconductor structure

Microwave voltage-impedance spectroscopy is used to study a semiconductor structure in the form of a doped n-GaAs film grown on a conducting n^+-GaAs substrate with a buffer sublayer. A system of concentric barrier contacts is formed on the structure surface. A technique has been developed for measuring complex impedance spectrum Z(f,U) of the sample as a function of DC bias voltage U. Spectra Z(f,U) were measured using a Cascade Microtech probe station in the frequency range 0.01-40 GHz with a lateral resolution of 15-30 μm at U=0-10 V. The main electrophysical characteristics of a semiconductor film were determined from the spectra: type, concentration and mobility of free charge carriers, electrical conductivity. An excess resistance was found in the range f=0.1-20 GHz. This effect is interpreted as the deep states (traps) recharging for two types of traps --- low-frequency l and high-frequency h with characteristic time tau_l=10-9 s, tau_h=4.2·10-11 s. A model description is proposed that explains the characteristic shape of the trap resistance spectrum, its dependence on the contact area and voltage U. Keywords: microwave band, near field, impedance, semiconductor, barier contact, deep states, electrophysical characteristics.

Микроволновая вольт-импедансная спектроскопия полупроводниковой структуры

Microwave voltage-impedance spectroscopy is used to study a semiconductor structure in the form of a doped n-GaAs film grown on a conducting n+-GaAs substrate with a buffer sublayer. A system of concentric barrier contacts is formed on the structure surface. A technique has been developed for measuring complex impedance spectrum Z(f,U) of the sample as a function of DC bias voltage U. Spectra Z(f,U) were measured using a Cascade Microtech probe station in the frequency range 0.01 – 40 GHz with a lateral resolution of 15 – 30 μm at U = 0 – 10 V. The main electrophysical characteristics of a semiconductor film were determined from the spectra: type, concentration and mobility of free charge carriers, electrical conductivity. An excess resistance was found in the range f = 0.1 – 20 GHz. This effect is interpreted as the deep states (traps) recharging for two types of traps – low-frequency l and high-frequency h with characteristic time τl = 10^(-9) s, τh = 4.2∙10^(-11) s. A model description is proposed that explains the characteristic shape of the trap resistance spectrum, its dependence on the contact area and voltage U.

Temperature Dependence of the Electrical Properties of Na2Ti3O7/Na2Ti6O13/POMA Composites.

The temperature dependence of the electrical properties of composites formed by biphasic sodium titanate and poly(o-methoxyaniline) (Na2Ti3O7/Na2Ti6O13/POMA) with different concentrations of POMA (0%, 1%, 10%, 15%, 35% and 50%) in the ceramic matrix was determined from measurements of complex impedance. The structural details were studied by means of X-ray diffraction, confirming the formation of the Na2Ti3O7/Na2Ti6O13/POMA composites. The displacement of the (200) reflection from 2θ = 10.45° to 11.15° in the composites with 10 and 15% of POMA suggested the partial replacement of H+ for Na+ in the Na2Ti3O7 structure. The thermal properties were investigated by Thermogravimetry and Differential Thermal Analysis. The Thermogravimetry curves of the composites with POMA content of 1, 10 and 15% presented profiles similar to that of pure sodium titanate sample. The composites with 35 and 50% of POMA showed a process at temperatures around 60–70 °C, which was associated with water absorbed by the polymer. The analysis of the complex impedance spectroscopy measurements revealed that the electrical resistivity of the composites in the range from 0 to 35% increased by two orders of magnitude, with different values for each concentration. This positive temperature coefficient of resistivity was less noticeable in the composite with highest POMA mass content (50%). The rapid increase in resistivity caused an increase in the relaxation time calculated from the time domain. The electrical response of the 50% of POMA compound changes in relation to what was observed in the other compounds, which suggests that there is a saturation limit in the increase in resistivity with POMA content.

Experimental study of wetting effect of surfactant based on dynamic wetting process and impedance response of coal.

Surfactant can improve the wettability of water to coal, which is beneficial to reduce the production of coal dust in coal seam water injection. Through the measurement and calculation of contact angle and its decay rate, the wettability differences of SDS (C12H25OSO3Na), AES (C14H29O5NaS), OP-10 (C18H30O10), and JFC (RO(CH2CH2O)nH) to anthracite were compared. In addition, the wetting modification effect and infiltration rate of anthracite by water, AES, and OP-10 were studied by infrared spectroscopy and complex impedance monitoring of coal pillar immersion process. The results show that when the concentration of surfactant is 0.1%, the contact angle decay time of OP-10 is very short, and the contact angle decay rate is as high as 19°/s. The decay rate can more obviously reflect the wettability difference of surfactants. And the wetting modification effect of OP-10 on anthracite is stronger than that of AES, and the peaks of oxygen-containing functional groups such as carboxyl and hydroxyl groups are stronger. Furthermore, the capillary force between OP-10 and anthracite is much larger than that of water, which shows the characteristics of fast water absorption and wide distribution in the infiltration experiment of columnar coal. The results of complex impedance measurement indicate that the impedance decay rate of coal is well correlated with capillary rise factor FC, contact angle decay rate, and contact angle. It is hoped that the research results can provide help for coal seam water injection and dust prevention.

The Effect of Bi2O3 and Fe2O3 Impurity Phases in BiFeO3 Perovskite Materials on Some Electrical Properties in the Low-Frequency Field

Pure bismuth ferrite (BFO) and BFO with impurity phases (Bi2O3 or Fe2O3) were synthesized by the hydrothermal method. Complex dielectric permittivity (ε) and electrical conductivity (σ) were determined by complex impedance measurements at different frequencies (200 Hz–2 MHz) and temperatures (25–290) °C. The conductivity spectrum of samples, σ(f), complies with Jonscher’s universal law and the presence of impurity phases leads to a decrease in the static conductivity (σDC); this result is correlated with the increased thermal activation energy of the conduction in impure samples compared to the pure BFO sample. The conduction mechanism in BFO and the effect of impurity phases on σ and ε were analyzed considering the variable range hopping model (VRH). Based on the VRH model, the hopping length (Rh), hopping energy (Wh) and the density of states at the Fermi level (N(EF)) were determined for the first time, for these samples. In addition, from ε(T) dependence, a transition in the electronic structure of samples from a semiconductor-like to a conductor-like behavior was highlighted around 465–490 K for all samples. The results obtained are useful to explain the conduction mechanisms from samples of BFO type, offering the possibility to develop a great variety of electrical devices with novel functions.

Enhanced rate performance and specific capacity in Ti-substituted P2-type layered oxide enabled by crystal structure and particle morphology modifications

In recent years, P2-type layered oxides have received considerable attention as potential cathodes for Na-ion batteries owing to their compositional diversity, good specific capacity, and cyclability. However, poor rate performance and low-capacity retention at high discharge rates have limited their use in commercial battery applications. This study aims to mitigate this issue by synthesizing a series of structurally engineered P2-type cathode materials, through Ti substitution, with high cyclability and improved rate performance. P2-type Na0.70Ni0.20Cu0.15Mn(0.65-x)TixO2 were prepared through a sol–gel route and were characterized for their structural, electrical, and electrochemical properties In the Ti-substituted samples, the Rietveld refinement of XRD data revealed an increased size of the bottleneck area of the Na-O6 prism planes, while the SEM images showed a decrease in the aspect ratio of hexagon-type morphology of particles which facilitates faster Na-ion conduction through the material. These changes in the crystal structure and particle morphology induced by Ti substitution have significantly improved electrical and electrochemical performance compared to the parent material. The sample with x = 7.5% exhibited a specific capacity of 126 mAh/g at a discharge rate of 0.1C in the 2.00–4.25 V window, which was about 25% more than that of the undoped material. At a discharge rate of 1C, the specific capacity of Na0.70Ni0.20Cu0.15Mn0.575Ti0.075O2 was 97 mAh/g compared to 74 mAh/g for Na0.70Ni0.20Cu0.15Mn0.65O2 sample. Na0.70Ni0.20Cu0.15Mn(0.65-x)TixO2 samples also exhibited excellent cyclability, with over 95% of original capacity retained after 300 cycles. Complex impedance measurements corroborated the improved Na-ion conductivity in Ti-doped samples and an associated increase in Na-ion transference number from 0.86 for the sample with x = 0 to 0.97 for the sample with x = 0.075.

Relationship Between the Conductivity of Human Blood and Blood Counts

Achieving a better characterization of human blood conductivity is of high relevance for medical applications. In this study we measured the complex impedance of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> = 10 human whole blood samples (from <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> = 10 oncology patients) at room temperature ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> = 22.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm\; 0.8\;^\circ \mathrm{C}$</tex-math></inline-formula> ) and at body temperature ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> = 36.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm\; 0.4\;^\circ \mathrm{C}$</tex-math></inline-formula> ). The complex impedance was measured using the measurement setup consisting of a custom made four-electrode probe and a commercially available galvanostat. The measured complex impedance data were used to calculate the conductivity of whole blood over the 631 Hz–100 kHz frequency range. The calculated conductivity data is presented and was compared with the literature data. The data from our study is in good agreement with the data available in the literature. Additionally, full blood counts were provided for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> = 8 samples and Pearson correlation coefficient was calculated between the conductivity and blood counts at different frequencies. The three blood count parameters with the highest correlation coefficient are haematocrit (Hct), haemoglobin (Hgb) and red cell count (RBC). The correlation coefficient was shown to decrease as the frequency increases and was the highest at <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> = 631 Hz, which is the lowest reported frequency. To our knowledge this is the first study to measure low-frequency (i.e. below 1 MHz) conductivity of whole human blood at body temperature using the four-electrode technique. The results of this study represent an important contribution to the literature, which is currently limited in this area and will help further medical device design.

Advanced Electrochemical Impedance Spectroscopy of Industrial Ni-Cd Batteries

Advanced electrochemical impedance spectroscopy (EIS) was applied to characterize industrial Ni-Cd batteries and to investigate the electrochemical redox processes. A two-term calibration workflow was used for accurate complex impedance measurements across a broad frequency range of 10 mHz to 2 kHz, resulting in calibrated resistance and reactance values. The EIS calibration significantly improved the measurements, particularly at high frequencies above 200 Hz, with differences of 6–8% to the uncalibrated impedance. With an electromagnetic finite element method (FEM) model, we showed that the impedance is strongly influenced by the cable fixturing and the self-inductance of the wire conductors due to alternating currents, which are efficiently removed by the proposed calibration workflow. For single cells, we measured the resistance and the reactance with respect to the state-of-charge (SoC) at different frequencies and a given rest period. For Ni-Cd blocks that include two cells in series, we found good agreement of EIS curves with single cells. As such, EIS can be used as a fast and reliable method to estimate the cell or block capacity status. For electrochemical interpretation, we used an equivalent electric circuit (EEC) model to fit the impedance spectra and to extract the main electrochemical parameters based on calibrated EIS, including charge-transfer kinetics, mass transport, and ohmic resistances. From the charge-transfer resistance, we computed the exchange current density, resulting in 0.23 A/cm2, reflecting high intrinsic rates of the redox electron transfer processes in Ni-Cd cells.

Development of a transient complex impedance measurement device used in quasi-isentropic compression experiments.

A complex impedance measurement device with a short response time and high noise immunity is presented in this paper. The device based on a radio-frequency reflectometer was specially developed for electro-physical property investigations of materials in quasi-isentropic compression experiments. The maximum operating frequency of the device is up to 600MHz for reducing intense low-frequency noises. Meanwhile, an off-line signal processing code was developed to improve the response time of the device to less than 10ns. Using the device, the complex impedance and electrical conductivity of water compressed by an explosive-driven magnetic flux compression generator were measured, and an abrupt change in the complex impedance of water caused by a liquid-solid transition was directly observed under intense electromagnetic interference.