High Frequency Conductivity Research Articles

Increased extra-neurite conductivity of brain in patients with Alzheimer's disease: A pilot study

The objectives of this study were to investigate how the extra-neurite conductivity (EC) and intra-neurite conductivity (IC) were reflected in Alzheimer's disease (AD) patients compared with old cognitively normal (CN) people and patients with amnestic mild cognitive impairment (MCI) and to evaluate the association between those conductivity values and cognitive decline. To do this, high-frequency conductivity (HFC) at the Larmor frequency was obtained using MRI-based electrical property tomography (MREPT) and was decomposed into EC and IC using information of multi-shell multi-gradient direction diffusion tensor images. This prospective single-center study included 20 patients with mild or moderate AD, 25 patients with amnestic MCI, and 21 old CN participants. After decomposing EC and IC from HFC for all participants, we performed voxel-based and regions-of-interest analyses to compare conductivity between the three participant groups and to evaluate the association with either age or the Mini-Mental State Examination (MMSE) scores. We found increased EC in AD compared to CN and MCI. EC was significantly negatively associated with MMSE scores in the insula, and middle temporal gyrus. EC might be used as an imaging biomarker for helping to monitor cognitive function.

Mapping High‐Frequency Conductivity Using MRI in the Brains of Alzheimer’s Disease Patients

AbstractBackgroundPrevious studies reported increased concentrations of metallic ions and imbalanced Na+ and K+ ions in patients with Alzheimer’s disease (AD) and the increased mobility of protons by microstructural disruptions in AD. The purposes of this study were: 1)to apply a high‐frequency conductivity (HFC) mapping technique using a clinical 3T MRI system and 2)to compare HFC values among participant groups.MethodThis prospective study included 74 participants (23 AD patients, 27 amnestic mild cognitive impairment (MCI) patients, and 24 cognitively normal (CN) elderly people) to explore the clinical application of HFC mapping in the brain. For the brain MREPT images, a multi‐echo turbo spin‐echo pulse sequence was used. A homemade software was used to map the HFC at the Larmor frequency of 128 MHz at 3T. We performed statistical analyses to 1) compare HFC maps between the three participant groups and 2) to evaluate the association of HFC maps with Mini‐Mental State Examination (MMSE) scores.ResultThe HFC value was higher in the AD group than in the CN and MCI groups. MMSE scores were negatively associated with HFC values, but age was positively associated with HFC values. The HFC value in the insula has a high area under the ROC curve (AUC) value to differentiate AD patients from the CN participants (Sensitivity (SE) = 82, Specificity (SP) = 97, AUC = 0.902, p < 0.0001), better than GMV in hippocampus (SE = 79, SP = 83, AUC = 0.880, p < 0.0001).ConclusionHFC values were significantly increased in the AD group compared to the CN group and increased with age and disease severity. HFC values of the insula along with the GMV of the hippocampus can be used as an imaging biomarker to improve the differentiation of AD from CN.

High-Frequency Conductivity of Amorphous and Crystalline Sb2Te3 Thin Films

The results of study of charge transfer processes in thin amorphous and crystalline Sb2Te3 films in a wide range of frequencies and temperatures are presented. The frequency spectra of conductivity were obtained by the dielectric spectroscopy method. The authors analyzed the frequency dependences of the conductivity in the electric field and the temperature dependences of the exponent s. A transition from the classical correlated barrier hopping (CBH) to quantum mechanical tunneling (QMT) was observed at a certain temperature Tt. The CBH model allowed the authors to calculate the conductivity parameters of two phases. Two areas with different types of conductivity were revealed on the conduction spectra, and the activation energies of charge transfer processes for amorphous and crystalline films were determined. The following features were discovered: the difference in the temperatures of the change of the charge transfer mechanism and the transition from the semiconductor region to the metal region on the temperature dependence of conductivity. They can help to identify the amorphous phase in the quasi-binary chalcogenide Sb2Te3-GeTe system.

High frequency conductivity decomposition by solving physically constraint underdetermined inverse problem in human brain

The developed magnetic resonance electrical properties tomography (MREPT) can visualize the internal conductivity distribution at Larmor frequency by measuring the B1 transceive phase data from magnetic resonance imaging (MRI). The recovered high-frequency conductivity (HFC) value is highly complex and heterogeneous in a macroscopic imaging voxel. Using high and low b-value diffusion weighted imaging (DWI) data, the multi-compartment spherical mean technique (MC-SMT) characterizes the water molecule movement within and between intra- and extra-neurite compartments by analyzing the microstructures and underlying architectural organization of brain tissues. The proposed method decomposes the recovered HFC into the conductivity values in the intra- and extra-neurite compartments via the recovered intra-neurite volume fraction (IVF) and the diffusion patterns using DWI data. As a form of decomposition of intra- and extra-neurite compartments, the problem to determine the intra- and extra-neurite conductivity values from the HFC is still an underdetermined inverse problem. To solve the underdetermined problem, we use the compartmentalized IVF as a criterion to decompose the electrical properties because the ion-concentration and mobility have different characteristics in the intra- and extra-neurite compartments. The proposed method determines a representative apparent intra- and extra-neurite conductivity values by changing the underdetermined equation for a voxel into an over-determined minimization problem over a local window consisting of surrounding voxels. To suppress the noise amplification and estimate a feasible conductivity, we define a diffusion pattern distance to weight the over-determined system in the local window. To quantify the proposed method, we conducted a simulation experiment. The simulation experiments show the relationships between the noise reduction and the spatial resolution depending on the designed local window sizes and diffusion pattern distance. Human brain experiments (five young healthy volunteers and a patient with brain tumor) were conducted to evaluate and validate the reliability of the proposed method. To quantitatively compare the results with previously developed methods, we analyzed the errors for reconstructed extra-neurite conductivity using existing methods and indirectly verified the feasibility of the proposed method.

Laboratory dielectric measurements to evaluate the conductivity change in the presence of chelating agent with different brines

In the oil and gas reservoirs, the interaction between the injected fluids and the reservoir fluids and rocks plays a major role in the productivity of any oil and gas field. Studying the ion exchange between reservoir fluids and the injected fluids for water flooding or chemical enhanced oil recovery purposes would help in optimizing the oil displacement process and hence the productivity form such secondary or tertiary recovery mechanisms. Chelating agents are used for enhance oil recovery to improve the oil displacement and sweep efficiency by altering the reservoir rock’s surface. When it comes to fluid-rock interaction, conductivity and ionic activity of the injected water will have a great impact on the rock’s surface charge and therefore in the reservoir’s wettability. Dielectric laboratory measurements have the ability to observe the change in conductivity at high frequency due to the presence of free ions and salts in fluids. In this work, observing the effect of chelating agent with different salts on high frequency conductivity using laboratory dielectric measurements has been conducted. Introducing laboratory dielectric measurement could be a valuable tool in the lab as an evaluation technique into the ion exchange that occurs between different fluids from the reservoir with different brines and additives to study the fluid–fluid interaction activities. It can be also utilized to investigate the maximum chelating capacity of different chelating agents with different cations which can be reflected by the change in conductivity.

High-frequency conductivity and temperature dependence of electron effective mass in AlGaN/GaN heterostructures

We present calculations of frequency and wavevector dispersion of conductivity of two-dimensional electrons confined in AlGaN/GaN heterostructures at the arbitrary level of the degeneracy and the presence of strongly inelastic scattering mechanism by the optical phonons. We found that the high-frequency conductivity deviates from that of the standard Drude–Lorentz model and changes significantly at elevated temperature. The results were applied for simulation of the spectral characteristics of the grating-based plasmonic structure for wide temperature range. We found that thermal activation of the inelastic scattering leads to significant red-shift of the plasmon resonances. This facilitates refinement of interpretation of experimental results published in series of recent papers. We concluded that for AlGaN/GaN heterostructures, the electron effective mass changes in narrower diapason of 0.22me,…,0.26me, when temperature varies from 77 to 300 K. We suggest that found peculiarities of the high-frequency conductivity and its spatial dispersion are of a general character and can be important for high-frequency and high power devices based on AlGaN/GaN heterostructures.

Application of High-Frequency Conductivity Map Using MRI to Evaluate It in the Brain of Alzheimer's Disease Patients.

BackgroundThe previous studies reported increased concentrations of metallic ions, imbalanced Na+ and K+ ions, and the increased mobility of protons by microstructural disruptions in Alzheimer's disease (AD).Purpose(1) to apply a high-frequency conductivity (HFC) mapping technique using a clinical 3T MRI system, (2) compare HFC values in the brains of participants with AD, amnestic mild cognitive impairment (MCI), and cognitively normal (CN) elderly people, (3) evaluate the relationship between HFC values and cognitive decline, and (4) explore usefulness of HFC values as an imaging biomarker to evaluate the differentiation of AD from CN.Materials and MethodsThis prospective study included 74 participants (23 AD patients, 27 amnestic MCI patients, and 24 CN elderly people) to explore the clinical application of HFC mapping in the brain from March 2019 to August 2021. We performed statistical analyses to compare HFC maps between the three participant groups, evaluate the association of HFC maps with Mini-Mental State Examination (MMSE) scores, and to evaluate the differentiation between the participant groups for HFC values for some brain areas.ResultsWe obtained a good HFC map non-invasively. The HFC value was higher in the AD group than in the CN and MCI groups. MMSE scores were negatively associated with HFC values. Age was positively associated with HFC values. The HFC value in the insula has a high area under the receiver operating characteristic (ROC) curve (AUC) value to differentiate AD patients from the CN participants (Sensitivity [SE] = 82, Specificity [SP] =97, AUC = 0.902, p < 0.0001), better than gray matter volume (GMV) in hippocampus (SE = 79, SP = 83, AUC = 0.880, p < 0.0001). The classification for differentiating AD from CN was highest by adding the hippocampal GMV to the insular HFC value (SE = 87, SP = 87, AUC = 0.928, p < 0.0001).ConclusionHigh-frequency conductivity values were significantly increased in the AD group compared to the CN group and increased with age and disease severity. HFC values of the insula along with the GMV of the hippocampus can be used as an imaging biomarker to improve the differentiation of AD from CN.

High-frequency conductivity at Larmor-frequency in human brain using moving local window multilayer perceptron neural network.

Magnetic resonance electrical properties tomography (MREPT) aims to visualize the internal high-frequency conductivity distribution at Larmor frequency using the B1 transceive phase data. From the magnetic field perturbation by the electrical field associated with the radiofrequency (RF) magnetic field, the high-frequency conductivity and permittivity distributions inside the human brain have been reconstructed based on the Maxwell’s equation. Starting from the Maxwell’s equation, the complex permittivity can be described as a second order elliptic partial differential equation. The established reconstruction algorithms have focused on simplifying and/or regularizing the elliptic partial differential equation to reduce the noise artifact. Using the nonlinear relationship between the Maxwell’s equation, measured magnetic field, and conductivity distribution, we design a deep learning model to visualize the high-frequency conductivity in the brain, directly derived from measured magnetic flux density. The designed moving local window multi-layer perceptron (MLW-MLP) neural network by sliding local window consisting of neighboring voxels around each voxel predicts the high-frequency conductivity distribution in each local window. The designed MLW-MLP uses a family of multiple groups, consisting of the gradients and Laplacian of measured B1 phase data, as the input layer in a local window. The output layer of MLW-MLP returns the conductivity values in each local window. By taking a non-local mean filtering approach in the local window, we reconstruct a noise suppressed conductivity image while maintaining spatial resolution. To verify the proposed method, we used B1 phase datasets acquired from eight human subjects (five subjects for training procedure and three subjects for predicting the conductivity in the brain).

Ultraslow electron-phonon scattering and polaron formation in magnetite

The comprehensive analysis of AC electrical conductivity in magnetite was performed in order to find relations between the formation of polarons, phonons and conduction by a virtual free electron gas. The analysis performed here for the first time shows experimental data for the behavior of electrons for magnetite with the scattering time shifted to the GHz region. According to our study, the DC electrical conductivity can be described by the virtual free electron gas model, and high frequency conductivity can be described by the combination of the Drude model for disordered materials and Jonscher's universal power law. The observed peak at the imaginary part of AC conductivity was related to the scattering time of the electron-phonon coupling. This interaction between electrons and thermally formed phonons results in the formation of large polarons, and these are responsible for high frequency conductivity in magnetite.

Evolution of High-Frequency Conductivity of Pure Water Samples Subjected to Mechanical Action: Effect of a Hypomagnetic Filed

Evolution of High-Frequency Conductivity of Pure Water Samples Subjected to Mechanical Action: Effect of a Hypomagnetic Filed

Evaluation of electrical conductivity and anisotropy in muscle tissues using conductivity tensor imaging (CTI)

Low-frequency conductivity of an anisotropic tissue is associated with its cellular structure. Imaging of the conductivity tensor inside the human body could prove invaluable to analyses of interactions between electromagnetic fields and biological systems, such as predictions of current pathways during electrical stimulation. Since the muscle is the most abundant anisotropic tissue in the human body, in vitro and in vivo muscle conductivity tensor assessment has been attempted. In this study, we conducted phantom imaging of biological tissues using conductivity tensor imaging (CTI) to validate its in vivo usefulness. We constructed phantoms using bovine and/or porcine muscles and performed a CTI experiment using MRI. High-frequency conductivity was first obtained using B1 mapping with a multi-echo spin-echo pulse sequence. Information about the cellular space was obtained with a multi-b diffusion tensor imaging sequence. We combined the data from these separate scans to reconstruct the conductivity tensor images of the phantoms. The low-frequency and high-frequency conductivities of the muscle tissues in the phantoms were compared with the corresponding values measured by an impedance analyzer. The anisotropy of each muscle tissue was quantified as an anisotropy ratio (AR), defined as the ratio of the eigenvalues of a conductivity tensor along the longitudinal direction to those along the transversal directions. The isotropic conductivity and conductivity tensor in bovine muscles were less than those of porcine muscles. However, the anisotropy was stronger in bovine muscles based on the AR values by fiber directions. Current CTI is a promising noninvasive tool for evaluation of the muscle microstructure.

High-Frequency Conductivity of Disordered Semiconductors in the Region of the Transition from the Linear to Quadratic Frequency Dependence

Features of the high-frequency conductivity of disordered semiconductors related to hopping electron transport over the impurity band are discussed. The frequency dependence of the real part of the low-temperature phononless conductivity in the region of the transition (crossover) from the linear to quadratic frequency dependence of the conductivity is calculated in the pair approximation. It is shown that the crossover in the terahertz-frequency range is related to the transition of the conductivity from the variable-range to fixed-range hopping regime with increasing frequency.

Water-Dispersed High-Quality Graphene: A Green Solution for Efficient Energy Storage Applications.

Graphene has been the subject of widespread research during the past decade because of its outstanding physical properties which make it an ideal nanoscale material to investigate fundamental properties. Such characteristics promote graphene as a functional material for the emergence of disruptive technologies. However, to impact daily life products and devices, high-quality graphene needs to be produced in large quantities using an environmentally friendly protocol. In this context, the production of graphene which preserves its outstanding electronic properties using a green chemistry approach remains a key challenge. Herein, we report the efficient production of electrode material for micro-supercapacitors obtained by functionalization of water-dispersed high-quality graphene nanosheets with polydopamine. High-frequency (terahertz) conductivity measurements of the graphene nanosheets reveal high charge carrier mobility up to 1000 cm-2 V-1 s-1. The fine water dispersibility enables versatile functionalization of graphene, as demonstrated by the pseudocapacitive polydopamine coating of graphene nanosheets. The polydopamine functionalization causes a modest, i.e., 20%, reduction of charge carrier mobility. Thin film electrodes based on such hybrid materials for micro-supercapacitors exhibit excellent electrochemical performance, namely a volumetric capacitance of 340 F cm-3 and a power density of 1000 W cm-3, thus outperforming most of the reported graphene-based micro-supercapacitors. These results highlight the potential for water-dispersed, high-quality graphene nanosheets as a platform material for energy-storage applications.

Deviations of High Frequency Conductivity of Disordered Granular Systems from Universality

The real part of high frequency phononless conductivity was calculated in the pair approximation for a disordered array of densely packed spherical nanogranules. The generalization of the theory of phononless conductivity for systems with hydrogenic impurities to the systems with finite size localization centers reveals that the high frequency conductivity depends on the distribution function [Formula: see text] of the distances between the surfaces of granules. This can lead to deviations from the linear frequency dependence of the real part of conductivity [Formula: see text]. In the vicinity of the frequency [Formula: see text] ([Formula: see text] is the pre-exponential factor of the resonance integral) for disordered granular systems, one should expect deviations of [Formula: see text] from universality ([Formula: see text]), associated with weakening of the frequency dependence of [Formula: see text], and its nonmonotonicity. With an increase in the size of the granules, nonmonotonicity of [Formula: see text] is expected at lower frequencies. This stems from a decrease in the pre-exponential factor [Formula: see text] of the resonance integral with an increase in the granule size.

Determining High-Frequency Conductivity Based on Shielding Effectiveness Measurement Using Rectangular Waveguides

Determining the high-frequency conductivity of a material is one of the critical steps in material characterization. In this paper, we introduce a new technique for determining the conductivity which is based on shielding effectiveness measurement in a rectangular waveguide. The proposed method has been validated using the simulation and measurement. The proposed method has shown a very good accuracy for determining the conductivity and has significant advantages over traditional techniques, such as simple setup/postprocessing, no need for vector measurement, high repeatability, and being capable of measuring high conductivities precisely. Besides these advantages, the proposed method has some limitation such as being suitable for good and/or thick conductors.

Determination of High-Frequency Conductivity of NaCl and KCl Solutions on the Basis of Their Heating Rate by Electric Field with the Frequency of 27 MHz

A method of determination of high-frequency conductivity in aqueous electrolyte solutions is suggested. It is based on measurement of the rate of simultaneous heating of two liquids in a capacitor gap by high-frequency electric field. The equality of electric field strengths was provided by placing the cuvettes with the solutions in parallel to each other and joining them together by two common electrodes. The ratio of the heating rates of the electrolyte and water measured under this condition allows determining the ratio of their conductivities. The device used for high-frequency heating was an ultrahigh-frequency treatment device that allowed exposing the objects both to electric and magnetic field with the frequency of 27.12 MHz. The regularities of experimental determination of the heating rate of the studied liquids by electric and magnetic fields are provided. The rates of heating water and NaCl and KCl solutions in the concentration range of 0.004–20% by high-frequency electric field are determined. It is found that the solution heating rate exceeds the heating rate of distilled water by many times. The maximum heating rate and limiting high-frequency conductivity corresponding to the concentration of 0.5% for the both solutions exceed these parameters by approximately 60 times. Direct proportionality between the static conductivity of the electrolytes and the rate of their heating by high-frequency magnetic field is confirmed in the concentration range of 1–20%.

Application of elements of quantum mechanics in analysing AC conductivity and determining the dimensions of water nanodrops in the composite of cellulose and mineral oil

The paper presents a model of hopping conductivity at direct and alternating current, developed on the basis of the quantum phenomenon of electron tunnelling between neighbouring potential wells. The tunnelling takes place between neutral potential wells, resulting in an electrical dipole and additional thermally activated polarisation. An important parameter of the model is the time of existence of the dipole (relaxation time). After this time, the electron with the probability p can hop to the third well, which causes a direct current flow, or return with the probability (1 − p) to the first well, thus causing the flow of high frequency current. The model shows that for direct or low frequency current, the current density and conductivity do not depend on the frequency. In the high frequency region, the current density does not depend on the frequency either. Low-frequency conductivity is 2p times smaller than high-frequency conductivity. In the transitional region there is a frequency dependence of the current density. A formula was derived for the dependence of the relaxation time on the distance between neighbouring potential wells and temperature. The dependence of the hopping probability on activation energy, potential dipole energy and temperature was determined. A model of hopping conductivity at direct and alternating current was used to analyse the experimental frequency dependence of conductivity and permittivity of the composite of electrotechnical pressboard, mineral oil and water nanoparticles. It was found that there is a high compatibility of the experimental course with the model. It was also found that in the composite of electrotechnical pressboard, mineral oil and water nanodrops there are at least two mechanisms of changes in low frequency and high frequency conductivity. For the low-frequency stage, the value of the probability of p1 ≈ 0.2 and the relaxation time τ1 ≈ 0.02 s. The probability of hops for the high-frequency growth stage is p2 ≈ 0.0005 and the value of relaxation time τ2 ≈ 2 × 10−7 s. The occurrence of two episodes of increase in conductivity is related to the presence in the composite of electrotechnical pressboard, mineral oil and water nanoparticles of differences in distances between neighbouring nanoparticles. For a large number of nanoparticles randomly distributed in the percolation channel, the distribution of the probability of the occurrence of the distance is in a good approximation normal. This means that in the percolation channel there are pairs of neighbouring wells, the distances between which are both much smaller than the average and much larger. A pair of wells, the distance between which is smaller than medium, participates in the conductivity on the high-frequency stage with a short relaxation time. On the other hand, a pair of neighbouring wells for which the distance is greater than average causes a high resistance to DC or low-frequency current flow as well as a higher relaxation time. On the basis of the dependences designated in laboratory tests of low-frequency conductivity (10−4 Hz) from the moisture content, using the elements of quantum mechanics, the dimensions of water nanoparticles in electrotechnical pressboard impregnated with mineral insulating oil were determined. It was found that nanoparticles contain on average about 200 molecules of water, and their diameters are about 2.24 nm. This approach allowed to calculate the dielectric permittivity of cellulose and to determine that water nanoparticles are located inside the cellulose fibres.Graphical abstract

Conduction properties of thin films from a water soluble carbon nanotube/hemicellulose complex

We have examined the conductive properties of carbon nanotube based thin films, which were prepared via dispersion in water by non-covalent functionalization of the nanotubes with xylan, a type of hemicellulose. Measurements of low temperature conductivity, Kelvin probe force microscopy, and high frequency (THz) conductivity elucidated the intra-tube and inter-tube charge transport processes in this material. The measurements show excellent conductive properties of the as prepared thin films, with bulk conductivity up to 2000 S cm−1. The transport results demonstrate that the hemicellulose does not seriously interfere with the inter-tube conductance.

Induced polarization response of porous media with metallic particles — Part 5: Influence of the background polarization

Very often, ore bodies are found in altered porous materials that are rich in clay minerals. These altered rocks are in turn characterized by a relatively high normalized chargeability (product of the chargeability by the high frequency conductivity) or electrical quadrature conductivity with respect to clay-free materials. We have performed 36 experiments in which dispersed pyrite grains were mixed with a background host material composed of some pore water (NaCl, [Formula: see text] at 25°C or tap water), Na-exchanged bentonite, and silica grains. The induced polarization spectra were obtained in the frequency range of 1 mHz to 45 kHz at room temperature ([Formula: see text]). The spectra of the background porous materials alone (i.e., without pyrite) were also measured. The normalized chargeability and the quadrature conductivity of the sand-clay mixtures are consistent with available theoretical relationships. These new data complete previous data sets showing a clear relationship among the normalized chargeability, quadrature conductivity, surface conductivity, and cation exchange capacity. Bentonite is characterized by very high quadrature and surface conductivities. The normalized chargeability and the quadrature conductivity of the sand-clay mixtures (no pyrite) increase with the clay content. In the presence of pyrite, the chargeability and the phase lag depend primarily on the volume content of pyrite in a predictable way. The Cole-Cole exponent, characterizing the particle size distribution of the pyrite grains, is independent of the clay content. Still, in the presence of pyrite, the magnitude of the phase peak and the phase peak frequency depend on the clay content in a way that is not explained by the current model. We have observed that the Cole-Cole relaxation time, in the presence of pyrite, is inversely proportional to the conductivity of the background material.

Opto-electronic characterization of three dimensional topological insulators

We demonstrate that the terahertz/infrared radiation induced photogalvanic effect, which is sensitive to the surface symmetry and scattering details, can be applied to study the high frequency conductivity of the surface states in (Bi1−xSbx)2Te3 based three dimensional (3D) topological insulators (TIs). In particular, measuring the polarization dependence of the photogalvanic current and scanning with a micrometre sized beam spot across the sample, provides access to (i) topographical inhomogeneities in the electronic properties of the surface states and (ii) the local domain orientation. An important advantage of the proposed method is that it can be applied to study TIs at room temperature and even in materials with a high electron density of bulk carriers.