Complex Relative Dielectric Permittivity Research Articles

Investigation of the Microwave Absorption Properties of Fe Based Nanocomposites

The objective of this work was to provide information about the behaviour of Fe-based nanocomposites when exposed to microwaves. It is about rectangular bulk samples of epoxy resin reinforced by nanocrystalline Fe powders and shaped in accordance to the internal section of the R100 metallic waveguide (8.2 to 12.4 GHz) at a fixed thickness of 7 mm. The nanocrystalline Fe powders were obtained by high-energy mechanical milling process using a planetary Retsch PM 400-ball mill. The milling speed was fixed at 200 rpm for three durations and the milling process were performed under Argon atmosphere. The bulk nanocomposites were obtained by dispersion of 30% vol. of the nanocrystalline Fe powders in the resin matrix. Electromagnetic parameters as complex relative dielectric permittivity and magnetic permeability, electric and magnetic loss tangent and reflection loss were calculated using reordered S parameters. The scattering parameters were characterized using a measure cell made off two metallic R100 wave-guides associated to an Agilent 8719 network analyser according to the reflection-transmission technique. The obtained spectra inform on the new electromagnetic properties as well as the absorption characteristic acquired by the bulk nanocomposites due to the presence of the nanocrystalline Fe powders.

Analysis of the Snow Water Equivalent at the AEMet-Formigal Field Laboratory (Spanish Pyrenees) During the 2019/2020 Winter Season Using a Stepped-Frequency Continuous Wave Radar (SFCW)

Snow makes a great contribution to the hydrological cycle in cold regions. The parameter to characterize available the water from the snow cover is the well-known snow water equivalent (SWE). This paper presents a near-surface-based radar for determining the SWE from the measured complex spectral reflectance of the snowpack. The method is based in a stepped-frequency continuous wave radar (SFCW), implemented in a coherent software defined radio (SDR), in the range from 150 MHz to 6 GHz. An electromagnetic model to solve the electromagnetic reflectance of a snowpack, including the frequency and wetness dependence of the complex relative dielectric permittivity of snow layers, is shown. Using the previous model, an approximated method to calculate the SWE is proposed. The results are presented and compared with those provided by a cosmic-ray neutron SWE gauge over the 2019–2020 winter in the experimental AEMet Formigal-Sarrios test site. This experimental field is located in the Spanish Pyrenees at an elevation of 1800 m a.s.l. The results suggest the viability of the approximate method. Finally, the feasibility of an auxiliary snow height measurement sensor based on a 120 GHz frequency modulated continuous wave (FMCW) radar sensor, is shown.

Magnetic field intensity and graphene concentration effects on electrical and rheological properties of MREs-based membranes

Electric capacitors are manufactured having as dielectric materials cotton fabric-based membranes reinforced with silicone oil (), carbonyl iron (), graphene nanoparticles (nGr; and respectively ) and silicone rubber ( and respectively ). The equivalent capacitance (Cp) and resistance (Rp) of the capacitors are measured using an RLC bridge at a frequency of and for magnetic field intensities . From the functions and we obtain the real () and imaginary () components of the complex relative dielectric permittivity , the electrical conductivity σ, and viscosity η of the membranes. We show that , , σ and η increase with H and they are sensibly influenced by Φ. We present and discuss the obtained results.

ClotChip: A Microfluidic Dielectric Sensor for Point-of-Care Assessment of Hemostasis.

This paper describes the design, fabrication, and testing of a microfluidic sensor for dielectric spectroscopy of human whole blood during coagulation. The sensor, termed ClotChip, employs a three-dimensional, parallel-plate, capacitive sensing structure with a floating electrode integrated into a microfluidic channel. Interfaced with an impedance analyzer, the ClotChip measures the complex relative dielectric permittivity, ϵr , of human whole blood in the frequency range of 40Hz to 100MHz. The temporal variation in the real part of the blood dielectric permittivity at 1MHz features a time to reach a permittivity peak, , as well as a maximum change in permittivity after the peak, , as two distinct parameters of ClotChip readout. The ClotChip performance was benchmarked against rotational thromboelastometry (ROTEM) to evaluate the clinical utility of its readout parameters in capturing the clotting dynamics arising from coagulation factors and platelet activity. exhibited a very strong positive correlation ( r = 0.99, p < 0.0001) with the ROTEM clotting time parameter, whereas exhibited a strong positive correlation (r = 0.85,p < 0.001) with the ROTEM maximum clot firmness parameter. This paper demonstrates the ClotChip potential as a point-of-care platform to assess the complete hemostatic process using <10μL of human whole blood.

Hysteresis of the NMR response and the complex relative permittivity of the quartz granules powders and solid sandstones during the water imbibition and drainage

This work provides the results of the complex relative dielectric permittivity ε = ε′ − iε″ (CRP) measurement and nuclear magnetic resonance (NMR) response of the quartz powders and cores of different porosity at stepped moistening and drainage. It is shown that CRP and NMR responses are ambiguous functions on water. The character of the CRP hysteresis depends on the electromagnetic field frequency, on the particle size and water salinity. The analysis of the NMR data shows that firstly the larger pores are filled with water at moistening and they are also drained first. Thus at the same water content, which is obtained in both the processes of moistening and drainage, pores of different sizes are filled. Herewith the water droplets have different curvature radiuses and different wetting contact angles which is the main cause of the hysteresis in the CRP and NMR response.

A PMMA microfluidic dielectric sensor for blood coagulation monitoring at the point-of-care.

This paper describes the design and construct of a fully biocompatible, microfluidic, dielectric sensor targeted at monitoring human whole blood coagulation at the point-of-care (POC). The sensor assembly procedure involves using sputtered electrodes in a microfluidic channel with a physiologically relevant height of 50μm to create a three-dimensional (3D), parallel-plate, capacitive sensing area. The sensor is constructed with biocompatible materials of polymethyl methacrylate (PMMA) for the substrate and titanium nitride (TiN) for the sensing and floating electrodes. The real part of the complex relative dielectric permittivity of human whole blood is measured from 10kHz to 100MHz using an impedance analyzer and under static conditions. The temporal variation in dielectric permittivity at 1MHz for human whole blood undergoing coagulation shows a peak in permittivity at 5 minutes, which closely matches our previously established results. This sensor can pave the way for monitoring blood coagulation under physiologically relevant shear flow rates in the future.

Monitoring time course of human whole blood coagulation using a microfluidic dielectric sensor with a 3D capacitive structure.

This paper reports on the design, fabrication, and testing of a microfluidic sensor for dielectric spectroscopy (DS) of human whole blood during coagulation. The sensor employs a three-dimensional (3D), parallel-plate, capacitive sensing structure with a floating electrode integrated into a microfluidic channel. Using an impedance analyzer and after a 5-point calibration, the sensor is shown to measure the real part of complex relative dielectric permittivity of human whole blood in a frequency range of 10kHz to 100MHz. The temporal variation of dielectric permittivity at 1MHz for human whole blood from three different healthy donors shows a peak in permittivity at ~ 4 to 5 minutes, which also corresponds to the onset of CaCl2-initiated coagulation of the blood sample verified visually.

High performance sintering NiCuZn ferrites absorber sheet for HF application

By using a new and simple blade casting method, a lighter, thinner and higher absorption property sintering NiCuZn ferrite electromagnetic wave absorber sheet was first demonstrated. The electromagnetic wave absorption ability was enhanced by sintered NiCuZn ferrite absorbent. The experiment results reveal that NiCuZn ferrite with thickness 0.1 mm absorber sheet achieves its maximum reflection loss value above 0.5 dB at 13.46 MHz. Meanwhile, the absorption bandwidth is larger than 1 MHz is profitable for wideband electromagnetic wave absorbing. The absorption properties could be controlled by the variation of Nickel content in the spinel lattice and the sintering temperature of NiCuZn ferrites. More homogenous microstructure of SEM micrograph and better crystalline XRD pattern may be responsible for the best absorption ability at 1,150 °C sintering temperature. The absorption properties were also successfully analyzed in this work, which took into account both the frequency at the reflection loss minimum (f r) shift correlated with the value of inductance (L) and impedance (Z) of sintered toroidal cores and the power reflection loss (Γ) depending on e″, μ″, and $$\sqrt {\varepsilon \mu }$$ , where e and μ are complex relative dielectric permittivity and permeability, respectively, for the attenuation material. The sintering NiCuZn ferrites could merit to be potential candidates as electromagnetic attenuation materials to meet the demands for miniaturization, broader relative bandwidths at HF (3–30 MHz) and beneficial for fabrication of radio frequency identification metal tag.

Electrospun magnetic carbon composite fibers: Synthesis and electromagnetic wave absorption characteristics

AbstractElectrospun polyacrylonitrile (PAN)‐based carbon composite fibers embedded with magnetic nanoparticles have been developed as materials for electromagnetic wave absorption. The nanocomposite fibers were prepared by electrospinning from a dispersion of magnetite (Fe3O4) nanoparticles stabilized by L‐glutamic acid in a solution of PAN and N, N‐dimethyl formamide. The Fe3O4‐embedded PAN nanofibers were stabilized at 270°C in air and carbonized at 800°C in nitrogen. The Fe3O4 nanoparticles were crystalline with a particle size of about 7 nm, most of which was reduced to Fe3C with agglomerates of up to 50 nm diameter in the carbon fibers. The carbon morphology was mostly disordered, but exhibited a layered graphitic structure in the vicinity of the nanoparticles. The carbon composite fiber exhibited ferromagnetic behavior, and the induced magnetic saturation per unit mass of fibers increased with increasing Fe3O4 content in the precursor. The complex relative dielectric permittivity was tuned by adjusting the amount of Fe3O4 in the carbon fiber precursor. With increasing Fe3O4 content, good electromagnetic wave absorption characteristics were observed below 6 GHz, even for samples with fiber loadings as low as 5 wt %. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

A simple method for solving the Bussian equation for electrical conduction in rocks

Abstract. One of the most general and effective models for calculating the complex electrical conductivity and relative dielectric permittivity of rocks saturated with pore fluids is that of Bussian. Unlike most models, it is non-linear and cannot be solved algebraically. Consequently, researchers use reiterating numerical routines to obtain a solution of the equation, and then only for the real part of the solution. Here we present a different approach to the solution that uses conformal mapping in the complex plane, and implements it within MapleTM. The method is simple and elegant in that it requires, for example, only 3 lines of code in MapleTM 11 and little programming experience. The approach has been shown to be as precise as using the classical reiterating bisection method for real data implemented in C++ on an ordinary desktop computer to within a probability over 1 in 109. However, the conformal mapping approach is 52 times as fast. We show once more that the Bussian equation breaks down for low fluid conductivities, but recommend it (with the modified Archie's law) for use with rocks saturated with high salinity fluids when the matrix is conductive.

Structural and dielectric properties of relaxor Sr0·5Ba0·5Bi2Nb2O9 ceramic

Sr0·5Ba0·5Bi2Nb2O9 ceramic was prepared by a conventional solid state reaction method and studied using X-ray powder diffraction and dielectric measurements. At room temperature, an orthorhombic structure was confirmed and their parameters were obtained using the Rietveld method. Dielectric properties were studied in a broad range of temperatures and frequencies. Typical relaxor behaviour was observed with strong dispersion of the complex relative dielectric permittivity. The temperature of the maximum dielectric constant Tm decreases with increasing frequency, and shifts towards higher temperature side. The activation energy Ea≍0·194±0·003 eV and freezing temperature Tf ≍371±2 K values were found using the Vögel-Fulcher relationship. Conduction process in the material may be due to the hopping of charge carriers at low temperatures and small polarons and/or singly ionised oxygen vacancies at higher temperatures.

Microwave absorption properties of barium titanate/epoxide resin composites

Nano-barium titanate (BT) was prepared by a sol–gel method. The prepared powders were characterized by x-ray powder diffraction and transmission electron microscopy. The complex relative dielectric permittivity (ε = ε′ − jε″) and magnetic permeability (μ = μ′ − jμ″) of the BT powders were measured in the frequency range 8 ∼ 18 GHz. The BT/epoxide resin (EP) composite with different volume contents was investigated. The effects of thickness on the BT/EP composite were studied. It was found that an optimum thickness and contents of the absorber can yield the maximum reflection loss which could be obtained over a broad frequency region in the X and Ku bands. Our results indicate that BT could be a promising microwave absorption material.

The microwave electromagnetic and absorption properties of some porous iron powders

The porous iron powders were prepared by an activated carbon assisted route involving impregnation of activated carbon with concentrated metal nitrate solutions, calcination and subsequent hydrogen reduction. The prepared samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and nitrogen physisorption (physical adsorption). The complex relative dielectric permittivity ( ɛ r = ɛ′ − j ɛ″) and magnetic permeability ( μ r = μ′ − j μ″) of the powder–resin matrix composites were measured in the frequency range of 2–18 GHz. Morphological studies show that the prepared powder is flaky in appearance with a high specific surface area. For the porous magnetic fillers, the electromagnetic waves may occur repeatedly energy dissipation in the free space (pore) of the material, and resonance loss may be occurred. The porous microstructure in the materials under study has been found to be the main factor for the abnormal microwave response characteristics. The calculated results indicate that the prepared powder–resin composites exhibit good absorption performance in the 2–18 GHz range.

Influence of charge-ordering on the dielectric response of La 1− xSr xMnO 3

The low- and audio-frequency dispersions of the complex relative dielectric permittivity of La 1− x Sr x MnO 3 ( x=0.15 and 0.33) have been investigated. It is found that La 0.85Sr 0.15MnO 3 shows an extra contribution to the dielectric response that is not present in La 2/3Sr 1/3MnO 3. This extra contribution makes the dielectric constant to be quite stable in a relatively high value to at least 1 MHz and up to around 270 K. The difference between the dielectric responses of both materials is attributed to the charge-ordered state and the polaronic regime that are present in La 0.85Sr 0.15MnO 3 below its Curie temperature.

Microwave electromagnetic and absorption properties of some LiZn ferrites

The complex scattering parameters ( S 11 *, S 21 *, S 12 * and S 22 *), microwave dielectric permittivity ( ε r *= ε r′−j ε r″), magnetic permeability ( μ r *= μ r′−j μ r″) and absorption characteristics of an undoped and doped samples (1 wt% CuO and MgO) of (Li 0.5Fe 0.5) 0.7Zn 0.3Fe 2O 4 (LiZn ferrite) were measured using a microwave vector network analyzer by means of coaxial two-port and short- S 11 techniques in the frequency range of 0.3–13.51 GHz. The addition of CuO and MgO has been found to increase the reflectivity and transmittivity relative to the pure LiZn ferrite. The dielectric constant and loss decrease with the oxide addition but the change in the magnetic permeability is minimal. The ferrites exhibit magnetic resonance due to domain wall movement in the low-frequency regime. The specular absorber method is used in determining the reflection loss ( R L) of the absorber at different thicknesses. This would provide a feasible means of determining the frequency and thickness configuration for minimum reflection from the surface of an absorber. Microwave absorption profiles for all samples calculated using the specular absorber method are in good agreement with those obtained experimentally from the short- S 11 technique. The ferrites show a low-frequency matching condition when the complex relative dielectric permittivity equals that of the complex relative magnetic permeability ( ε r *= μ r *). The matching frequency is found to increase but with a smaller matching thickness for the doped samples. The second matching condition at higher frequency, which occur due to a geometrical cancellation of the incident and reflected waves at the surface of the absorbers, can only be seen for the LiZn/MgO sample. The second matching condition for the other two samples is assumed to occur at higher frequency.

Electromagnetic and absorption properties of some microwave absorbers

Electromagnetic properties of a thermoplastic natural rubber (TPNR), a lithium–nickel–zinc (Li–Ni–Zn) ferrite and a TPNR–ferrite composite subjected to transverse electromagnetic (TEM) wave propagation were investigated. The incorporation of the ferrite into the matrix of the TPNR was found to reduce the dielectric loss but the magnetic loss increased. The absorption characteristics of all the samples subjected to a normal incidence of TEM wave were investigated based on a model of a single-layered plane wave absorber backed by a perfect conductor. It is evident from a computer simulation that the ferrite is a narrowband absorber, whereas the polymeric samples show broadband absorption characteristics. Minimal reflection of the microwave power or matching condition occurs when the thickness of the absorbers approximates an odd number multiple of a quarter of the propagating wavelength. This is discussed as due to cancellation of the incident and reflected waves at the surface of the absorbers. The Li–Ni–Zn ferrite exhibits another matching condition at low frequency when the magnitude of the complex relative dielectric permittivity (εr*) equals that of the complex relative magnetic permeability (μr*). The specular absorber method provides a simple theoretical graphic aid for determining the absorption characteristics and the location of the matching conditions in the frequency domain. The result for the ferrite sample was tested and confirmed directly from terminated one-port measurements.

Dielectric relaxation in Ba-based layered perovskites

Ferroelectric materials with Bi-layered structure such as SrBi2Ta2O9 and SrBi2Nb2O9 are now intensively investigated in view of their applications in nonvolatile computer memories and high-temperature piezoelectric transducers. When Sr2+ is substituted with Ba2+, a significant disorder is induced and the material exhibits broadening of the phase transition. Such broadening is essential for applications since it allows achieving smooth temperature characteristics while maintaining high dielectric and piezoelectric properties. In this work, stoichiometric dense BaBi2Nb2O9 (BBN) ceramics are sintered using a mixed oxide route. Dielectric and ferroelectric properties are investigated in a broad range of temperatures and frequencies. Strong dispersion of the complex relative dielectric permittivity is observed including typical relaxor features such as shift of the permittivity maximum with frequency and broadening of the relaxation time spectrum with decreasing temperature. The dielectric relaxation obeys the Vögel–Fulcher relationship with anomalously low freezing temperature (Tf≈100 K), which is much lower than the permittivity maximum in the radio-frequency range. Polarization hysteresis loops testify linear properties of BBN at all temperatures above Tf. The properties of BBN ceramics are compared to conventional relaxor systems such as Pb(Mg, Nb)O3 and (Pb, La)(Zr, Ti)O3.

ESTIMATION OF THE EFFECT OF AEROSOL PRECIPITATIONS ON RADAR EFFICIENCY

We estimate the error of target-detection probability by a radar, resulting from the effect of aerosol precipitations (AP) deposited on the elements of its antenna-feeder system. Within the framework of this problem, we develop a technique for calculating the radiation pattern (RP) of a mirror antenna in the case where its feeder and reflector are covered by a given AP layer with an arbitrary shape and the known complex relative dielectric permittivity. An example of estimation of the relative error (with respect to the 0.9 confidence level calculated neglecting AP) of target-detection probability is given for the typical case of antenna icing.

Structure and dielectric properties at 4.8 and 9.5 GHz of saline ice

The structure and salinity characteristics of saline ice slabs removed from ice sheets grown in an outdoor pool have been studied and related to the complex relative dielectric permittivity measured, utilizing free space transmission techniques, at 4.8 and 9.5 GHz. The saline ice closely simulated Arctic sea ice in its structural and salinity characteristics which were regularly monitored in a number of ice sheets grown during the winters of 1983–1984 and 1984–1985. In‐situ transmission measurements at similar frequencies were also made on the ice sheet itself using antennas located above and beneath the ice. The slab measurements were made during warming from −28°C to −2°C on slabs grown during the winter of 1983–1984 (4.75 GHz) and during a warming and cooling cycle over a slightly larger temperature range on slabs grown during the winter, 1984–1985 (4.80 and 9.50 GHz). The 1983–1984 results show both the real εr′ and imaginary εr″ parts to vary almost in direct proportion to the brine volume νB at values less than about 50‰. The 1984–1985 slabs showed only εr′ to depend strongly on νB while values of εr″ were considerably less than in the previous year. This difference may have been due to greater electrical conductivity in the 1983–1984 slabs, which could be related to distinct differences seen in brine pocket structure between the two winters. Values of εr′ as a function of νB were slightly lower on the average at 9.50 GHz than at 4.80, and most values of εr′ agreed with the results of others when their temperature and salinity values were converted to values of νB. Changes in εr′ due to thermal modification (laboratory warming) seem more strongly related to a gain in air volume due to brine drainage than to the redistribution of brine inclusions. The in‐situ measurements showed extremely high attenuation for the young (&lt;12 cm) brine‐rich ice. Good agreement was found between data for the more desalinated samples and theoretical values predicted by a previously proposed dielectric mixing model that was modified to account for the brine pocket geometry observed in thin sections, and also by including a bulk conductivity term to account for the observed loss.

Microwave Dielectric, Structural, and Salinity Properties of Simulated Sea Ice

The crystalline structure, salinity characteristics, and microwave dielectric properties of artificially grown saline ice are presented. The ice was grown in an outdoor pool containing salt water of 23-25%0 salinity. The structure and salinity profiles of this ice sheet closely simulated those found in arctic first-year sea ice. The complex relative dielectric permittivity of slabs removed from the ice sheet was measured at 4.75 GHz as a function of temperature. The slabs were placed between open-end waveguide radiators, and dielectic properties were calculated from the forward scattering coefficient. The results show both the real (?r') and imaginary (?r) parts to vary almost in direct proportion to the brine volume with values for ?r showing more variation, and are compared with the previous work of others on actual sea ice samples.