509 research outputs found
Understanding the electromagnetic interaction of metal organic framework reactants in aqueous solution at microwave frequencies
Preparation of metal organic frameworks (MOFs) via microwave heating is becoming increasingly popular due to reduced reaction times and enhanced control of MOF particle size. However, there is little understanding about the detailed interaction of the electric field portion of the wave with reactants during the synthesis of MOFs. In order to overcome this lack of fundamental understanding, information about the dielectric properties of the reactants is required. In this work the dielectric constants (ΔâČ) and loss factors (ΔâČâČ) of benzene-1,4-dicarboxylic acid (H2BDC; also known as terephthalic acid) and a number of M(III) (M = metal) salts dissolved in deionized water were measured as a function of frequency, temperature and concentration and with varying anions and cations. Dielectric data confirm the aqueous M(III) salts to be strong microwave absorbers, particularly at 915 MHz. M(III) salts with mono-anionic ligands (for example chlorides and nitrates) exhibit higher losses than di-anionic salts (sulfates) demonstrating that the former are heated more effectively in an applied microwave field. Of the M(III) salts containing either singly- or doubly-charged anions, those containing Fe(III) have the highest loss indicating that they will heat more efficiently than other M(III) salts such as Cr(III) and Al(III). Interestingly, H2BDC exhibits little interaction with the electric field at microwave frequencies
Surface characterization of clay particles via dielectric spectroscopy
Dedicated to Professor Emil Chibowski on the occasion of his 65th birthday.This work deals with the high frequency dielectric relaxation of clay
(sodium montmorillonite, or NaMt) suspensions. By high frequency it is
meant that the permittivity will be determined in the region where the
Maxwell-Wagner-OâKonski relaxation takes place, roughly, the MHz
frequency range. The applicability of dielectric determinations for the
characterization of the electrical properties of these complex systems is
demonstrated. In fact, standard electrophoresis measurements only allow to
detect that the charge of the particles becomes slightly more negative upon
increasing pH. Much more information is obtained from the high frequency
electric permittivity for different concentrations of solids and pHs. From the
characteristic frequencies of the relaxation it is possible to detect separate
processes for parallel and perpendicular orientations of the clay platelets,
modelled as oblate spheroids with a high aspect ratio. In addition, using a
suitable model the surface conductivity of the clay particles can be
estimated. Our data indicate that this quantity is minimum around pH 7,
which is admitted as representative of the isoelectric point of the edges of
the clay platelets. Data are also obtained on the amplitude (value of the
relative permittivity at low frequency minus that at high frequency) of the
relaxation, and it is found that it depends linearly on the volume fraction of
solids, and that it is minimum at pH 5. These results are considered to be a
manifestation of the fact that particle interactions do not affect the electric
conduction inside the electric double layer, while the special behaviour at
pH 5 is related to the existence of aggregates at pH 5, which increase the
effective size of the particles and provoke a reduction of their effective
conductivity.Financial support for this work by MEC (Spain) (Projects FIS2005-06860-C02-01, 02) and Junta de Andalucia (Spain) (Project FQM410)
is gratefully acknowledged
Correcting the polarization effect in low frequency Dielectric Spectroscopy
We demonstrate a simple and robust methodology for measuring and analyzing
the polarization impedance appearing at interface between electrodes and ionic
solutions, in the frequency range from 1 to Hz. The method assumes no
particular behavior of the electrode polarization impedance and it only makes
use of the fact that the polarization effect dies out with frequency. The
method allows a direct and un-biased measurement of the polarization impedance,
whose behavior with the applied voltages and ionic concentration is
methodically investigated. Furthermore, based on the previous findings, we
propose a protocol for correcting the polarization effect in low frequency
Dielectric Spectroscopy measurements of colloids. This could potentially lead
to the quantitative resolution of the -dispersion regime of live cells
in suspension
Frequency Dispersion of Sound Propagation in Rouse Polymer Melts via Generalized Dynamic Random Phase Approximation
An extended generalization of the dynamic random phase approximation (DRPA)
for L-component polymer systems is presented. Unlike the original version of
the DRPA, which relates the (LxL) matrices of the collective density-density
time correlation fumctions and the corresponding susceptibilities of polymer
concentrated systems to those of the tracer macromolecules and so-called broken
links system (BLS), our generalized DRPA solves this problem for (5xL)x(5xL)
matrices of the coupled susceptibilities and time correlation functions of the
component number, kinetic energy and flux densities. The presented technique is
used to study propagation of sound and dynamic form-factor in disentangled
(Rouse) monodisperse homopolymer melt. The calculated sound velocity and
absorption coefficient reveal substantial frequency dispersion. The relaxation
time is found to be N times less than the Rouse time (N is the degree of
polymerization), which evidences strong dynamic screening because of interchain
interaction. We discuss also some peculiarities of the Brillouin scattering in
polymer melts. Besides, a new convenient expression for the dynamic structural
function of the Rouse chain in (q,p)-representation is found.Comment: 37 pages, 2 appendices, 48 references, 1 figur
Impedance-Based Water-Quality Monitoring Using the Parallel-Plate Method
The application of electromagnetic (EM) waves to measure the electrical properties (dielectric constant and loss tangent) of materials is a well-known approach. The electrical properties can be used to indirectly measure several physical properties of solutions in water such as the concentration and chemical composition of contaminants in water, as a representative of the liquid phase in soil. A capacitive method of measuring dielectric properties of solutions is proposed to detect and determine low-concentration chemical and biological contaminations in water. The primary objective of this project is to design a low-cost sensor that would require small volumes of samples to detect low concentrations of dissolved contaminants in water. A forward model was developed using a finite-element method (FEM) to simulate the experimental setup (EXP). A calibration function was also developed to minimize deviations between FEM and EXP results for benchmark/reference solutions with known dielectric properties. The validated, calibrated forward model was then inverted to calculate the electrical properties of unknown solutions using the corresponding EXP results
Defect formation of lytic peptides in lipid membranes and their influence on the thermodynamic properties of the pore environment
We present an experimental study of the pore formation processes of small
amphipathic peptides in model phosphocholine lipid membranes. We used atomic
force microscopy to characterize the spatial organization and structure of
alamethicin- and melittin- induced defects in lipid bilayer membranes and the
influence of the peptide on local membrane properties. Alamethicin induced
holes in gel DPPC membranes were directly visualized at different peptide
concentrations. We found that the thermodynamic state of lipids in gel
membranes can be influenced by the presence of alamethicin such that nanoscopic
domains of fluid lipids form close to the peptide pores, and that the elastic
constants of the membrane are altered in their vicinity. Melittin-induced holes
were visualized in DPPC and DLPC membranes at room temperature in order to
study the influence of the membrane state on the peptide induced hole
formation. Also differential scanning calorimetry was used to investigate the
effect of alamethicin on the lipid membrane phase behavior.Comment: 11 pages, 7 figures, 1 tabl
Monitoring of lung edema by microwave reflectometry during lung ischemia-reperfusion injury in vivo
It is still unclear whether lung edema can be monitored by microwave reflectometry and whether the measured changes in lung dry matter content (DMC) are accompanied by changes in PaO(2) and in pro-to anti-inflammatory cytokine expression (IFN-gamma and IL-10). Right rat lung hili were cross-clamped at 37 degrees C for 0, 60, 90 or 120 min ischemia followed by 120 min reperfusion. After 90 min (DMC: 15.9 +/- 1.4%; PaO(2): 76.7 +/- 18 mm Hg) and 120 min ischemia (DMC: 12.8 +/- 0.6%; PaO(2): 43 +/- 7 mm Hg), a significant decrease in DMC and PaO(2) throughout reperfusion compared to 0 min ischemia (DMC: 19.5 +/- 1.11%; PaO(2): 247 +/- 33 mm Hg; p < 0.05) was observed. DMC and PaO(2) decreased after 60 min ischemia but recovered during reperfusion (DMC: 18.5 +/- 2.4%; PaO(2) : 173 +/- 30 mm Hg). DMC values reflected changes on the physiological and molecular level. In conclusion, lung edema monitoring by microwave reflectometry might become a tool for the thoracic surgeon. Copyright (c) 2006 S. Karger AG, Basel
A new method for the precise multiband microwave dielectric measurement using stepped impedance stub
This article presents a new method of wideband dielectric measurement at microwave frequencies. This method can be used to determine the complex dielectric properties of solid and semisolid materials from 0.9GHz to 4.5GHz, including the ISM bands of 915 MHz and 2450MHz. The new method is based on the scattering parameter measurement of a stepped impedance open circuited micro-strip stub, partly loaded with dielectric test material. Current microwave wideband spectroscopy techniques generally measure dielectric materials over a wide range of frequencies but their accuracy is limited. In contrast, narrowband techniques generally measure dielectric properties to a high accuracy but only at a single frequency. This new technique is capable of measuring dielectric properties over a wide range of frequencies to a high accuracy. The technique has been verified by the empirical characterisation of the dielectric properties of Teflon and Duroid 5880 materials. Empirical results were in good agreement with values in the manufacturerâs data sheets. The complex permittivity data will be useful for further microwave processing of the materials
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