Browsing by Subject "Conductivity"

Now showing 1 - 16 of 16

Open Access
bSSFP phase correction and its use in magnetic resonance electrical properties tomography
(International Society for Magnetic Resonance in Medicine, 2019) Özdemir, Safa; İder, Yusuf Ziya
Purpose Balanced steady‐state free precession (bSSFP) sequence is widely used because of its high SNR and high speed. However, bSSFP images suffer from “banding artifact” caused by B0 inhomogeneity. In this article, we propose a method to remove this artifact in bSSFP phase images and investigate the usage of the corrected phase images in phase‐based magnetic resonance electrical properties tomography (MREPT). Theory and Methods Two bSSFP phase images, obtained with different excitation frequencies, are collaged to get rid of the regions containing banding artifacts. Phase of the collaged bSSFP image is the sum of the transceive phase of the RF system and an error term that depends on B0 and T2. By using B0 and T2 maps, this error is eliminated from bSSFP phase images by using pixel‐wise corrections. Conductivity maps are obtained from the uncorrected and the corrected phase images using the phase‐based cr‐MREPT method. Results Phantom and human experiment results of the proposed method are illustrated for both phase images and conductivity maps. It is shown that uncorrected phase images yield unacceptable conductivity images. When only B0 information is used for phase correction conductivity, reconstructions are substantially improved, and yet T2 information is still needed to fully recover accurate and undistorted conductivity images. Conclusions With the proposed technique, B0 sensitivity of the bSSFP phase images can be removed by using B0 and T2 maps. It is also shown that corrected bSSFP phase images are of sufficient quality to be used in conductivity imaging.
Open Access
Does the donor-acceptor concept work for designing synthetic metals? III. theoretical investigation of copolymers between quinoid acceptors and aromatic donors
(2006) Salzner, U.; Karaltı, O.; Durdaǧi, S.
Homopolymers of quinoxaline (QX), benzothiadiazole (BT), benzobisthiadiazole (BBT), thienopyrazine (TP), thienothiadiazole (TT), and thienopyrazinothiadiazole (TTP) and copolymers of these acceptors with thiophene (TH) and pyrrole (PY) were investigated with density functional theory. Theoretical band-gap predictions reproduce experimental data well. For all but six copolymers, band-gap reductions with respect to either homopolymer are obtained. Four of the acceptors, BBT, TP, TT, and TTP, give rise to copolymers with band gaps that are smaller than that of polyacetylene. BBT and TTP copolymers with PY in 1:2 stoichiometry are predicted to be synthetic metals. Band-gap reductions result from upshifts of HOMO energies and much smaller upshifts of LUMO values. The smallest band gaps are predicted with TTP, since changes in LUMO energies upon copolymerization are particularly small. The consequence of the small interactions between LUMO levels of donor and acceptor are vanishingly small conduction bandwidths. © Springer-Verlag 2006.
Open Access
Electronic structure analysis of a new quinoid conjugated polymer
(Springer, 2000) Salzner, U.
The low lying unoccupied orbitals of oligomers of 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b'] dithiophene (CDM) are not delocalized over the whole molecule. Is such electron localization in the conduction band of poly-CDM responsible for its low n-type conductivity? Are polymers of the tricyclic thioketone (TCT) with more delocalized unoccupied orbitals a better alternative for stable n-dopable conducting polymers? Monomer through tetramer of TCT have been optimized with density functional theory. IP, EA, energy gap, and band width of the corresponding polymer were obtained by extrapolation. Comparison with data for oligomers of 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b'] dithiophene and of thiophene indicates that the novel polymer would have a small band gap and would fulfil the conditions for n-dopability and high mobility of n-type carriers. © Springer-Verlag 2000.
Open Access
Electrospinning of uniform nanofibers of polymers of intrinsic microporosity (PIM-1): the influence of solution conductivity and relative humidity
(Elsevier, 2019) Topuz, Fuat; Satılmış, Bekir; Uyar, Tamer
Polymers of Intrinsic Microporosity (PIMs) are ultra-permeable macromolecules, which can be cast as a dense membrane and exploited in a wide spectrum of applications, particularly for gas separation owing to their extremely large inner surface area, free volume and high gas permeability. While they are mostly intended to serve as membranes for gas separation, in recent years, they have been also employed in water treatment applications owing to their solution processability, which enables the production of fibrous membranes by electrospinning. The fibrous form provides an increase in sorption performance, water permeability and flux for their application in water treatment. However, owing to the low conductivity of PIM-1 solutions in 1,1,2,2-tetrachloroethane (TeCA) that is the ideal solvent for the electrospinning of PIM-1 solutions, a higher polymer concentration is required to produce bead-free fibers. Furthermore, the electrospinning of highly concentrated PIM-1 solutions leads to the formation of microfibers. To tackle these problems, we herein incorporated an ammonium salt (i.e., tetraethylammonium bromide, TEAB) to increase the conductivity of PIM-1 solutions and study the impact of solution conductivity on the electrospinning of PIM-1 solutions. In parallel to the conductivity study, the influence of relative humidity on the electrospinning and morphology of PIM-1 fibers was explored. The addition of TEAB significantly increased the solution conductivity and drastically enhanced the electrospinnability of PIM-1. The electrospinning of PIM-1 solutions (10% (w/v)) in the presence 7.5 wt% TEAB (with respect to PIM-1) led to bead-free fibers, while at the same concentration, electrosprayed beads and droplet splashing were observed in the absence of TEAB. On the other hand, increasing humidity did not influence the electrospinnability of PIM-1 and the fiber texture, however, less fibers were formed in a given time at very high humidity conditions (~80%). Overall, the experimental findings revealed that the addition of the salt drastically enhanced the electrospinnability of PIM-1 solutions owing to the enhanced conductivity and could lead to the formation of very thin PIM-1 fibers with 160 nm in diameter while no significant effect of relative humidity on the electrospinnability of PIM-1 solutions was observed.
Open Access
Fabrication of supramolecular n/p-nanowires via coassembly of oppositely charged peptide-chromophore systems in aqueous media
(American Chemical Society, 2017-07) Khalily, M. A.; Bakan, G.; Kucukoz, B.; Topal, A. E.; Karatay, A.; Yaglioglu, H. G.; Dana, A.; Güler, Mustafa O.
Fabrication of supramolecular electroactive materials at the nanoscale with well-defined size, shape, composition, and organization in aqueous medium is a current challenge. Herein we report construction of supramolecular charge-transfer complex one-dimensional (1D) nanowires consisting of highly ordered mixed-stack π-electron donor-acceptor (D-A) domains. We synthesized n-type and p-type β-sheet forming short peptide-chromophore conjugates, which assemble separately into well-ordered nanofibers in aqueous media. These complementary p-type and n-type nanofibers coassemble via hydrogen bonding, charge-transfer complex, and electrostatic interactions to generate highly uniform supramolecular n/p-coassembled 1D nanowires. This molecular design ensures highly ordered arrangement of D-A stacks within n/p-coassembled supramolecular nanowires. The supramolecular n/p-coassembled nanowires were found to be formed by A-D-A unit cells having an association constant (KA) of 5.18 × 105 M-1. In addition, electrical measurements revealed that supramolecular n/p-coassembled nanowires are approximately 2400 and 10 times more conductive than individual n-type and p-type nanofibers, respectively. This facile strategy allows fabrication of well-defined supramolecular electroactive nanomaterials in aqueous media, which can find a variety of applications in optoelectronics, photovoltaics, organic chromophore arrays, and bioelectronics.
Open Access
Feasibility of conductivity imaging using subject eddy currents induced by switching of MRI gradients
(John Wiley and Sons Inc., 2017) Oran, O. F.; Ider, Y. Z.
Purpose: To investigate the feasibility of low-frequency conductivity imaging based on measuring the magnetic field due to subject eddy currents induced by switching of MRI z-gradients. Methods: We developed a simulation model for calculating subject eddy currents and the magnetic fields they generate (subject eddy fields). The inverse problem of obtaining conductivity distribution from subject eddy fields was formulated as a convection-reaction partial differential equation. For measuring subject eddy fields, a modified spin-echo pulse sequence was used to determine the contribution of subject eddy fields to MR phase images. Results: In the simulations, successful conductivity reconstructions were obtained by solving the derived convection-reaction equation, suggesting that the proposed reconstruction algorithm performs well under ideal conditions. However, the level of the calculated phase due to the subject eddy field in a representative object indicates that this phase is below the noise level and cannot be measured with an uncertainty sufficiently low for accurate conductivity reconstruction. Furthermore, some artifacts other than random noise were observed in the measured phases, which are discussed in relation to the effects of system imperfections during readout. Conclusion: Low-frequency conductivity imaging does not seem feasible using basic pulse sequences such as spin-echo on a clinical MRI scanner. Magn Reson Med 77:1926–1937, 2017. © 2016 International Society for Magnetic Resonance in Medicine. © 2016 International Society for Magnetic Resonance in Medicine
Open Access
Gradient-based electrical conductivity imaging using MR phase
(John Wiley and Sons Inc., 2017) Gurler, N.; Ider, Y. Z.
Purpose: To develop a fast, practically applicable, and boundary artifact free electrical conductivity imaging method that does not use transceive phase assumption, and that is more robust against the noise. Theory: Starting from the Maxwell's equations, a new electrical conductivity imaging method that is based solely on the MR transceive phase has been proposed. Different from the previous phase based electrical properties tomography (EPT) method, a new formulation was derived by including the gradients of the conductivity into the equations. Methods: The governing partial differential equation, which is in the form of a convection-reaction-diffusion equation, was solved using a three-dimensional finite-difference scheme. To evaluate the performance of the proposed method numerical simulations, phantom and in vivo human experiments have been conducted at 3T. Results: Simulation and experimental results of the proposed method and the conventional phase–based EPT method were illustrated to show the superiority of the proposed method over the conventional method, especially in the transition regions and under noisy data. Conclusion: With the contributions of the proposed method to the phase-based EPT approach, a fast and reliable electrical conductivity imaging appears to be feasible, which is promising for clinical diagnoses and local SAR estimation. Magn Reson Med 77:137–150, 2017. © 2016 Wiley Periodicals, Inc. © 2016 Wiley Periodicals, Inc.
Open Access
Improvement and comparison of complex B₁ mapping techniques for use in MREPT
(2018-09) Özdemir, Safa
Impedance imaging, (i.e., conductivity, , and permittivity, ) provides helpful information about contrast between healthy and malignant tissues. As one of the impedance imaging techniques, Magnetic Resonance Electrical Properties Tomography (MREPT) uses the perturbation on B1 caused by electrical properties, and via solving the inverse problem with the help of measured B1 field, electrical properties are obtained. Therefore, to obtain conductivity using MREPT, the knowledge of B1 phase and magnitude is required. This thesis focuses on improvement and comparison of complex B1 mapping techniques for use in MREPT. In this manner, balanced steady-state free precession (bSSFP) imaging, which is one of the best candidates to obtain B1 phase, is investigated. bSSFP imaging has high speed, high signal-to-noise ratio (SNR), motion insensitivity and automatic eddy current compensation. On the other hand, it suffers greatly from B0 inhomogeneity and the concomitant "banding artifact". In regions of banding artifact, MR signal reduces significantly in magnitude, and also phase errors occur. The correction of phase errors is conducted by using three different techniques: Inserting B0 and T2 information, linearization for off-resonance estimation (LORE) algorithm, and PLANET method. In the next step, 2D version of phase-based convection-reaction equation based MREPT (phase-based cr-MREPT) technique is utilized to obtain conductivity maps from corrected phase images that are acquired from three aforementioned techniques. In order to verify the effects of correction techniques, an experimental agar-saline phantom with conductivity contrasts is constructed. It is shown that, for all phase correcting techniques, banding artifact is removed from phase images and accurate conductivity maps are obtained. Yet, inserting B0 and T2 information results in lengthy scanning time if both B0 and T2 information is acquired via traditional, reliable methods which are widely considered as golden truth. On the other hand, PLANET method suffers from B0 drift and propagation of error. Therefore, LORE algorithm is considered as the best candidate to obtain B1 phase images which is required to find conductivity maps. Besides phase-based MREPT methods, there also exists MREPT methods that requires both B1 phase and magnitude information. In the purpose of acquiring B1 magnitude images, three different methods are investigated, namely double angle (DA) method, actual ip-angle imaging (AFI) method, and Bloch-Siegert shift (BSS) based method. To analyze B1 magnitude mapping qualities of these methods, theoretical SNR calculations and phantom experiments are conducted. Both theoretical and experimental studies reveal that, based on SNR results, BSS based method is advantageous over AFI method and DA method. For each of B1 magnitude mapping methods, conductivity maps are obtained. It is found that, although standard MREPT method is indifferent to the choice of B1 magnitude mapping methods, high-SNR B1 magnitude maps provide better conductivity results for standard cr-MREPT method.
Open Access
Investigation of the effect of donor-acceptor substitution on band gap, band width, and conductivity
(Elsevier, 2001) Salzner, U.
Polymers of two donor-acceptor systems, 3-cyano,3′-hydroxybithiophene 1 and 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b′],3,4-ethylenedioxythiophene (CDM-EDOT) 2, were analyzed with density functional theory. As predicted by perturbation theory, interactions between donors and acceptors with very different energy levels are greatly reduced compared to those between fragments with similar energy levels. This leads to localized states and bands with little dispersion. For poly-1 these localized states lie below the valence band and above the conduction band. For 2 localized unoccupied levels lie within the band gap. These acceptor levels account for the high electron affinity of poly-2 and allow for self-doping. Self doping explains the increased intrinsic conductivity of poly-2, the localized nature of the low lying MOs rationalizes the low mobility of n-type carriers in poly-2.
Open Access
Iterative fitting approach to CR-MREPT
(2019-06) Boğa, Çelik
Electrical properties (conductivity, and permittivity, ) imaging, reveals information about the contrast between tissues. Magnetic Resonance Electrical Properties Tomography (MREPT) is one of the electrical properties imaging techniques, which provides conductivity and permittivity images at Larmor frequency using the perturbations in the transmit magnetic eld, B+ 1 . Standard-MREPT (std-MREPT) method is the simplest method for obtaining electrical properties from the B+ 1 eld distribution, however it su ers from the boundary artifacts between tissue transitions. In order to eliminate this artifact, many methods are proposed. One such method is the Convection Reaction equation based MREPT (cr-MREPT). cr-MREPT method solves the boundary artifact problem, however Low Convective Field (LCF) artifact occurs in the resulting electrical property images. In this thesis, Iterative Fitting Approach to cr-MREPT is developed for investigating the possibility of elimination of LCF artifact. In this method, forward problem of obtaining magnetic eld with the given electrical properties inside the region of interest is solved iteratively and electrical properties are updated at each iteration until the di erence between the solution of the forward problem and the measured magnetic eld is small. Forward problem is a di usion convection reaction partial di erential equation and the solution for the magnetic eld is obtained by the Finite Di erence Method. By using the norm of the difference between the solution of the forward problem and the measured magnetic eld, electrical properties are obtained via Gauss-Newton method. Obtaining electrical property updates at each iteration, is not a well conditioned problem therefore Tikhonov and Total Variation regularizations are implemented to solve this problem. For the realization of the Total Variation regularization, Primal Dual Interior Point Method (PDIPM) is used. Using the COMSOL Multiphysics, simulation phantoms are modeled and B+ 1 data for each phantom is generated for electrical property reconstructions. 2D simulation phantom, modeled as an in- nitely long cylindrical object, is assumed to be under the e ect of the clockwise rotating radio-frequency (RF) eld. Second phantom modeled, is a cylindrical object with nite length and z- independent electrical properties, that is placed in a Quadrature Birdcage Coil (QBC). Third phantom modeled is a cylindrical object placed in a QBC, with z- dependent electrical properties. In addition to the simulation phantoms, z- independent experimental phantoms are also created for MRI experiments. Conductivity reconstructions of 2D simulation phantom, do not su er from LCF artifact and have accurate conductivity values for both Tikhonov and Total Variation regularizations. While, 2D center slice reconstructions of the zindependent simulation and experimental phantoms do not have LCF artifact, resulting conductivity values are lower than the expected conductivity values. These low conductivity values are obtained because of the inaccurate solution of the forward problem in 2D for 3D phantoms. When Iterative Fitting Approach is extended to 3D, such that solution of the forward problem is also obtained in 3D, resulting electrical property reconstruction does not have LCF artifact and obtained conductivity values are as expected for both z- independent simulation and experimental phantom. Reconstructions obtained for the z- dependent simulation phantom shows that electrical properties varying all 3 directions can be accurately reconstructed using Iterative Fitting Approach. For Iterative Fitting Approach reconstructions, voxel size of 2 mm is used for the 3D experimental phantom and voxel size of 1.5 mm is used for all simulation phantoms and 2D experimental phantom. Reconstructions obtained for all phantom with Iterative Fitting Approach are LCF artifact free. Conductivity reconstructions obtained using Tikhonov and Total Variation regularizations have similar resolutions (1-2 pixels) but Total Variation regularization results in smoother conductivity values inside the tissues compared to the Tikhonov regularization.
Open Access
Modelling a resistive-reflector antenna by the complex source-dual series approach: the 2-D case of H-polarization
(IEEE, 1995) Altıntaş, Ayhan; Nosich, A. I.; Yurchenko, V. B.
The simulation of reflector antenna is normally conducted under an assumption of the perfect conductivity of reflector. This paper presents an analysis of resistive reflector beamforming using modified method of regularization for solving the scattering from a curve resistive strip. Besides, to simulate a directive feed in equally accurate manner, the Complex Source Method is used.
Open Access
Modulation behaviors, conductivities, and carrier dynamics of single and multilayer graphenes
(IEEE Computer Society, 2019) Kaya, E.; Kakenov, Nurbek; Kocabaş, Coşkun; Altan, H.; Esentürk, O.
Time domain and time resolved terahertz studies of single- and multi-layer graphene (SLG and MLG) samples and modulator devices will be presented. A high performance up to 100% of modulators were observed with the devices even at very low voltages. High modulation depth over such a broad spectrum and simple device structure brings significant importance toward application of this type of device in THz and related technologies. In addition, conductivities of SLG and MLG devices were also investigated and a change in behavior was observed as the layer thickness increased. The charge carriers dynamics of the samples with pulp fluence and color was also highly interesting.
Open Access
Numerically exact analysis of a two-dimensional variable-resistivity reflector fed by a complex-point source
(Institute of Electrical and Electronics Engineers, 1997-11) Nosich, A. I.; Yurchenko, V. B.; Altintaş, A.
Accurate numerical analysis of a two-dimensional (2-D) variable-resistivity reflector has been carried out by the method of regularization based on the analytical inversion of the corresponding static problem. The complex source-point model has been used to account for the directivity of the feeder and both the H- and E-polarization cases are considered. Far-field radiation patterns, directivity, and total radiative power have been computed for reflectors of uniform and nonuniform complex resistivities. The concept of edge loading for the control and improvement of antenna characteristics is confirmed by this numerically rigorous technique.
Open Access
Plasmon-LO-phonon coupling effects on the drag rate in double quantum-well systems
(Elsevier, 1997) Güven, Kaan; Tanatar, Bilal
We study the Coulomb drag rate for electrons in a double-quantum-well structure taking into account the electron-optical phonon interactions. The full wave vector and frequency dependent random-phase approximation (RPA) at finite temperature is employed to describe the effective interlayer Coulomb interaction. The electron-electron and electron-optical phonon couplings are treated on an equal footing. The electron-phonon mediated interaction contribution is investigated for different layer separations and layer densities. We find that the drag rate at high temperatures (i.e., T greater than or equal to 0.3 E-F) is dominated by the coupled plasmon-phonon of the system. Including the local-field effects in an approximate way we estimate the importance of intralayer correlations to be significant. (C) 1997 Elsevier Science Ltd.
Open Access
A synthetic biology approach for engineered functional biofilm
(2017-12) Kalyoncu, Ebuzer
Extracellular polymeric substances consist of molecules, DNAs, carbohydrates, and proteins that are secreted by microbial biofilms. These molecules assist in the synthesis of bacterial biofilms as highly ordered, complex and dynamic material systems, contribute to the adaptation of cells to their environment, and increase their flexibility and functionality under a broad range of conditions. Bacterial biofilms are promising tools for functional applications as bionanomaterials. They are synthesized by well-defined machinery, readily form fiber networks covering large areas, and can be engineered for different functionalities. One aspect of the present thesis focuses on controlling the expression of the curli proteins of Escherichia coli and functionalize the curli fibers by genetically fusing various peptide molecules. Biofilm proteins were functionalized with designed conductive aromatic aminoacids by using programmed cellular machines in order to develop electrically conductive protein nanofiber networks. It has been shown how biological conductivity can be used to control and direct metabolic activities of bacterial populations. Understanding and building conductive biological interfaces to merge living systems with electronic gadgets is a demanding subject. First time in the literature we succeeded to demonstrate living cells enabled bio-conductivity via a conductive nanofiber network formation. In E. coli, there are two proteins as backbones of the nano-fibers (CsgA and CsgB) responsible for the formation of biofilms. In this thesis, tunability of the morphology and mechanical properties of biofilm backbones were investigated by using protein engineering. The effect of minor and major proteins and their engineered form on the final mechanical properties of the biofilm structures were probed by scanning probe microscopy. The minor protein plays a crucial role in tuning the mechanical and morphological properties of the biofilm structures. Biofilm protein engineering for material science can be used through the genetically tunable biofabrication of self-assembling functional materials. Using synthetic biological tools, externally controllable biofilm patterns can be achieved. Recombinase based genetic logic gates encoding AND, and OR to control the expression of structural protein CsgA with 6x-Histaq modification were engineered with using two independent control signals. In this thesis, the opportunity to engineer bacterial biofilms using synthetic biology approaches was demonstrated.
Open Access
Theoretical analysis of poly(difluoroacetylene) PDFA
(Elsevier, 2003) Salzner, U.
Due to the π-donating ability of fluorine, fluorosubstitution has a remarkable effect on the band structure of polyacetylene (PA). Valence and conduction band edges decrease in energy, leading to narrower valence and wider conduction bands. Ionization potential and electron affinity of PDFA are predicted to be about 1.5 eV higher than those of PA. This indicates that PDFA is an excellent candidate for an n-type conductor. PDFA tends to adopt non-planar structures but the energy of planarization is only 5.44 kcal/mol. Alternating difluoroethylene and ethylene units yield planar polymers with decreased band gaps. Ionization potential and electron affinity of the mixed polymer are between those of the homopolymers. © 2003 Elsevier Science B.V. All rights reserved.