Sharif Digital Repository

The reaction kinetics of the dehydration step of the condensation reaction between phenylhydrazine and benzaldehyde was studied spectrophotometrically in aqueous solutions of ethanol, propan-2-ol and 2-methylpropan-2-ol at pH 11.5 at 25 °C. The apparent second-order rate constants k2app of the reaction increase with mole fraction of water in all the aqueous solutions. Single-parameter correlations of log k2app versus π* (dipolarity/polarizability), α (hydrogen-bond donor acidity) and ETN (normalized polarity parameter) were obtained in all the aqueous solutions. In all cases, the correlations versus ETN and π* are acceptable, but correlations versus α are poor (e.g. in aqueous solutions of... 

A new method for the shape reconstruction of conducting cylinders from the measured data based on the cubic uniform B-splines was discussed. B-Spline was defined by an ordered set of control points which determined the shape of the curve. A direct problem was formulated using standard integral equations that were solved using method of moments. Principal polarization were treated separately. The method was found less sensitive to noise than those using Fourier expansions  

An on-line automated method for phototransformation studies of alachlor, aldicarb, and methiocarb by means of liquid chromatography/mass spectrometry (LC/MS) with particle beam (PB) interface is described. Surface water samples were first spiked with 50 μg/l of each pesticide and then exposed to the radiation of a medium-pressure mercury lamp. Next, in regular intervals of 60 min, aliquots of 50-ml sample were enriched on a solid-phase extraction (SPE) cartridge and on-line eluted by a gradient of LC eluent onto analytical column for separation, followed by MS detection. A phototransformation experiment with each pesticide was performed twice, with MS operated in electron ionisation (EI) and... 

The second-order rate constants of the [2 + 2] cycloaddition reaction between diethyl azodicarboxylate and ethyl vinyl ether were obtained spectrophotometrically in various solvents and aqueous solutions of 1,4-dioxane and methanol at 30 ± 0.1 °C. In all media except aqueous solutions, a very good linear correlation of logk2 vs ETN (normalized polarity parameter) was obtained (n = 11, r = 0.991, s = 0.086). Because of the higher polarity of the activated complex relative to the reactants, the rate of the reaction increase with increasing solvent polarity parameter. Dual-parameter correlation of logk2 vs π* (dipolarity/polarizability) and a (hydrogen bonding acidity) also gives good results... 

The flux behavior during the industrial cross-flow ultrafiltration of apple juice in a batch process was modeled using a combination of the fouling and concentration polarization models. It was observed that the major flux reduction was at the beginning and at the end of operation due to fouling and increasing solute concentration in the feed tank, respectively. The fouling phenomenon was analyzed by classical and empirical models and it was shown that the empirical one has the best correlation within less than 0.3% error for each experiment. The most significant advantage of this model is its ability to predict a steady flux, while other models predict zero flux at infinite time, which is... 

Pure Zn and Zn matrix composite coatings containing nano-sized SiC particles with an average size of 50 nm were prepared from the zinc sulfate bath. The effects of the pulse frequency, maximum current density and duty cycle on the amount of particles embedded were examined. Electron microscopic studies revealed that the coating morphology was modified by the presence of SiC nanoparticles. In the presence of SiC nanoparticles deposit grows in outgrowth mode resulting in a very rough and porous microstructure. However, at very low and very high duty cycles a smooth and pore free microstructure was obtained. Corrosion resistance properties of the coatings were studied using potentiodynamic... 

The hydrogen-induced cracking (HIC) behavior of X70 pipeline steel was studied by means of Linear Polarization Resistance (LPR), hydrogen permeation tests, weight loss and Scanning Electron Microscopy (SEM). In this study, the dissolved HZS was created by chemical reactions in solution. The specimens were immersed into synthetic sea water saturated with HZS. The experimental results showed that the increase of exposure time and HZS concentration leads to an increase of the hydrogen content in X70 steel, which plays a key role in the initiation and propagation of HIC  

Copper corrosion in sodium dodecyl sulphate (SDS) solutions and carbon nanotube (CNT) nanofluids were studied by potentiodynamic polarization. For the corrosion current densities calculations, Koutecky-Levich equation was modified to model the combined charge and mass transport. 0.005. M SDS reduced the copper corrosion current density by 81%. Higher SDS concentrations enhanced corrosion. Stirring SDS solutions increased the corrosion current density by ∼75%. By adding CNT to SDS solution, the corrosion current density first decreased and then remained constant. Stirring CNT nanofluids didn't change the corrosion current density. An adsorbed CNT layer on copper controlled the corrosion... 

In this work, spin transport in corrugated armchair graphene nanoribbons (AGNRs) is studied. We survey combined effects of spin-orbit interaction and surface roughness, employing the non-equilibrium Green's function formalism and multi-orbitals tight-binding model. Rough substrate surfaces have been statistically generated and the hopping parameters are modulated based on the bending and distance of corrugated carbon atoms. The effects of surface roughness parameters, such as roughness amplitude and correlation length, on spin transport in AGNRs are studied. The increase of surface roughness amplitude results in the coupling of σ and π bands in neighboring atoms, leading to larger spin... 

1-D compound metallic grating (CMG) is a periodic structure with more than one slit in each period. When CMG is combined with a graphene sheet as its cover, the incident light is effectively coupled to the plasmons in graphene which in turn can result in strong manipulation of light for both major polarizations. We show that tunable phase resonance and perfect absorption of the incident light are interesting outcomes of this manipulation. In this paper, we demonstrate that fano-like phase resonances which can be observed in CMGs under transverse magnetic polarized incident wave are tuned by changing the Fermi level of graphene. It is shown that while the spectral position of the phase... 

Periodic metallic structures are known to support resonant extraordinary optical transmission (EOT). When covered with graphene, these structures can be employed to effectively manipulate the light. In this paper, we propose an analytical circuit model for graphene-covered 1-D metallic gratings. By using the circuit theory, we demonstrate that 1-D periodic array of cut-through slits, which are covered by a continuous graphene sheet, exhibits tunable EOT resonance for TM polarization whose amplitude, unlike its spectral position, can be dynamically tuned by varying the Fermi level of graphene. In this fashion, it is shown that placing a perfect reflector at the bottom of the graphene-covered... 

We show theoretically that unidirectional surface waves can propagate on the surface of homogeneous bi-anisotropic layers with an anti-symmetric chirality tensor. These materials mimic the electromagnetic behavior of an anisotropic medium with gyrotropic permittivity and permeability tensors that operate on pseudo-electromagnetic fields. The unidirectional waves have transverse pseudo-electric or magnetic polarizations and pass through an obstacle without backscattering if the obstacle does not cause polarization change. The bi-anisotropic medium can be realized as a metamaterial comprising omega particles tailored to achieve the constitutive parameters desired. © 2018 IEEE  

We propose an accurate analytical model for a circularly polarized antenna comprising a normally magnetized, grounded ferrite disk with a metalized top surface. The antenna is assumed to be excited by a conventional current probe. In the case of a thin ferrite disk, the electromagnetic fields of the dominant mode of the antenna do not vary in the direction normal to the disk. However, with increasing thickness of the ferrite disk, higher order modes start to affect antenna characteristics such as resonance frequency. This work presents an analysis of the structure that takes into account all higher modes of the antenna. To validate the theoretical calculations, HFSS simulations are... 

We propose a novel graphene-dielectric-based metasurface for manipulating the polarization of the incident light in the terahertz regime. The proposed structure comprised two orthogonally oriented periodic array of graphene ribbons (PAGRs) which are separated by a dielectric spacer and deposited on an Au-backed dielectric substrate. Based on the transmission line theory, an equivalent model with excellent accuracy is suggested for the proposed structure. By leveraging the simplicity of the model, we design a three-state quarter wave plate that is able to dynamically switch the polarization of the reflected wave to linear, right-, and left-hand polarizations while keeping the reflected... 

The sentiment analysis (SA) applications are becoming popular among the individuals and organizations for gathering and analysing user's sentiments about products, services, policies, and current affairs. Due to the availability of a wide range of English lexical resources, such as part-of-speech taggers, parsers, and polarity lexicons, development of sophisticated SA applications for the English language has attracted many researchers. Although there have been efforts for creating polarity lexicons in non-English languages such as Urdu, they suffer from many deficiencies, such as lack of publically available sentiment lexicons with a proper scoring mechanism of opinion words and modifiers.... 

A simple approach to acquire wide-band/angle blazing operation in one-dimensional metallic gratings is investigated by means of mode-matching and equivalent circuit analysis. The gratings are single-groove perfectly conducting gratings that support two propagating guided modes (opposed to typical single mode cases). It is shown under what conditions, one can achieve blazing over a wide range of frequencies and angles. Most importantly, it is identified what governs blazing, in particular the true matching condition that needs to be satisfied, and how the structures’ multiple resonances play role in achieving such matching and tailoring the bandwidth. Parameters of the proposed equivalent... 

Operation of solid oxide fuel/electrolysis cells (SOFC/SOEC) at high temperatures (T > 850 °C) is accompanied by degradation phenomena, which severely affect the operational lifetime of the cell. Degradation processes are expected to occur slower at low temperatures. However, significant reduction in electrocatalytic activity of the oxygen electrode, is one of the major challenges in decreasing the operating temperature down to 500 °C-650 °C. Recently, Pr6O11 infiltrated Ce0.9Gd0.1O2 (CGO) based electrodes have been proposed to realize high electrochemical performance at intermediate temperature. In this study, Pr-oxide has been infiltrated into a well performing sub-micro... 

We analyze the scattering of circularly polarized electromagnetic waves from a time-varying metasurface having a time-dependent surface susceptibility that locally mimics a rotating, anisotropic surface. Such virtually rotating metasurfaces (VRM) can be realized by means of electronically tunable surface elements and reach microwave-range rotation frequencies. It is shown that the scattered field contains the incident tone, as well as a single up-or down converted tone which differs by twice the rotation frequency of the surface. A simple full frequency converter is then proposed by augmenting the VRM with a metal screen separated by a proper distance. It is shown that after reflection from... 

Through using a three-dimensional finitedifference time-domain (3D-FDTD) scheme, we analyze nonlinear self-focusing in air as a Kerr medium. For sufficiently intense laser beams, the gaseous medium can behave as a strong positive lens which tends to focus the beam. Our simulations' major focus is on the propagation characteristics of an intense continuous-wave laser beam. Necessary relations have been introduced into the conventional FDTD formulation to account for the nonlinear behavior of air. The results clearly indicate that sufficient amounts of optical intensity may lead to self-focusing immediately in front of the laser aperture. ©2008 IEEE  

Polarity coincidence correlator (PCC), when used to estimate the covariance matrix on an element-by-element basis, may not yield a positive semi-definite (PSD) estimate. Devlin et al. (Devlin, 1975) claimed that element-wise PCC is not guaranteed to be PSD in dimensions p > 3 for real signals. However, no justification or proof was available on this issue. In this letter, it is proved that for real signals with p ≤ 3 and for complex signals with p ≤ 2, a PSD estimate is guaranteed. Counterexamples are presented for higher dimensions which yield invalid covariance estimates. © 2007 IEEE