Sharif Digital Repository

Cooling water is used to remove the heat from stator bar of large turbogenerators and prevent additional temperature rise. Blockage of these channels leads to heat traps along the stator bar and its temperature failure. In addition, transposition of strands in a bar causes accumulation of heat in some parts of stator bar because of non-uniform arrangement of cooling channels especially when some of channels are blocked. In this paper, effect of cooling water channels blockage is studied using a precise thermal model. It is shown that such heat traps cause additional temperature rise in stator bars and this phenomenon can not be detected by thermal sensors of turbogenerator. Laboratory and... 

Multimode macroscopic states consisting of a superposition of spin coherent states that are generated in a trapped ion system are introduced. The role of various parameters that control the entanglement of the system are exposed, and their effects are quantified. In particular, it is shown that the generated states exhibit different entanglement characteristics for odd and even 2nj, where j is the spin of each mode and n is the number of modes. © 2018 Optical Society of America  

Dye-sensitized solar cells (DSCs) with nanotubular TiO2 electrodes of varying thicknesses are compared to DSCs based on conventional nanoparticulate electrodes. Despite the higher degree of order in one-dimensional nanotubular electrodes, electron transport times and diffusion coefficients, determined under short-circuit conditions, are comparable to those of nanoparticulate electrodes. The quasi-Fermi level, however, is much lower in the nanotubes, suggesting a lower concentration of conduction band electrons. This provides evidence for a much higher diffusion coefficient for conduction band electrons in nanotubes than in nanoparticulate films. The electron lifetime and the diffusion length... 

CdS nanocrystals were prepared through a thermally activated (thermochemical) method. The synthesis was performed in two different oxygen-saturated and Ar purged atmospheres. Na2S2O3 was used as a heat-sensitive source of S in the experiments. Thioglycerol was also applied as the capping agent to restrain the growth. The growth was activated by heating and controlled by solution pH. The results demonstrated a considerably higher luminescence intensity for the synthesis in oxygen-saturated atmosphere. This could show the role of luminescent oxygen trap states created inside the CdS nanocrystals. The improved luminescence was observed for the synthesis at different solution pHs. Nevertheless,... 

We propose a dielectric structure which focuses laser light well beyond the diffraction limit and thus considerably enhances the exerted optical trapping force upon dielectric nanoparticles. Although the structure supports a Fabry–Perot resonance, it actually acts as a nanoantenna in that the role of the resonance is to funnel the laser light into the structure. In comparison with the lens illuminating the structure, the proposed structure offers roughly a 10,000-fold enhancement in the trapping force—part of this enhancement comes from an 80-fold enhancement in the field intensity, whereas the remaining comes from a 130-fold enhancement in the normalized gradient of the field intensity... 

Hydrogen diffusion into steel can embrittle the material in H2S environments, but this effect can be offset by suitable hydrogen trapping sites in the microstructure. Fine Ti(C,N) inclusions have been studied as the trapping sites in high strength low alloy (API X-70) welds, with Ti additions ranging from 0.004 to 0.16 wt.%. The trapping sites were investigated by electron microscopy and thermal desorption spectroscopy. Manganese sulphide particles were the main initiation sites for hydrogen induced cracking as expected. The optimum Ti addition was around 0.02% with no evidence of cracking in the weld. The estimated values of trapping activation energy for dislocations, microvoids, MnS and... 

Trapping a single aerosol using optical tweezers could be of great importance for environmental sciences. Though a single nanoparticle as small as 10 nm is successfully trapped in aqueous media using optical tweezers, due to spherical aberration only large clusters of nanoparticles were stably trapped in air. In this paper we provide our theoretical and experimental results on optimized trapping of aerosols as small as 400 nm in radius by the introduction of an extra spherical aberration source in order to minimize the total spherical aberration of the system. Our method allows for trapping of high refractive index particles such as polystyrene beads in air. It also provides considerably... 

Field-effect transistors have been widely used to study electronic transport and doping in colloidal quantum dot solids to great effect. However, the full power of these devices to elucidate the electronic structure of materials has yet to be harnessed. Here, we deploy nanodielectric field-effect transistors to map the energy landscape within the band gap of a colloidal quantum dot solid. We exploit the self-limiting nature of the potentiostatic anodization growth mode to produce the thinnest usable gate dielectric, subject to our voltage breakdown requirements defined by the Fermi sweep range of interest. Lead sulfide colloidal quantum dots are applied as the active region and are treated... 

In dual-beam optical traps, the precision of position/force measurements is often compromised by crosstalk, i.e. imperfect separation of the two signals. We devise a simple yet very efficient method to perform crosstalk eliminating spatial filtering  

In this paper a 3D numerical model was developed to study the complicated interaction between waves and a set of tandem fixed cylinders. The fluid was considered to be inviscid and irrotational. Therefore, the Helmholtz equation was used as a governing equation. The boundary element method (BEM) was adopted to discretize the relevant equations. Open boundaries were used in far fields of the study domain. Linear waves were generated and propagated towards tandem fixed cylinders to estimate the forces applied on them. Special attention was paid to consideration of the effect on varying non-dimensional cylinder radius and distance between cylinders, ka and kd on forces and trapped modes. The... 

We study various derivations of Hawking radiation in conformally rescaled metrics. We focus on two important properties, the location of the horizon under a conformal transformation and its associated temperature. We find that the production of Hawking radiation cannot be associated in all cases to a trapping horizon because its location is not invariant under a conformal transformation. We also find evidence that the temperature of the Hawking radiation should transform simply under a conformal transformation, being invariant for asymptotic observers in the limit that the conformal transformation factor is unity at their location  

In this paper a simulation of the transverse localization of light in 1D array of optical waveguides in the presence of off-diagonal disorder is presented. Effects of self-focusing and self-defocusing Kerr nonlinearity on the transverse localization of surface and bulk modes of the disordered waveguides array are taken into consideration. The simulation shows that in the off-diagonal disordered array at low nonlinear parameters, the transverse localization of light becomes more than that of the corresponding diagonal disordered array. However by increasing the nonlinear parameters the diagonal disordered array is localized more than the associated off-diagonal disordered array for both... 

A three–dimensional metal organic framework (3D–MOF) and a two–dimensional polyoxometalate (2D–POM), both incorporating nanostructured molybdenum (VI) oxide, were synthesized and implemented for headspace needle trap extraction of traces of chlorobenzenes (CBs). The 3D–MOF of type {(Mo2O6)(4,4′–bpy)}n and the 2D–POM of type [4,4′–bpy][Mo7O22] were synthesized by a solvothermal process and characterized by FT–IR, powder X–ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetry, energy dispersive X–ray, elemental mapping and Brunner–Emmet–Teller adsorption analyses. The 3D–MOF proved to be superior. Following thermal desorption, the CBs... 

In this research, three fibers including cellulose acetate (CA), CA–phenyltriethoxysilane (PTES) prepared via sol–gel electrospinning and sol–gel–based CA fibers immersed in PTES solution (dipped–CA–PTES) were prepared. The composition and morphology of the prepared fibers were evaluated by energy dispersive X-ray spectroscopy and field emission scanning electron microscopy. The prepared fibers were implemented in a home–made needle trap device, followed by thermal desorption of the selected chlorobenzenes (CBs) to a gas chromatography–flame ionization detector. Finally, parameters affecting the extraction methodology such as the amount of sorbent, extraction time and temperature, desorption... 

Three-dimensional (3D) polyamide scaffolds were fabricated by applying a solvent bath as the collecting element. Electrospun nanofibers were immersed into the solvent bath to give a material with a laminated 3D texture. In parallel, 2D nanofibers were synthesized and utilized as microextractive phases in a needle trap device to compare the capabilities of 2D and 3D materials in terms of headspace extraction of various chlorobenzenes (chlorobenzene, 1,2-dichlorobenzene, 1,4-dichlorobenzene, 1,2,4-trichlorobenzene and 1,2,3,4-tetrachlorobenzene). The results demonstrate the superiority of 3D nanofibrous scaffolds over 2D mats. The porosity, morphology, and thermal stability of the 3D scaffolds... 

A superhydrophobic extractive phase was prepared and packed in a needle trap microextraction device for extraction of volatile organic compounds, followed by their determination by gas chromatography. The superhydrophobic phase was prepared by surface modification of melamine formaldehyde (MF) sponge embedded by reduced graphene oxide (rGO). The overall properties of the modified MF along with its high sorption capacity and low cost fabrication were indicative of its feasibility to be highly suitable for extraction of organic pollutants. The determined water contact angle (>150o) from the surface of melamine formaldehyde–reduced graphene oxide (MF–rGO) revealed its high affinity toward... 

In this article, a time-domain non-linear model is proposed for quantum dynamics of interface traps. This model includes the effects of thermoionic emission, combined with the drift and tunneling currents. The model is simplified for the case of interface traps of the directly bonded Silicon wafers, and linearized. Therefore, an equivalent electrical circuit is obtained being composed of two resistive and capacitive branches. The theory predicts a close spacing between the corresponding zero and pole in the frequency response of the system, being justified by experiment on directly bonded n-type Silicon wafers. The low frequency response to a square-wave is quite in agreement with the...