Sharif Digital Repository

Topological defects are interesting phenomena which can be observed in ordered phases such as oriented active fluids or nematic liquid crystals. Topological defects are determined by the overall structure of the director field in an active fluid in nematic phase and by exerting force to the units of the active particles they can interact or cause motion in the environment. Studying them as particles with dynamical equations, instead of studying the director field of the nematic environment, would provide us the power to study the characteristics of their motion. The equations of motion for multi-defect systems have been previously studied and in this work we focus on the chaotic properties... 

This article examines the Weibull statistical analysis that was used for investigating the effect of melt filtration on tensile properties and defects formed inside the casting. Forming and entrapping of double oxide films have been explained by using the context of critical velocity of melt in the runner. SutCast software results were used to examine the amounts of the velocity of melt as such. SEM/EDX analysis is used to observe the presence of double oxide films in the fracture surfaces of the tensile specimens. The article goes on to propose that castings made with foam filters with smaller pores show higher mechanical properties and reliability due to higher Weibull modulus and fewer... 

In this paper, the effects of Stone-Wales (SW) defect on transport properties of armchair graphene nanoribbons (AGNRs) are studied using tight binding calculations combined with nonequilibrium Green's function (NEGF). We evaluate transmission and density of states (DOS) in two cases, pristine and defective AGNR, and we compare the results. Our results indicate that in the latter case, a larger bandgap is made due to symmetry breaking in GNR layer  

This paper deals with investigation of deformations and pull-in charges of the cantilever and doubly clamped carbon nanotubes (CNTs) with different geometries using molecular dynamics simulation technique. The well-known AIREBO potential for the covalent bonds between carbon atoms, Lennar-Jones potential for the vdW interaction and the Coulomb potential for electrostatic actuation are employed to model the nano electromechanical system. The results reveal that longer CNTs with smaller diameters have smaller pull-in charges in comparison with shorter CNTs possessing larger diameters. Furthermore, the pull-in charges of the doubly clamped CNTs are higher than the pull-in charges of the... 

This paper describes the photoluminescence study of titanium dioxide (TiO2) nanorods grown by a hydrothermal synthesis method on the surface of fluorine doped tin oxide (FTO) coated glass. The effects of growth conditions including: reaction time, precursor concentration and adding NaCl to hydrothermal solution on the structural and optical properties of productions are examined by using SEM, XRD, TEM and room temperature photoluminescence measurements. Also, the performance of the TiO2 nanorods as a photoanode of dye sensitized solar cells is investigated. The different excitation energies and intensities are chosen to verify the discrete electronic state of radiative recombination centers... 

Using non-equilibrium Green’s function method and density functional theory, we study the effect of line structural defects on the electron transport of zigzag molybdenum disulfide (MoS 2 ) nanoribbons. Here, the various types of non-stoichiometric line defects greatly affect the electron conductance of MoS 2 nanoribbons. Although such defects would be lead to the electron scattering, they can increase the transmission of charge carriers by creating new channels. In addition, the presence of S bridge defect in the zigzag MoS 2 nanoribbon leads to more the transmission of charge carriers in comparison with the Mo–Mo bond defect. Also, we find that the different atomic orbitals and their... 

Often, during fabrication processes of fiber-matrix composites, the pertinent interface may be made imperfectly bonded either deliberately or undesirably. The effect of electric capacitors and mechanical imperfections on the electro-mechanical fields associated with an anisotropic piezoelectric matrix containing a cylindrical inhomogeneity made of a different anisotropic piezoelectric material is of interest. In fact the interface imperfection condition presented in this paper is quite general, in the sense that any combination of mechanical and electrical imperfections may exist. The interface electrical imperfection is mimicked by the electric capacitors. The capacity of the capacitors is... 

In the present work, failure analysis of an automatic coupler in railway carriages which failed in service and caused derailment, was carried out. An evaluation of the failed coupler was undertaken to assess its integrity, including visual examination, photo documentation, fractography, chemical analysis, hardness measurement, tensile and charpy impact testing and microstructural characterization. Results show that some casting defects (e.g. mis-run, cold lap, hot crack and inclusion) resulted in poor mechanical properties. Mis-runs were also responsible for fatigue crack nucleation from two different locations. Beach marks can be observed on the fracture surface. According to the results of... 

The effect of strain-hardening exponent was studied on the occurrence of central burst defects in the aluminum wiredrawing process. The study showed that in addition to geometrical factors, strain hardening is also a significant parameter related to central burst defects. The possibility of formation of central burst increases with decrease in strain hardening exponent. The study also revealed that during multipass drawing, metals show different behavior with respect to the central burst due to a new yield stress in each pass, especially during cold drawing  

Preparation of the defective TiO2 nanoparticles (NPs) have been recently in particular interest for its outstanding applications in visible-light active photocatalysis. In order to make TiO2 visible-light absorber, optical band gap should be modified especially via introducing structural defects like oxygen vacancies as an effective pathway. In this research, we prepared defective rutile TiO2 NPs with highly efficient photocatalytic activity under visible light. The NPs were synthesized by electric arc discharge in water as a cost-effective and environmentally friendly method with capability of defect formation which followed by heat treatment. The photocatalytic activity improvement... 

Incorporating the finite-element method for the modeling of the SQUID NDE response to a predefined defect pattern, an adaptive algorithm has been developed for the reconstruction of unknown defects using an optimization algorithm for updating of the forward problem. The defect reconstruction algorithm starts with an initial estimation for the defect pattern. Then the forward problem is solved and the obtained field pattern is compared with the measured signal from the SQUID NDE system. The result is used by an optimization algorithm to update the defect structure to be incorporated in the FEM forward problem for the next iteration. Since the mentioned model based inverse algorithm normally... 

Rolling element bearing fault diagnosis is a very important part of condition-based maintenance. In this article, a new method for detection of rolling element bearing defects is proposed. The method is based on the concept of the cyclostationarity of the vibration signal to find periodicity in statistical features of the vibration signal. Various statistical features are examined to find the best choice. Several case studies including inner race, outer race, and rolling element defects are investigated in this article. Comparison with the envelope analysis showed that the proposed method benefits from clearer defect frequency identification  

An efficient frequency-domain method, the phase variation monitoring (PVM) method, is proposed to determine the electromagnetic eigenmodes in two-dimensional photonic crystal waveguides. The proposed method is based on monitoring the reflection and transmission coefficients of incident plane waves. It is successfully applied to an illustrative line-defect photonic crystal waveguide and proved to be capable of calculating the in-plane leakage through the finite-size photonic crystal surrounding the line-defect. Calculation of the leakage loss is not only important for proper understanding of wave propagation within the defect but also for its significant role in applications of photonic... 

We report a study on hydrogen storage in Ca, Co, Fe, Ni, and Pd decorated multiwalled carbon nanotubes (MWCNTs) by using two techniques: volumetric and electrochemical. The results showed that hydrogen molecules are adsorbed on the defect sites and transported to the spaces between adjacent carbon via diffusion through both defect sites and opened tips into the layers. Hydrogen storage capacity can be improved in the decorated MWCNT by Co, Fe, Ni, and Ca metals in two approaches: (i) H2 adsorption via Kubas interaction and (ii) dissociation of H2 molecules on the metal particles. The results reveal that Pd are more effective catalyst for hydrogen storage process. It was found that... 

Measurement of partial discharges (PDs) current pulses is found to be one of the most efficient methods to evaluate the high voltage electrical equipment insulation condition. Generators, transformers, cables and transmission lines are especially tested for the presence of PDs. In AC, the solution is well known. Phase-resolved method can be used to detect and determine partial discharge activities. Although phase-resolved method works well under AC voltage; due to lack of a cyclic variation in DC voltage and current this cannot be used when studying partial discharges in DC voltage systems. Therefore, other methods are employed to interpret the partial discharge in DC. For this purpose, the... 

Suggestion and application of a set of new features for on-line Partial Discharge (PD) monitoring, where there is no information about the type of PD is a challenging task for condition assessment of power equipments, such as a power transformer. This is looked for in this paper. So far, in past various techniques have been employed to develop a comprehensive PD monitoring system, however limited success has been achieved. One of the challenging issues in this field is the discovering of proper features capable of differentiating the involvement of possible types of PD sources. In order to examine the efficiency of the method established in this paper, which is based on application of a set... 

A continuum theory is employed to numerically study the equilibrium orientation and defect structures of a circular cylindrical particle with flat ends under a homeotropic anchoring condition in a uniform nematic medium. Different aspect ratios of this colloidal geometry from thin discotic to long rodlike shapes and several colloidal length scales ranging from mesoscale to nanoscale are investigated. We show that the equilibrium state of this colloidal geometry is sensitive to the two geometrical parameters: aspect ratio and length scale of the particle. For a large enough mesoscopic particle, there is a specific asymptotic equilibrium angle associated to each aspect ratio. Upon reducing the...