Sharif Digital Repository

A new discretization scheme for the current density in the hydrodynamic model is presented. This scheme is an extension of the Scharfetter-Gummel (S-G) method and is derived without neglecting the convective term in the current density. Simulation of two model n-i-n diodes shows that this method gives better numerical results than the conventional upwind and S-G methods. © 2004 Elsevier B.V. All rights reserved  

The Liouville equation for the distribution function is solved and a series solution for the Wigner distribution function is derived. In this solution, potential is nonlocal so that the distribution function in each point is influenced by the potential of the entire space. By computing the carrier density, an effective classical potential is defined. In a quantum system this effective potential replaces the classical potential. Based on the solution of the Liouville equation a novel set of three-dimensional quantum hydrodynamic equations (QHD) is derived. The form of the resulting QHD equations is similar to the classical hydrodynamic (HD) equations but there are explicit quantum corrections... 

In this paper, series expansion is used to solve Liouville equation and two terms of the series are derived. Using the solution, an effective classical potential is defined which contains quantum effects. It is shown for two classical potentials, step potential and barrier potential, that in the effective potential, discontinuities are smoothed. Furthermore, it is demonstrated that the difference between the classical potential, with slow changes over position, and its effective potential is small  

Refractive index (RI) provides valuable information about various reservoir engineering calculations, making it a key parameter for characterizing crude oils. Determination of this index through experiment is capital-intensive, time consuming, and also toil. Hence, it is essential to search for an efficient and accurate estimation of crude oil RI. In this study, an intelligent approach, based on optimized support vector regression (SVR), is introduced to find a quantitative correlation between crude oil RI and SARA (saturate, aromatic, resin, and asphaltene) fraction data. Optimization of SVR is implemented through three searching approaches, viz. hybrid of grid and pattern search (HGP),... 

A finite element formulation governing the geometrically non-linear thermoclastic behaviour of plates and shells made of functionally graded materials is derived in this paper using the updated Lagrangian approach. Derivation of the formulation is based on rewriting the Green-Lagrange strain as well as the 2nd Piola-Kirchhoff stress as two second-order functions in terms of a through-the-thickness parameter. Material properties are assumed to vary through the thickness according to the commonly used power law distribution of the volume fraction of the constituents. Within a non-linear finite element analysis framework, the main focus of the paper is the proposal of a formulation to account... 

An analytical thermoelasticity solution for hollow finite-length cylinders made of functionally graded materials exposed to thermal loads, internal pressure and axial loadings is presented. For this purpose, the governing differential equations of equilibrium are obtained using the principle of minimum total potential energy. A first-order shear deformation shell theory, which accounts for the transverse shear strains and rotations, is considered for expressing displacement field. The governing differential equations are reduced to a set of linear algebraic equations using Fourier expansion series of the displacement field components in the axial coordinate. Subsequently, solution of the... 

Bidirectional DC/DC converters are so prevalent in several applications. Regarding some applications, increasing their voltage gain ratios (VTR) are quite important. So, this paper proposes a novel extendable bidirectional boost-buck converter. In this converter, any isolation and coupled inductors are not employed. Each stage is derived from fundamental bidirectional boost-buck converter with the same switching algorithms. By embedding n stages in topology, VTRs for step-up-step-down are multiplied $n$ times for step-down and step-up modes. Furthermore, a comprehensive comparison have provided in case of VTR and total voltage stress across active power switches. It can be seen that total... 

In this paper, simulative study on the effects of lattice temperature on a GaInP/6H-SiC strained quantum-well (QW) laser's device is presented. Loss mechanisms are severe in the edge-emitting lasers. As a consequence, the wall-plug efficiency is sensitive to changes in temperature. The lasers used in this work were separate-confinement quantum-well lasers with a single strained GaInP, located in a latticematched waveguide core and cladding region of 6H-SiC. This paper provides the key results of the wall-plug efficiency upon the lattice temperature  

The production of protease by Bacillus licheniformis BBRC 100053 was studied. The most appropriate medium for the growth and protease production is composed of: lactose 1 %, yeast extract 0.5 %, peptone 0.5 %, KH 2PO4 0.1%, MgSO4.7H2O 0.02 %. Enzyme production corresponded with growth and reached a maximums level (589 U/ml) during the stationary phase at 35 °C, pH equivalent to 10 and with 150 rpm after 73 hours. Protease activity was highest at pH 8 and 45 °C. The best carbon sources are respectively lactose and maltose and the best nitrogen source is peptone The protease was highly active and stable from pH 7.0 to 11.0 with an optimum at pH 7-8.Thermo stability of the enzyme was considered... 

Urban transportation is one of the leading causes of air pollution in big cities. In-use emissions of vehicles are higher than the emission control certification levels. The current study uses a roadside remote sensing emission monitoring campaign to investigate (a) fraction of high emitters in the light-duty vehicle (LDV) fleet and their contributions to the total emissions, (b) emission inspection (I/M) programs' effectiveness, and (c) alternate fuel (natural gas) encouragement policy. LDVs consist of passenger or freight transport vehicles with four wheels equivalent to classes M1 and N1 of European union vehicle classifications. The motivation is to assess the current emission inspection... 

A kinetic model for growth of ZnO nanorods via vapor-liquid-solid (VLS) mechanism based on the bulk diffusion of Zn atoms through the Au-Zn droplet is presented. The dependences of the growth rate on size are given quantitatively. A general expression for the growth rate of nanorods during VLS process is derived. The derived formula shows the dependences of growth rate on lateral size of nanorods, concentration and supersaturation of Zn atoms in the liquid droplet. Based on the presented kinetic model the smaller nanorods have faster growth rate. Au-catalyzed ZnO nanorods are grown by chemical vapor transport and condensation (CVTC) process experimentally. Theoretical and experimental... 

This paper presents the results of an experimental and analytical study on high-strength, reinforced-concrete (RC) columns with different levels of initial ductility enhanced with carbon fiber reinforced polymer (CFRP) wraps. Six square columns 260 mm wide and 1650 mm long were tested under constant axial load and reversed cyclic lateral load. The test specimens were divided into three groups. Groups I and II were designed and detailed according to the requirements of American Concrete Institute ACI318-02 for intermediate-moment frames. Group I was wrapped with CFRP, group II was control specimens, and group III was designed and detailed according to the requirements of ACI318-02 for... 

A leaky wave antenna of uniform configuration based on nonradiative dielectric wave guide (NRD) is studied. Non radiative dielectric wave guide is a modification of H-guide suitable for millimetre wavelengths. Wave propagation, power leakage, radiation pattern and frequency scanning feature of this kind of antenna is analyzed. Analysis is held on U band of frequencies using HFSS software  

An analytical modified method is presented to investigate the clearance effects on the dynamic behavior of cantilever beams. In this work, apart from clearance, all other nonlinearities are avoided to be considered during the analytical solution. The beam is idealized using an equivalent single-degree-of-freedom structure. A tri-linear stiffness system is also adopted to simulate the beam together with the clearance. Subsequently the analytical solution is derived for a cantilever beam structure with and without considering structural damping. The experimental method is employed to verify the results from the presented analytical solutions. It is noted that the results obtained by the... 

The production of protease by Bacillus licheniformis BBRC 100053 was studied. The most appropriate medium for the growth and protease production is composed of: lactose 1%, yeast extract 0.5%, peptone 0.5%, KH2PO 4 0.1%, MgSO4.7H2O 0.02%. Enzyme production corresponded with growth and reached a maximums level (589 U/ml) during the stationary phase at 35°C, pH equivalent to 10 and with 150 rpm after 73 hours. Protease activity was highest at pH 8 and 45°C. The best carbon sources are respectively lactose and maltose and the best nitrogen source is peptone. The protease was highly active and stable from pH 7.0 to 11.0 with an optimum at pH 7-8.... 

In the present paper, a practical thermal treatment process has been introduced to stress relieve the iron powders in a soft magnetic composite. The composites were pressed and treated at different pressures and temperatures. Also, some of the samples were treated for various periods of time at 200°C. Density, electrical resistivity and transverse rupture strength of the samples were measured and fracture surfaces were studied by SEM. A series of tests were performed to determine the DC magnetic properties of the samples. It has been shown that there are optimum amounts of resin content and compaction pressure and temperature, rather than time is the dominant factor in stress relief of the... 

Based on the solution of the Wigner-Boltzmann (WB) equation, quantum correction in the drift-diffusion (DD) model is proposed. The new model, which is a form of the quantum DD model, can show quantum phenomena such as tunneling and quantum repulsion. To evaluate the proposed model, potential barrier and potential wells are simulated. Simulation results show the electron tunneling through and repulsion from the potential barrier. © 2007 The Japan Society of Applied Physics  

As an alternative to conventional fertilizers, e.g. NPK (the Nitrogen-Phosphorous-Potassium-containing chemical fertilizers) which release their nutrients in a short time glass fertilizers are ideal as they release macro-and micronutrients for crops and plant nourishment more gradually. Also, despite conventional ones, they have no ground-water pollution. In the present study, glass fertilizers were synthesized via polymer-derived ceramics (PDC) method. Despite the melt-casting procedure, PDC needs lower temperatures in heat treatment. The precursors consist of poly-siloxane and active fillers. Thus, thanks to gaseous release during heat treatment of the present active fillers, i.e. Ca(OH)2,... 

Damage tolerance improvement has been reported by laminating aluminum alloys and composites by researchers. Three-layer laminates comprising Al6061 outer layers and Al1050 interlayer have been roll bonded in this research. While most of the works done have focused on fracture properties of roll bonded Al laminates in crack arrester geometry, this study explores their behavior in crack divider configuration. Rolling strain is varied to control the interfacial bonding in laminates. The fracture behavior of laminates and the constituent material was examined via three-point bending and impact tests. This study presents significant improvement in damage tolerance of laminates compared to their... 

This paper presents a structured dictionary-based model for hyperspectral data that incorporates both spectral and contextual characteristics of spectral samples. The idea is to partition the pixels of a hyperspectral image into a number of spatial neighborhoods called contextual groups and to model the pixels inside a group as members of a common subspace. That is, each pixel is represented using a linear combination of a few dictionary elements learned from the data, but since pixels inside a contextual group are often made up of the same materials, their linear combinations are constrained to use common elements from the dictionary. To this end, dictionary learning is carried out with a...