Sharif Digital Repository

In this paper, the electrical characteristics of vertical tunneling bilayer graphene field effect transistor (VTBGFET) are theoretically investigated. We evaluate the device behavior using nonequilibrium Green's function (NEGF) formalism combined with an atomistic tight binding model. By using this method, we extract the most significant figures of merit such as ON/OFF current ratio, subthreshold swing, and intrinsic gate-delay time. The results indicate that using a bilayer graphene instead of a monolayer graphene as the base material for the source and drain regions leads to a larger ON/OFF current ratio due to the presence of an energy bandgap in biased bilayer graphene. Also, the... 

There are many engineering applications in which composite materials are required to satisfy two or more criteria regarding physical and mechanical properties. In this article, Al-matrix nanocomposites reinforced with different volume fractions of SiC nanoparticles (~ 50 nm; up to 6%) were processed by powder metallurgy (P/M) routes through mechanical milling and hot consolidation techniques. Microstructural studies showed that nano-metric Al2O3 particles with a size of ~ 20 nm and volume fraction of ~ 2% were formed and distributed in the metal matrix, owing to the surface oxides breaking. Microstructural analysis also revealed that the size of cellular structure and the density of... 

In this paper, the characteristics of an armchair graphene nanoribbon spin FET (SFET) are investigated in the presence and absence of surface roughness, by employing a multiorbital tight-binding method along with the nonequilibrium Green's function approach. It is found that the bandgap monotonically decreases with increasing the vertical electric field, since Stark effect enhances spin-flip rate under a high vertical electric field. Furthermore, spin transport in the presence of a random potential, which is induced by the concurrent effect of the applied vertical electric field and surface roughness, is carefully analyzed. This random potential strongly scatters carriers and reduces spin... 

The present paper outlines the application of the recently proposed heat-flux model (Mazaheri et al., 2017) to high blowing-ratio film-cooling and corrugated heat-exchanger simulations. Here, the focus is mainly on the accuracy of the predicted thermal fields, while to find out the sources of inaccuracy detailed analysis of the adopted second-moment-closure hydrodynamic model is provided. To do so, fundamental benchmarks which contain the dominant phenomena in the main cases are thoroughly analyzed to identify the anomalies. Then, the main cases including leading-edge film-cooling, antivortex film-cooling and corrugated heat-exchanger are investigated. The numerical predictions indicate that... 

We report a density functional non-equilibrium Green's function study of electrical transport in a single molecular conductor consisting of an ethane-dithiolate (C2H4S2) molecular wire with two sulfur end groups bonded to the Au(1 1 1) electrodes. We show that the current was increased by increasing the external voltage biases. The projected density of states (PDOS) and transmission coefficients T(E) under various external voltage biases are analyzed, and it suggests that the variation of the coupling between the molecule and the electrodes with external bias leads to the increase of the current. Furthermore, the investigation of the transport properties of the pentane-dithiolate (C5H10S2)... 

We use a direct simulation Monte Carlo (DSMC) method to simulate gas heating/cooling and choked subsonic flows in micro/nanoscale channels subject to either constant wall temperature or constant/variable heat flux boundary conditions. We show the effects of applying various boundary conditions on the mass flow rate and the flow parameters. We also show that it is necessary to add a buffer zone at the end of the channel if we wish to simulate more realistic conditions at the channel outlet. We also discuss why applying equilibrium-based Maxwellian distribution on molecules coming from the channel outlet, where the flow is nonequilibrium, will not disturb the DSMC solution. The current... 

This article presents a novel armchair graphene nanoribbon (AGNR) field-effect transistor with engineered nanopores for resonant tunneling. Two rectangular nanopores are punched to create two potential barriers and one quantum well. Channel and source and drain contacts are AGNR, indicating structure homogeneity. Nonequilibrium Green's function and Poisson's equations are used for structural analysis. The input variables are well width (WW), drain voltage (VD), and barrier width (BW). The effects of repositioning nanopores and AGNR type (i.e., semiconductor and semimetal) are also studied. The impact of the parameters on the density of states, transmission probabilities, peak current (IP)... 

One promising solution to achieve moderate or intense low-oxygen dilution (MILD) combustion under lower temperatures may be using non-equilibrium plasma discharge. Plasma assisted combustion can extend the flammability limits by reducing the auto-ignition temperature of the reactants and can be a great approach to achieve steady MILD combustion under extreme conditions through decreasing ignition delay time. In particular, the influence of the outlet flows from a co-flowing cylindrical dielectric barrier discharge (DBD) on the CH4/N2 combustion under MILD combustion has been investigated numerically through some simplifications on plasma discharge. A three streams coaxial burner are modeled... 

Using the first-principles procedure of density-functional-theory within tight-binding approximation and nonequilibrium Green's function formalism, this paper reports on the impact of vacancy defects on the structural, electronic and transport properties of hydrogen-passivated graphene atomic sheet. After the introduction of vacancy defects in graphene atomic sheet passivated with hydrogen atoms, apart from increase in band gap, a suppression is noted in the intensity of transmission channels and density of states arising from the long array deformations of the graphene sheet and a corresponding shift of the Fermi level. This in turn decreases the conductance of the defected graphene atomic... 

Our aim is to improve the switching performance of the graphene nanoribbon field-effect transistors (GNRFETs), exploiting the concept of energy filtering. Within the proposed scheme, a superlattice (SL) structure is used in the source of the transistor for filtering high-energy electron tail by engineering the density of states (DOS). According to simulation results, this can significantly decrease the OFF-current and the subthreshold swing (SS). A comparison of the proposed device with a conventional GNRFET and a graphene nanoribbon (GNR) tunneling field-effect transistor (GNRTFET) demonstrates a significant improvement. Therefore, a typical SL-GNRFET can reduce the average and the minimum... 

This study aims at understanding the metallurgical and mechanical response of Hastelloy X nickel-based superalloy to the thermal cycle of gas tungsten arc welding (GTAW), post welding solution treatment (ST), and aging. In the as-weld condition, the weldment was characterized by the formation of the M6C carbide, sigma phase, and micro-solidification cracks in the interdendritic zones of the weld metal, and constitutional liquation of M6C carbide in the heat-affected zone. It is demonstrated that neither the weld metal nor the heat-affected zone could adversely affect mechanical properties of the welded joint in as-weld condition. However, to alleviate the microstructural heterogeneity,... 

A precise heat transfer simulation of a three-dimensional impinging jet porous heat sink is presented and is analyzed from thermodynamics vantage point under local thermal non-equilibrium condition. To increase the computational efficiency of the analysis, pore-scale modeling based on lattice Boltzmann method (LBM) is used inside the porous media (at a meso-scale), whilst finite volume method (FVM) is employed around it (at a macro-scale). The effects of the Reynolds number, porous layer thickness, solid/fluid thermal conductivity ratio, and porosity on the critical heat transfer and entropy generation parameters are investigated. Additionally, the relations between viscous entropy... 

Ultrafine-grained (UFG) Al6063 alloy reinforced with 0.8 vol% nanometric alumina particles (25 nm) was prepared by reactive mechanical alloying and direct powder extrusion. Transmission electron microscopy and electron backscatter diffraction analysis showed that the grain structure of the nanocomposite composed of nanosize grains (<0.1 μm), ultrafine grains (0.1-1 μm) and micronsize grains (>1 μm) with random orientations. Mechanical properties of the material were examined at room and high temperatures by compression test. It was found that the yield strength of the UFG composite material is mainly controlled by the Orowan mechanism rather than the grain boundaries. The deformation... 

The microstructure and mechanical properties of the ultra-fine grained (UFG) Al6063 alloy reinforced with nanometric aluminum oxide nanoparticles (25 nm) were investigated and compared with the coarse-grained (CG) Al6063 alloy (∼2 μm). The UFG materials were prepared by mechanical alloying (MA) under high-purity Ar and Ar-5 vol pct O2 atmospheres followed by hot powder extrusion (HPE). The CG alloy was produced by HPE of the gas-atomized Al6063 powder without applying MA. Electron backscatter diffraction under scanning electron microscopy together with transmission electron microscopy studies revealed that the microstructure of the milled powders after HPE consisted of ultra-fine grains...