Sharif Digital Repository

Composite aluminum-SiC foam was manufactured by injection of air and addition of reinforcement particles into the liquid aluminum. Microstructure and mechanical properties of the Al/SiC foams were investigated by scanning electron microscopy and compression tests. Results showed that both the cell size and the wall thickness augmented with increasing of the SiC reinforcement particles; while SiC particles resulted in the reduction of the plateau border length. With more SiC particles, plateau stress became larger; but maximum plateau strain became smaller. The stress-strain curves exhibited serrations in the plateau region due to addition of the SiC particles  

This paper is aimed at investigating some computational aspects of different isoperimetric problems on weighted trees. In this regard, we consider different connectivity parameters called minimum normalized cuts/isoperimetric numbers defined through taking the minimum of the maximum or the mean of the normalized outgoing flows from a set of subdomains of vertices, where these subdomains constitute a partition/subpartition. We show that the decision problem for the case of taking k-partitions and the maximum (called the max normalized cut problem NCP^M), and the other two decision problems for the mean version (referred to as IPP^m and NCP^m) are NP-complete problems for weighted trees. On... 

Let LL be a reversible Markovian generator on a finite set View the MathML sourceV. Relations between the spectral decomposition of LL and subpartitions of the state space View the MathML sourceV into a given number of components which are optimal with respect to min–max or max–min Dirichlet connectivity criteria are investigated. Links are made with higher-order Cheeger inequalities and with a generic characterization of subpartitions given by the nodal domains of an eigenfunction. These considerations are applied to generators whose positive rates are supported by the edges of a discrete cycle ZNZN, to obtain a full description of their spectra and of the shapes of their eigenfunctions, as... 

A simple, rapid, and highly regioselective green protocols for the synthesis of β-hydroxylnitriles by the direct reaction of epoxides with trimethylsilyl cyanide in the presence of catalytic amount of lithium perchlorate under solvent free conditions in high to quantitative yield is reported  

Treatment of aliphatic and aromatic aldehydes with chiral amines and trimethylsilyl cyanide in the absence or in the presence of Lewis acids (including lithium perchlorate) under solvent-free conditions afforded the corresponding α-aminonitriles in good yields and with a diastereoselectivity of 68 to 86%  

A heat-transfer model was formulated to determine the distribution of temperature within a bioheap of chalcopyrite of Sarcheshmeh copper mine. Bioleaching employs mixed mesophilic and thermophilic microbes for Cu extraction. Thermophiles are better than mesophiles to dissolve CuFeS2. The solution irrigation and aeration rates were taken into account as the main operational factors. The model was validated by comparing the temperature profiles of test columns with those of bioheap. The model was used to find the optimal ratio of irrigation to aeration. It was found that when the solution was fed at a flow rate of 5 kg/m2 h and air was blown at a flow rate of 7.5 kg/m2 h, the transition from a... 

The unique properties of NiTi alloy, such as its shape memory effect, super-elasticity and biocompatibility, make it ideal material for various applications such as aerospace, micro-electronics and medical device. In order to meet the requirement of increasing applications, great attention has been given to joining of this material to itself and to other materials during past few years. Laser welding has been known as a suitable joining technique for NiTi shape memory alloy. Hence, in this work, a comparative study on laser welding of NiTi wire to itself and to AISI 304 austenitic stainless steel wire has been made. Microstructures, mechanical properties and fracture morphologies of the... 

Having studied the intergranular crack nucleation in bicrystalline materials under fatigue [1], the concepts are extended to innovative treatise of crack initiation in thin-film metallizations under coupled interaction of thermomechanical fatigue (IMF) and corrosion. The pile-up of dislocations under thermal cycling and diffusion of corrosive agents have interacting effects in accelerating crack nucleation. It is shown that the corresponding evolutionary partial differential equation (PDE), which governs diffusion of the corrosive agent, contains two terms that involve the first and second spatial derivatives of hydrostatic pressure. Through dependence of the hydrostatic pressure on normal... 

In this paper, we propose an analytical approach to compute the probability of connectivity for one-dimensional ad hoc networks. The proposed analysis gives the exact probability of connectivity for an arbitrary distribution of nodes, provided that nodes are independently and identically distributed. We conduct separate analyses for two cases; in the first case, the number of nodes varies by time under a stationary distribution and in the second case, there is a fixed (known) number of nodes in the network. Using the approaches presented in this work, we are able to derive closed-form formulas for the probability of connectivity for some spatial distributions, while for more complicated... 

Wall walking robots are designed for different purposes, rescue operations, wall inspections and jobs such as painting and cleaning and fire fighting for tall buildings. These are some cases that these types of robots are extensively used. This paper describes a design of a new serial mechanism for wall climbing job. In deed we are seeking to define a minimum degree of freedom mechanism to be applied in a robot moving vertically on a surface. This mechanism has 5 links, but at any point it works with its 4 links, actually in each cycle of motion the linkage will be interchanged. Copyright © 2006 by ASME  

Orthogonal frequency division multiplexing (OFDM) has a large peak to mean envelope power ratio (PMEPR), which can result in significant signal distortion in presence of nonlinear amplifiers. In this paper, we will show that peak of the continuous envelope of OFDM signal can be bounded by maximum of corresponding oversampled sequence. This fact is then used as the corner stone of our study in two parts. In the first part, statistical analysis is used to find probability upper bound for complementary cumulative distribution function of PMEPR. It is also proven that for large number of subcarriers, only one redundant bit is necessary to limit PMEPR within %10 of its maximum value. In the... 

Regulations of offshore structures suggest the application of Morison type equation for the estimation of forces induced by wave and current on the slender bodies of Jacket and Jack-up structures. However, common values of hydrodynamic coefficients are rarely defined in two different regulations. Estimation of global responses of Jack-up structure, the simplified geometrical model is used, therefore we will try to modify the DNV formulations in order to estimate the hydrodynamic forces on equivalent pile. Finally, the forces on the real structure and the equivalent pile model are compared and it has been shown that the approximation of the inertia forces has more accuracy comparing to the... 

The assessment of forest sustainability in the Asian and Pacific Region was carried out. It was stated that 61 countries are members of the largest United Nations regional commission, the Economic Commission for Asia and Pacific (ESCAP). The study showed that for about one third of these countries, the national forest and woodland coverage has steadily diminished and for more than 50 percent of the remaining countries the coverage has not improved significantly. The results confirmed the significance of the region 's forest sustainability problem and its forthcoming challenges for the 21st century  

A fast high-performance liquid chromatography-diode array detection (HPLC-DAD) approach combined to solid phase extraction (SPE) as a pre-concentration step is developed for simultaneous determination of five selected pharmaceuticals (carbamazepine, naproxen, diclofenac, gemfibrozil and mefenamic acid) in water samples. The effective factors on the efficiency of SPE procedure are optimized using faced-centered central composite design (FCCD). In addition, multi-response optimization by using Derringer's desirability function is used to find the optimum experimental conditions for extraction of analytes from well and river waters. Due to the complexity of water matrices and the presence of... 

In this brief, we present a fine-grained dark silicon architecture to facilitate further integration of transistors in static random access memory-based reconfigurable devices. In the proposed architecture, we present a technique to power off inactive configuration cells in nonutilized or underutilized logic blocks. We also propose a routing circuitry capable of turning off the configuration cells of connection blocks (CBs) and switch boxes (SBs) in the routing fabric. Experimental results carried out on the Microelectronics Center of North Carolina benchmark show that power consumption in configuration cells of lookup tables, CBs, and SBs can, on average, be reduced by 27%, 75%, and 4%,... 

In this paper, we revisit the structure of infeasibility results in network information theory, based on a notion of information state. We also discuss ideas for generalizing a known outer bound for lossless transmission of independent sources over a network to one of lossy transmission of dependent sources over the same network. To concretely demonstrate this, we apply our ideas and prove new results for lossy transmission of dependent sources by generalizing: 1) the cut-set bound; 2) the best known outer bound on the capacity region of a general broadcast channel; and 3) the outer bound part of the result of Maric, Yates, and Kramer on strong interference channels with a common message  

Recently, considerable attentions have been paid to alloy Mg-4Zn-0.2Ca for biomedical applications due to its suitable biocompatibility and acceptable mechanical properties. In this work, the effects of the addition of different amounts of Al on microstructure, mechanical properties, degradation behavior, and biocompatibility of this alloy were investigated. The corrosion behaviors of the alloys were investigated through polarization tests, chronoamperometry analysis, immersion tests, and EIS experiments. The mechanical properties were analyzed by using tensile tests and compression tests. The results showed that the addition of Al up to 3 wt.% considerably modifies the degradation behaviors... 

An effective and self-organized differentiation of human neural stem cells (hNSCs) into neurons was developed by the pulsed laser stimulation of the cells on graphene films (prepared by drop-casting a GO suspension onto quartz substrates). The effects of graphene oxide (GO) and hydrazine-reduced graphene oxide (rGO) sheets on the proliferation of hNSCs were examined. The higher proliferation of the cells on the GO was assigned to its better hydrophilicity. On the other hand, the rGO sheets, which have significantly better electrical conductivity than GO, exhibited more differentiation of the cells into neurons. The pulsed laser stimulation not only resulted in an accelerated differentiation... 

Recommender systems have become an important issue in network science. Collaborative filtering and its variants are the most widely used approaches for building recommender systems, which have received great attention in both academia and industry. In this paper, we studied the relationship between recommender systems and connectivity of users-items bipartite network. This results in a novel recommendation algorithm. In our method recommended items are selected based on the eigenvector corresponding to the algebraic connectivity of the graph - the second smallest eigenvalue of the Laplacian matrix. Since recommending an item to a user equals to adding a new link to the users-items bipartite... 

In the present work, adsorption of hydrogen molecules over a metal organic framework (MOF-5) has been investigated by using first principles density functional theory (DFT). Different strategies have been applied for improving hydrogen storage, i.e. metal doping, boron substitution and functionalization. The metal atoms used for enhancing hydrogen adsorption include Li, Ca and Sc. It is found that the binding energy between these metal atoms and MOF is not enough to prevent clustering. Therefore a number of carbon atoms are substituted by boron atoms and it is indicated that boron substitution enhances the binding energies, significantly. Also the results reveal that boron substituted MOF...