Sharif Digital Repository

Nowadays, there is no doubt about the importance of investigation of physicochemical properties of nucleosides in order to determine the structure of these biopolymers. Nucleotides have a variety of roles in cellular metabolism. They are the constituents of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), the molecular repositories of genetic information. The metal ions play a significant role in several biological processes. Synthesis, replication, and cleavage of DNA and RNA as well as their structure integrity are affected by the presence of these ionized metals in the cell nucleus. To understand the role of cations in the biophysics of DNA, it is necessary first to... 

Metal matrix composites (MMC) and metal foams (MF) have been the topic of numerous researches during the last decades. The appropriate mechanical and physical properties of these materials make them useful in various industries. In this research, a unique approach is introduced for producing MMCs and MFs in which accumulative roll bonding (ARB) is employed in order to achieve a homogeneous distribution of ceramic particles in the metal matrix. Also, foamed structures can be made by subjecting Al/TiH2 composite to an appropriate annealing heat treatment where TiH2 particles decompose to hydrogen and titanium. In this work, the effect of number of ARB cycles on the size and distribution of... 

Doping metal or semiconductor nanoparticles in transparent matrix have attracted much attention for practical application in recent years. From fundamental veiw point, limiting the charge carriers and exitons in nanometric scales resulted in new quantum phenomena. In this context, transparent films containing metal nanoparticles shows an absorption peak in visible region of spectrom. Thus, by choosing an appropriate metal and controlling the size and concentration of particles in corresponding matrix, we can provide coatings with a wide rang of colors. Furthermore, thin films containing metal or metal oxide nanoparticles had been considered as nanostructure materials in research and... 

In this research we focus on synthesis and analysis of metal and metal oxide nanoparticles and their photocatalytic activity. At the beginning a high current DC power supply and a reactor for the electrical arc discharge process designed and implemented. Several nanoparticles such as tungsten oxide, zinc oxide, silver and gold have synthesized by this method in deionized water, deionized water including surfactants and stabilizers and analyzed. Several characterization techniques such as X-ray diffraction (XRD), dynamic light scattering (DLS) X-ray photoelectron spectroscopy (XPS), BET, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV-Vis spectroscopy have... 

A Focal Plane Array (FPA) consists of one or two dimensional array of sensor elements and is widely used in imaging systems. Generally an FPA can be divided into two main parts: Sensor array and read-out/test circuit. In this thesis we design and implement a test circuit for an FPA that works in 77K. The circuit will be attached to the sensor to detect and amplify its very low level current that is in the range of 10 pA to more than 10 nA. Reducing MOSFET temperature from room temperature (300K) down to liquid nitrogen (77K) will greatly changes its parameters and characteristics such as Threshold Voltage and carrier mobility. However MOSFET models in low temperature are either... 

Temperature reduction from room temperature to liquid nitrogen temperature has a significant effect in improvement of semiconductor devices operation such as field effect transistors. Nowadays cryoelectronics as a considerable branch of electronics is concerned by several groups. In order to optimize the procedure of low temperature IC design and fabrication we investigated the temperature effects on different parameters of MOSFET in this thesis. Several transistors in a variety of channel lengths were fabricated in different technology files. Wide measurement experiments were done on these transistors in order to find their drain current vs. drain to source voltage characteristics. From... 

With regard to recent progress in decreasing of operating temperature of Solid Oxide Fuel Cell (SOFC), metallic interconnect use widely than ceramic interconnect. Metallic interconnect have interested due to their low cost, easy fabrication and high electronic and thermal conductivity.oxidation resistance of metals is big challenge, because of low conductivity of formed oxide layers that led to increasing of electrical conductivity. Ferritic stainless steel is among the most thriving candidate alloy because of BCC structure that its coefficient of temperature expansion is close to that of other SOFC. Nonetheless, the increase of electrical resistance over the time and the evaporation of... 

Excellent mechanical properties and high aspect ratio of carbon nanotubes have been the reason for extensive research on nanocomposites reinforced by this nanostructure, and recently metal matrix type of these nanocomposites, due to their advantages, have received great attention. Moreover, nanoporous materials are another type of nanostructured materials which due to their very high porosity have special applications in hydrogen storage, separating molecules, purification and energy absorption. Because of completely neglecting the details of nanoscale structure and not paying attention to the length scale, classical continuum mechanics methods are not precise enough for evaluating different... 

Using materials with better mechanical properties is always advantageous. Many sectors, including automotive and aerospace industries, are always seeking new materials with better mechanical properties and lower weight. A lighter vehicle means lower emission and higher fuel efficiency. Thus, developing new methods to investigate novel materials is very important. One of these novel materials are Metal Matrix NanoComposites (MMNCs). MMNCs are fabricated using nano scale particles embedded in metallic matrix. High strength, stiffness, and hardness are the three most important mechanical properties of MMNCs. Introducing a suitable way of modeling these materials is important since it will... 

Relying on current fuels and technologies in vehicle industry in recent years has led to an enormous challenge such as air pollution, climate change and energy security. In the other hand, fossil fuel reservoirs are being depleted with an intensive rate. Nowadays, hydrogen, as a substitute for current fuels and hybrid, fuel cell and electric vehicles, as substitutes for current vehicular technologies are under research and development. Because of abundant advantages such as high efficiency and long cycle life, no depreciation for parts and scalability, fuel cell vehicles is of interest for many car manufacturers throughout the world and lots of them have demonstrated their prototype fuel... 

In this research, aluminum composite foams reinforced with silicon carbides were manufactured using melt direct foaming with gas. The density of produced foams changed from 0.1 to 0.35 g cm-3. At first, simulation of bubble configuration was carried out in order to specify the effective parameters on foaming process. As the result, parameters such as ceramic particle volume percentage and gas type were selected. Results showed that, to process foam with acceptable stability, there was a critical minimum of ceramic particles concentration above which, stability of the foam was enhanced. Gas type was also showed to have a significant effect on foam stability. Argon, carbon dioxide and air were... 

In designing of sheet metal forming processes, knowledge of the maximum permissible strains in the sheet is of prime importance for the design engineer. Thus, a large amount of experimental and theoretical researches have been carried out to estimate these limiting strains. A renowned and prevalent theoretical method for prediction of forming limits is the Marciniak and Kuczynski model. In this model, it is assumed that a small groove exists in the sheet even before applying the loads; and due to the weakness of this region, necking starts from there. The major problem of the M-K model is requiring of an experimental point to “calibrate” the results. Furthermore, while experiments indicate... 

The extrusion process of flat dies in non-symmetrical sections is among processes which in spite of enormous applications, has not been thoroughly understood due to complexities of analyzing the section deformation process. In these dies, a zone is formed during egression of billet from die in which the material is stationary and accordingly the material on profile of this zone shears. This zone is called the dead metal zone. Dead metal zone is a natural non-linear die and it is pragmatic to obtain the value of the deviation. In the present work, determine the curvature of the exit profile by using prediction of the dead metal zone profile is target. The dead metal zone profile is assumed... 

The present study was aimed at investigating the use of platinum and platinum nanoparticles stabilized on zeolite as catalyst for the reduction reaction of p-nitrotoluene as one of the dangerous pollutants. The adsorption properties of platinum based on zeolite have been studied for some heavy metals. Monodispersed platinum nanoparticles were synthesized through reduction of H2PtCl6 by ethanol in the presence of polyvinyl pyrrolidone (PVP) as a stabilizer, and then were immobilized on four types of zeolites. The study then focused on elaboration of the catalytic activity of the nano catalysts under different operational conditions. In order to investigate the catalytic properties, operating... 

The Pulse Laser Deposition (PLD), Sputtering, Evaporation and Metal-Organic Chemical Vapor Deposition are the common methods for thin film fabrication. The mentioned methods are very costly for commercials application. Due to the importance of cost in large thin film fabrication, scientist tried to find a low-cost method. The low-cost non-hazardious Metal Organic Deposition (MOD), method which is based on chemical manufacturing method was developed. The type of the solvent and removal the destructive residual materials are the main challenge in this method. In this project, by utilizing acetyl acetone method, high quality MOD YBCO superconducting thin film has been fabricated and will be... 

In recent years, carbon nanotubes have been the focus of considerable researches. Numerous investigators have reported remarkable physical and mechanical properties for this new form of carbon. In particular, the exceptional mechanical properties of carbon nanotubes, combined with their low density, offer scope for the development of nanotube reinforced composite materials. The potential for nanocomposites reinforced with carbon tubes having extraordinary specific stiffness and strength represent tremendous opportunity for application in this century and recently metal matrix type of these nanocomposites, due to their advantages, have received great attention. Classical theories of... 

Due to recent progresses in lowering the temperature of SOFC, using metallic interconnects instead of ceramic interconnects was possible. Among the alloys used for interconnects, ferritic stainless steels (FSS) due to thermal expansion coefficient compatible with other cell components and cheap fabrication, have become the most widely used material for interconnects. Because of improvement of oxidation resistance of these FSSs, many studies have been made on coating of oxides on FSSs and results show that the spinel oxides have the best performance. In this study, the CuCo2O4 performance as coating in SOFC's interconnects was investigated. Coating process used was sol-gel dip coating and raw... 

Volatile Organic Compounds (VOCs) have become an important source of air pollution due to their extended use as solvents in many industrial processes. Aliphatic and aromatic VOC removal from contaminated air can be achieved by a variety of methods including catalytic oxidation, adsorption, and biological treatment. Catalytic oxidation is often the preferred process due to its low cost and high efficiency. Supported transition metal catalysts are effective and inexpensive catalysts for VOC removal from contaminated air. The objective of this project was to prepare low cost transition metal catalysts (Fe, Ni, Co, and Cu) supported on alumina for catalytic oxidation of cyclohexane (as a... 

Exchange rate is one of the key variables which affect different sectors of the economy. Volatile exchange rate causes uncertainty for investors, and inappropriate allocation of resources. Consequently, the high volatility of exchange rate should be considered for investment risk assessment. Base metals industry is one of the main industries in Iran which is largely affected by exchange rate volatility. In this thesis first we model the exchange rate volatility by an ARJI model. Then we use the estimated volatility in a GARCH model to derive the volatility and to estimate the VaR of base metals Index. We backtest the estimated VaR, and compare it with the VaR calculated by GARCH without... 

Hadfield’s manganese steel, nominally Fe–1.2%C–13%Mn, is an alloy with inherent toughness, work-hardening characteristics and excellent resistance to some types of adhesive and abrasive wear. However, due to its low yield strength, it may be deformed markedly before its work-hardening become effective. There have been many studies on increasing strenght by alloying manganese steel, but the effects some other alloying additions such as 1 to 2 percent Al, Cr, W, V despite increase strenght and wear resistance at the low stress abrasive condition, decrease work-hardening characteristics and wear resistance at the high stress abrasive condition. The aim of present work, is to modify...