Sharif Digital Repository

Designing a scalable protocol for the blockchain will reduce transaction fees and increase the use of cryptocurrencies for financial transactions. Increasing scalability means that if the investment of nodes increases or the number of network nodes increases, the security of the protocol and the decentralization of the network will be maintained and its throughput will increase. In this thesis, with the aim of increasing scalability, the scaling methods of layer one and layer two are examined and compared, and it is shown that sharding is a better method to achieve scalability. In sharding, nodes are divided into several shards (clusters) and each shard manages its own blockchain. The... 

This thesis encompasses two parallel, but independent subjects as follow: The first part of the thesis deals with synthesis of bimetallic Ni-Au nanoparticles (Bi Ni-Au), and application of them as electrodes in the methanol fuel cells. The Bi Ni-Au nanoparticles for this purpose were synthesized by controlled reduction of HAuCl3 and NiCl3 in the aqueous medium using NaBH4 as reductant and PVP as stabilizer, and immobilized on the surface of a glassy carbon (GC) electrode by means of Nafion polymer. The obtained electrode then was conditioned in an alkaline solution by cyclic voltammetric method in potential range 100 -700 mV (vs. Ag/AgCl). Physicochemical characterizations of the... 

In conventional grinding of hard to cut materials such as Ti6Al4V alloys, surface burning, redeposition and adhesion of chips to wheel and workpeice occur visibly unless it is carried out at low speeds and with high volume of cutting fluid. Ultrasonic assisted grinding (UAG) is an efficient machining process for hard-to-cut materials with poor machinability. UAG improves the machinability of such materials by changing the kinematics of the process. In this research, the effect of imposition of ultrasonic vibration on the grinding of Ti6Al4V is studied. Longitudinal vibration at ultrasonic frequency range (20 kHz) is applied on the workpiece and machining forces and surface roughness are... 

Laser Assisted Machining is a novel type of machining in which laser beam warms up the workpiece locally. Using laser beam in machining leads to a reduction in machining forces, an increase in tool life and an improvement in surface quality. This study introduces the process, reviews literatures related to it and then, simulates the process thermo-mechanically. A process in which a Gaussian pulsed ND-YAG laser beam of 700w power is used for machining of AISI 52100 is simulated, considering sufficient properties of material. Simulations are done using Abaqus®/Standard and Abaqus®/Explicit softwares. Also, Abaqus®/CAE visual interface, Intel® Visual Fortran compiler alongside with Microsoft... 

Due to high strength, hardness and high resistance to abrasion, ceramics are difficult for machining. Also as the conductivity coefficient of these materials is low, major amount of produced heat during machining process will be absorbed in the cutting tool which causes intensive wear in cutting tool edge. As a result employing traditional machining for these materials because of low machining speed, tool wear, high roughness and large cutting force, has little efficiency in practice. Therefore non-traditional machining method may improve efficacy. In this thesis effect of utilizing ultrasonic vibration in machining process of fused cast silica ceramic has been studied. Aim of this study at... 

One of the newest methods of rapid prototyping is SLS . In this day, SLS is more common in compare others methods, because it uses little time and man does not nearly interfere in the producing stages of part which is made by SLS method. Also, sophisticated parts, which cannot be built by other methods, can be produced by SLS method. One of the disadvantages of SLS method is low dimensions accuracy of parts produced in this way. This problem has two major reasons: Shrinkage and Warpage. These two phenomena are created because of ununiformed temperature distribution. Others features which have effect on dimensions accuracy are residual stress, porosity and removing space of between powder... 

Natural Boron consists of two stable isotopes, 10B (19.3%) and 11B (80.7%). 10B isotope has a large thermal neutron absorption and therefore, use widely in such aspects as modern industry, atomic energy industry (as control rods, chemical shim and etc.), Medical science and applications in disclosing neutrons to measure neutron reactors fluxes. Boron trifluoride is most importantly used as a reagent in organic synthesis, typically as a Lewis acid. In this study, we designed and built the related setup for preparation of high purity BF3 gas. And then we tried to optimize the efficiency with process design tools such as response surface methodology (RSM). At the end of this study, we were able... 

Hydrogen gas is considered as a clean fuel and energy carrier that its storage for future uses is a challenging issue. One of the methods to store it, is adsorption on a group of adsorbents called metal-organic frameworks. In this research, one of these frameworks called ZIF-8, has been selected to study hydrogen storage capability. To evaluate the effect of having two metals, a mixture of two metals was used in the synthesis process. To characterize the structure of the synthesized adsorbents, X-ray powder diffraction (XRD),adsorption and desorption of nitrogen at temperature of 77 K, Field Emission Scanning Electron Microscopy (FESEM), energy-dispersive X-ray Spectroscopy (EDS) and... 

In the nuclear industry, gas centrifuges are considered of high importance for the purpose of uranium enrichment. Today, with advances in science and technology, the use of gas centrifuges for separating isotopes other than uranium, is so common. With respect to the use of conventional methods such as Pancake equation in calculating the velocity profile in centrifuges, it is needed that A^2=(MV^2)/2RT≫1 (M: Feed molecular weight; V: Peripheral speed of centrifuge; T: Mean temperature of feed; R: The universal constant of gases). While the molecular weight of the feed mixture gets lower, the A2 parameter gets closer to one. So, other methods for modeling the behavior of the gas in the... 

Purpose of ongoing thesis is providing a tool for suitable modeling, increase efficiency and decrease construction cost of solar absorption heat pump. At first, the solar absorption heat pump is modeled by the thermodynamic laws. Then, this model is converted to the new standard mathematical programming model using exergy and thermodynamic Analysis with considering all technical and economic constrains. This mathematical model is optimized by Genetic algorithm (GA). Because of the nonlinear objective function and the constraint, GA is used as optimization algorithm. After applying GA, optimized parameters of cycle heat exchanger designing and its electricity consumption are calculated.... 

Grasping and manipulation of biological cells have an extensive applications in genetics ,cell proliferation ,cell injection and etc. In robotic manipulation ,it is often assumed that the objects being handled are rigid and only small deformations occurred during manipulation. this does not apply for the case of biological cells. biological cells are highly deformable structures, and their material properties are not well quantified. Therefore strategies for characterizing and manipulating of such deformable objects needs to develop. Different experimental techniques have recently been used and devised to study biological cells. On the one hand, although this experimental techniques have... 

Excessive water production in highly heterogeneous mature reservoirs (water flooded or affected by water zones) leads to increased production costs. Water production causes sediment formation, creation of corrosion, water treatment and disposal, and finally due to the inefficiency of production, it leads to closure of the well.Among the methods introduced to reduce water production, the use of polymers and gels as an affordable and effective method has been widely used in enhanced oil recovery processes. The main mechanism in this method is blocking or limiting the major paths of water flow (fractures or high permeability zones) and thereby increasing sweep efficiency in water injection....