Sharif Digital Repository

In this study, the ultrafine-nanometric yttria stabilized zirconia (YSZ) particles were used for reinforcing of copper metal powder. To producing of bulk nanocomposite specimens, the copper powder and the proper volume percent of reinforcement powders (i.e. 2, 3 and 5) were mixed using Turbula mixer for one hour. Before the mixing of two metallic powders and ceramic nanoparticles, for separation of agglomerated zirconium oxide nanoparticles that witch leads to producing of nanocomposite specimens with satisfactorily distributed secondary phase, the ultrasonic devise with enough quantity of ethanol as neutral liquid have been used for 15 minutes. The powders mixture have been placed under... 

The aim of this study was to evaluate the effects of applying low replacement ratios (0.75% and 1.50% of the binder weight) of nano-SiO2 particles with different specific surface areas (200 and 380 m2/g) on the properties of high-performance concrete (HPC). Mechanical (compressive and splitting tensile strengths), electrical resistivity, non-destructive (ultrasonic pulse velocity), and microstructural (mercury intrusion porosimetry, X-ray diffraction, and scanning electron microscopy) tests were conducted to investigate the macroscopic and microscopic effects of nano-SiO2 particles on HPC characteristics. The results indicated that the performance of nano-SiO2 particles significantly... 

Performance and mechanical properties of concrete such as tensile strength, compressive strength, conductivity, etc change with creation and emission of cracks. As health monitoring of important structures is a vital subject that demands a large amount of budget annualy. Studying problems and disadvantages of concrete is one of the most important branches of research.
There are many ways to detect damage in concrete. Electrical resistance method that has been used in this thesis. In this study, an instrument called Risistivity Meter is used to measuring the electrical conductivity. This instrument measured the electrical conductivity, that it can be appropriate way to detect cracks... 

This study was conducted in order to investigate on production and propeties of lead free solder alloy. For this purpose, Sn-Ag alloy with 3.5 weight percent of silver and sn-3.5Ag-0.7Cu at eutectic composition were produced with casting procedure.Single-lap Specimens were used to simulate real solder joints with the solder reflowed between two pure Cu substrate.Microstructural characterization, electrical resistance measurements and shear-tension test were performed.A deacrease of electrical resistance was observed in produced alloys in comparison with Sn-Pb . Experimental results showed that mechanical properties of produced alloys was better than common pb content alloy.The effect of... 

For application such as electrical contacts and spot welding electrodes, both high electrical conductivity and wear resistance are required. Additionally, such parts should have structural stability at evaluated temperature. Copper based composites containing carbides or oxides reinforcement can be the good choice for production of such parts. In present work, in-situ Cu/NbC composite via mechanical alloying with homogenous distribution of nano reinforcement particle has been produced. For this purpose, in first stage, copper powder was milled with 5, 8, 10 and 12 wt. % of niobium powder in two types of mills, i. e. vibratory dicky mill and planetary ball mill for 5 and 32 h respectively. At... 

One of the most common nondestructive tests is ultrasonic testing which has been paid great attention from specialists of this field. Apart from being economical and efficient, ultrasonic waves are sensitive to small changes in the structure and thus have a high degree of reliability. The most common method of ultrasonic testing is manual single-element A-scan inspection, carried out offline using longitudinal waves with pulse echo technique which is used in this thesis. The purpose of this thesis is to monitor structural health of thin metal joints and estimate quality of resistance spot welds by simulating ultrasonic testing using a finite element software. Initially, acoustic properties... 

Aluminum and steel are the most utilized metals in industry and joining of them has many applications including heat exchangers and parts in aluminum extraction process. In this research, Upset Resistance Welding of aluminum to stainless steel was studied. In this regard, the effect of current intensity, time of welding, in-situ post-heating operation and post-heating operation in the furnace were investigated on the microstructure, mechanical properties and electrical resistance of welds. The currents of 2.25, 2.30 and 2.35 kA and times of 55, 60 and 65 cycles were applied for welding. Then in-situ post-heating conducted under currents of 1.75, 2.25 and 2.75 kA and 10 cycles on the sample... 

Over the last few years, exploiting particle sensing systems for micro-nanoparticles has grabbed much attention. Attaining the physical properties of particles using resistive pulse sensing has been one of the utmost applicable methods of sensing particles. Pores are pivotal elements of systems based on resistive pulse sensing. Two electrodes are placed at both sides of the pore, filling the containers with an electrolyte solution. Pulses of particle translocation across the pore can be recorded by applying a voltage across the electrodes. In more developed versions of resistive pulse sensing systems, pore size is tunable to attain polydisperse particles within a dispersion. In subsequent... 

In this study, Butt resistance welding of copper to aluminum wires was studied. In this regard, the effect of current, pressure, post weld heat treatment and using intermediate layer of tin on the microstructure, strength and special electrical resistance of welds were examined. Based on a series of initial tests to establish the connection with the appropriate strength, the diameter of the aluminum and copper wires respectively 3 and 3.5 mm and their length 9 and 3.5 cm were selected. In order to weld samples, currents 4.4 to 4.7 KA and pressures 0.3 and 0.5 MPa were used. Then post weld heat treatment at currents 0.5, 1 and 1.5 KA on the welded sample in the pressure 0.3 MPa and the... 

It is well known that Cu/Al2O3 nanocomposites have high potential for use in structural, electrical applications which enhanced mechanical characteristics are required. In the present work, in order to improve the in-situ oxidation kinetic of solute element, chromium was used instead of aluminum. At the initial stage, elemental Cu and Cr powders were mechanically alloyed for 60h under argon atmosphere. It was followed by mechanically milling of Cu-Cr pre-alloyed and Cu2O powders. The nanocomposite specimens, containing different amounts of Cr2O3, depending on the Cr content in the range of 1-3 wt. %, were produced by in-situ oxidation of Cu-Cr pre-alloyed powders. To prevent oxidation of... 

Electrical connectors that used in high speed systems interrupt the connection at the effect of Inertia. Also connectors should be able to work in high vibration conditions without any problems. Vibration of the connector pins can reduce the contact area or even remove it at very short time intervals, which lead to increase connector resistance or disconnection of instantaneous current. These current fluctuations will result in unpredictable performance disturbanceof the electrical system in very sensitive systems and therefore it should be avoided. In this Project, impacts of shock and vibration loads on the connectors is modeled by using FEM and with consideration of this model, connection... 

Aluminum coated steel sheet is an excellent composite material in which; the strength and economy of steel and durability of aluminum are combined. This material is produced by several methods. However, there are two main technologies to produce aluminized steel sheets: 1) the hot dip method and 2) the aluminum clad method (cold roll welding process). The hot dip method is most widely used for producing the aluminum coated steel. The aluminum coated steel sheet manufactured by this method is believed to be inferior to an aluminum sheet with respect to surface appearance, corrosion resistance and formability of the coated metal. Consequently, the aluminum clad steel method is one of the most... 

This study was conducted in order to investigate the effect of difference in electrical resistivity between low carbon steel and austenitic stainless steel on the physical and mechanical properties and fracture mode of dissimilar resistance spot welds of these alloys. For this purpose, plastic deformation using constrained groove pressing combined with rolling applied to low carbon steel in three levels of 40%, 156% and 272%. Microstructural characterization, micro-hardness measurements and shear-tension test were performed. It was shown that by increasing the pre-strain in low carbon steel its electrical resistivity increases from 13.08 μΩ.cm to 19.5 μΩ.cm and because of reduction of... 

Friction stir soldering (FSS) or brazing (FSB) as an emerging joining process, was first developed to resolve some shortcomings in FSLW and FSSLW such as excessive IMC layer thickness and confined bond area. Although it has advantages over conventional furnace soldering and/or brazing, including localized heating, less detrimental thermal effects on the substrates and capability to reduce the amount of voids at the joint interface. The aim of this study is to investigate the effects of rotational speed, traverse speed and plunge depth of the pinless tool on the shear strength and electrical resistivity of the copper-coated graphite-copper FSSed joints with the aid of Sn-37Pb solder alloy. At...