Sharif Digital Repository

Diverse methods of producing semi-solid metal have been the subject of extensive study for the past 40 years. Among all these methods, electromagnetic stirring (EMS) and gas induced semi solid (GISS) have been used on a commercial scale. Considering the fact that in previous studies the intensity of electromagnetic field in the central part of the stirring chamber was negligible, a hollow cylinder with an almost pure iron core was utilized in the central part of the chamber in order to intensify the electromagnetic field in the center and to introduce gas bubbles into the melt. Aluminum A356 alloy was stirred by a three phase electromagnetic stirrer and introduction of gas bubbles under... 

It has been five years since the new education system was started, according to which fundamental changes have been applied to school text books. The mentioned changes in the text books are based on “the fundamental transformation document” and “Iran’s national educational curriculum”. In “Iran’s national educational curriculum”, eleven fields concerning pedagogy and learning are explicitly mentioned; one of these eleven fields is “training and learning work and technology”. One of the merits which has been stressed in this field, is “technological thinking”. Nowadays, methods of extending and reinforcing the technological thinking have come to be the subject of investigations in many... 

In this thesis we study the strongly correlated electron systems and investigate the interply between electron-electron and electron-phonon interactions using a non-perturbative method, known as Dy- namical Mean Field Theory (DMFT). This method is capable of going beyond standard perturbation techniques. DMFT is by construction a local theory which neglects spatial correlations. In a first step, we approach the problem of interplay between these two interactions in the normal state of cuprate materials using the Holstein-Hubbard model. We then introduce orbital degrees of freedom, and study the polaron crossover and the pairing transition for a two-orbital model with Jahn-Teller coupling to a... 

The process of energy transfer in quantum systems is an interesting phenomenon due to the emergence of quantum effects . This phenomena can be studied in one of two possible spaces : real space and energy space of the system . In addition , to evaluate how this process is successful , one can define an efficiency as the population of a target state . Finding the approaches to increase this quantity takes into account one of the main objectives of the current researches . On the other hand , isolating the quantum systems from the surrounding environment (bath) in practice is impossible and then interaction between them is inevitable . In this research , we provide a comprehensive... 

In this project we tried to examine the effective variables on rent to price ratio and how the variations in R/P influence households’ decisions. Using rent to price ratio, the efficiency of Housing market has been inspected. However, the results for Tehran are in contrast to efficiency assumptions. Moreover, having access to unique database which remarkably increase the accuracy of our estimation, the relation between rent risks and House price has been examined. The results show that rent fluctuations has positive impact on house price. Rent to Price ratio has different applications and we tried to determine the variables that can explain this ratio. Literature review demonstrate that... 

This thesis deals with a theoretical and experimental investigation of different approaches for reduction of electronic recombination in the semiconductor/Dye interface of dye sensitized solar cells (DSSC) including dye coverage, core-shell structures and energy level distributions. Interface of ZnO/TiO2 in the core-shell structure have been studied by density functional theory and the energy level distribution of surface atoms, recombination and Fermi level changes, open circuit voltage and surface dipole distributions have been discussed.
The presence of TiO2 shell increases the ZnO surface dipole moment, and shifts the ZnO conduction and valence bands to higher energies. Also, it... 

In this project, we have studied the geometric and electronic structures of CdS nano-particles. The bulk CdS is known to be a semiconductor with the experimental energy gap of 2.42eV which is more than twice the theoretical resuls based on LDA or GGA energy functionals used in the density functional theory. However the calculations based on Kohn-Sham formalism used in DFT (KS-DFT) predicts the geometric structure of bulk semiconductors fairly well. Therefore, we have employed the KS-DFT formalism for our CdS nano-particles and have used the Gaussian as well as SIESTA computer codes to predict the geometric structures of these nano-semiconductor-particles. For calculating their electronic... 

Since the classical continuum theory fails to deal with the problems associated with defects, stress concentrators, and relevant deformation phenomena in solids, alternative approaches that can detect the atomistic nature of materials' fracture are required. The deficiency of the capture the size effect which yields delusively high values for some components of the stress field right on the edge of the stress concentrators, and its weakness in describing the complex interaction between small inhomogeneities, cracks and the like when they are only a few nanometers apart, are among some of the disadvantages of the classical approach. In recent years, however, atomistic methods are emerging to... 

Aluminum Bronzes have many applications in marine environments. These alloys suffer from both hot cracking and cold cracking. In order to overcome the hot cracking and cold cracking, Semisolid Stir Joining method and a proper thermal cycle was used respectively. Effects of temperature, stirring rate, and tool type were investigated in Semisolid Stir joining method. In this method, butt joint design was used in order to place specimens, and the specimens were heated up to specific temperatures (920, 925, 930°C). A stirrer (Cylindrical and Grooved tool) with three rotational speeds (800, 1200, 1600 RPM) was introduced into the stir weld seam. Welded specimens were cooled to the 900°C... 

The low female labor force participation rate in the Iranian labor market relative to the global average and even to countries that do not differ significantly from the socio-economic characteristics of our country is one of the important questions of the Iranian labor market. In this study, we seek to answer the question of whether the economic participation rate of married women will increase if the gender gaps in wages, job findings and job losses among women and men are eliminated in the Iranian labor market. To answer this question using household expenditure and income data and labor force data, we show that between the two categories of occupations, in terms of the share of women's...