Sharif Digital Repository

Explaining the angular momentum of the cosmological structures is an essential part of the structure formation theory, and it is not fully understood yet. The Tidal Torque Theory (TTT) is the standard explanation of the initial angular momentum of the halos based on the gravitational instability picture. It predicts the angular momentum growth with linear approximation and has been confirmed for linear stages of proto-halos, but the final halo is a highly non-linear object. Nonlinear effects are hard to model, however, we propose a new mechanism for the nonlinear evolution of the halos. We suggest that when two halos pass by each other, their angular momentum changes with a similar mechanism... 

Pulsar timing arrays (PTAs) serve as a promising method for detecting gravitational waves through the analysis of timing residuals in a network of millisecond pulsars (MSPs). Hellings and Downs pioneered a technique to identify the gravitational wave background (GWB) utilizing PTAs. Given the low amplitude of these waves, the PTA signal represents one of the most viable prospects for the detection of GWB. While the GWB may have an astrophysical origin, its implications for early universe models and inflationary theory are significant and unavoidable. A central issue is whether the primordial gravitational waves (GWs) produced during the inflationary period are sufficiently strong to be... 

In the second half of the 20th century, the standard cosmology was encountered some problems like flatness problem and horizon problem. The inflation theory proposed by Alan Guth in the 1980s, not only solved these problems but also predicted some primordial fulctuations for the universe that as a source for the fluctuations of cosmic microwave background, could explain our observations.The fluctuations predicted by simple inflationary models, are Gaussain. Although we have not found a valid observational result suggesting Non-Gaussianity in the primordial fluctuations so far, but these simple single scalar field inflationary models, are very specific. So it is very probable that in the... 

In this project, in general, we have studied the particle production during and after the Inflation.Today, the Inflation is considered as a part of the standard model of cosmology and it solves so-called two cosmological problems: flatness and horizon problem. According to it,the universe after the inflation becomes supercool; however, the thing we observe today is in contradiction with this theory. The Reheating era is a time period in which the inflationary field gets to its minimum of potential and starts to socialite. Actually, here is the place in which the phenomenon of the particle production occurs. This period is called the Reheating era. therefore, it seems all particles which we... 

In 1917 Einstein used Λ as a cosmological constant in his theory of general relativity, but after a while he receded for some reasons. This constant has been reconsidered again recently. But using it in general relativity has caused some problems. In this thesis, after a short review of important elements in studying cosmological constant problem, like a short introduction to general relativity and cosmology in the first chapter as well as a short review of quantum field theory and supersymmetry in the second chapter, we will study the history and introduction of the problem in the third chapter.in the third chapter , different solutions to this problem ,like supersymmetry,anthropic... 

We know the matter can curve the space-time according to the general theory of relativity. On the other hand Quantum Field Theory has been formulated in a flat space-time and we cannot use most of its techniques within a curved space-time. Formulating the Quantum Field Theory in curved space-time is important for various subjects in physics like early universe or black hole physics. We will review the formulating QFT in cosmological background and will look for observable consequences on Cosmological Microwave Background in this thesis  

Semi-automated Continuous Speech system deals with collecting corpus speech. It has very different problems in various natural languages because of different types of speech, and collecting speech corpus containing continuous word is very time-and- cost consuming. To overcome this problem, human-based computing techniques are proposed to collect speech corpus. In this thesis, we describe influencing factors in collecting steps of continuous speech and provide a description of human-based techniques to solve problems specially collecting of continuous speech. To collect speech corpus using human-based techniques, a game has been used so that speech corpus of web users are collected during the... 

Titanium alloy Ti-6Al-4V ELI with high biocompatibility and corrosion resistance, has a lot of applications in biomedical engineering. Disadvantage of this alloy is it’s disability to create a fast and good contact with the host/bone environment, after implanting in the body. Beside that it has low wear resistance. Nowadays to optimize the wear resistance, bioactivity and osteoconduction of surface of implants which are made from this alloy, the surface morphology are optimized in size and distribution. Different surface treatments are used for producing rough and porous surfaces to improve bioactivity along with wear resistance. In this study, surface modification of Ti-6Al-4V ELI was done... 

Commercially pure titanium (CP-Ti) is an important groups of titanium family and because of high strength, low density, high corrosion resistance and biocompatibility, this group of titanium is a suitable choice for application at chemical, petrochemical and medicine industry. Nowadays surgeons have a tendency toward use of CP-Ti instead of Ti-6Al-4V alloy. Moreover, because of body anatomy limitation, use of thick sheets result in high volume and damage soft tissue. Therefore, for sheet thickness reduction, study of the effects of metallurgical and geometrical parameter on fracture behavior of thin sheets is important. In this study, the fracture behavior of CP-Ti thin sheets investigated.... 

The introduction of multi phase steels in recent years resulted in a considerable increase in development of some industries such as automobiles. By inducing soft and hard phases together, these steels could bring better mechanical properties than classical ones. Recent studies on 4340 ferritic- bainitic and ferritic- martensitic dual phase steels, indicate that 34 volume percent ferrite with hard phase, has the best combination of strength, toughness and fatigue properties. In present research, by proper heat treatment on a 4340 steel, it is tried to prepare specimens ferritic- bainitic- martensitic triple phase steels with 34 volume percent of ferrite and different percents of martensite.... 

In the research transient liquid phase bonding of AL6061-SiC composite was performed using copper as interlayer. The joining praocess was performed in various tetemperatures (560c and 580c and 600c) And times (6 and 8 hours). Optimum Joining condition was determined by using optical and of the joint, micro hardness test was performed from the bonding zone to the bulk of the base composite. The fatigue endurance limit of Al6061-SiC composite was determined by several fatigue tests and TLP bonded samples were loaded under achieved endurance limit stress. Finally fatigue fracture surface of both base composite and TLP bonded samples were studied by using of scanning electronic microscope  

In this research, transient liquid phase (TLP) bonding of FSX-414 superalloy was investigated using MBF-80 interlayer. The bonding was performed at different temperatures (1050-1200oC) for various times (1-120min). Also, different gap sizes were studied using the interlayer with various thicknesses (25-100µm). Homogenizing treatment was performed at different temperatures (1175-1225oC) and times (1-6h) for the samples with complete isothermal solidification. The microstructure of the samples were studied using the optical, scanning electron and transmission electron microscopes. Also, XRD, SEM/EDS, SEM/WDS and TEM/EDS analyses were used to analyse the observed phases at the joints. To... 

AISI 4340 steel bars were austenitized at 900°C for 1 hour followed by heating at 740°C (ferrite and austenite region) for 100 min and quenching into a salt bath at 300°C for different times followed by quenching into water to obtain triple phase microstructures with 34 Vol.% ferrite and various martensite (or bainite) contents. Presence of three phases in adjacent of each other confirmed by metallographic analysis and TEM technique. Volume fraction of different phases was measured by image analyser. The results of optical microscopy showed that by increasing VM, morphology of martensite varies from uniform distribution of small spherical network particles to large blocky islands with... 

Interlayer composition as one of the most important factors of TLP bonding could affect bonding region composition and microstructure that both of them govern the mechanical properties of the joint. Due to the risk of substrate microstructure degradation , welding of dual phase steel needs more cautions. TLP bonding ideally acquires joints having more similar microstructure to substrate. Moreover , using this method could avoid the change of base metal microstructure when heat treatment of carbon steel is postponed until after bonding. Fe-based , Ni-based and commercially pure Cu are three different composition has been studied in this work. The bonded samples were investigated by optic and... 

Dual Phase ferritic-martensitic steels are kind of high strength low alloy steels (HASLA) which are widely used in industry because of their strength and formability. Since welding of dual phase steels in conventional welding methods causes structural change and Subsequently alters mechanical properties of both weld zone and heat affected zone(HAZ), additional operations which are not commercially viable is needed to achieve a dual phase structure in structurally changed regions. In this study, transient liquid phase (TLP) bonding method is used during dual phasing process in order to preserve dual phase ferritic-martensitic structure and avoid extra heat treatment. Applied pressure during... 

Stainless steels have many applications in different industries like chemistry, medicine and food production. So for using these alloys it is necessary to join them with appropriate method. In this research, microstructure-mechanical properties relationship of 304LC austenitic stainless steel transient liquid phase bonded using a Co-based interlayer was investigated. For this purpose, bonding was conducted at 1180 ℃ for different holding times. Microstructure of the joints was evaluated by optical microscopy (OM) and Scanning Electron Microscopy (SEM). The results showed that isothermal solidification (IS) was completed within 1800s and no intermetallic compound was formed in the joint area... 

After World War II, steel bearings containing copper element have been developed by the Navy of the United States of America with the HSLA-100 brand. Since 1890 and in order to manufacture tanks and tools for the body of navy equipment, the steel HY 80 was used instead of HSLA-100. Because of the high carbon content, the welding properties of the HY 80 are low. Furthermore, increasing of hardness and decreasing of toughness in the heat-affected zone generated during the welding process lead to susceptibility of this alloy to hydrogen cracking and brittle fracture. Therefore, in order to minimize the cost of welding, low-alloy high strength steel (HSLA) is used which contain copper. Reducing... 

The inflation paradigm solves the so-called flatness problem of the standard cosmo- logical model and explains the homogeneity of the Universe by adding a period of exponential expansion at early times to the standard scenario. Moreover, it provides a mechanism for generating density perturbations, serving as the seed of large-scale structures. One of the successes of inflation is predicting primordial scale-invariant perturbations that agree with the observations of cosmic background radiation. Nev- ertheless, there will be a cold, matter-free universe after exponential expansion, so one needs a process that leads to a hot cosmos containing standard particles after the end of inflation, the... 

In this thesis, in order to improve the performance of the mobile robot navigation, a Fuzzy approach is used for making a safe path in the cluttered environment with hurdles in the work space of the robot. To this end, based on data collected from instantaneous location of the robot and location of the robot, heuristic rules are extracted. Also, in order to obtain optimal data fusion of the sensors, Kalman filter is used to localize the robot. In this regards, by using the kinematics of the robot and supposing the white noise in the process and measurements, the position and orientation of the mobile robot are estimated in a real-time and adaptive manner  

Cooling sandwich panels are one of the components of the heat exchangers used to cool the components of the sandwich panels . One of the applications of TLP Bonding for steel can be called cooling sandwich panels. In this research , TLP of 304 Stainless Steel , using Fe-base interlayer at 1200 C has been investigated . Cross section of bonded samples were examined by optical and scanning electron microscopes. Microscopy results revealed that 2h is sufficient for completion of isothermal solidification and there is no eutectic compound at bonding zone . Bonding in this condition resulted in the free precipitates in the diffusion affected area. By analyzing eutectic phase in bonding areas at...