Sharif Digital Repository

In this thesis the attitude dynamics, simulation and control of a flexible satellite with CMG (Control Moment Gyro) actuator in three-dimensional slew and rest-to-rest maneuvers have been studied. The flexible satellite is modeled as a rigid hub and two flexible appendages. The bending and torsion of the flexible panels have been considered. The equations of motion have been derived using the Lagrange method and assumed-modes approach is utilized for converting the partial differential equations of structural dynamics into an ordinary differential equation set. The MATLAB and SIMULINK software has been used for simulation and validation of the equations of motion. The simulation and control... 

Modern communication systems,while demanding high power efficiency, suffer from the inherent nonlinearty effects. Such a problem becomes more critical in LEO satellites, in which there are fundamental power limitations. Considering that the main source of nonlinear effects are power amplifiers, one of the most common methods to improve signal quality in a transmitter, meanwhile maintaining power efficiency, is to utilize linearization techniques for power amplifiers. This project consists of two parts. The first is to design and fabricate a power amplifier for the desired frequency (2.4 GHz) and power requirements (3 W). The other is design and fabrication of a linearizer and matching it to... 

Concrete is the second most structural material in the world. So attention and promotion of the concrete mix design with respect to variable issues and requirements, is very important. On the other hand, these days environment and its pollution has become very popular discussion. Waste-glass which is produced by human products is one of the environmental pollutants. In this thesis it replaces some aggregate in concrete and it can reduce the level of this material in the world. Also we study effects of silica-fume as cement replacement and properties enhancer in our concrete mix design and see how it can ... 

Resin Transfer Molding (RTM) which is one of the categories of Liquid Composite Molding (LCM) process, is considered as one of the most important process in polymer composite manufacturing. To improve this manufacturing method and reaching higher qualities, numerous modified methods has been developed. One of these modified methods is Vaccum Assisted Resin Trasnfer Molding with Flexible Tooling which is considerably applicable in manufacturing parts with high quality, large and complex geometry. Although, this manufacturing method has variety of advantages, there are some problems such as void formation making production difficult and dependent to experimental trial and error. So numerical... 

Interconnects in SOFC’s has oxidation problems which is Because of high temperature operation. Also Cr migration to cathode make chroumiom poisoning problems and all of this affects SOFC’s efficiency. To solve this problem researchers decided to use some protective coating on interconnect. The best group of coatings is Mn-Co Spinels. In this thesis (Mn,Co)3O4 spinels was synthesized via solid state reaction. MC reffered to Mn1.5Co1.5O4 was calcined at 850°C and then sintered at 1200°C. It’s electrical conductivity measured by 4-point probe DC method. It had the lowest ASR (17mΩ.cm2) among other stochiometries. To reduce the sintering temperature, Cu and Y was added as dopants. Among the... 

Because of the high operating temperature, interconnect surface tends to be oxidized and Cr2O3 oxide layer will be created on the surface. This phenomenon is undesirable due to decreasing electrical conductivity, so a preservative coating is needed to improve the interconnect properties. In this paper, a dense spinel thin film was successfully synthesized on a ferritic stainless steel (type 430) substrate by a sol-gel dip coating method. This coating improved chemical stability, oxidation behavior of the substrate and decreased cathode poisoning. The Nickel-Cobalt spinel was coated on the substrate by sol-gel coating process. First, during a hydrolyze process, a stable sol was made up from... 

Because of the high operating temperature, interconnect surface tends to be oxidized and Cr2O3 oxide layer will be created on the surface. This phenomenon is undesirable due to decreasing electrical conductivity, so a preservative coating is needed to improve the interconnect properties. In this paper, a dense perovskite thin film was successfully synthesized on a ferritic stainless steel (type 430) substrate by a sol-gel dip coating method. This coating improved chemical stability, oxidation behavior of the substrate and decreased cathode poisoning. The lanthanum chromite was coated on the substrate by sol-gel dip coating process. First, during a hydrolyze process, a stable sol was made up... 

Plasma electrolytic oxidation process (PEO) is now recognized as a relatively new method in surface engineering. In this process, formation of micro arcs resulted from electric discharges on the surface of the specimen in the appropriate electrolyte, is used to ionize the gas layer which has enshrouded the specimen. In the present study, ceramic coating on aluminized steel was produced by PEO process and the effects of the influential parameters were comprehensively investigated. For this purpose, coating on the aluminized steel with dimension of 25×30 mm² was produced under different condition of processing time from 1 to 15 minutes, current density from 10 to 20 A/dm², frequency from 50 to... 

Because of limitation of titania coating that can act as self-cleaning surface under UV light irradiation, introduction of transition metal ions into titania lattice can decrease band gap energy of titania in order to activate the coating under visible light irradiation. In this research, by using vanadium element and silica prelayer, a titania coating was deposited on the soda-lime glass substarte to produce self-cleaning surface. The scope of coating was to increase the photocatalytic activity and hydrophilicity of vanadium doped titania coating on the soda-lime glass and silica coated glass by sol-gel dip coating method. XRD and FTIR analysis were done on the powders with different... 

Gelcasting is a new ceramic forming technique getting worldwide attention. The process is based on casting of slurry; containing powder, water and water-soluble organic monomers. The premix solution was prepared using Acrylamide as a monomer and Methylene bisacrylamide as a crosslinker with AM:MBAM ratio of 25:1, and Poly Acrylic Acid Sodium as a dispersant (1 wt% based on ceramic powders weight). A series of Al2O3/ZrO2 ceramic nanocomposites with 15, 17.5 and 20 wt% content of ZrO2 and 40, 43, 45, 47, 50, 53 and 55 vol.% of solid loading were fabricated by gelcasting. After casting the mixture, the monomer was polymerized to form gel parts. Drying, burning out and sintering completed the... 

Ceramic coatings on metallic substrates are used in many different applications. In biomaterials science, hydroxyapatite coating due to its similarity to the mineral part of bone structure is used to modification of implants surface. In this research, the HA/TiO2 double layer was coated on Ti-6Al-4V by sol- gel dip-coating method. After preparation of stable TiO2 sol, the Ti-6Al-4V substrate was dipped into the TiO2 sol. The speed of dipping was 10cm/min. After drying at 80°C, the TiO2 layer heat-treated at different temperature (350-500°C) for 1h. The HA outer layer was coated by dipping of Ti-6Al-4V substrate with TiO2 layer into HA sol, followed by heat-treatment at different temperatures... 

Solid oxide fuel cells as high temperature electrochemical devices draw much attention in the last decades due to their fuel flexibility, high efficiency and low pollution. However, lowering operating temperature from about 850°C to around 650°C without significant overpotential loss, in order to lower costs and increase cells life time has remained a challenge. Recently, a new family of mixed ionic and electronic conducting ceramics (MIECs) which are formulated Ln2NiO4+δ (Ln= La,Nd,Pr) and crystallized in Ruddlesden–Popper structure, have been regarded as appropriate cathode materials for the low or intermediate temperature solid oxide fuel cells (IT-SOFC). Amongst these compounds,... 

Nowadays, one of the most important research goals is to develop intermediate-temperature solid oxide fuel cells (IT-SOFC) operated at 500–800 °C. However, the large cathode polarization resistance caused by the reduced temperature is a major barrier against such an urgent demand for commercialization. In this regard, it is necessary to select a proper material as a cathode working efficiently at reduced temperatures without losing its desired performance. Various mixed ionic electron conductors (MIECs), especially Ruddlesden–Popper-type oxides, are used to improve the cathode performance at intermediate temperatures. Among these layered oxides, Pr2NiO4 has been reported to possess the... 

Li7La3Zr2O12 (LLZO) solid-state electrolyte is a very promising electrolyte for a variety of ion lithium solid-state batteries, due to high conductivity of Li ions at room temperature as well as its chemical stability with Li metal anode. LLZO phase has two tetragonal and cubic symmetries. The conductivity of the cubic phase is two times higher than that for the tetragonal phase. To stabilize the cubic phase at room temperature, 0.2, 0.25, and 0.3 mol% Aluminum was doped to LLZO solid-state electrolyte. This powder was synthesized by sol-gel combustion method at 1000˚C. The cubic phase and lack of impurity of the samples doped with 0.25 and 0.3% Aluminum were determined by X-ray Diffraction... 

Refractory metals (W,Re,Ta,Mo,Nb) and their alloys can exhibit exceptional properties at elevated temperatures. However, refectory metals tend to be relatively heavy and, in some cases, can be relatively difficult to form in complex shapes at low cost. Light composites of refractory metals with high-melting ceramics, which can be fabricated into dense, near net shapes at low temperatures, would be particularly attractive for advanced aerospace applications. In this project, WC powders have been pressed and shaped to cylindrical preforms. Secondly, these performs have been sintered slightly(presintered) into porous, rigid ones with about 50% open porosity. Finally, in order to reach dense... 

Hydroxyapatite is the most substantial inorganic component of bone tissue which displays great biocompability and bioactivity. Nevertheless, its mechanical properties is not appropriate for a bone substitiues. Therefore, it is used to improve the mechanical properties of polymer matrix composite scaffolds. In the present work chitosan as a polymeric matrix was employed to fabricate hydroxyapatite- chitosan biocomposite scaffolds. Sol-Gel method was employed to synthesize hydroxyapatite nano particles. Porous scaffolds were fabricated via freeze-drying by introducing two different cross linkers, Glutaraldehyde and Sodium Tripolyphosphate. Mechanical (compressive strength), biocampability and... 

The fabrication of potassium sodium niobate piezoceramic with oriented microstructure and study of its piezoelectric properties is one of the main objectives of the present study. For this purpose, first the raw materials were synthesized. solid state reaction and topochemical reaction were used for the synthesis of potassium sodium niobate and sodium niobate respectively. The X-ray diffraction results determined that the powder synthesized by the solid state method in the binary system KNbO3-NaNbO3 is closer to the morphotropic phase boundary compared to the powder synthesized in the ternary system Na2CO3-K2CO3-Nb2O5. Particles morphology also includes coarse cubic shape and small equiaxed... 

Zirconia Toughened Alumina (ZTA) is a well-known ceramic matrix composite due to its high fracture toughness and good mechanical properties. Addition of spinel (MgAl2O4) to ZTA inhibits alumina grain growth and produces phase boundaries with low strength. This phenomenon leads to formation of a ceramic matrix composite with special mechanical properties such as high temperature super plastic deformation. However, the room temperature mechanical properties of alumina-spinel-zirconia (AMZ) composite such as fracture toughness were rarely investigated by researchers. In this research the AMZ nanocomposite powders were synthesized via solution combustion method. The dense AMZ composite samples... 

Plasma electrolytic oxidation (PEO) is currently recognized as an effective coating method on active metals such as magnesium. In this method, through occurrence of strong electric discharges on the surface of the anode accompanied by electrochemical and thermo-chemical reactions at plasma environment, a relatively thick ceramic coating with complex compounds grows on the metal surface which significantly improves its properties. Regarding the influence of PEO electrical parameters on the morphology and other properties of the coating, the objective of the present study is to prepare a quality ceramic coating on AZ31 magnesium alloy substrate via setting the process parameters, such as... 

In this study, the photocatalytic activity and hydrophilicity of coat improved using Nitrogen and Vanadium elements as dopant in TiO2 coating. By using vanadium and nitrogen elements and silica prelayer, a titania coating was deposited on the soda-lime glass substrate by sol-gel dip coating method to produce self-cleaning surface. XRD and FTIR analyses were done on the powders after heat treating at in 450˚C for 30 min. XPS analysis was done for investigation of N replacement in titania structure. The band-gap energy of formed anatase was derived using UV-Vis analysis. The coating morphology and thickness were also investigated by SEM. Hydrophilicity of coatings was determined by measuring...