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Multi-walled carbon nanotubes (MWCNTs) are a potential alternative to commonly used catalyst support structures in hydrotreating processes. Synthesis of MWCNTs with specific pore diameters can be achieved by chemical vapor deposition (CVD) of a carbon source onto an anodic aluminum oxide (AAO) template. AAO films consist of pore channels in a uniform hexagonal arrangement that run parallel to the surface of the film. These films are created by the passivation of an aluminum anode within an electrolysis cell consisting of certain weak acid electrolytes. Changing the concentration of the electrolyte (oxalic acid) and the electrical potential of the electrolysis cell altered the pore channel diameter of these AAO films. Controlling the pore diameter of these templates enabled the pore diameter of MWCNTs synthesized by CVD to be controlled as well. The produced MWCNTs were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Raman spectroscopy, and N2 adsorption analysis. Anodizing conditions of 0.40 M oxalic acid concentration and 40.0 V maximum anodizing potential were found to produce AAO films that resulted in MWCNTs with optimum surface characteristics for a catalyst support application. CVD parameter values of 650°C reaction temperature and 8.00 mL/(min·g) C2H2-to-AAO ratio were found to produce the highest yield of MWCNT product.<p> The MWCNTs were synthesized for the purpose of supporting hydroprocessing catalysts, with several grades of NiMo/MWCNT sulfide catalysts being prepared to determine the optimum pore size. These catalysts were characterized by techniques of TEM, CO chemisorption, N2 adsorption, and H2 temperature programmed reduction (TPR). A MWCNT grade with 67 nm inner diameters (found from TEM analysis) was found to offer the best hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activities for the treatment of coker light gas oil (CLGO). After determining the most suitable pore diameter, the optimum catalyst metal loadings were found to be 2.5 wt.% for Ni and 19.5 wt.% for Mo. The optimum catalyst was found to offer HDS conversions of 90.5%, 84.4%, and 73.5% with HDN conversions of 75.9%, 65.8%, and 55.3% for temperatures of 370°C, 350°C, and 330°C, respectively. An equal mass loading of commercial NiMo/ã-Al2O3 catalyst offered HDS conversions of 91.2%, 77.9%, and 58.5% with HDN conversions of 71.4%, 53.2%, and 31.3% for temperatures of 370°C, 350°C, and 330°C, respectively.<p> A kinetic study was performed on the optimum NiMo/MWCNT catalyst to help predict its HDS and HDN activities while varying the parameters of temperature, liquid hourly space velocity (LHSV), pressure, and gas-to-oil flow rate ratio. Rate expressions were then developed to predict the behavior of both the HDS and HDN reactions. Power law models were best fit with reaction orders of 2.6 and 1.2, and activation energies of 161 kJ/mol and 82.3 kJ/mol, for the HDS and HDN reactions, respectively. Generalized Langmuir-Hinshelwood models were found to have reaction orders of 3.0 and 1.5, and activation energies of 155 kJ/mol and 42.3 kJ/mol, for the HDS and HDN reactions, respectively.

Today, ceramic, polymer, and composite nanofibers are among the most charming materials for nanotechnology. Because of their small characteristic dimension, high surface area, and microstructural features, they provide unique mechanical, optical, electronic, magnetic, and chemical properties for an extensive variety of materials applications. Electrospinning provides an effective way of the nanofiber production in a nanometer scale. This technique utilizes a high voltage DC to create a strong electric field and a certain charge density in a viscous solution contained in a pipette. As a result, fibers with diameters ranging from the micrometer to nanometer are formed from this charged solution. This study deals with, the fabrication of alumina borosilicate ceramic nanofibers using electrospinning technique. Alumina borosilicates contain important components having intriguing characteristics for many applications and have been widely studied with different compositions. In this study, alumina borosilicate/PVA solution was prepared using the conventional sol-gel method. Polyvinyl alcohol (PVA) was added into this solution to increase the viscosity for electrospinning. After the alumina borosilicate/PVA solution was electrospun into fibers, high temperature sintering was carried to obtain ceramic alumina borosilicate fibers. The products were characterized by scanning electron microscopy (SEM), X-ray diffractometry (XRD), Fourier transform-infrared spectroscopy (FT-IR), and thermogravimetric/differential thermal analysis (TG-DTA) techniques.

The aim of this study was to produce partially reinforced aluminum metal matrix composite components by insertion casting technique and to determine the effects of silicon content, fiber vol% and infiltration temperature on the mechanical properties of inserts, which were the local reinforcement parts of the components. Silicon content of alloys was selected as 7 wt% and 10 wt%. The reinforcement material, i.e. Saffil fiber preforms, had three different fiber vol% of 20, 25 and 30 vol% respectively. The infiltration temperatures of composite specimens were fixed as 750 &deg / C and 800 &deg / C. In the first part of the thesis, physical and mechanical properties of composite specimens were determined according to the parameters of silicon content of the matrix alloy, infiltration temperature and vol% of the reinforcement phase. X-ray diffraction examination of fibers resulted as the fibers mainly composed of deltaalumina fibers and scanning electron microscopy analyses showed that fibers had planar isotropic condition for infiltration. Microstructural examination of composite specimens showed that appropriate fiber/matrix interface was created together with small amount of micro-porosities. Bending tests of the composites showed that as fiber vol% increases flexural strength of the composite increases. The highest strength obtained was 880.52 MPa from AlSi10Mg0.8 matrix alloy reinforced with 30 vol% Saffil fibers and infiltrated at 750 &deg / C. Hardness values were also increased by addition of Saffil fibers and the highest value was obtained as 191 HB from vertical to the fiber orientation of AlSi10Mg0.8 matrix alloy reinforced with 30 vol% Saffil fibers. Density measurement revealed that microporosities existed in the microstructure and the highest difference between the theoretical values and experimental values were observed in the composites of 30 vol% Saffil fiber reinforced ones for both AlSi7Mg0.8 and AlSi10Mg0.8 matrix alloys. In the second part of the experiments, insertion casting operation was performed. At casting temperature of 750 &deg / C, a good interface/component interface was obtained. Image analyses were also showed that there had been no significant fiber damage between the insert and the component.

In this study, effects of changes in various parameters of the Bayer process on the amount of &ldquo / red mud&rdquo / , which has many environmental drawbacks, were examined in accordance with the principles of &ldquo / sustainable development&rdquo / . The production process of SeydiSehir Aluminum Plant is modeled as a case study. First a steady-state mass balance calculation is carried out by incorporating sequential modular approach. Then a model of the Bayer process digester is programmed and several simulations are carried out using this model. Results of the mass balance calculation revealed that changes in the extent of the digestion reaction, which is a function of temperature and caustic concentration, and washing efficiency may have a 2.07% decrease on the amount of red mud produced,which implies nearly 10,000 tons of decline per annum, while amount of hydrate produced is increased by 4.52%. A 7.40 % decrease on the amount of red mud produced on dry basis per kg of hydrate was found to be achievable. Optimum operating temperature for the digester was calculated as 277.3 &deg / C.

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Previous studies on very high-cycle fatigue behavior of thin silicon films suggest a strong environmental dependence of the degradation mechanism, the precise nature of which is still subject to debate. In the present study, 2-micron-thick polycrystalline Si notched cantilever beam structures were used to investigate fatigue degradation in a high-temperature (80°C), high-humidity (90%RH) environment. The specimens were subjected to fully reversed sinusoidal loading at resonance (~40kHz) with stress amplitudes ranging from 1.46 to 1.6GPa, resulting in life-spans between 10⁶ and 10⁹ cycles. Comparison to a reference set of S-N data obtained at moderate environmental conditions (30°C and 50%RH) reveals a strong tendency for faster degradation with increasing temperature and humidity. The obtained damage accumulation rates in the 80°C, 90%RH environment exceed the reference by two orders of magnitude. Transmission electron microscopy (TEM) on vertical through-thickness slices reveals oxide thickening after cycling. The influence of ~20nm Al[subscript2]O₃ deposited on the surface of the fatigue specimens using Atomic Layer Deposition (ALD) technique was also studied. The presence of the alumina coating results in a higher fatigue resistance at 30°C and 50%RH, as well as a drastically different frequency evolution behavior. No oxide thickening was observed in the TEM for coated run-out specimens. A model is proposed to explain the different degradation behavior of the ALD-alumina coated samples. Thickened oxides after cycling appear consistent with the reaction-layer fatigue mechanism. Finite element modal analysis incorporating surface oxide layers and cracking was employed to relate the damage observed in TEM to the experimentally measured changes in resonant frequency. In conclusion, the reaction-layer mechanism seems capable of describing micron-scale polysilicon fatigue, even though the critical processes such as room-temperature, stress-assisted oxidation remain elusive.

There are many problems associated with the use of fossil fuels to produce fuels and chemicals, and lignocellulosic biomass stands as a promising alternative fuel/chemical feedstock. Large scale processing of biomass will likely take place in high temperature liquid water due to the low vapor pressure and polar nature of carbohydrates. However, little is known about the material stability of these catalysts in high temperature aqueous phase environments. This dissertation aims to investigate the structural integrity of some common catalytic materials under typical biomass reforming conditions. There are 3 main objectives of this study: 1) identify potentially stable candidates from commonly used materials, 2) understand the mechanism(s) by which these catalysts degrade, 3) design/modify catalysts in an effort to increase their hydrothermal stability. The two main materials investigated in this work are zeolites (faujasite, ZSM-5) and γ-Al2O3 as these are commonly used as catalysts and catalyst supports. A number of physicochemical techniques were used to characterize the materials as a function of treatment time at conditions relevant for biomass reforming. For zeolites, the major findings are that ZSM-5 framework is highly stable whereas faujasite stability depends on the Si/Al ratio, where silicon rich materials are less stable. For γ-Al2O3 based catalysts, it was found that the alumina support hydrates and undergoes a phase transformation to form crystalline boehmite (AlOOH) with a subsequent loss in surface area and Lewis acid sites. When metal particles are present on the support, the phase change kinetics are slowed. The role of metal precursor on the stability of γ-Al2O3 supported catalysts was also explored, and it was found that the precursor used in catalyst synthesis changes the boehmite formation kinetics and also affects alumina support dissolution. The final thrust aims to stabilize a Pt/γ-Al2O3 catalyst by depositing silicon on the catalyst surface. The silicon modification is effective in protecting the catalyst from boehmite formation upon exposure to hot liquid water while also stabilizing metal particles against sintering. Additionally, an increase in turnover number for hydrogen production via aqueous phase reforming of sorbitol was observed.

Creep resistance is an important design criterion at high temperatures especially when continuous attempts are made to increase the efficiencies by increasing the operating temperatures. Alumina is an important high temperature material and in addition to that it is used in wide variety of applications such as substrates for electronic packaging, spark plugs, envelopes for sodium vapour lamps, cutting tools (when reinforced with silicon carbide) and in artificial joint prostheses. Studies on creep in alumina. have started as early as 1961. There are differing mechanisms proposed to explain the creep behaviour of alumina in the literature, but until now there is no any unanimous decision as to what the rate controlling mechanism is. Magnesia doped at ppm levels can produce significant changes in the microstructure of alumina, the most important consequence of that being the grain growth inhibition, which renders alumina superplastic. However, in a stoichiometric oxide like alumina, small impurities can create extrinsic defects which would change the diffusivities and creep rates. Therefore the background impurities in alumina should be kept to a minimum, if small dopant effects have to be studied. The present study was undertaken making use of high purity alumina powder and comparing the grain growth and creep properties of pure and magnesia doped alumina, especially since no such investigation was carried out in the recent past with high purity alumina. Pure alumina was processed by cold compaction followed by cold isostatic pressing (CIP) and pressureless sintering in air at 1773 K for 1 hour. Magnesia doped alumina was prepared by calcining a mixture of alumina and magnesium nitrate at 973 K for 2 hours followed by cold compaction, CIPing and pressureless sintering in air at 1773 K. Both pure and magnesia doped alumina were further annealed at 1873 K for various times to get grain sizes in the ranges of 1-5 μm. Grain growth kinetics of pure and magnesia doped alumina were studied at 1823 and 1873 K. The parameter Kg which quantifies the mobility of the grain boundary was got. It was found that Kg had decreased in the magnesia doped alumina (in comparison with pure alumina) by a factor of about 3 to 4 which was marginal and insignificant. The grain sizes followed a log normal distribution in both the cases, indicative of normal grain growth. Creep studies were conducted on pure and magnesia doped alumina in three modes, namely, constant stress, temperature jump and stress jump test. The temperature range used was 1673 to 1773 K and the stress range used was 10 to 100 MPa. The creep parameters were found to be n~1.6, p~3.7 and Q-545 kJ mol"1 for pure alumina and n~l .3, p~3.0 and Q~460 kJ mol-1 for magnesia doped alumina. The creep rates in the case of magnesia doped alumina were found to have increased by a factor of 2 to 3, in comparison with pure alumina. The increase in creep rates were found to be insignificant. The creep data were analyzed and the possibility of the dislocation and interface reaction controlled creep mechanisms were ruled out since they were inconsistent with the data. It was found, from creep parameters and the comparison of theoretical Coble and Nabarro-Herring creep rates with the experimental rates, that Coble creep might be rate controlling. The activation energy values suggested that aluminium ion diffusing along grain boundary might be the rate controlling species. However, when the theoretical creep rates considering various species were compared, the rate controlling species turned out to be oxygen ion diffusing along the grain boundary.