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Browsing Materials Science by Author "Bekele, Mulugeta (PhD)"
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Item Bragg-Williams Model of Ordered B2, L12 and L10 Type Binary Intermetallic Compounds Containing Point Defects(Addis Ababa Universty, 2011-04) G.Egziabher, Daniel; Bekele, Mulugeta (PhD)Ordered intermetallic compounds draw great interest at the present time because of both possible applications and fundamental scientific importance. The scientific interest is derived from the recognition that many intermetallics exhibit considerable ranges of nonstoichiometry which correspond to the presence of constitutional point defects which are temperature independent. Additional to these constitutional point defects thermally activated point defects can occur. The total defect structure (i.e. the types and the concentrations of the point defects) determines important properties of the intermetallic compounds (e.g. all diffusion dependent properties). Hence, for understanding and control of these properties a detailed knowledge of the defect structure in intermetallic compounds as function of composition and temperature within the homogeneity range of the compounds is a prerequisite. This thesis is aimed at formulating a model which can be used to calculating the ordering energy parameters ΔHAB and ΔHB□, and the concentration of vacancy (z) and the concentration of Antistructure Defect as a function of the composition parameter χ and the temperature T. The ordering energy is related to but different from the formation energy, because the former is the energy change from the disordered alloy AB to the ordered alloy AB, while the latter is the energy change from the pure metals A and B to the ordered AB. Having obtained the model parameters (note that these are H ij and S ij for the fitting of the vacancy concentration and hij and sij for the fitting of the thermodynamic properties, respectively), the temperature dependence of the vacancy concentration can also be determined.Item Effects of Carbon Content in Ferrite and Martensite on Lattice Parameter and it’s Diffusion Process (Computer Simulation)(Addis Ababa Universty, 2011-01) Mengistu, Yilma; Bekele, Mulugeta (PhD)Molecular dynamics and molecular static simulation has been performed to investigate the structural transition of martensite and the diffusion properties of single carbon atom in the bulk of ferrite. For both types of simulations embedded atom potential was applied to describe atomic interaction. This potential considers short-range interaction of iron and non-interacting defects which can be valid only for small concentration. After carrying out molecular static simulation we found that octahedral sites are the preferential position of carbon that have lowest formation energy -6.277 eV with minimum energy configuration than tetrahedral sites which have -5.349 eV formation energy. The migration energy of carbon calculated using this method when carbon moves from one preferential position to the next preferential position is 0.89 eV. This result is in good agreement with Ab initio calculation of migration energy 0.90 eV and experimental results 0.83 eV. From direct molecular dynamics simulation result of equilibrium lattice constant at room temperature we found that ferrite that contains carbon less than 0.081 wt % have cubical structure (that have equal values of lattice constant in the three direction (100), (010) and (001)). When the amount of carbon contained reaches 0.081 wt % the equilibrium lattice constant of martensite started splitting in to two values a and c. This splitting shows ferrite-martensite structural transition. Thus this content in our case is the minimum concentration (critical point) at which structural transition of martensite from body centered cubic to body centered tetragonal occur at room temperature. The diffusion coefficient calculated using molecular dynamic method is 9.736 × 10−8m2/s.Item Model Computation of Water Clustering (H2O) n effect in the Negative Ion Proton Transfer Reactions(Addis Ababa Universty, 2010-06) Dereje, Gemechis; Bekele, Mulugeta (PhD)We report the effect of water clustering (H2O) n in the negative ion proton transfer reactions on reaction rate constants and the change in the Gibbs free energy of the corresponding reactions. Reaction rate constant of the reaction between the new proposed reagent acetate ion CH3COO- than the usual hydronium H3O+ and environmentally important molecules were computed. The computation was made using the hard sphere average dipole orientation theory (HSA) and the average dipole orientation theory (ADO) as an investigation tool. It was found that rate constant decreases for a range of cluster size and starts to below up steadily for the HSA. For ADO, it decreases approaching a lower limiting value. Rate constant were observed to rise as molecular size in enlarges and became non-sensitive to isomers of the same molecule for molecules which have similar dipole moments but different molecular size and structural configuration. Fifteen atmospherically potential molecules were chosen in the study scheme of the effect of the clustering to the reagent ion on the proton transfer reaction rate constant and the change in Gibbs free energy of these reactions. In both cases of the theories, the computed results of the rate constants of the clustered reagent ion were found to be less than that of the non-clustered. In contrary to this, change in the Gibbs free energy of the reactions was found to be greater. The rate constant of the reaction between clustered ions of these molecules and the neutral acetic acid molecule was also computed. These computed results facilitate the use of NI-PTR MS technique, which uses acetate ion as reagent ion, in the application of laboratory investigations and field measurement of environmentally important volatile organic compounds VOCs.Item Simulation of Three-Layer Solid State Memcapacitor(Addis Ababa Universty, 2010-06) Wami, Shiferaw; Bekele, Mulugeta (PhD)Memcapacitors are capacitors whose capacitance depends on the past states through which the system has evolved. The focus of this thesis is on the possible realization of a three-layer solid-state memory capacitive (memcapacitive) system. The functioning of this device is based on the slow polarization rate of a medium between plates of a regular capacitor. To achieve this goal, this work considers a three-layer structure embedded in a capacitor. The three layer structure is formed by metallic layers separated by an insulator so that non-linear electronic transport between the layers can occur. Unlike conventional capacitor, the capacitance of this device depends on the history of the system. Our calculation shows non-pinched hysteretic charge-voltage and capacitance-voltage curves, and both negative and diverging capacitance within certain ranges of the field. This property of memcapacitive system makes it a good candidate for non-volatile memory application.