Browsing by Author "Winkler, Christopher Reid"
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- The Effect of Milling Time on the Structure and the Properties of Mechanically Alloyed High Carbon Iron-Carbon AlloysKhalfallah, Ibrahim Youniss A. (Virginia Tech, 2017-11-22)The effects of mechanical alloying milling time and carbon concentration on microstructural evolution and hardness of high-carbon Fe-C alloys were investigated. Mechanical alloying and powder metallurgy methods were used to prepare the samples. Mixtures of elemental powders of iron and 1.4, 3, and 6.67 wt.% pre-milled graphite were milled in a SPEX mill with tungsten milling media for up to 100h. The milled powders were then cold-compacted and pressure-less sintered between 900°C and 1200°C for 1h and 5h followed by furnace cooling. Milled powders and sintered samples were characterized using X-ray diffraction, differential scanning calorimetry, Mossbauer spectroscopy, scanning and transmission electron microscopes. Density and micro-hardness were measured. The milled powders and sintered samples were studied as follows: In the milled powders, the formation of Fe_3 C was observed through Mossbauer spectroscopy after 5h of milling and its presence increased with milling time and carbon concentration. The particle size of the milled powders decreased and tended to become more equi-axed after 100h of milling. Micro-hardness of the milled powders drastically increased with milling time as well as carbon concentration. A DSC endothermic peak around 600°C was detected in all milled powders, and its transformation temperature decreased with milling time. In the literature, no explanation was found. In this work, this peak was found to be due to the formation of Fe_3 C phase. A DSC exothermic peak around 300°C was observed in powders milled for 5h and longer; its transformation temperature decreased with milling time. This peak was due to the recrystallization and/or recovery α-Fe and growth of Fe_3 C . In the sintered samples, almost 100% of pearlitic structure was observed in sintered samples prepared from powders milled for 0.5h. The amount of the pearlite decreased with milling time, contrary to what was found in the literature. The decrease in pearlite occurred at the same time as an increase in graphite-rich areas. With milling, carbon tended to form graphite instead of Fe_3 C. Longer milling time facilitated the nucleation of graphite during sintering. High mount of graphite-rich areas were observed in sintered samples prepared from powders milled for 40h and 100h. Nanoparticles of Fe_3 C were observed in a ferrite matrix and the graphite-rich areas in samples prepared from powders milled for 40h and 100h. Micro-hardness of the sintered samples decreased with milling time as Fe_3 C decreased. The green density of compacted milled powders decreased with milling time and the carbon concentration that affected the density of sintered samples.
- Formation of Meso-Structured Multi-Scale Porous Titanium Dioxide by Combined Soft-Templating, Freeze-Casting and Hard-Templating Using Cellulose NanocrystalsZahed, Nizar Bassam (Virginia Tech, 2019-01-28)This thesis identifies a facile and versatile technique for creating multi-scale porous titania with tunable meso-scale morphology. Three templating approaches were simultaneously utilized in achieving this; namely, soft-templating by template-free self-assembly of an aligned macroporous structure, freeze-casting for the preservation of particle dispersion found in suspension, and hard-templating by the use of cellulose nanocrystals (CNCs) as sacrificial material. A systematic study was conducted wherein three synthesis parameters (water content, alcohol solvent content, and drying method) were varied in the hydrolysis of titanium tetra-isopropoxide (TTIP) by the sol-gel method to determine their contribution to the formation of multi-scale porous titania exhibiting aligned macrochannels and mesoporosity. The optimal synthesis settings for producing multi-scale porous titania were identified as H2O/TTIP molar ratio of 30, without any isopropanol (acting as solvent), and freeze-drying after freezing at -40°C. Subsequently, CNCs were added in various quantities (0-50vol%) to the hydrolysis of TTIP using these optimized settings to achieve more direct and precise control of the final titania meso-structure. Morphological studies revealed that the final titania bodies maintained the formation of macrochannels 1-3 μm in diameter as a result of hydrolysis in excess water in the absence of an organic solvent and exhibited successful templating mutually affected by CNC addition and freeze-casting. Freeze-drying preserved particle dispersion in the colloid suspension, hindering agglomeration otherwise found after oven-drying and enhanced the CNCs' role of disrupting titania aggregation and increasing interconnectivity. Thus, meso-structured multi-scale porous titania was prepared by a combined templating strategy using template-free self-assembly, freeze-casting, and CNC hard-templating.
- Microstructural Controls on the Crystallization and Exhumation of Metamorphic RocksNagurney, Alexandra Bobiak (Virginia Tech, 2021-06-10)Microstructural data on the orientation and distribution of minerals can be utilized to better understand the processes controlling mineral crystallization during metamorphism and the extent to which equilibrium versus kinetic factors control the evolution of metamorphic rocks. Four studies in this dissertation address this, finding that: i) garnet crystals crystallize via epitaxial nucleation in which garnet crystallizes by templating on the crystal structure of muscovite; ii) the distribution of grain boundary void space at quartz-quartz and garnet-quartz grain boundaries is a function of the orientation of quartz crystals on either side of the grain boundary. There are more voids, and in some cases larger voids, at grain boundaries in which the a-axis of a neighboring quartz grain is perpendicular to the grain boundary than any other orientation; iii) the chemical potentials of garnet-forming components evolve differently in samples in which garnet growth either significantly or minimally overstepped equilibrium garnet-forming reactions; iv) the southwestern Meguma Terrane, Nova Scotia, experienced peak metamorphic conditions of ~630ºC and 4.0 kbar, likely resulting from regional metamorphism during the Neoacadian orogeny. A case study on the mechanisms controlling garnet crystallization in one Nova Scotian sample reveals that the rate limiting step of garnet crystallization was probably the diffusional transport of Al through the intergranular matrix. Taken together, this work has implications for understanding: i) the properties of grain boundaries in metamorphic rocks and ii) the extent to which equilibrium versus kinetic factors impact metamorphic petrogenesis.
- Relation between surface structural and chemical properties of platinum nanoparticles and their catalytic activity in the decomposition of hydrogen peroxideSerra Maia, Rui Filipe (Virginia Tech, 2018-09-26)The disproportionation of H₂O₂ to H₂O and molecular O₂ catalyzed by platinum nanocatalysts is technologically very important in several energy conversion technologies, such as steam propellant thrust applications and hydrogen fuel cells. However, the mechanism of H₂O₂ decomposition on platinum has been unresolved for more than 100 years and the kinetics of this reaction were poorly understood. Our goal was to quantify the effect of reaction conditions and catalyst properties on the decomposition of H₂O₂ by platinum nanocatalysts and determine the mechanism and rate-limiting step of the reaction. To this end, we have characterized two commercial platinum nanocatalysts, known as platinum black and platinum nanopowder, and studied the effect of different reaction conditions on their rates of H₂O₂ decomposition. These samples have different particle size and surface chemisorbed oxygen abundance, which were varied further by pretreating both samples at variable conditions. The rate of H₂O₂ decomposition was studied systematically as a function of H₂O₂ concentration, pH, temperature, particle size and surface chemisorbed oxygen abundance. The mechanism of H₂O₂ decomposition on platinum proceeds via two cyclic oxidation-reduction steps. Step 1 is the rate limiting step of the reaction. Step 1: Pt + H₂O₂ → H₂O + Pt(O). Step 2: Pt(O) + H₂O₂ → Pt + O₂ + H₂O. Overall: 2 H₂O₂ → O₂ + 2 H₂O. The decomposition of H₂O₂ on platinum follows 1st order kinetics in terms of H₂O₂ concentration. The effect of pH is small, yet statistically significant. The rate constant of step 2 is 13 times higher than that of step 1. Incorporation of chemisorbed oxygen at the nanocatalyst surface resulted in higher initial rate of H₂O₂ decomposition because more sites initiate their cyclic process in the faster step of the reaction. Particle size does not affect the kinetics of the reaction. This new molecular-scale understanding of the decomposition of H₂O₂ by platinum is expected to help advance many energy technologies that depend on the rate of H₂O₂ decomposition on nanocatalysts of platinum and other metals.