http://hdl.handle.net/1957/18466 Sat, 18 May 2013 09:03:02 GMT 2013-05-18T09:03:02Z http://hdl.handle.net/1957/37935 Degradation of graphite electrodes in acidic bromine electrolytes Bistrika, Alexander A. As the world's power needs grow, the demand for power from renewable resources, such as wind or solar is increasing. One major drawback associated with these renewable resources is that the power output is dependent on environmental factors, such as cloud cover and wind speeds. This allows the possibility of either power output exceeding or falling short of forecast levels that may lead to grid instabilities. Therefore, Large Scale Energy Storage (LSES) systems are critical to store excess power when the output exceeds demand in order to supplement output power when it falls short of demand.¹ The Zinc/Bromine Redox Flow Battery (RFB) is a promising technology because of previously reported long cycle-life (CL) capability, high efficiencies, low cost materials, and scalable operating conditions.² The excellent energy storage performance of the Zinc/Bromine system was confirmed by measuring both Faradaic and Coulombic electrochemical cell efficiency dependence on temperature of a bench scale Zinc/Bromine flow cell. At room temperature, near 75% Faradaic efficiency was measured when cycling the system between 20% and 100% State of Charge (SOC), which is in good agreement with published values,³ and was measured to be over 80% efficient when operating at an elevated temperature of 50°C. To elucidate capital and operational costs, key system operation parameters especially focused on degradation mechanisms were investigated. Since deep discharge cycling is perceived as highly damaging to electrochemical systems, a system was cycled between 0% and 5% (SOC) 10,000 times. Performance was quantified by measuring the frequency factor (i[subscript 0]) and relative activation energy (α) for the reactions using Tafel scans. No statistically significant degradation or change to the electrodes was observed during the zero point cycling experiment. However, it was found that under conventional operation damage to the electrodes does accumulate, presumably due to the highly oxidative environment caused by the presence of high concentrations of dissolved bromine or tri-bromide. While the performance of both electrodes shows decreases in frequency factor attributed to the damage process, the bromide oxidation process seems to be more damaging (i.e., at the positive electrode during the charging process). Long term measurements show a degradation of the electrocatalytic parameters at an applied overpotential of 100 mV from ca. 40 mA/cm² to ca. 5 mA/cm² at the positive electrode and from ca. 20 mA/cm² to ca. 10 mA/cm² for the negative electrode. A degradation rate model was proposed to predict the service life expectancy of graphite electrodes in a bromine system based on processes showing a combined second order reaction rate coupled with a negative first order reaction rate. The model can be used to predict the cost of energy when operating any device using graphite electrodes, based on the operating power ratio, defined here as the quotient between operating power and system rated power. This damage could be partially reversed by exposing the electrode surfaces to concentrated potassium hydroxide dissolved in isopropanol, presumably due to exfoliation of the electrocatalytic surface leading to the exposure of a clean surface with electrocatalytic performance close to the original. Further, a chemical pretreatment for the graphite surface imparting enhanced stability in aqueous bromine systems was developed that shows negligible damage when similar amounts of current have passed through the electrode surface. After bromide oxidation equivalent to passing ca. 10 Ah/cm² the treated surface showed a change in steady state current density at an applied overpotential of 100 mV from ca. 50 mA/cm² to ca. 48 mA/cm². Graduation date: 2013; Access restricted to the OSU Community at author's request from April 1, 2013 - April 1, 2015 Wed, 27 Mar 2013 00:00:00 GMT http://hdl.handle.net/1957/37935 2013-03-27T00:00:00Z http://hdl.handle.net/1957/37908 Industrially-situated project-based learning : a study of feedback and diffusion Gilbuena, Debra M. The Virtual Chemical Vapor Deposition (CVD) Process Development Project provides the context for the two areas of the research presented in this dissertation. The first area, generally referred to as feedback in this dissertation, focuses on student learning and the interactions of students and instructors that take place in the project, specifically focused on characterizing feedback and determining the influence of feedback as student teams progress towards completing the project. The characteristics of feedback found in this project are presented within a situative perspective using the analytical framework of episodes. The characteristics include: a list and categorization of episode themes, the structure and flow of episodes during the coaching session, the sub-structure present within individual episodes, and the types of feedback present. This dissertation shows how these characteristics frame participation in a community of practice and can be used as tools to scaffold instructor feedback in project-based learning. Episodes analysis is also used to investigate how feedback on professional skills can help to enculturate students into a community of practice and influence their fluency with professional skills and engagement in more technical activities. The second area examines the spread of this innovative project from its home institution to other institutions. In this area an analysis of the spread of the Virtual CVD Process Development Project in the high school setting is presented. The project was found to provide versatility for instructors and afford student learning in the areas of motivation, cognition, and epistemological beliefs. These two areas inform each other. As the project is assessed at different institutions, it is continually improved and the sensitivity of different aspects of the project is explored, e.g., the aspects of the project that are crucial to maintain effectiveness are identified. One of these aspects is the feedback that takes place in the project. As the project is further examined at the home institution in depth, more can be learned about the best ways it can be delivered. This information informs scaffolding that then can be provided to faculty at other institutions such that they can attend to crucial aspects of the project in the most efficient, effective manner, improving not only the probability of successful adaptation, but also the likelihood that the project will further diffuse to other institutions. Graduation date: 2013 Mon, 18 Mar 2013 00:00:00 GMT http://hdl.handle.net/1957/37908 2013-03-18T00:00:00Z http://hdl.handle.net/1957/37900 Synthesis of colloidal metal oxide nanocrystals and nanostructured surfaces using a continuous flow microreactor system and their applications in two-phase boiling heat transfer Choi, Chang-Ho Metal oxide nanocrystals have attracted significant interests due to their unique chemical, physical, and electrical properties which depend on their size and structure. In this study, a continuous flow microreactor system was employed to synthesize metal oxide nanocrystals in aqueous solution. Assembly of nanocrystals is considered one of the most promising approaches to design nano-, microstructures, and complex mesoscopic architectures. A variety of strategies to induce nanocrystal assembly have been reported, including directed assembly methods that apply external forces to fabricate assembled structures. In this study ZnO nanocrystals were synthesized in an aqueous solution using a continuous flow microreactor. The growth mechanism and stability of ZnO nanocrystals were studied by varying the pH and flow conditions of the aqueous solution. It was found that convective fluid flow from Dean vortices in a winding microcapillary tube could be used for the assembly of ZnO nanocrystals. The ZnO nanocrystal assemblies formed three-dimensional mesoporous structures of different shapes including a tactoid, a retangle and a sphere. The assembly results from a competing interaction between electrostatic forces caused by surface charge of nanocrystals and collision of nanocrystals associated with Dean vortices. The as synthesized colloidal ZnO nanocrystals or assembly were directly deposited onto a substrate to fabricate ZnO nanostructured surfaces. The rectangular assembly led to flower-like ZnO nanostructured films, while the spherical assembly resulted in amorphous ZnO thin film and vertical ZnO nanowire (NW) arrays. In contrast to the formation of flower structure or amorphous thin film, only colloidal ZnO nanocrystals were used as the building blocks for forming vertical ZnO NW arrays. This study demonstrates the versatility of the microreactor-assisted nanomaterial synthesis and deposition process for the production of nanostrucuturesres with various morphologies by tuning the physical parameters while using the same chemical precursors for the synthesis. ZnO flower structure was coated on a microwick structure to improve the capillary flow. The coated microwick structure showed an enhanced capillary rise, which was attributed to the hydrophilic property and geometrical modification of ZnO nanostructure. Two-phase boiling heat transfer was performed using ZnO nanostructured surfaces. ZnO nanocoating altered the important characteristics including surface roughness and wettability. Hydrophilic nature of the ZnO nanocoating generally enhanced the boiling heat transfer performance, resulting in higher heat transfer coefficient (HTC), higher critical heat flux (CHF), and lower surface superheat comparing to the bare surface. Octahedral SnO and porous NiO films, fabricated by a continuous flow microreactor system, were suggested as potential boiling surfaces for the high porosity and irregularity of their structures. Graduation date: 2013 Mon, 04 Mar 2013 00:00:00 GMT http://hdl.handle.net/1957/37900 2013-03-04T00:00:00Z http://hdl.handle.net/1957/37560 Residence time distribution of solids in a multi-compartment fluidized bed system Pongsivapai, Pajongwit In a conventional well-mixed fluidized bed the solids are almost completely mixed and the residence time for individual particles may vary between zero and infinity. For materials especially sensitive to the processing time, a wide distribution of residence time is extremely undesirable. A multi-compartment fluidized bed was proposed to minimize this problem. The two-compartment fluidized bed system was designed and experimentally investigated with positive results. Residence time distributions were measured with glass bead particles of 379 p.m mean diameter size. Tracer particles were colored glass beads which had exactly the same properties as the particles used for bed material. Two theoretical models were proposed to predict the flow behavior of solids through the two-compartment fluidized bed vessel. The results show the solids flow pattern can be described by the axial dispersion plug flow model and the tanks-in-series model. The parameters of both models can be determined by minimizing the sum of the squared differences between experimental data and model predictions curves. The influence of fluidization velocity, diameter of the orifice connecting the two adjacent compartments, and the height of the overflow exit orifice were investigated. The two-compartment fluidized bed can improve the residence time distribution of solids from a single fluidized bed. The residence time distribution of solids was also improved with decreasing fluidizing gas velocity. Orifice diameter and height of the overflow exit orifice did not influence significantly on particles flow in the two-compartment fluidized bed system. However, a solids mass flow through the partition orifice is predicted reasonably well, within moderate range of variance, by equation developed by de Jong (1965) [12]. Graduation date: 1994 Fri, 14 Jan 1994 00:00:00 GMT http://hdl.handle.net/1957/37560 1994-01-14T00:00:00Z