Current Search: DURABILITY (x)
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- Title
- PREPARATION AND CHARACTERISATION OF STABILIZED NAFION/PHOSPHOTUNGSTIC ACID COMPOSITE MEMBRANES FOR PROTON EXCHANGE MEMBRANE FUEL CELL (PEMFC) AUTOMOBILE ENGINES.
- Creator
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Agarwal, Rohit, Fenton, James, University of Central Florida
- Abstract / Description
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Membrane durability is one of the limiting factors for proton exchange membrane fuel cell (PEMFC) commercialisation by limiting the lifetime of the membrane via electrochemical / mechanical / thermal degradation. Lower internal humidity in the membrane at high temperature (>100 oC) and low relative humidity (25-50 %RH) operating conditions leads to increased resistance, lowering of performance and higher degradation rate. One of the promising candidates is composite proton exchange membranes ...
Show moreMembrane durability is one of the limiting factors for proton exchange membrane fuel cell (PEMFC) commercialisation by limiting the lifetime of the membrane via electrochemical / mechanical / thermal degradation. Lower internal humidity in the membrane at high temperature (>100 oC) and low relative humidity (25-50 %RH) operating conditions leads to increased resistance, lowering of performance and higher degradation rate. One of the promising candidates is composite proton exchange membranes (CPEMs) which have heteropoly acid (HPA) e.g. Phosphotungstic acid (PTA) doped throughout the Nafion® matrix. HPA is primarily responsible for carrying intrinsic water which reduces the external water dependence. The role of relative humidity during membrane casting was studied using surface analysis tools such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Thermo-gravimetric analysis (TGA), and Scanning electron microscopy (SEM) / Energy dispersive spectrometer (EDS). Membrane casting at lower relative humidity (30% approx.) results in finer size, and better PTA incorporation in the composite membrane. The effect of increase in PTA concentration in the Nafion matrix was studied with regards to conductivity, performance and durability. In-plane conductivity measurements were performed at 80 oC and 120 oC. During theses measurements, relative humidity was varied from 20% to 100% RH. Membrane conductivity invariably increases on increasing the relative humidity or operating temperature of the cell. Membrane conductivity increases with increasing PTA content from 3% to 25% PTA but never reaches the conductivity of membrane with 0% PTA. Possible explanation might be the role of cesium in PTA stabilisation process. Cesium forms a complex compound with PTA inside host matrix, rendering the PTA incapable of holding water. In plane conductivity measurements only measure surface conductivity, hence another reason might be the existence of a PTA skin on the membrane surface which is not truly representative of the whole membrane. XRD revealed that the structure of the composite membrane changes significantly on addition of PTA. Membrane with 3% PTA has structure similar to Nafion® and does not exhibit the characteristic 25o and 35o 2Ө peaks while membrane with 15% PTA and 25% PTA have strong characteristic PTA peaks. Also the membrane structure with 25% PTA matches well with that of PTA.6H2O. By applying the Scherer formula, PTA particle size was calculated from Full width half maximum (FWHM) studies at 17o 2Ө peak of the membranes. Particles coalesce on increasing the PTA concentration in the membrane leading to larger particles but still all particles were in nanometer range. Also the FWHM of membranes decreased at 17o 2Ө peak on increasing the PTA concentration, leading to higher crystallinity in the membrane. Structure analysis by FTIR indicated increase in PTA signature intensity dips, as the PTA concentration in membrane increases from 0-25%. Also by FTIR studies, it was found that some PTA is lost during the processing step as shown by comparison of as cast and protonated spectra. Possible reasoning might be that some amount of PTA does not gets cesium stabilized which gets leached away during processing. TGA studies were performed which showed no signs of early thermal degradation (temperature >300 oC); hence the assumption that all membranes are thermally robust for intended fuel cell applications. The membranes with different amounts of PTA were then catalyst coated and tested for 100-hour at open circuit voltage (OCV), 30% RH and 90 oC. By increasing the PTA in the host Nafion® matrix, the percent change in fuel crossover decreases, percent change in ECA increases, cathode fluoride emission rate decreases, and percent change in OCV decreases after the 100 hour test. Possible reasons for decreasing percentage of fuel crossover might be the increased internal humidity of the membrane due to increasing PTA incorporation. It is reported that during higher relative humidity operation, there is decrease in fuel crossover rate. Increasing ECA percentage loss might be due to the fact that HPA in the membrane can get adsorbed on the catalyst sites, rendering the sites inactive for redox reaction. Decrease in cathode fluorine emission rate (FER) might be due to the fact that there is more water available internally in the membrane as compared to Nafion®. It is reported that at higher relative humidity, FER decreases. ECA and crossover both contribute to the OCV losses. Higher component of OCV is crossover loss, which results in mixed potentials. Hence decreasing percentage of crossover might be the reason behind the decreasing OCV loss. Initial performance of fuel cell increases with increasing PTA concentration, but after the 100 hour test, higher PTA membrane exhibited highest performance loss. Increasing initial fuel cell performance can be due to the lowering of resistance due to PTA addition. Increasing ECA losses might be responsible for the increasing performance losses on adding more PTA to host membrane.
Show less - Date Issued
- 2008
- Identifier
- CFE0002486, ucf:47677
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0002486
- Title
- ON THE USE OF POLYURETHANE MATRIX CARBON FIBER COMPOSITES FOR STRENGTHENING CONCRETE STRUCTURES.
- Creator
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Haber, Zachary, Mackie, Kevin, University of Central Florida
- Abstract / Description
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Fiber-reinforced polymer (FRP) composite materials have effectively been used in numerous reinforced concrete civil infrastructure strengthening projects. Although a significant body of knowledge has been established for epoxy matrix carbon FRPs and epoxy adhesives, there is still a need to investigate other matrices and adhesive types. One such matrix/adhesive type yet to be heavily researched for infrastructure application is polyurethane. This thesis investigates use of polyurethane matrix...
Show moreFiber-reinforced polymer (FRP) composite materials have effectively been used in numerous reinforced concrete civil infrastructure strengthening projects. Although a significant body of knowledge has been established for epoxy matrix carbon FRPs and epoxy adhesives, there is still a need to investigate other matrices and adhesive types. One such matrix/adhesive type yet to be heavily researched for infrastructure application is polyurethane. This thesis investigates use of polyurethane matrix carbon fiber composites for strengthening reinforced concrete civil infrastructure. Investigations on mirco- and macro-mechanical composite performance, strengthened member flexural performance, and bond durability under environmental conditioning will be presented. Results indicate that polyurethane carbon composites could potentially be a viable option for strengthening concrete structures.
Show less - Date Issued
- 2010
- Identifier
- CFE0003073, ucf:48307
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0003073
- Title
- HIGH VOLTAGE BIAS TESTING AND DEGRADATION ANALYSIS OF PHOTOVOLTAIC MODULES.
- Creator
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Hadagali, Vinaykumar, Dhere, Neelkanth, University of Central Florida
- Abstract / Description
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This thesis mainly focuses on two important aspects of the photovoltaic modules. The first aspect addressed the high voltage bias testing and data and degradation analysis of high voltage biased thin film photovoltaic modules. The second aspect addressed the issues of reliability and durability of crystalline silicon module. Grid-connected photovoltaic systems must withstand high voltage bias in addition to harsh environmental conditions such as intermittent solar irradiance, high humidity,...
Show moreThis thesis mainly focuses on two important aspects of the photovoltaic modules. The first aspect addressed the high voltage bias testing and data and degradation analysis of high voltage biased thin film photovoltaic modules. The second aspect addressed the issues of reliability and durability of crystalline silicon module. Grid-connected photovoltaic systems must withstand high voltage bias in addition to harsh environmental conditions such as intermittent solar irradiance, high humidity, heat and wind. a-Si:H thin-film photovoltaic modules with earlier generation SnO2:F transparent conducting oxide (TCO) on the front glass installed on the FSEC High Voltage Test Bed were monitored since December 2001. The data was collected on a daily basis and analyzed. The leakage currents for some chosen time period were calculated and compared with the measured values. Current-voltage characteristic measurements were carried out to check any reduction in the power. Samples were cored and extracted for analysis from one of the -600 V biased modules. Leakage currents in high-voltage-biased laminates specially prepared with improved SnO2:F TCO are being monitored in the hot and humid climate in Florida. Negatively-biased modules showed clear signs of delamination. The leakage currents in high-voltage biased photovoltaic modules are functions of both temperature and relative humidity. Photovoltaic module leakage conductance was found to be thermally stimulated with a characteristic activation energy that depends on relative humidity. The adhesional strength was lost completely in the damaged area. Leakage current values from support to ground in new, unframed laminates fabricated with improved SnO2:F TCO layer were ~100 times lower under the high voltage bias in hot and humid environment. Information on the failure of field deployed modules must be complemented with why and how the modules fail while considering the issues of reliability and durability of crystalline silicon module. At present, all the failure modes have not been identified and failure mechanisms have not been understood. Experience has shown that as the materials and processes are changed, reliability issues that apparently had been resolved resurface. A multicrystalline silicon photovoltaic module that was manufactured by a non-US company and that had shown >50% performance loss in field-deployment of <2 years in hot and dry climate were studied for degradation analysis in comparison with a mc-Si module that was manufactured by the same company and that performed well after 10 years of field-deployment in hot and humid climate.. I-V measurements were carried out to analyze the reduction in photovoltaic parameters. Solder bond strength in mc-Si photovoltaic modules were measured to understand early degradation of performance. Samples were cored and extracted for further analysis. Adhesional strength between the busline metallization and the silicon cell in a newer generation mc-Si photovoltaic module was found to be considerably lower than that in the earlier vintage module. These results can be useful for early detection and diagnosis of field reliability issues and could assist in establishing correlation between long-term field data and observations and accelerated environmental stress testing. It is suggested that more detailed study should be undertaken using unencapsulated strings of crystalline silicon modules so as to avoid complication due to encapsulant creeping beneath the ribbons.
Show less - Date Issued
- 2005
- Identifier
- CFE0000798, ucf:46563
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0000798
- Title
- Three essays on the marketing strategies of a durable goods manufacturer.
- Creator
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Chau, Ngan, Desiraju, Ramarao, Krishnamoorthy, Anand, Joshi, Amit, Chintagunta, Pradeep, University of Central Florida
- Abstract / Description
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When purchasing durable goods, consumers not only pay for current but also future consumption; consequently, forward looking behavior is an important consideration in durable goods markets. For example, anticipating that prices will go down in the future, consumers may delay the purchase today; such behavior has a significant impact on the firm's marketing strategies. This dissertation investigates the impact of durability on two marketing strategies: new product introductions and supply...
Show moreWhen purchasing durable goods, consumers not only pay for current but also future consumption; consequently, forward looking behavior is an important consideration in durable goods markets. For example, anticipating that prices will go down in the future, consumers may delay the purchase today; such behavior has a significant impact on the firm's marketing strategies. This dissertation investigates the impact of durability on two marketing strategies: new product introductions and supply chain design. The first part of this dissertation (Chapter 3) examines a durable goods manufacturer's new product introduction strategy under different market environments where network effects and product compatibility are important. More specifically, this part explores the incentives of a firm to use either a replacement strategy or a skipping strategy---in the former, the firm commercializes the existing technology, while in the latter, it does not; in either case, an improved technology will be available in the future and the firm will introduce a new product at that time. Using a two-period analytical model with network effects, the analysis shows how the level of improvement in the new product, along with the type of compatibility between the products, interacts with network strength to determine the manufacturer's optimal strategy. Under gradual new product improvement, there is a strict preference for replacement. In contrast, under rapid new product improvement, that preference only holds in markets with relatively high levels of the network strength; at lower levels of the network strength, skipping is preferred; interestingly, for moderate values of the network strength, the level of product improvement affects the manufacturer's optimal choice differently under varying types of compatibility.The second part of this dissertation (Chapters 4 and 5) focuses on the supply chain design decisions of a durable goods manufacturer who is a sole supplier of an essential proprietary component for making the end product. Three different supply chain structures are considered. In the first, the manufacturer operates as a ``component supplier'' and sells the component to a downstream firm who then makes the end product. In the second structure, the manufacturer produces the end product using its component but does not make that component available to any other firms; here, the manufacturer operates as a ``sole entrant''. Finally, the manufacturer can operate as a ``dual distributor'' who not only makes the end product using its own component, but sells the component to a downstream firm who then competes against the manufacturer in the end product market.The extant literature on the optimal choice among the above supply chain structures has focused mainly on static settings in a framework of price competition. By contrast, researchers predominantly use quantity competition to examine durable goods markets in dynamic (i.e., multiple time period) settings. Moreover, the literature notes diversity in optimal firm behavior under the two types of (i.e., price and quantity) competition. Therefore, to transition from supply chain design in a static setting to a more dynamic one where consumers are forward-looking, this part utilizes Chapter 4 to analyze the manufacturer's choice using quantity competition in a static setting. This analysis (in Chapter 4) identifies precisely the shift in the manufacturer's choice of supply chain structure when moving from price competition to a quantity competition framework. With that analysis as a benchmark, the next chapter focuses on the manufacturer's choice in a dynamic setting. More specifically, Chapter 5 investigates the impact of durability on the optimality of the supply chain structures identified above. Using a two period setting, the analysis explores how the manufacturer's preference for different supply chain structures is modified. The findings reveal that, e.g., when durability is taken into account, the manufacturer's preference for the sole entrant role goes up, while the preference for the component supplier role goes down. Further, under certain conditions, the manufacturer may opt to be a dual distributor in the first period and then choose to become only a component supplier in the second period. The underlying rationale for such shifts in preference is directly linked to durability, which creates future competition and substantially reduces the manufacturer's profitability in the long run. Interestingly, this negative impact varies across different supply chain structures.Overall, this dissertation contributes to the current literature on durable goods and enhances our understanding of the impact of durability on the optimality of distinct marketing strategies, and provides insights that are valuable to both academics and managers.
Show less - Date Issued
- 2012
- Identifier
- CFE0004364, ucf:49428
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004364
- Title
- FLEXURAL MECHANICAL DURABILITY OF CONCRETE BEAMS STRENGTHENED BY EXTERNALLY BONDED CARBON FIBER REINFORCED POLYMER SHEETS.
- Creator
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Olka, Michael, Mackie, Kevin, University of Central Florida
- Abstract / Description
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About 77,600 bridges throughout the United States in the Federal Highway Association (FHWA) bridge database are listed as structurally deficient. This has created a need to either replace or strengthen bridges quickly and efficiently. Due to high costs for total replacement of deficient bridges, strengthening of existing bridges is a more economical alternative. A technique that has been developing over the past two decades is the strengthening of bridges using carbon fiber reinforced polymer...
Show moreAbout 77,600 bridges throughout the United States in the Federal Highway Association (FHWA) bridge database are listed as structurally deficient. This has created a need to either replace or strengthen bridges quickly and efficiently. Due to high costs for total replacement of deficient bridges, strengthening of existing bridges is a more economical alternative. A technique that has been developing over the past two decades is the strengthening of bridges using carbon fiber reinforced polymer (CFRP) sheets. The CFRP sheets are attached to the bottom of the bridge girders using structural adhesives so that the CFRP becomes an integral part of the bridge and carries a portion of the flexural loading. The CFRP sheets allow for an increase in the capacity of the bridge with minimal increase in the weight of the structure due to CFRP having a low density. Because the CFRP is expected to be an integral component and carry some of the long-term loading it is important to understand the long-term durability of the composite section. This thesis is part of a larger project, in which the long-term durability of the CFRP composite on concrete beams is investigated experimentally. The CFRP strengthened beams are exposed to fatigue testing and thermal-humidity cycling followed by failure testing. The testing scheme for this experiment allows for the investigation of the individual effects of fatigue and thermal-humidity loading as well as to explore the effects from combined fatigue and thermal-humidity loading. The investigation of the combined effects is a unique aspect of this experiment that has not been performed in prior studies. Results indicate that a polyurethane-based adhesive could provide a more durable bond for the CFRP-concrete interface than possible with epoxy-based adhesives.
Show less - Date Issued
- 2009
- Identifier
- CFE0002585, ucf:48252
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0002585
- Title
- experimental and numerical investigations on bond durability of cfrp strengthened concrete members subjected to environmental exposure.
- Creator
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Al-Jelawy, Haider, Mackie, Kevin, Gou, Jihua, Chopra, Manoj, University of Central Florida
- Abstract / Description
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Fiber reinforced polymer (FRP) composites have become an attractive alternative to conventional methods for external-strengthening of civil infrastructure, particularly as applied to flexural strengthening of reinforced concrete (RC) members. However, durability of the bond between FRP composite and concrete has shown degradation under some aggressive environments. Although numerous studies have been conducted on concrete members strengthened with FRP composites, most of those studies have...
Show moreFiber reinforced polymer (FRP) composites have become an attractive alternative to conventional methods for external-strengthening of civil infrastructure, particularly as applied to flexural strengthening of reinforced concrete (RC) members. However, durability of the bond between FRP composite and concrete has shown degradation under some aggressive environments. Although numerous studies have been conducted on concrete members strengthened with FRP composites, most of those studies have focused on the degradation of FRP material itself, relatively few on bond behavior under repeated mechanical and environmental loading.This thesis investigates bond durability under accelerated environmental conditioning of two FRP systems commonly employed in civil infrastructure strengthening: epoxy and polyurethane systems. Five environments were considered under three different conditioning durations (3 months, 6 months, and 1 year). For each conditioning environment and duration (including controls), the following were laboratory tested: concrete cylinders, FRP tensile coupons, and FRP-strengthened concrete flexural members. Numerical investigations were performed using MSC MARC finite element software package to support the outcomes of durability experimental tests. Precise numerical studies need an accurate model for the bond between FRP and concrete, a linear brittle model is proposed in this work that is calibrated based on nonlinear regression of existing experimental lap shear data.Results of tensile tests on FRP coupons indicate that both epoxy and polyurethane FRP systems do not degrade significantly under environmental exposure. However, flexural tests on the FRP strengthened concrete beams indicate that bond between FRP and concrete shows significant degradation, especially for aqueous exposure. Moreover, a protective coating suppresses the measured degradation. Also, experimental load-displacement curves for control beams show excellent agreement with numerical load-displacement curves obtained using the proposed bond model. Finally, a bond-slip model is predicted for concrete leachate conditioned beams by matching load-displacement curves for those beams with numerical load-displacement curves.
Show less - Date Issued
- 2013
- Identifier
- CFE0004971, ucf:49589
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004971
- Title
- The Behavior of Cerium Oxide Nanoparticles in Polymer Electrolyte Membranes in Ex-Situ and In-Situ Fuel Cell Durability Tests.
- Creator
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Pearman, Benjamin, Hampton, Michael, Blair, Richard, Clausen, Christian, Seal, Sudipta, Campiglia, Andres, Yestrebsky, Cherie, Mohajeri, Nahid, University of Central Florida
- Abstract / Description
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Fuel cells are known for their high efficiency and have the potential to become a major technology for producing clean energy, especially when the fuel, e.g. hydrogen, is produced from renewable energy sources such as wind or solar. Currently, the two main obstacles to wide-spread commercialization are their high cost and the short operational lifetime of certain components.Polymer electrolyte membrane (PEM) fuel cells have been a focus of attention in recent years, due to their use of...
Show moreFuel cells are known for their high efficiency and have the potential to become a major technology for producing clean energy, especially when the fuel, e.g. hydrogen, is produced from renewable energy sources such as wind or solar. Currently, the two main obstacles to wide-spread commercialization are their high cost and the short operational lifetime of certain components.Polymer electrolyte membrane (PEM) fuel cells have been a focus of attention in recent years, due to their use of hydrogen as a fuel, their comparatively low operating temperature and flexibility for use in both stationary and portable (automotive) applications.Perfluorosulfonic acid membranes are the leading ionomers for use in PEM hydrogen fuel cells. They combine essential qualities, such as high mechanical and thermal stability, with high proton conductivity. However, they are expensive and currently show insufficient chemical stability towards radicals formed during fuel cell operation, resulting in degradation that leads to premature failure. The incorporation of durability improving additives into perfluorosulfonic acid membranes is discussed in this work.Cerium oxide (ceria) is a well-known radical scavenger that has been used in the biological and medical field. It is able to quench radicals by facilely switching between its Ce(III) and Ce(IV) oxidation states.In this work, cerium oxide nanoparticles were added to perfluorosulfonic acid membranes and subjected to ex-situ and in-situ accelerated durability tests.The two ceria formulations, an in-house synthesized and commercially available material, were found to consist of crystalline particles of 2 (-) 5 nm and 20 (-) 150 nm size, respectively, that did not change size or shape when incorporated into the membranes.At higher temperature and relative humidity in gas flowing conditions, ceria in membranes is found to be reduced to its ionic form by virtue of the acidic environment. In ex-situ Fenton testing, the inclusion of ceria into membranes reduced the emission of fluoride, a strong indicator of degradation, by an order of magnitude with both liquid and gaseous hydrogen peroxide. In open-circuit voltage (OCV) hold fuel cell testing, ceria improved durability, as measured by several parameters such as OCV decay rate, fluoride emission and cell performance, over several hundred hours and influenced the formation of the platinum band typically found after durability testing.
Show less - Date Issued
- 2012
- Identifier
- CFE0004789, ucf:49731
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004789