bluuebunny/crossref_metadata_2025 huggingface dataset:
Cohesive Modeling of Quasi-Static Fracture in Functionally Graded Materials
Authors: Kandula Soma Sekhar V. , Abanto-Bueno Jorge , Lambros John , Geubelle Philippe H.
Published: August 2006

A spatially varying cohesive failure model is used to simulate quasi-static fracture in functionally graded polymers. A key aspect of this paper is that all mechanical properties and cohesive parameters entering the analysis are derived experimentally from full-scale fracture tests allowing for a fit of only the shape of the cohesive law to experimental data. The paper also summarizes the semi-implicit implementation of the cohesive model into a cohesive-volumetric finite element framework used to predict the quasi-static crack initiation and subsequent propagation in the presence of material gradients.
Quasi-Static and Dynamic Buckling of Thin Cylindrical Shape-Memory Shells
Authors: Nemat-Nasser Sia , Yong Choi Jeom , Isaacs Jon B. , Lischer David W.
Published: August 2006

To investigate the buckling behavior of thin and relatively thick cylindrical shape-memory shells, uniaxial compression tests are performed at a 295K initial temperature, using the CEAM/UCSD’s modified split Hopkinson bar systems and an Instron hydraulic testing machine. The quasi-static buckling response of the shells is directly observed and recorded using a digital camera with a close-up lens and two back mirrors. To document the dynamic buckling modes, a high-speed Imacon 200 framing camera is used. The shape-memory shells with an austenite-finish temperature of Af=281K, buckle gradually and gracefully in quasi-static loading, and fully recover upon unloading, showing a superelastic property, whereas when suitably annealed, the shells do not recover spontaneously upon unloading, but they do so once heated, showing a shape-memory effect. The thin shells had a common thickness of 0.125mm a common outer radius of 2.25mm (i.e., a common radius, R, to thickness, t, ratio, R∕t, of 18). A shell with the ratio of length, L, to diameter, D(L∕D) of 1.5 buckled under a quasi-static load by forming a nonsymmetric chessboard pattern, while with a L∕D of 1.95 the buckling started with the formation of symmetrical rings which then changed into a nonsymmetric chessboard pattern. A similar buckling mode is also observed under a dynamic loading condition for a shell with L∕D of 2. However, thicker shells, with 0.5mm thickness and radius 4mm(R∕t=8), buckled under a dynamic loading condition by the formation of a symmetrical ring pattern. For comparison, we have also tested shells of similar geometry but made of steel and aluminum. In the case of the steel shells with constrained end conditions, the buckling, which consists of nonsymmetric (no rings) folds (chessboard patterns), is sudden and catastrophic, and involves no recovery upon unloading. The gradual buckling of the shape-memory shells is associated with the stress-induced martensite formation and seems to have a profound effect on the unstable deformations of thin structures made from shape-memory alloys.
Fracture Toughness and Subcritical Crack Growth in Polycrystalline Silicon
Authors: Chasiotis I. , Cho S. W. , Jonnalagadda K.
Published: August 2006

The fracture behavior of polycrystalline silicon in the presence of atomically sharp cracks is important in the determination of the mechanical reliability of microelectromechanical system (MEMS) components. The mode-I critical stress intensity factor and crack tip displacements in the vicinity of atomically sharp edge cracks in polycrystalline silicon MEMS scale specimens were measured via an in situ atomic force microscopy/digital image correlation method. The effective (macroscopic) mode-I critical stress intensity factor for specimens from different fabrication runs was 1.00±0.1MPa√m, where 0.1MPa√m is the standard deviation that was attributed to local cleavage anisotropy and grain boundary effects. The experimental near crack tip displacements were in good agreement with the linearly elastic fracture mechanics solution, which supports K dominance in polysilicon at the scale of a few microns. The mechanical characterization method implemented in this work allowed for direct experimental evidence of incremental (subcritical) crack growth in polycrystalline silicon that occurred with crack increments of 1-2μm. The variation in experimental effective critical stress intensity factors and the incremental crack growth in brittle polysilicon were attributed to local cleavage anisotropy in individual silicon grains where the crack tip resided and whose fracture characteristics controlled the overall fracture process resulting in different local and macroscopic stress intensity factors.
An Experimental Investigation of the Motion of Flexible Strings: Whirling
Authors: Lin Bisen , Ravi-Chandar K.
Published: August 2006

Whirling of strings has been studied both theoretically and experimentally for many decades. According to linear theory, a heavy string can exhibit steady-state whirl only at its natural frequencies which form a discrete spectrum. The nonlinear theory, however, suggests that a string can undergo steady whirl at any frequency larger than the fundamental frequency and further that for each frequency between the nth and the (n+1)th eigenvalue, there exist n distinct whirling modes. Quantitative experimental observations on such whirling have never been reported, although anecdotal evidence suggests the possibility of such whirl. In this paper, we examine the whirling of a string with negligible bending stiffness through experiments utilizing a stereo-vision imaging system. It is shown that steady motion exists only when the string whirls at its natural frequencies and that whirling motions for other frequencies exhibit rich dynamics that needs further exploration.
A Comparative Evaluation of Three Isotropic, Two Property Failure Theories
Authors: Christensen Richard M.
Published: August 2006

Three fundamentally different failure theories for homogeneous and isotropic materials are examined in both the ductile and brittle ranges of behavior. All three theories are calibrated by just two independent failure properties. These three are the Coulomb-Mohr form, the Drucker-Prager form, and a recently derived theory involving a quadratic representation along with a fracture restriction. The three theories are given a detailed comparison and evaluation. The Coulomb-Mohr form and the Drucker-Prager form are found to predict physically unrealistic behavior in some important cases. The present form meets the consistency requirements.
The Effect of Molecular Mass Distribution on Time-Dependent Behavior
of Polyamides
Authors: Emri I. , von Bernstorff B. S.
Published: August 2006

In this paper we show that the time-dependent properties of polyamides may be significantly modified by altering the “initial kinetics” of the material through the modification of the molecular mass distribution. In our investigations we have used polyamide 6 (PA6) as a testing material. We have shown that the molecular weight distribution determines the time scale of material structure formation and, hence, its time-dependent properties. The melting temperature of the bimodal molecular mass distribution of polyamide 6 material is shifted for about 30°C towards higher temperatures in comparison to the conventional monomodal molecular mass distribution of PA6. Modification of the molecular mass distribution also improves the time and temperature dependence of the mechanical properties of PA6. It seems that understanding the effect of the molecular weight distribution on the process of material structure formation may well become a new approach in development of new generation polymeric materials.
Analysis of Single PEM Fuel Cell Performances Based on Current Density Distribution Measurement
Authors: Ghosh P. C. , Wüster T. , Dohle H. , Kimiaie N. , Mergel J. , Stolten D.
Published: August 2006

A new in situ measurement method of mapping the current density distribution in polymer electrolyte fuel cells (PEFC) is used to analyze the performance of a fuel cell under different operating conditions. The present method is useful in investigating the current density distribution in a single cell as well as a stack, which carries the information about the local reactant activity over the electrode area. It was found that the current density close to the gas inlets is strongly influenced by the reactants' relative humidity. The performance close to the gas outlets is greatly influenced by the inlet gas pressures and the stoichiometry factors of the reactant gases, mainly on the cathode side. It was also observed that the performance of the fuel cell drops with the increase in operating temperature if the reactant gases are not sufficiently humidified.
Measurement of Biaxial
Stress States in Silicon Using Micro-Raman
Spectroscopy
Authors: Gustafson Peter
A. , Harris Stephen
J. , O’Neill Ann E. , Waas Anthony M.
Published: August 2006

Micro-Raman spectroscopy is used to determine the multiaxial stress state in silicon wafers using a strategy proposed by Narayanan, et al. (J. Appl. Phys. 82, 2595–2602 (1997)) Previously, this strategy was validated when silicon was subjected to uniaxial stress in the laboratory frame (Harris, et al. J. Appl. Phys. 96, 7195–7201 (2004)). In the present work, silicon wafers have been analyzed that were subjected to biaxial stress states in the laboratory frame. The predicted curves for the initially degenerate F2g peaks were found to fall within the variability of the measured curves. Stress ratios were found to be predictable. Stress magnitudes were also found to be predictable, but are subject to uncertainty greater than 25%. To perform these tests, an apparatus has been developed which can provide controlled ratios of biaxial stress in a simple and compact test geometry. This fixture was used under a microscope, enabling in situ measurement of biaxial stress states.
Influence of Confining Pressure on the Crack Growth Behavior in a Highly Filled Elastomer
Authors: Liu C. T. , Ravichandran G.
Published: August 2006

In this study, the effect of confining pressure on the crack growth behavior in a highly filled elastomer was investigated. The material under investigation contains 86% by weight of hard particles embedded in a rubbery matrix. The specimens were tested at a constant strain rate under a confining pressure. Two confining pressures, 3.45MPa and 6.9MPa, and two types of loading, monotonically increasing and cyclic, were considered. The experimental data were analyzed, and the effects of confining pressure on the damage process as well as the constitutive and crack growth behavior in the material are discussed.
A Methodology for Assessing Fuel Cell Performance Under a Wide Range of Operational Conditions: Results for Single Cells
Authors: Baratella Andrea , Bove Roberto , Lunghi Piero
Published: August 2006

Testing the performance of fuel cells is an important key for verifying technology improvements and for demonstrating their potential. However, due to the novelty of this technology, there is not a standardized procedure for testing fuel cell performance. In order to fully investigate fuel cell performance, the behavior must be known under a wide range of operational conditions. Furthermore, in order to compare results coming from different test teams, a set of procedures and parameters to evaluate single cell performance should be defined. The research group of the Fuel Cell Laboratory of the University of Perugia is conducting performance tests on single cells, focusing on defining test procedures to find effective parameters to be used to compare tests performed by different teams. This work demonstrates how the testing parameters developed by the team allow one to perform advanced control on test procedures, to understand test results, and to compare them with tests carried out under different operational conditions. The entire analysis is easily conducted by using a single parameter variation hyperspace approach. The experimental results obtained on single fuel cells are reported.
Discussion: “A Paradox in Sliding Contact Problems With Friction” (Adams, G. G., Barber, J. R., Ciavarella, M., and Rice, J. R., 2005, ASME J. Appl. Mech., 72, pp. 450–452)
Authors: Hills D. A. , Sackfield A. , Churchman C. M.
Published: August 2006

In this interesting paper, the authors address an anomaly which arises when a rigid, square-ended block is pressed against a linear elastic half plane and slid along. The authors note that, within the framework of linear elasticity, the singularity in the contact pressure, and hence shearing traction, produces, adjacent to the edges, regimes in which the implied local relative slip direction dominates the rigid-body sliding velocity, and hence produces a violation of the Coulomb friction law. They seek to resolve the paradox by appealing to a more sophisticated strain definition. All of this is within the context of a quasistatic formulation. The authors recognize, of course, that in any real problem the paradox is unlikely to arise because of (a) the finite strength of the contact giving rise to a yield zone, and (b) the absence of an atomically sharp corner at the contact edge where there is, in all probability, a finite edge radius. Here, we wish to address these issues quantitatively, and so demonstrate that it is unlikely that the paradox described, though interesting, will have any bearing in a real contact.
Project ABSOLUTE: A ZEBRA Battery/Intermediate Temperature Solid Oxide Fuel Cell Hybrid for Automotive Applications
Authors: Brett D. J. L. , Aguiar P. , Brandon N. P. , Bull R. N. , Galloway R. C. , Hayes G. W. , Lillie K. , Mellors C. , Millward M. , Smith C. , Tilley A. R.
Published: August 2006

Project ABSOLUTE (advanced battery solid oxide fuel cell linked unit to maximize efficiency), aims to combine a sodium-nickel chloride battery and an intermediate temperature solid oxide fuel cell (IT-SOFC) to form an all-electric hybrid package that surpasses the efficiency and performance of a purely fuel cell driven vehicle, as well as extending the range of a purely battery driven electric vehicle. This paper discusses the project background, the ABSOLUTE hybrid concept, the methodology adopted, the vehicle types and drive cycles that best suit the hybrid and system control considerations. Results from a battery and IT-SOFC system model are presented.
Performance Improvement of (La,Sr)MnO3 and (La,Sr)(Co,Fe)O3-Type Anode-Supported SOFCs
Authors: Haanappel Vincent A. C. , Mertens Josef , Mai Andreas
Published: August 2006

Targets in the development of anode-supported or planar solid oxide fuel cells (SOFCs) are low operation temperatures, high durability, high reliability, high power density, and low production costs. During the past ten years steps have already been taken at Forschungszentrum Jülich to lower the operating temperatures while maintaining the power output. This was achieved by optimizing processing and microstructural parameters of the electrodes. This paper presents the latest results concerning performance improvement through variations of the processing route and the microstructure of La0.65Sr0.3MnO3 (LSM) and La0.58Sr0.4Co0.2Fe0.8O3−δ (LSCF)-type SOFCs. In the case of the LSM-type single cells, the following aspects relating to the electrochemical performance were investigated in more detail: (1) production of the anode substrate by tape casting versus warm pressing; (2) deposition of the anode functional layer (AFL) and electrolyte by screen printing versus vacuum slip casting; (3) use of noncalcined and non-ground YSZ for applying the cathode functional layer (CFL); and (4) sintering temperature of the CFL and cathode current collector layer (CCCL). In the case of LSCF-type cells, a systematic approach was initiated for optimizing the Ce0.8Gd0.2O2−δ (CGO) diffusion barrier layer: (1) deposition techniques of the CGO layer and (2) sintering temperature of the screen-printed CGO layer. Results have shown that certain modifications of the processing route led to a slightly lower electrochemical performance, whereas others did not affect the performance at all. Regarding LSCF-type SOFCs, a slight improvement of the performance was achieved by optimizing the sintering temperature of the CGO layer.
Dynamic Model of a Pressurized SOFC/Gas Turbine Hybrid Power Plant for the Development of Control Concepts
Authors: Wächter Christian , Lunderstädt Reinhart , Joos Franz
Published: August 2006

In a situation where fossil energy resources globally run short and the greenhouse effect increases, the interest in new technologies of energy conversion to reduce the demand of primary energy and emission of pollutants grows. The use of high temperature fuel cells like solid oxide fuel cells (SOFCs), especially in combination with gas turbines (GTs), promises remarkable room for improvement in the areas mentioned, compared to other state-of-the-art technologies. But design and handling of such complex plants require efficient control strategies to promote safe and reliable operation. The development of powerful control algorithms is based on an exact knowledge of the operating behavior, which can be obtained using dynamic system models. In this paper a nonlinear model with bulk parameters and 19 dynamic states is presented; the main assumptions and the underlying equations are given. The simulated system consists of a compressor, a SOFC, a turbine, a recuperator, an ejector with a diffusor, a reformer, and a load. Additionally, from the nonlinear model a linear one in state-space representation is derived at nominal conditions. The results of both models are compared. The agreement of the dynamic behavior and of steady state final values is satisfactory. Thus in future studies, methods of linear control theory could be used with the linear model to develop efficient control strategies.
Operating Microtubular SOFCS With Hydrogen Chloride and Hydrogen Sulfide Containing Fuels and Synthetic Wood Gas
Authors: Buchinger Gerhard , Hinterreiter Paul , Raab Thomas , Griesser Stefan , Claassen Richard , Claassen Dirk Peter , Sitte Werner , Meissner Dieter
Published: August 2006

Solid oxide fuel cells are known to be able to handle a large variety of different fuels. Because of the greenhouse effect the use of carbon dioxide neutral gases or liquids are of special interest. In this context wood-gas has a big potential to be an alternative fuel for solid oxide fuel cells (SOFCs). The gas is generated by a fluidized bed steam gasifier and consists of various components such as 25 Vol % carbon monoxide, 20 Vol % carbon dioxide, 10 Vol % methane, 2.5 Vol % ethylene, 0.5 Vol % propylene, 2 Vol % nitrogen, and the rest hydrogen (values in dry state). The water concentration of the original pyrolysis gas is about 35 Vol %. Besides these main ingredients there are of course many impurities like dust, tars, ammonia, hydrogen sulphide, and hydrogen chloride present in the product gas. Especially the last two ones may lead to degeneration of the fuel cell anode and must therefore be almost totally removed before feeding the gas into the cell. In order to reduce energy losses, hot gas cleaning systems are favored. This, however, limits the possibility to reduce the impurity concentrations to very low levels. Therefore the aim of this work is to define the maximum acceptable output concentrations for the hydrogen chloride adsorber also in combination with hydrogen sulphide, since for a micro-tubular SOFC there are as yet hardly any data available. In order to determine the influence of the hydrogen chloride on the performance of the fuel cell, different concentrations of this impurity were fed to the cell. Here, also the flow rate was changed while the electrochemical output was determined. In addition it was analyzed if there were any effects when changing from pure hydrogen to the HCl containing fuel. This was investigated at 1123 K and 1173 K, which are the preferred working temperatures for our cells. Cooling down as well as heating up procedures were tested with cells between 1173 K and 573 K. In a second series of experiments, combinations of hydrogen chloride and hydrogen sulphide of variable concentrations were tested. As before, changing between pure hydrogen and the acid containing fuel at above given temperatures was analyzed by determining the cell performance. In parallel to the above experiments, synthetic wood gas was used for operating the microtubular fuel cell while monitoring the electrochemical output with time.
Material Characterization and Modeling of Single-Wall Carbon Nanotube/Polyelectrolyte Multilayer Nanocomposites
Authors: Huang Gang , Wang Bo , Lu Hongbing , Mamedov Arif , Gupta Sachin
Published: August 2006

Strong single-wall carbon nanotubes (SWNTs) possess very high stiffness and strength. They have potential for use to tailor the material design to reach desired mechanical properties through SWNT nanocomposites. Layer-by-layer (LBL) assembly technique is an effective method to fabricate SWNT/polyelectrolyte nanocomposite films. To determine the relationship between the constituents of the SWNT/polymer nanocomposites made by LBL technique, a method has been developed to extend the recent work by Liu and Chen (Mech. Mater., 35, pp. 69–81, 2003) for the calculation of the effective Young’s modulus. The work by Liu and Chen on the mixture model is evaluated by finite element analysis of nanocomposites with SWNT volume fraction between 0% and 5%. An equivalent length coefficient is introduced and determined from finite element analysis. A formula is presented using this coefficient to determine the effective Young’s modulus. It is identified that the current work can be applied to SWNT loadings between 0% and 5%, while Liu and Chen’s approach is appropriate for relatively high SWNT volume fractions, close to 5%, but is not appropriate for relatively low SWNT volume fractions. The results obtained from this method are used to determine the effective Young’s modulus of SWNT/polyelectrolyte nanocomposite with 4.7% SWNT loading. The material properties are characterized using both nanoindentation and tensile tests. Nanoindentation results indicate that both the in-plane relaxation modulus and the through-thickness relaxation modulus of SWNT nanocomposites are very close to each other, despite the orientation preference of the SWNTs in the nanocomposites. The steady state in-plane Young’s relaxation modulus compares well with the tensile modulus, and measurement results are compared with Young’s modulus determined from the method presented.
Design and Testing of Ejectors for High Temperature Fuel Cell Hybrid Systems
Authors: Ferrari Mario L. , Bernardi Davide , Massardo Aristide F.
Published: August 2006

Our goal in this work is the improvement of the ejector performance inside hybrid systems supporting the theoretical activity with experimental tests. In fact, after a preliminary ejector design, an experimental rig has been developed to test single stage ejectors for hybrid systems at different operative conditions of mass flow rates, pressures, and temperatures. At first, an open circuit has been built to perform tests at atmospheric conditions in the secondary duct. Then, to emulate a SOFC anodic recirculation device, the circuit has been closed, introducing a fuel cell volume in a reduced scale. This configuration is important to test ejectors at pressurized conditions, both in primary and secondary ducts. Finally, the volume has been equipped with an electrical heater and the rig has been thermally insulated to test ejectors with secondary flows at high temperature, necessary to obtain values in similitude conditions with the real ones. This test rig has been used to validate simplified and CFD models necessary to design the ejectors and investigate the internal fluid dynamic phenomena. In fact, the application of CFD validated models has allowed us to improve the performance of ejectors for hybrid systems optimizing the geometry in terms of primary and secondary ducts, mixing chamber length, and diffuser. However, the simplified approach is essential to start the analysis with an effective preliminary geometry.
Numerical Modeling of Proton Exchange Membrane Fuel Cell With Considering Thermal and Relative Humidity Effects on the Cell Performance
Authors: Chi Pei-Hung , Weng Fang-Bor , Su Ay , Chan Shih-Hung
Published: August 2006

A three-dimensional (3D) model has been developed to simulate proton exchange membrane fuel cells. The model accounts simultaneously for electrochemical kinetics, current distribution, hydrodynamics, and multi-components transport. A single set of conservation equations of mass, momentum, energy, species, and electric current are developed and numerically solved using a finite-volume-based computational fluid dynamics technique (by computational fluid dynamics ACE+ commercial code). The physical model is presented for a 5cm×4.92cm×0.4479cm 3D geometry test cell with serpentine channels and counter flow. Subsequently, the model is applied to explore cell temperature effects in the cell environment with different relative humidity of inlet. The numerical model is validated and agreed well with the experimental data. The nonuniformity of thermal and water-saturation distributions is calculated and analyzed as well as its influence on the cell performance. As the cell is operated at low voltages (or high current densities), the thermal field of fuel cell tends to be nonuniform and exists locally in hot spots. The mechanism of thermal field and water content interacted with membrane dehydration and cathode water flooding will be discussed and revealed their influences on the cell performance, stability and degradation will be revealed.
Validation of a Mathematical Model Using an Industrial MCFC Plant
Authors: Gundermann M. , Heidebrecht P. , Sundmacher K.
Published: August 2006

The parameter identification of a mathematical model of a molten carbonate fuel cell (MCFC) is presented. The model is spatially distributed in two dimensions and describes the strongly non-linear coupling of the temperature field and reactions inside a MCFC. It contains a high number of unknown parameters that need to be validated by experimental means. Therefore, measurements from an industrial 250kW fuel cell power plant are used. This validation approach requires an elaborate strategy, which is presented in detail. The resulting set of parameters can predict the behavior of the MCFC over a broad operating range with a very good precision. The validated model is an important basis for further process optimization and for control system design.
Hybrid Proton-Conducting Membranes as Fuel Cells Solid Polyelectrolytes
Authors: Alberto Blázquez J. , Mecerreyes David , Miguel Oscar , Rodriguez Javier , Cendoya Ione , Ajuria Jon , Iruin Juan J. , Ignacio Santos J. , Marestin Catherine , Mercier Régis
Published: August 2006

A series of novel hybrid organic-inorganic membranes based on sulfonated naphthalimides and phosphotungstic acid N-methylpyrrolidone were prepared from a NMP solution. These materials, composed of two proton-conducting components, have good mechanical properties, high ionic conductivities (10.5×10−2S∕cm at 80°C) and good fuel cell performances at 70°C.
Diffusion and Chemical Reaction in the Porous Structures of Solid Oxide Fuel Cells
Authors: Haberman B. A. , Young J. B.
Published: August 2006

The Rolls-Royce integrated-planar solid oxide fuel cell (IP-SOFC) consists of ceramic modules with electrochemical cells printed on the outer surfaces. The cathodes are supplied with oxygen from air flowing over the outside of the module and the anodes are supplied with fuel diffusing from the internal gas channels. Natural gas is reformed into hydrogen in a separate reformer module of similar design except that the fuel cells are replaced by a reforming catalyst layer. The performance of the modules is intrinsically linked to the behavior of the gas flows within their porous structures. The multi-component convective-diffusive flows are simulated using a new theory of flow in porous material, the cylindrical pore interpolation model. The effects of the catalyzed methane reforming and water-gas shift chemical reactions are also considered using appropriate kinetic models. It is found that the shift reaction, which is catalyzed by the anode material, has certain beneficial effects on the fuel cell module performance. The shift reaction enables the fuel cells to make effective use of carbon monoxide as a fuel when the supplied fuel has become depleted of hydrogen. In the reformer module the kinetics of the reaction make it difficult to sustain a high methane conversion rate. Although the analysis is based on IP-SOFC geometry, the modeling approach and general conclusions are applicable to other types of SOFCs.
Plasma Surface Modification of Carbon Electrodes for Polymer Electrolyte Fuel Cells (EFC 2005-86319)
Authors: Chiu K.-F. , Hsieh M. Y.
Published: August 2006

Carbon electrodes are one of the key materials in polymer electrolyte fuel cells (PEFC), or proton exchange membrane fuel cells (PEMFC). The electrodes should allow water or water vapor, which is produced by the redox reactions, to flow out of the cells efficiently. In the meantime, the catalysis reactions are not interfered. In this study, the carbon electrodes for PEMFC have been modified in terms of the hydrophobic and hydrophilic properties by plasma irradiation. The process utilized inductively coupled plasma (ICP) driven by applying radio frequency (rf) power on an induction coil. A pure Ar, O2, and Ar∕O2 gas mixture were used as the plasma gas. Only one side of the sample has been treated. The material properties of the plasma treated and untreated carbon electrodes were investigated by Raman spectroscopy, Fourier transformed infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). FTIR results show the plasma treatments effectively modified the functional groups on the carbon surface, and therefore the hydrophilic and hydrophobic properties of the surface. SEM and Raman spectra data suggested that the ion bombardment during plasma treatments alters the surface morphology and carbon bonding structures of the samples, which also result in a hydrophilic surface. The treated carbon electrodes were used as cathodes and have been packed with commercial carbon anodes and catalyst coated membrane to form 5cm×5cm fuel cells. The current-voltage polarization curves of these fuel cells were measured and compared. The test results show the feasibility of improving the cell performance by plasma treated electrodes. The feasibility of altering the hydrophobic and hydrophilic properties by plasma treatment has been demonstrated. The capillary effect due to the unbalanced hydrophilicity between the treated and untreated electrode surfaces may be responsible for the improved cell performance.
Experimental Test Facility for the Analysis of Transient Behavior of High Temperature Fuel Cell/Gas Turbine Hybrid Power Plants
Authors: Taccani Rodolfo , Micheli Diego
Published: August 2006

Pressurized high temperature fuel cells and gas turbine integrated power systems are receiving growing attention as capable of reaching very high electrical conversion efficiency even in small size power plants. In this system the fuel and the oxidant (air) enter the cell after being compressed. The fuel oxidation reaction occurs predominantly within the fuel cell. The reaction is completed in a combustion chamber and the pressurized combustion products are exhausted through a turbine. The dynamic interdependences related to the integration of the fuel cell and the gas turbine are not completely understood and unexpected complications and dangers might arise. In fact as a consequence of both the relatively large volume of the pressurized portion of the plant and the shape of the stalled characteristic of available compressors, the plant could be affected by the inception of fluid-dynamic instabilities. In particular, surge could be detected in the transient off-design operational conditions occurring during plant regulation, start up and shut down. The paper presents a new experimental fuel cell gas turbine simulation facility that has been constructed at the Mechanical Engineering Department of the University of Trieste, Italy. The facility was designed to examine the effects of transient events on the dynamics of these systems. The theoretical analysis of the plant is completed using a dynamic model of the system purposely developed.
Studies on the Numerical Modeling of the Butterfly-type Unit Molten Carbonate Fuel Cell
Authors: Yoo Min-Jung , Kim Dong-Phil , Chung Gui-Yung , Lim Hee-Chun
Published: August 2006

In the butterfly-type molten carbonate fuel cell (MCFC), anode and cathode gases enter at the center of the gas channel and divide into two opposite directions in the gas channels, respectively. Mathematical modeling of the butterfly-type unit MCFC has been carried out and the results were compared with those of the co-flow type unit fuel cell, working at the same conditions and using the same amount of gases. In both the anode and the cathode gas channels, the local fuel conversions in the butterfly-type cell were slightly higher than those in the co-flow type fuel cell. Temperature distributions of the electrode-electrolyte plate in the butterfly-type cell were slightly lower than those of the co-flow type fuel cell. The more uniform temperature distributions could be obtained by using the butterfly-type fuel cell. However, distributions of current density and compositions become more nonuniform than those in the co-flow type fuel cell.
Dynamic Test and Real-time Control Platform of Anode Recirculation for PEM Fuel Cell Systems
Authors: Bao Cheng , Zhang Kexun , Ouyang Minggao , Yi Baolian , Ming Pingwen
Published: August 2006

Anode recirculation is essential to the pure-hydrogen proton exchange membrane fuel cell system. Keeping the pressure difference between the anode and the cathode is also important to the membrane health. In this paper, a dynamic platform was designed for the recirculation test of injection pump and real-time control of the anode pressure tracking. The test bench can work in a wide range of conditions for high- and low-pressure application. Based on the MATLAB/xPC Target environment, some S functions were written to drive the PC board for the hardware-in-loop application. Then an analytical full-order and a reduced-order model were built with good accuracy. By linearization of the nonlinear dynamic model, a linear quadratic Gaussian algorithm based on state feedback was used for set-point tracking. Moreover, an adaptive fuzzy neural network with an on-line neural network identifier was also designed to improve the control robustness. The foundation of the test bench and realization of the real-time control algorithms are meaningful to the future application in fuel cell systems.
Physical-Chemical and Thermodynamic Analyses of Ethanol Steam Reforming for Hydrogen Production
Authors: Caetano de Souza Antonio Carlos , Luz-Silveira José , Sosa Maria Isabel
Published: August 2006

Steam reforming is the most usual method of hydrogen production due to its high production efficiency and technological maturity. The use of ethanol for this purpose is an interesting option because it is a renewable and environmentally friendly fuel. The objective of this article is to present the physical-chemical, thermodynamic, and exergetic analysis of a steam reformer of ethanol, in order to produce 0.7Nm3∕h of hydrogen as feedstock of a 1kW PEMFC. The global reaction of ethanol is considered. Superheated ethanol reacts with steam at high temperatures producing hydrogen and carbon dioxide, depending strongly on the thermodynamic conditions of reforming, as well as on the technical features of the reformer system and catalysts. The thermodynamic analysis shows the feasibility of this reaction in temperatures about 206°C. Below this temperature, the reaction trends to the reactants. The advance degree increases with temperature and decreases with pressure. Optimal temperatures range between 600 and 700°C. However, when the temperature attains 700°C, the reaction stability occurs, that is, the hydrogen production attains the limit. For temperatures above 700°C, the heat use is very high, involving high costs of production due to the higher volume of fuel or electricity used. The optimal pressure is 1atm., e.g., at atmospheric pressure. The exergetic analysis shows that the lower irreversibility is attained for lower pressures. However, the temperature changes do not affect significantly the irreversibilities. This analysis shows that the best thermodynamic conditions for steam reforming of ethanol are the same conditions suggested in the physical-chemical analysis.
Preparation and Characterization of Pt/Superfine Mesocarbon Microbead Powers Electrocatalysts
Authors: Rong-Li Jia , Cheng-Yang Wang , Bin Zhu
Published: August 2006

Superfine mesocarbon microbead powders (SFMCMBs) as the new supports for platinum electrocatalysts were first investigated. The Pt∕SFMCMB electrocatalysts were prepared by an impregnation-reduction method, with hexachloroplatinic acid as the platinum precursor and formaldehyde as the reducing agent. The catalysts were characterized with x-ray diffraction (XRD), field emission gun transmission electron microscope (TEM), and electrochemical analysis. TEM photos showed the platinum particles were dispersed uniformly on the surface of SFMCMBs and there existed a little aggregation of platinum particles in the Pt∕SFMCMB catalysts. The TEM photos showed the existence of the platinum on the supports where the average platinum particle size were 4-6nm. The electrochemical analysis proved that SFMCMBs are excellent candidates to be used as the support of platinum electrocatalyst for methanol electrochemical oxidation as the potential catalyst candidate for direct methanol fuel cells (DMFCs).
MEAs for Polymer Electrolyte Fuel Cell (PEFC) Working at Medium Temperature
Authors: Gatto I. , Saccà A. , Carbone A. , Pedicini R. , Passalacqua E.
Published: August 2006

Recently, the CNR-ITAE activity has been addressed to the components development (electrodes and membranes) able to work in medium temperature PEFCs (80-130°C). One of the main problems to work at these temperatures is the proton conductivity loss due to a not full hydration of the membrane. For this reason a study on the modification of perfluorosulphonic membranes (like Nafion) was carried out by introducing different percentages of inorganic oxides (like SiO2, ZrO2) in the polymer matrix. These compounds have the function to improve the properties of the materials at high temperature due to their characteristics of softly proton conductor and/or hygroscopicity. The membranes were prepared by the Doctor-Blade casting technique that permits a good check of the thickness and a good reproducibility. A commercial ZrO2 was used to prepare the membranes varying the inorganic amount between 3 and 20wt%. The most promising results were obtained at 120°C with a Nafion-recast membrane loaded with a 10wt%ZrO2; a power density value of about 330mW∕cm2 at 0.6V was reached. On the other side, an optimization of the electrode structure was carried out, by introducing the inorganic oxide in the catalyst layer in order to improve the performance in the range of considered temperature. By using a spray technique, thin film electrodes with a Pt loading of 0.5mg∕cm2 in the catalyst layer, low PTFE content in the diffusion layer and a 30% Pt/Vulcan (E-Tek, Inc.) as an electro catalyst were prepared. Different amounts of ZrO2 were introduced in the catalytic layer of the electrodes to increase the working temperature and help the water management of the fuel cell. These electrodes assembled to the modified membrane have shown a better performance at higher cell temperature than standard MEA with a power density of about 330mWcm−2 at 130°C.
An Efficient Methodology for Fatigue Reliability Analysis for Mechanical Components
Authors: Park Young Ho , Tang Jun
Published: August 2006

This paper presents an efficient methodology to solve a fatigue reliability problem. The fatigue failure mechanism and its reliability assessment must be treated as a rate process since, in general, the capacity of the component and material itself changes irreversibly with time. However, when fatigue life is predicted using the S-N curve and a damage summation scheme, the time dependent stress can be represented as several time-independent stress levels using the cycle counting approach. Since, in each counted stress cycle, the stress amplitude is constant, it becomes a random variable problem. The purpose of this study is to develop a methodology and algorithm to solve this converted random variable problem by combining the accumulated damage analysis with the first-order reliability analysis (FORM) to evaluate fatigue reliability. This task was tackled by determining a reliability factor using an inverse reliability analysis. The theoretical background and algorithm for the proposed approach to reliability analysis will be introduced based on fatigue failure modes of mechanical components. This paper will draw on an exploration of the ability to predict spectral fatigue life and to assess the corresponding reliability under a given dynamic environment. Next, the process for carrying out this integrated method of analysis will be explained. Use of the proposed methodology will allow for the prediction of mechanical component fatigue reliability according to different mission requirements.
Low and High Cycle Fatigue and Cyclic Elasto-Plastic Behavior of the P355NL1 Steel
Authors: De Jesus Abílio M. P. , Ribeiro Alfredo S. , Fernandes António A.
Published: August 2006

A normalized fine grain carbon low alloy steel, P355NL1 (TStE355), intended for service in welded pressure vessels, where notch toughness is of high importance, has been investigated. Low and high cycle fatigue tests have been conducted on several series of smooth specimens under both strain and stress control. The monotonic and cyclic elasto-plastic behavior of the material is characterized and described using relations available in the literature. The shape of hysteresis loops are conveniently modeled, taking into account the observed non Masing behavior of the steel. Some important cyclic phenomena, observed for the studied steel, such as the cyclic creep and the cyclic stress relaxation, are illustrated. Strain, stress, and energy based relations for fatigue life prediction until crack initiation, are evaluated based on experimental results. The adequacy of several rules for damage accumulation is also investigated. Finally, along the paper, some comparisons are performed between the cyclic elasto-plastic and fatigue behaviors of the steels P355NL1 and ASTM A516 Gr. 70.
Statistical and Constraint Loss Size Effects on Cleavage Fracture–Implications to Measuring Toughness in the Transition
Authors: Rathbun H. J. , Odette G. R. , Yamamoto T. , He M. Y. , Lucas G. E.
Published: August 2006

A systematic investigation of the effects of specimen size on the cleavage fracture toughness of a typical pressure vessel steel is reported. Size dependence arises both from: (i) statistical effects, related to the volume of highly stressed material near the crack tip, that scales with the crack front length (B) and (ii) constraint loss, primarily associated with the scale of plastic deformation compared to the un-cracked ligament dimension (b). Previously, it has been difficult to quantify the individual contributions of statistical versus constraint loss size effects. Thus, we developed a single variable database for a plate section from the Shoreham pressure vessel using a full matrix of three point bend specimens, with B from 8 to 254 mm and b from 3.2 to 25.4 mm, that were tested at a common set of conditions. The University of California Santa Barbara (UCSB) b-B database was analyzed using three-dimensional finite element calculation of the crack tip fields combined with a cleavage model calibrated to the local fracture properties of the Shoreham steel. This paper focuses on the possible significance of these results to the Master Curve standard as formulated in ASTM E 1921. The statistical scaling procedure to treat variations in B used in E 1921 was found to be reasonably consistent with the UCSB b-B database. However, constraint loss for three point bend specimens begins at a deformation level that is much lower than the censoring limit specified in E 1921. Unrecognized constraint loss leads to a nonconservative, negative bias in the evaluation of To, estimated to be typically on the order of −10°C for pre-cracked Charpy specimens.
Effect of Charpy V Notch Position on Impact Toughness of all Weld Metal Welds and its Relevance With ASME Codes
Authors: Paranhos R. P. R. , Rosenthal R. , Bott I. S.
Published: August 2006

A neutral flux and four different wires were used to produce all weld metal submerged-arc multipass welds. Impact Charpy V transition curves were obtained from two testing series, each with a different notch position along the welded joint. The results show that, according to the notch position, distinct fractions of weld metal columnar and reheated regions are present at the notch tip. Impact toughness of the weld metal with predominance of reheated regions is always superior to that with predominance of columnar region. Modification of some of the ASME II Part C standards relating to the specification of welding consumables is recommended for improved consistency with ASME VIII code for boiler and pressure vessel fabrication.
Unified Interpretation of Crack Growth Rates of Ni-base Alloys in LWR Environments
Authors: Lu Zhanpeng , Shoji Tetsuo
Published: August 2006

Primary water stress corrosion cracking (PWSCC) of vessel penetrations (VP) fabricated from nickel based alloys such as alloy 600 and alloy 182 weld metal has created a great demand for elucidation of the cracking mechanism and for development of life prediction technologies. The generalized FRI crack growth rate (CGR) formulation was proposed, based on a deformation/oxidation mechanism and a theoretical crack tip strain rate equation derived by the authors. The effects of crack tip oxidation and crack tip mechanics and of their interactions on crack growth can be quantified. Experimental and actual plant data of CGR for alloy 600 in PWR primary water, which are sometimes scattered in CGR-K diagrams, are interpreted with the generalized CGR formulation, emphasizing the effects of temperature, K, yield strength and variations of K with time. It is suggested that it is essential to determine the type of dependency of CGR on K for accurate flaw disposition. The generalized formulation provides a unique parameter for interpreting CGRs as well as a unified method for predicting CGRs within a narrow scattered band even under various testing parameters, which is the basis for accurately predicting component life.
Sloshing Effects on the Seismic Design of Horizontal-Cylindrical and Spherical Industrial Vessels
Authors: Karamanos Spyros A. , Patkas Lazaros A. , Platyrrachos Manolis A.
Published: August 2006

The present paper investigates sloshing effects on the earthquake design of horizontal-cylindrical and spherical industrial vessels. Assuming small-amplitude free-surface elevation, a linearized sloshing problem is obtained, and its solution provides sloshing frequencies, modes, and masses. Based on an “impulsive-convective” decomposition of the container-fluid motion, an efficient methodology is proposed for the calculation of seismic force. The methodology gives rise to appropriate spring-mass mechanical models, which represent sloshing effects on the container-fluid system in an elegant and simple manner. Special issues, such as the deformability of horizontal-cylindrical containers or the flexibility of spherical vessel supports, are also taken into account. The proposed methodology can be used to calculate the seismic force, in the framework of liquid container earthquake design, and extends the current design practice for vertical cylindrical tanks stated in existing seismic design specifications (e.g., API Standard 650 and Eurocode 8). The methodology is illustrated in three design examples.