关键词： Solid Oxide Fuel Cell   Cermet Ni-YSZ   Redox stability   Kinetics   Modelling  

摘要： The 'redox' tolerance of a typical anode supported cell was evaluated for three temperatures of re-oxidation. For this purpose, an experimental work has been coupled to a modelling approach to estimate the risk of electrolyte failure during re-oxidation. A special attention has been paid to take into account both (i) the heterogeneity of oxidation and (ii) the cermet visco-plasticity in operation. Data required for the simulations - i.e. the oxidation kinetics rates, the cermet expansions and Young's modulus - were determined at T = 600, 700 and 800 degrees C. It has been found that the activation energy related to the kinetics of re-oxidation encounters a modification at high temperature (700-750 degrees C). This modification has been ascribed to a transition from a homogeneous oxidation process to a heterogeneous one. Local X-ray measurements have confirmed that an oxidation gradient in the cermet arises at T = 800 degrees C. Mechanical analysis has shown that the presence of an oxidised front at T = 800 degrees C: strongly impacts the cell 'redox' tolerance. Indeed, this phenomenon induces a significant cell bending, which adds a compressive stress component to the thin electrolyte. simulations have been carried out to determine both critical degree of oxidation and durations before electrolyte cracking. The effect of the cermet creep during operation on the cell 'redox' tolerance is also discussed. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

关键词： SWCNT   SWCNT bundle   MWCNT   Mechanical properties   MD simulation   Fracture  

摘要： Carbon nanotubes have caught tremendous attention of the researchers during the last decade due to their excellent mechanical, electrical, optical and thermal properties. The exploitation of these fibers as reinforcing agents in making strong fiber composites has been a primary research topic in the recent investigations on composite materials. Although the theoretical results are rather optimistic, the goal of achieving high strength of the carbon nanotube composites is still not satisfactorily realized. We report here a comparative study of the mechanical properties of single-walled, multi-walled and bundle of single-walled carbon nanotubes. Their mechanical behavior is investigated by molecular dynamics simulation, considering Brenner's second generation reactive empirical bond order interatomic potential between the carbon atoms making a tube. For a long range interaction, we have defined a weak van der Waals force which acts between different layers of a multi-walled tube or between different tubes of a bundle. Samples of three isolated armchair single-wall carbon nanotubes of different diameters, a multi-wall armchair carbon nanotube and finally a bundle of three armchair single-walled nanotubes of same diameter are taken. Their fracture pattern and buckling behavior are modeled and compared. Significant changes are observed in the mechanical properties of the samples of different types of carbon nanotubes which arise due to the interaction between the shells of a multi-walled tube or the tubes in a bundle.

关键词： Andosol   Temporal changes   Air and water conductivity   Precompression stress  

摘要： Soil physical properties continuously change as a result of wetting and drying cycles (WD), soil tillage and grazing events. In order to define sustainable grazing management, it is necessary to determine how the soil responds during and after applied external stresses. This is relevant in southern Chile where pastures are the main livestock feed throughout the course of the year and Andosols are exposed to great mechanical stresses by a wide range of soil water contents. Therefore, the purpose of this paper was to illustrate the effect of grazing. considering two grazing intensities in the winter: 50 and 200 cows ha(-1), on the temporal changes of structure-dependent properties of an Andosol in southern Chile. In order to characterize the impact of grazing, 15 soil samplings (5-10 cm depths) were conducted over a 2 year period. The penetration resistance (PR) and soil water content were determined in the field. Air capacity (AC), air (k(l)) and saturated hydraulic (k(s)) conductivity, precompression stress (Pc), and aggregate strength (AS) were measured. Changes in the soil mechanical stability and pore functions were observed as a consequence of grazing events and WD cycles, indicating that the structure-dependent properties are dynamic. Animal trampling induced an increase in PR;however, the increase in soil mechanical strength may not affect root growth, considering the changes in time of the PR and the specific properties of Andosol (e.g. bulk density <0.9 Mg m(-3)). Temporal variations in Pc did not exceed critical values for root growth even under high grazing intensities, indicating that the soil recuperates after the impact of trampling during grazing. The air capacity/conductivity values measured in our study reached critical values for gas exchange as a consequence of animal trampling;however, the Andosol showed its ability to recuperate its integral pore functionality. Finally, in spite of the low bulk density of the Andosol, the increasing grazing inten

关键词： STEEL metallurgy   POWDER metallurgy   NANOSTRUCTURED materials   TENSILE strength   ISOTHERMAL processes   HEAT treatment  

摘要： A PM approach is investigated in the context of extremely fine low temperature bainitic steels with a reasonable combination of ultimate tensile strength and uniform elongation. The strategy consists of three steps. First, ball milling is used for mechanical alloying and to create a large density of interfaces prior to sintering. In the second step, spark plasma sintering is used as a rapid consolidation process to retain a very fine and homogeneous structure after sintering. The isothermal heat treatment which leads to bainite can then be done at a low austenitisation temperature (860 degrees C) for a few minutes, leading to a small initial austenite grain size. A fine grain size promotes the rapid transformation of the austenite into bainite. The characteristics of a nanostructured Fe-Ni-C bainitic steel obtained after mechanical alloying and nanostructuring of the powder, spark plasma sintering and final heat treatment are reported.

关键词： Glasses,metallic   Internal friction   Thermal properties   Defects: theory  

摘要： Structural relaxation in Cu46Zr45Al7Dy2, Ti40Zr25Ni8Cu9Be18 and Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit 1) bulk metallic glasses has been studied by mechanical spectroscopy during annealing below the glass transition temperature T-g. Structural relaxation in bulk metallic glasses leads to an increase of the storage modulus G' and a decrease of the loss factor tan delta, which are observed after an incubation time. Kinetics of this phenomenon can be well described by a stretched exponential, similar to the Kohlrausch -Williams-Watts (KWW) equation used to fit differential scanning calorimetry (DSC) experiments. In the various investigated bulk metallic glasses the stretching exponent beta(KWW) does not depend on annealing temperature implying therefore that this exponent beta(KWW) is nearly independent of the microstructure. Decrease in the loss factor is due to a decrease in the atomic mobility. Various models have been proposed in the literature to describe this evolution: decrease in the free volume, decrease in the liquid-like regions or decrease in the defect concentration (flow defects or quasi-point defects). Present results are discussed using this concept of defect. In any case it corresponds to an increase of the short range order during annealing below the glass transition temperature T-g. (C) 2012 Elsevier Ltd. All rights reserved.

关键词： Metals   Fluids   Cavities   Flow (Dynamics)   Cycles   Computational fluid dynamics   Thermomechanics   Stators   Engines   Temperature   Geometry   Springs   Finite element analysis  

摘要： The optimization of heat transfer between fluid and metal plays a crucial role in gas turbine design. An accurate prediction of temperature for each metal component can help to minimize the coolant flow requirement, with a direct reduction of the corresponding loss in the thermodynamic cycle. Traditionally, in industry fluid and solid simulations are conducted separately. The prediction of metal stresses and temperatures, generally based on finite element analysis, requires the definition of a thermal model whose reliability is largely dependent on the validity of the boundary conditions prescribed on the solid surface. These boundary conditions are obtained from empirical correlations expressing local conditions as a function of working parameters of the entire system, with validation being supplied by engine testing. However, recent studies have demonstrated the benefits of employing coupling techniques, whereby computational fluid dynamics (CFD) is used to predict the heat flux from the air to the metal, and this is coupled to the thermal analysis predicting metal temperatures. This paper describes an extension of this coupling process, accounting for the thermo-mechanical distortion of the metal through the engine cycle. Two distinct codes, a finite element analysis (FEA) solver for thermo-mechanical analysis and a finite volume solver for CFD, are iteratively coupled to produce temperatures and deformations of the solid part through an engine cycle. At each time step, the CFD mesh is automatically adapted to the FEA prediction of the metal position using efficient spring analogy methods, ensuring the continuity of the coupled process. As an example of this methodology, the cavity flow in a turbine stator well is investigated. In this test case, there is a strong link between the thermo-mechanical distortion, governing the labyrinth seal clearance, and the amount of flow through the stator well, which determines the resulting heat transfer in the stator well. Th

关键词： Graphene   graphite   molecular dynamics   vacancy  

摘要： The mechanical properties of graphene and graphite containing vacancies under tensile loading were investigated using molecular dynamics simulations. Two types of potential functions were used in the simulations: the second-generation reactive empirical bond-order (REBO) potential for covalent C-C bonds, and the Lennard-Jones potential for the interlayer interaction in graphite. The influence of the size and distribution of the vacancies on the mechanical properties of graphene and graphite were studied. It was found that the tensile strengths of graphene and graphite are significantly decreased when they contain randomly distributed vacancies.

关键词： bonds (chemical)   fracture   graphene   molecular dynamics method   polymorphism   Young's modulus  

摘要： Graphyne is the allotrope of graphene. In this letter, four different graphynes (alpha, beta, gamma, and 6,6,12-graphenes) are investigated by molecular dynamics simulations to explore their mechanical properties and failure mechanisms. It is found that the presence of the acetylenic linkages in graphynes leads to a significant reduction in fracture stress and Young's modulus with the degree of reduction being proportional to the percentage of the linkages. This deterioration in mechanical properties stems from the low atom density in graphynes and weak single bonds in the acetylenic linkages where the facture is initiated. (C) 2012 American Institute of Physics. [http://***/10.1063/1.4747719]

关键词： Hydrolysis   Organopalladium   Mechanism   Activation parameters   Density functional theory   Solvent effects  

摘要： In this work, mechanism and kinetics of the hydrolysis of Organopalladium Complex [Pd(CNN)P(OMe)(3)](+) in low acidity range, has been theoretically investigated in detail. Using quantum mechanical approach, different probabilities were investigated and ultimately a model was presented in which an enol form is produced in the rate determining step and in continuation is converted into products. Considering the solvent effects, the activation energy, activation enthalpy and activation Gibbs free energy for the rate determining step were calculated, which are in good agreement with the experimental values. (C) 2012 Elsevier B.V. All rights reserved.

关键词： bicrystals   brittle fracture   chemical vapour deposition   failure (mechanical)   graphene   molecular dynamics method   nanofabrication   nanoribbons   nucleation   slip   surface cracks   tensile strength   tilt boundaries  

摘要： Recent experimental studies have shown that multi-grains are ubiquitously present in graphene grown with chemical vapor deposition method technique. The potential application of the unique two-dimensional material in future nanotechnology demands full understandings of their structure and properties. Using molecular dynamics simulations, we study the mechanical response of various bicrystal graphene consisting of symmetric tilt boundary subject to uniaxial tensile loading. Both bulk graphene and graphene nanoribbons (GNRs) are studied. We revealed that nano-crack initiated at grain boundaries (GBs) leads to brittle failure with no plasticity at room temperature. The mechanism that crack nucleated at the intersection of GB and free surface followed by fast advance of crack, preventing plasticity that involves dislocation slip or GB sliding that is seen in metals. Cleavage along GBs is observed to be dominant fracture behavior in the studied GNRs. Furthermore, the ultimate tensile strength decreases with increasing vacancy concentration, illustrating that the lowered strength of GB interface is primarily due to not well-bonded atoms, shedding light to the structure-properties relationship. Finally, local strain and atomic-level stress have been shown to be able to characterize the onset of crack nucleation and thereby good quantities for predicting the resulting ultimate tensile strength. (C) 2012 American Institute of Physics. [http://***/10.1063/1.4749812]