Layer Phenomena Research Articles

Interactions between Metallic Interconnects and Ceramic Electrodes in SOFC Operating Environments

Ferritic stainless steel bipolar plates are widely used to interconnect individual cells in planar Solid Oxide Fuel Cell stacks. In this study, thin films of electrode ceramic materials, LSM for the cathode and Ni/YSZ for the anode, were screen printed on the selected metallic alloys (with and without surface coatings). Samples were then placed in a test rig which allows simultaneous exposures to single (air/air) and dual atmospheres (air/fuel) at 800°C for 100 hours. This research is focused on the investigation of the thermally grown oxide layer and ionic transport phenomena for a better understanding of materials interactions during realistic SOFC exposures. Morphology of the surfaces and cross sections were analyzed by a Field Emission Scanning Electron Microscope (FESEM) and the composition was assessed using Energy Dispersive X-ray Spectroscopy (EDS).

Effect of Process Parameters on the Mechanical Properties of Friction Stir Welded Al-Li Alloy Butt Joints

Friction stir welding (FSW) is a new and promising welding process that can produce low-cost and high-quality joints of Al-Li alloy. In order to demonstrate the friction stir weldability of Al-Li alloy and determine optimum welding parameters, the effect of FSW welding parameters on the tensile properties of the Al-Li alloy butt joints has been studied in this paper. The results showed that, when the heat input increased, the ultimate strength and elongation of butt joints decreased. For the Al-Li alloy butt structure with the thickness of 2mm, the optimum parameters were the rotation speed of 600 rpm, the welding speed of 400mm/min, and the ultimate strength of butt structure was the maximum strength, arrived at 413MPa, equivalent to 83% that of the base material. Furthermore, it was found from the microstructural observation that, the welding parameters had obvious effect on the grain size of welded zone. As the heat input increasing, the grains of weld nugget and heat affected zone became larger accordingly. In addition, a special layered phenomenon appeared at tensile fracture surface, the tensile cracks initiated from the surface on welded zone near the thermomechanically affected zone(TMAZ) on the advancing side and then grew along the interface between the weld nugget and the TMAZ. The overall fracture mode was ductile - brittle hybrid fracture.

Measurement of turbulent water vapor fluxes using a lightweight unmanned aerial vehicle system

Abstract. We present here the first application of a lightweight unmanned aerial vehicle (UAV) system designed to measure turbulent properties and vertical latent heat fluxes (λE). Such measurements are crucial to improve our understanding of linkages between surface moisture supply and boundary layer clouds and phenomena such as atmospheric rivers. The application of UAVs allows for measurements on spatial scales complimentary to satellite, aircraft, and tower derived fluxes. Key system components are: a turbulent gust probe; a fast response water vapor sensor; an inertial navigation system (INS) coupled to global positioning system (GPS); and a 100 Hz data logging system. We present measurements made in the continental boundary layer at the National Aeronautics and Space Administration (NASA) Dryden Research Flight Facility located in the Mojave Desert. Two flights consisting of several horizontal straight flux run legs up to ten kilometers in length and between 330 and 930 m above ground level (m a.g.l.) are compared to measurement from a surface tower. Surface measured λE ranged from −53 W m−2 to 41 W m−2, and the application of a Butterworth High Pass Filter (HPF) to the datasets improved agreement to within +/−12 W m−2 for 86% of flux runs, by removing improperly sampled low frequency flux contributions. This result, along with power and co-spectral comparisons and consideration of the differing spatial scales indicates the system is able to resolve vertical fluxes for the measurement conditions encountered. Challenges remain, and the outcome of these measurements will be used to inform future sampling strategies and further system development.

On geometrically exact post-buckling of composite columns with interlayer slip—The partially composite elastica

This paper is focused on the geometrically exact elastic stability analysis of two interacting kinematically constrained, flexible columns. Possible applications are to partially composite or sandwich columns. A partially composite column composed of two inextensible elastically connected sub-columns is considered. Each sub-column is modeled by the Euler–Bernoulli beam theory and connected to each other via a linear constitutive law for the interlayer slip. The paper discusses the validity of parallel and translational kinematics beam assumptions with respect to the interlayer constraint. Buckling and post-buckling behavior of this structural system are studied for cantilever columns (clamped-free boundary conditions). A variational formulation is presented in order to derive relevant boundary conditions in a geometrically exact framework. The exact post-buckling behavior of this partially composite beam-column is investigated analytically and numerically. The Euler elastica problem is obtained in the case of non-composite action. The “partially composite elastica” is then treated analytically and numerically, for various values of the interaction connection parameter. An asymptotic expansion is performed to evaluate the symmetrical pitchfork bifurcation, and comparisons are made with some exact numerical results based on the numerical treatment of the non-linear boundary value problem. A boundary layer phenomenon, similar to that also observed for the linearized bending analysis of partially composite beams, is observed for large values of the connection parameter. This boundary layer phenomenon is investigated with a straightforward asymptotic expansion, that also is valid for large rotations. Finally, the paper analyses the effect of some additional imperfection eccentricities in the loading mode, that lead to some pre-bending phenomena.

Homogenization and boundary layers

This paper deals with the homogenization of elliptic systems with Dirichlet boundary condition, when the coefficients of both the system and the boundary data are e-periodic. We show that, as e → 0, the solutions converge in L2 with a power rate in e, and identify the homogenized limit system. Due to a boundary layer phenomenon, this homogenized system depends in a non trivial way on the boundary. Our analysis answers a longstanding open problem, raised for instance in [7]. It extends substantially previous results obtained for polygonal domains with sides of rational slopes as well as our previous paper [12] where the case of irrational slopes was considered.

Rheological studies of aqueous stabilised nano-zirconia particle suspensions

In the present investigation aqueous suspensions of nano- and colloidal range particles are stabilised by changing the ambient pH. Rheology is used to establish the stability of the suspensions and it is found that the rheology of the suspensions is strongly dependent on the pH values. The viscosity is highest close to the iso-electric point of the powders. At the iso-electric point the net surface charge on the powder particles is zero and is the cause of the high viscosity. Away from the iso-electric point, the particles are charged, giving rise to a double layer phenomenon and causing the reduction in viscosity. It is also found that increasing the solid contents of the suspensions reduces the pH region of low viscosity.

Vortex patch with wall interaction as new benchmark test

Abstract In this paper, a new computational benchmark test for fluid dynamics is presented. The new benchmark is based on the interaction of a single vortex structure (vortex patch) with a wall. It will be shown that it is possible to distinguish two critical or threshold values of the Reynolds number in the considered flow. The increase of the Reynolds number causes the appearance of the vortex bubble in the near-wall region first, and then next, the eruption of the boundary layer phenomenon. Further increase of the Reynolds number causes the flow to be more complex. The eruption phenomenon becomes more intense and also shows its regenerative nature.

Mechanisms of Slower Nitric Oxide Uptake by Red Blood Cells and Other Hemoglobin-containing Vesicles

Nitric oxide (NO) acts as a smooth muscle relaxation factor and plays a crucial role in maintaining vascular homeostasis. NO is scavenged rapidly by hemoglobin (Hb). However, under normal physiological conditions, the encapsulation of Hb inside red blood cells (RBCs) significantly retards NO scavenging, permitting NO to reach the smooth muscle. The rate-limiting factors (diffusion of NO to the RBC surface, through the RBC membrane or inside of the RBC) responsible for this retardation have been the subject of much debate. Knowing the relative contribution of each of these factors is important for several reasons including optimization of the development of blood substitutes where Hb is contained within phospholipid vesicles. We have thus performed experiments of NO uptake by erythrocytes and microparticles derived from erythrocytes and conducted simulations of these data as well as that of others. We have included extracellular diffusion (that is, diffusion of the NO to the membrane) and membrane permeability, in addition to intracellular diffusion of NO, in our computational models. We find that all these mechanisms may modulate NO uptake by membrane-encapsulated Hb and that extracellular diffusion is the main rate-limiting factor for phospholipid vesicles and erythrocytes. In the case of red cell microparticles, we find a major role for membrane permeability. These results are consistent with prior studies indicating that extracellular diffusion of several gas ligands is also rate-limiting for erythrocytes, with some contribution of a low membrane permeability.

Linear plane wave propagation and normal transmission and reflection at discontinuity surfaces in second gradient 3D continua

AbstractWe study plane waves in second gradient solids and their reflection and transmission at plane displacement discontinuity surfaces. The needed extension of the treatment adopted to study plane wave propagation in Cauchy continua is not straightforward and is developed here. In particular, the balance of mechanical energy valid for second gradient continua is deduced. The presented results may be of interest as many boundary layer phenomena can be accounted for by second gradient models. We prove that second gradient elastic moduli may influence, in a measurable manner, how planar waves behave at discontinuity surfaces: the novel results presented here can be the basis of experimental procedures to estimate some among these moduli. We explicitly remark that reflection and transmission coefficients which we have estimated show a significant dependence on frequency, which indeed makes easier to conceive effective measurement methods.

Nanostructures of liquid crystal phases in mixtures of bent-core and rod-shaped molecules

We report small angle x-ray scattering (SAXS) studies of isotropic, nematic, and smectic mesophases formed by binary mixtures of bent-core (BC) and rod-shaped (RS) molecules. While optical studies indicate that the components are fully miscible, SAXS reveals fascinating structures that are consistent with segregation on a nanoscopic scale. We find that tilted smectic clusters, which have been previously reported in both the nematic and isotropic states of the pure BC materials, are also present in mixtures with up to 50 wt% of the RS compound; this is consistent with previous dielectric and flexoelectric studies on such mixtures. Unexpectedly in this concentration range the clusters are present in the isotropic and in the induced smectic phase range, as well as throughout the nematic phase. The results in the smectic phase also reveal complex layering phenomena, providing important insight into the interaction between bent and rod-shaped molecules. These studies will be crucial in the design of promising new functional nanomaterials.

Felt and Enacted Stigma Among HIV/HCV-Coinfected Adults

The realization that many persons with HIV/AIDS are subjected to multiple layers of stigmatization because they belong to socially deviant and disenfranchised groups (e.g., injection drug users, racial/ethnic and sexual minorities) accounts for an increasing interest in the phenomenon of stigma layering. The stigma associated with hepatitis C virus (HCV) has also been conceptualized as layered. However, researchers have overlooked the fact that HCV adds a layer to the HIV stigma and vice versa. Qualitative interviews with 132 HIV/HCV-coinfected patients were analyzed to explore how they experience the two layers of stigma. Most participants hierarchically ordered the stigmas associated with each disease and regarded HIV as the more stigmatizing of the two. A small number perceived HIV and HCV as equally stigmatizing. The impact of the hierarchical and nonhierarchical ordering of the two stigmas on coinfected patients' felt and enacted stigmatization is explored and implications for interventions are discussed.

Filtering carbon dioxide through carbon nanotubes

Layering phenomena of carbon dioxide transported through carbon nanotubes are investigated through molecular dynamics simulations. The layering formation is examined for carbon nanotubes spanning from (8,8) to (20,20) subjected to pressures and temperatures that range from 1 to 20bar and 300 to 400K, respectively. Well defined layers are developed around the internal and external surface of the nanotubes for all the examined cases. It is shown that the number of layers along with their relative strength varies as a function of the nanotube’s diameter, size, carbon dioxide density and gas-structure interactions.

Nonlocal Four-Point Boundary Value Problem for the Singularly Perturbed Semilinear Differential Equations

This paper deals with the existence and asymptotic behavior of the solutions to the singularly perturbed second-order nonlinear differential equations. For example, feedback control problems, such as the steady states of the thermostats, where the controllers add or remove heat, depending upon the temperature detected by the sensors in other places, can be interpreted with a second-order ordinary differential equation subject to a nonlocal four-point boundary condition. Singular perturbation problems arise in the heat transfer problems with large Peclet numbers. We show that the solutions of mathematical model, in general, start with fast transient which is the so-called boundary layer phenomenon, and after decay of this transient they remain close to the solution of reduced problem with an arising new fast transient at the end of considered interval. Our analysis relies on the method of lower and upper solutions.

Boundary layer analysis for nonlinear singularly perturbed differential equations

This paper focuses on the boundary layer phenomenon arising in the study of singularly perturbed differential equations. Our tools include the method of lower and upper solutions combined with analysis of the integral equation associated with the class of nonlinear equations under consideration.

Nonlocal Four-Point Boundary Value Problem for the Singularly Perturbed Semilinear Differential Equations

This paper deals with the existence and asymptotic behavior of the solutions to the singularly perturbed second-order nonlinear differential equations. For example, feedback control problems, such as the steady states of the thermostats, where the controllers add or remove heat, depending upon the temperature detected by the sensors in other places, can be interpreted with a second-order ordinary differential equation subject to a nonlocal four-point boundary condition. Singular perturbation problems arise in the heat transfer problems with large Peclet numbers. We show that the solutions of mathematical model, in general, start with fast transient which is the so-called boundary layer phenomenon, and after decay of this transient they remain close to the solution of reduced problem with an arising new fast transient at the end of considered interval. Our analysis relies on the method of lower and upper solutions.

Study on local morphological changes of nickel in solid oxide fuel cell anode using porous Ni pellet electrode

Morphological change of nickel, especially the aggregation of nickel, in solid oxide fuel cell anode is an important deactivation mechanism which results in long-period degradation of anode. In order to study the mechanisms which cause local morphological changes of nickel in solid oxide fuel cell anode, porous nickel pellet, which is mechanically pressed against dense YSZ pellet with LSM cathode, was employed as the anode of the cell. The cell was tested by static-potential method in hydrogen with different humidities environment for 60 h. The study focused on the vicinity of three phase boundary which concentrated at nickel–YSZ interface. After the discharging test, the cell performance and the microstructure at nickel–YSZ contacting point of interface were studied and correlated to nickel morphological changes. The interlocking effect and the spreading of densified nickel layer phenomena were observed between nickel and YSZ substrate after discharging with an anode to cathode terminal voltage of 0.6 V. Humidity enhanced nickel surface diffusion and humidity gradient driving vaporization–deposition mechanism, which caused the growing and merging of independent nickel droplets, were used to explain the local morphological changes and redistribution of nickel inside and along the edge of YSZ surface bonded nickel cluster, respectively. The bulk nickel morphological changes were also studied to support the humidity enhanced nickel surface diffusion mechanism. The competition of interlocking effect, nickel redistribution at TPB and bulk nickel sintering finally determined the cell performance.

Contributions of fractional differentiation to the modelling of electric double layer capacitance

The electric double layer phenomenon is sometimes described by means of a specific element called “constant phase element”, instead of the usual electric double layer capacitance. This article deals with such an element, often associated with rough surfaces. It is first shown, using a lithium–ion battery and a PEM fuel cell, that this element can obviously improve accuracy in the modelling of electrochemical device harmonic spectrum. The article then focuses on an approximation method, which enables easy use of constant phase element in time domain. This method, which comes from fractional differentiation theory, is based on some properties of recursive transfer functions. The approximation procedure is explained, and comparisons between exact and approximated results are carried out.

Nanoparticle Self-Structuring in a Nanofluid Film Spreading on a Solid Surface

Liquids containing nanoparticles (nanofluids) exhibit different spreading or thinning behaviors on solids than liquids without nanoparticles. Previous experiments and theoretical investigations have demonstrated that the spreading of nanofluids on solid surfaces is enhanced compared to the spreading of base fluids without nanoparticles. However, the mechanisms for the observed enhancement in the spreading of nanofluids on solid substrates are not well understood. The complex nature of the interactions between the particles in the nanofluid and with the solid substrate alters the spreading dynamics [Wasan, D. T.; Nikolov, A. D. Nature 2003, 423, 156]. Here, we report, for the first time, the results of an experimental observation of nanoparticles self-structuring in a nanofluid film formed between an oil drop and a solid surface. Using a silica-nanoparticle aqueous suspension (with a nominal diameter of 19 nm and 10 vol %) and reflected light interferometry, we show the nanoparticle layering (i.e., stratification) phenomenon during film thinning on a smooth hydrophilic glass surface. Our experiments revealed that the film thickness stability on a solid substrate depends on the film size (i.e., the drop size). A film formed from a small drop (with a high capillary pressure) is thicker and contains more particle layers than a film formed from a large drop (with a lower capillary pressure). The data for the film-meniscus contact angle verses film thickness (corresponding to the different number of particle layers) were obtained and used to calculate the film structural energy isotherm. These results may provide a better understanding of the complex phenomena involved in the enhanced spreading of nanofluids on solid surfaces.

Bending Analysis of Thick Laminated Rectangular Plates Using a Boundary Layer Function

In this article, the governing bending equations of thick laminated transversely isotropic rectangular plates are derived based on third-order shear deformation theory (TSDT). Using a new function, called the boundary layer function, the three coupled governing equations are converted to two decoupled equations. These equations are in terms of the deflection of the plate and the mentioned boundary layer function, which are written in invariant form. By solving the decoupled equations, a Levy-type analytical solution is presented for bending of a transversely isotropic plate. Finally, numerical results are presented for boundary layer phenomenon and its effects in TSDT. It is shown that all of the boundary layer effects in Mindlin—Reissner theory appear in this theory. However, it is shown that the intensity of the boundary layer effects in TSDT exceeds that of the Mindlin—Reissner theory.

Modelling and simulation of the steady-state and dynamic behaviour of a PEM fuel cell

Abstract The performance of a fuel cell can be expressed by the voltage–load current (V–I) characteristics. In this study, two mathematical modelling for computing the steady-state and dynamic voltage–current (V–I) characteristics of PEM fuel cell stacks have been developed. For determining the humidity of the membrane in steady-state conditions, mathematical and theoretical equations are considered. This value is not an adjustable parameter. The goal of dynamic modelling is to find the response of the system against the load variations. In this research, in addition to the charge double layer phenomenon, the effects of temperature and gas flows are taken into account, then the fuel cell system is divided into three control volumes and thus a lumped-parameter model for these sub-systems is established using the mass and heat transfer equations. The proposed models are implemented in Matlab/Simulink environment. Additionally, these models were tested for the SR-12Modular PEM Generator, the Ballard Mark V FC, the BCS 500-W stack and various experimental data in open literature. They exhibit excellent agreement with other simulation and experimental results.