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Search Results: 1 - 10 of 200622 matches for " P. Skeldon "
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Porous anodic film formation on an Al-3.5 wt % Cu alloy
Páez, M. A.,Skeldon, P.,Thompson, G. E.,Sáez, M.
Revista de Metalurgia , 2003,
Abstract: The morphological development of porous anodic films in the initial stages is examined during anodizing an Al-3.5 wt % Cu alloy in phosphoric acid. Using transmission electron microscopy a sequence of ultramicrotomed anodic sections reveals the dynamic evolution of numerous features in the thickening film in the initial stages of anodizing. The morphological changes in the anodic oxide in the initial stages of its formation appears related to the formation of bubbles during film growth. From Rutherford backscattering spectroscopy (RBS) analysis of the film, the formation of the bubbles is associated with the enrichment of copper in the alloy due to growth of the anodic oxide. On the other hand, during constant current anodizing of Al-Cu in phosphoric acid, the current efficiency is considerably less than that for anodizing superpure aluminium under similar conditions. From the contrasting results between the charge consumed calculated from RBS and the real charge consumed during anodizing, oxygen gas bubbles generation and copper oxidation seem to be of less importance on the low efficiency for film formation. It is apparent that the main cause of losing efficiency for film growth on Al-Cu is associated with generation of oxygen at residual second phase, with the development of stresses in the film and, the consequence of these effects on film cracking during film growth. En este trabajo se examinó el desarrollo morfológico de películas anódicas porosas en los estados iniciales de la anodización de una aleación de aluminio Al-3,5 % p/p Cu. La observación de una secuencia de secciones ultramicrotomadas del metal y su película anódica, por microscopía electrónica de transmisión, revela la evolución dinámica de numerosos detalles morfológicos durante los inicios del crecimiento de la película anódica. Los cambios morfológicos en el óxido anódico, en los inicios de su formación, aparecen relacionados a la formación de burbujas durante el crecimiento de éste. A través de análisis por espectroscopia de reflexión Rutherford (RBS), la formación de burbujas, posiblemente de oxígeno, se asocia con el enriquecimiento de cobre en la aleación debido al crecimiento de la película anódica. Por otra parte, los cálculos de eficiencia anódica para la anodización de la aleación de aluminio a corriente constante, indican que ésta es bastante más baja que aquella determinada para la anodización de aluminio de alta pureza bajo condiciones similares. Sin embargo, la baja eficiencia anódica para la anodización de la aleación contrasta con los datos de carga consumida calculada
Semiconductive properties of anodic niobium oxides
,A.I. de; Rangel,C.M.; Skeldon,P.; Thompson,G.E.;
Portugaliae Electrochimica Acta , 2006,
Abstract: the semiconductive properties of anodic niobium oxides formed at constant potential and constant current density to different final voltages have been examined by mott-schottky analysis. thin anodic oxides were formed on sputtered niobium specimens at constant potential in the range of 2.5 to 10 vag/agcl in a borate buffer solution. thicker oxides were formed, also on sputtered niobium specimens, at a constant current density of 5 ma cm-2 in 0.1 m ammonium pentaborate solution to final voltages of 10, 50 and 100 v. capacitance measurements were performed in a borate buffer solution of ph 8.8, at a frequency range of 200 to 2000 hz, at a sweep rate of 5 mv s-1 from +2.5 to -1 vag/agcl. the results obtained show n-type semiconductor behaviour with a carrier density in the range of 8 ′ 1018 - 6 ′ 1019 cm-3 on films formed to 10 v. thicker films showed lower carrier densities in the range of 1 ′ 1018 - 2 ′ 1018 cm-3 with a calculated charge depletion layer of 33-36 nm.
Semiconductive properties of anodic niobium oxides
A.I. de Sá,C.M. Rangel,P. Skeldon,G.E. Thompson
Portugaliae Electrochimica Acta , 2006,
Abstract: The semiconductive properties of anodic niobium oxides formed at constant potential and constant current density to different final voltages have been examined by Mott-Schottky analysis. Thin anodic oxides were formed on sputtered niobium specimens at constant potential in the range of 2.5 to 10 V Ag/AgCl in a borate buffer solution. Thicker oxides were formed, also on sputtered niobium specimens, at a constant current density of 5 mA cm-2 in 0.1 M ammonium pentaborate solution to final voltages of 10, 50 and 100 V. Capacitance measurements were performed in a borate buffer solution of pH 8.8, at a frequency range of 200 to 2000 Hz, at a sweep rate of 5 mV s-1 from +2.5 to -1 V Ag/AgCl. The results obtained show n-type semiconductor behaviour with a carrier density in the range of 8 ′ 10(18) - 6 ′ 10(19) cm-3 on films formed to 10 V. Thicker films showed lower carrier densities in the range of 1 ′ 10(18) - 2 ′ 10(18) cm-3 with a calculated charge depletion layer of 33-36 nm.
The Inflammasomes: Molecular Effectors of Host Resistance Against Bacterial, Viral, Parasitic, and Fungal Infections
Alexander Skeldon,Maya Saleh
Frontiers in Microbiology , 2011, DOI: 10.3389/fmicb.2011.00015
Abstract: The inflammasomes are large multi-protein complexes scaffolded by cytosolic pattern recognition receptors (PRRs) that form an important part of the innate immune system. They are activated following the recognition of microbial-associated molecular patterns or host-derived danger signals (danger-associated molecular patterns) by PRRs. This recognition results in the recruitment and activation of the pro-inflammatory protease caspase-1, which cleaves its preferred substrates pro-interleukin-1β (IL-1β) and pro-IL-18 into their mature biologically active cytokine forms. Through processing of a number of other cellular substrates, caspase-1 is also required for the release of “alarmins” and the induction and execution of an inflammatory form of cell death termed pyroptosis. A growing spectrum of inflammasomes have been identified in the host defense against a variety of pathogens. Reciprocally, pathogens have evolved effector strategies to antagonize the inflammasome pathway. In this review we discuss recent developments in the understanding of inflammasome-mediated recognition of bacterial, viral, parasitic, and fungal infections and the beneficial or detrimental effects of inflammasome signaling in host resistance.
Parametrically Excited Surface Waves: Two-Frequency Forcing, Normal Form Symmetries, and Pattern Selection
Mary Silber,Anne C. Skeldon
Physics , 1998, DOI: 10.1103/PhysRevE.59.5446
Abstract: Motivated by experimental observations of exotic standing wave patterns in the two-frequency Faraday experiment, we investigate the role of normal form symmetries in the pattern selection problem. With forcing frequency components in ratio m/n, where m and n are co-prime integers, there is the possibility that both harmonic and subharmonic waves may lose stability simultaneously, each with a different wavenumber. We focus on this situation and compare the case where the harmonic waves have a longer wavelength than the subharmonic waves with the case where the harmonic waves have a shorter wavelength. We show that in the former case a normal form transformation can be used to remove all quadratic terms from the amplitude equations governing the relevant resonant triad interactions. Thus the role of resonant triads in the pattern selection problem is greatly diminished in this situation. We verify our general results within the example of one-dimensional surface wave solutions of the Zhang-Vinals model of the two-frequency Faraday problem. In one-dimension, a 1:2 spatial resonance takes the place of a resonant triad in our investigation. We find that when the bifurcating modes are in this spatial resonance, it dramatically effects the bifurcation to subharmonic waves in the case of forcing frequencies are in ratio 1/2; this is consistent with the results of Zhang and Vinals. In sharp contrast, we find that when the forcing frequencies are in ratio 2/3, the bifurcation to (sub)harmonic waves is insensitive to the presence of another spatially-resonant bifurcating mode.
New stability results for long-wavelength convection patterns
Anne C. Skeldon,Mary Silber
Physics , 1997,
Abstract: We consider the transition from a spatially uniform state to a steady, spatially-periodic pattern in a partial differential equation describing long-wavelength convection. This both extends existing work on the study of rolls, squares and hexagons and demonstrates how recent generic results for the stability of spatially-periodic patterns may be applied in practice. We find that squares, even if stable to roll perturbations, are often unstable when a wider class of perturbations is considered. We also find scenarios where transitions from hexagons to rectangles can occur. In some cases we find that, near onset, more exotic spatially-periodic planforms are preferred over the usual rolls, squares and hexagons.
Stability Results for Steady, Spatially--Periodic Planforms
B. Dionne,M. Silber,A. C. Skeldon
Physics , 1995,
Abstract: We consider the symmetry-breaking steady state bifurcation of a spatially-uniform equilibrium solution of E(2)-equivariant PDEs. We restrict the space of solutions to those that are doubly-periodic with respect to a square or hexagonal lattice, and consider the bifurcation problem restricted to a finite-dimensional center manifold. For the square lattice we assume that the kernel of the linear operator, at the bifurcation point, consists of 4 complex Fourier modes, with wave vectors K_1=(a,b), K_2=(-b,a), K_3=(b,a), and K_4=(-a,b), where a>b>0 are integers. For the hexagonal lattice, we assume that the kernel of the linear operator consists of 6 complex Fourier modes, also parameterized by an integer pair (a,b). We derive normal forms for the bifurcation problems, which we use to compute the linear, orbital stability of those solution branches guaranteed to exist by the equivariant branching lemma. These solutions consist of rolls, squares, hexagons, a countable set of rhombs, and a countable set of planforms that are superpositions of all of the Fourier modes in the kernel. Since rolls and squares (hexagons) are common to all of the bifurcation problems posed on square (hexagonal) lattices, this framework can be used to determine their stability relative to a countable set of perturbations by varying a and b. For the hexagonal lattice, we analyze the degenerate bifurcation problem obtained by setting the coefficient of the quadratic term to zero. The unfolding of the degenerate bifurcation problem reveals a new class of secondary bifurcations on the hexagons and rhombs solution branches.
Can weakly nonlinear theory explain Faraday wave patterns near onset?
A. C. Skeldon,A. M. Rucklidge
Physics , 2015,
Abstract: The Faraday problem is an important pattern-forming system that provides some middle ground between systems where the initial instability involves just a single mode and in which complexity then results from mode interactions or secondary bifurcations, and cases where a system is highly turbulent and many spatial and temporal modes are excited. It has been a rich source of novel patterns and of theoretical work aimed at understanding how and why such patterns occur. Yet it is particularly challenging to tie theory to experiment: the experiments are difficult to perform; the parameter regime of interest (large box, moderate viscosity) along with the technical difficulties of solving the free boundary Navier--Stokes equations make numerical solution of the problem hard; and the fact that the instabilities result in an entire circle of unstable wavevectors presents considerable theoretical difficulties. In principle, weakly nonlinear theory should be able to predict which patterns are stable near pattern onset. In this paper we present the first quantitative comparison between weakly nonlinear theory of the full Navier-Stokes equations and (previously published) experimental results for the Faraday problem with multiple frequency forcing. We confirm that three-wave interactions sit at the heart of why complex patterns are stablised but also highlight some discrepancies between theory and experiment. These suggest the need for further experimental and theoretical work to fully investigate the issues of pattern bistability and the role of bicritical/tricritical points in determining bifurcation structure.
Three-wave interactions and spatio-temporal chaos
Alastair M. Rucklidge,Mary Silber,Anne C. Skeldon
Physics , 2011, DOI: 10.1103/PhysRevLett.108.074504
Abstract: Three-wave interactions form the basis of our understanding of many pattern forming systems because they encapsulate the most basic nonlinear interactions. In problems with two comparable length scales, it is possible for two waves of the shorter wavelength to interact with one wave of the longer, as well as for two waves of the longer wavelength to interact with one wave of the shorter. Consideration of both types of three-wave interactions can generically explain the presence of complex patterns and spatio-temporal chaos. Two length scales arise naturally in the Faraday wave experiment with multi-frequency forcing, and our results enable some previously unexplained experimental observations of spatio-temporal chaos to be interpreted in a new light. Our predictions are illustrated with numerical simulations of a model partial differential equation.
Two-frequency forced Faraday waves: Weakly damped modes and pattern selection
Mary Silber,Chad M. Topaz,Anne C. Skeldon
Physics , 2000, DOI: 10.1016/S0167-2789(00)00102-0
Abstract: Recent experiments (Kudrolli, Pier and Gollub, 1998) on two-frequency parametrically excited surface waves exhibit an intriguing "superlattice" wave pattern near a codimension-two bifurcation point where both subharmonic and harmonic waves onset simultaneously, but with different spatial wavenumbers. The superlattice pattern is synchronous with the forcing, spatially periodic on a large hexagonal lattice, and exhibits small-scale triangular structure. Similar patterns have been shown to exist as primary solution branches of a generic 12-dimensional $D_6\dot{+}T^2$-equivariant bifurcation problem, and may be stable if the nonlinear coefficients of the bifurcation problem satisfy certain inequalities (Silber and Proctor, 1998). Here we use the spatial and temporal symmetries of the problem to argue that weakly damped harmonic waves may be critical to understanding the stabilization of this pattern in the Faraday system. We illustrate this mechanism by considering the equations developed by Zhang and Vinals (1997, J. Fluid Mech. 336) for small amplitude, weakly damped surface waves on a semi-infinite fluid layer. We compute the relevant nonlinear coefficients in the bifurcation equations describing the onset of patterns for excitation frequency ratios of 2/3 and 6/7. For the 2/3 case, we show that there is a fundamental difference in the pattern selection problems for subharmonic and harmonic instabilities near the codimension-two point. Also, we find that the 6/7 case is significantly different from the 2/3 case due to the presence of additional weakly damped harmonic modes. These additional harmonic modes can result in a stabilization of the superpatterns.
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