Non-uniform Layer Research Articles

Characteristics of surface layers formed on inconel 718 by laser shock peening with and without a protective coating

Coupons of a Ni base super alloy, Inconel alloy 718 (IN718) were laser shock peened with and without an ablative layer and resulting microstructure and residual stress state were studied. Laser shock peening without a protective overlay results in both mechanical and thermal loading of the material, leading to melting and re-solidification on the surface along with deformation due to laser induced shock wave. The surface shows presence of a non-uniform recast layer which increases the roughness of the surface and also results in a tensile state of residual stresses on surface. The recast layer has areas with modified chemistry and shows presence of nano particles deposited on top of the matrix. In this study, recast layer formed in Inconel alloy 718 (IN718) as a result of laser shock peening without protective overlay was characterized and compared with surface condition of a sample peened with a protective overlay. These results are presented and discussed in relation with those of previous studies.

Local frost behaviors of a scaled-up louvered fin heat exchanger

We investigated the local frost growth in a louvered fin heat exchanger using a scaled-up model and analyzed the characteristics of frost behaviors and variation in the heat transfer rate according to frost formation compared to a flat-fin heat exchanger. In contrast to a uniform frost layer of the flat-fin heat exchanger, highly non-uniform frost layer between the front and rear sides was observed on the louvered fin heat exchanger. The heat transfer rate of the louvered fin heat exchanger more rapidly decreased with the frost formation than that of the flat-fin heat exchanger. The blocking of the spaces between the louvers on the front side by frost mainly induced the significant decrease of the heat transfer rate of the louvered fin heat exchanger. On the louvered fin front side, the spaces between the louvers near the redirection region were first blocked by frost formation before those near the air inlet region due to the higher mass transfer driving force around the redirection region. Once the louvered fin front side was totally blocked by frost, the louvered fin rear side did not effectively work anymore.

Electroosmotic flow in the vicinity of a conducting obstacle mounted on the surface of a wide microchannel

A numerical study is made on the electroosmotic flow (EOF) near a polarizable metallic obstacle mounted on one of the non-conducting walls of a microchannel. The external electric field induces a Debye layer of non-uniform ζ-potential along the obstacle, which results in a non-linear electroosmotic flow. The combined effect of surface roughness and non-uniform electric double layer on the polarizable obstacle creates a vortical flow. The form of this vortical flow and its dependence on the bulk ionic concentration is analyzed. Our numerical model is based on computation of the Navier–Stokes, Nernst–Planck and Poisson equations. We have computed the governing non-linear coupled set of equations by the control volume method over a staggered grid system. Our results show that the form of the vortical flow, which develops in the vicinity of the obstacle, depends on the thickness of the Debye layer along the homogeneous part of the channel. The free-slip model, based on zero charge density in the bulk region is found to over-predict the result based on the present model. However, the present model approaches the free-slip model when the bulk ionic concentration is considered to be sufficiently strong.

Interaction of a finite-span synthetic jet near the tip of a sweptback wing

An experimental investigation was performed to study the three-dimensional flow interaction of a finite-span (aspect ratio of 18) synthetic jet located near the tip of a sweptback wing (cross-sectional profile of NACA 4421, aspect ratio of 4, and sweep angle of 30°) at a Reynolds number of 105 and at three angles of attack of 0°, 9°, and 15.5° (covering the range of attached to separated flow in the vicinity of the synthetic jet). Three blowing ratios were considered as 0.8, 1.2, and 2. Stereoscopic particle image velocimetry data were collected at multiple 2-D planes in the vicinity of the jet’s orifice, which were then used to reconstruct the flow volume, and the effect of the jet’s blowing ratio was analyzed using time-averaged and phase-averaged statistics. The study showed that the flow field in the vicinity of the synthetic jet orifice becomes highly three-dimensional and is governed by the streamwise structures that are associated with the finite span of the orifice (edge vortices). Furthermore, it was demonstrated that the baseline flow field that develops over a swept-back configuration (characterized by spanwise and streamwise vorticity components) is responsible for the immediate breakdown of the coherent structures that are introduced by the synthetic jet orifice and for the formation of the secondary flow structures that were seen in the time-averaged flow field. Moreover, the presence of a tip vortex results in the development of a non-uniform (in the spanwise direction) spanwise boundary layer that becomes more pronounced with increasing angle of attack. Consequently, the development of the flow structures is altered. Finally, the present work suggests that the location of the synthetic jet along the span is not as important (as the angle of attack and the blowing ratio) in the overall formation and evolution of the flow structures issued from the jet. However, the size and strength of these structures are affected by the jet’s spanwise location.

Multilayer coextrusion of rubber compounds

This research work presents the first continuous multilayer coextrusion system for high viscosity elastomeric materials. Three unvulcanized rubber materials were chosen to validate the system: two butyl rubbers and a polyisoprene. The elastomers were characterized under oscillatory shear and the results used to perform computational flow simulations to investigate the effect of geometry on the flow path and flow rate. Successful extrudates of 8 and 32 layers were extruded at two different throughput rates for rheologically matched and rheologically mismatched material pairs. The results show good layering performance for both systems with little existence of viscous encapsulation and acceptable pressure drops. Computational and experimental results both show a nonuniform layer thickness distribution due to the geometric design of the layer multiplier extrusion dies, which were designed to minimize pressure drop in the system. This nonuniformity decreases with increasing number of layers. POLYM. ENG. SCI., 55:1520–1527, 2015. © 2015 Society of Plastics Engineers

New X-ray insight into oxygen intercalation in epitaxial graphene grown on 4H-SiC(0001)

Efficient control of intercalation of epitaxial graphene by specific elements is a way to change properties of the graphene. Results of several experimental techniques, such as X-ray photoelectron spectroscopy, micro-Raman mapping, reflectivity, attenuated total reflection, X-ray diffraction, and X-ray reflectometry, gave a new insight into the intercalation of oxygen in the epitaxial graphene grown on 4H-SiC(0001). These results confirmed that oxygen intercalation decouples the graphene buffer layer from the 4H-SiC surface and converts it into the graphene layer. However, in contrast to the hydrogen intercalation, oxygen does not intercalate between carbon planes (in the case of few layer graphene) and the interlayer spacing stays constant at the level of 3.35–3.32 Å. Moreover, X-ray reflectometry showed the presence of an oxide layer having the thickness of about 0.8 Å underneath the graphene layers. Apart from the formation of the nonuniform thin oxide layer, generation of defects in graphene caused by oxygen was also evidenced. Last but not least, water islands underneath defected graphene regions in both intercalated and non-intercalated samples were most probably revealed. These water islands are formed in the case of all the samples stored under ambient laboratory conditions. Water islands can be removed from underneath the few layer graphene stacks by relevant thermal treatment or by UV illumination.

Miniature fuel cell with monolithically fabricated Si electrodes – Uniformity of Catalyst Layer Thickness -

Uniformity of catalyst layer thickness was improved to obtained higher output with our miniature fuel cells. Though the miniature fuel cells demonstrated high power density about 500mW/cm2 with 1 mm2 reaction area, it was difficult to maintain the power density with larger reaction area and careful observation of the prototypes revealed that the porous Si layer, which should be etched away completely, remained on the channel bottoms. Nonuniform porous Pt catalyst layer thickness was suspicious for the residual porous Si, and rotation of the plating vessel in the Pt deposition process was performed to mitigate variation of the catalyst layer thickness. Though the power density was still low with larger reaction area, the larger output was successfully obtained with the vessel rotation.

Directional flank spreading at Mount Cameroon volcano: Evidence from analogue modeling

AbstractMount Cameroon is characterized by an elongated summit plateau, steep flanks, and topographic terraces around its base. Although some of these features can be accounted for by intrusion‐induced deformation, we here focus on the contribution of edifice‐scale gravitational spreading in the structure of Mount Cameroon. We review the existing geological and geophysical data and morphostructural features of Mount Cameroon and surrounding sedimentary basins. Volcanic ridge gravitational spreading is then simulated by scaled analogue models on which fault formation is recorded using digital image correlation. Three sets of models are presented (i) models recorded in cross section (Type I), (ii) models recorded from above with a uniform (Type IIa), and (iii) nonuniform ductile layer (Type IIb). Type I models illustrate the formation of faults accommodating summit subsidence and lower flank spreading. Type IIa models favor displacement perpendicular to the long axis, with formation of a summit graben and basal folds, but fail to reproduce the steep flanks. Type IIb models investigate the effect of spatial variations in sediment thickness and/or properties consistent with geological evidence. Directional spreading of the volcano's central part perpendicular to the long axis is accounted for by a sediment layer with restricted lateral extent and increasing thickness away from the volcano axis. The later model closely reproduces key features observed at Mount Cameroon: steep upper flanks are accounted for by enhanced lateral spreading of the lower flanks relative to the summit. The relevance of these findings for understanding flank instabilities at large oceanic volcanoes is finally highlighted.

The Effect of Battery Potential and Charge Rate on the Decomposition Reaction on the Anode Electrode of Lithium Ion Polymer Battery

The breakdown of the protective solid electrolyte interface (SEI) layer on the anode electrode can initiate further electrolyte decomposition reaction and result in the formation of a non-uniform layer of electronically insulating reaction products on the graphite particles. The results from this study indicated that raising the state of charge (SOC) increased the rate of the electrolyte decomposition side reaction to form a thick deposit layer. The deposit surface layer formed at the electrode/electrolyte interface increased the interfacial charge transfer resistance and rise in the overall batteries impedance. A direct correlation between the capacity fade during cycling and the deposit layer growth was established. Analysis of the anode and the deposit layer showed an increasing amount of lithium in the deposit layer as well as “trapped” lithium in the graphite crystal structure as the batteries were aged at higher SOC.

Preparation and characterization of TiO2 barrier layers for dye-sensitized solar cells.

A TiO2 barrier layer is critical in enhancing the performance of dye-sensitized solar cells (DSSCs). Two methods to prepare the TiO2 barrier layer on fluorine-doped tin dioxide (FTO) surface were systematically studied in order to minimize electron-hole recombination and electron backflow during photovoltaic processes of DSSCs. The film structure and materials properties were correlated with the photovoltaic characteristics and electrochemical properties. In the first approach, a porous TiO2 layer was deposited by wet chemical treatment of the sample with TiCl4 solution for time periods varying from 0 to 60 min. The N719 dye molecules were found to be able to insert into the porous barrier layers. The 20 min treatment formed a nonuniform but intact TiO2 layer of ∼100-300 nm in thickness, which gave the highest open-circuit voltage VOC, short-circuit photocurrent density JSC, and energy conversion efficiency. But thicker TiO2 barrier layers by this method caused a decrease in JSC, possibly limited by lower electrical conductance. In the second approach, a compact TiO2 barrier layer was created by sputter-coating 0-15 nm Ti metal films on FTO/glass and then oxidizing them into TiO2 with thermal treatment at 500 °C in the air for 30 min. The dye molecules were found to only attach at the outer surface of the barrier layer and slightly increased with the layer thickness. These two kinds of barrier layer showed different characteristics and may be tailored for different DSSC studies.

Depolarization correction method for ellipsometric measurements of large grain size zinc-oxide films

A method allowing for the separation of scattering-caused depolarization from depolarization due to film thickness inhomogeneity on isotropic samples is presented. Mueller-matrix analysis shows that M22 matrix-element is correlated with the scattering-caused depolarization, and correction of the measured depolarization spectra with M22 element excludes this type of depolarization from the spectra. Ellipsometric measurements performed on polycrystalline ZnO samples with smooth and scattering surface, with uniform and non-uniform layer thickness in every possible combination confirm the suggested method. Besides depolarization, ellipsometric analysis and optical properties of the ZnO films are discussed.

On the influence of the magnetic field on the eigenmodes of thin laminated cylindrical shells containing magnetorheological elastomer

Laminated cylindrical sandwich shells composed by embedding magnetorheological elastomers (MREs) between elastic layers is the subject of this investigation. Physical properties of the magnetorheological (MR) layers are assumed to be functions of the magnetic field induction and curvilinear coordinates. A system of differential equations with complex variable coefficients depending upon the magnetic field and based on both the assumptions of the generalized kinematic hypothesis for the whole sandwich and experimental data for MREs is used as the governing one. To analyze damping capabilities of adaptive materials, free vibrations of a three-layered circular cylinder containing MRE core layer are studied at different levels of the magnetic field. Using the asymptotic approach, eigenmodes of free vibrations of a laminated cylindrical shell with variable physical characteristics of MRE are constructed in the form of functions decaying far from the weakest plot on the shell structure. It has been shown that applying constant magnetic field may result in localization of eigenmodes corresponding to low-frequency spectrum of three-layered circular thin cylinder with embedded nonuniform MRE layer. Dependencies of natural frequencies, damping decrement and parameter characterizing the power of the eigenmode localization on the intensity of applied magnetic field are analyzed.

Non-uniform time-to-corrosion initiation in steel reinforced concrete under chloride environment

Chloride-induced corrosion in reinforced concrete (RC) leads to non-uniform distribution of corrosion products around the rebar perimeter. The present study focuses on corrosion caused by sea-salt particles floating in the air, by considering diffusion as the dominant mode of chloride intrusion into concrete. Detailed parametric study is carried out involving critical chloride content, structural configuration, distance from the coastline, concrete cover thickness and rebar diameter, to understand their effects on chloride ingress into concrete. Qualitative comparison of damage induced on concrete cover as a result of non-uniform corrosion layer formation around rebars is presented through time-to-corrosion initiation (TCI) profiles.

Negative effect of Au nanoparticles on an IGZO TFT-based nonvolatile memory device

In this letter, the electrical characteristics of nonvolatile memory devices based on back gatetype indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) are investigated in terms of the Au nanoparticles (NPs) employed in the floating gate-stack of the device. The size of the Au NPs is controlled using a by 500 °C annealing process after the Au thin-film deposition. The size and the roughness of the Au NPs were observed by using scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. In order to analyze the electrical properties according to Au NP size, we measured the current-voltage (I D -V G ) characteristics of the nonvolatile memory devices fabricated without Au NPs and with Au NPs of various sizes. The size of the Au NP increased, so did the surface roughness of the gate. This resulted in increased carrier scattering, which subsequently degraded the on-current of the memory device. In addition, inter-diffusion between the Au and the α-IGZO through the non-uniform Al2O3 tunneling layer seemed to further degrade the device performance.

THE BEHAVIOR OF TRANSVERSE WAVES IN NONUNIFORM SOLAR FLUX TUBES. II. IMPLICATIONS FOR CORONAL LOOP SEISMOLOGY

Seismology of coronal loops using observations of damped transverse oscillations in combination with results from theoretical models is a tool to indirectly infer physical parameters in the solar atmospheric plasma. Existing seismology schemes based on approximations to the period and damping time of kink oscillations are often used beyond their theoretical range of applicability. These approximations assume that the variation of density across the loop is confined to a nonuniform layer much thinner than the radius of the loop, but the results of the inversion problem often do not satisfy this preliminary hypothesis. Here, we determine the accuracy of the analytic approximations to the period and damping time, and its impact on seismology estimates, when largely nonuniform loops are considered. We find that the accuracy of the approximations when used beyond their range of applicability is strongly affected by the form of the density profile across the loop, that is observationally unknown and so must be arbitrarily imposed as part of the theoretical model. The error associated with the analytic approximations can be larger than 50% even for relatively thin nonuniform layers. This error directly affects the accuracy of approximate seismology estimates compared to actual numerical inversions. In addition, assuming different density profiles can produce noncoincident intervals of the seismic variables in inversions of the same event. The ignorance about the true shape of density variation across the loop is an important source of error that may dispute the reliability of parameters seismically inferred assuming an ad hoc density profile.

Non-uniform Fracture in Three-stage Forming Process of Sheared Protrusion for Current Collector of Molten Carbonate Fuel Cells

The development of the non-uniform fracture layer and the effect of the non-uniform fracture on the forming process of the sheared protrusion were investigated through experiments and the finite element analysis. It was found that the non-uniform fracture occurred in the contact region between the slope of the slitting punch and the sheet due to the change of the slitting direction. In addition, the effect of the non-uniform fracture on the forming process of the sheared protrusion was investigated. Experiments involving microscopic and macroscopic observations were conducted to verify the finite element analysis of the three-stage forming process of the sheared protrusions.

Fiber-Optical Spectroscopy for Monitoring the Electro-Oxidation of Metals

A method of investigating phase-nonuniform metal-oxide surface layers using a fiber-optic spectrometer is presented. The reflection coefficients of the metal at different wavelengths are processed by algorithms of spectrotomography, modified to reflectometry, which enables the thickness and chemical composition of the nonuniform surface layers to be monitored. An example of a calculation of the parameters of oxide films on the surface of Fe–18Cr alloy, processed in a furnace and by electrical-contact heating, is given.

Ultrasonic Spot Welding of Aluminum to High-Strength Low-Alloy Steel: Microstructure, Tensile and Fatigue Properties

The structural applications of lightweight aluminum alloys inevitably involve dissimilar welding with steels and the related durability issues. This study was aimed at evaluating the microstructural change, lap shear tensile load, and fatigue resistance of dissimilar ultrasonic spot-welded joints of aluminum-to-galvanized high-strength low-alloy (HSLA) steel. Two non-uniform layers were identified in between Al and HSLA steel via SEM/EDS and XRD. One was an Al-Zn eutectic layer and the other was a thin (<2 μm) layer of intermetallic compound (IMC) of Al and Fe in the nugget zone. The lap shear tensile testing gave a maximum load of 3.7 kN and the sample failed initially in between the Al-Zn eutectic film and Al-Fe IMC, and afterward from the region containing Al on both matching fracture surfaces. The fatigue test results showed a fatigue limit of about 0.5 kN (at 1 × 107 cycles). The maximum cyclic stress at which transition of the fatigue fracture from transverse through-thickness crack growth mode to the interfacial failure mode occurs increases with increasing energy input.

The Galvanic Corrosion between Anodized 6061 Aluminum Plate and C1100 Copper Plate Couple

The galvanic corrosion behavior of anodic AA6061 Al and C1100 Cu couples is investigated. Anodized AA6061-T6 Al plates were produced at different thickness of 1μm, 5μm and 12μm, respectively. The galvanic corrosion current of anodized plate and C1100 copper plate couples was measured using a zero resistance ammeter for 8 hours in two solutions, 3.5wt.% NaCl solution and 1.0 wt.% NaClO solution under temperatures of 25°C, 40°C and 60°C. The results show that the AA6061-T6 aluminum plate without anodizing produced the highest galvanic corrosion current, 1200 μA/cm2, among the tested specimens under flowing NaClO electrolyte at 60°C. Anodized AA6061-T6 aluminum plates with a 12μm anodized layer produced the lowest galvanic corrosion current, 15 μA/cm2, in a static 3.5wt.% NaCl electrolyte at 25°C. Severe corrosion attack was observed at the aluminum specimen with thin and non-uniform anodized layer after the galvanic corrosion tests. The anodizing AA6061 Al plate process can effectively reduce the galvanic corrosion of the AA6061 Al and C1100 Cu couples.

THE BEHAVIOR OF TRANSVERSE WAVES IN NONUNIFORM SOLAR FLUX TUBES. I. COMPARISON OF IDEAL AND RESISTIVE RESULTS

Magnetohydrodynamic (MHD) waves are ubiquitously observed in the solar atmosphere. Kink waves are a type of transverse MHD waves in magnetic flux tubes that are damped due to resonant absorption. The theoretical study of kink MHD waves in solar flux tubes is usually based on the simplification that the transverse variation of density is confined to a nonuniform layer much thinner than the radius of the tube, i.e., the so-called thin boundary approximation. Here, we develop a general analytic method to compute the dispersion relation and the eigenfunctions of ideal MHD waves in pressureless flux tubes with transversely nonuniform layers of arbitrary thickness. Results for kink waves are produced and are compared with fully numerical resistive MHD eigenvalue computations in the limit of small resistivity. We find that the frequency and resonant damping rate are the same in both ideal and resistive cases. The actual results for thick nonuniform layers deviate from the behavior predicted in the thin boundary approximation and strongly depend on the shape of the nonuniform layer. The eigenfunctions in ideal MHD are very different from those in resistive MHD. The ideal eigenfunctions display a global character regardless of the thickness of the nonuniform layer, while the resistive eigenfunctions are localized around the resonance and are indistinguishable from those of ordinary resistive Alfv\'en modes. Consequently, the spatial distribution of wave energy in the ideal and resistive cases is dramatically different. This poses a fundamental theoretical problem with clear observational consequences.