Transition of the flow in a symmetric channel with periodically expanded grooves

Transition of the flow in a periodically grooved channel is numerically investigated for periodicity indices m=1 up to 6 by assuming the two-dimensional and fully developed flow field, where m is defined as a number of grooves in which the flow repeats periodically. Critical Reynolds numbers for the onset of a self-sustained oscillatory flow from a steady-state flow are evaluated by numerical simulations. It is found that the bifurcations occur at the critical Reynolds numbers as a result of Hopf bifurcation, and a period in the streamwise direction of the oscillatory flow is twice as long as the groove pitch of the channel. In addition, flow visualization with the aluminum dust method is carried out to confirm the results obtained from the numerical simulations. The experimental results are in good agreement with the numerical ones.     

Preparation of porous carbons from thermoplastic precursors and their performance for electric double layer capacitors

Porous carbons with high surface area were successfully prepared from thermoplastic precursors, such as poly(vinyl alcohol) (PVA), hydroxyl propyl cellulose and poly(ethylene terephthalate), by the carbonization of a mixture with MgO at 900 °C in an inert atmosphere. After carbonization the MgO was dissolved out using a diluted sulfuric acid and the carbons formed were isolated. The mixing of the PVA carbon precursor with the MgO precursors (reagent grade MgO, magnesium acetate or citrate) was done either in powder form or in an aqueous solution. The BET surface area of the carbons obtained via solution mixing could reach a very high value, such as 2000 m²/g, without any activation process. The pore structure of the resultant carbons was found to depend strongly on the mixing method; the carbons prepared via solution mixing were rich in mesopores, but those produced via powder mixing were rich in micropores. The size of mesopores was found to be almost the same as that of the MgO particles, suggesting a way of controlling the mesopore size in the resultant carbons. Measurement of capacitance was carried out in 1 mol/L H₂SO₄ electrolyte. The porous carbon with a BET surface area of 1900 m²/g prepared at 900 °C through solution mixing of Mg acetate with PVA showed a fairly high EDLC capacitance, about 250 F/g with a current density of 20 mA/g and 210 F/g with 1000 mA/g. The rate performance was closely related to the mesoporous surface area.     

Carbon-13 NMR spectroscopic analysis of 24-methyl-Δ5,22-sterols in rape and mustard seed oils

24-Methylcholesta-5,E-22-dien-3β-ol (C₂₈ Δ^5,22-sterol) was separated from the unsaponifiable matters of the following eight seed oils of Brassica species:Brassica campestris (candle I and II and torch),B. napus (tower and midas),B. juncea (brown and oriental mustards), andB. alba (yellow mustard). The configuration at C-24 methyl group of the respective sterols was evaluated by¹³C NMR spectroscopy. All the C₂₈ Δ^5,22-sterols in the Brassica seed oils were found to contain the C-24 epimer of brassicasterol,trans-22-dehydrocampesterol, in the range of ca. 10–30%.     

Anodic oxidation effects on pyrolytic graphite surfaces in acid

Anodic oxidation effects on the basal and edge surfaces of pyrolytic graphite in acid were studied by laser Raman spectroscopy, and by a gas-phase chemical modification method coupled with X-ray photoelectron spectroscopy. Surface covering fractions were also studied by -epichlorohydrin monolayer formation. It was found that the species of surface chemical groups added by anodic oxidation was closely related to the structure of the oxidized surface. In the case of the basal surface, carboxyl groups were added by anodic oxidation, being always accompanied by the destruction of its surface structure. On the other hand, in the case of the edge surface, hydroxyl groups were added without the destruction of its structure with a mild treatment. With more severe treatment, carboxyl groups were added and this addition was also accompanied by destruction of the surface structure. There was a range of treatment in which hydroxyl groups could be added without destroying the structure. Finally it was confirmed that both carboxyl and hydroxyl groups could make covalent bonds with epoxy groups of -epichlorohydrin.     

Some properties of Sb2Te3−xSex for nonvolatile memory based on phase transition

Composition dependence of properties of Sb₂Te_3–x Se _x in the range 0x<3 were studied using differential thermal analysis and X-ray diffraction. Sb₂Te_3–x Se _x form solid solution for 0<x1.25 and 2.75x<3. A systematic study of crystallization temperature in Sb₂Te_3–x Se _x (0x2.75) thin films prepared by flash evaporation was carried out. In preliminary experiments for some compositions, more than 10³ repetitions between amorphous and crystalline states were attained by the application of electric pulses.     

Effect of vanadium and chromium on the microstructural features of V–Cr–Mn–Ni spheroidal carbide cast irons

The objective of this investigation is to study the influence of vanadium（5.0wt%–10.0wt%） and chromium（0–9.0wt%） on the microstructure and hardness of Cr-V-Mn-Ni white cast irons with spheroidal vanadium carbides. The alloys＇ microstructural features are presented and discussed with regard to the distribution of phase elements. The structural constituents of the alloys are spheroidal VC, proeutectoid cementite, ledeburite eutectic, rosette-shaped carbide eutectic（based on M7C3）, pearlite, martensite, and austenite. Their combinations and area fraction（AF） ratios are reported to be influenced by the alloys＇ chemical composition. Spheroidized VC particles are found to be sites for the nucleation of carbide eutectics. Cr and V are shown to substitute each other in the VC and M7C3 carbides, respectively. Chromium alloying leads to the formation of a eutectic（γ-Fe ＋ М7С3）, preventing the appearance of proeutectoid cementite in the structure. Vanadium and chromium are revealed to increase the total carbide fraction and the amount of austenite in the matrix. Cr is observed to play a key role in controlling the metallic matrix microstructure.     

Prediction model of surface residual stress within a machined surface by combining two orthogonal plane models

The variation of surface residual stress within a machined surface layer caused by face turning was studied. The size of the tool’s corner radius and the feed rate affect residual stress. A process model using the finite element method is proposed and the mechanical effects of the corner radius and feed rate on a machined surface were discussed. When a tool with a small corner radius is used, surface residual stress perpendicular to the cutting direction becomes compression stress. As well, surface residual stress changes from tension to compression as the feed rate decreases. The process model consists of an orthogonal cutting simulation and an indentation-like simulation of a corner radius into a work piece surface. The simulated results show quantitative agreement with the residual stress measured experimentally. The integrity of the machined surface will be controlled more efficiently if the cutting conditions during finishing are determined with the proper consideration of the surface generating process.     

Generation of copper nanoparticles during alkaline etching of an Al-30 at.%Cu alloy

A mechanism of formation of copper nanoparticles is proposed for alkaline etching of a sputtering-deposited Al-30 at.%Cu alloy, simulating the equilibrium θ phase of 2000 series aluminium alloys. Their formation involves enrichment of copper in the alloy beneath a thin alumina film, clustering of copper atoms, and occlusion of the clusters, due to growth of alumina around the clusters, to form nanoparticles. The proposed mechanism is supported by medium energy ion scattering, Rutherford backscattering spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy of the alloy following immersion in the sodium hydroxide solution, which disclose the enrichment of copper and the generation of the nanoparticles in the etching product of hydrated alumina. The generation of the nanoparticles is dependent upon the enrichment of copper in the alloy in a layer of a few nanometres thickness, with no requirement for bulk de-alloying of the alloy.     

Grain orientation effects on copper enrichment and oxygen generation during anodizing of an Al-1at.%Cu alloy

Anodizing of solid-solution Al-1at.%Cu alloy in ammonium pentaborate electrolyte is shown to develop two distinct types of amorphous film. On alloy grains of {1 0 0} orientation, the alumina film is of uniform thickness and relatively featureless. For other grains, the film is of non-uniform thickness and contains oxygen bubbles. In both cases, copper species are distributed throughout the film. Copper is enriched in the alloy to ∼5.8×10¹⁵ Cu atoms cm⁻² for bubble-free grains, with similar or slightly lower levels for other grains. Evidently, copper enrichment alone does not lead to generation of oxygen. Other factors suggested to be involved, each dependent upon grain orientation, are the structure of the enriched alloy layer, the cyclic nature of the oxidation of copper, and the generation of modulated film compositions.     

Initial stages of plasma electrolytic oxidation of titanium

The initial stages of oxide growth on titanium are examined in a recently developed commercial alkaline pyrophosphate/aluminate electrolyte of interest for plasma electrolytic oxidation of light metal alloys. Constant current anodizing was employed, with resultant films examined by scanning and transmission electron microscopies and Rutherford backscattering spectroscopy. The initial film is relatively uniform and composed of TiO₂, with low concentrations of aluminium and phosphorus species incorporated from the electrolyte. With increase in voltage the film breaks down locally, and regions of original and modified film develop simultaneously, with the latter occupying more of the surface as the voltage rises. Porous regions due to dielectric breakdown also become increasingly evident. At 240 V, sparking commences, and the surface reveals extensive, relatively uniform porosity, with the coating now containing much enhanced concentrations of aluminium and phosphorus species compared with the coating at lower voltages. The films develop at low efficiency due to generation of oxygen. The oxygen is produced within the original film material and at sites of dielectric breakdown. The former type of film develops a two-layered morphology, with an outer layer of amorphous TiO₂ and an inner layer with numerous fine and course cavities. The cavities are due to the generation of oxygen that may be associated with the formation of anatase in the inner layer.