Porous Si layers—Preparation, characterization and morphology

Porous silicon layers (PSL) of nano- and micro-structures were prepared by metal-assisted electroless etching of silicon in HF-oxidizing agent aqueous solutions. The effect of oxidizing agent and HF content on the characteristics of the formed porous layers was investigated. A thin Pt film was electroless deposited on p-Si〈1 0 0〉 prior to immersion in the etching solution. The properties and morphology of the PSL formed by this method were investigated by electrochemical impedance spectroscopy (EIS) scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) technique. The characteristics of the PSL were found to be affected by the constituents of the etching medium and also, the etching time. Potassium bromate (KBrO₃), potassium iodate (KIO₃), and potassium dichromate (K₂Cr₂O₇) have been used as oxidizing agents. Pt-assisted etching of p-Si for 1 h in an etching solution consisting of 22.0 M HF and 0.05 M of KBrO₃, results in the formation of nano- and micro-pores on the Si surface. The use of 0.05 M KIO₃ or K₂Cr₂O₇ as oxidizing agent has led to the formation of a deposit on the silicon surface. At relatively higher concentration [>0.05 M] of K₂Cr₂O₇ the surface deposit becomes clear and was found to consist of an insoluble passive solid-phase of K₂SiF₆ which increases the film impedance and blocks the porous structure formation. The use of higher concentration [>22 M] of HF in the etching electrolyte is accompanied by an increase in the dissolution rate of the insoluble K₂SiF₆ layer and a decrease in the PSL passivity. The experimental impedance data were fitted to theoretical data according to a proposed equivalent circuit model which accounts for the mechanism of the porous film formation at the Si/electrolyte interface.     

Mixed convective heat transfer in rectangular enclosures driven by a continuously moving horizontal plate

The fluid flow and heat transfer induced by the combined effects of the mechanically driven lid and the buoyancy force within rectangular enclosures were investigated in this work. The fluid filled enclosures are heated and lid-driven either on the upper or on the lower horizontal wall, thermally isolated on the right vertical wall, and cooled on the other walls. The basis of the investigation was the numerical solutions of the equations for the conservation of mass, momentum, and energy transport using the finite difference method. The effects of the flow governing parameters including the Richardson and the Prandtl numbers, and the length-to-height aspect ratio, respectively, in the range 10⁻²  Ri  10², 10⁻³  Pr  10, and 1  AR  4 for a fixed Reynolds number, Re = 100, were studied. The results are presented in the form of the hydrodynamic and thermal fields, and the profiles for vertical and horizontal components of velocity, temperature, and the local heat flux. The fluid flow and energy distributions within the enclosures and heat flux on the heated wall are enhanced by the increase in the Richardson number. While an increase in the Prandtl number improves the heat flux on the heated wall, an increase in aspect ratio suppresses it. The results can be used as base line data in the design of systems in which mixed convection heat transfer in rectangular enclosures occurs.     

Effect of Zn and Pb contents on the electrochemical behavior of brass alloys in chloride-free neutral sulfate solutions

Different electrochemical methods such as open-circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical behavior of brass alloys with various Zn contents (5.5–38.0 mass%) and Cu–38.0Zn–Pb alloy with different Pb contents (1.0–3.4 mass%) in neutral sodium sulfate solutions. The influence of working conditions, e.g., immersion time, sulfate ions concentration and temperature on the electrochemical behavior of the different alloys was also studied. It was found that the initial corrosion rate is relatively high for alloys with the higher zinc content due to dezincification. The dezincification process initiates by selective dissolution of zinc and continues by a simultaneous dissolution of copper and zinc followed by re-deposition of copper. An increase in the lead content and immersion time in the sodium sulfate solution increases the corrosion resistance of the alloy and improves its stability. The stability of the leaded brass was considered to be due to the formation of an insoluble film of lead sulfate on its surface. The impedance data were fitted to theoretical data obtained according to an equivalent circuit model describing the electrode/electrolyte interface. The mechanism of the alloy dissolution was discussed in view of the obtained results.     

Phosphorylated oligosaccharides in lysosomal enzymes: identification of alpha-N-acetylglucosamine(1)phospho(6)mannose diester groups

In human fibroblasts, the recognition of lysosomal enzymes by cell surface receptors is mediated by mannose 6-phosphate residues located on oligosaccharides that can be cleaved by endo-beta-N-acetylglucosaminidase H. About half of these oligosaccharides, as isolated from beta-hexosaminidase and cathepsin D secreted by human skin fibroblasts, are anionic. Most of these are resistant to alkaline phosphatase. The resistance is due to alpha-N-acetylglucosamine residues linked to mannose 6-phosphate by a phosphodiester bond. The major phosphorylated oligosaccharides contain one and two and possibly three phosphate groups blocked by N-acetylglucosamine. Besides the blocked phosphate groups these oligosaccharides contain a common inner core consisting of Man alpha 1,6-(Man alpha 1,3)Man alpha 1,6(Man alpha 1,3)Man beta GlcNAc and either one or two alpha 1,2-linked mannose residues.     

Reverse running centrifugal pumps as hydraulic power recovery turbines for seawater reverse osmosis systems

Because of the ever increasing energy demand and costs, different recovery systems are becoming more and more useful and economical.On the Reverse Osmosis (RO) Systems about 70% of the energy for pumping is wasted at pressures of 800–1000 psi. This paper will discuss the application of reverse running centrifugal pumps as power recovery turbines to recover up to 80% of that energy. The economics of Hydraulic Power Recovery Turbine (HPRT) systems are explored with respect to the equipment cost, operating costs and payouts for the different sizes of R.O. Systems.HPRT's come in various sizes and types and configurations with efficiencies being a function of specific design, flow rate, pressure and speed. Typical design features of HPRT's are described for use in the R.O. System.Different schemes and equipment arrangements are available and require the use of steam turbines, motors, electric generators, clutches, and speed controls or a combination of the above for optimum results. Recommendation of the most economical equipment arrangement for the R.O. System is discussed.Head vs. capacity and BHP vs. capacity relationships are an essential criteria for selection and operation of HPRT's. An overview of the performance characteristics is illustrated. The conclusions will point to the use of reverse running pumps as hydraulic turbines for economical, reliable and efficient recovery of energy from the waste brine circuits of reverse osmosis system.     

Prospects,challenges, and latest developments in lithium–air batteries

Metal–air batteries are being envisioned as a clean and high energy fuel for the modern automotive industry. The lithium–air battery has been found most promising among the various practically applicable metal–air systems, that is, Al–air, Li–air, Mg–air, Fe–air, and Zn–air. The theoretical specific energy of the Li–air battery is ~12 kWh/kg, excluding the oxygen mass. This is comparable with the energy density of gasoline, which is ~13 kWh/kg. It has been hypothesized that the Li–air battery could supply an energy ~1.7 kWh/kg after losses from over potentials to run a vehicle ~300 miles on a single charge. During the first decade of this century, a fair amount of research has been conducted on Li–air battery system. Yet, Li–air batteries could not make an industrial breakthrough, and are still in the laboratory phase since their birth. In this article, we technically evaluated the recent developments, and the inferences have been analyzed from the practical/commercial point of view. The study concludes that low discharge rate, lower number of cycles, oxidation of lithium anode, discharge products at the cathode, and side reactions inside the battery are the key limiting factors in the slow progress of Li–air batteries on an industrial scale. The ongoing researches to overcome these hurdles have also been discussed. This analysis will help the reader to understand the current standing of the lithium–air battery technology. Copyright © 2014 John Wiley & Sons, Ltd.     

Effect of front air attack angles on heat transfer coefficient of the cross‐flow of four flat tube

Heat transfer coefficients are studied experimentally for airflow over a four‐flat‐tube configuration with different inlet attack angles. The range of Re numbers was from 1668 to 3782. The heat fluxes are 13.2, 38.5, and 99.8 W/m². In this paper, 45 samples of a flat tube with inlet airflow angles (30°, 45°, and 90°) are set up to investigate its thermal performance. The design of experimental approach was used to conduct the heat transfer parametric study by designing 45 experiments, analyzing the empirical model, and optimizing heat transfer. The response was modeled using a general factorial design model based on experimental data. From analysis of variance (ANOVA), we obtained significant coefficients by performing the 5% test of level of significance. The results indicated that both the Nu and Re numbers increase for all heat flux cases and these decrease with increasing the air attack. Also shown are that the Colburn j‐factor decreases with increase of air velocity. It was found that the ANOVA results are appropriate to the collected experimental data, as the R² and R²‐adjusted statistics are found to be 92.61% and 90.97%, respectively, for Nu number. In a similar manner, for Colburn j‐factor, the R² and R²‐adjusted statistic are 96.97% and 96.29%, respectively.     

Constructal design of multiscale elliptic tubes in crossflow

The optimal configuration of two‐scale elliptic tubes in crossflow is found on the basis of the constructal design. The larger tubes are installed inside a domain of fixed length and height. In the same domain, smaller tubes are inserted between the larger tubes in the entrance region at the mid leading edge to leading edge distance of the larger tubes. The spacing between the larger tubes, the semiminor axis of the larger tubes, the major axis of the smaller tubes, and the semiminor axis of the smaller tubes are varied inside the domain freely to find the optimal configuration. There are two optimal configurations: one without the smaller tubes and the other with the presence of the smaller tubes. Both the larger and the smaller tubes are heated at a constant surface temperature. The flow is induced by a fixed pressure difference. The equations for steady, laminar, two‐dimensional, and incompressible flow are solved by finite volume method. In the absence of the smaller tubes, the range of Bejan number (dimensionless pressure drop) is ${10}^3<\mathit{Be}<{10}^5$, and in the presence of the smaller tubes, Bejan number is Be = 10⁵. The range of the dimensionless larger tubes semiminor axis is 0.1 ≤ B ≤ 0.4. Air is used to cool the row of the tubes with Prandtl number equal to 0.7. The results show that for different semiminor axes of the larger tubes, the heat transfer rate is enhanced when the smaller tubes are placed between the larger tubes.     

Thermal evaporation and condensation synthesis of metallic Zn layered polyhedral microparticles

Metallic zinc layered polyhedral microparticles have been fabricated by thermal evaporation and condensation technique using zinc as precursor at 750 °C for 120 min and NH₃ as a carrier gas. The zinc polyhedral microparticles with oblate spherical shape are observed to be 2-9 μm in diameter along major axes and 1-7 μm in thickness along minor axes. The structural, compositional and morphological characterizations were performed by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). A vapour-solid (VS) mechanism based growth model has been proposed for the formation of Zn microparticles. Room temperature photoluminescence (PL) emission spectrum of the product exhibited a strong emission band at 369 nm attributed to the radiative recombination of electrons in the s, p conduction band near Fermi surface and the holes in the d bands generated by the optical excitation.