Effect of Alternating Pumping of Two Reactants into a Microchannel on a Phase Transfer Reaction

Alternating pumping of two reactants into a microchannel was found to provide, in comparison with batch processes, a more effective mixing and an increased yield for a particular organic reaction which uses a phase transfer catalyst. The superiority of the microreactor method increased with the frequency of alternation of the pumping of the two liquids.     

Bias voltage dependent electrochemical impedance spectroscopy of p- and n-type silicon substrates

For a comprehensive interpretation of the electrochemical processes occurring at a semiconductor/electrolyte interface, an accurate value of the flatband potential versus the reference electrode is needed together with the current/voltage response. The present work is mainly devoted to the analysis of impedance diagrams recorded with n- and p-type silicon electrodes in the dark, in a pure diluted HF solution under different bias values, the potential range being chosen so as the dc current is maintained at a low value, e.g. less than 5 μA cm⁻², and even less than 10 nA cm⁻² in the case of p-type Si under cathodic polarization.Two series of impedance diagrams were recorded. Firstly, the bias value was settled in the potential range where the depletion layer was generated within the semiconductor substrate. In this condition, the high resistance of the space charge region and the resulting low value of the dc current permitted to obtain well-defined semicircles leading to the derivation of both R and C values equivalent to the depletion layer charge distribution. The method based on the analysis of the voltage dependent R-value was compared to the usual Mott-Schottky plot treatment, and proved to be efficient for the determination of a reliable value of the flatband potential versus the reference electrode.In the second series, the analysis of the impedance diagrams was focused on the range of potentials corresponding to the onset of an accumulation layer within the semiconductor. In the case of p-Si, this condition corresponds to the rapid anodic etching of the silicon substrate. At a particular value of the bias, an important induction loop was observed in addition to the usual capacitive behaviour. A Fourier transform treatment demonstrated that the impedance components were interrelated to the time dependent current response under constant bias potential. Then, simultaneous experiments based on chronoamperometry measurements suggested that the electrochemical processes involved in the reaction of Si substrate with HF solution was correlated to a two-step charge transfer mechanism. This interpretation is valid for both the inductive loop obtained in electrochemical impedance spectroscopy and the rise of current versus time observed in chronoamperometry.     

Strategies for size reduction of microreactors by heat transfer enhancement effects

One of the likely aims of reactor miniaturization in the field of chemical production and energy generation is to increase the conversion to the desired product and the selectivity of the process through better control of heat and mass transfer. In addition to the effects related to miniaturization, a further increase of the transfer coefficients is achieved by applying microstructuring techniques. In this context, three different approaches for heat transfer enhancement in miniaturized reaction systems are presented. The ideas put forward rely on entrance flow effects, inertial flows in meandering channels, and suppression of axial heat conduction. Among these ideas the entrance flow effect, realized by an arrangement of microfins with a typical dimension of a few hundred micrometers, provides the most efficient heat transfer. It is found that a heat transfer enhancement of at least one order of magnitude can be achieved compared to unstructured channels. On this basis, a miniaturized heat-exchanger reaction system is investigated, where a kinetic model of an endothermic, heterogeneously catalyzed gas-phase reaction is used. The miniaturized heat-exchanger reactor, both with and without heat transfer enhancement, is subsequently benchmarked against conventional fixed-bed technology. It is shown that, for the reaction system under study, a substantial reduction of the required amount of catalyst can be achieved in microsystems.     

Experimental study of macropore formation in p-type silicon in a fluoride solution and the transition between macropore formation and electropolishing

Anodic dissolution of p-Si is studied in diluted fluoride solution (HF 0.05 M + NH₄F 0.05 M, pH 3), with special focus on the physico-chemical parameters which govern the morphology of pore formation (crystallographic orientation, applied potential, and etching time). The effect of potential has been investigated in the transition region between macropore formation and electropolishing. Upon increasing the anodization potential, the pore cross-section changes from circular to square shape, and the bottom of the pores changes from a rounded to a V-shaped profile. Prolonged etching of the contour of (1 1 0) p-Si disks in the regime of porous silicon formation allows for a comparison of the etching characteristics of the orientations. SEM observation indicates indeed different morphologies as a function of the crystal orientation, and the formation of fractal-like structures is obtained for some orientations. In the same geometry and at a potential just above the onset of the electropolishing regime, prolonged anodization allows for a direct measurement of the Si thickness removed as a function of the crystallographic orientation. We clearly observe the etching anisotropy, with etch depth τ(1 1 1) < τ(1 1 0) < τ(1 0 0). This sequence, similar to that observed for current density in more concentrated HF, differs from that observed for the chemical etching of Si in an alkaline solution.     

Porosification behaviour of LTCC substrates with potassium hydroxide

The demand to meet advanced substrate requirements in terms of electrical, mechanical, thermal, and dielectric properties has led to an increasing interest in low temperature co-fired ceramics (LTCC). However, LTCC materials suffer from high permittivity. We recently showed that the wet-chemical porosification under acidic conditions allows the reduction of the permittivity of LTCCs in the as-fired state. In the present study, potassium hydroxide solution was employed as an alternative etchant which features a suitable bearing plane for further metallization lines. Various characterization techniques, including scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction analysis, and electron energy loss spectroscopy were used for investigation of the morphology and chemical composition of the substrates. Three-dimensional information of the surface topography was acquired by means of MeX^® Alicona software and the obtained roughness parameters confirmed the advantage of the proposed approach over acid treatment when targeting an enhanced surface quality.     

Fabrication of patterned porous silicon using high-energy ion irradiation

P-type silicon has been patterned using high-energy protons beam prior to electrochemical etching in hydrofluoric acid. The ion beam selectively damages the silicon lattice, resulting in an increase in the local resistivity of the irradiated regions. It is found that the photoluminescence intensity of the irradiated regions increases with proton irradiation into a 0.02 Ω.cm resistivity p-type silicon. By immersing the etched sample into potassium hydroxide, the porous silicon is removed to reveal the underlying three-dimensional structure of the patterned area.     

Morphological and nanostructural features of porous silicon prepared by electrochemical etching

Porous layers were produced on a p-type (100) Si wafer by electrochemical anodic etching. The morphological, nanostructural and optical features of the porous Si were investigated as functions of the etching conditions. As the wafer resistivity was increased from 0.005 to 15 Ω·cm, the etched region exhibited ‘sponge’, ‘mountain’ and ‘column’-type morphologies. Among them, the sponge-type structured sample showed the largest surface area per unit volume. Silicon nanocrystallites, 2.0 to 5.3 nm in size, were confirmed in the porous layers. The photoluminescence peaks varied in the wavelength range of 615 to 722 nm. These changes in the maximum peak position were related to the size distribution of the Si crystallites in the porous silicon. The doping levels of the wafers significantly affect the size distribution of the Si crystallites as well as the light-emitting behavior of the etched Si, which contains nanoscale Si crystallites.     

Design of experiments in electrochemical microfabrication

Electrochemical material etching techniques have attracted a significant amount of attention in the “wet” metal etching arena, as the process typically involves neutral salt electrolytes and is relatively safe to operate. There are also economical and environmental advantages associated with these techniques compared with competing etching methods.A new concept of electrochemical microfabrication on substrates has been developed. In the technique the workpiece, which is the anode in the electrochemical reactor, is placed closely to a tool, which is the cathode containing the micro-pattern. Selective pattern transfer results in a higher etching rate on the areas opposing “exposed” regions of the cathode, and lower etching rates in the areas directly opposite to the areas, on the cathode, covered by an insulator.In this investigation the electrochemical micro-patterning process has been evaluated and characterised in a vertical flow system described previously in literature. The experiments were carried out using copper disk anodes and patterned cathodes in a 0.1 M copper sulphate electrolyte. A 2⁴ factorial experimental design procedure was adopted to determine the influence of process parameters on the electrochemical microfabrication process in terms of variability in pattern transfer over the electrode's surface area.     

Synthesis of Ti3C2Fx MXene with controllable fluorination by electrochemical etching for lithium-ion batteries applications

Ti₃C₂T_x MXene has attracted remarkable attention due to its promising applications in energy storage and sensors. However, traditional MXene preparation methods used HF as etchant, which was highly toxic and harmful to human and environment. Moreover, the aqueous etchants will also result in the combination of OH, O and F groups on the surfaces, making it difficult to control the varieties and contents of the surface terminations. In this paper, a green and mild electrochemical exfoliation method was proposed to synthesize Ti₃C₂F_x and synchronously control its fluorination degree on the surface. A non-aqueous ionic liquid, [BMIM][PF₆]-based solution was used as electrolyte. The as-prepared Ti₃C₂F_x was fluorinated with the CF and TiF₃ groups, which were electrochemically active and contributed to the excellent cycling stability of the MXene anode-based Li-ion batteries. These findings provided a facile strategy to prepare MXene materials and dope MXene with tailored property for MXene-based energy devices applications.     

On the modeling of shape evolution in through-mask electrochemical micromachining of complex patterned substrates

This study is about the optimization of the shape evolution of a printed circuit board (PCB) by means of an auxiliary electrode. Predicting the shape evolution of such complex workpieces calls for a significant reduction of the nodal points in discretization. The authors applied a hierarchic modeling strategy, which breaks the overall problem into three spatial scales: the macroscopic, the mesoscopic and the microscopic scale. Simulations on each scale are performed by considering influences from the other scales in a simplified way. The strategy yields the shape evolution of any chosen trench of the workpiece and allows for the consideration of an auxiliary electrode. The auxiliary electrode was optimized in terms of its geometry and potential. The uniformity of the etch rate was governed by an interplay of different extrema caused by the complex patterning of the workpiece. This interplay was cleared up by investigating the current density of each extremum as a function of the parameters.     

A novel approach for bulk micromachining of 4H-SiC by tool-based electrolytic plasma etching in HF-free aqueous solution

Bulk micromachining of single-crystal SiC has been challenging due to its extreme stability both mechanically and chemically. To address this issue, a novel tool-based electrolytic plasma etching method is proposed, with which micropatterns and micro-holes are fabricated in SiC in a hydrofluoric acid-free aqueous solution with no need for masks. The material removal is the result of the combined effects of electrolytic plasma chemistry and physics. The chemistry refers to the reaction of Si with hydroxyl radical to form various SiO_x and with H to form silanes, and the reactions of C to form volatile carbon oxides or hydrocarbons, all of which are accomplished and enhanced under the electrolytic plasma atmosphere. Besides, the local high temperature of plasma thermally promotes the evaporation or dissolution of SiO₂ in NaOH solution. The tool-based electrolytic plasma etching method provides alternative approaches for the fabrication of SiC-based MEMS and devices.     

Yield enhancement for the surface of solar-cell silicon wafers with electromechanical micromachining

Effective yield enhancement for the surface of silicon wafers of solar cells was developed using electrochemical micromachining and a design disc-form tool as a precision recycling module for Si₃N₄ thin-film microstructures and epoxy film. The low yields of epoxy films and Si₃N₄ thin-film deposition are important issues in semiconductor production. The current approach uses strong acid and grinding and may cause both damage to the physical structure of silicon wafers and pollution. Electrochemical micromachining allows the removal of the surface Si₃N₄ layer and epoxy film from the silicon wafers and may lead to the development of a mass production system for recycling defective or discarded silicon wafers of solar cells. A high feed rate of the silicon wafers of solar cells combined with enough electric power produces fast machining performance during etching. High rotational speed of the disc-form tool leads to high dreg discharge mobility and a good etching effect. A small height or a small bitter end radius of the anode corresponds to a high removal rate of the Si₃N₄ and epoxy. A small surface area of the cathode also corresponds to a high removal rate.     

Light room anodic etching of highly doped n-type 4H SiC in high concentration HF electrolytes: difference between C and Si crystalline faces

In this paper, we study the electrochemical anodization of n-type heavily doped 4 H-SiC wafers in a HF-based electrolyte without any UV light assistance. We present, in particular, the differences observed between the etching of Si and C faces. In the case of the Si face, the resulting material is mesoporous (diameters in the range of 5 to 50 nm) with an increase of the ‘chevron shaped’ pore density with depth. In the case of the C face, a columnar morphology is observed, and the etch rate is twice greater than for the one for the Si face. We''ve also observed the evolution of the potential for a fixed applied current density. Finally, some wafer defects induced by polishing are clearly revealed at the sample surfaces even for very short etching times.     

Tailored and deep porosification of LTCC substrates with phosphoric acid

Low temperature co-fired ceramics (LTCC) as an advanced technology for robust assembly of electronic components, has attracted significant attention in a wide application range such as in wireless communication or automotive radar systems. However, accurate designs of micromachined devices operated at high frequencies require substrates with regions of tailored permittivities. Introduction of controlled porosity into the substrate via wet-chemical etching procedure, is a promising approach for permittivity reduction which can be applied to commercially available LTCC without necessitating to alter their composition or sintering process. In the present study, by selective dissolution of celsian phase a very deep porosification (highest reported so far) could be realized while preserving the surface quality. Also, by a careful selection of the etching parameters, the depth of porosification and hence the permittivity reduction can be delicately tailored. Laser ablation inductively coupled plasma mass spectrometry was used for the investigation of chemical compositions of substrates.     

Microscale Modeling of an Anode‐Supported Planar Solid Oxide Fuel Cell

A microscale model of a solid oxide fuel cell (SOFC) involving the mass transfer together with the electrochemical reaction, the transportation of electrons and ions through the respective spherical shaped electron conducting and ion conducting particles inside the electrodes was mathematically developed. Couples of useful parameters were introduced in order to represent the characteristics of the cell. The predicted cell performance was showed according to various operating and design conditions. The effects of microscale electrode geometry on the cell performance were also taken into account. Parametric study according to the volumetric fraction of ionic and electronic conducting particles was conducted in order to examine the effects of operating conditions on the cell overpotentials. The study results substantiate the fact that SOFC overpotential could be effectively decreased by increasing the operating temperature as well as operating pressure. This present study reveals the working mechanisms of SOFC at the microscale level, while demonstrating the use of microscale relations to enhance the SOFC performance. The accuracy of the presented model was validated by comparing with already existing experimental results from the available literatures.     

Control of pit initiation sites on aluminum foil using colloidal crystals as mask

The electrochemical etching of aluminum foils using a physical mask of colloidal spheres was studied to directly control the initiation sites of pits independent of the surface activation state of the substrate. A two-dimensional array of colloidal spheres used as a mask was prepared by self-assembly on an aluminum substrate. The transfer of the hexagonally ordered pattern of self-assembled colloidal spheres to the aluminum substrate could be achieved by the selective electrochemical etching of an exposed aluminum surface. That is, etch pits were generated only in the triangular void space among the three spheres on the aluminum surface. Based on this process, the dispersibility of the initiation sites of pits was improved clearly in comparison with that for the conventional method. The density of pits could also be controlled by changing the diameter of spheres used as a mask.     

Mechanism of electroreduction of intermediates with and without a proton donor

Electrochemistry of various categories of intermediates was comparatively studied over wide range of electrode potentials, concentrations of proton donors (H₃O⁺, NH₄⁺), and temperatures. The suggested kinetic model considered two parallel pathways of electron transfer: either to an adsorbed intermediate (radical) or to the respective metastable complex with a proton donor. The electroreduction of all studied intermediates was found to obey the model. The formation of metastable complexes was shown to be facilitated for radicals having one or more functional groups. The particular reduction pathway is determined predominantly by the difference between overvoltages of electron transfer to a radical and to its metastable complex with a proton donor under the same experimental conditions (i.e. nature and concentration of a proton donor, electrode potential).     

Influence of the anion specificity on the electrochemical corrosion of anodized aluminum substrates

The electrochemical corrosion of anodized aluminum substrates in the presence of various electrolyte solutions at 0.01 mol L⁻¹ concentration has been investigated. The comparative results were found to exhibit a correlation between the aggressiveness of the anions and their cosmotrope/chaotrope nature. The origin of the observed behavior was assumed to result from a variable resistance against dehydration during the pitting process.     

Improving electrochemical performance of NiO films by electrodeposition on foam nickel substrates

NiO films for lithium-ion batteries were deposited on copper plates and foam nickel substrates by electrodeposition and subsequent heat treatment at 300 °C. At a discharge/charge rate of 0.1 C, foam NiO films delivered reversible capacity larger than 650 mAh g⁻¹ and capacity retention over 93% after 50 cycles. NiO films deposited on foam nickel exhibited higher reversible capacity, better cyclability, as well as higher rate capability than those on copper plates. The unique three-dimensionally porous morphologies of foam NiO films were responsible for the better electrochemical performance, which provided not only high electrode/electrolyte contact area but also a good electronic conduction matrix. The present finding offers a new pathway for the large scale fabrication of high-energy-density electrodes for lithium-ion batteries.     

Electrochemical nucleation and growth of Co and CoFe alloys on Pt/Si substrates

Electrochemical nucleation and growth of Co and CoFe alloys on Pt/Si(1 0 0) surface from watts (mixed chloride sulfate) baths were studied by voltammetric, chronoamperometric and AFM measurements. The CoFe alloys were deposited from solution with molar ratio of (1/1) and (10/1). The Scharifker and Hills model was employed to analyse the current transients. For both Co and CoFe (10/1) alloys the nucleation was a good agreement with the instantaneous model followed by 3D diffusion-limited growth. Inversely, for CoFe (1/1) alloy the nucleation was an agreement with the progressive model. It is evident that the compositions of the electrolyte influence greatly the type of nucleation. The atomic force microscopy (AFM) images revealed a compact and a granular structure of the electrodeposited Co layers and CoFe alloys.