Thermotreatment and chemical resistance of porous alumina membrane prepared by anodic oxidation

Porous alumina membranes were prepared by anodic oxidation of aluminum in an aqueous solution of oxalic acid. The thermal and chemical stabilities of porous alumina membranes were investigated by thermal analysis and corrosion test. Alumina membranes prepared in oxalic electrolyte had fairly good thermal stability, but they reacted with boiling water to form boehmite, which blocked pores on the surface of the membrane. For acid and alkali resistance, the membranes were estimated to have chemical stability in aqueous acidic and alkaline solution at 1< pH < 12. On the other hand, pore size and its distribution were not changed at the time of thermal treatment. Polycrystalline alumina membrane obtained by thermal treatment for 3 h at 900 ‡C could suppress corrosion reaction and enhance their resistance in acid and alkaline solution.     

Characteristics of gas permeation using two-layered alumina membrane prepared by anodic oxidation

The microporous alumina membrane with asymmetrical structure, having upper layer with pore diameter of 10 nm under and lower layer with pore diameter of 36 nm, was prepared by anodic oxidation using DC power supply of constant current mode in an aqueous H₂SO₄ solution as a electrolyte. The aluminum plate was pretreated with thermal oxidation, chemical polishing and electrochemical polishing before anodic oxidation. The membranes were prepared by controlling the current density such as a very low current density for upper layer and a high current density for lower layer of membrane. By controlling the cumulative charge density, the thickness of upper layer of membranes was about 6 Μm and the total thickness of membranes was about 80–90 Μm. We found from gas permeation experiments with the membranes prepared by above method that the mechanism of gas permeation of the all membranes prepared under each condition complied with model of the Knudsen diffusion.     

阳极氧化法制备大面积双面多孔氧化铝膜及其应用

室温下,以纯铝箔为基材,采用二步阳极氧化法,在0.3 mol/L草酸溶液中制备了大面积的双面氧化铝膜.在氧化过程中,电流呈现降低、升高、平稳、再降低的变化,最后接近于零.X射线衍射结果显示,双面氧化铝膜为无定型结构.扫描电镜结果显示:每面氧化铝膜的总厚度约为50 μm,孔径分布为50～100 nm.利用制得的双面氧化铝...     

The anodic and cathodic dissolution of Al and Al-Cu-Mg alloy

Atomic emission spectroelectrochemistry (AESEC) was used to monitor the release of Al from 99.99% aluminum (1199 alloy) and Al, Mg, and Cu from 2024 Al alloy in 30 g/l NaCl electrolyte as a function of pH. The cathodic dissolution of Al was demonstrated and attributed to an increase in the pH at the interface due to the water reduction reaction. The dissolution of Mg was also observed but was a more complex function of current probably depending on the interfacial pH and the Al dissolution rate. The detachment of copper-rich particles was observed as very rapid spectroscopic emission transients (peak width < 10 ms).     

稳定多晶氧化铝纤维质量之研究

通过改变胶体添加剂和热处理参数,提高纤维化器的转速,达到了稳定胶体粘度、改善成纤性能、降低纤维直径、稳定并提高氧化铝纤维质量、降低生产成本之目的。     

Role of metal ion impurities in generation of oxygen gas within anodic alumina

The generation of oxygen gas within an amorphous anodic alumina film is reported. The film was formed by anodizing aluminum, which was first electropolished and then chemically polished in CrO₃-H₃PO₄ solution, in sodium tungstate electrolyte. The procedure results in incorporation of mobile Cr³⁺ species, from the chemical polishing film, and mobile W⁶⁺ species, from the electrolyte, into the amorphous structure. The tungsten species are present in the outer 27% of the film thickness, while Cr⁶⁺ species occupy a thin layer within the tungsten-containing region. Above the Cr³⁺ containing layer, a band develops that contains oxygen bubbles of a few nanometres size. The oxygen is generated by oxidation of O²⁻ ions of the alumina. A mechanism of oxygen generation within the alumina is proposed based on the electronic band structure of the oxide, modified by the Cr³⁺ and W⁶⁺ species, and on the ionic transport processes during oxide growth.     

Control of the spatial distribution of porous alumina micro-domes formed during anodic oxidation

We found that micro-domes of porous alumina are self-assembled during anodic oxidation of an aluminum plate. We investigated the effects of the morphology of the initial aluminum surfaces on the formation of these micro-domes and found that the formation of micro-domes depends on the initial surface roughness of the substrate. We have also achieved spatial control over the distribution of these micro-domes through the use of artificial scratches on the initial surface. The origin of this control is the fact that micro-domes are preferentially formed inside hollow areas formed by the scratch. We investigated the inner structure of the micro-dome by separating it from the substrate. Inside the micro-domes, we observed nano-pore arrays similar to a porous alumina membrane, though the regularity of these pores is slightly worse than for the nano-pores around the micro-dome. These results indicate that the porous alumina micro-domes can be used as microscale nanoporous components.     

酸性溶液中阴阳极协同氧化强化铜的溶出

提出了一种阳极直接电化学溶解与阴极间接溶出耦合溶出铜的新方法,阴极间接氧化是基于氧气还原反应和类芬顿反应产生H2O2和?OH. 结果表明,在最佳电压0.25 V或电流密度6 mA/cm2下,电化学体系能产生大量活性物种,铜溶出率在直接阳极溶出的基础上大幅提升.     

Fabrication of ordered nanoporous anodic alumina prepatterned by mold-assisted chemical etching

In this article, a simple and cost-effective method to create patterned nanoindentations on Al surface via mold-assisted chemical etching process is demonstrated. This report shows the reaction-diffusion method which formed nanoscale shallow etch pits by the absorption/liberation behaviors of chemical etchant in poly(dimethylsiloxane) stamp. During subsequent anodization, it was possible to obtain the ordered nanopore arrays with 277 nm pitch that were guided by the prepatterned etch pits. The prepatterned etch pits obtained can guide the growth of AAO nanopores during anodization and facilitate the preparation of ordered nanopore arrays.     

Anodic processes in plasma electrolytic oxidation of aluminium in alkaline solutions

In this paper, basic electrochemical processes (such as oxide film growth, anodic dissolution and oxygen liberation) on an aluminium anode in a model alkaline solution are considered under conditions of galvanostatic DC plasma electrolytic oxidation (PEO). The experiments performed include: (i) recording and analysis of the main electrical characteristics of the process; (ii) determination of the oxide layer thickness; (iii) anodic gas collection and composition analysis and (iv) electrolyte analysis to determine dissolved aluminium. Four different stages of the PEO process have been identified, characterised by various rate proportions of the partial anodic processes. Overall current efficiency of the oxide film formation has been estimated to be in the 10-30% range. The film growth rate decreases significantly with increasing electrolyte concentration from 0.5 to 2 g l⁻¹ KOH, since the rate of anodic dissolution increases. Oxygen evolution is shown to be the main electrochemical process at the potentials corresponding to the plasma stages of the electrolysis. The overall rate of oxygen liberation at the anode exceeds the Faraday yield, which is probably due to the radiolytic effect of the plasma discharge on the adjacent electrolyte volume.     

Electrocatalysts for the anodic oxidation of borohydrides

Anodic performances of alkali borohydrides on several electrocatalysts such as Ni, Raney Ni, Pd, Pt, Cu, Au have been studied in an effort to develop suitable electrode materials for a borohydride-fueled fuel cell. The open-circuit potentials (OCPs), polarization performances, and hydrogen evolution behaviors were examined in a three-electrode system. The open-circuit potential was found to be dependent on borohydride concentration and also influenced by the electrocatalyst. In concentrated borohydride solutions used in this work, electrode polarizations were less influenced by the fuel concentration. Borohydrides on different electrocatalysts showed different hydrogen evolution behaviors. The relation of hydrogen evolution rate with the anode current was found to change not only with the concentration of borohydride and but also with the electrode material. Comparison of anodic behaviors of borohydride on different electrodes implies that the anodic oxidation of borohydride, as a multi-step process, may take different reaction paths, depending on the electrocatalyst and reaction conditions such as the borohydride concentration.     

氧化铝纤维增强红柱石基复合材料的制备与性能

在南非进口红柱石细粉中分别加入 5 %、10 %、15 %和 2 0 %的氧化铝纤维 ,研究了纤维加入量对材料显气孔率、体积密度、力学性能及显微结构的影响。结果表明 :当氧化铝纤维加入量为 5 %～ 10 %时 ,材料的体积密度大 ,力学性能好 ,超过 10 %后材料的性能显著下降     

Micro-electrochemical characterization of the effect of applied stress on local anodic dissolution behavior of pipeline steel under near-neutral pH condition

Local anodic dissolution behavior of smooth and pre-cracked X70 pipeline steel specimens under applied stress were investigated by micro-electrochemical measurements and numerical simulation. The results demonstrated that the anodic dissolution rate of steel is enhanced by applied tensile stress. Corrosion of the stressed steel is accompanied with the formation of a layer of corrosion product deposit on electrode surface. However, the deposit layer does not provide effective protection to the underlying steel for corrosion attack due to its loose, porous structure. Under small tensile stress, the stress-enhanced dissolution of steel is not significant. With the increase of stress level up to 80% of yielding strength of steel, the activity of the steel increases remarkably, resulting in a significant enhancement of the anodic dissolution of steel. A stress concentration is developed at the crack-tip and enhances significantly the local anodic dissolution of steel. The stress effect factor at the crack tip is as high as 3.6 when a tensile force of 3000 N is applied on the pre-cracked specimen, while that for the low-stress area is 1.102 only. The local dissolution rate at the crack-tip is accelerated with the test time, which would be attributed to the fact that, along with continuous propagation of the crack, the stress concentration at the crack-tip is further increased, which, again, enhances local dissolution. This result reflects exactly the interaction of stress and anodic dissolution at crack-tip during SCC of steel.     

Repair of thin thermally grown silicon dioxide by anodic oxidation

Anodic oxidation in 0.1 M HCl followed by a post-rapid thermal annealing process has been used to repair defects existing in thin thermally grown oxide layers (3 and 6 nm) on a p-type silicon substrate. The improved quality of the insulator layer is particularly useful for applications that require large gate areas in Metal-Insulator-Semiconductor (MIS) or Electrolyte-Insulator-Semiconductor (EIS) devices such as Light-Addressable Potentiometric Sensors (LAPS) and Scanning Photo-induced Impedance Microscopy (SPIM). Different methods have been used to characterize the oxide. High-frequency capacitance-voltage curves and ac impedance spectra showed that there was no significant change of the oxide thickness after repair, and the number of the interface states of the oxide was increased for both types of samples. Ramped voltage stress (RVS) measurements of Metal-Oxide-Semiconductor (MOS) structures with gate electrodes 2 mm in diameter showed leakage currents of 0.75 nA cm⁻² for the repaired and annealed 3 nm thick oxide and 1.31 nA cm⁻² for the repaired and annealed 6 nm thick oxide at accumulation voltage. XPS measurements confirmed that there was no change of the oxide thickness and no contamination with other ionic species after repair. AFM results showed a good agreement with the other characterization methods.     

Preparation of alumina/carbon nanotubes composites by chemical precipitation

Continuous alumina coating on multi-walled carbon nanotubes (MWCNTs) was successfully prepared by a new method of chemical precipitation using aluminum nitrate and ammonia as starting materials. Structure and morphology of the alumina/multi-walled carbon nanotubes (Al₂O₃/MWCNTs) composites were characterized by transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), infrared spectra (IR), thermo gravimetric analysis (TG), differential thermal analysis (DTA) and N₂ adsorption–desorption. The results show that polyvinyl alcohol (PVA) modification on the surface of MWCNTs contributes to form continuous alumina coating, γ-Al₂O₃ layers with thickness of 1–3 nm cover the surface of MWCNTs and the original structure of MWCNTs is retained during the coating process.     

Characterization and mechanical properties of TiO2 nanotubes formed on titanium by anodic oxidation

The well-ordered titanium dioxide (TiO₂) nanotube array surfaces were formed at different voltages such as 20 V, 40 V, 60 V, 80 V and 100 V for 1 h on cp-Ti by anodic oxidation (AO) technique. And then, to improve crystallinity of the surface, heat treatment was applied at 450 °C for 1 h to all surfaces without any morphological changing. The surface and cross sectional morphology, elemental structure, phase composition, functional groups, roughness and thickness, wettability and mechanical results were investigated by SEM, EDX, XRD, FT-IR, AFM, contact angle measurement device and nanoindentation tester, respectively. Mainly, anatase- and rutile-TiO₂ phases were obtained at post-heat treatment whereas only, Ti phase was detected on AO surfaces at pre-heat treatment. All nanotube structures and the elements of Ti and O were uniformly distributed through the whole surface. The roughness and thickness of tube structures usually increased with increasing voltage values and measured. The roughness and thickness values were measured as 10.67–111.97 nm and 0.21–1.92 μm, respectively. TiO₂ nanotube surfaces exhibited hydrophobic behaviors with respect to plain Ti surface. Furthermore, mechanical properties such as hardness and elastic modulus of the coating produced at minimum voltage were great compared to ones at higher voltage and plain Ti surface under a Berkovich indenter due to phase structure, homogeneity and density of nanotube structures.     

阳极氧化通电电流对碳纤维层剪强度及界面亲和性的影响

利用阳极氧化表面处理装置,碳酸氢铵作为电解质,以恒定的处理时间,对直流电源的电流进行调节,研究电流对直径5μm的6KPAN基碳纤维层间剪切强度和拉伸强度的影响,并通过SEM对拉伸试样的断面进行观察和对比,研究电流对界面亲和性的影响。结果表明,通电电流增大,层间剪切强度增大;合理调节通电电流,有效提高环氧树脂和碳纤维粘结强度,纤维与树脂的界面结合紧密,使纤维复材的整体承载能力增强,复丝强度较好。根据研究,直径5μm的6KPAN基碳纤维通常最佳电流强度应控制在20~25mA/K。     

The anodic dissolution of zinc and zinc alloys in alkaline solution. I. Oxide formation on electrogalvanized steel

The polarization behaviour of zinc in alkaline solution has been investigated using atomic emission spectroelectrochemistry. By independently measuring the oxidation rate of zinc (electrical current) and the rate of Zn²⁺ dissolution (partial elemental current) it is possible to calculate the amount of insoluble zinc cations produced at any instant. Assuming the insoluble cations are present as a zinc oxide film, the growth of this film as a function of potential and time was determined. On the basis of kinetic evidence, it was found that at least three forms of zinc based oxide/hydroxide films form during polarization experiments. Type I oxide formation occurs when the metal/electrolyte interface becomes locally saturated with Zn²⁺ ions. Type II oxide forms on the metal surface underneath the film of Type I oxide but has little inhibiting effect on zinc dissolution. Type III oxide is produced in much smaller quantity and results in a transition to the passive state. This may be due to a potential induced transition of Type II → Type III oxide.     

Application of alumina supported gold-based catalysts in total oxidation of CO and light hydrocarbons mixture

Gold nanoparticles supported on alumina have been produced using the anionic exchange method and ammonia washing procedure. The catalysts are tested in the reaction of total oxidation of a mixture of light hydrocarbons and carbon monoxide in order to study the possibility of application in the reduction of cold start emissions. The obtained results are promising according to the temperature range observed for the oxidation of unsaturated hydrocarbons. The results obtained for acetylene confirms the difference of oxidation of this hydrocarbon over gold catalysts. An ageing procedure has been employed. This procedure does not affect the comportment of the catalysts versus hydrocarbon oxidation.     

An approximate theoretical impedance analysis of the anodic dissolution of nickel across nickel(II) stabilised by means of competitive anions

A theoretical impedance function for the anodic dissolution of nickel is developed by considering the hypothesis that there are two competitive anions which participate in the stabilization of the electrogenerated Ni(II) species. The experimental effect of chloride ion concentration on nickel electrodissolution process is analyzed by means of a mechanism that considers this competency. It is observed that an increased Cl⁻ concentration causes an increase in Ni(I) surface concentration and a decrease in the kinetic constant rates of the considered mechanism. Moreover, an increased chloride concentration causes a flattening in the low frequencies inductive loop in the Nyquist plot recorded at relatively low stabilization potentials. In the active anodic dissolution/passive transition potential range chloride ions favours a salt layer precipitation which interferes with the passive layer formation.