Anodic Oxidation Process Research Articles

Anodic oxidation of lead in aqueous carbonate solutions. II. Film formation and dissolution in the pH range 9 to 14

The anodic oxidation of lead has been studied in aqueous carbonate solutions at pH values in the range 9 ≤ pH ≤ 14. The solid films formed on the electrode surface have been identified by X-ray diffractometry. The dissolution and film formation processes have been studied by a number of electrochemical techniques at rotating disk electrodes. The anodic oxidation process can be divided into two distinct regions. In the main anodic oxidation process, a variety of surface phases is formed. The nature of the phase formed is pH-dependent. Plumbonacrite (Pb10O(OH)6(CO3)6) formation is observed at all pH values and predominates at pH ≥ 13. At pH < 11, cerussite (PbCO3) is the predominant phase, whereas at intermediate pH values, hydrocerussite (Pb3(OH)2(CO3)2) predominates. The dissolution rate of Pb2+ species from the electrode surface is directly proportional to the solubility of the predominant phase present. At more positive potentials, a reactivation, involving increased dissolution and a further stage of film formation, is observed. Litharge (PbO) is observed to grow underneath the initially formed basic lead carbonates. Dissolution occurs either by the field-assisted dissolution of the base-layer on the electrode or by metal dissolution through faults in the base-layer.

Influence of the series resistance on the kinetics of the constant voltage stage during anodization—I

An approximate equation is derived for the time variation of the ionic current during the constant voltage stage of the anodic oxidation process. The equation is given in terms of the normalized variables, ie the variable referred to its value during the constant-current stage. The influence of the series resistance in the anodization cell on the kinetics of the oxide growth is explicitly taken into account in the calculations. Experimental results obtained for tantalum oxide show a good agreement with the theory.

Diaphragm Thickness Control in Silicon Pressure Sensors Using an Anodic Oxidation Etch‐Stop

An etch‐stop technique for silicon diaphragm formation has been developed. In the experiment, n/p epitaxial silicon wafers were used. The wafer was immersed in a hydrazine/water solution and a positive voltage was applied to an n‐type epitaxial layer. After the thick p‐type substrate was etched off, silicon etching was automatically stopped, and only a thin n‐type epitaxial layer diaphragm was left. The etch‐stop mechanism is considered to be as follows: after the p‐type substrate is chemically etched off, an oxide film is formed by an anodic oxidation process on the n‐type layer surface. The oxide film protects the n‐type silicon from being etched by the hydrazine/water solution. Piezoresistive silicon diaphragm pressure sensors with precise diaphragm thickness have been fabricated using this etch‐stop technique. Diaphragm thickness was controlled to .

Zinc Oxidation and Redeposition Processes in Aqueous Alkali and Carbonate Solutions: II . Distinction Between Dissolution and Oxide Film Formation Processes

The early stages of the process of anodic zinc oxidation in , , and Cl− solutions around pH 11.5, were studied by cyclic voltammetry, as in part I. Unusual peak current dependence on sweep rate is observed at the first anodic peak. This behavior is explained in terms of a mixed process involving coupled diffusion and film formation. By correlating the shapes of peaks and peak potentials for different Zn surface preparations, and the relation of the peak current to sweep rate and to electrode rotation rate, , the roles of film formation and diffusion of solution species are distinguished. A new method of plotting out cyclic voltammetry peak currents, , as a function of sweep rate, , is proposed that treats the anodic currents in terms of parallel components for dissolution into solution, with diffusion (proportional to ), and for film formation or reduction (proportional to ). This allows a separation of diffusion‐controlled and film formation currents to be made. The RDE experiments yield and sweep rate experiments showed relations apply to the for the peak. Two processes therefore take place: diffusion‐controlled dissolution and direct film formation. The importance of studies at single‐crystal surfaces is stressed where resolution of the various processes involved is much clearer. Thus, it is shown that only the direct oxide film formation process is predominant at the single‐crystal Zn(0001) face. This compact film is responsible for passivation. The results enable a model for dissolution and coupled‐film formation, associated with passivation, at the Zn electrode, to be suggested. The participation of these two types of process depends on pH and the types of ions present.

Electrochemical and spectroscopic characterisation of structural reorganisation in N, N′–dialkylpyridinium cation radical deposits

AbstractThe electrochemical reduction to form a deposit of the radical cation salt for a series of n‐alkyl asymmetrically substituted viologens and the corresponding anodic oxidation processes have been investigated using electrochemical methods in in situ uv‐visible reflectance spectroscopy. Particular attention was focussed on the nature of the restructuring involved in the ageing of the deposit. The use of both p‐ and s‐polarised radiation clearly showed that initially the viologen units were deposited with the major molecular axis parallel to the electrode and on ageing they reorganised into a preferred orientation perpendicular to the electrode. The viologens with the larger substituents were initially deposited as aggregates of mixed micelles containing both cation radical and dication units. The latter were reduced at a well defined potential, the process being seen as a highly‐reversible sharp current spike on the linear sweep voltammograms.

Theoretical Approach for the Constant Voltage Stage in Anodic Oxidation

Approximate equations are derived for the time variation of the residual current, electric field, and thickness during the constant voltage stage of the anodic oxidation process. The equations permit the calculation of the absolute value of thickness with an error of less than 1%. Experimental results for the residual current in the anodization of tantalum are also given showing a good agreement with the values predicted by the theoretical equations.

Oxidative degradation of aqueous phenol effluent with electrogenerated Fenton's reagent.

A novel process of an oxidative degradation of aqueous phenol effluent with hydrogen peroxide produced by electroreduction of oxygen dissolved in the effluent was studied. Effects of operational conditions such as pH, cathode potential, ferrous ion concentration and phenol concentration on the degradation rate of phenol were clarified. Phenol was successively degraded to carbon dioxide at pH 3 with higher efficiency rather than the other pH''s. The COD current efficiency at pH 3 was higher than that for the anodic oxidation process of phenol and was more than 60% for complete degradation in the range of 260-2600 ppm of initial COD.

Hydrogenated amorphous silicon position sensitive detector

The design, fabrication, and characterization of newly developed hydrogenated amorphous silicon position sensitive detectors (a-Si:H PSDs) which employ a tunnel metal-insulator-semiconductor (MIS) structure fabricated by anodic oxidation processes are described. Tunnel MIS structure is chosen as the a-Si:H PSD structure, since it offers high breakdown voltage compared to a-Si:H pin or Schottky-type structures. PSD process includes two-step anodic oxidation of a-Si:H: (1) oxidation in the dark to passivate defects and shunt paths introduced during the growth of a-Si:H layer and (2) oxidation with light to grow thin native oxide to provide barrier between metal and a-Si:H. The fabricated large-area, two-dimensional PSDs, as large as 3 cm×3 cm, show the incident beam position within error of less than 2% of the length of PSD.

Anodic oxidation of amines. IX. Anodic oxidation process of .ALPHA.-amino acids in aqueous buffer of pH 10.

Anodic oxidation of α-amino acids with and without a β-hydroxy group was investigated by cyclic voltammetry and controlled potential electrolysis in aqueous carbonate buffer of pH 10. The first wave of the α-amino acids is developed at nearly the same potentials as those observed for the corresponding simple aliphatic amines, showing that a β-hydroxy group does not significantly decrease the first oxidation potential, in contrast to the case of β-alkanolamines. Decarboxylation accompanied by dealkylation is the main reaction process, and a small amount of C-C bond fission also occurs only in the case of the β-hydroxy amino acids.

Le mecanisme du developpement de l'oxyde poreux d'aluminium—I. Sur la possibilite du contact direct aluminium-solution electrolytique au cours du processus de formation—transformation de la couche barriere

Experimental evidence on the electrolyte permeability of the barrier layer during the anodic oxydation of Al in H 2SO 4 are reported. On this basis, a new theoretic mecanism of anodic oxidation process is proposed. The current efficiency of the oxydation process η ox (determined by weighing the oxide) depends linearly on the barrier layer voltage, V, η ox = V/2 V x , were V x is the critical oxidation voltage.

Thermogravimetric studies on aluminium foils for electrolytic capacitors

Thermogravimetry was applied in studies of aluminium foils for electrolytic capacitors. Scanning electron microscopy, X-ray diffraction and surface area determination were also used in the interpretation of the results. Both the specific surface area and the capacity of the foils showed a linear relationship to the height of the oxidation step measured by TG, which was found suitable for following the processes of anodic oxidation and thermal treatment of the foils as well.

A-Si:H MIS position sensitive detector by anodic oxidation processes

The first fabrication of an amorphous silicon position-sensitive detector using an MIS structure by anodic oxidation processes is reported. The fabricated 3 mm × 26 mm one-dimensional position-sensitive detectors show excellent linearity with lateral photocurrent response, having a correlation coefficient of 0.996.

Anodic Oxide Films and Electrochemical Reactions on HgTe

Anodic oxidation processes on surfaces in acetate buffer pH 4.9, borate buffer pH 8.4, and pH 13.0, with and without ethylene glycol, have been studied. Pt‐split electrode experiments permit the simultaneous examination of formation and reduction of oxides and of soluble reaction products. The composition and thickness of the anodic oxide layers are evaluated coulometrically. XPS measurements yield the chemical composition of the surface after the different electrochemical treatments. They essentially confirm the electrochemical analysis. Optically homogeneous oxide layers of up to ∼1000Å thickness are grown in acetate buffer, borate buffer, and ethylene glycol. The layers are composed of mixed oxides, , with . The molar ratio levels off to a value of for high oxidation potentials in acetate buffer and reaches almost for the glycol mixture. The missing Hg is dissolved as during anodic oxidation with only a small amount of Te compounds. The first cathodic reduction step of the oxides in the same electrolytes reduces to metallic Hg. As a consequence is dissolved, demonstrating the importance of the for the chemical stability of the mixed oxide layer. Special cathodic reduction conditions (−0.6 VH in acetate buffer) produce a stoichiometrically clean surface, removing any oxide or elemental Te at the surface due to previous chemical treatments. The electrochemical reactions are similar for all three electrolytes mentioned above. However, in pure the disk and ring currents are more than one order of magnitude larger (>1 mA cm−2) than those in acetate and boric borate buffer (∼0.1 mA cm−2), indicating a higher dissolution rate of these oxide layers. The presence of 90% ethylene glycol in reduces the current density again to the low levels (<0.1 mA cm−2).

陽極酸化処理工程中における発生水素の回収用隔膜の性能

In the anodic oxidation process of aluminum, the hydrogen gas evolved at the cathode has not been utilized so far. This work is intended to establish a method to collect the gas for its effective utilization. In order to collect efficiently the hydrogen gas the cathode room was surrounded by a net, a plastics porous filter or asbestos filter. This technique made it possible to collect more than 90% of the gas generated. At the same time, the emission of acidic mist was reduced to a greater extent. Moreover, any unfavorable effect was not found neither in anodizing process nor in the characteristics of the anodized films, although the bath voltage slightly increased at constant current process.

Fabrication technology for an 80-ps normally-off GaAs MESFET logic

Fabrication technology of a high-speed normally-off GaAs MESFET logic has been described. Anodic Oxidation process is applied to control epitaxial layer thickness precisely. A SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> cap during alloying ohmic metal is used to prevent the ohmic layer surface from becoming uneven. A sloped mesa structure edge is used to avoid disconnection of metal interconnection. Electron-beam direct writing is employed to define a submicrometer gate. Applying these technologies, high-speed and small switching energy have been accomplished. The minimum delay timd and the associated switching energy were 77 ps and 75 fJ at room temperature and 51 ps and 97 fJ at 77 K.

Solar selective copper-black layers by an anodic oxidation process

Optical and structural properties of copper-black layers prepared by the anodic oxidation of copper in an alkaline medium have been studied. They have high visible absortion (α ≈0.95) and low infrared emission (ε ≈0.2) with cut-off at 3.5 μ. X-ray diffraction, electron diffraction and ESCA studies of these layers indicate that they are mixture of cuprous and cupric oxides. The crystallite size is between 100 to 200 Å.

High Efficiency MOS Solar Cells by Anodic Oxidation Processes

Preparation of GaAs, InP and Si single-crystal MOS solar cells by anodization in various modes is investigated. Both passive and active modes of anodization are found to increase the open-circuit voltage with respect to that of the bare Schottky cell. Best result is obtained by active anodization of n-GaAs. Passive anodization with a high formation voltage results in an anomalous reduction in photocurrent. A detailed analysis of cell currentvoltage characteristics indicates that cell behavior is dominated by interface-states.

Study of Anodization Process on GaAs by In Situ Differential Reflectance

The anodic film growth on during the anodization process and the dissolution process of the grown film in the open‐circuit condition have been observed with the in situ differential reflectance technique. The initial phase of anodic oxidation process has also been investigated in this study. The experimental results strongly suggest that the two‐dimensional growth of the anodic nuclei takes place in the initial phase and that the substrate‐dissolution process is controlling the film formation. A “less soluble region” has been observed in the dissolution curve. The film keeps growing in the open‐circuit transient region due to the residual ions within the grown film. It is also shown that the dissolution curve of the grown film is useful in characterization of the film rather than the anodization curve. The obtained value of parameter from the best‐fitting to the dissolution curve is 0.2Å sec−1 for the dissolution rate at pH 2.4 in tartaric acid‐water‐propylene glycol electrolyte and for the refractive index of the anodically grown film at 6328Å.

The Role of Surface Orientation in the Photoelectrochemical Behavior of Layer Type d‐Band Semiconductors

AbstractThe efficiency of photocurrents as well as the photopotential output of electrochemical solar cells based on molybdenum dichalcogenides depends on the crystalline quality of the electrode surface. This phenomenon has been investigated in a systematic way with n‐type crystals by varying the relative proportion of surfaces parallel to the c‐lattice‐vector (van‐der‐Waals surfaces) and surfaces not normal to the c‐vector. We report on anodic reactions studied in the presence of various redox systems and in such electrolytes where the current leads to oxidative dissolution of the solid. — It was found that the two regions react in a characteristically different way. Surfaces not parallel to the c‐axis provide favourable reaction centers for electron transfer reactions in the absence of light while photoreactions predominantly proceed at van‐der‐Waals surfaces. This can be explained by a pronounced difference in the electronic nature of the two sites which results in a different electron density distribution and a different position of electron energy levels in the surface. The metal atoms at surfaces non‐parallel to c have dangling bonds (d‐hybrides) which form surface states and interact strongly with reactants from the solution. At the van‐der‐Waals surfaces, however, the d‐levels are much more shielded from interaction with components of the electrolyte that electronic excitation (generation of holes) is needed to induce anodic oxidation processes.

The two-dimensional nucleation and growth of anodic oxide films on layered semiconductors

We demonstrate in this paper that layered semiconductors present a very favorable situation for the study of nucleation and growth of anodic oxide films on them: we can easily prepare semiconductor surfaces clean enough to have the two-dimensional nucleation and growth take place and we can investigate the growing process of the anodic oxide films under periodic surface conditions which are of great use to obtain information on the transient behavior of the film growth and the thickness of the grown film. Main discussion and conclusion are as follows: (1) periodically varying structures in the cell voltage versus time ( V c− t) and ellipsometric off-null signal versus time ( ΔI− t) curves, observed in anodic oxidation process of layered semiconductors Bi 2Te 3 under constant current condition, originate from the repetition of anodic oxidation of the monomolecular layer, and the unit lineshapes in the V c− t curves indicate the two-dimensional nucleation and growth. (2) In GaSe, a nonpassive film grows two-dimensionally in the initial stage of the anodic oxidation and transforms into a passive film in the final stage. (3) Transient behaviors in the V c− t or − d 2V c dt 2 −t characteristics are observed and discussed in terms of the rate determining process.