Anodic oxides growth kinetics

K. Ghowsi and R. J. Gale, Theoretical model of the anodic oxidation growth kinetics of Si at constant voltage, J. Electrochem. Soc. 136, 867, 1989. 

In conclusion, one can say that most anodic oxide films are of a duplex, or even triplex, character, with only the inner portion being composed of a pure anhydrous oxide. In the duplex films, the outer layer contains anions and often a degree of hydration. There could exist a third thin oxide layer at the surface, again with somewhat different properties, which may have a role in the kinetics of oxide growth. 

Figure 31. Correlation of the kinetics of oxide growth with kinetics of sulfate incorporation into the oxide during galvanostatic (a) and potentio-static (b) anodization of A1 in Ff2S04 solutions.166...

The second current maximum J3 corresponds to an oxide thickness at which tunneling of charge carriers becomes negligible, as shown in Fig. 4.7. At the bias corresponding to J3 the formation of anodic oxides in electrolyte-free HF shows a change of growth kinetics, as shown in Fig. 5.2. 

Addition of anions to NaOH solution affects the anodic film formation kinetics and morphology. These films are mainly a-PbO, with the exception of chloride ions solution, where 8-PbO is formed [157]. Tknodic oxidation of Pb electrode in hot alkaKne solution (containing NaOH) facilitates selective growth of a-PbO, /3-PbO, and Pb02- (x = 0-1) phases, depending... 

There are many types of silicon oxides such as thermal oxide, CVD oxide, native oxide, and anodized oxide. Only native oxide and anodic oxide are directly relevant in the context of this book. Anodic oxide film, which is involved in most of the electrochemical processes on silicon electrodes, has not been systematically understood, partly due to its lack of application in mainstream electronic device fabrication, and partly due to the great diversity of conditions under which anodic oxide can be formed. On the other hand, thermal oxide, due to its importance in silicon technology, has been investigated in extremely fine detail. This chapter will cover some aspects of thermal oxide such as growth kinetics and physical, electrical, and chemical properties. The data on anodic oxide will then be described relative to those of thermal oxide. 

Growth of Plasma-Deposited Phases. Plasma polymerization (22) and anodization are already used extensively in materials processing, although the mechanisms by which these reactions occur are not well characterized. However, they have their counterparts in aqueous electrochemistry in electrochemical polymerization and anodic film growth, respectively. In the case of plasma anodization, the mechanisms of growth of the oxide film are not well understood, nor are kinetic data available for a large number of systems. Considerable research effort is required to obtain the necessary data in order to develop viable mechanisms. 

N. Sato, M. Cohen, The kinetics of anodic oxidation of iron in neutral solution I. Steady growth region, J. Electrochem. Soc. Ill (1964) 512—519. 

In the present investigation, the kinetics of growth of anodic oxide films on niobium were studied in molten eutectic NaNOs - KNO3 (50 m/o) over the temperature interval 530-650 K. 

The growth kinetics of WO was studied in hydrochloric acid solution as the tungsten-oxide barrier film was growing as a response to anodic oxidation of tungsten (Eq. 12-11), with oxygen vacancies being treated as main charge carriers across the film [23] ... 

The kinetics of CO oxidation from HClOi, solutions on the (100), (111) and (311) single crystal planes of platinum has been investigated. Electrochemical oxidation of CO involves a surface reaction between adsorbed CO molecules and a surface oxide of Pt. To determine the rate of this reaction the electrode was first covered by a monolayer of CO and subsequently exposed to anodic potentials at which Pt oxide is formed. Under these conditions the rate of CO oxidation is controlled by the rate of nucleation and growth of the oxide islands in the CO monolayer. By combination of the single and double potential step techniques the rates of the nucleation and the island growth have been determined independently. The results show that the rate of the two processes significantly depend on the crystallography of the Pt surfaces. 

Fig. 4 shows a simple phase diagram for a metal (1) covered with a passivating oxide layer (2) contacting the electrolyte (3) with the reactions at the interfaces and the transfer processes across the film. This model is oversimplified. Most passive layers have a multilayer structure, but usually at least one of these partial layers has barrier character for the transfer of cations and anions. Three main reactions have to be distinguished. The corrosion in the passive state involves the transfer of cations from the metal to the oxide, across the oxide and to the electrolyte (reaction 1). It is a matter of a detailed kinetic investigation as to which part of this sequence of reactions is the rate-determining step. The transfer of O2 or OH- from the electrolyte to the film corresponds to film growth or film dissolution if it occurs in the opposite direction (reaction 2). These anions will combine with cations to new oxide at the metal/oxide and the oxide/electrolyte interface. Finally, one has to discuss electron transfer across the layer which is involved especially when cathodic redox processes have to occur to compensate the anodic metal dissolution and film formation (reaction 3). In addition, one has to discuss the formation of complexes of cations at the surface of the passive layer, which may increase their transfer into the electrolyte and thus the corrosion current density (reaction 4). The scheme of Fig. 4 explains the interaction of the partial electrode processes that are linked to each other by the elec-... 

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