Anodic oxide formation

Anodic oxide formation Lakhiani and Shreir have studied the anodic oxidation of niobium in various electrolytes, and have observed that temperature and current density have a marked effect on the anodising characteristics. The plateau on the voltage/time curve has been shown by electron microscopy to correspond with the crystallisation of the oxide and rupture of the previously formed oxide. It would appear that this is a further example of field recrystallisation —a phenomenon which has been observed previously during anodisation of tantalum" . No significant data on the galvanic behaviour of niobium are available however, its behaviour can be expected to be similar to tantalum. 

Figure 18. Dependence of activation barrier A f for the nucleation of a thin oxide film on the metal surface as a function of electrode potential. Ey is the equilibrium potential of anodic oxide formation.7 The solid line represents the value of A against and the dotted line corresponds to the critical potential for the film formation. AE = 0.2 V, Cd= -1Fm-2, am = 0.411 m 2, a -0.01 J m-2,...

Electronic conduction plays a limited role, if any, in anodic oxide formation, since under the anodization conditions and with a high... 

At higher anodic potentials an anodic oxide is formed on silicon electrode surfaces. This leads to a tetravalent electrochemical dissolution scheme in HF and to passivation in alkaline electrolytes. The hydroxyl ion is assumed to be the active species in the oxidation reaction [Drl]. The applied potential enables OH to diffuse through the oxide film to the interface and to establish an Si-O-Si bridge under consumption of two holes, according to Fig. 4.4, steps 1 and 2. Details of anodic oxide formation processes are discussed in Chapter 5. This oxide film passivates the Si electrode in aqueous solutions that are free of HF. 

The first of the four characteristic currents to J4 has a prominent position. It indicates the crossover from a charge supply limited reaction to a kmetically and mass transfer limited reaction. This crossover is accompanied by pronounced changes in charge state, chemical dissolution reaction, dissolution valence, pore formation and anodic oxide formation. Therefore its dependence on other parameters, such as crystal orientation, temperature or H F concentration deserves further investigation. In the literature Jt is usually termed /crl JPS or JPSL. In the following the symbol JPS will be used. 

If the same experiment is performed with an n-type Si electrode under identical illumination intensity the anodic photocurrent is found to be larger than for the p-type electrode under cathodic conditions. This increase is small (about 10%) for current densities in excess of JPS. Figure 3.2 shows that in this anodic regime injected electrons are also detected at p-type electrodes. This allows us to interpret the 10% increase in photocurrent observed at n-type electrodes as electron injection during anodic oxide formation and dissolution. 

Anodic oxide formation suggests itself as a passivating mechanism in aqueous electrolytes, as shown in Fig. 6.1a. However, pore formation in silicon electrodes is only observed in electrolytes that contain HF, which is known to readily dissolve Si02. For current densities in excess of JPS a thin anodic oxide layer covers the Si electrode in aqueous HF, however this oxide is not passivating, but an intermediate of the rapid dissolution reaction that leads to electropolishing, as described in Section 5.6. In addition, pore formation is only observed for current densities below JPS. Anodic oxides can therefore be excluded as a possible cause of pore wall passivation in PS layers. Early models of pore formation proposed a... 

The mechanism is determined from the competition between anodic oxide formation ... 

Conway and coworkers [372] have studied anodic dissolution of gold coupled with anodic oxide formation in HCIO4 solutions with addition of bromide ions. It was observed that gold initially dissolves in a 3e oxidation process. 

The formation or dissolution of a new phase during an electrode reaction such as metal deposition, anodic oxide formation, precipitation of an insoluble salt, etc. involves surface processes other than charge transfer. For example, the incorporation of a deposited metal atom (adatom [146]) into a stable surface lattice site introduces extra hindrance to the flow of electric charge at the electrode—solution interface and therefore the kinetics of these electrocrystallization processes are important in the overall electrode kinetics. For a detailed discussion of this subject, refs. 147—150 are recommended. 

A quantitative description of the diverse morphological features of PS requires the integration of the aspects discussed above as well as the fundamental reaction processes involved in silicon/electrolyte interface structure, anodic dissolution, and anodic oxide formation and dissolution as detailed in Chapters 2-5. Any mathematical formulation for the mechanisms of PS formation without such a global integration would be limited in the scope of its validity and in the power to explain details. In addition, a globally and microscopically accurate model would also require the full characterization of all of the morphological features of PS in relation to all of the... 

The oxide formation according to this two-step process is consistent with the thermodynamics. In the corresponding Pourbaix diagram, stable NbO and Nb02 modifications exist between Nb and Nb205. The formation of the suboxide in the first step can not be seen in the capacitance measurements due to its conductivity. Therefore, a more anodic oxide formation potential Uox is determined from the reciprocal capacitance curve. However, the slope of the curve used for determination... 

Cathodic stripping voltammetry has proved suitable for a number of organic compounds, including drugs and pesticides. These in general contain sulphur and again the deposition step involves anodic (oxidation) formation of an insoluble mercury salt. Clearly this is possible only with a mercury electrode. 

Eq.of partial react. Type part, react. Ions im oh. Rate det Descript, cf next spaces- Anodic oxide format. Catiiodic total reduction Diff. an. oxid./diff. cath. reduction Passive corr. (stead, state) Chem. diss. 

A good insight into the anodic oxide formation is gained from potentiostatic pulse measurements. Figure 19 shows current transients i t) of anodic oxide formation on aluminum at pH = 6.0. Various potential steps from 0 V (hess) were chosen to an oxide formation potential between 3.3 and 5.9 V [77]. This corresponds to an increase in field strength from 6.6 to 10.1 MVcm . The initial film thickness of 7.4 nm is given by a prepolarization to 3V (hess). Each experiment must be performed on a different sample with respect to the irreversible... 

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