Anodisation Time

R is electrolyte resistance, andRj andR2 are resistance of barrier aluminium oxide layer and the barrier layer with defect, respectively. CPE and CPEj are constant phase elements of the barrier layer and the defected barrier layer, respectively. 

The impedance response of the films was observed with just one time constant (Ri, CPEi), except for the film formed for 10900 s. This film showed two time constants similar to the films formed at voltages 30 V. In this case, the aluminium oxide film might be contaminated by anions migrating from the surface into the film. 

During the period of the current decay, there are two competitive processes, densification to form the barrier layer and dissolution of barrier layer to form the porous layer. Under the high electrical field strength (constant voltage mode), the densification of the aluminium oxide films is favoured for process durations shorter than 3700 s. When the constant voltage was kept for a long anodisation time (beyond 10900 s), the dissolution of the aluminium oxide film becomes more dominant. Thus, the film could be contaminated by inward migration of the electrolyte into the film or by formation of micro-voids. 

---

The dependence of the aluminium oxide film properties on the anodisation time was studied. Figure 23.10 shows the current-time transients during anodisation at the formation current density of 2.5 mA/cm. In the constant current... 

The film is formed on E-200-Al/glass at a current density of 0.5 mA/cm for the anodisation time of 1800 s. The film thickness was controlled by the formation voltage of 10 V. The electrical measurements were carried out on different electrode areas ranging from 0.0025 to 0.04 cm (see Figure 23.12). Below breakdown, low leakage currents of less than 13 nA/cm at 1.67 MV/cm could be observed. The breakdown of the dielectric film occurs when the electrical field strength approaches 8 MV/cm. 

Figure 23.12 Current density (/) vs. field strength (E) of the harrier aluminium oxide film/Al(200 nm)/glass formed at a current density of 0.5 mA/cm, a formation voltage of 10 V and an anodisation time of 1800 s.

FIGURE 4 Etched depth vs. anodisation time for the etching of p-type p-SiC in HF at a constant current... 

This is a variant of the standard sulphuric acid anodising procedure where the bath temperature, anodising time and current density parameters are somewhat modified to give the densest oxide film growth of all. The hardness of the oxide film increases and the pore size is significantly reduced. 

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. 

The anodising procedures in general use are shown in Table 15.1, sulphuric acid being the most commonly used electrolyte. Treatment time is IS min to 1 h. 

The behaviour of samples under the actual conditions of service is the final criterion, but unfortunately such observations take a long time to collect and assess, and the cautious extrapolation of data from accelerated tests must be relied on for forecasting the behaviour of anodised aluminium in any new environment. 

The aluminium oxide films formed for different times (marked in Figure 23.10) were characterised by electrochemical impedance spectroscopy. For the aluminium oxide film anodised for 160 s, the open circuit potential (OCP) is not stable. This can be explained by instability of the film structure. The processes of the film formation were not yet completed. The OCP is more stable and positive for films anodised for more than 700 s. This can be explained by the formation of the compact barrier aluminium oxide layer. 

The study by Lee et al. (2013b) confirmed that the morphology and size of elec-trochemically deposited calcium phosphate crystals on anodised cp-Ti substrates were strongly affected by the deposition time and electrolyte temperature. Flakelike CaP was observed at 25 °C, but needle-like CaP was present at 85 °C. A Ca/P ratio of 1.68, close to the stoichiometric ratio for HAp was found for coatings deposited at 85 °C. Crystalline OCP and HAp phases formed in CaP layers that were not heat treated, whereas TCP formed after annealing the CaP layers at 700 °C. 

The formation of photoactive films on metal electrodes is not restricted to inorganic materials. Copper, for example, can be anodised to form polymeric phenylacetylide [33] and acetylide [34] layers that appear to behave as p-type organic semiconductors. The photoconducting properties of the arylethynyl polymers have been known for some time, although the mechanism of photoconductivity is not well understood. It seems probable that charge carriers are created by the annihilation of mobile Frenkel excitons at electron traps such as adsorbed oxygen rather than by direct interband excitation. 

Fig. 4. Current-voltage-time relationships during anodisation, demonstrating progression from mild to hard anodisation conditions (left) and mUd-hard-mild conditions (right).

---