Formation Current Density

The anodisation factor can be considered as an indicator for the quality of the aluminium oxide films. In particular, the barrier aluminium oxide films formed with low anodisation factors exhibit high breakdown field strengths 

This replacement was necessary to adapt the equivalent circuit to the nonideal behaviour of the aluminium oxide film. The exponent n of the CPE element can be regarded as a measure of the inhomogeneity of the film structure [17]. For an ideal capacitor the exponent n is one. For the calculation of the CPE values, the fitting program in Ref [18] was used. 

The thickness of the initial aluminium oxide film is about 1.4 nm. 

For all aluminium oxide films the exponent n in the CPE was nearly constant, approximately 0.99 0.04, determined for current densities between 2.5 and 8.5 rciAJcnC. This value is almost one and significantly higher than values found in the literature [15]. The n value is related to the layer inhomogeneity. The high value of n confirms the formation of homogeneous aluminium oxide films. 

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While n.. shows no dependence on doping density, current density or electrolyte concentration in the electropolishing regime, it does in the PS regime [Le23, Fr6]. enerally n., increases with current density. This is shown for the mesoporous rein Fig. 6.9 a, and microporous regime in Fig. 4.6. From the data of the latter gure the dependence of n.. on formation current density J (in mA cm4) in etha-HF can be fitted to ... 

In general the porosity of PS layers increases with increasing formation current density, as shown for micro PS in Fig. 6.8 and for meso PS in Fig. 6.9 c. The porosity decreases with increasing HF concentration of the electrolyte [Un3, Be5]. The porosity is sensitive to the type of pores and therefore on substrate doping density, as shown in Fig. 6.9 c. For a given electrolyte composition, current density and tempera-... 

The broadening of the characteristic peaks of the silicon XRD signal provides information about stress and size of the crystallites. Figure 7.4 shows the diffraction pattern of microporous silicon powders scraped from p-type Si electrodes and of a bulk silicon powder sample. The peak broadening increases with increasing formation current density. For low formation current densities a superposition of... 

The formation of luminescent PS requires HF to be present in the electrolyte, while the presence of water is not essential [Pr7]. The intensity as well as the peak energy of the PL emission increases with the PS formation current density J for a fixed electrolyte concentration. If various electrolytes are compared, the ratio between formation current density J and the critical current density JPS is more relevant than the absolute value of J. Because the porosity itself depends on J/JPS, in many studies... 

For very high doping densities and large formation current densities, the pore dimensions approach the macroporous regime, as shown in the upper right of Fig. 8.3. In this regime the pore diameter depends approximately exponentially on current density. For p-type substrates of 1 mfi cm anodized in ethanoic F1F at 600 mA cnT2, pore diameters of 1 pm and porosities above 90% have been observed [Ja4]. 

The structural features of micro- and mesoporous silicon are much smaller than the wavelength of visible light, and so these materials may be treated according to the EMA. The dependence of the porosity of micro- and mesoporous silicon on formation current density and substrate doping density can be used to generate layers with a single or a periodic change in the dielectric constant... 

Upper right Electroluminescence from a micro PS film anodized in an O-ring cell viewed from the top (10% acetic acid, 10 mAcn-r2, 2.6 cm2 active area). Note that the luminescence appears orange in the center line, where the film has been formed under high current density (in 1 1 ethanoic HF at 200 mA cm"2), while it appears red for low formation current density (10 mAcirf2). After [Le3],... 

The current (z) measured during the growth of the porous film can be taken as the sum of the ionic current, due to the oxidation of the metal at the metal/oxide interface, and the electronic current, 4i, due to faradaic processes occurring at the oxide/electrolyte interface. The former can be described as the sum of the formation current density, associated to oxide formation, and the dissolution current, zmetal ions into the electrolyte at the pore bottom. Then,... 

SEM was used for morphological studies of anodic aluminium oxide films, formed at various formation current densities up to a formation voltage of 60 V. The total thickness of the films was determined by cross-section SEM micrographs as shown in Figure 23.1, but it should be emphasised that one can not clearly identify barrier and porous layers of the oxide film by using this technique. 

Figure 23.2 shows the dependence of the anodisation factor on the formation current density. The anodisation factor increases from 1.1 to 2.1 nmA when the current density is increased from 0.3 mA/cm to 8.5 mA/cm. This was explained by the transformation of the amorphous oxide into crystalline oxide at high current densities [11]. 

The capacitance values of the aluminium oxide films formed at various formation current densities are shown in Figure 23.5. The capacitance CPE ) decreases from 0.19 pF/cm, for the film formed at 0.3 mA/cm, to 0.13 pF/cm, for the film formed at 2.5 mA/cm. At higher current densities, the capacitance of the films increases. The capacitance of the aluminium oxide films is much... 

The transition voltage depends on the formation current density. At formation current densities above 2.5 mA/cm, the transition voltage was greater than 60 V. Chang et al. [11] reported that the transition voltage was 150 V when the films were formed at 25 mA/cm in 0.83 M ammonium adipate solution. 

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... 

It follows from all above said that the ratio o/j8-Pb02 in PAM is determined by a complex relationship between formation current density, cured paste density and H2SO4 concentration of the formation electrolyte. 

With increase of formation current density the polarization (voltage) of the cell increases, i.e. the potentials of the two types of plates rise. They may reach values higher than the evolution potentials of the H H2 and H2OIO2 electrodes. Consequently, flie rates of O2 and H2 evolution increase, i.e. greater volumes of gas are evolved per unit time. Adsorbed gas layers form on the surface of the lead and lead dioxide active materials, which increase the ohmic resistance of the cell and its polarization, and hence, the energy consumption for battery formation increases, too. The formed gas phase should be carried out of the cell, which can also be achieved by electrolyte re-circulation. 

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