494
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
system), but it had the advantage of faster anodization, leading to an about fivefold
increase in current between 0 and 20 °C. For providing the sufficient heat absorbance,
pulse anodization was recommended [117] in which the pore density and barrier layer
thickness correspond to the on-time voltage but the heat absorbance takes place also
during the off-time. When the anodization was performed at higher temperature,
the increase in acid concentration proved to be beneficial since it decreased the
barrier layer thickness and prevented the breakdown of the stable anodization process
even without a rigorous temperature control and agitation [133]. While the interpore
distance decreased with the increase in the acid concentration (in accord with the
reduced barrier layer thickness) [134], the interpore distance was invariant to the
temperature change [134, 135].
13.6.5 New Components of the Anodization Baths
Anodizing solutions with various acids—In the standard anodization processes elaborated between 1995 and 2000, three acid solutions were employed (see Table 13.1).
Since then, the ever-lasting pursuit to extend the well-established ranges of both
the pore diameter and the interpore distance resulted in novel processes that had
not been considered to be feasible earlier. A summary of the new bath formulations
with a single key component that yielded nanoporous alumina samples with mild
anodization is listed in Table 13.2.
For the solutions listed in Table 13.2, the proportionality of the interpore distance
and the anodization voltage is nearly the same as for the conventional anodizing solutions (sulphuric, oxalic and phosphoric acids). However, for many of these baths, the
pore array is significantly more disordered than in the well-established conventional
processes. It is noteworthy that typically multiprotic acids lead to the formation of
porous alumina.
Anodizing with mixed solutions of several acids—The conventional mild anodization yields PAA samples discrete intervals in the pore diameter (and correspondingly,
discrete intervals for both the interpore distance and the anodization voltage). In order
to overcome the limitation originating from the hardly tuneable pore diameter, various
successful attempts have been reported for aluminium anodization in acid mixtures.
The application of the mixtures of acids that yield nearly overlapping anodization
regimes (like sulphuric and oxalic acids) is straightforward [156, 157]. However,
such acid mixtures also proved to be applicable whose components alone would
yield faraway pore diameters [158–161]. The variation in the acid concentration
ratio opens the way for a nearly continuous pore diameter variation.
Anodizing with solutions containing alcohols—Various bath formulations have
been developed on the basis of classical anodization bath with various volume ratios
of alcohols as co-solvent [128, 162–169]. The list of the alcohols tested as co-solvent
includes methanol, ethanol, ethylene glycol and glycerol. Their application allows
one to perform anodization at temperatures formerly not available, both much below
0 °C and above 100 °C. Alcohols were found to stabilize the hard anodization process
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
system), but it had the advantage of faster anodization, leading to an about fivefold
increase in current between 0 and 20 °C. For providing the sufficient heat absorbance,
pulse anodization was recommended [117] in which the pore density and barrier layer
thickness correspond to the on-time voltage but the heat absorbance takes place also
during the off-time. When the anodization was performed at higher temperature,
the increase in acid concentration proved to be beneficial since it decreased the
barrier layer thickness and prevented the breakdown of the stable anodization process
even without a rigorous temperature control and agitation [133]. While the interpore
distance decreased with the increase in the acid concentration (in accord with the
reduced barrier layer thickness) [134], the interpore distance was invariant to the
temperature change [134, 135].
13.6.5 New Components of the Anodization Baths
Anodizing solutions with various acids—In the standard anodization processes elaborated between 1995 and 2000, three acid solutions were employed (see Table 13.1).
Since then, the ever-lasting pursuit to extend the well-established ranges of both
the pore diameter and the interpore distance resulted in novel processes that had
not been considered to be feasible earlier. A summary of the new bath formulations
with a single key component that yielded nanoporous alumina samples with mild
anodization is listed in Table 13.2.
For the solutions listed in Table 13.2, the proportionality of the interpore distance
and the anodization voltage is nearly the same as for the conventional anodizing solutions (sulphuric, oxalic and phosphoric acids). However, for many of these baths, the
pore array is significantly more disordered than in the well-established conventional
processes. It is noteworthy that typically multiprotic acids lead to the formation of
porous alumina.
Anodizing with mixed solutions of several acids—The conventional mild anodization yields PAA samples discrete intervals in the pore diameter (and correspondingly,
discrete intervals for both the interpore distance and the anodization voltage). In order
to overcome the limitation originating from the hardly tuneable pore diameter, various
successful attempts have been reported for aluminium anodization in acid mixtures.
The application of the mixtures of acids that yield nearly overlapping anodization
regimes (like sulphuric and oxalic acids) is straightforward [156, 157]. However,
such acid mixtures also proved to be applicable whose components alone would
yield faraway pore diameters [158–161]. The variation in the acid concentration
ratio opens the way for a nearly continuous pore diameter variation.
Anodizing with solutions containing alcohols—Various bath formulations have
been developed on the basis of classical anodization bath with various volume ratios
of alcohols as co-solvent [128, 162–169]. The list of the alcohols tested as co-solvent
includes methanol, ethanol, ethylene glycol and glycerol. Their application allows
one to perform anodization at temperatures formerly not available, both much below
0 °C and above 100 °C. Alcohols were found to stabilize the hard anodization process