492
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
nanochannels can also be induced with high-voltage anodic spikes during an otherwise stable steady-state anodization process [106]. When the voltage during the
spikes favours a pore diameter larger than the stable pore diameter plus the pore wall
thickness, the walls can be partly destroyed. However, the length of such spikes has
to be short enough so that the entire coating is not burnt (i.e., not the entire pore wall
is detached from the substrate).
13.6 Pursuits Related to the Extension of the Anodizing
Conditions
13.6.1 Anodization of Commercial Aluminium Samples
In the recipes of the preparation of highly ordered PAA samples, the advice
concerning the purity of the starting material tells dogmatically that the material
of highest possible purity has to be chosen. However, there are various applications
such as adsorbents, drug carriers and protective coatings, in which the pore regularity is of no importance, unlike the price of the starting material. Therefore, it is a
persistent pursuit to develop suitable anodization procedures for aluminium samples
of commercial quality. Grade AA1050 is of particular interest (composition: Al >
99.5%; Si, Fe < 0.25%; other metals < 0.05%) [107–112].
The general trends in the anodization of commercial grade aluminium can be
summarized as follows. The pore regularity is much lower than for high-purity
starting materials, even though a proper surface pretreatment can improve it significantly [107, 111]. Under the same anodization conditions, the pores are usually
somewhat wider [110]. As it was shown for pure aluminium, the pore diameter
increases with anodization voltage of commercial grade Al, too. It is common that
three-dimensional porous alumina is formed instead of well-directed parallel pore
arrays [113, 114]. The growth rate of the PAA structure is lower than pure Al because
of its lower corrosion rate due to the impurities that cause chemical dissolution [114].
This is particularly true when the commercial-grade aluminium contains precipitates
around which pore branching and pore bending takes place, hence encapsulating the
inhomogeneity by fully consuming Al around it [109, 115].
The means of improving the anodization of commercial grade aluminium include
various modifications of the conventional processes: (i) tuning the solution composition, anodization voltage and temperature [108, 110]; (ii) optimizing the steps of the
sequential anodization and layer removal processes [107]; (iii) application of pulse
anodization, partly in order to limit the heat flux in the oxidation zone [116–118];
(iv) application of constant-current anodization [109]; (v) application of various heat
treatment steps for stress relaxation and/or sample oxidation [113]. The best results
so far were achieved with sulphuric acid solutions more concentrated than in the
conventional anodization process [108].
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
nanochannels can also be induced with high-voltage anodic spikes during an otherwise stable steady-state anodization process [106]. When the voltage during the
spikes favours a pore diameter larger than the stable pore diameter plus the pore wall
thickness, the walls can be partly destroyed. However, the length of such spikes has
to be short enough so that the entire coating is not burnt (i.e., not the entire pore wall
is detached from the substrate).
13.6 Pursuits Related to the Extension of the Anodizing
Conditions
13.6.1 Anodization of Commercial Aluminium Samples
In the recipes of the preparation of highly ordered PAA samples, the advice
concerning the purity of the starting material tells dogmatically that the material
of highest possible purity has to be chosen. However, there are various applications
such as adsorbents, drug carriers and protective coatings, in which the pore regularity is of no importance, unlike the price of the starting material. Therefore, it is a
persistent pursuit to develop suitable anodization procedures for aluminium samples
of commercial quality. Grade AA1050 is of particular interest (composition: Al >
99.5%; Si, Fe < 0.25%; other metals < 0.05%) [107–112].
The general trends in the anodization of commercial grade aluminium can be
summarized as follows. The pore regularity is much lower than for high-purity
starting materials, even though a proper surface pretreatment can improve it significantly [107, 111]. Under the same anodization conditions, the pores are usually
somewhat wider [110]. As it was shown for pure aluminium, the pore diameter
increases with anodization voltage of commercial grade Al, too. It is common that
three-dimensional porous alumina is formed instead of well-directed parallel pore
arrays [113, 114]. The growth rate of the PAA structure is lower than pure Al because
of its lower corrosion rate due to the impurities that cause chemical dissolution [114].
This is particularly true when the commercial-grade aluminium contains precipitates
around which pore branching and pore bending takes place, hence encapsulating the
inhomogeneity by fully consuming Al around it [109, 115].
The means of improving the anodization of commercial grade aluminium include
various modifications of the conventional processes: (i) tuning the solution composition, anodization voltage and temperature [108, 110]; (ii) optimizing the steps of the
sequential anodization and layer removal processes [107]; (iii) application of pulse
anodization, partly in order to limit the heat flux in the oxidation zone [116–118];
(iv) application of constant-current anodization [109]; (v) application of various heat
treatment steps for stress relaxation and/or sample oxidation [113]. The best results
so far were achieved with sulphuric acid solutions more concentrated than in the
conventional anodization process [108].