480
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
The sample purity is the first but by far not the only criterion for regular pore
formation. Other factors of importance are the residual stress and dislocation density
in the specimens to be anodized [17]. Since unrelaxed stress can accumulate in the
sample during either the rolling of the aluminium foils or the in-house cutting process
prior to the anodization, a relaxation heat treatment is recommended (500 °C was
applied in several works). Here, a bifurcation occurs between bulk Al samples and
thin layers. Bulk samples contain relatively large crystals with diameter much above
the micrometre range, and the relaxation heat treatment affects the long-range pore
regularity but not the pore size and the porosity. However, the grain size of as-received
thin sputtered layers is comparable to the layer thickness, and one ultrasmall grain
may accommodate not more than one single pore. This leads to an apparent layer
thickness dependence of the pore diameter, although the real determining factor is
the grain size [18]. Annealing thin Al layers leads to grain coarsening, which also
leads to a change in pore size and porosity upon anodization.
Concerning the surface quality, the smaller the surface roughness of the
aluminium, the more regular pore formation can be expected. Mechanical polishing
of the surface may be necessary if the high-purity aluminium sheet purchased is too
rough, but electropolishing is practically an indispensable preliminary step prior
to the anodization. Besides diminishing the surface roughness, electropolishing
removes the surface layer that may be deformed during the preliminary mechanical
steps. The most common electropolishing solution contains ethanol and perchloric
acid (with 4…8:1 mixing ratio), and the electropolishing time at a fixed current
density is about 5 min.
13.3.2 Anodizing Cells: Cell Arrangement, Electrodes,
Thermal Management
Laboratory cells used for aluminium anodization are exclusively two-electrode cells.
The counter electrode is most often platinum (or a Pt–Ir alloy). Occasionally, inert
electrodes made of other materials such as carbon or lead are also applied, even
though the overvoltage of the hydrogen evolution on these electrodes can be relatively
large. In the big majority of the papers, the anodization is carried out at a preset cell
voltage, and this value is reported as the key electrical parameter of the process.
Working electrode potential is neither measured nor reported. Although the total cell
voltage includes both the ohmic drop across the cell and the cathode overvoltage,
the latter terms do not account for more than ±1 V difference in the overall cell
voltage that is in the 20–300 V range, depending on the electrolyte solution. The
major voltage drop is across the thin barrier layer of the pore bottom with a local
electric field of about 2 × 10
7 V cm
−1 ; hence, the electric field elsewhere in the cell
is small and the voltage drops can be practically neglected.
In the cells for PAA preparation, the aluminium anode is fixed to the bottom of the
cell and the cathode is in a facing position. The solution is chilled and stirred in order
13 Preparation of Nanoporous Oxides from Metals by Electrochemical …
The sample purity is the first but by far not the only criterion for regular pore
formation. Other factors of importance are the residual stress and dislocation density
in the specimens to be anodized [17]. Since unrelaxed stress can accumulate in the
sample during either the rolling of the aluminium foils or the in-house cutting process
prior to the anodization, a relaxation heat treatment is recommended (500 °C was
applied in several works). Here, a bifurcation occurs between bulk Al samples and
thin layers. Bulk samples contain relatively large crystals with diameter much above
the micrometre range, and the relaxation heat treatment affects the long-range pore
regularity but not the pore size and the porosity. However, the grain size of as-received
thin sputtered layers is comparable to the layer thickness, and one ultrasmall grain
may accommodate not more than one single pore. This leads to an apparent layer
thickness dependence of the pore diameter, although the real determining factor is
the grain size [18]. Annealing thin Al layers leads to grain coarsening, which also
leads to a change in pore size and porosity upon anodization.
Concerning the surface quality, the smaller the surface roughness of the
aluminium, the more regular pore formation can be expected. Mechanical polishing
of the surface may be necessary if the high-purity aluminium sheet purchased is too
rough, but electropolishing is practically an indispensable preliminary step prior
to the anodization. Besides diminishing the surface roughness, electropolishing
removes the surface layer that may be deformed during the preliminary mechanical
steps. The most common electropolishing solution contains ethanol and perchloric
acid (with 4…8:1 mixing ratio), and the electropolishing time at a fixed current
density is about 5 min.
13.3.2 Anodizing Cells: Cell Arrangement, Electrodes,
Thermal Management
Laboratory cells used for aluminium anodization are exclusively two-electrode cells.
The counter electrode is most often platinum (or a Pt–Ir alloy). Occasionally, inert
electrodes made of other materials such as carbon or lead are also applied, even
though the overvoltage of the hydrogen evolution on these electrodes can be relatively
large. In the big majority of the papers, the anodization is carried out at a preset cell
voltage, and this value is reported as the key electrical parameter of the process.
Working electrode potential is neither measured nor reported. Although the total cell
voltage includes both the ohmic drop across the cell and the cathode overvoltage,
the latter terms do not account for more than ±1 V difference in the overall cell
voltage that is in the 20–300 V range, depending on the electrolyte solution. The
major voltage drop is across the thin barrier layer of the pore bottom with a local
electric field of about 2 × 10
7 V cm
−1 ; hence, the electric field elsewhere in the cell
is small and the voltage drops can be practically neglected.
In the cells for PAA preparation, the aluminium anode is fixed to the bottom of the
cell and the cathode is in a facing position. The solution is chilled and stirred in order