Biomedical Engineering Reference
In-Depth Information
nanotubes by anodization follows
several distinct phases that can be observed during the anodization
experiment directly by following the change of colors of the titanium
layer or by following the current response of the anodization cell.
For the latter, the current response of the cell as function of the time
is shown in Fig. 5.4.
After applying the high potential, the current drops quickly in
the first seconds and a compact oxide layer on Ti surface is formed
(step I, inset, Fig. 5.4). The color of the titanium layer turns violet
within this time interval. When the experiment is performed in
fluoride-free electrolytes (1 M H
The formation of the TiO
2
+ 1 M NaOH), the anodization
should stop here. The compact oxide layer that is 40-60 nm thick
now acts as a very efficient barrier layer and no longer increases in
thickness. The violet color seems to be given by a phenomenon of
light dispersion as it does not change when observed under different
angles. The dispersion of light may occur on the small defects in the
oxide layer. When the current density reaches the first minimum,
the titanium surface turns blue for several seconds. The blue color
is a marker for the beginning of the second region of anodization
(step II, inset, Fig. 5.4). At this stage the defect sites present in the
compact oxide layer are attacked and dissolved by the fluoride
(F
PO
3
4
) ions present in solution. This process represents the pores
nucleation. Then, the current density increases (2.5 min), because
the pores are soaking through the barrier oxide layer exposing the
titanium to the anodization solution. The beginning of the third
region of anodization (step III, inset, Fig. 5.4) is characterized by the
maximum of the current density and by a change in color turning
to intense green. This corresponds to the maximum pore formation
rate. As the viable pores are selected (by a mechanism that will be
discussed in a following section), TiO
2
-
nanotubes begin to form. The
length of the tubes increases throughout the whole third region.
The diffusion length of the fluoride ions and of the anodization by-
products increase as well [37]. This is probably the reason for the
slow decay of the current density value. Stage IV corresponds to the
quasi-equilibrium state in which the growth rate of the tubes equals
their dissolution rate.
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