Biomedical Engineering Reference
In-Depth Information
fluoride ion concentration.
(D) Dissolution of surface film: Simultaneous to pore growth,
the initial compact surface film is by the same chemical
process (Eq. 5.2). However, the process is slow as it is not
enhanced by the autocatalytic effect that leads to local
acidification at the bottom of the pores. However, by the
end of the anodization, it is commonly found that the film is
etched away by chemical dissolution.
(E) Conversion to straight pores: The growth direction of the
strong pore is also affected by the neighboring pore. If the
oxidation area is larger on one side than on the opposite,
due to possible overlapping areas with neighbor pores, the
oxidation and H
production are larger on one side, leading
to the bending of the pore (Fig. 5.5c). This bent growth
produces a wormlike structure. When the competition
for oxidizable area is equilibrated, the formation of the
wormlike structure finishes and all the remaining pores
grow straight until the end of anodization, thus producing
a tubular morphology.
(F) Self-organization: The mechanism of self-organization was
proposed by Yasuda et al. [39], and it is based on a natural
selection process. Deeper pores grow at the expense of
shorter ones. There is a competence between pores for
the volume available for oxidation. When shorter pores
are surrounded by deeper pores, the oxide layer grows too
thick to allow the short pore to continue growing (Fig. 5.5c).
(G) Conversion to nanotubes: Self-organization of the porous
structures could lead to hexagonal arrangements of
pores similar to those found in anodic alumina oxide
(AAO) membranes commonly used as templates in the
synthesis of nanotubes and nanobars. On the contrary, the
separation of nanotubes takes place in anodized titanium,
which converts the self-organized porous system to a TiO
2
+
nanotube arrangement. The formation of cracks between
the pores that leads to the nanotube morphology results
from the mechanical stress in the oxide tubes. In turn, the
mechanical stress results from the volume expansion when
the metal phase is converted to the oxide phase during
 
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