Biomedical Engineering Reference
In-Depth Information
Complete porous oxide morphology is observed after 200 s
anodization. Extending time of anodization to 1000 s results in
separation of individual tubes from the nanopores and the outer
oxide layer is estimated to be about 150 nm thick. A clear self-
ordered nanotubular structure appears after 3000 s of anodization.
The thickness of the oxide layer was about 500 nm with about
100-120 nm outer diameter tubes.
In the acidic electrolyte solutions containing HF, the strong
dissolubility of hydroluoric acid limits the nanotube length to a
maximum of about 500 nm [58].
Raja
et al
. [74] tried to explain mechanism of self-ordered
nanotubular oxide formation. The sequence of nanotubular oxide
layer formation during anodization is described as follows:
(i) Formation of a passive inner barrier-type ilm during the irst
few seconds of anodization
(ii) Thickening of barrier layer and subsequent microissuring
(formation of easy paths)
(iii) Secondary oxide nucleation through these easy paths and
pore nucleation
(iv) Coverage of the secondary oxide on the entire surface and
growth of pores
(v) Pore separation to form individual and self-ordered
nanotubes.
Pore separation leading to formation of individual nanotubes
occurs under speciied environmental condition that results in
higher anodic current density. In acidic solutions, the current
density is higher and steady, leading to nanotubular structure
(Fig. 9.45a,b). When Ti
4+
ions dissolve into solution, cation
vacancies are generated [74]. These cation vacancies migrate
along the electric ield, reaching the metal/oxide interface. If the
vacancies are not annihilated, they condense to form voids. When
dissolution in acidic solution proceeds fast, the density of cation
vacancies is very high. Dissolution occurs mainly near the inner wall
of the pores containing adsorbed anions, but in this case, the ield
strength may not be as high as that in the barrier layer. Therefore,
Raja [74] considered vacancy transportation in the radial
direction, which results in rows of vacancies reaching centers
of the interconnected porewalls from the two neighbor pores
(Fig. 9.45c). If the charges have the same sign, they repel and keep
an equilibrium distance. To maintain electrical neutrality, oxygen
vacancies are generated. If the dissolution rate is much higher than
