Biomedical Engineering Reference
In-Depth Information
In stage 3, the nanopores continue to grow into the oxide layer
and there is an abrupt transition from nanopores to nanotubes,
so a bi-layered structure is visible [13]. The top and bottom layers
consist of the nanopores and the nanotubes, respectively. In
this stage, nanotubes grow deeper into the substrate due to the
competition of the oxide growth and dissolution at the bottom of the
nanotube. As nanotubes grow, the nanoporous layer is chemically
dissolved, resulting in thinning or disappearance. In Crawford
and Chawla's experiments, the nanoporous layer disappears after
90 min of anodization [13]. In stage 4, once the nanoporous layer has
undergone complete dissolution, the coating consists of an ordered
array of TiO
2
nanotubes. With anodization time, the nanotubes
continue to grow in length (Fig. 9.40), and the current density
continues to drop, until the both current density and coating
thickness stabilize. Evolution of four distinct layers within TiO
2
,
(barrier layer, nanoporous layer, TiO
2
nanotube layer, and precipitate
layer) measured by Crawford and Chawla [13] is shown in
Fig. 9.40. They have observed a thick, relatively dense precipitate
layer on the surface of the nanotube coating (Fig. 9.41). The
pre-cipitate layer grows in thickness rapidly between 2 and 4 h
(Fig. 9.40). The presence of this precipitate layer is likely associated
with the rapid dissolution of Ti. For samples anodized at more acidic
pH <5.0 they did not observe a precipitate layer [13]. For longer
etching times (20 h), the layer disappears.
Figure 9.40
Layer thickness versus anodization time for the barrier layer,
nanoporous layer, precipitate layer, and nanotube layer [13].
