Biomedical Engineering Reference
In-Depth Information
8.6
Example of cross-sectional HRTEM analysis of alumina-coated iron
nanoparticles (Hakim et al., 2007). Reproduced by permission of IOP
Publishing.
temperature-dependent oxidation behavior of uncoated Fe nanopowder and
the oxidation resistance of Fe nanopowder coated by 30 and 50 cycles of
Al
2
O
3
ALD is shown in Fig. 8.7. It is clear that the Fe nanopowder began to
oxidize almost instantaneously in air, and resulted in a mass gain of 20% at
a temperature of 250
6 nm film thickness), the
onset of oxidation was delayed until a temperature of approximately 350
8
C. For the 30 cycle film (
~
C.
The 10 nm film (50 cycles) showed no signs of oxidation when held at a
temperature of 400
8
C for over three hours. The resulting elemental analyses
from these experiments showed that the oxygen content of the 50 cycle
material matched the anticipated content for Al
2
O
3
coated on iron metal,
not an oxidized form. Similar results have been obtained for Al
2
O
3
ALD
films coated on the other core metals produced from the decomposition of
their respective oxalates. However, at much higher temperatures, the
mismatch between thermal expansion coefficients of the core and shell is too
great and the passivating layers may be fractured. This problem can be
overcome by depositing alternating multilayers of materials to perform
thermal expansion matching between the composite film and the core
particle. The magnetic moment of the micron-sized iron spheres coated by
various thicknesses of Al
2
O
3
remained unchanged, which was anticipated
due to the extreme thinness of each shell (Hakim et al., 2007).
8
