Biomedical Engineering Reference
In-Depth Information
from further oxidation. In this way, one can obtain a self-stabilized
oxide-metal structure. Very similar behavior was observed for
the microcrystalline Co thin ilms oxidized under atmospheric
conditions. From the peak-to-peak amplitude, we have calculated
a maximum atomic concentration of oxygen inside the sample as
2 at.%. The concentration of carbon impurities inside the sample
was below 0.5 at.%.
Figure 4.6b shows the element speciic Auger intensities of the
nanocrystalline LaNi 4.2 Al 0.8 sample as a function of the sputtering
time, converted to depth. Similarly to the results obtained for the
microcrystalline sample, there is relatively high concentration
of carbon and oxygen immediately on the surface. The carbon
concentration strongly decreases towards the interior of the
sample. We have found that at the oxide-metal interface, only iron
impurities and lanthanum atoms are present. As the escape-depth
of the Auger electrons from nickel and aluminum atoms is about
2 nm, we conclude that these elements are practically not present
on the metallic surface. In other words, lanthanum atoms and iron
impurities strongly segregate to the surface and form an oxide layer
under atmospheric conditions. The lower lying Ni atoms form a
metallic subsurface layer and are responsible for the observed high
hydrogenation rate in accordance with earlier indings. The above
segregation process is stronger compared to that observed for the
microcrystalline sample. The presence of the signiicant amount of
iron atoms in the surface layer of the nanocrystalline LaNi 4.2 Al 0.8
alloy could be explained by Fe impurities trapped in the MA powders
from erosion of the milling media. The amount of the Fe impurities
considerably decreases in the subsurface layer of the sample. From
the peak-to-peak amplitude we have estimated a maximum atomic
concentration of oxygen as ~2 at.% in the interior of the sample.
Similarly to the results reported for the polycrystalline LaNi 4.2 Al 0.8
alloy, the concentration of carbon impurities inside the sample was
below 0.5 at.%.
Concluding, the contamination with Fe based wear debris
in mechanically synthesized nanomaterials can generally be
reduced to less than 1-2 % and oxygen contamination to less than
300 ppm. But, contamination through the milling atmosphere can
have a positive impact on the milling conditions if one wants to
prepare metal or ceramic nanocomposites with one of the metallic
elements being chemically highly reactive with the gas (or luid)
environment. On the other side, main advantage of top-down
approach can be high production rates of nano powders.
 
Search WWH ::




Custom Search