Biomedical Engineering Reference
In-Depth Information
The cleanliness of the surface of nanocrystalline alloys prepared
by mechanical alloying method was studied by X-ray photoelectron
spectroscopy (XPS) and Auger electron spectroscopy (AES) [24].
In Fig. 4.6, we show 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. Data for microcrystalline LaNi
4.2
Al
0.8
sample are included, too.
Figure 4.6
AES spectrum of microcrystalline (a) and nanocrystalline (b)
LaNi
4.2
Al
0.8
alloy vs. sputtering time, as converted to depth. The
sample surface is located on the left-hand side.
As it can be seen, there is relatively high concentration of carbon
and oxygen immediately on the surface, which could be due to
carbonates or adsorbed atmospheric CO
2
. The carbon concentration
strongly decreases towards the interior of the sample. At the metal
interface itself, only oxygen is present, making it very likely that only
an oxide layer is formed, and no other compounds, the latter growing
apparently with a smaller probability. We have found that at the
oxide-metal interface, mainly lanthanum and nickel atoms are
present. Taking into account that the escape-depth of the Auger
electron from nickel and aluminum atoms is about 2 nm, the
concentration of these elements on the metallic surface is signiicantly
lower compared to the average bulk composition. Therefore, the
lanthanum atoms, which segregate to the surface form a La based
oxide layer under atmospheric conditions. The oxidation process is
depth limited such that an oxide-covering layer with a well-deined
thickness is formed by which the lower lying metal is prevented
