Chemistry Reference
In-Depth Information
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Figure 10.6 Atom maps of Al (left) and Pt-17.3%Rh-14.0%Ir (at%) (right) following
oxidation in a separate environmental cell under 20 mbar of O
2
for
30 min at 723 K. The relative thickness of the respective layers demon-
strates that Al much more readily oxidizes than the Pt-based alloy.
(Reprinted from ref. 17, copyright
(2014), with permission from
Elsevier).
10.2.3.2 Reaction Cells for Catalysis Studies
For catalysis studies, the topmost atomic layers of tip samples are the region
of interest whereas most applications of APT focus on the bulk composition
of materials. For this reason, adsorption studies are only relevant if reactive
surfaces are not exposed to air contamination during transfers between gas
exposures and atom probe analyses. Bagot et al. developed an experimental
set-up in which an environmental reaction cell is mounted on an atom probe
instrument.
41
In this catalytic atom probe (CAP), the reaction cell and gas-
dosing rig were designed and integrated into the machine to allow specimens
to be exposed to various gas mixtures at pressures of up to one atmosphere
and at temperatures of up to 873 K. Sample transfers are then performed
under high vacuum (HV) conditions. Among a number of studies on segre-
gation effects produced by physico-chemical treatments (Section 10.3.2), the
method has shown its capability to map nitric oxide molecules (NO) select-
ively adsorbed at the edge of a (012) region of a tip sample after exposure
to 10 mbar NO for 15 min at 292 K in the reaction cell.
41
Furthermore, the
detector area in this atom probe is comparable with the projected size
of specific crystallographic poles, allowing exploration of how different sur-
faces may react differently to the same exposure conditions.
41,42
More recently, an integrated environmental cell has been designed and
developed for the latest generation of APT LEAP
t
instruments, allowing
controlled exposure of samples to gases at pressures up to some tens of
millibars and at temperatures currently up to
.
800 K.
17
This new design for
an environmental APT offers a new dimension for studying the influences of
high pressure and temperature as well as the effect of surface crystallography
B
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