Chemistry Reference
In-Depth Information
O
2
ZrAl
3
3/2 Al
2
O
3
(amorphous)
T
>500°C
O
2
T
>800°C
+
ZrO
22-
x
3/2 (
γ, δ,θ,)
-Al
2
O
3
+t-ZrO
2
Scheme 3.16 Milling oxidation conduced in a Spex CertiPrep Freezer/Mill 6800.
oxygen pressure was found to be significant only at a low initial O
2
pressure
range. O
2
adsorption (macroadsorption) was evident at high O
2
pressures
(Z50 kPa), but disappeared with decreasing O
2
pressure. The oxidation was
mechanochemically limited at the initial O
2
pressure range of 152.0 to
33.4 kPa. Upon a further decrease in O
2
pressure to
r
27.4 kPa, oxidation was
found to be controlled by O
2
adsorption onto the active sites of ilmenite
particle surfaces (micro-adsorption). Mechanochemical oxidation (conduced
in a vertical planetary ball mill (QM-1SP2, Nanjing, China)) occurred mainly
at active sites that present c-axis lattice strain.
Geßwein and Binder have dealt with the heterogeneous oxidation of ZrAl
3
particles in a gaseous oxidant (oxygen-containing atmosphere).
17
The oxi-
dation kinetics of ball milled ZrAl
3
powder was investigated by thermo-
gravimetry at temperatures of up to 1100 1C. The non-selective oxidation of
ZrAl
3
results in the formation of a-Al
2
O
3
and tetragonal and monoclinic
ZrO
2
. According to X-ray, TG/DSC and kinetic results, the following simpli-
fied oxidation scheme for ZrAl
3
powder is proposed (Scheme 3.16).
Ultra-high temperature ceramics (UHTCs) have recently become the focus
of widespread attention due to their critical applications in the thermal
protection systems of hypersonic aerospace vehicles. Of the UHTCs avail-
able, zirconium diboride (ZrB
2
) is a particularly highly covalent refractory
material. Oxide impurity in ZrB
2
powder promotes coarsening, resulting in
lower sinterability. Because of this effect on sinterability, Ortiz et al. (2012)
have carried out an in-depth study into ZrB
2
oxidation in high-energy ball
milling conditions. They reported that high-energy grinding in air intro-
duces twice as much oxygen into the ZrB
2
powders as the more conventional
attrition milling. Furthermore, this oxygen does not form solid-solutions
with ZrB
2
, but amorphous oxides (i.e. ZrO
2
and B
2
O
3
) that preferentially lo-
cate onto the surface of the ultrafine agglomerates that result from the cold-
welding of the primary nanoparticles that form during the ball milling.
18
Aside from the case of CO
2
hydrogenation,
19
a few observations of catalytic
reactivity induced by the application of friction under high vacuum con-
ditions are described in the literature. Sch¨th et al. have hypothesized that it
may also be possible to activate a solid catalyst in situ to make it more ef-
ficient in catalyzing the reaction between gas-phase molecules.
20
They have
reported a case of catalytic CO oxidation under in situ ball-milling conditions
(reaction conduced in a planetary ball mill (Fritsch Pulverisette 6) where the
reaction rate was increased by three orders of magnitude by the milling. In
an initial exploratory study to assess the potential of in situ milling in het-
erogeneous catalysis, a set of batch experiments using various metal oxides,
Search WWH ::
Custom Search