of the as-received NiO powders was ~3.5 m2/g. After
480 min of milling, all of the dry milled samples have
BET surface areas > 10 m2/g. In contrast, the powders
that were slurry milled in Vertrel XF and methanol
exhibit more modest BET surface area increases to
~4.9 and 4.6 m2/g, respectively. The observation of a
more rapid decrease in particle size for dry versus wet
grinding is consistent with previous work, documenting
structural changes of α-Fe2O3 as a function of milling
environment [30]. In contrast, a more rapid decrease
in particle size is observed for slurry milling of ZrO2
as opposed to dry milling [31]. This apparent
inconsistency is resolved when we consider that
the zirconia study measured particle size using
sedimentation. As such, agglomerates and primary
particles are indistinguishable. It is expected that the
dry milled samples of ZrO2 contain smaller primary
particles, but they are more heavily aggregated as
compared to the corresponding slurry milled samples.
From Table 2, it is also clear that the surface area of
the NiO dry milled with carbon is greater than NiO dry
milled with no additive. If the BET surface area of NiO
milled with carbon is estimated to be a weighted
average of the BET surface areas of the starting Printex
L carbon material (150 m2/g) and NiO milled alone,
the estimated value is 14.5 m2/g, still less than the
measured value shown in Table 2. As such, it is
probable that the carbon acts as a grinding aid for NiO
[32,33] while itself undergoing comminution during
milling.
The SEM images of the powders provide insight
into the particle size distributions of the milled
samples (Fig. 1). The unmilled powders (Fig. 1(a))
are characterized by aggregated and irregular NiO
particles, with few particles below 200 nm in size.
After milling for 480 min with no additive (Fig. 1(b)),
the extent of aggregation increases while the particle
size distribution now includes many more particles in
the 100 nm range. Milling for 480 min with carbon
(Fig. 1(c)) also increases the frequency of small