high energy ball milling process for nanomaterial synthesis
It is a ball milling process where a powder mixture placed
in the ball mill is subjected to high-energy collision from
the balls. This process was developed by Benjamin and
his coworkers at the International Nickel Company in the
late of 1960. It was found that this method, termed
mechanical alloying, could successfully produce fine, uniform
dispersions of oxide particles (Al2O3, Y2O3, ThO2) in
nickel-base superalloys that could not be made by more
conventional powder metallurgy methods. Their
innovation has changed the traditional method in which production
of materials is carried out by high temperature synthesis.
Besides materials synthesis, high-energy ball milling
is a way of modifying the conditions in which chemical
reactions usually take place either by changing the reactivity
of as-milled solids (mechanical activation
increasing reaction rates, lowering reaction temperature of
the ground powders)or by inducing chemical reactions
during milling (mechanochemistry). It is, furthermore,
a way of inducing phase transformations in starting
powders whose particles have all the same chemical composition: amorphization or polymorphic transformations of
compounds, disordering of ordered alloys, etc.
The alloying process can be carried out using different
apparatus, namely, attritor, planetary mill or a horizontal
ball mill. However, the principles of these operations are
same for all the techniques. Since the powders are cold
welded and fractured during mechanical alloying, it is critical
to establish a balance between the two processes in order
to alloy successfully. Planetary ball mill is a most frequently
used system for mechanical alloying since only a very small
amount of powder is required. Therefore, the system is particularly
suitable for research purpose in the laboratory. The
ball mill system consists of one turn disc (turn table) and two
or four bowls. The turn disc rotates in one direction while
the bowls rotate in the opposite direction. The centrifugal
forces, created by the rotation of the bowl around its own
axis together with the rotation of the turn disc, are applied
to the powder mixture and milling balls in the bowl. The
powder mixture is fractured and cold welded under high energy
The figure below shows the motions of the balls and the powder.
Since the rotation directions of the bowl and turn disc are opposite, the centrifugal forces are alternately synchronized.
Thus friction resulted from the hardened milling balls and
the powder mixture being ground alternately rolling on the
inner wall of the bowl and striking the opposite wall. The
impact energy of the milling balls in the normal direction
attains a value of up to 40 times higher than that due to
gravitational acceleration. Hence, the planetary ball mill can
be used for high-speed milling.
During the high-energy ball milling process, the powder
particles are subjected to high energetic impact. Microstructurally, the
mechanical alloying process can be divided into four stages: (a) initial
stage, (b) intermediate stage, (c) final stage, and (d) completion
(a) At the initial stage of ball milling, the powder particles
are flattened by the compressive forces due to the collision of the balls. Micro-forging leads to changes
in the shapes of individual particles, or cluster of particles
being impacted repeatedly by the milling balls
with high kinetic energy. However, such deformation
of the powders shows no net change in mass.
(b) At the intermediate stage of the mechanical alloying
process, significant changes occur in comparison with
those in the initial stage. Cold welding is now significant.
The intimate mixture of the powder constituents
decreases the diffusion distance to the micrometer
range. Fracturing and cold welding are the dominant
milling processes at this stage. Although some dissolution
may take place, the chemical composition of
the alloyed powder is still not homogeneous.
(c) At the final stage of the mechanical alloying process,
considerable refinement and reduction in particle size
is evident. The microstructure of the particle also
appears to be more homogenous in microscopic scale
than those at the initial and intermediate stages. True
alloys may have already been formed.
(d) At the completion stage of the mechanical alloying
process, the powder particles possess an
extremely deformed metastable structure. At this
stage, the lamellae are no longer resolvable by optical
microscopy. Further mechanical alloying beyond this
stage cannot physically improve the dispersoid distribution.
Real alloy with composition similar to the
starting constituents is thus formed.
Theoretical considerations and explorations of planetary
milling process have been broadly studied in order to better
understand and inteprate the concept. Joisels work is
the first report to study the shock kinematics of a satellite
milling machine. This work focused on the determination
of the milling parameters that were optimized for
shock energy. The various parameters were determined geometrically
and the theoretical predictions were examined
experimentally using a specifically designed planetary mill. Schilz et al. reported that from a macroscopical
point of view, the geometry of the mill and the ratio
of angular velocities of the planetary to the system wheel
played crucial roles in the milling performance. For a particular
ductile-brittle MgSi system, the milling efficiency of
the planetary ball was found to be heavily influenced by the ratio of the
angular velocity of the planetary wheel to that of the system wheel as
well as the amount of sample load. Mio et al. studied the effect of rotational direction
and rotation-to-revolution speed ratio in planetary ball
milling. Some more theoretical issues and kinematic modeling
of the planetary ball mill were reported later in related
references. Because mechanical alloying of materials
are complex processes which depend on many factors,
for instance on physical and chemical parameters such as
the precise dynamical conditions, temperature, nature of the
grinding atmosphere, chemical composition of the powder
mixtures, chemical nature of the grinding tools, etc., some
theoretical problems, like predicting nonequilibrium phase transitions under
milling, are still in debate.
For all nanocrystalline materials prepared by high-energy
ball milling synthesis route, surface and interface contamination
is a major concern. In particular, mechanical attributed
contamination by the milling tools (Fe or WC) as well as
ambient gas (trace impurities such as O2, N2 in rare gases)
can be problems for high-energy ball milling. However,
using optimized milling speed and milling time may effectively
reduce the contamination. Moreover, ductile materials
can form a thin coating layer on the milling tools that
reduces contamination tremendously. Atmospheric contamination
can be minimized or eliminated by sealing the vial
with a flexible O ring after the powder has been loaded
in an inert gas glove box. Small experimental ball mills can
also be completely enclosed in an inert gas glove box. As a
consequence, the contamination with Fe-based wear debris
can be reduced to less than 12 at.% and oxygen and nitrogen
contamination to less than 300 ppm. Besides the
contamination, long processing time, no control on particle
morphology, agglomerates, and residual strain in the crystallized
phase are the other disadvantages of high-energy ball
Notwithstanding the drawbacks, high-energy ball milling
process has attracted much attention and inspired numerous
research interests because of its promising results,
various applications and potential scientific values. The synthesis
of nanostructured metal oxides for gas detection is
one of the most promising applications of high-energy ball
milling. Some significant works have been reported in recent
years. Jiang et al.
a-Fe2O3MO2 (M: Ti and Sn) solid
solutions by high-energy milling for C2H5OH detection.
The 85 mol%
a-Fe2O3SnO2 sample milled for 110 hours
showed the highest sensitivity among all the samples studied.
The best sensitivity to 1000 ppm C2H5OH in air at an
operating temperature of 250 C was about 20. Zhang et al. synthesized FeSbO4 for LPG detection. They found
that there were two-step solid-state reactions occurring in
the raw powders during the ball milling:
The response and recovery times of their sensor were less
than one second. The sensitivity to 1000 ppm C2H5OH at
an operating temperature of 375 C was about 45. Diguez et al. employed precipitation method
to prepare nanocrystalline SnO2 and planetary milling to
grind the obtained powder for NO2 detection. They found
that the grinding procedure of the precursor and/or of the
oxide had critical effect on the resistance in air. As a result,
the gas sensing properties to NO2 had been considerably
improved. Cukrov et al. and Kersen et al. synthesized
SnO2 powders by mechanochemical processing for
O2 and H2S sensing applications, respectively. The O2 sensor
exhibited stable, repeatable and reproducible electrical
response to O2. More recently, Yamazoes group
reported the sensing properties of SnO2Co3O4 composites
to CO and H2. A series of SnO2Co3O4 thick films
containing 0100% Co3O4 in mass were prepared from the
component oxides through mixing by ball-milling for 24 h,
screen-printing and sintering at 700 C for 3 h. The composite
films were found to exhibit n- or p-type response to CO
and H2 depending on the Co3O4 contents in the composites.
The n-type response was exhibited at 200 C or above
by SnO2-rich composites (Co3O4 content up to 5 mass%).
The sensor response to both CO and H2 was significantly
enhanced by the addition of small amounts of Co3O4 to
SnO2, and the response at 250 C achieved a sharp maximum
at 1 mass% Co3O4. The p-type response was obtained at
200 C or below by the composites containing 25100 mass%
Co3O4. The sensitivity as well as selectivity to CO over H2
could thus be increased by the addition of SnO2 to Co3O4.
Besides the above mentioned researches, significant
efforts on the synthesis of nanostructured metal oxide with
high-energy ball milling method for gas sensing have been
actively pursued by the authors of this chapter. In our research, we use the
high-energy ball milling technique to synthesize various nanometer
powders with an average particle size down to several nm, including nano-sized
a-Fe2O3 based solid
solutions mixed with varied mole percentages of SnO2, ZrO2
and TiO2 separately for ethanol gas sensing application,
stabilized ZrO2 based and TiO2 based solid solutions mixed
with different mole percentages of
a-Fe2O3 and synthesized
SrTiO3 for oxygen gas sensing. The synthesized powders
were characterized with XRD, TEM, SEM, XPS, and DTA.
Their sensing properties were systematically investigated and
sensing mechanisms were explored and discussed as well.
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