MECHANICAL ALLOYING OF Cu-Al2O3 NANOPARTICLES
K. Borodianskiy1,2, A. Basov1,2, A. Gedanken2, M. Zinigrad1
1
Natural Science Faculty, Ariel University Center of Samaria, Ariel, Israel
2
Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel
Abstract. Nano-sized materials formation by mechanical alloying (MA) is the interest
of this article. Results of Mechanical Alloying treatment of Al and CuO are presented.
Reduction of CuO by aluminum is very exothermic reaction where Cu9Al4 phase
forms first and it is the possibility of explosion during the reaction. To avoid Cu9Al4
phase formation and explosion possibility, pre-milling of CuO was prepared. This
pre-milled oxide was milled with Al for different times and after 15 h of milling, Cu
and Al2O3 nanoparticles with an average size of 125 nm are formed.
INTRODUCTION
Mechanical alloying has been one of the novel non-equilibrium methods of a solid
state powder processing technique involving repeated deformation, welding and
fracturing of powder particles. This technique was developed in the middle of 1960s
by John S. Benjamin at the Paul D. Mercia research laboratory of the International
Nickel Company to produce nickel-based oxide dispersion strengthening (ODS)
superalloys. This technique is an alternative for the traditional method where
materials production is carried out by high temperature synthesis.
The method of MA has been widely used to synthesize a various solid state
materials, such as amorphous alloys [1], quasicrystalline and nanocrystalline alloys
[2], intermetallic compounds [3], borides [4], or carbides [5].
Nanostructured materials may be formed by MA. During the process, the grain
size of materials decreases with milling time and reaches a saturation level when a
balance is established between the fracturing and cold welding events. The minimal
grain size is different depending on the material and milling conditions. This way Ye
et al [6] synthesized TiC with a grain size of 9 nm, Tavoosi et al [7] synthesized
Al2O3 which reached grain size about 40 nm. The main advantage of these synthesis
techniques are that they can be produced at relatively low temperatures and also in an
economical way through fewer steps than in the conventional way.
Nano-sized structures can be synthesized by a displacement reaction:
MeO  R  Me  RO ,
(1)
where a metal oxide (MeO) is reduced by more reactive metal (reductant R) to the
pure metal Me. The chemical reactions induced by high energy ball milling can be
either combustion or progressive reactions. The Al2O3 synthesis from pure Al and
metal oxide, ZnO [7], CuO [8] or Cu2O [9] are highly exothermic and the self
propagating reactions can be avoided by reducing the activities of reactants. This can
be achieved by intermetallic compounds formation.
Aluminum oxide can be synthesized by aluminothermic reaction where Al uses as
a reducing element at a high temperature. However, MA process is a simple and easy
process to achieve homogeneous nano-sized oxide.
In this study, we demonstrate the Al2O3 formation by mechanosynthesis
displacement reaction avoiding intermediate phase Cu9Al4 intermetallic compound
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which forms during the reaction. Additionally, we show the way to suppress a self
propagating combustion reaction.
EXPERIMENTAL
Copper oxide powder (Acros organics, 98%) was pre-milled in planetary ball mill
under air atmosphere for 5 h at 450 rpm. The ball-to-powder (BPR) ratio was 1:10.
Stearic acid (Sigma-Aldrich, 95%) was used as a process control agent (PCA) with
3wt.%, added to prevent powder spreading. A milling vial of chromium hardened
steel was of 125 ml.
Pre-milled CuO and Al (Metallisation, 99%+) with a molar ratio of 1:1 with
3wt.% of stearic acid which used as a PCA were milled in planetary ball mill at 500
rpm under Ar atmosphere. Vials and balls are made of chromium hardened steel, and
the volume of the vial was 125 ml, which contains 12 balls (10 mm in diameter). The
BPR was about 10:1. The milling process was interrupted at different times, i.e., 0, 5,
10, 15 h, and small amount of powders were removed from the vial for
characterization.
XRD measurements (X-ray powder diffractometer, D8 Bruker, 40kV, 40mA) with
Cu Kα radiation (λ=0.154 nm) was used to analyze the microstructure of as-milled
samples. The XRD patterns were recorded in the 2Θ range of 20-80º (step size 0.05º
and time per step 3s). The microstructure of powder particles was investigated by
scanning electron microscopy (SEM, FEI Inspect S).
RESULTS AND DISCUSSION
In this research we obtained Cu and Al2O3 nanoparticles using mechanosynthesis
displacement reaction between a pure Al and CuO powders. This red-ox reaction is
very high exothermic reactions:
3CuO  2 Al  3Cu  Al2O3 ,
(2)
H  1210kJ / molAl2O3 ,
where Cu9Al4 phase forms first. To avoid this phase formation we suggested CuO premilling where CuO becomes more reactive because of energy transformed to the
powder in this process.
Figure 1 shows XRD patterns of the CuO and Al powder mixture after different
stages of milling. The intensity of copper oxide and pure aluminum peaks reduced
within the first 5 h of the process. Pure copper phase peaks forms near 43º, 50º, and
74º. They become broader with increasing of mechanical milling time (Fig.1). As it is
known from X-ray diffraction theory, peak broadening becomes as the particle size
decreases to nano-size. Al2O3 (corundum) phase falls very close to that of Cu peak
near 35º.
Figure 2 illustrates Scanning Electron Microscopy (SEM) image of the obtained
powder. On this image we can find nanoparticles which average size was calculated
using a computer program “Image J”. According to this calculation, the nanoparticles
size was about 125 nm.
Alumina phase presence in the obtained powder was found using Energy
Dispersive X-ray analysis (EDX) and shown in Fig. 3.
As shown in Fig. 3, Al2O3 phase is present in the obtained powder. This is in
agreement with SEM image where we can find light zones which are explained by the
theory of SEM as having high charge, e.g. in ceramic materials.
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Fig. 1. XRD patterns after (a) 0 h, (b) 5 h, (c) 10 h, and (d) 15 h of milling.
Fig. 2. SEM image of obtained nanopowder.
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Fig. 3. EDX image of obtained nanopowder.
CONCLUSIONS
The red-ox synthesis by mechanical alloying on the example of reaction between
aluminum and copper oxide was conducted. As it is known from previous works, this
reaction is very exothermic and it is chance for explosion during the process. To avoid
this problem, CuO pre-milling process was prepared. The obtained pre-milled CuO
powder milled with Al and nanoparticles, with an average size of 125 nm of
aluminum oxide and copper, were obtained.
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