Role of process control agent on synthesis and consolidation behavior of nano-crystalline copper produced by mechano-chemical route

Article abstract of DOI:10.1016/j.jallcom.2007.10.062

Nano-crystalline copper was synthesized by mechano-chemical reduction of Cu₂O with carbon. Influence of stearic acid addition as a process control agent (PCA) on characteristics of synthesized and consolidated copper samples was investigated. The structural evolution and morphology of powders were evaluated using XRD and SEM, respectively. It was found that the nano-crystalline Cu formed after 35 h of milling and addition of stearic acid prolonged the process. On the other hand, the use of stearic acid in final stages of the process prevented excessive cold welding of ultra fine Cu particles during milling. In fact, it successfully inhibited the formation of coarse Cu particles and eventually decreased the crystallite size of the product to 19 nm. The hardness of sintered Cu sample increased due to the reduction of mean crystallite size.

Full text of DOI:10.1016/j.jallcom.2007.10.062

Journal of Alloys and Compounds 465 (2008) 78–82
Role of process control agent on synthesis and consolidation behavior of
nano-crystalline copper produced by mechano-chemical route
S. Sheibani∗, A. Ataie, S. Heshmati-Manesh
School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran
Received 11 August 2007; received in revised form 14 October 2007; accepted 16 October 2007
Available online 22 October 2007
Abstract
Nano-crystalline copper was synthesized by mechano-chemical reduction of Cu2O with carbon. Influence of stearic acid addition as a process
control agent (PCA) on characteristics of synthesized and consolidated copper samples was investigated. The structural evolution and morphology
of powders were evaluated using XRD and SEM, respectively. It was found that the nano-crystalline Cu formed after 35 h of milling and addition
of stearic acid prolonged the process. On the other hand, the use of stearic acid in final stages of the process prevented excessive cold welding of
ultra fine Cu particles during milling. In fact, it successfully inhibited the formation of coarse Cu particles and eventually decreased the crystallite
size of the product to 19 nm. The hardness of sintered Cu sample increased due to the reduction of mean crystallite size.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Nanostructured materials; Mechano-chemical processing; Microstructure; X-ray diffraction
1. Introduction
In our pervious works [6,7], synthesis of nano-crystalline
Cu through following solid-state reaction between Cu2O and
graphite using mechano-chemical route was investigated.
Mechano-chemical processing is a novel technique for prepa-
ration of nanosized materials. It has been shown that enhanced
reaction rates can be achieved and dynamically maintained dur-
ing milling as a result of microstructural refinement and mixing
processes accompanying repeated fracture, welding and defor-
mation of particles during collision events [1]. Depending on
which process is dominant during mechanical alloying, powder
particles may grow in size through agglomeration by cold weld-
ing, or become smaller in size through the fracture process. A
PCA is normally added in milling process in order to obtain a
balance between fracturing and welding processes. Stearic acid
is one of the most commonly used and effective PCA [2].
Chemical reduction of metal oxides could occur when milled
with a suitable reductant [3]. Recently, copper nanopowder with
various physical and mechanical properties has been used in
electronics industry [4,5]. The reduction of copper oxide with a
number of metallic reductants such as Fe, Al, Ti, Ni and Ca has
been already investigated [2].
2Cu2O + C → 4Cu + CO2|
(1)
Since the influence of PCA on this process has not been stud-
ied yet, the present research focuses on the influence of stearic
acid as a PCA on the displacement reaction and Cu nanopowder
formation. Furthermore, sintering behavior of copper powders
produced through conventional and mechano-chemical routes
was investigated.
2. Experimental procedure
Starting materials used in this research were commercially pure Cu2O (99%,
5–30 ␮m) and graphite (99.9%, 10–50 ␮m). Mixture of Cu2O together with
40 mol% of extra carbon according to the reaction (1) was subjected to intense
mechanical treatment for various periods of time using a Fritsch P5 planetary
ball mill with hardened steel balls and vial. A constant ball-to-powder weight
ratio of 35:1 was maintained during the operation. The milling speed was also
kept constant at 300 rpm.
To avoid the oxidation during milling, the vial was filled with pure argon
before milling. Various amounts of stearic acid, CH3(CH2)16CO2H, were added
in the beginning and final stage of the milling operation. Table 1 summarizes
the milling conditions.
The morphology and phase identification of the products were examined by
SEM (CamScan MV2300) equipped with an Energy Dispersive Spectrometer
∗
Corresponding author at: School of Metallurgy and Materials Engineering,
University of Tehran, P.O. Box: 14395-553, Tehran, Iran. Tel.: +98 912 1958219;
fax: +98 21 88006076.
E-mail address: Sheibani s@engmail.ut.ac.ir (S. Sheibani).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.10.062

S. Sheibani et al. / Journal of Alloys and Compounds 465 (2008) 78–82
79
comparison, a Cu powder produced by conventional route, i.e., micro-grained
sample, was also pressed and sintered under the same conditions.
The density of the sintered samples was measured by Archimedes’ principle
in accordance to ASTM B328. Brinell hardness was measured on the polished
samples.
Fig. 1. XRD patterns of milled powders with 0, 0.5 and 2 wt.% of stearic acid
for (a) 8 h and (b) 15 h.
(EDS) (Oxford Instrument) and XRD (Philips PW-1730) using Cu K␣ radiation,
respectively. The line broadening due to the instrument was calculated from
Warren’s method [8]. The mean crystallite size and internal strain of particles
were obtained using Williamson–Hall plot [9].
Mechano-chemically produced Cu powders were isostatically cold-pressed
at 600 MPa into cylinders with 20 mm diameter and 20 mm height. The com-
pacts were then sintered at 550 ◦C for 1 h in a vacuum furnace at 100 Pa. For
Fig. 2. XRD patterns of samples after 35 h milling.
Fig. 3. SEM images of Cu powders in S7 (a), S8 (b) and S9 (c) samples.

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S. Sheibani et al. / Journal of Alloys and Compounds 465 (2008) 78–82
Fig. 4. EDS analysis of particles in S9 sample after 35 h milling.
Table 1
Table 2
Mean crystallite size and internal strain of Cu particles in samples S7, S8 and
Milling conditions of the samples
S9
Sample PCA added (wt.%) Time of PCA addition (h) Milling time (h)
Sample
Crystallite size (nm)
Strain (%)
S1
S2
S3
S4
S5
S6
S7
S8
S9
0
0.5
2
0
0.5
2
0
0.5
0.5
–
0
0
–
0
0
–
25
30
8
8
8
15
15
15
35
35
35
S7
S8
S9
27
19
22
0.41
0.57
0.49
of 0.5 wt.% stearic acid, the milling duration required for the
reduction reaction is nearly doubled.
In the second set of experiments, 0.5 wt.% stearic acid was
added in final stages of the milling process, i.e., after 25 (S8) and
30 h (S9). Fig. 2 shows XRD patterns of S7, S8 and S9 samples.
Analysis of these patterns reveals that the reduction of Cu2O to
Cu for all three samples has been completed after 35 h and the
reaction process has not been affected by the addition of stearic
acid.
3. Results and discussion
3.1. Characteristics of nano-crystalline Cu powders
Analysis of the XRD pattern of sample milled for 8 h with-
out stearic acid indicates Cu2O as a dominant phase and Cu as
a minor phase. There is no distinct difference in the phase com-
position of samples milled for 8 h with 0.5 and 2 wt.% of stearic
acid (Fig. 1(a)). XRD patterns in Fig. 1(b) show different spec-
tra for samples milled for 15 h without and with stearic acid.
The difference tends to be more evident when more stearic acid
is used. The intensities of Cu peaks increased by milling up to
15 h in the absence of stearic acid whereas Cu2O peaks intensi-
ties are decreased significantly. By addition of 0.5 wt.% stearic,
Cu peaks appear after 15 h of milling whereas no detectable
reaction takes place for Cu production in the sample milled with
2 wt.% stearic acid.
The mean crystallite size and internal strain of Cu particles
in S7, S8 and S9 samples are presented in Table 2. As seen in the
It seems that the addition of stearic acid alters the behav-
ior of reactants significantly during the mechanical milling and
delays the displacement reaction. The suppression of the reac-
tion by stearic acid is primarily due to the decrease in contact
area between reactants [2]. Also, enhanced lubrication because
of higher acid content is believed to reduce the local temperature
during collisions. Similarity of the XRD patterns of samples S1
and S5 confirms the above point and indicates that by addition
Fig. 5. Relative green and sintered densities of S7, S8, S9 and micro-grained
samples.

S. Sheibani et al. / Journal of Alloys and Compounds 465 (2008) 78–82
81
table, the mean crystallite size is decreased and the internal strain
3.2. Characteristics of compacted and sintered samples
increased by addition of stearic acid. The larger crystallite size
calculated for sample S7 could be due to the relatively higher
temperature obtained during milling in the absence of stearic
acid.
Fig. 5showsrelativegreen andsintereddensitiesofS7, S8, S9
and micro-grained samples. In the micro-grained sample, green
and sintered densities were found to be 93.31% and 94.14% of
theoretical value, respectively. The sintered density in samples
S8 and S9 are almost the same (∼77% of the theoretical den-
sity); this value for sample S7 is only 73.69%. It has been already
shown that high green density in metal powders may be achieved
when plastic yielding is occurred [11]. The high plasticity of
micro-grained powders, based on the Hall–Petch dependence of
yield stress on grain size, lead to higher density. However, work
hardening due to the milling process resulted in lower green
density in S7, S8 and S9 samples. Moreover, sliding and rear-
rangement in nanopowders are severely restricted owing to large
frictional forces among powder particles [11]. As a result, lower
green densities are likely to be achieved in mechano-chemically
processed samples in comparison with micron size powders pre-
pared conventionally. Lower sinterability of nanopowders in S7,
S8 and S9 samples is probably due to their agglomerated nature
and surface oxidation [11]. Less agglomeration in S8 and S9
samples due to the addition of stearic acid could be responsible
for their relatively higher sintered density in comparison with
Fig. 3 shows SEM images of S7, S8 and S9 samples. It is
observed that the morphology of products processed in the pres-
ence of stearic acid differed considerably from that of processed
without stearic acid. As shown in Fig. 3(a), large particles of
copper exhibiting wide size distribution are formed in sample
S7. The formation of large Cu particles may be attributed to
its fcc crystal structure in which particles are easily deformed
and cold welded to each other to form large agglomerates [10].
As could be seen in Fig. 3(b) and (c), the mean particle size
of copper is considerably decreased in S8 and S9 samples. It
seems that by addition of stearic acid, the cold welding process is
suppressed.
EDS point chemical analysis of S9 sample after 35 h milling,
Fig. 4, showed that the product is purely Cu and no impurity
such as carbon and oxygen resulting from possible dissociation
of PCA was detected. Au peaks in this pattern has come from
the coating material. Similar results obtained from EDS analysis
of S7 and S8 samples.
Fig. 6. SEM images of sintered micro-grained (a), S7 (b), S8 (c) and S9 (d) samples.

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S. Sheibani et al. / Journal of Alloys and Compounds 465 (2008) 78–82
4. Conclusions
Nano-crystalline Cu powders were synthesized by mechano-
chemical reduction of Cu2O with graphite using high-energy
ball milling with and without addition of the stearic acid as a
PCA. It was found by XRD analysis that the mean crystallite
size of the copper produced after 35 h of ball milling decreased
from 27 nm in S7 sample to 19 nm in S8 sample. Moreover, the
PCA strongly affects the formation process of copper. A fast
reduction reaction occurred in the absence of PCA and presence
of PCA delayed the reduction process. SEM results showed that
addition of PCA efficiently reduced the particle size of Cu by
suppression of excessive cold welding. Also, addition of stearic
acid improved hardness of sintered nanopowder samples.
Fig. 7. Brinell hardness of sintered S7, S8, S9 and micro-grained samples.
S7 sample. Therefore, the influence of stearic acid on subse-
quent powder sintering is an indirect effect. On the other hand,
as indicated before via EDS analysis, stearic acid does not affect
the final chemical composition of the powders. Consequently,
differences in powder sintering are not due to the direct effect
of PCA.
Acknowledgement
The financial support of this work by the Iranian Nanotech-
nology Initiative is gratefully acknowledged.
References
SEM images of sintered micro-grained S7, S8 and S9 sam-
ples are shown in Fig. 6. Micro-grained sample shows a dense
microstructure, whereas large pores are observed between par-
ticles of S7 sample. Microstructures of S8 and S9 samples
indicating uniform grains together with some small pores con-
firm the positive role of stearic acid in densification process of
these samples.
The Brinell hardness of sintered samples is shown in Fig. 7.
It could be seen that S8 sample shows the maximum hardness
of 68 BHN, which is 1.4 times higher than that of micro-
grained sample. Increased hardness in S8 sample is due to
its smallest crystallite size and highest remained strain (see
Table 2). Moreover, higher hardness in S8 and S9 samples may
be caused by the small amount of remained copper oxides due
to the presence of the stearic acid during mechano-chemical
process.
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