Composites: Part A 38 (2007) 646–650

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Short communication

Dispersion of carbon nanotubes (CNTs) in aluminum powder

a,* b
A. Esawi , K. Morsi
a
Department of Mechanical Engineering and The Science and Technology Research Center (STRC), The American University in Cairo, Cairo, Egypt
b
Department of Mechanical Engineering, San Diego State University, 5500 Campanile drive, San Diego, CA 92182, USA

Received 15 February 2006; received in revised form 26 April 2006; accepted 26 April 2006

Abstract

In the present work, we use mechanical alloying (MA) for the first time to generate a homogenous distribution of 2 wt% CNT within
Al powders. The effect of milling time (up to 48 h) on the morphological development of the powders and dispersion of CNTs was inves-
tigated. The results show that the technique is effective in dispersing the nanotubes within the soft Al matrix which simultaneously pro-
tects the nanotubes from damage under the impact of the milling balls. The results can have important implications for the processing of
CNT-reinforced metal-matrix composites in general.
Ó 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Metal-matrix composites (MMCs); E. Powder processing

1. Introduction contributing to weight savings. Not surprisingly, carbon

nanotubes have emerged as new reinforcements for a num-
Carbon nanotubes (CNTs) have recently emerged as ber of material systems including polymeric [7,8], metallic
materials with exceptional properties exceeding those of [9–11] and ceramic [12] matrices. Numerous research
any conventional material. Defect-free carbon nanotubes, groups have added carbon nanotubes to polymer matrices
both single walled nanotubes (SWNTs) and multi-wall with the aim of producing conductive and high strength
nanotubes (MWNTs) have elastic modulii of 1 TPa and polymers. However, no significant improvement in
tensile strengths in the region of 150 GPa [1–4]. These mechanical properties has been reported. The challenges
exceptional properties (electrical, physical and mechanical) faced were many, including nanotube agglomeration, and
have earned carbon nanotubes the serious consideration of the lack of control over alignment.
industrial and scientific organizations for possible use in Few research groups have investigated either metal or
field emission applications (display panels) [5], nano- ceramic matrices. Unsuccessful attempts were carried out
devices [6], probes for SPM (scanning probe microscopy), to infiltrate the metal into a preform of nanotubes, so alter-
replacing silicon in microcircuits or in multilevel chips, native techniques had to be sought. A successful procedure
and hydrogen storage. Research efforts, however, have was recently reported [10,13] in which a small % of CVD
focused on developing their synthesis techniques, on pro- (chemical vapor deposition)-produced MWCNT were
ducing defect-free tubes, and on evaluating them in appli- incorporated in both aluminum (Al) and magnesium
cations that mainly take advantage of their electrical (Mg) matrices by powder metallurgy (PM) techniques.
properties. Their exceptional mechanical properties make Low-energy ball milling in a Turbula mixer was used to
CNTs ideal candidates as reinforcements in composite homogenize the mix followed by hot compaction into a
materials to increase both stiffness and strength while also disk shape, sintering and then HIPing. The samples tested
exhibited a ductile behavior, but the expected improvement
in Young’s modulus was only slightly achieved. Therefore,
*
Corresponding author. Tel.: +1 202 797 5786, fax: +1 202 795 7565. the powder metallurgy technique is promising but extensive
E-mail address: a_esawi@aucegypt.edu (A. Esawi). investigations are still needed.

1359-835X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.compositesa.2006.04.006
 A. Esawi, K. Morsi / Composites: Part A 38 (2007) 646–650 647

In a recent study [14], high-energy ball milling of a mix- the aluminum particles (10s of microns) and the nanotubes
ture of CNT and aluminum powder for 5 min was con- (140 nm). Fig. 1 below shows clustering of the nanotubes
ducted in order to break down the CNT clusters. The after dry mixing in a Turbula mixer at 46 rpm for 8 h. Sim-
authors limited the milling intensity to 200 rpm and the ilar clustering was observed for other mixing speeds and
milling time to 5 min to ensure that the nanotubes remain times with the minimum cluster size of about 20 lm
intact. However, no detailed investigation of the effect of reached when mixing for 46 rpm for 8 h or 67 rpm for 6 h.
milling time on the dispersion of the nanotubes was con- A more effective technique was thus used which is
ducted. Although some work also covered the ball milling mechanical alloying, as explained earlier. The FESEM
of just CNT [15], to-date mechanical alloying of CNT–Al was used to characterize the morphology and size of the
for investigating the dispersing effect of MA on the mechanically alloyed composite powders during the vari-
CNT–Al system has not been conducted, which is the focus ous stages of milling and to investigate the effect of milling
of the current paper. on the dispersion of the nanotubes within the aluminum
matrix. The results are presented in Fig. 2.
2. Experimental procedures It can be seen from Fig. 2 that the aluminum particles
were first flattened under the impact of the balls forming
Al (99.7 % pure—75 lm) and multi-wall carbon nano- flakes which then started welding together forming large
tubes (MWCNT) (approximately 140 nm in diameter and particles with a rough surface that became smoother as
3–4 lm in length, supplied by the MER corporation), were the milling continued. Some particles reached a size
used in the present study. approaching 3 mm after 24 h of milling with a very smooth
2 wt% CNT with 98 wt% Al powder were placed in surface. In MA there are two competing processes; one is
125 ml stainless steel mixing jars containing 25 stainless cold working of the powders which should lead to a
steel milling balls of 10 mm diameter (giving an initial decrease in ductility and eventual fracturing of the parti-
ball-to-powder weight ratio (BPR) = 10:1). The jars were cles, while the other is cold welding of particles which tends
filled with argon and were then agitated using a Planetary to increase the particle size [16]. Due to the overwhelming
ball mill (Retsch 400 MA) at 200 rpm for varying milling ductility of Al in the 2 wt% CNT–Al composite particles
times up to 48 h. Samples were extracted from the batch and possible dynamic recovery processes occurring, parti-
after 30 min, 1, 3, 6, 12, 18, 24, 36 and 48 h and used for cle welding may be more pronounced, leading to the very
field emission scanning electron microscopy (FESEM) large particles observed even after 48 h. The authors how-
analysis (using a LEO Supra 55 FESEM) to examine the ever expect that with an increase in CNT content the com-
dispersion of the CNTs within the Al matrix and to charac- posite particles will be less ductile and much lower particle
terize the composite particle morphology and size. sizes can be obtained, which would be more suitable for
subsequent sintering processes. Another possibility would
be to use process control agents which should reduce the
3. Results and discussion cold welding. These are the subject of ongoing research
by the authors.
Investigation into the effect of dry mixing on the disper- Investigation of the particle surface after various mill-
sion of the CNTs by the authors has confirmed that cluster- ing times revealed the presence of nanotubes—uniformly
ing is a major problem due to the large difference between distributed—on the flakes’ surfaces after 0.5 h of milling,
as shown in Fig. 3 below. The same dispersed presence
of nanotubes on the surface was observed for particles
milled for 1 and 3 h. However, after 6 h, at which time
smooth-surfaced large particles had been formed, no
nanotubes were observed on the surface. It was assumed
that by this stage the nanotubes were already embedded
in between the cold welded particles. This was confirmed
by deforming the large particles obtained after the maxi-
mum milling time of 48 h and then fracturing the particles
to reveal the fracture surface. High magnification FESEM
analysis was conducted and revealed the presence of
CNTs within the aluminum matrix as shown in Fig. 4.
It can be seen that the CNTs appear intact after being
subjected to the MA process. Preliminary characterization
of the CNTs on the surfaces of the mechanically alloyed
powders at the early stage of milling and fractured sur-
faces of mechanically alloyed powders at the later stages
of milling did not reveal any noticeable damage of the
Fig. 1. Clusters of nanotubes after dry mixing in a Turbula mixer. CNTs.
648 A. Esawi, K. Morsi / Composites: Part A 38 (2007) 646–650

Fig. 2. SEM micrographs of mechanically alloyed 2 wt% CNT/Al powder after (a) 0.5 h, (b) 1 h, (c) 3 h, (d) 6 h, (e) 12 h, (f) 18 h, (g) 36 h and (h) 48 h.
 A. Esawi, K. Morsi / Composites: Part A 38 (2007) 646–650 649

Fig. 3. CNTs dispersed on the surface of aluminum particles after 0.5 h milling time.

Fig. 4. 2 wt% CNT–Al (48 h MA) deformed and fractured particles, showing individual CNTs embedded in the aluminum particles after the longest
milling time of 48 h.

4. Conclusions much lower particle sizes should be obtained, which would

then be more suitable for subsequent sintering processes.
One of the key issues in the development of CNT/metal- Another way to reduce the excessive particle welding is to
matrix composites is controlling the agglomeration of the use a process control agent, which are typically used in
nanotubes. This has been a major impediment facing the the MA field. These are however the subject of ongoing
development of these new materials. The results presented investigations by the authors.
in this paper demonstrate that mechanical alloying is a
promising technique to overcome this problem. The SEM Acknowledgements
results showed that the usual CNT clustering often
observed when using Turbula mixing was eliminated; The authors wish to thank the following group of under-
moreover, individual nanotubes were observed embedded graduate students at the American University in Cairo for
in the aluminum matrix after 48 h of milling which did their assistance with the ball milling experiments: Abdel
not appear to be damaged by the selected milling intensity Rahman Reda, Ahmed Abdel Gawad, Ahmed Sayed Mo-
(200 rpm) and ball-to-powder ratio (10:1). hamed, Basel El-Thalathiny, Mahmoud El-Sarag and
Very large particle sizes were however obtained after Moataz Hamouda. Dr. Esawi also wishes to acknowledge
prolonged MA due to the high CNT–Al composite particle the financial support by the Science and Technology Re-
ductility which promotes excessive cold-welding. The search Center (STRC) at the American University in Cairo.
authors however believe that with an increase in CNT con- Grateful thanks to Dr. Raouf Lotfi of the MER Corpora-
tent the composite particles will become less ductile and tion for supplying the nanotubes used in this study and to
650 A. Esawi, K. Morsi / Composites: Part A 38 (2007) 646–650

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