Fabrication and Microstructural Analysis of CNTs Reinforced Copper Matrix Nanocomposites via MIM Technique

Ali Samer Muhsan1,a, Faiz Ahmad1,b, Norani M. Mohamed2,c, M. Rafi Raza3,d 1Mechanical Engineering Department, Universiti Teknologi PETRONAS (UTP), Malaysia

2Fundamental and Applied Sciences Department, UTP, Malaysia

3Universiti Kebangsaan Malaysia, 43600, Bangi, Selangor,Malaysia

aalisameer2007@gmail.com, bfaizahmadster@gmail.com, cnoranimuti_mohamed@petronas.com.my, drafirazamalik@gmail.com

Keywords: Cu-CNTs nanocomposites, CNTs dispersion, Functionalization, Ultrasonication.

Abstract. Carbon nanotubes (CNTs) reinforced copper (Cu) matrix nanocomposites prepared via

advanced fabrication method is presented. Functionalization and ultrasonication processes have

been applied to enhance the dispersion of purified CNTs and creates sidewalls groups that have the

potential to bond CNTs to the metal matrix. The main part of this approach was metal injection

molding (MIM) technique, which is a combination of powder metallurgy and plastic injection

molding technique. Preparation of MIM feedstock required a melting and mixing process of binder

system (polymers) with the solid loading, which has been carried out using a twin screw rotor

machine. This machine provides a viscous media of the molten binder with high shear forces that

allow the additives (carbon nanotubes/copper powder) to be mixed properly and exfoliate the CNTs

clusters with uniformdispersion inside the Cu matrix. Subsequently, to prove our expected results,

observation tests of TEM, SEM, FESEM and CNTS were employed and discussed literally.

Introduction

The extraordinary physical properties of carbon nanotubes (CNTs) led it to have very high impact

on developing new generation of nanocomposites with enhanced functionality and make it suitable

for wide range of applications. However, incorporating of CNTs in matrix nanocomposites has its

own barriers and difficulties that might destroy the desired propertied of CNTs [1]. Dispersion of

CNTs in metal matrix composite is one of the most important factor and difficult challenge in the

fabrication of these nanocomposites. CNTs have a large specific surface area that can reach up to

200 (m2/g), with the association of Van der Waals forces, CNTs tend to agglomerate and form CNTs

clusters [2]. CNTs clusters have low strength, high porosity and so they behave as discontinuities.

Thus, clusters increase the porosity of the fabricated composites and degrade the desired properties.

In addition, clustering and non-uniform dispersion of CNTs will lead to inhomogeneous property

distribution in the structural component because the properties like coefficient of thermal expansion,

thermal conductivity, strength and elastic modulus depend strongly on the volume friction of CNTs.

Uniform dispersion of CNTs in matrix composite is a key for efficient use of their properties as well

as for obtaining homogeneous properties [3].

Many researchers have shown decrease in thermal, electrical and mechanical properties of CNTs

based nanocomposites due to the clustering phenomena[4,5] Therefore, several techniques have

been proposed to improve the dispersion of CNTs in matrix composites which have their own

advantages and disadvantages.

Unfortunately, most of these techniques have been limited to the polymer nanocomposites, while

only few researchers have focused on improving CNTs dispersion in metal matrix nanocomposite

[6-10]. Even though, their results did not match their ambition. These techniques can be divided

into two classes: chemical approaches and physical approaches. In each class several methods have

been developed For example, Hagenmuellera et al. [10] have reported a physical approach to

improve the mechanical and physical properties of the PMMA/CNTs nanocomposite after

improving the dispersion of CNTs [11]. They cut PMMA/CNTs composite sample in which the

Applied Mechanics and Materials Vol. 459 (2014) pp 11-17© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.459.11

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CNTs were visibly not well dispersed into small pieces. They mixed these pieces and hot pressed

the mixture to create new sample. They repeated this cutting and mixing procedure 20 times and the

authors reported that the dispersion quality improved. Their comment about the quality was based

on observing sections of the sample under an optical microscope [11].

Meanwhile, Kyung et al. have suggested a new technique named molecular level mixing

approach to disperse CNTs homogenously in copper matrix [1]. They claimed that using technique

resulted in copper powders containing excellent dispersion of CNTs [13]. Despite, a lot of work is

needed to improve the dispersion of CNTs in composites, especially in metal matrix

nanocomposites where the wettability between CNTs and metal matrix is very low due to the high

difference in density [4].

Experimental

Raw materials. The raw materials used in this study were: (1) copper powder (99.95% purity)

having a spherical shape, produced by gas atomization and supplied by Sandvik Osprey LTD, UK.

The copper particle size distribution was ≥ 22 µm. Particle size analysis was performed using

CILAS 1190 DRY, (2) CNTs used in this experiment was supplied by Shenzhen Nano-Technologies

Port Co., Ltd., China. The dimensions of CNTs were below 80nm in diameter and length of 5-15

µm. The purity of CNTs was95-98 % with ash contents of ≤0.2 wt. % and (3) the binder system,

which consists of three ingredients of paraffin wax, polyethylene and stearic acid. These

components were supplied by Titan Pet chem. (M) SdnBhd, Johor, Malaysia [5, 8].

Functionalization and ultrasonication of CNTs. Functionalization has been applied to enhance

the dispersion of purified CNTs and creates sidewalls groups that have the potential to bond CNTs

to the metal matrix. Furthermore, this process has removed the catalyst of the pristine CNTs (mostly

Iron and Nickel) by dissolving in concentrated acid solutions. Liquid phase purification always

simultaneously removes both amorphous carbon and metal catalyst [1]. Therefore, CNTs were

immerged in an acid solution of H2SO4 (98%) and HNO3 (67%) with ratio of 3:1 under ultrasonic

treatment (Zhao et al., 2001). To test the effect of ultrasonication time on the dispersion and

purification of CNTs, a few parameters is set to make a substantial comparison. The parameters are

fixed as shown in Table 1.

Table 1 Parameters of acid treatment of raw CNTs

Ratioofacidsolution/

nicationTime

CNTs

150mlacid,1grawCNTs

0.5hours

150mlacid,1grawCNTs

2.75hours

150mlacid,1grawMWNT

5hours

Samples are then filtered with a microfiltration system, using a cellulose membrane filter of 0.47

µm pore size. Then the filtered CNTs are left to dry for12 hours at room temperature, and then put it

in a vacuum oven of 100ºC for 5 hours. Samples are taken for FESEM and EDX analysis and the

best sample is identified to be used for making of the nanocomposite (Fig 2 and 3).Both raw and

purified CNTs are then suspended in the Triton X-100 solution (5mg/ml), and sent for

ultrasonication for 1 hour. The ratio of CNTs to surfactant is 1g MWNT: 1.5g Triton X-100. The

carbon nanotubes are filtered, washed and dried in oven for 1 day at 100ºC to be used for

fabrication of the nanocomposites [2].

Metal Injection Molding of Cu-CNTs. Metal Injection Molding (MIM) technique concerns four

main stages: Feedstock preparation, injection molding of binder/solid powder, binder removal

(debinding process) to get a shaped porous metallic part, and sintering process to get the condensed

part in pure or composite metallic material [6].

12 Applied Mechanics and Mechanical Engineering IV

In principle, injection molding is a relatively simple process. A feedstock is plasticizing unit, in

which a binder system (usually polymers) and solid powder are mixed together at the melting point

of the binder components results homogenous viscos liquid that solidifies at room temperature [3,

7]. The prepared feedstock then granulate to small pieces that can be plasticized in the heated barrel

of injection molding machine, and then a controlled volume of the molten feedstock is injected from

the plasticator under pressure into a closed mold, with solidification beginning on the mold’s cavity

wall to get the desired shape [4, 5].

1) Feedstock preparation process

The feedstock components (Binder system, MWNT and Cu powder) were then progressed to the

melt processing in which comprises of melting polymer binder to form a viscous liquid, with

MWNT/Cu as an additive to be mixed into the melt through shear mixing. The feedstock

components of binder system (Polyethylene, Paraffin Wax and Stearic Acid), Cu powder and CNTs

were mixed using Haake Rheomix 600 mixer machine under temperatures of165ºC, the mixing

speed 30 rpm with mixing duration of 30 minutes. The binder system was allowed to be melted for

5 minutes prior to add CNTs, with 0, 1, 2, 5, 7.5 and 10 vol. % [8, 12].

2) Feedstock injection molding process

Subsequently, the mixture was granulated into pellets and sent to the next process of shaping

sample to be directly injection molded using vertical injection molding machine adhering to the

standard of ASTM D638. The injection molding parameters are as follows; barrel temperature: 165-

190ºC, mold temperature: 25ºC, pressure: 700bar, and speed: 55 rpm.

3) Debinding and sintering Processes

Solvent debinding was carried out using Heptane as a solvent at 60°C for 5 hours. It is

considered that after removing some percent of the binder, some interconnected capillary porosity

inside of the samples were created which make leaving of gaseous products in subsequent thermal

debinding easy in short time. Since approximately 70% of binder system was removed in solvent

debinding step, subsequent thermal debinding can be finalized with higher speed in comparison

with usual thermal debinding process [4, 10]. Thereafter, thermal debinding and sintering processes

were carried out at the same time. To optimize the sintering heating rates and environments,

different heating rates and environments were performed [7, 10].

The following procedures were used for both thermal debinding and sintering process: From

room temperature to 450°C, the heating rate was 1°C/min. To thermally debinding the binder, the

samples were held on that temperature for 1 hour. From 450°C to the sintering temperature, the

heating rate was performed to be 3°C/min, and then the samples were held at 900°C for 1 hour and

cooled down slowly to the room temperature [12]. Fig. 1 shows the whole MIM fabrication

processes of Cu-CNTs.

Fig. 1 The fabrication processes of Cu/CNTs nanocomposites using MIM technique

Applied Mechanics and Materials Vol. 459 13

Results and Discussion

Fig. 2 and 3 show the TEM micrographs of as received (pristine) CNTs. The TEM photograph and

CNTS results show some impurities in the pristine CNTs which were mostly Iron and Nickel. It is

now acknowledged that CNTs suffer from poor phase-purity since CNTs are generally grown from

metallic catalyst nanoparticles [2]. The pristine and functionalized CNTs samples were analyzed

FESEM and EDX with a clear distinction of purification and shortening of CNTs in different

ultrasonication time. Not much difference can be seen on the structure of CNTs after 0.5 hours of

ultrasonication, but more obvious changes are found from the samples that have undergone

ultrasonication for 2.75 hours and 5 hours. At 5 hours, the CNTs are severely shortened and

dispersed, with the risk of the CNTs being damaged under prolonged acid treatment.

Fig. 2 TEM micrographs of (a) typical pristine MWCNT, (b,c and d) of Ni and Fe catalyst

Impurities contained in raw entangled MWCNTs in different magnitudes

Fig. 2, shows the TEM image of the pristine and CNTs. It can be seen that many tubes loosely

entangled together with particle-like impurities. TEM micrographs (Fig. 3) confirm the

effectiveness of purification of the CNTs.

Fig. 3 TEM micrographs of (a) pristine MWCNT with impurities, (b) Purified MWCNTs after 2.75

hours of acid treatment

Table 2 shows the CNTS results of both raw and treated CNTs, where it can be clearly noted that

Fe catalyst was eliminated and Ni was reduced significantly after acid treatment. However,

oxidation also occurred more aggressively under longer periods of ultrasonication. Based on our

reference, the optimum parameter obtained is the ultrasonication time of 2.75 hours, in 150 ml of

acid solution, whereby a sufficient amount of Ni was reduced without a drastic oxidation of carbon.

The presence of Sulphur is due to the acid treatment, and can be removed with thorough washing.

14 Applied Mechanics and Mechanical Engineering IV

Table 2 EDX results of functionalized CNTs after different sintering time

Weight%

Atomic% 0.5 h Sonicationtreatment

C 94.22 96.52

O 4.04 3.11 Fe 0.46 0.10 Ni 1.29 0.27

2.75 h Sonicationtreatment

C 90.56 93.40

O 7.95 6.16 Fe - - S 0.73 0.28 Ni 0.76 0.16

5h Sonicationtreatment

C 88.93

91.82

O 10.26 7.95

Fe - -

S 0.32 0.12

Ni 0.50 0.10

While anionic, cationic and nonionic surfactants have been used in previous studies to disperse

carbon nanotubes, a nonionic surfactant was chosen for this experiment due to polypropylene’s

insolubility in water. Triton X-100 was used due to its availability in the market and previous

studies have shown that it was an effective dispersing agent [13].

Melt-mixing was chosen over solution mixing due to time constraints and the first being a

preferred choice for bulk production. It was also intended to combine the shear force of melt mixing

with the functionalized CNTs to achieve better dispersion of CNTs. However, it was found that the

functionalized CNTs form a hardened deposit after filtration and drying. A mortar grinder was used

to grind the deposit into powder-form again before adding into the melt-mixer.

Fig. 4 shows SEM micrographs of Cu-10vol. % CNTs nanocomposites after mixing, thermal

debinding and sintering processes with different magnifications. From Fig. 4-a, it can be noted that

the binder system is packing Cu particles together and filling up the voids in between. In this Fig.,

due to the low magnification, CNTs couldn’t be clearly observed. Fig. 4-b displays the sintered Cu-

10% vol. CNTs after polishing process. While Fig. 4-c illustrates the thermal debinded part showing

uniform and homogenous dispersion of CNTs in the Cu matrix composites. It also indicates that the

binder system has been removed completely.

Applied Mechanics and Materials Vol. 459 15

Fig. 4 SEM images of Cu-10vol.% MWNTs after different processes: (a) Cu-10vol.% MWNTs

feedstock, (b) polished surface of sintered Cu-10vol.% MWNTs, (c) fractography of thermal

debinded Cu-10vol.%MWNTs and (d) fractography of sintered Cu-10vol.% MWNTs

Meanwhile, Fig. 4-d demonstrates the fractography of the sintered Cu-10vol. % CNTs

nanocomposite which confirms uniform dispersion of CNTs in the Cu matrix, as well as it shows a

good bonding between each of Cu-Cu particles and Cu-CNTs.

Conclusion

In brief, this work presented a newly developed technique for dispersion CNTs in copper matrix

nanocomposites using multilevel mixing approach. This technique showed the ability in exfoliating

and dispersing of CNTs uniformly in the metal matrix by the influence of the created electrostatic

forces between individual CNTs that overcome the Van der Waals forces, which are the main reason

of the clustering phenomenon in carbon nanotubes. This process also provides a viscos liquid media

with high shear forces that allows the additives of Cu powder/CNTs to be mixed properly and

exfoliates the MWCNTs clusters with uniform dispersion into the metal matrix.

Acknowledgment

Authors would like to acknowledge Universiti Teknologi PETRONAS (UTP) for supporting this

work.

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