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(2008) 47–50www.elsevier.com/locate/matlet

Materials Letters 62

Single-walled carbon nanotube-reinforced copper composite coatingsprepared by electrodeposition under ultrasonic field

Y.L. Yang a, Y.D. Wang a,⁎, Y. Ren b, C.S. He a, J.N. Deng a, J. Nan a, J.G. Chen a, L. Zuo a

a Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110004, Chinab X-ray Science Division, Argonne National Laboratory, Argonne, I L 60439, USA

Received 6 June 2006; accepted 20 April 2007Available online 27 April 2007

Abstract

Single-walled carbon nanotube-reinforced Cu composite coatings prepared by electrochemical deposition under ultrasonic field exhibit smallercrystallite size and higher lattice micro-strain compared with a pure Cu coating. The as-deposited coatings retain a good electrical conductivitycomparable to pure copper and simultaneously show a significant enhancement in mechanical properties. This indicates that the presentelectrochemical deposition technique can be used for preparing the carbon nanotube-reinforced metals with enhanced mechanical and functionalproperties.© 2007 Elsevier B.V. All rights reserved.

Keywords: Carbon nanotubes; Electrical properties; Strengthening; X-ray diffraction

1. Introduction

Single-walled carbon nanotubes (SWNTs), possessingexcellent mechanical and physical properties [1–3], havebecome an attractive reinforcement filler in composite materialsfor enhancing mechanical and functional properties. In recentyears, great interest has developed in the fabrication and studyof carbon nanotube (CNT) reinforced metals. Bian et al. [4]pointed out that CNT-reinforced Zr displays high fracturestrength, strong ultrasonic attenuation characteristics andexcellent wave-absorption ability. The temperature dependenceof electrical resistivity in CNT-reinforced Al shows asurprisingly abrupt drop of more than 90% at 80 K [5]. TheCNT-reinforced metal composites prepared by electrodeposi-tion have high hardness, excellent wear resistance and goodresistance to corrosion [6,7]. However, the effective fabricationof CNT-reinforced metals depends on the homogenousdispersion of CNTs in the metal matrix and the interfacialadhesion between them. Indeed, the intrinsic van der Waals

⁎ Corresponding author.E-mail address: ydwang@mail.neu.edu.cn (Y.D. Wang).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.04.086

forces among CNTs always lead to agglomerate, which lowersthe reinforcement between CNTs and matrix.

Due to its excellent electrical conductivity, pure Cu waswidely used in the electronics industry, but its intrinsic softnessoften caused the failure of electronic components. Strengthen-ing metals, such as pure Cu, through various approaches usuallyleads to a pronounced decrease in conductivity [8]. It was foundthat SWNT-reinforced metals have prominent enhancement inmechanical properties [4–7], but their electrical conductivitiesare decreased [9,10]. In the present study, we used theelectrochemical deposition technique under ultrasonic field toprepare SWNT-reinforced Cu composite coatings (SWNT-Cu)for improving the interfacial adhesion between the SWNTs andthe Cu matrix. We found that the electrical conductivity of thecomposites is almost comparable to that of pure Cu, but theirmicro-hardness value is obviously higher than those of theirpure counterpart.

2. Experimental

SWNTs from Shenzhen Nanotech Port Co. Ltd. were used inthe present investigations. They were acid treated in a solution of1:3 nitric acid/sulfuric acid at 80 °C for 6 h to remove catalysis

Fig. 1. (a) Williamson–Hall plots, bi versus Si, for the as-deposited SWNTs-Cuprepared from different electrolytes containing 0 g/L, 1.5 g/L, 3 g/L SWNTs.The up-inset shows the {111} peak for X-ray diffraction patterns for thecorresponding composites. (b) Crystallite size, d, and micro-strain, ε, for theSWNT-Cu.

Fig. 2. (a) Dependence of electrical resistivity (r) for the as-deposited SWNT-Cu(with electrolyte containing 3 g/L SWNTs) in temperature range of 10 to 300 K(in red line). For comparison, electrical resistivity data of OFHC Cu [8] are alsoincluded (in black line). (b) Micro-hardness of as-deposited SWNT-Cu at roomtemperature.

Table 1Measured electrical resistivity, ρ, at 0 K and 293 K, and temperature coefficientof resistivity, α, at 293 K. For comparison, the electrical resistivity data ofoxygen-free high-conductivity (OFHC) Cu are also included [8]

Specimen ρ (0 K) (10−8 Ω·m) ρ (293 K) (10−8 Ω·m) α (10−1 K−1)

SWNTs-Cu 0.085±0.010 1.65±0.05 6.20±0.20OFHC Cu 0.068 1.67 6.80

48 Y.L. Yang et al. / Materials Letters 62 (2008) 47–50

particles and generate functional groups [11–13] on the surfacesof the tubes. Then they were further collected on 0.2 μm filter,rinsed with distilled water, and dried at 100 °C for 4 h. Thetreated SWNTs were added to electrolytes, containing 0 g/L,1.5 g/L and 3 g/L SWNTs, respectively, and an ultrasonic field(20 KHz, 60 W) was applied during mixing of the electrolytesfor at least 6 h. The copper sulfate electrolyte had the followingcomposition: 220 g/L CuSO4·5H2O, 64 g/L H2SO4, 3 g/Lcationic surfactant of cetyl-trimethylamine chloride (CTAC)[14]. The SWNT-Cu was deposited on aluminum sheet coatedby a Ni–P amorphous film with an exposed surface area of20×15 mm2. A pure electrolytic Cu (99.99%) plate with surfacearea of 60×50 mm2 was used as an anode. The ultrasonic field(20 KHz, 30W) was also added during the deposition accordingto the previously-described method [15]. The electrochemicaldeposition was performed at room temperature, and the pH valueof the electrolyte was maintained at 0.8 by adjusting the volumefraction of diluted H2SO4. The cathode direct current densitywas controlled at 3.0±0.1 A/dm2.

The micro-strain and crystallite size of the as-depositedcomposites were characterized using an X'Pert PRO MRD

diffractometer equipped with Ni-filtered Cu-Kα radiation.Diffraction data were collected by scanning each hkl reflection,with a step size of 0.02°. The Williamson–Hall (WH) integralbreadth method [16,17] was used to determine the crystallitesize, d, and micro-strain, εM, from {111} and {222} peaks, i.e.,βi = 1 / d+2ε

MSi (where Si = 2 sin(θi) /λ is the length ofdiffraction vector for the diffraction peak; βi =βi

0 cos(θi) /λ isthe integral breadth of the diffraction peak in the reciprocalspace; θi is the Bragg angle; λ is the wavelength; and βi

0 is theintegral breadth for the diffraction peak located at 2θi).

A conventional four-probe technique was used to measure thedependence of electrical resistance for the as-deposited compositein the temperature range of 10 K to 300 K. Microstructures of thecomposite were investigated by a TECNAI G220 transmissionelectronmicroscope at an operation voltage of 200 kV. Themicro-

Fig. 3. TEM micrograph of SWNT-Cu showing the two typical sites for coppernucleation at the end (A) and on the outer surface (B) of a SWNT.

49Y.L. Yang et al. / Materials Letters 62 (2008) 47–50

hardness measurements of the polished surfaces were carried outby a 401 MVD digital sclerometer using an applied load of 10 gfor 15 s.

3. Results and discussion

Fig. 1a shows the integral breadth of Cu {111} and {222}diffraction peaks for the three composite coatings, which weredeposited from electrolytes containing 0 g/L, 1.5 g/L, and 3 g/LSWNTs, respectively. The corresponding volume fractions of SWNTsin the three samples are about 0, 7%, and 10%, calculated from thedifference of density between SWNT-Cu and the pure counterpart. Thevolume fractions are not proportional to the content of SWNTs in theelectrolytes, which is in agreement with that reported by Wang et al.[14]. The up-inset in Fig. 1a shows the {111} peaks of the X-raydiffraction patterns (XRD) for the corresponding samples. The micro-strain of the composite coatings can be calculated from the integralbreadth of {111} and {222} diffraction peaks. Fig. 1b displays theaverage crystallite size and the value of micro-strain in the Cu matrixobtained from the XRD analysis of the different coatings. Comparedwith pure Cu coating, the average crystallite size of the Cu matrix isdecreased by ∼50% and the micro-strain is increased by ∼100% forthe SWNT-Cu. Also, an increase in the volume fraction of SWNT-doping leads to a reduction in crystallite size and an increase in micro-strain. The grain refinement in the SWNT-Cu suggests that the co-deposited SWNTs may become nucleation centers, which wouldincrease the nucleation rate during electrochemical depositions. Theultrahigh lattice micro-strain found in the SWNT-Cu may be wellexplained by the fact that the co-deposition of SWNTs (with a veryhigh aspect ratio of length to diameter) and Cu matrix causes a largemismatching stress between the matrix and the SWNTs, which cannotbe relaxed after deposition.

It has been reported that the electrical resistivity of a thin filmincreases with the increment of micro-strain and grain refinement[18,19]. This phenomenon is attributed to the fact that the higherdensity of grain boundaries or defects associated with the smallercrystallite size and larger strain field leads to more electron scattering.The temperature dependence of electrical resistivity for the as-deposited SWNT-Cu (with electrolyte containing 3 g/L SWNTs) isdisplayed in Fig. 2a. The figure shows that the resistivity of the sampledecreased linearly with temperature to about 75 K. From 75 K to 10 K,

a nonlinear temperature dependence of resistivity was detected. Forcomparison, the electrical resistivity data of oxygen-free, high-conductivity (OFHC) Cu [8] are also presented in Fig. 2a. It isobserved that the conductivity (reciprocal of resistivity) of SWNT-Cuis nearly the same as that of OFHC Cu throughout the temperaturerange, but does not decrease with the increment of micro-strain andgrain refinement.

The fitting parameters of the electrical resistivity are summarized inTable 1. The electrical resistivity of SWNT-Cu at room temperature(ρ293 K) is 1.65±0.05×10

−8 Ω·m, which is slightly lower than that ofOFHC Cu (1.67×10−8 Ω·m). The residual electrical resistivityapproaching 0 K, which is an indication of defects or doping elements[8], is about 8.5±1.0×10−10 Ω·m. This value is slightly higher thanthat of OFHC Cu (6.8×10−10 Ω·m). This may be explained by the factthat the SWNT-doping in Cu, which acts as scattering centers ofelectrons, increases the resistivity at lower temperature. Furthermore,the grain refinement and increase of micro-strain in the Cu matrix alsoplay important roles in the increase of electron scattering. The value oftemperature coefficient of resistivity, α, which illustrates the electron–phonon interaction [20], is 6.20±0.20×10−11 K−1 at 293 K, which isslightly lower than that of OFHC Cu (6.80×10−11 K−1). This slightdifference may be explained by the following three factors: First, theelectrical conductivity of SWNTs with∼108 S·m−1 [21,22] is the sameas or even better than that of pure Cu; Second, the high surface area ofthe nanotubes creates a large interface-to-volume ratio of SWNT/Cuinterfaces, resulting in large scattering during electron transfer and,hence, leading to an increase in resistivity [23]; Third, the majority ofthe SWNTs distribute in the grain boundaries and form a networkleading to the continuity of the nanotube phase, which provides aninterlinked electrical pathway for a reduction in resistivity [24]. Webelieve that the third factor, i.e., the electron scattering and electron–phonon interactions near the SWNT-concentrated grain boundaries,plays an important role in the decrease of electrical resistivity at roomtemperature for the SWNT-doped Cu.

The micro-hardness of the as-deposited coatings is displayed in Fig.2b. The micro-hardness is 1.43 GPa and 1.61 GPa, respectively, for theSWNT-Cu prepared from electrolytes containing 1.5 g/L and 3 g/LSWNTs. Those values are obviously higher than that of the pure Cucoating (1.18 GPa) by ∼21% and 36%, respectively. The hardnessobserved in SWNTs-Cu prepared by our method is also higher than thatin the composites fabricated by means of the powder metallurgytechnique (with the maximum hardness increment of 20%), as reportedby Dong et al. [9]. This indicates that the SWNT-Cu prepared by ourmethod has a good adhesion between the SWNTs and the Cu matrix.

For most materials, an increase in hardness is closely related to aninduction in grain size. Since crystallite-size values for SWNTs-Cu areabout half of those for pure Cu (as shown in Fig. 1b), the micro-hardness is expected to increase if only the grain refinement effect isconsidered. According to the experiment by Scattergood et al [25], thehardness of the composites can reach ∼1.50 GPa. Another factorinducing the strengthening effect is the effective load transfer from theCu matrix to the SWNTs during deformation, which was alreadysuggested by numerous experiments and simulations on CNT-polymercomposites [26–31]. Due to the intrinsic covalent bonds in CNTs, theCNT-containing specimens exhibit greater stress sharing than the Cumatrix during deformation.

The transmission electron microscopy (TEM) micrograph shown inFig. 3 displays that a good adhesion among the SWNTs and the Cumatrix was achieved. The activated defect sites of acid-treated SWNTsurfaces have many functional groups, such as carbonyl groups,oxygen–hydrogen bonds and C–O bonds [11], which greatly promotethe combinations between SWNTs and Cu, and with the help of

50 Y.L. Yang et al. / Materials Letters 62 (2008) 47–50

ultrasonic vibrations, induce the formation of a wettable SWNT/Cuinterface. The typical sites of Cu deposition on SWNTs are displayed inFig. 3. It is observed that Cu is deposited at the end (A site in Fig. 3)and on the activated outer surface (B site in Fig. 3) of SWNTs, which issimilar to that found in the co-depositions of nickel and multi-walledcarbon nanotubes (MWNTs) [32]. As SWNTs contain structuralvacancies [33,34] and additional defects on the outer surface of tubesgenerated by acid treatments, the active defect sites may become thepreferred nucleation sites of Cu (due to lower nucleation barrier at thedefect sites) than the defect-free sites on the outer surface of SWNTs[32]. It should be stressed that the use of ultrasonic vibration may alsoprovide some extra energy to reduce the nucleation barrier of SWNTsand Cu deposits and promote the homogeneous dispersion of SWNTsamong the Cu matrix.

4. Conclusions

In comparison to pure Cu coating, the as-deposited SWNT-reinforced Cu composite coatings, which were prepared by theelectrochemical deposition under ultrasonic field, exhibitsmaller crystallite size and larger lattice micro-strain. However,the electrical conductivity of the composite is comparable tothat of pure Cu. A significant enhancement in micro-hardness(by 36%) is observed in the as-deposited SWNT-Cu compo-sites. This enhancement is attributed to the large micro-strain,grain refinement, and effective load transfer from matrix toSWNTs. TEM observations show that the interface betweenSWNTs and matrix is wettable and in good adhesion. Webelieve that the present technique for preparing SWNT-Cu issuitable for fabrication of other CNT-reinforced metal compo-sites with advanced functional and structural properties.

Acknowledgements

The authors are very grateful for the financial support fromthe National Natural Science Foundation of China (Grant No.50471026) and the National Ministry of Education of China.This is also supported by the 111 Project (B07015).

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