Powder Metallurgy and Metal Ceramics, Vol. 47, Nos. 9-10, 2008
PROCESSING AND CHARACTERIZATION OF CNT/Cu NANOCOMPOSITES BY POWDER TECHNOLOGY Walid M. Daoush1 UDC 546.23:548.5 Carbon nanotube/copper (CNT/Cu) nanocomposite powders with different volume fractions of multiwall CNTs (MWNTs) are prepared using e electroless Cu deposition method in alkaline citrate bath. The prepared CNT/Cu nanocomposite powders are investigated with HRSEM. The CNT/Cu nanocomposite powders have morphology of implanted CNTs in the copper particles of about 200 nm in size. An electroless deposited pure Cu powder is also prepared by the same method to compare the properties of pure copper samples and the prepared CNT/Cu nanocomposites. The prepared powders undergo XRD and elemental analysis to evaluate the fabricated CNT/Cu nanocomposites. The XRD patterns and elemental analysis inform that the deposited copper is composed of Cu powder and traces of some elements, e.g., sulfur and oxygen. The CNTs/Cu nanocomposite, as well as the pure copper powders, is sintered by spark plasma at 600°C and compaction pressure of 20 MPa for one min under 10–3 torr vacuum condition. The microstructure of the sintered nanocomposites is investigated with SEM. The CNTs are homogenously distributed in the copper matrix in the case of CNT/Cu nanocomposites with CNT volume fraction up to 10 vol.% has a porosity of 3%, and the porosity increases by increasing CNT volume fraction due to the agglomeration of the CNTs in the copper matrix. The electrical resistivities of the prepared sintered copper and CNT/Cu nanocomposites are measured with four-probe method. The results show that the highest resistivity is 13.5 μΩ ⋅ cm for 40 vol.% CNT/Cu. Keywords: carbon nanotube, electroless copper deposition , sintering, electrical resistivit.
INTRODUCTION Carbon nanotubes (CNTs) are promising raw materials for industrial application due to their outstanding high length, diameter ratio, physical, electrical, and mechanical properties such as high tensile strength, high elastic modulus, high thermal conductivity and electric conductivity [1, 2]. Significant interest has been recently focused on carbon nanotube composites that enhance mechanical and electrical characteristics more than those of the host materials. Thus, by placing nanotubes into appropriate matrices, it is postulated that the resulting composites will have enhanced properties, such as in metal matrix materials, and CNTs enhanced mechanical properties (stiffness, wear, and fatigue resistance). Kuzumaki et al. [3] characterized the processing and mechanical properties of a carbon-nanotube-reinforced Al composite prepared by hot pressing followed by hot extrusion. This work indicates that the carbon nanotubes in the composites are not
1Central
Metallurgical Research & Development Institute, Department of Powder Technology, Cairo, Egypt; e-mail:
[email protected]. Published in Poroshkovaya Metallurgiya, Vol. 47, No. 9–10 (463), pp. 38–45, 2008. Original article submitted September 11, 2007. 1068-1302/08/0910-0531©2008 Springer Science+Business Media, Inc.
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damaged during the composite preparation and that no reaction products at the nanotube/Al interface are visible after annealing for 24 h at 983 K. However, CNTs demonstrate properties such as the poor wetting by metals like graphite. Therefore, the properties of their surfaces do not have catalytic effect [4, 5]. The authors have studied the wetting of carbon nanotubes in detail and reported that it is not easily for metals to wet the surface of CNTs. This means, if carbon nanotubes are used as reinforcing fibers for metal–matrix composites, it will be difficult to achieve high-strength interfacial adhesion [6]. Powder coating processes are good solution for increasing the wettability as well as the homogenous distribution of the reinforced powders in the matrix. Electroless coating technique has been widely used to prepare the composite coatings, in which nanometer particles are used as reinforcing phase. But up to now, it is described rarely about electroless nanometer composite coatings. The authors studied coating CNT by metals with Ni–P layer followed by mixing this composite powders with Cu power and powder metallurgy processing of these composites [8]. The present work aims to improve the homogeneous distribution of CNTs in the metal matrix by using electroless Cu deposition on the CNTs surfaces followed by powder metallurgy consolidation process. CNT/Cu composite powders with different CNT volume fractions were prepared in alkaline citrate bath by electroless copper deposition. The produced composite powders were heat-treated and sintered using spark plasma technique. The microstructure was investigated by scanning electron microscopy and also the density and electrical resistivity were measured for evaluating the fabricated CNT/Cu nanocomposites.
EXPERIMENTAL Carbon nanotube grade of 10–50 μm in length and 15–30 nm in diameter with BET about 200 m/g was supplied from “Nanotech Co. Ltd” (Korea). Highly pure chemicals were supplied from “Junsei Chemical Co. Ltd” (Japan). Copper (II) sulphate pentahydrate was used as a Cu metal source, tri-sodium citrate monohydrate as a complexing agent, and formaldehyde (38 vol.%) solution was used as a reducing agent: Copper sulphate pentahydrate . . . . . . . . . . . . . . . . . . Tri-sodium citrate monohydrate. . . . . . . . . . . . . . . . . . Sodium hydroxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . Formaldehyde. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . pH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bath temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
170 g/l 70 g/l 50 g/l 100 ml/l 11.5 Room
The calculated CNT wt.% for each Cu/CNT volume fraction was added to the solution of copper sulphate and trisodium citrate and the pH was adjusted at 11.5 by using sodium hydroxide. The solution was stirred with 500 rpm magnetic stirrer for 2 h at room temperature to suspend the CNTs in the solution [8–10]. The equivalent formaldehyde amount was added and the rate of formaldehyde addition to the reactor was adjusted to 0.1 ml/min. The electroless reaction was completed within 30 min. The solution were filtered and washed by acetone followed by vacuum drying at 100°C for 2 h. The above-mentioned chemical bath composition was used for preparing four CNT/Cu composite powders of 10, 20, 30, and 40 vol.% CNTs. In addition, a pure copper powder sample as a reference sample was also prepared by the same conditions. For increasing the bonding between CNTs and copper, the prepared nanocomposite powders were heat-treated at 400°C for 30 min under hydrogen atmosphere. The prepared composite powders underwent elemental analysis. Each composite powder was sintered by spark plasma using an SPS apparatus (Dr SINTER model). The “LAB Spark Plasma Sintering System” enables the powders to be sintered by joule heat and spark plasma generated by high pulsed electric current applied through the compact. Preliminarily experiments were conducted to select the optimum conditions of the spark plasma sintering technique for CNT/Cu nanocomposites. The observations indicated that the shrinkage of the CNT/Cu stopped in one minute when the sintering temperature is 600°C and the compaction pressure is 20 MPa under vacuum of 10–3 torr. The sintering occurred in a uniaxial graphite mold of 10 mm in diameter to produce a 2-mm thick sintered sample.
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a
b
CNT CNT
c
d
Fig. 1. SEM micrographs for (a) the prepared copper powder by electroless deposition, (b) the asreceived multiwall carbon nanotube, (c) the prepared 10 vol.% CNT/Cu nanocomposite powder, and (d) the heat-treated 10 vol.% CNT/Cu nanocomposite at 400°C under hydrogen
The sintered samples underwent mounting and grinding using 2400 and 4000 grade SiC papers, respectively. The grinding samples were polished down to 3 μm using diamond paste of 3 μm particle size. Etchant solution composed of ferric nitrate and hydrochloric acid was used for etching the CNT/Cu sintered samples. The microstructures of the unetched and etched samples were investigated by HRSEM microscopy (“Philips XL 30SFEG” model). Each CNT/Cu nanocomposite was investigated by X-ray diffraction (XRD) with Cu-Kα source using diffractometer of the RIGAKU D/Max-IIIC (3KW) model. The densities of the prepared sintered materials were measured by Archimedes method using water as a floating liquid. The electrical resistivity of the CNT/Cu sintered materials was measured with four-probe method by using ohmmeter of the Omega multimeter CL1084 model by applying one ampere current through the sample, and the output voltage was measured and the resistivity was calculated.
RESULTS AND DISCUSSIONS Figure 1a shows SEM micrograph for the prepared copper powder by electroless deposition process in citrate bath. The copper powder has morphology of spherical particle shape of 1–2 μm in size. The electroless copper deposition reaction mechanism occurred according to the following chemical equations: HCHO + H2O → HCOOH + 2H+ + 2e–,
(1)
Cu++ + 2e– → Cu.
(2)
The first equation explains the hydrolysis of the formaldehyde producing the nascent hydrogen which reduces the citrate/copper complex ions to copper metal (according to Eq. 2) and the citrate salt remained in the solution and was removed by filtration. The electroless copper deposition reaction was finished within about 30 min
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Intensity (counts)
a
Cu(111)
15.0 10.0
Cu(200)
5.0
Cu(311) Cu(222)
Cu(220)
-3
10
20 3000
30
40
b
50
60
70
80
90
Cu(111)
2500 2000 Cu(200)
1500
Cu(220) Cu(311)
1000 500 0
Cu(222)
CNT(002)
10
20
30
40
50
60
70
80
2q, deg
Fig. 2. XRD patterns for (a) the prepared Cu powder and (b) the prepared 10 vol.% CNT/Cu nanocomposite powder by electroless deposition technique
when the color of the solution converted from blue to colorless due to the reduction of the blue copper ion to the copper metal. On the other hand, Fig. 1b shows SEM micrograph for the as-received multiwall carbon nanotubes (MWNTs). CNTs have microstructure of agglomerated filaments with a diameter between 10 and 15 nm. Figure 2c shows SEM micrograph for 10 vol.% CNT/Cu nanocomposite powders. One can see that the CNTs were implanted in the copper particles and the bonding between the CNTs and copper metal was increased by the heat-treatment process under hydrogen at 400°C for 30 min as shown in the SEM micrograph in Fig. 1d. Figure 2a illustrates the XRD pattern of the as-prepared sintered copper. The pattern indicates only one kind of peak for copper metal. On the other hand, Fig. 2b shows the XRD pattern for 10 vol.% CNT/Cu nanocomposite that has two kinds of peaks, the first kind for the copper metal and the second kind for the CNT only. However, the CNT peak has small intensity due to the high amorphous structure of the CNTs. Because the XRD analysis cannot detect all the trace elements included in the CNT/Cu nanocomposites, an elemental analysis for the prepared CNT/Cu powders was carried out as shown in Table 1. It was observed from the results of the elemental analysis that the CNT/Cu nano-composite powder contains trace amounts of sulfur, which contaminates the prepared materials, and it is assumed that this sulfur content remains from the sulphate bath of electroless Cu deposition. In addition, trace amounts of hydrogen, nitrogen, and oxygen contaminate the produced powder. TABLE 1. Typical Elemental Analysis of the Produced CNT/Cu Nanocomposites
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CNT, vol.%
C
S
N
H2
O2
Cu
10 20 30 40
2.50 5.00 7.50 10.00
0.50 0.70 0.30 0.40
0.17 0.27 0.12 0.14
0.10 0.15 0.12 0.13
0.20 0.45 0.50 0.75
96.53 93.43 91.46 88.58
CNT
a
CNT
b
CNT
c
d
Fig. 3. SEM micrographs for the prepared pure copper and the related CNT/Cu nanocomposites: a) sintered copper; b), c), and d) nanocomposites CNT/Cu with 10, 20, and 40 vol.% CNT, respectively
CNT CNT
a
b
CuLa
CuKa
1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 c
Fig. 4. SEM micrographs with EDAX semi-quantitative analysis for CNT/Cu nanocomposite: a) unetched CNT/Cu nanocomposites, b) etched CNT/Cu nanocomposites, and c) EDAX analysis for the copper matrix of CNT/Cu nanocomposites
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Electrical resistivity, mW · cm
Cu-40 CNT
14
Cu-30 CNT
12
Cu-20 CNT
10
Cu-10 CNT
8 6 4 2
Cu
0 Composition, vol.%
Fig. 5. The effect of CNT volume fraction on the electrical resistivity of the nanocomposites TABLE 2. Sintered and Relative Density Values for the Prepared Copper and CNT/Cu Nanocomposites CNT, vol.%
Density, g/cm3
Relative density, %
Cu (pure) 10 20 30 40
8.80 8.00 7.20 6.65 6.01
98.80 97.91 97.00 96.70 96.00
Figure 3a shows SEM micrograph for the sintered copper. The micrograph shows low porosity content of 1.2% as calculated from the Archimedes density as shown in Table 2. The micrographs in Figs. 3b, c, and d show the distribution of CNTs in the copper matrix for the prepared CNT/Cu nanocomposites with different CNT volume fractions. It was observed that with increasing the CNT content the volume fraction of CNT increased and the homogeneous distribution of CNTs in the copper matrix decreased as well as the density of the sintered composite decreased. Table 2 lists the density values for the prepared nanocomposites; by increasing the CNT volume fraction, the density decreased due to the agglomeration of CNTs in the copper matrix. It was also observed that the relative densities indicating good sinterability of the prepared copper and the related CNT/Cu nanocomposites. As illustrated from the SEM micrograph in Fig. 4a for the prepared CNT/Cu nanocomposites. Some of the CNTs are located at the grain boundaries of the copper matrix. But by etching the samples the CNTs can be appeared in the copper grains as shown in Fig. 4b. Also, the copper matrix was investigated with semi-quantitative EDAX analysis as shown in Fig. 4c. It was found that the copper matrix was composed of Cu metal. It was observed from the microstructures illustrated in Fig. 3, that the spark plasma sintering technique is a good method for preparing sintered copper as well as CNTs/Cu composites with good CNT homogeneity and distribution in the Cu matrix with low segregation of the CNTs from the copper matrix and fine microstructure preventing grain coarsening. Figure 5 shows the effect of the CNT volume fraction on the electrical resistivity. It was observed that the values of the electrical resistivity increased with increasing the CNT volume fraction in the copper matrix from 6.9 ⋅ 10–6 Ω · cm in case of 10 vol.% CNT/Cu to 13.5 ⋅ 10–6 Ω · cm in case of 40 vol.% CNT/Cu nanocomposite. Because CNT has resistivity of the order of 10–3 Ω · cm [11] but copper of the order of 10–6 Ω ⋅ cm, i.e., CNT has 0.001 conductivity relative to the pure copper metal, which decreases the conductivity of copper in the CNT/Cu nanocomposites. In addition, some CNTs agglomerated at the copper grain boundaries, a kind of grain boundary phase may be formed, which will increase the scattering of the charge carrier at grain boundary, thus reducing the conductivity.
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CONCLUSIONS Electroless deposition of copper on CNT surface is a good method for preparing high volume fractions of CNT/Cu nanocomposites. The CNTs are implanted in the copper particles. The sintering process of high volume fractions of CNT/Cu nanocomposites was improved by spark plasma sintering technique producing homogeneous CNT/Cu nanocomposites with good sinterability, low porosity, and low segregation of CNTs in the copper matrix. In addition, the sintered density decreased with increasing the CNT volume fractions in the copper matrix. The electrical resistivities of the CNT/Cu nanocomposites increased with increasing the CNT volume fraction in the copper matrix.
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