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An approach to the uniform dispersion of a high volume fraction of carbon nanotubes in aluminum powder
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available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
An approach to the uniform dispersion of a high volume fraction of carbon nanotubes in aluminum powder Lin Jiang a, Genlian Fan a, Zhiqiang Li a,*, Xizhou Kai a, Di Zhang a, Zhixin Chen b, Sam Humphries c, Greg Heness c, Wing Yiu Yeung c a b c
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China The Faculty of Engineering, University of Wollongong, NSW 2522, Australia Department of Physics and Advanced Materials, University of Technology Sydney, P.O. Box 123, Broadway, NSW 2007, Australia
A R T I C L E I N F O
A B S T R A C T
Article history:
A slurry based process was developed to achieve the uniform dispersion of a high volume
Received 2 December 2010
fraction of carbon nanotubes (CNTs) in aluminum powder. Al powder was subjected to sur-
Accepted 11 January 2011
face modification to introduce a hydrophilic polyvinyl alcohol (PVA) membrane on its sur-
Available online 14 January 2011
face, which has good wettability and strong hydrogen bonding interactivity with functionalized CNTs. It was found that, when mixed with a CNT aqueous suspension, the PVA-modified Al (Al@PVA) powder resulted in much better adsorption uniformity of CNTs than the untreated Al powder. The adsorption capacity of CNTs was greatly enhanced by using nanoflake Al powder, which has better geometric compatibility with the CNTs and a larger surface area than spherical powder. Consequently, a uniform dispersion of 20 vol.% CNTs was achieved in the nanoflake Al@PVA powders. The advantage of this approach is that it provides easy control over adsorption uniformity and capacity of CNTs in a metal matrix, through the combination of surface modification and thickness adjustment of the metal flake powders. Ó 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
Since the landmark paper by Iijima [1–3], carbon nanotubes (CNTs) have aroused much research interest in the materials science and engineering community. The unique electrical, excellent thermal and mechanical properties [4–8], together with high aspect ratios [9], large surface areas and light weight, make CNTs as ideal reinforcements in the field of nanocomposites. However, until recently the strength enhancement achieved by CNTs in metal matrix composites (MMCs) was far from expectations [10]. This was mainly attributed to the great difficulty in obtaining the high volume fraction and uniform dispersion of the CNTs in the metal matrix, originating from the intrinsic incompatibility between CNTs and the metal [11,12]. In order to achieve the full poten-
tials of CNTs in MMCs, simple but effective methods must be developed to uniformly disperse a high volume fraction of CNTs in the metal matrix. Up to the present, most research efforts have resorted to high energy ball milling to disperse CNTs in the metal powders [13]. However, the morphology and structural integrity of the CNTs were often damaged by the impacting of the milling balls, which proved to be detrimental to the mechanical properties and thus the reinforcing effect of CNTs [14]. On the other hand, many reports have confirmed that entangled CNTs could be broken up and uniformly dispersed in polymer and ceramic matrices through simple solvent or slurry blending [15,16]. Recently, several attempts were made to disperse CNTs in metal powders based on solvent or slurry blending. For examples, Deng et al. [17] dispersed CNTs in ethanol first,
* Corresponding author: Fax: +86 21 3420 3913. E-mail address: [email protected] (Z. Li). 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.01.021
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and then mixed the resultant CNT-ethanol suspension with 2024Al powders, followed by 1 h ball milling. Agarwal et al. [18] dispersed CNTs in a polyvinyl alcohol (PVA) aqueous solution first, and then mixed the resultant CNT-aqueous suspension with Al powder, followed by spray drying. However, in both cases the CNTs tended to agglomerate and form concentrated zone on the surface of the Al powders, even when the volume fraction of the CNTs was less than 2.0 vol.% (about 1.0 wt.%) [19]. The large difference of zeta potential and poor bonding between CNTs and metal powders are considered the primary causes of the non-uniform distribution in these slurry based processes [20]. In addition, the adsorption capacity of CNTs is also limited by the spherical shape and resultant surface curvature of the metal powders. Therefore, the incompatibilities of metal powders with CNTs, both in terms of surface properties and geometries, are the obstacles to achieve the uniform dispersion of a high volume fraction of CNTs in a slurry based process. In this paper, a novel strategy for a slurry based process was developed and verified to address the incompatibilities between the metal powders and CNTs. Firstly, PVA, a highly hydrophilic polymer was used to modify the surface of the Al powders, to make their surface properties more compatible with the functionalized CNTs; Secondly, Al flake powders with large flat surfaces were employed to make the geometries of the Al powders more compatible with the 1-D nature of the CNTs. Through the combination of these two measures, significant improvements in both adsorption uniformity and adsorption capacity of CNTs have been achieved.
2. 2.1.
Experimental Materials
Multiwall CNTs, approximately 30–50 nm in diameter, 0.5– 2 lm in length, functionalized with carboxyl (–COOH), were supplied by Chengdu Organic Chemistry Co. Ltd., China. Sodium dodecyl benzene sulfonate (SDBS) and PVA, with a molecular weight of about 77,000 g/mol, were supplied by Sinopharm Group Co. Ltd., China. Pure gas-atomized Al spherical powder was used as the starting metal powder. Spherical Al powders of three mean particle sizes, 45, 10, 1.5 lm were used. The Al flake powders of four different mean thicknesses of 0.2, 0.4, 1.5 and 10 lm were made from the spherical Al powder by ball milling. The spherical Al powders (100 g) were placed in 750 ml stainless steel mixing jars containing 2 kg
stainless steel milling balls of 5 mm diameter (giving an initial ball-to-powder weight ratio (BPR) = 20:1). The jars were filled with argon and were then agitated using a stirring ball mill at 423 rpm for varying milling times to get the required Al flake powders. Fig. 1a shows the morphology of the spherical Al powder (10 lm). The Al flakes show a large aspect ratio (about 100) and tabular morphology as shown in Fig. 1b. On the surface of the Al powders, there is a naturally formed alumina film (about 5 nm in thickness), as shown in the inset of Fig. 1b.
2.2.
Preparation of CNTs/Al composite powders
Fig. 2 illustrates the CNTs/Al composite powder fabrication procedure used in this investigation. In a typical experiment, four steps were involved in our slurry-based powder process:
2.2.1.
Preparation of the aqueous dispersion of the CNTs
1.5 wt% SDBS, which was used to solubilize a high weight fraction of CNTs in water by the nonspecific physical adsorption [21], and 5 mg/ml of CNTs were added into deionized water and then sonicated for 2 h to get an ink-like dispersion. This treatment dispersed the as-received tangled CNTs (Fig. 1c) into separated single wires in water.
2.2.2.
Modifying the Al powder with PVA
Al powder (100 g) was added to a 3 wt.% PVA aqueous solution (500 ml) and stirred for 1 h, filtered and then rinsed with deionized water to obtain the PVA-modified Al powder (Al@PVA).
2.2.3.
Adsorption of the CNTs onto the Al powder
Al@PVA powder (about 100 g) was added into water (1000 ml) to make a powder slurry and then the CNT aqueous dispersion was added in a dropwise fashion. The mixed slurry was mechanically stirred until its colour changed from black to transparent, then filtered and rinsed with deionized water to get the CNTs/Al composite powder. Finally, the CNTs/Al composite powder was vacuum dried (about 10 1 pa) at 333 K for 12 h.
2.2.4.
Removal of the PVA by pyrolysis
The CNTs/Al composite powder (about 100 g) was heated in a flowing argon atmosphere at 773 K for 2 h to remove the PVA from the composite powder. Energy dispersive X-ray (EDX) spectra of the CNTs/Al powder after pyrolysis shows no C K peak, which indicates that all the PVA has been removed.
Fig. 1 – Morphology of powders: (a) spherical Al powder; (b) flake Al powder, the inset TEM picture shows the thickness of alumina on the Al surface; (c) as-received tangled CNT powder.
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Fig. 2 – Fabrication procedures for CNTs/Al composite powder.
In this investigation, CNTs/Al spherical composite powders and CNTs/Al flake composite powders loaded with various volume fractions of CNTs were prepared by the technique described above.
2.3.
Characterization
The naturally formed alumina film on the Al surface was confirmed by high-resolution transmission electron microscopy. The distributions of CNTs within the Al/CNTs composite powders were characterized by field emission scanning electron microscopy (FE-SEM) on a LEO Supra 55 FE-SEM. To confirm the surface modification of Al with PVA, fourier transform infrared spectroscopy analysis (EQUINOX55) was carried out on the untreated Al and the Al@PVA powders. The infrared absorption spectra (IR) of the untreated Al and Al@PVA powders were obtained on an EQUINOX55 fourier transform infrared spectroscope from 1000 to 4000 cm 1 at a resolution of 2 cm 1.
3.
the O–H bend vibration of adsorbed water, and the bands at 2850, 2925 and 1460 cm 1 correspond to the C–H stretch and bend vibrations, originating from the PVA modified Al surface [22]. The bands at 3425 and 1379 cm 1 are assigned to the O–H stretch and bend vibrations of PVA respectively; the band at 1098 cm 1 is related to the C–O stretch vibration, a characteristic of PVA [23,24]. Thus the IR analysis confirms that the Al surface was coated with the PVA membrane and also that the PVA membrane contains a high density of hydroxyl groups (–OH). The inset of Fig. 3 shows the mixture of the CNT black suspension with the untreated Al slurry and the Al@PVA slurry after mechanical stirring. For the CNTs and untreated Al mixture, the upper layer of the mixture remains black, suggesting that most of the CNTs were still in suspension and were not adsorbed onto the Al surface. In contrast, the upper layer of the CNTs and the Al@PVA powders mixture is almost transparent, suggesting that most of the CNTs in the Al@PVA slurry have been adsorbed onto the Al surface. Typical secondary electron images of the composite powders are shown in
Results and discussions
3.1. Effect of the surface property of the Al powder on the adsorption uniformity of CNTs PVA is a water soluble polymer and is widely used as a binder, surfactant, corrosion inhibitor, and processing agent in the field of metal powder processing and water-borne coatings and/or pigments. The wettability of the Al powders was significantly increased by the PVA treatment in view of the fact that the Al@PVA powders dispersed in water after only a few minutes of ultrasonic stirring, while the untreated Al powders required a much longer time of ultrasonic stirring to be dispersed in water. The significant improvement in the hydrophylicity of the Al@PVA powder over the untreated Al maybe explained in terms of the surface structure changes introduced by the PVA treatment. As shown in Fig. 3, the IR spectrum of the Al@PVA powder shows many additional bands compared to that of the untreated Al. The band at 1640 cm 1 is due to
Fig. 3 – IR patterns of treated and untreated Al powders, inset is the image of suspension after Al adsorbing CNTs.
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Fig. 4 – FE-SEM of CNT-Al composite powder: (a) CNT clusters in untreated Al powders, (b)–(c) uniform adsorption of CNTs achieved by Al@PVA powders, noted that all the PVA has eventually been removed by pyrolysis.
Fig. 5 – Schematic of the CNT adsorption mechanism on the Al@PVA surface: (a) PVA membrane formation; (b) CNT adsorption.
Fig. 4. For the untreated Al powders, only a few of CNTs were found attached on the surface of the Al. Most of the CNTs were agglomerated as clusters in the gaps between the Al particles as shown in Fig. 4a. Fig. 4b and c shows the uniform adsorption of CNTs achieved by Al@PVA powders. It can be seen that the CNTs are homogeneously distributed on the whole surface of Al particles without obvious agglomeration of the CNTs between the particles. This striking difference in the distribution of CNTs prepared in the untreated Al and Al@PVA powders indicates that the PVA modification on the Al surface plays a key role on the adsorption uniformity of the CNTs. The poor adsorption uniformity of CNTs in the untreated Al powder is due to the different surface properties between the functionalized CNTs and the untreated Al, i.e., the –COOH functionalized CNTs are hydrophilic, while the untreated Al encapsulated by a naturally formed alumina film is hydrophobic. The very different water wettability implies that the CNTs and the untreated Al have little small chance to adsorb effectively during the mixing stage. Even if the CNTs come in contact with the Al surface by chance through stirring or electrostatic attraction, most of the CNTs cannot be anchored onto the untreated Al surface because the alumina film on the untreated Al surface has a very low density of –OH groups. Consequently, the CNTs tend to migrate with water flow and agglomerate into clusters
during the subsequent processing, resulting in the poor adsorption uniformity of CNTs. The PVA surface treatment resulted in the formation of a thin hydrophilic membrane on the Al surface, which provides compatible water wettablity to that of the functionalized CNTs. The –OH groups of the naturally formed alumina film on the surface of Al play a crucial role in the formation of the PVA membrane by absorbing PVA molecules through hydrogen bonding, when the numbers are large enough and cross-linking of the PVA molecules, as illustrated in Fig. 5a. Once formed the PVA membrane is quite stable and cannot be separated from the Al by rinsing with water, mechanical stirring and sonication. This PVA membrane introduced large amounts of –OH groups onto the surface and facilitated the uniform adsorption uniformity of CNTs through two different mechanisms. Firstly, the PVA macromolecule chains extend into the solution and make the surface of the Al powder hydrophilic. The hydrophilic nature of the PVA membrane significantly improves the water wettability of the Al powder and makes the surface of the Al powder compatible to that of the functionalized CNTs, thus increasing the chance of adsorption chance of the CNTs to the Al powder during the mixing stage. Secondly, When the CNTs impinge against the PVA membrane through high speed stirring, the PVA macromolecule interact with and anchor the CNTs onto the Al sur-
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face by hydrogen bonding between the –OH groups of the PVA and the –COOH groups of the CNTs, as illustrated in Fig. 5b. In the subsequent drying, the adsorbed CNTs are not free to migrate, thus maintaining a uniform dispersion till the end of the process. So, the great improvement in adsorption uniformity can be attributed to the PVA modification of the Al surface, which enhances water wettability and interactivity with –COOH functionized CNTs.
3.2. Effect of geometry of Al powder on adsorption capacity of CNTs Fig. 6 shows the relationship between the maximum adsorption capacity without causing CNT entanglement of the excess or non-adsorbed CNTs and the diameter of the Al powders. When the diameter of the spherical Al (P10 m) is much larger than the length of the CNTs (0.5–2 m), the adsorptive capacity decreases with the increase of the diameter, whereas the opposite trend occurs when the diameter of the Al powder is smaller or equal to the length of the CNTs. For example, the saturation amount of CNTs that can be homogeneously distributed on the surface of the 45 m spherical Al powder was about 0.15 vol.% and was about 0.5 vol.% for the 10 lm spherical Al powder. This can be explained by
Fig. 6 – Uniform CNT adsorption capacity as a function of the diameter of the spherical Al and the thickness of flake Al, inset depicts the curvature effect on CNT adsorptive capability.
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the increase of the specific surface area with the decrease in diameter of the Al particles. Although the specific surface area continues to increase with the further decrease of the diameter of the spherical Al, the adsorptive capacity decreases rather than increases when the diameter of the Al powder is close to the length of the CNTs. The 1.5 lm spherical Al powder has the adsorptive capacity of only 0.1 vol.% CNTs. This indicates that the saturation of adsorptive capacity does not depend only on the specific surface area of the powder, but also on the geometry of the Al particles. For the spherical Al particles, when the diameter reduces below some critical size, the adsorptive capacity becomes sensitive to the curvature of the Al powder. In the case of these smaller diameter particles the CNTs have to bend significantly on the Al surface in order to be adsorbed. That is the smaller the diameter of the Al particle, the more bending required of the CNTs. Consequently, only relatively shorter CNTs will be adsorbed onto the Al surface as schematically shown in the inset of Fig. 6. Fig. 7a clearly shows that the long CNTs protrude from the small Al particles (1.5 lm) and entangle with other long CNTs. Only short CNTs were found attached on the spherical Al surface. But for the large Al particles (10 lm), no CNTs clusters were found when 0.5 vol.% CNTs was added, as shown in Fig. 7b. Therefore, for the spherical Al, the uniformly dispersion of the CNTs is also limited by the relative size of the Al particles to the length of the CNTs, and larger specific surface areas does not necessarily result in high adsorptive capacity. To overcome the curvature effect and increase the adsorption capacity of CNTs, spherical Al powders were ball milled into Al flakes and the CNT adsorption capacity was investigated and compared with that of the spherical Al powders. By ball milling the 45 lm spherical Al into 1.5 lm-thick flakes, the adsorptive capacity increased from 0.15 to 2.5 vol.% CNTs, as seen in Fig. 6. Fig. 8 shows the surface morphologies of the 1.5 lm-thick Al flakes loaded with various volume fractions of the CNTs prepared by the Al@PVA slurry process. It can be seen that discrete CNT wires are attached to the surface of the Al flakes and no CNT clusters are evident even when the volume fraction of CNTs reaches as much as 2.5 vol.% as shown in Fig. 8e– f. When the 45 m spherical Al powder was milled into 0.2 lmthick Al flakes, the specific surface area increased by 150 times. Correspondingly, the adsorptive capacity increased from less than 0.2 vol.% for the spherical 45 m powder to 20 vol.% CNTs for the 0.2 m flaky powder (Fig. 6). So the CNT
Fig. 7 – FE-SEM of Al spherical powder of different diameters with adsorbed CNTs by the Al@PVA slurry process: (a) 1.5 lm Al with 0.5 vol.% CNTs; (b) 10 lm Al with 0.5 vol.% CNTs.
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Fig. 8 – FE-SEM of Al flake powder (1.5 lm): (a) without CNTs; (b) thickness of flake; (c)–(d) loaded with 0.6 vol.% CNTs; (e)–(f) loaded with 2.5 vol.% CNTs.
Fig. 9 – FE-SEM of Al flakes with 2.5 vol.% CNTs by Al@PVA slurry process: (a) 1.5 lm-thick flakes; (b) 0.4 lm-thick flakes; (c) 0.2 lm-thick flakes; (d)–(f) illustrating the distribution of CNTs between Al flakes.
adsorption capacity of the PVA-modified nano-flake Al is very large, much higher than the spherical Al powder. Compared to the usually used spherical Al powder, the Al flakes do not only have higher specific areas, but more importantly have much larger flat surfaces which can accommodate the adsorption of the longer CNTs with greater ease, as schematically shown in the inset of Fig. 6. As a result, all the CNTs used in this investigation, regardless of the length, can be adsorbed onto the surface of the flat Al flakes. The saturation capacity of adsorptive CNTs on the Al flakes increases rapidly with the decrease of the thickness as shown in Fig. 6 and a resulting increase in flat surface area. The more flat surface, the higher the adsorptive capacity for the Al flakes. Fig. 9 shows the surface morphologies of the Al flakes of different thickness loaded with the same volume fraction of the CNTs (2.5 vol.%). As can be seen that the interval between the CNTs on the surface of the 0.2 lm-thick Al flake is much larger than on the 1.5 lm-thick Al flake, thus the chance of CNT entangling on 0.2 lm-thick Al flake is less than 1.5 lmthick Al flake. So, the thinner the Al flakes, the higher the uniformity of CNTs on the Al surface, because of the larger specific area. Moreover, the distribution of CNTs in the direction perpendicular to the Al surface is more uniform for the thin-
ner Al flakes when they were closely packed together, as illustrated in Fig. 9d–f. Therefore, the high surface area and flat morphology of nanoflake powder plays an important role in achieving the high adsorption capacity and uniformity of CNTs in our approach. The thinner the flakes used, the higher CNTs adsorption capacity and uniformity can be achieved. Up to 20 vol.% CNTs can be uniformly adsorbed by the 0.2 lmthick flaky Al powder. To the best of our knowledge, this is the highest CNT volume fraction ever dispersed uniformly in Al powder, much higher than those reported for spherical Al powders [17,19]. The advantage of our approach over the existing slurry based processes is that, by adjusting the thickness of Al flakes, easy control over both adsorption uniformity of CNTs and adsorption capacity is achieved. Moreover, it can be extended to other metallic matrix systems such as Cu, Ti and Mg with the appropriated modifications, provided that suitable hydrophilic polymers can be found to form a membrane of good interactivity with the CNTs. Since only a few mild chemicals and water as a solvent, were used, our approach is an environmentally friendly, energy effective and scalable process to fabricate CNTs reinforced metal matrix composites.
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Conclusions
An environmentally friendly slurry based process was developed to uniformly disperse high volume fractions of CNTs in Al powders. The great improvement in the adsorption uniformity of CNTs can be attributed to the formation of a hydrophilic PVA membrane on the surface of Al powder, which has good water wettability and strong hydrogen bonding interactivity with the –COOH functionalized CNTs. The maximum adsorption capacity of CNTs for the spherical Al powders is very limited (no more than 1 vol.%) due to the incompatibility of the stiffness of 1-D CNTs and the surface curvature of the spherical particles. However, for flake Al powder, the maximum adsorption capacity of CNTs was greatly increased due to the large surface area and flat morphology. We found that as much as 20 vol.% CNTs could be homogenously adsorbed by the PVA-modified Al nanoflakes about 0.2 lm in thickness. Thus, high volume fractions of CNTs can be uniformly distributed in Al powder through the combination of PVA surface modification and using nanoflake powders with large surface areas.
Acknowledgments The authors would like to acknowledge the financial support of the National Natural Science Foundation (No.50890174, No.51071100), the International S&T Cooperation Program of China (No.2010DFA52550), and the International Science Linkage Program (No.CG110069) of the Department of Innovation, Industry, Science and Research (DIISR) of Australia.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2011.01.021.
R E F E R E N C E S
[1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. [2] Boehm HP. The first observation of carbon nanotubes. Carbon 1997;35:581–4. [3] Monthioux M, Kuznetsov VL. Who should be given the credit for the discovery of carbon nanotubes? Carbon 2006;44:1621–4. [4] Ruoff RS, Lorents DC. Mechanical and thermal properties of carbon nanotubes. Carbon 1995;33:925–30. [5] Lopez Manchado MA, Valentini L, Biagiotti J, Kenny JM. Thermal and mechanical properties of single-walled carbon nanotubes-polypropylene composites prepared by melt processing. Carbon 2005;43:1499–505. [6] Kuzumaki T, Hayashi T, Miyazawa K, Ichinose H, Ito K, Ishida Y. Processing of ductile carbon nanotube/C60 composite. Mater Trans 1998;38:574–7.
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[7] Salvetat-Delmotte JP, Rubio A. Mechanical properties of carbon nanotubes: a fiber digest for beginners. Carbon 2002;40:1729–34. [8] Thostenson ET, Ren Z, Chou TW. Advanced in the science and technology of carbon nanotubes and their composites: a review. Comp Sci Technol 2001;61:1899–912. [9] Valentini L, Biagiotti J, Kenny JM, Santucci S. Morphological characterization of single-walled carbon nanotubes-PP composites. Comp Sci Technol 2003;63:1149–53. [10] Choi HJ, Kwon GB, Lee GY, Baea DH. Reinforcement with carbon nanotubes in aluminum matrix composites. Scripta Mater 2008;59:360–3. [11] Robertson J. Realistic applications of CNT. Mater Today 2004;7:46–52. [12] Bakshi SR, Lahiri D, Agarwal A. Carbon nanotube reinforced metal matrix composites – a review. Int Mater Rev 2010;55:41–64. [13] Ruiz-Navas EM, Fogagnolo JB, Velasco F, Ruiz-Prieto JM, Froyen L. One step production of aluminium matrix composite powders by mechanical alloying. Composites Part A 2006;37:2114–20. [14] Poirier D, Gauvin R, Drew Robin AL. Structural characterization of a mechanically milled carbon nanotube/ aluminum mixture. Composites Part A 2009;40:1482–9. [15] Balani K, Bakshi SR, Chen Y, Laha T, Agarwal A. Role of powder treatment and carbon nanotube dispersion in the fracture toughening of plasma-sprayed aluminum oxidecarbon nanotube nanocomposite. J Nanosci Nanotechnol 2007;7:3553–62. [16] Estili M, Kawasaki A. An approach to mass-producing individually alumina-decorated multi-walled carbon nanotubes with optimized and controlled compositions. Scripta Mater 2008;58:906–9. [17] Deng CF, Zhang XX, Wang DZ, Lin Q, Li AB. Preparation and characterization of carbon nanotubes/aluminum matrix composites. Mater Lett 2007;61:1725–8. [18] Bakshi SR, Singh V, Seal S, Agarwal A. Aluminum composite reinforced with multiwalled carbon nanotubes from plasma spraying of spray dried powders. Surf Coat Technol 2009;203:1544–54. [19] Deng CF, Zhang P, Ma YX, Zhang XX, Wang DZ. Dispersion of multiwalled carbon nanotubes in aluminum powders. Rare Met 2009;28:175–8. [20] Kwon H, Estili M, Takagi K, Miyazaki T, Kawasaki A. Combination of hot extrusion and spark plasma sintering for producing carbon nanotube reinforced aluminum matrix composites. Carbon 2009;47:570–7. [21] Islam MF, Rojas E, Bergey DM, Johnson AT, Yodh AG. High weight fraction surfactant solubilization of single-wall carbon nanotubes in water. Nano Lett 2003;3:269–73. [22] Yang Q, Deng Y, Hu W. Preparation of alumina/carbon nanotubes composites by chemical precipitation. Ceram Int 2009;35:1305–10. [23] Hamouda SB, Nguyen QT, Langevin D, Roudesli S. Poly(vinylalcohol) /poly(ethyleneglycol)/poly(ethyleneimine) blend membranes-structure and CO2 facilitated transport. C R Chim 2010;13:372–9. [24] Ranjan S, Jue ZF, Chen FL. Multi-walled carbon nanotube/ polymer composite: a nano-enabled continuous fiber. J Compos Mater 2010;44:1305–16.
4 9 ( 2 0 1 1 ) 1 9 6 5 –1 9 7 1
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
An approach to the uniform dispersion of a high volume fraction of carbon nanotubes in aluminum powder Lin Jiang a, Genlian Fan a, Zhiqiang Li a,*, Xizhou Kai a, Di Zhang a, Zhixin Chen b, Sam Humphries c, Greg Heness c, Wing Yiu Yeung c a b c
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China The Faculty of Engineering, University of Wollongong, NSW 2522, Australia Department of Physics and Advanced Materials, University of Technology Sydney, P.O. Box 123, Broadway, NSW 2007, Australia
A R T I C L E I N F O
A B S T R A C T
Article history:
A slurry based process was developed to achieve the uniform dispersion of a high volume
Received 2 December 2010
fraction of carbon nanotubes (CNTs) in aluminum powder. Al powder was subjected to sur-
Accepted 11 January 2011
face modification to introduce a hydrophilic polyvinyl alcohol (PVA) membrane on its sur-
Available online 14 January 2011
face, which has good wettability and strong hydrogen bonding interactivity with functionalized CNTs. It was found that, when mixed with a CNT aqueous suspension, the PVA-modified Al (Al@PVA) powder resulted in much better adsorption uniformity of CNTs than the untreated Al powder. The adsorption capacity of CNTs was greatly enhanced by using nanoflake Al powder, which has better geometric compatibility with the CNTs and a larger surface area than spherical powder. Consequently, a uniform dispersion of 20 vol.% CNTs was achieved in the nanoflake Al@PVA powders. The advantage of this approach is that it provides easy control over adsorption uniformity and capacity of CNTs in a metal matrix, through the combination of surface modification and thickness adjustment of the metal flake powders. Ó 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
Since the landmark paper by Iijima [1–3], carbon nanotubes (CNTs) have aroused much research interest in the materials science and engineering community. The unique electrical, excellent thermal and mechanical properties [4–8], together with high aspect ratios [9], large surface areas and light weight, make CNTs as ideal reinforcements in the field of nanocomposites. However, until recently the strength enhancement achieved by CNTs in metal matrix composites (MMCs) was far from expectations [10]. This was mainly attributed to the great difficulty in obtaining the high volume fraction and uniform dispersion of the CNTs in the metal matrix, originating from the intrinsic incompatibility between CNTs and the metal [11,12]. In order to achieve the full poten-
tials of CNTs in MMCs, simple but effective methods must be developed to uniformly disperse a high volume fraction of CNTs in the metal matrix. Up to the present, most research efforts have resorted to high energy ball milling to disperse CNTs in the metal powders [13]. However, the morphology and structural integrity of the CNTs were often damaged by the impacting of the milling balls, which proved to be detrimental to the mechanical properties and thus the reinforcing effect of CNTs [14]. On the other hand, many reports have confirmed that entangled CNTs could be broken up and uniformly dispersed in polymer and ceramic matrices through simple solvent or slurry blending [15,16]. Recently, several attempts were made to disperse CNTs in metal powders based on solvent or slurry blending. For examples, Deng et al. [17] dispersed CNTs in ethanol first,
* Corresponding author: Fax: +86 21 3420 3913. E-mail address: [email protected] (Z. Li). 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.01.021
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and then mixed the resultant CNT-ethanol suspension with 2024Al powders, followed by 1 h ball milling. Agarwal et al. [18] dispersed CNTs in a polyvinyl alcohol (PVA) aqueous solution first, and then mixed the resultant CNT-aqueous suspension with Al powder, followed by spray drying. However, in both cases the CNTs tended to agglomerate and form concentrated zone on the surface of the Al powders, even when the volume fraction of the CNTs was less than 2.0 vol.% (about 1.0 wt.%) [19]. The large difference of zeta potential and poor bonding between CNTs and metal powders are considered the primary causes of the non-uniform distribution in these slurry based processes [20]. In addition, the adsorption capacity of CNTs is also limited by the spherical shape and resultant surface curvature of the metal powders. Therefore, the incompatibilities of metal powders with CNTs, both in terms of surface properties and geometries, are the obstacles to achieve the uniform dispersion of a high volume fraction of CNTs in a slurry based process. In this paper, a novel strategy for a slurry based process was developed and verified to address the incompatibilities between the metal powders and CNTs. Firstly, PVA, a highly hydrophilic polymer was used to modify the surface of the Al powders, to make their surface properties more compatible with the functionalized CNTs; Secondly, Al flake powders with large flat surfaces were employed to make the geometries of the Al powders more compatible with the 1-D nature of the CNTs. Through the combination of these two measures, significant improvements in both adsorption uniformity and adsorption capacity of CNTs have been achieved.
2. 2.1.
Experimental Materials
Multiwall CNTs, approximately 30–50 nm in diameter, 0.5– 2 lm in length, functionalized with carboxyl (–COOH), were supplied by Chengdu Organic Chemistry Co. Ltd., China. Sodium dodecyl benzene sulfonate (SDBS) and PVA, with a molecular weight of about 77,000 g/mol, were supplied by Sinopharm Group Co. Ltd., China. Pure gas-atomized Al spherical powder was used as the starting metal powder. Spherical Al powders of three mean particle sizes, 45, 10, 1.5 lm were used. The Al flake powders of four different mean thicknesses of 0.2, 0.4, 1.5 and 10 lm were made from the spherical Al powder by ball milling. The spherical Al powders (100 g) were placed in 750 ml stainless steel mixing jars containing 2 kg
stainless steel milling balls of 5 mm diameter (giving an initial ball-to-powder weight ratio (BPR) = 20:1). The jars were filled with argon and were then agitated using a stirring ball mill at 423 rpm for varying milling times to get the required Al flake powders. Fig. 1a shows the morphology of the spherical Al powder (10 lm). The Al flakes show a large aspect ratio (about 100) and tabular morphology as shown in Fig. 1b. On the surface of the Al powders, there is a naturally formed alumina film (about 5 nm in thickness), as shown in the inset of Fig. 1b.
2.2.
Preparation of CNTs/Al composite powders
Fig. 2 illustrates the CNTs/Al composite powder fabrication procedure used in this investigation. In a typical experiment, four steps were involved in our slurry-based powder process:
2.2.1.
Preparation of the aqueous dispersion of the CNTs
1.5 wt% SDBS, which was used to solubilize a high weight fraction of CNTs in water by the nonspecific physical adsorption [21], and 5 mg/ml of CNTs were added into deionized water and then sonicated for 2 h to get an ink-like dispersion. This treatment dispersed the as-received tangled CNTs (Fig. 1c) into separated single wires in water.
2.2.2.
Modifying the Al powder with PVA
Al powder (100 g) was added to a 3 wt.% PVA aqueous solution (500 ml) and stirred for 1 h, filtered and then rinsed with deionized water to obtain the PVA-modified Al powder (Al@PVA).
2.2.3.
Adsorption of the CNTs onto the Al powder
Al@PVA powder (about 100 g) was added into water (1000 ml) to make a powder slurry and then the CNT aqueous dispersion was added in a dropwise fashion. The mixed slurry was mechanically stirred until its colour changed from black to transparent, then filtered and rinsed with deionized water to get the CNTs/Al composite powder. Finally, the CNTs/Al composite powder was vacuum dried (about 10 1 pa) at 333 K for 12 h.
2.2.4.
Removal of the PVA by pyrolysis
The CNTs/Al composite powder (about 100 g) was heated in a flowing argon atmosphere at 773 K for 2 h to remove the PVA from the composite powder. Energy dispersive X-ray (EDX) spectra of the CNTs/Al powder after pyrolysis shows no C K peak, which indicates that all the PVA has been removed.
Fig. 1 – Morphology of powders: (a) spherical Al powder; (b) flake Al powder, the inset TEM picture shows the thickness of alumina on the Al surface; (c) as-received tangled CNT powder.
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Fig. 2 – Fabrication procedures for CNTs/Al composite powder.
In this investigation, CNTs/Al spherical composite powders and CNTs/Al flake composite powders loaded with various volume fractions of CNTs were prepared by the technique described above.
2.3.
Characterization
The naturally formed alumina film on the Al surface was confirmed by high-resolution transmission electron microscopy. The distributions of CNTs within the Al/CNTs composite powders were characterized by field emission scanning electron microscopy (FE-SEM) on a LEO Supra 55 FE-SEM. To confirm the surface modification of Al with PVA, fourier transform infrared spectroscopy analysis (EQUINOX55) was carried out on the untreated Al and the Al@PVA powders. The infrared absorption spectra (IR) of the untreated Al and Al@PVA powders were obtained on an EQUINOX55 fourier transform infrared spectroscope from 1000 to 4000 cm 1 at a resolution of 2 cm 1.
3.
the O–H bend vibration of adsorbed water, and the bands at 2850, 2925 and 1460 cm 1 correspond to the C–H stretch and bend vibrations, originating from the PVA modified Al surface [22]. The bands at 3425 and 1379 cm 1 are assigned to the O–H stretch and bend vibrations of PVA respectively; the band at 1098 cm 1 is related to the C–O stretch vibration, a characteristic of PVA [23,24]. Thus the IR analysis confirms that the Al surface was coated with the PVA membrane and also that the PVA membrane contains a high density of hydroxyl groups (–OH). The inset of Fig. 3 shows the mixture of the CNT black suspension with the untreated Al slurry and the Al@PVA slurry after mechanical stirring. For the CNTs and untreated Al mixture, the upper layer of the mixture remains black, suggesting that most of the CNTs were still in suspension and were not adsorbed onto the Al surface. In contrast, the upper layer of the CNTs and the Al@PVA powders mixture is almost transparent, suggesting that most of the CNTs in the Al@PVA slurry have been adsorbed onto the Al surface. Typical secondary electron images of the composite powders are shown in
Results and discussions
3.1. Effect of the surface property of the Al powder on the adsorption uniformity of CNTs PVA is a water soluble polymer and is widely used as a binder, surfactant, corrosion inhibitor, and processing agent in the field of metal powder processing and water-borne coatings and/or pigments. The wettability of the Al powders was significantly increased by the PVA treatment in view of the fact that the Al@PVA powders dispersed in water after only a few minutes of ultrasonic stirring, while the untreated Al powders required a much longer time of ultrasonic stirring to be dispersed in water. The significant improvement in the hydrophylicity of the Al@PVA powder over the untreated Al maybe explained in terms of the surface structure changes introduced by the PVA treatment. As shown in Fig. 3, the IR spectrum of the Al@PVA powder shows many additional bands compared to that of the untreated Al. The band at 1640 cm 1 is due to
Fig. 3 – IR patterns of treated and untreated Al powders, inset is the image of suspension after Al adsorbing CNTs.
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Fig. 4 – FE-SEM of CNT-Al composite powder: (a) CNT clusters in untreated Al powders, (b)–(c) uniform adsorption of CNTs achieved by Al@PVA powders, noted that all the PVA has eventually been removed by pyrolysis.
Fig. 5 – Schematic of the CNT adsorption mechanism on the Al@PVA surface: (a) PVA membrane formation; (b) CNT adsorption.
Fig. 4. For the untreated Al powders, only a few of CNTs were found attached on the surface of the Al. Most of the CNTs were agglomerated as clusters in the gaps between the Al particles as shown in Fig. 4a. Fig. 4b and c shows the uniform adsorption of CNTs achieved by Al@PVA powders. It can be seen that the CNTs are homogeneously distributed on the whole surface of Al particles without obvious agglomeration of the CNTs between the particles. This striking difference in the distribution of CNTs prepared in the untreated Al and Al@PVA powders indicates that the PVA modification on the Al surface plays a key role on the adsorption uniformity of the CNTs. The poor adsorption uniformity of CNTs in the untreated Al powder is due to the different surface properties between the functionalized CNTs and the untreated Al, i.e., the –COOH functionalized CNTs are hydrophilic, while the untreated Al encapsulated by a naturally formed alumina film is hydrophobic. The very different water wettability implies that the CNTs and the untreated Al have little small chance to adsorb effectively during the mixing stage. Even if the CNTs come in contact with the Al surface by chance through stirring or electrostatic attraction, most of the CNTs cannot be anchored onto the untreated Al surface because the alumina film on the untreated Al surface has a very low density of –OH groups. Consequently, the CNTs tend to migrate with water flow and agglomerate into clusters
during the subsequent processing, resulting in the poor adsorption uniformity of CNTs. The PVA surface treatment resulted in the formation of a thin hydrophilic membrane on the Al surface, which provides compatible water wettablity to that of the functionalized CNTs. The –OH groups of the naturally formed alumina film on the surface of Al play a crucial role in the formation of the PVA membrane by absorbing PVA molecules through hydrogen bonding, when the numbers are large enough and cross-linking of the PVA molecules, as illustrated in Fig. 5a. Once formed the PVA membrane is quite stable and cannot be separated from the Al by rinsing with water, mechanical stirring and sonication. This PVA membrane introduced large amounts of –OH groups onto the surface and facilitated the uniform adsorption uniformity of CNTs through two different mechanisms. Firstly, the PVA macromolecule chains extend into the solution and make the surface of the Al powder hydrophilic. The hydrophilic nature of the PVA membrane significantly improves the water wettability of the Al powder and makes the surface of the Al powder compatible to that of the functionalized CNTs, thus increasing the chance of adsorption chance of the CNTs to the Al powder during the mixing stage. Secondly, When the CNTs impinge against the PVA membrane through high speed stirring, the PVA macromolecule interact with and anchor the CNTs onto the Al sur-
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face by hydrogen bonding between the –OH groups of the PVA and the –COOH groups of the CNTs, as illustrated in Fig. 5b. In the subsequent drying, the adsorbed CNTs are not free to migrate, thus maintaining a uniform dispersion till the end of the process. So, the great improvement in adsorption uniformity can be attributed to the PVA modification of the Al surface, which enhances water wettability and interactivity with –COOH functionized CNTs.
3.2. Effect of geometry of Al powder on adsorption capacity of CNTs Fig. 6 shows the relationship between the maximum adsorption capacity without causing CNT entanglement of the excess or non-adsorbed CNTs and the diameter of the Al powders. When the diameter of the spherical Al (P10 m) is much larger than the length of the CNTs (0.5–2 m), the adsorptive capacity decreases with the increase of the diameter, whereas the opposite trend occurs when the diameter of the Al powder is smaller or equal to the length of the CNTs. For example, the saturation amount of CNTs that can be homogeneously distributed on the surface of the 45 m spherical Al powder was about 0.15 vol.% and was about 0.5 vol.% for the 10 lm spherical Al powder. This can be explained by
Fig. 6 – Uniform CNT adsorption capacity as a function of the diameter of the spherical Al and the thickness of flake Al, inset depicts the curvature effect on CNT adsorptive capability.
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the increase of the specific surface area with the decrease in diameter of the Al particles. Although the specific surface area continues to increase with the further decrease of the diameter of the spherical Al, the adsorptive capacity decreases rather than increases when the diameter of the Al powder is close to the length of the CNTs. The 1.5 lm spherical Al powder has the adsorptive capacity of only 0.1 vol.% CNTs. This indicates that the saturation of adsorptive capacity does not depend only on the specific surface area of the powder, but also on the geometry of the Al particles. For the spherical Al particles, when the diameter reduces below some critical size, the adsorptive capacity becomes sensitive to the curvature of the Al powder. In the case of these smaller diameter particles the CNTs have to bend significantly on the Al surface in order to be adsorbed. That is the smaller the diameter of the Al particle, the more bending required of the CNTs. Consequently, only relatively shorter CNTs will be adsorbed onto the Al surface as schematically shown in the inset of Fig. 6. Fig. 7a clearly shows that the long CNTs protrude from the small Al particles (1.5 lm) and entangle with other long CNTs. Only short CNTs were found attached on the spherical Al surface. But for the large Al particles (10 lm), no CNTs clusters were found when 0.5 vol.% CNTs was added, as shown in Fig. 7b. Therefore, for the spherical Al, the uniformly dispersion of the CNTs is also limited by the relative size of the Al particles to the length of the CNTs, and larger specific surface areas does not necessarily result in high adsorptive capacity. To overcome the curvature effect and increase the adsorption capacity of CNTs, spherical Al powders were ball milled into Al flakes and the CNT adsorption capacity was investigated and compared with that of the spherical Al powders. By ball milling the 45 lm spherical Al into 1.5 lm-thick flakes, the adsorptive capacity increased from 0.15 to 2.5 vol.% CNTs, as seen in Fig. 6. Fig. 8 shows the surface morphologies of the 1.5 lm-thick Al flakes loaded with various volume fractions of the CNTs prepared by the Al@PVA slurry process. It can be seen that discrete CNT wires are attached to the surface of the Al flakes and no CNT clusters are evident even when the volume fraction of CNTs reaches as much as 2.5 vol.% as shown in Fig. 8e– f. When the 45 m spherical Al powder was milled into 0.2 lmthick Al flakes, the specific surface area increased by 150 times. Correspondingly, the adsorptive capacity increased from less than 0.2 vol.% for the spherical 45 m powder to 20 vol.% CNTs for the 0.2 m flaky powder (Fig. 6). So the CNT
Fig. 7 – FE-SEM of Al spherical powder of different diameters with adsorbed CNTs by the Al@PVA slurry process: (a) 1.5 lm Al with 0.5 vol.% CNTs; (b) 10 lm Al with 0.5 vol.% CNTs.
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Fig. 8 – FE-SEM of Al flake powder (1.5 lm): (a) without CNTs; (b) thickness of flake; (c)–(d) loaded with 0.6 vol.% CNTs; (e)–(f) loaded with 2.5 vol.% CNTs.
Fig. 9 – FE-SEM of Al flakes with 2.5 vol.% CNTs by Al@PVA slurry process: (a) 1.5 lm-thick flakes; (b) 0.4 lm-thick flakes; (c) 0.2 lm-thick flakes; (d)–(f) illustrating the distribution of CNTs between Al flakes.
adsorption capacity of the PVA-modified nano-flake Al is very large, much higher than the spherical Al powder. Compared to the usually used spherical Al powder, the Al flakes do not only have higher specific areas, but more importantly have much larger flat surfaces which can accommodate the adsorption of the longer CNTs with greater ease, as schematically shown in the inset of Fig. 6. As a result, all the CNTs used in this investigation, regardless of the length, can be adsorbed onto the surface of the flat Al flakes. The saturation capacity of adsorptive CNTs on the Al flakes increases rapidly with the decrease of the thickness as shown in Fig. 6 and a resulting increase in flat surface area. The more flat surface, the higher the adsorptive capacity for the Al flakes. Fig. 9 shows the surface morphologies of the Al flakes of different thickness loaded with the same volume fraction of the CNTs (2.5 vol.%). As can be seen that the interval between the CNTs on the surface of the 0.2 lm-thick Al flake is much larger than on the 1.5 lm-thick Al flake, thus the chance of CNT entangling on 0.2 lm-thick Al flake is less than 1.5 lmthick Al flake. So, the thinner the Al flakes, the higher the uniformity of CNTs on the Al surface, because of the larger specific area. Moreover, the distribution of CNTs in the direction perpendicular to the Al surface is more uniform for the thin-
ner Al flakes when they were closely packed together, as illustrated in Fig. 9d–f. Therefore, the high surface area and flat morphology of nanoflake powder plays an important role in achieving the high adsorption capacity and uniformity of CNTs in our approach. The thinner the flakes used, the higher CNTs adsorption capacity and uniformity can be achieved. Up to 20 vol.% CNTs can be uniformly adsorbed by the 0.2 lmthick flaky Al powder. To the best of our knowledge, this is the highest CNT volume fraction ever dispersed uniformly in Al powder, much higher than those reported for spherical Al powders [17,19]. The advantage of our approach over the existing slurry based processes is that, by adjusting the thickness of Al flakes, easy control over both adsorption uniformity of CNTs and adsorption capacity is achieved. Moreover, it can be extended to other metallic matrix systems such as Cu, Ti and Mg with the appropriated modifications, provided that suitable hydrophilic polymers can be found to form a membrane of good interactivity with the CNTs. Since only a few mild chemicals and water as a solvent, were used, our approach is an environmentally friendly, energy effective and scalable process to fabricate CNTs reinforced metal matrix composites.
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Conclusions
An environmentally friendly slurry based process was developed to uniformly disperse high volume fractions of CNTs in Al powders. The great improvement in the adsorption uniformity of CNTs can be attributed to the formation of a hydrophilic PVA membrane on the surface of Al powder, which has good water wettability and strong hydrogen bonding interactivity with the –COOH functionalized CNTs. The maximum adsorption capacity of CNTs for the spherical Al powders is very limited (no more than 1 vol.%) due to the incompatibility of the stiffness of 1-D CNTs and the surface curvature of the spherical particles. However, for flake Al powder, the maximum adsorption capacity of CNTs was greatly increased due to the large surface area and flat morphology. We found that as much as 20 vol.% CNTs could be homogenously adsorbed by the PVA-modified Al nanoflakes about 0.2 lm in thickness. Thus, high volume fractions of CNTs can be uniformly distributed in Al powder through the combination of PVA surface modification and using nanoflake powders with large surface areas.
Acknowledgments The authors would like to acknowledge the financial support of the National Natural Science Foundation (No.50890174, No.51071100), the International S&T Cooperation Program of China (No.2010DFA52550), and the International Science Linkage Program (No.CG110069) of the Department of Innovation, Industry, Science and Research (DIISR) of Australia.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2011.01.021.
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