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phys. stat. sol. (a) 201, No. 8, R53R55 (2004) /DOI 10.1002/pssa.200409045

2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

physica

statussolidi

Rapid

Research

NoteRapid Research Note

Carbon nanotube/magnesium composites

E. Carreo-Morelli*, 1, J. Yang2, E. Couteau2, K. Hernadi2, J. W. Seo2, C. Bonjour1,L. Forr 2, and R. Schaller21 University of Applied Sciences of Western Switzerland, Design & Materials Unit, 1950 Sion,

Switzerland2 Swiss Federal Institute of Technology Lausanne, Institute of Physics of Complex Matter,

1015 Lausanne, Switzerland

Received 23 April 2004, revised 17 May 2004, accepted 18 May 2004Published online 25 May 2004

PACS 61.46.+w, 81.05.Ni, 81.05.Zx, 81.07.De

Novel magnesium matrix composites reinforced with carbon nanotubes have been processed by powdermetallurgy. Blends of metal powders and multi-wall carbon nanotubes were compacted by uniaxial hotpressing followed by hot isostatic pressing. A uniform dispersion of nanotubes in the metal matrix wasobtained. A coating method of nanotubes is described, which is promising to improve the matrix-reinforcement bonding strength.


1 Introduction High stiffness and very low density make carbon nanotubes (CNTs) good candidatesfor use as reinforcements in composite materials [1]. CNTs can be produced by arc-discharge methods,laser ablation or catalytic chemical vapor deposition (CVD). Arc-discharge and laser ablation CNTs arethe ultimate high strength fibers with Youngs modulus up to 1.2 TPa [2], but they cannot yet be pro-duced in large-scale. Catalytic CNTs are one order of magnitude less stiff [3], but they offer the greatestpotential for production up-scaling. In recent years, much research has been devoted to the developmentof nanotube-reinforced polymers and ceramics. On the other hand, only few studies have been concernedwith nanotube-reinforced metals, which remains almost a virgin field [1, 4]. In this work, novel metalmatrix composites are processed by powder metallurgy of catalytic CNT/light-metal blends.

2 Experimental Multi-wall carbon nanotubes were processed by catalytic decomposition of acetylene

in a fixed-bed flow reactor at 720 C over 5% Co, Fe/CaCO 3 catalyst [5]. Commercially available mag-nesium powder (99.8% purity, average particle size 38 m, Alfa Aesar) was used as base metal powder.

Mg2wt%CNT powder mixtures where prepared by dry blending for 4 h in a Turbula T2C mixer.Ceramic balls were used during mixing to mill CNT agglomerates and improve mixture homogeneity.The blends were placed in a double-action graphite tooling consisting of a die and two cylindrical pis-

tons. Disk-shaped compacts ( 53 mm 5 mm) were obtained by hot pressing at 600 C in a vacuumatmosphere, under a pressure of 50 MPa for 30 min. Finally, the compacts where hot isostatic pressed at600 C for 60 min under an argon pressure of 1800 bar. The density of the sintered parts was measuredby the Archimedes method.

*Corresponding author: e-mail: [email protected], Phone: +41 606 88 37, Fax: +41 606 88 15


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phys. stat. sol. (a) 201, No. 8 (2004) / www.pss-a.com R55


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Ti K

Cu L

Cu K

Mg K

Intensity

(arb.

unit)

Energy (keV)

Fig. 3 a) TEM micrograph of a multi-wall CNT with Mg coating. White arrows indicate the homogene-

ous coating of Mg with a thickness of 2 nm. b) EDX-spectrum reveals a significant peak of Mg. Cu peaksare due to the TEM grid used as sample supporting grid; the Ti peak is an artefact of our EDX set-up. No

significant peak of O K

at 0.523 keV is visible.

of interface bonding strength to guarantee load transfer from the matrix to the reinforcement. In a previ-ous work [3], it has been shown by AFM measurements of the Youngs modulus that catalytic CNTsexhibit poor mechanical properties compared with arc-discharge CNTs. The problem is that, at the mo-ment, the arc-discharged technique cannot be used for large-scale production. On the other hand, inter-face bonding strength can be improved by applying appropriate surface treatments to the nanotubes.Recently, an homogeneous coating of nanotubes with various inorganic materials as alumina, silica andtitania has been obtained [7]. In the present research, the focus was on metallic Mg coatings. Purifiedmulti-wall CNTs were used as raw or modified by surfactant (sodium dodecyl sulfate, SDS). The im-

pregnation was carried out with and without solvent (isopropanol) for both types of CNTs. As magne-sium sources, three compounds, namely magnesium ethoxide (Mg(OC2H5)2, Mg(OEt)2) and MgCl2 wereapplied. When MgCl2 source was applied, a thick layer of amorphous magnesia was obtained over SDS-treated CNTs, whereas the use of plain CNTs resulted in a few tubes showing slight coverage. Moreoverhomogeneous coverage of Mg was obtained on plain CNTs when the impregnation was carried out usingorganometallic and inorganic sources. Figure 3a shows a representative TEM image of a plain multi-wallCNT after coating with Mg. The coating is about 2 nm thick and the nanotube is homogeneously cov-ered. EDX analysis clearly confirms the presence of Mg as coating material (Fig. 3b).

4 Concluding remarks Carbon nanotube reinforced magnesium has been processed by dry blendingof base powders followed by hot pressing and hot isostatic pressing. A uniform dispersion of nano-tubes in the magnesium matrix has been obtained. The same processing steps have been used to process

Al2wt%CNT up to 96% of the theoretical density. In addition, a coating method of nanotubes with mag-nesium was developed, which is intended to improve interface bonding strength in sintered compacts.

Acknowledgements This work was supported by the Swiss Innovation Promotion Agency under CTI Contract No.5990.3 TNS, and by the European TMR network NANOCOMP.

References

[1] E. T. Thostenson, Z. Ren, and T.-W. Chow, Compos. Sci. Technol. 61, 1899 (2001).[2] F. T. Fischer, R. D. Bradshaw, and L. C. Brindson, Compos. Sci. Technol. 63, 1689 (2003).[3] J.-P. Salvetat et al., Adv. Mater. 11, 161 (1999).[4] C. L. Xu, B. Q. Wie, R. Z. Ma, J. Liang, X. K. Ma, and D. H. Wu, Carbon 37, 855 (1999).[5] E. Couteau et al., Chem. Phys. Lett. 378, 917 (2003).[6] B. Vittoz, B. Secrtan, and B. Martinet, J. Appl. Math. Phys. 14, 46 (1963).[7] K. Hernadi, E. Ljubovic, J. W. Seo, and L. Forr, Acta Mater. 51, 1447 (2003).

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