Chemical Modification of Single-Walled Carbon Nanotubes for the Reinforcement of Precursor-Derived Ceramics Yongming Li, † Lucı ´a Fernandez-Recio, ‡ Peter Gerstel, † Vesna Srot, † Peter A. van Aken, † Gerhard Kaiser, † Marko Burghard, ‡ and Joachim Bill* ,† Nanoscale Science Department, Max-Planck-Institut fu ¨r Festko ¨rperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany and Institut fu ¨r Nichtmetallische Anorganische Materialien, UniVersita ¨t Stuttgart, PulVermetallurgisches Laboratorium and Max-Planck-Institut fu ¨r Metallforschung, Heisenbergstrasse 3, D-70569 Stuttgart, Germany ReceiVed April 24, 2008. ReVised Manuscript ReceiVed June 23, 2008 Single-walled carbon nanotubes (SWNTs) were incorporated into precursor-derived ceramics made from a polysilazane. To improve the dispersion of the nanotubes in the liquid precursor and finally in the ceramic matrix, the SWNTs were chemically modified by (iodomethyl)trimethylsilane via a radical reaction. The functionalization degree of the modified SWNTs was determined to be 3 atom %. Microscopic investigation combined with viscosity measurements and thixotropy tests demonstrated that the functionalized SWNTs are more homogeneously dispersed in the liquid SWNT/polymer mixtures and the solid cross-linked precursor, as compared to pristine nanotubes. SWNT/Si-C-N ceramics with nanotube contents of up to 1 wt % were obtained through pyrolysis of cross-linked SWNT/polymer composites at 1000 °C. The presence of intact nanotubes in these composites could be verified by scanning transmission electron microscopy. The high viscosity of the SWNT/polysilazane mixtures was identified as an important prerequisite for attaining good nanotube dispersion in the Si-C-N matrix. Introduction Carbon nanotubes have attracted strong attention since their discovery 1 owing to their excellent mechanical, electrical, and thermal properties. Incorporation of carbon nanotubes into polymeric, 2-4 metallic, 5,6 or ceramic 7,8 matrices has enabled to improve substantially their mechanical performance and implement useful thermal, electrical, or optical properties. In particular, mechanical studies of multiwalled carbon nanotubes (MWNTs)/Si-C-N composites reveal a remarkable increase in the fracture toughness of the ceramics. 9 Several studies on polymer matrix composites have shown that the reinforcement by SWNTs is often superior to that achievable with MWNTs. 2 This difference is mainly due to the substantially higher surface area per unit mass of the SWNTs, which allows them a greater interaction with composite matrices. Toward the production of composites with a ceramic matrix, conventional powder-based ceramic processing techniques might disrupt the integrity of the nanotubes besides the difficulty in achieving a homogeneous dispersion of the nanotubes throughout the matrix. Such dispersion is particularly difficult to achieve for SWNTs as a consequence of a strong cohesive force between individual tubes that leads to formation of bundles and agglomerates. Accord- ingly, full exploitation of the benefit of SWNTs for reinforce- ment requires their chemical modification in order to facilitate separation of bundles into individual nanotubes. At the same time, the attached functional groups can provide a suitable interfacial bonding to ensure efficient load transfer between the matrix and the nanotubes. Chemical modification schemes 10 that have been successfully used for unbundling the nanotubes and tuning their interaction with the matrix 2,11 comprise the covalent attachment of alkyl 12-14 or aryl groups, 15,16 amines, 17,18 amino acids and biomolecules, 19,20 as well as polymers 21 like polystyrene, 22 poly(methyl methacrylate), 23 and polyimide. 24 Moreover, siloxane modification has been achieved via reaction of chlorosilanes with oxidized MWNTs, 25,26 and aminosilanes have been covalently linked to fluorinated SWNTs. 27 An alternative approach to ceramics, which offers various advantages over powder processing for the task of producing * To whom correspondence should be addressed. Phone: +49 711 689 3228. Fax: +49 711 689 3131. † Universita ¨t Stuttgart. ‡ Max-Planck-Institut fu ¨r Festko ¨rperforschung. (1) Iijima, S. Nature 1991, 354, 56–58. (2) Coleman, J. N.; Khan, U.; Gun’ko, Y. K. AdV. Mater. 2006, 18, 689– 706. (3) Moniruzzaman, M.; Winey, K. I. Macromolecules 2006, 39, 5194– 5205. (4) Ajayan, P. M.; Shur, J.; Koratkar, N. J. Mater. Sci. 2006, 41, 7824– 7829. (5) Laha, T.; Liu, Y.; Agarwal, A. J. Nanosci. Nanotechnol. 2007, 7, 515– 524. (6) Chen, W. X.; Tu, J. P.; Wang, L. Y.; Gan, H. Y.; Xu, Z. D.; Zhang, X. B. Carbon 2003, 41, 215–222. (7) Burghard, Z.; Schoen, D.; Gerstel, P.; Bill, J.; Aldinger, F. Int. J. Mater. Res. 2006, 97, 1667–1672. (8) Vasiliev, A. L.; Poyato, R.; Padture, N. P. Scr. Mater. 2007, 56, 461– 463. (9) Katsuda, Y.; Gerstel, P.; Narayanan, J.; Bill, J.; Aldinger, F. J. Eur. Ceram.Soc. 2006, 26, 3399–3405. (10) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV. 2006, 106, 1105–1136. (11) Breuer, O.; Sundararaj, U. Polym. Compos. 2004, 25, 630–645. (12) Saini, R. K.; Chiang, I. W.; Peng, H.; Smalley, R. E.; Billups, W. E.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. Soc. 2003, 125, 3617. (13) Boul, P. J.; Liu, J.; Mickelson, E. T.; Huffman, C. B.; Ericson, L. M.; Chiang, I. W.; Smith, K. A.; Colbert, D. T.; Hauge, R. H.; Margrave, J. L.; Smalley, R. E. Chem. Phys. Lett. 1999, 310, 367. (14) Liang, F.; Sadana, A. K.; Peera, A.; Chattopadhyay, J.; Gu, Z.; Hauge, R. H.; Billups, W. E. Nano Lett. 2004, 4, 1257–1260. 5593 Chem. Mater. 2008, 20, 5593–5599 10.1021/cm801125k CCC: $40.75  2008 American Chemical Society Published on Web 08/12/2008