Microstructural investigations on zirconium oxide–carbon nanotube composites synthesized by hydrothermal crystallization F. Lupo * , R. Kamalakaran, C. Scheu, N. Grobert, M. R€ uhle Max-Planck-Institut f€ ur Metallforschung, Heisenbergstrasse 3, D-70569 Stuttgart, Germany Received 4 December 2003; accepted 25 March 2004 Available online 30 April 2004 Abstract Zirconium oxide (ZrO 2 )/carbon nanotube composites were prepared by hydrothermal crystallization at 200 °C for 8 h of zir- conium hydroxide [(Zr(OH) 4 nH 2 O; n ¼ 8–16] in the presence of carbon nanotubes. The hydrothermal crystallization of zirconium oxide, under autoclave conditions, occurred on the walls of the tubes. The process yielded a homogeneous composite powder consisting of microcrystalline ZrO 2 and multi-walled nanotubes, hence representing an alternative route for the synthesis of cera- mic–nanotube composites. Microstructural and qualitative characterizations were performed using different electron microscopy techniques. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Carbon nanotubes; B. Pyrolysis; C. Hydrothermal treatment; D. Transmission electron microscopy; E. Microstructure 1. Introduction Since their discovery by Ijima [1] carbon nanotubes (CNTs)havebeenattractivecandidatesforfundamental research studies. They occur as single-walled nanotubes (SWNTs) consisting of a single graphene sheet rolled-up into a cylinder with diameters as small as one nanome- tre,andmulti-wallednanotubes(MWNTs)composedof concentric graphene layers. Several applications were proposed for CNTs many of which are concerned with conductive or high strength composites [2–4]. The interest of using CNTs in ceramic composites arises from their formidable mechanical properties [5] i.e. Young’smodulusupto1470GPaforSWNT[6]andup to 950 GPa for MWNTs [7]. However, it has been demonstrated that the inner layers [8,9] of multi-walled CNTscontributelittletocarrytheload.Theinnerlayers of the nanotube ‘telescope’ out and fragment, when subjected to critical tensile stresses. On the other hand, single-walled CNTs, having higher curvatures and smaller diameters are predicted to lead to higher strengths and are therefore better suited for composites. Nevertheless MWNTs are often used for initial experi- ments due to easier availability. Recent literature [2,3] suggests the possibility to add of small amounts of nanotubes to ceramic powders to produce tougher ceramic materials. The inclusion of CNTs in a ceramic matrix is expected to create com- posites exhibiting high stiffness and improved mechani- cal properties compared to the single-phase ceramic material. Ideally, the CNTs dispersed in a ceramic ma- trix should serve as sites for fracture energy dissipation through mechanisms such as crack deflection and nanotube debonding/breakages [3]. Hot-pressing of ceramic powder mixed with CNT is the most common methodtopreparesuchcomposites.Inthiscontext,hot- pressed(2273K)CNTs–SiCcomposites,investigatedby Maetal.[10],showedanincreaseof10%inthestrength and fracture toughness compared to SiC ceramics pre- paredwiththesamemethod.Alumina–CNTcomposites werealsoinvestigatedinsomedepthsbyseveralauthors [11–16]. In contrast to reports of Peigney et al. [11]and Flahaut et al. [12], which did not observe an improve- ment of the mechanical properties of CNT–metal–alu- mina composites obtained by hot-pressing, Siegel et al. [13] measured an increase in fracture toughness of 24% (from 3.4 up to 4.2 MPam 1=2 ) in MWCNT–alumina composites (10% in vol) prepared by hot-pressing. * Corresponding author. Tel.: +49-711-689-3687; fax: +49-711-689- 3522. E-mail address: f.lupo@mf.mpg.de (F. Lupo). 0008-6223/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.03.037 Carbon 42 (2004) 1995–1999 www.elsevier.com/locate/carbon