Generation of carbon nanofilaments on carbon fibers at 550 °C Claudia C. Luhrs a, * , Daniel Garcia a , Mehran Tehrani a , Marwan Al-Haik a , Mahmoud Reda Taha b , Jonathan Phillips a,c a Department of Mechanical Engineering, University of New Mexico, Albuquerque, NM 87131, United States b Department of Civil Engineering, University of New Mexico, Albuquerque, NM 87131, United States c Los Alamos National Laboratories, Los Alamos, NM 87544, United States ARTICLE INFO Article history: Received 13 May 2009 Accepted 6 July 2009 Available online 9 July 2009 ABSTRACT Employing a relatively new method, in which carbon structures are grown from fuel rich combustion mixtures using palladium particles as catalyst, multi-scale diameter nanome- ter – micrometer filament structures were grown from ethylene/oxygen mixtures at 550 °C on commercial PAN micrometer carbon fibers. The filaments formed had a diameter roughly equal to the palladium particle size. At sufficiently high metal loadings (>0.05 wt.%) a bimodal catalyst size distribution formed, hence a bimodal filament size dis- tribution was generated. Relative short, densely spaced nanofilaments (ca. 10 nm diame- ter), and a slightly less dense layer of larger (ca. 100 nm diameter) faster growing fibers (ca. 10 lm/h) were found to exist together to create a unique multi-scale structure. A protocol was developed such that only nano-scale fibers or a mixture of nano and sub- micron fibers could be produced. No large range order was evident in the filaments. This work demonstrates a unique ability to create a truly ’multi-scale’ carbon structure on the surface of carbon fibers. This fiber structure potentially can enhance composite material strength, ductility and energy absorption characteristics. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Composites (e.g. fiber reinforced polymers) failure under high-energy loading, i.e. impact, can be traced to the limited bond strength between the matrix and the fibers [1–3]. Mul- ti-scale fiber systems that include high surface area ‘nano’ component clearly will have increased surface area, hence possibly increased shear strength. The most common ap- proach to creating a multi-scale system is simply to physically mix carbon nanotubes into a more traditional composite con- sisting of epoxy with embedded microscale fibers. The inclu- sion of carbon nanotubes (CNTs) clearly toughens different matrices [4,5]. Depositing CNTs in a brittle matrix increases stiffness by orders of magnitude [6]. This approach to create multi-scale composites is limited due to the difficulty of dis- persing significant amounts of nanotubes [7,8] and it has repeatedly been reported that phase separation occurs above relatively low weight percent loading (ca. 3%) due to the strong van der Waals forces between the CNTs compared with that between the CNTs and the polymer matrix. Hence, the nanotubes tend to segregate and form inclusions. One means to prevent nanotubes or nanofilaments agglomeration is to anchor one end of the nanostructure, thereby creating a stable multi-scale structure. This is most readily done by literally growing the CNTs directly on micron scale fibers. Recently, CNTwere grown on carbon fibers, both polyacrylonitrile- (PAN-) and pitch-based, by hot filament chemical vapor deposition (HFCVD) using H 2 and CH 4 as pre- cursors. Nickel clusters were electrodeposited on the fiber surfaces to catalyze the growth and uniform CNTs coatings were obtained on both the PAN- and pitch-based carbon fibers. Multi-walled CNTs with smooth walls and low 0008-6223/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.07.019 * Corresponding author: Fax: +1 505 277 1571. E-mail address: ccluhrs@unm.edu (C.C. Luhrs). CARBON 47 (2009) 3071 – 3078 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon