PERGAMON Carbon 38 (2000) 1933–1937 Preparation of carbon nanofibers by the floating catalyst method * Lijie Ci , Yanhui Li, Bingqing Wei, Ji Liang, Cailu Xu, Dehai Wu Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China Received 13 September 1999; accepted 26 January 2000 Abstract Carbon nanofibers with diameters of 10|100 nm were produced by the floating catalyst method, which has a high productivity due to its continuous production and has potential applications in industrial engineering. Several experimental parameters, such as sulfur additives, the evaporating temperature of feedstock ( T ) and the hydrogen flow amount, have bw evident effects on the growth of carbon nanofibers. The samples were observed by transmission electron microscopy. It is shown that only an optimal amount of sulfur in feedstock is needed to obtain filamentous carbon. It is also indicated that the T is a key factor for the production of carbon nanofibers. In order to obtain thinner, straighter carbon nanofibers, a low T bw bw of 205|2308C was needed. The amount of hydrogen flow also plays an important role in the floating catalyst method, and the effect of hydrogen is a complex problem. Our experiments showed that a lower hydrogen flow amount is needed to obtain carbon nanofibers with smaller diameters.  2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers, Carbon filaments, Catalytically grown carbon, Vapor grown carbon 1. Introduction small-diameter VGCFs are stiffer than those of a larger diameter. Other studies showed that the characteristics of Progress in the preparation of vapor grown carbon fibers VGCFs, as their diameter decreases, bear a close con- (VGCFs) has continued at a considerable pace during the nection to the carbon nanotube (CNT) [5], which has novel past few years because of their exciting prospective electronic and mechanical properties and was widely potential (low production costs, as-produced short fibers, investigated since its discovery by Iijima in 1991 [6]. high electrical and thermal conductivity) in the production However, the poor productivity of CNTs limits its en- of polymer–matrix and metal–matrix composites. VGCFs gineering application. We believe that the carbon nanofib- are produced from the catalytic decomposition of a hydro- ers can be a substitute for CNTs in some fields such as carbon such as benzene or methane in a hydrogen atmos- nanofiber-reinforcing composites because they can be phere within the temperature range of 700|12008C [1]. In commercially produced more easily. the laboratory, experimental methods that have been used In most of the earlier studies, carbon fibers with to prepared VGCFs include the seeded catalyst method diameters of several microns were prepared by the floating [1,2], which uses the catalysts seeded on a substrate within catalyst method. In our studies, we attempted mass pro- the reactor, and the floating catalyst method [2,3], which duction of carbon fibers with smaller diameters (i.e., uses the catalysts floating in the reactor space. The floating carbon nanofibers) [7]. During our preparation, we found catalyst method has a higher productivity than the seeded that the features of carbon fibers, depending on the reactor one because the former can collect products continuously conditions and experimental parameters, can be easily and has potential applications in industrial engineering. controlled. The earlier studies revealed that VGCFs prepared by the floating catalyst method are constructed of a catalytically grown filament of excellent graphitization (also known as 2. Experimental primary filaments), covered with varying layers of pyrocar- bon of lower graphitization [4]. It is also shown that the 2.1. Experimental apparatus Fig. 1 shows the schematic diagram of the apparatus. A *Corresponding author. E-mail address: cilijie@263.net (L. Ci) quartz tube (outer diameter568 mm, inner diameter558 0008-6223 / 00 / $ – see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(00)00030-0