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SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 39 3 Synthesis and Processing: Morphologically Specific Methods. FILAMENTARY STRUCTURES The growth of filamentous carbon by the catalytic decomposition of hydrocarbons has been the subject of intensive research worldwide for more than a decade. Most of the work, regardless of its source, has centered on the use of Fe, Ni, or their oxides as hydrocarbon decomposition catalysts. The preferred hydrocarbons are the C1âC3 alkanes and benzene. The primary motivation for this research has been the potential for the low-cost synthesis of carbon filaments for use in filament-reinforced composites (e.g., carbonepoxy and carbon-carbon composites). Growth mechanisms have been established by electron microscopy observations. The most revealing studies have been performed by controlled-atmosphere electron microscopy, in which a gas reaction cell is incorporated within an electron microscope to permit the continuous observation of the gas-solid reactions as they occur (Baker and Harris, 1978). Much of this work has involved filament synthesis from the decomposition of acetylene at temperatures between approximately 500 and 975°C in the presence of iron, cobalt, and chromium catalysts. Each of the filaments was observed to have a catalyst particle at its growing end, with the diameter of the filament fixed by that of the catalyst particle (Figure 10). The filaments varied from 0.01 to 0.15 µm in diameter and from 0.5 to 8.0 µm in length. Filament growth followed random paths, thereby forming loops, spirals, and interconnected networks. Growth rates varied inversely with catalyst particle size. A filament stopped growing when its catalyst particle became completely covered with a carbon layer (Figure 10). In studies (Baker et al., 1982) of the effect of pretreatment of iron surfaces on filament formation, it was observed that heating in steam at 700°C to form FeO produced an extremely active filament-forming catalyst when
SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 40 Figure 10 Baker mechanisms for filamentous carbon growth: (A) particle at the tip mechanism; (B) particle at the base mechanism (Baker and Harris, 1978).
SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 41 reacted with acetylene or ethane. The key to the high activity of FeO is its role as a precursor for the formation of a high-surface-area Fe catalyst. More recently, the performance of transition metal-based alloy catalysts has been examined (H. Witzke and B. H. Kear, private communication, 1989). The behavior of the alloy catalysts is distinguished from that of pure Ni by the directionality and the rate of filamentous carbon growth. Filaments formed on pure Ni typically grow in a single direction, with the particle located at the tip of the filament. Alloy particles generally exhibit bidirectional or multidirectional growth, with the particle positioned in the middle of the filaments. Alloy particles also yield significantly higher filament growth rates than Ni alone. High-resolution lattice imaging studies of the filaments have revealed a bimodal structure consisting of a graphitic skin and amorphous core. Attempts to exploit this filamentary growth mechanism have been frustrated by the inability to obtain as- grown carbon filaments of sufficient strength and stiffness to qualify them as reinforcing elements in composites. The problem seems to be that the preferred temperature range for catalytic growth of ultrafine filamentary carbon networks (500 to 900° C) is well below that necessary for producing fully graphitic material of high strength and stiffness. To overcome this problem, several different approaches are being investigated (Tibbets and Devour, 1986; Endo and Koyama, 1976). Heat treatments at temperatures above 2500° C are reported to give fully graphitized filaments of high strength and stiffness. After heat treatment at 2500° C, the tensile strength and elastic coefficient of the resulting fibers were, respectively, 1 GPa and 14 GPa. Graphite fibers have also been grown directly on a ceramic surface pretreated with a ferric nitrate solution by a two-stage process. At the start of the two-stage growth process a mixture of methane (5 to 15 volume percent) and hydrogen is passed over the surface of the ceramic heated to between 600 and 1200° C. In the presence of this reducing atmosphere, the iron compound decomposes to form micron-sized iron particles, which act as the nucleation sites for the fibrous carbon growth. Filament formation is completed in a relatively short time after nucleation. The second growth stage is then initiated by increasing the methane concentration in the gas to 15 volume percent or higher. The methane-enriched gas results in the thickening of the filaments into fibers with diameters between 5 and 15 µm. It is expected that these different approaches to the synthesis of ultrafine carbon filaments from gaseous precursors will in due course yield filaments with the desired mechanical properties, in which case the emphasis will shift to matrix infiltration processes and the control of matrix-filament adhesion for optimizing composite properties. The long-term goal of General Motors Corporation, for example, is to commercialize the technology for producing carbon fibers at a cost competitive with that for glass fibers currently used in fiber-reinforced plastic automotive bodies.
