Synthesis of catalyst particles in a vapor grown carbon fiber reactor

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Abstract

Carbon fiber has become an important reinforcement material in composite materials because of its low density, high strength (up to 7GPa) and tensile modulus up to 600GPa. In addition, high electrical and thermal conductivity of carbon fiber makes it useful in a wide variety of products. A variety of carbon fibers are currently being considered for use in composite materials. The focus of this paper is the synthesis of catalyst particles in a vapor grown carbon fiber (VGCF) reactor. These reactors produce carbon fiber by pyrolysis of hydrocarbons over catalytic transition metal particles. The VGCF reactor being studied synthesizes iron sulfide catalyst particles inside the reactor as a part of the carbon fiber production process. The sulfur content of these particles is critical to their function as catalyst. A model has been developed to study the formation of catalyst particles in a flow reactor. The model is based on the nucleation of particles by collision of molecules of catalyst, which in turn are produced by gas phase reactions. Calculations have been carried out to determine the nucleation rates of the catalyst particles in the mixture, and the composition of the catalyst particles as they grow. Results indicate that submicron catalyst particles are produced in the process; the smallest catalyst particles are predicted to contain higher fraction of sulfur. Experimental studies have been also carried out to determine carbon fiber growth as a function of residence time.

Introduction

Carbon fiber reinforced composites are being developed for high specific strength over a wide range of temperatures. However, conventional carbon fibers, such as PAN based and pitch based fibers, cannot compete with glass fibers in automotive and low cost applications due to their high production cost. High specific stiffness, strength, electrical and thermal conductivity would make carbon fiber a very attractive alternative if production costs could be reduced. Vapor grown carbon fibers (VGCF), produced in flow reactor by pyrolytic decomposition of hydrocarbon gases, are expected to provide economical and functional advantages in many applications.

The history of growing carbon fibers from hydrocarbon gases is more than 100 years old (Hughes & Chambers, 1889), but the detailed mechanisms of fiber formation and growth are still not completely understood. Since then, there have been many attempts to explore the underlying principles behind VGCF production. Koyama (1972), and later Koyama and Endo (1973) made significant developments in this area by studying the decomposition of benzene at 1200°C. Endo and Komaki (1983) have investigated the relationship between the fiber structure and the catalytic particles which initiate and enhance the growth of fibers. Katsuki, Matsunaga, Egashira, and Kawasumi (1981) grew fibers on many types of catalyst particles by thermally decomposing naphthalene–hydrogen mixture. Masuda, Mukai, and Hashimoto (1993) have studied the liquid pulse injection technique for producing VGCF. Oberlin, Endo, and Koyama (1976), and Hoque and Alam (1996) have described models to describe catalyst particle growth in the VGCF reactor. This paper describes an extension to the earlier model to determine the composition of the catalyst particle.

A model for catalyst particle growth for a pulse injection VGCF reactor has been described by Mukai, Masuda, Matsuzawa, and Hashimot (1998). The pulse injection method described in the study uses a solution of ferrocene as the catalyst precursor. The ferrocene solution is injected into the reactor in the liquid state and impinges on the hot wall of the reactor. The solution then evaporates and decomposes to produce the catalyst. The authors combined experimental and analytical approaches to determine the catalyst particle size range that would improve the VGCF yield. The analytical approach used a discrete-sectional approach to study the growth of catalyst particles and compared the analytical and experimental results.

This paper focuses on the experimental and theoretical study of a VGCF reactor. The reactor configuration is shown in Fig. 1. This is very similar to the typical VGCF reactor described by Tibbetts and Alig (1992). In this reactor, methane gas is introduced along with Fe(CO)5 vapor into a heated cylindrical reactor tube. Along with these reactants, H2S gas is generally added to improve the fiber yield. The decomposition of Fe(CO)5 occurs first, leading to the production of iron particles. The iron particles suspended in the gases are carried along with the flowing gases. Subsequently, methane decomposes in the reactor, and carbon is dissolved in the iron catalyst particles. The catalyst particles grow submicron carbon fibers from the dissolved carbon. The fibers grow thicker by chemical vapor deposition (CVD), and the final thickness is of the order of half a micron.

Experiments have shown that the size of the catalyst particle determines the diameter of the carbon fiber that grows out of the catalyst particle. Consequently, the catalyst size is an important parameter for the process. The yield of carbon fibers also depends on the number or concentration of the catalyst in the gases. The determination of catalyst particle size, composition, and fiber growth history is a primary objective of this study.

Tibbetts (1983) developed a process for the growth of carbon fiber from methane gas in a flow reactor. From these studies it was determined, as originally proposed by Oberlin et al. (1976), that fiber growth process consists of two separate steps—axial growth or lengthening process followed by the classical CVD process. In the second step, fiber grows in the radial direction by CVD, thereby producing a thicker fiber. Tibbetts, Devour, and Rodda (1987) later modeled the hydrocarbon adsorption and diffusion through the catalyst particles, and the CVD thickening process which appears to terminate the fiber lengthening. Benissad, Gadalle, Caulon, & Bonnetain 1988a, Benissad, Gadalle, Caulon, & Bonnetain 1988b studied the methane–hydrogen system, focusing on conditions of fiber growth and the annealing and melting of the catalyst. It has been shown that, when H2S is added to the reactant stream, sulfur from H2S enhances the catalytic action of the iron particles in producing carbon fibers (Tibbetts, Bernardo, Gorkiewicz, & Alig, 1994).

A critical step in the production of VGCF is the nucleation of iron particles in the gas phase, and the incorporation of the sulfur into the iron catalyst. The size and composition of the iron catalyst particles determine the characteristics of the fiber. It has been suggested that the fiber growth is caused by temperature gradients in the catalyst particle (Baker, 1989).

The primary objective of this paper is to model the processes of catalyst particle formation in the VGCF reactor, and the composition history of the particle. The analysis steps are similar to what was done for the liquid pulse injection reactor by Mukai et al. (1998), and the VGCF reactor model by Hoque and Alam (1996); the primary difference in the current model is the addition of species differentiation in the calculation. The species calculation gives the composition history of the particle as it grows in the reactor.

The analysis incorporates the techniques of multicomponent aerosol dynamics (Gelbard & Seinfeld, 1980) and is based on the discrete-sectional model of aerosol formation and growth (Wu & Flagan, 1988). In this approach, collisions of single molecules producing molecular clusters, and growth of the molecular clusters by inter-particle collisions are modeled by using particle dynamics. Such models have been used (Hoque & Alam, 1996) to model the iron catalyst formation in the VGCF reactor. The model presented here incorporates calculations of the catalyst composition along with catalyst size and concentration. The composition is determined by tracking collision rates of sulfur molecule clusters with iron molecule clusters. A similar approach has been used by Biswas, Wu, Zachariah, and McMillin (1997) to study the formation of iron–oxide–silica composites. The results from the particle collision model provide information on the particle sizes, concentration and composition of the catalyst particles. The analysis is applied to a VGCF reactor and the results are compared with experimental data.

Section snippets

VGCF reactor

A joint study of the VGCF reactor was undertaken by investigators from Ohio University, General Motors Research Laboratories (Warren, MI), and Applied Sciences Inc. (Cedarville, OH). This reactor (Fig. 1) consists of a vertical mullite tube, 91.4cm long and 1.45cm in diameter, heated externally by a resistive heating furnace. The controller of the heater was set at 1100°C, which results in a uniform temperature zone of about 50cm in the middle of the reactor. The catalyst particles and fibers

Catalyst nucleation

The catalyst nucleation and growth model to be discussed here is developed from a simpler model presented in an earlier study (Hoque & Alam, 1996) in which the catalyst particle size was calculated. In the current model, the size, as well as the composition of catalyst particle (sulfur and iron) is determined. The catalyst nucleation and growth calculations must therefore include the formation of iron particles as well as the inclusion of sulfur in the iron particles. The effect of sulfur is to

Model results for catalyst nucleation

The growth , , , were integrated to simulate the process of catalyst nucleation in the VGCF reactor with iron pentacarbonyl (one species only, i.e., s=1) in an inert carrier gas (helium) flow ranging from 2700 to 10,800sccm at atmospheric pressure. These flow rates correspond to residence times of 0.25–1s. The temperature history was obtained from the flow rates and the temperature profile shown in Fig. 3. The concentration of the iron pentacarbonyl in the inert carrier gas was varied from 2×10

Growth of carbon fibers

Experiments were also been carried out to determine the growth rate of carbon fiber in the VGCF reactor as a function of the residence time. The preliminary results from these experiments are summarized below. The reactor in which these fiber growth experiments were carried out is a vertical flow reactor, which has been described earlier. The change in the residence time in this reactor is accomplished by changing the total volume flow through the reactor volume. The results of these

Conclusions

The formation and growth of the iron catalyst in a VGCF reactor has been studied by using the particle collision theory. The model incorporates the calculation of sulfur and iron content in the catalyst particle. The results indicate that the size of catalyst particles is controlled by the residence time and the reactant concentration. The particle size increases rapidly with residence time until the reactant is depleted, or when the particle concentration decreases. The sulfur content is

Acknowledgements

The authors would like to acknowledge support from Edison Material Technology Center (EMTEC, Kettering, Ohio), and Applied Sciences Inc., through the EMTEC CT-46 program.

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