Properties of Novel Fullerene Tubule Structures: A Computational Study
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SUSAN B. SINNOTT%, CARTER T. WHITE* AND DONALD W. BRENNER" *Naval Research Laboratory, Surface Chemistry Branch, Code 6170, Washington, DC 203755342, "Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907
ABSTRACT Theoretical Young's moduli have been estimated for several fullerene carbon tubule fibers along the tubule axis. These results indicate that carbon fibers composed of nested tubules could have a modulus nearly 1.4 times that of a conventional graphite whisker. The results also indicate that the modulus of the tubule fiber can be further increased by decreasing the distance between the close-packed tubules in the fiber. This is true for both nested and single-shell tubules, but is only significant for the latter. Taking advantage of this property of single-shell tubules, we have examined a hypothetical tubule-diamond composite that, if it could be produced, would yield a modulus greater than diamond while at the same time stabilizing the fiber against shear distortions.
INTRODUCTION Since their discovery by Iijima in 1991', fullerene tubules have been targeted as potential building blocks for materials with a wide variety of new and novel properties. Electronic structure ' '5 , for example, have shown that the band gap for individual tubules depends strongly calculations2 34 on their helical structure and radius, with predicted conductivities ranging continuously from insulating to metallic. This suggests that carbon-based molecular wires with a range of electronic 6 78 properties could be produced using tubules with carefully tailored structures. Other calculations ' ' have examined the elastic properties of individual tubules and tubule crystals, with predictions of high strengths 7 and anomalous bulk moduli 8. Still other calculations 9 have suggested that tubules may act as molecular straws, providing nanometer-scale sieves for collecting and studying liquids at very small scales. With these predictions of new materials with unique properties, it is no wonder that fullerene tubules have been heralded as appropriate 'synthetic targets of the 1990's'". In this paper we explore the potential for fullerene tubules to yield new light-weight materials with high Young's moduli. The structure we examine, which is a natural one for this system, is a carbon fiber composed of tubules with their axes aligned. The calculations reported in this paper are restricted to Young's moduli in the direction of the tubule axis for fibers consisting of both close-packed individual and nested tubules. Based on a simple analysis, we show that fibers of this type can have large Young's moduli exceeding those of more traditional carbon whiskers provided that either the packing of individual tubules is very tight, or structures composed of nested tubules are used. We also suggest a novel diamond-tubule composite that may be able to maintain the very tight packing of tubules needed to achieve fiber moduli rivaling that of diamond, and at the same time provide resistance to shear
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