Heat treating carbon nanofibers for optimal composite performance
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G.G. Tibbetts, C. Kwag, and M.L. Lake Applied Sciences, Inc., Cedarville, Ohio 45314 (Received 13 June 2006; accepted 13 July 2006)
Partial graphitization of carbon nanofibers by high-temperature heat treatment can give improved composite properties. The intrinsic electrical conductivity of the bulk carbon nanofibers measured under compression is maximized by giving the fibers an initial heat treatment at 1500 °C. Similarly, for carbon nanofiber/polypropylene composites containing up to 12 vol% fiber, initial fiber heat treatments near 1500 °C give tensile modulus and strength superior even to composites made from fibers graphitized at 2900 °C. However, optimum composite conductivity is obtained with a somewhat lower heat-treatment temperature, near 1300 °C. Transmission electron microscopy (TEM) along with x-ray diffraction (XRD) explains these results, showing that heat treating the fibers alters the exterior planes from continuous, coaxial, and poorly crystallized to discontinuous nested conical crystallites inclined at about 25° to the fiber axis.
I. INTRODUCTION
The attractive properties of catalytically grown carbon nanofibers (CNF) have been known for some time,1 as they offer a reinforcement that is of low diameter, electrically conductive, stiff, and strong. These fibers, called vapor-grown carbon fibers in earlier papers, were produced as an additive to polymers for conducting applications such as static discharge, electrostatic painting, and radio frequency interference shielding. Very early in the development of these materials, researchers recognized that, because of the partially graphitic nature of the fibers, their mechanical and electrical properties could be improved by a high-temperature graphitization.2 In the earliest work, and in many following papers,3 it was assumed that graphitization temperatures above 2800 °C offered optimum improvements. In this article we show for the first time that, for a nanofiber with a complex morphology incorporating nonlongitudinally oriented graphitic basal planes, partial rather than full graphitization may offer optimal overall improvements in electrical conductivity, tensile strength, and modulus. This work grew out of our recent measurements on graphitized carbon nanofibers that demonstrated that, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0325 2646
J. Mater. Res., Vol. 21, No. 10, Oct 2006
while in some cases the electrical conductivity of composites made with high-temperature graphitized nanofibers was considerably improved,4 the tensile strength of composites made with graphitized nanofibers degraded compared with the as-grown fibers. It has been suggested that, by analogy with polyacrylonitrile (PAN)-based carbon fibers, optimum microstructure for mechanical properties of carbon nanofibers might be achieved by a partial graphitization near 1500 °C.5 This step could be designed to leave distorted, cross-linked graphene layers, in contrast to the more highly ordered flat graphene planes created above 28
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