Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
1999-11-22
2003-01-14
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
Reexamination Certificate
active
06506355
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to vapor grown carbon fibers generally, and more particularly to vapor grown carbon fibers having high surface energy and high surface area, and to methods of producing such fibers without post-manufacture treatment.
Commercial carbon fibers hold great promise as a high performance material for composites due to their high strength and high modulus. They are commonly made by elevating a precursor material such as polyacrylonitrile (PAN) or pitch in an inert atmosphere to a temperature around 1000° C. on continuous wind-up devices. They are generally continuous filaments and approximately 8 &mgr;m diameter.
This application is concerned with vapor grown carbon fibers (VGCF), which are a relatively recent entry in the field of carbon fibers and have similar or even superior physical properties, along with the potential for production at a lower cost.
The concept underlying the production of VGCF begins with metallic particles, with iron generally being the predominant constituent, which catalyze the growth of long slender partially graphitic filaments when exposed to a hydrocarbon gas in the temperature range of 600°-1500° C. It is known that sulfur enhances the growth (Kaufmann, U.S. Pat. No. 2,796,331) and that ammonia results in improved distribution in fiber diameter (Alig, et al. U.S. Pat. No. 5,374,415).
The iron catalyst required for the reaction is provided by using either liquid zero valent iron compounds such as iron pentacarbonyl or solid zero valent iron compounds dissolved in liquid hydrocarbons. An example of the latter is ferrocene. Liquid compounds are preferred since their introduction into the reactor is easier to control. They are most readily introduced by bubbling an inert gas through the liquid, thus carrying the catalyst into the reactor in the vapor state. The normal inert gases used for this method are argon, helium, and nitrogen.
U.S. Pat. No. 5,024,818 to Tibbetts et al. and U.S. Pat. No. 5,374,415 to Alig et al., the disclosures of which are hereby incorporated by reference, describe typical reaction processes and chambers. VGCF differ substantially from commercial carbon fiber in that VGCF are not continuous. They are about 0.001 to 0.04 mm long. Also, VGCF are much finer than their continuously grown counterparts. In addition, upon leaving the reactor, the fibers exist as an entangled mass that is very lightweight with a large apparent volume, from 5 to 50 ft
3
/lb. In other words, the fibers form a lightweight, fluffy entangled mass. In this state, the fibers are very difficult to ship and handle. Such a light and fluffy material is almost impossible to incorporate into mixing equipment that typically processes rubber or plastic. The fly loss and incorporation time are tremendous.
The VGCF produced by these processes have a surface energy in the range of 25 to 40 mJ/m
2
. This surface energy, although useful for some applications, is considered low with respect to the ability of resins to wet and, therefore, to adhere to the fiber for purposes of preparing useful composites. Due to the low surface tension, the fibers cannot be easily wetted out or mixed into liquid applications without prior surface treatments. These problems represent a severe limitation on the use of VGCF, as they cannot be readily dispersed into rubbers, plastics or the like. Thus, the development of methods by which the VGCF are wet out is important to the commercialization of these materials.
The conventional technique for providing the required surface modification of VGCF involves exposure to one or more of a number of oxidizing or etching agents in a post-manufacture processing step. “Surface Properties of Carbon Fibers” P. Ehrburger, in
Carbon Fibers and Filaments,
pp 147-161, J. L. Figueiredo, et al. (eds.) Kluwer Academic Publishers, 1990. “Effect of Surface Treatment on the Bulk Chemistry and Structure of Vapor Grown Carbon Fibers”, H.Darmstadt et al., CARBON, 35, no. 11, pp. 1581-1585, 1997. Such post-manufacture processing steps have also been demonstrated to provide desired modification of surface properties of VGCF. “Effect of Surface Treatment on the Bulk Chemistry and Structure of Vapor Grown Carbon Fibers”, H. Daimstadt et al., CARBON, 35, no. 11, pp. 1581-1585, 1997.
Carbon fibers may be activated by exposure to a number of oxidizing agents in the gas phase, including H
2
O and CO
2
. “Activation of Carbon Fibers by Steam and Carbon Dioxide”, S. K. Ryu, et al., CARBON 31, no. 5, pp. 841-842, 1993. Also, the use of ammonia as an additive gas during fiber synthesis, as taught by U.S. Pat. No. 5,374,415, has been shown to have beneficial impact on the morphology of the fiber, and to modify the surface of the fiber. Furthermore, the use of air containing O
2
as a purge gas following the synthesis of the fiber has been shown to provide some degree of oxidation of the fiber.
However, the known processes for increasing the surface energy and surface area of VGCF require post-manufacture treatment, which increases the cost and complexity of production. Accordingly, there is a need for methods of making high surface energy VGCF without post-manufacture treatment. There is also a need to produce such fibers in a way that maintains the physical integrity of the fiber, i.e., the inherent fiber strength is not compromised, and other inherent fiber properties, such as electrical and thermal conductivity, are maintained.
SUMMARY OF THE INVENTION
These needs are met by the present invention whereby high surface energy VGCF and methods of making such fibers are provided. The high surface energy VGCF of the present invention have a surface energy greater than about 75 mJ/m
2
without post-manufacture treatment. They preferably have a surface energy in the range of about 125 to about 185 mJ/m
2
and more preferably about 145 to about 185 mJ/m
2
. In addition, they preferably have a total surface area in the range of about 25 to about 200 m
2
/g, and an external surface area greater than 20 m
2
/g. The surface area of the high surface energy vapor grown carbon fibers is preferably increased by a factor of at least 2, more preferably 10 or greater.
The invention teaches the use of a gaseous oxidant, such as CO
2
, as an additive during fiber synthesis in order to provide the desired surface modification to the fiber, including increased surface area and surface energy. Because CO
2
is known to oxidize carbon as well as iron, the use of CO
2
during fiber synthesis might be expected to poison the fiber synthesis reaction, or to degrade the mechanical properties of the fiber. Thus, the successful use of CO
2
as a surface-modifying agent during a one-step fiber synthesis process without serious adverse effects on fiber synthesis rates or fiber structural properties was unexpected. This discovery reduces the cost and complexity of the production of high surface energy VGCF. It provides a one-step fiber synthesis process without the necessity of post-manufacture treatment.
In accordance with the present invention, methods of making a high surface energy VGCF are provided. One method involves forming a mixture comprising a gaseous hydrocarbon, ammonia, and an iron-containing compound decomposable to form iron nucleation sites. The hydrocarbon and the ammonia are present in an amount sufficient to provide a ratio of carbon atoms to nitrogen atoms in a range of from about 1:1 to 30:1. The gaseous oxidant is added as a separate stream to the mixture. The mixture is heated in a reactor for a time and at a temperature sufficient to cause decomposition of the decomposable compound to form particles of nanometer size iron nucleation sites dispersed and entrained in the gaseous mixture which induce growth of carbon fibers. High surface energy VGCF are formed, which have an average diameter of about 0.05 to about 0.5 micron, contain carbon and nitrogen, and have a surface energy greater than about 75 mJ/m
2
. The high surface energy VGCF are then recovered.
In another embodiment, the carbon dioxide is introduced into the mixture by us
Glasgow D. Gerald
Lake Max L.
Applied Sciences, Inc.
Hendrickson Stuart L.
Killworth, Gottman Hagan & Schaeff, L.L.P.
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