Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
2001-05-22
2004-07-13
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
C423S447100
Reexamination Certificate
active
06761871
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention involves the use of intense magnetic fields to accelerate the chemical changes associated with the formation of graphitic nanotubes. This invention pertains to a new process and apparatus for the massive synthesis of carbon nanotubes using strong external magnetic fields to enhance the formation and growth by reinforcing the intrinsic magnetic mechanisms occurring during the formation.
The carbon nanotube is a hollow tube with walls made of graphene and a capping dome containing C
6
conjugated aromatic rings and a few C
5
member rings to alleviate curvaceous strain on the bonds. Such macromolecules of graphene are known as fullerenes. The carbon nanotube is a fullerene of great importance.
The mass production of carbon nanotubes for diverse applications is currently among the most challenging problems in nanochemistry, potentially revolutionizing many important areas of material science, particularly nanoelectronics and structural materials. The importance of enhanced formation and mass production follows from the rich potential for applications.
For example, the potential applications in nanoelectronics result from the atomic level perfection. Future nanoelectronic materials will require atomic level perfection for optimum device performance. Such a demand is unrealistic for most materials. However, the nanotube is an exception to this impracticality. The nanotubes exhibit intrinsic perfection due to the chemically bonded network, where thermodynamics restricts defect formation. Such perfection in the carbon nanotube allows ideal exploitation of mesoscopic and molecular properties, where atomic defects are intolerable. The perfection allows nice thermal and electrical conductivity in the direction of the tube axis due to the extended electronic resonance. The chemically bonded network also leads to the maximum possible strength among materials. The strength of nanotubes will contribute to many future structural applications. This optimum strength reflects the extremely stable, doubly covalently bonded network. Carbon nanotubes therefore exhibit the largest strength/mass ratio known for fibers. This threshold for maximum strength requires the existence of atomic perfection in these nanotubes.
Because of these and other possibilities, the large scale production of carbon nanotubes is currently being investigated for applications in nano-computers, strong structural support, catalyst support, scan tunneling microscopy tips and hydrogen storage in order to make use of these unique properties. Unfortunately, the large-scale preparation of the carbon nanotubes has been a challenge with no breakthroughs, except for the new approach described here. With all the prospective applications, large-scale production will be necessary to realize these many new applications. First, the older techniques are considered along with problems associated with these techniques. Then the new invention is described with detailed explanations as to how the new art resolves difficulties of the older art.
The carbon nanotube, C
60
, and other fullerenes were initially discovered in the debris of DC arc discharges. Therefore, the earlier preparations of the nanotubes used DC arc discharges with the graphite and the catalytic metal buried within the anode. In this process, the anode is packed with the metal catalyst and a carbon source. The ablation during an intense pulse of electric current produces high temperature and generates plasma, which contains ions and atoms of the eroded anode material. These ions and atoms nucleate the fullerenes. Later strategies used modified arc discharge, which synchronously arched and helium quenched plasma in an effort to increase the formation of graphite encapsulated metal particles and the formation of carbon nanotubes. Laser ablation of the anode was the next effort used to increase production.
In recent years, with the discovery of catalytic chemical vapor deposition (CCVD), the emphasis for mass production has shifted from arc discharge to CCVD techniques. CCVD involves the formation of carbon nanotubes by the catalytic decomposition of hydrocarbons on the surface of Fe, Co, and Ni nanoparticles. The CCVD technique is a continuous process, whereas the arc discharge process is a batch process. The continuous CCVD process is an ideal approach for the industrial synthesis of nanotubes, since CVD allows more strict control of processing conditions and CVD has demonstrated itself to be a useful method for industrial production of many other materials. Although the CCVD effort has increased the production rate, current CCVD methods still do not achieve the desired bulky formation rates.
Unfortunately, past attempts to increase growth rates of nanotubes by CVD have failed due to the poisoning of the catalytic nanoparticles. These attempts involved the older upscaling methods. The older upscaling methods involved changing reactor parameters such as increasing concentration, increasing temperature, and changing catalyst content so as to increase growth rate. These efforts have not been very fruitful for achieving commercial growth rates. Higher hydrocarbon concentrations cause poisoning of the catalyst. Poisoning is the loss of catalytic activity due to carbiding. Carbiding is the formation of metal-carbon chemical bonds on and within the catalytic metal nanoparticle. Supersaturation is the state of the metal nanocatalyst in which the carbon concentration exceeds the equilibrium concentration for carbon precipitation. High concentrations also contribute to multi-walled carbon nanotubes and the accumulation of amorphous carbon along the tube backbone. High growth temperature leads to impractical production cost. The adulteration of the catalyst can introduce impurities into the nanotube and broaden the size distribution. These problems of traditional upscaling arise because the catalytic mechanism of carbon nanotube growth involves novel phenomena alien to the older art.
The following synopsis provides a condensed overall disclosure of references on many of these difficulties associated with trying to apply the traditional art to the carbon nanotube mass production. The current state of understanding of the nanotube formation in terms of the older chemical mechanisms and the history behind developing strategies for addressing these difficulties are disclosed. The current level of technology for nanotube formation and growth is disclosed. Finally, the new revelation from this invention for mass production is presented. The new revelation is considered in details, giving a detailed account of: 1) the new process in comparison to older processes, 2) the new mechanism based upon the intrinsic magnetic field and the consequences of superposing and reinforcing by an external magnetic field and 3) the new design and apparatus. But, first the references are given below:
R. T. K. Baker et al., “Nucleation and Growth of Carbon Deposits from the Nickel Catalyzed Decomposition of Acetylene,” Journal of Catalysis vol. 266, pp. 51-62 (1972) discloses the growth of graphite laminae from nickel particles (300 nm diameter) by the catalytic decomposition of acetylene at 1300K. Furthermore, they disclose terminated growth after 15 seconds at 870K upon the accumulation of amorphous carbon about the catalyst. The regeneration is reported after adding H
2
at 1100K or O
2
at 1000K. The filament formation is accounted for by acetylene adsorption and decomposition at one facet of the catalyst with subsequent internal carbon diffusion to and precipitation at an opposite facet of the particle. The diffusion process is disclosed as temperature and concentration driven.
J. C. Shelton et al., “Equilibrium Segregation of Carbon to Nickel(111) Surface: A Surface Phase Transition,” Surface Science vol. 43, pp. 493-520 (1974) discloses the precipitation on the (111) surface of Ni of at least three carbonaceous states: a high temperature dilute carbon phase, a condensed graphitic monolayer and a multilayered epitaxial graphite precipitate.
S. E. Stein and A. Fahr
Hendrickson Stuart L.
Lish Peter J
Little Reginald Bernard
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