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Method for Producing Complex Carbon Molecules (C-60) using an IEC Discharge

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A non-equilibrium "Star" mode of operation used in an IEC Device for complex carbon molecule production

Abstract

A device for producing fullerenes includes an IEC vacuum chamber which has a central grid-like electrode and a conductive outer shell that are connected to a pulsed source of high voltage and provide an electric field within the chamber. The applied voltage supports the creation of a plasma at the inner core of the chamber near the electrode. A carbon-based gas, which is introduced into the chamber, possibly along with an inert buffer gas, is dissociated into component carbon and hydrogen ions that are separated and the carbon ions recombined into fullerenes that appears as a soot. The device includes a soot extraction mechanism for removing and collecting the fullerenes.

Background

The present invention concerns a method and apparatus for producing complex  carbon molecules and, in particular, a method and apparatus that utilizes the plasma within an inertial electrostatic confinement (IEC) device to convert a carbon-based gas into "buckey-balls" or fullerene C-60 and sister molecules.

The IEC was originally developed as a neutron source for activation analysis as reported in G. H. Miley, J. B. Javedani, R. Nebel, J. Nadler, Y. Gu, A. J. Satsangi, and P. Heck, "An Inertial Electrostatic Confinement Neutron/Proton Source," Third International Conference on Dense Z-pinches, eds. Malcom Haines and Andrew Knight, AIP Conference Proceeding No. 299, AIP Press, New York, 675-689 (1994).  For such an application, when a gas is introduced into the chamber in the tens of mTorr pressure range, a plasma discharge is created by applying high voltage (10-100 kV) to the grid.  The grid also serves to extract ions from the discharge and accelerate them toward the center of the device, where a dense, high-temperature plasma is formed.  The potential surfaces are shaped such that ions are trapped and recirculated, creating a highly non-thermal plasma with energetic (kV) ions and lower-energy background electrons.  The resulting plasma provides several unique opportunities for plasma processing, either using in situ methods or employing radiation emitted from the dense core region.

The inertial electrostatic confinement device disclosed therein includes a metallic vacuum vessel which is held at ground potential and contains internally and concentric to the vessel, a wire grid which acts as a cathode.  The cathode may be made from a variety of metals having structural strength and appropriate secondary electron and thermionic electron coefficients.  The cathode wire grid is connected to a power source to provide a high negative potential (30 kV-70 kV), while the vessel itself is conductive and maintained at a ground potential.  The desired gas mixture is introduced into the vessel. A voltage is applied to the cathode wire grid and the pressure is adjusted in order to initiate a glow discharge.  The glow discharge generates ions which are extracted from the discharge by the electric field created by the cathode grid. These ions are accelerated through the grid openings and focused at a spot in the center of the spherical device.

Upon application of a potential to the cathode grid, under certain grid-voltage, gas pressure, gas type and grid-configuration conditions, high density ions and electron beams will form within the IEC device initiating a "star" mode of operation.  In this mode, high density space charged neutralized ion beams are formed into microchannels that pass through the open spaces between the grid wires.  As the ions avoid contact with the wires, this mode increases the effective grid transparency to a level above the geometric value.  These microchannels significantly reduce grid bombardment and erosion and increase power efficiency.  For conventional star mode operation, the grid and microchannel beams are symmetric so that a convergent high-density core develops. The inertial electrostatic confinement device serves as a valuable source of neutrons or protons.

Non-thermal plasma production in the IEC leads to several other quite different but possible applications.  One that has been explored to date is the production of ultraviolet (UV) radiation.  The device provides a high-intensity UV-radiation source if heavy gases, such as krypton or xenon, are used.   Another application is the use of the IEC to create thrust by flowing the plasma out through a channel created by an enlarged grid wire opening.

C-60 was discovered in 1985 and was found to have three-dimensional, cage-like, all-carbon molecules in a gas phase carbon cluster.  These seven-numbered soccerball-shaped robust molecules were named "fullerenes" after R. Buckminster Fuller, the American architect who pioneered geodesic design.  Since that time, there have been only a limited number of studies and papers presented on the subject of fullerene production and theory due to the relative unavailability of the all-carbon materials.  Nonetheless, it was also found that in addition to the originally identified C-60 and C-70, there were hosts of other stable carbon configurations ranging from C-24 up to C-240 and beyond. Moreover, within the past 5 years, there have been modest strides in the production of C-60 and C-70 and limited yields of the higher and lower order carbon molecules.

Recently, the demand for fullerenes has been growing due to their potential applications. Many advanced materials currently in use show only a single application, but fullerenes show a series of applications, which include their use as superconductors, anti-AIDS drugs, catalysts and catalyst supports, photoconductors, optical limiters, adsorbents, precursors to synthetic diamonds, and plant growth regulators. Additionally, a major thrust of fullerene research is to exploit its use for energy production.  Recent studies show that C-60 is a good hydrogen storage medium and can attach more hydrogen atoms (up to 48) per single storage molecule as compared with conventionally used storage material like palladium. 

Another area that is related to energy production is the use of C-60 as battery electrodes.  Fullerene-based electrodes would be light in weight and comparable with conventional nickel-oxide electrodes in efficiency.  Finally, C-60 has also been thought of as an excellent candidate for many new applications in the near future, such as molecular ball bearings for ships and as a propellant for electric thrusters on satellites.  By far the most advanced concept is in the realm of microstructures-the nanotube-wherein an all-carbon linked structure that is completely cylindrical and tubular, can have metallic and semiconductor properties.

Production of fullerene to achieve these results has been approached on both a theoretical and practical level.  Although the various production techniques developed to date have allowed the scientific community access to carbon molecules, a need for highly efficient methods with a reasonable production rate for economic manufacture of quality fullerene substances still remains.  The full utilization of the originally identified C-60 structure (fullerene) and its sister molecules will not be economically feasible for large scale applications until this problem is solved.

Accordingly, it is an object of the present invention to utilize an energetic non-thermal plasma discharge as a medium for efficient fullerene (C-60) production

Summary

A specific apparatus and method employing the IEC for fullerene production has been explored and involves the injection of a carbon-based gas with a buffer gas into an IEC operating in either a continuous or pulsed mode. During the pulse, a dense, energetic, non-thermal plasma is formed, disassociating the methane into carbon and hydrogen. The configuration of the IEC offers a very efficient way to form the desired plasma, which is non-Maxwellian in form such that the energetic ion component serves to effectively decompose the methane (or other carbon-containing gas feed). The potential field configuration in the core plasma region of the IEC is such that the higher Z carbon ions are preferentially concentrated in the core region of the plasma, while the hydrogen is moved towards the outer edge of the core. Due to the non-neutral character of the non-Maxwellian IEC plasma, a "double well" electronic potential profile is created in the core region of the plasma. This natural separation of the carbon atoms from other species due to the potential field structure provides a highly efficient mechanism for recombination of carbon to form fullerene in relatively large quantities. At the end of the pulse, the plasma quickly cools leading to recombination of the various species. Fullerene (C-60) formation is favored in the central core region, where the combination of a high carbon ion and low hydrogen ion concentration favors carbon linking with a minimum probability of hydrogen interference via chain termination. The buffer gas, (e.g., helium, xenon, argon) does not directly participate in this process but is selected to serve as an energy storage/transfer component of the plasma. Thus, use of the buffer gas allows further optimization of the process.

Some Possible Applications

The present invention makes it possible to utilize the Inertial Electrostatic Confinement (IEC) device for the non-thermal production of fullerene (C-60) and its sister molecules, and to take advantage of its strengths and uniqueness over other forms of production. This provides a method suitable for efficient production of fullerene (C-60) and its sister molecules, potentially on a commercially viable scale, utilizing a relatively simple but efficient device and process.

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