WO1993022239A1 - Selective process for making metallofullerenes and uranofullerene compositions - Google Patents

Abstract

This invention provides a composition of and method for making metallofullerenes, especially the uranofullerene (U @ C28). The metallofullerenes are made by vaporizing carbon and contacting the carbon vapor with a metal that is selective to the formation of fullerenes.

Description

SELECTIVE PROCESS FOR MAKING METALLOFULLERENES AND URANOFULLERENE COMPOSITIONS

I. BACKGROUND OF THE INVENTION

1. Field of the Invention This invention relates to a process for making metallofullerenes wherein the selectivity for metallofullerene product is greatly enhanced, and to the metallofullerene compositions resulting from the process. In particular, uranofullerenes may be produced in abundance by this process, especially the uranofullerene (U @ g).

2. Description of the Prior Art A fullerene is a third form of pure carbon and is different from graphite and diamond, the only two forms known before 1985, see "Fullerenes," Curl, R.F. and Smalley, R.E., Scientific American, October, 1991, pp. 54-63, incorporated herein by reference, and references cited therein.

A fullerene structure is generally characterized in that each carbon atom is bonded to three other carbon atoms. The carbon atoms so joined curve around to form a molecule with cage-like structure and aromatic properties. One fullerene molecule referred to as "bucknύmterfullerene'' contains 60 carbon atoms bonded together in a spherical relationship, a structural diagram of which resembles the familiar shape of a soccer ball. A structural diagram representing C₆₀ is shown in Figure 1. Fullerenes may contain even numbers of carbon atoms totalling from 20 to 500 or more.

Figure 2 shows the structural diagram of a Cj₀ fullerene and Figure 3 shows the structural diagram of a C^ fullerene.

Fullerenes are not necessarily spherical. They may take the form of long tubular structures with hemispherical caps at each end of the tube. Hyperfullerene structures also exist wherein one structure is contained within a second larger structure. For generally spherical molecular structures, these hyperfullerenes in cross-section resemble an onion layered structure. Tubular structures within larger structures are also possible.

The molecular structure for buckrninsterfullerene was first identified in 1985, see NATURE, C₆₀: "Buckminsterfullerene", Kroto, H.W., Heath, J.R., O'Brien, S.C., Curl, R.F. and Smalley, R.E., Vol. 318, No. 6042, pp. 162-163, November 14, 1985. The process for making fullerenes described therein involves vaporizing the carbon from a rotating solid disk of graphite using a focused pulsed laser. The carbon vapor was then carried away by a high-density helium flow. That process did not utilize a temperature controlled zone for the growth and annealing of fullerene molecules from the carbon vapor formed by the laser blast, and only microscopic quantities of fullerenes were produced.

The fullerene yield from that type of carbon vaporization by laser was improved by providing a temperature controlled space for the carbon atoms in the carbon vapor to combine in a fullerene structure, see, "Fullerenes with Metals Inside," Chai, Y., Ting, G., Changming, J., Haufler, R.E., Chibante, L.P.F., Fure, J., Wang, L., Alford, J.M. and Smalley, R.E., J. Phys. Chem., Vol. 95, No. 20, pp. 7564-7568 (1991). This method is also disclosed in a co-pending patent application having U.S. Serial No. 07/799,404 filed November 27, 1991, which is fully incorporated herein by reference.

Another method of making fullerenes was described in J. Phys. Chem. "Characterization of the Soluble All-Carbon Molecules C₆₀ and CV Ajie et al., Vol. 94, No. 24, 1990, pp. 8630-8633. The fullerenes are described as being formed when a carbon rod is evaporated by resistive heating under a partial helium atmosphere. The resistive heating of the carbon rod is said to cause the rod to emit a faint gray-white plume. Soot-like material comprising fullerenes is sai'd to collect on glass shields that surround the carbon rod. Another method of forming fullerenes in greater amounts is described in "Efficient Production of C₆₀ (Buckminsterfullerene), C₆₀H₃₆ And The Solvated Buckide Ion," Haufler, R.E., Conceicao, J., Chibante, L.P.F., Chai, Y., Byrne, N.E., Flanagan, S., Haley, M.M., O'Brien, S.C., Pan, C, Xiao, Z., Billups, W.E., Ciufolini, M.A., Hauge, R.H., Margrave, J.L., Wilson, L.J., Curl, R.F., and SmaUey, R.E., J. Phys. Chem., Vol. 94, No. 24, pp. 8634-8636 (1990). The method is also described in U.S. Serial No. 07/771,741 filed October 4, 1991 wherein carbon is vaporized in an electrical arc and the carbon vapor condenses into fullerenes. U.S. Serial No. 07/771,741 is incorporated herein by reference.

One disadvantage of-the prior art is the low yield of fullerenes containing metal atoms. Another disadvantage of the prior art is the inability to produce a molecule containing a metal atom inside a relatively small fullerene.

II. SUMMARY OF THE INVENTION The above-mentioned and other disadvantages of the prior art are overcome by the present invention which provides a highly selective method of producing metallofullerenes in macroscopic quantities. Although the present invention is primarily described herein by reference to an all carbon fullerene cage, atoms of elements other than carbon may also be incorporated in the fullerene cage network as described in J. Phys, Chem., Vol. 95, No. 20, pp. 7564-7568 (1991) and in U.S. Serial No. 07/799,404. The present invention also provides a composition comprising fullerenes with two or more fullerene selective metal atoms inside the fullerene cage and a method for making such a composition. This invention provides a process for making fullerene compounds by vaporizing materials containing carbon and a fullerene selective metal, using any known means of vaporizing carbon. Preferably, a focused laser beam is used to form a vapor of carbon and fullerene selective metal, and then a temperature controlled space is provided for the carbon atoms in the carbon vapor to combine in a fullerene structure. Atoms of the fullerene selective metal will contact carbon in the carbon vapor to yield fullerenes wherein the carbon-carbon bonded network encloses one or more atoms of the fullerene selective metal. The fullerene molecules, along with graphite carbon molecules, are then condensed and collected as solid soot material. The fullerenes may be purified by subliming the fullerenes from the soot and then condensing them, or by extracting the soot with an appropriate solvent followed by evaporation of the solvent to yield fullerene molecules in solid form.

These fullerene compounds may be useful as catalysts or as molecular carriers for drugs. Doped fullerenes may be useful as p-type of n- type dopants in fullerene semiconductor devices. Metal containing fullerenes may be useful as catalysts. Uranofullerenes can be used for making stable uranium/carbon composites. Uranofullerene compounds are also useful for catalysis and preparation of uranium containing polymers and glasses. Many other uses for these fullerenes are possible.

In one aspect of this invention, carbon and a fullerene selective metal are heated to form a vapor which is then condensed to yield a fullerene surrounding the fullerene selective metal. Optionally, the vapor may be retained in a temperature controlled space to provide for the growth and annealing of fullerene molecules. In another feature of this invention, the carbon source is vaporized and then contacted with fullerene selective metals to form fullerenes with at least one metal atom inside. A third feature of this invention is the vaporization of a carbon source containing fullerene selective metal atoms to form fullerenes with one, two or more metal atoms inside the fullerene cage. A fourth feature of this invention is the vaporization of a fullerene selective metal, a carbon source and other elements to form fullerenes with at least one fullerene selective metal inside wherein one or more of the atoms of the fullerene cage is an atom of an element other than carbon. These and other features of this invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings.

ffl. IN THE DRAWINGS Figure 1 is a diagram representing C₆₀, buclα nsterfullerene.

Figure 2 is a diagram representing a Cj₀ fullerene. Figure 3 is a diagram representing a C^ fullerene. Figure 4 is a graph representing the time of flight mass spectrum of carbon clusters produced in a supersonic beam by laser vaporization of graphite in a pulsed flow of helium. The clusters were ionized by direct 1- photon ionization with an F₂ excimer laser pulse (photon energy 7.9eV). Figure 5 is a diagram representing a g fullerene. Figure 6 is a cross-sectional view of a laser vaporization apparatus. Figure 7 is a diagram of a laser vaporization apparatus.

Figure 8 is a cross-sectional view of an electrical arc vaporization apparatus.

Figure 9 is a graph representing the FT-ICR Mass spectrum of positive carbon/uranium cluster ions produced by laser vaporization of a graphite/U0₂ composite disc, with conditions optimized to view the clusters in the 200-500 amu mass range.

Figure 10 is a diagram representing a uranofullerene, (U @ . Figure 11 is a graph representing the FT-ICR mass spectrum of carbon/uranium clusters in the larger mass region as produced by laser vaporization of a graphite/U0₂ composite disc.

Figure 12 is a graph representing a laser "shrinkwrapping experiment on U @ C_n uranofullerene ions levitated in the high magnetic field of an FT-ICR analysis cell. The top panel shows the initial selection of U @ C₅₀, U @ C₆₀, and U @ Cj₀ clusters. The bottom panel shows the cluster distribution obtained after irradiating these selected uranofullerenes with 150 pulses of a XeCl excimer laser (308 nm) at a fluence of roughly 60 mj cm^"2 pulse^"1.

Figure 13 is a graph representing the FT-ICR mass spectral analysis of sublimed film produced from DC arc vaporization of a graphite UO- composite rod in an oven at 1200° C.

Figure 14 is a graph representing the XPS analysis of the uranium content of (a) sublimed

@ g fullerene film as compared to (b) uranium oxide powder, U0₂, in binding energy region expected for the 4f core electrons of uranium. — Figure 15 is a graph representing the FT-ICR mass spectrum of hafnium/carbon clusters produced by laser vaporization of a graphite/Zr0₂ composite disc.

Table 1 shows theoretical predictions from SCF-HF calculations.

Table 1. Theoretical predictions from SCF-HF calculations"

Molecule Basis Energy ΗOMO ^UMO IP(eV) Λ(HOMO-

LUMO) g Ai) 280 -1058.18643 -0.262(t₂) -0.080(α_x) 7.1 0.182 C»(*r_)^β 280 -1058.15272 -0.239(_ttl) 0.044(e) q,₈(⁵T₂)^c 280 -1058.30392 -0.321(t_x) 0.054(e)

288 -1060.85293 -0.295(0 0.038(0

TKgQa Ai) 312 -1906.27657 -0.320(c._x) -0.020(0

Zr@C₂₈(¹A₁) 378 -4597.32717 -0.315(_αι) 0.002(e)

a All energies in Hartrecs except IP in cV. b Numbers of basis functions and basis set in parethesis. c For open shell system, the HOMO, LUMO do not have same meaning as that in closed shell case. So do the IP and HOMO-LUMO gap. IV. DETAILED DESCRIPTION OF THE INVENTION

Buckminsterfullerene, C₆₀, shown in Figure 1, is the most symmetrical and least chemically reactive member yet discovered of a potentially huge class of closed carbon cage molecules composed of 12 five- membered rings and any number (except 1) of six-membered rings. R. F. Curl and R. E. Smalley, Science 242, 1017 (1988); Sci. American 265, 54 (1991). The smallest of these, Q, has only one possible structure, the dodecahedron. Each carbon in such a ,, cage is bonded to three others with a bond angle of 108°, which is so close to the tetrahedral bond angle of 109.5° that the bare _Q molecule would be expected to be extremely reactive, with each carbon striving to passify its fourth "dangling" bond. In fact the fully hydrogenated C_JQH_JO molecule, dodecahedrane first synthesized in 1976 does turn out to be a highly stable, robust molecule, Leo A Paquette, R. J. Ternansky, D. W. Balogh, and G. J. Kentgen, J. Am. Chem. Soc. 105, 5446-5450 (1983), and no evidence has ever been found in carbon cluster beams for a particularly abundant bare (___ cluster.

With the incorporation of increasing numbers of hexagons, the average bond angle for carbons in the larger fullerene cages increases-lending to the perfect planar sp² graphite limit of 120° for cages of infinite size. But, except in unique case of C₆₀ in the perfect I_h symmetry of a soccerball, the strain of curvature necessary to close the carbon network in all larger fullerenes is distributed unequally. It tends to be concentrated at the vertices of the 12 pentagons, and this simple geometrical effect is increasingly pronounced as the cage size increases and particularly whenever two or more pentagons are adjacent. These carbon positions where the strain is concentrated will be susceptible to chemical attack as they are on C₂₀. As with oHjo, it should be possible to stabilize these more reactive fullerenes by attachment of hydrogen atoms or other monovalent radicals to the most strained sites on the outside surface of the cage. One particularly attractive possibility is the chemical stabilization of g. In some of the earliest supersonic carbon cluster beam work using gentle 1-photon ionization with an F₂ excimer laser to probe the cluster distribution. g appeared to be the smallest even-numbered cluster formed with special abundance. Q. L. Zhang, S. C. O'Brien, J. R. Health, Y. Iiu, R. F. Curl, H. W. Kroto, and R. E. SmaUey, J. Phys. Ch. 90, 525 (1986). Figure 4, for example, shows a particularly vivid example of such a cluster distribution as produced with a more advanced version of the laser- vaporization cluster beam source. R. E. Haufler, L-S Wang, L.P.F. Chibante, C. Jin J. Conceicao, Y. Chai, and R. E. SmaUey, Chem. Phys. Lett. 179, 449 (1991). Here the 28th cluster appears to be nearly as special as C₆₀ and quite a bit more intense than Q₂ which is generaUy the smaUest fuUerene found in laser "s____kwrapping" experiments of aU larger fuUerenes. R. F. Curl and R. E. SmaUey, Science 242, 1017 (1988); Sci. American 265, 54 (1991). Results such as Figure 4 suggest that g may be the smaUest fuUerene that forms in abundance in condensing carbon vapors. Modification of the cluster source conditions giving more opportunity for "chemical cooking" of the condensing carbon vapor prior to expansion showed that g does finaUy react away, leaving C₆₀ as the lone survivor, but it is one of the last such carbon clusters to do so. ^~

Figure 5 shows one possible structure for a C-_g fuUerene. H. W. Kroto, Nature 329, 529-531 (1987). It is the most symmetrical possible structure for 28 atoms ~ T_d symmetry with a triplet of pentagons arranged at each vertex of a tetrahedron. Direct self-consistent-field (SCF) Hartree Fock and analytic energy gradient methods, R. Ahlrichs, M. Ban, M. Haser, H. Horn and C. Kotmel, Chem. Phys. Lett. 162, 16 (1989), such as those recently used to study tetrahedral hydrogenated o, T. Guo and G. E. Scuseria, Chem. Phys. Lett. 191, 527 (1992), were appUed to g to obtain optimized geometry shown in Figure 5. The ground electronic state of the optimized structure was found to be 5T₂ with one electron in an aj orbital and three in an a- orbital corresponding to a dangling bond located at each of the four carbons in the center of the pentagon triplets. This open sheU electronic structure suggests that addition of 4 hydrogen atoms at the sites of these dangling bonds would produce a very stable closed-sheU molecule,

Accordingly, geometry optimization employing double-zeta basis sets were carried out on this molecule. As shown in Table 1,

does turn out to be an exceedingly stable hydrocarbon with a very large HOMO-LUMO gap, and it is also worth mentioning that the double-zeta SCF optimized C-C bond-lengths of the six- member ring in g and

correspond to aromatic values (1.412A and 1.391A respectively). Efforts to synthesize this molecule by addition of H₂ or some other hydrogen-bearing species at an appropriate stage in condensing carbon vapors are currently underway.

However, these calculations suggest there is another possible wa to chemicaUy stabUize g: put a tetravalent atom on the inside. The present invention provides a composition of and method for making such metaUofuUerenes.

The invention, in one aspect, provides a method for making metaUofuUerene molecules by a process wherein carbon and a fullerene selective metal are vaporized and then the vapor produced is condensed. Preferably, the vapor is maintained at a controUed temperature for a sufficient amount of time to aUow the fuUerene structure to form. The use of a fullerene selective metal in the process dramatically increases the metaUofuUerene yield over what has been obtained by the prior art, and provides for a fuUerene structure with a lower number of carbon atoms than ever produced before.

In this application, accepted symbols for elements and subscripts to denote numbers of elements wiU be utilized to describe molecules. In addition, a set of parentheses around the symbol "@", will be used to indicate that the atoms Usted within the parentheses are grouped to form a fullerene. Within the parenthetical group, all atoms listed to the right of the @ symbol are part of the fuUerene cage network, and aU atoms to the left of the @ symbol are situated inside the cage. Under this notation, buckmmsterfullerene is (@ C₆₀), and a C₆₀-caged metal species is written (M @ C_ω). A more complex example is K_(K @ C₅₉B), which denotes a 60-atom fuUerene cage with one boron atom substituted for a carbon in the fuUerene cage network, a single potassium atom trapped inside the fuUerene cage, and two potassium atoms adhering to the outside. 1. The Carbon Source Material

A carbon vapor may be provided by vaporizing any source of carbon. Diamond, graphite, fuUerene or combinations thereof may serve as the carbon source. Graphite is cheaper and is therefore preferred.

The carbon source may be pure carbon to result in unsubstituted fuUerene formation after vaporization. Alternatively, the carbon source may contain other materials selected to form a desired type of substituted or "doped" fuUerene after vaporization. For instance, the carbon source may contain boron nitride (BN) in addition to carbon. Upon vaporization, some of the boron atoms wiU be incorporated into the fuUerene cage network.

Atoms of nitrogen may be incorporated into the fuUerene cage by combining potassium cyanide (KCN) with carbon in the carbon source material and vaporizing the KCN concurrently with the carbon. Other sources of nitrogen may be used, for example polyacrylamide.

The product of any fuUerene generation process may be used as the carbon source for a subsequent fuUerene generation. The product to be used in the subsequent fuUerene generation may be the raw carbon soot or it may be the refined soot enriched in fuUerenes. For instance, the carbon source may comprise carbon in forms including graphite and fuUerenes generated in the electrical arc process described below and in U.S. Serial No. 07/771,741. The soot produced by the arc process may then be shaped into a target carbon source for a laser beam. Alternatively, soot produced by a laser vaporization process may be enriched in fuUerenes by extractive separation, and the enriched portion vaporized either by laser, electrical arc, or resistive heating.

2. The Fullerene Selective Metal

The fuUerene selective metal is any metal that, when contacted with a carbon vapor, interacts with the carbon vapor to selectively form fuUerenes with the fuUerene selective metal inside. Selectivity is a way of measuring the tendency of a particular reaction that yields multiple products to form one particular product in relation to other products. For instance, the prior art processes for forming fuUerenes with one or more metal atoms inside are not particularly selective because most of the fuUerene molecules produced are regular fuUerenes (@ , not metal-containing fuUerenes (M_x @ . Therefore, the prior art selectivity is low because the metal containing fullerenes are only a smaU portion of the total fuUerenes produced. The selectivity-of the present invention is higher, meaning that the proportion of metal containing fullerenes in the total fullerene product is higher than what is known in the prior art.

The fullerene selective metals are hafnium, tin, lead, radium and the actinide series of elements in the Periodic Table of the Elements, CRC Handbook of Chemistry and Physics, 57th edition, 1976-77, including thorium, protactinium, uramum, neptunium, plutonium, americium, curium, berkelium, and californium. Uranium is presently preferred.

The fullerene selective metal may be utilized in the pure metallic state or it may be chemically combined with other elements in fullerene selective metal compounds. In particular, oxides and haUdes of fuUerene selective metals, can be utilized in this invention. Particularly preferred fuUerene selective metal compounds are the oxides including U0₂, Th0₂, Pb₃0₄, and the haUdes including UC. Presently, U0₂ is most preferred.

3. The Carbon Source/Fullerene Selective Metal Combination The carbon source material and the fullerene selective metal may be combined prior to carbon vaporization. The combination may be made, for example, by mixing graphite with U0₂ and then processing the mixture into an electrode as described hereinafter and in Serial No. 07/771,741. The electrode containing the fuUerene selective metal would then be utilized in the electrical arc vaporization described herein. Similarly, the carbon source material and the fuUerene selective metal may be combined to form a target to be used in the laser vaporization apparatus described herein. The amount of fuUerene selective metal utilized in the combination is not critical but generaUy ranges from 0.1 to 40 wt%, preferably 1 to 20 wt% or more preferably, 2 to 10 wt% of carbon source/fuUerene selective metal combination.

Alternatively the fuUerene selective metal may be vaporized separately from the carbon source material. In this embodiment, the fuUerene selective metal vapor must then be contacted with the carbon vapor. Any means of introducing the fuUerene selective metal vapor to the carbon vapor and then contacting the two is sufficient to result in formation of fuUerenes with the fuUerene selective metal inside. It is also sufficient if the carbon vapor is contacted directly with the fuUerene selective metal. The heat from the carbon vapor wiU vaporize some of the fuUerene selective metal to yield fuUerenes with metal inside. 4. The. Inert Atmosphere

Vaporization of carbon should occur in an atmosphere selected to promote, or at least not hinder, the formation of fuUerenes. The atmosphere necessary for forming pure carbon fuUerene molecules from a carbon vapor includes inert gases such as helium, neon, argon, krypton, and xenon. Other gases may be useful in providing an inert atmosphere as long as the molecules of the atmosphere are not reactive with the carbon vapor. Other possibiUti.es for the atmosphere include carbon monoxide, carbon dioxide and nitrogen. Hydrogen containing gasses such as water vapor should not be used as they are beUeved detrimental to fuUerene formation. Currently, a heUum atmosphere is preferred. The carbon vapor is preferably formed in an inert gas at a low pressure ranging from 1 to 20,000 Torr, and preferably 5 to 2000 Torr. Absolute pressures of helium ranging from 10 to 700, preferably 50 to 500 To have been found to be particularly useful for generating a carbon vapor from which fullerenes may be condensed. 5. The Vaporization Process

The carbon must be heated to a temperature sufficient to form a carbon vapor. The energy necessary to heat carbon to a vapor may be provided by any means including a focused laser beam, an electrical arc (plasma); or by resistive heating.

A. Vaporization by a Focused Laser Beam. Any type of pulsed laser which produces a beam that will vaporize carbon wiU work. The energy fluence of the beam should be greater than 10^" joule/mm² with each pulse lasting between 1 and 100 nanoseconds. One type of laser which works weU is a Nd:YAG (Neodymium doped Yttrium Aluminum, Garnet) laser made by Quantel.^'. The green, second harmonic of the Nd:YAG laser may be used and the laser operated at 300 miUijoule (mJ) per pulse, each pulse having a duration of 5 to 10 ns. The laser is operated at 10 pulses per second (pps). Other types of lasers will work, including an Eximer XeCl laser with a wavelength of 308 nm which may produce fullerenes more economically than the Nd:YAG laser.

The laser beam should be aimed at a target (carbon source and fullerene selective metal) so that the energy from the laser will vaporize the carbon and/or the fuUerene selective metal. The carbon source may be in the form of carbon rods made as disclosed in J. Phys. Chem., Vol. 94, No. 24, pp. 8634-8636 (1990), and in U.S. Serial No. 07/771,741. Since the rods need not be electrically conductive, rods prepared by omitting the graphitization step wUl also be useful in this invention. Carbon may also be in the form of flat discs formed from carbon or a flat disc with a film of carbon deposited on the surface. Graphite rods are relatively inexpensive and easy to work with and are therefore preferred.

The fuUerene selective metal may be combined with carbon in the carbon source. The amount of metallic compound combined with carbon wiU vary depending on the metallic compound and the desired end product, but generaUy the carbon source may contain 0.1 to 40 wt% of the metaUic compound, preferably 1 to 20 wt% and more preferably 2 to 10 wt%. For instance, U0₂ may be combined with the carbon in the carbon source which upon vaporization wiU result in a carbon vapor which also contains atoms of uranium. Some of the fuUerene molecules wiU then grow and close around some of the uranium atoms.

The fuUerene structure may enclose one or more than one fuUerene selective metal. For example, two, three, four or more metal atoms may be enclosed by the fuUerene structure, including compounds represented by the formula (M_x @ where M is the fuUerene selective metal, x is an integer from 1 to 4, and n is an integer from 14 to 500 or more. Preferably x is 1 and n is 14-50, more preferably n is 14-17, most preferably 14. Representative uranofuUerene compounds formed by the vaporization of a mixture of graphite and U0₂ include (U @ g), (U @ C,₆), (U @ C_u), (U @ C₅₀), (U @ o), (U @ o), (U @ QJ and (U @ C₈₂).

Molecules with atoms other than carbon as part of the fuUerene cage network may also be formed enclosing one or more metal atoms. This may be accompUshed by including the metaUic compound and the compound other than carbon together in the carbon source material. For instance, a carbon rod or disk may be formed from carbon, U0₂ and BN. Alternatively, the carbon rod or disk may be formed from carbon, U0₂ and boron powder. A carbon source containing BN has been vaporized to produce such compounds as (@ C₅₅B), (@ C_5gB₂), (@ C₅₇B₃), (@ C₅₅B₅) and (@ C₅₄B₆). Any - number of different boron substituted fuUerenes may be formed in this manner and may be generaUy represented by the formula (©C^B_;) where n is an integer, from 14 to 500 or more, and i is an integer such that 2n/i is equal to or greater than 10. Other sources of boron may be used such as pure boron powder, boron hexaflouride (BF₆) and sodium borate (Na^O,). Further information about the preparation and analysis of boron doped fuUerenes may be found in "Doping Bucky: Formation and Properties of Boron-Doped Buckminsterfullerene," Guo, Ting; Changming, Jin; and Smalley, R.E., The Journal of Physical Chemistry, Vol. 95, No. 13, pp. 4948-4950, (1991), which is incorporated herein by reference.

Due to the limited atmospheres and pressures within which fuUerenes can be coUected in sufficient quantities, the process is usually carried out inside a fuUy enclosed chamber or system. The enclosed chamber or system may be evacuated by means of a vacuum pump thereby removing undesirable hydrogen-containing molecules such as water. After the chamber or system has been evacuated, it may be partiaUy refiUed vith the desired inert atmosphere such as heUum.

Although a high temperature is necessary to vaporize carbon, the resulting carbon vapor should not reside at this temperature for very long periods; however, it is also desirable to avoid cooling the carbon vapor too fast. Preferably, the carbon vapor is maintained at a temperature above 1,000°C for at least 0.1 millisecond after the vapor is formed. It is therefore desirable to move the carbon vapor from the immediate vaporization zone to a zone of controUed temperature that will allow the carbon vapor to grow into a network that wiU eventuaUy form a fuUerene molecule. Thereafter, the gas stream which wiU contain carbon vapor, fuUerenes, and graphitic carbon shoul be moved to a cooler zone to condense solid fuUerene molecules along with solid graphite soot. Suitable conditions for the fullerene growth and formation include residence times of 0.1 miUisecond (ms) to 100 seconds (s), 0.5 ms to 5 s, or 1 ms to 10' s, at temperatures of at least 1,000°C, preferably 1,100°C to 1,500°C and more preferably 1,100°C to 1,300 °C. The temperature may be controUed within these limits by means weU known in the art. For instance, the temperature and flow rate of the inert gas may be adjusted so that the fuUerene growth and formation zone is kept at the desired temperature without the need of further means for adding or removing heat. Alternatively, an external heater may be provided to keep the fuUerene growth and formation zone at the desired temperature, or, if the vaporization process adds more than enough heat to keep the system within the desired temperature limits, an external means of removing heat may be provided. Condensation of fuUerene molecules and graphite soot may be obtained by moving the mixture carbon vapor fuUerenes and inert gas away from the temperature controUed fuUerene growth and formation zone and cooling the mixture. The cooling may be accomplished by moving the mixture through a passageway and cooling the waUs of the passageway by means weU known in the art. Temperatures below 2000°C, preferably below 1000°C, wiU condense the fuUerenes and graphite soot.

In order to facUitate removal of the carbon vapor from the laser vaporization zone and passage of the carbon vapor into the temperature controUed fuUerene growth and formation zone, an inert gas flow may be provided. The inert gas may be directed across the laser vaporization zone and into the temperature controUed fuUerene growth and formation zone. The inert gas may then be directed into a condensing zone and then withdrawn from the condensing zone by means of a pump or fan. If inert gas is withdrawn from the system, fresh inert gas should be introduced at about the same rate as gas is withdrawn in order to keep the pressure of the system relatively constant. The inert gas flow velocity and temperature may be controUed and adjusted to maintain the desired residence time and temperature in the temperature controUed zone.

Preferably inert gas withdrawn from the condensing zone is conserved by recirculating it back to the vaporization zone. The recirculation may be effected by providing a gas circulating turbine which circulates the inert gas around from the vaporization chamber exit back to the chamber gas inlet, across the vaporization zone, through the fuUerene growth and formatio zone, the condensing zone, and back out the chamber exit. Preferably a filter or other separating means is provided to remove any graphite soot and fuUerene molecules that may remain in the gas stream after condensation before the gas stream is reintroduced to the vaporization zone.

Particulates in the inert gas stream may removed by any means of separating soUd particles from a gas, for instance cyclone separators may b used instead of or in addition to filtration. FuUerenes and any graphite soot may also be removed from the gas stream by means of a rotating cooled cylinder or drum located in the gas stream. The cooled rotating cylinder would condense fuUerenes on its surface exposed to the carbon vapor stream. Condensed fuUerenes could then be scraped or brushed off of the cylinder an recovered.

It may be advisable to add a protecting agent either before or during condensation or purification of fuUerenes containing boron in the fuUerene cage or one or more metal atoms inside the fullerene cage. The protecting agent wiU reduce the reactivity of the metal containing fullerene with oxygen and other reactive species. Undoped fuUerenes may serve as adequate protecting agents for fuUerenes with metals inside. Other protectin agents for fuUerenes with metals inside may be useful including boron trifluoride (BF₃). FuUerenes doped with boron are electron deficient at the boron site and may be protected with electron donating compounds such as ammonia, amines, and pyridines.

Recovery of fuUerene compounds from the carbon soot may be accompUshed by extracting the carbon soot with an extraction solvent which preferentially dissolves fullerene compounds but not graphite. Benzene, toluene or xylene are some acceptable solvents. The extract may then be filtered to remove the soUd graphite leaving a filtrate containing the solvent and fuUerenes. Evaporation of the solvent from the extract wiU aUow recovery of fuUerenes as the soUd residue. Separation of fuUerene from graphite may be accompUshed by other methods known to those skiUed in separation arts. For instance, separation may be accomplished by boiling solvents, ultrasonic sonication separation, supercritical fluid extraction, Soxhlet extraction and many other methods. FuUerenes may also be separated by heating the soot under vacuum to a temperature where the fuUerenes sublime as a vapor from the soot, and then condensing the fuUerene vapor.

The laser vaporization process may be better understood by reference to Figure 6 wherein laser source 10 emits a pulsed laser beam 12. The laser beam 12 passes through lens 14 and through window 16 and hits target 18. The target 18 may be comprised of carbon alone or carbon and one or more of the materials described above including boron, nitrogen, and fuUerene selective metals. The materials comprising target 18 are vaporized by laser beam 12 and are carried away from the vaporization area by inert gases flowing in conduit 20. Inert gases may be added to conduit 20 through conduit 22 and the rate of inert gas addition may be controUed by valve 24. It may be desirable to pre-heat the inert gas stream either by pre-heating the inert gas before it enters conduit 20 or by means of a heater 26 sunounding conduit 20 immediately upstream of target 18.

The materials vaporized from target 18 are carried by the inert gas stream into a temperature controUed zone 28. The temperature controUed zone 28 is the part of conduit 20 that is downstream of target 18. For optimum fuUerene generation, the temperature of zone 28 should be maintained between 1000° and 1500°C. One means of controlling the temperature in zone 28 is heater 30 through which the immediate downstream portion of conduit 20 passes. Alternatively, the temperature of zone 28 may be maintained within appropriate ranges without the use of heater 30 by maintaining the appropriate temperature and flow rate of the inert gas feed stream. Zone 28 should be large enough to provide for residence times of between 10^"4 and 100 seconds so that fuUerenes have an adequate opportunity to grow and form.

FuUerenes may be condensed and collected by cooling down the exit portion of conduit 20 past zone 28. Fullerenes will collect on the cool waUs of conduit 20 and may be recovered by brushing the inside walls of conduit 20. The method of recovery shown in Figure 5 also utihzes a filter assembly 32 into which conduit 20 empties. Filter assembly 32 contains a filter 34 disposed in the gas stream exiting conduit 20 in order to filter out particles of fullerene and graphite soot. The flowing inert gas stream may be removed from the system by conduit 36 exiting from filter assembly 32. Alternatively, the inert gas may be recirculated by gas recirculation assembly 38 containing a fan blade 40 for blowing the filtered inert gas stream through conduit 42 which is connected to the exit of gas recirculation assembly 38. The inert gas then recirculates around through conduit 42 back into conduit 20. It is preferable to rotate target 18 during the vaporization process to ensure even vaporization of the surface of target 18. One means of accompUshing the rotation is rotation assembly 44 which comprises target housing 46 attached to conduit 20 in a way to keep conduit 20 sealed from the external atmosphere. Rotation of the target 18 may be accompUshed by connecting target 18 to axle 48 which passes through target ϋousing 46. Since the apparatus is usuaUy operated below atmospheric pressure, axle 48 passes through target housing 46 by way of a rotary vacuum feed through 50 which prevents the outside atmosphere from entering target housing 46. Axle 48 terminates at positioning gear 52 which may be mechanicaUy rotated (by means not shown in this Figure) to spin axle 48 along its axis and thereby rotate target 18.

Since the surface of target 18 is eroded at the point of laser beam impact, it^* is necessary to advance target 18 toward window 16 if the process is to be continuous. This may be accomplished by mechanicaUy advancing positioning gear 52 toward window 16 (by mechanical means not shown) at a rate roughly equal to the rate of decrease of length of target 18.

In operation, target 18 may be a carbon rod prepared as described above mounted with the cross section of the rod face positioned to be bit by laser beam 12. Fresh inert gas may be added to the system through conduit 22 and, if necessary, heated by heater 26 to the desired temperature, which is usuaUy about 1200°C. At the same time, heater 30 should be activated, if needed, so that zone 28 is maintained at a temperature of 1000° to 1500°C. Once the temperatures in zone 28 have stabilized at a desired level, the laser source 10 should be energized so that laser beam 12 is focused on the surface of target 18. The materials. which comprise target 18 wiU be vaporized by laser beam 12 and carried away by the gas stream flowing in conduit 20 through zone 28. While in zone 28, the materials vaporized from target 18 wiU grow and form into fuUerene molecules. If, in addition to fuUerene selective metals and carbon, other elements are present in the vapor, some of the fuUerene selective metal atoms wiU be incorporated inside a fuUerene cage containing only carbon atoms, some of the fuUerene selective metal atoms wiU be incorporated inside a fuUerene cage containing mostly carbon atoms but also atoms of other elements, and some fuUerenes wiU be formed with nothing inside the cage but with atoms of other elements forming parts of the fuUerene cage. Many of these molecules wiU condense and be coUected on the cool down stream end of conduit 20 past zone 28. Molecules which condense but do not adhere to the walls of conduit 20 wiU be coUected in filter 34. The materials formed by the process may be coUected and purified by any of the methods described above.

The invention can also be understood with reference to Figure 7 which depicts a simple version of an apparatus suitable for forming the fuUerenes described herein. A quartz tube 100 may be mounted in furnace 102. The furnace 102 is maintained at a temperature of 1,000° to 1,500°C. An inert gas may be introduced into the end 104 of tube 100 so that the inert gas flows in the direction indicated by arrow 106.

An axle 108 may be mounted within quartz tube 100. The axle 108 should be mounted so that it may be rotated within the tube in a clockwis or counterclockwise motion as indicated by arrow 110. (The means for mounting and rotating axle 108 are not shown in the figure.) A target 112 is mounted on the end of axle 108 so that target 112 is held within a fixed position within tube 100 and target 112 may be rotated by the rotation of axle 108. In the embodiment shown in Figure 6, target 112 is located within furnace 102. However, target 112 may be mounted closer to end 104 of tube 100 and target 112 may even be outside of furnace 102.

In operation, axle 108 is rotated and a laser source is activated t send a pulse of a laser beam 114 down the central portion of tube 100 so that laser beam 114 impacts upon target 112. The energy from laser beam 114 wUl then vaporize the material contained in target 112 and the cloud of vaporized material will be carried by the inert gas toward end 116 of tube 100. A fullerene containing soot wiU coUect on the waUs of tube 100 at area 118. Further collection and purification of fullerene molecules may be accomplishe as described above. A film comprising fuUerenes and metal containing and/or boron doped fullerenes may be made by subliming the material produced in the vaporization process onto a crystaUine surface. Such a film may be used as a semiconductor device in the same manner as semiconductor lattices like sUicon. B. Vaporization by An Electrical Arc Process

Another method of vaporizing carbon utilizes the heat from an electrically induced plasma between two electrodes to produce a carbon vapor Preferably, each electrode is formed of carbon and therefore serves as both electrode and carbon source. Other types of materials may be used to form one or both electrodes, such as tungsten, molybdenum, tantalum, or osmium. The electrode material should be electricaUy conductive and selected to withstand high temperatures on the order of 1000° to 4000°C.

The carbon should be placed in close proximity to the electrical arc between the electrodes. For example, graphite dust and fuUerene selective metals could be passed through an electrical arc between two electrodes to form a carbon vapor. Alternatively, carbon may be placed close to the arc by constructing one or both electrodes of carbon.

If only one of the electrodes is constructed of carbon, the other electrode should be constructed of the materials described above for non- carbon electrodes. Preferably both electrodes are constructed of carbon and the carbon vapor is formed as the carbon from one or both electrodes erodes at areas in or in close proximity to the electrical arc and vaporizes due to the heat of the arc. Different combinations of carbon sources are contemplated within the scope of the invention. For instance, graphite dust may be blown through an electrical arc between two carbon electrodes or the arc between a carbon and a non-carbon electrode.

The graphite rods suitable for use as electrodes in this invention are graphite rods that conduct electricity. Carbon electrodes, which may be purchased commerciaUy, are completely acceptable. Suitable graphite rods may also be constructed by packing graphite powder together with a graphite cement binder inside a hoUow cylinder mold and then compacting the mixture at low to moderate pressure. Compaction pressures of 10 to 110 atm are sufficient. It may be necessary to heat the mixture to a temperature, usuaUy about 150°C, sufficient to melt the binder material. After the material has cooled and soUdifϊed, a soUd rod, commonly refened to as a green body, may be removed from the mold. This green body is not electricaUy conductive and therefore is not a suitable electrode. Compounds suitable as binder include pitch type compounds such as C Bond graphite cement produced by Dylon Industries Inc. The green body should be initiaUy cured at high temperatures in an inert atmosphere. For instance, the green body may be packed in fine san and heated to a temperature of about 1000°C in a helium atmosphere for about 24 hours. This process is commonly refened to as carbonization. Thereafter, the temperature .should be raised even higher for up to several days. For instance, the rod may be heated to a temperature of 2000° to 2500°C for about 1 week to complete the process of graphitization of the rod. After the rod has been processed in this manner, the rod will conduct electricity, and wiU be a suitable electrode. Although cylindrical graphite rods are used as electrodes in the prefened embodiment, the electrodes may be constructed in any shape. For instance, electrodes may be in the shape of rectangular prisms, flat discs or spheres^ Many other shapes are possible and within the scope of the invention. The two electrodes need not have the same shape. An electrical arc may be formed by connecting one of the electrodes to an electrical voltage source and connecting the other electrode t ground. Any method of generating an electrically conductive plasma between the electrodes may be used to start the arcing. An electrical arc between the two electrodes may be started by causing the two electrodes to touch each other, either before or after apphcation of electrical voltage to one of the _ electrodes, and then separating the two electrodes after electrical current is flowing through the electrodes. While the electrodes remain separated by the gap, the continued application of sufficient electrical voltage to one of the electrodes wiU maintain the plasma between the electrodes and allow an electrical arc to bridge the arc gap between the two electrodes. During this time, a substantial portion of the electric cunent is flowing across the arc gap, maintaining the electrically conductive plasma in place.

The amount of voltage necessary to produce an arc will depend on the size and composition of the electrodes, the length of the arc gap, and the ambient gas pressure. The electrical power source may provide either alternating or direct voltage to one electrode. CommerciaUy avaUable arc welding equipment is an acceptable power source. If DC voltage is used, the electric power may be suppUed in pulses rather than continuously. Optimum fuUerene generation may be obtained by adjusting the frequency and on/off time ratio as required by the particular circumstances such as electrode type, arc gap, atmosphere type and pressure among others. For instance, a 50% duty cycle wherein voltage is suppUed only one-half of the time may be optimum. It may also be beneficial to apply the voltage in a series of positive and negative pulses and to alternate those pulses between the two electrodes so that one electrode serves as the positive electrode for a time period and then serves as a ground in rapidly repeating sequence. Although the amount of cunent necessary may vary depending upon the circumstances, the power requirement generaUy ranges from 10 to 500 amps at 10 to 50 volts for 1/4- inch to 1/2-inch diameter electrodes. At higher power consumptions, the electrical conductors deUvering cunent to the electrodes may become very hot. In order to maintain the electrical conductors at appropriately low temperatures, it may be necessary to, cool the electrical conductors by some means. Cooling may be accompUshed by passing a cooling fluid over or through the electrical conductors in order to carry away some of the heat generated. To prevent electrical shorts, the electrical conductor should at aU times remain fiiUy electricaUy insulated from the cooling fluid.

If an electrode is made of carbon, the electrode wiU erode as the carbon is consumed to form the carbon vapor. To maintain a consistent arc gap between the electrodes, it is therefore necessary to provide some means for maintaining the arc gap within certain limits. A spring connected to one of the electrodes to urge one of the electrodes toward the other with a relatively constant force is one means of providing a relatively constant arc gap. One or both of the electrodes may be fitted with such a spring mechanism. Many other means for maintaining the arc gap within appropriate limit wUl be readUy apparent to those of ordinary mechanical skill, and are within the sco of this invention.

The optimum length of the arc gap between the two electrodes wiU depend upon the diameter and cross sectional area of the electrodes and other factors such as the operating cunent, voltage, and ambient conditions o inert gas flow and temperature. Generally, for graphite rods of circular cross section ranging from 1/8-inch to 1/2-inch, the arc gap should range between 0.01 mm and 10 mm. Maintaining the arc gap within this range will provide for the maintenance of the electrical arc between the electrodes when electrical voltage is appUed to one of the electrodes. Precise control of the gap length is not necessary and the gap length may vary during fuUerene generation.

The atmosphere necessary for forming pure carbon fullerene molecules from a carbon vapor generated in an electrical arc process include inert gases such as heUum, neon, argon, krypton, and xenon. Other gases ma be useful in providing an inert atmosphere as long as the molecules of the atmosphere are not reactive with the carbon vapor. Other possibiUties for th atmosphere include carbon monoxide, carbon dioxide and nitrogen. Hydroge containing gasses such as water vapor should not be used as they are believe detrimental to fullerene formation. Cunently, a helium atmosphere is prefened.

In the electrical arc process, carbon vapor is preferably formed a low pressure ranging from 1 to 20,000 Ton, and preferably 5 to 2000 Ton. Absolute pressures of heUum ranging from 10 to 700, preferably 50 to 500 T have been found to be particularly useful for generating a carbon vapor from which fuUerenes may be condensed.

Due to the limited atmospheres and pressures within which fuUerenes can be coUected in sufficient quantities, the electrical arc process i usually carried out inside a fully enclosed chamber or system. The enclosed chamber or system may be evacuated by means of a vacuum pump thereby removing undesirable hydrogen-containing molecules such as water. After the chamber has been evacuated, it may be partiaUy refilled with the desired atmosphere such as heUum.

Temperatures within the electrical arc may reach 6000°C or higher. Although a high temperature is necessary to vaporize carbon, the resulting carbon vapor should not reside at this temperature for very long periods. It is therefore desirable to move the carbon vapor away from the electrical arc and cool it to aUow fuUerene molecules to grow within the carbon vapor and to condense soUd fuUerene molecules along with graphite soot. Condensation of fuUerene molecules and graphite soot may be obtained by moving the carbon vapor away from the arc and cooling off the passageway through which it is moving. Temperatures below 3000°, preferably below 2000°C will condense the fuUerenes and graphite soot. Cooling and condensation may also be controUed by adjusting the flow velocity and temperature of any inert gas flow of the process more fuUy described below. In order to faciUtate removal of the carbon vapor from the electrical arc area and passage of the carbon vapor into the condensing area, a gas flow may be provided. The gas may be directed across the electrical arc and into the fuUerene condensing area. The gas may then be withdrawn from the condensing area by means of a pump or fan. If inert gas is withdrawn from the system, fresh inert gas should be introduced at about the same rate as gas is withdrawn.

Preferably the gas is recirculated in order to conserve the inert gas. The recirculation may be effected by providing a gas circulating turbine which circulates the inert gas around from the chamber exit back to the chamber gas inlet, across the arc area, through the fuUerene condensing zone, and back out the chamber exit. Preferably a filter or other separating means is provided to remove any graphite soot and fuUerene molecules that may remain in the gas stream after condensation before the gas stream is reintroduced to the chamber. Any means of separating soUd particles from a gas may be utiUzed, for instance cyclone separators may be used instead of or in addition to filtration. FuUerenes and any graphite soot may also be removed from the gas stream by means of a rotating cooled cylinder or drum located in the gas stream. The cooled rotating cylinder would condense fuUerenes on its surface exposed to the carbon vapor stream. Condensed fuUerenes could then be scraped or brushed off of the cylinder and recovered.

The electrical arc process for generating fuUerenes may be more fully understood by reference to Figure 8, which is a cross sectional view of one embodiment of a carbon arc fullerene generator. The fuUerene generato comprises a vaporization chamber 210 defined by the inside walls of enclosure body 212. The enclosure body 212 may be constructed of any material that is capable of withstanding the temperatures and pressures required. The enclosure body 212 is preferably constructed of stainless steel. Electrode 214 and electrode 216 are placed within the vaporization chamber 210. Electrode 216 is connected to an electrical voltage source 218 via electrical conductor

219 which passes through a water cooled cunent feedthrough 220. The cune feedthrough 220 passes through a wall of enclosure body 212 but is insulated from electrical conductor 219 so that there is no electrical contact between th electrical cunent source 218 and the enclosure body 212. The opening in enclosure body 212 through which cunent feedthrough 220 passes is sealed by seal 221 to prevent passage of the outside atmosphere into the vaporization chamber 210.

Electrical conductor 219 provides electrical contact between electrode 216 and electrical cunent source 218. The radiant energy from the arc required by the fuUerene generator wUl heat electrical conductor 219. In order to reduce power loss and prevent melting of electrical conductor 219, th cunent feedthrough 220 may be cooled. One method of coohng electrical conductor 219 is to provide a cooling fluid around the electrical conductor 21 by circulating cooling fluid through cunent feedthrough 220. Electrical contact between electrode 216 and electrical conductor 219 may be made by any means which wiU provide electrical conduction between the two. In a prefened embodiment, the electrodes are rotated and the electrical contact between electrical conductor 219 and electrode 216 is made by gimble wheel loaded rod contact 222. The gimble wheel loaded rod contact 222 provides for continuous electrical contact even though the electrode is rotating within the rod contact 222.

In the embodiment shown in Figure 8, electrode 216 passes through an opening in enclosure body 212 to faciUtate rotation of electrode 216. Electrode 216 need not pass through an opening in enclosure body 212 if the electrode is not rotated, or if electrode 216 is rotated by means placed within enclosure body 212. If electrode 216 passes through an opening in enclosure body 212, electrode 216 should be insulated from electrical contact with the enclosure body 212. Insulator 224 provides electrical isolation of the electrode 216 from the enclosure body 212 in tite embodiment shown in Figure 7. Insulator 224 also provides a seal to keep the outside atmosphere from entering vaporization chamber 210 which is usuaUy operated below atmospheric pressure.

Electrode 216 may be rotated by securely attaching a rotation gear 226 concentricaUy with electrode 216. Rotation gear 226 is connected to a rotational drive mechanism (not shown) that wiU rotate the gear around its longitudinal axis and thereby rotate electrode 216 along its longitudinal axis. The prefened method of spinning rotation gear 226 is accomplished by passing a continuous drive belt around rotation gear 226 and around a motorized gear which, when rotated, puUs the continuous drive belt around rotation gear 226 thereby spinning rotation gear 226 and electrode 216. Many other methods of rotating the electrode are possible and are within the scope of the invention.

Since the electrodes are consumed during fuUerene generation, it is desirable to provide some means for advancing the electrodes toward the gap area in order to maintain the desired gap for the electrical arc. The threaded rod feed mechanism 228 is one means of accomplishing this movement. The feed mechanism 228 comprises a threaded rod 230 fixed to gear 232 at one end. Threaded rod 230 passes through threaded nut 234 and engages insulator 236. Insulator 236 provides electrical isolation of the electrode 216 from threaded rod 230 and also provides a means for preventin the rotational motion of electrode 216 from being transferred to threaded rod 230. Insulator 236 also serves as a movable platform which fixes the position of electrode 216. Insulator 236 may either be securely fixed to electrode 216 and rotate freely around threaded rod 230 or the insulator 236 may be securel fixed to threaded rod 230 and rotate freely around electrode 216. In either instance, insulator 236, electrode 216, and threaded rod 230 should be cooperatively coupled so that electrode 216 may be pushed or puUed by rotation of threaded rod 230.

Electrode 214 is electrically connected to electrical ground 240 which passes out of vaporization chamber 210 through cunent feedthrough 24 and is connected to the ground of the electrical voltage source 218. The opening in enclosure body 212 through which cunent feedthrough 241 passes i sealed by seal 242 to prevent the outside atmosphere from entering into vaporization chamber 210. Since electrical ground 240 will be radioactively heated by the arc, it is preferable to cool electrical ground 240 in the same manner as electrical conductor 219 is cooled. It is also preferable to rotate electrode 214 in the same manner as electrode 216 is rotated. The mechanis for providing the rotation is the same as described for rotating electrode 216. Either or both of the electrodes may be rotated. If both of the electrodes are rotated, it is preferable to rotate each electrode in an opposite direction. Thi helps tnaintain an even erosion of the electrode faces. The speed of rotation generally ranges from 1 to 100 rpm.

Provisions should also be made for lateral movement of electrode 214 in the same manner as for electrode 216, i.e., threaded rod feed mechanism 228. Although one lateral adjustment mechanism would be capable of mamtaining the proper arc gap, it is prefened to lateraUy move both electrodes, when both are carbon electrodes, in order to maintain the arc gap in the appropriate location within vaporization chamber 210.

The apparatus described in Figure 8 further comprises feed conduit 244 which passes through an opening in enclosure body 212. The opening in the waU of enclosure body 212 through which feed conduit 244 passes is sealed by seal 246 to prevent the outside atmosphere from entering into vaporization chamber 210. The outlet end of feed conduit 244 opens within the vaporization chamber 210. The outlet end of feed conduit 244 may be flared or provided with a nozzle to focus the stream which may be flowing through feed conduit 244 and into vaporization chamber 210.

The fuUerene generator further comprises a chimney passage 250 passing through a waU of enclosure body 212. The chimney passage 250 extends down through the vaporization chamber 210 to a location near the gap between electrodes 214 and 216. Preferably, the chimney passage is relatively cool. Cooling may be affected by cooling coil 252 which sunounds chimney passage 250. During operation of the fuUerene generator, a fluid such as water may be introduced to the cooling coU through cooling coU inlet 254, passed through the cooling coils thus absorbing heat from the chimney passage 250, and then withdrawn through cooling coU outlet 256. The warmed fluid may be disposed of or cooled and recirculated back to the cooling coU inlet 254.

The chimney may be connected through chimney exit 260 to a gas recycle system 262. The chimney exit 260 is sealed to gas recycle system 262 by sealing together flange 264 and flange 266. The gas recycle system 262 comprises a gas coUection vessel 268 with a filter bag 270 secured against the inside of flange 266 by seal 272. FUter bag 270 is sealed so that gases and particulates entering the gas recycle system 262 through chimney exit 260 are separated into sόUd particles which remain inside filter bag 270 and clean gas which passes through the waUs of the filter bag into the remaining portion of gas coUection vessel 268. The clean gas is then circulated back to feed conduit 244 by means of a gas circulating turbine 274 connected to feed conduit 244. Other methods and means of separating soUd particles from gas streams may be utilized as gas recycle system 262 in addition to or in place of filtration. As described above, a cyclone separator or a cooled rotating drum could be utilized as weU as other devices known to those skUled in separation techniques.

AU passages through the walls of enclosure body 212 should be sealed so that vaporization chamber 210 may be isolated from the atmosphere outside of the fuUerene generator, AU gas circulation connections should be sealed so that the entire system is free from contamination by the outside atmosphere. Also, the electrodes should be insulated from enclosure body 212 so that electrical cunent may flow only through electrical conductor 219, electrode 216, the electrical arc between electrode 216 and electrode 214, electrode 214, and electrical ground 240. Before voltage is applied to the electrodes, the cooling mechanisms should be operating. It may also be necessary to cool enclosure body 212. Electrical conductor 219 and electrical ground 240 should both be cooled by circulating water around the cunent carrying devices. The cooling of chimney passage 250 should also begin before the cunent is applied. In normal operation, electrodes 214 and 216 are both electrically conductive graphite rods. If electrodes 214 and 216 are in the shape of long right cylinders (rods) they should be aligned on the same longitudinal axis. The vaporization chamber 210 should also be free of water. Preferably the vaporization chamber is evacuated to a pressure of less than 10^"2 Torr. After the vaporization chamber 210 has been evacuated, a small amount of inert gas may be added through gas addition inlet 276 and circulated via the gas circulating turbine 274. To start the device, electrodes 214 and 216 should be adjusted to barely touch. At this time, with the electrodes touching, the electrical voltage source 218 should be activated to apply voltage to electrode 216 in an amount sufficient to cause an electrical cunent to flow from electrode 216 to electrode 214. After cunent flows, the electrodes should be separated to achieve the desired arc gap. In practice, the gap may be very short and the electrodes may appear to touch. The position of each electrode should be adjusted, as the electrodes erode, to maintain the desired gap between them so that the arc wiU continue. Electrode 216 may be moved by tiirning threaded rod 230 by toning gear 232 in order to push electrode 216 toward electrode 214. Electrode 214 may be pushed toward electrode 216 in the same manner by rotating the threaded rod connected to electrode 214.

Feed conduit 244 is positioned so that the inert gas exits feed conduit 244 and passes around and through the arc gap area. If graphite dust is to be passed through the arc, the graphite dust may be mixed with the inert gas and introduced to the system through gas addition inlet 276. K the temperature of the inert gas is to be adjusted, this may be accompUshed by either heating or cooling feed conduit 44 by known heating or cooling means. Carbon vapor flows to chimney passage 250 where the temperature is low enough to condense a carbon soot on the waUs of chimney passage 250. This carbon soot comprises two different forms of pure carbon, graphite and fuUerene molecules. Soot particles remaining in the vapor but not condensed on the walls of the chimney passage 250 pass through chimney exit 260 and are separated from the gas stream by filter bag 270. The cleaned inert gas is then recirculated by gas circulating turbine 274 back to feed conduit 244 where it is reintroduced to the carbon vaporization zone. Any inert gas makeup that is necessary may be introduced through gas addition inlet 276. The process may be continued until the electrodes have been consumed, at which time the electrical voltage should be withdrawn. Carbon soot lining the walls of the chimney passage 250 and filter bag 270 may then be recovered. Recovery of fuUerene compounds from the carbon soot may be accomplished by extracting the carbon soot with an extraction solvent which preferentiaUy dissolves fuUerene compounds but not graphite. The extract may then be filtered to remove the soUd graphite leaving a filtrate containing the solvent and fuUerenes. Evaporation of the solvent from the extract wiU allow recovery of fuUerenes as the soUd residue. Separation of fuUerene from graphite may be accompUshed by other methods known to those skUled in separation arts. For instance, separation may be accomplished by boiling solvents, ultrasonic sonication separation, supercritical fluid extraction, Soxhlet extraction and many other methods known to those skiUed in separation arts.

Although the embodiment described utihzes a circulating gas stream, gas recirculation is not required. When gas circulation is not used, carbon vapor from the electrical arc passes into the condensing zone by convection cunents. Operation of the process in this manner may be accompUshed by sealing off chimney passage 250 at chimney cap 251. Carbon soot wiU condense on the cool chimney waUs until the voltage is withdrawn or the carbon source is completely vaporized. After carbon condensation stops, carbon soot may be recovered by opening the system, for instance by removing chimney cap 251, and gently scraping or brushing the carbon soot from the chimney waUs. FuUerenes may then be recovered from the soot as described above.

Although not required by the invention, it may be beneficial to provide a zone where the growth and formation of fuUerenes^" are promoted. This zone is refened to as a fuUerene annealing zone and provides an atmosphere where the temperature, pressure and residence time favor the growth and formation of fuUerenes. Temperatures within the fullerene annealing zone preferably range between 1,000° and 2,000°C. Although the optimum residence time within the fullerene annealing zone is not known with certainty, residence times between 1 miUisecond to 1 second are usually sufficient to aUow the growth and formation of fullerene molecules. The desired pressure? ranges and atmosphere types are the same as described for carbon vapor formation. The fuUerene molecules are then removed from the fuUerene annealing zone along with graphite soot and condensed in an area of lower temperature.

With reference to the drawing in Figure 8, the carbon vapors formed by the electrical arc between the electrodes are carried away by the inert gas into a fuUerene annealing zone above the electrical arc but below the chimney passage 50. In this area, the temperature is appropriate for the growth and formation of fuUerenes from the carbon vapor. This fuUerene annealing zone may be comparatively small and vapor passage through it relatively rapid. Substantial fuUerene growth and formation may occur in the carbon vapor generation zone as weU as the fuUerene condensing zone. 6. The MetaUofuUerene Products

When the fuUerene selective metal is contacted with the carbon vapor in any of the previously described methods, some of the resulting fuUerenes wiU contain one or more atoms of the fuUerene selective metal inside. For example, two electrodes formed primarily from graphite mixed with a U0₂ have been used in the electric arc process to generate fuUerenes with a uranium atom trapped inside the fuUerene carbon structure. Surprisingly, the vaporization of a mixture of carbon and a fuUerene selective metal compound, particularly U0₂, results in a greater proportion of the product fuUerenes having a metal inside. Even more surprising is the formation of (U @ g), to date, the smaUest fuUerene known to have been produced, measured by the number of atoms in the fuUerene structure. (U @ g) also has been produced by the laser vaporization process, also producing a greater proportion of fuUerenes with uranium inside than prior art processes utilizing other metals.

The metaUofuUerene products may be represented by the formula (Mx @ .) where M is the fuUerene selective metal, x is an integer from 1 to 4, and n is an integer from 14 to 500 or more. Preferably, x is 1 and n is 14-50, more preferably n is 14-17, most preferably n is 14. Preferably M is uranium. The metaUofuUerene compounds are initiaUy coUected from the soot of the fuUerene generation process. The soot wiU usually included non- fullerene carbon such as graphite and soot products of the starting fuUerene selective metal compound. The fullerenes and fullerenes with metal inside may be separated from the soot by any of the means previously described herein and in the prior art. Extraction of the soot with an aromatic solvent, for example benzene, toluene or xylene, is one way of separating the fuUerene from non-fuUerenes. The soot may be mixed or contacted with the aromatic solvent to dissolve the fuUerenes. The non-fuUerenes may be filtered out and the solvent evaporated to yield fullerenes in solid form. Alternatively, the fuUerenes may be kept in solution and utilized accordingly.

Figures 9 to 15 show experimental results involving production o metaUofuUerenes. Figure 9 shows a mass spectral analysis of clusters produce by laser vaporization of a graphite/U0₂ composite target to produce La@C_n and Y@C_n metaUofuUerenes. Y. Chai, T. Guo, C. Jin, R. E. Haufler, L.P.F. Chibante, J. Fure, L. Wang, J. M. Alford, and R. E., SmaUey, J. Phys. Chem. 95 7564 (1991); J. H. Weaver, Y. Chai, G. H. KroU, C. Jin, T. R. Ohno, R. E. Haufler, T. Guo, J. M. Alfred, J. Conceicao, L.P.F. Chibante, A Jain, G. Palmer, and R. E. SmaUey, Chem. Phys. Lett. 190, 460-464 (1991). Note that the most abundant species appears to be U @ g. Figure 10 is a diagra representing a uranofuUerene, (U @ g). Figure 11 shows the higher mass range of this same mass-spectral analysis. Here it is clear that essentially all the clusters produced are of the form U @ C_n. In fact it is clear that substantial amount of doubly-doped fullerenes, U₂ @ C_n have been made as weU, beginning with U₂ @ C₅₀.

Figure 12 shows the result of a laser "slninkwrapping" experime on a selected set of U @ C₅₀, U @ C₆₀ and U @ C-₀ clusters. Note that here, unlike any previous experiments with metaUofuUerenes, the endpoint of shrinkwrapping is actuaUy below the C^ level for an empty fullerene. Here the inernal uranium atom has apparently stabilized the fuUerene cage, and made U @ C^ the final result of laser shrinking.

This evidence that uranium had stabilized the smaU fuUerene cages led to attempts at bulk synthesis of U @ g using the methods we introduced previously for other metals. Y. Chai, T. Guo, C. Jin, R. E. Haufler, L.P.F. Chibante, J. Fure, L. Wang, J. M. Alford, and R. E., SmaUey, J. Phys. Chem. 95 7564 (1991); J. H. Weaver, Y. Chai, G. H. KroU, C. Jin, T. R. Ohno, R. E. Haufler, T. Guo, J. M. Alfred, J. Conceicao, L.P.F. Chibante, A. Jain, G. Palmer, and R. E. SmaUey, Chem. Phys. Lett. 190, 460-464 (1991). Figure 13 shows the analysis of a sublimed film of fuUerenes prepared by laser vaporization of a graphite/U0₂ composite rod in an oven at 1200°C. Similar results were obtained when a DC arc was used to vaporize this graphite/U0₂ composite rod in the oven instead of the pulsed laser. In either case it is clear that U @ g is present in the sublimed film, and that it has survived the sublimation process as an intact molecule at least as weU as U @ C_∞.

Figure 14 shows the result of an XPS analysis of the uranium content of these sublimed films. Much as in previous studies with La@C_n films, this probe shows the uranium to be in a caged state, completely immune to oxidation to form U0₂, even though the film had been extensively exposed to air and water. ^~

Figure 15 shows that this use of a tetravalent internal metal atom to stabilize the g works with other elements as weU. Similar results have been obtained with hafnium.

Many other variations and modifications may be made in the apparatus and techniques hereinbefore described, by those having experience in this technology, without departing from the concept of the present invention. Accordingly, it should be clearly understood that the apparatus and methods depicted in the 'accompanying drawings and refened to in the foregoing description are iUustrative only and are not intended as limitations on the scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A uranofullerene represented by the formula (U_x @ wherei x = 1-4, n = 14-50.

2. A uranofuUerene in accordance with claim 1 wherein x = 1 and n = 14-17.

3. A uranofuUerene in accordance with claim 2 represented by the formula (U @ g).

4. A process for making uranofuUerenes comprising: (a) providing carbon and a uranium compound to a vaporization zone, (b) passing an inert gas stream through the vaporization zone, _ (c) vaporizing carbon and the uranium compound, (d) condensing the vapor produced in step (c).

5. A process in accordance with claim 4, wherein the vaporizing of the carbon and the uranium compound is accompUshed by inducing an electrical arc between two electrodes one or both electrodes comprising carbon and uranium.

6. A process in accordance with claim 5, wherein the uranium compound is U0₂.