Commercial Production of Fullerenes and carbon nanotubes.pdf

Commercial Production of Fullerenes andCarbon Nanotubes

Raouf O. Loutfya, Timothy P. Lowea, Alexander P. Moravskya andS. Katagirib

aMER Corporation, Tucson Arizona USAbFIC Corporation, New York NY USA

It has been slightly over ten years since the development of a way to produce macroscopicquantities of fullerene, and the related discovery of fullerene nanotubes. As a result, over1500 worldwide patents have been filed for the production and applications of these newmaterials. These applications are so wide ranging that they extend across different indus-tries with products from additives to polymers, photoconductors, photo-resists, andbio-active agents to cosmetics. MER Corporation in Tucson, Arizona joined the ranks offullerene enthusiasts at the beginning of its discovery by immediately licensing theHuffman–Krätschmer patents. While we are widely recognized as a producer of fullereneand nanotubes, MER has also been active in developing applications for fullerenes and nano-tubes. The different applications investigated by MER will be reviewed in subsequentchapters. The overriding factor for the success of any of these applications, however, is theprice of fullerenes. However, the price cannot come down markedly until large-scale appli-cations are found. To introduce the first large-scale application an organization had to take aleap of faith and initiate the large-scale low-cost production. Mitsubishi/FIC Corporationshas been a leader and pioneer in recognizing the need to support large-scale productioneffort to realize the fullerene and nano-technology commercialization dream. It is now ouropportunity to realize the commercial applications. The present status of the scale-up pro-duction effort of fullerenes and the different nanotubes will be presented in this chapter.

1. Fullerene Production

The family of fullerenes includes the hollow cage all-carbon molecules having a convexclosed-shell structure containing arbitrary numbers of hexagonal and exactly twelvepentagonal faces. They are synthesized abundantly in the carbon arc, hydrocarbon flameor field-induced hot carbon plasma and are originally embedded in the soot product.Fullerenes can be extracted from the soot due to appreciable solubility in some organicsolvents. Separation of the various members of the fullerene family is accomplished bychromatography. The most stable fullerenes and have been produced in amountsof up to 23 wt % in the total harvested soot in the arc process [1], although typical tech-nological yields are on the order of 8–15% at different production facilities. The higher-order fullerenes, of which and are the most abundant, typically make upabout 2–4 wt % of the arc-produced soot. It is interesting to note that the molarratio remains constant at 5.06 within about 1% accuracy in any carbon arc process inhelium gas atmosphere, regardless of run conditions or graphite rod thickness [2].

Soon after the famous discovery of the arc process in 1990, MER Corporation wasthe first to offer fullerenes commercially. The mixed fullerenes that are extracted fromthe soot were the first product available, and after the development of chromatographic

E. (ed.,) Perspectives of Fullerene Nanolechnology, 35–46.© 2002 Kluwer Academic Publishers. Printed in Great Britain.

Raouf O. Loutfy, Timothy P. Lowe, Alexander P. Moravsky, et al.

separations on alumina with hexane eluent, purified and were offered.Subsequent improvement in chromatographic methods increased the separation capacityby a factor of several hundred. The first primitive arc reactors were operated manually,with one graphite rod at a time being loaded and vaporized. Automated computer-controlled reactors (Figure 1), which operate without any attendance for several days,were constructed in 1992, and the output of mixed fullerenes and purified andwas increased dramatically.

Further increases in fullerene production rate were expected by employing thickergraphite rods in larger, more powerful arc reactors. However, fullerene production in car-bon arcs was generally believed to be a non-scalable process, in the sense that the use ofthe rods of larger diameter leads to a rapid reduction in fullerene yield. Guided bydetailed mechanistic considerations of the carbon arc process [1], we have proved exper-imentally, that this is not the case, by vaporization of 3-inch (~76 mm) diameter graphiterods to routinely produce over 11 wt % fullerenes soot product. The scaled-up reactor(Figure 2) is currently in use for commercial fullerene production in Osaka, Japan.

With several applications of fullerenes about to flourish, the scale-up of the arcprocess has reduced the cost of fullerenes to a level that will stimulate these new uses offullerenes, and will make fullerene products viable alternatives to existing products thatuse other materials.

A further increase in vaporized rod diameter and in arc power is technically feasiblebut does not appear to be economically justified. The large-scale arc reactor developedby MER Corporation (Figure 2) has been demonstrated to be close to the optimum sizeand power for fullerene synthesis in an arc, and will probably remain the largest reactorin the world for a long time. In the arc process, the next logical step for increasingfullerene output will be the automation of large-scale arc reactors, which will render theproduction process nearly continuous.

While the arc process is presently the most efficient means for high fullerene produc-tion rates, other methods that have been investigated for fullerene synthesis may soon be

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Commercial Production of Fullerenes and Carbon Nanotubes

adapted for commercial production. To become competitive to the arc process, thesealternative techniques would have to make use of their potential to be operated continu-ously at a level of hundreds of kilowatts useful power per production unit, provided theirundesirable properties such as low inherent yield and specific production rate, highoperating cost and/or high capital expenditure are obviated.

The field-induced high-intensity plasmas are an attractive alternative to the arc process;inexpensive powder, non-conductive carbon or hydrocarbon gas could be used as startingmaterial, and product collection could be automated and continuous. MER is undertakingtechnical feasibility studies for this method. Other known techniques, such as the use oflow-pressure hydrocarbon flames or laser vaporization of carbon have only a slightchance of becoming competitive on a large scale, mainly because of technical difficulties.The flame technique possesses the potentially important advantage of being an exother-mic process, but it has too low a specific production rate to ever reach the fullerene out-put level available with a scaled-up arc process. The hundreds of kilowatt continuouspower lasers are prohibitively expensive to be used for fullerene production. In conclu-sion, it becomes obvious that fullerenes can be produced by virtually any process capableof generating and appropriate curing of carbon vapor, and that many opportunities are notyet explored. A new highly efficient process may be just around the corner.

2. Synthesis of Carbon Nanotubes (MWNTs, SWNTs and DWNTs) andVapor-Grown Carbon Fibers (VGCFs) by the Arc and CVD Techniques

2.1. Arc-Grown MWNTs

MWNTs are an all-carbon fullerene structure composed of concentric nested graphenecylinders forming an average misorientation angle of 0° to the tube axis. Arc-grownMWNTs are produced without metal catalysts, and are found in the core material of the

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deposit that is formed on the cathode during vaporization of the anode. A transmissionelectron microscopy (TEM) image of arc-grown MWNTs is shown in Figure 3. Theytypically have outer diameters in the range of 3–50 nm, with 15 nm as the average. Theinner diameter is in the 1–4 nm range, and is most commonly 2 nm. The aspect ratio isaround 100, and both ends are capped. Arc-produced MWNTs are perfectly straight andhave few defects. They therefore have high mechanical, thermal and electrical conduc-tivity. The Young’s modulus of MWNT is calculated to be ~1000–1500 GPa and hasbeen measured at 1000–3700 GPa. The measured tensile strength is 11–63 GPa for theouter layer of the MWNT. The thermal conductivity along the length of the tube is~1500 W/mK, and the resistivity is

2.1.1. Scaled-up production of arc-grown MWNTs

MWNTs can be regarded as a useful by-product of fullerene synthesis in the arc. Theirdistribution, orientation, quality and content in cathode deposits [3, 4] depend primarilyon the radial temperature distribution within the arc gap, which in turn is defined by theoperating conditions of the arc reactor. While limited amounts of MWNTs are found inthe cathode deposits produced during fullerene production, the optimized conditions forMWNT production give approximately 40 wt % MWNTs in the soft core material of thecathode deposit. TEM examination of representative samples show the major contami-nants to be multi-layer polyhedral particles (MPPs) and various kinds of graphitic parti-cles. The cathode deposit core is a soft, black fibrous material that is easily separatedfrom the fused carbon shell, which contains few nanotubes. MER/FIC offers the sepa-rated core material on a kilogram scale, and it has proven useful as a conductive additive

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for plastics, as electron emitters in electron guns, and as probe tips in atomic-forcemicroscopy, among many other uses under development.

The laboratory-scale process has proven to be scalable with the production of arc-grown MWNTs in a large-scale reactor built by MER. While the larger-scale reactorshave been found to produce lower yield products than small-scale reactors, MWNTs ofmore than 95% purity have been obtained in research scale samples (Figure 4) by purifi-cation of the cathode deposit core material using a procedure which decomposes thegraphitic carbon and MPP contaminants. Coincidentally, the purification process alsoopens the ends of many of the MWNTs. This procedure has clear potential for scale-upto process arc-grown MWNT, and may foster research into effective ways to fillMWNTs with useful materials.

2.2. Catalytic MWNTs

MWNTs are also produced by catalytic pyrolysis of hydrocarbons over metals (catalyticCVD method). Catalytically grown MWNTs are of about the same outer and innerdiameters (3–70 nm and 2–10 nm) as arc-grown nanotubes, but are usually much longer

CCVD-grown tubes are often capped with a metal nanoparticle at thegrowing end, and may contain metal inclusions in the inner channel. Catalytic MWNTsabound in structural defects, resulting in bends and kinks between straight segments.Therefore their mechanical strength and conductive properties are lower than those ofarc-produced MWNTs (sometimes by several orders of magnitude, depending on theamount of defects). Catalytic MWNTs are preferable to arc-produced MWNTs whenhigh aspect ratio is more important than high structural strength or high conductivity.

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2.2.1. Scaled-up production of MWNTs by the catalytic pyrolysis of hydrocarbons

At MER, a scaled-up version of a reported process [5] that employs the pyrolyticdecomposition of xylenes over a ‘floating’ ferrocene-derived catalyst produces nearlypure MWNTs, with small amounts of residual iron catalyst. A straightforward treatmentto remove iron catalyst nanoparticles yields a product that is nearly 100% MWNTs. Thelength of these catalytic tubes can reach 1 mm, which is two orders of magnitude longerthan the arc-produced MWNTs, while the tube diameter is about the same in bothproducts. The length and diameter of the mass-produced catalytic MWNTs can be con-trolled over a very wide range, 100nm–l mm and respectively, by varyingthe process parameters, as opposed to arc-produced tubes that have a fairly stable sizedistribution independent of arc conditions. A true continuous process for catalyticMWNT production has been recently developed at MER Corporation.

2.2.2. Synthesis of large arrays of aligned MWNTs by CVD using a floating catalyst

In a system similar to the one where catalytic MWNTs are synthesized, large area (up touniform arrays of MWNTs are grown that are perfectly aligned normal to a

flat support (Figure 5). It has been demonstrated that conductive supports, like n-dopedsilicon wafers, can be employed, and it was shown that there is good electrical contactbetween the MWNTs and the support. The thickness of the layer of aligned MWNTs

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can be varied from a few micrometers to one millimeter, with surface density of tubes ofapproximately 30 billion per Aligned MWNT arrays on conductive supports withthe tube ends free of iron catalyst particles have also been produced.

2.2.3. Vapor-grown carbon fibers (VGCFs)

The vapor-grown carbon fibers (VGCFs) are near axially symmetric lamellar structurescomposed of conical-shaped graphene-sheet-based elements. The average misorientationangle between a graphene plane and the tube axis can be in the rangeAccordingly, the cone angle may vary between 0 and 180°. The latter extreme corre-sponds to a ‘platelet’-type structure, ideally composed of a stack of flat graphene sheets.A ‘herring-bone’-type structure comprises either intact cones or truncated cones havingflat or convex caps. If the cap is absent, a hollow internal cavity is formed and the struc-ture is considered to be a conical layer nanotube (CLNT). In real ‘herring-bone’ andCLNT structures, the angle can vary within a layer in the radial direction, matchingto the shape of the metal catalyst particle from which it formed, and the layers maycomprise various defects and discontinuities. With the cone is transformed into acylinder and VGCFs are referred to as MWNTs, although these tubes can be muchthicker than arc-grown MWNTs (a few micrometers in diameter) due to sidewallgrowth, and they generally possess less orderly structured outer layers.

Multiple exposed edges appear on the outer walls of the VGCFs, and the lineardensity of edges increases with increasing The many exposed edges make VGCFsgood candidates for intercalation of the interlayer spaces with small molecules or ions,so they may find use in gas storage or Li-ion electrochemical cells. The edges can alsobind transition metal ions, making VGCFs a promising catalyst support. These arethe most promising areas for VGCF applications, since their mechanical and conduc-tive properties are poor compared to those of carbon nanotubes and polymer-basedcarbon fibers.

2.2.4. RF plasma-assisted CVD synthesis of aligned arrays of VGCFs

Perfectly aligned arrays (Figure 6) of VGCFs with ‘herring-bone’ internal structurehave been deposited normal to the surface of electrically conducting supports by theRF plasma-assisted CCVD method [6], which is similar to the microwave plasmaCCVD technique that was first reported in [7]. These practically pure VGCFs are typi-cally up to ten times thicker (20–200 nm in diameter) than concentric MWNTs. Theyare less conductive than MWNTs, but are much better adapted to the adsorption andrelease of chemicals from between the graphene layers. The growing ends ofas-produced VGCFs contain nickel or iron microparticles depending on the catalystemployed, which can be removed by treatment with a mineral acid. The diameterand surface density of fibers produced are controlled by the method of catalyst prepara-tion and by the process parameters. Under certain conditions some MWNTs areproduced as a component of aligned arrays whose major composition is ‘herring-bone’VGCFs.

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2.2.5. Bulk production of VGCFs with abundant random exposed edges

Randomly oriented pure VGCFs of ‘herring-bone’ and ‘platelet’ internal structure(Figure 7) are obtained at MER Corporation in bulk quantities (kg) by the catalyticpyrolysis of a mixture of ethylene or carbon monoxide with hydrogen over an iron-copper catalyst. This process is described in detail in [8].

2.3. Single-walled carbon nanotube (SWNT)

SWNTs are a fullerene structure made of a graphene sheet rolled into a seamless cylinderhaving carbon caps on both ends. The diameter of SWNTs produced by the arc, laser orcatalytic CVD process is in the range 1–5 nm, and can be controlled by experimentalconditions. The aspect ratio can be as large as Figure 8 is a TEM micrograph of arc-grown SWNTs. SWNTs made by any of the synthetic methods are essentially defect-freestructures that possess outstanding mechanical and conductive properties. The tubes arehighly elastic and are always buckled or kinked rather than broken if bent. Theoreticalestimates predict 1200–1700 GPa for the value of the Young’s modulus and 3000 W/m Kfor the axial thermal conductivity. The metallic type of SWNTs possess a very low resis-tivity of both by theoretical estimates and direct measurements.

2.3.1. Scaled-up arc synthesis of SWNTs

The laboratory-scale synthesis of SWNTs in the arc using Ni/Co based catalyst has beensubjected to an extensive set of mechanistic studies [9], leading to the consistent

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production of SWNT product with 40 vol % yield of SWNTs. As in the scale-up offullerene production by increasing the diameter of the reactant rod, there is a generalconsensus that yield will decrease as the rod diameter increases. However, research atMER has determined that 25 mm diameter rods packed with the Ni/Co based catalyst canbe used to increase production with a minimal decrease in yield. An efficient purification

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procedure employed at present at MER yields research quantities (~10g per month) ofmore than 90 wt % pure SWNTs (Figure 9). The procedure enables ~30% recovery ofpure tubes even from low-yield SWNT starting materials. Therefore, the scale-up of thepurification process will make these materials valuable, provided they are produced inabundance at low cost.

The control of SWNT properties has proven to be difficult. Variation of arc conditionsand/or catalyst composition has some effect on the tube parameters, but more experi-mentation is required to define the effects. A sulfur-promoted Fe/Co/Ni-based catalystwas recently found [10] that is capable of producing more than 60 wt % yieldof SWNTs with very large diameter (about 4.0 nm on the average), which could havespecial applications. Further optimization of the synthesis parameters and consequentscaling-up of the process are expected to finally give much more efficient production ofthese high-yield thick tubes.

2.4. Double-walled carbon nanotubes (DWNTs)

DWNTs are a fullerene structure made of two nested graphene cylinders with bothcylinders capped at the ends (see Figure 10). The outer diameter of DWNTs is in the rangeof 2–5 nm, while the inner diameter is 1–4 nm, depending on the synthesis conditions.The inter-wall distance (~0.39 nm) is slightly larger than the distance in MWNTs. Thelength of DWNTs can reach DWNTs are produced in a carbon arc in a techniquethat is similar to SWNT production, with modifications to the catalysts and atmosphereemployed [10]. The outer and inner walls of a tube are defect-free graphene sheets, andthe tube probably has similar high mechanical strength and conductive properties to thosefound in SWNTs. The tubes are highly elastic and buckle at bending, and since they arestiffer than SWNTs, they are typically found in a less curved and tangled state. The

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values for mechanical strength, thermal and electrical conductivity for DWNTs are notyet defined.

2.4.1. Selective synthesis of DWNTs in the arc

Selective synthesis of double-walled carbon nanotubes (DWNT) is efficiently accom-plished using a slight variation in the same Fe/Co/Ni catalyst composition and preparationprocedure used for SWNTs [10]. High-resolution TEM (HRTEM) analysis of typicalsamples has shown that the ratio of DWNT/SWNT is higher than 30 and the yield ofDWNTs in as-produced fibrous material exceeds 70 wt %. The outer diameter of theseDWNTs ranges from 3.2 to 4.9 nm, and the size distribution is sharply peaked at 3.8 nm[10]. The scaling-up of this arc synthesis method is underway to make DWNTs avail-able for extensive research work on anticipated applications in cold field emission,hydrogen storage, lithium batteries and other technologies.

3. Conclusion

Different methods were investigated experimentally and theoretically to producefullerenes and various types of nanotubes (SWNT, arc-MWNT, VG-MWNT, CVD-alignedMWNT, and arc-DWNT). Scale-up production of many of these products was accom-plished. Scale-up schemes for all other materials are underway. Accordingly, the limitingfactors for the commercialization of the fullerenes-based materials is now finding the right(killer) applications. In Chaps. 22–29 some of the MER/FIC applications developmentefforts are described.

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Kaouf O. Loutfy, Timothy P. Lowe, Alexander P. Moravsky, et al.

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