Commercial synthesis methods produce nanofibers of either single-walled carbon nanotubes (SWNTs) or multi-walled carbon nanotubes (MWNTs) as a soot-like material. The strength and elastic modulus of individual carbon nanotubes in this soot are known to be exceptionally high, ˜37 GPa and ˜0.64 TPa, respectively, for SWNTs of about 1.4 nm diameter SWNTs [R. H. Baughman, A. A. Zakhidov, and W. A. de Heer, Science 297, 787-792 (2002)]. Relevant for applications needing light structural materials, the density-normalized modulus and strength of individual SWNTs are even more impressive, i.e., higher than steel wire by factors of ˜19 and ˜54, respectively. A critical problem hindering applications of these and other nanofibers is the need for methods for assembling these fibers having nanoscale dimensions into long fibers and fiber-derived shaped articles, of macroscale dimensions, that effectively utilize the properties of the nanofibers. Since such nanofibers can impart functionalities other than mechanical properties, methods are needed for enhancing the mechanical properties of filaments made of the nanofibers without compromising these other functionalities. Important examples of these other functionalities, together with the mechanical functionality that make the fibers multifunctional, are electrochromism, electrical and thermal conductivity, electromechanical actuation, and electrical energy storage.
A carbon single-wall (single-walled) nanotube (SWNT) consists of a single layer of graphite that has been wound up on itself into a seamless tube having a nanoscale diameter. A carbon multi-wall nanotube (MWNT), on the other hand, comprises two or more such cylindrical graphite layers that are coaxially nested one within the other in a manner analogous to Russian nesting dolls. Both single-wall and multi-wall nanotubes have been obtained using various synthetic routes that typically involve the use of metallic catalysts and very high processing temperatures. Typical synthesis routes are those employing a carbon arc, laser evaporation of carbon targets, and chemical vapor deposition (CVD).
SWNTs are produced by the carbon-arc discharge technique using a pure carbon cathode and a carbon anode containing a mixture of graphite powder and catalytic metal(s), like Fe, Ni, Co and Cu [D. S. Bethune et al. Nature 363, 605-7 (1993) and S. Iijima and T. Ichihashi, Nature 363, 603-5 (1993)]. C. Journal et al. [Nature 388, 756-758 (1997)] have described an improved carbon-arc method for the synthesis of SWNTs that uses Ni/Y (4.2/1 atom %) as the catalyst. Co-vaporization of carbon and the metal catalyst in the arc generator was shown to produce a web-like deposit of SWNTs that is intimately mixed with fullerene-containing soot.
Smalley and co-workers [A. Thess et al., Science 273, 483-487(1996)] developed a pulsed laser vaporization technique for the synthesis of SWNT bundles from carbon targets containing 1 to 2% (w/w) Ni/Co. The dual laser synthesis, purification and processing of carbon single-wall nanotubes has been described in the following references: J. Liu et al., Science 280, 1253 (1998); A. G. Rinzler et al., Applied Physics A 67, 29 (1998); A. G. Rinzler et al., Science 269, 1550 (1995); and H. Dai et al., Nature 384, 147 (1996).
A CVD method described by Cheng et al. [Appl. Phys. Lett. 72, 3282 (1998)] involves the pyrolysis of a mixture of benzene with 1 to 5% thiophene or methane, using ferrocene as a floating catalyst and 10% hydrogen in argon as the carrier gas. The nanotubes form in the reaction zone of a cylindrical furnace held at 1100-1200° C. Depending on the thiophene concentration, the carbon nanotubes form as either multi-wall nanotubes or bundles of single-wall nanotubes. Another useful method for growing carbon single-wall nanotubes uses methane as the precursor, ferric nitrate contained on an alumina catalyst bed, and a reaction temperature of 1000° C. [L. C. Qin et al, Applied Physics Letters, 72, 3437 (1998)].
Another CVD synthesis process was described by R. E. Smalley et al. in PCT Patent Application Publication Nos. WO 2000026138 and WO 2000017102, and by P. Nikolaev et al. in Chem. Phys. Lett. 313, 91-97 (1999). This process, known as the HiPco process, utilizes high pressure (typically 10-100 atm) carbon monoxide gas as the carbon source, and nanometer sized metal particles (formed in situ within the gas stream from metal carbonyl precursors) to catalyze the growth of single-wall carbon nanotubes. Suitable catalyst precursors are iron carbonyl (Fe(CO)5) and mixtures of iron carbonyl and nickel carbonyl (Ni(CO)4). The HiPco process produces a SWNT product that is essentially free of carbonaceous impurities, which are a major component of the laser-evaporation and carbon-arc products. Furthermore, the process enables control over the range of nanotube diameters produced, by controlling the nucleation and size of the metal cluster catalyst particles. In this way, it is possible to produce unusually small nanotube diameters (e.g., about 0.6 to 0.9 nm).
The nanotube-containing products of the laser-evaporation and carbon-arc processes invariably contain a variety of carbonaceous impurities, including various fullerenes and less-ordered forms of carbon soot. The carbonaceous impurity content in the raw products of the laser and carbon arc processes typically exceeds 50 weight %. Purification of these products generally relies on a selective dissolution of the catalyst metals and highly ordered carbon clusters (called fullerenes), followed by a selective oxidation of the less ordered carbonaceous impurities. A typical purification process is described by Liu et al. [Science 280, 1253 (1998)]. This method involves refluxing the crude product in 2.6 M nitric acid for 45 hours, suspending the nanotubes in pH 10 NaOH aqueous solution using a surfactant (e.g., TRITON X-100 from Aldrich, Milwaukee, Wis.), followed by filtration using a cross-flow filtration system While the effects of these purification processes on the nanotubes themselves are not completely understood, the carbon nanotubes are typically shortened by oxidation.
As discussed by B. I. Yakobson and R. E. Smalley [American Scientist 85, 325, (1997)], SWNT and MWNT materials are promising for a wide variety of potential applications because of the exceptional physical and chemical properties exhibited by the individual nanotubes or nanotube bundles. Some SWNT properties of particular relevance include metallic and semiconducting electrical conductivity (such conductivity being dependent upon the specific molecular structure), an extensional elastic modulus of 0.6 TPa or higher, tensile strengths of about 37 GPa and possibly higher, and surface areas that can exceed 300 m2/g [R. H. Baughman, A. A. Zakhidov, and W. A. de Heer, Science 297, 787-792 (2002)].
The proposed applications of carbon nanotubes include mechanical applications, such as in high-strength composites, electrical applications, and multifunctional applications in which different physical properties of the carbon nanotubes are simultaneously utilized. Tennent et al. (U.S. Pat. No. 6,031,711) describe the application of sheets of carbon nanotubes as high performance supercapacitors. In this latter application, a voltage difference is applied to two high-surface-area carbon nanotube electrodes that are immersed in a solid or liquid electrolyte. Current flows, thereby injecting charge in the nanotubes, by creating an electrostatic double-layer near the nanotube surfaces.
The application of carbon nanotube sheets as electromechanical actuators has been recently described [R. H. Baughman et al., Science 284, 1340 (1999) and R. H. Baughman, C. Cui, J. Su, Z. Iqbal, and A. A. Zakhidov, U.S. Pat. No. 6,555,945]. These actuators utilize dimensional changes that result from the double-layer electrochemical charge injection into high-surface-area carbon nanotube electrodes. If carbon nanotubes can be assembled into high modulus and high strength assemblies (such as filaments, ribbons, or sheets) that maintain their ability to electrochemically store charge, then superior actuator performance should be obtainable. The problem has been that no methods are presently available for the manufacture of nanotube articles that have these needed characteristics.
Those applications described above, as well as other promising applications, require assembling the individual nanotubes into macroscopic arrays that effectively use the attractive properties of the individual nanotubes. Failure to effectively achieve this has created an obstacle that has thus far hindered applications development. A primary problem is that MWNTs and SWNTs are insoluble in ordinary aqueous solvents and do not form melts even at very high temperatures.
Nanotube sheets (called “nanotube paper” or “bucky paper”) comprising mostly nanotubes can be obtained by filtering a SWNT dispersion through a filter membrane, peeling the resulting sheet from the filter after washing and drying steps, and thermally annealing the sheet at high temperatures to remove impurities that convert to gases [A. G. Rinzler et al., Appl. Phys. A 67, 29 (1998) and Liu et al. in Science 280, 1253 (1998)]. This preparative method utilizes the fact that under certain conditions, and with the aid of surfactants and ultrasonic dispersion, bundles of SWNTs can be made to form a stable colloidal suspension in an aqueous medium. The obtained carbon nanotube sheets, which can range in conveniently obtainable thickness from 10-100 microns, possess mechanical strength derived from the micro-scale entanglement of the nanotube bundles. These nanotube sheets preserve the large accessible surface area of the nanotube bundles, but typically exhibit elastic modulus values (typically a few GPa) that are a very small fraction of the intrinsic elastic modulus of either the individual SWNTs or the SWNT bundles.
Glatkowski et al., in U.S. Pat. No. 6,265,466, teach a method for preparing an electromagnetic shielding composite having nanotubes, wherein the nanotubes are oriented when a shearing force is applied to the composite. The method includes a step of providing a polymer with an amount of nanotubes, and imparting a shearing force to the molten polymer containing carbon nanotubes to orient the nanotubes. Glatkowski et al. generically teach that the nanotube concentration can be as high as 15 wt %, but that it is preferable that the concentration is 0.1 to 1.5 wt %. These materials do not have properties that would render them useful in actual mechanical or electrochemical applications, since they are made with such a low loading of the carbon nanotubes.
Yashi et al. [Materials Research Society Symposium Proceedings, “Science and Technology of Fullerene Materials,” 359, pg. 81-6, 1995] have attempted to overcome these problems by using a method for forming fiber of aligned carbon nanotubes by extruding a mixture of carbon nanotubes and polypropylene through a small die having a diameter of 2 mm that is maintained at about 200° C., so that the polypropylene is in molten state. As in the case with Glatkowski above, these materials do not have properties that would render them useful in actual mechanical or electrochemical applications, since they are made with such a low loading of the carbon nanotubes and the polypropylene remains in the final product. Use of high nanotube concentrations results in very high viscosities for the nanotube mixture with molten polymer, which essentially prohibits extrusion.
A. Lobovsky, J. Matrunich, R. H. Baughman, I. Palley, G. A. West, and I. Golecki have described (U.S. Pat. No. 6,764,628) a melt spinning process that attempts to avoid the usual limitations caused by low concentrations of carbon nanotubes in melt spun fibers. This process involved melt compounding 30 weight percent of very large diameter multi-walled carbon nanotubes (150-200 nm in diameter and 50-100 microns in length) in a polypropylene matrix. This nanotube/polymer mixture was successfully spun as the sheath of a sheath/core polymer comprising polypropylene as the core. Despite the high viscosity of the nanotube/polymer mixture in the sheath and the brittleness of the solidified composition, the presence of the polymer core permitted this sheath-core spinning and the subsequent partial alignment of nanotubes in the sheath. Pyrolysis of the polypropylene left a nanotube fiber that is hollow (outer diameter=0.015 inches, inner diameter=0.0084 inches). To increase the strength of the hollow nanotube fiber, it was coated with carbon using a chemical vapor deposition (CVD) process. Even after this CVD coating process, however, the hollow nanotube fiber had low strength and low modulus and was quite brittle.
Although advances have been made in spinning polymer solutions in which carbon nanotubes are dispersed, the solution viscosity become too high for conventional solution spinning when the nanotube content rises above about 10%. Nevertheless, impressive mechanical properties have been obtained for solution spinning SWNTs in a polymer to provide a polymer nanotube composite, which in large part express the high mechanical properties of the polymer matrix for the nanotubes [T. V. Sreekumar, T. Liu, B. Min, G. Byung, H. Guo, S. Kumar, R. H. Hauge, R. E. Smalley, Advanced Materials, 16, 58-61 (2004) and S. Kumar, T. D. Dang, F. E. Arnold, A. R. Bhattacharyya, B. G. Min, X. Zhang, R. A. Vaia, C. Park, W. W. Adams, R. H. Hauge, R. E. Smalley, S. Ramesh, P. A. Willis, Macromolecules 35, 9039-9043, (2002)]. One problem with such approaches, however, is that the nanotubes are not present in sufficient quantities to effectively dominate such properties as mechanical modulus, mechanical strength, and thermal and electrical conductivity.
Methods are known for dry spinning multi-walled carbon nanotubes as yarns from MWNT forests [K. Jiang, Q. Li, S. Fan, Nature 419, 801 (2002)]. However, the fibers so obtained are so weak that they cannot be used for structural applications. In fact, such fibers are so weak that they cannot be processed into continuous lengths.
In another process [V. A. Davis et al, United States Patent Application Publication No. 2003170166 and L. M. Ericson et al., Science 305, 1447-1450, (2004)], single-walled carbon nanotubes were first dispersed in 100% sulfuric acid, or in another super acid, and then wet-spun into a coagulation bath comprising diethyl ether, water, or 5 wt % sulfuric acid. This method is referred as the super-acid coagulation spinning method (SACS), since the spinning solution used is a mixture of carbon nanotubes and a super acid. The resulting fibers have compromised properties, in part due to a partial degradation of the SWNTs and super acid intercalation, caused by prolonged contact with the super acid in the spinning solution. This creates a serious obstacle for practical applications. In order to partially reverse property degradation caused by prolonged exposure to super acid spinning solutions, and to thereby enhance electrical conductivity, it was necessary to anneal the as-spun fibers at high temperatures (typically 850° C. and higher), which would increase the cost of fiber production. Also, the use of 100% sulfuric acid or super acid in the spinning solution causes other problems that are not present for processes that do not use strongly acidic spinning solutions. These include the need to blanket the spinning solution with an inert atmosphere and the use of spinnerets, spinning solution containment means, and pumping means for applying pressure during spinning that are not corroded by the super acid in the spinning solution.
Polymer gel-based processes have been shown to enable the spinning of continuous fibers of SWNT/poly(vinyl alcohol) composites [B. Vigolo et al., Science 290, 1331 (2000); R. H. Baughman, Science 290, 1310 (2000); B. Vigolo et al., Applied Physics Letters 81, 1210-1212 (2002); A. Lobovsky, J. Matrunich, M. Kozlov, R. C. Morris, and R. H. Baughman, U.S. Pat. No. 6,682,677; and A. B. Dalton et al. Nature 423, 703 (2003)]. According to such processes, the carbon nanotubes are first dispersed in an aqueous or non-aqueous solvent with the aid of a surfactant. A jet of this nanotube dispersion is then injected into a viscous polymer solution that causes partial aggregation and alignment of the dispersed nanotube bundles to form a gel fiber, which is a dilute mixture of carbon nanotubes in an aqueous gel of the coagulation polymer. This gel fiber is weak; however, it has sufficient strength for slow manipulation leading to subsequent conversion of the gel fiber to a solid polymer fiber. In some processes, the wet gel fiber is washed in water or other liquid in order to remove some of the polymer binder, and the washed filament is subsequently withdrawn (drawn) from the wash bath and dried. During the draw-dry process, during which evaporation of the liquid occurs from the gel, capillary forces collapse the gel fiber into a solid fiber. This total process will henceforth be referred to as the polymer coagulation spinning (PCS) process.
In a typical PCS process, as described by Bernier and co-workers [Vigolo et al., Science 290, 1331 (2000)], the nanotubes are dispersed in water with the aid of sodium dodecyl sulfate (SDS) surfactant. The viscous carrier liquid is an aqueous solution of poly(vinyl alcohol) (PVA) in which the PVA serves to neutralize the effect of the SDS surfactant by directly replacing these molecules on the carbon nanotube surfaces during spinning. Bernier and co-workers describe preferred concentrations for the various ingredients, and viscosity ranges and flow velocities of the spinning solutions. Polarized light microscopy of the coagulated nanotube fibers confirms preferential alignment of the carbon nanotube along the fiber axis. Further evidence of carbon nanotube alignment is provided by the measured extensional elastic modulus, which is approximately 10-40 GPa for the final PCS fibers, as compared to typically 1 GPa for bucky paper.
Present problems with this process are that the nanotube fibers are inherently self-assembled in combination with PVA, and this PVA interferes with the electrical and thermal contacts between carbon nanotubes. Using existing technology, this polymer can only be completely removed from the gel fiber by thermal annealing that causes pyrolysis of the polymer. This removal of polymer by thermal pyrolysis degrades the mechanical properties of the nanotube fibers by decreasing strength and modulus and making them rather brittle.
Unfortunately, because of the presence of residual PVA, electrical and thermal conductivity of the fiber is smaller than that of nanofiber sheets. PVA, a typical insulating polymer, exhibits poor electrical and thermal conductivity as compared with carbon nanotubes. As a result, the conductivity of such composite fibers decreases with increasing PVA content; it also becomes substantially dependent on post-spinning washing, which does not remove all of the polymer. Other disadvantages of PVA-based fibers are poor thermal stability caused by decomposition of the polymer at 100-150° C., sensitivity to moisture, and reduced resistance to solvents. Also, the fibers made by the PCS process are not useful in applications as electrodes immersed in liquid electrolytes because of a surprising shape-memory effect. This shape-memory effect causes the PCS fibers to dramatically swell (by 100% or more) and lose most of their dry-state modulus and strength. Because of this structural instability of fibers made by the PCS process, they are unusable for critically important applications that use liquid electrolytes, such as in supercapacitors and in electromechanical actuators. In contrast, as-produced carbon nanotube sheets made from the same nanotubes can be used for both capacitor and actuator devices that use liquid electrolytes.
Thus, the prior art spinning processes are unsatisfactory for providing high loadings of underivatized polymer-free nanotubes in macroscopic nanotube fibers, which most desirably have continuous lengths. This absence of a suitable technology for spinning polymer-free nanotube fibers has been a barrier to application of carbon nanotubes and other nanotube fibers for such applications as mechanical elements, elements having high thermal conductivity, and as components in devices that provide electromechanical actuation, mechanical energy harvesting, mechanical dampening, thermal energy harvesting, and energy storage. Although the individual nanotube fibers have very attractive performance attributes, the prior art has not demonstrated processes whereby the properties of these individual nanotubes can be effectively used in macrofibers comprising the nanofibers. Additionally, no prior art technology has provided a method for spinning hollow carbon nanotube fibers, and such carbon nanotube fibers can be usefully employed for such applications as filtration, materials absorption, and materials transport. Methods that overcome the above-described deficiencies would be most desirable.