Abstract:
A pile of carbon nanotubes was prepared by placing a small amount of catalyst solution on a substrate, putting the substrate into a furnace, purging the furnace, and heating the substrate under a flow of gaseous carbon source. A pile of carbon nanotubes formed on the substrate. Nanotubes from the pile were spun into a fiber.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/735,032 filed Nov. 8, 2005, which is incorporated by reference herein. 
     
    
     STATEMENT REGARDING FEDERAL RIGHTS  
       [0002]     This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION  
       [0003]     The present invention relates generally to the preparation of carbon nanotubes and more particularly to the preparation of a pile of carbon nanotubes, and to fibers spun from the pile.  
       BACKGROUND OF THE INVENTION  
       [0004]     Carbon nanotubes (CNTs) are seamless nanometer scale diameter tubes of graphite sheets. They have shown promise for nanoscale electronic devices, chemical sensors, high strength materials, field emission arrays, tips for scanning probe microscopy, gas storage, and other important applications.  
         [0005]     CNTs may be multi-walled or single-walled. Multi-walled CNTs were discovered in the hard deposit formed on the graphite cathode of an arc-evaporation apparatus used to prepare carbon fullerenes C 60  and C 70 . Single-walled CNTs were reported shortly thereafter.  
         [0006]     Single walled CNTs have been prepared using arc and laser techniques. There has been some success in producing single-walled CNTs from the catalytic cracking of hydrocarbons. Single-walled CNTs have also been produced from the catalytic disproportionation of carbon monoxide (CO). In an example, the diameters of single walled carbon nanotubes (SWNT) were found to vary from 1 nm to 5 nm, and seemed vary as a function of the size of particle size of the metal catalyst.  
         [0007]     Rope-like bundles of single-walled CNTs have been generated from the thermal cracking of benzene using an iron catalyst and sulfur additive at temperatures between 1100-1200 degrees Celsius. These single-walled CNTs were roughly aligned in bundles and woven together, similar to those obtained using an electric arc or laser vaporization.  
       SUMMARY OF THE INVENTION  
       [0008]     In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes a method for preparing a pile of carbon nanotubes. The method involves heating a catalyst species on a substrate in an atmosphere comprising a gaseous source of carbon at a temperature sufficient to decompose the gaseous source of carbon and form a pile of carbon nanotubes.  
         [0009]     The invention also includes a pile of carbon nanotubes prepared by heating a catalyst species on a substrate in an atmosphere comprising a gaseous source of carbon at a temperature sufficient to decompose the gaseous source of carbon.  
         [0010]     The invention also includes a method for preparing a fiber comprising forming a pile of carbon nanotubes by a method comprising heating a catalyst species on a substrate in an atmosphere comprising a gaseous source of carbon at a temperature sufficient to decompose the gaseous source of carbon and form a pile of carbon nanotubes, and thereafter spinning a fiber from the pile of carbon nanotubes.  
         [0011]     The invention also includes a fiber prepared by heating a catalyst species on a substrate in an atmosphere comprising a gaseous carbon source at a temperature sufficient to decompose the gaseous carbon source and form a pile of carbon nanotubes, and thereafter spinning a fiber from the pile of carbon nanotubes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:  
         [0013]      FIG. 1  shows a schematic representation of an embodiment apparatus used for preparing a pile of carbon nanotubes.  
         [0014]      FIG. 2  shows an optical image of a pile of nanotubes on a substrate prepared using the apparatus of  FIG. 1 .  
         [0015]      FIG. 3  shows a side view of the pile of nanotubes of  FIG. 2 .  
         [0016]      FIG. 4  shows a transmission electron microscope (TEM) image of a carbon nanotube from the pile of  FIG. 2 .  
         [0017]      FIG. 5  shows an image of a carbon nanotube fiber that was spun from the pile of carbon nanotubes of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0018]     The invention relates to the preparation of a pile of carbon nanotubes and to fibers spun from the pile. Individual nanotubes from the pile are believed to have lengths of about 2 millimeters, about 3 millimeters, about 4 millimeters, and longer.  
         [0019]     The practice of the invention can be further understood with the accompanying figures. Similar or identical structure is identified using identical callouts.  FIG. 1  shows a schematic representation of an embodiment apparatus embodiment used for preparing a pile of carbon nanotubes. Apparatus  10  includes quartz tube  12  having inlet end  14  and outlet end  16 . Catalyst solution  20  is placed on the surface near an end of substrate  18 , and then substrate  18  is placed inside tube  12  such that the end of substrate  18  with the catalyst solution is near inlet end  14 .  
         [0020]     Substrates that may be used with the present invention include silicon; silicon having a top layer of silicon dioxide; silicon carbide; silicon carbide with a top layer of silicon dioxide; silicon nitride; silicon nitride with a top layer of silicon dioxide; quartz; and glass. In an embodiment, substrate  18  is a silicon (100) substrate with dimensions of about 5 millimeters in width and about 10 millimeters in length.  
         [0021]     Transition metal catalyst species are preferred. In an embodiment, solution  20  is a solution of ferric chloride (FeCl 3 ) catalyst (0.10 molar) and cobalt (III) chloride catalyst (0.1 molar) in ethanol solvent, and substrate  18  is a silicon substrate. It should be understood, however, that these materials are only exemplary and that other catalysts (nickel containing catalysts, for example) and catalyst/substrate combinations could also be used.  
         [0022]     The purpose of the solvent is to dissolve the catalyst and thereafter provide finely divided metal catalyst on the substrate after evaporation of the solvent. While an alcohol solution of FeCl 3  and CoCl 3  was used in a demonstration example, it should be understood any solvent capable of dissolving the transition metal containing species could also be used.  
         [0023]     Inlet end  14  of quartz tube  12  is connected via connector  22  to inlet gas manifold  24 , which is capable of sending gas through into tube  12 . Tube  12  includes outlet end  16 , which is connected via connector  26  to an outlet assembly that includes vacuum pump  28 . With substrate  18  inside tube  12 , inlet end  14  is connected to manifold  24 , and tube  12  is placed inside furnace  30 . Furnace  30  is then powered up, heating quartz tube  12  and substrate  18 , and causing evaporation of solvent from solution  20 .  
         [0024]     In an embodiment, a flowing gas mixture (about 10.5 cc/min) of argon and hydrogen (about 94 percent argon, about 6 percent hydrogen) was sent through end  32  of manifold  24 , into inlet end  14 , and into tube  12  while furnace  30  heated substrate inside to a temperature of about 900 degrees Celsius. After about 30 minutes, ethanol and acetone vapors were added to the hydrogen/argon gas mixture by sending hydrogen/argon gas through ends  34  and  40  of inlet gas manifold  24 . The hydrogen/argon gas bubbled through ethanol solution  36  at a flow rate of about 4 cc/min, and through acetone solution  38  at a flow rate of 8.5 cc/min. After about one hour more, the power to furnace  30  is turned off to allow quartz tube  12  to cool down. The substrate was removed from the tube. A pile of carbon nanotubes formed on the substrate.  
         [0025]     The flow rate of the argon/hydrogen gas mixture may be in the range of from about 1 cc/min to about 50 cc/min.  
         [0026]     The flow rate of the gas bubbled through the ethanol may be in the range of from about 1 cc/min to about 50 cc/min.  
         [0027]     The flow rate of the gas bubbled through the acetone may be in the range of from about 1 cc/min to about 50 cc/min.  
         [0028]     In an embodiment, the carbon source for preparing a pile of carbon nanotubes was a mixture of alcohol and acetone vapors. Other input gases that can be used with alcohol acetone vapors include hydrogen (H 2 ), inert gases (argon, helium, and nitrogen and mixtures thereof, for example), and mixtures of hydrogen and inert gas. These other input gases are used during the initial heating stages to provide an inert and/or reducing atmosphere, so that the solution of transition metal catalyst species would release finely divided metal catalyst particles after the solvent is evaporated from the catalyst solution. Hydrogen may also be used to provide this reducing atmosphere. However, it has been determined that the use of hydrogen is not critical because inert gases such as argon can be used instead.  
         [0029]     In an embodiment, the temperature used for decomposing the alcohol and acetone was about 900 degrees Celsius. It is expected that carbon nanotubes can be formed according to the invention when the substrate is heated to a temperature of from about 600 degrees Celsius to about 1200 degrees Celsius.  
         [0030]     Because of the length of the CNTs in the pile that are capable of being produced, the invention is expected to have a significant impact for applications in which shorter carbon nanotubes are inadequate. It is expected that the relatively long carbon nanotubes produced according to the present invention can be used to make fibers that are much stronger than any current engineering fibers, and that the carbon nanotubes and fibers could be used for applications that include, but are not limited to, neuronal growth, micro electric motors, neuronal implants, biological and chemical sensors, optical and electronic cables, and micro electromechanical systems.  
         [0031]     The following EXAMPLES illustrate embodiments of the invention.  
       EXAMPLE 1  
     Catalyst Preparation  
       [0032]     A catalyst solution was prepared by dissolving enough ferric chloride (FeCl 3 ) and cobalt (III) chloride (CoCl 3 ) in ethanol to produce a solution that was 0.1 molar in cobalt and 0.1 molar in iron.  
       EXAMPLE 2  
     Preparation of Pile of Carbon Nanotubes  
       [0033]     The catalyst solution of EXAMPLE 1 was applied with a pen to a short edge of a silicon (100) substrate having dimensions of about 5 mm×10 mm and a 0.1-micrometer thick surface layer of SiO2. The substrate was supported on a quartz plate having dimensions of about 15 mm×50 mm. The substrate and quartz plate were then placed into a 1-inch diameter quartz tube. The tube was placed in a tube furnace. The furnace was purged for about 0.5 hour with about 20 sccm of forming gas (Ar+6% H 2 ). As the furnace was being purged, it was heated at a rate of 60° C./min to a temperature of about 900° C. When the furnace reached this temperature, the forming gas was reduced to 10.5 sccm and a gaseous carbon source was added to the gaseous stream by bubbling 4 sccm of forming gas through ethanol, and bubbling 8.5 sccm of forming gas through acetone, and adding these to the stream that was already flowing through the quartz tube. The furnace temperature was maintained for about one hour, and the furnace was cooled down. After the furnace cooled down, the substrate was removed. A pile of carbon nanotubes formed on the substrate.  
         [0034]     An optical image of the pile is shown in  FIG. 2 . A side view image of the pile is shown in  FIG. 3 .  
         [0035]     A transmission electron spectroscopy (TEM) image of the end of one of the nanotubes from the pile is shown in  FIG. 4 . The diameter of this nanotube is about 100 nanometers.  
       EXAMPLE 3  
     Preparation of a Fiber from the Pile of Carbon Nanotubes  
       [0036]     A multi-CNT fiber of carbon nanotubes was spun from the pile of carbon nanotubes of EXAMPLE 2. A needle was used to pick up nanotubes from the pile. A fiber of nanotubes formed as the needle was rotated and pulled away from the pile. The fiber had a length greater than 5 centimeters.  
         [0037]     The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching.  
         [0038]     The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.