Abstract:
A horizontally-disposed reaction tube for generating multi-walled carbon nanotubes is described. Gaseous reactants and very fine solid catalyst particles are introduced into the horizontally-disposed reaction tube, and chemical reactions take place to grow multi-wall carbon nanotubes on the catalyst particles.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims priority to U.S. Provisional Application No.: 60/518,233, entitled SYSTEMS AND METHODS FOR MANUFACTURE OF CARBON NANOTUBES, filed Nov. 7, 2003, which is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to the field of materials science, and more particularly, to carbon nanotubes.  
       DESCRIPTION OF PRIOR ART  
       [0003]     Carbon may be instantiated in the form of nanotubes (CNTs), and this form of carbon has received much attention in recent years, as these materials possess a number of interesting properties, particularly related to their electrical conductivity/resistivity, and their ability to switch properties under different stimuli or environments. These materials appear to have particular applications in the emerging field of nanotechnology. Indeed the name “nanotubes” reflects the relative size of these materials, which ordinarily have diameters on the order of nanometers. Carbon nanotubes may be single-walled or double-walled.  
         [0004]     The prior art details a number of methods of producing carbon nanotubes and particularly single wall carbon nanotubes (SWCNTs). There remains a need for efficient, high-quality, and cost-effective techniques for the manufacture of multi-walled carbon nanotubes (MWCNTs). This inadequacy in the prior art is addressed by the present invention.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention is based on a horizontally-disposed reaction tube for the generation of carbon nanotubes. In embodiments of the invention, gaseous reactants and very fine solid catalyst particles are introduced into the horizontally-disposed reaction tube, and chemical reactions take place to grow Multi-Wall Carbon Nanotubes (MWCNTs) on the catalyst particles. In embodiments of the invention, the reactions include one or more of the following steps: (i) thermal decomposition of the reactant gases on the catalyst, (ii) accumulation of carbon in the catalyst, and (iii) the subsequent growth of the MWCNTs outwards from the catalyst particles. This is often referred to as chemical vapor deposition (CVD), whereby a material (MWCNT) is created by exposing a solid (the unsupported catalytic particles) to a specific composition of reactant gases at a prescribed temperature and pressure. Advantages of the present invention include rapid growth rate of the carbon nanotube materials, as well as the high product purity of the carbon nanotube end-product, both in terms of its structure and composition. These and other aspects of the invention are further described herein.  
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0006]      FIG. 1  illustrates an apparatus for manufacturing carbon nanotubes in accordance with embodiments of the invention.  
         [0007]      FIG. 2  illustrates an internal view of the carbon nanotube manufacturing apparatus, in accordance with embodiments of the invention.  
         [0008]      FIG. 3  illustrates a chemical vapor deposition furnace, in accordance with embodiments of the invention.  
         [0009]      FIG. 4  illustrates a side view of a chemical vapor deposition furnace in accordance with the embodiments of the invention.  
         [0010]      FIG. 5  illustrates a reaction tube for a carbon nanotube manufacturing apparatus, in accordance with the embodiments of the invention.  
         [0011]      FIG. 6  illustrates a catalyst feeder for inserting catalysts into a carbon nanotube manufacturing system in accordance with embodiments of the invention.  
         [0012]      FIG. 7  illustrates a feedstock feeder for combining gaseous components into a reaction tube for the CNT manufacturing system, in accordance with embodiments of the invention.  
         [0013]      FIG. 8  illustrates a method for synthesizing carbon nanotubes, in accordance with embodiments of the invention.  
         [0014]      FIG. 9  illustrates a system for collecting CNT from a CNT manufacturing system, in accordance with the embodiments of the invention.  
         [0015]      FIG. 10  illustrates an alternate system for collecting CNT from a CNT manufacturing system, in accordance with the embodiments of the invention.  
         [0016]      FIG. 11  illustrates yet another alternative system for collecting CNT from a CNT manufacturing system, in accordance with the embodiments of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIG. 1  illustrates an apparatus for manufacturing multi-walled carbon nanotubes. The apparatus includes a horizontally-disposed chemical vapor deposition (CVD) furnace, or “oven”. The CVD  1000  also includes a reaction tube  2000 , and supplies uniform heat for driving the reaction that generates the MWCNT. As shown in  FIG. 3  and  FIG. 4 , the CVD furnace  1000  may include a heating zone  1100 , which, in embodiments, comprises a region of constant elevated temperature. The heating zone  1100  may be heated by use of heating coils  1150 . In embodiments of the invention, these heating coils operate by transforming electrical energy to heat through coiled resistance wires. In some embodiments, the CVD finance  1000  may also include a door  1300  and a door bar, or handle  1200 , which may be used to access the contents  2000  of the CVD  1000 . In some embodiments of the invention, the CVD furnace  1000  may further include thermal isolation materials  1400 , which help maintain uniform temperature.  
         [0018]      FIG. 1  also depicts a reaction tube  2000 , in which the MWCNTs are grown. As shown in  FIG. 5 , some embodiments of the invention may include an optional gate valve  2101 , which allows chemical feedstock to flow in (gas and catalyst). Some embodiments may also include another optional gate valve  2103 , which allows gaseous byproducts and unconsumed reactant gases to exit the reaction tube. Embodiments may also include an additional optional gate valve  2105 , which allows product  6000  retrieval to take place, as further discussed herein.  FIG. 5  further depicts a tube cap  2200 . In some embodiments, this tube is normally closed, but may be opened for maintenance or alternative product retrieval.  
         [0019]     Embodiments of the invention also include a catalyst feeder  3000 , as depicted in  FIG. 2 . By way of non-limiting example, this catalyst feeder  3000  may comprise a “Hopper” style container, which feeds a little catalyst at a time into the incoming gas stream. Additional features of the catalyst feeder  3000  are shown in  FIG. 6 , such as a Catalyst container  3100 , which holds the catalyst for the MWCNT producing reaction, in accordance with embodiments of the invention. Also depicted are a container lid  3150  affixed to a top of the catalyst feeder  3000 . In embodiments, the catalyst feeder is attached to a catalyst flow controller  3200 , which controls a rate at which the catalyst is fed into a gas stream. Also depicted in  FIG. 6  are an optional holder, for supporting the catalyst container  3100 , container lid  3150 , and catalyst flow controller  3200 , in accordance with embodiments of the invention.  
         [0020]      FIG. 7  illustrates a feedstock feeder  4000 , often referred to as an “intake manifold.” As used in embodiments of the invention, the feedstock feeder  4000  may combine several gaseous components to allow one entry point into the reaction tube. The feedstock feeder  4000  may further include a gas flow controller  4101 , to control a rate at which a gas such as NH3 (ammonia) is entered into the reaction tube. In embodiments the feedstock feeder  4000  may also include a gas flow controller  4103  to control the rate of C2H2 (acetylene) addition to the reaction tube. In embodiments, the feedstock feeder also includes another gas flow controller  4105 , which controls the rate of Ar (argon) added to the reaction tube.  FIG. 7  also illustrates the addition of particular gases into the reaction tube, such as NH 3    4110 , C 2 H 2    4120 , and Ar  4130 . A tube connector  4200  may join the gas manifold to reaction tube  2000 .  
         [0021]      FIGS. 9, 10 , and  11  depict embodiments for collection of the end product MWCNT from the system. A product collector  5000  comprises a mechanism and container to collect and temporarily store the MWCNT product. By way of non-limiting examples, this collector  5000  may comprise a vacuum tube; other suitable containers shall be readily apparent to those skilled in the art. Some embodiments of the invention may include a product collector or container  5100  which allows for the product to transported out by vacuum (‘pneumatic transport’) and the process gases to be recycled through an optional recycling gas tube  5200  on the intake side. Another embodiment allows gas through  2105  to blow the product out, where it could be collected in  5100  on the exit end of the process tube. Also depicted in  FIG. 9  is a container lid  5150 , along with a one-way valve  5250 . As shown in  FIG. 11 , embodiments of the invention may include an expandable vacuum head  5300 . By way of non-limiting example, this vacuum head may be made of an expandable material, such as a metal. Other suitable materials for the vacuum head shall be apparent to those skilled in the art. Embodiments of the invention may also include vacuum intake holes  5350 , which vacuums up the product from the floor of the reaction tube. Alternatives to the vacuum process may include a mechanical device, such as an Archimedes screw, and other alternatives shall be apparent to those skilled in the art. Also depicted in  FIG. 11  is a vacuum device and controller, along with the end product MWCNTs  6000 .