Patent Publication Number: US-2007098623-A1

Title: Method for manufacturing carbon nanotubes

Description:
TECHNICAL FIELD  
      The present invention generally relates to methods for manufacturing carbon nanotubes. Specifically, the present invention relates to a method for manufacturing carbon nanotubes by chemical vapor deposition (CVD) using magnetic fluid.  
     BACKGROUND  
      Carbon nanotubes produced by arc discharge between graphite rods were first discovered and reported in an article by Sumio Iijima entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58).  
      Carbon nanotubes are electrically conductive along their length, are chemically stable, and can have very small diameters (much less than 100 nanometers) and large aspect ratios (length/diameter). Due to these and other properties, it has been suggested that carbon nanotubes can play an important role in fields such as microscopic electronics, field emission devices, thermal interface materials, etc.  
      The manufacture of carbon nanotubes has three main methods: arc-discharge, laser ablation, and chemical vapor deposition.  
      The arc-discharge method is performed in a stainless steel chamber. Two graphite rods are used as an anode and a cathode. A DC (Direct Current) voltage is applied between the two graphite rods. The arc-discharge between the two graphite rods deposits carbon nanotubes which grow on the cathode. However, the purity of the carbon nanotubes is low so that it is not suitable for mass production of carbon nanotubes. Therefore, a purification step is needed for high purity of carbon nanotubes.  
      The laser ablation method is performed by vaporizing carbon using laser impinging on a metal-graphite composite target. The vaporized carbon is swept up with a gas-flow and deposited onto a surface of a water-cooled copper collector positioned downstream thus growing carbon nanotubes. Purity of carbon nanotubes, especially single wall carbon nanotubes (SWCNTs), is high. However, productivity of SWCNTs is low so that it is not suitable for mass production.  
      The chemical vapor deposition (CVD) method involves manufacture of carbon nanotubes by a catalytic decomposition of hydrocarbons onto a metallic layer, made of a substance such as iron (Fe), cobalt (Co), nickel (Ni) or any appropriate alloy thereof. Since the CVD allows easier control of manufacturing carbon nanotubes and large area manufacture, the CVD is used widely to manufacture carbon nanotubes. However, in a conventional CVD method, a metallic layer must be deposited onto a substrate by sputtering or evaporation for forming a catalytic layer. Due to cost of the deposition system for the metallic layer is high and the deposition system is operationally complicated, it therefore is not a cheap process with which to manufacture carbon nanotubes.  
      What is needed, therefore, is a cheaper method with which to manufacture carbon nanotubes.  
     SUMMARY  
      In a preferred embodiment, a method for manufacturing carbon nanotubes includes the steps of: providing at least one substrate having a first surface and an opposite second surface; spin coating magnetic fluid onto the first surface and the second surface of the at least one substrate thereby forming first and second catalytic layers on the respective first and second surfaces; growing carbon nanotubes on the first and second surfaces of the at least one substrate by a chemical vapor deposition method.  
      Cost of manufacturing carbon nanotubes is reduced due to simplicity and low cost of the spin-coating technique.  
      Other advantages and novel features will become more apparent from the following detailed description of the present method, when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Many aspects of the present method for manufacturing carbon nanotubes can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present method for manufacturing carbon nanotubes. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.  
       FIG. 1  is a schematic view of a substrate fixed onto a spin-coating device in accordance with a preferred embodiment,  
       FIG. 2  is similar to  FIG. 1 , showing a catalytic layer spin coated on a surface of the substrate of  FIG. 1 ;  
       FIG. 3  is a schematic, sectional view of the substrate with the catalytic layers thereon in accordance with the preferred embodiment.  
       FIG. 4  is a schematic, sectional view of the substrate with catalytic layers placed into a chemical vapor deposition chamber in accordance with the preferred embodiment; and  
       FIG. 5  is a schematic, sectional view of a plurality of substrates with catalytic layers placed into a chemical vapor deposition chamber in accordance with the preferred embodiment. 
    
    
      Corresponding reference characters indicate corresponding parts throughout the drawings. The exemplifications set out herein illustrate at least one preferred embodiment of the present method for manufacturing carbon nanotubes, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.  
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      Reference will now be made to the drawings to describe preferred embodiments of the present method for manufacturing carbon nanotubes, in detail.  
      Referring to FIGS.  1  to  5 , successive steps of a method for manufacturing carbon nanotubes, in accordance with a preferred embodiment, are shown. The method includes the following steps:  
      (1) providing a substrate  40  having a first surface  40   a  and an opposite surface  40   b;    
      (2) spin coating magnetic fluid  302  onto the first surface  40   a  and the second surface  40   b  of the substrate  40  thereby forming a first catalytic layer  42   a  and a second catalytic layer  42   b  on the respective first surface  40   a  and second surface  40   b;    
      (3) growing carbon nanotubes on the first surface  40   a  and second surface  40   b  of the substrate  40  by a chemical vapor deposition method.  
      In step (1), a material of the substrate  40  is selected from the group consisting of silicon, quartz and glass.  
      Referring to  FIG. 1 , in step (2), a spin-coating device  100  is provided. The spin-coating device  100  includes a rotary plate  140  and a pair of positioning posts  160   a ,  160   b  extending from the rotary plate  140 . The rotary plate  140  rotates centrifugally with the pair of the positioning posts  160   a ,  160   b  when working.  
      The substrate  40  is attached to the spin-coating device  100  by extension of the pair of positioning post  160   a ,  160   b  through the substrate  40  in a manner that the second surface  40   b  of the substrate  40  faces the rotary plate  140 . In this preferred embodiment, the substrate  40  is attached to the spin-coating device  100  by matching screw caps  200  with screw threads on the pair of positioning posts  160   a ,  160   b . The spin-coating device  100  drives the rotary plate  140  to rotate. The magnetic fluid  302  is injected onto the first surface  40   a  of the substrate  40  by an injecting device  300 . A magnetic fluid layer is spin coated uniformly on the first surface  40   a , thereby a first catalytic layer  42   a  is formed on the first surface  40   a  of the substrate  40 .  
      Preferably, the substrate  40  is rotated at a speed in the range from 1000 to 5000 revolutions per minute (rpm). The magnetic fluid  302  mainly contains a solvent, magnetic nanoparticals dispersed in the solvent, and a surfactant. The magnetic nanoparticals are selected from the group consisting of ferroso-ferric oxide (Fe 3 O 4 ) nanoparticals, ferrite nanoparticals, iron (Fe) nanoparticals, cobalt (Co) nanoparticals, nickel (Ni) nanoparticals and mixture thereof. The ferrite nanoparticals include Co-substituted magnetite (CoFe 2 O 4 ) nanoparticals and Ni-substituted magnetite (NiFe 2 O 4 ) nanoparticals. Grain size of the magnetic nanoparticals is in the range from 10 to 100 nanometers. The solvent is selected from the group consisting of organic solvent, such as heptane, xylene, toluene and acetone, hydrocarbon, synthetic ester, polyglycols, halogenated hydrocarbon, styrene, or pure water. The surfactant may be capric acid (CH 3 (CH 2 ) 8 COOH). Preferably, a thickness of the first catalytic layer  42   a  is in the range from 100 to 900 nanometers.  
      Referring to  FIGS. 2 and 3 , the substrate  40  is reversed so as to let the first surface  40   a  with the first catalytic layer  42   a  face the rotary plate  140  and then be attached to the spin-coating device  100  in a manner such that a space is kept between the substrate  40  and the rotary plate  140 . Then a second catalytic layer  42   b  is formed on the second surface  40   b  by spin coating. Therefore, the substrate  40  having two surfaces with the catalytic layers  42   a  and  42   b  is formed according to  FIG. 3 . Preferably, a thickness of the second catalytic layer  40   b  is in the range from 100 to 900 nanometers. Damage to the formed first catalytic layer  42   a  can be avoided due to the space between the substrate  40  and the rotary plate  140 .  
      In order to avoid clumping of the coated magnetic fluid  302  on the first surface  40   a  and the second surface  40   b  on the substrate  40 , the magnetic fluid  302  further contains a binder, such as poly(vinyl alcohol) (PVA) to adjust a viscosity of the magnetic fluid  302  so as to form the uniform catalytic layers  42   a  and  42   b.    
      Referring to  FIG. 4 , in step (3), carbon nanotubes are manufactured on the substrate  40  with the catalytic layers  42   a  and  42   b  by a chemical vapor deposition method. A more detailed description follows.  
      Firstly, a CVD reactor  10  is provided. The CVD reactor  10  has a chamber  12 , a supporting stage  14  and a heating device  18 . The chamber  12  includes an inlet  122  and an opposite outlet  124 . The inlet  122  and the outlet  124  are arranged in a manner such that a flow direction of carbon-containing gases is perpendicular to or almost perpendicular to a growth direction of carbon nanotubes. The supporting stage  14  has a pair of positioning posts  142   a  and  142   b  extending therefrom. The substrate  40  is attached to the supporting stage  14  by extension of the pair of positioned pins  142   a  and  142   b  through the substrate  40 . Pads  16  are used for keeping space between the supporting stage  14  and the substrate  40  so that damage to the catalytic layer  42   b  by the supporting stage  14  is avoided. A heating device  18 , such as a high-temperature furnace and a high-frequency furnace, etc., is used for heating the catalytic layers  42   a  and  42   b.    
      Secondly, hydrogen is introduced into the CVD chamber  12  through the inlet  122 . Then the catalytic layers  42   a  and  42   b  are heated up to a temperature in the range from 800 to 900 degrees centigrade by the heating device  18 .  
      Thirdly, a mixture of carbon-containing gas and ammonia is introduced into the CVD chamber  12  simultaneously. Alternatively, after ammonia introduced into the CVD chamber  12  for five minutes, the carbon-containing gas is introduced into the CVD chamber  12 . Then carbon nanotubes grow perpendicularly to or almost perpendicularly to a flowing direction of the mixed gas. The carbon-containing gas is selected from the group consisting of acetylene, ethylene, methane and carbon monoxide. Preferably a flow ratio of the carbon-containing gas to ammonia is in the range from 1:2 to 1:10. The total flow amount of carbon-containing gas and ammonia is in the range from 90 to 200 standard cubic centimeters per minute (sccm).  
      Finally, after growing carbon nanotubes for 3 to 5 minutes, the carbon-containing gas and ammonia flow is stopped. Inert gas, such as nitrogen and argon, is introduced into the CVD chamber  12  and the substrate  40  is cooled to room temperature. The carbon nanotubes can then be collected.  
      Referring to  FIG. 5 , a plurality of substrates  40  with catalytic layers  42   a  and  42   b  are attached to the supporting stage  14  by extension of the pair of positioning posts  142   a  and  142   b . The plurality of substrates  40  are separated from each other at a certain distance. Carbon nanotubes grow on each substrate  40  so that mass production of carbon nanotubes may be achieved. The distance between the plurality of the substrates  40  is determined by a length of the carbon nanotubes, for example the average distance between neighboring substrates  40  is at least twice longer than the length of the carbon nanotubes. The distance between each plurality of the substrates  40  is set by a plurality of pads  16 .  
      In this preferred embodiment, the catalytic layers  42   a  and  42   b  are formed on the substrate  40  by spin coating the magnetic fluid  302  on the first surface  40   a  and the second surface  40   b  of the substrate  40 . Cost for manufacture of carbon nanotubes is reduced due to simplicity and low cost of the spin-coating technique. Moreover, since two surfaces of the substrate  40  are coated with catalytic layers  42   a  and  42   b , the area for manufacturing carbon nanotubes is doubled. Furthermore, mass production of carbon nanotubes can be achieved by attaching the plurality of substrates  40  to the supporting stage  14  by extension of the pair of positioning posts  142   a  and  142   b.    
      It is to be understood that the above-described embodiment is intended to illustrate rather than limit the invention. Variations may be made to the embodiment without departing from the spirit of the invention as claimed. The above-described embodiments are intended to illustrate the scope of the invention and not restrict the scope of the invention.