Patent Publication Number: US-2010119435-A1

Title: Processes for growing carbon nanotubes in the absence of catalysts

Description:
FIELD OF THE INVENTION 
     The present invention relates to processes for single-wall carbon nanotube production. By using a disordered carbon target, the processes can provide increased production rates as compared to known processes. 
     BACKGROUND OF THE INVENTION 
     In the field of molecular nanoelectronics, few materials show as much promise as nanotubes, and in particular carbon nanotubes, which comprise hollow cylinders of graphite. Nanotubes can be incorporated into electronic devices such as diodes and transistors, depending on the nanotube&#39;s electrical characteristics. Nanotubes are unique for their size, shape, and physical properties. Structurally, a carbon-nanotube resembles a hexagonal lattice of carbon rolled into a cylinder. 
     Besides exhibiting intriguing quantum behaviors at low temperature, carbon nanotubes exhibit the following important characteristics: a nanotube can be either metallic or semiconductor depending on its chirality (i.e., conformational geometry). Metallic nanotubes can carry extremely large current densities. Semiconducting nanotubes can be electrically switched on and off as field-effect transistors (FETs). The two types may be covalently joined (sharing electrons). These characteristics point to nanotubes as excellent materials for making nanometer-sized semiconductor circuits. 
     Nanotubes can be formed as single-wall carbon nanotubes (SWNTs) or multi-wall carbon nanotubes (MWNTs). SWNTs can be produced, for example, by arc-discharge and laser ablation of a carbon target. Local growth of tubes on a surface can also be obtained by chemical vapor deposition (CVD). The growth of the nanotubes is made possible by the presence of metallic particles, such as Co, Fe, and/or Ni, acting as catalyst. The resultant carbon nanotubes typically contain contaminants, e.g., catalyst particles. For some potential nanotube applications the use of clean nanotubes can be important, such as, for example, where nanotubes are incorporated as an active part of electric devices. The presence of contaminating atoms and particles can alter the electrical properties of the nanotubes. The metallic particles can be removed; however the process of cleaning or purifying the nanotubes can be complicated and can alter the quality of the nanotubes. 
     U.S. patent application Ser. No. 2004/0035355 discloses a method for growing single-wall nanotubes comprising providing a silicon carbide semiconductor wafer comprising a silicon face and a carbon face, and annealing the silicon carbide semiconductor wafer in a vacuum at a temperature of at least about 1,350° C., thereby inducing formation of single-wall carbon nanotubes on the silicon face. The disclosed process is carried out at 10 −9  Torr, which is not generally conducive to high material production rates. 
     SWNTs have been identified as potential components of electronic devices in varied applications. Therefore, a need exists for new and/or improved processes for growing single-wall carbon nanotubes. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is a process comprising: 
     a) providing a target comprising disordered carbon and a metal catalyst, wherein the carbon target has a density of 0.01 to 2 gm/cm 3 ; 
     b) vaporizing the target at a temperature from about 900° C. to about 1500° C. and at a pressure from about 10 −3  Torr to about 10 +3  Torr in the presence of a non-oxidizing gas; and 
     c) forming a product comprising at least one single-wall carbon nanotube. 
     In some preferred embodiments, the target comprises about 10 weight percent or less of the metal catalyst. 
     Another aspect of the present invention is a single-wall carbon nanotube made by a process comprising: 
     a) providing a target comprising disordered carbon and a metal catalyst, wherein the carbon target has a density of 0.01 to 2 gm/cm 3 ; 
     b) vaporizing the target at a temperature from about 900° C. to about 1500° C. and at a pressure from about 10 −3  Torr to about 10 +3  Torr in the presence of a non-oxidizing gas; and 
     c) forming a product comprising at least one single-wall carbon nanotube. 
     These and other aspects of the present invention will be apparent to those skilled in the art, in view of the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows the production rate as a function of target density according to an embodiment of present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     All documents cited herein are expressly incorporated herein by reference in their entirety. Applicants also incorporate herein by reference the co-owned and concurrently filed application entitled “PROCESSES FOR GROWING CARBON NANOTUBES IN THE ABSENCE OF CATALYSTS”. (Attorney Docket # CL 2626). 
     When an amount, concentration, or other value or parameter is given as either a range, preferred range, or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range. 
     The present invention provides processes for producing single-wall carbon nanotubes (SWNT). In preferred embodiments, the processes provide increased rates of production of single-wall carbon nanotubes. It has been found that the use of a relatively lower density carbon target (sometimes referred to as “softer” carbon target, e.g., about 2.2 g/cm 3  or less) can provide increased rates of production of nanotubes as compared with known methods such as conventional laser ablation of graphite, arc discharge, chemical vapor deposition, and high pressure carbon monoxide techniques. As examples only, using a target having a density of 1.6 g/cm 3 , it has been observed that the rate of production of nanotubes was from about 0.1 to 0.5 grams per hour, and with a density of 0.9 g/cm 3 , the rate observed was about 1.2-1.5 g/hour. It is not intended that the invention be limited by these recited rates, as variations in the process within the scope of the invention, which may be made by one skilled in the art, can result in varied rates of production of nanotubes. 
     In highly preferred embodiments, the processes include providing a target that is a mixture of a metal catalyst and disordered carbon. Suitable metal catalysts include yttrium, iron; nickel and cobalt and combinations thereof. By “disordered carbon” is meant a carbon material having a density less than 2.2 g/cm 3 . Forming the target can be accomplished, for example, by forming a mixture of catalyst, carbon and a graphite cement in a volatile solvent, allowing the solvent to evaporate, then compression molding the residual solid. The target thus can have a density of 0.01 to 2 gm/cm 3 . The compression-molded article can be optionally heated, preferably in an inert atmosphere, to substantially remove traces of the solvent. 
     Vaporization of the target can be carried out by laser ablation or other suitable methods, such as, for example, radio frequency induction heating and sputtering. The vaporization can be carried out at temperatures between about 900° C. and 1500° C., preferably from about 1000° C. to about 1300° C. Also, the vaporization can be carried out at a pressure of about 10 −3  Torr to about 10 +3  Torr, preferably from about 300 Torr to 600 Torr. The vaporization is carried out in the presence of a non-oxidizing gas, such as argon, neon, helium, nitrogen or mixtures thereof. It is generally desirable to grow the tubes at pressures preferably 1 millitor or higher, and more preferably at 500 Torr or higher. In some preferred embodiments, the pressure is about 1000 Torr. It is generally not desirable that the pressure be greater than about 1000 Torr. Although a reduction in pressure below about 500 Torr has not been observed to undesirably affect the rate of growth of nanotubes, pressures of about 500 Torr or greater are often practical. 
     In one embodiment of this invention, the SWNT-containing product can serve as a target for one or more additional cycles of vaporization and SWNT-formation. 
     The process can further include an annealing step. The annealing can be performed in an ultra-high vacuum (UHV) (e.g., at a pressure less than about 10 −9  Torr), or at higher pressures, even above atmospheric pressure (760 Torr). 
     According to an embodiment of the present invention, vaporizing the disordered carbon target can induce the growth of SWNTs. The vaporizing can be performed under reduced pressure (e.g., at a pressure of about 300-600 Torr) in non-oxidizing conditions and at temperatures between about 900° C. and 1500° C. The process produces nanotubes, which are preferably predominately SWNTs. The SWNTs can be very long and have a good crystalline quality. “Good crystalline quality” means substantially free of observable defects. By further increasing the extent of carbon disorder, which is reflected in a decrease in the density, the production rate of SWNTs can be increased. All of the compositions and processes disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and processes of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations can be applied to the compositions and processes and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit, and scope of the invention. All such substitution and modifications apparent to those skilled in the art are deemed to be within the spirit, scope, and concept of the invention as defined by the appended claims. 
     EXAMPLES 
     These Examples show the effects of the amount of disorder of the carbon target on the nanotube production rate. 
     Example 1   
     Carbon black powder (66 grams (g), Alfa Aesar, Ward Hill, Mass.), nickel metal catalyst powder (7.56 g, 2.2-3 μm stock # 10255, Alfa Aesar, Ward Hill, Mass.), cobalt metal catalyst powder (7.56 g, 1.6 μm stock # 10455, Alfa Aesar, Ward Hill, Mass.), and Dylon® graphite cement (grade C, 94.12 g, Dylan Industries, Inc., Cleveland, Ohio) were incorporated into a homogeneous mixture using 260 mL of methanol. The mixture was allowed to dry and then was broken up. The mixture as made provides targets with a high level of disorder and a density of 0.93 g/cm 3 . A portion of this mixture was placed into a stainless steel die (3.18 cm diameter by 7.62 cm high). The die was placed in a press and heated to 130° C. Force up to 40034 Newtons was then applied for 1 hour. The die was allowed to cool before the target was removed. The targets were then heated in an Ar atmosphere to 1150° C. The targets were then placed in a non-oxidizing atmosphere of Ar at 500 Torr, heated to 1100° C. and ablated with two Nd:YAG lasers operating at 1064 nm and 30 Hz. A felt-like material containing single-wall carbon nanotubes was formed and collected downstream in a cool zone. This felt-like material was generated at a rate of 1.3 g/hr. 
     Example 2   
     The target preparation described in Example 1 was repeated using 187.2 g of Dylan® graphite cement, 7.56 g of nickel catalyst powder, 7.56 g of cobalt metal catalyst powder, and 50 mL of methanol. The mixture as made provided targets with a density of 1.4 g/cm 3  and less disorder than the targets of Example 1. A portion of this mixture was placed into a stainless steel die (3.18 cm diameter by 7.62 cm high). The die was placed in a press and heated to 130° C. Force of 40034 Newtons was then applied for 1 hour. The die was allowed to cool before the target was removed. The targets were then heated in an Ar atmosphere to 1150° C. The targets were then placed in an atmosphere of Ar at 500 Torr, heated to 1100° C., and ablated with two Nd:YAG lasers operating at 1064 nm and 30 Hz. A felt-like material containing nanotubes was formed and collected downstream in a cool zone. The felt-like material was generated at a rate of 0.9 g/hour. This production rate was lower than that obtained with the targets of 0.93 g/cm 3  density of Example 1. 
     Example 3   
     The target preparation described in Example 1 was repeated using 94.12 g of Dylon® graphite cement, 7.56 g of nickel catalyst powder, 7.56 g of cobalt metal catalyst powder, 66 g of graphite powder (grade UCP-1-M, Carbone of America, Ultra Carbon Division, Bay City, Mich.), and 130 mL of methanol. The mixture as made provides targets with a density of 1.6 g/cm 3  and even less disorder than the targets of Examples 1 and 2. A portion of this mixture was placed into a stainless steel die (3.18 cm diameter by 7.62 cm high). The die was placed in a press and heated to 130° C. Force up to 40034 Newtons was then applied for 1 h. The die was allowed to cool before the target was removed. The targets were then heated in an Ar atmosphere to 1150° C. The targets were then placed in an atmosphere of Ar at 500 Torr, heated to 1100° C. and ablated with two Nd:YAG lasers operating at 1064 nm and 30 Hz. A felt-like material containing nanotubes was formed and collected downstream in a cool zone. The felt-like material was generated at a rate of 0.4 g/hour. This production rate was lower than that achieved with the targets of 0.93 g/cm 3  or 1.4 g/cm 3  density of Examples 1 and 2.