Abstract:
An RF-induction heated side-pumped synthesis chamber for the production of carbon nanotubes. Such an apparatus, while capable of producing large volumes of carbon nanotubes, concurrently provides a simplified apparatus that allows for greatly reduced heat up and cool down times and flexible flowpaths that can be readily modified for production efficiency optimization.

Description:
The United States of America may have certain rights to this invention under Management and Operating Contract DE-AC05-84ER 40150 from the United States Department of Energy. 

   FIELD OF THE INVENTION 
   The present invention relates to the synthesis of single-walled carbon nanotubes and more particularly to a novel apparatus for such synthesis. 
   BACKGROUND OF THE INVENTION 
   Since the first reports of the production of single-walled carbon nanotubes (hereinafter SWNT) in 1991 by researchers at NEC and IBM, a variety of synthesis routes have been developed to improve both the production rate and the fractional conversion of carbon feedstocks to SWNTs. Among the methods developed for this purpose, all of which are well known and well documented in the art are: Arc Discharge (AD); Pulsed Laser Vaporization (PLV); and Chemical Vapor Deposition (CVD). Much of the work with PLV techniques has been carried out at Rice University as reported by A. Thess et al. in Science 273, 483 (1996). While this work still represents, to the best of our knowledge, the state of the art in the production of high quality SWNTs, it must be noted that the production rates for the processes described by these researchers are only on the order of milligrams per hour. The results of this work and the application of the SWNTs thus produced indicate that there exist many applications for SWNTs, but only if adequately high production rates can be achieved while maintaining the quality of the SWNT, i.e. the integrity of the tube wall. 
   One such application is as reinforcement for lightweight polymeric structures, especially inflatable structures for use in outer space where in addition to the strength imparting properties of the SWNTs, their electrical conductivity provides a means of reducing static charge buildup on such devices. Other potential applications reside in the areas of hydrogen storage at “low”, i.e. about atmospheric pressure, although debate still rages as to this application and in NEMS or nano electro-mechanical structures useful in, for example, quantum computing devices. 
   It has been demonstrated that the longer the SWNT the better its properties as a reinforcing agent. Similarly, it is highly desirable that the SWNT “bundles” be small to permit better dispersal in the foregoing reinforcement applications. According to evaluations of SWNTs produced in accordance with the work at Rice University their SWNTs are on the order of 3-5 μm in length and occur in bundles about 10-25 nm in length. Evaluation of SWNTs produced in accordance with the present invention are generally 4-10 μm in length and occur in bundles of from 4-18 nm in length, thus making them more desirable candidates for application in reinforcing applications. 
   Thus, while there exist numerous areas of potential and actual application for SWNTs, what has been, and is currently lacking, is a method for the production of SWNTs of high quality in sufficient quantities as to provide a reliable and adequate source of desirable raw material for the development and implementation of such applications. 
   While numerous attempts have been made to improve the production rates of these materials, most such attempts have resulted in the design of carbon nanotube production apparatus and operating procedures that were extremely complex due to operating conditions below atmospheric pressure, required extended heat up and cool down times on the order of hours and/or provided limited flowpaths that could not be readily modified for optimal apparatus operation. 
   There therefore remains a significant need for an apparatus and operating method that while capable of producing large volumes of carbon nanotubes provide simplified apparatus, greatly reduced heat up and cool down times and flexible flowpaths that can be readily modified for production efficiency optimization. 
   OBJECTS OF THE INVENTION 
   It is therefore an object of the present invention to provide an apparatus for the production of carbon nanotubes that, while capable of producing large volumes of carbon nanotubes, concurrently provides a simplified apparatus that allows for greatly reduced heat up and cool down times and flexible flowpaths that can be readily modified for production efficiency optimization. 
   SUMMARY OF THE INVENTION 
   According to the present invention, there is provided an RF-induction heated side-pumped synthesis chamber for the production of carbon nanotubes. Such an apparatus, while capable of producing large volumes of carbon nanotubes, concurrently provides a simplified apparatus that allows for greatly reduced heat up and cool down times and flexible flowpaths that can be readily modified for production efficiency optimization. The RF-induction heated side-pumped carbon nanotube synthesis chamber of the present invention comprises: a generally T-shaped furnace housing defining a horizontal chamber and a vertical chamber for the admission of a laser beam; an RF coil about at least a portion of the horizontal chamber; a graphite core bisecting the horizontal chamber to form parallel inner upper and lower horizontal chambers and having a gap therein, the gap being aligned with the vertical chamber; a spindle capable of rotational and translational movement within the upper inner horizontal chamber; a graphite/catalyst target mounted on the spindle and moving therewith in alignment with the gap; the inner lower horizontal chamber defining a flowpath for the passage of a nanotube spray; a porous plug heater in the flowpath up stream of said vertical chamber for heating incoming inert gas; and an orifice plate intermediate the porous plug heater and the vertical chamber controlling the flow of inert gas to the flowpath. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic depiction of a front-pumped carbon nanotube production chamber of the prior art. 
       FIG. 2  is a schematic depiction of a side-pumped carbon nanotube production chamber of the prior art. 
       FIG. 3  is a schematic depiction of the RF-induction heated side-pumped carbon nanotube synthesis chamber of the present invention. 
   

   DETAILED DESCRIPTION 
   Referring now to the accompanying drawings, as shown in  FIG. 1 , the original front-pumped chamber  10  utilized to produce carbon nanotubes included a vacuum chamber  12  maintained at about 500 torr and 1000° C., a graphite/catalyst target  14 , an argon flow as depicted by arrow  16  and a laser beam  18  that interacted in well-known fashion to produce a plasma plume  20  that in turn resulted in the formation of a nanotube vortex  22  that, driven by argon flow  16 , resulted in movement of vortex  22  toward and onto a target at  24 . 
   While such an apparatus was capable of producing carbon nanotubes, its production levels were very low and demanded improvement. 
     FIG. 2  depicts one of the first side-pumped synthesis chambers for the production of carbon nanotubes designed to improve the level of production of carbon nanotubes over the levels achievable with the design of  FIG. 1 . 
   As shown in  FIG. 2  the prior art side-pumped synthesis chamber  26  comprised a chamber furnace  28  maintained at about 1000° C. and 760 torr; a rotatable graphite/catalyst target  30  mounted on a rotating and horizontally moveable spindle  32  as depicted in  FIG. 2  by arrows  31  and  33  respectively; an argon heater  34  terminating in a sonic nozzle  36  that generated a heated argon flow  38 ; a laser beam  40  impacting target  30  through a side chamber  42  to generate a plasma plume  44  that was driven by heated argon flow  38  to form a nanotube spray that was subsequently deposited on a target  41 . While this apparatus performed quite satisfactorily and resulted n the production of carbon nanotubes in volumes of 2-6 grams per hour as opposed to the 200 milligram per hour production rates of prior art devices, it possessed certain inherent limitations. 
   Because of its design and operating conditions, heat up of the device to 1000° C. required about 2 hours and cool down occurred over a 3-4 hour period, maintenance of the 760 torr vacuum greatly complicated plumbing and flow controls and flowpath variability was quite restricted. Thus, while carbon nanotube production was significant operating conditions remained somewhat constrained. To eliminate these operating constraints, the apparatus depicted in  FIG. 3  was designed, constructed and operated. 
   Referring now to  FIG. 3  that schematically depicts the RF-induction heated side-pumped chamber  48  of the present invention, comprises: a quartz purge vessel  50  surrounded by an RF heating coil  52  (preferably a 3.5 KW RF coil); a conventional generally T-shaped tube-type furnace  53  defining a horizontal chamber  55  and a vertical chamber  57  containing the various subsequently described elements of chamber  48 ; an induction heated graphite core  54  bisecting horizontal chamber  55  into target chamber  59  and flowpath  68  and defining a gap  74  also referred to herein as ablation zone  74 ; a graphite/catalyst target  56  that is mounted on a spindle  58  that rotates in the direction shown by arrow  60  and translates in the directions shown by arrow  62 ; a side-pumped laser beam  64  that enters chamber  48  through vertical chamber  57 , passes through gap  74  and strikes target  56  forming an ablation plume  66  and a flowpath  68  for the nanotube spray  70  to exit synthesis chamber  48  and be collected on a separate deposition target (not shown). As is well known in the art, the separate deposition target comprises a separate chamber containing water-cooled copper baffle plates on which the SWNT soot collects by thermophoresis as in any conventional SWNT deposition system. Spindle  58  is rotated and translated by means of a servo motors (not shown) in a conventional fashion. A graphite felt insulation layer  72  is preferably used to maintain temperature in the area of target  56  within synthesis chamber  48  and woven silica insulation  76  is also preferably used to retain heat in the entire assembly. According to a preferred embodiment, synthesis chamber  48  also includes a pyrometer port  78  in graphite core  54  for purposes of monitoring the temperature of graphite core  54  and the temperature within inner chamber  48 . A purge gas is also applied within area  51  to further insulate and maintain proper operating conditions within chamber  48 . 
   Within flowpath  68 , argon temperature and flow, as shown by arrow  69 , are controlled by the presence of a porous plug graphite heater  80  and a orifice plate  82 , preferably fabricated from Niobium, the former imparting heat to flowpath  68  and the latter, regulating the flow of argon in flowpath  68  and hence the size, shape and velocity of nanotube spray  70 . 
   The apparatus just described can be brought to operating temperature in about 8 minutes and requires only about 15 minutes to cool down to room temperature thus significantly shortening operating cycle time. 
   The apparatus just described is useful in the practice of a novel method for the production of SWNTs. Spinning target  56  and illuminating it with laser beam  64  can produce a variety of results depending primarily upon three variables. These three variables are: the temperature of ablation zone  74 ; the spin rate and pattern of movement of target  56 ; and laser fluence  64  in W/cm 2  which is inversely proportional to the focal spot size of laser beam  64 . Target grain size can also affect production rates and a fine grain target  56  relative to the size of the laser spot produces larger yields. 
   Regarding each of the variables just mentioned for optimum production the temperature of ablation zone  74  should be at or above 1000° C. At temperatures below 750° C. nanotube yield is trivially low. As to the spin rate of target  56  results indicate that a high spin rate is favorable so that a shallow depth of target material is removed on each track of laser beam  64 . In those cases where the grain size of the target relative to the depth of material removed with each pass was large, high spin rates produce unfavorable results. Thus, the finest grain possible should be used to obtain the best results. It has also been found that spin pattern can play an important role in the nanotube production process. Best results were obtained when a “barber pole” stripe of material was removed from target  56  with each pass through the rotational and translational action of spindle  58 . By tracking each subsequent strip carefully next to the previous stripe a continuous layer of material is removed, while the barber pole pattern allows distribution of waste heat from the plasma over the full length of the target thereby minimizing localized heating that produces undesirable results. 
   The fluence of the laser beam  64  also plays an important role in the successful practice of the method of the present invention. According to a preferred embodiment of the present invention, laser beam  64  is produced by a free electron laser. In conventional laser ablation processes used for the production of carbon nanotubes a Nd:YAG laser is generally used. Representative conditions for such a laser are: 3 W average power at 30 Hz repetition rate with ˜ 1 J/pulse. Thus, for a 10 nanosecond pulse and 1 cm diameter spot the average fluence is 4 W/cm 2  and the peak fluence is 1.3×10 e8  W/cm 2 . Using the preferred free electron laser of the present method, a 700 W beam at 9 MHz repetition rate with a 0.5 picosecond pulse focused to a 0.15 cm spot yields an average fluence of 9×10 e6  W/cm 2  and a peak fluence of 2.2×10 e11  W/cm 2  thus the fluence is about one million times greater due to the physics associated with the interaction of ultrafast pulses with a solid surface. 
   While fluence plays an important role, it has also been found that best yields are obtained using the lowest possible fluence (largest spot size) that will sustain a plume. In practice, this involves focusing laser beam  64  tightly to initiate a plume and then defocusing to the point just before extinction of the plume or just above the ablation threshold. This procedure allows the use of a larger spot size than can be achieved in equilibrium. It is theorized that the larger spot size produces higher yields as it reduces thermal coupling to the target in favor of ultrafast non-thermal coupling, which in turn, produces greater excitation and finer diminution of nanotube precursors. 
   There have thus been described a novel apparatus and method for the high yield production of carbon nanotubes. 
   As the invention has been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the appended claims.