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Large scale manufacturing techniques for single walled carbon nanotubes

• What is the most economical and practical methood for large scale manufacturing of single walled carbon nanotubes? Please provide comparisons between various methoods and clearly delineate your rationale for identifying the most economical and practical methood.

• Answer:

  Dear leonardodavinci, The three principal approaches to single-walled carbon nanotube (SWCNT) fabrication are chemical vapor deposition, laser ablation, and arc welding. All currently known methods consist of some variant of one of these approaches. Other fabrication mechanisms may be discovered in future, but for the time being we must restrict our attention to these three. The most recently developed production method is chemical vapor deposition (CVD), which grows nanotubes in an enclosed high-temperature environment permeated with a carbon-bearing gas. The decomposition of the gas results in the gradual deposition of carbon on a prepared surface. This type of manufacturing is especially well suited to electronic manufacturing applications in which nanotube structures must be grown in precise quantities and locations. Recent developments in the use of plasma as a CVD growth environent have made it possible to grow CNTs at temperatures below 100 degrees Celsius. CVD methods can also be harnessed to the production of long strands of carbon nanotube, typically of the multi-walled rather than single-walled variety, for use in ultra-strong rope and similar products. A thermal CVD reactor is simple and inexpensive to construct, and consists of a quartz tube enclosed in a furnace. Typical laboratory reactors use a 1 or 2" quartz tube, capable of holding small substrates. The substrate material may be Si, mica, quartz, or alumina. The setup needs a few mass flow controllers to meter the gases and a pressure transducer to measure the pressure. The growth may be carried out at atmospheric pressure or slightly reduced pressures using a hydrocarbon or CO feedstock. The growth temperature is in the range of 700-900 C. A theoretical study of CNT formation suggests that a high kinetic energy (and thus a high temperature, ? 900 C) and limited, low supply of carbon are necessary to form SWCNTs. NASA Ames Research Center: M. Meyyappan and D. Srivastava: Carbon Nanotubes: Section 4.2 http://www.nanoelectronics.engr.scu.edu/nanotechnology/NanoTech%20Downloads/crc_chapter_11_14_01(section3-supple1).doc It has been shown that CVD is amenable for nanotube growth on patterned surfaces, suitable for fabrication of electronic devices, sensors, field emitters and other applications where controlled growth over masked areas is needed for further processing. More recently, plasma enhanced CVD (PECVD) has been investigated for its ability to produce vertically aligned nanotubes. A variety of plasma sources and widely varying results have been reported in the literature. [...] The plasma enhancement in CVD first emerged in microelectonics because certain processes cannot tolerate the high wafer temperatures of the thermal CVD operation. The plasma CVD allows an alternative at substantially lower wafer temperatures (room temperature to 100 C) for many processes and hence has become a key step in IC manufacturing. The low temperature operation is possible because the precursor dissociation (necessary for the deposition of all common semiconductor, metal and insulator films) is enabled by the high-energy electrons in an otherwise cold plasma. Plasma Source Science and Technology: M. Meyyappan, L. Delzeit, A. Cassell, and D. Hash: Carbon Nanotube Growth by PECVD http://www.iop.org/EJ/article/0963-0252/12/2/312/ps3212.pdf Midway in SWCNT fabrication history between arc welding and CVD is the technique of laser ablation. In this process, a graphite rod inside a cylindrical furnace with a controlled gas flow is vaporized by a high-powered laser, throwing off carbon nanoparticles that coalesce downstream in a quartz tube to form SWCNT structures. Laser ablation requires costly apparatus to produce small quantities of high-quality SWCNT with purity ranging from 70% to 90%. In laser ablation, a target consisting of graphite mixed with a small amount of transition metal particles as catalyst is placed at the end of a quartz tube enclosed in a furnace [60]. The target is exposed to an argon ion laser beam which vaporizes graphite and nucleates carbon nanotubes in the shockwave just in front of the target. Argon flow through the reactor heated to about 1200 C by the furnace carries the vapor and nucleated nanotubes which continue to grow. The nanotubes are deposited on the cooler walls of the quartz tube downstream from the furnace. This produces a high percentage of SWCNTs (~70%) with the rest being catalyst particles and soot. NASA Ames Research Center: M. Meyyappan and D. Srivastava: Carbon Nanotubes: Section 4.1 http://www.nanoelectronics.engr.scu.edu/nanotechnology/NanoTech%20Downloads/crc_chapter_11_14_01(section3-supple1).doc Laser ablation products from fullerene materials have been studied by transmission electron microscopy and Raman spectroscopy. Using nickel and cobalt as a catalyst, single-wall carbon nanotubes were produced at an ambient temperature of 400 °C. The results were compared with those using graphite as starting materials. It is suggested that the formation of single-wall carbon nanotubes is controlled by both the availability of proper precursors and the activity of the metal catalyst. Applied Physics Letters -- November 15, 1999 -- Volume 75, Issue 20, pp. 3087-3089: Y. Zhang and S. Iijima: Formation of single-wall carbon nanotubes by laser ablation of fullerenes at low temperature http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=APPLAB000075000020003087000001&idtype=cvips&gifs=yes The earliest method of SWCNT production, indeed the one that resulted in the serendipitous discovery of carbon nanotubes, is arc welding. Here, graphite is vaporized by a strong electric current to produce structurally sound but relatively impure SWCNT. Although arc welding is not amenable to the application of precise quantities such as those required in nanoelectronics, it is a relatively inexpensive and high-volume production method. A recent refinement of this technique developed at NASA Goddard has resulted in much higher yield rates than the usual 30%, approaching the 70% range. The arc process involves striking a dc arc discharge in an inert gas (such as argon or helium) between a set of graphite electrodes [1,59]. The electric arc vaporizes a hollow graphite anode packed with a mixture of a transition metal (such as Fe, Co or Ni) and graphite powder. The inert gas flow is maintained at 50-600 Torr. Nominal conditions involve 2000-3000 C, 100 amps and 20 volts. This produces SWCNTs in mixture of MWCNTs and soot. The gas pressure, flow rate, and metal concentration can be varied to change the yield of nanotubes, but these parameters do not seem to change the diameter distribution. Typical diameter distribution of SWCNTs by this process appears to be 0.7-2 nm. NASA Ames Research Center: M. Meyyappan and D. Srivastava: Carbon Nanotubes: Section 4.1 http://www.nanoelectronics.engr.scu.edu/nanotechnology/NanoTech%20Downloads/crc_chapter_11_14_01(section3-supple1).doc High yields (70-90%) of SWNTs close-packed in bundles can be produced by laser ablation of carbon targets. The method (electric-arc) used here is cheaper and easier, but previously had only low yields of NTs. They show that it can generate large quantities of SWNTs with characteristics similar to those obtained by laser ablation. Northwestern University: F. Fisher and C. Brinson: Carbon Nanotubes Literature Review: page 6 http://www.tam.northwestern.edu/~ftf234/nano/LitReview/LitTry3/NanotubeReview022101web.pdf When considering which of the three SWCNT fabrication methods is best suited to large-scale manufacturing, our criteria consist of volume and quality. By definition, large-scale manufacturing requires that apparatus procured at a reasonable price be capable of producing significant quantities of SWCNT. This rules out laser ablation, which requires significant expenditures to produce small quantities of SWCNT. For similar outlay, both CVD and arc discharge methods have been shown to be capable of producing tens or hundreds of grams of carbon nanostructures daily in each enclosure. However, the criterion of product quality demands that we strike CVD methods for the time being. Although CVD yields can be very pure, meaning that the proportion of non-CNT contaminant is low compared to the number of CNT particles, the nanostructures themselves tend to be compromised by extensive defects. Since large-scale production requires consistent structural properties, CVD does not at present appear to be as suitable an approach as arc discharge. Furthermore, CVD is best equipped to fabricating multi-walled CNT structures rather than the single-walled variety. Therefore, the best currently known fabrication technique for large-scale SWCNT manufacturing is the arc discharge process. Thanks to recent advancements by nanostructure fabrication researchers at NASA Goddard Space Flight Center, it is now possible to take advantage of the high-yield properties of arc discharge while enjoying yield purities comparable to those of CVD and laser ablation. The new methods, which do without a metal catalyst, also reduce considerably the cost and complexity of SWCNT manufacturing. The early processes used for CNT production were laser ablation and an arc discharg approach. [...] Of the two, laser ablation is not amenable for scaleup whereas the arc discharge process has been used to produce large quantities of CNTs. Plasma Source Science and Technology: M. Meyyappan, L. Delzeit, A. Cassell, and D. Hash: Carbon Nanotube Growth by PECVD http://www.iop.org/EJ/article/0963-0252/12/2/312/ps3212.pdf Large-scale synthesis of SWCNT by the arc discharge method yielded quantities of tens of grams a day under arc conditions of 40~60 A d.c. and helium pressures of 500 to 700 torr. Results show that helium atmosphere strongly affects the yield of SWNTs, and that the diameter distribution of the SWNTs is affected by the catalyst. Northwestern University: F. Fisher and C. Brinson: Carbon Nanotubes Literature Review: page 3 http://www.tam.northwestern.edu/~ftf234/nano/LitReview/LitTry3/NanotubeReview022101web.pdf NASA scientists have developed an SWCNT manufacturing process that does not use a metal catalyst, resulting in simpler, safer, and much less expensive production. Researchers used a helium arc welding process to vaporize an amorphous carbon rod and then form nanotubes by depositing the vapor onto a watercooled carbon cathode. Analysis showed that this process yields bundles, or ?ropes,? of single-walled nanotubes at a rate of 2 grams per hour using a single setup. NASA?s process offers several advantages over metal catalyst production methods. For example, traditional catalytic arc discharge methods produce an ?as prepared? sample with a 30% to 50% SWCNT yield at a cost of approximately $100 per gram. NASA?s method increased the SWCNT yield to an average of 70% while significantly reducing the per-gram production cost. Fuentek: Available Technologies: Producing Lower-Cost Single-Walled Carbon Nanotubes Without Metal Catalysts http://www.fuentek.com/technologies/carbon-nantubes.htm It has been an interesting challenge to address this question on your behalf. If you have any concerns about my answer, please let me know through a Clarification Request so that I have the opportunity to fully meet your needs before you assign a rating. Regards, leapinglizard