AIST Research Team First to Selectively Produce High Purity Double Wall Carbon Nanotubes in Bulk

FIG. 10 exemplifies an external appearance of a vertically aligned double-walled carbon nanotube bulk structure obtained through growth by chemical vapor deposition (CVD) apparatus developed at AIST's Nanotube Research Center as shown in U.S. Patent Application 20090297846. 

Japanese researchers at the National Institute of Advanced Industrial Science and Technology (AIST) Nanotube Research Center (Ibaraki, JP) reveal a process to selectively produce aligned double-walled carbon nanotube bulk structure in U.S. Patent Application 20090297846. The process is capable of "realizing high purity, large scaling and patterning, an aspect of which has not hitherto been achieved," say inventors Kenji Hata Takeo Yamada. Motoo Yumura, and Sumio Iijima.

 The process can produce double-walled carbon nanotube, characterized by having an average outer diameter of 1 nm or more and 6 nm or less and a purity of 98 mass % or more can be produced in bulk. According to the process it is possible to produce a double-walled carbon nanotube and its bulk structure with high selectivity and high efficiency by extremely simple means inclusive of control of the particle size of fine particles of the catalyst metal, control of the thickness of a catalyst metal thin film enabling to realize it and the presence of an oxidizing agent such as water vapor in the reaction system.

The height (length) of the vertically aligned double-walled carbon nanotube varies depending upon the use; its lower limit is 0.1 .mu.m, and its upper limit is not particularly limited, from the viewpoint of actual use, it is preferably between 2.5 mm to 10 cm or more. In addition, it is possible to prolong the life of the metal catalyst, realize the efficient growth thereof at a high growth rate and achieve mass production. Also, the carbon nanotube grown on a substrate can be easily peeled off from the substrate or catalyst.

The inventors say, it is to be especially emphasized that according to the production process, the double-walled carbon nanotube coexisting with a single-walled carbon nanotube (SWCNT) and a multi-walled carbon nanotube of three or more, its proportion of presence following the growth can be freely selected and controlled by controlling the particle size of the catalyst metal and further the thin film of the catalyst metal. For example, the proportion of the double-walled carbon nanotube can be selectively controlled at 50% or more, 80% or more, and further 85% or more.

On the other hand, it is also possible to increase the proportion of the single-walled carbon nanotube or the multi-walled carbon nanotube of three or more walls. According to such control, the behavior of its application is largely expanded. The aligned double-walled carbon nanotube bulk structure can be expected to have various applications in heat dissipators, heat conductors, electric conductors, reinforcing materials, electrode materials, batteries, capacitors or super capacitors, electron emission elements, adsorbing agents, optical elements and in semiconductor devices.

The patterning method for semiconductor devices may be a wet process or a dry process. For example, in addition to patterning using a mask, patterning using nanoimpirnting, patterning using soft lithography, patterning using printing, patterning using plating, patterning using screen printing and patterning using lithography are all possible. As shown in FIG. 1, a thin film of a metal catalyst having a strictly controlled thickness is first provided on a substrate. For example, an iron chloride thin film, an iron thin film prepared by sputtering, an iron-molybdenum thin film, an alumina-iron thin film, an alumina-cobalt thin film and an alumina-iron-molybdenum thin film can be enumerated.

A carbon nanotube chemical vapor deposition (CVD) equipment is used with an apparatus for feeding an oxidizing agent to grow the DWNT. It is not particularly limited with respect to configuration and structure of other reaction apparatus and reactors for achieving the CVD method. Any of known conventional apparatus such as a thermal CVD furnace, a heating furnace, an electric furnace, a drying furnace, a thermostat, an atmospheric furnace, a gas replacement furnace, a muffle furnace, an oven, a vacuum heating furnace, a plasma reactor, a micro plasma reactor, an RF plasma reactor, an electromagnetic wave heating reactor, a microwave irradiation reactor, an infrared ray irradiation heating furnace, an ultraviolet ray heating reactor, an MBE reactor, an MOCVD reactor and a laser heating apparatus can be used.

FIG. 2 is a schematic view of a production apparatus for producing double-walled carbon nanotube or an aligned double-walled carbon nanotube bulk structure.

Figure 13 is a tunneling electron microscope (TEM) photographic image obtained by peeling off the vertically aligned double-walled carbon nanotube from its substrate using a pair of tweezers, dispersing it in a solution, placing it on a grid of an electron microscope (TEM) and observing it by an electron microscope (TEM). It is noted that neither the catalyst nor amorphous carbon is incorporated in the obtained carbon nanotube. The double-walled carbon nanotube of pictured is 99.95 mass % in a non-purified state

Frequently, the availability of materials limit the research and development of applications, and SWNTs are no different, where their mass production is the key factor in establishing a CNT industry. The Nanotube Research Center is committed to and actively engaged in developing methods for mass production of economical, pure, and high quality SWNTs based on its Super Growth methods. The Center is running a 5 year/ $20million national project to pursue this goal. For SWNTs to become a widely used industrial material, the cost must be reduced to the level of classic carbons, such as activated carbon or carbon-fibers. This translates to a 100-fold to 1000-fold reduction in production cost in the future. They believe that this problem will eventually be solved by the very high growth efficiency of Super-growth that can already produce more than 1 g of SWNTs in ten minutes on a A4 (20x30cm) substrate.

National Institute of Advanced Industrial Science and Technology (AIST)
Nanotube Research Center
AIST Tsukuba Central 5 1-1-1 Higashi Tsukuba Ibaraki 305-8565 Japan
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