SEM image of multi wall carbon nanotubes bundles (MWCNT)


Carbon nanotubes (CNTs) are molecular scale sheets of graphite (called graphene) rolled up to make a tube and can be described as a new member of carbon allotropes, between fullerenes and graphite. The Single Wall Nanotubes (SWCNT) consist of single graphene rolls, while the Multi Wall Nanotubes (MWCNT) consist of two or more coaxial tubes-within-a-tube. Properties of individual CNTs can be influenced significantly by their chirality (twist) and geometry.  Held together by the Van der Waals force, SWCNTs tend to bundle in ropes, while MWCNTs generally form “birds nest” agglomerates due to their growing mechanism in the catalytic chemical vapour deposition process. Both configurations can also be synthesized as “forests” in highly vertically aligned structures (VACNT) for use in electronics.




A  diagram showing the types of single wall carbon nanotube (SWCNT) and multi wall carbon nanotube (MWCNT)



Ever since their discovery in 1991 by Sumio Iljima, carbon nanotubes (CNT) have inspired scientists and developers of future technologies, yet, until recently, their practical application was limited by relatively high production costs. CNTs are particularly interesting for various applications in cutting edge electronics, optics and material engineering: they are approximately 50 000 times thinner than a human hair, and yet they are the strongest and the stiffest materials known to man, with an E-modulus 10 times greater than steel.




CNT manufaturing process at the Future Carbon GmbH in Bayeruth, Germany



Unlike traditional materials, CNTs conduct electricity balistically, so electrons, just like cars in a multiple lane highway, can be transported in high densities and speed with a minimal resistance and hence the electrical conductivity of CNT along the axis is very high (106 S m-2) and surpasses that of metals, such as copper. They are the best field emitters of any known material and in theory, metallic nanotubes can carry an electric current density of 4 × 109 A/cm2 which is more than 1,000 times greater than metals such as copper. While the thermal conductivity of CNTs along their axis is high, and has been measured as high as 3500 W m-1 K-1 (although in theory it could reach 6600 (W m-1 K-1), the thermal conduction is 100 times or so smaller in the direction perpendicular to its axis.   The overall conductivity is an average sum of the two. 


The remarkable electrical and thermal conductivity and unsurpassed mechanical strength make CNTs an outstanding material for the emerging IMAT technology and many other innovative applications. The material is not only extremely light and robust, but can also efficiently heat up surfaces of any size, and feature a very rapid thermal response, which is an important factor in maintaining ultra steady temperatures, and in reducing heating and cooling times. 




A  diagram showing  the  graphene  layer