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Posted: Jun 03, 2016

Birth and early history of carbon nanotubes (page 3 of 5)

Are CNTs formed in Space?
There are studies, which actually speculate an interstellar origin to carbon nanotubes as reported previously in the case of fullerenes. A study by Heymann and Vis14 put forth a hypothesis that closed structures such as carbon nanotubes can be the host carrier for heavy planetary gases of Q phase of carbonaceous meteorites. The results were based on the study conducted on Allende (C3V) meteorite and the hypothesis was supported by various findings related to high capacity for storing planetary gases in such a way that neither non-oxidising acids nor heating to 1100°C could release the stored gas from the carrier host.
Corroborating these characteristics of carrier host with that of carbon nanotubes, which are resistant against non-oxidising acids and become unstable above 1200°C, the authors concluded that “circumstantial evidence” points towards the possibility of CNTs in the carrier host. However, these arguments are not yet supported by direct observation of carbon nanotubes in meteorites.
Another study conducted by NASA scientists showed that in laboratory experiments, when graphite grains are repeatedly exposed to H2, CO, and N2 at 875 K, abundant carbon nanotubes were observed to form on or from the surfaces of the graphite grains, which eventually grew to graphite whiskers15.
Fries and Steele have previously discovered graphite whiskers in high-temperature phases in meteorites such as calcium aluminum inclusions formed near 2200 K, and chondrules formed above 1200 K16. The outcome of these studies shows that carbon nanotubes can be formed at 873 K, and indirectly implies that graphite whiskers can be formed throughout a larger volume of the nebula.
Using NASA’s infrared-sensing Spitzer telescope, Domingo Aníbal García-Hernández and coworkers have actually identified C70 and C60 fullerene molecules as well as signs of graphene (present as planar C24) among a complex mix of aliphatic and aromatic species such as hydrogenated amorphous carbon grains (HACs) and polycyclic aromatic hydrocarbon clusters in planetary nebulae - the material shed by dying stars17.
The team proposes that the graphene and fullerenes were formed in space from the shock-induced destruction of hydrogenated amorphous carbon grains (HACs). All these studies overwhelmingly suggest that carbon nanotubes can be formed in interstellar space provided the favorable conditions and prerequisite materials exist.
Recent History of CNTs
Filamentous form of carbon has a long history and been well known before Iijima’s landmark discovery of carbon nanotubes. The first scientific report mentioning a process for preparing carbon filaments using thermal decomposition of hydrocarbon was a patent filed by T.V. Hughes and C.R. Chambers way back in 188618. Studies conducted in 1950s showed that carbon filaments can be produced by the interaction of hydrocarbons and other gases in the presence of metal catalysts such as iron, cobalt and nickel.
However, none of these studies could establish the presence of carbon nanotubes because of the resolution limitations of then available microscopic tools. Subsequent to the invention of transmission electron microscope, several studies were followed to understand the morphology of carbon filaments.
First electron micrographs of carbon filaments showing the tubular morphology is believed to be reported by L.V. Radushkevich and V.M. Lukyanovich19 in the Journal of Physical Chemistry of Russia in 1952. Due to the limited access to Russian scientific publications to the western world, however, this report did not receive wider attention.
Roger Bacon20,21 at Union Carbide reported the growth of sub-micron diameter graphite whiskers by DC arc discharge of graphite electrodes. The graphite whiskers formed consist of one or more concentric tubes, each tube being in the form of a scroll, or rolled-up sheet of graphite layers, having diameters ranging from sub-micron to over five microns, extending continuously along the length of the whisker with recoverable lengths up to 3 cm. These graphite whiskers had demonstrated highest mechanical strength and for many years served as the performance benchmarks for carbon fibers used in aerospace industry.
Research into the catalytic formation of carbon filaments flourished in the 1970s, but with an objective to understand the growth mechanisms so that their formation in coal and steel industry processing and in the coolant channels of nuclear reactors could be prevented.
Later, the pioneering studies by Morinobu Endo and co-workers revealed the nanostructure of vapor grown carbon fibers produced from a hydrocarbon gas source using metal catalysts,22,23 which was later termed as single walled carbon nanotubes. They concluded that a nano sized primary fiber was formed by catalytic process, which was thickened further by the chemical vapor deposition of carbon layers to form the carbon fibers.
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