In this fourth lecture, we look at carbon nanotubes, analyzing their structures, scientific history, production methods, associated chemistry, purification and separation processes, and physical properties. Carbon nanotubes (CNTs) are nanomaterials that can be described as resulting from the winding of a graphitic sheet (graphene-like plane) onto itself. This winding gives rise to three main types of carbon nanotubes (chair, zigzag, helical-chiral), whose electronic properties are respectively metallic (chair CNTs) or semiconducting (chiral CNTs, zigzag CNTs). CNTs are often mixtures, but depending on the synthesis method, single-walled carbon nanotubes (SWCNT) or multi-walled carbon nanotubes (MWCNT) may be more or less preferred. Single-walled carbon nanotubes (SWCNT) are hollow carbon cylinders with diameters in the nanometer range, lengths from tens of nanometers to centimeters, and walls that are one atomic layer thick. Multiwall carbon nanotubes (MWCNTs) are made up of a few dozen concentric cylinders with regular periodic interlayer spacing of the order of 0.36 nm. Depending on the number of layers, the internal diameter of MWCNTs varies from 0.4 nm to a few nanometers, and the external diameter from 2 nm to 30 nm. The two ends of the CNTs are generally dome-shaped, as they are closed and capped by half-molecules of fullerenes (another carbon allotrope with a football-like structure of hexagonal and pentagonal panels). The existence of CNTs in very ancient materials has recently been demonstrated in swords made from Damascus steel dating back to ancient India. This discovery supports the hypothesis that these renowned blades owe their exceptional characteristics to the presence of carbon nanotubes. The scientific history of CNTs (MWCNTs) is more recent. As early as the early 1950s, numerous authors mentioned the presence of carbon threads or tubes observed by microscopy. These observations were solidly confirmed between 1973 and 1976. However, this field of research did not make its quantitative debut until 1991, with the publication in Nature by Sumio Iijima of Japan. It was undoubtedly the combined effect of the publication of a good article in a major multidisciplinary journal, a subject that resonated with the discovery of fullerenes and a certain maturity, or interest, in nanotechnology on the part of society that generated the scientific and technological tidal wave associated with CNT. However, the case of CNTs is more complicated than it seems, because they are not well-defined molecules. CNTs have different structures, masses and dimensions, and consequently different properties. Their polydispersity also leads to non-uniform properties that are difficult to predict. The manufacture of homogeneous materials requires control not only of the individual building blocks, but also of the higher-level architecture at which the elementary patterns fit together. Because of these complications, the fabrication of functional macroscopic structures that can fully utilize the exceptional properties of individual CNTs has been difficult. Indeed, most technological developments require predictable and uniform performance, and consequently many of the research strategies have focused on chemistry to prepare, in particular, SWCNTs with perfectly defined diameters, lengths, chiralities and electronic properties. In order to study their properties and incorporate them into devices or materials, it is therefore very important to separate, select and classify the different types of CNTs. We began by exploring the two main routes to CNT synthesis, starting with high-temperature routes such as graphite evaporation (T> 3,200°C), electric arc synthesis (T> 1,700°C) or laser ablation (T> 1,200°C), and ending with intermediate-temperature methods (T = 600-900°C) such as molten-salt electrolysis and plasma-assisted chemical vapor deposition (PA-CVD).
16:30 - 17:30
Lecture
Carbon nanotubes
Clément Sanchez
