BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 10

It has been realized that the fullerene-type materials and the carbon nanofibers
known from catalysis are relatives and this broadens the scope of knowledge and
of applications. This chapter describes the issues around application and produc-
tion of carbon nanostructures. Electro spinning is a simple and versatile method
for generating ultrathin fibers from a rich variety of materials that include poly-
mers, nanocomposites and ceramics. In a typical process, an electrical potential
is applied between a droplets of a polymer solution, or melt, held at the end of a
capillary tube and a grounded target. When the applied electric field overcomes
the surface tension of the droplet, a charged jet of polymer solution is ejected. The
following parameters and processing variables affect the electro spinning pro-
cess: (i) system parameters such as molecular weight, molecular weight distribu-
tion and architecture (branched, linear, etc.) of the polymer, and polymer solution
properties (viscosity, conductivity, dielectric constant, and surface tension, charge
carried by the spinning jet) and (ii) process parameters such as electric potential,
flow rate and concentration, distance between the capillary and collection screen,
ambient parameters (temperature, humidity and air velocity in the chamber) and
finally motion of the target screen. Morphological changes can occur upon de-
creasing the distance between the syringe needle and the substrate. Increasing the
distance or decreasing the electrical field decreases the bead density, regardless
of the concentration of the polymer in the solution. Elemental carbon in the sp 2
hybridization can form a variety of amazing structures. The nanotubes consisted
of up to several tens of graphitic shells (so-called multi-walled carbon nanotubes
(MWNT)) with adjacent shell separation of 0.34 nm, diameters of 1 nm and high
length/diameter ratio. Two years later, Iijima and Ichihashi synthesized single-
walled carbon nanotubes (SWNT). There are two main types of carbon nanotubes
that can have high structural perfection. Single walled nanotubes (SWNT) consist
of a single graphite sheet seamlessly wrapped into a cylindrical tube. Multi-walled
nanotubes (MWNT) comprise an array of such nanotubes that are concentrically
nested like rings of a tree trunk [54, 57].
Recent discoveries of fullerene, a zero dimensional form of carbon and car-
bon nanotube, which is a one-dimensional form, have stimulated great interest in
carbon materials overall. Fullerenes are geometric cage-like structures of carbon
atoms that are composed of hexagonal and pentagonal faces when a Bucky ball
is elongated to form a long and narrow tube of few nanometers diameter approxi-
mately, which is the basic form of carbon nanotube. This stimulated a frenzy of
activities in properties measurements of doped fullerenes. The discovery of fuller-
enes led to the discovery of carbon nanotubes by Iijima in 1991. The discovery of
carbon nanotubes created much excitement and stimulated extensive research into
the properties of nanometer scale cylindrical carbon networks. Many researchers
have reported mechanical properties of carbon nanotubes that exceed those of any
previously existing materials. Although there are varying reports in the literature
on the exact properties of carbon nanotubes, theoretical and experimental results