Biomedical Engineering Reference
In-Depth Information
8.1
Introduction
The recent discovery of novel carbon nanostructures has caused
many expectations mainly because of their possible applications in
the fabrication of nanodevices for gas adsorption and storage.
Thanks to their large surface/volume ratio and their peculiar
geometry, nanostructured materials can, in principle, adsorb large
quantities of gas, making them suitable for many applications: gas
storage, with particular emphasis to the hydrogen retention problem,
and chemical sensors are probably the most appealing ones. The
requirement for hydrogen storage in a solid substrate has been
determined by the U.S. Department of Energy (DOE) to be about 9
wt.% for the year 2015 [62]. Presently, however, none of the major
gas storage media, i.e., compressed gas cylinders, liquid hydrogen
tanks, and metal hydrides satisfy this condition.
Gas storage is important also for many other technological
applications: helium, for example, is employed as a coolant because
it is low-cost, safe, and chemically inert, it has low viscosity and does
not cause corrosion of the cooling system; also gaseous nitrogen has
a wide variety of applications in metallurgic industry.
Nanostructured sensors are needed for the detection of small
concentrations of chemical species in many fields: from medical
applications to environmental monitoring [11, 61, 96]. Unfortunately,
most of the experiments made on gas adsorption on carbon
nanostructures have evidenced many controversial results whose
interpretation may be sometimes affected by the partial knowledge
of the processes involved [34, 86]. Moreover, physisorption data
are highly sensitive to the experimental conditions (temperature,
pressure, humidity, etc.) [75], thus increasing the possible sources of
discrepancy.
Theoretical modeling and atomistic simulations are nowadays
crucial to understand many of the processes involved in the
adsorption/desorption mechanisms and to avoid possible
misinterpretations. Classical model potentials are used to calculate
statistical properties because large systems can be handled. However,
classical models are quite limited and inaccurate if quantum theory
is needed as in the case of chemisorption. Therefore, the phenomena
occurring during adsorption can be quite different depending
on whether chemisorption or physisorption is involved and, as a
consequence, they will be treated separately.
