BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 151

steps for property averaging. Shorter runs than this were not adequate to sample
desorption events. The average number of water molecules in the simulation cell
varied from a few molecules at the lowest pressures to a few hundred or thou-
sands of molecules when the pores were full; filled pores contained about 320
molecules for a pore width of 0.79 nm, 460 at0.99 nm, 830 at 1.69 nm, and 2100
at 4.5 nm. Calculations were carried out on the Cornell Theory Center IBM SP2.
In determining the adsorption isotherm, we commenced with the cell empty; a
value of the fugacity corresponding to a low pressure was chosen and the average
adsorption terminated from the simulation. The final configuration generated at
each stage was used as the starting point for simulations at higher fugacities. The
pressure of the bulk gas corresponding to a given chemical potential was deter-
mined from the ideal gas equation of state. Gas phase densities corresponding to
the range of chemical potentials studied were determined by carrying out simula-
tions of the bulk gas. These were found to agree with those calculated from the
ideal gas equation within the estimated errors of the simulations.
Researchers reported a review about fulleren-like models for micro porous
carbons that investigated structural evolution and adsorption of fulleren-like mod-
els. Modeling the structural evolution of microporous carbon including the forma-
tion mechanism of microporous carbon is not well understood at the atomic level.
A number of groups have attempted to model the process, and in several cases
these modeling exercises have produced structures, which contain fullerene-like
elements. On the starting point for the simulation was a series of all-hexagon
fragments, terminated with hydrogens, that the evolution of the structure showed
that the H/Cratio is reduced (the temperature is increased). During this evolution,
pentagons and heptagons form as well as hexagons, resulting in the formation
of curved fragments. In each case the final carbon was made up of a hexago-
nal network with 10-15% non-hexagonal rings (pentagons and heptagons). The
properties of the simulated carbons appeared to be generally consistent with ex-
perimental results. A different approach to modeling the evolution of microporous
carbon was used. Here, the initial system consisted of carbon gas atoms at very
high temperature. This choice of initial condition was intended to represent the
high temperature state in a pyrolysis process after the polymer chains break down
and most other elements have evaporated. The temperature was then decreased
so that the atoms condensed to form a porous structure composed of curved and
defected graphene sheets, in which the curvature was induced by non-hexagonal
rings. In 2009, some researchers described a comprehensive molecular dynam-
ics study of the self-assembly of carbon nanostructures. The precursor for these
simulations was highly disordered amorphous carbon, which was generated by
rapid quenching of an equilibrated liquid sample. It was found that, under certain
conditions, annealing the amorphous carbon at high temperature could lead to
the highly curved sp 2 sheet structure. In modeling adsorption using fullerene-like
models for microporous carbon, there have been relatively few attempts to use