handbook of carbon, graphite, diamond and fullerenes

130 Carbon, Graphite, Diamond, and Fuiierenes
It has been suggested that the structure of vitreous carbon includes,
in addition to the graphitic sp 2 (trigonal) bonds, some sp 3 (tetragonal) bonds
which are characteristic of diamond bonding. This would possibly contribute
to the isotropy, the high strength, and the hardness of the material (see
Ch. 2, Sec. 3.0) .Pl^ The presence of the diamond structure in a similar
material, diamond-like carbon, is well established (see Ch. 14).
These two factors, ribbon network and sp 3 bonds, would prevent
further ordering of the structure, regardless of the graphitization temperature.
3.2 Porosity
Vitreous carbon has low density (approximately two-thirds that of the
ideal graphite crystal) which implies that it has high porosity. Yet the
permeability of the material is exceptionally low. Helium permeability for
instance, measured by the vacuum-drop method, is only 10 11 cm 2 /s. This
means that the pores are extremely small and essentially inaccessible to
helium. Pore diameters of 0.1 to 0.3 nm are reported with an average of 0.25
nm after heat-treatment to 220°C and 0.35 nm to 3000°C.( 1 lPl
This extremely fine-pore structure gives vitreous carbon the characteristics
of a molecular sieve and allows the absorption of some very small
molecules as shown in Table 6.1 J 1 1 This table shows a minimal absorption
for water (in spite of the small diameter of its molecule). This contradicts
the results of Fitzer 16 ! who found the absorption to be similar to that of
methanol.
Table 6.1. Adsorption of Selected Molecules by Vitreous Carbon
Gas-kinetic % increase
molecular by wt. after
Compound diam., nm 20 days
Water
Methanol
Isopropanol
Isobutanol
0.289
0.376
0.427
0.475
0.03
1.34
1.16
0