Biomedical Engineering Reference
In-Depth Information
the graphene layers of the material [94, 95], because process III (as
shown by the analysis in Refs. [10, 17, 97] of the experimental data
reported in Refs. [12, 14, 53, 96] is characterized by much smaller
changes (by 2-7%) of the interplanar spacing for a similar adsorbate
content.
Hence, there are many reasons, including the results of the
analysis [52, 53, 60, 100, 101], to believe that process
in the GNF
and single-wall nanotube samples in Refs. [94, 95] corresponds to
chemisorption IV (Table 2.1) and that, similarly to process III, such
process under specific conditions can be described by the Sieverts
sorption isotherm (2.5a).
Process IV, in contrast to process III, may lead to the anomalous
increase in the interplanar spacing between the graphene layers
in the adsorbent, because it has the highest energy of the C-H
bond (models C and D in Fig. 2.8, Table 2.1) and it is localized in
the defective regions of the graphite lattice, at the edges of cluster
(dislocation) loops of interstitial type [53], which may induce in the
strained material the hydrogen saturation under ultrahigh pressures
(9 GPa) [94, 95].
The data in Refs. [94, 95] on the sorption capacity and the kinetics
of process
γ
in GNF and single-wall nanotubes can be described quite
well by the sorption isotherm and the thermodynamic and diffusion
characteristics of chemisorption process IV, as done in the analytical
study in Ref. [10] for the experimental data on nanostructured
graphite [52].
To establish the nature of process
γ
, it is useful to estimate the
diffusivity via Eq. (2.25). We can use the data in Ref. [94] on the
release of about 0.15 wt% of hydrogen [(H/C)
α
≈ 0.018] by GNF
α
samples heated at the rate of
from about 173 to
273 K. Assuming that the characteristic diffusion length is of the
order of the GNF samples thickness [94] and that the diffusion time
(near the mean temperature
υ
≈ 20 K min
-1
T
α
≈
223 K) is around ∆
T
α
/
υ,
where
-4
2
−1
∆
T
α
≈
100 K, we obtain the diffusivity
D
α
≈
10
cm
s
for process
α
.
This value of
D
α
is several orders of magnitude greater than the
value of
) corresponding to the chemisorption process I and
several orders of magnitude smaller than the gas diffusivity [68]. At
the same time, it is close (as order of magnitude) to the values of
D
(at
T
α
I
s
D
def
and
(Sections 3.3 and 5.1, Tables 2.1 and 2.2) characteristic of
the van der Waals interaction, which indicates the manifestation of
the physical mechanism characteristics for the sorption process
D
α
.
