Biomedical Engineering Reference
In-Depth Information
wall nanohorn samples. For oxidized samples, it can be assumed that
clusterization of hydrogen molecules occurs near the inner walls of
the nanotubes and that a pseudo-high-pressure effect manifests itself
near the nanoholes in the intertubular cavities; all this increases the
physical interaction between the internal surfaces of the tubes and
hydrogen.
Murata
[31] have also remarked on the fast kinetics of
establishing equilibrium (taking about 10 min) after single-wall
nanohorn samples saturation with hydrogen at 303, 196, and
77 K. According to the authors' opinion, the reproducibility and
reversibility of the adsorption-desorption isotherms is an indication
of the physical nature of sorption.
However, such kinetics (presumably diffusion kinetics) at 303
and 196 K may correspond to physical adsorption or to type-I
chemisorption. Assuming that the smallest linear size (thickness)
L
et al.
of single-wall horn samples varies from about 0.1 to 1 mm [31],
Eqs. (2.22) and (2.25) can be used to estimate the effective hydrogen
diffusivity as
2
−7
−5
2
−1
D
≈
L
/
t
≈ 2(10
- 10
) cm
s
. These values of
D
are
comparable (at 303 and 196 K) to the values of
D
and the hydrogen
I
s
,
surface diffusivity
D
with the relatively low van der Waals activation
s
energy
(Table 2.1). At 77 K, special diffusion of hydrogen in the
material may have occurred.
Unfortunately, Murata
Q
[31] have not studied the thermal
desorption spectra of single-wall nanohorn samples and have not
determined the activation energies of desorption or diffusion
of hydrogen for different TPD peaks, which could be helpful in
identifying the nature of sorption processes in the material and in
separating their contributions.
In such a context, it is reasonable to examine the data in Ref.
[29] regarding the kinetics of hydrogen desorption at 293 K
from electrochemically hydrogen-saturated composite samples
(cylindrical pellets with a diameter of about 7 mm and thickness
et al.
L
of about 0.1 mm) fabricated by cold pressing a mixture of single-wall
nanotube bundles (10 mg) and gold powder (90 mg). The adsorbate
content in single-wall nanotube samples reached 0.9 wt% (H
/C ≈
2
−2
5.4 × 10
). After samples ageing at 293 K, about half of the adsorbate
left the samples during the first hour and the relaxation time of this
diffusion process
≈ 5 min, according to Eqs. (2.21) and
(2.22). About 220 h was then required for the second half to leave
α
was
τ
α
