Biomedical Engineering Reference
In-Depth Information
firstly, of 623 K (21 h, first stage), then of 733-763 K (9 h, second
stage), followed by cooling to RT (third stage) and lowering the
pressure to 1 atm. The total hydrogen content obtained by burning
the samples in a current of oxygen reached 10.8 ± 0.1 wt% or,
calculated for hydrogen, 5.4 wt% [(H/C) ≈ 0.68].
The total content of hydrogen in single-wall nanotube samples,
studied by these two groups of researchers [94, 95], is proportional,
in a satisfactory approximation, to the square root of the hydrogen
pressure, corresponding to the Sievert's dissociative absorption
isotherm (Eq. (5a)).
The measurements [94] of the pressure of the gas released
during vacuum heating of hydrogen-saturated single-wall nanotube
samples and GNF (from 77 to 873-923 K, with the rate of 20 K min
-1
)
have shown that three desorption processes take place: (
) at about
175-280 K, with about 0.45 and 0.15 wt% of hydrogen released,
respectively; (
α
) at about 773-873 K, with about 5.3 and 4.8 wt%
of hydrogen released, without taking the possible release of a small
amount (about 0.05) of hydrocarbons into account [94, 95]; and (
β
+
γ
γ
)
at ≥823 K, the remainder, about 1 wt%.
Figure 2.18 shows the X-ray diffraction patterns of the samples
studied in Ref. [94]. The X-ray diffraction patterns of hydrogen-
saturated GNF samples contain a broad intense peak near 2
=
20° instead of the very strong, narrow reflection of (002) line of
graphite at 2
θ
= 26.5°, characteristic of the initial samples. This is a
clear indication of the increasing in interplanar spacing between the
graphene layers in all graphite nanofibers by approximately 40%,
from 3.36 Å in the pristine GNFs to 4.67 Å in the hydrogen-saturated
GNFs. It is also an indication that microstrains have to be present.
The next graphite reflections are noticeably shifted toward small
angles, and only the catalyst line remains in place.
A decrease in the total adsorbate content (6.3 wt% of hydrogen)
by about 43% in hydrogen-saturated GNF samples subjects to
vacuum annealing at about 773 K in the course of 1-10 h (i.e., the
removal of about 2.7 wt% of hydrogen during the annealing process)
leads to the emergence (see Fig. 2.18) of two very strong reflections
at 2
θ
≈ 19.9° and 24.0°, shifted toward each other and extremely
broadened. The authors of Ref. [94] believe that this constitutes the
diffraction pattern of a two-phase state.
These XRD features indicates that about 50% of nanofibers with
interplanar spacing of the graphene layers somewhat smaller than
θ
