Biomedical Engineering Reference
In-Depth Information
4.67 Å, and about 50% of nanofibers with interplanar spacing higher
than 3.36 Å are presented in such samples. In both cases a high level
of microstrain is also present.
After two cycles of heating at 873 K, reducing the total adsorbate
content by about 81%, corresponding to the removal of about
5.1 wt% of hydrogen, the peak at 2
≈ 19.9° in Fig. 2.18 either
virtually disappears or manifests itself as an extremely broadened
peak of low intensity. Moreover, a high peak appears at 2
θ
≈ 26°,
corresponding to the (002) reflection of the graphite line, shifted
toward smaller angles. This is in agreement with an incomplete
removal of adsorbed hydrogen (the remainder amounts to about
1.2 wt%) and suggests that the predominant fraction of nanofibers
in such samples has interplanar spacings close to the normal value
of 3.36 Å and a low level of microstrain in the graphene layers. For
the other nanofibers, the situation is quite diferent.
Prolonged (6 h) vacuum annealing of the hydrogen-saturated
GNF samples at 973 K has fully restored the initial pattern. This
treatment has led to a strong, narrow (002) reflection at 2
θ
= 26.5°,
which corresponds to the normal interplanar spacing between the
graphene planes and to the absence of microstrain in GNF.
The diffraction patterns of hydrogen-saturated single-wall
nanotube samples [94], in contrast to those of the initial single-
wall nanotube samples, exhibit a broad peak with a predominant
intensity near 18.5°, while the narrow reflection of the (002)
graphite line near 26.5° becomes very pronounced. It may happen
because such samples contain up to 40-50 wt% of graphitized
(under thermobaric treatment) multilayer nanoparticles, most
of which have an interplanar spacing larger by roughly 40%, and
an increased level of microstrain in graphene layers. The other
graphite nanoparticles have an interplanar spacing close to normal
(3.36 Å), with no essential microstrain in the graphene layers.
For the hydrogen-saturated single-wall nanotube and GNF
samples [94] subjected to vacuum annealing at about 773 K for
1-10 h, the decrease in the total adsorbate content amounts to
about 37% and about 43%, respectively (i.e., about 2.5 and about
2.7 wt% of hydrogen) and leads to the complete disappearance of
the narrow absorption lines in the IR spectra at 2860 and 2920 cm
θ
-1
,
a characteristic feature of C-H bond valence vibrations, and also to
the disappearance of the broad adsorption line at about 1200 cm
-1
(curves 2 and 3 in Fig. 2.19).
