674
M. C. Bouleklab et al.
The entire set of bands can be found on the areas with a significantly lower content
of nanotubes. So, the maximum positions of all the above bands are marked with
arrows for the area p3(0.5) (Fig. 19).
Thus, we can argue that, as a wide background W band, the D and G bands,
etc., observed in areas of the high MWCNT concentration, in fact, are the result of
the numerous spectral components superposition due to the presence of different or
modified carbon structures. The described results are consistent with existing ideas
regarding the manifestation of defects and structures in the Raman spectra of various
carbon forms. Indeed, it is known that at the gradual transition from macroscopic
material to particles up to 20 Å (e.g., from mono to nanocrystalline graphite), the
G band maximum is shifted toward high frequencies: up to 1600 cm
−1 , and this is
accompanied by an increase in the I D /I G ratio f to values greater than 1. In part, this
shift is a consequence of the intensity increase of another “defective” band, D / , that is
in the vicinity ~1610–1620 cm
−1 . The CNT grinding due to the process of mechanical
and chemical activation of composites can be accompanied by an increase of the subfine phase nanotubes contribution to the carbon material composition (such as the
amorphous component). The D and G band widening, the appearance and increase
of the background band, the I D /I G ratio increase up to value more than 1, and the
appearance of the T band can be the consequence of such “amorphization” [70, 72,
73]. These manifestations are characteristic for the areas p1(0.5), p3(2.0) and p4(2.0)
of the studied composites, whereas it has been explained above, there is a significant
amount of carbon material. However, the obtained data show that, on the background
of the noted W band, the D and G strips can be located, the characteristics of which
(positions of the maxima and half-width) are close to the characteristics of the initial
MWCNT (see, e.g., the data for the area p1(0.5)). This observation showed that a
noticeable amount of slightly deformed CNT may be in the interstitial space These
nanotubes are provided a low CTE for a composite containing 2% MWCNT.
It should also be noted here that, due to the specifics of the milling processes and
the subsequent stages of the composite samples preparation, amorphization occurs
not only with respect to the “soft” components of these composites, such as MWCNT,
but amorphization also concerns of the metal constituents. The presence of a rather
intense background in the XRD spectra should be considered as a manifestation
of amorphous phases (Fig. 15) [31, 74]. Small-sized crystallites make a dominant
contribution to the formation of the D band in the Raman spectra [75]. Moreover,
in addition to various types of defects (vacancies, doubling lines, etc.), there is an
influence of the carbon network breaking at the edges of the nanotubes and their
fragments [76]. The boundary atoms have unsaturated valences those can become
saturated by the impurity atoms or molecular groups binding [70, 72, 73]. This
mechanism is one of those that binds one together “oxide” and “carbide” mechanisms
and the influence of foreign atoms and groups on the dilatometric characteristics of
Fe–Cu-CNT composites. At the same time, such a linkage significantly complicates
dilatometric behavior and the analysis of the results of dilatometric studies.
In addition to defects, disordering of the bonds in the sp
2 rings, the small length of
the nanotubes, and the presence of sp
2 chains as a result of the nanotubes destruction
can influence the characteristics of Raman bands [74]. The reduction in the length
M. C. Bouleklab et al.
The entire set of bands can be found on the areas with a significantly lower content
of nanotubes. So, the maximum positions of all the above bands are marked with
arrows for the area p3(0.5) (Fig. 19).
Thus, we can argue that, as a wide background W band, the D and G bands,
etc., observed in areas of the high MWCNT concentration, in fact, are the result of
the numerous spectral components superposition due to the presence of different or
modified carbon structures. The described results are consistent with existing ideas
regarding the manifestation of defects and structures in the Raman spectra of various
carbon forms. Indeed, it is known that at the gradual transition from macroscopic
material to particles up to 20 Å (e.g., from mono to nanocrystalline graphite), the
G band maximum is shifted toward high frequencies: up to 1600 cm
−1 , and this is
accompanied by an increase in the I D /I G ratio f to values greater than 1. In part, this
shift is a consequence of the intensity increase of another “defective” band, D / , that is
in the vicinity ~1610–1620 cm
−1 . The CNT grinding due to the process of mechanical
and chemical activation of composites can be accompanied by an increase of the subfine phase nanotubes contribution to the carbon material composition (such as the
amorphous component). The D and G band widening, the appearance and increase
of the background band, the I D /I G ratio increase up to value more than 1, and the
appearance of the T band can be the consequence of such “amorphization” [70, 72,
73]. These manifestations are characteristic for the areas p1(0.5), p3(2.0) and p4(2.0)
of the studied composites, whereas it has been explained above, there is a significant
amount of carbon material. However, the obtained data show that, on the background
of the noted W band, the D and G strips can be located, the characteristics of which
(positions of the maxima and half-width) are close to the characteristics of the initial
MWCNT (see, e.g., the data for the area p1(0.5)). This observation showed that a
noticeable amount of slightly deformed CNT may be in the interstitial space These
nanotubes are provided a low CTE for a composite containing 2% MWCNT.
It should also be noted here that, due to the specifics of the milling processes and
the subsequent stages of the composite samples preparation, amorphization occurs
not only with respect to the “soft” components of these composites, such as MWCNT,
but amorphization also concerns of the metal constituents. The presence of a rather
intense background in the XRD spectra should be considered as a manifestation
of amorphous phases (Fig. 15) [31, 74]. Small-sized crystallites make a dominant
contribution to the formation of the D band in the Raman spectra [75]. Moreover,
in addition to various types of defects (vacancies, doubling lines, etc.), there is an
influence of the carbon network breaking at the edges of the nanotubes and their
fragments [76]. The boundary atoms have unsaturated valences those can become
saturated by the impurity atoms or molecular groups binding [70, 72, 73]. This
mechanism is one of those that binds one together “oxide” and “carbide” mechanisms
and the influence of foreign atoms and groups on the dilatometric characteristics of
Fe–Cu-CNT composites. At the same time, such a linkage significantly complicates
dilatometric behavior and the analysis of the results of dilatometric studies.
In addition to defects, disordering of the bonds in the sp
2 rings, the small length of
the nanotubes, and the presence of sp
2 chains as a result of the nanotubes destruction
can influence the characteristics of Raman bands [74]. The reduction in the length