30
3 Interaction of Molten Salts with Graphite
or impurities present on graphite surface [27, 28]. Along with this, some finer particles can additionally be seen in Fig. 3.7c, which could be residue remaining after
partial oxidation of more active components like binders or less graphitized carbon
phases. The inset in Fig. 3.7c is a high-magnification SEM micrograph taken from
the edge of an oxidized graphite flake, showing that the edge recession took part in
the degradation of graphite. It is established that while the basal plane of graphite is
very difficult to oxidize with molecular oxygen, the edges of basal planes are much
more active because of the presence of free bonded atoms and thus oxidize more
readily [29].
Considering all the results, it is known that the oxidation of graphite is, to some
degree, repressed at high heating rates. It can also be realized that the oxidation rate of
carbon nanomaterials decreases when the critical oxidation temperature is traversed
rapidly [18]. This conclusion can be interpreted by taking into account-independent
regimes of graphite oxidation. Findings show that the oxidation of graphite in air
is regulated by chemical reactions at medium temperatures below 600 °C. In contrast, the graphite oxidation is governed by in-pore diffusion at higher temperatures
between 600 and 800 °C and by boundary layer diffusion at temperatures over 800 °C
[30].
As maintained by the observations, heating the mixture of graphite and LiCl
at the heating rate of 80 °C min
−1 to 1250 °C not just induced the evaporation
of LiCl, but also the microstructural modifications in graphite. Correspondingly,
three microstructural features, which were largely unlike from the pristine and the
partially oxidized graphite samples, were recognized in the heat-treated powders.
These microstructures are portrayed in Figs. 3.8 and 3.9 and described accordingly.
Figure 3.8a presents the dominant microstructure made up of bent graphite grains.
These vary from the planar grains of the pristine graphite exhibited in Fig. 3.7a. The
charging effect, which can take place for non-conducting materials, was not able to
be detected in the SEM micrographs. This verifies the nonexistence of LiCl with
the graphite. Along with the lines of this observation, no traces of chlorine were
able to be found by EDX analysis. As a result, the evaporation of LiCl upon the
heat treatment of the graphite–LiCl mixture is apparent. The extensions of Fig. 3.8b
show the microstructure of the identical area in more details, from which it is noticed
that the material maintains a layered microstructure. Using the given observation,
it is apparent to suppose that this microstructure resulted from the intercalation of
LiCl into the pristine graphite and the ensuing exfoliation of graphite. It should be
mentioned that, derived from the thermodynamics and chemical band theory, molten
LiCl is less likely to be intercalated into graphite [31]. Accordingly, it is feasible to
assume that the layered structure of Fig. 3.8 can be formed by the intercalation of LiCl
vapor between the graphite layers. SEM micrographs of Fig. 3.8b show the general
characteristics of the exfoliated nanosheets in a higher magnification. Additionally,
there is evidence present that supports the existence of individual graphene sheets in
the heat-treated samples, for instance as presented in Fig. 3.8c. The figure displays
a TEM micrograph of a folded graphene sheet. The inset of the image presents the
selected area electron diffraction pattern taken from the edge of the sheet showing
the distinctive hexagonal structure of graphene.
3 Interaction of Molten Salts with Graphite
or impurities present on graphite surface [27, 28]. Along with this, some finer particles can additionally be seen in Fig. 3.7c, which could be residue remaining after
partial oxidation of more active components like binders or less graphitized carbon
phases. The inset in Fig. 3.7c is a high-magnification SEM micrograph taken from
the edge of an oxidized graphite flake, showing that the edge recession took part in
the degradation of graphite. It is established that while the basal plane of graphite is
very difficult to oxidize with molecular oxygen, the edges of basal planes are much
more active because of the presence of free bonded atoms and thus oxidize more
readily [29].
Considering all the results, it is known that the oxidation of graphite is, to some
degree, repressed at high heating rates. It can also be realized that the oxidation rate of
carbon nanomaterials decreases when the critical oxidation temperature is traversed
rapidly [18]. This conclusion can be interpreted by taking into account-independent
regimes of graphite oxidation. Findings show that the oxidation of graphite in air
is regulated by chemical reactions at medium temperatures below 600 °C. In contrast, the graphite oxidation is governed by in-pore diffusion at higher temperatures
between 600 and 800 °C and by boundary layer diffusion at temperatures over 800 °C
[30].
As maintained by the observations, heating the mixture of graphite and LiCl
at the heating rate of 80 °C min
−1 to 1250 °C not just induced the evaporation
of LiCl, but also the microstructural modifications in graphite. Correspondingly,
three microstructural features, which were largely unlike from the pristine and the
partially oxidized graphite samples, were recognized in the heat-treated powders.
These microstructures are portrayed in Figs. 3.8 and 3.9 and described accordingly.
Figure 3.8a presents the dominant microstructure made up of bent graphite grains.
These vary from the planar grains of the pristine graphite exhibited in Fig. 3.7a. The
charging effect, which can take place for non-conducting materials, was not able to
be detected in the SEM micrographs. This verifies the nonexistence of LiCl with
the graphite. Along with the lines of this observation, no traces of chlorine were
able to be found by EDX analysis. As a result, the evaporation of LiCl upon the
heat treatment of the graphite–LiCl mixture is apparent. The extensions of Fig. 3.8b
show the microstructure of the identical area in more details, from which it is noticed
that the material maintains a layered microstructure. Using the given observation,
it is apparent to suppose that this microstructure resulted from the intercalation of
LiCl into the pristine graphite and the ensuing exfoliation of graphite. It should be
mentioned that, derived from the thermodynamics and chemical band theory, molten
LiCl is less likely to be intercalated into graphite [31]. Accordingly, it is feasible to
assume that the layered structure of Fig. 3.8 can be formed by the intercalation of LiCl
vapor between the graphite layers. SEM micrographs of Fig. 3.8b show the general
characteristics of the exfoliated nanosheets in a higher magnification. Additionally,
there is evidence present that supports the existence of individual graphene sheets in
the heat-treated samples, for instance as presented in Fig. 3.8c. The figure displays
a TEM micrograph of a folded graphene sheet. The inset of the image presents the
selected area electron diffraction pattern taken from the edge of the sheet showing
the distinctive hexagonal structure of graphene.