20
3 Interaction of Molten Salts with Graphite
it was demonstrated that graphite in contact with molten salts undergoes degradation and considerable attack on the surface, and therefore, protective coatings have
been suggested for the prevention of corrosion [10, 11]. Bernardet et al. studied the
reactivity of nuclear graphite materials toward a LiF–NaF–ZrF 4 ternary molten salt
at 500 °C and indicated that molten salt, to some extent, penetrates in the graphite
through the surface porosity. Nevertheless, no structural disintegration of the graphite
materials was observed [10].
In contrast with these examples, there are molten salt technologies in which the
electrochemical reactivity of molten salts with graphite materials is desirable, leading
to the preparation of carbon nanostructures. The electrochemical corrosion, reaction
and/or exfoliation of graphite in molten salts [12, 13] are discussed in the Chap. 4.
Since new molten salt technologies are becoming more important, the reactivity of
graphite with molten salts at different conditions should be carefully investigated.
From this perspective, this chapter concerns the corrosion of graphite in molten salts.
Particularly, it is interesting to explore the feasibility of using molten salt corrosion
of graphite as a low-cost top-down approach for the production of carbon nanomaterials, including CNTs, carbon nanorods, graphene and graphene-based nanosheets.
Therefore, it would be of both fundamental and practical interests to explore whether
molten salt corrosion of graphite can be considered as a method for the production
of carbon nanomaterials. In the following sections, the structural and microstructural
changes which might occur during non-isothermal heating of graphite in the air with
and without the involvement of molten salts are discussed [14].
3.1 Thermal Analysis of Pristine Graphite Powder
Figure 3.1 exhibits the SEM micrograph as well as the differential scanning calorimetry (DSC) and thermal gravimetry (TG) thermographs performed on a pristine
graphite powder (EVC, Morgan USA) at varying heating rates, ranging from 20
to 80 °C min
−1 under an ambient airflow of 100 mL min
−1 . The DSC thermograms
are plotted so that a downward peak corresponds to an exothermic event. The DSC
thermogram recorded at the heating rate of 20 °C min
−1 shows a wide exothermic
peak with the maximum at 900 °C which is because of the oxidation of graphite.
The TG curve, Fig. 3.1c, demonstrates that the oxidation begins at around 630 °C
and ends at around 950 °C, resulting in a mass loss close to 98%. To characterize
the final residue, the pristine graphite powder was heated in a tube furnace with
similar conditions to the TG furnace, and the remaining ash was analyzed by EDX.
An outline of the analytical data is shown in Table 3.1.
The DSC thermogram of the pristine graphite powder acquired at the heating
rate of 40 °C min
−1 , Fig. 3.1b, reveals an even immense oxidation peak with the
maximum at 1018 °C [14]. The analogous TG curve, Fig. 3.1c, shows that the quick
oxidation of the graphite material took place at temperatures above 640 °C and was
3 Interaction of Molten Salts with Graphite
it was demonstrated that graphite in contact with molten salts undergoes degradation and considerable attack on the surface, and therefore, protective coatings have
been suggested for the prevention of corrosion [10, 11]. Bernardet et al. studied the
reactivity of nuclear graphite materials toward a LiF–NaF–ZrF 4 ternary molten salt
at 500 °C and indicated that molten salt, to some extent, penetrates in the graphite
through the surface porosity. Nevertheless, no structural disintegration of the graphite
materials was observed [10].
In contrast with these examples, there are molten salt technologies in which the
electrochemical reactivity of molten salts with graphite materials is desirable, leading
to the preparation of carbon nanostructures. The electrochemical corrosion, reaction
and/or exfoliation of graphite in molten salts [12, 13] are discussed in the Chap. 4.
Since new molten salt technologies are becoming more important, the reactivity of
graphite with molten salts at different conditions should be carefully investigated.
From this perspective, this chapter concerns the corrosion of graphite in molten salts.
Particularly, it is interesting to explore the feasibility of using molten salt corrosion
of graphite as a low-cost top-down approach for the production of carbon nanomaterials, including CNTs, carbon nanorods, graphene and graphene-based nanosheets.
Therefore, it would be of both fundamental and practical interests to explore whether
molten salt corrosion of graphite can be considered as a method for the production
of carbon nanomaterials. In the following sections, the structural and microstructural
changes which might occur during non-isothermal heating of graphite in the air with
and without the involvement of molten salts are discussed [14].
3.1 Thermal Analysis of Pristine Graphite Powder
Figure 3.1 exhibits the SEM micrograph as well as the differential scanning calorimetry (DSC) and thermal gravimetry (TG) thermographs performed on a pristine
graphite powder (EVC, Morgan USA) at varying heating rates, ranging from 20
to 80 °C min
−1 under an ambient airflow of 100 mL min
−1 . The DSC thermograms
are plotted so that a downward peak corresponds to an exothermic event. The DSC
thermogram recorded at the heating rate of 20 °C min
−1 shows a wide exothermic
peak with the maximum at 900 °C which is because of the oxidation of graphite.
The TG curve, Fig. 3.1c, demonstrates that the oxidation begins at around 630 °C
and ends at around 950 °C, resulting in a mass loss close to 98%. To characterize
the final residue, the pristine graphite powder was heated in a tube furnace with
similar conditions to the TG furnace, and the remaining ash was analyzed by EDX.
An outline of the analytical data is shown in Table 3.1.
The DSC thermogram of the pristine graphite powder acquired at the heating
rate of 40 °C min
−1 , Fig. 3.1b, reveals an even immense oxidation peak with the
maximum at 1018 °C [14]. The analogous TG curve, Fig. 3.1c, shows that the quick
oxidation of the graphite material took place at temperatures above 640 °C and was