34
3 Interaction of Molten Salts with Graphite
The reason why different carbon nanostructures are able to be formed in the same
sample is for the presence of local reactions in the system. It is worth noting that
the reaction of molten LiCl with the atmospheric moisture occurs at the melt/gas
interface. This reaction results in the formation of lithium oxides which eventually
react with the neighboring graphite surfaces to create lithium-carbon-oxide components bringing about the formation of corrosion pits. The lithium-carbon-oxides then
are portrayed as the catalysts to create carbon nanorods. As a result, the formation
of corrosion pits and carbon nanorods, as it can be observed in Figs. 3.9 and 3.10,
respectively, can be premeditated as local effects inducing contrasting microstructures. Meanwhile, the dominant microstructure incorporates layered grains such as
displayed in Fig. 3.8.
As mentioned previously, the microstructures shown in Figs. 3.8, 3.9 and 3.10
were brought by the attentive characterization of the graphite–LiCl mixture heated
at the rate of 80 °C min
−1 to 1250 °C. It should be noted that alike microstructures
could be observed in the samples heated at other heating rates (10–90 °C min
−1 )
as well. The interaction between molten LiCl and graphite can be considered as a
straightforward and fast approach for the synthesis of carbon nanostructures, having
the benefits of being inexpensive to produce and ascendable [14].
As a summary, the heating of the graphite powder at 20 °C min
−1 to 900 °C brings
about the intensive disintegration of graphite fakes into extremely fine fragments
because of the oxidation process. The intense oxidation of graphite powder can be
lowered by accelerating the heating rate to 80 °C min
−1 . In the latter case, the material
could be heated to a high temperature of 1250 °C in air atmosphere devoid of having
undergone the entirety of the oxidation. The heating of a mixture of graphite–LiCl
to 1250 °C results in the structural and microstructural modifications to the graphite.
The intense oxidation of graphite could be stopped by the combination of the high
heating rate employed, and the protective action brought about by molten LiCl.
Additionally, the mean crystallite size of the graphite rises notably upon molten salt
heat treatment. Three clearly contrasting carbon microstructures can be identified
in the heat-treated graphite powder incorporating of exfoliated carbon sheets and
nanosheets, pitted particles and carbon nanorods. These microstructural alterations
are implied to be attributable to (a) the intercalation of LiCl vapor into the graphite
structure, (b) the reaction of lithium oxides formed in the melt with graphite and (c)
the catalytic decomposition of carbon monoxide, formed by the oxidation of graphite.
Moreover, the evaporation of LiCl from the graphite–LiCl mixture was found to have
a greater activation energy in comparison with that of LiCl. It was assigned to the
interfacial adhesion energy between molten LiCl and graphite. These findings are
of interest for further development of emerging molten salt-based technologies for
the preparation of nanostructured carbon materials. The electrochemical erosion of
graphite in molten salts is discussed in the Chap. 4.
3 Interaction of Molten Salts with Graphite
The reason why different carbon nanostructures are able to be formed in the same
sample is for the presence of local reactions in the system. It is worth noting that
the reaction of molten LiCl with the atmospheric moisture occurs at the melt/gas
interface. This reaction results in the formation of lithium oxides which eventually
react with the neighboring graphite surfaces to create lithium-carbon-oxide components bringing about the formation of corrosion pits. The lithium-carbon-oxides then
are portrayed as the catalysts to create carbon nanorods. As a result, the formation
of corrosion pits and carbon nanorods, as it can be observed in Figs. 3.9 and 3.10,
respectively, can be premeditated as local effects inducing contrasting microstructures. Meanwhile, the dominant microstructure incorporates layered grains such as
displayed in Fig. 3.8.
As mentioned previously, the microstructures shown in Figs. 3.8, 3.9 and 3.10
were brought by the attentive characterization of the graphite–LiCl mixture heated
at the rate of 80 °C min
−1 to 1250 °C. It should be noted that alike microstructures
could be observed in the samples heated at other heating rates (10–90 °C min
−1 )
as well. The interaction between molten LiCl and graphite can be considered as a
straightforward and fast approach for the synthesis of carbon nanostructures, having
the benefits of being inexpensive to produce and ascendable [14].
As a summary, the heating of the graphite powder at 20 °C min
−1 to 900 °C brings
about the intensive disintegration of graphite fakes into extremely fine fragments
because of the oxidation process. The intense oxidation of graphite powder can be
lowered by accelerating the heating rate to 80 °C min
−1 . In the latter case, the material
could be heated to a high temperature of 1250 °C in air atmosphere devoid of having
undergone the entirety of the oxidation. The heating of a mixture of graphite–LiCl
to 1250 °C results in the structural and microstructural modifications to the graphite.
The intense oxidation of graphite could be stopped by the combination of the high
heating rate employed, and the protective action brought about by molten LiCl.
Additionally, the mean crystallite size of the graphite rises notably upon molten salt
heat treatment. Three clearly contrasting carbon microstructures can be identified
in the heat-treated graphite powder incorporating of exfoliated carbon sheets and
nanosheets, pitted particles and carbon nanorods. These microstructural alterations
are implied to be attributable to (a) the intercalation of LiCl vapor into the graphite
structure, (b) the reaction of lithium oxides formed in the melt with graphite and (c)
the catalytic decomposition of carbon monoxide, formed by the oxidation of graphite.
Moreover, the evaporation of LiCl from the graphite–LiCl mixture was found to have
a greater activation energy in comparison with that of LiCl. It was assigned to the
interfacial adhesion energy between molten LiCl and graphite. These findings are
of interest for further development of emerging molten salt-based technologies for
the preparation of nanostructured carbon materials. The electrochemical erosion of
graphite in molten salts is discussed in the Chap. 4.