2
1 Introduction
graphite-lined steel containers. In such a cell, a very high current, typically 150,000
amperes, is passed between a carbon anode, made of petroleum coke and pitch, and
a cathode, formed by the thick carbon or graphite-lined steel container, in which
molten Al is electrochemically deposited [3]. The chemical stability of graphite in
contact with the molten salt is a key technical issue in an Al smelter.
The application of molten salts in the extractive metallurgy is based on their capability of being thermally ionized to form metal cations, which can then be reduced to
the corresponding metals upon electrolysis. However, molten salts could offer more
than this. In the 1940s, the Oak Ridge National Laboratory began to investigate the
possibility of using fluid fuels in nuclear reactors to promote its Aircraft Nuclear
Propulsion plan. Molten fluoride salts appeared particularly interesting because they
have high solubility for uranium, chemical stability, low vapor pressure at high temperatures and reasonably high heat transfer properties. Moreover, these salts are not
damaged by radiation, do not react violently with air or water, and can be relatively
inert to structural metals [4]. Today, molten salt reactors use molten fluoride salts as
primary coolant at low pressure. In the fourth-generation molten salt breeder nuclear
reactors, a molten fluoride salt, used as both fuel and coolant, flows between the
graphite reactor core and the heat exchanger [5, 6]. In this application, the possible
interaction between molten salts and graphite is of importance.
In 2000, another considerable breakthrough was reported from Derek Fray’s group
at University of Cambridge. They reported that the oxygen from metal oxides cathodically charged in molten CaCl 2 can be ionized and dissolved in the molten salt and
then discharged at the anode, leaving pure metal at the cathode [7]. Other chloride
salts such as LiCl may also be used for the extraction of nuclear metals [8].
In these so-called FFC-based methods, graphite crucibles and graphite anodes are
often employed for the electroreduction of metal oxide cathodes. Graphite is one
of the few choices in these applications because of its relatively high physical and
chemical stability and electrical conductivity. The other candidates such as platinum
are very expensive to be employed. The use of graphite, particularly as the anode,
however, is a main source of metal contamination in FFC processes. Therefore, there
is an active research line to replace graphite with the so-called inert anodes.
In all discoveries mentioned above, graphite is used as a relatively reliable structural material or electrode, and is not supposed to be included in the final product.
In 1995, another discovery was reported from Hsu and his co-workers from University of Sussex, which expanded the applications of molten salts from metallurgy
and nuclear industry to materials science. It was reported that a mixture of curved
multiwalled carbon nanotubes and clustered carbon nanoparticles can be generated
from a graphite cathode in the electrolysis of molten lithium chloride [9]. Fundamental investigations, carried out mostly in University of Cambridge during 2002–2009
[10–14], suggested that the molten salt electrolytic formation of carbon nanomaterials commences with the intercalation of alkali metals (Li or Na) from the molten
LiCl or NaCl electrolyte into the graphite cathode. This is followed by the breakdown
of the graphite material into a variety of mostly nanostructured carbon constituents.
The scalability of the molten salt approach was demonstrated in subsequent investigations [15]. It should be mentioned that the electrochemical Li
+ intercalation and
1 Introduction
graphite-lined steel containers. In such a cell, a very high current, typically 150,000
amperes, is passed between a carbon anode, made of petroleum coke and pitch, and
a cathode, formed by the thick carbon or graphite-lined steel container, in which
molten Al is electrochemically deposited [3]. The chemical stability of graphite in
contact with the molten salt is a key technical issue in an Al smelter.
The application of molten salts in the extractive metallurgy is based on their capability of being thermally ionized to form metal cations, which can then be reduced to
the corresponding metals upon electrolysis. However, molten salts could offer more
than this. In the 1940s, the Oak Ridge National Laboratory began to investigate the
possibility of using fluid fuels in nuclear reactors to promote its Aircraft Nuclear
Propulsion plan. Molten fluoride salts appeared particularly interesting because they
have high solubility for uranium, chemical stability, low vapor pressure at high temperatures and reasonably high heat transfer properties. Moreover, these salts are not
damaged by radiation, do not react violently with air or water, and can be relatively
inert to structural metals [4]. Today, molten salt reactors use molten fluoride salts as
primary coolant at low pressure. In the fourth-generation molten salt breeder nuclear
reactors, a molten fluoride salt, used as both fuel and coolant, flows between the
graphite reactor core and the heat exchanger [5, 6]. In this application, the possible
interaction between molten salts and graphite is of importance.
In 2000, another considerable breakthrough was reported from Derek Fray’s group
at University of Cambridge. They reported that the oxygen from metal oxides cathodically charged in molten CaCl 2 can be ionized and dissolved in the molten salt and
then discharged at the anode, leaving pure metal at the cathode [7]. Other chloride
salts such as LiCl may also be used for the extraction of nuclear metals [8].
In these so-called FFC-based methods, graphite crucibles and graphite anodes are
often employed for the electroreduction of metal oxide cathodes. Graphite is one
of the few choices in these applications because of its relatively high physical and
chemical stability and electrical conductivity. The other candidates such as platinum
are very expensive to be employed. The use of graphite, particularly as the anode,
however, is a main source of metal contamination in FFC processes. Therefore, there
is an active research line to replace graphite with the so-called inert anodes.
In all discoveries mentioned above, graphite is used as a relatively reliable structural material or electrode, and is not supposed to be included in the final product.
In 1995, another discovery was reported from Hsu and his co-workers from University of Sussex, which expanded the applications of molten salts from metallurgy
and nuclear industry to materials science. It was reported that a mixture of curved
multiwalled carbon nanotubes and clustered carbon nanoparticles can be generated
from a graphite cathode in the electrolysis of molten lithium chloride [9]. Fundamental investigations, carried out mostly in University of Cambridge during 2002–2009
[10–14], suggested that the molten salt electrolytic formation of carbon nanomaterials commences with the intercalation of alkali metals (Li or Na) from the molten
LiCl or NaCl electrolyte into the graphite cathode. This is followed by the breakdown
of the graphite material into a variety of mostly nanostructured carbon constituents.
The scalability of the molten salt approach was demonstrated in subsequent investigations [15]. It should be mentioned that the electrochemical Li
+ intercalation and