6
2 Production of Advanced Materials in Molten Salts
In the last twenty years, molten salt-based processes have provided exciting new
opportunities for the preparation of various valuable materials which are difficult
and/or expensive to be fabricated by their alternative fabrication methods. These
include the molten salt production of metallic materials such as Ti [6], Nb [7], Cr
[8], W [9], U [10], Ni [11], Fe [12], V [13], Si [14] as well as other materials including
intermetallic compounds such as TiAl 3 [15], ZrSi and ZrSi 2 [16], nanocomposites
such as ZrC/ZrSi [17], complex alloys such as La(Ni 0.7 Co 0.3 ) 5 [18], ceramics such
SnO 2 nanostructures [19, 20], LiNbO 3 [21] and Cr 2 AlC nanostructures [22].
In addition to these, molten salt methods have created various strategies for the
preparation of carbon nanostructures. The focus of this book is to discuss the possible interactions between molten salts and carbonaceous materials leading to the
fabrication of valuable carbon nanostructures. In contact with carbonaceous materials, molten salts can either be relatively inert or reactive. Both behaviors have been
employed for the preparation of carbon or carbide nanomaterials. In this chapter, an
overview on these molten salt techniques is presented.
2.1 Inert Molten Salt Synthesis Methods of Carbon
Nanostructures
Molten salts can provide an inert medium for the occurrence of reactions without
becoming directly involved in the chemical or electrochemical interactions. In this
case, the use of molten salts is beneficial since they can provide a uniform ionically
conductive heating media in which reactive species may show an enhanced reactivity,
leading to a significant promotion of reaction kinetics. Cui et al. [23] used the eutectic
KCl–LiCl molten salt at 800–950 °C as the medium to conduct the reaction between
CVD-produced multiwall carbon nanotubes (MWCNT) and Ta powder leading to
the formation of tantalum carbide (TaC) nanofibers. In a different research, molten
KCl–LiCl was found to be an ideal environment for the carbonization of ZIF-8
polymers, which subsequently led to the formation of 2D N-doped amorphous carbon
nanosheets [24]. Here, the salt was proposed to act as a template to form 2D carbons
(Fig. 2.1).
Likewise, Li et al. [25] used a template concept in which molten LiCl–KCl–KF
salt system acted as the medium, supporting the reaction between transition metals
with carbon nanotubes, resulting in the formation of metal carbide nanofibers. Similarly, in another study, low-graphitized nitrogen-doped carbon hollow cubes could be
produced by carbonization of biomass L-lysine monohydrochloride (C 6 H 15 O 2 N 2 Cl)
in molten NaCl at 1000 °C (Fig. 2.2) [26].
Nita et al. [27] produced porous carbon materials by treatment of organic carbonaceous precursors in molten KCl, NaCl and LiCl. They suggested that the molten salt
can play an important role in textural properties of the carbon material produced. Ding
et al. [28] produced nitrogen-doped carbon material with a large number of active
sites by carbonization of PANI in molten NaCl. They found that there is a close
morphological relationship between the raw materials and the products (Fig. 2.3).
2 Production of Advanced Materials in Molten Salts
In the last twenty years, molten salt-based processes have provided exciting new
opportunities for the preparation of various valuable materials which are difficult
and/or expensive to be fabricated by their alternative fabrication methods. These
include the molten salt production of metallic materials such as Ti [6], Nb [7], Cr
[8], W [9], U [10], Ni [11], Fe [12], V [13], Si [14] as well as other materials including
intermetallic compounds such as TiAl 3 [15], ZrSi and ZrSi 2 [16], nanocomposites
such as ZrC/ZrSi [17], complex alloys such as La(Ni 0.7 Co 0.3 ) 5 [18], ceramics such
SnO 2 nanostructures [19, 20], LiNbO 3 [21] and Cr 2 AlC nanostructures [22].
In addition to these, molten salt methods have created various strategies for the
preparation of carbon nanostructures. The focus of this book is to discuss the possible interactions between molten salts and carbonaceous materials leading to the
fabrication of valuable carbon nanostructures. In contact with carbonaceous materials, molten salts can either be relatively inert or reactive. Both behaviors have been
employed for the preparation of carbon or carbide nanomaterials. In this chapter, an
overview on these molten salt techniques is presented.
2.1 Inert Molten Salt Synthesis Methods of Carbon
Nanostructures
Molten salts can provide an inert medium for the occurrence of reactions without
becoming directly involved in the chemical or electrochemical interactions. In this
case, the use of molten salts is beneficial since they can provide a uniform ionically
conductive heating media in which reactive species may show an enhanced reactivity,
leading to a significant promotion of reaction kinetics. Cui et al. [23] used the eutectic
KCl–LiCl molten salt at 800–950 °C as the medium to conduct the reaction between
CVD-produced multiwall carbon nanotubes (MWCNT) and Ta powder leading to
the formation of tantalum carbide (TaC) nanofibers. In a different research, molten
KCl–LiCl was found to be an ideal environment for the carbonization of ZIF-8
polymers, which subsequently led to the formation of 2D N-doped amorphous carbon
nanosheets [24]. Here, the salt was proposed to act as a template to form 2D carbons
(Fig. 2.1).
Likewise, Li et al. [25] used a template concept in which molten LiCl–KCl–KF
salt system acted as the medium, supporting the reaction between transition metals
with carbon nanotubes, resulting in the formation of metal carbide nanofibers. Similarly, in another study, low-graphitized nitrogen-doped carbon hollow cubes could be
produced by carbonization of biomass L-lysine monohydrochloride (C 6 H 15 O 2 N 2 Cl)
in molten NaCl at 1000 °C (Fig. 2.2) [26].
Nita et al. [27] produced porous carbon materials by treatment of organic carbonaceous precursors in molten KCl, NaCl and LiCl. They suggested that the molten salt
can play an important role in textural properties of the carbon material produced. Ding
et al. [28] produced nitrogen-doped carbon material with a large number of active
sites by carbonization of PANI in molten NaCl. They found that there is a close
morphological relationship between the raw materials and the products (Fig. 2.3).