152
8 Molten Salt-Assisted Preparation of Nanodiamonds at Atmospheric …
based on the laser irradiation, electron irradiation or hydrogen plasma treating of carbon nanostructures, although interesting, are microscale techniques with very limited
capability at larger scales. It is worth mentioning that with increasing shortage of
natural diamonds, there is an increasing demand for simpler and more economical
bulk diamond production methods.
As discussed in 4.6, carbon dioxide can be converted into Li 2 CO 3 nanosingle
crystals with particle sizes below 30 nm using a reactive LiCl–Li 2 O molten salt
method. The Li 2 CO 3 nanocrystals can then in situ encapsulated into carbon cages.
The carbon-encapsulated Li 2 CO 3 nanoparticles could act as self-pressurized diamond nanocatalysts upon heating under ambient external pressure to nucleate diamond crystallites within encapsulated Li 2 CO 3 crystals. The diamond crystallites
could grow to micrometer-sized octahedral crystals by further heating. It is suggested
that the very high pressure required for the catalytic phase transition of graphite to
diamond can be created in carbon-encapsulated Li 2 CO 3 nanocrystals. These “highpressure nanovessels” enable the nucleation of diamond crystallites upon heating in
atmospheric pressure of air, representing a much less severe nucleation condition than
those used in conventional technologies. Interestingly, these high-pressure nanovessels were fabricated in the consumption of CO 2 , which is regarded as the main source
of global warming and ocean acidification, with catastrophic consequences [68].
The experimental reactor used for fabrication of Li 2 CO 3 nanocrystals and carbonencapsulated Li 2 CO 3 nanoparticles is presented in Fig. 4.15a. The reactions taking
place in the reactor can be explained as follows. A moist CO 2 gas flow is directed
into the molten LiCl–2 wt% Li 2 O at 800 °C, resulting in triggering the reaction (8.3)
and hence the formation of lithium carbonate.
CO 2(taken from atmosphere) + Li 2 O (from the melt) = Li 2 CO 3 G
◦
(at 800
◦ C) = −66 kJ
(8.3)
Apart from this, the hydrolysis reaction occurs between water from the moist gas
and molten LiCl, described by the reaction (8.4).
H 2 O + LiCl = HCl + LiOH G
◦
at 800
◦ C
= 86 kJ
(8.4)
LiOH produced is then decomposed to form Li 2 O. It should be mentioned that
LiCl shows no considerable affinity for hydrolysis in the solid form due to the energy
barrier involved. However, the hydrolysis of LiCl becomes much more significant
in the molten state, because of the fact that the hydrolysis products (HCl [69–84]
and Li 2 O [75]) are readily soluble in molten LiCl. As a consequence, the hydrolysis
reaction can proceed, although its standard Gibbs free energy is positive. Since LiCl
is highly hygroscopic, the formation of HCl and Li 2 O during the practical use of
molten LiCl is common [76]. This reaction is likely to contribute to the diverse
detrimental or beneficial effects associated with the corrosion of metallic equipment
[77] or fabrication of high-value materials such as LiNbO 3 [78].
Lithium carbonate nanocrystals formed in the process explained above were
encapsulated in graphitic layers, by using a graphite cathode as the carbon source
8 Molten Salt-Assisted Preparation of Nanodiamonds at Atmospheric …
based on the laser irradiation, electron irradiation or hydrogen plasma treating of carbon nanostructures, although interesting, are microscale techniques with very limited
capability at larger scales. It is worth mentioning that with increasing shortage of
natural diamonds, there is an increasing demand for simpler and more economical
bulk diamond production methods.
As discussed in 4.6, carbon dioxide can be converted into Li 2 CO 3 nanosingle
crystals with particle sizes below 30 nm using a reactive LiCl–Li 2 O molten salt
method. The Li 2 CO 3 nanocrystals can then in situ encapsulated into carbon cages.
The carbon-encapsulated Li 2 CO 3 nanoparticles could act as self-pressurized diamond nanocatalysts upon heating under ambient external pressure to nucleate diamond crystallites within encapsulated Li 2 CO 3 crystals. The diamond crystallites
could grow to micrometer-sized octahedral crystals by further heating. It is suggested
that the very high pressure required for the catalytic phase transition of graphite to
diamond can be created in carbon-encapsulated Li 2 CO 3 nanocrystals. These “highpressure nanovessels” enable the nucleation of diamond crystallites upon heating in
atmospheric pressure of air, representing a much less severe nucleation condition than
those used in conventional technologies. Interestingly, these high-pressure nanovessels were fabricated in the consumption of CO 2 , which is regarded as the main source
of global warming and ocean acidification, with catastrophic consequences [68].
The experimental reactor used for fabrication of Li 2 CO 3 nanocrystals and carbonencapsulated Li 2 CO 3 nanoparticles is presented in Fig. 4.15a. The reactions taking
place in the reactor can be explained as follows. A moist CO 2 gas flow is directed
into the molten LiCl–2 wt% Li 2 O at 800 °C, resulting in triggering the reaction (8.3)
and hence the formation of lithium carbonate.
CO 2(taken from atmosphere) + Li 2 O (from the melt) = Li 2 CO 3 G
◦
(at 800
◦ C) = −66 kJ
(8.3)
Apart from this, the hydrolysis reaction occurs between water from the moist gas
and molten LiCl, described by the reaction (8.4).
H 2 O + LiCl = HCl + LiOH G
◦
at 800
◦ C
= 86 kJ
(8.4)
LiOH produced is then decomposed to form Li 2 O. It should be mentioned that
LiCl shows no considerable affinity for hydrolysis in the solid form due to the energy
barrier involved. However, the hydrolysis of LiCl becomes much more significant
in the molten state, because of the fact that the hydrolysis products (HCl [69–84]
and Li 2 O [75]) are readily soluble in molten LiCl. As a consequence, the hydrolysis
reaction can proceed, although its standard Gibbs free energy is positive. Since LiCl
is highly hygroscopic, the formation of HCl and Li 2 O during the practical use of
molten LiCl is common [76]. This reaction is likely to contribute to the diverse
detrimental or beneficial effects associated with the corrosion of metallic equipment
[77] or fabrication of high-value materials such as LiNbO 3 [78].
Lithium carbonate nanocrystals formed in the process explained above were
encapsulated in graphitic layers, by using a graphite cathode as the carbon source