154
8 Molten Salt-Assisted Preparation of Nanodiamonds at Atmospheric …
about 50 °C min
−1 . High-resolution electron micrographs of the product (Fig. 8.7c)
revealed the formation of diamond crystals.
As mentioned before, graphite can be transformed into diamond at very high
temperatures and pressures according to the carbon phase diagram. The addition of
specific transitional metals or their alloys can reduce the critical temperature and
pressure required to achieve the diamond nucleation in industry.
It is interesting to consider that natural diamonds are found in carbonaterich rocks or contain carbonate inclusions [85, 86]. Carbonates can catalyze the
oxidation of carbon. On the other hand, graphite–carbonate systems have been
studied in order to simulate the process of natural diamond formation, and their
solvent–catalytic properties have been demonstrated in a number of publications
[4, 87–99]. Palyanov et al. [4] treated capsules containing graphite pieces and Li 2 CO 3
at 7 GPa and 1700 °C, at which Li 2 CO 3 is in a molten state, for 2 h. After cooling down
the system, it was confirmed that diamond nucleation has occurred at the interface
between graphite and carbonate melt. It was further observed that diamond crystals
had octahedral, cubic or trapezohedral crystalline forms, separated from graphite by
carbonate films [4].
Despite the industrial and scientific importance, the mechanisms involved in the
HPHT catalytic transformation of graphitic materials into diamond are still not quite
clear, although in the case of metallic catalysts, it is believed to be based on the
dissolution of carbon from graphite into the molten metal catalyst used [1, 100]. On
the other hand, the detailed mechanisms involved in the diamond formation in carbonate–graphite systems are not available in the literature, which is mostly due to the
lack of sufficient knowledge on basic issues such as behavior of alkaline carbonates
at high pressures and temperatures [101], solubility and diffusion rate of carbon into
carbonates [95] and possible decomposition, carbothermic decomposition [102] or
reduction of carbonates into graphite or diamond [103, 104].
Li 2 CO 3 crystals of less than 30 nm encapsulated in graphitic shells acted as
C-catalyst cells, and the diamond formation was realized within the Li 2 CO 3 phase.
Moreover, octahedral diamond crystals could occasionally be detected in the product.
The overall observations are in line with those reported in the literature concerning
the diamond formation in C–Li 2 CO 3 system [101] with the exception of the core–
shell nanospherical geometry of the cells and the fact that the diamond formation
was achieved at external ambient pressure and a temperature of less than 600 °C.
The formation of diamond crystallites from the core–shell nanostructures formed in
molten salts can be explained based on the carbon-encapsulated morphology of the
Li 2 CO 3 nanocrystals. This morphology facilitates the diffusion of carbon from the
encapsulating graphitic layers into Li 2 CO 3 , leading to the saturation of carbon atoms
in Li 2 CO 3 . This saturation was likely to occur during the processing of the material
in molten LiCl [68]. In addition to the core–shell nanostructure, the presence of LiCl
may play an additional catalytic role in the overall process, particularly with respect
to the fact that alkali metal chlorides can be found in inclusions coated on natural
diamonds [105–107], suggesting the possible involvement of alkali metal chlorides
in the natural diamond formation.
8 Molten Salt-Assisted Preparation of Nanodiamonds at Atmospheric …
about 50 °C min
−1 . High-resolution electron micrographs of the product (Fig. 8.7c)
revealed the formation of diamond crystals.
As mentioned before, graphite can be transformed into diamond at very high
temperatures and pressures according to the carbon phase diagram. The addition of
specific transitional metals or their alloys can reduce the critical temperature and
pressure required to achieve the diamond nucleation in industry.
It is interesting to consider that natural diamonds are found in carbonaterich rocks or contain carbonate inclusions [85, 86]. Carbonates can catalyze the
oxidation of carbon. On the other hand, graphite–carbonate systems have been
studied in order to simulate the process of natural diamond formation, and their
solvent–catalytic properties have been demonstrated in a number of publications
[4, 87–99]. Palyanov et al. [4] treated capsules containing graphite pieces and Li 2 CO 3
at 7 GPa and 1700 °C, at which Li 2 CO 3 is in a molten state, for 2 h. After cooling down
the system, it was confirmed that diamond nucleation has occurred at the interface
between graphite and carbonate melt. It was further observed that diamond crystals
had octahedral, cubic or trapezohedral crystalline forms, separated from graphite by
carbonate films [4].
Despite the industrial and scientific importance, the mechanisms involved in the
HPHT catalytic transformation of graphitic materials into diamond are still not quite
clear, although in the case of metallic catalysts, it is believed to be based on the
dissolution of carbon from graphite into the molten metal catalyst used [1, 100]. On
the other hand, the detailed mechanisms involved in the diamond formation in carbonate–graphite systems are not available in the literature, which is mostly due to the
lack of sufficient knowledge on basic issues such as behavior of alkaline carbonates
at high pressures and temperatures [101], solubility and diffusion rate of carbon into
carbonates [95] and possible decomposition, carbothermic decomposition [102] or
reduction of carbonates into graphite or diamond [103, 104].
Li 2 CO 3 crystals of less than 30 nm encapsulated in graphitic shells acted as
C-catalyst cells, and the diamond formation was realized within the Li 2 CO 3 phase.
Moreover, octahedral diamond crystals could occasionally be detected in the product.
The overall observations are in line with those reported in the literature concerning
the diamond formation in C–Li 2 CO 3 system [101] with the exception of the core–
shell nanospherical geometry of the cells and the fact that the diamond formation
was achieved at external ambient pressure and a temperature of less than 600 °C.
The formation of diamond crystallites from the core–shell nanostructures formed in
molten salts can be explained based on the carbon-encapsulated morphology of the
Li 2 CO 3 nanocrystals. This morphology facilitates the diffusion of carbon from the
encapsulating graphitic layers into Li 2 CO 3 , leading to the saturation of carbon atoms
in Li 2 CO 3 . This saturation was likely to occur during the processing of the material
in molten LiCl [68]. In addition to the core–shell nanostructure, the presence of LiCl
may play an additional catalytic role in the overall process, particularly with respect
to the fact that alkali metal chlorides can be found in inclusions coated on natural
diamonds [105–107], suggesting the possible involvement of alkali metal chlorides
in the natural diamond formation.