8.2 Conversion of Carbon Nanostructures into Nanodiamonds
151
material in air at atmospheric pressure. It should be noted that although the CNTs
and nanoparticles are ignited at about 420 °C, registered in the DSC curve of Fig. 8.5,
the true local temperature of the carbon nanomaterials during the oxidation is likely
to exceed 4000 °C. At this temperature, the Li 2 CO 3 encapsulated in graphitic shells
is also likely to produce a considerable amount of internal pressure. This process
should be much cheaper than the HPHT approach and should be scalable to produce
a more economical product. The applied voltages and current densities and the rate
of production of carbon nanotubes in molten LiCl [41] are very similar to that of
aluminum in the Hall–Heroult cells which produce 45 M tonnes per annum worldwide. Upscaling should not be a problem. The cost of aluminum is around $2/kg,
while the present price of nanodiamonds is about $3/g. Using this novel technique
should result in a substantial drop in the cost of nanodiamonds and a widening of the
applications [39].
8.3 Conversion of CO 2 into Diamond Nanocrystals
Carbon is an interesting element due to its existence in more than ten million compounds [42] such as hydrocarbons, carbonates and CO 2 as well as various elemental
allotropes such as amorphous carbon, graphite and diamond. Among these, CO 2 is
considered to be a major greenhouse gas, contributing to global warming and climate change [43], and hence reduction in CO 2 emissions is currently a global effort
[44–46].
Along with this fact that the global generation of CO 2 may not be significantly
suppressed in the near future, the capture of CO 2 has increasingly been studied,
exploring the absorption of CO 2 by various materials such as Zr(OH) 4 [47], CaO
[48], TiO 2 [49], SiO 2 [50], Fe 2 O 3 [51], carbon nanostructures [52], activated carbon
[53], activated biocarbons [54], organic materials [55] and ionic liquids [56, 57].
Moreover, the conversion of CO 2 emissions produced into useful materials such as
hydrocarbon fuels [58], CO [59], O 2 [60], acids [61, 62] and other chemicals [63]
such as dimethyl carbonate [64] has been considered as a viable approach.
As discussed, diamond is considered as one of the most valuable and rather expensive materials with remarkable properties including the highest known hardness, thermal conductivity and chemical resistance [65]. Mentioned in the previous section, the
current technologies of diamond nucleation and growth, which are based on applying
huge external pressures and heats, are very complicated and expensive requiring the
use of enormously large hot presses weighing hundreds of tons [19]. Nanodiamonds
can also be fabricated by massive detonation of carbonaceous explosives in closed
metallic chambers, in which the pressure and temperature may rise instantaneously
to more than 25 GPa and 3500 °C, respectively [66]. It should be noted that in order to
accommodate an explosion arisen from only 20 kg of such explosives, a thick metallic chambers weighing over 100 tons may be required [67], indicating a high level
of complexity, particularly at larger scales. The other methods of forming diamonds,
151
material in air at atmospheric pressure. It should be noted that although the CNTs
and nanoparticles are ignited at about 420 °C, registered in the DSC curve of Fig. 8.5,
the true local temperature of the carbon nanomaterials during the oxidation is likely
to exceed 4000 °C. At this temperature, the Li 2 CO 3 encapsulated in graphitic shells
is also likely to produce a considerable amount of internal pressure. This process
should be much cheaper than the HPHT approach and should be scalable to produce
a more economical product. The applied voltages and current densities and the rate
of production of carbon nanotubes in molten LiCl [41] are very similar to that of
aluminum in the Hall–Heroult cells which produce 45 M tonnes per annum worldwide. Upscaling should not be a problem. The cost of aluminum is around $2/kg,
while the present price of nanodiamonds is about $3/g. Using this novel technique
should result in a substantial drop in the cost of nanodiamonds and a widening of the
applications [39].
8.3 Conversion of CO 2 into Diamond Nanocrystals
Carbon is an interesting element due to its existence in more than ten million compounds [42] such as hydrocarbons, carbonates and CO 2 as well as various elemental
allotropes such as amorphous carbon, graphite and diamond. Among these, CO 2 is
considered to be a major greenhouse gas, contributing to global warming and climate change [43], and hence reduction in CO 2 emissions is currently a global effort
[44–46].
Along with this fact that the global generation of CO 2 may not be significantly
suppressed in the near future, the capture of CO 2 has increasingly been studied,
exploring the absorption of CO 2 by various materials such as Zr(OH) 4 [47], CaO
[48], TiO 2 [49], SiO 2 [50], Fe 2 O 3 [51], carbon nanostructures [52], activated carbon
[53], activated biocarbons [54], organic materials [55] and ionic liquids [56, 57].
Moreover, the conversion of CO 2 emissions produced into useful materials such as
hydrocarbon fuels [58], CO [59], O 2 [60], acids [61, 62] and other chemicals [63]
such as dimethyl carbonate [64] has been considered as a viable approach.
As discussed, diamond is considered as one of the most valuable and rather expensive materials with remarkable properties including the highest known hardness, thermal conductivity and chemical resistance [65]. Mentioned in the previous section, the
current technologies of diamond nucleation and growth, which are based on applying
huge external pressures and heats, are very complicated and expensive requiring the
use of enormously large hot presses weighing hundreds of tons [19]. Nanodiamonds
can also be fabricated by massive detonation of carbonaceous explosives in closed
metallic chambers, in which the pressure and temperature may rise instantaneously
to more than 25 GPa and 3500 °C, respectively [66]. It should be noted that in order to
accommodate an explosion arisen from only 20 kg of such explosives, a thick metallic chambers weighing over 100 tons may be required [67], indicating a high level
of complexity, particularly at larger scales. The other methods of forming diamonds,