8.1 Transformation of Graphite into Diamond
143
8.1 Transformation of Graphite into Diamond
Since diamond is obviously far more valuable than graphite, the transformation of
graphite to diamond has been subjected to a large number of studies [2–14]. Diamond
crystals in nano- and micrometer size range have myriad applications in abrasives,
cutting and polishing tools, biomedicine, nanocomposites, and as seeding material
for the growth of diamond [13, 15].
The phase diagram of carbon (Fig. 8.1c) shows that diamond is the stable phase
if the pressure is beyond 1.7 GPa at room temperature although, once formed, it
is stable under ambient conditions. The industrial production of diamond crystallites is currently achievable by applying either a combination of enormous external
pressure and heat on graphite or graphite–catalyst systems or detonation of explosive
carbonaceous materials. The solid–solid phase transformation of hexagonal graphiteto-cubic diamond can be achieved by the so-called high pressure, high temperature
(HPHT) method at P > 12 GPa and T > 2000 °C [16]. Graphite can alternatively be
dissolved in a catalyst from which the solute carbon atoms reprecipitate as diamond
at slightly less severe conditions. Various catalysts such as nickel, cobalt, iron [17],
germanium [18], iron nitride [19], phosphorus [20] and sulfur [21] have been used in
the HPHT synthesis of diamond crystals. By using a catalyst, the diamond formation
was observed at P > 5 Pa and T > 1500 °C. During the heating at HPHT conditions,
the metal catalyst is melted to form a shell covering each diamond nucleus. Then, the
carbon from the graphite diffuses through the metallic melt to deposit itself onto the
surface of the diamond nucleus, and the diamond particle grows. The slowest step in
the growth of diamond is the diffusion of carbon through the metallic melt [22, 23].
The synthesis of diamond particles is thus a diffusion-controlled process.
Nonmetallic catalysts such as inorganic salts have also been used as solvent–
catalysts in the HPHT diamond growth process, although higher P–T conditions are
required as well as longer reaction times [3–7]. Using alkali carbonates, it was found
that the yield was related to the cation radius in the sequence Li 2 CO 3 > Na 2 CO 3
> K 2 CO 3 > Cs 2 CO 3 [4, 24]. Furthermore, it is known that carbonates are found as
inclusions in natural diamonds [8, 9] and, also, diamonds occur in carbonate-bearing
and carbonate-rich rocks [10]. At the pressure of 7 Gpa and temperature of 1700 °C,
Pal’yanov found that the required time for diamond to nucleate and grow in a splitsphere-type high-pressure cell (Fig. 8.2a) was 2 h [4]. For this, high-purity graphite
(99.99%) was used as the carbon source, and the nucleation of diamond was realized
at the interface between the graphite and carbonate melt. Diamond crystals were
separated from graphite by a carbonate film, demonstrating the catalytic activity of
the carbonate molten salts. The diamond growth on seeds was also performed through
the carbonate film. The diamond crystallization scheme is exhibited in Fig. 8.2b. The
diamond crystals formed in Li 2 CO 3 –C and Cs 2 CO 3 –C systems are shown in Fig. 8.2c
and d, respectively, exhibiting octahedral, cubic and trapezohedral shaped crystals.
The diamond nucleation on the graphite, diamond seeds and the colder part of the Pt
capsule was found to take place subsequently.
143
8.1 Transformation of Graphite into Diamond
Since diamond is obviously far more valuable than graphite, the transformation of
graphite to diamond has been subjected to a large number of studies [2–14]. Diamond
crystals in nano- and micrometer size range have myriad applications in abrasives,
cutting and polishing tools, biomedicine, nanocomposites, and as seeding material
for the growth of diamond [13, 15].
The phase diagram of carbon (Fig. 8.1c) shows that diamond is the stable phase
if the pressure is beyond 1.7 GPa at room temperature although, once formed, it
is stable under ambient conditions. The industrial production of diamond crystallites is currently achievable by applying either a combination of enormous external
pressure and heat on graphite or graphite–catalyst systems or detonation of explosive
carbonaceous materials. The solid–solid phase transformation of hexagonal graphiteto-cubic diamond can be achieved by the so-called high pressure, high temperature
(HPHT) method at P > 12 GPa and T > 2000 °C [16]. Graphite can alternatively be
dissolved in a catalyst from which the solute carbon atoms reprecipitate as diamond
at slightly less severe conditions. Various catalysts such as nickel, cobalt, iron [17],
germanium [18], iron nitride [19], phosphorus [20] and sulfur [21] have been used in
the HPHT synthesis of diamond crystals. By using a catalyst, the diamond formation
was observed at P > 5 Pa and T > 1500 °C. During the heating at HPHT conditions,
the metal catalyst is melted to form a shell covering each diamond nucleus. Then, the
carbon from the graphite diffuses through the metallic melt to deposit itself onto the
surface of the diamond nucleus, and the diamond particle grows. The slowest step in
the growth of diamond is the diffusion of carbon through the metallic melt [22, 23].
The synthesis of diamond particles is thus a diffusion-controlled process.
Nonmetallic catalysts such as inorganic salts have also been used as solvent–
catalysts in the HPHT diamond growth process, although higher P–T conditions are
required as well as longer reaction times [3–7]. Using alkali carbonates, it was found
that the yield was related to the cation radius in the sequence Li 2 CO 3 > Na 2 CO 3
> K 2 CO 3 > Cs 2 CO 3 [4, 24]. Furthermore, it is known that carbonates are found as
inclusions in natural diamonds [8, 9] and, also, diamonds occur in carbonate-bearing
and carbonate-rich rocks [10]. At the pressure of 7 Gpa and temperature of 1700 °C,
Pal’yanov found that the required time for diamond to nucleate and grow in a splitsphere-type high-pressure cell (Fig. 8.2a) was 2 h [4]. For this, high-purity graphite
(99.99%) was used as the carbon source, and the nucleation of diamond was realized
at the interface between the graphite and carbonate melt. Diamond crystals were
separated from graphite by a carbonate film, demonstrating the catalytic activity of
the carbonate molten salts. The diamond growth on seeds was also performed through
the carbonate film. The diamond crystallization scheme is exhibited in Fig. 8.2b. The
diamond crystals formed in Li 2 CO 3 –C and Cs 2 CO 3 –C systems are shown in Fig. 8.2c
and d, respectively, exhibiting octahedral, cubic and trapezohedral shaped crystals.
The diamond nucleation on the graphite, diamond seeds and the colder part of the Pt
capsule was found to take place subsequently.