How does graphite turn into diamond?

Historical facts of finding natural diamonds in kimberlite pipes indicate that diamonds are formed in the bowels of the Earth, where there are high temperatures and pressures. Since it is high pressure that favors an increase in density (Le Chatelier’s principle), graphite under these conditions (its density is 2,25 g/cm3) transforms into diamond (diamond density is 3,51 g/cm3). Early attempts (from the 20s of the 1893th century) to obtain diamonds from graphite and other carbon-containing substances were based only on the effects of high pressure without any serious calculation, so they ended in failure. One of the first attempts to synthesize diamond, which went down in history, was made by Professor Henri Moissan in 3. By that time, sufficient information had already been accumulated about the structure and hypothetical conditions of synthesis: – diamond, graphite, carbon – chemically identical substances, therefore, diamond can be obtained from graphite; – the specific gravity of diamond is greater than that of graphite, therefore, its compression is necessary to obtain diamond; – diamond in nature is born in the depths of the Earth, where high temperatures and high pressures reign, therefore, in addition to temperature, high compression pressure is required; – diamond is found in stone and iron meteorites, therefore, iron must be a suitable environment for the appearance of diamond. Unlike other metals, when iron supersaturated with carbon is cooled, its volume does not decrease, but increases. Therefore, if it is quickly cooled, the frozen outer shell will exert a compressive effect on the inner layers. In addition, Moissan had the highest temperature heating device of that time – an arc furnace. The essence of his experiment was as follows: he melted cast iron in a crucible, and the cooling device was a wooden tub with cold water. After quickly transferring the crucible with the melt into water, an ingot of cast iron was formed. The dissolution of the ingot in acids continued for several weeks and as a result, several grains with a specific gravity of 1953 g/cmXNUMX remained. They left scratches on ruby ​​and corundum. Attempts by Moissan’s students and followers to obtain diamonds using his method, as well as the search for diamonds in cast iron, were unsuccessful, since it was unknown at what temperatures, pressure and for what time the synthesis occurs. In Russia, a successful attempt to synthesize alma was made according to the Moissan scheme in XNUMX by Professor K. Hru Shchev, who instead of iron used silver, which expands when cooled, like iron. The scientific study of the process of transformation of carbon-containing substances into diamond began with the construction of a phase diagram. The existence of carbon modifications is determined by the p-T state diagram (pressure – temperature), represented by equilibrium curves for all phases that are stable under different conditions. From a thermodynamic point of view, polymorphism is caused by different curves of changes in the thermodynamic potential of possible atomic configurations of a given substance depending on temperature and pressure. Phase transformations occur when some of the possible structures in a certain temperature and pressure range have a minimum thermodynamic potential, and the equilibrium point of modifications (transition point) is determined by the equality of the thermodynamic potentials of these modifications.

Phase transformations graphite – diamond

Graphification* For a one-component system, the condition for phase equilibrium will be the equality of their thermodynamic potentials Gm = G r. During a phase transition, the Gibbs energy, calculated using the Clausius – Clayperon equation AG = E – TAS + PAV, where E is the internal energy of the system, T is the temperature, AS is the entropy, P is the pressure, AV is the volume of the system, must be negative AG < 0. Under equilibrium conditions, the enthalpy of graphite is less than the enthalpy of diamond (Rgr < #al), therefore AG M _ r = — 0. At any temperature AC7Gr-al >0, so the process is impossible:

AC/al _gr = ~ (G^al riSpp) p (Gad ~Vrp) ~~A

“^AN^.

Consequently, P must be negative, and this requires compression of the substance and a change in temperature. The general tendency of phase transitions in solids with increasing pressure is an increase in physical density and packing density or an increase in the degree of symmetry of the structure. The study of the thermodynamic properties of diamond and graphite has shown that at atmospheric pressure and at any temperature, graphite is a more stable modification of carbon than diamond, which under normal conditions is a metastable modification of carbon. The transformation of diamond into graphite does not occur due to the infinitesimal rate of this process (inhibited processes). As the temperature increases, the rate of this process increases, which causes diamond graphitization at T > 1500 K. The transformation of diamond into graphite at atmospheric pressure is an exothermic reaction with a small thermal effect. The enthalpy of the transformation AN diamond -> graphite is 1884 J/(g-atom). One of the first published phase diagrams of carbon transformations was the Roseboom diagram (1901), based on thermodynamic calculations. Tammann later produced a more detailed diagram according to the behavior of carbon known at the time. It shows triple points: graphite – liquid – vapor and graphite – diamond – liquid – in the p – T space, in which graphite and diamond can exist in equilibrium. In 1938, Rossini, based on all known thermodynamic quantities of graphite and diamond (heat capacity, entropy, compressibility, thermal expansion, etc.), quite accurately determined the temperature and pressure at which graphite and diamond exist. Based on the data obtained, a graphite–diamond equilibrium curve was constructed as the geometric location of the points where the differences in the thermodynamic potentials of the two mentioned phases are equal to zero. This equilibrium curve was plotted up to temperatures of about 1000 °C and pressures up to 5 GPa. In 1939 O.I. Leipunsky studied the thermodynamic and kinetic aspects of the problem of diamond synthesis. By extrapolation, the graphite–diamond equilibrium curve was calculated up to temperatures of 2700 K and the phase diagram was given in a very wide p–T range (Fig. 9). It was also assumed that diamonds could be formed by cooling a solution of carbon in a solvent such as molten iron at a pressure of at least 1 GPa. Based on newer thermodynamic data5″, a refined phase diagram of carbon was proposed in 1955. It was concluded that the pressure along the equilibrium line can increase almost linearly with increasing temperature up to the triple point graphite – diamond – liquid carbon. It has been established under what conditions graphite can turn into diamond. However, experiments conducted at low temperatures* * Berman R., Simon F. // Zei 1. Electrochem. – 1955. – Bd. 59. No. 5. – S. 333-338. and very high pressures, gave a negative result. Graphite, compressed at room temperature with a very high pressure of about 40 GPa, did not transform into diamond (Bridgeman experiment). Obviously, creating only thermodynamic favorable conditions for the desired process to occur is not enough; it is necessary to increase the temperature in order to eliminate the inhibition of the reaction. Further studies showed that to ensure a sufficient reaction rate for diamond production, a temperature of at least 1400-1500 K and, therefore, a pressure of 4,5-6,0 GPa are required. It follows from this that we need equipment in which it is possible to create and maintain specific

new	time		the above			Liquid carbon A V/V- 0,05 AV/V=0
pressure		and temperature.
For		building				phase
charts			carbon,			которая
It has	great value for
synthesis		diamond,			necessary
more	data			о	depending on
temperature				melting coal
kind of from pressure.
Application					accordingly
howling equipment allowed
lo study			mechanism			plavle	Rice. 9. Diagram graphite – diamond according to O.I. Lei-
carbon reduction under pressure and
							Punsky: minimal	Crystallization temperature
turning graphite into diamond							graphite diamond: 1 – from	iron solution;
in a wide range of p and T							2 – from solid solution
Due to			Togo,		that	diamond

can be obtained not only from graphite, but also from other carbon-containing materials, it is very important to determine the values ​​of p and T for the conditions of equilibrium coexistence of diamond with such materials. L.F. Vereshchagin determined the conditions for the thermodynamic equilibrium of diamond with graphite, pyrocarbon, glassy carbon, and coke* Taking into account the data on the enthalpy of combustion of these materials, the coefficients of thermal expansion, compressibility, and heat capacity, the values ​​of the thermodynamic potentials for these carbon-containing substances were calculated. Based on the calculations, a phase p – G diagram was constructed, from which the following important conclusions were made.* * Vereshchagin L.F., Yakovlev E.N., Buchnev L.M., Dymov B.K. // Thermophysics of high temperatures. – 1977. – T. 15. – No. 2. – P. 316-321. The graphite – diamond equilibrium line obtained by L.F. Vereshchagin, coincided with that received by I.O. Berman. For other carbon materials, the equilibrium line did not coincide. So, for example, for pyrolytic carbon the equilibrium pressure at a temperature T = 2000 K is 5,4 GPa versus 6,0 GPa for graphite. For coke and glassy carbon, the conditions of equilibrium with diamond differ significantly from the conditions of the graphite–diamond system due to the higher values ​​of the thermodynamic potentials of these substances compared to graphite. The conditions for equilibrium between diamond and these materials are also observed at normal atmospheric pressure. Thus, at p « 0 for the glassy carbon – diamond system the equilibrium temperature is T = 940 K, and for the coke – diamond system the equilibrium temperature is T = 960 K. However, the stable modification of carbon under these conditions is graphite and, therefore, the transformation of the starting substances should be In this case, give mainly the graphite phase. Other authors believe that, in addition to diamond and graphite, regions of thermodynamic stability on the phase diagram also have other structures: various modifications of carbyne, lonsdaleite, structures intermediate in density. A.V. Kurdyumov and A.N. Pilyankevich believe that only hexagonal graphite and diamond have regions of thermodynamic stability at pressures below the transition points to the metallic state. Romboed-P>Pa

Fig. 10.	Generalized phase
diagram		systems		graphite –
dense forms of carbon: 1 –
area	catalytic				pre
rotation graphite –				diamond; 2 –
area	Direct		transformations
graphite –	diamond		or	graphite –
lonsdaleite;		3 –	melting line
metastable				graphite;
4 – hypothetical area of ​​pre
rotation into metal fa
carbon memory;		5 –	area		hit
hardening of graphite from
cutting into diamond and lonsdaleite*
+1000 2000 3000	4000 G, K
* Kurdyumov A.V., Pilyankevich A.N.	// Phase transformations in carbon and bo nitride
r a – Kyiv: Naukova Dumka, 1979. –	188 s.

September 17, 2018 Do you want to turn graphite into diamond? Try it. The first experiments on the direct synthesis of diamonds from graphite were published in 1961 by Jameson and DeCarlne of the American company Allied chemical corporation. In the course of one microsecond, with the help of a strong vibration, a pressure of three hundred thousand atmospheres and a temperature of 1200 degrees Celsius were created. After carrying out the experiment in graphite samples, even fragmentary grains of diamond were found in the form of particles up to 100 angstroms (0,00001 mm) in size, similar to “carbonado” crystallized in meteorites that formed as a result of vibukhov collapsed due to the impact of a meteorite on the surface of the earth. Already in 1963, Francis Bundy and another American company, General Electric, succeeded in synthesizing diamond under a static pressure of one hundred and thirty thousand atmospheres. For what purpose was the Halfbelt installation created and then modified into the “Belt” installation in any heating graphite bar up to 2000 degrees Celsius was generated by pulses of electric current, and the temperature was kept up to several milliseconds, which is significantly lower than that of its predecessors. Francis Bundy was able to synthesize newly created particles three to five times larger than the size of the particles that were synthesized during visbukhov’s compression as a result of the research of Jamison and his colleagues. Two series of experiments provided the basic data for the process showing the pressures and temperatures that allow the synthesis of stable diamonds. It’s a pity that only technical diamonds have been produced so far. These experiments were carried out by Casper and Bandi in Vikoristan monocrystalline graphite. In their first studies, diamond crystals were synthesized into an original cubic crystal lattice. De Carli and Jameson, in their research, noted that the transformation is easier, since the graphite grains are aligned along the C axis, so that they are perpendicular to the graphite balls. When Bandi and Kasper applied pressure to the monocrystal along the C axis, the support of the crystal began to increase under the reached pressure of one hundred and forty thousand atmospheres. This was associated with the transition of graphite into diamond, although when the pressure was removed, a reverse transformation occurred, but after heating from 900 to 2000, crystals of a new phase of the high pressure hexagonal structure were synthesized. After the successful installation of the “Belt” installation in the General Electric company, it was replaced with a tetrahedral type installation, which was fragmented by Hall, in which the pressure was applied only in one area, and on others it was transmitted through the end piston-punchons. In the 1960s, the Radyansky Union at the Institute of High Vise Physics under the supervision of Professor Vereshchagin also developed a technology for holding piece diamonds at a temperature of 2000-2500 degrees Celsius and in a vise 50000 to 100000 atm. The greatest value and great crystals cut in the institute are small in diameter 0,5-0,8 mm Finally, in 1967, Robert Wentorf patented the technology of growing crystals into seeds to extract diamonds from jewelry. The essence of this lies in the placement of the acceptor crystal on the seeds, and the donor crystals on the acceptor crystal, and at a temperature of about 1500 C, the acceptor crystal is consumed by the diamond material of the donor crystals. It should be noted that the more the crystal grows, the more it grows. And in this way it is not possible to polish a great diamond with pleasant skill. If its diameter exceeds 5 mm, then the quality of cutting a piece diamond is equal to the quality of a natural diamond of the same diameter; the largest diamond cut in this way has a diameter of 6 mm. There is also a new method for growing diamonds in methane streams. The essence of this method lies in heating the diamond crystal to a temperature of 1111 C and supplying methane to the jet growth zone under a slight pressure. The growing process takes place on the heated surface of the diamond by distributing methane into water and carbon and adding the cut off carbon into the crystal lattice of the growing diamond. However, it should be noted that when the diamond is overheated to a temperature of 1200 C without access, the oxide will again transform into graphite. The liquidity of the build-up in this manner is not great and amounts to 0,2% of the surface of the fuse per year In this case, the shape of the crystals is cubic, and the color is black, although in value it is equal to natural diamonds. In the industry, the most widely used method is vibrator pressing, which produces a high yield of diamonds for technical needs with an average grain size of 30 to 50 microns. In this case, the greater the pressure of the impact blade, the greater the number of diamonds and the larger their size. In Ukraine, diamonds were minted using this method at the Poltava Diamond Factory, and the main raw material for them was high-purity piece or natural graphite Other articles Graphite has a promising alternative in lithium-ion batteries 07 June 2020 New version of statistics Tambov is developing an environmentally friendly method for removing nanographite May 17, 2020 New version of statistics