Minoru Akaishi

Synthesis of diamond from graphite-carbonate system under very high temperature and pressure

Abstract Although transition metals such as Fe, Co, Ni and their alloys have been used as diamond-producing solvent-catalysts under very high temperature and pressure, non-metallic compounds such as carbonates and oxides have also been claimed in the patents as the catalysts. In the present study, to make clear the catalytic effect of carbonates on the formation of diamond, high pressure experiments were carried out in the mixture of graphite and the carbonates of Li, Na, Mg, Ca and Sr. Diamond could reproducibly be synthesized from graphite in the presence of these carbonates at high pressure and temperature of 7.7 GPa and 2150°C. Although starting graphite was completely transformed to diamond in the presence of the carbonates, no transformation to diamond could be detected from graphite only at the same pressure and temperature condition. Therefore, it can be concluded that the carbonates have strong solvent-catalytic effect on the transformation of graphite to diamond.

Phosphorus: An Elemental Catalyst for Diamond Synthesis and Growth

As diamond-producing catalysts, 12 transition metals such as iron, cobalt, and nickel were first reported by General Electric researchers more than 30 years ago. Since then, no additional elemental catalyst has been reported. An investigation of the catalytic action of group V elements is of great interest from the viewpoint of producing an n-type semiconducting diamond crystal. In the present study, diamond was synthesized from graphite in the presence of elemental phosphorus at high pressure and temperature (7.7 gigapascals and 1800�C). Furthermore, single-crystal diamond was grown on a diamond seed crystal.

Diamond nucleation and growth by reduction of carbonate melts under high-pressure and high-temperature conditions

We report for the first time experimental evidence for the nucleation and growth of diamonds from carbonatitic melts by reduction in reactions with silicon metal or silicon carbide. Experiments were carried out in the CaMg(CO 3 ) 2 -Si and CaMg(CO 3 ) 2 -SiC systems at 7.7 GPa and temperatures of 1500-1800 °C. No graphite was added to the run powder as a carbon source; the carbonate-bearing melts supply the carbon for diamond formation. Diamond grows spontaneously from the carbonatitic melt by reducing reactions: CaMg(CO 3 ) 2 + 2Si = CaMgSi 2 O 6 + 2C in the CaMg(CO 3 ) 2 -Si system, and CaMg(CO 3 ) 2 + 2SiC = CaMgSi 2 O 6 + 4C in the CaMg(CO 3 ) 2 -SiC system. Our results provide strong experimental support for the view that some natural diamonds crystallized from carbonatitic melts by metasomatic reducing reactions with mantle solid phases.

New catalysts for diamond growth under high pressure and high temperature

Diamond has been found to grow from copper, zinc, and germanium when temperatures and pressures in excess of those usually used for growth via conventional catalysts are used. Around their melting temperatures these metals are inert with respect to graphite. However, under the conditions used in this study, namely temperatures of 1600 °C and pressures of 6 GPa, they exhibit catalytic action. The conventional catalysts, which were first discovered by General Electric, act as catalysts immediately after melting in the presence of graphite, and this distinguishes them from the catalysts used in this study which should therefore be placed in a different category. A new model of diamond growth is proposed in order to explain the behavior of these new catalysts.

Crystallization of diamond from a silicate melt of kimberlite composition in high-pressure and high-temperature experiments

In high-pressure and high-temperature experiments (1800- 2200 °C and 7.0-7.7 GPa), diamond crystallized and grew in a volatile-rich silicate melt of kimberlite composition. This diamond has well- developed {111} faces, and its morphologic characteristics resemble those of natural diamond but differ from those of synthetic diamond grown from metallic solvent-catalysts. The kimberlite melt has a strong solvent-catalytic effect on diamond formation, supporting the view that some natural diamonds crystallized from volatile-rich melts in the upper mantle.

Formation process of diamond from supercritical H2O-CO2 fluid under high pressure and high temperature conditions

Abstract The formation process of diamond from supercritical H 2 O–CO 2 fluid was studied using 13 C-graphitic carbon and oxalic acid dihydrate, (COOH) 2 ·2H 2 O, as starting materials under a diamond stable high pressure–high temperature (HP–HT) condition of 7.7 GPa and 1600°C. The exchange reaction between 13 C-graphitic carbon and 12 CO 2 in the supercritical H 2 O–CO 2 fluid, which was first formed by the decomposition of oxalic acid dihydrate, occurred very rapidly and became nearly equilibrated after 6 h. At the same time, graphite was recrystallized and coexistent with the fluid until traces of diamond were first observed after 8 h. All graphite transformed into diamond after 17 h, showing that a considerably long induction time was present for the formation of diamond in this fluid system.

High Pressure Synthesis of Diamond in the Systems of Grahpite-Sulfate and Graphite-Hydroxide

Diamonds were reproducively synthesized from graphite in the presence of Na2SO4, MgSO4, CaSO41/2H2O, Mg(OH)2 and Ca(OH)2 at high pressure of 7.7 GPa and a temperature of 2150°C. Although starting graphite was completely transformed to diamond in the presence of the sulfates or hydroxides, no transformation to diamond could be detected from graphite only at the same pressure and temperature condition. Therefore, we concluded that sulfates and hydroxides have a strong catalytic effect on the transformation of graphite to diamond.

Reaction between carbon and water under diamond-stable high pressure and high temperature conditions

Abstract Reactions between graphite and water were investigated at high pressures of 5.5 and 7.7 GPa and high temperatures from 1200 to 1500°C for a duration of 24 h in a platinum sealed capsule. At temperatures above 1200°C, at both pressures, graphite was recrystallized into well-crystallized flaky crystals. At 7.7 GPa, graphite partly transformed to diamond at 1400°C and almost completely transformed at 1500°C. At 5.5 GPa, no diamond with spontaneous nucleation was formed throughout the temperatures, but growth of diamond was observed on a diamond seed crystal at temperatures above 1300°C. Fluids coexisting with solid carbon were analyzed by a mass spectrometer, and a small amount of CO2 was found to be present with H2O, showing that H2O–CO2 fluid was formed in the above HP-HT conditions by the dissolution of graphite into water.

Nucleation of diamond in the system of carbon and water under very high pressure and temperature

Diamond could be nucleated spontaneously from graphite in high-pressure-high-temperature water condition at temperatures not less than 1900°C at 7.7 GPa. On the other hand, when diamond crystals were present as seeds in the above system, nucleation temperature decreased to 1700°C at 7.7 GPa, where numerous small diamond crystals were heterogeneously nucleated around the seed crystals.

Crystal growth of diamond in the system of carbon and water under very high pressure and temperature

Abstract Diamond grew considerably with a weight increase of about 70% on a natural octahedral diamond of about 2 mg weight embedded in a graphite capsule with a small amount of water at high pressure and temperature of 7.7 GPa and 2200°C for 17 min. Although much graphite was included in the grown diamond, epitaxial growth was realized with the layer growth patterns on the surface and the tightly bonded boundary between the newly grown region and the seed crystal.