Large Diamond Crystals Research Articles

Effect of nitrogen impurity on the dislocation structure of large HPHT synthetic diamond crystals

The real structure of the {111} sectors of large single synthetic diamond crystals with different nitrogen contents has been studied by the selective etching method: (1) nitrogen-gettered diamonds with nitrogen impurity content at a level of 1–2ppm, (2) diamond crystals grown without additives with nitrogen impurity of 180–220ppm and (3) diamonds nitrogen alloyed during growth up to the concentrations of 550–600ppm. All diamond crystals have been grown by the temperature gradient method using the high pressure apparatus of “split-sphere” type (BARS) at T=1350°C, P=5.7GPa and (100) seed crystal. It is found that the density of dislocations increases from 85cm−2 to 4.0×103cm−2 and the total length of planar defects increases from 30 to 300μm/cm2 as the nitrogen content in the crystals increases up to 600ppm. It is shown that partial dislocations are the dominant dislocations (60–90%) in all the crystals studied. A proportion of perfect dislocations increases as the nitrogen content in crystals, and correspondingly, the dislocation density increase. Possible causes for the increase of the extended defects density in the synthetic diamond crystals with the nitrogen impurity concentration rise are discussed.

Large Defect-Free Synthetic Type IIa Diamond Crystals Synthesized via High Pressure and High Temperature

Large high-quality type IIa diamond crystals measuring up to 12 mm in diameter were successfully synthesized by the temperature gradient method at high pressure and high temperature, using high-crystalline-quality (001)-oriented seed crystals, and by controlling the temperature conditions with high precision. The X-ray projection topograph using a synchrotron radiation beam revealed that the large synthetic diamond crystals are characterized by high-crystalline-quality, having no dislocation or stacking fault in the (001) growth sectors in the upper part of the crystals grown on seeds. Diamond crystals containing a large defect free area of 5 ×5 mm2 or more were successfully produced by controlling the temperature strictly on the low-temperature side in the synthesis region for growing type IIa diamond crystals to allow the (001) growth sectors to become dominant.

Effects of a carbon convection field on large diamond growth under high-pressure high-temperature conditions

Large diamond crystals were successfully synthesized by a FeNi—C system using the temperature gradient method under high-pressure high-temperature conditions. The assembly of the growth cell was improved and the growth process of diamond was investigated. Effects of the symmetry of the carbon convection field around the growing diamond crystal were investigated systematically by adjusting the position of the seed crystal in the melted catalyst/solvent. The results indicate that the morphologies and metal inclusion distributions of the synthetic diamond crystals vary obviously in both symmetric and non-symmetric carbon convection fields with temperature. Moreover, the finite element method was applied to analyze the carbon convection mode of the melted catalyst/solvent around the diamond crystal. This work is helpful for understanding the growth mechanism of diamond.

Large Defect-Free Synthetic Type IIa Diamond Crystals Synthesized via High Pressure and High Temperature

Large high-quality type IIa diamond crystals measuring up to 12 mm in diameter were successfully synthesized by the temperature gradient method at high pressure and high temperature, using high-crystalline-quality (001)-oriented seed crystals, and by controlling the temperature conditions with high precision. The X-ray projection topograph using a synchrotron radiation beam revealed that the large synthetic diamond crystals are characterized by high-crystalline-quality, having no dislocation or stacking fault in the (001) growth sectors in the upper part of the crystals grown on seeds. Diamond crystals containing a large defect free area of 5 ×5 mm2 or more were successfully produced by controlling the temperature strictly on the low-temperature side in the synthesis region for growing type IIa diamond crystals to allow the (001) growth sectors to become dominant.

HPHT Growth and IR Characterizaton of Large Diamond Crystals in FeNi-C-FeS Systems

By temperature gradient method (TGM), with Invar alloy as solvent catalyst, FeS and boron as additives, large diamond crystals are grown under high pressure and high temperature (HPHT) of about 5.4GPa and 1550K. With the content of FeS increased, the colors of grown crystals would be changed from transparent yellow to opaque gray-black, and the quality of diamond crystals could also be destroyed markedly. When both boron and FeS co-doped, the negative effect of FeS on crystal quality could be eliminated somewhat. To a certain content of FeS, with the boron content increased, the crystal color would be changed from opaque gray-black to transparent yellow, and boron content needed is related directly to FeS content in growth systems. The nitrogen content in diamond lattice decreases greatly, with FeS content increased in the growth system, and a 1050cm-1 absorption peak in IR spectrum is also present in diamond crystals grown in FeNi-FeS-C system.

Synthesis of large diamond crystals containing high-nitrogen concentration at high pressure and high temperature using Ni-based solvent by temperature gradient method

This paper reprots that with Ni-based catalyst/solvent and with a dopant of NaN3, large green single crystal diamonds with perfect shape are successfully synthesized by temperature gradient method under high pressure and high temperature in a China-type cubic anvil high-pressure apparatus (SPD-6 × 1200), and the highest nitrogen concentration reaches approximately 1214–1257 ppm calculated by infrared absorption spectra. The synthesis conditions are about 5.5 GPa and 1240–1300 °C. The growth behaviour of diamond with high-nitrogen concentration is investigated in detail. The results show that, with increasing the content of NaN3 added in synthesis system, the width of synthesis temperature region for growth high-quality diamonds becomes narrower, and the morphology of diamond crystal is changed from cube-octahedral to octahedral at same temperature and pressure, the crystal growth rate is slowed down, nevertheless, the nitrogen concentration doped in synthetic diamond increases.

Mechanism of diamond-to-graphite transformation at diamond-stable conditions

The diamond-to-graphite transformation at diamond-stable conditions is studied by temperature gradient method (TGM) under high pressure and high temperature (HPHT), although it is unreasonable from the view of thermodynamic considerations. It is found that, at diamond-stable conditions, for example, at 5.5 GPa and 1550 K, with fine diamond grits as carbon source and NiMnCo alloy as metal solvent assisted, not only large diamond crystals, but metastable regrown graphite crystals would be grown by layer growth mechanism, and the abundance of carbon source in the higher temperature region is indispensable for the presence of metastable regrown graphite crystals. From this transformation, it is concluded that, with metal solvent assisted, although the mechanism of crystal growth could be understood by the macro-mechanism of solubility difference between diamond and graphite in metal solvents, from the point of micro-mechanism, the minimum growth units for diamond or graphite crystals should be at atomic level and unrelated to the kinds of carbon source (diamond or graphite), which could be accumulated free-selectively on the graphite with sp2п or diamond crystals with sp3 bond structure.

Finite Element Analysis of Convection in Growth Cell for Diamond Growth Using Ni-Based Solvent

Thermal-electrical-fluid coupled finite element analyses are performed in the model of the growth cell in a high- pressure and high-temperature (HPHT) cubic apparatus in which the large diamond crystal can be grown by using Ni-based solvent with temperature gradient method (TGM). The convection in the Ni-based solvent with different thicknesses at 1700–1800K is simulated by finite element method (FEM). The experiments of diamond crystal growth are also carried out by using Ni-based solvent at 5.7GPa and 1700–1800K in a China-type cubic high pressure apparatus (CHPA). The simulation results show that the Rayleigh number in the solvent is enhanced obviously with the increasing solvent thickness. Good quality diamond single crystal cannot be grown if the Rayleigh number in the solvent is too high.

Recent Advances in High Pressure Apparatus for Diamond Synthesis

Recent progress of the technology of large-scale high pressure apparatus have permitted us to produce synthetic diamond powders in larger quantities at a lower cost. The advances in high-pressure and high-temperature control technique have enabled the commercialization of high-purity large synthetic diamond crystals. In this article, recent advances and future prospects in the high-pressure apparatus technology for synthetic diamonds are described.

RECENT DEVELOPMENT IN DIAMOND SYNTHESIS

The high pressure-high temperature (HPHT) method led to wide applications of artificial diamonds in industry. Low temperature coating of diamonds has been studied for some time, and chemical vapor deposition has been developed. In this contribution, we will focus on the most recent developments in diamond synthesis, and especially the important progress upon hydrothermal synthesis and the reduction of carbide (HSRC), which has attracted more and more attention due to its great commercial potential. Especially, we will discuss the synthesis of colorless, large diamond crystals from the reduction of carbon dioxide or carbonates. In addition, some recent progress on the theoretical studies of the metastable nucleation and growth of diamonds will also be reviewed. Finally, theories about the genesis of natural diamond were briefly reviewed, and a new theory based on our experimental results was proposed, which suggests that diamond within the Earth could be produced from carbon dioxide wherever carbon dioxide existed and the conditions (e.g., temperature, pressure, and reducing environment) are satisfied.

Revealing of planar defects and partial dislocations in large synthetic diamond crystals by the selective etching

Etching of synthetic diamond single crystals in NaNO 3 and KNO 3 melts at 750–800 °C has been carried out to study linear etch figures and related etch pits. Two types of planar defects in {1 1 1} planes in large synthetic diamond crystals have been found from the structure of linear etch figures. They are microtwin lamellae and stacking faults. It is established that linear etch figures, bound by the etch pits with α (1 1 1) =3° are formed at the emergence lines of stacking faults. The linear etch figures, bound by the etch pits with α (1 1 1) =2° or showing no etch pits are formed at the emergence lines of the microtwin lamellae or double stacking faults. Possible types of partial dislocations bounding stacking faults and triangular pits bounding the linear etch figures have been correlated. The analysis has shown that shallow etch pits with inclination angles α (1 1 1) =2° and 3° are formed at the emergence points of partial dislocations and etch pits with α (1 1 1) =4° at the emergence points of screw dislocations.

Are Archean lithospheric keels inverted?

Post-tectonic granite “blooms” in many Archean cratons manifest widespread crustal melting that cannot be explained by lithosphere removal, considering the long-term buoyancy of Archean cratonic keels and low mantle geotherms indicated by Archean diamondiferous lithosphere. Rather, heat may have been transferred from the mantle through a process of lithosphere inversion, driven by basal tractive forces acting on gravitationally unstable lithosphere. The top-heavy cell comprises eclogitic lower crust ( ρ = 3500 kg/m 3) lying above depleted mantle lithosphere ( ρ = 3300 kg/m 3), whose aggregate density remained less than that of surrounding asthenosphere ( ρ = 3340 kg/m 3). Parameterized numerical models of the inversion process show a > 40 m.y. pulse of maximum 1060 °C temperatures in the lower crust, sufficient to drive anhydrous melting, when overturned basal lithosphere reaches the Moho. In equilibrating to lower lithosphere conditions, the eclogitic cap may have yielded siliceous melts that infiltrated overlying, previously depleted mantle, producing the high Si/Mg characteristic of some cratonic peridotites. P–T paths calculated for the central part of the inverting cell criss-cross the graphite-diamond boundary, explaining development of large diamond crystals through numerous growth increments. The craton stabilization process follows terminal tectonism by about 50 m.y. as a result of initial cooling, eclogite formation and lithosphere stiffening. Related consequences in the crust include low-pressure regional metamorphism and widespread hydrothermal effects including some gold mineralization. Craton stability is attributed to the high strength of the depleted mantle lithosphere, its rectified density profile, and the presence of refractory compositions at the lithosphere–asthenosphere boundary.

Grow Large High-Quality Diamonds with Different Seed Surfaces

Large high-quality type Ib diamond crystals have been grown withdifferent seed surfaces by temperature gradient method at 5.5 GPa,1500–1600 K, with NiMnCo alloy as the metal solvent. Compared with{100} as the growth surface, the growth region of large high-qualitydiamond crystals with {111} as the growth surface at a higher growthrate shifts markedly from lower temperatures (suitable for{100}-facet growth) to higher temperatures (suitable for{111}-facet growth). However, regardless of different growthsurfaces, {100} or {111}, the grown crystals of sheet-shaped shapeare most difficult for metal inclusions to be trapped into, and whetheror not matched growth between the seed surfaces and the growthtemperatures determines the crystal shapes. In view of the growthrates, large high-quality diamond crystals of sheet-shaped shapes canbe grown at a growth rate of above 2.5 mg/h, while the growth rate oflarge high-quality diamond crystals should not be beyond 1.5 mg/h fortower-shaped crystals.

Numerical analyses of a microwave plasma chemical vapor deposition reactor for thick diamond syntheses

We have carried out simulations of microwave plasmas inside a reactor for thick diamond syntheses. In a model reactor used in the calculation, a diamond substrate with finite thickness and area is taken into account. Distributions of electric field, density of microwave power absorbed by the plasma, temperatures and flow field of gas have been studied not only in a bulk region inside a reactor but also a local region around the substrate surface. Numerically predicted distributions of (1) microwave power density, (2) temperature on the top surface of the substrate, and (3) gas flow around the substrate imply that the adopted arrangement of the substrate is not desirable for continuous growth of large diamond crystals.

Effect of Regrown Graphite on the Growth of Large Gem Diamondsby Temperature Gradient Method

Generally, when growing high-quality large gem diamond crystals bytemperature gradient method under high pressure and high temperature,the crystal growth rate is only determined by the temperature gradient.However, we find that the seed crystal cannot completely absorb all thediffused carbon sources, when growing gem diamonds under a highertemperature gradient. Other influence factors appear, and the growthrate of growing diamonds is partly dependent on the crystalline form ofsuperfluous unabsorbed carbon source, flaky regrown graphite or smalldiamond crystals nucleated spontaneously. The present form isdetermined by the growth temperature if the pressure is fixed.Different from spontaneous diamond nuclei, the appearance of regrowngraphite in the diamond-stable region can retard the growth rate of gemdiamonds substantially, even if the temperature gradient keepsunchanged. On the other hand, the formation mechanism of metastableregrown graphite in the diamond-stable region is also explained.

Diamond growth on an array of seeds: The revolution of diamond production

The consumption of saw diamond grits is a measure of a nation's constructional activities. The per capita consumption for the world is about 0.7 carats in 2004, and in China, about 3 carats. The manufacture of large saw diamond grits requires stringent control of pressure and temperature that only a few companies can master. However, with the implementation of a novel diamond seeding technology, large saw diamond grits of extreme quality can be mass produced. With this breakthrough, the prices of saw grit will plummet in the near future that should benefit the constructional industry worldwide. Moreover, electronic or thermal grade of large diamond crystals may be produced for applications in semiconductor, electronic or optical industry.

Reply to Comment on “Growth of Large Diamond Crystals by Reduction of Magnesium Carbonate with Metallic Sodium”

Reply to Comment on “Growth of Large Diamond Crystals by Reduction of Magnesium Carbonate with Metallic Sodium”

Reply to Comment on “Growth of Large Diamond Crystals by Reduction of Magnesium Carbonate with Metallic Sodium”

In reply, some of the large diamonds synthesized in CO2/Na and MgCO3/Na systems are irregular in shape (see picture), and therefore different from diamonds made by high-pressure high-temperature techniques or by chemical vapor deposition. In addition, the Raman spectra of diamonds prepared from the new systems have features that do not appear in those of diamonds obtained by other techniques, thus indicating that the diamonds have different origins.

Diamante variedade carbonado na serra do Espinhaço (MG/BA) e sua enigmática gênese

Carbonado, uma variedade policristalina do diamante, tem sua gênese alvo de intensas controvérsias. A discussão das principais hipóteses propostas para a questão, em associação com o enquadramento geológico das sequências pré-cambrianas que portam o material na serra do Espinhaço, parece favorecer uma dessas hipóteses. Na mesma, os microcristais que compõem o carbonado se formariam ordinariamente no manto, seriam trazidos para a crosta junto com os diamantes monocristalinos encontrados nessas sequências e, em estágio posterior, se agregando num ambiente sedimentar rico em material radioativo.

26aC12 Present state and future issues of growth technology of high-purity large diamond crystals(NCCG-34)

26aC12 Present state and future issues of growth technology of high-purity large diamond crystals(NCCG-34)