Direct Conversion Of Graphite Research Articles

Nanocrystals of face-centred cubic carbon, i-carbon and diamond obtained by direct conversion of graphite at high temperatures and static ultra-high pressures

Until recently graphite, diamond and amorphous carbon were considered to be the only three carbon modifications of a terrestrial origin present in nature. Recently numerous publications indicated the existence of two other uncommon metastable carbon allotropes - face-centred cubic carbon and i‑carbon – on Earth. Nanoparticles of these carbon allotropes were found in carbon spherules of samples taken from soils and rocks in different countries of Europe and North America, crude oil, ice sheets from Greenland, etc. The existence of these metastable carbon allotropes in nature appeared to confirm the Younger Dryas extraterrestrial impact hypothesis proposing such an impact over North America, that caused a global catastrophe and climate change. This hypothesis suggests an extraterrestrial impact origin of these carbon allotropes due to dynamic shock compression, since to date they have only been simultaneously produced in ultra-high pressures dynamic shock experiments. We report the first synthesis of nanocrystals of the both metastable carbon allotropes along with diamond at elevated temperatures and static ultra-high pressures. This provides evidence that nanocrystals of face-centred cubic carbon and i‑carbon present in nature are likely to form in Earth's mantle, so that they have a terrestrial origin. Results of electron diffraction and high-resolution transmission electron microscopy clearly indicate that face-centred cubic carbon and i‑carbon are characterized by unique crystal lattices, which have nothing common with the diamond crystal lattice. These two carbon allotropes are thought to be characterized by unconventional chemical bonds due to unusual combinations of their crystal lattice types and lattice parameters. The nature of chemical bonds in face-centred cubic carbon and i‑carbon is supposed to be different from that of conventional basic chemical bonds (covalent, ionic and metallic bonding as well as the van der Waals forces). These carbon allotropes can therefore possess unique and potentially valuable properties suitable for different applications.

Natural occurrence of pure nano-polycrystalline diamond from impact crater.

Consolidated bodies of polycrystalline diamond with grain sizes less than 100 nm, nano-polycrystalline diamond (NPD), has been experimentally produced by direct conversion of graphite at high pressure and high temperature. NPD has superior hardness, toughness and wear resistance to single-crystalline diamonds because of its peculiar nano-textures, and has been successfully used for industrial and scientific applications. Such sintered nanodiamonds have, however, not been found in natural mantle diamonds. Here we identified natural pure NPD, which was produced by a large meteoritic impact about 35 Ma ago in Russia. The impact diamonds consist of well-sintered equigranular nanocrystals (5–50 nm), similar to synthetic NPD, but with distinct [111] preferred orientation. They formed through the martensitic transformation from single-crystal graphite. Stress-induced local fragmentation of the source graphite and subsequent rapid transformation to diamond in the limited time scale result in multiple diamond nucleation and suppression of the overall grain growth, producing the unique nanocrystalline texture of natural NPD. A huge amount of natural NPD is expected to be present in the Popigai crater, which is potentially important for applications as novel ultra-hard material.

Wear characteristics of nano-polycrystalline diamond tool in cutting of tungsten carbide

Nano-polycrystalline diamond is a binderless diamond synthesised by the direct conversion of graphite under high pressure and high temperature. This diamond has the same or higher hardness than synthesised monocrystalline diamond and is expected to be an effective material for cutting tools. In this study, the cutting of tungsten carbide was conducted using two monocrystalline diamond tools, a chemical vapour deposition (CVD) diamond tool and a nano-polycrystalline diamond tool to investigate their wear characteristics in terms of the diamond structure and composition. A supply of chemical solution coolant resulted in a stable machined surface, and a negative back rake angle of 30° was shown to provide the minimum tool wear and machined surface roughness. The nano-polycrystalline diamond tool shows excellent wear resistance superior to those of the two monocrystalline diamond tools and CVD diamond tool in continuous cutting as well as better wear resistance than the CVD diamond tool in intermittent cutting.

Experimental Methods of Synthesis of Nano-/Macro Mineral Materials

This work is devoted to development of synthesis methods of nano- and macrofunctional materials for industry, electronics, and modern science and technology. The authors have developed advanced methods of obtaining various carbon, silicate and chalcogenide materials by melt and hydrothermal methods with using of optimized chemical and thermodynamic parameters of synthesis. As a result a new super hard material - carbonate-synthetic diamond polycrystals were obtained. Nanodiamonds were synthesized by direct conversion of graphite into diamond. New species of macro- and fine crystalline piezoelectric tourmaline containing 3d-elements were produced by hydrothermal method. Using a flux technique the single crystals of different chalcogenide phase with unique superconducting properties were grown.

Distinctive mechanical properties of nano-polycrystalline diamond synthesized by direct conversion sintering under HPHT

The mechanical properties of nano-polycrystalline diamond (NPD) synthesized by the direct conversion of graphite under high pressure and high temperature have been investigated. Indentation hardness and bending strength tests revealed that NPD has considerably high hardness and high transverse rupture strength (TRS) at high temperature, far surpassing those of conventional polycrystalline diamond (PCD) and single-crystal diamond (SCD). The hardness remained higher than 100GPa even at 800°C, while the hardness of SCD sharply decreased to 60GPa above 300°C. The TRS remained at about 3GPa up to 1000°C, above which it showed a positive temperature dependence, while the TRS of PCD decreased rapidly at about 500°C. Wear tests using a diamond wheel indicated that the abrasive wear resistance of NPD is equivalent to those of the high wear-resistance directions on SCD, and from 10 to 50 times higher than those of PCD. These results suggest that NPD has outstanding potential for cutting tools.

Nanohardness and Young’s modulus of nanopolycrystalline diamond

We present the measurement of the Young’s modulus ( E ) of a nanopolycrystalline diamond (NPD) sample synthesized by the direct conversion of graphite under high pressure and temperature. E has been measured along with the nanohardness ( H N ) using the nanoindentation technique ( E = 707 GPa and H N = 62 GPa). Comparing E and H N of NPD with other commercially available superhard anvils and polycrystalline diamonds, we demonstrate its greater hardness and elastic properties.

Elastic properties of sintered diamonds with and without Co binder

Elastic properties of sintered diamonds with Co binder and binderless nano-polycrystalline diamond were determined by ultrasonic measurement at ambient condition. Elastic wave velocities of the sintered diamonds increase with increasing diamond content, and the nano-polycrystalline diamond, which was made by direct conversion of graphite and contains no binder, has the elastic wave velocities and elastic moduli significantly higher than those of the sintered diamonds with Co binder. The data suggest a great potential of the nano-polycrystalline diamond for industrial and scientific applications.

B06 バインダレスナノ多結晶ダイヤモンドの機械特性と切削性能(OS3 工具・ツーリング(2))

Single-phase (binderless) nano-polycrystalline diamond synthesized by direct conversion of graphite under high pressure and high temperature has a very fine texture consisting of small diamond grains (several tens of nanometers in size), having extremely high hardness, no cleavage and high thermal stability. In order to clarify its potentiality for industrial uses in cutting tools, the mechanical properties of nano-polycrystalline diamond were systematically investigated. In addition, the cutting performances of the nano-polycrystalline diamond tool for some materials were evaluated. These results revealed that the nanopolycrystalline diamond has significantly higher cutting performance than conventional sintered diamond tools or mono-crystal diamond tools.

高圧直接変換ナノ多結晶ダイヤモンドの微細構造・物性・応用

高圧直接変換ナノ多結晶ダイヤモンドの微細構造・物性・応用

Micromachining and surface processing of the super-hard nano-polycrystalline diamond by three types of pulsed lasers

Laser beam micromachining was applied to super-hard nano-polycrystalline diamond (NPD) synthesized by the direct conversion of graphite at high pressure and high temperature. Three types of pulsed lasers were tested: nanosecond near-infrared, nanosecond near-ultraviolet, and femtosecond near-infrared lasers. The latter two were also applied for synthetic single crystal of diamond to compare the results with those of the NPD. It was demonstrated that the nanosecond near-infrared laser was the most efficient device for rough shaping of the NPD, while the ultraviolet and femtosecond lasers give satisfactory results for precise surface finishing of it. The properties of the laser-processed surfaces were analyzed by scanning and transmission electron microscopy, laser scanning microscopy, and micro Raman spectroscopy. These analyses demonstrated that the three types of lasers play different and complementary roles, and that their combination is the best suitable solution for micromachining of the hardest diamond into any desired shapes.

Origin of unique microstructures in nano-polycrystalline diamond synthesized by direct conversion of graphite at static high pressure

The influence of the local stress state and crystallinity of graphite on the graphite-diamond transformation process was investigated using a laser-heated diamond anvil cell and two types of graphite samples with contrasting crystalline characteristics. The samples recovered from the experiments using highly oriented graphite (HOG) have layered structures composed of lonsdaleite and diamond arranged in a coaxial relation, indicative of their martensitic formation from graphite. The compression of HOG perpendicular to the graphite c axis results in a partial fragmentation of the original layered structure, but no clear evidence of diamond formation by the diffusion process is found even in the fragmented regions. On the other hand, the sample recovered from the experiment using mechanically milled graphite powder consists of euhedral diamond nanocrystals (size: 20-50 nm), indicating that the diamond is formed by the diffusion process. The results of the present study suggest that the graphite-diamond transformation process and the resulting microstructures are controlled mainly by the initial structure characteristics (such as crystallinity and crystallite size) of the graphite, while the influence of the local stress state in the graphite under compression is likely less significant for the formation process and the microtexture.

Characterization of large nano-polycrystalline diamond synthesized by direct conversion of graphite

Characterization of large nano-polycrystalline diamond synthesized by direct conversion of graphite

Formation of pure polycrystalline diamond by direct conversion of graphite at high pressure and high temperature

Formation of pure polycrystalline diamond by direct conversion of graphite at high pressure and high temperature

Formation of pure polycrystalline diamond by direct conversion of graphite at high pressure and high temperature

Polycrystalline diamond aggregates have been synthesized by direct conversion of graphite at pressures of 12–25 GPa and temperatures 1800–2500 °C using a multianvil apparatus. The synthesized diamond was optically transparent and colorless, and was of cubic symmetry as determined by micro-focus X-ray diffraction. TEM analyses revealed that the sample consists of minute crystals of typically 10–20 nm, and only a very weak and broad band at ∼1332 cm −1 was observed by Raman spectroscopy. An indentation test demonstrated that the present polycrystalline diamond possesses a Knoop hardness of up to 140 GPa, which is equivalent to or even higher than those of natural and synthetic single-crystal diamonds (∼60–130 GPa) and nearly twice as high as those of synthetic polycrystalline diamonds containing binders (∼50–70 GPa). The present results imply that natural polycrystalline diamonds may have been formed by a rapid transformation from metastable graphite present in cold subducted crust, when encountering in warmer regions, such as rising plumes in the mantle transition region.

Materials: Ultrahard polycrystalline diamond from graphite.

Polycrystalline diamonds are harder and tougher than single-crystal diamonds and are therefore valuable for cutting and polishing other hard materials, but naturally occurring polycrystalline diamond is unusual and its production is slow. Here we describe the rapid synthesis of pure sintered polycrystalline diamond by direct conversion of graphite under static high pressure and temperature. Surprisingly, this synthesized diamond is ultrahard and so could be useful in the manufacture of scientific and industrial tools.

超高圧下でのダイヤモンド直接変換合成

Polycrystalline diamond was successively synthesized recently at very high pressure and temperature using a Kawai-type apparatus, which turned out to be extremely hard when compared with currently available sintered diamond products and natural single crystal diamonds. Here the author summarizes the results of experiments and reviews aspects of the synthesis of polycrystalline diamonds by the direct conversion of graphite. Some future perspectives and related unsolved problems are also discussed on the basis the results of our and other experimental studies on the synthesis of polycrystalline diamond.