The laboratory confirmed natural crystallization of diamond from carbonized organic compounds

Abstract

In the paper the conception of the natural diamond growth as a result of the organic compounds reaction in the presence of noncompensated spin was checked. They might have played the essential role in the nucleation of diamond crystals. In the experiments, the partly carbonized phenolformal-dehyde resin instead of graphite was used as a carbon source. The final result of the process of the diamond growth depended on the temperature of resin carbonization as well as on the temperature and pressure of synthesis. The 3,8 GPa pressure, which is less than needed for graphite to diamond transformation in the classic industrial process, was sufficient to produce transparent and colourles diamond crystal of the size up to 0,7 mm.     

Nano-polycrystalline diamond synthesized through the decomposition of stearic acid

ABSTRACT

Here we report a novel route for synthesizing nano-polycrystalline diamond (NPD) using stearic acid (C₁₈H₃₆O₂) as a starting material under high pressure and high temperature. The obtained NPD shows a transparent dark-yellowish color similar to the standard NPD synthesized from graphite and consists of extremely fine diamond grains (～10?nm). The temperature required for the present synthesis of pure transparent NPD is as low as 1000°C at 13 and 17?GPa, which is surprisingly lower than that for conventional NPD synthesis (1800–2000°C). The amorphous-like, extremely poorly crystalline graphite produced by the thermal decomposition of stearic acid likely provides preferential nucleation sites for diamond and significantly lower the activation energy. The removal of volatile components such as H₂O generated through the decomposition from the system is a key to obtain pore-free transparent NPD. Magnesite, MgCO₃ and periclase, MgO can be used as an efficient H₂O remover through the hydration reaction.     

Effect of Carbon Source with Different Graphitization Degrees on the Synthesis of Diamond

Using three kinds of graphites with different graphitization degrees as carbon source and Fe-Ni alloy powder as catalyst, the synthesis of diamond crystals is performed in a cubic anvil high-pressure and high-temperature apparatus （SPD-6 × 1200）. Diamond crystals with perfect hexoctahedron shape are successfully synthesized at pressure from 5.0 to 5.5GPa and at temperature from 1570 to 1770K. The synthetic conditions, nucleation, morphology, inclusion and granularity of diamond crystals are studied. The temperature and pressure increase with the increase of the graphitization degree of graphite. The quantity of nucleation and granularity ofdiamonds decreases with the increase of graphitization degree of graphite under the same synthesis conditions. Moreover, according to the results of the M6ssbauer spectrum, the composition of inclusions is mainly Fe3 C and Fe-Ni alloy phases in diamond crystals synthesized with three kinds of graphites.     

Structural changes in graphite-to-diamond transition on the P-T link of minimum concentration jump in carbon

Abstract

Structural aspects of the direct graphite-diamond transition have been studied along with the reaction P-T-hysteresis arising during phase transformations in carbon. The direct transition has been found to be a high-molecular reaction of cross-linking of two-dimensional graphite networks resulting in their transformation into three-dinensions1 covalent networks of diamond. Crystal-oriented barochemical reaction of cross-linking is described by two fundamental matrices: the matrix of atomic coordinates transformation and the matrix of atomic concentration Jump. A method of hi&-molecular thermomechanics developed in this paper is used to discuss P-T-hysteresis that was experimentally observed in, the direct and inverse transitions in carbon.     

Bulk diamond formation from graphite in the presence of C-O-H fluid under high pressure

Abstract

C-O-H fluids have been successfully applied as catalysts for bulk diamond formation under high pressure. New insight into C-O-H fluids extends the understanding of the origin of natural diamond, which is presently of interest in materials and geological sciences. This review presents current literature data concerning the synthesis and characterization of bulk diamond formation assisted by C-O-H fluids at high pressure and high temperature. Based on a general survey of this subject, the pressure-temperature regime for diamonds formed in these fluids was established and the mechanism of conversion from graphite to diamond is discussed. Finally, a few questions are put forward that may be useful for the continued development of this research area.     

再结晶石墨对金刚石成核的影响

 本文结合实验现象，分析了在合成初期，碳源在溶剂-触媒金属中的分散，溶解及形成再结晶石墨的过程。并根据再结晶石墨与金刚石成核量的实验结果，初步定性地研究了膜生长法中，再结晶石墨对金刚石成核的影响。     

Novel techniques for the prepartion of synthetic diamond

Abstract

The objectives of this work are two fold: (1) to study the effect of using oxygen-acetylene flame grown synthetic diamond as seed crystals for the high pressure-high temperature conversion of graphite into diamond and (2) to demonstrate the ability to produce small crystallites of diamond by a simple, electron beam evaporation technique. In each case, the production of diamond from graphite was confirmed.     

Effect of thermobaric treatment on crystallinity of carbon materials

Abstract

The process of ordering of different carbon materials in the presence of solvent metal Mn₆₀-Ni₄₀ under the pressure of 3.9 GPa and temperature of 1250–1370° C was studied. The rearrangment of imperfect materials structure into the graphite one occurred through an intermediate layer modification together with diamond formation, the intensity of the latter being significantly higher on carbon materials having the most imperfect structure.     

Enhanced diamond nucleation on copper substrates by graphite seeding and CO2 laser irradiation

Diamond nucleation on copper (Cu) substrates was investigated by graphite seeding and CO₂ laser irradiation at initial stages of the combustion-flame deposition. A graphite aerosol spray was used to generate a thin layer of graphite powders (less than 1 μm) on Cu substrates. The graphite-seeded Cu substrates were then heated by a continuous CO₂ laser to about 750 °C within 1 min. It was found that diamond nucleation density after this treatment was more than three times as much as that on the virgin Cu substrates. As a consequence, diamond films up to 4 μm were obtained in 5 min. The enhancement of diamond nucleation on the graphite-seeded Cu substrates was attributed to the formation of defects and edges during the etching of the seeding graphite layers by the OH radicals in the flame. The defects and edges served as nucleation sites for diamond formation. The function of the CO₂ laser was to rapidly heat the deposition areas to create a favorable temperature for diamond nucleation and growth.     

Melting behaviors of carbon underhigh pressures

Abstract

The melting behaviors of graphite and diamond were investigated at pressures up to 25 GPa using flash-heating method. By rapid heating, the metastable graphite was melted in the diamond stable P-T field, competing with its conversion to diamond in the rate of reaction. For the diamond the pressure dependence of inserted energy required to reach the molten state suggested that the melting temperature of diamond increases with pressure.     

Electronic structure of graphite nanopipes

A recursion method is used for calculating the electron-state density n(E) of carbon in diamond, graphite, C₆₀ fullerene, and graphite nanopipes of various structures and diameters. The calculated n(E) for diamond, graphite, and fullerene are compared with experimental data. The distinctive features of the electron-state density n(E) in graphite nanopipes are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1118–1121 (June 1997)     

Monocrystalline on seed diamond growth into HP/HT lpe autoautoclaves

Abstract

In the paper Authors analyse the possible ways of phase nucleation and the mechanisms of crystal growth, which suggest that, if we use as sp³ orbitals organized carbon source, diamond nucleation and growth by the way of spiondal decomposition and volumetic coalescence may take place. Such a process may be analysed as semihydrothermal-metalotheric coalescence may take place. Such a process may be analysed as semihydrothermal-metalothermal high pressure liquid phase epitaxy (MHPLPE) in separated autoautoclaves.     

Electron microscopy analysis of debris produced during diamond polishing

This paper deals with an analysis of debris produced during the polishing of diamond. The debris is carefully collected 'as ejected' to shorten the history of the freshly removed material. Using high-resolution electron microscopy as well as electron-energy-loss spectroscopy, the structure of the material is revealed and analysed in terms of density, percentage of sp 2 hybridized carbon, and oxygen content. Debris from polishing in the so-called hard and soft directions were involved in this investigation. Overall the structure of all debris is amorphous carbon. The material appears to be composed of small clusters, some nanometres in diameter, in which the graphite basal planes can be recognized. Very few and very small nanometre-sized diamond particles were found in the debris from polishing in the hard direction. The results support a polishing mechanism based on a mechanically induced transformation of diamond to graphite, after which material removal easily occurs. The well-known anisotropy observed in polishing can be explained satisfactorily on the basis of this model. Finally, in appendices, the art of polishing and the role of the black powder during preparation of the scaife are discussed.     

Diamond-like amorphous carbon and nanocrystalline diamond obtained by detonation method

Abstract

The diamond-like amorphous carbon phase was obtained by means of detonation compression of mixtures of an explosive with soot or graphite. The pycnometric density of powders after compression and treatment in boiling acids achieved 3.42 g/cm³. The X-ray and TEM study showed that fine-grained crystalline and nanocrystalline diamond was also present in the samples in addition to the diamond-like amorphous phase.     

Structure of carbon dendrites obtained in an atmospheric-pressure gas discharge

The influence of growth conditions on the carbon dendrite structure has been investigated. The threshold values of the ratio between electron temperature T _e and kinetic temperature T of the gas near a needle electrode and of the discharge current density, which are necessary for dendritic growth, have been determined. It has been shown that the hexagonal structure of submicron carbon particles arises when a number of hydrocarbons are used to synthesize dendrites. It has been found that the degree of order in the carbon structure can be controlled by applying external actions at the stage of graphite particle nucleation. The characteristic frequencies of inertial actions that may be energetically appropriate must exceed 10 kHz.     

Synthesis of nano-polycrystalline diamond from glassy carbon at pressures up to 25 GPa

ABSTRACT

Nano-polycrystalline diamond (NPD) with various grain sizes has been synthesized from glassy carbon at pressures 15–25?GPa and temperatures 1700–2300°C using multianvil apparatus. The minimum temperature for the synthesis of pure NPD, below which a small amount of compressed graphite was formed, significantly increased with pressure from ～1700°C at 15?GPa to ～1900°C at 25?GPa. The NPD having grain sizes less than ～50?nm was synthesized at temperatures below ～2000°C at 15?GPa and ～2300°C at 25?GPa, above which significant grain growth was observed. The grain size of NPD decreases with increasing pressure and decreasing temperature, and the pure NPD with grain sizes less than 10?nm is obtained in a limited temperature range around 1800–2000°C, depending on pressure. The pure NPD from glassy carbon is highly transparent and exhibits a granular nano-texture, whose grain size is tunable by selecting adequate pressure and temperature conditions.     

Structural transformations in carbon nanoparticles induced by electron irradiation

This paper reviews the electron-irradiation effects in graphitic nanoparticles. Irradiation-induced atomic displacements cause structural defects in graphite lattice forming the basis of carbon nanoparticles such as nanotubes or carbon onions. Defects of the type of non-six-membered rings induce topological alterations of graphene layers. The generation of curvature under electron irradiation leads to the formation of new structures, such as spherical carbon onions or coalescent nanotubes. At high temperatures, the self-compression of carbon onions can promote the nucleation of diamond cores or phase transformations of foreign materials that are encapsulated by onionlike graphitic shells. Under the nonequilibrium conditions of intense irradiation, the phase equilibrium between graphite and diamond can be reversed. It is shown that graphite can be transformed into diamond even if no external pressure is applied. All electron-irradiation and electron-microscopic studies described here were carried out using in situ transmission electron microscopy.     

Phase Diagrams and Synthesis of Diamond

For reasons of phase equilibria, the lowest temperatures T min , above which at high pressures the diamond crystallization from melt solutions is allowable in terms of thermodynamics, have been found for a number of metal-carbon systems. In the Ta-C and Nb-C systems, the diamond synthesis is possible at temperatures below T min , while to synthesize diamond in the Mg-Zn-C system, the temperatures much higher than T min , are required because of the necessity to overcome the kinetic difficulties.     

用过剩压法生长金刚石过程石墨再结晶现象的研究

 描述了在过剩压驱动下金刚石晶种外延生过程中，大量伴生的石墨再结晶现象。再结晶石墨抑制了金刚石的自发成核；它们分布于合成腔触媒金属的低温区，结晶数量多，晶粒片状分层，尺寸大，但出现乱层晶体结构；同时产生一定数量的无定形碳。分析认为，这与长时间的低过剩压驱动，触媒金属内有足够的碳源供给，并具备在高温高压下石墨充分结晶但又达不到完全石墨化条件有关。还讨论了在低过剩压驱动下，促进金刚石晶体外延生长的碳源可能是活化的碳原子，而不是具有乱层结构特征的再结晶石墨。     

Investigation on the formation of lonsdaleite from graphite

Structural stability and the possible pathways to experimental formation of lonsdaleite—a hexagonal 2H polytype of diamond—have been studied in the framework of the density functional theory (DFT). It is established that the structural transformation of orthorhombic Cmmm graphite to 2H polytype of diamond must take place at a pressure of 61 GPa, while the formation of lonsdaleite from hexagonal P6/mmm graphite must take place at 56 GPa. The minimum potential barrier height separating the 2H polytype state from graphite is only 0.003 eV/atom smaller than that for the cubic diamond. The high potential barrier is indicative of the possibility of stable existence of the hexagonal diamond under normal conditions. In this work, we have also analyzed the X-ray diffraction and electron-microscopic data available for nanodiamonds found in meteorite impact craters in search for the presence of hexagonal diamond. Results of this analysis showed that pure 3C and 2H polytypes are not contained in the carbon materials of impact origin, the structure of nanocrystals found representing diamonds with randomly packed layers. The term “lonsdaleite,” used to denote carbon materials found in meteorite impact craters and diamond crystals with 2H polytype structure, is rather ambiguous, since no pure hexagonal diamond has been identified in carbon phases found at meteorite fall sites.