Graphitization of diamond powders of different sizes at high pressure–high temperature (original) (raw)
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Journal of Applied Physics, 2000
CN x samples subjected to a high pressure-high temperature treatment were studied by micro-Raman spectroscopy, microhardness, and x-ray diffraction techniques. After treatment the CN x material leads to the formation of some highly ordered diamond crystals, showing an extraordinarily low broadening of the 1332 cm Ϫ1 Raman line (⌬ϭ2.43 cm Ϫ1 ). We propose that the graphite-diamond phase-structural transformation takes place through the formation of rombohedral graphite inclusions in the treated sample and the corresponding effect of puckering of the graphite layers. The observation of the band at ϳ1621 cm Ϫ1 in the Raman spectra of the matrix surrounding the diamond crystals may be considered as an evidence of the proposed model.
Atomistic Evidence of Nucleation Mechanism for the Direct Graphite-to-Diamond Transformation
2021
The direct graphite-to-diamond transformation mechanism has been a subject of intense study and remains debated concerning the initial stages of the conversion, the intermediate phases, and their transformation pathways. Here, we successfully recover samples at early conversion stage by tuning high-pressure/high-temperature conditions and reveal direct evidence supporting the nucleation-growth mechanism. Atomistic observations show that intermediate orthorhombic graphite phase mediates the growth of diamond nuclei. Furthermore, we observe that quenchable orthorhombic and rhombohedra graphite are stabilized in buckled graphite at lower temperatures. These intermediate phases are further converted into hexagonal and cubic diamond at higher temperatures following energetically favorable pathways in the order: graphite -> orthorhombic graphite -> hexagonal diamond, graphite -> orthorhombic graphite -> cubic diamond, graphite -> rhombohedra graphite -> cubic diamond. Th...
Doklady Earth Sciences, 2020
Data on the interaction of the Fe-Ni melt with CaCO 3 and graphite at 5 GPa and 1400°С under the thermogradient conditions used in experiments on the growth of diamond on the BARS high-pressure apparatus are presented. The phase composition and component composition of the fluid captured by diamonds in the form of inclusions were studied by gas chromatography-mass spectrometry (GC-MS). Diamonds were synthesized from graphite. During the interaction of the Fe-Ni melt with CaCO 3 , CaFe oxides and (Fe, Ni) 3 C carbide were formed. The stability of heavy hydrocarbons under the experimental conditions was confirmed. It was established that the composition of the fluid in synthesized diamonds is close to the composition of the fluid from inclusions in some natural diamonds. Nevertheless, it was concluded that crystallization of large diamonds under natural conditions is hardly possible due to the filling of the main crystallization volume with refractory oxide phases.
physica status solidi (RRL) – Rapid Research Letters, 2020
This paper presents experimental evidence on the catalytic role of boron in the conversion of graphite to diamond. Highly ordered graphite with up to 1.5 % of carbon atoms substituted with boron, was obtained from a mixture of nano-globular carbon and amorphous boron at 5.0 GPa and 1400÷1700 °C. It transformed completely into heavily boron doped diamond at pressure of 7.5÷8.0 GPa and temperature of 1600÷1650 °C, which is well below the melting temperature of boroncarbon eutectic. To convert different carbons into diamonds, pressures above 10 GPa and temperatures above 2000 °C are required. In particular, graphite transforms into transparent polycrystalline diamond at 12÷25 GPa and 2300÷2500 °C. [1] At the same time, the pressure and temperature of the synthesis of heavily boron-doped (HBD) diamond can be significantly reduced. For instance, the synthesis of HBD polycrystalline diamonds from a graphite/B 4 C mixture takes place at pressure of 8 GPa and temperatures of 2300÷2500 °C that still exceed the melting point of boron-carbon eutectic. [2] Lower temperatures of boron-doped diamond synthesis are also reported. HBD diamond microcrystals
physica status solidi (a), 2001
Polycrystalline diamond thick films were subjected to annealing in vacuum at temperatures of 1350-1450 C. The films were examined by optical absorption, Raman spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. The formation of amorphous carbon and/or of well-crystallized graphite layers up to 20 nm thick was evidenced along grain boundaries. Intra-granular nanometer-sized graphite islands were also observed, sometimes as transformed micro-twin bands. The diamond-to-graphite transition occurs in such a way that three (111) diamond planes transform into two (0002) graphitic sheets. The internal graphitization causes a severe degradation of the optical quality of the diamond films.
Journal of Applied Physics, 1999
In recent high resolution transmission electron microscopic studies we have found that high temperature vacuum annealing ͑1200-1800 K͒ of ultradispersed ͑2-5 nm͒ and micron size diamond produces fullerene-like graphitic species, namely, onion-like carbon and closed curved graphite structures ͑multilayer nanotubes and nanofolds͒, respectively. Here we undertake theoretical studies to help in the understanding of the experimental data for these systems. ͑1͒ Calculations of cluster models by a standard semiempirical method ͑MNDO a software package͒ are used to explain the preferential exfoliation of ͕111͖ planes over other low index diamond planes. ͑2͒ The same approach suggests the likelihood that the graphitization is initiated by a significant thermal displacement of a single carbon atom at temperatures close to the Debye temperature. ͑3͒ At the diamond-graphite interface we have observed the formation of two curved graphitic sheets from three diamond ͕111͖ planes. We suggest that the evolution of this interface proceeds by a ''zipper''-like migration mechanism with the carbon atoms of the middle diamond layer being distributed equally between the two growing graphitic sheets. ͑4͒ The observed mosaic packaging of closed curved graphite structures during the diamond surface graphitization is suggested to be a self-assembling process. This process is explained in terms of the ''stretching'' of a bowed graphite hexagonal network. The stretch is due to the fact that, if relaxed, the network would be smaller than the initially transformed hexagonal diamond ͑111͒, and to the increased separation between the separated sheet and the surface. The initial phase of the process is studied quantitatively using a molecular mechanics simulation.
Heliyon, 2019
Herein, we describe the multi-step synthesis and characterization of monodisperse cubic-structured nanocrystalline diamond particles, showing that they can be easily prepared from graphite flakes under ambient conditions. The above synthesis features the conversion of graphite flakes into graphene oxide (via a modified Hummer's method) and its subsequent transformation into nanodiamond under the action of ultrasonication-induced cavitation, with the nucleation and growth of nanodiamond particles being strongly influenced by the incorporation of a specific metal oxide spacer material. Overall, the developed method is demonstrated to be superior to conventionally used ones, exhibiting the advantages of simplicity, high yield, and upscaling potential.